Vinyl Record Display

Retractable Banner Stands

If you are looking into investing in a banner for your marketing campaigns, and to enhance your presence at trade shows, you will certainly want to consider, if you have not done so already, the retractable banner stand, aka the pull-up or roll-up stand.

Here are 3 reasons why the Retractable Banner Stand is as Popular as it Is:

The Retractable Banner is ‘Retractable’

The no. 1 selling feature of the retractable banner stand is quite simply that it’s retractable.  Retractable means that it is easy to set up and store.  Retractable makes for portable.  The last thing you want to do is attend a marketing event, with a display the size of a flat tv screen that you have to carry in, with your briefcase.

Retractable Banner Stands are Inexpensive

Just think of the marketing alternatives for a moment.  You’ve got the traditional business cards, fliers, posters, newsletters, samplers, to name a few.  None of these are likely to attract a prospect, with the exception of the samplers, long enough, for a real business dialogue to take place.  Flyers make it, with record time, into briefcases.  Business cards often get slipped into a wallet without a second glance.  And the samplers, all too often, are consumables.  The banner stand invites a conversation around a business need, and it’s this dialogue that will be memorable and make and close deals.

Retractable Banner Stands are a One-Man Show

If your company is marketing your products or services, your no. 1 goal is likely to generate revenue.  Maybe you cannot spare, several of your staff to man a booth at a trade show, and if this is the case, the banner stand can serve as an extra body.  And you don’t need to send your most technically-inclined staff member to set up the your banner stand.  After all, the Retractable Banner Stand, is known to be user-friendly and to only take under a minute to be set up.  It is of interest to note that retractable banner stands are often motorized.  The in-built motor serves to scroll the banner in a loop.

If you set up an appointment with a Banner Stand Specialist, you will want to talk about extension poles, telescoping legs, quick-change banner rails, and replacement banners.  It goes without saying that you’ll need to talk about the height and weight of your banner stand, as well as the ideal thickness of the banner vinyl.  Don’t forget as well to ask about the custom carrying cases.

http://www.skdisplays.com.au Visit website to know more about Roll Up Banner Stands and Scrolling Banner Stands

The Varied Use Of A Label

Where would we be without the simple label. In todays world we rely on information at our fingertips and labeling has become an integral part of everyday life, informing us with all the informtation we require. Below we look at a few uses we have for the humble label (or if you prefer stickers):
Product Labeling: Labels are used extensively for all products. Without a label a product would not be able to illustrate its function or allow you to use it. Commonly labels are used for membrane switch panels on your television remote, or a visual sign of what make your fridge is. Every product you and I buy will be labeled in some way, whether it is a typical vinyl label displaying a company logo or a direct printed label to the surface of the product – product labeling is essential.
Food Labels: How would we recognise the food we wish to buy without labels and labeling. We are guided by our choice of food by the colourful labeling and packaging that todays food is wrapped in. Not only does it convey the brand we are buying but also delivers the required information such as ingredients and health requirements. Furthermore, labels have made purchasing food considerably easier since the invention of barcodes and their subsequent use across almost every food product.
Industrial Labels: All industry use some sort of labeling systems. Whether it is a hazard warning label to indicate potential hazards or way-finding labels to direct people to the required information or place, they all are important. Perhaps the greatest use of labels within industry would be the instructional label, in forming the user of the particular use of a machine or how exactly to fulfill a task.
Packaging Labels: Without packaging labels, none of us would receive the goods we purchase for delivery. Labels are essential for guiding the postman or courier to deliver to the correct address and keep the consignment in a good condition. Without the address label how would the delivery address be clearly conveyed and without clear labeling indicating for instance a fragile consignment we all would receive broken and damaged goods.
Laboratory Labels: The medical profession rely heavily on labeling, especially laboratories. Labs generally label everything so they can track and keep a record of all samples and testing material. Without barcode labels and other thermally printed labels the medical profession would struggle to operate.
Todays world is saturated by labels but over the years the production of labels has modernised and now we are capable of printing onto almost any substrate with a host of printing possibilities – including screen printed labels, hot foil printing, thermally printed labels and possibly the greatest advancement in modern times… digitally printed labels.

Sony Ericsson W960i and Nokia N82, Media Marvels

Let’s check out 2 fabulous media phones, the Sony Ericsson W960i and Nokia N82.

The vinyl black coloured casing of the Sony Ericsson W960i gives it an extremely stylish look and feel. This music phone has a whopping 8 Gbytes of memory and can store up to 8,000 songs that you can carry with you wherever you go. It is one feature that is surely going to give the iPods a run for their money. The large and lucid screen measuring 2.6 Inches provides a screen resolution of 240 x 320 pixels which displays up to 262k colours. The viewing pleasure is enhanced through this wide screen that also acts as a viewfinder for its camera.

The 3.2 megapixel camera with 3 x digital zoom comes adorned with all the features that a stand alone digital camera possesses. It can click great quality stills and well as moving pictures that can be cherished by the user later on. The Sony Ericsson W960i comes with a built in Walkman music player that supports album art which can be displayed on the large touch screen of the phone. Be it your favourite music channel or information from around the world, the Fm radio in the handset fills your day with fun and entertainment.

Next up is the Nokia N82.

Nokia is going great guns these days with its spectacular handset called the Nokia N82. It’s a mighty product with an enormous assortment of features. The Nokia N82 has a sexy, shiny body, as well as a bright and clear 16 million colour TFT screen with screen resolution of 240 x 320 pixels. With an integrated 5 megapixel autofocus camera, the phone delivers awesome stills and videos under any light condition. Music quality is at par with Nokia standards.

The integrated music player not only supports a a variety of formats (MP3, AAC, AAC+, eAAC+ and WMA) but also offers crystal clear sound output. Connectivity is brilliantly controlled via 3G, Pop Port, EDGE, Bluetooth, USB 2.0, and WLAN Wi-Fi. Other notable features in the phone are GPS Navigation, voice commands, document viewer, push to talk, second camera (CIF), voice recording, voice dialing, handsfree speaker, Nokia Music Manager etc.

Both the Sony Ericsson W960i and Nokia N82 are available for pre-order now.

Get A Free Car Or Get Paid For Driving Your Car

Advertising is a multi-billionaire business. Ads could be seen anywhere nowadays; in the subway, on the TV, on the public transportation, even on the benches of the pubic parks.

In 2008 anyone could take part in the advertising and make an extra income.

Several companies across the world offer Free Cars or Getting paid for driving your own car – if you agree to advertise.

So how does it work?

Get free Car. Advertising companies offer brand new cars to almost anyone who is willing to drive – however, there are certain conditions;

- The Car is Covered with advertising decals (fully or partially)
- The Car is paid by the advertiser
- Car’s maintenance is paid by the advertiser
- Gas is paid by the driver
- Insurance is paid by the driver

You have certainly seen BMW’s Mini Cooper advertising Red Bull. Snickers often chooses Jeep to advertise their chocolate bars. In some cases these advertising companies even offer you to drive a Hummer or a very economic Smart.

Get paid for driving your car – the principle is the same as described above. The advertiser covers your car with vinyl decals and you get paid from $200 to $3000 a month for simply driving your car to work or wherever else you drive it.
The decals are relatively easy to take off the car and do not damage your car’s paint.

If you decide to get paid for driving, you must NOT take off the decals and you might be required to drive a minimum amount of mileage on a daily base – which is normal because the ads must be seen – this is the point of advertising.

Unfortunately this opportunity is not available across the globe. Only residents of the United States, Canada, United Kingdom, France and Spain have this kind of service available to them. Certainly advertising companies in other countries will pick up on this practice, but at this moment only the countries listed above offer free cars.

If you want to get free car or get paid for driving a car, you must submit an application to one of the advertisers. The companies usually require a one time small fee of under $50, there are two (2) main reasons for charging the fee; 1) The company wants to make sure they are dealing with a serious applicant, 2) The fee is used to pay off the expenses such as phone calls, mailing costs… etc.

Remember, if the fee is recurring or over $50 – it might be a scam. Serious company will not ask you to keep on paying them so they could eventually pay you – this simply doesn’t make since. The same business model is used at the modeling agencies – you pay a one time fee and the agency distributes your name to all the other agencies who might be interested.

What Can I do to qualify?

If you would like to get a free car or get paid for driving, make sure that:

- You are at least 18 years old
- You have a valid drivers license
- You do not have DUI (Driving Under Influence) or other criminal records
- You are a citizen or permanent resident of USA, Canada, UK, France or Spain.

You must take note that sometimes you do not have a choice of a vehicle but most of the time companies allow you to chose the advertising you would like to display.

Festive Stocking Fillers for Fellas

As Christmas edges slowly towards us, and the sound of those Jingle Bells ringing grows increasingly louder; a divide across the nation begins to widen. On one side, those overly organised people who have already bought all their Christmas gifts, and on the other, a panic-stricken crowd who perhaps got a little bit distracted by the Santa’s Grotto, and are desperately wondering what gifts they could buy for their loved ones this year.

If we go by the stereotypes then buying gifts for men should be quite simple: socks, whiskey, a sporting biography and a DVD. But no one likes to be predictable and whilst these Christmas gifts are all nice and easy to wrap up, what are you supposed to do for stocking fillers?

Buying stocking fillers should be easy when the shops are filled with so many wonderful gifts, some more useful than others. But for anyone who has ever stood still in the middle of a busy high street in mid-December and drawn a blank as to where to go next, then here is a little guide to help you on your way…

Stocking Fillers for the Sophisticated Man

The sophisticated man is a true gent who knows how to appreciate the finer things in life. The choice of stocking fillers can be difficult but it’s always best to indulge them in their little pleasures. If the man in your life likes to cook then you could always treat him to a ‘Grow it…’ gift box. Let him grow his own chilli plants or herbs and these pocket-sized gifts are ideal as stocking fillers. He’s most likely to be an organised sort of chap who likes to keep his things in order. If this sounds familiar, then why not treat him to a ‘Vehicle Document file Folder’ or perhaps go for something a little more personal such as an ‘Engraved City Desk Set’?

Stocking Fillers for the Connoisseur

Christmas morning is the only day of the year when it’s considered okay to have a drop of alcohol with breakfast (a small drop of Bucks Fizz or Champagne perhaps). For the wine-buff in the family then an engraved ’5 Piece Wine Bar Set’ can make a truly personal gift – these superior stocking fillers will out-last any hangover. If he knows his Pinot’s from his Chardonnay’s then you could always pop a ‘Wine Tasting Tour’ voucher inside his stocking – remarkable Christmas gifts that you can enjoy as well.

Stocking Fillers for the Sporting Pundit

So you already have the sporting biography wrapped and under the Christmas tree with the other gifts, why not go for a hat trick and throw in a couple of sporting stocking fillers too? From mugs and cufflinks, to journals and posters, there are a number of gifts, trinkets and keepsakes to honour his favourite sport. But for something a little more special, then opt for those stocking fillers that he’ll never want to throw away. Choose from a variety of personalised calendars that will display your loved one’s name on all twelve sporting-themed images. These unique gifts will make him the envy of all his friends.

Stocking Fillers for the Real Romantic

Christmas can be a very romantic time of year and if you’re lucky enough to have a ‘real romantic’ in your life, this is the time to make him feel special with some ‘luvvy duvvy’ gifts. For those smaller stocking fillers, a ‘Lover Duck’ or even fridge magnet gifts can easily spell out what you want to say. For a real romantic treat then you could present your man with a special record that honours the song of your choice – the first dance from your wedding perhaps or his favourite tune, even the song that was at number one when he was born. He’s bound to be bowled over when you present him with this mounted vinyl and plaque.

Stocking Fillers for The Big Kid

Perhaps the most familiar character of them all, the big kid is certainly catered for when it comes to Christmas gifts and stocking fillers. Whether they be gadgets or toys, the Big Kid is a man who likes to play and is the sort of chap who is usually found ‘showing’ the kids how to play with their gifts on Christmas morning. For handy stocking fillers, how about a ‘Cork Screw Key Ring’ or a ‘Rubber Band Ball’? For the man who likes to play, then indulge him with his favourite toys this year or go for a twist on the traditional ones such as a ‘Devil Duck’ rubber duck that will glow red with fury when he puts it in water – a little bit of fun at bath time!

With so many ideas for gifts and stocking fillers, this year is sure to be a very merry Christmas indeed.

How To Pick An Outdoor Grill

I went into my regional garden centre just to find that the undivided place have been completely re-vamped and an immense topic keen to barbecues and smokers. What did this tell me?……..Two equipment:-

Barbecue grills and outside smokers remain to become more common recreational equipment.There’s money to be made! Let’s face it; the smartest “in your face” displays have to be for the food that have the prime margins, right? Having spoken to the boss I think perhaps it’s the accessories where a carnage can be made but whatever the lawsuit, you can utterly easily spend an acute total of notes on a new sear quiz and that will be completely wasted money if you buy the criminal thing.

Consider the spot twenty odd time ago before fresh manufacturing techniques, globalization and low charge country sourcing, the spot was even inferior for someone with no income and masses of leisure time (you guessed it, I was a learner!). CD’s had just appeared on the song shelves (a big drain on notes compared to vinyl), I don’t think the word download was in our vocabulary and sure the thought of receiving limitless question recipes off the internet could be nothing more than a nightmare.

I burden what students do best – having one long gathering! Nevertheless the opportunity to have a roast celebrate was nil because we simply couldn’t offer to buy a flame, and to an inclusive outside cooking enthusiast. It was at this spot that I absolute to body my homemade brick flame out of 16 support hand bricks and two cake cooling trays.

Not only is this a great low detriment introduction to charcoal barbecue grills it’s also a wonderfully portable structure too. Simply lay 8 bricks on the ground in two rows of four to make an honestly. On two sides place 2 bricks (on their sides) and place another 2 bricks on top so that in angry piece you’ve now got a U character. Throw the charcoal on the brick center a span two cake cooling grids across the top and there you have it.

The cake cooling trays are genuinely shoddy in the supermarket and (as a rightful scholar has to) the bricks were acquired one night from a citizen housing development. So with the barbecue complete, how did I get manage of some good open BBQ recipes?

Well that’s where it all happening to plunge apart and a method of enthusiastic audition and fault began. I’m delighted to say that those early seminal days have salaried off handsomely; all my contacts still love my outdoor cooking and so much so that BBQ smoker recipes.com was natural to record wealth and my contacts’ juicy creations.

If you do give the low rate barbecue a try, remark how the bricks globular up and generate an “all round” ardor. Not only is it cheap, it’s actually very operative too! I now have a clay barbecue at home and yes, it did charge more than $1, and this takes the roast retention to the basic. If you get hooked on outdoor cooking after wearisome my low cost grill then maybe it’s time to recoil cutback?

How To Watch Movie Everyone Else 2010

Watch Movie Everyone Else 2010

visit

http://www.movie25.tk

In this edgy comedy-drama from director Maren Ade, Chris (Lars Eidinger) and Gitti (Birgit Minichmayr) are a couple whose relationship has more than its share of ups and downs; she works as a publicist for a rock group whose career is going nowhere in particular, while he’s an architect who hasn’t been able to persuade anyone to build one of his designs just yet. While Gitti’s career isn’t much, it’s enough to give her head of the household status, to Chris’s chagrin. Chris and Gitti are spending some time at his well-to-do family’s summer home in Sardinia, and they seem to be getting along relatively well until they meet another couple vacationing nearby, Hans (Hans-Jochen Wagner) and Sana (Nicole Marischka). Hans is an architect like Chris, but unlike Chris his career is in high gear, while Sana is a well-respected artist. Hans isn’t afraid to display his alpha male status in their relationship, and Chris’s attempts to emulate him add to the tension between him and Gitti, while she isn’t sure what to make of a couple who seem so outwardly happy. Alle Anderen (aka Everyone Else) was an official selection at the 2009 Berlin International Film Festival.

Everything Louder than Everyone Else is the third live album by the British band Motörhead. The title refers to a remark repeated by Ian Gillan of Deep Purple during a concert in Japan: “Could we have everything louder than everything else?” (see Purple’s live album Made in Japan at the start of The Mule). Recorded at a show in Hamburg, Germany  on May 21, 1998. Everything Louder than Everyone Else is produced ‘undubbed’ in a two disc format. It was first released as a digipak version, later followed by a slimline 2CD jewel-case run.

Lemmy recalls making this during the ‘Snake Bite Love’ tour, having decided to include an entire show on the release, which they hadn’t done before because of the limitations of vinyl. There was some debate over the inclusion of “Overkill” but it being a different lineup and the fans would approve, it was considered valid.[1] Lemmy says he chose to record it in Germany because;

the Germans have been such loyal fans of ours. They always rescued our ass when we were going down for the third time. They stuck with us, and we knew Hamburg would be a great audience. It’s like Liverpool — a seafaring town, and you know where you are with a sailor!.[1]

The band’s performance on the album is reported[citation needed] to be good enough to have a hard time telling the band consists of only three people — the presence of second guitarist is not missed as Lemmy uses his bass much like a rhythm guitar, plugging any “holes” in the sound. At the start of the concert Lemmy announces “We are Motörhead and we’re gonna kick your ass” and commenters pick this theme up, some advocating that those thinking of buying a first record will be disappointed with studio versions of the songs after listening to the show.[

Thoroughly Cool & Ultra-Sensitive Reaction Phone Samsung M7600-Myluxphone

Now we will get down to the superb music technologies and capabilities that have been especially designed for the Samsung M7600. The user can enjoy Bang Louse and ICEP power amplifier which is very energy efficient and with the 5.1 virtual surround sound on-off control the user can mange the manage the sound control with ease. Obviously there are other applications we would expect from the Beat DJ which include shuffle, repeat, fast-forward and rewind so you can be in total control over how you listen to your music. There is a Find Music application which is part of the music entertainment centre similar to the of Sony’s Track ID service. With the other Samsung music phones from the Beat range which include the Samsung M3510 Beat and the Samsung M3200 Beat s, the Beat DJ provides more enhanced music capabilities with fantastic touch-sensitive controls with a superb music mixer technology taking a music mobile phone to whole new level. The reason the word “DJ” is part of the Samsung M7600 name is that the handset allows the users mix and scratch their music in the same way a DJ would on the virtual vinyl image that is on the touch display screen.

Samsung W699 is one of those mobiles that stake everything on a unique and exclusive feature. Designed to stand out, the M7600 takes enhanced music playback capabilities to unprecedented heights. Allowing you to mix your own music right the way you like it, the Beat DJ opens up an empty market niche and makes the M7600 a standard-setter. The entertainment package is complete with the nice touch user interface and relevant design. The mixer controls include REC, Filters, Scratch and samples. To explain the REC starts recording the adjustments they has user has made to the music – Scratch enables the user to spin the virtual vinyl to add the scratches. There are 10 Filters enabling the user to adjust and tweak the music to name a few echo, looping and tempo change. Just to add only two filters can be used at any one time. For the Samples Library there are 20 pre-installed which enables the user to add other sounds while a track is actually playing.

Samsung SCH-W579 allows for scrolling between the samples and filters providing an amazing interactive mobile music experience. What a fantastic and fun element to feature on a phone to have your mixing deck along with all the other superb music features in your pocket. The user can choose from a selection of Polyphonic, MP3 and WAV ring-tones or the user can simply download their favorite tune and add it as a ring-tone. There is only 50 mega-bytes of internal memory on the Beat DJ mobile phone, but not to worry this can be increased with a Micro-SD Trans-Flash with up to a massive 16 gigabytes which will provide a huge amount of storage space for all the users music files and downloads.

By appleplmltd

Beverage Coasters Come in a Wide Variety of Designs

Although beverage coasters are round or square, they can be of any size. Some are only small round discs to fit the size of a glass, while others are large enough to provide ample room for a large beer stein.

They come in glass, wood, marble, sandstone, cork, cardstock paper, leather and many other materials. The designs that you can have imprinted on them are too many to mention. You can also have coasters with photos that you provide to the manufacturer or ones with a place to insert a photo of your choice.

There are also unusual designs in beverage coasters (http://www.thirstycoasters.com/servlet/-strse-Collegiate-dsh-Car-Coasters/Categories). The unusual coasters will no doubt cause your visitors to ask about the design and even where you purchased them so that they too can have similar coasters in their homes.

If you are of Irish descent, you will be proud to have Celtic coasters on your coffee table. These coasters have the intricate knots and spirals associated with the Celtic tradition and the Celtic cross. They are made of resin and stone so that they are very absorbent. A set of four coasters measure about 4.5 inches across and have rubber backing to protect your furniture.

Do you have a collection of old records? If so, you will likely enjoy using beverage coasters shaped like record labels. These are made from recycled records that are no longer playable. The vinyl is laminated to seal in the labels and the hole in the spindle as well as offering protection from the moisture that collects on the glasses. A set of 6 such coasters each measuring 4 inches in diameter will contain 6 different labels and comes in a clear display case.

Marble drink coasters (http://www.thirstycoasters.com/servlet/-strse-Dining-&-Entertaining/Categories) are perhaps the most elegant of all. They add to the decor of any room and add a touch of class to the furniture. There are various colors and designs to choose from and all have cork bottoms so that the stone does not come in contact with the furniture and cause scratches.

For relaxing on the patio in the summer while you have a drink with your friends, you can have a great laugh when you provide beverage coasters in the shape of flip flops. The straps on the coasters will hold the glass in place. Each set contains 4 different colored coasters so if you lay down your drink you will easily know which one is yours.

For large gatherings you will need to have several sets of beverage coasters. If you choose to buy English pub design coasters you can have 25 coasters all in one set. These coasters display popular English and Irish beer labels that may inspire you to have an assortment of beers to offer your guests as you may want to sample all of the different kinds displayed.

These coasters are not just for beer though and you can use them for any beverage. They are made of cardboard and only cost about 45 cents each which means you can order several sets of cocktail coasters at one time to make sure you always have them on hand.

You can also get personalized beverage coasters containing your initials or that of another person. This makes a perfect gift for a wedding shower, birthday or anniversary. They are made of glass but have protective padding on the bottoms. Each of these coasters measures 4 inches square and the glass is 3/16 inches thick. You can have many hours of fun choosing beverage coasters for every occasion you can think of.

polymer science

Introduction:  Polymer Morphology

Two different states or forms can be identified in which a polymer can display the mechanical or thermomechanical properties that can be associated with solids, viz., the form of a crystal or the form of a glass. It is not really the case that all polymers are able to crystallize. As a matter of fact, a high degree of molecular symmetry and microstructural regularity within the polymer chains are a prerequisite for crystallization to occur.  Even in those polymers, which do crystallize in any rate, the ultimate degree of crystallinity developed is mostly less than 100%.

Studies of physical form, arrangement and structure of the molecules or the molecular aggregates of a material system relates to what is known as its morphology.  Polymer morpho-logy covers the study of the arrangement of macromolecules over the crystalline, amorphous and the overlapping regions and the overall physical clustering of the molecular aggregates.

When cooled from, the molten states, different polymers exhibit different tendencies to crystallize at different rates depending on many factors including prevailing physical conditions, chemical nature of the repeat units and of the polymer as a whole, their molecular or segmental symmetry and structural regularity or irregularity, as referred to above.  Bulky pendent groups or chain branches of different lengths hinder molecular packing and hence crystallization.  The nature of the crystalline state of polymers is not simple and it should not be confused with the regular geometry of the crystals of low molecular weight compounds such as sodium chloride or benzoic acid.  There are polymers, which are by and large amorphous, and they have very poor tendency to get transformed into ordered or oriented structures on cooling to near or even below room temperature.  Natural or synthetic rubbers and glassy polymers such as polystyrene, acrylate and methacrylate polymers belong to this class.

In a crystalline polymer, a given polymer chain exists in or passes through several crystalline and amorphous zones.  The crystalline zones are made up of intermolecular and intramolecular alignment or orderly and hence closely packed arrangement of molecules or chain segments, and a lack of it results in the formation of amorphous zones.

Glass Transition and Melting Transition

On the basis of following the changes in a mechanical property parameter such as shear modulus with changes (rise) in the temperature of observation for polymer material systems, one can readily observe successively – (i) glass transition and  (ii) melting transition phenomena, more easily from a graphical plot , and may also have a measure of the glass transition temperature, Tg and the melting temperature,  Tm.

The glass transition and the melting transition may also be observed and ascertained from a plot of specific volume ( Vsp )  versus temperature.  Let us consider the various possibilities as a melt is cooled from the position A at a high temperature that corresponds to a relatively high Vsp value as well, fig. 1.  The path ABDG shows how the specific volume drops down as a low molecular weight compound is frozen.  As the melting temperature Tm is reached at the point B, a sharp discontinuity in Vsp is observed (BD).  The slopes AB and DG give measures of coefficients of thermal expansion of the liquid and the solid respectively.  The thermal expansion coefficient also suffers a discontinuity at Tm.

Fig.1:Schematic diagram highlighting possible changes in the specific volume (Vsp)

of a polymer with change in temperature .

We may however, start with a molten polymer material at A and observe volume change as described by the path ABHI and there is no discontinuity notable at Tm. The liquid line AB gets further extended beyond Tm with lowering of temperature and it is seen to suffer a change in slope at a much lower temperature, Tg and finally, turns into a different linear portion (HI) of a much lower constant slope.  Here, actually, the slope-change occurs over a small range of temperature (which may usually range about 5 – 100C), but extrapolation of the two linear parts allows right assessment of Tg by this method.  The zone HI represents the glassy state that ensues as the glass transition temperature is reached or just crossed as we go down in temperature.  Transition to the glassy state is also commonly termed as vitrification.  The region BH represents the existence of a super cooled liquid state or rubbery state of relatively poor dimensional stability, even under the influence of a low stress.

For all polymers, the glassy state is always attained finally on cooling, irrespective of whether the polymer being tested is crystallizable or not.  Even under situations favouring crystal formation, it does not necessarily mean that crystallization occurs rapidly or completely.  There still remains in most cases significant portions of amorphous zones after the primary crystallization process is completed.

The path ABCEFG in fig. 1 represents the case of a partly crystalline, partly amorphous polymer system.  On cooling down to Tm, crystallization begins and the characteristic discontinuity in Vsp becomes apparent even though the sharpness at which Tm is revealed is not as pronounced for polymers as for a low molecular weight compound, and this can be appreciated from the curvature of the portion of the path BCEF.  For such a system, FG represents the glassy zone and BA the melt or liquid zone and BCEF zone is by and large the amorphous rubbery (super cooled liquid) zone.  The point F, where slope between the segments EF and FG changes corresponds to the glass transition point, Tg, and the polymer in such a case remains by and large amorphous.  If partial crystallization would occur on cooling below Tm , the amorphous content decreases and in that case, the change in slope at Tg may be much smaller and harder to detect.

The path ABJK may appear as a variation of the path ABHI and here, AB describes the liquid state, BJ the super cooled liquid or the rubbery state and JK describes the glassy state.  The path ABHI shifts to ABJK under the condition of a higher cooling rate; it is likely that Tg is also displaced to a higher temperature (Tg?) for a faster cooling rate.

Thus, the temperature response of linear polymers may be viewed as divided into three distinctly separate segments:

1. Above Tm :

In this segment, the polymer remains as a melt or liquid whose viscosity would depend on molecular weight and on the temperature of observation.

2. Between Tm and Tg :

This domain may range between near 100% crystalline and near 100% amorphous chain molecular clusters depending on the polymer structural regularity and on experimental conditions.  The amorphous part behaves much like super cooled liquid in this segment.  The overall physical behaviour of the polymer in this intermediate segment is much like a rubber.

3. Below Tg :

The polymer material viewed as a glass is hard and rigid, showing a specified coefficient of thermal expansion.  The glass is closer to a crystalline solid than to a liquid in behavioural pattern in terms of mechanical property parameters.  In respect of molecular order, however, the glass more closely resembles the liquid.  There is little difference between linear and cross linked polymer below Tg .

The location of Tg depends on the rate of cooling.  The location of Tm is not subject to this variability, but the degree of crystallinity depends on the experimental conditions and on the nature of the polymer.  If the rate of cooling is higher than the rate of crystallization, there may not be an observable change at Tm, even for a crystallizable polymer.

The simple device used to follow volume changes upon cooling or heating is called a dilatometer, having a glass bulb or ampoule at the bottom fitted with a narrow bore capillary at the top, as in fig. 2.  A dilatometer may also be used in studying progress of polymerization with time at a given temperature by following volume contraction of liquid monomer system (the polymer being formed having a higher density than the monomer being polymerized).  For studies with a polymer say, polystyrene, the sample is placed in the bulb, which is then filled with an inert liquid, usually mercury and the volume changes with change of temperature (or sometimes at a constant temperature for a phase change, such as at Tm ) are then registered, as in a thermometer.  The expansion / contraction of mercury due to change of temperature is to be duly accounted for during experimentation for a volume change of the polymer sample.  The experiments are required to be accomplished by placing the dilatometer in a thermostated bath.  The sample must be immiscible with the displacement fluid and degreased to eliminate air entrapment.  Specific volume – temperature plot for polystyrene showing a distinct change in slope at 95.60C, indicates glass transition temperature, fig. 3.

Fig.2:A dilatometric arrangement for                 Fig. 3:Temperature dependence of

measurement of volume change of a                  specific volume for polystyrene indicating

the glass transition temperature, Tg.

(Courtesy: Tata McGraw –Hill, New Delhi)

Thus, it is a common experience that raising or lowering of temperature, just as application or withdrawal of stress, greatly influences the physical structure and properties of polymers.  With change of temperature a high polymer material passes through two distinct transitions characterized by (i) melting point or first order transition, denoted by Tm and (ii) the glass transition or second order transition, denoted by Tg .

Melting Point or First Order Transition

Melting of a crystalline solid or boiling of a liquid is associated with change of phase and involvement of latent heat.  Many high polymers possess enough molecular symmetry and/or structural regularity that they crystallize sufficiently to produce a solid-liquid phase transition, exhibiting a crystalline melting point.  The melting is quite sharp for some polymers such as the nylons, while in most other cases as for different rubbers and polystyrene, etc., the phase change takes place over a range of temperature.  Phase transitions of this kind, particularly in low molecular weight materials, being associated with sharp discontinuities in some primary physical properties, such as the density or volume, V,  [ V = (?G / ?P)T ] and entropy,  S,  [–S = (?G / ?T)P ] , which are first derivatives of free energy, are commonly termed first order transitions.  Although  we observe  melting,  a true first order  transition or  ideal  melting  in high polymers is frequently absent or missing, in view of the distribution of molecular weight and entanglements of chain molecules giving rise to the complex phenomenon of retarded flow or viscoelasticity.

Glass Transition or Second Order Transition

Glass transition or second order transition is not a phase transition and almost every polymeric or high polymeric material is characterized by a specific glass transition temperature (Tg) or second order transition point (SOTP), appearing well below its (crystalline) melting point, Tm.

At Tg, the thermodynamic property parameters S, V and H merely undergo change of slope when plotted against temperature, without, however, showing sharp discontinuities as observed in the case of first order transitions, such as the idealized plot shown in fig. 4.

Fig. 4: First order transition showing an idealized phase transition (melting or freezing): Trend of change of volume or entropy with rise of temperature, showing discontinuity at the transition point. (Courtesy: Tata McGraw –Hill, New Delhi)

The properties that suffer discontinuities at the glass transition temperature are:  heat capacity CP,  [ CP = (?H / ?T)P ], coefficient of thermal expansion ? ,

1                                 1          ?

?  =           (?V / ?T)P = . { (?G / ?P)T } P

V                                 V         ?T

and isothermal compressibility  K,

1                                    1

K  = – (?V / ?P)T = – (? 2G / ?P 2)T

V                                    V

which are second derivatives of free energy and it is for this reason that the glass transition temperature, Tg is commonly referred to as the second order transition temperature, fig. 5.  Refractive index (R1) also shows a sharp change at the glass transition point (Tg).

Fig.5: Trends of change in (a) specific volume, (b) coefficient of thermal expansion (?) or isothermal compressibility (K) and (c) refractive index (RI) of polymers with temperature indicating the glass transition  (Courtesy: Tata McGraw- Hill, New Delhi)

The glass transition is not a phase transition and therefore, it involves no latent heat.  Below this temperature normally rubber – like polymers lose flexibility and turn rigid, hard and dimensionally stable and they are then considered to be in a glassy state, while above this temperature, all normally rigid, stiff, hard glassy polymers turn soft and flexible, become subject to cold flow or creep and as such turn into a rubbery state.  The difference between the rubbery and glassy states lies not really in their geometrical structure, but in the state and degree of molecular motion.

Below the glass transition temperature, Tg, the chain segments or groups, as parts of the chain molecular backbone, can undergo limited degrees of vibration; they do not possess the energy required to rotate about bonds and change positions with respect to segments of the neighbouring chains.At or slightly above Tg, rotation sets in, particularly of side groups or branch units, and it is conceivable that only short range molecular segments rather than the entire high polymer molecule would rotate at this point.  The much higher coefficient of thermal expansion just beyond Tg is indicative of much greater degree of freedom of rotation.

At the respective glass transition or second order transition temperatures, different polymers may be viewed to be in an isoviscous state, and in reality, Tg is a common reference point for polymers of diverse nature, below which all of them behave as stiff rigid plastics (glassy polymer) and above which they appear leathery and rubbery in nature.  As we understand, a useful rubber is a polymer having its Tg well below room temperature, while a useful plastic is one whose Tg is well above the room temperature. Table 4.1 lists the Tm and Tg values of some common polymers.

Table 1:          Tm and Tg Values of Several Polymers

Polymer

Repeat Unit

Tm, 0C

Tg, 0C

Polyethylene

– CH2 – CH2 –

137

-115,-60

Polyoxymethylene

– CH2 – O –

181

-85,-50

Polypropylene (isotactic)

– CH2 – CH (CH3) –

176

- 20

Polyisobutylene

– CH2 – C (CH3)2 –

44

- 73

Polybutadine (1, 4 cis)

– CH2 – CH = CH – CH2 –

2

- 108

Polyisoprene (1, 4 cis), (NR)

– CH2 – C(CH3) = CH – CH2 –

14

- 73

Poly (dimethyl siloxane)

– OSi (CH3)2 –

- 85

- 123

Poly (vinyl acetate)

– CH2 – CH (OCOCH3) –

—

28

Poly (vinyl chloride)

– CH2 – CH Cl –

212

81

Polystyrene

– CH2 – CH (C6H5) –

240

95

Poly (methyl methacrylate)

– CH2 – C(CH3)( COOCH3) –

200

105

Poly tetrafluoroethylene

– CF2 – CF2 –

327

126

Poly caprolactam (Nylon 6)

– (CH2)5 CONH –

215

50

Poly(hexamethylene adipamide)

(Nylon 66)

–HN(CH2)6-NHCO–(CH2)4CO –

264

53

Poly (ethylene terephthalate)

– O(CH2)2 – OCO – (C6H4) CO –

254

69

Poly (ethylene adipate)

– O(CH2)2 – OCO – (CH2)4 CO –

50

-70

Molecular weight and molecular weight distribution, external tension or pressure, plasticizer incorporation, copolymerization, filler or fibre reinforcement, and cross linking are some of the more important factors that influence the glass transition temperature, melting point or heat – distortion temperature of a matrix polymer.  The comparative lowering of Tm and Tg for modification of polymer by external plasticization (plasticizer incorporation) and by internal plasticization (comonomer incorporation) is shown in fig. 6. Generally, a comonomer incorporation i.e. copolymerization is more effective than external plasticization in lowering the melting point, while the latter process (external plasticizer incorporation) is more effective than the former (copolymerization) in lowering the glass transition temperature.  Cross-linking causes significant uprise in Tg, as cross-links hinder rotation of chain elements, thus necessitating a higher temperature for inception of rotation of segments between cross-links.  Likewise, higher molecular weight, leading to complex, long range chain entanglements, restricts scope for segmental rotation and thereby causes a rise in the Tg value with a notable levelling off effect for molecular weight > 105.

Fig. 6: Schematic plots showing relative lowering of Tm and Tg of a polymer by separately incorporating (a) an external plasticizer.and (b) a comonomer by copolymerization.  (Courtesy: Tata McGraw –Hill, New Delhi)

Brittle Point

A polymer is also characterized by a temperature called the brittle point1 or brittle temperature (Tbr) which is close to or somewhat higher than its glass transition temperature (Tg ) for most high polymers.  As the temperature of the polymer in its rubbery state is lowered, the flexible nature and rubbery properties are gradually lost and the polymer stiffens and hardens; at an intermediate stage, a temperature called the brittle point is attained at or below which the polymer specimen turns brittle and breaks or fractures on sudden application of load.

For comparison of brittle points of different polymers, it is necessary to do the testing under specified conditions, including specified sample size and thickness, degree and rate of cooling, etc. as the test is empirical in nature.  The brittle point corresponds to a temperature at which the time interval of load application just matches or equals that needed by the test specimen to undergo the necessary deformation.  At a lower temperature, the specimen is unable to deform as rapidly, and hence it fails to withstand the load and thus breaks; above the brittle temperature, the time of load application is more than adequate for the specimen to absorb the applied energy and deform to escape fracturing or breakage.  Lower molecular weight limits the scope for long-range molecular interactions and chain entanglements and hence leads to a higher brittle temperature. Changes in Tg and Tbr with polymer molecular weight, as schematically illustrated in fig. 7, clearly shows that the trends of change for the two parameters are just the opposite.  The difference between the two is much narrower in the higher molecular weight range, but it gets progressively wider as the molecular weight decreases.

Fig. 7: Typical plots showing dependence of brittle temperature (Tbr) and glass transition temperature (Tg) on polymer molecular wieght.

(Courtesy: Tata McGraw –Hill, New Delhi)

Development of Crystallinity in Polymers

Polymer morphological studies primarily relate to molecular patterns and physical state of the crystalline regions of crystallizable polymers. Amorphous, semi-crystalline and prominently crystalline polymers are known.  It is difficult and may be practically impossible to attain 100% crystallinity in bulk polymers.  It is also difficult according to different microscopic evidences, to obtain solid amorphous polymers completely devoid of any molecular or segmental order, oriented structures or crystallinity.  A whole spectrum of structures, spanning near total disorder, different kinds and degrees of order and near total order, may describe the physical state of a given polymeric system, depending on test environment, nature of polymer and its synthesis route, microstructure and stereo – sequence of repeat units, and thermomechanical history of the test specimen.  Further, the collected data for degree of crystallinity may also vary depending on the test method employed.  The degree of crystallinity data shown in Table 2 must therefore be taken as approximate.

Polymers showing degrees of crystallinity > 50% are commonly recognized to be crystalline.  The cellulosics (cellulose acetate) and also regenerated cellulose (viscose) used as fibres have crystallinity degree lower than that of native cellulose, the base fibre.  The predominantly linear chain molecules of high-density polyethylene (HDPE) show a degree of crystallinity that is much higher than any other polymer known (even substantially higher than that for the low-density polyethylene (LDPE).  For HDPE, the attainable crystallinity degree is close to the upper limit (100%).  Atactic polymers in general (including those of methyl methacrylate and styrene bearing bulky side groups), having irregular configurations fail to meaningfully crystallize under any circumstances.

Table 2: Approximate Degree of Crystallinity (%) for Different Polymers.

Polymer

Crystallinity (%)

Polyethylene (LDPE)

60 – 80

Polyethylene (HDPE)

80 – 98

Polypropylene (Fibre)

55 – 60

Nylon  6 (Fibre)

55 – 60

Terylene (Polyester fibre)

55 – 60

Cellulose (Cotton fibre)

65 – 70

Regenerated cellulose (Viscose rayon fibre)

35 – 40

Gutta  Percha

50 – 60

Natural rubber (Crystallized)

20 – 30

Figure 8 provides a comprehensive idea about crystallization rate (volume change with time) at different selected temperatures.  For high density polyethylene (HDPE), as the temperature is lowered, the volume changes proportional to the rates of crystallization rapidly increase and well below the actual melting point (1270C), the volume change soon becomes so rapid that measurements and observation become uncertain and difficult, if not practically impossible.  The obvious consequence of the very high rate of crystallization in polyethylene is that it is virtually impossible to obtain and isolate the polymer in the amorphous state at room temperature i.e., under ambient conditions.  Sudden chilling or quenching of the melt to below room temperature results in a material which is still largely crystalline, though expectedly with the likelihood of a somewhat lower degree of crystallinity than otherwise developed on normal melt – cooling.  The reason for this state of affairs is that the time required for crystallization is far shorter than the time taken for cooling the test polymer specimen.

Fig. 8: Plot of relative volume with time (min) showing densification of polylethylene on development of crystallinity at different specified temperatures.

(Courtesy: Tata McGraw –Hill, New Delhi)

For practical reasons, therefore, the process of polymer crystallization is very conveniently studied and measured with confidence using a polymer that is by and large amorphous; natural rubber is one such polymer.  The merit of using rubber as a model material for study of polymer crystallization is that the crystallization process is slow to allow due measurements with easy manipulations and it takes place in a convenient range of temperature.

It is worthy of mention that all rubbers (particularly those which are copolymers) are not crystallizable.  Only those built up of chains characterized by chemically identical and regular repeat units, such as natural rubber, 1, 4 cispolyisoprene and certain grades of polychloroprene are capable of crystallization.

Crystallilzation of Rubber on Cooling

If unvulcanized natural rubber (NR) is held at a fixed low temperature, say 00C, it slowly gets somewhat stiffened and hard, and loses flexibility and softness proportionately.  However, the material still retains some degree of flexibility and toughness.  The observed physical change is also associated with some enhancement in density or lowering in volume; the associated changes are consequences of slow development of crystallinity in the material.

Crystallization in an ordinary low molecular weight liquid on cooling to or below the freezing point takes place very rapidly, consequent to ready and fast molecular rearrangement from a disordered state to a very regular state of packing.  A polymer melt system is, however, much more complicated due to chain entanglements, restricting free mobility of the chain segments, and consequently, hindering and delaying the desired rearrangement process on cooling.  For rubber – like polymers, the time scale of crystallization is commonly much longer than for liquids of low molecular weight materials.

Fig. 9: Densification on crystallization of natural rubber,

plot of relative volume vs. time (hour) at different temperatures.

(Courtesy: Tata McGraw –Hill, New Delhi)

Trends of change in relative volume of natural rubber (NR) with time due to crystallization at different low temperature are shown in fig. 9.  The attainable maximum crystallinity and the time span required for this to happen are very much dependent on the temperature of observation6.  In each case, the volume contraction rate is relatively slow initially; the volume contraction (or crystallization) rate shows an increasing trend with time, passes through a higher steady zone at an intermediate time period and then finally drops down, decays or levels off giving a maximum attainable development of crystallinity degree at a given temperature.  Lowering of temperature causes enhancement in the steady rate of crystallization of NR till about –250C, where the steady rate vs. temperature plot, fig. 10 passes through a maximum.  Further reduction in the temperature of crystallization causes a falling trend in the steady rates of crystallization as in fig.10.   The crystallization is (nearly) completed in about five hours at –250C.  In natural rubber, the degree / extent of crystallinity under the most favourable situation does not exceed 30%.

Fig. 10: Plot indicating trend of change in steady rate of crystallization with change in temperature for natural rubber (Courtesy: Tata McGraw –Hill, New Delhi)

Mechanism of Crystallization

As the polymer melt is kept at a temperature close to or slightly above its melting range, the initial slowness in crystallization rate build up (delayed crystallization) is linked with the initial process of nucleation.  Growth of crystallites is contingent upon the development and existence of a certain number of very tiny growth centers or nuclei for the deposition of oriented chain segments.  The growth centers are initially formed on extended cooling or holding of the melt at the specified temperature by coming together of a small number of chain segments in the course of their random motion (micro Brownian motion) under the prevalent situation.  Nucleation is, however, common to all processes that turn an initially homogeneous medium into a heterogeneous system as a consequence of deposition of a separate phase.

As the growth is sustained and continued, the opposing effect of chain entanglements becomes increasingly severe and ultimately critical, thus imparting severe restrictions on the mobility of chain segments and thus making it difficult for them to get to a position for attachment to any one of the crystallites formed.  Beyond this stage, the crystallization rate diminishes sharply and finally, the process dies down.

Lower temperature favours nucleation and lower thermal energy of the chain segments makes it less likely that a nucleus once formed would disappear again, the net result being a gain in the number of nuclei and an increase in the overall rate of crystallization with progressive lowering of temperature. At progressively lower temperatures, however, the overall energy of the polymer system including that available to chain segments tend to get so much lowered that the segments seem to practically lose much of their mobility and hence their deposition on a nucleus formed is progressively hindered much more effectively and there appears a sharp dropping trend in the rates of crystallization.  For natural rubber, the crystallization process gets effectively frozen out below – 500C, fig. 10.

Stress – Induced Crystallization of Rubber

It is a common knowledge and a matter of wide experience that stretching of a strip of vulcanized rubber makes it develop a temporary crystallinity by axial orientation of the chain molecules along the direction of stretching and that the orientational effect disappears instantly on withdrawal of the stretching force.  A strip of raw or unvulcanized rubber also develops crystallinity when subjected to high extensions on application of a stretching force, but it remains more or less in the extended state (in view of the absence of restraining cross links) without notable retraction to its original state on stress release.  However, when heated carefully in the subsequent stage, such as by dipping the test strip into slightly warm water (temperature > 300C) the crystals melt and allow the strip to revert largely to its unstrained state.

The cross links in the vulcanized rubber act as points of reinforcement and are responsible for accumulation of the strong retracting or restoring force that comes into play in breaking the stress – induced orientation (or the crystalline structure) on withdrawal of the applied stress.  In the unvulcanized system, the absence of cross links allows varied degrees of chain uncoiling if not chain slippage on low/moderate extensions and whatever elastic restoring force accumulates is far too insufficient or inadequate to break the crystalline structure and induce dimensional recovery.  Raising the test strip temperature to 300C or slightly above this level, allows melting of the axially oriented crystallites, causing the rubber chain molecules to coil up and the test strip to retract to its initial or near initial (random / unoriented) state.

Fig. 11: Time-dependency of stress-induced crystallization (densification) of unvulcanized rubber held at 00C for different indicated orders of fixed extensions, plot of density change (%) vs. time (min). (Courtesy: Tata McGraw –Hill, New Delhi)

Fig.11shows the time-dependency of crystallization of unvalcanized rubber at a low temperature (here 00C) on application of different fixed extensions revealing trends of % change (increase) of density with time of specified stretch application. Moderate extensions produce effects as observed for lowering of temperature.  For extensions > 100%, however, the crystallization rates are very high, such that only final stages are practically observable.

Melting of Rubber

Beyond this point, further enhancement in temperature gives a linear plot much in tune with the thermal volume expansion of the amorphous rubber. Fig.12:‘Melting curve’ showing increase in              Fig. 13: Melting curve showing a plot

specific volume (cm3/g) vs. temperature (0C)          of relative volume vs. temperature for rise for natural rubber                                                             polyethylene.

(Courtesy: Tata McGraw –Hill, New Delhi)

The melting curve of the highly crystalline polymer polyethylene characteristically shows a sharp volume change and the temperature of the beginning and end of the melting process is usually limited well within a range of 100C or to be more precise, within a span of 50C.  If after melting the rubber, the temperature is lowered again, fig. 12, the linear volume contraction for the amorphous rubber continues to much lower temperatures and the melting curve is not retraced in the reverse direction simply because, measurable recrystallization fails to occur in the time – span of the experiment.  For the highly crystallizable polymer, polyethylene, however, the melting and crystallization / recrystallization processes are by and large reversible in a practical sense and the recrystallization curve is mostly a retrace of the melting curve, fig. 13 from the opposite direction.

For the amorphous polymer, natural rubber, whereas melting occurs over an extended range of temperature, the beginning of melting and the temperature range over which the melting process is accomplished and completed are also largely dependent on the temperature at which the preceding crystallization was done.  Usually, melting begins at a temperature that is 4–60C higher than the temperature at which the preceding crystallization was accomplished, fig. 14.

Fig. 14: Plot indicating dependence of melting range of natural rubber on temperature of crystallization, the diagonal line below the melting range (shaded zone) indicating temperature of crystallization. (Courtesy: Tata McGraw –Hill, New Delhi)

Thus, it is possible to have simultaneous or consecutive melting and recrystallization in a given piece of rubber as it is slowly heated over the melting range (shaded area in fig. 14) after initial crystallization and then held at a specific temperature within that (melting) temperature range.

Polymer Single Crystals

Single crystals of different readily crystallizable polymers can be grown by slow cooling and precipitation from very dilute solutions.  They appear in the form of very thin plates or lamellae, usually diamond shaped with spiral growth pattern and showing step – like formation on the surface.

The single crystals are very small in size and can not be examined by x-ray diffraction.  However, they can be readily and conveniently studied by electron microscopy.  Electron diffraction pattern and electron micrographs reveal certain interesting features about polymer single crystals.  The thickness of the lamellae is very small (100 – 200 Å) compared to the usual polymer chain length.  The diffraction pattern reveals with no uncertainty that the chain axis is directed perpendicular to the plane of the lamellae.  The structural pattern of the single crystal is thus understood well on the basis of the well known folded chain theory.  This theory envisages that a single molecule of the polymer must bend or fold forwards and backwards many numbers of times across the thickness of the lamellae.  Such folded chains are readily stacked in the crystal lattice with ease.  It is widely believed that the single crystal comprises an array of folded chains packed individually and successively between the top and bottom surfaces or planes and on the growing edges of the lamellae as schematically shown in fig. 15.

Fig. 15: Chain folding to yield polymer single crystal (schematic)

This kind of oriented structure or crystal formation involving whole individual polymer molecules discretely without interference or interposition of other molecules is apparently made possible due to large distances that exist to ideally separate the individual molecules in very dilute solutions, fig. 16.  The wide – distance separation ensures practical elimination of chain entanglements.  Hence, when one segment of a polymer molecule gets attached to one of the thin edges of the growing crystal, it faces practically no competition from other far away molecules for occupation of the close by, adjacent lattice site.  There will be little hindrance to the successive occupation of immediately adjacent sites by segments of the same molecule by a chain folding mechanism that would continue till the whole molecule is drawn and arranged and oriented into the folds.

Fig. 16: Separation between polymer chain molecules in (a) very dilute solution and (b) concentrated solution (schematic). (Courtesy: Tata McGraw –Hill, New Delhi)

Structure of Bulk Polymers

Crystalline polymers obtained on cooling of their melts likewise produce electron micrographs showing the lamellae structure for the crystallites and providing little direct evidence for the presence of major amorphous regions.  An idealized model of the lamellae structure as in fig. 17(a) is probably far from the real state of affairs and it may not be applicable to all types of polymers.  Most polymers other than the polyethylenes (HDPE and LDPE) contain amorphous regions to the extent of 20 – 50% or even more, distributed in the material along with the crystalline domains.  In the structural model for a real system, a provision has to be made to accommodate the amorphous material.  In a fringed – micelle or fringed – crystallite model, fig. 17 (b), the disoriented, amorphous material fractions are shown interspaced between the randomly distributed and positioned crystallites.  This model explains and reveals the morphological features in such materials as rubbers and some cellulosic or other non-crystalline or semi-crystalline polymers with isotropic property pattern.  For different polymers of intermediate orders of crystallinity, random mix of fringed micelle model and regularly stacked lamellae model may represent the overall structural pattern. These structural concepts make allowances for imperfections commonly encountered, such as the interlamellar entanglements, molecular loops of diverse dimensions, irregular fold lengths and interconnecting chains passing through different lamellae.

Fig. 17: Schematic representation of (a) ideal stacking of lamellar crystals (discrete folded chains), (b) fringed – micelle model showing randomly distributed amorphous and crystalline zones, and (c) interlamellar amorphous model. (Courtesy: Tata McGraw –Hill, New Delhi)

A model consisting of stacks of lamellae interspaced with and connected by amorphous regions may be referred to as the interlamellar amorphous model, fig. 17(c).  This unique model provides the most useful approach to the understanding of the mechanical property profile of bulk crystallized polymers of moderate to high degrees of crystallinity.  The different degrees of ductility and cohesive character are direct consequences of the existence of interlamellar ties.  Somewhat like stacks of bricks without clay or sand – cement interlayers as the mortar, stacks of lamellae (crystals) without the existence of interlamellar tie molecules such as those obtained by slow cooling of a very dilute solution, would prove relatively fragile and brittle.  The tie molecules reduce brittleness and infuse ductility and stability.

Spherulites

The most distinctive, prominent and common feature of bulk crystallized (melt cooled) polymers is the development of spherulites, i.e. spherical crystallites. A spherulite is characteri-zed by a symmetrical structure build – up arising as a consequence of the cooperative growth of oriented chain segments called crystallites radially outward from a core or nucleus in three dimensions, fig. 18.  Bulk crystallized polymers are, in fact, not merely a series of stacked lamellae separated and interconnected by amorphous regions; the lamellae units are intricately organized in a radial fashion within the spherulites.  The crystallization process through which the spherulites are formed follows sequential steps beginning with nucleation.  The nucleation process may be aided by intentional addition of a foreign substance, called the nucleating agent.  The nucleating agents by their presence reduce the size of the spherulites by increasing the number of nuclei.  Growth of large spherulites contributes to enhanced brittleness.

Fig. 18: State of spherulite growth for polypropylene [(a) and (b)] and (c) schematic structure of a spherulite (radial growth and branching of the lamellae with an enlarged portion showing chain folding perpendicular to the spherulitic radius). (Courtesy: Tata McGraw –Hill, New Delhi)

It is generally observable that most polymers continue to slowly densify long after spherulite growth is complete.  The post – primary crystallization densification occurs both in the interspherulitic regions and intraspherulitic regions.  The densification due to secondary crystallization slowly taking place after the primary process of spherulite growth leads to thickening of the lamellae, as chain segments are gradually pulled in from the amorphous zones.  One more consequence of the secondary crystallization is the trend toward increase in brittleness.  The whole after-effects on mechanical and related properties of the polymer are recognized to be complex and they depend largely on many factors including the rate and span of cooling, annealing, cold – drawing or stretch – cooling.

Thermal Analysis

The thermal properties of polymers are conveniently studied by employing such techniques as differential thermal analysis (DTA) and differential scanning calorimetry (DSC).  The DTA technique usually allows detection of thermal response and effects that

Fig.19: A block diagram for a DTA apparatus    Fig. 20: A typical DTA thermogram indicating

thermal changes of a crystallizable polymer (schematic)

(Courtesy: Tata McGraw –Hill, New Delhi)

accompany chemical or physical changes in a material system when it is heated or cooled in a programmed manner through a zone of transition, phase change, chemical transformation or decomposition. It allows location and measurement of glass transition temperature, Tg, the crystallization temperature (Tc), the (crystalline) melting point (Tm), and the temperatures of thermal / oxidative degradation, cross linking and other types of reactions.  Figures 19 and 20 show respectively a block diagram of a DTA equipment and schematic representation of a DTA thermogram.

In practice, the material sample and a thermally inert reference material placed in the respective holders of the DTA cell are heated in a programmed manner.  Any physical or chemical change in the test material at a specific temperature, which is the characteristic feature of the material under study, is usually associated with thermal change leading to a notable difference in temperature (?T), between the test and reference materials held in the furnace temperature.  ?T is recorded as a function of temperature, T.  For no thermal change / transition, in the test sample, ?T remains nearly unchanged (constant).  In DTA, the correlation between ?T and energy changes over a specific transition or transformation (reaction) is uncertain and unknown, thereby making the conversion of the endotherm or exotherm peak areas to energies also uncertain.  However, the DTA technique is applicable to virtually all polymers and many other material systems, revealing in most cases qualitative information about the thermal effects giving clear indications of the transition (endothermic or exothermic) temperatures, fig. 20.  The technique is commonly unsuitable for quantitative measurements of parameters such as heat capacity, heat of fusion or heat of crystallization (for crystallizable polymers) or change in specific heat associated with glass transition for amorphous polymers; quantitative measurements are, however, readily done employing differential scanning calorimetry (DSC).  In DSC, the test sample and the reference material are heated separately by individually controlled units.  The power or electrical energy inputs to those heaters are controlled and continuously adjusted consequent to any thermal effect in the test sample in such a manner as to maintain the two at identical temperatures.  The differential power or heat energy needed to achieve this state of affairs is recorded against the programmed temperature of the system. For transition involving latent heat such as for fusion, the heat of the transition (fusion) is determined by integrating the (heat) energy input over the time interval covering the transition in question.

Different polymers decompose over different ranges of temperature releasing some volatiles and leaving some residues.  Thermogravimetric analysis (TGA) is a useful analytical technique for recording weight loss or weight retained of a test sample as a function of temperature, which may then be used for an understanding of the chemical nature of the polymer.  Along with the analysis of the released volatiles and the residue left behind, TGA provides information about thermal stability, and decomposition of the material in an inert atmosphere or in air or oxygen and about moisture content and other volatiles or plasticizer content, ash content and extent of cure for cross linked polymer.  The test sample is placed in a furnace while it remains suspended from one arm of a precision balance.  The TGA thermograms are obtained by recording change in the weight of the test sample as it is held at a fixed temperature or as it is dynamically heated in a programmed manner.  TGA thermograms of some selected polymers are shown in fig.21.

Fig. 21:TGA thermograms of some selected polymers

(Courtesy: Tata McGraw –Hill, New Delhi)

References

Ghosh, P., Polymer Science and Technology – Plastics, Rubbers, Blends and Composites, 2nd ed., Tata McGraw Hill, New Delhi, 2002. Hiemenz, P.C., Polymer Chemistry – The Basic Concepts, Mercel Dekker, New York, 1984. Billmeyer, Jr., F.W., Text Book of Polymer Science, 3rd ed., Wiley – Interscience, New York, 1984. Schmidt, A.X., and C.A. Marlies, Principles of High Polymer – Theory and Practice, McGraw-Hill, New York, 1948. Mandelkern, L., Crystalization of Polymers, McGraw-Hill, New York, 1964. Wood, L.A., Advances in Colloid Science, H. Mark and G.S. Whitby Eds., Wiley Interscience, New York 1946, Vol. 2, pp. 57 – 95. Bekkedahl, N. and L.A. Wood, Ind. Eng. Chem. 23 (1941) 381. Geil, P.H., Polymer Single Crystals, Interscience, New York, 1963.

Selected Readings

1. Maiti, S., Analysis and Characterization of Polymers, Anusandhan Pub., Midnapore,

2003.

2. Turi, E.A. Ed., Thermal Characterization of Polymeric Materials, Academic Press,

New York, 1981.

3. Fried, J.R., Polymer Science and Technology, Prentice – Hall, Englewood Cliffs, 1995.

4. Treloar, L.G.R., Introduction to Polymer Science, Wykeham Pub., London, 1970.