Duratrax Undercarriage
Duratrax Undercarriage
Gas Rc Truck Kits
Gas Rc Truck Kits
5 Quick Factors When Buying A New Or Used RC Model
Buying a new or used RC model car is a rewarding and exciting process. Gas or electric? Truck or car? Helicopter or plane? There are many considerations you must address before making these decisions. However, there are 5 top factors that will help you on your decision.
Battery or gas?
The major factors determining your choice here are run time and noise. Gas RC cars, trucks, planes, or helicopters are usually loud, even with a muffler installed. The advantage of gas RC’s are that they can run a lot longer with a tank of fuel versus a battery powered RC. The average battery life on a RC vehicle will give you roughly 10 to 25 minutes of run time – at full speed. Some radio control trucks and cars deplete energy faster due to accessories like lights, horns and other gadgets.
Kit vs RTR?
When buying a new RC car or truck or even a plane, there are 2 directions you can take based on your patience, budget, and skill level. If you’re the adventurous type and enjoy working on a project from beginning to end then a Kit package is your best option. Kits involve putting together the entire RC car, truck, or plane together from a box of pieces into the final product, which you can be proud of your accomplishment! Kit packages are usually more expensive but are more modifiable. RTR or “Ready to Run” packages are ready to go outside of the box. Charge the battery or fuel up the tank and you’ll be enjoying your new RC within minutes, versus hours or even days compared to kits.
Location, Location, Location
Gas powered RC cars and trucks require space and are not ideal for apartment complexes or homes which could have neighbors calling in noise complaints to your local authorities. Radio controlled planes and helicopters need wide open spaces with little obstruction for safe operation. Electric RC’s can normally be operated anywhere as they are considerably less noisy than gas RC’s. Another consideration is terrain. Is your RC truck or car an off-road buggy or stomper truck or is it a track racer? For example, street RC’s perform better and tires last longer on pavement versus asphalt.
Wear and Tear
There are great deals out there on unwanted RC models, however it is important to understand the wear-and-tear of these vehicle before you shell out your hard-earned cash. Start with the heart. That is check the motor, the most important piece to the model. If it’s gas, check for oil leakage around the head and base of the motor. Shocks when worn out tend to leak oil as well so look at these with a close eye as well.
What’s your Budget
With prices for used buggies around $75 to fully loaded USED RC Helicopters peaking over $1,500, it’s best to have an understanding of your purchasing budget, but also your operational budget as well. After you buy your new or used radio controlled model the spending doesn’t stop there as you will have maintenance costs for tires, batteries, fuel, bushings, blades, servos, and more. Depending on how serious you are involved with your RC model will determine how much of an operating budget you will need.
About the Author
Contributor and frequent shopper of
RC gas trucks
, cars, and helicopters on
www.kyosho-rc-store.com
the leader in new and used rc cars, trucks, planes, and helicopters by Kyosho.
RC ADVENTURES – PT1 – AXIAL SCX10 HONCHO SCALE RC TRUCK KIT BUILD – INTRO
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Matchbox® Mega Rig® Pirate Ship! Building System Commercial
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Axial Crawler Upgrades
Redcat RS10 vs Axial AX10 – Can a low cost crawler compete?
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Cen Racing Engines
Cen Racing Engines
ACER Racing
History
ACER Racing was founded in 1989 and introduced Silicon Nitride balls to the racing scene that same year. Their Ceramic Nitride Pro Series balls made from pure silicon nitride material were a huge contrast to earlier ceramic balls made by Dan’s RC Stuff from white aluminum oxide which were not as hard but were very lightweight. Racers were complaining of brittle differential balls. Silicon nitride changed that. It was unbreakable and harder than traditional tungsten carbide balls yet weighed 79% less and lasted forever. This reduction in rotational mass in the differential contributed to faster lap speeds and quicker acceleration.
The 1996 release represented an improvement for ACER Racing as their Ceramic Nitride Pro Series ceramic ball bearings were released to the public. They incorporated diamond polished silicon nitride balls inside the ceramic ball bearings, used specific internal bearing tolerances, applied their SIN synthetic lightweight ball bearing oil to the ceramic balls in a proprietary process, engineered an HSR inner ball bearing cage and sealed the bearing with no contact dual seals. The HSR ball bearing cage tolerated much higher bearing RPMs with minimal friction and low heat and noise. It also allowed for precision ball alignment inside the ball bearing.
ACER Racing Team
Greg Degani
2002 IFMAR World Champion driving his Kyosho MP 7.5 with OS engine
Andrew Smolnik
ROAR 1/10 Pro Stock Buggy Champion and CEN Racing Engineer
Chad Bradley
Nevada State Champion
Jeremy Kortz
Placed third at 2004 IFMAR World Championships
Overall winner at the Best of the West Championships
Swiss Driver Shasa Lackner
Current European Champion
Youngest A Main driver at age 11
Youngest expert driver at age 12
Lee Martin
UK Champion and NEO Finalist
Current Releases
2008
A Line of Pro Ceramic skateboard bearings
2009
ACER SK8 Pro Skating ball bearings made from pure silicon nitride and silicon nitride balls
Notable achievements
First company to introduce ceramic ball bearings to the radio controlled car market.
First company to introduce silicon nitride balls to the radio controlled car market.
First USA IFMAR World Champion in 2002 was ACER Racing factory driver Greg Degani.
First RC company to use supermodels in their print ads (including July 2005 Playboy cover girl and Dancing with the Stars’ Joanna Krupa).
First RC company to design a model car with titanium turnbuckles and ceramic bearings (ACER Racing AR-8 pro 1:8 offroad car)
References
^ http://www.accuratus.com/silinit.html
^ http://www.serpent.com/news/Massimo Fantini
http://www.accuratus.com/silinit.html
http://www.serpent.com/news/Massimo Fantini
Categories: Radio controlled cars | 1989 establishmentsHidden categories: Orphaned articles from January 2010 | All orphaned articles
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Kyosho Mini Z MR03 First Drive 1280×720 HD MR-03
Axial Scorpion Video
Axial Scorpion Video
Boyd Rice – Sheet Metal cabinet Supplier – China Sheet Metal Fabrication
Biography
Rice became widely known through his involvement in V. Vale’s RE/Search books. He is profiled in RE/Search #6/7: Industrial Culture Handbook and Pranks!. In Pranks, Rice described his experience in 1976 when he tried to give President Ford’s wife, Betty Ford, a skinned sheep’s head on a silver platter. In this interview, he emphasized the consensus nature of reality and the havoc that can be wreaked by refusing to play by the collective rules that dictate most people’s perception of the external world.
In the mid-1980,s Rice became close friends with Anton LaVey, founder and High Priest of the Church of Satan, and was made a Priest, then later a Magister in the Council of Nine of the Church. The two admired much of the same music and shared a similar misanthropic outlook. Each had been inspired by Might is Right in fashioning various works: LaVey in his seminal Satanic Bible and Rice in several recordings.
Rice’s Social Darwinist outlook eventually led to him founding the Social Darwinist think tank called The Abraxas Foundation, named after the ancient Gnostic god Abraxas. The organization promotes authoritarianism, totalitarianism, misanthropism, elitism, is antidemocratic, and has some philosophical overlap with the Church of Satan. During an interview with Christian talk show host Bob Larson, Rice described the basic philosophy of the foundation as being “The strong rule the weak, and the clever rule the strong”.[citation needed]
Rice has documented the writings of Charles Manson in his role as contributing editor of The Manson File. Rice was a featured guest on Talk Back, a radio program hosted by the Evangelical Christian Bob Larson. In total, Rice made three appearances on Larson’s program.
Although Rice was sometimes reported to possess the world’s largest Barbie collection, he confessed in a 2003 interview with Brian M. Clark to owning only a few.
In 2000, along with Tracy Twyman, editor of Dagobert’s Revenge, Rice filmed a special on the Rennes-le-Chateau for the program In Search of… on Fox television. (The segment was later included in the 2002 version of In Search of… on the Sci Fi Channel.) Rice has done extensive research into Gnosticism as well as Grail legends and Merovingian lore, sharing this research in Dagobert’s Revenge and The Vessel of God.
Rice was involved in creating a Tiki bar called Tiki Boyd’s at the East Coast Bar in Denver, Colorado. Rice decorated the entire establishment out of his own pocket due to his fondness of Tiki culture, asking an open tab at the bar in return. Boyd has long expressed a love of Tiki culture, in contrast to the other elements of his public persona.[citation needed]
Tiki Boyd’s was given its name in his honor. Due to disagreements between Rice and the owners, Rice pulled out of the deal and reclaimed all of his Tiki decorations. The future of the bar as it remains now is uncertain. Rice plans to re-establish another Tiki Bar elsewhere in Denver.[citation needed] Music
Rice creates music under his own name, as well as under the moniker of NON and with contributors under various other project names. Early sound experiments
Rice started creating experimental noise recordings in 1975, drawing on his interest in tape machines and bubblegum pop sung by female vocalists such as Little Peggy March and Ginny Arnell. One of his earliest efforts consisted entirely of a loop of every time Lesley Gore sang the word “cry”. After initially creating recordings simply for his own listening, he later started to give performances, and eventually make records. His musical project NON grew out of these early experiments; he reportedly selected the name because “it implies everything and nothing”. Techniques and implementations
From his earliest recordings, Rice has experimented with both sound and the medium through which that sound is conveyed. His methods of expanding upon the listening possibilities for recorded music were simple. On his second seven-inch, he had 2-4 extra holes punched into the record for “multi axial rotation”. Another early LP was titled Play At Any Speed. While working exclusively with vinyl, he employed locked grooves that allowed listeners to create their own music. He was one of the first artists, after John Cage, to treat turntables as instruments and developed various techniques for scratching. Rice has been treating sounds from vinyl recordings as early as 1975. NON
Under the name NON, originally with second member Robert Turman, Rice has recorded several seminal noise music albums, and collaborated with experimental music/dark folk artists like Current 93, Death In June and Rose McDowall. Most of his music has been released on the Mute Records label. Rice has also collaborated with Foetus, Tony Wakeford of Sol Invictus and Michael Moynihan of Blood Axis. His later albums have often been explicitly conceptual.
On Might! (1995), Rice layers portions of “Ragnar Redbeard”‘s Social Darwinist harangue, Might is Right over sound beds of looped noise and manipulated frequencies. 1997′s God and Beast explores the intersection in the soul of man’s physical and spiritual natures over the course of an album that alternates abrasive soundscapes with passages of tranquility.
In 2006, Rice returned to the studio to record raw vocal sound sources for a collaboration with Industrial percussionist/ethnomusicologist Z’EV. In addition he and long time friend Giddle Partridge are recording an album titled LOVE/LOVE-BANG/BANG!, under the band name of Giddle & Boyd. Crowd control
Early NON performances were designed to offer choice to audience members who might otherwise expect only a prefabricated and totally passive entertainment experience. Rice has stated that he considers his performances to be “de-indoctrination rites”. Rice has performed using a shoe polisher, the “rotoguitar” (an electric guitar with an electric fan on it), and other homemade instruments. He has also used found sounds, played at a volume just below the threshold of pain, to entice his audiences to endure his high decibel sound experiments.
Rice coupled his aural assaults with psychological torture on audiences in Den Haag, the Netherlands, by shining exceedingly bright lights in their faces that were deliberately placed just out of reach. As their frustration mounted, Rice states that he:
..continued to be friendly to the audience, which made them even madder, because they were so mad and I didn’t care! They were shaking their fists at me, and I thought that at any minute there’d be a riot. So I took it as far as I thought I could, and then thanked them and left. Controversy
In 1989, Rice and Bob Heick of the American Front were photographed for Sassy Magazine wearing uniforms and brandishing knives. While Rice would later recall it as a prank, the photo has caused boycotts and protests at many of Rice’s appearances. When asked if he regrets the photo, Rice stated, “I don’t care. I don’t think I ever made a wrong move. The bad stuff is just good. America loves its villains”.
This photograph was additionally published in the book Blood in the Face: The Ku Klux Klan, Aryan Nations, Nazi Skinheads, and the Rise of a New White Culture by James Ridgeway.
Rice has responded to accusations of fascism by stating:
I’ve always done everything at my disposal to avoid labeling what I do, or to avoid being labeled myself… To be beyond any existing classification has always pleased me. Unfortunately, I have learned over the years that when you refuse to be categorized, there’s a world full of people (all entirely less well qualified) who are only too eager to pigeonhole what you do or think. That the pigeonholing is generally more a reflection of what they think, or assume, is fairly obvious The will to label will always prevail over what’s being labeled, usually at the expense of either truth or understanding… I have never made any secret of any of my thoughts or areas of interest. I’ve always been honest, open, and upfront. I have never pretended to be a nice guy, because I’m not. It’s fairly impossible to remain true to oneself and still be a “nice guy.” Similarly, only people as misanthropic as myself can be counted on not to have to lie to others, since we have the unique luxury of not caring what sort of opinions others formulate about us… When all is said and done, I have no great quarrel with being labeled a “fascist.” While it is not the whole story, it implies (to me) a sort of Marquis De Sade worldview that sees life in terms of master and slave, strong and weak, predator and prey. I know such views are highly unfashionable, but to me they seem fairly consistent with what I’ve seen to be true. If others choose to see the world in terms of sugar, spice and everything nice, that’s certainly their prerogative, and I would never dream of trying to tell them otherwise. However, I might suggest that they always keep a loaded pistol on the off chance that they could possibly be mistaken.[citation needed] Discography
Year
Title
Under
1977
The Black Album
Boyd Rice
1977
Mode of Infection/Knife Ladder – 7″
NON
1978
Pagan Muzak – 7″ with multiple locked grooves
NON
1982
Rise – 12″
NON
1982
Physical Evidence
NON
1984/1981
Easy Listening For The Hard Of Hearing
Boyd Rice and Frank Tovey
1987
Blood and Flame
NON
1990
Music, Martinis and Misanthropy
Boyd Rice and Friends
1991
Easy Listening for Iron Youth – The Best of NON
NON
1992
In the Shadow of the Sword
NON
1993
Ragnarok Rune
Boyd Rice
1993
Seasons In The Sun
Spell
1994
The Monopoly Queen – 7″
The Monopoly Queen (w/ Mary Ellen Carver & Combustible Edison)
1995
Might!
NON
1995
Hatesville
The Boyd Rice Experience
1996
Heaven Sent
Scorpion Wind (w/ Douglas P. & John Murphy)
1997
God & Beast
NON
1999
Receive the Flame
NON
2000
The Way I Feel
Boyd Rice
2000
Solitude – 7″ with locked grooves on B-side
NON
2001
Wolf Pact
Boyd Rice and Fiends
2002
Children of the Black Sun
NON
2004
Baptism By Fire (Live)
Boyd Rice and Fiends
2004
Terra Incognita: Ambient Works 1975 to Present
Boyd Rice/NON
2004
Alarm Agents
Death In June & Boyd Rice
2008
Going Steady With Peggy Moffitt
Giddle & Boyd Films
Pranks! TV! (1986, VHS), directed by V. Vale, RE/Search Publications
Charles Manson Superstar (1989)
Speak of the Devil (1995, VHS), about Anton LaVey, directed by Nick Bougas, Wavelength Video
Pearls Before Swine (1999), directed by Richard Wolstencroft
Nixing The Twist (2000, DVD), directed by Frank Kelly Rich, High Crime Films
The Many Moods of Boyd Rice (2002, VHS), Predatory Instinct Productions
Church of Satan Interview Archive (2003, DVD), Purging Talon
Iconoclast (2009 release date) Directed by Larry Wessel (www.iconoclastmovie.com)
Modern Drunkard (In Production), directed by Frank Kelly Rich Performance
Live in Osaka (DVD), features concert performance from Osaka, Japan, in 1989, with Michael Moynihan, Tony Wakeford, Douglas P., and Rose McDowall. Also includes Rice-made films Invocation (One) and Black Sun Print
RE/Search No. 6: Industrial Culture Handbook, RE/Search Publications (1983, ISBN 0-940642-07-7)
RE/Search No. 10: Incredibly Strange Films: A Guide to Deviant Films, RE/Search Publications (1986, ISBN 0-940642-09-3) (joint author)
RE/Search No. 11: Pranks!. RE/Search Publications (1986, ISBN 0-9650469-8-2)
The Manson File edited by Nikolas Schreck, Amok Press (1988, ISBN 0-941693-04-X)
Apocalypse Culture: Expanded & Revised Edition edited by Adam Parfrey, Feral House, (1990, ISBN 0-922915-05-9).
ANSWER Me!, issue #3 (1993, ISBN 0-9764035-3-6)
ANSWER Me!, issue #4 (1994)
Apocalypse Culture II, edited by Adam Parfrey, Feral House (2000, ISBN 0-922915-57-1).
Paranoia: The Conspiracy Reader, issue 32, Spring 2003. References
^ As stated by Shaun Patridge on the Unpop website:
^ Modern Drunkard Magazine Online staff writer list: ^ a b Vale, V. Juno, Andrea. Re/Search #6/7: Industrial Culture Handbook (1983) ISBN 0-940642-07-7
^ Juno, Andrea (Editor), Ballard, J. G. (Editor), Re/Search #11: Pranks (1987) ISBN 0-940642-10-7
^ “My Dinner with Bob Larson”, Snake Oil magazine (1994)
^ From The Black Pimp Speaks, 2003 interview with Boyd Rice appearing in Rated Rookie magazine #6, 2004. Viewable online: ^ Rice’s Official website for the project can be found here: ^ The official website for Tiki Boyd’s can be found here:
^ “Laugh til it hurts”. The Wire magazine (256).
^ “With Pity Towards None (interview)”. Tangents. 1997. http://www.boydrice.com/interviews/tangents.html.
^ official website, www.boydrice.com
Persondata
NAME
Rice, Boyd
ALTERNATIVE NAMES
SHORT DESCRIPTION
Author
DATE OF BIRTH
PLACE OF BIRTH
Lemon Grove, California, United States
DATE OF DEATH
PLACE OF DEATH Categories: 1956 births | Living people | Social Darwinists | Noise musicians | American industrial musicians | Sound artists | American SatanistsHidden categories: Articles needing cleanup from September 2008 | All pages needing cleanup | BLP articles lacking sources | Articles lacking reliable references from September 2008 | All articles lacking sources | All articles with unsourced statements | Articles with unsourced statements from November 2007 | Articles with unsourced statements from February 2008
About the Author
I am an expert from cnc-machiningparts.com, while we provides the quality product, such as Sheet Metal cabinet Supplier , China Sheet Metal Fabrication, machining parts supplier,and more.
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Axial Force Coefficient
Axial Force Coefficient
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.
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