An Overview of the Glass Transition Temperature of Synthetic Polymers Keith R. Beck1 Department of Consumer Sciences and Retailing, Textile Science. Purdue University, W. Lafayette, IN 47907 Richard Korsmeyer2 School of Chemical Engineering, Purdue University, W. Lafayette, IN 47907 Rosemary J. Kunz Rohm and Haas Company, 5750 W. Jarvis Ave., Niles. IL 60648 The growing importance of polymer chemistrv in hiaher education hasbeen underscored by recent articies in THIS JOURNAL. An initial series of papers on the development of polymer chemistry ( 1 3 ) was foliowed by an entire issue devoted to that subiect (4). In addition to several articles on "basic principles of polymer chemistry," there were papers dealing with "aids to learning and teaching- -polymer chemistry." Athird excellent series dealt with some areas of current research in polymer characteristics and properties, e.g., molecular weight and molecular weight distribution (5), morphology (6),rheology (7), mechanical properties (S), and characteristic of rubber elasticitv. (9). . . A verv . imoortant . amorphous polymers, the glass trnnsition temperature (T,). was discussed brieflv ( 8 ) .The intent of t his article is to oresent general glass transiGoi temperature information in hopes that it can be used in the same educational mahner as that found in the previously referenced papers. Detailed reviews on this subiect can be found elsewhere (10-13). Why is the glass-u-ruhher transition temperature so imoortant? It is one of the orooenies that dictates oossible uses L r polymers. lloly(dim&h;i siloxane) has aver; low 'I.,(see Table 1 ) and therefore retains its lubrirarinc oronrrties even under extremely cold conditions. The loweriiktof use of an elastomer, e.g., cis-1,4-polyisoprene (natural rubber), is determined by its T,, i.e., below its T, i t is no longer elastic. Processing conditions may also be dependent on T,, e.g., both nylon 6,6 and poly(ethy1ene terephthalate) fibers are drawn to reduce filament diameter and increase strength just above their glass trsnsiriun temperatures.'l'he T,of'pdy&rs, from arrqlic hinders for non-woven fahrics to . polvethvlene for en. . gineering applications, is obviously a very important property. ~
Background The degree of molecular order in polymers may range from totally amorphous (disordered) to completely crystalline (highly ordered). Most polymers are partially crystalline and when heated, will pass through two major transitions, Tg and T,, the melting point. Other minor transitions are discussed elsewhere and will not be included here (10,12). Below T,, the macromolecules exist in the glassy state and hehave as a solid. Thermal energy is adequate only for motion by atoms and small groups of atoms restrained hy bonding forces in the amorphous regions. Movement by bond rotation is quite limited. Atoms in the crystalline regions vibrate about their equilibrium positions in the lattice. As the temperature is raised, more energy becomes available and, at the glass transition temperature, large scale (2&50 chain atoms) segmental movement by bond rotation becomes possible. This large scale
' Author to whom correspondence should be addressed.
Present address: B. F. Goodrich R&D Center, Brecksvitle, OH
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Table 1. Glass Transltlon Temperatures of Various Polymers (from Brydson ( 10)) Polvmer
TX1
cis-1.4-polyisoprene trans-1.4-po1yisoprene nylon 6.6 poly(ethy1ene terephthalate) poly(ethyleneadipate) polyethylene polypropylene poly(viny1chloride) poly(1-butene) poly(rnethy1methacrylate) (atadic) poly(rnethy1methacrylate) (isotactic) ~ ~ l v l r n e t hmethacwlate) vl lsvndiotacticl
movement causes an increase in the specific volume of the polymer. The transition is also accompanied by an increase in the free volume of the polymer. The free volume is the difference between the specific volume of the polymer and the space actually occupied by its atoms, and may he thought of as "elbow room" for the chains (14). For most glass-forming polymers, the free volume fraction, the ratio of the free volume to the specific volume is nearly constant (0.025) a t or below T,,and increases as the temperatue rises above T, (15). As the temperature of the polymer continues t o rise, T, of the crystalline regions will be reached, and the solid will become liquid. At temperatures between T, and T,, a semicrystalline polymer usually will behave as a tough flexible thermoplastic. An obvious demonstration of the difference between the properties of the glassy state and the rubbery state is the shattering of natural rubber or Tygonmtubing that has been cooled below its glass transition temperature. In such a state, the polymer molecules are rigidly held in the glass matrix and, since the chains cannot move to absorb a sudden shock, the material shatters. Above the T,, the shock can be absorbed by transformation into large scale molecular movement and the polymeric tubing will recover from the blow. The changes which occur on the molecular level a t the glass transition have a number of macroscopic consequences. The change in volume may be observed by dilatometry; changes in thermal properties such as Cp may be observed by differential scanning calorimetry (DSC); and changes in physical properties may be observed by appropriate mechanical tests. The latter is one of the more sensitive methods for detecting T,. While changes in volume or heat capacity may be rather subtle and difficult to detect, the stress-relaxation modulus-a measure of the "stiffness" of the ~olvmer-commonlv decreases by a factor of approximateiy ~ O O Oas the polymer is heated above T, (16).Variations in experimental conditions
combined with the difficulty of preparing reproducible polymer samples, can lead to fairly large discrepancies in the reported values of T , for a given type of polymer. Values from -130°C to +60°C have been reported for polyethylene, an otherwise well-characterized polymer (10). Thermodynamics of the glass transition have been discussed by a number of authors including DiMarzio (17, la), Rehage (13,191, and Kovacs (20). However, the glass transition as determined in the laboratory is strongly affected by kinetic factors. Choy and Young observed that the transitions of poly(viny1 chloride) depended on the heating rate (21). Furthermore, dvnamic measurements show that the temperature of the r&xations associated with 71g is a function of the wst frequency (161.Complete understanding of the glass transition may deprnd upon thedevelopment of theoriesthat take into awount hoth thermodynamic and kinetic factors. Factors that Affect T, In addition to the previously mentioned factors of heating rate and method of measurement, there are sevrral structural characteristics that affect the T , of polymers. These include: 1. Structure of the monomer 2. Ratio of co-monomers 3. Sequence distribution of co-monomers 4. Stereochemistry of the polymer 5. Percent crystallinity of the polymer 6. Amount of crosslinking I . Molecular weight 8. Concentration of plasticizer
The structure of the monomer is a major factor in determining the T , of a homopolymer because i t will determine chain stiffness. The more rigid a polymer chain, the greater the resistance to large scale movement, and the higher the temperature (T,) required for such movement. Chain stiffness or flexibility can be engineered into the polymer by varying the size and the nature of the side chain. If one of the hydrogens in ethylene is replaced by a methyl group, the resulting chains of polypropylene will have a higher harrier to rotation than those of the unsubstituted polyethylene. The methyl groups increase the rotational energy barriers of the chain and increase the T , 25OC (see Tahle 1). If, in addition to therestricted rotation, a group adds polarity and the attendant repulsive effects to the chain, the T. will be increased even more. A chlorine atom has about thi! same size as a methyl group, hut the T , of poly(viny1 chloride) is 75' above that of polypropylene (see Table 1). Side chain alterations also may he employed to lower T,. If the pendant group becomes so large as to force the chains apart and decrease intermolecular attractions, then T , will he decreased. In the case of poly(1butene). the additional carbon atom and its attached hvdrogens in'the side chain (relative to polypropylene) have caused an internal nlasticizine effect that has counteracted the chain-stiffening hy the'first carbon in the group, and the T, is the same as that nf odvethvlene (seel'ahle 1). If the chain " " becomes sufficiently iong (greater than twelvecarbons), T g mav aeain beein to increase with chain leneth as a result of ent&aemen&d increased interside chain keractions (22). The chain-stiffening effect also can be observed in step reaction polymers by the substitution of a rigid benzene ring in poly(ethy1ene terephthalate) for the flexible butylene chain in poly(etby1ene adipate) (see Tahle 1). The ratio of co-monomers will also influence the T gof a polymer. The sequence distribution of monomer units in a polymer chain may also be important. A copolymer whose structure is -A-B-A-B-A-B- will be different from one whose structure is -A-A-A. . .-B-B-B-. The glass transition of these polymers is not a simple weighted average of the T,'s of the co-monomers. Reasonably accurate methods for predicting T , under these conditions are given by Johnston (23). Table 2 shows the T , and approximate composition for a series of
commercially available acrylic emulsions that can be used to demonstrate this influence as well as side-chain effects. Ethyl acrylate is common to all four polymers, and the variations in the glass transition temperature and film stiffness are created by varying the amount and type of co-monomer. T o decrease the T , of (B), butyl acrylate is added, giving side chains that exert the aforementioned plasticizing effect. T o elevate T , and make the films generated by these emulsions more rigid, methyl methacrylate is added as a co-monomer. In this system, the amount of co-monomer with the chainstiffening methyl group seems to correlate very well with the elass Films of these "~~transition temnerature of the conolvmer. . emulsion^ generated by drying tur 24 h a t ambient tempcrature in a Petri dish provide vivid evidence of these monomer effects on 7;. I4lm (A) is very soft, elastic, and pliable hecause its T. is below n n m temoerature. while film (1)) is toueh and rigidehecause it is in a giassy state. End uses f i r these films nro~erties are reauired. dictate which T. and attendant . . Stereochemistry of macromolecules may also affect their T.. but t h ~ w relationshios are more difficult to exvlain and than the effects. A number of different explanations for the lower T,of trans-1,4-polyisoprene (gutta percha) relative to cis-14-polyisoprene (natural rubber) (see Tahle 1) have been offered, hut there does not appear to he a consensus as to which is most correct (10). The effect of tacticity on T gvaries from polymer to polymer. Fairly large differences in T , may be noted in Tahle 1for three different poly(methy1 methacrylate) polymers. These differences are probably caused by the varying degrees of interactions hetween the pendant . groups in the stereoisomorphs. In other polymers, e.g., polystyrene and polypropylene, T g may he independent of stereoisomorphic structure (10). Increasing crystallinity should decrease chain mobility and increase T,. As the volume of crystalline domains increases, the length6f chain segments existing in amorphous regions decreases and the molecules may experience increased tension. In either case. decreased mohilitv should increase T.. This has been ohserveb with poly(ethylene terephthalate) (54),hut an increase in the crystallinity of poly(4-methyl-1-pentene) was accompanied by a decrease in T , (25). I t is apparent that the relationship between crystallinity and T , is not a simple one. Crosslinkine" is still another feature that should restrict the mohility of polymers in amorphous regions. Increased crosslinkine ahould decrease free volume and increase T,. For low to moierate levels of crosslinking with agents simcar to the dominant monomers in the chains. this assertion seems to hold. Ct)polymeriration of 8.5% divinylbenzene with styrene rrsults in a crosslinked oolvmer that h a a T , that is higher (by 4OC) than that of poiysiyrene (10). At hiiher concektrations of crosslinkine aeent.. a similar increase in the amount of divinylhenzene would produce a much smaller increase in T.. As with the effects of crvstallinitv, the changes in T. caused by crosslinking are sometimes &re complex than thg example given here. In addition, increased crosslinking or crystallinity makes T , more difficult to observe, since chain mobility in the rubbery state is reduced. ~
~
~.
.
Table 2.
Glass Transnion Temperature and Composltlons Of Acrylic Emulsions
Polymern
Approximate Composition
T,
(A) K-3
so:so ethyl acrylate: butyl acrylate ethyl acrylate 67:33 ethyl acrylate: memyl methacrylate 5050 ethyl acrylate: methyl methacrylate
-3Z°C
(B) H A 4
(C) HA-1 2 (D) HA-16
-lO°C 13% 29'6
RDhm and Hsas Canpamy.
Volume 61 Number 8 August 1984
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The effect of molecular weieht on T. should be a direct one. Az the molecular we~ghtincreases, there should he fewer cham ends. lcadine to a lower free volume and a h~eher7'".Fox and lor; (15) slggested the following relationship hefween molecular weight and T,:
Tg= Tg,-KIM,, where M. = the number-average molecular weight, T, = the glass transition temperature of the sample, and T,,, = the elass transition temoerature of that nolvmer with M.. = m. " ?his relationship was shown to hold for polystyrene. Structural modifications have been mentioned as means of altering the T, of polymers. A physical means of changing the level of intermolecular interactions and the T, is the addition of a plasticizer to the polymer. Plasticizers are added to improve processibility by reducing viscosity and to change the glass transition temperature. Water acts as a hydrogenbonded ~lasticizerin nvlon and a small Dercentaee of i t will decrease the T, of the bolymer to the pbint where it can he drawn at room tem~erature.Di(2-ethvlhexvl)~hthalate.also known as dioctylphthalate (DOP), and: hutil benzylphthalate are plasticizers commonly used to modify the T,of poly(viny1 chloride). Addition of 10%DOP decreases the transition from 80°C to 60°C, while 50% of this plasticizer will lower T, t o -30°C (26).Tygonm tubing is an example of a well-plasticized polvmer with a T, lower than room temoerature. while the Bame poly(viny1 choride) in an unplastikzed state is sufficiently rigid and strong to he used in pipe and drainage tile. The plasticizer does not have to be an additive. I t may tome from the environment of the polymer. In other words, water or o r g ~ n i ciolvents may penetrate a polymer and chnngt. its properties during use. This is ol~viouslyan important factor when a polymer is hving used as a structural rnemher or as a barrier (packaging films, rigid containers, or semipermeahle membranes) sincc the permeability of a ruhhery polymer is considerably higher than that of a glassy one. The rtveae may also occur when a plasticizer evaporates or is leached out hy a solvent. 'l'hus, flexible parts may become brittle with age. (Degradation of the polymer under the influence of oxygen and UV light is usually a more important factor.) An example freauentlv observed in the lahoratorv occurs when Tveona "" tubing is used to transfer a solvent such as petroleum ether. The hvdrncarbon swells the tuhine onlv sliehtlv but the tuhine is much more rigid upon drying,\eca"use~hebo~(or what-
.
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Journal of Chemical Education
..
ever plasticizer is used) has been partially extracted. In addition, most polymers are plasticized by their monomers. Therefore, the amount of unreacted monomer remaining in a resin is of some importance, not only due t o the toxicity of monomers like vinyl chloride, hut also for the final properties of the plastic. This article has attempted t o present an overview of the glass-to-rubber transition, what it is, why i t is important, and the major factors that influence it. With polymers destined to play an even more significant role in the future, i t seems logical that more information concerning them should find its way into our curricula. Acknowledgment The helpful suggestions of Nikolaos Peppas and John Aklonis are gratefully acknowledged.
18) Aklonis. J. J.. J. CHEM.E ~ ~ ~ . . 5 8 , 811981). 92 (9)Mark,J.E.. J.C~eM:'Eo~~.,58,898(1981). I101 Brydson, J.A..in "PoIymerSEien&,.l Vol. 1. (Editor; JcnLins.A. n.).Nortb-Hollmd
(Editors: Kaufmsn, H. S., and Fdcetta, J. J.), Wik-lnteteienca. New York,
.~~. ~~.
(18j ~ i ~ a n ~:~,in.,strun&aod~obilityinG~~&and~tcanic~~asaas,"(mito~s: i* O'ReiUy, d. M..and Goldstein, M.), New York Academy of Science. New York. IPS,.
119) ~ehapLG.and Bouohard, W., in"% PhyeicaofGhyPolymarr" (Editor:Haward. R. N.). John Wiley & &m, New York, 1973. 120) Kovae, J., J. Phyr. Chem., 85,2060 (1981). (21) Chov,C.L,andYoung,K.,Polymer, IY,lW1(1978). (22) Overbeww,C.G.,Amnd.L. H., Wiley,RH..andCanen,R. R., J.Polym. Sei.,7,431
