E
Polymer Glass Transition Temperatures David R. Burfield University of Malaya. Kuala Lumpur 22-1 1, Malaysia
A recent article ( I ) in this Journal, in providing an overview of the glass transition temperature (T,), discusses a t some length the factors that determine the actual value of the same. Unfortunately the authors chose to exemplify the paper . - with outdated experimental values that lead t o the perpetuation of certain erroneous conclusions concerning the effect of microstructure and morphology on the value of
7;. For example, in discussing the effert of cisttrans isomerism un 7'".values are auoted that show that the transition tempera&e of trans-i,4-polyisoprene is 20 OC higher than the cis analoeue. However, recent DSC studies (2) have shown conclu~ivelythat the values of the cis and trans isomers are comparativelv close with the trans form having a slightly lowe; glass transition temperature (see tahle).It would seem therefore that there is little difference in the rotational freedom of these two isomers, and consequently ex~lanations(3) put forward to rationalize the supposed substantial differences are redundant. In contradistinction, i t is asserted that for certain polymers. such as oolvoroovlene. -. ." . T. mav be independent of the sterebisomordhic structure. DQC ieasure&ents ( 4 ) show the reverse to he true. Admittedly the differences are less pronounced than observed with disubstituted monomers such as methvl methacrvlate, nevertheless, the observed Tg,.b,t;, > T,j,,t.,ts is clearly the same. The trend T,,,,ai, series demonstrates increasing segmental rotational freedom in the transformation from syndiotactic through to isotactic configurations. Precise measurements on wellcharacterized stereoisomorphs of other monosubstituted polymers will presumably lead to similar trends. Furthermore, the effect of morphology on polymer Tg values is probably not as arbitrary as the authors appear to sueeest in citing examoles where crvstallinitv variouslv en"hances (5) or riduces 76) the glass transition temperature. The sole auoted e x a m ~ l of e the latter trend is deduced from studies df poly(4-methyl-1-pentene).This is perhaps a uniaue case in that the density of the crystalline regions is apparently lower than that of the amorphous polymer a t temperatures in the vicinity of T,. In addition, the experimental data showing the relationship of T, to percent crystallinity was drawn from a series of distinct samples and the authors of the original paper suggested that the Tgvariation was in fact a consequence of differences in tacticity rather than crystallinity per se. Recent studies (2) with both cis- and trans-1,4-polyisoprene have found that the crystallized polymers show only very marginal T, increases (0.3 to 1.4 O C ) compared with their completely amorphous counterparts. On the other hand, an examination (7) of the thermal properties of polypropylene show that the presence of crystallinity shifts the ohsewed T, to much higher values. It seems likely that the effect of crystallization is largely determined by the size distribution of the crystalline and amorphous regions. Morphologies characterized by relatively large undisturbed amorphous regions hounded or interspersed by crystallites will show little change in values of T.. This is typical of the hehavior of the polyisoprenes. For these polymers, the crystalline regions will act as inert fillers similar ro the case of frozen natural rubber latires (8)where the ice rrystals have no effect on the rubber T, hecause of the relatively large size of the rubher particles (amorphous regions). Ry contrast,
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Glass Transition Temperatures of Various Polymers
G
Polymer
('C)*
cis-1.4-poiyisoprene natural rubber
(-73)
n.s.
n.6
1
-67 -67 -55
DSC DSC DSC
amorphous ~emicrystalline amorphous
2 2 2
n.s. DSC DSC DSC DSC
ns.
1
amorphous semicrystalline amorphous ~emicry~lalline
2
n.s.
n.6.
DSC DSC DSC
amorphous amorphous liquid nitrogen qmched Sample
NATSYN 2200
Techniqueb
Mwphologyb
Reference
(98.4%CIS) mmt.4polylsapene (-53) gutta percha -70 -59 -71 -71
balata polypropylene syndiolactic PP atactic PP isotactie PP
(+5) -4 -6 -18
'
2 2 2 1 4 4 4
polymers such as polypropylene, which undergo rapid crystallization on cooling from the melt, will have amorphous regions frequently interrupted by crystallites. The constraints put on the noncrystalline chain segments connecting the crystalline areas (tie molecules) will effectively reduce chain mobility and raise observed T, values. In this case the actual value of T, will probably depend on the length of the tie molecules and the abundance of free chain ends. Much of the confusion that arises in comparative discussion of polymer T, values stems from the failure to appreciate that a given polymer structure or even a given sample does not haveaunique value that can be assigned as the glass transition temperature. This arises for two reasons: 1. The determined value of T,is grossly dependent on the method of measurement because of variation in test frequency and the
time-denendentnature of the class transition. For example, the elass transition temoerature of nolv(methvlm~thacrvanoarkt .. . . latr) hns hem dwrved to increme from 1 I0 to 160 'C on rhanging from a dilatumctrir ton rehound elasticity terhnrque (91. 2. F m a given polymer structure thr 7; value depends upon many v s r i a l h rrlating to both the morphology and microstructure of the macromolerule. The morphology in turn may b~ crucially dependent on the thermal history of theramplr.
~.. ~
For these reasons. meaningful comparisons can be made only u,hen rmplovingdataobtained hy similar terhniqueson well characterbed samples of known rhermal hktory.
Literature CRed 1. Beck, K. R.; Korsmeyer, R.: Kunz, R.J. J. Chem. Educ. 1984,61,868. 2. Rurfirld, O.R.; Lim,K. L. Morromolacuiaa 1983,16,1170. 3. Brydson. J. A. In Polymer Science; Jenkins, A. D.. Ed.; North-Holland: Amsterdam.
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7*"".
4. Burfield. D.R.:Doi. Y. Mocromoleeules 1983,16.702
Volume 64 Number 10 October 1987
875
