Unsymmetrical polymers

A third factor influencing the value of Tg is backbone symmetry, which affects the shape of the potential wells for bond rotations. This effect is illustrated by the pairs of polymers polypropylene (Tg=10 C) and polyisobutylene (Tg = -70 C), and poly(vinyi chloride) (Tg=87 C) and poly(vinylidene chloride) (Tg =- 19°C). The symmetrical polymers have lower glass transition temperatures than the unsymmetrical polymers despite the extra side group, although polystyrene (100 C) and poly(a-meth-ylstyrene) are illustrative exceptions. However, tacticity plays a very important role (54) in unsymmetrical polymers. Thus syndiotactic and isoitactic poly( methyl methacrylate) have Tg values of 115 and 45 C respectively. 

Unsymmetrical polymers were defined as those containing a main-chain atom that does not have two identical substituents. Other polymers are regarded as symmetrical. Since then many workers have used this relationship as a rule of thumb. 

The majority of the polymers have Tg/Tm ratios between 0.56 and 0.76 with a maximum number around 2/3 both symmetrical and unsymmetrical polymers belong to this group. 

There exists a fairly good correlation between the Tm and Tg for a large number of polymers. A useful rule of thumb is that the ratio Tg/Tm is 1 /2 for symmetrical polymers, i.e. those containing a main-chain atom having two identical substituents, and 2/3 for unsymmetrical polymers. 

The stmctural dependence of the crystalline melting temperature is essentially the same as that for the glass transition temperature. The only dilTerence is the effect of structural regularity, which has a profound influence on crystallizability of a polymer. T is virtually unaffected by structural regularity. From a close examination of data for semicrystalline polymers it has been established that the ratio Tg/T , (K) ranged from 0.5 to 0.75. The ratio is formd to be closer to 0.5 in symmetrical polymers (e.g., polyethylene and polybutadiene) and closer to 0.75 in unsymmetrical polymers (e.g., polystyrene and polychloro-prene). This behavior is shown in Figure 4.9. 

While Tjn is a first-order transition, Tg is a second-order transition and this precludes the possibility of a simple relation between them. There is, however, a crude relation between T and Tg. Thus the ratio Tg/Tn, range from 0.5 to 0.75 when the temperatures are expressed in Kelvin. The relation is represented in Figure 1.22 where a broad band covers most of the results for linear hompolymers and the ratio (Tg/Tm) lies between 0.5 and 0.75 for about 80% of these [ 17,18]. The ratio is closer to 0.5 for symmetrical polymers such as polyethylene and polybutadiene, but closer to 0.75 for unsymmetrical polymers, such as polystyrene and polyisoprene. The difference in these values may be related to the fact that in unsymmetrical chains with repeat units of the type -(CH2-CHX-)- an additional restriction to rotation is imposed by steric effects causing Tg to increase, and conversely, an increase in symmetry lowers Tg. 

Helfand, E. and Sapse, A.M. (1975) Theory of unsymmetric polymer-polymer interfaces. J. Chem. Phys., 62 (4), 1327-1331. 

The ratio generally is higher for symmetrical polymers such as poly(vinylidene chloride) than it is for unsymmetrical polymers such as isotactic polypropylene. 

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