Tacticity glass transition effects

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. 

The glass transition temperature of amorphous polymers is a function of the chemical structure of the polymer chain. It varies widely with the types of skeletal atoms present, with the t T)es of side groups, and with the tacticity of side groups along the polymer backbone. Table 14.11 demonstrates the effects of structural variations on the crystalline melting temperature and glass transition temperature for several polymers. 

Grohens, Y., Hamon, L., Carriere, R, HoU, Y., and Schultz, J., Tacticity and surface chemistry effects on the glass transition temperature of thin supported PMMA films. Mater. Res. Soc. Symp., 629, FF171-FF177 (2000). 

Table 8.12 Effect of tacticity on the glass transition temperatures of polyacrylates and polymethacrylates (141)...

In practice TMA is mostly used for measurement of the glass transition temperature by measuring CLTE and on occasion volume expansion. Therefore, it is appropriate to review those chemical factors (effects of pendant groups, molecular mass, tacticity, crystallinity, crosslinking, etc.) that have the largest influence on the glass transition temperature and the expansion of a polymer. 

Tsuji (2002) conducted an investigation into the hydrolysis of an amorphous form of PLA, to determine the effects of L-lactide content, tacticity and enantiomeric polymer blends. In this work four samples were prepared—poly(D,L-lactide), poly(L-lactide), poly(D-lactide) film and the blend sample of poly(L-lactide) and poly(D-lactide). The results are sununa-rized in Table 7.4, which also covers a complementary study that explored the effects of hydrolysis in terms of molecular weight and its distribution, glass transition temperature, crystallization temperature, melting temperature and mechanical properties. 

Relaxations below the glass transition have received widespread study, using principally dynamical mechanical or dielectric relaxation techniques (2). Some effects of tacticity have been noted but will not be discussed here. The effect of configuration on the properties of the quasi-isolated macromolecule, i.e. in dilute solution under e-condition, is also of great interest but is outside the scope of this report. 

Vinyl monomers, such as styrene, and alkenes with a side group, such as propylene, can polymerize in several molecular forms whose crystallization behaviors are quite different from each other. If the side groups are all on one side of the backbone, the structure is called isotactic, and if they are on alternating sides, it is called syndiotactic. If they are distributed in a random fashion, the polymer is said to be atactic. The isotactic and syndiotactic forms are crystallizable, often in a helical structure, while the atactic form does not crystallize and solidifies only at its glass transition temperature. Figure 2.3 illustrates the tacticities mentioned above for the case of polypropylene. It has been found that polypropylene tacticity can also have an important effect on chain dimensions [10] and on the rheological behaviour of the melt [11]. 

Biswas CS, Patel VK, Vishwakarma NK, etal. Effects of tacticity and molecular weight of poly(N-isopropylacrylamide) on its glass transition temperature. Macromolecules. 2011 44 5822-5824. 

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