Glass transition temperatures factors affecting

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. 

Chemical structure factors affect the melting point and glass transition temperature in much the same manner. A good empirical rale for many polymers is (142-144)... 

Table 1 lists the glass transition temperatures for the pertinent siloxane oligomers as a function of TFP and DP contents. The percent of each comonomer is recorded with reference to the siloxane units as well as the entire oligomer. One notes the difference that this creates between the two nominally 100% TFP siloxanes of different molecular weight. Note also the higher Tg values for the DP series at equal weight percents, a factor which limits their ease of synthesis and may affect their mobility during cure. 

The influence of factors such as chemical structure, molecular weight, cross-linking and plasticizers in the glass transition of polymers can be related to the changes that they provoke on the free volume fraction, which, as we already know, reaches a critical value at the glass transition temperature. The factors affecting the glass transition can be classified into two types (1) molecular factors, i.e., those related to the chemical structure of the polymer chain, and (2) external or controllable factors. 

Controllable Factors Affecting the Glass Transition Temperature... 

Both the dipole-relaxation time and the ionic conductivity are related to the glass-transition temperature Fg. As a material is heated through its glass-transition temperature, static dipoles gain mobility and start to oscillate in an electric field. This causes an increase in permittivity and a loss-factor peak is noted. Obviously this motion is affected by frequency (lower frequencies have greater effects). This effect is shown in Figure 3.62 (Prime, 1997a), which shows the peaks in permittivity and loss factor at Tg. 

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