Polymers with High Tg Values

For thermoplastic polymers with high glass temperatures, the value of at room temperature must be astronomical. So ft, calculated by Eq. (30), will be large. Hence polymers with high Tg values will exhibit clean detachment from the surfaces of hard solids interfacial crack opening will occur when a small force is applied. This prediction agrees with experiment. 

THERMAL CONTROL MECHANISMS IN ADHESION ADIABATIC THEORY 4.1. Pressure-Sensitive Adhesives 

Gruntfest(4i) has introduced an adiabatic catastrophe theory of the temperature rise in fracture. He used a model that made no hypotheses about the microscopic structure of the separation front. 

Physically, the temperature rise in a lament must be due to a combination of viscous dissipation and elastic (i.e., entropic) heating the Gough-Joule effect.(D The former is no doubt dominant in the case of fracture of high polymers(40) see below. But it is not possible at present to estimate the proportions of these two causal factors. It is to be expected that their 

FIGURE 12. Temperature-time curve for aluminum-backed tape at peeling speed 100 cm/min. (From Reference 1.) 

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Figure 5.9 shows the effect of EMA addition into PMMA on tan 8 of blends. The EMA has maximum dampness at around -4.1°C, whereas PMMA has dampness at 98.6°C (Table 5.11). The PMMA/EMA blends have maximum dampness values at 106.8,112.4,104.9, and 100.7°C, for 5,10,15, and 20% EMA blends, respectively. The magnitude of the peak is more or less characteristic of the relative concentration of the components and whether or not the phases are dispersed or continuous [40,52-55]. For instance, at a given concentration, tan 5 is greater if the polymer with high Tg value is in continuous phase [52]. 

The above reactions yield polymers with high Tg values. The materials, however, exhibited lowered thermooxidative stability. The same was found to be true when the ethynyl moieties were replaced by phenylethynyl groups... 

Banerjee et al, also prepared several diamine monomers [74-76] (3-20, 3-21, 3-22 in Table 3,3) with high FFV that were used for gas separation, and studied properties such as gas permeation and gas selectivities for several gas pairs, Banerjee et al, prepared high-Tg polymers by incorporating imido aryl [77] moiety (3-23 in Table 3,3) and further studied the gas separation properties, A fluorinated diamine monomer with anthracene moiety [78] (3-24 in Table 3,3) was also synthesized by Banerjee et al, the resulting polymers had high Tg values in the range 256-330 °C,... 

It is possible to distinguish between SBR and butyl rubber (BR), NR and isoprene rubber (IR) in a vulcan-izate by enthalpy determination. In plastic-elastomer blends, the existence of high Tg and low Tg components eases the problems of experimental differentiation by different types of thermal methods. For a compatible blend, even though the component polymers have different Tg values, sometimes a single Tg is observed, which may be verified with the help of the following equation ... 

Other interesting examples of the behaviour of the considered samples are given in Fig. 32. Samples with high Tg which are chemically less defective have lower E25 values. These anomalies in the mechanical properties of epoxy polymers prepared at low Tcure are very difficult to understand in terms of conventional concepts of the relationship between the structure of polymer glasses with their mechanical properties. 

Figure 8 presents the temperature dependence of the dynamic viscoelasticity of the Si-containing polyacetylenes (23, 24). Poly(2c) shows a dispersion at low temperature because of the presence of the long n-pentyl group. From the sharp increase in tan 8, the glass transition temperature (Tg) of this polymer is about 150 °C. In contrast, poly(3) and poly(4a) hardly show dispersions at low temperature, and their TgS are about 200 or higher. The high Tg values of these polyacetylenes compared with those of most vinyl polymers can be attributed to their stiff main chain. 

It can be stated that polymers with flexible backbone chains have low Tg values and those with stiff chains have high Tg values. 

Within Class I, the same structural features that promote high crystalline melting points also yield high values of Tg. Consequently, there exists a rough correlation between T and Tg for this class of polymers (2. By controlling chain stiffness and intermolecular forces, polymers with high Tf and Tg or low T. j and Tg can... 

The following tables also fist values of Tg in order to facilitate the identification of polymers with high or low gas permeabilities and high or low gas selectivities. The TgS were either stated in the fisted references or, in some cases, obtained from other sources values of Tg reported by different investigators for a given polymer sometimes may differ by 10 °C or more, depending on the method and time frame of the measurements and on other factors. 

Both polymers can be solution cast onto Cu- or glass plates and crosslinked photochemically (e.g. with the Ta-catalyst C8, Scheme 9) or thermally (with arene ruthenium-phosphines of the type C3, Scheme 4, to give strong transparent films with low Tg values for P24 (-50°C) and P25 (-70°C) and relatively high Tg values for P26... 

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