Glass transition main-chain polarity

FACTORS AFFECTING THE GLASS TRANSITION IN AMORPHOUS POLYMERS 197 Main-chain polarity... 

This combination of monomers is unique in that the two are very different chemically, and in thek character in a polymer. Polybutadiene homopolymer has a low glass-transition temperature, remaining mbbery as low as —85° C, and is a very nonpolar substance with Htde resistance to hydrocarbon fluids such as oil or gasoline. Polyacrylonitrile, on the other hand, has a glass temperature of about 110°C, and is very polar and resistant to hydrocarbon fluids (see Acrylonitrile polymers). As a result, copolymerization of the two monomers at different ratios provides a wide choice of combinations of properties. In addition to providing the mbbery nature to the copolymer, butadiene also provides residual unsaturation, both in the main chain in the case of 1,4, or in a side chain in the case of 1,2 polymerization. This residual unsaturation is useful as a cure site for vulcanization by sulfur or by peroxides, but is also a weak point for chemical attack, such as oxidation, especially at elevated temperatures. As a result, all commercial NBR products contain small amounts ( 0.5-2.5%) of antioxidant to protect the polymer during its manufacture, storage, and use. 

Figure 6.4. Power factor-temperature curves for three polar polymers whose polar groups are integral with or directly attached to the main chain. The rise in power factor above the glass transition point is clearly seen in these three examples...

The dielectric properties of polar materials will depend on whether or not the dipoles are attached to the main chain. When they are, dipole polarisation will depend on segmental mobility and is thus low at temperatures below the glass transition temperatures. Such polymers are therefore better insulators below the glass temperature than above it. 

The increase in the length of the side chain results normally in an internal plasticization effect caused by a lower polarity of the main chain and an increase in the configurational entropy. Both effects result in a lower activation energy of segmental motion and consequently a lower glass transition temperature. The modification of PPO with myristoyl chloride offers the best example. No side chain crystallization was detected by DSC for these polymers. 

The temperature position of the secondary fi relaxation (about 290 K 1 Hz), generally attributed to partial rotations of the side chains COOR, is only slightly affected by the polarity and volume of the substituent R but decreases markedly (by 120 K) on removal of the a-methyl group on the main chain. The experimental data obtained contradict the assumption that there is a certain relationship between this temperature and the glass transition temperature. Nevertheless, we can infer that the pertinent molecular mechanism in polymethacrylates differs from that in polyacrylates, probably due to the different participation of the main chains. The values of the individual contributions to the activation energy were estimated by employing a procedure similar to that used in the y relaxation process, and their sum was found to agree approximately with the experimental values. 

The influence of a stiff and bulky substituent on the thermal and the solution properties is governed by the size, polarity, position, and number of the substituents [24]. The main differences between flexible and bulky substituents are their influence on the thermal stability and the glass transition temperature. Whereas flexible chains act as an internal plasticizer and lower the thermal stability, most bulky substituents increase Tg and, depending on the chemical nature, do not lower the thermal stability. 

The photoluminescence of soluble l,3-bis(phenyl-1,3,4-oxadiazole)s with polar groups in the main chain such as —NO2, —OH, —CH3 and —Cl has been studied [45]. The photoluminescence exhibits a significant shift in the emission wavelengths in solid as well as in solution depending on the groups attached in the main chain, both a red-shift and a blue-shift and change in the intensity of the emission in comparison to those of the simple polyoxadiazoles at around 300 nm. The polymers are thermally stable up to 350-400 °C and the glass transition temperature is of 100-150 °C. 

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