Glass transition temperature steric effects

Steric factors - branching at or close to the hydantoin ring - raised the glass transition temperature while maintaining the shielding effect. Amines of different structures were used as room temperature curatives with a few representative resins, to observe the effect on hydrophilic-hydrophobic balance. Solvent effects were examined on aromatic amine-cured resins the most hydrophilic cured system proved to have the broadest range of lyophobicity. 

In discussing the influence of steric features on mechanical properties, it is convenient to consider the side chains and the main chain separately. The effects of flexible side chains differ completely from those of stiff side chains. Long, flexible side chains reduce the glass transition temperature, while stiff side chains increase it. Long, flexible side chains increase the free volume and ease the steric hindrance from neighboring chains and as such facilitate the movement of the main chain. Figure 13.28 illustrates... 

On the other hand, cis or trans and cyclic 1,2-disubstituted ethylenes have been well-known till 1980 to be not homopolymerized with a radical initiator owing to the much more increased steric effect of the substituents except for a few exceptions, e.g., fluoro-substituted ethylenes and cyclic derivatives, vinylene carbonate (VCa), maleic anhydride (MAn), yV-substituted maleimides (RMI), and acenaphthylene. However, if 1,2-disubstituted and tri- or tetra-substituted ethylenes can homopolymerize, the polymers with a substituted polymethylene structure, which is different from the above polyethylenes, would be obtained (eqs. 1 and 2), i.e., the polymers would be less flexible (more rigid), and have higher glass transition temperatures than those for the polyethylenes, although their processabilities would decrease. 

By chromophore-polymer composite materials, we refer to chromophores physically incorporated (dissolved) into commercially available polymer materials such as amorphous polycarbonate (APC) [58] from Aldrich Chemicals. Chromophore and polymer are dissolved in a suitable spin-casting solvent, such as cyclopentanone. Spin-cast thin films are heated to near the glass transition temperature of the composite material (which will vary with chromophore concentration due to the plasticizing effect of the chromophore). Acentric chromophore order is induced by electric field poHng. If one assumes that the presence of the polymer host does not sterically hinder the reorientation of the chromophores under the influence of the poling field, the order parameter can be readily calculated. We have already noted that if chromo-phore-chromophore intermolecular electrostatic interactions are neglected then (cos 0) = p,F/5kT and the order parameter will be independent of chromophore concentration (or number density, N). Intermolecular electrostatic interactions can be treated at several levels of sophistication. 

As of steric effects the unsubstituted cyclopentadienyl compound is more active than the substituted ones. The fluorinated compounds are much more active (up to a factor of 30) than the chlorinated ones. The glass transition temperature of the polyisoprenes is about —52°C. 

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