P-Phenylene ring

The effect of the chemical nature of the main chain of the polymer on the glass transition temperature is similar to the effect that it has on the melting temperature, T. The chemical structure has a determining influence on the flexibility of the chain. For example, polymers such as polyethylene, (—CH2 — CH2—) , and polyoxyethylene, (—CH2—CH2 — O—) , have relatively flexible chains as a result of the ease of rotation around their chain bonds. Thus they have low values of Tg and as can be seen in Table 2.3. The incorporation into the main chain of units that hinder rotation and consequently increase the rigidity of the chain clearly causes a large increase in Tg. For example, the incorporation of a p-phenylene ring (Ph) into the monomeric unit of polyethylene gives poly(p-xylylene), which has a Tg of around 353 K (see Table 2.3). 

The value of Tg is raised markedly by the insertion of groups which stiffen the chain by impeding rotation, so that more thermal energy is required to set the chain in motion. Particularly effective in this respect is the p-phenylene ring. Thus, a chain consisting entirely of p-phenylene rings, namely, poly(p-phenylene) (XVI)... 

An X-ray crystal structure of the analogous silver perchlorate complex shows that the silver ion is not situated exactly in the center of the cavity but is displaced outwards for about 240 pm [20]. Boekelheide et al. [21] confirmed this phenomenon by carrying out an X-ray crystal structure of the silver triflate complex 19 where the silver ion is likewise not situated in the center between the three p-phenylene rings. 

The use of monomers having additional p-phenylene rings, i.e. terphenyl or quaterphenyl derived, leads to polymers with increased rigidity and increased degree of crystallinity. TTius, terphenyl diol (11) was condensed with a number of dihalobenzenoid compounds. ... 

CHi—)n and polyoxyethylene (—CH2—CH2—O—) which have relatively flexible chains because of easy rotation about main chain bonds, tend to have low values of Tg, The value of the Tg of polyethylene is a matter of some dispute because samples are normally highly crystalline which makes measurement of Tg rather difficult. Values have been quoted between 140 K and 270 K. However, the incorporation of units which impede rotation and stiffen the chain clearly cause a large increase in Tg. For example, the presence of a p-phenylene ring in the polyethylene chain causes poly(p-xylylene) to have a Tg of the order of 80 C. 

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