Molecular orientation of hairy-rod polyesters: Effects of side chain length

Polarized laser Raman microscopy was employed in order to investigate the effects of side chain length on the orientability of uniaxially drawn films of a model hairy-rod polymer, as a function of draw ratio; an aromatic polyester containing terphenyl units in the main chain and aliphatic side chains (hexyl and dodecyl units) was used in the experiments. The second and fourth moments of the segment orientation distribution function, Pt and Pq, respectively, determined from the analysis of the Raman spectra, indicate that the polyester with longer (dodecyl) side chains clearly exhibits higher molecular orientation compared to the one with shorter (hexyl) side chains, for the same draw ratio. Complementary FTIR measurements provide detailed information on the segmental orientation and confirm the effects of side chain length in inducing orientation; the latter result strongly suggests that the longer aliphatic side chains are oriented toward the polyesteric main chain, much more effectively than the shorter ones, apparently due to their higher flexibility. Raman measurements at high wavenumbers corresponding to the CH2 and CH3 stretching modes of the aliphatic side chains corroborate this suggestion. Further evidence comes from independent measurements of optical anisotropy of p-phenylene oligomers bearing hexyl and dodecyl side chains. The implications to the processing of hairy polymers are evident.