POLY(DI-N-ALKYLSILOXANE)S WITH LONG ALKYL SIDE-GROUPS

High molecular weight samples of poly(di-n-pentylsiloxane)(PDPeS), poly(di-n-hexylsiloxane) (PDHS), and poly(di-n-decylsiloxane) (PDDS) have been prepared by means of anionic (PDPeS, PDHS) and cationic (PDDS) ring-opening polymerization of the corresponding hexa-n-alkylcyclotrisiloxanes. The phase behavior of the polymers has been studied by polarizing microscopy, differential scanning calorimetry, wide-angle X-ray scattering, and Si-29 solid-state NMR spectroscopy. Like the homologues with shorter allyl substituents, all three new polysiloxanes showed two disordering transitions and formed a conformationally disordered mesophase before isotropization. Elongation of the alkyl side groups from ethyl to n-hexyl improved the stability of the hexagonal columnar mesophase, which is reflected in an increase in the mesophase temperature interval from DETLAT = 43-degrees-C for poly(diethylsiloxane) (PDES) to more than 310-degrees-C in the case of PDHS. Optical polarizing microscopy indicated the formation of extended chain lamellae within this state. When the alkyl side groups were elongated to n-decyl, however, the conformationally disordered state was only observed by temperature-dependent Si-29 chemical shift anisotropy measurements as an undercooled phase in coexistence with the high-temperature crystalline phase.