Structure-property relationships in side chain liquid crystal polymers
Knowledge-based rules relating polymer structure to thermal behaviour for side chain liquid crystal polymers are established and tested by the syntheses and characterisation of carefully selected homologous series of homo- and copolymers. The effects of the length and parity of the flexible spacer and the flexibility of the polymer backbone on thermal behaviour are considered in detail. The mesogenic properties of the poly-(4-methoxbipheny1-4'-yloxy)alkyl methacrylates are described; the clearing temperatures (Tcl) show a distinct odd-even effect while the glass transition temperatures (T g) decrease without alternation. Structural investigations of this series show that SA1 phases are exhibited. The transitional behaviour of the poly-4'-cyanobiphenyl-4'- yloxy)alkyl methacrylates shows that increasing backbone flexibility results in decreased Tgs but enhanced T cls. These trends are rationalised in terms of the ability of the anisotropic environment to confine the backbone. These empirical rules are tested using the poly-(4-methoxybiphenyl-4'-yloxy)alkyl acrylates. It is expected that the polyacrylate-based series will exhibit higher Tcls than the corresponding polymethacrylate-based series. The odd members of this series, however, display anomalously low T cls. This behaviour suggests that for the polyacrylates, the clearing transition is entropically driven whereas for the polymethacrylates it is enthalpically driven. The thermal and mechanical behaviour of the poly-(4-cyanoazaobenzene-4'-oxy)alkyl methacrylates are studied. The Tgs lie intermediate between those of the analogous methoxybiphenyl- and cyanobiphenyl-bases series. This is rationalised in terms of the smectic phase structure and the interaction strength parameter between mesogens. Dynamic mechanical thermal analysis detects a weak relaxation for members of this series which is considered to be associated with 180° reorientational jumps of the long axis of the mesogenic unit about the polymer backbone.