The thermo-structural charaterisation of a main chain liquid crysal polymer

Abstract

The structural properties and transitions occurring in a random liquid crystalline polymer were studied over a range of thermal conditions using a combination of complementary characterisation techniques, including differential scanning calorimetry, wide angle x-ray scattering and optical microscopy. From the results, ordering in the polymer is observed to increase as a function of temperature and time prior to melting. An anisotropic phase develops at 330 C and degradation cxxurs at 400 C. The polymer displays a nematic phase on heating and hence is said to be thermotropic. Any order induced by annealing is destroyed on heating well above the polymer's melting point The material reverts back to its original random structure. This behaviour is atttibuted to transesterification reactions occurring, over relatively short annealing times (120 seconds) at elevated temperatures. Although order and crystallinity are observed to increase as a function of annealing conditions, very little structural rearrangement is observed to occur for samples annealed over die time range, 0-2 hours. In general, die behaviour of the polymer is very dependent upon its thermal history. The synthesis of a chemically ordered version of the liquid crystalline polymer of identical composition, was attempted to enable a back to back characterisation with the original random version. Attempts at the synthesis proved, unsuccessful. Small angle neutron scattering techniques were used to study the mechanism and kinetics of transesterification. The activation energy calculated is identical to the value stated in the literature for PET, suggesting that the reaction mechanism is identical for both die rigid and the flexible polyester. The rate constants calculated for a range of polymers are inversely related to their initial molecular weights thus transesterification appears to proceed via an active chain end mechanism. A unique route to the formation of highly blocky structures via a process termed crystallisation induced reorganisation (C.I.R.), based upon die transesterification reaction, is discussed in the literature. The present study aims to establish whether a C.I.R. reaction occurs in the random polymer. Samples were annealed at selected temperatures for reaction times ranging from 0-72 hours. An ester-interchange catalyst was impregnated into selected samples. The polymer was observed to undergo the C.I.R. reaction both above and below the melt, leading to a block structure. The presence of the catalyst did not influence the rate of structure development