Synthesis of novel siloxane containing block copolymers for impact modification of polybutyleneterephthalate.

The aim of this work was to synthesise poly(butyleneterephthalate) (PBT) poly(dimethylsiloxane) (PDMS) block copolymers, containing the hydrolytically stable Si-C linkage. A number of routes involving mutually reactive oligomers were studied. Once synthesised the effect of the copolymer as a toughening agent for the PBT matrix was investigated. Siloxane copolymer precursors were synthesised by an equilibration reaction. The relative molar mass (RMM) of the precursor was governed by the initial ratio of end blocker to cyclic species. The functionality of the precursor determined by the end blocker. alpha, to hydroxy PBT oligomeric precursors were chemically modified, if necessary, to form mutually reactive species.Initial block copolymer synthetic routes concentrated on chloroplatinic acid catalysed hydrosilations in solution. This involved alpha, to di (hydrosilane) PDMS and alpha, to divinyl functionalised PBT. The lack of a suitable common solvent together with competing side reactions limited the progress of this route. Melt hydrosilation reactions proved ineffective also, because of the thermal instability of the catalyst. Further block copolymer experiments involving mutually reactive oligomers were performed in the melt. The most promising of these was one of transesterification. This used alpha,o-hydroxypropyl PDMS (RMM 1000) and alpha,o-hydroxy PBT (RMM 2000) precursors. Analysis indicated successful reaction to form a copolymer of low RMM. A higher RMM copolymer was desirable, for improved mechanical properties, and a number of approaches to achieve this were followed. Problems of competing reactions and ineffective catalysis were encountered. However, a material with promising mechanical properties was formed when using a diisocyanate as a chain extender.A PBT-PDMS copolymer was blended by itself, and also together with high RMM PDMS, in the PBT matrix. The mechanical properties of the blends were studied and compared. An improvement in impact properties, as compared to PBT, was achieved when the copolymer was used as an emulsifying agent in a PBT-PDMS blend.