Block copolyurethanes containing poly(12-hydroxy stearate) and poly(oxy tetramethylene) segments: some aspects of synthesis

Abstract

Block copolyurethane networks containing blocks of poly(12-hydroxy stearate) (PTHS) and poly(oxy tetramethylene) (POTM) were realized by two synthesis strategies. In a single-step synthesis, a PTHS-based polyol, viz. ISRO polyol and hydroxy telechelic poly(oxy tetramethylene), were co-cured in the presence of tolylene diisocyanate (TDI) using trimethylol propane (TMP) as crosslinker. In this case, increasing POTM content resulted in an initial improvement in mechanical properties, but enhancing its concentration beyond 20% by weight was detrimental to the properties. At this stage, the cure reaction was found to be incomplete and the partly cured system showed visible phase separation. This has been attributed to the differential reactivities of the OH groups of varying nature present on the polyols towards TDI that could lead to chain-extended homopolyurethane segments accelerating the phase separation. The possible non-uniform distribution of the crosslinker in the two phases and consequent imbalance in OH/NCO rapport might lead to incomplete crosslinking. This problem was overcome by pre-reacting PTHS and hydroxy functional POTM through an esterification reaction to result in a block copolyol. The composition of POTM in the block copolyol was varied using hydroxy functional POTM of different molecular weights, which were synthesized through the cationic ring opening polymerization of tetrahydrofuran. Curing the block copolyol using TDI and TMP resulted in a homogeneous block copolyurethane network. Increasing the concentration and chain length of POTM block was conducive to enhancing both the tensile strength and elongation of the polyurethane network. The phase miscibility of the formed block copolyurethane was inferred from the occurrence of a single glass transition temperature (Tg) for the polymer. Enhancing POTM content decreased the Tg of the copolyurethane in conformation with Fox’s law for a homogeneous copolymer.