No abstract

ABSTRACT:Polyester based polyurethanes were synthesized from a low molecular weight polyester (M. -2000) and 4,4'-methylene bis(phenyl isocyanate) (MDI) with butanediol as a chain extender and glycerol as crosslinker. The polyester was synthesized from adipic acid and glycol, a mixture of I ,6-hexanediol and I ,2-propanediol. The effect of the crosslinker content on the phase segregation of soft-and hard-segments was studied by DSC (differential scanning calorimetry) and SAXS (small angle X-ray scattering). The introducing of trio! functional group into the hard segments increased the aggregation of hard segments through crosslink covalent bonds. Owing to highly steric hindrance, the presence of tri-o! crosslinker in the hard segments decreased the aggregation of hard segments through hydrogen bonding. The phase segregation of soft-and hard-segments is thus due to the aggregation of hard-segments through crosslinked covalent bonds and H-bonding.KEY WORDS Polyurethane I Differential Scanning Calorimetry I Small Angle X-Ray Scattering I Segmented polyurethanes are thermoplastic elastomers which consist of hard and soft segment units. They are thermoplastic elastomers with a wide range of mechanical properties depending on the number of two structural components. It is generally agreed that the unique mechanical properties of polyurethanes, as compared to other types of elastomers, are predominantly the result of a two-phase morphology. 1.2 Polyester based urethanes consist of an aromatic diisocyanate with a glycol chain extender as the hard segment and low molecular weight polyester as the soft segment. They are considered segmented block copolymers, made up of alternating hard and soft block segments. Compositional variables and processing conditions affect the degree of phase segregation, phase mixing, hard segment domain organization, and subsequent polyurethane properties. 2 -4 Depending on relative incompatibility of hard and soft segments, phase segregation occurs during processing and post cure annealing. The effects of polyurethane composition and structure on properties have been investigated, with attention to model compounds based on aromatic diisocyanates, such as toluene diisocyanate (TDI) 5 • 6 or diphenyl methane diisocyanate (MDI). 5 -8 The phase segregation of hard and soft segment domains has been demonstrated by: small and wide angle X-ray scattering, 9 -20 differential scanning calorimetry, 9 -18 • 21 -24 infrared spectroscopy, 24 -29 microscopy, 16 • 30 • 31 small angle laser light scattering (SALS), 16 and dynamic mechanical thermal analysis (DMT A)Y In the present work, soft segment polyester-diols were synthesized from adipic acid and mixed glycols of l ,2-propanediol and I ,6-hexanediol. The polyurethanes were synthesized from polyester and 4,4'-methylene bis(phenyl isocyanate) (MDI) with mixtures of glycerol and 1,4-butanediol as a crosslinking agent and chain extender, respectively. The effect of the concentration of glycerol on the morphology of polyurethanes was studied t To whom co...