T. Leon Yu

Physical Properties of Tri-isocyanate Crosslinked Polyurethane

Polyester based polyurethanes were synthesized from low molecular weight polyester (Mn∼2000) and 4,4′-methylene bis(phenyl isocyanate) (MDI) with butanediol as a chain extender and tris(6-isocyanatohexyl) isocyanurate (HDT-LV), a tri-NCO terminated compound, as a crosslinker. The polyester was synthesized from adipic acid and glycol, which was a mixture of 1,6-hexanediol and 1,2-propanediol. Two series of crosslinked polyurethanes were prepared. One series were prepared using HDT-LV crosslinked on hard segments and the other series were crosslinked on soft segments. The effect of crosslinker content on the degree of hard segment H-bond formation and phase segregation of polyurethanes was investigated using DSC (differential scanning calorimetry) and FTIR (Fourier transform infrared spectroscopy). The experimental results revealed that the incorporation of tri-NCO crosslinker into the hard-segments of polyurethane resulted in a decrease of hard segment H-bond formation with increasing crosslinker content. However, only a slight decrease of hard segment H-bonding was observed with increasing tri-NCO crosslinker content while it was incorporated into soft segments.

Weak Gel Behaviour of Poly(vinyl alcohol)-Borax Aqueous Solutions

Thermal transition of PVA-borax aqueous gels with a PVA concentration of 60 g/L and a borax concentration of 0.28 M was investigated at temperatures ranging from 15 to 60 ○C using static light scattering (SLS), dynamic light scattering (DLS), and dynamic viscoelasticity measurements. Three relaxation modes, i.e. two fast and one slow relaxation modes, were observed from DLS measurements. Two fast relaxation modes located around 10−3∼101 sec, with one fast mode (τf1) being scattering vector q-dependent and the other fast mode (τf2, with τf2>τf1) being q-independent. The τf1 mode was attributed to the gel mode whilst the τf2 mode could be due to the hydrodynamics of intra-molecular hydrophobic domains formed by uncharged segments of polymer backbones. The slow relaxation mode with relaxation time located around 101∼103 sec in DLS data was due to the motion of aggregated clusters and was observed only at temperatures above 40 ○C. The amplitude and relaxation time of slow mode decrease as temperature is increased from 40 to 60 ○C. At temperatures below 40 ○C, no slow relaxation mode was observed. The SLS measurements showed PVA-borax-water system had fractal dimensions Df∼2.4 and Df≤2.0 as temperature was below and above 40 ○C, respectively. The simple tilting test indicated gel behaviour for the PVA-borax aqueous system at temperatures below 40 ○C with a creep flow after a long time exposure in the gravity field. But the dynamic viscoelasticity measurements demonstrated a solution behaviour for PVA/borax/water at temperatures below 40 ○C, the critical gel point behaviour for G′(ω) and G″(ω) was not observed in this system as those reported for chemical crosslinked gels. These results suggest that the PVA-borax aqueous system is a thermoreversible weak gel.