Basic rheological features of block polyurethane solutions: Entanglements, crystallization, and gelation

Three block polyurethane samples [differing in hard (urethane) to soft (macroglycol) segments ratio] dissolved in 2-butanone, are investigated. Rheological results obtained in the range 10-25°C led to the hypothesis of an order-disorder transition associated with H bonds being discarded. The critical concentration and the characteristic molecular weight for entanglements, M_c, are estimated approximately from linear viscosity results. M_c decreases as the percent of hard segments in the polyurethane increases, a result which is correlated with short-distance chain parameters such as length and molecular weight per bond and characteristic ratio. Annealing clear solutions at temperatures around 0°C causes haziness; on heating, a clearing temperature is detected at T_c ≈ 15°C. This transition coincides with a maximum observed in complex viscosity η* versus temperature curves. These results are probably due to incipient crystallization of soft segments domains. This hypothesis is compatible with differential scanning calorimetry results. Opaque solutions give rise to thermoreversible gels: we assume that the network branch points necessary for gelation involve microcrystalline domains which result from crystallization of soft segments from solution.