Phase separation segmented polyurethane

Figure 8. Effect of soft segment content on point of phase separation during polyurethane polymerization as indicated by discontinuity in DSC reaction profile. Dotted line shows theoretical prediction assuming separation at constant number-average sequence length (1.3) of hard segments ("see text).

Taylor JE, Laity PR, Wong SS, Norris K, Khunkamchoo P, Cable M, et al. Examination of hard segment and soft segment phase separation in polyurethane medical materials by electron microscopy techniques. Microsc Microanal 2006 12(2) 151-5. 

Chen W-P and Schlick S (1990) Study of phase separation in polyurethanes using paramagnetic labels effect of soft-segment molecular weight and temperature. Polymer 31 308-314. 

Reaction-induced phase separation is certainly also the reason for which an inhomogeneous structure is observed for photocured polyurethane acrylate networks based on polypropylene oxide (Barbeau et al., 1999). TEM analysis demonstrates the presence of inhomogeneities on the length scale of 10-200 nm, mostly constituted by clusters of small hard units (the diacrylated diisocyanate) connected by polyacrylate chains. In addition, a suborganization of the reacted diisocyanate hard segments inside the polyurethane acrylate matrix is revealed by SAXS measurements. Post-reaction increases the crosslink density inside the hard domains. The bimodal shape of the dynamic mechanical relaxation spectra corroborates the presence of a two-phase structure. 

The variation of Tg of the soft matrix in segmented polyurethanes as a function of composition or segmental chemical structure has been monitored and used as an indicator of the degree of microphase separation. Factors influencing the phase-separation process in these MDI-based polyurethanes have been summarized by Aitken and Jeffs (69) as follows (a) crystallization of either component, (b) the steric hin-... 

Wilkes and Emerson (97) studied the time-dependent behavior of a polyester polyurethane (MDI-BD based 40% hard segment) which was heated to 160°C for 5 min, then rapidly quenched to room temperature. To monitor changes in phase separation, SAXS intensity values (at a fixed angle) were recorded as a function of time. Furthermore, the elastic modulus and soft-segment Tg were followed with time. The results, shown in Figure 14, reveal an approximately exponential decay toward equilibrium with a good correlation between properties (Tg and modulus) and structure (inferred by SAXS intensities). 

DSC, WAXS, electron diffraction, and electron microscopy support a phase-separated morphology for all five of the random copolymer PCP/MDI/BDO-segmented polyurethanes studied. The slight increase observed by Seefried et al. (9) in the lower temperature Tg with hard-segment concentration is therefore likely attributable to increased reinforcement of the soft-segment matrix by an increased number of dispersed hard-segment domains. 

This strong dependence of the order of both the soft- and hard-segment phases on the sample preparation technique may account for some of the previous disagreement on the extent of phase separation in a given polyurethane. 

Figure 1. Molecular schematics of some possible segmented polyurethane morphologies. For convenience, good phase separation is assumed, (a) Hard-segment domains dispersed within a soft-segment matrix (b) interconnected hard-segment phase giving rise to continuous phases of both hard and soft segments (c) soft-segment phase is dispersed within a continuous matrix of hard segments.

Several studies have been published in recent years on the properties of RIM polyurethanes and polyurethane-ureas (1-7). Unlike most studies, however, we have restricted our attention to simplified linear systems in order to establish the effect of reaction rates and mold temperatures on the phase separation and molecular weight of segmented polyurethane elastomers produced by RIM(6). 

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