Microphase Separation and Rheological Properties of Polyurethane Melts. 3. Effect of Block Incompatibility on the Viscoelastic Properties

Abstract

The accompanying paper described the structural characterization of polyurethane multiblock copolymers that varied block incompatibility at fixed block length and composition. Their linear viscoelastic properties are presented in this paper. Dynamic mechanical experiments in temperature sweep mode confirmed that the materials ranged from almost homogeneous to highly microphaseseparated. Dynamic mechanical frequency sweep experiments showed a Rouse-like frequency response in all materials at high temperatures, including those polyurethanes that were highly microphaseseparated. This is in stark contrast to the numerous reports on microphase-separated block copolymers that show nonliquidlike terminal behavior at low frequencies. We attribute this homopolymer-like response of microphase-separated polyurethanes to a lack of long-range order in their microphase-separated structure and regard it to be a crucial feature of most commercial multiblock copolymers. The apparent activation energy for terminal flow was found to be insensitive to the extent of microphase separation, indicating that the thermodynamic penalty N expected for chain motion in block copolymers plays an insignificant role in the dynamics of the present materials. We demonstrate that bare effects (primarily proximity to the T g) and dynamic asymmetry (friction in one block much larger than in the other) play a major role in the dynamics of polyurethanes. The latter is expected to be especially important in the dynamics of elastomeric block copolymers whose blocks usually have widely separated Tg's. IntroductionPolyurethanes are (AB) n type multiblock copolymers composed of "hard" and "soft" segments that microphaseseparate due to thermodynamic incompatibility. The resulting microstructure of low-T g soft-segment-rich domains reinforced by rigid hard-segment-rich domains has excellent elastomeric properties, making polyurethanes useful as thermoplastic elastomers, coatings, textiles, etc. 1 The most important parameters that influence the properties of multiblock copolymers are block length N, block incompatibility , and composition. The first paper in this series investigated the effect of block length on the structure and the viscoelastic properties at constant incompatibility and composition. 2 In that work, we studied the "E-series" of materials with polycaprolactone soft segments and isophorone diisocyanate/1,4-butanediol hard segments as a function of block length. There were three primary conclusions regarding the viscoelastic properties of polyurethanes in that paper. First, block length had a very large effect on the terminal properties of polyurethane melts with terminal viscosity varying as approximately (block length) 4.5 . Second, at sufficiently high temperature the polyurethanes had a relaxation spectrum similar to that of a homopolymer of low molecular weight, indicating that either the polyurethanes were homogeneous at high temperatures or microphase separation did not affect the relaxation spectrum. Finally, the shift factors at suffic...

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“…In this system the incompatibility is found to be the maximum. Our results are very consistent with experimental studies, which show that the thermodynamic immiscibility between hard and soft segments influence the formation of the microphase-separated microstructure of the segmented copolymers from nearly mixed to strongly microphase separated ones [68]. These results also are supported by the FTIR spectroscopic studies, which showed that kinetics and extent of microphase separation is directly related with the structure of the HS [69,70].…”

Microphase Separation and Rheological Properties of Polyurethane Melts. 2. Effect of Block Incompatibility on the Microstructure

“…Compositions of the materials were found using quantitative 13 C nuclear magnetic resonance (NMR) spectroscopy on a 240 I725-0 48 0 I725-12 725 40000 47 12 I725-25 46 25 I725-40 45 40 I1000-0 47 0 I1000-13 1000 36000 46 13 I1000-32 45 32 I1000-40 45 40 I2000-0 57 0 I2000-14 2000 29000 55 14 I2000-29 54 29 IPDI/DMP 36700 0 a Estimated error (2%. b Estimated error (3%.…”

The phase mixing studies on moisture cured polyurethane-ureas during cure

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