Effect of intersegmental interactions on the morphology of segmented polyurethanes with mixed soft segments: a coarse-grained simulation study

Abstract

Segmented thermoplastic polyurethanes, polyureas and polyurethaneureas (TPU) based on a given hard segment and two chemically different soft segments display interesting microphase morphologies and thermal, mechanical and surface properties. In these systems the final TPU morphology is mainly controlled by the structure, amount and molecular weight of the soft segment oligomers and the nature and extent of specific intermolecular interactions between the mixed soft segments themselves and with the urethane hard segments. These interactions lead to variable compatibilities between the soft and hard segments resulting in interesting TPU morphologies. The proper choice of the two chemically different soft blocks provides more flexibility in controlling the extent of microphase separation, size and shape of the microphase domains and offers new possibilities for controlling the properties of TPUs. In this study coarse grained computer simulations were carried out to better understand the nature of intermolecular interactions and to elucidate the equilibrium microphase morphologies of TPUs with two different soft segments at 300 K. Model TPU systems investigated are comprised of poly(tetramethylene oxide) (PTMO) or poly(hexylethyl carbonate) (PHEC) and polydimethylsiloxane (PDMS) or polyisobutylene (PIB) soft segments with molecular weights in the range of 500-2500 g/mol. Hard segments consisted, in all cases, of diphenylmethane diisocyanate (MDI) based urethane repeat units and ranged from 25 to 50% by weight. Through coarse grained Dissipative Particle Dynamics (DPD) simulations it was demonstrated that by varying the composition and the chain lengths of the soft and hard blocks, quite different morphologies from homogeneous (or mixed) to gradient and to completely microphase separated structures were attainable. As expected, fairly hydrophobic soft blocks such as PIB and PDMS favored strong microphase separation when compared with relatively hydrophilic PHEC and PTMO segments. For comparison, morphologies of the TPUs based on single soft segments (PTMO, PHEC, PDMS and PEO) with varying molecular weights and hard segment contents were also simulated.