Comprehensive Structural Study of Pre- and Post-Heat Treated Compression Molded Polyurethane Samples of Varying Composit
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COMPREHENSIVE STRUCTURAL STUDY OF PRE- AND POST-HEAT TREATED COMPRESSION MOLDED POLYURETHANE SAMPLES OF VARYING COMPOSITION STUDIED BY SCANNING PROBE TECHNIQUES M.E. Hawley*, E.B. Orler*, D.A. Wrobleski*, R.P. Hjelm**, and G.W. Brown* * Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545 ** LANSCE, Los Alamos National Laboratory, Los Alamos, NM 87545 ABSTRACT Only a limited number of structural studies have been performed on polyurethanes using scanning probe techniques to determine both the microstructure and the corresponding distribution of hard and soft segments within samples. This type of information is needed to better understand the mechanical properties of these materials and to facilitate modeling. In order to address these issues, we have fabricated a series of compression molded segmented poly(ester urethane) samples with hard (HS) to soft segment ratios from 19 to 100%. Samples were examined using scanning probe phase imaging techniques to obtain the topography and corresponding distribution of hard domains before and after heating at 100°C. A number of significant differences were observed between the pre- and post-heat treated samples. Variations in structure and heat-induced morphological changes were directly related to HS content. Fine strand- or fibril-like structures were most prominent in the 23 and 19% HS sample but first appeared at 30% HS. Harder, thicker elongated structures dominated the surface of the100% HS sample and were seen to a limited extent on all samples, especially after annealing and quenching. The 23% HS sample surface structure depended on quenching rate and time after treatment. INTRODUCTION Thermoplastic polyurethanes derive their elastomeric properties from the thermodynamic incompatibility of low glass transition temperature (Tg) soft segments (SS’s) from covalently attached hard segments (HS’s) (Fig. 1). The phase separation of the HS s into domains greatly increases the modulus and strength of the elastomer. Above 40°C, the polyurethane mechanical properties decrease rapidly with increasing temperature. This correlates with disruption of the hard domains (Fig. 1), which is complete above 80°C, where the elastomer becomes thermoplastic. Upon cooling to ambient temperatures, the hard segments are expected to phase separate and reform the network structure and the material once again is a tough elastomer. The recovery of the mechanical properties can take several days. While some general aspects of phase separation in segmented polyurethanes are known, details of the morphology and phase behavior of these complex materials are far from understood. Earlier studies primarily employed small angle x-ray and small-angle neutron scattering techniques to determine the morphology of these materials [1-4]. These studies indicated that at the higher HS densities the structures have characteristic length scales of the order of 10 nm. Recently, scanning force microscopy (SFM) tapping mode techniques have begun to play an increasingly importa
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