Small-Angle Neutron Scattering of a Solvent-Swollen Segmented Polyurethane as a Probe of Solvent Distribution and Polymer Domain Composition

Abstract

Segmented polyurethanes consist of alternating rigid aromatic (hard) and flexible (soft) segments that phase separate into hard and soft segment-rich nanodomains. The domain structure results in what has been presumed to be an extended, cross-linked network, giving these materials unique mechanical properties. Because of poor contrast and volume fractions of the discontinuous phase, previous scattering studies have focused on polyurethanes with hard segment contents greater that 30 wt %. This study focused on a commercial poly(ester urethane), Estane 5703, containing only 23 wt % hard segment content. To probe the domain structure and composition, we used small-angle neutron scattering (SANS) with contrast variation methods on bulk samples swollen with deuterated/protonated mixtures of benzene, toluene, or o-, m-, or p-xylene isomers. We used the fluid model and Pedersen’s micelle model to describe the SANS data as a function of contrast. Both the fluid and micelle models consider Estane as a fluid of spherical, discrete domains in a soft segment-rich matrix. The discrete domains have a closest center-to-center approach defined by the Percus−Yevick hard-sphere potential. The micelle model divides the discontinuous domain into a core and a surrounding corona. We find that the micelle model completely described the data, whereas the fluid model required the ad hoc addition of an additional length scale. The models, which were in good agreement with each other, show that in Estane the discrete domains comprised a small fraction, 0.05−0.06, of the sample volume and have a radius of about 5 nm, giving a number density of the order 1017 cm−3. Furthermore, the discrete domains contain a higher hard segment concentration than the matrix, but incorporated only 0.06−0.13 of the hard segment volume. The results also show there is considerable mixing of the hard and soft segments, and there is a significant amount of soft segment in the discrete domains. Using these results, we were able to determine some important factors behind the thermomechanical properties of Estane. Furthermore, we make a critical assessment of the view that the discrete domains are cross-links for a continuous network and forward the hypothesis that the domains are more like filler particles coupled to the matrix by polymer entanglements.