The thermal response of the polyether soft segment chain conformation in a polyurethane block copolymer measured by small-angle neutron scattering

Abstract

The conformation of the soft segment chains in a pair of polyether polyurethanes was studied as a function of temperature using small angle neutron scattering. The samples were synthesized from 3 moles of methylene bis( p-phenyl isocyanate), 2 moles of butanediol, and one mole of a poly(tetramethylene oxide) (PTMO) blend. The PTMO blend was composed of 0.325 moles of deuterated PTMO (d-PTMO) and 0.675 moles of hydrogenous PTMO. This degree of deuterolabelling was chosen so that there would be no interphase scattering in the final sample; only intrachain scattering from the labelled soft segments comprised the coherent part of the total scattering. At room temperature, the average soft segment was found to be in an extended conformation. As the temperature was raised from room temperature, the radius of gyration of the soft segments decreased. This was attributed to the stress exerted by the extended soft segments on the adjoining hard segments increasing as the temperature was increased. The increased stress causes some of the hard segments to pull out of the hard domain into the soft phase, thereby allowing the soft segments adjacent to the extracted hard segment to relax to a more compact conformation. As the temperature was increased above 160°C, the soft segment radius of gyration increased slightly. This behaviour is ascribed to an increased degree of mixing between the phases. The presence of substantial amounts of hard segment material in the soft phase causes the increase in the soft segment R g due to the greater compatibility between the soft and hard segments in the soft phase at these elevated temperatures. This effect is similar to a homopolymer being swollen by a small amount of a good solvent, where the chain conformation is a random coil, but the radius of gyration is larger than that measured for the pure material.