Thermoplastic segmented polyurethanes (PUs) can exhibit shape memory behavior, if they feature multiple kinds of physical cross-links that can be dissociated at different temperatures. This is the case if the hydrogen-bonded hard phase is joined with soft segments that can partially crystallize, so that the melting transition acts as the memory switch. For applications in the biomedical field, it is important that the fixation and recovery temperatures can be minutely controlled. We show here that this tailoring can be easily achieved by formulating a commercial PU featuring poly(1,4-butylene adipate) (PBA) as a crystallizable segment (PBA-PU) with either PBA or poly(ε-caprolactone) (PCL) of moderate molecular weight. We show that the nature of the end groups and the processing conditions dictate if there is any reaction between the components or if the product is merely a blend. Interestingly, in either case, the addition of PBA or PCL causes nucleation and thereby a noteworthy increase of the crystallization temperature of the switching element from below to above ambient temperature, so that excellent shape fixity (∼98%) can be achieved at 37 °C. The melting temperature is maintained above 50 °C and significant increases in strength and modulus are achieved. The new materials platform is well suited for applications in which a shape is to be fixed at physiological temperature.
Keywords: blends; melt-mixing; nucleation; poly(ester urethane); shape fixation; shape memory polymer.