Tailoring the Properties of a Shape-Memory Polyurethane via Nanocomposite Formation and Nucleation

Thermoplastic polyurethanes (PUs) can display shape-memory (SM) characteristics if their microphase-separated structure, consisting of domains formed by hard blocks and soft segments, respectively, is complemented with the ability of the soft segments to partially crystallize, so that the third phase thus formed can serve as the switching element for the shape-memory effect. While property modifications of SMPUs usually require de novo synthesis, we show at the example of a commercially available poly­(ester urethane) consisting of crystallizable poly­(1,4-butylene adipate) soft/switching segments and hard segments composed of 4,4-methylene­diphenyl diisocyanate and 1,4-butanediol that the thermomechanical properties can also be modified by formulating nanocomposites and/or influencing the crystallization of the soft/switching segments via the addition of a nucleating agent. The incorporation of cellulose nanocrystals (CNCs) by simple melt-mixing allowed increasing the tensile storage modulus from 150 MPa (neat polymer) to 572 MPa (15% w/w CNCs) while the shape fixity at a specific fixing temperature was increased from 47 to 75%. The temperature at which good fixity (>97%) could be rapidly achieved was increased from 10 to 25 °C upon addition of 1% w/w dodecanoic acid, which served to nucleate the poly­(1,4-butylene adipate) crystallization.