Impact of the Combined Use of Magnetite Nanoparticles and Cellulose Nanocrystals on the Shape-Memory Behavior of Hybrid Polyurethane Bionanocomposites.

Hybrid bionanocomposites with shape-memory behavior are reported. The materials were accessed by combining a polyurethane matrix with a highly renewable carbon content, cellulose nanocrystals (CNCs), and magnetite nanoparticles (MNPs). The integration of the two nanoparticle types resulted in tough materials that display a higher stiffness and storage modulus in the glassy and rubbery state, thus contributing to the structural reinforcement, as well as magnetic properties, reflecting a synergistic effect of this combination. A quantitative characterization of the thermoactivated shape-memory effect made evident that the addition of CNCs increases the shape fixity, due to the higher glass transition temperature (Tg) and the higher stiffness below Tg than the neat PU, while the addition of MNPs made it possible to activate the shape recovery by applying an alternating magnetic field. Moreover, the new hybrid bionanocomposites showed good bio- and hemocompatibility.

Biocompatible thermo- and magneto-responsive shape-memory polyurethane bionanocomposites.

Thermo- and magneto-responsive shape-memory bionanocomposites based on a bio-based polyurethane and magnetite nanoparticles were prepared. Due to the structure of the reactants, the behavior of the polyurethane matrix differs from common polyurethanes, since the soft segment was formed by a diisocyanate and a chain extender, whereas the macrodiol served as hard segment. The influence of the magnetite nanoparticles on the thermal and mechanical properties and the shape-memory behavior was studied. It was observed that magnetite nanoparticles interacted with macrodiol-rich domains and decreased the overall crystallinity of the material, although their presence did not affect the mechanical properties to a great extent. At the same time, the magnetite nanoparticles increased the shape fixity and contributed to shape recovery. The bionanocomposites exhibited magnetic behavior and could be easily heated in an alternating magnetic field, allowing fast and almost complete shape recovery. Preliminary cytotoxicity, hemocompatibility, and cell adhesion analysis suggest that the new materials are benign and potentially useful for biomedical applications.

Tailoring the Shape Memory Properties of Segmented Poly(ester urethanes) via Blending.

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.

Shape Memory Composites Based on Electrospun Poly(vinyl alcohol) Fibers and a Thermoplastic Polyether Block Amide Elastomer.

The present study aimed at developing new thermally responsive shape-memory composites, that were fabricated by compacting mats of electrospun poly(vinyl alcohol) (PVA) fibers and sheets of a thermoplastic polyether block amide elastomer (PEBA). This design was based on the expectation that the combination of the rubber elasticity of the PEBA matrix and the mechanical switching exploitable through the reversible glass transition temperature (Tg) of the PVA filler could be combined to create materials that display shape memory characteristics as an emergent effect. Dynamic mechanical analyses (DMA) show that, upon introduction of 10-20% w/w PVA fibers, the room-temperature storage modulus (E') increased by a factor of 4-5 in comparison to the neat PEBA, and they reveal a stepwise reduction of E' around the Tg of PVA (85 °C). This transition could indeed be utilized to fix a temporary shape and recover the permanent shape. At low strain, the fixity was 66 ± 14% and the recovery was 98 ± 2%. Overall, the data validate a simple and practical strategy for the fabrication of shape memory composites that involves a melt compaction process and employs two commercially available polymers.