A Structural Approach to Establishing a Platform Chemistry for the Tunable, Bulk Electron Beam Cross-Linking of Shape Memory Polymer Systems

Abstract

The synthetic design and thermomechanical characterization of shape memory polymers (SMPs) built from a new polyurethane chemistry that enables facile, bulk and tunable cross-linking of low-molecular weight thermoplastics by electron beam irradiation is reported in this study. SMPs exhibit stimuli-induced geometry changes and are being proposed for applications in numerous fields. We have previously reported a polyurethane SMP system that exhibits the complex processing capabilities of thermoplastic polymers and the mechanical robustness and tunability of thermomechanical properties that are often characteristic of thermoset materials. These previously reported polyurethanes suffer practically because the thermoplastic molecular weights needed to achieve target cross-link densities severely limit high-throughput thermoplastic processing and because thermally unstable radiation-sensitizing additives must be used to achieve high enough cross-link densities to enable desired tunable shape memory behavior. In this study, we demonstrate the ability to manipulate cross-link density in low-molecular weight aliphatic thermoplastic polyurethane SMPs (Mw as low as ~1.5 kDa) without radiation-sensitizing additives by incorporating specific structural motifs into the thermoplastic polymer side chains that we hypothesized would significantly enhance susceptibility to e-beam cross-linking. A custom diol monomer was first synthesized and then implemented in the synthesis of neat thermoplastic polyurethane SMPs that were irradiated at doses ranging from 1 to 500 kGy. Dynamic mechanical analysis (DMA) demonstrated rubbery moduli to be tailorable between 0.1 and 55 MPa, and both DMA and sol/gel analysis results provided fundamental insight into our hypothesized mechanism of electron beam cross-linking, which enables controllable bulk cross-linking to be achieved in highly processable, low-molecular weight thermoplastic shape memory polymers without sensitizing additives.

Visualisation of the insertion of a membrane for the treatment of preterm rupture of fetal membranes using a synthetic model of a pregnant uterus

“…There was evidence of some pitting of the surface of the PLA fibres post e-beam sterilisation. On the other hand, it has been shown previously that e-beam irradiation has a crosslinking effect on polyurethane materials, 11 and although that was not measured in this study, this may explain the slight increase in the UTS shown on tensile testing. However, the mechanical properties of the bilayer membrane were not significantly affected and remained above or within the range of values for human fetal membranes, 6 which can be interpreted as an indicator of a good mechanical performance when subjected to pressures experienced during pregnancy.…”

A Processable Shape Memory Polymer System for Biomedical Applications

“…Covalent crosslinking fixes dispersed additive positions in polymer matrices and reduces dispersed particle rearrangements associated with thermoplastic polymer mobility. From a material performance standpoint, covalent crosslinking often enables more high integrity shape memory behavior than physical crosslinking because covalent crosslinking sites constitute more permanent netpoints than physical crosslinks . The device in this study is predicted to exhibit better shape fixity and shape recovery over a prolonged storage time in a programmed geometry than the 2002 device.…”

Shape memory properties of dynamically vulcanized poly(lactic acid)/nitrile butadiene rubber (PLA/NBR) thermoplastic vulcanizates: The effect of ACN content in NBR

“…At the same time, the deformed elastic component can provide enough recoverable force to recover to the original shape. Additionally, the cross‐linked networks with higher recovery force bring about better shape memory properties, including higher recovery speed and lower recovery time . Chang et al prepared shape memory polymer networks based on carboxylated telechelic poly(ε‐caprolactone) (XCPL)/epoxidized natural rubber (ENR) blends.…”