title: 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 (Mwas 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.
authors.affiliation: Department of Biomedical Engineering, Texas A&M University, College Station, Texas 77843, United States ; Chemical Sciences Division, Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, California 94550, United States.
authors.forename: Keith
authors.initials: K
authors.lastname: Hearon
authors.affiliation: Department of Chemistry, Texas A&M University, College Station, Texas 77842, United States.
authors.forename: Celine J
authors.initials: CJ
authors.lastname: Besset
authors.affiliation: Department of Chemistry, Texas A&M University, College Station, Texas 77842, United States.
authors.forename: Alexander T
authors.initials: AT
authors.lastname: Lonnecker
authors.affiliation: Department of Materials Science & Engineering, The University of Texas at Dallas, Richardson, Texas 75080, United States.
authors.forename: Taylor
authors.initials: T
authors.lastname: Ware
authors.affiliation: Department of Materials Science & Engineering, The University of Texas at Dallas, Richardson, Texas 75080, United States.
authors.forename: Walter E
authors.initials: WE
authors.lastname: Voit
authors.affiliation: Chemical Sciences Division, Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, California 94550, United States.
authors.forename: Thomas S
authors.initials: TS
authors.lastname: Wilson
authors.affiliation: Department of Chemistry, Texas A&M University, College Station, Texas 77842, United States.
authors.forename: Karen L
authors.initials: KL
authors.lastname: Wooley
authors.affiliation: Department of Biomedical Engineering, Texas A&M University, College Station, Texas 77843, United States.
authors.forename: Duncan J
authors.initials: DJ
authors.lastname: Maitland