Effect of chemical structure and crosslinking density on the thermo-mechanical properties and toughness of (meth)acrylate shape-memory polymer networks

The objective of this work is to characterize and understand structure- mechanical property relationships in (meth)acrylate networks. The networks are synthesized from mono-functional (meth)acrylates with systematically varying sidegroup structure and multi-functional crosslinkers with varying mole fraction and functionality. Fundamental trends are established between the network chemical structure, crosslink density, glass transition temperature, rubbery modulus, failure strain, and toughness. The glass transition temperature of the networks ranged from -29 to 112 °C, and the rubbery modulus ranged from 2.8 to 129.5 MPa. At low crosslink density (Er 10 MPa), network chemistry has little influence on material toughness. The characteristic ratio of the mono-functional (meth)acrylates components is unable to predict trends in thermoset toughness as a function of chemical structure, as is accomplished for thermoplastics. The cohesive energy density is a better tool for prediction of network mechanical properties. Due to superior mechanical properties, networks with phenyl ring sidegroups are further investigated to understand the effect of phenyl ring distance on toughness. This work provides a fundamental basis for designing (meth)acrylate shape memory polymer networks with specific failure strain, toughness, glass transition temperature, and rubbery modulus.M.S.Committee Chair: Kenneth Gall; Committee Member: David Bucknall; Committee Member: Karl Jaco

Tough Semicrystalline Thiol–Ene Photopolymers Incorporating Spiroacetal Alkenes

We report a tough, semicrystalline, ternary thiol–ene polymer system containing linear dithiols, cross-linking trithiols, and spiroacetal alkene units in the main chain backbone that is synthesized by “click” ultraviolet photopolymerization in a one-step, solvent-free process. We varied the cross-link density to tune crystallinity and microstructure; in turn, thermomechanical properties such as yield strength, glass transition temperature, failure strain, and stress–strain behavior could be modified and controlled. Thiol–enes containing 7.5 and 10 thiol mol % cross-linker resulted in networks that balanced crystallinity, elasticity, and cross-linking to maximize toughness. These materials demonstrate how the presence of spiro units throughout a polymer’s backbone creates semicrystalline networks of substantial toughness from traditionally weak chemistries such as thiol–enes. This system can be synthesized in a neat, one-step photopolymerization process; as such, it illustrates the power of spirochemistry in designing photopolymers with tunable, robust thermomechanical properties