UNDERSTANDING AND ADVANCING SHAPE MEMORY IN SEMI-CRYSTALLINE POLYMER NETWORKS

Semi-crystalline polymer networks are widely used as shape memory materials, given that there are two distinctive networks each in charge of a distinctive shape: a chemical network dictating the as-prepared primary shape, and a crystalline scaffold fixing the programmed secondary shape. The two networks are under constant competition, and delicate balance is required to achieve desirable shape memory pathways. One-way irreversible shape memory can be easily realized by fixing the secondary shape upon cooling and recovering back to the primary shape upon complete removal of crystals. However, to achieve two-way reversible shape memory is very challenging for most semi-crystalline elastomers. We have discovered and established a universal mechanism of reversible shape memory in conventional semi-crystalline elastomers. This mechanism relies on a new partial melting procedure and self-seeding crystallization. At partial melting state, the molten network strands are confined by both the chemical crosslinks and the remaining crystals. Upon cooling, the molten strands undergo self-seeding crystallization, readily restoring the secondary shape. This mechanism can be potentially applied to any types of semi-crystalline elastomers. It enables a novel two-way reversible, free-standing, multiple-times and reprogrammable shape shifting pathway of the conventional irreversible semi-crystalline elastomers. We have further investigated the effect of various chemical network topologies on reversible shape memory performance. We optimized the reversibility, the repeatability, and durability over time. It is shown that only with sufficient chemical crosslinks (crosslinking densities), would there be enough network confinements to ensure a two-way reversible shape memory behavior. We have explored new applications using this new mechanism in semi-crystalline elastomers, including a dynamic optical grating. With the primary shape being a flat surface, we programmed grating structure as the secondary shape on the surface. Therefore by applying partial melting and reversible shape memory protocol, we realized a reversible change in grating’s height, which result in a reversible change in diffraction intensity. The optical reversibility is sufficiently high (>95%), and can be repeated for many cycles (>10). So far, most semi-crystalline shape memory elastomers consist of linear network strands. However, there is an intrinsic lower modulus limit for essentially all linear networks, which is on the order of 105 Pa. To break this lower boundary, and to better mimic soft biological tissues which are usually on the order of 103 Pa, we adopt a new network architecture: bottlebrush network. The resulting modulus change of the semi-crystalline bottlebrush network is unprecedentedly high, covering from GPa at crystalline state, all the way down to kPa or ever lower at amorphous state (six orders of magnitude change). At the rigid state, the material is easy to handle and sharp to penetrate; at the soft state, the material is tunable to mimic the exact modulus of surrounding tissues, even for soft tissues such as brain. We are targeting at two practical applications, brain implant and microneedles drug delivery. In all, semi-crystalline polymer networks show great potential as a versatile shape memory material, which can incorporate both one-way irreversible and two-way reversible shape shifting at the same time. In terms of application, it brings significance in fields such as robotics, actuators, dynamic surface and optics. Also, with the help of bottlebrush network architecture, the semi-crystalline elastomers can possess huge modulus change, which provides a promising candidate for bio-medical implants.Doctor of Philosoph