Inna Gurevitch

Conformational behavior of polymers adsorbed on nanotubes

The importance of hydrophobic interactions in determining polymer adsorption and wrapping of carbon nanotubes is still under debate. In this work, we concentrate on the effect of short-ranged weakly attractive hydrophobic interactions between polymers and nanotubes (modeled as an infinitely long and smooth cylindrical surface), neglecting all other interactions apart for chain flexibility. Using coarse-grained Monte Carlo simulation of such simplified systems, we find that uniform adsorption and wrapping of the nanotube occur for all degrees of chain flexibility for tubes with sufficiently large outer radii. However, the adsorbed conformations depend on chain stiffness, ranging from randomly adsorbed conformations of the flexible chain to perfect helical or multihelical conformations (in the case of more concentrated solutions) of the rigid chains. Adsorption appears to occur in a sequential manner, wrapping the nanotube nearly one monomer at a time from the point of contact. Once adsorbed, the chains travel on the surface of the cylinder, retaining their helical conformations for the semiflexible and rigid chains. Our findings may provide additional insight to experimentally observed ordered polymer wrapping of carbon nanotubes.

Shape memory polymer foams from emulsion templating

Shape memory polymers (SMPs) change their shape under a stimulus (thermal, chemical, light) and return from an imposed temporary shape to their permanent, original shape. SMPs usually contain “permanent” domains that determine the permanent shape (chemical or physical crosslinks) and “reversible” domains that determine the temporary shape, usually by heating above a glass transition temperature or a melting point (Tm). Compared to fully dense SMPs, SMP foams can undergo higher temporary deformations and can exhibit higher deformations when they recover. In this paper, SMP foams based upon (meth)acrylates with crystallizable long side-chains were synthesized through emulsion-templating within nanoparticle-stabilized high internal phase Pickering emulsions where the nanoparticles also served as crosslinking centers. The nature of the polymer backbone affected the nature of the crystalline phase for identical side chains. The SMP foams at room temperature maintained the temporary shape (a strain of 0.7) imposed above the Tm and exhibited good recovery upon reheating for all four compression–recovery cycles. While the methacrylate-based SMP exhibited a single-stage recovery, the acrylate-based SMP, with identical side-chains, exhibited a two-stage recovery that can be associated with the existence of two crystalline phases. The recovery behavior was described using Kelvin–Voigt units in series with the dependence of viscosity on temperature described using a WLF-like relationship.