Research Projects

Vitrimers are a tunable class of reprocessable and adaptable materials, which combine mechanical resilience and solvent resistance at low temperature (T) with stress relaxation and reprocessability at high T. This paradoxical behavior originates from their ability to rearrange their topology without depolymerization, e.g., via the formation of reversible crosslinks or by having exchangeable crosslinks. This behavior contrasts that of permanently crosslinked polymers, which cannot fully relax stress, and are hard to recycle and reprocess. Controlling the viscoelasticity of vitrimers, wherein associative/dissociative crosslinks are statistically distributed along polymer chains, has primarily relied on optimizing crosslink exchange. The exchange kinetics exhibit much slower rates than the fast segmental motions of network strands, which follow the Williams-Landel-Ferry (WLF) behavior. The Arrhenius T-dependence of the relatively slow crosslink exchange furnishes a wide temperature window for viscosity control, motivating substantial work on externally or internally catalyzed exchange reactions. Another salient, less appreciated feature, is that these crosslink groups are chemically quite dissimilar to the chain monomers. These chains with groups having significantly different polarizabilities can thus exhibit microphase separation leading to the formation of hierarchical nano- and microstructures. The segregated domain formation at varied length scales driven by backbone/crosslink incompatibility remains an under-appreciated means of controlling vitrimer dynamics. Moreover, the spatial localization of dynamic crosslinks within specific microdomains also tends to enhance segregation in these nanostructured melts, which exhibit the rheology of structured solids. Consequently, strongly segregated vitrimers remain solid-like over a range of temperatures up to their thermal decomposition points. We posit that this combination of reversible crosslinking and microphase separation leads to strongly enhanced mechanical behavior relative to the base, unfunctionalized polymer. We hypothesize that the molecular design of strong yet reprocessible materials involves positioning the vitrimer temperature Tv (a “topological freezing temperature”) below the accessible order-to-disorder transition temperature (TODT) of the segregated morphology. This will enable liquid-like flow above TODT. Successful design of such material requires navigation of a multiparameter design space, which includes control variables such as the fraction of active groups, in a polymer chain, special arrangement of active groups in a chain, temperature-dependent crosslink lifetimes, effect of crosslinking on TODT and the critical segregation strength for the microphase separation. Thus, we propose large-sale computer simulations across this multiparameter space to establish universal correlation between relaxation modulus and segregation of crosslinkers in a vitrimer. We posit that controlling the segregation of crosslinkers is a key to tune the flow characteristic of vitrimers with immediate relevance to the development of recyclable and reusable plastics.