Controlling Polymer Material Structure during Reaction-Induced Phase Transitions

Conspectus Living systems are composedof a select number of biopolymers andminerals yet exhibit an immense diversity in materials properties.The wide-ranging characteristics, such as enhanced mechanical propertiesof skin and bone or responsive optical properties derived from structuralcoloration, are a result of the multiscale, hierarchical structureof the materials. The fields of materials and polymer chemistry haveleveraged equilibrium concepts in an effort to mimic the structureof complex materials seen in nature. However, realizing the remarkableproperties in natural systems requires moving beyond an equilibriumperspective. An alternative method to create materials with multiscalestructures is to approach the issue from a kinetic perspective andutilize chemical processes to drive phase transitions. ThisAccount features an active area of research in our group,reaction-induced phase transitions (RIPT), which use chemical reactionssuch as polymerizations to induce structural changes in soft materialsystems. Depending on the type of phase transition (e.g., microphaseversus macrophase separation), the resulting change in state willoccur at different length scales (e.g., nm-& mu;m), thusdictating the structure of the material. For example, the in situformation of either a block copolymer or a homopolymer initially ina monomer mixture during polymerization will drive nanoscale or macroscaletransitions, respectively. Specifically, three different examplesutilizing reaction-driven phase changes will be discussed: 1) in situpolymer grafting from block copolymers, 2) multiscale polymer nanocomposites,and 3) Lewis adduct-driven phase transitions. All three areas highlighthow chemical changes via polymerizations or specific chemical bindingresult in phase transitions that lead to nano- and multiscale changes. Harnessing kinetic chemical processes to promote and control materialstructure, as opposed to organizing presynthesized molecules, polymers,or nanoparticles within a thermodynamic framework, is a growing areaof interest. Trapping nonequilibrium states in polymer materials hasbeen primarily focused from a polymer chain conformation viewpoint,in which synthesized polymers are subjected to different thermal andprocessing conditions. The impact of reaction kinetics and polymerizationrate on final polymer material structure is starting to be recognizedas a new way to access different morphologies not available throughthermodynamic means. Furthermore, kinetic control of polymer materialstructure is not specific to polymerizations and encompasses any chemicalreaction that induces morphology transitions. Kinetically driven processesto dictate material structure directly impact a broad range of areas,including separation membranes, biomolecular condensates, cell mobility,and the self-assembly of polymers and colloids. Advancing polymermaterial syntheses using kinetic principles such as RIPT opens newpossibilities for dictating material structure and properties beyondwhat is currently available with traditional self-assembly techniques.