(581h) Thermodynamic Manipulation of Polymerization Induced Phase Separation: Influence of Entropic Versus Enthalpic Driving Forces

The design of heterogeneous polymer blends and composites allows for potentially synergistic combination of properties from individual polymer constituents. This design is often exploited for the optimization of mechanics, toughness, flexibility or compatibility of a material. The final morphology and the differential in properties between distinctive phases are important in the development of heterogeneous polymers and polymer networks, as both parameters can drastically alter bulk performance. Approaches such as blending or physical mixing have been historically utilized to achieve this control; another method of interest is to utilize phase separation during polymerization. With the latter, thermodynamic instabilities that result from enthalpic or entropic variations are exploited to drive the formation of heterogeneous domains. When utilized during a polymerization reaction, such instabilities allow for the in situ, rapid formation of heterogeneous polymer blends and networks. While this is appealing for a variety of in situ applications such as the design of coatings, adhesives, and biomaterials, a challenge with this approach is understanding precisely how to manipulate the resultant blend morphology and property differential between phases, as the dynamic interplay between thermodynamically driven phase separation and a simultaneous polymer (network) formation evolves rapidly and it is difficult to arrest both the phase evolution and network development if needed. Here we study the phase separation that occurs between poly(n-alkyl methacrylates) and a multi(methacrylate) monomer during the formation of a densely cross-linked network; this approach is aimed at improving overall performance of these typically brittle materials. We show that the proclivity of the linear precursor to undergo nanophase segregation influences the ultimate driving force for macroscopic phase separation, which transitions from being an entropy-driven (repeat unit side chain of pentyl length or shorter) to an enthalpy-driven process (repeat unit side chain longer than pentyl). Variations in the side chain repeat unit allow for direct manipulation of resulting phase compositions, so that as the alkyl chain length increases, a phase that is richer (more pure, ~85%) of the linear additive forms. This control and phase composition was previously unattainable in rapidly forming, phase separated networks. Via manipulation of heterogeneous phase composition, we also show that it is possible to optimize performance properties such as fracture toughness, modulus, and strength.