Simulations of Morphology Evolution in Polymer Blends during Light Self-Trapping

Simulations are presented for binary phase morphologies prepared via coupling the self-trapping properties of light with photopolymerization induced phase separation in blends of reactive monomer and inert linear chain polymer. The morphology forming process is simulated based on a spatially varying photopolymerization rate, dictated by self-trapped light, coupled with the Cahn–Hilliard equation that incorporates the free energy of polymer mixing, degree of polymerization, and polymer mobility. Binary phase morphologies form with a structure that spatially correlates to the profile of the self-trapped beam. Attaining this spatial correlation emerges through a balance between the competitive processes entailed in photopolymerization-induced decreases in diffusion mobility and the drive for the blend components to phase separate. The simulations demonstrate the ability for a self-trapped optical beam to direct binary phase morphology along its propagation path. Such studies are important for controlling the structure of polymer blends, whereby physical properties and critical physical and chemical phenomena may be enhanced

Synthesis of Micropillar Arrays via Photopolymerization: An in Situ Study of Light-Induced Formation, Growth Kinetics, and the Influence of Oxygen Inhibition

We report a study on the growth kinetics and resultant structures of arrays of pillars in photo-cross-linkable films during irradiation with a periodic array of microscale optical beams under ambient conditions. The optical beams experience a self-focusing nonlinearity owing to the photopolymerization-induced changes in refractive index, thereby concentrating light and driving the concurrent, parallel growth of microscale pillars along their path length. We demonstrate control over the pillar spacing and pillar height with the irradiation intensity, film thickness, and the size and spacing of the optical beams. The growth of individual pillars in a periodic array arises from the combination of intense irradiation in the beam regions and oxygen inhibition afforded by the open, ambient conditions under which growth is carried out. We propose a kinetic model for pillar growth that includes free-radical generation and oxygen inhibition in thick films of photoinitiated media in order to interpret the experimental results. The model effectively correlates micropillar array structure to the oxygen inhibition effects. This approach of growing micropillar arrays through photopolymerization is straightforward and scalable and opens opportunities for the design of textured surfaces for applications

Optical Autocatalysis Establishes Novel Spatial Dynamics in Phase Separation of Polymer Blends during Photocuring

We report a fundamentally new nonlinear dynamic system that couples optical autocatalytic behavior to phase evolution in photoreactive binary polymer blends. Upon exposure to light, the blend undergoes spontaneous patterning into a dense arrangement of microscale polymer filaments. The filaments’ growth in turn induces local spinodal decomposition of the blend along their length, thereby regulating the spatially dynamics of phase separation. This leads to the spontaneous organization of a large-scale binary phase morphology dictated by the filament arrangement. This is a new mechanism for polymer blend organization, which couples nonlinear optical dynamics to chemical phase separation dynamics, and offers a new approach to light-directed patterning and organization of polymer and hybrid blends

Tunable Nonlinear Optical Pattern Formation and Microstructure in Cross-Linking Acrylate Systems during Free-Radical Polymerization

We report cross-link-tunable, nonlinear optical pattern formation of transmitted light in a photopolymer undergoing free-radical polymerization. Photopolymerization induces microscale filamentation of a uniform, broad transmitted beam, which corresponds to a concurrent spatial evolution in cross-linked morphology in the photopolymer. Because the photopolymerization is permanent, the ensemble of filaments imprint a microstructure comprising a cross-link gradient pattern. Tuning the system’s capability to cross-link and branch changes the magnitude of the refractive index change (Δ<i>n</i>), which both induces nonlinear conditions and also changes the strength of the optical nonlinearity. Only a monomer with sufficient functionality shows stable optical pattern formation, and its nonlinear regime exists for a specific range of exposure intensities. A monomer of lower functionality can be pushed into the nonlinear regime by formulating it with higher functional monomers, whereby Δ<i>n</i> is increased to provide a stronger response to light. In such formulations, the strength of the nonlinearity, as evidenced by changes in light confinement in the optical pattern, is tuned by varying this monomer’s functionality or its relative weight fraction. The strong correlation among polymerization-induced refractive index change, optical pattern feature size, and cross-linked morphology demonstrates tunable optical nonlinearity through variations in the inherent polymer structure