(617e) Optochemical Self-Organization in Cross-Linking Polymer Systems

Optochemical self-organization is a fundamentally new non-linear mechanism of polymer pattern formation that couples nascent photopolymerizable systems with optical modulation instability (MI). MI is a process whereby tiny phase and amplitude perturbations that are always present in a wide input beam grow exponentially during propagation under the interplay between divergence and medium nonlinearity. In a photopolymerizable medium, non-linearity arises from photo-induced increases in the local refractive index (Δn) arising from densification due to monomer-monomer bonding.

As a result, a broad, uniform beam of light launched into a photopolymerizable system spontaneously divides into a densely packed population (> 10,000 cm²) of identical filaments. Each filament is a non-linear waveform (i.e., optical soliton) characterized by divergence free, light-guided propagation through the medium, and all of which permanently inscribe, and become self-trapped in, their own microscale polymer channel along their propagation path. The channels can be on upwards of several centimeters long and potentially cover an unlimited area (> 1 m²). Hence, photopolymerizing systems in general provide rich opportunities to elicit and study spontaneous pattern forming processes, and open routes to optochemical organization of large-scale, functional polymer microstructures.

As the non-linearity arises from photopolymerization induced densification, and consequent refractive index increase, MI is dependent not only on the optical field, but also the spatial and temporal evolution of the underlying polymer structure during photopolymerization. Yet, controlling MI-based pattern formation by tuning the photopolymerizable medium remains to be explored.

In this work, we examine the role of cross-linking in optical MI by studying multifunctional acrylate systems undergoing free-radical photoinitiated polymerization upon exposure to an incandescent light source. We find that MI-based pattern formation arises from a balance between chain growth and cross-linking, which determines the degree of densification (correlated to Δn). We demonstrate in two experimental photopolymerizable systems that cross-linking is in fact an essential factor in establishing non-linearity in the medium whereby MI may occur. 

The first system comprises a triacrylate and a visible light titanocene photoinitator (λ_max: 393, 460 nm) that is exposed to systematically varied light intensity (1-60 mW/cm²). We employ a spatial amplitude mask to both seed MI as well as coax the ensuing filaments into an ordered lattice. Permanently inscribed patterns form within a specific range of exposure intensities.  Below this range, systems undergo gelation with no observable pattern over the examined exposure durations. Above this range, only weak patterns form, or else no patterns at all. Using correlations between the glass transition (T_g) of the microstructured samples and the refractive index difference (Δn), we show that optical MI occurs under exposure intensities that yield the strongest difference in cross-linking between the micro channels and their surroundings.

The second system consists of acrylate or diacrylate monomer (and photoinitator) either in neat form or mixed with systematically varied weight fractions of triacrylate as a cross-linker (0-30%). Importantly, only when triacrylate is included in the mixture does either system undergo MI-induced pattern formation. The patterns are especially clear with increasing weight fraction of cross-linker — even better than patterns formed from pure triacrylate. The cross-linker inhibits the shrinkage tendencies of the acrylate, allowing stable patterns to form, and enables the diacrylate to undergo pattern formation due to the enhanced densification.

We explain the importance of cross-linking in terms of both increased densification to yield a greater refractive index difference, as well as increased viscosity necessary so pattern formation may occur without any disturbances (i.e., mass flow, exothermic heat convection, etc.) that might cause a nascent structure to prematurely “wash out”.

This work informs on suitable formulations, structural evolution, and processing conditions that lead to non-linearity in photopolymerizable media, whereby MI-based, spontaneous pattern formation can occur, to produce large-scale, microstructured materials. Optochemical self-organization can be employed using a wide range of photopolymerizable systems important to optics, electronics, biomaterials, and microfluidics.