(505f) Phase–Separation in Photopolymer Composite Media Under Non–Uniform Irradiation

Background: The addition of filler material such as nanoparticles (NP) to a polymer matrix significantly enhances the thermomechanical properties of the polymer composite material [1], making nanoparticle (NP)–containing polymer composite materials ubiquitous in daily life. Controlling NP distribution in polymer matrices is critical to controlling the thermal, mechanical, and chemical properties of these materials, as has been shown for rubber tires, for example [2]. This applies not only to homogenous materials such as polymer films, but also to structured polymer composite materials such as micropillar arrays used in magnetic actuation [3], microfluidics [4], and reentrant structures for anti–wetting applications [5].

Problem: Typically, these structures are on the length scale of a few hundred micrometers, and incumbent manufacturing techniques such as lithography and imprinting do not offer control over NP distribution during fabrication, severely limiting their use in directly organizing the nanofiller phase in polymeric systems. Moreover, these techniques require the use of molds and templates, and often consist of multiple processing steps involving toxic chemicals such as HF [6].

Motivation: Motivated by the importance of controlling the spatial distribution of NPs in polymer composites and the limitations of incumbent techniques in doing so, we present here a photopolymerization–based technique known as light–induced self–writing (LISW) that allows directly controlling NP distribution in a single processing step [7].

Methods and Results: In this technique, optical beams passing through a photomask elicit photopolymerization and consequent photopolymerization–induced phase–separation (PhIPS) of NPs along their pathlength in the composite photopolymer medium [8]. Adding another layer of complexity and richness to the entire process, the optical beams themselves undergo divergence–free propagation across the film thickness, generating pillar–like structures with tunable height [9]. The competing processes of thermodynamically favorable phase–separation and kinetics of photopolymerization in these polymerizing regions are leveraged directly, specifically using light intensity as the tool to control polymer morphology. First, we explored the phase–separation dynamics of a model Silicon nanoparticle (SiNP)–acrylate photopolymer system using in–situ Raman spectroscopy, which revealed that high light intensity (~20 mW/cm²) arrests NP phase–separation, whereas low light intensity (~4 mW/cm²) allows NP phase–separation to evolve relatively more freely from the polymerizing regions to the non–polymerizing regions created by the optical photomask. This occurs due to the intensity–dependent nature of the photopolymerization rate. To confirm the final morphologies obtained, we used ex–situ X–ray mapping and Raman spectroscopy, which both confirmed phase–separated structures with a core–shell–like morphology at low light intensity, and NP–embedded structures with arrested NP phase–separation at high light intensity [8]. Since the presence of NPs in these morphologies imparts exciting properties to the structured composite materials, we show one application each for the phase–separated and embedded structures. Highlighting the versatility of this technique, we demonstrate phase–separated superhydrophobic structures for anti–wetting applications using TiO₂ nanoparticles [6], and magnetically–responsive Fe₃O₄–containing composite pillars [10]. In the former, the pattern generated by the optical photomask and the phase–separated NPs synergistically impart a dual–scale roughness to the material surface, enabling excellent superydrophobicity and self–cleaning ability. In the latter, the Fe₃O₄ NPs embedded within acrylate pillars impart magnetic properties to the final composite material. It is noteworthy that NP organization and overall polymer structure growth occur and evolve simultaneously herein, which is a highlight of this technique that can be very useful in fabricating materials of this nature.

Implications: Since this work combines nonlinear optics and polymer composite processing, it provides understanding of important aspects such as the influence of nanoparticles on depth of cure and photopolymerization kinetics. Moreover, this technique can be extended to a variety of photoresponsive composite formulations to obtain structured composite materials for applications such as chemical and gas sensing, CO₂ capture, and stimuli–responsive structures, among others.

Conclusions: We first demonstrate control over NP spatial distribution during photopolymerization leveraging the fundamental relationship between incident light intensity and photopolymerization rate. We then extend and apply this process to formulations that are relevant from an applications standpoint, and demonstrate that materials with functionalities can be obtained using this technique.

References:

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[10] Wellington, N., Pathreeker, S., & Hosein, I. D. (2022). Light–induced Self–Writing of polymer composites: A novel approach to develop core–shell–type structures. Composites Communications, 101058.