Revealing structural evolution occurring from photo-initiated polymer network formation

Photopolymerization is a widely used technique in materials science, enabling the creation of materials with diverse applications, from surface coatings to bone glues. The chemical transformations during photopolymerization, involving the transition from liquid monomers to a cross-linked solid, are well-understood at the molecular level, based on photochemistry and polymerization kinetics. However, a comprehensive understanding of the simultaneous physical transformations occurring at the nanoscale during this process has been lacking, limited primarily to hypothetical approaches and theoretical simulations. While microscopic-scale changes have been partially elucidated, the nanoscale domain remains largely unexplored experimentally. Advancements in photopolymerization techniques have allowed for materials with tailored optical, chemical, and mechanical properties, including complex 3D architectures. Examples include dynamically actuated Origami-like structures and the photopolymerization of two-dimensional polymers to create molecularly thin sheets. Despite these achievements, a thorough understanding of the film formation process at the nanoscale remains a significant challenge. Current techniques like GISAXS have been successfully applied to monitor film formation in physically drying dispersed systems, but the specific nanoscale dynamics of photopolymerization necessitate novel approaches. This research aims to bridge this knowledge gap by developing and applying a combination of established and advanced techniques to provide a detailed, real-time, nanoscale view of the morphological evolution during photopolymerization.

Previous studies have investigated the inhomogeneities that arise during the photopolymerization of diacrylates, often observing density fluctuations within the material. These density fluctuations are thought to originate from nano-size phase separation of fast-growing network fragments, which are poorly soluble in the unreacted monomers. These fragments continue to grow via a reaction-diffusion mechanism, leading to differences in reactive shrinkage and resultant local density variations if vitrification occurs before full conversion. The reaction-diffusion process also contributes to chain stretching and limited rotational freedom. While techniques like atomic force microscopy (AFM) have provided insights into these heterogeneities at a microscopic level, the nanoscale details and the role of the precursor’s initial structure remain largely unclear. Studies on polymer blends have used GISAXS to reveal reoccurring domains within thin films, offering a potential approach for investigating photopolymerization. Previous GISAXS applications have largely focused on larger size ranges and time scales, such as in the study of physically drying dispersed systems, highlighting the need for a high-resolution, real-time technique like the one presented here.

The study employed two model photopolymer formulations, designated LT (low T_g) and HT (high T_g), to compare systems polymerizing in the non-vitrified and vitrified states, respectively. Both were acrylate-based resins with a chain-growth polymerization mechanism. Real-time Fourier-transform infrared spectroscopy (FTIR) was used to monitor the photo-initiated conversion of double bonds, providing information about the chemical reaction kinetics. Grazing-incidence small-angle X-ray scattering (GISAXS) and X-ray reflectivity (XRR) measurements, conducted at the P03/MiNaXS beamline at PETRA III/DESY, allowed for in situ and real-time observation of nanoscale morphology evolution. GISAXS probed the in-plane morphology, providing information on the size and distribution of nanoscale heterogeneities. The X-ray penetration depth was controlled using a suitable incident angle (α_i = 0.4°) to ensure the full film thickness was probed. XRR, with an attenuated X-ray beam, complemented GISAXS by examining the internal interfaces and density profiles across the film thickness. Atomic force microscopy (AFM) was used to characterize the surface topography and determine film thickness. Finally, dynamic mechanical analysis (DMA) was performed to investigate the viscoelastic properties of the resins. The GISAXS data was reduced using DPDAK software, and the scattering patterns were analyzed to determine the in-plane length scales of heterogeneities using fitted q_xy positions and the relation d = 2π/q_xy. The correlation length characterizing the replication of substrate morphology was determined by analyzing the decrease in amplitude of oscillations from resonant diffuse scattering. XRR profiles were fitted using a multilayer model to study interface roughness and density profiles. AFM was used in tapping mode to determine the root-mean-square roughness of the film surfaces. DMA was conducted in a temperature sweep to assess the glass transition temperatures (T_g) and other viscoelastic parameters.

The real-time FTIR measurements confirmed rapid polymerization in both LT and HT systems, with LT achieving near-complete conversion and HT reaching approximately 80% due to vitrification and restricted mobility. GISAXS revealed that initial nanoscale heterogeneities were present in both formulations before polymerization. These heterogeneities grew significantly larger in LT (from 47.2 nm to 189.2 nm within 1 second of UV exposure) compared to HT (13.4 nm to 17.2 nm), reflecting the difference in their segmental mobility and glass transition temperatures (T_g: LT = -17.3 °C, HT = 40.5 °C). The in-plane heterogeneity length scale in LT was comparable to the film thickness, while in HT, it remained significantly smaller than the film thickness, suggesting homogeneous distribution throughout the film. XRR measurements showed that the roughness at the air-resin interface decreased in HT during polymerization, while it increased in LT. AFM measurements revealed that the fully polymerized LT film had a higher root-mean-square roughness (σ_LT = 0.74 nm) than the HT film (σ_HT = 0.55 nm). The study clearly demonstrated a strong correlation between the initial nanoscale structures of the liquid precursors, the dynamics of the polymerization process (including vitrification), and the final morphology and physical properties of the resulting polymer network. The initial state of the system, significantly influenced by Tg, dictated the overall heterogeneity size and distribution.

The findings highlight the critical role of initial nanoscale structures and molecular mobility in determining the final morphology and properties of photopolymerized materials. The observed differences between LT and HT demonstrate that controlling the glass transition temperature of the precursor can effectively tailor the nanoscale morphology. The rapid growth of heterogeneities in LT, reaching a size comparable to the film thickness, contrasts with the limited growth and homogeneous distribution in HT. This suggests that controlling the rate of polymerization and preventing premature vitrification can lead to larger, less homogeneous structures. This understanding is crucial for the design and fabrication of photocured materials with desired macroscopic properties. The in situ real-time approach offers significant advantages over traditional characterization methods, providing a more complete and accurate picture of the complex processes involved in photopolymerization.

This study successfully demonstrated in situ and real-time monitoring of both chemical and physical transformations during photopolymerization at the nanoscale. The results revealed a strong correlation between initial precursor morphology, polymerization dynamics, and the final material properties. The ability to tailor nanoscale morphology by controlling precursor properties opens new avenues for designing materials with precise macroscopic properties. Future research could focus on exploring a wider range of photopolymerizable systems and investigating the influence of other factors, such as cross-linking density and monomer functionality, on nanoscale morphology evolution.

The study focused on two specific model acrylate-based resin systems. The generalizability of the findings to other types of photopolymerizable systems requires further investigation. The experiments were conducted under specific conditions (UV-LED irradiation, nitrogen atmosphere). Variations in these conditions might influence the observed nanoscale morphology and reaction kinetics. The resolution of the employed techniques is limited and could not fully resolve the fine details of the polymerization process at a molecular level.