Morphological Characterization and Microstructure Formation in  Interpenetrating Polymer Networks

Heyl, Tyler

doi:https://doi.org/10.21985/n2-6fjn-1e12

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Morphological Characterization and Microstructure Formation in Interpenetrating Polymer Networks

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Interpenetrating polymer networks (IPNs) are multicomponent materials that enhance the compatibility of otherwise immiscible polymers by trapping the microstructure in a non-equilibrium state. By combining polymers with vastly different moduli, IPNs effectively disperse rigid polymers within a soft matrix, resulting in a reinforced elastomer. This approach significantly increases the modulus and toughness of the subsequent elastomer without sacrificing its elasticity, and it has been observed that these properties are influenced by the microstructure of the IPN. Despite numerous studies exploring the properties and microstructure of IPNs, our ability to predict these properties and microstructure along with understand how properties vary on a nanometer length scale remains limited. This thesis aims to develop a comprehensive understanding of the microstructure of polydimethylsiloxane/poly(methyl methacrylate) (PDMS/PMMA) IPNs by establishing the relationship between the microstructure and macroscopic properties, investigating local properties, and establishing mechanisms responsible for microstructure formation.To begin, we investigated macroscopic structure-property relationships in PDMS/PMMA IPNs by varying composition and synthesis routes, either through simultaneous or sequential curing. At high PMMA compositions (up to 50 wt% PMMA), we observed an order of magnitude increase in modulus and fracture toughness in the resulting IPN. The Krieger-Dougherty equation was used to model the modulus increase, and we concluded that the primary cause of the increased modulus was due to the increased PMMA fraction. The optical properties and microstructure were also studied, and we found significant variation between the synthesis routes. In sequentially cured IPNs, the material became more transparent as the PMMA fraction increased. However, the opposite trend was observed in simultaneously cured IPNs. We explained this phenomenon by extracting domain size from small angle x-ray scattering (SAXS) and atomic force microscopy (AFM) images, which showed that in sequentially cured IPNs, the PMMA size decreased with increasing PMMA fraction. However, in simultaneous IPNs, domain size remained constant with increasing PMMA fraction. We concluded that both size and composition affect the transmittance of IPNs. To gain a deeper understanding of mechanical reinforcement and phase separation, we investigated local properties in simultaneously cured PDMS/PMMA IPNs. The degree of polymer mixing is traditionally explored in IPNs using macroscopic techniques that fail to capture local details. To bridge this gap, we correlated single-molecule localization microscopy (SMLM) with nanomechanical mapping (NM) in the same spatial area to establish a relationship between local composition fluctuations (determined from SMLM) and modulus (determined from NM). Our findings revealed significant variations in composition and modulus across individual PMMA domains, and the relationship between local composition and modulus was modeled by the mixing equation for modulus. Upon examining the relationship between domain size and PMMA composition, we observed that domains larger than 250 nm were composed of pure PMMA, whereas smaller domains had a composition range of 50-80 wt% PMMA. Based on these observations, we proposed that the microstructure forms via a combination of nucleation and growth and spinodal decomposition. Continuing our investigations of microstructure formation in IPNs, we varied the composition and curing intensity in PDMS/PMMA IPNs to understand how different morphologies develop. In most cases, a spherical microstructure with a monomodal domain size distribution was observed. However, when 30 wt% PMMA IPNs were cured at low intensities (0.2 mW/cm2), an irregularly shaped morphology was formed. Additionally, 50 wt% PMMA IPNs cured at high intensities (≥2 mW/cm2) produced a spherical morphology with a bimodal domain size distribution. We then used in situ Fourier-transform infrared spectroscopy and small-angle X-ray scattering experiments to correlate polymerization kinetics with microstructure development. Based on the results, we proposed different phase separation mechanisms. We concluded that a spherical microstructure with a monomodal domain size distribution is created via spinodal decomposition and the final microstructure is kinetically trapped. The irregularly shaped morphology is also produced by spinodal decomposition, but the microstructure is not kinetically trapped, causing domain to aggregate after gelation. Finally, the bimodal domain size distribution is produced by a mechanism that is a combination of nucleation and growth and spinodal decomposition phenomena as previously mentioned. We believe that the various phase separation mechanisms are driven by differences in PMMA molecular weight. These results, combined with our previous studies, form a greater understanding of microstructure formation, local composition, and resulting properties of PDMS/PMMA IPNs. Our findings may allow for the prediction and deliberate design of the microstructure and macroscopic properties of future interpenetrating polymer networks.

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