UCLA

Soft materials, such as elastomers, hydrogels, and liquid crystal elastomers (LCEs), exhibit unique mechanical properties like hyperelasticity, poroelasticity, and anisotropy, stemming from their molecular structures. Elastomers are rubber-like materials composed of cross-linked long-chain polymer networks, while hydrogels consist of cross-linked polymer networks immersed in a solvent. Unlike hydrogels, LCEs combine polymer networks with liquid crystal mesogens, exihibiting semisoft elasticity where a finite, though small, stress is required to rotate the mesogens. These materials find applications in both natural and artificial structures, including biological tissues, soft robots, and flexible sensors.Under extreme external loading conditions, soft materials can display various deformation behaviors such as mechanical instability, phase separation, and fracture. These behaviors are highly nonlinear and not fully understood. For instance, mechanical instability under extreme compression can significantly alter the shape and load-bearing capacity of the materials. Dramatic environmental changes can induce phase separation in a homogeneous mixture, causing it to split into different phases. Additionally, extreme tension can cause soft materials to fracture into multiple pieces. This dissertation aims to study these phenomena and uncover the underlying mechanisms driving these complex behaviors. First, we study mechanical instability through elastomeric tube structures. Specifically, we conduct three-dimensional buckling and postbuckling analysis for thick hyperelastic tubes subjected to axial compression under finite deformation by the asymptotic expansion method. Our theoretical results successfully predict the deformation and stress-strain curves of buckled tubes near the critical loading, which are well validated by finite element analysis. Depending on the geometry, three kinds of postbuckling paths, including continuous buckling, snap-through and snap-back, are discovered. Our work provides understanding and insights into the buckling and postbuckling of thick tubes, and bridges the knowledge gap between postbuckling of thick columns and tubes. Second, we investigate the underpinning role of mechanical constraints and dynamic loading on triggering volume phase transitions and phase separation of hydrogels. Using the Flory-Rehner free energy, which does not predict phase separation of hydrogels under equilibrium free swelling, we show that mechanical constraints can lead to coexistence of multiple phases. We systematically obtain the states of equilibrium for hydrogels under various mechanical constraints, and unravel how mechanical constraints change the convexity of the free energy and monotonicity of the stress-stretch curves, leading to phase coexistence. Using a phase-field model, we predict the pattern evolution of phase coexistence, and show many features cannot be captured by the homogeneous states of equilibrium due to large mismatch stretch between the coexisting phases. We further reveal that the system size, quenching rate, and loading rate can significantly influence the phase behavior, which provides insights for experimental studies related to morphological patterns of hydrogels. Lastly, we investigate the fracture behavior of liquid crystal elastomers (LCEs). We begin by developing a modified semisoft constitutive model to accurately capture their unique mechanical responses. Next, we address the gap in understanding the effect of deformation-director coupling on LCE fracture paths and the lack of established fracture criteria. By combining experimental and theoretical approaches, we aim to elucidate fracture propagation in LCEs. We stretch edge-cracked monodomain LCE samples, recording their stress-strain responses and crack paths under varying initial directors and stretching rates. Our findings reveal that crack propagation paths are highly dependent on both the initial director and the stretching rate. To further understand LCE fracture behavior, we develop a rate-dependent phase-field fracture model, which is validated through experiments and demonstrates the ability to predict complex fracture paths. Our study paves the way for designing LCEs with enhanced fracture properties, beneficial for future applications.