Large amplitude oscillatory shear (LAOS) is an emerging technique for examining the way fluids and soft solids fracture or deform beyond the minimum strain for structural distortion. With potential to more accurately simulate various processing steps and applications, LAOS is beginning to give practical insights into the microstructure changes undergone during large strains in gels, colloids, and solutions. These methods are also useful for discovering differences between samples that display only subtle or no differences in strain sweeps (small amplitude oscillatory shear) by looking at the progression of shear stress, elastic modulus (G’) and viscous modulus (G”) under large strain in an extended amplitude sweep. Lissajous plots also measure the influence of strain on the microstructure application of repeated oscillatory stains. Furthermore, thixotropic recovery methods have also implemented with LAOS to evaluate materials for processing and product design.
Polymer Gels Examined with LAOS Data
LAOS research on non-covalent gels is continuing to improve the characterization of polymer chain entanglement, self-assembly, and aggregation. Polybutadiene star polymers have been studied under large strains for their liquid to gel to colloidal glass transitions with increased concentration in an athermal solvent. (1) Correlations were made between the Lissajous plot shape and extended amplitude sweep at large strains to distinguish the LAOS signature for a gel versus a tightly-packed colloidal glass. (1) Seen in the general schematic in Figure 1, the extended amplitude sweeps for colloidal gels showed a smooth G’ decrease after the LVE (Figure 1a), while colloidal glasses have a signature G’ peak centered near the G’ = G” crossover point (Figure 1b).
In Lissajous plots, these characteristics manifest themselves as ellipsoids for gels
(Figure 2a) and rectangles for glasses (Figure 2b). Such a strong difference in the large strain data for the gel and glass may be attributed to the colloidal glass experiencing particle caging, which results from steric forces confiding particles (in this case, star polymers) to their respective place in the network. (1) Contrastingly, the gel sample does not have a high enough concentration to induce particle caging, and therefore the star polymers do not sterically repel each other. Therefore, the Lissajous plot displayed an ellipse because the gel-like polymers can move more freely within the solvent without specific ordering, which is a characteristic of liquid-like materials. The clear differences shown between the gel and the colloidal glass can be utilized for quality control measurements as well as formulation for creating a gel or glass with the desired particle interactions.
Similar to the gel- and glass-forming star polymers, LAOS plots displayed the influence of temperature rather than concentration on the microstructure of Pluronic (PEO-PPO-PEO triblock copolymers) micelles though the presence or absence of G’ peaks at large strain with elliptical or rectangular Lissajous plots. (2) Application of a high temperature was seen to induce a gel-like microstructure marked by no G’ peak (Figure 1a) and a rectangular Lissajous curve (Figure 2b), whereas the lower temperature samples displayed soft gel characteristics of an elliptical Lissajous curve (Figure 2a). (2) This study demonstrated how the thermal impact on gelation may be examined with more insight into gel network hardening or softening under high strains. Industrial testing of these conditions could be used to predict gel durability and fracture under various environmental conditions.
LAOS settings have also been incorporated in time-dependent structural recovery tests, also referred to as thixotropic measurements. Gel strength and resilience have been examined with repeated exposure to both low and high strains. To investigate how the gels endured after the onset of deformation under LAOS, Wang et al. tested clay hydrogels linked with macromolecular dendrites (multi-armed polymer chains) with a thixotropic test under large strains. (3) A gel-to-liquid transition was observed, followed by rapid recovery of the shear moduli when repeatedly cycled between 5,100% and 50% strain. (3) Figure 3 displays this concept of LAOS thixotropy with G’ and G” responses that represent the microstructural endurance of the gel over the recurring exposure to the extreme strains. This method may be adapted for other gels to examine how easily the structure could fall apart beyond the LVE strain.
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Truzzolillo, D., et al., “Depletion gels from dense soft colloids: Rheology and thermoreversible melting”, Journal of Rheology, 2014, 58, 1441.
Hyun, K., and Ahn, K. H., “Large amplitude oscillatory shear behavior of PEO-PPO-PEO triblock copolymer solutions”, Rheologica Acta, 2006, 45, 239.
Wang, Q., et al., “High-water-content mouldable hydrogels by mixing clay and a dendritic molecular binder”, Nature Letters, 2010, 463, 339.