Effect of polymer architecture on structure and dynamics of polymer nanocomposites

Polymer nanocomposites (PNCs) possess exceptional physical properties that make them highly desirable for various applications. While the investigation of these systems has predominantly focused on linear polymer chains, the influence of polymer matrix architecture on local dynamics, bulk rheology, and nanoparticle (NP) motion remains largely unexplored. In the first phase of this research, we utilized quasi-elastic neutron scattering, bulk rheology, and X-ray photon correlation spectroscopy to examine nanocomposites comprising spherical silica nanoparticles well dispersed in poly(ethylene oxide) matrices with distinct architectures (linear, stars, and hyperbranched). Our findings demonstrate a profound alteration in the mechanical reinforcement of nanocomposites with nonlinear polymers, surpassing conventional counterparts with linear polymers by orders of magnitude. The pivotal roles of polymer compactness and interpenetrability in determining bulk rheology were identified. Moreover, at the microscopic level, the average segmental dynamics were significantly impeded by attractive NPs in matrices with a high degree of branching, contrasting the negligible effect observed in linear polymer matrices at equivalent NP loadings. Additionally, nanoscale dynamics in compact nonlinear matrices exhibited strong decoupling from bulk viscoelasticity, enabling rapid relaxation even at approximately 30% by volume. In PNCs, the outstanding rheological performance is largely attributed to a significant fraction of interfacial polymers. Nevertheless, strategies to control the structure and dynamics of interfacial polymers have been limited. In the second phase, we propose a facile approach centered on varying the macromolecular architecture of interfacial polymers. Our results demonstrate that altering the topology of bound polymers from linear to star and hyperbranched structures effectively modifies polymer-NP interactions and chain interpenetration in interphases, without altering the type, molecular weight of the polymer, or NP surface chemistry in attractive PNCs. The dependency of dispersion and rheological behavior of PNCs on the functionality and arm length of polymers at interfaces is evident. Distinct polymer chain architectures lead to fundamentally different rheological responses and internal dynamics, offering opportunities to rationalize advanced thermoplastic nanocomposites with tunable mechanical behavior. In the third phase, we investigated three model poly(methyl methacrylate) (PMMA) polymers with linear, bottlebrush, and star architectures, each having the same total molar mass, in their neat form and as nanocomposites with well-dispersed silica nanoparticles. Linear chains formed an entangled polymer network, while branched bottlebrush and star chains exhibited a viscoelastic response without a rubbery entanglement plateau and a weak arm relaxation regime between Rouse and terminal flow, akin to other branched polymers. The addition of nanoparticles primarily influenced the terminal relaxation regime, hindering the overall chain motion in the presence of attractive nanoparticles. Broadband dielectric spectroscopy results revealed over 10 times slower segmental relaxation for star homopolymers and a slowdown in the α-relaxation process for all three architectures in their composite form. In the fourth phase, utilizing small angle light scattering (SALS), UV-vis spectroscopy, and rheology, we examined the effect of polymer chain architecture on the dispersion and viscosity of polymer-NP solutions. Our findings suggest that, compared to linear polymers with the same molecular weight, star polymers with short arms create a more compact polymer layer at the interface. Moreover, the viscosity of solutions exhibited a direct response to additional branching, with up to a sixfold higher viscosity observed in solutions with linear polymers compared to those with hyperbranched polymers. In the fifth and final phase, we investigated the structure and dynamics of NPs in solutions with different polymer architectures using X-ray photon correlation spectroscopy (XPCS) and rheology. A direct relationship was observed between increasing branching of polymer chains and disordering in the structure of NPs. Hydroxyl end groups in highly branched architectures protonated the surface of silica NPs, resulting in decreased effective surface charge and weakened electrostatic repulsion among NPs. This led to a decrease in excluded volume between NPs and a broader spacing range among them. Furthermore, increasing branching directly affected the diffusivity of NPs in the medium, where NP motion was subdiffusive in solutions with less-branched linear and four-armed star polymers and diffusive in solutions with hyperbranched chains. In conclusion, this comprehensive study provides critical insights into the intricate interplay between polymer architecture and the physical properties of nanocomposites. It establishes a foundation for designing tailored materials with enhanced performance and tunable mechanical behavior across diverse applications.