Abstract
Molecular design provides powerful opportunities to tailor polymer dynamics, mechanics, and functionality. Associative polymers represent a simple yet versatile platform, incorporating reversible "stickers" that mediate interchain and intrachain interactions. For more than three decades, the sticky Rouse model has been widely accepted to describe the dynamics of associative polymers. The model predicts that reversible associations introduce a rubbery plateau in the linear viscoelastic spectra, at which associations have not yet relaxed and thus effectively act as crosslinks.
In this thesis, we design and synthesize new classes of unentangled associative polymers carrying unprecedentedly high fractions of stickers, up to eight per Kuhn segment, that can form strong pairwise hydrogen bonding of ~20k_B T without microphase separation. We experimentally show that reversible bonds significantly slow down the polymer dynamics but nearly do not change the shape of linear viscoelastic spectra. This behavior can be explained by a renormalized Rouse model that highlights an unexpected influence of reversible bonds on the structural relaxation of associative polymers. This unexpected behavior raises a fundamental question: under what molecular conditions do reversible associations recover the classical sticky Rouse behavior?
To address this question, we next develop a model system in which sticker fraction and placement can be independently controlled. We show that randomly distributed stickers neither form clusters nor change flow properties, whereas stickers placed at chain ends drive nanocluster formation even at low concentrations. Adding more end-stickers produces a rubbery plateau spanning eight decades in frequency with two distinct relaxation timescales. These results demonstrate that sticker distribution dictates whether associative polymers undergo nanocluster formation or microphase separation, while significant alterations in dynamics and viscoelasticity require both sticker aggregation and thermomechanical stability of associated domains. Our findings resolve a longstanding debate on associative polymer dynamics and provide molecular design rules for programmable soft materials.
Building on these molecular design principles, homogeneous associative polymers are combined with block copolymer self-assembly to create dual-crosslinked linear-associative-linear (LAL) triblock copolymers. These materials form soft, melt-reprocessable elastomers with independently tunable stiffness and energy dissipation. This architecture establishes a versatile molecular design strategy for tough, deformable materials and additive manufacturing.
Beyond associative interactions, this dissertation explores polymer architecture and topological constraints as additional routes to regulate material behavior. Highly entangled bottlebrush polymer networks are developed by increasing molecular weight beyond the entanglement threshold, revealing an ultralow entanglement modulus and producing elastomers that combine tissue-like softness with exceptional extensibility, toughness, and fatigue resistance. The bottlebrush architecture is further extended to ion-conducting polymer systems, where suppression of crystallization enables soft, stretchable, solvent-free polymer electrolytes while maintaining high ionic conductivity at room temperature.
Taken together, these studies establish molecular design principles that link associative interactions, polymer architecture, and chain entanglements to polymer dynamics, mechanics, and functionality, providing a framework for engineering advanced soft materials.
