Sustainable Polyurethane and Polyurethane-like Network Materials with Excellent Reprocessability and Stress Relaxation and Stiffness in Acrylic Random Copolymer FilmsPublic
Conventional polymer networks are composed of strong, fixed covalent cross-links. The covalent cross-links render polymer networks with outstanding mechanical properties, heat stability, and chemical resistance; however, they also prevent polymer networks from being decross-linked or/and recycled into similar-value products at the end of their life, leading to environmental and economic concerns. Polyurethanes (PUs) are one of the most widely used commodity polymers. However, the effective recycling of PUs, especially the cross-linked ones, has never been well established. The first part of this dissertation describes two approaches to improve the recyclability of PU and PU-like network materials. In the first approach, the intrinsic dynamic nature of urethane linkages is utilized to achieve excellent reprocessability of conventional PU networks. The property recovery of PU networks after reprocessing is enhanced by judicious molecular design, off-stoichiometry reactions, and proper choice of catalyst. In the second approach, polyhydroxyurethanes (PHUs), which are a class of non-isocyanate PU-like materials, are synthesized from bio-based precursors. Hydroxyurethane dynamic chemistry involves dual dissociative and associative mechanisms. Exploiting this dynamic chemistry, we develop bio-based PHU networks that exhibit complete recovery of cross-link density and tensile properties after multiple reprocessing cycles. We also demonstrate that the mechanical and thermal properties of reprocessable PHU networks can be enhanced by using polyhedral oligomeric silsesquioxanes (POSS) with cyclic carbonate end groups as reactive nanofillers to fabricate dynamic PHU network composites. The intrinsic reprocessability of PHU networks is not impacted by POSS incorporation. With up to 10 wt% POSS loading, PHU–POSS network composites can undergo multiple reprocessing cycles with 100% recovery of cross-link density. The second part of this dissertation focuses on understanding the residual stress relaxation behavior and stiffness of supported acrylic random copolymer films. The residual stress relaxation process in spin-coated poly(styrene/n-butyl acrylate) (P(S/nBA)) random copolymer films is characterized by ellipsometry and fluorescence. Both techniques show that stress relaxation occurs over a period of hours in the rubbery state and that the presence of a very small amount of nBA units vastly changes the relaxation behavior from neat polystyrene films. Fluorescence characterizations show that the interfacial stiffness of supported P(S/nBA) films is enhanced by attractive hydrogen bonding interactions between polymer films and substrates. The length scale over which substrate perturbations modify the average stiffness of a P(S/nBA) film near a hydrophilic substrate interface increases with increasing nBA molar content. In contrast, studies associated with supported poly(styrene/2-ethylhexyl acrylate) (P(S/EHA)) random copolymer films reveal that the interfacial stiffness and stiffness gradient length scales in supported P(S/EHA) films are independent of interfacial hydrogen bonding interactions. For P(S/EHA)s and P(S/nBA)s with similar bulk glass transition temperatures, the stiffness gradient length scales associated with P(S/EHA) films are substantially longer than those of P(S/nBA) films.