Despite significant past efforts to exploit green, renewable precursors in polymeric materials and to improve the recyclability and reprocessability of nonisocyanate polyurethane (NIPU) networks, no single study has previously investigated biobased polyhydroxyurethane (PHU) network reprocessability. Renewable, dynamic PHU networks were synthesized by reacting bioderived polyfunctional cyclic carbonates, carbonated soybean oil (CSBO), and sorbitol ether carbonate (SEC), with either a synthetic diamine or a biobased diamine. Network reprocessability was studied by dynamic mechanical analysis. With relatively mild reprocessing conditions, CSBO-based PHU networks exhibit complete recovery of crosslink density and associated properties after multiple melt-state recycling steps. In addition to the presence of reversible cyclic carbonate aminolysis and transcarbamoylation exchange reactions, CSBO-based networks were shown via a model reaction to undergo a third dynamic chemistry based on a transesterification exchange reaction. In contrast to the excellent property recovery achieved by CSBO-based PHU networks, as a result of disadvantageous monomer molecular design, SEC-based networks exhibit poor reprocessability even with increased catalyst load and reprocessing temperature and time. This work reveals the effect of monomer structure on the reprocessability of dynamic polymer networks and highlights the effectiveness of CSBO to serve as a precursor of robust, sustainable NIPU networks with excellent reprocessability.

Dynamic urethane chemistry, which involves both associative and dissociative mechanisms, can be used to achieve reprocessability in cross-linked polyurethane (PU) networks. We discovered that property recovery and thermal stability of dynamic covalent PU networks can be enhanced by incorporating excess free hydroxyl groups, thus performing polymerization reactions at slight stoichiometric imbalance of hydroxyl and isocyanate groups, and by increasing the cross-linker functionality from three to four. While free hydroxyl groups suppress the reversion of urethane links and minimize side reactions associated with liberated isocyanate groups under reprocessing conditions, tetrafunctional cross-linkers help to maintain network integrity in the presence of small levels of side reactions. Using these strategies, we developed a PU network that, within experimental error, exhibits 100% recovery of cross-link density and tensile properties after multiple reprocessing steps, which has not been reported before. We also demonstrated the potential of recovering monomer from PU networks by alcoholysis under relatively mild conditions. Our study not only offers a solution to the long-standing issue of PU network recycling but also provides simple strategies to improve property recovery of reprocessable networks.

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Understanding nucleation dynamics is important both fundamentally and technologically in materials science and other scientific fields. Two-dimensional (2D) nucleation is the predominant growth mechanism in colloidal crystallization, in which the particle interaction is attractive, and has recently been regarded as a promising method to fabricate varieties of complex nanostructures possessing innovative functionality. Here, polymers are added to a colloidal suspension to generate a depletion attractive force, and the detailed 2D nucleation process on the terrace of the colloidal crystals is investigated. In the system, we first measured the nucleation rate at various area fractions of particles on the terrace, ϕ. In situ observations at single-particle resolution revealed that nucleation behavior follows the framework of classical nucleation theory (CNT), such as single-step nucleation pathway and existence of critical size. Characteristic nucleation behavior is observed in that the nucleation and growth stage are clearly differentiated. When many nuclei form in a small area of the terrace, a high density of kink sites of once formed islands makes growth more likely to occur than further nucleation because nucleation has a higher energy barrier than growth. The steady-state homogeneous 2D nucleation rate, J, and the critical size of nuclei, r*, are measured by in situ observations based on the CNT, which enable us to obtain the step free energy, γ, which is an important parameter for characterizing the nucleation process. The γ value is found to change according to the strength of attraction, which is tuned by the concentration of the polymer as a depletant.

The heterogeneous nucleation of colloidal crystals with attractive interactions has been investigated via in situ observations. We have found two types of nucleation processes: a cluster that overcomes the critical size for nucleation with a monolayer, and a method that occurs with two layers. The Gibbs free energy changes (ΔG) for these two types of nucleation processes are evaluated by taking into account the effect of various interfacial energies. In contrast to homogeneous nucleation, the change in interfacial free energy, Δσ, is generated for colloidal nucleation on a foreign substrate such as a cover glass in the present study. The Δσ and step free energy of the first layer, γ, are obtained experimentally based on the equation deduced from classical nucleation theory (CNT). It is concluded that the ΔG of q-2D nuclei is smaller than of monolayer nuclei, provided that the same number of particles are used, which explains the experimental result that the critical size in q-2D nuclei is smaller than that in monolayer nuclei.