Polymerization/Depolymerization-Induced Self-Assembly under Coupled Equilibria of Polymerization with Self-Assembly.

Depolymerization breaks down polymer chains into monomers like unthreading beads, attracting more attention from a sustainability standpoint. When polymerization reaches equilibrium, polymerization and depolymerization can reversibly proceed by decreasing and increasing the temperature. Here, we demonstrate that such dynamic control of a growing polymer chain in a selective solvent can spontaneously modulate the self-assembly of block copolymer micellar nano-objects. Compared to polymerization-induced self-assembly (PISA), where irreversible growth of a solvophobic polymer block from the end of a solvophilic polymer causes micellization, polymerization/depolymerization-induced self-assembly presented in this study allows us to reversibly regulate the packing parameter of the forming block copolymer and thus induce reversible morphological transitions of the nano-objects by temperature swing. Under the coupled equilibria of polymerization with self-assembly, we found that demixing of the growing polymer block in a more selective solvent entropically facilitates depolymerization at a substantially lower temperature. Taking ring-opening polymerization of δ-valerolactone initiated from the hydroxyl-terminated poly(ethylene oxide) as a model system, we show that polymerization/depolymerization/repolymerization leads to reversible morphological transitions, such as rod-sphere-rod and fiber-rod-fiber, during the heating and cooling cycle and accompanied by changes in macroscopic properties such as viscosity, suggesting their potential as dynamic soft materials.

Patchwork Metal-Organic Frameworks by Radical-Mediated Heterografting of Star Polymers for Surface Modification.

We report a synthetic methodology for decorating a surface of metal-organic frameworks (MOFs) with polymers through postsynthetic modification. Well-defined polymers with reversibly deactivated radical species at their chain end were reacted with vinyl-functionalized MOFs in the presence of a radical initiator. The radical addition forms a C-C bond between the polymer end with the functional group at the MOF ligand. We used sterically bulky star polymers containing electron-deficient maleimide chain ends, which facilitated modification of the external surface, yielding polymer-grafted MOF composite particles. A patchy MOF particle can also be obtained by simultaneously grafting two polymers and jammed at the immiscible liquid-liquid interface. We further show that the selective removal of a sacrificial polymer would partially expose the surface of MOFs to external environment, which hinders the uptake of macromolecular guests above the critical hydrodynamic size. Overall, four polymer@MOF composites have successfully been achieved through the present postsynthetic patchworks on MOFs with star polymers and selective etching process.

Side-Chain Density Driven Morphology Transition in Brush-Linear Diblock Copolymers.

We report the synthesis and self-assembly of brush-linear diblock copolymers with variable side-chain length and density. Poly(pentafluorophenyl acrylate-g-ethylene glycol)-b-polystyrene ((PPFPA-g-PEG)-b-PS) brush-linear diblock copolymers are prepared by sequential reversible addition-fragmentation chain transfer (RAFT) polymerization of PPFPA and PS, followed by postpolymerization reaction between the precursor PPFPA-b-PS diblock copolymer and amine-functionalized PEG. By controlling the PEG chain length and the degree of substitution, we obtained brush-linear diblock copolymers with different side-chain lengths and densities. The solid-state morphologies of the diblocks are then examined by small-angle X-ray scattering (SAXS). At low PEG side-chain density, the segregation of PEG and PS away from PPFPA leads to the formation of PEG and PS lamellar domains with PPFPA in the interface. At high PEG side-chain density, the segregation is between the PPFPA-g-PEG brush block and the PS linear block, and the domain morphology is determined by the composition of the brush block. A partial experimental phase diagram is presented, and it illustrates the importance of both side-chain length and density on the microdomain morphology of brush-linear diblock copolymers.

Fabrication and Analysis of Sepiolite/Glass Microcapsules/Liquid Crystal Polymer Composites.

Liquid crystal polymer (LCP) composites filled with sepiolite and glass microcapsules were prepared by melt compounding. The composites were extruded using a twin-screw extruder and injection-molded. The objective of this study is to check a possibility of producing a polymeric composite with a low dielectric constant. Physical characteristics of the composites, such as morphological, rheological, mechanical, and electrical properties were analyzed. In particular, the glass microcapsule-reinforced LCP composites showed a significant improvement in lowering the dielectric constant due to its high air content. Additionally, sepiolite could act as an effective filler to improve the mechanical properties of the composites.

Effects of light-, self-, and tack-curing on degree of conversion and physical strength of dual-cure resin cements.

PURPOSE: To evaluate the polymerization degree of conversion (DC) and physical strength of dual-cure cements with tack-curing, and compare them to those with light-curing and self-curing resins. METHODS: Four dual-cure resin cements were evaluated by DC and diametral tensile strength (DTS) tests with three different polymerization methods: Light-cure (photo-polymerization 40 seconds, self-curing 30 minutes); Self-cure (self-curing 30 minutes); and Tack-cure (photo-polymerization 3 seconds, self-curing 30 minutes). Polymerization degree of conversion was determined using Fourier transform infrared spectroscopy, and calculated based on the ratio changes of aliphatic-to-aromatic C=C IR absorption peaks before and after polymerized. Specimens for DTS (n = 10) were prepared using circular molds (6.0 mm in diameter and 3.0 mm in height) and tested after 24-hour water storage. Data were analyzed by two-way ANOVA. Multiple post-hoc pairwise comparisons were performed by t-test when significant effects were found across the factors (α = 0.05). Results: The Self-cure groups had slow initial curing rate, resulting in the lower DC than both the Light-cure and Tack-cure groups. After 30 minutes of polymerization, only in the RelyX Ultimate group, light-curing resulted in higher DC than tack-curing, which resulted in higher DC than self-curing (P < 0.05). The self-cure of resin cements resulted in a significantly lower DTS only for RelyX Ultimate cement (P < 0.05). There was no significantly different DTS between the Tack-cure and Light-cure groups for all of the resin cements. For all of the three curing modes, RelyX Ultimate cements had the lowest DTS among the four cements tested in this study.