Yingwu Luo

A General Approach Towards Thermoplastic Multishape‐Memory Polymers via Sequence Structure Design

The chain sequence of a poly(styrene-co-methyl acrylate) copolymer is designed to form a V-shaped gradient sequence via controlled/living radical emulsion copolymerization. This specially designed chain sequence gives this common copolymer the capacity of multishape memory. The copolymer can sequentially recover to its permanent shape from three or more previously programmed temporary shapes with the stimulus of temperature.

Shape memory polymer network with thermally distinct elasticity and plasticity

Shape memory polymer with thermally distinct elasticity and plasticity enables highly complex shape manipulations. Stimuli-responsive materials with sophisticated yet controllable shape-changing behaviors are highly desirable for real-world device applications. Among various shape-changing materials, the elastic nature of shape memory polymers allows fixation of temporary shapes that can recover on demand, whereas polymers with exchangeable bonds can undergo permanent shape change via plasticity. We integrate the elasticity and plasticity into a single polymer network. Rational molecular design allows these two opposite behaviors to be realized at different temperature ranges without any overlap. By exploring the cumulative nature of the plasticity, we demonstrate easy manipulation of highly complex shapes that is otherwise extremely challenging. The dynamic shape-changing behavior paves a new way for fabricating geometrically complex multifunctional devices.

Supramolecular Lego Assembly Towards Three‐Dimensional Multi‐Responsive Hydrogels

Inspired by the assembly of Lego toys, hydrogel building blocks with heterogeneous responsiveness are assembled utilizing macroscopic supramolecular recognition as the adhesion force. The Lego hydrogel provides 3D transformation upon pH variation. After disassembly of the building blocks by changing the oxidation state, they can be re-assembled into a completely new shape.

Dynamic Covalent Polymer Networks: from Old Chemistry to Modern Day Innovations

Dynamic covalent polymer networks have long been recognized. With the initial focus on the unintended impact of dynamic covalent linkages on the viscoelasticity of commercial rubbers, efforts in modern times have transitioned into designing dynamic covalent polymer networks with unique adaptive properties. Whereas self-healing and thermoset reprocessing have been the primary motivations for studying dynamic covalent polymer networks, the recent discovery of the vitrimeric rheological behavior and solid-state plasticity for this type of material have opened up new opportunities in material innovations. This, coupled with the revelation of the dynamic characteristics of commercially relevant polymer building blocks such as esters and urethanes, suggests a promising future for this class of materials.

Pushing the mechanical strength of PolyHIPEs up to the theoretical limit through living radical polymerization

High internal phase emulsion (HIPE) living polymerization was proposed in current work. Reversible addition-fragmentation chain transfer polymerization (RAFT), one of the living radical polymerization methods, was introduced to the polymerization of HIPE for the preparation of polystyrene PolyHIPEs. Living polymerization changed the crosslinking process giving a highly homogeneous polymeric wall of PolyHIPEs. The resolving of the heterogeneity problem increased the mechanical strength of PolyHIPEs significantly with the Youngs modulus reaching the theoretical value of ∼45 MPa, which was more than 3 times higher than that by the conventional method. Not only the open-cellular structure of PolyHIPEs was maintained but also a higher connectivity was achieved. Both the type and the concentration of RAFT agents were found to have influence on the morphology and mechanical strength of PolyHIPEs.

Fast-moving soft electronic fish

A soft robotic fish can quickly swim and turn with a fully integrated onboard system for power and remote control. Soft robots driven by stimuli-responsive materials have unique advantages over conventional rigid robots, especially in their high adaptability for field exploration and seamless interaction with humans. The grand challenge lies in achieving self-powered soft robots with high mobility, environmental tolerance, and long endurance. We are able to advance a soft electronic fish with a fully integrated onboard system for power and remote control. Without any motor, the fish is driven solely by a soft electroactive structure made of dielectric elastomer and ionically conductive hydrogel. The electronic fish can swim at a speed of 6.4 cm/s (0.69 body length per second), which is much faster than previously reported untethered soft robotic fish driven by soft responsive materials. The fish shows consistent performance in a wide temperature range and permits stealth sailing due to its nearly transparent nature. Furthermore, the fish is robust, as it uses the surrounding water as the electric ground and can operate for 3 hours with one single charge. The design principle can be potentially extended to a variety of flexible devices and soft robots.

pH Responsivity and micelle formation of gradient copolymers of methacrylic acid and methyl methacrylate in aqueous solution.

A series of gradient copolymers of methacrylic acid (MAA)/methyl methacrylate (MMA) with four end-to-end composition profiles (uniform, linear gradient, triblock with linear gradient midblock, and diblock) but all having an average chain composition of ̅F(MMA) ≈ 0.5 and an average chain length of 200 were synthesized via model-based, computer-programmed, semibatch atom-transfer radical copolymerization (ATRcoP). These samples allowed us to investigate systematically the effects of the gradient composition profile on the pH responsivity and micelle formation of the copolymers in an aqueous solution. Measurements included light transmittance, TEM, AFM, DLS, (1)H NMR, and pH titration. It was found that linear gradient, triblock, and diblock copolymers formed spherical micelles at high pH. The micelles of the linear gradient copolymer contained MMA units in their hydrophilic shells, and those of the triblock and diblock copolymers had all of their MMA units residing in their cores. The composition profile showed a strong effect on the degree of acid dissociation at a given pH. The conformational transition of the copolymer chains was determined by both the pH value and composition profile. Copolymers having sharper gradients required a lower pH to trigger the conformational transition and a narrower pH range to complete the transition.

Limiting Conversion Phenomenon in Hybrid Miniemulsion Polymerization

A phenomenon seemingly unique to hybrid miniemulsion polymerization was observed where monomer conversion would either plateau at a limiting value or quickly switch to a dramatically lesser rate. This phenomenon has been attributed to a combination of three factors. The first factor is the degree to which the monomer and resinous component are compatible. Second is the resultant particle morphology after circa 80% monomer conversion, which roughly corresponds to the portion of reaction where this morphology is established. The third factor is the degree of interaction between the growing polymer and resin (grafting). Of these three, the first two factors were found much more significant in contributing to the limiting conversion. When particle morphology was found to be core/shell, a hard shell (high Tg polymer, PMMA) was found to form a barrier against newly formed initiator radicals derived in the aqueous‐phase after appropriate conversion. Residual unreacted monomer solubilized in the resin‐dominated particle core was thereafter unreachable by new radicals; hence a limited monomer conversion. In cases where the acrylic polymer (PBA) exhibited a glass transition significantly below the reaction temperature, instead of a core/shell morphology one where the acrylic polymer (and monomer) comprised the continuous particle‐phase with small internal resinous island domains was observed. A portion of the monomer concentration was again found to be solubilized within the resin domains, yet in this case newly formed initiator radicals encountered a viscous environment instead of an effective barrier. Rate was found to be limited by the feed of monomer to local polymerization in the continuous particle phase from those resinous islands where residual monomer is solubilized. This is what led to continued polymerization, but at a considerably lesser rate. †This paper is dedicated to Professor Gary W. Poehlein, colleague, mentor and friend.

Fabrication of non-collapsed hollow polymeric nanoparticles with shell thickness in the order of ten nanometres and anti-reflection coatings

The non-collapsed hollow polymeric nanoparticles with shell thicknesses in the order of 10 nm are prepared by interfacially confined reversible addition fragmentation transfer (RAFT) miniemulsion polymerization. The void fraction and average diameter of the hollow polymeric nanoparticles could be largely tuned up to 0.58 and from 68 nm to 180 nm, respectively. The non-collapsed hollow polymeric nanoparticles could be a building block for nanoporous materials, which hold promise in many fields such as anti-reflection coatings, ultra-thermal insulation materials, catalysis and sensors.

Tailor-made compositional gradient copolymer by a many-shot RAFT emulsion polymerization method

A many-shot RAFT emulsion polymerization method is proposed to synthesize gradient copolymers with high molecular weight and a tailor-made compositional gradient. In this method each shot consisting of comonomers with pre-set different fractions and targeting the molecular weight of 10 000 g mol−1 was added in a stepwise manner during the reaction. High conversions over 95% were achieved in 35 min after each shot. The compositional variation along the polymer chain was then directly determined by the comonomer fractions added at each shot. Styrene/n-butyl acrylate gradient copolymers (including linear and V-shaped gradient) with molecular weights as high as 90 000 g mol−1 were prepared by this method. The composition profiles along the polymer chains agreed well with the theoretical predictions, and the composition distribution among the polymer chains was narrow. The gradient copolymers showed different thermal and phase separation properties from their block counterparts, as expected. These results demonstrated the successful tailor-making of the gradient copolymers. The current strategy will act as a facile method to prepare tailor-made gradient copolymers with high molecular weights and within a short time.