The unique capability of topological rearrangement for dynamic covalent polymer networks has enabled various unusual properties (self-healing, solid-state plasticity, and reprocessability) that are not found in conventional thermosets. Achieving these properties in one network in a synergetic fashion can open up new opportunities for shape memory polymer. To accomplish such a goal, the freedom to tune topological rearrangement kinetics is critical. This is, however, challenging to achieve. In this work, two sets of dynamic bonds (urethane and hindered urea) are incorporated into a hybrid network for synthesizing shape memory poly(urea-urethane). By changing the bond ratio, networks with highly tunable topological rearrangement kinetics are obtained. Combining self-healing, solid-state plasticity, and reprocessability in one such shape memory network leads to unusual versatility in its shape-shifting performance.Keywords: dynamic covalent bond; plasticity; reprocessing; self-healing; shape memory polymer; thermoset;

Thermoset shape memory polymer (SMP) with dynamic covalent bonds in the network is a new class of SMPs for which the permanent shape can be reconfigured via topological rearrangement (plasticity). Catalyzed transcarbamoylation has recently been established as an effective exchange reaction for plasticity in cross-linked polyurethane networks. However, ensuring the plasticity severely constrains the network design which adversely affects the ability to tune other classical shape memory properties for practical applications. Facing this new challenge, we design an amorphous polyurethane system for which the cross-linking density can be adjusted in a wide range. We discovered that the use of an aromatic diisocyanate in the synthesis of the polyurethanes facilitates achieving plasticity without requiring any catalyst. The overall network design leads to tunable recovery stress and shape memory transition temperatures without sacrificing the plasticity. The versatility of our polyurethane SMP is further reflected in its triple-shape memory performance. We anticipate that our tunable polyurethanes will benefit a variety of potential SMP device applications.

Polymer foams with interconnected open macropores have been widely utilized in industry and our daily lives. Cryopolymerization is a simple yet efficient method to prepare open porous materials. However, in most cases, this technique was used in an aqueous phase to synthesize macroporous hydrogels; moreover, diversity in these materials needs to be further developed to achieve multi-functionality. In this study, we present organo-phase cryopolymerization as a facile and general method to synthesize acrylate-based shape-memory foams with controllable pore architecture and highly tunable properties including thermomechanical behavior and swelling capability. Pore orientation and porosity can also be controlled through manipulating the conditions of cryopolymerization such as freezing routes and monomer concentrations. By varying the monomer compositions (methyl methacrylate/butyl acrylate), shape-memory foams with a highly tunable transition temperature (Tg from approximately −43 °C to 123 °C) were obtained. Amongst various potential applications, we demonstrated that the shape-memory foams can be programmed into a rather compact form and can expand only during the solvent absorption process to allow simultaneous fast and large absorption, an advantage over the intrinsic bulkiness of classical porous absorbents. Poly(lauryl methacrylate) foam was also prepared, which enabled fast absorption of various solvents including gasoline (ADf = 10.2 g g−1).

AbstractSolid-state plasticity by dynamic covalent bond exchange in a shape-memory polymer network bestows a permanent shape reconfiguration ability. Spatio-selective control of thermally induced plasticity may further extend the capabilities of materials into unexplored domains. However, this is difficult to achieve because of the lack of spatio-control in typical polymer network synthesis. Metal–ligand interactions possess the high strength of covalent bonds while maintaining the dynamic reversibility of supramolecular bonds. Metallosupramolecular shape-memory polymer networks were designed and prepared, which demonstrated solid-state plasticity. The metallo-coordination bonds within these networks permit facile tuning of the plasticity behavior across a wide temperature range, simply by changing the metal ion. By controlling the diffusion of two different metal ions during preparation of a polymer film, a plasticity behavior with a spatial gradient was achieved, providing a unique shape-morphing versatility with potential in shape-memory devices.

Device applications of shape memory polymers demand diverse shape changing geometries, which are currently limited to non-omnidirectional movement. This restriction originates from traditional thermomechanical programming methods such as uniaxial, biaxial stretching, bending, or compression. A solvent-modulated programming method is reported to achieve an omnidirectional shape memory behavior. The method utilizes freeze drying of hydrogels of polyethylene glycol networks with a melting transition temperature around 50 °C in their dry state. Such a process creates temporarily fixed macroporosity, which collapses upon heating, leading to significant omnidirectional shrinkage. These shrunken materials can swell in water to form hydrogels again and the omnidirectional programming and recovery can be repeated. The fixity ratio (R f) and recovery ratio (R r) can be maintained at 90% and 98% respectively upon shape memory multicycling. The maximum linear recoverable strain, as limited by the maximum swelling, is ≈90%. Amongst various application potentials, one can envision the fabrication of multiphase composites by taking advantages of the omnidirectional shrinkage from a porous polymer to a denser structure.

The reversible and click nature of Diels–Alder (DA) reactions has made them ideal candidates to design materials with nonconventional properties. Most commonly, the reversibility of DA is utilized for designing thermosets that can be liquefied for reprocessing and self-healing, yet the dynamic equilibrium nature has been largely neglected. In this work, shape memory polymers (SMP) containing DA moieties in the networks were synthesized. In addition to its remoldability at the liquid state at sufficiently high temperatures (above 110 °C), we show uniquely and surprisingly that such a network can undergo plastic deformation in its solid state at intermediate temperatures (60–100 °C) by taking advantage of its dynamic equilibrium for network topological rearrangement. The liquid state remoldability and solid state plasticity represent two distinct yet complementary mechanisms to manipulate the permanent shape of an SMP, leading to unprecedented versatility that can benefit a variety of applications in the future.

Enzyme-degradable hydrogels with aligned pores were prepared via unidirectional freezing of a mixed solution of oxidized dextran (oxDex) and chitosan (CHI) followed by cryo-gelation. At the frozen state, imine bonds as the crosslinking points of the hydrogels were formed as a result of the reaction between the aldehyde groups on the oxDex and the amino groups on the CHI. The morphologies of the obtained hydrogels were studied by confocal laser scanning microscopy (CLSM). Microtubular pores aligned along the freezing direction were obtained in the swollen state of the hydrogels. The pore size was adjustable ranging from 20 to 100 μm by controlling the freezing temperature in the unidirectional freezing step. Upon exposure of the hydrogels to chitosanase solution, chitosan can be hydrolyzed into short oligosaccharide fragments leading to collapse and degradation of the gel network. The degradation rate can be tuned by adjusting the gel composition and enzyme concentration.

Biodegradable macroporous hydrogels were prepared using a biodegradable crosslinker based on chitosan derivatives. The chitosan crosslinker was synthesized through the amidating reaction of amino groups in the chitosan oligosaccharide (CSO) chains and carboxyl groups in acrylic acid (AA) under the catalysis of 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimidehydro-chloride (EDC) and N-hydroxysuccinimide (NHS). The amidating reaction is able to provide polymerizable carbon–carbon double bonds for CSO. The chemical structure of the resulting compound chitosan oligosaccharide-graft-acrylic acid (CSO-g-AA) was characterized by Fourier transform infrared spectroscopy (FTIR) and 1H nuclear magnetic resonance (1H NMR) spectroscopy. The content of double bonds was measured by titration analysis. The measurement showed a grafting ratio of AA on CSO of about 3.5% (on average two acryoyl groups per CSO-g-AA chain) when the applied molar ratio of AA to CSO was 1.0. Polyacrylamide (PAM) cryogels were synthesized by the free radical polymerization of acrylamide using a modified freezing polymerization method crosslinked by CSO-g-AA. The obtained cryogels were macroporous hydrogels which could be degraded into solutions of linear polymer chains when treated with appropriate enzymes. Then we used snailase as a model enzyme to study the biodegradation process. The degradation was monitored by morphological studies using a confocal laser scanning microscope (CLSM) and scanning electron microscope (SEM), mechanical strength, and swelling ratio. The duration of the degradation process was adjustable from one month to two months when using different concentrations of snailase solutions.

Polymer foams with interconnected open macropores have been widely utilized in industry and our daily lives. Cryopolymerization is a simple yet efficient method to prepare open porous materials. However, in most cases, this technique was used in an aqueous phase to synthesize macroporous hydrogels; moreover, diversity in these materials needs to be further developed to achieve multi-functionality. In this study, we present organo-phase cryopolymerization as a facile and general method to synthesize acrylate-based shape-memory foams with controllable pore architecture and highly tunable properties including thermomechanical behavior and swelling capability. Pore orientation and porosity can also be controlled through manipulating the conditions of cryopolymerization such as freezing routes and monomer concentrations. By varying the monomer compositions (methyl methacrylate/butyl acrylate), shape-memory foams with a highly tunable transition temperature (Tg from approximately −43 °C to 123 °C) were obtained. Amongst various potential applications, we demonstrated that the shape-memory foams can be programmed into a rather compact form and can expand only during the solvent absorption process to allow simultaneous fast and large absorption, an advantage over the intrinsic bulkiness of classical porous absorbents. Poly(lauryl methacrylate) foam was also prepared, which enabled fast absorption of various solvents including gasoline (ADf = 10.2 g g−1).