Linearly-Changed Thermal Behavior and Depressed Brill Transition in Long-Chain Polyamides Substituted by Methyl Side Groups.

Side substitution is an effective way of functionalizing and modifying the properties of polyamides. Meanwhile, side substitution would significantly influence the crystallization kinetics and polymorphic phase transition of polyamides, which, however, has not been well elucidated. Herein, we synthesized the side-substituted long-chain polyamides with various content of methyl pendent groups and investigated their crystallization and phase transition behaviors. We find that the thermal parameters of side-substituted polyamides vary linearly with the side group content, analogous to the isomorphic crystallization of random copolymers. All the solution-crystallized polyamides experience the α-γ Brill transition during heating, with the Brill transition temperature linearly decreasing as the side group content increases. Intriguingly, the γ-α transition of polyamides during cooling is suppressed with the presence of side methyl groups due to the difficulty in H-bond reorganization and gauche-trans conformational changes. This work has demonstrated the critical role of side substitution in the polymorphic crystallization and phase transition of long-chain polyamides.

Spatially and Temporally Programmable Transparency Evolutions in Hydrogels Enabled by Metal Coordination toward Transient Anticounterfeiting.

Hydrogels have emerged as promising candidates for anticounterfeiting materials, owing to their unique stimulus-responsive capabilities. To improve the security of encrypted information, efforts are devoted to constructing transient anticounterfeiting hydrogels with a dynamic information display. However, current studies to design such hydrogel materials inevitably include sophisticated chemistry, complex preparation processes, and particular experimental setups. Herein, a facile strategy is proposed to realize the transient anticounterfeiting by constructing bivalent metal (M2+)-coordination complexes in poly(acrylic acid) gels, where the cloud temperature (Tc) of the gels can be feasibly tuned by M2+ concentration. Therefore, the multi-Tc parts in the gel can be locally programmed by leveraging the spatially selective diffusion of M2+ with different concentrations. With the increase of temperature or the addition of a complexing agent, the transparency of the multi-Tc parts in the gel spontaneously evolves in natural light, enabling the transient information anticounterfeiting process. This work has provided a new strategy and mechanism to fabricate advanced anticounterfeiting hydrogel materials.

Ultrasensitive Ionic Conductors with Tunable Resistance Switching Temperature Enabled by Phase Transformation of Polymer Cocrystals.

Phase-transformable ionic conductors (PTICs) show significant prospects for functional applications due to their reversible resistance switching property. However, the representative design principle of PTICs is utilizing the melt-crystallization transition of ionic liquids, and the resistance switching temperatures of such PTICs cannot be tuned as desired. Herein, a new strategy is proposed to design PTICs with on-demand resistance switching temperatures by using the melt-crystallization transition of polymer cocrystal phase, whose melting temperature shows a linear relationship with the polymer compositions. Owing to the melt of polymer cocrystal domains and the tunable migration of ions in the resistance switching region, the obtained PTICs display ultrahigh temperature sensitivity with a superior temperature coefficient of resistance of -8.50% °C-1 around human body temperature, as compared to various ionic conductors previously reported. Therefore, the PTICs can detect tiny temperature variation, allowing for the intelligent applications for overheating warning and heat dissipation. It is believed that this work may inspire future researches on the development of advanced soft electrical devices.

Construction of a Soft Antifouling PAA/PSBMA Hydrogel Coating with High Toughness and Low Swelling through the Dynamic Coordination Bonding Provided by Al(OH)3 Nanoparticles.

Marine biofouling, resulting from the adhesion of marine organisms to ship surfaces, has long been a significant issue in the maritime industry. In this paper, we focused on utilizing soft and hydrophilic hydrogels as a potential approach for antifouling (AF) coatings. Acrylic acid (AA) with a polyelectrolyte effect and N-(3-sulfopropyl)-N-(methacryloxyethyl)-N,N-dimethylammonium betaine (SBMA) with an antipolyelectrolyte effect were selected as monomers. By adjusting the monomer ratio, we were able to create hydrogel coatings that exhibited low swelling ratio in both fresh water and seawater. The Al(OH)3 nanoparticle, as a physical cross-linker, provided better mechanical properties (higher tensile strength and larger elongation at break) than the chemical cross-linker through the dynamic coordination bonds and plentiful hydrogen bonds. Additionally, we incorporated trehalose into the hydrogel, enabling the repair of the hydrogel network through covalent-like hydrogen bonding. The zwitterion compound SBMA endowed the hydrogel with excellent AF performance. It was found that the highest SBMA content did not lead to the best antibacterial performance, as bacterial adhesion quantity was also influenced by the charge of the hydrogel. The hydrogel with appropriate SBMA content being close to electrical neutrality exhibits the strongest zwitterionic property of PSBMA chains, resulting in the best antibacterial adhesion performance. Furthermore, the pronounced hydrophilicity of SBMA enhanced the lubrication of the hydrogel surface, thereby reducing the friction resistance when applied to the hull surface during ship navigation.

Crystal Polymorphism of Isodimorphic Polyesters Tuned by cis- and trans-C═C Comonomer Units.

Polymorphism is ubiquitous in polymer crystallization due to the diversified chain conformations and interchain packings in polymer crystals. Controlling chain conformation is effective in tailoring the crystal polymorphism of polymers, which, however, is challenging at the molecular level. Herein, we have synthesized poly(butylene adipate) (PBA)-based copolymers containing C═C units and demonstrated the important role of trans/cis-C═C units in tuning the chain conformation and crystal polymorphism of polymers. Both PBA-based trans- and cis-copolymers show isodimorphic crystallization behavior with the partial inclusion of C═C units in PBA crystals. The presence of trans-C═C units favors the formation of metastable ß-crystals of PBA and retards the ß-to-α crystal transition upon heating due to the highly conformational matching between trans-C═C units and ß-crystals. Conversely, the incorporation of cis-C═C units destroys the regularity of the trans conformation and favors the growth of α-crystals of PBA. This work has elucidated the crucial role of local chain conformation in the crystal polymorphism of polymers.

Tunable Crystallization Kinetics and Crystal Polymorphism in Semicrystalline Polymers: Insight into the Multilevel Structural Order of Polymer Melts.

The melting of semicrystalline polymers is a typical multistep process and involves a series of intermediate melt states. However, the structural characteristics of the intermediate polymer melt is unclear. Herein, we choose polymorphic trans-1,4-polyisoprene (tPI) as a model polymer system and elucidate the structures of the intermediate polymer melt and their strong effects on the following crystallization process. We find that the metastable ß crystals of the tPI melt first into an intermediate state and then recrystallize in new crystals upon thermal annealing. The intermediate melt shows multilevel structural order at the chain level depending on the melting temperature. The conformationally ordered melt can memorize the initial crystal polymorph and accelerate the crystallization process, while the ordered melt without the conformational order can only enhance the crystallization rate. This work provides deep insight into the multilevel structural order of polymer melts and its strong memory effects on the crystallization process.

An Efficiently Doped PEDOT:PSS Ink Formulation via Metastable Liquid-Liquid Contact for Capillary Flow-Driven, Hierarchically and Highly Conductive Films.

With commercial electronics transitioning toward flexible devices, there is a growing demand for high-performance polymers such as poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS). Previous breakthroughs in promoting the conductivity of PEDOT:PSS, which mainly stem from solvent-treatment and transfer-printing strategies, remain as inevitable challenges due to the inefficient, unstable, and biologically incompatible process. Herein, a scalable fabrication of conducting PEDOT:PSS inks is reported via a metastable liquid-liquid contact (MLLC) method, realizing phase separation and removal of excess PSS simultaneously. MLLC-doped inks are further used to prepare ring-like films through a compromise between the coffee-ring effect and the Marangoni vortex during evaporation of droplets. The specific control over deposition conditions allows for tunable ring-like morphologies and preferentially interconnected networks of PEDOT:PSS nanofibrils, resulting in a high electrical conductivity of 6,616 S cm-1 and excellent optical transparency of the film. The combination of excellent electrical properties and the special morphology enables it to serve as electrodes for touch sensors with gradient pressure sensitivity. These findings not only provide new insight into developing a simple and efficient doping method for commercial PEDOT:PSS ink, but also offer a promising self-assembled deposition pattern of organic semiconductor films, expanding the applications in flexible electronics, bioelectronics as well as photovoltaic devices.

Multi-Level Information Encryption/Decryption of Fluorescent Hydrogels Based on Spatially Programmed Crystal Phases.

The growing urgence of information protection promotes continuously the development of information-encryption technique. To date, hydrogels have become an emerging candidate for advanced information-encryption materials, because of their unique stimulus responsiveness. However, current methods to design multi-level information-encrypted hydrogels usually need sophisticated chemistry or experimental setup. Herein, a novel strategy is reported to fabricate hydrogels with multi-level information encryption/decryption functions through spatially programming the polymorphic crystal phases. As homocrystalline and stereocomplex crystal phases in fluorescent hydrogels have different solvent stabilities, the transparency and fluorescence of the hydrogels can be regulated, thereby enabling the multi-level encryption/decryption processes. Moreover, the structural origins behind these processes are discussed. It is believe that this work will inspire future research on developing advanced information-encryption materials upon programming the polymer crystal structure.

Thermoresponsive Water-in-Oil-in-Water Pickering Double Emulsions Stabilized with Biodegradable and Semicrystalline Poly(ethylene glycol)-b-poly(ε-caprolactone-co-δ-valerolactone) Diblock Copolymer Micelles for Controlled Release.

Water-in-oil-in-water (W/O/W) Pickering double emulsions are promising materials for the construction of carriers for water-soluble and oil-soluble molecules or drug delivery systems if the contradictive trade-off between their extreme stability and controlled release properties can be resolved. In this study, biodegradable and biocompatible poly(ethylene glycol)-b-poly(ε-caprolactone-co-δ-valerolactone) (PEG-b-PCVL) diblock copolymers with predesigned hydrophilic to hydrophobic block length ratios and nearly identical ε-caprolactone/δ-valerolactone molar ratio (8/2), were synthesized by ring-opening copolymerization. Then, they self-assembled to create semicrystalline micelles. The melting points of PEG-b-PCVL copolymers and their lyophilized micelles were within a physiological range of temperatures, as determined by differential scanning calorimetry. Water contact angle measurements provided evidence that the surface wettability of PEG-b-PCVL micelles could be tuned by the PCVL block mass fractions or temperature stimulus. Such PEG-b-PCVL micelles were employed as a single particulate stabilizer to develop Pickering double emulsions through a one-step emulsification technique. W/O/W Pickering double emulsions could be generated using relatively hydrophobic PEG-b-PCVL micelles with high mass fractions (exceeding about 89%) of PCVL blocks, and they displayed excellent long-term physical stabilities at room temperature. However, the Pickering double emulsions underwent a rapid microstructural transition into simple oil-in-water Pickering emulsions instead of complete demulsification at elevated temperature (37 °C), which was attributed to the hydrophilicity of micelles enhanced when the core-forming PCVL melted realized by temperature stimulus. Consequently, such W/O/W Pickering double emulsions stabilized solely with semicrystalline PEG-b-PCVL micelles exhibit thermal responsiveness, enabling them to release vitamin B12 encapsulated within the internal aqueous phase rapidly.

Effect of Hydrogen Bonding on Dynamic Rheological Behavior of PVA Aqueous Solution.

The rheological behavior of polyvinyl alcohol (PVA) aqueous solution is crucial to optimizing the processing technology and performance of PVA products. In this paper, the dynamic rheological behavior of PVA aqueous solution was investigated in detail. PVA solution with a concentration of 10 wt% showed unnormal rheological behaviors, that is, the liquid-like behavior in the high frequency (ω) region and the solid-like behavior in the low ω region. A storage modulus (G') plateau appears in the relatively low ω region as a gel with a network structure. Different from conventional hydrogel, this plateau has a low modulus, and the corresponding size of the relaxation unit is estimated to be 554 nm, being higher than the size of a whole PVA chain. It is believed that the network mesh is formed by the intermolecular hydrogen bonding interactions among PVA chains. The relaxation time of these meshes is longer than the reptation time of a PVA chain. Based on the relaxation spectrum and calculation analysis, it is found that the destruction of intermolecular hydrogen bonds, such as by heating up, adding sodium dodecyl sulfate, and shear operation, will make the relaxation unit (mesh) larger and lead to the left shift of the intersection of G' and loss modulus (G″). In a PVA solution with a high concentration, multiple meshes of various sizes could be formed and thus generate multiple relaxation peaks. The large-sized meshes mainly contribute to the left shift of the intersection of G' and G″, and the small-sized meshes contribute to the high plateau modulus. The results in this paper offer a new angle to analyze polymer solutions with strong intermolecular interaction.

Light-Induced Crystalline Size Heterogeneity of Polymers Enables Programmable Writing, Morphing, and Mechanical Performance Designing.

Constructing the spatio-selective crystalline structures has been an effective strategy to diversify the functions and applications of polymers. However, it is still challenging to program the crystalline heterogeneity into commercialized polymers and realize associate functions by a simple yet generalizable method. Herein, we propose a facile approach to fabricate multifunctional materials by programming the spatial distribution of crystal size in semicrystalline polymers. Various crystal size patterns in both plane and depth directions are introduced by the photothermal effect of printed ink and subsequent crystallization at different temperatures, which can be reprogrammed by repeated melting and crystallization. These obtained materials with well-defined crystal size heterogeneities exhibit diverse and regulable optics, mechanical and swelling properties, as manifested in applications including rewritable polymer paper, programmed mechanics, and advanced morphing devices. The light-induced crystal size heterogeneity of polymers has provided insights into developing advanced multifunctional materials.

Microstructurally tunable pickering emulsions stabilized by poly(ethylene glycol)-b-poly(ε-caprolactone) diblock biodegradable copolymer micelles with predesigned polymer architecture.

Poly(ethylene glycol)-b-poly(ε-caprolactone) diblock copolymers (PEG-b-PCL) with predesigned hydrophilic/hydrophobic block length ratios have been synthesized and self-assembled to form micelles, then used to emulsify medium-chain triglycerides with an aqueous phase. The morphologies and sizes of PEG-b-PCL copolymer micelles have been characterized by transmission electron microscopy and dynamic light scattering. Interfacial tension testing between micellar dispersions and oil, combined with water contact angle measurements, have been performed to assess the ability of these micelles to adjust interfacial tension and micellar hydrophobicity, respectively. Relationship between the wettability of PEG-b-PCL copolymer micelles and their emulsification properties has been proved through phase diagram, optical microscopic observation, droplet sizes evolution and phase separation behavior of Pickering emulsion samples. Results show that both oil-in-water and water-in-oil Pickering emulsions, as well as water-in-oil-in-water (W/O/W) double-Pickering emulsions, may be controllably prepared through one-step homogenization. Double microstructure of W/O/W Pickering emulsion has proved to be extremely stable during long-term storage.

Self-evolving materials based on metastable-to-stable crystal transition of a polymorphic polyolefin.

Living organisms can self-evolve with time in order to adapt to the natural environment. Analogically, self-evolving materials also show similar properties based on non-equilibrium structural transformation. The common design of these materials tends to rely on solutions and hydrogels, yet only little attention has been paid to dry materials. To break this limitation, a new principle for developing self-evolving materials from a commercialized polymorphic polyolefin via programmable crystal transition is proposed. The self-evolving materials can encode information on patterns and morphing by the metastable crystal phase. Dynamically, this phase transforms to the stable crystal phase so that the encoded information self-evolves with time, displaying the autonomous characteristic. Moreover, this process can be interrupted at an arbitrary time through solvent-induced recrystallization. These advantages have been demonstrated by fabricating an edible period indicator and imitating sophisticated human body language. It is believed that this work may inspire future research studies on self-evolving materials based on the non-equilibrium process of dry materials.

Polymorphic Phase Formation of Liquid Crystals Distributed in Semicrystalline Polymers: An Indicator of Interlamellar and Interspherulitic Segregation.

Amorphous and melted components can segregate into the interlamellar or interspherulitic regions of polymer crystals in their blends/mixtures; this phase behavior strongly influences the physical properties and functions of materials. However, it is experimentally difficult to evaluate the spatial distributions of the other components in polymer crystals. Herein, we use a small-molecule liquid crystal (LC) as a probe and find that it forms different solid phases when mixed with the semicrystalline polymer poly(l-lactic acid) (PLLA). The LC can form the metastable phase at the lower PLLA crystallization temperature but the stable phase at the higher PLLA crystallization temperature in the PLLA/LC mixture. The formation of LC metastable and stable phases is attributed to the segregation of the LC material in the interlamellar and interspherulitic regions of polymer crystals, respectively. This study provides a potential way to evaluate the spatial segregation in the crystallization-induced microphase separation of polymer blends/mixtures.

Role of Chain Entanglements in the Stereocomplex Crystallization between Poly(lactic acid) Enantiomers.

Stereocomplex (SC) crystallization between polymer enantiomers has opened a promising avenue for preparing high-performance materials. However, high-crystallinity SCs are difficult to achieve for high-molecular-weight (HMW) enantiomeric blends of chiral polymers [e.g., poly(lactic acid)]. Despite extensive studies, why HMW enantiomeric blends have difficulty in SC crystallization has not been clarified. Herein, we chose the HMW poly(l-lactic acid)/poly(d-lactic acid) (PLLA/PDLA) 1/1 blend as the model system and demonstrated the crucial role of chain entanglement in regulating SC crystallization. PLLA/PDLA blends with various entanglement degrees were prepared by freeze-drying. We observed that disentangling promoted not only the crystallization rate but also the crystallinity of SCs in both the nonisothermal and isothermal processes. The less-entangled samples crystallized exclusively as the high-crystallinity SCs at different temperatures, in contrast to the predominant homocrystallization that occurred in the common entangled samples. This study provides deep insight into the SC crystallization mechanism of polymers and paves the way for future research attempting to prepare SC materials.

Fast photothermal poly(NIPAM-co-ß-cyclodextrin) supramolecular hydrogel with self-healing through host-guest interaction for intelligent light-controlled switches.

A graphene oxide/poly(N-isopropylacrylamide-co-ß-cyclodextrin) (GO/poly(NIPAM-co-ß-CD)) hydrogel has been synthesized through host-guest interaction between ß-cyclodextrin (ß-CD) and the isopropyl group of N-isopropylacrylamide (NIPAM). The product exhibits rapid responses to the stimuli of temperature and near-infrared (NIR) irradiation, self-healing properties, and excellent mechanical properties. The host-guest interaction serves as the main physical cross-linker, while a hydrogen bond between the hydroxyl group of ß-CD, GO sheets and amide group of NIPAM acts as a secondary cross-linker. The volume phase transition temperature and NIR response rate of such a hydrogel are controlled by its contents of ß-CD and GO. The obtained hydrogels showing excellent properties might be applied in remote contactless control devices in advanced smart technologies. Based on the excellent characteristics of the hydrogels, remote light-controlled switches have been designed, and more applications will be explored, such as intelligent light-controlled drivers and soft robots.

Stereocomplexed and homocrystalline thermo-responsive physical hydrogels with a tunable network structure and thermo-responsiveness.

The widespread application of thermo-responsive hydrogels requires materials with robust mechanical properties and tunable responsiveness. Herein, we report robust thermo-responsive physical hydrogels with a tunable network structure and responsiveness by controlling the manner of crystallization of hydrophobic blocks. Biocompatible, stereocomplexable poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA) were introduced into thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) to obtain the enantiomeric grafted copolymers PNIPAM-g-PLLA and PNIPAM-g-PDLA and their corresponding hydrogels. The hydrophobic PLLA/PDLA domains served as physical crosslinking junctions in the hydrogels. The crystalline structure of the hydrogels can be facilely tuned by varying the ratio of PLLA/PDLA enantiomeric blocks. Stereocomplex (SC) crystallization between PLLA and PDLA facilitates the formation of H-bonded hydrophobic domains with denser chain packing, which endows the racemic hydrogels with a stronger network structure, higher mechanical strength, and better solvent resistance compared to enantiopure examples. The hydrogels exhibit good thermo-sensitivity in water; the stronger racemic hydrogel network restricts volume shrinkage and water desorption at high temperatures, enabling the facile control of thermo-responsiveness. The crystallization-tuned thermo-responsiveness of racemic and enantiopure hydrogels also allows for the design of assembled bilayer hydrogels capable of thermally triggered reversible shape morphing.

Polyether-modified siloxane stabilized dispersion system on the physical stability and control release of double (W/O/W) emulsions.

In this work, three different polyether-modified siloxanes (PMS1, PMS2, and PMS3) were applied to stabilize water-in-oil emulsions, and sodium caseinate (SC) was used to establish water-in-oil-in-water (W/O/W) emulsions. Here, PMS polymers were modified by Isolan GPS and SC by Tween 80. The impact of modifications on the physical stability and controlled release of W/O/W emulsions were investigated. It was found that the storage stability and control release of double emulsions were dependent on the types of PMS used, percent of Isolan GPS, and Tween 80. When PMS1 and PMS2 were combined with low percent of Isolan GPS and Tween 80, the dispersed droplet sizes were reduced, lower percent in the gravitational sedimentation were achieved than using PMS3 emulsions. The controlled releases of Mg2+ from W/O/W emulsions by using PMS3 were slower than using other PMS. PMS3 had a strong influence in controlling the release of Mg2+ from the double emulsions.

Programmable Reversible Shape Transformation of Hydrogels Based on Transient Structural Anisotropy.

Stimuli-responsive shape-transforming hydrogels have shown great potential toward various engineering applications including soft robotics and microfluidics. Despite significant progress in designing hydrogels with ever more sophisticated shape-morphing behaviors, an ultimate goal yet to be fulfilled is programmable reversible shape transformation. It is reported here that transient structural anisotropy can be programmed into copolymer hydrogels of N-isopropylacrylamide and stearyl acrylate. Structural anisotropy arises from the deformed hydrophobic domains of the stearyl groups after thermomechanical programming, which serves as a template for the reversible globule-to-coil transition of the poly(N-isopropylacrylamide) chains. The structural anisotropy is transient and can be erased upon cooling. This allows repeated programming for reversible shape transformation, an unknown feature for the current hydrogels. The programmable reversible transformation is expected to greatly extend the technical scope for hydrogel-based devices.

Thermoresponsivity, Micelle Structure, and Thermal-Induced Structural Transition of an Amphiphilic Block Copolymer Tuned by Terminal Multiple H-Bonding Units.

Constructing noncovalent interactions has been a benign method to tune the stimuli responsivity and assembled structure of polymers in solution; this is essential for controlling the functions and properties of stimuli-responsive materials. Herein, we demonstrate a novel supramolecular strategy to manipulate the cloud point (Tcp) and assembled structure of thermoresponsive polymers in solution by using H-bonding interactions. We use poly(lactide-co-glycolide)-b-poly(ethylene glycol)-b- poly(lactide-co-glycolide) (PLGA-PEG-PLGA) as a model thermoresponsive polymer and functionalize its chain terminals by the self-complementary quadruple H-bonding motif, 2-ureido-4[1H]-pyrimidinone (UPy). UPy end functionalization and increasing PLGA block length decrease the Tcp of copolymer. Both UPy- and nonfunctionalized copolymers form the spherical micelles at low temperature. They undergo the intermicellar aggregation and form large compound micelles during heating; this thermally induced structural transition causes the presence of Tcp. Due to the UPy-UPy H-bonding interactions, UPy end functionalization leads to more copolymer chains to associate in one micelle, thus, enhancing the hydrodynamic, gyration radii, core size, as well as the packing density of PLGA in micelle core and grafting density of PEG on core-shell interface. The decreased Tcp of UPy-functionalized copolymer stemmed from the stronger intermicellar attractions at high temperature. Furthermore, UPy-functionalized copolymers exhibit higher drug loading content, slower drug release rate, and better separation efficiency in removing the hydrophobic substances from water than PLGA-PEG-PLGA precursors.