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.

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.

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.

Dual-Crosslink Physical Hydrogels with High Toughness Based on Synergistic Hydrogen Bonding and Hydrophobic Interactions.

Constructing dual or multiple noncovalent crosslinks is highly effective to improve the mechanical and stimuli-responsive properties of supramolecular physical hydrogels, due to the synergistic effects of different noncovalent bonds. Herein, a series of tough physical hydrogels are prepared by solution casting and subsequently swelling the films of poly(ureidopyrimidone methacrylate-co-stearyl acrylate-co-acrylic acid). The hydrophobic interactions between crystallizable alkyl chains and the quadruple hydrogen bonds between ureidopyrimidone (UPy) motifs serve as the dual crosslinks of hydrogels. Synergistic effects between the hydrophobic interactions and hydrogen bonds render the hydrogels excellent mechanical properties, with tensile breaking stress up to 4.6 MPa and breaking strain up to 680%. The UPy motifs promote the crystallization of alkyl chains and the hydrophobic alkyl chains also stabilize UPy-UPy hydrogen bonding. The resultant hydrogels are responsive to multiple external stimuli, such as temperature, pH, and ion; therefore, they show the thermal-induced dual and metal ion-induced triple shape memory behaviors.

Stereocomplexed physical hydrogels with high strength and tunable crystallizability.

Physical hydrogels crosslinked by non-covalent interactions have attained increasing attention due to their good mechanical properties and processability. However, the use of feasible and controllable non-covalent interactions is highly essential for preparing such hydrogels. In this article, we report on stereocomplexed physical hydrogels prepared by simple casting and swelling of amphiphilic graft copolymers bearing a poly(acrylic acid) (PAA) backbone and poly(l-lactic acid) (PLLA) or poly(d-lactic acid) (PDLA) stereocomplexable side chains. The microstructure, swelling behavior, and mechanical and shape memory properties of the obtained hydrogels can be tuned by varying the copolymer composition and stereocomplex (SC) crystallization of PLLA/PDLA enantiomeric chains. The long PLLA or PDLA chains segregate to form hydrophobic, crystallized domains in water, serving as physical crosslinking junctions for hydrogels. SC crystallization between PLLA and PDLA further enhances the number density of physical crosslinkers of enantiomerically mixed hydrogels. The SC content increases as the PLLA/PDLA ratio approaches 1/1 in enantiomerically mixed hydrogels. The average distance between crosslinking junctions declines for the hydrogels with a high PLLA (or PDLA) mass fraction (MPLA) and SC content, due to the increased number density of physical crosslinkers. Accordingly, the tensile strength and the Young's modulus increase but the swelling ratio and the elongation-at-break of the hydrogels decrease with an increase in MPLA and SC content. The hydrogels exhibit shape memory behavior; the shape fixing ability is enhanced by the SC crystallization of PLLA/PDLA side chains in the hydrogels.

Thermoresponsive physical hydrogels of poly(lactic acid)/poly(ethylene glycol) stereoblock copolymers tuned by stereostructure and hydrophobic block sequence.

CBABC-type poly(lactic acid) (PLA)/poly(ethylene glycol) (PEG) pentablock copolymers composed of a central PEG block (A) and enantiomeric poly(l-lactic acid) (PLLA, B), poly(d-lactic acid) (PDLA, C) blocks were synthesized. Such pentablock copolymers form physical hydrogels at high concentrations in an aqueous solution, which stem from the aggregation and physical bridging of copolymer micelles. These gels are thermoresponsive and turn into sols upon heating. Physical gelation, gel-to-sol transition, crystalline state, microstructure, rheological behavior, biodegradation, and drug release behavior of PLA/PEG pentablock copolymers and their gels were investigated; they were also compared with PLA-PEG-PLA triblock copolymers containing the isotactic PLLA or atactic poly(d,l-lactide) (PDLLA) endblocks and PLLA-PEG-PLLA/PDLA-PEG-PDLA enantiomeric mixtures. PLA hydrophobic domains in pentablock copolymer gels changed from a homocrystalline to stereocomplexed structure as the PLLA/PDLA block length ratio approached 1/1. The gel of symmetric pentablock copolymer exhibited a wider gelation region, higher gel-to-sol transition temperature, higher hydrophobic domain crystallinity, larger intermicellar distance, higher storage modulus, and slower degradation and drug release rate compared to those of the asymmetric PLA/PEG pentablock copolymers or triblock copolymers. SAXS results indicated that the PLLA/PDLA blocks stereocomplexation in pentablock copolymers facilitated the intermicellar aggregation and bridging. Cylindrical ordered structures were observed in all the gels formed from the PLA/PEG pentablock and triblock copolymers. The stereocomplexation degree and intermicellar distance of the pentablock copolymer gels increased with heating.

Core-shell structure, biodegradation, and drug release behavior of poly(lactic acid)/poly(ethylene glycol) block copolymer micelles tuned by macromolecular stereostructure.

Poly(ethylene glycol)-b-poly(L-lactic acid)-b-poly(D-lactic acid) (PEG-b-PLLA-b-PDLA) stereoblock copolymers were synthesized by sequential ring-opening polymerization. Their micelle formation, precise micelle structure, biodegradation, and drug release behavior were systematically investigated and compared with the PEG-b-poly(lactic acid) (PEG-b-PLA) diblock copolymers with various PLA stereostructures and PEG-b-PLLA/PEG-b-PDLA enantiomeric mixture. Stereoblock copolymers having comparable PLLA and PDLA block lengths and enantiomerically-mixed copolymers assemble into the stereocomplexed core-shell micelles, while the isotactic and atactic PEG-b-PLA copolymers formed the homocrystalline and amorphous micelles, respectively. The PLA segments in stereoblock copolymer micelles show smaller crystallinity than those in the isotactic and enantiomerically-mixed ones, attributed to the short block length and presence of covalent junction between PLLA and PDLA blocks. As indicated by the synchrotron radiation small-angle X-ray scattering results, the stereoblock copolymer micelles have larger size, micellar aggregation number, core radius, smaller core density, and looser packing of core-forming segments than the isotactic and enantiomerically-mixed copolymer micelles. These unique structural characteristics cause the stereoblock copolymer micelles to possess higher drug loading content, slower degradation, and drug release rates.

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.

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.

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.

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.

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.

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.

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.

Expansion Properties and Diffusion of Blowing Agent for Vinylidene Chloride Copolymer Thermally Expandable Microspheres.

Vinylidene chloride copolymer microspheres were synthesized by in situ suspension copolymerization of vinylidene chloride (VDC), methyl methacrylate (MMA), and/or acrylonitrile (AN) in the presence of a paraffin blowing agent. The effects of shell polymer properties including compositions, glass transition temperature (Tg), crosslinking degree, blowing agent type, and encapsulation ratio (Er) on the expansion properties of copolymer microspheres were investigated. Moreover, the diffusion properties of blowing agent in copolymer microspheres were studied. The results show that VDC-MMA-AN copolymer microspheres exhibited excellent expansion properties, and the volume expansion ratio (Ev) and the apparent density were decreased over 40 times, but it was difficult to expand for the VDC-MMA copolymer microspheres. In addition, the moderately crosslinked inside of the polymer shell enhanced the Ev more than 30 and the stable expansion temperature range (Tr) was about 30 °C by adding 0.2-0.4 wt% of divinyl benzene. The Tg of the shell polymer must be higher than the boiling point of the blowing agent as a prerequisite; the lower the boiling point of the blowing agent, the higher the internal gas pressure driven microsphere expansion, and the wider the Tr. By increasing the Er of blowing agent improved the Ev of the microspheres. The diffusion of pentane blowing agent in VDC-MMA-AN copolymer microspheres were divided into Fick diffusion and non-Fick diffusion.

Stress-Free Two-Way Shape Memory Effects of Semicrystalline Polymer Networks Enhanced by Self-Nucleated Crystallization.

Stress-free two-way shape memory polymers (2W-SMPs) capable of reversible shifting between two distinct shapes are versatile platforms for the development of future smart devices. However, it is challenging to prepare stress-free 2W-SMPs with good actuation performance and shape programmability from single-component semicrystalline polymers. Herein, we demonstrate a straightforward and universal strategy for preparing 2W-SMPs through self-nucleated crystallization (SNC) of semicrystalline polymers. SNC enables the formation of two types of crystals in the 2W-SMPs, annealed and primary crystals, which function as the skeleton phase and actuation phase, respectively. We achieved a high reversible actuation strain of 17.6% and a good reprogrammability of the SNC-treated polymer networks. Complex shape transformations were obtained, and smart devices were fabricated from the SNC-treated networks by using a locally designed folding and kirigami structure. The SNC strategy provides a generalized approach to improve the 2W-shape memory behavior of semicrystalline polymers.

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.

Sequence-Rearranged Cocrystalline Polymer Network with Shape Reconfigurability and Tunable Switching Temperature.

Switching temperature (Tsw) is a key parameter governing the service condition of shape memory polymers (SMPs). However, tuning Tsw of SMPs often requires sophisticated synthesis or intricate processing. Herein, we report a simple yet effective strategy to prepare the SMPs with tunable Tsw and good reconfigurability by using the cocrystalline polyesters as the reversible phase. The cocrystallizable copolyesters with rearranged sequences were prepared by the transesterification of mixed polyester diols and then photo-cross-linked to achieve the SMP networks. Cocrystallization of copolymer blocks endows the SMP networks tunable melting point and relatively high crystallinity, affording the network good shape fixing and recovery ability at body temperature. Besides, the dynamic nature of transesterification, that enables the network to have good shape reconfigurability, allows for the easy processing of SMPs with complicated shapes. The reconfigurable SMPs capable of actuating at the body temperature show great potential for use as biomedical devices.

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.