Chemical Crosslinking Density Research Articles

Dynamically Securing the Data by 1O2 Sensitization of Fluorescent Composites with a High Latency and Uncrackable Features

Comprehensive SummaryDynamic fluorescent materials play a crucial role in secure inks for data encryption; however, they are still plagued by issues such as photodegradation, poor latency, and susceptibility to unauthorized access. Herein, we propose a photochemically modulated dynamic fluorescent encryption system based on 1O2 sensitization of fluorescent composites, comprising a 1O2‐sensitive fluorophore (F2) and non‐emissive polymers. After UV irradiation, in‐situ generated 1O2 from the polymer effectively binds with F2 to form endoperoxides (F2EPO), resulting in a significant redshift in emission, up to 150 nm. The 1O2 concentration is closely related to the irradiation time, rendering different fluorescent colors in a time‐gated fashion. Moreover, the emission of F2EPO can be regulated by polymer chemical structure, molecular weight, and crosslinking density. Relying on these merits, we develop a dynamic data encryption method with various non‐emissive polymers as the data storage media, UV light irradiation as the data encoder, and F2 as the data decoder. UV light irradiation of diverse polymer solutions generates 1O2 at different concentrations, effectively encoding the data, which remains invisible under both UV and natural lights. The addition of F2 to these irradiated polymer solutions produces different redshifted fluorescence, enabling secure data decryption. Attributing to the non‐emissive nature of the polymers, time‐gated readout fashion, excellent latency of 1O2, and subtle interactions between 1O2 and F2, this data encryption is nearly undecipherable. This work offers an advantage data encryption approach beyond the reach of conventional fluorophores.

Elucidating the Role of Lignin Type and Functionality in the Development of High-Performance Biobased Phenolic Thermoset Resins.

In this work, a series of biobased phenolic resins were developed starting from kraft and soda lignin, suitably functionalized through esterification by means of succinic anhydride. As a result of an extensive optimization study of the functionalization and curing reactions, clear correlations between lignin type and chemical-physical characteristics and the properties of the resulting phenolic resin systems were described. In particular, the esterification reaction through succinic anhydride was found to play a key role in enhancing the chemical reactivity and in facilitating the successful incorporation of lignin into the resin formulations. The obtained high-lignin-content thermoset materials were shown to exhibit tunable chemical (functionality, gel content, and cross-linking density), thermal (glass transition temperature and thermo-oxidative stability), and mechanical (surface hardness, indentation modulus, and creep behavior) characteristics, which could outperform those of fully oil-based reference phenolic resins by judicious control of lignin concentration and chemical characteristics. In particular, succinylated kraft lignin was found to enable more efficient incorporation into the cured systems. This work provides the first demonstration of the incorporation of succinic-anhydride-modified-lignin in the formulation of high-performance phenolic resins, ultimately contributing to the definition of structure-property-performance correlations for rational biobased material design in the context of advanced and sustainable manufacturing.

Study on thermal aging properties of PBT energetic elastomer

AbstractAccelerated aging experiments of 3,3‐bis(azidomethyl)oxetane (BAMO) and tetrahydrofuran (THF) copolymers (molar ratio 1 : 1, marked as PBT prepolymer) and PBT elastomer at elevated temperatures were performed to explore the aging mechanism of PBT energetic elastomer during long‐term storage. The evolution of functional groups, plasticizer content, element chemical state and crosslinking density were studied by FT‐IR, HPLC, XPS and swelling method, respectively. Results show that: (1) Oxidation of terminal hydroxyl, oxidative crosslinking between binder macromolecules, chain breaking of polymers and degradation of −CH2N3 occurred during the thermal aging of PBT prepolymer, and antioxidant (BHT) could effectively delay the thermal aging process of PBT prepolymer by absorbing the active oxidation substances. (2) The relative contents of −C=O showed a cubic function with time, and the relative contents of −NO2 and −N3 changed linearly with time during the thermal aging of PBT elastomer. The relative contents of BDNPF and BDNPA declined linearly with time, and the thermal stability of BDNPF was better than that of BDNPA. (3) The thermal aging of PBT elastomer experienced three main reaction stages: degradation of carbamate, the oxidative crosslinking of molecular chain caused by the decomposition products of BDNPF/BDNPA, and chain breaking of the polymer system. In addition, a small amount of −CH2N3 decomposed throughout the whole aging process.

Cell response to substrate energy dissipation outweighs rigidity sensing.

The mechanical properties of the extracellular matrix (ECM) are essential for tissue homeostasis, as they determine cell spreading, differentiation, proliferation and migration through the concerted action of mechanoresponsive proteins such as YAP. However, interpretation of the mechanical crosstalk between cells and the ECM using hydrogel mimetics has been limited by the intricacies to engineer viscoelasticity typical of the ECM, the difficulty in discerning viscoelastic from viscoplastic behaviors, and the convolution of their mechanical and non-mechanical properties. As a result, whether cells sense ECM energy dissipation (i.e. viscoelasticity) independently of stiffness remains unknown. Here, we exploit fundamental polypeptide conformational changes under force to produce titin-based protein hydrogels with tailored viscoelasticity and preserved chemical composition, ultrastructure, crosslinking density, and biochemical environment. Using our system, we demonstrate that cells respond to changes in viscoelastic cues only, as nuclear YAP decreases with substrate energy dissipation. Furthermore, the rate of YAP translocation to the nucleus is more sensitive to changes in energy dissipation than in stiffness; indicating that the response to the viscous component engineered into our ECM mimetics supersedes rigidity effects. This observation implies that cells perceive ECM rigidity and energy dissipation by non-overlapping mechanisms, illustrating the relevance of ECM viscoelasticity in the biology of the cell.

Butyl rubber as a macro-cross-linker in the preparation of a shape-memory and self-healing polymer

Recently, a simple strategy was developed for preparing interconnected interpenetrating polymer networks (IPNs) based on butyl rubber (IIR) and poly(n-octadecyl acrylate) (PC18A). Solvent-free UV polymerization of n-octadecyl acrylate (C18A) monomer in the melt of IIR at ambient temperature resulted in IPNs with self-healing and shape-memory functions. Here, we demonstrate that the use of IIR grafted with acrylic acid, methacrylic acid, and 10-undecenoic acid instead of unmodified IIR provides a significant improvement in the mechanical properties of IPNs. Differential scanning calorimetry, small-angle x-ray scattering, and wide-angle x-ray scattering analysis reveal side-by-side packing of C18 side chains of PC18A to form lamellar crystals with a melting temperature Tm between 46 and 52 °C. Transmission electron microscopy analysis indicates the existence of quasispherical nanoparticles composed of crystalline domains, which are dispersed in a continuous interpenetrating rubber-PC18A matrix. This microstructure provides them a complete self-recovery behavior induced by heating and an efficient shape-memory function. IPNs exhibit around tenfold higher chemical cross-link density as compared to those prepared from the native IIR, reflecting the effect of pendant vinyl groups on the extent of covalent interconnections between the IIR and PC18A components. The type of the grafted monomers significantly affects the mechanical performance of IPNs, which can be explained with the individual contributions of chemical and physical cross-links to the total cross-link density. The amount of the grafted rubbers in IPN could be further increased up to 80 wt. % by the incorporation of toluene into the reaction system, resulting in IPNs with a wide range of tunable thermal and mechanical properties.

Effect of Nonisoprene Degradation and Naturally Occurring Network during Maturation on the Properties of Natural Rubber.

It well-known that the superior performance of natural rubber (NR) compared to its synthetic counterpart mainly derives from nonisoprene components and naturally occurring network, which varies during the progress of the maturation and thereby results in technically graded rubber with different properties. However, identifying the roles of these two factors in the forming of excellent performance of NR is still a challenge as they change simultaneously during the maturation process. Here, influences of naturally occurring networking and nonisoprene degradation on the components, structures and properties of NR were systematically investigated by tailored treatments of maturation. It was found that the maturation-induced formation of natural network structure contributes to the increase in initial plastic value, Mooney viscosity and gel content for un-crosslinked NR, while the decomposition of nonisoprene components plays a dominant role in improving the mechanical properties of vulcanized NR. Stress-strain curve and Mooney-Rivlin analysis demonstrate that the biodegradation of the nonisoprene components significantly boost the vulcanization process, which significantly increases the number of chemical cross-link networks and effective cross-link density of the material, greatly improving the mechanical properties of NR vulcanizates. This resulted in the tensile strength of TSR 10CV being able to reach 22.6 MPa, which is significantly improved compared to 15.8 MPa of TSR 3CV. Evidenced by tubular model fitting, the increase in chemical cross-linking points effectively reduces the movable radius of the molecular chain under dynamic loading, making the molecular chain more difficult to move, which suppresses the entropy change under dynamic loading and consequently endows NR excellent dynamic mechanical properties. This resulted in a significant decrease in the temperature rising of TSR 10CV to 3.3 °C, while the temperature rising of TSR 3CV was still as high as 14.5 °C. As a minor factor, the naturally occurring network improves the mechanical properties of vulcanizates in the form of sacrificial bonds.

A programmable bilayer hydrogel actuator based on the asymmetric distribution of crystalline regions.

A novel strategy to fabricate bilayer hydrogel actuators based on the asymmetric distribution of crystalline regions across the bilayer structures was proposed. By employing PVA polymer chains into an alkali solvent-derived chitosan hydrogel matrix, chitosan/PVA hybrid bilayer hydrogels with both excellent responsive bending and mechanical properties were obtained as pH-controlled manipulators. In the design, the chitosan/PVA hydrogels upon treatment with freeze-thawing cycles were taken as the first monolayer, where excessive crystalline regions appeared. The original chitosan/PVA hydrogel as the second monolayer was then integrated into one bilayer device through the chemical-crosslinking of epichlorohydrin at the interface. The results showed that the resultant chitosan/PVA bilayer hydrogel actuator with a weight ratio of 3 : 1 displayed better sensitivity upon exposure to stimuli. The actuation behaviors are strongly dependent on experimental parameters such as the pH, PVA content and the chemical-crosslinking density. It is proposed that the driving force originates from the asymmetric distribution of crystalline regions, thus resulting in differential swelling ratios between the monolayers. In addition, programmable 3D shape transformations were achieved by using the bilayer hydrogel with designed 2D geometric patterns, and the tailored gripper-like hydrogel actuator can successfully capture and transport the cargo. Moreover, this actuation behavior can be erased and re-written on demand under certain conditions. Taking advantage of this universal strategy, more attractive actuators derived from synthetic or natural polymers in combination with PVA are highly expected, which can be used as smart soft robots in various fields such as manipulators, grippers, and cantilever sensors.

Tough Hydrogels with Dynamic H-Bonds: Structural Heterogeneities and Mechanical Performances

The recently developed tough and self-healing hydrogels, containing a large number of physical bonds, have found widespread applications in bioengineering and soft electronics. Owing to the sophisticated physical interactions and organization, these hydrogels often demonstrate structural heterogeneities, which strongly influence their mechanical performances. Using poly(N,N-dimethyl acrylamide-co-methacrylic acid) hydrogels (P(DMAA-co-MAAc)) with dynamic hydrogen bonds (H-bonds) as a model system, the structural characterization has been carried out in this study using small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) analyses, along with exploring the influence of the structure on the mechanical properties. By systematically tuning the polymer composition, including the monomer fraction and chemical cross-linking density, two different hydrogel regions with distinct mechanical properties were observed: the swollen regions and shrunk regions. The former was mechanically weak, whereas the latter exhibited a tough behavior. Both types of hydrogels were noted to be highly heterogeneous, which results from the nanoscale spatial polymer concentration fluctuations at ∼100 nm scale. Combined with the contrast variation utilizing SANS, the structure parameters, including the polymer volume fraction of the dense and sparse regions, volume fraction occupied in the space, and average correlation length of the long-range and short-range heterogeneous structures, were explored using a scaling model based on a two-phase system composed of the densely and sparsely cross-linked regions. Using in situ SAXS, the microscopic deformation of the tough and shrunk hydrogels was noted to follow the affine deformation, while a significantly non-affine deformation was observed in the swollen hydrogels, which might have led to the different mechanical performances of these materials.

Quantifying Chemical Composition and Cross-link Effects on EPDM Elastomer Viscoelasticity with Molecular Dynamics

To understand the effect of chemical composition, cross-link density, and microstructure on the linear and nonlinear viscoelasticity of ethylene propylene diene monomer (EPDM) rubber, we carried out high-frequency oscillatory shear molecular dynamics simulations at varying shear strain rates. Sweeping through different EPDM compositions with varying ethylene, propylene, and diene ratios, a positive correlation was observed between the ratio of the propylene monomer and the complex shear modulus of EPDM in the high-frequency glassy regime. For small deformations in this regime, we found that the simplest measure of local molecular stiffness, namely, the Debye–Waller factor, is predictive of the complex shear modulus and loss modulus of 20 unique systems with distinct compositions and cross-link densities. Polymer design parameters that reduce the Debye–Waller factor, including cross-linking or increased propylene content generally, result in higher moduli. Remarkably, large-amplitude oscillatory shear simulations revealed that dissipation becomes strongly influenced by polymer entanglements, which results in divergent optimal compositions for small-strain vs large-strain applications of EPDM. Utilizing time–temperature superposition and varying strain rates in simulations, we were able to capture rheological properties over 6 orders of magnitude in frequency. The data was captured well using a Rouse model superposed with a stretched exponential function, which was used to predict key constants that determine the mechanical behavior in these regimes. Our findings establish a chemistry-specific molecular simulation approach for capturing the constitutive behavior of elastomers and pave the way for multiscale analyses linking composition and microstructure to performance.

Experimental Investigation on the Effect of Graphene Oxide Additive on the Steady-State and Dynamic Shear Properties of PDMS-Based Magnetorheological Elastomer.

Isotropic polydimethylsiloxane (PDMS)-based magnetorheological elastomers (MREs) filled with various contents of graphene oxide (GO) additive were fabricated by the solution blending-casting method in this work. The morphologies of the produced MREs were characterized, and the results indicate that the uniform distribution of GO sheets and carbonyl iron particles (CIPs) becomes difficult with the increase of GO content. The steady-state and dynamic shear properties of the MREs under different magnetic field strengths were evaluated using parallel plate rheometer. It was found that the physical stiffness effect of GO sheets leads to the increase of the zero-field shear modulus with increasing GO content under both the steady-state and dynamic shear loads. The chemical crosslinking density of PDMS matrix decreases with the GO content due to the strong physical crosslinking between GO and the PDMS matrix. Thus, the MREs filled with higher GO content exhibit more fluid-like behavior. Under the dynamic shear load, the absolute MR effect increases with the GO content due to the increased flexibility of the PDMS matrix and the dynamic self-stiffening effect occurring in the physical crosslinking interfaces around GO sheets. The highest relative MR effect was achieved by the MREs filled with 0.1 wt.% GO sheets. Then, the relative MR effect decreases with the further increase of GO content due to the improved zero-field modulus and the increased agglomerations of GO and CIPs. This study shows that the addition of GO sheets is a possible way to prepare new MREs with high MR effect, while simultaneously possessing high zero-field stiffness and load bearing capability.

Changing compliance of poly(vinyl alcohol) tubular scaffold for vascular graft applications through modifying interlayer adhesion and crosslinking density.

Poly(vinyl alcohol) (PVA) is a water-soluble polymer and forms a hydrogel that has been studied as a potential small-diameter (<6 mm) vascular graft implant. The PVA hydrogel crosslinked using sodium trimetaphosphate (STMP) has been shown to have many beneficial properties such as bioinert, low-thrombogenicity, and easy surface modification. Compared to conventional synthetic vascular graft materials, PVA has also shown to possess better mechanical properties; however, the compliance and other mechanical properties of PVA grafts are yet to be optimized compared to the native blood vessels. Mechanical compliance has been an important parameter to be studied for small-diameter vascular grafts, as compliance has been proposed to play an important role in intimal hyperplasia formation. PVA grafts are made using dip-casting a cylindrical mold into crosslinking solution. The number of dipping can be used to control the wall thickness of the resulting PVA grafts. In this study, we hypothesized that the number of dip layer and wall thickness, the chemical crosslinking, and interlayer adhesive strength could be important parameters in the fabrication process that would affect compliance. This work provides the relationship between the wall thickness, burst pressure, and compliance of PVA. Furthermore, our data showed that interlayer adhesion as well as chemical and physical crosslinking density can increase the compliance of PVA grafts.

Effect of Sulfur Variation on the Vulcanizate Structure of Silica-Filled Styrene-Butadiene Rubber Compounds with a Sulfide-Silane Coupling Agent.

The vulcanizate structure of filled compounds is affected by filler–rubber interactions (FRI) and the chemical crosslink density (CCD) of the matrix rubber. In particular, in filled compounds using a silica–silane system, FRIs due to silica–rubber coupling are a major influencing factor for the vulcanizate structure and physical properties. In this study, the effect of sulfur variation on the vulcanizate structure of silica-filled solution styrene–butadiene rubber compounds using a sulfide–silane coupling agent was studied. The vulcanizate structure according to sulfur variation was quantitatively analyzed using the swelling test and Flory–Rehner and Kraus equations. As the sulfur content increased, both FRI and the CCD increased, and it was confirmed that sulfur variation influenced the silica–rubber coupling efficiency through increased FRI. In addition, field emission scanning electron microscope images showed that increased FRI contributed to improvements in silica dispersion, abrasion resistance, and energy loss characteristics.

Effects of the curing atmosphere on the structures and properties of polybenzoxazine films

The effects of four different curing atmospheres, i.e., static air, circulating air, nitrogen and vacuum, on the polymerization mechanism, chemical structure, hydrogen bonding, mechanical property, thermal property, water contact angle, etc., of the polybenzoxazine films were systematically studied. It was found that curing in air caused more oxidation and decomposition, generating more benzoquinones, carbonyl groups and iminium ions in the resultant polybenzoxazine films. Consequently, the films cured in air were less cross-linked, darker in color and more brittle. The films cured in static air (SA) showed tensile strength and elongation at break of 12.59 MPa and 2.16%, respectively, which were 61.24 MPa and 2.74% lower than those of the films cured in nitrogen (N2). Moreover, it was demonstrated that more intramolecular rigid –OH···N hydrogen bonding was formed in the films cured in air, which was believed to be the fundamental reason for the lower chemical cross-linking density and poorer toughness. Nevertheless, these films showed higher char yield due to the formation of more thermally stable groups in oxidation process. Further investigation on chemical structures, hydrogen bonding and water contact angles of the upper and lower surfaces of the films revealed that the upper surfaces were more inclined to be oxidized and decomposed during curing and more intramolecular hydrogen bonding (–OH···N HB) was formed on upper surfaces, which led to increased water contact angles.

Meso-Reconstruction of Wool Keratin 3D "Molecular Springs" for Tunable Ultra-Sensitive and Highly Recovery Strain Sensors.

Wool keratin (WK) consists of a large number of α-helices, which are just like many molecular-scale springs. Herein, the construction of 3D WK molecular spring networks are reported by cross-linking individual WK molecules via a Michael addition reaction. The as-prepared springs display a superior recovery capability with unusual nonlinear elasticity, very low dissipative energy, and turntable elastic constant achieved by adjusting the chemical crosslinking density of WK networks. Owing to these unique characteristics, the 3D WK networks based flexible strain sensors reveal a high sensitivity, broad sensing ranges, and extremely long and stable performance. While normal highly sensible strain sensors, obtained by highly sophisticated surface or bulk patterning, often exhibit a relatively narrow range of measurements and limited life cycles. Such the WK mediated sensing materials have widespread applications in wearable electronics, such as detection and tracking of different human motions, and even discern voice during speaking.

Mussel-Inspired Tough Double Network Hydrogel As Transparent Adhesive

Hydrogels with excellent adhesive and mechanical properties have attracted extensive attraction, whereas few “tough and sticky” hydrogels have been successfully prepared with desirable and tunable performance. In this work, dopamine-modified hyaluronic acid (HA-DM) was first introduced to the double network (DN) system, which greatly improved the adhesive and mechanical performance of the original HA-acrylamide (AAm) DN hydrogel. The prepared mussel-inspired hydrogel features instant and flexible bonding capability (adhesive strength up to 90 kPa) and high transparency (transmittance up to nearly 100%), which could serve as a novel type of transparent adhesive for various uses. Moreover, by modulating external parameters such as pH, Fe3+ and HA-DM concentration of the precursor solution, protonation state of hyaluronic acid and chemical cross-linking density of PAAm chains within the hydrogel network could be controlled to achieve well-tuned mechanical and adhesive properties. Meanwhile, the structure–pro...

Effects of the Density of Chemical Cross-links and Physical Entanglements of Ultraviolet-Irradiated Polystyrene Chains on Domain Orientation and Spatial Order of Polystyrene-block-Poly(methyl methacrylate) Nano-Domains.

Ultraviolet irradiation (UVI) of varied duration caused cross-linking and neutralization of polystyrene (PS) homopolymers of molar mass (Mn) from 6 to 290 kg mol-1 on a silicon-oxide surface. An optimal neutral skin layer on the surface of the PS was obtained via brief UVI in air (UVIA), by which the PS had no preferential interaction with either block in the copolymer. UVI in an inert environment (gaseous dinitrogen) (UVIN) stabilized the PS layers via cross-linking and enabled the PS networks to have an effective adhesive contact with the underlying substrate. Thorough examination of domain orientations and spatial orders of a series of block copolymer, polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA), thin films deposited on these UVI-treated PS support layers yielded clear evidence that a dense layer of neutralized PS chains was required for the perpendicular orientation of PS-b-PMMA nanodomains. In particular, in addition to neutralization, two factors-the densities of physical entanglements and of chemical crosslinks-both in UVI-treated PS should be considered for the perpendicular orientation of nanolamellae and nanocylinders in symmetric and asymmetric PS-b-PMMA thin films. The density of physical entanglement in PS depends intrinsically on Mn of the PS, whereas the density of chemical cross-links was controlled with a varied duration of UVIN. Sufficiently large densities of physical entanglements and chemical cross-links can prevent PS-b-PMMA chains from penetrating through the neutral skin layer. The total density of physical entanglements and chemical cross-links required for the perpendicular orientation is correlated with the dimensions of the PS-b-PMMA chains.

Particle packing in bimodal size carbon black mixtures and its effect on the properties of styrene-butadiene rubber compounds

Particle packing, together with other filler characteristics, can be considered as an important structural parameter of fillers, affecting on the properties of rubber compounds. For this purpose, N220 and N550 carbon blacks with nearly similar structure and surface chemistry but different particle sizes were selected. N220/N550 showed the maximum packed density at the composition 35/65 according to the accepted theory for bimodal packing of spheres. A correlation between the packing behavior of carbon blacks mixtures and physical, electrical and rheological behaviors of filled rubber compounds was investigated. A synergic effect in compounds properties was observed before the vulcanization reaction at the maximum packed composition of carbon-blacks. The synergic effect was attributed to the dense and highly immobilized rubber trapped inside packed structure of carbon blacks aggregates, but it didn't appear for the vulcanizates due to the adverse effect of immobilized rubber on the chemical cross-link density formed during vulcanization reaction. • Particle packing is an important structural parameter in rubber reinforcement. • There is a correlation between particle packing and the properties of filled rubber compounds. • A synergic effect in properties was observed at the maximum packed composition of fillers before vulcanization. • The synergic effect was due to the highly immobilized rubber trapped in the filler network. • The synergic effect was absent in some properties after vulcanization.

Facile preparation of polybenzoxazine/graphene nanocomposites for electromagnetic interference shielding

Novel nanocomposites with good electromagnetic interference (EMI) shielding efficiency were prepared in-situ from thermosetting polybenzoxazine (PBZ) and trichlorophenylsilane modified graphene (GS). The pristine globular graphene (G) was intercalated by trichlorophenylsilane and formed more single layer graphene, which was verified by TEM images. Good dispersion of GS decreased the percolation threshold to less than 1 wt%. The conductivity increased greatly and reached 0.0032 S/cm with the addition of 20 wt% of GS, and the corresponding nanocomposites showed the highest shielding effectiveness (SE) of 40.9 dB over the range of X-band. And the SE was found to be dominated by absorption, which could effectively reduce the secondary pollution of electromagnetic wave. Additionally, the mechanical strength, storage modulus at rubbery state, and chemical cross-linking density were highly improved by the addition of GS.

A Very Simple Strategy for Preparing External Stress-Free Two-Way Shape Memory Polymers by Making Use of Hydrogen Bonds.

Development of two-way shape memory polymers that operate free of external force remains a great challenge. Here, the design criteria for this type of material are proposed, deriving a novel fabrication strategy accordingly, which employs conventional crosslinked polyurethane (PU) containing crystalline poly(ε-caprolactone) (PCL) as the proof-of-concept material. Having been simply trained by stretching and thermal treatment without additional ingredients and chemicals, the PU is coupled with a two-way shape memory effect. The core advancement of this study lies in the successful conversion of the inherent hydrogen bond network, which is often the easiest to overlook, into an internal stress provider. The temperature-dependent reversible melting/recrystallization of the crystalline phases elaborately works with the tensed hydrogen bond network, leading to implementation of the two-way shape memory effect. An average reversible strain of as high as ≈20% along the stretch direction is obtained through cooperation adjustment of chemical crosslinking density, crystallinity, and concentration of hydrogen bonds. Meanwhile, the highest internal tension offered by the hydrogen bond network is determined to be 0.10 MPa. Owing to the great convenience characterized by material selection, preparation, programming, and application, the current work may open up an avenue for production and usage of the smart material.

Entrapped Styrene Butadiene Polymer Chains by Sol-Gel-Derived Silica Nanoparticles with Hierarchical Raspberry Structures.

A sol-gel transformation of liquid silica precursor to solid silica particles was carried out in a one-pot synthesis way, where a solution of styrene butadiene elastomer was present. The composites, thus produced, offered remarkable improvements of mechanical and dynamic mechanical performances compared to precipitated silica. The morphological analysis reveals that the alkoxy-based silica particles resemble a raspberry structure when the synthesis of the silica was carried out in the presence of polymer molecules and represent a much more open silica-network structure. However, in the absence of the polymer, the morphology of the silica particles is found to be different. It is envisaged that the special morphology of the in situ synthesized silica particles contributes to the superior reinforcement effects, which are associated with a strong silica-rubber interaction by rubber chains trapped inside the raspberry-like silica aggregates. Therefore, the interfaces are characterized in detail by low-field solid-state 1H NMR spectroscopy, 29Si solid-state NMR spectroscopy, and energy-dispersive X-ray spectroscopy. Low-field 1H NMR-based double-quantum experiments provide a quantitative information about the cross-link density of the silica-filled rubber composites and about the influence of silane coupling agent on the chemical cross-link density of the network and correlates well with equilibrium swelling measurements. The special microstructure of the alkoxy-based silica was found to be associated with the interaction between alkoxy-based silica and rubber chains as a consequence of particle growth in the presence of rubber chains.