Increase In Crosslink Density Research Articles

Effect of Polymer Properties on the Biodegradation of Polyurethane Microplastics.

The release of fragments from plastic products, that is, secondary microplastics, is a major concern in the context of the global plastic pollution. Currently available (thermoplastic) polyurethanes [(T)PU] are not biodegradable and therefore should be recycled. However, the ester bond in (T)PUs might be sufficiently hydrolysable to enable at least partial biodegradation of polyurethane particles. Here, we investigated biodegradation in compost of different types of (T)PU to gain insights into their fragmentation and biodegradation mechanisms. The studied (T)PUs varied regarding the chemistry of their polymer backbone (aromatic/aliphatic), hard phase content, cross-linking degree, and presence of a hydrolysis-stabilizing additive. We developed and validated an efficient and non-destructive polymer particle extraction process for partially biodegraded (T)PUs based on ultrasonication and density separation. Our results showed that biodegradation rates and extents decreased with increasing cross-linking density and hard-segment content. We found that the presence of a hydrolysis stabilizer reduced (T)PU fragmentation while not affecting the conversion of (T)PU carbon into CO2. We propose a biodegradation mechanism for (T)PUs that includes both mother particle shrinkage by surface erosion and fragmentation. The presented results help to understand structure-degradation relationships of (T)PUs and support recycling strategies.

Construction of a cellulose-based high-performance adhesive with a crosslinking structure bridged by Schiff base and ureido groups

Biomass-based adhesives are considered to be the preferred alternative to formaldehyde-type wood adhesives due to their wide range of sources, low cost, and sustainability. Herein, an environmentally friendly Schiff base cross-linked compact three-dimensional network structure bio-adhesive (DAC-PEI-U) derived from polyethyleneimine (PEI), urea, and cellulose was successfully prepared, verifying by detailed FTIR, NMR, and XPS analysis. Schiff base bridging between aldehyde groups in dialdehyde cellulose (DAC) and amino groups in polyurea (PEIU) not only constructed crosslinking networks but also endowed adhesives with good adhesion property. The dry bond strength of DAC-PEI-U adhesive reached 2.71 MPa, and the wet shear strength was 1.51 MPa (hot water) and 1.34 MPa (boiling water), respectively. It not only improves the water resistance and bonding process, but also displays simple synthesis and low cost. The improved performance of DAC-PEI-U adhesive is attributed to the generation of hyperbranched cross-linking structure in the adhesive system, which results in increased cross-linking density and promotes the formation of dense cross-sections in the curing adhesive. This work paves a solid way for developing cellulose-based wood adhesives with wet bonding properties, thus holding great potential as an alternative to formaldehyde-type adhesives in wood-based panel and indoor panel bonding industries.

Influences of Crystallinity and Crosslinking Density on the Shape Recovery Force in Poly(ε-Caprolactone)-Based Shape-Memory Polymer Blends.

Shape-memory polymers (SMPs) show great potential in various emerging applications, such as artificial muscles, soft actuators, and biomedical devices, owing to their unique shape recovery-induced contraction force. However, the factors influencing this force remain unclear. Herein, we designed a simple polymer blending system using a series of tetra-branched poly(ε-caprolactone)-based SMPs with long and short branch-chain lengths that demonstrate decreased crystallinity and increased crosslinking density gradients. The resultant polymer blends possessed mechanical properties manipulable across a wide range in accordance with the crystallinity gradient, such as stretchability (50.5-1419.5%) and toughness (0.62-130.4 MJ m-3), while maintaining excellent shape-memory properties. The experimental results show that crosslinking density affected the shape recovery force, which correlates to the SMPs' energy storage capacity. Such a polymer blending system could provide new insights on how crystallinity and crosslinking density affect macroscopic thermal and mechanical properties as well as the shape recovery force of SMP networks, improving design capability for future applications.

Preparation of functional fiber hybrid enhanced high strength and multifunctional protein based adhesive

Protein based adhesive is limited by poor mechanical properties and water resistance. Inspired by the structure of muscle tissue, a complex structure of layered double hydroxide (LDH) anchored chicken feather fiber (CFF) and tannic acid (TA) was constructed by self-assembly process, forming a novel organic–inorganic hybrid reinforcement structure (CFF-LDH-TA). CFF-LDH-TA was closely combined with soybean meal (SM) through covalent and hydrogen bond multiple interactions. TA simulated the function of connective tissue around fibers, significantly improving the cohesion of SM based adhesive through effective energy dissipation and load transfer. Due to the increased crosslinking density, the dry and wet shear strengths of SM/CFF-LDH-TA reached 2.12 and 1.60 MPa, respectively, which were 50 % and 214 % higher than original SM adhesive, and significantly exceeded the strength of several commercialized formaldehyde adhesives. The existence of bridging effect and sacrificial structure improved toughness of the adhesive, the work of adhesion increased by 802 %, reached 830 mJ. In addition, the SM/CFF-LDH-TA adhesive had better anti-mold and flame retardant ability. More importantly, the cost was lower than the commercial SM based adhesive. This simple and green preparation method provided a new strategy for the production of high-performance, multifunctional and economically effective SM based adhesive.

Effects of crystallization temperature and cooling rate on the polymorphism of trans‐1,4‐polyisoprene vulcanizate

AbstractThe effects of crystallization temperature and cooling rate on the polymorphism of trans‐1,4‐polyisoprene vulcanizate (TPI‐V) with different sulfur dosages were monitored by WAXD in real time. The results showed that both the crystallization temperature and the cooling rate play decisive roles in determining the polymorphism and their relative amounts, whereas their effects are interfered by the crosslinking density which restrains the crystallization kinetics. Only α crystal is formed when crystallization is conducted at 40°C or greater and mixed α/β crystals are obtained in the temperature range of 10–30°C. β crystal is predominate when crystallized at 10°C while α crystal is limited at 30°C. Both the lower temperature and increased crosslinking density are beneficial to β crystal formation. A high crosslinking density causes a prolonged time is needed for pure α crystal formation, and hinders the TPI‐Vs from forming pure α crystal even if crystallization is conducted at 45°C. A lower cooling rate favors the formation of α crystal while a higher cooling rate favors β crystal formation. The β spherulites are much brighter than α spherulites under the observation of polarized microscopy.

Simultaneously improving the fire safety and mechanical properties of epoxy resin with iron phosphonated grafted polyethylenimine

Endowing epoxy resins with enhanced flame retardancy, toughness and strength as well as glass transition temperature (Tg) simultaneously is a big challenge in academia and industry fields. Herein, a hyperbranched multifunctional additive (PEIP-Fe) was designed, which was prepared through a Mannich condensation reaction of polyethyleneimine and phosphorous acid and followed by combining with ferric. When 3 wt% of PEIP-Fe was incorporated into epoxy resin, the impact strength, tensile strength, flexural strength and Tg were all enhanced due to the multiple toughening mechanisms, good compatibility and the increased cross-linking density. Simultaneously, fire safety such as flame retardancy, smoke suppression and toxicity were all improved. The flame-retardant mechanism was verified in both condensed-phase and gas-phase by catalytic charring and free radical scavenging, forming compact and continuous char layer and releasing phosphorus-containing species respectively. This work provides new thinking for the design of multifunctional and high efficiency additives for epoxy resin.

Increasing strengths of liquid crystalline polymers while minimizing anisotropy via topological rearrangement assisted bi-directional stretching of reversibly interlocked macromolecular networks

To prepare oriented crosslinked liquid crystalline polymers (LCPs) while preventing the unwanted anisotropy, the authors fabricated interlocked networks from liquid crystalline epoxy and amorphous polyurethane, which are crosslinked by reversible Diels-Alder (DA) bonds and Schiff base, respectively. By making use of dynamicity of the reversible bonds as well as relative mobility of the parent networks of the interlocked networks, the material is subjected to stepwise solid-state bi-directional drawing. Owing to the directional alignment of the mesogens and molecular chains and the increased crosslinking density during stretching caused by the networks reshuffling, the strengths of the material in orthogonal directions are simultaneously improved and gradually converged to the values comparable to the reported highest level of unidirectionally orientated crosslinked LCPs. Furthermore, the biaxially oriented interlocked networks have inherited the intrinsic self-healing capability from the included reversible bonds. The proposed method is easy to use and expected to contribute to the practical applications of crosslinked LCPs, which is becoming more and more important in modern microelectronics and communication technology.

Thermodynamics incompatibility-driven covalent crosslinking network in situ phase separation from biomimetic design

Mussel byssus is an elegant example with high strength and high toughness, inspiring the design of multiphase network structure. At present, many works focus on the design of noncovalent multiphase network. However, such noncovalent bonds (for example, hydrogen bonds) are sensitive to temperature and solvent, which limits the application in the engineering field. To overcome the shortcoming of noncovalent multiphase network, we construct the covalent multiphase network through thermodynamic incompatibility-driven covalent network in situ phase separation. Herein, this work increases the content of sulfur network in the polyolefin matrix. Due to the thermodynamics incompatibility between sulfur network and polyolefin chain, crosslinking network-rich domain and crosslinking network-poor domain appear in the matrix, forming the covalent multiphase network. The improvement in strength can be attributed to the increase in crosslinking density, while the improvement in toughness originates from the preferential rupture of crosslinking network-rich domain. Such covalent multiphase network opens up an innovative strategy for the design of robust materials.

VISCOELASTICITY AND DYNAMIC FATIGUE CRACK GROWTH BEHAVIOR OF NATURAL RUBBER/CIS-POLYBUTADIENE RUBBER COMPOSITES

ABSTRACT Because cis-polybutadiene rubber (BR) can improve the fatigue crack growth (FCG) resistance of natural rubber (NR) in the low-tearing-energy (G) range, a blending system of NR/BR is often used in tire materials. In this study, based on relationship of (FCG rate (dc/dN) − G) established from the perspective of fracture mechanics and then inspired by Persson's theory and starting from the perspective of viscoelasticity, the FCG behavior and mechanism of NR/BR were investigated. NR/BR with different blending ratios were prepared, and dc/dN under different G inputs (500/1500 J/m2) was measured. According to the viscoelastic parameters (storage modulus E′, loss factor tan δ, and loss compliance modulus J″) recorded in situ, energy dissipation distribution at the crack tip (energy consumption for FCG inside crack tip: G0 and energy loss in linear viscoelastic zone near crack tip: G0f [v, T]) was determined, and the relationship of (viscoelasticity − dc/dN) was finally set up. When G = 500 J/m2, blending BR can reduce dc/dN as compared with pure NR. On one hand, with a higher BR fraction, an increased cross-linking density and enhanced filler network provided greater rigidity, which increased E′; on the other hand, a low glass transition temperature and flexible chain of BR reduced hysteresis, which decreased tan δ. The joint action led to a decrease in J″, which caused more G0f (v, T) and less G0, resulting in the ultimate reduction of dc/dN. In contrast, for G = 1500 J/m2, when the BR content was >50 phr, dc/dN showed a significant increase. Although more BR evidently decreased J″ and then led to a large amount of G0f (v, T), due to absence of strain-induced crystallization, the chain orientation of BR was hard to resist FCG when G increased. Finally, the morphology of the crack tip propagation path was captured to corroborate the different orientation characteristics of NR and BR and their effects on FCG behavior.

Improvement in toughness and flame retardancy of bismaleimide/diallyl bisphenol A resin with a eugenol allyl ether-grafted polysiloxane

Despite bismaleimide/2,2′-diallyl bisphenol A (BD) resin has been successfully used as a high-performance thermosetting resin, it is needed to further improve its toughness and flame retardancy to meet the requirements of application in harsh environments. In this study, we synthesized a eugenol allyl ether grafted polysiloxane (PMES-Allyl), and used it as an allyl compound to modify BD resin to obtain a new BDPA resin. The eugenol structure helped to improve the compatibility of the polysiloxane with the BD resin. When the addition amount of PMES-Allyl was 15 wt%, PMES-Allyl could be well dispersed in the resin matrix, leading to a significant increase of crosslinking density. As a result, the cured BDPA15% resin had better toughness than the cured BD resin, with the impact strength and flexural strength reaching 17.9 kJ/m2 and 177.1 MPa, respectively. Compared to the cured BD resin, the cured BDPA15% resin showed a comparable glass transition temperature and thermal stability, while an improved char yield. The active role of PMES-Allyl in helping the resin form an expanded, dense and stable char during heating, further made the cured BDPA15% resin exhibit an excellent flame retardancy. Compared with the cured BD resin, the cured BDPA15% resin showed a decrease of peak heat release rate (PHRR), total heat release (THR), and total smoke production (TSP) by 58.6 %, 33.1 %, and 41.9 %, respectively, as well as a reduced release amount of the gasified pyrolysis products. This study provides a new insight and method for improving the properties of BD resin.

Study on properties of copolymers based on different types of benzoxazines and branched epoxy resins

Previous studies on linear epoxy (bisphenol A epoxy) resin/benzoxazine composites showed that with the addition of epoxy (EP) resin, the resulting copolymers exhibited an increased glass transition temperature ( Tg) ( Tg reached a maximum value at a specific content), improved flexural strength, lower heat resistance, and reduced tensile strength. Herein, branched EP resin (AG-80)/benzoxazine copolymers featuring novolac (N-box) and siloxane (Si-box) chains were prepared without any external curing agent. In both systems, the EP resin endowed the copolymers with an increased crosslinking density; however, Tg continued to increase with increasing EP content. In addition, the heat resistance of the copolymers gradually enhanced. Different types of benzoxazines have various effects on the properties of copolymers. In terms of mechanical properties, AG-80/N-box copolymers exhibited brittle fracture characteristics; with increasing EP content, the flexural strength of the copolymer decreased while the tensile strength increased. AG-80/Si-box copolymers exhibited ductile fracture characteristics, with gradual increases in flexural and tensile strengths. Furthermore, with increasing EP content, the molecular chain migration ability and network homogeneity of the AG-80/N-box copolymers decreased gradually. Alternatively, in the case of the AG-80/Si-box copolymers, the molecular chain migration ability remained unchanged and network homogeneity improved. Hence, the developed copolymers can be used as resin matrices for the fabrication of advanced composites.

Sustainable lignin precursors for tailored porous carbon-based supercapacitor electrodes

Sustainable materials are attracting a lot of attention since they will be critical in the creation of the next generation of products and devices. In this study, hydrogels were effectively synthesized utilizing lignin, a non-valorised biopolymer from the paper industry. This study proposes a method based on utilizing lignin to create highly swollen hydrogels using poly(ethylene) glycol diglycidyl ether (PEGDGE) as a crosslinking agent. The influence of different crosslinker ratios on the structural and chemical properties of the resultant hydrogels was investigated. Pore size was observed to be lowered when the amount of crosslinker was increased. The inclusion of additional hydrophilic groups in the hydrogel network decreased the swelling capacity of the hydrogels as the crosslinking density increases. These precursor materials were carbonised and electrochemically tested for application as electrodes for supercapacitors with capacitance characterized as a function of crosslinker ratio.

Effects of glutaric anhydride functionalization on filler-free benzoxazine/epoxy copolymers with shape memory and self-healing properties under near-infrared light actuation

We have developed new types of thermosets from bio-based benzoxazine/epoxy copolymers with shape memory and self-healing properties that could be triggered by using near-infrared (NIR) light, without using any filler. The copolymers were prepared from bio-based raw materials, including vanillin, furfurylamine, and epoxidized castor oil, along with functionalized with glutaric anhydride (GA), due to which self-healing properties of the copolymers were enabled through dynamic covalent bonds. Effects of GA content (0.00, 0.02, 0.04, 0.06, 0.08, and 0.10 mol) on molecular structure, mechanical properties, dynamic mechanical properties, and thermal stability of the copolymers were investigated. The results from infrared spectroscopy showed the formation of ester linkages and free carboxylic acids upon functionalization of copolymers with GA. The increased crosslinking density by GA was observed from increased tensile strength (from 11 to 16 MPa), glass transition temperature (72–150 °C), and degradation temperature at 5% weight loss (382–405 °C) of the copolymers. However, excess contents (0.08 and 0.10 mol) of GA might result in the existence of free components, hindering the formation of the copolymer network and diminishing the mechanical properties of the copolymers. Shape memory properties of GA-functionalized V-fa/ECO copolymers were investigated under NIR actuation; the results showed a 100% of shape recovery ratio within 20 s of actuation. Self-healing properties of GA-functionalized V-fa/ECO copolymers were investigated under NIR actuation; the results showed 94% of restored storage modulus after 20 min of actuation. Lastly, the self-healing mechanism under NIR actuation was proposed that NIR photothermal effects of the copolymers induced the crack closing due to their shape memory effects. Consequently, the copolymer chains could diffuse across the crack interface along with transesterification through GA functionalities, leading to crack healing. The findings of this study suggested a new way to produce NIR light-responsive self-healing polymer without using NIR photothermal filler. • Filler-free benzoxazine/epoxy copolymers with shape memory and self-healing effects triggered by NIR light were developed. • Copolymers were synthesized from bio-based raw materials and functionalized with glutaric anhydride (GA). • Copolymers showed 100% shape recovery within 20 s and 94% restored storage modulus after 20 min of NIR light actuation. • Self-healing mechanism involved shape memory effects and transesterification induced by NIR photothermal effects.

Improving Mechanical Properties of Starch-Based Hydrogels Using Double Network Strategy.

This work aims to improve the mechanical properties of starch-based hydrogels using a double-network (DN) strategy. The single network (SN) starch hydrogel was first prepared using glutaraldehyde as a crosslinker. The compressive properties of the SN hydrogels were influenced by both crosslinker content and crosslinking time. The SN starch hydrogel possessing the best mechanical properties was then fabricated into DN hydrogels. Poly(vinyl alcohol) (PVA) and borax were used as a secondary polymer and a crosslinker, respectively. The PVA–borax complexation partly enhanced the DN hydrogel’s compressive modulus by 30% and its toughness by 39%. DN hydrogels were found to have denser microstructures than SN hydrogels. To be specific, their walls thickened and grew more continuous while their pores shrank. The increased crosslinking density resulted in changes to the microstructure, which were well correlated with their porosity and water uptake capacity. An in vitro cytotoxicity test of the DN hydrogels revealed that they were non-toxic to chondrocytes. This work demonstrated that double networking is a simple but effective strategy for improving mechanical properties of starch-based hydrogels without sacrificing their biocompatibility. This approach can be used to tailor hydrogel properties to fulfill requirements for biomedical applications, such as tissue engineering and other related fields.

Study on Thermo-oxidative Aging Mechanism of Air Spring Rubber Cord Composites

To study the thermo-oxidative aging mechanism of air spring rubber cord composites, observe the changes in surface micro-morphology, study the thermo-oxidative aging mechanism of air spring rubber cord composites, observe the changes in surface micro-morphology, infrared spectrum, quality, hardness, and mechanical properties during the process of thermo-oxidative aging, and analyze the aging mechanism. The results show that the micro-morphological changes of the rubber surface of the rubber cord composites go through two stages: at 0-72 h, the rubber surface The results show that the micro-morphological changes of the rubber surface of the rubber cord composites go through two stages: at 0-72 h, the rubber surface develops from no cracks to a large number of cracks; at 96-336 h, the rubber surface changes from a cracked state. The size of the crack on the rubber surface affects the rate of change of IRHD hardness. The larger the crack, the greater the hardness growth rate, otherwise the opposite. In the aging process, the rubber is in a state of dynamic development. The increase of cross-linking density causes changes in microscopic The increase of cross-linking density causes changes in microscopic morphology and accessible volume, which affects the IRHD hardness. The aging mechanism is more complicated. Material failure is dominated by the failure of the rubber matrix.

A phosphaphenanthrene‐based derivative for simultaneously improving flame retardant and smoke suppression of epoxy resin composites

AbstractEpoxy resin (EP) have the disadvantage of high flammability and large amount of smoke, which greatly restricts its wide application in basic material. In order to obtain EP with excellent smoke inhibition and flame retardance, the flame retardant called as PBO, which with both condensed phase and gas phase flame‐retardant effects, was synthesized by the reaction among p‐phenylenediamine, DOPO, and indole‐3‐carboxaldehyde. From combustion results, EP containing PBO obtained V‐0 grade with a significantly improvement from 22.5% to 30.2% in limiting oxygen index. In comparison with EP, total smoke production, total heat release and the peak of heat release of EP/PBO‐1.0 were declined by 32%, 41.5%, and 37.4%, respectively. Besides,Tgof EP composites also had a certain degree of improvement, which due to the increase of rigidity and crosslinking density of EP caused by the addition of PBO. Meanwhile, the analysis of chars and pyrolysis products of PBO presented that the improvement was assigned to two phase synergy through barrier effect of chars in condensed phase and interrupt chain reaction in gaseous phase.

Use of Self-Assembled Colloidal Prodrug Nanoparticles for Controlled Drug Delivery of Anticancer, Antifibrotic and Antibacterial Mitomycin.

Herein we present the synthesis of a polymeric prodrug nanomaterial capable of spontaneous, self-assembled nanoparticle formation and of the conjugation (encapsulation) of drugs with amino and/or carboxyl and/or hydroxyl groups via ester and/or amide linkage. Mitomycin C (MMC) a versatile drug with antibiotic, antibacterial and antineoplastic properties, was used to prove this concept. The in vitro drug release experiments showed a fast release for the pure MMC (k = 49.59 h−n); however, a significantly lower MMC dissolution rate (k = 2.25, 1.46, and 1.35 h−n) was obtained for the nanoparticles with increased cross-link density (3, 10, 21%). The successful modification and conjugation reactions were confirmed using FTIR and EDX measurements, while the mucoadhesive properties of the self-assembled particles synthesized in a simple one-pot reaction were proved by rheological measurement. The prepared biocompatible polymeric prodrugs are very promising and applicable as a drug delivery system (DDS) and useful in the area of cancer treatment.

Super-strong and anti-tearing poly(vinyl alcohol)/graphene oxide nano-composite hydrogels fabricated by formation of multiple crosslinking bonding network structure

As load-bearing material in biomedical-applications as cartilage replacement, artificial meniscus and tendons, etc., poly(vinyl alcohol) (PVA)/graphene oxide (GO)-tannic acid (TA) nano-composite hydrogels with multiple-crosslinking network were fabricated by establishing freezing/thawing-annealing-swelling method. By TA anchoring, PVA molecules were grafted onto GO surface efficiently, and strong interfacial interaction led to exfoliation and uniform distribution of GO in matrix. By introducing annealing process, the crystallinity and crystallite size of PVA increased and introducing GO-TA led to more perfect and denser crystalline structure, while physical crosslinking network centered on GO-TA and hydrogen bond-abundant crystalline phase formed, resulting in increasing crosslinking density of hydrogel. By further swelling in CaCl2 aqueous solution, hydroxyl-Ca2+ coordination formed, and multiple-crosslinking network was constructed with high crosslinking density. The tensile strength and fracture toughness of composite hydrogel were remarkably improved, reaching 14.38 MPa/27.93 MJ/m3, approximately 11-/26-fold higher than those of neat PVA hydrogel, while tearing strength was significantly enhanced, attributed to high energy dissipation through unzipping multiple interactions.

Diselenide as a Dual Functional Mechanophore Capable of Stress Self-Reporting and Self-Strengthening in Polyurethane Elastomers

Diselenide as a Dual Functional Mechanophore Capable of Stress Self-Reporting and Self-Strengthening in Polyurethane Elastomers

Poly‐cardanol as plasticizer and compatibilizer on styrene butadiene rubber with improved processability and silica dispersion

AbstractNatural products used as plasticizer can inhibit its migration in rubber and improve the dispersion of inorganic power. Herein, styrene butadiene rubber (SBR) was plasticized by poly‐cardanol (PCA) originated from the Friedel–Crafts alkylation without solvent of cardanol which was extracted from abandoned cashew nut shell. Results show that the Mooney viscosity of SBR/PCA compounds decreases from 45.8 to 34.7 and Tg from −52°C to −56.3°C when PCA is added from 0 to 30 phr. Moreover, the elongation at break of SBR contained 30 phr PCA increases by 515% compared with SBR. PCA also shows amphiphilic, which is verified by the improved dispersion of SiO2 in SBR/PCA compounds. The swelling ratio of SBR/PCA/SiO2 vulcanized rubber decreases and the crosslinking density increases. Meanwhile, the elongation at break of SBR70‐40 vulcanized rubber is 17.3%, which is higher than that of SBR100‐40 vulcanized rubber. PCA, acted as plasticizer and compatibilizer, provides a simple and green way for the application of natural products in the rubber industry.