Changes In Crosslink Density Research Articles

Effects of Peroxide and Sulfur Curing Systems on Physical and Mechanical Properties of Nitrile Rubber Composites: A Comparative Study.

This study compares the effect of sulfur and dicumyl peroxide (DCP) vulcanizing systems on the physical and mechanical properties of rubber compounds based on acrylonitrile butadiene rubber (NBR). NBR compounds cured by different amounts of DCP and NBR vulcanizates filled with various concentrations of carbon black (CB) and a constant amount of sulfur or DCP were prepared. The vulcanizates were characterized by tensile testing, dynamic mechanical thermal analysis (DMTA), and cross-link density determination. The tensile strength and Young's modulus were found to increase with the rising amount of DCP and CB, while elongation at break decreased. The samples vulcanized by the sulfur system and filled with CB show a substantial increase in tensile strength from 13.1 to 21.2 MPa. Higher storage modulus and glass transition temperature were observed with the increase in the amount of peroxide and filler, and consequently, the increase in cross-link density, indicating rigidity increase and lower molecular mobility. The changes in the physical and mechanical properties of the NBR vulcanizates were in correlation with the changes in solvent uptake and cross-link density.

Developing 3D-network gels from polysaccharide gums for biomedical applications

Keeping in view the recent advancements in polysaccharide materials, gum acacia (GA) - tragacanth gum (TG) was investigated for designing polymeric network structures impregnated with cephalexin antibiotics for drug delivery (DD). Grafted product of polyvinylimidazole [poly(VIm)] and GA-TG was characterized by field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC). Change in mesh size (from 17.73 to 32.98 nm) and crosslinking density (between 1.64 × 10−5 to 5.06 × 10−5 mol cm−3) was found with change in poly(VIm), gum and crosslinker NNMBA content which revealed that these parameters can be tailored made as per requirement and applications. Swelling properties illustrated pH sensitivity in copolymers because of composition which is useful for colon or site-specific DD of cephalexin. FESEM and AFM demonstrated heterogeneous morphology with the rough surface topology of grafted product. XPS and FTIR confirmed the grafting of poly(VIm) onto polysaccharide chains. Drug diffusion was non-Fickian and release was fitted in Higuchi kinetic model from copolymers. The release was slow and prolonged due to drug-polymer interactions. Copolymers exhibited 31.29% scavenging ability in DPPH assay and haemolysis was less than 2% that inferred biocompatible nature. Bio-surface interactions with intestinal membrane indicated the muco-adhesive nature of copolymers. Antimicrobial properties of hydrogel were evaluated against E. coli, P. aeruginosa and S. aureus. All observations revealed that GA-TG crosslinked copolymers could be explored for colon DD of antibiotic drugs with tailored crosslinking density.

Evaluation of Effective Crosslinking Density for Pseudo-Semi Interpenetrating Polymer Networks Based on Polyether Glycols Mixture by Dynamic Mechanical Analysis

Pseudo-semi interpenetrating polymer networks (pseudo-semi IPNs) are a special example of topological isomerism in macromolecules, which have attracted significant attention in recent years with a high potential in a variety of engineering applications of polymeric materials. In this article, pseudo-semi IPNs were synthetized by sequential polymerization of thermoplastic polymers (TPEs) in the presence of thermosetting elastomer (TSEs) with contents of 10, 20, 30, 40 and 50 wt.% in a vacuum oven at 60 °C for about 72 h. In addition, this article describes a method for researching the elastic modulus, effective crosslinking density and physical crosslinking density of TSEs and pseudo-semi IPNs. The inherent interactions and entanglements of pseudo-semi IPNs were discussed by analyzing the changes in elastic modulus and effective crosslinking density at different temperatures. The results show that after the TPE was added to the TSE matrix as a plastic-reinforced material, the ductility increased from 89.6% to 491%, the effective crosslinking density was increased by 100% at high temperatures and the strength of the material matrix was significantly improved. Two physical events take place in our pseudo-semi IPNs as result of energy dissipation and polymeric chains mobility.

Dynamic mechanical characteristics of aged silicone rubber blend

The effect of aging on the mechanical properties of silicone rubber (SR) was investigated by means of ultrasonic, dynamic mechanical analysis, and FTIR techniques. Both longitudinal and shear (Ultrasonic wave velocities) were measured at room temperature and at frequencies of 2 MHz. Density, molar volume, ultrasonic wave velocities, tensile strength, mechanical properties, and FT–IR showed the improvement of the silicone rubber network with aging time from 0 to 70 days, while loosening of the network structure was observed at 14 days and 50 days aging. These behaviours were explained in terms of the change in cross-link density and average stretching force constant of bonds with aging. Thermogravimetric analysis and differential scanning calorimetric techniques showed quite low thermal stability and temperature performance for aged SR at 14 and 50 days than virgin SR which was confirmed by the cracks and voids appeared under scanning electron microscope.

Behavior of an FG temperature-responsive hydrogel bilayer: Analytical and numerical approaches

Sensitive hydrogel bilayers have recently been used for many purposes, such as sensors and actuators etc. In this paper, an innovative analytical technique is introduced to study the finite bending of a bilayer, including a thermally-activated functionally graded hydrogel layer which is connected to a neutral incompressible elastomeric one. In the hydrogel layer, the amount of cross-linking density changes linearly or exponentially in the thickness direction. The governing equations of the bilayer are developed and solved considering the additive decomposition of the free energy for the hydrogel and incompressibility of the elastomer using appropriate boundary conditions. To check the validity of the analytical technique, the deformation of the bilayer occurred by the virtue of temperature variations is modelled using the finite element method (FEM), and the results are extracted and compared with the analytical approach. The results of both approaches were in perfect agreement, demonstrating that the presented method is reliable and robust. In addition, the effects of some geometrical and material characteristics on the bending behavior of the bilayer were examined, and it was discussed whether there are any optimum values for these parameters for the bilayer under various conditions.

Mechanistic insights into the ageing of EPDM micro/hybrid composites for high voltage insulation application

In this work, we explore the mechanistic insights into the aging of EPDM based micro/hybrid composites under following environment namely acid rain, cryogenic temperature, high temperature and UV irradiation. Physical properties and cross-link density changes were measured before and after aging which confirms the degradation of EPDM samples under high temperature and UV environment. Surface properties were also measured for the samples under various aging conditions. High-temperature aging resulted in an increase in contact angle whereas, UV has led to a major loss in contact angle for all the samples. Based on the chemical changes predicted by FTIR and NMR we propose a photo-oxidative degradation mechanism under UV irradiation and the influence of nano-micro filler at cryogenic and high temperature aging. Surface morphology and dielectric property of the samples were found to undergo major changes under high temperature and UV aging which directly influences the electrical insulation property of the composite. Finally, the study gives insights into the factor affecting the shelf life of the EPDM composites which are exposed mainly under the mentioned environment for its application in outdoor high voltage insulation applications and can help in overcoming the aging issues mainly faced by polymer composites.

Supramolecular Hydrogel Based on Pseudopolyrotaxane Aggregation for Bacterial Microenvironment-Responsive Antibiotic Delivery.

Due to the extensive use of antibiotics, the variety and number of drug-resistant pathogens have increased dramatically and have become a major global health problem. This imposes significant demands on the rational and effective use of antibiotics. To this end, a supramolecular hydrogel based on pseudopolyrotaxanes aggregation is proposed for antibiotic delivery. Supramolecular cross-linking strategies allow hydrogels to be obtained under mild conditions that facilitate the encapsulation of antibiotics. The presence of pH-sensitive imine bonds allows for the reversible detachment of PEG residues from the PEGylated hyaluronic acid backbone in an acidic environment, which leads to reversible changes in hydrogel crosslink density and thus controls antibiotic release behavior. Antimicrobial assessments indicated that the hydrogel exhibited good antimicrobial efficiency against both Gram-positive and negative bacteria, while responding to the bacterial microenvironment and enabling a burst release of antibiotics in severe infections. The proposed hydrogel also has excellent biocompatibility and thus possesses great potential for biomedical applications.

Reinforcing DNA Supramolecular Hydrogel with Polymeric Multiple-Unit Linker

Reinforcing DNA Supramolecular Hydrogel with Polymeric Multiple-Unit Linker

Understanding the Effect of Interfacial Interphase on the Elastic Response of Hollow Glass Microsphere Reinforced Microcomposites

The hollow glass microspheres (HGMS) has been recently used in the fabrication of low-density polymeric composites due to rather high stiffness nature of the fillers together with their lightweight that in turn results in the development of micro-composites of engineered properties with enhanced mechanical properties. Interfacial interactions at the filler/polymer interface control the load transfer and, thus, bulk properties of composites leading to unpredictable performance of composites embedded with inclusions. Nevertheless, useful analytical models are required to estimate the mechanical behavior of the HGMS based composites with the incorporation of the effect of interfacial interactions and possible agglomeration of fillers. No studies so far have reported the analytical modeling of HGMS reinforced thermosetting composites emphasizing the role of the interphase shaped at the vicinity of fillers. This study aims at the fabrication of 0-20 wt% HGMS/polyester micro-composites followed by micromechanical modeling of the fabricated parts whilst the effect of the interphase region is emphasized by models modification. The results indicated a strong correlation between the interphase characteristics and Young’s modulus of the specimens revealing the dependency of the modulus on the thickness and modulus of the interphase as well as the level of agglomeration and interfacial debonding of the HGMSs. The results demonstrated that with considering no interphase, the models underestimate the modules of the parts, which suggests the presence of stiff interphase around the HGMS governed by changes in the interfacial cross-link density of the parent polymer as hypothesized supported by the mechanical response of the parts.

Preparation and properties of CNTs loaded bisphenol F epoxy nanocomposites modified by noncovalent dispersant and nonionic surfactant

AbstractEpoxy resin is one of the most important matrix materials in the field of high performance composites. Its mechanical strength, toughness, and thermal stability are very important to the properties of composites. In this paper, the improvement of epoxy resin properties by carbon nanotubes (CNTs) is mainly studied. First, aniline trimer (AT) and polyethylene glycol octyl phenyl ether (Triton X‐100) are combined to modify CNTs. Then the dispersed CNTs (treated‐CNTs) are used to prepare bisphenol F epoxy nanocomposites. The dispersion of CNTs and fracture surface of composites are analyzed by SEM and TEM. The combination of AT and Triton X‐100 with CNTs is analyzed by UV–vis and FTIR. The mechanical, thermodynamic, tribological properties, and crosslinking density changes of epoxy composites are evaluated. The results show that AT and Triton X‐100 are successfully bonded to the surface of CNTs. The dispersion effect of treated‐CNTs in the composites is better than that of the original CNTs, which have a better reinforcing and toughening effect on the composites. The introduction of treated‐CNT reduces the crosslinking density and glass transition temperature of the composites, but does not affect the thermal stability of the composites before 300°C.

Microneedle Patch Prepared from a Hydrogel by a Mild Method for Insulin Delivery

AbstractMost reported microneedle patches still suffer from a complicated preparation process and insufficient loading capacity for the glucose‐responsive delivery system. Herein, based on the reversibility of the boronate ester bond, a novel gel was formed by mixing phenylboronic acid grafted sodium hyaluronate and polyvinyl alcohol. The insulin‐loaded microneedle patch could be directly obtained by simply blending high‐dose insulin and gel into the mold. The introduction of cellulose nanofiber was also found to be in favor of mechanical strength. The glucose responsibility could be achieved when the boronate ester bond was broken by glucose, leading to the change of crosslinking density and further controlling insulin release. In the hypoglycemic experiment of diabetic rats, the microneedle patches could effectively puncture the skin and maintain normal blood glucose levels for an extended period. The facile preparation process and high loading capacity give this microneedle patch a great chance in mass production.

Network former mixing effects in alkali germanotellurite glasses: A vibrational spectroscopic study

Alkali germanotellurite glasses of composition 0.3M2O–0.7[(1-x)GeO2–xTeO2], M=Li, Na and 0 ≤ x ≤ 1, were investigated by Raman and infrared vibrational spectroscopic techniques to search for the origins of the alkali ion-dependent network former mixing (NFM) effect in these ion-conducting glasses. The vibrational spectra measured on mixed network-former glasses, and the spectral comparison between equimolar-mixed glasses (x = 0.5) and pellet-mixtures of the endmember glasses, 0.3M2O–0.7GeO2 and 0.3M2O–0.7TeO2, provided evidence for the formation of hetero-atomic Ge–O–Te linkages and structural rearrangements in the germanate and tellurite components of the glass. The mixing-induced structural rearrangements were expressed in terms of chemical equilibria between the network-building units and were used to make qualitative predictions for changes in the network cross-linking density and the related network-strain energy, as well as in the binding energy part of the activation energy for ion conduction. Thus, it is proposed that the mixing-induced structural modifications in the germanate and tellurite parts of glass cause the cancelation of changes in the binding energy and the network-strain energy contributions to the activation energy for ion transport. These qualitative predictions were discussed in the context of the previously found absence of an NFM effect in ionic conductivity for M=Na and the presence of a weak positive NFM effect for M=Li.

Anti-Oxidative Activity of Alcohol-Water Extracts from Field Horsetail (Equisteum arvense) in Elastomer Vulcanizates Subjected to Accelerated Aging Processes.

The article aimed to highlight the antioxidant potential of natural extracts obtained from Equisteum arvense in elastomer vulcanizatec made of natural rubber. Horsetail extracts were prepared using four solvent systems at different volume ratios: methanol–water (50/50 and 70/30) and ethanol–water (50/50 and 70/30), which were then lyophilized and added to the polymer matrix. A deep characterization of the obtained bio-additives was performed. The obtained vulcanizates were subjected to various aging processes: thermo-oxidative, ultraviolet and solar. Then, the resistance and susceptibility of vulcanizates to selected processes of accelerated, simulated degradation were determined based on the changes in the spatial structure (network nodes), material strength and physical properties (color). The research showed the presence of polyphenols in the extracts of horsetail, which resulted in their high anti-oxidative activity. Moreover, the extracts were able to absorb UV radiation. As a consequence, they were active additives that protected rubber vulcanizates against the damaging effects of the aging processes, securing their functional properties. Samples containing natural extracts were characterized by better stability of: mechanical strength parameters, the increase in cross-linking density and color changes after simulating aging processes in comparison with the control sample.

Effects of acrylonitrile content on thermal characteristics and thermal aging properties of carbon black-filled NBR composite

In seal applications, thermal characteristics and thermal aging properties of rubber are essential factors in determining the applicable temperature range and lifetime of the seal. In this study, thermal characteristics and thermal aging properties of the acrylonitrile butadiene rubber composite in response to the acrylonitrile (ACN) content were investigated. Thermal stability, glass transition temperature ( T g), and dynamic property were determined by thermogravimetric analysis, differential scanning calorimetry, and dynamic mechanical analysis, respectively. The results showed that T g, thermal decomposition temperature, and tan δ were increased as the ACN content increased. To investigate the thermal aging properties of the composite, the composite was subjected to an accelerated thermal-oxidative and thermal aging process at 100°C for 168 h. The aged composites were evaluated by investigating the change in cross-link density, mechanical properties, and elastic recovery. The change in the chemical functional group was also studied using attenuated total reflectance Fourier transform infrared spectroscopy. The results indicate that subjecting the composite to thermal-oxidative and thermal aging process decreased the elongation at break and elastic recovery, while the cross-link density, Shore A hardness, and 100% modulus were increased. All of the properties were also dependent on the ACN content as well as the aging conditions.

Storage life prediction under pre-strained thermally-accelerated aging of HTPB coating using the change of crosslinking density

In order to predict the storage life of a certain type of HTPB (hydroyl-terminated polybutadiene) coating at 25 °C and analyze the influence of pre-strain on the storage life, the accelerated aging tests of HTPB coating at 40 °C, 50 °C, 60 °C, 70 °C with the pre-strain of 0%, 3%, 6%, 9%, respectively were carried out. The variation regularity of the change of crosslinking density was analyzed and the aging model of HTPB coating under pre-strained thermally-accelerated aging was proposed. The storage life of HTPB coating at 25 °C was estimated by using the Berthelot equation as the end point of the aging life with a 30% decrease in maximum elongation. The results showed that the change of crosslinking density of HTPB coating increased with the increase of aging temperature and aging time, and decreased with the increase of pre-strain. Under 0% pre-strain, the relationship between the change of crosslinking density of HTPB coating and the aging time can be described by the logarithmic model with the confidence probability greater than 99%.The stress relaxation phenomenon existed under 3%, 6% and 9% pre-strained aging. The aging model considering chemical aging and pre-strain was established with the confidence probability greater than 90%. The storage life of HTPB coating was 15.2935 years at 25 °C under 0% pre-strain, which was reduced by 13.9007%, 75.6949% and 89.7859% under 3%, 6% and 9% pre-strain, respectively. The existence of pre-strain has a serious impact on the storage life of HTPB coating, therefore, the pre-strain should be avoided as much as possible during the actual storage.

ENTANGLED NETWORK INFLUENCED BY CARBON BLACK IN SOLUTION SBR VULCANIZATES REVEALED BY THEORY AND EXPERIMENT

ABSTRACTThe contribution of carbon black (CB) on changes in cross-link density (n) and physical entanglement has been quantified by swelling and uniaxial stress–strain measurements considering Mooney–Rivlin parameters. Solution SBR (SSBR) vulcanizates with varying content of high abrasion furnace (HAF) CB were studied. Rubber–filler networks increase the n values, which were determined by using Flory–Rehner and modified Guth–Gold equations by equilibrium swelling study. The Mooney–Rivlin parameter C1 was quantified using n, whereas parameter C2, representing physical entanglement, was determined for filled rubber by correlating with tensile results. The parameter C2 is monitored to be decreased with increasing CB (HAF) loading. A simple parabolic trend for physical entanglement parameter with increasing CB loading is proposed, and it shows a correlation coefficient (R2) of 0.99595. Atomic force microscopy study confirms the generation of filler networking in the rubber matrix. The current findings elucidate a way for quantifying physical network changes due to fillers in an unfilled rubber system.

The sulfur reversion process in natural rubber in terms of crosslink density and crosslink density distribution

Unfilled natural rubber compounds composed of conventional (CV), semi-efficient (SEV), efficient (EV) and sulfur donor (SD) vulcanization systems were heat aged to promote sulfur reversion. Rheometry, hardness, strain-strain characteristics including Mooney-Rivlin analysis, equilibrium solvent swell and Double Quantum (DQ) Nuclear Magnetic Resonance (NMR) were used to monitor crosslink density changes. A loss of crosslink density was observed by rheometry, C1, equilibrium swelling and by DQ NMR as a function of cure extent. No chain scission reactions were operating in the time/temperature conditions used. All crosslink distributions were unimodal and the network homogeneity followed the order of EV > SD > SEV > CV. The crosslink distribution narrowed during the curing process for the CV and SEV systems. Non-oxidative maturation reactions were advantageous in promoting a more random distribution of crosslinks in the polymer matrix.

Double‐crosslinked reversible redox‐responsive hydrogels based on disulfide–thiol interchange

ABSTRACTWe present novel redox‐responsive hydrogels based on poly(N‐isopropylacrylamide) or poly(acrylamide), consisting of a reversible disulfide crosslinking agent N,N′‐bis(acryloyl)cystamine and a permanent crosslinking agent N,N′‐methylenebisacrylamide for microfluidic applications. The mechanism of swelling/deswelling behavior starts with the cleavage and reformation of disulfide bonds, leading to a change of crosslinking density and crosslinking points. Raman and ultraviolet‐visible spectroscopy confirm that conversion efficiency of thiol–disulfide interchange up to 99%. Rheological analysis reveals that the E modulus of hydrogel is dependent on the crosslinking density and can be repeatedly manipulated between high‐ and low‐stiffness states over at least 5 cycles without significant decrease. Kinetic studies showed that the mechanical strength of the gels changes as the redox reaction proceeds. This process is much faster than the autonomous diffusion in the hydrogel. Moreover, cooperative diffusion coefficient (Dcoop) indicates that the swelling process of the hydrogel is affected by the reduction reaction. Finally, this reversibly switchable redox behavior of bulky hydrogel could be proven in microstructured hydrogel dots through short‐term photopatterning process. These hydrogel dots on glass substrates also showed the desired short response time on cyclic swelling and shrinking processes known from downsized hydrogel shapes. Such stimuli‐responsive hydrogels with redox‐sensitive crosslinkers open a new pathway in exchanging analytes for sensing and separating in microfluidics applications. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 2590–2601

Correlative roles of silica as a blowing aid and a Pb(II) adsorbent for natural rubber composite foams

The present work aimed to study two correlative behaviors of rubber composites and rubber composite foams filled with SiO2 that affect the cure characteristics and the adsorption capacity of Pb(II) from an aqueous solution; these behaviors have never been reported before as a conjoint analysis. A two-stage curing technique was used to vulcanize the rubber samples. The effect of NaHCO3 as well as SiO2 content on the cure characteristics, physical properties, and Pb(II) adsorptivity of the rubber compounds was examined. The addition of SiO2 had a more pronounced effect on the cure characteristics and crosslinking density of rubber compounds than that of NaHCO3. Changes in t90, CRI, and crosslinking density as a function of SiO2 loading played an important role in the foaming behavior of the rubber samples. The SiO2 addition in the rubber compounds enhanced the blowing efficiency, which facilitated the adsorption of Pb(II). The adsorption mechanism for the samples containing SiO2 was mainly based on electrostatic attraction, as evidenced by a shifting wavenumber for the silanol functional group and as observed in FTIR results. The experimental results showed promising potential for the utilization of rubber filled with SiO2 for Pb(II) removal beyond its service lifetime.

Effect of surface modification of titanium dioxide on the UV‐C aging behavior of silicone rubber

ABSTRACTSurface modification of titanium dioxide (TiO2) nanoparticles (NPs) by silane coupling agents and the ultraviolet‐C (UV‐C) aging behavior of silicone rubber (SiR) incorporated with the modified TiO2 NPs were investigated in this work. The SiR samples incorporated with TiO2 NPs displayed excellent stability against the UV‐C radiation. In order to improve the dispersion of TiO2 NPs in the SiR matrix, the surface of TiO2 NPs was modified with 3‐aminopropyl triethoxysilane and 3‐trimethoxysilyl propyl methacrylate, respectively. The surface modification of TiO2 NPs was characterized by thermal gravimetric analysis and Fourier transform infrared spectroscopy. The dispersion stability of the pristine TiO2 NPs and surface silane‐modified TiO2 NPs was evaluated in an organic solvent (toluene). Effect of surface modified TiO2 NPs on the UV‐C aging behavior of SiR was evaluated in terms of the change of surface morphology, tensile properties, hardness, crosslinking density, and surface microstructure before and after the UV‐C aging. The results showed that surface modification of TiO2 NPs with silane coupling agents could improve the dispersion of TiO2 NPs in the SiR matrix. Moreover, the SiR with modified TiO2 NPs showed an improved aging resistance to the UV‐C radiation, compared with the samples with pristine TiO2 NPs. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47170.