Rate Of Crosslinking Research Articles

Investigating the performance of non-damaging hybrid gel to mitigate lost circulation in fractured formations: A comprehensive study using Box–Behnken design

Gels that have a low viscosity at the time of mixing and form a three-dimensional gel structure after some time delay can be applied successfully in controlling lost circulation. Cross-linked gels play an important role in sealing high permeable and fractured formations. This research aims to evaluate a novel gel that can mitigate lost circulation during drilling in fractured formations. The hybrid gel consists of a cross-linked polymer gel as the continuous phase and an oil as the internal phase. The purpose of designing hybrid gels is to reduce the possibility of formation damage caused by the presence of gel in the fractures. In addition, the use of an internal phase in the gel can increase the durability of the gel and reduce the final cost of the operation.This article consists of two parts, experimental investigation and empirical modeling using response surface methodology. A number of parameters such as initial gelation time, final gelation time, and cross-linking rate were defined to evaluate the gel behavior under different conditions. First, the effect of polymer type, internal phase viscosity, pH, temperature, salinity, and shear rate on the behavior of the hybrid gel was investigated. Furthermore, in order to increase cross-linking efficiency, the cross-linker was added onto the reactive nano-silica surface to achieve a nano-crosslinker, and then the performance evaluation was accomplished. Thereafter, the sealing pressure test was examined to measure the amount of pressure that the hybrid gels could withstand in the fractures to prevent further fluid loss. Finally, the amount of gel rupture in 15% and 28% hydrochloric (HCl) was studied over time at 25 and 60 °C so as to evaluate the formation damage of hybrid gels.Response surface methodology (RSM) was employed based on Box-Behnken design to determine a correlation for predicting the final viscosity of hybrid gel as a function of temperature, pH, and CaCl2 concentration. The predicted model was obtained by solving the quadratic regression model. The value of the correlation coefficient (R2 = 0.90) for the present mathematical model indicated good relation between experimental data and predicted values. Therefore, the proposed model is able to predict the final viscosity of hybrid gel adequately within the limits of input parameters being used.The experimental results show that hybrid gels, due to their flexible viscosity response, long-term stability, and non-damaging properties, can have proper performance and application in industry to combat lost circulation. Nano-crosslinker can effectively increase the cross-linking rate and final viscosity of the gel and also improve the gel performance at high temperatures. Based on dynamic sealing pressure experiments, the value of maximum sealing pressure for the guar-based hybrid gel is higher than that of xanthan. In addition, the nano-crosslinker enhances the maximum tolerable pressure of the gels to seal the fractures.

Effect of Free Cross-Linking Rate on the Molding of Bulk SiOC Ceramics

Polymer-derived ceramics (PDCs) have many advantages in ceramic molding and ceramic properties, but because of the obvious volume shrinkage in the process of precursor transformation into ceramics, it is easy for defects to appear in the forming process of bulk PDCs. Herein, theoretical analyses and experimental studies were carried out to improve the quality of sintered samples and realize the parametric design of raw materials. Firstly, based on the HPSO/D4Vi cross-linking system, the mathematical model of the free cross-linking ratio was established, and the theoretical value was calculated. After that, the samples with different free cross-linking rates were heated at 450 °C and 650 °C for different holding times. It was found that the free cross-linking ratio (α) had a significant impact on the weight loss of the samples. When the difference of the α value was 10%, the difference of the samples' weight loss ratio could reach 30%. Finally, the morphology of sintered products with different α values was analyzed, and it was found that obvious defects will occur when the free cross-linking ratio is too high or low; when this value is 40.8%, dense and crack-free bulk ceramics can be obtained. According to analysis of the chemical reaction and cross-linking network density during sintering, the appropriate value of the free cross-linking ratio and reasonable control of the cross-linking network are beneficial for reducing the loss of the main chain element and C element, alleviating the sintering stress, and thus obtaining qualified pressureless sintered bulk ceramic samples.

Microstructure–properties relationship in ethylene-propylene diene monomer (EPDM)/nitrile butadiene rubber (NBR)/halloysite nanotubes (HNTs) nanocomposites

AbstractStructure–properties relationship in complex rubber nanocomposites is a key for enlarging the performance window. Herein, halloysite nanotubes (HNTs) are added at variable content to ethylene-propylene diene monomer (EPDM)/nitrile butadiene rubber (NBR) rubber blends compatibilized with maleic anhydride grafted HNTs to evaluate cure characteristics, along with microstructure, and mechanical and swelling behavior. The crosslinking rate increased by HNTs loading, but the scorch time decreased. Moreover, a 45% rise in tensile strength was observed for systems containing 10 wt% HNTs. SEM and TEM micrographs revealed a rough fracture surface with proper dispersion of HNT within EPDM/NBR. The modulus of EPDM/NBR/HNTs nanocomposites is theoretically estimated by modified Kolarik model, demonstrating a good agreement with experimental value. Dynamic mechanical thermal analysis (DMTA) revealed a higher storage modulus up to 2.27 GPa with the introduction of HNTs into EPDM/NBR compound. Correspondingly, lower solvent uptake (decreased by 38%) is reported. Thermogravimetric analysis (TGA) revealed higher thermal stability for highly-loaded systems.

Circadian clock proteins drive structural oscillations in colloidal fluids.

Cyanobacteria rely on a circadian oscillator system of proteins, KaiA, KaiB and KaiC, to regulate a variety of cellular processes. Powered by ATP phosphorylation, KaiA and KaiB proteins alternately bind to KaiC, resulting in the cyclical assembly and disassembly of KaiC-KaiB complexes. By biotinylating KaiB monomers, we repurpose the KaiC-KaiB complexes to function as crosslinkers with unique time-dependent properties. To demonstrate the efficacy of these ‘circadian crosslinkers’, we design suspensions of streptavidin-coated colloids that exhibit time-dependent structural changes driven by the oscillatory assembly and disassembly of KaiB-KaiC complexes. Further, using fluorescence microscopy, image analysis and biochemical assays, we show that the rate, efficacy and oscillatory signature of crosslinking can be tuned by altering the phosphorylation state of KaiC. Our findings demonstrate that a model circadian oscillator can drive time-varying restructuring of abiotic colloidal suspensions, providing new biochemical machinery for developing self-directed, programmable, and reconfigurable biomaterials. This work may one day enable engineers to incorporate biochemical circadian clocks into future exosuits, infrastructure, and deep sea and space exploration equipment.

An in situ inhibition strategy: Forming a physical barrier around ionic crosslinkers to toughen double-network hydrogels

Ionically crosslinked networks are extremely effective in reinforcing double-network (DN) hydrogels. But the simultaneous improvement of both homogeneity and ionic crosslinking density remains as a challenge for toughing DN hydrogels. Herein, an in situ inhibition method is proposed to solve this problem. In this method, inhibitor is generated in situ to form a physical barrier around the ionic crosslinker, thereby decreasing the ionic crosslinking rate and further toughening DN hydrogels in a controllable manner. The chemical structure, micromorphology and mechanical properties were investigated, and molecular dynamics simulations were performed to reveal the inhibition mechanism. The transmission electron microscopy (TEM) images confirm the formation of inhibitors. It is revealed that the introduction of inhibitor can decrease ionic crosslinking rate, leading to an improvement in homogeneity and mechanical properties. The fracture energy of alginate-Ca2+/polyacrylamide (PAAm) DN hydrogels is up to 19,638 ± 947 J/m2, which is notably larger than the previously reported values (9000–16,000 J/m2). The inhibitor does not affect the chemical structures, thermal stability and micromorphology. This in situ inhibition method has significant potential to become a general method to toughen DN hydrogels containing ionically crosslinked networks, which will greatly promote the usage of hydrogels in diverse engineering applications.

Evaluation of Fatigue Life of Recycled Opaque PET from Household Milk Bottle Wastes.

Polyethylene terephthalate (PET) is among the most used thermoplastic polymers in large scale manufacturing. Opaque PET is increasingly used in milk bottles to save weight and to bring a glossy white aspect due to TiO2 nanoparticles. The recyclability of opaque PET is an issue: whereas the recycling channels are well established for transparent PET, the presence of opaque PET in household wastes weakens those channels: opaque bottles cannot be mixed with transparent ones because the resulting blend is not transparent anymore. Many research efforts focus on the possibility to turn opaque PET into resources, as one key to a more circular economy. A recent study has demonstrated the improvement of the mechanical properties of recycled PET through reactive extrusion. In the present work, the lifespan of recycled opaque PET has been evaluated throughout tensile–tensile fatigue loading cycles at various steps of the recycling process: The specimens are obtained from flakes after grinding PET wastes (F-r-OPET), from a subsequent homogenization step (r-OPET-hom) and after reactive extrusion (Rex-r-OPET). Virgin PET is also considered as a comparison. First, tensile tests monitored by digital image correlation have been carried out to obtain the elastic modulus and ultimate tensile stress of each type of PET. The fatigue properties of reactive REx-r-OPET increase, probably associated with the rise of cross-linking and branching rates. The fatigue lifespan increases with the macromolecular weight. The fracture surface analysis of specimens brings new insight regarding the factors governing the fatigue behavior and the damaging mode of recycled PET. TiO2 nanoparticles act as stress concentrators, contributing to void formation at multiple sites and thus promoting the fracture process. Finally, the fatigue life of REx-r-OPET is comparable to those of virgin PET. Upcycling opaque PET by reactive extrusion may be a relevant new route to absorb some of the growing amounts of PET worldwide.

Controlled Covalent Self-Assembly of a Homopolymer for Multiscale Materials Engineering.

Polymer self-assembly is a crucial process in materials engineering. Currently, almost all polymer self-assembly is limited to non-covalent bonding methods, even though these methods have drawbacks as they require complicated synthesis techniques and produce relatively unstable structures. Here, a novel mechanism of covalent polymer self-assembly is discovered and employed to address drawbacks of non-covalent polymer self-assembly. A simple ketone homopolymer is found to self-assemble into nano- to macroscale hydrogels during covalent crosslinking. In contrast to non-covalent self-assembly, the covalent self-assembly is independent of and unaffected by solvent conditions (e.g., polarity and ionic strength) and does not require additional agents, e.g., organic solvents and surfactants. The covalent polymer self-assembly is subjected to a new mechanism of control by tuning the covalent crosslinking rate. This leads to nanogels with an unprecedented and tightly controlled range of dimensions from less than 10nm to above 100nm. Moreover, the crosslinking rate also regulates the assembly behavior of microgels fabricated by microfluidics. The microgels self-assemble into granular fibers, which is 3D printed into stable porous scaffolds. The novel covalent polymer assembly method has enormous potential to revolutionize multiscale materials fabrication for applications in drug delivery, tissue engineering, and many other fields.

Molecular Simulations of Thermomechanical Properties of Epoxy-Amine Resins.

All-atom molecular dynamics (MD) simulations were performed with the CHARMM force field to characterize various epoxy resins, such as aliphatic and bisphenol-based resins. A multistep cross-linking algorithm was established, and key properties such as density, glass temperature, and elastic modulus were calculated. A quantitative comparison was made and was proven to be in good agreement with experimental data, with average absolute deviations between experiments and molecular simulation comprised between 2% and 12%. Additional findings on structure–property relationships were highlighted such as the effect of the cross-linking rate and oligomerization of the resin.

Polyethylene crosslinking using the epoxy‐anhydride reaction I: A strategy for a curing process with high thermal sensitivity

AbstractThe initiated epoxy‐anhydride reaction was examined as a crosslink motif for effecting cure in functionalized polyethylene for electrical cable insulation applications. A specific challenge for this application is that little to no crosslinking can occur during the processing steps (~140°C), but crosslinking must be rapid and complete within a few minutes during the curing process (~200°C). To achieve this, we coupled the kinetics of the formation of an initiator for the epoxy‐anhydride reaction to the crosslinking step to achieve a phenomenological temperature sensitivity or “latency” that would be difficult to access via a single simple reaction. Evaluations of different imidazolium and phosphonium salts as initiator precursors were conducted with model compounds in solution, and specific salts were chosen for polymer studies based on the ratio of the phenomenological rates measured for the model reaction at ~140°C and ~200°C. In polymer studies using epoxide‐functional poly(ethylene), high crosslinking rates were observed at ~200°C while crosslinking was minimal at ~140°C. However, inclusion of the anhydride‐functional polyethylene in the formulation led to loss of initiator latency, and significant crosslinking was observed at ~140°C. A reaction between the anhydride or an impurity and initiator precursor is postulated to produce a kinetically‐competent initiator at lower temperatures that destroys latency.

Phenolic resin gel suitable for medium-temperature and high-salinity reservoirs

Gel has been widely applied as a plugging agent for enhanced oil recovery. In this study, partially hydrolyzed polyacrylamide (HPAM) or co-polymers of acrylamide and 2-acrylamido-2-methylpropanesulfonic acid (AM-AMPS) cross-linked with water-soluble phenolic resin (WSPR) forming two phenolic resin gels with mineralization of 41529 mg/L at 90 °C. The properties of the two gels were systematically evaluated in this work. Under the same dosage of polymer and cross-linker conditions, HPAM gel exhibits a faster crosslinking rate and higher strength than AM-AMPS gel, but is less thermal stable than AM-AMPS gel. Part of the two phenolic resin gels was prone to premature syneresis at higher crosslinker dosages, which has not been reported in published literature. Severe syneresis was observed in the HPAM gel with the cross-linker concentration ranges from 0.8∼1.0 wt% when heated up to 60 days. Fortunately, stable gels were developed when the loading of HPAM and cross-linker in the gelant ranges from 0.8∼1.0 wt% and 0.4∼0.6 wt% respectively. Besides, the dehydration ratio of some stable HPAM gels was still less than 8% when these gels aged for 150 days. For AM-AMPS gel, syneresis was observed in the gels with 0.8∼1.0 wt% cross-linker and 0.4 wt% polymer when heated up to 120 days. Luckily, when the polymer dosage is over 0.4 wt%, the crosslinker loading had little effect on thermal stability, so some stable AM-AMPS gels can be acquired. Premature syneresis of higher cross-linker loading gels, which is counter to our goal of increasing crosslinker dosage to improve the crosslinking density and further enhance thermal stability. Based on XPS, FT-IR, and Cryo-SEM testing results, two syneresis mechanisms of the higher crosslinker gels were proposed, which ascribes to the decrease in hydrophilicity and excessive crosslinking of gel systems. A piece of advice should bear in mind when design a phenolic resin gel: keep the cross-linker dosage appropriate to avoid overloading it. As a result, our findings may guide the development of stable phenolic gels under similar conditions, and part of the two gels could provide a solution for plugging in medium-temperature and high-salinity reservoirs.

Preparation of Various Nanomaterials via Controlled Gelation of a Hydrophilic Polymer Bearing Metal-Coordination Units with Metal Ions.

We investigated the gelation of a hydrophilic polymer with metal-coordination units (HPMC) and metal ions (PdII or AuIII). Gelation proceeded by addition of an HPMC solution in N-methyl-2-pyrrolidone (NMP) to a metal ion aqueous solution. An increase in the composition ratio of the metal-coordination units from 10 mol% to 34 mol% (HPMC-34) increased the cross-linking rate with AuIII. Cross-linking immediately occurred after dropwise addition of an HPMC-34 solution to the AuIII solution, generating the separation between the phases of HPMC-34 and AuIII. The cross-linking of AuIII proceeded from the surface to the inside of the HPMC-34 droplets, affording spherical gels. In contrast, a decrease in the ratio of metal-coordination units from 10 mol% to 4 mol% (HPMC-4) decreased the PdII cross-linking rate. The cross-linking occurred gradually and the gels extended to the bottom of the vessel, forming fibrous gels. On the basis of the mechanism for the formation of gels with different morphologies, the gelation of HPMC-34 and AuIII provided nanosheets via gelation at the interface between the AuIII solution and the HPMC-34 solution. The gelation of HPMC-4 and PdII afforded nanofibers by a facile method, i.e., dropwise addition of the HPMC-4 solution to the PdII solution. These results demonstrated that changing the composition ratio of the metal-coordination units in HPMC can control the gelation behavior, resulting in different types of nanomaterials.

Experimental study on thermotropic side-chain liquid crystalline photo-sensitive polymers

When the chalcone moiety and methylene spacers are bonded to polymer backbone, they are expected to incorporate photosensitive and liquid crystalline properties, respectively. The thermotropic side-chain liquid crystalline photosensitive polymers were synthesized using 4-phenyl styryl carbonyl (phenyl-4-oxy-4-hexyl) methacrylate (PSCPOHMA) and 4-phenyl styryl carbonyl (phenyl-4-oxy-4-decyl) methacrylate (PSCPODMA) monomers by free-radical polymerization using benzoyl peroxide (BPO) as an initiator. The structures of the synthesized monomers and polymers were confirmed by FT-IR, 1H NMR and 13C NMR techniques. The thermal stability and glass transition temperature of the poly (PSCPOHMA) were higher than that of the poly (PSCPODMA). The photocrosslinking of the polymers on irradiation with UV light established that the polymers followed 2π + 2π cyclo addition. The rate of crosslinking increased with increase in the length of the methylene chain. The mesophase of the polymers was also characterized using x-ray diffraction analysis. The liquid crystalline behavior of the polymers was confirmed by DSC and Hot stage optical polarized microscopy (HOPM).The polymers exhibited nematic phase in HOPM. So the polymers have high potential for application in photo resists and electro optic technology.

Immobilization of poly(vinyl pyrrolidone) in Polysulfone Membranes by Radically-Initiated Crosslinking Using Potassium Persulfate.

Polysulfone (PSU) membranes with poly(vinyl pyrrolidone) (PVP) as a pore-forming and hydrophilic additive were prepared using the non-solvent-induced phase separation (NIPS) technique. PVP immobilization by radical-initiated crosslinking using potassium persulfate (KPS) was studied in view of obtaining membranes with high and long-lasting surface hydrophilicity. A method based on the ATR-FTIR technique was developed to discriminate crosslinked PVP from unreacted PVP in the membrane. The crosslinking progress was investigated as a function of temperature, KPS concentration, and reaction time. The results showed that temperature was the main factor influencing the crosslinking reaction since radical formation is temperature-dependent. Increasing the concentration of KPS and the reaction time led to an increase in the crosslinking rate. The effect of the degree of PVP crosslinking on the structure and properties of the prepared membranes was examined by studying mechanical properties, morphology by SEM, surface hydrophilicity by contact angle measurements, and water permeability.

Rapid curing of polysilazane coatings at room temperature via chloride-catalyzed hydrolysis/condensation reactions

Moisture curing of polysilazane coatings at room temperature is of advantage to industrial applications, though it often takes a longer time than the other curing methods. To tackle this problem, tetrabutylammonium chloride (TBAC) is applied in this work as an efficient catalyst for the hydrolysis and condensation reactions of polymethyl(hydro)/polydimethylsilazane coatings under ambient conditions. In the suggested catalytic mechanism, the Si–H group is nucleophilically attacked by chloride ions to form a transient Si–Cl group, which is then nucleophilically substituted by a hydroxyl group from the moisture environment. It is found that by adding TBAC in the polysilazane coating, 1.6 times more of the reactive groups (Si–H, Si–NH) are consumed during the curing process in a shorter time (the dry-to-touch time for the uncatalyzed and the TBAC-catalyzed polysilazane coatings are ca. 19 h and 1 h, respectively), while 2 times more of the Si–O–Si groups are formed. Owing to the significantly enhanced crosslinking rate and degree, the coating quality and the mechanical properties are greatly improved. The hardness and the elastic modulus of the coating are increased by a factor of two and four, respectively.

A reaction–diffusion model for grayscale digital light processing 3D printing

Digital light processing (DLP) 3D printing is an additive manufacturing process that utilizes light patterns to photopolymerize a liquid resin into a solid. Due to the accuracy of modern digital micromirror devices (DMD) and recent advances in resin chemistry, it is now possible to create functionally graded structures using different light intensity values, also known as grayscale DLP (g-DLP). Different intensities of light lead to differences in the polymer crosslinking density after curing, which ultimately produces a part with gradients of material properties. However, g-DLP is a complicated process. First, the DLP printing is a highly coupled chemical and physical process that involves light propagation, chemical reactions, species diffusion, heat transfer, volume shrinkage, and changes in mechanical behaviors of the curing resin. Second, in g-DLP, light gradients create strong in plane gradients of chemical species concentrations in the curing liquid resin due to the strong dependence of light intensity on the rate of monomer crosslinking. Furthermore, light gradients through the depth create concentration gradients due to the degree of cure dependent light absorption and the use of photoabsorbers. These complex physical features of the printing process must be understood in order to properly control printing parameters such as light exposure time, printing speed, and grayscale variations to achieve accurate mechanical properties. In this paper, a photopolymerization reaction–diffusion model is developed and used in conjunction with experiments to investigate the coupled effects of light propagation, chemical reaction rates, and species diffusion during g-DLP 3D printing. The model is implemented numerically utilizing the finite difference method and simulation results are compared to experimental findings of simple printed structures. The agreement between experimental and model predictions of simple quantities of interest, such as geometric feature sizes, shows that the model can capture the overcure due to free-radical and other species diffusion during printing when grayscale patterns are employed. This model lays the groundwork for future extensions that can incorporate more complex coupled physics such as heat transfer, volume shrinkage, and material property evolution, which are critically important in utilizing g-DLP 3D printing for the fabrication of high-performance parts which excellent geometric and material property tolerances.

Injectable metronidazole-eluting gelatin-alginate hydrogels for local treatment of periodontitis.

Infection of the periodontal pocket presents two major challenges for drug delivery: administration into the periodontal pocket and a high fluid clearance rate in the pocket. The current study aimed to develop and study a novel hydrogel system for delivery of the antibiotic drug metronidazole directly into the periodontal pocket via injection followed by in situ gelation. The natural polymers gelatin and alginate served as basic materials, and their crosslinking using a carbodiimide resulted in a dual hydrogel network. The study focused on the effects of the hydrogel's formulation parameters on the drug release profile and the hydrogel's physical and mechanical properties. A cell viability test was conducted on human fibroblasts. The metronidazole-loaded hydrogels demonstrated a decreasing release rate with time, where most of the drug eluted within 24h. These hydrogels exhibited fibroblast viability of at least 75% after 24 and 48h, indicating that they are highly biocompatible. Although the alginate concentration used in this study was relatively low, it had a strong effect on the physical as well as the mechanical properties of the hydrogel. An increase in the alginate concentration increased the crosslinking rate and enabled enhanced entanglement of the 3D structure, resulting in a decrease in the gelation time (less than 10s) and swelling degree, which are both desired for the studied periodontal application. Increasing the gelatin concentration without changing the crosslinker concentration resulted in significant changes in the physical properties and slight changes in the mechanical properties. Metronidazole incorporation slightly decreased the hydrophilicity of the hydrogel and therefore also its viscosity, and affected the sealing ability and the tensile and compression moduli. The developed hydrogels exhibited controllable mechanical and physical properties, can target a wide range of conditions, and are therefore of high significance in the field of periodontal treatment.

Preparation, characterisation and comparison of glabridin-loaded hydrogel-forming microneedles by chemical and physical cross-linking

Poly(vinyl alcohol) (PVA) and carbomer were used as the hydrogel system to fabricate glabridin-loaded hydrogel-forming microneedles (HFMNs) by chemical cross-linking (CCMNs) and physical cross-linking (PCMNs). The properties and drug permeation effects of glabridin-loaded HFMNs with different methods were compared. They both owned excellent shapes, mechanical and insertion properties. PCMNs showed collapsed shapes during swelling due to the low cross-linking rate and high porosity, which probably results in resealing of skin pores during transdermal drug delivery. However, CCMNs could rapidly swell within 2 h with slightly bending. The infrared spectra indicate that CCMNs and PCMNs might form the hydrogel network by generating hydrogen and covalent bonds, respectively. The in vitro release studies showed that cumulative permeation amount within 24 h (1654 μg/cm2) of CCMNs significantly higher than that (372 μg/cm2) achieved by PCMNs and that (118 μg/cm2) achieved by glabridin-loaded gel. The skin barrier recovery test suggests the desirable security of both microneedles (MNs), notwithstanding the presence of mild erythema in the mouse skin applied CCMNs. These results indicate that CCMNs were more desirable for glabridin delivery using PVA and carbomer as a skeleton of the hydrogel network.

Effect of temperature and inorganic salts concentration on syneresis rate of AM/DAC hydrogel

Compared with ordinary polyacrylamide, acrylamide copolymer of acryloyloxyethyl trimethylammonium chloride (AM/DAC) hydrogel is easier to reside in large pores and fractures in sandstone formation through electrostatic adsorption. However, the syneresis mechanism of AM/DAC hydrogel caused by high temperature and inorganic salts has not been clear. In this paper, the type of water in gelling solutions and hydrogels was analyzed by the DSC method, the results showed that the bound water in the hydrogel was transformed into free water and then the free water was separated from the hydrogel during the syneresis of AM/DAC hydrogel. Experimental investigations, including dynamic light scattering (DLS), differential scanning calorimetry (DSC), high-performance liquid chromatography (HPLC), phase-analysis light-scattering (PALS), and scanning electron microscope (SEM), have been conducted to elucidate the mechanism of temperature and inorganic salts on syneresis rate of AM/DAC hydrogel. The result showed that with the increase of temperature, the polymer in the AM/DAC hydrogel degraded and the molecular chain broken, which destroyed the grid of hydrogel and reduced the water holding capacity. At the same time, the formation of the hydrophobic group in the molecular chain led to the decrease of the hydrophilicity so that the AM/DAC hydrogel was easy to syneresis. In addition, the increase of temperature will promote the formation of crosslinking intermediate (di/trimethylol), improve the crosslinking rate between crosslinking intermediate and polymer, which makes the AM/DAC hydrogel easy to over crosslink. Inorganic salts diffused the electric double layer by compressing the polymer and competed with the hydrophilic groups on the polymer to absorb water, which will reduce the elongation and hydrophilicity of the polymer molecular chain. This will lead to the decrease of the hydrodynamic radius of polymer molecules in AM/DAC hydrogel, which is the main reason why inorganic salts promote hydrogel dehydration. Besides, inorganic salts had little effect on the formation rate and crosslinking rate of di/trimethylol. Therefore, reducing the molecular hydrodynamic radius of AM/DAC is the main reason why inorganic salts promote the syneresis of AM/DAC hydrogel. The research demonstrated the mechanism of high temperature and inorganic salts on syneresis rate of AM/DAC hydrogel. This study also provides technical guidelines for inhibiting hydrogel syneresis.

DEPENDENCE OF THE PROPERTIES OF SULFUR AND PEROXIDE VOLCANISES BASED ON A MIXTURE OF ISOPRENE WITH POLYVINYL ACETATE ON THE STRUCTURE OF THE VOLCANIZATION NETWORK

The effect on the crosslinking process of modified isoprene rubber (SKI-3-PVA) with the participation of sulfur and dicumyl peroxide (PD) in the presence of hexachloroparaxylene (HCPA) and 2,4-dichloro-6-dimethyl-amino sym triazine was studied. It was found that, depending on the length of time, the number of crosslinks in these systems (SKN-3-PVA + sulfur + GHPK + ZnO; SKI-3-PVA + PD + GHPK) is 2.5x105 and 1.9x105 mol / cm³, respectively, per Based on the observed changes in the parameter of the spatial grid of the crosslinked samples, the yield of effective crosslinks was calculated. It was shown that in the presence of dicumyl peroxide in filled systems, the number of crosslinks decreases, and a decrease in crosslinks is also observed. Comparative tests of elastomeric materials based on SKI-3 - PVA copolymers in liquid aggressive media were carried out. Sol-gel analysis found that with an increase in the content of the copolymer in isoprene, the concentration of the number of cross-linked molecules in the copolymer increases. It is shown that the physical and mechanical properties of filled vulcanizates are inferior to sulfuric ones in strength properties, but surpass peroxide ones in thermal aging and the action of solvents. It was found that an increase in aging with an increase in the heating time of the samples indicates the formation of additional cross-links. The role of zinc oxide in the peroxide crosslinking of the SKI-3 - PVA copolymer has been studied. It was found that the introduction of zinc oxide has a noticeable effect on the rate of sulfur and peroxide crosslinking of mixtures based on SKI-3 - PVA copolymers, an increase in the structuring time of mixtures, destructive processes are observed, and the number of crosslinks and crosslinks decreases. It was found that the physical and mechanical properties of peroxide vulcanizates are inferior to sulfuric ones in strength properties, but they are superior to peroxide ones in thermal aging. It is shown that an increase in the resistance to aging with an increase in the heating time of the samples indicates the formation of additional cross-links. As a result of the reaction with oxygen and the accumulation of interacting polar groups. A decrease in the degree of swelling is associated with the formation of C-C chemical bonds in elastomers, it effectively participates in the formation of cross-links during peroxide vulcanization. The nature of the interaction of sulfur PD, HCPC, and DCDEAST with elastomers was judged by the change in the conditional number of crosslinks (n´c) and the number of crosslinked molecules (1 \ Ms) in unfilled sulfur and peroxide vulcanizates. It is shown that in sulfur peroxide mixtures based on SKI-3-PVA with the proposed agents at room temperature it causes the formation of effective crosslinks, and with an increase in the duration of heating in systems 1, 2, the number of crosslinked molecules increases. An increase in resistance to aging with an increase in the heating time of the samples indicates the formation of additional cross-links as a result of the reaction with oxygen and the accumulation of interacting polar groups. A decrease in the degree of swelling is associated with the formation of C - C chemical bonds in elastomers, effectively participating in the formation of a network of crosslinks during peroxide vulcanization. Thus, the analysis of the results of the studies performed allows us to conclude that elastomers based on the isoprene copolymer SKI-3-PVA obtained with the participation of vulcanizing agents of sulfur and peroxide (PD) in the presence of hexachloroparaxylene (HCP) and triazine compounds are superior to those known in terms of thermal stability. aging in strength and accumulation of permanent deformation in aggressive environments. Keywords: isoprene, polyvinyl acetate, hexachloroparaxylene, peroxide dicumil, vulcanization, modification

A methylcellulose/agarose hydrogel as an innovative scaffold for tissue engineering.

In situ crosslinked materials are the main interests of both scientific and industrial research. Methylcellulose (MC) aqueous solution is one of the representatives that belongs to this family of thermosensitive materials. At room temperature, MC is a liquid whereupon during temperature increase up to 37 °C, it crosslinks physically and turns into a hydrogel. This feature makes it unique, especially for tissue engineering applications. However, the crosslinking rate of MC alone is relatively slow considering tissue engineering expectations. According to these expectations, the crosslinking should take place slowly enough to allow for complete injection and fill the injury avoiding clogging in the needle, and simultanously, it should be sufficiently fast to prevent it from relocation from the lesion. One of the methods to overcome this problem is MC blending with another substance that increases the crosslinking rate of MC. In these studies, we used agarose (AGR). These studies aim to investigate the effect of different AGR amounts on MC crosslinking kinetics, and thermal, viscoelastic, and biological properties. Differential Scanning Calorimetry (DSC) and dynamic mechanical analysis (DMA) measurements proved that AGR addition accelerates the beginning of MC crosslinking. This phenomenon resulted from AGR's greater affinity to water, which is crucial in this particular crosslinking part. In vitro tests, carried out using the L929 fibroblast line and mesenchymal stem cells (MSCs), confirmed that most of the hydrogel samples were non-cytotoxic in contact with extracts and directly with cells. Not only does this type of thermosensitive hydrogel system provide excellent mechanical and biological cues but also its stimuli-responsive character provides more novel functionalities for designing innovative scaffold/cell delivery systems for tissue engineering applications.