Polymer Crosslinking Research Articles

Influence of cross-link density on the non-isothermal crystallization kinetics of polybutylene terephthalate

Kinetics of crystallization of the samples of cross-linked polybutylene terephthalate with different cross-link densities was studied experimentally using conventional differential scanning calorimetry (cooling rates 5 to 40 K·min–1) and fast scanning calorimetry (cooling rates 0.5 to 5000 K·s–1). The samples were obtained by copolycondensation of terephthaloyl chloride, 1,4-butanediol, and glycerol. The cross-linking degree was characterized using 1H NMR spectroscopy. An increase of the number of glycerol cross-links in polymer leads to a decrease in the crystallization and melting temperatures and suppression of crystallization process. The critical rate of crystallization decreases monotonically with increasing cross-linking degree from 2700 K·s–1 for linear polybutylene terephthalate to 110 K·s–1 for the sample with 270 mol·m–3 cross-link density. At the cross-link densities higher than 270 mol·m–3, the polymer does not crystallize even at 5 K·min–1 cooling rate. The observed dependence of the cold crystallization enthalpy on the previous cooling rate suggests that the homogeneous nuclei formation process also slows down with an increase in the cross-linking degree.

Granular Hydrogels for Harnessing the Immune Response.

This review aims to understand the current progress in immune-instructive granular hydrogels and identify the key features used as immunomodulatory strategies. Published work is systematically reviewed and relevant information about granular hydrogels used throughout these studies is collected. The base polymer, microgel generation technique, polymer crosslinking chemistry, particle size and shape, annealing strategy, granular hydrogel stiffness, pore size and void space, degradability, biomolecule presentation, and drug release are cataloged for each work. Several granular hydrogel parameters used for immune modulation: porosity, architecture, bioactivity, drug release, cell delivery, and modularity, are identified. The authors found in this review that porosity is the most significant factor influencing the innate immune response to granular hydrogels, while incorporated bioactivity is more significant in influencing adaptive immune responses. Here, the authors' findings and summarized results from each section are presented and suggestions are made for future studies to better understand the benefits of using immune-instructive granular hydrogels.

Photoinitiator-free light-mediated crosslinking of dynamic polymer and pristine protein networks

This study reports a photoinitiator- and radical-free biocompatible crosslinking method based on ortho-nitrobenzyl alcohol, which provides a versatile approach to generate dynamic hydrogels and crosslink pristine proteins for biomedical applications.

Evaluation of Cross-linking of Rubber and Polymers by TDNMR

Evaluation of Cross-linking of Rubber and Polymers by TDNMR

Effect of polymerization on free water in polyacrylamide hydrogels observed with Brillouin spectroscopy.

Brillouin spectroscopy is used to determine the effects of polymer concentration, crosslinking density, and polymerization on the longitudinal storage and loss moduli of polyacrylamide hydrogels. The model established by Chiarelli et al. is implemented to calculate the speed of sound in the free water [Chiarelli et al., The Journal of the Acoustical Society of America, 2010, 127(3), 1197-1207]. The polymer concentration has the greatest effect on the moduli of the polymer matrix. We determined that the crosslink density has no measurable effect on the logitudinal storage or loss modulus of polyacrylamide hydrogels when measurements are made at GHz frequencies, in contrast to measurements made at kHz frequencies as documented by other studies. However, the moduli are independent of monomer concentration if the acrylamide is not polymerized. We show at the GHz frequency that the incorporation of acrylamide polymer chains affects the mechanical properties of the free water. The speed of sound in the free water is reduced by the introduction of polymerized acrylamide. The long polymer chains and their interactions with the bounded water disrupt the bonding organization of the unbound water, causing a reduction of the average hydrogen bond strength between free water molecules. This results in a decreased speed of sound in the free water and an increase in the longitudinal storage modulus of the hydrogel.

Strong, tough and conductive single-network hydrogels based on deswelling and the salting-out effect.

In recent years, the continuous attention given to increasing the fracture toughness and Young's modulus of polymeric gels has gradually shifted from toughening strategies on double-network (DN) gels to single-network (SN) gels. The salt-soaking method has been adopted to realize the toughening of SN gels through the salting-out effect and deswelling, constructing dense network structures with simultaneously precipitated polymer chains and cross-links. By comparing the mechanical properties between salt-treated hydrogels and air-dried hydrogels, the increased polymer chain concentration is proved to promote energy transfer by enlarging the dissipation region size due to the unwinding and slippage of coiled chains during stretching. The newly formed cross-link points in salt-treated hydrogels are considered to consume more deformation energy during stretching. The synergistic effect in energy transfer and dissipation arising from increases in polymer fraction and cross-linking plays an indispensable role in toughening SN hydrogels. In addition, the soaking process introduces abundant free ions to endow hydrogels with prominent conductivity. Thus, this salt-soaking method provides a general approach to synthesize strong, tough and conductive hydrogels with applications in flexible electrical devices.

Dual aggregation-induced emission enhancement (AIEE) and crosslink-enhanced emission (CEE) driven via halogen-bond-assisted cocrystallization and radical solid-phase polymerization

Concurrent emission enhancement was attained by halogen-bond-driven cocrystallization (aggregation-induced emission enhancement, AIEE), followed by halogen-bond-driven (non)covalent crosslinking of polymer (crosslink-enhanced emission, CEE).

In Ukrainian

Thermal destruction of composites of ureaformaldehyde (UPR) and polyester resins (PER) with silicon dioxide nanoparticles having a specific surface area of 280 m2/g, titanium dioxide and titanosilicate with a specific surface area of 40 and 48 m2/g, respectively, when a filler content is no more than 5.0 wt% have been studied. The investigations were performed using The thermally programmed mass spectrometry method with registration of the masses of desorbed atomic fragments in the 10 ‒ 200 m/z range. It was established that during the main polymer mass destruction at 150 ‒ 350 oC, along with low temperature decomposition products, anomalously high thermal resistance of a number of atomic fragments of polymer chains and cross-links are recorded. The atomic composition of destruction fragments and their desorption temperature in the range 400 ‒ 700 oС were determined. It was established that in composites of ureaformaldehyde resin with SiO2 and TiO2 nanoparticles the high temperature resistance of fragments with m/z 27 exhibits due to the formation of strong bonds among the Si and Ti surface sites and the nitrogen atoms of the polymer. Such thermal stability is not realized in resin loading with (Si/Ti)O2 nanoparticles. In composites of polyester resin with silica a high-temperature destruction of oxygen atoms from polyester chains realizes at temperatures of 290 ‒ 400 oC and a low-intensity wide destruction band takes place in the temperature range 400 ‒ 700 oC. In addition, in the temperature range of 400-700 oC cross-links are destroyed with the release of benzene rings and styrene molecules. It was established that anomalously high-temperature desorption is typical for atomic fragments of the polymer structure attached to surface Si and Ti sites through nitrogen or carbon atoms from the polymer structure. Thus, in UPR composites with silicon and titanium oxides, strong chemical nitride bonds of the form Si-N≡C-H and Ti-N≡C-H are formed, which demonstrate anomalously high heat resistance. It is shown that in composites of polyester resin with silicon dioxide nanoparticles, the high-temperature destruction of fragments is due to their desorption from the surface of silicon dioxide particles when breaking their bonds with silicon atoms. Thus, polymer matrices have been determined, in which atomic fragments of the macromolecule, binding to the surface centers of fillers, significantly weaken the thermal destruction of composites due to the formation of strong chemical and coordination bonds.

In Ukrainian

The influence of the electronic subsystem of nanographene particles modified with titanium dioxide nanoparticles on the thermal destruction of the polymer matrix is analyzed. The variation in the conductivity of the modifiers made it possible to vary the effect of the metallic electronic system of graphene on thermal destruction by attenuating its effect through the transfer of energy to the electronic subsystem throughout the semiconductor film of anatase and excluding its effect on thermal destruction through a wide-band TiO2 dielectric of rutile form, which is opaque for energy transfer to the electronic subsystem. The application of such films does not affect the thermal processes in which the phonon subsystems of polymer, graphene and crystal modifiers are involved. The method of thermally programmable desorption mass spectrometry investigated the thermal destruction of hybrid composites of epoxy resin with graphene, namely the effect of surface modification of multilayer graphene particles by nanoparticles of pyrogenic titanium dioxide (anatase and rutile forms) on the thermal stability of hybrid epoxy composites in the temperature range of 40 − 800 °C. The activation energies Ed of destruction of atomic fragments of the polymer structure with the value m/z in the range of 16 ‒ 140 D were determined. It has been established that the thermal destruction of fragments of the polymer structure of epoxy resin composites with graphene modified with anatase and rutile forms of titanium dioxide is determined by the peculiarities of thermal transport at the polymer-modifier interfaces. Modification of graphene particles by the anatase form of titanium dioxide leads to increased thermal stability of polymer chains and cross-links in epoxycomposites in the entire range of m/z values of desorbed fragments. Filling the resin with graphene particles coated with the rutile form of TiO2 practically does not affect the intensity of thermal destruction of the composite matrix. The Ed values of different destruction fragments in composites with unmodified and modified graphene are in the range of 50 ‒ 130 kJ/mol. The value of Ed increases by 5 ‒ 10 kJ/mol when the resin is filled with graphene nanoparticles, while the coating of particles with titanium dioxide of the anatase and rutile phases practically does not change the value of Ed of the vast majority of fragments.

A photocurable polyurethane elastomer for digital light processing with comprehensive reusability based on dynamic hindered urea bonds

Digital Light Processing (DLP) 3D printing has emerged as a highly promising technique, enabling the fabrication of intricate geometric structures with ease and precision, without the need for complex mold-making processes. However, the printed objects are typically crosslinked photopolymers that cannot be reshaped, reprocessed and recycled. Herein, we successfully synthesize a dynamic soft polymer crosslinker containing hindered urea bonds (HUBs) composed of polyurethane acrylate prepolymer and tri-functional for recyclable DLP 3D printing. Contributing to the dynamic thermal response of HUBs, the printed objects can be recycled in the initial printing inks at 80 °C in 2h and the obtained recycled ink can be reprinted by DLP achieving a low viscosity and high printing resolution. In addition, the printed objects can be reprocessed by hot-press at 110 °C for 30 min upon a pressure of 5 MPa. By a reasonable proportion of monomer and crosslinker, a series of photocured elastomers were successfully prepared, in which mechanical properties could be tailored by changing the content of the crosslinker. The resulting products exhibited some fascinating attributes, such as excellent fatigue resistance, thermodynamic stability, and transparency. Significantly, the resin can regain initial 3D printing capability after undergoing solvent recycling, which maintains its printing resolution and excellent mechanical properties. This innovative design strategy not only alleviates environmental impact but also plays a pivotal role in developing the DLP 3D printing industry by tackling the long-standing issue of resin recyclability.

Enhanced DC Dielectric Properties of Crosslinked Polyethylene Comprehensively Modified by the Grafting of a Multifunctional Voltage Stabilizer.

In this paper, a new multifunctional compound, 1,1'-(oxalylbis(4,1-phenylene))bis(1H-pyrrole-2,5-dione) (BVM), is grafted onto crosslinked polyethylene (XLPE) by radical-initiated grafting to play triple roles as a voltage stabilizer, space-charge inhibitor and crosslinking auxiliary and to achieve the purpose of comprehensively enhancing the DC dielectric properties of polymers while decreasing the type and number of additives. By analyzing the DC breakdown field strength, current density and space-charge distribution of the materials at different temperatures, it is demonstrated that BVM grafting can comprehensively and effectively enhance the electrical properties of the materials, with little dependence on temperature. The BVM molecule has two polar groups and an effective molecular structure that acts as a voltage stabilizer, thus enabling the introduction of dense, uniform, deeply trapped energy levels within the material to inhibit the space charge and to capture high-energy electrons to prevent damage to the material structure; however, the two functions do not affect each other. This is also consistent with first-principles calculations and quantum-chemical calculations. Gel content testing shows no effect on polymer crosslinking, even with a 27.8% reduction in the amount of the crosslinking agent di-isopropyl peroxide (DCP), which reduces the damage to the polymer's electrical resistance caused by the byproducts of DCP decomposition. Therefore, grafting multifunctional BVM compounds to improve the dielectric characteristics of polymers is a viable area of study in the development of high-voltage direct current (HVDC) cable materials.

Fluorinated‐polyhedral oligomeric silsesquioxane (F‐POSS) functionalized halloysite nanotubes (HNTs) as an antifouling additive for epoxy resin

AbstractAntifouling epoxy resin serves as a member of advanced nanocomposite coatings and engineered surfaces. Nevertheless, developing an antifouling epoxy composites is a state‐of‐the‐art technology. Herein, a complex nanostructure is tailored to be served as an advanced additive giving the antifouling characteristics to the epoxy resin. F‐POSS@HNT hybrid nanostructure, that is, halloysite nanotubes (HNTs) decorated with fluorinated polyhedral oligomericsil sesquioxane (F‐POSS), was synthesized and characterized. Chemical structure changes, thermal stability and morphology of hydroxylated HNTs (hHNTs) intermediate and F‐POSS@HNT ultimate nanostructures were analyzed by FTIR spectroscopy, TGA and TEM, respectively. The F‐POSS@HNTs catalyzed the epoxy‐amine crosslinking reaction, taking “Good” or “Excellent” crosslinking tags an quantified by the “Cure Index.” The apparent activation energy values were calculated for the epoxy (reference), epoxy/hHNTs, and epoxy/F‐POSS@HNTs systems (26, 51, and 47 kJ mol−1, respectively). Contact angle measurements were performed via dynamic tests demonstrating improved hydro‐ and oleophobicity of the thermoset composites. The advancing contact angle with diiodomethane increased 36% and 30% for nanocomposites containing 5 and 10 wt.% of the developed hybrid nanostructure, respectively, compared with the neat epoxy. Likewise, 54% and 67% reduction in paraffin fouling in the same order confirmed their antifouling ability. Regarding self‐cleaning characteristics, 24% and 33% surface recovery were observed, respectively.Highlights Antifouling surface was achieved in epoxy thermoset nanocomposites for potential industrial applications. Self‐cleaning behavior correlated with low wetting and antifouling properties. F‐POSS@HNT nanofillers were synthesized and successfully added to epoxy matrix. Both hHNT and F‐POSS@HNT showed a beneficial autocatalytic effect on epoxy‐amine reaction yielding satisfactory polymer crosslinking. Activation energy was reduced by incorporating F‐POSS@HNT compared with hHNT, signature of facilitated crosslinking.

Rapeseed Oil as Feedstock for Bio-Based Thermoset Foams Obtained via Michael Addition Reaction.

Rapeseed oil was used to develop thermoset foams via Michael addition reaction by mixing two liquid components, Michael donor and Michael acceptor. The foaming of the curing thermoset was achieved by the physical blowing agent which expanded from the reacting foam mass due to an exothermic curing reaction. The influence of the rapeseed oil-based Michael donor functionality on the foaming process and the characteristics of the obtained thermoset foams was studied. The 1,1,3,3-tetramethylguanidine catalyst's influence on the foaming process kinetics was studied using FOAMAT equipment. The curing of the bio-based thermoset was analysed using a dielectric polarisation sensor. The morphology of the developed thermoset foam was analysed using a scanning electron microscope and the obtained foams were characterized using TGA, DSC, DMA and mechanical analysis tests. A direct correlation between the thermoset foam polymer crosslinking density and foaming reactivity, mechanical properties and glass transition temperature were determined. Obtained rapeseed oil based thermoset foams had a relatively low thermal conductivity of 33.9-35.4 mW/(m·K) which allows their use as thermal insulation material in civil engineering applications.

PH-responsive chitosan/poly (vinyl pyrrolidone) based hydrogel composites: Antibacterial properties and release kinetics of diclofenac sodium

Development of effective hydrogels based anti-inflammatory drug delivery materials remains great challenge. In this study, diclofenac sodium (DS) was successfully encapsulated into chitosan (CS)/poly (vinyl pyrrolidone) (PVP) with glutaraldehyde crosslinked hydrogels. The characteristics of the hydrogel composites were characterized by swelling behavior, FTIR spectroscopy, hydrophilicity, and porosity test, SEM-EDX, X-ray diffraction, thermal stability, and mechanical strength methods. pH-dependent drug (DS) release behavior and the antibacterial activity of the hydrogel composites were examined. Typically, the interaction of polymer precursors, crosslinker, and drug compound occurred via hydrogen bonding. The cross-sectional SEM images demonstrated the pore formation in hydrogel matrix. Mechanical properties of hydrogel improved from 8.9 to 13.1 MPa due to crosslinking process and DS encapsulation. Insertion of diclofenac sodium improved the hydrophilicity as shown in the lowering of contact angle to 55.47°. Hydrogel samples showed promising antibacterial activity against Pseudomonas aeruginosa and Staphylococcus aureus with the zone of inhibition (ZoI) up to 21.8 ± 0.05 mm and 18.7 ± 0.09 mm, respectively. The controlled DS release from hydrogel composites at pH 4, 7 and 10 has maximum release amount of approximately around 10 %, 80 %, and 90 %, respectively. The kinetics and mechanism of drug release from hydrogels was followed Kosmeyer-Peppas and Higuchi models, indicating that the DS release from hydrogel was predominantly controlled by Fickian diffusion or swelling process. These results indicated that the pH-responsive hydrogels based on CS/PVP can be a promising candidate for drug delivery due to its high susceptibility to antibacterial activity.

Nanosponges- A Versatile Approach of Drug Delivery System

With the escalating number of diseases, an opportunistic target drug delivery system is needed. Nanosponges (NS) are nanosized drug delivery systems having three-dimensional (3D) configurations, fabricated by crosslinking of polymers to ensnare a wide assortment of drugs. These small particles move inside the body till they arrive at the target site and deliver the medication in a specific and controlled way. They can ensnare (entrap) both hydrophilic and hydrophobic moieties in a smarter manner and improve the solubility and bioavailability of drugs. NS have been utilized to examine the medication conveyance by oral, parenteral, and topical routes. Another significant property of NS is that they can work on the solvency of ineffectively water-soluble moieties and are utilized for the storage of gases, and proteins as well as for the removal of toxins from the systemic circulation. The present review highlights the types of nanosponges, their methods of preparation, characterization, and applications in drug delivery.

Synthesis and Characterization of Responsive Antimicrobial Materials for Chronic Wound Management

Chronic wound treatment requires the ability to regulate the wound healing process and mitigate the propagation of bacterial infection with constant monitoring, in real-time. The need for multi-functional wound management materials has emerged as a significant research area with promise shown in the incorporation of antimicrobial agents within responsive polymeric materials to elicit signals providing dynamic information about the healing process. Traditional wound dressings provide a barrier between the outside environment and the wound; however, these methods are increasingly becoming less effective. Hydrogels synthesized from natural biopolymers have emerged due to their intrinsic properties such as biodegradability and non-cytotoxicity and enhanced from their well-defined, three-dimensional, non-covalent networks ideal for modification and functionalization to create “micro-reactors.” Previous work focused on the strategic design of an alginate-based hydrogel containing dual functional additives (i.e., amino acids) paired with photochemically prepared Ag- and Cu-TiO2 nanoparticles (Ag/Cu-NPs) to understand the synergistic interactions of the polymer chains, cross-linkers, and additives. The antimicrobial effectiveness of the composite was examined using an epi-fluorescent optical tweezer to measure the rates of bacterial cell death. Results showed that with the incorporation of the NPs there is a decrease in green signal (live cells) within 1 hour of exposure. Furthermore, the healing rates of simulated wounds using human dermal fibroblasts were explored along with changes in the expression of growth factors that promote wound healing, such as collagen and TGF-b. This work focuses on a two-layer responsive material with a wound-healing layer and a conductive layer formed through the secondary functionalization of the composite system through a metal-coordination-assisted photopolymerization of conductive polymers (i.e. aniline). Preliminary results show there is uniform polymeric chain growth seen in enhanced charge transfer through the matrix upon an applied voltage. Studies have also been conducted to measure the impact of saturation of the bandage through simulated wound exudate that revealed once saturation occurs, the passing of electricity between the ions found in wound exudate carried current through the composite, increasing the conductivity, leading to the signaling of an LED prompting replacement. The resulting two-layer nanocomposite system then functions as a responsive, antimicrobial wound healing material capable of eliminating constant monitoring with application in healthcare and home settings.

Bio-valorization of Tagetes floral waste extract in fabrication of self-healing Schiff-base nanocomposite hydrogels for colon cancer remedy.

The drastic boom in floriculture and social events in religious and recreational places has inevitably led to generation of tremendous floral waste across the globe. Marigold (Tagetes erecta) is one of the most common loose flowers offered for the same. Generally discarded, these Tagetes floral wastes could be valorized for biogenic syntheses. In this study, we have utilized the floral extract towards green synthesis of nano ZnO, the formation of which was affirmed from different analytical techniques. Bionanocomposite Schiff-base hydrogel composed of chitosan and dialdehyde pectin was fabricated by the facile strategy of in situ polymer cross-linking, and the ZnO nanoparticles were embedded in the hydrogel matrix. The hydrogel exhibited remarkable self-healing ability. The antioxidant and anti-inflammatory activities were enhanced owing to nano ZnO. Furthermore, it was hemocompatible and biodegradable. A controlled release drug profile for 5-fluorouracil from the hydrogel was accomplished in the colorectum. The exposure of the drug-loaded nanocomposite hydrogel demonstrated improved anticancer effects in HT-29 colon cancer cells. The findings of this study altogether put forth the successful biovalorization of Tagetes floral waste extract for colon cancer remedy.

Novel biodegradable polymers modified by humic acids

The possibility of using humic acids (HA) as hybrid modifiers is considered. They were taken from lignite using first sodium pyrophosphate solution, then a 1 % solution of NaOH for extraction, and next mineral acid for precipitation. It has shown that in gelatin–HA hybrid systems, the latter mainly join water molecules and “sequester” them to prevent their interaction with gelatin chains. In the hydroxypropyl methylcellulose (HPMC)-HAs systems, the processes of polymer cross-linking through multipoint chelate interaction with the carboxylate group of HAs have been determined. Hybrid polyvinyl alcohol (PVA)-HAs structures are obtained by the matrix synthesis mechanism. Also, during the hybrid modification of PVA with HAs, additional supramolecular interactions, in addition to hydrogen bonds, occur between their functional groups as a result of crystallization. It was found that hybrid modification of HAs within the matrix synthesis of gelatin-based biopolymer hydrogels allows obtaining polymer hydrogels with an increased swelling degree and gives them antibacterial qualities. The optimal content of HAs in biodegradable polymers based on gelatin, PVA and HPMC is 10 % by mass.

Role of Polymer Concentration on the Release Rates of Proteins from Single- and Double-Network Hydrogels

Controlled delivery of proteins has immense potential for the treatment of various human diseases, but effective strategies for their delivery are required before this potential can be fully realized. Recent research has identified hydrogels as a promising option for the controlled delivery of therapeutic proteins, owing to their ability to respond to diverse chemical and biological stimuli, as well as their customizable properties that allow for desired delivery rates. This study utilized alginate and chitosan as model polymers to investigate the effects of hydrogel properties on protein release rates. The results demonstrated that polymer properties, concentration, and crosslinking density, as well as their responses to pH, can be tailored to regulate protein release rates. The study also revealed that hydrogels may be combined to create double-network hydrogels to provide an additional metric to control protein release rates. Furthermore, the hydrogel scaffolds were also found to preserve the long-term function and structure of encapsulated proteins before their release from the hydrogels. In conclusion, this research demonstrates the significance of integrating porosity and response to stimuli as orthogonal control parameters when designing hydrogel-based scaffolds for therapeutic protein release.

Molecular docking-based screening of methicillin-resistant Staphylococcus aureus FEM proteins with FDA-approved drugs.

Antibiotic resistance stands as one of the most significant public health challenges in recent decades. FEM proteins are responsible for the synthesis of pentaglycine cross-bridge, a primary constituent of bacterial peptidoglycan polymer crosslinking during cell wall biosynthesis. Since they are necessary for bacterial survival and antibiotic resistance, they were considered as significant antibacterial targets. We report herein, the virtual screening and selection of FDA-approved drugs and their potent similar molecules as FEM protein inhibitors and analyzed for inhibiting affinity and their ADMET pharmacokinetic properties. This data provide a foundation for the development and optimization of structurally innovative antimicrobial drugs.