Increase In Crosslink Density Research Articles

The Effect of Heat Treatment toward Glycerol-Based, Photocurable Polymeric Scaffold: Mechanical, Degradation and Biocompatibility.

Photocurable polymers have become increasingly important for their quick prototyping and high accuracy when used in three dimensional (3D) printing. However, some of the common photocurable polymers are known to be brittle, cytotoxic and present low impact resistance, all of which limit their applications in medicine. In this study, thermal treatment was studied for its effect and potential applications on the mechanical properties, degradability and biocompatibility of glycerol-based photocurable polymers, poly(glycerol sebacate) acrylate (PGSA). In addition to the slight increase in elongation at break, a two-fold increase in both Young’s modulus and ultimate tensile strength were also observed after thermal treatment for the production of thermally treated PGSA (tPGSA). Moreover, the degradation rate of tPGSA significantly decreased due to the increase in crosslinking density in thermal treatment. The significant increase in cell viability and metabolic activity on both flat films and 3D-printed scaffolds via digital light processing-additive manufacturing (DLP-AM) demonstrated high in vitro biocompatibility of tPGSA. The histological studies and immune staining indicated that tPGSA elicited minimum immune responses. In addition, while many scaffolds suffer from instability through sterilization processes, it was proven that once glycerol-based polymers have been treated thermally, the influence of autoclaving the scaffolds were minimized. Therefore, thermal treatment is considered an effective method for the overall enhancement and stabilization of photocurable glycerol-based polymeric scaffolds in medicine-related applications.

REINFORCEMENT EFFECT OF IN SITU DEVELOPED ITACONIC ACID BASED METAL SALT NANO-CRYSTALS IN ACRYLONITRILE-BUTADIENE COPOLYMER

ABSTRACT Itaconic acid has been employed as a special facilitator to construct divalent metal ion based ionic crosslinking framework in the acrylonitrile butadiene rubber matrix. Readily accessible double bonds in itaconic acid could directly react with the elastomer to form effective covalent bonds. On the other hand, presence of easily dissociable protons in itaconic acid enables them to form ionic bonds that leads to an increase in crosslinking density of the vulcanizates. The synergistic effect of covalent crosslinking induced by peroxide and ionic crosslinking induced by metal carboxylate could effectively enhance the overall mechanical and dynamic mechanical properties of the rubber composites. In this study, three metal oxides, that is, zinc oxide, magnesium oxide, and calcium oxide, have been selected for this purpose. Tensile strength of nitrile rubber composites depends on the strength of ionic crosslinks, which in turn is influenced by the size of the alkaline earth metals, such as Mg, Ca, etc., and stoichiometric quantity of itaconic acid, which is at par in the formulation of this study. The novelty of this study is that the introduction of a dicarboxylic acid in combination with metal oxides enhances the crosslink density and tensile strength of nitrile rubber composites which could result from the metal organic framework.

Mechanical and Structural Consequences of Associative Dynamic Cross-Linking in Acrylic Diblock Copolymers

The composition of low-Tg n-butylacrylate-block-(acetoxyaceto)ethyl acrylate block polymers is investigated as a strategy to tune the properties of dynamically cross-linked vinylogous urethane vitrimers. As the proportion of the cross-linkable block is increased, the thermorheological properties, structure, and stress relaxation evolve in ways that cannot be explained by increasing cross-link density alone. Evidence is presented that network connectivity defects such as loops and dangling ends are increased by microphase separation. The thermomechanical and viscoelastic properties of block copolymer-derived vitrimers arise from the subtle interplay of microphase separation and network defects.

Synthesis, characterization, anti-inflammatory and anti-Helicobacter pylori activities of novel benzophenone tetracarboxylimide benzoyl thiourea cross-linked chitosan hydrogels

Chitosan (Cs) was cross-linked with four various quantities of 4,4′-(5,5′‑carbonylbis(1,3-dioxoisoindoline-5,2-diyl))dibenzoyl isothiocyanate. Elemental analysis, FTIR and 1H NMR spectroscopy assured that the amino groups of chitosan reacted with the isothiocyanate groups of the cross-linker producing four new hydrogels namely as BBTU-Cs-1, BBTU-Cs-2, BBTU-Cs-3, and BBTU-Cs-4 according to the increment of their cross-linking content, respectively. SEM showed their porous structures and XRD indicated their amorphous nature. Their swell ability increased with decreasing the medium pH value and with increasing cross-linking density. In comparison with the popular COX inhibitor Celecoxib, these hydrogels showed an inhibition activity towards COX enzymes with selective inhibition towards COX-2. Their inhibition activity could be arranged as follows: Celecoxib > BBTU-Cs-4 > BBTU-Cs-3 > BBTU-Cs-2 > BBTU-Cs-1. BBTU-CS-4 hydrogel exhibited a potent inhibition against COX-2 (IC50 0.42 μg/ml) compared with that observed for the standard Celecoxib (IC50 0.26 μg/ml). BBTU-Cs-4 is more potent against H. pylori compared to the other hydrogels. BBTU-Cs-4 at a concentration of 7.81 μg/ml is able to kill 100% of the H. pylori and exhibits a preferential ability to inhibit 89.35% of COX-2 than COX-1 (0%). These findings make BBTU-Cs-4 a promising anti-H. pylori and selective anti-inflammatory agent.

HEAT AGING OF A BROMOBUTYL TIRE INNER LINER UNDER AEROBIC AND ANAEROBIC CONDITIONS

ABSTRACT A bromobutyl tire inner liner compound was prepared and subjected to aerobic and anaerobic heat aging at a temperature of 100 °C for seven aging times up to 8 weeks. Hardness and mechanical properties were monitored, and the evolution of the crosslink density was followed using equilibrium solvent swell and low field double quantum (DQ) nuclear magnetic resonance (NMR). The hardness and the 300% tensile stress increased with heat aging, while both tensile strength and elongation at break dropped. Both chain scission and crosslinking reactions were taking place. Equilibrium swelling and DQ NMR results confirmed that a larger crosslink density increase was seen under aerobic versus anaerobic aging conditions. The network distribution consisting of a dominant low crosslinking zone and small areas of higher crosslinking slowly broadened and shifted toward higher crosslink densities upon heat aging. The compounds aged heterogeneously. Attenuated total reflectance–Fourier transform infrared spectroscopy confirmed the presence of an oxidized surface layer, and therefore diffusion-limited oxidation effects, but only under aerobic aging conditions. Reaction mechanisms are proposed to explain the net crosslink rise with heat aging.

Study of crystallization behaviour of electron beam irradiated polypropylene and high-density polyethylene.

The influence of electron-beam irradiation on polypropylene (PP) and high-density polyethylene (HDPE) was investigated with a focus on crystallization. A high-temperature (200°C) creep test revealed that the HDPE gradually increased cross-linking density in the range 30–120 kGy, while the PP underwent a chain scission which was quantitatively evaluated by gel permeation chromatography. The mechanical properties were measured in the range -150 to 200°C by a dynamic mechanical analysis. A small presence of C=C and C=O bonds was found in the irradiated PP by a Fourier transform infrared spectroscopy. Crystallization kinetics measured by differential scanning calorimetry and hot-stage optical microscopy results were influenced tremendously by irradiation for HDPE and to a lesser extent for PP. Irradiation caused a decrease in both the number of nucleation centres and the growth rate of individual spherulites. Crystallization was analysed in detail with the help of Hoffman–Lauritzen, Avrami and Arrhenius equations. Interestingly an increasing β-crystal formation with an increasing irradiation level was discovered for PP by X-ray diffraction. A generation of defects in the crystalline structure owing to irradiation was discussed.

Biologically controlled gelatin actuators

Gelatin is a natural and environment-friendly polymer that can be cross-linked to various degrees with glutaraldehyde (GTA) and swollen with water to form a hydrogel. As the gelatin cross-link density increases, gel stiffness increases and swelling in water decreases with the cross-link density saturating at about 0.01 mol of GTA. Forming gelatin hydrogel bilayers with layers of differing cross-link densities results in a gelatin actuator that bends toward the layer of higher cross-link density when swollen in water. The extent of bilayer bending is dependent on the difference between the volume swelling ratio of the layers, which is directly determined by the cross-link density difference. When the gelatin bilayers are close in swell ratio or when the swell ratios are significantly different, no bending occurs. The highest bending is found when the higher cross-linked passive layer swells to about 60% of the swelling of the lower cross-linked active layer. Filling the active layer with pre-gelatinized starch increases the bilayer curvature, but it returns to its original shape after hydrolyzing the starch with α-amylase, demonstrating shape change in response to a biochemical reaction. The results show that simple gelatin bilayers can serve as actuators when stimulated with water or sugar and enzyme.

Sepiolite-embedded binary nanocomposites of (alkyl)methacrylate-based responsive polymers: Role of silanol groups of fibrillar nanoclay on functional and thermomechanical properties

Binary nanocomposites filled with fibrillar nanoclay sepiolite (SP) was manufactured using a hydrophilic terpolymer matrix containing 2-(diethylamino)ethyl methacrylate, hydroxyethyl methacrylate and 2-acrylamido-2-methyl-1-propanesulfonic acid. A direct correlation between dispersion of nanofillers, matrix-filler local interaction and improvement of macroscopical mechanical properties was obtained. Mechanical testing revealed that incorporation of SP nanofillers impart to terpolymer matrix a higher degree of reinforcement. Besides fibrous structure of SP, the interactions between silanol (-SiOH) groups present along fibers and terpolymer chains are responsible of extent of reinforcement of nanocomposites. The increase in modulus imparted by incorporation of -SiOH groups reflected a continuous increase in crosslinking density created by terpolymer-nanofiller interactions. Effective crosslink density of nanocomposites was expressed by a cubic polynomial function of SP loading. Swelling ratio of SP nanofiller containing nanocomposites was found to highly decrease at CSP > 3.50%(w/v), with regard to clay-free terpolymers reflecting strong interactions between pendant hydroxyl, carbonly, tertiary amino groups of terpolymer matrix and silanol groups of SP. Excellent pH-responsive swelling was observed in the range of pH 2.1–10.7 and the nanocomposites swelled in acidic conditions depending on the amount of protonated diethyl amino groups. Sensitivity sequence of terpolymer nanocomposites to mono- and di-valent cations with a common anion (Cl−) on swelling was Na+ > K+ > Mg2+. The control of quantity of SP fibers and polymerization temperature allows to manage mechanical properties by just tuning chemical architecture of terpolymer/clay nanocomposites. SP reinforcement of terpolymers can enrich various further characteristics such as targeted delivery of biomolecules or drugs.

The Effect of Electron Beam Irradiation on Elastomers Used in Tooling for Composites Manufacturing

Abstract Background: This technical brief discusses material degradation issues and potential remedies related to advanced thermoset composites manufacturing using a new out-of-autoclave consolidation and curing process called Electron Beam processing with Specialized Elastomeric Tooling for Resin Infusion (EB-SETRI). To provide context, the design process for EB-SETRI tooling based on finite element structural analysis and Monte Carlo simulations of EB attenuation within tooling materials is briefly described. Of particular interest in this paper is the elastomeric mask, since it exhibits significant changes in mechanical properties based on prior work. Methods: Samples of five different silicone blends (four different durometers and two different catalysts) and one urethane (elastomeric mask materials of choice) were irradiated by an EB source with 3.0-MeV maximum power to simulate the conditions experienced by EB-SETRI tooling during processing. Changes in surface hardness and compression modulus were measured using ASTM D575 and D2240 as a function of dosage. Results: Urethane embrittles and becomes unusable even at low dosages, whereas silicone generally hardens to a maximum level at higher dosages, presumably due to increased crosslinking density, and stiffens (modulus increases) linearly. The embrittlement of silicone is shown to be a result of the EB irradiation and not due to a temperature increase from energy absorption. Conclusions: Changes in elastomer mechanical properties confound process performance as a result, and several concepts for dealing with these changes are suggested. Although the experimental focus is on EB-SETRI, results apply to any manufacturing process that combines the use of EB irradiation and elastomers.

Low cost highly efficient natural polymer-based radiation grafted adsorbent-I: Synthesis and characterization

High energy Co-60 gamma radiation was used to graft cationic and anionic monomers onto radiation crosslinked chitosan through mutual radiation grafting technique. The absorbed dose for synthesizing adsorbent of good strength and adsorption capacity was monomer specific. The grafting extent increased with the increase in radiation dose, dose rate and monomer concentration. While a decrease in grafting extent was observed with the increase in crosslinking density of chitosan backbone. The grafted products were characterized for water uptake capacity, surface morphology, thermal stability and elemental analysis by SEM, TGA and EDXA respectively.

Facile synthesis of a novel zinc-triazole complex for simultaneous improvement in fire safety and mechanical properties of epoxy resins

A novel zinc-triazole complex (ZTC) was synthesized by a co-precipitation method. The addition of 4 wt% of ZTC showed slight change in the thermal stability of the epoxy resin (EP) matrix, whereas the inclusion of 4 wt% of 3,5-diamino-1,2,4-triazole deteriorated the thermal stability. The glass transition temperature (Tg) of EP/ZTC-4% was dramatically enhanced to 162 °C from 150 °C for the unmodified epoxy, which could be ascribed to the increased cross-linking density and the structural rigidity of ZTC. The flame-retardant property of EP/ZTC-4% was greatly improved, achieving UL-94 V-0 classification and a LOI value of 29.0%. The peak heat release rate, total heat release and smoke production rate of EP/ZTC-4% were dramatically declined by 72.9%, 86.0% and 73.8%, respectively, compared to the unmodified EP. Such desirable features including high fire retardant efficiency and proper smoke suppression make zinc-triazole complex a promising candidate for high fire safety polymers.

Soft solution in situ synthesis of chitosan/iron oxide nanocomposites and their magnetic properties.

Chitosan/iron oxide nanocomposites (CS/IO) were synthesized by using soft solution in situ synthesis. An aqueous mixture of iron(ii), iron(iii) and chitosan was added drop by drop to a solution of a sodium tripolyphosphate crosslinker with stirring for 30 min, resulting in in situ ionically crosslinked chitosan, with incorporated Fe2+ and Fe3+ (CS/Fe2+Fe3+). The CS/Fe2+Fe3+ precursors were then treated in alkaline solution by two different methods, i.e. hydrothermal and refluxing, where the Fe2+ and Fe3+ ions reacted to form quasi-spherical magnetite-maghemite nanocrystals in the constrained space of the crosslinked chitosan CS/IO nanocomposites. The pressurized hydrothermal system promoted the growth of iron oxide nanocrystals, leading to slightly larger crystallites (3.9-4.3 nm), compared to 3.9 nm from the refluxing system. The iron oxide crystallites also became smaller with increased crosslinking density of the chitosan matrix. The resultant CS/IO nanocomposites exhibited superparamagnetism with Mmax in the range of 9.6-15 emu g-1 and low coercivity and magnetic remanence. In addition, they showed high cell viability, 82-96%, indicating them as potential candidates for medical applications.

Engineering of TeO2-ZnO-BaO-Based Glasses for Mid-Infrared Transmitting Optics.

In this paper, the glass systems, TeO2–ZnO–BaO (TZB), TeO2–ZnO–BaO–Nb2O5 (TZB–Nb) and TeO2–ZnO–BaO–MoO3 (TZB–Mo), were fabricated by the traditional melt-quench protocol for use as mid-infrared (mid-IR) transmitting optical material. The effect of Nb2O5 and MoO3 on the key glass material properties was studied through various techniques. From the Raman analysis, it was found that the structural modification was clear with the addition of both Nb2O5 and MoO3 in the TZB system. The transmittance of studied glasses was measured and found that the optical window covered a region from 0.4 to 6 μm. The larger linear refractive index was obtained for the Nb2O5-doped TZB glass system than that of other studied systems. High glass transition temperature, low thermo-mechanical coefficient and high Knoop hardness were noticed in the Nb2O5-doped TZB glass system due to the increase in cross-linking density and rigidity in the tellurite network. The results suggest that the Nb2O5-doped TZB optical glasses could be a promising material for mid-infrared transmitting optics.

Functionally Graded Adhesives via High-Energy Irradiation

A thermally cured epoxy/anhydride system is shown to exhibit dose-dependent increases in cross-link density, glass transition temperature, and modulus. The addition of various sensitizers further b...

Semi-interpenetrating polymer networks prepared from castor oil-based waterborne polyurethanes and carboxymethyl chitosan

A series of vegetable oil-based waterborne polyurethane composites were prepared through construction of novel semi-interpenetrating polymers network using carboxymethyl chitosan (CA) as the secondary polymer phase. The effects of CA contents on storage stability, and particle size distribution of the composite dispersions and thermal stability, mechanical properties and surface wettability of composite films were investigated and discussed. The results showed that the composite dispersions displayed excellent storage stability and the biomass contents of resulting films were high up to 80 %. A significant increase in crosslinking density and glass transition temperature of the composite films were observed as the CA contents increased, which was attributed to the increasing hard segment of films and strong hydrogen bonding interaction between polyurethanes and CA. This work provided a simple method to tailor the performance of environmentally friendly vegetable oil-based waterborne polyurethane, which could find application in the field of coatings, adhesives, ink and so on.

DNA-crosslinked alginate and layered microspheres to modulate the release of encapsulated FITC-dextran

Alginate can be gently crosslinked by calcium into hydrogels and microspheres for the encapsulation and release of proteins and drugs. However, the release is often over short periods unless alginate is also covalently modified or crosslinked. This research aims to sustain the release of encapsulated model drug FITC-dextran by covalently crosslinking alginate with short oligomers DNA because evidence suggests that DNA may also interact with alginate to further increase effective crosslinking. Furthermore, modulating the release of drugs from alginate in response to specific proteins could tailor release profiles to improve patient treatment. This research develops a DNA-crosslinked alginate hydrogel and layered alginate microspheres to encapsulate and then sustain the release FITC-dextran (model drug). An aptamer sequence to hen egg-white lysozyme is included in one DNA strand to allow for the disruption of the crosslinks by interactions with human lysozyme.Alginate was covalently modified with complementary strands of DNA to crosslink the alginate into hydrogels, which had increased crosslinking density when re-swollen (in comparison to controls crosslinked with PEG) and could sustained the release of encapsulated FITC-dextran. When an aptamer sequence for hen lysozyme was included in the DNA crosslinks, the hydrogels decrosslinked when incubated in human lysozyme for 60 days. In addition, calcium alginate microspheres were coated with 3 alternating layers of poly-Lysine, DNA-crosslinked alginate, and poly-L-lysine. FITC-dextran loaded into the microspheres released in a sustained manner past 30 days (into PBS at 37 °C) and would likely continue to release for far longer had the studies continued. When incubated with 3 μM of human lysozyme, a burst release of FITC-dextran occurred from both the hydrogels and microspheres, with no changes in the controls. The increased release was in bursts followed by similar sustained release rates suggesting that the human lysozyme temporarily disrupted the DNA crosslinks which were then re-established or were influenced by interactions between DNA and alginate. Importantly, covalently bound complementary strands of DNA could crosslink the alginate and additional interactions appeared to further sustain the release of encapsulated therapeutics.

Assessment of transport kinetics and chemo-mechanical properties of GF/Epoxy composite under long term exposure to sulphuric acid

Glass Fibre-Epoxy (GF/Epoxy) composites are being the choice of material when exposed to inorganic acids such as sulphuric acid due to its superior corrosion resistance properties. Water diffusion into GF/Epoxy composites is well-reported. However, inter-relation between acid diffusion mechanism and its analogous influence on chemo-mechanical properties is never investigated with greater details. In this work, the transport behaviour of aqueous 10% sulphuric acid solution into GF/Epoxy composite and its complex inter-relations with the cross-linking density, morphology and mechanical properties is presented. The transport of aqueous solution of 10% sulphuric acid in the composite was quantified by gravimetric analysis and confirmed by the Energy Dispersive X-Ray Spectroscopy (EDX). The kinetic parameters were evaluated and compared using Fickian and Langmuir diffusion models, where the Langmuir model described the transport behaviour better. Analysis of thermomechanical properties such as storage modulus is characterised by dynamic mechanical thermal analysis (DMTA). The free volume distributions are estimated using positron annihilation lifetime spectroscopy (PALS). The indication of an increase in cross-link density with the acid exposure is further validated by Fourier-transform infrared spectroscopy (FT-IR) analysis. In the absence of substantial chemical degradation and with increased cross-linking density, the deterioration in the mechanical properties is attributed to the erosion of epoxy resin confirmed by the morphology performing Field Emission Scanning Electron Microscope (FE-SEM) analysis. The acid diffusion mechanism results show promising prospects of selecting and designing GF/Epoxy composites for industrial storage and manufacturing facilities containing aggressive environments

Norbornene-Functionalized Plant Oils for Biobased Thermoset Films and Binders of Silicon-Graphite Composite Electrodes.

We herein report the functionalization of plant oil with norbornene (NB) and subsequent polymerization to prepare biobased thermoset films and biobased binders for silicon/mesocarbon microbead (MCMB) composite electrodes for use in lithium-ion batteries. A series of NB-functionalized plant oils were prepared as biobased thermoset films via ring-opening metathesis polymerization (ROMP) in the presence of a second-generation Grubbs catalyst with tunable thermomechanical properties. Increasing the catalyst loading and cross-linking agent increased cross-link density, storage modulus (E′), and glass transition temperature (Tg), while the numbers of unreacted or oligomeric components in the films were reduced. High number of NB rings per triglyceride in the plant oil encouraged monomer incorporation to form a polymer network, therefore accounting for the high Tg and E′ values. Furthermore, the NB-functionalized plant oil and 2,5-norbornadiene (NBD) were copolymerized as bioderived binders for silicone/MCMB composite electrodes of lithium-ion batteries via ROMP during electrode preparation. Cell performance investigation showed that the silicone/MCMB composite electrode bearing the NBD-cross-linked NB-functionalized plant oil binder exhibited a higher C-rate and cycle-life performance than that using a conventional poly(vinylidene fluoride) (PVDF) binder. Finally, the electrode based on the bioderived binder exhibited a high specific charge capacity of 620 mA h g–1 at 0.5 C.

DEVELOPMENT OF BIPHENYL MONOMERS AND ASSOCIATED CROSSLINKED POLYMERS WITH INTRAMOLECULAR PI-PI INTERACTIONS.

Monomers containing biphenyl moieties were employed to create two sets of covalently crosslinked polymers that displayed noncovalent interactions in their 3-dimensional network. The biphenyls (precursors) used were 2-phenylphenol, 4-phenylphenol and 4,4'-dihydroxybiphenyl, and their acrylated forms were synthesized and named as 2-phenylphenolmonoacrylate (2PPMA), 4-phenylphenolmonoacrylate (4PPMA), and 4,4'-dihydroxybiphenyldiacrylate (44BDA), respectively. These were characterized by differential scanning calorimetry (DSC), nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR) to confirm the successful acrylation reaction. Polymers were synthesized via free radical polymerization reactions with varying crosslinker contents, and their network properties were characterized using swelling studies and compressive modulus tests. Interestingly, swelling studies did not show the expected decreasing swelling ratio with increasing crosslinker content, while compression testing indicated the expected trend of increasing modulus with increasing crosslinking density. The unexpected swelling results are hypothesized to result from the intramolecular interactions between the biphenyl side groups that result in noncovalent crosslinks.

Cross-Linking of Rubber Matrices with Dicumyl Peroxide and Zinc Dimethacrylate. Part I: Effect of Co-Agent Content

The work is focused on the investigation of the influence of the amount of co-agent on cross-linking and properties of rubber compounds. The in-situ radical polymerization of zinc dimethacrylate occurs during the curing process of rubber compounds with organic peroxide. Polymerized zinc dimethacrylate tends to form different types of chemical and physical linkages within the rubber matrices. As the amount of dimethacrylate ranged from 10 to 50 phr, the conversion of zinc dimethacrylate remained low. Residual zinc dimethacrylate formed micro-dispersion in the rubber matrix. The dispersion and distribution of zinc dimethacrylate in the rubber depends on the viscosity of matrix, shear stress and polarity of the matrix. Despite low conversion of zinc dimethacrylate, the application of co-agent resulted in the increase of cross-link density and improvement of physical-mechanical properties of vulcanizates.