Effect Of Crosslink Density Research Articles

Enhancing the antimony sorption properties of nano titania-chitosan beads using epichlorohydrin as the crosslinker

Antimony is classified as a pollutant of priority importance by USEPA. We have earlier reported the synthesis of nano-titania impregnated epichlorohydrin crosslinked chitosan (TA-Cts-Epi) beads, in a format suitable for large scale applications with high sorption capacity for antimony. To understand the sorption mechanism, and to fine tune the bead composition, the effect of crosslinking density on the swelling and sorption properties of the beads was investigated in detail. Epichlorohydrin effected significant changes in physical and sorption properties of the beads. The antimony sorption capacity of the TA-Cts-Epi beads prepared by crosslinking 0.3g non-crosslinked titania-chitosan beads (TA-Cts-NCL) with 6.4mmol epichlorohydrin was 493μmol/g, while those crosslinked with 0.64mmol showed a capacity of 133μmol/g. Whereas, TA-Cts-NCL beads showed a capacity of 75μmol/g. The increase in uptake capacity with increase in crosslinking demonstrated the active involvement of the epichlorohydrin moieties in antimony binding leading to enhanced sorption. Apart from altering the stability, swelling behaviour and sorption kinetics of the beads, crosslinking significantly increased the uptake of the anionic species via electrostatic interactions. Epichlorohydrin crosslinked chitosan beads prepared without TiO2 also showed similar behaviour. The results demonstrated the involvement of chitosan, TiO2 and epichlorohydrin in sorption.

Effects of Cross-Link Density on Structures and Properties of Dual-Sensitive Semi-Interpenetrating Polymer Networks Hydrogel Microspheres

Semi-interpenetrating polymer network (semi-IPN) strategy is employed to fabricate uniform microspheres with temperature and pH dual-responsive behavior, which is composed of poly(N-isopropylacrylamide) and poly(acrylic acid) in the presence of N,N-methylenebisacrylamide (MBA) as the cross-linker. The influences of MBA amount (M-m) on the structures and properties of the microspheres are investigated in terms of particle size, surface morphology, pH sensitivity, and thermo-sensitivity with low critical solution temperature in the range M-m = 2.5-15%. Additionally, functional group distributions in the microspheres are probed by titration and employing the Henderson-Hasselbalch equation. The results show that all the properties strongly change depending on M-m. Based on transmission electron microscopy and confocal laser scanning microscopy measurements, and these properties, the M-m-induced changes in the structures of the semi-IPN microspheres are discussed.

Synthesis of aminosilane crosslinked cationomeric waterborne polyurethane nanocomposites and its physicochemical properties

A series of cottonseed oil based waterborne polyurethane–silica (CSOWPU–Si) hybrid materials were prepared via the chemically interaction between NCO groups of polyurethane prepolymer and the surface of 3-aminopropyl trimethoxysilane (APTMS). The biobased triglyceride oils were first epoxidized cottonseed oil followed by oxirane ring-opened with varying polyhydroxy (OH) fatty acid numbers. The attached modified APTMS thus performed the dual roles not only chain extender filler but including reinforcing agent in the resulting hybrid coatings. Fourier transform infrared spectra expose the crosslinking interaction between polyurethane and the appearance of silica nanoparticles in the polymer matrix. The 29Si solid-state NMR spectra sign the formation and the behavior of siloxane linkage in the CSOWPU–Si nanocomposites films. Dynamic mechanical analysis (DMA) studies show that the appearance of silica nanoparticles provides rigidity to CSOWPU–Si film, thereby enhancing the storage modulus. The effect of crosslinking density and thermophysical and mechanical properties of the polyurethane and the resulting hybrid materials films was evaluated by the differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), and measurement of the mechanical properties. This work renders a new effective and promising route of utilizing biorenewable for the development of CSOWPU–Si hybrid materials with high performance coating applications.

Effect of crosslinking density on adhesion performance and flexibility properties of acrylic pressure sensitive adhesives for flexible display applications

The use of acrylic pressure sensitive adhesives (PSAs) in flexible displays involves their attachment to each layered device. Due to the high industrial demand of flexible displays, acrylic PSAs must necessarily exhibit high flexibility. In this study, the effect of the crosslinking density of acrylic PSAs on their adhesion and flexibility properties has been investigated by incorporating a diisocyanate crosslinking agent into the PSA structure. As the content of the crosslinking agent increased, the measured peel strength and tack of the synthesized PSAs decreased, while the maximum value of the lap shear stress increased. In addition, the maximum stress and shear strain determined for the crosslinked PSA specimens decreased with an increase in the crosslinking agent concentration (although, the stress measured at low strain values was initially increasing until the crosslinking agent content reached 1 phr). The results of stress relaxation testing showed a stress increase at specified strain levels with an increase in the crosslinking agent content up to 0.5 phr, while the results of creep testing revealed that the measured strain was inversely proportional to the degree of elastic recovery. The obtained data indicate that crosslinking generally improves the PSA properties related to their use in flexible display applications; however, very high crosslinking densities produce mostly a negative effect on the PSA flexibility and adhesion characteristics.

Effect of Cross-linking Density on Creep and Recovery Behavior in Epoxy-Based Shape Memory Polymers (SMEPs) for Structural Applications

Epoxy-based shape memory polymers (SMEPs) are gaining importance in the area of aerospace structures due to their high strength and stiffness which is a primary requirement for an SMEP in structural applications. The understanding of viscoelastic behavior of SMEPs is very essential to assess their shape memory effect. In the present work, three types of SMEPs with varying cross-linking densities were developed by curing an aromatic epoxy resin with aliphatic amines. Glass transition temperature (Tg) was measured for these SMEPs using advanced rheometric expansion system, and from the Tg measurements, a range of temperatures from glassy to rubbery regimes were chosen. At selected temperatures, creep-recovery tests were performed in order to evaluate the viscoelastic behavior of SMEPs and also to investigate the effect of temperature on creep-recovery. Further, a three-parameter viscoelastic model (Zener) was used to fit the data obtained from experiments. Model parameters like moduli of the springs and viscosity of the dashpot were evaluated by curve fitting. Results revealed that Zener model was well suited to describe the viscoelastic behavior of SMEPs as a function of test temperatures.

Hydrogels reinforced with nanoclays with improved response rate

ABSTRACTTwo series of semiinterpenetrating networks (SIPN) based on linear hydrophilic poly(vinyl alcohol) (PVA) and thermo‐responsive poly(N‐isopropylacrylamide) (PNIPA), physically crosslinked with inorganic clay, are presented. The hydrogels with different crosslinking densities were prepared by varying the content of clay from 1 to 6 wt % and contained linear interpenetrant, PVA in the range of 0.5–1.5 wt %. The effect of clay content on swelling/deswelling behavior and phase transition in PNIPA gels, as well as the feasibility of reinforcing the gels with high molecular weight PVA, were analyzed. The thermal response of hydrogels, followed by DSC, confirmed that the insertion of hydrophilic PVA did not have a significant effect on the onset of the volume phase transition temperature, while the response was faster. The equilibrium degree of swelling of SIPNs and PNIPA hydrogels was in the range of 9–79 and decreased with increasing content of clay. The internal morphology and surface wettability of the hydrogels were investigated by scanning electron microscope analysis and contact angle measurements, respectively. The network structural parameters of the PNIPA and SIPN nanocomposites hydrogels, such as the average molecular weight between crosslinks, Mc, and effective crosslinking density, Ne, were determined by dynamic mechanical analysis. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44535.

Renewable Epoxy Networks Derived from Lignin-Based Monomers: Effect of Cross-Linking Density

Three modification methods, which either improved molecular weight, orientation or the number of functional groups, were employed to increase the cross-linking density of biobased epoxy networks based on 2-methoxy-4-propylphenol (dihydroeugenol, DHE). The modifications were realized through o-demethylation and phenol–formaldehyde reactions. Structures of DHE-based monomers and cured networks were characterized by NMR and FTIR spectroscopy. Cross-linking densities of cured networks were calculated using rubber elasticity theory from dynamic mechanical analysis (DMA). Networks with higher cross-link density were found to exhibit greater mechanical and thermal performance as measured by DMA and thermogravimetric analysis. GEDHEO-NOVO, an epoxy monomer featuring all three modification processes, exhibited significant improvements in cross-linking density (0.39 to 9.77 mol/dm3), α-relaxation temperature (Tα, 40 to 139 °C) and statistic heat-resistant index temperature (Ts, 125 to 153 °C) compared to the unmodi...

Poly (acrylic acid-co-acrylamide)/cellulose nanofibrils nanocomposite hydrogels: effects of CNFs content on the hydrogel properties

Nanocomposite hydrogels based on cellulose nanofibrils (CNFs) and poly(acrylic acid-co-acrylamide) were synthesized via in situ free radical polymerization in an aqueous suspension of CNFs. Effects of the inclusion of CNFs with content up to 10 wt% on the swelling properties at different pH and on the compression strength were investigated. The presence of CNFs was shown to strongly reinforce the hydrogel without deteriorating its elasticity. Nanocomposite hydrogel with a content of 10 % CNF supported respectively a strength and deformation about 13 and 2 times higher than that of the neat hydrogel. This effect was explained by the grafting of polymers on the CNFs surface, contributing to an increase in the effective crosslinking density, and improving the interfacial adhesion between polymer chains and CNFs. Moreover, the addition of CNF enhanced the water holding capacity of the hydrogel and caused the system to release urea simulating a fertilizer in a more controlled manner than that with neat hydrogel.

Interpenetrating anion exchange membranes using poly(1-vinylimidazole) as bifunctional crosslinker for fuel cells

There is a balance between ionic conductivity and dimensional stability in anion exchange membranes (AEMs). High conductivity resulting from densely charged groups is normally accompanied with high water uptake and excessive swelling. Herein, a series of novel AEMs with interpenetrating network was prepared by the incorporation of poly(vinylbenzyl chloride) (PVBC) and poly(1-vinylimidazole)(PVIm) into the PVA matrix. PVBC and PVIm act as bifunctional macromolecular crosslinking agents to provide both imidazolium cations and to form crosslinked network in the membranes. The resulting AEMs were characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The effect of crosslinking density on conductivity, water uptake, swelling ratio and stability was investigated. The results reveal that the addition of PVBC-c-PVIm could be simultaneously conducive to higher conductivity and lower swelling ratio. Notably, the membrane containing 50wt% PVBC-c-PVIm had IEC of 1.86meqg−1, water uptake of 39.4%, and hydroxide conductivity of 21.9mScm−1 at 30°C. A maximum power density of 37.1mWcm−2 is achieved at a current density of 64mAcm−2. The crosslinked AEMs also exhibit excellent alkaline stability in a 4M KOH solution at 60°C for 1320h.

The effect of crosslinking density on molecularly imprinted polymer morphology and recognition

In this report, the crosslinking density of bupivacaine molecularly imprinted methacrylic acid (MAA)–ethylene glycol dimethacrylate (EGDMA) copolymers was investigated through replacement of EGDMA by methyl methacrylate (MMA). The effects were examined using a series of full-scale MD simulations of pre-polymerization mixtures, equilibrium rebinding studies on the corresponding synthesized polymers and morphology characterization through nitrogen sorption measurements. While the extent of hydrogen bonding between the functional monomer MAA and bupivacaine observed in the MD pre-polymerization mixtures was comparable in each of the systems studied, the decrease in degree of crosslinking impacted directly on polymer morphology as observed in BET and BJH studies of surface area and porosity. Further, decreases in the crosslinking density induced reductions in template rebinding capacity as seen from a series of radio-ligand binding studies, demonstrating the importance of crosslinking on the performance of molecularly imprinted MAA–EGDMA copolymers, the polymer system most commonly used in molecular imprinting science and technology.

Molecular simulation guided constitutive modeling on finite strain viscoelasticity of elastomers

Viscoelasticity characterizes the most important mechanical behavior of elastomers. Understanding the viscoelasticity, especially finite strain viscoelasticity, of elastomers is the key for continuation of their dedicated use in industrial applications. In this work, we present a mechanistic and physics-based constitutive model to describe and design the finite strain viscoelastic behavior of elastomers. Mathematically, the viscoelasticity of elastomers has been decomposed into hyperelastic and viscous parts, which are attributed to the nonlinear deformation of the cross-linked polymer network and the diffusion of free chains, respectively. The hyperelastic deformation of a cross-linked polymer network is governed by the cross-linking density, the molecular weight of the polymer strands between cross-linkages, and the amount of entanglements between different chains, which we observe through large scale molecular dynamics (MD) simulations. Moreover, a recently developed non-affine network model (Davidson and Goulbourne, 2013) is confirmed in the current work to be able to capture these key physical mechanisms using MD simulation. The energy dissipation during a loading and unloading process of elastomers is governed by the diffusion of free chains, which can be understood through their reptation dynamics. The viscous stress can be formulated using the classical tube model (Doi and Edwards, 1986); however, it cannot be used to capture the energy dissipation during finite deformation. By considering the tube deformation during this process, as observed from the MD simulations, we propose a modified tube model to account for the finite deformation behavior of free chains. Combing the non-affine network model for hyperelasticity and modified tube model for viscosity, both understood by molecular simulations, we develop a mechanism-based constitutive model for finite strain viscoelasticity of elastomers. All the parameters in the proposed constitutive model have physical meanings, which are signatures of polymer chemistry, physics or dynamics. Therefore, parametric materials design concepts can be easily gleaned from the model, which is also demonstrated in this study. The finite strain viscoelasticity obtained from our simulations agrees qualitatively with experimental data on both un-vulcanized and vulcanized rubbers, which captures the effects of cross-linking density, the molecular weight of the polymer chain and the strain rate.

Influence of Novel Crosslinker on the Properties of the Degradable Thermosensitive Hydrogels

SummaryA novel degradable crosslinker, N, N′‐bis(acryloyl) cystamine (BACy), was synthesized with cystamine dihydrochloride (Cy) and acryloyl chloride. Then three series of degradable thermosensitive hydrogels with different crosslinker contents were prepared from N‐isopropyl acrylamide (NIPAAm), BACy (Bx‐series) or N, N′‐methylene‐bis‐acrylamide (NMBA)(Nx‐series) or equimolar BACy and NMBA (BNx‐series) as crosslinker, and diethoxyacetophenone (DEAP) as photoinitiator under UV‐irradiation polymerization. The effect of crosslinker content on the properties of the hydrogels such as swelling behavior and mechanical properties was investigated. At the same time, their degradabilities were investigated by dissolving them into different concentrations of cysteine solution. The results show that the swelling ratios for the Bx‐series hydrogels are lower than those for the Nx‐series hydrogels. But, after degradation in the cysteine solution, the swelling ratios for the Bx‐series hydrogels were greater than those for the Nx‐series hydrogels. When the crosslinker content was increased, all values of shear stress (τ), shear modulus (G), and effective crosslinking density (ρX) increased. But, the ρX values for the reduced Bx series and BNx series hydrogels were lower than those for original hydrogels. Results also showed that the ρX values for the Nx series hydrogel were not affected by cysteine. The disulfide bonds of BACy in these novel hydrogels were decomposed (reduced) in the cysteine solution and converted into thiol group. These thiol groups could be used to absorb some metal ions such as silver ion. Hence, the absorption of silver ion for the hydrogel with thiol group was also investigated in this research.

Nanoporous Polymers from Cross-Linked Polymer Precursors viatert-Butyl Group Deprotection and Their Carbon Dioxide Capture Properties

A two-step synthetic strategy has been developed to achieve functionalized nanoporous polymers via the deprotection of the cross-linked polymer precursors containing tert-butyl carboxylate-functionalized stilbene or styrene and N-phenylmaleimide alternating sequences. Three different deprotection methods to generate nanoporosity were examined in this work. The resulting nanoporous polymers showed a significant increase in BET surface area. The effects of cross-linking density and the stiffness of the alternating sequences on the nanoporosity of these polymers were studied. The resulting nanoporous polymers were also investigated as potential CO2 sorbents.

Effect of crosslinking density of polymers and chemical structure of amine-containing monomers on the neutralization capacity of dentin adhesives

ObjectivesNeutralization of the acidic micro-environment at the tooth/material interface is expected to provide enhanced durability for dental composite restorations. The objective of this study is to explore the effect of amine-containing monomer formulations and the crosslinking density of the resultant polymers on the neutralization capacity. Materials and methodsThe co-monomer system was varied systematically to obtain different proportions of Bisphenol A glycerolate dimethacrylate (BisGMA) and 2-hydroxyethyl methacrylate (HEMA), while maintaining a constant amount of amine-containing methacrylate monomer. A series of amine-containing monomers covering a range of pKa values were examined. Crosslinking density of formed copolymers was controlled by adjusting the weight content of the dimethacrylate monomer BisGMA. Lactic acid (LA) was used as a probe to analyze the effectiveness of the basic polymers to neutralize acid. The neutralization capacity of each amine-containing crosslinked copolymer was characterized by measuring pH as a function of time when the specimens were soaked in 1-mM LA solution, and the results were compared to the control formulations composed solely of BisGMA and HEMA. Polymer surfaces were examined using the methyl orange (MO) assay to quantify the amount of accessible amine groups. ResultsFor each amine-containing crosslinked co-polymer, the neutralization capacity is enhanced by decreasing crosslinking density (e.g., by reducing BisGMA concentration in the formulation). In addition, more amine groups were accessible when crosslinking density was decreased. For different amine-containing polymers with the same BisGMA concentration, the neutralization capacity is higher when the amino monomers with higher pKa values were used in the formulations. SignificanceThis is the first time that the neutralization capacity based on crosslinked dental polymers has been studied. The information is important for future development of durable dentin adhesives.

Dual responsive antibacterial Ag-poly(N-isopropylacrylamide/itaconic acid) hydrogel nanocomposites synthesized by gamma irradiation

Thermo- and pH-sensitive Ag-P(NiPAAm/IA) hydrogel nanocomposites were prepared by in situ reduction of Ag+ ions with gamma irradiation. P(NiPAAm/IA) copolymer hydrogel matrices have been previously synthesized by radiation induced copolymerization. The copolymer composition was determined by elemental analysis of the dried gels, and was found to be close to that of the feed. The dependence of swelling properties and phase transitions on the IA comonomer content, temperature, pH values and presence of AgNPs were investigated. The hydrogels showed both thermo- and pH responses. By introducing small amounts of IA, swelling capacity increase, and the volume phase transition temperature (VPTT) of P(NiPAAm/IA) copolymers is transferred to higher values. Presence of AgNPs causes decrease of equilibrium swelling degree and VPTT, and allows of fine-tuning of these properties. Both the average values of compressive elastic modulus and Young’s modulus of elasticity were decreased by addition of IA as a consequence of looser structure of the network (increase in molar mass between two neighboring crosslinks and decrease in effective crosslinking density). The introduction of AgNPs promoted not only improvement of these mechanical parameters, but an improvement of stress at failure especially in the systems with the highest content of IA. In addition, Ag-P(NiPAAm/IA) hydrogel nanocomposites showed good antibacterial potentials.

페닐기 함유 열경화성 폴리유기실록산 코팅제 특성

Dimethyldimethoxysilane(DMDMS), methyltrimethoxysilane(MTMS)과 phenyltrimethoxysilane(PTMS)을 이용하여 폴리유기실록산 열 경화성 코팅액을 제조하고 가교도와 페닐기 함량이 코팅액과 도막 물성에 미치는 영향을 살펴보았다. Si-NMR 측정에서 얻어진 D <TEX>$T^{Me}$</TEX> <TEX>$T^{Ph}$</TEX> 구조 코팅액 조성비가 사용된 DMDMS, MTMS, PTMS의 몰 비로부터 얻어진 이론치와 일치함을 확인할 수 있었다. 가교밀도가 높을수록 그리고 페닐 함량이 낮을수록 실록산 결합의 증가에 의하여 코팅액 분자량이 증가됨을 알 수 있으며 페닐기 증가는 코팅액 굴절률 증가에 매우 큰 영향을 미침을 알 수 있었다. 가교밀도가 높을수록, 페닐기 함량이 감소할수록 코팅도막 경도는 증가되었으며 내열특성 또한 우수해짐을 확인할 수 있었다. 이상의 결과로부터 같은 실록산 결합을 갖는 폴리유기실록산 코팅제라도 치환도에 따른 가교도 조절과 페닐기 함량에 따라 코팅액 물성이 달라짐을 확인할 수 있었다. Polyorganosiloxane having controlled cross-linking density and phenyl group content were prepared by dimethyldimethoxysilane (DMDMS), methyltrimethoxysilane (MTMS) and phenyltrimethoxysilane (PTMS). The effect of cross-linking density and phenyl group content on the physical properties of siloxane resin and its coated film have been invetigated. Si-NMR results confirmed that synthesized siloxane resins have equivalent D <TEX>$T^{Me}$</TEX> <TEX>$T^{Ph}$</TEX> structures according to applied mole ratios of DMDMS, MTMS and PTMS. Polyorganosiloxane having higher cross-linking density with high phenyl content showed the high molecular weight and increasing phenyl content resulted in higher refractive index as well as better thermal stability. Cross-linking density is more important factor than phenyl content to obtain higher pencil hardness of coated film on the glass. Our results concluded that even polyorganosiloxanes having similar siloxane structures show different physical properties as function of cross-linking density and phenyl content in polyorganosiloxane.

Shape-Memory Polymers with Adjustable High Glass Transition Temperatures

Shape-memory polymers (SMPs) are synthesized with adjustable glass transition temperature (Tg) ranging from 299 to 322 °C, higher than those reported previously. The polyimide containing thermal stable but flexible linkages within the backbone act as reversible phase, and high molecular weight (Mn) is necessary to form physical cross-links as fixed phase of thermoplastic shape-memory polyimide. The critical Mn is 21.3 kg/mol, and the relationship between Mn and Tg is explored. Thermoset polyimides show higher storage modulus and better shape-memory effects than thermoplastic counterparts due to covalent cross-linking, and the effective cross-link density with the influence on their physical properties is studied. The mechanism of high-temperature shape-memory effect of polyimide on the basis of chain flexibility, molecular weight, and cross-link density is proposed, which will benefit further research on high-temperature SMPs.

Nitro-Group Functionalization of Dopamine and its Contribution to the Viscoelastic Properties of Catechol-Containing Nanocomposite Hydrogels.

Linear polyacrylamide (PAAm) is modified with dopamine or nitrodopamine (PAAm-D and PAAm-ND, respectively) to evaluate the effect of nitro-group modification on the interfacial binding properties of polymer-bound catechol. Nanocomposite hydrogels are prepared by mixing PAAm-based polymers with Laponite and the viscoelastic properties of these materials are determined using oscillatory rheometry. The incorporation of a small amount of catechol (≈0.1 wt% in swollen hydrogel) drastically increases the shear moduli by 1-2 orders of magnitude over those of the catechol-free control. Additionally, PAAm-ND exhibits higher shear moduli values than PAAm-D across the whole pH range tested (pH 3.0-9.0). Based on the calculated effective crosslinking density, effective functionality, and molecular weight between crosslinks, nitro-group functionalization of dopamine results in a polymer network with increased crosslinking density and crosslinking points with higher functionality. Nitro-functionalization enhances the interfacial binding property of dopamine and increases its resistant to oxidation, which results in nanocomposite hydrogels with enhanced stiffness and a viscous dissipation property.

Effect of Cross-Linking Density of Silicone Encapsulant on Sulfur Compound Gas Permeability of Light-Emitting Diode

To study the effect of cross-linking density of silicone encapsulant on gas permeability, a light-emitting diode (LED) package was produced with two kinds of silicone, methyl-group (silicone A) and phenyl-group (silicone B), and a sulfur compound gas permeability test of the fabricated packages was performed. We found that the main penetration route of sulfur compound gas into the Ag-plated layer in an LED package could be directly observed by inserting Ag wire in an encapsulant. The persistency rate of radiant flux increased as the curing time of the silicone encapsulant became longer. When electron beam is irradiated on an LED package, the persistency of radiant flux is also increased.

Biobased polyurethanes prepared from different vegetable oils.

In this study, a series of biobased polyols were prepared from olive, canola, grape seed, linseed, and castor oil using a novel, solvent/catalyst-free synthetic method. The biobased triglyceride oils were first oxidized into epoxidized vegetable oils with formic acid and hydrogen peroxide, followed by ring-opening reaction with castor oil fatty acid. The molecular structures of the polyols and the resulting polyurethane were characterized. The effects of cross-linking density and the structures of polyols on the thermal, mechanical, and shape memory properties of the polyurethanes were also investigated.