Effect Of Crosslink Density Research Articles

Effect of Loop Defects on the High Strain Rate Behavior of PEGDA Hydrogels: A Molecular Dynamics Study.

The high strain rate behavior of nonideal poly(ethylene glycol) diacrylate hydrogels under uniaxial tension and transient-state shear deformations is investigated using molecular dynamics (MD) simulations. This work specifically focuses on the influence of first-order loop defects, including their effect on topological evolutions. Two approaches are proposed to systematically introduce first-order loops, allowing separate and controllable investigations of effective cross-link functionality and cross-link density. MD simulations confirm that first-order loop defects are elastically inactive, but the topological disruptions caused by the presence of loop defects influence mechanical behavior. For decreasing effective cross-link functionality but constant cross-link density, a weaker tensile stress-strain response and decreasing shear-thickening behavior are observed. This is due to nonaffine translation of cross-link junction positions during deformation. Hydrogels with lower cross-link density but constant effective functionality show a stronger stress-strain response and an earlier transition between entropic and enthalpic deformation regimes. This behavior is correlated to changes in mesh size caused by the introduction of loops within an elastically active network. However, the resulting range of cross-link densities is not sufficient to cause measurable changes in shear-thickening behavior. To conclude, reductions in effective cross-link functionality are more important to high strain rate behavior than reductions in cross-link density.

Effect of crosslinking density on thermoresponsive nanogels: A study on the size control and the kinetics release of biomacromolecules

Thermoresponsive nanogels emerged as a robust and efficient platform for the delivery of a number of therapeutic agents. The need to deliver a wide range of biomolecules from different sizes requires a structural system with the ability to be tuned according to the specific applications. In this study, we design a library of nanogels using dendritic polyglycerol (dPG) as a crosslinker and poly (N-isopropyl acrylamide) as connecting thermoresponsive chains. Nanogels with diameters in a range of 70–210 nm were obtained by precipitation polymerization with low polydispersity. We provide a detailed study on how the flexibility of the network and size of the nanogels can be tuned by the modification of functionalities of the dPG, monomer feed and the size of the crosslinker. Additionally, we use different molecular weight proteins (6–66 kDa) as an indicator of the density of the structure by encapsulation and release experiments. We prove that the versatility of the system relays on the correct design of the nanogels and the careful selection of the starting materials and its final proportion in the reaction. By controlling the network density, a wider range of application can be achieved from a system with a short-term release triggered by temperature to a long-term release that could serve as reservoir platform.

Charge-balanced terpolymer poly(diethylaminoethyl methacrylate-hydroxyethyl methacrylate-2-acrylamido-2-methyl-propanesulfonic acid) hydrogels and cryogels: scaling parameters and correlation with composition.

The scaling laws relating the preparation conditions to the swelling degree, reduced modulus and effective crosslinking density of poly(diethylaminoethyl methacrylate-co-hydroxyethyl methacrylate-co-2-acrylamido-2-methyl-propanesulfonic acid), henceforth designated as PDHA, gels prepared by radical crosslinking copolymerization in a solvent mixture were reported. Charge-balanced terpolymer PDHA hydrogels and cryogels (PDHA-Hgs and Cgs) were prepared in different monomer feed compositions. The swelling dependence of the reduced modulus was described by a power law relationship Gr≈ (φV)m with an exponent of m = -0.30 at low swelling degree, while in the high swelling region the scaling becomes 0.21, indicating the finite extensibility of the network chains. The scaling exponent for the swelling degree and terpolymer composition, φV≈ (Nν)m, was found to be -0.13, indicating the increasing extent of the topological constraints arising from the trapped entanglements. By combining elasticity and swelling results, the scaling relationship between the apparent crosslink density and HEMA content used in the terpolymer feed was obtained as a cubic polynomial of the mol% of HEMA. In the HEMA-rich terpolymer PDHA Hgs and Cgs, the swelling degree was possibly controlled by the HEMA part of the terpolymer network, while the presence of DEAEM units in the network triggered the thermoresponsive swelling behavior. The dependence of interaction parameter χ on the volume fraction of the crosslinked terpolymer network in the swollen gel ν2 was evaluated and the results revealed extremely strong concentration dependence of χ for all terpolymer samples. Because of their inherent properties, the resulting terpolymer gels might contribute to the improvement of the loading capacity of polymers used in anticancer drug delivery systems.

Fair Investigation of Cross-Link Density Effects on the Bond-Exchange Properties for Trans-Esterification-Based Vitrimers with Identical Concentrations of Reactive Groups

In this research, we demonstrate effects of cross-link density on the bond-exchange property of trans-esterification-based vitrimers. In most of the past literature, the change of cross-link densit...

Experimental Evidence for Universal Behavior of Ion-Induced Volume Phase Transition in Sodium Polyacrylate Gels.

Introduction of high valence counterions into polyelectrolyte gels results in a reversible volume phase transition. In the present work new experimental results are reported for the volume transition induced by calcium/sodium exchange in sodium polyacrylate gels. The effects of cross-link density, concentration of ionized groups on the network chains, composition of the equilibrium salt solution containing both mono- and divalent cations, and temperature on the swelling degree of these gels are systematically investigated. It is demonstrated that the normalized swelling data fall on a master curve, indicating that the ion-exchange-induced volume transition exhibits universal behavior in sodium polyacrylate gels. Model calculations made on the basis of the classical Flory-Rehner theory are in reasonable agreement with the measured dependencies.

Charge‐Functionalized and Structurally Enhanced (Cryo)Hybrids from Acrylic (Co)Monomers and Kaolin: Altering Physicochemical Properties via Cryotropic Gelation

ABSTRACTThe potential to improve mechanical, structural, and mechanochemical properties of charge‐functionalized poly(N,N‐dimethylaminoethyl methacrylate) (PDMAEMA)‐based hybrid cryogels is investigated. The simple and versatile synthesis of hybrid cryogels with high strength and toughness using cationic DMAEMA and ionic comonomer 2‐acrylamido‐2‐methyl‐1‐propane sulfonic acid has been proposed via in situ free‐radical crosslinking (cryo)polymerization by which the properties of virgin polymer can be modulated to required applications by incorporation of inorganic filler kaolin (KLN). Two factors affecting swelling and elasticity of hybrid gels (referred as PDA/KLNm), KLN content and gel preparation temperature, are studied. The optimum KLN concentration for desired swelling and modulus of elasticity is determined as 0.80% (w/v). Effective crosslinking density of hybrid hydrogels increases with KLN addition and this dependence is expressed by a quadratic polynomial as a function of KLN concentration. The results show that obtained hybrid gels with multiresponsive properties could be regarded as “smart materials” in sensing and actuation applications. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 1758–1778

Diffusion coefficient of solute in heterogeneous and macroporous hydrogels and its correlation with the effective crosslinking density

The diffusion coefficient, Dg, of a solute in hydrogels is a key parameter in their applications. Dg has been correlated with the volume fraction, φp, of polymer in gel. In this study, the effective crosslinking density, νe, was determined from the rubber network theory using compression tests, and was applied as a model parameter for the characterization of the polymer networks and gel structures. It was subsequently examined to estimate Dg for various types of gels, including heterogeneous and macroporous gels. The Dg values of NaCl and trisodium citrate were determined with a permeability test using the gel membrane. The diffusion of the solute in all the tested gels was accurately represented by both conventional (Dg/D0 = exp(-kφpn)) and novel (Dg/D0 = exp(-aνeb)) models.

Influence of gel preparation concentration on statistical mechanics of poly(dialkylaminoethyl methacrylate) gels on the basis of scaling concept: Toward tunable elasticity and thermomechanical parameters

ABSTRACTIn order to evaluate the scaling parameters of weakly cationic crosslinked network structures, poly(dialkylaminoethyl methacrylate)‐based hydrogels were synthesized via free‐radical crosslinking in aqueous solution varying systematically concentration of pregel solution. Based on the gel‐preparation concentration, variation in structural properties, effective crosslinking density, average molecular weight of polymer chains, and thermodynamic parameters from combined swelling and elasticity results were discussed using the scaling theory to predict various exponent identities. The concentration dependence of compressive elastic modulus as‐prepared state was described by a power‐law relationship with the exponent of m = 3.55 indicating the importance of the trapped entanglements. Two structural characteristics, the network chain length N and the average molecular weight of polymer chains have inverse dependence on the gel‐preparation concentration in the matrix, while the compressive moduli and effective crosslinking density show completely direct dependence. Experimentally determined N values of PDMAEMA hydrogels first decrease with increasing up to 0.2972 and the dependence of N on the gel‐preparation concentration gives the relation with a scaling parameter n = −1.80, which coincides with the prediction of scaling theory. Acceptable agreement was found between the estimate of crosslink density fluctuations deduced from mechanical measurements and the results derived from independent swelling observations. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48350.

Color-Switchable Polar Polymeric Materials.

Spiropyran is an important mechanophore, which has rarely been incorporated as a cross-linker in polar polymer matrices, limiting its applications in innovative mechanochromic devices. Here, three spiropyrans with two- or three-attachment positions were synthesized and covalently bonded in polar poly(hydroxyethyl acrylate) (PHEA), to achieve color-switchable materials, triggered by light and when swollen in water. The negative photochromism in the dark and mechanical activation by swelling in water were investigated. Measurements of negative photochromism were conducted in solution and cross-linked PHEA bulk polymers, with both showing color reversibility when stored in the dark or on exposure to visible light. The force of swelling in water was sufficient to induce the ring-opening reaction of spiropyran. It was found that tri-substituted spiropyran (SP3) was less influenced by the polar matrix but showed the fastest color activation during swelling. SP3 also showed accelerated ring opening to the colored state during the swelling process. Bleaching rates and color switchability were investigated under swollen and dehydrated conditions. The effect of cross-link density on the swelling activation was explored to better understand the interaction between the mechanophore and the polar environment. The results demonstrated that influences from both the polar environment and the mechanochromic nature of spiropyran had an impact on the absorption intensity, rate of change, and the decoloration rate of the materials. This study provides the opportunity to manipulate the properties of spiropyrans to afford materials with a range of color-switching properties under different stimuli.

Scratch behavior of model epoxy resins with different crosslinking densities

The scratch behavior of model epoxy resins with varying degrees of crosslink density was investigated according to the ASTM D7027/ISO 19252 scratch test methodology. Three model epoxy systems with low, medium, and high crosslinking densities were prepared to systematically investigate how the scratch behavior might be influenced. The effect of crosslinking density on the onset of groove, crack and plowing formation during scratching was investigated. Coefficient of friction measurements, uniaxial tensile and compressive stress-strain curves, and dynamic mechanical analyses were conducted to correlate the intrinsic material properties to the observed scratch-induced deformation of the model epoxy systems. The findings indicate that the scratch behavior of epoxy resins can be directly correlated to their material properties. It is found that both the tensile strength and compressive yield stress determine the resistance against damage formation during scratch. Additionally, a comparison between pencil hardness and scratch test is presented to assess the suitability of both methods for evaluating scratch performance of polymers with significantly different mechanical properties.

The effect of cross-linking density on water vapor and oxygen permeability of hybrid UV cured nano coatings

In this study, the effect of cross-linking density of organic contents, precursor ratio and proportion of inorganic to organic phases on water and oxygen permeability of sol-gel based hybrid UV cured coating on biaxially oriented polypropylene (BOPP) substrate was evaluated. Multi-functional acrylic monomers including, Isobornyl acrlate (IBOA), Tripropylene glycol diacrylate (TPGDA), Trimethylol propane triacrylate (TMPTA), Di-penta-erythritol hexa-acrlate (DPHA) and tri-functional aliphatic urethan acrylate oligomer were used as organic phase, while inorganic phase contained tetraethyl orthosilicate (TEOS) and 3-methacryloxy propyltrimethoxy silane (MEMO) precursors. A mixture technique utilized as designing the experimental processes. A high cross-linked hybrid enhanced the barrier property and reduced considerably both the water vapor transmission rate (WVTR) and oxygen transmission rate (OTR) of uncoated BOPP from 88 ± 0.89, to 0.88 ± 0.05 (g / m2.day) and from 2010 ± 26–317 ± 11 (cm3 / m2.day), respectively. It was observed that in constant TEOS to MEMO ratio (I/O = 50), by altering the ratio of inorganic /organic from 20 to 80 water vapor depreciated from 3.9 ± 0.10 to 0.88 ± 0.05 g/ (m2.d). These results were in agreement with the presence of a dense network. Phase images of atomic force microscopy analysis showed a homogenous distribution of inorganic particles in the organic network. The presence of a high cross-linked organic phase can replace the presence of 30% of an inorganic phase in hybrid coatings. The correlation between WVTR and the multiplication of the surface energy (ε) in the cross-linking density (ʋ) has shown that increasing the diffusion coefficient had considerable influence on reduction of WVTR than increasing the solubility coefficient.

Molecular Modeling of Complex Cross-Linked Networks of PEGDA Nanogels.

Poly(ethylene glycol) (PEG)-based nanogels are attractive for biomedical applications due to their biocompatibility, versatile end group chemistry, and ability to sterically shield encapsulated drug molecules. The characteristics of a hydrogel network govern the encapsulation and efficient delivery of drug molecules for a target application. A molecular-level description of network topology can complement experimental investigations to understand its effects on the structural properties of these nanogels. In this work, atomistic molecular simulations of heterogeneous, nonideal PEG-diacrylate (PEGDA) nanogels are presented. The effects of cross-linking density and topological features on the structural properties of PEGDA nanogels were studied. The average functionality was controlled to systematically study the effect of cross-linking density on the radius of gyration, shape, and mesh size of the nanogels. For a given average functionality, the impact of distinct network topologies on the structural properties was also studied. The aspect ratios, based on the gyration tensor, were calculated to characterize the shapes of these nanogels for different topologies. Nanogel structures with higher cross-linking densities showed a globular shape, while structures with lower cross-linking density showed shape anisotropy. The distribution and connectivity of the cross-linked junctions played a key role in determining the size and shape anisotropy of PEGDA nanogels; the number of unreacted chain ends and their connectivity directly affected the anisotropy. The mesh size, denoted by the limiting "free volume element" present in the nanogel samples, does not show a significant change with increasing average functionality. This work provides insight into the structural properties of heterogeneous hydrogels that aid the design of nonideal nanogel networks for a targeted drug delivery application.

In situ synthesis of magnetic iron oxide nanoparticles in chitosan hydrogels as a reaction field: Effect of cross-linking density

Magnetic iron oxides such as magnetite and γ-hematite have attracted considerable attention as thermoseeds for hyperthermia treatment because of their ability to generate heat under an alternating magnetic field. Control of the particle size and their combination with biocompatible polymers are expected to be beneficial for optimization of the nanoparticles. These processes can be accomplished through the synthesis of magnetite in gels, as the network structure of the polymer gel can control the grain growth of the magnetite. However, the effect of the cross-linking density of the gels remains unclear. In this study, we synthesized magnetic iron oxides in situ in chitosan hydrogels with different cross-linking densities and examined the crystalline structure and heat generation under alternating magnetic field. The crystalline phase and amount of magnetite were observed to be dependent on the cross-linking density of the gel, and the heat generation of the nanoparticles was governed by their crystalline structure and particle size rather than solely the amount of formed iron oxide.

Insight into (alkyl)methacrylate-based copolymer/sepiolite nanocomposite cryogels containing amino and sulfonic acid groups: Optimization of network properties and elasticity via cryogelation process

(Alkyl)methacrylate-based copolymer/sepiolite nanocomposite gels containing amino and sulfonic acid groups have been prepared via (cryo) gelation of N,N-dimethylaminoethyl methacrylate (DMAEMA) and 2-acrylamido-2-methyl-propanosulfonic acid (AMPS) in the presence of fibrous clay mineral sepiolite (SEP). The morphology, thermal stability, compression performance, and swelling behavior of nanocomposite gels were systematically studied with various SEP contents. X-ray diffractometry results indicated that SEP had been participated in (cryo)gelation, and the resulting thermal stability and surface morphology were improved by the addition of inorganic clay SEP. Owing to dispersion of SEP, the prepared nanocomposite cryogels exhibited dramatic improvements on their elasticity while low water absorption capacity was maintained. The presence of low amount of SEP significantly enhanced the mechanical strength and the cross-link density of DMAEMA-based nanocomposite gels due to its fibrous morphology and crystalline structure acting as cross-links. Addition of 0.4% (w/v) SEP led to five-fold increase in the compressive modulus of nanocomposite cryogels. As SEP concentration in the feed was increased from 0 to 2.40% (w/v), the effective cross-link density in the nanocomposite matrix increased continuously from 2.84 to 22.9 × 103 mol/m3. The prepared nanocomposite gels showed enhanced swelling response and on-off switching characteristics in aqueous KBr solutions. The parameters of equilibrium swelling, initial swelling rate, diffusional exponent, and diffusion coefficient were evaluated and the swelling kinetics of the nanocomposite gels in aqueous KBr solution followed the pseudo second order model. Since DMAEMA and inorganic clay SEP were effectively combined in a nanocomposite network, the obtained results would expand the range of responsive properties and related industrial applications available to DMAEMA and SEP individually.

A Model Study for Prediction of Performance of Automotive Interior Coatings: Effect of Cross-Link Density and Film Thickness on Resistance to Solvents and Chemicals

A Model Study for Prediction of Performance of Automotive Interior Coatings: Effect of Cross-Link Density and Film Thickness on Resistance to Solvents and Chemicals

Polymer Nanogels as Reservoirs To Inhibit Hydrophobic Drug Crystallization.

The effects of cross-link density and composition on the loading and in vitro dissolution of the drug phenytoin as amorphous solid dispersions in emulsion polymerized poly( N-isopropylacrylamide) (PNIPAm) and poly( N-isopropylacrylamide- co- N, N-dimethylacrylamide) nanogels were investigated near the lower critical solution temperature (LCST). Nanogel size and particle density in phosphate buffered saline were quantified by dynamic light scattering (DLS) and viscometry experiments, while drug-nanogel interactions were revealed by cross peaks in aqueous-state nuclear Overhauser effect spectroscopy measurements. Spray-dried dispersions (SDDs) of drug-loaded PNIPAm nanogel particles ( R ≈ 43 nm) were directly visualized by cryogenic transmission electron microscopy and further quantified by small-angle X-ray scattering during in vitro dissolution. SDD dissolution profiles were highly dependent on the nanogel cross-link density and directly correlated with the state of dispersion of the drug-loaded nanogel particles. A balance between net particle hydrophobicity and hydrophilicity along with the distance in temperature from the LCST are shown to dictate the in vitro dissolution of the amorphous solid dispersions. Solubility enhancement mechanisms disclosed in this study provide essential guidance for the design of effective nanogels for oral drug delivery applications.

Effects of Crosslink Density on Plastic Flow of Glassy Epoxy Networks

Effects of Crosslink Density on Plastic Flow of Glassy Epoxy Networks

Effects of Cross-Link Density and Distribution on Static and Dynamic Properties of Chemically Cross-Linked Polymers

Molecular dynamics simulations have been performed to study the effects of the cross-link density and distribution on the network topology, the dynamic structural properties, the tensile mechanical properties, and the viscoelastic properties of chemically cross-linked polymers. Simulation results show that the introduction of cross-links slows the chain dynamics down and thus leads to a slight increase in the glass transition temperature. Improving the cross-link dispersion state is found to be essentially equivalent to increasing the effective cross-link density, as reflected in the network topological features. The structural relaxation behavior is analyzed in terms of the incoherent intermediate dynamic structure factor, and the characteristic α-relaxation time is examined by the MCT and VFT equation. The results indicate that the time–temperature superposition principle holds on the segmental length scale but fails on the chain length scale, where it can be valid at sufficiently high temperature above...

Effects of Crosslink Density on Relaxation Modulus of Aged Polymer

In order to study the effects of crosslink density on relaxation modulus of aged polymer materials, an analytic expression was obtained based on the deformation kinetics model of polymer and the assumption that the Helmholtz free energy of materials can be represented as a quadratic form of internal variables on isothermal condition. This expression shows that crosslink density has effects on equilibrium modulus, transient modulus and relaxation rate in different ways. In a special case, a crosslink density-time corresponded principle exists when the polymer is lightly cross-linked and the crosslink density has a common influence on equilibrium and transient parts of relaxation modulus.

Effect of Cross-Linking Density on the Structures and Properties of Carbodiimide-Treated Gelatin Matrices as Limbal Stem Cell Niches.

Given that human amniotic membrane is a valuable biological material not readily available for corneal epithelial tissue engineering, gelatin is considered as a potential alternative to construct a cellular microenvironment. This study investigates, for the first time, the influence of cross-linking density of carbodiimide-treated gelatin matrices on the structures and properties of artificial limbal stem cell niches. Our results showed that an increase in the carbodiimide concentration from 1.5 to 15 mM leads to an upward trend in the structural and suture strength of biopolymers. Furthermore, increasing number of cross-linking bridges capable of linking protein molecules together may reduce their crystallinity. For the samples treated with 50 mM of cross-linker (i.e., the presence of excess N-substituted carbodiimide), abundant N-acylurea was detected, which was detrimental to the in vitro and in vivo ocular biocompatibility of gelatin matrices. Surface roughness and stiffness of biopolymer substrates were found to be positively correlated with carbodiimide-induced cross-link formation. Significant increases of integrin β1 expression, metabolic activity, and ABCG2 expression were noted as the cross-linker concentration increased, suggesting that the bulk crystalline structure and surface roughness/stiffness of niche attributed to the number of cross-linking bridges may have profound effects on a variety of limbal epithelial cell behaviors, including adhesion, proliferation, and stemness maintenance. In summary, taking the advantages of carbodiimide cross-linking-mediated development of gelatin matrices, new niches with tunable cross-linking densities can provide a significant boost to maintain the limbal stem cells during ex vivo expansion.