Varying Crosslinking Densities Research Articles

UV-curable PUDs based on sustainable acrylated polyol: Study of their hydrophobic and oleophobic properties

Abstract UV-curable polyurethane dispersions (UV-PUDs) are fast expanding commercial applications since they combine benefits of both water-borne as well as UV-curing technologies while addressing many technical, environmental and performance benefits. Varying the compositions and cross-link density of UV-PUD polymeric chain backbone can control the film properties of UV-PUDs. There are many design restrictions posed by availability of commercial materials. In the present research work we demonstrate synthesis and application of a multi-functional acrylate polyol derived from soybean oil, as soft-segment of UV-PUDs. A series of UV-PUDs have been designed for high performance coatings that are specifically hydrophobic and oil-resistant. To this end, UV-PUDs based on acrylated soy-polyol have been further modified by siloxane and perfluoro compounds and their films with varying cross-link density have been investigated. The UV-PUD films were characterized for their film properties, particle size, contact angle and solvent swelling ratio. The outcome of this study provides useful insights into design considerations for hydrophobic and oil-resistant UV-curable coatings.

Highly swellable ultrathin poly(4-vinylpyridine) multilayer hydrogels with pH-triggered surface wettability

We report on the synthesis of a novel type of ultrathin hydrogel with remarkably high, fast and reversible swelling/shrinkage and sharp surface wettability transitions in response to pH changes. The poly(4-vinylpyridine) (PVP) hydrogels are produced by the selective cross-linking of PVP copolymer layers in PVP/poly(methacrylic acid) (PVP/PMAA) layer-by-layer (LbL) films assembled by a spin-assisted method on surfaces. These multilayer hydrogels exhibit drastic and reversible 10-fold swelling when pH is switched from neutral to acidic. Importantly, hydrogels at pH > 5 collapse to the thickness of dry films, evincing sharp hydrophilic-to-hydrophobic transitions. The swelling amplitude of these multilayer hydrogel coatings is controlled by varying cross-link densities within the hydrogel. Further, the pH-triggered swelling properties of PVP hydrogels are significantly affected by the multilayer assembly method. PVP films produced using the spin-assisted method experienced almost a 4-fold greater swelling enhancement over their conventionally “dipped” counterparts. Finally, PVP hydrogels dehydrated from basic and acidic solutions show drastically different surface morphology and wettability properties. The hydrogel contact angles vary from 70° to 20° for the films dried from neutral and acidic solutions, respectively. Our study provides better understanding of regulated volume and surface wettability transitions in polymer networks which is critical for developing novel materials for a wide variety of applications including sensing, transport regulation, and self-cleaning coatings.

Biomimetic hydrogels gate transport of calcium ions across cell culture inserts

Control of the in vitro spatiotemporal availability of calcium ions is one means by which the microenvironments of hematopoietic stem cells grown in culture may be reproduced. The effects of cross-linking density on the diffusivity of calcium ions through cell culture compatible poly(2-hydroxyethyl methacrylate) [poly(HEMA)]-based bioactive hydrogels possessing 1.0 mol% 2-methacryloyloxyethyl phosphorylcholine (MPC), 5 mol% N,N-(dimethylamino)ethylmethacrylate (DMAEMA) and ca. 17 mol% n-butyl acrylate (n-BA) have been investigated to determine if varying cross-link density is a viable approach to controlling transport of calcium across hydrogel membranes. Cross-linking density was varied by changing the composition of cross-linker, tetraethyleneglycol diacrylate (TEGDA). The hydrogel membranes were formed by sandwich casting onto the external surface of track-etched polycarbonate membranes (T = 10 μm, φ = 0.4 μm pores) of cell culture inserts, polymerized in place by UV light irradiation and immersed in buffered (0.025 HEPES, pH 7.4) 0.10 M calcium chloride solution. The transport of calcium ions across the hydrogel membrane was monitored using a calcium ion selective electrode set within the insert. Degree of hydration (21.6 ± 1.0%) and void fraction were found to be constant across all cross-linking densities. Diffusion coefficients, determined using time-lag analysis, were shown to be strongly dependent on and to exponentially decrease with increasing cross-linking density. Compared to that found in buffer (2.0-2.5 × 10⁻⁶ cm²/s), diffusion coefficients ranged from 1.40 × 10⁻⁶ cm²/s to 1.80 × 10⁻⁷ cm²/s and tortuosity values ranged from 1.7 to 10.0 for the 1 and 12 mol% TEGDA cross-linked hydrogels respectively. Changes in tortuosity arising from variations in cross-link density were found to be the primary modality for controlling diffusivity through novel n-BA containing poly(HEMA)-based bioactive hydrogels.

Hydrogel crosslinking density regulates temporal contractility of human embryonic stem cell-derived cardiomyocytes in 3D cultures

Systematically tunable in vitro platforms are invaluable in gaining insight to stem cell-microenvironment interactions in three-dimensional cultures. Utilizing recombinant protein technology, we independently tune hydrogel properties to systematically isolate the effects of matrix crosslinking density on cardiomyocyte differentiation, maturation, and function. We show that contracting human embryonic stem cell-derived cardiomyocytes (hESC-CMs) remain viable within four engineered elastin-like hydrogels of varying crosslinking densities with elastic moduli ranging from 0.45 to 2.4 kPa. Cardiomyocyte phenotype and function was maintained within hESC embryoid bodies for up to 2 weeks. Interestingly, increased crosslinking density was shown to transiently suspend spontaneous contractility. While encapsulated cells began spontaneous contractions at day 1 in hydrogels of the lowest crosslinking density, onset of contraction was increasingly delayed at higher crosslinking densities up to 6 days. However, once spontaneous contraction was restored, the rate of contraction was similar within all materials (71 ± 8 beats/min). Additionally, all groups successfully responded to electrical pacing at both 1 and 2 Hz. This study demonstrates that encapsulated hESC-CMs respond to 3D matrix crosslinking density within elastin-like hydrogels and stresses the importance of investigating temporal cellular responses in 3D cultures.

Acoustic Spectroscopy of Colloids Dispersed in a Polymer Gel System

The technique of acoustic spectroscopy offers some significant advantages over conventional techniques, such as dynamic light scattering and differential sedimentation (centrifugation), for the characterization of colloidal dispersions in that it does not require that the systems be highly dilute and transparent. Another advantage of the method may derived from the fact that in applications, the relative motion between any particle and the medium is very small, at the most being comparable to the particle size. It may thus be suited, within limits, to the study of dispersions in polymer gels, without the additional limitation of conventional methods to transparent media (matching refractive index of polymer and liquid). The present work seeks to probe experimentally the limits of the technique and its current theory for the determination of particle size distributions in gel media. Experiments measuring acoustic attenuation have been conducted on dispersions of silica particles of varying size in aqueous hydroxylpropyl cellulose (HPC) gels of varying cross-link density. The particle size distribution (PSD) was successfully measured by acoustic attenuation theory for dispersions in Newtonian media provided that the hydrodynamic particle diameter was less than the hydrodynamic mesh size of the gel, as given by simple rubber elasticity theory (mesh size/particle size ≳1.5). The same results were obtained at particle loadings of up to 15 wt %. If the particles are larger than the mesh size, then a viscoelastic response from the gel matrix is observed that cannot be interpreted to yield the particle size using the existing theoretical framework.

加硫ゴム材料の伸長変形過程におけるＥＳＲ情報

Electron spin resonance (ESR) and wide angle X-ray diffraction (WAXD) measurements under the tensile deformation were carried out for H-NBR, NBR and SBR vulcanizates with varying cross-link densities to discuss on the changes of network structure during tensile deformation. The H-NBR vulcanizates showed a strain-induced crystallization at room temperature, but the NBR and SBR vulcanizates did not. At a given stress, the radical concentration determined by ESR measurements was higher for H-NBR vulcanizates than for NBR and SBR vulcanizates. The degree of strain-induced crystallization at a given strain was likely to increase with the decrease of crosslink density. These results suggested that the strain-induced crystallization of H-NBR vulcanizates was accompanied by the partial breakdown of network structure in the vulcanizates.

The Softer the Better: Fast Condensation on Soft Surfaces

Condensation on soft elastic surfaces differs significantly from condensation on hard surfaces. On polymeric substrates with varying cross-linking density, we investigate the nucleation and the growth of condensing water drops. With increasing softness of the substrates, we find (1) increasing nucleation density, (2) longer relaxation times for drop shape equilibration after merging of two drops, and (3) prevention of merging on very soft surfaces. These effects lead to higher surface coverage and overall condensed volume on soft surfaces.

Polybasic Nanomatrices Prepared By UV-initiated Photopolymerization.

A novel method for synthesizing nanoscale polymer networks that swell in acidic media is described here using photoinitiated emulsion polymerization. These nanomatrices consist of a crosslinked core of poly[2-(diethylamino)ethyl methacrylate] surface grafted with poly(ethylene glycol) (PDGP) with an average diameter of 50-150 nm. Control over mesh size, surface charge, encapsulation efficiency and in vitro biocompatibility was obtained by varying crosslinking density. The ability to image nanomatrices in their dry state using conventional scanning electron microscopy was made possible by increasing crosslinking density. Theoretical calculations of matrix mesh sizes were supported by the encapsulation of both insulin and colloidal gold 2-5 nm in diameter. The ability to sequester and control the aggregation of an inorganic phase confirmed their use as a nanocomposite matrix material. These networks could be used as imaging agents, drug delivery devices or as components of sensing devices.

Effect of Cross-Link Density on Interphase Creation in Polymer Nanocomposites

The thermal and dynamic behavior of epoxy nanocomposites with varying cross-link density was tested via dynamic scanning calorimetry (DSC) in order to determine the influence of cross-link density on the creation of “interphase”, zones of altered polymer properties near polymer−particle interfaces. The results show that the inclusion of nanoparticles creates an increase in the glass transition temperature (Tg) at low cross-link density and a decrease in Tg at higher cross-link densities. This phenomenon suggests that two mechanisms work in tandem to alter the Tg of epoxy systems, with relative magnitudes determined by cross-link density: (1) network disruption at the nanotube−polymer interfaces leading to lower Tg and (2) interphase creation leading to retarded dynamics, resulting in higher Tg. Results show that as cross-link density increases, the length scale of cooperatively rearranging regions (CRRs) decreases. This decrease hinders communication of the dynamics between adjacent CRRs, thereby reducing...

Effects of PEG hydrogel crosslinking density on protein diffusion and encapsulated islet survival and function

The rational design of immunoprotective hydrogel barriers for transplanting insulin-producing cells requires an understanding of protein diffusion within the hydrogel network and how alterations to the network structure affect protein diffusion. Hydrogels of varying crosslinking density were formed via the chain polymerization of dimethacrylated PEG macromers of varying molecular weight, and the diffusion of six model proteins with molecular weights ranging from 5700 to 67,000 g/mol was observed in these hydrogel networks. Protein release profiles were used to estimate diffusion coefficients for each protein/gel system that exhibited Fickian diffusion. Diffusion coefficients were on the order of 10(-6)-10(-7) cm(2)/s, such that protein diffusion time scales (t(d) = L(2)/D) from 0.5-mm thick gels vary from 5 min to 24 h. Adult murine islets were encapsulated in PEG hydrogels of varying crosslinking density, and islet survival and insulin release was maintained after two weeks of culture in each gel condition. While the total insulin released during a 1 h glucose stimulation period was the same from islets in each sample, increasing hydrogel crosslinking density contributed to delays in insulin release from hydrogel samples within the 1 h stimulation period.

Optical Detection of Polyacrylamide Swelling Behavior in a Porous Silicon Sensor

Abstract This work proves that a 1-D porous silicon (PSi) sensor is capable of monitoring the optical changes in a polyacrylamide (PAAm) hydrogel that correlate with swelling capacity. The PSi device was impregnated with PAAm hydrogel with varying crosslinking density and total solids. The optical response of the PSi sensor was utilized to distinguish the changes in refractive index of hydrogels with varying cross-linking densities. Refractive index values calculated by the composite hydrogel-PSi sensor response agree well (≤1% difference) with values measured using a bench-top refractometer. This work serves to build a foundation for creating a composite biochemical-responsive hydrogel-PSi sensor in which competitive binding of a target analyte would cause a reduction in hydrogel cross-linking density. Long-term goals of this work are to exploit the volume sensitivity of a PSi sensor and leverage the added optical response of the responsive hydrogel to increase target detection sensitivity in an affinity based biosensor.

Characterisation of macroporous poly(methyl methacrylate) coated with plasma-polymerised poly(2-hydroxyethyl acrylate)

Macroporous poly(methyl methacrylate) networks with varying cross-linking density and porosity were coated with plasma-polymerised poly(2-hydroxyethyl acrylate) grafted on the pores surface. The result is a mechanically reinforced hydrogel (PMMA- gr- plPHEA) whose properties are characterised in this work using several experimental techniques. Bulk PMMA and bulk PHEA were also characterised as reference materials. The diffusion and water sorption properties of these hydrogels were studied through equilibrium water sorption isotherms and desorption starting with the sample equilibrated in immersion in liquid water or in a vapour atmosphere. Glass transition, dynamic-mechanical relaxation and thermal degradation were characterised in order to study the interphase interaction in these biphasic systems. All these experimental techniques suggested that plasma-polymerised PHEA is more homogeneously interpenetrated with highly cross-linked macroporous PMMA than if the porous substrate is a loosely cross-linked polymer network.

Nanomechanics of collagen fibrils under varying cross-link densities: Atomistic and continuum studies

Collagen is a protein material with intriguing mechanical properties — it is highly elastic, shows large fracture strength and plays a crucial role in making Nature’s structural materials tough. Collagen based tissues consist of collagen fibrils, each of which is composed out of a staggered array of ultra-long tropocollagen molecules extending to several hundred nanometers. Albeit the macroscopic properties of collagen based tissues have been studied extensively, less is known about the nanomechanical properties of tropocollagen molecules and collagen fibrils, their elementary building blocks. In particular, the relationship between molecular properties and tissue properties remains a scarcely explored aspect of the science of collagen materials. Results of molecular multi-scale modeling of the nanomechanical properties of the large-strain deformation regime of collagen fibrils under varying cross-link densities are reported in this paper. The results confirm the significance of cross-links in collagen fibrils in improving its mechanical strength. Further, it is found that cross-links influence the nature of its large-deformation and fracture behavior. Cross-link deficient collagen fibrils show a highly dissipative deformation behavior with large yield regimes. Increasing cross-link densities lead to stronger fibrils that display an increasingly brittle deformation character. The simulation results are compared with recent nanomechanical experiments at the scale of tropocollagen molecules and collagen fibrils.

Immobilization of mushroom polyphenol oxidase on poly(allyl glycidyl ether-co-ethylene glycol dimethacrylate) macroporous beaded copolymers

Functional, macroporous, beaded copolymers containing epoxy groups were synthesized for immobilization of polyphenol oxidase (PPO) from edible mushroom ( Agaricus bisporus). The effect of incorporation of two different sets of monomers such as glycidyl methacrylate (GMA) and allyl glycidyl ether (AGE) and the effect of cross-linking agent ethylene glycol dimethacrylate (EGDM) with varying cross-link densities on binding and expression of mushroom PPO activity were studied. The effect of porogen viz. cyclohexanol and hexanol on PPO immobilization was studied. AGE copolymers with hexanol as a porogen were found to give higher binding and expression of PPO activity than GE polymers. Cross-linking of amino groups of enzyme with 5% glutaraldehyde for 6 h gave a stable binding of PPO on AGE-75(Hex) polymer with storage half-life of approximately 25 days. Under optimum conditions, AGE-75(Hex) polymer gave 70.3% of activity yield while percent retention of PPO activity was found to be 83.5%. Immobilized PPO showed a broader pH, higher temperature and excellent storage stability.

Monte Carlo Simulation of Polyelectrolyte Gels: Effects of Polydispersity and Topological Defects

Volumes and other structural properties of polyelectrolyte gels in equilibrium with pure water have been determined by Monte Carlo simulations. The role of chain length polydispersity and topological network defects of four different networks with varying cross-linking density, monomer charge, and chain stiffness have been investigated. Generally, a chain length polydispersity reduced the gel volume, whereas the presence of chains with one end detached from the cross-linker (severed chains) led to an increased gel volume. Polyelectrolyte networks displayed the largest and uncharged polymer networks the smallest dependence on chain length polydispersity. The effect of severed chains was strongest for flexible polyelectrolyte gels and weakest for uncharged networks and stiff polyelectrolyte gels. Mechanical properties of uncharged and charged polymer gels were also investigated through uniaxially stretching and compared with theory.

Preparation and characterization of surfactant‐free stimuli‐sensitive microgel dispersions

AbstractA surfactant‐free method to produce responsive polymer microgels is introduced. As an example, poly(methacrylic acid) hydrogels with varying crosslinking density have been synthesized in bulk and then chopped using a high shear mechanical cutter to form microgel particles dispersed in water. The mechanical cutting technique enables the concentration and particle size distribution of the microgel suspensions to be easily controlled, therefore making the rheology of the suspensions tuneable. The particle size distribution of the dispersions, characterized using light scattering, was dependent on the speed and duration of mechanical cutting. The particle size distribution also depended on the degree of crosslinking of the hydrogel. The higher the crosslinking density, the lower the average mean diameter of the resulting microgel particles. The lower the crosslinking density of the hydrogel, the larger the difference between the maximum and minimum particle size. The time to complete swelling of the particles upon change in pH was measured to be up to 45 s, depending on the particle size. The rheology of the resulting suspensions as a function of pH was investigated. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 104: 1912–1919, 2007

Plasma-induced polymerisation of hydrophilic coatings onto macroporous hydrophobic scaffolds

Macroporous poly(methyl methacrylate) scaffolds with a well-interconnected pore architecture were coated with poly(2-hydroxyethyl acrylate) following a particular protocol of plasma-induced polymerisation. Plasma-polymerised PHEA (plPHEA) was grafted onto two macroporous poly(methyl methacrylate) scaffolds with varying cross-linking density showing significant differences in the interpenetration of the coating and the PMMA substrate. Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) proved that the surface of the pore structure in the whole volume of the sample was coated. The stability of the plPHEA coating was studied by extraction with distilled water at different temperatures. After the extraction, these samples were observed by SEM and analysed by differential scanning calorimetry (DSC) and FTIR showing that only in very drastic conditions plPHEA suffers hydrolytic degradation.

Effects of Cross-linking Type II Collagen-GAG Scaffolds on Chondrogenesis In Vitro: Dynamic Pore Reduction Promotes Cartilage Formation

Articular cartilage tissue-engineering investigations often implement bioassays for chondrogenesis in vitro using articular chondrocytes or mesenchymal stem cells in cell pellets that contract with time in culture, suggesting an association between the processes of contraction of the cell pellet and cartilage formation. The objective of the present study was to investigate this relationship further using adult canine articular chondrocyte-seeded type II collagen-GAG scaffolds. The collagen-GAG scaffolds were chemically cross-linked to achieve a range of cross-link densities. Chondrocyte-seeded scaffolds of varying cross-link densities were then cultured for 2 weeks to evaluate the effect of crosslink density on scaffold contraction and chondrogenesis. Scaffolds with low cross-link densities experienced cell-mediated contraction, increased cell number densities, a greater degree of chondrogenesis (viz., chondrocytic morphology of cells, synthesis of type II collagen), and an apparent increase in the rate of degradation of the scaffold compared to more highly cross-linked scaffolds that resisted cellular contraction. The results of this study suggest the promise of "dynamic pore reduction" for scaffolds for articular cartilage tissue engineering. In this approach, scaffolds would have an initial pore diameter large enough to facilitate cell seeding and a mechanical stiffness low enough to allow for cell-mediated contraction to yield a reduced pore volume to favor chondrogenesis. This approach may provide a useful alternative to traditional means of increasing cell number density and retention of synthesized molecules that promote cartilage formation in tissue-engineered constructs.

Influence of the cross-linking density on the main dielectric relaxation of poly(methyl acrylate) networks

A series of polymer networks of varying cross-linking density was prepared by copolymerization of methyl acrylate and ethyleneglycol dimethacrylate. The aim of this work is to study the influence of cross-linking on the conformational mobility of the polymer chains using dielectric relaxation spectroscopy (DRS) in the temperature range of the main dielectric relaxation. As expected, the temperature range in which glass transition takes place became wider with increasing crosslinking density. DRS results were analyzed using the Havriliak-Negami equation. Master Cole-Cole arcs could be drawn for all the networks. The arcs become more symmetric as cross-linking density increases, as a consequence of the different effect of cross-links on large and small scale mobility. The conformational mobility that produces the main relaxation is drastically reduced when the cross-linking density increases what reduces the relaxation strength, but it also gives a qualitative change of behavior, as shown by the temperature dependence of the relaxation strength. In the loosely cross-linked networks the relaxation strength decreases monotonously as temperature increases, as in the main dielectric relaxation of linear polymers. Nevertheless, in highly cross-linked networks the curve of relaxation strength against temperature presents a maximum. POLYM. ENG. SCI., 45:1336–1342, 2005. © 2005 Society of Plastics Engineers

Polyaniline/polyurethane networks. II. A spectroscopic study

The interconnection of polyaniline (Pani) chains through polyurethane (PU) blocks of the same length but with different spacing has originated a series of new semi conducting networks of varying crosslinking density, with good mechanical properties. FTIR, UV–Vis–NIR, XPS and EPR were used to probe the molecular features of the conducting species, and to test the morphological model proposed in a previous publication, where a continuous Pani phase percolates a PU matrix, linked together by a mixed interphase. Blends with the same composition of the networks and a model structure of crosslinked Pani were also prepared for comparison purposes. The proposed morphological model was confirmed by the new findings obtained by the spectroscopic techniques. The differences between the blends and networks of the same composition were discussed in terms of the morphology presented by each of these systems.