Varying Crosslinking Densities Research Articles

Redox-responsive degradable prodrug nanogels for intracellular drug delivery by crosslinking of amine-functionalized poly(N-vinylpyrrolidone) copolymers

HypothesisFacile approaches for the development of new tailored drug carriers are of high importance for the controlled administration of drugs. Herein we report a method for the synthesis of water-soluble reactive copolymers with well-defined architectures for fabrication of redox-sensitive degradable prodrug nanogels for intracellular drug release. ExperimentsPrimary amine-functionalized statistical copolymers were obtained by hydrolysis of poly(N-vinylpyrrolidone-co-N-vinylformamide) copolymers which were synthesized via Reversible Addition–Fragmentation chain-Transfer (RAFT) polymerization. Redox-sensitive degradable nanogels with varying crosslinking densities were synthesized with a redox-sensitive cross-linker. Doxorubicin (DOX) was loaded to form prodrug nanogels (DNG) with hydrodynamic radius from 142 nm to 240 nm. FindingsThe nanogels demonstrated slower degradation and retarded drug release rate with increased crosslinking density in the presence of 10 mM reduced glutathione (GSH) at 37 °C. The in vitro release studies revealed that maximum 85% DOX was released in 24 h under a reductive environment. Intracellular drug release profiles in HeLa cells indicated that the DOX delivery rate was tunable via varying crosslinking density of the nanogels. Cell viability assay demonstrated that the blank nanogels were biocompatible in wide concentrations up to 0.5 mg/mL while the DOX-loaded nanogels displayed medium antitumor activity with IC50 (half-maximal inhibitory concentration) of 1.80 μg/mL, 2.57 μg/mL, 3.01 μg/mL for DNG5, DNG10 and DNG15 respectively.

AB070. 125. Evaluation of a hydrogel scaffold seeded with mature adipocytes for breast reconstruction post-mastectomy

Background: Hydrogels are suitable for adipose tissue engineering for breast regeneration due to their ability to mimic native extracellular matrix. The aim of this study was to assess the suitability of adipose tissue seeded hyaluronic acid (HA) hydrogels at varying crosslinking densities and cell loading densities as adjuncts to implant breast reconstruction post-mastectomy.

Confinement-Induced Stiffening of Elastomer Thin Films.

While entropy is understood to dictate the elastic mechanics of elastomers, a consensus has not been reached regarding why they stiffen when confined to thin films or even the magnitude of this effect. Here, we present a combined computational and experimental approach to measure the confinement-induced stiffening of thin elastomer films using nanoindentation. First, we use finite element computations to modify the adhesion-free contact model to include the presence of a rigid substrate and verify this correction using indentation experiments on macroscopic polydimethylsiloxane (PDMS) films. Next, we validate this correction factor in indentation experiments where adhesion plays a major role through a series of experiments on micrometer-scale PDMS films. With this correction factor in hand, we perform an extensive series of nanoindentation experiments on submicrometer thickness films of varying cross-link density using a variety of probe radii and probe surface energies. We observe a consistent stiffening of all PDMS thin films that, interestingly, converges to a consistent elastic modulus at thicknesses ∼100 nm regardless of the cross-link density. To explain these results, we propose a surface cross-linking model where the density of crosslinks between chains increases near the surface of the film because of oxidation-induced bonds between chains. Importantly, the apparent moduli predicted by this entropic model are in good agreement with all experimental results. Collectively, these results shed new light on the nanomechanics of elastomers and provide a general method for predicting the size-dependent effective moduli of soft polymers.

Injectable degradable PVA microgels prepared by microfluidic technology for controlled osteogenic differentiation of mesenchymal stem cells

The direct injection of bone marrow mesenchymal stem cells (hMSCs) is a promising strategy for bone tissue engineering applications. Herein, we have developed injectable degradable poly(vinyl alcohol) (PVA) microgels loaded with hMSCs and growth factors and prepared by a high-throughput microfluidic technology. The PVA-based microgels with tunable mechanical and degradable properties were composed of vinyl ether acrylate-functionalized PVA (PVA-VEA) and thiolated PVA-VEA (PVA-VEA-SH) through a Michael-type crosslinking reaction under mild conditions. The hMSCs sustain high viability in PVA microgels, and cell proliferation and migration behaviors can easily be adjusted by varying crosslinking densities of PVA microgels. Additionally, bone morphogenetic protein-2 (BMP-2) co-encapsulated into the microgel environments enhanced osteogenic differentiation of hMSCs as indicated by a significant increase in alkaline phosphatase activity, calcium content, and Runx2 and OPN gene expression levels. These results demonstrate the degradable PVA microgels with tailored stem cell microenvironments and controlled release profile of the growth factor to promote and direct differentiation. These PVA-based microgels have promising potential as ideal cell vehicles for applications in regenerative medicine. Statement of SignificanceStem cell transplantation by an injectable, minimally invasive method has great and promising potential for various injuries, diseases, and tissue regeneration. However, its applications are largely limited owing to the low cell retention and engraftment at the lesion location after administration. We have developed an injectable degradable poly(vinyl alcohol) (PVA) microgel prepared by a high-throughput microfluidic technology and co-loaded with bone marrow mesenchymal stem cells (hMSCs) and growth factor to protect the stem cells from harsh environmental stress and realize controlled cell differentiation in well-defined microenvironments for bone regeneration. We demonstrated that these degradable PVA microgels can be used as stem cell scaffolds with tailored cell microenvironments and controlled release profile of growth factor to promote and direct differentiation. We are convinced that these PVA-based microgels have promising potential in the future as cellular scaffolds for applications in regenerative medicine.

Effect of Chemistry and Process Parameters on Shape Recovery Behaviour of Shape Memory Epoxies in Flexure

Epoxy based Shape Memory Polymers (SMEPs) of varying crosslinking densities were prepared by reacting an aromatic epoxy resin, Diglycidyl Ether of Bisphenol A (DGEBA) with different aliphatic amines varying in chain lengths, triethylenetetramine (TETA), 2,2′ethylenedioxybisethylamine (EDEA) and 4,7,10-Trioxa-1, 13-Tridecanediamine (TTD). Shape memory tests were conducted for all three SMEPs by deforming the samples at temperatures greater than their Tg in flexure (3 point bending). The effect of crosslinking density (CLD), shape fixing temperature (Tfix) and shape recovery temperature (Trec) on shape memory behaviour of SMEPs were studied systematically. In terms of the chemical structure, the results showed that with the increase in CLD, glass transition temperature (Tg) increased with an associated reduction in the shape recovery behaviour. However, superior shape memory behaviour was observed for lower CLD SMEPs. With respect to the process parameters, as the Tfix was lowered below the Tg, % shape fixity increased and the recovery curves shifted towards higher temperature. When Trec was increased above the Tg, recovery rate increased.

The mechanics of phantom Mikado networks

The elasticity of networks comprised of semi-flexible polymers plays a vital role in regulating the mechanics of both intra- and extra-cellular matrices. The behaviour of this polymer scaffold will depend on the nature and density of cross-linking between constituent fibres. While modelling efforts have investigated the effects of cross-link density in biopolymer networks, this is often accompanied by changes in both the fibre density and the network structure. We investigate the elasticity of a quasi two-dimensional Mikado network of elastic rods, in which cross-link density is allowed to vary while polymer density is held constant. In particular, this model is extended by allowing constituent rods to cross without forming cross-links, while polymer density and network geometry are preserved. In doing so, the competing contributions to the shear modulus from cross-link density, mesh size, geometry and polymer density are decoupled. We find that previous scaling laws fail to capture the well-studied transition from bend- to stretch- dominated elasticity as cross-link density is varied. We identify a length scale which relates cross-link density to the transition between affine and nonaffine regimes, and which provides a collapse of simulation data curves for varying cross-link densities.

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.

Tunable Enzymatic Activity and Enhanced Stability of Cellulase Immobilized in Biohybrid Nanogels

This paper reports a facile approach for encapsulation of enzymes in nanogels. Our approach is based on the use of reactive copolymers able to get conjugated with enzyme and build 3D colloidal networks or biohybrid nanogels. In a systematic study, we address the following question: how the chemical structure of nanogel network influences the biocatalytic activity of entrapped enzyme? The developed method allows precise control of the enzyme activity and improvement of enzyme resistance against harsh store conditions, chaotropic agents, and organic solvents. The nanogels were constructed via direct chemical cross-linking of water-soluble reactive copolymers poly(N-vinylpyrrolidone-co-N-methacryloxysuccinimide) with proteins such as enhanced green fluorescent protein (EGFP) and cellulase in water-in-oil emulsion. The water-soluble reactive copolymers with controlled amount of reactive succinimide groups and narrow dispersity were synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization. Poly(ethylene glycol) bis(3-aminopropyl) and branched polyethylenimine were utilized as model cross-linkers to optimize synthesis of nanogels with different architectures in the preliminary experiments. Biofluorescent nanogels with different loading amount of EGFP and varying cross-linking densities were obtained. We demonstrate that the biocatalytic activity of cellulase-conjugated nanogels (CNG) can be elegantly tuned by control of their cross-linking degrees. Circular dichroism (CD) spectra demonstrated that the secondary structures of the immobilized cellulase were changed in the aspect of α-helix contents. The secondary structures of cellulase in highly cross-linked nanogels were strongly altered compared with loosely cross-linked nanogels. The fluorescence resonance energy transfer (FRET) based study further revealed that nanogels with lower cross-linking degree enable higher substrate transport rate, providing easier access to the active site of the enzyme. The biohybrid nanogels demonstrated significantly improved stability in preserving enzymatic activity compared with free cellulase. The functional biohybrid nanogels with tunable enzymatic activity and improved stability are promising candidates for applications in biocatalysis, biomass conversion, or energy utilization fields.

Core/shell microgels with �switchable� deformability

Event Abstract Back to Event Core/shell microgels with “switchable” deformability Kimberly Clarke1, Erin Sproul2 and Ashley Brown2 1 Georgia Institute of Technology, School of Chemistry and Biochemistry, United States 2 North Carolina State University and University of North Carolina at Chapel Hill, Joint Department of Biomedical Engineering, United States Introduction: Microgels are colloidal hydrogels that have been utilized for a range of biomaterial applications due to their tunable mechanical and chemical properties. Thermoresponsive poly(N-isopropylacrylamide) (pNIPAM) microgels have been well characterized and are fabricated by precipitation polymerization, typically in the presence of a crosslinking agent, such as N,N’-methylenebisacrylamide (BIS). The mechanical properties of the resulting particles can be tuned by varying cross-linking density. Interestingly, pNIPAM microgels can be formed under "crosslinker free" conditions, which results in the formation of “ultra-low” crosslinked (ULC) particles. These particles are softer than traditionally crosslinked microgels, with a Young’s Modulus of ~10 kPA, and are highly deformable as evidenced by their ability to spread extensively on surfaces and translocate through nanopores significantly smaller than the hydrodynamic diameter of the particles [1]. Due to these unique properties, ULC particles are useful for a variety of biomedical applications, including a recent demonstration that this deformability enables clot contraction by ULC-based platelet-like particles [2]. Here, we aim to create microgels with “switchable” deformability. To that end, we synthesized core/shell microgels, comprised of a N,N’-(1,2-dihydroxyethylene)bisacrylamide (DHEA) crosslinked core, which can be degraded to produce a hollow microgel. We hypothesized that, like ULCs, hollow microgels would display high degrees of deformability. Materials and Methods: Core/shell microgels were constructed by first producing a core containing 90% N-Isopropylmethacrylamide (NIPMAM) and 10% DHEA through precipitation polymerization. Following purification of the core particles, shells containing 88% NIPAM, 2% BIS and 10% acrylic acid or 91% NIPAM, 4% BIS and 5% acrylic acid were added in a second precipitation polymerization reaction. To create hollow particles, cores were degraded by the addition of sodium periodate (NaIO4), which cleaves the DHEA crosslinks (diols). Particle hydrodynamic radii were then determined through dynamic light scattering (DLS) and particle deformability was determined with atomic force microscopy (AFM) to analyze spreading on a glass surface. Results and Discussion: DLS revealed an increase in hydrodynamic diameter in hollow microgels vs core/shell microgels. After core degradation, the diameter of core/shell microgels increased 23% (675±5 vs 829±9 nm) in 2% BIS crosslinked hollow microgels. Similarly, 4% BIS crosslinked hollow microgels exhibited a 34% increase in diameter (576±7 vs 774±19 nm). AFM demonstrated increased particle spreading in 2% BIS hollow microgels (Figure 1); a 119% increase in diameter (490±40 vs 1090±230 nm) with an 87% decrease in height (73±12 vs 10±1 nm) was observed. Similarly, AFM showed a 268% increase in diameter (404±76 vs 1490±423 nm) and a 93% decrease in height (86±18 vs 6±1 nm) in 4% BIS hollow microgels. These results demonstrate that hollow microgels display a high degree of deformability and spread much more extensively on surfaces than core/shell microgels. Conclusions: Like ULC microgels, hollow microgels display a high degree of deformability. Furthermore, unlike ULC microgels, because hollow microgels are created by degrading the core of core/shell microgels, this particle architecture allows for switchable deformability.

Prediction of the “in vivo” mechanical behavior of biointegrable acrylic macroporous scaffolds

This study examines a biocompatible scaffold series of random copolymer networks P(EA-HEA) made of Ethyl Acrylate, EA, and 2-Hydroxyl Ethyl Acrylate, HEA. The P(EA-HEA) scaffolds have been synthesized with varying crosslinking density and filled with a Poly(Vinyl Alcohol), PVA, to mimic the growing cartilaginous tissue during tissue repair. In cartilage regeneration the scaffold needs to have sufficient mechanical properties to sustain the compression in the joint and, at the same time, transmit mechanical signals to the cells for chondrogenic differentiation. Mechanical tests show that the elastic modulus increases with increasing crosslinking density of P(EA-HEA) scaffolds. The water plays an important role in the mechanical behavior of the scaffold, but highly depends on the crosslinking density of the proper polymer. Furthermore, when the scaffold with hydrogel is tested it can be seen that the modulus increases with increasing hydrogel density. Even so, the mechanical properties are inferior than those of the scaffolds with water filling the pores. The hydrogel inside the pores of the scaffolds facilitates the expulsion of water during compression and lowers the mechanical modulus of the scaffold. The P(EA-HEA) with PVA shows to be a good artificial cartilage model with mechanical properties close to native articular cartilage.

Programmable Liquid Crystal Elastomers Prepared by Thiol-Ene Photopolymerization.

The spontaneous conversion of a flat film into a 3-D shape requires local programming of the mechanical response. Historically, the ability to locally program the mechanical response of high strain (>30%) liquid crystalline elastomers (LCEs) has been limited to magnetic or mechanical alignment techniques, which limits spatial resolution. Recently, we reported on the preparation of LCEs capable of 55% strain with spatial control of the mechanical response at scales as small as 0.01 mm2. Here, we report a distinct formulation strategy to realize programmable stimulus-response in LCEs. Photopolymerization of thiol-ene/acrylate formulations yields materials that exhibit large reversible strain up to 150%. The photopolymerization reaction is extremely rapid, reducing preparation time from days to minutes. The mechanical behavior of these materials can be tuned by varying cross-link density. Spatial and hierarchical programming of the director profile is demonstrated, enabling 3-D shape change, including twisting ribbons and localized Gaussian curvature.

Macroporous thin membranes for cell transplant in regenerative medicine

The aim of this paper is to present a method to produce macroporous thin membranes made of poly (ethyl acrylate-co-hydroxyethyl acrylate) copolymer network with varying cross-linking density for cell transplantation and prosthesis fabrication. The manufacture process is based on template techniques and anisotropic pore collapse. Pore collapse was produced by swelling the membrane in acetone and subsequently drying and changing the solvent by water to produce 100 microns thick porous membranes. These very thin membranes are porous enough to hold cells to be transplanted to the organism or to be colonized by ingrowth from neighboring tissues in the organism, and they present sufficient tearing stress to be sutured with surgical thread. The obtained pore morphology was observed by Scanning Electron Microscope, and confocal laser microscopy. Mechanical properties were characterized by stress–strain experiments in tension and tearing strength measurements. Morphology and mechanical properties were related to the different initial thickness of the scaffold and the cross-linking density of the polymer network. Seeding efficiency and proliferation of mesenchymal stem cells inside the pore structure were determined at 2 h, 1, 7, 14 and 21 days from seeding.

Poly(ethylene glycol)-poly(lactic-co-glycolic acid) core–shell microspheres with enhanced controllability of drug encapsulation and release rate

Poly(lactic-co-glycolic acid) (PLGA) microspheres have been widely used as drug carriers for minimally invasive, local, and sustained drug delivery. However, their use is often plagued by limited controllability of encapsulation efficiency, initial burst, and release rate of drug molecules, which cause unsatisfactory outcomes and several side effects including inflammation. This study presents a new strategy of tuning the encapsulation efficiency and the release rate of protein drugs from a PLGA microsphere by filling the hollow core of the microsphere with poly(ethylene glycol) (PEG) hydrogels of varying cross-linking density. The PEG gel cores were prepared by inducing in situ cross-linking reactions of PEG monoacrylate solution within the PLGA microspheres. The resulting PEG-PLGA core–shell microspheres exhibited (1) increased encapsulation efficiency, (2) decreased initial burst, and (3) a more sustained release of protein drugs, as the cross-linking density of the PEG gel core was increased. In addition, implantation of PEG-PLGA core–shell microspheres encapsulated with vascular endothelial growth factor (VEGF) onto a chicken chorioallantoic membrane resulted in a significant increase in the number of new blood vessels at an implantation site, while minimizing inflammation. Overall, this strategy of introducing PEG gel into PLGA microspheres will be highly useful in tuning release rates and ultimately in improving the therapeutic efficacy of a wide array of protein drugs.

Mechanical characterization of oligo(ethylene glycol)-based hydrogels by dynamic nanoindentation experiments

Oligo(ethylene glycol)-based (OEG) hydrogel samples of varying cross-link densities and degrees of swelling were characterized through dynamic nanoindentation testing. Experiments were performed using a non-standard nanoindentation method, which was validated on a standard polystyrene sample. This method maximizes the capability of the instrument to measure the stiffness and damping of highly compliant, viscoelastic materials. Experiments were performed over the frequency range of 1 to 50Hz, using a 1mm diameter flat punch indenter. A hydration method was adopted to avoid sample dehydration during testing. Values of storage modulus (E′) ranged from 3.5 to 8.9MPa for the different OEG-hydrogel samples investigated. Samples with higher OEG concentrations showed greater scatter in the modulus measurements and it is attributed to inhomogeneities in these materials. The (E′) values did not show a strong variation over frequency for any of the samples. Values of loss modulus (E″) were two orders of magnitude lower than the storage modulus, resulting in very low values of loss factor (E″/E′<0.1). These are characteristics of strong gels, which present negligible viscous properties.

Macroporous Poly(Vinyl Acetate-Co-Divinyl Benzene) Copolymer Beads as Adsorptive Support for the Direct Immobilization of Candida antarctica Lipase B

A series of macroporous poly(vinyl acetate-co-divinyl benzene) [poly(VAc-co-DVB)] copolymer beads with varying cross-link density (CLD) were synthesized by suspension polymerization and used as support for the immobilization of Candida antarctica lipase B (CALB) with and without saponification. These copolymer beads have average pore diameter and BET (Brunauer-Emmett-Teller) surface area ranging between 69 nm to 115 nm and 89 m2/g to 204 m2/g respectively. On immobilization, non-saponified poly(VAc-co-DVB) copolymer beads with CLD 50% (PVAc-DVB-50-UH) showed higher tributyrin hydrolysis activity (2647.35 TBU/g dry beads) than that saponified beads (PVAc-DVB-50-H) (1706.54 TBU/g dry beads) indicating that direct immobilization without saponification is better for CALB immobilization. PVAc-DVB-50-UH showed 94.82% residual activity after 100 days of storage.

Cross-linked polymers of diethynylbenzene and phenylacetylene as new polymer precursors for high-yield synthesis of high-performance nanoporous activated carbons for supercapacitors, hydrogen storage, and CO2capture

We demonstrate in this article the first use of a family of cross-linked polymers synthesized from acetylenic monomers (diethynylbenzene and phenylacetylene) as new polymer precursors for high-yield synthesis of nanoporous activated carbons of tuneable textural properties. A range of cross-linked polymers of varying cross-linking densities has been tailor synthesized via catalytic polymerization of the monomers. Their carbonization in the presence of KOH as the activation agent renders effectively high-surface-area, predominantly microporous activated carbons of unique textural properties (surface area up to 1418 m2 g−1; pore volume up to 0.78 cm3 g−1) at high yields (ca. 70 wt%). It is discovered that the textural properties of the resulting carbon materials (including surface area, pore volume, pore size and pore size distribution) depend sensitively on the cross-linking density of the polymer precursors and are thus tuneable through polymer design. With their unique textural properties, the high-surface-area activated carbons exhibit superior performance in applications both as electrode materials in supercapacitors and as sorbents for H2 storage and CO2 capture. High specific capacitance (up to 446 F g−1 at 0.5 A g−1 in 3-electrode cell; 334 F g−1 at 0.1 A g−1 and 287 F g−1 at 0.5 A g−1 in 2-electrode cell) has been obtained for the electrodes fabricated with these carbons. Gas sorption study confirms that these carbons show remarkably high capacities for the adsorption of both H2 and CO2 (up to 2.66 wt% for H2 adsorption at 77 K and 1 bar; up to 6.95 mmol g−1 along with a high CO2/N2 selectivity of 11 for CO2 adsorption at 0 °C and 1 bar). These performance properties in all three applications are well comparable to or even better than the best results obtained with various activated carbons to date. The nanoporous activated carbons obtained from this new family of polymer precursors are thus the rare carbon materials that show combined superior performance in all three important applications.

EPDM RECYCLED RUBBER POWDER CHARACTERIZATION: THERMAL AND THERMOGRAVIMETRIC ANALYSIS

ABSTRACT Investigations on ethylene–propylene–diene rubber (EPDM) with varying cross-link densities and carbon black contents, as well as commercial EPDM waste ground rubber (WGR), have been performed to improve characterization. The aim is to provide additional quality-control methods in the field of rubber recycling, as WGR is transformed and reused in the form of rubber mats mainly for playground, agricultural, sport, and automobile applications. Inductively coupled plasma optical emission spectrometry, atomic absorption spectrometry, and elemental (CHNS) analyses were used to determine the content of various chemical elements. Thermogravimetric analysis (TGA) was used to determine the amount of carbon black and inorganic material in the sample. Mass loss at 400 °C was related to cross-link density. TGA coupled with mass spectrometry showed that this mass loss corresponds to the loss of SO2 as cross-links are destroyed during heating. Melt point and glass transition temperatures determined by differential scanning calorimetry are mainly proportional to the ethylene/propylene weight ratio.

Crosslinking effect on polydimethylsiloxane elastic modulus measured by custom‐built compression instrument

ABSTRACTA macroscopic compression test utilizing a simple custom‐built instrument was employed to measure polydimethylsiloxane (PDMS) elastic modulus. PDMS samples with varying crosslinking density were prepared with the elastomer base to the curing agent ratio ranging from 5 : 1 to 33 : 1. The PDMS network elastic modulus varied linearly with the amount of crosslinker, ranging from 0.57 MPa to 3.7 MPa for the samples tested. PDMS elastic modulus in MPa can be expressed as 20 MPa/PDMS base to curing agent ratio. This article describes a simple method for measuring elastic properties of soft polymeric materials. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 41050.

Photoswitchable ratchet surface topographies based on self-protonating spiropyran-NIPAAM hydrogels.

In this work, self-protonating spiropyran-based poly(N-isopropylacrylamide) polymer networks are prepared. These photoresponsive hydrogel coatings can change their surface topography upon exposure with visible light in a neutral environment. Photoresponsive surface-constrained films have been fabricated for which the swelling behavior can be controlled in a reversible manner. In a first step, symmetrical switchable surface topologies with varying cross-link density are obtained by polymerization-induced diffusion. Under light exposure, the areas with low cross-link density swell more than the areas with high cross-link density, thus forming a corrugated surface. Asymmetric ratchet-like photoresponsive surfaces have been prepared on prestructured asymmetric substrates. As a result of thickness variation of the surface-confined hydrogel layer, an asymmetric swelling behavior is obtained. Depending on the cross-link density of the hydrogel, it is possible to switch between a ratchet and flat surface topography or even an inverse ratchet surface by light.

Effect of crosslinking density on resilient performance of low-resilience flexible polyurethane foams

Low-resilience flexible polyurethane foams (FPUFs) with varying crosslinking densities, were synthesized from polyols, 4, 4′-diphenyl methane diisocyanate (MDI-100), and water. The effects of crosslinking agent content, the molecular weight (Mw) of polyether diol, and the ratio of (polyether triol)/(polyether diol) on the resilience performance of FPUFs were investigated. Results indicate that higher crosslinking density was beneficial to the increasing of recovery time. The recovery time of the FPUF using polyether diol with Mw of 400 was 24.3 s. It was 3.2 times longer compared with FPUF using polyether diol with Mw of 3000. Dynamic mechanical analysis (DMA) results showed that, FPUFs with high crosslinking density displayed viscoelasticity in a wider range of temperature. However, when molecular chains were longer enough, FPUFs with low crosslinking density also demonstrated significant viscoelasticity, which is owing to the excess of physical crosslinking points. The results in stress–strain cycling and recovery time experiments were consistent with the results of DMA. POLYM. ENG. SCI., 55:308–315, 2015. © 2014 Society of Plastics Engineers