Controlled Formation of Microheterogeneous Polymer Networks: Influence of Monomer Reactivity on Gel Structure

Closed-Loop Controlled Photopolymerization of Hydrogels

Here, we present a closed-loop controlled photopolymerization process for fabrication of hydrogels with controlled storage moduli. Hydrogel crosslinking was associated with a significant change in the phase angle of a piezoelectric cantilever sensor and established the timescale of the photopolymerization process. The composition, structure, and mechanical properties of the fabricated hydrogels were characterized using Raman spectroscopy, scanning electron microscopy (SEM), and dynamic mechanical analysis (DMA). We found that the storage moduli of photocured poly(ethylene glycol) dimethacrylate (PEGDMA) and poly(N-isopropylacrylamide) (PNIPAm) hydrogels could be controlled using bang-bang and fuzzy logic controllers. Bang-bang controlled photopolymerization resulted in constant overshoot of the storage modulus setpoint for PEGDMA hydrogels, which was mitigated by setpoint correction and fuzzy logic control. SEM and DMA studies showed that the network structure and storage modulus of PEGDMA hydrogels were dependent on the cure time and temporal profile of UV exposure during photopolymerization. This work provides an advance in pulsed and continuous photopolymerization processes for hydrogel engineering based on closed-loop control that enables reproducible fabrication of hydrogels with controlled mechanical properties.

Enhanced Mechanical Properties by Ionomeric Complexation in Interpenetrating Network Hydrogels of Hydrolyzed Poly (N-vinyl Formamide) and Polyacrylamide

Tough hydrogels were made by hydrolysis of a neutral interpenetrating network (IPN) of poly (N-vinyl formamide) PNVF and polyacrylamide (PAAm) networks to form an IPN of polyvinylamine (PVAm) and poly (acrylic acid) (PAAc) capable of intermolecular ionic complexation. Single network (SN) PAAm and SN PNVF have similar chemical structures, parameters and physical properties. The hypothesis was that starting with neutral IPN networks of isomeric monomers that hydrolyze to comparable extents under similar conditions would lead to formation of networks with minimal phase separation and maximize potential for charge-charge interactions of the networks. Sequential IPNs of both PNVF/PAAm and PAAm/PNVF were synthesized and were optically transparent, an indication of homogeneity at submicron length scales. Both IPNs were hydrolyzed in base to form PVAm/PAAc and PAAc/PVAm IPNs. These underwent ~5-fold or greater decrease in swelling at intermediate pH values (3-6), consistent with the hypothesis of intermolecular charge complexation, and as hypothesized, the globally neutral, charge-complexed gel states showed substantial increases in failure properties upon compression, including an order of magnitude increases in toughness when compared to their unhydrolyzed states or the swollen states at high or low pH values. There was no loss of mechanical performance upon repeated compression over 95% strain.

Direct Observation of Spatiotemporal Heterogeneous Gelation by Rotational Tracking of a Single Anisotropic Nanoprobe

Polymer network gels usually exhibit spatial heterogeneity of local defects and cross-link density, which can affect their elasticity on the microscopic scale differently. The ability to evaluate the formation and distribution of these heterogeneities is important for guiding the application of gels in biology, medicine, and separation science. Previously, it has been reported that single-particle tracking based microrheology could provide local properties of gel networks with high resolution; however, the particle probes have been limited to spherical micro/nanotracers undergoing translational motions. In this work, we used single gold nanorods (AuNRs) as rotational microrheology probes to study the polyacrylamide gelation process by dual-channel polarization dark-field microscopy. The AuNRs were in Brownian motion during the initial stages of the gelation. As the reaction continues, individual AuNRs are confined locally and almost lost translational motion, but still maintained rotational motion. As the reaction proceeded further, the rotation state of the AuNRs gradually changed from free rotation in 3D to restricted rotation in 2D and eventually stopped completely. The appearance of the intermediate 2D plane indicated the existence of localized anisotropic compression of the gel during the heterogeneous gelation process. Our method can be further applied to investigate the formation of different polymer gels and a wide variety of heterogeneous biophysical and soft material systems.

Origin of nanostructural inhomogeneity in polymer-network gels

Polymer-network gels often display nano- to microstructural inhomogeneity; this article reviews multiple types of origin of this structural feature.

Hydrogel microphones for stealthy underwater listening

Exploring the abundant resources in the ocean requires underwater acoustic detectors with a high-sensitivity reception of low-frequency sound from greater distances and zero reflections. Here we address both challenges by integrating an easily deformable network of metal nanoparticles in a hydrogel matrix for use as a cavity-free microphone. Since metal nanoparticles can be densely implanted as inclusions, and can even be arranged in coherent arrays, this microphone can detect static loads and air breezes from different angles, as well as underwater acoustic signals from 20 Hz to 3 kHz at amplitudes as low as 4 Pa. Unlike dielectric capacitors or cavity-based microphones that respond to stimuli by deforming the device in thickness directions, this hydrogel device responds with a transient modulation of electric double layers, resulting in an extraordinary sensitivity (217 nF kPa(-1) or 24 μC N(-1) at a bias of 1.0 V) without using any signal amplification tools.

Nanostructural heterogeneity in polymer networks and gels

Many polymer gels display network defects and crosslinking inhomogeneity. This review reflects and interrelates investigations on the characterization of such polymer-network heterogeneity and on its impact on the swelling, elasticity, and permeability of polymer gels.

A molecular dynamics simulation study on polymer networks of end-linked flexible or rigid chains

The differences in formation and structural properties of polymer networks consisting of end-linked flexible or rigid chains were studied by molecular dynamics simulation. Networks were formed from monodisperse, linear, short, flexible or rigid chains with functional end groups and a stoichiometric ratio of trifunctional cross-linkers. The rigid chains had a rodlike shape defined by an angle potential, while the flexible chains had no angle potential. In order to understand the influence of chain rigidity, all parameters of precursor chains (length, reactivity, bond potential, nonbonding potential) were the same, with the exception of the angle potential. The system density rho, corresponding to the concentration of monomer in solvent, was varied from 0.01 to 0.11. Different network structures resulting from the different processes of network formation were observed. Simulations showed that the flexible chains created an inhomogeneous network on a large scale via microgel cluster formation, in agreement with experimental observations, whereas the rigid chains rapidly created a homogeneous network in the entire system volume without first generating microgel clusters, with the additional difference that they gave rise to mutually interpenetrating networks at the local scale.

Interaction between lysozyme and poly(acrylic acid) microgels

The interaction between lysozyme and oppositely charged poly(acrylic acid) microgels was investigated by micromanipulator-assisted light microscopy, confocal microscopy and circular dichroism. Lysozyme uptake and distribution within the microgel particles, and its effect on microgel deswelling, was studied regarding effects of pH, ionic strength and lysozyme concentration. For a range of conditions, lysozyme distributes nonuniformly within the microgels, forming a lysozyme/microgel shell in the outer parts of the microgel. This shell formation is associated both with increased lysozyme loading to the microgels and with increased lysozyme-induced microgel deswelling. At high microgel charge density, the shell formation displays nonmonotonic ionic strength dependence. The shells formed are characterized by a net positive charge, and by relatively fast exchange of lysozyme between shell and solution, although the exchange kinetics decreases strongly with decreasing ionic strength. At conditions of slower exchange kinetics, the shells are characterized by an effective pore size of less than about 4 nm.

Nanomagnetic sponges for the cleaning of works of art

This letter reports the synthesis and characterization of functionalized magnetic nanoparticles associated with chemical gels and their application to the conservation of cultural heritage. Magnetic nanoparticles, which are associated with acrylamide ethylene oxide polymers, produce a sponge that can be loaded with oil-in-water microemulsions, forming a magnetically responsive gel-like system and acting as a permanent hydrogel. The magnetic gel-like system can be used for specific applications in detergents or in the release of the loaded material. The system can be magnetically manipulated and cleaned from the loaded materials and then dried and reused for a different application. We report an important application of this new nanomagnetic responsive material in the field of cultural heritage conservation.

Using in situ rheology to characterize the microstructure in photopolymerized polyacrylamide gels for DNA electrophoresis

Photopolymerized cross-linked polyacrylamide hydrogels are attractive sieving matrix formulations for DNA electrophoresis owing to their rapid polymerization times and the potential to locally tailor the gel pore structure through spatial variation of illumination intensity. This capability is especially important in microfluidic systems, where photopolymerization allows gel matrices to be precisely positioned within complex microchannel networks. Separation performance is also directly related to the nanoscale gel pore structure, which is in turn strongly influenced by polymerization kinetics. Unfortunately, detailed studies of the interplay among polymerization kinetics, mechanical properties, and structural morphology are lacking in photopolymerized hydrogel systems. In this paper, we address this issue by performing a series of in situ dynamic small-amplitude oscillatory shear measurements during photopolymerization of cross-linked polyacrylamide electrophoresis gels to investigate the relationship between rheology and parameters associated with the gelation environment including UV intensity, monomer and cross-linker composition, and reaction temperature. In general, we find that the storage modulus G' increases with increasing initial monomer concentration, cross-linker concentration, and polymerization temperature. The steady-state value of G', however, exhibits a more complex dependence on UV intensity that varies with gel concentration. A simple model based on rubber elasticity theory is used to obtain estimates of the average gel pore size that are in surprisingly good agreement with corresponding data obtained from analysis of DNA electrophoretic mobility in gels cast under identical polymerization conditions.

The effect of curing modes on polymerization contraction stress of a dual cured composite

Although a lower curing rate is often cited as the reason why a chemical cured (CC) dental composite produces lower polymerization contraction stress (PCS) than a light cured (LC) composite, the exact mechanism is still unclear. In addition, the comparison is often made by using different brands of composites. The comparison's fairness is questionable because the two composites have different compositions and preparation procedures. The goal of the present work was to determine if the curing mode alone can produce different PCS. We formulated a dual cured composite and prepared it the same way for both CC and LC modes. We measured PCS by a strain gauge method, shrinkage by a video-imagining technique, degree of conversion (DC) by infrared spectroscopy, and flexural modulus by the three-point bending test. The CC specimens showed lower PCS and lower flexural modulus than the LC specimens, although both possessed an identical chemical composition and physical texture before cure. This finding indicates that the curing mode alone can affect PCS. Because the CC and LC specimens produced a similar shrinkage and DC, the lower modulus is considered to be one of the reasons for the lower stress. Using a structural inhomogeneity model, we explained how a resin composite with an identical DC can have different physical properties such as the modulus.

Protein Transfer through Polyacrylamide Hydrogel Membranes Polymerized in Lyotropic Phases

A way to control the average pore size in cross-linked polyacrylamide-based membranes is by altering the ratio of cross-linker to acylamide monomer. Larger pore sizes are prepared with a minimum amount of cross-linker, resulting in membranes that are mechanically weak and have short lifetimes. The aim of this study was to prepare cross-linked polyacrylamide membranes with large pore sizes and with good mechanical integrity. The methodology was to carry out the polymerization in a template, formed from the self-aggregation of surfactant. Two surfactant templates were used, and their pore size was examined with proteins of different sizes. The surfactants chosen for this study were sodium dodecyl sulfate (SDS, ionic surfactant) and TERIC BL8 (nonionic surfactant), both of which have very different aggregation properties. The data showed that at 10% and greater of TERIC BL8, a very different and open gel structure is formed, in which the pore size was significantly increased. SDS seemed to have little effect on the pore size. The data suggests that the gel structures for both surfactants up to 4% (w/v) are similar and micellular, because SDS is known to favor a micelle structure. Above 4% (w/v), TERIC BL8 then goes through a change in its lyotropic phase, thus, producing membranes of a large pore size. In conclusion, the pore size and gel structure of polyacrylamide hydrogel membranes can be significantly increased using TERIC BL8 (nonionic) surfactant. This allows large-pore-size membranes with a high cross-link density and consequently high mechanical strength to be prepared for the separation of large biomolecules.

Novel cross-linked homogeneous polyacrylamide gels with improved separation properties: investigation of the cross-linker functionality

Polyacrylamide (PAAm) gels were synthesized using cross-linkers with their potential functionality (twice the number of double bonds of a cross-linker) varying from six to sixteen. Improved electrophoretic separation and highly desirable porosity and sieving properties were observed for most of the PAAm gels containing novel cross-linkers. An increase in the potential functionality of cross-linkers used in PAAm gels was an important factor, influencing the pore size and pore size distribution of the network.