Cross-linked Polymer Gels Research Articles

Modeling of Partially Hydrolyzed Polyacrylamide-Hexamine-Hydroquinone Gel System Used for Profile Modification Jobs in the Oil Field

The cross-linked polymer gel systems are being used increasingly to redirect or modify reservoir fluid movement in the vicinity of injection wells for the purpose of permeability/profile modification job in the oil field due to their high temperature stability and capability to provide rigid gel having high mechanical strength. In this study, a partially hydrolyzed polyacrylamide-hexamine-hydroquinonegel is used for the development of polymer gel system. The experimental investigation demonstrates that the gelation time varies with polymer and crosslinker concentration and the temperature. The mathematical model is developed with the help of gelation kinetics of polymer gel and using Arrhenius equation, which relates the gelation time with polymer, crosslinker concentrations, and temperature. The developed model is solved with the help of multivariate regression method. It is observed in this study that the theoretical values of gelation time have good agreement with the experimental values.

Photoinduced Volume Transition in Liquid Crystalline Polymer Gels Swollen by a Nematic Solvent.

Nematic liquid crystalline (LC) cross-linked polymer gel sheets containing 3 mol % azobenzene (Az) unit were prepared and swollen by a nematic solvent of 4'-pentyl-4-cyanobiphenyl (5CB). This 5CB-swollen gel sheet exhibited a discontinuous volume change around the nematic-isotoropic phase transition temperature of the LC gel (TNIG). UV irradiation at a temperature slightly lower than TNIG provoked a large volume transition (expansion) due to a loss of nematic order within the gel sheet caused by the trans-to-cis photoisomerization of Az. The volume was reverted by irradiation with 436 nm light. By templating the colloidal crystal film of monodispersed silica particles, a LC gel sheet possessing a microporous structure was also prepared. Due to the facilitated diffusion of 5CB, the microporous LC gel exhibited significant enhancements in the extent and rate of the photoinduced volume transition.

Rheological Images of Dynamic Covalent Polymer Networks and Mechanisms behind Mechanical and Self-Healing Properties

In our previous work [Macromolecules2010, 43, 1191–1194], we synthesized dynamic covalent cross-linked polymer gels through condensation of acylhydrazines at the chain ends of poly(ethylene oxide) (A2) and aldehyde groups in tris[(4-formylphenoxy)methy]ethane (B3) and reported reversible sol–gel transition and self-healing properties of the gels. For those dynamic gels, this paper examines the gelation kinetics and rheological behavior in pre- and postgelation stages and discusses the molecular mechanism underlying the mechanical and self-healing properties. The results showed that the condensation reaction before the critical gelation point can be treated as the pseudo-second-order reaction. The scaling exponent n (=0.75) for the frequency dependence of the complex moduli at the critical gel point, the exponent γ (=1.5) for the concentration dependence of the viscosity in the pregel regime, and the exponent z (=2.5) for the concentration dependence of the equilibrium modulus in the postgel regime were found to not exactly obey the relationship for covalent gels, n = z/(z + γ), possibly because of the dynamic nature of the gels. The terminal relaxation of the dynamic gels at high temperature (125 °C) accorded with the Maxwellian model, as often observed for transient associating networks. In contrast, at low temperature (25 °C) where this transient network reorganization was essentially quenched in a time scale of experiments (∼50 s), the uniaxial stress–strain behavior of the gel was well described by the classical model of rubber elasticity σeng = G(λ – 1/λ2) up to 300% stretch (as similar to the behavior of usual gels chemically cross-linked in a swollen state). Ultimately, the gel cut into two pieces was found to exhibit self-healing under ambient conditions in 8 and 24 h, respectively, when the edges of those pieces were coated and not coated with acid (catalyst for dynamic covalent bond formation).

Interactions between Colloids Induced by a Soft Cross-Linked Polymer Substrate

Using videomicroscopy imaging, we demonstrate the existence of a short-ranged equilibrium attraction between heavy silica colloids diffusing on soft surfaces of cross-linked polymer gels. The intercolloid potential can be tuned by changing the gel stiffness or by coating the colloids with a polymer layer. On sufficiently soft substrates, the interaction induced by the polymer matrix leads to large-scale colloidal aggregation. We correlate the in-plane interaction with a colloid-surface attraction.

Biofilms as complex fluids

Abstract

PH-induced metal-ligand cross-links inspired by mussel yield self-healing polymer networks with near-covalent elastic moduli.

Growing evidence supports a critical role of metal-ligand coordination in many attributes of biological materials including adhesion, self-assembly, toughness, and hardness without mineralization [Rubin DJ, Miserez A, Waite JH (2010) Advances in Insect Physiology: Insect Integument and Color, eds Jérôme C, Stephen JS (Academic Press, London), pp 75-133]. Coordination between Fe and catechol ligands has recently been correlated to the hardness and high extensibility of the cuticle of mussel byssal threads and proposed to endow self-healing properties [Harrington MJ, Masic A, Holten-Andersen N, Waite JH, Fratzl P (2010) Science 328:216-220]. Inspired by the pH jump experienced by proteins during maturation of a mussel byssus secretion, we have developed a simple method to control catechol-Fe(3+) interpolymer cross-linking via pH. The resonance Raman signature of catechol-Fe(3+) cross-linked polymer gels at high pH was similar to that from native mussel thread cuticle and the gels displayed elastic moduli (G') that approach covalently cross-linked gels as well as self-healing properties.

A crosslinked “polymer–gel” rechargeable lithium-ion battery electrolyte from free radical polymerization using nonionic plastic crystalline electrolyte medium

A cross-linked polymer–gel soft matter electrolyte with superior electrochemical, thermal and mechanical properties obtained from free radical polymerization of vinyl monomers in a semi-solid organic nonionic plastic crystalline electrolyte for application in rechargeable lithium-ion batteries is discussed here.

Investigation of Damage Mechanisms in PMMA during ToF-SIMS Depth Profiling with 5 and 8 keV SF5+ Primary Ions

Cluster secondary ion mass spectrometry (cluster SIMS) has been proven to be a useful technique for the surface and in-depth characterization of molecular films. In this study, an SF5+ polyatomic primary ion source is utilized for depth profiling in poly(methyl methacrylate) (PMMA) bulk and thin films (200 nm). The effects of SF5+ ion beam energy are discussed in detail. Both 5 and 8 keV ion beam energies are utilized for depth profiling experiments, where the chemistry of sputtering is investigated using surface analytical tools such as X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) in conjunction with SIMS. Thin film depth profiles acquired with 5 keV SF5+ display evidence of significant damage accumulation at the interface in the form of a highly cross-linked polymer gel. There is very little evidence of similar damage accumulation at the interface for the corresponding 8 keV SF5+depth profile. AFM and XPS analysis of the sputtered crater bottoms also indicates that very different chemistries and morphologies are present at the interface when employing 5 keV vs 8 keV SF5+. For PMMA bulk samples, greater erosion depths were achieved when employing higher beam energies, similar to what has been observed previously with C60n+ depth profiling.(1) These increased erosion depths are attributed to the increased sputter rates of the PMMA at 8 keV SF5+ as compared to 5 keV SF5+, thus allowing for increased amounts of material to be removed prior to the approach of the gel point of the PMMA (dose at which a 3-D cross-linked structure is formed). Despite these increased erosion depths, the 8 keV SF5+ beam imparts greater initial structural damage, as indicated by decreased C═O contents in the C1s XPS spectra and increased amounts of graphitic type peaks in the corresponding SIMS spectra. Overall, the results indicate that, for thicker samples, one should employ higher beam energies for optimum results. However, for thinner films, in which the gel effect does not play a significant role, lower beam energies are preferred.

Reversibly Cross-Linked Polymer Gels as Healing Agents for Epoxy-Amine Thermosets

Reversibly Cross-Linked Polymer Gels as Healing Agents for Epoxy-Amine Thermosets

New functional copolymers of N-acryloyl-N′-methyl piperazine and 2-hydroxyethyl methacrylate: synthesis, determination of reactivity ratios and swelling characteristics of gels

Copolymers of N-acryloyl-N′-methylpiperazine (AcrNMP) and 2-hydroxyethyl methacrylate (HEMA) were synthesized by free radical solution polymerization in dioxane at 70 ± 1 °C, using 2,2′-azobisisobutyronitrile (AIBN) as initiator. The copolymer compositions were analyzed by the methods of FTIR spectroscopy and elemental analysis. Both the method of analysis yielded results that agreed reasonably well. The monomer reactivity ratios of the copolymerization were determined by the linearization methods of Finemann–Ross (FR) and Kelen–Tudos (KT). The reactivity parameter results derived using FTIR analysis showed that the copolymerization yielded mainly alternating structure with reactivity ratios, r1(AcrNMP) = 0.263 ± 0.011 and r2(HEMA) = 0.615 ± 0.097 by F–R method and r1 = 0.227 ± 0.074 and r2 = 0.53 ± 0.15 by KT method. Microstructure data calculated by the method of Igarashi also supports the alternating structure (tendency) of the copolymer. Crosslinked polymer gels of this system exhibited remarkably high swelling of more than 500% in water at ambient temperature.

Covalent Cross-Linked Polymer Gels with Reversible Sol−Gel Transition and Self-Healing Properties

Covalent Cross-Linked Polymer Gels with Reversible Sol−Gel Transition and Self-Healing Properties

Interaction of Polymer and Solvent in Chemical Crosslinked Polymer Gel

Interactions of polymer and solvent in a highly chemical crosslinked polystyrene (PS) and polymethyl acrylate (PMA) gels are investigated. Crystal thermo of the benzene in PS-benzene gels were analyzed using differential scanning calorimeter (DSC). It was noted that benzene molecules in the gels can be categorized into three types, which is free benzene, weak bonded benzene and strong bonded benzene. In average, one PS structure unit interacted with three benzene molecules in the gel system. The conformation of dichloroethane (DCE) in PMA-DCE gels was studied using Fourier transfer infrared (FTIR). In PMA-DCE gels, DCE molecule bonded with PMA chains in a trans mode through a hydrogen bond. And the amount of trans DCE molecules increased with the increase in concentration of PMA in the gel system.

Solvent Programmable Polymers Based on Restricted Rotation

Solvent programmable polymers (SPPs) were developed that can modulate their recognition properties by heating in different solvents. These highly cross-linked polymer gels were able to respond to differences in solvent polarity at elevated temperatures via rotation about a C(aryl)-N(imide) bond of a carboxylic acid monomer. When heated in polar solvents such as water, the number of solvent accessible carboxylic acids in the polymers increases. When heated in nonpolar solvents such as toluene, the number of solvent accessible carboxylic acids decreases. On cooling to rt, these changes are preserved and maintained even when the polymer is removed from the solvent imprinting environment. The solvent memory is due to the reestablishment of restricted rotation around that C(aryl)-N(imide) bond, which locks the carboxylic acid recognition groups into either a solvent accessible or inaccessible orientation. The solvent programmability was also shown to be reversible. The fidelity of the SPP switching process did not decrease after five cycles of heating in polar and nonpolar solvents.

Reversibly Cross-Linked Polymer Gels as Healing Agents for Epoxy−Amine Thermosets

The Diels-Alder reaction was used to develop a reversibly cross-linking gel as a healing agent for traditional epoxy-amine thermosets. Direct application of the reversibly cross-linking network to a crack surface in an epoxy-amine thermoset resulted in the recovery of 37% of the initial epoxy-amine network's strength. Composites in which the reversibly cross-linking gel was incorporated as a secondary particulate phase recovered 21% of the initial composite strength after the first healing cycle, with healing possible up to five times.

Copolymer-Based Progressive Addition Lens with Graded Index Designed for Astigmatism and Distortion Correction

In this research, we propose to reduce astigmatism and distortion in a progressive addition lens (PAL) by employing a polymer-based graded-index (GRIN) material. We adopted the diffusion copolymerization method for the GRIN-PAL fabrication. The GRIN material was prepared by partial diffusion of methyl methacrylate (MMA) and 2,2,2-trifluoroethyl methacrylate (3FMA) monomers into the cross-linked benzyl methacrylate polymer gel. GRIN-PAL was prepared by polymerization of the GRIN material in a mold of commercially available PAL. As a result, methods to control Deltan, the diffusion length, and the order of the GRIN profile in polymeric GRIN materials were demonstrated. In a combination of the PBzMA gel and the MMA/3FMA diffusion monomer, 50/50 in volume provides the highest Deltan. The diffusion period mainly affects the diffusion length. These two parameters consequently determine the order of the GRIN profile and the focusing power that the GRIN provides. The difference in the ease of diffusion caused by the difference in the cross-linking density provides a method for positional control of the GRIN profile. Furthermore, GRIN-PAL using a polymeric material was successfully fabricated. Improvement in astigmatism and distortion was demonstrated using GRIN. Therefore, the GRIN material is suggested to be applicable for the further modification of the PAL with low astigmatism and distortion.

Influence of Pressure on Boron Cross-Linked Polymer Gels

Using steady-shear rheometry in combination with high-pressure 11B nuclear magnetic resonance spectroscopy (11B NMR), we have found that gels formed from water-soluble polymers containing vicinal hydroxyl groups cross-linked with various boron-containing compounds undergo significant structural changes that result in a pronounced loss of viscosity when placed under pressure. Importantly, gels from other cross-linking agents tested, including Ti(IV) and Zr(IV), did not show this loss in viscosity. The experimental study probed pressure-induced changes to both galactomannan and polyvinyl alcohol (PVA) gels cross-linked with either aryl boronic acids or alkali metal boron-containing salts using pressure conditions that ranged from atmospheric to 680 bar and temperatures that ranged from 20 to 65 °C. Significantly, the pressure-induced losses in viscosity and, to a somewhat lesser extent, the concomitant pressure-induced 11B NMR spectral changes were found to be reversed upon lowering the pressure.

Colloid Aggregation Arrested by Caging within a Polymer Network

The aggregation of colloidal particles within the confines of a polymer network has been studied. An isorefractive covalently cross-linked polymer gel in dimethyl sulfoxide was formulated so that the multicomponent system that is the gel is essentially invisible to light scattering. The high dielectric solvent was chosen so that electrostatics could be used to control the state of aggregation of a colloid dispersed within the gel matrix. Smoluchowski's population balance equations were solved for the case where aggregates larger than the gel's mesh spacing are immobile. Light scattering intensities predicted from the evolution of the aggregate population were calculated. The observed asymptotic increase in scattering intensity is consistent with this model and indicates that the aggregation process becomes arrested by the spatial constraints imposed by the mesh of the polymer network. Essentially once the aggregates reach a certain size, they become caged within the mesh of the gel network and thus no longer aggregate. Evidence is also given that indicates that formulating for specific gel properties can lead to controlled final aggregate size.

Temperature-responsive glass coverslips with an ultrathin poly( N-isopropylacrylamide) layer

A temperature-responsive cross-linked polymer gel was covalently grafted onto glass coverslips by electron beam irradiation. The grafted thickness and amount of polymer as well as the surface wettability increased with the initial monomer concentration. When the monomer concentration was 5 wt.%, the grafted polymer density was 0.84 μg cm −2, and cells adhered and spread on the surface at 37 °C, but detached at 20 °C. In contrast, when the monomer concentration was 35 wt.%, the polymer density was 1.28 μg cm −2, and the surfaces were cell repellent even at 37 °C. These results show a remarkable contrast to those obtained from temperature-responsive polymer-grafted tissue culture polystyrene dishes, since various types of cells showed temperature-dependent cell adhesion/detachment when the grafted density was around 2 μg cm −2 on these surfaces. We discuss the possible molecular mechanisms underlying this discrepancy.

Model of Microphase Separation in Two-Dimensional Gels

We have recently introduced a simple model which contains the basic features of phase separation in cross-linked polymer gels: a stretched elastic network of Lennard-Jones particles [Peleg, O.; et al. EPL 2007, 77, 58007]. While the original work clearly demonstrated the phenomenology of the model at a limited set of system parameters, this paper explores quantitative details and gives answers to many of the questions which remained open. We define and analyze an order parameter, explore the phase diagram, and identify the range of elastic constants in which microphase separation is observed and analyze the microphase separation patterns. We discuss the microscopic origin of the formation of linear rather than globular filaments as well as monitor and quantify the reorganization of the network, not only in equilibrium but also its dynamics upon applying a shear deformation. These simulations provide us with the elastic properties, most importantly, the shear modulus, whose magnitude is shown to correlate with the order parameter.

Impact of Solvent Quality on the Density Profiles of Looped Triblock Copolymer Brushes by Neutron Reflectivity Measurements

Preferential adsorption of poly(2-vinylpyridine)-deuterated polystyrene-poly(2-vinylpyridine) (PVP-dPS-PVP) triblock copolymers from toluene onto silicon leads to the formation of dPS loops tethered by the PVP end blocks. Using neutron reflectometry, we have determined the segment density profiles of these looped polymer brushes in toluene, a good solvent for the dPS block, and in cyclohexane at 20 C (poor solvent), 32 C, (near- solvent), and 50 C (marginal solvent). While the swelling behavior qualitatively agrees with that observed for singly grafted brushes, there are interesting differences in the local structural details: In a good solvent, the segment density profiles are composed of an inner parabolic region and a long, extended tail. In cyclohexane, the profiles are described by exponential decays. We ascribe these features to a novel polydispersity effect that arises due to tethering the PS loops by both ends. The results also show that the less dense layers undergo more significant changes in swollen height as solvent quality is changed and that the looped brushes of different molecular weight, asymmetry, and tethering density adhere to scaling relationships derived for lightly cross-linked polymer gels.