Effects of guest microparticles on the swelling behavior of polyacrylamide gels

We report the effects of guest particles on the swelling properties of bulk polyacrylamide gels. The guest particles were the spheres of poly(N‐isopropylacrylamide) gel with submicrometer diameter, which were synthesized by an emulsion‐polymerized reaction in water. Polyacrylamide gels were prepared by a free radical polymerization reaction, immobilizing the gel microparticles with different concentrations at gelation. The macroscopic swelling ratio of this hybrid gel in a cylindrical shape was measured as functions of temperature and acetone concentration. The presence of guest particles was found to strongly affect the swelling behavior in the bulk gels when the concentration of incorporated particles exceeded a threshold. The experimental results indicated that the macroscopic volume in response to the temperature change should be determined by the guest particles above the threshold. On the other hand, the hybrid gel could not evidently shrink by adding acetone when the concentration of guest particles exceeded the threshold. To make clear the distribution of guest particles in the bulk networks, the fractured surfaces of dried gels were imaged by tapping mode atomic force microscopy. The guest particles were found to aggregate in the bulk homogeneous networks to form microdomains with densely connected structure, which became larger with increasing particle concentration. The roles of bulk networks as well as guest particles on the swelling behavior of hybrid gels were qualitatively discussed on the basis of the incorporated structure of guest particles, depending on the concentration of guest particles. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1696–1704, 2005     

水解聚丙烯酰胺柠檬酸铝体系成胶行为与形态结构的研究

采用光学显微镜、扫描电镜及流变性能测试等手段 ,研究了部分水解聚丙烯酰胺 (HPAM )与柠檬酸铝 (AlCA)的成胶行为与形态结构 .结果表明 ,当AlCA浓度超过 10 0mg/L时 ,随HPAM浓度由低向高变化 ,HPAM AlCA交联体系可形成三种不同形态结构的凝胶 :分散凝胶 (由交联聚合物颗粒形成的分散体 )、两相(分散凝胶相与连续网状凝胶相 )共存凝胶和连续网状凝胶 .HPAM AlCA形成分散凝胶时 ,无明显的粘度升高现象 ,但体系中存在由HPAM大分子交联在一起的颗粒结构 .HPAM AlCA在形成连续网状凝胶时 ,体系复模量和复粘度大幅度提高 ,网状凝胶中含有粒状凝胶颗粒 .     

Pore Structure Simulation of Gels with a Binary Monomer Size Distribution

The pore size distribution in silica gels can be tailored by the addition of silica soot particles during the gel formation. We introduce a numerical model in order to simulate the structure of this “composite gel”. The algorithm is based on Diffusion-Limited Cluster-Cluster Aggregation model with an initial binary distribution of monomer sizes. The textural properties of the simulated gels are calculated using a simple triangulation method. Nitrogen adsorption-desorption experiments show that with the powder addition the mean pore size is shifted towards larger pore size and the specific surface area decreases. Numerical results of the mean pore size, specific surface area, and particles are in good agreement with experimental data. Because of these textural properties this new type of gels and aerogels has larger permeability and interesting properties as host matrix. The composite gels and the numerical model could also be helpful to simulate the natural allophanic gel found in volcanic soils.     

Synthesis and characterization of polyhedral oligomeric silsesquioxane hybrid co‐crosslinked poly(N‐isopropylacrylamide‐co‐dimethylaminoethyl methacrylate) hydrogels

Polyhedral oligomeric silsesquioxane hybrid temperature and pH double‐responsive hydrogels with organic–inorganic co‐crosslinked networks are synthesized by in situ, free‐radical polymerization of N‐isopropylacrylamide and dimethylaminoethyl methacrylate in the presence of both organic crosslinker N,N′‐methylenebis(acrylamide) (BIS) and inorganic crosslinker octavinyl polyhedral oligomeric silsesquioxane (OvPOSS) in tetrahydrofuran media. The resulting hydrogels (OR‐OvP gels) display obvious temperature and pH double responsiveness, OvPOSS particles dispersed in polymer make a dominant effect on the properties of gels. With the increase of OvPOSS, the aggregation of particles on nano‐ or microscale happens and causes a considerable change on the properties of gels, such as the lower critical solution temperature and better compression strength. Specially, the interconnected microporous structure of gels ascribed to the microphase separation results in faster deswelling rate, which makes the gel become attractive. Besides, the crosslink by BIS intensifies the heterogeneity of gels significantly, which could also be used to adjust the properties of gels. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1494–1504     

Electro‐conductive double‐network hydrogels

Novel electro‐conductive and mechanically‐tough double network polymer hydrogels (E‐DN gels) were synthesized by polymerization of 3, 4‐ethylenedioxythiophene in the presence of a double network hydrogel (DN gel) matrix. The E‐DN gels showed not only excellent mechanical performance, having a fracture stress of 1.4–2.1 MPa, but also electrical conductivity as high as 10^?3 S cm^?1, both under dry and water‐swollen states. The fracture stress and fracture energy of the E‐DN gel was increased by 1.7 and 3.4 times, respectively, as compared with the DN gel. From scanning electron microscope and AFM observations, it was found that electro‐conductive poly(3,4‐ethylenedioxythiophene) (PEDOT) was incorporated into DN gel matrix, apparently due to the formation of a poly‐ion complex with sulfonic acid group of the DN gel network. Thus, PEDOT incorporated into the DN gel matrix greatly improves not only electronic conductivity, but also mechanical properties, reinforcing the double network gel matrix. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Supramolecular gels formed from multi-component low molecular weight species

Low molecular weight supramolecular gels consist of small molecules (gelators) that in an appropriate solvent self-assemble into nano- or micro-scale network structures resulting in the formation of a gel. Most supramolecular gels consist of two parts, namely the solvent and the gelator. However, the concept of multi-component supramolecular gels, in which more than one compound is added to the solvent, offers a facile way (e.g. by changing the ratio of the different components) to tailor the properties of the gel. The simplest multi-component gels consist of two components added to the solvent and are the most widely studied to date. There are three general classes of such multi-component gels that have been investigated. The first class requires all the added components to access the gel; that is, no component forms a gel on its own. A second class uses two (or more) gelators which can either co-assemble or self-sort into distinct assemblies and the final class consists of one (or more) gelator and one (or more) non-gelling additive which can impact the assembly process of the gelator and therefore the gel's properties.     

Mechanical Properties and Failure Behavior of Physically Assembled Triblock Copolymer Gels with Varying Midblock Length

Mechanical properties including the failure behavior of physically assembled gels or physical gels are governed by their network structure. To investigate such behavior, we consider a physical gel system consisting of poly(styrene)‐poly(isoprene)‐poly(styrene)[PS‐PI‐PS] in mineral oil. In these gels, the endblock (PS) molecular weights are not significantly different, whereas, the midblock (PI) molecular weight has been varied such that we can access gels with and without midblock entanglement. Small angle X‐ray scattering data reveals that the gels are composed of collapsed PS aggregates connected by PI chains. The gelation temperature has been found to be a function of the endblock concentration. Tensile tests display stretch‐rate dependent modulus at high strain for the gels with midblock entanglement. Creep failure behavior has also been found to be influenced by the entanglement. Fracture experiments with predefined cracks show that the energy release rate scales linearly with the crack‐tip velocity for all gels considered here. In addition, increase of midblock chain length resulted in higher viscous dissipation leading to a higher energy release rate. The results provide an insight into how midblock entanglement can possibly affect the mechanical properties of physically assembled triblock copolymer gels in a midblock selective solvent. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1014–1026     

Effect of homopolymer solubilization on triblock gel structure and mechanical response

The linear modulus, swelling behavior, and high strain response of a set of well-characterized model triblock gels were investigated to understand the effect of homopolymer solubilized within the micelle core on gel structure and mechanical properties. Structural parameters were obtained from small-angle X-ray scattering (SAXS) as well as from self-consistent field theory (SCFT) calculations. Experimental results are compared with Neo-Hookean and exponentially strain hardening models for gel behavior and rigid filler effects are discussed. The main conclusion is that the addition of homopolymer to the micelle core increases the chain stretching in both the core and coronal blocks. The total extension of a chain for a given external load is fixed by its length; however, the initial prestretch imparted to the chain due to micellization changes with the size of the micelle core and can greatly reduce the amount of extension observed for a given external force. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1395–1408, 2010     

Effects of pigments on the volume‐phase‐transition properties of N‐isopropylacrylamide gels as light‐modulation materials

The effects of pigments contained in N‐isopropylacrylamide (NIPAM) gels on their volume‐change properties were investigated. All the NIPAM gel particles, containing various kinds and concentrations of pigments, showed a volume phase transition at 34 °C. No pigment affected the volume‐phase‐transition temperature of the NIPAM gels. As the concentration of the pigment in the NIPAM gels was increased, the amount of the volume change of the NIPAM gels was reduced. The water absorptivity of the NIPAM gels in the swollen state decreased as the pigment concentration increased, whereas the water absorptivity in the shrunken state was almost constant. Reducing the initial monomer concentration of the polymerization of the NIPAM gel increased the water absorptivity in the swollen state. With an increase in the water absorptivity, the volume changes of the NIPAM gels containing pigments were increased. Prototype light modulators in which the NIPAM gel particles containing pigment were dispersed between glass plates were fabricated. The light modulator using the gel particles with improved diameter change (d/d₀ = 2.3, where d and d₀ are the equilibrium diameter and the diameter of the fully shrunken state at 50 °C, respectively) exhibited a larger transmittance change from 8 to 79% than that using the gel particles before the improvement (d/d₀ = 1.7; from 38 to 79%) according to temperature changes. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4644–4655, 2006     

Synthesis and characterization of magnetoelectric polymer nanocomposites

Magnetoelectric polymer nanocomposite structures are synthesized using conducting polyaniline and nanosized BFO particles through in situ sol–gel polymerization. The effect of nanosized BFO in polyaniline matrix is studied. The SEM, XRD, VSM, FTIR, and UV–Vis studies were made to understand the morphology, crystalline structure, magnetic, and optical properties of PANI/BFO composites with various concentrations of nanofiller. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2418–2422, 2008     

Hyaluronic acid‐poly(vinyl alcohol) composite cryo‐gel for biofunctional material application

Recently, applications of hyaluronic acid (HA) as a biomaterial were investigated. However, the weak structure of HA gel and the effects of using cross‐linker raised concerns during in vivo resolution. In this study, we investigated the method to solve these two problems using physical cross‐linking and compositing with poly(vinyl alcohol) (PVA). Various weight ratios of HA and PVA solutions were mixed, adjustment of pH to 1.8 using HCl then used to fabricate HA‐PVA cryo‐gel by freezing‐thawing. Young's modulus of the prepared gel rose with the increase of both HA and PVA concentrations or either one of them. We estimated that HA and PVA have exhibited these mechanical properties due to forming a double network. HA‐PVA gel showed kinetic friction force of approximately 10 times of PVA gel, while water contact angle and protein adsorption of HA‐PVA gels were remarkably decreased. The properties of the prepared gel suggest that it can be used for postoperative adhesion prevention applications.     

Friction coefficient and structural transition in a poly(acrylamide) gel

The friction coefficient between the polymer network of an opaque poly(acrylamide) gel and water is measured as a function of the mole fraction of cross linker. The friction coefficients of opaque gels are 4 to 5 orders of magnitude smaller than those of the transparent gels. This drastic decrease in friction occurs when the mole fraction of cross linker is 0.2. In opaque gels, the friction coefficient of gels and the mole fraction of cross linker are related by a power law. The network structure of the opaque gels used in the friction measurements is examined with a confocal laser scanning microscope. The opaque gel network consists of a fractal aggregate of colloidal particles. The radius of particles and the volume occupied by the particles depend on the mole fraction of cross linker. Both relationships are well described by the power laws. The power law of the friction coefficient is well explained in terms of the power laws of the structural parameters and the Stokes equation of the hydrodynamic friction for the spherical particle. It indicates that the friction of the opaque gel is determined simply by the structure of the polymer network.     

Effect of sodium poly(styrene sulfonate) on thermoreversible gelation of gelatin

The effect of an added polyanion, sodium poly(styrene sulfonate) (NaPSS), on the thermoreversible gelation and remelting of gelatin gels has been investigated by polarimetry and rheology. The presence of NaPSS can either enhance or reduce collagenlike helix formation, depending on the polymer concentration relative to that of gelatin and the gelation temperature. At temperatures < 20°C, the helical content is reduced by increasing the amount of added NaPSS, demonstrating the disruption of helical structure of gelatin by the polyanion. Synchronous measurements of optical rotation and modulus at 25°C, in both gelation and remelting, indicate that the optical rotation at the gel point for the pure gelatin is lowered on addition of NaPSS. At low frequency, the storage modulus of gelatin is increased by the addition of a small amount of NaPSS relative to that of gelatin, but decreased with excess NaPSS. The mechanical properties of gelatin with and without NaPSS will be discussed in light of the competition between network junction formation by strands of triple helices among gelatin chains and temporary ionic crosslinking between gelatin and the polyanion. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2287–2295, 1999     

Clustering and mechanics in dense depletion and thermal gels

We report on the microstructure and mechanical properties (elastic modulus) of concentrated depletion and thermal gels of octadecyl-coated silica particles for different values of the strength of interaction--polymer concentration for depletion gels and temperature for thermal gels. The depletion gels are composed of dense clusters and voids, while the thermal gels are devoid of clusters. Shear breaks up clusters in depletion gels while it induces clustering in the thermal gels. In both of these gels, the microstructure recovers to the presheared state upon cessation of shear. The recovery of the elastic modulus mimics the microstructure in the sense that the elastic modulus recovers to the presheared sheared state after shearing is stopped. Calculations of the gel boundary by modeling the interactions with an effective one-component square-well model reveals that suspensions with similar ranges of attraction gel at the same volume fraction at a fixed strength of attraction. Calculations of the elastic modulus using the na?ve mode coupling theory for depletion gels are in good agreement with experimental measurements provided clustering is taken into account and have the same magnitude as the elastic moduli of thermal gels with similar strengths of attraction. These calculations, in addition to the experimental observations reinforce the point that the microscopic parameter determining the elastic modulus of dense gels and its recovery is the localization length which is only a fraction of the particle diameter and not the structure on the length scale of the particle diameter and larger.     

Swelling kinetics of anisotropic filler loaded PDMS networks

The kinetics of swelling of spherical poly(dimethyl siloxane) (PDMS) composite networks in cyclohexane has been studied. The PDMS composite gels contain magnetite and iron particles built in the network either in randomly distributed form or in chain-like structure. The composite gel becomes anisotropic in the terms of both, mechanical and swelling properties. As a consequence the swelling kinetics can be characterized by direction dependent relaxation time. The swelling kinetics of these samples has been compared to that of pure PDMS gels. It was found that Li-Tanaka theory can be applied to describe the swelling kinetics of PDMS network.     

Parametric study of factors affecting melamine-resorcinol-formaldehyde xerogels properties

Resorcinol-formaldehyde (RF) xerogels are organic materials have been widely studied due to their industrially relevant characteristics, through which, RF gels have significant potential to be tailored to specific applications. Xerogel properties have been tailored, within this study, by altering the synthesis procedure with a focus on monomer concentrations, catalyst to monomer ratio, and the introduction of a nitrogen-rich precursor, thereby incorporating nitrogen into the structure to additionally affect the chemical properties of the final gel. Melamine (M) is used as the source of nitrogen, partially replacing the resorcinol (R) typically used, and resulting in a melamine-resorcinol-formaldehyde (MRF) gel; synthesis was facilitated by a sodium carbonate catalyst (C), as often used in RF gel production. R/C and R/F molar ratios, and M concentration ([M]), were chosen as parameters to study in-depth, as they have previously been shown to markedly influence sol-gel formation. The MRF gels produced were subsequently characterized to determine porous structure and chemical functionality. The results indicate that, texturally, increasing [M] produces a similar effect as increasing R/C values: increasing pore size, while decreasing surface area. Pore volume tends to increase when R/C or M increase individually but pore volume and surface area decrease drastically when both variables increase concurrently. Microporosity also tends to increase as R/C decreases, and as the concentration of M is decreased. Altering the gel matrix, by replacing M for R, results in a weakening of the gel structure, as the bridges formed during curing are reduced in quantity, which indicates a maximum level of substitution that can occur within these materials. Combined, these results suggest that nitrogen can be successfully incorporated into organic gel structures but that the interplay between process variables is crucial in determining final gel characteristics for specific applications.     

Microgel particles: The structure-property relationships and their biomedical applications

Microgels are crosslinked soft particles with a three-dimensional network structure that are swollen in a good solvent. They have frequently been termed “smart materials” since the size, softness, and interaction forces between particles are tunable by external stimuli such as temperature, pH, or magnetic and electric fields. It is this unique feature that has captured the interest of many scientists across a wide range of disciplines. This brief review covers the basic aspects of the relationships between the network structure and gel properties of the thermally sensitive poly(N-isopropylacrylamide) (pNIPAM) microgels including the phase transition process, the internal structure of microgels, and the phase behavior. Additionally, we highlight the impacts of microgels on the biomedical applications, especially in the gene delivery, cell matrix and differentiation of stem cells. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2995–3003     

Antimicrobial loading into and release from poly(ethylene glycol)/poly(acrylic acid) semi‐interpenetrating hydrogels

Electrostatic interactions within a semi‐interpenetrating network (semi‐IPN) gel can control the postsynthesis loading, long‐term retention, and subsequent release of small‐molecule cationic antibiotics. Here, electrostatic charge is introduced into an otherwise neutral gel [poly(ethylene glycol) (PEG)] by physically entrapping high‐molecular‐weight poly(acrylic acid) (PAA). The network structure is characterized by small‐angle neutron scattering. PEG/PAA semi‐IPN gels absorb over 40 times more antibiotic than PAA‐free PEG gels. Subsequent soaking in physiological buffer (pH 7.4; 0.15 M NaCl) releases the loaded antibiotics for periods as long as 30 days. The loaded gels elute antibiotics with diffusivities of 4.46 × 10^?8 cm²/s (amikacin) and 2.08 × 10^?8 cm²/s (colistin), which are two orders of magnitude less than those in pure PEG gels where diffusion is controlled purely by gel tortuosity. The release and hindered diffusion can be understood based on the partial shielding of the charged groups within the loaded gel, and they have a significant effect on the antimicrobial properties of these gels. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 64–72     

Structural origin of the thixotropic behavior of a class of metallosupramolecular gels

Complexation, between a ditopic ligand, consisting of a 2,6-bis-(1′-methylbenzimidazolyl)-4-oxypyridine moiety (O-Mebip) attached to either end of a penta(ethylene glycol) core, with transition metal and lanthanide ions, results in the formation of metallosupramolecular polymers, soluble in acetonitrile at high temperatures. Cooling the hot sol to room temperature causes phase separation and crystallization, and produces mechanically-strong gels, which exhibit a highly thixotropic behavior. Optical microscopy indicates that the gel morphology consists of spherulitic particles, which are easily broken by mechanical shear. Reproducible gel properties are produced when the gel is formed by cooling in a sonication bath, which produces a finely-divided globular morphology, and increases the modulus of the gels. Wide angle X-ray diffraction study shows that the crystalline structures of the gels are strongly dependent on the thermal history of gel formation and the nature of the metal ion. The gel properties are a result of the interactions between the colloidal particles produced by the phase separation and crystallization process. These interactions, which may reflect electrostatic forces and possibly metal-ligand binding, in addition to the usual van der Waals interactions, give rise to the formation of a network structure. The disruption of this network by mechanical shear, and its facile reformation when shear is removed, are the origin of the pronounced thixotropic behavior of the gels.     

Preparative isolation of protein complexes and other bioparticles by elution from polyacrylamide gels

Due to its unmatched resolution, gel electrophoresis is an indispensable tool for the analysis of diverse biomolecules. By adaptation of the electrophoretic conditions, even fragile protein complexes as parts of intracellular networks migrate through the gel matrix under sustainment of their integrity. If the thickness of such native gels is significantly increased compared to the analytical version, also high sample loads can be processed. However, the cage-like network obstructs an in-depth analysis for deciphering structure and function of protein complexes and other species. Consequently, the biomolecules have to be removed from the gel matrix into solution. Several approaches summarized in this review tackle this problem. While passive elution relies on diffusion processes, electroelution employs an electric field to force biomolecules out of the gel. An alternative procedure requires a special electrophoresis setup, the continuous elution device. In this apparatus, molecules migrate in the electric field until they leave the gel and were collected in a buffer stream. Successful isolation of diverse protein complexes like photosystems, ATP-dependent enzymes or active respiratory supercomplexes and some other bioparticles demonstrates the versatility of preparative electrophoresis. After liberating particles out of the gel cage, numerous applications are feasible. They include elucidation of the individual components up to high resolution structures of protein complexes. Therefore, preparative electrophoresis can complement standard purification methods and is in some cases superior to them.