Structure and protein separation efficiency of poly(N-isopropylacrylamide) gels: Effect of synthesis conditions

Crosslinked poly(N-isopropylacrylamide) (PNIPA) gels with different crosslink densities in the form of rods and beads were prepared by free-radical crosslinking copolymerization. Solution and inverse suspension polymerization techniques were used for the gel synthesis. The gels were utilized to concentrate dilute aqueous solutions of penicillin G acylase (PGA), bovine serum albumin (BSA), and 6-aminopenicillanic acid (6-APA). The discontinuous volume transition at 34°C observed in the gel swelling was used as the basis of concentrating dilute aqueous protein solutions. PNIPA gels formed below 18°C were homogeneous, whereas those formed at higher temperatures exhibited heterogeneous structures. The water absorption capacity of PNIPA gels in the form of beads was much higher, and their rate of swelling was much faster than the rod-shaped PNIPA gels. It was also found that the polymerization techniques used significantly affect the properties of PNIPA gels. The separation efficiency decreased when the protein molecules PGA or BSA in the external solution were replaced with small-molecular-weight compounds, such as 6-APA. The protein separation efficiency by the gel beads increased to 100% after coating the bead surfaces with BSA. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67: 805–814, 1998     

Soy protein gelation

Heat-induced protein gels are of importance for the structure and properties of many food products. Gel formation is a complex process which often involves several reactions such as denaturation, dissociation-association, and aggregation. The kinetics of the reactions involved will determine the type of structure formed. Protein gels can be divided into two types: gels formed by random aggregation and gels formed by association of molecules into strands in a more ordered way. The two soy proteins glycinin and conglycinin both have the ability to form ordered structures consisting of strands 10–15 nm thick. The glycinin gel strands formed in distilled water are regular, and cross sections of strands showed a hollow cylindrical structure. In the presence of sodium chloride, glycinin forms an aggregated gel structure at 85 C, but at 95 C a regular structure similar to that found in distilled water was formed. The aggregated structure was interpreted as a transient state similar to the soluble aggregate formed on heating dilute solutions prior to dissociation into subunits. Conglycinin gels are more irregular and more cross-linked than gels of glycinin. Also, the strands of conglycinin showed a complex mode of aggregation possibly in the form of double spirals. The addition of salt does not affect the microstructure of conglycinin gels as dramatically as in the case of glycinin gels. Commercially produced soy protein isolates may behave quite differently from native soy proteins, due to processing conditions causing denaturation and various states of aggregation.     

Zur Struktur von konzentrierten Polymerlösungen und Gelen. I. Reaktion von OH-Gruppen enthaltenden Polyacrylsäurebutylestern mit Isocyanatgruppen oder Säurechloridgruppen enthaltenden Polyacrylsäurebutylestern

The mutual penetration of random coils in concentrated solutions and gels was investigated by comparison of two crosslinking reactions, which have been carried out under similar conditions: 1. Reaction of an OH-groups containing copolymer of n-butylacrylate with hexamethylenediisocyanate 2. Reaction of an OH-groups containing copolymer of n-butylacrylate with a NCO-groups containing copolymer of n-butylacrylate. Reaction 1 leads in a short time to a conversion of nearly 100°o, while reaction 2 does not exceed conversions lying between 5 and 20%, even after very long reaction times. The influence of crosslinking and incompatibility, which restricts the diffusion of chain segments and thus the reaction 2 was eliminated by extrapolation of the conversion rates to very small OH and NCO concentrations in the polymer chains. The conversion rates of the limiting conversion [U,], obtained in this way, correspond to the zones of mutual penetration in concentrated polymer solutions and gels. The depth of these contact zones depends on the polymer concentration and reaches at 30% polymer concentration about 18% of the whole coil volume, which means, that in such solutions the polymer coils are arranged in a closed package of individual particles, which do not, excepting the small border zones, penetrate each other. Concentrated polymer solutions and gels have therefore the morphological structure of cellular tissues or soap foams, where the thickness of the ?cell walls”? increases with increasing polymer concentration. The diameter and the volume of the single coil decreases with increasing polymer concentration, caused by the decreasing space per macromolecule.     

Syntheses of hydroxypropyl methylcellulose phthalate gels in organic solvents by radiation crosslinking

Cellulose gels were prepared through the crosslinking of hydroxypropyl methylcellulose phthalate (HPMCP) with electron‐beam irradiation in concentrated organic solvent solutions. The effects of the solvent species, polymer concentration, and irradiation dose on the formation of the gels were investigated. Some organic solvents, such as alcohols with short alkyl chains, alkyl acetates, and ketones, were found to be suitable as media for the radiation crosslinking of the polymer. The prepared HPMCP gels showed excellent swelling in various organic solvents with medium hydrogen‐bonding abilities, such as pyridine, cresol (meta), acetic acid, dimethylformamide, dimethyl sulfoxide, tetrahydrofuran, 1,4‐dioxane, acetone, methyl ethyl ketone, methyl acetate, and chloroform. In an acetone/water mixture, the swelling ratio was significantly dependent on the solvent composition because of the coexistence of both hydrophilic and hydrophobic moieties in HPMCP. These results suggest that HPMCP gels have the potential to be superabsorbents for various kinds of organic solvents. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3002–3007, 2004     

Laws of the Formation and Diffusion Properties of Silica and Agarose Gels

The physicochemical and mass-transfer properties were studied for two gels that can be applied in the fabrication of bioreactors using additive manufacturing. The fundamental difference between these gels lies in the mechanisms of their formation from solutions. The first one is a silicic acid gel, which is formed in a chemical reaction and has an irreversible gelling process. The second one is an agarose gel, which is formed from a water solution of agarose upon cooling and turns back into liquid when heated. Studies were performed by optical methods that include the spectroscopy of gel samples during their formation and the visualization of the diffusion fronts of transferred substances. Being nonintrusive, these methods have no effect on the state of samples and do not distort the dynamics of processes. Characteristic patterns of changes in the spectral parameters during gelling were studied for both gels depending on the dispersed phase concentration, which was relevant to the practical application of the gels. New experimental data were obtained on the structure of the gels and the mechanisms of its change during gel aging. The main laws are determined for changes in the diffusion coefficients in the gels with increasing concentration of the dispersed phase.     

Crosslinked poly(ethylene glycol) networks as reservoirs for protein delivery

Poly(ethylene glycol)s are cross-linked by tris(6-isocyanatohexyl)isocyanurate via urethane/allophanate bond formation to obtain polymeric networks capable of swelling in phosphatebuffered saline or ethanol, resulting in gels. The protein loading of the gels swollen in ethanol is enhanced by formation of hydrophobic ion-paired complexes of proteins with sodium dodecyl sulfate. The protein release profile from loaded gels through biomimetic membranes impregnated with phosphatidylcholine is governed by the gel crosslinking density. Spectroscopic data and enzymatic activity of the proteins released from the gels into aqueous solutions indicate that proteins are recovered unchanged upon their redissolution in water. © 1996 John Wiley & Sons, Inc.     

Synthesis of thermosensitive gel by living free radical polymerization mediated by an alkoxyamine inimer

Thermosensitive gel is synthesized through controlled/“living” free radical copolymerization of styrene and DVB mediated by an alkoxyamine inimer, 2,2,6,6-tetramethyl-1-(1′-phenylethoxy)-4-(4′-vinylbenzyloxy)-piperidine (V-ET). The inimer plays the role of both incorporating “T-shaped” inter-chain linkages and mediating the polymerization. First order kinetics is observed for crosslinking polymerizations before gel point, indicating a constant concentration of propagating radicals. Monomer conversion at the gel point depends on the feed ratio of DVB to V-ET. Higher amount of V-ET results in later gel point due to smaller molecular weight of the primary chains that depends inversely on the concentration of nitroxide. The resulting gel contains permanent and labile crosslinking points formed by DVB units and alkoxyamine moieties, respectively. Therefore, the gels exhibit gel-sol transition within a narrow temperature range. The gel properties, such as the swelling ratio and gel-sol transition temperature, can be controlled by changing the feed ratio of DVB to V-ET. The microenvironments in different gels, or at different temperatures, are investigated by ESR spectroscopy.     

Bioactive factor delivery strategies from engineered polymer hydrogels for therapeutic medicine

Polymer hydrogels have been widely explored as therapeutic delivery matrices because of their ability to present sustained, localized and controlled release of bioactive factors. Bioactive factor delivery from injectable biopolymer hydrogels provides a versatile approach to treat a wide variety of diseases, to direct cell function and to enhance tissue regeneration. The innovative development and modification of both natural- (e.g., alginate (ALG), chitosan, hyaluronic acid (HA), gelatin, heparin (HEP), etc.) and synthetic- (e.g., polyesters, polyethyleneimine (PEI), etc.) based polymers has resulted in a variety of approaches to design drug delivery hydrogel systems from which loaded therapeutics are released. This review presents the state-of-the-art in a wide range of hydrogels that are formed though self-assembly of polymers and peptides, chemical crosslinking, ionic crosslinking and biomolecule recognition. Hydrogel design for bioactive factor delivery is the focus of the first section. The second section then thoroughly discusses release strategies of payloads from hydrogels for therapeutic medicine, such as physical incorporation, covalent tethering, affinity interactions, on demand release and/or use of hybrid polymer scaffolds, with an emphasis on the last 5 years.     

Gelation of soybean oil with polybutadiene

In this study self‐supporting, resilient, load bearing polybutadiene ‐ soybean oil gels were obtained. The gels were made by dissolving polybutadiene (PBD) in soybean oil (SO) and selectively crosslinking PBD with a free radical source. PBD concentration, free radical source concentration, and the temperature and time of the crosslinking reaction were varied, and the effects of these changes on the mechanical properties of the gels were examined. Our experiments show that successful gelation is possible within PBD concentration limits of 7.5 to 12%, peroxide concentration between 25 to 100% (based on PBD), temperature between 110°C and 130°C and reaction times of 3 hours with tert.butyl‐peroxybenzoate as the free radical source. The crosslinking reaction was followed by IR and H‐NMR spectra, and the crosslinking density was followed by compression testing and swelling behavior. Higher radical source concentration and higher PBD concentration gave gels with better mechanical properties. The spectra and the viscosity increase of SO extracted from the gels indicate that there is dimer and trimer formation of SO during the reaction. The spectra of the PBD extracted from gels indicate that SO was added to PBD in a small but measurable amount. Integration of peak intensities in the NMR spectrum of methylene groups of PBD and methylene groups of triglyceride indicated one triglyceride molecule for approximately 45 repeating units in PBD. The modulus of the best gel sample (PBD 10%. peroxide 50%) was 1.96 × 10^?2 MPa. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 2240–2246, 2005     

Enzyme immobilization onto renewable polymeric matrixes: Past,present, and future trends

In this review, we present an overview of the different renewable polymers that are currently being used as matrixes for enzyme immobilization and their properties and of new developments in biocatalysts preparation and applications. Polymers obtained from renewable resources have attracted much attention in recent years because they are environmentally friendly and available in large quantities from natural sources. Different methods for the immobilization of enzymes with these matrixes are reviewed, in particular: (1) binding to a prefabricated biopolymer, (2) entrapment, and (3) crosslinking of enzyme molecules. Emphasis is given to relatively recent developments, such as the use of novel supports, novel entrapment methods and protocols of polymer derivatization, and the crosslinking of enzymes. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42125.     

Reversible gelation of acrylonitrile–vinyl acetate copolymer solutions. II. Mechanical properties

Concentrated solutions of acrylonitrile polymers exhibit reversible gelation. The rate of gelation at 25°C. was determined for various solutions of an acrylonitrile copolymer containing 7.7% vinyl acetate in mixtures of dimethylacetamide (solvent) and water (nonsolvent) by measuring the shear modulus of the forming gel as a function of time. The mechanical properties were also measured on a series of gels formed by cooling solutions to ?78°C. It was found that both the rate of gelation at 25°C. and the modulus of gels formed at ?78°C. increase very rapidly as either the solids level of the solution of the water content of the solvent is increased. The gelation rate data wree correlated with the gel melting points of the gels. The results are discussed and compared with the analogous but limited data available for other systems.     

Differential scanning calorimetry of aqueous polymer solutions and gels

The melting phenomena of aqueous polymer solutions and gels have been investigated by differential scanning calorimetry (DSC). The polymers used were synthetic polyacrylamide and poly(vinyl alcohol) samples as well as guar and xanthan gums. By using an empirical relation, the energy measured from the area under the melting peak yielded heats of mixing and sorption, when fitted by an association factor computed from the data. This factor (independent of the concentration) is a measure of the water fraction associated with the polymer and has a definite and characteristic value for a given polymer in water. When a crosslinking agent (potassium pyroantimoniate or chromic nitrate) was added to the water–polymer system, the association factor varied with the polymer concentration; the macromolecular chains thus become less accessible to penetrating water. If a branched gel was obtained owing to the formation of chemical crosslinks, a hump appeared on the melting peak.     

Dependence of properties of swollen and dry polymer networks on the conditions of their formation in solution

A marked difference between the properties of networks first crosslinked and then swollen, and those synthesized in a solvent medium, has been shown. The present investigation was carried out using two different types of regular networks, polysiloxane and polyisocyanurate networks, which had different chain flexibilities, crosslinking densities, and intermolecular interactions. The concentration at which the network is formed v₀ and the quality of the solvent were varied over a wide range. It was found that with a decrease of v₀ from 1-0.05, the elastic modulus of both gels and dry networks obtained from these gels after removing the solvent decreased by a factor of 2–4 and equilibrium swelling decreased a few times.     

Effect of method of preparation on properties of temperature and pH-sensitive gels: Chemical crosslinking versus irradiation with e-beam

Two problems that have impeded a broader application of polyelectrolytic smart polymers are their low speed of response and their faulty behavior in the presence of salts. To contribute to the solution of those problems we prepared two series of gels based on N-isopropylacrylamide (NIPAAm) containing ampholytic units using chemical crosslinking methods and electron beam irradiation. In the first method, the gels prepared contain NIPAAm, N-N′-dimethylaminoethylmethacrylate (DMAEM) and a methacrylate of the ortho-, meta- or para-benzoic acid, respectively, to impart amphoteric behavior. They were crosslinked using N-N′-methylenebisacrylamide (BIS) as crosslinker by standard free-radical techniques. In the second method aqueous solutions containing mixtures of polyNIPAAm and charge balanced copolymers of DMAEM with the methacrylates of the ortho-, meta- and para-benzoic acid, respectively, prepared previously, were irradiated by e-beam. Both series were characterized by DSC, mechanical compression and swelling–shrinking tests in deionized water, buffers solutions at different pH-values and in water having different salt concentrations. The results show that the gels prepared by irradiation have a swelling degree up to four times higher than that of the gels prepared by chemical crosslinking, furthermore, their shrinking velocity is up to twenty times higher than that of gels prepared by chemical crosslinking. The results also showed that the gels prepared by chemical crosslinking using monomers show only polyelectrolyte behavior, while those prepared by e-beam irradiation of polymer solutions show in fact amphoteric behavior; while all gels retained their temperature sensitivity between 32 and 39 °C.     

Preparation and characterization of nanocomposite hydrogels based on polyacrylamide for enhanced oil recovery applications

In the present work, attempts have been made to prepare nanocomposite type of hydrogels (NC gels) by crosslinking the polyacrylamide/montmorillonite (Na‐MMT) clay aqueous solutions with chromium (III). The X‐ray diffraction patterns of the NC gels exhibited a significant increase in d₀₀₁ spacing between the clay layers, indicating the formation of intercalated as well as exfoliated type of morphology. Exfoliation of the clay layers through out the gel network was found to be predominated, which evidences the high interaction between the polyacrylamide segments and montmorillonite layers. Gelation time as well as variation of viscoelastic parameters such as storage modulus (G′) of the gel network during gelation process at 75°C was studied and followed by rheomechanical spectroscopy (RMS). The NC gels prepared with lower crosslinker concentration showed higher strength and elastic modulus compared with the similar but unfilled polyacrylamide gel. This distinct characteristic of the NC gels yields a gel network structure with high resistance towards syneresis at high temperature in the presence of the oil reservoir formation water. The effects of the composition, such as clay content, crosslinker concentration, and also water salinity upon the gelation rate, gel strength as well as rate of syneresis have been investigated. To optimize the injectivity of the intercalated polyacrylamide solution before the onset of gelation with the gel strength of the final developed gel, sodium lactate was employed as retarder. This was found to be effective to balance these two characteristics of the NC gels, which are aimed to be used for water shut‐off and as profile modifier in enhanced oil recovery (EOR) process during water flooding process. The nanocomposite gels showed much more elasticity and extensibility at low crosslinker concentration compared with the similar but unfilled gel, which makes the NC gels suitable as an in‐depth profile modifier, and also as an oil displacing agent in the heterogeneous oil reservoir in chemical EOR. Effects of the clay content on the thermal stability of the gel network have also been investigated by thermogravimetric analysis (TGA). Scanning electron microscopy (SEM) has been performed upon the NC‐gel samples. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2096–2103, 2006     

Struktur und Gelierverhalten verdünnter PVC-Dioxan-Lösungen

The melting temperatures of gels that were formed from dilute solutions of dioxane and suspension polyvinylchloride of different preparation were measured. Dependent on the temperature of polymerization of the polyvinylchloride different degrees of syndiotacticity could be shown by the measurement of the heat of crosslinking and the examination of the length of crystallized sequences in the gel. Fractionation of polyvinylchloride by precipitation led to products which were not only different in molecular weight but in the stereoregular structure too.     

Thermal gelation properties of methyl and hydroxypropyl methylcellulose

Aqueous solutions of methyl and hydroxypropyl methylcellulose are known to gel upon heating. These gels are completely reversible in that they are formed upon heating yet will liquefy upon cooling. The precipitation temperature, gelation temperature, and gel strength of these methylcellulose solutions were determined as a function of molecular weight, degree of methyl and hydroxypropyl substitution, concentration, and presence of additives. The precipitation temperature of these polymer solutions decreases initially with increasing concentration until a critical concentration is reached above which the precipitation temperature is little affected by concentration changes. The incipient gelation temperature decreases linearly with concentration. The strength of these gels is time dependent, increases with increasing molecular weight, decreases with increasing hydroxypropyl substitution, and depends on the nature of additives. Hydrophobe–hydrophobe interaction or micellar interaction is postulated to be the cause of gelation. This thermal gelation property of the polymers is utilized in many end uses including food, pharmaceuticals, ceramics, tobacco, and other industrial applications.     

The role of dimethyldithiocarbamic acid in accelerated sulfur vulcanization

Polyisoprene/tetramethylthiuram disulfide (TMTD)/sulfur compounds were vulcanized under a variety of conditions. TMTD does not decompose to tetramethylthiourea (TMTU) at vulcanization temperatures as has been suggested, neither is it formed as an integral part of the crosslinking process. Instead, it results from the attack of dimethylamine, released on decomposition of dimethyldithiocarbamic acid (Hdmtc), on TMTD. It is demonstrated that the formation of TMTU in vulcanizates may be overlooked, as it is readily lost in the work‐up for HPLC analysis. Hdmtc is shown to play an essential role in the crosslinking process in polyisoprene/TMTD/sulfur formulations, and its removal from the system during vulcanization severely impedes crosslinking. Polysulfidic thiuram‐terminated pendent groups are formed, in part, by the interaction of tetramethylthiuram polysulfides with the polymer chain, but largely by an exchange between Hdmtc and polysulfidic thiol pendent groups. The latter are formed when sulfurated Hdmtc reacts with the polymer chain. Crosslinking of thiuram‐terminated pendent groups is slow, and in the absence of ZnO crosslinking results from reaction between polysulfidic thiuram pendent groups and thiols. Crosslinking is delayed until the bulk of the accelerator is bound to the polymer chain, at which point the concentration of free thiuram groups, in the form of Hdmtc, is low, and exchanges between newly formed thiol pendent groups and Hdmtc is less frequent, permitting crosslinking of thiuram pendent groups with these newly formed thiol pendent groups. Data to support the proposed reaction mechanism is presented. Hdmtc on its own accelerates sulfur vulcanization and acts as a catalyst for the reaction, being regenerated in the crosslinking process. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1371–1379, 1999     

VISCOMETRIC MEASUREMENT OF CHROMIUM(III)-POLYACRYLAMIDE GELS†

Gelled polymers are being used increasingly to modify the movement of injected fluids in secondary and enhaced oil recovery processes. A common gelation process involves the reduction of Cr(VI) to Cr(III) in the presence of polyacrylamide. The Cr(III) reacts or interacts with the polymer to form a gel network. Although correlations of gelation time with principal process variables have been obtained, viscometric data have not been reported during or after gelation. These data are needed for fluid flow calculations in surface equipment and estimation of flow behaviour in reservoir rocks.

A Weissenberg Rheogoniometer, with cone and plate geometry, was used to obtain viscometric data for the gelation of polyacrylamide and chromium (III). Solutions consisting of polyacrylamide polymer, sodium dichromate-dihydrate and sodium bisulfite were gelled under a steady shear field at constant temperature. The shear stress versus time profile for the galation process was interpreted to define a gelation time and to determine the apparent viscosity of the gelled fluid. The gelation time decreased as the applied shear rate increased up to about 14.25 sec^?1 and was affected by shear rate history. Viscometric properties of the gelled solutions were determined. Apparent viscosity of the gelled solutions decreased as the shear rate under which they were formed increased.

Post gelation studies indicated that gels exhibited a residual stress at zero shear rate and behaved as Bingham plastics under steady shear. Gels formed at low shear rates were more viscous than gels formed at high shear rates. However, the structure of these gels was susceptible to shear degradation.