Equilibrium swelling of thermo-responsive core-shell microgels

Design of new routes for preparation of hydrogels with fast response and enhanced mechanical and physical properties requires adequate modeling of their swelling. A model is developed for the equilibrium swelling of thermo-responsive gels. A characteristic feature of the model is that it accounts for a strong increase in the elastic moduli above the volume phase transition temperature T_c driven by aggregation of hydrophobic segments into clusters that serve as extra physical bonds between chains. The model is applied to the analysis of swelling diagrams on poly(N-isopropylacrylamide) macroscopic gels, microgel latices, and core-shell microgels with rigid cores. Good agreement is shown between the experimental data and results of simulation. It is demonstrated that the elastic moduli of microgels are higher, while their degrees of swelling in the stress-free state are lower compared with those of macroscopic gels.     

Equilibrium swelling and solvation studies on crosslinked polyacrylamides

Polyacrylamide (PA) crosslinked with four different crosslinking agents, triethyleneglycol dimethacrylate (TEGDMA), N,N′‐methylene bisacrylamide (NNMBA), hexanediol dimethacrylate (HDDMA) and divinylbenzene (DVB), with mole percents ranging from 5 to 20, was prepared by solution polymerization and subjected to swelling and solvation studies. Solubility parameters and cohesive energy density were determined from swelling studies. Molecular weight between crosslinks for these systems were determined by Flory–Rehner analysis. There is a discontinuous volume change for 10% NNMBA and HDDMA crosslinked PA, 15% TEGDMA crosslinked PA and 10 and 15% DVB crosslinked PA in solvent mixtures of acetic acid and water due to phase change occurring at this stage. The hydrogels exhibit inhomogeneous crosslink distribution due to multiple crosslinking, cyclization and network irregularity owing from arising from entanglements. As the percentage of crosslinking increases, crosslinks become more homogeneous due to a decrease in entanglements. Copyright © 2004 Society of Chemical Industry     

The dynamic swelling behaviour of chitosan-based hydrogels

A comparative study of the swelling behaviour of crosslinked chitosan (cr-CS) and its semi-interpenetrating polymer network (IPN) with poly(oxypropylene glycol) (cr-CS/PE semi-IPN) was carried out by DSC, NMR and positron annihilation lifetime spectroscopy (PALS). The results reveal that cr-CS/PE semi-IPN containing hydrophobic moieties has more free volume for water at a definite water content and restricts its mobility less. NMR and PALS data indicate that water clusters coexist with water molecules hydrogen-bonded to the network at the beginning of swelling. © 1998 SCI.     

Equilibrium swelling studies of highly swollen acrylamide/mesaconic acid hydrogels

Acrylamide/mesaconic acid (AAm/MA) hydrogels were prepared by free radical solution polymerization in aqueous solution of acrylamide (AAm) with mesaconic acid (MA) as comonomer and two multifunctional crosslinkers such as ethylene glycol dimethacrylate (EGDMA) and 1,4‐butanediol dimethacrylate (BDMA). Swelling experiments were performed in water at 25°C, gravimetrically. The influence of mesaconic acid content in hydrogels was examined. Swelling of AAm/MA hydrogels was increased up to 2301% (for containing 20 mg MA and crosslinked by EGDMA) to 3296% (for containing 80 mg MA and crosslinked by BDMA), while AAm hydrogels swelled up to 1330% (crosslinked by BDMA) to 1400% (crosslinked by EGDMA). The values of equilibrium water content of the hydrogels are 0.9301–0.9706. Diffusion behavior was investigated. Water diffusion into hydrogels was found to be non‐Fickian in character. Diffusion coefficients of AAm/MA hydrogels were calculated by the short time approximation and found to be from 38.01 × 10^?6 cm² s^?1 to 182.73 × 10^?6 cm² s^?1. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 2253–2259, 2005     

Equilibrium swelling behavior and elastic properties of polymer–clay nanocomposite hydrogels

Nanocomposite hydrogels were prepared by free‐radical polymerization of the monomers acrylamide (AAm), N,N‐dimethylacrylamide (DMA), and N‐isopropylacrylamide (NIPA) in aqueous clay dispersions at 21°C. Laponite XLS was used as clay nanoparticles in the hydrogel preparation. The hydrogels based on DMA or NIPA monomers exhibit much larger moduli of elasticity compared with the hydrogels based on AAm monomer. Calculations using the theory of rubber elasticity reveal that, in DMA‐clay or NIPA‐clay nanocomposites, both the effective crosslink density of the hydrogels and the functionality of the clay particles rapidly increase with increasing amount of Laponite up to 10% (w/v). The results suggest that DMA‐clay and NIPA‐clay attractive interactions are stronger than AAm‐clay interactions due to the formation of multiple layers on the nanoparticles through hydrophobic associations. It was also shown that, although the nanocomposite hydrogels do not dissolve in good solvents such as water, they dissolve in dilute aqueous solutions of acetone or poly(ethylene oxide) of molecular weight 10,000 g/mol, demonstrating the physical nature of the crosslink points. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Equilibrium swelling and mechanical properties of hydrogels of acrylamide and itaconic acid or its esters

Summary The equilibrium swelling and the plateau elastic modulus of a family of hydrogels made by the polymerization of acrylamide with itaconic acid or some of its esters were investigated as a function of composition and crosslinking degree to find materials with satisfactory swelling and elastic properties. We show that an appropriate selection of the comonomers and the concentration of the crosslinking agent is very important to produce hydrogels with large swelling capacity and good mechanical attributes. Tailoring of mechanical properties and swelling can also be achieved by this method. Received: 7 September 1999/Revised version: 3 December 1999/Accepted: 6 December 1999     

Miscibility of SAN with polyacrylates and polymethacrylates

Styrene-acrylonitrile copolymers, SAN, are known to be miscible with poly(methyl methacrylate). Based on the presence of a single composition dependent glass transition and lower critical solution temperature behaviour, it is shown that SAN is also miscible with poly(ethyl methacrylate). However, the SAN does not appear to be miscible with other polymethacrylates or polyacrylates.     

Equilibrium swelling of polystyrene by alkanes and carboxylic acids: A simple quantitative correlation model

In this study, we show that the equilibrium saturation swelling of polystyrene by linear alkanes and linear carboxylic acids, as reported earlier (Bernardo and Vesely, Eur Polym J 2007, 43, 4983, Bernardo and Vesely, J Appl Polym Sci 2008, 110, 2393) can be correlated quantitatively using a model where only the alkyl‐chain lengths and the saturated vapor pressures of the organic liquids are considered. According to this model, at very low vapor pressures, the number of polystyrene monomer units per solvent molecule is directly proportional to the alkyl chain length of the solvent molecule under consideration. At higher vapor pressures, the number of polystyrene monomer units per solvent molecule tends to become independent of its alkyl chain length. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Thermodynamics of high swelling of polymer particles with a large amount of monomer utilizing the dynamic swelling method

Recently, we suggested a novel swelling method to make polymer seed particles absorb a large amount of monomer prior to the seeded polymerization for preparing monodispersed polymer particles having diameter above 5μm, which we named the “dynamic swelling method.” This article discusses the thermodynamic background of this method, both theo-retically and experimentally. © 1996 John Wiley & Sons, Inc.     

Equilibrium swelling properties of weakly lonizable 2-hydroxyethyl methacrylate (HEMA)-based hydrogels

Hydrogles, sensitive to pH, were prepared by copolymerizing 2-hydroxyethyl methacrylate (HEMA) with 2-dimethylaminoethyl methacrylate (DMA). The mole fraction of the DMA monomer present during synthesis was fixed at 0.3, while the volume fraction of total monomer varied from 0.29 ( I ) to 1.0 ( II ). Swelling equilibria were measured in citrate of phosphate buffer for pH from 3.0 to 7.6 Swelling of the hydrogels depends on the volume fraction monomer present during synthesis, solution, pH, and identity of the buffer, Hydrogel I reached a four-fold larger swelling ratio (g swollen hydrogel/g dry hydrogel) in pH 3.0 citrate buffer, as compared to hydrogel II . Experimental results are interpreted on the basis of ideal Donnan equilibria. © 1994 John Wiley & Sons, Inc.     

The deformation behavior of poly(dimethyl siloxane) networks. II. Equilibrium swelling

The equilibrium swelling behavior of end-linked silicone networks has been studied, using low molecular weight silicone oils as diluents. Using the phantom and affine versions of the Flory–Rehner elasticity theory, the predictability of the equilibrium swelling has been examined. It was found that a modified version of the Flory–Rehner equation can be used to predict the equilibrium swelling behavior, provided the interaction parameter is known or measured. The modification is the use of the measured elastic shear modulus of the network prior to swelling, to calculate the density of network chains needed in the equation for swelling. This implies that both entangled and chemically linked chains contribute to the swelling behavior, since the modulus itself is predicted by the use of an equation that incorporates the contribution of entanglements. © 1994 John Wiley & Sons, Inc.     

Equilibrium swelling behavior of thermally responsive metal affinity hydrogels, Part I: Compositional effects

Copolymer hydrogels were synthesized from N-isopropylacrylamide (NIPAAm) and vinyl iminodiacetic acid (VIDA) monomers, incorporating thermally responsive swelling and metal affinity properties. Compared to pure NIPAAm hydrogels, the copolymer hydrogels showed significantly increased swelling due to the hydrophilic VIDA groups while still retaining their sharp phase transition behavior. However, excessive amounts of VIDA caused the gels to lose this behavior and not fully collapse even at temperatures as high as 80 °C. When chelated with copper the VIDA groups became less hydrophilic, partially reversing the increased swelling due to VIDA, enabling the gels to regain their phase transition behavior. Increasing the crosslinking density in the gels generally had the effect of decreasing their swelling. However, for gels with higher VIDA amounts, increasing the crosslinking density unexpectedly caused the hydrophilic groups with bound waters to resist the hydrophobic group-induced collapse at high temperatures. The results suggest that the NIPAAm, VIDA and crosslinker amounts and copper chelation are essential elements in the molecular design of the gel to retain a sharp phase transition. These variables were used to develop a phase transition phase diagram.     

Equilibrium swelling behavior of thermally responsive metal affinity hydrogels, Part II: Solution effects

In a previous study the effect of compositional variations on the equilibrium swelling of co-polymer hydrogels designed and synthesized from N-isopropylacrylamide (NIPAAm) and vinyl iminodiacetic acid (VIDA) monomers was investigated [Iyer G, Tillekeratne LMV, Coleman MR, Nadarajah A. Polymer, in this issue, doi:10.1016/j.polymer.2008.06.037]. The gels have both thermally responsive swelling and metal affinity properties and the effect of solution conditions on the equilibrium swelling of copper chelated and unchelated gels is studied here. In contrast to their sharp phase transition behavior in DI water, buffer solutions unexpectedly caused swelling of both gels to be the same and lose the sharp phase transition. Imidazole solutions had the expected phase transition behavior with increasing swelling and loss of phase transition of the unchelated gels which were partially reversed by copper chelation. Other small non-binding molecules, such as phenol, had minimal effects on the swelling behavior. Chicken egg white lysozyme solutions caused both gels to have reduced but equal equilibrium swelling and lose their sharp phase transition. These solution effects are explained in terms of salting in/salting out phenomena, the polarization of amide groups in VIDA, the solution pH and protein adsorption on hydrogel surfaces.     

Equilibrium swelling properties of a novel ethylenediaminetetraacetic dianhydride (EDTAD)-modified soy protein hydrogel

A novel pH and ionic strength-sensitive protein-based hydrogel was synthesized via cross-linking ethylenediaminetetraacetic dianhydride-modified soy protein isolate (EDTAD–SPI) with glutaraldehyde. Incorporation of ionizable carboxyl groups into soy proteins increased the net negative charge of the protein and caused extensive unfolding of the protein structure. The EDTAD–SPI hydrogel was capable of imbibing 80-300 g water per g dry gel after centrifuging at 214g, depending on the extent of modification, protein structure, crosslinking density, protein concentration during the crosslinking step, gel particle size, and environ-mental conditions, such as temperature, pH, and ionic strength. The protein concentration used during the crosslinking step was found to be the most important factor affecting the water uptake of the gel. The lower the protein concentration, the higher was the water uptake at 214g. The hydrogel was highly sensitive to pH and exhibited reversible swelling when sequentially exposed to water and 0.15M NaCl. © 1996 John Wiley & Sons, Inc.     

Synthesis of telechelic polyacrylates with unsaturated end-groups

A convenient, one-pot synthesis of telechelic and semi-telechelic polyacrylates with unsaturated end-groups has been developed. Atom transfer radical polymerization of methyl or n-butyl acrylate was initiated with either ethyl α-bromomethylacrylate or methyl dichloroacetate, as a monofunctional or difunctional initiator, respectively, and mediated with various Cu/amine complexes. Addition of excess ethyl 2-bromomethylacrylate was found to not only immediately quench the polymerization but also installed 2-carbethoxyallyl moieties on the ends of the polymer chains. The telechelic polymers were found to undergo thermally or photochemically induced crosslinking reactions.     

Synthesis and characterization of polyacrylates containing charge transfer groups

Summary Polyacrylates containing para-nitro azobenzene have been synthesized by free radical polymerization. The influence of the length of the spacer of the homopolyacrylates (HPn, n=3,4,6), content of methyl acrylate in the copolyacrylates (CP6) with para-nitro azobenzene groups on the thermal properties, such as liquid crystallinity, Tg and Tm, was studied by DSC, WAXD and polarized optical microscopy. Among the polymers studied, only the homopolyacrylate (HP6) with six carbon atoms in the spacer exhibited a nematic phase. The second-harmonic generation (SHG) signal of the poled HP6 film was detected qualitatively by Maker-fringer method.     

Swelling of crosslinked polyacrylates in isotropic and anisotropic solvents

The purpose of this study was to examine the swelling and deswelling of photochemically crosslinked poly(n‐butylacrylate) networks in isotropic and anisotropic solvents. The phase diagrams were established in terms of composition and temperature for five isotropic solvents, acetone, cyclohexane, methanol, tetrahydrofuran, and toluene, and two low‐molecular‐weight nematic liquid crystals, 4‐cyano‐4′‐n‐pentyl‐biphenyl and an eutectic mixture of cyanoparaphenylenes. Networks were formed by ultraviolet curing in the presence of 0.5 wt % difunctional monomer (hexane diol‐di‐acrylate) and 0.5 wt % photoinitiator (Darocur 1173). Immersion in excess solvent allowed us to measure the solvent uptake by weight and to determine the size increase by optical microscopy in terms of temperature. We calculated weight and diameter ratios considering the swollen‐to‐dry network states of the samples. Phase diagrams were analyzed with the phantom network model according to the Flory–Rehner theory of rubber elasticity, and for the anisotropic solvents, modeling was supplemented with the Maier–Saupe theory of nematic order for free energy. The polymer–solvent interaction parameter was deduced as a function of temperature, but the values were in discrepancy with Fedors's model of solubility parameters, which overestimated the interaction. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1–9, 2004     

交联型聚硅氧烷/聚丙烯酸酯复合乳液的制备

在甲基丙烯酰氧丙基三甲氧基硅烷存在下，以交联的聚硅氧烷为种子，丙烯酸酯单体为第二单体，偶氮二异丁腈为引发剂，分别采用间歇法、溶胀法和半连续法制备了聚硅氧烷／聚丙烯酸酯复合乳液。采用透射电子显微镜对乳胶粒子形态进行了表征，发现乳液中形成了以聚丙烯酸酯为核、聚硅氧烷为壳的反相核壳结构乳胶粒子；放置90天后，乳胶粒子的形态未发生翻转；将复合乳液破乳后分析，聚合物凝胶量达到96％以上，说明聚合体系交联很好。     

Dynamic mechanical properties of some liquid crystalline polyacrylates

Summary The dynamic mechanical properties of some LC (nematic and smectic) polyacrylates with biphenyl mesogens with different spacers and tails are described. From plots of G and G against frequency and temperature the phase transitions could be clearly detected, in agreement with DSC analysis. Furthermore, the dynamic viscosities of the LC polyacrylates compared with an isotropic model polymer of similar stucture showed a strong shear rate and temperature dependence. Surprisingly it was found that the viscosity of the LC polymers was higher in the nematic phase than in the isotropic phase.