Discontinuous Volume Transition Research Articles

Strong Cationic Radical Initiator‐Based Design of a Thermoresponsive Hydrogel Showing Drastic Volume Transition

AbstractIn this study, a millimeter‐sized thermoresponsive poly(N‐isopropylacrylamide) (PNIPAAm) gel is prepared by free radical polymerization using 2,2′‐azobis‐[2‐(1,3‐dimethyl‐4,5‐dihydro‐1H‐imidazol‐3‐ium‐2‐yl)]propane triflate (ADIP), a radical initiator possessing a strong cationic group that is newly developed. By comparing this gel with a PNIPAAm gel prepared using a conventional radical initiator, 2,2′‐azobisisobutyronitrile, the following properties are found: 1) the cationic residue of ADIP is introduced in the gel, and 2) the PNIPAAm gel prepared using ADIP exhibits a discontinuous volume transition as a function of temperature and rapid shrinking in response to temperature jump. This thermoresponsive behavior is attributed to the low polymer fraction in the gel prepared using ADIP. The ADIP‐based design of the thermoresponsive hydrogel investigated herein may contribute to the design of functional soft materials.

Salt-Induced Swelling and Volume Phase Transition of Polyelectrolyte Gels

A theoretical model of polyelectrolyte gels is presented to study continuous and discontinuous volume phase transitions induced by changing salt concentration in the external solution. Phase diagrams are constructed in terms of the polymer–solvent interaction parameters, external salt concentration, and concentration of fixed charges. Comparisons with previous experiments for an ionized acrylamide gel in mixed water–acetone solvents are made with good quantitative agreement for a monovalent salt (NaCl) but fair qualitative agreement for a divalent salt (MgCl2), using a simple set of parameters for both cases. The effective polymer–solvent interactions vary with the volume fraction of acetone in the mixed solvent, leading to either continuous or discontinuous volume transitions. The presence of divalent ions (Mg2+) in addition to monovalent ions in the external solution reduces the critical salt concentration for the discontinuous transition by several orders of magnitude. Moreover, a secondary continuous transition is predicted between two highly swollen states for the case of a divalent salt. The present model may be further extended to study volume phase transitions of polyelectrolyte gels in response to other stimuli such as temperature, pH and electrical field.

Hysteresis in the surfactant-induced volume transition of hydrogels.

The discontinuous uptake and release of surfactants by hydrogels and the accompanying discontinuous volume transition is known to occur with a hysteresis. We have performed a theoretical analysis in order to find the mechanistic origin of this phenomenon. Using a mean-field model, we have quantitatively reproduced the experimental behavior by considering the cost of elastically deforming the gel material to allow phase coexistence. The major part of the hysteresis is due to the high phase coexistence cost of the swelling transition, since in this direction the coexistence cost depends not only on the elasticity of the network (being a weak force in comparison) but also on the entropy of the monovalent nonsurfactant electrolyte present in the system.

Volume phase transition of polyelectrolyte gels: effects of ionic size.

Although the volume transition of the polyelectrolyte gel has been studied for decades, less study on the finite size effects of the mobile ions has been conducted. In the present paper, Tanaka's classical theory of polyelectrolyte gel is extended to the cases of mobile ions of finite volume. In the salt free limit, the theoretical results show that the discontinuous volume transition of the polyelectrolyte gel will become a continuous one for counterionic size larger than a critical value. When a significant amount of salt is added, the critical value for the volume transition increases as a result of electrostatic screening. An increase in salt concentration can also make the polyelectrolyte gel in poor solvent collapse. Poorer solvent is needed to trigger the salt-induced collapse in polyelectrolyte gel with larger mobile ions than that with smaller ones. The effects of ionic size on the critical points and phase diagram of the volume transition are also discussed. The theoretical results suggest that the swelling behavior of polyelectrolyte gel might be tuned with salt of different volumes.

Dynamic light scattering studies of ionic and nonionic polymer gels with continuous and discontinuous volume transitions

AbstractWe have performed dynamic light scattering experiments on poly(acrylamide)‐poly(acrylic acid) copolymer gels with controlled crosslink density and copolymer composition, by varying the temperature, amount and valency of added salt, pH, and solvent quality. Our systematic study provides several insights. The correlation length for the monomer density fluctuations, as inferred from the measured diffusion coefficient, is too small to be identified as the mesh size of the gel. The correlation length in an ionic gel, which is found to be smaller than that for an equivalent gel without ionization. Comparison of swelling ratio with the diffusion coefficient shows that these quantities are not simply geometrically related to each other. When a discontinuous volume phase transition is induced by gradually varying the solvent quality, the diffusion coefficient exhibits a pretransitional reduction by two orders of magnitude even before the gel collapse. These findings provoke a need for new theoretical approaches for describing the elastic modes of polyelectrolyte gels. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010

Anisotropic Swelling and Phase Behavior of Monodomain Nematic Networks in Nematogenic Solvents

Equilibrium swelling behavior of uniaxially oriented nematic networks in nematogenic solvents has been studied in the temperature (T) range covering the two independent nematic−isotropic transition temperatures for the swollen network (TNIG) and the surrounding solvent (TNIS; TNIG > TNIS). The swollen isotropic state smoothly changes into the shrunken nematic state with an elongation along the director axis within a narrow temperature range around TNIG. In the region of TNIS < T < TNIG, where the surrounding solvents are in the isotropic state, the shrunken nematic gels reswell with a further increase in shape anisotropy upon cooling. In the totally nematic phase of T < TNIS, the shape anisotropy increases without significant volume change with decreasing T. A mean field theory satisfactorily describes the features of experimental data, although a sharp transition predicted disagrees with the smooth transition observed. The polydomain nematic network without global director orientation shows no anisotropy in swelling but exhibits substantially the same T dependence of swelling degree as the corresponding monodomain system. A striking difference is that the polydomain system undergoes a discontinuous volume transition, unlike the continuous transition in the monodomain system.

Terpolymer smart gels: synthesis and characterizations

Two smart terpolymer gels, MS-1 and MS-2, were synthesized such that the same gel can respondto more than one external environmental condition, such as pH, temperature, solvent composition,electric field. So two terpolymers gels of vinyl monomers such as sodium acrylate, acrylamide andN-isopropyl acrylamide were synthesized by using ammonium persulfate (APS) as an initiator,N,N,N',N'-tetramethyl ethylene diamine (TMEDA) as an accelerator and methylene bisacrylamide as a cross-linker.These terpolymers were characterized by elemental and Fourier transform infrared analysis.The swelling behavior of these terpolymer smart gels was evaluated by changing the pH,temperature and solvent composition. The variation of the swelling behavior with time wasevaluated in an aqueous medium at room temperature. The time taken for maximum swelling(tm) was about 20 min for the gel MS-2. However thetm value for the gel MS-1 is higher than that of MS-2. The swellingbehavior remains almost unchanged over a temperature range of22–50 °C for both the gels. The discontinuous volume transitions were observed at pH 7.6 and 8.2 forthe two gels, MS-1 and MS-2, respectively. The gel MS-1 suddenly shrinks below and swellsabove pH 7.6. Correspondingly, the pH is 8.2 for the case of MS-2. Volume transitions in anacetone–water mixture were also observed for these gels. The swelling behaviors ofthese two smart gels are almost parallel above the 40% acetone concentration.

Synthesis and characterization of hydrophobically modified poly(vinyl alcohol) hydrogel membrane

Hydrophobically modified poly(vinyl alcohol), [PVA] was synthesized by graft copolymerization of N-tertiary butyl acrylamide [NTBA] onto PVA by free radical polymerization. The incorporation of NTBA onto PVA chains was confirmed by elemental analysis, FT-IR and NMR spectroscopy. A series of graft copolymers with different contents of NTBA were prepared and membranes were casted from these copolymer solutions in dimethyl sulfoxide. The increase in hydrophobicity with an increase in NTBA content was investigated by contact angle measurements. The swelling behaviour of membranes as a function of temperature, hydrophobic content, annealing temperature and period was studied. Permeability of solutes through these membranes was investigated as a function of solute size, membrane hydrophobicity and temperature. The swelling behaviour of the copolymer membranes showed that the lower content of NTBA gives discontinuous volume transition with respect to temperature whereas, the presence of higher amounts of NTBA showed decreased swelling ratios with very little influence of temperature on the swelling. The permeabilities of solutes through these membranes were strongly dependent on the size of the solute, solution temperature and hydrophobicity of the membrane. The copolymer membranes were further characterized using DSC, DMA and XRD. The tan δ peak becomes broader as the NTBA content increases.

Discontinuous Volume Transitions in Cross-Linked Polyelectrolyte Gels Induced by Short-Range Attractions and Strong Electrostatic Coupling

The swelling behavior of polyelectrolyte gels with and without short-range attraction was studied at variable permittivities using Monte Carlo simulations. The primitive model, with explicit treatment of all charged species augmented with an attractive square-well potential between network particles, was used. Discontinuous volume transitions appeared at critical values of the depth of the square-well potential at fixed permittivity and at critical values of the permittivity at fixed short-range potential. In particular, we observed such a transition also without the short-range potential. The collapsed state was favored by the short-range attraction and/or at low permittivity. The volumes of the collapsed states were 1-2 orders of magnitude smaller than those of the swollen states.

Volume Transition of Liquid Crystalline Gels in Isotropic Solvents

The equilibrium swelling and phase behavior of the side chain liquid crystalline (LC) networks in isotropic (nonmesomorphic) solvents has been investigated as a function of temperature. The LC networks swollen in di-n-amyl phthalate or di-n-butyl phthalate exhibit discontinuous volume transition driven by nematic ordering: The swollen isotropic gel is discontinuously transformed into the shrunken nematic gel at a characteristic temperature (TNIG). The transition temperature TNIG depends on the swelling solvent, and TNIG decreases as the degree of equilibrium swelling in the isotropic phase increases. The volume transition phenomena observed are satisfactorily described by a mean field theory for nematic gel.

Thermoreversible swelling behaviour of hydrogels based on N-isopropylacrylamide with a hydrophobic comonomer

Hydrogels were prepared by free radical polymerisation in aqueous solution of N-isopropylacrylamide (NIPA) and of NIPA with di- n-propylacrylamide (DPAM), di- n-octylacrylamide (DOAM) or di-dodecylacrylamide (DDAM) as hydrophobic comonomer. N, N-methylene bisacrylamide (BIS) and glyoxal bis(diallyacetal) (GLY) were used as crosslinkers. A series of copolymers with three different comonomer contents was synthesised and for some polymers three different crosslinker concentrations were employed. The swelling equilibrium of these hydrogels was studied as a function of temperature, hydrophobic comonomer species and content in aqueous solutions of the anionic surfactant sodium dodecyl sulfate (SDS). In pure water the gels showed a discontinuous volume phase transition at 33 and 30 °C for PNIPA and hydrophobically modified PNIPA copolymeric hydrogels, respectively. The swelling ratio r and the transition temperature (LCST) increased at low temperatures with the addition of SDS, this is ascribed to the conversion of non-ionic PNIPA gels into polyelectrolyte gels through the binding of SDS. At SDS concentration below 0.5 wt%, gels exhibited a single discontinuous volume transition at 36–38 °C. However, for SDS concentration above 0.5 wt%, two discontinuous volume transitions at 36–40 and 70 °C were observed. Additionally, the replacement of BIS by the novel octafunctional crosslinker glyoxal bis(diallylacetal) (GLY) yielded an increase in the swelling ratio.

Designing new thermoreversible gels by molecular tailoring of hydrophilic-hydrophobic interactions

We have shown that the lattice fluid hydrogen bond (LFHB) model can successfully quantify the first-order volume transition in hydrogels. The model predicts that a critical balance of hydrophilic and hydrophobic interactions is required for a gel to exhibit a discontinuous volume transition. In this work we will report the swelling behavior of a new thermoreversible copolymer hydrogel, which has been synthesized from two monomers, whose homopolymers do not show any volume transition in water in the observable range of temperatures. The discontinuous volume transition phenomenon in the copolymer gel was observed only at a critical balance of hydrophilic-hydrophobic interactions. The discontinuous nature of the volume transition is lost with a subtle change in the hydrophilic-hydrophobic balance. The copolymer gel was synthesized from 2-acrylamido 2-methyl propane sulfonic acid (AMPS), which is a hydrophilic monomer, and N-tertiary butylacrylamide (N-t-BAm), which is a hydrophobic monomer. The hydrophilic-hydrophobic balance in the gel was altered by either changing the composition of the co-monomers or by substituting the N-t-BAm with another hydrophobic monomer, N-isopropylacrylamide (NIPAm).

The volume transition in thermosensitive core-shell latex particles containing charged groups

An investigation of the volume transition in thermosensitive core–shell particles by dynamic light scattering (DLS) is presented. The core of the particles consists of polystyrene (diameter 118 nm), whereas the thermosensitive shell is composed of a network of poly (N-isopropylacrylamide) containing 2 mol% acrylic acid counits. The hydrodynamic radius of these particles as determined by DLS decreases in a continuous manner when raising the temperature. It is shown that the volume transition in the core–shell microgels remains continuous for a wide range of ionic strengths and pH values. This behavior is opposite to that of macrogels of the same chemical composition, which undergo a discontinuous volume transition. The present investigation therefore demonstrates that affixing the network to solid colloidal particles profoundly alters the volume transition of thermosensitive networks. The reason is that shrinking can take place only along the radial direction of the particles. The solid core thus exerts a strong spatial constraint onto the network, which leads to the observed behavior.

Evidence for phase transition in nerve fibers, cells and synapses

By examining rapid changes in the volume and temperature during excitation of a large number of excitable cells and tissues, we have shown that the excitation process is invariably accompanied by swelling and heat production in the superficial protoplasmic layer. By comparing the behavior of a model system (synthetic anionic gels containing both Ca- and Na- ions) with that of nerve fibers under comparable environmental conditions, we have demonstrated that the onset of an action potential is a reflection of a discontinuous volume transition in the superficial layer of nerve cells and fibers. Evidence for the existence of a first- order phase transition (involving volume and temperature changes) in nerve cells, fibers and synapses is presented.

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

Zein−Poly(N-isopropylacrylamide) Conjugates

Stable cross-linked gels were synthesized by polymerization of N-isopropylacrylamide (NIPA) blended with a seed protein (zein) and 4,4‘-diazido-2,2‘-stilbenedisulfonic acid (DAS) in dimethyl sulfoxide following irradiation. Grafting of DAS−zein chains on polyNIPA backbone was proposed. The gels were shown to undergo discontinuous volume transition at around 32 °C corresponding to lower critical solution temperature of NIPA. The gel swelling was pH-sensitive. Sharp maximum on viscosity vs temperature plots was observed in suspensions of the gel particles at 32 °C due to interparticle network formation caused by hydrophobic interactions between polyNIPA domains.

Volume Transitions, Phase Separation, and Anisotropic Surface Phases in Charged Gels

Thermodynamic analysis of phase transitions in charged gels in poor solvents is presented. At sufficiently high degree of ionization, the gel undergoes a discontinuous volume transition to a new homogeneous and isotropic phase. When the two-phase coexistence line is reached under fixed volume conditions, the new phase forms a thin anisotropic layer on the surface of the gel. The concentration and the deformation of the surface phase are calculated and the results are generalized to the case of uniaxially deformed gels. The stability of the surface phase is studied and it is shown that a dilute surface phase is unstable against long wavelength fluctuations with wave vectors oriented along the surface of the gel. The connection between the theory and experiments on phase transitions in gels is discussed.

Phase transition in polymer gels due to local heating by illumination of light

Abstract The volume phase transition in gels induced by visible light and its related properties are presented, the mechanism of which is based on local heating of a polymer network by illumination of light. The gels consist of a covalently cross-linked copolymer network of thermosensitive N-isopropylacrylamide and a chromophore. Without light illumination, the gel volume changes sharply, but continuously at approximately 34°C when the temperature is varied. At a fixed temperature of an appropriate value, a discontinuous volume transition is observed when the light intensity is gradually changed. The phase transitions can be understood in terms of the temperature increment at the immediate vicinity of polymer chains due to the local heating via light absorption and subsequent thermal dissipation of light by the chromophore. The results can be qualitatively described by the Flory-Huggins mean-field equation of state of gels. In order to make clear the mechanism of the light-induced phase transition in the ...

Discontinuous volume transitions induced by calcium-sodium ion exchange in anionic gels and their neurobiological implications.

Discontinuous volume transitions in poly (acrylic acid) gels were studied by determining equilibrium swelling curves of small gel beads immersed in salt solutions containing varying concentrations of Ca2+ and Na+. Reversible contractions of gel membranes associated with Ca(2+)-Na+ exchange were examined using both isometric and isotonic recording devices. A discontinuous change in electric conductance was demonstrated in association with Ca(2+)-Na+ exchange in gel membranes. These experimental findings provide a sound physicochemical basis for elucidating the mechanism of nerve excitation.

Thermo-sensitiveN-n-propylacrylamide gels

Abstract Thermo-sensitive gels were prepared by irradiating aqueous solutions of poly(N-n-propylacrylamide) (NNPA) and its copolymers with acrylic acid (AA) with γ-rays from a 60Co source. The equilibrium swelling volume of the gel in water was determined as a function of temperature. NNPA gel showed a discontinuous and reversible volume phase transition. The transition temperature and the volume change at the transition decreased with irradiation time. The transition temperature was approximately 12°C lower than that of poly(N-isopropylacrylamide) gel. A discontinuous volume transition was also observed in the copolymer gels of NNPA and AA. The dependence of the transition temperature on the concentration of carboxyl groups revealed a marked difference depending on whether they were protonated or dissociated. For gels having side groups of COONa, the transition temperature rose and the volume change at the transition was elevated as the COONa concentration increased. In contrast, an increase in the COOH concentration resulted in a decrease in the transition temperature. These results are discussed in terms of an equation of state constructed based on scaling theory.