Volume Phase Transition Of Gels Research Articles

Phase Transition of Gels-A Review of Toyoich Tanaka's Research.

In 70’s, the extensive studies about the gel science has begun with the discovery of the volume phase transition of gel at the physics department of Massachusetts Institute of Technology. After the discovery of the volume phase transition of gel, the phenomenon was extensively studied and advanced by the discoverer, the late Professor Toyoichi Tanaka, who deceased on 20 May 2000 in the halfway of his research. In this paper, we would like to review his research to clarify his deep insight into the science of gels.

Controlling Syneresis of Hydrogels Using Organic Salts

AbstractSupramolecular hydrogels can spontaneously undergo syneresis through fibre–fibre interactions and expel significant amounts of water upon aging. In this process, the hydrophobicity of fibres which regulates the 3D‐rearrangement of the self‐assembled structures during syneresis is important. Here, we show that we can control the hydrophobic microenvironment of gels by incorporating organic salts into the co‐assembled gel fibres thereby enabling control of the macroscopic gel volume phase transition.

Controlling Syneresis of Hydrogels Using Organic Salts.

Supramolecular hydrogels can spontaneously undergo syneresis through fibre–fibre interactions and expel significant amounts of water upon aging. In this process, the hydrophobicity of fibres which regulates the 3D‐rearrangement of the self‐assembled structures during syneresis is important. Here, we show that we can control the hydrophobic microenvironment of gels by incorporating organic salts into the co‐assembled gel fibres thereby enabling control of the macroscopic gel volume phase transition.

Re-Entrant Conformation Transition in Hydrogels.

Hydrogels are attractive materials not only for their tremendous applications but also for theoretical studies as they provide macroscopic monitoring of the conformation change of polymer chains. The pioneering theoretical work of Dusek predicting the discontinuous volume phase transition in gels followed by the experimental observation of Tanaka opened up a new area, called smart hydrogels, in the gel science. Many ionic hydrogels exhibit a discontinuous volume phase transition due to the change of the polymer–solvent interaction parameter χ depending on the external stimuli such as temperature, pH, composition of the solvent, etc. The observation of a discontinuous volume phase transition in nonionic hydrogels or organogels is still a challenging task as it requires a polymer–solvent system with a strong polymer concentration dependent χ parameter. Such an observation may open up the use of organogels as smart and hydrophobic soft materials. The re-entrant phenomenon first observed by Tanaka is another characteristic of stimuli responsive hydrogels in which they are frustrated between the swollen and collapsed states in a given solvent mixture. Thus, the hydrogel first collapses and then reswells if an environmental parameter is continuously increased. The re-entrant phenomenon of hydrogels in water–cosolvent mixtures is due to the competitive hydrogen-bonding and hydrophobic interactions leading to flow-in and flow-out of the cosolvent molecules through the hydrogel moving boundary as the composition of the solvent mixture is varied. The experimental results reviewed here show that a re-entrant conformation transition in hydrogels requires a hydrophobically modified hydrophilic network, and a moderate hydrogen-bonding cosolvent having competitive attractions with water and polymer. The re-entrant phenomenon may widen the applications of the hydrogels in mechanochemical transducers, switches, memories, and sensors.

(Invited) Semi-Interpenetrating Polymer Networks Based on N-Isopropylacrylamide and 2-Acrylamido-2-Methylpropane Sulfonic Acid for Hydrogel-Based Chemical Sensors

(Invited) Semi-Interpenetrating Polymer Networks Based on N-Isopropylacrylamide and 2-Acrylamido-2-Methylpropane Sulfonic Acid for Hydrogel-Based Chemical Sensors

Volume Phase Transition in Gels: Its Discovery and Development.

The history of volume phase transition of responsive gels from its theoretical prediction to experimental discovery was described and the major role of mixing Gibbs energy function in theoretical models was stressed. For detailed analysis and fine tuning of the volume phase transition, the generalized Flory–Huggins model with concentration and temperature dependent interaction function coupled with Maxwell construction as a tool is very suitable. Application of expansive stresses can uncover the potential of various swelling gels for volume phase transition. Experimentally, the abrupt, equilibrium-controlled phase transition is often hard to achieve due to passage of gel through states of mechanical instability and slow relaxation processes in macroscopic objects.

Semi-interpenetrating Polymer Networks Based on N-Isopropylacrylamide and 2-Acrylamido-2-methylpropane Sulfonic Acid for Hydrogel-based Chemical Sensors

Stimulus-responsive hydrogels represent sensor elements with a high application potential for the detection of the concentration of a solved species. In combination with a piezoresistive pressure sensor, the hydrogel's analyte-dependent swelling pressure is transformed into an electrical output signal. In the past, numerous sensors with sensitivities to various analytes, like e.g. ethanol, glucose and pH, have been researched using this sensor principle [1]. However, the time span until a final steady-state value of the sensor's output signal is reached is typically in the range of minutes to hours due to the excessive diffusion processes associated with the gel's volume phase transition and visco-elastic behaviour.

Construction of a Universal Gel Model with Volume Phase Transition

The physical principle underlying the familiar condensation transition from vapor to liquid is the competition between the energetic tendency to condense owing to attractive forces among molecules of the fluid and the entropic tendency to disperse toward the maximum volume available as limited only by the walls of the container. Van der Waals incorporated this principle into his equation of state and was thus able to explain the discontinuous nature of condensation as the result of instability of intermediate states. The volume phase transition of gels, also discontinuous in its sharpest manifestation, can be understood similarly, as a competition between net free energy attraction of polymer segments and purely entropic dissolution into a maximum allowed volume. Viewed in this way, the gel phase transition would require nothing more to describe it than van der Waals’ original equation of state (with osmotic pressure replacing pressure P). But the polymer segments in a gel are networked by cross-links, and a consequent restoring force prevents complete dissolution. Like a solid material, and unlike a van der Waals fluid, a fully swollen gel possesses an intrinsic volume of its own. Although all thermodynamic descriptions of gel behavior contain an elastic component, frequently in the form of Flory-style rubber theory, the resulting isotherms usually have the same general appearance as van der Waals isotherms for fluids, so it is not clear whether the solid-like aspect of gels, that is, their intrinsic volume and shape, adds any fundamental physics to the volume phase transition of gels beyond what van der Waals already knew. To address this question, we have constructed a universal chemical potential for gels that captures the volume transition while containing no quantities specific to any particular gel. In this sense, it is analogous to the van der Waals theory of fluids in its universal form, but although it incorporates the van der Waals universal equation of state, it also contains a network elasticity component, not based on Flory theory but instead on a nonlinear Langevin model, that restricts the radius of a fully swollen spherical gel to a solid-like finite universal value of unity, transitioning to a value less than unity when the gel collapses. A new family of isotherms arises, not present in a preponderately van der Waals analysis, namely, profiles of gel density as a function of location in the gel. There is an abrupt onset of large amplitude density fluctuations in the gel at a critical temperature. Then, at a second critical temperature, the entire swollen gel collapses to a high-density phase.

Deswelling Induced Morphological Changes in Dual pH and Temperature Responsive Ultra-Low Crosslinked Poly (N-isopropyl acrylamide)-co-Acrylic Acid Microgels.

Poly(N-isopropylacrylamide) microgels prepared without exogenous crosslinker are extremely "soft" as a result of their very low crosslinking density, with network connectivity arising only from the self-crosslinking of pNIPAm chains. As a result of this extreme softness, our group and others have taken interest in using these materials in a variety of bioengineering applications, while also pursuing studies of their fundamental properties. Here, we report deswelling triggered structural changes in poly (N-isopropylacrylamide-co-acrylic acid) (ULC10AAc) microgels prepared by precipitation polymerization. Dynamic light scattering suggests that the deswelling of these particles not only depends on the collapse of the pNIPAm chains but is also influenced by the ionization state of the acrylic acid moieties present in the copolymer. The ULC10AAc microgel behaves like a traditional crosslinked pNIPAm microgel at pH 3.5, showing a sharp decrease in the hydrodynamic diameter around the lower critical solution temperature (LCST) of pNIPAm. As the pH is increased to 4.5 we observe multiple transitions in the deswelling curve, suggesting inhomogeneity in the structure and/or composition of the microgels. At pH 6.5 the microgels cease to be thermoresponsive over the studied temperature range due to increased charge repulsion between the fully deprotonated AAc groups and an increase in gel osmotic pressure due to solvated counterion ingress. Atomic force microscopy images of particles deposited at different temperatures reveal a temperature induced morphological change, with punctate structures forming inside microgels at pH 4.5 and 6.5 and temperature above the gel volume phase transition temperature (VPTT).

Necklace-like microstructure in shallow-quenched aqueous solutions of poly(n-isopropylacrylamide), detected by advanced small-angle neutron scattering methods.

The microstructure of aqueous poly(N-isopropyl acrylamide) (PNIPA) gel and solution was investigated by small-angle neutron scattering (SANS) in the vicinity of the gel volume phase transition at TV (= 34 °C). The SANS technique was reinforced by refractive neutron lenses and perfect single crystals in order to get access to μm length scales. At 31 °C SANS shows Ornstein-Zernike (OZ) type scattering in the swollen gel which at 32 °C starts to deviate from the OZ-formalism, exhibiting excess scattering and at the wave number qc≅ 5 × 10-3Å-1 a crossover to Porod's asymptotic q-4 power law. For shallow quenches of ΔT < 1.0 K above TV the excess scattering intensity is further increasing whereas qc is shifting toward lower values. Based on this observation and analysis of the SANS q-profiles, we propose a necklace-like microstructure consisting of PNIPA-rich globules of R≅ 100 Å size which are connected by swollen PNIPA chains and stabilized for more than a day by pinning of chain connectivity. The formation of PNIPA globules near TV is discussed in terms of partially cooperative dehydration which is crucial to explain the "miscibility square phase behavior" of aqueous PNIPA solutions. Globule-like structure was also found in aqueous PNIPA solution of size slightly larger than in gels. At deeper quenches of gels above TV (ΔT > 1.0 K) the globules are aggregating to larger objects of R≅ 0.24 μm size as determined from a strong intensity upturn in the small q-region of USANS.

Generative Force of Self-Oscillating Gel

We succeeded in measuring the generative force of a self-oscillating polymer gel in an aqueous solution comprising the three substrates of the Belousov-Zhabotinsky (BZ) reaction (malonic acid, sodium bromate, and nitric acid) under constant temperature. In this study, we developed an apparatus with a microforce sensor for measuring the generative force of small-sized gels (1 mm(3)). The self-oscillating polymer gel directly converts the chemical energy of the BZ reaction into mechanical work. It was determined that the generative force of the self-oscillating gel was 972 Pa, and the period of self-oscillation was 480 s at 18 °C. We demonstrated that the generative force of the gel was about a hundredth the generative force of a muscle in the body. We analyzed the time dependence of the color change in the self-oscillating polymer gel. The color of the gel changed periodically owing to the cyclic change in the redox state of the Ru moiety, induced by the BZ reaction. The peaks of the waveforms of the generative force and color change were almost identical. This result showed that the generative force was synchronized with the periodical change in the oxidation number of the Ru catalytic moiety in the gel. To understand a theoretical basis for the generative force of a self-oscillating gel, we considered a general theory that is based on the volume phase transition of gel and the two-parameter Oregonator model of the BZ reaction.

Effects of strongly selective additives on volume phase transition in gels

We investigate volume phase transition in gels immersed in mixture solvents, on the basis of a three-component Flory-Rehner theory. When the selectivity of the minority solvent component to the polymer network is strong, the gel tends to shrink with an increasing concentration of the additive, regardless of whether it is good or poor. This behavior originates from the difference of the additive concentration between inside and outside the gel. We also found the gap of the gel volume at the transition point can be controlled by adding the strongly selective solutes. By dissolving a strongly poor additive, for instance, the discontinuous volume phase transition can be extinguished. Furthermore, we observed that another volume phase transition occurs far from the original transition point. These behaviors can be well explained by a simplified theory neglecting the nonlinearity of the additive concentration.

Dual-responsive and super absorbing thermally cross-linked hydrogel based on methacrylate substituted polyphosphazene

Stimuli-responsive hydrogels have played a crucial role in biomaterials being extensively studied for applications such as tissue engineering and drug delivery. Here, we report on the synthesis and characterization of a new hydrogel based on methacrylate substituted polyphosphazene (PMAPhos). The hydrogel was obtained spontaneously after the polyphosphazene substitution reaction and its mechanical properties were improved after annealing. By annealing the hydrogel in the presence of a 6% w/w methacrylic acid solution, we could obtain a poly(methacrylic acid) (PMAA) branched and slightly cross-linked polyphosphazene network, which showed ultra-high absorbency being able to absorb distilled water, at maximum, as much as 870 times of its own weight. Moreover, PMAPhos showed dual-responsive behaviour responding to both pH and temperature changes. Unlike many other similar systems, the hydrogel structure collapsed not only at lower but also at higher pHs. The gel volume phase transition as a function of temperature was determined by both differential scanning calorimetry (DSC) and by swelling experiments. However, after the hydrogel annealing, the thermal response was suppressed and overridden by the pH-response behaviour of the PMAA branches. Hydrogel morphology and swelling behaviour were also analyzed directly by Cryo-transmission electron microscopy, which revealed the fibrous polymer network. Remarkable features of this new hydrogel are: the biodegradation offered by the polyphosphazene backbone and simplicity of preparation where no co-polymerizations were used to reach the dual-responsive behaviour. Besides, the particular pH-responsive behaviour found for this hydrogel is a useful characteristic for applications aiming for controlled delivery of substances in specific sites of the human body.

Chemical-to-Electrical-Signal Transduction Synchronized with Smart Gel Volume Phase Transition.

A stimulus-responsive polymer gel designed on a field-effect transistor gate undergoes a reversible volume phase transition in response to a specific biomolecule. An abrupt permittivity change at the gel/gate interface during the transition gives rise to a chemical to electrical signal conversion; the signal is thus detectable via a transistor without the limit of the Debye length.

A correlation between mechanical and electrical properties of the synthetic hydrogel chosen as an experimental model of cytoskeleton

The correlation between the electrochemical (Donnan) potential and volume swelling was studied for synthetic polyelectrolyte hydrogels considered as models of cytoskeleton gel-forming biopolymers. Hydrogels involving polyacrylic and polymethacrylic acids with varying network density were synthesized by a radical polymerization in aqueous solution. Electrical charge was introduced into the gel network by partial neutralization of monomer acids with several alkali and alkali earth (hydr)oxides. The electrochemical (Donnan) potential of synthetic gels was determined using conventional microelectrode tools for cell potential determination. It was demonstrated that the negative electrical potential of many anionic gels with various charges and network densities decreased with the decrease of equilibrium swelling, i.e., with the decrease in water content in the gel. It was shown that a drastic phase transition in the gel structure from a swollen to a compressed state induced by K+/Ca2+ exchange is accompanied by an analogous decrease in the absolute Donnan potential of the gels. A kinetic study demonstrated that the gel volume changed ahead of its electrical potential. This suggests that the volume phase transition in gel is the main cause of the electrical response. A similarity between the swelling/compression transition in synthetic gels and the volume changes in the cytoskeleton in the vicinity of the cell membrane was demonstrated. Based on the universal analogy between the properties of synthetic and natural polymer gels, a possible involvement of swelling of the gel-like cytoskeleton structures in electrical regulation in the cell was postulated.

Chemical sensors based on multiresponsive block copolymer hydrogels

In order to realize a chemical sensor with a high signal reproducibility, a thermo-shrinking photo cross-linkable co- and terpolymer of N-isopropylacrylamide (NIPAAm), 2-(dimethyl maleimido)- N-ethylacrylamide (DMIAAm) as the chromophore and poly(2-vinylpyridine) (P2VP) (or N, N-dimethylacrylamide (DMAAm), respectively) has been used as chemo-mechanical transducer. Commercially available pressure sensor chips with a flexible thin silicon bending plate have been employed as mechano-electrical transducer for the transformation of bending plate deflection into an corresponding electrical output signal. A thin film of the photo cross-linkable hydrogel has been deposited onto the backside of the silicon bending plate and irradiated with UV light. The aqueous solution to be measured has been pumped through inlet tubes into the silicon chip cavity and induces swelling or shrinking processes of the hydrogel. Changes in the polymer composition have allowed to vary the gel volume phase transition temperature ( T c) from 25 °C for P2VP-block-P(NIPAAm-co-DMIAAm) to 43 °C for NIPAAm-DMIAAm-DMAAm terpolymer. The combination of the temperature-sensitive PNIPAAm with a pH-sensitive P2VP component allowed us to realize a pH sensor with a long-term stable signal in the pH range between 2 and 5. In addition, the sensitivity of the sensors with regard to concentration changes of different inorganic solutes has been studied. The influence of an initial gel-conditioning procedure on the signal value as well as the sensitivity of the proposed chemical sensors was investigated. Measurement conditions necessary for high signal reproducibility and a long-term stability were determined.

Graphical analysis for gel morphology. III. Gel size and temperature effects on the volume phase transition of gels

Just after poly(N-isopropylacrylamide) (PNIPA) gels are immersed into hot water over the critical temperature, they undergo a two- or three-step shrinking accompanied with the whitening. The size of gels and the destination temperature were changed and their effects on the shrinking process of PNIPA gels were studied. The following important knowledge in the volume phase transition process of PNIPA gels was obtained. The first is the fact that the fraction of first shrinking is determined by the shrinking rate in the first shrinking stage. The second is the fact that PNIPA gels are divided into small parts by the pinning of density fluctuation in the final stage of the first shrinking and that the shrinking process of each small part undergoes in the secondary shrinking stage. The volume phase transition process of PNIPA gels is briefly discussed from the viewpoint of the scaling theory.

Roles of Hydrophobic Interaction in a Volume Phase Transition of Alkylacrylamide Gel Induced by the Hydrogen-Bond-Driving Alkylphenol Binding

It was found that a degree of the binding of alkylphenols to N-alkylacrylamide gel increased transitionally to induce the volume phase transition of gel. Binding isotherms of nonylphenol (n-Ph), propylphenol (p-Ph), ethylphenol (e-Ph), methylphenol (m-Ph), and phenol (Ph) to N-isopropylacrylamide (NIPA), N,N-diethylacrylamide (DEA), N,N-dimethylacrylamide (DMA), and acrylamide (AM) gels were examined. Two types of binding, the site binding at beta < 1 and the multimolecular binding at beta > 1, were observed, where beta was a degree of binding to a monomeric unit of the chain. The former binding was analyzed with the Hill equation applicable to the cooperative binding and the latter binding with the Brunauer-Emmett-Teller (BET) equation applicable to the multilayer adsorption. The binding constant, K, and the Hill coefficient, N, decreased and increased, respectively, in the order of DEA, NIPA, and DMA gels in the case where the binding alkylphenol was the same. The K value increased in the order of Ph, m-Ph, e-Ph, p-Ph, and n-Ph that bound to the same type of gel. The N value was found to change little with the type of binding alkylphenol. The complexes of N-alkylamide with alkylphenol were condensed to form the ordered nanostructures that were observed as broad scattering peaks in small-angle X-ray scattering experiments. The fluorescence excimer emission was observed for the phenol-binding DMA gel, which corresponded to the condensed state of phenol.

Thermal-Swing Extraction of Cd(II) by Thermosensitive Gel Crosslinked with Nitrogen-donor Ligands

A thermal swing extraction system using a thermosensitive gel, poly-N-isopropylacrylamide (NIPA) crosslinked with 2,6-di(3-vinylbenzyl-1,2,4-triazol-5-yl)pyridine (BTP) was proposed. Extraction of Cd(II) in an aqueous nitrate solution and elution with acidic solutions (pH = 1 to 5) were tested by two thermosensitive gels prepared by the ultrasonic polymerization and the ordinary radical polymerization, called US-gel and FR-gel, respectively. cd(II) was extracted in the gel shrunken at higher than LCST (lower critical solution temperature, 31°C) and released from the gel swollen at less than LCST. Such temperature-change of extractability for the US-gel was larger than that for the FR-gel. The US-gel, in which aggregates of BTP molecules are not formed, is suitable to the thermal-swing extraction system. The elution of Cd(II) into a weakly acidic solution (pH > 2) was promoted by the volume phase transition of gel from the shrinking state to the swelling one. Even in a very weakly acidic solution of pH 5, more than 50% of Cd(II) extracted was eluted by the volume phase transition of gel. These results suggest that the elution operation with the volume phase transition of gel is very effective for reducing a quantity of eluting agent.

Nematic ordering-induced volume phase transitions of liquid crystalline gels

We present a mean field theory to describe swelling behaviors of a nematic gel immersed in a low molecular weight liquid crystal solvent under an external field such as magnetic and electric fields. We calculate the equilibrium concentration of the gel, orientational order parameters, and elongation of the gel. As a result of a nematic ordering between the gel and solvent inside the gel, the shape of the gel is discontinuously (or continuously) elongated with increasing the strength of the external field. We examine the condition for a nematic ordering-induced first- and second-order volume phase transition of the gel.