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New Era in the Volume Phase Transition of Gels

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A special issue of Gels (ISSN 2310-2861).

Deadline for manuscript submissions: closed (28 February 2021) | Viewed by 42613

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Prof. Dr. Masayuki Tokita

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Guest Editor

Department of Physics, Graduate School of Science, Kyushu University, Fukuoka 819-0395, Japan
Interests: gel; phase transition; critical phenomena; sol-gel transition; phase separation; morphogenesis
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Prof. Dr. Masahiko Annaka

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Department of Chemistry, Faculty of Science, Kyushu University, Fukuoka 819-0395, Japan
Interests: gels; polyethylene glycols; neutron scattering
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Prof. Dr. Gerald S. Manning

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Department of Chemistry and Chemical Biology, Rutgers University, Piscataway, NJ 08854-8087, USA
Interests: gels; polyelectrolytes; bubbles and droplets; osmosis
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Special Issue Information

Dear Colleagues,

It is now well recognized that the discovery of the volume phase transition of gels in 1978, by Toyoichi Tanaka, marked the beginning of a new era in the science and technology of gels. Following this finding, many new phenomena related to the volume phase transition of gels have been found and discussed, one by one, in terms of the theory of the volume phase transition of gels. The theory of the volume phase transition of gels, which was proposed in the early stage of these studies, is constructed on the basis of analogy with the liquid–gas transition of a van der Waals gas, where the gas phase and the liquid phase correspond to the swelling state and the collapsed state of gel, respectively. This theory adequately demonstrates the universality of the volume phase transition of the gel and can be easily understood by a general audience. Consequently, such a depiction of the volume phase transition of the gel has been widely disseminated. From a scientific point of view, however, it is clearly an oversimplification. The crucial point is that the gel consists of a polymer network and solvent. In other words, the gel usually consists of at least two components, a solute and a solvent. On the other hand, the van der Waals gas is a single-component system consisting only of gas molecules. Moreover, the solute, that is, the polymer network, cannot dissolve into the solvent freely because the polymers are connected by crosslinks to form a network. These points may appear trivial but are the essence of gels. Although many aspects of the volume phase transition of gel have been clarified so far, many phenomena are still left unsolved. We believe it is time to revisit the volume phase transition of gels in marking a possible second beginning of a new era in the science of gels. We look forward to the submission of new results on the volume phase transition of gels. The submission of both theoretical and experimental studies is welcome.

Prof. Dr. Masayuki Tokita
Prof. Dr. Masahiko Annaka
Prof. Dr. Gerald S. Manning
Guest Editors

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Keywords

volume phase transition of gels
swelling behaviors of gels
phase equilibrium of gels
chemical and/or physical studies related to the volume phase transition of gels

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Published Papers (12 papers)

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Editorial

Jump to: Research, Review

2 pages, 161 KiB

Open AccessEditorial

Editorial on Special Issue “New Era in the Volume Phase Transition of Gels”

by Masayuki Tokita, Masahiko Annaka and Gerald S. Manning

Gels 2022, 8(7), 411; https://doi.org/10.3390/gels8070411 - 30 Jun 2022

Viewed by 1070

Abstract

The Special Issue of gels titled “Advancements in Gel Science” has been published from MDPI in 2019 [...] Full article

(This article belongs to the Special Issue New Era in the Volume Phase Transition of Gels)

Research

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12 pages, 2053 KiB

Open AccessArticle

Accounting for Cooperativity in the Thermotropic Volume Phase Transition of Smart Microgels

by Simon Friesen, Yvonne Hannappel, Sergej Kakorin and Thomas Hellweg

Gels 2021, 7(2), 42; https://doi.org/10.3390/gels7020042 - 8 Apr 2021

Cited by 11 | Viewed by 3125

Abstract

A full quantitative description of the swelling of smart microgels is still problematic in many cases. The original approach of Flory and Huggins for the monomer–solvent interaction parameter

χ

cannot be applied to some microgels. The reason for this obviously is that the [...] Read more.

A full quantitative description of the swelling of smart microgels is still problematic in many cases. The original approach of Flory and Huggins for the monomer–solvent interaction parameter

χ

cannot be applied to some microgels. The reason for this obviously is that the cross-linking enhances the cooperativity of the volume phase transitions, since all meshes of the network are mechanically coupled. This was ignored in previous approaches, arguing with distinct transition temperatures for different meshes to describe the continuous character of the transition of microgels. Here, we adjust the swelling curves of a series of smart microgels using the Flory–Rehner description, where the polymer–solvent interaction parameter

χ

is modeled by a Hill-like equation for a cooperative thermotropic transition. This leads to a very good description of all measured microgel swelling curves and yields the physically meaningful Hill parameter

ν

. A linear decrease of

ν

is found with increasing concentration of the cross-linker N,N

^{'}

-methylenebisacrylamide in the microgel particles p(NIPAM), p(NNPAM), and p(NIPMAM). The linearity suggests that the Hill parameter

ν

corresponds to the number of water molecules per network chain that cooperatively leave the chain at the volume phase transition. Driven by entropy,

ν

water molecules of the solvate become cooperatively “free” and leave the polymer network. Full article

(This article belongs to the Special Issue New Era in the Volume Phase Transition of Gels)

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18 pages, 2148 KiB

Open AccessArticle

Volume Phase Transitions of Heliconical Cholesteric Gels under an External Field along the Helix Axis

by Akihiko Matsuyama

Gels 2020, 6(4), 40; https://doi.org/10.3390/gels6040040 - 16 Nov 2020

Cited by 2 | Viewed by 2396

Abstract

We present a mean field theory to describe cholesteric elastomers and gels under an external field, such as an electric or a magnetic field, along the helix axis of a cholesteric phase. We study the deformations and volume phase transitions of cholesteric gels as a function of the external field and temperature. Our theory predicts the phase transitions between isotropic (I), nematic (N), and heliconical cholesteric (

C h_{H})

phases and the deformations of the elastomers at these phase transition temperatures. We also find volume phase transitions at the

I - C h_{H}

and the

N - C h_{H}

phase transitions. Full article

(This article belongs to the Special Issue New Era in the Volume Phase Transition of Gels)

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19 pages, 2544 KiB

Open AccessArticle

Microgel Particles with Distinct Morphologies and Common Chemical Compositions: A Unified Description of the Responsivity to Temperature and Osmotic Stress

by Andrea Ruscito, Ester Chiessi, Yosra Toumia, Letizia Oddo, Fabio Domenici and Gaio Paradossi

Gels 2020, 6(4), 34; https://doi.org/10.3390/gels6040034 - 16 Oct 2020

Cited by 8 | Viewed by 3053

Abstract

Poly(N-isopropylacrylamide) (PNIPAM) hydrogel microparticles with different core–shell morphologies have been designed, while maintaining an unvaried chemical composition: a morphology with (i) an un-crosslinked core with a crosslinked shell of PNIPAM chains and (ii) PNIPAM chains crosslinked to form the core with a shell consisting of tethered un-crosslinked PNIPAM chains to the core. Both morphologies with two different degrees of crosslinking have been assessed by confocal microscopy and tested with respect to their temperature responsivity and deformation by applying an osmotic stress. The thermal and mechanical behavior of these architectures have been framed within a Flory–Rehner modified model in order to describe the microgel volume shrinking occurring as response to a temperature increase or an osmotic perturbation. This study provides a background for assessing to what extent the mechanical features of the microgel particle surface affect the interactions occurring at the interface of a microgel particle with a cell, in addition to the already know ligand/receptor interaction. These results have direct implications in triggering a limited phagocytosis of microdevices designed as injectable drug delivery systems. Full article

(This article belongs to the Special Issue New Era in the Volume Phase Transition of Gels)

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13 pages, 1014 KiB

Open AccessArticle

Gel Volume Near the Critical Point of Binary Mixture Isobutyric Acid–Water

by Takao Yamamoto, Motoki Noguchi, Yasuyuki Maki and Toshiaki Dobashi

Gels 2020, 6(3), 30; https://doi.org/10.3390/gels6030030 - 12 Sep 2020

Cited by 1 | Viewed by 2521

Abstract

The volume of a cylindrical polyacrylamide gel was measured when immersed in a binary mixture of isobutyric acid–water at different temperatures and weight fractions of isobutyric acid. Near the upper critical solution temperature of the binary mixture, the curve for gel volume vs. isobutyric acid weight fraction has a shoulder or a peak near the critical weight fraction. On the other hand, in a region away from the critical temperature, the gel volume decreased monotonically with increasing isobutyric acid weight fraction. The cloud point temperature of the binary mixture inside the gel was lower than that outside the gel. Thermodynamic description for the gel in the critical mixture is derived on the basis of the Ising model. By the description, the experimental results are explained consistently. The theoretical analysis shows that the shoulder and the peak appearing in the swelling behavior of the gel are respectively induced by the criticalities of the binary mixture outside and inside the gel. It also shows that the cloud point temperature lowering of the binary mixture inside the gel is attributed to the effective enhancement of the temperature of the binary mixture inside the gel induced by the presence of the gel polymer. Full article

(This article belongs to the Special Issue New Era in the Volume Phase Transition of Gels)

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10 pages, 2234 KiB

Open AccessArticle

Crosslinker-Based Regulation of Swelling Behavior of Poly(N-isopropylacrylamide) Gels in a Post-Polymerization Crosslinking System

by Shohei Ida, Akimitsu Katsurada, Mitsuhiro Tsujio, Motoharu Nakamura and Yoshitsugu Hirokawa

Gels 2020, 6(1), 2; https://doi.org/10.3390/gels6010002 - 21 Dec 2019

Cited by 8 | Viewed by 4186

Abstract

A fundamental understanding of the effect of a crosslinker on gel properties is important for the design of novel soft materials because a crosslinking is a key component of polymer gels. We focused on post-polymerization crosslinking (PPC) system utilizing activated ester chemistry, which is a powerful tool due to structural diversity of diamine crosslinkers and less susceptibility to solvent effect compared to conventional divinyl crosslinking system, to systematically evaluate the crosslinker effect on the gel properties. A variety of alkyldiamine crosslinkers was employed for the synthesis of poly(N-isopropylacrylamide) (PNIPAAm) gels and it was clarified that the length of alkyl chains of diamine crosslinkers strongly affected the gelation reaction and the swelling behavior. The longer crosslinker induced faster gelation and decreased the swelling degree and the response temperature in water, while the crosslinking density did not significantly change. In addition, we were able to modify the polymer chains in parallel with crosslinking by using a monoamine modifier along with a diamine crosslinker. This simultaneous chain modification during crosslinking (SMC) was demonstrated to be useful for the regulation of the crosslinking density and the swelling behavior of PNIPAAm gels. Full article

(This article belongs to the Special Issue New Era in the Volume Phase Transition of Gels)

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Review

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17 pages, 3070 KiB

Open AccessReview

Re-Entrant Conformation Transition in Hydrogels

by Oguz Okay

Gels 2021, 7(3), 98; https://doi.org/10.3390/gels7030098 - 20 Jul 2021

Cited by 5 | Viewed by 3148

Abstract

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. Full article

(This article belongs to the Special Issue New Era in the Volume Phase Transition of Gels)

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24 pages, 2839 KiB

Open AccessEditor’s ChoiceReview

On Going to a New Era of Microgel Exhibiting Volume Phase Transition

by Haruma Kawaguchi

Gels 2020, 6(3), 26; https://doi.org/10.3390/gels6030026 - 17 Aug 2020

Cited by 18 | Viewed by 5356

Abstract

The discovery of phenomena of volume phase transition has had a great impact not only on bulk gels but also on the world of microgels. In particular, research on poly(N-isopropylacrylamide) (PNIPAM) microgels, whose transition temperature is close to body temperature, has made remarkable progress in almost 35 years. This review presents some breakthrough findings in microgels that exhibit volume phase transitions and outlines recent works on the synthesis, structural analysis, and research direction of microgels. Full article

(This article belongs to the Special Issue New Era in the Volume Phase Transition of Gels)

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15 pages, 3627 KiB

Open AccessReview

Application of a Clapeyron-Type Equation to the Volume Phase Transition of Polymer Gels

by Toshikazu Takigawa and Jun-ichi Horinaka

Gels 2020, 6(3), 25; https://doi.org/10.3390/gels6030025 - 14 Aug 2020

Cited by 4 | Viewed by 2842

Abstract

The applicability of the Clapeyron equation to the volume phase transition of cylindrical poly(N-isopropylacrylamide)-based gels under external force is reviewed. Firstly, the equilibrium conditions for the gels under tension are shown, and then we demonstrate that the Clapeyron equation can be applied to the volume phase transition of polymer gels to give the transition entropy or the transition enthalpy. The transition enthalpy at the volume phase transition obtained from the Clapeyron equation is compared with that from the calorimetry. A coefficient of performance, or work efficiency, for a gel actuator driven by the volume phase transition is also defined. How the work efficiency depends on applied force is shown based on a simple mechanical model. It is also shown that the force dependence of transition temperature is closely related to the efficiency curve. Experimental results are compared with the theoretical prediction. Full article

(This article belongs to the Special Issue New Era in the Volume Phase Transition of Gels)

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36 pages, 10921 KiB

Open AccessReview

Volume Transition and Phase Coexistence in Polyelectrolyte Gels Interacting with Amphiphiles and Proteins

by Per Hansson

Gels 2020, 6(3), 24; https://doi.org/10.3390/gels6030024 - 13 Aug 2020

Cited by 14 | Viewed by 3619

Abstract

Polyelectrolyte gels have the capacity to absorb large amounts of multivalent species of opposite charge from aqueous solutions of low ionic strength, and release them at elevated ionic strengths. The reversibility offers the possibility to switch between “storage” and “release” modes, useful in applications such as drug delivery. The review focuses on systems where so-called volume phase transitions (VPT) of the gel network take place upon the absorption and release of proteins and self-assembling amphiphiles. We discuss the background in terms of thermodynamic driving forces behind complex formation in oppositely charged mixtures, the role played by cross-links in covalent gels, and general aspects of phase coexistence in networks in relation to Gibbs’ phase rule. We also briefly discuss a gel model frequently used in papers covered by the review. After that, we review papers dealing with collapse and swelling transitions of gels in contact with solution reservoirs of macroions and surfactants. Here we describe recent progress in our understanding of the conditions required for VPT, competing mechanisms, and hysteresis effects. We then review papers addressing equilibrium aspects of core–shell phase coexistence in gels in equilibrium. Here we first discuss early observations of phase separated gels and results showing how the phases affect each other. Then follows a review of recent theoretical and experimental studies providing evidence of thermodynamically stable core–shell phase separated states, and detailed analyses of the conditions under which they exist. Finally, we describe the results from investigations of mechanisms and kinetics of the collapse/swelling transitions induced by the loading/release of proteins, surfactants, and amphiphilic drug molecules. Full article

(This article belongs to the Special Issue New Era in the Volume Phase Transition of Gels)

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14 pages, 1822 KiB

Open AccessEditor’s ChoiceReview

Volume Phase Transition in Gels: Its Discovery and Development

by Karel Dušek and Miroslava Dušková-Smrčková

Gels 2020, 6(3), 22; https://doi.org/10.3390/gels6030022 - 31 Jul 2020

Cited by 25 | Viewed by 5371

Abstract

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. Full article

(This article belongs to the Special Issue New Era in the Volume Phase Transition of Gels)

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11 pages, 368 KiB

Open AccessReview

Construction of a Universal Gel Model with Volume Phase Transition

by Gerald S. Manning

Gels 2020, 6(1), 7; https://doi.org/10.3390/gels6010007 - 27 Feb 2020

Cited by 3 | Viewed by 3388

Abstract

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

Π

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. Full article

(This article belongs to the Special Issue New Era in the Volume Phase Transition of Gels)

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