Search

Poly(N-isopropylacrylamide) (PNIPAM) nanogels were prepared by emulsion polymerization using sodium dodecyl sulfate (SDS) and employed as a capping agent in platinum nanoparticle (Pt NP) synthesis by liquid-phase reduction with hydrogen gas. When the PNIPAM nanogels were used without removing SDS, that is, a slight amount of SDS was included in the reaction solution, Pt nanocubes (NCs) were predominantly produced (>80%). The proportion of the resultant Pt NCs was much higher than that obtained using the PNIPAM linear polymer (∼60%). To clarify the effects of the three-dimensional polymer network and SDS, we synthesized Pt NPs using the PNIPAM nanogel without SDS (SDS-free PNIPAM nanogel) and found that Pt NCs are rarely formed, and most NPs obtained have an irregular shape. When only SDS was used as a capping agent, NCs were hardly obtained, but other polyhedral NPs were formed. Furthermore, the use of SDS together with the PNIPAM polymer led to the decrease in the proportion of the Pt NCs compared with that obtained using only the linear polymer. These results indicate that the enhancement of the Pt NC proportion using the PNIPAM nanogel with SDS is attributable to not only the three-dimensional polymer network of the PNIPAM nanogel but also the assist of SDS as a capping agent.

Poly(

A designed amphiphilic conetwork (APCN) having thermoresponsive polymer chains is attractive for the development of novel stimuli-responsive materials with a controlled responsive behavior. We have recently proposed a novel APCN having crosslinked domain (CD) structures prepared by the post-polymerization crosslinking of controlled triblock precursor polymers with reactive sites in the outer blocks. In the current study, we evaluated the effects of the structures of the triblock precursors including the sequence, molecular weight, and composition on the gelation reaction and the swelling properties of the obtained gels in detail. The gelation reaction and the volume at the swelling state at a low temperature were strongly affected by the molecular weight of the middle block of a precursor, whereas the temperature and the sharpness of the response were controlled by the composition of a precursor. Interestingly, the gel consisting of thermoresponsive CDs and hydrophilic bridging chains had improved elastic modulus and elongation ability upon heating in air without external water, probably because water flowed between the thermoresponsive CDs and the domains of the hydrophilic bridging chains in response to temperature change.

Research to improve the dimensional properties of silver nanowires (Ag NWs) for transparent conductive film (TCF) applications are being carried out intensively. However, the protocol for the designed synthesis of high-quality Ag NWs is yet to be developed due to the inadequacy of knowledge on the role of parameters. Here, we attempt to elucidate the role played by the parameters and propose a monoalcohol-copolymer based system for the designed synthesis of Ag NWs superior in quality to the one synthesized using conventional ethylene glycol (EG)-polyvinylpyrrolidone (PVP) system. The key findings of the study are as follows: (1) the solubility of Ag source and the partially formed AgCl particles in monoalcohols was found to play an important role not only in the reduction to metallic Ag but also on the uniaxial growth, (2) the adsorption of capping agents on Ag NWs was carried through O and N atoms in the base molecule and their binding energies indicated that the strength is the key parameter to obtain Ag NWs with high aspect ratio, (3) the properties of nanowire could be enhanced through copolymerization of VP and base molecules that have O- and N-based ligands, and (4) the influence of copolymerization on the physical and chemical properties of the surface active agent has been theoretically and experimentally verified. Consequently, we succeeded in the development of a new technique to synthesize high yield of Ag NWs with improved aspect ratio than EG-PVP system by using benzyl alcohol as reducing solvent and N-vinylpyrrolidone/N,N-diethylaminoethyl metacrylate copolymer as a capping agent. The optical transmittance and electrical resistivity of TCFs prepared using the Ag NWs with an average diameter of 43 nm, and an average length of 13 μm were 98.6% and R: 49.1 Ω/□, respectively.

For the fine control of the thermoresponsive properties of polymer hydrogels, we focused on the monomer and crosslinker sequence in a network. In this paper, we designed novel thermoresponsive/hydrophilic polymer co-networks with functional crosslinked domain (CD) structures such as a thermoresponsive network with hydrophilic CDs (DCDN gel) and a hydrophilic network with thermoresponsive CDs (NCDD gel). The gel synthesis was based on post-polymerization crosslinking of triblock prepolymers with reactive sites in the outer blocks. The obtained gels showed larger, sharper, and more rapid volume transitions in response to temperature change as compared to the corresponding gel with a random monomer/crosslinker sequence as well as a randomly crosslinked polymer co-network with the same composition. In addition, significant differences were observed in the transition temperature and shrinking kinetics between DCDN and NCDD gels with the same composition and a similar network structure, and the swelling behavior was also controlled by the composition of the triblock prepolymers. These results indicated that the domain structure in the network could effectively function and the monomer/crosslinker sequence exerts a strong effect on the swelling behavior of the gels.

In this paper, the effect of gelation solvent and temperature on poly(N-isopropylacrylamide) (PNIPAAm) gels synthesized by post-polymerization crosslinking (PPC) was discussed in detail. PPC was two-step synthesis performed by (1) radical copolymerization of NIPAAm and activated ester monomer (NHSA) and (2) crosslinking reaction of the obtained prepolymer and diamine crosslinker. In PPC, various solvents could afford gels, while the gelation time was strongly dependent on solvent polarity. The obtained gels showed similar swelling curves in water, indicating that the network structure of PPC gels was dominantly determined by the prepolymer structure. It was quite contrasting to conventional divinyl crosslinking (DVC), by which gelation could be observed only in water and not in organic solvents under the same concentration condition as PPC gelation system. We also examined the effect of gelation temperature in PPC synthesis in water. The PPC-PNIPAAm gel prepared in water at high temperature was consistently white turbid as same as DVC-PNIPAAm gel, suggesting that this turbidity was derived from polymer aggregation and entanglement fixed by chemical crosslinking.

We demonstrated the effect of poly(N,N-diethylacrylamide) (PDEAAm) microgels on the regulation of shape and size of gold nanoparticles (Au NPs), which were prepared by reduction of Au ion adsorbed on PDEAAm microgels. The reaction solvent used for the reduction changed the swelling state of the microgels to affect the shape of the Au NPs. In addition, the size of Au NPs could be regulated by adsorbed amount of Au ion in the microgels and the crosslinking density of the microgels.

Telechelic poly(N-isopropylacrylamide) (PNIPAAm) was designed to prepare homogeneous and self-healable gels by end-crosslinking. The PNIPAAm with carboxyl groups at both polymer ends and trithiocarbonate (TTC) in the main chain was obtained by reversible addition-fragmentation chain transfer (RAFT) polymerization of NIPAAm with a symmetric TTC compound (CTA-1) as a RAFT agent. The obtained polymers of a series of molecular weights were well-defined within a narrow molecular weight distribution (Mw/Mn~1.1). Then, the end groups of the resultant PNIPAAms were quantitatively transformed into activated esters as reactive sites for gelation. These designed PNIPAAms were employed for an end-crosslinking reaction with trifunctional amine crosslinker, and it was found that gelation condition was closely related to the molecular weight and the concentration of the telechelic prepolymers. Furthermore, the obtained gel showed the characteristic ultraviolet-induced self-healing owing to the chain exchange reaction of TTC groups. Self-healable gel was designed by end-crosslinking of controlled telechelic polymer carrying a trithiocarbonate group in the middle of the chain. The prepolymers, which were precisely synthesized by RAFT polymerization, were crosslinked with branched crosslinker using activated ester chemistry to produce the network with same molecular weight network chain between crosslinking points. The obtained gel showed UV-induced self-healing owing to the function of the trithiocarbonate groups.

Summary The novel thermoresponsive polymer gels were found to be designed by copolymerization of hydrophilic and hydrophobic monomers. Thermoresponsive properties of these novel copolymer gels could be easily regulated by the monomer structures and the feed ratio. First, we copolymerized hydrophilic N,N-dimethylacrylamide (DMAAm) and hydrophobic N-n-butylacrylamide (NBAAm) in various feed ratio, and found that certain compositions (DMAAm : NBAAm = 70 : 30 ∼ 40 : 60) gave the thermoresponsive gels with sharp volume phase transition. It was because of appropriate hydrophilic/hydrophobic balance in the network as same as poly(N-isopropylacrylamide) gel, in which the balance of hydrophilic amide group and hydrophobic isopropyl group produces thermoresponsive property. Then, we examined the effect of comonomer structures on thermoresponsive behavior of the copolymer gels by utilizing butyl acrylamide isomers (s-butyl, i-butyl, and t-butyl) as hydrophobic component. As a result, all the gels copolymerized by butylacrylamide having isomeric butyl groups with DMAAm were found to show the thermoresponsive properties such as sharp change of the degree of swelling and collapsed temperature, which would be related to the hydration structure of water.

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.

We report a new insight into the chemical gelation process of poly(N-isopropylacrylamide) (PNiPAAm) hydrogel elucidated on the basis of the in situ observations with time-resolved Fourier-transform...

We have prepared silica particles grafted with poly(N-isopropylacrylamide) (PNIPA) copolymers as adsorbent for the temperature-swing adsorption of bovine serum albumin (BSA) in water, where vinylbe...

The adsorption and desorption behaviors of bisphenol-A in water have been studied using the temperature-responsive polyampholyte gels composed of sodium styrene sulfate (SSS), vinylbenzyl trimethylammonium chloride (VBTA), and N,N′-dimethylacrylamide (DMAA). The resultant behaviors of the SSS-VBTA-DMAA gels of three different monomer compositions are compared with those of the SSS-VBTA gel that can adsorb bisphenol-A at room temperature and desorb the pre-adsorbed bisphenol-A at higher temperature as in our previous report (Morisada et al., Adsorption 2008, 14, 621), to elucidate the effect of DMAA on the adsorption property of the polyampholyte gel. The adsorption isotherms, which were described by Henry's law, indicated that the adsorption amounts of bisphenol-A onto the SSS-VBTA-DMAA gels at 298 K are much larger than those at 353 K. At 298 K, the Henry's adsorption constant for the SSS-VBTA-DMAA gel with the lowest mole fraction of DMAA was larger than that for the SSS-VBTA gel, whereas a further increase in the DMAA mole fraction resulted in a decrease in the adsorption constant. Moreover, the temperature-swing adsorption experiments demonstrated that the SSS-VBTA-DMAA gel can repeatedly adsorb bisphenol-A at 298 K and desorb some of the pre-adsorbed bisphenol-A at 353 K, where the desorption amount of bisphenol-A is larger than that from the SSS-VBTA gel. The results obtained here show that the addition of DMAA to the SSS-VBTA gel slightly improves the performance of the gel as a temperature-responsive adsorbent. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Internal structures of the opaque poly(N-isopropylacrylamide) gel obtained at the preparation temperature (TP) higher than 24.5 °C was found to have hierarchical structures by means of a combined small-angle-scattering (CSAS) method of small-angle light scattering (SALS), ultra-small-angle neutron scattering (USANS), and small-angle neutron scattering (SANS) with an aid of laser scanning confocal microscopy (LSCM). The combined CSAS and LSCM studies revealed that a global structure of the gels had a sponge-like two-phase structure. The polymer-rich phase of the sponge was filled with microgels having highly cross-linked network (“tight network”) chains that are interconnected by loosely cross-linked network (“loose network”) chains swollen with water, while the other phase of the sponge was mostly composed of water. These two phases were cocontinuous in three-dimensional space, hence, forming macroscopic gels. The SALS and USANS studies clarified that the sponge had a characteristic spacing of ≈12 μm and ...

The adsorption property of the polyampholyte gel composed of sodium styrene sulfate (SSS) and vinylbenzyl trimethylammonium chloride (VBTA) has been investigated with several hydrophobic aromatic compounds as adsorbate. Using the N-isopropylacrylamide (NIPA) gel, the corresponding experiments were also performed for comparison. At room temperature, the NIPA gel hardly adsorbed the aromatic compounds, while it adsorbed them at higher temperatures. As for the SSS-VBTA gel, the adsorption amounts of the polyaromatic compounds decreased with increasing temperature, while the adsorption amounts of the monoaromatic compounds were almost independent of temperature and smaller than those of the polyaromatic compounds. These results indicate that the aromatic rings in the SSS-VBTA gel may play an important role in the adsorption of the aromatic compounds. Also, it has been demonstrated that the SSS-VBTA gel can repeatedly adsorb bisphenol-A at room temperature and desorb it at higher temperature by the temperature-swing operation: this behavior is diametrically opposite to that of the NIPA gel. This shows that the SSS-VBTA gel is much more suitable for the adsorption removal of the hydrophobic aromatic compounds from very dilute aqueous solutions, because a vast amount of energy is required for heating a large amount of water when using the NIPA gel.

SSS-VBTA gel was synthesized with sodium styrene sulfate (SSS) and vinylbenzyl trimethylammonium chloride (VBTA). The SSS-VBTA gel was found to show the thermo-reversible changes in its volume and properties, that is, shrunken and hydrophobic state at around room temperature while swollen and hydrophilic state at above room temperature. It was revealed that the SSS-VBTA gel concentrated trace amounts of Bisphenol-A present in very dilute aqueous solutions at around room temperature and released the concentrated Bisphenol-A above room temperature.

Polymer gels consist of three-dimensional cross-linked polymer network and fluid. Internal structures relating to inhomogeneities of cross-linked polymer network are important factors characterizing the polymer gels. Internal structures of poly(N-isopropylacrylamide) gel obtained at various temperatures were observed by means of confocal laser scanning microscope and found to be bicontinuous structures consisting of dense and sparse cross-linked polymer networks. Generation process of the bicontinuous structures was also discussed on the basis of the results observed on temperature quenched gels.

Laser scanning confocal micrographs for blends of poly(styrene-ran-butadiene) (SBR) and polybutadiene (PB) demonstrated the existence of fine structures inside one of the well-characterized macroscopically phase-separated bicontinuous domains, in the depth range of the blends belonging to the bulk. The average size of the fine structures was on the submicrometer length scale, while the characteristic spacing of the macroscopically phase-separated domains was on the order of 10 μm. Fluorescence laser scanning confocal micrographs revealed that the fine structures exist only in the SBR-rich phase. Although the driving force for the formation of the fine structures needs to be investigated further, a small amount of PB molecules in the SBR-rich domains seems to play an important role. Illumination and highlights were shed on the advantage of using laser scanning confocal microscopy for investigation of fine structures in the polymer blends.

Influences of casting solvent on the microphase-separated structures of poly(2-vinylpyridine)-block-polyisoprene films were studied by transmission electron microscopy and small-angle X-ray scattering. The variations of the structures obtained were consequences of vitrification of microdomain structures developed during the solvent casting processes. The various microphase-separated structures were properly understood by considering the Hansen's solubility parameters of the solvent and the copolymer, which quantify the polymer–solvent interactions in the system.

Internal structures of translucent poly(N-isopropylacrylamide) gel were for the first time observed by laser scanning confocal microscopy (LSCM). The observed image reflected the mesoscopic internal structures in the gel due to the “fixed concentration fluctuations” of network polymers as described in the text. Three-dimensional internal structures in the gel constructed from a series of the cross section images taken by the LSCM revealed that the gel had a bicontinuous domain structure consisting of regions dense and sparse in polymer networks.

Fine palladium (Pd) particles, which have a diameter of 4−5 nm and are coordinated with the poly(2-vinylpyridine) homopolymer (P2VP) or poly(2-vinylpyridine)-block-polyisoprene diblock copolymer (P2VP-b-PI), were prepared by the reduction of palladium acetylacetonate (PdII(acac)2) with 1-propanol in the dilute benzene solution of P2VP or P2VP-b-PI. These Pd “nanoparticles” were selectively incorporated into a particular region of microdomains by casting a solution comprised of the Pd nanoparticles, P2VP-b-PI, and chloroform as a solvent into film specimens. The nanoparticles coordinated with P2VP were selectively incorporated into the P2VP domains, while those coordinated with P2VP-b-PI were around the interface between the P2VP and PI domains.

高分子ゲルの内部構造は,ゲルの性質や機能を考える上で非常に重要である.これまで,高分子ゲルの不均質性について研究され,その重要性が指摘されてきているが,実空間解析手法を用いて,ゲルの内部構造を明らかにした例はない.今回,共焦点レーザースキャン顕微鏡を用いて初めて,高分子ゲルの三次元的な内部構造を捉えることができた.今後,高分子ゲル研究への応用が期待される.

Acetal additions to β-substituted vinyl ethers having a variety of substituents (alkenyl ethers) were stereochemically investigated as model reactions for their cationic polymerization. The reactions catalyzed by BF3O(C2H5)2 in CH2Cl2 at O°C gave 1:1 adducts, the steric structure of which was determined by means of 13C-NMR spectroscopy. trans-Alkenyl ethers always gave adducts with a single structure stereospecifically, indicating that the intermediate carbocation attacks a trans-alkenyl ether from a definite direction independent of the bulkiness of substituents. On the other hand, cis-alkenyl ethers formed adducts with two steric structures, and the direction of cation addition was found to depend on the bulkiness of the alkoxy group involved. The above trends were in agreement with the results for poly(alkenyl ether)s and allowed detailed discussion of the stereochemistry of the propagation processes in alkenyl ether polymerizations.

Ethyl propenyl and butenyl ethers were polymerized in toluene at −78°C by a variety of catalysts, and effect of a counter-anion on steric structure of the polymer was investigated. The trans monomer produced stereoregular polymer with threo-meso structure and its content was independent of kind of the catalysts. In the polymerization of the cis monomer, the contents of the threo-meso and racemic structures of the poly(cis-propenyl ether) depended on the kind of the catalysts whereas those of the poly (cis-butenyl ether) having bulky β-ethyl groups did not. The difference of the counter-anion effect was discussed on the basis of the difference of the β-substituent in bulkiness.

The stereochemistry in the cationic polymerization of anethole, a β-substituted styrene, was investigated to clarify the propagation mechanism. The addition of acetaldehyde dimethyl acetal to anethole (acetal addition) was also studied as a model reaction. 13C NMR spectra showed that trans-anethole gave stereoregular polymers and that cis-isomer did not, indicating that the steric structure of the polymers depended on the geometrical structure of the monomer rather than on the polymerization conditions employed. On the other hand, the stereospecificity of the acetal addition was not very high even in the best case with trans-anethole (76% specificity). These results suggest that the structure of the attacking carbocation plays an important role in controlling the steric structure of products.

Equilibrium swelling curves of N‐isopropylacrylamide (NIPA) gel and a series of its ionized counterparts were measured as a function of temperature. Nonionic NIPA gels underwent a sharp yet continuous volume change, whereas incorporation of a small amount of ionizable groups into the gel network drives the transition toward a discontinuous one. The critical ionic concentration and polymer network density at which the get undergoes a critical phase transition were determined. The results were analyzed using a mean field theory. Discrepancy between experimental data and theory is nontrivial, and may require the formulation of a more elaborate theory.

Reentrant volume–phase transitions are observed in N‐isopropyoacrylamide gels in the methanol–water mixtures. When the solvent composition is varied systematically, the gel undergoes two transitions: a discontinuous collapsing followed by a discontinuous swelling. The reentrant transition defines a closed‐loop instability phase boundary having both upper and lower critical points. The closed‐loop phase boundary depends on temperature and diminishes to a point at approximately 0 °C. A simple mean field theory is presented to describe the phenomenon, which reveals an alteration of free energy of alcohol–water interaction by presence of polymer network. In the case of ethanol–water mixtures, there appear two closed‐loop phase boundaries, whose physico‐chemical basis are not yet clear.

In order to elucidate the possibility of stereoelective cationic polymerization (asymmetric selective polymerization) of olefinic monomers, racemic cis- and trans-1-methylpropyl propenyl ether and racemic 1-methylpropyl vinyl ether were polymerized by asymmetric alkoxyaluminum dichlorides. In the polymerization of racemic cis-1-methylpropyl propenyl ether with (−)-menthoxyaluminum dichloride in toluene at −78°C, the polymer obtained showed a positive optical activity, and the residual monomers were converted by BF3OEt2 into a polymer having a negative optical activity. Thus, the stereoelective polymerization of racemic cis-1-methylpropyl propenyl ether was beyond any doubt attained in homogeneous cationic polymerization. In the polymerization of the trans isomer by the same catalyst, an optically active polymer was hardly formed. In the polymerization of racemic 1-methylpropyl vinyl ether which has no β-methyl group, stereoelectivity was not observed at all. The cis-1-methylpropyl propenyl ether did not produce an optical active polymer in the polymerization catalyzed by (S)-1-methylpropoxyaluminum dichloride or (S)-2-methylbutoxyaluminum dichloride under the same polymerization conditions.

In order to elucidate the nature of the propagating species in cationic polymerization, isobutyl vinyl ether was polymerized by iodine in solvents of different polarities, both in the presence and absence of a common-ion salt (tetra-n-butylammonium tri-iodide). The molecular weight distribution (MWD) of polymers produced in salt-free methylene chloride at 0°C was broad. With increasing salt concentration, the amount of high-molecular-weight polymer decreased, and the MWD was very similar to that for the polymers produced in carbon tetrachloride. The polymerization rate was also reduced by the salt and approached a constant value. These results indicate the existence of two kinds of propagating species in different dissociation states in methylene chloride: dissociated and non-dissociated species. In spite of the existence of the two propagating species, however, the polymers did not show a bimodal MWD in methylene chloride. This, coupled with the uniformity in tacticity of the polymers, suggests a rapid exchange between the two species. For comparison, polymerizations with acetyl perchlorate were carried out in a variety of solvents.

Cationic Polymerization of (S)-2-Methylbutyl Propenyl Ether and Optical Rotatory Properties of Its Polymer in Solutino

The formation of polymers with erythro-meso structures, which could not be obtained from propenyl ethers with BF3O(C2H5)2, was studied by 13C-NMR spectroscopy on poly(s-substituted vinyl ether)s obtained under a variety of conditions of polymerization. It was established that poly(cis-ethyl propenyl ether) obtained with Al2(SO4)3–H2SO4 complex in toluene at 0°C was a highly stereoregular polymer with an erythro-meso structure. Cis-2-chlorovinyl ethyl ether and cis-methyl and ethyl butenyl ethers also yielded polymers with erythro-meso structures under the same conditions. In addition, with BF3O(C2H5)2 at −78°C these three cis isomers produced amorphous polymers with threo-meso, racemic, and, in a few cases, erythro-meso structures, whereas cis-ethyl propenyl ether produced polymers with only threo-meso and racemic structures by the same catalyst. On the other hand, all trans isomers produced stereoregular polymers with threo-meso structures with BF3O(C2H5)2 at −78°C, regardless of their s-substituents; no erythro-meso structures were found in the polymers obtained.

Gel forme d'acrylamide N,N'-methylene bisacrylamide et chlorure de (methacrylamidopropyl) trimethylammonium

Polymer gels, consisting of a cross-linked polymer network immersed in liquid, undergo a volume phase transition: when external conditions such as temperature or solvent composition change, a gel reversibly swells or shrinks, but does so discontinuously1–5. The volume change at the transition can be as large as a factor of one thousand2, and the phenomenon occurs in all gels6,7. The equilibrium aspects of the phase transition have been extensively studied, but its kinetics have not yet been fully explored. In particular, the appearance of patterns on the originally smooth surface of a gel during the transition makes the kinetic process difficult to understand. Here we elucidate the physical basis underlying the formation and evolution of the pattern.

Transition de phase de spheres de gels ionises de N-isopropylacrylamide avec des diametres compris entre 0,2 et 1 μm

We report the observation of sharp volume phase transition in the non‐ionic N‐isopropylacrylamide gel. The finding demonstrates that the phase transition of gels may be general and not confined to ionic gels as was previously considered. It will allow us to separate the electrostatic effects and network‐structural effects on the phase transition.

The volume phase transition of submillimeter spherical isopropylacrylamide gels shows critical kinetic behavior near the critical point; the transition takes an infinite time, and the thermal expansion becomes infinitely large. At the critical point the gels also undergo internal critical density fluctuations as revealed by photon-correlation spectroscopy. The observation of critical kinetics along with critical dynamics clarifies the physical principle of the kinetics of gel volume change.