Free‐volume change during the volume phase transition of polyacrylamide gel studied by positron annihilation: Solvent‐composition dependence

Changes in the free‐volume parameters of polyacrylamide (PAAm) gels during the volume phase transition (VPT) were studied with the positron annihilation lifetime technique. The VPT was induced through the variation of the solvent composition in a mixture of acetone and water. The PAAm gels containing 0 and 4 mol % carboxyl groups in their polymer chains were adapted to compare the effect of the presence of ionic groups on the microscopic environment. The change of the free‐volume property is discussed on a nanoscopic scale, with attention paid to the interactions between the polymer chains and the solvent molecules. It is proven that the variations of the free‐volume parameters correlate significantly with the VPT phenomenon. The results of the free volume for both gels are well‐explained when an interaction parameter, ε_g, is assumed. The interpretation suggests that the state of the interactions among the components (the polymer chain, acetone, and water molecules) plays an important role in the change of the free volume of PAAm gels during the VPT. An increase of the dispersion of the free‐volume size near the VPT point was observed for the ionized PAAm gel. The broadened size distribution of the free volume of the ionized PAAm gel around the VPT point lay between those of pure water and the corresponding mixed solvent, suggesting that a local minimum of the average free‐volume size at the VPT point is caused by the increase of a specific interaction, hydrogen bonding. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 922–933, 2000     

Positron annihilation lifetime study of continuous volume phase transition of poly(N-isopropylacrylamide) gel induced in methanol–water mixed solvent

The macroscopic volume shrinkage and swelling of poly(N-isopropylacryl-amide) (PNIPA) gel induced by the compositional change in the methanol–water mixed solvent is correlated to the change in the nanoscopic free volume size and numerical concentration formed in the PNIPA gels. The free volume size and numerical concentration are estimated from the longest component appearing in the positron annihilation lifetime curves. The apparent free volume fraction calculated by the free volume size and numerical concentration, and dispersion of the free volume deduced by the size distribution are utilized to analyze the origin and location of the free volumes. The free volume parameters obtained by analysis of the positron annihilation data show various nanoscopic phases occuring within the PNIPA gels during the volume change, implying the variation of the strength of the interactions among the solvent molecules and the polymer chains of the PNIPA. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1141–1151, 1998     

Synthesis and properties of organic–inorganic hybrid liquid crystal gels

Organic–inorganic hybrid liquid crystal (LC) gels have been synthesised by the thiol-ene reaction of a multifunctional cyclic siloxane, 1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane (TVMCTS) and alkane dithiols, 1,6-hexanedithiol (HDT) or 1,9-decanedithiol (DDT), in LC matrices, 4-cyano-4?-pentylbiphenyl (5CB) or 4′-n-octyl-4-cyano-biphenyl (8CB). The LC gels were prepared in an isotropic phase at 70°C or mesophases at 25°C using radical initiators. The phase transition temperatures from a mesophase to an isotropic phase of the resulting gels were lower than those of the original LCs. The gels containing 8CB (8CB gels) prepared at 25°C showed two phase transitions: smectic-to-nematic and nematic-to-isotropic transitions. By contrast, the 8CB gels synthesised in the isotropic phase showed only one phase transition from smectic phase directly to isotropic phase. Reaction conversions in the LC gels prepared at 70°C were higher than that in the gels prepared at 25°C. Scanning microscopic light scattering analysis of the LC gels cleared homogeneous small size mesh with a small amount of large defect. Polarisation micrographs of the LC gels showed framed optical textures derived from the LC molecules at room temperature. The LC gels containing more than 90 wt% of LC showed electro-optic response.     

Use of environmentally-responsive smart glasses in controlled release: Diffusion of molecules under applied stimuli

The use of organosilica sol-gels as environmentally-responsive materials for stimuli-controlled release of encapsulated molecules is reported. These sol-gels exhibit bulk volume changes with respect to applied stimuli such as pH, temperature, salt, and solvents. These volume changes result in expulsion/intake of solvent molecules which can be utilized as a means for externally-regulated release and delivery of dopant molecular entities. For these materials, which undergo a volume transition in response to environmental changes, the magnitude and response time of the material is related to diffusion of solvent molecules in and out of the material. The release of encapsulated molecules and diffusion of solvent accompanies the volume transitions, which can be initiated by changes in temperature or ionic concentration. The kinetics and mechanism of volume changes of organosilica network consisting of hydrophilic and hydrophobic segments are investigated which provide critical insights into the nature of underlying factors that influence release of encapsulated entities. The diffusion coefficient of water molecules in these gels is determined to be considerably faster as compared to organic polymer gels and silica gels indicating rapid volume responses which can be utilized as efficient trigger mechanisms for release of molecules. The results validate preliminary feasibility of stimuli-controlled release of molecules with these gels initiated by the diffusional flow of solvent established by the volume changes.     

Application of Fluorescence Depolarization Method to Monitor Free Volume in Gels of Stereoregular Polystyrene

Summary: A fluorescence depolarization technique was applied to get the information on free volume among polymer chains in gel form. Four fluorescent molecules with different molecular sizes were doped throughout the gels of syndiotactic polystyrene (sPS) and isotactic polystyrene (iPS) physical gel system, and their fluorescence anisotropy values were examined in detail for a range of polymer concentrations. Consequently, the free volume among sPS chains in sPS/chloroform gels is as large as the size of molecules smaller than 1,5-dimethylnaphthalene and is consistent with that of the cavity size in the δ-empty crystalline form of sPS solids. The cause to produce δ-empty crystalline form of sPS solids and to form cocrystals between sPS and guest molecules is discussed by comparing the molar size of guest molecules with the free volume among sPS chains in gel form.     

Free‐volume change in volume phase transition of polyacrylamide gel as studied by positron annihilation: Salt dependence

Variation of free‐volume parameters—average radius size, number concentration, and size distribution—of a polyacrylamide (PAAm) gel containing 4 mol % carboxylate anions is studied during a volume phase transition (VPT) caused by a change of sodium chloride (NaCl) concentration. A positron annihilation lifetime technique is used for the determination of the free‐volume characteristics. The measurement is performed in an acetone–water 3 : 2 (v/v) [0.27 : 0.73 (mol/mol)] mixed solvent at 20°C, and the free‐volume parameters deduced from the analysis of a positron annihilation curve are utilized. An average free‐volume size of the swollen PAAm gel, ∼ 0.32 nm in radius, almost agrees with that of the mixed solvent for a corresponding salt concentration, while the size of the collapsed gel, which is ∼ 0.28 nm in radius, is smaller than that of the mixed solvent. The results for the collapsed gel indicate that the hydrogen bond plays a significant role in the nanoscopic environment. The radius of the free‐volume of the swollen PAAm gel seems to be influenced by the composition between acetone and water. An inhomogeneity of the nanoscopic structure inside the PAAm gels is discussed in terms of a dispersion of a size distribution of the free‐volume. It is concluded that a change of the nanoscopic environment of the PAAm gel during the VPT can be monitored through the free‐volume parameters obtained by the positron annihilation lifetime technique. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2634–2641, 1999     

Fluorescence studies on volume phase transition in poly(acrylamide) and poly(N-isopropylacrylamide) gels

The changes in microenvironments during the volume phase transition of poly(acrylamide) and poly(N-isopropylacrylamide) gels induced by pH change or the change in solvent composition were studied by using dansyl or pyrenyl fluorescent probes.     

Novel tetrapodal molecules and reticular polyamides based on tetraphenylmethane

A series of tetrapodal derivatives of tetraphenylmethane were synthesized and characterized. Crystals obtained from tetrakis(4-acetamidophenyl)methane (1c) and from tetrakis[4-(4-aminobenzamido)phenyl]methane (2b) were analyzed by X-ray diffraction. The analyses pointed to the crystal packing problems faced by molecules of this kind by showing that the crystals, with composition1c·2DMF·2H₂O and2b·2DMSO, respectively, contained cocrystallized solvent molecules. The solvent molecules were found in both cases to be held in place by H bonds; in the case of2b·2DMSO they occupied channels running along theb axis. Tetrakis(4-aminophenyl)methane (1b) was used in polycondensation reactions with terephthalic acid, under modified Yamazaki conditions, to produce rigid aromatic polyamide networks. The networks were obtained as gels encompassing the whole volume of the reaction mixture. The volume of the gels did not vary noticeably upon changing the solvent (1-methyl-2-pyrrolidone) with less polar solvents, but the gels collapsed upon drying. No crystallinity was observed.     

Polymer-solvent interactions in thermoreversible gels of isotactic poly(methyl methacrylate) as studied by measurement of 1h NMR relaxation of the solvent

Dynamic structure and polymer-solvent interactions in solutions and gels of isotactic (i) poly(methyl methacrylate) (PMMA) in butyl acetate (BAC) were characterized by measurements of nonselective and selective ¹H NMR spin-lattice relaxation times of the solvent. In thermoreversible gels, the existence of i-PMMA-BAC complex, where the life-time of the bound solvent is − 100 ms or shorter, was confirmed. Although the correlation time of the solvent bound in i-PMMA-BAC complex is 5-10 times longer than for the free solvent, there is still a relative motional freedom for complexed solvent molecules. The same behaviour observed recently for thermoreversible gels of syndiotactic PMMA indicates the same character of polymer-solvent complexes in gels of both stereoregular forms of PMMA.     

Effect of solvent type on formation of PVC‐solvent complex

The effect of solvent type on the structural features of PVC gels prepared from the solutions of dibutyl phthalate(DBP) and butyl benzoate(BB) were investigated. The influence of solvent type, i.e., diester solvent or monoester solvent, on the mechanical responses was characterized by dynamic viscoelastic measurements as a function of temperature, aging time, and PVC concentration. In the case of PVC/DBP aged gels, the transition phenomenon appeared close to 50°C, whereas no such transitions were observed for PVC/BB aged gels. This transition is considered to be associated with polymer‐solvent complex which can be accounted for electrostatic interaction between C=O bond of solvent and H‐C‐Cl part of PVC. While BB solvent molecules can not form polymer‐solvent complex due to no capacity to act as a bridge between PVC molecular chains. FT‐IR spectroscopy and DSC investigation on either PVC/DBP gels or PVC/BB gels was reported and discussed together with the mechanical behavior. Electron micrographs of the dried gel prepared from the critical point drying technique reveal that the gel morphology consists of well developed three dimensional fibrous network structure independent of solvent type.     

Specific swelling behaviors of alkali‐metal poly(styrene sulfonate) gels in aqueous solvent mixtures

Counterion‐ and solvent‐specific swelling behaviors were investigated for alkali‐metal poly(styrene sulfonate) (PSSM) gels having different degrees of sulfonation in aqueous organic solvent mixtures [water plus methanol, ethanol, 2‐propyl alcohol, t‐butyl alcohol, dimethyl sulfoxide (DMSO), acetone, acetonitrile, tetrahydrofuran, or dioxane]. With an increasing organic solvent concentration, most gel systems, except for DMSO, showed a volume phase transition. The transition abruptly occurred without significant deswelling in the lower solvent concentration region. Such swelling behavior contrasted with that of other common charged gel systems, including alkali‐metal polyacrylate (PAAM) gels, which showed gel collapse after gradual deswelling with an increasing organic solvent concentration. The dielectric constant at the critical transition point (D_cr) for most mixed solvent systems decreased in the order of PSSK ≥ PSSCs ≥ PSSNa > PSSLi; that is, larger counterion systems were favorable for the transition. The counterion specificity also contrasted with our previous results for PAAM gels: PAANa > PAAK > PAALi ～ PAACs. On the other hand, the solvent specificity for the PSSM gels was similar to that for the PAAM gels; the higher the dielectric constant was of the organic solvent, the higher the D_cr value was at which the transition occurred. These specificities were examined on the basis of the solvation properties of the counterions and polymer charged groups and the solvent properties such as the Gutmann–Mayer donor number and acceptor number. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1166–1175, 2007     

Organogel from L-leucine-containing surfactant in nonpolar solvents

 Alkyl (S)-2-ammonium-2-isobutylacetate p-toluenesulfonate formed organogel in nonpolar solvents. The gels exhibited thermally reversible sol–gel phase transitions. UV spectroscopic study suggested that dodecyl (S)-2-ammonium-2-isobutylacetate p-toluenesulfonate forms reversed micelle-like aggregate at low concentration in a nonpolar solvent. Circular dichroism spec-troscopy indicated that component molecules of the reversed micelle-like aggregate are cooperatively organized and result in chiral aggregate. The huge fibrous aggregate responsible for gelation was observed with transmission electron microscopy. The accumulation and rearrangement of reversed micelle-like aggregate resulted in the formation of huge fibrous aggregates. A gathering of numerous fibrous aggregates formed the three-dimensional network to immobilize the isotropic liquid. Received: 24 June 1997 Accepted: 3 October 1997     

Structural Phase Transitions of a Molecular Metal Oxide

The structural phase of a metal oxide changes with temperature and pressure. During phase transitions, component ions move in multidimensional metal–oxygen networks. Such macroscopic structural events are robust to changes in particle size, even at scales of around 10 nm, and size effects limiting these transitions are particularly important in, for example, high-density memory applications of ferroelectrics. In this study, we examined structural transitions of the molecular metal oxide [Na@(SO₃)₂(n-BuPO₃)₄Mo^V₄Mo^VI₁₄O₄₉]⁵⁻ (Molecule 1 ) at approximately 2 nm by using single-crystal X-ray diffraction analysis. The Na⁺ encapsulated in the discrete metal-oxide anion exhibited a reversible order–disorder transition with distortion of the Mo–O molecular framework induced by temperature. Similar order–disorder transitions were also triggered by chemical pressure induced by removing crystalline solvent molecules in the single-crystal state or by substituting the countercation to change the molecular packing.     

Deuterium substitution and fluorescence studies on polymer hydrogels and complexes

Deuterium isotope effects on swelling kinetics and volume phase transition in typical polymer hydrogels (poly(N-isopropylacrylamide) and polyacrylamide gels) are discussed. Deuterium substitutions affect on the swelling kinetics and volume phase transition of the polymer hydrogels. The slower swelling kinetics of hydrogels in D₂O than in H₂O arises mainly from the high viscosity of the medium. The deuterium isotope effect on the swelling-shrinking curve of hydrogels would come from the different polymer-solvent interaction. The microenvironments of hydrogels studied by solvatochromic fluorescence probe are compared with the bulk state. The zipper-type hydrogen-bonding inter-polymer complexes (poly(acrylic acid)-polyacrylamide and poly(acrylic acid)-poly(N-acryloylglycineamide)) are also investigated and show the huge isotope effect on the phase separation temperature.     

Investigation of the effect of type and concentration of ionizable comonomer on the collapse behavior of N-isopropylacrylamide copolymer gels in water

This work was done to investigate the effect of three different ionizable components (acrylic acid, AA; itaconic acid, IA; maleic acid, MA) on the volume phase transitions and swelling equilibria of thermoshrinking type N-isopropylacrylamide (NIPAAM) gels in water. NIPAAM copolymer gels were synthesized by free radical crosslinking copolymerization of NIPAAM with each of AA, IA, and MA, the difference being both between configurations and carboxyl group numbers, and pK values, in the presence of N,N′-methylene-bis-acrylamide (MBAAM). The influence of comonomer concentrations (1, 5, and 10 mol %), MBAAM content (0.0096, 0.0193, and 0.0288 g), and comonomer type (AA, IA, and MA) on the external views, the percentages of equilibrium mass, and volume swellings [S %(m), S % (v)], the number-average molecular weight between crosslinks (M̄_c), effective crosslinking densities (ν_e), the change of the collapse temperatures, and swelling ratios in the swelling–shrinking process of the gels were examined. It was observed that phase transition temperature and swelling degree in the case of MA having a cis configuration and higher pK value are larger than those of the samples containing IA and AA. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1847–1855, 1999     

Characteristics of migration patterns of DNA oligomers in gels and the relationship to the question of intrinsic DNA bending

We have developed a methodology that is capable of quantitatively describing the electrophoretic mobility patterns of oligomeric B-DNA through polyacrylamide gels (PAG) in the presence of varying concentration of the organic solvent 2-methyl-2,4-pentanediol (MPD), used routinely to induce DNA crystallization. The model includes the ion atmosphere and its polarization, electrostatic excluded volume, hydrodynamic interactions, and fluctuation effects that characterize the overall size of the migrating polyion. Using this model, and by critically examining the mobility patterns of linear random-sequence B-DNA molecules in PAG as a function of MPD, we address the question of the discrepancy between current models used to explain the molecular origins of A-tract-induced DNA bending. Direct analysis of the mobility of B-DNA oligomers on PAG, and comparison to the mobility of A-tract-containing oligomers, shows a significant apparent effect of MPD on the mobility of generic B-DNA sequences, which is larger than the effect on A-tract-containing oligomers. The effect is chain-length dependent, especially at lower MPD concentration. Thus, the apparent reduction in gel mobility, as a function of MPD, is not unique to A-tract regions or A-tract-containing molecules. However, our analysis suggests that MPD molecules are probably excluded from the surface of both B-DNA and A-tract molecules. This is supported by circular dichroism studies on A-tract and B-DNA molecules in solutions containing various MPD concentrations.     

Hydrogels from diacylphosphatidylcholine

The formation of hydrogels from diacylphosphatidylcholine (PC) and water/glycerol mixtures and the properties of the gels are reported. The gels are formed when L_α phases from the PC in the solvent mixtures are cooled from T >55 °C below the Krafft temperature of the PC (T _m ∼52 °C). The glycerol can also be replaced by other co-solvents like butylenglycol. Above T _m, the PC spontaneously forms L_α phases with multilamellar vesicles that show a strong stationary birefringence. On cooling below T_m, the L_α phases jellify to transparent gels. DSC measurements of the gels show that the PC molecules undergo a phase transition into the crystalline state. This transition does not seem to be accompanied by a change of the morphological structure of the liquid L_α phase. The hydrogels also have a stationary birefringence. The vesicles in the gels have been imaged by the CryoTEM method. The hydrogels are already formed with as little as 1% of PC in the mixed solvent. The rheological properties of the gels were determined from oscillating rheological measurements. Samples with 10% of PC have a storage modulus of >10,000 Pa.     

Compound formation and solvent (dis)ordering in syndiotactic polystyrene/benzylmethacrylate systems

In-situ small- and wide-angle X-ray scattering (SAXS and WAXS) experiments combined with Raman spectroscopy have been performed to study the phase behaviour of syndiotactic polystyrene (sPS) and benzylmethacrylate (BzMA). In the quenched gels, sPS adopts a helical conformation which is stabilised by the solvent molecules, in fact compound formation occurs. From the combined experimental data it was concluded that two different structural modifications exist within the solvent-included helical δ-phase, respectively the δ' phase in which the solvent molecules are intercalated and ordered between the phenyl rings of sPS and a δ” phase where this solvent ordering is lost.     

Formation of Hierarchical Pore Structure in Silica Gel

By inducing a phase separation parallel to the sol-gel transition of alkoxy-derived silicate systems, gels with well-defined macroporous structure can be prepared. Depending on the post-gelation treatment such as aging and solvent exchange, the final pore structure in the nanometer range of dried and heat-treated gels exhibits a considerable variation. With an aim of completely controlling the hierarchical pore structure in the discrete size ranges of nanometers and micrometers, systematic experimental studies have been performed. The macroporous nature of the wet gels allows an efficient solvent exchange process compared with conventional gels only with mesopores. In addition, the surface chemistry of the wet gel skeleton affects the mesopore formation process by the solvent exchange to a great extent. The median size of mesopores larger than 5 nm can be controlled by adjusting the basic solvent exchange conditions such as pH value, temperature and bath ratio for any kind of macroporous silica gel. On the other hand, the control of pore volume independent of the mesopore size is possible only in the system incorporated with the micelle-forming surfactant. Some examples of the effects of controlled mesopores on the analytical performance of monolithic-type chromatographic columns are also presented.     

Effect of solvent–matrix interactions on structures and mechanical properties of micelle‐crosslinked gels

Previous studies on hydrogels crosslinked by acrylated PEO₉₉–PPO₆₅–PEO₉₉ triblock copolymer (F127DA) micelles demonstrate outstanding strength and toughness, which is attributed to the efficient energy dissipation through the hydrophobic association in the micelles. The current study further focuses on how the solvent property affects the structures and the mechanical properties of F127DA micelle crosslinked polyacrylamide gels. Binary solvents comprised of dimethyl sulfoxide (DMSO) and water are used to adjust the polymer/solvent interactions, which consequently tune the conformations of the polymer chains in the network. The presence of DMSO significantly decreases the strength but increased the stretchability of the gels, whereas the overall tensile toughness remained unchanged. In situ small‐angle X‐ray scattering measurements reveal the deformation of micelles along with the stretching direction. A structure evolution mechanism upon solvent change is proposed, according to the experimental observations, to explain influence of solvent quality on the mechanical properties of the micelle‐crosslinked gels. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 473–483