Formation Of Thermoreversible Gels Research Articles

Poly(vinylidene fluoride) Aerogels with α, β, and γ Crystalline Forms: Correlating Physicochemical Properties with Polymorphic Structures.

Strategic customization of crystalline forms of poly(vinylidene fluoride) (PVDF) aerogels is of great importance for a variety of applications, from energy harvesters to thermal and acoustic insulation. Here, we report sustainable strategies to prepare crystalline pure α, β, and γ forms of PVDF aerogels from their respective gels using a solvent exchange strategy with green solvents, followed by a freeze-drying technique. The crucial aspect of this process was the meticulous choice of appropriate solvents to enable the formation of thermoreversible gels of PVDF by crystallization-induced gelation. Depending on the polymer-solvent interactions, the chain conformation of PVDF can be modulated to obtain gels and aerogels with specific crystalline structures. The crystalline pure α-form and piezoelectric β-form aerogels were readily obtained by using cyclohexanone and γ-butyrolactone as gelation solvents. On the other hand, the γ-form aerogel was obtained using a binary solvent system consisting of dimethylacetamide and water. These aerogels with distinct crystalline structures exhibit different morphologies, mechanical properties, hydrophobicities, acoustic properties, and electrical properties. Measurement of thermal conductivity for these aerogels showed exceptionally low thermal conductivity values of ∼0.040 ± 0.003 W m-1 K-1 irrespective of their crystal structures. Our results showcase the fabrication approaches that enable PVDF aerogels with varied physicochemical properties for multifunctional applications.

Formation of thermo-reversible gels from whey proteins after combined thermal and ethanol pretreatment

Τhis study investigates the ability of whey proteins, which were pre-treated with ethanol (0–70%) and subjected tothermal treatment (70 °C for 60 min), to form thermo-reversible gels after removing the alcohol through freeze-drying. The ethanol-free gels were monitored by means of rheology and confocal laser scanning microscopy. Moreover, the role of ethanol during heating of whey proteins was investigated by UV difference spectroscopy. The results showed a retention of the denatured character (30–35%) of whey proteins after removal of ethanol, regardless of whether heating was applied. Intermolecular disulfide connections increased in whey protein solutions pretreated with 30 and 50% w/w ethanol. The determination of free –SH groups demonstrated that their number was increased by heating in the presence of ethanol and after ethanol's removal. Dynamic rheological measurements showed that ethanol-free gels, previously heated with 50% w/w ethanol, formed thermo-reversible gels upon cooling to 5 °C and exhibited significantly higher storage modulus values (G’∼300 Pa) than the counterparts without pretreatment of the proteins with ethanol (G’ = 0.0002 Pa). Generally, the partial retention of the denaturing effect of ethanol on whey proteins after its removal and their ability to form thermo-reversible gels could lead to new applications in the food and pharmaceutical industries.

Decisive properties of solvent able to form gels with syndiotactic polystyrene

The thermoreversible gel formation of syndiotactic polystyrene (SPS) was studied in detail. Even if SPS solutions with high concentrations did not form gels, there were the cases where SPS formed gels at lower concentrations of SPS, which favor the formation of a polymer-solvent molecular compound. Sixty-two solvent compounds were examined in regard to whether they could form SPS gels. All the compounds whose molecular volume is between 69 and 153 Å3 and whose Fedors' solubility parameter value is between 8.8 and 12.3 (cal/cm3)1/2 were found to produce SPS gels as solvent. Since a solvent molecule that can form SPS gel can be a guest of SPS δ co-crystalline phase, it has become convenient to find functional molecules able to co-crystallize with SPS by taking into account these conditions of gelation solvent molecules.

Release properties of gel-type W/O/W encapsulation system prepared using enzymatically-modified starch

A gel-based encapsulation system was developed by incorporating W/O/W emulsions and 4-α-glucanotransferase (4αGTase) treated starch capable of thermoreversible gel formation, and its physical and release characteristics were investigated as functions of preparation conditions and temperature. Release properties of the W/O/W gels were affected by stability and encapsulation efficiency (EE) of W/O/W emulsions embedded within. Lower EE caused by longer sonication time increased fast release dye portion, which resulted in higher dye release rate, even though emulsion stability improved at longer sonication time. Lower dye release rate of W/O/W gels prepared with relatively higher W/O volume fractions slightly increased as temperature increased from room temperature to 90°C. However, samples prepared with relatively lower W/O volume fractions showed higher dye release rate and a larger increase at 90°C. The 4αGTase-treated starch gel improved W/O/W emulsion stability and thus retarded dye release even at 90°C.

Elucidation of the structure of poly(γ-benzyl-l-glutamate) nanofibers and gel networks in a helicogenic solvent

The synthesis, characterization, self-assembly, and gel formation of poly(γ-benzyl-l-glutamate) (PBLG) in a molecular weight range from ca. 7,000–100,000 g/mol and with narrow molecular weight distribution are described. The PBLG is synthesized by the nickel-mediated ring-opening polymerization and is characterized by size-exclusion chromatography coupled with multiple-angle laser light scattering, NMR, and Fourier transform infrared spectroscopy. The self-assembly and thermoreversible gel formation in the helicogenic solvent toluene is investigated by transmission electron microscopy, atomic force microscopy, small-angle X-ray scattering, and synchrotron powder X-ray diffraction. At concentrations significantly below the minimum gelation concentration, spherical aggregates are observed. At higher concentrations, gels are formed, which show a 3D network structure composed of nanofibers. The proposed self-assembly mechanism is based on a distorted hexagonal packing of PBLG helices parallel to the axis of the nanofiber. The gel network forms due to branching and rejoining of bundles of PBLG nanofibers. The network exhibits uniform domains with a length of 200 ± 42 nm composed of densely packed PBLG helices. Electronic supplementary materialThe online version of this article (doi:10.1007/s00396-012-2866-9) contains supplementary material, which is available to authorized users.

Self-organization of amide dendrons with focal dipeptide units

Dendron–dipeptide conjugates were synthesized to investigate their self-assembly characteristics not only in the aqueous phase but also in organic media. In the aqueous phase, the self-organized structure of 2G-PhePhe and 2G-GlyGly was influenced predominantly by the self-assembly characteristics of the amide dendron unit to exhibit vesicular structures. For 1G-PhePhe and 1G-GlyGly, however, the role of the focal dipeptide unit became important to show donut-like assemblies and rod-like structures, respectively. In HFIP/H2O (1 : 100 v/v), 2G-PhePhe and 1G-PhePhe exhibited donut-like structures, while 2G-GlyGly and 1G-GlyGly formed rod-like structures. The secondary structures of the dendron–peptide conjugates were remarkably influenced by the dendron unit. The circular dichroism (CD) spectra showed that 1G-PhePhe in H2O adopted the β-sheet structure. On increasing the generation of the dendron unit, on the other hand, 2G-PhePhe did not adopt the β-sheet structure but exhibited random structures. In addition, the curvature of the assembled structure of the dendron–peptide conjugates was affected by the dendron unit. In organic media such as toluene, all the dendron–dipeptide conjugates formed fibrous structures which could induce thermoreversible gel formation. Therefore, the modulation of the dendritic moiety and the focal peptide unit provided an opportunity to produce diverse self-organized structures in various conditions.

Bicomponent Hydrogels of Lumichrome and Melamine: Photoluminescence Property and Its Dependency on pH and Temperature

Lumichrome (L) and melamine (M) produce thermoreversible hydrogels in LM31 and LM11 compositions, but LM13 composition does not produce hydrogel (the numbers indicate the respective molar ratio of the components). The formation of thermoreversible gels is confirmed from morphology, DSC, and rheological experiments where LM13 system does not meet the required characteristics of thermoreversible gels. FTIR spectra suggest that H-bonding between L and M produces the supramolecular complex, and (1)H NMR spectra suggest that pi-stacking of the complex produce fibrillar network structure entrapping a large amount of water producing the hydrogels. The nonplanar structure of LM13 complex probably causes difficulty in pi-stacking, prohibiting the gel formation. The UV-vis spectra show a blue shift of the pi-pi* transition band (354 nm) indicating H-aggregate formation but the pi-pi* band coupled with n-pi* transition (386 nm) shows a constant red shift by 7 nm, indicating independency of pi-stacking on the n-pi* transition in the different LM systems. The PL intensities of LM11 and LM31 gels become more quenched than the intensity of pure L due to formation of nonfluorescent complex (static quenching) in the gels. In the LM13 sol the degree of quenching is less than that of the gels because of absence of energy transfer through the junction points of gels. The increased lifetime values of LM gels compared to that of pure L is also indicative of H-aggregate formation. The PL intensity increases linearly with increase of temperature due to thinning of the fibers decreasing the exciton energy transfer. The emission peak shows a red shift with rise in temperature, indicating H- to J-aggregate transformation, and at the melting temperature it shows a sharp decrease. With both increase and decrease of pH from the neutral pH 7, the gels exhibit higher PL intensity because of sol formation.

New Possibilities of the Acetosulfation of Cellulose

AbstractRegioselectively substituted cellulose sulfates in C2/3‐, C2/6‐, or C6‐position of the anhydroglucose unit are accessible by certain synthesis routes. Thereby, products with different properties and various application areas are resulted. Important characteristics of cellulose sulfates regarding their applications are solubility (e.g. in water), rheological behavior, different interaction with low or high molecular cations, thermo reversible gel formation, enzymatic degradability, anticoagulant and antiviral activity. In C6‐position substituted cellulose sulfates can be synthesized in principle by acetosulfation. The acetosulfation is a quasi‐homogeneous synthesis proceeding under gradually dissolution of the cellulose by using different reactivity of the primary and secondary OH‐groups as soon as converting cellulose acetate sulfates. After precipitation of the polymer the acetyl groups are cleaved in alkaline solution. The focus of our study was firstly the investigation of the acetosulfation in different polar aprotic solvents by various sulfating and acetylating agents. In general it should be investigated if C6 substituted cellulose sulfates can be obtained by acetosulfation with different solvents and agents. The products were characterized by 13C‐NMR and Raman spectroscopy.

Morphology, Structure, Rheology, and Thermodynamics of Piezoelectric Poly(vinylidene fluoride)−Ethylene Carbonate Thermoreversible Gel

Poly(vinylidene fluoride) (PVF2) produces a thermoreversible gel with the important solvent, ethylene carbonate, which is used as a dispersing medium in a lithium ion battery. This is evidenced from the fibrillar network morphology and reversible first-order phase transition of the gels. The invariance of the storage and loss modulus with frequency at lower temperatures (≤110 °C) and the existence of a crossover point between them at higher temperatures further supports thermoreversible gel formation in the system. Solvent subtracted FTIR spectra of the gels indicate formation of a β polymorph, which is retained even after drying using cyclohexane as a guest solvent. The wide-angle x-ray scattering (WAXS) diffractogram of the dried gel also indicates formation of the piezoelectric β polymorph, giving a first time report of thermoreversible piezoelectric gel formation. The temperature−concentration phase diagram in conjunction with Tamman's plot indicates the presence of a polymer−solvent molecular compoun...

Gelling characteristics of curdlan aqueous dispersions in the presence of salts

Gelling characteristics of curdlan aqueous dispersions were investigated in the presence or absence of various inorganic salts by dynamic viscoelasticity measurements and puncture tests. Increases in dynamic storage modulus G′ for curdlan aqueous dispersion arising from (a) moisture uptake (i.e. swelling) upon heating, (b) formation of thermo-irreversible gels upon heating occurring at higher temperatures than swelling, and (c) formation of thermo-reversible gels upon cooling were all depressed by the addition of salts, which was validated by puncture tests elucidating that failure stress and strain of curdlan gels were decreased with increasing addition level. These results suggested that hydration or disordering of curdlan molecules, involving breakages of inter- or intra-molecular hydrogen bonds, was repressed by salts added, leading to decrease in the molecular mobility and subsequently to inhibition of hydrophobic interactions between (or partially within) the molecules upon heating that induce the formation of irreversible gels. The results also suggested that new or re-natured hydrogen bonds between (or partially within) the molecules upon cooling that induce the formation of reversible gels was depressed by salts added through decrease in the number of the molecules participating in the junction zones. Some hypotheses were drawn to deduce changes in the gelation behavior of curdlan occurring in the presence of salts.

Role of the Surface State and Structural Feature in the Thermoreversible Sol−Gel Transition of a Zirconyl Aqueous Precursor Modified by Sulfuric Acid

The sols produced by admixture of ZrOCl2 acidified solutions to hot H2SO4 aqueous solutions were studied to clarify the effects of Cl- and SO42- ions on the kinetic stability of nanoparticles and to obtain some new evidence concerning the mechanism of a thermoreversible sol−gel transition observed in this system. The study of suspensions prepared with different molar ratios RS = [Zr]/[SO42-] and RCl = [Zr]/[Cl-] revealed domains of composition of formation of thermoreversible gels, thermostable sols, and powder precipitation. The effects of RS and RCl on the structural features of nanoparticles and on the particle−solution interface were systematically analyzed for samples of thermoreversible and thermostable sol domains. Small-angle X-ray scattering measurements revealed the presence of small fractal aggregates in all samples of thermoreversible domains, while compact packing aggregates of primary particles are present in the thermostable sol. Extended X-ray absorption fine structure and elemental chemic...

A thermophysical approach to the behaviour of gels and complex micellar solutions

AbstractDSC analysis, conductimetric and rheological techniques have been used to investigate the structural changes able to develop in the [water + hexadecyltrimethyl ammonium bromide (CTAB) + phenol] ternary micellar system. Specific, mainly electrostatic, interactions at the micellar interface between the surfactant polar head groups and phenol give rise to a swelling of aggregates into worm‐like micelles as evidenced by large changes of viscosity of the mixture in some phenol/CTAB ratio and temperature ranges. A thermophysical approach allowed us to determine the formation of a strong 1:1 complex between CTAB and phenol, as evidenced by either the viscoelastic properties of the solution or the formation of thermoreversible gels. At low temperature, a crystallized molecular surfactant complex is also observed. Its melting point differs from the Krafft temperature of CTAB.© 2003 Society of Chemical Industry

Encapsulation of a self-assembling bicopper complex in polymer nanowires

We report on a new type of composite material where the filaments of a self-assembling bicopper complex (copperll-2-ethylhexanoate) are encapsulated in a polymer matrix. The encapsulation is achieved in the fibrils created during the formation of thermoreversible gels. Two gelling systems have been studied: isotactic polystyrene/trans-decalin and poly[n-hexylisocyanate]/n-heptane. In the former system, addition of bicopper complex trigger gelation by heterogeneous nucleation while in the latter formation of a solid solution is thought to occur. In both cases, however, the bicopper complex filaments are encapsulated in the polymer fibrils of the gel. Characterization by differential scanning calorimetry (thermodynamic study) and by atomic force microscopy (morphology study) will be presented.

Theoretical and Computational Study of Thermoreversible Gelation

Abstract This paper reviews recent theoretical and computational studies of thermoreversible gelation with multiple cross-link junctions in associating polymers. Paying special attention to the multiplicity and sequence length of the network junctions, we derive phase diagrams with coexisting gelation and phase separation. Local and global structures of the gel networks are studied. The conventional Eldridge–Ferry method is extended to allow for simultaneous evaluation of multiplicity and sequence length (or enthalpy of the cross-links), and applied to the experimental gel melting curves of poly(vinylalcohol) in water. On the basis of the Scanlan–Case criterion, we calculate the number of elastically effective chains and of dangling ends in the network as functions of polymer concentration and temperature. The effect of added surfactants on the formation of thermoreversible gels (surfactant-mediated gelation) in hydrophobically modified associating polymer solutions is also studied under the assumption of the existence of a minimum multiplicity required for stable cross-links. Competition between intramolecular micellization and intermolecular cross-linking is also studied with special attention to the formation of flower micelles.

Rheological properties of dibenzylidene sorbitol networks in poly(propylene oxide) matrices

We have systematically investigated gels of dibenzylidene sorbitol (DBS) in amorphous poly(propylene oxide) matrices by temperature, strain, and frequency dependent rheological measurements. The concentration and the substitution pattern of DBS as well as the molecular weight of the matrix have been varied. Even very small amounts below 0.3 wt % of the low molecular weight gelators lead to a significant increase in the viscosity and elasticity as compared with the neat matrix. This reinforcing effect has been attributed to the formation of an internal DBS network, also proven by our morphological investigations. By means of temperature dependent tests, the temperature Tf of the thermoreversible gel formation has been detected. Strain dependent measurements establish a comparatively small range of linear viscoelastic response and show the extreme deformation sensibility of the gels above a critical strain. Because the principle of time temperature superposition is unexpectedly valid for these heterogeneous systems, master curves can be constructed from frequency dependent measurements at temperatures well below Tf. In the case of higher DBS concentrations above 0.4 wt %, a power law behavior with a small exponent instead of an equilibrium modulus Ge has been observed in the low frequency range of the master curves. We relate this peculiar feature to the dynamic break and reformation of the weak network junctions.

Aluminum Polyphosphate Thermoreversible Gels: A Study by31P and27Al NMR Spectroscopy

The formation of thermoreversible gels from aqueous aluminum polyphosphate solutions was previously assigned to hydration-dependent interionic associations, but these were not elucidated. In the present work, aluminum-phosphate interactions were examined by31P- and27Al-NMR spectroscopy of aqueous aluminum polyphosphate solutions and gels, at various P/Al ratios and temperatures. The fraction of aluminum and phosphate groups actually involved in the sol–gel transition is small, as evidenced by the comparison of spectra of solutions at various P/Al ratios and temperatures, in which only minor spectral changes are actually associated with gel formation. Chemical groups responsible for network formation are mainly Al- (H2O)4(PO3)2groups, in which the phosphate ligands are polyphosphate chain-ends. Within the gels, a large fraction of total aluminum is found as hexaquoaluminum ions, as evidenced by27Al-NMR spectra; on the other hand,31P-NMR spectra show that most phosphate ions are associated. These data indicate that bonding and bridging involve both inner- and outer-coordination sphere interactions. The former depend on ligand exchange reactions with half-life on the order of magnitude of a few seconds, which explains the time dependence of thermoreversible sol–gel transformations, in this system.

Thermoreversible Gel Formation from Aqueous Aluminum Polyphosphate Solutions

Admixture of sodium polyphosphate and aluminum salt aqueous solutions leads to the formation of thermoreversible gels. Gels formed above room temperature change back to clear solutions upon cooling. Gel formation was observed using various polyphosphate fractions obtained by van Wazer's method of fractional precipitation and characterized by 31P NMR spectra. Gel formation is dependent on polyphosphate chain size and is more pronounced when oligomers (e.g., pentamer to decamer) are used. The rate of syneresis was determined, and was found to be higher under centrifugation. Glassy, dried gels were examined by energy-loss spectroscopic imaging; this revealed the existence of aluminum-richer domains dispersed in a phosphate-rich matrix. Thermoreversible gel formation is rationalized by considering the thermodynamics of ion hydration, together with the large connectivity of both cations and anions, in aluminum-polyphosphate systems.

Phase behaviour and structure formation in solutions of vinyl polymers

The supramolecular structure formation in solutions of syndiotactic polystyrene is discussed from the viewpoint of the temperature-concentration behaviour of this system. The phase diagrams that result from these investigations localize the different temperature-concentration domains in which the β -phase (planar zigzag chain conformation) and the δ -phase (helix chain conformation) are formed. The study of these diagrams informs also on the relationship between the different phases and the nature of the supramolecular structure that is formed. The shape of the diagrams is characteristic of the formation of an incongruent melting compound ( δ -phase) between the polymer and the solvent. The formation of this compound at not to high polymer concentrations leads to the formation of thermoreversible gels. The solvent quality has a different influence on the stability of the β - and the ( δ -phase. An increase of the solvent quality lowers the melting point of the β -phase, but increases the stability of the ( δ -phase. The diagrams are interpreted on the basis of schematized equilibrium diagrams.

Molecular Structure and Thermal Behavior of Poly(methyl methacrylate) Thermoreversible Gels and Aggregates

The formation of thermoreversible gels and aggregates of poly(methyl methacrylate) (PMMA) has been investigated as a function of temperature in two different solvents, i.e., bromobenzene and o-xylene. The molecular structure and the thermal behavior have been determined and correlated. It is again shown that the systems are composed of rigid, interconnected cylinders characterized by a two-population distribution as far as the transverse cylinder radius, r H , is concerned: a majority of cylinders with r H =1.5±0.1 nm and a minority of cylinders with r H =3.5-4.0 nm. When the gelation sets in, the proportion of the second population is seen to increase substantially at the expense of the first population

Study of polymer gelation by fluorescence spectroscopy

AbstractA fluorescence technique is used to study the mechanism and dynamics of thermoreversible gel formation in concentrated polymer solutions. Two examples of these ordered structures were studied: isotactic polystyrene (iPS) in decalin and syndiotactic poly(methyl methacrylate) (sPMMA) in toluene. iPS can form two types of gels in decalin solutions. Turbid suspensions are formed between 60°C and 130°C. When the solution is quenched beneath 20°C, transparent gels are formed. In the former solution the polymer has a 31 helix structure; in the latter a 121 helix is formed. These structures formed by iPS in decalin solution have different fluorescence spectra. The ratio (Im/Ie) of the intensity of the monomer fluorescence band in the spectrum over the intensity of the excimer emission band is higher in the spectra of transparent gels. The formation of the transparent gels from a solution can be followed by plotting Im/Ie as a function of temperature. The kinetics of the gel formation are studied by following Im/Ie as a function of time at a fixed temperature. sPMMA forms gels in toluene by a mechanism called “conformational gelation.” This mechanism consists of two steps: a fast conformational change followed by a slower aggregation of polymer chains. Both steps were followed by measuring fluorescence of a pyrene label attached to the polymer. The fluorescence lifetime of this probe is sensitive to the polarity of the environment. When the polymer conformation goes from a random coil to an all trans conformation, the environment of the fluorescent label becomes increasingly richer in solvent. This change in polarity is reflected in a change in fluorescence lifetime. The change in fluorescence anisotropy of the label reflects the second step of the mechanism and correlates with the occurrence of chain aggregation.