Search

We utilize dynamic light scattering (DLS)-based passive microrheology to probe the dynamics and structural evolution of laponite® and laponite®-polymer glasses and dispersions at the microscale. The results reveal an increase in the dynamic heterogeneity of laponite® dispersions with an increase of laponite® concentration and aging time. In neat laponite® dispersions, the degree of stiffness is enhanced and the dynamics are retarded at higher laponite® concentration due to the formation of a repulsive glass. In the presence of PEO with a moderate molecular weight of 20 kg/mol, the microviscoelastic properties of 2 wt% laponite® dispersions show non-monotonic effects with PEO concentration upon aging, which agrees with the results obtained previously from bulk rheology. However, the magnitudes of the viscoelastic moduli (G’ and G”) of dispersions beyond the gel point obtained from DLS-microrheology is lower than that obtained from conventional rheology. Our results suggest that the DLS-microrheology can be used to qualitatively study dynamic transitions and the microviscoelastic properties of gels and soft solids.

The radiation-induced gelation of two monomers, acrylic acid and acrylamide, crosslinked using four model compounds was investigated using Fourier transform mechanical spectroscopy. The gel point characteristics of the resulting hydrogels were compared. The gelation characteristics of the two monomers were very similar. The fractal dimensions of the critical gels ranged between 1.77 and 1.82 when the reactions were controlled. The patterns of viscoelastic events of the products at the gel point were also similar, with the matching reactivity of the monomers and the comparable molecular weight of the crosslinkable species, providing a foundation for such events. All of the reactions exhibited low mutation numbers, indicating that the samples tested were quasi-stable during the measurements. The extent of the UV irradiance during crosslinking reactions is an important parameter in the gelation process, with an increase in UV irradiance decreasing the gel times, decreasing the network relaxation exponents (increased fractal dimensions), leading to stiffer critical gels. The evolution of δ with G*, undertaken as van Gurp–Palmen plots, shows that in the fully formed gels, however, the monomer choice and the crosslinker variation can dictate the viscoelasticity of the materials. In particular, the potential for greater steric effects/greater stabilities of the radical active centres of the dimethacrylate species, in comparison with the diacrylate species, resulting in lesser viscoelasticities in the fully formed gels.

The synthesis of glycidyl methacrylate–based polyHIPEs by free-radical polymerization in the presence of 1,8-diaminoctane or tris(2-aminoethyl)amine-enriched high internal phase emulsions is presented. This innovative “one-pot” synthetic route was developed to produce the so-called in situ hyper-cross-linked polyHIPEs without the use of any additional catalyst, cross-linker, or solvent. In situ hyper-cross-linking was performed through the amine-epoxy reaction before the gel point had been reached, resulting in the formation of the β-amino alcohol derivatives that represent cross-linking knots between the neighboring epoxy repeating units within the poly (glycidyl methacrylate) backbone. In this way, the volume of the mesopores smaller than 3 nm significantly increased. Thus, by changing the amounts of ethylene glycol dimethacrylate and amines in the HIPE templates, the porous structure and the pore volume of the hyper-cross-linked polyHIPEs were systematically altered in order to amplify the polyHIPE’s specific surface area.

This study investigated the effects of carboxyl content and mechanical treatment intensity on the morphological characteristics of carboxymethylated cellulose nanofibrils (CM CNFs) and on the rheological properties of CM CNF suspension. The mechanical properties of self-standing CM CNF film were also examined. CM CNFs produced under different conditions had similar, uniform widths of about 5 nm, as measured using transmission electron microscope images and Image J software. The aspect ratios of three CM CNFs were evaluated using gel point analysis and the crowding number theory. Higher carboxyl content in the CM CNFs reduced the amount of mechanical energy required and increased the aspect ratio. The rheological properties and the tensile properties of CNF film were all strongly influenced by the aspect ratio of the CM CNFs. The CM CNF samples with higher aspect ratios produced stronger suspension network structures and had greater tensile properties.

The conventional free radical polymerization (FRP) of multivinyl monomers (MVMs) inevitably leads to gelation even at low monomer conversion resulting in difficulties to control and monitor the reaction process. Flory and Stockmayer (F-S theory) studied it based on two fundamental assumptions: (1) independent and equivalent vinyl groups; (2) no intramolecular cyclization. However, until now its applicability to FRP of MVMs (especially regarding the extent of intramolecular cyclization) is still controversial. In this paper, Monte Carlo simulations are used to study FRP of divinyl monomers by two kinetic models: with/without cyclization models. The results of the simulations are compared with the calculated gel points based on F-S theory and the experimental data. It is found that the intramolecular cyclization has a negligible impact on the polymerization process and the gel point before gelation, which are in agreement with the prediction by F-S theory, but the effect becomes significant above the gel points.

This study examines and quantifies the effect of adding polyelectrolytes to cellulose nanofibre suspensions on the gel point of cellulose nanofibre suspensions, which is the lowest solids concentration at which the suspension forms a continuous network. The lower the gel point, the faster the drainage time to produce a sheet and the higher the porosity of the final sheet formed. Two new techniques were designed to measure the dynamic compressibility and the drainability of nanocellulose–polyelectrolyte suspensions. We developed a master curve which showed that the independent variable controlling the behaviour of nanocellulose suspensions and its composite is the structure of the flocculated suspension which is best quantified as the gel point. This was independent of the type of polyelectrolyte used. At an addition level of 2 mg/g of nanofibre, a reduction in gel point over 50 % was achieved using either a high molecular weight (13 MDa) linear cationic polyacrylamide (CPAM, 40 % charge), a dendrimer polyethylenimine of high molecular weight of 750,000 Da (HPEI) or even a low molecular weight of 2000 Da (LPEI). There was no significant difference in the minimum gel point achieved, despite the difference in polyelectrolyte morphology and molecular weight. In this paper, we show that the gel point controls the flow through the fibre suspension, even when comparing fibre suspensions with solids content above the gel point. A lower gel point makes it easier for water to drain through the fibre network, reducing the pressure required to achieve a given dewatering rate and reducing the filtering time required to form a wet laid sheet. We further show that the lower gel point partially controls the structure of the wet laid sheet after it is dried. Halving the gel point increased the air permeability of the dry sheet by 37, 46 and 25 %, when using CPAM, HPEI and LPEI, respectively. The resistance to liquid flow was reduced by 74 and 90 %, when using CPAM and LPEI. Analysing the paper formed shows that sheet forming process and final sheet properties can be engineered and controlled by adding polyelectrolytes to the nanofibre suspension.

The polyvinyl alcohol (PVA) as-spun fibers were prepared by dry-wet gel spinning, with dimethylsulfoxide (DMSO) as solvent and methanol as the cooling medium. The rheological properties of spinning solutions, matter exchanges, sonic orientation factor and entanglement of PVA as-spun fibers were investigated. Solution with lower PVA content at lower temperature has a same viscosity with the more concentrated solution at higher temperature. Solution with lower concentration has a higher G′ and a lower Tan δ, which leads to a lower sonic orientation factor for the fiber. The calculating results from the optical microscope show that solution with lower PVA content loses less DMSO due to the weaker gel ability and the possible phase separation during the coagulation. Moreover, solution more closed to the gel point bounds more DMSO, which makes the DMSO exchange difficult to process. The entanglement in the as-spun fiber decreases with the decreasing PVA content in the solution. Suitable crystallization and entanglement can obtain fibers with higher strength.

Linear viscoelastic complex modulus (reported by Weiss and Zhao, 2009) of three ionomers, obtained through random sulfonation of oligomeric styrene at low ionic contents, p = 2.5, 4.8, and 6.5mol%, were analyzed using mean-field gelation theory with the fraction of repeat units that are sulfonated the effective extent of reaction p. Oligomeric styrene with low M = 4000 g/mol, having N = 38 repeat units, ensures absence of entanglement effects. The dynamics change in a complicated way with ion content p because the gel point pc = 1/(N−1) = 2.7 mol%. For ionomers having p = 2.5 mol%, the system is below but quite close to the gel point. The terminal relaxation is governed by the effective breakup of large clusters into subclusters of comparable sizes, as anticipated by Rubinstein and Semenov. The samples with p = 4.8 mol% and 6.5 mol%, are beyond the gel point and exhibit properties of reversible gels.

Sol–gel transition properties play a key role in various applications of thermosensitive hydrogels, but conventional methods for studying the sol–gel transition have some limitations. For extensive characterization of the water–polymer interaction and microstructure change during sol–gel transition, we propose a rapid and nondestructive method based on monitoring water mobility through low field NMR (LF-NMR), and this was applied to chitosan/β-glycerophosphate (CS/GP) hydrogels. The spin–spin relaxation time (T2) that depicted water mobility was measured by LF-NMR within 90 s. The T2 component corresponding to water protons trapped in polymer networks (T21) was very sensitive to sol–gel transition. A remarkable decrease of T21 value indicated obvious variations of water mobility when CS/GP was heated, and a turning point was observed on the T21–time curve. The gel point associated with this turning point could be easily determined by fitting the T21–time curves to a bilinear regression model, and the results showed good accuracy and repeatability owing to the nondestructive nature of LF-NMR. Variations in water components and microstructure of CS/GP caused by water migration after solidification were also analyzed by monitoring dynamic changes of T2. This rapid, nondestructive method provides a powerful tool for studying the sol–gel transition of hydrogels.

A molecular dynamics simulation method is presented and used in the study of the formation of polymer networks. We study the formation of networks representing the methylene repeating units as united atoms. The network formation is accomplished by cross-linking polymer chains with dedicated functional end groups. The simulations reveal that during the cross-linking process, initially branched molecules are formed before the gel point; approaching the gel point, larger branched entities are formed through integration of smaller branched molecules, and at the gel point a network spanning the simulation box is obtained; beyond the gel point the network continues to grow through the addition of the remaining molecules of the sol phase onto the gel (the network); the final completion of the reaction occurs by intra-network connection of dangling ends onto unsaturated cross-linkers. The conformational properties of the strands in the undeformed network are found to be very similar with the conformational properties of the chains before cross-linking. The uniaxial deformation of the formed networks is investigated and the modulus determined from the stress-strain curves shows reciprocal scaling with the precursor chain length for networks formed from sufficiently large precursor chains (N ≥ 20).

Water-soluble cyanoethyl cellulose (CEC) samples were synthesized by homogeneous reaction of cellulose with acrylonitrile in LiOH/urea aqueous solutions. The dynamic viscoelastic property of CEC with different degree of substitution (DS) in water at different temperatures and concentrations were investigated. At low concentrations, the CEC solutions displayed liquid-like behavior with G′ smaller than G′′ at low frequencies; while the curves of G′ and G′′ intersected at the middle of the frequency range at higher concentration, indicating an existence of chain aggregation and entanglement network. The gelation temperature (T gel) was determined from the point of intersection in tan δ versus temperature (T) at different frequencies, indicating the validity of Winter–Chambon criteria. It was found that T gel increased with increasing DS, and that the exponent (n) values at the gel point decreased with increasing DS. The heating–cooling process proved that the sol–gel transition for CECs in water was thermally irreversible. The AFM images revealed that the particles packed loosely together to form gel aggregates. Moreover, the Maxwell model with four elements was used to describe the frequency dependencies of G′ and G′′ for CEC in aqueous solution.

The crosslinking behaviour of a silicone resin which is interesting from a technical point of view was investigated by means of rheology. In order to accelerate the crosslinking process, zinc acetylacetonate and aluminium acetylacetonate were applied as latent catalysts. The effect of the type of catalyst, its concentration, and the temperature on crosslinking was determined by isothermal dynamic-mechanical measurements. A radial gradient in crosslinking causes the gel point to be reached earlier at the outer edge of the sample in the rheometer. This radial gradient is averaged when measuring G′ and G″. Therefore, since the physically well-defined state of critical gelation (gel point) could not be obtained from the data, the time at which a distinct crosslinking state is reached was determined by the crossover of the moduli G′ and G″. For this distinguished point, the denotation gelation index GI is introduced. The gelation indices measured at different temperatures follow an Arrhenius-type relationship. Activation energies between 89 and 126 kJ/mol were determined. They were found to be dependent on the type of catalyst used but independent of its concentration. The activation energies of the crosslinking processes enable the calculation of the gelation index at temperatures not measured directly.

We report the preparation and rheological characterization of interpenetrating polymer network (IPN) hydrogels made from alginate and hydrophobically modified ethyl hydroxyl ethyl cellulose (HMEHEC). To our knowledge, there have been no studies of the gelation behavior of IPNs. We found that the rheology of these systems can be easily tuned, with the elastic modulus of the IPN strongly dependent on the relative ratio of HMEHEC to alginate. The sol–gel transition of these systems was found to satisfy the Winter–Chambon criterion for gelation at various crosslinker densities. From the power law relationship of the dynamic moduli (G ′ ~G ″ ~ω n), the exponent n appears to be dependent on both the crosslinker density and relative amount of two polymers. The value of n was found to be ~0.5 for all samples for stoichiometric amounts of crosslinker. The effect of molecular weight of HMEHEC on the gel point and viscoelastic exponent has also been reported. Alginate seems to dominate the kinetics of the process but the effect of high molecular weight HMEHEC on the gel point, especially at lower proportion was also evident.

The polycondensation reaction between silicate and hydroxoaluminate in alkaline aqueous solution has been experimentally studied. The time required for the solution to transform to a gel was measured to determine the reaction rate. The addition of alkali hydroxide to the silicate solution was found to strongly accelerate the subsequent reaction of silicate with hydroxoaluminate. The results indicate that the acceleration was caused by depolymerisation of the oligosilicates contained in the water glass, which was used as the source of soluble silicates. The course of oligosilicate depolymerisation in aqueous alkaline solution was experimentally observed by measuring the electrical conductivity of the solution.

The present study is an extension of our preceding work on gelation behavior. We observed no substantial difference in the actual gel points among three isomeric diallyl phthalates: diallyl phthalate, diallyl isophthalate and diallyl terephthalate. The resulting network polymer precursors (NPPs) were characterized by size-exclusion chromatography with both multiangle laser light scattering and viscometry. It is of note that the structure of NPP, consisting of oligomeric primary polymer chains, becomes core-shell type dendritic or nanogel-like with the progress of polymerization. The nanogel-like NPPs can then collide with one another to form cross-links, eventually leading to gelation. Although the concentration of NPP should be high at the conversion close to the gel point, the dilution of NPP by adding monomer could prevent the cross-link formation among NPPs, and consequently lead to the successive growth of high-molecular-weight NPP from a nanogel to a microgel. The further growth of the microgel as an inhomogeneous NPP with high cross-link density could eventually reach an extremely inhomogeneous network polymer. These processes were pursued as typical examples using the bulk polymerization of DAT.

The early stage of polymer crystallization may be viewed as physical gelation process, i.e., the phase transition of polymer from liquid to solid. Determination of the gel point is of significance in polymer processing. In this work, the gelation behavior of poly(butylene succinate) (PBS) at different temperatures has been investigated by rheological method. It was found that during the isothermal crystallization process of PBS, both the storage modulus (G′) and the loss modulus (G″) increase with time, and the rheological response of the system varies from viscous-dominated (G′ G″), meaning the phase transition from liquid to solid. The physical gel point was determined by the intersection point of loss tangent curves measured under different frequencies. The gel time (t c) for PBS was found to increase with increasing crystallization temperature. The relative crystallinity of PBS at the gel point is very low (2.5%–8.5%) and increases with increasing the crystallization temperature. The low crystallinity of PBS at the gel point suggests that only a few junctions are necessary to form a spanning network, indicating that the network is “loosely” connected, in another word, the critical gel is soft. Due to the elevated crystallinity at gel point under higher crystallization temperature, the gel strength S g increases, while the relaxation exponent n decreases with increasing the crystallization temperature. These experimental results suggest that rheological method is an effective tool for verifying the gel point of biodegradable semi-crystalline polymers.

This study reports an automated numerical method for the location of the gel point in oscillatory shear data and demonstrates its potential application in measurements on therapeutically modified (heparinised) samples of healthy coagulating blood. Heparin prolongs the onset of clot formation and has a significant effect on the microstructure of the clot which is eventually formed. The results demonstrate the potential of this rheometrical technique as a new tool for monitoring the effect of heparin on samples of whole blood with significantly better linearity of response within the therapeutic range than another global coagulation monitoring technique (thromboelastography) which has previously been used.

Thermal-induced gelation process of different polyacrylonitrile (PAN) solutions was investigated through dynamic rheological measurements. The rheological material parameters characterizing the gelation behavior of the PAN solutions were measured and the effects of such factors as water content, solvent type and concentration on the gelation process were analyzed. It is found that the gel point T gel of PAN/DMSO (dimethyl sulfoxide)–water solution increases with increased water content in the solution. Also, T gel becomes higher when the solvent has a lower solvency for PAN. A linear relationship between logG′ and logω, logG″ and logω indicates the beginning of gelation. The PAN concentration of the solution affects T gel in a way similar to that of the water content. The relaxation exponent n is found to relate to the fractal dimension of the polymer gel. For PAN/DMSO–water solution, this exponent n remains almost constant when the water content differs. The solvent type and concentration respectively influences n value in their own way. The PAN/DMSO–water solution systems in our work exhibit similar characteristics of gel structure, concluded from the n and d f values of these systems. A logarithmic plot between G′ and G″ can be used to evaluate the structural evolution of the solution, and a comparatively steep slope of the curve is an indication of gelation.

Studies on the gelation behaviors of the reactive blends of nylon1212 and functional elastomer were carried out. The results show that the curves of the storage modulus(G′)–frequency (ω) exhibit a gel plateau in the low ω region, and the transition from liquid-like to solid-like viscoelastic behaviors emerges with the concentration of SEBS-g-MA increasing. There exist the gelation behaviors in the blending process similar to those of crosslinking polymer. Based on Winter’s method, the gel point of blends is determined to be, φg = 17.45 wt%, and the corresponding value of tanδ is 1.44. The gel index n calculated is 0.61 and the gel strength Sg is 1.08 × 104 Pa s0.61. However, the non-reactive blends of nylon1212 and elastomer have no emergence of gelation behaviors. The morphology analysis shows that the gel point for the reactive blends is a threshold of cocontinuous morphology, and morphology analysis can also be a method to determine the gel point.

Dynamic viscoelastic properties of polyacrylonitrile(PAN)/DMSO/H2O solutions with different H2O contents were studied as a function of temperature. These PAN solutions gradually became gel with decreasing temperature. The sol-gel transition took place at a critical gel temperature, at which the scaling law of G′(ω)∼G″(ω)∝ωn held, allowing an accurate determination of the critical gel temperature by means of the frequency independence of the loss tangent. The gel point of PAN solutions increases with increasing H2O content. The scaling exponent n (=0.86) at the gel point is confirmed to be universal for PAN gels, which is independent of temperature, suggesting the similarity of the fractal structure in the critical PAN gels.

Hydrogels are hydrophilic macromolecular networks that are capable of retaining a large amount of water. A precise description of these systems is actually quite complex and the practical use of hydrogels for drug delivery and biomedical applications is often not supported by a well-defined knowledge of the overall structure of the polymeric network. In this paper, we report the characterization of two different systems: a chemical network based on Guar Gum (GG) and a physical gel prepared with Xanthan (Xanth) and Locust Bean Gum (LBG). The dynamo-mechanical properties of the gels were analysed: the cohesiveness and the adhesion of the networks were strongly dependent on time, temperature, and composition. The kinetics of the chemical crosslinking was followed by means of rheological measurements, i.e. recording the mechanical spectra of the gelling system, and the power law exponent at the gel point was evaluated. Furthermore, the networks, loaded with model drugs with different steric hindrance, were used as matrices for tablets and the rate of release of such model drugs was studied. The diffusion of the guest molecules was deeply dependent on their dimensions; in the case of Xanth-LBG tablets the release profiles were almost independent from the different cohesion properties of the starting hydrogel composition.

The preceding theory of gelation is extended to the multilink system of the R–Ag+R–Bf−g model, where the branching process proceeds forming junction points each comprising of J/2 A FU’s and J/2 B FU’s. We show that there exists a neat solution of the ring concentration for this generalized system; the mathematics is identical with the previous finding, but the expression is greatly simplified. Making use of this result together with the random distribution assumption of cyclic bonds, we derive the closed expression of the gel point for this new gelling system which is expected to have wider application.

Sol-gel polymerization of methyltrimethoxysilane (MTMS) in ethanol using a two-step acid/base catalyzed procedure (B2) is followed by 29Si NMR spectroscopy. Analysis of the structural evolution of the B2 system shows that esterification of monomeric and end silicon species is rate-limited while that of linear and cyclic species is able to reach pseudoequilibrium in the second basic step. Condensation reactivity is reduced with increasing network connectivity, however, to a much less degree under B2 conditions than MTMS polymerization under acidic conditions. Steric effects as well as many other factors are attributed to this trend. The concentration of cyclic and polycyclic species of the B2 system is nearly 3 times lower compared to the acid-catalyzed system. The empirical degree of condensation at the gel point is determined to be 0.88. The effects of cyclization and phase separation on MTMS gelation are discussed for both B2 and acid-catalyzed systems. Based on these results it is believed that MTMS-based gels form for B2 and not acid-catalyzed conditions due to reduced cyclization, rapid hydrolysis and condensation, effective use of functional groups, and effective contribution of branched and polycyclic species as crosslinking points to connect polymeric chains in the B2 system.

Gel points, accounting for intramolecular reaction, are predicted using Ahmed-Rolfes-Stepto (ARS) theory. They are compared with experimental gel points for polyester (PES)-forming, polyurethane (PU)-forming and poly(dimethylsiloxane) (PDMS) polymerisations. The PES and PU polymerisations were from stoichiometric reaction mixtures at different initial dilutions and the PDMS ones were from critical-ratio experiments at different fixed dilutions of one reactant. The ARS predictions use realistic chain statistics to define intramolecular reaction probabilities and employ no arbitrary parameters. Universal plots of excess reaction at gelation versus ring-forming parameter are devised to enable the experimental data and theoretical predictions to be compared critically. Significant deviations between experiment and theory are found for the PU systems and the PDMS systems with higher molar-mass reactants. However, ARS theory fits well the results for the PES systems and the PDMS systems having lower molar-mass reactants. Possible reasons for these differences in behaviour are discussed in terms of the PU-forming reaction mechanism and the effects of entanglements in PDMS. Whilst ARS theory provides a good basis for gel-point predictions and can be applied to many types of polymerisation, more experimental systems at different initial dilutions and ratios of reactants still need to be studied and the various methods used for detecting gel points still need to be compared to enable a definitive assessment of its performance to be made.

Gel point conversions and equilibrium tensile moduli of polyester-based polyurethane networks were determined for different dilutions during network formation. Also, the limiting dilution beyond which the network was not formed at full conversion of functional groups was determined. It ranged between 92 and 94% of diluent. The scaling dependences against the reference states – the ring-free case and the limiting dilution case – showed that the excluded volume effects were operative and structural changes associated with cyclization affected the rates of both the intramolecular and the intermolecular reactions.

Hybrid siloxane-polymethylmethacrylate (PMMA) nanocomposites with covalent bonds between the inorganic (siloxane) and organic (polymer) phases were prepared by the sol gel process through hydrolysis and polycondensation of 3-(trimethoxysilyl)propylmethacrylate (TMSM) and polymerization of methylmethacrylate (MMA) using benzoyl peroxide (BPO) as initiator. The effect of MMA, BPO and water contents on the viscoelastic behaviour of these materials was analysed during gelation by dynamic rheological measurements. The changes in storage (G′) and loss moduli (G′′), complex viscosity (η*) and phase angle (δ) were measured as a function of the reaction time showing the viscous character of the sol in the initial step of gelation and its progressive transformation to an elastic gel. This study was complemented by 29Si and 13C solid-state nuclear magnetic resonance (NMR/MAS) measurements of dried gel. The analysis of the experimental results shows that linear chains are formed in the initial step of the gelation followed by a growth of branched structures and formation of a three-dimensional network. Near the gel point this hybrid material demonstrates the typical scaling behaviour expected from percolation theory.

Summary Turbidity and rheological features during gelation of semidilute solutions of pectin via the Ugi multicomponent condensation reaction have been investigated at different polymer and cross-linker concentrations at ambient temperature. The gelation time of the system decreased with increasing polymer and cross-linker concentrations. At the gel point, a power law frequency dependence of the dynamic storage modulus (G'∼ω n' ) and loss modulus (G''∼ω n'' ) was observed with n'=n''=n. The value of the power law exponent is about 0.6~ 0.7 for all the gelling systems, which is close to that predicted (0.7) from the percolation model. The elastic properties of the gels continue to grow over a long time in the post-gel region, and a solidlike response is observed at later stages during the gelation process. The turbidity of the gelling system is higher as the polymer and cross-linker concentrations rise, but the turbidity is virtually unaffected during an extended time after the formation of the incipient gel. It was demonstrated that polymer and cross-linker concentrations could be utilized to tune the physical properties, such as transparency and viscoelasticity, of the Ugi hydrogels.

The long-time stress relaxation was examined for poly(styrene)-poly(ethylene- co-butylene)-polystyrene triblock copolymer with asymmetric composition in the microphase separated state. Stress relaxation data from a parallel plate rheometer were extended to very long times by using a newly built compression instrument. Three properties were measured, the relaxation time spectrum, the equilibrium modulus, and the instantaneous modulus. Relaxation experiments over several months (without zero drift) allowed determination of extremely long relaxation times as found in soft solids and in liquids near the gel point. In this way, we can distinguish solid from liquid behavior within the experimental time frame. The BCC lattice structure of the test polymer behaves as liquid except at low temperature where an equilibrium modulus was observed in an experiment that lasted over a period of 4 months. The corresponding instantaneous modulus was observed to be constant for the same material and for the same time period.

A wide range of hydrocarbons were rapidly gelled by adding a polysiloxane copolymer in the presence of divinylbenzene and a platinum catalyst. The gel point was measured over a range of concentrations for hydrocarbons/solvents and organogels, using three separate methods: rheology, visual (tilt-tube) and FTIR. As the fraction of solvent was increased, the rate of reaction decreased, leading to an increase in the gelation time. The absolute value of the gel point depends upon the techniques used to measure it. For any particular system the gel point values always followed the order: rheology > visual > FTIR. The crosslink densities of the gel systems were determined using both swelling and thermomechanical analysis. The swelling measurements confirmed that the addition of large quantities of solvent markedly reduced the crosslink density of the obtained chemical gel networks, which helped in designing gels with suitable critical strength for effective field work. Also the effectiveness of gelation with the proposed gelling system for different hydrocarbons/solvents was evaluated, and discussed in relation to their dielectric properties.

Sodium silicate solutions were mixed with boric acid in order to form sodium borosilicate gels. The gelation process was observed by dynamic oscillatory experiments. The effects of composition and temperature on the gel point are discussed. Furthermore, an indicator of the gel point for the water-based sodium borosilicate gels was found. The turbidity was investigated by UV/VIS spectroscopy and optical microscopy. Infrared spectroscopic measurements were applied to characterise the gelation process, too.

Fourier transform mechanical spectroscopy technique (FTMS) is utilized as a powerful tool to study the sol-gel transition of covalent bonded polymeric network. Winter and Chambon criteria resulting from the fractal-geometry characteristic of the gel networks allow the determination of the gel point with only single experiment using this technique. The gelation behaviors of low melt viscosity ternary systems of benzoxazine, epoxy, and phenolic resins are investigated and analyzed by the technique in order to study the effect of epoxy diluent on the rheological property development before and after the gel points. The gel time at 140 °C ranges from 5 min to 30 min and less than 5 min at 180 °C for all tested ternary system compositions. The gelation of the ternary mixture shows an Arrhenius-type behavior and the gel time can be well-predicted by the Arrhenius equation.

We are very grateful for the financial aid from the Ministry of Education of Korea through the Brain Korea 21 Program at Seoul National University.

A large body of literature is focused on the accurate determination of a gel point for systems undergoing a sol-gel phase transition. Investigation into the limiting strain and stress for linear viscoelastic behaviour at various stages of a phase transition such as gelation is a subject that is rarely commented on. The small amplitude oscillatory rheological behaviour of a biopolymer cross-linker system through a thermally activated sol-gel transition is presented. Mechanical spectra were interpreted through application of the gelation criteria of Chambon and Winter (Winter and Chambon 1986; Chambon and Winter 1987), where the (so-called gel strength) parameter S, and relaxation exponent, n are obtained. A detailed study of the limit of linear viscoelasticity yields important trends in critical stress (σ°c) and critical strain (γ°c) limits highlighting the possible experimental difficulties associated with mechanical measurements obtained in close proximity to the gel point.

Physical gelation can be induced in various organic and silicone-based liquids, as well as in polymeric melts, upon addition of 1,3:2,4-dibenzylidene sorbitol (DBS). Such gels are stabilized by the formation of a percolated DBS network composed of highly interconnected nanofibrils. In this study, we explore several factors affecting the rheological properties of poly(propylene glycol) (PPG) gelled by DBS. To ascertain the effect of PPG molecular weight (MPPG) on gel formation and rheology, we have investigated three series of DBS-induced PPG gels in which MPPG varies from 425 g/mol to 4000 g/mol. Dynamic stress measurements reveal that the DBS concentration identifying the onset of PPG gelation decreases with increasing MPPG. Since the solubility parameter (δs) of PPG decreases sharply as MPPG increases over this MPPG range, this observation suggests that DBS gelation is sensitive to δs of the matrix liquid, in agreement with previously reported data collected from DBS-gelled solvents. Moreover, the elastic modulus and yield stress are found to increase with increasing (i) DBS concentration for the three series of PPG/DBS gels examined here and (ii) recovery time after cessation of an introductory shear.

Free radical co-polymerization of methyl methacrylate (MMA) and ethyl glycol dimethyl methacrylate (EGDMA) in solution leads to the formation of polydisperse branched PMMA which grows in size until the system gels. The structure and the size distribution of the PMMA aggregates were characterized at infinite dilution using static and dynamic light scattering and size exclusion chromatography (SEC). The reaction extent was measured using SEC and Raman spectroscopy. The results show that the structure and size distribution of PMMA aggregates formed close to the gel point are compatible with those of percolating clusters. The structure factor of semi-dilute solutions of PMMA aggregates is the same as that of dilute solutions at distance scales much smaller than the correlation length of the concentration fluctuations ( ). However, the cut-off function of the pair correlation function at for semi-dilute solutions is more gradual than the cut-off function at for dilute solutions.

Free radical co-polymerization of methyl methacrylate (MMA) and ethyl glycol dimethyl metacrylate (EGDMA) was investigated in solution at different molar ratios R = [EGDMA]/[MMA] between 0 and 0.05. Initially mainly linear PMMA was formed with weight average molar mass 7.5 g/mol independent of R. At larger reaction extents branched polymers were formed and the systems gelled. The scattering intensity rose initially with the reaction extent, but reached a plateau value at larger reaction extents. The plateau value increased strongly with R. Dynamic light scattering showed the appearance of a slow relaxation not observed in linear PMMA solutions. The data can be interpreted by assuming that the excess scattering originates from the branching points and relaxes through self diffusion of the branched particles. The results agree with predictions of the percolation model for gelation and Rouse dynamics. Viscosity measurements corroborate this interpretation. Measurements on a progressively diluted sample quenched close to the gel point again showed quantitative agreement with the percolation model for gelation.

We develop a new theory of gelation which takes into account (i) delay of the gel point and (ii) change of functionality due to ring formation. We show that the problem of finding the gel point in real polymer solutions reduces to the problem of calculating the total ring concentration and the extent of reaction of intermolecular reaction at the gel point. In this paper, we solve a special case of this problem, on the basis of the independence assumption between intermolecular reaction and cyclization which takes into account only (i) the delay of the gel point: making use of the asymptotic equality of the total ring concentration, we acquire an approximate solution for the gel point Dc as a function of the inverse concentration , the relative frequency of cyclization and dimension d. Applying the observed values of in linear polyesters, the theoretical result reproduces well the Wile and the Gordon-Scantlebury observations, showing the existence of a critical dilution beyond which gelation can not occur, and an asymptote . As the classical gel point is approached, the present theory reduces to the linear equation, which makes one-to-one correspondence with the real slope , suggesting the inequality which is just what polymer chemists have quested for so far, with the physical meaning having remained unknown.

While the rheological behavior at the gel point (GP) itself is well known, many questions remain about the approach to the gel point (from either side). For studying this phenomenon, rheological experiments were performed on crosslinked polydimethylsiloxanes (PDMS) and characteristic patterns in the relaxation behavior were abstracted into a phenomenological model for the relaxation time spectrum. The well known power law with slope-n and cutoff at the longest relaxation time λmax governs the relaxation near the gel point. Beyond the gel point, an additional box-like contribution appears in the spectrum. The relaxation exponent n decreases with increasing extent of crosslinking. The longest relaxation time λmax and equilibrium modulus Ge show power-law scaling with the distance from the gel point, |p − pc|. For samples with imbalanced stoichiometric ratios r ≥ 1, the function n (p − pc) is independent of r. Samples with strong crosslinker deficiency (r = 0.5) exhibit a different functional form for n (p − pc) from those with r ≥ 1. The power law relaxation with decreasing n in the terminal zone was found previously for a different crosslinking system. It seems to be a common pattern for partially crosslinked materials from polymeric precursors.

Oscillatory shear and viscosity measurements have been carried out on semidilute acid aqueous systems of unmodified chitosan (UM-chitosan) and of hydrophobically modified chitosan (HM-chitosan) with three different degrees of C12-aldehyde substitution. These systems form physical gels at higher concentrations. The gel point, determined by the observation of a frequency independent loss tangent, was found to be shifted toward lower concentrations with increasing hydrophobicity. At the gel point, a power law frequency dependence of the dynamic moduli (G′∼G″∼ωn) was observed, with n values of 0.36 and 0.46 for the UM-chitosan and the hydrophobically modified chitosans, respectively. These values are interpreted in the framework of a fractal model. The gel strength parameter S increased with decreasing hydrophobicity. The UM-chitosan and HM-chitosan systems exhibited a non-Newtonian shear thinning behavior. This effect is promoted by increasing polymer concentration and hydrophobicity. Both the linear and nonlinear rheological properties elucidate the intricate interplay between hydrophobic associations and entanglement effects.

We have studied the dynamics of poly(vinyl alcohol) (PVA) in aqueous borax solution by dynamic light scattering (DLS) and dynamic viscoelastic (DVE) measurements. DLS measurement showed the presence of two dominant modes with decaying rates of Γ f and Γ s (Γ f>Γ s). Different dynamical behaviors were observed above and below a critical concentration, C N. The slow mode was manifested to be the diffusive mode for PVA concentration C C N. Dynamical correlation length, ξ H, estimated from Γf exhibited a jump at C N with increasing C. Detailed analysis revealed the applicability of the dynamic scaling theory to Γ s for C C N. From these results, we drew the picture on a growing process of associating PVA chains to a temporally cross-linked network with increasing C and proposed that C N corresponded to the gel point of the chemically cross-linked gel in the short-time domain. The concept is found to be useful for interpretation of the concentration dependence of the plateau modulus.

The existing models describing the kinetics of aggregation in the presence of an infinite cluster, orgel, are reviewed, and a new class of post-gel models is proposed. In this new class of models, clusters are assumed to be acyclic and the rate constant for reactions involving the gel can be varied. The model is called thegeneralized acyclic model (GAM) since it generalizes Stockmayer's model and Ziff's third model. It is shown that the GAM can be solved exactly in terms of standard (hypergeometric) functions. The solutions are analyzed in detail, both asymptotically in various limits and numerically.

Dynamic viscoelasticity of α,ω-dihydroxyl polybutadiene/p-xylene system is studied through gelation process by end-crosslinking reaction with crosslinker which has three isocyanate groups. The gel point is determined as the reaction time t=t c 1 at which the storage and the loss shear moduli, G'(ω) and G(ω), become both proportional to ω over the whole ω range measured, where ω is the angular frequency and n is a critical exponent at the gel point. The relative extent of chemical reaction p(t) is estimated from a decrease in the absorption peak of the isocyanate residue by FT-IR measurement to estimate precisely e=(p(t)-p|/p c 1 the relative distance from the gel point (p=p c ). Effects of polymer concentration C on the increase of zero shear viscosity η o before the gel point and on the increase of equilibrium modulus G eq after the gel point are examined in terms of critical exponents of k and z for power laws, η o ∼ e -k and G eq ∼ e z , respectively. The exponents n and z are found to take universal values of n=0.67±0.02, and z= 2.5 ± 0.1 independent of C in the lower concentration of C

Functionalised triblock copolymers based on poly((lactic acid)-co-(glycolic acid)) and poly(ethylene glycol) (PLGA-PEG-PLGA) further modified with 3-methylenetetrahydrofuran-2,5-dione (itaconic anhydride; ITA) exhibited sol-gel transition induced by increasing temperature. Rheological properties of a series of ITA/PLGA-PEG-PLGA/ITA copolymer concentrations (6–24 mass %) in deionised Milli-Q water were studied by both the test tube inverting method (TTIM) and rheometer. The gel stiffness increased with the polymer concentration shifting the gel point of the copolymer to the lower temperature. The present study demonstrates that each method describes a sol-gel transition, but the combined method gives comprehensive information about changes in colour, viscosity, elastic and loss moduli. Characterisation of such a gel is necessary for its further use, in order to determine whether the material is appropriate as an injectable biomedical hydrogel.

Based on an infinite network gel model, using Graph theory and Probability as investigative tools, first of all, advanced a new theory about predicting the gel point Pc The critical state equations of gelation for three kinds of non-linear polymerization have been derived. The Pc obtained by these equations lies between the results from Carothers equation and Statistical theory, and is in good agreement with experimental results.

Methyl methacrylate-ethylene glycol dimethacrylate (MMA-EGDM) copolymerization has been investigated in toluene at a monomer concentration of 22 w/v %. The kinetic models developed recently along with the experimental conversion curve and gel point data provided the calculation of the average reactivity of pendant vinyl groups. It was found that the pendant vinyl reactivity for intermolecular links is much less than the monomeric vinyl reactivity and it decreases further as the EGDM concentration increases. At 5–15 mol % EGDM, the average pendant reactivity is 1–2 orders of magnitude lower than the monomeric vinyl reactivity. The reduced pendant reactivity is mainly responsible for the shift of the gel point towards higher conversions.

Silica-titania gels, prepared in acidic conditions, with contents of TiO2 up to 6 mol%, have been studied by small angle neutron scattering (SANS) as wet gel with increasing aging times. In all samples, small primary particles have been found with a diameter of about 3 nm. At the gel point, cylindrical clusters, ca. 10 nm long, were observed as well. These grow with aging time, becoming branched. The cluster-cluster diffusion limited aggregation model is found to be consistent with the SANS results. The growth process is independent of the TiO2 content. It is mainly influenced by the ratio of the aging time to the gelation time.

The dynamical viscoelasticity of ABS melts with different particle size was investigated at various levels of rubbery phase contents. The effects of the rubber are more pronounced in the terminal zone: a transition from viscoelastic liquid to viscoelastic solid behavior was observed which can be interpreted as a physical gelation occurring at a critical rubbery phase content. This critical content resulted in being smaller in the case of smaller particles. A quantitative explanation of the experimental findings was proposed in terms of the average interparticle distance and overlapping of the chains grafted onto the neighboring rubber particles. The gel-like transition appeared to correspond to an approximately constant level of grafted chains overlapping.

The structure evolution of montmorillonite gels, during drying by evaporation, has been studied by scanning electron microscopy (SEM) after hypercritical drying with liquid CO2. Fresh Fe3+1 electrolytes allow clay particles to aggregate to floes with a structure consistent with diffusion-limited aggregation. Compression under the capillary stresses, during evaporation, leads to gelation. During further drying by evaporation, the Fe bonding is strong and prevents the complete collapse of the gel into a parallel stacking of clay plates. On the contrary, such a collapse occurs more easily when fresh Fe electrolyte has not been added.

Study of Polyolefin Gel in Organic Solvents VIII. Size of Crystalline Junction and Structure of Ethylene-Propylene Random Copolymer Gel

In this work we investigate the gel formation of EVA/recycled motor oil systems as a route to obtain synthetic binders which could be used instead of natural bitumen, as well as mixed with natural bitumen to modify adequately its viscoelastic response. The EVA copolymer studied in this work has a vinyl acetate content of 18 wt% and solutions of EVA/motor oil have been prepared up to a concentration of 50 wt%. Dynamic viscoelastic frequency sweeps between 10−2 Hz and 102 Hz have shown that above 3 wt%, at 25 °C, EVA/motor oil systems form gels. It has been possible to define an elastic equilibrium modulus, Ge, for each gel. The dependence of Ge on concentration has been compared with that of PVC/DOP and SBS/oil gels on the basis of the De Gennes model. Thermal stability of EVA/motor oil gels has also been analyzed, indicating that for the highest polymer concentrations gel point is above 80 °C.