Activation Energy For Gelation Research Articles

The dual effects of non-solvent on the sol-gel transition of polyacrylonitrile solution: The promotion in thermodynamics and hindrance in kinetics

The effects of non-solvent on the sol-gel transition of polyacrylonitrile (PAN) solution were investigated in thermodynamics and kinetics by the means of rheology method. The facilitating effects of non-solvent in thermodynamics was illustrated by the variety of viscoelasticity and gel transition temperature (Tgel). With the addition of non-solvent, the mobility of PAN molecular chains are limited due to its increased entanglement. As a result, the PAN solution with higher non-solvent content has lower conformation entropy to satisfy the occurrence of sol-gel transition at higher temperature. It means that less thermodynamic driving is demanded. The kinetical characteristics of PAN solution with different non-solvent content in sol-gel transition process were also discussed. The kinetic hindrance of non-solvent was proved by the gradually stronger time-dependence of loss tangent with the addition of non-solvent during sol-gel transition. This apparent extending of relaxation time is resulted from the more restriction of the mobility of PAN molecules. The gelation activation energy (Egel) was calculated according to Ozawa method. The results also indicated that the activity of PAN molecules in this transition became weaken with the addition of non-solvent since a relative tight connection between PAN molecules was there. Therefore, the formation of physical junction points for sol-gel transition was hindered due to the decreasing of the mobility of self-adjustment for PAN molecules. This is the fundamental reason for the kinetic hindrance role of non-solvent.

Alkynyl-functionalized benzoxazine containing phthalide side group: Synthesis, characterization and curing mechanism

Alkynyl-functionalized benzoxazine (A-BZ) resin containing phthalide side group with excellent heat resistance has been successfully synthesized with phenolphthalein, meta-aminophenylacetylene and formaldehyde solution as starting materials. Its chemical structure was characterized and analyzed by Fourier transform infrared (FT-IR) spectroscopy and hydrogen nuclear magnetic resonance (1H-NMR) spectroscopy. A-BZ can easily dissolve in the common organic solvents including ethanol, acetone et al. The gelation time under different temperature of A-BZ resin was measured by dynamic mechanical analysis (DMA) according to the intersection time of the isothermal curves of the storage modulus and loss modulus. The gelation activation energy was determined by Arrhenius formula (Ea = 48.51 kJ/mol). The optimal curing procedure was established by combing non-isothermal differential scanning calorimetry (DSC) and isothermal DMA. The curing mechanism was analyzed in in-situ FT-IR method by monitoring the change of characteristic peaks under different temperature. The reaction temperatures of ring-opening reaction of benzoxazine (170 °C) and trimerization cyclization of alkynyl groups (170–200 °C) were determined. The glass transition temperature (Tg) of A-BZ cured product tested by DMA equals to 382 °C, which is much larger than those of counterparts (251–259 °C).

The interplay between gelation and phase separation in PAN/DMSO/H2O blends and the resulted critical gels

Isothermal aging experiments are carried out in Polyacrylonitrile/Dimethylsulfoxide/H2O (PAN/DMSO/H2O) blends, and rheological and optical methods are used to investigate the interplay between phase separation and gelation. Two types of gel structures are found. In most samples, gelation is induced by phase separation in PAN concentrated regions, yet it will subsequently slow the kinetics of phase separation down by altering the concentration balance and fluctuation. As a result, the drop of light transmittance becomes slower and a phenomenon called “RT reversal” is observed. At last, the phase separated morphology is replaced by gel structure, which inherits the inhomogeneity in PAN content distribution. Another type of gel grows from homogeneous solution and its structure disperses uniformly in space. Moreover, the temperature dependence of critical gel properties, like its size limits and thermodynamics, is discussed. It is found that the self-similar structure in homogeneous critical gel covers wider length scales and possesses stronger gel stiffness than those of inhomogeneous ones. The activation energy for gelation is also calculated, which reveals no obvious dependence on original PAN concentration or temperature.

Phase Behavior of Aqueous Suspension of Laponite: New Insights with Microscopic Evidence.

Investigating microstructure of suspensions with particles having anisotropic shape that share complex interactions is a challenging task leading to competing claims. This work investigates phase behavior of one such system: aqueous Laponite suspension, which is highly contested in the literature, using rheological and microscopic tools. Remarkably, we observe that over a broad range of Laponite (1.4 to 4 wt %) and salt concentrations (0 to 7 mM), the system overwhelmingly demonstrates all the rheological characteristics of the sol-gel transition leading to a percolated network. Analysis of the rheological response leads to fractal dimension that primarily depends on the Laponite concentration. We also obtain the activation energy for gelation, which is observed to decrease with increase in Laponite as well as salt concentration. Significantly, the cryo-TEM images of the postgel state clearly show the presence of a percolated network formed by interparticle bonds. The present work therefore conclusively establishes the system to be in an attractive gel state resolving a long-standing debate in the literature.

Effect of PEGDE addition on rheological and mechanical properties of bisphenol E cyanate ester

AbstractCyanate esters are a group of resins with exceptional thermal and mechanical properties and are often blended with lower cost epoxy resins. These blends are often brittle, and a toughening method, such as the addition of soft segments, is necessary. Poly(ethylene glycol) diglycidyl ether, or PEGDE, was added to bisphenol E cyanate ester as a toughening agent. PEGDE was added at loadings of 0–30 wt %, and rheological and mechanical properties were analyzed after curing. As PEGDE loading was increased, the gel point increased accordingly while the activation energy of gelation decreased, indicating PEGDE accelerated the gelation process. Dynamic mechanical analysis results indicated a decrease in glass transition temperature as the PEGDE loading increased. Three‐point bending tests yielded similar toughness to the neat BECy at low PEGDE loadings, with a dramatic increase at loadings of 25 and 30 wt % PEGDE. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Rheological Behaviors Characterization and Modeling of a Novel Three-Branched Phenylethynyl-Terminated Imide Oligomer

To prepare novel polyimides with enhanced thermal stability and low melt viscosity, a novel three-branched phenylethynyl-terminated imide oligomer was introduced. The oligomer can be used to prepare high performance resin-based composite material via resin transfer molding (RTM) due to its low melt viscosity (<2Pa.s) between 250°C and 320°C. The cured resin exhibits excellent thermal stability and higher glass transition temperature than PETI series as a result of the introduction of star-branched units. In this research, the rheological properties of the oligomer were measured and numerically fit with the dual Arrhenius model to predict the progression of the viscosity during cure. The calculated kinetic activation energies for gelation with two different Arrhenius equations, 120.8kJ/mol and 164kJ/mol, respectively,had some differences. The numerical results were compared with the experimental measurements, and it was found that the model predicts the experimental observations quite well.

Colloidal and chemical aspects of nanosized hydrated zirconium dioxide synthesized via a sol–gel process

The processes of coagulation and gelation of sols in zirconium oxychloride aqueous solutions and the properties of hydrated zirconium dioxide sols (pH, particles size, dynamic viscosity) depending on the aging conditions of partially neutralized zirconium oxychloride feedstock solution–zirconyl hydroxychloride used for the synthesis and molar ratio of initial reagents (Zr/NaOH, Zr/CH 3COO −) were first studied in the paper. The concentration and temperature limits of hydrated zirconium dioxide sols stability were determined. The coagulation constant, gelation activation energy, and gelation thermal effect values were found.

Kinetic studies of a composite carbon nanotube-hydrogel for tissue engineering by rheological methods

Here we used rheological methods to study the gelation kinetics of silanized hydroxypropylmethylcellulose (HPMC-Si) hydrogel for tissue engineering. Firstly, the gelation time was determined from the independence of tan delta on frequency, and the Arrhenius law was applied to obtain the apparent activation energy of gelation, which was found to be about 109.0 kJ/mol. Secondly, the gelation process was monitored by measuring the sample storage modulus. The results showed that the gelation process could be well classified as a second-order reaction. In addition, a composite HPMC-Si/MWNTs hydrogel system for potential cartilage tissue engineering was investigated. The comparison of pure HPMC-Si hydrogel and composite HPMC-Si/MWNTs systems indicated that the addition of MWNTs could increase the mechanical strength of hydrogel without changing the gelation mechanism of the system.

Crosslinking of mixtures of diglycidylether of bisphenol‐A with 1,6‐dioxaspiro[4.4] nonan‐2,7‐dione initiated by tertiary amines: III. Effect of hydroxyl groups on network formation

AbstractBACKGROUND: Blends of epoxy resin oligomers, diglycidylether of bisphenol‐A (DGEBA), and a bislactone, 1,6‐dioxaspiro[4.4]nonan‐2,7‐dione (s(γBL)), were anionically copolymerized using two tertiary amines as anionic initiators. Their curing rheology and gelation behaviour were studied to provide a more comprehensive knowledge of the curing of these previously studied systems.RESULTS: The activation energy for gelation was found to be similar to that previously measured using differential scanning calorimetry and appeared to increase in the presence of the bislactone. The reaction rate during copolymerization of DGEBA with s(γBL) was slower than DGEBA homopolymerization alone because the alkoxide attack on the epoxide is faster than the reaction of the carboxylate ion and the epoxy group. The effect of the initiator type on the gel conversion was small and was presumably due to differences in the kinetic chain length caused by amine regeneration from the quaternary amine. For the same initiator and at a constant ratio of DGEBA/s(γBL), an increase in the hydroxyl concentration of the DGEBA oligomer raised the gel conversion. For a DGEBA oligomer with low hydroxyl levels, an increase in the concentration of s(γBL) increased the gel conversion; however, for a DGEBA oligomer with high hydroxyl levels, increasing s(γBL) concentration decreased the gel conversion.CONCLUSION: These results are interpreted in terms of the effect of initiation rate and chain transfer rate on the kinetic chain length. The glass transition temperature of the gel was found to be controlled by the fraction of the aliphatic s(γBL) and the amount of plasticizing sol in the matrix. Copyright © 2009 Society of Chemical Industry

Caustic‐induced gelation of β‐lactoglobulin

SummaryThe gelation kinetics of β‐lactoglobulin (βLg) solutions has been determined in the alkaline regime over a wide range of protein concentrations, gelling temperatures and gelation pH (pHgel), set using NaOH. The behaviour is compared with caustic‐induced gelation of whey protein concentrate and the alkaline dissolution of heat‐induced whey gels. The gelation time decreases significantly between neutral conditions and pHgel 9, because of the activation of the free cysteine groups and displacement to the monomeric form, and between pHgel 10 and 11, due to the base denaturation of βLg. Both transitions are associated with a significant decrease in the activation energy of gelation. At pHgel >11.5 the gelation time is observed to increase with pHgel, owing to destruction of interprotein crosslinks. These results are consistent with the recently reported observation that a minimum pH for the dissolution of βLg gels and aggregates exists around 11.6 [Biomacromolecules8 (2007) 1162]. This phenomenon has been assigned to the destruction of non‐covalent interactions that would inhibit the final percolation of the gel.

Effects of Concentrations of Methyl Ethyl Ketone Peroxide on Gelation Characteristics and Shrinkage of Solventless Polyester Varnish

The effects of concentrations of methyl ethyl ketone peroxide (MEKP) on the gelation characteristics and the shrinkage properties of solventless polyester varnishes were studied. Activation energies for gelation at different concentrations (0.75, 1.0, 1.5, 2.0, 2.5, 3.0 and 5.0 phr) of MEKP were 56.2, 54.3, 53.9, 51.5, 49.4, 44.0, and 41.6 kJ/mole, respectively. The mechanism of gelation and the kinetic rate equation are proposed to define the possible reasons for the variation of the above-mentioned varnish properties in the presence of the catalyst.

Effect of urea on solution behavior and heat-induced gelationof chitosan-β-glycerophosphate

In this study, we have investigated the effect of urea on the physicochemical (pH and conductivity) and rheological properties of the chitosan- β-GP system in order to assess the main polymer–polymer interactions at low and high temperature. The pH of the solutions was slightly increased due to the consumption of H + in solution through the hydrolysis of urea. Furthermore, the addition of urea considerably decreased the conductivity, and therefore the ionic strength of the solutions, and this effect was more important at high temperature. It indicated that urea strongly affects polymer–polymer interactions by weakening hydrogen bonds at low temperature, but in addition can hinder hydrophobic effect at high temperature since the reduction of ionic strength results in less screening of electrostatic repulsion between protonated glucosamine groups. At 15 °C, the addition of urea to chitosan- β-GP solutions decreased their elasticity, shortened relaxation times and simplified the relaxation process due to the disruption of hydrogen bonds. Heat-induced gelation of the chitosan system in the presence of urea showed higher gelation temperature ( T gel) in non-isothermal tests and longer gelation time ( t gel) in isothermal conditions. The activation energy for gelation also increased with increasing urea concentration. We concluded that the detrimental effect of urea on the gelation process was mainly related to a decrease in polymer-polymer hydrophobic effect, as shown by the decrease in conductivity.

Thermal Cross‐Linking of Poly(Vinyl Methyl Ether). II. Rheological Behavior at the Gel Point

Isothermal time evolution measurements at different constant temperatures (170°C, 180°C, and 190°C) over a wide range of frequency for the thermal cross‐linking process of poly(vinyl methyl ether), PVME, have been investigated rheologically. At the onset of cross‐linking (t onset) the elastic storage modulus, G′, increases abruptly. The magnitude of the elevation in G′ and the value of t onset were found to be temperature‐dependent. Similar behavior was observed for both the viscous loss modulus, G″ and the complex dynamic viscosity, η*; however, the value of G″ shows a very low sensitivity to the cross‐linking process compared to G′ and η* at the same experimental conditions. The gel point, t gel, was evaluated from the point of intersection in plots of tan δ vs. curing time for different constant shear frequencies. At the gel point tan δ is no longer frequency‐dependent, and all curves cross‐over, indicating the validity of the Winter–Chambon criterion. The value of t gel obtained from the coincidence of G′ and G″ was about 10 min longer than that determined from tan δ vs. t, indicating that the crossover of G′ and G″ is not be considered as a general method for evaluation of t gel. The value of the apparent activation energy of gelation determined from the temperature dependence of t gel was 74 kJ mol−1 in good agreement with literature values for other different systems. At the gel point G′ and G″ showed a power law with shear frequency, i.e., G′ ∼ G″ ∼ ω n with critical exponents equal to 0.64 and 0.75, respectively, in close agreement with the percolation theory (n = 2/3). The zero shear viscosity, η0, and the equilibrium shear modulus, G eq, can also be expressed in power low scaling functions with the relative distance from the gel point, ϵ i.e., η0 ∼ ϵ−k and G eq ∼ ϵ z with k = 1.3 and z = 2.4 in good agreement with the predicted values based on the percolation theory.

Thermoreversible Gelation of Poly(Vinylidene Fluoride) in Propylene Carbonate#

Thermoreversible gelation of poly(vinylidene fluoride) (PVDF) in propylene carbonate (PC) solutions at relatively low concentrations (0.5–3.5 wt.%) has been studied in terms of the gelation time (or rate) as a function of temperature and polymer concentration. A master curve could be made for the gelation time–temperature curves using the temperature shift based on the difference in the gel melting temperature (T m g) at different PVDF concentrations. The apparent activation energy of gelation was estimated to be proportional to (T m g − T)−1.8. The gel melting enthalpy (ΔH m g) determined from the modified Eldrige–Ferry equation was 18.67 kJ/mol. In dried gel films many spherulites connected by tie molecules were observed by scanning electron microscopy (SEM). From the wide angle x‐ray diffraction (WAXD) measurement, crystallites from PVDF gels were found to consist only of the γ‐type crystal with the TTTGTTTG¯ conformation, irrespective of the polymer concentration. As judged from the results obtained, the gelation of PVDF in PC seems to proceed mainly via liquid–liquid phase separation (even if followed by crystallization in the final stage) within the concentration range covered. #Dedicated to Prof. Won‐Jei Cho on the occasion of his retirement.

A novel thermally-activated crosslinking agent for chitosan in aqueous solution: a rheological investigation

The use of 2,5-dimethoxy-2,5-dihydrofuran (DHF) as a temperature-controlled gelation agent for chitosan under acidic conditions has been examined by dynamic oscillatory and viscometry techniques. In particular, the rate and extent of gelation have been examined over a range of different temperatures (40–98 °C), DHF concentrations (10–100 mM) and pH conditions (0.9–2.1). The gelation time, tG, decreases, and the maximum gelation rate increases substantially as a function of rising temperature. When fit with a simple Arrhenius function, the tG data yield an activation energy for gelation of 55±8 kJ mol-1. Gelation is found to occur on the shortest time-scale, and the strongest gels result, at the highest DHF concentrations investigated. Similarly, the gelation rate and gel strength are highest for the most acidic solution conditions examined. Experimental findings are interpreted in terms of a competition between the crosslinking reaction (which drives gel formation, and is initially dominant) and protolytic decomposition of chitosan (which disrupts the gel structure, and becomes increasingly important as time progresses). Syneresis phenomena additionally impact results obtained at DHF concentrations ≥50 mM.

Suspension polymerization casting of lead zirconate titanate, part I: Acrylamide hydrogel system

Suspension polymerization casting of lead zirconate titanate (PZT) filled acrylamide systems was studied. A high solid loading (51 vol%) PZT slurry with low viscosity (about 280 mPa·s at shear rate 10 s−1) was obtained by optimizing the dispersant amount. The polymerization process for the monomer solution and PZT slurry were characterized with the help of the storage modulus measurement. For the monomer solution, the overall activation energy of gelation was calculated as 60–76 kJ/mol, while for the PZT slurry, this energy increased to 91 ± 9 kJ/mol. The drying, burnout and sintering processes were also addressed.

Development of a two‐stage, dual‐Arrhenius rheology model for a high‐performance phenylethynyl‐terminated poly(etherimide)

AbstractA two‐stage, dual‐Arrhenius rheology model was successfully utilized to model the isothermal complex viscosity of a phenylethynyl‐terminated poly(etherimide) as a function of time and temperature over the experimental temperature range of 325°C to 350°C. Union of the dual‐Arrhenius model with a previously developed combination reaction kinetics model provided a chemoviscosity model giving viscosity as a function of degree of cure. A two‐stage, versus a single, dual‐Arrhenius model was used to accommodate a transition in the reaction process from chain growth to branching and network forming that was found to onset around a degree of cure of 0.37. The reduction in chain mobility brought on by branching and networking was quantified by the almost order of magnitude increase in the apparent activation energy for initial viscosity from the chain growth stage to the networking stage. The calculated apparent activation energies for gelation for the first and second stages, 2.14 × 105 J/mol and 2.05 × 105 J/mol, respectively, agree well with results presented elsewhere from differential scanning calorimetry measurements. The experimental procedures followed to obtain the rheological data and the construction of the model from these data are described; the predictions of the model are compared with experimental results.

Rheological analysis of the gelation behavior of tetraethylorthosilane/ vinyltriethoxysilane hybrid solutions

The gelation behavior of tetraethylorthosilane (TEOS) sol in the presence of a silane coupling agent vinyltriethoxysilane (VTES) was investigated experimentally. Specifically, the gel time and gel structure were analyzed by the rheological multiwave test of Fourier transform mechanical spectroscopy (FTMS) for the mixtures of TEOS/ VTES with various molar ratios at different temperatures. Gelation was accomplished through the sol-gel reactions of the silicon alkoxide TEOS in aqueous acidic solution. The results showed that, at elevated temperatures, the gel point was scattered and obscured by the noise in low frequencies owing to the enhanced thermal agitations. In this case, the statistical method was used to find the exact gel time. The activation energy of gelation and the gel exponent ranged from 25 to 30 kJ/mol and from 0.581 to 0.771, respectively, depending on the TEOS/VTES composition. The fractal dimension was estimated from the gel exponent and indicated the gel structure and that the TEOS/VTES solutions formed a relatively open and coarse gel structure.

In situ monitoring of energy release in real time during gelation of acrylamide (AAm)

Temperature variation and optical transmission was monitored in real time during the free radical crosslinking copolymerization (FCC) of acrylamide (AAm) and N,N′-methylenebis(acrylamide) (Bis). Gelation experiments were initiated at various onset temperatures (Ton). During gelation the temperature rose then decreased to its initial value as the gelation time increased. That is explained with the energy release (Gelation activation energy) during the polymerization process. Gelation activation energies, ΔEG were measured and found to be increased as the onset temperature, Ton was increased. It was observed that transmitted photon intensities, Itr, decreased dramatically at the reaction time, which were attributed to the increase in scattered light intensity, Isc, during the formation of interconnected microgels in the system. Increase in Isc was modeled using Rayleigh's equation where reaction time was found to be proportional to the volume of the microgels. It was observed that a number of microgels increased as the onset temperature, Ton was increased.

Rheological study of crosslinking and gelation in bismaleimide/cyanate ester interpenetrating polymer network

AbstractIn searching for high performance polymer resins that have a combination of low dielectric constant and loss, high temperature resistance, ease of being processed, and other desirable properties, an interpenetrating polymer network (IPN) based on cyanate ester (CE) and 2,2′‐diallylbisphenol A (DBA) modified bismaleimide resin (BMI) was prepared via prepolymerization followed by thermal curing. This work discusses the use of multiple waveform rheological technique to investigate the crosslinking and gelation behavior of this resin system at various temperatures. The gel point can be accurately determined from a single experiment using this technique. At the point of gelation, both the storage modulus (G′) and loss modulus (G″) of the IPN follow a similar power law equation with oscillation frequency used in the rheological measurement. Both the relaxation exponent n, a viscoelastic parameter related to the cluster size of the gel, and gel strength S, related to the mobility of the crosslinked chain segments, were determined via a curve fitting method. Both n and S were found to be temperature dependent in this BMI/DBA–CE IPN system. The apparent activation energy of gelation or curing reaction was found to be approximately 47.6 kJ/mol. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 2437–2445, 2001