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The effects soybean oil volume fraction on the rheological characteristics and gelation behavior of acid and salt-induced soft tofu-type gels was investigated. High oil volume fraction resulted in lower gel point temperature and shorter gelation time (tgel) while G′ at the gel point increased as the oil volume fraction in the gels increased. A power law relationship tgel, gel point temperature and oil concentration was found and also the plot of G′ versus oil concentration fitted well in the model: G′∼CA. The exponent “A” was positive in salt-induced gels but negative in acid-induced gels. This gelation behavior trend was demonstrated by estimating the composite gel shear modulus, which showed that, in salt-induced gels the stiffness of the oil droplets was higher than the one of the gel matrix while it was the opposite in acid-induced gels. Furthermore, the weak gel model was used to analyze and describe the network extension and its strength. Practical Applications In this comparative study, soybean oil volume fraction effects on the rheological characteristics and gelation behavior of acid and salt-induced soft tofu-type gels was established. Different coagulation temperatures and coagulants were used. The study gives new insights on the protein-to-oil droplets interactions during gelation at different pHs in a tofu gel like product. The better understanding of the gelation mechanism, small and large scale deformations of soy protein gels with additional oil, is industrially relevant, as it will help in better process line design, monitoring and quality control which would potentially lead to the development of novel soy-based and dairy free cheeses products. Additionally, the parameters and conditions used were closely related to the ones used during soft silken tofu making, therefore, ensuring the process implementation much easier.

A three-dimensional DNA hydrogel was generated by self-assembly of short linear double-stranded DNA (dsDNA) building blocks equipped with sticky ends. The resulting DNA hydrogel is thermoresponsive and the length of the supramolecular dsDNA structures varies with temperature. The average diffusion coefficients of the supramolecular dsDNA structures formed by self-assembly were determined by diffusion-ordered NMR spectroscopy (DOSY NMR) for temperatures higher than 60 °C. Temperature-dependent rheological measurements revealed a gel point of 42±1 °C. Below this temperature, the resulting material behaved as a true gel of high viscosity with values for the storage modulus G' being significantly larger than that for the loss modulus G''. Frequency-dependent rheological measurements at 20 °C revealed a mesh size (ξ) of 15 nm. AFM analysis of the diluted hydrogel in the dry state showed densely packed structures of entangled chains, which are also expected to contain multiple interlocked rings and catenanes.

Three room-temperature ionic liquids (ILs), differing only in anion identity [1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate (EMI FAP), 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMI TFSI), and 1-ethyl-3-methylimidazolium tetracyanoborate (EMI TCB)], are employed to directly compare the mechanical and electrical responses of cross-linked, poly(ethylene glycol) diacrylate (PEGDA)-supported ionogel solid electrolytes. It is observed that the choice of IL influences the minimum amount of polymer required to create a non-flowing gel, but not the value of the Young’s modulus for a given weight fraction of PEGDA above the gelation point. Furthermore, the relative decrease in ionic conductivity above the gel point is also found to be IL-independent. These observations suggest that IL contributions to the character of these ionogels can be distinguished from the contributions of the supporting polymer scaffold. In addition, an extremely large ionic conductivity value (13.1 mS cm−1) is achieved at room temperature using EMI TCB at its gelation point (7.2 wt % PEGDA).

Copolyimides were synthesized in N-methyl-2-pyrrolidone (NMP) to prepare polyimide (PI) solutions with various concentrations, and gels can form in PI/NMP solution with a suitable concentration or at a low temperature. Gel–sol transition in the PI/NMP solution has been investigated through a combination of polarized optical microscope, differential scanning calorimeter measurement, and rheological measurements. According to Winter–Chambon theory, the critical gel point is 12 wt %, and the loss tangent (tan δ) shows frequency independence. Gel–sol transition temperature (Tgel) for the 13 wt % solution is around 65 °C. Below the Tgel, the gel exhibits strong optical planar texture, indicating the formation of a fully anisotropic phase, which has a great importance for the fabrication of PI fibers by gel-spinning. Dynamic temperature sweep measurements show that the formation of the gel from solution is thermoreversible and is significantly delayed in the cooling process. Under the favorable conditions, the tensile strength and modulus of the fibers derived from the solution are about 0.7 and 12.5 GPa using gel-spinning method, which is believed to afford broad application potential in preparing high-performance PI fibers. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 450–459

In the present work, a kinetic model of crosslinking free-radical copolymerization based on multidimensional population balances and accounting for multiradicals is developed. The model is applied to the simulation of bulk copolymerization of methyl-methacrylate (MMA)/ethylene-glycol-dimethacrylate (EGDMA). A literature criterion proposed to elucidate the model type best suited for a given system (i.e., with or without MRs) is extended to the industrially relevant case of diffusion limited systems. Moreover, a master plot for the system under investigation is proposed: given the reaction recipe, the error on the gel point prediction employing the monoradical assumption is identified, thus allowing more conscious model selection.

The modeling approach called “numerical fractionation” has been incorporated into a PREDICI model to simulate crosslinking copolymerization. In order to take inhomogeneities of the considered copolymerization into account, the kinetic parameters of the model are proposed to be different for each generation of the numerical fractionation. Using this approach the chain-length dependence of the termination has been incorporated into the model so that even the method of moments could be applied to study crosslinking copolymerization. The styrene/m-divinylbenzene crosslinking copolymerization at low crosslinker content has been simulated. The chain-length dependence of the termination has been found to accelerate the gel point in monovinyl/divinyl copolymerization and must be taken into account for a correct description of the gel points.

The dynamic behaviour during the crosslinking of an epoxy polymer near the gel point was monitored using rheological multiple frequency experiments. The influence of a needle-shaped inorganic nanofiller, sepiolite, either non-modified or organically surface modified during the cure process in the presence of an aliphatic and an aromatic hardener was investigated. The validity of various criteria for determining the gel point was examined for the crosslinking of these filled thermosets. The Winter–Chambon criterion at the gel point is obeyed by the unfilled and by the non-modified sepiolite-filled epoxy matrix with either of the two hardeners. However, physical gels are formed in the presence of the organically modified sepiolite and the Winter–Chambon criterion is not valid. For all the systems investigated, the nanofiller reduces the time to reach gelation. Critical relaxation exponents and gel strength have been determined indicating a more elastic and stronger gel in the presence of the aliphatic hardener. © 2012 Society of Chemical Industry

Gel time of a glass/epoxy prepreg, HexPly®1454, was investigated by a parallel plate rheometer. The prepreg is based on dicyandiamide (DICY)-cured diglycidyl ether of bisphenol A epoxy resin system. It is found that the application of the G′-G″ crossover method for gel time determination is not suitable for this system. A new approach was proposed in which the maximum tan δ is regarded as the gel point. This can accurately define the gel point at various temperatures. The results proved that this point is independent of the applied frequency. The activation energy for the cure reaction of the system was determined via gel time determination of the prepreg at different isothermal temperatures and found to be 75.0 ± 10.2 kJ/mol. This is in good agreement with the activation energy obtained from the dynamic DSC studies. The steady-shear rheology experiment was used to study the viscosity profile and subsequently to determine the gel point and verify the new approach for gel time determination. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

The photoreactivity of polystyrene bearing photoreactive benzil (BZ) pendant groups was studied. Upon irradiation (λ > 370 nm), BZ groups were transformed to benzoyl peroxide (BP) groups that caused crosslinking of the polymer upon heating. The main goal of this study was to characterize the final network. Melt viscoelasticity is a powerful method to assess some specific parameters of a crosslinking process (gel point, crosslink density). Classical methods based on low-frequency slope variations, such as the Winter and Chambon method, were successfully applied. A good correlation was established between the content of BP groups after the first irradiation and the final density of the network after crosslinking. It was determined that at least two peroxidic species (BP groups) per macromolecular chain are necessary to reach the gel point. The concentration of BZ groups and their conversion into BP groups are the factors supporting a denser network. Copyright © 2010 Society of Chemical Industry

In this work, the rheological characterization of the gel point in polymer-modified asphalts is carried out. The viscoelastic properties of polymer-modified asphalts, in which the polymer is styrene–ethylene butylene–styrene (SEBS) with grafted maleic anhydride (MAH), were measured as a function of MAH concentration. The crosslinking reaction that leads to gelation is characterized by power-law frequency-dependent loss and storage modulus (G″ and G′). The relaxation exponent n (a viscoelastic parameter related to the cluster size of the gel) and gel strength S (related to the mobility on the crosslinked chain segments) were determined. The value of the power-law exponents depends on the composition of polymer, ranging from 0.30 to 0.56, while the value of the rigidity modulus at the gelation point (S) increases with the amount of reactive groups of the modifier polymer. Both n and S are temperature-dependent in the blends. The blends containing gels present a coarse morphology, which is related to the rheological properties of the matrix and dispersed phase. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

A new, nickel-coated graphite resistance-change-based method for gel-point determination for epoxy-based thermoset resins is presented and compared with {DSC} and rheological methods. Gelation times determined by this new method are in very good agreement with conventional techniques; this new method is potentially simpler and less time consuming than existing ones. ?? 2010 {WILEY-VCH} Verlag {GmbH} \& Co. {KGaA}, Weinheim.

We investigate the phase transition behavior and dissolution resistant properties of thermo-sensitive nanocomposite hydrogels made from PEO-PPO-PEO triblock copolymer (Pluronic F127) and Laponite silicate nanoparticles. The rapid dissolution properties of F127 copolymer hydrogels usually limit their use as sustained release drug carriers. We overcome this limitation by synergistic combination of nanoparticle gelation characteristics with polymer thermo-sensitivity. We present a proof of concept that the temperature-dependent phase transitions can be shifted as a function of hydrogel composition and that the dissolution of the polymer hydrogels as well as the release of a model drug, albumin, can be significantly slowed down by addition of nanoparticles. The dissolution resistant properties generated will prove useful in the future formulation, processing and application of our polymer hydrogels for sustained release drug delivery carriers.

No difference in the actual gel points was substantially observed among three isomeric diallyl phthalates such as diallyl phthalate (DAP), diallyl isophthalate, and diallyl terephthalate (DAT); this interesting gelation behavior was discussed further in terms of the correlation between gelation and the difference in cyclization modes, and also, the difference in reactivity between the uncyclized and cyclized radicals for cross-linking. In the present work, we tried to extend the preceding discussion to the polymerization of triallyl trimellitate (TAT) because the molecular structure of TAT is presumed to essentially involve the characteristics of three isomeric diallyl phthalates and, therefore, the enhanced gelation was expected in TAT polymerization. However, no enhancement of gelation was observed. For a full understanding of the gelation in multiallyl cross-linking polymerization, we explored further the polymerizations of DAP, DAT, and TAT, especially focusing on the characterization of resultant network polymer precursors (NPPs) using SEC-MALLS-viscometry providing the correlation of [η] versus Mw of fractionated samples. Notably, the structure of NPP consisting of oligomeric primary polymer chains generated from specific allyl polymerization would become core-shell type dendritic with the progress of polymerization. The correlation between delayed gelation and decreased reactivity of dendritic NPP for intermolecular cross-linking is discussed. Conclusively, the reactivity for intermolecular cross-linking between NPPs decreased with the progress of polymerization leading to a delayed gelation. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2871–2881, 2009

An investigation was reported here with an aim to prepare nanocomposite thermoplastic elastomer gels by dissolving polystyrene-b-poly(ethylene/butylene)-b-polystyrene (SEBS) triblock copolymer in selective hydrocarbon oils with the presence of multiwalled carbon nanotubes (MWCNTs). The properties related to morphology, viscoelasticity, electrical and mechanical properties, and thermal stability were explored and discussed. Dynamic rheological measurements of the resultant nanocomposite thermoplastic elastomer gels (NCTPEGs) confirmed that addition of MWCNTs affects the linear viscoelastic properties in which dynamic storage and loss moduli increase to some extent. At a temperature between 30°C and 40°C below the gel point the NCTPEGs have dynamic storage modulus greater than loss modulus (G′ and G″), thereby indicating that at room temperature a physical network is still present despite the addition of MWCNTs. The morphological properties revealed that MWCNTs were dispersed and exfoliated within the swollen TPE. The incorporation of small quantity of MWCNTs improved the thermal stability and mechanical properties of NCTPEGs. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers

In this work, we investigated the effect of expandable graphite (EG) on the property of triblock copolymer prepared from a poly(styrene-b-(ethylene-co-butylene)-b-styrene) (SEBS) imbibed with an EB-compatible hydrocarbon oil. The rheological properties showed that at a temperature between 30 and 40°C below the gel point, the triblock copolymer gels had a dynamic storage modulus (G′) greater than loss modulus (G‵), thereby indicating that at ambient temperature, a physical network is still present in spite of the addition of nanoparticles. Dynamic rheological measurements of the resultant nanocomposite triblock copolymer gels confirmed that the addition of EG affects the linear viscoelastic properties and maximum operating temperature of the parent triblock copolymer gels. The mechanical properties showed only marginal increase, which can be attributed to the poor dispersion that leads to agglomeration of particle into micrometer size stacks, and thus the particles behave only as inorganic fillers. The morphology and X-ray diffraction revealed that the EG used to generate nanocomposite triblock copolymer gels is dispersed generally within the swollen copolymer and/or solvent. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers

The confined space produced during the polymerization has access for all small organic molecules or oligomers with small size to enter this confined space; however, it can prevent the macromolecules with big size from entering. Therefore, the reaction between two branched macromolecules is excluded in A2+B3 polymerization system, resulting uncrosslinked branched polymers, and there was no gel point observed throughout the polymerization. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1730–1737, 2008

ABA and BAB triblock thermoresponsive copolymers (A ¼N-isopropylacrylamide, B ¼2-hydroxyethyl methacrylate) have been synthesized by atom transfer radical polymerization (ATRP). BAB is shown to exhibit a higher transition temperature and a good solubility in water comparedtoABAwith thesamecomposition andconcentration.Thedifferences areattributed to the distinct micellar structure, which is regulated by the block order of the copolymers. It is speculated that ABA and BAB copolymers separately form branch and flower micelles in water, which mainly influence the course of phase transition. The moduli of the ABA copolymer solution are higher than those of the BAB above gel point. In terms of hypothesized micelle models, it is proposed that the ‘branch’ micelles of the ABA solution preferably form more physical interconnections.

The random end linking of different amounts of trifunctional crosslinkers with 3 000 prepolymer linear chains, with length varying from 10 to 30 monomers, to form networks at different system number densities was dynamically simulated by the molecular dynamics method. Investigation of the crosslinking kinetics shows that, with a stoichiometric number of crosslinkers present, the time evolution of free ends fraction decays as a power law in time t -3/4 . This scaling behavior is different from the one in a dense polymer melt. Structural analyses of the resulting networks indicate that the imperfections of the network structure strongly depend on the initial synthesis conditions including the initial system number density and the ratio of crosslinkers to precursor polymer chain ends.

The effect of several network-forming nanoscale materials such as two different types of graphite and multiwalled carbon nanotube on the property development of thermoplastic elastomer (TPE) gels prepared from microphase- ordered poly(styrene-b-(ethylene-co-butylene)-b-styrene) (SEBS) triblock copolymer dissolved in paraffin oil was studied. Dynamic rheological measurements of the resultant nanocomposite TPE (NCTPE) gels showed that at temperature between 30 °C to 40 °C below the gel point, the NCTPE gels have dynamic storage modulus greater than loss modulus (G′ and G″), thereby indicating that at ambient temperature a physical network is still present despite the addition of nanoparticles. In general, the nanoparticles lower the gelation temperature. The X-ray diffraction of NCTPE gels showed that EG2 system exhibited intercalation, those with CNTs exhibited exfoliation and EG1 did not change at all.

A simple method of determining crosslink yields and examining crosslink statistics in polymers is described. In this method the soluble fraction of materials crosslinked to well above their gel point is examined with gel permeation chromatography and compared with the original material. The results of such an examination of the soluble fractions of irradiated polyolefins indicate that the random statistical crosslinking mechanism first suggested by Charlesby is followed closely by low-density polyethylenes. The high-density polyethylenes examined show deviations from statistical behavior, which may be attributed to dimerization or endlinking through vinyl decay. However, other factors may also be involved. The results suggest that G (crosslinks) does not change significantly with dose and that the ratio of chain scission to crosslinks is less than 0.03 in the range up to 80 Mrad in a low-density polyethylene.

Use of the Wesslau molecular weight distribution equation in the equations relating production of gel to total radiation dose leads to equations that can be solved only by means of computer techniques. This solution has been carried out for a number of initial values of the weight-average to number-average molecular weight ratios and for a number of ratios of chain degradation G values to crosslink G values. It is shown that the initial slope of the curve of s + s1/2 versus rg/r, where s is the soluble fraction after a dose r, and rg is the dose to the gel point, can be related to the type of initial molecular weight distribution in the polymer. For the first time definite evidence of an increase of the crosslinking rate with dose has been obtained.

The gel properties of two-component mixed polymer gels are investigated using a cascade model, which assumes that the gel network is formed via the self-association of one of the two components and the cross-association of the two components. The effects of the model parameters, such as the equilibrium constants and the functionalities for cross-associations and self-associations, on the composition dependence of the modulus and gel point curves are examined to elucidate the contribution of self-associations to the gel network. The results show that the characteristics of self-associations become pronounced when the equilibrium constant or the functionality for self-associations is comparable to that for cross-associations. The model is applied to analyze the critical gelling concentration data for xanthan/locust bean gum mixed gels, which shows significant self-associations at high xanthan compositions. The resulting model curves agree well with the experimental data at all temperatures. The analysis of the temperature dependence of the best-fit equilibrium constant yields values of enthalpy change that are consistent with previous findings. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 80–91, 2008

A computer-based simulation for rigid polyurethane foam-forming reactions was compared with experimental data for six blowing agents including methyl formate and C5-C6 hydrocarbons. Evaporation of blowing agent was modeled as an overall mass transfer coefficient times the difference in activity of the blowing agent in the gas foam cells versus the resin walls of the cells. Successful modeling hinged upon use of a mass transfer coefficient that decreased to near zero as the foam resin approached its gel point. Modeling on density agreed with experimental measurements. The fitted parameters allowed for interpretations of the final disposition of the blowing agent, especially, if the blowing agent successfully led to larger foam cells versus being entrapped in the resin. The only component-specific fitted parameters used in the modeling was the activity coefficient that was lower for methyl formate than the value used for hydrocarbons. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42454.

Dynamic viscoelastic properties of poly(vinyl chloride) (PVC)/bis(2-ethylhexyl) phthalate (DOP) and PVC/di-n-butyl sebacate (DBS) gels with molecular weight distribution (M w /M n ), of 2.16 and various polymer concentrations c, have been studied as a function of temperature. These PVC gels exhibited an elastic solid at room temperature T, and gradually became liquid (sol) with increasing temperature. The sol-gel transition took place at a critical gel temperature at which the scaling law of G'(ω) ∼ G(ω) ω n held, allowing an accurate determination of the critical gel temperature by means of the frequency w independence of the loss tangent. In this study the scaling exponent n, was 0.75-0.77. This is in good agreement with the previous results observed at different temperatures and suggests the formation of a similar fractal structure of the PVC gels. The gel strength S g , at the gel point increased with increasing PVC concentration. These results suggest a unique character and structure for the gel points of PVC-plasticizers.

In our previous study [J. Yang, L. Jia, L. Yin, J. Yu, Z. Shi, Q. Fang, A. Cao, Macromol. Biosci. 2004, 4, 1092.], new biodegradable copolymers of diblock methoxy poly(ethylene glycol)-block-poly(epsilon-caprolactone) and methoxy poly(ethylene glycol)-block-poly(delta-valerolactone), and triblock poly(epsilon-caprolactone)-block-poly(ethylene glycol)-block-poly(epsilon-caprolactone) and poly(delta-valerolactone)-block-poly(ethylene glycol)-block-poly(delta-valero-lactone) bearing narrow molecular weight distributions and well-defined block architectures were reported to be prepared with our original aluminum metal complex templates. This work will continue to report new investigations on their water solubility, and reversible thermal responsive micellization and solution to gel transition in distilled water. Among the new synthesized copolymers (P1-P23), seven diblock or triblock samples (P3, P6, P7, P11, P12, P19, and P21) with higher hydrophilic building block populations were revealed to be water soluble under ambient temperature. By means of UV spectrophotometer attached with a thermostat, important parameters as critical micellization mass concentrations (CMCs) and critical micellization temperatures (CMTs) were characterized for these new amphiphile dilute aqueous solution with the aid of an lipophilic organic dye probe of 1,6-diphenyl-1,3,5-hexatriene (DPH). Furthermore, the critical gelation temperatures (CGTs) were simultaneously investigated for these water-soluble block copolymers via a tube tilting method. It was found that the CMC, CMT, and CGT were strongly affected by the population and nature of the hydrophobic building blocks, and a higher hydrophobicity of the new amphiphilic block copolymer finally led to lower CMC and CMT, and higher CGT. In addition, the salts of KBr and NaCl were found to play as a salt-out effect on the solution to gel transition for the diblock P6 and triblock P11, exhibiting an interesting tunable gelation temperature close to 35-42 degrees C. These results will pave new possibility for the synthesized block structural amphiphiles as potential biomaterials to be applied in vivo.

The properties of several types of organogels that can undergo a chemical transition during gelation or while in the gel phase are described. The transitions can be physically induced by light or chemically triggered by the addition of an acid or a neutral molecule such as CO 2 or CS 2 . In some cases, the gelation properties of the new species formed are markedly different from those of the precursors. The link between molecular structure and the nature of the gel networks as well as results obtained from the multidisciplinary tools used to study them are discussed.

A kinetic model is presented for thiol-ene crosslinking photopolymerizations including the allowance for chain growth reaction of the ene, i.e., homopolymerization. The kinetic model is based on a description of the average chain lengths derived from differential equations of the type of Smoluchowski coagulation equations. The method of moments was applied to obtain average properties of thiol-ene reaction systems. The model predicts the molecular weight distribution of active and inactive species in the pregel regime of thiol-enes, as well as the gel points depending on the synthesis parameters. It is shown that, when no homopolymerization is allowed, the average molecular weights and the gel point conversion are given by the typical equations valid for the step-growth polymerization. Increasing the extent of homopolymerization also increases the average molecular weights and shifts the gel point toward lower conversions and shorter reaction times. It is also shown that the ratio of thiyl radical propagation to the chain transfer kinetic parameter (k p1 /k tr ) affects the gelation time, t cr Gelation occurs earlier as the k p1 /k tr ratio is increased due to the predominant attack of thiyl radicals on the vinyl groups and formation of more stable carbon radicals. The gel point in thiol-ene reactions is also found to be very sensitive to the extent of cyclization, particularly, if the monomer functionalities are low.

A new method has been developed to prepare smart microgels that consist of well-defined temperature-sensitive cores with pH-sensitive shells. The microgels were obtained directly from aqueous graft copolymerization of N-isopropylacrylamide and N,N'-methylenebisacrylamide from water-soluble polymers containing amino groups such as poly(ethyleneimine) and chitosan. The gel diameters ranged from 300 to 400 nm with narrow size distribution. The unique core-shell nanostructures exhibited tuneable responses to pH and temperature.

Poly(methyl methacrylate) (PMMA) gels with varying amounts of silicone and solvent and constant amounts of crosslinker were prepared by solution free radical crosslinking copolymerization of methyl methacrylate (MMA), ethylene glycol dimethacrylate (EGDM), tetraethoxysilane (TEOS) and vinyltriethoxysilane (VTES) comonomer systems. They were then studied in benzene at a total monomer concentration of 3.5 mol L−1 and 70 °C. The conversion of monomer, volume swelling ratio, weight fraction and gel point were measured as a function of the reaction time, silicone concentration and benzene content up to the onset of macrogelation. Structural characteristics of the gels were examined by using equilibrium swelling in benzene, gel fraction and Fourier-transform infrared (FTIR) analysis. The morphology of the copolymers was also investigated by SEM. Based on the obtained results, it was concluded that the FTIR data did not have the capacity to show the presence of the VTES or TEOS moiety in these kinds of copolymers. On the other hand, the variation of weight fraction of gel, Wg, and its equilibrium volume swelling ratio in benzene, qv, exhibited the same behaviour as that of MMA/EGDM copolymers. Also, the dilution of the monomer mixture resulted in an increase in the gel point and swelling degree and a decrease in the percent of conversion and gel fraction. Finally, TEOS is not an ideal silicone compound for reaction in the MMA/EGDM copolymerization system, whereas VTES is a suitable silicone comonomer for this system and it has been proved useful. Copyright © 2005 Society of Chemical Industry

Polymer dispersed liquid crystals (PDLC) were obtained by polymerization-induced phase separation of the nematic E7 liquid crystal in a reactive diepoxide–diamine thermosetting matrix prepared by polycondensation (in two thermal steps). The evolution of some important electrooptical characteristics (transmittance in the off-state, threshold voltage, maximum transmittance of the on-state, transmittance at maximum applied voltage) were studied at three different reaction conversions of the polymer matrix beyond the gel point. The electrooptical curves greatly depended on the reaction conversion, especially beyond the gel point of the polymer matrix. It was found that the transmission in both the off-state and the on-state decreased with the extent of cure. In addition, threshold and saturation voltages increased with the reaction conversion. The electrooptical curve showed unusual behavior at approximately 60% of reaction conversion (i.e., near the gel conversion). At higher conversions, the expected normal mode was recovered. We discussed these electrooptical characteristics in the light of the very significant evolution of some properties of the crosslinked polymer matrix (glass-transition temperatures, concentration of crosslink points, molar mass, and weight fraction of the residual uncrosslinked oligomers), whereas the droplet morphology did not exhibit dramatic changes. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2621–2628, 2004

Mass balance equations in terms of the moment generating function of the distribution of mole concentrations of polymer species for free radical copolymerizations of mono/divinyl monomers could be numerically solved after gel point using open source code ACDC, needed for extremely stiff two-point boundary value problems. For the first time, it became possible to compare the error of earlier well-known approximated estimation methods for the weight fraction of sol and average molecular weights to this accurate mathematical solution. It turns out that predictions by the pseudo-kinetic method are reasonable only when equal reactivity of double bonds prevails, causing early gelation in the batch reactor. Otherwise the discrepancies between the exact and approximate solutions are quite important.

Phenomenological cure kinetic relations for a commercially available amine-cured epoxy were developed independently using differential scanning calorimetry (DSC) in isothermal and dynamic (constant heating rate) modes and using numerical optimization techniques. The resulting model was found to be independent of the method used to collect reaction rate data. The model was modified at high temperature to account for an observed deviation from Arrhenius behavior. The cure dependence of the glass transition of the material was also investigated with DSC. The gel point as measured by parallel plate rheometry was found to occur at a degree of cure of 0.71, a value significantly advanced from a simple Flory prediction based on the functionality of the cross-linking agent.

At the start of our research program concerned with the elucidation of the crosslinking polymerization mechanism leading to interpenetrating polymer network (IPN) formation, in which IPNs consist of both polymethacrylates and polyurethane (PU) networks, this article deals with the polyaddition crosslinking reaction leading to PU network formation. Therefore, 2-methacryloyloxyethyl isocyanate (MOI) was radically copolymerized with methyl methacrylate (MMA) in the presence of CBr 4 as a chain-transfer agent. The resulting poly(MMA-co-MOI)s, having pendant isocyanate (NCO) groups as novel multifunctional polyisocyanates, were used for polyaddition crosslinking reactions with ethylene glycol as a typical diol. The second-order rate constants depended on both the functionality of poly(MMA-co-MOI) and the NCO group concentration. The actual gel points were compared with the theoretical ones calculated according to Macosko's equation; the deviation of the actual gel point from the theoretical value became more remarkable for a greater functionality of poly(MMA-co-MOI) and at a lower NCO group concentration or at a lower poly(MMA-co-MOI) concentration. These are discussed mechanistically, with consideration given to the significance of intramolecular cyclization and intramolecular crosslinking reactions leading to the shrinkage of the molecular size of the prepolymer, along with the data of the intrinsic viscosities of resulting prepolymers and the swelling ratios of resulting gels.

Aqueous xanthan solutions form gels when aluminium salts are added and the solutions are heated above 45 °C. The gelation process was followed by dynamic light scattering. Characterization was based on the heterodyne and nonergodic approaches. Both techniques gave the same fast relaxation times, but for the slow motion much larger values were found in the heterodyne than in the nonergodic approach. The heterodyne fraction 1-X was found to correlate closely with the plateau height of the time correlation function (TCF) at large delay times in the nonergodic experiments. Three methods of gel point determination are demonstrated: (i) onset of heterodyne/nonergodic behavior, (ii) observation of a sharp maximum for the fast relaxation time at the gel point, (iii) observation of power-law behavior of the TCF. The statistics of nonergodic fluctuations were examined and evaluated. The potential of this procedure for detailed structure evaluation of inhomogeneities in the gel is emphasized.

Because of the familiarity of gelation theories in polycondensation reaction of multifunctional groups, often the gel-point is defined as the point of diverging weight averaged molar mass. The authors present an industrially relevant counter-example to this common perception. Chain-growth polymerization in realistic reactors introduces history dependent crosslinking probability. For copolymerization of a two functional monomer (ethylene) with a four functional comonomer (nonconjugated diene), the authors show from a Monte Carlo scheme that standard gelation scaling exponents remain valid for a semibatch reactor. However, for syntheses in a continuous stirred tank reactor (CSTR), all commonly measured molar mass moments (number, weight and “z”-averaged moment) remain finite at the gel-point; the first moment to diverge is the fourth moment. Hence, identification of the gel point from experimental observations is difficult, and cannot be achieved through monitoring of the weight averaged molar mass. The authors use a numerical scheme based on the tube model of polymer melts to predict the rheology of the generated molecules. Stress relaxation follows a power-law decay, but due to dynamic dilution effects the CSTR resins exhibit much slower increase in the zero shear viscosity as the gel point is approached as compared to the semibatch reactor resins.

A dicyanate ester resin was modified by blending with polysulfone (PSF) and cured at different temperatures with or without cobalt catalyst. Size exclusion chromatography was used to determine the cyanate conversion until the gel point. The morphology of the cured samples, characterised by scanning electron microscopy, varied from PSF particle structure to a combined particle structure.

The thermal and dynamic mechanical behaviour of ordered stoichiometric as well as off-stoichiometric polyurethane networks based on mesogenic diol - 6,6'-[ethylenebis(1,4-phenyleneoxy)]dihexanol (D), 2(4)-methyl-1,3-phenylene diisocyanate (DI) and poly(oxypropylene) triol (T) is reviewed. The effects of various initial molar ratios of the reactive groups ([OH] T /[NCO] DI /[OH] D ) and of the ordered or isotropic state of the diol on the critical gel structure at the sol-gel transition and a power-law mechanical behaviour is discussed. The effects of amount of mesogenic diol in the elastically active network chains (EANCs) on the conformational rearrangements required for ordering, the concentration of permanent physical interactions (trapped entanglements) and on the intensity of the slow relaxation process in the rubbery region are shown.

Gel points, predicted using Ahmed-Rolfes-Stepto (ARS) theory and a Monte-Carlo (MC) simulation method accounting for intramolecular reaction, are compared with experimental data for polyester (PES)-, polyurethane (PU)-and poly(dimethyhl siloxane) (PDMS)-forming polymerisations. The PES and PU polymerisations were from stoichiometric reactions at different initial dilutions and the PDMS ones were from critical-ratio experiments at different dilutions of one reactant. The 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. It is shown that various gel points can be predicted by MC simulations, depending on the criterion for gelation used. Due to the lengthy computations needed and the uncertainties in the predictions, MC simulation is not a viable approach. Although inconsistencies are noted in the measured gel points, so that a unified interpretation of the data cannot be achieved, ARS theory is shown to be the preferred basis for gel-point prediction. It is also concluded that, before one can be certain of agreement between experiment and predictions, more experimental systems at different initial dilutions and ratios of reactants need to be studied and the various methods used for detecting gel points need to be compared.

A study of the dynamic viscoelastic properties of gels of Ethylene Vinyl Acetate (EVA) and Styrene-Butadiene-Styrene (SBS) copolymers in recycled motor oil is presented. Both systems form gels with enhanced elastic moduli, with respect to SBS/aromatic oil gels which have been used to develop synthetic binders. Although the procedure described by Winter is conveniently applied to obtain gel-sol 1 transition of EVA/oil gels, it is not suitable for SBS/oil gels which do not give rise to a homogeneous liquid when they melt. For EVA/oil gels the relaxation exponent at the gel point is A=0.5, which according to Muthukumar's model corresponds to a fractal dimension d f =2. The variation of the elastic modulus with polymer concentration follows the scaling law G e -c n , with n=2.8 for EVA/oil and n=1.3 for SBS/oil. In the case of EVA/oil gels the validity of theoretical models relating fractal dimension to n exponent is discussed.

Equilibrium phase diagrams in composition-conversion space have been calculated for stoichiometric blends of an epoxy resin with a diamine hardener and dendritic hyperbranched polymers, to which various numbers of epoxy groups have been grafted. An attempt has been made to incorporate both the effects of the polydispersity of the resin and the reactivity of the functionalized hyperbranched polymers. The results suggest that below the gel point, the resin-rich regions of the phase diagrams are only weakly affected by reaction of the hyperbranched polymers for any given value of the interaction parameter. However, if the reactivities of all the epoxy groups are comparable, the presence of a highly functionalized modifier is expected to lead to a reduction of the gel point conversion and a narrowing of the composition-conversion window available for cure induced phase separation. The experimental cloud point data are consistent with the model, although they suggest that the functionalized hyperbranched polymers may be significantly less reactive than the resin. Moreover, in the present system, the influence of functionalization on the phase behavior is also strongly linked to the accompanying changes in the interaction parameter.

The cloud-point temperatures (Tclo's) of poly(N-isopropyl acrylamide) (PNIPAM)/water solutions with NaCl, NaBr, or NaI were measured. All these salts reduced the Tclo's of PNIPAM/water solutions to different extents, in the following order: NaCl > NaBr > NaI. The higher the concentration of the added salt was, the more greatly Tclo dropped. A dynamic viscoelasticity investigation of the PNIPAM/water solutions with the salts indicated that during phase separation, the system changed from a homogeneous fluid into a physically crosslinked network, and the addition of salts also reduced the temperature at which this change began. The gelation temperature (Tgel) and the scaling exponent of the PNIPAM/water solutions with NaBr were obtained with dynamic scaling theory, and Tgel was found to be close to Tclo. That the addition of salts to the solution decreased Tclo and Tgel to the same extent further proved that the network structure was formed with the phase separation in the PNIPAM/water solutions. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 901–907, 2001

A log-log plot of sol fraction versus dose has been found to be an effective graphical technique to treat sol-gel analysis data. The treatment procedure is described in detail with an example of radiation crosslinking of linear low-density polyethylene. Radiation chemical yields both for crosslinking and scission of macromolecules were determined using Monte Carlo simulation. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2621–2625, 2001

The acrylamide/N,N′-methylenebisacrylamide gelation process was studied by time dependent measurement of real ϵI and imaginary ϵII parts of the complex dielectric permittivity. The measurements were carried out in the frequency interval 500 Hz to 13 MHz. Strong relaxation behavior of ϵI and ϵII was observed in all dielectric spectra during the transformation from the sol to the gel state. At all frequencies a maximum in the imaginary part of dielectric constant ϵII was observed. The gel point was found by extrapolating this maximum to zero frequency. The behavior of ϵI and ϵII related to the gelation mechanism is discussed. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 463–466, 2000

Full Paper: To understand the fundamental aspects of the polycondensation reaction of hydrolyzed tetraethoxysilane (TEOS) and silanol-terminated poly(dimethylsiloxane) (PDMS), we modeled the reaction system as a stepgrowth polymerization of A4 and polydisperse A2 , assuming the reactivities of all functional groups are equal. The analytical solution for the weight-average molecular weight is developed, and in addition, a Monte Carlo simulation is conducted to investigate the detailed structural development. It was found that as long as the molecular weight of PDMS is much larger than TEOS, the apparent behavior is significantly different from usual gelling systems. The gel point is relatively insensitive to the weight fraction of crosslinker (TEOS), the polydispersity index may decrease during polymerization before the rapid increase to infinity, and the molecular weight distribution profile may not show a significant broadening toward gelation. Even though the present model assumes a complete random reaction process among functional groups, formation of a heterogeneous structure in which a tight core consisting of TEOS-based molecules is surrounded by soft PDMS chains was observed in the Monte Carlo simulation.

In a previous article, a general model for the condensation copolymerization of mono and bifunctional monomers was developed in which the sequence length distribution was calculated statistically from the concentrations of linkages (e.g., CONH) labeled by the identities of the participating monomer units. A set of balance equations for the effect of each major reaction upon these concentrations, those of the end groups, and the moments of the chain-length distribution completed the general model framework for linear polymers. In this article, this technique is extended to the case of nonlinear polymerizations with multifunctional monomers capable of branch or gel formation. This modification is required because in nonlinear polymerization, the moments of the chain-length distribution diverge at the gel point, and the traditional description of the sequence length distribution is only well defined for macromolecules consisting of a single backbone chain. Although the balance equations for the end group and linkage concentrations presented in the previous article are completely transferable, the statistical techniques must be modified to accommodate branch and network chain architectures. By coupling the general kinetic model with the recursive approach of Macosko and Miller for the calculation of sol/gel properties, one can describe the microstructures of a wide variety of systems such as those in which the copolymer has a blocky microstructure caused by interchange reactions between multiple components. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 266–274, 2001