Gelation behavior of near‐zero shrinkage polybenzoxazines

A new class of phenolic thermosetting resins is developed that is based on the ring‐opening polymerization of a benzoxazine precursor. These new materials seek to combine the thermal properties and flame retardance of phenolics with the mechanical performance and molecular design flexibility of advanced epoxy systems. These materials overcome many of the traditional shortcomings of conventional novolac and resole‐type phenolic resins, while retaining their benefits. The viscoelastic behavior of the polybenzoxazines during isothermal cure is monitored by dynamic mechanical analysis. Isochronic measurements show that although the aniline‐based benzoxazine has a lower activation energy for the gelation process than the methylamine‐based resin, it has a slower rate of reaction. The purified monomer and as‐synthesized precursor for each benzoxazine are found to polymerize by the same mechanism, despite the absence of an initiating species in the purified resins. The chemical gelation phenomenon of the methylamine‐based resin is probed by a multifrequency dynamic cure analysis that allows determination of the instant of chemical gelation, as well as the network relaxation exponent, n. The constant value of the exponent regardless of cure temperature demonstrates that chemical gelation is, in fact, an isoconversion event for the methylamine‐based benzoxazine. The multifrequency and isochronic analyses are shown to produce very similar gel times and activation energies for the gelation process. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 406–417, 2001     

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

In 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     

Elongational behavior of epoxy during curing

Elongational behavior of epoxy (epoxy/curing agent = 100/0.5, w/w) cured at various conditions over the critical gelation time was investigated. Dynamic viscoelastic measurements of the epoxy system were performed and the critical gelation time of epoxy was determined according to the frequency dependence of G′ and G″ proposed by Winter and Chambon. Elongational behavior of epoxy cured for various times were measured. Epoxy, cured over the critical gelation time, showed strain hardening and elongational behavior similar to a crosslinked rubber. Increase of elongational viscosity of the sample occurred early, and the sample broke at small strain as curing time increased. The effect of strain rate on the elongational stress of epoxy cured near the critical gelation time was measured at various strain rates. For epoxy cured for critical gelation time only, high stress at a small strain rate was represented as strain rate increased. When increasing curing time further, the tensile stress converged on a single curve regardless of strain rate, and samples broke at nearly the same stress and strain. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Gelation via cationic chelation/crosslinking of acrylic‐acid‐based polymers

The gelation characteristics of acrylic‐acid‐based polymers in the presence of a range of cationic species, namely Ca²⁺, Mg²⁺ and Al³⁺, were investigated using in situ rheological measurements during photo‐polymerisation. Fourier transform mechanical spectroscopy was used to identify the gel point, using the Winter–Chambon criteria which allow the gel point to be pinpointed by establishing the sample spanning network and quantitatively determining stiffness, relaxation exponent, gel stiffness and fractal dimensions. The results showed that the gelation processes were greatly influenced by the type of cationic species that was used in the syntheses. At the gel point, more open network clusters were formed when Al³⁺ cations were used instead of Ca²⁺ cations or Mg²⁺ cations, all relating to chloride salts. Although the concentrations of the chelating/crosslinking aluminium species affected the kinetics of the gelation, the critical gel characteristics were hardly affected. Also the solubility of the chosen aluminium salt was shown to dictate the crosslinking rates and the properties of the critical gels. The extents of the reactions and the types of network formed at the gel point and beyond indicated that reactions between the Al³⁺ ions and COOH sites, from growing poly(acrylic acid) molecular chains, differ from those exhibited by Mg²⁺ and Ca²⁺ ions. All of the chelation/crosslinking reactions met the criteria of low mutation number (N_mu), such that in all cases N_mu ? 1. © 2019 Society of Chemical Industry     

Rheological study of the gelation of cross-linking polyhedral oligomeric silsesquioxanes (POSS)/PU composites

The gelation process of N-Phenylaminomethyl-POSS/PU (polyurethane) nanocomposites during curing and at the stable state after curing was investigated by rheology. An increase in dynamic shear moduli, G′ and G″, was observed during the dynamic temperature ramps of the sample. In time-resolved mechanical spectroscopy (TRMS), G′ and G″ increased with curing time at constant curing temperatures over a wide of frequencies. The gelation time of the composites decreased with the rise of curing temperature or with the increase of POSS concentration. The relaxation exponent n at the critical gel was around 0.73. The gel stiffness S decreased as curing temperature increased or as POSS concentration increased. After the completion of the curing reaction, the critical concentration of POSS beyond which the gelation of POSS/PU composites would happen was found around 2.5 wt%. The viscoelastic properties of crosslinking POSS/PU fitted time–temperature-superposition well which implied the incorporation of multifunctional POSS into PU increased the homogeneity of crosslinking POSS/PU composites. Surprisingly, the modulus of the fully cured materials between 2.7 wt% and 6 wt% could also be supposed onto a master curve at high temperature, which implied self-similarity of network near the critical gel. The similar microstructure of POSS/PU at stable state was also confirmed by TEM. The network formation mechanism and the fine structure of the crosslinking POSS/PU were firstly investigated which would provide technical and theoretical basis for other hybrid crosslinking systems.     

Rheological study of carbon nanofiber induced physical gelation in polyolefin nanocomposite melt

The rheological behavior of nanocomposites based on elastomeric ethylene-propylene (EP) random copolymer (84.3 wt% P) and well-dispersed modified carbon nanofibers (MCNFs) with concentrations from 0.5 to 20 wt% were studied by oscillatory-shear rheometry at varying temperatures. At relatively low temperatures, the entanglement density of the polymer chains appeared to increase with filler concentration, ensuring strong MCNF-polymer interactions. At elevated temperatures, pronounced deviations from the ideal melt behavior in the low frequency domain (i.e. positive slopes of the tan δ(ω) curves) were observed, indicating the formation of a three-dimensional percolated network. Following the Winter-Chambon criterion, the transition from pseudo-solid-like to liquid-like behavior (i.e. the critical gel point) upon cooling was determined. The physical gelation induced by MCNFs is a thermo-reversible phenomenon and its origin can be traced to the interactions of nanofillers, where the percolation temperature decreases with filler concentration. Abrupt changes in the critical gelation temperature, the stiffness of the gel and the relaxation exponent were observed in nanocomposites with MCNF loading above 10 wt%. This behavior indicates a change of the mechanism of physical gelation at high MCNF loadings that can be explained by the concept of bridge formation of polymer segments between two adjacent nanofillers in concentrated nanocomposites.     

Rheological properties of syndiotacticity-rich ultrahigh molecular weight poly(vinyl alcohol) dilute solution

Viscoelastic behavior of the dilute solution of ultrahigh molecular weight syndiotactic poly(vinyl alcohol) (UHMW s-PVA)/dimethyl sulfoxide (DMSO)/water was investigated through rheological response. Below a critical temperature, the dynamic storage modulus (G′) of the solution became greater than the dynamic loss modulus (G″) and the viscoelastic exponent for G′ became smaller than that for G″ before macroscopic gelation, which indicates the evolution of viscoelastic solid properties at the sol state. Also, the loss tangent (tan δ) of the solution below the critical temperature increased with increasing frequency. Consequently, the dilute solution of UHMW s-PVA/DMSO/water showed the rheological behaviors as can be observed in general chemical or physical gel systems below the critical temperature. These results suggest that solid-like heterogeneity prevailed in the solution before macroscopic gelation with decreasing temperature. Such heterogeneity was considered as phase-separated domains attributed to spinodal decomposition. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 569–576, 2001     

Gelation of attractive particles in polymer melt

The physical gelation of the attractive carbon black particles in the isotactic polypropylene melt is studied. Following the method proposed by Winter and Chambon, oscillatory rheology is used to study the kinetic gelation process. The gel time, the relaxation exponent of the critical gel, the gel stiffness and the activation energy for the gelation are determined. The influences of the filler concentration and the experimental temperature on the gelation process are also investigated. The results suggest that the gelation of the filler particles in the polymer melt mainly follows second-order kinetics, and the critical gel should be formed by the rigid particles and the inter-particle polymer layers.     

多糖的共价键合对7S球蛋白凝胶性能的影响

引入细胞学说中的大分子拥挤体系并在该体系通过Maillard反应制备了大豆7S球蛋白(b-conglycinin,7S)和葡聚糖(dextran,Dex)的共价复合物,利用转谷氨酰胺酶(transglutaminase,TGase)的交联作用制得蛋白-多糖共价复合物凝胶.对其流变学、质构特性及微结构进行分析.结果表明,在大分子拥挤环境下,7S-葡聚糖比例为2:1的共价复合物制得具有致密且孔洞分布均匀的凝胶网络结构.单纯的蛋白自聚集或者与多糖的共混状态时形成的凝胶弹性模量都较高,但蛋白过度的聚集并不能得到孔洞均匀的凝胶网络结构,共价复合物制备的凝胶中葡聚糖的共价键合作用抑制了TGase交联过程中蛋白质分子间发生过度的相互作用.形貌学观察表明,共价复合物制备的凝胶形成了网络孔径均匀且非常致密的凝胶网络结构.     

Effect of initial crystallinity on the response of high‐density polyethylene to high‐energy radiation

Samples of each of two high‐density polyethylenes with various initial degrees of crystallinity, but otherwise identical, were exposed under a vacuum to moderate doses of gamma irradiation. The results indicate that, for otherwise initially identical polymer samples, the dose required to reach the gel point increases with increase of the initial degree of crystallinity. Above the critical dose for gelation, the gel content decreases with higher degrees of crystallinity at equal radiation doses. The mechanical behavior of the polymers changed progressively from ductile to brittle as the crystallinity was increased. The extensibility of originally ductile samples decreases with increasing radiation dose. The irradiation of samples having intermediate behavior produces a change to ductile behavior. Mechanical behavior is not modified substantially when brittle samples are irradiated. The initial modulus is little altered by irradiation, while the yield stress shows a slight increase with irradiation. The mechanical properties, such as draw ratio at break and ultimate tensile stress, decrease with dose in ductile samples. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1375–1384, 1999     

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.     

Gelation studies on acrylic acid‐based hydrogels via in situ photo‐crosslinking and rheology

The gelation characteristics of acrylic acid (AA)‐based hydrogels were investigated using real time in situ photocrosslinking and rheological measurements. The gel point and gelation times were established using Winter–Chambon criteria. Frequency independence of tan δ was observed in all cases, such that G′ and G″ scaled as ～ωⁿ. The Flory–Stockmayer theory was used alongside other indices in order to probe the gelation and the post‐gelation characteristics of the critical gels and the fully formed hydrogels. Network relaxation exponents (n) were influenced by the concentrations of AA and methylene bis‐acrylamide. The gel stiffness (S) decreased with an increase in the concentration of the monomer and of the concentration of the crosslinker, while network branching decreased (lower fractal dimensions) at the gel point. The conversion at the gel point was found to be iso‐conversion with respect to the intensity of the UV irradiance used in the photocrosslinking reactions (1–20 mW/cm²). Thus, network clusters and the crosslinking reaction mechanism were the same irrespective of radiation intensity, although the rates of the reactions were affected. Having sufficient amounts of reactive species at the time of cure drive the crosslinking reactions beyond the gel point to greater crosslink density and smaller mesh sizes. The effects of auto‐acceleration and free‐volume were observed and shown to have key effects on the gelation mechanisms and the branching topographies of the network, when the concentration of the known polyacrylamide medium were not controlled. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46691.     

Synthesis and linear viscoelastic behavior of poly(amic acid)–organoclay hybrid

Synthesis and rheology of poly(amic acid)–organoclay hybrids were studied in order to address the molecular chain ordering and its relationship with clay particles in the prepolymer. The poly(amic acid) was prepared from oxydianiline and pyromellitic dianhydride solution in N‐methylpyrollidinone with exfoliated montmorillonite. The composite solution was mixed to the nanoscale level, and the extent of defoliation and phase separation were studied using X‐ray diffraction, transmission electron microscopy, and differential scanning calorimetry. Linear viscoelasticity was used to examine the influence of increasing extent of organoclay. The gelation time of the prepolymer decreased exponentially with increasing clay content. Scaling behavior by power law frequency independence at gel point was found (G′ ∝ G″ ∝ ω″) from slopes of storage and loss moduli. The power law exponent n was found to be about 0.5. At low organoclay concentrations, the gel becomes stiffer and gel strength rises with the concentration. At higher concentrations, the gel becomes weak, gel strength decreases, and phase separation seems to take place, possibly due to organoclay aggregation. The study suggests that linear viscoelasticity can be used to understand the evolution of molecular order in prepolymers of polyimide and its clay hybrids. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 592–603, 2001     

PAN／DMSO溶液凝胶化行为的流变学研究

通过动态流变测试,研究PAN／DMSO溶液的热致变凝胶化过程和测定PAN／DMSO溶液的凝胶点温度。主要通过测量动态流变学参数（动态模量G′i和G″ii、损耗角正切tanδ、复数粘度η^＊）来表征PAN溶液的凝胶化行为,分析了含水量对溶液凝胶化行为和凝胶结构的影响。结果发现,虽然PAN溶液的凝胶点温度随着溶液中含水量的增加而升高,但是由松弛指数n和分形维数值df表征的凝胶结构几乎不受溶液中含水量的影响。     

Dielectric investigation of the sol‐gel transition in the acrylamide/N,N′ methylenebisacrylamide system

The acrylamide/N,N′‐methylenebisacrylamide gelation process was studied by time dependent measurement of real ϵ^I and imaginary ϵ^II parts of the complex dielectric permittivity. The measurements were carried out in the frequency interval 500 Hz to 13 MHz. Strong relaxation behavior of ϵ^I and ϵ^II 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 ϵ^II was observed. The gel point was found by extrapolating this maximum to zero frequency. The behavior of ϵ^I and ϵ^II related to the gelation mechanism is discussed. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 463–466, 2000     

Study of crosslinking copolymerization and its fractal behavior of styrene-divinylbenzene

The crosslinking copolymerization of styrene-divinylbenzene (DVB) with 6% (w/w) DVB was investigated quantitatively by liquid chromatography (LC), viscosity, GPC, and static and dynamic laser light. Through these experiments, reaction order, mechanism, and evolution of molecular weight, size, polydispersity, and branching of the copolymer chains during the gelation were measured. The critical exponents γ, β, δ, τ, and ν_p of the gelation were calculated. Furthermore, fractal behavior of the copolymer was evidenced firmly; the progression of fractal dimensions of these species following the gelation were also obtained using the dependence of intensity scattered on angles by synchrotron small angle X-ray scattering. A methodology for characterization of gelation forming network polymer is provided objectively. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68:363–371, 1998     

Thermoreversible gelation of nitrocellulose solutions

Thermoreversible gelation of concentrated solutions of nitrocellulose was examined. The sol–gel transition diagrams were established using the ball‐drop method. The critical gel concentrations (C_gel), defined as the lowest concentration for gelation to occur, are approximately proportional to the reciprocal of the molecular weight of nitrocellulose. In addition, the values of C_gel[η] lie in the concentrated regime where entanglement effects are present. These facts suggest that the entanglement of polymer chains is a necessary condition for the formation of nitrocellulose gels. Moreover, the association of polymer segments is identified as another factor affecting the gel stability. This is evidenced by both the enhancement of the gel stability in poor solvents and the large enthalpy of mixing at low temperatures. Because of the presence of the association, the number of polymer chains in a junction point is larger than two, which is the value predicted by assuming entanglements as the only factor responsible for gelation. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 4000–4008, 2003     

Enhanced strain hardening in elongational viscosity for HDPE/crosslinked HDPE blend. I. Characteristics of crosslinked HDPE

Blending a crosslinked high‐density polyethylene (xHDPE) enhances melt strength and strain hardening behavior in elongational viscosity of high‐density polyethylene (HDPE) to a great degree. Gel fraction of xHDPE has a stronger effect on the strain hardening than sol fraction, although sol fraction also enhances the strain hardening to some degree. Further, the xHDPE crosslinked by peroxide in a compression mold exhibits more pronounced effect than xHDPE by radiation, which is attributed to the difference in the amount of the gel fraction. The xHDPE, which enhances the strain hardening, has sparse crosslink points in the network. Moreover, it was found from linear viscoelastic measurements, such as oscillatory modulus and relaxation modulus, that the xHDPE is characterized as a critical gel, which was also supported by the result of tensile testing. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 73–78, 2002     

Crystallization and the mechanical and rheological properties of decrosslinked–crosslinked‐high‐density polyethylenes using a supercritical fluid

The crystallization, and mechanical and rheological properties of decrosslinked–crosslinked‐high‐density polyethylenes using supercritical methanol were investigated by DSC, WAXS, DMTA, and UDS. Crosslinked high‐density polyethylenes were successfully decrosslinked in a supercritical methanol condition. The residual gel content of the decrosslinked samples decreased with the reaction temperature. The crystallization behavior, mechanical, and rheological properties of the decrosslinked samples were influenced considerably by the gel content. As the gel content increased, the network gel structure restricted the chain mobility of polymer molecules in the melt state and hindered their crystallization. Thus, the nonterminal yield behavior in the melt state was enhanced and the crystallinity decreased. The dynamic elastic modulus of the decrosslinked sample in solid state increased with the increase in the crystallinity. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Electromagnetic and electromagnetic wave‐absorbing properties of the SrTiO3–Epoxy composite

The effects of SrTiO₃ content on the electromagnetic properties and electromagnetic wave‐absorbing characteristics of SrTiO₃–epoxy composites were investigated. Also, the frequency dispersion behavior of the complex permittivity of composites was demonstrated. The complex permittivity and permeability were measured using a network analyzer in the frequency range of 130 MHz to 10 GHz. As the SrTiO₃ content increased, it was found that the complex permittivity and permeability of the composites increased and the resonance frequency moved toward low frequency range. The logarithmic model coincided with the effective permittivity of composite as a function of SrTiO₃ content comparatively well. The resonance frequency of composites was found to show good agreement with the theoretical values calculated by the equation proposed in this article. The electromagnetic wave‐absorbing behavior showed that the center frequency of attenuation curve was shifted to a lower frequency band with increasing the amount of SrTiO₃ and the thickness of composite. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 75–83, 1999