Weight Fraction Of Gel Research Articles

Study on gel weight fraction of ultraviolet-cured acrylic adhesives

Ultraviolet (UV)-cured adhesives have attracted plenty of attention in the fields of health care and electronic components thanks to low energy loss and environment pollution. UV-cured adhesives bearing epoxy acrylate prepolymers have been extensively utilized, ascribed to low cost and high stability. However, the gel content of UV-cured adhesives has hardly been investigated in detail. The gel content has been found to strongly affect the mechanical properties and serving life of adhesives. We have found the strong composition dependence of adhesive performances of UV-cured adhesives before. To clarify the influencing factors of gel content in UV-cured adhesives containing epoxy acrylate prepolymers, in this work, a family of UV-cured adhesives with epoxy acrylate constituents have been elaborately designed and further fabricated. The influences of prepolymer kind, UV photoinitiator kind, UV photoinitiator concentration, reactive diluent concentration, feed style during preparing prepolymers and film thickness on gel weight fraction of cured adhesive films were deeply researched. This work might open the door to the large-scale preparation of promising high-performance UV-cured adhesives with a higher gel weight fraction based on regulating the formula.

STUDIES ON BOUND RUBBER OF CB BEFORE/AFTER VULCANIZATION OF ISOPRENE RUBBER

ABSTRACT The high abrasion furnace (HAF), fast extrusion furnace (FEF), and fine thermal (FT) loadings used in this study were adjusted to keep the carbon black (CB)–specific area of cetyl trimethyl ammonium bromide adsorption to 2490 m2g−1. Their polyisoprene (IR) compounds and vulcanizates were investigated. Regarding the dependence of the CB gel fraction (FCB gel) on the IR gel/CB weight ratio (WRIR/CB), the amount of CB in the CB gel increased with a larger CB aggregate size, whereas the amount of the IR gel decreased. With regard to the dependence of the activation energy (ΔE(T2)) of the spin–spin relaxation time (T2) on the CB aggregate size, an immobilized rubber layer around CB formed more easily in the order of FT < FEF < HAF. Moreover, it became clear from the relationship between WRIR/CB and ΔE(T2) that the amount of the IR gel in the CB gel increased with decreasing ΔE(T2). The unique ratio of the surface area to the volume of CB aggregates (SCB/VCB) was obtained from results of three-dimensional transmission electron microscopy observation. The dependence of the amount of dibutyl phthalate (DBP) adsorption and compressed DBP adsorption on SCB/VCB was approximately linear, which suggested that SCB/VCB was closely related to CB aggregates and CB agglomerates. The linear relationship seen between SCB/VCB and ΔE(T2) revealed that an immobilized rubber layer was present around CB. The dependence of (qCB/qIR)(WRIR/CB) on SCB/VCB was also investigated using the densities (ρIR and ρCB) of IR and CB. The slope (tirl) of their approximately linear relationship was about 3.9 nm, which almost agreed with the bound rubber thickness reported in the literature to date. It is also inferred that because the intercept (α), which reflects the change in the IR gel weight fraction, was about 0.04 (4%), the IR gel weight fraction changed very little between before and after vulcanization.

Mathematical Modeling of NMRP of Styrene–Divinylbenzene over the Pre‐ and Post‐Gelation Periods Including Cyclization

Nitroxide‐mediated polymerization of styrene‐divinylbenzene has been modeled using generating functions of length distributions, pseudo‐kinetic propagations, and numerical fractionation with the crosslinking rate depending on generation. Cyclization reactions are tackled by balances of sequences, yielding fair predictions of the measured pendant double bond concentration. With reduction in crosslinking, agreement for the experiments at 90 °C between predicted and measured weight‐average, molecular weight, and weight fraction of gel is observed. A much higher relative crosslinking reactivity is observed at 130 °C as compared to 90 °C, likely an effect of the chain mobility.

Modeling Studies on the Synthesis of Superabsorbent Hydrogels Using Population Balance Equations

AbstractThe synthesis of super‐absorbent hydrogels is simulated using a kinetic model based upon population balance equations of generating functions. Dynamics in a batch reactor of properties such as the weight fraction of gel or average molecular weights of the soluble fraction can be predicted. This kinetic model neglects intramolecular cyclization reactions for simplicity (hence predictions can be valid only for very small amounts of crosslinker) but it can accommodate the operation with different kinds of crosslinking agents, namely bifunctional (e.g. N,N′‐methylenebisacrylamide), trifunctional (e.g. trimethylolpropane triacrylate) and tetrafunctional (e.g. tetraallyloxyethane). The influence of the use of such different kinds of crosslinkers on the dynamics of gelation is discussed. It is also assessed the impacts of the rate propagation coefficient of the monofunctional monomer (typically acrylic acid), of the reactivity of the pendant double bonds (PDB) and of the initial composition on the dynamics of gel production. Some crucial details concerning the numerical solution of the two‐point boundary value problems (TPBVP) associated with this simulation tool are also presented. Predictions of the proposed kinetic approach are compared with those obtained using the Theory of the Branching Processes which is not strictly valid with kinetically controlled polymerization systems such as those here considered. Important differences between the predictions of the two approaches are shown. Superabsorbent hydrogels were synthesized with a 2.5 L batch reactor and the experimental data are used to show that the simple kinetic model developed is able to capture the main features of this polymerization system.

Free-radical cross-linking copolymerization of methyl methacrylate and ethylene glycol dimethacrylate in the presence of trimethoxyvinylsilane

Free-radical cross-linking copolymerization of methyl methacrylate (MMA), ethylene glycol dimethacrylate (EGDM), and trimethoxyvinylsilane (TMVS) has been investigated in toluene at a total monomer concentration of 4 mol L −1 and at 70 °C. Conversion of monomer, volume swelling ratio, weight fraction, and gel point were measured as a function of the reaction time, TMVS concentration, toluene content, temperature, and chain transfer agents up to the onset of macrogelation. Structural characteristics of the gels were examined by using equilibrium swelling in toluene, gel fraction, and Fourier transform infrared (FTIR) analysis. The morphology of the copolymers was also investigated by scanning electron microscopy (SEM). Based on the obtained results, it was concluded that the FTIR data did not have the capacity to show the presence of the TMVS moiety in these kinds of copolymers. On the other hand, the variation of weight fraction of gel, W g , and its equilibrium volume swelling ratio in toluene, q v , exhibited the same behaviors 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, TMVS is a suitable silicone co-monomer for this system and it has been proved useful.

Preparation and properties of silicone‐containing poly(methyl methacrylate) gels

AbstractPoly(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

Phase separation during the formation of poly(acrylamide) hydrogels

The crosslinking polymerization of acrylamide (AAm) was investigated at a monomer concentration of 5 w/v% in water with N, N′-methylenebis(acrylamide) (BAAm) as the crosslinker. The crosslinker ratio (mole ratio of BAAm to AAm) was set to 1/82. The fractional monomer conversion x, the weight fraction of gel W g, and the equilibrium weight swelling ratio q w of the hydrogels were measured as a function of the reaction time. Experimental results indicate that the first gel molecule in the reaction system appears after a reaction time t c =4±1 min or after a monomer conversion of x c=0.07±0.02. At this critical point the homogeneous reaction system turned into a heterogeneous one consisting of gel and separated solution phases. The network formation process took place in this heterogeneous system within the first 60 min of the reaction time. Experimental x, W g, and q w data indicate that, during the gel growth process, the growing hydrogel molecule cannot occupy the entire polymerization system, which is responsible for the observed phase separation. It was shown that the volume of the separated liquid phase decreases as the polymerization and crosslinking reactions proceed and finally, the reaction system becomes homogeneous again due to the decreasing dilution degree of the hydrogel.

Synthesis and prperties of peroxy derivatives of epoxy resins based on Bisphenol A. 1. Effects of the presence of inorganic bases

AbstractOligomers containing peroxy groups have been synthesized from epoxies and aliphatic and aromatic hydroperoxides. The dependence of reaction rates upon ratios of starting materials, the presence of an inorganic base as a catalyst, nature of solvent, quantity of water, and temperature have been investigated. The hydroperoxide + epoxy reactions proceed very slowly in the presence of NaOH but at satisfactory rates in the presence of KOH. The reactions do not proceed in acetone, methylethylketone, dioxane, or toluene, but do proceed in secondary and tertiary alcohol media. We have studied in particular the reaction between tert‐butyl‐hydroperoxide (tBHPO), diglycidyl ether of diphenylpropane and an epoxy resin based on Bisphenol A and epichlorohydrin. Structures of the products were confirmed by IR and 1H‐NMR spectroscopy. Mechanisms of reactions have been defined and an empirical equation that describes kinetics of the reaction between tBHPO and the epoxy has been developed. The peroxy oligomers so obtained were used to create networks based on a polyesteric resin. Going from 10 wt% of the peroxy oligomer to 25% increases significantly the rate of gelation as well as the final gel weight fraction. However, a further increase of the peroxy oligomer concentration to 50% affects only slightly the gelation kinetics and the plateau gel fraction.

Molecular weight distribution of crosslinked polymers produced in emulsion

A kinetic model for evaluating the chain length distribution of a branched polymer produced in emulsion was developed. Chain branching occurring through any intermolecular mechanism is considered, namely, crosslinking, chain transfer to polymer and propagation to terminal double bond. The model accounts for active chain compartmentalization and, when coupled to a model able to describe the evolution of the polymerization system, allows evaluation of the cumulative properties of the produced polymer both in the pregel and postgel phases. The numerical difficulties related to the description of a rather wide polymer chain population and of gel formation are overcome by using the ‘numerical fractionation’ technique. A parametric analysis of both instantaneous and cumulative properties is reported and discussed, with special attention to the role of radical compartmentalization in determining the molecular weight properties of a polymer produced in emulsion. Significant differences with the molecular weights computed using models developed for homogeneous (non compartmentalized) systems have been found. A comparison with the predictions of Flory's statistical theory is also reported in terms of gel point and gel weight fraction. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1127–1156, 1998

Processing and morphology of polystyrene/ethylene‐propylene rubber reactive and nonreactive blends

AbstractThe difference between reactive and nonreactive polymer blends in terms of processing and morphology has been investigated. The glassy phases for the blends are an oxazoline functional polystyrene (PS‐Ox) and a similar non‐functional polystyrene (PS). The rubbery phases are an ethylene‐propylene rubber (EP) and a similar ethylene‐propylene rubber (EP‐MA) with 07% grafted maleic anhydride. In the case of PS‐Ox/EP‐MA blends, the oxazoline group may react with the grafted rubber functionality at the interface between the two immiscible components during blending to form a compatibilizing agent in‐situ. The nonreactive blends systems of PS‐Ox/EP and PS/EP‐MA were used for comparison to the reactive system. The blend components are rheologically matched to simplify the interpretation of the process and morphological data. The blends were prepared in a batch mixer with roller blades. The torque required for mixing was measured during the blending process. The torque traces for the reactive blends exhibited a peak in torque, attributed to the chemical reaction at the interface. The weight fraction of gel in the blends was used to measure the extent of reaction. It correlates well with the mixing torques and rheological properties. The nonreactive PS‐Ox/EP and PS/EP‐MA blends show poor interfacial adhesion between the two phases. In contrast, the reactive PS‐Ox/EP‐MA blends show excellent during annealing is also much greater for the reactive blends. Varying the functionality concentration in the PS phase shows that the dispersed phase rubber particle size is reduced by increasing the concentration of oxazoline in the matrix. Blends with no or small amounts of functionality in the PS phase exhibit yield behavior in tension. However, a level of concentration of reactive functionality may be reached where the material becomes brittle.

Computer simulation of network formation in free-radical crosslinking copolymerization

Abstract A new Monte Carlo simulation model for the kinetics of network formation in free-radical copolymerization of vinyl and divinyl monomers is used to investigate various important property changes during polymerization, such as the whole molecular weight distribution profile, the distribution of crosslink points among various polymer molecules, weight fraction of gel and sol as well as the spatial distribution of the crosslinked polymer chains. This simulation model does not assume a finite reaction system as an approximation for the infinite system, but employs a sampling technique in which polymers are sampled from an infinite number of polymer molecules in the reaction system. The structure of each crosslinked polymer is simulated based on the crosslinking density distribution of the primary polymer molecules that results from a kinetically controlled network formation. The proposed simulation method is quite general and can be applied to various complex polymerization reactions that involve crosslinking and branching in a kinetically controlled system irrespective of reactor types used.

Kinetics of network formation in free-radical crosslinking copolymerization

A new kinetic theory for network formation in free-radical copolymerization of vinyl and divinyl monomers is used to investigate various important property changes in the postgelation period, such as the weight fraction of gel, the molecular weight distribution, the cross-linking density within sol and gel fractions, the partition of the unreacted pendant double bonds between sol and gel, etc. This theory is based on the cross-linking density distribution among primary polymer molecules, and the history of the generated network structure is fully accounted for. It is shown clearly that the deviation from the Mory-Stockmayer theory becomes significant as the variance of the cross-linking density distribution increases

Electron beam effects on polymers: Structure–property behavior of radiation‐cured bis‐GMA

AbstractStructure–property relationships were investigated for the diglycidly methacrylate derivative of bisphenol‐A crosslinked by electron beam irradiation. This material, commonly called bis‐GMA, is a viscous liquid at room temperature which crosslinks to form a glassy network. The major parameters which were systematically varied in this study were radiation dosage, dose rate, aging time after irradiation, and post‐cure annealing at higher temperatures. Measurements were conducted to quantify the crosslinking reaction and to characterize the physical properties of the resulting networks. Solvent extraction was done to determine the relative degree of network formation through the equilibrium swelling and the gel weight fraction after drying. Another method utilized FTIR to monitor the disappearance of double bonds as the crosslinking reaction proceeded. To characterize the thermal and physical properties, differential scanning calorimetry (DSC) and dynamic mechanical spectroscopy were utilized. Network density was found proportional to the irradiation dosage, with an upper limit reached above some critical dosage. Over the range of dose rate studied, this variable was not found to influence the degree of cure greatly. The crosslinking reaction became diffusion limited as vitrification occurred. These phenomena were discussed in terms of the well‐known time–temperature–transformation diagram. Free radicals trapped in these reacting networks due to vitrification exhibited a finite lifetime. Post‐curing could be achieved by annealing at a temperature above the Tg of the initially cured network, as shown by the increase of the glass transition temperature from both DSC and dynamic mechanical results.

Monte Carlo results for random coagulation

This paper gives a Monte Carlo algorithm for stochastic coagulation when the coagulation kernel K ij = f i g j + f j g i . For a monodisperse system with M = 10 6 monomers, each realization takes 7 min on a CDC 7600. On the basis of Monte Carlo experiments performed with this algorithm, the following conclusions are inferred. If w g = w g ( t) = the gel weight fraction = the proportion of monomers contained in the largest cluster at time t, then, as M → ∞, for K ij = ( ij) ω : w ≦ 1 2 , w g → 0 ; 1 2 < w ≦ 1; w g approaches a well-defined limit, nonzero after a critical time ([i]and [ii]support a conjecture of Ziff); and w > 1, w g(t) → 1, t > 0 . (This last result was known to Domilovskii et al. in 1978.) For ω < 1, the Smoluchowski coagulation equation is the limit of random coagulation. For 1 2 < ω < 1 , this equation gives rise to mass nonconservation; the mass deficit w g ( t) is contained in a single cluster. For K ij = [( f − 2) i + 2][( f − 2) j + 2], due to correlations, the coagulation equation is not the limit of random coagulation. Instead, its Flory version, incorporating gel reactivity, is the appropriate limit.

Network Structure in Terms of Prepolymer and Curative Parameters

Abstract The structure of polymer networks is considered. The networks are assumed to be formed from the reaction of functional groups on low molecular weight telechelic polymers with a cross-linking agent. Relationships are derived between cross-link density and properties of the prepolymer mix such as functional group equivalent weight, functionality, and including the maximum or final extent of reaction of the functional groups and the gel content of the cured stock. At an equivalent reactant ratio of 1.0 to 1.0 and with the branch points uniting no more than three chain ends, the cross-link density in the gel was found to be proportional to the product of a concentration term, q, and a geometrical term, (1 − 1/fa − 1/fp), which is called the branching function. The absolute value of the network cross-link density in moles/g was found to be where q = equivalents of prepolymer functional groups per gram, fp = functionality of prepolymer, fa = functionality of cross-linking agent, CDF = cross-link decrease factor, a constant having a value between 0.42 and 0.66, and Wg = weight fraction of gel. The CDF term is the factor by which the cross-link density in the gel is decreased because of failure of the reaction to go to completion at the maximum degree of cure. At a reactant ratio of 1.0 this term was found to have the form where e is the maximum or final extent of the curing reaction in the gel, and ec is the extent of reaction at the gel point. At a reactant ratio of 1.0 the CDF term was constant over a wide range of functionalities. The extent of reaction in the gel was determined from a previously published equation of the form e2 = (1 − k)ec 2 + k. The value of the constant k was estimated to be 0.3 ± 0.1.

Molecular weight distributions from gel‐sol ratios

AbstractA method for determining molecular weight distributions is presented, based upon the partition of polymer molecules between gel and sol in a partly crosslinked system. It is assumed that the polymer sample consists of a series of molecular weight species, and analysis of previously developed theories covering this situation yields an expression which equates the weight fraction of gel to a summation involving the weight fraction of the material in each species and the number of crosslinked units in the sample. The data are presented in the form of a graph showing how the weight fraction of gel varies as a function of the number of crosslinked units in the sample. Data for the graph are obtained from swelling measurements made in the usual manner. The procedure is outlined for using the information from the graph to make a step‐by‐step evaluation of the fractional weight of each molecular weight species in the sample for as many fractions as may be arbitrarily chosen. Both a first approximation method and a more refined second approximation method are outlined. The procedure has been applied to samples of natural rubber, synthetic rubbers, and to two known molecular weight distributions.