Rheokinetic Studies Research Articles

Chemorheological Kinetic Modeling of Uncatalyzed Hydroxyl‐Terminated Polybutadiene and Isophorone Diisocyanate

AbstractCatalysts are often employed to enable nonisothermal reaction kinetic studies when tracking cure progression of polyurethane reactions. However, when quickly reacting, catalyzed materials are prepared through mixing followed by the addition of fillers or additives, subsequent processing of the partially reacted material becomes difficult. Here, chemorheology is used to track changes in viscoelastic properties of uncatalyzed polybutadiene‐diisocyanate reactions for multiple days, which quantifies the degree of cure during polymerization. Viscosity profiles are accurately captured by the two‐stage Arrhenius model (r2>0.95), and conversion progress is adequately represented by the Kamal–Sourour model (r2>0.76). Transition state analysis via Wynne–Jones–Eyring–Evans theory (WJEE) reveals the presence of an associative mechanism according to entropic and enthalpic activation energies ΔS# = −96.9 J K−1 and ΔH# = 36.4 kJ, respectively. These rheokinetic modeling results align with Fourier transform infrared spectroscopy findings. To the knowledge, no rheokinetic studies have tracked cure progress of this uncatalyzed system for the extended timeframe presented here. The cure profile also presents unique viscosity growth, representative of molecular weight buildup, compared to catalyzed systems. This finding emphasizes the novelty of applying previously developed chemorheological models to multiday thermosetting reactions within a flow field in a manner that is generalizable to many long‐term curing systems.

Rheokinetic studies on the curing process of energetic systems containing RDX, HTPB with high content of 1,2-vinyl groups and hydantoin-based bonding agent

Series of rheokinetic studies involving pure HTPB with high content of 1,2-vinyl groups, binder systems with HTPB, IPDI and DBTDL, and energetic systems containing RDX, HTPB binder system and hydantoin-based bonding agent were conducted. Using the viscosity-temperature dependency of pure HTPB the VFT parameters were calculated. Next, pot-life and curing rate constants of HTPB/IPDI binder systems and kinetic parameters of Arrhenius and Eyring equations were determined. The influence of RDX and bonding agent on curing process of energetic systems was examined.

Rheokinetic studies and compressive response of high performance polybenzoxazine syntactic foams

ABSTRACTPolybenzoxazines are finding increasing usage in demanding applications where high temperature stability is required, especially in the field of aerospace. In this work, thermally stable bisphenol F‐based polybenzoxazine [poly(BF‐a)] syntactic foams containing varying volume fractions (30–60%) of hollow glass microballoons (HGMs) were prepared and their mechanical response in the quasi‐static regime was established. The effect of introducing glass microballoons on the curing profile of benzoxazine resin was studied using both nonisothermal differential scanning calorimetry and rheometry. Temperature‐sweep experiments were performed to arrive at the optimal processing window of the benzoxazine‐glass microballoons formulations, particularly in terms of viscosity, gelation temperature, and time. Thermally accelerated ring‐opening polymerization of the benzoxazine resin led to complete curing of the syntactic foam formulations, as assessed by calorimetric studies. The thermal degradation behavior of the poly(BF‐a)/HGM was studied using thermogravimetric analysis. As expected, the density of the syntactic foam specimens decreased with increasing microballoon content. Maximal increase in the specific compressive properties of the poly(BF‐a)/HGM samples was observed in formulations containing 40% volume fraction of glass microballoons. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci.2019,136, 47234.

Calorimetric and rheokinetic analyses merged to capture crystallization kinetics in polyamide/clay nanocomposites: Revisiting predictability of models

ABSTRACTCrystallization kinetics of polymer/clay systems was the subject of numerous investigations, but still there are some ambiguities in understanding thermal behavior of such systems under isothermal and nonisothermal circumstances. In this work, isothermal rheokinetic and nonisothermal calorimetric analyses are combined to demonstrate crystallization kinetics of polyamide6/nanoclay (PA6/NC) nanocomposites. As the main outcome of this work, we detected different regimes of crystallization and compared them by both isothermal dynamic rheometry and nonisothermal differential scanning calorimetry (DSC), which has not been simultaneously addressed yet. A novel analysis, somehow different from the common ones, is used to convert the storage modulus data to crystallinity values leading to more reasonable Avrami parameters in isothermal crystallization. It was found based on isothermal rheokinetic studies that increase of NC content and shear rate are responsible for erratic behavior of Avrami exponent and crystallization rates. Optimistically, however, isothermal crystallization by rheometer was confirmed by DSC. Nonisothermal calorimetric evaluations suggested an accelerated crystallization of PA6 upon increasing NC content and cooling rate. The crystallization behavior was quantified applying Ozawa (r2 between 0.070 and 0.975), and combinatorial Avrami–Ozawa (r2 between 0.984 and 0.998) models, where the latter appeared more appropriate for demonstration of nonisothermal crystallization of PA6/NC nanocomposites. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46364.

Cure kinetics of epoxy/MWCNTs nanocomposites: Isothermal calorimetric and rheological analyses

Abstract A combinatorial route has been applied in cure kinetics study of epoxy nanocomposites containing multi-walled carbon nanotubes (MWCNTs) based on differential scanning calorimetry and rheokinetic analyses under isothermal conditions. Pristine and amine-modified MWCNTs bearing primary and secondary amines were used at very low concentrations (0.1 and 0.3 wt.% based on epoxy weight). Model-free and model-fitting methods were applied on calorimetric data, and Kamal autocatalytic model revealed good agreement with the experimental data. In case of epoxy filled with amine-modified MWCNTs, activation energy took a value lower than that obtained for sample containing pristine MWCNTs at an identical loading because of less hindrance brought about by curing. Isothermal rheokinetic studies supported formation of a filler network during processing, which hindered epoxy-anhydride cure reaction at higher MWCNTs loadings leading to a highly crosslinked epoxy network.

New multi-block isophorone diisocyanate-based copolymers with urethane urea hard segments

Segmented polyurethane ureas (SPUUs) were synthesized on the base of oligopropylene oxide (PPO), oligotetramethylene oxide (PTMO), 2,4-toluene diisocyanate, isophorone diisocyanate (IDI), and methylene- o-chloroaniline. Structure of polymer chains with thermodynamically compatible PTMO and PPO soft segments (SS) was identified using data from rheokinetic studies of reactionary mixes with different diisocyanates. The significant swelling value in low-polar solvent toluene and non-elevated softening temperature indicates the formation of loose (defect) structure of the hard phase in compositions, synthesized using IDI. Thermal, thermomechanical, and physicomechanical properties of SPUU with variable compositions of hard segment and SS (blocks) were investigated. An interrelation between structure and properties of block copolymers of new type was estimated. Investigation results showed a significant increase in the strength of polyurethane urea at the equimolar ration between various hard blocks and SS on the base of PPO and PTMO. High strength values were obtained for SPUU in a wide diapason of stretching rates (0.56–0.003 s−1). Structural factors contributing to stability of strength properties of tetrablock-copolymers, namely: stronger inter-chain interaction in the soft phase and a “loose” structure of the hard phase. were revealed. The data relating to elevation of thermostability of these materials were given. New materials can be applied for various conditions of mechanical loading.

Specific features of curing of oligodieneurethane isocyanate prepolymer with glycerol in the presence of an organotin catalyst at twofold excess of isocyanate groups

The results of rheokinetic studies of catalytic curing of oligo dieneurethane prepolymer with glycerol at double excess of isocyanate groups are presented. The curing is described in the initial step by the polymerization equation with varying number of active sites (up to conversion x ≈ 20%); then, by an equation of an autocatalytic reaction with adiabatic self-acceleration (up to x ≈ 40%); and in the final step (x > 40%), by a second-order equation with self-acceleration effect.

Rheokinetic features of the initial steps of curing of oligodiene-urethane prepolymer with a hydroxyl-containing curing agent at equimolecular ratio of the reactive groups

Rheokinetic studies of curing of oligodiene-urethane prepolymer with a hydroxyl-containing curing agent at the molar ratio NCO/OH = 1.0 were performed. In the initial step of curing, catalyltic formation of polydiene-urethane follows a second-order equation, whereas the urethane formation step is adequately described by a kinetic equation with an autoacceleration effect.

Rheokinetic studies on the formation of urethane networks based on hydroxyl terminated polybutadiene

Rheokinetic studies on the formation of copolyurethane networks based on hydroxyl terminated polybutadiene, poly(12-hydroxy stearic acid-co-TMP) ester polyol and different isocyanates, such as toluene-di-isocyanate, hexamethylene-di-isocyanate, isophorone-di-isocyanate and methylene-di-cyclohexyl di-isocyanate were undertaken by following the build up of viscosity of the reaction mixture during the bulk polymerization reaction between the hydroxyl telechelic pre-polymers and the di-isocyanate compounds. Linear rheokinetic plots were obtained by plotting ln(viscosity) versus time and the slopes of which, are the rate constants for the viscosity build up. The rate constants range from 0.038×10 −2 to 27.1×10 −2 min −1 and were found to have strong dependence on the isocyanate type, NCO/OH equivalent ratio ( r-value) and copolyurethane composition as well as the structural characteristics, such as functionality distribution of the pre-polymers and the nature of the hydroxyl group.

Rheokinetics and the influence of shear rate on the Trommsdorff (gel) effect during free radical polymerization

AbstractSome rheokinetic studies of free radical polymerization have been published In the past. They generally make modeling of the time dependent increase of viscosity possible at low degrees of conversion, where Newtonian behavior holds and the rheokinetic equations can be linearized. In this paper we try to extend the rheokinetic approach to the region of high degrees of conversion, where the reacting solution is non‐Newtonian and the shear rate at which the polymerization is performed becomes a fundamental parameter. Moreover, nonlinear phenomena such as the Trommsdorff effect and phase separations are likely to occur‐ The principal result of this study is that the Trommsdorff effect is reduced and sometimes eliminated when the polymerization is performed at high shear rate. Instabilities of the time dependent increase of viscosity occurred when the conversion was higher than 85%. They were attributed to phase separation of the macromolecules under shear flow.

Rheo-kinetic evaluation on the formation of urethane networks based on hydroxyl-terminated polybutadiene

Rheo-kinetic studies on bulk polymerization reaction between hydroxyl-terminated polybutadiene (HTPB) and di-isocyanates such as toluene-di-isocyanate (TDI), hexamethylene-di-isocyanate (HMDI), and isophorone-di-isocyanate (IPDI) were undertaken by following the buildup of viscosity of the reaction mixture during the cure reaction. Rheo-kinetic plots were obtained by plotting ln (viscosity) vs. time. The cure reaction was found to proceed in two stages with TDI and IPDI, and in a single stage with HMDI. The rate constants for the two stages k1 and k2 were determined from the rheo-kinetic plots. The rate constants in both the stages were found to increase with catalyst concentration and decrease with NCO/OH equivalent ratio (r-value). The ratio between the rate constants, k1/k2 also increased with catalyst concentration and r-value. The extent of cure reaction at the point of stage separation (xi) increased with catalyst concentration and r-value. Increase in temperature caused merger of stages. Arrhenus parameters for the uncatalyzed HTPB-isocyanate reactions were evaluated. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1869–1876, 2001

Design of morphology in PMMA-modified epoxy resins by control of curing conditions. I. Phase behavior

Rheokinetic and phase separation behavior of diglycidylether of bisphenol-A–4,4′-diaminodiphenyl methane epoxy mixtures, modified with a constant amount (15 wt %) of poly(methyl methacrylate) (PMMA), have been investigated. Stoichiometric epoxy/amine mixtures precured at 80°C several times presented various levels of miscibility. Differential scanning calorimetry (DSC) and dynamic mechanic thermal analysis were used for rheokinetic studies of curing and also for testing the thermal behavior of the fully cured mixtures. Phase separation, through curing, was simultaneously studied by transmission optical microscopy and DSC, showing an excellent correlation between the results obtained with both techniques. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 772–780, 1999

Rheokinetic studies for the reaction injection molding of polydicyclopentadiene

AbstractDicyclopentadiene (DCPD) based liquid engineered resins are emerging among the newest commercial reaction injection molding (RIM) systems. Since the filling stage is coupled with chemical reaction, an understanding of the chemorheological changes is critical for establishing moldability criteria. An adiabatic reactive viscometry technique which involves the simultaneous measurement of viscosity and temperature changes was used to investigate the chemorheological changes during the adiabatic copolymerization of a DCPD based RIM system. The influence of impingement mixing conditions and initial material temperature on the reaction kinetics, rheology, as well as the resulting final physical properties of the system were investigated. The “apparent gel time” under adiabatic conditions is estimated from the intersection of the tangents to the initial and final portions of the viscosity curve. An autocatalytic kinetic model with an Arrhenius rate constant was used to model the reaction kinetics.

Some features of dimethyl diallyl ammonium chloride high conversion polymerization in aqueous solutions

The kinetic features of the high conversion radical polymerization of N, N-dimethyl- N, N-diallyl ammonium chloride (DMDAAC) in aqueous solutions have been studied for the first time. Kinetic measurements have been carried out by calorimetry. Special rheokinetic studies of the DMDAAC polymerization under conditions of small amplitude cyclic non-destructive deformation (dynamic measurements) produce independent information on the high conversion reaction.

Rheokinetic studies of the synthesis and properties of elastomers based on the oligodiene dihydrazides and unsaturated dicarboxylic imides

Rheokinetic methods were used to investigate the synthesis of elastomers from oligoisoprene dihydrazide and maleic acid bis-imides. The effect of the bis-imide structure, temperature-time parameters and the ratio of initial compounds in the rheokinetics of synthesis of cured compositions and on their deformation-strength properties has been investigated. A crosslinking mechanism is proposed, taking account of the reaction between terminal hydrazide and maleimide functional groups, as well as homopolymerization of maleimide. The feasibility of reinforcement of the obtained elastometer by active fillers has been ascertained.