Isothermal Polymerization Research Articles

Calorimetric studies on an anhydride-cured epoxy resin from diglycidyl ether of bisphenol-A and diglycidyl ether of poly(propylene glycol). I. Onset of diffusion control during isothermal polymerization

Calorimetric studies on a series of anhydride-cured epoxy resins, in which the epoxy oligomer is a mixture of diglycidyl ether of bisphenol-A (DGEBA) and diglycidyl ether of poly(propylene glycol) (DGEPPG) in different mole ratios, were carried out. DGEPPG is a flexible epoxy oligomer that was used to tune glass transition temperature for the fully reacted epoxy resin. Conversion versus time curves for the systems with different DGEBA/DGEPPG mole ratios (not including the neat DGEPPG system) were found to overlap with each other in mass-controlled reaction regime, indicating similar reactivities of epoxy groups in both epoxy oligomers. Onset of diffusion-controlled reaction regime for different systems was estimated by fitting the conversion versus time data using a phenomenological kinetic equation, as well as from direct comparison of the conversion versus time curves. For the systems (i.e., 0, 10, and 30% DGEPPG) that vitrify during reaction, the crossover from mass-controlled to diffusion-controlled reaction occurs close to the onset of the vitrification, where Tg is about 25–30 K below the reaction temperature. For the system (i.e., 50% DGEPPG system) that does not vitrify during the reaction, such crossover still occurs when the Tg of the mixture reaches a value about 25 K below the reaction temperature. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2155–2165, 2008

Toughening of polymerized cyclic butylene terephthalate with carbon nanotubes for use in composites

Low viscosity cyclic butyleneterephthalate (CBT) is a promising material for thermoplastic composites, however after isothermal polymerization (to PBT) and crystallization the material is rather brittle. In the present work, high aspect ratio nanofiller, carbon nanotubes (CNTs), are added to improve ductility. The dispersion of CNTs in CBT is achieved by rotational mixing and appears optimal. Concentrations up to 0.1 wt.%CNT are added to investigate their influence on the mechanical behavior. The addition leads to an increase in stiffness, strength and energy to failure, however, the failure strain slightly decreases. Thermal measurements indicate that the CNTs have no influence on the crystallinity. However, when the mixture of CBT and CNTs is processed in fiber reinforced composites processed by vacuum infusion, the CNTs show a tendency of filtration through the fabric.

New insight into isothermal frontal polymerization models: Wiener's method to determine the diffusion coefficients for high molecular-weight poly(methyl methacrylate) with neat methyl methacrylate

We used Wiener's method (a laser sheet deflection technique abbreviated LLD) to determine how well LLD (laser line deflection) could detect different diffusive layers within a glassy polymer system of poly(methyl methacrylate) with its own monomer of methyl methacrylate. LLD showed distinct boundaries that defined the layers that existed within the swelling/diffusing system, i.e. pure polymer, penetrated polymer, and swollen layer as identified and defined by Ueberreiter in the 1960s. Analysis of the LLD images showed that for early times within the diffusion that the layers between the pure, unpenetrated polymer and the pure monomer possessed a continuous refractive-index gradient, and thus, these layers formed one continuous layer, the “swollen layer”. The width of this swollen layer widened over time, and its widening was halted by the appearance of a second swollen layer formed from the pure polymer underneath the first swollen layer. Using the boundary positions of the swollen layer over time, we determined the solvent penetration velocity and the induction period, and we used these two parameters to calculate the average mutual diffusion coefficient ( D ¯ ). In addition, we studied the diffusion at three different temperatures to determine if the diffusion coefficients had a measurable temperature dependence. This paper also discusses how LLD relates to other techniques that have been used to monitor glassy polymer diffusion.

Crystallization features of oxidized isotactic polypropylene

The effect of the initial molecular weight parameters of polypropylene on the kinetics of isothermal polymerization is shown. It is found that the feed polymer with a wider molecular weight distribution (P-1) is characterized by lower values of the half-crystallization time as compared to the polymer with a narrower molecular weight distribution (P-2). Comparison between the crystallization parameters and the kinetics of accumulation of oxidation products showed that as the degree of oxidation of the polypropylene increases, the crystallization rate for both samples decreases and the concentration of the oxidation products increases. In this case, the degradation of tie chains accelerates the formation of crystallites. It is shown that P-1 has a higher nucleation energy as compared with P-2 and that this parameter increases during the oxidation of both polymers.

Rheological behavior of thermoset/thermoplastic blends during isothermal curing: Experiments and modeling

The evolution of the viscoelastic properties of a molten thermoplastic/thermoset system during the course of the isothermal polymerization of the thermoset precursors has been investigated and modeled. Such systems are initially homogenous and phase separate upon polymerization of the monomers. In the present study, atactic polystyrene (85 and 60wt%) is blended to a stoichiometric mixture diglycidyl ether of bisphenol A with 4,4′-methylenebis(2,6-diethylaniline). During the polymerization, polystyrene becomes the thermoplastic-rich matrix and an epoxy-rich dispersed phase appears. Both phases experience changes in their composition and viscoelastic properties. A rheokinetic model is proposed to take into account four contributions to the viscoelastic behavior: progressive deplastification of the polystyrene matrix involving a modification of the glass transition and thus of free volume, dilution of the network of entanglements of the matrix by the non yet converted low molar weight molecules, emulsion behavior after the separation of the epoxy-rich phase and finally interparticular interactions being assimilated to a mechanical percolation. Provided that the glass transition temperature of the matrix and the dynamic moduli of the neat components are known, the changes in the viscoelastic behavior of the system with time can be predicted with no ad hoc parameter and model calculations are in good agreement with the experimental data.

Study of the isothermal free radical polymerization of some higher n-alkyl methacrylates

The free radical polymerizations of higher n-alkyl methacrylates were not investigated in detail until now. In this work, the courses of the isothermal free radical bulk polymerization of dodecyl, quatrodecyl and hexadecyl methacrylates were investigated by differential scanning calorimetry. The effects of the polymerization temperature and the alkyl group length in the esters on the monomer conversions during polymerization were studied. It was found that the polymerization rate vs. time curves have two maxima. The free radical polymerizations of above-mentioned monomers proceed with slightly expressed gel effect at the temperatures below 90°C, at initiator concentration 1 mass% in monomer.

Quasi-stationary concentration principle for auto-wave processes

Quasi-Stationary Concentration (QSC) principle was used to study radical polymerization of vinyl monomers during reaction traveling front. Based on the experimental results and numerical calculation of one-dimensional front it was shown that the reaction zone could be both narrow and wide. Full and quasi-stationary problems were solved for different sets of kinetic and thermo-physical parameters typical for vinyl monomers' radical polymerization. Polymerization proceeds practically at a constant temperature for narrow-reaction-zone case as was shown by comparing the dimensionless values of heat-wave velocities and the final temperatures. Hence, criterial conditions obtained for isothermal polymerization permit to use QSC principle accurately.

Temperature modulated DSC study of the kinetics of free radical isothermal network polymerization

Temperature modulated differential scanning calorimetry (TMDSC) is used to study the kinetics of the free radical isothermal polymerization of triethyleneglycol dimethacrylate (TEGDMA). Azo-bis-isobutironitrile was used as initiator. The polymerization’s temperature is lower than the final glass transition temperature of the polymer network. The measurement of the average heat flow released and the heat capacity during the reaction allows identifying the different stages of the reaction. The presence of double peaks in the heat flow is ascribed to the autoacceleration. The influence of temperature, measuring conditions and oxygen are described. Vitrification is detected by the drop in heat capacity. It occurs at increasing conversion rates for increasing temperatures. After vitrification, the diffusion-controlled reaction continues.

Analysis of the Phase Separation Induced by a Free-Radical Polymerization in Solutions of Polyisobutylene in Isobornyl Methacrylate

We analyze a polymerization-induced phase separation taking place in solutions of a polymer dissolved in a vinyl monomer undergoing an isothermal free-radical polymerization. The selected system was a solution of polyisobutylene (PIB) in isobornyl methacrylate (IBoMA). Cloud-point curves for PIBs of different molar masses were fitted with the Flory-Huggins equation stated for a 3-component system. Phase separation was analyzed for two different PIBs at off-critical compositions. Comparing experimental values of the volume fraction of dispersed phase with those predicted by the coexistence curves it was found that the system evolved close to equilibrium during an initial stage. But phase separation stopped at intermediate monomer conversions as assessed by scanning electron micrographs of partially converted materials. The evolution of the size distribution of dispersed domains was obtained from the fitting of light scattering (LS) spectra. Average sizes predicted by LS and measured in scanning electron micrographs were in rough agreement.

Chemorheological studies on a thermoset PU/clay nanocomposite system

Chemorheology of neat polyurethane (PU) system, composed of a diisocyanate and a polyol, and a PU/clay nanocomposite system with an organoclay 5 phr was investigated. Differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR) were used to study the polymerization behavior of both systems. The PU/clay nanocomposite system showed a faster polymerization rate than neat PU system. The rheological property change during isothermal and dynamic polymerization of both systems was measured using a rheometer to analyze chemorheology of them. A viscosity function that can describe viscosity change of a thermoset resin system during polymerization was adopted to analyze the viscosity data obtained from the rheometer. The PU/clay nanocomposite system had shorter gelation time than neat PU system. Structural analyses of the PU/clay nanocomposite were performed by X-ray diffractometer (XRD) and transmission electronic microscope (TEM).

Isothermal frontal polymerization: Confirmation of the isothermal nature of the process and the effect of oxygen and polymer seed molecular weight on front propagation

AbstractIsothermal frontal polymerization is a directional polymerization that utilizes the Norish‐Trommsdorff (gel) effect to produce optical gradient materials. When a solution of methyl methacrylate and thermal initiator contacts a polymer seed (a small piece of poly(methyl methacrylate), a viscous region is formed in which the polymerization rate is faster than in the bulk solution. We obtained definitive evidence of the isothermal nature of the process by placing thermocouples above the propagating front. Using the optical technique of laser line deflection (Weiner's method), we studied the front propagation to determine the induction period, and the maximum distance propagated as a function of the molecular weight of the seed. We determined that the polymer seed must have a minimum molecular weight to initiate a front. We also determined that oxygen would act as a bulk polymerization inhibitor and increase the front propagation distance, but after purging the monomer–initiator solution with oxygen for several hours, the distance was shortened. We ascribed this behavior to the formation of peroxy radicals from the slow decomposition of the initiator and subsequent reaction with oxygen. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3601–3608, 2006

Modeling isothermal free-radical frontal polymerization with gel effect using free volume theory, with and without inhibition

Frontal Polymerization is a process that converts monomers into polymers by means of a propagating spatially localized reaction front. Such fronts exist with free-radical polymerization, where in the simplest case, a mixture of monomers and initiator is placed into a test tube and upon initiation of the reaction at one end of the tube, a self-sustained wave develops and propagates through the tube. Isothermal Frontal Polymerization (IFP), often referred to as interfacial gel polymerization, occurs due to the coupling of mass diffusion of the species and the gel effect. Utilizing the free volume theory of Vrentas and Duda for describing the self-diffusive behavior of the gel effect, we mathematically model and study this IFP process. We determine, both numerically and analytically, characteristics of the process including the propagation velocity of the reaction zone, the structure of the wave, and the distance traveled by the front before it breaks down due to reactions ahead of the front.

Isothermal frontal polymerization: Confirmation of the mechanism and determination of factors affecting the front velocity, front shape, and propagation distance with comparison to mathematical modeling

AbstractIsothermal frontal polymerization (IFP) is a directional polymerization that uses the Trommsdorff, or gel, effect to produce gradient materials for optical applications. When a solution of methyl methacrylate and a thermal initiator contacts a polymer seed (a small piece of polymer), a viscous region is formed in which the polymerization rate is faster because of the Trommsdorff effect. Using the optical techniques of laser line deflection (Weiner's method) and shadowgraphy along with controls, we obtained definitive experimental evidence of IFP. Moreover, we were able to measure accurately and precisely the front position and front concentration profile as a function of time by monitoring IFP systems and controls of various initiator concentrations and cure temperatures. The experimental data were compared with theoretical predictions from a model using mass‐diffusion and radical polymerization kinetics. The model reproduced the decrease of the propagation time and showed an increase in the propagation velocity for an increase in the initiator concentration and/or cure temperature. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5774–5786, 2005

ε-Caprolactam polymerization in presence of polyhedral oligomeric silsesquioxanes (POSS)

Polyhedral oligomeric silsesquioxanes (POSS) have been covalently linked to polyamide 6 (PA6) chains with the aim of synthesizing hybrid organic/inorganic polymer materials. The synthesis has been achieved by in situ polymerization of ε-caprolactam (CL) in presence of increasing amounts of POSS molecules, using two polymerization mechanisms (hydrolytic and anionic). The latter method has been carried out by three different approaches, in order to get PA6 samples characterized by various morphologies and content of structural defects: (i) quasi-adiabatic bulk polymerization; (ii) isothermal bulk polymerization; (iii) quasi-isothermal suspension polymerization. The products obtained have been characterized in term of structure, morphology, thermal properties and molecular mass.

Kinetic studies on the thermal polymerization of N‐chloroacetyl‐11‐aminoundecanoate potassium salt

AbstractA potassium salt of N‐chloroacetyl‐11‐aminoundecanoate was thermally polymerized to obtain the corresponding poly(glycolic acid‐alt‐11‐aminoundecanoic acid). A kinetic study was then performed that was based on isothermal and nonisothermal polymerizations performed in a differential scanning calorimeter. The complete kinetic triplet was determined (the activation energy, pre‐exponential factor, and integral function of the degree of conversion). A kinetic analysis was performed with an integral isoconversional procedure (free model), and the kinetic model was determined both with the Coats–Redfern method (the obtained isoconversional value being accepted as the effective activation energy) and through the compensation effect. The polymerization followed a three‐dimensional growth‐of‐nuclei (Avrami) kinetic mechanism. Isothermal polymerization was simulated with nonisothermal data. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1166–1176, 2005

EFFICIENT INPUT SIGNAL DESIGN FOR THIRD-ORDER VOLTERRA MODEL IDENTIFICATION

The present work addresses the identification of third-order Volterra models from input-output process data. Building on previous studies regarding input sequence design for third-order Volterra models (Soni and Parker, 2004), a new reduced-length sequence is designed to identify third-order sub-diagonal kernels. This input sequence leads to a 30% reduction in the data required to accurately estimate the sub-diagonal kernel. Identification of second-and third-order off-diagonal kernels is carried out using a random binary sequence (RBS). These input sequences exploit the third-order Volterra model structure and use the prediction error variance expression as a measure of model fidelity. The utility of the proposed approach is demonstrated on an isothermal polymerization reactor case study. The reduced length sequence shows excellent performance and results in an 47% improvement in the sum-squared error (SSE) value of the third-order sub-diagonal kernel.

Changes in molecular dynamics of the tri-ethylene glycol dimethacrylate monomer upon polymerization

The dielectric behavior of the tri-ethylene glycol dimethacrylate monomer was investigated by dielectric relaxation spectroscopy after isothermal polymerization at different temperatures and compared with the unreacted monomer. The relaxation process associated with the monomer glass transition slightly deviates to lower frequencies/higher temperatures in partial polymerized samples. Additionally, it is stronger depleted the higher the polymerization temperature, vanishing after a temperature where the polymerization reached completion. A secondary relaxation process related with localized mobility with the appearance of an excess wing in the bulk monomer previously studied in a narrower frequency range, becomes better defined after partial polymerization due to the depletion of the main relaxation process. Its location and activation energy is independent of the polymerization temperature. Whereas its intensity does not change significantly upon polymerization, the decrease of the intensity of the glass transition process could be taken as a degree of conversion. A αβ splitting scenario was drawn that maintains its pattern in the partial polymerized systems relatively to the bulk monomer. The fragility parameter of the bulk monomer was recalculated (m=85), slightly increasing when the monomer left unreacted co-exists with the newly formed polymer.

Multiobjective Optimization of a Semibatch Epoxy Polymerization Process Using the Elitist Genetic Algorithm

Multiobjective Pareto optimal solutions for a semibatch isothermal epoxy polymerization process are obtained by adapting the binary-coded nondominated sorting genetic algorithm II (NSGA II). The nu...

Effects of vitrification on the isothermal polymerization of acrylate blends under radiation

The radiation-induced polymerization of some acrylate oligomers was investigated by means of FT-IR spectroscopy applied to films of various thickness that were cured under isothermal conditions. Different types of oligomers, one polyethyleneglycol diacrylate (PEGDA), one polyurethane triacrylate (PURTA), and one epoxy diacrylate derived from bis-phenol A (EPDA) were selected for their contrasting networks properties, with a low, intermediate, and high T g of the radiation-cured materials, respectively. In order to gain a deeper insight into the effect of gradual vitrification on the reactivity of the polymerizable systems, the conversion - dose profiles of the epoxy diacrylate were recorded at various temperatures. The dependence of the limiting conversion on the polymerization temperature was examined under various experimental conditions and compared with the conversion - T g relation, which was deduced from dynamic thermo-mechanical analysis. The correlation between resulting T g measured in the samples at maximal conversion for each curing temperature was shown to be slightly deviating from linearity. A phenomenological model of the reaction kinetics was shown to depict satisfactorily the observed photopolymerization profiles. From the well-determined T g - conversion relation, a semi-mechanistic kinetic model taking into account the gradual vitrification of the network can be foreseen.

Presence of Two Maxima in the Isothermal Free-Radical Polymerization Rate of Isobornyl Methacrylate Retarded by Oxygen

The kinetics of the free-radical polymerization of isobornyl methacrylate (IBoMA) at 80C, initiated by benzoyl peroxide (BPO), was determined by differential scanning calorimetry (DSC), using sample pans previously sealed in air or in nitrogen. The polymerization was arrested by vitrification at a final conversion close to 0.80. The kinetics in nitrogen could be fitted using initiation and propagation rate constants reported in the literature, a functionality of the termination rate constant controlled by translational diffusion based on the free-volume model, and an efficiency factor for the initiator decomposition that decreased with conversion in the medium-high conversion range. The presence of oxygen led to a significant decrease in the polymerization rate, an effect that was enhanced when decreasing the sample size (increasing the surface-to-volume ratio). The original finding was the presence of two maxima in the isothermal polymerization rate (a double gel effect), a fact that was confirmed by replicating experiences in two different DSC devices and changing the amount of initiator in the formulation. The first maximum in the polymerization rate was explained by the decrease of the diffusion rate of macromolecular radicals to the oxygen-rich boundary, an effect that starts at low conversions. The second maximum was related to the decrease in the solubility/diffusion of oxygen in the reaction medium, a phenomenon that is particularly severe at high conversions.