Semibatch operation and primary cyclization effects in homogeneous free-radical crosslinking copolymerizations

A theoretical analysis is presented on the use of semibatch reactors for producing non-linear copolymers through the homogeneous free-radical polymerization of mono- and divinyl-monomers. The kinetic scheme distinguishes two kinds of pendant double bonds and five kinds of polymer radicals, as it also takes into account the primary cyclization of terminal divinyl units. In spite of the fairly large number of groups and reactions, the generating function of vector chain length distribution of polymer species can still be accurately computed by a numerical implementation of the method of characteristics and thus average molecular weights before and after gelation can be reliably obtained. The influence of feed policies of the monomers, initiator and transfer agent on gelation, average chain lengths and weight fraction of sol is discussed, as well as their sensitivity to some kinetic parameters.     

GEORDNETE EPOXIDNETZWERKE AUS 4-GLYCIDYLPHENYL-4-GLYCIDYL-BENZOAT und 3- BZW. 4-AMINOPHENYL-4-AMINOBENZOAT

The introduction of mesogenic groups in main- and sidechains of epoxy thermosets results in an ordered multiphase network with cellular structure, if gelation occurs below the maximal cure temperature T_max. T_max is individual for a given combination of monomers. The multiphase network consists of relatively soft anisotropic cell nuclei and hard isotropic cell walls. If gelation occurs above T_max single phase networks are obtained. The size of the cell nuclei strongly depends on the used monomers and varies up to two orders of magnitude. The multiphase structure was found to have no impact on the tensile elastic modulus.     

Prediction of mean square radius of gyration of tree-like polymers by a general kinetic approach

This paper describes a kinetic method to predict the z-average molecular mean square radius of gyration of tree-like polymers formed by irreversible reactions, assuming Gaussian chains. It is based on the population balance equations for the two-sided molecular distributions of pendant chains associated with every chemically distinguishable kind of bonds. An automated method for the solution of those equations is valid both before as well as after gelation for complex kinetic schemes. Examples of its use are presented with polycondensation systems leading to hyperbranched polymers, the anionic polymerization of mono- and divinyl monomers and a radical polymerization with terminal branching and transfer to polymer.     

Simultaneous interpenetrating networks of a polyurethane and poly(methyl methacrylate). I. Metastable phase diagrams

Any simultaneous interpenetrating network (SIN) synthesis contains three key events. These are gelation of polymer I, gelation of polymer II, and phase separation of polymer I from polymer II. Metastable phase diagrams of SINs are developed, in which the time occurrence of these three events is represented. A polyurethane/poly(methyl methacrylate) (PU/PMMA) system was chosen as a model. Polymerization kinetics were followed in situ for both PU and PMMA using Fourier Transform Infrared Spectroscopy (FTIR) with the aid of a heated demountable cell. Glass transitions of fully cured samples were determined by dynamic mechanical spectroscopy (DMS) and differential scanning calorimetry (DSC). Phase separation was determined by the onset of turbidity, and gelation of the first gelling polymer was determined by the sudden resistance of the system to flow. As a result, a metastable phase diagram was constructed for the four-component SIN system (the two monomers and their respective polymers) as a tetrahedron in three dimensions with the two monomers and two polymers at the four apexes. Phase separation and gelations of the two polymers are indicated by various surfaces. These surfaces intersect at lines and curves, representing unique conditions of an SIN synthesis, e.g., simultaneous gelation of both polymers, or simultaneous phase separation and gelation of polymer I, etc. These conditions are critical in terms of the development of the SIN morphology, dividing the reaction space into specific regions. Finally, it is shown how the tetrahedron diagram helps visualize the course of the three key events during SIN synthesis, and provides direction for controlling them. © 1995 John Wiley & Sons, Inc.     

Theoretical investigation on the polyaddition of A2 and CB2 monomers with non-equal reactivity

Taking account of the difference of reactivities between C and B group, the evolution of the monomers and the various structural units formed from the polyaddition of A₂ and CB₂ monomers was investigated by the kinetic mechanism. The calculated results theoretically explain the experimental data observed in our previous works very well, if the reactivity ratio of C to B groups is 200. The critical conversion of gelation for the A₂ + CB₂ type polymerization depends on the reactivity ratio. The lower the ratio, the earlier the gelation appears. Compared with the A₂ + B₃ type polymerization, the disappearance of gelation for the A₂ + CB₂ type polymerization should attribute to the much higher reactivity of C group than that of B one. The analytical expression of the degree of branching was derived as well. If the reactivity of C group is much higher than that of B group and the substitution effect is neglected, the value of the degree of branching for the hyperbranched polymers obtained is equal to the value of conversion of B groups and it may exceed 0.5, with the feed ratio of monomers varying from 1 to 3/2.     

A general kinetic method to predict sequence length distributions for non-linear irreversible multicomponent polymerizations

A method to predict the sequence length distribution (SLD) for homogeneous non-linear irreversible multicomponent polymerizations is described. With more than two monomers, it also predicts chain length distributions of the sub-domains containing a prescribed sub-set of the repeating units, but in all possible orderings. Its goal is the analysis of polymerization systems involving complex kinetic schemes in an automated way. The radical terpolymerization of two vinyl monomers with a divinyl monomer and a radical copolymerization including branching by transfer to polymer and by propagation on terminal double bonds are considered as case studies showing the interest of our approach. Finally, the concept of gelation of sequences is presented and discussed in comparison with the related and better known gelation of the population of polymer molecules.     

Stochastic modeling and simulation of photopolymerization process

In this study, the effect of photoinitiator concentration on the gelation time of different resins were studied in the absence of oxygen in the reaction volume by using passive microrheology technique. Four different monomers which are ethoxylated pentaerythritol tetraacrylate (SR494), trimethylolpropane triacrylate (SR351), triethylene glycol diacrylate (SR272), and 2(2‐ethoxyetoxy) ethyl acrylate (SR256) were used in these experiments. Resins were prepared from these four different monomers by mixing them with various amount of 2,2‐dimethoxy 1,2‐diphenylethanone photoinitiator molecule with high absorption coefficient at the frequency of UV light used in these experiment. The simulations of the results obtained from microrheology experiments were carried out with the new model based on the stochastic Monte Carlo approach in order to account for the inherently random and discrete nature of the photopolymerization reactions. The model captures the nonlinear decrease of gelation time with increasing photoinitator concentration and number of acrylate fragments on each monomer. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Preparation of a Porous Cermet SOFC Anode Substrate by Gelcasting of NiO–YSZ Powders

A porous NiO–YSZ substrate for anode-supported solid oxide fuel cells has been prepared by gelcasting of NiO–YSZ powders using urea–formaldehyde monomers, followed by humidity-controlled drying, binder removal, and sintering of the gelled bodies. The gelled bodies had sufficient strength to remove even 2-mm-thick samples from the mold immediately after gelation. A gelcast NiO–YSZ sample sintered at 1450°C for 2 h showed an open porosity of ∼53 vol%, and the porosity increased to ∼58% upon reduction with hydrogen. Pore sizes measured on the scanning electron microscopy photomicrograph of NiO–YSZ and Ni–YSZ cermet substrates are in the range of 2–5 μm. Urea–formaldehyde polymer, present in a high amount (∼13 wt%) in the gelcast body, acts as a template for pores.     

Hyperbranched poly(methyl methacrylate)s prepared by miniemulsion polymerization and their (non)-Newtonian flow behaviors

Miniemulsion polymerization is most suitable for the targeted synthesis of vinyl copolymers than the conventional emulsion polymerization, because in miniemulsion polymerization each monomer nanodroplet is a nanoreactor, and the monomers in each droplet are in situ converted to the corresponding polymers. Soluble and hyperbranched poly(methyl methacrylate)s (PMMA) were prepared with quantitative monomer conversion and without gelation by the miniemulsion copolymerization with di- and tri-acrylate and mediated with 1-dodecyl thiol (DDT). DDT acted both as a gelation prohibitor and as a reactive cosurfactant. The PMMAs with varied “X” or “Ж” shaped branches, depending on the di- and tri-functional acrylate used as the branching agent, are characterized and interpreted in terms of the repeating units per part, parts and branches per macromolecule, average molecular weight, latex particle size and size distribution. Effects of topology changes of the branched PMMAs on the rheological behaviors are observed for the first time: from Newtonian flow for the densely branched PMMAs to the non-Newtonian flow with pronounced shear thickening for the PMMA samples with high-molecular-weight and longer parts.     

Viscoelasticity and mechanical properties of reactive PVC plastisols

Reactive poly(vinyl chloride) (PVC) plastisols have been developed to substitute the hydrocarbon diluents generally used in low viscosity PVC plastisols. For this purpose, methacrylate monomers (5–15%) were added in the PVC suspension (based on diisononyl phtalate plasticizer) to reduce the viscosity at room temperature and to polymerize (by radical polymerization initiated by an organic peroxide) during the gelation process. Both the reactive processing and the gelation process were carried out between the plates of a rheometer cell in the linear viscoelasticity domain (small deformation) and under increasing temperature from room temperature up to 160°C (ω = 6.283 rad s^?1, = 5°C min^?1). A temperature criterion was proposed to define the right balance between the polymerization and the gelation to get the best mechanical properties (i.e., elongation and stress at break). The polymerization process must be slower than the gelation process as the polymerization must take place when PVC grains have fused together to form a homogeneous medium at least at the microscale. Actually, the polymerization kinetics can be controlled by the decomposition kinetics of the organic peroxide. Finally, triethylene glycol dimethacrylate and lauryl methacrylate monomers and dicumyl peroxide as initiator turned out to be the best reactive system for some potential industrial applications. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Chemorheological behavior of β‐SiAlON aqueous suspensions in gelcasting process

The chemorheological behavior of an aqueous suspension of monofunctional and difunctional monomers containing β‐SiAlON was investigated for gelcasting process. The effect of various parameters on gel and processing times was also studied. The time–temperature–transformation (TTT) diagram was used to correlate the pertinent processing parameters (time and temperature) with the gelation and processing times during in situ polymerization. The significant difference noticed between TTT diagrams of gel system containing β‐SiAlON and a pure gel system was a sharp shift of castable region to lower temperatures. This shift indicates that at the same monomers and initiator concentrations, the gelation can occur at lower temperature in the presence of SiAlON. These results clearly imply that the ingredient has accelerating effect on the polymerization reaction of acrylamide/methylene bisacrylamide. Reduction of polymerization activation energy from 72 kJ/mol for the monomer solution to 61 kJ/mol for a suspension containing SiAlON (35 vol%) confirmed this result. The impurities and surface groups of SiAlON are responsible for the accelerating effect of SiAlON. Furthermore, all suspensions showed shear thinning behavior, which were enhanced by β‐SiAlON rate of concentration increase. POLYM. ENG. SCI., 53:2123–2128, 2013. © 2013 Society of Plastics Engineers     

Passive microrheology for measurement of gelation behavior of a kind of polymer gel P(AM‐AA‐AMPS)

One kind of polymer gel P(AM‐AA‐AMPS) was prepared by radical aqueous copolymerization, using acrylamide (AM), acrylic acid (AA) and 1‐acrylanmido‐2‐methylpropanesulfonic acid (AMPS) as monomers, N,N‐methacrylamide (MBA) as crosslinker and ammonium persulfate (APS) as initiator. The microstructure and molecular structure of the polymer gel were characterized by environmental scanning electron microscope (ESEM), infrared spectrometer (IR) and thermal gravity analysis (TGA). Main factors affecting the gelation behavior of P(AM‐AA‐AMPS) were qualitatively and quantitatively studied by multi‐speckle diffusion wave spectroscopy (MS‐DWS) technology, and the elasticity index (EI) and macroscopic viscosity index (MVI) were introduced to evaluate the elasticity and viscosity of the polymer gel. The results show that the synthesized P(AM‐AA‐AMPS) polymer gel has three‐dimensional network structure gel with thermally resistant and salts tolerant groups. The EI and MVI of solution increase abruptly during the gelation time and the two indexes tend to stabilize. Under certain conditions, with the increase of reaction temperature and concentration of monomers and initiator, the gelation time of polymer gel gets shorter and the gel strength increases; with the increase of concentration of crosslinker, the strength of polymer gels increases, while the gelation time remains almost unchanged. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43364.     

Association Equilibrium and Gelation in Solutions of Cross-Associating Chains Containing Inactive Fragments

Solutions of two cross-associating chain molecular species (n-chains and p-chains) containing both associative (“sticky”) and non-associative monomer links have been simulated by molecular dynamics. The inactive monomers increase the stickers’ association equilibrium constant due to the excluded volume effect. In a wide concentration range, the equilibrium constant can be approximated as a simple function of the total volume concentration of the inert monomers, the volume effectively excluded by one inert monomer being about the same at different mixture compositions. Characteristics of molecular aggregates formed at pre-gelation and gelation conditions are examined, including effects of local cyclization.     

Effects of a Protic Ionic Liquid on the Reaction Pathway during Non-Aqueous Sol–Gel Synthesis of Silica: A Raman Spectroscopic Investigation

The reaction pathway during the formation of silica via a two-component “non-aqueou” sol-gel synthesis is studied by in situ time-resolved Raman spectroscopy. This synthetic route is followed with and without the addition of the protic ionic liquid 1-ethylimidazolium bis(trifluoromethanesulfonyl)imide (C₂HImTFSI) in order to investigate its effect on the reaction pathway. We demonstrate that Raman spectroscopy is suitable to discriminate between different silica intermediates, which are produced and consumed at different rates with respect to the point of gelation. We find that half-way to gelation monomers and shorter chains are the most abundant silica species, while the formation of silica rings strongly correlates to the sol-to-gel transition. Thus, curling up of linear chains is here proposed as a plausible mechanism for the formation of small rings. These in turn act as nucleation sites for the condensation of larger rings and thus the formation of the open and polymeric silica network. We find that the protic ionic liquid does not change the reaction pathway per se, but accelerates the cyclization process, intermediated by the faster inclusion of monomeric species.     

Simulation of models for multifunctional photopolymerization kinetics

In this study, two different models of photopolymerization of multifunctional monomers are developed. The first is a kinetic model and is based on first order nonlinear ordinary differential equations describing the rates of photoinitiator, monomer, and live and dead radicals of different chain lengths. In the second model, a stochastic Monte Carlo approach is used to predict the time evolution of the photopolymerization process. In this model the chain affects are incorporated both in the propagation and termination rate constants. The simulations of these models for the predictions of gelation times are compared with the experimental values measured in the absence of oxygen in the reaction volume by using passive microrheology technique. The nonlinear decrease of gelation time with increasing photoinitator concentration and number of acrylate fragments on each monomer is best captured with the second model. Furthermore, the second model is successfully verified with kinetic data generated from FTIR and DSC experiments. POLYM. ENG. SCI., 54:1737–1746, 2014. © 2013 Society of Plastics Engineers     

Effect of intramolecular reactions in multifunctional step growth polymerizations in cascades of continuous-flow,stirred-tank reactors

A simple kinetic model describing intramolecular reactions in multifunctional step growth polymerizations of RA_j monomers is presented. The model is far simpler than more detailed kinetic or Monte Carlo models which pose severe numerical difficulties close to gelation. The formulation presented is quite general, but it requires a relationship for the ensemble-cum-time average number of rings, r _n, per molecule having n RA_j units. A very simple yet intuitively correct equation has been chosen for r _n, which involves an empirical parameter ?? Results on cascades of homogeneous continuous-flow stirred-tank reactors (HCSTRs) show trends which are physically expected. The present formulation could possibly be used for scale-up of HCSTRs, with ?? obtained experimentally. It is hoped that future studies using rigorous models would lead to more fundamental and improved relationships for r _n.     

Investigation of chitosan gelation mechanisms by a modeling approach coupled to local experimental measurement

The formation of chitosan hydrogels using experimental and modeling approaches are described. Chitosan gelation was induced by an ammonia intake from a liquid phase (wet process) or a gaseous phase (vapor process), involving three steps: (i) external mass transfer at the interface, (ii) internal transport within the polymer solution, and (iii) chemical reactions inducing gelation. The experimental study allowed quantifying gelation fronts speeds from local measurement. The main resistance to gelation was investigated for both gelation processes. Experimental results exhibited that internal diffusion through the chitosan matrix was the main resistance in the whole gelation mechanism. The numerical model involved a coupling between mass and heat transfer phenomena and chemical reactions, in transient conditions. Numerical results were first validated and then used as a predictive tool to investigate (i) coupled mechanisms localized in the chitosan matrix and (ii) the influence of operating conditions on gelation rates. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Thermally and anionically initiated cure of bismaleimide monomers

A kinetic and thermodynamic study of the thermally or anionically induced cure of aromatic bismaleimide monomers (namely 2,4-bismaleimidotoluene, 4,4′-bismaleimidodiphenylmethane and their eutectic mixture) is presented. Calorimetric investigations allowed the determination of heats of polymerization and activation energies for both types of initiations, and the values found are compared with data in the literature. Isothermal cures were also carried out with and without anionic initiator (typically imidazole). SEC measurements were used to monitor the conversion of the monomers as a function of time. The thermally induced polymerization seems to proceed in a rather heterogeneous way, owing to an initiation that is too slow: gelation appears at first only as the formation of microgels, with no immediate increase in the viscosity; macrogelation occurs some time after. On the contrary, the anionically induced polymerization is more homogeneous. A complete study of a particular reacting system revealed no interference between radical and anionic polymerization over a wide range of temperatures, and in this case gelation results in an immediate increase in viscosity. However, the conversion at the gel point is unexpectedly high, which can be interpreted as the result of a very fast initiation as well as of transfer reactions. Glass transition temperatures were also measured as a function of time and curing temperature in order to study the influence of vitrification on the kinetics and to check for the existence of a unique relation between T_g and conversion. This actually proved to be correct as long as diffusion does not slow down the reaction; in this case Gillham's treatment of T_g versus in t was successfully applied and led to the correct value of the activation energy. All the measurements allowed us to establish the time-temperature-transformation cure diagram for a specific reacting system. © 1993 John Wiley & Sons, Inc.     

Transition diagrams for a liquid crystalline thermoset containing a rigid‐rod epoxy

The liquid crystalline thermoset monomers 4,4′‐diglycidyloxy‐α‐methylstilbene (DOMS) and D2A1 that resulted from the reaction between DOMS and aniline were synthesized. DOMS was cured with sulfanilamide and D2A1 was cured with catalytic curing agent 1‐methyl imidazole. Transition diagrams have been constructed and the percent conversion data were determined from dynamic differential scanning calorimetry scan to examine the applicability of Flory's gelation theory. The results reveal that even though the amount of reaction that occurs in the liquid crystalline phase is different at different cure temperatures, the isoconversion theory of gelation fits quite well. POLYM. ENG. SCI. 46:623–629, 2006. © 2006 Society of Plastics Engineers