Free Radical Bulk Polymerization Research Articles

Viscosity of bulk free radical polymerizing systems under near-isothermal and non-isothermal conditions

A viscometer-reactor assembly is used to generate data on the viscosity, η( t), of an example polymerizing system exhibiting the Trommsdorff effect, namely, the bulk free radical polymerization of methyl methacrylate (MMA), at different temperature conditions [near-isothermal and non-isothermal (near-step increase and near-step decrease in temperature)] and at two different initiator, 2,2′-azoisobutyronitrile (AIBN), concentrations. Two types of cup and bob assemblies, viz., the Haake ® SV-2 and the Haake ® HV-DIN, have been used to measure η( t) of the reaction mass, until reasonably high values of viscosity, well into the gel effect region. Only three sets of experimental data on x m( t), M w( t) and η( t) under near-isothermal conditions, are used to develop general correlations for the viscosity. These tuned correlations predict the values of the viscosity for a whole variety of other experimental conditions, including non-isothermal cases, reflecting that the physics of the system is well represented by them. Hence, these correlations can be used for other systems after tuning their parameters. The feasibility of on-line soft sensing is demonstrated for a few cases.

Experimental Study of a Tetrafunctional Peroxide Initiator: Bulk Free Radical Polymerization of Butyl Acrylate and Vinyl Acetate

This investigation examines the use of a tetrafunctional peroxide initiator in the bulk free radical homopolymerization of butyl acrylate and vinyl acetate. Rate of polymerization, molecular weights and in certain cases, gel fraction data were collected for selective experiments and compared to the results obtained with a monofunctional counterpart. In order to avoid the formation of insoluble gel material, further experiments were completed with varying concentrations of a chain transfer agent. Experimental results showed that when used at identical concentrations, the tetrafunctional initiator produced a faster rate in the polymerization of both butyl acrylate and vinyl acetate compared to the monofunctional initiator. Without the use of a chain transfer agent, runs with butyl acrylate produced significant amounts of gel material isolated by Soxhlet extraction. For a particular reaction time, the tetrafunctional initiator produced higher levels of gel compared to its monofunctional counterpart. When a chain transfer agent was used, the molecular weights were determined by its concentration and in turn, initiator functionality did not have an impact. In the polymerization of vinyl acetate, similar observations were made and no discernable difference could be seen between the molecular weight results of samples produced with either initiator.

The synthesis and characterization of polystyrene/magnetic polyhedral oligomeric silsesquioxane (POSS) nanocomposites

Fc–CH CH–C 6H 6–(C 5H 9) 7Si 8O 12 (POSS1, Fc: ferrocene) which contain both metal and C C double bond was firstly synthesized by Wittig reaction. The chemical structure of POSS1 was characterized by FTIR, 1H, 13C and 29Si NMR, mass spectrometry and elemental analysis, and the magnetic property of POSS1 have also been studied. Polystyrene composites containing inorganic–organic hybrid polyhedral oligomeric silsesquioxane (POSS1) were prepared by bulk free radical polymerization. XRD and TEM studies indicate that POSS1 is completely dispersed at molecular level in PS matrix when 1 wt% POSS1 is introduced, while some POSS1-rich nanoparticals are present when content of POSS1 is beyond 3 wt%. GPC results show that molecular weight of the PS/POSS1 nanocomposites are increased with addition of POSS1. TGA and TMA data show the thermal stabilities of PS/POSS1 nanocomposites have been improved compared to neat PS. The PS/POSS1 nanocomposites also display higher glass transition temperatures ( T g) in comparison with neat PS. Viscoelastic properties of PS/POSS1 nanocomposites were investigated by DMTA. The results show the storage modulus ( E′) values (temperature> T g) and the loss factor peak values of the PS/POSS1 nanocomposites are higher than that of neat PS. Mechanical properties of the PS/POSS1 nanocomposites are improved compared to the neat PS.

Effect of acrylic copolymer and terpolymer composition on the properties of in-situ polymer/silica hybrid nanocomposites

Acrylic copolymer- and 5% acrylic acid (AA) modified terpolymer-silica hybrid nanocomposites were synthesized by free radical bulk polymerization of ethyl acrylate (EA), butyl acrylate (BA) and acrylic acid (AA) with simultaneous generation of silica from tetraethoxysilane by sol-gel reaction. The pure polymers were analyzed by using Fourier Transform infrared (FTIR) spectroscopy and solid state nuclear magnetic resonance (NMR) spectroscopy. The hybrid samples were characterized by scanning electron microscopy (SEM), FTIR spectroscopy, NMR spectroscopy, dynamic mechanical, mechanical and thermal properties. SEM images confirmed the presence of nanosilica particles within the polymer matrices, whose dispersion and particle size distribution and visual appearance were dependent on the relative polarity (hydrophilicity) of the polymer matrices and the concentration of the filler. There was no evidence of strong chemical interaction between the polymers and the dispersed silica phase, as confirmed from the FTIR results. Terpolymer-silica hybrids demonstrated superior mechanical properties compared to the copolymer-silica hybrids. They also showed higher dynamic storage modulus and positive shift in the loss tangent peaks. The thermal stability of the nanocomposites was marginally higher which was possibly due to the dipolar interaction at the organic-inorganic interface.

Poly( N, N-dimethylaminoethyl methacrylate)/polysulfone composite membranes for gas separations

Composite membranes comprised of a thin cationic poly( N, N-dimethylaminoethyl methacrylate) (PDMAEMA) layer and a microporous polysulfone (PSF) substrate were prepared by coating a layer of PDMAEMA onto the PSF substrate. The homopolymer PDMAEMA was synthesized by free radical bulk polymerization using 2,2′-azobisisobutyronitrile as an initiator. The membrane morphology was examined under scanning electron microscopy. The effects of parameters involved in the membrane preparation procedure on the permselectivity of the resulting membrane were investigated. The permeability of the membrane to H 2, N 2, O 2, CO 2, and CH 4 was tested, and the membrane showed a high permselectivity to CO 2. For example, at 23 °C and 412 kPa of CO 2 feed pressure, the permeance of CO 2 was 30 GPU and the CO 2/N 2 ideal separation factor was 53. The high CO 2/N 2 permselectivity of the membranes make them particularly suitable for CO 2 capture from flue gas, a system relevant to greenhouse gas emission control. The effect of temperature on the performance of the membrane was also evaluated.

Bulk free radical polymerizations of methyl methacrylateunder non-isothermal conditions and with intermediate additionof initiator: Experiments and modeling

Experimental data on the monomer conversion, x m, and the weight average molecular weight, M w, have been generated under several isothermal and non-isothermal conditions for the polymerization of methyl methacrylate in a rheometer-reactor assembly. The non-isothermal results, in particular, can be used to provide more stringent tests of kinetic models than isothermal data alone. A simple empirical model has been used to describe this system that accounts for the gel (Trommsdorff) and glass effects. The model involves only x m and the temperature, and is quite general. The model parameters are tuned using only three sets of isothermal data. Good agreement is found between the experimental results and model predictions for a whole variety of experimental conditions, including non-isothermal operation and with intermediate addition of initiator. Because of its generality, this model is quite suitable for use for on-line optimizing control as well as for describing industrial reactors.

Accessing Chain Length Dependent Termination Rate Coefficients of Methyl Methacrylate (MMA) via the Reversible Addition Fragmentation Chain Transfer (RAFT) Process

AbstractSummary: The RAFT‐CLD‐T methodology is demonstrated to be not only applicable to 1‐substituted monomers such as styrene and acrylates, but also to 1,1‐disubstituted monomers such as MMA. The chain length of the terminating macromolecules is controlled by CPDB in MMA bulk free radical polymerization at 80 °C. The evolution of the chain length dependent termination rate coefficient, k, was constructed in a step‐wise fashion, since the MMA/CPDB system displays hybrid behavior (between conventional and living free radical polymerization) resulting in initial high molecular weight polymers formed at low RAFT agent concentrations. The obtained CLD of kt in MMA polymerizations is compatible with the composite model for chain length dependent termination. For the initial chain‐length regime, up to a degree of polymerization of 100, kt decreases with α (in the expression k = k · i−α) being close to 0.65 at 80 °C. At chain lengths exceeding 100, the decrease is less pronounced (affording an α of 0.15 at 80 °C). However, the data are best represented by a continuously decreasing non‐linear functionality implying a chain length dependent α. magnified image

Polystyrene-grafted Carbon Fibers: Surface Properties and Adhesion to Polystyrene

It is highly desirable to improve attractive interactions between carbon fibers and unreactive thermoplastic matrices to the possible maximum. This could be achieved by a simple grafting process to create a covalently bonded interface or interlayer, which should result in cohesive interactions between the polymer-grafted fibers and the same matrix material, leading to a better adhesion strength in the obtained composite material. Here, we are describing the grafting of styrene onto unmodified and unsized carbon fibers via free-radical bulk polymerization in the presence of fibers. After grafting, the surface properties of the carbon fiber approach those of pure polystyrene which was proven by contact angle and zeta (ζ) potential measurements. As indicated by the water contact angle, the carbon fiber surface becomes more hydrophobic. Scanning electron microscopy (SEM) provides evidence of grafted polymer. This simple procedure results in a continuous polystyrene coating. The fiber diameter increases significantly after polymer grafting. The adhesion and fracture behavior between the original and polystyrene-grafted carbon fibers to a polystyrene (VESTYRON®) matrix was characterized using the single-fiber pull-out test. There is a considerable increase in the measurable adhesion, i.e., the interfacial shear strength IFSS, by almost 300% between the grafted fibers and polystyrene as compared to untreated original fibers. Two planes of interfacial failure could be distinguished; first in the fiber coating interface leading to lower interfacial shear strength and second in the PS-matrix-PS-coating interphase resulting in a higher interfacial shear strength. In addition to the improved adhesion, there are also clear differences in the pull-out behavior between the nongrafted and grafted fibers. After the initial debonding process corresponding to the maximal pull-out force is completed, the pull-out force is increasing again.

The effect of camphorsulfonic acid in TEMPO-mediated bulk and dispersion polymerization of styrene

The TEMPO-mediated living free-radical bulk and dispersion polymerization of styrene in the presence of camphorsulfonic acid (CSA) are investigated. In the absence of TEMPO and CSA in the bulk polymerization, a conversion of 93% is achieved within 6 hr of polymerization. When only TEMPO is involved in this polymerization, the pseudo-living free-radical polymerization is well achieved, however, the polymerization rate becomes quite slow. This retardation of the polymerization rate is solved by the addition of a low concentration of CSA. In the TEMPO-mediated dispersion polymerization in the presence of CSA, similar trends in the conversion, kinetics, and PDI are observed as those observed in the case of bulk polymerization. When only TEMPO is used in the dispersion polymerization, the resulting particle size becomes quite broad, due to the prolonged polymerization time. However, when a 1.0 molar ratio of CSA to TEMPO is added to the TEMPO-mediated dispersion polymerization, fairly monodisperse PS microspheres having an average size of 5.83 μm and a CV of 3.4% are successfully obtained, due to the narrow molecular weight distribution of the intermediate oligomers and shortening of the polymerization time. This result indicates that the addition of CSA to the TEMPO-mediated bulk and the use of dispersion polymerization not only shortens the polymerization time, but also greatly improves the uniformity of the microspheres.

Synthesis and characterization of branched polymers by a free radical approach using a novel ‘macroiniferter’

Free radical bulk polymerization of styrene and methyl methacrylate (MMA) was carried out using a novel ‘macroiniferter’ which resulted in branched polymers with relatively narrow molecular weight distribution. This approach involving the novel macroiniferter; poly[3-( t-butylperoxy)propyl disulfide] (PBPPDS) that has side chain peroxide groups and main chain disulfide linkages was developed to prepare soluble branched polymers as well as to control the extent of branching in vinyl polymers synthesized via a free radical route. The synthesis, characterization and thermal degradation studies of PBPPDS are reported here for the first time. The resulting polystyrene (PS) and poly(methyl methacrylate) (PMMA) polymers were characterized using gel permeation chromatography (GPC), intrinsic viscosity [ η] measurements and the degree of branching was studied by the determination of g′ factor.

Synthesis and rheological properties of polystyrene/layered silicate nanocomposite

Exfoliated polystyrene (PS)/organo-modified montmorillonite (MMT) nanocomposites were synthesized through in situ free radical bulk polymerization by dispersing a modified reactive organophilic MMT layered silicate in styrene monomer. The original MMT was modified by a mixture of two commercial cationic surfactants, [2-(acryloyloxy)ethyl](4-benzoylbenzyl)dimethylammonium bromide (ADAB) and cetyltrimethylammonium bromide (CTAB), with the former containing a polymerizable vinyl group. The exfoliating and intercalating structures were probed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Comparing with pure PS, the nanocomposites show much higher decomposition temperature, higher dynamic modulus, stronger shear thinning behavior and a smaller die swell ratio. The leveling-off of the storage modulus at low frequencies in the oscillatory shear measurements, as well as the observed yield like behavior, implies that the formation of a percolating nanoclay network is the origin of the enhanced viscoelasticity in these composites.

Access to Chain Length Dependent Termination Rate Coefficients of Methyl Acrylate via Reversible Addition−Fragmentation Chain Transfer Polymerization

The reversible addition−fragmentation chain transferchain length dependenttermination (RAFT-CLD-T) method is employed to map out the chain length dependence of the termination rate coefficient in methyl acrylate (MA) bulk free radical polymerizations at 80 °C. Methoxycarbonylethyl phenyldithioacetate (MCEPDA)a novel RAFT agent carrying a methyl acryl leaving groupis identified as suitable for the RAFT-CLD-T method applied to methyl acrylate, as interfering inhibition and rate retardation effects are avoided. The chain length dependency of the termination rate coefficient was constructed in a stepwise fashion since the MA/MCEPDA system displays hybrid behavior (between conventional and living free radical polymerization), resulting in initial high molecular weight polymers formed at low RAFT agent concentrations. The chain length dependency of kt in the MA system for chain lengths, i, ranging from 5 to 800 at 80 °C may be described by a value for α of 0.36 ± 0.05 (where α is the slope of the associated log kti,i vs log i plot). An alternative RAFT agent, dimethoxycarbonylethyl trithiocarbonate (DMCETC), may not be as ideally applicable to map CLD dependent kti,i in MA polymerizations. Since the leaving group of both RAFT reagents is identical to the propagating methyl acrylate radical, the addition rate coefficient of the methyl acrylate propagating radicals to the initial and polymeric RAFT agent, kβ, was determined and found to be close to 1.4 × 106 L mol-1 s-1 for MCEPDA and 2.1 × 106 L mol-1 s-1 for DMCETC at 60 °C.

Dendrimers as Scaffolds for Reversible Addition Fragmentation Chain Transfer (RAFT) Agents: a Route to Star-Shaped Block Copolymers

Star-shaped block copolymers of styrene and n-butyl acrylate having three, six, and twelve pendent arms were successfully synthesized via reversible addition fragmentation chain transfer (RAFT) polymerization. Dendritic cores (based on 1,1,1-trimethylolpropane) of generation 0, 1, and 2 have been functionalized with 3-benzylsulfanylthiocarbonylsulfanylpropionic ester groups and have subsequently been employed to mediate the polymerization of styrene and n-butyl acrylate to generate macro-star-RAFT agents as starting materials for chain extension. The chain extension of the macro-star-RAFT agents with either styrene or n-butyl acrylate by bulk free radical polymerization at 60°C gives narrowly distributed polymer (final polydispersities close to 1.2) increasing linearly in molecular weight with increasing monomer-to-polymer conversion. However, with an increasing number of arms (i.e., when going from three- to twelve-armed star polymers), the chain extension becomes significantly less efficient. The molecular weight of the generated block copolymers was assessed using 1H NMR spectroscopy as well as size exclusion chromatography calibrated with linear polystyrene standards. The hydrodynamic radius, Rh, of the star block copolymers as well as the precursor star polymers was determined in tetrahydrofuran by dynamic light scattering (90°) at 25°C. Interestingly, the observed Rh–Mn relationships indicate a stronger dependence of Rh on Mn for poly(butyl acrylate) stars than for the corresponding styrene polymers. Rh increases significantly when the macro-star-RAFT agent is chain extended with either styrene or n-butyl acrylate.

Study of the calcification of PHEMA hydrogels using a two compartment permeation cell

To simulate the process of calcification in hydrogel implants, particularly calcification inside hydrogels, in vitro experiments using two compartment permeation cells have been performed. PHEMA hydrogel membranes were synthesized by free radical polymerization in bulk. The permeability and diffusion coefficient for Ca 2+ ions at 37 °C were determined using Fick's laws of diffusion. It was evident that Ca 2+ ions either from CaCl 2 or SBF solutions may diffuse through PHEMA hydrogel membranes. The formation of calcium phosphate deposits inside the hydrogel was observed and attributed to a heterogeneous nucleation from diffusing calcium and phosphate ions. The morphology of the deposits both on the surface and inside the hydrogels was found to be similar, i.e. spherical aggregates with a diameter of less than one micron.

Synthesis and characterization of copolymers of bromoacrylated methyl oleate

AbstractIn this study, methyl oleate was bromoacrylated in the presence of N‐bromosuccinimide and acrylic acid in one step. Homopolymers and copolymers of bromoacrylated methyl oleate (BAMO) were synthesized by free radical bulk polymerization and photopolymerization techniques. Azobisisobutyronitrile (AIBN) and 2,2‐dimethoxy‐2‐phenyl‐acetophenone were used as initiators. The new monomer BAMO was characterized by FTIR, GC‐MS, 1H, and 13C‐NMR spectroscopy. Styrene (STY), methylmethacrylate (MMA), and vinyl acetate (VA) were used for copolymerization. The polymers synthesized were characterized by FTIR, 1H‐NMR, 13C‐NMR, and differential scanning calorimetry (DSC). Molecular weight and polydispersities of the copolymers were determined by GPC analysis. Ten different feed ratios of the monomers STY and BAMO were used for the calculation of reactivity ratios. The reactivity ratios were determined by the Fineman–Ross and Kelen–Tudos methods using 1H‐NMR spectroscopic data. The reactivity ratios were found to be rsty = 0.891 (Fineman–Ross method), 0.859 (Kelen–Tudos method); rbamo = 0.671 (Fineman–Ross method), 0.524 (Kelen–Tudos method). © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 2475–2488, 2004

Analysis of the Molecular Weight Distribution of Polystyrene

In this paper are reported molecular weight distributions (MWD) of polystyrenes obtained by bulk free-radical polymerization of styrene under isothermal conditions. MWD of polystyrenes were determined by gel-permeation chromatography. From analysis of the results it is apparent that samples synthesized at 80 °C and 90 °C have a bimodal MWD whereas those obtained at 100 °C and 110 °C have unimodal MWD. By two-peak deconvolution of experimental curves (at 80 °C and 90 °C) it is possible to separate the contribution of polystyrene obtained before and after onset of the gel effect.

High‐Temperature Propagation and Termination Kinetics of Styrene to High Conversion Investigated by Electron Paramagnetic Resonance Spectroscopy

AbstractSummary: The free radical bulk polymerization of styrene at 120 °C has been investigated over almost the entire conversion range using electron paramagnetic resonance spectroscopy, Fourier‐transform near‐infrared spectroscopy and gel permeation chromatography. The free radical concentration went through a sharp maximum that coincided with the peak in the rate of polymerization during the gel effect, and shifted to higher conversion with increasing initiator concentration. The termination rate coefficient (kt), decoupled from the initiator efficiency (f) by use of the instantaneous degree of polymerization, remained close to constant up to as high as approximately 80% conversion, at which a dramatic decrease occurred. Both the propagation rate coefficient (kp) and f fell orders of magnitude near 80% conversion in spite of the temperature being above the glass transition temperature of the system. The value of kp increased with the initiator concentration at a given conversion in the highest (diffusion‐controlled) conversion range.Termination rate coefficient (kt) versus conversion for bulk free radical polymerization of St initiated by TBP at 120 °C. [TBP] = 0.15 (○), 0.10 (▴) and 0.05 M ().magnified imageTermination rate coefficient (kt) versus conversion for bulk free radical polymerization of St initiated by TBP at 120 °C. [TBP] = 0.15 (○), 0.10 (▴) and 0.05 M ().

Facile Access to Chain Length Dependent Termination Rate Coefficients via Reversible Addition−Fragmentation Chain Transfer (RAFT) Polymerization: Influence of the RAFT Agent Structure

A recently developed methodology for determining chain length dependent termination rate coefficients, 〈kti,i〉, via reversible addition−fragmentation chain transfer (RAFT) polymerizations has been extended and validated for 1-phenylethyl phenyldithioacetate (PEPDA) and 3-benzylsulfanylthiocarbonylsulfanylpropionic acid (BSPA) mediated styrene (bulk) free radical polymerizations at 80 °C. While the use of cumyl phenyldithioacetate (CPDA) enables a highly precise mapping of the chain length dependence of the termination rate coefficient, employment of PEPDA and BSPA leads to considerable information loss for short chain lengths (i < 10). Careful simulations demonstrate that such behavior is caused by a substantial decrease in the initial transfer effectiveness of the RAFT agents when going from CPDA to BSPA, leading to hybrid behavior between conventional and living free radical polymerization. The observed hybrid behavior is quantifiable via (overall) transfer rate coefficients for the individual RAFT agen...

Direct and inverse neural network modeling in free radical polymerization

Abstract The first part of this paper reviews of the most important aspects regarding the use of neural networks in the polymerization reaction engineering. Then, direct and inverse neural network modeling of the batch, bulk free radical polymerization of methyl methacrylate is performed. To obtain monomer conversion, number and weight average molecular weights, and mass reaction viscosity, separate neural networks and, a network with multiple outputs were built (direct neural network modeling). The inverse neural network modeling gives the reaction conditions (temperature and initial initiator concentration) that assure certain values of conversion and polymerization degree at the end of the reaction. Each network is a multi-layer perceptron with one or two hidden layers and a different number of hidden neurons. The best topology correlates with the smallest error at the end of the training phase. The possibility of obtaining accurate results is demonstrated with a relatively simple architecture of the networks. Two types of neural network modeling, direct and inverse, represent possible alternatives to classical procedures of modeling and optimization, each producing accurate results and having simple methodologies.

A rheokinetic study of bulk free radical polymerization performed with a helical barrel rheometer

AbstractThe Trommsdorff effect is a non‐linear autoacceleration phenomenon in bulk free radical polymerizations. It is undesired in processes on industrial scale, as is causes hot spots and erratic behavior. In this paper a rheokinetic investigation of the Trommsdorff effect is performed with a helical barrel rheometer. This rheometer can be considered a small‐scale model of large reactors, particularly extruders. The rheokinetic measurements show that the Trommsdorff effect can be reduced when the polymerization is performed at a high rotational velocity of the screw. This confirms a previous study performed with a cone and plate rheometer and is encouraging with a view to designing better‐controlled bulk free radical polymerization processes on industrial scale.