G. R. Meira

BAND BROADENING IN SIZE EXCLUSION CHROMATOGRAPHY OF POLYMERS. STATE OF THE ART AND SOME NOVEL SOLUTIONS

ABSTRACT This work reviews the main problems of band broadening (BB) in SEC, and also presents some novel and state-of-the-art solutions. The first part of the work describes Tungs equation and presents an overview on the expected order-of-magnitude of BB effects. The second part deals with the experimental determination of the broadening function using ultra-narrow standards. The third part describes several algorithms for inverting the mass chromatograms. To allow for the simultaneous occurrence of narrow and broad regions in the same chromatogram, a novel regularization filter with a position-dependent parameter was introduced. When comparing different inversion procedures, the maximum entropy method has proven effective for recuperating structural details narrower than two times the width of the kernel. For broader and smoother regions, most other inversion procedures are equally effective. The fourth part of the work aims at producing unbiased estimates in multidetection SEC. For a derived variable obtained by linear combination of two or more chromatograms, it is preferable to directly perform the calculation from the raw measurements, and then to correct the (broadened) derived variable with the standard system kernel. When estimating molar masses from in-line molar mass sensors, intolerable errors are produced near to the baselines. To cope with this problem and to simultaneously correct for BB, a theoretical procedure is reviewed that is based on recuperating the linear (unbiased) molecular weight calibration.

Synthesis of polystyrene with mixtures of mono- and bifunctional initiators

This work studies the bulk and nonisothermic polymerization of styrene (S) using mixtures of mono- and bifunctional initiators. The effects on polymerization rate and on molecular weight averages after changes in the global initiator concentration and in the nature of the initiator mixtures were experimentally and theoretically analyzed. The mathematical model was adapted from the literature to admit initiator mixtures and to simulate the applied temperature profiles. Compared to the standard use of monofunctional initiators, the use of initiator mixtures that include bifunctional initiators presents the advantage of reducing the polymerization time, while not deteriorating the final polymer quality. The simulation program accurately describes the evolution of conversion, but exhibits some deviations with the average molecular weights.

Bulk polymerization of styrene in the presence of polybutadiene. The use of bifunctional initiators

This work experimentally and theoretically investigates the use of bifunctional initiators in the synthesis of high-impact polystyrene (HIPS). The experimental design involved a series of nonisothermal bulk polymerizations of styrene (St) in the presence of polybutadiene (PB). The performance of three commercial initiators [2,5-dimethyl −2,5 bis(2-ethylhexanoyl peroxy] hexane or L–256; 2,5 bis(benzoyl peroxy) hexane or L–118; and ethyl 3,3 di(t-butyl peroxide) butirate or L–233] were compared to the performance of a standard monofunctional initiator (terbutylperoctoate or TPBO), and to the blank case (i.e., without initiator). From samples taken along the prepolymerization period, the phase inversion point and the 30% conversion point were estimated. For the final product, the free polystyrene (PS) molecular weights and the St grafting efficiency were measured. A mathematical model was developed that predicts the evolution of the MWDs for the free PS the residual PB, and the graft copolymer, together with the chemical composition distribution for the total graft copolymer. Compared to the monofunctional case, the L–256 initiator induces phase inversion and rubber grafting at low conversions. Also, it shortens the prepolymerization times by around 38%, without affecting the molecular characteristics of the final product. L–118 also shortens prepolymerization time with respect to TBPO; but is not as effective as L–256 or TBPO in promoting rubber grafting. At the polymerization end, the final molecular characteristics are practically independent of the initiator type because most of the polymerization in induced by monomer initiation. Due to its slow decomposition rate, the L–233 initiator is less effective that TBPO for reducing prepolymerization times and for promoting phase inversion.

Bulk polymerization of styrene in presence of polybutadiene: Calculation of molecular macrostructure

In our previous publication the detailed molecular macrostructure generated in a solution polymerization of styrene (St) in the presence of polybutadiene (PB) at 60°C, was theoretically calculated. In this work, an extended kinetic mechanism that incorporates monomer thermal initiation, chain transfer to the rubber, chain transfer to the monomer, and the gel effect is proposed, with the aim of simulating a bulk high-impact polystyrene (HIPS) process. The mathematical model enables the calculation of the bivariate weight chainlength distributions (WCLDs) for the total copolymer and for each of the generated copolymer topologies and the univariate WCLDs for the free polystyrene (PS), the residual PB, and the crosslinked PB topologies. These last topologies are characterized by the number of initial PB chains per molecule; copolymer topologies are characterized by the number of PS and PB chains per molecule. The model was validated with published literature data and with new pilot plant experiments that emulate an industrial HIPS process. The literature data correspond to a dilute solution polymerization at a constant low temperature with chemical initiation and a bulk polymerization at a constant high temperature with thermal initiation. The new experiments consider different combinations of prepolymerization temperature, initiator concentration, and solvent concentration. One of the main conclusions is that most of the initial PB is transformed into copolymer. For example, for a prepolymerization temperature of 120°C with addition of initiator, the experimental measurements indicate that the final total rubber mass is approximately three times higher than the initial PB. Also, according to the model predictions, the final weight fractions are: free PS, 0.778; graft copolymer, 0.220; initial PB, 0.0015; and purely crosslinked PB, 0.0005. The final graft copolymer exhibits the following characteristics: average molecular weights, Mn,C = 492,000 and Mw,C = 976,000; average weight fraction of St, 0.722; and average number of PS and PB branches per molecule, 5.19 and 1.13, respectively.

EMULSION COPOLYMERIZATION OF ACRYLONITRILE AND BUTADIENE. SEMIBATCH STRATEGIES FOR CONTROLLING MOLECULAR STRUCTURE ON THE BASIS OF CALORIMETRIC MEASUREMENTS

A semibatch emulsion copolymerization of acrylonitrile and butadiene is theoretically investigated, with the aim of controlling the molecular structure of the produced NBR. An open-loop estimator based on calorimetric measurements is proposed for monitoring the chemical composition, the average molecular weights, and the average degree of branching. With little effect on the other quality variables, the intermediate addition of acrylonitrile allows to produce a polymer with a constant chemical composition. Similarly, the intermediate addition of the chain transfer agent produces a polymer with either a fixed or with a prespecified linear variation of the average branching. The required feed profiles are obtained from a numerical inversion of a discrete process model. By increasing the initiator loads, the semibatch strategies also allow to increase the final conversion between 3% and 6%, without altering the reaction time nor deteriorating the polymer quality with respect to the batch. The numerical procedures were tested by inputing (relatively noisy) heat measurements from an industrial batch reactor.

Particle Size Distribution of SBR and NBR Latexes by UV-VIS Turbidimetry near the Rayleigh Region

Abstract This paper deals with data treatment problems that arise when turbidimetry is employed to estimate the particle size distribution (PSD) of soft polymer latexes with low diameter limits around 40 nm. Scanning electron microscopy and dynamic light scattering were used as comparison techniques. Industrial latexes of styrene-butadiene rubber (SBR) and of acrylonitrile-butadiene rubber (NBR) were investigated. The data treatment involved the use of Mies Model to obtain an average diameter and/or the complete PSD. For estimating the complete PSD, a least squares optimization (with an imposed distribution shape) and a numerical deconvolution procedure (without assumptions on the distribution shape) were attempted. A synthetic example was solved to investigate the limits of the applied numerical methods. For the polymer refractive index functions, Cauchys Law was used — and its adequate adjustment proved essential for good turbidimetric estimations. A reasonable agreement between the turbidity measurem...

Inverse optimal filtering method for the instrumental spreading correction in size exclusion chromatography

Abstract The Kalman filter based techniques are adapted to solve the most general form of Tungs integral formula, i. e. when a non-uniform, non-symmetric calibration model is employed to correct chromatograms obtained in size exclusion chromatography from instrumental broadening errors. Through this method, the inverse smoothing of a chromatogram contaminated with measurement noise of known statistics is optimally performed by minimizing the estimation error variance. The method is numerically very “robust”, improves the signal to noise ratio, provides good validation checks, and does not involve any previous parameter estimation procedure.

Emulsion polymerization of styrene. Use of n -nonyl mercaptan for molecular weight control

Abstract The unseeded emulsion polymerization of styrene with n-nonyl mercaptan (nNM) as chain transfer agent (CTA) was investigated, with the aim of producing a PS latex of a low molecular weight polydispersity at high conversion and in short reaction times. To this effect, starved and minimum time semibatch reactions were investigated and compared to equivalent batch polymerizations. The high reactivity of nNM with respect to the monomer makes it feasible to implement a minimum time policy with intermediate addition of CTA only. The MWDs of the minimum time runs were intermediate between the broader distributions of the batch reactions and the narrower distributions of the starved experiments. The conversion profiles of minimum time experiments almost coincided with those of equivalent batch polymerizations. For controlling MWDs through semibatch operations, it seems preferable to use nNM instead of other (less reactive and more common) CTAs like tert-dodecyl mercaptan or CCl4.

Emulsion Copolymerization of Acrylonitrile and Butadiene in an Industrial Batch Reactor. Estimation of Conversion and Polymer Quality from On-line Energy Measurements

ABSTRACTA method for monitoring an industrial emulsion copolymerization of acrylonitrile and butadiene for the production of two NBR grades is presented. From an on-line calculation of the reaction heat (proportional to the mass flow rate of evaporated refrigerant), three open-loop estimators of increasing complexity were developed for monitoring conversion, copolymer composition, and average molecular weights. A good agreement between on-line estimates and off-line measurements was obtained.

Polymerization of styrene in the presence of polybutadiene: determination of the molecular structure

This work experimentally and theoretically determines the molecular macrostructure of the polymer mixture that is developed (at relatively low conversions) in a solution polymerization of styrene (St) in presence of polybutadiene (PB). The reaction was carried out at 70°C in a batch-stirred tank reactor. From samples taken along the reaction, the three polymeric components of high-impact polystyrene (HIPS) (i.e., polystyrene PS, residual PB, and graft copolymer) were first separated from each other by solvent extraction. Then, the graft copolymer was ozonized to isolate the St branches. The molecular weight distributions (MWDs) of the total HIPS, the three HIPS components, and the grafted St branches were determined by the size exclusion chromatography (SEC). For the graft copolymer and the total HIPS, the variation of the St mass fraction with molecular weights was also determined by SEC. All measurements were compared with theoretical estimates, and a reasonable agreement is observed. For the theoretical estimates, the mathematical model of Estenoz, D. A.; Valdez, E.; Oliva, H. M.; Meira, G. R. (J Polym Sci 1996, 59, 861) was extended to compare the MWD of the St branches with the MWD of the free PS. For the sought experimental conditions, these two distributions had very similar results but in a bulk industrial process, larger discrepancies are to be expected.