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Macromolecules 1998, 31, 2848-2856
Dispersion Polymerization in Supercritical CO2 with Siloxane-Based Macromonomer. 2. The Particle Formation Regime M. L. O’Neill, M. Z. Yates, and K. P. Johnston* Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712-1062
C. D. Smith and S. P. Wilkinson Air Products and Chemicals, Inc., Allentown, Pennsylvania 18195-1501 Received September 3, 1997; Revised Manuscript Received February 9, 1998
ABSTRACT: Dispersion polymerization of methyl methacrylate in supercritical CO2 is studied in situ by turbidimetry at 65 °C from 2000 to 5000 psia for various concentrations of a poly(dimethylsiloxane) monomethacrylate (PDMS-mMA) macromonomer stabilizer. The average particle size, particle number density, and overall surface area are reported vs time during particle formation. Coagulative nucleation and controlled coagulation regions have been identified. They are governed by the amount of stabilizer available relative to the total surface area of the dispersion. Near the end of the controlled coagulation region, which can last tens of minutes, the particle number density approaches the final value. The time in this region is longer than predicted by the model proposed by Paine (Macromolecules 1990, 23, 3190) due to incomplete incorporation of stabilizer, solubility limitations of polymerized stabilizer in the continuous phase, and plasticization of the particles by CO2, which increase particle coagulation. Threshold values of pressure and stabilizer concentration are required to achieve a solvent quality and surface coverage sufficient to prevent uncontrolled coagulation during particle formation.
Introduction Heterogeneous polymerizations in CO2 have been performed on various monomer systems including methyl methacrylate,1-4 styrene,5 2,6-dimethyl phenol,6 and acrylamides.7 Recently PDMS-based macromonomers with a reactive methyl methacrylate group, which form graft copolymer stabilizers in situ during polymerization, have been employed successfully in the dispersion polymerization of methyl methacrylate in supercritical CO2.8 On the basis of experimental results2,9-11 and modeling of dispersion polymerization,10 it is assumed that the particle formation regime is complete very soon after polymerization commences, e.g., before 0.1% conversion. After the completion of the formation stage, the particle number density remains essentially constant for the remainder of the polymerization, without the formation of new stable particles.2,9,10,12 One of the more comprehensive mechanisms of particle formation, the coagulative nucleation mechanism, incorporates the basic phenomenology of homogeneous and aggregative nucleation combined with heterocoagulation of small nuclei and unstable aggregates with larger stabilized particles.11 This mechanism may be used to describe the effects of stabilizer concentration and solvent strength of the continuous phase. While the effects of these variables have been well studied experimentally in the particle growth regime,9,11 their roles in the particle formation regime are ill-defined. Experimental studies have verified the coexistence of unstable nuclei and stable particles during the initial stages of particle formation in dispersion polymerization.12-14 It has proven difficult to follow the particle formation regime with either electron microscopy2 or dynamic light scattering(DLS)12,13 due to the rapid increase in dispersed phase volume fraction and the changing composition of the system. Recently DLS has been used to observe the particle formation stage during
dispersion polymerization of MMA and styrene.12,13 Particle size measurements were limited to the first 3-5 min beyond phase separation, after which multiple scattering began to affect the measurements due to the increase in dispersed phase concentration. Turbidimetry has been used to study a variety of heterogeneous systems for the determination of particle size,15-20 stabilization and flocculation of emulsions,21-23 and particle nucleation in emulsion polymerization.24 Recent studies of emulsions in supercritical fluids used dynamic light scattering and turbidimetry to quantify emulsion stability, concentration, and particle sizes in situ.23,25 Compared to other techniques used to measure micron and submicron sized particles,19,24,26 turbidimetry allows for a higher concentration threshold and can provide very rapid, in situ measurements. For dispersion polymerization, higher conversions may be studied than in the above studies by dynamic light scattering. Turbidimetry may also be performed on systems which are quiescent or turbulent, unlike techniques which measure diffusion coefficients to obtain hydrodynamic radii. In part 1. of this series,4 the details of chronology of molecular weight, particle size, product morphology, and stability of dispersions were determined for the particle growth regime in the dispersion polymerization of MMA in supercritical CO2 with PDMS-mMA macromonomer. The mediocre solvent quality of pure CO2 for PDMS dictated that monomer need be present in the continuous phase at all pressures studied (1500-4000 psia) to help solvate the PDMS tails and prevent flocculation of the dispersion. Particle size was strongly dependent on stabilizer concentration yet only mildly dependent on pressure above 3000 psi at 65 °C, in agreement with previous studies.2,8 Finally, it was determined that both a threshold pressure (∼3000 psia) and stabilizer concentration (∼2 wt % stabilizer/monomer) exist below which there is significant change in the polymerization
S0024-9297(97)01315-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/17/1998
Macromolecules, Vol. 31, No. 9, 1998
Dispersion Polymerization in Supercritical CO2
as indicated by reduced polymerization rates and increased particle coagulation due to insufficient steric stabilization. The objective of this study is to determine the mechanism of particle formation in the dispersion polymerization of MMA in supercritical CO2, especially the role of stabilizer, to complement our study of the particle growth regime in part 1. Polymerizations conducted at 65 °C and approximately 20 wt % MMA/CO2 and 1 wt % AIBN/MMA are monitored at pressures from 2000 to 5000 psia and stabilizer concentrations from 0 to 14 wt % stabilizer/MMA. Turbidimetry measurements as a function of wavelength are used to determine average particle sizes, dispersed phase volume fraction, particle number density, and overall surface area of the dispersion. The pressure is varied to manipulate the solvent quality, which influences the solvation of the PDMS grafts, the solubility of in situ formed graft copolymer stabilizer, and the partitioning of this stabilizer to the interface. The macromonomer concentration is varied to adjust the amount of surface area which may be stabilized. Reaction rates are estimated from dispersed phase volume fractions, and combined with results from part 1 on particle growth,4 to describe the changing locus of polymerization. Explanations are given for why the results are more complex than those predicted by the model of Paine.10 Theory The turbidity τ is defined in terms of the attenuation of electromagnetic radiation at a scattering angle of 0°
τ ) (1/l) ln(Io/I)
(1)
where l is the path length, and Io and I are the intensities of the incident and transmitted light. In dispersions which scatter but do not absorb the incident light, the turbidity is a function of particle concentration and size and may be calculated from the equation for specific turbidity of Kourti24
τ/φ ) 3K*/2D
(2)
where D is the droplet diameter, and φ is the dispersed phase volume fraction. The scattering coefficient K* is the ratio of the scattering cross-section to the geometric cross-section. It is a complex function of R, a dimensionless size parameter equal to πD/λ (where λ is the wavelength of the incident light), and m, the ratio of the refractive index of the dispersed to continuous phase.15,27 If R and m are known, K* may be calculated exactly using the Mie theory.28 The ratio of turbidities at two different λ is a direct measure of the ratio of scattering coefficients for a monodisperse latex15,24
τ1/τ2 ) K*1/K*2
(3)
The turbidity ratio method shown in eq 3 is limited to size regimes where the exponential dependence of K* on λ is high and continuously changing. In this study the measurements are taken in the m range from 1.10 to 1.20 and 1 < R < 10, where unique solutions for K* and particle size can be obtained. For polydisperse latexes, the diameter measured from the turbidity ratio can be related to different moments of the distribution depending upon the values of R and m.18,24,29 For 1 < R < 10, the measured diameter is
Figure 1. paratus.
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Schematic of high-pressure turbidimetry ap-
closely related to the weight average particle size (Dw ) (∑(NiDi)4/∑(NiDi)3). For mildly polydisperse size distributions (Dw/Dn < 1.1), the various moments of the distribution are close in magnitude and provide a very close estimate of the weight average so long as particle size and light wavelength are comparable.16,24 Once particle sizes have been determined from turbidity ratios, they can be inserted into eq 2 to calculate dispersed phase volume fractions and particle number densities. Errors introduced from experimental uncertainties in the measured particle sizes are magnified in the determination of dispersed phase volume fractions and number densities. Knowledge of the refractive indices of both phases is required for the determination of particle size. In dispersion polymerization, both dispersed and continuous phases are mixtures of several components. The refractive indices of both phases were estimated using the Lorenz-Lorentz mixing rule30
(n12 - 1)2/(n12 + 2)2 )
∑φi(ni - 1)2/(ni + 2)2
(4)
where φ is the volume fraction of a given component of the mixture and n is the refractive index of the pure component at the same temperature and pressure. Experimental Section Materials. Methyl methacrylate (MMA), poly(dimethylsiloxane)-monomethacrylate (PDMS-mMA) with Mn ∼ 10 000, and 2,2′-azobis(isobutyronitrile) (AIBN) were obtained from Aldrich and used as received. MMA and PDMS-mMA were purged with nitrogen (Liquid Carbonics, >99.999%) before use. Carbon dioxide obtained from Liquid Carbonics (instrument grade,
