Dispersion polymerization of styrene in polar solvents. 7. A simple mechanistic model to predict particle size

Abstract

An integrated model for dispersion polymerization is developed to predict particle size from first principles. The key components of this model are a multibin kinetic model for unstabilized particle coalescence, the grafting mechanism of stabilization, and the radius of gyration of the grafted stabilizer chains. A critical point is defined where similar-sized particles stop coalescing with one another because the graft available equals the minimum graft required to stabilize the particles. Examples indicate that the particle count is determined within the first 1% of conversion, in agreement with experiment, and that the critical size particles are larger than 0.1 pm-significantly bigger than nuclei. In cases where full adsorption of graft is likely, absolute prediction of particle size is remarkably consistent with experiment. More generally, however, partial adsorption of graft is likely to affect the predicted scaling of diameter with stabilizer, initiator and monomer concentrations, and stabilizer molecular weight. Above the critical size, the size distribution narrows because the growing particles continue to capture dead polymer formed in solution. Observed loss of monodispersity by stabilization of secondary particles or coalescence of large particles is readily explained with this model. A critical factor influencing the final size distribution is the locus of polymerization and how it changes during the reaction.During dispersion polymerizations latex particles are formed from an initially homogeneous reaction mixture by polymerization in the presence of a suitable steric stabilizer p01ymer.l-l~ In hydrocarbon solvents, poly(dimethylsiloxane), poly(isobutylene), poly(l2-hydroxystearic acid), and poly(2-ethylhexyl methacrylate) have been employed as stabilizers for polymerization of methyl metha~rylate.~-" Styrene monomer has been polymerized in alcohols with steric stabilizers such as (hydroxy-propy1)cellulose (HPC), poly(acry1ic acid) (PAA), or poly(N-vinylpyrrolidone) (PVP).1s4s6J2-17 In favorable circumstances, these particles can have a very narrow, or even monodisperse, size distribution.Our previous work on the dispersion polymerization of styrene in alcohols had focused primarily on HPC and, more recently, on PVP as steric stabilizer.l* These efforts continued work initiated by Ober, Lok, and Hair (HPC and PAA),13-15 Almog e t al. (PVP and others),17 and Vanderhoff and El-Aasser et al. (PVP).16 Although a costabilizer (anionic or nonionic surfactant) was employed with P V P in t h e investigations by Vanderhoff and El-Aasser, recent evidence suggests that this additive has no influence on the outcome of the reaction in most of the parameter space inve~tigated.~ The gross features of the HPC-and PVP-stabilized reactions are substantially similar and are comparable to reactions stabilized by PAA, poly(viny1 butyral), and poly(N-vinylpyridine).18The preceding work by us and others has shown the mechanism of dispersion polymerization to be complex and poorly understood. It is known that the particle c...

Polythiophene microspheres synthesized by transition metal mediated oxidative dispersion polymerization

“…Nonetheless, particle size distribution also increased considerably (D w /D n ¼ 1. 19) and some agglomeration was observed.…”

“…But if appropriate macromolecular stabilizers with large excluded volume and good compatibility with oligomer chains are used, the coalescence of unstable particles will stop when there are enough stabilizer chains absorbed on the particle surface to provide steric stabilization. 18,19 This procedure is similar to conventional dispersion polymerization.…”

From Euclidean to fractal geometry: The case of disintegration of polyaniline colloid particles

“…The inner structure of the colloid particles is also in accord with the dispersion polymerization mechanism proposed by Paine (ref. 16). The particulates that are produced at the onset of phase separation in dispersion polymerization may be identified with the nuclei which reduce their surface energy by way of aggregation with themselves bringing their bare surfaces in contact and perhaps sharing the adsorbed stabilizer.…”

“…The aggregation of nuclei with themselves (homocoalescence) ceases when the aggregates become stable particles (ref. 16). The latter grow subsequently by absorption of the nuclei (heterocoalescenec) (ref .…”

Non-equilibrium particle morphology in dispersion-polymerized polystyrene particles

“…An important mechanistic feature is that the balance between the amount of polymerization occurring inside the particles vs. that occumng in the continuous phase depends on the particle size (5,6). The low-volume fraction of solids after nucleation tends to favour extensive initiation in the continuous phase, rather than in the nuclei.…”