Non-aqueous poly(methyl methacrylate) dispersions: Radical dispersion polymerization in the presence of the diblock copolymer poly(styrene -b-methyl methacrylate)

Eur. Polym. J. Vol. 23, No. 2, pp.173-175, Printed Jn Great Britain

1987

0014-3057/87 $3.00+0.00 Pergamon Journals Ltd

NON-AQI]EOUS POLY(METHYL METHACRYLATE) POLYMERIZATION

DISPERSIONS:

RADICAL DISPERSION

IN THE PRESENCE OF THE DIBLOCK COPOLYMER

-~-METHYL

POLY(STYRENE

METHACRYLATE)

by J.V.DAWKINS! a)* S.A.SHAKIR (a) and T.G.CROUCHER (b) (a) Department of Chemistry, The University, Loughborough, Leics., U.K. (b) Polymer Laboratories Ltd., Church Stretton, Shropshire SY6 6AX, U.K. * author for correspondence (Received

18 November

]986)

ABSTRACT Poly(methyl methacrylate) particles stabilized in cyolohexane by surface layers of polystyrene chains have been produced by radical dispersion polymerization in the presence of the diblock copolymer poly(styrene-_b-methyl methacrylate). The preparation of stabilized particles with a narrow particle size distribution and a mean particle diameter of 0 . 1 ~ m required a diblock copolymer concentration of 5% and the incorporation of a seed stage into the dispersion polymerization. The particles retained stability when the dispersion medium was a theta system or better than a theta system for the polystyrene chains. KEY WORDS Dispersion polymerization; poly(methyl methacrylate) particlesl stabilizing polystyrene chains; diblock copolymer; poly(styrene-~methyl methacrylate).

INTRODUCTION Non-aqueous

polymer dispersions

are conveniently

a preformed block (or graft) copolymer ant for the polymer

(i), Dispersions

polymer dispersions

ing chains in free solution

uently,

dispersions

studies,

properties

stabilized with polystyrene

graft copolymers,

styrene block copolymer,

poly(styrene-~-dimethyl

for the stabiliz-

having stabilizing chains

in solution are well documented.

Conseq-

chains should be preferred for flocculation

(6) and poly(vinyl

and particles of polyacetylene

have been reported.

siLoxane)

(3,4). Because the critical flocculation

(5), there is a need for dispersions

and thermodynamic

(PMMA) in n-alkanes have

is close to the theta conditions

and particles of polyacrylonitrile

polystyrene

methacrylate)

diblock copolymers

(2) and poly(styrene-~-[ethylene-co-propylene])

whose conformational

in the presence of

a monomer dissolved in a diluent which is a precipit-

of poly(methyl

been stabilized with the well-defined

point for non-aqueous

prepared by polymerizing

acetate)

(7), stabilized with

(8), stabilized with a poly-

Here, we describe the preparation

of a dispers-

ion of PMMA particles

stabilized in organic media by the diblock copolymer poly(styrene-~-

methyl methacrylate),

abbreviated

to PS-PMMA,

in which PS is the stabilizing block and P M ~

is the anchor block. EXPERIMENTAL Diblock Copolymer The PS-PMMA diblock copolymer was synthesized by anionic polymerization (9) performed under conditions of rigorous purity using a high vacuum technique similar to that described elsewhere (2). Solvents and monomers were extensively dried and purified. The polymerization of styrene in tetrahydrofuran was initiated with cumyl caesium at 213K. The solution was then warmed to 298K. Part of the "living" polystyryl caesium solution was removed and terminated with methanol. Diphenylethylene (DPE) was added to polystyryl caesium and again part of the solution was terminated with methanol. The remaining anions formed on addition of DPE were cooled to 178K to initiate the polymerization of MMA, and the resulting "living" copolymer 173

174

was terminated by adding methanol. Molar mass characterization was performed by gel permeatchromatography

(GPC) with dual refractive index and ultraviolet detectors.

Dispersion Polymerization Methyl methacrylate monomer was destabilized by standard procedures for removing inhibitor and then distilled.

The initiator,

azobis(isobutyronitrile),

ethanol. The PS-PMMA sample (lg) was dispersed in cyclohexane

was recrystallized

from

(15g) which had previously

been dried over molecular sieves, degassed and distilled under vacuum, by first leaving the mixture overnight at room temperature and then raising the temperature of the stirred mixture to 338K for 30 min. The apparatus contained N 2 gas throughout. The seed stage of the dispersion polymerization was then performed by adding MMA (0.8g which represented 20% by weight of the total monomer with the equivalent proportion of the initiator). After this addition, the seed dispersion was allowed to form for ing monomer

two hours, following which the remain-

(3.2g MMA with initiator) was added incrementally as a feed over a period of one

hour. The total reaction time for dispersion polymerization was 48 hours. The dispersion was stored at ambient temperature in a mixture of cyclohexane/dichloromethane

(90:10 v/v) and

this liquid mixture was used in repeated centrifuge/diluent exchange cycles to remove unadsorbed copolymer and unconverted MMA. The final redispersion with pure cyclohexane required storage of the PMMA dispersion at 313K. Values of the mean diameter of the PMMA particles were estimated from transmission electron micrographs according to the method reported previously (2).

RESULTS AND DISCUSSION GPC characterization showed that there was no difference between the PS samples (numberaverage molar mass ~

= 32000 g mo1-1 and polydispersity = 1.07) before and after the additn -i ion of DPE. The PS-PMMA sample had M = 55000 g mol (assuming PS and PMMA have the same GPC n calibration (I0)) and polydispersity = 1.13. The ratio of these values of ~ is in reasonable n agreement with the PS fraction of 0.45 estimated from UV spectrophotometry of PS-PMMA in chloroform. GPC suggested that PS-PMMA did not contain PMMA homopolymer but did contain a very small quantity of preterminated PS homopolymer.

The transmission electron micrograph in Figure i indicates that the PMMA particles were spherical and that narrow particle size distributions were obtained by incorporating a seed

Figure 1. Transmission electron micrograph of PMMA partciles.

O

O

O

,

i

175

stage into the dispersion than the dimensions

polymerization.

The mean particle diameter of 0 . 1 ~ m

of PS-PMMA micelles having diameters of about 300 ~. The stabilization

the particles in Figure 1 required a minimum concentration polymerization,

is higher

as lower concentration

produced coarse particles and coagulation

of PMMA. The

PS content of a PMMA particle was estimated to be 10% (w/w) by UV spectrophotometry icles dissolved

in chloroform.

Dichloromethane

was added to cyclohexane

cycles because PMMA particles

for storing the dispersions

flocculated

in cyclohexane

301.6K. With a dispersion medium of cyclohexane/carbon

of part-

and during redispersion

(theta temperature

tetrachloride

= 307K) at

(0.869:0.131,

v/v), it

was shown that PME~ particles on cooling lost stability at 283.8K. This flocculation ature is near to the theta temperature ure (11), confirming

effective

of

of PS-PMMA of 5% in the dispersion

temper-

of 288K for PS chains in the same binary liquid mixt-

stabilization

of particles when the dispersion medium is a

theta system or better than a theta system for the PS chains at the surfaces of the P~ZA particles.

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