Polymer Latexes - American Chemical Society

Chapter 27

Nonaqueous Polymer Colloids Stabilization of Poly(vinyl acetate) Particles with the Diblock Copolymer Poly(styrene-b-[ethylene-co-propylene])

Downloaded by UNIV OF GUELPH LIBRARY on August 6, 2012 | http://pubs.acs.org Publication Date: June 3, 1992 | doi: 10.1021/bk-1992-0492.ch027

J. V. Dawkins and S. A. Shakir Department of Chemistry, Loughborough University of Technology, Loughborough, Leicestershire LE11 3TU, United Kingdom

Nonaqueous dispersions of poly(vinyl acetate) in n-alkanes have been prepared in the presence of the diblock copolymer poly(styrene-b[ethylene-co-propylene]) (S-EP) as steric stabilizer. The particle size over the range 0.13 to 0.31µm may be varied by performing seeded radical dispersion polymerizations of vinyl acetate as a function of monomer content and concentration of S-EP diblock copolymer. Estimates of surface coverage of the stabilizing EP copolymer on particles have been obtained. Controlled studies of flocculation with particles in a dispersion medium of a binary liquid mixture of nheptane and n-propanol as a function of temperature suggest that the steric stabilization mechanism operates for particles stabilized with surface layers of EP copolymer. The thickness of the surface layer was determined from viscosity studies of the dispersions at 298, 308 and 318 K. The dimensions were observed to be somewhat larger than the dimensions offreeEP copolymer chains in solution.

The term polymer colloids to describe polymer dispersions with particle diameters below 10 |im has been used for twenty years following an ACS Symposium organised by Fitch (1,2). A later Symposium Volume illustrated the substantial interest in dispersions of polymer particles in water (3). Nonaqueous polymer colloids are prevented from flocculation by steric stabilization in which each particle is surrounded by a surface layer of adsorbed polymeric stabilizer (4). Initial studies of nonaqueous polymer colloids involved dispersion polymerization in the presence of a graft copolymer of a monomer dissolved in a diluent which is a precipitant for the polymer (5). The stabilizing copolymer contains A blocks which are insoluble in the dispersion medium and act as anchors for the B blocks which are swollen by the diluent and which extend away from the particle surface. Much of this early work was concerned with the preparation and properties of nonaqueous poly(methyl methacrylate) (PMMA) dispersions (5), and Fitch and Kamath reported on the interfacial behaviour of graft copolymers stabilizing PMMA particles (6), Our work has involved the preparation of non-aqueous PMMA dispersions in aliphatic hydrocarbons stabilized with well-defined AB diblock copolymers based on

O097-6156/92AM92-O432$06.00/0 © 1992 American Chemical Society In Polymer Latexes; Daniels, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


27. DAWKINS AND SHAKIR

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Nonaqueous Polymer Colloids

polystyrene anchor blocks (7,6V. Extensive studies of the properties of these PMMA dispersions stabilized by AB poly(styrene-b-dimethyl siloxane), abbreviated to PSPDMS, and by AB poly(styrene-b-[ethylene-co-propylene]), abbreviated to S-EP, have been reported (9-12). The preparation of non-aqueous polymer colloids other than PMMA in the presence of stabiliser containing A blocks of PS is of interest because effective anchoring of the copolymer may be influenced by the degree of compatibility between the PS anchor blocks and the polymer chains in the particle core. Interpretation of scattering intensities from small angle neutron scattering experiments on PMMA particles stabilized in n-alkanes with PS-PDMS indicated segregation of the PS blocks into domains at or near the surfaces of the particles (13). It would appear that effective anchoring arises because the PS blocks which are incompatible with PMMA chains become trapped within a hard polymer matrix. The present study on polyvinyl acetate) (PVAC) particles was therefore undertaken in order to examine stabilization for a core polymer having a glass transition temperature below the dispersion polymerization temperature. Whilst there have been studies of PVAC particles stabilized in hydrocarbon media by grafted chains (14-17), the only work involving block copolymers appears to be preliminary studies involving PS-PDMS (18,19). These initial experiments on dispersion polymerisation of VAC with PS-PDMS in aliphatic hydrocarbons had indicated that effective anchoring for soft particles might require covalent grafting of the PS blocks to some core PVAC chains (18,19). Here, we report on the dispersion properties of non aqueous polymer colloids of PVAC stabilized by S-EP. Experimental Block copolymer and dispersions. Samples of the S-EP diblock copolymer were kindly provided by Dr B. Wright, Shell Research Centre, Thornton, Chester, Dr A. Bull, Shell Research BV, Amsterdam and Ms K.F. Churchley, Shell Centre, London. Characterization data for S-EP provided by Dr Wright were number average molar mass M = 104,000 g mol" , weight average molar mass M = 118,000 g mol , and a styrene content of 38.5% by weight, indicating a molar mass M = 64,000 g mol" for the EP copolymer block. Our characterization experiments by gel permeation chromatography (GPC) indicated that S-EP had a polydispersity M / M ~1.1. This S-EP diblock copolymer has, therefore, a well-defined structure and had been produced by hydrogenating the polyisoprene block in a diblock copolymer of polystyrene-polyisoprene synthesized by anionic polymerization (20). Vinyl acetate (VAC) monomer (Aldrich Chemical Co. Ltd., stabilized with 4 ppm hydroquinone and 300 ppm diphenylamine) was degassed, and distilled under vacuum when required for use. The initiator azobisisobutyronitrile (AIBN) was double recrystallised from ethanol. The S-EP sample was dispersed in an nalkane, typically n-heptane (which had previously been dried over molecular sieve, degassed, and distilled under vacuum), by first leaving the mixture overnight at room temperature and then raising the temperature of the stirred mixture to 343 K for 30 min. The apparatus consisting of a round-bottomed flask with a side arm and equipped with stirrer, stopper and condenser contained nitrogen gas throughout. A seeding technique was incorporated into the dispersion polymerization because single stage ("one-shot") polymerizations provided somewhat broader particle size distributions. A typical seed stage involved adding monomer (20% by weight of the total VAC with the equivalent proportion of AIBN) very quickly to the dispersion medium containing the S-EP stabilizer at the polymerization temperature 343 K. After this addition, the seed dispersion was allowed to form for 13 hours. When the seed stage had been accomplished, the growth stage was started by adding the remaining VAC (with AIBN) incrementally as a feed over a period of 2 hours. A typical small-scale dispersion polymerization consisted of S-EP (lg), heptane (20g), 1

1

n

£ p

1

In Polymer Latexes; Daniels, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


434

POLYMER LATEXES

VAC (3.3 g) and AIBN (0.06 g), with a monomer conversion to PVAC of about 70 per cent Dispersions obtained with polymerization times up to 50 hours were stored at ambient temperature. Dispersions were washed by repeated centrifuge/diluent exchange cycles to remove unadsorbed S-EP and unconverted monomer. Analysis of the supernatant by infrared spectroscopy demonstrated that six such redispersion cycles were usually sufficient to reduce the excess stabilizer to negligible proportions. Redispersion also provided a way of exchanging the dispersion medium for a different one, and products prepared were redispersed in hexane and heptane. Determinations of the critical flocculation temperature (CFT) on cooling were performed with dispersions in a cell designed and constructed to be accommodated in a Unicam SP600 UVvisible spectrophotometer operating at 600 nm. The sample compartment containing the dispersion was surrounded by a jacket containing water circulated from an external thermostated bath. A rotating magnet beneath the cell rotated a bar stirrer in the sample compartment to ensure constant and efficient mixing of the dispersion. A dispersion in n-heptane (polymer content 2x10 g cm* in 10 cm ) was added to the cell at 298 K. Addition of n-propanol (99.9% Aristar grade from B.D.H.) dropwise to the stirred dilute dispersion provided a dispersion medium of n-heptane/npropanol (70:30, v/v). The stirred contents of the cell were heated to at least 5°C above the CFT and then allowed to cool at the rate 1°C per 360 s, and the temperature at which a significant change in transmittance was observed was recorded as the CFT. Typically, a significant change in transmittance occurred over a temperature range as the dispersion was cooled, and so the estimate of CFT has an experimental error < ±0.4 K. Flocculation was noticed to be reversible, and addition of further n-heptane or an increase in temperature produced deflocculation. Stopping the stirrer at the flocculation point caused a remarkable decrease in the turbidity as the floes settled. Values of mean particle diameter D were estimated from transmission electron micrographs (TEM) by the method described previously (7). The polydispersity of the particles was estimated to be less than 1.03. The size of a particle core was assumed constant on heating from 298 to 318 K. The surface coverage of a particle by S-EP was estimated from the PS content determined by UV spectrophotometry (Kontron UVIKON810) on dry particles at 272 nm. During the preparation of dispersion samples for TEM examination, the swollen surface layer of EP chains collapses onto the surface of a particle when the dispersion medium is removed. From the surface coverage data, the thickness of this interfacial layer of bulk EP copolymer was estimated to be about 2 nm which represents 1.5% increase in the diameter of the smallest particles. Thus, the thickness of the collapsed layer could be neglected, and the TEM diameter was taken as the core diameter. 3

3

3

Viscometry. The relative viscosity ti of dispersions with a volume fraction | of dispersed phase in the range 0.02-0.12 was measured with a Cannon-Fenske type of capillary viscometer having a capillary diameter 0.55 mm. This diameter was large compared to the diameters of the dispersion particles; thus, corrections for walleffects could be neglected (10). Flow times for the pure dispersion medium were typically of the order of 200 s and reproducibility was better than ±0.1%. Relative viscosities were determined for dispersion particles in n-heptane at three different temperatures, 298, 308 and 318 ± 0.02 K. The relative viscosity was converted to an absolute viscosity with literature data for the viscosity of the dispersion medium (21). Cumulative errors arising from dilution procedures were avoided by gravimetrically determining the polymer content of samples at each dilution. The viscometer was washed with filtered heptane and filtered chloroform and dried between each determination. Possible deposition of dispersion particles on the walls of the viscometer was avoided by silylating the viscometer and all glassware with a solution of chlorotrimethylsilane (10% w/v) in chloroform (10). Glassware was r




435


baked for several hours at 373 K before cooling, then treated with the silylating agent for 24 h, and finally washed thoroughly with filtered chloroform before drying. The silylation of the viscometer in such a manner remained effective for at least six months. Results and Discussion Dispersion Polymerization. The progress of polymerization for the growth stage was followed for a range of polymerizations with the AIBN concentration fixed at 1 weight per cent based on monomer and VAC concentration in the range 10-30 weight per cent based on diluent in the presence of S-EP having a concentration in the range 1-6 weight per cent based on diluent. From these polymerizations, a major aim was a controlled particle size coupled with a narrow particle size distribution. Seeded polymerizations were employed to produce PVAC particles for studies of flocculation and for characterization of dispersions by viscometry. The concentration of S-EP stabilizer is an important factor controlling nucleation in the seed stage. Figure 1 demonstrates the variation of particle diameter of dispersions prepared in the presence of different concentrations of S-EP for [VAC] held constant at 20 weight per cent. As the stabilizer concentration increased, smaller particles were produced, as predicted by the theories of particle formation (5). The data in Figure 1 plotted on logarithmic axes gave a straight line which obeyed the relationship 0

Dae -

62

(1)

in which D is the average particle diameter and c is the concentration of the S-EP stabilizer in solution. A similar relationship for dispersions of PMMA stabilized by graft copolymers has been reported by Barrett (5) who found that the exponent was in the range -0.5 to -0.6. Dawkins and Taylor (7) reported an exponent of -0.77 for dispersions of PMMA stabilized with PS-PDMS. Dispersion polymerizations of VAC were usually performed in the presence of 5 weight per cent S-EP in solution. It should be noted that despite this relatively high concentration, only up to 50% of the S-EP stabilizer was actually incorporated onto PVAC particles. It appeared that higher concentrations were required to prepare stable dispersions since the adsorption of the S-EP block copolymer onto the particle surface might not be readily achieved owing to the low miscibility of PS anchor blocks with the PVAC chains in the particle core. Figure 2 shows the variation of mean particle diameter prepared with an increasing proportion of monomer in the seed stage with [S-EP] held constant at 5 weight per cent. As the monomer content of the seed stage increased, larger particles were produced, until in the limit all the monomer in the seed stage corresponds to a one-shot polymerization when PVAC particles with a mean diameter of 0.48 |im were produced. The process of particle formation begins when a polymer chain (oligomer) grows in solution until it reaches a threshold molar mass at which it will precipitate and contribute to nucleus formation. The results in Figure 2 for a total overall monomer concentration fixed at 20% for each dispersion are consistent with the proposal that a lower monomer content in the seed stage produces low solvency of the dispersion medium for the propagating chains which precipitate forming more initial nuclei. Since the total overall [VAC] concentration is fixed, smaller particles must result. Our seed-feed method was to employ 1/5 of the total overall [VAC] in the seed stage. The variation of mean particle diameter with the total overall monomer concentration for these variable seed (1/5 of VAC) experiments is shown in Figure 3. These results demonstrate that somewhat higher concentration of VAC during the seed stage together with an increase in VAC concentration at the feed stage produced larger particles.


436

POLYMER LATEXES

Log (Copolymer concentration) 10

0

0-2

0-6

(K

0-8


1 1 1 1 1 1 1

I

1

1

1

1

i

3

r—

1

5

Copolymer concentration /%w/w

Figure 1. Dependence of mean particle diameter on the concentration of S-EP diblock copolymer in dispersion polymerizations of vinyl acetate.

I

0

1

i

40

i

i

- U

80

Monomer in the seed stage /wt%

Figure 2. Dependence of mean particle diameter on monomer content in the seed stage in seeded dispersion polymerizations of vinyl acetate.





437

Particle Characterization and Stability The surface coverage of PVAC particles by EP copolymer chains can be estimated from the copolymer content and particle diameter. In Figure 4 we display the percentage of block copolymer stabilizer for PVAC and PMMA dispersions which had been freed of excess non-adsorbed S-EP and then dried. It follows that the amount of S-EP surrounding PVAC particles is higher than for PMMA particles and that the block copolymer concentration increased as the particle size decreased. The content of EP copolymer per polymer particle can be calculated from Figure 4 since the block copolymer composition of S-EP was known. Surface coverage results can be interpreted in terms of the surface area A occupied or stabilized by each EP copolymer chain and the mean separation distance d between adjacent EP copolymer chains, assuming that the PS block in the S-EP stabilizer does not extend significantly into the dispersion medium, that each EP copolymer chain is terminally anchored at the particle surface, and that an EP copolymer chain is anchored at the centre of a regular hexagon of area A. The data for A in Table I indicate that there is no obvious dependence of the area stabilized per chain on particle size, so it can be assumed that total surface coverage is attained. The value of A = 30 nm for an EP copolymer chain is lower for PVAC particles than A = 44 nm previously reported for PMMA particles (11) which follows from the block copolymer contents displayed in Figure 4. It may be concluded that the EP stabilizing chains are packed more closely around the surface of a PVAC particle than a PMMA particle, and we calculate that d = 6 nm for PVAC particles compared with the result d = 7 nm previously reported for PMMA particles (12). For a free EP copolymer chain it is calculated that the root-mean-square radius of gyration is at least 9 nm (12). With d < 2 it is indicated that close-packing of the stabilizing chains promotes overlaps between neighbouring EP copolymer chains in order to attain total surface coverage of particles. PVAC dispersions free of excess S-EP exhibited excellent long-term stability (over many months). Values of CFT for PVAC dispersions are shown in Table I. It was demonstrated that there was no variation in CFT with particle concentration up to 2 x 10" g cm" , in agreement with previous observations for PMMA particles (9,11). As with the surface coverage data in Table I, there is little or no dependence of the CFT on D over the particle diameter range 0.13 to 0.31 |im. In determinations of the CFT with the liquid mixture having a composition n-heptane/n-propanol (70:30, v/v), PVAC dispersions retained stability at experimental conditions when PMMA dispersions flocculated with CFT = 315.4K (11). Theta conditions for this same liquid mixture have been determined for EP copolymer chains, and 0 was found to be 317K (11). These results indicate that these dispersions retained stability at theta conditions, with flocculation occurring when the dispersion medium was just worse than a theta system for the EP copolymer chains. Further work is required to explain the different flocculation behaviour of PVAC and PMMA particles. One possible contribution might be preferential sorption of one liquid component by the stabilizing chains or by the particle core which may change the solvency of the dispersion medium around the stabilizing chains. This has been considered previously in connection with the proposal that preferential sorption of n-propanol on polymer particles may occur (11), but the significant quantity of PS blocks per particle (see Figure 4 and Table I) could provide a barrier to uptake of liquid components from the dispersion medium into the particle core. Another explanation may follow from how S-EP adsorbs onto the two types of polymer particles. We have observed that d is somewhat higher for EP copolymer chains on PMMA particles than on PVAC particles, and the surface layer thickness is higher for EP copolymer chains on PVAC particles than on PMMA particles (see Table II). Figure 4 indicates that the segment density of EP copolymer chains is higher for PVAC particles. Preliminary studies (22) indicate that the mode of anchoring of S-EP through PS blocks is different for the two types of particles which 2

2

2

2

2

05

3


0,5


438

POLYMER LATEXES

0-10 I 5

1

1 1 1 15 25 Monomer /wt %

1

1—I 35

Figure 3. Dependence of mean particle diameter on total monomer concentration in seeded dispersion polymerizations of vinyl acetate.

Particle diameter /jum

Figure 4. Dependence of concentration of stabilizing S-EP copolymer (%, w/w) on mean particle diameter, o, PVAC particles; PMMA particles from references 11 and 12. In Polymer Latexes; Daniels, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.



439


Table I. Dependence of flocculation behaviour on particle size and surface coverage D/nm

S-EP/ % (w/w)

A/nm

136 158 180 203 250 309

19.8 18.3 16.5 13.6 12.5 10.6

30.7 29.1 29.0 32.3 28.9 28.1

CFT/K*

2

312.9 313.1 313.1 313.0 312.9 313.0

where D is mean particle diameter, A is the surface area stabilized by each EP copolymer chain, and CFT is the critical flocculation temperature. particles in n-heptane/n-propanol (70:30, v/v)

Table II. Chain dimensions of EP copolymer in n-heptane

Tempera ture/K

SflMMAynm''

8(PVAC)/nm

298 308 318

36 41 45

25 25 24

26 28 30

where 8 is the hydrodynamic thickness for the EP surface layer for PVAC and PMMA particles and ° is the root-mean-square end-to-end distance for an EP copolymer chain. 2

5

Reference 12


440

POLYMER LATEXES

may contribute to these observations, and a more detailed study of block copolymer attachment will be considered in a subsequent paper. Viscosity Plots of the dependence of the absolute viscosity of PVAC particles in n-heptane at 298 K against the volume fraction of the polymer particle core are shown in Figure 5. The value of was calculated from the total polymer°content in the dispersion (obtained gravimetrically), the EP content (obtained from surface coverage data for dry particles), and the density of the particle core (assumed to be 1.19 g cm" for bulk PVAC (23)). The value of the hydrodynamic thickness 8 for the EP surface layer may be calculated from the limiting slope of the curves at = 0 in Figure 5 according to the method employed by Walbridge and Waters (24). However, this method may not be accurate because of errors in locating the curves in Figure 5 at low values of , and so we have preferred to use the procedure involving the relative viscosity T| reported by Barsted et al. (25) The dependence of r\ on is given by Equation (2) which was proposed by Saunders (26) who modified the relation suggested by Mooney (27) o

o

3


o

o

j

4>/ln r i ^ G / a ^ - W y a ^ .

o

(2)

0

In Equation (2) oc is the Einstein coefficient (equal to 2.5 for a dilute suspension of rigid non-interacting spheres), k is a crowding factor and the factor f, as defined by Maron et al. (28) is given by o

3

f=n>

where < > | is the total volume fraction of the dispersed phase. Viscosity data plotted according to Equation (2) are shown in Figures 6 and 7 for PVAC particles stabilized in n-heptane with S-EP at 298, 308 and 318 K. The quantity o f in Equation (2) may be thought of as an effective Einstein coefficient which may be determined from the intercepts in Figures 6 and 7. For dispersions of particles having a value of 8 which is significant compared with D, Goodwin (29) proposed that the magnitude of f was correctly given by 3

f=[l+(287D)] .

(4)

and so the effective Einstein coefficient is given by 3

a f = a[l+(2o7D)] .

(5)

1/3

Values of ( a f ) determined from the intercepts in Figures 6 and 7 were plotted against the reciprocal of D, as shown in Figure 8 which confirms the linear behaviour predicted by Equation (5) for the range of particle core diameters (130-300 nm) examined. As D" tends to zero, the surface layer becomes negligible relative to the core diameter and the effective Einstein coefficient approaches the true Einstein coefficient of 2.5. From the intercept in Figure 8 the value of a was found to be in good agreement with the true Einstein value, suggesting that die PMMA particles were spherical and free from aggregation. The sphericity of the particles was indicated from TEM observations. The linearity of the plots in Figure 8 implies that 8 at each temperature was essentially constant over the particle size considered. Therefore, from Equation (5) values of 8 at each temperature may be foundfromthe ratio of the slope to the intercept for each plot in Figure 8. The results are shown in Table II, where the error in 8 considering the experimental data for ri and D is about 1 nm. 1

r r





004

441

008

Figure 5. Variation of viscosity of PVAC particles in n-heptane at 298 K with the volume fractionty of core polymer, o, D = 137 nm; D = 292 nm. o

016

C

012

008

"

0OA

O08

Figure 6. Plots of viscosity data according to Equation (2) for PVAC particles in n-heptane at 298 K (A,A), 308 K (•,•), and 318 K (o,#). Open symbols, D = 202 nm;filledsymbols, D = 137 nm



442

POLYMER LATEXES

006

1

1

'

0

1

1

0-04

1

1

1

— 012

008 •o

Figure 7. Plots of viscosity data according to Equation (2) for PVAC particles in n-heptane at 298 K ( A , A ) , 308 K (•,•), and 318 K (o,m). Open symbols, D = 292 nm; filled symbols, D = 170 nm.

2-6

10

I 0

1

1

20

1

1

1

L0

"

60

1

1

—

1

8 0

D-VjjnrT

1

1/3

Figure 8. Dependence of the function (a f) on the reciprocal of mean particle diameter for PVAC particles in Figures 6 and 7. A, 298 K; • , 308 K; o, 318 K. o




443


2

5

Predicted values for the root-mean-square end-to-end distance ° have been made elsewhere (12) and are shown in Table II. This comparison of chain dimensions indicates that EP copolymer chains terminally anchored at the interface between PVAC and n-heptane appear to be somewhat extended over random coil dimensions forfreechains in solution. The surface layer thickness for EP copolymer chains on PVAC particles is larger than for PMMA particles. The dimensions of EP copolymer chains are not expected to change markedly with temperature because nheptane may be considered to be a good solvent for free copolymer chains. The increase in o with temperature could be overestimated because D has been estimated by TEM on dried particles. If the particles remained glassy (e.g. PMMA), then the volume change for thermal expansion on heating from 298 to 318 K would be extremely small. If the particles were at or near the glass transition temperature (e.g PVAC), then a larger volume change might be expected together with possible diffusion of n-heptane into the particle core. Consequently, a possible explanation of the data at 318 K is that 8 is overestimated for PVAC particles because expansion of the PVAC core has not been incorporated into the values of D. Overall, it does appear that the surface layer is thicker than expected on the basis of the dimensions of free EP copolymer chains, which is consistent with results reported for PDMS steric stabilizers (10). Furthermore, values of 8 in Table II exceed 10 nm and so attractive forces between particle cores will not influence stabilization/flocculation behaviour. Consequently, the results of our controlled flocculation experiments are consistent with Napper's view (30) on steric stabilization of polymer colloids with long chains in a thick interfacial layer. Conclusions Our results demonstrate that S-EP diblock copolymer stabilizes PVAC particles during non-aqueous dispersion polymerization. Flocculation/stabilization behaviour can be interpreted in terms of surface layers of EP copolymer chains around particles. It was observed that data for surface coverage and surface layer thickness of the stabilizing EP copolymer chains were somewhat different for PVAC and PMMA particles. Literature Cited 1. Amer. Chem. Soc. Symposium, "Polymer Colloids", 1970. 2. Fitch, R.M., Ed. "Polymer Colloids"; Plenum: New York, 1971. 3. Fitch, R.M., Ed. "Polymer Colloids II"; Plenum: New York, 1980. 4. Vincent, B. Adv. Colloid Interface Sci., 1974, 4, 193. 5. Barrett, K.E.J., Ed. "Dispersion Polymerization in Organic Media"; Wiley: New York, 1975. 6. Fitch, R.M.; Kamath, Y.K. J. Indian Chem. Soc., 1972, 49, 1209. 7. Dawkins, J.V.; Taylor, G. Polymer, 1979, 20, 599. 8. Dawkins, J.V.; Maghami, G.G.; Shakir, S.A.; Higgins, J.S. Colloid Polym. Sci., 1986, 264, 616. 9. Dawkins, J.V.; Taylor, G. Colloid Polym. Sci., 1980, 79, 258. 10. Dawkins, J.V.; Taylor, G. J.C.S. FaradayI,1980, 76, 1263. 11. Dawkins, J.V.; Shakir, S.A. Colloid Polym. Sci., 1987, 265, 329. 12. Dawkins, J.V.; Shakir, S.A. Colloids Surfaces, 1988, 32, 345. 13. Higgins, J.S.; Dawkins, J.V.; Taylor, G. Polymer, 1980, 21, 627. 14. Napper, D.H. Trans. Faraday Soc., 1968, 64, 1701. 15. Croucher, M.D.; Hair, M.L. Colloids Surfaces, 1980, 1, 349. 16. Croucher, M.D.; Hair, M.L. J. Colloid Interface Sci., 1981, 81, 257. 17. Croucher, M.D.; Lok, K.P.; Wong, R.W.; Drappel, S.; Duff, J.M.; Pundsack, A.L.; Hair, M.L. J. Appl. Polym. Sci., 1985, 30, 593. 18. Dawkins, J.V.; Taylor, G.; Baker, S.P.; Collett, R.W.R.; Higgins, J.S. Amer. Chem.Soc.Symp. Ser., 1981, 165, 189.



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POLYMER LATEXES

19. Dawkins, J.V.; Taylor, G.; Ghaem-Maghami G.; Higgins, J.S. Amer. Chem. Soc. Symp. Ser., 1984, 240, 267. 20. Noshay, A.; McGrath, J.E. "Block Copolymers: Overview and Critical Survey", Academic Press, New York, 1977. 21. Weast, R.C. "Handbook of Chemistry and Physics", 53rd edition, CRC Press, Cleveland, Ohio, 1972. 22. Shakir, S.A. Ph.D. thesis, Loughborough University of Technology, 1987 23. Broens, O.; Muller, F.H. Kolloid Z., 1955, 140, 121. 24. Walbridge, D.J.; Waters, J.A. Discuss. Faraday Soc., 1966, 42, 294, 25. Barsted, S.J.; Nowakowska, L.J.; Wagstaff, I.; Walbridge, D.J. Trans. Faraday Soc., 1971, 67, 3598. 26. Saunders, F.L., J. Colloid Sci., 1961, 16, 13. 27. Mooney, M., J. Colloid Sci., 1951, 6, 162. 28. Maron, S.H.; Madow, B.P.; Krieger, I.M. J. Colloid Sci., 1951, 6, 584. 29. Goodwin, J.W., In "Colloid Science"; Everett, D.H., Ed.; Specialist Periodical Report, The Chemical Society, London, 1975; Vol. 2, Chapter 7. 30. Napper, D.H. "Polymeric Stabilization of Colloidal Dispersions", Academic Press, New York, 1983. RECEIVED December 4, 1991


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