Electron microscopic study of the locus of a latex reaction

ELECTRON

MICROSCOPIC LATEX

STUDY OF THE LOCUS OF A REACTION

Edward Crampsey, Manfred The

Royal

Received

Technical August

Gordon, and John W. Sharpe

College, $0, 1953;

Glasgow revised

C. 1, Scotland January

29, 195.4

The outstanding example of the study of reaction loci in an emulsion system was Harkins’s work (1) in the field of emulsion polymerization. A simpler case presents itself for study when chemical reactions are carried out with preformed polymer emulsions (latices), as in the hydrochlorination of polyisoprene latex (2, 3). Here the reaction consists in the progressive conversion of the individual unsaturated isoprene units to their hydrochlorinated form : -CHr-C(CH3)=CH-CHs-

HCl -+

-CH+(CHs)-CHz-CHt-

Cl

the polymer particles preserving their identity throughout the react,ion. The hydrochloric acid arrives at the site of reaction by a series of diffusion processes from the vapor phase and through the continuous aqueous phase. In principle there are still two loci at which the isoprene units may react with this reagent: in the bulk of the particle and at (or near) the interphase between the aqueous acid and the polymer particle. We were led by kinetic experiments (3) to conclude that the two loci do react at different rates and by different mechanisms, and it is the purpose of this report to provide quantitative evidence for this conclusion. At the same time we wish to put on record some observations connected with the electron microscopic technique used in the investigation. The situation in the latex hydrochlorination reaction bears some resemblance to that in Smith’s work (4) on emulsion polymerization “seeded” with preformed polymer particles. This proved that the polymerization took place in the preformed particles, after the relevant reagent (viz. the monomer) had arrived there by diffusion. However, no distinction has arisen in emulsion polymerization between the bulk locus and the surface locus of a particle of polymer. The technological importance of this new distinction in rubber chemistry is stressed below. Our kinetic observations (3) suggested that a fast conversion of isoprene 5

185

186

E.

CRAMPSEY,

M.

GORDON,

AND

J.

W.

SHARPE

II

0 FIG.

1. Rate

curves

for

30 the

60

hydrochlorination and 2 atm.

90 of polyisoprene of HCl.

latices

at

300”abs.

units near the polymer particle surface is responsible for observed abnormalities near the beginning of the rate curves. This is quantitatively confirmed by correlating the extent of the initial “jump” in the rate curves, measured by the intercepts P (Fig. I), with measurements of the specific surface of the particles by means of electron micrographs (Figs. 2-4). Details of the preparation of the latices and of the kinetics and mechanism of the hydrochlorination in the bull< of the polymer particles, whose rate is measured by the slope of the straight lines in Fig. 1, have been published (3). The lines refer to natural (Hevea) latex and two synthetic polyisoprene latices. Two further synthetic latices are reported on in Table I, and the total range of average particle diameters studied is seen to be tenfold. The correct method of averaging n, particles of diameters Di so as to obtain a measure of the specific surface of a given latex, and thus of its total locus for the surface reaction, is to calculate the surface average diameter D defined thus: aD2 = (c

rDf)/n;

i.e. D = I($

D:),n]?

TABLE Surface

Hydrochlorination

D (unsh~~wed),

D' (shafowed),

P%

2.0 6.25 15 20 21

(4500) 1175 670 467 460

1400 700 -

I

Reaction

(300”

abs.,

T, A.

T’, A.

15 12.5 15 17 17.4

14.7 24.9 -

2 atm.

of HCl) Latex

Natural Synthetic Synthetic Synthetic Synthetic

I III II V

LOCUS

OF

A

LATEX

187

REACTION

The thickness 7 of the outer shell of a sphere of diameter D, which comprises P % of the sphere by volume, may then be computed thus:

PI

7 = D(1 - 0.2155(100 - pyy2.

The values of 7 found for the one natural and four synthetic latices in Table I shorn very satisfactory constancy at 15 =t 2.5 A. This confirms the kinetic inference that units privileged for fast reaction, react in the vicinity of the particle surface, i.e., the interphase with the aqueous phase. The result can be expressed by saying that the extent of the fast initial hydrochlorination reaction is governed merely by the percentage P

FIG.

FIG.

2. Natural

rubber latex

4. Synthetic chloride All

hydrochloride

polyisoprene latex II. samples

hydro-.

shadow-cast

FIG.

FIG.

3.

Synthetic chloride

5. Synthetic

at approximate

angle

polyisoprene latex I.

polyisoprene cot?

2.5

hydro-

latex

I.

188

E.

CRAMPSEY,

M.

GORDON,

AND

J.

W.

SHARPE

of the polymer units which lies within 7 = 15 A. of the particle surfaces. This shows indirectly that the subsequent main reaction, corresponding to the linear rate plots (Fig. l), occurs at the other possible reaction locus, namely, inside the bulk of the polymer particle. The kinetic evidence shows that the rate of the surface reaction, which is here too fast to be measured, must fall off rapidly with the progress of the surface reaction, and once the first four layers have reacted, the surface rate becomes negligible. This may be expressed by saying that the surface reaction has a very high reaction order. With natural rubber the thickness r increases with temperature at constant HCI pressure. This, and other reasons, suggest that the falling off of the surface reaction is governed by a diffusion rate. If so, the rate of the surface reaction should be sensitive to prior crosslinking of the rubber. This has been investigated in vulcanization studies to be reported to the Third Rubber Technology Conference (London, 1954). All latex samples (saturated with HCl in the case of hydrochlorinated samples) were prepared for examination in the electron microscope by diluting with distilled water and then placing a single drop of the dispersion on the standard collodion-covered specimen grids used in electron microscopy. The specimen was then allowed to dry in a desiccator. In some cases the samples were shadow-cast with gold/palladium alloy in order to enhance contrast. Owing to their narrow size ranges, the synthetic latices (Figs. 3, 4) allowed sufficiently accurate values of the diameter D to be obtained by averaging a few particles chosen at random on the electron micrographs. In natural latex the particles are far from uniform in size (Fig. 2), but two different samples were found by van den Tempel (5) to agree closely in distribution curve and D, so that his value of D = 4500 A. was accepted.’ All our micrographs were taken at a magnification of 8000 X . Table I lists separately the surface average particle diameters D measured on nonshadow-cast specimens and those, denoted by D', measured on shadowed specimens of synthetic latices I and II. The maximum width of the shadow was taken as D', and this can be measured with somewhat higher precision than D, because of the higher definition of micrographs of shadowed specimens. The value of D' is seen to be larger than D by about 230 A. in both cases. The effect of shadow-casting in increasing the apparent particle size has been repeatedly noted by other workers, but its explanation is not quite certain. Kern and Kern (7), and Cosslett (8) independently 1 Rough estimates from our own micrographs led to somewhat lower values (25003000 A), while preliminary measurements by Cosslett quoted by Cockbain (6) are slightly in excess of van den Tempel’s figure. In view of the limited relative accuracy of the small intercept P for natural latex (2 f 0.5’%), a full size analysis of our natural latex did not seem warranted.

LOCUS

OF

A

LATEX

REACTION

189

explain the effect in terms of the surface charge of the particle producing the action of an electrostatic lens. The former authors imagine the lens to act on the electron beam in the microscope. A spuriously enlarged picture of the unshadowed particle is thus produced, while the shadowed particle, which allows its surface charge to leak away, gives a correct measure. Cosslett, however, considers the lens to affect the metal atoms in the shadowing process, in which case we must assume that the unshadowed particle gives the correct measure of the diameter. The chemical evidence based on our measurements of the intercepts P (Fig. 1) appears to support Cosslett’s explanation, inasmuch as the ‘%mshadowed” diameters D lead via Eq. [2] to noticeably more constant values of T’, when compared to the values r’ obtained via the same equation from the “shadowed” diameters D’ (as shown in Table I). Whatever the explanation of the effect, it is apparent from micrographs shown in Kern and Kern’s paper that the effect is relatively more marked with smaller particles (such as bacteriophages). Our measurements confirm this and suggest in fact that D’ - D = 230 A.

[31

The almost identical difference of 250 A. between shadowed and unshadowed particles was also observed by Kern and Kern on much larger (polystyrene) latex particles. The suggestion of the simple law of Eq. [3] may assist in the final theoretical explanation of the phenomenon. Comparison of electron micrographs before and after hydrochlorination of our latices reveals several interesting features (cf. Figs. 3 and 5). The change from the rubbery amorphous to the glassy state (with or without embedded crystalline regions2) during hydrochlorination is reflected in a change of appearance of shadow-cast samples. Rubbery particles are flattened (Fig. 5) and appear disclike rather than spherical, owing to wetting of the collodion support by the deformable particle. The glassy particles are so stiff as not to be visibly flattened, and are thus much more useful for measuring the particle diameters. Although much irreversible aggregation occurs in drying out the rubbery samples, the ultimate particle size before the hydrochlorination reaction is seen to be essentially the same as after the reaction. This confirms that the particles preserve their identity during the reaction without dissolution or coagulation. (The volume change due to the actual reaction can, of course, be accurately computed from density data, but changes the diameter by only a few per cent.) The contrast in the micrographs, particularly in the case of unshadowed particles, increases considerably in the course of the reaction owing to the 2 Electron diffraction experiments by the authors confirm that natural hydrochloride shows crystallinity (cf. van Veersen (9)), whereas synthetic isoprene hydrochloride is amorphous (cf. D’Ianni et al. (10)).

latex poly-

190

E. CRAMPSEP,

M. GORDON, AND J. W. SHARPE

relatively high scattering power of the chlorine atoms introduced thereby. This suggests the usefulness of reactions like hydrohalogenation or halogenation of latices, after suitable stabilization against coagulation, when it is desired to study size distributions by electron microscopy. The demonstration here given of the occurrence of a polymer particle surface reaction locus, controllable by such variables as particle size and temperature, has colloid-chemical and technical importance. That the hydrochlorination reaction is not merely an isolated instance of this effect is suggested by similar rate curve “jumps” in the cyclization of Hevea latex (cf. ref. 3). Preliminary work suggests that in vulcanization of latex a surface locus can also be observed. In this reaction the technical effect would be expected to be particularly marked. For example, a lightly and uniformly cross-linked particle would give a useful prevulcanized latex. If, however, the reaction (interrupted after 1 per cent or so of conversion in practice) were found to be largely concentrated in the surface layer, as the present work shows the hydrochlorination reaction to be at such low conversions, one would obtain what might be described as an unvulcanized particle surrounded by a shell of ebonite, which would be technically useless. Differences in film strength of prevulcanized latices at constant combined sulfur are thought to be connected with the variability of the contribution of the surface locus from one vulcanization recipe to another. The authors are indebted work.

to Professor

P. D. Ritchie

for his kind interest

in this

REFERENCES 1. HARKINS, W. D., J. Am. Chem. Sot. 69, 1428 (1947). 2. GORDON, M., AND TAYLOR, J. S., J. Appl. Chem. (London) 3, 537 (1953). 3. CRAMPSEY, E., GORDON, M., AND TAYLOR, J. S., J. Chem. Sot. 1963, p. 3925. 4. SMITH, W. V., J. Am. Chem. Sot. 70, 3695 (1948). 5. VAN DEN TEMPEL, M., Trans. Inst. Rubber Ind. 27, 290 (1950). 6. COCKBAIN, E. G., Trans. Inst. Rubber Ind. 29, 297 (1952). 7. KERN, S. F., AND KERN, R. A., J. Appl. Phys. 21, 705 (1950). 8. COSSLETT, V. E., Proc. Conf. Electron Microscopy, Delft (1950). 9. VAN VEERSEN, J., Proc. %nd Rubber Technol. Conf. 1948, p. 87. 10. D’IANNI, J. D., NAPLES, F. J., MARSH, J. W., AND BARNEY, J. L., Ind. Eng. Chem.

38, 1178 (1946).