light-scattering studies of phase-demixing of polystyrene/poly(o-chlorostyrene) blends

Light-Scattering Studies of Phase-Demixing of Polystyrene/Poly( o-Chlorostyrene) Blends

JOHN GILMER, NIEL GOLDSTEIN, and RICHARD S. STEIN,* Polymer Research Institute and Materials Research Laboratory, University of

Massachusetts, Amherst, Massachusetts 01003

Synopsis

The phase separation of blends of polystyrene and poly(o-chlorostyrene) has been studied by observing the changing small-angle light-scattering profile with time. As a blend is heated to a temperature at which it undergoes phase separation, a light-scattering maximum is observed which grows in intensity and moves to smaller angles with time. This maximum is associated with a characteristic spacing which increases at a rate which becomes greater a t higher temperature or with lower molecular weight. This spacing varies with a power of time as might be expected for domain growth occurring by a viscous flow mechanism. The integrated scattering intensity (invariant) is found to increase initially with time and then remain constant, as is characteristic of phase separation followed by phase ripening.

INTRODUCTION

Studies by Alexandrovich,' Ryan: Karasz, and MacKnight have shown that compatible blends of atactic polystyrene (aPS) and atactic poly(o-chlorostyrene) (aPOCS) can be formed. The precise phase diagram for these two polymers depends on their molecular weights and molecular weight distributions. Ryan estimated the location of the binodal for different aPS/aPOCS systems by ana- lyzing the change in their characteristic DSC curves with phase separation. More recent studies using cloud points for PS/POCS/solvent systems have been carried out by Zachariu~.~

The phase separation behavior of homogeneous blends of aPS and aPOCS with initial composition well within the spinodal region was investigated. The observed behavior of these blends was compared with that predicted by the theories of spinodal decomposition developed by Cahn4 and by Langer5 and was compared as well with the experimental results of McMaster6 for phase separation of blends containing a styrene-acrylonitrile copolymer and poly(methy1 methacrylate). Visible light scattering was used to observe the changing morphology of polymer blends undergoing phase separation.

EXPERIMENTAL

The PS obtained from the Monsanto Company had the following specified molecular weights: M , = 92,000 f 3000; M , = 274,000 f 14,000; M, = 485,000 f 15,000. All POCS was obtained by free-radical polymerization carried out

*To whom correspondence should he addressed.

Journal of Polymer Science: Polymer Physics Edition, Vol. 20, 2219-2227 (1982) 0 1982 John Wiley & Sons, Inc. CCC 0098-1273/82/122219-09$01.90

2220 GILMER, GOLDSTEIN, AND STEIN

in toluene at 6OoC with azobisisobutyronitrile serving as the initiator. The POCS used in studies performed on the light-scattering apparatus equipped with an optical multichannel analyzer7 (OMA) had the following GPC-determined polystyrene equivalent molecular weights*: M , = 57,000; M , = 120,000; M, = 200,000. The POCS used in studies performed on the photometric light- scattering apparatus9 had a viscosity molecular weight of Mu = 221,000 f 3000.10

Blends of aPS and aPOCS were prepared by coprecipitation and subsequent melt pressing. A toluene solution of the two polymers was added dropwise to the nonsolvent, methanol. The dry precipitate was then placed inside a thin foil shim between a glass microscope slide and a cover slip. The sample was then melt pressed cyclically a t 140-15OOC and at 1200 lb/in.2 until the prepared film showed no transmittance of light when placed between crossed polaroids.

It is not certain whether the films used for these light-scattering experiments were initially homogeneous at the molecular level. These films were optically clear before being heated to the experimental temperature and became markedly cloudy shortly after the commencement of the experiment. Differential scanning calorimetry performed on some of the blend powders indicated the presence of only one glass transition temperature, the expected result for a one-phase system.

When the photometric apparatus was used to analyze the light-scattering data from phase-demixing blends, the following procedure was employed: A sample blend was heated in a constant-temperature sand bath for a set of period of time and was then quenched to room temperature. The variation of the light-scattering intensity with angle was then measured for the phase-separated blend with the blue line (435.8 nm) of a mercury arc serving as the light source.

A different procedure was followed when the light-scattering apparatus equipped with an OMA and a helium-neon laser (632.8 nm) was used. Since the OMA collects data simultaneously over the entire angular range, quenching the phase-separated blend to room temperature was not necessary. By placing the blend on a Mettler hot stage which had been preheated to 190°C, the light- scattering profile was taken while the sample blend was in the process of phase demixing. There was no noticeable difference between the results obtained in this manner and those obtained from the quenched samples.

THEORY AND RESULTS

The scattering of small-angle monochromatic light from phase-demixing blends of PS/POCS gives rise to a circular amorphous halo which is associated with a spatial periodicity in concentration. Photomicrographs of these phase- separated films (Fig. 1) showed a mottled interconnected appearance in unpo- larized light characteristic of a heterogeneity in refractive index. An optical diffraction pattern obtained from these photomicrographs exhibited a halo characteristic of a periodicity identical with that deduced from the direct scattering pattern from the polymer film itself.

It has been observed that polymers may phase separate by nucleation and growth or by spinodal mechanisms, depending on the composition of the sample and the degree of supercooling [for systems exhibiting an upper critical solution temperature (UCST)] or superheating [for systems exhibiting a lower critical

LIGHT-SCATTERING STUDIES OF PHASE DEMIXING 2221

Figure 1. Photomicrograph for a blend of 60% PS/40% POCS which had been heated to 190°C for 1 h. The marker indicates 100 fim.

solution temperature (LCST)] .11J2 This has been experimentally demonstrated by McMaster6 on poly(methy1 methacrylate)/styrene-acrylonitrile blends and Nishi et al.13 for the PS/poly(vinyl methyl ether) (PVME) system. The mor- phological difference resulting from these two modes of phase separation is that (i) the nucleation and growth mode initially arises through randomly located nuclei which then grow as expanding spheres in a manner influenced by the concentration depletion of their surroundings, whereas (ii) spinodal decomposition is initially associated with a periodic concentration fluctuation of constant “wa~elength~~ but which grows in amplitude with time. While the following description is phrased in terms of the spinodal model, we do not intend to suggest that this is a unique interpretation of our results. As is indicated, it will be seen that the studies we have carried out are at too late a stage of phase separation to adequately discriminate among mechanisms.

To analyze the light-scattering data we employ a theory similar to that developed by van Aartsen and Smolders14 for a polymer solution undergoing spinodal decomposition. The concentration fluctuation at a given location i can be described as a three dimensional Fourier series:

where 41 is the volume fraction of component 1, B is the fluctuation wave number, and ri is the position vector of volume element i. The intensity of V, polarized scattered light is described by


0 5 10 8 (Degrees)

Figure 2. Variation of the Rayleigh factor with scattering angle for blends of 40 wt % PS/60 wt % POCS which had been heated to 190°C for the indicated periods of time.

where rij = rj - ri, k = 27r/X, ai is the polarizability a t point i, and s = SO - S‘ = (2 sinl/zO)[i sin1/20 - j c0s1/20 - k(cosl/zO)cosp] with s - s = h2/k2 = (2 sin1/zO)2. In evaluating (ai aj ) we can approximate the value of ai by

We obtain the following expression for the average of the product (ai - aj ) :

We can now describe the light-scattering intensity a t a given angle by

where A ( k s ) = [A’ 2(ks) + B’ 2(ks)] indicates the amplitude of a given and a single p of value ks contributes to the light-scattering intensity a t that angle. We can approximate the concentration profile of the phase-demixing blend to be that of the Fourier component corresponding to the light-scattering intensity maximum. We can now define a fluctuation spacing D, given by

(6)

where 0, is the fluctuation wave number of the angle of maximum intensity.

D, = 2 ~ ( & - Pm)-’/2


TIME (Hours)

Figure 3. Variation of scattering periodicity D, with time for a 40 wt 70 PS/60 wt 9% POCS blend quenched from the indicated temperatures.

From the relation 6 = ks we obtain the following relation for D,:

D, = A012 sin(W3,) (7)

where 6, is the angle of the intensity maximum. This equation represents es- sentially a Bragg condition for a diffraction from the periodic concentration variation. l4

Our measurements were taken at times greater than those for which the Cahn theory of spinodal decomposition would be applicable. The observed light- scattering pattern resulting from phase-demixing PS/POCS blends (Fig. 2 ) was a single circular halo which decreased in angle and increased in intensity with time. It was found that the change of the fluctuation spacing with time can be described by the empirical expression

D, = a t b (8) where t is time and a and b are adjustable parameters (Figs. 3-5). The values for a and b vary with the temperature, the molecular weights, and the composition of a sample blend (Table I). The coefficient a increases both with in-

TABLE I Characteristics of Phase-SeDarated Blendsa

Temperature wt. '7O ("C) POCS MW POCS b a ( d h )

160 M" = 221,000 40 1.60 f 0.10 0.0027 220 M , = 221,000 40 1.13 f 0.10 22 190 M , = 221,000 40 0.85 f 0.10 1.9 190 M,, = 120,000 40 0.80 f 0.05 4.9 190 Mu. = 120,000 50 0.85 f 0.05 5.9 190 M,,, = 120.000 60 0.87 f 0.05 5.4

a Values of parameters a and b for different samples.


TIME (Hours)

Figure 4. Variation of scattering periodicity D, with time for a 40 wt 70 PS/60 wt % POCS blend quenched from 190°C for molecular weights of the POCS of 120,000 and 221,000.

creasing temperature and with decreasing molecular weight, as would be expected for a diffusion-dependent process. The major change in exponent b occurs with changes in the experimental temperature. Blends investigated at 190 and 220°C have an initial composition well within the spinodal,15 whereas the blend investigated at 16OOC probably has a composition near the spinodal. These changes in b could be due to the occurrence of slight changes with temperature in the mechanism of phase separation.

Langer et a1.16 used a computer model to predict the scattering intensity for a decomposing two-component homogeneous mixture whose composition lies in the spinodal region. They found that the rate of growth of the fluctuation spacing can be described by eq. (8) where b = 0.2. In the Cahn theory of spinodal

50 w t % P S / S O w t % POCS 407 I I I I ! 1 1 . 1 I

I 5 I0 TIME (Hours)

Figure 5. Variation of scattering periodicity D, with time for blends quenched from 190°C for different compositions.

three


decomposition the fluctuation spacing remains constant with time; Langer5 at- tributes the shift of Pm with time to the nonzero value of b4f(co)/bc4 where f is the free energy per unit volume, c is the concentration of component 1, and co is the value of c for the homogeneous mixture. However, the domain growth rate for the PS/POCS system depends more strongly on time than the Langer theory would predict.

Following the initial stages of phase separation, regions coalesce to form larger domains, decreasing their surface area. This process occurs a t later stages of phase separation and leads to a modification of the initial morphology.

For a system in which droplets of one phase are distributed in a matrix of another, coarsening is predicted to occur by Ostwald ripening, a process in which the particle diameter increases at t 1/3.6J7 Using the transmission electron microscope, McMaster6 observed the phase separation behavior for blends of poly(methy1 methacrylate) and styrene-acrylonitrile (SAN/PMMA). These blends exhibited a high degree of interconnectivity and thus underwent coarsening by a viscous flow mechanism driven by interfacial tension. After bulk thermodynamic equilibrium was achieved, the size of phase-separated domains grew initially in a linear manner, which is described by eq. (8) with the exponent b equal to one. During the later stages of growth, the SAN/PMMA displayed an exponential growth rate described by

D, = aebt (9)

McMaster accounted for the observed behavior in both the linear and exponential growth regions by utilizing the predictions of Tomotikal8 for the behavior of a thin thread of one viscous, Newtonian fluid suspended in a matrix of another viscous Newtonian iluid. The results which we obtained for the PS/POCS system, where the value of b in eq. (8) ranges from 0.8 to 1.6, are similar to the results of McMaster (in the linear region) for b = 1.0. This indicates that the mechanism of domain growth in the POCS/PS system probably involves viscous flow where the occurrence of network breakup was minimal during the time of observation.

In order to determine whether a blend was undergoing phase separation or merely domain ripening (where the larger domains are growing at the expense of the smaller ones), the light-scattering invariant or total integrated intensity was observed as a function of time. The light-scattering invariant QLS which is analogous to the x-ray invariant, was defined by Koberstein, Russell, and Steinlg by

QLS J R h 2 d h

where R is the Rayleigh factor (a measure of the scattered intensity) and h is the scattering vector equal to ks. The light-scattering invariant can be expressed in terms of the mean-square polarizability fluctuation ( q2) by the equation

For an isotropic two-phase system separated by an interfacial layer with a

(12)

where 41,42, a1, a2 are the volume fractions and polarizabilities of the two phases,

linear polarizability gradient the mean-square polarizability can be written20

( q 2 ) = (4142 - '/64i)(al - a2)2


T I M E (Hours)

Figure 6. Variation of the scattering invariant QSALS with time for a 40 wt % PS/60 wt % POCS blend.

respectively, and 4i is the volume fraction of the interfacial region. For a system with a sharp interfacial region, the contribution of 4i to ( q2> can be ignored and the light-scattering invariant can be expressed in the form19

QLS = (32T6/~d4i4z(ai - ad2 (13)

If the mechanism of phase separation is spinodal, the composition of each of the two phases changes, and thus (a1 - increases with time. With a nucleation and growth mechanism, as the amount of phase-separated material increases with time, the product 4142 also increases. Thus with either mode of phase separation we would see an increase in QLS with time. If domain ripening is the only process occurring, both $142 and (a1 - &# remain constant, and thus QLS also remains constant.

In phase-demixing PS/POCS blends QLS was observed to increase initially and then level off (possibly even decreasing slightly) (Fig. 6). Thus, according to the above interpretation, phase separation is complete a t 190°C after ap- proximately five hours. However, no accompanying change in the rate of growth of D, is observed at this time.

CONCLUSIONS

Homogeneous aPS/aPOCS blends undergo phase separation in a manner qualitatively similar to that predicted by Langer where the phase-separated domains grow according to eq. (8). Domain growth probably occurs by a viscous flow mechanism similar to that observed by McMaster. In order to determine whether the kinetics of the early stages of phase separation follow the prediction of the Cahn theory of spinodal decomposition, the light-scattering pattern must be observed in the appropriate angular range during the initial stages of phase separation. Preliminary light-scattering measurements at early stages of phase separation for PSPVME by Hashimoto21 show a scattering maximum which


initially remains constant in position with time, consistent with early stages of spinodal decomposition. Attempts are in progress by C. Williams22 to observe early stages by small-angle x-ray scattering using synchrotron radiation. Finally, the effect of polydispersity on the kinetics of phase separation could be investigated by using narrow-molecular-weight fractions of each polymer.

References

1. P. Alexandrovich, Ph.D. Thesis, University of Massachusetts, 1979. 2. C. L. Ryan, Ph.D. Thesis, University of Massachusetts, 1979. 3. S. L. Zacharius, Ph.D. Thesis, University of Massachusetts, 1982. 4. J. W. Cahn, J . Chem. Phys., 42,93 (1965). 5. J. S. Langer, in Fluctuations, Instabilities and Phase Transitions, T. Riste, Ed., Plenum,

6. L. P. McMaster, Adv. Chem. Ser. 142, American Chemical Society, Washington, DC, (1975),

7. A. Wasiak, D. Peiffer, and R. S. Stein, J . Polym. Sci. Polym. Lett. Ed., 14,381 (1976). 8. T. P. Russell, Ph.D. Thesis, University of Massachusetts, 1978. 9. A. Plaza, F. H. Norris, and R. S. Stein, J . Polym. Sci., 24,455 (1957).

New York, 1957, pp. 19-42.

p. 43.

10. K. Matsumura, Makromol. Chem., 124,204 (1969). 11. D. R. Paul and S. Newman, Polymer Blends, Academic, New York, 1978, Vol. 1. 12. 0. Olabisi, L. M. Robeson, and M. T. Shaw, Polymer-Polymer Miscibility, Academic, New

13. T. Nishi, T. T. Wang, and T. K. Kwei, Macromolecules, 8,227 (1975). 14. J. J. van Aartsen and C. A. Smolders, Eur. Polym. J . , 24,455 (1957). 15. C. L. Ryan, private communication, 1980. 16. J. S. Langer, M. Bar-on, and H. D. Miller, Phys. Rev. A , 11,4 (1975). 17. I. M. Lifshitz and V. V. Slyozov, J . Phys. Chem. Solids Lett., 19,35 (1961). 18. S. Tomotika, Proc. R. Soc. London, 150,322 (1935). 19. J. Koberstein, T. P. Russell, and R. S. Stein, J. Polym. Sci. Polym. Phys. Ed., 17, 1719

20. F. H. Khambatta, Ph.D. Thesis, University of Massachusetts, 1976. 21. T. Hashimoto, private communication, 1981. 22. C. Williams, private communication, 1981.

York, 1979.

(1979).

Received February 15,1982 Accepted August 23,1982

light-scattering studies of phase-demixing of polystyrene/poly(o-chlorostyrene) blends

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