polyisoprene AB block copolymers

Micelle Forming Properties of Polystyrene/Polyisoprene AB Block Copolymers R O L F HILFIKER, *'2 BENJAMIN C H U , * ' t

'1 A N D

Z H O N G D E XU{

*Chemistry Department and t Department of Materials Science and Engineering, State University of New York at Stony Brook, Long Island, New York 11794-3400; and {.Department of Polymer Materials, East China University of Chemical Technology, Shanghai 200237, People's Republic of China Received September 23, 1988; accepted January 9, 1989 Static and dynamic light scattering, small-angle X-ray scattering, and viscosity measurements were used to characterize solutions ofpolystyrene-polyisoprene AB block copolymers with constant molecular weight of polyisoprene and various molecular weights of polystyrene in aniline. Aniline is a selective solvent for polystyrene, is isorefractive with polystyrene, and also has the same electron density as polystyrene. This fact enabled us to measure independently both the core and the total size of the micelles formed. From the data, we were able to obtain a picture of the micelle structure. The hard-sphere model and a model proposed by S. Alexander (J. Physique38, 983 ( 1977 )) were applied successfully to interpret the data. © 1989AcademicPress,Inc. 1. INTRODUCTION

If block copolymers (e.g., AB block copolymers) are dissolved in a solvent which is a good solvent for one block and a poor solvent for the other block, aggregates are formed under certain conditions. Commonly, it is assumed that spherical micelles are formed in which the insoluble block forms the micelle core and the soluble block forms the shell ( 1 ). The structure of these micelles is of interest: e.g., the density of the core consisting of polymer A and the shell consisting of polymer B. A c o m m o n method of investigating structures of aggregates is light scattering. In the case of light scattering of copolymers, however, considerable complications are encountered, so that it is very difficult or often impossible to obtain conclusive results using light scattering measurements alone. These difficulties can be Author to whom requests for reprints should be addressed, Chemistry Department, State University of New York, Stony Brook, NY 11794-3400. 2 Present address: Institut Fuer Physikalische Chemie, Universitaet Basel Klingelbergstr. 80 4056 Basel, Switzerland.

avoided if one of the blocks of the polymer is isorefractive with the solvent. In contrast to other authors ( 2 - 4 ) , we were working with a solvent which is isorefractive with the polymer for which it is a selective solvent. The behavior of three different polystyrenepolyisoprene AB block copolymers was studied. In all three cases the molecular weight of the polyisoprene block was the same (19 kg/ mole), whereas the molecular weight of the polystyrene block was 28, 41, and 61 kg/mole, respectively. We will label these copolymers CO1, CO2, and CO3 in the following. The samples were all rather monodisperse with Mw/Mn ranging from 1.06 to 1.08. Mw is the weight average; Mn, the number average molecular weight. In Section 2, a brief outline of the theory of light scattering of copolymers is provided. In Section 3 the experimental methods are described and, finally, in Section 4 the experimental results obtained by static and dynamic light scattering, small-angle X-ray scattering, and viscosity measurements are presented. The results are compared with existing theories and the conclusions summarized.

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JournalofColloidandInterfaceScience,Vol. 133,No. 1, November1989

MICELLE

2. T H E O R E T I C A L

FORMATION

177

OF COPOLYMERS

increment of a copolymer molecule with a weight fraction WAof polymer A is

BACKGROUND

Intensity of Scattered Light (On/Oc) = WA(On/OCA) The excess Rayleigh ratio (R'v) extrapolated to zero concentration and scattering angle (0) for vertically polarized incident and scattered light has the following form for a system which is monodisperse in molecular weight (M) and refractive index increment (On/Oc)r,p (hereafter the subscripts T and P are dropped).

(R%)c~o,o-,o = K*(On/Oc)2cM.

[1]

K* = 4~rZn2/NAXg, with n the refractive index of the solution, NA Avogadro's number, and Xo the wavelength of the incident light in vacuo, c is the concentration of solute in grams per cubic centimeter. For a system which is monodisperse in (0n/ Oc), but polydisperse in M, c M is replaced by (ciMi), with ci the concentration of the species with molecular weight M~. In this case Eq. [ 1] changes to (R%)c~O,O~O = K*(On/Oc)ZcMw,

[2]

with Mw ( = ( ~ ( c i M i ) ) / Z c i ) the weight average molecular weight. Copolymers are in general polydisperse in both composition and molecular weight. A composition polydispersity leads to a polydispersity in refractive index increment. In this case Eq. [ 1] will change to

(5)

+ (1 - Wa)(On/OcB).

[4]

The apparent molecular weight can then be expressed by

M* = ( O n / Oc) -2 { WAMAw(On/ OcA)2 + (1 - WA)MBw(On/OcB)2 + 2(On/Oca)(On/OcB) X ~ [(Ci/C)MiWA,i(1 --

WA,I)I} •

[51

i

M A and M~ are the weight average molecular weights of the A and B parts, respectively. From an inspection of Eq. [ 5 ], it can be seen that the apparent molecular weight is only equal to the weight average molecular weight if(On/OcA) or (On/OcB) is equal to zero (or if the sample is ideally monodisperse). Otherwise light scattering measurements must be performed in a series of solvents with different refractive indices in order to obtain the true molecular weight. This procedure cannot be applied if aggregation phenomena are studied since the aggregation behavior is certainly affected by the nature of the solvent. The measured radius of gyration ((s2,)0.5) of a monodisperse block copolymer is (5)

( s2, ) = ( On/ Oc) -2 { w2 (On/ OcA)2 ( s~ )

(R%)c-~o,o~o = K* [ Z { (On/Oci) 2ciM, } ] i

= K*(On/Oc)ZcM *.

[3]

M* ( = ( O n / O c ) - z [ z (ci/c)mi(On/Oci)2]) is the apparent molecular weight and depends on both the copolymer and the solvent and therefore has no direct physical meaning. If the A and B segments of the copolymer are sufficiently long that their refractive index is not dependent on the chain length and if the partial specific volumes of the polymer segments are not affected by the overall composition of the copolymer, the refractive index

+ (1 - w A ) 2 ( O n / O c ~ ) 2 ( s ~ )

+ WA(1 -- WA)(On/OCA)(On/OcB) × [(S 2 ) + (Sg) + f 2 ] } ,

[6]

where (S2A) and ( s ~ ) are the mean-square radii of gyration of the A and B parts, respectively, a n d f 2 = ((GAGB) 2) is the mean square of the distance GAGB separating the two centers of gravity. Again, it is apparent that the measured radius of gyration is dependent on the refractive index of the solvent. However, if the solvent is isorefractive with polymer A or B, the measured radius of gyration is equal to the Journal of Colloidand InterfaceScience,Vol. 133, No. 1, November 1989

178

HILFIKER, CHU, AND X U

radius or gyration of the B or A part, respectively. From these considerations, the big advantage of using a solvent which is isorefractive to A or B becomes apparent.

Polystyrene standards were purchased from Waters Associates, Massachusetts. Aniline (Gold Label, 99.5+%) was obtained from Aldrich Chemical Co. and was distilled under vacuum prior to use.

3. EXPERIMENTAL METHODS

1. Materials Polystyrene-polyisoprene AB block copolymer samples were synthesized via the usual anionic polymerization technique. Purified styrene monomer, cyclohexane solvent, and sec-butyl lithium initiator were allowed to react at 40 to 50°C under high vacuum for 4 to 6 h. Then purified isoprene monomer was added and allowed to react completely to obtain the polystyrene-polyisoprene diblock copolymer sample. The SEC measurements were carried out by the use of a Varian HPLC with a UV detector ( ~o = 254 n m ) and a 3 X 30-cm styragel column manufactured by the Chemistry Department, Jilin University, People's Republic of China. The porosity range of this column set ranged from 103 to 10 7 (polystyrene molecular weight). Purified T H F was the carrier solvent at a flow rate of 1 cm 3 min -1 . The temperature of the measurement was 25 °C. The number average molecular weights of the polystyrene-polyisoprene samples were measured with a Hewlett-Packard 503 osmometer equipped with an S and S 08 membrane. Toluene was the measurement solvent at a temperature of 37 oc. The dilute solution viscosities in dioxane and T H F were measured in Ubbelohde viscometers having negligible kinetic energy correction, The intrinsic viscosities were obtained from extrapolation to zero concentration of nso/c by means of leastsquares computation. The compositions of the samples were determined by 60-MHz NMR. The microstructure of the polyisoprene segment was not determined for these samples. Typical microstructures obtained from such anionic polymerizations contain approximately 70% cis-l,4; 23% trans-l,4; and 7% 3,4. JOurnal of Colloid and Interface Science~ VoL 133,NO. l, NovemberI989

2. Refractive Index Increment The refractive index increment (On/Oc) was determined using a Brice-Phoenix differential refractometer. Measurements were made at 25.0°C and X0 = 436 and 546 nm. From the measured values, the refractive index increment at 488 nm was obtained by interpolation. The absolute value of the refractive index increment at 488 nm for polystyrene in aniline was smaller than 2 × 10 -3 g-i cm 3, which is at least 10 times smaller than the refractive index increment of the copolymer solutions.

3. Light Scattering Measurements A standard, laboratory-built light scattering spectrometer (6) was used. With this device, it was possible to measure the scattered light intensity and to perform photon correlation measurements at angles between 15 and 140 °. The cell was thermostated at 25 + 0.05°C. A Spectra-Physics argon ion laser (model 165) was operated at 488 n m and an output power of 0.1 W. The photoelectron autocorrelation function was measured with a Brookhaven BI-2030 64channel digital correlator. All measurements, where the calculated and measured baselines differed by more than 0.1%, were discarded and repeated subsequently. The measured autocorrelation function in the self-beating mode has the form G(2l(q, r ) = A ( 1 + fllg(1)(q, ~_)12),

[7]

with q = ( 47rn / X0) sin ( O / 2 ) the wave vector, r the delay time, A the background, and 13the coherence factor. The measured first-order electric field correlation function ] g(l)(q, r ) I was fitted with a cumulants expansion (7),

g(1)(q, r ) = e x p { - F ( q ) r q- 0 . 5 u 2 ( q ) 7 "2 q- • • " } .

[8]

MICELLE FORMATION OF COPOLYMERS is the mean linewidth and tz2/ r 2 is the variance of the linewidth distribution function. The cumulants fit is applicable if the distribution is not too broad (t~z/r 2 < 0.4). From the z-average translational diffusion coefficient (/)) is obtained:

19 = p/q2.

[9]

By extrapolating the values for the diffusion coefficient to zero concentration (Do), the hydrodynamic radius (Rh) can be calculated by using the Stokes-Einstein relation

Rh = kBT/(67r•Do).

[10]

kB, T, and n are the Boltzmann constant, the absolute temperature, and the solvent viscosity. The solutions for the light scattering experiments were prepared by dissolving the polymer under gentle agitation over a period of a few days in aniline. The solution was kept in an argon atmosphere during this process. The solutions were filtered through Millipore FG filters (pore size: 0.2 t~m) into the cylindrical sample cell with a 14-ram inner diameter. The cells were sealed under vacuum in order to avoid oxidation of aniline.

4. Small-Angle X-Ray (SAXS) Measurements The X-ray spectrometer consists of a standard Kratky camera (8) and a 0.154-nm (CuK~) X-ray source. The measured scattering intensities were desmeared, with the effect of the slit width taken into account (9). The excess scattered intensity of a 2% solution of polystyrene (MW: 40,000 g mole -1 ) was zero within the experimental accuracy. This is due to the fact that polystyrene and aniline have the same electron density. Thus "isorefractivity" is also achieved in X-ray scattering.

5. Viscosity Viscosities of the solutions and solvent were determined with a Wells-Brookfield c o n e / plate digital viscometer. Shear rates between

179

230 and 10 s -1 were applied. The results were extrapolated to zero shear rate. 4. RESULTS AND DISCUSSION

(i) Static Light Scattering Extrapolated to Zero Scattering Angle For the interpretation of light scattering data of micelles whose formation can be described by the closed association model, Debye's formula (10) can be used as an approximation:

K( c - CMC) Ro - Ro(CMC )

1 (MN)w

+2A2(c-CMC)+

'''.

[11]

K ( = K * ( d n / d c ) 2) is the optical constant; CMC, the critical micelle concentration; (Mx)w, the weight average molecular weight of the aggregate with an aggregation number N; and A2, the second virial coefficient. By fitting the experimental data to Eq. [ 11 ], the CMC can be determined. This determination is accurate only if measurements were performed in the neighborhood of CMC. This requirement was fulfilled in the case of CO1 and CO2 but not with CO3, where the CMC could not be determined. If the molecular weight of the m o n o m e r is known, the aggregation number is obtained from (MN)w. The data determined for N, (mN)w,and CMC are displayed in Table I. A2 is positive--therefore a repulsive potential between the particles prevails. To obtain more physical insight, we applied one of the simplest models for repulsive interact i o n - t h e so-called hard-sphere (HS) model ( 11, 12). This model can be solved exactly in the Percus-Yevick approximation for the intensity of the scattered light at zero angle (Eq. [ 12 ]) as well as for the angular dependence of the scattered light intensity. R'o~o(C)/C {R~-~o(C)/C}~o = ( 1 - - ~ I ) ) 4 ( 1 q- 2 ~ )

-2.

[121

~I, is the volume fraction of the hard spheres and is related to the concentration of the solute Journal of Colloid and Interface Science, Vol. 133, No. I, November 1989

180

HILFIKER, CHU, A N D X U

"-'I

TABLE I

0 E U~

Experimental Results for Copolymers

MW of polystyrene block (1000 g mole -l ) MW of polyisoprene block (1000 g mole -l ) (M~c)w (106 g mole -l) N C M C (10 -5 g cm -3) R e (nm) (core) Rl (nm) (core) Rh (nm) (d/2) (nm) (Rh - Re) (rim) 1 (nm) 1/0" (g-i cm 3) (viscosity) 1/o (g-l c m 3) (HS model)

COl

co2

co3

28

41

61

19 30.5 650 2.5 60 17 67 -7 7

19 6.2 103 3.3 9 10 41 -32 18

16

7

12

12

4

11

T= ~o

-,g El:

19 6.1 75 -10 9 47 61 37 23

by cI, = c/o, where # is the density of the hard spheres. Aggregates consisting of polymers will not be compact (in contrast to, e.g., microemulsions (13, 14)) and therefore o is not a priori known but is treated as an adjustable parameter, which is inversely proportional to the degree of swelling of the aggregate. Figure 1 shows a fit of Eq. [12] to the measured values, where R' and c were replaced by R' - R'(CMC) and c - CMC, respectively, according to Eq. [ 11 ]. The 1/0 values are also listed in Table I.

(ii) Angular Dependence of Scattered Light From a Zimm plot (Fig. 2) of the copolymer sample with the smallest molecular weight of the polystyrene portion (CO1), the radius of gyration (Rg) of the polyisoprene core can be determined. In order to make clear that the radius of gyration of the polyisoprene core is measured, we will label it Rg instead of s . . The core of the micelles formed by the other two copolymer samples (CO2, CO3) is too small to be measured by the asymmetry of the Journal of Colloid and Interface Science,Vol. 133,No. 1, November1989

40

1.0

310

610

9.I 0

12

c ' / i 0 - 3 gcm-3 FIG. 1. Plots ofR'/Kc' (R' is the Rayleigh ratio of the solution minus the Rayleigh ratio of the solution at the critical micelle concentration ( C M C ) ; c' is the concentration minus the CMC) extrapolated to zero angle versus c' for CO 1 (full squares), CO2 ( diamonds ), and CO3 (open squares) in aniline. The solid lines are fits according to Zq. [121: R'/Kc' = {R'/Kc'}c,-o (1 - c'/0)4(I + 2c'/ o) -2. The values of 1/0 are listed in Table I. T = 25.0°C.

scattered light. Under the experimental conditions the lower detectable limit of the radius of gyration is ~_15 nm. An interesting feature is encountered with the copolymer sample having the largest polystyrene part (CO3). At moderate concentrations (>0.005 g cm-3), the Rayleigh ratio increases with scattering angle. This behavior is rarely encountered in light scattering measurements of dilute solutions and occurs only if long-range interactions between the particles, such as Coulomb interactions in polyelectrolyte solutions, are present ( 15 ). It is obvious that no charges are present

o E

7

12

~

9.0

c~ ~

a.o

F1-

3.0

u -,/

o

1:5 sin

310 2(/9/2)

415

6.

+500C

FIG. 2. Zimm plot of CO 1 in aniline. R' and c' have the same meanings as in Fig. 1. T = 25.0°C. The c' values are 1.0 × 10 -s, 1.9 X 10 -3, 4.9 × 10 -3, and 9.7 X 10 -3 g cm-3.

MICELLE FORMATION OF COPOLYMERS in the system investigated. However, a longrange HS interaction is m i m i c k e d since the scattering entity (the polyisoprene core) is m u c h smaller than the HS radius, which is the s u m o f the polyisoprene core radius and the thickness o f the polystyrene shell which is "invisible" for the light. The structure factor ( S ( q ) ) for HS can be calculated analytically (16) and extended in a power series for small qd, with d the hardsphere diameter,

S ( q ) = S ( q = O) X [1 + x ( ~ ) q Z d 2+ . - . ] ,

[13]

181

4.0

HC~3.2 -

~

O 2.4 r--~ 1.6

o

0.8 0

015 1t.0 i~.5 q2 / lO-lnm-2

2.0

FIG. 4. Logarithm of desmeared excess scattered intensity in SAXS measurements (arbitrary units) versus the square of the scattering vector for a 9.6 X 10-3 g cm-a solution of CO2 in aniline. T = 25.0°C.

with ×(~)

=

4/5ff-

ll/20~z + 1/5~ 3 (1 + 2q~):

[14]

F r o m a fit o f E q . [ 13 ] to the experimental data (Fig. 3), the hard-sphere radius ( d / 2 ) is obtained. Within the experimental accuracy, the hard-sphere radius thus determined was independent o f the concentration, as one would expect. The value is displayed in Table I.

(iii) Small-Angle X-Ray Scattering

small-angle X-ray scattering (see Table I). A Guinier plot (Eq. [ 15 ]) o f the excess scattered intensity ( I ) yielded the radii o f gyration (Rg) o f the polyisoprene cores o f the two samples. A n example o f this plot is displayed in Fig. 4.

log(I(0))

=

log(I(0 ~ 0)) -(log(e)/3)q=R~.

[15]

(iv) Dynamic Light Scattering

T h e radii o f gyration o f the two c o p o l y m e r samples CO2 and CO3 was determined by

o~

P h o t o n correlation m e a s u r e m e n t s were performed with solutions o f all three copolymers in aniline at different scattering angles 2.5 t i i and concentrations. F r o m an extrapolation o f the m e a n linewidth to zero scattering angle II and zero concentration, the h y d r o d y n a m i c CO radii can be calculated according to Eqs. [9] 1.7 and [ 10 ] (cf. Table I). The variance o f the linewidth distribution r~ 1.3 determined with a second-order cumulants fit was smaller than 0.1 and often smaller than 0.9 ~ i 4.0 8 0 12 16 0.05 for all solutions. Therefore, the size disq2 / 10-4nm-2 tribution function o f the micelles is very narFIG. 3. Structure factor of CO3 in aniline normalized row: i.e., the particles are nearly monodisperse. to zero scattering angle versus the square of the scattering This finding justifies a posteriori the use o f Eq. vector at Cl = 5.1 X 10-3 gcm - 3 (squares) and c2 = 1.1 [ 11 ] for which a closed association m o d e l is X 10-2 g cm -a (diamonds). The solid line is a fit according to the hard-sphere model (Eq. [13]): S(q) = S(q = 0)[1 assumed. F r o m static light scattering results, + x(c'/p)qZd2]. X(ff,) and dare 3.5 X 10-2 and 124 nm it is often impossible to decide whether the for q; and 5.7 X 10-2 and 119 nm for c2. T = 25.0°C. system can be better described by an open or Journal o f Colloid and Interface Science, Vol. 133, No. 1, November 1989

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HILFIKER, CHU, AND XU

a closed association model, since the concentration range at which the measurements would have to be performed is often inaccessible (10). However, the fact that the formed aggregates are monodisperse excludes the possibility of an open association model.

(v) Viscosity (a) Polystyrene solutions. In order to determine the solvent quality of aniline with respect to polystyrene, intrinsic viscosities of solutions of polystyrene standards with molecular weights ranging from 10 4 to 2.7 X 10 6 g mol -~ in aniline were determined. According to the Mark-Houwink equation ( 17, 18), intrinsic viscosity ( [ ~/]) and molecular weight of the solute are related by [n] = k m ~.

[161

k and c~ are constants, a does not fall below 0.5 and is seldom larger than 0.8. For a (0) solvent, a = 0.5 and for a very good solvent = 0.8. The measured values for k and a were 7.6 × 10 -3 and 0.75, respectively. Aniline is thus a good solvent for polystyrene. (b) Copolymer solutions. For dilute solutions, the concentration dependence of the specific viscosity (~sp) of a solution of spheres can be described by the Einstein formula ~sp = 2.5~vis.

(vi) Comparison of Hydrodynamic Radius and Radius of Gyration Since the polystyrene shell is isorefractive with the solvent, the measured radius of gyration is the radius of gyration of the polyisoprene core, whereas the measured hydrodynamic radius is the hydrodynamic radius of the micelle as a whole. The difference of Rh and Rg is proportional to the thickness of the polystyrene layer (1). It is interesting to note that l is about five times smaller for CO 1 than for CO2 and CO3, although the molecular weights of the polystyrene blocks differ only by factors of 1.5 and 2.2. This behavior can be understood if the model of Alexander ( 19, 20) is applied. It is stressed by the authors that the model is only semiquantitative and mainly predicts qualitative features. If flexible, long chains are grafted at one end on a solid surface, the thickness of the polymer layer (1) can be calculated in the framework of this model if the mesh size of the lattice (a), the fraction of surface sites grafted (cr), and the number of m o n o m e r s (Nm) per polymer chain are known, a is the m o n o m e r length in the polymer chain and a is the number of polymer chains grafted to the surface per a 2. If o-is very small, the thickness of the layer will be comparable to the Flory radius (RF) for a coil in a good solvent:

[17] l - RE = N~6a.

~vis is the volume fraction of the solute and is proportional to the concentration of the solute (i.e., ffvi~ = c/p*, with p* the density of spheres) if the structure of the aggregates is independent of concentration. From viscosity measurements of the copolymer solutions, p* has been determined and is listed in Table I. It is interesting to compare the measured p* values with the values of p obtained from the fit of Eq. [ 12 ] to the light scattering data. If the hard-sphere model described the actual situation exactly, p* and p would be equal. As can be seen from Table I, the values are not equal. The trend however is correct. Journal of Colloid and Interface Science, Vol. 133, No. 1, November 1989

[18]

This situation will be prevalent if aRF2 < a 2 or equivalently if a < Nm 12. If this requirement is not fulfilled, the polymer coils will be compressed in the direction parallel to the surface and the surface layer thickness will increase. In this case, the model predicts a layer thickness according to Eq. [ 19 ].

l "~ Nmatr 1/3.

[19]

From the measured aggregation number and by assuming the surface area to be 4~rR 2, a n d / c a n be calculated by taking a = 0.25 nm.

MICELLE FORMATION OF COPOLYMERS It turns out that a N ~ 2 is 0.75, 8.1, and 5.6 for CO 1, CO2, and CO3, respectively. In the case of CO1 the polystyrene layer thickness can therefore be calculated from Eq. [ 18 ], whereas Eq. [ 19 ] applies to CO2 and CO3. By comparing the calculated layer thickness (Table I) with the difference of Rh and Rg it can been seen that the theory predicts qualitatively the big increase ofRh - Rg, for CO2 and CO3 as compared to CO 1. (vii) Discussion (a) Hard-sphere model. Obviously, the hard-sphere model which has been used extensively in the interpretation of the results is an oversimplification. However, it is surprising how well the observed results can be explained in the framework of this model. The hardsphere model has been used very successfully in colloidal systems ( 13, 21 ) but is seldom applied in the description of polymer solutions. The reason for that is that homopolymers form very loose structures in good solvents, as is evidenced by high intrinsic viscosities. These structures are very different from hard spheres. However, if the polymers form aggregates, m u c h more compact structures can be formed. The investigated system is an example of a formation of comparatively dense structures. This fact explains the relative success of the hard-sphere model. (b) Structure o f the micelles. By comparing the radius of gyration of the polyisoprene core with the radius of a liquid-like droplet (R1) of polyisoprene having the molecular weight of the corresponding polyisoprene core (see Table I), it can be seen that CO2 and CO3 form the expected structure of a micelle formed by block copolymers. They consist of a core which is made up of the polymer which is insoluble in the solvent and whose density is close to the density of the corresponding solid (or liquid) h o m o p o l y m e r and a shell with a loose structure which consists of the other polymer of the block copolymer. CO 1 however forms an entirely different structure. The po-

183

lyisoprene core is m u c h less compact than liquid polyisoprene and must therefore be penetrated by solvent molecules. For copolymers with even shorter polystyrene parts, it can be expected that they are no longer soluble in aniline. Thus CO 1 is expected to be close to the phase separation line in the phase diagram. In principle, it is also possible that CO1 forms vesicles. In that case a spherical layer of polyisoprene would be covered by polystyrene on both the inside and the outside. Such a structure would yield the same experimental results; however, the polyisoprene layer could have a liquid-like configuration. With the measurements performed it is not possible to distinguish between the two structures. By measuring the structure factor very precisely to large q values, a distinction could be made. SAXS experiments with synchrotron radiation are planned for that purpose and will be reported in a forthcoming Communication. ACKNOWLEDGMENTS We gratefully acknowledge support of this project by the PolymersProgram of the National ScienceFoundation (DMR 8617820) and the U.S. Army Research Office (DAAL 0387 K0136). REFERENCES 1. Tuzar, Z., and Kratochvil, P., Adv. Colloid Interface Sci. 6, 201 (1976). 2. Price, C., Chan E. K. M., Hudd, A. L., and Stubbersfield, R. B., Polym. Commun. 27, 196 (1986). 3. Price, C., McAdam, J. D. G., Lally,T. P., and Woods, D., Polymer 15, 228 (1974). 4. Bahadur, P., Sastry, N. V., Marti, S., and Riess, G., Colloids Surf. 16, 337 (1985). 5. Benoit, H., and Froelich, D., in "Light Scatteringfrom Polymer Solutions" (M. B. Huglin, Ed). Academic Press, New York, 1972. 6. Chu, B., Onclin, M., and Ford, J. R., J. Phys. Chem. 88, 6566 (1984). 7. Koppel, D. E., J. Chem. Phys. 57, 4814 (1972). 8. Kratky, O., and Stabinger, H., Colloid. Polym. Sci. 262, 345 (1984). 9. Hendricks, R. W., and Schmidt, P. W., Acta Phys. Austriaca 26, 7 (1967). 10. Elias, H.-G., in "Light Scattering from Polymer SoJournal of ColloM and Interface Science, Vol. /33, No. 1, November 1989

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