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Structure of a styrene-butadiene-styrene block copolymer by light scattering.

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Die Angewandte Makromolekulare Chemie 27 (1972) 219-222 ( N r . 425)
From the Research Division, The Goodyear Tire & Rubber Co.,
Akron, Ohio, U.S.A. 44316
Short Communication *
Structure of a Styrene-Butadiene-StyreneBlock
Copolymer by Light Scattering**
By I?. S. PILLAI,D. I. LIVINGSTON,
and J. D. STRANG
(Eingegangen am 20. Juli 1972)
SUMMARY:
The phase structure of a styrene-butadiene-styreneblock copolymer (mol. wt.
21 000-98 0000-21 000) film cast from tetrahydrofuran/methyl ethyl ketone is
studied by light scattering. The scattered intensity I corresponding to V,, H,, vh,
and Hh polarization directions is measured as a function of scattering angle. From
the In I vs sin2(0/2) plots the domain sizes are found to be about 0.4and 1.Op.The
a values are independent of polarization directions. This proves that the domains
are geometrically isotropic. The two sizes correspond to two correlation distances ;
the smaller value represents the extent of heterogeneity within a spherical domain
and the larger value the inter-domain separation distance.
The heterophase structure of styrene-butadiene-styrene(SBS) block copolymers has been established from electron microscope, viscoelastic, and small
angle x-ray studiesl. These experiments show phase dimensions of the order
of a few hundred angstroms. B u t light scattering studies of these materials
also reveal the presence of much larger domains. STEINand WILKES~
photographed H, scattering patterns from Kraton 1101, a commercial block polymer.
Though the corresponding domain sizes could not be calculated from their
results because of lack of angle data, the fact that such patterns were obtained
indicates that the domains are of micron size. STEIN3 explains this scattering
as due t o optical anisotropy in the domains caused by the form birefringence
of packed polystyrene cylinders.
I n the present investigation the authors investigated the phase structure of
one SBS block copolymer (mol. wt. 21 000-98000-21 000) containing 30 %
styrene. This sample was synthesized4 a t the University of Akron and kindly
supplied by Prof. M. MORTON.
*
**
Contribution No. 492.
Based on a paper presented at the Meeting of the American Physical Society,
Division of High Polymer Physics, Cleveland, Ohio, April 29 - May 1, 1971.
219
P. S. PILLAI,
D. I. LIVINGSTON,
and J. D. STRANG
Thin films of the material were cast on a mercury surface from a 10% solution
in tetrahydrofuran/methyl ethyl ketone (90jl0) and were left to dry overnight.
They were then lifted from the mercury dish and vacuum dried for about 5 days.
The film was held perpendicular t o the laser beam and the scattered intensity
obtained as a function of angle using the experimental setup described before5.
By rotating the laser 90", the scattered intensity I was measured for the two
perpendicular polarization directions of the incident beam and are denoted IvV,
IH", I H ~ ,
and I V h . where the capitalized subscript refers t o the polarization of t h e
scattered light (fixed by the analyzer setting) and the lower case subscript the
polarization of the incident beam.
The intensities IvV and 1~~ were nearly 6 times greater than 1~~ and I v ~
respectively. These were plotted as a function of scattering angle 8 corrected
for refraction. Using the statistical method of DEBYEand BUECHE~,
and
assuming a Gaussian correlation function, it has been shown that the scattered
intensity I can be written as7
I = CV , 2 , 3 1 2
a3 exp (-4 ~2 sin2 (812) aZ/P),. . .
(1)
where C is a constant, V the scattering volume, 7 the mean scattering power,
8 the scattering angle, and a the mean size of the scattering domain.
The present data were analyzed using expression (1).The plot of log Ivv
against sin2 (8/2)is shown in Fig. 1. Similar plots were drawn for I H ~IVh
, and
IHh measurements. The graphs are all very similar in shape, having two h e a r
regions. The first region extends up to about 15" and has a higher slope than
1
200
400
sin28/2 x lo4
Fig. 1. Ivv (arbitrary units) vs sin2 (0/2) plot.
220
600
Styrene- Butadiene-Styrene Block Copolymer
the second region beyond 18". I n all cases the mean values of a obtained from
the slopes are about 1.0p (ag) and 0.4p (al) as shown in Table 1 . The a1 and
a2 values are considered t o be the short range and long range correlation
distances, respectively. Since they are independent of polarization direction,
it is concluded that there is no geometrical anisotropy in these domains, or, in
other words, the scattering domains are spherical in shape. The sizes obtained
here are much larger than the polystyrene block domains ( M 200 A) seen by
electron microscopy or by small angle x-ray scatterings.
Table 1. Domain sizes.
Polarization of
incident beam
Analyzer setting
Domain size
Separation
distance
It is known that the H, scattering arises only from optical anisotropy in the
domains while Vv scattering arises from both density fluctuation and optical
anisotropy. It has been mentioned earlier that the Iv, intensity was nearly
6 times the IH, values. STEINand WILSONg have shown that the ratio IVv/IHv
due t o optical anisotropy alone is 413.We attribute the additional contribution
to Iv, to scattering from density fluctuation within the domains.
The domain dimensions of micron scale obtained here are too large t o be
associated with molecular structure and must be connected with inhomogeneity
in a higher state of aggregation. X-ray scattering measurements8 have shown
structure on a much smaller but still supermolecular scale due t o microsegregation of the component blocks into polystyrene domains of about 200 A size in
a substantially polybutadiene matrix. Such regions of microheterogeneity
could grow radially during solvent evaporation to give spherical domains of
much larger dimensions. Upon growing together, such domains would leave a
certain amount of disordered material which could not be regularly accommodated, thus terminating the process. LEWIS and PRICE^^ report similar
micron-size grains in a commercial styrene-butadiene-styrenesample.
From the foregoing, we suggest that the phase structure of the solvent cast
films of the block polymer studied here consists of heterogeneous spherical domains of well developed, microsegregated polystyrene regions in a matrix of less
segregated material. These domains are the light scattering entities of approximately 0.4 p radius whose next nearest neighbors are located on the
circumference of a sphere of approximately 1.0 p radius.
221
P. S. PILLAI,D. I. LIVINGSTON,
and J. D. STRANG
2
3
4
5
6
7
8
9
10
J. F. BEECHER,
L. MARKER,
R. D. BRADFORD,
and S. L. AGGARWAL,
J. Polym.
Sci. C 26 (1969) 117;
T. INOUE,
T. SOEN,T. HASHIMOTO,
and H. KAWAI,J. Polym. Sci. A-2 7 (1969)
1283;
H. HENDUS,
K. H. ILLERS,and E. ROPTE,Kolloid-Z. Z. Polym. 216 (1970) 110;
G. S. FIELDING-RUSSELL,
Rubber Chem. Technol. 45 (1972) 253.
R. S. STEINand G. L. WILKES,J. Polym. Sci. A-2 7 (1969) 1695.
R. S. STEIN,J. Polym. Sci. B 9 (1971) 747.
M. MORTON,J. E. MCGRATH,
and P. C. JULIANO,
J. Polym. Sci. C 26 (1969) 99.
P. S. PILLAI,
D. I. LIVINGSTON,
and J. D. STRANG,
Rubber Chem. Technol. 45
(1972) 241.
P. DEBYEand A. M. BUECHE,J. Appl. Phys. 20 (1949) 518.
P. S. PILLAI
and D. I. LIVINGSTON,
Paper presented a t a meeting of the American
Chemical Society, Akron, Ohio, (1968) May 9-10.
P. S. PILLAI
and D. I. LIVINGSTON,
Paper presented a t a meeting of Ohio Section
of the American Physical Society, Dayton, Ohio, (1971) May 28.
R. S. STEINand P. R. WILSON,J. Appl. Phys. 33 (1962) 1914.
P. R. LEWISand C. PRICE,
Polymer 13 (1972) 20.
222
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structure, block, scattering, copolymers, light, styrene, butadiene
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