Die Makrornolekulare Chernie 148 (1971) 305-309 (Nr. 3795)

Centre National de la Recherche Scientifique, Centre de Recherches sur les Macromolhcules, 6 , rue Boussingault, 67 Strasbourg, France

Kurzmitteilung

Solubilization and Chain Conformation in a Block Copolymer System

ANTOINE SKOULIOS, PIERRE HELFFER, YVES GALLOT, and JOSEPH SELB

(Eingegangen am 10. Mai 1971)

It is now well established that block copolymers produce mesomorphic phases in the presence of moderate amounts of a selective or a preferential solvent of one of the blocksl). Being incompatible with one another, the two blocks segregate and fit into distinct microdomains which have well defined geometrical shapes and are periodically distributed in space.

Recently, the question has arisen as to whether these block copolymer/ solvent systems are capable of solubilizing homopolymers, as soap/water systems are in the case of low molecular weight compoundsz). More specifically, can they solubilize homopolymers of the same chemical na- ture as the insoluble blocks? In the present paper we describe some pre- liminary results we have obtained which answer this question.

The mesomorphic phase considered is a 5 0 % by weight mixture of a polystyrene/polyvinyl-2 pyridin two-block copolymer with octanol which dissolves the pyridin blocks. The copolymer has been synthesized under vacuum through anionic polymerization. Its polydispersity was very low, and the molecular weight of its blocks was 9,500 for the polystyrene and 10,600 for the polyvinylpyridin. The structure of this mesomorphic phase has been found to correspond to a two-dimensional hexagonal packing of cylinders containing the insoluble polystyrene blocks imbedded in a matrix formed by the mixture with the solvent of the soluble polyvinyl- pyridin blocks3). The radius of the cylinders is equal t o 53,9 8, and the specific surface of the macromolecules, i.e., the mean lateral packing area of one polymer chain a t the interface, equal to 557 Az.

Initially, we studied the solubility of highly monodisperse homopoly- styrenes of different molecular weights, ranging from 3,820 t o 22,400, in this particular gel. For this purpose, we prepared a series of mixtures containing 11.1 yo by weight of homopolystyrene. Since this polymer is insoluble in octanol, the gels were homogenized by adding 2 volumes of tetrahydrofuran (which is a good solvent of all the species present in the

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A. SKOULIOS, P. HELFFER, Y. GLLLOT, and J. SELB

system), by heating in air-tight cells a t 80°C during 4 hours, and by cautious evaporation of T H F (which is much more volatile than octanol).

All the samples containing low molecular weight homopolystyrene, i.e., Mn = 3,820, 4,800, 6,200, 7,500, and 9,500, were clear, while those con- taining high molecular weight homopolystyrene, i.e., M, = 13,700, 14,200, 16,400, and 22,400, were opaque. The transition between these two optical aspects was very sharp. I n addition, low-angle X-ray diffraction patterns of these gels also showed a very sharp transition. The opaque gels gave X-ray patterns identical t o those corresponding to the gel free from homo- polystyrene. The transparent samples gave X-ray patterns which were different as far as the BRAGG spacing o f their diffraction lines is concerned, indicating tha t homopolystyrene was actually included in the meso- morphic phase.

From these observations, it is concluded tha t the mesomorphic gel examined solubilizes small amounts of homopolystyrene, provided its molecular weight is not higher than the molecular weight of the insoluble polystyrene blocks of the copolymer.

Secondly, we considered the effect of the concentration of the homo- polymer added in the solubilization process, using a homopolystyrene of the same molecular weight as the polystyrene blocks of the copolymer,

Concentrations being of about 20-25 yo by weight of homopolymer in the system, the gels were homogeneous and transparent. Above this limit, they were opaque, indicating a phase separation of the excess homopdy- mer. This critical concentration roughly corresponds to the concentration of the insoluble blocks in the system.

It is worth noting tha t the diffraction lines of the transparent gels are sharp, indicating a highly ordered structure (which is the same type as for the pure solvent/copolymer gel), while those of the opaque gels are increasingly diffuse. Moreover, the spacing d of the first order of diffrac- tion varies in a continuous way with the homopolymer content of the system (Fig. 1).

Let x be the polystyrene content of the block copolymer, Ti and Tm

the specific volumes of the insoluble polystyrene blocks (0.952 cm3g-1) and of the swollen matrix (Tm can be deduced from the values of the specific volumes, 0.952 and 1.212 cm3g-1, of polyvinylpyridin and oc- tanol), q the quantity of solvent referred to the quantity of copolymer present in the system, p the quantity of homopolymer referred to the quantity of insoluble blocks, Mi the molecular weight of the insoluble

i.e., Mn = 9,500.

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Solubilization and Chain Conformation in a Block Copolymer System

30C

- "2 -3

20c

0 4 5 P '10

Fig. 1. Structural parameters as a function of homopolymer content

blocks and N AVOGADRO'S number. Among the geometrical parameters which characterize the structure of the gel, we have calculated

R = ( L y d (1 + Pm (1 - x + q) )-1/2

,I/? BiX (1 + p)

which represents the radius of the cylinders in which are located the in- soluble blocks and the homopolymer added to the system, and

which is the lateral packing area of one block copolymer chain a t the interface.

Knowing the importance of the molecular weight of the homopolymer in the solubilization process, and more specifically the existence of a critical value of this molecular weight equal t o the molecular weight of the insoluble blocks themselves, it is tempting to suggest the following without further proof. The solubilized homopolymer is not located in the very central part of the cylinders away from the insoluble blocks, bu t in

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A. SKOULIOS, P. HELFFER, Y. GALLOT, and J. SELB

Fig. 2. Schematic view of the palisade model : full lines represent the poly- styrene blocks and homopolymer chains, dotted lines the polyvinylpyri- din blocks, and circles the solvent molecules (see also Fig. 5 of ref.4))

fact, is molecularly dispersed between the blocks to form with them a palisade (Fig. 2). With this in mind, a third geometrical parameter char- acterizing the structure can be calculated, i .e . ,

which represents the mean lateral packing area of one polymer chain, regardless of whether it belongs to the copolymer or to the solubilized homopolymer.

It is interesting to note (Fig. 1) that Stop, which represents the mean lateral packing area of one soluble block a t the interface, is little affected by the addition of homopolymer to the system. Instead, Schain decreases rapidly when p increases. The packing of the insoluble blocks becomes tighter because the homopolymer chains are inserted in between the block copolymer chains while the packing of the soluble blocks remains approximately constant. As a result, the radius of the cylinders increases, translating the elongation of polymer chains. This elongation is important since it can attain a value corresponding to half the total length of poly- mer chains.

The results we report here certainly require a more detailed examin- ation. However, they strongly suggest, first, that the interface is very sharp since the homopolymer chains which are longer than the insoluble blocks (and which, therefore, starting from the center of the cylinders, should enter the matrix between the cylinders) cannot be solubilized;

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and second, that the conformation of the insoluble polymer chains is not so chaotic as one could think it of a t first sight, but in fact is willingly elongated, the polymer chains tending to orient almost parallel one to another.

A. SKOULIOS, G. FINAZ, and J. PARROD, C. R. Acad. Sci. 251 (1960) 739; H. AILEAUD, Y. GALLOT, and A. SKOULIOS, Makromol. Chem. 140 (1970) 179.

2, J. FRANCOIS, B. GILG, P. SPEGT, and A. SKOULIOS, J. Colloid Interface Sci. 21 (1966) 293; A. S. C. LAWRENCE, Trans. Faraday SOC. 33 (1937) 815.

3, P. GROSIUS, Y. GALLOT, and A. SKOULIOS, Makromol. Chem. 127 (1969) 94. 4, A. SKOULIOS and G. FINAZ, J. Chim. Phys. 59 (1962) 473.

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