POLYMER SECTION
Properties of PolystyrenePolydimethylsiloxane Block Copolymers John C. Saam and F. W. Gordon Fearon Polymer Research Laboratories, Dou, Corning Corp., Midland, M i c h . 48640 Factors influencing the micromorphology of polystyrene (A), polydimethylsiloxane (B), AB diblock copolymer, and (BAB), multiblock copolymers are examined. Electron micrographs of solvent-cast films of AB copolymers show that solvent, molecular weight, and composition alter the type and dimensions of the microphases formed. The (BAB), multiblock copolymers are less sensitive to these factors. The morphology of these systems i s apparently influenced b y a relatively high interfacial surface free energy and a large difference in solubility parameter between the t w o polymeric blocks. The (BAB), multiblock copolymers are strong thermoplastics or thermoplastic elastomers, depending on their composition. As with other thermoplastic elastomers, the later copolymers require neither crosslinking nor reinforcing fillers to develop mechanical strength.
T h e recent synthesis of well defined AB diblock copolymers of polystyrene (A) and polydimethylsiloxane (B) provides an opportunity to examine the morphological properties of this unusual copolymer system. The present investigation extends the synthesis and broadens the previous observation (Saam et al., 1970) that these copolymers form microphases resembling those seen in the polystyrenepolydiene block copolymers (Bradford et al., 1968; Inoue et al., 1969). Included herein are (BAB), multiblock copolymers which should show properties characteristic of thermoplastic elastomers if microphases do form in the bulk (Holden et al., 1969). Thermoplastic elastomers containing crystalline silphenylenesiloxane block (Merker and Scott, 1964), or glassy poly(bispheno1 A carbonate) blocks (Kambour, 1969; Vaughn, 1969) alternating with blocks of polydimethylsiloxane have been reported, but because of the short lengths of the blocks. these systems may often resemble random rather than true block copolymers. For example, in the polysiloxane-poly(bispheno1-A carbonate) system it is necessary to postulate extensive distribution of the polycarbonate blocks in the rubbery polysiloxane phase in order to explain certain mechanical and optical properties (LeGrand, 1969). In the present study, therefore, well defined (BAB) polystyrene-polydimethylsiloxane block copolymers whose block lengths are a magnitude higher than previous systems, as well as AB block copolymers, are prepared and morphological properties are examined. Materials
The AB copolymers were synthesized by the previously described polymerization of hexamethylcyclotrisiloxane, D with polystyryllithium in the presence of promoting solvents such as tetrahydrofuran or 1,2-dimethoxyethane 10
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 1, 1971
(Saam et al., 1970). This general method was extended to the polymerization of D 3 with living a,w-dilithiopolystyrene prepared from difunctional initiators. This gives BAB block copolymers now terminated a t each end with the lithium siloxanolate moiety. These or the corresponding silanol-ended polymers are condensed by known non-bond-rearranging methods of siloxane chain extension (Noll, 1968) to give the (BAB), block copolymers. The BAB prepolymers as well as the derived (BAB), species were judged to be essentially free of homopolymer based on the failure of either polystyrene or polydimethylsiloxane to separate from their solutions in selective solvents such as bromobenzene or methylcyclohexane (Saam et al., 1970). Gel phase chromatography also shows the BAB prepolymers to be of relatively narrow molecular weight distribution and free of other polymeric species. The same analysis shows increased molecular weight as well as a broadened molecular weight distribution in the condensed (BAB), copolymers, as might be expected of a polymer obtained from a step-growth process. In some
Table I. Polystyrene-Polydimethylsiloxane (BAB). Multiblock Copolymers O/O
Wt % Me.SiO
Yield,
%"
M n X 10
80 70 50 50
93 87 92 95
85.5 137 . 110 . 303 .
I h
Si
x'
Found
Calcd
2.3 3.2 2.3 6.4
30.1 26.0 18.6
30.2 26.6 19.0
...
...
'Based on both steps of synthesis. 'Over all Mn as measured by membrane osmometry. 'Based on initial Mn of prepolymer as measured by membrane osmometry.
Figure 3. Electron micrographs of films of A B block copolymers of varied molecular weight but at constant composition A. a n , 47,600; 68 wt ?.A polystyrene. 8. En, 28,600; 70.9% polystyrene. C. fin, 7,100;
71.6% polyifyrene
The effect of changing composition on morphology is shown in Figure 2. I n tihis sequence the films are all cast from toluene and the molecular weights of the polymers are nearly constant. I i n apparent inversion of phases occurs when one or the otkier of the COI nponents predominates but the spaghetti-like morpholog y is retained. The absence of basic changes in morpho1ogy over a broad range in composition was unexpected in view of current theory dealing with the 6bffects of COInposition on morpbology (Inoue e t al., 19159; Meier, 1970) and previous observations in diene systems (Matsuo et al., 1969). The effect of molecular weight at constant composition in films of AB block coliolymers cast from toluene is shown in Figure 3. Here FIronounced effects are observed where the structures changle from spaghetti-like to lamellae to ill-defined suheres. Tht? latter appear to be close to :"h+ P-" ..h--.. . : & . , a the lower limit of moleculh. wr16LLb pL1aUr urp,alabluE,. These observations are also apparently a t variance with current theory which predicts that over-all molecular weight of the block copolymer should play a relatively minor role in determining the morphological structures (Inoue et al., 1969; Meier, 197(1). These deviations can be ascribed to film formation uncler nonequilibrium couditions, but another important fa< :tor is a high interfacial contact energy, y, arising from differences in solubility between the two blacks. This sho uld significantly influence the molecular weight dependenc e of the end-to-end distance of the chains within the n iicelles and consequently alter entropy requirements in pl acing a copolymer chain within a particular type of micellrdar structure. This factor can be ignored in the polystyren,e-diene systems, presumably because the interfacial C O ntact I energies and suhsequent "mirror effects" can be neglected. Estimates of y for a molten polystyrene-polydimethylsiloxane interface are 11 to 13 dynes per cm (Fo!vkes, 1964; Ryong-Joon, 19681, whereas for polystyrene-1Iolybutadiene this value is in the range of 1 dyne per cm IIMeier, 1969). The original theoretical approa(:h by Meier (1969) allows an estimate of domain dimensionsi, provided the molecular weight of the blocks and the intczfacial tension or energy per unit area of micelle surface are known. Comparison ~
n*
12
...-
Ind. Eng. Chem. Prod. Res. Develoi>.,Val. 10, No. 1 , 1971
of the observed micelle dimensions for some of the present AB block copolymers with dimensions calculated by this method gives good agreement when a value of 15 dynes per cm is chosen for the interfacial tension, y. This holds over a broad range of compositions and molecular weights for the AB block copolymers deposited from toluene. These comparisons are summarized in Table 11. The relatively high value of y necessary to calculate the dimensions of microphases in the AB block copolymers is roughly of the magnitude approximated above for the corresponding homopolymers. Electron micrographs of the multiblock, (BAB),, copolymers show that morphology does not change over a broad range of compositions. The absence of any significant changes in micelle structure is attributed to the importance of the number of blocks in determining ~~
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