Spherical polybutadiene domains

Craze Growth by Cavitation of Spherical Polybutadiene Domains 315... 

Fig. 1. Two-phase model for S-B-S triblock copol3nner where S = polystyrene and B = polybutadiene where polystyrene segments form spherical domains in polybutadiene continuum.

Thermoplastic Elastomers. These represent a whole class of synthetic elastomers, developed siace the 1960s, that ate permanently and reversibly thermoplastic, but behave as cross-linked networks at ambient temperature. One of the first was the triblock copolymer of the polystyrene—polybutadiene—polystyrene type (SheU s Kraton) prepared by anionic polymerization with organoHthium initiator. The stmcture and morphology is shown schematically in Figure 3. The incompatibiHty of the polystyrene and polybutadiene blocks leads to a dispersion of the spherical polystyrene domains (ca 20—30 nm) in the mbbery matrix of polybutadiene. Since each polybutadiene chain is anchored at both ends to a polystyrene domain, a network results. However, at elevated temperatures where the polystyrene softens, the elastomer can be molded like any thermoplastic, yet behaves much like a vulcanized mbber on cooling (see Elastomers, synthetic-thermoplastic elastomers). 

Shen and Kaelble (29) found the same linear dependence in the region —60° and 60°C but state that below —50°C and above 80°C the temperature dependence of Kraton 101 could be described by the WLF equation with cx = 16.14, C2 = 56, and Tr — — 97°C below —50°C, and Tr — 60°C above 80°C. They ascribe the temperature dependence below —50 °C to the pure polybutadiene phase and that above 80 °C to the pure polystyrene phase. They then assume that at temperatures between —50° and 80°C the molecular mechanisms for stress relaxation are being contributed by an interfacial phase visualized as a series of spherical shells enclosing each of the pure polystyrene domains and characterized... 

In spite of (Mw/Mn)s = 2.9, the morphology appears to be basically spherical, albeit with considerable connectivity of polybutadiene domains. Moreover, for this blend tan 8max = 0.028, T(tan 8max) = — 88 °C, with virtually no anisotropy in storage modulus, consistent with spherical or short rod-shaped polybutadiene domains. 

FIGURE 13.8 Schematic illustration of phase separation in a thermoplastic elastomer based on a styrenic triblock copolymer such as SBS. The isolated spherical domains containing the polystyrene end blocks form the hard phase, which acts as both intermolecular tie point ("physical crosslinks ) and filler. The continuous phase from the polybutadiene midblock imparts the elastomeric characteristics to this polymer. 

Bates, F. S., Cohen, R. E., and Argon A. S. (1983) Dynamic mechanical properties of polystyrene containing micro spherical inclusions of polybutadiene influence of domain boundaries and rubber molecular weight, Macromolecules, 16, 1108 1114. 

Fig. 6. A graphical representation of SB block copolymer structure at ambient temperature, with styrene content of 15% or less. Polystjrrene phase (dark short lines) is presented as spherical domains dispersed in polybutadiene phase.

Styrenic block copolymers are special polydiene elastomers (polyisoprene or polybutadiene) of moderate molecular mass (7x10 to 2.5x10 ) with end blocks of polystyrene [(7-25)x 10 g/mol). They are thermoplastic elastomers At low temperature the polystyrene blocks form spherical hard domains that act as cross-links in the adhesive and result in very high cohesion. Above about 90 °C they soften to give a melt of comparatively low viscosity like a thermoplastic [211, pp. 317-373). 

It was pointed out in Section 2.16.9 that anionic living polymerisation can be used to prepare ABA tri>block copolymers suitable for use as thermoplastic elastomers. In such copolymers the A blocks are normally of a homopolymer which is glassy and the B block is of a rubbery homopolymer (e.g. a polydiene such as polybutadiene or polyisoprene). The characteristic properties of these materials stems from the fact that two polymers which contain repeat units of a different chemical type tend to be incompatible on the molecular level. Thus the block copolymers phase separate into domains which are rich in one or the other type of repeat unit. In the case of the polystyrene-polydiene-polystyrene types of tri-block copolymers used for thermoplastic elastomers (with about 25% by weight polystyrene blocks), the structure is phase-separated at ambient temperature into approximately spherical polystyrene-rich domains which are dispersed in a matrix of the polydiene chains. This type of structure is shown schematically in Fig. 4.36 where it can be seen that the polystyrene blocks are anchored in the spherical domains. At ambient temperature the polystyrene is below its Tg whereas the polydiene is above its Tg. Hence the material consists of a rubbery matrix containing a rigid dispersed phase. 

If sufficient numbers of diffraction peaks are observed, simple inspection for the absence or weakening of particular peaks in an observed sequence of d spacings can provide a useful estimate of the radius (or thickness) of domains. To obtain accurate values, however, it is necessary to compare the whole of the observed intensity profile (after correcting for polarization, the Lorentz factor, etc.) with intensity profiles predicted by theoretical models. When there are only a few peaks (which is almost always the case with spherical domains, and is often the case with cylindrical domains) then a fitting procedure must be used. With the scattering profile obtained for the polystyrene-6/ock-polybutadiene-Wock-polystyrene (10000 71000 10 000 M ) copolymer this approach yielded a value of 12 nm for the cylindrical diameter. 

Figure 10 shows a 3D surface-rendered image of an ultrathick specimen of an acrylonitrile butadiene styrene (ABS) resin. The ABS resin (TECNO ABS 330) was purchased from Technopolymer Co. Ltd., Japan. The thickness of the micro-tomed section is ca. 1pm. The spatial distribution of 0s04-stained polybutadiene (PB) spherical domains (often... 

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