Styrene/butadiene diblock

Block copolymer chemistry and architecture is well described in polymer textbooks and monographs [40]. The block copolymers of PSA interest consist of anionically polymerized styrene-isoprene or styrene-butadiene diblocks usually terminating with a second styrene block to form an SIS or SBS triblock, or terminating at a central nucleus to form a radial or star polymer (SI) . Representative structures are shown in Fig. 5. For most PSA formulations the softer SIS is preferred over SBS. In many respects, SIS may be treated as a thermoplastic, thermoprocessible natural rubber with a somewhat higher modulus due to filler effect of the polystyrene fraction. Two longer reviews [41,42] of styrenic block copolymer PSAs have been published. 

Table 19.3 Typical reaction conditions for the hydrogenation of polybutadiene (PB), styrene-butadiene diblock copolymer (SB), styrene-butadiene-styrene triblock copolymer (SBS) and nitrile butadiene rubber (NBR).

Block copolymers -A-A-A-A-A-A-B-B-B-B- Styrene-butadiene diblock copolymers Styrene-butad iene-styrene triblock copolymers Polyurethane multiblock copolymers (elastomeric yams)... 

It is claimed that styrene/butadiene diblock polymers bring about an improvement in the hardness, strength, and processability of polybutadiene elastomers (27), as well as an improvement in the ozone resistance of neoprene rubber (28). Styrene diblock polymers have also been made with isoprene, a-methyIstyrene, methyl methacrylate, vinylpyridine, and a-olefins. Block copolymers of ethylene, propylene, and other a-olefins with each other have been made as well. Heteroatom block copolymers based on styrene or other hydrocarbons and alkylene oxides, phenylene oxides, lactones, amides, imides, sulfides, or slloxanes have been prepared. 

Figure 5.19 Small-angle neutron scattering intensity obtained with a styrene-butadiene diblock copolymer having spherical butadiene microdomains. The peaks at very small q are due to a body-centered cubic lattice structure of ordered microdomains. The solid curve is the calculated intensity of independent scattering from solid spheres of mean radius 124 A. (From Bates etal.34)...

Krause, S., Lu, Z.-H., and Iskander, M., Rroperties of low molecular weight block copolymers 4. Differential scanning calorimetry and refractive index-temperature measurements on styrene-butadiene diblock copolymers and styrene-butadiene-styrene triblock copolymers. Macromolecules, 15,1076-1082 (1982b). 

Lazzeri and Bucknall [131] have proposed that the pressure dependence of yield behaviour caused by the presence of microvoids can explain the observation of dilatation bands in rubber-toughened epoxy resins [132], rubber-toughened polycarbonate [133] and styrene butadiene diblock copolymers [134]. These dilatation bands combine in-plane shear with dilatation normal to the shear plane. Whereas true crazes contain interconnecting strands, as described in Section 12.5.1 above, dilatation bands contain discrete voids that, for rubber-toughened polymers, are confined to the rubber phase. 

Fig. 2. The observed cloud points are plotted against the volume fraction

Fig. 15. Reciprocal of the peak intensity obtained from SAXS measurement of styrene/butadiene diblock copolymer plotted against the reciprocal of temperature T. Linear extrapolation of high temperature data to zero gives the spinodal temperature, while the first deviation of the observed intensity from the straight line gives the microphase separation temperature. (From Zin and Roe...

Fig. 16. Phase diagram of a mixture containing polystyrene (M = 2400) and styrene/butadiene diblock copolymer (27 % styrene, M = 28000). Liquid phases L, and Lj represent mixtures of disordered block copolymer and polystyrene. Mesophase M, consists of ordered microdomains of the block copolymer swollen with polystyrene. Mesophase Mj probably contains aggregates of block copolymer micelles within the medium of polystyrene. The features on the lower right, drawn in broken lines, are more speculative. (From Roe and Zin...

The micelles formed when styrene-butadiene diblock copolymer is mixed with a large excess of low molecular weight polybutadiene were studied by Rigby and Roe (by SAXS) and by Selb et al. (by SANS). The first group employed three block... 

Fig. 18. The critical micelle concentrations of three styrene/butadiene diblock copolymer samples in the matrix of low molecular weight polybutadiene are plotted against temperature. The approximate compositions (stytene vs. butadiene) of the block copolymers are indicated.. (From Rigby and Roe )...

The electron micrographs in Fig. 1.10 show the (left) (100) and (right) (111) projections of the BCC macrolattice for a styrene-butadiene diblock... 

SB (styrene-butadiene) diblock copolymer (about 50/50 composition) ... 

Fig. 5.46 A transmission electron micrograph of an osmium tetroxide stained thin section of a poly(styrene-butadiene) diblock copolymer (16.1 wt% polybutadiene) shows the (100) projection of a body centered cubic lattice. (From Kinning et al. [214] reproduced with permission.)...

Early morphological studies to determine the nature of multiphase polymers and blends were reviewed by Folkes and Keller [363]. Many studies were of extruded block copolymers of materials such as SBS where the dispersed phase, an unsaturated rubber stained with OSO4 (see Section 4.4.2), was observed in the form of spheres, cylinders, or lamellae [364]. An excellent example is shown in a TEM micrograph of a thin section of a poly(styrene-butadiene) diblock copolymer, stained with OSO4 [365], which depicts the (100) projection of a body centered cubic lattice (Fig. 5.79). 

This method is industrially utilized to produce styrene-butadiene diblock copolymers (SB) by anionic polymerization. The corresponding triblock copolymer (SBS), which is a thermoplastic elastomer, is obtained by coupling the precedent living diblock copolymer using dimethyldichlorosilane ... 

As discussed previously, thermoplastic elastomers are materials which have the functional properties of conventional vulcanized rubbers but which may be processed as normal thermoplastics (see section 2.9). Effects of this kind are shown by styrene-butadiene block copolymers. Two types of styrene-butadiene block copolymers are produced commercially, namely triblock and radial block copolymers. The triblock copolymers (denoted by SBS) consist of a centre block of butadiene units with two terminal blocks of styrene units. The radial block copolymers (denoted by (SB) X) consist of three or more styrene-butadiene diblock copolymers radiating from a central... 

The radial block copolymers are also prepared by anionic polymerization. Firstly, a styrene-butadiene diblock which is active at the butadiene end is formed. A polyfunctional coupling agent such as silicon tetrachloride is then added to produce the radial structure. The process may be represented as follows ... 

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