Isoprene block, glass transition temperature

The standard molecular structural parameters that one would like to control in block copolymer structures, especially in the context of polymeric nanostructures, are the relative size and nature of the blocks. The relative size implies the length of the block (or degree of polymerization, i.e., the number of monomer units contained within the block), while the nature of the block requires a slightly more elaborate description that includes its solubility characteristics, glass transition temperature (Tg), relative chain stiffness, etc. Using standard living polymerization methods, the size of the blocks is readily controlled by the ratio of the monomer concentration to that of the initiator. The relative sizes of the blocks can thus be easily fine-tuned very precisely to date the best control of these parameters in block copolymers is achieved using anionic polymerization. The nature of each block, on the other hand, is controlled by the selection of the monomer for instance, styrene would provide a relatively stiff (hard) block while isoprene would provide a soft one. This is a consequence of the very low Tg of polyisoprene compared to that of polystyrene, which in simplistic terms reflects the relative conformational stiffness of the polymer chain. 

Important examples of the ABA type are the styrene-butadiene-styrene (SBS) and styrene-isoprene-styrene (SIS) triblocks in which the outer blocks of the glassy styrene are much shorter than the inner elastomeric blocks. The usual phase arrangement of these materials is shown schematically in fig. 12.13. SBS copolymers have two glass-transition temperatures, as expected for a segregated structure, at —90 and -1-90 °C. The morphology depends on the precise composition and on the method of preparation samples of SBS containing either cylinders in a hexagonal array or spheres in a body-centred cubic array have been obtained. 

Toporowski, R, and Roovers, J. E. L., Glass transition temperature of low molecular weight Styrene-isoprene block copolymers,/. Polym. Sci. Polym. Chem. Ed., 14,2233-2242(1976). 

The simplest dependency exists between composition and glass transition temperature Independent from the ratio A/B one finds two values for Tg, one for the block from monomer A and one for the block of B. More complex are the dependencies with the mechanical properties. Here, parameters like the ratio A/B, number of blocks, block length, and alternation of the blocks play a decisive role. This is shown in Examples 3.47 and 3.48 with triblock copolymers of butadiene or isoprene with styrene. If the content of the diene blocks is around 20%, a stiff and elastic, transparent thermoplastic material is obtained. Instead, if the diene content is raised to about 70%, a highly elastic but still rather stiff thermoplastic elastomer is obtained. It has to be stressed that these properties can only be reached, when the polystyrene blocks are the terminal ones. 

There have been very few studies reported on the viscoelastic properties of rubber-resin pressure sensitive adhesive systems. In 1973, M. Sherriff and co-workers (1) reported on the effect of adding poly (j3-pinene) resin to natural rubber. Based on a G master curve, they showed that the resin shifted the entry to the transition zone to a lower frequency and reduced the modulus in the rubbery plateau. G. Kraus and K.W. Rollman (2) reported in 1977 on their study of resins blended with styrene-isoprene-styrene block copolymers. They showed that the addition of a resin increased the glass transition temperature of the rubbery mid-block and decreased the plateau modulus. Accordingly, a satisfactory tackifying resin should produce these changes. 

One way to approach the problem with the hysteretic loss associated with the onset of the polystyrene glass transition in vulcanized S-I-S polymers is with block copolymers having small random styrene-isoprene copolymer end-blocks, and a pure polyisoprene center block, IS-I-IS. The copolymer composition is chosen to produce a Tg of 40-45 C, at about 60% styrene content. At temperatures experienced in "green processing or tire building the block copolymer would be below Tg and strong, while at the elevated temperatures typically experienced by the vulcanized product in actual use, the end-blocks would be above their glass transition temperature... 

Block copolymers of styrene with either butadiene or isoprene are used in pressure-sensitive adhesives. These are three-block copolymers of type ABA where A are polystyrene blocks and B are polydienes. The two phases are incompatible, each phase having a separate glass transition temperature. The rubbery phase is continuous and the styrene phase consists of small particles 20-30pm diameter. The styrene content is generally above 25%. The Tg of the polybutadiene phase is about —80°C, and it is about — 50°C for polyisoprene. The Tg of polystyrene is about 100°C, and this temperature must be exceeded in thermal processing. They are also used as thermoplastic rubbers. 

The production and properties resulting from the anionic terpolymerization in solution incorporating styrene, butadiene, and isoprene have been reviewed by Halasa [7]. The product, SIBR, can be tailored by monomer composition to give polymers with single or multiple glass transition temperatures. As noted earlier, block copolymers produced by rhythmic alternation of styrene and butadiene in the feed of anionic polymerization are quite different in mechanical behavior from conventional SBR (see Section 4.5). 

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