Butadiene block polymers, modification

A relatively new development which promises to gain in importance in the future is the modification of asphalt by butadiene-styrene block polymers.44 The block polymers help reduce the low temperature brittleness and impart resistance to flow at elevated temperatures. Applications in mastics, automobile body undercoatings and waterproofing materials such as high quality roofing membranes are envisaged. 

Polystyrene is one of the most widely used thermoplastic materials ranking behind polyolefins and PVC. Owing to their special property profile, styrene polymers are placed between commodity and speciality polymers. Since its commercial introduction in the 1930s until the present day, polystyrene has been subjected to numerous improvements. The main development directions were aimed at copolymerization of styrene with polar comonomers such as acrylonitrile, (meth)acrylates or maleic anhydride, at impact modification with different rubbers or styrene-butadiene block copolymers and at blending with other polymers such as polyphenylene ether (PPE) or polyolefins. 

We use poly sty rene-f>-poly butadiene block copolymers as the starting material with preformed polymer architecture. These polymers are comparatively cheap and easily accessible. For the present problems a series of narrowly distributed polystyrene-Zr-polybutadiene block copolymers with rather different molecular weights were synthesized via anionic polymerization (Figure 10.4, Table 10.1). As a test for the modification of technological products, a commercial triblock copolymer was also used. 

In a sense, the styrene-butadiene block copolymers, SB or SBS, (first reported in 1956) constituted the next stage of PS modification. The triblock styrene-diene thermoplastic elastomers were patented in 1962, and soon incorporated in blends with PS, PP, LDPE, HDPE, PPE, PET, PBT, or PC, either as impact modifiers or compatibilizers [Bull and Holden, 1977]. In the 1977-78 patents (applications in 1976) it was disclosed that selective hydrogenation of these copolymers leads to new materials, with properties particularly attractive for polymer blends. For example, blending hydrogenated-SBS, or SEES, generated phase co-continuity in blends with PP, PA, PC, PBT, PES, etc. [Gergen et al., 1987]. More recent modification of these copolymers involved incorporation of acidic or acid-anhydride moieties. 

Rubber vulcanization crosslinking was an early chemical modification method. Block and graft methods are also widely used in polymer modification. One of the successful examples of a block copolymer is a thermoplastic elastomer. It is a new material that can be processed like plastic and has elasticity like rubber. Among graft copolymers, the most widely used one is the acrylonitrile butadiene and styrene copolymer... 

Significant modification in properties of polymers and block copolymers containing isoprene and/or butadiene have been reported following hydrogenation of these macromolecules.(9,13-18)... 

As stated earlier, S/DPE copolymers are compatible with GPPS up to a DPE content of about 15wt%. This means that on modifying S/DPE(>15) with S-B(H)-S the impact modification decreases owing to decreased compatibility between the S/DPE matrix and the styrene blocks of the S-B(h>-S impact modifier. S-DPE, however, is prepared anionically and using this polymerization mechanism S/DPE-butadiene S/DPE block copolymers can be prepared. Thus the S/DPE blocks can be tailor-made to be compatible with the S/DPE polymer matrix. For compatibility, the DPE content of the blocks and the matrix should not differ by more than about 15 %. As with the S-B(H)-S block copolymers, the double bonds of the butadiene phase should be removed by hydrogenation. 

The main interest of this work concerns the question whether the reaction conditions can also be applied to the block copolymers. The experiments showed that no modifications of the reaction conditions had to be made. Two polystyrene-block-1,2-poly butadiene with a PS/PB ration of 89/11 and a molecular weight of55 000 g/mol for PSPB I and 118 000 g/mol for PSPBII were used as starting materials. High conversion as well as no change in polydispersity is found for both polymers (Table I). This ratio will lead to a lamellar morphology of the block copolymers if phase separation occurs. 

Mechanical properties of asphalt can be improved [16] by modification with some polymers, especially stirene-butadiene-stirene (SBS) tri-block copolymer. SBS/ asphalt blends are prepared via dynamic vulcanization. Recycled tire crumb rubber is incorporated as a major component in its multi-layer construction [17] for a stadium field-turf. 

Developments in the anionic polymerization of butadiene were adopted for manufacture of solution SBR. While the emulsion process gave primarily 1,4-cis microstructure in the final product, the solution process gave a lower level of 1,4-cis level, typically around 45%. Furthermore the cis content as well as 1,2-vinyl content could be modified. In addition, better control of branching and molecular weight distribution attainable with anionic process made solution SBR suitable for tire applications, challenging the established use of cold SBR. Developments in the anionic process also led to new copolymer structures in which blocks of polybutadiene can be coupled to blocks of polystyrene, generating a imique class of polymers. Developments in SB block copolymers led to new materials which were thermoplastic in character, unlike SBR which is an elastomer. Solution-processes-based thermoplastic SB block copolymers form the basis of the transparent impact polystyrene (TIPS) as well as the other block copolymers used in plastics modification. The block copolymers of styrene and butadiene are the subject of the second part of this article. 

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