Block polymers rubbers

In Chapters 3 and 11 reference was made to thermoplastic elastomers of the triblock type. The most well known consist of a block of butadiene units joined at each end to a block of styrene units. At room temperature the styrene blocks congregate into glassy domains which act effectively to link the butadiene segments into a rubbery network. Above the Tg of the polystyrene these domains disappear and the polymer begins to flow like a thermoplastic. Because of the relatively low Tg of the short polystyrene blocks such rubbers have very limited heat resistance. Whilst in principle it may be possible to use end-blocks with a higher Tg an alternative approach is to use a block copolymer in which one of the blocks is capable of crystallisation and with a well above room temperature. Using what may be considered to be an extension of the chemical technology of poly(ethylene terephthalate) this approach has led to the availability of thermoplastic polyester elastomers (Hytrel—Du Pont Amitel—Akzo). 

Interesting products may also be produced by introducing boron atoms into the chain. The amount of boron used is usualy small (B Si 1 500 to 1 200) but its presence increases the self-adhesive tack of the rubber, which is desirable where hand-building operations are involved. The products may be obtained by condensing dialkylpolysiloxanes end-blocked with silanol groups with boric acid, or by reacting ethoxyl end-blocked polymers with boron triacetate. 

Hybrids of block copolymer rubbers and acrylics have also been used to increase the low-temperature impact resistance of the adhesive used for body-side molding attachment [127]. To further enhance performance, a new type of hybrid adhesive has been developed, which combines an adhesive polymer, like an... 

Olefins are the basic building blocks for many chemical syntheses. These unsaturated materials enter into polymers, rubbers, and plastics, and react to form a wide variety of chemical compounds such as alcohols, amines, chlorides and oxides. 

Puskas, J.E., Pattern, W.E., Wetmore, P.M., and Krukonis, A. Synthesis and characterization of novel six-arm star polyisobutylene-polystyrene block copolymers. Rubber Chem. TechnoL, 72, 559-568, 1999. Puskas, J.E., Wetmore, P.M., and Krukonis, A. Supercritical fluid fractionation of polyisobutylene-polystyrene block copolymers, Polym. Prepr., 40, 1037-1038, 1999. 

One type of block polymer is known as thermoplastic elastomers. They consist of a number of rubber blocks tied together by hard crystalline or glassy blocks. These materials can be processed in injection molding and extrusion equipment since the crystalline blocks melt or the glassy ones soften at high temperatures. However, at lower temperatures, such as at room temperature, the hard blocks behave very much as cross-links to reduce creep and stress relaxation. Thermoplastic elastomers have creep behavior between that of very lightly cross-linked rubbers and highly cross-... 

ABA-type triblock copolymerization of MMA/BuA/MMA should give rubberlike elastic polymers. The resulting copolymers should have two vitreous outer blocks, where the poly(MMA) moiety (hard segment) associates with the nodules, and the central soft poly(BuA) elastomeric block provides rubber elasticity. Ihara et al. [35] were the first to synthesize an AB-type block copolymer, with MMA (190 equivalents of initiator) first polymerized by... 

Elastomeric block polymers of styrene and butadiene or iso-prene and their products of hydrogenation are finding increasing use in a variety of fields.44. Linear and radial block polymers are used extensively in injection molded rubber goods, footwear, pressure sensitive and hot melt adhesives and in mechanical rubber goods such as hose, tubing, cove base, toys, drug sundries, rubber bands, stoppers, erasers, etc. 

An outstanding property of these polymers is their shear stability. The sonic shear stability testsfci indicate that these polymers are superior to some of the currently used polymers of ethylene-propylene or methacrylate type. The excellent stability of the hydrogenated diene-styrene polymers is attributed to their relatively low molecular weight and narrow distribution consistent with the established theory of shear degradation of polymers. The most recent developments in this field are block polymer VI improvers with dispersancy properties, built into the molecule by chemical modification of the rubber block. 2... 

Almost all isoprene produced is used in the preparation of polymers and copolymers. cis-Polyisoprene, primarily for vehicle tyres, is the largest application, with styrene-isoprene-styrene (SIS) block polymers being a rapidly growing secondary application. Butyl rubber is a significant third application. World demand for isoprene for monomer use in 1992 was (thousand tonnes) polyisoprene, 827 SIS, 95 butyl rubber, 25 and other uses, 10 (Weitz Loser, 1989 Lybarger, 1995). 

Mastication of rubber. VIII. Preparation of block polymers by... 

Sonic Modulus. If crack or craze branching is the operative mech-nism in toughening, toughness should be directly related to the difference in sonic speeds in matrix and dispersed phases. Experiments to confirm this effect were undertaken using three commercial ABS resins. These were selected to represent the three main rubber types encountered commercially an acrylonitrile/butadiene copolymer rubber, a butadiene rubber with grafted styrene/acrylonitrile copolymer, and a block polymer of... 

Van Henten, at the Shell Plastic Laboratories (II), showed that styrene-butadiene block polymers can be blended with commercial HIPS to upgrade its impact strength to 5.8 ft-lbs/inch. Childers, at Phillips Petroleum (12), blended commercial polystyrene with block polymers in a Brabender plastograph. To control rubber particle size he added a peroxide during the blending operation, thereby creating crosslinks. With this technique he achieved an impact strength of 5.9 ft-lbs/inch. 

Initially various rubbery butadiene and styrene-butadiene block polymers were screened as impact-modifying agents for polystyrene. Commercial polystyrene and various rubbers were blended by dissolving the polymers in benzene and by subsequently precipitating them with isopropyl alcohol. The solid polymer blends were dried and molded into test bars. Laboratory and commercial polybutadiene and polystyrene were used in several combinations with the block polymer prepared in our laboratory. 

With high shear mixing equipment such as the Banbury or continuous mixer, the use of block polymers with a number average molecular weight (Mw) below 150,000 did not give the expected improvement in impact over polystyrene. This anomalous behavior occurred because the particle size of the discrete rubber phase averaged less than 0.2 pm compared with an average of 1 pm for blends with optimum impact obtained in the solution blends. 

The modulus and yield kinetic parameters of the block polymer B can be related to those of the homopolymer in terms of a microcomposite model in which the silicone domains are assumed capable of bearing no shear load. Following Nielsen (10) we successfully applied the Halpin-Tsai equations to calculate the ratio of moduli for the two materials. This ratio of 2 is the same as the ratio of the apparent activation volumes. Our interpretation is that the silicone microdomains introduce shear stress concentrations on the micro scale that cause the polycarbonate block continuum to yield at a macroscopic stress that is half as large as that for the homopolymer. The fact that the activation energies are the same however indicates that aside from this geometric effect the rubber domains have little influence on the yield mechanism. 

S-B-S and S-I-S. Much of our discussion will refer directly to data for S-B-S and S-I-S block polymers. We justify this on several counts. These block polymers can be clearly defined as to structures, molecular weights, and compositions. They have served as model systems for much of the recent work in block polymers. They also comprise the largest volume of commercial block polymers. Finally, we believe that the discovery (20) of the S-B-S and S-I-S thermoplastic rubbers which are strong, resilient rubbers without vulcanization, and the concomitant, readily understood theory (21). provided a paradigm (terminology of T. S. Kuhn (2 2)) that significantly accelerated the scientific work on these polymers in recent years. 

We suggest that the S-B-S thermoplastic rubbers and the domain theory have produced a paradigm upon which the block polymer field advanced significantly. In reviewing the technological discovery of the S-B-S thermoplastic rubbers and the virtually simultaneous formation of a theory that enabled us to move rapidly toward commercialization, it was of interest to us to trace a pathway to discovery for the relatively small group concerned with it. We have set forth in Table II some selected events that we believe enabled us to very rapidly understand the physical phenomena and proceed. Table II is, then, not intended in any way to be a comprehensive illustration of the history of block polymers but is rather a discrete list of events leading to the discovery of S-B-S and S-I-S thermoplastic rubbers in our laboratory. 

In 1961 Crouch and Short (38) discussed the use of S-B block polymers, although not identified as such, in vulcanized rubber applications. Commercial production of the S-B block polymer commenced late in 1962. Identification of the S-B block structure was presented by Railsback, Beard, and Haws (39) in 1964. 

Characteristic Features of A-B-A Thermoplastic Elastomers Three-Block Polymer High-Strength Rubber No Vulcanization Required Completely Soluble Reversible Melt-Bulk Properties Two Glass Transition Temperatures Two Phases... 

This discovery culminated in the commercial production and the announcement (41) in 1965 of thermoplastic elastomers from block polymers of styrene and butadiene (S-B-S) and of styrene and isoprene (S-I-S). To rubber scientists and technologists the most outstanding property of S-B-S and S-I-S was the unvulcanized tensile strength compared to that of vulcanized NR and vulcanized SBR carbon black stocks. Stress-strain curves, to break, of these latter materials are compared to that of S-B-S in Figure 2. It was pointed out that the high strength of S-B-S must be due to physical crosslinks. 

Although we did not publish our domain theory in 1965, it was fairly evident to those acquainted with the historical developments in Table II that the new products were three-block polymers. Researchers in the field very rapidly commenced to use these polymers as models for comparison and as subject for physicochemical studies. Evidence for this may be seen in the work reported by Cooper and Tobolsky (42) in 1966, in which they correlated the behavior of polyester-polyurethane thermoplastic rubbers (Estane products) (37) with those of a Shell S-B-S polymer. They concluded that the presence of segregated hard and soft phases in the Estane... 

The Impetus given to the block polymer field by discovery of the diene-based thermoplastic rubbers, S-B-S and S-I-S, is suggested by the data of Figure 3. Here we show a histogram of the number of... 

U.S. patents on block polymers generally related to the thermoplastic rubbers, over the period of 1961-80. By taking into account the lag in patent issuance, the sharp increase of patents issued from 1966-1970 and subsequently confirms the upsurge of interest. 

For these reasons, we feel that the discovery of A-B-A thermoplastic rubbers and the domain theory gave the block polymer field a paradigm (22) that greatly accelerated research on a worldwide basis. The content as well as the volume of literature support this view (Figure 3). The usefulness of the paradigm was based on the well-characterized block polymers that can be produced from the anionic polymerization system as shown in the box and on the readily understood basics of the domain theory. 

---