Thermoplastic polystyrene elastomers

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). 

Thermoplastic elastomers are most commonly formulated from elastomeric polyurethane or block copolymers of polystyrene-elastomer, polyamide-elastomer, or polyether-elastomer bases. Thermoplastic elastomers are provided as a raw material in pelletized form for subsequent compounding. The internal domain structure that is required for thermoplastic-elastomeric performance has been established by specific considerations of blending and structural-chemical interactions. In compounding operations, specific temperature ranges are required to assure that phase separation does not occur in the TPE base polymer. 

Styrene Copolymer Dispersions. The and hardness of polystyrene can be adjusted over a wide temperature range by copolymerization of styrene with soft monomers such as butadiene and acrylate esters. Styrene-butadiene (SB) dispersions are quantitatively the most important. With a styrene-butadiene weight ratio of 85 15 the is ca. 80 C, at a ratio of 45 55 the T, is ca. — 25 C. On account of the cross-linking capability of butadiene, SB copolymers are not thermoplastics, but elastomers. Elasticity can be modified by controlling the molecular mass and degree of cross-linking. 

The carrier resins are polyethylene, polypropylene, polystyrene, ABS, polyamide 6 and 66, polyacetal, and thermoplastic polyester elastomer. 

Casting resin Thermoplastic elastomer Cast resin, flexible Mineral- and/or glass-filled Epoxy molding and encapsulating compound Polystyrene... 

Gun Propellents. Low sensitivity gun propeUants, often referred to as LOVA (low vulnerabUity ammunition), use RDX or HMX as the principal energy components, and desensitizing binders such as ceUulose acetate butyrate or thermoplastic elastomers (TPE) including poly acetal—polyurethane block copolymers, polystyrene—polyacrjiate copolymers, and glycidyl azide polymers (GAP) to provide the required mechanical... 

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). 

The particular type of thermoplastic elastomer (TPE) shown in Figure 3 exhibits excellent tensile strength of 20 MPa (2900 psi) and elongation at break of 800—900%, but high compression set because of distortion of the polystyrene domains under stress. These TPEs are generally transparent because of the small size of the polystyrene domains, but can be colored or pigmented with various fillers. As expected, this type of thermoplastic elastomer is not suitable for use at elevated temperatures (>60° C) or in a solvent environment. Since the advent of these styrenic thermoplastic elastomers, there has been a rapid development of TPEs based on other molecular stmctures, with a view to extending their use to more severe temperature and solvent environments. 

Considerable amounts of EPM and EPDM are also used in blends with thermoplastics, eg, as impact modifier in quantities up to ca 25% wt/wt for polyamides, polystyrenes, and particularly polypropylene. The latter products are used in many exterior automotive appHcations such as bumpers and body panels. In blends with polypropylene, wherein the EPDM component may be increased to become the larger portion, a thermoplastic elastomer is obtained, provided the EPDM phase is vulcanked during the mixing with polypropylene (dynamic vulcani2ation) to suppress the flow of the EPDM phase and give the end product sufficient set. 

Proportion of Hard Segments. As expected, the modulus of styrenic block copolymers increases with the proportion of the hard polystyrene segments. The tensile behavior of otherwise similar block copolymers with a wide range of polystyrene contents shows a family of stress—strain curves (4,7,8). As the styrene content is increased, the products change from very weak, soft, mbbedike materials to strong elastomers, then to leathery materials, and finally to hard glassy thermoplastics. The latter have been commercialized as clear, high impact polystyrenes under the trade name K-Resin (39) (Phillips Petroleum Co.). Other types of thermoplastic elastomers show similar behavior that is, as the ratio of the hard to soft phase is increased, the product in turn becomes harder. 

Legge, N.R., Holden, G. and Schroeder, H.E., Thermoplastic elastomers based on polystyrene-polydiene block copolymers. Thermoplastic Elastomers, Hanser Publishers, New York, 1987. 

SBS (linear or star) Polystyrene Polybutadiene Polystyrene Polyethylene Thermoplastic elastomer... 

Plastics, such as PE, PP, polystyrene (PS), polyester, and nylon, etc., and elastomers such as natural rubber, EPDM, butyl rubber, NR, and styrene butadiene rubber (SBR), etc., are usually used as blend components in making thermoplastic elastomers. Such blends have certain advantages over the other type of TPEs. The desired properties are achieved by suitable elasto-mers/plastic selection and their proportion in the blend. 

Asathana S., Majoros I., and Kennedy J.P., TPEs Star-block comprising multiple polystyrene-b-PIB arms radiating from a crossUnked polydivinylbengene core. Rubber Chem. TechnoL, 71, 949, 1998. Shim J.S. and Kennedy J.P., Novel thermoplastic elastomers. II. Properties of star-block copolymers of PST-b-PlB arms emanating from cyclosiloxane cores, J. Polym. Set, Part A, Polym. Chem., 37, 815, 1999. 

Plastomer, a nomenclature constructed from the synthesis of the words plastic and elastomer, illustrates a family of polymers, which are softer (lower hexural modulus) than the common engineering thermoplastics such as polyamides (PA), polypropylenes (PP), or polystyrenes (PS). The common, current usage of this term is reshicted by two limitahons. First, plastomers are polyolehns where the inherent crystallinity of a homopolymer of the predominant incorporated monomer (polyethylene or isotactic polypropylene [iPP]) is reduced by the incorporahon of a minority of another monomer (e.g., octene in the case of polyethylene, ethylene for iPP), which leads to amorphous segments along the polymer chain. The minor commoner is selected to distort... 

Puskas, J.E. et al. Synthesis and characterization of novel dendritic (arborescent) polyisobutylene-polystyrene thermoplastic elastomers, J. Polym. Set A, 43, 1811, 2005. 

St. Lawrence, S., Shinozaki, D.M., Puskas, J.E., Gerchcovich, M., and Myler, U. Micro-mechanical testing of polyisobutylene-polystyrene block-type thermoplastic elastomers. Rubber Chem. TechnoL, 74, 601-613, 2001. 

Puskas, J.E., Pattern, W.E., Wetmore, P.M., and Krukonis, A. Multiarm-star polyisobutylene-polystyrene thermoplastic elastomers from a novel multifunctional initiator, Polym. Mater. Set Eng., 82,42 3, 1999. Brister, L.B., Puskas, J.E., and Tzaras, E. Star-branched PIB/poly(p-t-bu-Styrene) block copolymers from a novel epoxide initiator, Polym. Prepr., 40, 141-142, 1999. 

Chattopadhyay, S., Kwon, Y., Naskar, A.K., Bhowmick, A.K., and Puskas, J.E. Novel Dendritic (Arborescent) Pol3dsobutylene-Polystyrene Thermoplastic Elastomers. Paper 27, ACS Rubber Division, 162th Technical Meeting, October 8-11, Pittsburgh, PA, 2002. 

Kwon, Y., Antony, P., Paulo, C., and Puskas, J.E. Arborescent polyisobutylene-polystyrene block copolymers—a new class of thermoplastic elastomers, Polym. Prepr., 43, 266-267, 2002. 

El Fray, M., Puskas, J.E., Tomkins, M., and Altstadt, V. Evaluation of the Eatigue Properties of a Novel Polyisobutylene-Polystyrene Thermoplastic Elastomer in Comparison with other Rubbery Biomaterials. Paper 76, ACS Rubber Division, 166th Technical Meeting, October 5-8, Columbus, OH, 2004. Puskas, J.E. and Chen, Y. Novel Thermoplastic Elastomers for Biomedical Applications. Paper 40, ACS Rubber Division, 163nd Technical Meeting, April 28-30, San Erancisco, CA, 2003. 

The use of lightly crosslinked polymers did result in hydrophilic surfaces (contact angle 50°, c-PI, 0.2 M PhTD). However, the surfaces displayed severe cracking after 5 days. Although qualitatively they appeared to remain hydrophilic, reliable contact angle measurements on these surfaces were impossible. Also, the use of a styrene-butadiene-styrene triblock copolymer thermoplastic elastomer did not show improved permanence of the hydrophilicity over other polydienes treated with PhTD. The block copolymer film was cast from toluene, and transmission electron microscopy showed that the continuous phase was the polybutadiene portion of the copolymer. Both polystyrene and polybutadiene domains are present at the surface. This would probably limit the maximum hydrophilicity obtainable since the RTD reagents are not expected to modify the polystyrene domains. 

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