Polystyrene Domains Research Articles

Solvent Effects in Styrene—Butadiene—Styrene Block Copolymer Cast Films and Evidence for Supermolecular Ordering by X-Ray and Light Scattering

Abstract It has been found that solution-cast films of an SBS block copolymer (Kraton 1101) have two orders of phase structure. The first is of molecular dimension and consists of elongated polystyrene domains (≈ 200 A˚) dispersed in a polybutadiene matrix. There is also a supermolecular heterogeneous structure of spherical domains of fractional micron size. A somewhat larger nearest neighbor distance is also seen. The domain size and separation are larger for the polymers cast from solvents for polystyrene than those cast from a solvent for polybutadiene alone. Furthermore, SBS films cast from good solvents for polystyrene exhibit additional supermolecular ordering in contrast to the more random arrangement of the others.

Stress–birefringence study of styrene–butadiene multiblock copolymer

AbstractThe styrene–butadiene block copolymers of SB, SBS, and SBSBS types were investigated by means of the stress–birefringence technique to elucidate the effects of the block multiplicity on the mechanical properties of copolymers. The SBS‐type and the SBSBS‐type block copolymers exhibit nonlinear stress–birefringence relations, which were classified into three regions. They are a low stress‐optical coefficient region (optical creep), an increasing coefficient region, and a region of retarded increase in coefficient, as pointed out by Henderson et al.8 It was found that optical creep is associated with the polystyrene continuum. The third region, however, originates not only from the phenyl groups of the polystyrene block but also from a form‐birefringence due to the orientation of the polystyrene domain. The multiblock copolymer of SBSBS type was found to be rheo‐optically similar to the SBS type, both being affected solely by the details of the terminal styrene block.Pure gum vulcanizates of the SB‐type and SBS‐type block copolymers also exhibit optical creep. In a vulcanized block copolymer having both chemical and physical crosslinks, the latter seemed to disappear at elevated temperatures. At decreasing temperatures, the vulcanizates of block copolymers showed higher modulus of elasticity and lower stress‐optical coefficients.

A rheological characterization of SBS block copolymers

AbstractThe dynamic viscosities of styrene–butadiene–styrene (SBS) block copolymer melts with different end‐ and midblock sizes are given as a function of frequency at several different temperatures. The data exhibit two distinct regions, a high frequency region where the response is typical of thermoplastics and a low frequency region where the viscosity continually increases with decreasing deformation rates. These results are successfully explained in terms of the formation of polystyrene domains in the low frequency region. The correlations given for the rheological properties should provide additional insight to the processing of these materials.

Fracture Behaviour of Binary Styrene/Butadiene Block Copolymer Blends

Morphology and fracture behavior of binary block copolymer blends (triblock and star block copolymers being made up of styrene and butadiene) have been studied via transmission electron microscopy (TEM) and instrumented Charpy impact testing. The toughness of the ductile blends has been characterized by dynamic crack resistance curves (R-curves). The lamellar star block copolymer shows nearly elastic behavior of small scale yielding and unstable crack growth. Addition of 20 wt % of a triblock copolymer comprising polystyrene domains dispersed in rubbery matrix leads to a strong increase in toughness. The mechanism of toughness modification, based on the nanophase-separated structures, represents a specific morphology-toughness correlation which differs fundamentally from toughening mechanisms observed in conventional polymer blends.DOI: 10.3126/jncs.v22i0.523Journal of Nepal Chemical Society Vol. 22, 2007, pp. 55-66

Developments in self‐reinforced elastomers

AbstractUntil recently it has been believed that either stress‐crystallization or the presence of reinforcing filler is necessary for the development of high strength in elastomers. Now it has been observed that gum vulcanizates of isoprene/acrylonitrile copolymers have tensile strengths in the order of 3000–4000 psi in the absence of fillers. In this case crystallization does not occur and the strength is associated with the ability of the polymer molecules to become highly oriented on stretching. Similar observations have been made with ABA block copolymers polystyrene‐polybutadiene‐polystyrene which are highly elastomeric in the absence of chemical crosslinks. The elastomeric properties and high strength of the block copolymers are attributed to the mutual incompatibility of the A and B blocks which leads to phase separation on a microscopic scale. The effective network structure can be temporarily destroyed by heating the polymer above the softening temperature of the resinous blocks which makes the material readily processable. The initial decrease in retractive force with increasing temperature up to 60–70°C is attributed to an increase of mobility of the resinous domains in the rubbery matrix. At higher temperatures the crosslinking effectiveness decreases markedly due to softening of the resinous regions. The process of yielding, hysteresis effects, and delayed elastic creep arise because of viscous forces which oppose the motions of the colloidal polystyrene domains. The strength and effectiveness of physical crosslinks in block copolymers shows a similar temperature dependence as has been observed for other filled and unfilled vulcanizates. The process of self‐reinforcement in isoprene‐acrylonitrile copolymers and polystyrene‐polybutadiene‐polystyrene block copolymers is attributed to viscous effects which make possible the alignment of a large number of polymer chains in the stretching direction.

Structure‐property relationships for styrene‐diene thermoplastic elastomers

AbstractA study has been carried out on block polymers of the A‐B‐A type, the thermoplastic elastomers, where A represents polystyrene and B polybutadiene or polyisoprene. The objective was to relate the mechanical properties of these elastomers to their molecular architecture. For this purpose a series of styrene‐butadiene and styrene‐isoprene block polymers were synthesized by means of organolithium initiators, in which the block lengths of the polystyrene and polydiene were varied. It was found that the stress‐strain properties of styrene‐butadiene polymers are mainly dependent on the polystyrene content, regardless of the block sizes, and that the center, elastic block does not appear to behave as the “molecular weight between crosslinks” of these networks. However, the monodispersity of the chain length of these elastic blocks does appear to contribute substantially to higher tensile strengths. The polystyrene appears to be aggregated in small domains, of the order of several hundred angstrom units, and these undergo an irreversible deformation under stress, which is, however, completely recoverable above the Tg of the polystyrene. At high polystyrene contents (∼40%) these elastomers exhibit an irreversible yield point at very low elongations, and this is ascribed to the presence of a continuous phase of connected polystyrene domains, which can be re‐formed by raising the temperature above the Tg of the polystyrene. Although the stress‐strain curves are not affected by the thermal history of the sample, the tensile strength, especially at high styrene contents, is strongly dependent on “annealing” of any frozen stresses in the polystyrene phase. The useful range of block molecular weights is about 10,000–20,000 for polystyrene and 40,000–80,000, respectively, for polybutadiene. The lower limit is probably governed by the minimum polystyrene chain length required to insure the formation of a heterogeneous phase; while the upper limit is set by the high viscosity of both blocks, which might seriously hamper domain formation.

Properties of Random and Block Copolymers of Butadiene and Styrene. III. Three Sequence Styrene-Butadiene-Styrene Block Polymers

Abstract In butadiene styrene copolymers containing long block sequences chain segments associate with like segments to form a two phase structure. Properties of such polymers are dependent not only on composition and molecular weight but also on block sequence along the chain. Polymers containing two or more polystyrene blocks per molecule form networks and exhibit elastomeric properties in the uncured state resembling those of filler reinforced vulcanizates. This behavior is shown both by linear styrene-butadiene-styrene elastomers and multichain block copolymers branched in the polybutadiene blocks. A prominent loss tangent peak was observed around —40° C for the multichain polymers. Stress strain following prestretching and stress relaxation measurements indicate some shifting of polystyrene associations during stretching. Tensile strength is reduced by increasing temperature and addition of plasticizers. Reinforcement by polystyrene domains in vulcanized block copolymers is evident from tensile strength, dynamic modulus, and swelling measurements, but decreases with increased crosslinking. The number of styrene sequences in the primary molecules is less important after vulcanization as crosslinking destroys the individuality of the original polymer chains.

Stress–birefringence properties of styrene–isoprene block copolymers

AbstractStress–birefringence measurements have been made on a number of styrene–isoprene block copolymers, and their mechanical properties have been interpreted in terms of their molecular structures. Essentially, the polymers consist of highly dispersed phases of polystyrene and polyisoprene and undergo elastic deformation with the polystyrene domains acting as crosslinks. Hysteresis measurements have been carried out to interpret the stress–strain behavior of the polymers in more detail and the conformational properties of films prepared with selective solvents have also been examined with similar birefringence techniques.