Block copolymers melt viscosity

Styrenic block copolymers (SBCs) are also widely used in HMA and PSA appHcations. Most hot melt appHed pressure sensitive adhesives are based on triblock copolymers consisting of SIS or SBS combinations (S = styrene, I = isoprene B = butadiene). Pressure sensitive adhesives typically employ low styrene, high molecular weight SIS polymers while hot melt adhesives usually use higher styrene, lower molecular weight SBCs. Resins compatible with the mid-block of an SBC improves tack properties those compatible with the end blocks control melt viscosity and temperature performance. 

Holden et al. (47) first noted the peculiar characteristics in the steady shear behavior of the SBS block copolymer melts. For a certain composition of styrene and butadiene, no limiting Newtonian viscosity was found at low shear rates. For some of the others, there exist two distinct viscosity vs. shear rate relationships (Figure 10). Arnold and Meier (73) carried out the experiments in oscillatory shear and found the same... 

PCO/PE Block copolymers with Ethylene or Propylene and norborene blocks low melt viscosity thus good processability, high elongation at break, impact strength, toughness, hardness, and modulus Epple and Brekner, 1994... 

Nanophase separation of block copolymer melts introduces interesting rheological behavior. For example, the transient elongational viscosity behavior of a triblock copolymer of styrene and olefins was found to be strongly dependent on the initial orientation of the cylindrical domains [103]. 

Block (Star) Arrangement. The known star polymers, like their linear counterparts, exhibit microphase separation. In general, they exhibit higher viscosities in the melt than their analogous linear materials. Their rheological behavior is reminiscent of network materials rather than linear block copolymers (58). Although they have been used as compatibiUzers in polymer blends, they are not as effective at property enhancements as linear diblocks... 

ABA and ) n block polymers exhibit higher melt viscosities than do AB diblock copolymers with similar molecular weights. The former two... 

Blends with styrenic block copolymers improve the flexibiUty of bitumens and asphalts. The block copolymer content of these blends is usually less than 20% even as Httie as 3% can make significant differences to the properties of asphalt (qv). The block copolymers make the products more flexible, especially at low temperatures, and increase their softening point. They generally decrease the penetration and reduce the tendency to flow at high service temperatures and they also increase the stiffness, tensile strength, ductility, and elastic recovery of the final products. Melt viscosities at processing temperatures remain relatively low so the materials are still easy to apply. As the polymer concentration is increased to about 5%, an interconnected polymer network is formed. At this point the nature of the mixture changes from an asphalt modified by a polymer to a polymer extended with an asphalt. 

Microdomain stmcture is a consequence of microphase separation. It is associated with processability and performance of block copolymer as TPE, pressure sensitive adhesive, etc. The size of the domain decreases as temperature increases [184,185]. At processing temperature they are in a disordered state, melt viscosity becomes low with great advantage in processability. At service temperamre, they are in ordered state and the dispersed domain of plastic blocks acts as reinforcing filler for the matrix polymer [186]. This transition is a thermodynamic transition and is controlled by counterbalanced physical factors, e.g., energetics and entropy. 

Next, we present experimental evidence for the electric-field-induced decrease of Todt in a block copolymer. Because of the high melt viscosities, temperatures close to the decomposition temperature and extremely high electric field strengths are required to achieve a measurable effect. In recent studies, we have demonstrated that concentrated block copolymer solutions in a neutral solvent act like a melt, thereby effectively circumventing the above-mentioned limitations [31, 57, 70],... 

Styrene Copolymers. The so-called thermoplastic rubbers based on styrene-butadiene-styrene and styrene-isoprene-styrene block copolymers can be used for hot-melt adhesives, particularly when extended with tackifying resins and oils. They can be made into pressure-sensitive adhesives, as melts with low viscosity—being applied from fine spinnerets which are oscillated to make a... 

Based on thermodynamic considerations, criteria for the existence of domains in the melt in simple shear fields are developed. Above a critical shear stress, experimental data for the investigated block copolymers form a master curve when reduced viscosity is plotted against reduced shear rate. Furthermore the zero shear viscosity corresponding to data above a critical shear stress follow the WLF equation for temperatures in a range Tg + 100°C. This temperature dependence is characteristic of homopolymers. The experimental evidence indicates that domains exist in the melt below a critical value of shear stress. Above a critical shear stress the last traces of the domains are destroyed and a melt where the single polymer molecules constitute the flow units is formed in simple shear flow fields. 

Previous investigations (4, 5,6,7,8) have shown that block copolymers exhibit unusual melt rheological properties such as a very high viscosity, elasticity, and non-Newtonian behavior even at very low shear rates which are all attributed to the multiphase structure resulting from the incompatibility between the two copolymer units in the melt state. 

The fact that the melt of the block copolymer above a critical shear stress shows the same structure-viscosity relations as found for linear melts in simple shear flow is taken as an indication of a monomolecular melt state. 

Figure 21.4 Corrected melt viscosity as a function of shear stress and temperature for the three block copolymers studied. Reproduced with permission from Legge, Holden and Schroeder, Thermoplastic Elastomers A Comprehensive Review, Hanser Verlag, Munich, 1987...

Hydrogenated SBCs are often used to modify polyolefins such as polypropylene, polybutylene and polyethylene. One of the unique characteristics of strongly phase-separated block copolymers such as high molecular weight (>50 000) SEBS and SEPS is their response to shear in the melt. These polymers retain their phase-separated structure well above the Tg of the polystyrene because their order-disorder transition temperatures are above processing temperatures. This phase separation strongly inhibits flow in the absence of shear resulting in infinite viscosity at zero shear rates. The application of shear... 

Not only molecular weight influences the interfacial activity of the PS-hPBD diblock copolymers but also the internal structure. Instead of a single covalent bond between PS and hPBD, a more or less random copolymer sequence can form the transition between the two blocks resulting in the so-called tapered diblock copolymers. The effect of the tapered structure (12,22) and its superiority over the pure diblock (18) have been documented elsewhere and discussed in terms of a change in the interface itself and the melt viscosity of the interfacial agent. 

Such final block copolymer products are described and used as compatibilised chain extender additives for the modification of melt viscosity of polyamides during compounding by reactive extmsion [10] (Fig. 6). 

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