Dynamic mechanical properties of block copolymer blends--a study of the effects of terminal chains in elastomeric materials

Citation

Cohen, Robert Edward (1972) Dynamic mechanical properties of block copolymer blends--a study of the effects of terminal chains in elastomeric materials. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/J0FN-DS74. https://resolver.caltech.edu/CaltechETD:etd-03292004-152803

Abstract

Blends of well characterized polystyrene-polybutadiene SB diblock and polybutadiene continuous SBS triblock copolymers provide rubbery network systems with controlled amounts of terminal chains of known molecular weight. Such systems also provide quantitative information on the concentrations of trapped and untrapped chain entanglements which is not available in conventional elastomers. Three different SB diblocks were synthesized using homogeneous anionic polymerization techniques. These diblocks were blended in various-amounts with a single research grade SBS triblock to form three series of samples for mechanical testing.

The mechanical properties of these materials were studied (1) in free oscillation at about 0.2 Hz over a temperature range from -150?C to 100?C, and (2) in dynamic uniaxial compression from 0.1 to 1000 Hz at various temperatures between -87 and 85?C.

The effect of terminal chains on the mechanical properties depends upon their length and concentration in the network. The terminal chains act as a diluent, lowering the storage modulus in the rubbery region. Above a critical molecular weight, the untrapped entanglements provided by the terminal chains can be coupled into a temporary stress-bearing portion of the network; the amount of entanglement coupling is dependent upon temperature. Several low frequency viscoelastic mechanisms appeared as a result of entanglement slippage, and their effect was enhanced as terminal chain content increased. At very high frequencies in the glassy and transition regions the presence of terminal chains had no effect on the mechanical behavior.

The various mechanisms associated with the terminal chains have been incorporated into a mathematical model whose parameters are given in terms of the structural and compositional features of an entanglement network. The model successfully predicts the level and the location of the low frequency mechanical response for the various materials studied here.

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