This invention provides a composition of matter composed of a substantially uniform mixture of a thermoplastic elastomeric hydrocarbon black copolymer and 0.1 to 12 percent by weight of the composition of a polysiloxane. The composition exhibits improved surface, elasticity, and tensile strength characteristics as well as superior processibility. The block copolymer may comprise blocks of styrene-ethylene-butylene-styrene in which the styrene blocks have a molecular weight of 5,000 to 40,000 and the ethylene-butylene block has a molecular weight of 20,000 to 500,000. Mineral oil and polypropylene are preferably dispersed in the composition. Combining the copolymer with the polysiloxane typically occurs under the influence of a pressure of 1,500 to 2,500 p.s.i. or above, such as provided by extrusion blending. The resulting elastomeric composition may be extruded into sheets as thin as 0.015, 0.010, or even 0.005 inch, which a puller may thin even further. The resulting composition has a surface smoothness microscopically characteristic of silicone rubber. As one of its many varied uses, the elastomer may find use as an inflatable cuff or balloon on such medical devices as endotracheal tubes. The high elasticity of the composition permits it to act, when in the trachea, as a high residual volume type endotracheal tube, except that it avoids the folds which permit aspiration fluids to channel to the patient's lungs. The pressure it exerts against the tracheal wall falls even below the high volume device. When not inflated, the material returns to a sufficiently small volume that it acts as a low volume device for ease of insertion and withdrawal.