Phase behavior and viscoelastic properties of entangled block copolymer gels

Triblock copolymers in midblock-selective solvents can form physical gels. However, at low triblock contents (near the percolation threshold), the bri dging of chains between micelles can lead to macrophase separation. Adding a styrene-isoprene diblock to a styrene-isoprene-styrene triblock copolymer in squalane can eliminate macrophase separation, yielding a wide range of stable, single-phase gels with a disordered arrangement of micelles. The pl ateau modulus of these triblock gels scales with the 2.2 power of polymer c ontent, indicating the importance of entanglements in dictating the modulus . Comparing gels made from the midblock-saturated derivative of the same po lymer [styrene-(ethylene-alt-propylene)-styrene] in squalane reveals that t he modulus differences in the gels are a direct consequence of the differen ce in the entanglement molecular weight of the midblock homopolymer in bulk . Finally, the broad relaxation spectrum of these triblocks is well-describ ed by a recent theory for the dynamics of entangled star polymers, with the breadth of the relaxation spectrum dictated by the number of entanglements per midblock in the gel. (C) 2001 John Wiley & Sons, Inc.