In a preferred first embodiment, the polyether block copolymer is selected from the group consisting of polymers of formulas ##STR1## wherein A and A' are A-type linear polymeric segments, B and B' are B-type linear polymeric segments, and R1, R2, R3 and R4 are either block copolymers of formulas (I), (II) or (III) or hydrogen and L is a linking group, with the proviso that no more than two of R1,