Abstract: An assembly comprising a film bounded by a frame, the film having a first thermal expansion coefficient along a first direction parallel to the film and a second thermal expansion coefficient along a second direction parallel to the film, wherein thermal expansion of the film compared to that of the frame is greater along the first direction than along the second direction, and wherein the film has a shape at an ambient reference temperature different from that of the frame, the shape of the film being selected to reduce clearance while allowing sufficient room between the film and the frame for thermal expansion in the first direction for temperatures up to a predetermined elevated reference temperature.

Abstract: The invention provides a two-step process for increasing the modulus of a film in a desired direction, by drawing the film at between about 10.degree. C. above T.sub.g and 40.degree. C. below T.sub.m at a draw ratio of between 1.05 and 5.5 and then drawing the film in the same direction at between about 5.degree. C. and 35.degree. C. below T.sub.m at a draw ratio of between about 1.05 and 2.5.