The authors gratefully acknowledge the support of EPSRC (UK) under the "Microscale Polymer Processing" consortium grant, and the essential additional support of BP Chemicals, DuPont Films, DuPont Teijin, Dow Chemical, BASF, DSM, and Lucite International. Helpful discussions with Nat Inkson, Ralph Colby, and Daniel Read assisted the preparation of the manuscript, as did helpful comments from referees.