Inflation of polymer melts into elliptic and circular cylinders

A thin sheet (membrane) of the polymeric material is clamped between a Tefl on-coated thermostated plate and a thermostated aluminium cylinder. By appl ying thermostated air through the plate, the polymer membrane deforms into an elliptic or a circular cylinder. The position of the top of the inflatin g membrane is detected by fibreoptic sensors positioned in the cylinder. Th e pressure difference across the inflating membrane is measured as well. Me asurements were performed on a polyisobutylene melt. As the deformation in this device is highly non-uniform, the response of the material is modelled by a finite element method (the 3D Lagrangian integral method). Here, the non-linear properties are modelled with a constitutive equation of the Fact orised K-BKZ type, using a potential function F((ln u')), where (In u') rep resents the potential function from the Doi-Edwards reptation theory. The L inear viscoelastic properties of the material (the time dependence) have be en obtained from oscillatory shear measurements and modelled with a continu ous relaxation spectrum (BSW) as a memory function, using a leastsquare met hod. This allows the strain dependence in the constitutive equation to be e stimated, based on numerical simulations and experimental measurements of t he membrane inflation. (C) 2000 Published by Elsevier Science B.V.