Study of equibiaxial extensional flow of polymer melts using lubricated squeezing flow technique

Equibiaxial extensional (EBEX) flow is a shear-free deformation in which a material element is stretched equally in two directions while it contracts in the third direction. EBEX flow is a predominant deformation in many polymer processing technologies, such as blow molding, compression molding and foamed polymer production. Despite the technological significance of EBEX flow, simple and reliable methods for obtaining rheological flow data in this type of flow are presently lacking. EBEX flow data are also useful in examining the predictive capabilities of rheological constitutive equations.; LSF is a promising method for generating rheologically-controlled EBEX flows (Cha81). In LSF, a disk shape polymer melt is squeezed between two solid plates which are lubricated by a low viscosity lubricant. The distance between the plates is decreased, and, ideally, the polymer melt undergoes extension, while shear flow takes place in the lubricant films. LSF is a simple technique which requires relatively simple device and it can be used to conduct EBEX flow experiments with various strain histories. Despite the many advantages of LSF over other existing techniques, it is restricted in some respects. The most severe drawback of LSF is lubricant thinning.; The main objective of the present study is to better understand the LSF technique, examine its limitations, and to find ways of resolving them. In this study the effect of lubricant viscosity and test geometry on the viability of LSF was studied, experimentally and theoretically. Experimental and modeling results show that the lubricant viscosity, initial melt thickness and sample aspect ratio play an important role in the LSF technique. Experimental results have been collected in constant strain rate and in constant stress test modes on two low density Polyethylene melts and a Polydimethylsiloxane.; A mathematical model for LSF has been developed, which takes the viscoelasticity of the polymer melt into account. Results from this model, which were in qualitative agreement with experimental results, show that uncontrolled lubricant thinning amplifies the pressure in the lubricant film and leads to anomalous measurements of the melt stress.; To resolve lubricant thinning and at the same time provide a uniform pressure in the lubricant film, the LSF technique has been modified. In modified LSF, lubricant is continuously injected through "perforated" plates and into the lubricant film between the plates and polymer melt. This method provides a uniform pressure in the lubricant film and is able to reduce lubricant thinning. Experimental results on the low density Polyethylene show that this modified technique enhances lubricant pressure film uniformity and expands the operational limits of LSF from a Hencky biaxial strain of 0.2 to 1.5.