Surface and interfacial tension measurements of polymer melts with pendant drop apparatus: Effect of structural and material properties on the surface tension of LLDPE. An improved experimental method.

摘要

The pendant drop apparatus is the most reliable and accurate method of measuring the interfacial tension of polymer melts. In spite of its widespread use, a detailed experimental procedure has yet to be published. This study examined all practical aspects of pendant drop experiments with viscous polymer melts and establishes procedures for material preparation, syringe diameter selection, experimental protocol, determination of equilibrium, and the most accurate measurement of interfacial tension. In addition, the effects of both drop volume and unstable drops on interfacial tension measurements are discussed. Comparing the surface tension and interfacial tension values for several polyethylene (PE) and polystyrene (PS) resins to literature data tested the effectiveness of the experimental procedure.;The values of surface tension of several experimental linear low-density polyethylene (LLDPE) resins were determined and related to the copolymer structural and material properties. It was determined that the surface tension of the LLDPE hexene copolymers is primarily a function of the chain branching density rather than number average molecular weight. This occurs because the properties of a branched end are believed to be similar to those of a chain end. Therefore, bulk polymer properties of a branched polymer should depend primarily on the concentration of branched ends. Studying the relationship between the surface tension and the free volume of the LLDPE copolymers corroborated the relationship between the surface tension and the branching density. The values of surface tension and surface tension temperature coefficients of the LLDPE copolymers were found to be similar to literature values of linear polyethylene of equal effective molecular weight. However, the findings are inconclusive due to inconsistencies in the literature data. Also, the effects of polydispersity were believed to have caused a significant surface tension reduction and increased the value of the surface tension temperature coefficient in one of the samples.