Novel Techniques for Interfacial Tension and Contact Angle Measurements in Polymer/CO2 systems

The use of supercritical CO2 is a leading candidate as a replacement for current foaming agents as they are phased out by the Montreal Protocol. Numerous studies have found that adding CO2 to polymer melts lowers the viscosity and enhances many foam properties. Despite this, there are many obstacles that need to be cleared, as the traditional foaming agents make much higher quality foams than are currently being made using CO2. Because of the necessity to create these higher quality foams, work must be done in order to attempt to improve the foams. This work is based on nucleation theory and depends mainly on interfacial tension between the polymer melt and the gas and on the contact angle between the gas, the melt, and the substrate which is usually added to polymer melts. Literature data is limited when it comes to contact angles with high pressure and high temperature polymer melts. This research attempted to find contact angle measurements for commonly used polymer melt systems and find the conditions that result in the best foaming conditions, based on thermodynamic properties. This was done using a high pressure temperature controlled vessel with plate glass windows to allow observation of the system inside. Nearly all of the common methods of drop analysis were carried out, as was the capillary rise technique. The drop methods involving a polymer drop in the gas environment led to many problems, resulting from viscosity problems as well as the drop wetting the entire surface. This was an unpredicted phenomenon that showed the glass had a very strong attraction to the polymers. The only method that showed much promise was the captive bubble method, in which the interfacial tension and contact angle can be measured simultaneously. Because of this, the results were compared to the literature interfacial tensions for polystyrene, and large errors were found. This led to the design of a new apparatus to hopefully minimize any errors that the sides of the apparatus or other bubbles in the system may cause