THE INFLUENCE OF THE CHEMICAL-STRUCTURE ON THE SURFACE AND INTERFACIAL-TENSION OF LOW-MOLECULAR-WEIGHT RUBBERS

The pendant drop method allows the determination of the surface and interfacial tension of polymer melts and fluids in the range between room temperature and ca. 300-degrees-C. A digital on-line contour analysis was developed which allows a continuous drop shape analysis and thus the exact determination of the equilibrium condition. The experiments on low molecular weight rubbers are presented. To allow a direct comparison, non-polar polymers of similar backbone structures and their polar versions were studied. One group of polymers (polyoctenamer, polybutadiene, ethylen-propylen-copolymer (EPM)) is of almost non-polar structure, while the second group (CH2OH-polybutadiene, MSA-polybutadiene, butadiene-acrylnitrile-copolymer) shows some polarity. A linear dependence of the surface tension with temperature is observed for polyoctenamer, the surface entropy ranged from 0,084 to 0,1 mN/mK. For polyoctenamer the surface tension is a function of M(n)-213. Extrapolation yields a surface tension of 39 mN/m for a polyoctenamer of infinite molecular weight at 20-degrees-C. In blends of EPM and polyoctenamer the low-energy-component EPM is enriched in the polymer-air interface; fitting the experimental data, an occupied area of 2 nm2 per chain can be deduced. The polar contribution to the interfacial tension is the governing factor for incompatibility. Applying the harmonic mean, the fraction of polarity is 1,9% for the MSA-polybutadiene, 2,7% for CH2OH-polybutadiene and 9,5% for the butadiene-acryl-nitrile-copolymer. The interfacial thickness is a function of temperature. For the polymer pair CH2OH-polybutadiene/polyoctenamer a thickness of 10,9 nm at 20-degrees-C and of 350 nm at 200-degrees-C was determined; compatibility is expected begond 220-degrees-C. Light microscopy confirms these results.