Influence of temperature, molecular weight, and molecular weight dispersity on the surface tension of PS, PP, and PE. I. Experimental

In this work, the influence of temperature, molecular weight ((M) over bar (n)), and molecular weight dispersity (MWD) on the surface tension of polys tyrene (PS) was evaluated using the pendant drop method. The influence of t emperature on the surface tension of isotatic polypropylene (i-PP) and of l inear low-density polyethylene (LLDPE) was also studied here. It was shown that surface tension decreases linearly with increasing temperature for all the polymers studied. The temperature coefficient -dgamma/dT (where gamma is the surface tension, and T, the temperature) was shown to decrease with increasing molecular weight and to increase with increasing MWD. The surfac e tension of PS increased when the molecular weight was varied from 3400 to 41,200 g/mol. When the molecular weight of PS was further increased, the s urface tension was shown to level off. The surface tension was shown to dec rease with increasing molecular weight distribution. Contact angles formed by drops of diiomethane and water on films of PS with different molecular w eights were measured at 20 degreesC. The surface energies of those polymers were then evaluated using the values of the different pairs of contact ang les obtained here using two different models: the harmonic mean equation an d the geometric mean equation. It was shown that the values of the surface energy obtained are slightly less than are the ones extrapolated from surfa ce-tension measurements in the rubbery state. (C) 2001 John Wiley & Sons, I nc.