Influence of Temperature, Molecular Weight, and Molecular Weight Dispersity on the Surface Tension of Polystyrene, Polypropylene, and Polyethylene. II. Theoretical NICOLE RAYMONDE DEMARQUETTE, 1 JOSE ´ CARLOS MOREIRA, 1 RENATO NORIO SHIMIZU, 1 * MAZEN SAMARA, 2 MUSA R. KAMAL 2 1 Metallurgical and Materials Engineering Department, Cidade Universita ´ria, Avenida Prof. Mello Moraes 2463, CEP 05508-900, Sa ˜o Paulo, SP, Brazil 2 Chemical Engineering Department, McGill University, 3610 University Street, Montreal, Quebec, Canada H3A 2B2 Received 16 November 2000; accepted May 20 2001 Published online 21 December 2001 ABSTRACT: Experimental data for the surface tension of polystyrenes of different molecular weights (3400 –200,000) and different molecular weight dispersities (1–3) and of different polyolefins are compared with the predictions of the Patterson–Rastogi and Dee–Sauer cell theories, which infer the surface tension from pressure–volume– temperature (PVT) data. PVT data for these polymers were obtained from the literature and experimentally and are fitted to the Flory–Orwoll–Vrij equation of state. Both theories predict that the surface tension will decrease linearly with increasing temper- ature and increase with molecular weight, thereby corroborating the experimental data. However, both theories underestimate the entropy change in the surface forma- tion per unit area at a constant volume for low molecular weight and polydisperse systems and underestimate the effect of molecular weight dispersity on surface tension. Both theories feature two parameters, m and b, that quantify the enthalpic and entropic contributions to surface tension. The theoretical predictions are fitted to the experimental data for monodisperse polystyrene (with a molecular weight above the molecular weight of entanglement), polypropylene, and linear low-density polyethylene to quantify the enthalpic contribution to surface tension. b is then evaluated as a function of molecular weight and molecular weight dispersity and is found to decrease with increasing molecular weight and to increase with increasing molecular weight dispersity, showing that end-group excess at the surface has some effect on surface tension. © 2002 John Wiley & Sons, Inc. J Appl Polym Sci 83: 2201–2212, 2002 Key words: surface tension; polystyrene; polyethylene; polypropylene INTRODUCTION Knowledge of the surface tension of molten polymers is very important for plastics, inks, films, textiles, and adhesion technology. How- ever, very little experimental evidence concern- ing the surface tension of molten polymers has been published because of experimental diffi- culties encountered in the determination of this parameter. Therefore, it would be interesting to be able to predict theoretically the surface tension of molten polymers. 1–11 Several thermodynamic theories have been developed for that purpose. They basi- cally can be divided into two groups: (1) theories based on the square-gradient theory first devel- oped by Cahn and Hilliard that correlates the Correspondence to: N. R. Demarquette (nick@usp.br). *Present address: Embraer, Empresa Brasileira de Aero- na ´ utica S.A., Avenida Brigadeiro Faria Lima, 2170, 12227- 901, S.J. dos Campos, SP, Brazil. Contract grant sponsor: Fundac ¸a ˜ o de Amparo ao Estado de Sao Paulo; contract grant numbers: 97/06071-2 and 00/02744-7. Journal of Applied Polymer Science, Vol. 83, 2201–2212 (2002) © 2002 John Wiley & Sons, Inc. DOI 10.1002/app.10187 2201