The network density in the surface layers of three-dimensional polymers

Abstract

THE presence of groups of different polarity in polyurethane chains gives rise to the formation of stable intermolecular bonds of different types in these polymers [1]. Therefore stable physical crosslinks are formed along with a threedimensional networks in polyurethanes. The result is in some cases that the physical crosslinks are the predominating factor in the overall density of the threedimensional network [2]. When polyurethanes are used for coatings it must be taken into account that additional physical bonds are formed in them through the reaction of the polyurethane film with the solid surface, and this must affect the overall density of the three-dimensional network. In previous investigations the authors showed that the interaction of polymers with hard surfaces limits the mobility of the polymer chains, and markedly alters some of the physicochemical properties of the polymers [3, 4]. The surface also affects the effective crosslink density if the hardening takes place in the presence of filler particles [5]. From this point of view study of tile surface properties of three-dimensional polymers is of major importance when the chain segments of the polymers between the chemical crosslinks of the network possess considerable flexibility so that there is the possibility of their being adapted to the surface, with the result that additional physical crosslinks are formed. The substances investigated were coatings based on polyesters and polyethers and toluylenediisocyanate with different ratios of N C O : O H groups. Trimethylolpropane¢ was used as the crosslinking agent. The effective crosslink density was determined for the free films and for films on a support consisting of aluminium foil 14/~ thick. A relative estimate of the effective crosslinking density was made using the Flory-Rener [6] equation derived in a s tudy of the swelling of natural rubber in low molecular weight liquids; this equation is used by investigators to find the effective crosslink density of various polymers [7, 8]. The calculations were based on the following equation p V1 (v~-2v2/F ) M e = ~ . . . . • O ) ~0v 2 -~ v 2 + In (1-v,,)