Direct Observation of Swelling of Non-Cross-Linked Latex Particles by Scanning Angle Reflectometry

Abstract

Introduction. The swelling of latex particles by plasticizers has been the subject of many studies over the past few decades. The most comprehensive thermodynamic analysis of this problem was made by Bindschaedler1 in 1985. The often observed non-Fickian behavior of the swelling led to the concept of a diffusion/ relaxation mechanism, of which Berens and Hopfenberg gave a phenomenological description.2 The technique they used consisted of measuring the weight of the sample as a function of time, from which they calculated the uptake of solvent as a function of time. They saw an initial large uptake of plasticizer (diffusion controlled) by the particle, followed by a much slower uptake (controlled by network relaxations). Their conclusion was that the initial stage is totally dependent on the free volume in the particle, only resulting in a weight gain of the particle, without an increase in particle diameter. We investigated the swelling of non-cross-linked polystyrene particles (size ≈ 410 nm) by methyl ethyl ketone (MEK), dissolved in water, using scanning angle reflectometry. With this technique, we measured the diameter of the particles as a function of time, from which we deduce the uptake of MEK. A great advantage of scanning angle reflectometry is that it is very sensitive to small increases in particle size (<0.1% of the diameter of the particle). Another advantage is the stability of the setup, allowing us to measure for more than a week without losing accuracy. In contrast to the conclusion of Berens and Hopfenberg,2 we observed an increase in the diameter of the particle in the initial stage of swelling. Although observation of very rapid growth in the beginning of a swelling experiment has also been observed by other researchers,5-8 the consequences for the interpretation of the Berens and Hopfenberg model were never discussed. Based on our observations, we will propose a modification to the model of Berens and Hopfenberg. Theory. In this section, we briefly summarize the main results of the thermodynamic equilibrium analysis of Bindschaedler and of the diffusion/relaxation model of Berens and Hopfenberg. For a detailed description of the analysis of the reflectometry data, we refer to recently published material.3 The thermodynamics of polymer particle swelling by a mixture of an organic solvent and water is discussed by Bindschaedler et al.1 In the case of swelling of polystyrene, water does not enter the polymer phase, and the requirement of thermodynamic equilibrium yields where the indices 1, 2, and 3 refer to the solvent, water, and polymer, vi,p is the volume fraction of species i in the polymer phase, vi,a is the volume fraction of species i in the aqueous phase, and xi is the ratio of the molar volume of species i and the molar volume of the solvent; o13 and