Relaxation phenomena of hydrolyzed polyvinylamine molecules adsorbed at the silica/water interface

Abstract

Surface area exclusion chromatography (SAEC) was employed to determine the individual relaxation of polymer molecules within a saturated heterogeneous layer composed of two polymers of different molecular characteristics. The investigations focused on three systems differing in molecular weight and/or hydrolysis grade. The molecular relaxation process was determined to be different within the heterogeneous layer when compared with the behavior of the same polymer in the homogeneous layer. The modifications in the relaxation process of a given polymer were imposed by the interfacial characteristics of the second polymer. Finally, in heterogeneous layers, the relative variation of the interfacial area of the two polymers is expressed in a single relationship.

Introduction

In Part I we reported that the interfacial reconformation of charged macromolecules within a saturated homogeneous layer is a function of the molecular weight of the hydrolyzed polyvinylamine and to a lesser degree, the hydrolysis grade [1]. The absence of such correlations might be a result of the given polydispersity in hydrolysis grade of the different samples and, to a lesser degree, in molecular weight. Therefore in Part II, heterogeneous layers were built on the stationary phase of the chromatography column composed of stacked glass fiber filters by concomitantly injecting two polymers of different mass and/or hydrolysis characteristics. Elution with mixtures of radiolabeled and nonlabeled polymers (50%/50% w/w) allowed investigation of the relaxation characteristics of each component of the solution when the interfacial layer is equilibrated with the mixed polymer solution. Thus, the change in the apparent interfacial molecular characteristics belongs to a given adsorbed polymer that is entangled within the heterogeneous layer. Changes in the interfacial molecular area were determined as a function of time only because the polymer concentration in the eluant and the rate of elution were held constant [2].

In addition to this special fundamental problem relative to the influence of linear charge density on the “rate” and extent of layer relaxation, it was interesting to determine how it is possible to control such interfacial processes and to elaborate strategies for such a control. The SAEC method is a powerful tool with which to investigate adsorption phenomena when the ratio of surface area to bulk volume is great or under strong homogenization conditions where bulk-to-surface transfer does not limit the adsorption process of macromolecules of different molecular weight [3], [4], [5]. For polyvinylamine adsorbing on cellulose fibers (oppositely charged surfaces), mixing of polymer and fibers did not lead to an equilibrium situation as the time required to establish equilibrium adsorption was determined to be greater than 200 min [6]. Because mixing periods in paper making processes are relatively small, it is expected that thermodynamic equilibrium is rarely established and that the adsorption process may resemble the adaptive random sequential adsorption model developed by Douglas et al. for polymethylmethacrylate adsorbing on silica [7]. This model serves to interpret the stepwise adsorption of polyvinylpyridine on glass beads [8]. For partially protonated polyvinylpyridine molecules adsorbing on glass-fiber filters, the SAEC method provided information on the composition of the interfacial layer established at short terms resulting from the injection of a mixture of two polymers of different molecular weight [9]. Interest in polyvinylamine systems resulted from the possible influence of molecular reconformation during adsorption of polymer of two different average masses and/or hydrolysis grade.

As in Part I, the global adsorption characteristics were first determined from the balance between the mass of polymer injected and the mass of polymer adsorbed/retained on the stationary phase. Then, from the adsorption histogram—the mass of polymer adsorbed per glass-fiber filter as a function of position i of the filter along the column—we derived the interfacial relaxation characteristics of individual polymers.