Depth profiling of small molecules in dry latex films by confocal Raman spectroscopy F. Belaroui a , Y. Grohens a , H. Boyer b , Y. Holl a, * a Institut de Chimie des Surfaces et Interfaces, CNRS, 15, rue J. Starcky, BP 2488, 68 057 Mulhouse, France b Jobin Yvon, 16-18 rue due Canal, 91165 Longjumeau, France Received 15 December 1999; received in revised form 1 February 2000; accepted 11 February 2000 Abstract The distribution of small molecules in dry latex films is the result of driving forces competing to displace these molecules at the interfaces or to retain them distributed inside the film. In order to provide a better insight into the process leading to segregation of surfactant molecules, confocal Raman spectroscopy was used. This technique was powerful in the quantitative determination of the distribution profiles of small molecules, namely sodium dodecyl sulfate (SDS) and sulfate anion SO 2- 4 ; active in Raman mode, in the overall thickness of polymeric films. The concentration of the SO 2- 4 ion is higher at the film/substrate interface than in the bulk whereas the film/air interface exhibits a depletion, which propagates at 50 mm from the surface in the film thickness. An enrichment of SDS is found at both interfaces; and aggregates of small molecules inside the film were detected for both SO 2- 4 and SDS.  2000 Elsevier Science Ltd. All rights reserved. Keywords: Confocal Raman spectroscopy; Latex films; Butyl acrylate 1. Introduction The growing importance of synthetic latex in domains such as adhesives, paints or non-weaved textiles is owing to their film formation and adhesion properties, and also because of ecological and economical considerations. Film formation corresponds, when water evaporates, to a change in the system from independent particles dispersed in water to a continuous film. Three steps can be distin- guished during the process of film formation [1–3]: (1) concentration of the latex; (2) deformation of the particles; and (3) diffusion of the macromolecules across the particle boundaries. During these processes the migration of surfac- tant molecules through the film towards the interfaces is controlled by water fluxes [4], coalescence [5], miscibility with the polymeric matrix [6–14] and ability of the surfac- tant molecules to lower the interfacial tension at the film/air or film/substrate interfaces [6,12]. However, during the segregation process a part of the surfactant molecules is trapped in the polymeric film and does not reach the inter- faces. Properties of the latex films such as adhesion, mechanical strength and permeability are strongly influenced by the distribution of the surfactant in the film. This explains the numerous works, which have been devoted to the fate of surfactant molecules in latex films [5,6,10–12,15–19]. Atte- nuated total reflection (ATR) [5,6,10–12,15] and step scan photoacoustic spectroscopy (PAS) [20] have been exten- sively used, but have shown some limitations in the depth probed and the quantitative interpretations, respectively. Confocal Raman spectroscopy has shown its efficiency for depth profiling of laminates, coatings, membranes and composites [21–25]. In this paper, confocal Raman spectro- scopy is shown to be a powerful quantitative technique for the depth profiling of small molecules in the overall thick- ness of dry latex films. The distribution profiles of sodium dodecyl sulfate (SDS) and of sulfate ion SO 2- 4  in the films are investigated. 2. Experimental 2.1. Materials Butyl acrylate (BuA) was copolymerised with 1 or 4 wt% of acrylic acid (AA) by radical emulsion polymerisation with 2.5% of surfactant (SDS). The mean particle diameter was around 100 nm, with a narrow particle size distribution. The solids content was adjusted at 25% by weight. The latex was systematically purified by dialysis at pH 10 to eliminate the surfactant used in the synthesis. Defined amounts of SDS and Na 2 SO 4 were then introduced in the purified Polymer 41 (2000) 7641–7645 0032-3861/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S0032-3861(00)00145-2 * Corresponding author.