Depth profiling of small molecules in dry latex films by confocal Raman spectroscopy
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 distinguished during the process of film formation [1], [2], [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 surfactant molecules through the film towards the interfaces is controlled by water fluxes [4], coalescence [5], miscibility with the polymeric matrix [6], [7], [8], [9], [10], [11], [12], [13], [14] and ability of the surfactant 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 interfaces.
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], [11], [12], [15], [16], [17], [18], [19]. Attenuated total reflection (ATR) [5], [6], [10], [11], [12], [15] and step scan photoacoustic spectroscopy (PAS) [20] have been extensively 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], [22], [23], [24], [25]. In this paper, confocal Raman spectroscopy is shown to be a powerful quantitative technique for the depth profiling of small molecules in the overall thickness of dry latex films. The distribution profiles of sodium dodecyl sulfate (SDS) and of sulfate ion (SO42− ) in the films are investigated.
Section snippets
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 Na2SO4 were then introduced in the purified latex. The dispersions were cast on a
Results and discussion
Fig. 3shows the Raman spectra recorded from a BuA/AA 1% film containing 6% of SDS. The peaks at 1084 and 1063 cm−1 are assigned to the SDS υ(SO) and the polymer υ(C–C) vibrations, respectively. The plane of focus was positioned at the film/air interface and the diameter of the confocal pinhole was tuned at 1000, 400, 200 μm. Decreasing the pinhole reduces the analysed volume. The relative intensity of the SDS peak at 1084 cm−1 increases comparatively to the polymer peak with the decrease of the
Acknowledgements
We thank Rhodia for financial support. It is also a pleasure to thank Drs M. Dorget, J.F. d'Allest, C. Bonnet-Gonnet and B. Amram (Rhodia, Aubervilliers) for helpful discussions.
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