Small-angle neutron scattering study of particle coalescence and SDS desorption during film formation from carboxylated acrylic latices
Introduction
The complexity of the thin film formation mechanism from a polymer colloid (latex) is unanimously recognized. This subject was already extensively studied in the academic and industrial scientific community. Excellent review articles can be found in the literature [1], [2], [3]. Several aspects of the process remain insufficiently understood as attested by examples of works recently published. Some of the main ones are (i) drying [4] and particle deformation [5], (ii) particle coalescence (in this subtopic, latices and nonpolymeric emulsions share many common features [6], and (iii) distribution of low molecular weight species [7]. All these aspects are interdependent and can hardly be discussed independently. This explains the complexity referred to above.
We intended to re-address two important issues related to the mentioned topics. The first one is the rupture of the hydrophilic shell surrounding particles in many types of latices. This hydrophilic shell was called a membrane by some authors [8]. In nascent films, these membranes form a continuous phase. They have to rupture to allow proper particle coalescence. Upon rupturing, they are expelled in separated domains, forming a discontinuous phase. The final morphology of the film will depend on this phase-inversion mechanism. The process is kinetically controlled and it is interesting to investigate the parameters influencing this kinetics. Our second topic of interest pertains to the problem of distribution of small molecules in latex films. To gain better insight into the complex phenomena leading to the distribution of surface-active small molecules, it is important to determine at what time and to what extent these species are desorbed from the particle–water interface when particles become very close in the film formation process.
Only a few experimental methods are available for such kinds of problems. Small-angle neutron scattering (SANS) is a powerful technique well suited for this purpose and is already extensively used in studies of film formation mechanisms, as reviewed by Chevalier [9]. Another, more recent review by Ballauff [10], devoted to studies of the structure of latex particles and including X-ray scattering data, is also available. The main topics where SANS was used are as follows (only some representative and, when possible, recent examples are quoted): disorder–order transition upon drying (formation of a colloidal crystal) [11]; drying mechanisms [8]; structure of adsorbed layers at the particle surface [12] and consequence on latex stability [13]; most importantly, interdiffusion of chains across particle boundaries [14]; studies on structure of films made from latex blends or core–shell particles [15], of films containing inorganic particles (close to real coatings) [16], or of films under mechanical stress [17]. To our knowledge, no SANS studies exist on the problem of surfactant desorption from the particle–water interface during film formation.
For this study, suitable core–shell latices were carefully synthesized. Coalescence of their particles was studied by SANS versus shell thickness and composition, core softness, pH, time, and temperature. Some details on the film structure after coalescence could also be obtained. In a second part, desorption of sodium dodecyl sulfate (SDS) was followed, again by SANS, during drying of the latex.
Final remark in this introduction: there is an unfortunate ambiguity in the literature with the term “latex”. For most authors in the field of polymer colloids, latex designates the whole colloidal system (polymer particles plus water). For some authors, latex refers to the polymer particles only. As already apparent above, in this paper, latex is used in the former sense.
Section snippets
Latices
All reagents were from Merck (Darmstadt, Germany). Monomers: n-butyl acrylate (BuA), methyl methacrylate (MMA), and acrylic acid (AA) were purified by filtration through basic alumina powder. The surfactant, sodium dodecyl sulfate (SDS) (purity over 99%), and the initiator, ammonium persulfate ((NH4)2S2O8) (purity 99%), were used directly from the bottle. Distilled water was used throughout.
The latex syntheses were performed by a semicontinuous process, in a double-wall 1.5-liter glass reactor,
Coalescence
Neutron scattering spectra of the films formed from surfactant-free latices are shown in Fig. 1. When structure peaks are present at q values around 0.01 Å−1, the expected value for particle diameters close to 100 nm (see, for example, BuA 4, pH 7), the system has not coalesced and the hydrophilic acrylic acid-rich phase is continuous. When coalescence has occurred, no peak is visible (see, for example, BuA 1 or 4 at pH 2). Table 4 summarizes the pH, acrylic acid content, and core mobility
Coalescence
In the kinds of systems used in this work, coalescence can be divided into three subprocesses closely following each other: membrane rupturing, membrane expulsion, and fusion of the adjacent particle cores. This is analogous to the phase inversion occurring in concentrated emulsions [29]. We have shown that the whole process is hindered by an increase of the core Tg, of the thickness and acrylic acid concentration of the particle shell, and of the dispersion pH (Fig. 1 and Table 4). The first
Conclusion
The results presented in this paper will be very useful in the interpretation of distribution data of low molecular weight species (surface active or not) in these kind of latex films [41].
On the other hand, a precise knowledge of the morphology of the films, whether the rigid hydrophilic phase remains continuous despite its low volume fraction or becomes discontinuous, and the thermal conditions required to switch from one to the other, will allow us to establish nice structure–property
Acknowledgements
We thank Rhodia for financial support. It is also a pleasure to thank Drs. J.F. d'Allest, C. Bonnet-Gonnet, and B. Amram (Rhodia, Research Center Aubervilliers) for helpful discussions.
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