André Herzog Cardoso

Easy polymer latex self-assembly and colloidal crystal formation: the case of poly[styrene-co-(2-hydroxyethyl methacrylate)]

Abstract Poly[styrene-co-(2-hydroxyethyl methacrylate)] core and shell latex particles easily undergo self-ordering, as evidenced by the iridescence of dispersions and dry solids. Microscopic (scanning electron microscopy) observation of the dry latex and fracture surfaces reveals the coexistence of different crystallographic arrangements. Many domains are observed, in which particle positions are correlated for tens of crystallographic planes. Isotherms of latex monolayers were also obtained, showing a strong interparticle repulsion. The ease of particle self-assembly in this latex is assigned to two factors: (i) the existence of strong repulsive interparticle interactions, which allows particle ordering at long distances, while particle diffusion throughout the dispersion is still possible; (ii) the hydrophilic surface layer, which allows for strong capillary adhesion during the whole drying process, according to Nagayamas model (Nature, 361 (1993) 26).

Latex macrocrystal self-assembly dependence on particle chemical heterogeneity

Abstract A self-arrayed poly(styrene-co-hydroxyethylmethacrylate) film surface was examined using microscope techniques possessing microchemical capabilities, together with standard (control) techniques. Secondary electron (SEI) and backscattered electron (BEI) images obtained in a field-emission scanning microscope, as well as scanning atomic force (AFM) and electric potential (SEPM) images, show an accumulation of hydrophobic particles around point and line macrocrystal defects. Defects in self-arrayed latex films are thus largely due to latex particle chemical heterogeneities. This result is consistent with the capillary adhesion-dependent models for macrocrystal formation and it shows the importance of latex purification, prior to macrocrystallization.

Latex coagulation induced by alcohol vapors: an unusual factor for colloidal instability

Abstract A polystyrene (PS) latex prepared by redox initiation is coagulated by exposure to the vapors of lower alcohols (methanol to 2-butanol) as well as to alcohol–water (>30% w/w) solutions, but not to the vapors of higher alcohols and many other solvents. Latex particle size determinations by PCS, under various exposure conditions, show that the vapor-induced coagulation takes place at the liquid latex surface, rather than within the liquid, but the effect of co-solvent alcohols on particle diameters under non-coagulating concentrations is significant (∼10%). Two major fractions of the latex are discerned by centrifugation in density gradients as well as by SEM examination of dry coaguli formed under ethanol vapor.

PS-HEMA latex fractionation by sedimentation and colloidal crystallization

A poly(styrene-co-hydroxyethylmethacrylate) latex underwent sedimentation under gravity followed by an spontaneous and extensive colloidal crystallization. It was then fractionated in three visually distinguishable layers. Latex aliquots layers were sampled at different heigths and the particles were characterized by PCS, microelectrophoresis, infrared spectra and analytical electron microscopy. The major fraction was opalescent and contained the colloidal crystals settled in the bottom of the liquid. Two other latex fractions were obtained, which differed in their chemical compositions, particle sizes and topochemical features from the self-arraying particles. Macrocrystallization of the fractionated latex yielded high quality crystals with a low frequency of defects, which confirms that particle chemical homogeneity is an important factor for particle self-arraying.