Control of Particle Morphology

C. Schellenberg, I. Stapff, K. Tauer, M. Antonietti

Heterogen structured latex particles, like core-shell latex particles, are perfect systems for investgating the material properties of polymer blends. By latex film formation, the process by which an aqueous dispersion of polymer particles is transformed into a continuous material, we know the possibility to generate structured polymer films and coatings with advantageous material properties. The particle morphology is controlled by two major types of influences concomitatly acting in the system: thermodynamic and kinetic. The thermodynamic factors determine the equlibrium morphology of the final composite particle, while the kinetic factors determine the ease with which the thermodynamically favoured morphology can be achieved.

The preparation of core-shell polymer dispersions is usally a two-stage emulsion polymerization via seed latexes and a physical or a time separation of the two stages. For the second stage polymerization different addition modes as batch (swelling method) and the continuous addition process (drop-wise method) are reported. The structured latexes in this paper were prepared via first stage polymerization and a semi-batch second stage polymerization in the same reactor under controlled kinetic conditions. The polymerization was followed and optimized by use of a reaction calorimeter.

A typical calorimetric record of the complete core-shell polymerization is presented in Fig. 1. Knowledge of the reaction rate profile and the reaction heat flow curve allows to start the addition of the second stage monomer after 80-90 % conversion of the first monomer for core-shell latex preparation.

Schematic of core-shell latex synthesis via reaction calorimetry: A - adjustment of estimated power baseline; B - the calibration power was switched on; C - the initiator solution was injected; D - polymerization heat of first monomer; E - the monome

Atomic force microscopy (AFM) is a family of techniques that has grown rapidly since the invention of the original ‘contact mode’ AFM experiment in 1986. Today we know a number of contact and noncontact scanning force microscopes (SFM), like the chemical SFM or the tapping-mode SFM. For the investigations of soft materials (also soft composite latex particles) the tapping-mode SFM has become a widely used technique. Beside the transmission electron microscope (TEM) normally used for the characterization of composite latex particles we established also the tapping-mode SFM as analytical technique for quantifying the particle morphology in this case.

The figure set below shows an area of the samples in the same magnifications in the topography mode. The observed nanostructuration of optimized core-shell latex films includes both a controlled topographic roughness on the 100 nm scale as well as a control of the network superstructure.

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