(138e) Dynamic Evolution of the Internal Structure of a Drying Colloid-Polymer Film

Drying process is essential to various industrial applications including pharmaceutical drug-release films for dermal and oral drug delivery. One of the primary concerns is the distribution of active ingredients within dry films. In colloid-polymer mixtures, the drying mechanism is complicated and its study is limited. Prior studies show the drying behavior of the colloid-polymer mixtures at various pH, including particle dispersion and flocculation due to depletion interactions between colloidal particles and non-adsorbing polymers by using small-angle X-ray scattering (Kim et al., Macromolecules, 49, 2016) and stress development (Kim et al., Langmuir, 25, 2009). Also, recent simulation suggests stratifications in drying colloid-polymer mixtures depending upon polymer chain length and colloid diameter (Michael et al., Langmuir, 33, 2017). This work is an experimental in-situ investigation of the drying mechanism and film formation of colloid-polymer mixtures using confocal laser scanning microscopy to visualize the internal structural evolution of surrogate “active” ingredients. The mixtures prepared at high and low pH start as colloidal liquids or gels and are investigated over various colloid and polymer concentrations. Suspensions are drawn into thin films on glass substrates and dried at room temperature. During drying process, the film is evaluated by monitoring the dynamic evolution of the distribution and 3D structure of colloidal particles by analyzing the concentration profile, the number of nearest neighbors, the radial distribution function, and the Voronoi polyhedra distributions. In between these 3D scans, 2D scanning to determine the microrheology and internal flow is also obtained. The concentration and gel formation lead to the differences in particle distribution of the dry films. In gel samples, the time evolution of the structure is highly complex, sometimes experiencing periodic gel formation, gel breaking, flow and restructuring before the final structure is permanently immobilized. Moreover, the particle aggregation between liquid and gel mixtures during drying shows distinguishing patterns which are more comprehensible and useful for the industrial process and determining the final structure that dictates drug delivery.