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Published on: June 30, 2018
Directed stratification in polymer-latex film blends via pH and drying control
Yang Liu1, Ilias Halimi1, Tejveer Singh Plaha1
1Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada.
Hypothesis:
Stratification in polymer-colloid films is governed by interfacial interactions, component miscibility, and drying kinetics. We hypothesize that introducing a pH-responsive, acid-rich oligomer (ARO2) into latex films enables controlled vertical phase separation through electrostatic tuning and evaporation rate modulation.
Experiments:
We formulated latex-polymer films by blending ARO2 (8 wt% acrylic acid, Mₙ ≈ 4000 Da) with latex particles (dh = 274 nm), varying pH (8.2-9.5), relative humidity (RH: 15-45 %), and ionic strength (0-50 mM NaCl). Förster Resonance Energy Transfer (FRET) was used to quantify ARO2-latex interdiffusion, while vertical distribution was characterized using confocal laser scanning microscopy (CLSM) and cross-sectional atomic force microscopy (AFM).
Findings:
FRET measurements indicated limited molecular mixing in dispersion (ΦET ≈ 0.06), creating conditions favorable for stratification during drying. At pH 9.5 and 15 % RH, electrostatic repulsion between ionized ARO2 and latex particles, combined with size ratio (α ≈ 18) and moderate Péclet numbers (Pe ≈ 2.1), drove diffusiophoretic transport of ARO2 toward the evaporating interface. CLSM and AFM revealed a distinct ARO2-rich surface layer (10-20 μm thick) under these conditions, while slower drying or increased salt concentration suppressed segregation. This controllable stratification mechanism enables design of structured polymer-latex coatings: pH > 9 + low RH promotes surface enrichment, while pH < 8 or high RH yields uniform distribution.
