Influence of deposition time on layer-by-layer growth of clay-based thin films
The influence of deposition time on film growth of montmorillonite (MMT) or laponite (LAP) clay paired with branched polyethylenimine (BPEI) is studied here using layer-by-layer (LbL) assembly. Deposition times from 5 to 300 s in each ingredient were evaluated with respect to thickness, mass deposited, film structure, and oxygen barrier. Ellipsometry and quartz crystal microbalance show the BPEI/LAP system to be more dip-time dependent than the BPEI/MMT system because LAP platelets have a much smaller aspect ratio than MMT. Similar thickness is achieved for all recipes when only the BPEI deposition time is varied. Cross-sectional TEM images confirm thicker growth of BPEI/LAP in 300 s relative to 10 s. X-ray diffraction (XRD) suggests the gallery spacing between clay layers is insensitive to dip time, which is a key structural parameter linked to both mechanical and transport properties of these types of films. Additionally, oxygen transmission rates (OTR) are below the detection limit of commercial instrumentation (<0.005 cc/m2 day) in both 10 s/300 s and 300 s/300 s BPEI/MMT assemblies, suggesting similar defect-free structures in both cases. This study indicates that LbL assemblies could be produced more quickly, which is a key hurdle currently facing their commercial use.