Background of Layer-by-layer assembly
Layer-by-layer (LbL) Assembly uses complementary interactions (ionic interaction, hydrogen bonding, van der Waals forces, etc )between different components to deposit materials one layer at a time. Different from conventional polymer complexes that obtained using mechanical mixing, the LbL assembly features a bottom-up fabrication approach, which allows us to fully control the property of final products. LbL assembly is highly versatile. By selecting appropriate ingredients and assembling parameters, numerous desirable properties can be imparted to multilayer thin film coatings, such as gas barrier, fire retardant, anti-microbial, anti-reflection, drug delivery, etc. LbL assembly features properties that unmatched by traditional materials. For instance, permeability of polyethylenimine/poly(acrylic acid) bilayer assembly is over 3 orders of magnitude lower than that of poly(ethylene terephthalate), which is most popular gas barrier material for soda bottles. LbL assembly is also industrial friendly. Almost all of the LbL assembly process can be conducted under ambient conditions, and the equipment involved are easily accessible. For instance, dip coating can be performed by hand dipping a substrate into cups holding different polymer solutions. Spray coating can be performed by spray polymer solution onto a substrate using plant mister. In all, LbL assembly is an extremely powerful yet simple technique. Multilayer thin films with multiple exceptional properties can be obtained through rational design of the assembly process. |
My Reasoning and Findings
Conventional gas barrier polymer nanocomposites obtained using mechanical mixing of polymer and clay features mediocre gas barrier and low transparency due to random alignment and aggregation of clay platelets. Although multiple methods have been used to improve the dispersion and alignment of clay platelets in these conventional polymer nanocomposites, the level of improvement is very limited. The advent of Layer-by-Layer assembly technique allows highly exfoliated clay platelets to be uniformly aligned within polymer/clay LbL thin films. This type of structure is coined as "nanobrick wall" by my adviser, Professor Jaime Grunlan. In this nanocomposite, exfoliated clay platelets act as nanobrick while polymer act as mortar. Gas molecules are forces to follow a tortuous path between clay layers when diffuse through the assembly, resulting in much improved gas barrier. According to the pioneering model predicted by Cussler in 1988, the diffusion path of gas molecules can be extended by increasing the spacing between parallel clay platelets. Previous research by other group member already showed that gas barrier could be improved by inserting more polymer layers between clay sheets, which in turn increased clay spacing. However, adding too many polymer layer between clay sheet is detrimental to gas barrier due to lower clay loading. So, a method which increases clay spacing without adding more polymer layers is needed to further improve gas barrier. During my research, I find that adsorption and desorption coexist during the deposition of PEI/PAA bilayers (which act as mortar), with adsorption dominates the initial stage and desorption happens at a later time. By reducing the excessively long deposition time that commonly used for LbL assembly, desorption of polymer can be suppressed, leading to thicker film, which can be used to enlarge clay spacing. Polymer/clay multilayer thin films assembled with clay of different aspect ratios (25, 200, 1100) were obtained, and all multilayer prepared using shorter dipping time were found to feature better gas barrier. |