Using cross-sectioned multilayer polymer film and surface modification to form chemically patterned substrates

Highly layered structures are important to micro- and nanofabrication technologies for understanding and controlling surface structures through manipulation of chemical and physical interactions. The objective of this work was to develop a new approach to create micro- and nanopatterned surfaces using multilayer polymer films of commercially available and inexpensive polymers instead of inorganic substrates. As an example, linear low density polyethylene (LLDPE) and ethylene-co-acrylic acid copolymer (EAA) were used as alternating inert and reactive polymers, respectively. Thin cross-sections of the multilayer molded sheets were prepared by ultra-microtoming and the highly layered microstructure was verified by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and polarized optical microscopy. The length scale of the micro- and nanolayer thickness varied from 7 mum to around 500 nm depending on the extent of layer multiplication with compression molding.; As a precursor to the multilayer work, surface modification of EAA was conducted to carefully control the chemical functionality on the surface by a variety of methods. Dansyl cadaverine and polyethylene glycol (PEG) derivatives were grafted on the surface of EAA film and in its subsurface region through formation of amides and esters, respectively. A two-step reaction was conducted. First, EAA film was activated with PCl5 and then the acid chloride was reacted with dansyl cadaverine or a PEG derivative. It was found that dichloromethane yielded the highest grafting efficiency, with the dansyl cadaverine penetrating throughout the ATR-FTIR analysis region (∼ 400 nm) in two minutes. Moreover, two other reaction schemes were developed to covalently graft PEG chains on EAA surfaces. The schemes involved surface grafting of linker molecules l-lysine or polypropyleneamine dendrimer (AM64), with subsequent covalent bonding of PEG chains to the linker molecules. NHS and EDC were used to activate the carboxylic acid groups of the EAA in the outermost region of the film, estimated to be 20 nm by ATR-FTIR spectroscopy. Combining the data from ATR-FTIR, XPS, and contact angle goniometry, it was found that the PEG chains were grafted on the surface of the EAA film and larger surface coverage was achieved when the dendrimer was used as intermediate layer. This surface also had the lowest water contact angle.; Research was then conducted on the EAA-LLDPE multilayer cross-sectioned templates. Regionally confined chemical functionality was confirmed by grafting an amine-terminated biotin to the alternating layers of EAA. Subsequently, fluorescently labeled streptavidin selectively adsorbed on the biotin-modified EAA layers. As a further development, polyelectrolyte multilayers (PEM) were adsorbed on the nanopatterned surfaces to significant increase the areal density of reactive groups. Using PAH and PAA as the polyelectrolytes, the EAA nano-stripes were successfully modified by PEM films, forming a nanopatterned template with alternating hydrophilic and hydrophobic regions. This kind of nano-striped surface could serve as a template for many applications, including biomedical, separation, and electronics.