Conductive thin films on functionalized polyethylene particles

Hyperbranched polyethylenimine (PEI) is covalently grafted to the surface of polyethylene (PE) particles in an effort to promote the growth of conductive thin films deposited using layer-by-layer (LbL) assembly. Growth of the grafting process is monitored with infrared spectroscopy and titration. LbL films are then deposited using dilute aqueous mixtures containing carbon black stabilized with PEI or poly(acrylic acid). Deposition of carbon black-filled bilayers on PEI-grafted PE shows uniform surface coverage and strong bonding after just two bilayers, while neat PE shows patchy film growth and poor adhesion, requiring eight bilayers to achieve full surface coverage. Acid-oxidized PE shows intermediate behavior with regard to deposition but shows weak bonding like neat PE. LbL films are characterized using electron microscopy and thermogravimetric analysis, which show linear growth for PEI-grafted particles and nonlinear growth for neat and acid-oxidized particles. Following carbon black deposition, the electrical conductivities of films made by compressing the coated particles were compared. No conductivity can be measured for films made with neat PE particles containing two and four bilayers, but acid-oxidized and PEI-grafted systems exhibit conductivities of 0.000 004 5 and 0.01 S/cm, respectively. Plotting conductivity as a function of carbon black concentration reveals a percolation threshold below 0.01 wt % and a conductivity of 0.2 S/cm with just 6 wt %. This combination of covalent polyelectrolyte grafting and LbL deposition could potentially be used to impart useful properties to a variety of polyolefin surfaces.