Amine functionalization by initiated chemical vapor deposition (iCVD) for interfacial adhesion and film cohesion

Abstract

Amine functional polymer thin films provide a versatile platform for subsequent functionalization because of their diverse reactivity. Initiated chemical vapor deposition (iCVD) is a polymer chemical vapor deposition technique that utilizes the delivery of vaporphase monomers to form chemically well-defined polymer films with tunable conformality and property. In this thesis work, amine functional iCVD poly(4-aminostyrene) (PAS) thin films were synthesized for the first time. The pendant amine groups enable the formation of a robust nanoadhesive with complementary epoxy functional groups. Bonded devices able to withstand >150 psi were achieved by combining polydimethylsiloxane (PDMS) and a wide variety of polymeric materials. Additionally, the all-iCVD nanoadhesive bonding process displays high resistance against hydrolytic degradation (>2 weeks). In addition to bonding, the iCVD layers remaining in the microfluidic channels provide functional groups for subsequent reaction and also act as diffusion barriers against oxygen permeation into the devices. Two applications utilizing this nanoadhesive bonding technique were introduced, including for growth of E. coli in the iCVD-bonded chips and fabrication of gas impermeable microchannels for microparticle synthesis from organic solvents. Another amine functional conformal coating has been designed, synthesized, and characterized. The novel alternating copolymer thin film synthesized from maleic anhydride and aminostyrene via iCVD extensively self-crosslinks after gentle heating. The annealed copolymer films display an elastic modulus exceeding 20 GPa, far greater than typical polymers (0.5~5 GPa). Moreover, the cross-linked films maintain their flexibility, neither cracking nor delaminating with repeated flexing. This achievement represents a significant advance in the fabrication of tough, durable, conformal, functional coatings. Furthermore, the highly crosslinked coating material has oxygen permeability lower than leading commercially available permeation barrier films, making it an attractive material for electronics or food industries. Also described is the utility of a new initiator, tert-butyl peroxybenzoate (TBPOB), for the iCVD synthesis. Using TBPOB instead of tert-butyl peroxide (TBPO), the rate of iCVD film growth increased by a factor of up to ~8 at comparable conformality and lower the filament temperature from ~250 °C to ~150 °C at a comparable deposition rate. The faster deposition rates improve the economics of the iCVD process and the ability to initiate polymerizations at a much lower filament temperature reduces heat load to substrate, which is advantageous for temperature sensitive polymeric substrates or monomers that decompose at high temperatures.