Nanoporous low dielectric cyclosiloxane bearing polysilsesquioxane thin films templated by poly(epsilon-caprolactone)

Abstract

Dielectric cyclosiloxane bearing polysilsesquioxanes (CS-PSSQs) were prepared by acid catalyzed polymerization using 2,4,6,8-tetramethyl-2,4,6,8-tetra(trimethoxysilylethyl)cyclotetrasiloxane. The molecular weight, and content of the functional end-groups of the CS-PSSQs were found to be dependent on the process parameters, such as the molar ratio of water and catalyst to the -OCH3 group of the silane monomer, the amount of solvent, and the poly(e-caprolactone) (PCL) content, etc. Based on these CS-PSSQ prepolymers, nanoporous organic/inorganic hybrid thin films were fabricated by spin-coating mixtures of these prepolymers with star-shaped PCL on the silicon substrate, and subsequently heating them at 150 and 250 degreesC for 1 minute each and then at 420degreesC for 1 hour. A dielectric constant as low as 2.28 was achieved for the nanoporous PCL/CS-PSSQ (3:7 v/v) film, along with low moisture absorption in ambient and water conditions, primarily due to the presence of hydrophobic moieties such as ethylene and methyl groups of the polymer and nano-sized hydrophobic pores inside the matrix, resulting in the film having stable dielectric properties. Moreover, the PCL/CS-PSSQ (3:7 v/v) nanohybrid film revealed a nanoporous structure containing ca. 1.52 nm of average-sized mesopores, as measured by the N-2 adsorption method. The CS-PSSQ-only film showed high mechanical strengths having an elastic modulus and hardness of 6.64 and 0.88 GPa, respectively, for the 7 500 Angstrom thick film and 2.41 and 0.38 GPa, respectively, for the PCL/CS-PSSQ (3:7 v/v) film. In addition, the crack velocity of the CS-PSSQ-only film was found to be ca. 10(-11) m . s(-1) in ambient conditions and an aqueous environment, probably due to the enhanced hydrophobicity and mechanical toughness of the incorporated methyl group, siloxane unit and ethylene moieties in the polymer matrix.