Relation between toughness and molecular coupling at cross-linked polymer/solid interfaces

The relationship between the interfacial fracture toughness (Gc) and molecular coupling between epoxy polymers and silicon wafers was studied using the asymmetric double cantilever test (ADCT). In order to investigate the molecular coupling, the coupling molecules had to be applied along the polymer/substrate interface in various concentrations. The grafting from technique by means of the �living� free radical polymerisation techniques, namely nitroxy-mediated �living� free polymerisation (NMP) and atom transfer radical polymerisation (ATRP), were used to prepare suitable coupling molecules. Unfortunately, these techniques did not produce satisfactory results, and the new route of grafting to was investigated. This method involved chemical attachment of brominated polystyrene chains (PS) to the silicon substrate. The bromine functional groups of the brominated polystyrene (PS(Br)) were then further converted to amino (-NH2) groups using the Heck reaction, producing PS(NH2). The conversion method was first tested using the free (unattached) polystyrene (MW = 200K) which was brominated (molar fraction of brominated units, x = 0.1). The conversion from PS to PS(Br) and then to PS(NH2) was monitored by proton and carbon nuclear magnetic resonance techniques (1H-NMR and 13C-NMR) and gel permeation chromatography. For the grafting to experiments, monochlorosilane end-functionalised polystyrene (PS-SiClMe2) (MW = 8000) was used. Various ratios of PS and PS(Br) ranging from 0 to 20% PS(Br) were deposited onto silicon substrates. Applying the Heck reaction, Br groups were converted to NH2, producing surfaces with different concentrations of amino groups which were expected to react with the epoxy polymer. ADCT was adapted to investigate the dependence of the interfacial fracture toughness on the degree of coupling. The dependence of Gc on the concentration of the coupling molecules was not directly confirmed. This was attributed mainly to the challenges in deposition of the polymer chains and the final conversion to PS(NH2). However, the differences in Gc values between experiments suggested that Gc�s measured by ADCT reflect the differences in surface properties of the polymer modified surfaces, and therefore this technique is suitable for interfacial adhesion measurements between epoxy polymers and solid substrate.