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Modelling plasma-induced reactions on polymer surfaces using aliphatic self-assembling and LB layers
(1998)
Monotype functionalizations with different types of functional groups (OH, NH2, COOH) on polypropylene and poly(tetrafluoroethylene) surfaces were synthesized using pulsed plasma-initiated homo- or copolymerization of functional group-carrying monomers. The maximum concentrations of functional groups were 31 OH, 18 NH2 or 24 COOH groups per 100 C atoms using allyl alcohol, allylamine or acrylic acid respectively as the monomer. The measured peel strengths of aluminium deposits increased linearly with the concentration of functional groups. Near the maximum concentration of OH (>27 OH/100 C atoms) or at moderate concentrations of COOH groups (>10 COOH/100 C atoms), constant (maximum) peel strengths were measured due to the mechanical collapse of one component in the composite (cohesive failure). Interface failures in Al-PP composites were found with COOH, NH2 and OH groups and cohesive failures were seen when higher concentrations of COOH groups were applied (>10 COOH/100 C atoms).
The peel strength of aluminium to polypropylene and poly(tetrafluoroethylene) was determined in dependence on the type and the concentration of functional groups on the polymer surface. For this purpose the polymer surface was equipped with monotype functional groups. The first method to produce monotype functionalized surfaces was an introduction of O functional groups using an oxygen plasma treatment and converting these groups to OH groups applying a wet chemical reduction. In result of this two-step treatment the hydroxyl group concentration at the polymer surface could be increased from 34 to 1014 OH groups/100 C atoms. The second method consists in the deposition of a 150 nm adhesion-promoting layer of plasmapolymers or copolymers onto the polymer surface using the pulsed plasma technique. For that purpose functional groups carrying monomers as allyl alcohol, allylamine and acrylic acid were used. Applying the plasma-initiated copolymerization and using neutral monomers like ethylene or butadiene the concentration of the functional groups was varied.
A correlation of peel strength with the ability of forming chemical interactions between Al atoms and functional groups was found: COOH > OH >> NH2 > H(CH2CH2).
In contrast to other plasma modification processes of polymer surfaces, the bromination is very selective and shows a high yield in C—Br groups. The most convenient bromination process was found using bromoform, which was thus preferred to elemental bromine, allyl bromide, vinyl bromide or tert-butylbromide. The bromoform process give yields in C—Br up to 40 C—Br or more, with only 2-3% co-introduction of O-functionalities whereas allyl bromide results in yields of about 20 C—Br and more, but in more than 10% oxygen-containing by-products. C—Br groups serve as anchoring points for grafting of molecules, oligomers and pre-polymers of diole or diamine character.