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- Spacer (4)
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- Plasma bromination (3)
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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.
A new concept for molecular interface design in metal-polymer systems is presented. The main features of this concept are the replacement of weak physical interactions by strong covalent bonds, the flexibilization of the interface for compensating different thermal expansions of materials by using long-chain flexible and covalently bonded spacers between the metal and the polymer as well as its design as a moisture-repellent structure for hindering diffusion of water molecules into the interface and hydrolysis of chemical bonds.
For this purpose, the main task was to develop plasmachemical and chemical techniques for equipping polymer surfaces with monotype functional groups of adjustable concentration. The establishing of monotype functional groups allows grafting the functional groups by spacer molecules by applying usual wet-chemical reactions. Four processes were favoured for production of monotype functional groups by highly selective reactions: the plasma bromination, the plasma deposition of plasma polymers, the post-plasma chemical reduction of O-functionalities to OH-groups, and the chemical replacement of bromine groups by NH2-groups. The grafting of flexible organic molecules as spacers between the metal layer and polymer improved the peel strength of the metal. To obtain maximal peel strength of aluminium coatings to polypropylene films and occurrence of cohesive failure in the polypropylene substrate, about 27 OH groups per 100 C-atoms or 6 COOH groups per 100 C-atoms were needed. Introducing C6-11-aliphatic spacers 1 OH or COOH group per 100 C-atoms contributed about 60% of the maximal peel strength of the Al-PP system, i.e. 2 or 3 spacer molecules per 100 C-atoms were sufficient for maximal peel strength.
The production of chemically-defined plasma polymers and the introduction of monotype functional groups onto polymer surfaces are described. One method is to lower the energetic level of low-pressure plasmas. Pressure- and plasma-pulsed plasmas were successfully tested for the production of chemically-defined plasma polymers by increasing the monomer supply during the plasma-off period. Well-defined ultra-thin polymer films with regular structure were deposited from atmospheric plasmas by electrospray techniques. Post-plasma wet-chemical processing was also applied, as were gas/liquid-based aerosols and underwater plasmas.
Plasma-chemical bromination has been shown to be well-suited for highly selective and efficient C–Br-functionalization of polyolefin surfaces and has now been applied to graphitic materials, including highly oriented pyrolytic graphite (HOPG), natural graphite, carbon nanotubes, and graphitized carbon fibres. In contrast to the radical substitution of hydrogen by bromine at hydrogen-terminated sp3-hybridized carbon, bromination of all-carbon bound sp2-hybridized carbon in graphene requires addition to aromatic double bonds. The related change of sp2 to sp3 hybridization of the carbon atoms involved helps to explain experimental results of a loss in surface planarity on brominated HOPG. Bromine concentrations of up to 50 bromine atoms per 100 C-atoms were achieved in elemental bromine vapour under low-pressure plasma conditions using low-energetic inductively coupled radio-frequency plasma excitation. The time scale of the bromination progress was found to increase with the specific surface area of the material. The results of plasma-chemical bromination are compared to that of wet-chemical electrophilic addition reactions, which achieved 28 Br per 100 C for MWCNT. The resulting C–Br bonds from plasma-chemical processing of graphitic materials were shown to be well-suited for hydroxyl functionalization and for grafting of organic molecules by nucleophilic substitution with 1,6-diaminohexane and (3-aminopropyl)triethoxysilane.
Polyglycerol (PG) coated polypropylene (PP) films were synthesized in a two-step approach that involved plasma bromination and subsequently grafting hyperbranched polyglycerols with very few amino functionalities. The influence of different molecular weights and density of reactive linkers were investigated for the grafted PGs. Longer bromination times and higher amounts of linkers on the surface afforded long-term stability. The protein adsorption and bacteria attachment of the PP-PG films were studied. Their extremely low amine content proved to be beneficial for preventing bacteria attachment.
Activators regenerated by electron transfer–atom radical transfer polymerization (ATRP) as a controlled living polymerization are distinguished by their acceptance of small amounts of transition-metal complexes and oxygen and by their tolerance of reducing agents at a high concentration. The precondition of all ATRP applications is the use of homolytic or heterolytic cleavable halides as a dormant species; this allows the propagation of monomer chains. Hence, alkyl bromides are slightly cleavable and are the preferred initiators for ATRP. The bromination of polymer slides used as macroinitiators was carried out under gentle bromoform plasma conditions. This led to an oxidation-resistant stable bromine layer. More than 20 bromines per 100 carbons on the polymer scaffold were permanently bound to the substrate after plasma treatment. The resulting amounts of secondary and tertiary bromines on the polymer scaffold exhibited a suitable macroinitiator concentration for the surface-initiated polymerization of methyl methacrylate and glycidyl methacrylate.