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- Polymer-Oberflächenfunktionalisierung (2)
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- Carbon nanofibres (1)
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Eingeladener Vortrag
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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.
Underwater plasma and glow discharge electrolysis are interesting new methods for polymer surface functionalization. The achievable content of O-containing functional groups exceeds that of oxygen glow discharge gas plasmas by a factor of two (up to ca. 56 O/100 C). The percentage of OH groups among all O-containing groups can reach 25 to 40%, whereas it is about 10% in the gas plasmas. Addition of hydrogen peroxide increases the fraction of OH groups to at most 70% (27 OH/100 C). The liquid plasma systems are also able to polymerize acrylic acid and deposit the polymer as very thin film on substrate surfaces or membranes, thereby retaining about 80% of all COOH functional groups (27 COOH/100 C).
Carbon nanotubes (CNTs) and carbon nanofibers (CNFs) have electron affinities similar to those of fullerenes C60 and C70 and they are therefore capable of acting as radical scavengers in free radical chain reactions, including polymerisation and the thermo-oxidative degradation of polymers. It is assumed that the CNTs and CNFs used as integral part of polymer composites are able to exhibit an antioxidant effect in these materials because of their radical accepting capacity.
To examine this presumption the antioxidative activity of original and purified commercial multiwall carbon nanotube MWCNT and carbon nanofibre of platelet structure CNF-PL has been studied by means of a model oxidation reaction of cumene initiated (2,2'-azobisisobutyronitrile, AIBN) in liquid phase. This model reaction was designed to simulate the thermo-oxidative processes in carbon-chain polymers and allows comparison and transfer of obtained results to a polymer system.
Kinetic measurements of oxidation rates showed that the effect of inhibition for the model oxidative reaction in the presence of the original and purified MWCNT and CNF-PL strongly depends on the presence of metals (Co, Fe) in the nanoparticles. Rates of oxidation Wo2 (CNT;CNF) observed for the unrefined samples are result of the two competing rates - rate of inhibition Winh.(CNT; CNF) caused by structures of the CNT or CNF and the rates of initiation Wi(M) due to the following interaction: ROOH + M (Co;Fe) i.e, Wo2 ~ W inh (CNT;CNF) + Wi(M).
The effective rate constants for the addition of cumyl radicals (R.) to MWCNT and CNF-PL have been determined. These constants reduced to the same concentration (0.5wt.%) and temperature (60°C) units have magnitudes: k1(MWCNT) [MWCNT] = (2.8 ± 0.3) x104 s-1and k1(CNF) [CNF] = (6.0 ± 1.0) x103 s-1. Thus, the effective rate constant, reflecting the antioxidative activity for the CNT, is five times higher than that for the CNF, is about equal to the rate constant for HAS Chimassorb 2020: k1(Chim.2020)[Chim. 2020] = (2.2 ± 0.3) x104 s-1, is ten times less than that for the HAS Chimassorb 119FL: k1(Chim.119FL)[Chim. 119FL] = (2.8 ± 0.3) x105 s-1 and is about forty times less than that for the case of fullerene C60: k1(C60)[C60](353K) = (1.2 ± 0.2) x106 s-1.
The kinetic data obtained specify the level of original antioxidative activity of MWCNTs and CNFs and scope of their rational use in polymer composites. It is believed that the results will be helpful for designing optimal profile of polymer composites filled by the CNT/CNFs.