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- Functional groups (3)
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Modelling plasma-induced reactions on polymer surfaces using aliphatic self-assembling and LB layers
(1998)
The influence of different types of low and atmospheric pressure plasma on poly(ethylene terephthalate) (PET) has been studied in terms of changes in molar mass and molar mass distribution. Apart from a variation of plasma gases (oxygen, helium) different types of plasma (microwave, radio frequency, corona discharge) were used for the plasma surface modification. The changes in molar mass and types of functional end groups of lower molar mass products were investigated by means of matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOFMS), whereas the high-molar mass fraction was analyzed by means of size-exclusion chromatography (SEC). The formation of crosslinked products during exposure to a helium plasma, which emits preponderately energy-rich and intense ultraviolet radiation, was proved by means of thermal field-flow fractionation (ThFFF). This method combined with a multiangle laser light scattering (MALLS) detector allows detection of weakly crosslinked polymers and microgels. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1639-1648, 1998
Several possibilities exist to produce a modified polymer surface with a high density of only one sort of functional group such as: (i) the plasma grafting of unfragmented monomer molecules and their polymerization forms OH, NH2, COOH groups, etc. in concentrations of approximately 25 groups per 100 C atoms; (ii) selective plasma bromination provides 1025 CBr groups; (iii) the plasma oxidation of polymer surfaces in an O2 plasma followed by the chemical reduction of all O-containing groups to OH groups by diborane, vitride (Na complex) or LiAlH4 yields 914 OH groups per 100 carbon atoms; and (iv) the grafting of spacers with different endgroups onto OH or CBr groups produces 710 spacer molecules/100 C. This work was focused on the formation of thin plasma deposited polymer layers with a maximum of (homo)functional groups and with a minimum of chemical irregularities using the pulsed plasma technique. The monomers were allylalcohol, allylamine, acrylonitrile and acrylic acid. The further intent was to study the interactions of functional groups (OH, COOH, NH2) and deposited metals (Cr, Al, Ti). It was expected that more basic (NH2), weakly basic or neutral (OH) or more acidic (COOH) groups would show different interactions and chemical reactions with metal atoms.
Functional groups bearing monomers were pulsed plasma polymerised with a degree of retained functional groups of 5591%. The following functional groups could be produced at a maximum of: 30 OH, 18 NH2, and 24 COOH per 100 C atoms. A plasma-initiated radical copolymerisation could also be realised using the functional group bearing monomers as a source of functionalities, olefins as chain-extenders and dienes as chemical cross-linkers. The peel strengths of Al layers on such plasma polymers were correlated to the type and density of functional groups.
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).
Polymers synthesized with plasma techniques are very interesting materials for electronic, optic, and bio compatible applications. Thin films of plasma polymers shows a good adhesion to metals, glass, or other polymers. But the supramolecular structure, the durability, and the chemical and mechanical behavior of these polymers is poorly understood. Therefore dielectric investigations are carried out to study the dynamic behavior of the plasma polymers. As polymer system allyl alcohol/alkene is chosen to get polymers with a defined concentration of hydroxyl groups. The dielectric investigations shows several relaxation processes and a dependency of the dielectric parameters from the ratio of allyl alcohol in the polymer is observed. This results indicated that the alkene monomers were assembled continuous into the polymer matrix.
The chemical and morphological stabilities of polymer segments in the near-surface layer were investigated by spectroscopic methods such as X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy. Model studies were undertaken with LangmuirBlodgett films, self-assembled monolayers and oligomer films. For thin polymer layers (30 to 500 nm), the changes in molecular-weight distributions of some polymers were investigated systematically by size exclusion chromatography, matrix-assisted laser desorption/ionizationtime-of-flight mass spectrometry and thermal-field flow fractionation for oxygen- and helium-plasma exposures. The polymer surfaces were found to be relatively stable at exposure to an oxygen low-pressure plasma up to ca. 2 s. This is important information to get maximum adhesion to metals in composites. In correlation to their redox potentials, potassium, aluminium and chromium react with oxygen functional groups at the polymer/metal interface. In a dedicated study, chromium was found to attack aromatic rings and form different reaction products.
Technical waxes and paraffins were investigated using supercritical fluid chromatography (SFC), matrix-assisted laser-desorption/ionization mass spectrometry (MALDI-MS) and size-exclusion chromatography (SEC). SEC enables the simultaneous determination of molecular masses up to high values and molecular mass distributions. The resolution of homologous species, however, is poor. In this respect the use of SFC is advantageous. Proper resolution of homologues may be provided up to Mr~1200 without great difficulty. To compare these chromatographic results MALDI-MS was applied for the determination of molecular masses and mass distributions in technical waxes for the first time. Using reflectron mode excellent MALDI mass spectra could be achieved in the molecular mass range up to 1000 Da. The mean values of the molecular masses calculated by MALDI-MS were in sufficient agreement with those of SFC. In the linear mode of MALDI-MS molecular masses could be determined to nearly 3000 Da. Beyond this mass range SEC still remains the method of choice. Advantages and difficulties of the used methods are discussed in this paper.
Polyethylene foils, used as greenhouse foils stabilized with various types of sterically hindered amine light stabilizers (HALS), showed an extraordinary prolonged lifetime on exposure to natural or artificial weathering if a gas-phase fluorination under low-pressure conditions was applied. The fluorination was performed using F2/N2 mixtures and provided ca. 50 F/100 C (fluorination degree ap25%). The lifetimes of fluorinated PE foils on exposure to artificial or natural weathering were increased at least by a factor of 2-4 as measured in terms of tensile strength and elongation at break.
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).
The hydrolytic degradation of technical poly(ethylene terephthalate) (PET) was investigated by means of different methods such as size-exclusion chromatography (SEC), viscometry, light-scattering, thin-layer chromatography, end-group titration, and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). The long-term degradation was simulated by exposing PET filament yarns to aqueous neutral conditions at 90°C for up to 18 weeks. By means of MALDI-MS and thin-layer chromatography, the formation of different oligomers was obtained during polymer degradation. As expected, an ester scission process was found generating acid terminated oligomers (H-[GT]m-OH) and T-[GT]m-OH and ethylene glycol terminated oligomers (H-[GT]m-G), where G is an ethylene glycol unit and T is a terephthalic acid unit. Additionally, the scission of the ester bonds during the chemical treatment led to a strong decrease in the number of cyclic oligomers ([GT]m). The occurrence of di-acid terminated species demonstrated a high degree of degradation.