Renate Mix

Fluorescence spectroscopic studies on plasma-chemically modified polymer surfaces with fluorophore-labeled functionalities.

Molecular engineering of polymer surfaces that includes the attachment of functional molecules to existing or previously generated reactive chemical groups like e.g., − OH, − NH2, or − CHO requires simple strategies and tools for the controlled generation of surface functionalities and their derivatization as well as for their identification and eventually quantification. Here, we systematically investigate the plasma-chemical surface modification of polypropylene films in combination with dansyl labeling chemistry and conventional, yet costly, XPS and highly sensitive fluorescence spectroscopy for the detection of surface groups. Based on these results, the potential of and requirements on the fluorometric characterization and quantification of surfaces functionalities are discussed.

A New Concept for Adhesion Promotion in Metal–Polymer Systems by Introduction of Covalently Bonded Spacers at the Interface

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.

Ultra-Thin Polymer Layer Deposition by Aerosol–Dielectric Barrier Discharge (DBD) and Electrospray Ionization (ESI) at Atmospheric Pressure

Polyolefins are chemically inert and do not adhere well to metals, polymers or inorganics. To overcome this problem, polyolefin surfaces were modified thermally, plasmachemically, or by flame treatment with different oxygen-containing groups, however, unfortunately, such treatments were accompanied by undesired, adhesion lowering polymer degradation. To solve this dilemma, solutions of synthetic polymers and copolymers were prepared, sprayed into the barrier discharge or electrosprayed without discharge and deposited as thin adhesion-promoting layers. The deposited polymer layers from poly(vinylamine), poly(ethylene glycol)–poly(vinyl alcohol) copolymers and poly(acrylic acid) were endowed with monotype functional groups. Using the aerosol — dielectric barrier discharge only a fraction of functional groups survived the deposition process in contrast to the electrospray in which all functional groups were retained.

Surface controlled fire retardancy of polymers using plasma polymerisation

Fire retardant coatings are deposited on polyamide-66 using plasma polymerisation. Chemical composition and thickness of deposits are adjusted varying the plasma treatment based on hexamethyldisiloxane mixed with oxygen. The fire retardancy performances are evaluated using a cone calorimeter. The correlation between fire retardancy and thickness as well as chemical composition is discussed.

Surface and Bulk Structure of Thin Spin Coated and Plasma-Polymerized Polystyrene Films

Polystyrene (PS) spin coated thin films were modified by O2 and Ar plasma as well as by UV irradiation treatments. The modified PS samples were compared with plasma polymerized and commercial polystyrene. The effects of plasma (O2 and Ar) and UV irradiation treatments on the surface and the bulk properties of the polymer layers were discussed. The surface properties were evaluated by X-ray Photoelectron Spectroscopy and Contact angle measurements and the bulk properties were investigated by FTIR and dielectric relaxation spectroscopy. As a result only one second treatment time was sufficient to modify the surface. However, this study was also dedicated to understand the effect of plasma and plasma irradiation on the deposited layers of plasma polymers. The dielectric measurements showed that the plasma deposited films were not thermally stable and underwent an undesired post-plasma chemical oxidation.

Influence of Differently Structured Aluminium–Polypropylene Interfaces on Adhesion

Polar groups were introduced on polypropylene surfaces for increasing the surface energy and the peel strength to evaporated aluminium layers. Three kinds of plasma processes were used for introducing such functional groups to polyolefin surfaces: low-pressure radio-frequency (RF) O2 plasma exposure, atmospheric-pressure dielectric-barrier discharge (DBD) treatment in air, and the deposition of allylamine plasma polymer. The amino groups of the allylamine plasma polymer were also used as anchoring points for chemical introduction of covalently bonded spacer molecules equipped with reactive endgroups. Thus, silanol endgroups of a covalently bonded spacer were able to interact with the evaporated metal layer. The Al–PP composites achieved a maximal peel strength of 470 N/m by exposing the polymer to the lowpressure O2 plasma and 500 N/m on exposure to the atmospheric DBD plasma. After allylamine plasma polymerization and grafting of spacers, the peel strength was usually higher than 1500 N/m and the composites could not be peeled.

Characterization of plasma-polymerized allyl alcohol polymers and copolymers with styrene

Pulsed plasma was used to initiate chemical copolymerization of allyl alcohol and styrene. In this way the concentration of OH groups at the surface of the copolymer layer could be adjusted from 0 (styrene homopolymerization) to 31 OH groups/100 C atoms (allyl alcohol homopolymerization). The copolymerization kinetics correspond to those of a pure chemical (radical) copolymerization. Thus, the variation of OH group density is only possible in a small range of comonomers ratio, otherwise styrene homopolymerization dominates.

Effect of Br gassing after Ar plasma treatment of polyolefins

For simulation and acceleration of artificial polymer ageing, polyolefin foils were exposed to low-pressure Ar plasma. Plasma particle bombardment and irradiation induce C–C and C–H bond scissions by σ → σ ∗ excitations on the surface and in near-surface layers. Consequently, radicals are generated. They react by recombination, cross-linking, metastable trapping of the radical site or formation of olefinic double bonds. The long-living and metastable trapped C-radicals as well as double bonds in polyolefins were immediately exposed to bromine vapour without breaking the vacuum after switching-off the plasma. These reactive sites rapidly react with the molecular bromine under formation of C–Br bonds. For 5 min of argon plasma exposure, the elemental concentration of bromine was 13% for polyethylene and 22% Br/C for polypropylene as analysed by X-ray photoelectron spectroscopy. Nevertheless, not all C radical sites have reacted with bromine. Later on, when the polyolefins brought in contact with ambient air, an additional post-plasma reaction of the remaining trapped radicals with oxygen was observed. The oxygen concentrations were lower after bromine gassing, thus repressing partially the post-plasma oxidation in the analysed layer (ca. 6 nm) by radical quenching. Such bromination took place either at the surface or in near-surface layers because the Attenuated Total Reflectance (ATR)-FTIR spectra (sampling depth ca. 2500 nm) did not show significant changes for argon plasma-treated PE foils with and without bromine vapour exposure. Further addition of bromine may also occur on C=C double bonds.

Anchoring of Fluorophores to Plasma-chemically Modified Polymer Surfaces and the Effect of Cucurbit[6]uril on Dye Emission

Polypropylene supports were functionalized by plasma-deposition of polymeric allylamine layers. The surface amino groups generated were wet-chemically reacted with xanthene dyes resulting in fluorescent polymer films. The effect of polymer-attachment of the dyes on their emission features was studied fluorometrically and different methods were tested to improve the fluorescence properties of the films. Modification with cucurbit[6]uril (CB6) yields a moderately enhanced fluorescence as well as an improved photostability. The observed effect is most likely due to CB6-induced rigidization of the linker molecules which seems to reduce fluorescence quenching dye–dye and fluorophore–surface interactions.

Fluorescence Measurements on Functionalized Polymer Surfaces—Problems and Troubleshooting

Plasma‐chemically tailor‐made polymer surfaces are of ever‐increasing importance to control surface properties in material science, as well as for (bio)analytical and biomedical applications. For the characterization of such systems, sensitive fluorescence techniques are attractive tools. To underline the potential and drawbacks of these strategies, this article addresses different problems that complicate fluorometric analysis. To overcome some of these limitations, such as nonspecific adsorption of unreacted fluorescent probes, we discuss potential troubleshooting, including the use of a chromogenic and fluorogenic pyrylium dye for the detection of amino functionalities at polypropylene surfaces.