St. Weidner

Selective surface functionalization of polyolefins by plasma treatment followed by chemical reduction

Abstract Polymer surfaces can be finished with functional groups upon exposure to a plasma. Species of the plasma gas are attached at surface carbon atoms, forming functional groups of different composition. To produce a modified polymer surface with a high density and homogeneity of hydroxyl groups only, the oxygen-plasma-formed oxygen functional groups were chemically reduced by diborane and LiAlH4 with yields of 10 to 11 OH groups per 100 carbon atoms in the 3 to 5 nm near-surface layer as detected by X-ray photoelectron spectroscopy (XPS). The identification of hydroxyl groups was performed by means of attenuated total reflectance–Fourier transform infrared spectroscopy and XPS.

Chemical reactions at polymer surfaces interacting with a gas plasma or with metal atoms — their relevance to adhesion

Abstract 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 Langmuir–Blodgett 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/ionization–time-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.

Plasmaoxidative and Chemical Degradation of Poly(ethylene terephthalate) Studied by Matrix-assisted Laser Desorption/Ionization Mass Spectrometry

A model system of linear ethylene glycol terminated poly(ethylene terephthalate) oligomers of the general formula H–[GT]n–G (where G is an ethylene glycol unit and T represents a terephthalic acid unit) was synthesized and exposed to an oxygen plasma. The degradation of the oligomers was investigated by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). The results were compared to a chemical degradation of commercial poly(ethylene terephthalate) which had been exposed to an aqueous solution of natural chalk for many years. In both cases an ester scission process was found which generates terephthalic acid terminated oligomers (H–[GT]m–OH) and decomposes linear ethylene glycol terminated oligomers (H–[GT]m–G). The scision of the ester bonds during the chemical treatment additionally leads to the formation of T–[GT]m–OH like oligomers and to a strong decrease of the number of cyclic oligomers ([GT]m). Furthermore during the plasma treatment an additional formation of [GT]m–G like cyclic oligomers was observed.

Improvement of bonding properties of polypropylene by low-pressure plasma treatment

Abstract Bonding of rapidly expanded polypropylene (PP) remains poor unless a special pretreatment of the PP substrate is performed. In the past mainly chemical primers were used. A replacement of this type of pretreatment by plasma pretreatment offers advantages from an environmental viewpoint. From the present investigations, where the plasma parameters were systematically varied, it can be concluded that a very short exposure to the plasma and new curing conditions led to the best results. The lap shear (bond) strength after plasma pretreatment became even higher than after a primer pretreatment. ESCA analyses and ultrasonic C-scan analyses of the fracture areas were performed. It was found that a long exposure to the plasma weakened the boundary layer. The C-scan analyses showed that the failure of the plasma pretreated bonds appears different from that at the primer pretreated specimens. Another important finding — especially from a practical viewpoint — was that the beneficial effect of the plasma pretreatment before bonding lasted in a laboratory environment much longer than after a primer pretreatment.

Liquid Adsorption Chromatography near Critical Conditions of Adsorption Coupled with Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry

Abstract Chemical heterogeneities and molecular weight distributions of poly(ethylene oxide) (PEO)-co-polymethylene (PM) model oligomers, which are relevant to the synthesis of commonly used tensides, were investigated. For analytical characterization, the well-known principle of liquid adsorption chromatography at ‘critical conditions’ (LACCC) was modified. Near the critical conditions of adsorption of the PEO unit, e.g., at slight adsorption conditions of PM, the copolymers could be separated according to their PM chain length. The eluates were separated and single fractions of each peak were continuously transferred onto the MALDI target by means of a commercially available device. Simultaneously, the MALDI matrix solution was continuously added with a second pump. This procedure offers the possibility of the formation of homogeneous matrix-polymer textures. By MALDI-MS a complete characterization of the chemical composition (PEO and PM chain length) of each peak could be achieved. The obtained MALDI mass spectra of the eluates at different retention times could be used for the molecular weight calibration of the LAC system. In this way, an additional application of SEC, as in conventional 2D-chromatography, was avoided by using the MALDI method as quasi chromatographic separation.

Modelling plasma-induced reactions on polymer surfaces using aliphatic self-assembling and LB layers

Abstract Oxygen functional groups can be formed at the surface of aliphatic monolayers, oligomers and polymers by short-time oxygen low-pressure glow discharge plasma treatment without degradation of chains and building blocks. The molecular orientation is nearly preserved if not more than ~25% of all carbon atoms are functionalized. Longer exposure to oxygen plasma results in further increases in functionalization accompanied by cross-linking, degradation and disorientation of molecular chains. Polymer-specific degradation was exemplified by the cracking and splitting up of aromatic rings as seen in the NEXAFS spectra and by random cracking or depolymerization of macromolecular chains evidenced by an occurrence of new fragment peaks and by shifts in the molecular weight distribution as detected by MALDI mass spectrometry.

Plasmaoxidative Degradation of Polymers Studied by Matrix‐assisted Laser Desorption/Ionization Mass Spectrometry

Different low molecular mass model polymers, such as poly(ethylene terephthalate), poly(methylmethacrylate), poly(ethylene glycol) and aliphatic poly(carbonate) were exposed to an oxygen radio frequency plasma for different times. The change of the molecular weight distribution and the formation of different functionalized fragments of these low molecular mass polymers were investigated by matrix-assisted laser desorption/ionization mass spectrometry. The majority of the polymers show the expected strong decrease of the molecular masses in the spectra of their soluble fraction indicating a degradation of the polymers. Simultaneously, different types of degradation products were found, which can give detailed information on the chain scission processes occuring in an oxygen plasma.

Epoxidation of Polydienes Investigated by MALDI-TOF Mass Spectrometry and GPC-MALDI Coupling

Abstract Polybutadienes (PB) and polyisoprenes (PI) with various molecular weights and polydispersities were epoxidized applying two different synthetic routes. The degree of epoxidation was determined by means of matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF MS). It has been found that the degree of functionalization of polybutadiene is generally lower than that for polyisoprenes. Additionally, PB shows lower degrees of epoxidation at higher masses, whereas the level of functionalization of PI was unaffected by the molar mass. Coupling of gel permeation chromatography (GPC) with MALDI-TOF mass spectrometry was successfully used for the investigation of epoxidized polymers with higher masses. Using mass spectra of single fractions, the degree of epoxidation could be determined and applied for a calibration of GPC.

Derivatization of Double Bonds Investigated by Matrix‐assisted Laser Desorption/Ionization Mass Spectrometry

Hexatriacontane (C36) was exposed to vacuum ultraviolet radiation. The formation of double bonds was investigated by derivatization, i.e. these olefinic structures were transformed by epoxidation in order to determine their number in the molecule. The extent of the reaction was determined by means of matrix-assisted laser desorption/ionization mass spectrometry. In order to confirm the results a model compound (squalene) containing six double bonds was also investigated. As a result of the derivatization an easier ionization of functionalized substances caused by an increase in their polarity was found. As a result, the sample preparation was changed and the use of silver salts for a better ionization of non-polar alkanes could be avoided.

Polymer Reference Materials: Round-Robin Tests for the Determination of Molar Masses

Abstract This article reports round-robin tests dealing with static light scattering (LS), size exclusion chromatography (SEC), and viscometry measurements of 10 reference materials of various polymer classes: poly(styrene)s (PS), poly(methyl methacrylate)s (PMMA), poly(ethylene oxide)s (PEO), and poly(lactide)s (PLA). In the certificates, molar masses and intrinsic viscosities as well as their uncertainties are specified. Additional values from nuclear magnetic resonance (NMR), infrared spectroscopy (IR), differential scanning calorimetry (DSC), melt flow index (MFR), and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS), which were determined exclusively in BAM laboratories are given, as well. These results represent the base for the BAM certification work of creating polymer reference materials according to the BAM Guidelines for the Production and Certification of BAM Reference Materials, a version of the European guidelines (BCR/01/97). These samples are available for third-party user.