Nicolai Aust

Studies on the effect of electron beam radiation on the molecular structure of ultra-high molecular weight polyethylene under the influence of α-tocopherol with respect to its application in medical implants

Ultra-high molecular weight polyethylene (UHMW-PE) is being used successfully for articulating surfaces in joint endoprostheses, especially for cups of total hip endoprostheses. Sintered specimens containing various amounts of α-tocopherol (vitamin E) as a biocompatible stabilizer, were irradiated in nitrogen atmosphere as well as in air with various dosages of electron beam radiation. Size exclusion chromatography (SEC) was used to analyze the soluble fractions of the UHMW-PE samples according to their molecular weight distribution prior to and after irradiation. In nitrogen atmosphere the radiation-induced crosslinking showed to be dependent on the added amount of α-tocopherol in the sintered specimens. With an increasing content of α-tocopherol, the stabilizer acted as a scavenger for free radicals. Thus, the crosslinking was more and more hindered. The same effect was observed on the samples irradiated in air, where, in addition to the crosslinking process, oxidative molecular degradation occurred. The highest extent of crosslinked material was yielded with unstabilized samples in nitrogen atmosphere.

SEC of Ultra-high Molar Mass Polymers: Optimization of Experimental Conditions to Avoid Molecular Degradation in the Case of Narrow Polystyrene Standards

Abstract Size exclusion chromatography (SEC) of commercially available polystyrene standards with narrow distribution of molar mass and weight average molar mass (Mw ) up to 17,000 kg/mol was carried out in order to determine the influence of the concentration, injected volume of sample solutions, average size of the gel particles of the stationary phase within the columns, flow rate, as well as the system filters on the detected molar mass and its distribution. Low-angle laser light-scattering (LALLS) was used online to measure Mw and detect the drop in molar mass in the case of molecular degradation due to mechanical forces acting in the SEC system. For samples with Mw < 5,000 kg/mol, no molecular degradation could be observed at “standard conditions” (flow rate = 0.5 mL/min, concentration = 0.25 g/L, columns with 10 and 20 μurn diameter gel particles and exclusion limits varying from 10,000 to 100,000 kg/mol for polystyrene (PS), injection volumes from 100 to 300 μL). Analysing ultra-high molar mass polystyrene with Mw > 5,000 kg/mol under these conditions, molecular degradation occurred. By systematic variation of the experimental conditions, the extent of molecular degradation could be minimized. These conditions were flow rate ≤ 0.2mL/min, diameter of the gel particles of 20 μm, injected volume of about 100 μL and concentration of injected sample solution of ≤ 0.1 g/L.

Thermal cure kinetics of epoxidized linseed oil with anhydride hardener

The thermal cure kinetics of an epoxidized linseed oil with methyl nadic anhydride as curing agent and 1-methyl imidazole as catalyst was studied by differential scanning calorimetry (DSC). The curing process was evaluated by non-isothermal DSC measurements; three iso-conversional methods for kinetic analysis of the original thermo-chemical data were applied to calculate the changes in apparent activation energy in dependence of conversion during the cross-linking reaction. All three iso-conversional methods provided consistent activation energy versus time profiles for the complex curing process. The accuracy and predictive power of the kinetic methods were evaluated by isothermal DSC measurements performed at temperatures above the glass transition temperature of the completely cured mixture (Tg∞). It was found that the predictions obtained from the iso-conversional method by Vyazovkin yielded the best agreement with the experimental values. The corresponding activation energy (Ea) regime showed an increase in Ea at the beginning of the curing which was followed by a continuous decrease as the cross-linking proceeded. This decrease in Ea is explained by a diffusion controlled reaction kinetics which is caused by two phenomena, gelation and vitrification. Gelation during curing of the epoxidized linseed/methyl nadic anhydride system was characterized by rheological measurements using a plate/plate rheometer and vitrification of the system was confirmed experimentally by detecting a significant decrease in complex heat capacity using alternating differential scanning calorimetry (ADSC) measurements.

Application of size-exclusion chromatography to polymers of ultra-high molar mass

This article describes the experimental conditions that should be applied to avoid molecular degradation in size-exclusion chromatography of polymers of ultra-high molar mass (weight-average molar mass M(w)>5000 kg/mol). The applicability of the optimized experimental conditions is demonstrated using polystyrene as a model substance, but also by using polymers of biochemical and biophysical interest, such as polyethylene of ultra-high molar mass, suitable, e.g., for articulating surfaces in joint endoprotheses, and natural rubber, the most important commercial source for products like septa or medical gloves.

Molecular Characterization of Ultrahigh Molar Mass and Soluble Fractions of Partially Cross-Linked Polyethylenes

A reliable method is described for analyzing ultrahigh molar mass polyethylene (UHMW-PE) with a weight-average molar mass of 5700 kg/mol and molar mass fractions up to 50,000 kg/mol by size exclusion chromatography (SEC) without molecular degradation as was proved by comparison of on-line and off-line viscometry. The use of low-bleed SEC columns (PLgel 20 µm Mixed-A LS), especially designed for on-line light-scattering detectors, allowed SEC to be coupled to a low-angle laser light-scattering detector without a filter ahead of the light-scattering cell. With this procedure, molded specimens of UHMW-PE, which are used for medical implants, such as hip endoprostheses, containing various amounts of f -tocopherol as a biocompatible stabilizer, were investigated. Furthermore, the effect of sterilization of the samples by n -irradiation ( 60 Co, 1.33 MeV, absorbed dose: 25 kGy) was studied with respect to the content of insoluble cross-linked material and the molar mass distribution of the soluble fraction.

Molecular Characterization of Polyolefins

Abstract The results of a round-robin test carried out by IUPAC Working Party IV. 2.2 on Molecular Characterization of Commercial Polymers with two grades of polypropylene with narrow (“controlled rheology” PP) and broad molar mass distribution (MMD) are summarized. The interlaboratory agreement for M w expressed by the relative standard deviation, σ(%), is ± 6 for PP with broad MMD (M w/M n = 5.7) and is better than for the controlled rheology grade [σ(%) = ± 11.7, M w/M n = 2.55]. This finding is explained by the influence of peak broadening in size exclusion chromatography (SEC). The measured MMDs are discussed with respect to the frequency dependence of the complex shear modulus and to differentiation between the respective grades by process control data from injection molding. A comparison of procedures for the determination of MMD and distribution of short-chain branching in polyethylene of medium and high density (PE-MD and PE-HD) with an alternative sequence of fraction is presented. The direct ex...

Determination of the Distribution of Molar Mass and Comonomer Content of Medium Density Polyethylene. Comparison of Procedures with Alternative Sequence of Fractionation

Abstract A polyethylene copolymer of medium density, synthetized from ethene and 1-hexene has been characterized according to its molar mass M distribution and comonomer mass content w 0 by means of Holtrup fractionation, size exclusion chromatography coupled with low-angle laser light scattering (SEC/LALLS), and preparative and analytical temperature rising elution fractionation (TREF). The sequence of these analytical methods has been varied systematically, and the results obtained are represented by contour diagrams in the plot w 0 vs. log m. Preparative TREF with SEC/LALLS measurements on the respective fractions (conventional Procedure) leads to poor resolution of the distribution of M and w 0. Better results were obtained by applying a Holtrup fractionation (including SEC/LALLS) with subsequent preparative TREF fractionation of the Holtrup fractions. Replacing the preparative TREF fractionation by analytical TREF gave more detailed information of the comonomer content, which resulted in a refined co...

Synthesis, characterization, and properties of isocyanate-free urethane coatings from renewable resources

Isocyanate-free urethane coatings were prepared from renewable resources through the aminolysis reaction of five-membered cyclic carbonate from linseed oil and phenalkamine from cashew nut shell liquid. The five-membered cyclic carbonate functional reactants were synthesized from epoxidized linseed oil through carbon dioxide insertion. The porous-free urethane coatings were prepared on aluminum substrates using synthesized carbonated linseed oil and phenalkamine at three different mixing ratios (1:0.5, 1:0.75, and 1:1.25) and at three different cure temperatures (60, 80, and 100°C). The formation of urethane linkages and the rate of cyclic carbonate conversion during the aminolysis reaction were characterized using FTIR spectroscopy. The thermal behavior of the cured urethane films was characterized using thermogravimetry. The effects of amine concentration and cure temperature on the optical, mechanical, and end-use properties of the coatings were evaluated by measuring specular gloss, solvent resistance, crosshatch, solvent swelling, and impact strength. The increase in cure temperature was directly proportional to the rate of aminolyis reaction and also to the coating performances.

Distribution of molar mass and branching index of natural rubber from hevea brasiliensis trees of different age by size exclusion chromatography coupled with online viscometry

Natural rubber from hevea brasiliensis trees (Thailand, RRIM 600 clone) of different age (8, 20, and 35 years) were characterized by size exclusion chromatography (SEC) coupled with online viscometry with respect to their distribution of molar mass and branching index at a temperature of 70 °C using cyclohexane as solvent. The natural rubber samples were purified by washing with an aqueous solution of sodium dodecylsulfate and subsequent saponification. With this procedure physical branching points caused by phospholipids, proteins and hydrophobic terminal units, mainly fatty acids, of the natural rubber (cis-1,4-polyisoprene) molecule, could be removed leading to completely soluble polymer samples. All samples investigated possess a very broad (10 to 50,000 kg/mol) and distinct bimodal molar mass distribution. With increasing age of the trees the peak area in the low molar mass region decreases favoring the peak area in the high molar mass region. By plotting the branching index as a function of the molar mass it could be shown that branching of the natural rubber increases with both, the molar mass and the age of the trees.

Processing and Characterization of Natural Fibre Reinforced Composites Using Lignin Phenolic Binder

Natural fibre-reinforced composites were prepared using a lignin-phenolic resol binder, in which 30 wt.-% of the phenol was substituted by a modified lignosulfonate from renewable resources. The reinforcement of the lignin phenolic binder matrix was accomplished by different natural fibres. The non-woven flax, flax/wood and flax/kenaf fibre mats were impregnated with the lignin-phenolic binder and they were compression-moulded at elevated temperature. The curing condensations of the synthesized lignin phenolic resins were characterized by differential scanning calorimetry. Water absorption, thickness swelling and flexural strengths of the natural fibre-reinforced composites were determined, as well as their thermal stability using thermogravimetric analysis. The flexural strengths of the flax/wood and flax/kenaf composites were higher than those of the flax reinforced composite. Morphological analysis using scanning electron microscopy showed an enhancement in fibre/matrix interaction for the flax/wood and flax/kenaf fibre mixtures.