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Bei der Abschätzung der Beständigkeit von Kunststoffen gegenüber niedermolekularen Flüssigkeiten ist die Kohäsionsenergiedichte CED ein wichtiger Parameter auf molekularer Ebene. Die Vernetzungsdichte ν eines polymeren Festkörpers bildet hierbei einen bedeutsamen Kennwert, der darüber hinaus neben der Löslichkeit wichtige Eigenschaften wie thermische Stabilität und Steifigkeit des Materials charakterisiert. Für die experimentelle Bestimmung von CED und ν eignet sich die Untersuchung des Quellverhaltens in unterschiedlichen Lösungsmitteln. Es werden zwei Messmethoden miteinander verglichen und die Ergebnisse unter Berücksichtigung des molekularen Aufbaus von Proben aus Polyurethan (PU) und hochmolekularem – (HMWPE) und ultrahochmolekularem Polyethylen (UHMWPE) diskutiert. Zum Verständnis des Quellverhaltens dieser Polymerproben in niedermolekularen Lösungsmitteln werden neben den Messergebnissen Literaturdaten zu Grunde gelegt.
Due to their extreme high hydrogen contents, high molecular weight (HMW-) and ultra-high molecular weight (UHMW-) polyethylene (PE) are a comprehensible choice as neutron radiation shielding material in casks for storage and transport of radioactive materials. But as a direct consequence of inserting radioactive material in such casks, gamma radiation occurs. Hence, the impact of gamma radiation on the molecular structure of polyethylene has to be taken into consideration. Consequently, PE has to withstand any type of gamma radiation induced degradation affecting safety relevant aspects in order to be applicable for long term neutron radiation shielding purposes during the whole storage period (in Germany, for instance, up to 40 years). The scope of our investigation comprises an estimation of the impact of gamma radiation and temperature on the molecular and supra molecular structure of the two types of PE used as neutron radiation shielding cask components. A further point which is worth exploring is to what extent these changes are detectable by conventional analysis methods. Therefore, thermoanalytical measurements were performed such as differential scanning calorimetry (DSC), thermo mechanical analysis (TMA), dynamic mechanical analysis (DMA), and thermo gravimetric analysis (TGA). Additionally optical and weighing methods were applied. With those methods it is possible to detect structural changes in polyethylene induced by exposure to gamma radiation. The observed amounts of changes of the irradiated material are not safety relevant for the application of polyethylene as neutron radiation shielding material; moreover, some properties actually improve via irradiation.
Nanoparticulate networks as fillers for polymer based nanocomposites offer an optimized, surfactant free distribution of the inorganic component. Additionally, their non-aggregated but interconnected nanoparticulate subunits can lead to new properties owing to special inherent solid state properties and/or spacial restrictions to the polymeric parts. In particular, polymer nanocomposites based on poly(methyl methacrylate) (PMMA) or poly(hydroxyethyl methacrylate) (PHEMA), filled with different nanoparticulate networks of AlOOH were synthesized using two advanced synthesis techniques. An ex situ method, in which a preformed AlOOH-gel was solvent exchanged by methyl methacrylate (MMA) and an in situ method, in which the AlOOH scaffold was prepared within hydroxyethyl methacrylate (HEMA), have been used to produce the materials described here through polymerization. On the MMA based composites the particles were removed by etching, yielding porous polymers. Promising for a broad variety of crystallisable networks within polymers, the phase transition of amorphous AlOOH within PMMA towards crystalline boehmite was performed using a hydrothermal method. The materials were characterized using transmission electron microscopy, X-ray diffraction, gel permeation chromatography, differential scanning calorimetry, IR-spectroscopy, UV/Vis spectroscopy and hardness measurements. Special material properties were investigated using thermogravimetry, thermomechanical analysis and dynamic mechanical analysis. Furthermore, interactions between PMMA segments and the AlOOH network were detailed studied using dielectric spectroscopy.
The special properties of high molecular weight polyethylene (HMW-PE) and ultra high molecular
weight polyethylene (UHMW-PE) result basically from their extreme chain lengths and their high
degree of crystallinity. As high-performance polymers, they are used for a variety of applications.
UHMW-PE in particular is often utilized for endoprothesis (due to its excellent slip and wear
properties) and due to its high hydrogen content as a neutron moderator in casks for storage and
transport of radioactive materials.
To prepare the material for instance for its use as a total joint replacement, it is exposed to radiation
for several reasons, such as sterilization and crosslinking, leading to partial improvement of the
mechanical properties (e.g. fracture toughness, crack propagation resistance, wear resistance) and
better chemical stability.
To be applicable for long term radiation shielding purposes for instance over a period of 40 years,
PE has to withstand any type of degradation affecting safety relevant aspects.
The scope of our investigation comprises an estimation of the radiation impact on the molecular and
supra molecular structure of two types of PE and to what extent these changes are detectable by
thermo-analytical (TA) methods, such as Differential Scanning Calorimetry (DSC), Thermo
Mechanical Analysis (TMA), Dynamic Mechanical Analysis (DMA) and Thermo Gravimetric
Analysis (TGA). Additionally FT-IR spectroscopy as well as density and gas sorption
measurements were carried out.
Due to the poor solubility of HMW-PE and UHMW-PE, some classical analytical techniques are
not applicable. But TA-methods represent a feasible approach to detect structural and
morphological features of these materials as well as changes caused by external influences, such as
thermal treatment and/or irradiation. With the combination of the applied TA-techniques it is
possible to distinguish between crosslinking and degradation.
Nanocomposites with a predominantly exfoliated morphology were prepared by melt blending of organically modified doubled layered hydroxide (LDH) and low density polyethylene (PE). The prepared materials were investigated by differential scanning calorimetry (DSC) and in detail by dielectric spectroscopy. The DSC experiments show that the degree of crystallinity decreases linearly with increasing content of the LDH. The extrapolation of this dependence to zero results in a limiting concentration of ca. 50 wt % LDH were the crystallization of PE is completely suppressed by the nanofiller. The dielectric response of pure polyethylene shows different weak relaxation processes. The intensity of dynamic glass transition (β-relaxation) increases with the concentration of LDH. This is attributed to the increasing concentration of the exchanged anion dodecyl benzene sulfonate (SDBS). The SDBS molecules are strongly adsorbed at the exfoliated LDH layers. Therefore, a detailed analysis of the β-relaxation provides information about the structure and the molecular dynamics in the interfacial region between the exfoliated LDH layers and the polyethylene matrix. This analysis shows that the β-relaxation region consists of two processes: both of them obey glassy dynamics. The difference in corresponding Vogel (ideal glass transition) temperatures is about 30 K. The relaxation process at lower frequency is assigned to polyethylene segments with a reduced molecular mobility close to LDH layers. The process at higher frequencies is related to polyethylene segments in a farer distance for the surface of the nanofiller. In the nanocomposite materials an additional process due to Maxwell-Wagner-Sillars (MWS) polarization was observed. The time constant of this MWS process can be correlated with characteristic length scales in nanocomposites and therefore provides additional information on dispersion and delamination/exfoliation of the LDH layers in these materials.
Radiation induced structural changes of HMW-PE and UHMW-PE as detected by thermo-analytical methods
(2009)
Thermosetting moulding compounds are synthetic materials which can be easily formed in the molten state and achieve high temperature stability due to a cross-linking process which takes place during manufacture. To ensure thermal and mechanical properties, post-curing of moulded phenolic resin components is necessary for high quality applications. In the industrial practice, post-curing time-temperature-programs are heuristically acquired. In this paper, dynamical mechanical thermal analysis is employed to determine optimal post-curing conditions for injection moulded parts from phenolic resin.
Nanocomposites of bisphenol A polycarbonate with 2, 4, 6, and 15 wt% multiwall carbon nanotubes (MWNT) and their use in fire retardancy are investigated. Their thermal behavior and pyrolysis are characterized using thermogravimetry, differential scanning calorimeter, oscillatory shear rheology, and dynamic mechanical analysis. The flammability is addressed using LOI and UL 94; the fire behavior, with a cone calorimeter using different irradiation. With increasing MWNT content the storage modulus is increased (10-20%) and melt viscosity increases by several orders of magnitude, particularly for low shear rates. The melt flow, dripping, and deformation during fire are hindered, which influences UL 94 and cone calorimeter results. The peak heat release rate is reduced up to 40-50% due to an improved barrier for small amounts (2 wt%) of MWNT and for low irradiation, whereas the effect is reduced for increasing irradiation and nearly vanishes for increasing filling. Adjuvant but also deleterious mechanisms result in the complex dependency on the MWNT content. Significant flame retardancy effects are specific and limited to only some fire properties. This study allows the materials' potential for implementation in different fire scenarios and tests to be assessed and provides insight into active mechanisms.