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Blends of poly(ethylene terephthalate) (PET) and poly(ethylene naphthalate) (PEN) were processed into biaxially drawn films, and samples taken from the bi-oriented films were then investigated by dynamic rheology experiments in the melt state. Storage modulus G and loss modulus G were determined in the frequency range of 10-2-102 rad/s at temperatures between 260 and 300°C. Although the time-temperature superposition (TTS) principle was found to hold in the high frequency regime, a breakdown of TTS was observed at low frequencies, and the terminal behavior of the storage modulus G of the blends departs drastically from the terminal behavior observed for the blend components. This is caused by interfacial surface tension effects. The results indicate that despite the effect of transesterification reactions, the PET/PEN blend systems investigated consist of a microseparate phase of PEN platelets in a matrix of PET. This morphology is produced when the blends are processed into biaxially oriented PET/PEN films, and droplets of PEN are deformed into a lamellar structure consisting of parallel and extended, separate layers. The large interfacial surface area of the bi-oriented PET/PEN blends leads to remarkably strong interfacial tension effects in dynamic rheology measurements.
Study of carboxylic functionalization of polypropylene surface using the underwater plasma technique
(2009)
Non-equilibrium solution plasma treatment of polymer surfaces in water offers the possibility of more dense and selective polymer surface functionalization in comparison to the well-known and frequently used low-pressure oxygen plasma. Functional groups are introduced when the polymer surface contacts the plasma moderated solution especially water solutions. The emission of ions, electrons, energy-rich neutrals and complexes, produced by the ion avalanche are limited by quenching, with the aid of the ambient water phase. The UV-radiation produced in plasma formation also helps to moderate the reaction solution further by producing additional excited, ionized/dissociated molecules. Thus, monotype functional groups equipped polymer surfaces, preferably OH groups, originating from the dissociated water molecules, could be produced more selectively. An interesting feature of the technique is its flexibility to use a wide variety of additives in the water phase. Another way to modify polymer surfaces is the deposition of plasma polymers carrying functional groups as carboxylic groups used in this work. Acetic acid, acrylic acid, maleic and itaconic acid were used as additive monomers. Acetic acid is not a chemically polymerizing monomer but it could polymerize by monomer/molecular fragmentation and recombination to a cross linked layer. The other monomers form preferably water-soluble polymers on a chemical way. Only the fragmented fraction of these monomers could form an insoluble coating by cross linking to substrate. The XPS analysis was used to track the alterations in -O-CO- bond percentage on the PP surface. To identify the -COOH groups on substrate surface unambiguously, which have survived the plasma polymerization process, the derivatization with trifluoroethanol was performed.
Among new types of plasma processes, the underwater plasma is one of the most attractive methods for functionalization of polymer surfaces. The interesting features of plasma solution system are that the material surfaces to be modified remain in contact with the plasma-moderated solution. The role of plasma-moderated liquids, allows the reach of the reactive species through solution onto the geometrically hindered sites. The UV radiation produced in plasma formation helps in generating additional excited, ionized, and dissociated molecules and species in the reaction solution. An interesting feature of the technique is its flexibility to use a wide variety of additives as or in solution system. This allows us to create a selective or monotype functionalization of material surfaces. Such system was studied for the selective hydroxyl functionalization of polypropylene surface. The oxidation of polymer surfaces and the introduction of O-containing functional groups by underwater plasma was found to exceed concentrations typically achieved in oxygen low-pressure gas discharge plasmas up-to two-folds (maximal 56 O/100 C). The fraction of OH groups among all O-containing moieties amounts from 25 to 40% in comparison to that in the gas plasma of about 10% OH groups. Addition of hydrogen peroxide into this same system increases the fraction of C—O bonds up to 75% (27-OH/100 O). A study was focused to optimize the role of hydrogen peroxide on the efficiency of oxidation and selectivity with chemical derivatization with respect to the formation of mono-sort hydroxyl functionalities, calculated using a chemical derivatization technique.
Subjecting a thin NiTi specimen to uniaxial tension often leads to a localized martensitic transformation: macroscopic transformation bands form and propagate through the specimen, separating it into regions of fully transformed martensite and original austenite. In the present study, the alternating current potential drop (ACPD) technique is used to analyze the change in electrical resistance of ultra-fine grained NiTi wires subjected to a broad range of thermo-mechanical load cases: (i) uniaxial tensile straining at constant temperatures (pseudoelastic deformation); (ii) cooling and heating through the transformation range at constant load (actuator load case); (iii) a combination of mechanical and thermal loading. We monitor the ACPD signals in several zones along the gauge length of specimens, and we demonstrate that a localized type of transformation is a generic feature of pseudoelastic as well as of shape memory deformation. Moreover, the ACPD signals allow to differentiate between temperature-induced martensite (formed during cooling at no or relatively small loads), stress-induced martensite, and reoriented martensite (formed under load at low temperatures).
Plasma chemical methods are well suited for introducing functional groups to the surfaces of chemically inert polymers such as polyolefins. However, a broad variety of functional groups are often formed. Unfortunately, for further chemical processing such as grafting of molecules for advanced applications a highly dense monotype functionalized polyolefin surface is needed. Therefore, the main task was to develop a selective surface functionalization process, which formed preferably only a single type of functional groups at the surface in high concentration. Amongst the novel plasma methods, the underwater plasma process (UWP) is one of most attractive options to solve the problem of monotype functionalization. Such plasma is an efficient source of ions, electrons, UV-radiation, high-frequency shock waves, radicals such as hydroxyl radical, and reactive neutral molecules such as hydrogen peroxide. In contrast to established gas phase glow discharge processes, the water phase limits the particle and radiation energies and thus the energy input into the polymer. By virtue of the liquid water environment, which moderates highly energetic plasma species, extensive oxidation, degradation, cross-linking and radical formation on the polymer are more limited as compared to gas plasma exposure. The variety of plasma produced species in the water phase is also much smaller because of the limited reaction possibilities of the plasma with water. The possibility to admix a broad variety of chemical additives makes underwater plasma even more attractive. Hydrogen peroxide and the catalyst (Fe-ZSM5) should influence and increase the equilibrium concentration of OH radicals in the underwater plasma process. It was found that these radicals played a very important role in OH functionalization of polyolefin surfaces. Hydrogen peroxide was identified to be the most prominent precursor for OH group formation in the UWP. The catalyst would affect the steady state of OH radical formation and its reaction with the substrate surface and thus accelerates the functionalization process.
Combinations of different common flame retardants in wood–plastic composites (WPCs) are tested to identify synergistic or antagonistic effects with the goal of improving the fire performance of WPCs. Flame retardants investigated are expandable graphite (EG), ammonium polyphosphate (APP), and red phosphorus (RP) and combinations of two of them are used. The fire behavior is studied by cone calorimetric measurements. Additional thermogravimetry is used for further investigations. The fire tests show that EG has the highest potential for flame retardancy, but due to its expansion it cracks the formed residue layer. Combinations of EG with RP or with high amounts of APP are able to suppress this cracking.
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.