A series of amorphous photochromic homo- and copolymethacrylates with an azobenzene moiety in the side group is investigated systematically by optical and dielectric spectroscopy. The aliphatic ester component of the comonomer unit and the concentration of the azobenzene groups within the copolymer are varied. The kinetics and the temperature dependence of the E/Z (trans/cis) photoisomerization and of the thermal Z/E (cis/trans) isomerization is studied for spin-coated films of the polymers. To understand the polarity of the polymeric materials as well as the matrix dependence of the reactions and of the photoinduced reorientation processes in the steady state molecular dynamics is investigated by dielectric spectroscopy. A variety of relaxation processes is observed: a γ-relaxation at low temperatures followed by a β-, an α-, and a δ-process. Additionally at room temperature a new -relaxation is detected for polymers containing azobenzene moieties. The temperature dependence of the reaction rate of the thermal Z/E isomerization process measured by optical spectroscopy seems to correlate with the values of the β'-process. A correspondence between dielectric and photochemical behavior is discussed.
The crosslinking kinetic of a thin polymeric layer based on a prepolymer of a phthalic aciddiallylester was studied by real-time dielectric spectroscopy in the frequency range from 10-1 to 105 Hz. With increasing reaction time the real part of the dielectric function ε ´ decreases. The time dependence of ε ´ can be described by a stretched exponential function with a stretching exponent of 0.5. This means that the influence of the chemical reaction on ε ´ cannot be described by a first order kinetic. From the temperature dependence of the characteristic time constant an activation energy of 71 kJ/mol could be estimated for the reaction. From the dielectric loss data the change of the relaxation rate of the dynamic glass transition fpα with the reaction time is obtained. After a temperature dependent induction period fpα decreases very strongly. No plateau value which corresponds to a glass transition in the crosslinked system is obtained for long reaction times.
Polymers synthesized with plasma techniques are very interesting materials for electronic, optic, and bio compatible applications. Thin films of plasma polymers shows a good adhesion to metals, glass, or other polymers. But the supramolecular structure, the durability, and the chemical and mechanical behavior of these polymers is poorly understood. Therefore dielectric investigations are carried out to study the dynamic behavior of the plasma polymers. As polymer system allyl alcohol/alkene is chosen to get polymers with a defined concentration of hydroxyl groups. The dielectric investigations shows several relaxation processes and a dependency of the dielectric parameters from the ratio of allyl alcohol in the polymer is observed. This results indicated that the alkene monomers were assembled continuous into the polymer matrix.
Dielectric spectroscopy has been used to study poly(ethylene naphthalene 2,6 dicarboxylate) (PEN) samples of different morphologies obtained by thermally treating bi-axially stretched PEN films. Neat and thermally treated samples of PEN films have been characterised by differential scanning calorimetry in order to measure the glass transition and melting temperatures as well as the degrees of crystallinity. Dielectric analysis has allowed the observation of the evolution of molecular relaxation phenomena with morphology changes and has revealed three relaxation processes: ?-, ?*- and ?-relaxation (increasing temperature). The ?-relaxation is associated with local motions of ester and the ?*-relaxation with partially cooperative motions of naphthalene groups. The latter has been shown to be related to the morphology of the materials under study. The ?-relaxation associated to the glass transition of PEN corresponds to cooperative motions induced by conformational rearrangements of the main chain and depends also on the morphology of the PEN films. Dielectric relaxation behaviours were compared using the activation energies calculated from Arrhenius equation formalism for the two sub-glass processes. VogelFulcherTammann fits were performed on the ?-relaxation. Our important contribution for the bi-axially stretched PEN films study is related to the assignment of the ?*-relaxation that can be attributed to naphthalene aggregates.
The retention of chemical structure and functional groups during pulsed plasma polymerization was used for producing adhesion-promoting plasma polymer layers with high concentrations of exclusively one kind of functional groups, such as OH, NH2, or COOH. The maximum content of functional groups was 31 OH using allyl alcohol, 18 NH2 using allylamine, or 24 COOH per 100 C atoms using acrylic acid. To vary the density of functional groups, chemical co-polymerization with ethylene as 'chain-extending' co-monomer, or butadiene as 'chemical crosslinker' was initiated in the pulsed plasma. The composition of these co-polymers was investigated by XPS and IR spectroscopy. The concentrations of functional groups were measured by derivatizing with fluorine-containing reagents and using XPS. A set of plasma parameters was found to be a good compromise between a high number of functional groups and complete insolubility in water, ethanol or THF,which is needed for further chemical processing. Here, these monotype-functionalized surfaces were used in metal-polymer systems as adhesion-promoting interlayers to examine the influence of type and density of functional groups on adhesion. As expected, COOH- and OH-group-terminated interlayers showed maximum peel strengths to evaporated aluminium layers. The adhesion increased linearly with the number of OH groups to a maximum at about 27 OH per 100 C atoms. Higher concentrations of OH groups did not increase the peel strength further.