We performed systematic dewetting experiments on isotactic poly(para-methylstyrene) (iPpMS) films to explore the temperature dependence of the viscoelastic behavior of these films. We quantified the amount of residual stresses σ res induced through film preparation by spin-coating. As anticipated, σ res was found to be independent of the temperature T dew at which dewetting was done. A particular focus was on the temperature dependence of the relaxation time τ of σ res , which was measured with the help of three independent dewetting parameters. Within error, all three values of τ were identical and followed an Arrhenius behavior yielding an activation energy of 60 ± 10 kJ/mol. The initial dewetting velocity, being proportional to the ratio of surface tension of iPpMS and the corresponding viscosity, increased significantly with T dew . Assuming a linear stress−strain response, we deduced that the elastic deformation responsible for the maximum height of the dewetting rim increased with temperature, although σ res did not vary with temperature. Correspondingly, the shear modulus of iPpMS films was found to decrease monotonically with increasing temperature. Using a Maxwell-type model, the corresponding viscosity of the film showed the expected decrease with increasing temperature. Our experiments suggest that preparationinduced residual stresses affect material properties such as elastic modulus or viscosity of iPpMS as a function of temperature.
Nanocomposites based on sequential semi-interpenetrating polymer networks (semi-IPNs) of crosslinked polyurethane and linear poly(2-hydroxyethyl methacrylate) filled with 1-15 wt % of nanofiller densil were prepared and investigated. Nanofiller densil used in an attempt to control the microphase separation of the polymer matrix by polymer-filler interactions. The morphology (SAXS, AFM), mechanical properties (stress-strain), thermal transitions (DSC) and polymer dynamics (DRS, TSDC) of the nanocomposites were investigated. Special attention has been paid to the raising of the hydration properties and the dynamics of water molecules in the nanocomposites in the perspective of biomedical applications. Nanoparticles were found to aggregate partially for higher than 3 and 5 wt % filler loading in semi-IPNs with 17 and 37 wt % PHEMA, respectively. The results show that the good hydration properties of the semi-IPN matrix are preserved in the nanocomposites, which in combination with results of thermal and dielectric techniques revealed also the existence of polymer-polymer and polymer-filler interactions. These interactions results also in the improvement of physical and mechanical properties of the nanocomposites in compare with the neat matrix. The improvement of mechanical properties in combination with hydrophilicity and biocompatibility of nanocomposites are promising for use these materials for biomedical application namely as surgical films for wound treatment and as material for producing the medical devises.
We
examined changes in rheological properties of nonequilibrated
spin-coated thin films of isotactic polystyrene (iPS) and isotactic
poly(para-methylstyrene) and blends thereof by dewetting
experiments. For 200 nm thick films and a systematic variation of
dewetting temperature, we focused on changes in the amount of residual
stresses and the corresponding relaxation times as a function of iPS
content. We found that both parameters increased in an approximately
logarithmic fashion with iPS content, suggesting an increased tendency
for the formation of transient clusters in blends with increased iPS
content. Assuming linear elasticity, we determined a nonequilibrium
shear modulus and a nonequilibrium viscosity, which we compared with
published data of the equilibrium properties. Particularly for the
viscosity, huge deviations from published values were observed. We
provide a tentative explanation.
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