Δομή και ρεολογία μακρομοριακών συστημάτων σε διεπιφάνειες
Structure and rheology of macromolecular systems at interfaces
This thesis focuses on the structural and rheological properties of viscoelastic films at the air-water interface. Our aim is to tailor the rheological properties of fluid in¬terfaces and link them with their structure at the molecular scale. Langmuir quasi-monolayers were built by employing three different types of molecular systems, a series of semifluorinated alkanes that consist of two hydrophobic segments, a homopolymer, Poly(methyl methacrylate) (PMMA), and block copolymers of PEO-PDMS with differ¬ent molecular architectures. The techniques employed in this thesis include Langmuir-Pockels trough, to measure surface pressure/ area isotherms, and the magnetic rod in-terfacial stress rheometer, double wall ring fixture and bi-conical device in commercial rheometers for interfacial rheology studies. For structural studies of the films at the air-water interface we performed neutron reflectivity measurements and scanning force microscopy.
We observed great tunability of the structural and rheological properties of the semifluorinated alkanes moving from simple linear molecules to more complex and branched architectures, showing that molecular modification can change dramatically the viscoelastic response of the layers. Regardless of the molecular architecture effect, the external light stimulus effect was investigated for photo-sensitive semifluorinated alkanes that could change their conformation reversibly from trans to cis. One of the main findings of this study was the transition from ordered to mixed (disordered) struc¬tures whose onset is signaled by the drop of the interfacial storage modulus of about one order of magnitude.
The predominantly elastic layers of PMMA were used to investigate the presence of even harmonics among different rheometry techniques. By analysing the strain signal in the Fourier space, it was found that the bi-conical and the double wall ring fixture
do not generate even harmonics while the magnetic rod showed the presence of 2nd harmonics that are connected with asymmetry in the stress-strain profile, apparently stemming from ununiformity in flow and the subphase contribution.
Finally, the diblock and triblock copolymers of PEO-PDMS exhibit perfectly re¬versible layers at the air-water interface showing a reproducible PDMS phase transition for all the different molecular weights. Moreover, the average Flory radius of the poly¬mers scale as RF~NV, with ν=0.56±0.01. Such result are in agreement with predicted scaling for 2D-polymers in bad solvent conditions.