Structure and rheology of macromolecular systems at interfaces

Abstract

This thesis focuses on the structural and rheological properties of viscoelastic filmsat the air-water interface. Our aim is to tailor the rheological properties of fluid interfacesand link them with their structure at the molecular scale. Langmuir quasimonolayerswere built by employing three different types of molecular systems, a seriesof semifluorinated alkanes that consist of two hydrophobic segments, a homopolymer,Poly(methyl methacrylate) (PMMA), and block copolymers of PEO-PDMS with differentmolecular architectures. The techniques employed in this thesis include LangmuirPockelstrough, to measure surface pressure/ area isotherms, and the magnetic rod interfacialstress rheometer, double wall ring fixture and bi-conical device in commercialrheometers for interfacial rheology studies. For structural studies of the films at theair-water interface we performed neutron reflectivity measurements and scanning forcemicroscopy.We observed great tunability of the structural and rheological properties of thesemifluorinated alkanes moving from simple linear molecules to more complex andbranched architectures, showing that molecular modification can change dramaticallythe viscoelastic response of the layers. Regardless of the molecular architecture effect,the external light stimulus effect was investigated for photo-sensitive semifluorinatedalkanes that could change their conformation reversibly from trans to cis. One of themain findings of this study was the transition from ordered to mixed (disordered) structureswhose onset is signaled by the drop of the interfacial storage modulus of aboutone order of magnitude.The predominantly elastic layers of PMMA were used to investigate the presence ofeven harmonics among different rheometry techniques. By analysing the strain signalin 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 2ndharmonics that are connected with asymmetry in the stress-strain profile, apparentlystemming from ununiformity in flow and the subphase contribution.Finally, the diblock and triblock copolymers of PEO-PDMS exhibit perfectly reversiblelayers at the air-water interface showing a reproducible PDMS phase transitionfor all the different molecular weights. Moreover, the average Flory radius of the polymersscale as RF∼N ν , with ν=0.56±0.01. Such result are in agreement with predictedscaling for 2D-polymers in bad solvent conditions.