BEGIN:VCALENDAR VERSION:2.0 PRODID:-//CERN//INDICO//EN BEGIN:VEVENT SUMMARY:Keynote 8 - Studying the influence of molecular conformation on ex tensional rheology by combining filament stretching rheometry with ex-situ SANS DTSTART;VALUE=DATE-TIME:20181211T130000Z DTEND;VALUE=DATE-TIME:20181211T134000Z DTSTAMP;VALUE=DATE-TIME:20260523T152842Z UID:indico-contribution-90-492@lindico453.srv.lu.se DESCRIPTION:Speakers: Qian Huang (Chemical Engineering\, Technical Univers ity of Denmark)\nExtensional flow is a major type of deformation in many p olymer processing operations such as blow molding and fiber spinning. Exte nsional rheology of polymer melts is highly sensitive to molecular archite cture and conformation. In the past four decades\, people have made a grea t effort to study the dynamics of polymer chains in extensional flow in or der to predict the rheological behaviour by using constitutive equations b ased on molecular theories (e.g. the tube model). However\, while producin g pure extensional flow in a highly controlled manner (e.g. constant strai n rate) is already a challenge\, rheology measurements give indirect infor mation of molecular conformation in terms of stress-strain responses only. Thus combining state-of-the-art extensional rheometry with scattering tec hniques provides a powerful tool to understand polymer dynamics.\n\nIn thi s work\, we measure the extensional rheology of polystyrene (PS) melts usi ng a filament stretching rheometer which is able to produce a well-defined extensional flow. We show that the rheological behaviour of PS melts with different molecular architectures (linear and star shaped) is identical i n fast extensional flow\, indicating the same molecular conformation. The hypothesis of the same molecular conformation is confirmed in small angle neutron scattering (SANS). We also measure a bi-disperse PS blend containi ng linear chains of two molecular weights\, and a monodisperse PS melt con taining the short linear chains only\, in stress relaxation following a fa st extensional flow. By combing rheological measurements with SANS measure ments\, we show that there are nematic effects and strain coupling between the short and long chains in the blend. Furthermore\, we show that the ch ain stretch idea in the tube model can be critically tested through analys ing the SANS data at different time in stress relaxation process.\n\nhttps ://lindico453.srv.lu.se/event/67/contributions/492/ LOCATION:Elite Hotel Ideon Tera URL:https://lindico453.srv.lu.se/event/67/contributions/492/ END:VEVENT BEGIN:VEVENT SUMMARY:Contr. Talk 9 - Poised between order and disorder: sheared DNA sol utions across the Isotropic-Nematic transition DTSTART;VALUE=DATE-TIME:20181211T134000Z DTEND;VALUE=DATE-TIME:20181211T140000Z DTSTAMP;VALUE=DATE-TIME:20260523T152842Z UID:indico-contribution-90-491@lindico453.srv.lu.se DESCRIPTION:Speakers: Giuliano Zanchetta (University of Milano)\nHydrogels built from DNA are an attractive class of materials\, whose structure and mechanical properties can be tuned by the design of “physical” cross- links based on selective Watson-Crick base pairing [Pan\, Soft Matter 12\, 5537 (2016)]. Even simple systems of entangled DNA helices can display com plex mechanical response\, such as shear banding [Boukany\, Soft Matter 5\ ,780 (2009)] and strain hardening [Orakdogen\, Macromolecules 43\, 1530 (2 010)]\; however\, in discussing such intriguing features\, it is often dis regarded that long DNA strands also undergo a transition to Nematic (N) li quid crystalline phase at concentrations as low as few mg/ml [Merchant\, B iophys.J. 73\,3154 (1997)].\nWe investigate the correlation among phase be haviour\, structure and rheological properties in DNA helices of thousands of base pairs. In a wide concentration range\, the solutions are viscoela stic while also displaying a continuous transition from Isotropic to N\, w ithout detectable phase separation. These solutions exhibit very low degre e of ordering at rest\, but strong\, transient birefringence when sheared. \nWe measure the mechanical and optical response to continuous\, oscillato ry and step deformations across the transition region. To this aim\, we co mbine rheology to real-space measurement of shear-induced alignment in a m icroscope shear cell\, or through polarized reflection directly on the rhe ometer [Mykhaylyk\, J. Polym. Sci. B 54\,2151 (2016)]. Furthermore\, throu gh rheo-SAXS experiments we gain access to the local degree of alignment a nd structure. \nWe find that both shear stress and birefringence relax wit h two different time scales\, a slow decay related to spontaneous disentan glement and a faster one\, which we interpret as the result of the frustra ted phase separation and local heterogeneity of the system. Regions with l arger N order are more easily aligned and fluidized than the isotropic\, v iscoelastic matrix in which they are embedded\, resulting in a faster opti cal and mechanical relaxation.\n\nhttps://lindico453.srv.lu.se/event/67/co ntributions/491/ LOCATION:Elite Hotel Ideon Tera URL:https://lindico453.srv.lu.se/event/67/contributions/491/ END:VEVENT END:VCALENDAR