ΘΕΜΑ: Παρουσίαση PhD Λετζάκη Ελένης
ΤΕΤΥ ΑΠΟΣΤΟΛΕΑΣ: Charalampos
Stratigis [mailto:stratigis@xxxxxxxxxxxxxxxx] Πρόσκληση σε
Δημόσια Παρουσίαση της Διδακτορικής Διατριβής της κα. Λεντζάκη Ελένης (Σύμφωνα με το άρθρο 12 του Ν.
2083/92) Την Δευτέρα 28 Απριλίου 2014 και ώρα
11:00 στην αίθουσα Σεμιναρίων 3ου ορόφου Τμήματος Φυσικής στο κτίριο
του Φυσικού θα γίνει η δημόσια παρουσίαση και υποστήριξη της
Διδακτορικής Διατριβής της υποψηφίας διδάκτορος του Τμήματος Επιστήμης και
Τεχνολογίας Υλικών κα. Λεντζάκη Ελένης με θέμα: «Δυναμική και Ρεολογία Πρότυπων
Διακλαδωμένων Πολυμερών» «Dynamics
and Rheology of Model Branched Polymers»
ABSTRACT The thesis focuses on the determination of
the physical origin of chain stretch in complex branched polymers. The
uniaxial extensional rheology of linear polymers is rate dependent and the
onset rate of experimental strain hardening (the macroscopic consequence of
chain stretch) is equivalent to the theoretical prediction of the inverse Rouse
time. The molecular dynamic picture becomes more complicated when introducing
two or more branch points. We study well-defined comb polymers (Roovers, 1979)
with long molar mass of backbone Mb and rather short arms. When
systematically doubling the number of entanglements of the arms while keeping
the Mb constant, the onset of chain stretch occurs at earlier rates.
This can be rationalized by accounting for the effect of dynamic tube dilation
and extra drag from the arms which results in an effectively slower stretch
relaxation time. We modify the original differential pom-pom model by
introducing drag strain coupling and by specifying the coupling of stretch
between adjacent backbone segments. The model is validated successfully by
comparison with a wide variety of combs (with different molecular features) and
a wide range of extensional rates. At high rates, the maximum stretch condition
is reached and branch point withdrawal occurs, when arms are first oriented and
then withdrawn into the stretched backbone tube segments, first from the free
ends and then gradually progressing towards the centre. Moreover, our study focuses on the effects of the
environment on the reptation and fluctuations of model H and comb polymers. By
systematically varying the length of the linear chains, we study the
acceleration factor related to the arm and backbone relaxation times. The
acceleration factor has a strong dependence on the length of the linear chains.
We model the SAOS data using the Time Marching Algorithm by estimating a priori
whether the linear polymer would be taken as a theta solvent or whether the
reptation occurs in a skinny tube. The criterion for this estimation is based
on the relaxation time scale separation between the linear matrix and the H or
comb. -- Charalampos Stratigis Secretariat Dept. of Materials Science and Technology University of Crete P.O. Box 2208, GR-710 03 HERAKLION, GREECE Tel. +30 2810 394272 Fax. +30 2810 394273
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