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Freiburger Materialforschungszentrum
Stefan-Meier-Str. 21
79104 Freiburg, Germany

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You are here: Home Events Prof. Adem Levent Demirel "Poly(2-alkyl-2-oxazoline)s: Bulk Crystallization and Solution Self-Assembly"

Prof. Adem Levent Demirel "Poly(2-alkyl-2-oxazoline)s: Bulk Crystallization and Solution Self-Assembly"

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Koç University, Chemistry Department, Istanbul, Turkey

What
  • Seminar
When May 28, 2014
from 02:15 PM to 03:00 PM
Where Seminarraum A, FMF, Stefan-Meier-Str. 21, Freiburg
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Poly(2-alkyl-2-oxazoline)s (PAOX) have recently gained attention in especially biological applications due to their lower critical solution temperature (LCST) in aqueous solutions being close to the body temperature and their biocompatibility. The physicochemical properties of PAOX depend on the alkyl side chain length. As the alkyl side chain length decreases, LCST of aqueous solutions increases, glass transition temperature (Tg) increases and bulk crystallization is significantly hindered. Among PAOX having shorter side chains, Poly(2-ethyl-2-oxazoline) (PEOX) is the only member whose bulk crystallization was previously not reported. We have shown that self-assembled crystalline PEOX fibers form when aqueous PEOX solutions are kept above the cloud point temperature (Tc) for long times (days to weeks). In order to understand the role of alkyl side chain length in PAOX crystallization and the factors affecting the formation of crystalline fibers in aqueous solutions, we have investigated PAOX crystallization in bulk and the self-assembly in aqueous solutions as a function of alkyl chain length, temperature, molecular weight and ionic strength. We identified different crystallization behavior depending on the length of the alkyl side chain. The self-assembly into water insoluble fibers above Tc, a slow crystallization process, was enhanced by the addition of various sodium salts. The specific anion effects on Tc of aqueous PEOX solutions were correlated with the hydration entropy and the surface tension increment of anions to identify the contribution of different mechanisms to phase separation at Tc and self-assembly. We argue that the enhanced crystallization of PAOX with increasing alkyl side chain length is due to increase in average distance between the polymer backbones and thus decrease in the average strength of amide dipole interactions. In aqueous solutions, solvation of the amide dipole increases the mobility of the rather rigid PEOX backbones which is essential in crystallization and in promoting equilibration of the system, thus leading to the formation of very regular nanofibers with smooth edges and well-defined dimensions.

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