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Weiche Materie Poster: Do., 13:00–15:30 D-P323
Real time evolution of structural and morphological features in quenched
polymers during heating
Giovanni C. Alfonso 1 , Fiorenza Azzurri 1 , Matteo Alessi 1 , Sergio S. Funari 2
1 Department of Chemistry and Industrial Chemistry, University of Genova, Via Dodecaneso,
– 2 HASYLAB at DESY, Notkestraße 85, D-22603
A simple experimental approach to gain information on structuring of polymeric materials
in very fast cooling processes, mimicking solidification under processing conditions,
has been recently developed in our laboratory. It consist in sandwiching a thin wire
thermocouple type K (wire diameter 50 microns) between two thin films (ca. 200
micron thick) of the polymer and in recording its signal at fairly high sampling rate
(5-10 readings/s) while the sample cools due to its immersion in a thermostatic liquid
bath at a given constant temperature. The analysis of the cooling curve, T(t), is
then carried out to obtain semi-quantitative information on the latent heat released
by the crystallization of the sample. A set of experiments performed with i-PP, PVDF
and HDPE at several quench temperatures indicate that crystallization takes place
at increasingly higher undercooling and to a progressively lower extent on increasing
cooling rate and that the sensitivity of crystallization to the imposed quench conditions
decreases on increasing the intrinsic crystallization rate of the polymer, i.e. in
the order i-PP>PVDF>HDPE. In general, samples obtained by quenching at rates of
the order of several hundreds ◦ C/s contain metastable highly defective crystals which
may undergo important structural and morphological reorganization before melting
during a typical DSC scan aimed at establishing the correlation between crystallization
conditions and characteristics of the semicrystalline state. In this context, the
unique potential of simultaneous acquisition of SAXS/WAXD during heating provided
by synchrotron facilities enables one to obtain an unambiguous picture of the evolution
of crystals with different initial features. This will be demonstrated, for example,
by comparing the reorganization and melting behaviour of i-PP samples quenched to
various temperatures, from 20 to 90 ◦ C, and initially exhibiting different structures,
from pure smectic to a mixture of smectic and α-monoclinic. The following observations
can be made: - the weakly exothermal smectic-monoclinic transition takes place
in the solid state and, upon heating, it begins at temperatures which are slightly lower
for samples solidified at very high cooling rates. - the structural and morphological
features of differently quenched samples progressively change during heating and, at
temperatures around 120 ◦ C, the systems eventually attain an identical type and extent
of ordering despite the appreciable differences in their initial state. This implies
that, consistent with the recorded DSC signals, their behaviour in the melting range
is independent from the initial morphology and structure and, therefore, it cannot be
used to differentiate the state of order in products obtained under typical processing
conditions.