Dynamics of surface crystallization and melting in polyethylene and poly(ethylene oxide) studied by temperature-modulated DSC and heat wave spectroscopy
T. Albrecht et al., Dynamics of surface crystallization and melting in polyethylene and poly(ethylene oxide) studied by temperature-modulated DSC and heat wave spectroscopy, MACROMOLEC, 34(24), 2001, pp. 8456-8467
Polymers with a high longitudinal diffuse mobility within the crystallites
are known to show a continuous, reversible surface melting and crystallizat
ion; temperature changes are accompanied by shifts of the crystalline-amorp
hous interface, resulting in a crystal thickening on cooling and a thickeni
ng of the amorphous layers on heating. In measurements of the dynamic heat
capacity c*(omega), the process shows up as a strong excess contribution wh
ich increases up to the temperature of the final irreversible crystal melti
ng. Experiments were carried out for linear polyethylene (LPE) and poly(eth
ylene oxide) (PEO). Employing both a temperature-modulated differential sca
nning calorimeter (TMDSC) and a heat wave spectrometer (HWS), thereby cover
ing the frequency range from 10(-3) to 10(2) Hz, we could analyze the proce
ss dynamics. The times required for the surface melting or crystallization
were deduced from the change at the signal amplitude with frequency. They a
re remarkably long. For temperatures near to the respective final melting p
oints we found about 12 s for the LPE sample and 120 s for PEO. The dynamic
heat capacity of PE measured by TMDSC at low frequencies corresponds to th
e temperature dependence of the crystallinity. A comparison with the result
s of a SAXS structure analysis showed perfect agreement. For PEO the quasi-
stationary conditions were not reached. Even at the lowest frequencies prob
ed by TMDSC, the dynamic heat capacity was still below the value expected o
n a basis of the temperature dependence of the crystallinity determined by
SAYS. In determinations of the dynamic heat capacity by TMDSC and HWS, it i
s in general necessary to correct the raw data to account for the inner hea
t flow resistance, additional heat capacities, and delay times introduced b
y the electronics. The corrections can be accomplished by an appropriate mo
deling of the measuring devices.