Dynamics of Surface Crystallization and Melting in Polyethylene and Poly(ethylene oxide) Studied by Temperature-Modulated DSC and Heat Wave Spectroscopy

Abstract

Polymers with a high longitudinal diffuse mobility within the crystallites are known to show a continuous, reversible surface melting and crystallization; temperature changes are accompanied by shifts of the crystalline-amorphous interface, resulting in a crystal thickening on cooling and a thickening of the amorphous layers on heating. In measurements of the dynamic heat capacity c*(ω), the process shows up as a strong excess contribution which increases up to the temperature of the final irreversible crystal melting. Experiments were carried out for linear polyethylene (LPE) and poly(ethylene oxide) (PEO). Employing both a temperature-modulated differential scanning calorimeter (TMDSC) and a heat wave spectrometer (HWS), thereby covering the frequency range from 10 -3 to 10 2 Hz, we could analyze the process dynamics. The times required for the surface melting or crystallization were deduced from the change at the signal amplitude with frequency. They are remarkably long. For temperatures near to the respective final melting points 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 the temperature dependence of the crystallinity. A comparison with the results of a SAXS structure analysis showed perfect agreement. For PEO the quasi-stationary conditions were not reached. Even at the lowest frequencies probed by TMDSC, the dynamic heat capacity was still below the value expected on a basis of the temperature dependence of the crystallinity determined by SAXS. In determinations of the dynamic heat capacity by TMDSC and HWS, it is in general necessary to correct the raw data to account for the inner heat flow resistance, additional heat capacities, and delay times introduced by the electronics. The corrections can be accomplished by an appropriate modeling of the measuring devices.

Crystallization and melting behavior of three biodegradable poly(alkylene succinates). A comparative study

“…In general Wunderlich et al, interpreted reversible melting using the concept of molecular nucleation [48]. Strobl et al studied reversible melting in PE, poly(-ethylene oxide) and other polymers like iPP, polycaprolactone and PET, and concluded that polymers with high longitudinal diffuse mobility within crystallites show a continuous, reversible surface melting and crystallization [49,50]. This also may be the explanation for the observations in the present paper.…”

Crystallization of Polymers Investigated by Temperature-Modulated DSC

“…This could be the mechanism of reversing melting in polymers that do not exhibit sliding diffusion. In this case, the phenomenon could take place at the lateral surfaces [2,6,7], whereas polymers characterized by sliding diffusion could undergo reversing melting at the fold surfaces, as changes in the fold-length can occur without melting [8,9]. …”

Kinetics of reversible surface crystallization and melting in poly(ethylene oxide): Effect of crystal thickness observed in the dynamic heat capacity

“…We found a particularly strong excess signal for polyethylene and poly(ethylene oxide) and thus exactly for those two polymers out of the series which exhibit surface crystallization and melting. In a second work [8] it could be demonstrated by a comparison with results of small-angle X-ray scattering experiments that the excess part in the dynamic heat capacity had indeed the magnitude expected from the structural changes due to surface crystallization 238 The European Physical Journal E and melting. A high inner mobility of PEO is also demonstrated by works of Kovacs et al [9] and Cheng et al [10] on oligomer crystals.…”

“…These are temperature-modulated differential scanning calorimetry (TMDSC) used in the range from 10 −3 s −1 to 10 −1 s −1 , and "heat wave spectrocopy" (HWS) applied from 10 −1 s −1 to 10 2 s −1 . Detailed descriptions of the two methods have been given in other recent papers [8,16], so that we can restrict ourselves here to a brief explanation of the basic features. TMDSC measurements were carried out with a standard differential scanning calorimeter which was in our case a Perkin Elmer DSC4; the temperature oscillation is realized by an appropriate software.…”

“…Crystal thicknesses can be varied by a change of the crystallization temperature, and the dependence can be characterized by small-angle X-ray scattering. On the other hand, by employing dynamic calorimetry one can measure the times associated with the surface melting and crystallization in PEO [8]. The two techniques used by us to carry out the experiments, temperature-modulated differential scanning calorimetry and heat wave spectroscopy, cover together just the frequency range of the process.…”