Quasi-isothermal temperature-modulated differential scanning calorimetry wa
s employed in a study of thermoreversible structural changes in the melting
range of semicrystalline polymers. The results indicated a reversible melt
ing and crystallization process occurring at the fold surfaces of crystalli
tes of polyethylene and poly(ethylene oxide). For polyethylene, good agreem
ents were found with reported small-angle X-ray scattering data and Fischer
's theory. Surface melting and crystallization depend on the ability of the
chains in the crystals to carry out a sliding diffusion. This was shown by
a comparison of polyethylene and poly(ethylene oxide) with other polymers
such as poly(ethylene terephthalate), polycaprolactone, isotactic polypropy
lene, and syndiotactic polypropylene. When the longitudinal chain mobility
in the crystals is much reduced or completely absent, only a small or no ex
cess reversing heat capacity is observed. The special performance of short-
chain poly(ethylene oxide) is indicative for the metastability of the cryst
als built of once-folded chains.