The thickening process of lamellar crystals from two folded-chain (2-FC) lamellae through one folded-chain (1-FC) lamellae finally to extended chain (EC) ones in bulk samples of a poly(ethylene oxide) (PEO) with a molecular weight of 5000 was studied.The samples with 2-FC crystals were prepared by quenching them from their melt to liquid nitrogen.During reheating them or treating them at selected annealing temperatures

we tracked the lamella-thickening process using small-angle X-ray scattering (SAXS).After analyzing the SAXS data and the corresponding one-dimensional correlation functions

we found that below 52℃ 2-FC crystals primarily thickened into 1-FC crystals; but above 52℃ 2-FC crystals mainly thickened into EC crystals.In the temperature region between 58℃ and melting point of the EC crystals

the newly formed 1-FC crystals will further thicken into EC crystals.Our observations using polarized light microscopy (PLM) and scanning electron microscopy (SEM) show no changes of crystalline superstructures

such as spherulites

formed by these crystals.Therefore

we can conclude that the thickening process

we observed in this crystalline PEO samples

do not cause any destruction of large crystalline superstructures

so it is a process only containing melting of thinner lamella crystals and then recrystallization of the molecules into thicker lamella crystals.More importantly

this process is strictly restricted within some local regions of large crystalline superstructures.