1 research outputs found
Lamellar Melting, Not Crystal Motion, Results in Softening of Polyoxymethylene on Heating
We probe temperature-dependent changes in the semicrystalline
microstructure
of polyoxymethylene using a combination of modulated DSC, SAXS, and
solid-state NMR to characterize macroscopic behavior, lamellar-level
structure, and molecular environments, respectively, and correlate
these with the change in mechanical properties probed using DMA and
AFM. Two model samples are investigated: a melt crystallized sample
prepared by injection molding and a sample obtained by crystallization
from dilute solution. Our investigations reveal that, for both samples,
there is an increase in crystalline motions and in the amorphous content
on heating. DMA and AFM measurements reveal that the modulus of the
molded sample decreases on heating to about 100 °C; however,
there is a significant difference in behavior of the solution crystals,
where we observe no significant decrease in stiffness (from AFM measurements).
Thus, in contrast to previous reports, we demonstrate that the decrease
in modulus on heating polyoxymethylene does not correlate with chain
motions in the crystalline regions. We use SAXS to probe the semicrystalline
morphology for the samples on heating and show that, for the molded
sample, there is a distribution of lamellar thickness at room temperature
and that the thin lamellae in this distribution melt on heating. In
contrast to the behavior of the melt crystallized samples, the solution
crystals exhibit no change in the lamellar stacking on heating to
150 °C. We also demonstrate that, on heating, the amorphous regions
in the solution crystals always appear to have restricted mobility
while there are mobile and low mobility amorphous regions in the molded
samples. Our results suggest that, contrary to conventional belief,
the decrease in modulus on heating polyoxymethylene arises not from
motions in the crystalline lamellae but primarily from melting of
thin lamellae