Scanning transmission electron microscopy (STEM) dark field images rev
eal that within the essentially amorphous matrix of quench-cooled isot
actic polypropylene (iPP) there exist microcrystalline regions, 100 to
200 angstrom in size, that have the alpha-monoclinic crystal form (a
= 6.67 +/- 0.01, b = 20.89 +/- 0.05, c = 6.48 +/- 0.01 angstrom, beta
= 98.4 +/- 0.6-degrees-). Regions of lower crystalline order are also
observed which, according to micro-micro diffraction, also possess the
alpha-monoclinic crystal structure but with substantially larger unit
cell constants (a = 7.03 +/- 0.01, b = 21.1 +/- 0.1, c = 6.47 +/- 0.0
2 angstrom, beta = 99 +/- 2-degrees-). Differential scanning calorimet
ry studies indicate that during quench cooling to the glassy state iPP
samples attain a low degree of crystalline order to an extent that de
pends on sample mass. Upon heating from the glassy state, these sample
s exhibit exothermic transitions, between -15 and 120-degrees-C, that
correspond to the formation and subsequent perfection of the microcrys
talline regions observed by STEM dark field.