THE USE OF TRANSMISSION ELECTRON-MICROSCOPY TO STUDY THE BLEND MORPHOLOGY OF STARCH POLY(ETHYLENE-CO-VINYL ALCOHOL) THERMOPLASTICS

Transmission electron microscopy (TEM) was used to examine the morphol ogy of extruded thermoplastic starch/poly(ethylene-vinyl alcohol) (EVO H) blends. Both the starch and EVOH polymers were found to be very bea m sensitive: the maximum allowable beam dosages for the crystalline fr action of both components are estimated at less than 3.5 x 10(-3) C cm (-2) at 200 kV and room temperature. In addition, both polymers experi enced significant mass thickness contrast variations which at moderate beam dosage ( > 6.0 x 10(-3) C cm(-2)) led to contrast-reversal of im ages of the blend morphology. Noting the effect of the electron beam o n the materials, low dose techniques were used with conventional TEM t o reliably image the blends. For as-processed starch/EVOH blends, EVOH was observed to be the matrix component even at very high starch conc entrations (up to 70%). Domain sizes of starch were observed to range from less than 0.1 mu m to greater than 3 mu m indicating that all of the starches became destructurized during the preparation of the blend s. Variation in the blend structure was also observed at similar starc h compositions (50%) for the three corn starch types. An analysis of c ontrast differences in blends containing Waxy Maize, Native Corn, and high amylose Hylon VII starches with EVOH suggested increasing miscibi lity with increasing amylose content in the starch component. Finally, structural gradients in thermoplastically formed articles (e.g. a sta rch/EVOH melt-spun fibre) were observed. (C) 1998 Published by Elsevie r Science Ltd. All rights reserved.