Morphological Study by TEM on Thin Films of Nylon-6, Nylon-6,6, and Their Blends As-cast from Formic Acid Solution

Abstract

KEY WORDSNylon-6 / Nylon-6,6 / Polymer Blend / Solution-Cast Thin Film / Transmission Electron Microscopy / Electron Diffraction / Morphology / Nylon-6 (polycaprolactam) (N6) and nylon-6,6 (poly(hexamethylene adipamide)) (N66) are very important commercial polymers in the nylon family. 1 By blending these two polymers, a remarkable improvement on their inherent mechanical properties can be expected. Extensive efforts have been, therefore, made for mechanical, thermodynamic and/or structural studies of the blends. 2-11 N6 and N66 are miscible in their molten state, 1 and are also expected to be miscible in their solution state. 7 Some morphological studies by transmission electron microscopy (TEM) of as-solution-cast crystalline thin films of each pure component have been reported so far (e.g., refs 6, 12, and 13 for N6, and refs 6, 14, and 15 for N66; mostly, cast from solutions in formic acid), but there have been few reports on morphology of their blend films as-cast from solution. 6 In particular, although selected-area electron diffraction (SAED) patterns obtained from the same specimenarea tilted at a series of angles are very important to identify the crystal modifications and/or crystallite orientation in a given specimen film, such SAED experiments by specimen-tilting have been done in a few reports only on N66 14, 15 and in no report on their blends, to our knowledge. In this communication, we report some results on morphology of the thin films of N6, N66 and their blends, all of which were prepared by casting the respective solutions in formic acid onto water surface, and also on a set of SAED patterns recorded from the un-tilted and tilted specimen-area for each of the thin films. EXPERIMENTAL Materials and Preparation of Thin FilmsN6 (M w = 16000) and N66 (M w = 22000) are respectively the products of Unitika Ltd. (A1030BRL) and BASF Co. (R8270), both of which were kindly supplied by Research and Development Center, Unitika Ltd. and were the same materials investigated by Hirami and his co-workers 7-10 and by Bedia et al. 11 The procedure to make solutions in formic acid is similar to that reported previously: 11 Solutions (0.2 wt%) of N6 and N66 were separately prepared at room temperature (RT) by dissolving each nylon in formic acid (this concentration, 0.2 wt%, was determined by trial and error to be just appropriate for our purpose). In order to make blends of desired wt% compositions (N6/N66 = 30/70, 50/50 and 70/30), the corresponding volume ratios of N6 and N66 solutions were mixed together at RT. Thin polymer films were prepared from each of the resulting solutions by casting onto the surface of water thermostated at 55, 65, or 75 • C. Of course, these three temperatures are much lower than the melting temperature of N6 crystal and also that of N66 one. 11 Rybnikár and Geil 6 prepared a specimen film for TEM by evaporating a drop of solution directly on a carbon-coated TEM grid or on a glass slide. Direct evaporation of a solution in formic acid on the TEM grid, however, may leave a residue...