JOURNAL

OF MATERIALS

SCIENCE

LETTERS

3 (1984) 693--694

Transmission electron microscopy of polyamides J. M A R T I N E Z - S A L A Z A R * , C. G. CANNON

H. H. Wills Physics Laboratory, University of Bristol, UK

Despite a few early attempts to reveal the details of the morphology of polyamide specimens by heavy atom staining of thin films [1,2] and sections [3] the technique has proved to be very difficult to apply successfully and results have been disappointing. The problem is basically one of controlled diffusion of the heavy atom carriers into the disordered regions of the polymer structure without disturbance of the crystalline regions. The relatively high concentration of the highly polar amide groups along each molecular chain means that the disordered regions are tightly packed, since infrared spectroscopy shows that the a m i d e amide interactions are as strongly developed in the disordered regions as in the crystal lattice [4]. Very small molecules such as water are able to diffuse into the non-crystalline regions as moisture uptake and H~D exchange with heavy water demonstrate [4]. In addition to their small size, water molecules can hydrogen bond to amide groups and swell the non-crystalline regions. We have tried reacting allylamine with the polyamide to create double bond sites for reaction with osmium tetraoxide. Infrared spectra showed the presence of some terminal vinyl groups. Very dense staining of the interspherulitic regions was observed by optical microscopy. Electron microscopy of ultra thin sections showed no penetration of the spherulite morphology so this method was abandoned. Other methods tried included thallium in ethanol, aqueous copper sulphate, uranyl acetate in water and in ethanol, iodine in aqueous potassium iodide, a chrome dyestuff (Irgalam) in water, aqueous lead citrate and aqueous phosphotungstic acid. Despite the presence of small proton donor solvent molecules, which should have swelled the polymer, the staining was negligible with most

systems and poor even with phosphotungstic acid, which has been very successfully used with biological specimens [5]. In commercial dyeing processes of fabrics or yarns of synthetic fibres a dyeing assistance reagent is often added to the aqueous dye bath in small quantities. Benzyl alcohol has been used for polyamides and polyesters. The dyeing assistant is preferentially adsorbed into the fibres and the resulting swelling creates diffusion paths for the relatively large dye molecules thus increasing the rate of dyeing. The diffusion of heavy atom staining might similarly be facilitated. We have therefore explored such mixed solutions and found that for nylons 6 and 6.6, 2% phosphotungstic acid + 2 % benzyl alcohol in water gave the best results. Thin films of polyamide specimens less than 100nm thick are captured on a specimen grid which is floated on a drop of the mixed solution at room temperature. After short times of the order of 10min the grid is removed and immediately washed free of solution in distilled water with two or three rinses. Two examples are presented here to illustrate the degree of resolution achieved by this technique. Thin films of nylon 6 and nylon 6.6 were cast on water from benzyl alcohol and formic acid solutions, respectively. Small fragments of each polymer were collected on carbon coated grids in order to melt and recrystallize the samples. Fig. 1 is an electron micrograph of a typical melt crystallized nylon 6 film where the chain folded ribbon lamellae are clearly resolved. Measurement shows the average thickness to be 7nm, fairly regular over the whole field of view. Despite the multiple branching which can be clearly seen most of the lamellae appear to be edge on. Fig. 2 shows an electronmicrograph of a nylon 6.6 melt crystal-

*Permanent address: Instituto de Estructura de la Materia (CSIC), Serrano 119, Madrid, Spain. 0261 --8028/84 $03.00 + .12

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Figure 1 Transmission electronmicrograph of a melt crystallized thin film of nylon 6.

Figure 2 Transmission electronmicrograph of a melt crystallized thin film of nylon 6.6.

lized film. Again the chain folded ribbon lameUae are clearly resolved. In this sample the stained regions occupy a much larger area than the specimen shown in Fig. 1 indicating that the total crystallinity is lower. We hope that these two examples will show the value of this new staining method for studying the morphology of polyamides in detail. One of us (J. M-S) has experimentally applied the technique to melt blends of dissimilar polymers, details of which will be published elsewhere.

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References 1. K. HESS, E. GOTTER and H. MAHL, Kolloid Z. 168

(1960) 37. 2. B. J. SPIT, Proceedings of the Fifth International Congress for Electron Microscopy, Vol. 1 (Academic Press, New York, 1962) BB-7. 3. J. A. RUSNOCK and D. HANSEN, J. Polyrn. Sei. A

3 (1965) 647. 4. C. G. CANNON, Spectroehim. Aeta 16 (1960) 302.

Received 31 January 1984 and accepted 15 Feburary 1984