LOW-ANGLE X-RADIOGRAMS AND STRUCTURE OF ORIENTED POLYETHYLENE FILMS*t

D. YA. TSVANKIN

Heteroorganic Compounds Institute, U.S.S.R. Academy of Sciences

(Received 7 January 1967)

THE low-angle diffraction of X-rays provides various information about the morphology of the supermolecular structures of crystalline polymers [1, 2].

The X-radiograms of oriented samples, which may be termed low-angle texture patterns, are of great interest in that they reveal a number of peculiar diffraction effects due to the morphology of the polymers, i.e. connected with the dimensions and relative arrangement of the regions into hundreds and thou- sands of angstroms differing in regard to their internal structure.

In a previous paper [3] a s tudy was made of oriented films of poiyethylene- terephthalate by means of wide- and low-angle diffraction, and the conclusion reached was that the long period characteristic of crystalline polymers in the oriented state is always parallel to the direction of the molecular chains. This conclusion refers to those samples where the direction of the macromolecular axes coincides with that of the texture axis (axial or horizontal). In this case the long period is apparently due to the internal structure of polymer fibrils stretched along the texture axis. I t characterizes the crystallites and disordered regions of varying density alternating along the polymer fibril.

Films and fibres obtained by orienting drawing in one or two directions have structures of this kind; there are in addition oriented samples of another type where the direction of the macromolecular axes differs from that of the texture axis, a typical example being polyethylene films produced by extrusion. In these films the macromolecular axes are approximately perpendicular to the texture axis [4]. The morphology and origin of low-angle scattering in these films have yet to be explained, and a s tudy of their structure would be of great interest at the present time.

Some of the structural changes occurring in the stretching of films of this kind were described in [5[. The main aim in the present paper is to s tudy and interpret low-angle texture patterns of the initial and stretched films.

All the low-angle radiograms were obtained with copper radiation in a cam- era with a collimator consisting of two 0.1 mm apertures 100 mm apart.

The initial films of high-pressure polyethylene [5] were 260 and 40/1 thick, t

*Vysokomol. soyed. A9: No. 12, 2668-2071, 1967. ¢ The author wishes thank Yu. B. Zimin of the Dorkhimzavod for supplying the film

samples.

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3022 D. YA. TSVA~KIN

In both cases there was a 200 reflections on the meridian Of the X-radiogram oriented along the texture axis so that the texture axis coincided with the a axis (a-texture). The thicker the film the greater was the dispersion of orientation and with the thick film (260/~) there was practically isotropic intensity distribu- tion over on the Debye arc. Figure la shows an X-radiogram of the initial

Fio. 1. Wide-angle X-radiograms: a--initial film; b--maximally stretched film.

thick film. I f we stretch this film along the texture axis the wide-angle diffraction characterizes the gradual transition from a-texture to ordinary c-texture where the macromolecular axes are oriented along the texture axis (Fig. lb) [4, 5]. Our aim was to examine changes in the low-angle texture patterns undergoing transition from a-texture to c-texture.

FIG. 2. Low-angle texture patterns. Values of 2: a--0; b--l ' l; c--1"5; d--l'8; e-- 2"4; f-- 3"5; g-- 4"0.

• Figure 2a shows the low-angle texture pattern of the initial thin film, while Fig. 2b shows tha t of a film after undergoing 10% stretching (~1 -1 ) . As the wide-angle texture was unchanged after this stretching (Fig. la) this film may also be taken as the initial. The low-angle X-radiograms obtained from stretched samples of the thick and thin films are basically similar.

Low-angle X-radiograms and structure of oriented polyethylene films 3023

Figure 2c-g shows pictures of the most characteristic low-angle X-radio- grams of the samples obtained by systematic stretching of the initial films along the texture axis. Radiogram 2, g is that of the final sample possessing c-texture (Fig. lb).

Figures 2a and 2b show that the low-angle texture patterns of the initial films are very pecurliar. There is scattering on the meridian in the form of a slightly diverging fan (Fig. 2a) or in the form of a spherical reflection (Fig. 2b). Since the samples possess a-texture and both reflections are on the meridian, it follows that they are oriented in the direction perpendicular to the molecular chains. This is very different from the usual meridional streaks (Fig. 2g) cor- responding to the long period in the X-radiograms of the oriented samples when the low-angle reflection is on the texture axis parallel to the macromolec- ular axes.

On stretching the meridional reflection splits up into four central streaks forming a certain angle with one another. With further stretching the streaks gradually approach the equator of the X-radiogram (Fig. 2c, 2d, 2e) and their intensity is reduced (Fig. 2f); at maximum stretching they disappear (Fig. 2g). Irrespective of the streaks there is the usual low-angle reflection on the merid- ian in the form of a weakly defined four-pointed figure (Fig. 2d, d=130 /~), the position of which changes little in the stretching process (Fig. 2e, 2f, 2g).

The central streaks in the low-angle X-radiograms are characteristic of helical fibrils. Thus in the X-radiograms of cellulose samples there is either strictly equatorial scattering--in the case of oriented fibres of ramie or flax--or several central streaks when the fibrils in the sample form helices [6, 7]. The streaks in X-radiograms 2c-g are identical to those seen in the X-radiograms of cellulose.

In addition to the central streaks the X-radiograms of the polyethylene films have the usual long period (Fig. 2d-g), which is not found with cellulose. The appearance of the central streaks is a clear indication that there are helical fibrils in the polyethylene films. As already noted, the angle of the helix de- termined by means of the streaks is in complete agreement with that determined from the wide-angle reflections. The molecular chain is oriented along the heli- cal fibril which is gradually straightened out in the stretching process [5].

Let us now return to the initial films. The meridional scattering in Figs. 2a and 2b changes immediately to the central streaks due to helical fibrils. I t fol- lows naturally from this that the meridional reflection in the initial sample is "equatorial" scattering from coiled helical fibrils. In the initial sample the angle betwaen the fibril axis (the molecular axis) and the texture axis is close to 90 °, and so the central streak is on the meridian.

I t is difficult to determine the exact direction of the texture axis in the initial film: the 200 reflection cannot be used for this purpose owing to the dispersion of the orientations. We therefore used the 110 reflection for approximate deter- mination of the position of the texture axis. By measuring the X-radiograms of a

3024 D. YA. TSVANXI~¢

n um be r of films i t was found t ha t a ~ 58 ° (Fig. la). Hence the vector angle of Hl l 0 with the t ex tu re axis f l~58.5 °. I f the t ex tu re axis were the a axis, fl would be 56.5 °. The difference is quite small, al though, if the t ex tu re axis is in the plane ac, the angle be tween the molecular axis (the c-axis) and the t ex tu re axis for fl----58.5 ° will be 70 °. I f the t ex tu re axis does not lie in the plane ac this angle will be diff- erent , bu t likewise less t han 90 ° . Consequent ly the initial film is not s tr ict ly speak- ing an a- texture . The t ex tu re axis is a t a certain small angle to the a-axis and the helical fibril along the axis of which the axis of the macromolecule (the c-axis) is or iented m a y be di rected at an angle of 70-80 ° to the t ex tu re axis.

Thus in the initial ex t ruded polye thylene film there are p robab ly helical spirals along the axis parallel to the macromolecular axes. The meridional low- angle scat ter ing is in this case "equa to r i a l " scat ter ing from one fibril or f rom a sys- t em of helical fibrils. I t differs f rom the usual long period caused by a l ternat ing dens i ty wi thin the fibril. I t is hard to believe t h a t helical fibrils are formed f rom other morphological format ions during the s t re tching process. I t is more likely t ha t t h e y are present in coil form in the initial film in the same way as t h ey m a y be found in the radius of a spherulite. I t is na tura l to make this analogy as the radius of a spheruli te is always a t ex tu re in which the macromolecular axis is o r ien ted at a certain angle to the radius which is the t ex tu re axis.

CONCLUSIONS

1. A s tudy has been made of low-angle X-radiograms of polye thylene films possessing a- texture . I t has been shown tha t as a resul t of s t re tching along the t e x t u r e axis the meridional scat ter ing in the initial films changes into central s treaks character is t ic of helical fibrils. The usual long period also appears on stretching.

2. These changes can be explained if it is assumed t h a t the initial film consists of s t rongly coiled fibrils which gradual ly s t ra ighten out when the film is s t re tched.

3. The low-angle scat ter ing of films with a - t ex tu re is due to equator ia l scat- ter ing of helical fibrils.

Translated by R. J. A. HENDRY

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1. Latest methods of polymer research (Russ.) edited by B. Ki, publ. by 'Mir', 1966 2. Small Angle Scattering from Fibrous and Partially Ordered Systems. Edited by R. It.

Marchessault, J. :Polymer Sci. C1 3:1966 3. A. I. KITAIGORODSKII, D. Ya. TSVANKIN and Yu. M. PETROV, Vysokomol. ~oyed.

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8: 2060, 1966 (Translated in Polymer Sci. U.S.S.R. 8:42 12, 2274, 1966) 6. A. N. J. HEYN, J. Amer. Chem. Soc. 70: 3138, 1948; 72: 2284, 1950 7. M. KANTOLA, Faserforsch. und Textiltechnik, 15: 587, 1964