Determination of the crystallinity of polyvinylidene fluoride

DETERMINATION OF THE CRYSTALLINITY OF POLYVINYLIDENE FLUORIDE* YE. L. GAL'PERIIg, B. P. KOSMYNIN and V. K. SMIRNOV Moscow Textile Institute (Received 16 June 1969)

ACCORDING to l i t e r a t u r e results the crystallinity k of p o l y v i n y l i d e n e fluoride (PVF~) is b e t w e e n 40 and 70% [1-3]. M a k a r e v i c h [1] used I R spectroscopy for d e t e r m i n i n g k of PV'F 2. A m o r p h o u s bands a t 600 a n d 740 cm -1 were used for the m e a s u r e m e n t s for the fl-form a n d a v e r y weak b a n d at 905 em -1 i0r the a-form [4]. The k values obtained, which were 50 to 550/0 for t h e fl- a n d 40% for the a-form, do n o t agree w i t h N a t t a ' s results [2] concerning the crystallinity of PVF~, calculated using the X - r a y m e t h o d a n d representing a m i n i m u m of 700/0 . I t should be noted, however, t h a t t h e r e is no i n f o r m a t i o n concerning the d e t e r m i n a t i o n of the crystallinity o f PVF2 in N a t t a ' s p a p e r [2]. Doll a n d L a n d o [3] d e t e r m i n e d t h e k value b y a n X - r a y m e t h o d for II- or the a-form (68%) a n d a new I I I - f o r m P V F 2 (62%). I n b o t h forms t h e a m o r p h o u s halo is u n d e r the group of main crystalling reflections a n d separation o f the diffraction curve into crystalline a n d a m o r p h o u s c o m p o n e n t s involves considerable difficulty. I t is not indicated in t h e s t u d y how this is being done, b u t owing to the u n c e r t a i n t y of this separation a n error of 4-10% is i n t r o d u c e d in d e t e r m i n i n g k. Using the k value o b t a i n e d for the aform a n d knowing t h e d e n s i t y of its crystallites ( p c r = l ' 9 0 g/cm 3) [5] and t h e d e n s i t y of the sample ( p = 1 . 7 7 g/cm3), t h e a u t h o r s d e t e r m i n e d a value o f pare =1"48 g/cm a for PVF2. The aim o f this s t u d y was to develop m e t h o d s for d e t e r m i n i n g the crystallinity o f PVF2 samples of different forms b y X - r a y a n d from results of density measurements. EXPERIMENTAL A Hilger Y-140 X-ray diffractometer, copper radiation, monoohromatized by a Ross differential filter and a scintillation counter were used iu the study. A rcfleetiorL method was employed. Instead of a standard holder for the sample, a high-temperature attachment was set up on the goniometer. The temperature of the sample during observation of the diffraction curve was maintained with an accuracy of ± 2°. The density of the powders was determined pyertometrieally in alcohol at 25°; the density of samples obtained by compression moulding was established using gradient tubes at 30 °. To eliminate air from the surface and from the open pores of the samples, they were previously retained under a vacuum for 3 to 4 hr. The error of determining the density was less than -t- 0. 002 g/era 3. * Vysokomol. soyed. A12: No. 8, 1880-1885, 1970.

2133

2134

YE.

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GAL'PERII~"

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PVF2 powders obtained both by radiation and chemically initiated polymerization of vinylidene fluoride were examined. The powders were compressed into pellets under a pressure of 200 arm. Diffraction curves were also obtained for samples produced by compression moulding of the polymer at temperatures of 170 to 220 ° (according to the molecular weight of PVF2) followed by subsequent slow cooling or quenching in liquid nitrogen. RESULTS

Separation of the amorphous halo. The X - r a y m e t h o d of determining crystallinity i n d e p e n d e n t l y of its various modifications is based in principle on the comparison of the areas u n d e r the crystalline reflections a n d the amorphous halo [6]. The chief problem arises in separating the areas under the diffraction curve into crystalline a n d amorphous components.

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I~G. 1. Diffraction curves of PVF~: a--fl-form; b--ar-fonn, c--eh-form, d--y-form, e--~h-form after irradiation with a dose of 109 rad in vacuo, f--melt at 220°. PV-F~ crystallizes in several forms. According to the polymerization conditions the polymer m a y assume two forms: fl (Fig. la) or r a n d o m a (ar) (Fig. lb) [7]. During crystallization of P V F 2 from the melt the crystallites normally assume a highly ordered a-form (ah) (Fig. lc) independent of the initial polymer structure [4, 7, 8]. Ho~rever, in some cases during compression moulding of comparatively low molecular weight P V F 2 ~: mptes ~,t t~mperatures of 170 to 185 ° t h e y crystal-

Determination of crystallinity of polyvinylidene fluoride

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lize in a third form (7) [9-11] or in a mixture of (~ -~7). Films obtained from a polymer solution in dimethylformamide or dimethylsulphoxide also crystallize in the y-form [9]. It is possible that PVF~ also assumed this form during crystallization from the melt under a pressure of 3000-5000 arm [3]. Figure 1 indicates that the amorphous halo is only clear in the diffraction curve of the fl-form; in diffraction curves of other PVF2 modifications crystalline reflections are superimposed, and consequently the separation of amorphous and crystalline constituents is complicated. For reliable separation of the amorphous halo a cempletely amorphous sample of this polymer is required. In some cases this can be obtained b y sudden quenching of the polymer melt [12, 13]. As a consequence of rapid crystallization PVF2 cannot be made amorphous b y this method. X-ray diffraction patterns and the density of these samples and slowly cooled samples hardly differ. Diffraction curves for amorphous P V F 2 were obtained ~ i t h samples heated above the melting point of PVF2 crystallites ( ~ 170°). Figure If shows a diffraction curve of a PVF~ melt at 220 °. It can be seen that the position of the maximum of the amorphous halo is displaced b y 2-2.5 ° in the direction of lower angles 20, compared with the position of the halo in Fig. la. Since in the diffraction curve of Fig. la the accurate position of the maximum of the amorphous

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FIG. 2. Dependence of the maximum position of the amorphous halo of PVF~ or~ the temperature of the sample. FIG. 3. Curve showing the dependence of intensity of "crystalline" (Ser) on "amorphous" (Sam)scattering for PVF2. halo remains uncertain owing to the superposition of the crystalline reflection, t h e problem of determination arises. As in a former study [14], we determined this b y two methods. First, a series of diffraction curves was obtained at different sample temperatures (155 to 290 °) and the dependence of the position of the maximum of the amorphous halo on sample temperature plotted (Fig. 2). I t can be seen that the experimental points are satisfactorily situated on a straight line, the extrapolation of which to 25 ° for the maximum position produces,

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YE. L. GAL'P~.m~ et al.

28~=18°40 '. The second method of determining the m a x i m u m position of the amorphous halo involves the use of a satisfactorily oriented PVF~ sample. Crystalline reflections in the range of angles 28=17-21 ° [(200) (ll0)B, (010)~, (200)~ and (110)~] of this sample are situated on the equator, whilst the amorphous halo is weakly oriented. Taking in the aperture an intensity distribution along the meridian, or at an angle of 40-50 ° to it, we obtain an amorphous halo (Fig. la, dashed curve), the m a x i m u m of which corresponds to the angle 28=18°30 '. Thus, in both cases we obtain approximately the same value for 28. It should be noted, however, that the information concerning the shape and position of the amorphous halo obtained using an oriented sample should be applied with care to unoriented samples: a) owing to the different geometry of plotting (clearance and reflection) and b) since differences between the dimensions and structure of amorphous sections in oriented and unoriented samples are not excluded: An analysis of diffraction patterns of PVF2 obtained at sample temperatures of 155 to 290 ° shows that the shape of the amorphous halo remains practically unchanged. It was hence assumed that even at room temperature the amorphous halo retains its shape and diffraction curves of a melt were used (Fig. l f ) to determine the amorphous halo in the diffraction curves of partially crystallized PVF 2 samples (Fig. la-e) assuming that its maximum corresponds to the angle 28= 18°40 '. The next difficulty in determining the area of the amorphous halo is in construtting the base of the halo. In our case the effect of the amorphous halo on the X-ray scattering intensity at angles 28 of 10 and 30 ° can conventionally be assumed as zero in practice and by joining these points of the straight line one can take it to be the base of the amorphous halo. Figure la and b shows that the effect of crystalline reflections on the scattering intensity at angles 28 of 16 and 24 ° can be ignored. From the curve in Fig. i f (considering thermal displacement) we find that in these points intensity 116o=0"53 I~o and I24o-=0"21 I6o, where I 0 is the intensity of the amorphous halo at the maximum. For diffraction curves of Fig. lb and c the effect of crystalline reflections can be ignored at angles of 16 aud 22 ° (I2~o=0.42 I0o). We used these ratios to separate the amorphous component (broken line in diffraction curves of Fig. la-e). It should be noted that because of these assumptions we will probably obtain only a proportional value and not the true area of the amorphous halo. Determination of crystallinity. It has been pointed out in m a n y papers [6, 14, 15] t h a t the polymer model normally accepted for calculating crystallinity (division into the crystalline and amorphous regions without considering first and second order deviations) does not wholly reflect the real structure of crystallizing polymers: Furthermore, it is difficult in practice to use the whole diffraction pattern to evaluate the effect of amorphous and crystalline constituents and to consider various factors (thermal and incoherent scattering, absorption, etc.). However experience shows that often, even when using only part of the

Determination of erystallinity of polyvinylidene fluoride

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diffraction curve and without allowing for these factors, the crystallinity values can be sastisfactorily correlated with results obtained by other methods [6]. Bearing all this in mind, to determine k for PVF, we were restricted to the range of 20 of 10-30 ° and only introduced a correction for X-ray scattering b y air. Crystallinity k 1 was calculated from the formula: ~1 =[Scr/(Scr ~-Sam)]" 100%

(1)

where Scr is the area of crystalline reflection in the range of 20 of 10 ¢o 30 ° and Sam is the area of the amorphous halo. kl values thus calculated for certain PVF2 samples of different crystalline structure and density are tabulated. The crystallinity of the PVF2 powders studied, independent of the shape of the crystallites, is within the range of 38-48~o. After compression moulding the density of PVF2 remains almost unchanged within the range of 1.765-1.78 g/cm2; the crystallinity kl of these samples is also approximately the same (45-50%). To obtain PVF2 samples with low crystallinity parts of the samples were exposed to y-radiation with a dose of 109 rad i n v a c u o and in air. The diffraction curve of the sample irradiated i n v a c u o is shown in Fig. le. The crystallinity is less than 20~o. The crystallinity of the sample irradiated in air is lower still (Table). Diffraction curves of these samples which in the initial state have the ah structure from a qualitative point of view are close to the diffraction pattern of the at-form of PVF2 [8]. The difference is in the displacement after irradiation of the (ll0) and (200) reflections in the direction of smaller angles 20 [(110) by ~20', (200) by ~8'] and in the much more considerable diffusion of the (110) reflection compared with the (200). The first reflection disappears completely. All this proves that in a-crystallites dm'ing irradiation bonds in the direction of the crystallographic c-axis disintegrate first. In spite of all these differences the at-form was attributed to irradiated samples in the Table. It is interesting to note that similar changes in the X-ray diffraction pattern of the an form of PVF 2 are also observed when heating the samples. Thus, on increasing the temperature of the sample from room temperature to 160 ° the (ll0) reflection is displaced by 15' in the direction of lower 20, and the (200) reflection by 3-5'; the (110) reflection is also more diffuse than the (200). However, the position of the amorphous halo in the diffraction curve remains unchanged during irradiation, whereas on heating to 160 ° it is displaced by 1° 20' (Fig. 2). Thus, there is much similarity between the mechanisms of disintegration of the crystalline lattice of the %-form during heating and irradiation. This is also confirmed by the fact that the I R spectrum of the irradiated sample in the 400-1500 cm -1 range almost completely coincides with the spectrum of the PVF 2 melt [1, 10]. To compare results for determining the crystallinity of PVF2, we also used the Hermans method [13, 16, 17]. The method requires the standardization of

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YE. L. GAL'PERIN et aZ. C R Y S T A L L I N I T Y A N D D E N S I T Y OF V A R I O U S

Characteristics of the sample Powder obtained by polymerization with chemical initiation Powder obtained by radiation polymerization in the gaseous phase of the monomer Pellet crystallized from the melt at a pressure of 100 arm at 200° Same at 180° Pellet with ~n structure after irradiation with 10~ rad i n vacuo Same, in air

Shape of crystals

PVF2 S A M P L E S

Density, g/cm 3

kl ~o

k3, °/o

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1.774

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46.5

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experimental results a n d the reduction of the integral i n t e n s i t y of X - r a y scattering on a single scale./ks in a former paper [12], we were unable to carry o u t strict standardization a n d we standardized the overall scattering in the 20 range of 10 to 30 °, i.e. Set q-Sam-kSba~kg (where Sbackg is the area on the diffraction curve u n d e r the base of the amorphous halo) to a certain conventional value. Figure 3 shows a curve of the dependence of S c r = f (Sam) after the setting the rate of experimental results. The crystallinities t h u s obtained (k2) are t a b u l a t e d a n d a r e n o r m a l l y somewhat u n d e r e s t i m a t e d compared w i t h the kl values. To determine the crystallinity k3 from density measurements (p) it is necessary to know the densities of the crystalline pcr a n d amorphous regions pare. Then,

k,3=[(p--pam)l(Per--p~)]"100% per depends on the form of the PVF2 crystallites, whereas pare can a p p a r e n t l y be assumed to be the same for all samples. F o r crystallites of the a-form per

= 1"89 g/em 3 [8, 18], for the fl-form--1.925 g/cm 3 [8]. The density of crystallites of the ?-form is u n k n o w n since so far n9 information is available concerning elem e n t a r y cell parameters of this form. On changing the parameters cell (e.g. after irradiation) a correction should be introduced in per [19]. T h u s , for the PVF~ sample shown in the Table a n d irradiated i n v a c u o the parameters are: a = 9 - 6 6 , b = 5 . 1 1 a n d c = 4 . 6 5 A, from which p e r = l ' 8 4 g/cm 8. Using this value of per a n d t a b u l a t e d result for k 1 a n d p of the irradiated sample, we obtain pare= 1"675 g/cm 3. The k~ values calculated from formula (2) are tabulated. F o r a sample with a ?-structure two ]ca values are given which were calculated from per for a a n d flerystallites, as there is reason to assume t h a t the density of ?-crystallites has an i n t e r m e d i a t e value. The authors are v e r y grateful to D. Ya. Tsvankin for his valuable comments on reading the manuscript a n d to R. A. B y c h k o v for his help in determining the d e n s i t y of bulk samples.

Determination of crystallinity of polyvinylidene fluoride

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CONCLUSIONS

(1) X - r a y m e t h o d s were d e v e l o p e d for d e t e r m i n i n g t h e c r y s t a l l i n i t y o f p o l y v i n y l i d e u e fluoride (PVF2) s a m p l e s o f different crystalline s t r u c t u r e s . (2) F r o m e x p e r i m e n t a l d e n s i t y values of PVF~ a n d X - r a y results, considering t h e d i m e n s i o n a l v a r i a t i o n o f t h e e l e m e n t a r y cell of i r r a d i a t e d samples, t h e d e n s i t y of a m o r p h o u s P V F 2 was calculated (1.675 g/cm3). This facilitates the use o f t h e d e n s i t y v a l u e for d e t e r m i n i n g t h e c r y s t a l l i n i t y of s a m p l e s c o n t a i n i n g crystallites of ~- or /?-form. (3) I t was f o u n d t h a t i n d e p e n d e n t of t h e s h a p e of PVF2 crystallites t h e crystallinity of initial p o w d e r s a n d s a m p l e s crystallized f r o m the m e l t is a p p r o x i m a t e l y

4o-50% (4) I t was s h o w n t h a t d u r i n g y-radiation o f PVF2 w i t h a dose of l09 r a d b o t h t h e c r y s t a l l i n i t y a n d t h e ordering o f t h e crystallites t h e m s e l v e s c o n s i d e r a b l y decreases. D i s i n t e g r a t i o n o f a-crystallites d u r i n g i r r a d i a t i o n a n d h e a t i n g o f P V F 2 s a m p l e s first t a k e s place in t h e direction of t h e c r y s t a l l o g r a p h i c b-axis. Translated by E. SEMERE REFERENCES

1. N. I. MAKAREVICH, Zh. prikl, spektroskopii 2: 341, 1965 2. G. NATTA, G. ALLEGRA, $. W. BASSI, D. SIANESI, G. CAPORICEIO and E. TORTI,

J. Polymer Sei. A3: 4263, 1965 3. W. W. DOLL and J. B. LANDO, J. Macromolee. Sci. B2: 219, 1968 4. Ye. L. GAL'PERIN, Yu. V. STROGALIN and M. P. MLENIK, Vysokomol. soyed. 7: 933, 1965 (Translated in Polymer Sci. U.S.S.R. 7: 5, 1031, 1965) 5. J. B. LANDO, H. G. OLF and A. PETERLIN, J. Polymer Sci. 4, A-l: 941, 1966 6. W. O. STATTON, J. Polymer Sci. C18: 33, 1967 7. Ye. L. GAL'PERIN, S. S. DU:BOV, Ye. V. VOLKOVA, M. P. MLENIK and L. A. BULYGINA, Vysokomol. soyed. 8: 2033, 1966 (Translated in Polymer Sei. U.S.S.R. 8: 11, 2246, 1966) 8. Ye. L. GAL'PERIN and B. P. KOSMYNIN, Vysokomol. soyed. All: 1432, 1969 (Translated in Polymer Sci. U.S.S.R. 11: 7, 1624, 1969) 9. Ye. L. GAL'PERIN, B. P. KOSMYNIN and R. A. BYCHKOV, Vysokomol. soyed. BI2: 555, 1970. (Not translated in Polymer Sci. U.S.S.R.) 10. G. GORTILI and G. ZERBI, Spectrochim. acta A23: 285, 1967 11. G. GORTILI and G. ZERVI, Spectrochim. acta, A23: 2216, 1967 12. V. P. LEBEDEV, N. A. OKLADNOV, K. S. MINSKER and B. P. SHTARKMAN, Vysokotool. soyed. 7: 655, 1965 (Translated in Polymer Sci. U.S.S.R. 7: 4, 724, 1965) 13. G. CHALLA, P. H. HERMANS and A. WEIDINGER, Makromolek. Chem. 56: 169, 1962 14. N. KAKUDO and R. ULLMAN, J. Polymer Sci. 45: 91, 1960 15. V. A. MOSKALENKO and D. Ya. TSVANKIN, Vysokomol. soyed. All: 383, 1961 (Translated in Polymer Sci. U.S.S.R. 11: 2, 429, 1961) 16. P. H. HERMANS and A. WEIDINGER, Makromolek. Chem. 44-46: 24,, 1961 17. P. H. HERMANS, Materia Plastiche ed Elastomeri 5: 464, 1963 18. K. OKUDA, T. YOSHIDA, M. SUGITA and M. ASAHINA, J. Polymer Sci. B5: 465, 1967 19. L. G. KAZARYAN and D. Ya. TSVANKIN, Vysokomol. soyed. A9: 377, 1967 (Translated in Polymer Sci. U.S.S.R. 9: 2, 423, 1967)