Makromol. Chem. 193,549- 558 (1992) 549

On the conformation and the crystalline structure of an ethylene-chlorotrifluoroethylene alternating copolymer

Gaetano Guerra *, Claudio De Rosa, Mauro Iu liano, Vittorio Petraccone, Paolo Corradini

Dipartimento di Chimica, Universita di Napoli, via Mezzocannone 4, Napoli, 1-801 34, Italy

Rachele Pucciariello, Vincenzo ViIIani

Dipartimento di Chimica, Universita della Basilicata, via N. Sauro 85, Potenza, 1-85100, Italy

Giuseppe Ajroldi

Centro Ricerche Montefluos, via S. Pietro 50, Bollate, Milano, Italy

(Date of receipt? April 22, 1991)

SUMMARY The conformation and the packing of the chains of the alternating ethylene-chlorotrifluoro-

ethylene copolymer (ECTFE), in the crystalline phase, are investigated by X-ray diffraction analysis, by calculations of the conformational energy and by calculations of the Fourier-trans- forms of models. These analyses suggest that the crystalline form of ECTFE is a mesomorphic form, with a pseudo-hexagonal packing of the axes of nearly trans-planar chains (interchain distances of 5,69 A) and a nearly complete translational and rotational disorder along the chain axis.

Introduction

Ethylene-chlorotrifluoroethylene copolymers (ECTFE), with high degrees of one- to-one alternation, have been prepared many years ago These materials were found to be crystalline with melting temperatures (240 "C-265 "C) higher than those of the corresponding homopolymers2s 3).

Some X-ray diffraction patterns of unoriented 2, ') as well as oriented samples 3, 4,

have been reported. However, a detailed description of the diffraction intensities for oriented samples has not been presented. A hexagonal unit cell with a = 9,86 A, containing three chains, has been also suggested3), which appears to be unusual, particularly for a non-stereoregular (although possibly regioregular) copolymer. On the basis of the identity period along the chain axis (c = 5,02 and assuming average bond angles along the chain close to 113", as well as on the basis of the information from polarized infrared spectra of oriented samples, a "kinked" confor- mation for the crystalline phase has been suggested 3). This chain conformation, which presents along the main chain a succession of trans and anticlinaldihedral angles of the kind TA+TA- (more precisely: 180", +135", 180", -135") is shown in two different views in Fig. 1.

0 1992, Huthig & Wepf Verlag, Basel CCC 0025-1 16X/92/$05.00

550 G. Guerra et al.

A trans-planar conformation of the chains, with average values of the bond angles along the main chain close to tetrahedric, has been instead suggested for the ortho- rhombic form of the analogous alternating ethylene-tetrafluoroethylene copolymer (ETFE)s), which shows a similar c value (5,M A). In a recent paper of our group, through a comparison of the observed diffraction intensities with the calculated Fourier-transforms of models 6 ) , the trans-planar conformation for the ETFE chains in the orthorhombic form has been confirmed, and the disorder in the relative heights of neighbouring chains has been studied.

In this contribution, the conformation and the packing of ECTFE copolymer molecules in the crystalline phase are investigated by X-ray diffraction analysis, by calculations of conformational energy and by calculations of the Fourier-transforms of models.

Experimental part

The poly(ethy1ene-ult-chlorotrifluoroethylene) (ECTFE) fibers were supplied by Ausimont U. S. A. (Halar 300). The X-ray diffraction patterns for stretched fibers were obtained by using a Nonius automatic X-ray diffractometer with Ni-filtered Cu-K, radiation. The Lorentz-polariza- tion (LP) correction was applied for all the layer lines, according to the diffraction geometry [LP = (1 + cos2 20)/sin 201.

The corrected diffraction intensities (I) are reported in Fig. 2A for the equator and for the four layer lines, as a function of the reciprocal coordinate 4. The analogous pattern for poly- (ethylene-ult-tetrafluoroethylene) (ETFE) 6, is shown, for comparison, in Fig. 2 B.

For the same layer lines the diffraction intensities integrated over the reciprocal cylindrical coordinate p, (I’), obtained by multiplying the corrected intensity I by (, are reported in Figs.3 A and 3 B, for ECTFE and ETFE, respectively.

Fig. 1. Side view and projection along the chain axis of a model of the alternating ethylene- chlorotrifluoroethylene copolymer (ECTFE), in the “kinked” conformation proposed in ref. 3, A value of chain repetition of 5,02 A is obtained for an average value of the bond angles C-C-C of nearly 113”

On the conformation and the crystalline structure of an . 551

A B

0 0.2 0.L 0.6 i y A - 1

0 0.2 0.L 0.6 tyA-1

Fig. 2. Experimental diffraction intensities (I), corrected by the Lorentz-polarization factor, for the equator (I = 0) and four layer lines (1 5 I 5 4), as a function of the reciprocal coordinate (: A) poly(ethy1ene-ult-chlorotrifluoroethylene) (ECTFE); B) poly(ethy1ene-ult-tetrafluoro- ethylene) (ETFE)

The presence of a single intense peak on the equator of the pattern for ECTFE (Fig. 2A) suggests the presence of a pseudohexagonal arrangement of the chain axes, as for the high- temperature form of ETFE (stable above 100°C)637). The corresponding d spacings, measured for ECTFE and ETFE at a temperature of loO"C, are 4,65 A and 4,93 A , respectively, corresponding to interchain distances of 5,69 A and 5,37 A.

The similarity of the diffraction patterns of the two alternating copolymers on the non- equatorial layer lines is apparent (cf. Figs. 3 A and 3 B). Very broad reflections, located at similar ( values, are observed for both copolymers. The diffraction peak on the 2nd layer-line is for ECTFE also broader than for ETFE.

The diffracted intensity on the meridian (that is the intensity at ( = 0 for variable values of 0 is reported, for both copolymers, in Fig. 4. By the location of the peaks, a value of the chain repetition of 504 A is obtained for both copolymers.

552 G. Guerra et al.

0 ’j-L 1

l&lL==- 0 0 0.2 0.L 0.6

C / a - i

0 U L 1

0 l d c k = L l

lL 0 0 0.2 0.L 0.6

Fig. 3. Diffraction intensities integrated over the reciprocal cylindrical coordinate 9, obtained by multiplying the measured intensity (2) by <, as a function of the reciprocal lattice coordinate r: A) ECTFE; B) ETFE

0.2 0.1 0.6 0.8 a -1

Fig. 4. Experimental diffraction intensities (Z), corrected by the LP factor, along the meridian of the spectrum, that is for < = 0, as a function of the reciprocal lattice coordinate [: A) ECTFE B) ETFE

On the conformation and the crystalline structure of an . . . 553

Calculation methods

The conformational-energy calculations were performed using for the non-bonded interactions the 6-12 potential with Hopfinger's set of parameters8). The fluorine and chlorine atoms, as well as the carbon atoms directly bonded to halogen atoms, were treated as point charges, and Coulombic interactions between these charges were considered. The charges have been calculated through the semi-empirical quantum-chemical CNDO method ') on the model molecule

CH,-CF,-CFCl-CH,

in the trans-planar conformation and are listed in Etb. 1.

Tab. 1. values are expressed as fractions of the electron charge

CNDO net atomic charges for the model molecule de,scribed in the text. The absolute

-C1 I I

F(CC1) F(CF) I I I I

C(FC1) I

C(F2) I 0,42 0,24 -0,17 -0,19 -0,ll

The C-C bond length was taken to be 1,54 A, the one of C-H as 1,07 A, C-F as 1,35 A lo)

and C-CI as 1,78 A lo). For the sake of simplicity the bond angles, in the energy analysis, have been all assumed as tetrahedral. This choice is not critical: in fact, similar energy maps and minimizations are obtained for the cases of average bond angles along the chain in the range

The procedure for the calculation of the Fourier-transforms is that already used for ETFE6). The square of the modulus ( 1 F(<, p, Q I ') of the Fourier-transform was calculated for each model as a function of the cylindrical reciprocal lattice coordinates and [ for a fixed value of the third coordinate 9. The mean value with respect to 9, henceforth indicated as ( I F(<, Q I '), was obtained by averaging the results for 90 different rotations of the models around the chain axis. The integral

109" 113".

which is the quantity to be compared with experimental data such as those of Fig. 3, is then obtained through multiplication of ( I F(<, Q I ') by 2nr.

The calculated intensities Z(& Q were multiplied by a thermal factor of the kind:

exp (- Bc * <'/2) exp (- B( * ['/2)

For the sake of simplicity, Bt and Bc have been fixed equal to those assumed for ETFE6), that is 12 and 0, respectively.

The broadness along (of the meridional diffraction on the fourth I$yer line (Fig. 4A) indicates an average coherent length along the chain axis of the order of 50 A. Calculations of Fourier- transform were then performed on chains with a length along the axis of nearly 50 A .

554 G. Guerra et al.

Results and discussion

Conformationai-energy caiculations

The conformational-energy calculations reported in this paper refer to the chain stretches of Figs. 5 A- C, which correspond to the two possible configurational repeating units (depending on the chirality of the carbon atom with two different substituents (-CFCl-)) for the constitutional repeating unit of ECTFE (Fig. 5A and 5B) and to ETFE (Fig. 5C). It has been verified that the reported results remain substantially unaltered if longer chains (e. g. three constitutional repeating units) are considered, irrespective of the succession of the configurational repeating units (C. R. U.) as well as of the presence of possible regioirregularities in the enchainment of the chlorotrifluoroethylene monomer.

In order to have an easy comparison of the energy of the trans-planar conformation (observed for ETFE) and of the "kinked" conformation (suggested for ECTFE), conformational-energy maps with 8, = 8, = 180" and variable values of O2 and O4 are reported in Figs. 5D-F, for the models of Figs. 5A-C, respectively. The absolute energy minimum corresponds to the tians-planar conformation for ETFE while it is slightly deviated for ECTFE (8, = 8, = 170" or B2 = O4 = 190", depending on the configuration of the repeating unit).

The "kinked" conformation (indicated by a triangle in Figs. 5 D and 5 E) does not correspond to an energy minimum and has an energy of nearly 2 or 5 kcal/mol higher than the absolute energy minimum, depending on the absolute configuration of the repeating unit. Conformational sequences which would allow changes in the succession of the anticlinai dihedral angles, depending on the configuration of successive constitutional units [i. e. irregular conformational sequences of the kind

. . . (A+T A-T) (A-T A+T) (A-T A+T) . . .

rather than regular sequences

. . . (A+T A-T) (A'T A-T) (A'T A-T) . . . ]

would reduce the increase of the energy of the isolated "kinked" conformation with respect to the minimum energy conformation to nearly 2 kcal/(mol of repeating unit). This conformation, however, would maintain straight the chain axis only if large distorsions of dihedral and bond angles in the junctions are involved. Of course, these distorsions would increase again the energy content of the conformation.

Energy minimizations relative to all the dihedral angles 8,, 02, 03. 8., indicate conformations of absolute minima close to the trans-planar one. For instance, for fixed tetrahedral bond angle values, the energy minimum corresponds to the sequences of dihedral angles 175", IW", 175", 185" and l85", 1704 185", 1753 for the chain stretches of Figs. 5A and 5B, respectively*).

*) It is, however, worth to note that also small differences in the values of the consecutive bond angles can produce significative deviations of internal rotation angles from 180" (still with periodicities typical of nearly fully extended chains). This effect has been recently shown in detail for the case of vinyl polymers 11* 12).

On the conformation and the crystalline structure of an . . . 555

3'

300' - I

A

32

6O0

00 Oo 60° 120' 180° 2L0° 300°

I 91

Oo 60° 120° 180° 2LO0 300' 00 1

Fig. 5 . Chain stretches, considered in the energy calculations, corresponding to: A) and B) the two possible configurational repeating units of the constitutional repeating unit of ECTFE, C) of the constitutional repeating unit of ETFE. D-F) Conformational-energy maps, with = 8, = 180" and variable values of 8, and 04, relative to the chain models A- C. The stars indicate the absolute minima, the dots the relative minima and the triangles the "kinked" conformations. The numbers indicate the relative conformational energies in kcal/mol for the minima and for the kinked conformations

556 G. Guerra et al.

Fig. 6. axis of the nearly trans-planar model of ECTFE corresponding to the absolute energy minimum found in the present analysis. A value of chain repetition close to 5,04 A is obtained for an average value of the bond angles C-C-C of nearly 110'

Side view and prbjection along the chain

Random sequences of these two sets of dihedral angles, corresponding to a random succession of the configurational repeating units, can easily maintain the chain axis ucaltered (with only small distorsions of the dihedral and bond angles in the junctions), since in this case all the dihedral angles are nearly trans.

In the assumption of average bond angles along the main chain close to 110" (as supposed for ETFE ' s 6 ) ) , trans-planar and nearly trans-planar conformations are able to account for the observed identity period along the main chain, as shown in Fig. 6.

Fourier-transform calculations of isolated model chains

The Fourier-transforms on the non-equatorial layer lines for a perfectly alternating ECTFE chain in the "kinked" conformation and in the trans-planar conformation are shown in Fig. 7 A and 7B, respectively The results for the nearly trans-planar conformation corresponding to the energy minimum (calculated in the framework of our geometrical assumptions) are substantially equal to those of Fig. 7 B.

It is apparent that, for the trans-planar model chains, the position of the intensity maxima on the layer-lines are in fair agreement with the experimental data (cf. Fig. 7 B with Fig. 3A). On the contrary, for the kinked conformation, the positions of the intensity maxima on the 3rd and 4th layer lines are located at too high ( values (Fig. 7A).

Concerning calculated intensities, an appropriate value of the thermal factor B, (taken as zero in the shown calculations) adjusts the ratio between the values of the intensity maxima on the 2nd and 4th layer lines as well as the ratio between the values of the intensity maxima on the 1st and 3rd layer lines to the experimental results. However, as previously found for ETFE", the calculated values of the intensities are too high for the odd layer lines. As for ETFEQ, the intensities on the odd layer lines

On the conformation and the crystalline structure of an . . 557

ul

C r 3 = 2 > l z 2 2 ._ c l

c

0

+ o Fig. 7. Results of the

3 calculations of Fourier- transforms on the non- 2 equatorial layer lines ( I = 1 1 to 4), for stretches of 0 isolated chains: A) in the 2

1 “kinked” conformation of Fig. 1; B) in the trans- ~

A

1. L 1

0 0 0.2 0.L 0.6 planar conformation

t / R - ’

B

0. 0.2 0.1 0.6 CIA- ’

in the calculated patterns can be easily reduced (leaving unaltered the intensities on the even layer lines) by introducing in the isolated chain some defects in the regular comonomer alternance.

On the basis of this kind of analysis an approximate evaluation of the amount of defects in the comonomer alternance was attempted for ETFE. This kind of evaluation is, for ECTFE, further complicated by the presence of stereoirregularities and regio- irregularities in the enchainment of the CTFE monomer. It will be possibly subject of our future studies.

The similarity between the Fourier-transform of an isolated nearly trans-planar chain and the observed diffraction intensities (and, in particular, the similar broadness of the reflections) indicates that correlations in the positions and orientations of the chains, besides the pseudo-hexagonal arrangement of the chain axes, are substantially absent in the mesomorphic phase of ECTFE. At variance, in the case of ETFE, an analogous comparison has suggested that the translational displacements of the chains, in the direction of the chain axis, are not completely random6).

Conclusions

In summary, the present analysis suggests that the crystalline form of ECTFE is a mesomorphic form, with a pseudo-hexagonal packing of the axes of nearly frans- planar chains (interchain distances of $69 A) and a nearly complete translational and rotational disorder along the chain axis.

Similar structures have been proposed for the hexagonal form of ETFE7), as well as for the phase I of polytetrafluoroethylene at high temperatures I3 - l5 ) .

558 G. Guerra et al.

The authors wish to thank Dr. S. Chandrasekaran of Ausimont U. S. A. for supplying the ECTFE samples. The financial support of the “Ministem dell’llniversitti e della Ricerca Scientifca e Tecnologica” of Italy is also gratefully acknowledged.

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