Polymorphism and Phase Transitions of Linear Oligomers of

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J. Phys. Chem. 1995,99, 4609-4619

4609

Polymorphism and Phase Transitions of Linear Oligomers of Polytetrafluoroethylene Revealed by Vibrational Spectroscopy Masamichi Kobayashi" and Toshihisa Adachi Department of Macromolecular Science, Faculty of Science, Osaka University, Toyonaka, Osaka 560, Japan Received: June 17, 1994; In Final Form: October 11, 1994@

As for the polymorphism of a linear oligomer of polytetrafluoroethylene (PTFE),perfluoro-n-eicosane (c20F42), Strobl and co-workers have clarified by means of X-ray and neutron diffraction that this compound crystallizes in three crystal modifications designated, from the higher to lower temperature range, as the rhombohedral (R), intermediate (I), and monoclinic (M) phases, and the molecules assume essentially the same ordered conformation corresponding to the (15/7) uniform helix of PTFE in the I and M phases. In the present work conformation and aggregation state of the molecules in the crystalline phases of c 2 8 4 2 and C24F50 (perfluoron-tetracosane) as well as their mixtures were investigated by means of vibrational spectroscopy and DSC. In infrared spectrum of the R phase there were observed satellite absorption bands associated with the molecules having gauche-type conformational defects at the chain ends. The amount of the end-gauche conformation decreased with lowering temperature and vanished in the I and M phases. The enthalpy difference between the gauche and nearly trans conformations at the chain ends was estimated as ca. 8.0 kJ/mol per bond. Presence of the same end-gauche conformation was confirmed in the high-temperature phase of C24F50. The conformational defects at the chain ends in C Z O F & ~ ~ Fmixed ~ O crystals were also investigated. Formation of the end-gauche in the shorter molecules was found to be restricted by being surrounded with the straight chain stems of the longer molecules. On the contrary, the chain ends of the longer molecules were squeezed out of the lamellae into the interfacial area and assumed gauche-like conformation. IR spectrum indicated that the conformational defects in the mixed crystals were frozen even in the low-temperature phase. Phase transition behavior and thermodynamic stability of the M phase were found to depend on the domain size of crystallites. IR spectral changes on the phase transitions suggested that organization of the terminal CF3 groups at the lamellae interface played an important role for the appearance of the M phase.

Introduction Polytetrafluoroethylene (FTFE) is a typical linear polymer having a simple chemical formula, (-CFZ-)~, and is of considerably commercial importance. It is also interesting from a scientific viewpoint on account of the polymorphism and solidstate phase transitions. It appears in three crystal modifications within different temperature ranges. Below 19 "C the molecules assume a (13/6) uniform helix (13 CF2 groups are arranged in six tums of the helix) forming a monoclinic or triclinic lattice. Between 19 and 30 "C, the conformation changes to a (15/7) helix, the chains being arranged in a trigonal crystal structure. Above 30 "C, the molecules are believed to contain conformational defects even though the hexagonal alignment of the molecular chains is As for the detailed molecular structure in the respective modifications and the molecular mechanism of the phase transitions at 19 and 30 "C, studies based on vibrational spectroscopy and lattice dynamics have been made by Zerbi and co-worker~.~,'However, there still remain some unsettled problems. Quite recently, extensive works on the polymorphism and thermal phase transitions of perfluoro-n-eicosane c 2 8 4 2 , a linear oligomer of PTFE, were performed by a German group8-I0 by means of X-ray and neutron diffraction as well as molecular dynamical computation. They showed that this compound appeared in three crystal forms in different temperature ranges at atmospheric pressure designated as, from higher to lower temperature range, R (rhombohedral), I (intermediate), and M (monoclinic) phases. It was clarified that the molecules in the M and I phases assumed essentially the same ordered conforma@

Abstract published in Advance ACS Abstracts, March l , 1995.

0022-3654/95/2099-4609$09.00/0

tion corresponding to the (15/7) uniform helix of PTFE, and in the R phase the molecules were perturbed by the formation of helix reversal defects and were also disordered with respect to their longitudinal position and the azimuthal angle around their axis. Although the temperature ranges of the appearance of three crystal modifications of the oligomer are different from those of PTFE, detailed analyses of the phase transition behaviors of this compound will give us valuable new information for understanding the polymorphism of PTFE. The crystal structure of the high-temperature phase of C24F50 has been determined by Zhang and Dorset by means of electron diffraction." As for the vibrational spectra of PTFE oligomers, many important results have been reported so far.I2-l8 However, in most of the previous works, efforts were concentrated to elucidate the vibrations of the molecules as well as the molecular structures, and little was taken into consideration on the aggregation states of the oligomer molecules. In the present work, we are concerned with polymorphism and behaviors of the solid-state phase transitions of perfluoron-eicosane and perfluoro-n-tetracosane C24F50 revealed by infrared and Raman spectroscopy. Compared with studies on polymeric samples of PTFE, the use of the oligomers has the following advantages. (1) We are able to get spectra of singlecrystal specimens free from interference by the signals due to the amorphous part. (2) Assignments of the symmetry species of the bands are definitely made from polarization data measured on single crystals. (3) In addition to the optically active zonecenter modes which are observed in polymeric samples, many progression bands appear in the oligomers with finite chain length. Some of them exhibit substantial changes in their

0 1995 American Chemical Society

Kobayashi and Adachi

4610 J. Phys. Chem., Vol. 99, No. 13, 1995

profiles, reflecting the changes in the state of molecules far sensitively compared with the bands due to the zone-center modes. Therefore, we are able to derive more convincing information about the structures and their changes through quantitative analysis of the spectral data. (4) The number of the observed spectral bands of the oligomers is far larger than that of PTFE,and each band gives us its own information. Therefore, more comprehensive discussion can be made. Our attention is focused to the following problems. (1) The type and amount of the conformational defects formed in the molecules and their temperature dependencies are evaluated by quantitative analyses of the profiles of various progression bands. (2) The lateral packing of the chain molecules and the intermolecular potential field in the three crystal phases are investigated from the split pattern of the infrared bands in connection with the rotational motion of the molecules. (3) Infrared and Raman spectral changes induced by the change in the state of the terminal CF3 groups are studied in order to elucidate the role of the interfacial structure in the lamellar crystals. (4) The effect of contamination of homologous impurity on the phase transition behaviors is investigated from the spectral data obtained for mixed crystals of c 2 8 4 2 and c 2 8 5 0 of various compositions. (5) The effect of crystallite size on the phase transition behaviors is considered by comparing the thermal and spectral data between polycrystalline and singlecrystal specimens.

C20F42

I I

143.1 K A H =0.69 kJ/mol I

I20

I

-

-

Results and Discussion 1. Polymorphism of c 2 8 4 2 and C24F50. Figure 1 shows the DSC thermograms (on heating) measured on powder samples of c2OF42 and C24F50 in the temperature range below the melting points (T, = 437 K for C&42,462 K for C24F50) at atmospheric pressure. In C20F42 two endothermic peaks for the first-order solid-state transitions appear at T,I = 199.9 K (AH!= 3.90 kJ/mol) and Tc2 = 143.1 K ( A H 2 = 0.69 kJ/mol). The three crystal modifications are designated, from the higher to lower temperature range, as R(rhombohedral, Tm T > T,I), I (intermediate, TCl1 T L Tc2), and M (monoclinic, T < Tc2) in accordance with the notations in ref 7. In C24F50 there appears only one distinct endothermic peak at T,I = 214.7 K (AH1 = 6.15 kJ/mol) accompanied with a weak shoulder at the low-

II

Y

I

I50

199.9 K AH. 3.90kJ/mol 1

I

210

I80

C24F50

240

Powder

1

214.7K

I I20

Experimental Section Samples of C2oF42 and C24F50 of a commercial source (Aldrich Chem. Ind.) were used without further purification. Plateshaped single crystals were grown from solutions dissolved in trichlorotrifluoroethane. Powder mixture of C20F42 and c 2 8 5 0 with various compositions (represented here by the mole ratios of C ~ O F ~ ~ / C were ~ ~ Fsealed S O ) in aluminum pans, molten at 250 “C, and then cooled to room temperature. DSC thermograms were taken with a SEIKO DSC type 120 in the temperature range of 500-100 K. Measurements were carried out with various heating (or cooling) rates on each sample, and the transition temperatures for I R (TCl) and M I (Tc2)as well as the corresponding transition enthalpies AH1 and A H 2 were evaluated through extrapolation to the zero rate. Infrared spectra were measured using a JASCO 8300 Fourier transfondinfrared (FTRR) spectrometer equipped with a wiregrid polarizer. Polarized Raman spectra were taken with a JASCO NRlOOO double monochromator using the 514.5 nm line from an Ar+ laser for the excitation. For the spectral measurements at low temperatures an Oxford flow-type cryostat was used. The temperature controlled within the fluctuation of f0.5 K was measured by two sets of thermocouples attached in the vicinity of the sample.

Powder

AH.6.15 kJ/mol I

I50

I

I

I80 T / K

210

240

Figure 1. DSC thermogram (on heating) of powder samples of C20F42 and C M F ~ ~ .

temperature wing of it. The modification above Tcl is named here as the R phase, although the unit cell belongs to a facecentered orthorhombic Bravais cell and contains four molecules arranged in two molecular layers. In each layer the molecules pack in a hexagonal lattice as in the case of the R phase of C20F42.I0 The crystal structure below Tcl is not known. As for the presence of another phase corresponding to the M phase of C2oF42 will be discussed in section 5. 2. Conformational Defects Formed in R Phase. The zonecenter normal modes of the (15/7) uniform helix of PTFE are classified into nine symmetry species (r = AI 3A2 8E1 9E2 ... 9E7). The AI and E2 species are Raman-active, the A2 is IR-active giving rise to parallel absorption bands, the El is active in both IR (perpendicular bands) and Raman spectra, and the other species are optically inactive. The optically active fundamentals are designated here as Al(m)(m = 1 - 4), Az(m)(m= 1 - 3) and El(m)(m = 1 - 8) and Ez(m)(m = 1 - 9), the numbering m being made in the order of higher to lower frequency. In the infrared spectra of the oligomers with finite chain lengths, there appear many absorption bands, referred to as “progression band series” which correspond to the vibrational modes on the dispersion curves of the infinitely extended uniform helix of PTFZ at particular allowed phase angles between the neighboring CF2 groups. For the present case, there are nine branches of the vibrational dispersion V I3 9 , and the phase angles allowed for CnFzn+2are given by the equation

+

+ +

d,=md(n-l),

+

m = l , 2 , 3 ,..., n - 2

+

(1)

in the approximation of simply coupled oscillators. The progression bands are referred to as vj(m). Among them, the

J. Phys. Chem., Vol. 99, No. 13, 1995 4611

Polymorphism of Polytetrafluoroethylene

t-

C24F50 U3( I O ) 988

I

1000 IO00

I

I

I

900

800

700

Wavenumber Icm-1

Figure 2. Temperature dependence of infrared spectrum of a powder sample of C2oF42. The assignments of some v3 and v4 progression bands as well as the positions of the Al(2) and Az(2) zone-center modes of PTFE are indicated.

modes with the phase angles closest to those of the zone-center modes of PTFE (#m = 0 for A1 and Az, #m = 0 . 9 3 for ~ El,and #m = 0 . 1 3 for ~ E2) are named as the most-in-phase modes and they give rise to intense infrared and/or Raman bands similar to the zone-center modes of PTFE. Figure 2 shows infrared spectra of CZOF42 taken on a powder sample (KBr disk) at various temperatures covering the ranges of the three crystal modifications. In the frequency region shown here, there appear some progression bands of v3 [symmetric CF2 stretch v,(CFz) CF2 wagging w(CF2)I and v~[w(CFZ)] branches. The positions of the zone-center modes [A1(2) (Raman-active) and A2(2)] of PTFE are also indicated. The corresponding spectra of C&0 (powder) are reproduced in Figure 3. The Raman spectra of the oligomers at room temperature resemble that of FTFE, having no distinct progression bands. At low temperatures some of the progression bands are sharpened and become detectable as shown in Figure 4 with the examples of the progression bands in the vicinity of the Al(2) and Al(3) bands. With lowering temperature from 300 K (R phase) to 30 K (M phase), no distinct changes are observed in the main spectral features of the oligomers, except for significant reduction in

+

I

I\

Powder

.

.

.

.

I

.

900

.

.

.

I

.

.

.

.

I

.

000 7 00 Wovenumber/cm- 1

.

.

.

60 0

Figure 3. Temperature dependence of infrared spectrum of a powder sample of C24F50. The assignments of some v3 and v4 progression bands are indicated.

bandwidth, suggesting that the nearly trans conformation of the (W7) helix (with the C-C intemal rotation angle of z = 165.7") almost remains unaltered in the whole temperature range investigated. Zerbi and Sacchi showed by lattice dynamical calculation of conformationally disordered PTFE molecule that the vibrational spectrum of PTFE was not greatly changed by the introduction of conformational defects.6 The presence of conformational defects, if any, is considered to be reflected to the progression bands on the branches with steep dispersion. Here, we have monitored the temperature dependencies of the infrared progression bands of the v3 branch, since it shows the steepest dispersion among the nine branches, in particular in the range of = 30-120". As shown in Figures 2 and 3, there appear small satellite bands on the high-frequency wing of the v3(6) and v3(8) bands of C20F42 and the v3(4), v3(6), and 148) bands of C24F50 at room temperature, and their intensities decrease with lowering temperature, vanishing below Tcl.The v3 branch of PTFE is shown in Figure 5 with the data points of the v3(m) bands measured on c 2 8 4 2 and c24F50, the phase angle for each band being given by the ratio m/(n - 1) in eq 1. The frequencies of the three satellite bands of C20F42 are indicated by the horizontal bars, and the phase angles of 6d17, 6n/18, 8 M 7 , and 8 d 1 8 by the vertical bars. From their positions the satellite bands are assigned to the vibrations of the straight stems of the helical molecules shortened by one (the 841 cm-' band with # = 6d18) or two carbon atoms (the 854 cm-' band

Kobayashi and Adachi

4612 J. Phys. Chem., Vol. 99, No. 13, 1995

A .

,

,

.

.

.

.

.

A

I\

139K

n

7 0

.

7 40

Figure 4. Temperature dependence of polarized Raman spectra measured on a single crystal specimen of v7 progression bands are indicated.

with 4 = 6d17) or their superposition (the 970 cm-' band with 4 = 8 d 1 8 4 = 8d17) of the C20F42 molecules. Here, we consider the type of conformational defects present in PTFE oligomers. The potential energy calculated for the uniform helix of F"FE gives mutually symmetric three pairs of minima at the internal rotation angles of z = f60", f90", and f165".19 The deepest minima lie at z = f165" that correspond to the right- or left-handed ( W 7 ) uniform helix of PTFE (z = 165.7'). Hopping of the conformation between the two deepest minima through a very small potential barrier causes

+

C20F42.

.

The assignments of the

v3

and

a helix reversal motion that has been mentioned by Zerbi and Sacchi6 and Albrecht et aL9 Conformational defect of this type might not be localized to a particular site of the molecule but be distributed uniformly in the whole molecule, and the defect site moves along the chain. This arises broadening of IR and Raman bands. In contrast, the potential minima at t = 60" and 90' are very steep, and the respective conformations are able to be localized at particular sites of the molecule. The similar steep minima appear in the calculated potential energy of p01yethylene.I~ In fact, in crystalline phases of n-alkanes

Polymorphism of Polytetrafluoroethylene

J. Phys. Chem., Vol. 99, No. 13, 1995 4613 20-

C20F42 l4/ 23

/

I100

815\ I-

Q

-

\

I

+= m r / ( n - I)

IOOOt

10-

10/234

0 -

7

a m

LL

Q

/

0 C20F42

OC24F50

T/ K Figure 6. Temperature dependence of the integrated intensity fractions AJATof the satellite bands measured for the 146) and ~$3) progression bands of C20F42.

C20F42

I

0

I

I

I

30

60

90

I

120

1

150

Powder

I

180

+/deg.

-1.ot

Figure 5. Dispersion curves of the v3 branch and the positions of the v3 progression bands measured on C20F42 and C24F50 samples. For C20F42, the frequencies of the satellite bands of the 146) and v3(8) progression bands and the phase angles for the molecules having gauche-formed chain-end@) are indicated with horizontal and vertical bars, respectively.

presence of the gauche conformations at the chain ends (endgauche) or the central part of the chain (kink) has been recognized. In the case of PTFE oligomers, we considered that the shortening of the molecular stems was caused by the introduction of the gauche-type conformational defects at one or both chain-ends by the analogy of n-alkanes. In what follows we named tentatively the conformational defects of this type as the end-gauche defect. The ratio of the integrated intensity of the ith satellite component Ai to the total integrated intensity AT of the progression band is regarded as the weight fraction of the respective shortened chain stems assignable to the ith satellite band. The temperature dependencies of AJAT for the abovementioned three satellite bands of C20F42 are shown in Figure 6. For every case the AJAT decreases with lowering temperature and almost vanishes below Tcl, i.e., as the crystal goes into the I phase. This suggests that the conformational defects at the chain ends (in other words, at the lamellae interface) cease to occur in the I and M phases. For the ~ 3 ( 6 )and ~ 3 ( 8progression ) bands, logarithm of the intensity ratio of the ith satellite to the main component, ln(AJ Ao) is plotted against the reciprocal of the absolute temperature in Figure 7. Each plot gives a straight line with the slope of -AHi/R, where AHi denotes the increase in enthalpy due to the introduction of gauche conformation to the chain end(s) and R the gas constant. The 840 and 854 cm-' satellite bands of the v3(6) progression band give AH = 7.16 and 17.79 kJ/mol, respectively. The 970 cm-' satellite band of the 148) progression band gives AH = 11.03 kJ/mol. Based on the assignments of these satellite bands, the AH (=Hgauche - Hnearly sans) value at the chain-end is evaluated as ca. 8.0 kJ/mol per bond. Similar

AH=7.16 kJ/mol

-4.0-

-5.0

-

3.0

4.0 50 I / Tx103 Figure 7. Temperature dependence of the integrated intensity ratios AJAo of the satellite bands to the respective main component measured for the v3(6) and ~ 3 ( 8 )progression bands of C20F42.

intensity analysis of the satellite band for the ~ 3 ( 8 )mode of C24F50 gave AH = 7.8 kJ/mol per bond (Figures 8 and 9). The 748 cm-' band in the vicinity of the 744 cm-' v3(4) band is not the satellite band of this sort, because it differs from other satellite bands in polarization and in temperature dependence. 3. Conformational Defects in Mixed Crystals of CZOF~Z and C24F50. In mixed crystals of c20F42 and C24F50, the chain ends of the longer oligomer are considered to be squeezed out of the molecular lamellae into the interfacial area, so that the amount of the end-gauche conformation is predicted to increase compared with that in pure C24F50. On the contrary, occurrence of the conformational defects of the shorter component might be little affected or even restricted by being surrounded with

Kobayashi and Adachi

4614 J. Phys. Chem., Vol. 99, No. 13, 1995

-

I

I

TC = 86.5 ' C

I

I

50/50

- Io

o

T / K

o

v

30/70

Figure 8. Temperature dependence of the integrated intensity fraction AJAr of the satellite band measured for the 1 4 8 ) progression band of

C24F50.

c

C24F50

I

TC= -81.2 O C

b3(81

AH: 7.83kJ/mol

C2o/C24

200 -

0

Mixed Crystal

Tm

OA TC

3.0

I 4.0

i 5.0

I / TxlO'

Figure 9. Temperature dependence of the integrated intensity ratio A,/Ao of the satellite band to the main component of the v3(8) progression band of C24F50.

longer molecules in the mixed crystals. In order to verify this prediction, temperature dependencies of the intensities of the satellite bands associated with both C20F42 and C24F50 were investigated for their mixed crystals of various compositions. For this purpose, occurrence of co-crystallizationin the mixed samples prepared was checked by DSC as shown in Figure 10. For every case, there appears only one broadened endothermic peak on heating at the position in-between the T,l's of C20F42 and C24F50. The melting and transition temperatures measured for the mixed crystals vary with composition as shown in Figure 11, suggesting that co-crystals are formed in the present mixed samples. In what follows the composition of the mixed samples is represented by the mole ratio of (c2&24). We mention first the bands associated with the shorter component. In the IR spectrum of the mixed crystals, the

I

-1201

f -144

1'0

i o i o 40

5b

$0

7'0

8b do

,bo

C24F50 /mol%

Figure 11. Composition dependence of the melting point T,,, and the C ~ ~ Fcrystals. ~O transition point T, of C Z O F ~ ~ / mixed

absorption profiles due to C2oF42 are almost the same as those of pure C2oF42 (Figure 12). The satellite bands (marked with

Polymorphism of Polytetrafluoroethylene

I

PURE

C20

I

J. Phys. Chem., Vol. 99, No. 13, 1995 4615

C20/C24

MIX( 90/10)

c I

.

4

1000

,

.

.

,

I

,

.

.

,

900

I

-

.

,

I

,

(10/90)

.

,

.

1000

800

Wavenumber/cm-1

.

.

.

,

.

.

.

.

900

,

,

.

.

800

Wavenumber/cm-1

(a)

(b>

Figure 12. Infrared spectra of C20F42/C24F50 mixed crystals with various compositions. The satellite bands of the v3(6) and ~ ( 8progression ) bands of C20F42 are marked with arrows.

0

C20/C24 MIX(90/10)

v)

a U

1000

I I I I I / I ( I ( I

OO

50

IO0

C20F42/ mol % Figure 13. Composition dependence of the integrated intensity fractions AJAT of the satellite bands measured at room temperature for the 1 4 6 )and v3(8) progression bands of c2OF42 accommodated in c2&/ C24F50 mixed crystals.

990

980

Wavenumber/cm-I

Figure 14. Temperature dependence of the v3( 10) infrared band profile of C24F50 measured for pure sample (upper) and a C ~ O F ~ ~ / C mixed ~~FSO crystal with (90/10) composition (lower).

arrows) of the v3(6) and ~ 3 ( 8 )progression bands behave in the same manner as in pure C20F42with variation in temperature, giving nearly the same AH = 7.39 kJ/mol per bond for the

Kobayashi and Adachi

4616 J. Phys. Chem., Vol. 99, No. 13, 1995 1.5

50

c20/c24

MIX (80/20)

c 20

Powder

8

2 ?

20

Wavenumber i cm-1

Figure 17. Temperature dependence of infrared absorption profile due to the E1(4)mode of powder samples of CZOF42 (upper) and C24F50 (lower).

t Tc I

I

1

IO0

200

300

T/K Figure 15. Temperature dependence of the integrated intensity fraction AJAT of the satellite band of the v3(10) infrared band of C24F50 accommodated in a C20F42/Cz4F50mixed crystal with (80/20) composition.

out-of-plane

in-plane

Figure 18. Splitting of the doubly degenerate El mode in the M lattice consisting of right(R)- and left(L)-handed helices. El Ai,A2 E2 I

I

I

I

I200

Rhombohedral

aR = bR = 5.70A CR = 85.00A p =120" R 3 m - Dd:

Monoclinic

am = 9.65 A pu.970

bu = 5.70A cu = 28.3 A Pa-Ci

Figure 16. Correlation between the unit cells (end-view) of the rhombohedral (R) and monoclinic (M) lattices of C20F42. The positions of the molecular chains at each layer of the triple-layered R cell are distinguished by solid, broken, and dotted circles. The symmetry elements of the space groups of R3m (R) and Pa (M) are described. gauche-trans enthalpy difference at the chain ends. The fraction of the integrated intensity of a (say, ith) satellite band Ai to the total absorbance of the progression band AT was measured at room temperature as a function of composition for the 854 [for v3(6)] and 970 cm-I [for v3(8)] satellite bands (Figure 13). They decrease linearly with an increase in degree of dilution with longer ((224) component, suggesting that occurrence of the end-gauche conformation is more or less restricted for c & 4 2 molecules surrounded by the ordered stems of C24F50 molecules. W e mention next the conformational defects in the longer component in the binary system. Unfortunately, the progression

I

30

1

I

60

90

4 / deg.

I 120

I

150

I

I

Figure 19. Dispersion curves of the v3 and v4 branches of F'TFE and the frequency of the v,(CF3) mode. The positions of the progression bands measured on C2OF42 and Cz&o are indicated.

J. Phys. Chem., Vol. 99, No. 13, 1995 4617

Polymorphism of Polytetrafluoroethylene

Wsvsnuaber/cm-l

Figure 20. Temperature dependence (on cooling) of infrared spectrum of a C20F42 single crystal measured with radiation incident normal to the lamella face. Discontinuous spectral changes are seen at the transition points. 65(

C20F42

Single Crystal

C20 Powder

108K

c 2 0 Single-Crystal

105K

184.7K 201.1 K AH811.49 AH.3.28 kJ/mol I 1

3 3. \

0.0

0 v)

0.0 108K

P

C I30

I55

180

205

230 Wavenumber I cm-1

heating ‘

l

0 I30

o

I

I55

o

, I80

Wavenumber i cm-1

Figure 22. Split patterns of the El(4) infrared band of C&2 and C24F50 measured on powder and single crystal samples. I

201.6K AH=7.33kJ/moI 2 05

1lOK

230

T/ K Figure 21. DSC thermograms, on cooling (upper) and heating (lower), of C20F42 measured on a single crystal sample.

bands that carry satellite bands are overlapped by the absorption due to C2fi42. Therefore, we investigated the ~ ~ ( 1progression 0) band around 990 cm-I, because it was free from the interference by the other bands. This band appears as a singlet carrying no satellite bands in pure C ~ &even J at room temperature (Figure 14, upper). In a (90/10) mixture, on the contrary, there appears another band as a shoulder appearing at the high-frequency side of the main band. It becomes clearer with lowering temperature (Figure 14, lower). This shoulder is not detectable for the mixtures containing c2OF42 less than 70% and can be regarded as the satellite band due to the C24F50 molecules having conformational defects at the chain end(s). Fraction of the integrated intensity of this shoulder to the total v3(10) band intensity measured for a (80/10) mixture is plotted against

temperature in Figure 15. The ordinate represents approximately the weight fraction of C24F50 molecules having conformational defect at the chain ends. At room temperature, the content of the end-gauche in the (80/10) mixture is twice as large as in pure C24F50 (cf. Figure 8). Although the end-gauche content decreases with lowering temperature, it does not vanish but approaches a finite value even below TClin contrast to the case of pure oligomers. This fact suggests that a part of the conformational defects is frozen even in the low-temperature phase, presumably concentrated in the lamellae interface. In the DSC thermogram of mixed samples, there is no distinct peak corresponding to the transition between the I and M phases of C2oF42 (see Figure 10). As will be discussed in the next section, rearrangement of the terminal CF3 groups in the lamellae interface plays an important role in this phase transition. 4. Crystal-Field Splitting of IR Bands in M Phase. The unit cell of the M phase of C2oF42 is of a single-layered type (in contrast to the triple layered unit cell of the R phase) and contains two molecules which are correlated with each other by the a-glide symmetry, one being the left-handed and the other right-handed helix. The unit cell correlation between the R and M phases viewed along the chain axis is illustrated in Figure 16 where the space group symmetry elements are described. The molecules positioned at different heights in the triple layered unit cell of the R phase are distinguished by the solid, broken, and dotted circles. In the R phase, the molecules are in a highly mobile state around the chain axes (by librational and/or helix

Kobayashi and Adachi

4618 J. Phys. Chem., Vol. 99, No. 13, 1995 0.50

C24F50

a

u 3(4)

Single- Crysta I

AI(2)

ABS

0 . 0 0 1 1 . . . . I 910 e50

. . . .

. . . .

I

. . . .

1

750

BOO

I

1

690

Wavenumber /cm-i

Figure 23. Temperature dependence (on heating) of infrared spectrum of a normal to the lamella face.

reversal motion). Therefore, crystallographically the molecules are regarded as structure-less cylindrical rods. The unit cell of R contains one molecule per layer, and there is no chirality that distinguishes the handedness of the helix. The high-temperature phase (above Tcl) of C24F50 belongs to a face-centered orthorhombic Bravais lattice with the unit cell containing four molecules arranged in two molecular layers, and in each layer, molecules pack in a hexagonal lattice and are in a highly mobile state like in the R phase of C20F42.I1 In IR spectra of the R phase of C20F42 and the high-temperature phase of C24F50, no split pattem caused by intermolecular coupling is observed for every molecular mode investigated, as predicted from the structures mentioned above. Group theoretical consideration predicts that in the M phase of C2#42 each of the doubly degenerate E1 molecular vibrations, which give rise to perpendicular IR bands, splits into a quartet via the breaking of degeneracy (the site splitting) and further via the intermolecular coupling (the Davydov splitting). Each of the A2 bands as well as the parallel progression bands is expected to split into a doublet via coupling of the two molecules in the unit cell, consisting of one strong parallel (due to the symmetric combination with respect to the glide plane) and one weak perpendicular component (antisymmetric combination). Among the four IR perpendicular bands associated with the El(1) through E1(4) zone-center modes, the medium intense El(4) band is suitable for measuring the change in split pattern on cooling process. Figure 17 (upper) shows temperature dependence of the El (4) absorption profile of c2OF42 measured on a powder sample. The singlet (at 555 cm-I) seen at 296 K (R phase) starts to split at Tcl (R I) and splits clearly into a doublet (at 553 and 558 cm-') below Tc2 (M). The doublet pattem, rather than quartet, can be interpreted as follows (Figure 18); one doubly degenerate molecular mode splits into the inplane and out-of-plane modes via the site splitting. For the inplane mode, the in-phase combination (with respect to the glide plane) of the two neighboring molecules gives rise to absorption strong enough to be observed, while the out-of-phase combination might be too weak to be detected. The reverse is the case for the out-of-plane mode. The magnitude of the band gap in the doublet represents the extent of anisotropy in the molecular field that hinders the librational or helix reversal motion of the molecules. Figure 17 (lower) shows the result measured on a powder sample of C24F50. Even at a temperature as low as 31 K, the El(4) band does not give a clear doublet, and the profile looks like a superposition of the singlet at 555 cm-' and the

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c2&0

of a single crystal sample measured with radiation incident

doublet with peaks positioned at 553 and 558 cm-', indicating that a large part of the high-temperature phase is frozen. At present the crystal structure of the low-temperaturephase (below Tcl)of C24F50 is not clarified yet. The split pattem of the El(4) mode depends significantly on the state of the sample specimens as described in the next section. The spectral changes on the I M transition of C20F42 is observed in the Raman spectrum in the vicinity of the v3(l) band at 735 cm-' [corresponding to the Al(2) zone-center mode]. The v3(2) band shifts discontinuously from 743 to 740 cm-I as shown in Figure 4. This spectral change is interpreted from the shapes of the v3 and v4 dispersion curves. The two branches strongly couple with each other in the vicinity of 4 = 40°, so that the modes positioned in the frequency gap of 720780 cm-' might be very sensitive to a change in state of molecules. In the vicinity of this frequency gap there exists the symmetric stretch mode of the terminal CF3 groups v,(CF3) (around 778 cm-I) (see Figure 19). Therefore, the discontinuous Raman spectral change is considered to be caused by structural change in the lamellae interface through strong coupling between the Y&!) and vS(CF3)modes. In order to confirm this, temperature dependence of the vs(CF3) mode and its coupling with other modes was investigated by IR measurement. Figure 20 shows transmission IR spectra taken with normal incidence of radiation to the lamella face of a C20F42 single crystal. Here, only the perpendicular bands are strongly enhanced. Very clear discontinuous changes are observed on the I M phase transition for the vs(CF3) band (from doublet to singlet) along with the ~3(6),v3(4), and 142) bands (discontinuous low-frequency shifts). The Al(2) band becomes IR-active in the M phase, and the v3(7) band disappears in I and M phases. In other frequency ranges no discontinuous spectral changes are observed. These facts suggest that a coherent rearrangement of the terminal CF3 groups constructing the lamellae interfaces plays an important role in the formation of the M phase. In fact, for C20F42/C24F50 mixed crystals no discontinuous spectral changes are observed in the temperature range investigated. 5. Structure and Thermal Behavior of Intermediate Phase. For powder samples of c2#42 the I phase appears in the temperature range between Tcl = 200 K and Tc2 = 143 K (see Figure 1). The transition temperatures as well as the AH values are not so much influenced by the measuring conditions: cooling or heating and its rate. For single-crystal samples, the Tc2 exothermic peak in DSC thermogram on cooling appears

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Polymorphism of Polytetrafluoroethylene

J. Phys. Chem., Vol. 99, No. 13, 1995 4619

at 184.7 K, much higher than that of powder samples. The corresponding endothermic peak is not detected on reheating the same sample after holding it at 130 K for several hours; the M phase transforms directly to the R phase at TCl = 201 K (Figure 21). The same result is obtained by IR spectral changes measured on a single crystal specimen. The AH values of the first (AH,) and second transitions (A&) depend on the state of samples: on cooling A H 2 = 0.42 kJ/mol for powder and AH2 = 1.49 kJ/mol for single crystal, and on heating AH1 AH2 = 3.9 0.69 = 4.57 kJ/mol for powder and AH1 f AH2(=0) = 7.33 kJ/mol for single crystal. These experimental facts suggest that stability of the M phase depends on the domain size of crystallites. For well-grown crystals, the M phase transforms directly to the R phase on heating. On the reverse transition from R to M on cooling, there are three possible ways for selecting the b axis (the unique axis) of the M phase from three equivalent orientations of the a axis of the R phase. This induces cracking in crystal domains and reduces their size. For the crystals with reduced domain size, the M phase is unstabilized, more or less, and instead the I phase is generated in a temperature range below TCl. On further cooling beyond Tc2, some part of the I phase transforms to M, but the remaining part is frozen in the I phase. The amount of M phase generated depends on crystal domain size. This is demonstrated by the split pattern of the El(4) IR band characteristic of the M phase. Figure 22 shows the El(4) absorption profiles measured on powder and single crystal specimens of c2$42 and C24F50 at low temperatures far below Tc2. For C20F42, crystallites are transformed perfectly to M in single crystal specimen, while in the powder sample a small part is frozen in the I phase. The same tendency is observed for C24F50, although a fairly large amount of the crystallites is frozen in I compared with the case of c20F42. The most conspicuous spectral feature characteristic of the I phase is doubling of the perpendicular components of the IR bands due to the v,(CF3) and 144) modes (see Figure 20). The origin of the doubling is considered to be caused by a specific interfacial structure formed on the cooling process from R to M phase. For the case of C24F50, the similar doubling of the v,(CF3) band is detectable in a very narrow temperature range

+

+

immediately below Tcl(= 215 K) as shown in the spectrum measured at 214 K (Figure 23). In the DSC thermogram of this compound (for both powder and single crystal deformed a little by compression) exhibits a small shoulder at the lowtemperature wing of the T,l peak (see Figure 1). This shoulder corresponds to the Tc2 signal of C24F50, although a large amount of the high-temperature phase is frozen below Tc2 as seen in the split pattern of the El(4) band (see Figure 22). On the basis of the experimental facts mentioned above, it is inferred that the I phase is regarded as a transient phase formed on cooling process from the R to the M phase.

Acknowledgment. The authors are indebeted to Daikin Industrials, Ltd. for kind supply of trichlorotrifluoroethane. References and Notes (1) Bunn, C. W.; Howells, E. R. Nature (London) 1954, 174, 549. Clark, E. S . ; Muus, L. T. Z. Kristallogr. 1962, 117, 119. Clark, E. S . J . Macromol. Sci., Phys. 1967, I , 795. Kilian, H. G. Kolloid-Z. 1962, 185, 13. McCall, D. W.; Douglass, D. C.; Falcone, D. R. J . Phys. Chem. 1967, 71, 998. (6) Zerbi, G.; Sacchi, M. Macromoiecules 1973, 6 , 692. (7) Masetti, G.; Cabassi, F.; Morelli, G.; Zerbi, G. Macromolecules 1973, 6 , 700. (8) Schwickert, H.; Strobl, G.; Kimmig, M. J . Chem. Phys. 1991, 95,

(2) (3) (4) (5)

2800. (9) Albrecht, T.; Elben, H.; Jaeger, R.; Kimmig, M.; Steiner, R.; Strobl, G.; Stuehn, B.; Schwickert, H.; Ritter, C. J . Chem. Phys. 1991, 95, 2807. (10) Albrecht, T.; Jaeger, R.; Petry, W.; Steiner, R.; Strobl, G.; Stuehn, B. J . Chem. Phys. 1991, 95, 2817. (11) Zhang, W. P.; Dorset, D. L. Macromolecules 1990, 23, 4322. (12) Koenig, J. L.; Boerio, F. J. J . Chem. Phys. 1969, 50, 2823. (13) Hannon, M. J.; Boerio, F. J.; Koenig, J. L. J . Chem. Phys. 1969, 50, 2829. (14) Boerio, F. J.; Koenig, J. L. J . Chem. Phys. 1970, 52, 4826. (15) Rabolt, J. F.; Fanconi, B. Macromolecules 1978, 11, 740. (16) Rabolt, J. F.; Fanconi, B. Polymer 1977, 18, 1258. (17) Hsu, S. L.; Reynolds, N.; Bohan, S. P.; Strauss, H. L.; Snyder, R. G. Macromolecules 1990, 23, 4565. (18) Cho, H.-K.; Strauss, H. L.; Snyder, R. G. J . Phys. Chem. 1992, 96, 5291. (19) De Santis, P.; Giglio, E.; Liquori, A. M.; Ripamonti, A. J . Polym. Sci.: Part A 1963, I , 1383. Jp941507X