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A Partial Phase Diagram and Crystal Solvate for the Poly(p-Phenyleneterephthalamide)/Sulfuric Acid System K. H. GARDNER, R. R. MATHESON, P. AVAKIAN, Y. T. CHIA, and T. D. GIERKE Central Research and Development Department, E. I. du Pont de Nemours and Company, Experimental Station, Wilmington, DE 19898 It is now well established that certain aromatic polyamides form ordered complexes with their solvents (i.e., crystal solvates). A summary of the occurrence of these crystal solvates has been compiled in a recent review article by Iovleva and Papkov1. Among the crystal solvates that have been identified are poly(m-phenylene isophthalamide) with N-methylpyrrolidone and with hexamethylphosphortriamide ; poly(p-benzamide) with sulfuric acid ; and poly(p-phenylene terephthalamide) (PPTA) with sulfuric acid and with hexamethylphosphortriamide . These solvates are all characterized by a discrete melting point and a crystalline diffraction pattern. 2
2
3
4
5
Although the presence of solvate phases has been established and qualitative phase diagrams have been published, to our knowledge, a detailed model for a polymer solvate and its phase behavior has not been presented. At this time we would like to present a partial phase diagram for the poly(p-phenylene terephthalamide) (PPTA)/sulfuric acid system and a model for the crystal solvate formed. In addition the structure of a model complex will be described. Experimental Experiments were carried out with low molecular weight poly(p-phenylene terephthalamide) (PPTA) produced by the acid ^ degradation of commercial Kevlar aramid fibers (6N H C l , 20 hr) . The inherent v i s c o s i t y of the degraded polymer was ca. 1.0 which, based on an empirical correlation between molecular weight and inherent v i s c o s i t y , corresponds to a molecular weight of ca. 7000. Solutions of PPTA i n 99.7% s u l f u r i c acid were prepared with concentrations ranging from 2% 24% (w/w) under ^ i n a dry box. Homogeneous solutions were obtained by intensive mixing at
0097-6156/ 84/0260-0091 $06.00/0 © 1984 A m e r i c a n C h e m i c a l Society
Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
POLYMERS FOR FIBERS A N D E L A S T O M E R S
50-70°C for short times. This low molecular weight polymer was chosen for the study because of i t s ( r e l a t i v e l y ) easy d i s s o l u t i o n behavior and low solution v i s c o s i t i e s as compared to those of higher molecular weight polymer. D i f f e r e n t i a l Scanning Calorimetry (DSC) of the PPTA/ s u l f u r i c acid system was carried out i n order to establish the melting behavior of the system. Samples (5-10 mg) were sealed i n gold-coated aluminum pans under N2 and cooled at 10°C/min to -150°C. Subsequently the samples were heated to 100°C at 10°C/min and the melting thermograms were recorded. The d i f f r a c t i o n patterns of various PPTA/sulfuric acid solutions as a function of temperature were obtained using a Rigaku Theta-Theta Diffractometer run in the horizontal mode (CuK« r a d i a t i o n ) . D i g i t a l data were taken at intervals of 0.02° in 2®. The 2 scale was calibrated with diamond and the temperature was measured by a thermocouple embedded i n the sample. e
RESULTS Model Solvate Structure In order to determine the manner i n which s u l f u r i c acid interacts with PPTA we are investigating the structure of PPTA oligomers complexed with s u l f u r i c acid. We have recently determined the structure of a N,N'-(p-phenylene)dibenzamide (PPDB)/sulfuric acid complex using single c r y s t a l x-ray methods and similar studies with longer PPTA oligomers are underway now. We expect that the general p r i n c i p l e s for the interaction of s u l f u r i c acid and PPTA w i l l become apparent from these studies. The PPDB/sulfuric acid complex c r y s t a l l i z e s i n a t r i c l i n i c unit c e l l with dimensions a_ = 9.75A, b_ = 10.31A, c^ = 7.88A, a_ = 108.7°, 6_ = 111.4° and jy = 89.2°. The space group i s PI and the c e l l contains one PPDB molecule and four s u l f u r i c acid moieties. A view of the molecule together with the four nearest 2 4 groups i n the plane of the phenyl groups i s presented i n Figure 1. Two of the s u l f u r i c acid molecules can be seen to have protonated the carbonyl oxygens of the amide groups, thus the structure i s actually a s u l f u r i c acid/bisulfate salt of PPDB. H
S 0
The neutral compound PPDB exists i n twg phases, and structural studies have been reported for each. ' A comparison with these two structures and with the structure of N,N -diphenylterephthalamide (DPTA) i s presented i n Table 1. The protonated oxygens cause an expected lengthening of the C-0 bonds from 1.22A (avg) i n the neutral compound to 1.30A (avg) i n the s u l f u r i c acid s a l t . In addition the C-N amide bond i s shortened from 1.36A (avg) to 1.30A (avg), indicating a substantial increase of double bond character i n this bond. f
Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
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Figure 1.
PPTA/Sulfuric
Acid System
93
f
Molecular structure of N,N -(p-phenylene) dibenzamide (PPDB) and the four nearest s u l f u r i c acids; (a) atomic numbering, (b) with hydrogens.
Table 1
Structure
9
PPDB
PPDB
DBTA
1.222 1.355 28.9 35.7 64.4
1.229 1.356 23.8 55.4 79.2
1.226 1.359 30.7 30.4 60.9
8
10
present
o
C-0 distance(A) O N distance(A) amide...outer ring angle(°) amide...central ring angle(°) outer...central ring angle (°)
1.30, 1,293 1.301, 1.303 36.6, 37.9 -41.2, -44.4 5.7, 8.2
Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
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A pronounced flattening of the protonated PPDB structure i s observed i n comparison to the unprotonated compound. In the neutral compound the phenyl rings are rotated by 64° and 79° from each other, while i n the present structure the phenyl rings are nearly coplanar. However, i n a l l cases, the amide groups are severely rotated out of the plane of the phenyl groups. The torsional angles about the amide groups are ca. +30, +30 i n the neutral compound and ca. +30, -30 i n the acid s a l t . These conformers would be iso-energetic i n the neutral state and, i t would seem, of close i f not i d e n t i c a l energy i n the protonated form. Both previous studies of the neutral compound indicate a s i g n i f i c a n t influence of c r y s t a l packing on the conformation (see Table 1). It i s possible that the conformer found i n the acid s a l t was directed by c r y s t a l packing considerations which may not be a factor i n the PPTA/sulfuric acid complex. In the c r y s t a l , the s u l f u r i c acid and b i s u l f a t e groups are arranged i n sheets with an extensive network of hydrogen bonding (Figure 2). These sheets alternate with sheets of PPDB consisting of stacks of the molecules with p i - p i * ring interactions (allowed by the planarity of the molecule). The sheets are inter-connected by strong amide to sulfate hydrogen bonds. A l t e r n a t i v e l y , the structure can be thought of as the close-packing of hydrogen bonded sheets containing PPDB and s u l f u r i c acid/bisulfates (Figure 3). We believe a sheet structure of this or similar nature i s the structural unit of the PPTA/sulfuric acid solvate phase. Phase Diagram The phase diagram for the (quasi-binary) PPTA-sulfuric acid system was compiled based on the melting point/composition information gathered from DSC-melting thermograms. In Figure 4, melting thermograms of a number of polymer-acid mixtures are presented. These thermograms were recorded for the temperature range -150° to 100°C at 10°C/min. The various thermograms were used to construct the melting point/composition diagram, which i s shown i n Figure 5. It can be seen that we are dealing with an apparent eutectic polymer-diluent system with a eutectic composition of 11-12% (w/w) of PPTA and a eutectic temperature of -8°C. The clearing temperature of the solutions as determined by o p t i c a l microscopy have been indicated i n Figure 5. Figure 6 shows the d i f f r a c t i o n pattern of a 22% PPTA/sulfuric acid solution as a function of temperature. The d i f f r a c t i o n patterns can be seen to consist of two components sharp d i f f r a c t i o n peaks superimposed on a broad component. This pattern i s consistent with a two phase system containing semicrystalline PPTA/sulfuric acid solvate and disordered components. As the temperature i s raised the portion of the d i f f r a c t i o n patterns attributable to the solvate phase decreases and f i n a l l y disappears at temperatures consistent with the melting endotherm observed by DSC. This d i f f r a c t i o n pattern agrees wjtjj that previously reported for the PPTA/sulfuric acid solvate. '
Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
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GARDNER ET AL.
Figure 2.
Figure 3.
PPTA I Sulfuric Acid System
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In the c r y s t a l , the s u l f u r i c acids are arranged i n sheets with an extensive network of hydrogen bonding. These sheets alternate with sheets of PPDB.
A l t e r n a t i v e l y , the complex can be viewed as sheets consisting of PPDB molecules alternating with s u l f u r i c acids. Sheets of this nature are believed to exist i n the PPTA solvate.
Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
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P O L Y M E R S FOR FIBERS A N D E L A S T O M E R S
Figure 5.
Phase diagram f o r PPTA/I^SO^. Experimental points were obtained by DSC (m) and o p t i c a l microscopy ( c ) .
Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
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PPTA/Sulfuric
G A R D N E R E T AL.
Acid System
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Solvate Structure The c r y s t a l l i n e component o f the d i f f r a c t i o n pattern can be indexed by a m e t r i c a l l y orthorhombic unit c e l l with dimensions £ = 16.35A, _b = 9.59A and £ = 12.9A. The c e l l contains two chain fragments and ca. 9 s u l f u r i c acid molecules per chemical repeat of the polymer. The quantity and organization of the s u l f u r i c acid i n the c e l l w i l l be d i s c u s s ^ below. Projections down the chain axis of the PPTA unit c e l l and the proposed solvate structure are shown i n Figure 7. Both structures have PPTA chains located at the corner and center of their respective unit c e l l s . In the neutral structure, the chains form hydrogen bonded sheets p a r a l l e l to the be plane. The c r y s t a l solvate can be thought of as a "swollen form of the neutral structure. The hydrogen bonded sheets p a r a l l e l to the bc_ plane have been preserved but now s u l f u r i c acid molecules participate i n the sheet while other acid molecules l i e i n i n t e r s t i c e s between the hydrogen bonded sheets. We have modeled the "sheet" with two s u l f u r i c acids per amide as was found i n the PPDB/sulfuric acid complex. [The chain-chain separation distance i n the polymer sheet is 9.59A as compared with 9.75A i n the model structure.] However, with this chain-chain separation distance i t i s also possible to construct a plausable sheet structure with one s u l f u r i c acid molecule per amide where a single s u l f u r i c acid molecule bridges two amide groups (N-H...0-S-O...H...0-C). In both models the s u l f u r i c acids between the sheets are not d i r e c t l y attached to the PPTA molecules. 77
°3 The volume of the solvate unit c e l l i s 2006A . If we assume that the volume occupied by the PPTA chains in the solvate i s tlje same as i t would be i n the pure PPD-T c r y s t a l , i . e . , 526A , and the volume of the s u l f u r i c acid molecules i n the hydrogen bonded s^eet i s the same as that i n the PPDB/sulfuric acid complex (78A ) we can calculate the volume of the intersheet s u l f u r i c acid molecules. These volume considerations and density measurements on undegraded samples (unpublished data) lead us to conclude that ( i ) there i s a nonstoichiometric amount of s u l f u r i c acid between the sheets and ( i i ) the acid i s disordered with a m o l e c u l a r volume equivalent to that of l i q u i d s u l f u r i c acid (ca. 87A ). These observations lead us to believe that the space group of the solvate i s a c t u a l l y monoclinic (or even t r i c l i n i c ) and the structural unit i s a sheet. Discussion Our present understanding of the phase diagram i n Figure 5 i s at a semiquantitative l e v e l . If the e u t e c t i c - l i k e pattern of the points labeled m are interpretable as such, then the l e f t hand branch r e f l e c t s the freezing point depression of s u l f u r i c acid (mp * 10°C). The fjope of this branch might then be expected to obey the familiar equation ,
AT =
km f
l
(1)
Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
POLYMERS FOR FIBERS A N D E L A S T O M E R S
15
25 35 SCATTERING ANGLE
45
D i f f r a c t i o n pattern of a 22% (w/w) solution of PPTA/H SO^ as a function of temperature. 2
ab Projections of the PPTA and PPTA/IUSO. solvate unit cells. (a) PPTA structure using Northolt coordinates, molecules ^orm hydrogen bonded sheets p a r a l l e l to the bc_ plane. (b) Proposed structure for the PPTA/^SO^ solvate. Sulfuric acid molecules have been incorporated i n the hydrogen bonded sheets and between the sheets (not shown).
Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
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PPTA / Sulfuric Acid System A
in the l i m i t of m * 0. Here, T i s the difference i n melting points between the pure acid and a solution of^molarity m; and i s th^ cryoscopic constant. For^ s u l f u r i c acid k^ = 68.1 [K*10 g acid*(moles of solute) ]. When the low concentration points i n Figure 5 are f i t with eq. 1 and this value of k^, the desired number average molecular weight of the PPD-T solute i s only ca. 500. This value i s much smaller than the molecular weight derived from an empirical c o r r e l a t i o n between molecular weight and inherent v i s c o s i t y ; ca. 7000. We believe that this d i s p a r i t y r e f l e c t s , in part, the e l e c t r o l y t i c character of the dissolved PPTA . The r i g h t hand branch of the locus of 'm' points in F i g . 5 i s also interpretable as a freezing point depression. In this case, the most that we can say i s that the data are consistent with plausible values for the heat of fusion and melting points of a stoichiometric sheet-like structure. If we extrapolate the l i m i t i n g slope of the data [and suppose that this remains l i n e a r to the sheet melting point], and presume that the sheet s t o i c h i o metry i s 2 acids/amide as suggested by the x-ray observations on PPDB (see Figure 3), then the sheets are computed to melt at 162°C with a heat of fusion of ca. 5.0 kcal/mole of amides. Assumed stoichiometries of 1 to 1 and 3 to 1 r e s u l t i n melting points of 205°C or 126°C and heats of fusion of ca. 4 or ca. 8 kcal/mole, respectively. A l l three sets are p l a u s i b l e . Their exact quantitative significance i s questionable, since they are based on the i m p l i c i t assumption that a l l of the PPTA present i s incorporated into sheets. This assumption i s suspect on the general grounds of inevitable imperfections in these quite viscous solutions, and the rather long extrapolation required; as well as on the s p e c i f i c grounds discussed in the next paragraph. The points marked as 'c' i n Figure 5 correspond to observed clearing points. We cannot claim that they are equilibrium c l e a r i n g points; but, rather, indicate the temperature at which s i g n i f i c a n t quantities^of an i s o t r o p i c solution phase form on the time scale of 10 -10 seconds. Nevertheless, the biphasic character of the solutions lying near the locus of 'c'-points i s indisputable. Conse^engly, i t i s of interest to ask about the predictions of theory * for nematic-isotropic biphasic s t a b i l i t y . The appropriate equations have been published. The appropriate m o ^ l i s one of polydisperse chains of s p e c i f i e d Kuhn length i n a nonathermal solvent. The r e s u l t s of some i l l u s t r a t i v e calculations are presented i n Figure 8a and 8b, with the r e q u i s i t e parameters noted i n the legends of that figure. It i s important to note that the Flory-Huggins solvation parameter x has been treated as a disposable parameter. The intention here i s to merely demonstrate the q u a l i t a t i v e correspondence between theory and observation; no claim i s made for the actual value of x« There i s c l e a r l y predicted to be a biphasic regime i n the concentration range corresponding to Figure 5. This i s a d i r e c t consequence of the PPTA polydispersity, and i s subject to only a minor degree to
Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
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P O L Y M E R S FOR FIBERS A N D
1
e.e ——— e.e
•
"
. WEIGHT FRACTION
Figure 8a.
«
——i .3
POLYMER
The isotropic fraction of the t o t a l solution volume i s plotted versus the o v e r a l l polymer-concentration (w/w). Parameters used i n the c a l c u l a t i o n are: most probable d i s t r i b u t i o n of chain lengths with p = 0.9881; the longest a x i a l ^ r a t i o of the system, n = 100; the 1956 approximation , C = 0; and solvation parameters, X = X' of (a) +0.005, (b) 0.0, (c) -0.005.
WEIGHT FRACTION
Figure 8b.
1 .2
.1
ELASTOMERS
POLYMER
The same plot as i n Figure 8a but with x = 0 and the longest a x i a l r a t i o of the system, n = (a) 200, (b) 100, (c) 50.
Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
6. G A R D N E R E T A L .
PPTA I Sulfuric Acid System
101
the influence of the somewhat arbitrary parameterization of the calculations. It i s not unreasonable that the isotropic phase w i l l act d i f f e r e n t l y than the predominent nematic phase at the freezing point. This v i t i a t e s the exact v a l i d i t y of the estimates of sheet melting parameters based on the data presented i n Figure 5 and analyzed i n the preceding paragraph. In summary, a l l of the qualitative features of the phase diagram are explicable i n terms of established physical p r i n c i p l e s . A quantitative description of the data i s tentative, because the PPTA-sulfuric acid solution i s very complicated. However, with plausible values for the various parameters a satisfactory semiquantitative description i s attainable.
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M. M. Iovleva, S. I. Banduryan, N. I. Ivanova, V. A. Platonov, L. P. Milkova, Z. S. Khanin, A. V. Volokhina and S. P. Papkov, Vysokomol. soyed. (1979) B21, 351.
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Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
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R E C E I V E D M a y 4, 1984
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