19280
J. Phys. Chem. 1996, 100, 19280-19288
Molecular Organization of Poly(p-phenyleneterephthalamide) in Sulfuric Acid: An NMR Study Min Zhou, Veronica Frydman, and Lucio Frydman* Department of Chemistry (M/C 111), UniVersity of Illinois at Chicago, 845 W. Taylor Street, Chicago, Illinois 60607-7061 ReceiVed: June 14, 1996; In Final Form: October 1, 1996X
Solutions prepared by dissolving synthetic poly(p-phenyleneterephthalamide) (PPTA) in 99.8% H2SO4 were analyzed using natural abundance NMR methods as a function of the polymer concentration, molecular weight, and temperature. Concentration and molecular weight-driven transitions between isotropic, nematic, and solidlike phases could be clearly distinguished from the 13C NMR spectra of the solute and from 1H NMR spectra of the solvent. The 13C solute NMR spectra point toward a distribution in the order parameter of the liquid crystalline director and could be quantitatively reproduced using 13C shielding tensor elements measured by solid NMR in polycrystalline PPTA. Thermodynamic parameters for the nematic a isotropic equilibrium were obtained from the temperature dependence of the liquid crystalline 13C NMR spectra, and 2D NMR methods were employed to retrieve information about the kinetics of PPTA and H2SO4 migration between isotropic and nematic domains. The results obtained from these spectroscopic studies compare well with previous observations obtained using non-NMR methods; the significance of the new NMR measurements is briefly discussed.
1. Introduction Liquid crystalline polymers constitute an important class of macromolecules whose valuable applications have made them the center of extensive theoretical and experimental research.1-4 This work has been driven to a large extent by the success with which certain members of this polymer family, particularly parasubstituted aromatic polyamides, have been shown to lead to ultrastrong, heat-resistant fibers. An important example of the ability of these aramide molecules to yield materials with high mechanical strength is furnished by poly(p-phenyleneterephthalamide) (PPTA, 1), a polymer which constitutes the basis of the Kevlar brand of fibers developed and commercialized by the DuPont corporation.5,6 One of the most interesting physical properties of PPTA is the strong affinity of its molecules to spontaneously align with respect to each other when dissolved in a suitable solvent such as absolute sulfuric acid. This orientational arrangement is a consequence of the extreme anisometry that characterizes the shape of these rigid polymer molecules and of disruptions between the intermolecular hydrogen bonds that are brought about by the extreme acidity of the solvent. The microscopic nematic domains that may then arise in these PPTA/H2SO4 solutions can be further aligned by the application of external fields. This behavior is key toward the generation of high-modulus fibers from PPTA, which are industrially spun from flowing sulfuric acid solutions subject to intense shearing fields and then coagulated into highly ordered fibrilar arrangements.6,7
1
By virtue of this interplay between the liquid crystalline behavior of the precursors and the mechanical properties of the final materials, studies on the nature of PPTA/H2SO4 solutions have become a continuing field of active investigations. X
Abstract published in AdVance ACS Abstracts, November 15, 1996.
S0022-3654(96)01767-4 CCC: $12.00
Experimental tools of this research have included viscosity measurements, calorimetric methods, optical spectroscopy, birefringence determinations, and X-ray scattering analyses,5-11 techniques that have provided ample proof of the presence of transitions in PPTA/H2SO4 systems between isotropic, liquid crystal, and solid phases. These measurements have also been accompanied by improvements in the theory underlying liquid crystalline polymer solutions, which began with Onsager and Flory’s entropy-based rigid rod approach and have evolved into more sophisticated thermodynamic models.12-14 In spite of these successes in the characterization of PPTA/ H2SO4 solutions, the nonlocal character of the experimental probes employed in the majority of the measurements has failed to provide a direct quantitative picture of the environments characterizing solute and solvent molecules under different phases and conditions. These molecular level measurements, on the other hand, are feasible with the aid of nuclear magnetic resonance (NMR), a method that has had a decisive impact in the field of low molecular weight liquid crystals.15 There have actually been in the past a number of solution NMR studies on PPTA/H2SO4 systems. The first of these determinations failed to observe any phase transitions but reported a series of peak splittings that were analyzed in terms of an equilibrium process of PPTA molecules between cis and trans conformations.16 These results were challenged by a later 13C NMR investigation that ascribed these multiplicities to oligomers resulting from sample degradation and which concluded that solution NMR methods cannot be used to detect either isotropic or anisotropic phases of high molecular weight PPTA.17 Still a very recent report published during the course of this work succeeded in observing the nematic NMR spectrum of ≈19% w/w PPTA solutions in H2SO4, by applying 2H NMR methods on an isotopically labeled sample.18 In the present study we demonstrate that natural abundance NMR investigations of the different phases adopted by PPTA/H2SO4 solutions are actually possible and that new aspects about their transitions and molecular properties can be obtained from 13C and 1H NMR studies. Our attention was mainly focused on the effects that polymer © 1996 American Chemical Society
Molecular Organization of PPTA in H2SO4 SCHEME 1
J. Phys. Chem., Vol. 100, No. 50, 1996 19281 TABLE 1: Spin-Lattice Relaxation Times Measured on the 1H and 13C Resonances of PPTA/H SO Samplesa 2 4 sampleb, % w/w PPTA/H2SO4
concentration, polymer molecular weight, and temperature have on the phase transitions of the PPTA/H2SO4 system, as viewed through the changes introduced in the 13C NMR spectrum of the solute. A series of solid/nematic/isotropic equilibria could be clearly discerned from these spectra, whose quantitative analysis was undertaken with the aid of solid phase NMR data on the various shielding tensors that are involved. This allowed us to determine in an accurate way average nematic order parameters characterizing PPTA/H2SO4 solutions, as well as the temperature dependence of the nematic a isotropic equilibrium. Finally, the distinct spectral features characterizing the different PPTA/H2SO4 phases were exploited in a series of 2D exchange NMR experiments which provided the first dynamic details about the rates at which molecular migrations between isotropic and anisotropic environments take place in these systems. 2. Experimental Section Since 13C NMR studies of phase equilibria in PPTA/H2SO4 solutions have been unsuccessfully attempted in the past, we dwell here briefly on the details that were involved in our sample preparations and NMR determinations. In spite of our initial intention to carry out the present study by dissolving commercial Kevlar fibers in sulfuric acid, this approach was abandoned upon noticing that the resulting solutions were usually considerably darker than the starting solid material and that upon standing they showed the appearance of small black suspended particles. This was probably the result of an H2SO4-driven decomposition of either the PPTA itself or of an additive to the fiber, an effect which may have played a role in the lack of success of previous 13C NMR studies to detect significant phase transitions in these systems. Consequently, all determinations described in the present study were carried out on PPTA batches synthesized in our laboratory using the procedure described by Bair et al. (Scheme 1).5 The average molecular weights (MW) of these PPTA batches were controlled by varying the temperature at which polymerizations were carried out and were estimated from viscosity measurements in 96% H2SO4 using calibration curves described in the literature.19 According to these measurements, the molecular weights of the samples that were here analyzed are approximately half as large as those reported in the literature for commercial PPTA fibers.7 The solutions to be studied by NMR were prepared by dissolving in the 5 mm NMR tubes on which the measurements were carried out various quantities of synthetic PPTA in 99.8 ( 0.1% H2SO4 aliquots. These aliquots were prepared by mixing appropriate amounts of concentrated (≈95%) and fuming (≈20% SO3) sulfuric acid and were checked by titration right before their use. Since the unassisted dissolution of PPTA is very slow, we attempted to accelerate this process by heating the H2SO4 solutions up to 60-75 °C. 13C NMR spectra recorded on these systems, however, showed splittings in the peaks arising from the different sites which resembled those reported in ref 16; these splittings were not reproducible from sample to sample and as noted earlier17 are probably the result of polymer degradation by excessive heating. The practice of aiding the dissolution of the polymer samples by heating was consequently discontinued, and all solutions employed in the
9.6 9.6 9.6 11.9 11.9 11.9 11.9
nucleus 13C 13C 13C 13C 13
C H (solvent) 1H (solvent) 1
chemical shift (ppm)
T1 (ms)c
126.1 130.6 171.6 155.4d 160.7d 10.9d 11.2
200 ( 30 225 ( 20 1100 ( 50 200 ( 30 150 ( 20 950 ( 20 880 ( 10
a Determined by nonlinear fits of inversion-recovery NMR data. b MW ) 15 kg mol-1 for all samples. c Error margins denote the standard deviations of the fit. d Peaks in a nematic phase.
present study were prepared by letting the sealed NMR tubes equilibrate at room temperature for long periods of time. Solutions prepared in this manner were of a pale yellow color and have hardly changed their aspect or their NMR spectra during the course of a year. Moreover, in no case have we succeeded so far in preparing a solution showing anisotropic 13C NMR resonances in systems that have been left to equilibrate for less than a month. NMR experiments were carried out at 7.1 T using a twochannel home-built NMR spectrometer. Measurements were performed on vertically spinning samples employing a doubletuned Nicolet probe head and a spinning stack generously donated to us by the Amoco Corporation. 13C NMR spectra were collected using a 1H-decoupling Waltz-16 sequence20 involving 15 µs 90° pulses during the acquisition time (≈100 ms long) and no NOE enhancement during the relaxation delay. Although higher decoupling powers were assayed, no additional narrowing of the peaks could be observed and only an onset of sample decompositionsprobably the result of long-term radiation heatingsresulted. Conversely, a series of preliminary experiments acquired on a commercial liquids NMR spectrometer (Bruker AM400) using a WALTZ-16 decoupling sequence characterized by long (≈100 µs) 90° pulses, showed only a broad, featureless peak for the protonated 13C sites in the nematic phase. Special care was also taken to achieve a constant and reliable reading of temperatures, which were calibrated with an ethylene glycol standard and controlled using an Omega-based home-built variable temperature system. An average of 3000 scans separated by 2 s relaxation delays and excited by 60° pulses (≈ 9 µs) were added in the collection of the 13C data. Given the spin-lattice relaxation times that characterize the various carbon resonances of PPTA (Table 1), these conditions imply a substantial relaxation of the spin system that allowed us to interpret the 13C results in a quantitative manner. Thirtytwo scans were usually collected in the 1H 1D acquisitions. All chemical shifts were referenced to δTMS ) 0 ppm using benzene as an external reference and are estimated accurate to within (0.25 ppm for the 13C and (0.1 ppm for the 1H data on the basis of repetitive measurements. 3. Results and Discussion 3.1. Concentration and Molecular Weight Dependence of PPTA/H2SO4 NMR Spectra. The sensitivity of natural abundance 13C NMR to the phase equilibria undergone by PPTA/H2SO4 solutions as a function of solute concentration can be appreciated from Figure 1, which shows a series of spectra collected for a MW ≈ 15 kg mol-1 polymer sample at room temperature for different w/w percentage ratios. When collected at low concentrations, the 13C NMR spectrum yields five welldefined isotropic peaks at 126.1, 130.6, 131.8, 133.0, and 171.6 ppm downfield from TMS; on the basis of literature values, we
19282 J. Phys. Chem., Vol. 100, No. 50, 1996
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Figure 2. Concentration dependence observed for the 13C NMR spectra of a MW ≈ 8 kg mol-1 PPTA solution at room temperature.
Figure 1. Concentration dependence of the 1H-decoupled 13C NMR spectrum arising from PPTA (MW ≈ 15 kg mol-1) in 99.8% H2SO4 at 25 °C. Resonances indicated by the letter i denote signals corresponding to molecules in the isotropic phase; the letter n denotes resonances generated by polymer in a nematic phase.
assign these resonances to the protonated diaminophenyl, protonated terephthaloyl, nonprotonated terephthaloyl, nonprotonated diaminophenyl, and carbonyl carbons of 1, respectively. These assignments are further aided by the nearly fully relaxed conditions under which these data were acquired and agree well with previous results reported on 0.1% w/w Kevlar solutions in 96% H2SO4.17 Except for an increase in signal-to-noise this trace remains essentially unchanged as the polymer concentration increases, until the latter reaches a value of approximately 11% w/w. At this point a new set of peaks possessing an intensity pattern similar to the one observed for the molecules in the isotropic phase but appearing at 155.4, 160.7, 194.5, 197.4, and 205.8 ppm emerges, which can be ascribed to polymer molecules located within a new liquid crystal phase. Indeed although the shifts of these peaks are relatively unusual for an isotropic scale, they can be readily explained by invoking the onset of a chemical shift contribution arising from anisotropic shielding components. Moreover, all changes that occur in these 13C NMR spectra are reversible, and isotropic peaks could be retrieved if aliquots of the more concentrated samples were diluted with 99.8% H2SO4. It is interesting to note that in spite of their considerable chemical shift displacements neither the longitudinal nor transverse relaxation times of these nematic peaks differ substantially from those characterizing their isotropic counterparts (Table 1 and Figure 11, Vide infra). This coincidence is probably a result of the restricted tumbling motions that rigid PPTA molecules are allowed to execute even when placed in the isotropic phase as a consequence of their high molecular weight and of the solvent’s viscosity. A second set of PPTA solutions corresponding to a MW ≈ 8 kg mol-1 polymer sample was also analyzed in this manner and led to the 13C NMR spectra presented in Figure 2. Although an anisotropic phase could not be detected for this sample at room temperature at any of the assayed concentrations, the isotropic 13C NMR signals arising from the polymer became unobservable as the solute concentration increased beyond ca. 15% w/w. This “disappearance” of all resonances can be explained in terms of a self-assembly process whereby polymer molecules aggregate into larger particles, whose slow correlation times cause the NMR signals to broaden over several kilohertz due to relaxation and anisotropic coupling effects. This fluidto-solid transition is unlike the behavior observed in conven-
Figure 3. Molecular weight dependence observed for the 13C NMR spectra of PPTA. Data were acquired at room temperature using 1213% w/w solutions in H2SO4.
tional solutions, where once a saturation point is reached the excess solute forms a separate solid phase, and resembles instead certain aggregation processes that have been observed for elongated or disklike molecules and proteins.14 Also interesting to observe are the changes occurring in the 13C NMR spectra of concentrated PPTA solutions when recorded as a function of the polymer’s average molecular weight. Samples with low MW show only isotropic resonances, since their short chains are unable to generate aligned phases and the solid-like particles into which they eventually aggregate do not yield observable signals. As the polymer average chain length increases, peaks corresponding to an anisotropic phase begin to emerge, and their intensities increase until becoming the sole spectral feature at the highest molecular weight (Figure 3). Noteworthy, in spite of the high sensitivity of the anisotropic resonance intensities on the concentration and MW of the polymer, their chemical shifts remain essentially constant regardless of these changes (top traces, Figures 1 and 3). A very similar behavior is observed when variations occur in the temperature of PPTA/H2SO4 solutions (see below), thus indicating a remarkable independence of the nematic order parameter on most environmental factors. The changes occurring in these aramide solutions as a function of polymer concentration and molecular weight also affect the appearance of the corresponding 1H NMR spectra. These proton traces are dominated by a signal arising from the sulfuric acid, which in the isotropic solutions appears at 11.2 ppm and has a line width dominated by magnetic field inhomogeneities. In the case of the 15 kg mol-1 sample, this solvent peak shifts slightly upfield (less than 0.1 ppm) as the polymer concentration increases until a second feature, associated to the appearance
Molecular Organization of PPTA in H2SO4
J. Phys. Chem., Vol. 100, No. 50, 1996 19283
Figure 4. Room-temperature 1H NMR spectra arising from the PPTA/ H2SO4 solutions employed in the measurements shown in Figure 1, as a function of polymer concentration. Arrows indicate the H2SO4 peaks arising from the isotropic (i) and nematic (n) phases.
Figure 6. (A) Generic phase diagram of lyotropic liquid crystalline polymers derived from microscopy and calorimetry measurements on PPTA/H2SO4 solutions. Abbreviations denote isotropic (iso) and liquid crystal (lc) phases; polymer in the solid phase may take the form of crystal solvate or of crystalline material.21 The areas dominated by each of the phases are drawn only approximately to scale, and their sizes are polymer dependent. (B) Schematic MW vs concentration dependence expected for PPTA/H2SO4 solutions on the basis of the thermodynamic phase diagram (A) and on phase transition studies reported in the literature. The dashed lines indicate (qualitatively) the regions explored by the spectra shown in Figures 1-3, as marked by the corresponding circled numbers. Figure 5. Room-temperature 1H NMR spectra of the PPTA/H2SO4 solutions employed in the acquisitions presented in Figure 2, collected for the indicated polymer concentrations.
of an anisotropic phase, emerges at about 10.9 ppm (Figure 4). The displacement of this new solvent resonance from its isotropic value probably reflects the onset of an anisotropic shielding contribution induced by the quenching of random reorientations. This phenomenon should also bring about a broadening of the H2SO4 resonance due to 1H-1H dipolar coupling effects; the fact that the latter only amounts to ca. 30 Hz suggests the presence of a considerable self-decoupling averaging process, probably arising from intermolecular hydrogen migrations. This makes a quantitative analysis of the average alignment of H2SO4 molecules in these systems by either 1H or 2H NMR considerably uncertain. Notably, a similar solvent peak becomes the dominant spectral feature in 1H NMR spectra recorded at high polymer concentrations for the lower molecular weight PPTA sample (Figure 5). This suggests that in spite of the molecular aggregation process in which 1 is involved the bulk of the solvent remains essentially a fluid, albeit one that does not sense a truly isotropic environment. As previous attempts to observe phase transition processes in PPTA/H2SO4 solutions using natural abundance NMR have met only limited success, it is worth discussing how the spectral data shown in Figures 1-5 compare with expectations born out from non-NMR determinations described in the literature. Although a complete room-temperature phase diagram describing the dependence of the various PPTA/H2SO4 transitions on concentration and MW has to our knowledge not been described, it is possible to use as starting point for these comparisons the
detailed analysis that for a MW ) 31 kg mol-1 PPTA sample in 99.8% H2SO4 has been reported as a function of temperature and concentration.11 This diagram, schematized in Figure 6A, shows regions dominated by isotropic, nematic, and solid phases separated by zones of binary phase coexistence. It is possible to transform the quantitative thermal dependence afforded by this diagram into a qualitative MW description of the various equilibria, with the aid of recent susceptibility and birefringence studies that explored how isotropic a nematic transition temperatures (Tin) are affected by polymer chain lengths.22,23 These studies observed a depression in the values of Tin with decreasing MW, thus implying that, at constant temperature and within certain limits of minimum molecular size compatible with the onset of phase transitions, the MW vs concentration dependence of PPTA/H2SO4 solutions will behave as depicted in Figure 6B. This diagram is in turn in very good agreement with the changes that we observe by natural abundance NMR under different conditions; for high molecular weight samples, for instance, it predicts the appearance of concentration-driven isotropic a nematic equilibria (line 1 in Figure 6B), while for lower MW values it forecasts isotropic-to-solid phase transitions such as those observed in Figure 2. Weight-driven transitions for constant polymer concentrations are also predicted by this model, in a sequence that agrees well with the data observed by 13C NMR (Figure 3 and line 3 in Figure 6B). The significant changes detected by the NMR spectra can therefore be unambiguously traced to phase transition phenomena that have already been observed for PPTA/H2SO4 solutions; we turn next to analyses of the unique quantitative information afforded by NMR about order parameters and site concentrations at the nematic interphase.
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3.2. PPTA/H2SO4 Solutions at the Nematic-Isotropic Interface. Important advantages that result from applying NMR spectroscopy to the analysis of aramide solutions include the method’s capability of quantifying in a simple manner the relative abundance of molecules in the different phases and its ability to characterize the degree of order that various chemical sites in the polymer possess in the nematic phase. The quantitative aspects of NMR are well-known: provided that, as we did in the present work, data are collected under nonsaturating conditions and without the use of signal enhancement techniques, the area underneath each resonance can be assumed to reflect the relative abundance of its corresponding chemical site. The determination of average nematic order parameters using NMR relies on measuring the shift differences ∆δ that affect a particular chemical site, when it ceases to tumble isotropically and becomes aligned in an anisotropic phase due to the magnetic coupling between a molecule and the external field Bo.24 This difference between isotropic and liquid crystal chemical shifts can be conveniently expressed in terms of the Saupe matrix {Sij}i,j ) 1,2,3 describing the degree of alignment of a site as15
∆δ ) δiso - δlc )
2
∑
3 i,j)1,2,3
Sijδij
(1)
where {1, 2, 3} represent the directions of an arbitrary orthogonal axis system, {δij}i,j)1,2,3 are the elements that in the same system describe the anisotropic components of the chemical shielding tensor, and the expression assumes a time average over all possible molecular conformations. In the particular case of PPTA, the effective cylindrical symmetry introduced by fast overall molecular motions about the main polymer axis and the preferential alignment of these chain axes parallel to the external magnetic field allow one to rewrite eq 1 in terms of a single order parameter Szz that defines the degree of alignment of polymer chains with respect to B0:
∆δ ) Szzδzz
(2)
In this equation δzz represents the contribution that the anisotropic chemical shielding components of a particular site will have to the overall NMR chemical shift when the main polymer chain axes are perfectly aligned with the external magnetic field. Its knowledge requires solid phase NMR determinations where the principal values of the various chemical shielding tensors are measured and their orientations within a molecular reference frame established. We have recently carried out these determinations for polycrystalline PPTA using bidimensional 13C NMR methods;25 these results are comparable to measurements described elsewhere26 and are summarized for the sake of completion in Table 2. The uncertainties involved in these solid phase chemical shift determinations put a limit on the accuracy with which nematic order parameters can be measured; we have numerically estimated the upper limit of these errors at (5%. In addition to a marked difference in their chemical shift, nematic and isotropic 13C NMR PPTA line shapes differ in the magnitudes of their resonance line widths. This can be clearly appreciated from Figure 1, which shows that all chemical sites corresponding to molecules in the liquid crystalline phase yield broader resonances than their isotropic counterparts. Although these differences could be ascribed to the presence of nematic 1H-13C dipolar couplings, this mechanism becomes unlikely when considering that the bandwidth of the decoupling field used in the present measurements exceeds by a factor of 4 the magnitude of the largest motionally averaged heteronuclear coupling and that the nematic line broadening affects with equal
TABLE 2: 13C Chemical Shielding Parameters in Polycrystalline PPTAa
C
asymmetry parameterb
θc
80
0.8
90
34
-120
0.6
90
6
110
0.8
90
14
-140
0.5
90
74,134
-110
0.5
90
66,126
shielding anisotropyb
site
φc
O
C
N
C
C
C
H(terephthaloyl)
C
H(diaminophenyl)
a Adapted from ref 25. b Under the conventions: |δzz| g |δxx| g |δyy|; anisotropy ) δzz - δiso; asymmetry ) (δxx - δyy)/(δzz - δiso). Principal values estimated accurate within (5 ppm. c Polar (θ) and azimuthal (φ) angles in degrees with respect to the main polymeric chain.
strength both protonated and nonprotonated carbons. Twodimensional NMR determinations further described below reveal that the homogeneous line widths of isotropic and anisotropic resonances are actually very similar; the additional line broadening affecting peaks of nematic PPTA must therefore be reflecting an inhomogeneous distribution of anisotropic spectral parameters within the sample. Given the heterogeneous nature of the polymer solute, it is natural to ascribe this dispersion to a nonuniform distribution of the order parameter Szz over the bulk liquid crystal.21 This feature can be taken into account in a total line shape analysis of the 13C NMR spectra by assuming that the nematic director of PPTA is distributed over a range of values, characterized by an average order parameter S°zz and a standard deviation ∆Szz. The total 13C NMR spectrum I(δ) arising from each chemical site in the polymer can then be computed as a weighted sum of isotropic and anisotropic components according to
I(δ) ) I0{xisoL(δ-δiso) + xlc∫L[δ-δlc(Szz)] exp[-(Szz - S°zz)2/2(∆Szz)2] dSzz} (3) where I0 is a normalization constant which takes into account the relative abundance of the site and which can be directly extracted from spectra measured on dilute solutions, xiso, xlc are the fraction of molecules present in the isotropic and liquid crystalline phases at a given temperature, and
L[δ - δ0] )
1 1 + 4π2(T2*)2[δ - δ0]2
(4)
represents a Lorentzian line shape centered at δ0 and depending on an effective transverse relaxation time T2* which for the sake of simplicity we assumed identical in both phases and corresponding to the value that can be extracted from the isotropic peaks. The Lorentzian and super-Lorentzian line shapes that result from this formalism closely follow the experimental isotropic and anisotropic resonances; their assumption, however, is not crucial, and similar results would have been obtained if alternative line shapes (e.g., Gaussians) were assumed. Equipped with this formalism we set out to investigate the information that 13C NMR can provide about the relative abundance and orientational order of PPTA molecules in the region of the nematic/isotropic coexistence. The center column of Figure 7 presents a series of experimental results obtained on a MW ≈ 15 kg mol-1, 12% w/w PPTA sample as a function of temperature. In order to reproduce these experimental
Molecular Organization of PPTA in H2SO4
J. Phys. Chem., Vol. 100, No. 50, 1996 19285
Figure 7. 1H-decoupled 13C NMR spectra arising from a 12% w/w PPTA/H2SO4 solution as a function of temperature. The center column presents the actual set of data collected at the indicated conditions on a MW ≈ 15 kg mol-1 polymer sample. The column on the right corresponds to best fit spectral simulations calculated on the basis of the shielding anisotropy parameters determined in the solid phase. Spectra in the left-hand column were calculated assuming that the diaminophenyl 13C anisotropies measured in the solid were partially averaged by fast ring librations. Also indicated are the S°zz and ∆Szz values employed in the simulations. According to these calculations the nematic peaks arise (in lower to higher ppm order) from the diaminophenyl protonated, terephthaloyl protonated, diaminophenyl nonprotonated, terephthaloyl nonprotonated, and carbonyl carbons, respectively.
recordings, we carried out a series of simulations employing the shift anisotropy parameters in Table 2 and using the ratio xlc/xiso, the average order S°zz, and the dispersion ∆Szz as the only adjustable parameters to reproduce simultaneously the changes observed for all resonances in the 13C NMR spectra. The best fit traces that could be obtained under these assumptions are presented on the right-hand column of Figure 7. These simulations can correctly reproduce the main changes observed in the experimental spectra using essentially constant values for S°zz and ∆Szz, and temperature-dependent ratios between the abundance of the liquid crystalline and isotropic phases. Although this temperature dependence of the nematic a isotropic equilibrium is natural, the constant value adopted by the order parameter is unexpected, as theoretical models backed by susceptibility measurements on poly(benzanilidyleneterephthalamide) solutions predict a decay of the average molecular alignment with increasing temperatures.23 The constant 13C shifts that we observe as a function of temperatures are, on the other hand, consistent with the quadrupolar behavior that has been measured by variable temperature 2H NMR on labeled PPTA/H2SO4 solutions.18 In spite of their good overall agreement with experiments, the simulations shown in the right-hand column of Figure 7 overestimate the predicted separation between protonated aromatic 13C resonances in the nematic phase. This discrepancy, which can be more clearly appreciated from the superimposed traces show in the top of Figure 8, originates in the larger in-
Figure 8. Comparison between experimental (s) and calculated (---) 13C NMR line shapes in the region of isotropic a nematic coexistence of PPTA/H2SO4 (T ) 47 °C, Figure 7). Simulations were obtained assuming either static (top) or dynamically averaged (bottom) anisotropic shielding parameters, as described in the text.
plane shielding anisotropy that characterizes terephthaloyl over diaminophenyl carbons and which for a given nematic order parameter will cause the former peaks to move downfield at a quicker rate. Although this discrepancy could be solved if
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Figure 9. Van’t Hoff plot resulting from the experimental xlc/xiso ratios made available by the simulations in Figure 7 (black dots). The straight line indicates a least-squares fit of the data to the equation ln Keq ) -∆G/RT; error bars reflect the experimental uncertainties in xiso, xlc.
shielding anisotropies were to change substantially as molecules go from the solid to the fluid phase, this is an unlikely possibility given the magnitude of the change that would be required (ca. 40 ppm) coupled to the similarity between the isotropic 13C shifts that are observed for PPTA in the solid and solution.25,26 An accurate reproduction of the experimental nematic shifts can also be achieved by introducing considerable deviations of the polymer structure from its idealized geometry; again, however, this is an unlikely possibility given the large bond distortions that would have to be involved. A more plausible explanation for the experimentally observed shifts can be proposed by assuming that the shielding tensors measured in the solid are actually partially averaged values, reduced from their full magnitude by the motions that are known to occur in solid PPTA.27,28 Although the molecular dynamics of polycrystalline PPTA is heterogeneous, multimodal, and as of yet incompletely characterized, some of the features that it is known to include are a higher mobility of diaminophenyl compared to terephthaloyl rings as well as the presence of fast librational ring motions. These features can be incorporated into a theoretical model according to which the diaminophenyl parameters listed in Table 2 are actually averaged shielding values resulting from fast ring librations about the para-axes; although admittedly oversimplified, this model leads to an almost perfect agreement with the experimental variable temperature spectra when the solid phase librational motions assume an amplitude of (30° (Figure 7, left-hand column; Figure 8, bottom traces). Further analyses regarding the validity of these assumptions are currently under way. In addition to orientational information, the data presented in Figure 7 yield the relative abundance of isotropic and nematic phases as a function of temperature. These fractions are quantified by our simulations via the xlc, xiso parameters and can be used to calculate an equilibrium constant
Keq )
xlc xiso
(5)
defining the distribution of polymer molecules between the two phases. The temperature dependence of this Keq can in turn be used to obtain the Van’t Hoff plot shown in Figure 9, whose linear fit yields the parameters ∆H ) 1.7 kcal mol-1 and ∆S ) 4.7 cal K-1 mol-1 controlling the thermodynamics of the
Figure 10. 1H NMR spectra arising from a 12% w/w PPTA/H2SO4 solution as a function of temperature. The solvent peaks are shown in the inset but have been taken out of scale in the main traces, leaving visible an isotropic polymer peak (i), a smaller resonance associated to the H2SO4 sample at 3 ppm (probably H2O), and a broad nematic doublet (n) split by ca. 14 kHz.
nematic a isotropic equilibrium. These parameters have to our knowledge not been quantified previously for aramide solutions, and their determination should prove useful for future models on the loss of orientational entropy and the strength of intermolecular interactions characterizing the phase equilibria in PPTA/H2SO4 solutions under the reported conditions. An independent confirmation on the interpretation given to the anisotropic 13C spectral features arising from PPTA’s isotropic a nematic equilibrium can be obtained, albeit in a less accurate manner, from 1H NMR spectra obtained on PPTA solutions as a function of temperature. Although dominated by the H2SO4 solvent peaks (Figure 10, inset), a vertical scale magnification of these spectra reveals a structured shoulder below 9 ppm, which is symmetrically flanked by a broad doublet split by approximately (7 kHz (Figure 10). The origin of the sharp peaks can be traced to the aromatic protons of polymer molecules dissolved in the isotropic phase of the solution, and the broader doublet to signals arising from PPTA molecules that are magnetically aligned in a nematic phase. This can readily explain the highly anomalous positions of the latter peaks with respect to TMS, as well as their apparent decrease with increasing temperatures. These 1H NMR spectra of solute molecules in a nematic phase will be dominated by intramolecular proton couplings defined by a dipole-dipole spin
Molecular Organization of PPTA in H2SO4
J. Phys. Chem., Vol. 100, No. 50, 1996 19287
Hamiltonian
Hd-d ) ∑Dij[3IziIzj - Ii‚Ij]
(6)
i
