Chapter 25
Molecular Motion in the Homopolyester of 4-Hydroxybenzoic Acid 1
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J. R. Lyerla , J. Economy , G. G. Maresch , A. Mühlebach , C. S. Yannoni , and C. A. Fyfe Liquid-Crystalline Polymers Downloaded from pubs.acs.org by YORK UNIV on 12/02/18. For personal use only.
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IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, CA 95120-6099 University of Illinois, Urbana, IL 61801 Max Plank Institute für Polymerforschung, Postfach 3148, D-6500 Mainz, Federal Republic of Germany CIBA-Geigy AG, Forchungszentrum, 180.053, CH-1701, Fribourg, Switzerland Department of Chemistry, University of British Columbia, Vancouver, British Columbia V6T 1Y6, Canada 2
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Proton and C NMR have been utilized to characterize the motional processes occurring in the homopolymer of 4-hydroxybenzoic acid, PHBA, over the temperature range 27-400°C. Changes in the line shape of the ring protons and of the chemical shielding anisotropy pattern of the carboxyl carbon provide a picture of motion in PHBA in which: 1) phenyl ring motion in the polymer occurs about the C - C axis well-below the 350°C phase transition and thus is not tied to the transition; 2) the onset of motion for some of the rings occurs at ca. 120°C while all the rings show motion (180° ring flipping) by 300°C; 3) the motion of the ester group only ensues with the phase transition and involves occurrence of a second motion about the chain axis, again involving jumps by ca. 180° but with the entire repeat unit participating in the motion. 1
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The homopolyester of 4-hydroxybcnzoic acid and its copolyesters with 6-hydroxy-2-naphthoic acid and with 4,4'-biphenol and terephthalic acid are of continuing commercial and academic interest. (1,2) From the applications side, interest stems from the high moduli and high temperature properties of these polymers, while on the fundamental side, interest arises from the main-chain thermotropic liquid crystalline 0097-6156/90/0435-O359$06.00A) © 1990 American Chemical Society
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character of these polymers. The homopolymer, PHBA, is itself a highly crystalline material that has been the subject of a number of structural investigations by X-ray and electron-diffraction. (3-8) Of particular interest has been the nature of the reversible DSC transition at ca. 350°C (Figure 1). The diffraction data, taken below and above the transition, are described in terms of a transition from an orthorhombic unit cell to a "pseudo-hexagonal" structure. Recent high-temperature X-ray diffraction data show that the unit cell expands significantly along one direction (a-axis) perpendicular to the chain axis (c-axis) between -100°C and 340°C At the transition, the unit cell volume increases sharply (ca. 9.5%) (7) due to further expansion perpendicular to the chain axis. The reduced density of the high temperature phase suggests that torsional degrees of freedom may be introduced for the phenyl and ester groups at the transition. Indeed, the interpretation derived from recent X-ray studies (6-8) suggests that the "pseudo-hexagonal" phase has a 180° rotationally degenerate, disordered structure. However, the X-ray results do not distinguish whether the disorder is dynamic or static. Thus, to examine directly the question of whether molecular motion accompanies the 350°C phase transition in PHBA, we have carried out solid state proton and C N M R lincshape measurements to probe the motions of the phenyl ring and the carboxyl carbon units of this homopolyester in the temperature range of 27 - 400°C. 6
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Experimental 1 3
The homopolyester, enriched to 60% in C at the carboxyl carbon, was prepared from labelled acetoxy-benzoic acid monomer as described by Muhlebach, et al.. (9) The polymer was characterized by DSC, T M A , and X-ray diffraction. Number-average molecular weight was determined to be >30K from the *H - N M R spectrum of hydrolyzed polymer (method of Krichcldorf and Schwarz). (H)) Solid state N M R experiments were carried out on a spectrometer (described previously) (jT) operating at 60 MHz for proton observation and 15.1 M H z for C observation. Broad-line proton spectra were obtained using a standard n/2 pulse sequence while proton-decoupled C spectra were obtained by standard cross-polarization (CP) techniques. Temperature dependent spectra were obtained in a range of 27 - 400°C using an inductive heating method that employs N M R tubes coated with a thin film of metal (Pt) as described by Maresch et al.. (12) Temperature was measured with thermocouples placed inside the sample. Simulation of the chemical shielding anisotropy (CSA) patterns in PHBA was carried out to derive the values of the principal elements of the shielding tensor at each temperature. Because of the magnitude of the C - C dipolar interactions in the enriched sample, this source of non-averaged dipolar broadening affects the CSA pattern. A nutation experiment (H) was 1 3
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Figure 1. DSC (heating rate, 20°C/min) of PHBA, 60% "C-labcIlcd at the carboxyl position, showing the large endotherm between 340-350°C.
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carried out at 27°C to determine the magnitude of the broadening which was found to be ca. 440 Hz. Only upon inclusion of the broadening is satisfactory fitting of the patterns obtained. A CSA pattern has been obtained at high field (100 M H z for C observation) (14) where the dipolar broadening has less effect on the pattern shape. An axial pattern is observed at room temperature having the same values found for the elements of the shielding tensor as derived from the fitting of the 27°C pattern at 15.1 MHz. In the high temperature phase, inclusion of a C C dipolar broadening of 240 Hz (having been reduced by motion) provides suitable fitting of the patterns at 15.1 MHz. , 3
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Results and Discussion 1 3
Previously, we have reported C cross-polarization, magic-angle spinning (CPMAS) data (9,15) on PHBA over the temperature range -196°C to 130°C (the highest temperature that could be reached in the MAS experiment with the available equipment). A chemical shift difference of 3.2 ppm is observed between the two ring carbons ortho to the carboxyl group in the spectra obtained over this temperature range. This persistence of non-equivalency of ring carbon resonances suggests that, even at moderately high temperature, there is no motion of large amplitude occurring with a frequency comparable to the splitting, ca. 48 Hz at 15 MHz and 320 Hz at 100 MHz. This result supports the very rigid nature of this highly crystalline polymer. A H spectrum of the polymer (labelled at the 3,5 positions) at 25°C was found to be consistent with a rigid Pake spectrum (9) and further supported the carbon results on ring motion. However, a H spectrum at 140°C suggested there is a small fraction of rings that undergo rapid motion at this temperature. (9) These mobile rings (correlation times ca. 10' s), could arise from defects in the crystal structure, from end groups and/or amorphous material. Apparently, the mobile rings are not readily distinguished in the C - C P M A S spectrum at 130°C because they do not cross-polarize efficiently and/or are not resolved from under the crystalline resonances. The previous N M R results on PHBA are augmented in Figure 2 which shows several proton spectra in the temperature range 27°C to 400°C. The spectrum at 27°C is the same as those observed at sub-ambient temperatures and reflects the rigid structure of the polymer over this temperature range. Above ca. 120°C, significant narrowing of the proton resonance line ensues. Narrowing of the proton spectrum occurs over a broad range of temperatures as indicated in Figure 3 which displays a summary of the lincwidth data over the temperature range of interest. Above ca. 260°C, the features of a Pake doublet become well-defined 2
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25.
LYERLA ETAL.
Molecular Motion in Homopolyester
H-NMR Spectra — 60 MHz
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Figure 2. Proton spectra at 60 MHz of PHBA at various temperatures. Note, the probe has a background proton signal (as shown) and this gives rise to the sharp center line seen in the spectra.
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Q6) (e.g. in the 313° C spectrum in Figure 2). The observed splitting of the doublet is ca. 12 kHz and is consistent with a spatial separation between interacting protons of .25 nm. This value is the separation of adjacent protons on the phenyl ring of the repeat unit which arc also the protons that give rise to the dominant H - *H dipolar interaction in the polymer. Since the Pake pattern persists in the presence of the line narrowing, the molecular motion giving rise to line narrowing must occur about an axis (nearly) parallel to the two-fold axis of the phenyl unit. This motion accounts for the observed overall narrowing (Figure 2) in that it reduces intermolecular dipolar interactions and cross-ring and ring-ring intramolecular dipolar interactions as the time-scale of the motion becomes fast relative to the magnitude of these interactions (ca. 1-10 kHz). The proton data establish directly that the phenyl rings in PHBA have large amplitude motion in the mid-kHz region well below the 350°C DSC transition; however, the data cannot be used to distinguish whether the motion is in the form of 180° jumps or continuous rotation. Preliminary high temperature H - N M R results (9,14) demonstrate that the motion is in the form of 180° flips about the Cj - C axis of the ring and is augmented by ring oscillations. Also, the H data provide definite evidence of heterogeneity in the ring motion in that the spectra consist of composites of lineshapes from rings both fast and slow on the H timescale. These data on motional heterogeneity support a similar finding in the proton data - i.e. while we have characterized the proton spectra by their fullwidth at half-height, the spectra in the temperature range 120° - 260°C are best represented by a composite of rings whose motion appears as rigid, intermediate, or fast on the relevant proton dipolar timescale (ca. 100 //sec). However, by 300°C, the entire population of rings are in rapid motion on the proton dipolar timescale. The question arises as to whether the carboxyl group moves in concert with the phenyl ring reorientation. Insight is provided by the carboxyl carbon chemical shift anisotropy (CSA) patterns shown in Figure 4. C enrichment of this carbon allows it to be isolated in the carbon spectrum - the ring carbon resonances, at natural abundance, being at the level of the noise in the spectrum. The spectrum observed at 27°C (Figure 4) is characteristic of the CSA pattern below the transition. In fact, the same spectrum is observed from -196 to 320°C and such a large range of temperature independence indicates that the axial symmetry observed is not due to any motional averaging, but to a fortuitous coincidence of two elements of the chemical shielding tensor. Above 320°C and through the phase transition, narrowing of the CSA pattern occurs and deviation from axial symmetry develops until above the transition a narrowed, non-axial pattern evolves (Figure 4) which is unchanged up to 400°C. The onset of ]
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Molecular Motion in Homopolyester
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Figure 3. Proton linewidths (full-width at half-maximum) in PHBA as a function of temperature.
T = 357°C
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I I | I I I I | I II II II I|III f I 200 0 a/ppm
Figure 4. C spectrum at 15.1 M H z of the carboxyl carbon in PHBA at 27°C and 357°C obtained by cross-polarization techniques. Note the change of width and shape of the CSA pattern.
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significant narrowing of the CSA pattern occurs above 340°C which provides direct evidence that motion of the carboxyl unit only occurs with the phase transition (in accord with recent dielectric data in the region of the 350°C DSC transition) (7) and that the motion, at much lower temperatures, involves only the flipping of the phenyl ring. Literature data on CSA patterns of static carbonyl and carboxyl carbons (17-19) have established that, in most cases, the direction of greatest shielding (a ) is perpendicular to the sp plane for these carbons while the least shielded direction (a ) is in the sp plane. The intermediate value of shielding (
