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Characterisation of poly(p-phenylenebenzobisoxazole) fibres by solid state NMR
European Polymer Journal 38 (2002) 1645–1651 www.elsevier.com/locate/europolj
Characterisation of poly(p-phenylenebenzobisoxazole) fibres by solid state NMR Serge Bourbigot a
a,*
, Xavier Flambard a, Bertrand Revel
b
Laboratoire de G enie et Mat eriaux Textiles (GEMTEX), UPRES-EA 2461, Ecole Nationale Sup erieure des Arts et Industries Textiles (ENSAIT), 9 rue de l’Ermitage, BP 30329, 59056 Roubaix Cedex 01, France b Centre Commun de Mesures RMN, Universit e des Sciences et Technologies de Lille, 59650 Villeneuve d’Ascq, France Received 21 June 2001; received in revised form 13 November 2001; accepted 11 January 2002
Abstract This work investigates the chemical composition and the structure of poly(p-phenylene-2,6-benzobisoxazole) (PBO) fibres using solid state NMR. CP-DD-MAS 13 C NMR including dephasing delay experiments permits to determine the chemical composition of PBO. Studying molecular dynamic of PBO (wide line 1 H NMR), the structure of PBO is examined. According to the model of Kitagawa et al., the structure of PBO is then explained. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: PBO; Solid state NMR; Molecular dynamic; Structure
1. Introduction The field of high performance fibres has witnessed considerable growth in the last three decades [1]. A large number of high performance polymeric fibres are on the market today and they exhibit many enhanced properties in comparison with the traditional polymeric materials, such as high mechanical properties, heat resistance or good flame retardancy. Their applications are numerous (fragmentation barrier, protective clothing, heat barrier, . . .). Poly(p-phenylene-2,6-benzobisoxazole) (PBO) registered under the trademark Zylonâ is a new high performance fibre [2]. It is a polybenzazole containing aromatic hetero-cyclic ring (Fig. 1). It is a rigid rod isotropic crystal polymer. The polybenzazoles have been developed by US Air Force researchers as super heat resistant polymer which could surpass the traditional aramide fibres. PBO has
*
Corresponding author. Tel.: +33-3-20-25-89-84; fax: +33-320-27-25-97. E-mail address: [email protected] (S. Bourbigot).
superior tensile strength and modulus compared to the classical p-aramide fibres. In our laboratory we have recently shown that this fibre, as a knitted fabric, has excellent properties in cutting and stab resistance [3]. The combination of various knitted layers gives exceptional results with PBO fibres (stab-resistance of 25 J for a textile structure of 3 kg/m2 with an English blade). PBO has also good flame resistance and thermal stability among organic fibres. As an example, the limiting oxygen index (LOI measured according to NF G 07-128) of PBO is 68 vol% [1]. In previous works [4–7], we studied the unique flame and heat resistance of PBO fibre. We used the cone calorimeter as fire model [8]. Rate of heat release (RHR) curves of knitted PBO fibres at two external heat fluxes (50 and 75 kW/m2 ) showed that PBO fibres present a very good fire behaviour. RHR peaks of PBO at 50 and 75 kW/m2 were respectively only 60 and 150 kW/m2 . It demonstrated that the contribution to fire of PBO was very low even in severe conditions (post flashover conditions). The heat resistance of PBO is high in comparison with the other organic polymers. Under air, PBO fibres degrade at about 600 °C and form a 3 wt.% residue at 1200 °C (Fig. 2). Under nitrogen, the degradation
0014-3057/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 2 ) 0 0 0 4 9 - 6
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S. Bourbigot et al. / European Polymer Journal 38 (2002) 1645–1651
Fig. 1. Repeat unit of PBO.
Fig. 2. TG curves of PBO fibres under nitrogen (dashed lines) and air (plain lines) (heating rate ¼ 10 °C/min).
of fibres started at higher temperature (about 700 °C) (Fig. 2). Larger amount of residue was observed at 1200 °C (65 wt.% for PBO). It was then shown the strong influence of oxygen on the thermal degradation of the fibres. Previous works [9–13] have been reported on the morphology and the structure of PBO fibres. Crystal size and the morphology by wide-angle X-ray scattering, dark field images of transmission electron microscope (TEM) and images of scanning electron microscope were studied [9]. It also was discussed the shape of the crystal by lattice images of TEM and it was measured smallangle X-ray scattering patterns [10]. The structure of PBO was then discussed [11]. Finally, Kitagawa et al. [2] surveyed and studied the morphology and the structure of commercial PBO fibres. The study was then refined by Tashiro et al. [14]. A fibre structure model was proposed. PBO fibre is formed by microfibrils and contain many capillary-like microvoids, which exist between microfibrils before drying. These microvoids are connected each other through cracks or openings between microfibrils. There is void-free region in the very surface of the fibre. The microfibril is made of extended PBO molecules, highly oriented to the fibre axis. After analysing crystal structure of PBO fibres by X-ray diffraction, the polymeric chains are assumed to form layers extending along the 1 1 0 planes with the relative heights of the adjacent chains in the sheets almost confined to
either þc=4 or c=4 (c: fibre identity period) and these sheets are stacked together in the b-axis direction with random heights along the chain axis. Solid state nuclear magnetic resonance (NMR) is a powerful tool for characterising and studying the structural and dynamical properties of oriented polymers [15]. The possibility of performing selective experiments renders this spectroscopy particularly interesting for analysing heterogeneities in polymers. Moreover, the relaxation times of 1 H nucleus contain a large amount of information on the dynamics. According to previous X-ray diffraction studies [2,14], PBO is a rigid-rod polymer and is highly crystalline. It can then be expected by measurement of relaxation times to get information on local structure of PBO isolating phases with different mobility. In particular, spin–lattice relaxation times are very sensitive to the short spatial proximity of interacting dipole moments of the protons [16] and they can be used to determine the local structure of PBO. It can be done by non-selective measurements of relaxation times [15,17,18]. Wide-line 1 H NMR permits the determination of different phases, interphases and the measurement of the crystallinity in polymers [16,19,20]. Using high resolution solid state NMR, the chemical determination can be examined. As far as we know, PBO has never been characterised by solid state NMR and the purpose of this work is to get further information on the structure of PBO using this technique. In this study, the chemical composition of PBO is investigated by cross-polarisation (CP)–– magic angle spinning (MAS)––high power dipolar decoupling (DD) NMR 13 C and by use of the interrupted decoupling experiment for selecting non-protonated carbon resonances [21]. Then, the structure of PBO is discussed using relaxation times (spin–lattice relaxation, spin–lattice relaxation in the rotating frame and spin– spin relaxation) measured by low resolution 1 H NMR.
2. Experimental 2.1. PBO samples PBO fibres (as spun fibres) were supplied by the company Toyobo (Japan) and is registered under the trademark Zylonâ. The yarns used in this study have the following characteristics: Nm 2=34 spun yarn (60 Tex). 2.2. High resolution
13
C NMR
13 C NMR measurements were performed on a Bruker ASX100 operated at 25.2 MHz (2.35 T) with MAS, high power 1 H DD and 1 H–13 C CP using a 7 mm probe. The Hartmann–Hahn matching condition was obtained by adjusting the power on 1 H channel for a maximum 13 C FID signal of adamantane. Spectra were acquired with
S. Bourbigot et al. / European Polymer Journal 38 (2002) 1645–1651
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contact times of 1 ms. A repetition time of 10 s was used for all samples. The reference used was tetramethylsilane and the spinning speed was 15000 Hz. The values of chemical shift are verified using adamantane and adamantanone before starting a new experiment. For these products, the chemical shifts were within 0.2 ppm. Interrupted decoupling experiment [21] was carried out by gating off the decoupler before the 13 C free induction decay (FID) acquisition time. The decoupler was turned on again during the acquisition period. The delay between the last 180° 13 C pulse and acquisition time allows the dephasing delay to vary from 0 to 120 ls. The delay is long enough to suppress the signals from carbons with attached protons. Fig. 3. CP-DD-MAS NMR
13
C of PBO fibres.
2.3. Wide line 1 H NMR Proton NMR studies were carried out using a Bruker ASX 100 spectrometer, operating at a proton frequency of 100.13 MHz and with a 7 mm solenoid probe. The method of inversion recovery ½p–s–p=2 was used to measure proton spin–lattice relaxation (T1 (1 H)) time [15] and T1q (1 H) was determined in a 12 G rotating field using the spin locking pulse sequence [22]. The computation of spin–spin relaxation time (T2 (1 H)) was made using a solid echo sequence ½ðp=2Þx s ðp=2Þx [23]. After the first 90° pulse, a second 90° pulse is applied to the system after an interval that is slightly longer than the dead time of the spectrometer (15 ls). This generates an echo which would retain the shape of the FID. We may consider that the FID lineshape after a solid echo pulse is a reasonable approximation of the true FID.
3. Results and discussion
Fig. 4. CP-DD-MAS NMR dephasing delay.
13
C spectra of PBO fibres versus
3.1. Chemical composition CP-DD-MAS 13 C NMR spectrum of PBO is shown in Fig. 3. Six bands at 162, 149, 139, 128, 111 and 95 ppm can be distinguished on the spectrum which correspond to different magnetically non-equivalent carbons. The DD techniques rely on a fundamental principle that in absence of 1 H decoupling field, the 13 C magnetisation will dephase or decay as a result of 1 H–13 C dipolar interaction. The rate of dephasing of the 13 C signal is related to the magnitude of dipolar interaction which depends on geometrical factors, namely, the 1 H–13 C 3 internuclear distance, rCH , as rCH and the angle between the internuclear vector and the static magnetic field [21]. Therefore, in the DD spectrum with long dephasing delay, the resonances observed are mainly of non-protonated and weakly coupled carbons. Fig. 4 shows 13 C NMR spectra of PBO versus dephasing delay. It can be seen that the bands at 111 and 95 ppm are totally removed at 40 ls and the band at 128 ppm decreases
versus dephasing delay till 60 ls. The other bands seem to be unchanged even at very long dephasing delay (>100 ls). This result suggests that the carbons at 111 and 95 ppm are protonated carbons and that the others are non-protonated carbons. According to the literature [24–27] and to the experiment of Fig. 4, the different carbons can be assigned. 160–100 ppm is the zone corresponding to the aromatic and polyaromatic compounds. This is in agreement with the repeat unit of PBO. The assignments of carbons are given in Table 1. The experiment of Fig. 4 and previous works [25,27] confirms that the band at 128 ppm corresponds to protonated and non-protonated carbons. 3.2. Wide line 1 H NMR Relaxation times of solid polymers are not only determined by dynamic phenomena. There may exist a
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Table 1 Assignments of PBOs bands Repeat unit of PBO
contribution from the static mechanism of spin diffusion [15]. When two proton populations have different spin temperatures at a given time, they will tend to a common spin temperature by spin diffusion. Such a situation occurs during T1 (1 H) and T1q (1 H) determinations in systems which may contain heterogeneities. According to these considerations, it has been estimated that for polymeric systems, spin diffusion will be efficient at length scales of the order of 10 and 1 nm during T1 (1 H) and T1q (1 H) measurements respectively [28–31]. It is to note that because of the low duration of the free induction, the spin diffusion cannot occur and then, T2 will be not affected by this phenomenon. The 1 H spin–lattice relaxation of PBO is characterised by the sum of two exponentials Fig. 5. The magnetisation, MðtÞ, is simulated according to the Eq. (1): t MðtÞ ¼ M0s As Bs exp T1s ð1 H Þ t ð1Þ þ M0l Al Bl exp T1l ð1 H Þ M0s , M0l , M1s (1 H) and T1l (1 H) are the initial magnetisations and the spin–lattice relaxations times of the short and long components respectively. Parameters As , Al , and, Bs , Bl are close to one and two respectively and adjustable values. They take into account that the in-
Bands (ppm)
Ref.
(1) 95 (2) 111 (3)–(8) 128 (9) and (10) 139 (11) and (12) 149 (13) and (14) 162
[24–27]
version is not perfect. The values of these parameters are reported in Table 2. The short T1 (1 H) (0.05 s) can be assigned to the noncrystalline component of PBO. Indeed, the non-crystalline part can exhibit molecular motions that favour relaxation. The long T1 (1 H) (1.1 s) can be assigned to the crystalline part of PBO because these protons are weakly coupled to the lattice and diffuse slowly. According to the considerations mentioned above, one can therefore predict that the crystal sizes of PBO are over 10 nm. The evolution of the magnetisation for measuring T1q (1 H) after a spin––locking experiment can be simulated by the sum of two exponentials (Fig. 6). The magnetisation, MðtÞ, is simulated according to the Eq. (2): t t MðtÞ ¼ M0s exp þ M exp 0l T1qs ð1 H Þ T1ql ð1 H Þ ð2Þ
Table 2 Measurement of T1 (1 H) using the inversion recovery sequence Short component PBO
1
Long component
M0s (%)
T1s ( H) (s)
M0l (%)
T1l (1 H) (s)
0.49
0:05 0:005
0.51
1:1 0:2
Fig. 5. Evolution of the normalised magnetisation of PBO versus time during an inversion recovery pulse experiment (plain line: short component; dashed line: long component).
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Fig. 6. Evolution of the normalised magnetisation of PBO versus time during a spin-locking pulse experiment (plain line: long component; dashed line: short component).
M0s , M0l , T1s (1 H) and T1l (1 H) are the initial magnetisations and the spin–lattice relaxations times in the rotating frame of the short and long components respectively. Two T1q (1 H) are observed (Table 3). For the same considerations as above, the short T1q (1 H) (3 ms) can be assigned to the non-crystalline component of PBO and the long T1q (1 H) (39 ms) can be assigned to the crystalline part of PBO. From these considerations, one can predict that the crystal sizes of PBO are over 1 nm. The deductions made from the T1 (1 H) measurement are then confirmed. The free precession of PBO have been recorded after the spin-echo pulse sequence (Fig. 7). It can be distinguished two decays: one fast up to 50 ls and the other slower. The two decaying components are assumed to be described by an intermediary shape between Lorentzian and Gaussian represented by Weibullian functions. This latter functions was proposed for the first time by Kaufman and Bunger [32] to a better approximation of the
Table 3 Measurement of T1q (1 H) using the spin-locking pulse sequence Short component PBO
1
Long component
M0s (%)
T1qs ( H) (ms)
M0l (%)
T1ql (1 H) (ms)
0.48
31
0.52
39 6
FID than that obtained with a single monoexponential relaxation. The transverse magnetisation MðtÞ can be therefore simulated by the Eq. (3):
MðtÞ ¼ M0s exp þ M0l exp
as t T2s ð1 H Þ al t T2l ð1 H Þ
ð3Þ
where M0s , M0l , T2s (1 H), T2l (1 H) and as , al are the initial magnetisations, the spin–spin relaxations times and the
Fig. 7. Evolution of the normalised magnetisation of PBO versus time during a spin echo pulse experiment (plain line: long component; dashed line: short component).
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Table 4 Measurement of T2 (1 H) using the spin-echo pulse sequence Short component PBO
Long component 1
M0s (%)
T2s ( H) (ls)
as
M0l (%)
T2l (1 H) (ls)
al
0.9
21 4
2 (Gaussian decay)
0.1
138 20
1.5 (Weibullian decay)
Weibullian coefficients (1 < a < 2) of the short and long components respectively. Two T2 (1 H) are observed (Table 4). The spin–spin relaxation times depend on the mobility of the different phases of the material. Thus it means that two phases are present in PBO. According to the Weibullian coefficient, the short decay is Gaussian and the long decay is intermediate between Lorentzian and Gaussian. The Gaussian function is a good approximation for the decay of a material whose nuclear dipoles rigidly in space while an exponential decay is observed for motionally narrowed materials [23,29]. The short relaxation time, T2s (1 H), can be associated with the crystalline region of PBO fibres as suggested by the Gaussian form of the decay. The long relaxation time, T2l (1 H), which derives from an intermediate decay (Gaussian–Lorentzian decay), is assigned because of the intermediate character of the decay, to relaxation of molecules in the non-crystalline regions (amorphous region) that are motionally activated on the NMR time scale and to imperfect crystalline structures produced during drawing of PBO fibres. The initial magnetisations, M0s , M0l , of the fast and slow components correspond to the number of hydrogen in the two phases. 90% of protons are in the crystalline of PBO fibres and 10% of protons are in the amorphous region. This result is not surprising because PBO fibres are highly crystalline [2,14]. Fig. 8. PBO structure model proposed by Kitagawa et al. (from [2]).
4. Discussion Recently, Kitagawa and co-workers proposed a fibre structure model of PBO fibre (Fig. 8) [2]. They propose that the fibre is formed from microfibrils (preliminary 10–50 nm in diameter) and contain many capillary-like microvoids. These microvoids are connected each other through cracks or openings between microfibrils. There is void-free region in the very surface of the fibre. The microfibril is made of extended PBO molecules, highly oriented to the fibre axis. The model proposed by Kitagawa et al. allows to complete the interpretations of our NMR results. T1 considerations have predicted that the crystal sizes of PBO are over 10 nm. Kitagawa et al. measured by wide angle X-ray diffraction crystal sizes from 2.7 to 9.6 nm depending on the orientation of the diffraction plane. Nevertheless, lattice images obtained from ultrathin section of PBO fibre indicated that the length of the co-
herency could continue over 40 nm. Their interpretation was that PBO fibre is composed of many extended chains. This is consistent with our T1s and T1qs predictions because if we assume that the non-crystalline regions (amorphous region and/or defects) are located at the surface of the microfibrils, spin diffusion diffuses slowly over the highly oriented PBO chains and two regions are distinguished. Considerations on T2s allow to propose two different phases in the PBO fibre. The long value of T2l (1 H) suggests that protons are very mobile in the amorphous/ defect region, and because of the low quantity of these protons (10%) and of the structure of PBO, we can propose that they are located at the interfaces microfibril––microvoid and/or at the end of the chains. This consistent with the apparent inconstancies between the proportions of the ‘‘rigid’’ and ‘‘mobile’’ components
S. Bourbigot et al. / European Polymer Journal 38 (2002) 1645–1651
determined by T1 (1 H), T1q (1 H) and T2 (1 H) measurements. The balance between the two components is much more even for the T1 (1 H) and T1q (1 H) measurements than for T2 (1 H), suggesting that there is some degree of averaging of relaxation by the spin diffusion at the interface.
5. Conclusion PBO fibres have been characterised by solid state NMR. CP-DD-MAS 13 C NMR including dephasing delay experiments has permitted to determine the chemical composition of PBO. Studying molecular dynamic of PBO, the structure of PBO has been investigated and the model of Kitagawa et al. has been completed. The structure of PBO is then explained.
References [1] Hongu T, Philips GO. New fibers. Cambridge, England: Woodhead Publishing Limited; 1997. [2] Kitagawa T, Murase H, Yabuki K. J Polym Sci Part B: Polym Phys 1998;36:39–48. [3] Flambard X. In: Ferreira FN, editor. Fiber society spring conference on sustainability and recycling of textile materials. Guimares, Portugal: Fiber Society; 2000. [4] Bourbigot S, Flambard X, Duquesne S. Polym Int 2001;50:157–64. [5] Bourbigot S, Flambard X, Duquesne S, Poutch F. In: Wilkie CA, Nelson G, editors. ACS Symposium Series No. 797. American Chemical Society; 2001. p. 63–75 [Chapter 6]. [6] Bourbigot S, Flambard X, Poutch F, Duquesne S. Polym Degrad Stab 2001;74(3):481–6. [7] Bourbigot S, Flambard X, Poutch F. Polym Degrad Stab 2001;74(2):283–90.
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[8] Babrauskas V. Development of cone calorimeter––a bench scale rate of heat release based on oxygen consumption, NBS-IR 82-2611, US Nat. Bur. Stand., Gaithersburg; 1982. [9] Krause SJ, Haddock TB, Vezie DL, Lenhert PJ, Hwang WF, Price GE, Helminiak TE, O’Brien JF, Adams WW. Polymer 1988;29:1354. [10] Martin DC, Thomas EL. Macromolecules 1991;24:2450. [11] Kumar S, Warner S, Grubb DT, Adams WW. Polymer 1994;35:5408. [12] Hunsaker ME, Price GE, Bai SJ. Polymer 1992;33:2128. [13] Bai SJ, Price GE. Polymer 1992;33:2136. [14] Tashiro K, Yoshino J, Kitagawa T, Murase H, Yabuki K. Macromolecules 1998;31:5430–40. [15] Ibbett RN, editor. NMR spectroscopy of polymers. London: Blackie Academic & Professional; 1993 (an imprint of Chapman & Hall). [16] Smith ME, Strange JH. Meas Sci Technol 1996;7:449–75. [17] McBrierty VJ. Polymer 1974;15:503–20. [18] Voelkel R. Angew Chem Int Ed Engl 1988;27:1468–83. [19] Assink RA. J Appl Polym Sci 1981;26:3689–98. [20] Soltani R, Laupr^etre F, Monnerie L, Teyssie P. Polymer 1998;39:3297–309. [21] Opella SJ, Frey MH. J Am Chem Soc 1979;101:5854–6. [22] Solomon I. CR Acad Sci, Paris 1959;248:92–102. [23] Powels JG, Strange JH. Proc Phys Soc 1963;82:6–20. [24] Earl WL, Vanderhart DL. J Magn Reson 1982;48:35. [25] Maciel GE, Bartuska VJ, Miknis FP. Fuel 1979;58:391. [26] Bourbigot S, Le Bras M, Delobel R, Descressain R, Amoureux JP. J Chem Soc––Faraday Trans 1996;92(1): 149. [27] Duncan TM, Douglass DC. J Chem Phys 1984;87:339. [28] Bloembergen N. Physica 1949;15:386. [29] Abragam A, Goldman M. Nuclear magnetism: order and disorder. New York: Oxford University Press; 1982. [30] McBrierty J, Douglass DC. J Polym Sci, Macromol Rev 1981;16:295. [31] McBrierty J, Douglass DC. Phys Rep 1980;63:63. [32] Kaufman S, Bunger DJ. J Magn Res 1970;3:218.
Characterisation of poly(p-phenylenebenzobisoxazole) fibres by solid state NMR Serge Bourbigot a
a,*
, Xavier Flambard a, Bertrand Revel
b
Laboratoire de G enie et Mat eriaux Textiles (GEMTEX), UPRES-EA 2461, Ecole Nationale Sup erieure des Arts et Industries Textiles (ENSAIT), 9 rue de l’Ermitage, BP 30329, 59056 Roubaix Cedex 01, France b Centre Commun de Mesures RMN, Universit e des Sciences et Technologies de Lille, 59650 Villeneuve d’Ascq, France Received 21 June 2001; received in revised form 13 November 2001; accepted 11 January 2002
Abstract This work investigates the chemical composition and the structure of poly(p-phenylene-2,6-benzobisoxazole) (PBO) fibres using solid state NMR. CP-DD-MAS 13 C NMR including dephasing delay experiments permits to determine the chemical composition of PBO. Studying molecular dynamic of PBO (wide line 1 H NMR), the structure of PBO is examined. According to the model of Kitagawa et al., the structure of PBO is then explained. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: PBO; Solid state NMR; Molecular dynamic; Structure
1. Introduction The field of high performance fibres has witnessed considerable growth in the last three decades [1]. A large number of high performance polymeric fibres are on the market today and they exhibit many enhanced properties in comparison with the traditional polymeric materials, such as high mechanical properties, heat resistance or good flame retardancy. Their applications are numerous (fragmentation barrier, protective clothing, heat barrier, . . .). Poly(p-phenylene-2,6-benzobisoxazole) (PBO) registered under the trademark Zylonâ is a new high performance fibre [2]. It is a polybenzazole containing aromatic hetero-cyclic ring (Fig. 1). It is a rigid rod isotropic crystal polymer. The polybenzazoles have been developed by US Air Force researchers as super heat resistant polymer which could surpass the traditional aramide fibres. PBO has
*
Corresponding author. Tel.: +33-3-20-25-89-84; fax: +33-320-27-25-97. E-mail address: [email protected] (S. Bourbigot).
superior tensile strength and modulus compared to the classical p-aramide fibres. In our laboratory we have recently shown that this fibre, as a knitted fabric, has excellent properties in cutting and stab resistance [3]. The combination of various knitted layers gives exceptional results with PBO fibres (stab-resistance of 25 J for a textile structure of 3 kg/m2 with an English blade). PBO has also good flame resistance and thermal stability among organic fibres. As an example, the limiting oxygen index (LOI measured according to NF G 07-128) of PBO is 68 vol% [1]. In previous works [4–7], we studied the unique flame and heat resistance of PBO fibre. We used the cone calorimeter as fire model [8]. Rate of heat release (RHR) curves of knitted PBO fibres at two external heat fluxes (50 and 75 kW/m2 ) showed that PBO fibres present a very good fire behaviour. RHR peaks of PBO at 50 and 75 kW/m2 were respectively only 60 and 150 kW/m2 . It demonstrated that the contribution to fire of PBO was very low even in severe conditions (post flashover conditions). The heat resistance of PBO is high in comparison with the other organic polymers. Under air, PBO fibres degrade at about 600 °C and form a 3 wt.% residue at 1200 °C (Fig. 2). Under nitrogen, the degradation
0014-3057/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 2 ) 0 0 0 4 9 - 6
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S. Bourbigot et al. / European Polymer Journal 38 (2002) 1645–1651
Fig. 1. Repeat unit of PBO.
Fig. 2. TG curves of PBO fibres under nitrogen (dashed lines) and air (plain lines) (heating rate ¼ 10 °C/min).
of fibres started at higher temperature (about 700 °C) (Fig. 2). Larger amount of residue was observed at 1200 °C (65 wt.% for PBO). It was then shown the strong influence of oxygen on the thermal degradation of the fibres. Previous works [9–13] have been reported on the morphology and the structure of PBO fibres. Crystal size and the morphology by wide-angle X-ray scattering, dark field images of transmission electron microscope (TEM) and images of scanning electron microscope were studied [9]. It also was discussed the shape of the crystal by lattice images of TEM and it was measured smallangle X-ray scattering patterns [10]. The structure of PBO was then discussed [11]. Finally, Kitagawa et al. [2] surveyed and studied the morphology and the structure of commercial PBO fibres. The study was then refined by Tashiro et al. [14]. A fibre structure model was proposed. PBO fibre is formed by microfibrils and contain many capillary-like microvoids, which exist between microfibrils before drying. These microvoids are connected each other through cracks or openings between microfibrils. There is void-free region in the very surface of the fibre. The microfibril is made of extended PBO molecules, highly oriented to the fibre axis. After analysing crystal structure of PBO fibres by X-ray diffraction, the polymeric chains are assumed to form layers extending along the 1 1 0 planes with the relative heights of the adjacent chains in the sheets almost confined to
either þc=4 or c=4 (c: fibre identity period) and these sheets are stacked together in the b-axis direction with random heights along the chain axis. Solid state nuclear magnetic resonance (NMR) is a powerful tool for characterising and studying the structural and dynamical properties of oriented polymers [15]. The possibility of performing selective experiments renders this spectroscopy particularly interesting for analysing heterogeneities in polymers. Moreover, the relaxation times of 1 H nucleus contain a large amount of information on the dynamics. According to previous X-ray diffraction studies [2,14], PBO is a rigid-rod polymer and is highly crystalline. It can then be expected by measurement of relaxation times to get information on local structure of PBO isolating phases with different mobility. In particular, spin–lattice relaxation times are very sensitive to the short spatial proximity of interacting dipole moments of the protons [16] and they can be used to determine the local structure of PBO. It can be done by non-selective measurements of relaxation times [15,17,18]. Wide-line 1 H NMR permits the determination of different phases, interphases and the measurement of the crystallinity in polymers [16,19,20]. Using high resolution solid state NMR, the chemical determination can be examined. As far as we know, PBO has never been characterised by solid state NMR and the purpose of this work is to get further information on the structure of PBO using this technique. In this study, the chemical composition of PBO is investigated by cross-polarisation (CP)–– magic angle spinning (MAS)––high power dipolar decoupling (DD) NMR 13 C and by use of the interrupted decoupling experiment for selecting non-protonated carbon resonances [21]. Then, the structure of PBO is discussed using relaxation times (spin–lattice relaxation, spin–lattice relaxation in the rotating frame and spin– spin relaxation) measured by low resolution 1 H NMR.
2. Experimental 2.1. PBO samples PBO fibres (as spun fibres) were supplied by the company Toyobo (Japan) and is registered under the trademark Zylonâ. The yarns used in this study have the following characteristics: Nm 2=34 spun yarn (60 Tex). 2.2. High resolution
13
C NMR
13 C NMR measurements were performed on a Bruker ASX100 operated at 25.2 MHz (2.35 T) with MAS, high power 1 H DD and 1 H–13 C CP using a 7 mm probe. The Hartmann–Hahn matching condition was obtained by adjusting the power on 1 H channel for a maximum 13 C FID signal of adamantane. Spectra were acquired with
S. Bourbigot et al. / European Polymer Journal 38 (2002) 1645–1651
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contact times of 1 ms. A repetition time of 10 s was used for all samples. The reference used was tetramethylsilane and the spinning speed was 15000 Hz. The values of chemical shift are verified using adamantane and adamantanone before starting a new experiment. For these products, the chemical shifts were within 0.2 ppm. Interrupted decoupling experiment [21] was carried out by gating off the decoupler before the 13 C free induction decay (FID) acquisition time. The decoupler was turned on again during the acquisition period. The delay between the last 180° 13 C pulse and acquisition time allows the dephasing delay to vary from 0 to 120 ls. The delay is long enough to suppress the signals from carbons with attached protons. Fig. 3. CP-DD-MAS NMR
13
C of PBO fibres.
2.3. Wide line 1 H NMR Proton NMR studies were carried out using a Bruker ASX 100 spectrometer, operating at a proton frequency of 100.13 MHz and with a 7 mm solenoid probe. The method of inversion recovery ½p–s–p=2 was used to measure proton spin–lattice relaxation (T1 (1 H)) time [15] and T1q (1 H) was determined in a 12 G rotating field using the spin locking pulse sequence [22]. The computation of spin–spin relaxation time (T2 (1 H)) was made using a solid echo sequence ½ðp=2Þx s ðp=2Þx [23]. After the first 90° pulse, a second 90° pulse is applied to the system after an interval that is slightly longer than the dead time of the spectrometer (15 ls). This generates an echo which would retain the shape of the FID. We may consider that the FID lineshape after a solid echo pulse is a reasonable approximation of the true FID.
3. Results and discussion
Fig. 4. CP-DD-MAS NMR dephasing delay.
13
C spectra of PBO fibres versus
3.1. Chemical composition CP-DD-MAS 13 C NMR spectrum of PBO is shown in Fig. 3. Six bands at 162, 149, 139, 128, 111 and 95 ppm can be distinguished on the spectrum which correspond to different magnetically non-equivalent carbons. The DD techniques rely on a fundamental principle that in absence of 1 H decoupling field, the 13 C magnetisation will dephase or decay as a result of 1 H–13 C dipolar interaction. The rate of dephasing of the 13 C signal is related to the magnitude of dipolar interaction which depends on geometrical factors, namely, the 1 H–13 C 3 internuclear distance, rCH , as rCH and the angle between the internuclear vector and the static magnetic field [21]. Therefore, in the DD spectrum with long dephasing delay, the resonances observed are mainly of non-protonated and weakly coupled carbons. Fig. 4 shows 13 C NMR spectra of PBO versus dephasing delay. It can be seen that the bands at 111 and 95 ppm are totally removed at 40 ls and the band at 128 ppm decreases
versus dephasing delay till 60 ls. The other bands seem to be unchanged even at very long dephasing delay (>100 ls). This result suggests that the carbons at 111 and 95 ppm are protonated carbons and that the others are non-protonated carbons. According to the literature [24–27] and to the experiment of Fig. 4, the different carbons can be assigned. 160–100 ppm is the zone corresponding to the aromatic and polyaromatic compounds. This is in agreement with the repeat unit of PBO. The assignments of carbons are given in Table 1. The experiment of Fig. 4 and previous works [25,27] confirms that the band at 128 ppm corresponds to protonated and non-protonated carbons. 3.2. Wide line 1 H NMR Relaxation times of solid polymers are not only determined by dynamic phenomena. There may exist a
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Table 1 Assignments of PBOs bands Repeat unit of PBO
contribution from the static mechanism of spin diffusion [15]. When two proton populations have different spin temperatures at a given time, they will tend to a common spin temperature by spin diffusion. Such a situation occurs during T1 (1 H) and T1q (1 H) determinations in systems which may contain heterogeneities. According to these considerations, it has been estimated that for polymeric systems, spin diffusion will be efficient at length scales of the order of 10 and 1 nm during T1 (1 H) and T1q (1 H) measurements respectively [28–31]. It is to note that because of the low duration of the free induction, the spin diffusion cannot occur and then, T2 will be not affected by this phenomenon. The 1 H spin–lattice relaxation of PBO is characterised by the sum of two exponentials Fig. 5. The magnetisation, MðtÞ, is simulated according to the Eq. (1): t MðtÞ ¼ M0s As Bs exp T1s ð1 H Þ t ð1Þ þ M0l Al Bl exp T1l ð1 H Þ M0s , M0l , M1s (1 H) and T1l (1 H) are the initial magnetisations and the spin–lattice relaxations times of the short and long components respectively. Parameters As , Al , and, Bs , Bl are close to one and two respectively and adjustable values. They take into account that the in-
Bands (ppm)
Ref.
(1) 95 (2) 111 (3)–(8) 128 (9) and (10) 139 (11) and (12) 149 (13) and (14) 162
[24–27]
version is not perfect. The values of these parameters are reported in Table 2. The short T1 (1 H) (0.05 s) can be assigned to the noncrystalline component of PBO. Indeed, the non-crystalline part can exhibit molecular motions that favour relaxation. The long T1 (1 H) (1.1 s) can be assigned to the crystalline part of PBO because these protons are weakly coupled to the lattice and diffuse slowly. According to the considerations mentioned above, one can therefore predict that the crystal sizes of PBO are over 10 nm. The evolution of the magnetisation for measuring T1q (1 H) after a spin––locking experiment can be simulated by the sum of two exponentials (Fig. 6). The magnetisation, MðtÞ, is simulated according to the Eq. (2): t t MðtÞ ¼ M0s exp þ M exp 0l T1qs ð1 H Þ T1ql ð1 H Þ ð2Þ
Table 2 Measurement of T1 (1 H) using the inversion recovery sequence Short component PBO
1
Long component
M0s (%)
T1s ( H) (s)
M0l (%)
T1l (1 H) (s)
0.49
0:05 0:005
0.51
1:1 0:2
Fig. 5. Evolution of the normalised magnetisation of PBO versus time during an inversion recovery pulse experiment (plain line: short component; dashed line: long component).
S. Bourbigot et al. / European Polymer Journal 38 (2002) 1645–1651
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Fig. 6. Evolution of the normalised magnetisation of PBO versus time during a spin-locking pulse experiment (plain line: long component; dashed line: short component).
M0s , M0l , T1s (1 H) and T1l (1 H) are the initial magnetisations and the spin–lattice relaxations times in the rotating frame of the short and long components respectively. Two T1q (1 H) are observed (Table 3). For the same considerations as above, the short T1q (1 H) (3 ms) can be assigned to the non-crystalline component of PBO and the long T1q (1 H) (39 ms) can be assigned to the crystalline part of PBO. From these considerations, one can predict that the crystal sizes of PBO are over 1 nm. The deductions made from the T1 (1 H) measurement are then confirmed. The free precession of PBO have been recorded after the spin-echo pulse sequence (Fig. 7). It can be distinguished two decays: one fast up to 50 ls and the other slower. The two decaying components are assumed to be described by an intermediary shape between Lorentzian and Gaussian represented by Weibullian functions. This latter functions was proposed for the first time by Kaufman and Bunger [32] to a better approximation of the
Table 3 Measurement of T1q (1 H) using the spin-locking pulse sequence Short component PBO
1
Long component
M0s (%)
T1qs ( H) (ms)
M0l (%)
T1ql (1 H) (ms)
0.48
31
0.52
39 6
FID than that obtained with a single monoexponential relaxation. The transverse magnetisation MðtÞ can be therefore simulated by the Eq. (3):
MðtÞ ¼ M0s exp þ M0l exp
as t T2s ð1 H Þ al t T2l ð1 H Þ
ð3Þ
where M0s , M0l , T2s (1 H), T2l (1 H) and as , al are the initial magnetisations, the spin–spin relaxations times and the
Fig. 7. Evolution of the normalised magnetisation of PBO versus time during a spin echo pulse experiment (plain line: long component; dashed line: short component).
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Table 4 Measurement of T2 (1 H) using the spin-echo pulse sequence Short component PBO
Long component 1
M0s (%)
T2s ( H) (ls)
as
M0l (%)
T2l (1 H) (ls)
al
0.9
21 4
2 (Gaussian decay)
0.1
138 20
1.5 (Weibullian decay)
Weibullian coefficients (1 < a < 2) of the short and long components respectively. Two T2 (1 H) are observed (Table 4). The spin–spin relaxation times depend on the mobility of the different phases of the material. Thus it means that two phases are present in PBO. According to the Weibullian coefficient, the short decay is Gaussian and the long decay is intermediate between Lorentzian and Gaussian. The Gaussian function is a good approximation for the decay of a material whose nuclear dipoles rigidly in space while an exponential decay is observed for motionally narrowed materials [23,29]. The short relaxation time, T2s (1 H), can be associated with the crystalline region of PBO fibres as suggested by the Gaussian form of the decay. The long relaxation time, T2l (1 H), which derives from an intermediate decay (Gaussian–Lorentzian decay), is assigned because of the intermediate character of the decay, to relaxation of molecules in the non-crystalline regions (amorphous region) that are motionally activated on the NMR time scale and to imperfect crystalline structures produced during drawing of PBO fibres. The initial magnetisations, M0s , M0l , of the fast and slow components correspond to the number of hydrogen in the two phases. 90% of protons are in the crystalline of PBO fibres and 10% of protons are in the amorphous region. This result is not surprising because PBO fibres are highly crystalline [2,14]. Fig. 8. PBO structure model proposed by Kitagawa et al. (from [2]).
4. Discussion Recently, Kitagawa and co-workers proposed a fibre structure model of PBO fibre (Fig. 8) [2]. They propose that the fibre is formed from microfibrils (preliminary 10–50 nm in diameter) and contain many capillary-like microvoids. These microvoids are connected each other through cracks or openings between microfibrils. There is void-free region in the very surface of the fibre. The microfibril is made of extended PBO molecules, highly oriented to the fibre axis. The model proposed by Kitagawa et al. allows to complete the interpretations of our NMR results. T1 considerations have predicted that the crystal sizes of PBO are over 10 nm. Kitagawa et al. measured by wide angle X-ray diffraction crystal sizes from 2.7 to 9.6 nm depending on the orientation of the diffraction plane. Nevertheless, lattice images obtained from ultrathin section of PBO fibre indicated that the length of the co-
herency could continue over 40 nm. Their interpretation was that PBO fibre is composed of many extended chains. This is consistent with our T1s and T1qs predictions because if we assume that the non-crystalline regions (amorphous region and/or defects) are located at the surface of the microfibrils, spin diffusion diffuses slowly over the highly oriented PBO chains and two regions are distinguished. Considerations on T2s allow to propose two different phases in the PBO fibre. The long value of T2l (1 H) suggests that protons are very mobile in the amorphous/ defect region, and because of the low quantity of these protons (10%) and of the structure of PBO, we can propose that they are located at the interfaces microfibril––microvoid and/or at the end of the chains. This consistent with the apparent inconstancies between the proportions of the ‘‘rigid’’ and ‘‘mobile’’ components
S. Bourbigot et al. / European Polymer Journal 38 (2002) 1645–1651
determined by T1 (1 H), T1q (1 H) and T2 (1 H) measurements. The balance between the two components is much more even for the T1 (1 H) and T1q (1 H) measurements than for T2 (1 H), suggesting that there is some degree of averaging of relaxation by the spin diffusion at the interface.
5. Conclusion PBO fibres have been characterised by solid state NMR. CP-DD-MAS 13 C NMR including dephasing delay experiments has permitted to determine the chemical composition of PBO. Studying molecular dynamic of PBO, the structure of PBO has been investigated and the model of Kitagawa et al. has been completed. The structure of PBO is then explained.
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