Insights into the Molecular Dynamics in Polysulfone Polymers from (13)C Solid-State NMR Experiments.

Article pubs.acs.org/JPCB

Insights into the Molecular Dynamics in Polysulfone Polymers from 13 C Solid-State NMR Experiments Kavalakal Mathai Eldho,† P. R. Rajamohanan,† Ralf Anto,‡ Neelima Bulakh,‡ Ashish K. Lele,*,‡ and T. G. Ajithkumar*,† †

Central NMR Facility and ‡Polymer Science and Engineering Division, CSIR-National Chemical Laboratory, Pune 411 008, India

Downloaded by UNIV OF NEBRASKA-LINCOLN on August 25, 2015 | http://pubs.acs.org Publication Date (Web): July 21, 2015 | doi: 10.1021/acs.jpcb.5b03103

S Supporting Information *

ABSTRACT: The molecular and segmental motions in three different grades of ductile polysulfone polymers; poly(ether sulfone) (PESU), polysulfone (PSU), and poly(phenyl sulfone) (PPSU) are probed using 13C solid-state NMR experiments. Polarization inversion spin exchange at magic angle (PISEMA) experiments indicates that the phenyl rings in the polymers are undergoing π-flip motions on the order of 100 kHz. The temperature dependent PISEMA experiments show that the fraction of mobile regions that undergoes aromatic π-flips is higher in PPSU than in the other two polymers. The center band only detection of exchange (CODEX) experiments was carried out and was unable to detect any slow segmental motions in the chains. A combination of 13C spin−lattice relaxation time (T1) and T1-filtered PISEMA experiments show that the mobile regions in all the polymers are dynamically heterogeneous.

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INTRODUCTION

The origin and nature of different intra- and intermolecular segmental motions and their influence on mechanical properties of amorphous polymers is an intense area of research in polymer science and engineering. Various studies using dynamic mechanical analysis (DMA),1−5 dielectric analysis (DEA), 6,7 neutron scattering (NS),7−11 and solid-state NMR12−20 on high-end engineering amorphous polymers such as polycarbonates and polysulfones, having phenyl or biphenyl rings, reveals that both the aromatic π-flips and small or large amplitude oscillatory motions of functional groups result in segmental motions that contribute to the γ relaxation transitions, which are thought to be responsible for the high impact strength of these polymers. Aromatic polymers containing the sulfone functional group such as PESU (poly(ether sulfone) with the trade name VERADEL), PSU (polysulfone with the trade name UDEL), and PPSU (poly(phenyl sulfone) with the trade name RADEL) (Figure 1) are known for their ductility, thermal stability, and hydrolytic resistance and are therefore used in medical, engineering, and food processing applications.21 Among these, PSU is an extensively studied polymer in terms of its thermomechanical properties. It has a glass transition temperature (Tg) of 185 °C assigned as the α relaxation, a sub-Tg β relaxation observed around 85 °C and another sub-Tg γ relaxation observed over a wide range centered around −80 °C.3 PESU and PPSU are polysulfone polymers, which have similar or even better thermomechanical properties than PSU. They have a Tg of 225 °C, which is 40 °C higher than that of PSU.21 No β transition has been reported for either of them, but a sub-Tg γ © XXXX American Chemical Society

Figure 1. Chemical structure of three different polysulfone polymers (a) PESU, (b) PSU, and (c) PPSU.

transition is observed in the region centered around −75 to −80 °C, which is almost similar to that of PSU.3 There are only a few reports that investigate the correlation of the molecular motions in polysulfones to their mechanical property.8−10,15 The neutron scattering experiments on PSU polymers by Arrese-lgor et al. show that along with aromatic πflip motions, small angle chain oscillations are also present.8−10 Aitken et al. monitored the dynamical mechanical relaxation behavior of PSU and BPA-PC by substituting a bulky tetramethylbisphenol A group alternatively for bisphenol A.3 They observed two separate γ-relaxation peaks for the copolymer of polysulfone−tetramethylbisphenol A−polycarbonate, whereas a single broad peak was observed for Special Issue: Biman Bagchi Festschrift Received: March 31, 2015 Revised: June 27, 2015

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DOI: 10.1021/acs.jpcb.5b03103 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

ωeff,H = (Δω2 + ω1,H2)1/2, with an offset Δω = ω1,H/ tan(54.74°). The basic PISEMA experiment, which was used extensively to study oriented systems was modified for spinning samples by Dvinskikh et al.,23 where the mismatch between the 1 H and 13C channel due to spinning rate is compensated by the RF condition ωeff,H − ω1,C = ±ωr. SIMPSON simulations24 for the PISEMA experiment showed that there is a decrease in the zero frequency with a small change in Hartmann−Hahn condition to ω1,C = ω1,H tan(54.74°), while keeping the resonance condition at ±ωr. As a result, all the PISEMA experiments were carried out using this condition. The theoretical scaling factor of the PISEMA experiment is calculated by the formula sin(54.74°)DCH/√2, (DCH is the 1 H−13C heteronuclear dipolar coupling), which is constant at spinning rates ≥12 kHz.23,25 We have observed a decrease in the scaling factor of PISEMA with a decrease in the spinning rate from 12 kHz (Figure S1, Supporting Information). The scaling factor of the sequence at a spinning rate of 8 kHz is observed as 0.52, and all the spectra have been scaled by this so that the actual dipolar coupling can be directly read out from the spectra. The heteronuclear dipolar coupling of the CH group in crystalline alanine is 21.5 kHz (DCH), which is measured from the PISEMA spectrum. A reduction in the heteronculear dipolar coupling from this value is an indication of molecular motions. Using the LG-SLF experiment, which is similar to the PISEMA experiment, Hong et al.26 showed that the dipolar coupling of the aromatic groups of polycarbonate is 11.7 kHz, which proves that they are undergoing motions on the order of 100 kHz. In this study, the heteronuclear dipolar coupling of aromatic CH groups in polysulfone polymers are measured using the PISEMA experiments to extract information about aromatic π-flip motions. CODEX. The slow motions on the order of milliseconds to seconds can be monitored using the CODEX experiment which was introduced by Schmidt-Rohr and co-workers.27,28 The pulse sequence of the CODEX experiment is shown in Figure 3. In the CODEX experiment, a change in the orientation of the chemical shift anisotropy is used to determine the motions in the polymer. The CODEX sequence contains a preparation period during which 1H cross-polarized (CP) 13C magnetization is placed in the xy plane, which is allowed to evolve. Two rotor-synchronized 180° pulse trains are used to refocus the chemical shift anisotropy (CSA) in the xy plane and a mixing time is introduced between the 180° pulse trains. A z filter is used at the end of the second 180° pulse train to compensate the loss in signal due to T1 relaxation during the mixing time. A CODEX measurement is a set of a CODEX and a Reference experiment where the Reference experiment is the same as the CODEX experiment but with the mixing and zfilter times interchanged. A change in the CSA during the mixing time due to motions, result in a decrease in CODEX signal intensity. The ratio of CODEX to Reference spectrum intensity (S/S0) removes the effect of signal loss due to relaxation from the CODEX spectrum and hence gives information about motions alone in the system. If there are no motions during the mixing time, CODEX and Reference spectrum is the same because there will be no decrease in the signal intensity for the CODEX spectrum with respect to the Reference spectrum. The S/S0 obtained for different mixing times is fitted with a stretched exponential function called the Kohlrausch−Williams−Watts (KWW) equation29,30 to extract the correlation time. The KWW equation is

polycarbonate−tetramethylbisphenol A−polycarbonate. This indicates that intramolecular correlated motions are present in polycarbonates, which is in agreement with Xiao et al., who proposed that a correlation distance of seven repeat polycarbonate units is necessary for the restoration of the γ relaxation peak in BPA-PC.4 However, such long-range correlations are suppressed in polysulfones due to the flexibility of polymer units around the oxygen linkage. The only reported solid-state NMR study on the PSU polymers is by Dumais et al., using deuterium NMR, which showed the presence of phenyl π-flips, similar to that observed in the BPA-PC, with dynamical heterogeneity.15 However, there have been no reports on NMR studies on the PESU and PPSU. In this study, insights into the molecular motions of all the three polymers were obtained using advanced 13C solid-state (SS) NMR experiments. Our focus has been on studying these polymers using 13C SS-NMR techniques that avoid complicated sample preparation methodologies like deuteration and 13C isotopic labeling. Specifically, we have used two advanced 13C SS-NMR techniques, namely, the polarization inversion spin exchange at magic angle (PISEMA) experiment to detect the motions of the phenyl groups on the order of 100 kHz and the center band only detection of exchange (CODEX) experiments for probing motions in the range 0.1 Hz to 1 kHz. A combination of spin−lattice (T1) relaxation and T1-filtered PISEMA experiments were carried out to probe dynamic heterogeneity in the mobile region.

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METHODS PISEMA. The pulse sequence of the PISEMA experiment, which recouples the heteronuclear dipolar coupling is shown in Figure 2a. PISEMA uses the Lee−Goldberg sequence,22 which suppress the 1H−1H homonuclear dipolar coupling, and simultaneously recouples the 1H−13C heteronuclear dipolar coupling. Homonuclear dipolar decoupling is achieved by spin locking the 1H magnetization at the magic angle 54.74° using continuous phase inverted 360° pulses. The magic angle pulse in the 1H channel is achieved by an RF pulse with the power

Figure 2. Pulse sequence used for (a) PISEMA23 and (b) T1-filtered PISEMA25 experiments. B


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Figure 3. Pulse sequence used for CODEX experiment.27,28


S /S0 = (1 − M ) + M exp[−(τm/τc)β ]

dimension. The total time of each PISEMA experiment varied from 12 h to 3 days depending on the T1 filter used. 13 C T1 relaxation times were measured using the Torchia experiment.31 The CODEX experiments were carried out at a spinning rate of 4.5 kHz. The Nτr dependence experiments were carried out to determine the plateau of the CODEX dephasing measurements. Using the optimum Nτr value that was obtained from the Nτr dependence experiment, the CODEX mixing time (τm) dependence experiments were carried out. In the mixing time dependence experiment, the mixing time (τm) was varied from 1 ms to 8 s with 16 points between them. The CODEX experiment for mixing times of 1 ms to 1 s were carried out with 1024 number of scans, the remaining experiments were performed with 2048 scans. The total acquisition time of one set of CODEX experiment at 26 °C was ∼7 days, and at 100 °C it was ∼15 days. The temperature of the sample was calibrated at spinning rates of 4.5 and 8 kHz, using methanol and PbNO3.34,35 Solid bars of the samples PESU, PSU, and PPSU were received from SOLVAY Specialty Polymers and were used as such. Cylindrical rods of the samples were machined from the solid bars to a diameter of 2.9 mm so as to snuggly fit into the 4 mm NMR rotor. The entanglement molecular weights of PESU, PSU, and PPSU used in our studies are 3600, 3020, and 2550, respectively.21

(1)

where M represents the relative fraction of mobile components active in the measurement and β is the nonexponentiality parameter that represents the distribution in the motional process. Fast motions that are out of CODEX window cannot be measured by the CODEX experiment. CODEX experiments have been carried out for all the three polymers PESU, PSU, and PPSU. 13 C T 1 Relaxation and T 1 13 C-Filtered PISEMA Sequence. The spin−lattice relaxation time was measured using the inversion recovery Torchia method.31 The relaxation data are fitted using modified biexponential decay equation * )βM ] + AR exp[− (t /T1R )] S = AM exp[− (t /T1M

(2)

where T1R is the T1 relaxation time of the rigid component. The relaxation time of the mobile region is a distribution determined by the parameter T*1M, and βM, a constant between 0 and 1, which is the nonexponential parameter that determines the distribution. AM and AR are the amplitude of the respective regions. The T1-filtered PISEMA experiment was introduced by Brus et al., where a T1 relaxation window was added to the PISEMA experiment with the aim of suppressing the contribution from the mobile components and hence used for obtaining selective information about crystalline regions in semicrystalline polymer nanocomposites, PA6/MMT/EMA and PPO/DGEBA/ POSS.25,32 Using this experiment, Zhang et al. was able to separate different chemical environments in polyamide-6 and silk fibroin on the basis of the difference in 13C T1 relaxation values.33 In the present study, this experiment is used for separating the overlapping spectral regions that have dynamically distinct regions with similar chemical shifts. Separation is done on the basis of the difference in 13C T1 relaxation time. The pulse sequence of T1-filtered PISEMA experiment is shown in Figure 2b.

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RESULTS AND DISCUSSION Assignment of the 13C spectra of all the three polymers PESU, PSU, and PPSU were carried out with the help of the spectra obtained from standard solution-state NMR experiments (1H and 13C, COSY, HMBC, and HSQC). The solution state HMBC spectrum of all the three polymers with 1H and 13C assignments are shown in Figure S2, in the Supporting Information. The solid state 13C CP/MAS NMR spectrum (Figure 4) assignments were made on the basis of solution state NMR and the values of aromatic carbons having directly attached protons are tabulated in Table 1. The detailed assignments are given in the Table S1 in the Supporting Information. It is observed that in PSU and PPSU, 13C solidstate NMR is unable to differentiate between different CH environments. Thus, the peaks from the CH environment ortho to the isopropylidene and sulfone groups are indistinguishable in PSU. Similarly, the CH environments near the biphenyl and sulfone groups in PPSU are also indistinguishable. The results from the PISEMA, CODEX, and T1-filtered PISEMA experiments are discussed below. PISEMA Results. As explained in the Methods section, the 1 H−13C heteronuclear dipolar coupling is obtained from the PISEMA experiment. Therefore, this experiment is relevant only to the carbons directly attached to the protons (carbons at 120 and 129 ppm). The PISEMA spectra of PESU, PSU, and PPSU at 29 °C, for the carbons at 120 ppm (CH environment

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EXPERIMENTAL SECTION All the solid-state NMR experiments were carried out on a Bruker Avance 300 MHz spectrometer with a 4 mm magic angle spinning probe head. For all the NMR experiments, the cross-polarization contact time of 800 μs was used. 1H and 13C 90° pulses of RF frequency 85 and 68 kHz were used, respectively. 1H longitudinal relaxation time (T1) of all the three polysulfone samples were measured and 4−5 times of this was used as the recycle delay for each experiment. The PISEMA, 13C T1 relaxation, and T1-filtered PISEMA experiments were carried out at a spinning rate of 8 kHz. An effective field of 73.5 kHz was used during LG decoupling. Each PISEMA experiment is acquired by coadding 128 scans in the direct dimension. A total of 120 points were observed to be enough for the complete decay of the signal in the indirect C


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the rigid part is absent which implies that all the phenyl regions are activated for π-flips and hence only the mobile region is observed. The full PISEMA spectra of PESU, PSU, and PPSU at 29 °C are shown in Figure S3 in the Supporting Information. We carried out the PISEMA experiments at higher temperatures to investigate the changes in the mobility with temperature. The PISEMA spectra of PESU, PSU, and PPSU at 60 °C for the carbons at 120 ppm are shown in Figure 5d−f. The outer contours (rigid region) in the spectrum of the PPSU sample almost vanished at this temperature, whereas in the spectra of PESU and PSU, some traces of the immobile regions are present. This indicates that at 60 °C almost all the aromatic units in the PPSU have been activated for π-flip motions, but in PESU and PSU the full activation of the immobile region has not happened. This is an interesting observation due to the fact that PPSU is known to have better impact strength than PESU and PSU, and this could be attributed to the phenyl groups that are more activated for the π-flips in PPSU than PESU and PSU. In the PISEMA spectra recorded at 100 °C (Figure 5g,h,i), the rigid regions in all the three polymers have almost disappeared. This indicates that at 100 °C, all the aromatic groups in PESU, PSU, and PPSU, which were rigid at 29 °C, are undergoing πflips with a frequency on the order of 100 kHz. Similar results are seen for the carbons at 129 ppm (CH groups close to sulfonyl/isopropylidene/biphenyl environments in PESU, PSU, and PPSU) and are shown in Figure S4 in the Supporting Information. The sum projection of PESU, PSU, and PPSU PISEMA at different temperatures for peak at 120 and 129 ppm is shown in Figure S5 in the Supporting Information. The PISEMA experiment cannot be used for a quantitative estimation in a dynamically hetrogenious sample, because 13C magnetization generated by cross-polarization and dipolar evolution in the indirect dimension is not quantitative. However, a semiquantitative estimation can be carried out by keeping the cross-polarization and dipolar evolution condition constant throughout all the measurements. A semiquantitative estimation of the fraction of the mobile region in these samples at different temperatures is obtained by calculating the volume integral of the PISEMA peaks of the rigid and the mobile regions and the values obtained are plotted in Figure 6. (The fraction of mobile region in all the polymers is tabulated in Table S2 in the Supporting Information.) It is seen that at 29 °C, PESU have the least mobile fraction followed by the PSU, whereas PPSU has the highest mobile fraction. At 60 °C, in PPSU the mobile fraction is ∼0.88, whereas in PESU and PSU it is only 0.8 and 0.83, respectively. At 100 °C for all the polymers, viz. PESU, PSU, and PPSU, the mobile fraction is

Figure 4. 13C CP/MASNMR spectrum of PESU, PSU, and PPSU at 29 °C. The carbons at 120 and 129 ppm in the spectrum (shown by dashed lines) are used as the probe for dynamics, in both PISEMA and CODEX experiments.

Table 1. 13C Structural Assignment of PESU, PSU, and PPSUa δCS (ppm) 120 129

PESU

PSU

ortho to ether functional group ortho to sufone functional group

ortho to ether functional group

ortho to ether functional group

PPSU

ortho to sulfone/ isopropylidene functional group

ortho to sulfone/ biphenyl functional group

a

The carbons directly attached to the protons are listed here. The full assignment is given in Table S4 in the Supporting Information.

in the aromatic ring ortho to the ether linkage, Table 1), are shown in Figure 5a−c, respectively. Two sets of contours with dipolar coupling of 21.5 kHz for the outer region and 11.7 kHz for the inner region are seen. The dipolar coupling of 21.5 kHz indicates that all the three polymers contain rigid regions (immobile or of very slow motions), whereas the reduced dipolar coupling of 11.7 kHz indicates the presence of mobile regions, which originates from the π-flips of the aromatic rings and are on the order of 100 kHz. These results are in close agreement with the results reported by Dumais et al., on PSU where two dynamically distinct regions, mobile and rigid were distinguished on the basis of deuterium NMR and relaxation experiments at 23 °C.15 They have also reported that at 74 °C

Figure 5. 13C PISEMA spectra of the carbons centered at 120 ppm at 29 °C (a, b, c), 60 °C (d, e, f), and 100 °C (g, h, i). In each group, the leftmost data are for PESU, the center are for PSU, and the rightmost are for PPSU. D


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shift to the right side of 26 °C, indicating that the origin of dephasing is not a motional process. Therefore, the CODEX experiment clearly shows that slow segmental motions are absent in polysulfone polymers even though phenyl flips on the order of 100 kHz are present. This means that the high frequency motions of the phenyl groups are not translated to the main chain due to poor cooperativity in segmental motions. A similar trend is observed for the carbons at 129 ppm and is shown in Figure S7 in the Supporting Information. 13 C T1 Relaxation and 13C T1-Filtered PISEMA Results. To get deeper insights into the dynamical heterogeneity in the polysulphones, a combination of 13C T1 relaxation and 13C T1filtered PISEMA experiments were carried out. Because the PISEMA experiment clearly shows mobile and rigid regions, the data obtained from the 13C relaxation experiment using the Torchia method are initially fitted using a biexponential decay equation (Figure S8, Supporting Information). The T1 values that were extracted are tabulated in table S3 in the Supporting Information. According to this model, the three polymers have fast relaxing components with T1 on the order of 1 s, which are attributed to the mobile regions. Slow relaxing components with T1 on the order of 16 s were also observed, which are attributed to the rigid regions. To verify the correctness of this model, T1-filtered PISEMA experiments were carried out on the basis of these T1 values. If the polymers contain only mobile and rigid regions with distinct relaxation times for each, a T1 filter of 3−4 times of the T1 of mobile region should be enough to filter out it is contribution in the T1-filtered PISEMA spectra. However, even with a T1 filter of 5 s (>4T1 relaxation time of mobile region) the inner peaks of carbons centered at 120 ppm are still present, though with reduced intensity, in the T1filtered PISEMA spectra (circled region, Figure S9b,d,f, Supporting Information). Similar spectra were obtained for the carbons centered at 129 ppm, shows contours with a dipolar coupling of 13.5 kHz (circled region, Figure S9h,j,l, Supporting Information). From these results, it is clear that the mobile region is not experiencing uniform motions and the biexponential model is inadequate to capture the dynamical heterogeneity in the polysufones. Further, it appears that there could be at least a third component with intermediate mobility or a distribution of mobility in the mobile region. Because a component with intermediate mobility would have manifested as a distinct peak

Figure 6. Fraction of mobile component of PESU, PSU, and PPSU plotted against the temperature (°C) for the carbons at (a) 120 ppm and (b) 129 ppm.

∼0.88, indicating that almost all the rigid regions have converted into mobile region. These results clearly show that, as reported in the case of BPA,13,14 the phenyl group flips on the order of 100 kHz are important in the polysulfone polymers and could be responsible for the ductility of these polymers. However, although the BPA polycarbonates show only mobile regions at ambient temperature, the polysulfones have mobile as well as rigid regions at ambient temperature, which are almost converted into mobile region at 100 °C. CODEX Results. The CODEX experiments were carried out on all the three polymers to detect slow segmental motions in the polymer chains at 26 °C. The results of the Nτr dependence experiment with a mixing time (τm) of 4 s resulted in a dephasing curve with a plateau at Nτr of ∼1.33 ms (Figure S6, Supporting Information). The τm dependence experiments were carried out using this Nτr value for all the polymers. The dephasing curve obtained for the carbons (CH) near the ether linkage (120 ppm) for PESU, PSU, and PPSU at 26 °C and for PESU and PPSU at 100 °C are shown in Figure 7. Although the CODEX curves show a gradual dephasing, this does not indicate the presence of slow motions because the spin diffusion also contribute to the dephasing.36,37 The CODEX dephasing curves at 100 °C for PESU and PPSU

Figure 7. CODEX τm dependence of CH moiety near the ether linkage (120 ppm) in (a) PESU, (b) PSU, and (c) PPSU at 26 and 100 °C, obtained with an Nτr of 1.33 ms. For PSU the experiment is carried out only at 26 °C. The line is shown only as a guide to the eye and is not a fit. E


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The Journal of Physical Chemistry B between the mobile and the rigid region, we believe it is more appropriate to consider a model with a distribution of mobility in the mobile region. Therefore, the T1 data are fitted using a modified biexponential model with exponential decay for the rigid region and a stretched exponential decay for the mobile region, as shown in eq 2. The parameters extracted for the carbons at 120 and 129 ppm using the modified biexponential model are tabulated in Table 2. It can be seen that the mobile Table 2. Parameters Extracted by Fitting the Spin−Lattice Relaxation Data Using the Modified Biexponential Model Eq 2 CS (environment)


120 ppm (ortho to ether link) 129 ppm (ortho sulfone, aliphatic, biphenyl)

sample PESU PSU PPSU PESU PSU PPSU

βM

T1M * (s) 1.71 1.48 1.12 2.24 1.14 1.27

± ± ± ± ± ±

0.22 0.18 0.11 0.28 0.10 0.13

0.69 0.69 0.70 0.66 0.73 0.68

± ± ± ± ± ±

T1R (s) 0.03 0.03 0.03 0.02 0.03 0.03

25.09 ± 1.80 20.3 ± 1.34 18.27 ± 1.11 29.03 ± 1.94 17.31 ± 0.74 20.91 ± 1.45

region has the parameters T*1M ∼1.5 s with βM ∼0.7. Therefore, according to the stretched exponential model, this means that the mobile region has spin−lattice relaxation times in the range 0.2T1M * to 1.5T1M * , as shown by Johnston.38 The experimental data points with the fitted curves and their residuals are shown in Figures S8 in the Supporting Information. The data were also fitted using a triexponential decay equation, and they are also shown in Figure S8, Supporting Information. To analyze the correctness of the modified biexponential model, T1-filtered PISEMA experiments were carried out with T1 filters of 2.5, 5, 10, and 15 s. The results of the T1-filtered PISEMA experiments for the carbons at 120 ppm (peak ortho to the ether linkage) are shown in Figure 8. A T1 filter of 2.5 s completely removes the highly mobile regions with lower relaxation times ∼0.3 s (Figure 8b,g,l, respectively, for PESU, PSU, PPSU). It can be seen that the center of gravity of inner contour of the spectra has moved apart from each other and hence a dipolar coupling of 13.5 kHz is observed for this region. The T1-filtered PISEMA spectra with a T1 filter of 5 s for the carbons at 120 ppm (Figure 8c,h,m) is not sufficient to remove the contours with a dipolar coupling of ∼13.5 kHz because the T1 of the mobile region is distributed over 0.3−2.1 s. It is observed that at a T1 filter duration of 10 s (Figure 8d,i,n for 120 ppm signal) the inner contours have vanished almost completely indicating the T1 relaxation time of mobile phase have a maximum relaxation time of around 2.1 s, which is in good agreement with the distribution value estimated for the mobile region. Finally, only the contours from the rigid region with a dipolar separation of 21.5 kHz are seen on increasing the T1 filter time to 15 s (Figure 8e,j,o). Similar observations for the carbons at 129 ppm are shown in Figure S10 of Supporting Information. The observation of dynamic heterogeneity in the mobile region is one of the important results of this study. The earlier deuterium NMR studies15 were able to show that two distinct dynamical regions were present in PSU. However, the present study clearly show dynamical heterogeneity in all the polysulfones studied, including PSU. The exact measurements of the frequency of motions in the mobile region are beyond the scope of this study. Reports from neutron scattering experiments also indicated the presence of π-flip motions along

Figure 8. T1-filtered PISEMA spectra of the carbons at 120 ppm in PESU (top), PSU (middle), and PPSU (bottom) at 29 °C with a T1 filter of 0 s (without T1 filter) (a, f, k) 2.5 s, (b, g, l), 5 s (c, h, m), 10 s (d, l, n), and 15 s (e, j, o), respectively.

with small angle oscillation and they have also observed a large distribution in motional phase.10

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CONCLUSION The functional group and segmental motions in PPSU, PESU and PSU were monitored using 13C solid-state NMR experiments; PISEMA, CODEX, 13C T1 relaxation, and 13C T1-filtered PISEMA. The PISEMA experiments at 29 °C revealed the presence of two dynamically distinct regions: the mobile and rigid, with further T1-filtered PISEMA experiments showed that the mobile region is dynamically heterogeneous. Temperature dependent PISEMA experiments carried out on the three polymers revealed that the rigid components of PPSU almost vanish at 60 °C whereas traces of the immobile component are present in PESU and PSU at this temperature. This is attributed to the presence of higher amount of the mobile region in PPSU and could be the reason for the higher impact strength observed in PPSU than in PESU and PSU. F


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The Journal of Physical Chemistry B However, at 100 °C, the outer contours almost vanished in the PISEMA spectra of PESU, PSU, and PPSU, indicating that the most of the immobile regions are converted to mobile regions, which are also supported by a semiquantitative estimation of mobile regions. The CODEX experiments carried out to monitor slow segmental motions in the polymers were unable to detect the presence of slow motions. The absence of slow motions indicate that the π-flip motions on the order of 100 kHz by the phenyl groups do not activate the slow segmental motions; i.e., π-flip motions and chain segmental motions are strongly uncoupled. Finally, it may be noted that the PISEMA and CODEX techniques described here are useful because they do not require deuteriated or isotope labeled polymers, which are often not easy to make. These techniques provide useful insights into the order of magnitudes of the molecular motions though it is not straightforward to quantify the exact nature of the motions.

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(3) Aitken, C. L.; McHattie, J. S.; Paul, D. R. Dynamic Mechanical Behavior of Polysulfones. Macromolecules 1992, 25, 2910−2922. (4) Xiao, C.; Yee, A. F. Scale of Cooperative γ-Relaxation of Bisphenol A Polycarbonate. Macromolecules 1992, 25, 6800−6809. (5) Horth, F. J.; Kuhn, K. J.; Mertes, J.; Hellmann, G. P. On the Mechanical γ-Relaxation Modes of Polycarbonate. Polymer 1992, 33, 1223−1227. (6) Merenga, A. S.; Papadakis, C. M.; Kremer, F.; Liu, J.; Yee, A. F. Comparative Broad-Band Dielectric Study on Poly(ester carbonate)s with 1,4-Cyclohexylene Linkages. Macromolecules 2001, 34, 76−81. (7) Arrese-Igor, S.; Mitxelena, O.; Arbe, A.; Alegría, A.; Colmenero, J.; Frick, B. Molecular Motions in Glassy Polycarbonate Below its Glass Transition Temperature. J. Non-Cryst. Solids 2006, 352, 5072− 5075. (8) Arrese-Igor, S.; Arbe, A.; Alegría, A.; Colmenero, J.; Frick, B. Secondary Relaxation in Two Engineering Thermoplastics by Neutron Scattering and Dielectric Spectroscopy. Appl. Phys. A: Mater. Sci. Process. 2002, 74, S454−S456. (9) Arrese-Igor, S.; Arbe, A.; Alegría, A.; Colmenero, J.; Frick, B. Short-Time Dynamics of Phenylene-Rings in Bisphenol Based Engineering Thermoplastics. Chem. Phys. 2003, 292, 363−370. (10) Arrese-Igor, S.; Arbe, A.; Alegría, A.; Colmenero, J.; Frick, B. Phenylene Ring Dynamics in Bisphenol-A-Polysulfone by Neutron Scattering. J. Chem. Phys. 2004, 120, 423−436. (11) Arrese-Igor, S.; Arbe, A.; Alegría, A.; Colmenero, J.; Frick, B. Sub-Tg Dynamics in Polycarbonate by Neutron Scattering and its Relation with Secondary γ Relaxation. J. Chem. Phys. 2005, 123, 014907−014910. (12) Spiess, H. W. Molecular Dynamics of Solid Polymers as Revealed by Deuteron NMR. Colloid Polym. Sci. 1983, 261, 193−209. (13) Schaefer, J.; Stejskal, E. O.; McKay, R. A.; Dixon, W. T. Molecular Motion in Polycarbonates by Dipolar Rotational Spin-Echo 13 C NMR. Macromolecules 1984, 17, 1479−1489. (14) Schaefer, J.; Stejskal, E. O.; Perchak, D.; Skolnick, J.; Yaris, R. Molecular Mechanism of the Ring-Flip Process in Polycarbonate. Macromolecules 1985, 18, 368−373. (15) Dumais, J. J.; Cholli, A. L.; Jelinski, L. W.; Hedrick, J. L.; McGrath, J. E. Molecular Basis of the β-Transition in Poly(arylene ether sulfones). Macromolecules 1986, 19, 1884−1889. (16) Roy, A. K.; Jones, A. A.; Inglefield, P. T. Phenylene Ring Dynamics in Solid Polycarbonate: An Extensive Probe by Carbon-13 Solid-State NMR Line-Shape Studies at Two Field Strengths. Macromolecules 1986, 19, 1356−1362. (17) Poliks, M. D.; Gullion, T.; Schaefer, J. Main-Chain Reorientation in Polycarbonates. Macromolecules 1990, 23, 2678− 2681. (18) Henrichs, P. M.; Nicely, V. A. 13C NMR Study of the Conformation and Dynamics of Bisphenol A Polycarbonate. Macromolecules 1991, 24, 2506−2513. (19) Shi, J. F.; Inglefield, P. T.; Jones, A. A.; Meadows, M. D. SubGlass Transition Motions in Linear and Cross-Linked Bisphenol-Type Epoxy Resins by Deuterium Line Shape NMR. Macromolecules 1996, 29, 605−609. (20) Kaji, H.; Tai, T.; Horii, F. One- and Two-Dimensional MAS 13C NMR Analyses of Molecular Motions in Poly(2-hydroxypropyl Ether of Bisphenol-A). Macromolecules 2001, 34, 6318−6324. (21) Solvay Design Guide - Radel PPSU, Veradel PESU, Acudel Modified PPSU. http://www.solvaysites.com/sites/solvayplastics/EN/ specialty_polymers/SulfonePolymers/Pages/Overview.aspx. (22) Lee, M.; Goldburg, W. I. Nuclear-Magnetic-Resonance Line Narrowing by a Rotating rf Field. Phys. Rev. 1965, 140, A1261. (23) Dvinskikh, S. V.; Zimmermann, H.; Maliniak, A.; Sandström, D. Heteronuclear Dipolar Recoupling in Liquid Crystals and Solids by PISEMA-type Pulse Sequences. J. Magn. Reson. 2003, 164, 165−170. (24) Bak, M.; Rasmussen, J. T.; Nielsen, N. C. SIMPSON: A General Simulation Program for Solid-State NMR Spectroscopy. J. Magn. Reson. 2000, 147, 296−330. (25) Brus, J.; Urbanová, M.; Kelnar, I.; Kotek, J. A Solid-State NMR Study of Structure and Segmental Dynamics of Semicrystalline

ASSOCIATED CONTENT

S Supporting Information *

Sum projections of the Alanine PISEMA spectra used for the calculation of the scaling factor, liquid state HMBC NMR spectra of all the polymers, full PISEMA spectra, temperature dependent PISEMA spectra of all the polymers for the carbons at 129 ppm, sum projections of the PISEMA spectra, CODEX Nτr dependence curves, CODEX τm dependence curves for the carbons at 129 ppm, fitting of the T1 relaxation data using the biexponential, triexponential, and modified biexponential models, and the T1 filtered PISEMA spectra. Tables with the detailed assignments of the 13C CP/MAS NMR spectra, fraction of the rigid and mobile components, values of the T1 obtained using the biexponential and triexponential models. Tables of 13C structural assignments, fractions of mobile components, and 13C spin−lattice relaxation times. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b03103.

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AUTHOR INFORMATION

Corresponding Authors

*A. K. Lele. E-mail: [email protected]. Phone: +91-2025902199. *T. G. Ajithkumar. E-mail: [email protected]. Phone: +91-20-25902569. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by Solvay Speciality chemicals. Discussions with Vito Leo of Solvay about the ductility of the polymers are gratefully acknowledged. Prof. Detlef Reichert, University of Halle, Germany, is gratefully acknowledged for his comments on CODEX results. The reviewers are gratefully acknowledged for their suggestions to improve the manuscript.

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REFERENCES

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