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EPJ 4755                                                                                              No. of Pages 11, Mod...
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EPJ 4755                                                                                                   No. of Pages 11...
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European Polymer Journal

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European Polymer Journal

  1. 1. EPJ 4755 No. of Pages 11, Model 3G 14 October 2008 Disk Used ARTICLE IN PRESS European Polymer Journal xxx (2008) xxx–xxx 1 Contents lists available at ScienceDirect European Polymer Journal journal homepage: www.elsevier.com/locate/europolj 2 Temperature dependence of molecular dynamics and supramolecular F 3 aggregation in MEH-PPV films: A solid-state NMR, X-ray and fluorescence OO 4 spectroscopy study 5 A.A. Souza a, R.F. Cossiello b, T.S. Plivelic c, G.L. Mantovani a, G.C. Faria a, T.D.Z. Atvars b, 6 I.L. Torriani c,d, T.J. Bonagamba a, E.R. deAzevedo a,* 7 a Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, SP, Brazil PR 8 b Instituto de Química, Universidade Estadual de Campinas, Caixa Postal 6154, 13084-971 Campinas, SP, Brazil 9 c Laboratório Nacional de Luz Síncrotron, Caixa Postal 6192, 13083-970 Campinas, SP, Brazil 10 d Instituto de Física, Universidade Estadual de Campinas, Caixa Postal 6165, 13084-971 Campinas, SP, Brazil 11 a r t i c l e i n f o a b s t r a c t 1 3 2 8 14 15 16 Article history: Received 29 June 2008 Received in revised form 23 August 2008 D This article presents an investigation of the temperature induced modification in the microstructure and dynamics of poly[2-methoxy-5-(20 -ethylhexyloxy)-1,4-phenylenevin- ylene] (MEH-PPV) cast films using Wide-Angle X-ray Scattering (WAXS), solid-state 29 30 31 TE 17 Accepted 16 September 2008 Nuclear Magnetic Resonance (NMR), and Fluorescence Spectroscopy (PL). MEH-PPV chain 32 18 Available online xxxx 19 motions were characterized as a function of temperature by NMR. The results indicated 33 that the solvent used to cast the films influences the activation energy of the side-chain 34 motions. This was concluded from the comparison of the activation energy of the toluene 35 20 Keywords: 21 MEH-PPV films cast film, Ea = (54 ± 8) kJ/mol, and chloroform cast film, Ea = (69 ± 5) kJ/mol, and could be 36 EC 22 NMR attributed to the higher side-chain packing provided by chloroform that preferentially sol- 37 23 DIPSHIFT vates the side chain in contrast to toluene that solvates mainly the backbone. Concerning 38 24 WAXS the backbone mobility, it was observed that the torsional motions in the MEH-PPV have 39 25 Molecular aggregation average amplitude of $10° at 300 K, which was found to be independent of the solvent 40 26 Fluorescence used to cast the films. In order to correlate the molecular dynamics processes with the 41 27 changes in the microstructure of the polymer, in situ WAXS experiments as a function of 42 RR temperature were performed and revealed that the interchain spacing in the MEH-PPV 43 molecular aggregates increases as a function of temperature, particularly at temperatures 44 where molecular relaxations occur. It was also observed that the WAXS peak associated 45 with the bilayer spacing, narrows and their by increases intensity whereas the peak asso- 46 ciated with the interbackbone planes reduces its intensity for higher temperatures. This 47 last result could be interpreted as a decrease in the number of aggregates and the reduction 48 CO of the interchain species during the MEH-PPV relaxation processes. These WAXS results 49 were correlated with PL spectra modifications observed upon temperature treatments. 50 Ó 2008 Elsevier Ltd. All rights reserved. 51 52 53 UN 54 1. Introduction tron and hole transport properties [1–4]. The large-scale 58 electroluminescent devices can be feasible due to their 59 55 Conjugated polymers have potential to be employed in simple production. However, the efficiency of the electro- 60 56 several applications such as materials for light-emitting luminescence (EL) and photoluminescence (PL) is reduced 61 57 diodes, lasers and thin-film transistors due to their elec- when the conjugated polymers form interchain species 62 that arise as a result of molecular aggregation [5,6]. Molec- 63 ular aggregation and, consequently, the electro- and 64 * Corresponding author. Tel.: +55 16 33738086; fax: +55 1633739876. E-mail address: azevedo@ifsc.usp.br (E.R. deAzevedo). photo-luminescence emissions and charge transport 65 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.09.030 Please cite this article in press as: Souza AA et al., Temperature dependence of molecular dynamics and supramolecular ..., Eur Poly J (2008), doi:10.1016/j.eurpolymj.2008.09.030
  2. 2. EPJ 4755 No. of Pages 11, Model 3G 14 October 2008 Disk Used ARTICLE IN PRESS 2 A.A. Souza et al. / European Polymer Journal xxx (2008) xxx–xxx 66 properties may be partially controlled by the processing 2. Experimental 126 67 conditions which change the polymer microstructure and 68 morphology [7–9]. MEH-PPV with average molar weight Mn = 86 kg/mol 127 69 Photoluminescence and electroluminescence are corre- and polydispersity Mn/Mw = 4.9 was obtained from 128 70 lated phenomena not only strongly dependent on the Sigma–Aldrich Co. Toluene and chloroform solvents with 129 71 microstructure of the material but also on the dynamics spectrophotometric grade were purchased from Acros. All 130 72 of the polymer chains [7–9]. For example, the temperature materials were used as received. 131 73 dependence of the electroluminescent devices with MEH- MEH-PPV solutions were prepared by dissolving the 132 74 PPV showed changes [10–12] associated with specific polymer samples in each solvent. After that, the solutions 133 75 movements of the polymer chain such as motions of the were maintained in the dark in a sealed flask. Films were 134 76 lateral groups at 220 K and related with the glass transition prepared by casting the solutions in a Petri dish, with slow 135 77 (Tg) at 330 K [13,14]. In our previous reports [13,14] it was evaporation under a saturated solvent atmosphere, at F 136 78 postulated that the blue shift of the PL spectra at temper- room temperature, for 30 h. Later, the films were dried in 137 79 atures above the glass transition could be explained by an oven under dynamic vacuum at a temperature of ca. 138 OO 80 the interchain dissociation induced by thermal motions, 323 K for 24 h. Film thicknesses were approximately 139 81 although no further experimental evidence have been 30–40 lm. For WAXS experiments, multilayer samples of 140 82 given. up to 400 lm thicknesses were prepared by stacking 141 83 Wide-Angle X-ray Scattering (WAXS) and Transmis- pieces cut from a single film. 142 84 sion Electron Microscopy (TEM) showed that MEH-PPV Steady-state fluorescence spectra of MEH-PPV films 143 85 films form molecular aggregates that present local nano- were recorded using a PC1TM Photon Counting Spectroflu- 144 PR 86 scopic order with structural anisotropy [15,16]. The dif- orimeter from ISS Inc. The spectral range was from 600 to 145 87 fraction patterns obtained in stretched films were 800 nm for the emission spectra. Slits were selected for a 146 88 indexed proposing an orthorhombic unit cell with spectral resolution of ±0.5 nm. Excitation wavelength was 147 89 parameters a = 7.12 Å, b = 16.05 Å and c = 6.47 Å [16]. A kex = 490 nm. 148 90 further description of the molecular aggregates was pre- WAXS experiments were performed at the D11A-SAXS 149 91 sented recently [2], where it was shown that MEH-PPV beamline of the LNLS (Brazilian Synchrotron Light Labora- 150 92 93 94 chains are locally ordered with phenyl rings partially or- ganized parallel to each other and also parallel to the film plane. This molecular packing was described as a D tory). The wavelength used was 1.608 Å and the sample detector distance was approximately 182 mm in all cases. The films were set-up in two configurations: with the inci- 151 152 153 TE 95 superstructure unit cell consisting of a bilayer arrange- dent X-ray beam perpendicular () and near-parallel (||) to 154 96 ment with repeating distance of 24.2 Å along the b-axis, the film plane. 155 97 a characteristic distance of 6.3 Å assigned to repeated The samples were first examined at room temperature 156 98 unit along the backbone (c-axis), and the regular spacing for two-dimensional (2D) patterns. Data were recorded in 157 99 between the backbones of the coplanar phenylene rings Fuji film image plates and 30 min exposures were taken 158 EC 100 (a = 4.3 Å). Moreover, the size of the chain-packed in all cases. Average radial intensity profiles were obtained 159 101 MEH-PPV nanostructured domain was estimated from integrating an arbitrary 30° angular sector in the case of 160 102 the diffraction line profile and assumed to consist of no the isotropic scattering pattern ( incidence) and a similar 161 103 more than 4d-spacings in each crystallographic direction. sector centered around the maximum in the oriented scat- 162 104 Although the structure of MEH-PPV films and the con- tering ring (|| incidence). Intensities were normalized by 163 RR 105 formation of the polymer chains have been well de- the integrated intensity incident on the sample during 164 106 scribed in the literature and their influence on the PL the exposure and by sample absorption. Parasitic scatter- 165 107 and EL were also well understood, the relationship be- ing was subtracted from each pattern. 166 108 tween structure and dynamics of the polymer chains Afterwards, one-dimensional (1D) patterns for the 167 109 are not well known [16]. Therefore, the aim of this re- in situ thermal treatment were recorded using a linear po- 168 110 port is the study of the temperature dependence of the sition sensitive detector (PSD). The samples were placed in 169 CO 111 dynamics, supramolecular organization, and short-range a hot stage cell specially designed for X-ray scattering mea- 170 112 chain ordering of MEH-PPV films, when the film is cast surements (THM 600, Linkam Ltda [25]). For each of the 171 113 from two solvents, chloroform and toluene, with distinct experimental geometries ( and || incidences) the films 172 114 solvation abilities. The polymer chains dynamics was were placed in a sample support adapted to the hot stage. 173 115 studied by a set of very convenient solid-state NMR tech- The difference between the values of the temperature on 174 UN 116 niques to detect possible differences in the molecular the sample and the values set on the controller was less 175 117 dynamics due to molecular aggregation: DIPSHIFT than 5 K for all scans in the range 123–423 K, allowing a 176 118 (DIPolar-chemical SHIFT correlation) [17–20] and CODEX fairly precise determination of the thermal state of the 177 119 (Centerband-Only Detection of EXchange) [21–24]. Tem- sample. The in situ measurements were performed allow- 178 120 perature evolution of the supramolecular and the ing 5 min of stabilization and 15-min data acquisition for 179 121 short-range structures was studied by WAXS as well as each desired temperature. From room temperature the 180 122 Steady-state Fluorescence Spectroscopy allows to check samples were rapidly quenched (60 K/min) down to 181 123 whether structural changes had happen (due to its sensi- 123 K. Next, after each step of a heating ramp (10 K/min), 182 124 tivity to short range interaction among the polymer WAXS patterns were obtained for the temperatures 198, 183 125 chains). 273, 313, 348 and 423 K. The samples were kept at the 184 Please cite this article in press as: Souza AA et al., Temperature dependence of molecular dynamics and supramolecular ..., Eur Poly J (2008), doi:10.1016/j.eurpolymj.2008.09.030
  3. 3. EPJ 4755 No. of Pages 11, Model 3G 14 October 2008 Disk Used ARTICLE IN PRESS A.A. Souza et al. / European Polymer Journal xxx (2008) xxx–xxx 3 185 highest temperature during 1 h. After this isothermal treat- 186 ment, additional data were taken. Finally, the samples 187 were cooled down to 348 and 293 K at the rate of 188 10 K/min, with additional exposures being obtained at 189 these temperatures. The 1 h annealing at 423 K, was 190 chosen well above the MEH-PPV glass transition tempera- 191 ture (Tg = 330 K) [13]. 192 NMR experiments were performed using a VARIAN 193 INOVA spectrometer at 13C and 1H frequencies of 100.5 194 and 400.0 MHz, respectively. A VARIAN 7-mm MAS dou- 195 ble-resonance probe head with variable temperature (VT) 196 was used. The spinning speeds, varying between 4 and F 197 6 kHz, were controlled by a VARIAN pneumatic system that 198 ensures a rotation stability of ±2 Hz. Typical p/2 pulses OO 199 lengths of 3.5 and 4.5 ls were applied for 13C and 1H, 200 respectively. Time Proportional Phase Modulated (TPPM) 201 proton decoupling with field strength of 70 kHz, cross- 202 polarization time of 1 ms and recycle delays varying be- 203 tween 3 and 5 s were used. Amplitude of slow molecular 204 motions were investigated using CODEX technique PR 205 [21,22] with mixing time tm of 200 ms and evolution times 206 (Ntr) ranging from 333 to 2500 ls. The temperature depen- 207 dence of 13C–1H dipolar coupling were measured using 208 DIPSHIFT technique [18], where 1H–1H homonuclear 209 decoupling was achieved by the Phase-Modulated-Lee- 210 Goldburg (PMLG) sequence [26,27], using field strengths 211 212 of approximately 80 kHz. 3. Results and discussions D TE 213 This article is organized as follow. The room tempera- 214 ture photoluminescence spectra of MEH-PPV films cast 215 form toluene and chloroform is presented and compared together with a short discussion about the temperature EC 216 Fig. 1. Steady-state fluorescence spectra of MEH-PPV films. (a) Room 217 dependence of the PL. In sequence, WAXS are analyzed as temperature spectra of chloroform (square symbols) and toluene (circle 218 a function of temperature in order to give some insight symbols) cast films before and after annealing by 12 h at 363 K (full or 219 about the temperature dependence of the microstructure empty symbols, respectively). The arrow stands for the blue shift 220 of the MEH-PPV aggregates. Then, to elucidate the charac- observed as a function of temperature. (b) Steady-state fluorescence 221 teristics of the molecular dynamics processes in the poly- spectra of MEH-PPV films cast from chloroform as a function of RR temperature from 53 to 393 K. The inset show the integrated intensity 222 mer chains, solid-state NMR measurements are of the corresponding spectra of MEH-PPV films cast from chloroform as a 223 presented. Finally, a correlation between the strucutural function of temperature. 224 and dynamics results are presented and discussed in the 225 context of the change in the PL spectra. range of 2000 cmÀ1 which are orders of magnitude larger 241 CO 226 3.1. Photoluminescence spectroscopy than expected for homogeneous broadening [33]. As it 242 can be observed in Fig. 1a, there is a slightly difference in 243 227 Steady-state fluorescence spectra (PL) of MEH-PPV films the intensity of the band assigned to the excimeric/inter- 244 228 cast from toluene and chloroform solutions before and chain species upon the different solvents, which are mostly 245 229 after annealing at 363 K for 12 h are shown in Fig. 1a. In erased with thermal annealing. We also noticed that there 246 UN 230 general, these spectra are composed by a higher intensity is an increase of the FWHM with the annealing, which can 247 231 band around 650 nm and a second overlapped band be attributed to the increase of the relative amount of 248 232 around 700 nm (Fig. 1). This spectrum, usually observed aggregates compared with the non-annealed sample. 249 233 for the MEH-PPV films, is assigned to the interchain species In order to provide information about the tempera- 250 234 separated by a backbone interplanar distance of $4.05 Å ture dependence of the photoluminescence, the PL spec- 251 235 [28–31]. The longer red-edge tail is also observed which tra was also recorded from 53 to 393 K for MEH-PPV 252 236 results from the overlap of two contributions, the vibronic films cast from chloroform, Fig. 1b. The behavior of the 253 237 progression of the aggregate emission and from the emis- spectra as a function of temperature is similar to the 254 238 sion of excimeric/interchain species [32]. Spectral broad- one reported in Ref. [13], i.e., at temperatures below 255 239 ening was estimated and the full-width at the half- 150 K little variation is observed either in the integrated 256 240 maximum (FWHM) showed that those values are in the intensity (see figure inset) or in the position of the PL 257 Please cite this article in press as: Souza AA et al., Temperature dependence of molecular dynamics and supramolecular ..., Eur Poly J (2008), doi:10.1016/j.eurpolymj.2008.09.030
  4. 4. EPJ 4755 No. of Pages 11, Model 3G 14 October 2008 Disk Used ARTICLE IN PRESS 4 A.A. Souza et al. / European Polymer Journal xxx (2008) xxx–xxx 258 bands. Between 150 and 300 K there is an increase in the For the interpretation of the structural changes, the 280 259 integrated intensity, which is accompanied by a blue curves were fitted using Gaussian functions for the peaks 281 260 shift. Above 300 K a stronger intensification and also a and a logarithmic baseline for the background. The individ- 282 261 blue shift is observed. Toluene cast films presents a sim- ually fitted contribution of each peak is plotted on the bot- 283 262 ilar behavior [13]. tom of the Fig. 2a and b whereas the total fit is represented 284 as a continuous red line in the same plots. The most impor- 285 263 3.2. Wide-Angle X-Ray Scattering (WAXS) tant wide angle peaks were referred to the molecular pack- 286 ing parameters proposed by Jeng et al. [2]. The first 287 264 The microstructure of the MEH-PPV films as well as reflection observed for q = 0.27 ÅÀ1 (d1 = 2p/q = 23.2 Å) is 288 265 the modifications induced by temperature was studied assumed to represent the bilayer chain packing distance. 289 266 by WAXS. Fig. 2 shows integrated intensity scans (left) The next important characteristic length observed for 290 267 from the 2D-WAXS images (right) for as cast MEH-PPV q = 1.03 ÅÀ1 (d2 = 6.1 Å) corresponds to the monomeric re- 291 F 268 free-standing films obtained from chloroform solution peat unit. These two parameters define the chain packed 292 269 using perpendicular and parallel incidences, at 293 K. planar layers of the MEH-PPV films. Finally, the peak in 293 OO 270 The anisotropy features of these films are already con- our patterns located at q = 1.49 ÅÀ1 (d3 = 4.2 Å) would cor- 294 271 firmed comparing the 2D patterns. These patterns are respond to the inter-backbone distance, in the direction 295 272 similar in shape, showing differences in relative peak normal to the coplanar phenylene rings, as reported by 296 273 intensities and scattering contributions according to the Jeng et al. [2]. It is worth mentioning that, due to the exist- 297 274 geometry of the experiment. Similar to Ref. [2], preferen- ing disorder, the scattering peaks are quite broad, but their 298 275 tial orientation of the ordered domains can be clearly positions could be well determined from the fittings. From 299 PR 276 noted in the 2D images of the parallel (||) incidence pat- the aforementioned results, is possible to conclude that the 300 277 terns, in which the reflections corresponding to q values major difference with the data reported in the literature 301 278 $0.27 ÅÀ1 and $1.5 ÅÀ1 present arcs of stronger [2,16] is that corresponding to the d1 parameter. Nonethe- 302 279 intensity. less, discrepancies in the bilayer packing parameter are not 303 D TE EC RR CO UN Fig. 2. MEH-PPV films cast from chloroform: WAXS intensity profiles obtained from the corresponding 2D patterns (shown at right). (a and b) Perpendicular and (c and d) parallel incidence to the film plane. Red line: fit of the experimental data using Gaussian functions for the peaks and a logarithmic baseline for the background. Please cite this article in press as: Souza AA et al., Temperature dependence of molecular dynamics and supramolecular ..., Eur Poly J (2008), doi:10.1016/j.eurpolymj.2008.09.030
  5. 5. EPJ 4755 No. of Pages 11, Model 3G 14 October 2008 Disk Used ARTICLE IN PRESS A.A. Souza et al. / European Polymer Journal xxx (2008) xxx–xxx 5 304 surprising, since more than one type of chain packing may also observed (see also Fig. 3b), indicating a decrease in 332 305 coexist for the as-cast films. the number of the scattering objects or chain dissocia- 333 306 Fig. 3 shows the WAXS intensity profiles at some se- tion in the perpendicular direction to the ring planes. 334 307 lected temperatures, below and above Tg, obtained for Finally, the behavior of the d2 parameter can only 335 308 MEH-PPV films cast from chloroform at parallel and per- be clearly followed in the perpendicular incidence pat- 336 309 pendicular incidences. All the intensity profiles were nor- terns (see Fig. 3b and d). Its value of (approximately 337 310 malized by an arbitrary scale factor to observe more 6.0 Å) for all temperatures ranging from 123 to 423 K, 338 311 clearly the changes in intensity and peak positions. The confirms the stability of this molecular packing param- 339 312 main modifications of the first diffraction maximum are eter. Note that this parameter represents the distance 340 313 better seen in the parallel incidence geometry (Fig. 3a). between two benzene units and is only related to bond 341 314 A shift in the peak position of this reflection, associated lengths. 342 315 with the bilayer spacing d1, and a narrowing of the line The explicit dependence of d1, d2 and d3 parameters 343 F 316 width as a function of increasing temperature is ob- with the temperature and the annealing time is shown in 344 317 served. The d1 values varied from 19.3 to 21.3 Å. A shoul- Fig. 4a for the chloroform cast film. The most noticeable ef- 345 OO 318 der in this peak is hardly noticed at the lower fect is observed for d1 parameter. It increases continuously 346 319 temperatures, but becomes more evident for 423 K. The as the temperature is raised from 123 to 318 K. Such 347 320 maximum intensity values and narrowest line profiles behavior is in good agreement with the increase in side- 348 321 were obtained for 423 K and after the samples were kept chain mobility, as we are going to show in sequence. The 349 322 at that temperature for 1 h (see Fig. 3c). The peak area subsequent increase in d1 values from $20.5 to $21.5 Å 350 323 increases as a function of thermal treatment (tempera- when the temperature reaches 348 K, can be correlated 351 PR 324 ture and annealing time). This result indicates an in- with the onset of the polymer glass transition at 323 K, 352 325 crease in the number of molecular aggregates in the that produces higher free volume for the relaxation of 353 326 bilayer normal direction. the aggregate supramolecular structures. From 348 to 354 327 Furthermore, in Fig. 3a, the peak associated with the 423 K and after 1 h annealing at 423 K, the d1 parameter 355 328 d3 parameter seems to shift to lower q values, which decreases slightly. After cooling to room temperature, a va- 356 329 would indicate an increase in the inter-backbone spacing lue of $20.9 Å is obtained. Thus, the overall increase of this 357 330 331 perpendicular to the plane of the phenylene rings. As a reduction in the peak intensity with the temperature is D parameter is $0.4 Å. A similar behavior in the WAXS profiles and d1, d2 and d3 parameters was observed for 358 359 TE EC RR CO UN Fig. 3. WAXS intensity curves of MEH-PPV film cast from chloroform for in situ thermal treatment. (a) Parallel and (b) perpendicular incidence scattering curves for temperatures from 123 to 423 K before annealing. (c) Parallel and (d) perpendicular incidence scattering curves at 423 K before and after 1 h annealing and for 293 K at the end of the temperature cycle. Please cite this article in press as: Souza AA et al., Temperature dependence of molecular dynamics and supramolecular ..., Eur Poly J (2008), doi:10.1016/j.eurpolymj.2008.09.030
  6. 6. EPJ 4755 No. of Pages 11, Model 3G 14 October 2008 Disk Used ARTICLE IN PRESS 6 A.A. Souza et al. / European Polymer Journal xxx (2008) xxx–xxx F OO PR D TE EC RR CO Fig. 4. Temperature dependence of the structural parameters d1, d2 and d3 for MEH-PPV: (a) cast from chloroform, (b) cast from toluene. (c) Average size hDi of the ordered domains in the bilayer normal direction. Dotted lines: visual guide of the behavior. 360 the toluene cast films except for the higher d1 variation at form casting, but bigger values (around 100 Å) are attained 372 UN 361 higher temperatures (see Fig. 4b). after the temperature of 423 K is reached. 373 362 Using Scherrer equation [34], the average size of the or- 363 dered domains <D> in the bilayer normal direction could 3.3. Solid-state NMR 374 364 be roughly estimated from the line width (FWHM) of the 365 d1 diffraction profile. As shown in Fig. 4c for chloroform Fig. 5 shows the repeat unit, the orientation of the 375 366 cast samples, almost constant values of <D> (around principal values of the CSA tensor, and typical 13C CP/ 376 367 60 Å) are found for temperatures up to 348 K. An increase MAS spectra of MEH-PPV at 303 K from chloroform and 377 368 of 20% is obtained for 423 K and a maximum value is toluene cast films. The line assignments are also shown. 378 369 reached after 1 h of annealing (90 Å). For the films obtained All the spectra are basically identical (including those at 379 370 from toluene solution, initially lower <D> values are low temperatures, 233 K), showing that there are no 380 371 achieved compared with the films obtained from chloro- drastic different in the chemical conformational state of 381 Please cite this article in press as: Souza AA et al., Temperature dependence of molecular dynamics and supramolecular ..., Eur Poly J (2008), doi:10.1016/j.eurpolymj.2008.09.030
  7. 7. EPJ 4755 No. of Pages 11, Model 3G 14 October 2008 Disk Used ARTICLE IN PRESS A.A. Souza et al. / European Polymer Journal xxx (2008) xxx–xxx 7 F OO PR 13 Fig. 5. (a) MEH-PPV chemical structure; (b) chemical shift tensor principal axis orientation and typical C CP/MAS spectra of MEH-PPV at 303 K for films cast from (c) chloroform, and (d) toluene. *denotes the spinning sidebands. 382 the polymer chains the films prepared with the different Fig. 7. It is possible to observe that the effective 417 383 384 solvents. NMR can provide specific information about the side- D hmdip ðTÞi=mrigid is smaller for films where toluene was used dip as solvent, confirming the higher degree of side-chain 418 419 TE 385 chain molecular dynamics in MEH-PPV [14]. The degree mobility in films cast from toluene. Comparing the CH 420 386 of molecular dynamics of a particular molecular group in and CH2 DIPSHIFT curves (Fig. 6a–b), it is also seen that 421 387 the side-chain can be characterized by measuring the there is a clear difference for chloroform than for toluene 422 388 strength of the effective 13C–1H dipolar coupling for that cast films. This also indicates a more loosely packed side- 423 389 segment, which can be provided by DIPSHIFT technique chain in the toluene cast films. This feature can be better 424 EC 390 [17,20]. Such experiments, performed under Magic-An- observed in Fig. 7 that shows the temperature behavior 425 391 gle-Spinning (MAS), provide a measurement of the of the CH and CH2 hmdip ðTÞi=mrigid parameters for both films. dip 426 13 392 C–1H magnetic dipolar coupling for each chemical group. For films cast from chloroform the CH group S parameter is 427 393 This is done by measuring the dependence of the signal higher than for CH2 in the whole temperature range, indi- 428 394 amplitude with the evolution period (t1), used for codifying cating a significant difference in the degree of mobility be- 429 the 13C–1H dipolar coupling, which produces a typical tween the head and the tail of the side-chain. In contrast, RR 395 430 396 curve that depends on the strength of the averaged dipolar for the film cast from toluene the S parameters for the 431 397 coupling, hmdip i. Since motions with correlation times CH and CH2 are mostly identical in the temperature range, 432 398 shorter than $100 ls average the dipolar coupling be- suggesting that the mobility in the head and tail of the 433 399 tween 1H and 13C, from the measurement of this parameter side-chain are much similar. This corroborates the above 434 400 it is possible to distinguish rigid from mobile segments and findings that point to a looser packing in the side-chain 435 CO 401 estimate the amplitude of the molecular rotation. Molecu- in films cast from toluene. Note that the packing of the 436 402 lar order parameters S for each chemical group can also be side-chain would avoid motion of the whole side-chain, 437 403 obtained as the ratio between this averaged dipolar cou- but not anisotropic motion of specific segments in the 438 404 pling and its respective rigid-lattice value S ¼ hmdip i=mrigid . dip side-chain. Moreover, the fact that the effective S parame- 439 405 Besides, measuring the ratio hmdip ðTÞi=mrigid versus temper- dip ters tend to a plateau different from zero at higher temper- 440 406 ature allows qualitatively monitoring the increase of the atures is due to the presence of a residual dipolar coupling, 441 UN 407 molecular dynamic rate as a function of T. Typical DIPSHIFT indicating that the side-chains in MEH-PPV do not rotate 442 408 curves for the CH (labeled 11 in Fig. 5a) and CH2 (labeled isotropically, but execute rotations around a local axis. This 443 409 14 and 15 in Fig. 5a) side-chain groups of MEH-PPV films residual coupling is observed as a plateau even at temper- 444 410 at 293 K cast from chloroform and toluene are shown in atures well above Tg, which indicates that the motions ob- 445 411 Fig. 6. These chemical groups were chosen as probes to served are not really associated with free side-chains in the 446 412 the molecular dynamics because they are placed in the amorphous region of the polymer, but with ‘‘trapped” side- 447 413 head (close to the backbone) and tail (at the end) of the chains in the aggregated regions of the polymer. To con- 448 414 side-chain, respectively, allowing detecting possible differ- firm that this effect is really solvent related we performed 449 415 ences in the dynamics at different side-chain positions. The thermal annealing of the samples at 363 K during 12 h un- 450 416 corresponding hmdip ðTÞi=mrigid parameters are also shown in dip der dynamic vacuum. As it can be observed in Fig. 7b and d, 451 Please cite this article in press as: Souza AA et al., Temperature dependence of molecular dynamics and supramolecular ..., Eur Poly J (2008), doi:10.1016/j.eurpolymj.2008.09.030
  8. 8. EPJ 4755 No. of Pages 11, Model 3G 14 October 2008 Disk Used ARTICLE IN PRESS 8 A.A. Souza et al. / European Polymer Journal xxx (2008) xxx–xxx characteristics, which can be easily erased by thermal 457 annealing. However, this was only a qualitative discussion 458 and it would be worth to quantify the change in the kinetic 459 parameter that characterizes the molecular dynamics. 460 Concerning the geometry of the motions, the fact that the 461 S parameter does not go to zero at high temperatures (fast 462 motion limit) indicates that the motion is anisotropic, i.e., 463 occurs about a specific axis or only part of the segments 464 take part of the motion in the DIPSHIFt time scale. How- 465 ever, the DIPSHIFT data of the CH2 group attached to the 466 backbone (labeled 10 in Fig. 5a) shows that this group is 467 rather rigid (no temperature dependence were detected 468 F in the DIPSHIFT curves of this group from 213 to 353 K). 469 This is largely consistent with a motional model where 470 OO the CH group executes n-site jumps around the 471 H2C10AC11H bond, i.e., n-site jumps on a cone with an 472 opening defined by the relative orientation of the CH bond 473 with respect to the CAC bond (ideally 109°). As described 474 in Ref. [35], using this previous knowledge of the motional 475 geometry one can use a spin dynamics simulation program 476 PR [36] to simulate the experimental DIPSHIFT curves for car- 477 bon 11 and extract the correlation times as a function of 478 temperature. Besides, the shape of the DIPSHIFT curve 479 can also be related with the non-exponentiality of the mo- 480 tion correlation function, which in some cases can be 481 translated as a distribution of correlation times. In Fig. 6a 482 D we show the temperature dependence of DIPSHIFT curves for the MEH-PPV CH group in films cast from toluene and chloroform with the corresponding simulations. The corre- 483 484 485 TE sponding correlation times as a function of temperature 486 are shown in the Arrhenius plot of Fig. 6c. The activation 487 energy was evaluated as (69 ± 5) kJ/mol for films cast from 488 chloroform and (54 ± 8) kJ/mol for films cast from toluene. 489 Thus, the results show that the energy barrier for the onset 490 EC of side-chain motion is higher for chloroform than for tol- 491 uene cast films. This seems the only effect of the solvent 492 memory interferes only on the local side-chain motion, 493 since the results do not point to a modification on the 494 geometry of the motion (same S parameter at high temper- 495 RR ature and rigid CH2 attached to the backbone). 496 Despite DIPSHIFT experiments are sensitive to motions 497 in the kHz frequency range, the presence of molecular mo- 498 tions that occur with rates beyond the detection limit of 499 these experiments cannot be ruled out. Thus, to better 500 characterize the dynamics of these systems it is attractive 501 CO to perform experiments capable of providing information 502 about motions in other frequency ranges. One of such 503 experiments is the CODEX [21,22] technique that makes 504 possible to characterize the slow motion (with rates in 505 the Hz scale) of different chemical groups with a consider- 506 UN able degree of details. Essentially, the experiment detects 507 Fig. 6. (a) and (b) Typical 13C DIPSHIFT curves for the CH and CH2 side- the signal reduction resulting from changes in the orienta- 508 chain groups of MEH-PPV films cast from chloroform and toluene at tion-dependent chemical-shift frequencies due to segmen- 509 293 K, respectively. (c) Arrhenius plot of the correlation time (sc) tal reorientations during a waiting time also denoted as 510 extracted from the CH DIPSHIFT curves for the samples cast from chloroform and toluene. mixing time tm. Information about the amplitude (mean 511 reorientation angles) of the motion is obtained by the 512 452 the behavior of both films becomes much similar, confirm- dependence of E(tm, Ntr), as a function of Ntr. 513 453 ing that the differences observed are associated with mem- In previous work, it was observed the presence of slow 514 454 ory effect due to the solvent. molecular motions in the backbone of MEH-PPV at 293 K 515 455 The above statements reveal that the side-chain [13,14]. These motions were assigned as small angle ring 516 456 dynamics of MEH-PPV films can be affected by the solvent rotations around the 1–4 axis. Fig. 8 shows E(tm, Ntr) as a 517 Please cite this article in press as: Souza AA et al., Temperature dependence of molecular dynamics and supramolecular ..., Eur Poly J (2008), doi:10.1016/j.eurpolymj.2008.09.030
  9. 9. EPJ 4755 No. of Pages 11, Model 3G 14 October 2008 Disk Used ARTICLE IN PRESS A.A. Souza et al. / European Polymer Journal xxx (2008) xxx–xxx 9 F OO PR D TE Fig. 7. Thermal behavior of the order parameter S for CH and CH2 side-chain groups in MEH-PPV cast from (a) chloroform before annealing, (b) chloroform after annealing at 363 K by 12 h, (c) toluene before annealing, and (d) toluene after annealing at 363 K by 12 h. EC 518 function of Ntr for the para-carbons (labeled 1 and 4 in served. The fact that no solvent effects were observed in 542 519 Fig. 5a) of MEH-PPV films cast from chloroform and tolu- the CODEX measurements suggests that the overall tor- 543 RR 520 ene at 293 K. The average rotation angles were obtained sional motions are similar in all cases. If we associate the 544 521 by simulating the experimental curves based on principal amplitude of these torsional motions with the conforma- 545 522 values and orientation of the chemical shift tensor. For tional disorder along the polymer backbone, we might con- 546 523 that, the chemical shift anisotropy principal values were clude that the motional induced conformational disorder is 547 524 measured using a standard Herszfeld and Berger analysis. similar for both studied films. However, it should be 548 525 [37] The orientation of the chemical shift principal axis pointed out that the average rotation angle obtained by 549 CO 526 systems of phenyl rings para-carbons is similar in different CODEX cannot be associated with a particular segment in 550 527 systems. Thus, we assumed the same orientation of p-xy- the polymer backbone, but reflects the average behavior 551 528 lene, i.e., the principal value r33 (z-axis) is 1° tilted from of all backbone segments, including those poorly conju- 552 529 the normal to the phenylene ring plane and the x-axis, cor- gated that do not contribute to the photoluminescence. Be- 553 530 responding to r11 is 1° away from the 1–4 axis, as shown in sides that, because the motional amplitude is rather small, 554 UN 531 Fig. 5b. Because the molecular motions in polymers do not proton-driven spin-diffusion [38] may have a non-negligi- 555 532 involve single but distributed rotation angles, the simula- ble contribution to the CODEX exchange intensity, which 556 533 tions were performed considering Gaussian shaped distri- masks the possible differences that may exist among the 557 534 butions of reorientation angles centered at 0° and with torsional motion of the samples. 558 535 full-width at half-maximum (FWHM), hwi, that represents 536 the average reorientation angle. 4. Conclusions 559 537 It can be observed in Fig. 8 that, within the experimen- 538 tal uncertainty, all curves can be fitted using the same dis- This work provided new results of distinct aspects of 560 539 tribution of reorientation angles with hwi $10°. This means the dynamics and supramolecular organization in MEH- 561 540 that no significant differences among the overall motional PPV films cast from two different solvents. The NMR exper- 562 541 amplitudes of the backbone phenylene rings could be ob- iments were particularly useful for describing general and 563 Please cite this article in press as: Souza AA et al., Temperature dependence of molecular dynamics and supramolecular ..., Eur Poly J (2008), doi:10.1016/j.eurpolymj.2008.09.030
  10. 10. EPJ 4755 No. of Pages 11, Model 3G 14 October 2008 Disk Used ARTICLE IN PRESS 10 A.A. Souza et al. / European Polymer Journal xxx (2008) xxx–xxx a direct evidence of the presence of interchains dissocia- 589 tion and increased disorder of the phenyl rings. Consider- 590 ing that the increase of conformational disorder may 591 quench some interchain processes that contributes to the 592 luminescence, the results confirms our previously pro- 593 posed model (dissociation of the interchain species during 594 the MEH-PPV relaxation processes) based on PL and NMR 595 results [13]. More specifically, the intensity increase ob- 596 served at 200 K is associated with in appearing of motional 597 ring torsions due to the onset of the side-chain motions 598 and the stronger intensification at 300 K to the increasing 599 of these motions due to the onset of the glass transition. 600 F In both relaxation processes the blue shift is attributed to 601 the increase of the motional induced conformational 602 OO disorder. 603 Acknowledgements 604 Authors thank FAPESP, CNPq, CAPES and MCT/PADCT/ 605 IMMP for the financial support and fellowships. TDZA 606 PR and RFC thank FAEPEX/Unicamp for financial support and 607 fellowships. ERdA thanks Prof. Kay Saalwachter for provid- 608 ing the spin dynamics simulation program and for helpful 609 discussions. WAXS data were collected under proposals 610 D11A-SAXS1 #4247 and #4293 of the Brazilian Synchro- 611 tron Light Laboratory (LNLS). 612 D References 613 TE [1] Inigo AR, Chiu H-C, Fann W, Huang Y-S, Jeng U, Linn TL, et al. 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