Kress, Roman

Schmidt, Hans-Werner

Schmidt, Marko

Senker, Juergen

Wittmann, Johannes J.

Chemical Communications

English

Marko Schmidt

Johannes J. Wittmann

Roman Kress

Hans-Werner Schmidt

Jürgen Senker

Crossref

Probing self-assembled 1,3,5-benzenetrisamides in isotactic polypropylene by<sup>13</sup>C DQ solid-state NMR spectroscopy

Senker, Jürgen

EPub Bayreuth

This journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 267--269 267Cite this: Chem. Commun.,2013, 49, 267Probing self-assembled 1,3,5-benzenetrisamides inisotactic polypropylene by 13C DQ solid-state NMRspectroscopy†Marko Schmidt,a Johannes J. Wittmann,a Roman Kress,b Hans-Werner Schmidtband Ju¨rgen Senker*aUsing 13C double quantum solid-state NMR spectroscopy, wewere ableto observe nuclei of a supramolecular BTA based additive on thenanoscale in amatrix of i-PP at a concentration of only 0.09 wt%. Thesenuclei exhibit the analogous structural features as the crystalline phaseof the neat additive.As 1,3,5-benzenetrisamides (BTAs) are well known to form supra-molecular structures via self-assembly,1 BTAs become more andmore important inmany research fields.2 Their accessibility togetherwith their supramolecular behaviour renders BTAs into a versatileclass of advanced materials with potential to be used as hydrogela-tors3 and organogelators.4 Besides, several research groups demon-strated the benefits of BTAs as nucleating agents in semi-crystallinepolymers such as isotactic polypropylene (i-PP),5,6 poly(ethylene-co-propylene),7 polybutylene terephthalate,8 polylactides,9 and poly-vinylidene fluoride.10 In addition, some derivatives of this class arehighly efficient as electret additives11 or as clarifying agents for i-PP.5In contrast to other nucleating agents such as inorganic salts, claysand organic pigments,12 supramolecular polymer additives aresoluble in the polymer melt and self-assemble upon cooling intosupramolecular nano-objects, which act as heterogeneous nucleiinitiating the crystallisation of the polymer.In particular, 1,3,5-tris(2,2-dimethylpropionylamino)benzene hasbeen proven to feature outstanding clarifying properties for i-PP.5The crystal structure of this BTA has been recently determined byNMR-crystallographic strategies revealing a hydrogen-bond mediatedpseudo-hexagonal columnar rod-packing where every molecule pos-sesses a high intrinsic dipole moment.13 This dipole moment gainsrapidly within one rod leading to a markedly increased potentialenergy which can be stabilised in strong electric fields.14 In thecrystalline phase the potential energy due to the macroscopic dipoles,however, is balanced by neighboring rods which are aligned antiparallel. The possibility to compensate dipole moments due to therod-packing is supposed to play a crucial role in the nucleatingprocess of the polymer since it allows an extremely fast one dimen-sional crystal growth process.13 However, there is practically noexperimental evidence of the nucleation mechanism within thepolymer melt since standardised techniques like X-ray diffraction,for example, fail due to the small size of the nuclei and the lowadditive concentrations.Here, we communicate the first experimental data on the struc-tural properties of this special BTA in a matrix of i-PP by means of13C double quantum (DQ) solid-state NMR. For this purpose we firstsynthesised and characterised 1,3,5-tris(2,2-dimethylpropionyl-amino)benzene 13C labelled at all three carbon atoms of theamide linkage (1).13 Subsequently, 0.09 wt% of the additive wasincorporated into i-PP using a twin screw mini compounder at280 1C. The extrudate of the binary mixture was finally crushedin a cryogenic laboratory mill at low temperature to generatepowder for the solid-state NMR experiments.From a 1H–13C CP-MAS spectrum of the binary additive–i-PPmixture all signals corresponding to i-PP and one of the 13C enrichedcarbonyl groups of the additive molecules (d E 177 ppm) can beobserved (Fig. S1, see ESI†). The 13CQO chemical shift in the solidstate does not show any difference compared to the chemical shift inthe liquid state. As a consequence, the occurrence of the resonancein the MAS spectrum cannot give sustainable evidence whether itbelongs to single molecules being randomly distributed in thepolymer matrix or to the supramolecular columns that are knownfrom the crystal structure of the bulk material.13For this reason 13C DQ experiments were performed. Theseexperiments are based on the dipolar coupling between two spinswhich is proportional to 1/r3 where r represents the spin–spindistance. To identify the closest connectivities between the additivemolecules and the polymer matrix, if existent, we recorded several2DDQ spectra with different excitation times. Here, every correlationsignal can clearly be assigned to its distinct chemical shift in theindirect dimension.Since the 13C resonance frequencies of the polymermatrix and theadditive (13CQO) are spread over roughly 12.5 kHz, the transmittera Inorganic Chemistry III, University of Bayreuth, Universita¨tsstr. 30, 95447,Bayreuth, Germany. E-mail: juergen.senker@uni-bayreuth.de;Fax: +49 (0)921/55-2788; Tel: +49 (0)921/55-2531bMacromolecular Chemistry I, University of Bayreuth, Universita¨tsstr. 30, 95447,Bayreuth, Germany. E-mail: hans-werner.schmidt@uni-bayreuth.de;Fax: +49 (0)921/55-3206; Tel: +49 (0)921/55-3200† Electronic supplementary information (ESI) available. See DOI: 10.1039/c2cc37384jReceived 16th August 2012,Accepted 8th November 2012DOI: 10.1039/c2cc37384jwww.rsc.org/chemcommChemCommCOMMUNICATIONPublished on 22 November 2012. Downloaded by UNIVERSITAT BAYREUTH on 8/27/2020 10:31:09 AM. View Article OnlineView Journal  | View Issue268 Chem. Commun., 2013, 49, 267--269 This journal is c The Royal Society of Chemistry 2013frequency offset was placed in the middle of these resonances. Thisleads to an off-resonant excitation of about 6 kHz which does haveonly a minor influence on the spectrum due to the robustness of thepulse sequence against offset effects.15 A rotation frequency of 6.25kHz was used with the non-g-encoded SR26114 sequence16 (see ESI†)since previous results showed that faster spinning does not lead to anefficient DQ build-up, especially for large distances ofE5 Å which weexpect from the structure model of the neat additive. In addition, toobtain pure absorptive 2D spectra without any rotational sidebandsin the DQ-domain, t1 must be an integer multiple of the rotorperiod and, moreover, possess equal values in the direct andindirect dimension (in our case two times trot). As a consequencethe spectral width in the indirect domain is just 41.5 ppm(19.5–61 ppm) causing a refolding of the DQ signals in theindirect dimension which were theoretically placed outside theNyquist frequencies (Nyquist theorem17).Fig. 1 displays the DQ-SQ 13C spectrum of the i-PP–additivematerial at B0 = 7.04 T, nrot = 6.25 kHz and texc = 15.36 ms. Thespectrum exhibits DQ correlations between all resonancescorresponding to the i-PP and the auto-correlation signal ofthe 13CQO group of the additive. The auto-correlation peak ofthe CH-group served as internal shift calibration.The chemical shift of the CH3-group in the direct domain ofdSQE 22.5 ppm leads at the same time to a signal at dDQE 45 ppmin the indirect dimension. This can also be observed for theCH-group where the peak in the DQ domain at dDQ E 53 ppmcorresponds to the signal at dSQE 26.5 ppm in the SQ domain.The signal at dDQ E 49 ppm can be identified as the one-bond correlation between the methyl and the methine group.The auto-correlation signal of the CH2-group with a chemical shiftdSQ E 44 ppm (dDQ,exp E 88 ppm) occurs at dDQ,obs E 46.5 ppmafter being refolded once into the spectrum. However, the intensityof this signal at texc = 15.36 ms is very weak, in comparison,for instance, with the auto-correlation peak of the CH-group.We additionally observe refolded correlation peaks betweenthe CH2-group of the polymer and the CH- and CH3-group atdDQ E 25 ppm and dDQ E 29 ppm. At dSQ E 177 ppm, a signalcorresponding to the carbonyl group of the additive occurs. After aneightfold refolding of the signal into the spectrum, the peak atdDQE 22 ppm can clearly be identified as the auto-correlation signalof the 13CQO group (dDQ,exp E 354 ppm), while other correlationsthat include the carbonyl groups, i.e. interaction with the polymermatrix, are not observed. This fact can also be seen in 2D DQ-SQspectra with lower and even higher excitation times, respectively (seeFig. S2 in the ESI†). In Table S2 (see ESI†) the expected and observedchemical shifts after refolding of all correlation signals in theindirect domain are summarized.The existence of the 13CQO auto-correlation indicates a spatialconnectivity of the additive carbonyl groups but, still, is no unequi-vocal evidence. To get a more detailed picture of the structuralfeatures of the additive clusters in the polymermatrix, we recorded asymmetric DQ build-up curve. The absence of correlations betweenthe additive and the polymer matrix allows treating the signal of thecarbonyl group independently. Thus, the intensities being extractedfrom the build-up curve of the i-PP–BTA material can be directlycompared to one of the neat 13C labelled additive 1.Fig. 2 displays the experimental symmetric 13C DQ build-upcurves of the labelled carbonyl groups of the bulk material (redcircles) and the additive within the polymer (blue squares witherror bars according to equation S3 in the ESI†). Both build-upcurves are normalised according to equation S1 (see ESI†).Within the experimental error, the progression of bothcurves matches perfectly. In this respect, the slight shift ofthe maximum of the DQ build-up curve for the i-PP–additivemixture is negligible. As a consequence, the 13C13C DQ dataprovide solid evidence that the structural motifs for the additiveFig. 1 Experimental 2D DQ-SQ 13C spectrum of the additive–i-PP mixtureusing the SR26114 sequence at B0 = 7.04 T, nrot = 6.25 kHz and texc = 15.36 ms.The F2 projection is the sum of all signals in the SQ domain – signalassignment included. Additional information is given in the ESI.†Fig. 2 Experimental symmetric 13C DQ build-up curves of the labelled carbonylgroups within neat 1,3,5-tris(2,2-dimethylpropionylamino)benzene (red circles,nrot = 5.5 kHz) and the i-PP–additive binary mixture (blue squares with error bars,nrot = 6.25 kHz). Both measurements were performed using the SR26114 pulsesequence (B0 = 7.04 T) with a maximum excitation time texc of 21.76 (bulk) and20.48 ms (mixture), respectively. Further information is given in the ESI.†Communication ChemCommPublished on 22 November 2012. Downloaded by UNIVERSITAT BAYREUTH on 8/27/2020 10:31:09 AM. View Article OnlineThis journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 267--269 269clusters in the polymer matrix and in the bulk material are similar.As demonstrated in ref. 13 the applied DQ filtered 13C NMRexperiment is able to probe the columnar arrangement of themolecules but not the alignment of neighbouring rods. This couldalso be successfully demonstrated by evaluating the root-sum-squaredipolar coupling drss for different scenarios (see ESI†).18 For dilutedmolecules, drss accounts to roughly 37 Hz and thus a slower DQbuild-up would be observed, while a single molecular strandand the rod packing in the crystal lead to nearly identical valuesof 127 Hz and 128 Hz.The DQ-SQ spectra (Fig. 1 and Fig. S2, ESI†) display intensivepeaks for all correlations with the exception of the signal betweenthe BTA carbonyl group and the polymer units. Although a quanti-tative analysis is demanding due to the influence of the DQ intensityon the nuclear distances,19 the geometry of the aggregates, the lowadditive concentration and possible motional processes its absencehints at the formation of larger clusters consisting of several rods.Nevertheless, the significant broadening of the DQ signal of thecarbonyl groups in the BTA–i-PP mixture where the fine structure ismasked – compared to the neat BTA (Fig. 3) – suggests a less welldefined long-range order and crystallite sizes in the lower nanometreregime within the polymer matrix.Our experiments demonstrate that the additive molecules formcolumns within the polymer matrix which are similar to the onesobserved for the neat material. The compensation of themacrodipole arising in one rod requires a pseudo-hexagonalrod packing accounting also for the fast 1D growth behaviour.Thus, even at low concentrations the BTAs can form long andthin needle-like objects which provide enough surface area toinitiate heteronuclear nucleation of the polymer. Interestingly,the spacing of the additive molecules within one rod leads toa surface modulation with similar characteristic distances(6.8 Å)13 as compared to neat i-PP (6.5 Å) indicating that thegrowth mechanism of the i-PP is indeed epitactic in nature.In conclusion, we have presented a powerful approach to visualisenuclei on a nanoscale of an efficient BTA-based nucleating andclarifying agent for i-PP at a concentration below 0.1 wt% withinthe polymer matrix. We successfully used 13C DQ solid-stateNMR to clearly demonstrate the structural analogy of the supra-molecular aggregates of the additive within the binaryi-PP–additive material compared to the neat bulk material.With this approach it will be possible to study nucleationprocesses from solutions and melts in an early stage for a widerange of molecular compounds including pharmaceuticals inthe future. This is important especially for systems with poly-morphism where probably small structural changes of thenuclei may lead to a different polymorph.We gratefully acknowledge financial support that was grantedby the Deutsche Forschungsgemeinschaft DFG (SFB 840).Notes and references1 (a) M. P. Lightfoot, F. S. Mair, R. G. Pritchard and J. E. Warren, Chem.Commun., 1999, 1945; (b) L. Brunsveld, A. P. H. J. Schenning, M. A. C.Broeren, H. M. Janssen, J. A. J. M. Vekemans and E. W. Meijer, Chem.Lett., 2000, 292; (c) M. Kristiansen, P. Smith, H. Chanzy, C. Baerlocher,V. Gramlich, L. McCusker, T. Weber, P. Pattison, M. Blomenhofer andH.-W. Schmidt, Cryst. Growth Des., 2009, 9, 2556; (d) D. Kluge,F. Abraham, S. Schmidt, H.-W. Schmidt and A. Fery, Langmuir, 2010,26, 3020; (e) M. Wegner, D. Dudenko, D. Sebastiani, A. R. A. Palmans,T. F. A. de Greef, R. Graf and H. W. Spiess, Chem. Sci., 2011, 2, 2040;( f ) D. Kluge, J. C. Singer, J.W. Neubauer, F. Abraham,H.-W. Schmidt andA. Fery, Small, 2012, 8, 2563.2 S. Cantekin, T. F. A. de Greef and A. R. A. Palmans, Chem. Soc. Rev.,2012, 41, 6125.3 (a) N. Shi, H. Dong, G. Yin, Z. Xu and S. Li, Adv. Funct. Mater., 2007,17, 1837; (b) A. Bernet, R. Q. Albuquerque, M. Behr, S. T. Hoffmannand H.-W. Schmidt, Soft Matter, 2012, 8, 66.4 (a) Y. Yasuda, E. Iishi, H. Inada and Y. Shirota, Chem. Lett., 1996,25, 575; (b) K. Hanabusa, C. Koto, M. Kimura, H. Shirai and A. Kakehi,Chem. Lett., 1997, 26, 429; (c) J. J. van Gorp, J. A. J. M. Vekemans andE. W. Meijer, J. Am. Chem. Soc., 2002, 124, 14759.5 M. Blomenhofer, S. Ganzleben, D. Hanft, H.-W. Schmidt, M. Kristiansen,P. Smith, K. Stoll, D. Maeder and K. Hoffmann, Macromolecules, 2005,38, 3688.6 (a) P. M. Kristiansen, A. Gress, P. Smith, D. Hanft and H.-W. Schmidt,Polymer, 2006, 47, 249; (b) F. Abraham, S. Ganzleben, D. Hanft,P. Smith and H.-W. Schmidt, Macromol. Chem. Phys., 2010, 211, 171.7 J. Wang, Q. Dou, X. Chen and D. Li, J. Polym. Sci., Part B: Polym.Phys., 2008, 46, 1067.8 F. Richter and H.-W. Schmidt, Macromol. Mater. Eng., 2012, DOI:10.1002/mame.201200034.9 (a) H. Nakajima, M. Takahashi and Y. Kimura, Macromol. Mater.Eng., 2010, 295, 460; (b) H. Bai, W. Zhang, H. Deng, Q. Zhang andQ. Fu, Macromolecules, 2011, 44, 1233; (c) P. Song, Z. Wei, J. Liang,G. Chen and W. Zhang, Polym. Eng. Sci., 2012, 52, 1058.10 F. Abraham and H.-W. Schmidt, Polymer, 2010, 51, 913.11 (a) N. Mohmeyer, N. Behrendt, X. Zhang, P. Smith, V. Altsta¨dt, G. M.Sessler and H.-W. Schmidt, Polymer, 2007, 48, 1612; (b) J. Hillenbrand,T. Motz, G. M. Sessler, X. Zhang, N. Behrendt, C. von Salis-Soglio,D. P. Erhard, V. Altsta¨dt and H.-W. Schmidt, J. Phys. D: Appl. Phys.,2009, 42, 65410; (c) D. P. Erhard, D. Lovera, C. von Salis-Soglio, R. Giesa,V. Altstaedt and H.-W. Schmidt, Adv. Polym. Sci., 2010, 228, 155.12 (a) F. Binsbergen, Polymer, 1970, 11, 253; (b) F. Binsbergen and B. deLange, Polymer, 1970, 11, 309; (c) F. Binsbergen, J. Polym. Sci., Polym.Lett. Ed., 1973, 11, 117.13 M. Schmidt, J. J. Wittmann, R. Kress, D. Schneider, S. Steuernagel,H.-W. Schmidt and J. Senker, Cryst. Growth Des., 2012, 12, 2543.14 C. F. C. Fitie´, W. S. C. Roelofs, M. Kemerink and R. P. Sijbesma,J. Am. Chem. Soc., 2010, 132, 6892.15 P. E. Kristiansen, M. Carravetta, J. D. van Beek, W. C. Lai andM. H. Levitt, J. Chem. Phys., 2006, 124, 234510.16 P. E. Kristiansen, M. Carravetta, W. C. Lai and M. H. Levitt, Chem.Phys. Lett., 2004, 390, 1.17 J. Keeler, Understanding NMR Spectroscopy, Wiley, Chichester, 2ndedn, 2010.18 V. E. Zorin, S. P. Brown and P. Hodgkinson,Mol. Phys., 2006, 104, 293.19 J. P. Bradley, C. Tripon, C. Filip and S. Brown, Phys. Chem. Chem. Phys.,2009, 11, 6941.Fig. 3 Bottom: experimental (black circles) and simulated (black line) 13C DQfiltered spectra of the neat BTA with deconvolution into three distinct resonances(blue, green and red lines). Their fine structure is caused by a second order14N quadrupolar interaction on a dipolar coupled 13C14N spin pair (compareESI†). Top: experimental 13C DQ filtered spectrum of the binary mixture(as sum over all 1D spectra extracted from the build-up curve).ChemComm CommunicationPublished on 22 November 2012. Downloaded by UNIVERSITAT BAYREUTH on 8/27/2020 10:31:09 AM. View Article Online

Probing self-assembled 1,3,5-benzenetrisamides in isotactic polypropylene by C-13 DQ solid-state NMR spectroscopy

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Probing self-assembled 1,3,5-benzenetrisamides in isotactic polypropylene by C-13 DQ solid-state NMR spectroscopy

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