Reversible Phase Transition Triggered by Order−DisorderTransformation of Carboxyl Oxygen Atoms Coupled with DistinctReorientations in [HN(C4H9)3](fumrate)0.5·(fumaric acid)0.5Muhammad Adnan Asghar,†,‡ Zhihua Sun,*,† Tariq Khan,†,‡ Chengmin Ji,† Shuquan Zhang,† Sijie Liu,†,‡
Lina Li,† Sangen Zhao,*,† and Junhua Luo*,†
†Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, ChineseAcademy of Sciences, Fuzhou 350002, China‡University of the Chinese Academy of Sciences, Beijing 100039, China
*S Supporting Information
ABSTRACT: A new reversible molecular phase transition material [HN-(C4H9)3](fumrate)0.5·(fumaric acid)0.5 (1) has been successfully synthesized.Differential scanning calorimetry measurement shows a pair of reversible peaks at181.9 and 178 K on heating and cooling modes, respectively. The large thermalhysteresis of ∼3.9 K discloses its reversible first-order structural phase transition.Specific heat capacity and dielectric constant measurements around Tc furtherconfirm its phase transition behaviors. The detailed structural analyses of 1 atvariable temperatures reveal that its structural phase transition is mainlyaccomplished by the order−disorder transformations of carboxyl oxygen atomsand distinct reorientations among cations and anion-acid infinite sheets togetherwith proton-dynamic motion. All these results open a new way to constructpotential phase transition materials through the selection of flexible aliphaticcations with simple carboxyl-based anions.
■ INTRODUCTION
Reversible phase transition materials, of which the optoelec-tronic responses can be systematically converted by applicationof external pressure, heat or light, have generated wide interestdue to their potential applications in nonlinear optical (NLO)switches, ferroelectrics, switchable dielectrics, phase shifters,sensing, signal processing and data storage, etc.1−6 Engineeringof solid-to-solid temperature-dependent reversible phasechange materials are not only significant for the searching oftechnologically potential materials, but also helpful for theinvestigation of structure−property relationships.7−9 Recently,breakthroughs in this field have been made by the discovery ofa correlation between the order−disorder transformation ofmoieties and switchable dielectric/ferroelectric properties.10−14
For example, Xiong et al. explored an amphidynamic crystal of[(CH3)2NH2]2[KCo(CN)6] exhibiting tunable and switchabledielectric constants, where the order−disorder transformationof dimethylammonium cation affords the driving force for itsreversible structural changes.15 In addition, some haloaceticacids, such as trichloroacetic acid, trifluoroacetic acid, dichloro-acetic acid, and difluoroacetic acid, also play an imperative roleduring the phase transitions of molecular crystals by theorderings of halogen atoms.16,17 For instance, the dichlor-oacetate-anion-based salt of potassium hydrogen bis-(dichloroacetate)-18-crown-6 undergoes a dielectric phasetransition triggered by the ordering of unique pendulum-likemotions of the dichloroacetate anion.18 Moreover, structural
phase transitions of several classic ferroelectrics are ascribed tothe proton transfer from the hydroxyl group, along with thedisordering or displacement of other parts.19−21 Take the firstknown ferroelectric of Rochelle salt as an example: its phasechanges are primarily induced by the proton transfer ofhydrogen bonds between water and L-tartrate molecules,together with the molecular displacements around potassiumatoms.22
However, searching for new phase transition materialscomposed of carboxyl and hydroxyl-based anions remainsquite interesting, because simple acids containing carboxylgroups are easy to form hydrogen bonds which play a vital rolefor structural changes. To the best of our knowledge, phasetransitions induced by order−disorder transformation ofcarboxyl oxygen atoms are comparatively scarce,23 whereas itshould be emphasized that sometimes only ordered−disordered transformation of carboxyl-based anions is notadequate to stimulate structural phase transitions.24,25 Theintroduction of other molecular motions, such as twisting and/or reorientations, would enhance the possibilities to assemblephase transition materials. In this context, we propose apotential strategy to design new phase transition compounds,that is, combining the carboxyl-based acids with the branched/
Received: October 11, 2015Revised: December 14, 2015Published: December 31, 2015
flexible aliphatic amines. Particularly, the aliphatic amines withlong and flexible alkyl chains have more opportunities to triggermolecular twisting and/or reorientations. Here, the combina-tion of flexible tripod-like tri-n-butyl amine with fumaric acidyields fruitful results. As continuing work of our group toexplore phase transition materials,26−32 we report that a newmolecular crystal of [HN(C4H9)3](fumrate)0.5·(fumaric acid)0.5(1) undergoes a reversible first-order phase transition at ∼182K, which is confirmed by the differential scanning calorimetry(DSC), specific heat capacity (Cp), and dielectric constantmeasurements. Variable-temperature single-crystal structureanalyses reveal order−disorder transformations of carboxyloxygen atoms that stimulate its phase transition, as well as thedistinct reorientations between cations and the anion-acidchains parallel with proton-dynamic motion.12 This findingaffords a potential pathway to assemble phase transitioncompounds by combining carboxyl-based acids withbranched/flexible aliphatic amines.
■ EXPERIMENTAL SECTIONSynthesis. All chemical reagents were purchased from Sigma-
Aldrich with high purity and used without further purification.Colorless polycrystalline material of 1 was harvested through a slowevaporation method by dissolving tri-n-butylamine and fumaric acid inethanol with a molar ratio of 2:1 at ambient conditions. Repeatedrecrystallization of 1 yields needle-like single crystals after several days.The phase purity of 1 was verified by comparing the room-temperature experimental and simulated powder X-ray diffractionpatterns (Figure S1). The stoichiometry of 1 was determined by X-raydiffraction and elemental analyses. Calcd for C16 H31 N O4; C: 63.76,H: 10.39, N: 4.65. Found C: 63.74, H: 10.38, N: 4.66.Thermal Measurements. Specific heat capacity and DSC
measurements of 1 were recorded on a Netzsch DSC 200 F3instrument with a heating−cooling rate of 5 K min−1 under a nitrogenatmosphere in the temperature range of 150−220 K using aluminumcrucibles.Dielectric Measurements. The complex dielectric permittivity (ε
= ε′ − iε″) of 1 was recorded on TH2828 A impedance analyzer at thefrequencies of 500 Hz, 10 kHz, 100 kHz, and 1 MHz in thetemperature range of 150−200 K with the measuring AC voltage fixedat 1 V.Single-Crystal Structure Determination. Variable single-crystal
X-ray diffraction were performed on Super Nova diffractometer loadedwith Cu-Kα radiation (λ = 1.54184 Å). A colorless crystal ofapproximate dimensions 0.33 × 0.29 × 0.25 mm was used in datacollections at 160 and 260 K. The SHELXS97 was used to solve thecrystal structures by direct method and refined by full-matrix methodon F2 using SHELXLTL software package.33 All hydrogen atoms weregenerated geometrically and all non-hydrogen atoms were refinedanisotropically. Crystallographic data and details of data collection andrefinement are listed in Table S1. Crystallographic data and details ofdata collection and refinement are listed in Table S1. CCDC1064095−1064096 for 1 contains the supplementary crystallographicdata for this paper, which can be obtained free of charge from TheCambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
■ RESULTS AND DISCUSSIONThermal Properties of 1. DSC is reliable thermodynamic
technique to detect the reversible phase transition nature of acompound with respect to temperature change. Here, thepolycrystalline samples of 1 were subjected to DSC measure-ments in an aluminum container under nitrogen conditions.Upon cooling and heating, an exothermic anomaly at 178 Kand an endothermic anomaly at 181.9 K were observed,respectively (phase transition point, Tc = 181.9 K, Figure 1).
The large thermal hysteresis of ∼3.9 K during cooling andheating suggests its first-order phase transition behavior. Inaddition, an entropy change (ΔS) is calculated with a value of3.27 J·mol−1·K−1 at about 182 K from the Cp measurements.From the Boltzmann equation ΔS = R ln N, where R is the gasconstant and N is the ratio of the numbers of respectivegeometrically distinguishable orientations, N is calculated as1.48, which signifies an order−disorder transformation of 1.34,35
Crystal Structural Analysis. A comprehensive structuralanalysis is imperative for understanding the microscopicmechanism of the reversible phase transitions. For 1, variable-temperature single-crystal X-ray diffraction analyses wereperformed at 260 K (above Tc, high-temperature phase,HTP) and 160 K (below Tc, low-temperature phase, LTP),respectively. Compound 1 crystallizes in the triclinic class withcentrosymmetric space group of P1 at both phases. In the HTP,the cell parameters are a = 8.425 Å, b = 9.71 Å, c = 12.537 Å, α= 100.236°, β = 109.249°, γ = 95.15°, and V = 940.75 Å3, whilein the LTP, its cell parameters are a = 8.188 Å, b = 9.887 Å, c =11.973 Å, α = 100.63°, β = 108.452°, γ = 94.113°, V = 894.86Å3. As shown in Figure. 2, the unit cell parameters of 1 wererecorded as a function of temperature from 240 to 100 K. Thecell constants show considerable change at about 182 K,suggesting its phase change behaviors, which agree fairly wellwith the results of DSC and dielectric measurements.
Crystal Structures at HTP and LTP. The asymmetric unitof 1 is composed of one protonated tri-n-butyl ammoniumcation, half part of a deprotonated fumarate anion, and halfmolecule of fumaric acid at HTP and LTP (Figures 3 and 4).The packing structures of 1 in both phases are characterized byan extensive hydrogen bonding arrangement. In the ellipsoidalstructure at HTP, the observed thermal values of oxygen atomsare found to be higher than that of the neighboring carbonatoms, which would reveal the disrodering feature of oxygenatoms (Figure S4-b). For example, the terminal oxygen atomsof fumarate groups acquire a higher thermal ellipsoidal state,which will be more suitable if split into two occupied sites asO2A and O2B with the occupancies of 0.5 and 0.5, respectively.In contrast, the flexible cations are found to be in the normalordered state, although the terminal carbon atoms of threeflexible arms of cations exhibit relatively large thermalvibrations. The detailed Ueq values of 1 are given in theTable S2.In the structure of HTP, the fumarate anions and fumaric
acid molecules are connected with each other infinitely throughrelatively weak O−H···O− hydrogen bonding interactions
between carboxylate moieties and adjacent acid molecules(distance between donor and acceptor is 2.498 Å, Figure 3b).The carboxyl atoms of acids act as donors to the adjacentcarboxylate groups in intra anion-acid hydrogen bonds, whichenhance the establishment of infinite sheets (as shown inFigures 3c and 4c). Moreover, each fumarate anion not onlyconnects with two different fumaric acids, but also interlockswith two tri-n-butyl ammonium cations through N−H···Ohydrogen-bonding interactions, facilitated by the protonatednitrogen atoms of cations and carboxyl oxygen atoms of anions.This type interlocking of cations creates the distinct layersabove and below the anion-acid infinite sheets, respectively.Structure Difference between HTP and LTP. It is
noteworthy that the disordered oxygen atoms of fumarateanions do not participate in the construction of H-bondinginteractions at both phases. It means that the disordered oxygenatoms might possess a higher dynamic freedom in the crystallattice, thus affording possibilities for the order−disordertransformation at HTP. With the temperature decreasingfrom HTP to LTP, the disordered motions of carboxylate
oxygen atoms become frozen, corresponding well to its moreordered low-temperature phase (as shown in Figure 5).
Such a fascinating order−disorder transformation can also bededuced from the tilting of angles between acid and anionicmoieties. As discussed previously, the one-dimensional infinitehydrogen-bonding sheets are formed between the anions andacid molecules at both HTP and LTP. However, these sheetsdisplay distinct reorientations at two states, as shown in Figure6. The observed torsion angle among C13−O1−O3−C15 iscalculated as 163.65° at HTP, which deviates abruptly to 157.0°at LTP. Besides, the flexible protonated tri-n-butyl ammoniumgroups show different geometrical configurations for bothphases. As shown in Figure 7, three n-butyl parts of the cations(named as C1−C2−C3−C4, C5−C6−C7−C8, and C9−C10−
Figure 2. Temperature-dependence of (a) cell parameters for axis lengths and (b) cell angles and volume in the range from 240 to 100 K for 1.
Figure 3. Asymmetric unit (a), hydrogen bonding (b) and packingdiagram (c) of 1 at HTP.
Figure 4. Asymmetric unit (a), hydrogen bonding (b) and packingdiagram (b) of 1 at LTP.
Figure 5. Order−disorder transformation of fumarate anions in 1 atHTP (left) and LTP (right).
Figure 6. Reorientations in anion-acid sheet at HTP and LTP.
C11−C12) bear the torsion angles of 178.44°, 179.65°, and−68.59° at HTP, while 179.0°, 178.58°, and −67.65° at LTP,respectively. These obvious reorientations at both phases unveilthe involvement of cations during phase change process. Thesedetailed structural comparisons of 1 disclose that the order−disorder transformation of carboxyl oxygen atoms would affordthe driving force for this first-order phase transition, togetherwith the relative reorientations between anionic-acid sheets andthe flexible cationic n-butyl parts.The deuterated analogue 2 was synthesized to investigate the
proton dynamic motions during phase-transition. It is obviousthat the deutrated compound exhibits an obvious change ofDSC shapes, compared to that of compound 1 in DSCmeasurements (Figure S3). The DSC peaks became broaderand displaced from the original position to 183.9 K on heatingand 175.3 K on cooling. However, the quite small change ofphase transition point (∼2 K) further confirms that order−disorder transformation of fumarate anions still dominates itsstructure changes, although the proton dynamic motions in theO−H···O hydrogen bond may also involve the phase transition.Dielectric Properties. The structural phase transitions not
only lead to thermal entropy change, but also help thestimulation of anomaly during dielectric measurements. Thepowder-pressed pellet of 1 was subjected to temperature-dependent dielectric measurements at 500 Hz, 10 kHz, 100kHz, and 1 MHz. As shown in Figure 8, the step-like anomalies
are clearly observed at about 182 K upon heating, which revealsthe occurrence of phase transition. In detail, the sample exhibitsan initial dielectric constant value of approximately 2.3 at 160K.Around Tc, a sharp variation is recorded, and the dielectric
constants jump to 6.1 at 182 K ( f = 500 Hz). As far as we are
aware, such temperature-dependent step-like dielectric anoma-lies around Tc are reliable with the characteristics of switchabledielectric materials.36 Besides, the comparative dielectricrelaxation near Tc shows no obvious responses, which indicatesthe fact that dipolar motion is very fast during phasetransition.37,38 Furthermore, the step-like dielectric anomaliestogether with the variable-temperature crystallographic studiesof 1 disclose that the phase transition is neither ferroelectric norantiferroelectric.It is noteworthy that dielectric constants of 1 also exhibit the
frequency-dependent characteristics. Above Tc, the dielectricconstants demonstrate an obvious decline with the frequencyincreasing, which change from ∼8.1 (at 500 Hz) to 5.4 (at 1000kHz). That is, the larger values of dielectric constants could beexpected at even lower frequency. Such a frequency-dependentdielectric response mainly results from the orientationalmotions of the structural groups in 1, coinciding well withthe above structural analyses of reorientations between cationsand anion-acid infinite sheets, which are responsible for thepresence of the motions of dipolar moments during the order−disorder phase transition.39,40
■ CONCLUSIONS
In conclusion, we have discovered a new phase transitioncompound of [HN(C4H9)3](fumrate)0.5·(fumaric acid)0.5,which exhibits a reversible second-order phase transition atabout 182 K, which is confirmed by the step-like dielectricanomalies, DSC and Cp measurements. Variable-temperaturestructural analyses of 1 further disclose that the origin of phasetransition is ascribed to the synergetic corporations betweenorder−disorder transformations of carboxyl oxygen atoms anddistinct torsions among cations and anion-acid infinite chaintogether with proton-dynamic motion. Thus, we believe thatthe present finding would afford an opportunity for the designof potential phase transition materials through selecting theflexible aliphatic amines with simple carboxyl-based acids.
■ ASSOCIATED CONTENT
*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.cgd.5b01452.
PXRD patterns, TG-DTA, deuterated sample DSC,thermal ellipsoids diagrams, and tables of compound 1(PDF)
Accession CodesCCDC 1064095−1064096 contains the supplementary crys-tallographic data for this paper. These data can be obtained freeof charge via www.ccdc.cam.ac.uk/data_request/cif, or byemailing [email protected], or by contacting TheCambridge Crystallographic Data Centre, 12, Union Road,Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
This work was financially supported by the National NatureScience Foundation of China (21222102, 21373220, 51102231,21171166, and 21301171), the One Hundred Talents Programof the Chinese Academy of Sciences, the 973 Key Programs ofthe MOST (2010CB933501, 2011CB935904), and Key Projectof Fujian Province (2012H0045). Dr. Sun and Zhao is thankfulfor the support from “Chunmiao Project” of Haixi Institute ofChinese Academy of Sciences (CMZX-2013-002 and CMZX-2013-003). T.K. is thankful to the CAS-TWAS Presidentprogram of the University of the Chinese Academy of Sciencesfor financial support.
■ REFERENCES(1) Wuttig, M. Nat. Mater. 2005, 4, 265−266.(2) Yu, Y.; Nakano, M.; Ikeda, T. Nature 2003, 425, 145.(3) Champagne, B.; Plaquet, A.; Pozzo, J. L.; Rodriguez, V.; Castet, F.J. Am. Chem. Soc. 2012, 134, 8101−8103.(4) Samoc, M.; Gauthier, N.; Cifuentes, M. P.; Paul, F.; Lapinte, C.;Humphrey, M. G. Angew. Chem., Int. Ed. 2006, 45, 7376−7379.(5) Zheng, H. M.; Rivest, J. B.; Miller, T. A.; Sadtler, B.; Lindenberg,A.; Toney, M. F.; Wang, L.-W.; Kisielowski, C.; Alivisatos, A. P. Science2011, 333, 206−209.(6) Wuttig, M.; Yamada, N. Nat. Mater. 2007, 6, 824−832.(c) Zhang, W.; Xiong, R. G. Chem. Rev. 2012, 112, 1163.(7) Zhang, Yi; Liao, W.-Q.; Ye, H.-Y.; Fu, D.-W.; Xiong, R.-G. Cryst.Growth Des. 2013, 13, 4025−4030.(8) Ge, J. Z.; Fu, X. Q.; Hang, T.; Ye, Q.; Xiong, R. G. Cryst. GrowthDes. 2010, 10, 3632.(9) Ye, Q.; Akutagawa, T.; Hoshino, N.; Kikuchi, T.; Noro, S.; Xiong,R. G.; Nakamura, T. Cryst. Growth Des. 2011, 11, 4175.(10) Fu, D. W.; Zhang, W.; Cai, H. L.; Ge, J. Z.; Zhang, Y.; Xiong, R.-G. Adv. Mater. 2011, 23, 5658−5662.(11) Ji, C. M.; Sun, Z. H.; Zhang, S. Q.; Chen, T. L.; Zhou, P.; Tang,Y. Y.; Zhao, S. G.; Luo, J. H. J. Mater. Chem. C 2014, 2, 6134−6139.(12) Zhou, Y.; Chen, T.; Sun, Z.; Zhang, S.; Ji, C. M.; Song, C.; Luo,J. Chem. - Asian J. 2015, 10, 247−251.(13) Fu, D. W.; Cai, H. L.; Liu, Y. M.; Ye, Q.; Zhang, W.; Zhang, Y.;Chen, X. Y.; Giovannetti, G.; Capone, M.; Li, J. Y.; Xiong, R.-G. Science2013, 339, 425−428.(14) Fu, D. W.; Cai, H. L.; Li, S. H.; Ye, Q.; Zhou, L.; Zhang, W.;Zhang, Y.; Deng, F.; Xiong, R.-G. Phys. Rev. Lett. 2013, 110, 257601−1−257601−5.(15) Zhang, W.; Ye, H.-Y.; Cai, H.-L.; Ge, J.-Z.; Xiong, R.-G.; Huang,S.-D. J. Am. Chem. Soc. 2010, 132, 7300−7302.(16) Sun, Z. H.; Luo, J. H.; Chen, T. L.; Li, L. N.; Xiong, R.-G.;Tong, M.-L.; Hong, M. C. Adv. Funct. Mater. 2012, 22, 4855−4861.(17) Shi, X. J.; Luo, J. H.; Sun, Z. H.; Li, S. G.; Ji, C. M.; Li, L. N.;Han, L.; Zhang, S. Q.; Yuan, D. Q.; Hong, M. C. Cryst. Growth Des.2013, 13, 2081−2086.(18) Li, S. G.; Luo, J. H.; Sun, Z. H.; Zhang, S. Q.; Li, L. N.; Shi, X. J.;Hong, M. C. Cryst. Growth Des. 2013, 13, 2675−2679.(19) Mishima, N. J. Phys. Soc. Jpn. 1984, 53, 1062−1070.(20) Itoh, K.; Mishima, N.; Nakamura, E. J. Phys. Soc. Jpn. 1981, 50,2029−2036.(21) Suzuki, E.; Shiozaki, Y. Phys. Rev. B: Condens. Matter Mater. Phys.1996, 53, 5217−5221.(22) Frazer, B. C.; Mckeown, M.; Pepinsky, R. Phys. Rev. 1954, 94,1435−1439.(23) Tang, Y. Y.; Ji, C. M.; Sun, Z. H.; Zhang, S. Q.; Chen, T. L.;Luo, J. H. Chem. - Asian J. 2014, 9, 1771−1776.(24) Tilborg, A.; Leyssens, T.; Norberg, B.; Wouters, J. Cryst. GrowthDes. 2013, 13, 2373.(25) Sarma, B.; Thakuria, R.; Nath, N. K.; Nangia, A. CrystEngComm2011, 13, 3232.(26) Sun, Z. H.; Luo, J. H.; Zhang, S. Q.; Ji, C. M.; Zhou, L.; Li, S. H.;Deng, F.; Hong, M. C. Adv. Mater. 2013, 25, 4159−4163.
(27) Sun, Z. H.; Li, S. H.; Zhang, S. Q.; Deng, F.; Hong, M.; Luo, J.H. Adv. Opt. Mater. 2014, 2, 1199−1205.(28) Sun, Z. H.; Wang, X.; Luo, J.; Zhang, S.; Yuan, D.; Hong, M. C.J. Mater. Chem. C 2013, 1, 2561.(29) Tang, Y. Y.; Sun, Z. H.; Ji, C. M.; Li, L.; Zhang, S. Q.; Chen, T.L.; Luo, J. H. Cryst. Growth Des. 2015, 15, 457−464.(30) Zhou, P.; Sun, Z. H.; Zhang, S. Q.; Ji, C. M.; Zhao, S. G.; Xiong,R. G.; Luo, J. H. J. Mater. Chem. C 2014, 2, 2341−2345.(31) Ji, C. M.; Sun, Z. H.; Zhang, S. Q.; Chen, T. L.; Zhou, P.; Luo, J.H. J. Mater. Chem. C 2014, 2, 567−572.(32) Asghar, M. A.; Ji, C. M.; Zhou, Y.; Sun, Z. H.; Khan, T.; Zhang,S. Q.; Zhao, S. G.; Luo, J. H. J. Mater. Chem. C 2015, 3, 6053.(33) Sheldrick, G. M. SHELXL97, Program for Crystal StructureRefinement; University of Gotttingen, Gottingen, 1997.(34) Besara, T.; Jain, P.; Dalal, N. S.; Kuhns, P. L.; Reyes, A. P.;Kroto, H. W.; Cheetham, A. K. Proc. Natl. Acad. Sci. U. S. A. 2011, 108,6828−6832.(35) Jain, P.; Ramachandran, V.; Clark, R. J.; Zhou, H. D.; Toby, B.H.; Dalal, N. S.; Kroto, H. W.; Cheetham, A. K. J. Am. Chem. Soc.2009, 131, 13625−13627.(36) Cai, H. L.; Zhang, Y.; Fu, D. W.; Zhang, W.; Liu, T.; Yoshikawa,H.; Awaga, K.; Xiong, R.-G. J. Am. Chem. Soc. 2012, 134, 18487−18490.(37) Jakubas, R.; Piecha, A.; Pietraszko, A.; Bator, G. Phys. Rev. B:Condens. Matter Mater. Phys. 2005, 72, 104−107.(38) Kulicka, B.; Kinzhybalo, V.; Jakubas, R.; Ciunik, Z.; Baran, J.;Medycki, W. J. Phys.: Condens. Matter 2006, 18, 5087−5104.(39) Williams, G. Chem. Rev. 1972, 72, 55−69.(40) AlegrIa, A.; Mitxelena, O.; Colmenero, J. Macromolecules 2006,39, 2691−2699.
