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Polymer 52 (2011) 3671e3676

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Polymer

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Chirality amplification of syndiotactic polystyrene induced circular dichroismchiral film in d form upon annealing

Kai Zheng a, Ruigang Liu a,*, Hongliang Kang a, Xia Gao b, Deyan Shen a, Yong Huang a,c,**

a Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory of Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, ChinabBeijing Centre of Physical and Chemical Analysis, Beijing Academy of Sciences and Technology, Beijing 100089, ChinacNational Research Center for Engineering Plastics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China

a r t i c l e i n f o

Article history:Received 14 December 2010Received in revised form3 June 2011Accepted 3 June 2011Available online 12 June 2011

Keywords:Syndiotactic polystyreneInduced circular dichroismChirality

* Corresponding author. Tel.: þ86 10 82618573; fax** Corresponding author. Laboratory of Polymer PhNational Laboratory of Molecular Sciences, Institute ofof Sciences, Beijing 100190, China.

E-mail address: rgliu@iccas.ac.cn (R. Liu).

0032-3861/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.polymer.2011.06.007

a b s t r a c t

The induced circular dichroism (ICD) syndiotactic polystyrene (sPS) chiral films in sPS d phase wereprepared and the chirality of the ICD sPS films upon annealing at different temperature was investigated.The sPS d crystals in the ICD films transform into g and then into a crystals as those of achiral sPSd crystals during annealing at different temperature. The CD signal intensity at l of 200 nm remainsunchanged during the d to g crystalline transformation, but enhanced tremendously during the g toa crystalline transformation. The amplification CD signal attributes to the closer package of the phenolrings in sPS a phase than that in sPS d and g phases. The lamellae grown at a temperature above the g toa transition follow the chiral aggregation structure, which also contribute to the amplification of the CDsignal.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

The interaction between chiral and achiral compounds canresult the induced circular dichroism (ICD) for the achiral coun-terpart [1]. The ICD has been investigated extensively. In fact, thissuitable cavity method has been extensively used in cyclodextrin[2,3] and some p-conjugated oligomers and biomimetics oligomers[4e8]. In general, upon noncovalent binding to a non-racemicguest, the chirality is transferred to the receptors, resulting in thegeneration of one of the enantiomeric or diastereomeric twisted orhelical conformers, thus producing a characteristic induced circulardichroism (ICD) in the absorption region of the receptors [9].

Syndiotactic polystyrene (sPS) shows a complex polymorphismbehavior, and four crystal forms, denoted as a, b, g and d, have beenreported [10,11]. Following the nomenclature proposed by Guerraet al. [12], the a and b forms are characterized by chains in thetrans-planar conformation, whereas the g and d forms containchains in the s(2/1)2 helical conformation [12e15]. The d form canbe obtained by casting films from solution or exposing the glassy

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sample to organic solvent molecules and forming a complexbetween the polymer and the solvent [12,16e24]. The g form can beobtained by annealing the d form to purge away the solventmolecules [17,18,25e31]. In particular, the d crystalline phase (s(2/1)2 helices) of sPS is nanoporous since it contains two identicalcavities and eight styrene monomeric units per unit cell, whichsupply a volume of nearly 0.12 nm3 cavity for selectively absorbingsuitable guest molecules [32e35]. The specific character of sPSallows to prepare chiral sensing sPS films via the induction of non-racemic compounds, by which ICD sPS thin films can be obtainedthrough guest-induced re-crystallization of sPS d form leading tonon-racemic crystallites. The chirality of aggregates can becontrolled by the choice of guest chirality [36]. Such ICD sPS filmcould be applied in chirooptical devices and data storage systems[37,38]. The presence of significant p-electron interactions amongphenyl groups in the dissymmetric environment of the one screwsense helical sPS molecule aggregates leads to the Cotton effect inthe ultraviolet region [39e42]. The macromolecular helicitymemory is stable and lasted unless the molten or dissolved.

The way chirality expressed depends not only on the chiralinformation encoded in the molecules or on the level of organiza-tion. It may also be triggered or altered by changes in the localenvironment of the aggregate, such as polarity, the presence ofspecific ions, pH, or temperature [43]. It is necessary to consider theeffect of these changes on induced chiral structures for clarifyingthe ICD mechanism. It is well-known that the sPS d form crystals

Fig. 1. CD spectra of the as-spin-coated sPS film and sPS film treated with (R)-carvonevapor.

K. Zheng et al. / Polymer 52 (2011) 3671e36763672

transform to the g form by heating or annealing above the glasstransition temperature, and the g form transforms into the a formby heating above 190 �C [12,27,28]. The solvent is removed whilethe TTGG conformation is retained during the d-g phase transition,while the helical TTGG conformation transforms into TTTT confor-mation during the g-a phase transition. The effects of rearrange-ment of the sPS chains during the heating or annealing on thechirality of the ICD sPS films is still absent in literatures.

In this work, we investigated the chirality of the ICD sPS filmsduring the heating procedure. The influence of the chain confor-mation of sPS on the chirality of sPS film will be discussed. Ourresults may be helpful in assignment of the chirality of molecules atmolecular and supramolecular levels, and application as chiroop-tical devices and chirality-responsive polymer materials.

2. Experiment part

2.1. Materials

Syndiotactic polystyrene (sPS, Mw 317,000 g/mol, IdemitsuPetrochemical Co. Ltd.) was used as received. (R)-carvone (98%, ee98%, Alfa Aesar) was used without further purification. Otherchemicals and solvents were obtained from local suppliers and areall analytical grade.

2.2. Samples preparation

The chiral sPS thin films were prepared as follows. Certainamount of sPS and chloroform were mixed together and heatednear the boiling point of chloroform in a sealed test tube to preparethe homogenous sPS/CHCl3 solution with sPS content of 0.8 wt.%.The sPS thin films were prepared by spin-coating (2000 rpm) fol-lowed by exposing to (R)-carvone vapor for several minutes atroom temperature. Then the thin films dried under vacuum toremove the absorbed (R)-carvone. The films was-spin-coated onquartz plate for CD, Laser-Raman, and AFM experiments, gold-coated glass for reflection-absorption infrared (RAIR) experi-ments, carbon-coated mica for TEM observation, and mica forWAXD and DSC measurements. The thickness of thin filmsmeasured by AFM was about 100 nm (Fig. S1). The thermalannealing experiments were performed using a Linkam (C1 94) hotstage. During the process of thermal annealing, the samples wereheated to 140, 160, 180, 200, 220 �C at the heating rate of 2 �C/min,holding for 30 min prior to slowly cooling to room temperature forcharacterizations.

2.3. Measurements

Circular dichroism (CD) spectra were measured using a JASCO J-815 spectropolarimeter. All measurements were performed asfollows [36]: single scan, continuous scanning mode (350-190 nmrange), 200 nm/min scanning speed, 2 nm slit width, 0.2 nm datainterval, vertical scale in autoranging mode, no baseline correction.The CD data were expressed as ellipticity (one mdeg equals0.001deg). Reflection-absorption FTIR spectra were collected witha Bruker EQUINOX 55 FTIR spectrometer equipped with a mercurycadmium telluride (MCT) detector. The measurements wereobtained by average 1024 scans and at a resolution of 2 cm�1. Theincidence angle was fixed at 83� for the best signal recording.Raman spectroscopic measurements were carried out ona Renishaw InVia Raman microscope. The excitation lines are514 nm with a power of 10 mW.

Differential scanning calorimetric (DSC) were carried out ona TA differential scanning calorimeter (Q2000 series) undera flowing nitrogen atmosphere. For DSC experiments, the chiral sPS

thin films on mica were collected by peeling from the micasubstrate and about 200 thin films are needed for each DSCmeasurement. The achiral sPS films were also prepared by solutioncasting for comparison.

Wide angle x-ray diffraction (WAXD) experiments were per-formed on a Bruker D8 Focus diffractometer with a ceramic X-raytube (Cu Ka, 2.2 kW) as the X-ray source. The samples for WAXDmeasurements were prepared as follows: the as-prepared chiralsPS thin films on mica, or the chiral films annealed at 160 or 220 �Cfor 30 min and cooled to room temperature, were collected bypeeling from the mica substrate. About 200 films were collected foreach sample. Transmission electron microscopy (TEM) was carriedon a JEOL JEM-2200FS (Japan) at an accelerating voltage of 200 kV.

3. Results and discussion

The as-spin-coated sPS films from chloroform solution aregenerally resulted sPS d form crystals, inwhich the sPS chains are inTTGG helical conformation [12,16e21]. Fig. 1 shows the typical CDspectra of the as-spin-coated sPS films and those treated with (R)-carvone vapor. The results show that no CD signal was observed as-spin-coated sPS films, which indicates that the as-spin-coated filmis non-chirality. Whereas the films treated with the (R)-carvonevapor show CD signals on the CD spectra (Fig. 1). The major Cottonband locates at about 200 nm and a minor Cotton band withopposite sign locates at 223 nm. The main Cotton band at about200 nm comes from the helicity of syndiotactic polystyrene chainsof the crystalline phase [36,39], while the minor Cotton band withopposite sign at 223 nm is the split Cotton effect of the majorCotton band. The chiral sPS films prepared by this method are in itsd form phase [37] and there is no axial orientation [36,44]. Theabove results are similar to those in literature [36].

The sPS d form crystals will transform to the g form by heatingor annealing above the glass transition temperature, and the g formtransforms to the a form at around 200 �C [12,27,28]. Fig. 2 showsthe DSC heating curves of the as-spin-coated sPS thin films andchiral sPS films. For the thin chiral sPS films (Fig. 2a), there is a wideendo peak at about 90 �C followed by an exdo peak at around109 �C on the DSC heating curve, which attributes to the meltingand re-crystallization during the d to g transition of sPS crystals.Whereas for the sPS thin achiral sPS films, the endo and exdo peaks

Fig. 2. DSC curves of the (a) sPS chiral films and (b) achiral casting sPS films.

K. Zheng et al. / Polymer 52 (2011) 3671e3676 3673

are at around 99 and 106 �C, respectively during the d to g transi-tion. The difference the chiral thin films have been recrystallizedduring the treatment of (R)-carvone vapor, in which the d crystalsare much more perfect than those in untreated achiral thin films.Moreover, there should be some solvent molecules trapped in theclathrate d phase, which is generally evaporated above the glasstransition temperature of sPS. The chiral and achiral sPS films havethe similar g to a transition temperature and the melting point, atabout 200 and 270 �C, respectively. The crystal transition of sPS inthe ICD sPS film was confirmed by WAXD experiments of the as-prepared sPS thin films and those films annealed at 160 and220 �C for 30 min as shown in Fig. 3. At least 200 sPS thin films areneeded for each WAXD measurement. The result in Fig. 3 confirmsthe crystal transition of sPS in the chiral thin films according to thecharacteristic diffraction peak of sPS crystals [12]. The results showthat the as-prepared chiral sPS films have a quite low crystallinity(Fig. 3a), and the crystallinity of the sPS increased upon theannealing (Fig. 3b and c).

Fig. 3. WAXD curves of (a) as-prepared chiral sPS films and the chiral films annealed at(b) 160 and (c) 220 �C for 30 min.

Fig. 4 shows the CD spectra of the chiral sPS film annealed atdifferent temperature and the peak intensity at 200 nm as a func-tion of annealing temperature. Same sPS film was used for the CDmeasurements in order to avoid the influences of the thickness andvariation of the sPS films on the CD signal. The results show that thechirality of the ICD sPS film remains unchanged at all annealingtemperature below the melting point of sPS. Moreover, when thechiral sPS film was annealed at a temperature below 180 �C, theintensity of the Cotton band at 200 nm remains unchanged,whereas the intensity of the Cotton band at 200 nm increasesobviously when the film was annealed at 200 �C or above. Theintensity is nearly doubled to its original value when the film wasannealed at 220 �C (Fig. 4b). While the Cotton band at 223 nm shiftsslightly to the higher wavelength (Fig. 4a). When the chiral sPS filmwas melted, e.g. at a temperature of 270 �C, the chirality of the filmwere destroyed and cannot be recovered after the films werecooled to room temperature (Fig. 4). The results show that the d-gcrystal transformation has no effect on the CD signal of the ICD sPSfilm, even though the crystallinity increased when the sPS chiralfilms were annealed at a temperature between 100 and 190 �C(Fig. 3). However, after the g-a crystal transformation, duringwhich

Fig. 4. CD spectra of chiral sPS films annealed at different temperature (a) and thepeak height of Cotton band of 200 nm as a function of annealing temperature.

Fig. 6. Side view of models of sPS in (a) TTGG helical conformation (d form), (b) trans-planar zig-zag TTTT conformation (a form).

K. Zheng et al. / Polymer 52 (2011) 3671e36763674

the sPS chains in the TTGG helical conformation transferred intothe zig-zag planar (TTTT) conformation, the intensity of CD signal at200 nm increases with the annealing temperature may be due tothat the crystallinity of the sPS a form crystals increased with theincreasing annealing temperature.

The changes of the chain conformation of sPS during theannealing of the ICD sPS films were investigated by the laser-Raman and RAIR spectra. Fig. 5a shows the laser-Raman spectraof the ICD sPS films annealed at different temperatures. The bandsat 1253 and 802 cm�1 correspond to TTGG conformation of sPSchains, and the bands at 1320 and 773 cm�1 correspond to the all-trans conformation of sPS chains [20,45]. The results indicate thatthe Raman spectra of the ICD sPS film remains unchanged atannealing temperature below 190 �C, whereas the bands at 1320and 773 cm�1 appear with the simultaneously disappear of thebands at 1253 and 802 cm�1 (Fig. 5a). RAIR spectra also indicatesthat the absorbed peak at 2850 cm�1 shifts to 2845 cm�1 anda shoulder peak at 2917 cm�1 appears at an annealing temperatureabove 190 �C (Fig. 5b). It is known that the absorption bands at 2917

Fig. 5. Laser-Raman (a) and RAIR (b) spectra of the as-prepared sPS chiral film andthose films annealed at the indicated temperature. The as-prepared films were spin-coated on quartz surface and treated with (R)-carvone vapor.

and 2845 cm�1 relate to the trans-planar sPS chains in the sPSa form crystals, whereas the absorption peak at 2850 cm�1 comesfrom the sPS TTGG conformation [46,47]. Both Raman and IR resultsindicated that the conformation of sPS molecular chains changedfrom helix to trans-planar when the films were annealed ata temperature above 190 �C. Whereas there is no chain conforma-tion changes was detected by Raman and RAIR experiments.

Fig. 7. CD spectra (a) and the peak height of Cotton band of 200 nm as a function ofannealing time for the chiral sPS films annealed at 220 �C.

Fig. 8. TEM images of (a) achiral as-spin-coated sPS film from chloroform solution, (b) chiral sPS film treated with (R)-carvone vapor, (c) achiral and (d) chiral sPS film heated to220 �C at heating rate of 2 �C/min.

K. Zheng et al. / Polymer 52 (2011) 3671e3676 3675

The above results indicate that the enhancement of the CDsignal upon annealing temperature may be attributed the followingtwo points. One is attributed to the chain conformation. The helicald phase is dominated by the formation of parallel layers of s(2/1)2helices [32,48,49], leading to short intermolecular contactsbetween only few phenyl CH group, as a result of the tendency ofrows of parallel s(2/1)2 helices with minimum interaction andmaximum interplanar distances to be parallel to the film plane [50].The distance between two neighbored phenyl rings, identityperiod, is 7.7 Å (Fig. 6a). While the trans-planar a phase is domi-nated by the parallel stacking of the phenyl rings, leading tostronger interactions between phenyl groups than helical confor-mation phase. The distance between two neighbored phenyl ringsis 5.1 Å (Fig. 6b). In organic stereochemistry, the amplitude of splitCotton effects is inversely proportional to the square of inter-chromophoric distances [51]. Therefore, the enhancement of theintensity of the CD signal at 200 nm for the sPS chiral films ina form than those in d form could attribute to the closer package ofthe phenol rings in sPS a form crystals. Moreover, the ICDphenomena and their high thermal stability are due to guest-induced re-crystallization of the sPS host to form non-racemiccrystallites. The morphology of the non-racemic crystallitesremains nearly unaltered also after the helix / trans-planartransformation of sPS [36]. However, more sPS non-racemic crys-tallites were formed by the induction of the original non-racemic

crystallites after the g/a, which lead to the obvious enhance-ment of the ICD signal. furthermore, the package of the sPS chainsin a form crystals is more dense than that in d and g form [50],which also can contribute to the enhancement of the intensity ofCD signal.

For the purpose to clarify the effect of the sPS chain package onthe CD signal of the sPS ICD film, the ICD sPS films were annealeddirectly at 220 �C, at with the sPS d crystals transform into a formcrystals directly. The CD spectra at different of annealing time andthe CD signal intensity at wavelength of 200 nm are shown inFig. 7. The results suggest that the chirality of the ICD sPS filmremains unchanged during the annealing procedure. However, theintensity of the CD signal at wavelength of 200 nm was enhancedsharply first and then increased slowly upon the processing of theannealing.

The effects of the annealing on the chirality of the sPS ICD filmswere further investigated. The annealing temperature was set at220 �C. During the annealing procedure, the sPS d form crystalstransform into a form directly and then follow by the growth of thesPS a crystals. Fig. 7 shows the CD spectra and the intensity of theCD signal at 200 nm as a function of annealing time. The resultssuggest that the chirality of the ICD sPS film remains unchangedduring the annealing procedure (Fig. 7a). Whereas the peakintensity at wavelength of 200 nm were increased sharply at thebeginning, which could attribute the close package of the neighbor

K. Zheng et al. / Polymer 52 (2011) 3671e36763676

phenol rings in sPS a form than that in sPS d form. Furtherannealing the sample leads to slowly increase in the CD signalintensity at 200 nm, which suggests that more sPS units packed ina chiral supramolecular structure induced by the original chiralstructure during the growth procedure of sPS a crystals. It should benoted that the crystallinity and the anisotropy of the crystals mayalso contribute to the variation on the CD signal. Therefore, lineardichroism (LD) experiments may help for the clarification thatthere is no molecular orientation effect on CD signal. However,there is no specific orientation of the molecular in the spin-coatingthin films as that clarified in literatures [36,37]. Moreover, the TEMmicrophotograph also shows that there is no specific molecularorientation in the spin-coating thin films (Fig. 8). Therefore, themolecular orientation effect on CD signal can be neglected.

Fig. 8 shows the morphology of the sPS films at different stages.It is clear that rod-like crystallites of sPS can be observed in the as-spin-coated achiral film from chloroform solution (Fig. 8a), which issimilar to those in literature [36]. After the film was treated by (R)-carvone vapor, more sPS larger crystallites formed in the ICD sPSfilm (Fig. 8b). After the chiral and achiral sPS films were annealed at220 �C for 30 min, long branch-like lamellae can be observed(Fig. 8c). Whereas in chiral sPS films, worm-like crystals can beobserved (Fig. 8d), which suggests that the ICD structure in thespin-coated film after treated with (R)-carvone vapor is remained.The new crystal lamellae formed during the annealing procedurefollow with the chiral structure of the ICD supramolecular aggre-gations, which lead the less perfection of the resultant sPS a crys-tals. Combined with the above results and discussion, it can beconcluded that the amplification of CD signal of ICD sPS film beingannealed at different temperature could attribute follow reasons.One is the closer package of the phenol rings sPS a form crystalsthan that in d crystals, which is mainly contribute to the trans-formation of the s(2/1)2 helical conformation into trans zig-zagplanar conformation. The other is attributed to the inducedpackage of the sPS lamellae into chiral supramolecular structure bythe precursor chiral supramolecular aggregation during the growthof the sPS a crystals.

4. Conclusion

The ICD sPS chiral films were prepared by treating the thin sPSspin-coated from chloroform solution with (R)-carvone vapor atroom temperature. The ICD chiral sPS films are in d form crystallinephase. The sPS d form crystalline phase transform into g and theninto a form crystalline phase as normal sPS d crystals. The chiralityof the ICD sPS chiral film remains unchanged upon heatingprocedure before the melting of the sPS a crystals. The CD intensityat the wavelength of 200 nm remains unchanged during the d to gcrystalline transformation, but enhanced tremendously during theg to a crystalline transformation. The enhancement of the CDintensity attributes to the closer package of the phenol rings in sPSa crystals than that in sPS d and g crystals. Moreover, the crystallinelamellae grown at a temperature above the g to a transition followthe chiral aggregation structure and also enhanced the CD intensityupon the crystalline growth procedure. The lamellae of the sPSa crystalline phase in the chiral sPS film are less in perfection thanthat in sPS achiral film.

Acknowledgements

We are thankful for grants from the National Natural ScienceFoundation of China (Grant No. 50821062).

Appendix. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.polymer.2011.06.007.

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