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Preparation of Oriented Liquid-Crystalline/IsotropicBlock Copolymer Films with Parallel or PerpendicularArrangement of Smectic Layers and LamellarMicrodomains

Pedro Figueiredo,1 Wolfram Gronski,* 1 Michael Bach2

1 Institut für Makromolekulare Chemie, Albert-Ludwigs-Universität Freiburg, Stefan-Meier-Straße 31,D-79104 Freiburg, GermanyFax: +49-761-203-6319; E-mail: wolfram.gronski@makro.uni-freiburg.de

2 Max-Planck-Institut für Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany

Keywords: block copolymers; liquid-crystalline polymers (LC); morphology; orientation;

IntroductionOne of the most interesting aspects of microphase-sepa-rated block copolymers is the possibility to obtain highlyoriented morphologies. Lamellar and cylindrical domainsin block copolymers have been macroscopically alignedinto a parallel, or a transient transverse orientation bycontrolling frequency and strain in oscillatory shear.[1, 2]

The solvent-roll-casting technique has been successfullyapplied to a number of block copolymer systems[3] toobtain perpendicular orientations or single crystal-oriented block copolymers. The synthesis of LC/I blockcopolymers consisting of a liquid crystalline and an iso-tropic polymer opened a new area of research in the studyof orientation phenomena of these systems.[4] The mainquestion of interest is how the order on the length scale ofthe microphase-separated lamellar or cylindrical blockcopolymer morphology and the order on the length scaleof a layered smectic LC phase are interrelated and under

which conditions well-oriented systems can be generatedthat are macroscopically ordered on both length scales.Investigations of LC/I block copolymers having smecticLC phases and lamellar microdomains suggest that thepreferred orientation of the smectic layers of the LCphase in bulk is perpendicular to the domain interface.[4–6]

A review summarizing the results of several researchgroups on this topic was given by Fischer and Poser.[7] Anorientation of the morphology parallel to the smecticlayers was found only in cases with oriented elliptical(deformed spherical),[8] cylindrical,[7] and lamellardomains.[9] Recently, a mixed orientation of smecticlayers parallel and perpendicular to the interface wasobserved in a lamellar LC block copolymer.[10]

In this work we show that the hierarchical morphologi-cal order of a given LC/isotropic block copolymer can bemanipulated by preparation conditions to yield either asystem in which the smectic layers are oriented parallel

Communication: Films of a symmetric liquid-crystalline/isotropic block copolymer consisting of a smectic LCside-chain polymer and polystyrene were prepared by sol-vent casting from solution and from the isotropic melt. Byannealing the solvent-cast film in the SA phase an orientedmicrophase-separated film of lamellar morphology wasobtained in which both the lamellae of the block copoly-mer and the smectic layers of the LC block were orientedparallel to the film surface. A lamellar morphology withperpendicular orientation of lamellae and smectic layerswas generated by cooling the block copolymer from themelt.

Macromol. Rapid Commun. 2002, 23, No. 1 i WILEY-VCH Verlag GmbH, 69451 Weinheim 2002 1022-1336/2002/0101–0038$17.50+.50/0

Macromol. Rapid Commun. 2002, 23, 38–43

Preparation of Oriented Liquid-Crystalline/Isotropic Block Copolymer Films ... 39

to the internal interfaces or a system with perpendicularorientation.

Experimental Part

Materials and Preparation of Samples

A hybrid liquid-crystalline/isotropic (LC/I) diblock copoly-mer was synthesized via the conversion of the poly(1,2-buta-diene) (PB) block of polystyrene-block-polybutadienediblock (PS-PB) copolymer prepared by living anionic poly-merization. Side-chain mesogens were introduced by hydro-boration of the PB block and subsequent esterification withactivated mesogenic units.[11] The copolymer has a chiral (S)-2-methylbutyl-4-biphenylcarboxylate mesogenic side groupas shown in Scheme 1.

The molecular weight is M—

n = 46.0 kg/mol and M—

w/M—

n =1.07 as measured by means of osmometry and SEC, respec-tively. The molecular weight of the LC block is M

—n =

26.1 kg/mol, as estimated by means of 1H NMR spectro-scopy. From the molecular weight data and densities of PS(1.061 g/cm3) and the LC homopolymer (1.407 g/cm3) thevolume fraction of the blocks was determined to be 0.5. Thisvalue was corroborated by the extinction of the second orderreflection of the SAXS pattern of the microphase-separatedsystem.

Solvent-cast films with a thickness of 1.1 mm were pre-pared by slow evaporation of the solvent in a Petri dish (u =2.2 cm) from a 4 wt.-% toluene solution during severalweeks at room temperature. The films were further dried invacuum for 2 weeks to remove residual solvent. The dryfilms will be referred to as sample I throughout this work.Films prepared by annealing sample I at 1008C for 105 hwill be designated as specimen II. The preparation of films Iand II by deswelling resulted in oriented samples with PSlamellae or smectic layers oriented parallel to the surfaces.In addition to these preparations, films of 2.0 mm thicknesswere prepared from the isotropic melt by slowly cooling thediblock from 1758C to 20 8C at a rate of 0.38C/min. Thisspecimen was not oriented and will be referred to as speci-men III. Specimen IV was obtained by oscillatory shear ofspecimen III during cooling from the melt at 1508C betweenmicroscope slides coated with Teflon. The period of coolingwas about 4 min. Shearing was carried out by hand as longas the increase of viscosity allowed.

Methods

The global morphology of block copolymer samples wasinvestigated with small-angle X-ray scattering (SAXS) usingCuKa radiation with wavelength k = 1.54 nm. Measurements

on unoriented samples were performed at room temperaturewith a Kratky compact camera (PAAR, Graz, Austria)equipped with a one-dimensional detector. The nature anddegree of microphase orientation were investigated with aSAXS equipment with pinhole collimation and 2D detection.An area gas detector with 5126512 pixels and 0.117 mmspatial resolution in each direction was used, and the sampleto detector distance was set to 119 cm. The acquisition timewas 1 h. 2D patterns were obtained and processed by meansof general Area Detector Diffraction Software from Siemens.Wide-angle X-ray scattering (WAXS) experiments were car-ried out with a Philips PW 1730 and CuKa radiation filteredby a graphite monochromator. The incident beam was paral-lel to the surface of the film. The scattered X-ray intensitywas detected by image plates (Fa. Schneider, Freiburg, Ger-many). The order parameter, S, of the smectic layers was cal-culated by an azimuthal scan applied to the smectic layerreflections.[12] WAXS and SAXS intensities were recorded inarbitrary units as function of scattering angle 2H and theabsolute value of the scattering vector s = 2(sinH)/k, respec-tively.

Results and Discussion

Microphase Separation and LC Phase Behavior

The thermal behavior of the specimens prepared by dif-ferent routes is displayed by the differential scanningcalorimetry (DSC) traces in Figure 1. While the annealedsolvent-cast specimen II, and specimen III, obtained byslow cooling from the melt, exhibit LC phase transitionsSC* e SA at 498C and SA e I at 1068C, closely to those ofthe LC homopolymer, specimen I of the solvent-cast non-annealed sample exhibits a crystalline to smectic transi-tion at 688C and a broad transition to the isotropic phaseat about 908C. The latter occurs at a temperature lowerthan the isotropization temperatures of samples II and III,indicating that the smectic phase is less stable than in theas-cast specimen.

Scheme 1.

Figure 1. DSC heating scans at 48C/min: (a) solvent-cast sam-ple I, (b) annealed solvent-cast sample II, (c) sample III obtainedby slow cooling from the melt at 175 8C (SA, SC*, Cr denotesmectic A, chiral smectic C, and crystalline phases, respec-tively).

40 P. Figueiredo, W. Gronski, M. Bach

SAXS measurements in Figure 2 display a first-orderreflection for block copolymers prepared from the melt(sample III) and from solution after annealing in the SA

phase (sample II). This indicates microphase separationinto a microdomain morphology with a periodicity of25 nm and 28 nm for samples III and II, respectively. Atthe volume fraction of 50% the morphology is lamellar aswas also proven by means of transmission electron micro-scopy (TEM). It should be noted that the block copolymerunder investigation is not microphase-separated in theisotropic melt.[13] Upon cooling from the melt, micro-phase separation is induced by the transition from the iso-tropic melt to the SA phase. In contrast to samples III andII, the specimen prepared from solution without anneal-ing (sample I) does not show a Bragg peak in theobserved scattering range and is therefore not micro-phase-separated. Unlike samples III and II specimen I isoptically clear showing the absence of a grain structure,which is typical of microphase-separated block copoly-mers.

Although a microphase-separated lamellar morphologyon the typical scale of the dimensions of the block copo-lymer is absent in the as-cast film of sample I, the filmexhibits a layered morphology of smectic/crystallinestructure according to Figure 1a. The 2D WAXS patternof this film in Figure 3, taken at room temperature,demonstrates that also the smectic/crystalline layers areoriented parallel to the film surface. Orientation is causedby anisotropic deswelling of the film during solvent eva-poration.[14] This occurs because the dimensions of thefilm parallel to the film surface remain constant due toadhesion at the walls of the Petri dish. The order param-eter of smectic/crystalline layers derived from the azi-muthal intensity distribution is S = 0.60. After annealingthe smectic order increased to S = 0.68. The improvementof order is accompanied by microphase separation of thePS and LC blocks.

Information of the state of order of the liquid-crystal-line phase in the as-cast film is given by a radial scan ofthe 2D WAXS pattern in Figure 4a. We see reflections ofsmectic layers of first and second order (reflections 1 and2) in all samples, corresponding to a layer thickness of3.5 l0.1 nm. The same distance is calculated on the basisof a model for the mesogenic side group having an all-trans conformation of the spacer. Reflection 3 appearingin all samples is characteristic of a correlation of polymerbackbones of the LC and PS block. In microphase-sepa-rated specimens II and III (Figure 4b and 4c) the broadreflection at large angles originates from mesogen/meso-gen correlation. In non-annealed solvent-cast film I (Fig-ure 4a) this reflection is split into two reflections 4 and 5.

Reflection 4 corresponding to a distance of 0.41 nm isshifted to larger distance, while reflection 5 (0.39 nm) isshifted to a smaller distance with respect to the mesogen/mesogen correlation peak of samples II and III (Figure4b and 4c). Since specimen I is not microphase-sepa-rated, the observed splitting is probably caused by a dis-turbance of mesogen/mesogen correlation by interveningpolystyrene chains.

The disturbance of the smectic structure of specimen Iis also evidenced by the depression of the isotropizationtemperature and transition enthalpy in Figure 1a. Thepresence of a transition with high transition enthalpy inFigure 1a must then be associated with a smectic crystal-line phase that is coexisting with the disturbed smecticphase. The presence of crystallinity in specimen I is alsodemonstrated by reflection 6 in Figure 4a correspondingto a distance of 0.34 nm, which is believed to be a crystal-line reflection. This reflection is absent in samples II andIII (Figure 4b and 4c) showing microphase separation ofthe block copolymer (Figure 2).

Figure 2. 1D-SAXS curves at 208C in dependence of the abso-lute value of the scattering vector s = 2(sinH)/k of solvent-castsample I, annealed solvent-cast sample II, and sample III pre-pared by slow cooling from the melt. Oriented samples I and IIwere measured as powders, sample III was measured in bulk. Figure 3. 2D-WAXS pattern in dependence of scattering angle

2H of solvent-cast film I. The direction of the X-ray beam wasparallel to the film surface.

Preparation of Oriented Liquid-Crystalline/Isotropic Block Copolymer Films ... 41

Interrelation of Layered Structures on Two LengthScales

The lamellar microdomains and the smectic layers formlayered structures on two length scales. In the followingwe will discuss how these structures are interrelated andin which way a structured morphology is formed fromdifferent initial states from solution and from the melt.Starting from the isotropic melt of a LC block copolymer,two possibilities have to be considered: (i) microphaseseparation occurs prior to the LC transition or (ii) micro-phase separation is induced by the LC transition. For theblock copolymer discussed here it was shown that case(ii) is realized.[13] Coming from high temperature, thesmall interaction parameter of the present system isenhanced in a discontinuous way by the entropy decreaseat the LC phase transition. The stretching of the polymerchains in the pre-transitional state meets the favorableoblate chain conformation of the LC polymer chainsbetween the smectic layers. Therefore, in the phase-sepa-rated state, the main chain of the block copolymer inter-

sects the interface perpendicularly and the mesogens ori-ent parallel to the interface. This is the general situationthat has been observed in smectic block copolymers inmost cases. The proof that this situation is also found inthe present case is seen in Figure 5, showing the patternof smectic and lamellar reflections of specimen IV, whichwas sheared during cooling from the melt. It is seen thatthe smectic layers are oriented parallel to the shearingdirection and the polystyrene lamellae are oriented per-pendicular to the surfaces. This means that the mesogensare oriented parallel to the lamellar interface. A model ofthe respective orientation of polystyrene lamellae andsmectic layers is depicted in Figure 5. Because thevolume fraction of the blocks is exactly 0.5 the lamellarthickness of the LC lamellae is 12.5 nm according to theSAXS curve of the corresponding unoriented sample IIIin Figure 2. This thickness is incommensurate with thesmectic layer distance of 3.5 nm preventing the layers tobe oriented parallel to the polystyrene lamellae.

When the system is prepared from solution and annealedin the SA phase the thickness of the LC lamellae is 14 nmcorresponding to the symmetrical composition and to theobserved periodicity of 28 nm in Figure 2. In this case,

Figure 4. Radial scans of the 2D-WAXS patterns in depen-dence of scattering angle 2H of (a) solvent-cast film I, (b)annealed solvent-cast film II, and (c) of sample III obtained byslow cooling from the melt. For oriented samples I and II sym-bols F and f denote scans in meridian and equatorial direction,respectively.

Figure 5. 2D WAXS and 2D SAXS patterns of sample IVobtained by slow cooling from the melt under shear. The modelshows the perpendicular orientation of PS lamellae and smecticlayers, with the smectic layers oriented parallel to the film sur-face.

42 P. Figueiredo, W. Gronski, M. Bach

exactly 4 smectic layers fit into the lamellae of the LCblock, which would allow the parallel arrangement ofsmectic layers and block copolymer lamellae. Thisarrangement is in fact realized as proved by the 2D SAXSpattern of the oriented block copolymer film in Figure 6showing that both smectic layers and polystyrene lamellaeare oriented parallel to the film surfaces. For obtaining anoriented film the film had not to be sheared as in the pre-vious case because the deswelling of the block copolymerduring evaporation of the solvent is equivalent to uniaxialcompression and, therefore, leads to an oriented speci-men.[14] The corresponding model is also depicted in Fig-ure 6. We refrain from visualizing the chain backbones ofthe LC block because we have no information about theconformation of the polymer chains. By entropic reasons itis improbable that the chains are preferentially locatedbetween the smectic layers. More probable is that thechain flux at the interface is oriented perpendicular to theinterface as in isotropic block copolymers, i.e. the LCchains cross the smectic layers. The long spacer of theside-chain LC polymer allows for ample ways of packingmesogens and chain backbones.

The question is why not the same perpendiculararrangement of smectic layers and block copolymerlayers is formed by annealing the solution-cast film as bycooling from the melt. We propose the following expla-nation. During evaporation phase separation into a sol-vent-rich and a solvent-poor phase occurs. At the begin-ning, both phases are homogeneous. Because toluene is apreferential solvent for the polystyrene block the solvent-poor phase will segregate at some stage. We assume thatthis segregation occurs on a molecular scale and that theLC block crystallizes or forms highly ordered smecticlayers between segregated polystyrene blocks. We further

assume the formation of a smectic phase from the sol-vent-rich phase in which polystyrene chains are inter-mixed with the smectic layers.

Important for the final establishing of the annealedmorphology is the orientation induced by the equibiaxialstress imposed by the deswelling of the system. Duringannealing the orientation is preserved and the crystalline/smectic two-phase morphology reassembles into theoriented morphology in which PS lamellae and smecticlayers are parallel to each other (Figure 6). The initiallyparallel arrangement can be transformed into the perpen-dicular arrangement by heating to the melt and subse-quent cooling through the isotropic/smectic transitionaccompanied by shear (Figure 5).

ConclusionWe have demonstrated that LC/I block copolymers hav-ing lamellar morphology and a smectic LC phase can beprepared in two orientational states. In one state thesmectic layers are oriented parallel to the lamellae of theisotropic block, in the other they have perpendicularorientation. In each case the smectic layers are orientedparallel to the substrate. This means that in one case thelamellae of the isotropic block are oriented parallel and inthe other case they are oriented perpendicular to the filmsurfaces. We have not investigated yet whether the twosystems can also be prepared in an oriented state in whichthe smectic layers are oriented perpendicular to the sub-strate (book-shelf geometry), which would result in a per-pendicular arrangement of the lamellae of the isotropicblock in the first case and in a parallel arrangement in thesecond. Possible ways to achieve this would be to changethe temperature or frequency of shearing and therebychanging the response of the liquid-crystalline directorand of the polystyrene lamellae. This geometry is particu-larly interesting as it has been suggested that microdo-mains in LC block copolymers having SC* phases breakup the helical superstructure of this phase. If an orientedblock copolymer film is prepared in the bookshelf geo-metry by shear, a ferroelectric system may be obtainedthat is stabilized by the block copolymer microdo-mains.[15] In analogy to the well-known surface-stabilizedferroelectric LC (SSFLC) the microphase-stabilized fer-roelectric LC (MSFLC) may be an interesting alternativeto bistable ferroelectric switching devices.

Acknowledgement: The authors are grateful to the FreiburgMaterials Research Center and to the SFB 428 for financial sup-port. They also gratefully acknowledge the help of Mrs. B. Heckof the Department of Experimental Polymer Physics with SAXSmeasurements.

Received: April 10, 2001Revised: September 21, 2001Accepted: November 9, 2001

Figure 6. 2D WAXS and 2D SAXS pattern of annealed sol-vent-cast film II, and a model showing the orientation of PSlamellae and smectic layers parallel to the film surface.

Preparation of Oriented Liquid-Crystalline/Isotropic Block Copolymer Films ... 43

[1] U. Wiesner, Macromol. Chem. Phys. 1997, 198, 3319.[2] Z. R. Chen, J. A. Kornfield, S. D. Smith, J. T. Grothaus, M.

M. Satkowski, Science 1997, 277, 1248.[3] R. J. Albalak, E. L. Thomas, J. Polym. Sci., Part B: Polym.

Phys. 1994, 32, 341.[4] J. Adams, W. Gronski, Makromol. Chem., Rapid Commun.

1989, 10, 553.[5] G. C. L. Wong, J. Commandeur, H. Fischer, W. H. de Jeu,

Phys. Rev. Lett. 1996, 77, 5221.[6] M. Yamaha, T. Iguchi, A. Hirao, S. Nakahama, J. Wata-

nabe, Polym. J. 1998, 30, 23.[7] H. Fischer, S. Poser, Acta Polym. 1996, 47, 413.[8] A. Omenat, R. A. M. Hikmet, J. Lub, P. van der Sluis,

Macromolecules 1996, 29, 6730.

[9] W. Y. Zheng, R. J. Albalak, P. T. Hammond, Macromole-cules 1998, 31, 2686.

[10] A. Schneider, J. J. Zanna, M. Yamada, H. Finkelmann, R.Thomann, Macromolecules 2000, 33, 649.

[11] J. Sänger, W. Gronski, Macromol. Chem. Phys. 1998, 199,555.

[12] G. R. Mitchell, A. H. Windel, “Developments in Crystal-line Polymers”, Vol. 2, Elsevier, Essex, England 1988,p. 115.

[13] P. Figueiredo, Ph.D. Thesis, Freiburg 2001.[14] S. T. Kim, H. Finkelmann, Macromol. Rapid Commun.

2001, 22, 429.[15] G. Mao, J. Wang, K. Ober, M. Bremer, M. J. E. O’Rourke,

E. L. Thomas, Chem. Mater. 1998, 10, 1538.