Meridional Orientation in Biaxially Aligned Thin Films of Hairy-Rod

Aug 11, 2007 - Matti Knaapila , Helen L. Vaughan , Thomas P. A. Hase , Rachel C. ... Andrew J. Moad , Lee J. Richter , Michael F. Toney , Martin Heene...
0 downloads 0 Views 2MB Size
Download PDF
Meridional Orientation in Biaxially Aligned Thin Films of Hairy-Rod Polyfluorene Knaapila,*,†,‡

Hase,†

Torkkeli,#

Stepanyan,§

Matti Thomas P. A. Mika Roman Laurence Bouchenoire,⊥ Hyeun-Seok Cheun,| Michael J. Winokur,X and Andrew P. Monkman†

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 9 1706-1711

Department of Physics, UniVersity of Durham, South Road, Durham DH1 3LE, UK, MAX-lab, Lund UniVersity, POB 118, SE-22100 Lund, Sweden, Department of Physical Sciences, POB 64, FI-00014 UniVersity of Helsinki, Finland, Material Science Centre, DSM Research, POB 18, NL-6160 MD Geleen, The Netherlands, XMaS, European Synchrotron Radiation Facility, BP 220, F-38043, Grenoble, France, Department of Materials Science and Department of Physics, UniVersity of Wisconsin-Madison, Madison, Wisconsin 53706 ReceiVed October 19, 2006; ReVised Manuscript ReceiVed May 24, 2007

ABSTRACT: The uniaxial chain alignment and equatorial patterning of poly[9,9-bis(2-ethylhexyl)fluorene-2,7-diyl] (Mn ) 29 kg/ mol, Mw ) 68 kg/mol) thin films atop rubbed polyimide substrates have been studied by grazing incidence X-ray diffraction. This specific molecular weight yields among the highest observed levels of chain alignment and optical anisotropy and, after thermal annealing, the sample undergoes transformation to a highly textured crystalline hexagonal phase. The two dominant equatorial orientations (i.e., types I and II crystallites) are found to have almost identical meridional orientation distributions. The X-ray deduced orientation is in quantitative agreement with that obtained by optical absorption measurements. It is also shown that the equatorial ordering is paracrystalline in nature, and both type I and type II crystallites are similar in this respect. This equatorial scattering is superimposed on a background of a hexagonal phase polymer with a cylindrically isotropic orientation (type III). 1. Introduction The inherent molecular level anisotropy of π-conjugated polymers1-4 and π-conjugated discotic molecules5 leads to highly textured liquid crystalline or crystalline phases. Manipulation of this ordering process is an important feature in the design of these materials. In this context, a branched side chain poly[9,9-bis(2-ethylhexyl)fluorene-2,7-diyl] (PF2/6) (Figure 1) polymer is pertinent for the following reasons. First, it serves as an ideal model system for aligned self-organized structures.6 Second, it can be uniaxially aligned, resulting in a strong optoelectronic anisotropy.7 Technologically important examples can be found in polarized electroluminescence in light emitting diodes (LEDs)8 or enhanced charge carrier mobility in field effect transistors (FETs).9 PF2/6 is inherently a stiff helical polymer.10,11 At room temperature, monodisperse F2/6 oligomers adopt a frozen smectic liquid crystalline state,12 whereas polydisperse low molecular weight PF2/6 samples appear in a frozen nematic state. Once the molecular weight exceeds a threshold value, M* ≈ 10 kDa,13 PF2/6 then forms hexagonal unit cells.10 This phase exhibits a relatively large coherence length and is dominated by lattice imperfections of the second kind.14 Thus, the hexagonal structure is seen to be paracrystalline rather than genuinely crystalline.13 The structure is further affected by the thin film geometry and the presence of interfacial forces, which results in a measurable reduction of lattice constants in the outof-plane direction.15 These surface interactions also give rise to a distinctive biaxial alignment marked by uniaxial (meridional) chain alignment and * To whom correspondence should be addressed. Tel: +46-46-22-24306. Fax: +46-46-22-24710. E-mail: [email protected]. † University of Durham. ‡ Lund University. # University of Helsinki. § Material Science Centre, DSM Research. ⊥ XMaS, European Synchrotron Radiation Facility. | Department of Materials Science, University of Wisconsin-Madison. X Department of Physics, University of Wisconsin-Madison.

Figure 1. Chemical structure of PF2/6.

Figure 2. Experimental geometry and schematics of the aligned PF2/6 microstructure with crystallite types I-III. Assuming chain alignment (i.e., the c-axis) along the rubbing direction, then the equatorial and meridional directions may be defined by the (xy0) plane and z-axis, respectively.

a pronounced equatorial anisotropy. In this situation, which corresponds to the classification of Heffelfinger and Burton16 (Figure 2), the c-axis is defined as the direction along the rodlike backbone. The two dominant equatorial crystallite orientations, types I and II, have their respective a-axes parallel and perpendicular to the surface normal.15 These two dominant orientations form a mosaic texture and, in thin films, the crystallites extend through the entire thickness of the film.17 This paper has three objectives. First, both the lattice parameters and the meridional crystallite size have been shown

10.1021/cg0607295 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/11/2007

Chain Orientation in Polyfluorene Crystallite Types

Crystal Growth & Design, Vol. 7, No. 9, 2007 1707

to be qualitatively similar in types I and II crystallites,17 but the extent of their (uniaxial) meridional orientation has not been rigorously measured. If types I and II actually represented distinct crystal polymorphs (and their preferred orientation reflected this difference), one could potentially identify processing conditions that would yield single equatorial orientation films with better uniaxial alignment. Second, we wish to fully characterize both the equatorial and the meridional alignment of PF2/6 samples at a molecular weight (or, equivalently, a phase point) where the highest uniaxial chain alignment is achieved. This is critical from the standpoint of optoelectronic anisotropy. Finally, we note that grazing-incidence X-ray diffraction (GIXD) is a powerful complementary probe of chain alignment, and here we further demonstrate its utility in assessing uniaxial alignment in conjugated polymer thin films. The layout of this paper is as follows: First we outline a simple model that quantifies the extent of uniaxial alignment. Next, we present GIXD data from an oriented PF2/6 thin film using the molecular weight (MW) of Mn ) 29 kg/mol and Mw ) 68 kg/mol. This MW is situated just above the crossover point from a nematic liquid crystal state to the hexagonal phase and corresponds to the MW that yields the highest level of uniaxial alignment. Additional GIXD data and subsequent analysis quantitatively assess the meridional orientation distribution. The GIXD-derived meridional orientation results are then compared to results deduced from optical spectroscopy experiments using the above mentioned model. 2. Theoretical Section The primary goal of this section is to show how the degree of uniaxial orientation measured directly from the X-ray azimuthal scans can be compared with optical absorption measurements. Generally speaking, optical density (OD) and X-ray diffraction (XRD) probe the molecular arrangement on quite different length scalessthe former on the level of polymer segments, whereas the latter on the level of crystallites. Yet the information about the molecular alignment obtained from these two techniques is not completely independent. In the first place, OD gives the dichroic ratio for absorption, R, which is related to the orientational order parameter, s, as

s)

R-1 R+2

(1)

Then, in order to connect OD data to that of XRD, we connect the order parameter s to the mosaic distribution defined through the azimuthal rotation angle about the surface normal (φ). Such a calculation is quite elaborate in general but can be significantly simplified by assuming a two-dimensional (2D) liquid crystalline structure. In other words, we assume the rodlike molecules to be always parallel to the (0yz) plane; see Figures 2 and 3, where θ denotes the angle between the c- and z-axes. This approximation is assumed to be valid for thin films. Hence, a 2D order parameter has to be used

s ) 〈2 cos θ - 1〉 ) 2

∫f(θ) cos 2θ dθ

(2)

For a high degree of orientation, one can simplify eq 2 using cos R = 1 - R2/2, thus

s = 1 - 2θ02

(3)

where θ0 ) x〈θ2〉 has the physical meaning of the angle, accessible for the rotational motion of a molecule (Figure 3).

Figure 3. An illustration of the degree of alignment. Here the vectors z and c represent the overall alignment direction and, more locally, the backbone of a rigid molecule, respectively.

Next we consider distribution functions Fk (φ), where k ) I, II corresponds to types I or II crystallites. For simplicity, we ignore type III. These in-plane mosaic functions are measured by XRD and in fact originate from the fluctuations in the orientations of the crystal planes. Because of the assumed 2D orientational order, the orientation distribution function in the whole sample can be written as a superposition,

f(θ) ) gIFI(θ) + gIIFII(θ) + gnc fnc(θ)

(4)

where gI, gII, and gnc are the fractions of type I, type II, and the noncrystalline (amorphous) material, respectively, and where fnc(θ) describes the orientational order in noncrystalline phase, not observed by XRD. Equations 2 and 4 yield

sOD ) gI sI + gII sII + gncsnc

(5)

where we have introduced sk ) 〈2 cos2 φ - 1〉k and sOD which is the order parameter as measured by OD. If the ordering parameters for types I and II are similar, sI = sII (we denote it as scr) eq 5 reduces to

sOD ) gcrscr + gncsnc

(6)

where gcr ) gI + gII is the total fraction of the crystalline phase. Now, if the approximation sI = sII holds, two important conclusions can be made. First, if the fraction of noncrystalline material is small, i.e., gnc = 0 and gcr ) 1 - gnc = 1, then the angle θ0 ) x(1-sOD)/2 measured from OD should be close to that estimated from the dispersion of the mosaic distribution about the surface normal φ in XRD. Second, if the order parameter snc in the noncrystalline phase is known, the volume fraction of this phase can be estimated as

gnc )

scr - sOD scr - snc

(7)

In this study, we moreover assume that the PF2/6 molecule is rigid and that the transition moment is parallel to the c-axis. In the experiment, the temperature was low (room temperature) and the degree of alignment (and R) was sufficiently high to justify the approximations. 3. Experimental Section The synthesis of PF2/6 (Mn ) 29 kg/mol and Mw ) 68 kg/mol) has been detailed elsewhere.18 The samples were thermotropically aligned using rubbed polyimide (PI) substrates as described in ref 17. In the adopted geometry (Figure 2), the x-axis is normal to the surface with

1708 Crystal Growth & Design, Vol. 7, No. 9, 2007

Knaapila et al.

Figure 4. GIXD images from the PF2/6 film studied. (a) (xy0) plane, φ ) 0° and (b) (x0z) plane, φ ) 90°. The GIXD patterns were measured with the incident beam along the z and y-axes, respectively. Blue and red indices show the primary reflections of the types I and II, respectively. the y- and z-axes parallel to the surface with z pointing along the rubbing direction. The GIXD measurements were performed at the XMaS beamline19 at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. The beam was monochromatized using a double Si(111) crystal and focused onto the sample through 0.2 × 2 mm (vert × hort) slits. Additional antiscatter slits placed close to the sample and a helium atmosphere were used to reduce the background and prevent radiation damage. To maximize the signal from the thin PF layer, experiments were conducted at a grazing angle close to the critical angle of the PF2/6 layer (θ ) 0.13° at 11.215 keV). The scattering was measured using a two-dimensional MarCCD detector as a function of azimuthal rotation about the surface normal (φ). The zero of the rotation angle, φ ) 0, was defined when the scattering plane was co-incident with the rubbing direction of the PI layer (z-axis in Figure 2). The data were normalized to the incident flux using an ionization chamber mounted just before the sample. For a given azimuthal angle, the detector records the scattered intensity as a function of reciprocal space perpendicular (q⊥ ) qx) and parallel q// ) x(qycos(φ))2+(qzsin(φ))2 to the film surface. No change in the scattering patterns was observed on repetition of φ-scans, and therefore no overt radiation damage was detected. Higher q resolution data were obtained at the W1.1 (ROEWI) beam line at Hamburger Synchrotronstrahlungslabor (HASYLAB) in Hamburg (Germany). A similar geometry to that employed at the ESRF was used but with an energy of 10.5 keV. A scintillation detector was scanned as a function of vertical and horizontal scattering angles. With the incident beam along the z- or y-axis these scans corresponding to straight lines extending from the origin of the qx-qy or qx-qz scattering maps observed at the ESRF. In the (xy0) plane these scans were recorded at intervals of χ′ ) 15° with χ′ ) 0° corresponding to when qy ) 0. The data were normalized to incident flux measured with an ionization chamber. The instrumental resolution function was found to be negligible. All X-ray experiments were made at room temperature. Polarized optical absorption measurements were carried out using a setup containing a Xe Arc lamp (Oriel 6255), a quartz optical fiber, a Glan-Thomson polarizer, focusing optics, and a linear array photodetector (Ocean Optics, USB2000 with 1.8 nm spectral resolution). The sample was mounted in an evacuated optical chamber. This consisted of a modified Burleigh T-1000 xyz translation stage, Melles Griot precision Nanomovers and a Newport ESP700 motor controller. The nominal on sample spot size was 200 µm in diameter.

4. Results and Discussion In Figure 4 we show examples of the GIXD patterns of the aligned PF2/6 film at two azimuthal directions, φ ) 0° and φ

Figure 5. 1D 2θ scans of aligned PF2/6 film along directions corresponding to different χ′-angles. Inset displays the average FWHM of the main GIXD reflections of type I (open blue squares) and type II (red squares) as well as linear fits for comparison.

) 90°, respectively, recorded at the ESRF. The data are only slightly contaminated by the reflections seen in the crosswise directions, indicating a high degree of meridional alignment (cf. ref 17). In Figure 4a, two sets of hexagonal reflections are clearly shown, which index to the two equatorial crystallite types I and II, as depicted in Figure 2. The intensity of the type I reflections is significantly higher than those originating from type II crystallites, and the relative fractions are around 4:1. Figure 4a shows also shallow arcs in between the hexagonal GIXD peaks. This equatorial scattering superimposed on a background of hexagonal phase points to a weak cylindrically isotropic crystallite orientation (denoted as type III in Figure 2). The reflections seen in Figure 4a are in the (xy0) plane, and those in Figure 4b at or close to the (x0z) plane and the actual scattering vectors lie slightly off these planes. While the magnitude of the scattering vector is given by q2 ) qx2 + qy2, the q vector is turned in an angle as in (qλ/4π) toward the reader. These reflections appear by virtue of a small amount of mosaic in-plane spread in the c-axis and out-of-plane spread in the a directions. The intramolecular structure of PF2/6 is peculiar with several helical models having been proposed and studied at length. The indexation in Figure 4b is according to the recently proposed 21-helicity11 contrary to the previously proposed 5-helical (i.e., 20-helical) model.10,15,20,21 However, if the structure is either 21 or 20 helix, this fine intramolecular detail does not influence the larger scale crystallite orientation discussed in the present paper. We note additionally that the faint arcs impinging on the 0021 may be a vestige of the 010 reflection. For comparison, Figure 5 plots 2θ scans along selected χ′-directions, and the inset plots the corresponding average fullwidth half-maxima (FWHM) of the equatorial primary GIXD reflections for corresponding scans in the (xy0) plane. The peak

Chain Orientation in Polyfluorene Crystallite Types

broadening of the hk0 reflections scales as q2, indicative of paracrystalline order. Because of the limited number of data points, we cannot yet give a rigorous analysis between microstrains and paracrystallinity. Flattening of hexagons along the x-axis probably driven by microstrains is also present and appears as a mixing of the positions of the GIXD reflections of the types I and II (inset of Figure 5). The equatorial crystallite size (extrapolated to q f 0) is 35 nm for both types. In this respect, there is basically no difference between types I and II, which is in agreement with the assumption that the two types represent the same local structure but different biaxial alignments. The meridional crystallite size is also 35 nm. This value is obtained from the 0021 reflections, and here we cannot distinguish between the different orientation types. These values are not much below typical film thicknesses (40-60 nm).22 We also note that the scaling and crystallite sizes are consistent with fiber XRD patterns13 where 00l reflections indicate additionally true crystalline order, whereas hkl peaks remain weak. The data shown in Figure 4 do not allow for quantitative determination of the degree of meridional alignment. To probe uniaxial distribution of crystallite types within the plane we show in Figure 6 integrated intensities of the representative hexagonal peaks as seen in Figure 4a as a function of azimuthal rotation, φ. Figure 6a shows selected first-order and Figure 6b secondorder GIXD reflections. Each data point (φ angle) represents a single GIXD image like that shown in Figure 4a. At first sight, two observations can be made. First, the angular distribution of the mosaic spread is narrow and falls within 15-20° (the instrument resolution is significantly less than 1° and can be ignored). Second, all peaks show rather identical distributions centered on the Bragg angle of 2.05 degrees. Therefore, the degree of meridional alignment seems to be similar for types I and II. As the types are not resolvable in the (x0z) plane, we did not make φ-scans in this sector around the y-axis. Figure 7 plots integrated intensities of the second (x3) hexagonal reflections representing types I and II. Also plotted are a putative weak type III and corresponding Gaussian fits for comparison. FWHM () 2.35σ) values of 10.3 ( 0.2°, 11.7 ( 0.2°, and 17.8 ( 0.3° are obtained for the fits of types I-III, respectively.23 Hence, the degree of uniaxial meridional alignment of types I and II is similar. The weak angular dependence of the possible type III crystallites differs from our earlier study of doped PF2/6 films where all crystallites were suggested to be equally aligned.24 As the present study concerns a different material, better aligned films, and a considerably lower percentage of type III crystallites, these results do not contradict each other. The observed similarity of the degree of meridional orientation and the crystallite size between the types may have implications on how the crystalline structure is formed. If the surface plays no crucial role in the crystallization but is only responsible for the uniaxial alignment (as it would be in the case of the nematic phase), there are no apparent reasons for the orientational differences between types I and II. Alternatively, if the surface acts as a dominant “nucleation center”, type II might be preferential, and we could expect it to be more ordered. We also underline that in our case the crystallite types are formed due to the transition from the nematic (i.e., not disordered) phase13 and, probably, have the same uniaxial order as the parent nematic had. Although extreme care was taken to remove background scattering, the GIXD data show a measurable background. The intensity of a nominal amorphous halo is 10% or less of that of the first-order reflections. If this arose from the sample, this

Crystal Growth & Design, Vol. 7, No. 9, 2007 1709

Figure 6. Integrated intensities of selected hexagonal GIXD reflections of aligned PF2/6 film corresponding to the sample for which data are shown in Figure 4. The sample is rotated about the x-axis, and φ is the angle between the incident beam and the z-axis. Labels denote counterclockwise χ′-angle (cf. Figure 2). (a) First hexagonal reflection. Open spheres: reflections 010, 110, 100, and 01h0 of type I. Open squares: 1h00 and 110 of type II. (b) Second hexagonal reflection. Open spheres: Reflections 120 and 11h0 of type I. Open squares: 210, 210, 1h10 and 120 of type II. Solid lines are a guide to the eye.

implies that a large fraction of polymers are not ordered (but amorphous). It is of course plausible that all polymers cannot be ordered, but it is not yet straightforward to make a distinction between contamination and diffuse scattering of the sample itself. On the other hand, all polymers are observed in optical probes whether crystalline or amorphous, and a comparison between the alignment as measured using X-rays and optical spectroscopy enables the amount of amorphous material to be estimated as shown in section 2. Therefore, the alignment was next studied using polarized optical absorption. Figure 8a plots the ex-situ absorption spectra vs polarizer angle parallel and perpendicular to the z-axis measured of the sample as discussed previously. The spot size was of the same order of magnitude as the footprint of the X-ray beam. More local information was obtained by mapping the area studied using X-rays through a microfocus optical configuration. Figure 8b shows examples of the more local measurement where a 200-micron spot size was mapped in a 3 × 3 grid with 1 mm spacing over the X-ray trace. All data are similar with small

1710 Crystal Growth & Design, Vol. 7, No. 9, 2007

Figure 7. Integrated intensities of the second hexagonal GIXD reflections of aligned PF2/6 film corresponding to the sample of which data are shown in Figure 4. (a) Type I. (b) Type II. (c). Type III. Solid lines are corresponding Gaussian fits.

Figure 8. (a) Polarized absorption spectra of a PF2/6 film corresponding to the sample of which data are shown in Figures 4-7. The upturn below 2.8 eV is a feature of the spectrometer. (b) Examples of mapping of the area illuminated in the GIXD measurements (see text for details). Solid, dashed, and dotted lines correspond to the same spot on the film. The radius of the spot is 200 µm.

systematic changes occurring over the studied area. The dichroic ratio for these absorption measurements ranges from 20:1 to 40:1. Following section 2, we immediately see that these dichroic ratios correspond to order parameter values s ≈ 0.86-0.92 and by implication θo ≈ 11°-15° (Figure 3). To compare these values with the recorded φ scans, we estimate the magnitude of the angle θ0 by two sigma (95% deviation from the mean), i.e., θI0 ) 8.8 ( 0.2°, θII0 ) 9.9 ( 0.2°, and θIII 0 ) 15 ( 1° for types I-III, respectively. Hence, the optical data are surprisingly

Knaapila et al.

well in accord with the values based on the GIXD data, and it is fully logical that OD results in a somewhat wider angular distribution. Even though this estimation is rather phenomenological, its basic assumptions (i.e., low temperature, stiffness of the molecule, high degree of alignment, and transition moment along the c-axis) are plausible, and therefore the direct comparison to GIXD data seems to be relevant. The OD averages over both amorphous and crystalline constituents, and so its higher value suggests poorer alignment of chains within amorphous regions. Furthermore, from the variance of the φ-distribution, we obtain scr = 0.98. To get an impression of the magnitude of the noncrystalline material volume fraction, gnc, we take snc ) 0, which yields gnc = 0.06-0.12. This is in accord with the order of magnitude estimated from the diffuse scattering (∼10%). It is noted that the degree of uniaxial alignment varies greatly as a function of both molecular weight and processing conditions (see discussion in ref 17), and therefore the absolute values presented here are physically meaningful only for the sample prepared as in this study. To assess these values, it should be recalled what is measured. As the rodlike PF2/6 forms an oriented lattice, the angle θ0 is directly related to the angular spreads of the Bragg reflections over φ. This is measured if instrumental factors contributing the angular spreads are either explicitly known or negligible. The latter assumption is employed here. R describes the anisotropy of the optical absorption process, and the transition probability is maximized when the transition moment of the molecule lies parallel to the electric vector of the light. Measured values of R can be connected to uniaxial alignment if the relation between the transition moment and the rigid backbone is known.25 Here it is assumed that the transition dipoles are parallel to the c-axis. The order parameter obtained from the OD is qualitatively less than that derived from XRD if the highest ordered crystallites, which GIXD weights most strongly, are also uniaxially best aligned. This is assumed here. The GIX reflections (like those shown in Figure 4) arise from PF2/6 within hexagonal crystallites with meridional trajectory located in-plane. Any possible amorphous phase or (hypothetical) hexagonal crystallites with the c-axis out-of-plane are not detected. From Figure 4, it is clear that there are no other major crystallite types as those illustrated in Figure 2. Moreover, the amorphous halo appears weak, and therefore we argue that the degree of crystallinity is very high.26 The photoabsorption data (shown in Figure 8) arise from all polymers (whether crystalline or amorphous) of which the c-axis lies in-plane. Possible chains with the c-axis out-of-plane are not detected. Therefore, as the polymers are known to be relatively rigid on the nanometer scale, our result suggests that the average degree of meridional alignment of polymers having their c-axis in-plane (whether amorphous or in crystallites) is close to the average degree of those forming hexagonal crystallites of types I and II. The potential PF2/6 fraction with backbone trajectory deviating from the (0yz) plane is not probed. As the relative fractions of types I and II are known (4:1 in the present case), one might calculate the average angle, which could be then compared to the optical data. However, as the difference between types I and II is subtle, and as the error in the determination of the dichroic ration is large, this conclusion cannot be made. We finally note that R values vary from position to position, but whether this is connected to the microphase separation of types I and II in-plane remains an open question. 5. Conclusions Summarizing, we have simultaneously determined both the uniaxial and biaxial alignment of the PF2/6 film at a specific

Chain Orientation in Polyfluorene Crystallite Types

molecular weight (Mn ) 29 kg/mol, Mw ) 68 kg/mol), a little over the crossover from nematic liquid crystal to the hexagonal phase regimes, a phase position corresponding to the highest uniaxial alignment. We find that the multiple oriented crystallite types I and II of aligned PF2/6 films have an essentially identical meridional orientation distribution, despite their 30° equatorial rotation with respect to the rubbing direction in the substrate. Furthermore, the degree of meridional orientation obtained from the GIXD measurements was found to follow that obtained from the optical spectroscopy, both methods yielding similar values (∼10°) for an angle θ0 defining a cone of the c trajectories of rigid backbones around the ideal orientation distribution z. For the sample showing this, the equatorial peak broadening and scaling as a function of diffraction order are similar for types I and II. An equatorial crystallite size of 35 nm was found, and the order is paracrystalline. These observations strongly indicate that the orientation types are not distinct polycrystalline forms but differ only in their equatorial orientation. Putative type III without surface preference was also found in pure PF2/6 films. In general, the work is also an example of how synchrotron radiation and GIXD can be used as simple tools to separately address both equatorial and meridional alignment for distinct crystallite types in polymer films. The similarity in the degree of meridional orientation and the equatorial coherence length of crystallite types might imply that the PI surface does not prefer either one and thus does not act as a dominant nucleation center. Acknowledgment. This study was funded by One NorthEast UIC Nanotechnology Grant (UK). M.K. and T.P.A.H. thank support from the XMaS project. We also thank B. P. Lyons of the Institute of Materials Research and Engineering (Singapore), R. Serimaa of the University of Helsinki, and O. H. Seeck of HASYLAB for discussions. Thanks are also due to R. Gu¨ntner and U. Scherf of the University of Wuppertal for providing PF2/6. References (1) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Nature 1999, 401, 685-688. (2) Chen, S. H.; Chou, H. L.; Su, A. C.; Chen, S. A. Macromolecules 2004, 37, 6833-6838. (3) Kline, R. J.; McGehee, M. D.; Toney, M. F. Nat. Mater. 2006, 5, 222-228. (4) Kline, R. J.; McGehee, M. D. J. Macromol. Sci.-Polym. ReV. 2006, 46, 27-45. (5) Nolde, F.; Pisula, W.; Mu¨ller, S.; Kohl, C.; Mu¨llen, K. Chem. Mater. 2006, 18, 3715-3725.

Crystal Growth & Design, Vol. 7, No. 9, 2007 1711 (6) Knaapila, M.; Stepanyan, R.; Lyons, B. P.; Torkkeli, M.; Monkman, A. P. AdV. Funct. Mater. 2006, 16, 599-609. (7) Lyons, B. P.; Monkman, A. P. J. Appl. Phys. 2004, 96, 47354741. (8) Miteva, T.; Meisel, A.; Grell, M.; Nothofer, H. G.; Lupo, D.; Yasuda, A.; Knoll, W.; Kloppenburg, L.; Bunz, U. H. F.; Scherf, U.; Neher, D. Synth. Met. 2000, 111-112, 173-176. (9) Yasuda, T.; Fujita, K.; Tsutsui, T.; Geng, Y.; Culligan, S. W.; Chen, S. H. Chem. Mater. 2005, 17, 264-268. (10) Lieser, G.; Oda, M.; Miteva, T.; Meisel, A.; Nothofer, H.-G.; Scherf, U.; Neher, D. Macromolecules 2000, 33, 4490-4495. (11) Knaapila, M.; Torkkeli, M.; Monkman, A. P. Macromolecules 2007, 40, 3610-3614. (12) Chi, C.; Lieser, G.; Enkelmann, V.; Wegner, G. Macrom. Chem. Phys. 2005, 206, 1597-1609. (13) Knaapila, M.; Stepanyan, R.; Torkkeli, M.; Lyons, B. P.; Ikonen, T. P.; Alma´sy, L.; Foreman, J. P.; Serimaa, R.; Gu¨ntner, R.; Scherf, U.; Monkman, A. P. Phys. ReV. E 2005, 71, 041802. (14) Balta´-Calleja, F. J.; Vonk, C. G. X-ray Scattering of Synthetic Polymers; Elsevier Publishers: Amsterdam, 1989. (15) Knaapila, M.; Lyons, B. P.; Kisko, K.; Foreman, J. P.; Vainio, U.; Mihaylova, M.; Seeck, O. H.; Pålsson, L.-O.; Serimaa, R.; Torkkeli, M.; Monkman, A. P. J. Phys. Chem. B 2003, 107, 1242512430. (16) Heffelfinger, C. J.; Burton, R. L. J. Polym. Sci. 1960, 47, 289-306. (17) Knaapila, M.; Stepanyan, R.; Lyons, B. P.; Torkkeli, M.; Hase, T. P. A.; Serimaa, R.; Gu¨ntner, R.; Seeck, O. H.; Scherf, U.; Monkman, A. P. Macromolecules 2005, 38, 2744-2753. (18) Nothofer, H.-G.; Meisel, A.; Miteva, T.; Neher, D.; Forster, M.; Oda, M.; Lieser, G.; Sainova, D.; Yasuda, A.; Lupo, D.; Knoll, W.; Scherf, U. Macromol. Symp. 2000, 154, 139-148. (19) Brown, S. D.; Bouchenoire, L.; Bowyer, D.; Kervin, J.; Laundy, D.; Longfield, M. J.; Mannix, D.; Paul, D. F.; Stunault, A.; Thompson, P.; Cooper, M. J.; Lucas, C. A.; Stirling, W. G. J. Synchrotron Rad. 2001, 8, 1172-1181. (20) Tanto, B.; Guha, S.; Martin, C. M.; Scherf, U.; Winokur, M. J. Macromolecules 2004, 37, 9438-9448. (21) Marcon, V.; van der Vegt, N.; Wegner, G.; Raos, G. J. Phys. Chem. B 2006, 110, 5253-5261. (22) Knaapila, M.; Lyons, B. P.; Hase, T. P. A.; Pearson, C.; Petty, M. C.; Bouchenoire, L.; Thompson, P.; Serimaa, R.; Torkkeli, M.; Monkman, A. P. AdV. Funct. Mater. 2005, 15, 1517-1522. (23) Note that these values correspond to the different χ angles. (24) Knaapila, M.; Torkkeli, M.; Lyons, B. P.; Hunt, M. R. C.; Hase, T. P. A.; Seeck, O. H.; Bouchenoire, L.; Serimaa, R.; Monkman, A. P. Phys. ReV. B 2006, 74, 214203. (25) Gather, M. C.; Bradley, D. D. C. AdV. Funct. Mater. 2007, 17, 479485. (26) The quantitative determination of the degree of crystallinity is not straightforward due to the nominal N6 enhancement of Bragg scattering. Furthermore, the origin of the amorphous halo is at least partly due to the PI layer and Kapton windows of the vacuum tubes.

CG0607295