Birefringence Control of Semicrystalline Block Copolymers by

Oct 21, 2010 - Institute for Soldier Nanotechnologies, Massachusetts Institute of Technology, Cambridge,. Massachusetts 02139, United States. Received...
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Birefringence Control of Semicrystalline Block Copolymers by Crystallization under Confinement )

Ming-Chia Li, † Guang-Wei Chang, † Tao Lin,‡ Rong-Ming Ho,*,† Wei-Tsung Chuang,§ and Steven Kooi

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† Department of Chemical Engineering, ‡Institute of Nanoengineering and Microsystems, National Tsing-Hua University, Hsinchu 30013, Taiwan, §National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan, and Institute for Soldier Nanotechnologies, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States

Received August 30, 2010. Revised Manuscript Received October 4, 2010 A series of semicrystalline block copolymers (BCPs), poly(4-vinylpyridine)-block-poly(ε-caprolactone) (P4VP-PCL), with lamellar phases have been synthesized. P4VP-PCL BCP thin films with large-scale, oriented lamellar microdomains were obtained by rimming coating process followed by oscillated shearing using a homemade shear device. Owing to the vitrified P4VP microdomains and strongly segregated microphase separation, specific PCL crystalline chain orientation can be formed from the growth of anisotropic PCL crystallites under confinement so as to uniformly increase the birefringence of the BCP thin films. The enhanced birefringence corresponds well with the increase of PCL crystallinity. Consequently, the birefringence of the P4VP-PCL thin-films can be fine-tuned by PCL crystallization. The variation on the birefringence of the BCP thin films attributed to crystallization and melting is a reversible process with respect to temperature. The BCP thin films can thus be used as temperature-stimulated materials with controllable birefringence via crystallization kinetics.

Introduction Applications of the birefringence of polymeric materials have been employed in the display industry for many years. External forces from flow, magnetic, or electric fields can be used to affect structural rearrangements in a wide range of polymeric materials so as to generate birefringence thin films.1-7 Since the early work of Stein and co-workers,8 birefringence has been employed to follow the chain orientation and crystallization of polymers subjected to flow fields. Kornfield and co-workers employed simultaneous measurements of birefringence to study the crystallization of polymeric melt subjected to extensional flow.9 Recently, semicrystalline diblock copolymers have been employed to explore the crystallization behavior of polymer chains confined in nanoscale space provided that crystallizable blocks crystallize within microdomains driven by microphase separation, *To whom correspondence should be addressed. E-mail: [email protected]. edu.tw. Tel: 886-3-5738349. Fax: 886-3-5715408. (1) L€owik, D. W. P. M.; Shklyarevskiy, I. O.; Ruizendaal, L.; Christianen, P. C. M.; Maan, J. C.; van Hest, J. C. M. Adv. Mater. 2007, 19, 1191–1195. (2) Murata, K.; Haraguchi, K. J. Mater. Chem. 2007, 17, 3385–3388. (3) Russell, T. P.; Kim, J.; Chin, I.; Smith, B. A.; Mays, J. W. Macromolecules 1993, 26, 5436–5440. (4) Seki, M.; Thurman, D. W.; Oberhauser, J. P.; Kornfield, J. A. Macromolecules 2002, 35, 2583–2594. (5) Floudas, G.; Hilliou, L.; Lellinger, D.; Alig, I. Macromolecules 2000, 33, 6466–6472. (6) Martins, C. I.; Cakmak, M. Macromolecules 2005, 38, 4260–4273. (7) Koike, Y.; Cakmak, M. Polymer 2003, 44, 4249–4260. (8) Stein, R. S. Newer Methods in Polymer Characterization; Wiley-Interscience: New York, 1964. (9) Kornfield, J. A.; Lucia, F. B.; Gough, T. J. Synchrotron Rad. 2008, 15, 185. (10) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: New York, 1998. (11) Nojima, Kato; S.; Yamamoto, K.; S.; Ashida, T. Macromolecules 1992, 25, 2237–2242. (12) Rangarajan, P.; Register, R. A.; Adamson, D. H.; Fetters, L. J.; Bras, W.; Naylor, S.; Ryan, A. J. Macromolecules 1995, 28, 1422–1428. (13) Rangarajan, P.; Register, R. A.; Fetters, L. J. Macromolecules 1993, 26, 4640–4645.

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for which the crystallization temperature TC lies below the temperature TODT for the order-disorder transition.10-32 Semicrystalline block copolymers (BCPs) possess inherently rich morphologies that offer various nanoscale confined environments to tailor the crystallization kinetics of crystallizable blocks in (14) Loo, Y. L.; Register, R. A.; Ryan, A. J. Macromolecules 2002, 35, 2365– 2374. (15) Loo, Y. L.; Register, R. A.; Ryan, A. J. Phys. Rev. Lett. 2000, 84, 4120– 4123. (16) Chen, H. L.; Hsiao, S. C.; Lin, T. L.; Yamauchi, K.; Hasegawa, H.; Hashimoto, T. Macromolecules 2001, 34, 671–674. (17) Chen, H. L.; Wu, J. C.; Lin, T. L.; Lin, J. S. Macromolecules 2001, 34, 6936– 6944. (18) Zhu, L.; Chen, Y.; Zhang, A.; Calhoun, B. H.; Chum, M.; Quirk, R. P.; Cheng, S. Z. D.; Hsiao, B. S.; Yeh, F.; Hashimoto, T. Phys. Rev. B 1999, 60, 10022– 10031. (19) Zhu, L.; Cheng, S. Z. D.; Calhoun, B. H.; Ge, Q.; Quirk, R. P.; Thomas, E. L.; Hsiao, B. S.; Yeh, F.; Lotz, B. J. Am. Chem. Soc. 2000, 122, 5957–5967. (20) Huang, P.; Zhu, L.; Gau, Y.; Ge, Q.; Jing, A. J.; Chen, W. Y.; Quirk, R. P.; Cheng, S. Z. D.; Thomas, E. L.; Lotz, B.; Hsiao, B. S.; Avila-Orta, C. A.; Sics, I. Macromolecules 2004, 37, 3689–3698. (21) Huang, P.; Gau, Y.; Quirk, R. P.; Ruan, J.; Lotz, B.; Thomas, E. L.; Hsiao, B. S.; Avila-Orta, C. A.; Sics, I.; Cheng, S. Z. D. Polymer 2006, 47, 5459–5466. (22) Hamley, I. W.; Fairclough, J. P. A.; Terrill, N. J.; Ryan, A. J.; Lipic, P. M.; Bates, F. S.; Towns-Andrews, E. Macromolecules 1996, 29, 8835–8843. (23) Lotz, B.; Kovacs, A. J. Kolloid-Z. Z. Polym. 1966, 209, 97. (24) Douzinas, K. C.; Cohen, R. E. Macromolecules 1992, 25, 5030–5035. (25) M€uller, A. J.; Albuerne, J.; Marquez, L.; Raquez, J. M.; Degee, P.; Dubois, P.; Hobbs, J.; Hamley, I. W. Faraday Discuss. 2005, 128, 231–252. (26) M€uller, A. J.; Albuerne, J.; Esteves, L. M.; Marquez, L.; Raquez, J. M.; Degee, P.; Dubois, P.; Collins, S.; Hamley, I. W. Macromol. Symp. 2004, 215, 369–382. (27) Koo, C. M.; Wu, L.; Lim, L. S.; Mahanthappa, M. K.; Hillmyer, M. A.; Bates, F. S. Macromolecules 2005, 38, 6090–6098. (28) Ho, R. M.; Lin, F. H.; Tsai, C. C.; Lin, C. C.; Ko, B. T.; Hsiao, B. S.; Sics, I. Macromolecules 2004, 37, 5985–5994. (29) Ho, R. M.; Chiang, Y. W.; Lin, C. C.; Huang, B. H. Macromolecules 2005, 38, 4769–4779. (30) Chung, T. M.; Ho, R. M.; Kuo, J. C.; Tsai, J. C.; Hsiao, B. S.; Sics, I. Macromolecules 2006, 39, 2739–2742. (31) Ho, R. M.; Chung, T. M.; Tsai, J. C.; Kuo, J. C.; Hsiao, B. S.; Sics, I. Macromol. Rapid Commun. 2005, 26, 107–111. (32) Sun, Y. S.; Chung, T. M.; Li, Y. J.; Ho, R. M.; Ko, B. T.; Jeng, U. S.; Lotz, B. Macromolecules 2006, 39, 5782–5788.

Published on Web 10/21/2010

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Article Table 1. Characterization of P4VP-PCL Diblock Copolymers a

sample

Mtotal n

(kg/mol)

MP4VP (kg/mol)b n

MPCL (kg/mol)b n

Ntotalc

Mw/Mnd

fPCLe

morphology

VP/CL 202/116 34.4 21.2 13.2 318 1.26 0.36 lamellae VP/CL 54/94 16.4 5.7 10.7 148 1.28 0.63 lamellae VP/CL 80/59 15.2 8.4 6.8 139 1.21 0.42 lamellae a VP/CL m/n, where m and n represent the degree of polymerization of the constituted P4VP (NP4VP) and PCL (NPCL) blocks, respectively. b MP4VP and n were characterized by proton nuclear magnetic resonance (1H NMR). c Ntotal = NP4VP þ NPCL. d Polydispersity index (PDI) in the final diblock MPCL n copolymers was determined by GPC using standard calibration. e fPCL = NPCL/(NP4VP þ NPCL).

BCPs.10-32 However, an incorporation of crystallizable moiety within a BCP often adds morphological complexities due to the interplay among crystallization of the crystallizable block, microphase separation, and vitrification of the amorphous block. Depending on the segregation strength of microphase separation at the crystallization temperature, the microphase-separated microdomains can be preserved, templated, undulated, or even broken upon crystallization. Furthermore, the investigation of crystalline orientation under confinement within an existing microphaseseparated microdomains is of interest not only for an understanding of the crystal packing of polymer crystals but also for exploring optical properties in various confined geometries. For example, on varying TC from low to high temperature, the c-axis orientation of poly(ethylene oxide) crystals within confined lamellar microdomains in a poly(ethylene oxide)-block-polystyrene (PEO-PS) diblock copolymer altered from random to parallel to the confined lamellar surface (a homogeneous configuration), then to inclined, and eventually to perpendicular (a homeotropic configuration).19 In our previous work, a series of semicrystalline block copolymer, poly(4-vinylpyridine)-block-poly(ε-caprolactone) (P4VP-PCL), with lamellar microstructure has been synthesized.32 Systematic studies of crystallization of PCL block within microphase-separated lamellar microdomains were carried out to experimentally examine the confined size effect on the crystallization of P4VP-PCL diblock copolymers. An interesting crystalline orientation has been identified at which the crystalline PCL chains are perpendicular to the lamellar microdomains under appropriate confined environment.33 For lamellar block copolymers, the birefringence is composed of two contributions: (1) intrinsic birefringence induced by chain orientation along the lamellar normal34 and (2) form birefringence caused by the periodic variation in bulk refractive index in the microphase separated alternative layers.35 The calculations indicate that the intrinsic birefringence is surprisingly large and, for the well-aligned systems, is often comparable to, or even greater than, the estimated form birefringence. By contrast, the sample exhibits near zero bulk birefringence, when there is no macroscopic alignment or optical axis, despite the high local degree of anisotropy. This result reflects that the orientation of polymeric chains, i.e., the tendency of the block end-to-end vectors to be aligned, is much more significant than the stretching of polymeric chains. Concerning potential applications, macroscopic anisotropy is often necessary so that external fields such as electric, magnetic, mechanical force, or flow fields have been used to achieve the nanoscale structures into the macroscopic alignment so as to obtain favorable anisotropic properties. Bates and co-workers have employed oscillation shear flow of polymeric material and construct a so-called alignment diagram, which maps the macroscopic alignment of the lamellar structure as a function of the oscillation parameters: temperature, (33) Sun, Y. S.; Chung, T. M.; Li, Y. J.; Ho, R. M.; Ko, B. T.; Jeng, U. S. Macromolecules 2007, 40, 6778–6781. (34) Folkes, M. J.; Keller, A. J. Polym. Sci. 1976, 14, 833–846. (35) Lodge, T. P.; Fredrickson, G. H. Macromolecules 1992, 25, 5643–5650.

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Figure 1. (a) TEM micrograph of VP/CL 202/116 and (b) 1D SAXS profiles of VP/CL 202/116, 54/94, and 80/59 at room temperature. All of the samples were stained by RuO4.

frequency, time, and strain amplitude.36,37 In other words, the induced anisotropic property plays a critical role in terms of applications. In this study, the crystallization of PCL blocks within the microphase-separated lamellar microdomains in P4VP-PCL diblock copolymers will be carried out to enhance intrinsic birefringence due to confined crystallites grown within the lamellar microstructures. Here we aim to explore the confined size effect on crystallization so as to examine the induced crystalline orientation and the corresponding variation in birefringence. Also, it is noted that this birefringence variation is a reversible process by simply controlling isothermal crystallization temperature. Consequently, this block copolymer thin film can provide a temperature-controlled birefringence film and also a controllable birefringence film with the crystallization time because of the unique anisotropic crystallites of confined crystallization.

Experimental Section Materials. Three P4VP-PCL BCPs designated as VP/CL 202/ 116, 54/94, 80/59 were used to explore the optical behavior under nanoscale confinement; in the sample code VP/CL m/n, m and n represent the degree of polymerization of each constituent block, respectively. The P4VP-PCL BCPs of various molecular weights (MWs) were obtained with PCL volume fractions ranging from 0.36 to 0.63. The PCL domains had thicknesses of 9.5 (VP/CL 80/59), 17.2 (VP/CL 54/94), and 17.9 nm (VP/CL 202/116). The characterization of the P4VP-PCL BCPs is summarized in Table 1. The detailed procedures for the synthesis of the P4VP-PCL BCPs were described in our previous study.32 Sample Preparation. Bulk samples of block copolymers with large-scale, oriented microphase-separated lamellar microdomains (36) Koppi, K. A.; Tirrell, M.; Bates, F. S.; Almdal, K.; Colby, R. H. J. Phys. II France 1992, 2, 1941–1959. (37) Koppi, K. A.; Tirrell, M.; Bates, F. S. Phys. Rev. Lett. 1993, 70, 1449–1452.

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Figure 2. (a) Geometry of the shear-induced P4VP-PCL with a lamellar microdomain. Sets of 2D SAXS patterns of VP/CL 202/116 at 20 C with the X-ray beam along directions (c) X, (d) Y, and (e) Z and (b) Schematic of the 2D SAXS pattern and corresponding azimuthal profiles for the 2D SAXS pattern with the X-ray along directions (f) X, (g) Y, and (h) Z.

Figure 3. 2D WAXD patterns of VP/CL 202/116 at 20 C with the X-ray beam along directions (a) X, (b) Y, and (c) Z and corresponding azimuthal profiles for the 2D WAXD pattern with the X-ray along directions (d) X, (e) Y, and (f) Z.

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Figure 4. 2D WAXD patterns of VP/CL 202/116 at 40 C with the X-ray beam along directions (a) X, (b) Y, and (c) Z and corresponding azimuthal profiles for the 2D WAXD pattern with the X-ray along directions (d) X, (e) Y, and (f) Z.

Figure 5. 2D WAXD patterns of VP/CL 202/116 at -10 C with the X-ray beam along directions (a) X, (b) Y, and (c) Z and corresponding azimuthal profiles for the 2D WAXD pattern with the X-ray along directions (d) X, (e) Y, and (f) Z.

were prepared by a rimming coating method.28 P4VP-PCL dichloromethane solution at concentration of 10 wt % (w/v) was first placed in an open test tube fixed on a spin coater, and then the solution was spun at 1200 rpm to form a thin-film sample on the wall of the test tube. To eliminate possible effects of PCL crystallization and residual solvent on microdomains during the rimming coating process and also achieve better orientation of the forming lamellar microdomain, shearing was carried out at 190 C by using a homemade shear device. The shear device was aerated with nitrogen gas to prevent thermal degradation. The shear frequency was 0.5 Hz, and the shear amplitude was Langmuir 2010, 26(22), 17640–17648

150%. The shear, vorticity and gradient directions were designated as X, Y, and Z, respectively. By carefully controlling shear strain, frequency, temperatures, and shear time, the thin-film BCP samples with well-ordered nanostructures were obtained.36-38

Small Angle X-ray Scattering (SAXS) and Wide Angle X-ray Diffraction (WAXD). Before we applied X-ray to characterize the microphase-separated lamellar morphology and crystal orientation of P4VP-PCL diblock copolymers, we subjected the (38) Chen, Z.-R.; Kornfield, J. A.; Smith, S. D.; Grothaus, J. T.; Satkowski, M. M. Science 1997, 277, 1248–1253.

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Figure 6. (a) Schematic of the 2D WAXD pattern and (b) molecular disposition of PCL crystalline stems corresponding to the lamellar interface.

2sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi3 λ I^ 5 sin - 1 4 Δn ¼ πd I^ þ I== 17644 DOI: 10.1021/la1034432

ð1Þ

Figure 7. Dependence of the isothermal crystallization half time for VP/CL block copolymers with different confined sizes. where 4n is the birefringence, d is the thickness of the sample, λ is the wavelength of the light source, I^ is the intensity of transmitted light between crossed polarizers, and I is the intensity of transmitted light between parallel polarizers. In this study, a He-Ne laser of λ=632.8 nm was used as the light source, and the thickness of the samples was approximately 10 μm as measured by the caliper. )

samples to thermal treatments in a DSC. 2D SAXS/WAXD experiments were performed at BL01B and BL01C SWLS beamline at the National Synchrotron Radiation Research Center (NSRRC), Taiwan. The incident X-ray beam was focused vertically by a mirror and monochromated to the energy of 10.5 keV by a Germanium (111) double-crystal monochromator. The wavelength of the X-ray beam was λ=1.18095 A˚. The 2D SAXS/ WAXD patterns were collected on CCD using X-rays from a synchrotron. The scattering vectors, q (q = (4π/λ) sin(θ/2), where θ is the scattering angle) in these patterns were calibrated using the two standard samples: silver behenate (SAXS) and R-Al2O3 (WAXD), respectively. Contribution of air scattering was subtracted from both the 2D SAXS and WAXD patterns. Differential Scanning Calorimeter (DSC). The crystallization kinetics of isothermally crystallized PCL blocks within the lamellar microdomains was monitored by using DSC (DSC-7 Perkin-Elmer) equipped with a mechanical intracooler to trace the corresponding enthalpy change. The DSC instrument was calibrated appropriately with an In standard at 10 C/min. Samples were annealed at 150 C for 3 min, followed by quenching to preset temperatures for isothermal crystallization; a scan rate 10 C/min was then employed to record the endotherms of PCL crystals in P4VP-PCL diblock copolymers. Transmission Electron Microscopy (TEM). Before TEM characterization, the bulk P4VP-PCL samples were isothermally melt-crystallized in the DSC, followed by ultracryomicrotoming at -120 C using a Reicher Ultracut microtome (equipped with a Reichert FCS cryochamber and a diamond knife). Staining was achieved by exposing the microtomed thin films to the vapors of a 4% aqueous RuO4 solution for 3 h. A JEOL JEM-2100 transmission electron microscope was used (accelerating voltage: 200 kV). Polarizing Light Microscopic (PLM). An Olympus BX-60 equipped with a CCD camera was utilized to examine the morphologies of thin-film BCP samples so as to analyze corresponding thermal transitions. The micrographs were taken at the prescribed temperature. Birefringence Measurement. The P4VP-PCL BCP thin film was placed between crossed polarizers oriented at (45 with respect to the shear direction for the birefringence measurement. The birefringence may be calculated using:

Results and Discussion Oriented Lamellar Nanostructure. Bulk samples of BCPs were first prepared by solution casting. The microphase-separated nanostructures of the P4VP-PCL BCPs were examined by TEM and SAXS. For TEM observation, the samples were sectioned by cryo-microtome, and then stained by RuO4. As illustrated (Figure 1a), for VP/CL 202/116, the microdomains of P4VP phase appear dark due to the staining while the microdomains of PCL appear bright. As expected from the composition, lamellar nanostructure can be observed. Similar TEM results for VP/CL 80/59 and VP/CL 54/94 can be found. Corresponding SAXS results examined at room temperature further confirm the identified Langmuir 2010, 26(22), 17640–17648

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Figure 8. Observation of VP/CL 202/116 thin film by polarized light microscopy between crossed polarizers with the transmitted light passing through the thin film along the shear direction of the thin film after introducing a gypsum plate during the thermal treatment cycle for crystallization and melting; The gypsum plate is inserted into the PLM in the 45 position.

lamellar phase at which the scattering peaks appear at q* ratio of 1:2:3:4 (Figure 1b). Furthermore, the SAXS results provide the long periods of microphase-separated lamellae according to an expression dlam = 2π/q* in which q* is the position of the primary peak. Considering the immerging primary peak with the central beam of X-ray, the long periods were determined by using the secondary peak. Combining the long periods and the volume v ), the PCL lamellae (dPCL) are fractions of PCL blocks ( fPCL determined as 9.5 (VP/CL 80/59), 17.2 (VP/CL 54/94), and 17.9 nm (VP/CL 202/116) therein, respectively. As a result, the microphase-separated lamellae having various long periods can be obtained upon varying the molecular weight of the BCPs. To achieve large-scale, oriented microphase-separated lamellar microdomains, bulk samples of the P4VP-PCL BCPs were prepared by a rimming coating process.28 To further orient the samples, oscillatory shearing was carried out by using a shear device.38 The oriented samples were examined by SAXS with X-ray beams along the shear, vorticity and gradient directions, designated as X, Y, and Z, respectively (Figure 2). The orientation of the microphase-separated lamellar microdomains induced by the shear field through shear device can be identified by the strongly anisotropic patterns along both X and Y directions, showing typical scattering results for distinct microphaseseparated lamellar phase with a q* ratio of 1:2:3:4 being observed, whereas only isotropic rings with weak intensities are observed Langmuir 2010, 26(22), 17640–17648

along the Z direction. The X and Y directions are notably identical. All the P4VP-PCL examined display reflections at multiple integrals of the first principal peak, indicating lamellar phase with long-range orientation and order. Crystalline Orientation and Kinetics under Confinement. In order to truly study the crystallographic details of crystalline anisotropic of PCL crystallites under confinement, the oriented samples of microphase-separated microdomains were prepared as described above. The shear-aligned and then crystallized samples were examined by the combined two-dimensional (2D) SAXS and WAXD measurements. Figure 2c-e shows 2D SAXS patterns taken along the shear, the vorticity, and the gradient directions designated as X, Y, and Z, respectively. As identified, the microphase-separated lamellar normal of shear aligned samples appears as preferred orientation along the gradient direction (namely, Z direction). It is also noted that the amorphous P4VP matrix has a glass transition temperature of 105 C, which is much higher than the melting point of PCL crystallites ranging from 40 to 60 C (see Figure S1 of Supporting Information) (i.e., hard confinement). Consequently, a well-defined system with strong segregation limits for polymer crystallization under vitrified nanoscale confinement could be obtained by controlling crystallization temperature ranging from 20 to 40 C. The positions and principal features of diffraction peaks in the SAXS profiles of the P4VP-PCL after isothermal crystallization (see Figure 2c,d) are DOI: 10.1021/la1034432

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Figure 9. (a) Schematic illustration of the P4VP-PCL thin film with oriented microphase-separated lamellar microdomains probed by He-Ne laser along the shear direction; Birefringence versus crystallinity evolution of VP/CL 202/116 isothermally crystallized at different TC: (b) 20, (c) 30, and (d) 40 C.

almost identical to the diffraction results of amorphous P4VPPCLs (see Figure 1b), revealing that the lamellar nanostructure is faithfully preserved after the crystallization of PCL segment. Corresponding 2D WAXD results along the X, Y, and Z directions (Figures 3a-c) on the aligned sample were also obtained. Similar to the SAXS results, the 2D WAXD patterns reveal strongly anisotropic patterns (like a fiber pattern) for X-ray beams along the X and Y directions but isotropic patterns (like powder rings) along the Z direction for the oriented samples, VP/ CL 202/116, crystallized at various Tc in a range from -10 to 40 C (as shown in Figures 3-5 for representative patterns). For the azimuthally integrated results of the SAXS pattern (for instance Figure 2f), broad diffraction peaks can be observed and the maximum intensities of all the observed scatterings appear at the azimuthal angle, Φ = 90 and 270. By contrast, as for the azimuthally integrated results of the WAXD pattern (Figure 4d), the maximum intensities of all the observed reflections appear at the azimuthal angle, Φ = 0 and 180. The observed reflections are indexed as (110), (200), (210), and (020) on the basis of an orthorhombic crystal structure of PCL. These results indicate that the PCL crystallites under hard confinement form specific crystalline orientation with respect to the shear-induced oriented microphase-separated lamellar microdomains. Similar results have been found for VP/CL 54/94 and VP/CL 80/59 samples crystallized at the same temperature (see Figures S2-S9 of Supporting Information). The azimuthal analysis indicates that the diffraction arcs in the WAXD patterns appear along the orthographic direction with respect to the scattering arcs in the 17646 DOI: 10.1021/la1034432

corresponding SAXS patterns. As a result, the crystallographic fiber pattern of the oriented PCL crystallites is shown in Figure 6a. Combining the corresponding SAXS and WAXD scattering results, the specific scattering features suggest that the molecule chain (c axis) of the PCL crystalline lamellae is oriented parallel to the microphase-separated lamellar surface normal whereas a and b axes are in the X-Y plane, as illustrated in Figure 6b. All PCL orthorhombic crystals within the 17.9-nm lamellar microdomains adopt the perpendicular-type orientation with respect to the lamellar plane regardless of Tc in the typical region of crystallization temperature -10 to þ40 C. As PCL crystallites adopt a perpendicular orientation with respect to the lamellar microdomains, we speculate that the lamellar crystallites might form a periodically ordered structure within the 17.9-nm confined space, i.e., a structure-within-structure morphology. To systematically examine the crystallization kinetics under confinement, isothermal crystallization of P4VP-PCL BCPs was carried out. The relationship between crystallization halftime (t1/2) on TC and dPCL was further examined.17 The upward shift of t1/2 on TC upon decreasing dPCL from 17.9 (VP/CL 202/116) to 9.5 nm (VP/CL 80/59) reflects a significant effect of confined size on the rate of crystallization (Figure 7). As a result, the increase in t1/2 (namely decrease in crystallization rate) can be attributed to the retardation of crystallization in PCL block under confinement. Moreover, this effect becomes more pronounced for high crystallization temperatures. Generally, crystallization at high temperatures, close to Tmo of the PCL crystallites, favors the growth of thinner crystalline stems with less chain folding. Langmuir 2010, 26(22), 17640–17648

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Figure 10. Birefringence versus crystallinity evolution of VP/CL 54/94 isothermally crystallized at various temperatures (a) 20 and (b) 30 C, and VP/CL 80/59 isothermally crystallized at various temperatures (c) 20 and (d) 30 C.

However, we speculate that this process is limited by the energetic barriers arising from the existing confinement. As a result, by varying the molecular weight of the P4VP-PCL BCP, confined PCL crystallization could be obtained so as to give a polymeric system with controlled crystallization kinetics that is dependent upon the confined size, and most importantly with specific anisotropic chain orientation that is a perpendicular orientation. Correspondence of Crystallization and Birefringence. Birefringence is an inherent property of anisotropic materials. For polymeric system, the birefringence arises from the anisotropic orientation of polymeric chains. Because of the unique anisotropic PCL crystalline chain orientation in the oriented semicrystalline P4VP-PCL BCPs, both the amorphous PCL chains and crystalline PCL chains in phase-separated microdomains may contribute to the birefringence. According to the SAXS and WAXD results, the PCL crystallites adopt perpendicular orientation corresponding to the microphase-separated lamellar interface. Figure 8 shows the PLM micrographs of the thin-film sample at different thermal stages. As illustrated, blue to yellow colors can be found in crystallized P4VP-PCL thin films by rotating the oriented samples for 90. This observation further confirms the formation of specific PCL crystallites chain orientation at which the color change from blue to yellow colors is attributed to the change of PCL crystallites chain orientation from parallel to perpendicular with respect to the slow direction of the gypsum plate (long axis of the ellipsoid). By contrast, orange color can be observed while the melting of the PCL crystallites occurs by heating above the melting temperature of the PCL crystallites. On the basis of the sensitive tint of Newton’s color sequence, the PLM observations suggest that the PCL crystallization indeed gives rise to obvious increase in birefringence of the BCP thin films whereas the birefringence of amorphous P4VP-PCL thin films (i.e., P4VP-PCL microphaseseparated lamellae after the melting of PCL crystallites) examined Langmuir 2010, 26(22), 17640–17648

is insignificant under PLM. As a result, the formation of thin-film samples with uniform birefringence can be achieved and the birefringence is appreciably induced by PCL crystallization. Moreover, the uniform birefringence patterns can be clearly identified in the oriented samples; it further confirms that the BCP thin-film samples with long-range well-ordered microstructures can be achieved by the shearing process. According the PLM results, the increase of the birefringence corresponds with the increase of the crystallinity. To systematically investigate the correspondence of crystallization and birefringence in the P4VP-PCL thin films, isothermally crystallized samples were examined by an optical train system to quantitatively measure the birefringence variation with crystallinity evolution using the incident laser beam along the shear direction (Figure 9a). The P4VP-PCL thin films were placed between crossed polarizers oriented at (45 with respect to the velocity gradient direction for birefringence measurement. As shown in Figures 9 and 10, the experimental results demonstrate that the birefringence enhancement during crystallization indeed corresponds well with the increase on crystallinity at different crystallization temperatures. On the basis of experimental observation mentioned above, a mechanism depicting the structural evolution from the amorphous state to the final crystalline state was proposed. At the state of melting, the polymer chains are unoriented and amorphous. Because the main effect of crystallization is to assist the formation of nuclei by the alignment of polymer chains in the supercooled melt. The oriented precursors were formed in the initial state of crystallization. Then, the precursors act as templates for the growth of oriented crystallites in which the polymer chains are aligned preferentially under confinement. When this occurs, the birefringence of the sample starts increasing as the increase of crystallinity. Therefore, the evolution of orientation-dependent birefringence via crystallization is contributed from anisotropic chain orientation along the DOI: 10.1021/la1034432

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normal direction under confinement. As observed, the birefringence resulting from the periodic variation in refractive index due to microphase separated layers is relatively small, suggesting that the enhanced birefringence is intrinsically attributed to the crystallization event. The increase in birefringence due to crystallization can be five times or even order of magnitude than that of amorphous BCPs. It is also noted that the increase in birefringence, resulting from the growth of the highly oriented crystallites, reaches a maximum once the crystallization process completes. The birefringence enhancement well corresponds to the increase of crystallinity, suggesting that the birefringence variation can be controlled by changing crystallization temperature and time. As a result, the birefringence variation in the semicrystalline P4VPPCL block copolymers can be templated by controlling the orientation and crystallinity of the crystalline microdomains due to the formation of preferential crystalline orientation.

Conclusions In this study, P4VP-PCL thin films with large-scale, oriented microphase separated lamellae were prepared to examine the variation on birefringence induced by PCL crystallization under confinement at which the crystalline PCL chains were perpendicular

17648 DOI: 10.1021/la1034432

to micrphase-separated lamellae. According to the PLM observations, the anisotropic orientation of crystalline PCL chains exhibits unique optical properties at which significant enhancement of birefringence can be found after crystallization. Also, the birefringence variation corresponds well with the increase of crystallinity. Consequently, the birefringence of the P4VP-PCL thin films can be fine-tuned by PCL crystallization. The BCP thin films can thus be used as temperature-stimulated materials with controllable birefringence via crystallization kinetics. Acknowledgment. We thank the National Science Council for support. The X-ray experiments were conducted at beamline BL01B and BL01C at the National Synchrotron Radiation Research Center (NSRRC), Taiwan. Supporting Information Available: 2D WAXD/SAXD patterns of VP/CL 54/94 and VP/CL 80/59 at different temperature with the X-ray beam along directions (a) X, (b) Y, and (c) Z and corresponding azimuthal profiles for the 2D WAXD/SAXD pattern with the X-ray along directions (d) X, (e) Y, and (f) Z. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2010, 26(22), 17640–17648