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Brewster Angle Microscopy Study of Poly(E-caprolactone) Crystal Growth in Langmuir Films at the Air/Water Interface Bingbing Li,† Yitian Wu,‡ Minghua Liu,‡ and Alan R. Esker*,† Department of Chemistry (0212), Virginia Polytechnic Institute and State UniVersity, Blacksburg, Virginia, 24061, and CAS Key Laboratory of Colloid and Interface Science, Institute of Chemistry, Chinese Academy of Sciences, 100080, P.R. China ReceiVed January 5, 2006. In Final Form: April 9, 2006 Surface pressure-induced crystallization of poly(-caprolactone) (PCL) from a metastable region of the surface pressure-area per monomer (Π-A) isotherm in Langmuir monolayers at the air/water (A/W) interface has been captured in real time by Brewster angle microscopy (BAM). Morphological features of PCL crystals grown in Langmuir films during the compression process exhibit four fully developed faces and two distorted faces. During expansion of the crystallized film, polymer chains slowly detach from the crystalline domains and diffuse back into the monolayer as the crystals “melt”. Typical diffusion-controlled morphologies are revealed by BAM during the melting process as the secondary dendrites melt away faster, that is, at a higher surface pressure than the principal axes. Electron diffraction on Langmuir-Schaefer films suggests that the lamellar crystals are oriented with the polymer chain axes perpendicular to the substrate surface, while atomic force microscopy reveals a crystal thickness of ∼7.6 nm.
Introduction Physical properties of polymers confined in thin or ultrathin films have attracted widespread interest because of their potential applications in semiconductors,1,2 antireflection coatings,3 electrochemical devices,4 and drug delivery systems.5 With decreasing film thickness, polymers in confined geometries exhibit differences from bulk behavior with respect to properties such as molecular mobility,6,7 the glass transition temperature,8,9 chain orientation,10-12 and so forth. In particular, the confinement of semicrystalline polymers to thin film geometries may alter the transport properties of chain segments to the growth fronts of crystallizing lamellae, resulting in changes in the growth rates, crystallinity, morphologies, and melting behavior of crystals.10-24 * To whom correspondence should be addressed. Address: Department of Chemistry (0212), Virginia Polytechnic Institute and State University, Blacksburg, VA, 24061. Telephone: 540-231-4601. Fax: 540-231-3255. E-mail:
[email protected]. † Virginia Polytechnic Institute and State University. ‡ Chinese Academy of Sciences. (1) Frank, C. W.; Rao, V.; Despotopoulou, M. M.; Pease, R. F. W.; Hinsberg, W. D.; Miller, R. D.; Rabolt, J. F. Science 1996, 273, 912-915. (2) O ¨ sterbacka, R.; An, C. P.; Jiang, X. M.; Vardeny, Z. V. Science 2000, 287, 839-842. (3) Walheim, S.; Scha¨ffer E.; Mlynek J.; Steiner, U. Science 1999, 283, 520522. (4) Lu, W.; Fadeev, A. G.; Qi, B. H.; Smela, E.; Mattes, B. R.; Ding, J.; Spinks, G. M.; Mazurkiewicz, J.; Zhou, D. Zh.; Wallace, G. G.; MacFarlane, D. R.; Forsyth, S.; Forsyth, A. M. Science 2002, 297, 983-987. (5) Li, Y. Y.; Cunin, F.; Link, J. R.; Gao, T. R.; Betts, E.; Reiver, S. H.; Chin, V.; Bhatia, S. N.; Sailor, M. J. Science 2003, 299, 2045-2047. (6) Zheng, X.; Sauer, B. B.; van Alsten, J. G.; Schwarz, S. A.; Rafailovich, M. H.; Sokolove, J.; Rubinstein, M. Phys. ReV. Lett. 1995, 74, 407-410. (7) Frank, B.; Gast, A. P.; Russell, T. P.; Brown, H. R.; Hawker, C. Macromolecules 1996, 29, 6531-6534. (8) Forrest, J. A.; Dalnoki-Veress, K.; Dutcher, J. R. Phys. ReV. E 1997, 56, 5705-5716. (9) Forrest, J. A.; Mattson, J. Phys. ReV. E 2000, 61, R53-56. (10) Mareau, V. H.; Prud’homme, R. E. Macromolecules 2003, 36, 675-684. (11) Mareau, V. H.; Prud’homme, R. E. Macromolecules 2005, 38, 398-408. (12) Mareau, V. H.; Prud’homme, R. E. Macromolecules 2002, 35, 53385341. (13) Sakai, Y.; Imai, M.; Kaji, K.; Tsuji, M. Macromolecules 1996, 29, 88308834. (14) Reiter, G.; Sommer, J. U. Phy. ReV. Lett. 1998, 80, 3771-3774. (15) Reiter, G.; Sommer, J. U. J. Chem. Phys. 2000, 112, 4376-4383. (16) Sommer, J. U.; Reiter, G. J. Chem. Phys. 2000, 112, 4384-4393. (17) Reiter, G.; Castelein, G.; Sommer, J. U. Phys. ReV. Lett. 2001, 86, 59185921. (18) Wang, M. T.; Braun, H.-G.; Meyer, E. Polymer 2003, 44, 5015-5021.
Thus, crystallization in thin films can provide an opportunity for the study of polymer chain organization in a confined geometry.10-24 Poly(-caprolactone) (PCL), a model semicrystalline polymer like polyethylene,25 is a hydrophobic polyester with a bulk glass transition temperature of Tg ∼ -60 °C, a melting point of Tm ∼ 50 °C, excellent biocompatibility, and low toxicity. In the past decade, PCL-based systems have attracted considerable interest for controlled-release drug delivery and as scaffolds for tissue engineering, which require a fundamental understanding of PCL’s degradation mechanisms and crystallization properties.26,27 Previous studies of PCL’s crystalline properties include shearinduced crystallization,28 molecular weight-dependent crystallization,29 and highly oriented surface-induced crystallization.30 Spherulitic crystal structures were commonly observed in these cases. The isothermal crystallization of PCL in PCL/poly(vinyl chloride) (PVC) spin-coated films with various thicknesses has recently been reported.10,12 Studies by Mareau et al. 10,12 indicated that isothermal crystallization rates decrease when the film thickness is less than 1 µm, and this behavior is independent of the composition of the blend or the crystallization temperature. Electron diffraction studies of PCL isothermally crystallized in ultrathin films (1-200 nm) suggest a flat-on orientation of lamellae relative to the substrate surface, while diffusioncontrolled growth morphologies were observed for film thick(19) Ferreiro, V.; Douglas, J. F.; Amis, E. J.; Karim, A. Macromol. Symp. 2001, 167, 73-88. (20) Ferreiro, V.; Douglas, J. F.; Warren, J. A.; Karim, A. Phys. ReV. E 2002, 65, 042802/1-042802/4. (21) Ferreiro, V.; Douglas, J. F.; Warren, J. A.; Karim, A. Phys. ReV. E 2002, 65, 051606/1-051606/16. (22) Beers, K. L.; Douglas, J. F.; Amis, E. J.; Karim, A. Langmuir 2003, 19, 3935-3940. (23) Scho¨nherr, H.; Frank, C. W. Macromolecules 2003, 36, 1188-1198. (24) Scho¨nherr, H.; Frank, C. W. Macromolecules 2003, 36, 1199-1208. (25) Bittiger, H.; Marchessault, R. H. Acta Crystallogr. 1970, B26, 19231927. (26) Zhu, Y. B.; Gao, C. Y.; Liu, X. Y.; Shen, J. C. Biomacromolecules 2002, 3, 1312-1319. (27) Kweon, H.-Y.; Yoo, M.-K.; Park, I.-K.; Kim, T.-H.; Lee, H.-C.; Lee, H.-S.; Oh, J.-S.; Akaike, T.; Cho, C.-S. Biomaterials 2003, 24, 801-808. (28) Lellinger, D.; Floudas, G.; Alig, I. Polymer 2003, 44, 5759-5769. (29) Chen, H.-L.; Li, L.-J.; Ou-Yang, W.-C.; Hwang, J.-C.; Wong, W.-Y. Macromolecules 1997, 30, 1718-1722. (30) Liu, J. C.; Li, H. H.; Yan, S. K.; Xiao, Q.; Petermann, J. Colloid Polym. Sci. 2003, 281, 601-607.
10.1021/la060048b CCC: $33.50 © 2006 American Chemical Society Published on Web 04/21/2006
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Langmuir, Vol. 22, No. 11, 2006 4903
nesses less than 15 nm.11 For the aforementioned studies, the thin films of PCL or its blends were usually prepared on solid substrates, such as silicon wafers, glass, or mica by spin-coating. The film thicknesses were usually adjusted by varying the concentrations of the polymer solutions used for spin-coating. At the air/water (A/W) interface, PCL Langmuir films formed at low surface pressures are uniform monomolecular layers, which can be thought of as the thinnest possible uniform film of PCL. The ultrapure water surface minimizes surface defects compared to solid substrates, which may affect the nucleation mechanism. While the crystallization of semicrystalline polymers in thin films on solid substrates has been widely studied,10-24 the nucleation and growth of PCL crystals grown in Langmuir monolayers at the A/W interface represent a unique model system. This study provides a detailed examination of the nucleation and growth of linear flexible PCL in Langmuir monolayers at the A/W interface. In situ Brewster angle microscopy (BAM) studies were carried out simultaneously during hysteresis experiments to monitor both the crystallization and “crystal melting” processes of PCL in monolayers. Analyses of surface pressure (Π) and the static elastic moduli (S) in terms of surface area per monomer (A) indicate possible conformational changes and phase transitions occurring in PCL Langmuir films at the A/W interface.
Figure 1. Π-A isotherm of PCL obtained at 22.5 °C during compression (solid line) and expansion (dashed line) at 0.010 nm2‚monomer-1‚min-1. The circle indicates the starting and end points of the hysteresis loop used for crystallization studies. A second spreading under more dilute conditions was used to measure the isotherm out to Π ∼ 0 mN‚m-1. The letters on the graph correspond to the BAM images in Figure 3. The inset shows the static elastic moduli, s, as a function of A.
Experimental Section PCL (weight average molar mass, Mw ) 10 kg‚mol-1; polydispersity, Mw/Mn ) 1.25), purchased from Polymer Source, Inc., was used as received. Chloroform (HPLC grade) solutions of PCL prepared at concentrations of ∼0.4 mg‚g-1 were spread onto the surface of ultrapure water (18.2 MΩ, Milli-Q Gradient A-10, Millipore) in a standard Langmuir trough (500 cm2, 601BAM, Nima Technology). The trough is maintained at 22.5 °C in a Plexiglas box with a relative humidity of 70-75%. The surface pressure, Π, was determined by the Wilhelmy plate technique using a paper plate. Polymer solutions were spread onto the water surface using a Hamilton gastight glass syringe, and the spreading solvent was allowed to evaporate by waiting for a suitable amount of time (∼20 min). For compression and hysteresis loop experiments, the surface layer was compressed at a rate of ∼0.010 nm2‚monomer-1‚min-1 to an arbitrary average area per monomer value and was immediately expanded at the same rate back to the initial trough area. To investigate the morphological features of PCL films at different stages during hysteresis experiments, simultaneous BAM (MiniBAM, NanoFilm Technologies GmbH, linear resolution: e20 µm) studies were carried out and BAM micrographs were taken with a charge-coupled device (CCD) camera. The Langmuir trough, BAM, and Plexiglas box rest on a floating optical table to minimize vibrations. PCL crystal samples for electron diffraction studies were transferred onto a carbon-coated copper grid at a surface pressure of ∼11 mN‚m-1 using the Langmuir-Schaefer (LS) method. A JEOL TEM-2010 electron microscope (Japan) equipped with a CCD camera operating at 200 kV was used to obtain electron diffraction patterns of PCL crystals.
Results and Discussion Figure 1 shows a Π-A isotherm of PCL with a weight average molecular weight of 10 kg‚mol-1. The Π-A isotherm was measured at 22.5 °C with a compression and expansion rate of ∼0.010 nm2‚monomer-1‚min-1. Because of our desire to grow larger crystals, PCL was spread to an initial Π value of 1.2 mN‚m-1 (A ∼ 0.63 nm2‚monomer-1) corresponding to a homogeneous liquid-expanded (LE) monolayer for the hysteresis experiment. A separate experiment was carried out to obtain the compression isotherm out to Π ∼ 0 mN‚m-1. The LE regime reflects the amphiphilic nature of PCL molecules since most of carbonyl groups are able to adsorb to the water subphase with the ethylene groups, preventing the molecules from dissolving into the subphase. Upon compression, there is a gentle rise in
Figure 2. Schematic diagram of PCL crystallization at the A/W interface: (a) the PCL repeat unit, (b) a condensed monolayer prior to nucleation at A ∼ 0.37 nm2‚monomer-1 that ultimately forms folded chain lamellae, (c) PCL crystals (this 1.28 × 0.96 mm2 image is seen upon compression), and (d) electron diffraction pattern of PCL crystals after LS transfer onto a copper grid.
Π and an increase in the film’s static elastic modulus, defined as s ) -A(dΠ/dA)T, until s reaches a maximum value of ∼15 mN‚m-1 at A ∼ 0.37 nm2‚monomer-1, as seen in the inset of Figure 1. At this point, it is likely that some of the carbonyl groups of the polymer backbone start to leave the A/W interface. Looking at the repeat unit structure in Figure 2a, and noting that “two-dimensional” (2D) monolayers of methyl esters of fatty acids become unstable at A ∼ 0.185 nm2‚monomer-1, and undergo a 2D f 3D transition upon further compression, the structure depicted in Figure 2b (bottom) ideally describes the monolayer structure corresponding to the maximum s value at A ∼ 0.37 nm2‚monomer-1. Further compression leads to a nearly linear decrease in s corresponding to the collapse of the structure in Figure 2b (bottom). Upon further compression, the ester groups could be forming hairpin turns because of the greater rotational freedom of ester linkages around the polymer backbone, thereby allowing the chain to fold back and forth in a fashion similar to the folded chain structure of polyethylene single crystals formed from dilute solution.31,32 Finally, the observable nucleation event from a “supersaturated solution”, that is, an overcompressed (31) Sperling L. H. Introduction to Physical Polymer Science, 3rd ed.; WileyInterscience: New York, 2001. (32) Kajiyama, T.; Ohki, I.; Takahara, A. Macromolecules 1995, 28, 47684770.
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polymer monolayer, was captured by BAM and corresponds to point a on the Π-A isotherm. These features are also consistent with the orthorhombic unit cell parameters of PCL crystals determined from X-ray data (a ) 0.748 ( 0.002 nm, b ) 0.498 ( 0.002 nm, c ) 1.726 ( 0.003 nm).33,34 During compression to point a, BAM shows the monolayer is homogeneous. Upon further compression from point a to point b in Figure 1, the number of stable nuclei increases, and crystals grow around the stable nuclei. The kink at point b in Figure 1, called a dynamic “collapse point” (Πc ∼ 11.3 mN‚m-1, Ac ∼ 0.20 nm2‚monomer-1), corresponds to the transport of polymer chains from the monolayer to crystal lamellae, as seen in Figure 2b (top). Thus, crystal growth becomes a dominant contributor to the phase behavior of PCL Langmuir films during the following plateau region. The above conclusion is further supported by the observation that the plateau region corresponding to crystal growth can be lengthened if one spreads to a higher surface pressure prior to compression. Figure 2c exhibits the morphology of a PCL crystal grown in the plateau region. The development of symmetrically distorted hexagonal shapes could be caused by the reduced diffusion coefficient of polymer chains on the water surface. Mareau et al. recently indicated that PCL crystallized in spin-coated films with thicknesses between 30 and 200 nm have a shape reminiscent of flat-on single crystals obtained from solution.10-12,35 For films on solid surfaces with thicknesses in the range of 1 ∼ 6 nm, dendritic morphologies are observed, and the shapes of the lamellar crystals are distorted with the disappearance of two of the four {110} sectors.11 Interestingly, atomic force microscopy images from LS films of PCL crystals formed at the A/W interface reveal a lamellar thicknesses of 7.6 nm (Supporting Information). This observation suggests that the lamellar thicknesses of PCL crystals grown in Langmuir monolayers at A/W are consistent with the lamellar thickness for flat-on PCL lamellae grown in spin-coated films, even though the shear forces resulting in the distortion of hexagonal crystals at the A/W interface are different from those on a solid substrate.10-12 These factors lead to the novel morphology of PCL crystals observed in Langmuir films, as shown in Figure 2c. More details regarding crystal morphology are discussed later in this paper, and morphologies are better illustrated in Figure 3. At the A/W interface, the growth of flaton lamellae is confirmed by the electron diffraction pattern of PCL crystals transferred by the LS method, as seen in Figure 2d. Based on the previous paragraph, the chain folds occur at the A/W interface, and the crystals grow parallel to the surface upon further compression. The apparent mechanism for the crystal growth at the A/W interface could be analogous to the nucleation and growth mechanism proposed by Vollhardt et al. for small molecule amphiphiles.36,37 As seen in Figure 3b (circle), 10 kg‚mol-1 PCL already shows anisotropic crystal growth at very early stages. As the crystals continue to grow, this anisotropy becomes more apparent (Figure 3e), ultimately revealing the dendritic nature of the crystals during the early stages of expansion, corresponding to the melting process for the crystals (Figure 3f). In bulk systems, dendritic structures frequently arise during crystallization from a failure to dissipate the latent heat of crystallization.36-39 However, as Vollhardt et al. pointed out, (33) Chatani, Y.; Okita, Y.; Tadokoro, H.; Yamashita, Y. Polym. J. 1970, 1, 555-562. (34) Hu, H. L.; Dorset, D. L. Macromolecules 1990, 23, 4604-4607. (35) Iwata, T.; Doi, Y. Polym. Inter. 2002, 51, 852-858. (36) Vollhardt, D.; Retter, U. J. Phys. Chem. 1991, 95, 3723-3727. (37) Vollhardt, D.; Ziller, M.; Retter, U. Langmuir 1993, 9, 3208-3211. (38) Witten, T. A.; Sander, L. M. Phys. ReV. Lett. 1981, 47, 1400-1403. (39) Vollhardt, D.; Gutberlet, T.; Emrich, G.; Fuhrhop, J.-H. Langmuir 1995, 11, 2661-2668.
Letters
Figure 3. BAM images obtained during a hysteresis experiment at 22.5 °C with a compression and expansion rate of ∼ 0.010 nm2‚monomer-1‚min-1. Compression (A/nm2‚monomer-1): (a) 0.243, (b) 0.200, (c) 0.170, (d) 0.120, and (e) 0.080. Expansion (A/nm2‚monomer-1): (f) 0.082, (g) 0.173, (h) 0.272, and (i) 0.387. All images are 1.28 × 0.96 mm2.
direct coupling of the monolayer to the subphase at the A/W interface alleviates this problem, and dendritic crystals arise from a difference in the rate of crystallization compared to the diffusion of new material to the growing crystal front.37,39 Even though the PCL crystals have irregular shapes (Figure 3a-e), dendritic structures are not observed during compression. However, dendritic crystal morphologies are clearly observed during the analogous crystal melting process. To observe this, the compressed film is expanded (dashed curve in Figure 1), and BAM is used to image the melting process accompanying the large hysteresis loop. During the early stage of expansion, the arrangements of PCL chains in the crystals undergo rapid relaxation as the external forces that were applied during compression are removed. The metastable crystals formed during compression become loose, and the polymer chains attached on the unstable edges along the growth fronts have greater mobility than the remainder of the crystal at the A/W interface. As a result, dendritic morphologies are seen in Figure 3f as the edges of the crystals melt first. Upon further expansion, parts of PCL chains apparently fall from the unstable edges back to the water surface, which is analogous to the crystal melting process as revealed in Figure 3f-i. The lighter secondary dendrites melt away faster than the principal axes. At lower surface pressure (analogous to a higher melting temperature) the principal axes also melt. The contrast between the principal axes and the secondary dendrites is consistent with the contrast mechanism for BAM, where both the thickness and refractive index differences contribute to the reflectivity of p-polarized light. Hence, even though the macroscopic measure of hysteresis is large (Π-A isotherm), the microscopic details (BAM) suggest crystal melting is still transport-limited. Nonetheless, the melting process is not completely reversible. Smaller crystals are observed during the second compression process (See Figure S2 in the Supporting Information). This feature means that the original state of molecular arrangement at the water surface prior to the initial compression of the films is not completely recovered following the first hysteresis cycle. Some locally well-ordered structures that are too small to be observed by BAM must remain after the first hysteresis cycle. These locally well-ordered aggregates serve as heterogeneous nucleation centers during the second compression step. As a result, there are more nuclei for crystallization during the second compression step. Hence, there are more, but smaller, crystals in the second compression step
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Langmuir, Vol. 22, No. 11, 2006 4905
Figure 4. Compression rate dependence of crystal growth in Langmuir monolayers at 22.5 °C. BAM images were obtained at compression rates of (a) 0.010, (b) 0.013, and (c) 0.026 nm2‚ monomer-1‚min-1 for A ∼ 0.08 nm2‚monomer-1. All images are 1.28 × 0.96 mm2.
than there are during the first compression step. In addition, increasing the compression rate during the initial constant compression rate experiment also leads to more and smaller crystals, as seen in Figure 4. Increasing the compression rate corresponds to an increase in the degree of undercooling because structural relaxation is hampered at higher compression rates. Since nucleation from the supersaturated monolayer with a higher degree of undercooling should be easier, the number of nuclei formed increases with increasing compression rate. Such an observation is consistent with the nucleation and growth mechanism of PCL crystals grown in bulk.
Conclusions In summary, “crystallization” processes for linear flexible PCL from Langmuir monolayers at A/W have been captured in real time by BAM. Moreover, the lamellar thicknesses of PCL crystals grown in monolayers correspond to the thickness for the crossover
from single-crystal morphologies to dendrites observed by Mareau et al. in spin-coated PCL films with film thicknesses less than 6 nm.11 Electron diffraction analysis indicates that the lamellae of the crystals are oriented with the polymer chain axes perpendicular to the substrate surface. The occurrence of crystallization during the “collapse” of PCL monolayers could make the system particularly suitable for studying the effects of substrate-film interactions on crystal morphology and testing models for crystallization kinetics in thin films. Furthermore, during the expansion of crystallized films, the polymer chains slowly detach from crystalline domains and diffuse back into the monolayer. Typical transport-limited growth morphologies are revealed during expansion. The secondary dendrites melt away faster than the principal axes are able to redissolve at higher pressure, analogous to a “lower melting temperature”. The morphologies and preferred direction of crystal melting were clearly revealed in real time by BAM, suggesting that further insight into the crystallization of Langmuir films could be studied through hysteresis experiments. Acknowledgment. Financial support by the National Science Foundation (CHE-0239633) and useful discussions with Herve Marand and Jianjun Deng are greatly appreciated. Supporting Information Available: Figure S1: AFM images and cross-section analysis for PCL (10 kg‚mol-1) crystals transferred onto Si substrates. Figure S2: BAM images of PCL (10 kg‚mol-1) crystals observed during two hysteresis loops. This material is available free of charge via the Internet at http://pubs.acs.org. LA060048B
