Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
Interface-Induced Crystallization of Polycaprolactone on Graphite via First-Order Prewetting of the Crystalline Phase Ann-Kristin Flieger, Martha Schulz, and Thomas Thurn-Albrecht* Experimental Polymer Physics, Institute of Physics, Martin Luther University Halle-Wittenberg, Halle 06120, Germany S Supporting Information *
ABSTRACT: Interface-induced crystallization of a liquid on a solid substrate can occur either via heterogeneous nucleation or via prefreezing, i.e., the formation of a crystalline layer, which is stable already above the bulk melting temperature. Upon cooling this layer acts as an “ideal crystal nucleus”. Using in situ AFM measurements, we here show that polycaprolactone exhibits prefreezing on graphite. Because of the special wetting behavior of the liquid phase, which is different from the previously investigated system polyethylene on graphite, it is possible to directly measure the thickness of the prefrozen layer as a function of temperature. A thin crystalline layer forms with a finite thickness which increases and theoretically diverges upon approaching the phase transition liquid−solid. The results of our experiments show that prefreezing is a first-order transition.
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INTRODUCTION
On the other hand, the crystal structures of PCL and PE are rather similar, even prefreezing was assumed to take place for PCL on PE.23 PCL is known to crystallize epitaxially on graphite24,25 like PE, and epitaxy was discussed as a mechanism for an efficient polymer−filler interaction.26 However, microscopic observations of nucleation processes at buried interfaces are generally difficult, and experimental findings for PCL are limited to an accelerated crystallization observed on a macroscopic scale. We therefore here follow the experimental approach introduced recently14 and use attractive mode atomic force microscopy (AFM) at high temperatures to observe melting and crystallization of ultrathin films of PCL on graphite substrates. We were able to demonstrate prefreezing as for PE on graphite and in addition to directly measure the thickness of the prefrozen layer as a function of temperature. The jump-like change of the thickness of the prewetting layer at the transition shows that prefreezing is a first-order surface phase transition.
It is well-known that surfaces can induce crystallization and in this way influence crystallization kinetics, structure, morphology, and properties of semicrystalline polymers.1 On a fundamental level, a solid surface can induce crystallization either by heterogeneous nucleation2 or by prefreezing.3,4 While nucleation is a nonequilibrium phenomenon taking place at a finite supercooling below the melting temperature Tm, the lessknown process of prefreezing can be understood as the formation of a crystalline prewetting layer occurring under equilibrium conditions above Tm. Prewetting5 was originally predicted and observed in liquid−vapor systems and binary liquid mixtures.6 For polymer blends, prewetting was studied in simulations.7 In liquid crystals prewetting of the nematic phase was demonstrated.8 Prefreezing, the prewetting of a crystalline layer at the melt−solid transition, was studied theoretically,3,4,9−12 but a direct observation has only been published for a colloidal system13 and recently for polyethylene (PE) on graphite.14 A direct measurement of the thickness of the prefreezing layer as a function of temperature, from which one could conclude on the order of the wetting transition, has to our knowledge not yet been reported. We here show experimental investigations of prefreezing using in situ AFM measurements on the system poly(εcaprolactone) (PCL) on graphite. On the one hand, PCL on graphite is a model system for composites of PCL and graphitic fillers. Composites of the biodegradable and biocompatible PCL with graphitic fillers are interesting materials for applications. Many studies show the nucleating effect and the mechanical reinforcement of graphitic material on semicrystalline PCL.15−18 Recently, these composites were also studied for applications in the medical area.19−22 © XXXX American Chemical Society
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THEORY: INTERFACE-INDUCED CRYSTALLIZATION AND WETTING Wetting phenomena have been most thoroughly studied for the transition liquid−vapor, although the description can be generalized to other transitions occurring in the presence of a wall, interface, or substrate. We give a short review using the system liquid−vapor as an example and then apply the description to the transition liquid crystal. A liquid coexisting with its vapor forms a certain contact angle with a solid surface (Figure 1a). The contact angle θ is Received: September 29, 2017 Revised: December 1, 2017
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DOI: 10.1021/acs.macromol.7b02113 Macromolecules XXXX, XXX, XXX−XXX
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different processes can then lead to interface-induced crystallization. If prefreezing occurs, the prefrozen layer will grow further and act as a kind of “ideal nucleus” for bulk crystallization. Without prefreezing crystallization has to be initiated by nucleation. Nucleation is an activated process requiring some finite supercooling, but the presence of the substrate can cause a substantial lowering of the activation barrier and speed up nucleation significantly.
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Figure 1. (a) Contact angle θ of a droplet of phase b (liquid, crystal) on a solid substrate in the presence of phase a (vapor, liquid). The interfacial energies γ between a, b, and substrate determine θ. (b) Schematic phase diagram showing the coexistence line of crystal and melt with a first-order wetting transition at the wetting temperature Tw and a prewetting line (dashed). The dotted horizontal line corresponds to an experiment in which crystallization occurs during cooling at ambient pressure. Crystallization can either be induced by prefreezing starting at Tmax or by nucleation below Tm.
determined by the three involved interfacial energies/surface tensions γ via Young’s equation. γsub,vap = γsub,liq + γliq,vap cos θ
SAMPLES AND EXPERIMENTAL METHODS
Materials and Thin Film Preparation. Poly(ϵ-caprolactone) was synthesized28 with an average molecular weight of Mn = 23 kg mol−1 and a polydispersity index of Mw/Mn = 1.8. The melting temperature as measured by DSC is about 57 °C; an examplary measurement is shown in Figure S1. Graphite substrates (highly oriented pyrolitic graphite (HOPG)) of type ZYB were purchased from NT-MDT (Moscow, Russia). For comparison and for thickness measurements silicon wafers from Siegert Wafer (Aachen, Germany) were used as substrates. Thin PCL films were prepared by spin coating a PCL solution on freshly cleaved graphite or on silicon wafers with 2000 rpm for 60 s. Toluene was used as a solvent; the concentration was adjusted according to the desired thickness (0.1−0.2 wt % for very thin films below 15 nm, 0.5 wt % for 15 nm, and 3 wt % for 127 nm). After preparation the films were heated to 85 °C for 30 min in a vacuum oven and then slowly cooled (rate ≈ 1 K/min). AFM. AFM intermittent contact mode measurements were performed with the NanoWizard 1 and 4 from JPK Instruments (Berlin, Germany). For imaging at room temperature NSG30 cantilevers from NT-MDT with ωres = 320 kHz and k = 40 N m−1 were chosen. A heatable sample holder (JPK HTST) was used for measurements at elevated temperatures. For these images softer cantilevers MPP-21100 (75 kHz, 3 N m−1) purchased from Bruker (Santa Barbara, CA) and NSG03 (ωres = 90 kHz, k = 1.75 N m−1) from NT-MDT were employed. An excitation frequency ω > ωres and set point ratios of about 30 nm/35 nm were chosen for stable imaging in the net attractive regime of the intermittent contact mode.29 In this regime the tip has less contact with the sample and is therefore more stable against contamination with molten material. Also, the height information is less distorted by indentation compared to the standard net repulsive regime. For net attractive AFM measurements we show height images and the corresponding amplitude signal which is sensitive to changes in height and able to image the semicrystalline morphology. X-ray Diffraction. For wide-angle X-ray scattering (WAXS) a PANalytical (Amelo, Netherlands) Empyrean X-ray diffractometer (λ = 1.54 Å) was used. The WAXS measurements were performed at room temperature. For θ−2θ scans of PCL thin films on graphite the incoming radiation is parallelized and monochromatized using a hybrid monochromator consisting of a parabolic X-ray mirror and a Ge crystal. The scattered beam passes a parallel plate collimator and soller slits before it is detected with a PIXcel 3D detector operated as a receiving slit. To observe the main Bragg reflections of PCL, a scan range of 20.5° ≤ θ ≤ 24.5° and a step size of 0.03° were chosen. The PCL powder sample was measured in Bragg−Brentano geometry with Soller slits. In this case the detector was used as a position-sensitive detector with 0.0131° step size.
(1)
With increasing temperature the contact angle can decrease to zero and a wetting transition can occur, at which the substrate changes from being not wet to being wet, i.e., covered by a macroscopically thick film of liquid. The corresponding transition temperature Tw is called wetting temperature. Upon crossing the wetting transition along the coexistence line the thickness of the liquid layer can increase continuously or with a jump in thickness at Tw.6,27 In the first case the process is called critical wetting, and the wetting transition is of second order; in the second case one speaks of a first-order wetting. A first-order wetting transition is associated with prewetting.5 That means a mesoscopically thick liquid prewetting layer is stable off-coexistence in an area between the coexistence line and the prewetting line (cf. Figure 1b). Upon crossing this prewetting line, the prewetting layer is formed and upon approaching coexistence its thickness diverges. The process of prewetting is well-known for liquid− vapor systems and binary liquid mixtures, and also for polymer blends prewetting was studied theoretically.6,7 Analogous to prewetting of the liquid phase, prewetting of the crystalline phase can occur at the melt−crystal transition. This process is called prefreezing. The knowledge about wetting transitions and prewetting can be generalized to a large extent to prefreezing with some smaller differences as recently discussed by Archer and Malijevský.12 The resulting phase diagram describing the process of prefreezing is shown in Figure 1b. Normally crystallization of a liquid occurs during cooling at ambient pressure. Depending on the existence and location of a wetting transition two thermodynamically
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RESULTS AND DISCUSSION Uniform Epitaxial Crystallization of PCL on Graphite. The effect of a graphite substrate in comparison to the amorphous oxide layer of a silicon wafer on the crystallization of a thin PCL film is shown in Figure 2. Here the semicrystalline morphology was imaged by AFM in the net repulsive mode. Although the same crystallization conditions (heated to 85 °C and slowly cooled) were applied, completely different structures were found for the two substrates. While the B
DOI: 10.1021/acs.macromol.7b02113 Macromolecules XXXX, XXX, XXX−XXX
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Figure 3. WAXS experiments on PCL. The powder diffraction in Bragg−Brentano geometry shows (110) and (111) as well as (200) reflection of the PCL unit cell. The θ−2θ scan of a PCL film with 127 nm thickness crystallized on graphite, however, shows the (110) reflection of PCL only, indicating the directed growth of the (110) planes parallel to the graphite surface.
Melting of PCL Thin Films on Graphite. Melting of bulk PCL takes place close to 60 °C (cf. Figure S1). We observed a similar melting temperature in a PCL film on graphite with a thickness of 127 nm, as shown in Figure 4. In the net attractive Figure 2. Morphology of thin melt-crystallized PCL films on silicon and graphite. Large scale AFM height images (color scale from 0 to 200 nm) of semicrystalline PCL films with 80 nm thickness on (a) silicon and (b) graphite. Smaller scale AFM images of films with 15 nm thickness on silicon ((c), height scale 0−30 nm) and on graphite (phase image in (d), 3-fold symmetry of graphite indicated by red lines). Schematic pictures illustrating the start of crystallization in thin films by the formation of isolated nuclei on silicon (e) and by the formation of a prefreezing layer on graphite (f).
large scale image on silicon (Figure 2a) reveals a 2D spherulitic structure, on graphite (Figure 2b) no spherulites are visible. The smaller scale AFM images show different lamellar orientations. On silicon (Figure 2c) flat-on lamellae are dominant, but on graphite (Figure 2d) only edge-on lamellae can be seen. The azimuthal lamellar orientation reflects the 3fold symmetry of the graphite substrate as indicated by the red lines in Figure 2d. The WAXS data shown in Figure 3 confirm the directed growth of PCL on graphite. The (111) and (200) reflections of PCL are missing in the thin film measurement because of the epitaxial orientation in the film. The graphite surface induces a directed growth of the PCL with the (110) planes being parallel to the graphite surface. The morphology of thin PCL films as presented in Figure 2 is well-known and was reported before for both substrates.25,30 The point here is that the crystallization of PCL is initiated differently on both substrates. While nucleation takes place randomly in thin PCL films on silicon leading to spherulitic lamellar growth (Figure 2e), the highly oriented morphology in the PCL films on graphite suggests a different mechanism. These results resemble the behavior of polyethylene on graphite,14 where we were able to show that the special lamellar structure on graphite is initiated by prefreezing. To prove the existence of a prefrozen layer for PCL on graphite, we used in situ high temperature AFM measurements.
Figure 4. AFM observation of bulk melting. Height and amplitude images taken in the net attractive regime. The melting of a PCL film with 127 nm thickness on graphite can be observed by the disappearence of crystalline lamellae at the indicated temperatures. Color scale of height images from 0 to 9 nm.
image at 59 °C the surface is mostly covered by lamellar crystals. The epitaxial symmetry is obvious. At 61 °C the surface is to a large extent already covered by molten material and much less lamellae, separated by molten material are visible. Note that the thickness of the lamellae does not significantly increase during the melting process. The film is more or less completely molten at 63 °C. The melting temperature of these thick films is therefore in good agreement with DSC results; the difference of a few degrees only is caused by a remaining calibration error. However, thinner PCL films on graphite melt at noticeably higher temperatures. Figure 5 shows AFM amplitude images of a series of measurements recorded during heating of a film with a thickness of about 13 nm. The image at 60 °C still shows the same structure as at room temperature. The film almost completely covers the graphite surface, besides some small C
DOI: 10.1021/acs.macromol.7b02113 Macromolecules XXXX, XXX, XXX−XXX
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Figure 5. Heating of a thin PCL film (ca. 13 nm) on graphite; series of net attractive AFM amplitude images. With increasing temperature melting and dewetting of liquid PCL is observed, leaving behind a thin prefrozen layer of semicrystalline PCL that is stabilized by the interaction with the substrate. The complete series of measurement can be found in Figure S2. The schematic pictures illustrate the morphologies of PCL found at different temperature.
Figure 6. Selected AFM height images (color scale: 0−60 nm) of the measurements shown in Figure 5. Height profiles extracted at 74 and 84 °C at the indicated positions.
crystalline PCL−molten PCL−air, while at 84 °C these are graphite−molten PCL−air. The contact angles of the molten PCL droplet in contact with the crystalline PCL layer at 74 °C and with the graphite at 84 °C are ca. 11° and 15°, respectively. The contact angles were determined from the slope of a linear fit (shown as red lines) to the profile of the droplet. Although the absolute error for the contact angle values amounts to several degrees, the relative difference between the two cases above is clearly visible. It is important to realize that the increased melting temperature of the PCL adjacent to graphite is not caused by an increased thickness of the lamellar crystals. The apparently larger semicrystalline structure observed at 78 and 80 °C in Figure 5 is caused by partial covering of the crystalline layer by molten material. The long period itself does not increase with decreasing film thickness or during heating above the bulk melting point. A more detailed analysis of this point can be found in the Supporting Information (Figures S4 and S5). Temperature Dependent Thickness of the Crystalline Layer. Having established the existence of a prefrozen crystalline layer at temperatures above the bulk melting point, we now turn to the analysis of the layer thickness. The corresponding values were determined from the height images of the above series of AFM measurements in a temperature range between 64 and 84 °C. The result is shown in Figure 7. The complete series of underlying AFM images is shown in Figures S2 and S3. For each temperature, the layer thickness was evaluated at four different positions of the image from three independent profile lines; i.e., 12 individual measurements were performed at each temperature. The procedure is illustrated in Figure S5. The error bars in Figure 7 correspond to the resulting standard deviation. Starting from a crystalline layer thickness of about 13 nm at 64 °C, the thickness decreases continuously during heating and
holes as the one in the lower left corner. The film has a thickness of about 10−15 nm and is covered by some elevations of similar height. These elevations formed during an autophobic dewetting process at higher temperatures and crystallized in this shape as previously observed.14 Everywhere the films shows a semicrystalline morphology. Like in the thicker films the lamellar orientation corresponds to the symmetry of graphite. With further increasing temperature the material in the thin film starts melting, at first only in the highest droplets (66 °C). Next to the molten droplets the lamellar structure is still clearly visible. At the same time, starting at around 74 °C the size of small holes in the thin crystalline layer increases; i.e., the crystalline PCL films dewets from HOPG, and the liquid PCL preferably sits on the crystalline layer but also autophobically dewets from it. Throughout 78 and 80 °C both processes proceed, and at 82 °C only a few crystalline islands are still visible. Finally, at 84 °C the PCL is completely molten forming large round droplets, which sit directly on the substrate. A schematic representation of the structural changes of the polymer during heating is shown in Figure 5 (the dewetting of the crystalline layer from the substrate is not included in this scheme). Obviously the interaction with the graphite substrate stabilizes the crystalline PCL layer up to a temperature of about 82 °C, which is about 20 °C higher than the bulk melting point. The final melting of the underlying crystalline layer also shows up in the shape of the droplets. Selected AFM height images from the series of measurements shown in Figure 5 are displayed in Figure 6. With increasing temperature the droplet volume clearly increases due to the melting of the crystalline layer. This effect becomes even more obvious in the extracted height profiles. Furthermore, the contact angles at 74 and 84 °C are different. As discussed above, Young’s equation describes three phases in contact. At 74 °C the three phases are D
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melting and dewetting. Some samples, like the one shown here, are caught in an intermediate state and still show a more or less complete thin layer with droplets on top (like at 60 °C in Figures 5 and 6), and these were selected for our study. The melting of the crystalline layer can be followed much more easily in this way by AFM. In comparison to our previous results obtained for PE on graphite,14 it is the fact that PCL is less stable on graphite, which makes the direct observation of the crystalline layer thickness possible and therefore allows to directly prove the first-order nature of the prefreezing transition. A last remark concerns the relation of the prefreezing process observed here with the phenomena reported by Asada et al.32 Although in that case also the stabilization of an ordered layer above the melting temperature was observed, we think that the phenomena are not directly related. Stabilization was in that case observed on silicon, a substrate for which we found a completely different morphology not consistent with prefreezing but classical nucleation.
Figure 7. Thickness of the prefrozen layer for PCL on graphite as a function of temperature. Black dots: values determined from the measurements shown in Figures 5 and 6. Unfilled colored symbols: measurements on another three PCL thin films on graphite. The guideline to the eye corresponds to a logarithmic divergence of the layer thickness.4 The two dashed blue vertical lines indicate the bulk melting temperature Tm = 62 °C measured in a thick PCL film on graphite (Figure 4) and the maximum temperature Tmax = 82.5 °C, at which the complete melting of the thin PCL film was observed (average value of all four series of measurements).
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CONCLUSIONS AND OUTLOOK
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ASSOCIATED CONTENT
In this work we demonstrated that the epitaxial crystallization of PCL on graphite is induced by prefreezing. The crystalline phase prewets the substrate in a temperature range above the melting point. We were able to directly measure the thickness of the prewetting layer as a function of temperature and observe the expected jump in thickness at the prewetting line. This result experimentally establishes the first-order nature of the prewetting transition. During further cooling the prewetting layer increases in thickness and finally acts as an “ideal crystal nucleus”, since it already exists at zero undercooling. The fact that crystallization starts from the prefreezing layer leads to the highly oriented semicrystalline morphology observed in thin PCL films on graphite. We suppose that, generally, prefreezing is the underlying mechanism for the enhanced crystallization observed for PCL filled with graphitic materials. With the methodology of in situ AFM experiments being established for the study of interface induced crystallization of polymeric liquids, a number of relevant fundamental questions in this field could be addressed in the future. Examples are the role of epitaxy for interface-induced crystallization and the question which material parameters determine the temperature range of prefreezing. In this context it would also be interesting to perform scattering experiments on the prefrozen film to characterize its crystal structure for different substrates. Furthermore, it would be attractive to study a case showing classical heterogeneous nucleation using the experimental approach introduced here.
saturates at around 5 nm, until it suddenly disappears around 84 °C. We record a thickness of 0 nm. The same experiment was performed with another three samples. The resulting values for the crystalline layer are depicted as unfilled symbols in Figure 7. Apart from small differences in the exact transition temperature, the same behavior including the sudden transition to zero layer thickness was observed.
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DISCUSSION The experimental results of Figure 7 correspond perfectly to the scenario of prefreezing as described in the Theory section (cf. Figure 1b). During cooling from temperatures above the bulk melting temperature the thickness of the wetting layer jumps to a mesoscopic value upon crossing the prewetting line and diverges at approaching coexistence.3,31 Prefreezing or precrystallization can be understood as a first-order surface transition,31 which is in line with the experimentally observed sudden jump of the crystalline layer thickness. A real divergence at lower temperatures can of course not be observed in our sample due to the limited film thickness. One could argue that our experiments were performed during heating only and not during cooling. However, in contrast to nucleation, which is a nonequilibrium process and requires a certain undercooling below Tm, prefreezing is a reversible equilibrium process and heating and cooling are equivalent, although there might exist a certain hysteresis.14 Experimentally, observations during cooling are difficult for PCL, since the molten material irreversibly dewets from the graphite substrate and forms large round droplets at high temperatures. It is very difficult to image the crystalline layer below the molten material. Nevertheless, the reversibility of crystallization in the thin layer was directly observed by cooling the sample again before the complete melting. Exemplary measurements are shown in Figure S7. Generally, the morphology of the samples studied here is derived from the nonequilibrium structure produced during spin-coating. The thermal treatment during sample preparation (heating to 85 °C in an oven) did not always lead to a complete
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02113. Bulk DSC measurements, complete series of AFM measurements (partially shown in Figures 5 and 6), description of thickness evaluation of the prefreezing layer, analysis of long period and reversibility of melting and recrystallization (PDF) E
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nanoplatelets: Relations between nucleation and platelet thickness. Thermochim. Acta 2015, 612, 25−33. (19) Wan, C.; Chen, B. Poly(ε-caprolactone)/graphene oxide biocomposites: mechanical properties and bioactivity. Biomed. Mater. 2011, 6, 055010. (20) Mattioli-Belmonte, M.; Vozzi, G.; Whulanza, Y.; Seggiani, M.; Fantauzzi, V.; Orsini, G.; Ahluwalia, A. Tuning polycaprolactonecarbon nanotube composites for bone tissue engineering scaffolds. Mater. Sci. Eng., C 2012, 32, 152−159. (21) Ramazani, S.; Karimi, M. Aligned poly(ε-caprolactone)/ graphene oxide and reduced graphene oxide nanocomposite nanofibers: Morphological, mechanical and structural properties. Mater. Sci. Eng., C 2015, 56, 325−334. (22) Holmes, B.; Fang, X.; Zarate, A.; Keidar, M.; Zhang, L. G. Enhanced human bone marrow mesenchymal stem cell chondrogenic differentiation in electrospun constructs with carbon nanomaterials. Carbon 2016, 97, 1−13. (23) Chang, H.; Zhang, J.; Wang, Z.; Yang, C.; Takahashi, I.; Ozaki, Y.; Yan, S. A Study on the Epitaxial Ordering Process of the Polycaprolactone on the Highly Oriented Polyethylene Substrate. Macromolecules 2010, 43, 362−366. (24) Sano, M.; Sasaki, D. Y.; Yoshimura, S.; Kunitake, T. Polymerization-induced epitaxy of polylactones on graphite as probed by scanning tunnelling microscopy. Faraday Discuss. 1994, 98, 307− 317. (25) Kikkawa, Y.; Takahashi, M.; Aoyagi, M.; Suga, H.; Kanesato, M.; Abe, H. Surface Patterning of Poly(-caprolactone): Epitaxial Crystallizaion and Enzymatic Degradation. Macromol. Chem. Phys. 2010, 211, 2480−2483. (26) Ning, N.; Fu, S.; Zhang, W.; Chen, F.; Wang, K.; Deng, H.; Zhang, Q.; Fu, Q. Realizing the enhancement of interfacial interaction in semicrystalline polymer/filler composites via interfacial crystallization Topical Issue on Polymer Physics. Prog. Polym. Sci. 2012, 37, 1425−1455. (27) Schick, M. In Les Houches, Liquids at Interfaces; Charvolin, J., Joanny, J. F., Zinn-Justin, J., Eds.; North Holland: Amsterdam, 1990. (28) Schäler, K.; Ostas, E.; Schröter, K.; Thurn-Albrecht, T.; Binder, W. H.; Saalwächter, K. Influence of Chain Topology on Polymer Dynamics and Crystallization. Investigation of Linear and Cyclic Poly(ε-caprolactone)s by 1H Solid-State NMR Methods. Macromolecules 2011, 44, 2743−2754. (29) Henze, T.; Schröter, K.; Thurn-Albrecht, T. Investigation of the different stable states of the cantilever oscillation in an atomic force microscope. Nanotechnology 2012, 23, 245702. (30) Mareau, V. H.; Prud’homme, R. E. In-Situ Hot Stage Atomic Force Microscopy Study of Poly(ε-caprolactone) Crystal Growth in Ultrathin Films. Macromolecules 2005, 38, 398−408. (31) Sear, R. P. Continuity of the nucleation of bulk and surface phases. J. Chem. Phys. 2008, 129, 164510. (32) Asada, M.; Jiang, N.; Sendogdular, L.; Gin, P.; Wang, Y.; Endoh, M. K.; Koga, T.; Fukuto, M.; Schultz, D.; Lee, M.; Li, X.; Wang, J.; Kikuchi, M.; Takahara, A. Heterogeneous Lamellar Structures Near the Polymer/Substrate Interface. Macromolecules 2012, 45, 7098− 7106.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (T.T.-A.). ORCID
Thomas Thurn-Albrecht: 0000-0002-7618-0218 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Elena Ostas for the synthesis of PCL and Katrin Herfurt for the DSC measurement. Funding was provided by Deutsche Forschungsgemeinschaft SFB TRR 102. Fruitful discussions with Wolf Widdra, Wolfgang Paul, and Kay Saalwächter are gratefully acknowledged.
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