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Morphology-Driven Surface Segregation in a Blend of Poly(E-caprolactone) and Poly(vinyl chloride) Zhuo-Lin Cheung,† Lu-Tao Weng,‡ Chi-Ming Chan,*,† Wei Min Hou,§ and Lin Li*,§ Department of Chemical Engineering and Materials and Characteristic Preparation Facility, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, and State Key Laboratory of Polymer Physics and Chemistry, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China Received March 10, 2005. In Final Form: June 16, 2005 A blend of poly(-caprolactone) (PCL) and poly(vinyl chloride) (PVC) with 90 wt % PCL was prepared. Two films of this blend, which were grown at 35 and 45 °C, showed the absence and presence of banded spherulites, respectively. A detailed examination conducted with time-of-flight secondary ion mass spectrometry (ToF-SIMS) found that the surface composition of the film grown at 45 °C was related to its structure, which was shown to contain ridges and valleys. Phase images obtained using atomic force microscopy (AFM) indicated that the ridges and valleys consisted of edge-on and flat-on lamellae, respectively. ToF-SIMS imaging revealed that PVC and PCL were located mainly on the surface of the valleys and ridges, respectively. This morphology-driven surface segregation was caused by the difference in the surface energy between the flat-on and edge-on lamellae.
1. Introduction 1-3 and Keith and Padden4-6 suggested that the
Bassett formation of circular birefringence extinction patterns called “bands” results from lamellar twisting. Based on atomic force microscopy (AFM) results, Xu et al.7 reported that the morphology of the lamellae on the surface of banded spherulites of chiral poly(R-3-hydroxybutyrateco-R-3-hydroxyhexananoate) changed from the edge-on to flat-on orientation consecutively along the radii. It is known that blends of poly(-caprolactone) (PCL) and poly(vinyl chloride) (PVC) form banded spherulites as PCL crystallizes. Previous studies have shown the effects of crystallinity on the surface chemical composition of blends and copolymers.8-13 Clark et al.8,9 studied the effects of crystallinity and molecular weight on the surface chemical composition of blends of PCL and PVC. Their results showed that the chemical composition of the surface was similar to that of the bulk for the blends containing PVC
Figure 1. SED image of the blend of PCL and PVC grown at 45 °C.
* To whom correspondence should be addressed. Phone: 8522358-7125. E-mail:
[email protected]. † Department of Chemical Engineering, Hong Kong University of Science and Technology. ‡ Materials and Characteristic Preparation Facility, Hong Kong University of Science and Technology. § Chinese Academy of Sciences. (1) Bassett, D. C.; Hodge, A. M. Proc. R. Soc. London 1981, A377, 25. (2) Bassett, D. C.; Olley, R. H.; Al Raheil, A. I. M. Polymer 1988, 29, 1539. (3) Bassett, D. C. Philos. Trans. R. Soc. London 1994, A348, 20. (4) Keith, H. D.; Padden, F. J., Jr. Polymer 1984, 25, 28. (5) Keith H. D.; Padden, F. J., Jr.; Lotz, B.; Wittmann, J. C. Macromolecules 1989, 22, 2230. (6) Keith, H. D.; Padden, F. J., Jr. Macromolecules 1996, 29, 7776. (7) Xu, J.; Guo, B. H.; Zhang, Z. M.; Zhou, J. J. Jiang, Y.; Yan, S.; Li, L.; Wu, Q.; Chen, G. Q.; Schultz, J. M. Macromolecules 2004, 37 (11), 4118. (8) Clark, M. B., Jr.; Burkhardt, C. A.; Gardella, J. A., Jr. Macromolecules 1991, 24, 799. (9) Clark, M. B., Jr.; Burkhardt, C. A.; Gardella, J. A., Jr. Macromolecules 1989, 22, 4495. (10) Schmitt, R. L.; Gardella, J. A., Jr.; Magill, J. H.; Chin, R. L. Polymer 1987, 28, 1462. (11) Gardella, J. A., Jr.; Chen, J. S.; Magill, J. H.; Hercules, D. M. J. Am. Chem. Soc. 1983, 105, 4563. (12) Thomas, H. R.; O’Malley, J. J. Macromolecules 1981, 14, 1316. (13) Thomas, H. R.; O’Malley, J. J. Macromolecules 1979, 12, 323.
Figure 2. Ion map of the blend of PCL and PVC grown at 45 °C.
with a molecular weight of 7.73 × 104 g/mol and l0 wt % PCL. The molecular weight of the PVC used by Clark et al.8,9 in this blend was similar to that of the PVC used in this study. For the blends containing 50 to 75 wt % PCL,
10.1021/la050649n CCC: $30.25 © 2005 American Chemical Society Published on Web 07/19/2005
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Figure 3. AFM images of the blend PVC and PCL grown at 45 °C. (a) A height image showing several bands of a spherulite and selected areas, marked with boxes b, c, and d, which are shown in panels b, c, and d of Figure 3, respectively; (b) A phase image showing the interface between ridge and valley at which edge-on and flat-on lamellae are interwoven together; (c) and (d) Phase images showing the morphologies of the ridges and valleys, containing the edge-on and flat-on lamellae, respectively.
the PCL concentration was much higher at the surface than at the bulk. This is explained by the fact that the surface energy of PCL was lower than that of PVC and an increase in the crystallinity was detected in these blends. However, surface enrichment of PVC was observed for the blend containing 90 wt % PCL. Similar results were found on the blends containing PVC with lower molecular weights. Based on the surface energy argument and the increase in crystallinity, it is not possible to explain the dramatic drop in the surface PCL concentration in the blend containing 90 wt % PCL. In this study, the mechanism of the surface segregation of PVC in the blend of PCL (90 wt %) and PVC (10 wt %) was studied using AFM, X-ray photoelectron spectroscopy (XPS), and timeof-flight secondary ion mass spectrometry (ToF-SIMS). 2. Experimental Section The PCL was obtained from Aldrich Chemical Co., Inc. The weight average molecular weight, M h w, and polydispersity index, M h w/M h n, were 65 000 g/mol and 1.53, respectively. The melting point, Tm, was measured to be about 60 °C using differential scanning calorimetry. The PVC was obtained from Scientific Polymer Products Inc. The weight average molecular weight, M h w, was 90 000 g/mol and the chlorine content is about 56 wt %. Films of the PCL/PVC blend with 90 wt % PCL were prepared by casting from THF solution onto glass plates or silicon wafers. The thickness of the films was measured to be about 0.03 mm using a profilometer. The film was placed on the hot stage of an optical microscope (OM). It was melted at about 100 °C for 30 min to evaporate THF under nitrogen purge and then quenched to 45 °C and annealed for about a week. The film was studied using AFM, contact angle measurements, ToF-SIMS, and XPS measurements directly. AFM height and phase images were collected with a NanoScope III AFM (Digital Instruments) at room temperature. The exact
surface temperature was probably a little bit higher than the room temperature due to the heating of the laser. Si tips with a resonance frequency of ∼300 kHz were used, and the scan rate was 0.8 Hz. The set-point amplitude ratio was set at 0.8. For each image, 512 lines were collected. Contact angles were measured on a Kru¨ss goniometer at room temperature. The volume of the droplet was 15 µL. Three films were used for the contact angle measurements to obtain an average value. The surface energy was determined by using the Owens-Wendt method.14 ToF-SIMS measurements were performed on a Physical Electronics PHI 7200 ToF-SIMS spectrometer. The chemical images of the PCL/PVC films were acquired in the negative mode using a Ga+ liquid metal ion source operating at 25 keV. The mapped area was 200 µm × 200 µm with a maximum of 50 frame scans. The total ion dose was lower than 4 × 1012 ions/cm2. The vacuum was about 1.5 × 10-9 Torr. XPS spectra were recorded on a PHI 5600 multi-technique system equipped with an Al monochromatic X-ray source. A pass energy of 58.7 eV was used. The spectra were obtained at a takeoff angle of 45°.
3. Results and Discussion The XPS result on the film of a blend containing 90 wt % PCL and 10 wt % PVC annealed at 45 °C for 1 week showed that the surface contained 17 wt % PCL. This result agrees with that of Clark et al.8 Figure 1 shows a secondary electron micrograph of the blend obtained in a ToF-SIMS equipped with a secondary electron detector (SED). Few spherulites with diameters approximately 500 µm are observed. The surface is corrugated. A small area near the center part of Figure 1 was chosen for ion mapping. The unique negative ions from PCL and PVC are O- and Cl-, respectively. Figure 2 is an ion map (14) Owens, D. K.; Wendt, R. C. J. Appl. Polym. Sci. 1969, 13, 1731.
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Figure 4. Ion map of the blend of PCL and PVC grown at 35 °C.
showing the distribution of O and Cl. The red and green areas represent the regions containing higher concentrations of O and Cl, respectively. It is easy to see that the distributions of the O and Cl are not uniform on the surface. Red and green concentric rings, which are clearly visible, may be related to the surface morphology of the banded spherulites. In banded spherulites, the ridges and valleys have been shown to represent the edge-on and flat-on lamellae, respectively.7 Figure 3a is a height image showing the surface contains concentric ridges and valleys. Selected areas showing the interface between the ridges and valleys, ridges and valleys are marked with boxes b, c, and d, respectively, in Figure 3a. Figure 3b shows a higher magnification phase image of the interface between the ridges and valleys. It shows the transition from the edgeon lamellae to the flat-on lamellae when moving from the ridges to the valleys. Figures 3c and b are phase images showing that the ridges and valleys contain the edge-on and flat-on lamellae, respectively. From the combined ToF-SIMS and AFM results, we conclude that the Cl concentration in the valleys is much higher than that in the ridges, implying that the PVC concentration in the valleys is higher. To confirm that this concentric-ringlike distribution of PVC at the surface of the blend is related to the corrugated structure revealed by the secondary electron micrograph, as shown in Figure 1, an ion image, as shown in Figure 4, was obtained on another blend sample containing 90 wt % PCL and 10 wt % PVC prepared at 35 °C for a day. The ion map shows that Cl is distributed uniformly on the surface. OM micrographs show that many small spherulites were present in this sample. An AFM phase image obtained on the surface of this sample did not reveal the corrugated surface. The
Cheung et al.
unique Cl ion pattern observed in the ion image, as shown in Figure 2, is related to the morphology of the surface. A higher PVC concentration is found on the surface of flat-on lamellae than on the surface of edge-on lamellae. In general, the surface segregation of the component of a polymer blend is related to its surface energy. The lower surface energy component tends to segregate to the surface. The surface energies of PCL and PVC were reported to be 42.9 and 44.0 J/m2, respectively.8 However, it is not clear that the reported surface energy of PCL is for the surface with the edge-on or flat-on lamellae. An AFM study of a pure PCL film, which was prepared and annealed at 45 °C for 100 hours revealed that the surface consisted of only edge-on lamellae. The surface energy of this PCL film was measured to be 41.9 J/m2. By comparing the surface energy of this PCL film that contains only edge-on lamellae with that of PVC, it is logical to deduce that the surface of the ridges is covered with PCL because PCL has a lower surface energy. It is well-known that the surface energy of flat-on lamellae is 3-6 times higher than that of edge-on lamellae because of the presence of the folding surface.15 Therefore, it follows that the surface of the valleys that consist of mainly flat-on lamellae has a surface energy that is much higher than that of PVC. Consequently, the surface of the valleys was found to have a higher concentration of PVC, as shown in Figure 2. As a result of the segregation of the PVC to the surface of the flat-on lamellae, the PCL concentration as measured by XPS is reduced to a level that is lower than that of the bulk. The distribution of Cl on the surface of the PCL/PVC blend as concentric rings was caused by the change in the surface morphology as the PCL lamellae twisted during their radial growth. The PVC segregated on the surface of the valleys which consist of PCL flat-on lamellae. On the surface of the ridges, which consist mainly of edge-on lamellae, PCL was the dominant species. In summary, this work is an example to show that the surface chemical composition of a blend consisting of amorphous and semicrystalline polymers is controlled by the periodic change in the surface morphology driven by crystallization of the semicrystalline component. Acknowledgment. This work was supported by the Hong Kong Research Grants Council under Grant Nos. HKUST6176/02 and 600503, the National Science Foundation of China, and the Hong Kong Research Grants Council Joint Research Scheme under Grant No. N_HKUST 618/01. This work was also supported by the Outstanding Youth Fund and the National Science Foundation of China (Grant Nos. 20174049 and 20131160730). LA050649N (15) Muthukumar, M. Adv. Chem. Phys. 2004, 128, 1.
