Morphology-Driven Surface Segregation in a Blend ofPoly(E-caprolactone) and Poly(vinyl chloride)

Zhuo-Lin Cheung,† Lu-Tao Weng,‡ Chi-Ming Chan,*,† Wei Min Hou,§ andLin 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 ofChemistry, Chinese Academy of Sciences, Beijing 100080, China

Received March 10, 2005. In Final Form: June 16, 2005

A blend of poly(ε-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 bandedspherulites, respectively. A detailed examination conducted with time-of-flight secondary ion massspectrometry (ToF-SIMS) found that the surface composition of the film grown at 45 °C was related toits structure, which was shown to contain ridges and valleys. Phase images obtained using atomic forcemicroscopy (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 andridges, respectively. This morphology-driven surface segregation was caused by the difference in the surfaceenergy between the flat-on and edge-on lamellae.

1. Introduction

Bassett1-3 and Keith and Padden4-6 suggested that theformation of circular birefringence extinction patternscalled “bands” results from lamellar twisting. Based onatomic force microscopy (AFM) results, Xu et al.7 reportedthat the morphology of the lamellae on the surface ofbanded spherulites of chiral poly(R-3-hydroxybutyrate-co-R-3-hydroxyhexananoate) changed from the edge-onto flat-on orientation consecutively along the radii. It isknown that blends of poly(ε-caprolactone) (PCL) and poly-(vinyl chloride) (PVC) form banded spherulites as PCLcrystallizes. Previous studies have shown the effects ofcrystallinity on the surface chemical composition of blendsand copolymers.8-13 Clark et al.8,9 studied the effects ofcrystallinity and molecular weight on the surface chemicalcomposition of blends of PCL and PVC. Their resultsshowed that the chemical composition of the surface wassimilar to that of the bulk for the blends containing PVC

with a molecular weight of 7.73 × 104 g/mol and l0 wt %PCL. The molecular weight of the PVC used by Clark etal.8,9 in this blend was similar to that of the PVC used inthis study. For the blends containing 50 to 75 wt % PCL,

* To whom correspondence should be addressed. Phone: 852-2358-7125. E-mail: kecmchan@ust.hk.

† Department of Chemical Engineering, Hong Kong Universityof Science and Technology.

‡ Materials and Characteristic Preparation Facility, Hong KongUniversity 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. Macro-molecules 1991, 24, 799.

(9) Clark, M. B., Jr.; Burkhardt, C. A.; Gardella, J. A., Jr. Macro-molecules 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 1. SED image of the blend of PCL and PVC grown at45 °C.

Figure 2. Ion map of the blend of PCL and PVC grown at45 °C.

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the PCL concentration was much higher at the surfacethan at the bulk. This is explained by the fact that thesurface energy of PCL was lower than that of PVC andan increase in the crystallinity was detected in theseblends. However, surface enrichment of PVC was observedfor the blend containing 90 wt % PCL. Similar resultswere found on the blends containing PVC with lowermolecular weights. Based on the surface energy argumentand the increase in crystallinity, it is not possible to explainthe dramatic drop in the surface PCL concentration inthe blend containing 90 wt % PCL. In this study, themechanism of the surface segregation of PVC in the blendof PCL (90 wt %) and PVC (10 wt %) was studied usingAFM, X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS).

2. Experimental SectionThe PCL was obtained from Aldrich Chemical Co., Inc. The

weight average molecular weight, Mh w, and polydispersity index,Mh w/Mh n, were 65 000 g/mol and 1.53, respectively. The meltingpoint, Tm, was measured to be about 60 °C using differentialscanning calorimetry. The PVC was obtained from ScientificPolymer Products Inc. The weight average molecular weight,Mh 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 preparedby casting from THF solution onto glass plates or silicon wafers.The thickness of the films was measured to be about 0.03 mmusing a profilometer. The film was placed on the hot stage of anoptical microscope (OM). It was melted at about 100 °C for 30min to evaporate THF under nitrogen purge and then quenchedto 45 °C and annealed for about a week. The film was studiedusing AFM, contact angle measurements, ToF-SIMS, and XPSmeasurements directly.

AFM height and phase images were collected with a NanoScopeIII AFM (Digital Instruments) at room temperature. The exact

surface temperature was probably a little bit higher than theroom temperature due to the heating of the laser. Si tips witha resonance frequency of ∼300 kHz were used, and the scan ratewas 0.8 Hz. The set-point amplitude ratio was set at 0.8. Foreach image, 512 lines were collected.

Contact angles were measured on a Kruss goniometer at roomtemperature. The volume of the droplet was 15 µL. Three filmswere used for the contact angle measurements to obtain anaverage value. The surface energy was determined by using theOwens-Wendt method.14 ToF-SIMS measurements were per-formed on a Physical Electronics PHI 7200 ToF-SIMS spec-trometer. The chemical images of the PCL/PVC films wereacquired in the negative mode using a Ga+ liquid metal ion sourceoperating at 25 keV. The mapped area was 200 µm × 200 µmwith a maximum of 50 frame scans. The total ion dose was lowerthan 4 × 1012 ions/cm2. The vacuum was about 1.5 × 10-9 Torr.XPS spectra were recorded on a PHI 5600 multi-technique systemequipped with an Al monochromatic X-ray source. A pass energyof 58.7 eV was used. The spectra were obtained at a takeoff angleof 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 weekshowed that the surface contained 17 wt % PCL. Thisresult agrees with that of Clark et al.8 Figure 1 shows asecondary electron micrograph of the blend obtained in aToF-SIMS equipped with a secondary electron detector(SED). Few spherulites with diameters approximately500 µm are observed. The surface is corrugated. A smallarea near the center part of Figure 1 was chosen for ionmapping. The unique negative ions from PCL and PVCare O- and Cl-, respectively. Figure 2 is an ion map

(14) Owens, D. K.; Wendt, R. C. J. Appl. Polym. Sci. 1969, 13, 1731.

Figure 3. AFM images of the blend PVC and PCL grown at 45 °C. (a) A height image showing several bands of a spherulite andselected areas, marked with boxes b, c, and d, which are shown in panels b, c, and d of Figure 3, respectively; (b) A phase imageshowing the interface between ridge and valley at which edge-on and flat-on lamellae are interwoven together; (c) and (d) Phaseimages showing the morphologies of the ridges and valleys, containing the edge-on and flat-on lamellae, respectively.

Morphology-Driven Surface Segregation Langmuir, Vol. 21, No. 17, 2005 7969

showing the distribution of O and Cl. The red and greenareas represent the regions containing higher concentra-tions of O and Cl, respectively. It is easy to see that thedistributions 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 bandedspherulites.

In banded spherulites, the ridges and valleys have beenshown to represent the edge-on and flat-on lamellae,respectively.7 Figure 3a is a height image showing thesurface contains concentric ridges and valleys. Selectedareas showing the interface between the ridges andvalleys, ridges and valleys are marked with boxes b, c,and d, respectively, in Figure 3a. Figure 3b shows a highermagnification phase image of the interface between theridges and valleys. It shows the transition from the edge-on lamellae to the flat-on lamellae when moving from theridges to the valleys. Figures 3c and b are phase imagesshowing that the ridges and valleys contain the edge-onand flat-on lamellae, respectively. From the combinedToF-SIMS and AFM results, we conclude that the Clconcentration in the valleys is much higher than that inthe ridges, implying that the PVC concentration in thevalleys is higher. To confirm that this concentric-ring-like distribution of PVC at the surface of the blend isrelated to the corrugated structure revealed by thesecondary electron micrograph, as shown in Figure 1, anion image, as shown in Figure 4, was obtained on anotherblend sample containing 90 wt % PCL and 10 wt % PVCprepared at 35 °C for a day. The ion map shows that Clis distributed uniformly on the surface. OM micrographsshow that many small spherulites were present in thissample. An AFM phase image obtained on the surface ofthis sample did not reveal the corrugated surface. The

unique Cl ion pattern observed in the ion image, as shownin Figure 2, is related to the morphology of the surface.A higher PVC concentration is found on the surface offlat-on lamellae than on the surface of edge-on lamellae.In general, the surface segregation of the component ofa polymer blend is related to its surface energy. The lowersurface energy component tends to segregate to thesurface. The surface energies of PCL and PVC werereported to be 42.9 and 44.0 J/m2, respectively.8 However,it is not clear that the reported surface energy of PCL isfor the surface with the edge-on or flat-on lamellae. AnAFM study of a pure PCL film, which was prepared andannealed at 45 °C for 100 hours revealed that the surfaceconsisted of only edge-on lamellae. The surface energy ofthis PCL film was measured to be 41.9 J/m2. By comparingthe surface energy of this PCL film that contains onlyedge-on lamellae with that of PVC, it is logical to deducethat the surface of the ridges is covered with PCL becausePCL has a lower surface energy. It is well-known that thesurface energy of flat-on lamellae is 3-6 times higherthan that of edge-on lamellae because of the presence ofthe folding surface.15 Therefore, it follows that the surfaceof the valleys that consist of mainly flat-on lamellae hasa surface energy that is much higher than that of PVC.Consequently, the surface of the valleys was found to havea higher concentration of PVC, as shown in Figure 2. Asa result of the segregation of the PVC to the surface of theflat-on lamellae, the PCL concentration as measured byXPS is reduced to a level that is lower than that of thebulk.

The distribution of Cl on the surface of the PCL/PVCblend as concentric rings was caused by the change in thesurface morphology as the PCL lamellae twisted duringtheir radial growth. The PVC segregated on the surfaceof the valleys which consist of PCL flat-on lamellae. Onthe surface of the ridges, which consist mainly of edge-onlamellae, PCL was the dominant species.

In summary, this work is an example to show that thesurface chemical composition of a blend consisting ofamorphous and semicrystalline polymers is controlled bythe periodic change in the surface morphology driven bycrystallization of the semicrystalline component.

Acknowledgment. This work was supported by theHong Kong Research Grants Council under Grant Nos.HKUST6176/02 and 600503, the National Science Foun-dation of China, and the Hong Kong Research GrantsCouncil Joint Research Scheme under Grant No. N_H-KUST 618/01. This work was also supported by theOutstanding Youth Fund and the National ScienceFoundation of China (Grant Nos. 20174049 and20131160730).

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(15) Muthukumar, M. Adv. Chem. Phys. 2004, 128, 1.

Figure 4. Ion map of the blend of PCL and PVC grown at 35°C.

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