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Concentric-Ringed Structures in Polymer Thin Films Yong Wang,† Chi-Ming Chan,*,† Lin Li,*,§ and Kai-Mo Ng†,‡ Department of Chemical Engineering and AdVanced Engineering Materials Facility, Hong Kong UniVersity of Science and Technology, Clear Water Bay, Kowloon, 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 31, 2006. In Final Form: May 18, 2006 Novel polymer crystalline structures containing micrometer-sized concentric rings (or bands) were observed in thin poly(bisphenol A hexane ether) (BA-C6) films. The origin of the banded structures was found to be different from that of traditional banded spherulites in polymer systems. Analyses based on optical microscopy (OM) and atomic force microscopy (AFM) revealed that the banded structures contained alternating ridge and valley bands of polymer crystals in the flat-on orientation. No lamellar twisting was observed within the concentric-ringed structures, which were developed as a result of the formation of a depletion zone during crystallization. The formation of a depletion zone was determined to be caused by the specific volume decrement between the crystal and the melt and by the diffusion of polymer chains to the fold surfaces of the flat-on lamellae. The height of the ridges and the interband widths could be adjusted by controlling the diffusion rate. Time-of-flight secondary ion mass spectrometry ion images showed higher concentrations of low-molecular-weight polymer chains on the surfaces of the ridges than in the valleys.
Introduction Naturally occurring, concentric-ringed (or banded) patterns on the macroscopic scale in mineralogical, geological, biological, and physicochemical systems have attracted the scientific interest of many researchers.1-7 Well-known examples are found in agate and malachite, the annual rings in plants and trees, and the Liesegang rings in reaction-diffusion processes. Polymer scientists are familiar with the concentric-ringed patterns formed in banded spherulites of semicrystalline polymers.8-10 It is wellknown that, in such polymers, there is regular and smooth lamellar twisting along the radial direction, which causes periodic bands within spherulites, resulting in the concentric-ringed pattern. In addition to this kind of traditional banded spherulite, banding patterns can also be found in other crystalline structures. In the 1950s, Keith,11 Schuur,12 and Schramm13 reported that the banding patterns of crystalline structures appeared in sharp contrast under either unpolarized or polarized light, but that they disappeared under plane-polarized light when the plane of polarization was perpendicular to the bands in thin films. Because of the limitation of optical microscopy (OM) (its low resolution), the detailed structure of the crystal bands could not be described. Spherulites, including banded spherulites, are common structures of the crystal aggregates in polymer bulk and thick films.8-16 * To whom correspondence should be addressed. † Department of Chemical Engineering, Hong Kong University of Science and Technology. ‡ Advanced Engineering Materials Facility, Hong Kong University of Science and Technology. § Chinese Academy of Sciences. (1) Bottinga, Y.; Kudo, A.; Weill, D. Am. Mineral. 1966, 51, 792. (2) Haase, C. S.; Chadam, J.; Feinn, D.; Ortoleva, P. Science 1980, 209, 272. (3) Allegre, C. J.; Provost, A.; Jaupart, C. Nature 1981, 294, 223. (4) Pearce, T. H.; Russell, J. K.; Wolfson, I. Am. Mineral. 1987, 72, 1131. (5) Reeder, R. J.; Fagioli, R. O.; Meyers, W. J. Earth Sci. ReV. 1990, 29, 39. (6) Peter, J. H.; Andrew, M. D. Science 1995, 269, 1562. (7) Liesegang, R. E. Naturwiss. Wochenschr. 1896, 11, 353. (8) Keith, H. D.; Padden, J. F. J. Polym. Sci. 1959, 39, 101. (9) Keith, H. D.; Padden, J. F. J. Polym. Sci. 1959, 39, 123. (10) Keller, A. J. Polym. Sci. 1959, 39, 151. (11) Keith, H. D.; Padden, J. F. J. Polym. Sci. 1958, 31, 415. (12) Schuur, G. J. Polym. Sci. 1953, 11, 385. (13) Schram, A. Kolloid-Z. 1957, 151, 18. (14) Keller, A. J. Polym. Sci. 1955, 17, 291.
Table 1. Physical Properties of the BA-C6 Samples
c
sample
(g/mole)
PDIa
Tgb (°C)
Tmc (°C)
BA-C6-60F BA-C6-67 BA-C6-124 BA-C6-164
6000 6700 12 400 16 400
1.88 1.80 1.76 1.65
22.2 21.9 33.7 35.2
92.2 92.5 97.1 101.4
a PDI ) polydispersity index. b T ) glass transition temperature. g Tm ) melting point.
According to the most recent reports,17-20 the formation of spherulites can be described as a simple process. The formation of a structure starts with nucleation; then the nucleus develops into a founding lamella through repeated chain folding with continuous branching and splaying of lamellae; the founding lamella then grows into a lamellar sheaf and finally into a mature spherulite. However, in polymer thin films, spherical crystal aggregates cannot form because of the mass and spatial confinements. In addition to a thickness confinement, dynamic confinements in mass and space around the growth fronts of the crystals (or crystal aggregates) play a vital role in governing the crystal growth and the subsequent development of crystalline structures. It is well-known that the specific volume of a crystal is smaller than that of the amorphous phase of a polymer. During crystallization, the polymer melt that resides slightly away from the crystal growth front must continuously flow into the depletion zone to sustain the growth process. If the diffusion rate of the polymer is lower than the rate at which the mass is used for crystallization, a depletion zone will be created. In polymer films, the formation of a crystalline structure from a flat-on founding (15) Phillips, P. J.; Andrews, E. H. Polym. Lett. 1972, 10, 321. (16) Phillips, P. J.; Edwards, B. C. J. Polym. Sci., Polym. Phys. Ed. 1975, 13, 1819. (17) Li, L.; Chan, C. M.; Li, J. X.; Ng, K. M.; Yeung, K. L.; Weng, L. T. Macromolecules 1999, 32, 8240. (18) Lei, Y. G.; Chan, C. M.; Li, J. X.; Ng, K. M.; Wang, Y.; Jiang, Y.; Li, L. Macromolecules 2002, 35, 6751. (19) Hobbs, J. K.; McMaster, T. J.; Miles, M. J.; Barham, P. J. Polymer 1998, 39, 2437. (20) Wang, Y.; Chan, C. M.; Ng, K. M.; Jiang, Y.; Li, L. Langmuir 2004, 20, 8220.
10.1021/la060863r CCC: $33.50 © 2006 American Chemical Society Published on Web 07/15/2006
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stages, resulting in molecular fractionation.22,23 Polymer chains with high MWs continuously diffuse onto the crystal growth surface and crystallize, while the lower MW polymer chains and the noncrystallizable components are excluded from the growth surface and diffuse onto the fold surface of the crystals to minimize the interfacial free energy. As a result of this fractionation process, it is suggested that the surfaces of the ridges have a higher concentration of low-MW polymer chains than the surfaces of the valleys have. To this end, we observed concentric-ringed structures forming in thin films of poly(bisphenol A hexane ether) (BA-C6). OM and atomic force microscopy (AFM) were used to capture the images of the structures, and time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used to study the surfaces of the ridges and the valleys. The effects of several parameters, including MW, crystallization temperature (Tc), and film thickness, which can affect the diffusion of the polymer, on the formation of these concentric-ringed structures were studied. Experimental Section Figure 1. Optical micrographs of crystal aggregates of a 185-nmthick BA-C6-124 film annealed at 80 °C for 2 days. (a) Spherulites and concentric-ringed crystalline structures. The bar line represents 50 µm. (b) A concentric-ringed crystalline structure. The bar line represents 10 µm.
lamella near or at the polymer/substrate interface creates a depletion zone around the lamella due to the accumulation of crystals growing in the upward and radial directions. Recently, a depletion zone-induced, concentric-ringed crystalline structure was observed in thin isotactic polystyrene films.21 Practically all polymers are polydispersed. Polymer chains with different molecular weights (MW) crystallize at different
BA-C6 was synthesized by condensation polymerization of bisphenol A and 1,6-dibromohexane.17,18,24 A slight excess of 1,6dibromohexane was used to maintain Br as the end group. The surface concentration of Br, which can be determined using ToFSIMS, was used as a measure of the concentration of the end groups. The physical properties of BA-C6 polymers are summarized in Table 1. Thin films with various thicknesses were prepared using 3 to 20 mg mL-1 polymer-chloroform solutions at spin-coating speeds varying from 3000 to 5000 rpm. Glass slices were used as the substrates for the OM studies, and silicon wafers were used as the substrates for AFM and ToF-SIMS studies. The samples were dried under vacuum at room temperature for 15 min before being heated to the desired temperature for the crystallization study. The thickness of the amorphous polymer films was estimated using an ellipsometer or a profilometer.
Figure 2. A series of AFM images showing the detailed structures of a typical concentric-ringed crystalline structure formed in a 190-nm-thick BA-C6-124 film annealed at 80 °C for 2 days. (a,b) AFM topographic images of a concentric-ringed crystalline structure and its height profile, respectively. The scale bar in panel a represents 10 µm. (c,e,g,i) AFM height images. (d,f,h,j) Phase images of the detailed structures at the central concave region, the first ridge band, the first valley band, and the second ridge band, respectively. The scale bar in panel c represents 400 nm.
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Figure 3. A concentric-ringed crystalline structure. (a) An AFM height image of concentric-ringed structures. (b-e) AFM height images and their height profiles recorded at the initial stage of the formation of concentric-ringed structures. (f-j) Schematic diagrams showing the construction of the central valley and the first ridge band. (k-o) Schematic diagrams showing the birth and development of a growth ridge band into a mature one. Tapping-mode AFM images were obtained using a NanoScope III MultiMode AFM (Digital Instruments) equipped with a hightemperature heater accessory (Digital Instruments). Both topographic and phase images were recorded simultaneously using the retrace signal. Si tips with a resonance frequency of approximately 300 kHz and a spring constant of about 40 N m-1 were used, and the scan rate was in the range of 0.4 to 1.2 Hz/sec with a scanning density of 512 lines/frame. The set-point amplitude ratio ranged from 0.7 to 0.9. A polarized optical microscope (Olympus BX50) equipped with a hot stage and a Sony DXC-950P video camera was used. The ToF-SIMS measurements were performed on a Physical Electronics PHI 7200 ToF-SIMS spectrometer. The chemical images of the BA-C6 polymer films were acquired in the negative mode using a 69Ga+ liquid metal ion source operating at 25 keV. The mapped area was 100 × 100 µm with a maximum of 50 frame scans. The total ion dose was lower than 4 × 1011 ions/cm2. The vacuum was about 1.5 × 10-9 Torr. (21) Duan, Y. X.; Jiang, Y.; Jiang, S. D.; Li, L.; Yan, S. K.; Schultz, J. M. Macromolecules 2004, 37, 9283. (22) Wunderlich, B.; Mehta, A. J. Polym. Sci., Polym. Phys. Ed. 1974, 12, 255. (23) Cheng, S. Z. D.; Wunderlich, B. J. Polym. Sci., Polym. Phys. Ed. 1986, 24, 595. (24) Li, L.; Chan, C. M.; Yeung, K. L.; Li, J. X.; Ng, K. M.; Lei, Y. G. Macromolecules 2001, 34, 316.
Results and Discussion We observed concentric-ringed patterns in BA-C6-124 thin films using an optical microscope when the crystallization had occurred at relatively high temperatures (80-95 °C). Figure 1 shows two optical micrographs obtained from a BA-C6-124 film crystallized at 80 °C for about 2 days. The film thickness was measured to be about 185 nm. In Figure 1a, regular spherulites consisting of radially grown edge-on lamellae and crystal aggregates exhibiting alternating bright and black bands can be clearly seen. A magnified image of a concentric-ringed structure is shown in Figure 1b. Concentric-ringed patterns are commonly observed in melt-crystallized polymers. The regular and smooth lamellar twisting along the radial direction causes periodic bands to form within the spherulites and subsequently leads to the concentric-ringed pattern. In addition, the banded spherulites also display the typical Maltese-cross extinction pattern and optical birefringence among the bands under cross-polarized light. However, Figure 1b shows neither a Maltese-cross extinction pattern nor obvious birefringent ringed/banded structures under cross-polarized light. It is quite clear that the concentric-ringed patterns, as shown in Figure 1, are not from traditional banded spherulites.
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Figure 4. A series of AFM height images displaying the effect of film thickness on the morphology of concentric-ringed crystalline structures at 80 °C. (a) 70 ( 5 nm, (b) 120 ( 10 nm, (c) 200 ( 10 nm, and (d) 305 ( 10 nm.
OM cannot provide detailed information on the crystal structures because of its low resolution. AFM substantially complements OM with its high spatial resolution. Moreover, AFM provides the height profiles and surface information of the crystalline structure in real time and in situ, owing to its topographic and phase-imaging functions. Silicon wafers were selected as the film substrate for the AFM studies because of their smooth surfaces. In this case, the effect of film thickness on the formation of the concentric-ringed structures could be investigated. Figure 2 shows a series of AFM images of a typical concentric-ringed structure formed on an approximately 190nm-thick BA-C6-124 film annealed at 80 °C for about 48 h. It should be pointed out that observed concentric-ringed structures are not an artifact caused by a particular substrate. On various substrates, such as glass slices and silicon wafers, similar crystalline structures were observed. An AFM topographic image of this structure is shown in Figure 2a. The height profile along the white line spanning the cross-section of the central region of this structure is shown in Figure 2b. The results shown in Figure 2a,b indicate that the periodic variation in the heights, forming alternating ridges and valleys, is the source of the concentric-ringed pattern. In addition, the height profile, as shown in Figure 2b, indicates that the height of the two ridges at the center of the structure is much higher than that of the rest of the ridges. Among all the other mature bands, the ridges and valleys have similar heights. However, near the growth front, the height of the growing ridge, labeled E and E′ in Figure 2b, are lower than the heights of the mature ridges. The presence of E and E′ confirms the existence of a depletion zone during the formation of the structure. Panels c and d of Figure 2 are the height and phase images, respectively, of the central concave region (labeled A in Figure 2b). Only flat-on lamellar terraces are clearly seen. Near the center of the valley, flat-on lamellar terraces, which are scale-like multilayers, as shown in Figure 2d, gradually increase in height through lamellar accumulation in the radial and outward directions. As a result, the central part of the valley is black, while the surrounding areas are bright in the height image, as shown in Figure 2c. Panels e, g, and i of Figure 2 are the height images for the locations labeled B, C, and D, respectively, in
Figure 5. The effects of temperature and film thickness on the concentric-ringed structures of BA-C6-124 films. (a) A plot of interband width versus crystallization temperature. (b) A plot of the height difference versus crystallization temperature. Curves i, ii, and iii were obtained on samples with thicknesses of 70 ( 5, 120 ( 10, and 200 ( 10 nm, respectively.
Figure 2b. Panels f, h, and j of Figure 2 are the corresponding phase images. On the surface of the concentric-ringed structure, no edge-on lamellae and no lamellar twisting are observed. Also, as shown in Figure 2b, the first two ridges (labeled B) near the center, which are about 200 nm higher than the melt surface, are very interesting features in this structure. The formation of valleys (the depletion bands) between the ridge bands is due to the migration of the polymer chains to the ridge bands. On the basis of the above structural information, the formation of the concentric-ringed crystalline structures in BAC6 thin films can be described using the schematic diagrams shown in Figure 3. The formation of the central valley point and the first ridge band is illustrated schematically in Figure 3f-j. The birth and development of a growing ridge into a mature one at the boundary of a concentric-ringed structure are illustrated schematically in Figure 3k-o. A concentric-ringed crystalline structure begins with a flat-on primary nucleus, which forms near or at the polymer/substrate interface. The nucleus grows into a founding lamella, which grows radially, forming a disklike object. Large molecules crystallize first, while relatively small ones diffuse to the fold surface of the founding crystal. Continuous upward branching occurs on the founding and subsidiary lamellar layers. A small depletion zone emerges around the growth front of the crystal aggregate mainly because of the accumulation of low-specificvolume crystals (cf. Figure 3g). The upward migration of polymer chains causes the surface of the central region of the object to become slightly convex, as shown in Figure 3c,e. As the first central ridge becomes taller and wider, the diffusion of the polymer chains to the central region of the ridge becomes more difficult. The first central ridge therefore develops into the first ridge band with a central valley, as shown in Figure 3h. The continuous upward branching and diffusion of polymer chains to the fold surface and the increase in the density of the crystalline
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Figure 6. AFM images (left column: height images; middle column: phase images; right column: height profiles) of the crystal morphology of two different MW samples and their mixture (1:1 in mass) at 80 °C: (a-c) Sample BA-C6-164, (d-f) BA-C6-67, and (g-i) mixture of BA-C6-164 and BA-C6-67. The film thicknesses of the samples are about 175 ( 10 nm. The scale bar represents 5 µm.
phase cause the formation of a depletion zone (the first valley band; cf. Figure 3i). As the amorphous layer at the depletion zone becomes thinner, the supply of polymer to the first ridge band is stopped (cf. Figure 3j). As more polymer diffuses from the far field to replenish the depletion zone, crystallization starts again, leading to the formation of the second ridge band (cf. Figure 3k). The first ridge is the tallest ridge in the concentricringed structure due to the fact that more material can flow into the central region before the narrowing of the depletion zone stops the material transport because polymer chains can diffuse to the central location from all directions. If there is no formation of a depletion zone, the ridge can become taller and wider with time. Both the height and width of the ridge are limited because of the development of the depletion zone. The height of the ridge and the interband width are determined by the diffusion rate of the polymer and the time (ts) at which the narrowing of the depletion zone stops any polymer transport. The birth and development of the second ridge band begin soon after more polymer chains replenish the depletion zone (cf. Figure 3k). This process more or less repeats itself, except that the succeeding ridges are shorter than the first one (cf. Figure 2b). This is because, when the first ridge band develops, the film is uniform in thickness and the polymer chains can diffuse to the central region from all directions, whereas all other ridge bands always start in the depletion zone, which is lower in height than the melt surface (cf. Figure 3j,k), and the polymer chains can only diffuse to the second and other ridge bands from more or less one direction. The width of all succeeding ridges is very similar, indicating that the values of ts for all bands are quite similar. Concentric-ringed structures only form in thin films because the transport of polymers is limited in thin films. As the film thickness becomes smaller and smaller, the diffusion rate of the polymer is reduced. Figure 4 shows a series of AFM height images displaying the effect of film thickness on the morphology
of the concentric-ringed structures of BA-C6-124 at 80 °C. Panels a-d of Figure 4 correspond to samples with film thicknesses of 70 ( 5, 120 ( 10, 200 ( 10, and 305 ( 10 nm, respectively. The results indicate that a decrease in the film thickness decreases the interband width (the distance between the neighboring peaks of the ridge bands, not including the first ridge band) and the height difference (the height difference between the peak of the first ridge band and the lowest point of the first valley band). These results can easily be explained by the fact that a decrease in the film thickness decreases the diffusion rate of the polymer. Hence, ts decreases, resulting in smaller band widths. To further understand how the concentric-ringed structures are formed, we studied the effects of several parameters including crystallization temperature (Tc), film thickness, and MW, which have significant influence on the diffusion rate of a polymer. As mentioned above, the concentric-ringed structures in BA-C6124 only appeared at Tc values ranging from 80 to 95 °C. The interband widths and the height differences were measured at various Tc’s and with different film thicknesses. Ten measurements were made to determine the average value. Panels a and b of Figure 5 display the plots of the interband widths and height differences versus Tc, respectively. The sample thickness varied from 70 to 200 nm. It can be clearly seen in Figure 5 that both the interband width and the height difference increased with Tc and film thickness. As the temperature increased, the diffusion rate increased and the crystallization rate decreased.. Consequently, more polymer was transported to the ridge band, resulting in larger height differences and greater interband widths. Two other BA-C6 samplessBA-C6-67 and BA-C6-164s with M h n values of 6700 and 16 400 g/mol, respectively, were studied. Their physical properties are summarized in Table 1. The AFM images and height profiles of BA-C6-67, BA-C6-164, and their mixture (weight ratio of 1:1) thin films with similar thicknesses (about 175 ( 10 nm) annealed at 85 °C for about 72 h are shown in Figure 6. Panels a-c, d-f, and g-i of Figure
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Figure 7. AFM images of BA-C6-67 and BA-C6-164 crystallized at 75 and 100 °C, respectively. Panels a, b, and c are the height image, the phase image, and the cross-section of the height image for BA-C6-67, respectively. Panels d, e, and f are the height image, the phase image, and the cross-section of the height image for BA-C6-164, respectively. The film thicknesses of the samples are all about 175 ( 10 nm. The bar line represents 5 µm.
6 were obtained from BA-C6-164, BA-C6-67, and the mixture, respectively. Panels a, d, and g are height images; panels b, e, and h are phase images; and panels c, f, and i are the corresponding height profiles of the cross-sections passing through the center of the individual concentric-ringed structure. As shown in Figure 6a-c, a tall first ridge band and a short second ridge band are observed with BA-C6-164. The interband widths are fairly narrow, indicating that the ts values are small because of the high MW of the polymer. Only a relatively tall first ridge band is formed because the polymer can diffuse into the central area from all directions. However, when the temperature is increased to 100 °C, a concentric-ringed structure with a taller first ridge band and a wider interband width appears, as shown in Figure 7a-c. Again, these results confirm the hypothesis that an increase in ts due to an increase in the diffusion rate is necessary to develop a taller ridge and wider interband width. Figure 6d-f shows the AFM results for BA-C6-67 with an M h n of 6700 g/mol. It is obvious that a higher diffusion rate is anticipated in this sample compared with that of BA-C6-164. An increase in ts is expected in this sample, leading to taller ridge bands and wider interband widths. When the temperature is lowered to 75 °C, much smaller height differences and interband widths are observed. The results of BA-C6-67 and BA-C6-164 indicate that low diffusion rates reduce the height of the ridges and the interband widths, while high diffusion rates increase the height of the ridges and the interband widths. We anticipated that the height of the ridges and the interband width of a concentricringed structure of a mixture of BA-C6-67 with BA-C6-164 would be between those of BA-C6-67 and BA-C6-164. Figure 6g-i shows that the AFM results of the mixture (the M h n and polydispersity index of the mixture are 10 800 g/mol and 2.46, respectively) were as we expected. Molecular fractionation is a direct consequence of the crystallization process. The segregation of molecules occurs in such a way that different molecular species crystallize at different stages of the crystallization. During the formation of the concentric-ringed crystalline structures, the relatively lower MW chains, which have lower surface energies,25-27 migrate to the surface of the melt and preferentially to the fold surface of the (25) Jalbert, C.; Koberstein, J. T.; Yilgor, I.; Gallagher, P.; Krukonis, V. Macromolecules 1994, 27, 2448. (26) Kajiyama, T.; Tanaka, K.; Ge, S. R.; Takahara, A. Prog. Surf. Sci. 1996, 52, 1. (27) Tanaka, K.; Takahara, A.; Kajiyama, T. Macromolecules 1998, 31, 863.
Figure 8. Negative ToF-SIMS ion images: (a) the total negative characteristic fragments and (b) the characteristic fragments of the end group 79Br- and 81Br- peaks at m/z ) 79 amu and m/z ) 81 amu, respectively.
flat-on crystals because the fold surface has a much higher surface energy. If the formation of the ridge and valley structures is due to the diffusion of the polymer chains, it is reasonable to infer that the surfaces of the ridges have a higher concentration of low-MW chains than do the surfaces of the valleys because polymer chains with different MWs crystallize at different stages, resulting in molecular fractionation. The low-MW chains will be the last ones to crystallize. ToF-SIMS imaging can be used to investigate the distribution of end groups. Two ToF-SIMS experiments were preformed to verify whether our hypothesis was correct. First, in the synthesis of BA-C6-124, an excess of 1,6-dibromohexane was used to ensure that Br was present at both ends of the polymer chains. Therefore, the surface with a high concentration of low-MW chains should show a high surface Br concentration. ToF-SIMS
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should decrease the topographic effect on ToF-SIMS imaging. Panels a and b of Figure 8 show the ion images using the total intensity of all negative ions as well as the intensity of the 79Brand 81Br- ions at m/z ) 79 amu and m/z ) 81 amu, respectively. Figure 8b shows a higher Br concentration at the topmost surface of the ridges. To eliminate the topographic effect between the ridges and the valleys, the relative intensity between the characteristic ion (Br-) and the fragment (CH2-) was used. The values of Br-/CH2- at the surfaces of the valleys and ridges were calculated. On the basis of 10 measurements, the average value of Br-/CH2- at the surfaces of the ridges was about 3.7 times greater than that at the surfaces of the valleys. This result indicates that the enrichment of relatively small chains occurred at the topmost surface of the ridge bands. The experiment was repeated with a blend of two BA-C6 samples with different MWs. The low (BA-C6-60F) and high (BA-C6-164) MW samples, which had CH2CF3 and Br as their end groups, respectively, were present in the weight ratio of 2:1. ToF-SIMS ion images were obtained on a 220-nm-thick film of the BA-C6-60F/BA-C6-164 mixture annealed at 85 °C for about 48 h. Panels a-c of Figure 9 show the ion images using the total negative ion intensity, the intensity of the 69CF3- ions, and the intensity using the Br- ions, respectively. As shown in Figure 9b, high concentrations of CF3ions were detected at the surface of the ridges. The average value of the 10 CF3- /CH2- measurements at the surfaces of the ridges was about 9 times that at the valleys. The average CF3-/Br- ratio was about 0.7 and 3.8 at the surfaces of the valleys and ridges, respectively. These results further indicate that there are higher concentrations of low-MW polymer chains at the surfaces of the ridges. ToF-SIMS results undoubtedly show the upward migration of smaller-MW chains during the continuous branching of flat-on lamellae.
Conclusion
Figure 9. Negative ToF-SIMS ion images: (a) the total negative characteristic fragments; (b) the characteristic fragments of the end group CF3- peak at m/z ) 69 amu; and (c) the characteristic fragments of end group 79Br- and 81Br- peaks at m/z ) 79 amu and m/z ) 81 amu, respectively.
Br imaging was performed on the surface of a 140-nm-thick BA-C6-124 film annealed at 85 °C for about 48 h. The choice of an intermediate annealing temperature for crystallization was to minimize the height difference between the ridges and valleys and to increase the interband width. The AFM height profile obtained on this film indicated that the height difference between the first ridge and the next valley (the largest height difference in this structure) was about 220 nm and that the mean horizontal distance between the neighboring mature ridges and valley bands was about 4200 nm. The reduction in topographic fluctuations
Novel self-organized structures of polymer crystals containing micrometer-sized concentric rings (or bands) were observed in BA-C6 thin films. The origin of the banded structure differs totally from the origin of traditional banded spherulites in polymer systems. These concentric-ringed crystalline structures consist of alternating bands of multilayer polymer crystals with a flat-on orientation. The formation of depletion zones due to specific volume decrements between the crystal and melt, and the diffusion of the polymer chains to the fold surface of the flat-on lamellae are found to lead to the construction of the concentric-ringed structures. The diffusion rate of the polymer controls the heights of the ridges and the interband widths. Acknowledgment. We are grateful for support from the Hong Kong Government Research Grants Council under Grant Nos. 600405 and 600503. LA060863R
