Atomic-Force and Optical Microscopy ... - ACS Publications

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Ind. Eng. Chem. Res. 2010, 49, 12084–12092

Atomic-Force and Optical Microscopy Investigations on Thin-Film Morphology of Spherulites in Melt-Crystallized Poly(ethylene adipate) Andreas Meyer,† Kai Cheng Yen,‡ Shu-Hsien Li,‡ Stephan Fo¨rster,*,† and Eamor M. Woo*,‡ Institute for Physical Chemistry, UniVersity of Hamburg, Grindelallee 117, 20146 Hamburg, Germany, and Department of Chemical Engineering, National Cheng Kung UniVersity, Tainan 701-01, Taiwan

The thin-film morphologies of double-ring-banded and ringless spherulites in melt-crystallized poly(ethylene adipate) (PEA) were examined using atomic-force microscopy (AFM) and polarizing optical microscopy techniques with phase image and depth profiles. For the PEA ring-banded spherulites (Tc ) 30 °C), edge-on lamellae, with top views shaped like a figure 8, are arranged in an orderly fashion on the ridges of ring bands, and the valley region of the ring bands is filled mainly with flat-on oval-shaped platelets. Between the edgeon and flat-on band regions, some ribbonlike long lamellae are present that propagate across these two regions. However, these three lamellar types are discrete, discontinuous, and independent, and they are not necessarily inter-related by physical turning. By contrast, in the ringless Maltese-cross spherulite (PEA crystallized at Tc ) 35 °C), similarly figure-8-shaped edge-on lamellae and flat-on platelets are also present, but they are scattered randomly, and no long lamellae are present. Near or within the edge-on lamellae, cavities are present as a result of densification and/or reorientation. In contrast with some single-banded spherulites where the valleys usually have a featureless amorphous texture, the valleys in the double-ring-banded PEA spherulites are filled with plateletlike lamellae, whereas the ridges contain mainly figure-8-shaped lamellae. Introduction Recent advances in atomic-force microscopy techniques provide unique capabilities for directly viewing the air-solid interfacial morphology of crystallized polymers with open interfaces to air, in contrast with polarized-light optical or transmission electron microscopies that require samples to be placed in contact with glass solids or microtomed. Issues of ring bands in various types of polymer spherulites have been intriguing and extensively studied.1-7 Ring patterns in spherulites can appear as single rings, where alternating bright and extinction (dark) bands are either regularly concentric or slightly deformed to a zigzag order. Double-ring-banded spherulites, in contrast to those with single bands, are cases where two bands of different contrasts or colors but none with full extinction are visible when viewed by polarized-light microscopy. Murayama reported and discussed the optical properties of double-ringbanded spherulites in poly(ethylene adipate) (PEA)8 and proposed a few models including one with lamellar twisting, but concluded that the later model was not yet possible to explain the formation of double bands. Poly(1,4-butylene adipate) (PBA), a polyester that is closely related to PEA, exhibits single-ring patterns or ringless Maltesecross spherulites when crystallized at different temperatures. In contrast with the spherulite patterns in PBA, PEA exhibits double-ring spherulites or ringless Maltese-cross spherulites when crystallized at different temperatures. Unlike PBA, which has polymorphic crystal forms (termed R and β) depending on temperature of crystallization,9-12 PEA has only one crystal cell13 of monoclinic packing, with a ) 0.547 nm, b ) 0.724 nm, c ) 1.155 nm, and a monoclinic angle of R ) 113.5°, when crystallized at all temperatures. PEA, with double rings and without complexity from the crystal-cell polymorphism and complex melting peaks seen in PBA, serves as an ideal model * To whom correspondence should be addressed. E-mail: emwoo@ mail.ncku.edu.tw (E.M.W.), [email protected] (S.F.). † University of Hamburg. ‡ National Cheng Kung University.

for comparisons between these two different types of ringbanded polyesters. Apparently, the lamellar textures in doublebanded spherulites can differ in certain ways from those in their single-banded counterparts. Finer details in the lamellar texture, however, cannot be discerned by polarized-light optical microscopy (POM) analysis; thus, atomic-force microscopy (AFM), with greater magnification and a nondestructive nature that preserves the original surface topography of samples, would be a suitable technique for analysis. The free-surface nature of polymer samples, with no requirement of vapor-sputtering or microtoming, for AFM characterization is well-suited for dealing with the technical difficulties that can be encountered with the fine and complex ring-banded crystals. As a powerful analytical tool, AFM has been widely applied to investigate surface topography and lamellar and crystal structures, including banded rings in spherulites.14-19 An earlier study investigated the thermal behavior of ring-banded spherulites in PEA to reveal subtle differences from that of ringless Maltese-cross spherulites.20 The issues of ring bands in spherulites are overly complicated, as the mechanisms for ring band formation might not be universal and the mechanism of ring bands for one polymer might be different from that for other polymers. More recently, Wang et al.21,22 proposed that the concentric rings in birefringent ringbanded spherulites, as well as those in the nonbirefringent ringbanded spherulites, are a manifestation of periodic variations in thickness along the radius for poly(ε-caprolactone) (PCL) grown from solution. Other investigators have proposed yet different mechanisms for ring bands in chiral polymers such as poly(L-lactic acid) (PLA),23 reporting that edge-on lamellae are present in PLA, exhibiting a curvature related to polymer chirality. Chen and Yang24 proposed that, for crystalline polymer blends of liquid-crystalline poly(aryl ether ketone) and poly(aryl ether ether ketone) (PEEK), the formation of ring-banded spherulites can be attributed to structural discontinuity caused by rhythmic radial growth. The view of structural discontinuity,

10.1021/ie901356q  2010 American Chemical Society Published on Web 01/21/2010

Ind. Eng. Chem. Res., Vol. 49, No. 23, 2010

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apparently, is quite dramatically in contrast to other prevailing proposals that ring bands are a result of coordinated lamellar twisting. These various studies point out that ring bands can be of many different types and that the ring bands in various different types of polymers (or blends) can involve complex and possibly different mechanisms, with no simple or uniform rules governing the formation of ring bands. An earlier study using AFM revealed that, in the single-ring-banded spherulites of poly(butylene adipate) (PBA),19 straight-stalk lamellae, tapering to pointed needlelike stalks, are on ridges that monotonically protrude from a plateau region forming the valleys. In that work, the high resolution and high magnification of AFM techniques were used to analyze and compare the geometrical and orientational differences between the crystalline lamellae in ringless versus double-ring-banded spherulites in PEA. Furthermore, atomic-force microscopy techniques provide a unique capability for directly viewing the air-solid interfacial crystalline morphology of crystallized polymers. Although studies of ring bands in polymers have been extensively reported in the literature, finer details of lamellar patterns in ringless Maltese-cross spherulites in comparison with those in ring-banded samples, which appear in PEA crystallized at temperatures (Tc) differing by only a few degrees Celsius, have yet to be analyzed to shed new light on and lead to better interpretations of lamellae selfassembly into peculiar patterns. For comparisons between surface morphology and interior bulk, future work will also probe lamellar structures in the interior bulk of polymers. Experimental Section Materials and Procedures. Poly(ethylene adipate) (PEA), a low-melting semicrystalline polyester, was purchased as a research-grade material from Aldrich (St. Louis, MO) The weight-average molecular weight (Mw) determined by gel permeation chromatography (GPC) was nearly 10000 g/mol. Molecular weights were determined by gel permeation chromatography (Waters 410) using tetrahydrofuran (THF) as the eluent at a flow rate of 1.0 mL/min. The polymer was purified by reprecipitation from chloroform into cold methanol (ca. -10 °C). Thin-film PEA samples (cast with thicknesses of 4-5 µm) were prepared by dissolving polymer in THF (∼2 wt %) and casting the solution on a glass slide (uncovered). Then, the PEA cast films on glass were dried in a vacuum oven at 40 °C for 7 days to remove the solvent. PEA cast from solution and spread in thin films on glass slides was crystallized at Tc ) 30 or 35 °C, to prepare samples with spherulites of either double-ring bands or ringless Maltese-cross types, respectively. After solution casting and drying, the PEA cast films on glass were briefly melted at ∼90 °C and then rapidly brought to isothermal conditions at Tc ) 30 or 35 °C for full crystallization. PEA samples with double-ring-banded spherulites were developed by melt-crystallization at Tc ) 30 °C (for long enough for full crystallinity). The PEA samples with double-ring (Tc ) 30 °C) and ringless Maltese-cross (Tc ) 35 °C) spherulites are designated as samples A and B, respectively. These two PEA samples (double-ring-banded and ringless) were analyzed for comparisons. Apparatus. A polarized-light optical microscope (Nikon Optiphot-2) equipped with a charge-coupled-device (CCD) digital camera and a microscopic heating stage (Linkam THMS6000 with TP-92 temperature programmer) was used for preliminary confirmation of ringless and ring-banded spherulites in samples prepared by crystallization using the procedures described in the previous section.

Figure 1. POM images for PEA crystallized at (A) Tc ) 30 °C, ring-banded spherulite, and (B) Tc ) 35 °C, ringless/Maltese-cross spherulite.

Atomic-force microscopy (AFM) investigations were performed in intermittent contact mode, using a JPK Nanowizard microscope with a silicon tip (ν ) 280 kHz, r ) 10 nm) installed. The largest scan range was 100 µm × 100 µm × 15 µm. The scan rate was kept at 0.2 Hz for the overview scan and at 1.0 Hz for zoom regions. Thin films of PEA were deposited on substrates of glass slides, with an open face up for AFM characterization. Preliminary optical microscopy characterization was performed to confirm the presence of regular Maltese-cross or ring-band crystals in PEA samples intended for subsequent AFM characterization. Results and Discussion Optical microscopy at lower magnification was first performed to identify spots of interest for later AFM characterization. Figure 1 shows preliminary POM characterizations revealing the general features of ring-banded and ringless spherulites in PEA. Two samples are presented: sample A (ringbanded at Tc ) 30 °C) and sample B (ringless Maltese-cross spherulite at Tc ) 35 °C). The ring-banded spherulites display a double-ring pattern superimposed with Maltese-cross extinction in sample A. Henceforth, the double-ring bands in PEA will be referred to simply as ring bands, as invariably PEA exhibits either ringless spherulites when crystallized at Tc ) 35 °C or higher or only double-ring bands and no single-ring bands when crystallized at or near 30 °C. Sample B exhibits a typical POM image for ringless Maltese-cross spherulites in PEA crystallized at 35 °C. It is known that the polymer chains inside the lamellae are perpendicular to the radial direction of the spherulite and the polarized light is scattered in such a way

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Figure 2. AFM overview scan of regions 1 and 2 of double-ring-banded spherulite in PEA sample A crystallized at Tc ) 30 °C: (A) height image, (B) phase image, (C) height profile obtained by scanning the radial direction.

that a Maltese cross appears. Because of the radial symmetry in spherulites, the sample can be rotated without affecting the Maltese cross. In sample B, the streaks of bright lamellae are interspersed with tiny dark spots, and the streaks of dark lamellae are littered with tiny bright spots. Consequently, AFM characterization was further performed to expound the phenomenon, as discussed later. Spots specifically intended for further AFM characterization are marked on the POM images for correlation. On each of the POM images, three regions (1, 2, 3) of interest are marked, where 1 is the center (eye), 2 is the dark-cross area (reverse birefringence), and 3 is the bright-cross area. AFM Characterization of Ring-Banded Spherulites. AFM was further performed to characterize the PEA ring bands. The AFM results were directly compared to the OM images in Figure 1 as a basis for pinpointing regions of interest. Note here that, although scanning or transmission electron microscopy might be able to reveal finer details, SEM or TEM would require goldsputtering or microtoming, which would deform the surface topography to some extent, especially for soft or low-melting PEA. Figure 2 shows an AFM overview of the scanning regions for PEA film sample A (Tc ) 30 °C), including a height image, a phase image, and a height profile in the radial direction as marked. Note that AFM characterization provides morphological details in the ring bands that cannot be as finely revealed using OM. For differentiation of regions covered by the AFM morphology, three regions (1, center; 2, ring band in a bright region; and 3, ring band at the extinction cross) of different crystal patterns are marked in this overview image. The height image was measured along the line labeled “cross section”, where apparent changes in the thickness along the radial direction are noted. The phase image shows a different contrast of colors, reflecting the hardness of regions on the surface of the spherulites. In the AFM phase images, in contrast to the height images, the brighter regions are not related to greater height but are normally associated with harder or more crystalline solid areas, whereas the darker regions are associated with softer materials (such as noncrystalline domains or crystals of different orientations). The center near the eyes of the crystals was examined. In the center (eyes) of both spherulite types, there exists a seed crystal that has a dramatically different height and shape from the subsequently growing lamellae radiating from the central

eye. Enlarged images of specific different regions of the spherulite follow. Figure 3 shows an AFM scan of region 1 (central eye-seed crystal) of sample A (ring band at Tc )30 °C). Alternating dark and bright bands, somewhat zigzag-shaped, are circled near the eye. The dark region displays larger holes and mostly vertical crystals, and the bright region features mixed thorny needles and flat-on pestles. Some smaller cavities are present in the bright region as well. A closeup view of the very center was obtained. Higher-magnification AFM micrographs for a closeup view of the central eye region of the PEA ringbanded spherulite (Tc ) 30 °C) are shown in Figure 4, where zoomed-in AFM phase and height images for region 1 (central eye of the ring-band crystal) are compared directly. The height image is not clear in finer details, but the phase image shows that the eye region is filled with both edge-on and flat-on lamellae in a mixed pattern. On and near the eye, the edge-on lamellae are oriented side by side with the flat-on platelets. Ring bands farther from the central eyes were then examined. Figure 5 shows an AFM scann of Region 2 (ring-band lamellae) of sample A (Tc ) 30 °C). In both the height and phase images, alternating bright and dark rings are apparent; however, bright and dark bands are almost opposite in the height versus phase images. In the height image, brighter bands indicate a higher region, and darker bands indicate a lower region. The pattern of lamellar orientation along the radial direction is yet to be explained later with respect to the zoomed-in images to be shown. The depth profile Figure 5C shows periodic up-anddown peaks and valleys along the radial direction. The vertical drop from a peak to a valley is a change of about 80 nm in thickness once every 5-µm spacing in the radial direction. Considering that the film thickness is about 4-5 µm, the depth change is about 2% of the film thickness. That is, valleys are lower than ridges by 2% of the total film thickness. Alternating bright and dark bands are also apparent in the AFM phase images. The dark bands in the phase image (corresponding to the bright bands in the height image) are mainly edge-on lamellae, which are characterized by figure-8-shaped crystals surrounded by cavities. The bright bands in the phase image (corresponding to the dark bands in the height image) are mainly flat-on lamellae shaped like oval platelets. For easy identification, the lowest spots on the depth profile are marked with solid triangles placed on the line marked as the radial direction of


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Figure 3. AFM scan of region 1 (central eye-seed crystal and surrounding area) of PEA sample A crystallized at Tc ) 30 °C (double-ring-banded spherulite): (A) height image and (B) phase image.

Figure 4. AFM scan of region 1 (central eye area of double-ring spherulite) at greater magnifications for PEA sample A crystallized at Tc ) 30 °C: (A) height image and (B) phase image.

the spherulites as viewed in Figure 5A and B (height and phase images, respectively). The corresponding depth profile shows that the dark band in the phase image (mainly edge-on figure8-shaped crystals) is roughly located on or near the ridges and has a higher height; oppositely, the bright band (flat-on platelets) has a lower height (on or near the valleys). The ridge (dark band in the phase image) is composed of figure-8-shaped edge-on lamellae with cavities. This fact indicates that the figure-8-shaped edge-on lamellae are highly crystalline and will likely shrink in volume upon crystallization or reorientation. A more plausible interpretation would be that the dark band (figure-8-shaped crystals) is crystallized into a highly dense band, but shrinks to create cavities. The alternating bright bands in the phase image are composed of flat-on platelets (or even amorphous species), so they are lower in height than the vertical figure-8-shaped edge-on crystals. The fact that the dark bands in the phase images of PEA correspond to higher ridges of edge-on crystals might require some justification. Note that numerous cavities are found in the dark-band regions (phase image of Figure 5), where edge-on lamellae (perpendicular to the paged) are shaped like a figure 8 when viewed vertically on top views. The figure-8-shaped edge-on lamellae create cavities of larger sizes between the vertical pestles. The cavities surrounding the figure-8-shaped edge-on lamellae might be a

major reason for the darker bands in the phase images. By comparison, the bright bands in the phase images are mainly composed of flat-on lamellae, which also show cavities but of smaller sizes. The smaller-size cavities in the bright bands might originate from stack-to-stack overlapping spaces between the flat-on pestles. Also note that the edge-on and flat-on lamellae are not arranged in an orderly fashion in the respective bands, but they are to some extent interpenetrating into each other’s band regions. The bright bands are not entirely from the flat-on lamellae type; some edge-on-like lamellae are interspersed among the flat-on ones. The same can be said of the dark bands. That is, there are no clear-cut borders separating the alternating bands. Closer zoomed-in views on finer details reveal the lamellar structures in the ridges and valleys of the ring bands. Figure 6 shows high-resolution phase image scans revealing finer details of the main lamellar types in region 2 of PEA sample A (Tc ) 30 °C). The schemes on the bottom illustrate possible lamellar orientations. Note that the schemes contain idealized crystal plates, whereas the actual lamellar entities are quite irregularly shaped. As discussed earlier, the edge-on lamellae are mostly shaped as figure 8’s, and the flat-on lamellar platelets are of an oval or flower-pestle shape, corresponding to the bright and dark bands observed by POM, respectively. Along the radial direc-

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Figure 5. AFM scan of region 2 (reverse birefringence) of double-ring-banded spherulites in sample A (Tc ) 30 °C): (A) height, (B) phase, and (C) height profile along radial direction marked “cross section”.

Figure 6. Higher-resolution scans revealing edge-on/flat-on lamellar region 2 of sample A (Tc ) 30 °C), phase image 8 µm × 2 µm. Scheme blocks illustrate several possible lamellar orientations along the radial direction.

tion, the two types of lamellae are roughly in alternating layers but not exactly so. In the band of edge-on figure-8-shaped lamellae, some flat-on platelets are also present. Similar intermingled plates of edge-on and flat-on orientations can also be seen in a ringless Maltese-cross spherulite, as discussed later. AFM Characterization of Ringless Maltese-Cross Spherulites. Ringless Maltese-cross spherulites in PEA were examined in comparison to the ring-banded ones. Figure 7 shows an AFM overview scan at lower magnifications of regions 1 and 2 of sample B (ringless Maltese-cross spherulites, Tc ) 35 °C). Note the numerous tiny cavity holes in the image. Such tiny cavities were also found in the AFM images of ring-banded spherulites (Figure 2), although the cavity holes were arranged differently in the ring-banded spherulites. Also note that the intertwining bright and dark streaks resmeble the branches of sea corals. Larger-magnification AFM can reveal the finer textures of lamellae that are responsible for the cavities in the ringless Maltese-cross spherulites. Figure 8 shows an AFM overview scan at greater magnifications of region 2 in sample B (Tc ) 35 °C, ringless spherulite). The AFM image reveals numerous

edge-on lamellae scattered or intertwined among flat-on ones in a nonorderly pattern; no ring bands can be found. Although not aligned in a ring-band pattern, two types of lamellae (edgeon and flat-on) are spread in streaks, where the bright streaks are mainly the edge-on lamellae and the dark streaks are mainly the flat-on lamellae (phase images). Numerous cavities can be seen in the dark regions where edge-on figure-8-shaped lamellae (perpendicular to the page) are generated. The bright streaks of flat-on lamellae also exhibit some cavities but of much smaller size in comparison to those in the dark streaks. The much smaller cavities might originate from stack-to-stack overlapping of the flat-on platelets. The bright streaks are placed side by side with the dark ones, but there seems to be no correlation between the edge-one and flat-on crystals. The dark and bright streaks radiate out side by side, and these features cannot be taken as lamellar twists or spirals into another. The edge-on and flat-on types of lamellae are separate and discrete. That is, assuming that the spherulite growth is roughly in the radial direction, there is hardly any way that the edge-on lamellae can twist or spiral into flat lamellae, as these two streaks are growing


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Figure 7. AFM overview scan at lower magnifications of regions 1 and 2 of ringless Maltese-cross spherulites in PEA sample B crystallized at Tc ) 35 °C.

Figure 8. Zoomed-in AFM scan at greater magnifications of region 2 of ringless spherulites in PEA sample B crystallized at Tc ) 35 °C.

out independently, roughly side by side. Note another interesting feature as exemplified earlier. The bright streaks of edge-on lamellae are not composed of parallel continuous vertical plates; instead, the morphology shows that the bright streaks are loose assemblies of many discrete edge-on plates, each shaped as a distorted figure 8. Each figure-8-shaped edge-on lamellar entity is about 5 µm square or in diameter. Similarly, the dark streaks are assemblies of many discrete flat-on platelets shaped like flower pestles or oval plates. The flat-on streaks are not of parallel continuous horizontal plates, but rather are composed of many discrete flat-on platelets, each of about 2-3 µm in diameter. Figure 9 shows an enlarged AFM phase image at higher magnifications of sample B (Tc ) 35 °C). Similarly figure-8shaped edge-on crystals and oval or circular flat-on plates are seen in the ringless Maltese-cross spherulite, but the spatial arrangement of these two types of crystals is completely different from that in the ring-banded PEA spherulites discussed earlier. In the ringless spherulites (sample B), the lamellae do not aggregate in ordered rings such as those seen in the ring-banded spheruite (sample A). Instead, the edge-on lamellae tend to scatter throughout the entire range. As revealed in the AFM image, the edge-on and flat-on lamellar stacks in the ringless spherulites can alternately appear side by side in a random way, rather than appearing in a regular alternating pattern along the main growing directions as seen in the ring-banded spherulites.

Figure 9. AFM height image (6 µm × 6 µm) at greater magnifications on ringless spherulites in PEA sample B (crystallized at Tc ) 35 °C).

As shown earlier in the corresponding POM images in Figure 1, no ring bands are seen in sample B. The AFM image clearly reveals that the numerous cavities result from crystalline lamellae (perpendicular to the page) being generated in the perpendicular edge-on crystals, thus contracting and leaving porosity. The streaks of flat-on lamellae, embedded among the streaks of cavity-filled edge-on lamellae, also show some cavities to lesser extents and of much smaller sizes. Discussion of Lamellar Orientation. The AFM-measured data on depth profiles in going through the ridges and valleys of PEA ring bands were used to check against common assumptions of lamellar twisting to create the periodic flat-on plates on valleys and edge-on crystals on edges. Figure 10 shows

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Figure 10. Scheme showing twisting from flat-on to edge-on, creating height profile difference of at least 2.8 µm. The actual AFM height profile measurement shows only an 80-nm difference from valley to ridge.

a scheme for illustrating the extents of depth profiles that would have been seen had the horizontal flat-on plates (dimensions of plate width and thickness as marked: 3.0 × 0.2 µm) continuously twisted from 0° position eventually to 90° to a vertical edge-on figure-8-shaped crystals (as viewed from the top by AFM, with an edge thickness of ∼0.2 µm and a height of ∼3.0 µm). If the horizontal plate twisted completely, with its original dimensions preserved in twisting, it would create a vertical uprising of 2.8 µm (or 2800 nm) in height. However, earlier Figure 5 showed that the measured vertical height difference from a peak to a neighboring valley is only about 80 nm with 5-µm spacing in the radial direction. Thus, the significant contradiction between the height created by twisting and the AFM-measured data on height profiles from valleys to ridges has to be resolved before the proposal of rhythmic lamellar twisting into ring bands can be validated. For the ring bands in PEA, at least, alternative mechanisms other than rhythmic twisting might work better. It would be interesting to compare the single-ring bands in PBA to the double-ring bands in PEA. The lamellar patterns are dramatically different between the PBA single-ring-banded spherulites19 and the PEA double-ring-banded spherulites in this study. Straight-stalk crystals are seen to stack to form the ridges in PBA ring bands, but figure-8-shaped crystals are responsible for the ridges of PEA ring bands. In addition, no cavities are present in the PBA spheruites,19 but numerous cavities are seen in both the PEA ring-banded and ringless spheruites. The AFM images of PEA samples A and B reveal some interesting features that are significantly different from those observed in PBA as reported earlier.19 The edge-on and flat-on lamellae in the ringless Maltese cross spherulite (sample B) are discrete and not interconnected. That is, the edge-on and flat-on lamellae are present side by side, resembling twigs in corals, but do not necessarily spiral/twist into one another. Similarly, in the ringbanded spherulites, the edge-on and flat-on lamellae are arranged in alternating layers. In addition, the AFM depth profile of the ring bands points out that, on average, the valleys are lower than the ridges by only 2% of the total film thickness. Considering the large difference in geometry of the edges versus the top-surface width of a typical lamella, coordinated twisting of lamellae within the film thickness of ca. 5 µm would be expected to generate depth profile changes from ridges to valleys much in excess of 2% of the film thickness. PEA exhibits some unique morphological features in the valley and ridge regions of the bands. Oval-shaped flat-on (2-3 µm) lamellae are the main species in valleys, and 8-shaped edgeon crystals account for the ridges of ringless Maltese-cross spherulites. Amazingly, similar fine textures of oval-shaped flat-on and figure-8-shaped edge-on lamellae are also in ringless Maltese-cross spherulites, except that the arrangements of the lamellae infrastructures differ from those in ringless spherulites. If the edge-on lamellae spiraling/twist into flat-on ones is taken as the mechanism for the formation of double-ring bands in PEA, then it would take a great deal of imagination to explain these facts: Why, for crystallization at Tc ) 35 °C (sample B),

Figure 11. Illustration of the changes in the lamellar orientation relative to the optical path upon tilting to 75-90°.

Figure 12. POM micrographs of ring-banded spherulite of PEA film tilted from 0°, 15°, and 45°, to 75°.

does the spiraling not lead to ring bands whereas crystallization at Tc ) 30 °C does (sample A)? Furthermore, the twsiting of a flat-on lamella into an edge-on one would create a height difference of at least 2800 nm, which is not justified by the AFM measurements on actual height differences between the valleys and ridges of the bands. In addition, tilting of the samples in the OM stage might create a similar effect of lamellar twisting for proving or disapproving lamellar twisting in PEA. Figure 11 shows a schematic plot of flat-on and edge-on sections of a lamella placed horizontally on the POM stage, creating dark and bright ring bands, respectively. On tilting the sample to approach 70-90°, as long as the POM light-beam focus can be manipulated, the flat-on and edge-on sections of the lamella can be rotated the same angles (70-90°). In the mean time, the tilted sample should show a reversed pattern of bright and dark bands on the tilted flat-on and edge-on sections, respectively, if indeed the lamellae are spiraling continuously from the flat-on to the edge-on configuration. POM was then used to verify this assumption. Experimentally, tilting to 90° made it impossible to get any POM images; however, tilting to ∼75 °C was feasible to obtain a thin stripe of sample under focus. Figure 12 shows POM micrographs (images sectioned to well-focused thin stripes) for PEA samples (all crystallized at Tc ) 30 °C) tilted on the stage to various degrees: 0° (i.e., untilted), 15°, 45°, and 75°. The arrow indicates the same band (fourth bright band from the center) showing bright color, and upon tilting from 0° to 75°, it is clearly observed that there are no changes in the interference color of the indicated band; the same is true for all other bands. Thus, the lamellar orientation relative to the optical path remains unchanged, regardless of tilting or spiraling. Apparently, the continuous tilting of the sample from 0° to 75° does not create


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reversion of the original dark bands into bright bands, or vice versa. All originally bright bands remain bright, and all dark bands remain dark regardless of tilting degree, which contradicts the assumption that the lamellae continuously and rhythmically switch from flat-on to edge-on to create alternating dark and bright bands, respectively. According to these POM results on sample tilting, lamellar twisting might not be an applicable mechanism for interpreting the ringed spherulites of PEA. Sample tilting experiments were also similarly performed on PBA films. Similarly to the results for PEA film tilting as discussed with regard to Figure 12, tilting of PBA films yielded no color change or dark/bright reversion in POM. In contrast to the POM results demonstrated for PEA and PBA, similar POM tilting experiments were performed by Ye et al.,25 who reported the discovery of left- or right-handed lamellar twists in an eyelike region of banded spherulites of chiral poly[(R)3-hydroxyvalerate] (PHV). Upon tilting to 30°, they claimed that PHV lamellae exhibited the opposite twist senses in the eyelike region compared to the remaining area of the bands. However, they did not discuss the alteration of dark versus bright bands on tilting to sharp angles. The objective for tilting in this study was different. For one thing, PEA is not a chiral polymer, and there exists no eyelike region in the PEA ring-banded spherulites; second, lamellar twisting could be tested only by tilting the samples to sharp angles such as 75°.

bright bands. However, these three crystal types seem discrete and unrelated to each other. That is, it does not necessarily take a spiral turn of an edge-on to flat-on lamella to account for the ring bands. The figure-8-shaped crystals and oval-shaped lamellae can very well be in discontinuous presences whose origin is driven by different mechsnism. Such a view that the transition from edge-on to flat-on lamellae is not necessarily related by turning becomes quite clear if one inspects the special arrangement of crystals in the ringless Maltese-cross spherulites (PEA sample B). For the ringless Maltese-cross spherulites (Tc ) 35 °C), similar figure-8-shaped edge-on lamellae are scattered randomly, instead of being arranged in ordered rings. Near or within the edge-on lamellae, cavities are present as a result of densification of crystallization. The 8-shaped edge-on lamellae in the ringless spherulites are amazingly similar in appearance as those seen in the ridges of ring-banded spherulites. However, in the ringless Maltese-cross spherulites, 8-shaped edge-on lamellae and flat-on plates are scattered randomly, and no long stalks of lamellae are present. The edge-on and flat-on lamellae in the ringless Maltese-cross spherulites are discrete and are not interconnected by visible turning. That is, the edge-on and flat-on lamellae are indeed present in both ringless and ringbanded spherulites, but these two types of lamellae are not necessarily related to each other by spiraling/twisting into one another.

Conclusions

Acknowledgment

Optically, PEA exhibits either double-ring bands (Tc ) 30 °C) or ringless Maltese-cross spherulites (Tc ) 35 °C). As revealed by the extensive AFM and POM data reported in this study, amazingly similar fine textures of lamellae are present in the double-ring bands and ringless Maltese-cross spherulites, except that the arrangements of the lamellar infrastructures differ. Conventional beliefs that edge-on lamellae form ridges of bands in the single-ring bands whereas flat-on lamellae form valleys might be just the opposite for the case of double-ring bands. The double-ring bands in PEA are quite intriguingly different in morphology from the single-ring bands more commonly seen in most other polymers. Central eyes of ringbanded spherulites are composed mainly of flat-on lamellae, but are occasionally interwined with some streaks of edge-on lamellae. No or very few cavities are seen near the central eye areas. For PEA ring-banded spherulites (Tc ) 30 °C), crystals with cavities whose top views are shaped like a figure 8 are arranged in an orderly fashion on the ridge region and thus they are considered as edge-on lamellae. The bands of the edge-on lamellae appear darker in the AFM phase images, mainly because all edge-on lamellae are surrounded by or sprinkled with numerous cavities. These cavities surrounding the figure8-shaped crystalline patterns are present as a result of densification of crystallization, or orientation flipping of lamellae. By contrast, the AFM images show that the valleys of ring bands are mainly flat-on platelets with fewer or almost no cavities. These two kinds of bands are arranged in such an way as to form alternating concentric rings, which appear as double-ring bands in POM images. The interfaces between the dark and bright bands are not as clear-cut; that is, intermingling of edgeon and flat-on lamellae occurs. In addition, between the dark and bright regions, some long stalks of lamellae are present that protrude across these two regions, with occasional turning. That is, three main types of lamellae are seen in ring-banded PEA spherulites: figure-8-shaped edge-on crystals on the ridges, oval-shaped flat-on platelets on the valleys, and ribbonlike lamellae with occasional turning that spread across dark and

This work was financially supported by a basic research grant (NSC 96-2221-E-006-011) from Taiwan’s National Science Council (NSC), for which the authors express sincerest gratitude. In addition, one of the coauthors, S.-H.L., was sponsored by an academic exchange student/scholar program (“sandwich program”) between Germany and Taiwan (DAAD visa-vis NSC) at the laboratory of Prof. S. Fo¨rster at the University of Hamburg (Germany). Verification by POM tilting experiments was performed upon the helpful suggestions of Dr. F. Khoury of NIST (Gaithersburg, MD) during conference discussions. Literature Cited (1) Bassett, D. C. Polymer Spherulites: A Modern Assessment. J. Macromol. Sci. B 2003, 42, 227. (2) Keith, H. D.; Padden, F. J. Twisting Orientation and the Role of Transient States in Polymer Crystallization. Polymer 1984, 25, 28. (3) Schultz, J. M. Self-induced Field Model for Crystal Twisting in Spherulites. Polymer 2003, 44, 433. (4) Kyu, T.; Chiu, H. W.; Guenther, A. J.; Okaba, Y.; Saito, H.; Inoue, T. Rhythmic Growth of Target and Spiral Spherulites of Crystalline Polymer Blends. Phys. ReV. Lett. 1999, 83, 2749. (5) Toda, A.; Arita, T.; Hikosaka, M. Three-dimensional Morphology of PVDF Single Crystals Forming Banded Spherulites. Polymer 2001, 42, 2223. (6) Lotz, B.; Cheng, S. Z. D. A Critical Assessment of Unbalanced Surface Stresses as the Mechanical Origin of Twisting and Scrolling of Polymer Crystals. Polymer 2005, 46, 577. (7) Wang, Y.; Ge, S.; Rafailovich, M.; Sokolov, J.; Zou, Y.; Ade, H.; Luning, J.; Lustiger, A.; Maron, G. Crystallization in the Thin and Ultrathin Films of Poly(ethylene-vinyl acetate) and Linear Low-Density Polyethylene. Macromolecules 2004, 37, 3319. (8) Murayama, E. Optical Properties of Ringed Spherulites. Polym. Prepr. Jpn. 2002, 51, 460. (9) Gan, Z.; Kuwabara, K.; Abe, H.; Iwata, T.; Doi, Y. The Role of Polymorphic Crystal Structure and Morphology in Enzymatic Degradation of Melt-crystallized Poly(butylene adipate) Films. Polym. Degrad. Stab. 2005, 87, 191. (10) Minke, R.; Blackwell, J. Single Crystals of Poly(tetramethylene adipate). J. Macromol. Sci. B 1980, 18, 233.

12092


(11) Pouget, E.; Almontassir, A.; Casas, M. T.; Puiggali, J. On the Crystalline Structures of Poly(tetramethylene adipate). Macromolecules 2003, 36, 698. (12) Wu, M. C.; Woo, E. M. Effects of R-form or β-form nuclei on polymorphic crystalline morphology of poly(butylene adipate). Polym. Int. 2005, 54, 1681. (13) Hobbs, S. Y.; Hobbs, F. W., Jr. Crystal Unit-cell Dimensions and Densities of Linear Aliphatic Polyesters. J. Polym. Sci. Part A-2 1969, 7, 1119. (14) Sasaki, S.; Sakaki, Y.; Takahara, A.; Kajiyama, T. Microscopic Lamellar Organization in High-density Polyethylene Banded Spherulites Studied by Scanning Probe Microscopy. Polymer 2002, 43, 3441. (15) Markey, L.; Janimak, J. J.; Stevens, G. C. Modelling and Simulation of the Permanganic Etching of Banded Spherulitic Polyethylene: Correlation with AFM Observations. Polymer 2001, 42, 6221. (16) Ivanov, D. A.; Jonas, A. M. Isothermal Growth and Reorganization upon Heating of a Single Poly(aryl-ether-ether-ketone) (PEEK) Spherulite, As Imaged by Atomic Force Microscopy. Macromolecules 1998, 31, 4546. (17) Li, L.; Chan, C. M.; Li, J. X.; Ng, K. M.; Yeung, K. L.; Weng, L. T. A Direct Observation of the Formation of Nuclei and the Development of Lamellae in Polymer Spherulites. Macromolecules 1999, 32, 8240. (18) Chen, J.; Yang, D. Nature of the Ring-Banded Spherulites in Blends of Aromatic Poly(ether ketone)s. Macromol. Rapid Commun. 2004, 25, 1425. (19) Fro¨msdorf, A.; Woo, E. M.; Lee, L. T.; Chen, Y. F.; Fo¨rster, S. Atomic Force Microscopy Characterization and Interpretation of Thin-Film

Poly(butylene adipate) Spherulites with Ring Bands. Macromol. Rapid Commun. 2008, 29, 1322. (20) Woo, E. M.; Wu, P. L.; Wu, M. C.; Yen, K. C. Thermal Behavior of Ring-Band versus Maltese-Cross Spherulites: Case of Monomorphic Poly(ethylene adipate). Macromol. Chem. Phys. 2006, 207, 2232. (21) Wang, Z.; Hu, Z.; Chen, Y.; Gong, Y.; Huang, H.; He, T. Rhythmic Growth-Induced Concentric Ring-Banded Structures in Poly(ε-caprolactone) Solution-Casting Films Obtained at the Slow Solvent Evaporation Rate. Macromolecules 2007, 40, 4381. (22) Wang, Z.; Alfonso, G. C.; Hu, Z.; Zhang, J.; He, T. Rhythmic Growth-Induced Ring-Banded Spherulites with Radial Periodic Variation of Thicknesses Grown from Poly(ε-caprolactone) Solution with Constant Concentration. Macromolecules 2008, 41, 7584. (23) Maillard, D.; Prud’homme, R. E. Crystallization of Ultrathin Films of Polylactides: From Chain Chirality to Lamella Curvature and Twisting. Macromolecules 2008, 41, 1705. (24) Chen, J.; Yang, D. Phase Behavior and Rhythmically Grown RingBanded Spherulites in Blends of Liquid Crystalline Poly(aryl ether ketone) and Poly(aryl ether ether ketone). Macromolecules 2005, 38, 3371. (25) Ye, H.-M.; Xu, J.; Guo, B.-H.; Iwata, T. Left- or Right-Handed Lamellar Twists in Poly[(R)-3-hydroxyvalerate] Banded Spherulite: Dependence on Growth Axis. Macromolecules 2009, 42, 694.

ReceiVed for reView August 29, 2009 ReVised manuscript receiVed November 28, 2009 Accepted December 7, 2009 IE901356Q

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