Crystal Lamellae of Mutually Perpendicular Orientations by Dissecting

Jan 24, 2012 - Three-dimensional dissecting onto interior lamellar structures of spherulites in ... Hiroaki Imai. Crystal Growth & Design 2017 17 (7),...
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Crystal Lamellae of Mutually Perpendicular Orientations by Dissecting onto Interiors of Poly(ethylene adipate) Spherulites Crystallized in Bulk Form E. M. Woo,* Lai-Yen Wang, and Siti Nurkhamidah Department of Chemical Engineering, National Cheng Kung University, Tainan 701-01, Taiwan ABSTRACT: Three-dimensional dissecting onto interior lamellar structures of spherulites in poly(ethylene adipate) (PEA) in bulk forms crystallized at 28 ± 3 °C were studied in correlation with thin-film forms. Interior morphology in bulk PEA samples revealed onion-like alternating shells. Interior spherulites clearly exposes that the lamellae within the shell interior are composed of stacked plate lamellae in radial direction while the shell surface region are arranged by tangential lamellae. Shell interior and surface correspond to the valley and ridge of ring-banded on the top surface, respectively. Morphology characterization on fracturing patterns further evidence that the shells could be cleaved apart along the shell surface, indicating weak physical connection between these two perpendicularly oriented shells. Further thermal analysis and X-ray results also supported existence of highly ordered lamellar packing into two mutually perpendicular chain orientations in the 28 °C-crystallized PEA bulks, which differed significantly from the randomly oriented lamellae in either 2 or 33 °C-crystallized PEA bulks.

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INTRODUCTION Interpretations on the formation of ring-banded morphology in polymer spherulites have been debated for decades. Lamellar twisting among all the proposed mechanisms is the most adapted model to account for the formation of the concentric ring bands in spherulites.1−8 Explanations provided to account for lamellar twisting may be divided into two views. One view proposes that lamellar twisting is associated with features that exist during growth or are generated by the growth process itself,9 while another describes that lamellar twisting is associated with the morphology or structures of lamellae themselves. These may be associated with screw dislocations6,8,10 or surface stresses at the fold surface.11 These complex issues have long been explored by some outstanding work in the literature. By following the classical work of Keith and Padden’s hints,2,3 Bassett et al.12,13 have explored further possibilities of lamellar assembly in relation with the ringbanded patterns in polymer spherulites. If the polymer chains are tilted in the lamellae and screw dislocations are present, they have shown that the development of the lamellae in the screws can be asymmetric. In addition, if the twist of the leading lamellae is influenced by these additional lamellae in the screws, then the whole stack of leading lamellae would be asymmetrical. Lustiger and Lotz et al.5 have further suggested that polyethylene (PE) lamellae undergo continuous twisting rather than successive misalignment of essentially untwisted segments. Crystals of C-shapes (abbreviated Cs), S-shapes (abbreviated Ss), or a combination of both of shapes can be observed on the top surface of PE ring-banded spherulites in the bulk samples depend on the projection of helicoids on a plane. It has been suggested that Cs and Ss are caused by geometrical effects as the lamellae are projected onto a surface at various angles.5 © 2012 American Chemical Society

Microscopic observation on thin-film samples has been the most often used method to study the ring-banded morphological features. Xu et al.14 have characterized banded spherulites of poly(3-hydroxybutyrate-co-hydroxyhexanoate) (P(HB-co-HHx) using in situ atomic force microscopy (AFM) and found that the lamellae in the banded spherulites of P(HB-co-HHx) exhibit complicated growth behaviors: twisting, bending, backward growth, and branching. In addition, the lamellae of P(HB-co-HHx) spherulites continuously twist to show alternating edge-on and flat-on views along the radial direction, and Xu et al. concluded that the lamellae of P(HB-coHHx) exhibited right-handed twisting.14 However, hypothesis of stem helical handedness may be no longer adequate, and a chiral polymer can have right- and left-handed lamellae within the same spherulites or upon blending according to Xu et al.’s later work. 15 Apart from AFM, transmission electron microscopy (TEM) has commonly been adapted to analyze the banded morphology in polymer spherulites. Ho et al.16 have explained that the banded morphology in poly(trimethylene terephthalate) (PTT) is attributed to lamellar twisting which is evidenced by the observations of wavy morphology from reflected light microscopy and TEM. Furthermore, the lamellar twisting in PTT may be caused by the tilted chain stems which are nonorthogonal to fold surface.16 Recently, microfocus (microbeam) wide-angle X-ray diffraction (WAXD) was used to analyze the ring-banded morphology in spherulites.17−20 A single poly(3-hydroxybutyrate) (PHB) spherulite has been mapped by means of microbeam X-ray diffraction using a synchrotron source with 7 μm beam, and it is concluded that Received: October 4, 2011 Revised: January 13, 2012 Published: January 24, 2012 1375

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samples (50−150 μm). Ring-banded patterns have conventionally been reported for the some polymers in thin films, but in this study, fractured interiors of bulk-crystallized PEA were observed instead for revealing inner lamellae in comparison with its top free-surface ring bands. In this study, using PEA as a convenient model, the crystalline morphology in the interior of bulk form was expounded by three-dimensional dissecting. There were two reasons for using PEA as a model system in this study: (1) the surface ring bands in PEA in thin-film forms have been well studied, and (2) PEA exhibits well-defined ring bands in a narrow temperature window (28 ± 3 °C); (3) PEA bulk samples could be easily fractured to expose the interiors. PEA samples were prepared in (a) films in comparison with (b) thicker bulks for stereo three-dimensional dissecting and ultimate construction of the periodical patterns underneath the surface rings. Furthermore, three-dimensional structures of ring bands were expounded to more fully understand the complex factors in lamellar assembly in crystallization and to shed new light on the architecture or mechanisms of the ringbanded spherulites in semicrystalline polymers. Correlation of surface ring bands with inner concentric 3-D shells (in fracture surface) was attempted by dissecting across several different interior spots of spherulites in bulk crystallized PEA.

the ringed spherulite of PHB is attributed to the lamellar twisting.16 Other researchers, however, propose mechanisms different from the lamellar twisting in expounding the formation of ringed spherulites.21−23 Wang et al.21 have proposed a nonlinear diffusion process induced rhythmic growth mechanism to interpret the concentric ring-banded structures of poly(ε-caprolactone) (PCL) produced during solvent evaporation, and the nonlinear diffusion process is due to the periodically change in the concentration gradient of the polymer solution. Ring-banded spherulites in poly(1,4-butylene adipate) (PBA) have attracted many researchers’ attention, and different mechanisms have ever been provided. Poly(ethylene adipate) (PEA) also shows apparent extinction double-ring bands at a very narrow crystallization temperature range (Tc = 28 ± 3 °C), whose surface morphological characterization on the ring-banded spherulites of PEA was performed using AFM.24 It has been revealed that the crystal species on the ridges and valley seem discrete and unrelated with each others. To be complete, the surface morphology PEA may have to be further extended by interior 3-D structural characterization. Thus, the frequently adapted lamellar twisting mechanism may have to be refined to account for the ring-banded spherulites in different polymers. Keith and Padden25 have stated in conclusion that it needs emphasis that a supposed “unique cause” for twisting orientation in banded polymer spherulites is a chimera and that twisting can be attributed to one or other of two factors: (i) chain tilt which promotes a generation of surface stresses in lamellae and (ii) isochiral helicity. Even though these two factors have been assumed by many investigators, however, Keith had reservation that they universally account for ring bands in all polymers. In addition, the lamellar packing in thin films may differ significantly from thick/bulk polymers as growth constraints and surface stress may be completely different in the bulk state. So far, attempts on interpreting the ring-banded spherulites of polymers have been based on the characterizations of surfaces of thin-film polymer samples. As pointed out in a preliminary study,26 interiors of 3-D ring bands of spherulites would be essential and informative for probing more details in the complex growth mechanisms of these periodical lamellar assembly patterns in semicrystalline polymers. As cited above, interpretations on the ring banded spherulites of polymers have been mostly based on the characterization on thin-film samples, and interiors of 3-D pictures of ring bands are yet to be probed for more detailed views on understanding the fine structures of ring bands in spherulites. In this study, scanning electron microscopy (SEM), wide-angle X-ray, and differential scanning calorimetry analyses were collectively applied to observe the crystalline morphology in the interior of bulk form using PEA as a model, which is know to exhibit ring bands in thin film when crystallized at 28 °C. Furthermore, three-dimensional structures of ring bands were expounded to more fully understand complex factors in lamellar assembly in crystallization and to shed new light on the architecture or mechanisms of the ringed spherulites in semicrystalline polymers. When crystallized at the 25−32 °C window, PEA is widely known to show ring bands on thin-film samples (nanometers to less than 10 μm),23,24 whose ring-band mechanism has been widely discussed and was not the objective of this study. This study for the first time addresses three-dimensional dissecting into interiors of lamellar assembly in spherulites of bulk PEA

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EXPERIMENTAL SECTION

Materials and Procedures. Poly(ethylene adipate) (PEA), a lowmelting semicrystalline polyester with Tg = −52 °C and Tm = 43 °C, was purchased as research-grade materials from Aldrich Co. The weight-average molecular weight (Mw) was nearly 10 000 g/mol, determined by gel permeation chromatography (GPC, Waters 410) using tetrahydrofuran (THF) as eluent at a flow rate of 1.0 mL/min. The polymer was purified by reprecipitation from chloroform into cold methanol (ca. −10 °C). PEA pellets/powder were heated, melted, and then brought to crystallization immediately by quenching into an isothermal bath. Alternatively, PEA was also dissolved in CHCl3 (chloroform) in higher concentrations, and a few drops were repeatedly deposited on glass substrate to stacked into thick bulks. Degassing was performed at 45 °C in a vacuum oven for 24 h. Crystallization conditions were as follows. PEA in thick bulk forms (25−50 μm) was typically heated to Tmax = 90 °C, rapidly brought to Tc = 2, 26, 28, 29, and 33 °C, etc., and held for time = 1−24 h (longer time for higher Tc) for full crystallization. Alternatively, another set of PEA samples in even thicker bulk forms were prepared by melting the PEA polymer deposited on aluminum foil or in wrapped with aluminum foil (for molding) at 90 °C for premelting, and then the molten PEA in foil was quickly dipped into water bath set precisely at controlled temperature of 2, 26, 28, 29, 33, and 35 °C for at least 2−4 h for full crystallization. During sample crystallization, the temperature of the bath was kept exactly isothermal within ±0.5 °C. For water bath, washing of samples was not needed. Bulk PEA samples had thickness from 50 to 75 μm. In this condition, nucleation was expected to initiate from (1) bottom substrate surface, (2) top surface, and (3) occasionally from interior. As cleavage across spherulites tended to be randomly initiated, fracture was performed on PEA bulk samples several times to acquire sufficient number of specimens for morphology analyses. Cleavage of samples was controlled to fracture near the poles of spherulites and across equator line of spherulites. Apparatus. Polarizing Optical Microscopy (POM). A polarizing optical microscope (Nikon Optiphot-2, POM), equipped with a digital camera charge-coupled device (CCD), was used to confirm the crystal morphology as crystallized at several Tc (2, 26, 28, 29, and 33 °C) of thin-film PEA samples. Scanning Electron Microscopy (SEM). Fractured surfaces of bulkform PEA samples were examined and characterized using scanning electron microscopy (FEI Quanta-400F, SEM) for revealing lamellar structure in the fracture and top free surface of bulk PEA. Samples 1376

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were coated with gold vapor deposition using vacuum sputtering prior to SEM characterization. Differential Scanning Calorimetry (DSC). Thermal transitions of bulk crystallized PEA samples were characterized with a differential scanning calorimeter (Diamond, Perkin-Elmer Corp.) equipped with an intracooler for quenching and cooling. For measurement of melting temperatures, a scanning rate of 10 °C/min was used. Prior to DSC runs, the temperature and heat of transition of the instrument were calibrated with indium or zinc standards. A continuous nitrogen flow in the DSC sample cell was maintained. Wide-Angle X-ray Diffraction (WAXD). PEA bulk samples (crystallized at specified Tc = 2, 26, 28, 29, and 33 °C) were characterized with X-ray instrument (Shimadzu/XRD-6000) with Cu Kα radiation (30 kV and 40 mA). The scanning angle (2θ) covered a range between 10° and 30° at a rate of 2°/min.

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RESULTS AND DISCUSSION Figure 1 shows SEM graphs (8000×) on top surfaces of bulk PEA sample (50−75 μm) crystallized at Tc = 28 °C. The ridge Figure 2. (A) SEM graphs for interior layered lamellae and their correspondence to top surface bands as viewed in a partially fractured bulk PEA samples, (B, C) zoom-in area as indicated in graph A, and (D) scheme for lamellar arrangement underneath the top surface.

is equivalent to “linear” of the corrugated board. In addition, each of the underneath shell cassettes corresponds to one ring band on the top surface, and the width of the interior cassette is exactly identical to the inter-ring spacing (∼6 μm) on the top surface. In each cassette shell, the lamellae are structured like a laminated corrugated board, where the radial lamellae are directly underneath the valley band on the top surface and the lamellae are oriented horizontal to the top surface. In between the laminated shells of the radial lamellae, there is an interfacial layer filled with tangential lamellae, which protrude out vertically to form the ridge band on the top surface. That is, each of the corrugate boards is packed with alternating shells of radial lamellae and tangential lamellae that are oriented at 90° angle to each other. Ideally, interior morphology of ring-banded PEA would be clearer if the interfaces of the neighboring laminated cassettes could be torn off from each other. Such viewed would be further shown and discussed on following results based on PEA bulk samples that were fractured in patterns suited for this objective. Side Views (Fracture across Thickness) on Internal Lamellae of Bulk PEA. Figure 3 shows SEM micrographs (A) top and fracture surfaces of an entire spherulite and (B) zoomin to portion of spherulite revealing concentric tangential lamellae-stacked plate lamellae underneath the top surface ring bands. Figure 3B shows zoom-in magnification SEM graph of the top free surface and interior fracture surface morphologies of bulk PEA samples. Ridge band of the top surface ringed spherulites corresponds to the ark-shaped tangential lamellae on the fracture surface, while the valley band of the top surface corresponds to the stacked plates lamellae on the fracture surface. The top surface displays evident ring-banded patterns similar to those observed in thin cast PEA films under POM. Concentric ring bands in the spherulites of PEA appear in twodimensional thin films, but alternating shell lamellae of two opposite orientations form in the interior of three-dimensional bulk PEA. The ring-banded patterns in top free surface of the spherulites in thick bulk PEA are quite similar to those typically seen thin-film morphology crystallized of PEA at the same Tc. However, the interior fracture exhibits a corrugate-board shell structure, which is not same as the top-surface ring but is

Figure 1. SEM graphs (8000×) of the top surface of bulk PEA crystallized at 28 °C.

band is packed with discrete triangular crystals, and the valley is apparently flat. It should be noted that such 3-D stereo ridge morphology with triangular protrusion crystals on the ridge band is not seen in thin-film PEA sample crystallized at the same Tc . Naturally crystallized bulk PEA would have amorphous constituents expelled to partially fill or cover up the lamellae in ridges and/or valley; i.e., the crystal plates associated with ridge and valley might both be partially or fully covered in samples as crystallized. To dig into interior of bulk lamellae, fracturing on bulk PEA samples (crystallized at Tc = 28 °C) was attempted. Acetone-solvent vapor etching was performed on PEA bulk (50−75 μm in thickness) to induce splitting of the specimens. Splitting of specific locations of PEA samples such that the interiors of the ring-banded spherulites were exposed. SEM was then performed such that both the top surface and interior fractured surface in bulk PEA crystallized at 28 °C could be revealed simultaneously for comparison and correlation of the top surface with the interior lamellar structure. Figure 2 shows SEM graphs on partially fractured PEA bulk correlating from top surface into interior surface perpendicular to each other. By this way, correlation between the interior layered lamellae and their correspondence to the ring band pattern on top surface can be viewed in a direct way. Two interesting morphological features are critical. First of all, the SEM micrograph onto the interior PEA bulk reveals clearly that the lamellae underneath the top surface are aligned in layered and laminated cassettes resembling a corrugated board. The layer of radial lamellae is equivalent to “fluting”, and the layer of tangential cilia lamellae 1377

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Figure 4. Schemes illustrating two different nucleation sites: interior nucleation (left) and nucleation from top surface or bottom substrate (right).

to ring bands on top surfaces and concave bowls associated with exterior surface growth and interior growth in 3-D patterns. Interior nucleation also leads to alternating shells of corrugated board structure, with a convex dome bulging out of the surface when fully grown. Both nucleation types can lead to a ring-banded structure on top surfaces, but the interior lamellae are packed differently. Fractured surface for the interior of ringless Maltese-cross spherulites in PEA crystallized at Tc = 35 °C and was characterized using SEM for direct comparison with the alternating-shell interior of ring-banded spherulites in PEA crystallized at 28 °C. For comparison, the interiors of bulk PEA sample crystallized at Tc = 35 °C where ringless spherulites are present on top surface were compared with bulk PEA sample crystallized at Tc = 28 °C where ring-banded spherulites are present on top surface. Figure 5 shows SEM graphs on the

Figure 5. SEM graphs on fracture interior of (A) bulk PEA crystallized at Tc = 35 °C and (B) 28 °C-crystallized PEA.

fracture surface of (A) interiors of bulk PEA crystallized at 35 °C for 24 h (for full crystallinity) and (B) interiors of bulk PEA crystallized at 28 °C for 4 h. The SEM graph in Figure 5A (35 °C-crystallized bulk) reveals that lamellae radiate out straightly from center, but no alternating shells are seen in the fractured surface. Such interior morphology corresponds to no ring bands found in the top (outer) surface of the PEA sample crystallized at Tc = 35 °C. Lamellae go straightly from center to edge of the spherulite with no periodical turns, twists, or sudden changes in orientations. For direct comparison, 28 °Ccrystallized bulk PEA samples were fractured and similarly characterized using SEM. As the fracturing would randomly cut across the 28 °C crystallized PEA samples, bulk PEA samples were fractured at several different locations for SEM examinations of interiors for dissecting a full picture of entire 3-D spherulites and lamellar structure therein. Note that depending on fracturing across poles or equators of 3-D spherulites the interiors might differ slightly. Figure 5B shows SEM graphs of 28 °C-crystallized PEA, revealing onion-like alternating shells near the center of 3-D spherulites. Corrugateboard shells of arc or hemisphere curves are clearly seen in the SEM graphs. The lamellae within the shell boundary are apparently perpendicular to the shell boundary. The shell boundary is easily fractured away, indicating that the interlayer is not interconnected by wandering chains from one layer to

Figure 3. SEM micrographs of (A) top and fracture surfaces of entire spherulite and (B) zoom-in portion of spherulite revealing concentric tangential lamellae-stacked plate underneath the top-surface ring bands. (C) Schemes for fracture across a 3-D truncated sphere.

apparently correlated with the periodical ring-banded pattern on the outer surface. The intershell spacing in the fracture surface is about 6.4 μm, which is same as the inter-ring spacing on the top surface. Interestingly, the corrugated crystals of two opposite orientations as revealed by SEM correspond to the alternating bright (ridge region) and dark bands (valley region) observed using POM, respectively. Figure 4 shows schemes illustrating two different nucleation sites: interior nucleation (left) and nucleation from top surface or bottom substrate (right). Top free surface nucleation leads 1378

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tangential lamellae exist). Figure 7 shows SEM graphs: (A) 1500× and (B) 3000× for fractured bulk PEA crystallized at 28

another. A closer look reveals that the shell surface is composed of tangential lamellae that are mostly oriented in the tangential direction. That is, within each shell of the onion structure, the lamellae are oriented in radial direction, similar to stacked plates. But the shell surface boundary reveals dramatically different lamellae, which differ from the stacked plates lamellae in shapes and orientation. The lamellae on the shell surface region are not plate-like but resemble tangential lamellae with orientation perpendicular to the stacked plates. Owing to such perpendicular orientation noninterconnecting of these two lamellae, fracturing of the interior exposes that the alternating shells are cleanly cleaved apart. Apparently, for bulk PEA (Tc = 33−35 °C) with top surface showing ringless spherulites, the corresponding interiors are dramatically different from PEA bulk crystallized at Tc = 28 °C with top surface showing apparently ring bands. The main differences are that alternate corrugate-board lamellar assembly is present only in PEA bulk crystallized with ring bands on top free surface. Figure 6A−C shows the SEM graphs of the interior versus exposed surface of PEA spherulite crystallized at 28 °C. The

Figure 7. SEM graphs: (A) top surface and fractured cross section and (B) magnified cross section, for bulk PEA crystallized at 28 °C, revealing alternating shells (corrugated board structure).

°C, revealing interiors with alternating shells. Figure 7A (1500×) shows the top-surface morphology (upper portion) in correlation with the interior morphology by dissecting into the thickness cross section, which exhibits distinct alternating hemisphere layers with the center near bottom substrate (lower portion). As the hemisphere is confined by the thickness (∼50 μm) on the top surface, the hemisphere layers are confined into an arc shape; however, layers of two opposite orientations are clearly visible on the thickness cross section. The morphology of alternating shells on the thickness cross section is correlated to the ring bands on the top surface. On the top surface (upper portion of Figure 7A) of bulk PEA, alternating concentric ring bands are clear. The ridge band (whiter contrast) of the surface bands again is lined with tangential lamellae. Note that in thin PEA films of crystallized PEA similar ring bands are present; however, the ridge bands in thin-film PEA are not lined with tangential lamellae like those seen in the top surface of bulk PEA crystallized at same Tc = 28 °C. Albeit similar ring bands are seen, the mechanisms in bulk PEA top surface ring bands apparently are subtly different from those in thin films. This can be reasonably expounded in greater details as the interior bulk morphology is further dissected. Figure 7B (3000×) shows magnified SEM image for the morphology of the thickness cross section. Between the two alternating hemisphere shells, apparently, the outer shells of the layers were incidentally fractured and exposed in this sample for ideal SEM characterization. The SEM graph in Figure 7B clearly shows that the shell surfaces are mainly composed of the bundles of tangential lamellae that run perpendicularly to the transverse stacked plates within the shells. These tangential thin lamellae on the shell surface in the interior bulk may reach the top surface and become the discrete triangular lamellae seen on the ridge band of the top-surface rings. Thus, the tangential lamellae correspond to the ridge band on the top surface, while the stacked plates lamellae (radial from sphere center) eventually packed like stacked cards to the surface and form the valley band of the top surface. Fractured surfaces in alternative bulk-crystallized PEA samples were examined for more thorough analysis. This was necessary as assurance of evidence should be made that the fractured interior morphology patterns in previous discussions were not sporadic or incidental. During bulk crystallization of PEA, it may be heterogeneously nucleated on the top or bottom surface or homogeneously nucleated in interior. Bulk crystallized PEA samples with nucleation near the bottom (or top) surface were fractured for SEM characterization. Figure 8 shows SEM graphs for top- or bottom-nucleated 3-D

Figure 6. SEM graphs for (A, B) surface vs fractured interior; (C) close-up view of interior of ring-banded spherulites in fractured bulk PEA sample crystallized at 28 °C.

spherulite of PEA is composed of multiple shells of crystals, which appear as multiple shells of corrugated boards stacked alternately. In between the longitudinal shells of crystals, there are transverse-oriented crystal species, with these two types of crystals orienting as a “T” joint. Growth direction of the spherulites is indicated on the graph. Note that the shell surface distance of the interior corrugate-board shell structure is exactly equal to the inter-ridge distance of the surface ring bands (∼6 μm). The growth direction of the short transverse-oriented crystals is along the radial direction while that of the long longitudinal crystals is perpendicular to the growth direction. A clear correlation between interior lamellae assembly and surface ring-banded textures of PEA spherulite can be established. Closer inspection in Figure 6C reveals that the transverseoriented crystal layers in spherulite interior correspond to the flat/valley regions in the surface and longitudinal straight-stalk crystal layers correspond to the ridge bands on the surface. Note that the 3-D periodical assembly leads to not only the noticeable ring band on exterior surfaces but also corrugateboard-like multishell structures in the interior of the spherulites. The orientations of the crystals forming the corrugated boards are discretely perpendicular to each other, with no gradual turning from one to another. These two crystal species are discrete and contact with one to the other like a “T”-shaped joint. It is clear that the interior of bulk PEA (Tc = 28 °C) is composed of concentric multishells of two opposite lamellar directions (radial from sphere center and tangential to radial direction). Bulk PEA (Tc = 28 °C) samples were fractured in several possible locations so that some of the dissections cut through the samples to expose the tangential regions (where 1379

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right) plate. In addition to the obvious geometric difference in these two lamellae in the mutually perpendicular tangential lamellae and stacked plates, the chain orientations in these two oppositely oriented lamellae are also different, with one being parallel and the other perpendicular to polarized light. Note that the chain orientations in these two lamellae in bulk PEA are perpendicular to each other, and this opposite orientation in the bulk PEA form would correspond to the two birefringence colors as viewed in POM if it is prepared in thin films crystallized at 28 ± 3 °C. Most evidently, thickness (50 μm) of the 3D interior lamellae shells of opposite orientations in PEA bulks as revealed in SEM perfectly agrees with the observed pitch of the thin-film ring bands (10 μm) [at Tc = 28 ± 3 °C] in POM characterization. This evidence from 3D dissecting on the bulks fully explains and justifies the quantitative birefringence rings and their pitches in POM characterization on thin films. In addition, it has been reported by Wang et al.27 and Chuang et al.28 that the growth of ring-banded spherulites in thin-film polymers, such as poly(ε-caprolactone) copolymers or poly(trimethylene terephthalate), is not uniform along the radial direction, and the growth is typically faster in one band and slower in the next band, i.e., intermittent growth across the two bands of opposite birefringence They inferred that the ridge band of spherulites in thin-film samples grows more slowly than the valley bands.27,28 The 3-D dissecting analyzed on the interior lamellae of PEA in this study could fully justify and explain this phenomenon of intermittent growth in ringbanded spherulites. Growth rates of spherulites are typically measured in plane along radial direction. The 3-D interior morphology has revealed that the growth of the radial lamellae (the fluting layer) can be measured at their true pace; however, as they turn 90° to form the tangential lamellae in the linear layer of the corrugated cassettes, they are slower as the lamellar direction is perpendicular to the radial growth measurement. In addition, the alternating ridge and valley band on top surface can be nicely explained. The interior of tangential lamellae are aligned to the top surface, and they grow and protrude outward to the top surface to form the ridged band; by contrast, the radial lamellae in the fluting layer are parallel to the top surface, and they are stacked to form the flat valley bands between two ridges on top surface. To summarize such complex 3D views in a simpler context, Figure 9 shows a scheme for correlations between the interior

Figure 8. SEM graphs for top- or bottom-nucleated 3-D spherulites in bulk PEA crystallized at 28 °C: (A) top surface and fractured cross section, (B) magnified cross section, (C) scheme for correlation between top-surface and interior lamellar morphology, and (D) edgeon lamellae in tangential lamellae (top) and flat-on lamellae in stackedplate (bottom).

spherulites in bulk PEA crystallized at 28 °C: (A) top surface and fractured cross section and (B) magnified cross section by zoom-in to interior spots. Again, the top-surface ring-band morphology (upper portion) is in correlation with the interior corrugate-board morphology on the fractured thickness cross section. The interior exhibits distinct alternating hemisphere shells. Shells of two opposite orientations are clearly visible on the thickness cross section. Figure 8B shows by zoom-in to interior spots the SEM graphs for the fractured interior of this PEA sample clearly evidence that the shell surface are mainly composed of the bundles of tangential lamellae that run perpendicularly to the stacked plates within the shells. Additionally, in order to vividly summarize the arc-shaped tangential lamellae-stacked plate shells in the fractured interior, Figure 8C shows a scheme for the interior corrugate-board lamellar morphology in the 3-D truncated sphere. The tangential lamellae are located in the shell surface regions sandwiched between the stacked plates. Figure 8D shows two schemes for edge-on lamellae in the tangential lamellae (bottom left) and flat-on lamellae in the stacked plate (bottom

Figure 9. Schemes for correlations between the interior layered lamellae and top-surface ring bands showing onion-like alternating shell. 1380

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WAXD intensity curves for the 26, 28, and 29 °C-crystallized PEA samples. All PEA samples show diffraction peaks at (111), (110), and (020) planes; however, only the 26, 28, and 29 °Ccrystallized PEA samples (ring-banded) show a much weaker diffraction intensity of the (111) plane. Additionally, the 26, 28, and 29 °C-crystallized PEA samples (ring-banded on top surfaces) show much a stronger intensity at the (020) diffraction (2θ = 24.5°) than that of the 2 and 33 °Ccrystallized PEA bulk samples (ringless). Furthermore, WAXD diffractograms for 28 °C-crystallized PEA show an extra peak at 2θ = 21.0°, which is absent in all PEA samples crystallized at 2 or 33 °C (i.e., outside the 28 ± 3 °C window). The WAXD diffractograms for 26 and 29 °C-crystallized PEA bulks also have this extra peak at 2θ = 21.0°. Such WAXD data of distinctly different diffraction patterns support that the highly ordered lamellar packing into two perpendicular chain orientations in the 28 °C-crystallized PEA sample differ significantly from the randomly distributed lamellae in 2 and 33 °C-crystallized PEA bulk samples. Inoue et al.31 have pointed out that PEA of high enough Mw may exhibit different intensities of the (111) peak (2θ = 20.5°) depending on the spherulite orientations or Tc in high-molecular-weight PEA, but for low-Mw PEA, the effect of orientation is relatively less. In addition, it has been claimed that highly oriented spherulites in PEA would exhibit much less intensity of (111) WAXD diffraction. To briefly summarize, the WAXD data apparently show that 26, 28, and 29 °C-crystallized PEA bulk samples have much lower (111) diffraction intensity but slightly sharper (020) diffraction peak, indicating that the tangential lamellae may be highly oriented in a uniform direction in the bulk PEA samples that exhibit apparent corrugate-board lamellae in interior assembly and ridge-valley ring bands on exterior surfaces. Thermal behavior of the oriented tangential lamellae in bulk PEA samples crystallized at 28 °C was investigated. The tangential lamellae, being almost fully extended into thin needle-like crystals, are supposed to have a higher Tm than the stacked plate lamellae. Thermal analysis was performed on 2, 28, and 33 °C-crystallized bulk PEA samples. It has been earlier proposed that the multiple melting peaks in crystallized PEA are associated with the different lamellae in the ring-banded spherulites.32 Figure 11 shows DSC thermograms (10 °C/min) for 2, 28, and 33 °C-crystallized PEA bulk samples. The 28 °Ccrystallized PEA sample exhibits DSC curve that is distinctly different from those for the 2 and 33 °C-crystallized PEA bulk samples. For the 28 °C-crystallized PEA sample, there are two peaks: one lower Tm (∼45 °C) and one higher Tm (52 °C), not counting the minor shoulder peak. The first lower Tm melting peak (P1) is more prominent than the second higher Tm peak (P2) when scanned at 10 °C/min. The tangential lamellae (chain more extended with more regular re-entry) are likely associated with the higher melting peak P2, while the lower melting P1 lamella (Tm = 45 °C) are associated with the stacked plate lamellae (chain folding with irregular re-entry) in the spherulites. For the 28 °C-crystallized PEA, the higher melting P2 (with Tm2 = 52 °C) may be attributed to the more highly oriented tangential lamella in the shell surface regions. The high-melting peak (higher P2 with Tm2 = 52 °C) indicates that the molecular chains in the tangential crystals on the shell surface are more highly oriented than those in the stacked plates (lower P1 with Tm1 = 45 °C). In addition, the relatively smaller fraction of the tangential crystals in comparison to the larger fraction of the stacked plate lamellae, which is also

layered lamellae and top-surface ring bands showing onion-like alternating shell. Brief explanations are offered. The 3-D interior morphology of bulk PEA has revealed that ring-banded spherulites are composed of onion-like alternating shells. Depending on which direction of the morphology of fractured surfaces was dissected, it shows slightly different morphology. Two regions of different view angles are of special interest: (A) Zoom-in graph in the left side, which shows alternating shells. Shell interior and surface correspond to the valley and ridge of ring-banded on the top surface, respectively. Shell interior is arranged by stacked lamellae in radial direction. The lamellae on the shell surface are perpendicular to the stacked lamellae in shell interior and ridge in the top surface. These two crystal species are discrete and contact with one to the other like a “T”-shaped joint. In addition, another view angle is (B) zoomin graph in the right side, which shows that ridge and valley on the top surface are correlated with tangential and radial lamellae in the fractured surface. That is, 3D morphology changes and switches slightly as the lamellae grow away from the center to peripherals, and as they are viewed from different perspectives. X-ray and Thermal Evidence for Highly Oriented Tangential Lamellae. Furthermore, preferred lamellar orientation could be discerned in WAXD diffractions peaks. PEA normally has only one crystal form of monoclinic cell, with a = 0.547 nm, b = 0.724 nm, c = 1.172 nm (fiber axis), and a monoclinic angle of α = 113.5°.29,30 For this form of crystallized PEA, three prominent diffraction peaks for (111), (110), and (020) are generally seen at 2θ = 20.5°, 21.7°, and 24.6°, respectively.31 Figure 10 shows WAXD diffractograms

Figure 10. WAXD diffractograms for PEA bulk samples crystallized at Tc = 2, 26, 28, 29, and 33 °C.

for 2, 26, 28, 29, and 33 °C-crystallized PEA bulk samples. Again, similar to the DSC differences among the various samples, the 28 °C-crystallized PEA sample (ring-banded) exhibits a WAXD diffractogram that is distinctly different from those for the 2 and 33 °C-crystallized PEA bulk samples (ringless). The 28 °C-crystallized PEA sample, similar to 26 and 29 °C-ones, exhibits an additional peak at 21.0°, and the (111) peak intensity (2θ = 20.4°) is much lower for PEA crystallized at 26−29 °C than that for PEA crystallized outside this Tc range (2 and 33 °C). The WAXD curves for the 2 and 33 °C-crystallized PEA bulk samples (with ringless spherulites) are almost identical, but they both differ significantly from the 1381

dx.doi.org/10.1021/ma202222e | Macromolecules 2012, 45, 1375−1383

Macromolecules

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Figure 11. DSC (10 °C/min) traces for PEA bulk samples crystallized at Tc: (A) 28 °C and (B) 2 and 33 °C.

form of PEA can also differ from the mechanism of lamellar packing or twisting in thin-film PEA. In addition, an aryl polyester poly(nonamethylene terephthalate) (PNT), crystallized into ring-banded spherulites in bulk form, has been proven to exhibit similar interior corrugate-board shells of alternating orientations corresponding to ridge/valley of the top-surface ring bands.39 This study, using PEA as a another model dissected parallel and vertical to the top-surface planes, further clearly proves that fine tangential lamellae are highly oriented in the tangential direction and emerge to top surface to form ridge bands.

correctly reflected in the smaller melting endotherm peak area of P2 to that of P1. On the other hand, there is only a single melting peak (Tm = 48 °C) for the 2 or 33 °C-crystallized PEA samples, which differ significantly from the 28 °C-crystallized PEA. For the 2 or 33 °C-crystallized PEA samples, the fractured surfaces of spherulites show no alternating corrugated layers of linear and fluting and that their corresponding POM micrographs would show ringless spherulites in thin-film forms of PEA crystallized at 2 and 33 °C. The DSC curves for the 2 and 33 °C-crystallized PEA bulk samples are almost the same with a single melting peak with Tm (=48 °C) intermediate to the values of the double melting peaks of Tm1 and Tm2 of the 28 °Ccrystallized PEA sample. The heats of fusion for all three samples are comparable at ΔHf = 56−58 J/g, indicating that all three samples have been crystallized almost to their maximum at respective Tc’s (2, 28, and 33 °C). Such DSC results suggest that the lamellae in the 2 or 33 °Ccrystallized PEA spherulites are randomly distributed to exhibit a single melting peak, but those in 28 °C-crystallized PEA sample are likely packed into two distinctly different lamellar types. Rijke and Mandelkern33 have shown that annealed stirred crystallized polyethylene at 142 °C with a melting tail of 152 °C is an indication of a small portion of fully extended chain system in PE. Similarly, Aida et al.34 have shown in extrusion polymerization via mesoporous silica using catalyzed synthesis of crystalline linear polyethylene, and they were able to produce PE nanofibers, which show a portion of crystals being packed in fully extended chains as evidenced by a trailing shoulder melting peak at 158 °C (in addition to the normal major melting peak at ∼140 °C). Clearly, the DSC result proves that the extra melting peak (the higher P2 peak) is associated with the melting of the tangential lamellae that are present only in 28 °C-crystallized PEA bulk samples, but such tangential lamellae packed in shell surface are absent in the 2 and 33 °C-crystallized ones. The tangential lamellae in the corrugate-board layers of PEA 3-D spherulites serve to connect the tangentially adjacent stacked plates, and they act like the linking fibrillar crystals reported earlier in PE lamellae, which have extended chains with some imperfect folded chains. An analogy can be drawn between the phase-separation morphology of amorphous polymer blends and ring-band spherulites of crystalline polymers or blends in thin films vs bulk states. It has been amply proven that the phase-separation morphology in polymer blends of bulk states are different with that in thin films of the same blend systems.35−38 This study based on bulk-state PEA has yielded critical evidence that the assembly of lamellae leading to alternating morphology in bulk

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CONCLUSION For the first time, 3-D spherulites in crystallized bulk polymers were dissected in aims to correlate the interior lamellar assembly with the outer surface ring-band patterns by directly tracing the morphology patterns from top surfaces onto interior fractured surfaces. Three-dimensional dissecting on spherulites in bulk PEA (50−75 μm) crystallized at Tc = 2−35 °C was performed and analyzed. Numerous attempts were made in order to achieve that 3-D PEA bulk spherulites were cleaved in several possible planes to expose details of interior structures, and morphology analyses were performed by dissecting several fracture surfaces across the specimen thickness. The interior morphology of bulk PEA crystallized at 28 ± 3 °C exhibits distinct corrugate-board layers stacked into 3-D spherulites or truncated hemisphere, depending on sample thickness confinement. SEM evidence reveals that interior of 3-D spherulites is composed of onion-like alternating shells. The shell interior is arranged by stacked lamellae in radial direction. The lamellae on the shell surface are perpendicular to the stacked plate lamellae in shell interior and ridge in the top surface. Shell interior and surface correspond to the valley and ridge of ringbanded on the top surface, respectively. Fracturing morphology of the 3-D spherulites further proves that the stacked shells could be cleanly cleaved apart along the shell surface lamellae, exposing that these two oppositely oriented crystal species are discrete and not physically interconnected. Thus, on the top surface, there are two types of free-surface ring band patterns depending on top-surface or interior nucleation. Both nucleation types in bulk PEA crystallization lead to multiple shell as interior of the spherulites. The interior of bulk PEA shows corrugate-board layers and the top free surface exhibits a ring-banded pattern. DSC and WAXD characterization results on PEA bulk samples crystallized at 28 °C vs 2 °C and 33 °C fully support that the lamellae in the 28 1382

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°C-crystallized PEA bulk are orderly assembled into two types of crystals with mutually perpendicular orientations, which yield distinctly different melting behavior and crystal diffractions from those in 2 or 33 °C-crystallized PEA bulks that contain only one type of randomly distributed lamellae.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Fax: +886 6 234-4496; Tel: +886 6 275-7575 ext 62670. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been financially supported by a basic research grant (NSC-97-2221-E-006-034-MY3) for three consecutive years from Taiwan’s National Science Council (NSC), to which the authors express their gratitude. The authors especially express their gratitude to Prof. B. Lotz for better and further refining the models and views in this work via many informal and casual but extremely helpful communications. Finally, this paper is dedicated to remember late Professor Jean-Jacques Point (retired from the University of Mons, Belgium, and passed away in November 2011) for his critical and original pioneering work in PEA ring bands and opening up an interesting field in polymer morphology so that others may pursue further by following his footsteps, to diversify and refine.

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