Multishell Oblate Spheroid Growth in Poly(trimethylene terephthalate

Jul 25, 2017 - Interior lamellae are alternatingly intersected with mutually perpendicular orientations and clear discontinuity, which evidently disag...
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Multishell Oblate Spheroid Growth in Poly(trimethylene terephthalate) Banded Spherulites Graecia Lugito and Eamor M. Woo* Department of Chemical Engineering, National Cheng Kung University, Tainan 701-01, Taiwan ABSTRACT: Unique interior dissection coupled with selective etching techniques for exposing the interiors of poly(trimethylene terephthalate) (PTT) banded spherulites. Three banded PTT spherulite types are present, originating from different nuclei geometries and corresponding to different assemblies of interior lamellae, but all possess similar multishell spheroid layered structures, each with their layer thickness exactly equal to the optical interband spacing. Interior lamellae are alternatingly intersected with mutually perpendicular orientations and clear discontinuity, which evidently disagrees with the conventional models of continuous helical twisting for banding. Interior 3D lamellae assemblies also have been fittingly correlated with top-surface banding patterns.

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INTRODUCTION Properties of semicrystalline polymers depend on their crystalline structures and spherulitic morphologies. Among many varieties of polymer morphologies, banded spherulites have attracted considerable attention due to their periodic arrangements. For half a century, the formation of polymer banded spherulites has been associated with the continuous twisting of chain folded lamellae during their growth in the radial directions. Proposed driving forces that impose lamellae to twist are screw dislocations, thermal or concentration or mechanical fields, anisotropic surface stresses, and autodeformation.1−3 It is interesting that banded spherulites only appear in crystallization of some (and not all) polymers, at specific ranges of crystallization temperatures (Tc), composition, and sample thickness, while all the proposed driving forces are always present in the dynamic process of polymer crystallization. Polymer lamellae may indeed twist during its growth; however, whether this lamellar twisting is the main reason for the banded spherulite formation is still a controversy. With technology advancement, discrepancies are found between the pitches of twisted lamella and the interference band spacing.4−14 It is quite impractical for polymer crystals to accurately maintain their helicoid shapes with exact pitches (twisted ribbon-like lamellae) as they grow along the amorphous species or other polymers, yet banded spherulites are even more pronounced with the existence of such disturbances.15−18 Although twisted crystals are widely observed in small molecules banded spherulites,19−24 the most convincing evidence of lamellar twisting in polymers banded spherulites only comes from methylamine (MA)-etched poly(trimethylene terephthalate) (PTT) and poly(3-hydroxybutyric acid-co-3hydroxyvaleric acid) (PHBV).17,18,25 One should take caution that MA easily degrades the polyesters by attacking the ester © XXXX American Chemical Society

linkage, converting the polymers into smaller amide compounds, and thus deforms the crystals.26,27 Chemical alteration and crystal deformation by MA have to be considered in attempting interpretations of mechanisms. On the other hand, hierarchical structures of continuous main stalk crystals with numerous adjoining branches have been discovered in many polymer spherulites, either from melt crystallization or in cast films from solutions. High-density polyethylene (HDPE) cast from hot solutions, as an example, has been reported to show a complex arrangement of main stalks with thousands ribbon-like lamellae transversely grow on the sample surface.28 Similarly, Pennings et al.29 reported the shish-kebab structure in flow-induced fibrillary polyethylene (PE) crystallized at 101.8 °C during Couette stirring of 5% solution in p-xylene. Such hierarchical structures composed of main stalks with numerous branches, though not termed as “shish-kebab”, have been collectively demonstrated in poly(ethylene oxide) (PEO), PHBV, and poly(L-lactic acid) (PLLA) blends as well as in neat poly(ethylene adipate) (PEA) and PTT.10−12 Unlike the classical shish-kebab fibrous lamellae, the hierarchical structures in these ring-banded spherulites have the “main crystal stalks” ordered as repetitive circular rings, but similarly with the branch crystals growing out from the main stalks. Rosenthal et al.30−32 attempted to correlate the lamellar twisting with the periodic oscillation of X-ray diffraction intensity as a function of distance from the center of banded spherulite. They come to conclusions that the value of chain tilt (4° angle) is not sufficient to generate surface stresses required for continuously twisted lamellar growth and, instead, supposed Received: April 23, 2017 Revised: June 20, 2017

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(FE-SEM, Hitachi SU8010) were conducted for both virgin and etched samples. Gold sputtering was performed on the PTT samples before the SEM characterization.

the local inclination of terminal segment of the crystalline stem at the surface as key factor controlling the surface stresses generated lamellar twisting. X-ray is indeed a powerful technique to show the crystal lattice and orientations of lamella based on the average diffraction and/or scattering patterns received by the detector. However, upon coming to bulk samples (composed of hundreds to thousands of lamellae), the interpretation of X-ray data is rather tricky because the intensity collected by the detector is the average one. A periodic oscillation plot of diffracted X-ray intensity as a function of radial distance does not solely correspond to lamellar twisting. Perhaps, the branching lamellae or other unprecedented proposed mechanisms may also true for such oscillation. The mechanism of banded spherulites should not be justified by only several dots of X-ray exposure on it. Without any direct supporting evidence of the real interior lamellar assembly, such interpretation would only be an argument. Banded structure in polymer spherulite is not always concentric rings; sometimes it assembles Archimedean spiral bands. The phenomena of Archimedean spirals in banded spherulites are rarely discussed although they are frequently found in polymer crystallizations. In attempts to study the crystal assembly of banded spherulites in corresponding to their unique and diversified structures and birefringence, Woo et al. conducted experiments and preliminary analyses on PTT thin and thicker film samples.12,13 These pioneering exhaustive attempts suggested that contrast colors displayed in the polarized optical microscope (POM) indeed relate to two mutually perpendicular-oriented crystals inside the PTT spherulites. Discontinuous packing into layered shells was visible that interlamellae boundaries could be exposed, which matched with the interband regions of the PTT spherulites. Nevertheless, details of lamellae assembly accounting for the entire banded spherulite structures are yet to be probed, which is the main aim of this work. Only by direct observation of the interior lamellar structure, the actual crystal assembly of PTT could be successfully revealed.

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RESULTS AND DISCUSSION Top Surface Observations. Before interior analysis, AFM characterization was performed on the crystallized films. PTT films, when crystallized at 165 °C, are known to pack into not just a single type of ring band but three, which suggests the complexity of the lamellae crystals being assembled into periodically repetitive patterns. Figure 1 shows AFM images

EXPERIMENTAL SECTION

Materials and Preparation. The aromatic polyester poly(trimethylene terephthalate) was obtained from Industrial Technology Research Institute (ITRI) (Taiwan). The glass transition (Tg) and melting temperatures (Tm) of PTT were measured to be 45 and 228 °C, respectively. Thick bulk samples of PTT (thickness ca. 50 μm) were prepared by directly pressing PTT pellets on a glass substrate at 260 °C. The PTT samples were then remelted at 260 °C for 5 min before crystallization at 165 °C. The degree of crystallinity of PTT samples was approximately 45%. The fully crystallized PTT samples were then carefully fractured across the film thickness to expose the interior lamellar structure. Permanganate enchants were prepared by dissolving potassium permanganate (KMnO4) into a 1:1 volume ratio of sulfuric acid (H2SO4 ):phosphoric acid (H3PO4 ) solution. Permanganate etching was performed on the fractured sample.12 Proper etching techniques with suitable etchants for the fractured and top surfaces of PTT samples are a critical key to successfully analyzing the lamellae assembly responsible for the observed banding phenomena. Unlike MA, permanganate tended to leave intact the real microstructures with the original skeletal crystals less likely to be chemically altered.33,34 Apparatus and Procedures. Top surface topological and morphological observations of the crystallized PTT banded spherulites were conducted using atomic force microscopy (AFM, diCaliber from Veeco Co.); a silicon tip ( f = 410 kHz, r = 10) was installed, and scanning was performed in intermittent tapping mode. Interior observations using field-emission scanning electron microscopes

Figure 1. AFM height and phase images as well as 2-D and 3-D schemes illustrating the development of various banded patterns/ structures in PTT spherulites and their corresponding SEM graphs: (a) concentric rings, (b) single spiral, and (c) double spiral.

and schemes of three common types of PTT banded spherulites (i.e., concentric-ring, single-spiral, and double-spiral structures). The concentric-rings banded spherulite is developed from concentric arrangements of the tangential and radial crystals. The single- and double-spiral banded spherulites are developed from the splayed spiral arm(s) composed of tangentially oriented lamellae and radially oriented branches connecting the adjacent bands. In-situ video recording of the spherulitic development, observed under a POM, has been reported for each type of the banded spherulite.13 Different types of banded spherulites are indeed originated from different nucleation shapes. SEM observation to the fully grown banded spherulites after permanganate etching also offers information about their growing mechanisms. Figure 2 shows schemes of gradual development of different kinds of PTT banded spherulite, from initial nuclei to their final banded structures, traced from the B

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Figure 2. Schemes illustrating the development of various banded patterns/structures in PTT banded spherulite and their corresponding SEM graphs: (a) concentric rings, (b) single spiral, and (c) double spiral. The samples were etched by 2% w/v permanganate enchant for 60 min prior the SEM observation.

lamellar assembly requires more delicate analyses in alternative approaches. In a bulk sample, the banded spherulite grows as an oblate spheroid. Even with considerably low crystallinity, the virgin PTT banded spherulite has shown a clear layer-by-layer structure on its fractured surface as shown in Figure 3. Distinct borders between the layers testify to the discontinuous growth of crystals. From the zoomed-in SEM image (Figure 3b) the concentric borders are composed of shish crystals (tangential lamellae). Arranging transversely from the shish crystals, the majority of kebab crystals (radial lamellae) are pointing inward, and only minor crystals are pointing outward, connecting the shish crystals of the successive bands. The spacing of the intertangential crystals is approximately 8 μm, which is proportional to the optical band spacing in POM graph.13 Resembling the classical “shish-kebab” structure in solutionsheared PE morphology,29 the crystal lamellae of PTT banded spherulite also evolve in two perpendicular directions, except that the framework is curved into concentric rings or Archimedean spirals. These results in PTT testify against the “continuous spiral lamellae” that are classically viewed as the principal way of lamellae packing in banded spherulites. The fractured samples were treated with permanganate enchants. The composition of the etchants and etching times were carefully controlled to optimize the etching extent without altering the crystal assembly. Figure 4 shows SEM images and scheme of the interior lamellar arrangement of fractured PTT spherulite etched by 0.7% w/v permanganate enchant for 40 min. As shown in Figure 4a, there are two kinds of fractured surface. At half-diagonal left, the sample was vertically fractured; at half-diagonal right, the upper segment of the sample was peeled off leaving the lower one exposed for characterization.

corresponding SEM graphs of the etched ones. After 60 min of permanganate etching, the concentric hemisphere (Figure 2a) loses its interband connections and a couple of its innermost bands; Archimedean spiraling arm(s) spherulites (Figure 2b,c), however, only lose their interband connections. Permanganate enchant, therefore, can penetrate through and wash the exposed amorphous PTT at interband connections and hence disintegrate the detached crystals from the rest of spherulite, yet leaving the inert skeletons of crystals intact. The development of spherulites would be highly dependent on the position and geometry of the nuclei. When the nucleation takes place parallel to the free top surface, the spherulitic growth toward all radial directions would be uniform, and a concentric banded spherulite would be formed. When the nucleation takes place at a certain angle from the free top surface, however, the spiral arm(s) would evolve from the nuclei, and a single-spiral or a double-spiral banded spherulite would be formed in accordance with the number of growing arm(s). Filling the spaces, side branches develop transversely from concentric tangential lamellae or the spiral arm(s). The interfaces between the tangential lamellae of the former band and the radial lamellae of the next band are supposed to be the area where amorphous PTT are accumulated and can be easily washed by the permanganate enchant. These hierarchical structures of PTT banded spherulites are found resembling the classical “shish-kebab” structure in PE, except that in PTT banded spherulites, the “shish-kebab structures” are not straightly aligned shish crystals but circumferentially curved accordingly to form concentric rings or Archimedean spirals. Interior Lamellar Assembly in Correlation to the TopSurface Banded Structures. It is essential that the interior C

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Figure 3. SEM images of fractured virgin PTT (unetched) meltcrystallized at 165 °C: (a) full image, (b) zoomed-in image of the white square area in (a), and (c) scheme simplifying the interior lamellar arrangement of the spherulite.

There exists clear discontinuity (as exposed crevices) from layer to layer. Crevices are developed along the hemisphere’s longitude on vertical fractured surfaces or along its latitude on horizontal fractured surfaces, separating the successive bands. In each of the layers, there are radial crystals and tangential crystals that embed one to another into a coherent band with dimension that matches the optical bands viewed under POM.12,13 This empirical fact is strong evidence that the periodic optical banding corresponds to a layered structure in the SEM interior analyses of the banded PTT spherulites. Figure 4c shows a 3D schematic model depicting the crystal assembly in the interior of banded PTT spherulites, where most of the radial lamellae are branching inward from the tangential lamellae and crevices segregate one layer from the next. On the top surface, however, these phenomena are hardly seen due to the tendency of lamellae to cover everything beneath when they emerge from interior onto the top surface, where they immediately bend 90° from tangential to radial direction. Within each layer comprising the tangential and radial crystal species, the mutual perpendicular intersections are created in two ways: (1) the tangential lamellae that protrude on the top surface keep on growing outward by bending toward the radial direction to form a convex plateau (called ridges), and (2) underneath the perpendicularly oriented radial crystals are side branches grown mostly inward from the tangential lamellae to fill the spaces. The intersections between the surface radial oriented tangential lamellae and the protruding tangential lamellae of the next layer appear lower and therefore called valleys. The cycles of layered structures repeat themselves to create periodic ridge and valley on the top surface and optical banding of opposed birefringence when viewed under POM. In a previous publication,12 we disclosed the interior lamellar assembly of PTT banded spherulites treated by 2% w/v permanganate enchant for 20 min, shown in Figure 5. The spherulite has dual-spiral arms covering each other as they grow from the center. Along the spiraling arms, fibrous branches are

Figure 4. SEM images of fractured PTT spherulite etched by 0.7% w/ v permanganate enchant for 40 min: (a) full image, (b) zoomed-in image, and (c) scheme illustrating the key assembly of interior lamellae in the banded spherulite.

growing in perpendicular to the spiral direction. The spiraling arms (the “shish” crystals) appear brighter, while the fibrous branches (the “kebab” crystals) appear darker. Different contrasts in the obtained SEM graph may indicate two things related to secondary electron (SE) topology mapping and backscattered electrons (BSE) density mapping, where the crystal portions that are more exposed to the electron or having higher crystal density will appear brighter. The brighter regions are projected higher as they are constituted by the tangential crystal lamellae (shish), while the darker regions are projected lower and likely constituted by the radially arranged branch lamellae (kebab). The 3D scheme in Figure 5b points out three D

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Figure 5. PTT spherulites etched by 2 wt % permanganate enchant for 20 min: (a) full SEM image, (b) scheme simplifying the interior lamellar arrangement of the banded spherulite, (c) zoomed-in SEM image of the left portion, (d) zoomed-in SEM image of the right portion. Parts a and b adapted with permission from ref 12. Copyright 2016 The Royal Society of Chemistry.

Figure 6. SEM image (a) and scheme (b) of fractured PTT spherulites etched by 2 wt % permanganate enchant for 20 min showing the detailed lamellae arrangements in two adjacent bands.

critical and novel features for the authentic interior lamellae in the repetitively banded PTT spherulites. These three features are (1) the existence of double spiraling arms (shish) composed of tangentially arranged crystals, (2) along the spiraling arms fibrous branches (kebab) grow as loosely packed and radially arranged lamellae, and (3) discontinuous crevices developed at the interfaces between two successive bands are exposed upon permanganate etching. These unique features become even more pronounced after zoom-in analyses on the successive bands. Figure 5c shows that the darker regions are composed of radial crystals; Figure 5d shows that the brighter regions consist of tangential lamellae (arranged along the longitude on the fractured surface and along the latitude on the top surface). This study, using the FE-SEM, zoomed into the adjacent bands of the fractured PTT spherulite etched by 2 wt % permanganate enchant for 20 min. Figure 6 shows the SEM image and 3D scheme illustrated the detailed lamellae arrangements in PTT banded spherulite. The tangential lamellae (composing the arms) are projected outward (appear brighter) on top and fractured surfaces. The radial lamellae are arranged transversely from these tangential lamellae fulfilling the spaces between the two arms. Thus, crevices that segregate two successive bands are developed near the interface between tangential and radial lamellae where amorphous PTT is supposedly accumulated. Using 3D bulk PTT samples, more strikingly clear evidence of inner layer-like structure is seen in an alternative fracturing of the banded PTT spherulite. Figure 7 shows SEM images and 3D scheme of the interior lamellar arrangement of PTT spherulite etched by 2% w/v permanganate enchant for 60 min. The SEM results clearly reveal a multishelled spheroid structure in ring-banded PTT spherulites. Layers with distinct interlayer crevices are evident in Figure 7a. These layered crystals are tangentially oriented, with the radial crystals (which are branching crystals growing 90° from the tangential ones) being etched out to expose the interfacing crevices between layers. The fracturing also exposes a central spherical core that

Figure 7. 3D spheroid layered structures of PTT spherulite: (a) SEM image, (b) scheme of the entire concentric sphere and scheme of the truncated hollow spheres forming: (c) concentric and (d) doublespiral banded spherulite.

remains intact and whose surface appears to be not deformed/ deteriorated by the etching. Interestingly, in the central core, there is a small piece of fracture tangential crystals (marked by the arrow), suggesting that the 3D PTT banded spherulite may pack with an Archimedean spiral. Fractured surface further justifies the existence of dual Archimedean spiral packing. The scheme in Figure 7b demonstrates the full concentric hallow spheres, while Figures 7c and 7d demonstrate the truncated− concentric hollow spheres right across the center forming the concentric and at a slight angle forming dual spiral, respectively. That is, Archimedean spiraling bands are seen not just on the E

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Macromolecules 2D top surface in thin films but also in the 3D interior of banded PTT spherulite. In comparison to etching by permanganate, alternative etching using MA on PTT was attempted, and the results are shown in Figure 8, which shows SEM images and an illustrating

hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH)36 and finally proclaimed to revert the original conclusion in the 2004 AFM old work that his reanalyzed results in 2013 could not fit with any existing models, and there was discontinuity in lamellae, i.e., meaning that the lamellae in banded PHBH spherulites are not continuously twisting, but interspersed with periodic discontinuity. These studies by seasoned investigators, on banded spherulites in two different polymer thin films, both came to either inconclusive validity of the long controversial model of continuous twisting or even reversion opinions. Thus, it is only timely that this work on PTT, using alternative etching agents, KMnO4 in comparison to MA, via 3D dissection on interiors in correlating with top-surface banding morphology, sheds new light on finer details of mechanisms of banded optical birefringence and lamellae assembly for such periodic repetition in crystalline spherulites that has puzzled many investigators in the past decades.

Figure 8. PTT spherulite etched by MA vapor for 16 h: (a) full SEM image and (b) scheme emphasizing the layered structure at the peripheral boundary of the spherulite.

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CONCLUSION Novel findings with clear evidence in this work have proved that in the banded PTT spherulites of three different types the lamellae do not helical-twist monotonously from a common center. Instead, the interior analyses coupled with selective etching techniques evidently and collectively show that the lamellae are packed as orderly aggregations spiraling and fanning out into multishelled spheroids with dominant discontinuity between the aggregated shells. Secondary branches grow perpendicularly from the main lamellae of the shells to fill the intershell space, and they intersect each other perpendicularly. Such a structure resembles the classical “shishkebab” morphology, where the interior lamellae of banded PTT spherulites evolve into two perpendicular directions, mutually packed into cabbage-like Archimedean-spiraling spheroids of multiple layers, with clear discontinuity between the shells. The interior microstructures and thus morphologies of PTT spherulites formed under the identical conditions from different sample thicknesses may differ. Thus, awareness that the actual self-assembly of the polymer should not be discerned from a restricted system, such as in a pseudo-two-dimensional system or top surface examination as what most of the researchers have done hitherto. We believe these new findings utilized from interior dissection of bulk sample may revolutionize the paradigm of crystallization and self-assembly of polymer. Scientific progress is evolutionarily step-by-step, with each step being improved and proven by different angles of observation.

scheme for the interior lamellar arrangement of PTT banded spherulites (Tc = 165 °C) that had been etched by 40 wt % MA vapor for 16 h. Observation is directed to the peripheral boundary of the PTT spherulite. The interface between the layers could also be observed in this MA-etched PTT spherulite, verifying the clear discontinuity in the banded PTT spherulite. The crystals in the MA-etched PTT banded spherulites appear to be “twisted” into a “U”-shape. As discussed earlier, MA easily caused crystal disorientation/ deformation (as well as a chemical alteration); such twisting in MA-etched samples must be checked rigorously with alternative etching methods. Nevertheless, the MA etching on PTT spherulites still revealed a layer-like structure with tangential and branching radial orientations, and the result of MA-etched PTT spherulites is still mostly in support of the results of PTT banded spherulites with permanganate etching. Once again, for the MA-etched PTT spherulites, the thickness of each of the interior layer crystals (the hollow spheres) measured ca. 10 μm, which corresponds exactly to the optical band spacing in PTT films. The MA-etching exposed layer thickness in the interior of PTT spherulite is ca. 10 μm, which is exactly the inter-ring spacing in POM graphs for PTT films crystallized at this Tc (165 °C). Although different etching techniques might have led to slight differences in finer details of interior morphologies, the general main features of mechanisms of lamellae assembly are similar. MA-etching results are in good agreement with the permanganate-etching results on the PTT ring-banded spherulites in general that the layered shells are always present, except that MA-etching tended to deform the lamellar geometry significantly and caused twists to severe extents, likely leading to misinterpretations of true morphology if due care was not taken. To sum up briefly, it is worthy to mention two cases of recently updated studies on banded spherulites in polymer thin films. As mentioned, Rosenthal et al.,31 in their recent work of PTT banded spherulites in thin films focusing on revisit of the classical Keith−Padden (K−P) model, finally concluded their data from a lot of analytical characterization could not appear to justify the chain-tilting-induced surface stresses being responsible for continuous twisting, and 3D analyses in the future were necessary to more properly resolve the issue. In addition, Schultz35 in 2013 revisited their own 2004 old atomic force microscopy (AFM) data on banded spherulites of poly(3-

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (E.M.W.). ORCID

Graecia Lugito: 0000-0003-1785-8321 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Graecia Lugito (postdoc) conducted the experiments, produced and analyzed the data, and wrote the draft manuscript; E. M. Woo (Professor) conceived the original research ideas, advised/guided the experiments and analyses, and further revised and reshaping the manuscript into its current form. F

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been financially supported by a basic research grant (MOST-105-2221-E-006-246-MY3) for three consecutive years from Taiwan’s Ministry of Science and Technology (MOST).

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REFERENCES

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DOI: 10.1021/acs.macromol.7b00838 Macromolecules XXXX, XXX, XXX−XXX