Interior Lamellar Assembly in Correlation to Top-Surface Banding in

Aug 27, 2014 - Interior Lamellar Assembly in Correlation to Top-Surface Banding in Crystallized Poly(ethylene ... Growth Des. , 2014, 14 (10), pp 4929...
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Interior Lamellar Assembly in Correlation to Top-Surface Banding in Crystallized Poly(ethylene adipate) Graecia Lugito and Eamor M. Woo* Department of Chemical Engineering, National Cheng Kung University, No. 1, University Road, Tainan, 701-01, Taiwan S Supporting Information *

ABSTRACT: Novel approaches and schematic models have been used to provide insights in viewing lamellar assembly in ring-banded spherulites of a model polymer poly(ethylene adipate) (PEA) via correlations between the outer-surface and three-dimensional interior morphology. When crystallized at 28 °C, PEA in thick bulk forms exhibits a ring-banded top surface with bowl-like and dome-like height profile centering on its nucleus. The three-dimensional periodical assembly leads to not only the noticeable ring bands on exterior surfaces but also corrugated-board-like multishell structures in the interior of the spherulites. The corrugated-board structure has been found to be composed of plate-like lamellae, which first grow in a tangential direction and then turn to a radial direction; the radial plates taper to form thinner cilia-like lamellae due to the polymer chain concentration gradient periodically precipitated during the growth. Alternating sequences of plate-like lamellae in two perpendicular orientations (bending from tangential to radial) in PEA ring-banded spherulites during growth in a radial direction account for the spherulites confined in thin films to display two contrast circumferential rings with alternating interference colors. Scanning electron microscopy dissection graphs clearly revealed interior corrugated layer thickness in bulk forms, which matches well with the inter-ring spacing in thin films.

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INTRODUCTION Spherulite in polymer science is generally defined as a highly anisotropic material consisting of crystal lamellar stacks with nearly spherical geometry which is formed in the process of polymer solidification. When the spherulite is examined under the polarized optical microscope with a sensitive tint plate, placed in between the sample and analyzer as shown in Figure S-1, Supporting Information, it is often shown in colors due to its anisotropic feature. The colors can be correlated to the actual retardation value, thickness, and birefringence of the spherulite. The birefringence of spherulite could be defined as the difference between high and low refractive indices which is related to the interaction between light photons and “free” electrons within that spherulite. The greater the free electron number, the slower the light-motion and the higher refractive index would be obtained in that direction. In the case of most polymers, free electrons are particularly found within covalent bonding along the polymer chain axis direction. Scheme 1 shows the polymer chain folding arrangement in (a) flat-on lamella; (b) tangentially growth edge-on lamellae; and (c) radially growth edge-on lamella. These three extreme lamellar arrangements are sometimes found tilted with respect to either the y or z direction. However, the type of spherulite is determined according to the axis with the highest refractive index; for most polymers, it is determined according to the chain axis (x) direction. On the other hand, it was claimed that spherulite would be called positive when the refractive index parallel to the radial direction (nr) of the spherulite is greater than that perpendicular to the radial direction (nt), and it would © 2014 American Chemical Society

Scheme 1. Polymer Chain Folding Arrangement in Three Ideal States: (a) Flat-on Lamella; (b) Tangentially Growth Edge-On Lamella and (c) Radially Growth Edge-On Lamella

be called negative when the converse is true (i.e., nt > nr).1,2 Thus, to be positively birefringent, the spherulite should contain polymer chains arranged in a radial direction more than in tangential ones,3,4 or intraspherulitic amorphous segregation that causes lamellae prefer to array regularly in the tangential direction.5 Otherwise, it will be called negatively birefringent spherulite. Illustrations of the directions of slow- and fastcomponent of the light transmitted by 530 nm tint plate, negative-, and positive-type spherulites are shown as Supporting Information in Figure S-2, and explanations on how negative- and positive-type spherulites show contrary blue and orange colors arrangements are provided. In the case of ring-banded morphology, birefringence and interference colors also take a role. According to the optical Received: February 19, 2014 Revised: August 26, 2014 Published: August 27, 2014 4929

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science rigorousness. As a matter of fact, twisted lamellae/fibers can be found in any type of spherulitesregardless of bands or not.3,15 This fact means that lamellae twist (regular or irregular) may be a habit that is not necessarily limited to occur in only banded spherulites. Moreover, there is a gap between the size of the transmission electron microscopy (TEM) evidence lamellae twist pitch versus polarized optical microscopy (POM) revealed banding pitch of polymers’ spherulites that could not be explained and has not been satisfactorily settled.22−25 The discrepancy between the TEM evidence for lamellae spiral pitch and POM optical banding pitch, as reported in various studies typically ranges from some 40 times to 300 times. For poly(ethylene adipate) (PEA), there has been no direct TEM evidence for lamellae spiral to date, although it is widely known that the optical banding pitch is about 6−7 μm, and widely it has been proposed that lamellae spiral is the mechanism for optical banding. For classical polyethylene (PE), the known literature reports23 the lamellae twist/spiral at 100 nm pitch, while the optical banding pitch for PE is known to be about 4 μm; thus, the discrepancy between TEM and POM evidence for PE is 40 times. Similarly, for poly(trimethylene terephthalate) (PTT), the discrepancy between TEM and POM evidence for lamellae spiral and optical banding pitch for banded PTT is about 50 times.24 Finally, for poly(L-lactic acidblock-styrene) (PLLA-b-PS), the TEM evidence for lamellae twist is about 100 nm, while the optical banding pitch is 30 μm (Figures 9 and 5 of the reference, respectively), and the discrepancy between the TEM lamellae twist and POM banding pitches for PLLA-b-PS is an enomous 300-times difference.25 For these enormous differences, some investigators argued that banding pitches in POM characterization might be larger because the lamellae are thicker, and thus the twist pitch in thicker lamellae is correspondingly larger. However, it should be pointed out that, based on numerous SAXS studies in many literature reports, the thickness of lamellae in most polymers, either in bulk states or thin films, all ranges from 5 to 10 nm at most, with long periods (crystalline + amorphous layers) being ca. ∼10−15 nm. Apparently, the difference in lamellae thickness is merely two or three times for most polymers either in bulk or thin-film states, which cannot feasibly justify the huge 10-fold or 100 times differences in the banding pitch scales between TEM and POM results. Such long-existing discrepancy between typical TEM observations and POM banding pitches demands that alternative approaches must be sought. There are several critical questions to be answered for further advancing the scientific understanding: (a) why and how the lamellae align themselves and twist synchronously at a temperature while by increasing one degree Celsius for crystallization, the synchronizing twists in the packed spherulites all suddenly disappear? (b) if spiral is a consequence of chain folding-induced surface stress, it seems odd that ringbanding easily occurs in some polymers but does not occur at all in other polymers (bear in mind that all polymer chains fold in lamellae) and finally (c) the pitch in POM banding usually does not match and differ as much as hundreds times in scales from TEM evidence of lamellae plate spirals. In addition, the issue of birefringence (the appearance interference colors) in ring-banded spherulite is also interesting to be discussed since there should be some correlation between crystal packing and the light motion passing through these crystals. The conventional analyses via views only on top surfaces with the crystallized sample systems confined in thin films have inherent

interference, the morphology of ring-banded spherulites can be classified into (a) zero-birefringent (non-birefringent), (b) single banded with extinction border, and (c) double ringbanded. Definitions are provided here. The “zero-birefringent ring-banded spherulite” shows only concentric bands of alternating heights with a purple color (generated from blue and red colors) and periodical light extinction. “Single ringbanded spherulite” shows vivid colors of only a single type of circumferential rings (either positive or negative) with periodically concentric light extinction between the neighboring bands. The third type of ring bands, “double ring-banded spherulite”, shows alternating colors of positive- and negativetypes in one spherulite with barely any extinction rings between two neighboring bands. Different types of ring-banded morphologies should be derived from different types of etiologies and likely must be interpreted differently. Thus, interior dissection is critical, because without interior anatomy of the spherulites, the top surface of spherulites in thin films tends to show only outer surface morphology that is inclined to be covered with bent or protruded lamellae whiskers or impurity rejected to the top surface. Rhythmic deposition and lamellar twisting are the two most widely accepted theories for the modulated pattern of ring-banded morphology. The rhythmic deposition theory has been believed as the consequence of competition between nonlinear interface kinetics and material supply by diffusion, resulting in a crystal-rich band and crystal-poor band in non-birefringent spherulite.6,7 By contrast, the lamellar twist model is another commonly proposed interpretation of mechanisms for optical contrast concentric bands. Several fundamentally different models have been proposed and explained briefly in many published papers.8−16 The banding phenomenon in crystallized long-chain polymers has long been observed; however, many low-Mw organic compounds and low-Mw liquid crystals, with no long chains to fold and thus no unbalanced lamellar surface stresses, are also known to exhibit such banding phenomena, with exactly the same ring-band patterns as that in polymers.15−21 Distinguished examples include classical and modern issues on Liesegang rings in gels with diffusion reactants,17 rhythmic precipitation into concentric rings of crystallized organic compound, phthalic acid [C6H4-1,2-(COOH)2],18,19 inorganic compound potassium dichromate (K2Cr2O7),20 and banded spherulitic growth in liquid crystalline material 4-cyano-4′decyloxybiphenyl,21 etc. Banding in phthalic acid, is particularly of interest, which has been reported to exhibit regular and concentric polycrystalline rings with ring-spacing ca. 20 μm.18 That is, small molecules, organic or inorganic compounds, and not only limited to long-chain polymers, all can assemble themselves into periodically repetitive ring-band patterns in crystallized (or diffusion-reacted products) assemblies when suitable conditions (chain structures and kinetics) are provided. Kahr et al. have recently reviewed the growth actuated bending and twisting of single crystal in many related objects and provided explanations about mechanisms that could be considered right some of the time for some substances under some conditions.16 Twisting is a credible mechanism to ringbanded morphology in a number of polymers and small molecules as what many researchers have reported. However, there are many different types of ring bands, and not all ringbanded materials have the same crystallization mechanism of twisting. Twist/spiral may be a working model for banding, but one should also probe other possibilities (and proofs) for 4930

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because it is in the midrange of 25−30 °C, at which PEA has been known to show banded spherulites.2,3,26−29 The fully crystallized bulk PEA samples (Tc = 28 °C for over 1 h) were washed with petroleum ether before observation. Scanning electron microscopy (SEM, FEI Quanta-400F) was used to characterize both fractured and top free surfaces of bulk form PEA samples. The samples were fractured into halves and then coated with gold vapor deposition using vacuum sputtering prior to SEM characterization. A polarized optical microscope (POM, Nikon Optiphot-2), equipped with a Nikon Digital camera system for microscopy Digital Sight (DS-U1) and a microscope hot stage (Linkam THMS-600 with TP-95 temperature programmer), was used to confirm the crystal morphology prior to the SEM observation. Atomic-force microscopy (AFM, diCaliber, Veeco Corp., Santa Barbara, USA) (merged into Bruker Corp.) investigations were made in intermittent tapping mode with a silicon-tip (fo = 70 kHz, r = 10 nm) installed. In addition to bulk form sample, PEA was also prepared in thin-film forms for observing the top-surface morphology for correlating with interior lamellae assembly as exposed in fracture surfaces of bulk samples. AFM measurements were carried out to determine the phase images and height profiles in the morphology topology of thin-film samples. Shimadzu XRD-6000 X-ray diffractometer with copper Kα radiation (at 30 kV and 40 mA) and a chromatized wavelength of 1.542 Å was used to characterize the crystal structure of PEA. The scanning 2θ angles ranged from 10° to 30° with a scanning rate of 2°/min.

limitations for addressing these critical questions. Mechanisms for such interpretations, to be universal, need to take the essential nature of crystal plates into account. Thus, it is emphasized that such nature should not be viewed simply from the top surfaces or limited to only thin films; interior dissection is vital for revealing more information on crystal assembly into ordered and repetitive patterns. Such dilemma need to be straightened out by alternative approaches drastically different from the conventional ones. Banding in polymers has conventionally and unilaterally been studied only in thin films with characterization on the top surface of thin films; no attempts, except for earlier studies from our laboratory,3,26 have ever approached the issues from interior views on polymer lamellae in correlating to the top surface banding patterns. Thus, this study aimed to probe these questions in further depth and to analyze the complex issues by novel approaches, which were (1) to dissect the interiors of banded spherulites in bulk-form samples and (2) to trace from the interior growth of lamellae to outer crystal morphology in exposed top surfaces. PEA, with double ring-banded morphology in clearly defined temperatures and well exemplified in thin-film states widely reported in the literature,2,3,26−30 was chosen as an ideal model for dissecting into bulk interiors. Scheme 2 shows the

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Scheme 2. Double Ring-Banded Spherulite in PEA Showing Alternating of Negative−Positive Signs of Birefringence along the Growth to Radial Directiona

a

RESULTS AND DISCUSSION Figure 1 shows SEM micrograps of top surfaces of PEA ringbanded spherulites crystallized in bulk sample at 28 °C: (a)

POM figure was taken at Tc = 28 °C.

alternating negative−positive birefringence along the growth to the radial direction in the double ring-banded morphology of PEA (POM figure was taken at Tc = 28 °C). Such correlations and analytical results on interiors of bulk PEA were then compared with characterization results on thin-film samples of the same PEA polymer crystallized at exactly the same parameters. The detailed analyses from both 3D and top surface views on bulk versus thin-film samples hopefully could bring us some steps closer to expound these long illusive scientific issues on crystallization.

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Figure 1. SEM micrographs of top surfaces of PEA spherulites crystallized in bulk sample at 28 °C: (a) bowl-like and (b) dome-like spherulites.

bowl-like and (b) dome-like spherulites. The top free surfaces of the bulk-crystallized PEA may exhibit either a regular concentric ring-banded spherulite with concave core (bowllike) convex-up bulge (i.e., dome-like). The graph of the domelike spherulite shows an apparently ring-banded pattern in the outer periphery of the center dome; however, the center region (appearing as a dome) of the spherulites is ringless. This domelike spherulite contains no well-defined nucleation center, which suggests that it may be the upper part of a spherulite, and the nucleation occurs and originates from the bottom free surface. The banded patterns in the spherical surface as shown are composed of cilia-like crystals pointing outward from the top surface of the spherulite. The surface morphology of both regular concentric spherulite and dome-like spherulite were then examined by using AFM. Figure 2 shows the AFM height image and height profile of ring-banded spherulite of PEA crystallized in thickfilm samples at 28 °C: (a) bowl-like and (b) dome-like spherulites. The film thickness of PEA was made to be about 20−30 μm, which was thinner than the SEM bulk samples, to

EXPERIMENTAL SECTION

PEA was purchased as research-grade material from Aldrich Co. (USA) with Tg = −52 °C and Tm = 43 °C. The weight-average molecular weight (Mw) was nearly 10 000 g/mol, determined by gel permeation chromatography (GPC, Waters 410) using tetrahydrofuran (THF) with an eluent flow rate of 1.0 mL/min. PEA was selected for study owing to its prominent banding patterns when crystallized at a specific narrow temperature range. The PEA polymer was purified by precipitation from chloroform into a large quantity of cold methanol (at ca. 10 °C). PEA was dissolved in CHCl3 (chloroform) in higher concentrations, and a few drops were repeatedly deposited on a glass substrate stacked into a thick bulk sample (∼100 μm). Degassing was performed at 45 °C in a vacuum oven for 24 h. PEA in thick bulk forms was heated to be melted at 90 °C and rapidly dipped in a silicone oil bath set at controlled temperature of 28 °C for crystallization. A crystallization temperature of 28 °C was chosen 4931

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interior lamellae actually show alternating shells just like those near the outskirt of the spherulite. The above AFM analyses, as discussed, provided only topsurface views, which might not be complete for full interpretations. Some investigators claimed that PEA showed a continuous twisting/spiral lamellae when it was observed from the top surface of thin films and that unbalanced stressinduced spiraling lamellae are responsible for the optical banding patterns in POM characterization.28−30 However, it should also be noted here that direct AFM or TEM evidence for spiraling PEA lamellae with exactly same pitch (ca. 7 μm) as POM birefringence is still missing in the literature so far. Dissection into the interior, viewed in SEM, of bulk PEA samples that had been crystallized into banded patterns (Tc = 28 °C) was deemed to be a critical approach. The formation of such a ring-banded pattern comprising cilia-like crystals on the spherical surface is attributed to the fact that the available growth space normal and tangential to the nucleation center may be different from the situation when the nucleation occurs near the top surface. Once the nucleation occurs near the surface, the normal distance along the radial growth direction in between the nucleation center and the top free surface is small, and growth is more confined in comparison to that along the sideways. However, when the radial growth distance is significantly different, the number of ring bands produced along the radial direction is also different. Thus, the growth of the lamellar crystals along different radial directions does not end in the same surface, and the excessive lamellae reach the outer surface with the cilia-like crystals pointing outward as those appearing in the outer circle of the central dome-like spherulite. Bowl-like and dome-like spherulites are different in the position of the nucleus with respect to the air−sample boundary. Assuming that the nucleus is close to the top surface, it cannot grow upward, resulting in a bowl-like spherulite. But when there is enough space and crystal chain for a nucleus to grow upward, it will grow spherically forming some corrugated-board layers, subsequently, before finally reaching the surface and shown as a dome-like spherulite (Figure S-3, Supporting Information). However, the mechanism of spherulite’s growth should be the same for both bowllike and dome-like, which are explained in the next section. Three dimensions observation of a fractured bulk sample of PEA was performed to expose the interior architecture in relation with the outer surface morphology. Figure 4 shows SEM graphs of the fracture surface of bulk sample of PEA crystallized at 28 °C. Taking 3D sphere parameters as a parallel comparison, there are radius and variation in theta (θ) and phi (φ) angles in a spherulite. Figure 4a shows the correlation between the interior and outer surface of the entire spherulite cut in variable θ- and φ-angles with 800× magnification. Figure 4b,c shows the interior lamellar packing of spherulite after being cut along the φ-angle or after its top layer was lifted up (magnifications are 3000× and 6000× for panels b and c, respectively). PEA ring-banded spherulite is composed of multiple scale-like layers of crystals, which appear as multiple corrugated boards stacked alternately. The 3D periodical assembly leads to not only the noticeable ring band on exterior surfaces but also to corrugated-board like multishell structures in the interior of the spherulites. In the interior, there is neither a ridge nor valley; there are only radially arranged lamellae vs tangentially arranged lamellae. The arrow in Figure 4c represents the radial direction of spherulite, and “tangential”

Figure 2. AFM height image and height profile of ring-banded spherulite of PEA crystallized from bulk at 28 °C: (a) bowl-like and (b) dome-like spherulite.

avoid the AFM tip from being jammed owing to much height variation. The height profile of the dome-like spherulite appears to be concave, just similar to the bowl-like spherulite. It does not show the convex profile as the thicker sample does in the SEM image. However, they (Figures 1b and 2b) can be considered as the same kind of spherulite which shows no ring bands in the central part, suggesting the nuclei is buried underneath the central dome. The top surface morphology of the dome-like and bowl-like ring-banded spherulite appears to be similar, with thorny needle-like lamellae (cilia) on the ridge band that radiate out straight from a common center in the interior of the sample. Figure 3 shows the AFM height and phase images of the center part of dome-like ring-banded spherulite. Even though

Figure 3. AFM height (a) and phase (b) images of the center part of dome-like ring-banded spherulite.

they are not as clear as the SEM image, the AFM images in Figure 3 clarify the existence of cilia-like lamellae with diameters of 0.3−0.5 μm at the central dome. Cilia here is not a single lamella crystal but a bundle of lamellae with a cilialike shape (its diameter is much smaller than its length). These cilia lamellae are what is seen when they emerge from the interior. These cilia-like lamellae might be viewed as “edge-on lamellae” when observed on the top suface of thin films; however, it should be noted that by dissecting on bulk interiors of banded PEA polymer, the cilia lamellae (edge-on lamellae) and radialy lamellae (flat-on crystals) clearly are not connected in a continuous radial spiraling sense, but they are respectively located in two discrete layers (onion-like structures). It will be shown later that underneath the ringless dome region, the 4932

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lamellae on the top-surface views may cause misinterpretation, especially when polymer samaples were confined in thin films. Through the interior observation by dissecting the bulk samples of PEA, one could see more insights that not all ring-banded materials have necessarily same mechanisms of continuous lamellar twisting/spiraling. Interior dissection on bulk polymer samples with banding patterns might provide new insights. Some preliminary analyses of results of PEA have been reported in previous publications.3,26 This work further aimed at focusing on how the PEA lamellae grow in interior bulks and why they grow in such manner to form double ring-banded spherulites with alternating negative−positive bands when confined in thin films and viewed in POM. Figure 5 shows the proposed growth mechanism of PEA ring-banded spherulite. At the melting state, the concentration density of crystalline polymer chains in the whole sample could be considered as a homogeneous state (x0). When the sample is being brought to a crystallization state, nucleation occurs due to fluctuation in density or order in the supercooled melt.31 After the critical size is achieved, the nucleus starts to grow in a radial direction; x2 is the concentration density of crystalline polymer chains in the crystal state. The concentration density of crystalline polymer chains in the growth front (x1) will change gradually with respect to the radius because of competition between nonlinear interphase kinetics and the supply of material by diffusion. The crystallization of the crystalline polymer chains will attract other crystalline polymer chains to come closer; however, the slow diffusion rate of PEA crystalline chains and the rejection of PEA noncrystalline (amorphous) make the value of x1 gradually decrease with respect to the radius. After some distance (depending on how fast the concentration density in the growth front is depleted; for PEA crystallized at 28 °C, it is about 6 μm), an insufficient value of x1 prevents the crystal from growing in a plate-like form, and only cilia-like lamellae can survive to grow and cover the whole spherical crystal. However, this condition only lasts for a while before the growth front meets the crystal chains reservoir and x1 is replenished to the original x0. The SEM figures show that it is more preferable for the polymer chains to sporadically entangle with the cilia-like lamellae and grow in a tangential direction forming a plate-like lamellae at the beginning of the replenishment x1. However, when the tangential lamellae have encountered each other, no more cilia-like lamellae could be entangled for further growth in the tangential direction, yet the plate-like lamellae start to bend and continually grow in the radial direction. After this massive crystallization, the deficiency of concentration density of x1 reoccurs, and cilia-like lamellae

Figure 4. SEM graphs of the fracture surface of bulk sample of PEA crystallized at 28 °C: (a) interior and outer surface of entire spherulite, (b) interior lamellar plates, and (c) zoom in of the square area in (b) (graphs b and c cut in φ-angle direction).

lamellae, therefore, are a layer sandwiched between, and perpendicular to, thinner and thicker radial layers of lamellae. Inspecting more detail in larger magnification, each corrugatedboard layer is composed of plate-like lamellae, which first grow in a tangential direction and then turn about 90° to a radial direction and followed by cilia-like lamellae. Bending lamellae occur not only in PEA ring-banded spherulite but also in PEA positive-type ring-less spherulite.3 However, the good arrangement of alternating sequences of bending plate-like lamellae in two perpendicular orientations (tangential to radial) in PEA ring-banded spherulite allows the spherulite (in thin film sample) to be observed in two contrast colors under the POM. Note that the interlayer distance of the interior corrugate-board shell structure, the inter-ridge distance of the surface ring bands, and the pitch distance of a pair of orange and blue interference colors observed by POM,3 etc., are all comparably similar and mutually in agreement (about 6.0 to 6.5 μm). The transition from plate-like lamellae into cilia-like lamellae might be the reason some of earlier investigators took as “twisting” from flat- to edge-on lamellae at the top surface.28−30 The “edge-on lamellae” typically observed on the top surfaces of polymer films are apparently the cilia-like lamellae which emerge to the surface, and the “flat-on lamellae” in the valley are apparently the plate-like lamellae either radially arranged or tangentially oriented in bulk interiors but bend in emerging to the top surface. The transition radial plates to tangential cilia

Figure 5. Scheme of the proposed growth mechanism of PEA ring-banded spherulite. The scheme was drawn based on the SEM images of the interior lamellar arrangement of PEA. 4933

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will again point outward and cover the massive spherical crystal. These sequences constantly recur until the whole space is filled up. This mechanism is consistent with the order of birefringence which was observed during the growth of PEA spherulite (refer to Figure S-4, Supporting Information). In addition to the constraint by the polymer-chain supply, confinement by the available growth space also has an important role in the growth of PEA spherulite. In a confined growth space, such as near the air−sample boundaries, only cilia-like lamellae could survive, and they eventually cover up the outer surface of the central dome of the dome-like spherulite. The ring-banded pattern in the outer periphery of the center dome indicates that the crystal no longer has space to grow as a sphere, and yet the growth sequences do not change. The above internal views are prominently apparent in revealing: (a) radial lamellae taper gradually and form thinner cilia-like lamellae before being attached sporadically by the next crystalline polymer chains tangentially in next layer; and (b) when the tangential lamellae have encountered each other, there is no more cilia-like lamellae could be entangled for further growth in tangential direction, yet the plate-like lamellae start to bend, curve, or twist 90° to continue the growth in the radial direction. The sequences of tapering, attaching, and bending repeated themselves on and on, resulting to the banded morphology in spherulites. But no matter if they bend or curve, the radial lamellae are always positioned 90° to the tangential skin layers. It could be because of the fact that tangential lamellae were covered by the cilia-like lamellae on the surface, which was detected as a continuous bend, curve, or twist in previous analysis on thin-film confinement. These limited views on the top surface of thin films might have pointed to twisting; however, internal views on PEA bulk samples have more accurately disclosed there are always perpendicular radial lamellae plates between two tangential layers. The two distinctly different types and shapes of the lamellae constituting the radial and tangential crystals deserved special attention to discern their lattice and thermal characteristics. The crystal lattices in ring-banded spherulites of PEA were characterized and further analyzed in comparison with those of ringless spherulites of the same PEA polymer crystallized at different Tc. Figure 6 shows graphs of (a) WAXD patterns of bulk PEA crystallized at different crystallization temperatures (20−35 °C) as indicated on traces; (b) Gaussian peak fitting for X-ray diffraction peaks of bulk PEA crystallized at 28 °C. Samples and spherulites therein covered from ringless to ringbanded spherulites with different extents of ring regularity (from none to less regular to very regular). PEA with ringbanded morphology (Tc = 28−31 °C) is clearly distinguishable from PEA with ring-less morphology (Tc = 20 and 35 °C) by the broadening peak of (110). However, PEA exhibits only a single crystal form. PEA packs in monoclinic crystal packing, with a = 0.547 nm, b = 0.724 nm, c = 1.155 nm, and a monoclinic angle of α = 113.5°.32−34 The broadening peak in X-ray pattern is identical with a smaller crystal size.35 Thus, these cilia-like fibrous lamellae in ring-banded PEA spherulites have given a plausible interpretation for this broadening X-ray peak. Repeated characterizations in this study on numerous PEA samples crystallized at temperatures of 28−31 °C, where the banded spherulites are packed with banding patterns, always showed these broadened X-ray peaks; in contrast

Figure 6. Graphs of (a) WAXD patterns of bulk PEA crystallized at different crystallization temperatures (20−35 °C) as indicated on traces, (b) Gaussian peak fitting for X-ray diffraction peaks of bulk PEA crystallized at 28 °C.

ringless PEA samples (crystallized at Tc = 20 or 35 °C) showed no such broadened diffraction peaks at all. The cilia-like lamellae in the outer surface were formed likely due to a periodically insufficient amount of crystalline polymer chain supply, and these lamellae tend to bend sharply to come to a periodical stop. These cilia-like lamellae, resembling fiber patterns, in the banded PEA spherulites may exhibit additional diffraction peaks, which are close to each other and all jammed to appear as a broadened peak with the original (110) at 2θ = 21.3° seen in nonbanded spherulites. For deconvolution of the broad and overlapped peaks, Gaussian peak fitting was performed on X-ray diffraction peaks to trace the additional peak/s under the broadened (110) peak (refer to Figure S-5, Supporting Information). The additional diffraction peak in the fiber cilia-like lamellae apparently is located at 2θ = 20.7°, which is lower than that of (110). All together, they appear as a broad peak to typify the lamellae crystal assembly in banded spherulites with cilia-like lamellae, which run strictly within tiny spaces and are sandwiched between layers of plate-like lamellae. To further exemplify the band formation mechanisms, Figure 7 illustrates three sections of the circumstances in the growth of PEA in developing into ring-banded spherulite patterns. Crystallization of crystalline polymer chains will attract the nearest polymer chains to come forward, approaching the crystal state in order to supply the crystal packing demand in the growth front. In the case of ring-banded spherulites of PEA, the growth rate of the crystal is higher than the diffusion rate of the crystalline polymer chain, resulting in a gradient concentration density of crystalline polymer chain in the growth front (x1). In forming the periodic rings, the three sections repeat themselves. Section 1 is the beginning of new band formation, where the growth front meets the crystal chains reservoir and x1 replenished to the original x0, allowing 4934

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Crystal Growth & Design

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in many recent studies have more and more clearly demonstrated that the interior exposed that the lamellae do not spiral synchronizingly in continuous spiral to generate the optical banding. Instead, the interior lamellae are packed in periodical two layers of mutually perpendicular orientations.4,26,36−39 Although lamellar twisting (i.e., continuous twisting/spiraling from the nuclei center all the way to peripheral of spherulite) in PEA thin films has been postulated as a mechanism in some classical studies such as those by Keith and Padden in 1959,8,9 they nevertheless cautioned that this was a possibility, but other mechanisms may also be possible and should not be overlooked. This study further expounded that views confined in PEA thin films might not be fully sufficient in properly accounting for the optical banding behavior as observed in POM characterization on thin films. When viewed from interior dissection of bulk PEA samples using SEM, there are apparently two perpendicular layers of opposite lamellar orientations (radial and tangential) alternately observed in PEA ring-banded spherulite displayed two contrast optical interference colors, with pitch (∼6.5 μm) matching perfectly with the interior dissected corrugated layer thickness (∼6.5 μm). That is, interior exposure of lamellar assembly did not reveal monotonous or continuous spiraling (gradual twisting from edge-on to flat-on) of a lamella from the nuclei center to edge of spherulites; instead, interior assembly of bulk PEA samples using SEM characterization clearly demonstrated a structure composed of alternately corrugated layers of two opposite orientations (radial and tangential). The lamellae in the radial layer differ not only in the orientation but also in geometric shapes and dimensions from those in the tangential layers. The opposite orientations in the corrugated layers of lamellae, as viewed in SEM interior analysis, are amply capable of interpreting the positive/negative birefringence as viewed in POM characterization on thin films. Although monotonous and continuous twisting from edge-on to flat-on is equally capable of explaining the opposite positive/negative birefringence in POM graphs, it is not seen or proven in the interior dissection of bulk PEA samples. Kinetically, it is also hard to justify the continuous growth of a continuous spiraling lamella from nuclei to edge of spherulites, uninterrupted continuity for ∼500 μm (radius of PEA spherulites) to achieve such spiraling. Diffusion limits of the long polymer chains simply prohibited such long continuity of ∼500 μm with no interruption or draining out of molten species for packing into continuous lamella crystals. In this case of corrugated layers of radial and tangential orientations, as revealed by SEM characterization on PEA interior, the rhythmical growth of PEA lamellae in two perpendicular orientations (radial and tangential) manifested in 3D interior observations gives a better explanation for the formation of “double ring-banded” spherulites, i.e., those with alternating bands of positive and negative birefringence (blue-orange colors with 530 nm tint plate) along the radius direction. Such double ring bands in spherulites are to be distinguished from an alternate type of ring bands with an extinction border between subsequent bands. Interior assembly of lamellae is considerably different in spherulites with doubly tinted ring bands from that in spherulites containing bands with an extinction border (viewed in POM graphs).

Figure 7. Illustration of some likely circumstances for growth of PEA lamellae in packing into ring-banded spherulites. The scheme is aimed to illustrate how the depletion of crystalline polymer chain concentration at the growth front (x1) causes a transition from plate-like lamellae to the cilia-like lamellae.

the crystal to grow in the form of plate-like lamellae. Section 2 is the middle part of the band, where the x1 decreases gradually but is still enough to form tapered plate-like lamellae. Section 3 is the end part of the band, where x1 becomes slightly constant and is only sufficient to supply the growth of cilia-like lamellae. This section does not last for long before section 4 takes over again. Those three sections are located in the middle state between the crystalline and melting states as illustrated by two gray rectangles. Unlike Figure 5, Figure 7 aims to explain how the depletion of crystalline polymer chain concentration at the growth front (x1) causes the transition from plate-like lamellae to cilia-like lamellae. The column which is fully filled with crystal indicates the plate-like lamellae; on the other hand, the column which is not fully filled with crystal indicates the cilialike lamellae. The abscissa of growth direction (indicated by the increasing of radius, r) and ordinate of x1 was indicated at the bottom. As previously explained, x0 is the crystalline polymer chains concentration at the melting state, x1 is the crystalline polymer chains concentration at the growth front, and x2 is the concentration density of crystalline polymer chains in the crystal state. In this scheme, we only intend to explain the profile of x0 (gray straight line) and x1 (gray dash line). x0 is only valid in the melting state, and x1 is only valid in the middle state; however, the value of x1 of the previous band in the crystalline state can be interpreted since we know that the profile should be the same for each of the bands. Classical views on twisting were mainly based on the observation of thin films, except for Lustinger et al.11 who did preliminary work of dissecting the bulk specimen and exposed the 3D interior of banded spherulite of polyethylene. However, there was no explanation about the lamellar arrangement/ assembly on the fracture surface in relation to the spherulic surface. Instead, a direct computer model of twisting helicoids was used to represent the spherulitic surface projection and claimed to be useful in assessing the spherulitic architecture. It should be pointed out that lamellar twisting does indeed occur in irregular fashion during the crystallization of polymers, even in spherulites without bands (i.e., ring-less spherulites).3 So, it is a critical question whether or not the classically addressed regular twist (or spiral from flat-on to edge-on in continuous growth) with a pitch matched the optical bandwidth. The facts 4935

dx.doi.org/10.1021/cg5002539 | Cryst. Growth Des. 2014, 14, 4929−4936

Crystal Growth & Design

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CONCLUSIONS Interior dissection is critical, because without interior anatomy of the spherulites, the top surface of spherulites in thin films tends to show only outer surface morphology covered with bent or protruded lamellae whiskers or impurity rejected to top surface. The 3-D periodical assembly leads to not only the noticeable ring band on exterior surfaces but also corrugatedboard like multishell structures in the interior of the spherulites. Each corrugated-board is composed of plate-like lamellae, which first grow in a tangential direction, then turn to radial direction, and followed by the cilia-like lamellae due to the growth front crystalline polymer chain concentration density (x1) gradient during the growth. When the x1 is very low only cilia-like lamellae could survive to grow and cover up the whole crystal; however, after a small distance, the x1 is replenished to the original x0, and it becomes more preferable for the polymer chains to entangle with the cilia-like lamellae and grow in a tangential direction. Alternating sequences of bending plate-like lamellae in two perpendicular orientations (tangential to radial) in PEA ringbanded spherulite displayed two contrast radial alternating interference colors with pitch matching perfectly with the interior dissected corrugated layer thickness. In addition, in packing into banded spherulites, lamellae growth from interior leads to the formation of the cilia-like lamellae in the outer surface due to an insufficient amount of crystalline polymer chain supply; these cilia-like lamellae, rambling fiber patterns in banded PEA spherulites are responsible for additional diffraction peaks that are jammed into a broadened peak (110).

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ASSOCIATED CONTENT

S Supporting Information *

Growth observation of PEA ring-banded spehrulite, observed at Tc = 28 °C, and Gaussian peak fitting of X-ray diffraction peaks of PEA crystallized at 29 and 31 °C. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +886 6 234-4496. Phone: +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 102-2221-E-006-268-MY3) for three consecutive years from Taiwan National Science Council (NSC), to which the authors express their gratitude.

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REFERENCES

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dx.doi.org/10.1021/cg5002539 | Cryst. Growth Des. 2014, 14, 4929−4936