Article pubs.acs.org/Macromolecules
Hierarchically Diminishing Chirality Effects on Lamellar Assembly in Spherulites Comprising Chiral Polymers Eamor M. Woo,*,† Graecia Lugito,† Jia-Hsuan Tsai,† and Alejandro J. Müller*,‡,§ †
Department of Chemical Engineering National Cheng Kung University, Tainan, 701, Taiwan POLYMAT and Polymer Science and Technology Department, Faculty of Chemistry, University of the Basque Country UPV/EHU, Paseo Manuel de Lardizabal 3, 20018 Donostia-San Sebastián, Spain § IKERBASQUE, Basque Foundation for Science, 48013 Bilbao, Spain ‡
ABSTRACT: Three-dimensional interior structures of poly(lactic acid) (PLA) spherulites have been examined in crystallization with poly(ethylene oxide). A high concentration of water-soluble PEO is used as diluent to obtain spherulitic morphologies of PLA in thick enough films (20−50 μm) for interior observation. Two different but closely related spherulite patterns (dendritic and spiral banded) of both PLA enantiomers have been studied in details, after removal of PEO by water etching. The interior of both curved dendrites and spiral banded spherulites are not connected by continuous twisting, as conventional models proposed; instead, the ridge band is composed of multiple polycrystals in parallel alignment and the valley band is also composed of polycrystalline branching lamellae. At increased film thickness, the molecular chirality may not be entirely responsible for the lamellar bending sense (i.e., clockwise or counterclockwise) in all hierarchical crystal structures. Film thickness also contributes to curvature sense and banding. The approaches taken and results presented here are ground breaking for interpreting the mechanisms of periodic patterns that can be observed in spherulites.
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INTRODUCTION Hierarchical crystal structures in polymer spherulites have been extensively studied. The crystallization of polymers into various morphological structures is influenced by many inherent parameters. Polymers are composed of repeating units of different chemical structures, whose physical properties differ accordingly. Thus, sensitivity to crystallization parameters, such as film thickness,1,2 temperature,3,4 composition of diluents, tacticity or chirality,5−9 preparation procedures,10 varies among different polymers. Polymer spherulites can display rich patterns of optical birefringence. The origin of periodic bands in the forms of rings (i.e., banding) has been debated for decades in achiral or chiral polymers from classical polyethylene (PE), aliphatic polyesters, poly(trimethylene terephthalate) (PTT), to more recent biodegradable poly(3-hydroxybutyrate) (PHB), etc.11−25 Chiral polymers have been studied seeking relationships between molecular chirality and higher levels of crystal organization. However, results may differ with respect to Tc, type of diluents, or film thickness.5−9 Biodegradable PLA has two optically isomeric forms (L-form and D-form) known as PLLA and PDLA, which otherwise have identical physical properties if their average molecular weights are equivalent. Maillard and Prud’homme6 prepared ultrathin (∼15 nm) films of PLLA and PDLA and observed single edge-on lamellae upon crystallization from the melt at 145 °C. PLLA and PDLA lamellae exhibit S-shaped (clockwise direction) and Z-shaped © XXXX American Chemical Society
(counterclockwise direction) bending near their nuclei center, respectively. They claimed that lamellae bending directions are opposite to the chirality of PLA. They also observed dendritic spherulites with S-shaped and Z-shaped dendrites (for PLLA and PDLA, respectively) on 10 μm thickness films of 75/25 PLA/poly(ethylene glycol) blend prepared by solvent evaporation (dichloromethane) at ambient temperature.7 In the case of PLLA/PDLA blends, they reported that triangular dendritic crystals with counterclockwise or clockwise bending are observed on 20 nm films of nonequimolar blends with excess PLLA or PDLA, respectively.8 Interestingly, they observed inversion of the sense of curvature in thicker films with large enantiomeric excesses (i.e., deviations from enantiomeric ratios).9 Growth mechanisms of polymers leading to final spherulite patterns have been highly debated. Many researchers have chosen to investigate ultrathin films for simplicity, although they may not reflect actual 3D polymer spherulites. In ultrathin films, crystals growth is limited in two dimensions (2D) to resemble morphologies of single crystals, which are easier for modeling.26−28 However, the actual 3D growth of polymer spherulites can be more complex and deserves in depth study. This approach (top surfaces of thin films) can lead to erroneous Received: February 18, 2016 Revised: March 21, 2016
A
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Figure 1. Lozenge-shape single-crystal with counterclockwise helical packing in straight PLLA dendrites (a) in PLLA/PEO (95/5) blend, (b) PLLA/ PBA (50/50) blend vs (c) fish-scale shingle packing in bending PLLA dendrites, and (d) both counterclockwise and clockwise helical assemblies in straight dendrites of achiral PESu. Reproduced with permission from refs 32 (a, copyright 2015 Wiley), 5 (b and c, copyright 2014 Wiley), and 27 (d, copyright 2011 Wiley), respectively. polydispersity index (PDI) 1.43. For this grade of PLLA, the glass transition temperature (Tg) = 54.2 °C and melting temperature (Tm) = 171 °C. Poly(D-lactic acid) (PDLA) was purchased from Aldrich (USA) with weight-average molecular weight of Mw = 53 100 g/mol and Tg = 54 °C. L- and D-forms were chosen so that they have comparable Mw’s. Poly(ethylene oxide) (PEO) was purchased from Aldrich (USA), with Mw = 200 000 g/mol, Tg = −60 °C, and Tm = 64 °C. Binary blends of PEO and PLA with intended compositions (PEO/ PLLA = 80/20−90/10) were dissolved and stirred in chloroform with polymer concentrations of 2 and 4 wt % then cast onto glass slides at 45 °C. The solvent was fully evaporated for 24 h and then dried in vacuum at 40 °C for 2 days. The two different levels of film thickness: 20−30 μm (thinner films) and 40−50 μm (thicker films) were employed to demonstrate that two entirely different spherulitic morphologies could be formed when the samples were crystallized at the same Tc. PEO contents in PEO/PLLA (or PEO/PDLA) blends for crystallization were kept high and PEO was employed for two reasons. First, PEO (at 20−80 wt % contents) is able to enhance the formation of ring bands in PLLA. Second, it can dilute the PLLA to extents of leaving intact the skeletons of PLLA crystals after etching out water-soluble PEO for convenience of analyzing PLLA crystal plates in association with ring band structures. PEO is water-soluble, and has a low melting temperature (lower than the crystallization temperature Tc of PLA [PLLA or PDLA] used in this study); thus, PEO can remain liquid and will not interfere with PLLA crystal growth. Degradation of neat PLLA under static air was analyzed using thermogravimetric analysis (TGA). Results showed that in air, degradation of neat PLLA initiated at ∼200 °C. As a result, 190 °C was used as Tmax in this study for avoiding degradation. The samples were heated to 190 °C for 2 min and then quenched to 120 °C to isothermally crystallize the PLA component in the blend (with PEO as diluent). Then, the samples were quenched to room temperature,
interpretations, as the top-surface crystal assembly can differ significantly from the inner one. Direct imaging of 3D interiors of polymer spherulites is thus necessary for correct assesment of lamellar assemblies. In the present study, blends of poly(ethylene oxide) with poly(lactic acid) of different chirality senses (i.e., PEO/PLLA and PEO/PDLA) have been prepared. As a novel approach, we studied the 3D interior of polymer spherulites by direct observation thanks to the water solubility of the poly(ethylene oxide) (PEO) component. Neat PLLA does not form ordered rings of distinct birefringence contrast, but adding 10 to 50% PEO is known to induce and intensify banding in PEO/PLLA (or PEO/PDLA) blends.29−32 In the present work, PLLA (or PDLA) is diluted with a large quantity of PEO (typically 80% or more) to study lamellar packing and banding. In addition, larger quantities of PEO facilitate the formation of samples with large enough film thickness (20−50 μm) for interior observation. The blend is melt-crystallized into 3D spherulites, where PLLA (or PDLA) lamellae are fixed into an assembly pattern according to their growth. After full crystallization, the water-soluble PEO constituent is etched out, without altering the solidified PLLA crystals. In this way, the inner morphology leading to specific birefringence patterns of periodic packing can thus be fully exposed and examined. Correlations between the top-surface morphology and interior crystal assembly can thus be established, leading to novel observations on the threedimensional structure building of polymeric spherulites.
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EXPERIMENTAL SECTION
Materials and Preparation. Poly(L-lactic acid) (PLLA) was purchased from Fluka (Switzerland). Size exclusion chromatography yielded values of molecular weight (Mw) 51 600 g/mol and B
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Macromolecules where the crystallization of the PEO component took place. After complete crystallization at room temperature, samples were cryogenically fractured in liquid nitrogen. One set of samples remain ascrystallized without water-etching while an additional set of PEO/PLA blend samples was water-etched. Water can only dissolve PEO, exposing the morphology of the PLA component. Apparatus and Procedures. Polarized optical microscopy and optical microscopy (POM and OM) observations were carried out on a Nikon Optiphot-2 instrument, equipped with a Nikon Digital Sight (DS-U1) digital camera and a microscopic heating stage (Linkam THMS-600 with T95 temperature programmer), and were used to characterize the superstructural morphology of the polymers. Environmental-scanning electron microscopy (E-SEM) (FEI Quanta-400F) was used to characterize both fractured and/or top free surfaces of samples. Samples were respectively crystallized, fractured, and then sprayed with deionized water to perform etching (exposing PLA lamellar structures). The samples were then washed and dried, then coated with gold vapor deposition using vacuum sputtering prior to SEM characterization. Note that the digital camera coupled (CCD) with different microscopy apparatus (AFM, POM, SEM) might register opposite sense of images. Ignorance of such differences can add to complexity and confusion in analysis in trying to establish relationships between the chirality and crystal bending senses. Therefore, in this study, caution has been taken in determining and comparing the curvatures, or bending, or spirals of the PLA spherulites as revealed in different apparatus.
PLLA straight dendrites, in either miscible blend with 95 wt % PEO (Figure 1a) or partially miscible blend with 50 wt % PBA (Figure 1b), are composed of lozenge-shaped singlecrystal-like lamellae with counterclockwise spirals resembling a flattened pyramid.5,32 Similar hexagonal dendritic morphology with lozenge-spiral dislocation single crystals has also been reported in ultrathin films of PLLA-b-PEO copolymers.24,34 The spiral rotation of the lozenge-shaped lamellae that compose the straight dendrites of PLLA is always and clearly “counterclockwise”, in agreement with the chirality of PLLA. On PLLA curved dendrites (Figure 1c), the fish-scale-like crystals with 3−5 μm wide and 10−15 nm thick are piled up along the long-axis of the dendrites. These curved dendrites display a counterclockwise rotation sense in PBA/PLLA blends.5 In achiral poly(ethylene succinate) (PESu), however, the lozenge-shaped single-crystal-like lamellae that compose the PESu straight dendrites randomly spiral in both clockwise and counterclockwise directions.27 Therefore, helical rotations of single crystals are not limited only to chiral polymers but can be displayed by achiral PESu, however, the helicity can be in either way (clockwise and counterclockwise). This comparison yields a hint that chirality in polymers may influence the directions of crystal packing but only in a limited range, and not likely in all hierarchical crystal structures. The results described above indicate that PLLA can exhibit a wide range of spherulitic and lamellar morphologies. An important question arises from examining these results and those cited in the Introduction. If chirality is the driving force behind banding sense, then, why are there so many different (and sometimes opposed or invisible) crystal habits in PLLA or PDLA polymers as morphology progressively develops from initial single crystals to polycrystalline spherulites? It should also be noted that polymer spherulites can be initiated by nuclei with different geometries, some conforming to chiral influences but others not. As polymer spherulites grow from single-crystallike crystals to 3D structures, the influence of chirality on crystal packing into higher orders may vary. Figure 2 shows PEO/PLA POM micrographs of thinner film samples (20−30 μm), (a) 80/20 PEO/PLLA and (b) 80/20 PEO/PDLA, and in comparison to thicker samples (40−50 μm), (c) 80/20 PEO/PLLA, (d) 80/20 PEO/PDLA, (e) 85/15 PEO/PLLA, and (f) 90/10 PEO/PLLA. All the micrographs were taken at Tc = 120 °C where PEO is in a molten state and acts as a diluent. In thinner samples, both PLLA (Figure 2a) and PDLA (Figure 2b) crystals grow as dendritic spherulites with curved dendrites. However, the curvature of PLLA and PDLA crystals are opposite (or mirror images) to one another, PLLA shows Sshaped (clockwise) dendrites, on the other hand PDLA shows Z-shaped (counterclockwise) dendrites. In thicker samples, both PLLA (Figure 2c) and PDLA (Figure 2d) crystals grow as spiral-banded spherulites with counterclockwise and clockwise spiral, respectively. Interestingly, the bending curvatures of the spiral bands in thicker samples are opposite to those of the dendrites in thinner samples. Compared to those in lozenge-shaped lamellae on the hexagonal straight dendrites and those in the similar curved dendrites observed in the partially miscible blend of PBA/PLA shown earlier in Figure 1, the bending curvatures of PLA (both L and D) crystals in dendritic spherulites of thinner PEO/PLA samples are in reverse directions, while in the spiral-banded spherulites of thicker samples are in same directions.
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RESULTS AND DISCUSSION Chiral Effects on Hierarchical Superstructures. Neat PLLA at regular micrometer thickness has been reported to form ringless spherulites (that only exhibit the Maltese-cross extinction pattern) or ring-banded spherulites. In order for PLLA to show curved dendrites or straight dendrites, the sample thickness should be as low as 15 nm.6,7 At regular thickness, PLLA films can form dendrites if PLLA is diluted or blended (or block copolymerized) with other polymers such as PEO (in a miscible blend system) or PBA (in a UCST blend system), and crystallized with the existence of the diluent.5,32,33 In our recent work,32 we have reported systematic variation of PLLA morphological patterns in PEO/PLLA blends of various compositions (uncovered film samples of 40−50 μm thickness) at Tc,PLLA = 120 °C. At 0 wt % PEO (neat PLLA), the spherulites melt-crystallized at 120 °C are regular spherulites with Maltese cross extinction patterns. With PEO contents of 25−80 wt %, the PLLA spherulites are distinctly spiral-banded (with counterclockwise spiral rotation). Increasing PEO contents to 85−90 wt % in PEO/PLLA blends, leads to PLLA dendritic spherulites with lamellae radiating out from a common center and slightly curving/bending in clockwise direction. Finally, with PEO content 95% (or higher), the highly diluted PLLA forms hexagonal dendritic spherulites with straight dendrites. Six main stalks of lamellae straightly radiate out from a center, with numerous side branches (also straight) growing from the main stalks as spherulites grow. The effect of adding PEO (i.e., a diluent) on PLLA spherulite patterns is equivalent to that of increasing temperature. If one fixes the PEO/PLLA composition and film thickness, but varies the crystallization temperature (Tc), similar trends of spherulite patterns can also be seen. Figure 1 shows schemes illustrating the detailed lamellar packing that characterize dendritic superstructures (either with straight or curved dendrites) of the chiral PLLA in comparison to the achiral poly(ethylene succinate) (PESU), obtained from AFM observations. C
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bending or straight) dominate for thinner films. The crystal morphology in PEO/PLLA and PEO/PDLA are similarly dendritic or spiral-banded when compared at same compositions and Tc’s; however, the curvatures of PEO/PLLA appear to be mirror images to those of PEO/PDLA. Two important questions arise from the results presented in Figure 2: (a) which factors do drive the spherulitic morphology from the original dendritic to spiral-banded as the thickness is increased or the Tc is decreased? (b) What are the crystal assembly differences between PLLA and PDLA spherulites that account for the opposite curvatures in these two systems? For understanding the mechanism of transition from ring bands to dendrites, or the connection between curved dendrites and spiral bands with varying thickness (or Tc), it is necessary to examine the interior crystal assembly of the spherulites. Water is able to dissolve the PEO component without affecting the shape and morphology of preformed PLA crystals. Wateretched samples thus provide better contrasts for observing inner 3D structures using SEM. Prior to SEM characterization, POM observations are performed on water-etched samples where the original birefringence patterns are preserved. S-Shaped vs Z-Shaped Dendrites in Thinner Films of 20−30 μm. Figure 3 shows POM (top) and OM (bottom)
Figure 2. PEO/PLA POM micrographs of thinner film samples (20− 30 μm): (a) PEO/PLLA (80/20), (b) PEO/PDLA (80/20). Micrographs in comparison to thicker samples (40−50 μm): (c) PEO/PLLA (80/20), (d) PEO/PDLA (80/20), (e) PEO/PLLA (85/ 15), and (f) PEO/PLLA (90/10). All micrographs are taken at Tc = 120 °C.
When PLLA content of the thicker sample is decreased to 15 or 10 wt %, both spiral-band and dendritic morphologies coexist in the spherulites as shown in Figure 2e or Figure 2f, respectively. The spherulites appear as counterclockwise spiral bands near the core, before it suddenly changes its curvature into clockwise dendrites toward the edge. There are transitions from the dendritic morphology in 80/20 PEO/PLA film samples (obtained from 2 wt % solution) to the dualmorphology in thicker 90/10 and 85/15 film samples (obtained from 4 wt % solution) to the spiral-banded morphology in thicker 80/20 film samples. These results imply that the amount of the polymer forming the crystallites (PLLA or PDLA) in the sample takes a considerable role in morphological development of the final superstructure. Similar phenomena of changing morphologies and curvatures of PLA spherulites could also be observed by changing the Tc of 80/20 PEO/PLLA blend of similar thickness (20−30 μm) from lower to higher temperatures. POM results indicate that spiralbanded patterns only exist at relatively low temperatures of crystallization (Tc < 100 °C); with a transitional temperature (Tc = 110 °C) where both spiral-banded and dendritic patterns coexist. At a high enough temperature (Tc = 120 °C), the spherulites of PEO/PLLA (or PEO/PDLA) are completely dendritic. Therefore, the crystallization temperature (Tc) is another governing factor in determining whether spiral bands or dendrites are formed in the final spherulites of PEO/PLLA (or PEO/PDLA). Spiral-banded patterns always dominate at lower Tc’s while dendrite patterns dominate at higher Tc. Conversely, at a fixed Tc = 120 °C, spiral-banded patterns dominate for thicker films while dendritic patterns (either
Figure 3. POM and OM micrographs for 20−30 μm thickness wateretched samples (Tc = 120 °C): (a) 80/20 PEO/PLLA and (b) 80/20 PEO/PDLA blends. (The arrow marks indicate main lamellae in the radial direction, versus side branches in the tangential direction.)
results of etched samples of (a) PEO/PLLA and (b) PEO/ PDLA, whose curved dendrites are exposed more clearly. The main branching lamellae are seen to bend along the radial direction into an arc-shape (marked by black arrows). Tiny side branches are seen to evolve from their main branch on either side at an angle of ca. 60° (marked by blue arrows). The optical birefringence of the side branches might also be different from the main branches thus the overall spherulite displays circumferentially alternating rings of blue and orange colors as drawn in the schemes. These POM micrographs show only superficial top morphology at low magnifications; nevertheless, they show clearly the opposed bending sense in PLLA vs PDLA (Figure 3a vs Figure 3b). Thus, the radiating rays in the dendritic spherulites (at Tc = 120 °C) in chiral PLLA or PDLA polymers are found to bend in two opposite direction. For PLLA, the bending is toward the clockwise direction (Figure 3a); while for PDLA, it is D
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another to form the main stalks (or “ridges”, because they are higher) of the dendrites. Between the main stalks is the valley (lower portion of concave) region. The valley is populated with finer side branches that evolve from the main lamellae and these side branches also occasionally twist into flat-on orientation as they grow away from the main stalks. Note also the edge-on lamellae in the main stalks can bend significantly to become flat-on as they emerge to the top surface, or as they make contact with the bottom substrate. Such evidence points out that the inner lamellae in spherulites can twist/bend into entirely different orientations as they emerge into the top surface (or alternatively impinge with the bottom substrates). Thus, when analyzing the spherulites in films, perception of crystal morphology on the top surface of spherulites may be entirely different from what is hidden underneath the top layer. Parts a3 and b3 of Figure 4 show the interior structures of the crystal assembly of the valley region, where the side branches grow from the main stalks. They are parallel aligned and lean at an angle with the substrate surface; that is, they are neither fully edge-on nor entirely flat-on. One thing is very clear: the ridge and valley of the dendritic spherulites are not connected by continuous twist of a monotonous single crystal. Instead, the ridge band is composed of multiple polycrystals in parallel alignment; the valley band is also composed of polycrystalline branching lamellae that align closer to the tangential direction. The hierarchical structures in PLLA and PDLA are similar; however, the way how PLLA and PDLA arrange themselves appears to be opposite to one another. The contrasted Sshaped and Z-shaped dendrites (for PLLA and PDLA, respectively) differ in the bending sense of the main lamellae that constitute the ridge bands of the dendrites. Furthermore, the branches and their inclination angles in the valley bands are also opposite to each other as illustrated by the schemes in Figure 4, parts a1 and b1. Such morphological details cannot be ascertained without analyzing the interiors of the spherulites. Figure 5 shows schemes for dendritic spherulites of PLLA within 80/20 PEO/PLLA blend to illustrate the relationships between the main branches composing the ridge band and side branches composing the valley band. PEO (remaining in molten state during PLLA crystallization at 120 °C) tends to fill the interlamellar spaces. Under the main branches, lamellae are mostly edge-on and in parallel alignment, but they can bend into flat-on configurations as they surface to the top or make contact with the bottom substrate. Side branches intersect with the main stalks at ca. 60°, and align parallel with one another along the main stalks. Under the side branches, lamellae are partially flat-on (inclining at a sharp angle to the substrate). In other words, the side branches (valley band) differ from the main lamellae (ridge band) in the crystal axes in two perspectives: (1) they mutually intersect at an angle, and (2) the side branches are partially flat-on while the main stalks are fully edge-on. Thus, their resultant positions may cause a contrast optical birefringence (interference colors) between the ridge and valley bands of the dendritic spherulites, which is demonstrated earlier in Figure 3. To better appreciate the lamellae assembly from initial nucleation to final dendritic PLLA spherulites, the scheme on the bottom of Figure 5 shows the evolution of the side branches from the main-stalk lamellae as crystals grow to fill the space. Just like the main lamellae originally grow from the center of the nucleus, the side
counterclockwise (Figure 3b). The patterns of these curving rays from a common center resemble curved “Catherine wheels” of opposed bending. It is worth mentioning that the curved Catherine wheel dendrites in PLLA or PDLA (blends with PEO) have also been similarly reported for other chiral polymers. In 2002, Shahin and Olley35 reported such a phenomenon in poly(3-hydroxybutyrate) (PHB)a chiral polymerwith bending in counterclockwise direction. In 1995, Singfield,et al.36 also reported that optically chiral R- or S-forms of poly(epichlohydrin) display clockwise (R-form) and counterclockwise bending (S-form) of what they called “pinwheel like” dendrites; but in an equimolar blend of Rand S-forms poly(epichlorohydrin), the dendrites become straight without any bending. They stated that in the chiral poly(epichlorohydrin) of two chiral forms (R and S), the backbone chirality of the polymer chains imposes significant restrictions on the lamellar organization. For getting finer 3D details, SEM characterization on crystallized films was performed. Figure 4 shows SEM
Figure 4. SEM micrographs and schemes for watered-etched dendritic spherulites in (a) PEO/PLLA and (b) PEO/PDLA (80/20) blends (Tc = 120 °C, 20−30 μm): (1) top surface, (2) fractured surface of main stalks (ridge), and (3) fractured surface of side branches (valley).
micrographs for water-etched film sample of the dendritic spherulites in (a) PEO/PLLA and (b) PEO/PDLA (80/20) crystallized at 120 °C. The top surfaces of PLLA (Figure 4a1) and PDLA (Figure 4b1) spherulites are packed with S-shaped and Z-shaped dendrites, respectively, by checking to ensure that the digital imaging of CCD in SEM is in agreement with that in POM. The interiors of the dendrites (Figure 4, parts a2 and b2) show that the edge-on lamellae are packed parallel to one E
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the OM micrographs (without polarization), the relative height of crystals can be preliminarily judged. The bands of bulge (ridge) vs bands of concave (valley) and the lamella orientations in the bands are marked on the OM micrographs. Black arrows indicate the orientation of lamellae at ridges (dominantly in radial direction) and blue arrows indicate the orientation of lamellae at valleys (dominantly in circumferential direction. Apparently the orientations of the lamellae on the top surface of spiral-banded spherulites are about the same to those on the dendritic spherulites. However, when they are observed under POM with addition of 530 nm tint plate, the bending sense of these two kinds of spherulites are opposite to each other (i.e., once the dendrites are in S-shape, the spiral bands are counterclockwise as seen in PLLA spherulites, and vice versa as seen in PDLA spherulites). Figure 7 shows SEM micrographs for spiral-banded spherulites in 80/20 PEO/PLLA vs. 80/20 PEO/PDLA blends that were crystallized at Tc = 120 °C. The top surface shows alternating rough ridges (with microporous structures) and flattened valleys along the radial direction of the spherulite. The spiral sense is quite pronounced at the center as indicated by red arcs. A similar radially dominant orientation is also found on every ridge. However, on every valley the orientation of lamellae changes; the lamellae are rather circumferentially oriented instead of radially oriented, as indicated by blue arcs. The lamellae underneath the top surface were exposed by fracture for further analysis. Most of the crystals underneath the ridge-bands are radially aligned, normal to the substrate (ignoring the localized random plate distortion), and appeared fuller as compared to the valley. Underneath the valley, the lamellae are quite randomized with inclinations from the substrate and positioned normal to the fractured surface. The empty spaces were once filled by molten PEO during the crystallization of PLA. Figure 8 shows schemes for 3D illustration of the lamellar assembly in thicker films of (a) PLLA and (b) PDLA crystals in spiral-banded spherulites (in PEO/PLLA blends). The schemes were made for both PLA enantiomers in accordance with the SEM images. From top views as illustrated above the dashed line, one sees only fibrous crystals periodically pointing toward the radial direction on the “ridge” bands, and these fibrous crystals are highly aggregated as fan-like bundles with microvoids (where the extracted PEO used to be) between the fiber bundles. Then, immediately adjacent to the ridge band, there are thinner and more sparsely populated cilia-like crystals collectively bend/curve to tangential direction on the valley bands. Note that although the fibrous crystal bundles on the ridges do not differ between PLLA and PDLA (both pointing toward the radial direction), the tangential crystals on the valleys consistently bend in two opposite directions in PLLA (Figure 8a) vs PDLA (Figure 8b). These results suggest that for the lamellae bundles conforming the ridge bands, chirality effects decrease. On the other hand, for thin and sparsely populated crystals conforming the valley bands, chirality effects should be more important. Below the dash lines in Figure 8, the interior lamellae crystals in PLLA and PDLA are represented. Periodical repetition of wider bands of radial crystal plates (underneath the ridges) and narrower bands of tangential crystal plates (underneath the valleys) are represented for both PLLA and PDLA. Under the ridges, a lighter shade is used to represent the regions that are filled with curved edge-on lamellae collectively pointing to the
Figure 5. Scheme for 3D illustration of lamellar assembly (top) and growth mechanism (bottom) of PLLA dendritic spherulites and correlation between the interior lamellae of main stalks and side branches and top-surface dendritic morphology in the 20−30 thickness PLLA/PEO (80/20) sample.
branches grow from the main lamellae as row nucleated parallel aligned daughter crystals. Clockwise vs Counterclockwise Spiral Bands in Thicker Films of 40−50 μm. Figure 6 shows POM and
Figure 6. POM and OM micrographs for 40−50 μm thickness wateretched samples of: (a) 80/20 PEO/PLLA and (b) 80/20 PEO/PDLA. (Yellow indicates the lamellae orientation at ridge is pointing in radial direction, and green indicates that crystal at valley is bending in the tangential direction.)
OM micrographs for 40−50 μm thickness water-etched samples of (a) 80/20 PEO/PLLA and (b) 80/20 PEO/ PDLA blends; PLLA and PDLA in these blends were crystallized at 120 °C prior to the etching treatment. The micrographs were taken at room temperature. The POM micrographs show that PLLA displays counterclockwise spiral-banded spherulites (with 2, 4, 6, or more arms); on the contrary, PDLA displays clockwise spirals. From F
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Figure 7. SEM micrographs for water-etched ring-banded spherulites in (a) PEO/PLLA (80/20) and (b) PEO/PDLA (80/20) blends (Tc = 120 °C, 40−50 μm): (1) top surface, (2) fracture surface, (3) zoom-in to the area indicated in part 2.
Figure 8. Schemes for 3D illustration of lamellar assembly of (a) PLLA vs (b) PDLA spiral-banded spherulites in PEO/PLA blend showing correlations between the interior layered lamellae and top-surface of periodic wave-like ridges to be followed with a flat region with lamellae bending.
Correlation between Thickness, Growth Rates, and Banding Patterns. The spherulitic growth rates of thicker films (40−50 μm) are found faster than those of thinner films (20−30 μm), as shown in Figure 9 for both 80/20 PEO/PLLA and 80/20 PEO/PDLA blends. The increase in film thickness not only leads to entirely different morphological patterns in the PEO/PLLA spherulites (spiral-banded vs curved-dendritic) but also increases the growth rate of the spherulites.
radial direction. Under the valleys, a darker shade is used to represent regions composed of crystals pointing upward, sideways, or downward, normal to the fractured surface, separated by voids (empty spaces). The crystals of the valleys are branches that grew from the crystals of the ridges. Owing to impingement between two successive ridges, the valley crystals are squeezed and collectively point to tangential directions (seen upward, downward, or sideways from the fractured surface). G
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Macromolecules
crystals (which correlate with film thickness) with curvature senses and spherulite growth rates. Please notice that the composition of crystallites (PLA) is kept the same (20 wt %) for both thinner and thicker film samples; therefore, the amount of PLLA crystallites increases with thickness. For further illustrating the effects of film thickness and chirality on spherulite patterns and crystal assembly, Figure 10 shows SEM micrographs for interior crystals of 80/20 PEO/PLA blend crystallized at 120 °C: (a) PLLA and (b) PDLA, packed as (1) dendritic (20−30 μm) and (2) spiral-banded (40−50 μm) spherulites. Insets are top surface images and color distribution schemes of the corresponding spherulites. The general main features of interior crystals are the same for L-form or D-form systems. The lateral cross-section (i.e., fractured surface) of the PLA spherulites shows some consistency in the lamellar bending sense with respect to molecular chirality. The lamellae bending sense difference in the L-form and D-form system is more noticeable on the top surface. Periodically wavy structures are observed in both curved-dendritic spherulites and spiralbanded spherulites. In curved dendrites, the wavy structures are periodically arranged along the circumferential direction, while in spiral bands, they are arranged along the radial directions. To illustrate the close similarity between the lamellar assemblies in leading to dendritic or spiral banded PLLA spherulites, a plausible mechanism is proposed below. Figure 11 shows schemes for lower and higher degree of curvatures of main-stalk lamellae, which grow originally from sheaf-like nuclei and evolve into dendritic and spiral-banded spherulites, respectively. In the range of film thickness used in this study (20−50 μm), the sheaf-like crystals are basically edge-on, which grow and continue to be edge-on to become the main lamellar stalks. In the case of 20−30 μm film samples (see Figure 11a), the PLLA lamellae first develop into early stage hexagon crystals, with a number of arms that can further grow and curve slightly into S-shaped dendrites (clockwise). These arms (called “ridge”) appear higher than other regions and are composed of main lamellae (edge-on). As the side branches evolve from the main stalks by intersecting at ca. 60° angle, they also flip into partial flat-on, which account for the lower topological
Figure 9. Growth rates of spherulites in (a) 80/20 PEO/PLLA and (b) 80/20 PEO/PDLA blends in two different thickness ranges of 20− 30 and 40−50 μm at Tc = 120 °C.
The above results indicate clear correlations between the dimensions available for superstructural growth and number of
Figure 10. SEM micrographs for interior crystals of 80/20 PEO/PLA blend crystallized at 120 °C: (a) PLLA and (b) PDLA, packed as (1) dendritic spherulites (20−30 μm) and (2) ring-banded (40−50 μm). Insets are the top surface images and color distribution schemes of the corresponding spherulites. H
DOI: 10.1021/acs.macromol.6b00350 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
thicker films or at low-to-intermediate Tc′ − with relatively higher growth rates), the lamellae are mostly bent to a high curvature, into spiral-banded spherulites, partially influenced by the chirality of the lactide unit. As a result, the optical patterns of counterclockwise or clockwise spiral-banded spherulites are characterized with periodic blue/orange bands that vary in the radial direction. Such finding is ground-breaking for interpreting the mechanism of periodic patterns in spherulites−not only spiral-banded (periodicity in radial direction) but also curveddendritic (periodicity in circumferential direction) spherulites that are commonly seen in various crystallization conditions. Another key feature in the spiral bands vs curved dendrites should be compared and emphasized. By referring to the two growth mechanisms illustrated in schemes of Figure 11, it should be emphasized here that the transition from curved dendrites to spiral bands in PLLA is associated with the changing degree of curvature of lamellae growing from the nuclei center. The two are closely related by the nature and share similar origins, but the bending curvature of main stalk lamellae during growth is governed by kinetic and/or thermodynamic influences. As an example, this study shows that larger available amount of molten PLA chains in thicker samples (in comparison to thinner film samples) increases the driving force and maintains a longer period of availability of PLA chains for crystallization to continue before draining out, thus higher degree of curvature results. However, when the degree of curvature is high enough and the growth is fast enough as in the thicker PEO/PLA film samples, the pathways of crystal growth are more likely distorted or rerouted (without bending preference in the ridge) and periodically terminated, allowing chirality to take firmer control in the valley bands as in the case of spiral-banded spherulites (Figure 11b). This work deals with films of PLA of L- or D-form in the presence of PEO at a certain level of film thickness, where spherulites are distinctly polycrystalline with various hierarchical levels of crystal structures. Reducing the thickness of film samples can have a significant effect on the morphology and crystallization rate of polymeric materials, see for instance a recent review of Prud’homme37 on the crystallization of morphology of ultrathin films as confinement, interactions with substrates and surface effects can become important. In the present case, an increase in film thickness can account for the changes in the bending sense that cannot be attributed solely to chirality effects. By comparison, in an earlier work on another crystallization system,5 it was found that a 50/50 PBA/ PLLA blend crystallized at 110 °C (with film thickness 170− 200 nm) versus a 50/50 PBA/PDLA blend crystallized at 120 °C developed counterclockwise Z-shape and clockwise S-shape curved-dendritic spherulites, respectively. The bending sense in this case (PBA/PLA blends) is just opposite to that for PEO/ PLLA (or PEO/PDLA). The results obtained here demonstrate that observation and analysis of only the top surface morphology and optical birefringence patterns of spiral bands may be misleading without taking into account the inner morphology of the sample. Interior dissection (after PEO extraction by water etching) using SEM was conducted on curved dendrites (20−30 μm) and spiral-bands (40−50 μm) samples of PEO/PLLA and PEO/PDLA. Interior dissection indicated that ridge and valley bands of curved-dendritic spherulites (20−30 μm) or spiral-banded spherulites (40−50 μm) are not connected by continuous twisting of a monotonous single crystal as conventional models proposed. Instead, the ridge band is composed of multiple
Figure 11. Schemes for nucleus-geometry induced growth with two curvatures of main-stalk lamellae of PLLA leading to dendritic (top) and helical banded spherulites (bottom).
height profiles of the valley bands. Optically, the schemes reflect that the alternating orange/blue bands are representative of the different crystal axes of main lamellae (ridge band) and side branches (valley band) according to the axes of the tint plate as shown in Figure 11a. For thicker samples (40−50 μm films), Figure 11b shows a scheme where growth begins with nucleation and the formation of PLA hexagonal crystals. Since the growth rate of spherulites is higher in thicker films, the bending occurs with a larger curvature as the crystals radiate out from the hexagons and such a larger curvature coupled with faster growth induces the formation of spiral growth. The curved lamellae thus make counterclockwise (PLLA) or clockwise (PDLA) spiral bands. Similar to the curved dendrites in Figure 11a, the side branches (blue arcs) within the spiral bands in Figure 11b continuously grow from the main stalks (red arcs). The bending main lamellar stalks constitute the “ridge” (collectively arranged in radial fashion) while the side branches from the main stalks, originally intersecting with the main stalks at 60− 90° angle, also bend into a curvature to form the valley bands (collectively arrange in tangential fashion). Optically, they display periodic blue/orange color bands under POM with a tint plate (or bright/extinction without tint plates). Depending on the composition, temperature, and film thickness, lamellae stalks of PLA can grow with either no curvature, medium or high curvatures. With no bending curvature (ρ = 0), the main stalks radiate out as straight lines from center to periphery; side branches fill the interlamellae regions by growing at ca. 60° angle to the main stalks. The optical patterns of straight dendritic spherulites are characterized by blue/orange stripes according to their long axes compared to the long axis of the tint plate (orange if perpendicular or blue if parallel). This usually occurs at high Tc (>120 °C) or ultrathin films (
