Anatomy into Interior Lamellar Assembly in Nuclei-Dependent

Sep 24, 2018 - Sophisticated dissection into the interiors of the three different birefringent types of poly(l-lactic acid) (PLLA) spherulites via del...
0 downloads 0 Views 3MB Size
Download PDF
Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

pubs.acs.org/Macromolecules

Anatomy into Interior Lamellar Assembly in Nuclei-Dependent Diversified Morphologies of Poly(L‑lactic acid) Yu-Ting Yeh and Eamor M. Woo* Department of Chemical Engineering, National Cheng Kung University, No. 1, University Road, Tainan 701-01, Taiwan

Downloaded via UNIV OF LOUISIANA AT LAFAYETTE on September 24, 2018 at 14:17:11 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Sophisticated dissection into the interiors of the three different birefringent types of poly(L-lactic acid) (PLLA) spherulites via delicate fracturing across the thickness sections, coupled with proper etching techniques, was adopted to reveal the correlations between the diversified birefringence patterns and interior lamellae assembly. The three types of spherulites are circularly ringed spherulite (type 1), hexagon-shaped axialite (type 2), and circularly core−stripe dendrites (type 3). Such morphological diversification originates from different nuclei geometries. For all three different types (circularly ringed, hexagonal, and core−stripe) of PLLA aggregation into spherulites or dendrites/axialites, the dissected inner lamellar arrangement shares universal commonality of intersecting at a 60° angle in the intersection between different lamellar species, with distinct discontinuity in the grating structures being characteristic of all three types of aggregated PLLA spherulites. This kind of grating assembly of the interior lamellae appears to be universally justified among many ring-banded polymer spherulites.

■

acid) (LMW-PLLA)/poly(ethylene adipate) (PEA),13 LMWPLLA/poly(trimethylene adipate) (PTA),13 etc. PLLA has been known to possess diversified morphology patterns when subjected to various crystallization conditions. When confined to nanometer thickness, PLLA display a singlecrystal morphology of lenticular (diamond) shape or hexagonal packing.14,15 Diamond-shaped single crystals of PLLA are usually solution grown, while hexagonal single crystals are by melt crystallization. For PLLA crystallized with film thickness in the ranges of micrometers or up to thick bulk, various other factors can influence the final crystalline morphology, such as molecular weight, Tc, film thickness, solvent evaporation rates, or blending with other components with varying interaction strength.16−19 Periodically banded morphology is often encountered in crystallized polyesters including PLLA; in addition to this interesting morphology, other peculiar patterns of morphologies also attract researchers’ attention. Nurkhamidah et al.20 employed a spin-coating technique to prepare PLLA/PBA blend of various compositions into thin films of 700 nm thickness, and for PLLA/PBA = 70/30 (w/w) and blend

INTRODUCTION Spherulites exist in a wide range of material systems: for example, organic polymers,1 polymeric selenium,2,3 minerals,4 synthetic ceramics, geological and biologically grown minerals and by ice, etc. Spherulites can also form many different morphologies. Commonly there are Maltese cross spherulites, ring-shaped spherulites, dendritic spherulites, and even more special morphologies such as hexagonal spherulites with central star-shaped spherulites, feather-like spherulites, leaf-shaped spherulites, flower-shaped spherulites, etc. Depending on levels of molecular weights of the biodegradable polymers, highmolecular-weight (HMW)-PLLA are used for surgery, while low-molecular-weight (LMW)-PLLA are used for drug delivery systems, tissue engineering, etc. The properties of the thermoplastics are used for the textile fiber industry. All those properties are highly correlated to the crystallization behavior of the polyester. In addition to mechanical and physical properties, biomedical compatibility with living cells is one of the main concerns for PLLA to be suited for intended purposes.5,6 Blends of PLLA with other polymers have been intensively investigated for miscibility and phase behavior, such as poly(L-lactic acid) (PLLA)/poly(D-lactic acid) (PDLA),7,8 PLLA/poly(methyl methacrylate) (PMMA),9 PLLA/poly((R)-3-hydroxybutyrate) (PHB), 10,11 HMW-PLLA/poly(ethylene oxide) (PEO),12 low-molecular-weight poly(L-lactic © XXXX American Chemical Society

Received: August 10, 2018 Revised: September 12, 2018

A

DOI: 10.1021/acs.macromol.8b01726 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

diluent for PLLA in crystallization to induce more regular assembly for enhancing the analysis of interior lamellae assembly of crystallized PLLA. As for sample preparation, PLLA and PTA were dissolved into CHCl3 (chloroform), made into 4 wt % polymer solution, and then cast on the glass slide as films of controlled film thickness for intended characterization/analysis purposes. Degassing was executed at 45 °C in a vacuum oven for 24 h. Samples were categorized as three different thicknesses: superthin film (∼1 μm), thin film (10−15 μm), and bulkier film (20−25 μm) samples. Superthin film samples (defined here as films of submicrometer thickness 30 wt %, the crystallized blend exhibited a phase-separated morphology, where PBA is crystallized into a ring-banded pattern but PLLA forms axialites. More interestingly, AFM analysis revealed that the PLLA axialites are composed of three layers of PLLA single crystals of lenticular shapes. Branches evolve straightly from the main stalks, making a 60° angle in consistency with hexagonal-shaped morphology. Ni’mah et al.21 introduced an ionic liquid (IL) into PLLA, and they found that crystallization of the PLLA/IL mixtures started from a tiny hexagon crystal nuclei into a star-shaped morphology with six protruded stalks. But finally all blank regions between the six protruded stalks are gradually filled by secondary crystallization, and the final fully crystallized morphology is reverted to a hexagon-shaped axialite. That is, the final PLLA spherulites are not spherical-shaped, but distinctly hexagonal with six stalks oriented exactly 60° to each other. Crystallized PLLA can display an even more puzzling morphology that might appear at first sight to be beyond comprehension. Such a dual-face PLLA spherulite is appropriately termed a “Janus-faced” spherulite,22−25 with “Janus” referring to the antient Roman myth god with two opposite faces. In 2014, Woo et al.23 investigated the PLLA/ PMMA (80/20) blend and found that PLLA crystallized in the blend can display three entirely different types of spherulites termed with codes type 1, type 2, and type 3, where type 3 PLLA spherulites are composed of one-half of type 1 and the other half of type 2, i.e., exactly a Janus-faced spherulite. Type 1 spherulites are composed of mainly strong-birefringence lamellae while type 2 ones are composed of weak-birefringence lamellae. The growth rate of the strong birefringence lamellae is faster than that of the weak birefringence ones. In 2017, Lugito et al.25 further investigated the details of the lamellae that comprises the three types of PLLA spherulites; they found that the Janus-faced PLLA spherulite might be related to different crystal lattices, and the lamellae species in these two halves of spherulites differ significantly in sizes and orientations. The growth mechanism of polymer spherulites is an important part for understanding polymer crystallization even though it is a very complicated process from nucleus to spherulite.26,27 For purposes of better dissecting the interior morphology and easier etching off the amorphous constituents to expose the crystalline skeletons, PLLA was blended with poly(trimethylene adipate) (PTA) for crystallization with PTA as an interacting diluent. PTA was chosen as it acts as a miscible diluent during PLLA crystallization to induce complex diversification of spherulite morphologies that otherwise are not present in neat PLLA, and PTA could be easily etched off the crystallized films to expose the interior PLLA crystalline lamellae for enhancing capacity of analysis on the finer details of lamellae assembly. Eventually, exact 3-D models are demonstrated for the interior and top-surface relief patterns of the periodic patterns of crystalline lamellae assembly to illustrate the mechanisms.

■

EXPERIMENTAL SECTION

Materials and Procedures. Poly(L-lactic acid) (PLLA) is a semicrystalline biopolymer purchased from Polysciences, Inc. (USA), with Mw = 11000 g mol−1, Tg = 45.3 °C, and Tm = 155 °C. Poly(trimethylene adipate) (PTA) was obtained from Scientific Polymer Products, Inc. (USA), with Mw = 8900 g mol−1, Tg = −63 °C, and Tm = 38 °C. PTA is miscible with PLLA, and it was used as a B

DOI: 10.1021/acs.macromol.8b01726 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 1. POM micrographs of PLLA/PTA blend fully crystallized at Tc = 110 °C of compositions (weight ratio): (a) 100/0, (b) 90/10, (c) 85/ 15, (d) 80/20, (e) 75/25, (f) 60/40, (g) 50/50, (h) 40/60, (i) 25/75, (j) 20/80, (k) 15/85, and (l) 10/90 by quenching from Tmax = 190 °C. All specimen film thicknesses kept in the range of ca. 10−15 μm.

°C for 2 min to a uniformly homogeneous molten state. They were then rapidly removed to the microscopic heating stage preset at a fixed Tc = 110 °C until full crystallization, and comparison of morphology was made for the PLLA/PTA blend of various compositions = 100/0 to 10/90 (i.e., the PTA diluent content in PLLA/PTA blend was increased from 0 to 90 wt %). Figure 1 shows that POM micrographs for PLLA/ PTA = 100/0−60/40 (at Tc = 110 °C) all display ring-banded optical patterns, where the banded spherulites’ size and interband spacing both decrease, and not increase, with higher PTA contents in the PLLA/PTA blend. This is quite opposite to commonly expected trends where higher contents of amorphous diluent should normally increase, and not decrease, the spherulite size and interband spacing of spherulites of the crystalline component with which the amorphous constituent is blended. This abnormality actually reflects that the spherulitic morphology of the PLLA/PTA blend can go through a transition from a circularly banded one into a straight dendritic pattern by reducing the banded core with increase of the amorphous PTA constituent. The “stripe spherulites”, upon closer and more careful observation, actually are composed of a well-rounded banded core, with the outer region filled with radiating striped lamella; thus, this peculiar PLLA spherulite has a resemblance of “a sun with radiating rays”. Such a morphology is appropriately termed as “bandedcore/stripe” mixed spherulites. As the central well-rounded core shrinks to minimal size, it becomes simply a “stripe” spherulite. Interestingly, with increase of PTA constituent from 10 to 40 wt % in PLLA/PTA blend, the banded core shrinks in size steadily, and so does the interband spacing in the banded core; conversely, the stripe pattern grow more pronouncedly with increase of PTA composition. With further increase of PTA content to 50 wt % or higher in the PLLA/PTA blend, the crystallized PLLA specimens are

of PLLA/PTA samples were etched by methanol for 3 min and then dried at room temperature. Wide-Angle X-ray Diffraction (WAXD). The XRD patterns for bulk samples of neat PLLA and the blends were obtained using a wideangle X-ray diffraction (Shimadzu, XRD-6000) with Cu Kα radiation (30 kV and 40 mA), measured in the 2θ range of 14°−24°, and the rate was set at 2° min−1. Differential Scanning Calorimetry (DSC). Thermal analysis of the PLLA/PTA blend was characterized with a differential scanning calorimeter (PerkinElmer, Diamond DSC) equipped with a mechanical intracooler for quenching. Samples were uniformized in DSC cells by heating to 190 °C, held for 2 min, rapidly quenched to various crystallization temperatures (Tc) for different crystallization times (tc), and then scanned from Tc to above Tm at a heating rate of 10 °C min−1. A continuous nitrogen flow in the DSC sample cell was maintained. Atomic Force Microscopy (AFM). The top-surface morphology of crystallized PLLA/PTA blend films was investigated with an atomic force microscope (diCaliber, Veeco Corp., AFM) in intermittent tapping mode with a silicon tip (f 0 = 70 kHz, r = 10 nm) installed to observe the top-surface topology as well as measurement of height profiles.

■

RESULTS AND DISCUSSION An earlier work13 preliminarily investigated the PLLA/PTA blend of various compositions to understand the basic phase behavior and crystallization kinetics, and it has been revealed that there is a single phase of miscibility with Tg−composition dependence. The melting points of PLLA and PTA (Tm) are respectively 155 and 38 °C. PTA is actually low melting and barely crystallizable even in its neat form; thus, it can be feasibly regarded as being noncrystalline and remaining fully amorphous when blended with PLLA at a 50/50 ratio. Effects of compositions of the amorphous PTA in PLLA/PTA blend on the crystalline PLLA morphology were first examined. Samples of various compositions were all heated to Tmax = 190 C

DOI: 10.1021/acs.macromol.8b01726 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules no longer circularly banded or “banded-core/stripe” mixed spherulites. For the PLLA/PTA blend of higher PTA contents, from Figure 1, one sees clearly that for PLLA/PTA = 50/50− 10/90 compositions the PLLA/PTA (50/50) is the onset composition where the banded PLLA spherulites turn into completely stripe-dendritic ones. With the PTA content increasing in the PLLA/PTA blend, a transitional PLLA spherulite pattern is noted at an intermediate PLLA/PTA composition = 50/50 (wt ratio), where the banded core shrinks to its minimal (one band) completed with the radiating stripes being the dominant optical patternnamed as “stripe spherulite”. With the PTA content further increasing to 60 wt % (i.e., PLLA = 40 wt %), an additional type of PLLA spherulites is recorded in the crystallized blend specimens, which is a ringless hexagonal pattern coexisting with the preexisting banded core/stripe PLLA spherulites. With PTA = 75 wt %, a more striking phenomenon is observed in the crystallized PLLA/PTA specimen, which is that all previous core/stripe and hexagonal spherulites both disappear and only a Janus-faced (dual-face-mixed) PLLA spherulite is present in the crystallized PLLA/PTA specimens. Here, the “Janus-face” spherulite refers to the ancient Roman Janus god’s two oppositely different faces. The Janus-face PLLA spherulite is actually a half-and-half mix of the banded core and hexagonal PLLA spherulites on two roughly equal but opposite faces, and these two optical patterns in a single spherulites are merged half-and-half (Figure 1i). For the PLLA/PTA blend with PTA = 90 wt % (10% PLLA), the crystallized spherulites are all of a regular hexagonal shape and are better named here as “axialites” to distinguish from “spherulites”. Note that for the PLLA/PTA (10/90) blend the crystalline PLLA content is quite low, and thus the optical birefringence retardation of the hexagonal PLLA axialites is naturally low but still recognizable by POM. To summarize the above variation of spherulites birefringence patterns with respect to compositions, Figure 2 shows

only by increase of the amorphous PTA constituent in the PLLA/PTA blend. The variety of the PLLA spherulites’ morphology is only caused by presence and amount of the amorphous diluent PTA in the PLLA/PTA blend. There are at least five major different types of PLLA spherulites (all at Tc = 110 °C) in the PLLA/PTA blend of various compositions: fully circularly ring-banded (type 1), fully ringless/hexagonal (type 2), stripe (type 3), ringed core/stripe, and Janus-face (half-stripe/half-hexagon), as summarized in Figure 2. Note that the ringed core/stripe and Janus-face PLLA spherulites are just a combination of two of the three main types (types 1−3), and thus these two composite types are not coded with numbers. With the exception of the Janus-mixed type that is actually a combination of two known types (stripe and hexagonal types), the other three different types of PLLA spherulites/axialites are coded as type 1 (circularly ring-banded), type 2 (hexagonshaped), and type 3 (core/stripe dendrites). Further analysis on the lamellae assembly will focus on these three main different types of PLLA spherulites. Lamellar assembly must be correspondingly different for these wildly diversified PLLA spherulites. Driving forces and factors accounting for the morphology have to be probed systematically. Analysis needs to involve characterization from the early stages of nucleation as well as the later stages of fully packed spherulites. As shown earlier, the PLLA spherulitic morphology goes through a systematic transition with respect to increasing PTA contents in the PLLA/PTA blend, with 50/50 being a cutoff composition (between circularly ring-banded and stripe spherulites). This work thus used the PLLA/PTA (50/50) blend as a focus of deeper examination. The specimen was heated to Tmax = 190 °C for 2 min to erase all prior thermal histories/nuclei and then quenched to various Tc = 95−120 °C until full crystallization. Figure 3 shows that at Tc = 95 °C the PLLA spherulites are ringless and of a negative-type birefringence; at Tc = 100 and 105 °C, the PLLA spherulites become distinctly circularly ringed type. At Tc = 110 °C, the crystallized specimens are still of a ring-banded types, yet there are straight-stripe lamellae emerging and growing in the radial direction on the periphery of the circularly ringed central core. The composite of outer stripes and central circular core makes them resembling a sun with radiating rays, and this type is coded as “circular core−stripe” spherulite. One can easily discern from Figure 3 that with increasing Tc the central ringed core steadily shrinks; finally at higher Tc (115 °C), the central ringed core shrinks to the nuclei region, and the entire spherulites are now fully straight-striped without any ringed core. At Tc = 115 and 120 °C, an additional type of aggregation (hexagonal PLLA axialite) appears that coexists with the pre-existing stripe PLLA spherulites. At even higher Tc = 125 and 130 °C, the PLLA spherulites become all fully dendritic and are properly termed as “dendrites” where the hexagonal and striped types (types 2 and 3, respectively) all disappear. As shown, both Tc and blend composition could influence the final PLLA spherulitic morphologies, and it would not be realistic to fumble through various Tc or compositions for all possibilities. Thus, following interior analysis to understand the inner lamellae assembly would be focused on specimens of PLLA/PTA (50/50) crystallized at Tc = 105 and 115 °C, respectively, as these two intermediate Tc’s led to three uniquely main different types of PLLA spherulites covering the circularly ringed, hexagonal, and striped (with various sizes of

Figure 2. Schemes of diversified lamellae aggregation into five patterns of PLLA spherulites/axialites in PLLA/PTA blends of various weight ratios all crystallized at the same Tc = 110 °C.

illustrative schematics (drawn according to respective POM data) for the composition-dependent evolution into five morphologies of the PLLA spherulites or axialites, all crystallized at a fixed Tc = 110 °C, which vary from a wellrounded and ring-banded optical pattern to the core−stripe one, then to coexistence of core−stripe and hexagonal patterns, further to a Janus-face pattern (half-stripe/half-hexagon), and finally to a fully hexagonal pattern. Note that Tc was fixed at the same 110 °C, and the morphology evolution from ringbanded to fully hexagonal (also ringless) pattern was induced D

DOI: 10.1021/acs.macromol.8b01726 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

comparably much smaller than that of type 3 (circularly core− stripe). The number fraction of type 2 spherulites is ca. 1.7− 5.8%, which was estimated by counting the number of type 2 divided by the total number of all PLLA spherulites (type 2 + type 3). As Tc is further increased to 120 °C, there are still two types of PLLA spherulites, but these two types at Tc = 120 °C differ from the two types at Tc = 110−115 °C. Two different types of PLLA spherulites crystallized at Tc = 120 °C are the stripe type (type 3) and the Janus-faced type (composed of a half type 2 and another half type 3). At the higher Tc = 125− 130 °C, all type 1, 2, and 3 spherulites disappear, and the only PLLA spherulites are irregularly dendritic. The complex evolutional changes of the PLLA spherulites with Tc are better represented by simplified schemes. Figure 4A summarizes the systematic evolution of optical birefrin-

Figure 3. POM micrographs of PLLA/PTA (50/50) blends crystallized at a series of Tc = (a) 95, (b) 100, (c) 105, (d) 110, (e) 115, (f) 120, (g) 125, and (h) 130 °C by quenching from Tmax = 190 °C. Insets on top of each of micrographs are schematics for birefringence patterns. All specimen film thicknesses kept in the range of ca. 10−15 μm.

central ringed cores), respectively. As discussed, crystallization of PLLA/PTA (50/50) at Tc = 105 °C resulted in circularly ring-banded PLLA spherulites only (coded as type 1), but crystallization of the PLLA/PTA (50/50) blend at Tc = 115 °C both led to two coexisting but entirely different types of PLLA spherulites: hexagon-shaped axialite (type 2), and circularly core−stripe spherulite (coded as type 3). POM data were recorded on the crystallized PLLA/PTA (50/50) blend by varying Tc for each 1 °C increment, and the spherulite patterns were recorded, as shown in the Supporting Information (Figures S-1 and S-2). Images were recorded on specimens crystallized at Tc starting from 110 to 125 °C for every 1 °C increment. From the closely spaced morphology evolution, one can easily discern that between Tc = 110 and 115 °C the ringed core−stripe spherulites contain a ringed core, but the core size and number of rings steadily decrease with every 1 °C increase. Finally, the core shrinks to a minute size, and the PLLA spherulite becomes fully striped (type 3). In the meantime, hexagonal type 2 PLLA axialites appear along with type 3 ones when crystallized at these Tc’s. It should be noted that although two types of PLLA spherulites coexist at these Tc’s, the number fraction of type 2 (hexagon axialite) is

Figure 4. (A) Schemes of PLLA spherulite birefringence morphology of PLLA/PTA (50/50) blends crystallized at various Tc ranges: (A, top) 100−130 °C and (A, bottom) 110−119 °C. (B) Schemes of variation of PLLA birefringence patterns with respect to sample film thickness (y-axis) and crystallization temperature (x-axis).

gence patterns of PLLA spherulites in crystallized PLLA/PTA (50/50) at various Tc’s. Figure 4A (top row) shows the evolving birefringence patterns of PLLA spherulites by varying Tc from 100 to 130 °C. A systematic transition from circularly banded type to ringless dendritic types is obvious, with the ringed portion steadily shrinking from full spherulites to only near the central nuclei region and in the meantime the outer stripe lengthening to occupy the entire spherulites. Figure 4A (bottom row) shows details of Tc dependence of the central ringed core between Tc = 110 and 119 °C, where the ringed E

DOI: 10.1021/acs.macromol.8b01726 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules core decreases in size with increasing Tc but the straight-stripe lamellae grow longer to replace the retracting ringed-core portion. At Tc = 119 °C, a peculiar Janus-face spherulite appears, which is a half-and-half mix of the hexagon-shaped and stripe PLLA spherulite. Figure 4B further summarizes the evolution of the optical birefringence patterns of the PLLA spherulites in the PLLA/PTA (50/50) blend, where the y-axis displays a range of film thicknesses from 1 to 25 μm and the xaxis displays a range of Tc from 100 to 120 °C. For the film thickness kept within the two levels of 10−15 or 20−25 μm, the morphologies of the PLLA spherulites are the same at the same Tc, which is ring-banded at Tc = 100−105 °C, striped (or stripe/core) at Tc = 110 °C, and finally, hexagon-shaped at Tc = 115 °C or higher. By distinct contrast, the PLLA/PTA (50/ 50) specimens in film thickness range of very thin level (ca. 1 μm) crystallized at all Tc = 100 , 105, and 110 °C, the PLLA spherulites have very low birefringence intensity/retardation, and no diversified PLLA morphology is present. This work did not attempt to deal with ultrathin films in range of a few nanometers, as specimens in the ultrathin range do not display visible optical birefringence, and no diversified PLLA spherulites patterns can be present. Apparently, the PLLA spherulites’ morphology is intimately influenced by various kinetic factors that governs the inherent way the lamellae are packed to final spherulites. Tc certainly is one of the most dominant kinetic factors governing the growth and packing of the spherulites. The film thickness of the PLLA/PTA specimens might also influence the final morphology. All specimens used in this work had film thickness controlled within the 10−15 μm range. Figure S3 shows detailed POM results for two other levels of film thickness of the PLLA/PTA specimens: very thin and thick films being 1 and 20−25 μm, respectively. In situ recordings of growth of type 1, 2, and 3 PLLA spherulites in PLLA/PTA (50/50) thin-film specimens at selected Tc = 105 and 115 °C were done using POM CCD. In situ images of type 1 PLLA spherulites at Tc = 105 °C are summarized in Figure S4, and in situ monitoring of POM patterns of type 2 and type 3 PLLA spherulites were done at a higher Tc = 115 °C, which are shown in Figure S5. The in situ recordings of growth of three different PLLA spherulites are condensed with representative schematics. The growth rates of these three types of PLLA spherulites were measured using in situ POM recording, whose results are summarized in Figure S6. From the plots, the growth rates of type 1 spherulites were measured at Tc = 105 °C (type 1 could appear only at Tc = 105 °C), but type 2 and 3 spherulites were both measured at same Tc = 115 °C. The growth rates were 1.14, 1.15, and 1.20 μm/ min, respectively, differing insignificantly for these three types of PLLA spherulites. This results suggests that the growth rate does not differ much, which can be ruled out as a possible influencing factors for leading to different PLLA spherulite morphology. What governs the final spherulite morphologies may be just the different nuclei geometries that were allowed to form at different Tc’s. Figure 5 summarizes schematics for PLLA morphological evolution (from initial nuclei to fully filled spherulites) of three major types (types 1−3) of PLLA spherulite. The nuclei crystals for different types of PLLA spherulites were initially very different in geometry shapes. Nuclei for type 1 PLLA are initially spherical dots, while those for type 2 spherulites are of a windmill geometry with hexagonal rim; those for type 3 spherulites are initially dots with a zigzag rim. As they grow,

Figure 5. Schematics for PLLA morphological evolution (from initial nuclei to fully filled spherulites) of (a) type 1 spherulite at Tc = 105 °C and (b) type 2 and (c) type 3 spherulites at Tc = 115 °C by quenching from Tmax = 190 °C.

finally the nuclei of different geometry lead to three entirely different types of PLLA spherulites: type 1 (circularly ring banded), type 2 (hexagonal axialite), and type 3 (core−stripe). It was expected that the initial nuclei geometric shapes, as formed at different Tc’s, might govern the growth, lamellae assembly, and final spherulites’ morphologies. We thus utilized a technique to “freeze” the initial nuclei for subsequent POM characterization. The PLLA/PTA (50/50) specimens (film thickness = 10 μm) were heated to Tmax = 190 °C for 2 min and rapidly quenched to Tc = 105−115 °C. Crystallization was allowed until early emergence of the nuclei identifiable in POM; subsequently, the specimens were dipped into liquid N2 to freeze the growth. Note that once the PLLA/PTA specimens were rapidly quenched from Tc (105−115 °C) to ambient or liquid nitrogen, the polymer chains were immediately “frozen”, and no secondary or postcrystallization growth could proceed further. Figure 6 shows (I) POM/SEM micrographs and (II) schematics of the initial nuclei for leading to type 1, type 2, and type 3 PLLA spherulites. As viewed in POM micrographs (insets on top of SEM micrographs), the nuclei have different birefringence patterns for type 1, type 2, and type 3 PLLA spherulites. Noticeably, the nuclei leading to eventually three different types of PLLA spherulites are distinctly different in their emerging geometric shapes from the early stages of nucleation. The nuclei for type 1 PLLA spherulites were further analyzed using SEM, which revealed a bent sheaf-like lamellae in the nuclei center (Figure 7A-I). The nuclei lamellae twist in circular circumferential direction, whose continuing growth could be expected to result in periodic banded patterns. The nuclei for type 2 spherulite display a windmill shape with blades twisting in the counterclockwise direction. Correspondingly, in SEM micrograph (Figure 7B-I), the nuclei for type 2 PLLA spherulite are also distinctly different from that for nuclei of type 1 (Figure 7A-I). Upon further growth from the respective nuclei, we will show later that the nuclei for type 1 grow into circularly ringbanded spherulites while the nuclei for type 3 will grow into ringless circularly striped spherulites (though with zigzag periphery). Before the lamellae assembly into aggregation patterns was analyzed, the crystal lattices in these various aggregated PLLA F

DOI: 10.1021/acs.macromol.8b01726 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 6. (I) SEM micrographs (POM graphs as insets for birefringence patterns) and (II) schematic illustration, of the initial nuclei for (A) type 1 (Tc = 105 °C), (B) type 2 (Tc = 115 °C), and (C) type 3 (Tc = 115 °C) PLLA spherulites/axialites in the crystallized PLLA/PTA (50/50) blend. All specimen film thicknesses kept in the range of ca. 10−15 μm.

Figure 7. WAXD diffractograms of the (A) neat PLLA and (B) PLLA/PTA (50/50) blend fully crystallized at different Tc (70−130 °C) by quenching from Tmax = 190 °C.

spherulites/axialites/dendrites were first probed. Small-angle X-ray scattering (SAXS) analysis, via microbeam, was not considered in this work, as for the 3D lamellae assembly in the diversified PLLA spherulites, the hierarchical orders are hundreds of nanometers up to micrometers; yet, SAXS is useful typically for orders of a few to tens of nanometers. However, the crystal lattices in lamellae packed in type 1−3 spherulites in the PLLA/PTA (50/50) blend were analyzed using WAXD on specimens crystallized at Tc = 70, 80, 90, 105, 115, and 130 °C. For comparison, crystal lattices in in the neat PLLA crystallized at the same Tc’s were also examined. It has been known that neat PLLA crystallized at Tc < 100 °C exhibits only the α′-form,28−31 while above Tc > 120 °C, it shows α-form modification, with the peaks of α′-form to αform differing by 2θ = 0.2°. Figure 7A shows WAXD characterization result for neat PLLA at Tc (70, 80, 90, and 105 °C); for comparison, the WAXD results for the crystallized PLLA/PTA (50/50) blend are shown in Figure 7B. For specimens at all lower Tc’s (70, 80, and 90 °C), the (110/200)

planes show diffraction peaks 2θ = 16.60°; for specimens crystallized at Tc = 105 °C, the 2θ value for these same planes upshifts a little bit. For the PLLA/PTA (50/50) blend crystallized at 115 and 130 °C, the 2θ values are almost same as the specimens crystallized at Tc = 105 °C; that is, the 2θ values of (110/200) planes for specimens crystallized at Tc = 105, 115, and 130 °C are all 16.66° (α-form) with almost no difference. The inclusion of amorphous PTA in crystallizing with PLLA has an effect of more easily packing directly into the ordered α-form even at Tc lower than 110 °C. Other than that the crystal lattices in neat PLLA and PLLA/PTA do not differ; thus, the higher hierarchical lamellae assembly differences in leading to three spherulite morphologies are not a result of the lattice form difference. Plausibly, the PLLA conformations in the PLLA/PTA (50/50) mixture at Tc = 105 vs 115 °C allowed the PLLA chains, via interactions with PTA diluent, to be nucleated into more diversified geometries. In addition, thermal behavior of the three types of PLLA spherulites/axialites crystallized from the PLLA/PTA blend G

DOI: 10.1021/acs.macromol.8b01726 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules (50/50) at various Tc’s (105−130 °C) was also analyzed using DSC, which are collected in the Supporting Information. Figure S7 shows DSC thermograms, with the measured Tm and ΔHf values listed at the bottom of the figure. For PLLA/PTA specimens crystallized at Tc = 105 and 115 °C, the DSC scanning reveals double melting peaks (split between 142 and 146 °C); for specimen crystallized at the higher Tc = 130 °C, the double melting peaks merges into a single one (147.4 °C). However, these phenomena are typical as semicrystalline polymers are annealed at different Tc’s. The ΔHf values differ very little among these three specimens crystallized at Tc = 105, 115, and 130 °C. Thus, the melting peaks appear to be not much associated with the complexity of the types of PLLA spherulites in the crystallized PLLA/PTA blend. The lamellae in high hierarchical PLLA spherulites are complex, and the correlation between the thermal behavior and types of PLLA spherulites is only regarded as preliminary and needs future justification using more refined characterization and analysis. Lamellae Assembly in Type 1 (Circularly Ringed) PLLA Spherulites. Specimens for SEM characterization were properly etched with methanol to remove PTA constituent overlayer from the PLLA lamellae. Figure 8 shows the SEM

lamellae directly underneath the top-surface ridge and valley regions might be assembled in a completely different fashion from that of those lamellae on the top surface. Dissection into the interiors of the ring-banded PLLA spherulites via delicate fracturing across the thickness section was thus necessary, and results are to be discussed in following figures. Specimens of crystallized PLLA/PTA (50/50) blend were fractured to expose the lateral interior of the banded PLLA spherulites, which were further etched with methanol prior to sputter-coating for SEM characterization. Note the crystallized specimens of PLLA/PTA (50/50) were carefully etched with solvent to remove amorphous PTA or PLLA and to expose the interior intact PLLA lamellae. Without solvent etching, results for the specimens upon SEM characterization are shown in Figure S8, which apparently does not reveal much of the interior details. Figure 9a shows the SEM micrographs for the

Figure 9. SEM micrograph of the type 1 spherulite (circularly ringed type) in PLLA/PTA (50/50) blends after isothermally crystallization at 105 °C and quench to ambient temperature: (a) methanol-etched fracture and top surface; (b) schematic illustration of interior lamellae for Type 1 PLLA spherulite. All specimen film thicknesses kept in the range of ca. 20−25 μm.

Figure 8. SEM graphs of the type 1 spherulite in PLLA/PTA (50/50) blends after isothermally crystallized at 105 °C and cooled to ambient temperature: (a) top-surface morphology of methanol-etched samples, (b) zoom-in of the area marked by square in (a), (c) schematic illustration of top surface for type 1 spherulite (circularly ringed type), (d) POM graph. All specimen film thicknesses kept in range of ca. 10−15 μm.

exposed interiors of the circularly ring-banded PLLA spherulites (solvent-etched), which exhibit a grating lamella structure. From the micrograph, the interior PLLA grating-like lamellae are assembled periodically alternating parallel/vertical to the glass substrate, with the periodicity equal to ca. 70 μm (exactly equal to the optical interband spacing). Lenticular cracks are oriented parallel to the interior lamellae (or lamellar bundles), which assist in revealing the assembly patterns of the interior lamellae underneath the top-surface-relief morphology. Apparently, the interior vertically oriented lamellae (or lamellar bundles) are situated directly underneath the top-surface ridge band, and the interior horizontally aligned lamellae (or bundles) are situated directly underneath the top-surface valley band. Note that the lamellae “parallel” to the glass substrate are not exactly straightly flat but actually curve into a concave shape (or bowl shape). The interface between the vertical and parallel lamellae are characterized by a rough 60° angle intersection, which suggest that the horizontal lamellae

micrographs of type 1 (circularly ringed) PLLA spherulite in PLLA/PTA (50/50) blend isothermally crystallized at Tc = 105 °C and then cooled to ambient (28 °C). Characteristic radial-oriented cracks are located on the ridge band of the ringbanded PLLA spherulites, and the radial-oriented slender cracks suggest the lamellae in the ridge band are also radially oriented. The interband spacing (measured from midline of a valley band to midline of next valley band) is ca. 70 μm (Tc = 105 °C). Figure 8b shows that the thicker and radially oriented ridge lamellae, upon reaching the valley region, bend immediately 90° angle to align along the circumferential valley, which is also illustrated in scheme of Figure 8c. However, it must be cautioned that although superficially “bending 90° angle” is characteristic of the lamellae on the top surface of the ring-banded PLLA spherulites, the interior H

DOI: 10.1021/acs.macromol.8b01726 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 10. (a) SEM graph of top surface of type 2 [hexagonal-shaped] PLLA axialite for methanol-etched in PLLA/PTA (50/50) blends after isothermally crystallized at 115 °C and cooled to ambient temperature. (b) Schematic of top-surface relief for type 2 spherulite. (c) POM graph. Specimen film thicknesses kept in range of ca. 10−15 μm.

(underneath the valley band) are likely branching crystals evolving from the vertical-oriented lamellae (underneath the ridged). Obviously, the lamellae in the ridge and valley are not continuous, and a distinct interface between the ridge and valley is present. Figure 9b illustrates the schematic of the interior lamellae assembly in type 1 circularly banded PLLA spherulites, as viewed and analyzed from the microscopic SEM evidence. On the basis of this dissected microscopic evidence on the type 1 banded PLLA spherulites, we can now propose that the growth starts with sheaf-like nuclei that twist and bend at both ends continuing into a double-spirals (or mathematically “Fermat’s spirals”), which constitute the vertical lamellae stalks in the ridge band. As the vertical lamellae stalks grow spirally, finer fractal branches evolve by initially intersecting at 60° angle with the vertical lamellae, but finally bending slightly upward to become roughly parallel (except for the two concave ends) to the glass substrate, forming a concave U-shaped between two successive lamellae spirals. These branching lamellae constitute the parallel lamellae underneath the “valley band”. Such similar grating structure in the type 1 ringed PLLA spherulite is also found in ring-banded poly(ethylene adipate) (PEA) spherulites32 crystallized at Tc = 28−29 °C; however, the periodicity of interband spacing for the ring-banded PEA spherulites is much smaller at ca. 6.7 μm (and is also exactly equal to its optical interband spacing). Thus, the grating assembly of interior lamellae appear to be universally common among many ring-banded polymer spherulites,33−36 where the only major difference may be just in the interband spacing for different polymers. Lamellae Assembly in Type 2 (Hexagon-Shaped Axialite) PLLA Spherulites. Top-surface relief morphology of the hexagon-shaped PLLA axialite (type 2) was first analyzed using SEM. Figure 10 shows SEM and POM micrographs for the top surface of type 2 PLLA spherulite (hexagon-shaped axialite) of thin-film specimen (methanoletched) of PLLA/PTA (50/50) blend isothermally crystallized at 105 °C. The SEM micrograph in Figure 10a clearly shows that six main stalks hexa-sectioning the entire PLLA spherulites, with the interstalk regions being filled with finer lamellae that actually are branches evolving from the main stalks. To simplify the results, Figure 10b shows a traitsimplified scheme for the hexagonal-shaped PLLA axialite, with alternating birefringence color as viewed in POM micrograph (Figure 10c), where six main-stalk lamellae spray out from the nuclei center in six directions and bisect each other with almost

exactly 60° angle. Note that the evolving branch lamellae always intersect at 60° angle with the right-hand-side main stalk but are parallel with the left-hand-side main stalk. Thus, optically, the birefringence color of the secondary fine branches is always the same as that of the left-hand-side main lamellae but strikingly differs from that of the right-hand-side main lamellae (POM in Figure 10c). Via delicately controlled solvent etching, the nuclei crystals of the hexagon-shaped axialites were more visibly exposed, showing a tiny windmilllike ensemble that further extends into larger/longer six main stalks. The dominant straight growth of the six stalks, with the secondary and slower growth of the interstalk branches to fill the expanding space, leads to a regular hexagon-shaped axialites. The fractured interior lamellae in type 2 PLLA spherulite (hexagon-shaped) were then analyzed. Fracturing randomly cut across any section of a spherulites, and a specimen happened to have a fracture across region near nuclei, as shown in SEM micrographs in Figure 11. The lamellae in the interior of type 2 PLLA spherulites, resembling that in type 1 PLLA, display a grating structure. However, the grating ensemble structure of type 2 axialites (hexagon-shaped) differs from that of the earlier discussed type 1 (circularly ringed type).

Figure 11. (a) SEM graph of top surface of type 2 PLLA spherulite [hexagon-shaped axialite type] methanol-etched in PLLA/PTA (50/ 50) blend after isothermal crystallization at 115 °C and cool to ambient temperature. (b) Schematic illustration of top surface of type 2 spherulite. Specimen film thicknesses kept in range of ca. 10−15 μm. I

DOI: 10.1021/acs.macromol.8b01726 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

(or inter-“lamellar bundles”) regions are voided with slender cracks, which are due to solvent-etching out the amorphous constituents. The etching has enhanced the morphological contrast for better viewing the interior crystalline lamellae assembly. Each of the stripes is composed of actually a main stalk lamella (oriented vertically to the substrate) and numerous side branches growing at a 60° angle with respect to the main stalk and thus also a 60° angle to the substrate. Figure 12c,d shows the AFM height image and height profile, and each of the stripes has a main lamella stalk protruding out to form a ridge. Figure 12d reveals that the interstripe region has a depth drop to form a valley between neighboring main ridge stalks. By combining the POM, SEM, and AFM characterization results, the alternate grating structure is responsible for the stripe optical colors of the type 3 PLLA spherulite. The results have proved that the lamellae in these three different PLLA spherulites types (types 1−3) all share similarity on the lamellae assembly mechanisms to display a variety of optical birefringence patterns; yet, the lamellae assemble themselves in different ways that might be driven by the initial nuclei geometric shapes at different Tc or blend compositions. With the interior lamellae assembly clearly revealed, we now can propose a growth mechanism for type 3 (circularly striped) PLLA spherulites as follows. The growth starts from bundled strawlike sheaves that twist (in counterclockwise direction), and the initial geometric shape of the nuclei sheaves resemble sunflower petals, which evolve into multiple main stalks, with branches growing in the lateral sides of the stalks intersecting again at a 60° angle. All branches grow at counterclockwise direction at a 60° angle intersection with the main stalks. The 60° angle intersection between the branches and main stalks is in agreement with the nature of the orthorhombic lattices of PLLA single crystals.20 Both main stalks (in radial direction) and side branches (in circumferential direction) may further grow in fractal patterns to evolve into thicker bundles. Thus, each of the main stalks and lateral branches form a gradually fan-out pattern resembling a slender triangle with the base being ca. 35 + 13 = 48 μm (refer to Figure 12b schematic) and the tip being the origin of the nuclei. Multiples of these slender triangles, if aligned around the origin, form the stripe (type 3) PLLA spherulite. The lamellae assembly from initial nuclei geometry into the type 3 stripe PLLA spherulites are summarily illustrated in Figure 13a−c, where Figure 13a shows the central nuclei and their gradual evolution into a grown stripe spherulite with multiple main stalks (blue color) and branches (orange color) filling the interstalk regions all intersecting at 60° angle and fanning out in counterclockwise direction from the main stalks. Figure 13b shows when type 3 (circularly striped) PLLA spherulite is viewed in POM with a tint plate, the main stalks display blue-birefringence color and the branches display orange color, whose actual POM micrograph is shown as Figure 13c. The general features of interior assembly with grating lamellae structures of type 3 (circularly striped) PLLA spherulite are similar to that of type 2 hexagon-shaped axialite, except that the stalks in type 3 core−stripe spherulite are much more numerous (ca. 30−40 main stalks radiating out from a common center) and highly jammed/crowded, leading to a circularly stripe spherulite with a diamond-tip zigzag periphery of the spherulite. By contrast, type 2 PLLA hexagonal axialite has only six main stalks that grow preferentially in six

Schematic in Figure 11b illustrates that the branching lamellae in one of the six sections are oriented at roughly 60° angle with those in the neighboring section. Such birefringence color alteration in the six sections of the hexagon-shaped PLLA axialites is clearly demonstrated in the POM micrographs (and schemes) as discussed earlier (Figure 4). The analysis on the dissected interior lamellae of the hexagon-shaped (type 2) PLLA axialites clearly exemplifies the universally applicable mechanism of crystal assembly behind the optical birefringence patterns. By starting from the nuclei of crystallization at Tc = 115 °C, the growth from initial to final fully packed type 2 (hexagonal) PLLA axialites can now be summarized. The growth starts from a tiny aggregate of windmill-shaped crystal sheaves (nuclei), which dominantly grow in six directions equally sectioned into six main stalks, followed with secondary branches growing at 60° angle with respect to the main stalks to continuously fill the ever expanding space until exhaustion of available PLLA molten-chain species. When fully grown and completed, a nearly regular hexagonal axialites with six main stalks resulted, with the interstalk regions being filled with finer branches all collectively aligned at a fixed angle (60°) with respect to the main stalk. Lamellae Assembly in Type 3 (Circular Core−Stripe) PLLA Spherulites. Interior lamellae of type 3 (circular core− stripe type) PLLA spherulites was similarly analyzed using the interior-dissection approaches. Figure 12a shows SEM micro-

Figure 12. (a) SEM graph of fracture surface of type 3 spherulite (circularly striped spherulite) in methanol-etched PLLA/PTA (50/ 50) blends after being isothermally crystallized at 115 °C and cooled to ambient temperature. (b) Schematic illustration of fracture surface for type 3 spherulite. (c) AFM height image. (d) AFM height profile for unetched sample. Specimen film thicknesses kept in the range of ca. 20−25 μm.

graphs for the fractured surface of Type 3 PLLA spherulites. The interior lamellae of Type 3 (circular core−stripe) again show a similarly grating-like structure; however, the details of the grating assembly differ from those of Type 1 (circularly ringed) spherulites or Type 2 (hexagon) PLLA axialites as discussed earlier. Across the two optical-birefringent stripes, the interior grating lamellae underneath the top-surface stripes are aligned alternately to be vertical (90°) to the substrate and inclined at 60° angle to the substrate, respectively, which is demonstrated in Figure 12b scheme. Note the interlamellae J

DOI: 10.1021/acs.macromol.8b01726 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 13. (a) Schematic of top-surface relief for type 3 spherulite (stripe with tiny core). (b) Single triangle-like stalk made of two birefringence stripes. (c) POM graph.

lamellae stalks and branching lamellae assembled into a grating structure. All branches grow at 60° angle intersection with the main stalks, where the 60° angle intersection between the branches and main stalks is in agreement with the nature of the orthorhombic lattices of PLLA single crystals.

directions, intersecting at a 60° angle mutually, leading to a hexagon-shaped geometry.

■

CONCLUSION The top surface vs interior lamellae assembly in three unique types of lamellae aggregation into diversified PLLA spherulites (or axialites/dendrites) by crystallization from PLLA/PTA blend of various compositions at specific Tc’s are analyzed. When crystallized at same Tc = 110 °C, the PLLA spherulitic morphology in the PLLA/PTA blend of various compositions (wt ratios = 90/10 to 10/90) systematically changes from a circularly ringed pattern to ringless striped or ringless hexagonal pattern. At several intermediate compositions, the ringed core and stripe patterns can merge into core−stripe PLLA spherulites or the stripe and hexagon patterns can merge into half-and-half Janus-face (half stripe and half hexagon) PLLA spherulites. These morphology results demonstrate the complexity and diversified varieties of pathways for lamellae assembly into aggregated PLLA spherulites. Dissection into the interiors of the three types of PLLA spherulites via delicate fracturing across the thickness sections, coupled with proper etching techniques, was adopted to reveal the correlations between the diversified birefringence patterns and interior lamellae assembly. Specifically, PLLA/PTA (50/ 50) crystallized at Tc = 105 and 115 °C are the main focuses, where there are three main different types of PLLA spherulites: type 1 (circularly ring-banded), type 2 (hexagonal axialite), and type 3 (core−stripe) spherulites. The results have proved that the lamellae in these three different PLLA aggregation types (types 1−3) all share similarity on the lamellae assembly mechanisms to display variety of optical birefringence patterns; yet, the lamellae assemble themselves in different ways driven by the initial nuclei geometric shapes at different Tc or blend compositions. Top-surface-relief morphology and interior dissections into all three types of PLLA spherulites yield interestingly consistent results for constructing workable mechanisms to account for the periodic ringed or ringless striped or hexagonal PLLA spherulites. For all three types of PLLA spherulites (including axialites or dendrites), regardless of different ways of assembly, there are ridge and valley portions, and the lamellae in the ridge and valley are all not continuous, with distinct interfaces between the lamellae constituting the ridge and valley portions. There are main

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01726.

■

Additional POM micrographs for illustrating the diversified variations of PLLA morphology by many kinetic factors; data/plots of growth rates of three different types of PLLA spherulites; DSC traces; SEM micrographs for PLLA specimens without solvent etching to contrast with the much clearer images on specimens properly etched, as adopted in this work (PDF)

AUTHOR INFORMATION

Corresponding Author

*(E.M.W.) E-mail [email protected]; Fax +886 6 2344496, Tel +886 6 275-7575 ext 62670. ORCID

Eamor M. Woo: 0000-0002-3653-2549 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by a basic research grant (MOST 105-2221-E-006-246-MY3) in three consecutive years funded by Ministry of Science and Technology (MOST) of Taiwan.

■

REFERENCES

(1) Geil, P. H. Polymer Single Crystals; J. Wiley & Sons: 1963; pp 421−490. (2) Melillo, L.; Wunderlich, B. Extended-chain crystals. Kolloid Z. Z. Polym. 1972, 250 (5), 417−425. K

DOI: 10.1021/acs.macromol.8b01726 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

(23) Woo, E. M.; Ni’mah, H.; Wang, Y.-H. Anisotropic Nucleation and Janus-Faced Crystals of Poly (l-lactic acid) Interacting with an Amorphous Diluent. Ind. Eng. Chem. Res. 2014, 53 (23), 9772−9780. (24) Zhang, J.; Li, L.; Song, S.; Feng, H.; Chen, P.; Wang, Z.; Gu, Q. Synchronous architecture of ring-banded and non-ring-banded morphology within one spherulite based on in situ ring-opening polymerization of cyclic butylene terephthalate oligomers. RSC Adv. 2016, 6 (97), 94524−94530. (25) Lugito, G.; Woo, E. M. Asymmetric Growth of Co-Crystallized Nano-and Micrometer-Sized Lamellae to Janus-Faced Spherulites in Poly(l-lactic acid) with Amorphous Poly(methyl methacrylate). Cryst. Growth Des. 2017, 17 (10), 5034−5037. (26) Bryant, W.; Pierce, R., Jr.; Lindegren, C.; Roberts, R. Nucleation and growth of crystallites in high polymers. Formation of spherulites. J. Polym. Sci. 1955, 16 (82), 131−142. (27) Schuur, G. Mechanism of the crystallization of high polymers. J. Polym. Sci. 1953, 11 (5), 385−398. (28) Lotz, B. Crystal Polymorphism and Morphology of Polylactides. In Synthesis, Structure and Properties of Poly(lactic acid); Springer: 2017; pp 273−302. (29) De Santis, P.; Kovacs, A. Molecular conformation of poly(Slactic acid). Biopolymers 1968, 6 (3), 299−306. (30) Eling, B.; Gogolewski, S.; Pennings, A. Biodegradable materials of poly(l-lactic acid): 1. Melt-spun and solution-spun fibres. Polymer 1982, 23 (11), 1587−1593. (31) Kawai, T.; Rahman, N.; Matsuba, G.; Nishida, K.; Kanaya, T.; Nakano, M.; Okamoto, H.; Kawada, J.; Usuki, A.; Honma, N.; Nakajima, K.; Matsuda, M. Crystallization and melting behavior of poly(L-lactic acid). Macromolecules 2007, 40 (26), 9463−9469. (32) Woo, E. M.; Yen, K.-C.; Yeh, Y.-T.; Wang, L.-Y. Biomimetically Structured Lamellae Assembly in Periodic Banding of Poly(ethylene adipate) Crystals. Macromolecules 2018, 51, 3845. (33) Woo, E. M.; Lugito, G.; Chang, S. Three-dimensional interior analyses on periodically banded spherulites of poly(dodecamethylene terephthalate). CrystEngComm 2018, 20 (14), 1935−1944. (34) Lugito, G.; Woo, E. M. Multishell Oblate Spheroid Growth in Poly(trimethylene terephthalate) Banded Spherulites. Macromolecules 2017, 50 (15), 5898−5904. (35) Su, C.-C.; Woo, E. M.; Hsieh, Y.-T. Perpendicularly oriented lamellae in poly(3-hydroxybutyric acid-co-3-hydroxyvaleric acid) blended with an amorphous polymer: ultra-thin to thick films. Phys. Chem. Chem. Phys. 2013, 15 (7), 2495−2506. (36) Schultz, J. M. Unusual “twisting” morphology in poly(3hydroxybutyrate-co-3-hydroxyhexanoate) spherulites. Macromolecules 2013, 46 (10), 4227−4229.

(3) Bisault, J.; Ryschenkow, G.; Faivre, G. Spherulitic branching in the crystallization of liquid selenium. J. Cryst. Growth 1991, 110 (4), 889−909. (4) Browne, G.; Kingston, D. Early diagenetic spherulitic siderites from Pennsylvanian palaeosols in the Boss Point Formation, Maritime Canada. Sedimentology 1993, 40 (3), 467−474. (5) Niederauer, G. G.; Slivka, M. A.; Leatherbury, N. C.; Korvick, D. L.; Harroff, H. H., Jr.; Ehler, W. C.; Dunn, C. J.; Kieswetter, K. Evaluation of multiphase implants for repair of focal osteochondral defects in goats. Biomaterials 2000, 21 (24), 2561−2574. (6) Merolli, A.; Gabbi, C.; Cacchioli, A.; Ragionieri, L.; Caruso, L.; Giannotta, L.; Leali, P. T. Bone response to polymers based on polylactic acid and having different degradation times. J. Mater. Sci.: Mater. Med. 2001, 12 (9), 775−778. (7) Yamane, H.; Sasai, K. Effect of the addition of poly (D-lactic acid) on the thermal property of poly(L-lactic acid). Polymer 2003, 44 (8), 2569−2575. (8) Tsuji, H.; Fukui, I. Enhanced thermal stability of poly(lactide) s in the melt by enantiomeric polymer blending. Polymer 2003, 44 (10), 2891−2896. (9) Shirahase, T.; Komatsu, Y.; Tominaga, Y.; Asai, S.; Sumita, M. Miscibility and hydrolytic degradation in alkaline solution of poly(Llactide) and poly(methyl methacrylate) blends. Polymer 2006, 47 (13), 4839−4844. (10) Focarete, M. L.; Scandola, M.; Dobrzynski, P.; Kowalczuk, M. Miscibility and mechanical properties of blends of (L)-lactide copolymers with atactic poly(3-hydroxybutyrate). Macromolecules 2002, 35 (22), 8472−8477. (11) Malinová, L.; Brožek, J. Mixtures poly((R)-3-hydroxybutyrate) and poly(L-lactic acid) subjected to DSC. J. Therm. Anal. Calorim. 2011, 103 (2), 653−660. (12) Nijenhuis, A.; Colstee, E.; Grijpma, D.; Pennings, A. High molecular weight poly(L-lactide) and poly(ethylene oxide) blends: thermal characterization and physical properties. Polymer 1996, 37 (26), 5849−5857. (13) Hsieh, Y.-T.; Kuo, N.-T.; Woo, E. M. Thermal analysis on phase behavior of poly(l-lactic acid) interacting with aliphatic polyesters. J. Therm. Anal. Calorim. 2012, 107 (2), 745−756. (14) Lee, W.-K.; Lee, J.-K.; Ha, C.-S. Growth of monolayered poly(llactide) lamellar crystals on a substrate. Macromol. Res. 2003, 11 (6), 511−513. (15) Kikkawa, Y.; Abe, H.; Iwata, T.; Inoue, Y.; Doi, Y. Crystallization, stability, and enzymatic degradation of poly(L-lactide) thin film. Biomacromolecules 2002, 3 (2), 350−356. (16) Nurkhamidah, S.; Woo, E. M. Correlation of crack patterns and ring bands in spherulites of low molecular weight poly(l-lactic acid). Colloid Polym. Sci. 2012, 290 (3), 275−288. (17) Woo, E. M.; Lugito, G.; Tsai, J.-H.; Müller, A. J. Hierarchically diminishing chirality effects on lamellar assembly in spherulites comprising chiral polymers. Macromolecules 2016, 49 (7), 2698− 2708. (18) Chen, H.-P.; Woo, E. M. Dendritic lamellar assembly in solution-cast poly(L-lactic acid) spherulites. CrystEngComm 2017, 19 (40), 6002−6007. (19) Nurkhamidah, S.; Woo, E. M. Unconventional Nonbirefringent or Birefringent Concentric Ring-Banded Spherulites in Poly(L-lactic acid) Thin Films. Macromol. Chem. Phys. 2013, 214 (6), 673−680. (20) Nurkhamidah, S.; Woo, E. M. Phase-separation-induced singlecrystal morphology in poly(l-lactic acid) blended with poly(1,4butylene adipate) at specific composition. J. Phys. Chem. B 2011, 115 (45), 13127−13138. (21) Ni’mah, H.; Woo, E. M. A novel hexagonal crystal with a hexagonal star-shaped central core in poly(L-lactide) (PLLA) induced by an ionic liquid. CrystEngComm 2014, 16 (23), 4945−4949. (22) Nurkhamidah, S.; Woo, E. M. Mechanisms of Multiple Types of Lamellae and Spherulites in Poly(l-lactic acid) Interacting with Poly(4-vinyl phenol). Macromol. Chem. Phys. 2013, 214 (20), 2345− 2354. L

DOI: 10.1021/acs.macromol.8b01726 Macromolecules XXXX, XXX, XXX−XXX