Handedness of Twisted Lamella in Banded Spherulite of Chiral

Jun 30, 2017 - Banded spherulite resulting from lamellar twisting due to the imbalanced stresses at opposite fold surfaces can be formed by isothermal...
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Handedness of Twisted Lamella in Banded Spherulite of Chiral Polylactides and Their Blends Hsiao-Fang Wang,† Chen-Hung Chiang,† Wen-Chun Hsu,† Tao Wen,† Wei-Tsung Chuang,‡ Bernard Lotz,∥ Ming-Chia Li,*,§ and Rong-Ming Ho*,† †

Department of Chemical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan § Department of Biological Science and Technology, National Chiao Tung University, Hsinchu 300, Taiwan ∥ Institut Charles Sadron, CNRS, Université de Strasbourg, 23, Rue du Lœss, F67034 Strasbourg, France ‡

S Supporting Information *

ABSTRACT: Banded spherulite resulting from lamellar twisting due to the imbalanced stresses at opposite fold surfaces can be formed by isothermal crystallization of chiral polylactide and its blends with poly(ethylene glycol) (PEG). Using a polarized light microscope, the handedness of the twisted lamella in banded spherulite is determined. With the same growth axis along the radial direction as evidenced by wide-angle X-ray diffraction (WAXD) for isothermally crystallized samples at different temperatures, the twisted lamellae of chiral polylactides (poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA)) display opposite handedness. The split-type Cotton effect on the C O stretching motion of vibrational circular dichroism (VCD) spectra helps determine the helix handedness (i.e., conformational chirality). The results indicate that the conformational chirality can be defined by the molecular chirality through intramolecular chiral interactions. Moreover, the preferred sense of the lamellar twist in the banded spherulite corresponds to the twisting direction identified by the C−O−C vibration motion of VCD spectra, reflecting the role of intermolecular chiral interactions in the packing of polylactide helices. Similar results are obtained in the blends of chiral polylactides and poly(ethylene glycol) (PEG, a polymer compatible with polylactide), indicating that the impact of chirality is intrinsic irrespective of the specific crystallization conditions. In contrast to the chiral polylactides, the spectrum of the crystalline stereocomplex that associates PLLA and PDLA shows VCD silence. The spectroscopic results are in line with the morphological observations. No banded spherulites are observed in the stereocomplex crystallites due to the symmetric packing of mirror L- and D-chain conformations in the fold surfaces and the crystallites core.

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(that is, tacticity) but flexible chains (for example, isotactic or syndiotactic C−C single bonds), the lamellar twisting appears to result from the imbalanced stresses at opposite fold surfaces due to different fold structures or conformations at these fold surfaces.5,9,10 By contrast, the origin of in the twist β sheets of fibrous proteins stems from the configurational chiral centers in the crystalline core of the lamellae and its transfer to higher structural levels via the strong structural identity of the hydrogen-bonded β sheets.11−13 When the interactions between polymers having different tacticities or configurations prevail over those between polymers with the same tacticity or configuration, a stereoselective association of the polymer pair takes place to form a stereocomplex. A well-known and typical stereocomplex is the one between isotactic and syndiotactic poly(methyl methacrylate) (PMMA).14 Equimolar binary

INTRODUCTION Self-assembly involves the spontaneous organization of molecules or macromolecules into stable, well-defined aggregates by secondary interactions (noncovalent forces).1,2 Nature uses the self-assembly of molecules and supramolecules to structure substances and materials. Diverse biological architectures are established by the interplay of secondary interactions.3 Helical morphology is probably one of the most fascinating textures among self-assembled architectures in nature. A banded spherulite (that is, a spherulitic crystalline morphology with extinction rings) is commonly found in polymeric crystallites when observed under a polarized light microscope (PLM).4−6 It is generally accepted that the banding in spherulites reflects the radial growth of twisted lamellae due to the imbalanced stresses at opposite fold surfaces.7,8 The difference of surface stresses can be related to different factors including chain tilt (e.g., polyethylene), chemical structure of chain folds (e.g., polyamide), and main-chain chirality (e.g., biopolymers).5 For chiral polymers with regular configuration © XXXX American Chemical Society

Received: February 14, 2017 Revised: June 2, 2017

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°C to remove the residual solvent.23 The polydispersity ĐM (Mw/Mn) was determined by GPC, and the number-average molecular weight (Mn) was determined by 1H NMR. Table S1 summarizes the characteristics of the samples used in this study. The molecular weights (Mn) of PLLA, PDLA, and PLA are 15 600, 12 100, and 20 000 g/mol with corresponding polydispersities 1.30, 1.28, and 1.22, respectively. PEG (Mn = 20 000 g/mol with polydispersity of 1.06) was purchased from Alfa Aesar (USA). Sample Preparation. Solutions for chiroptical measurements were 0.1 wt % in acetonitrile (AcCN) for CD and 2 wt % in dichloromethane for VCD measurements. Films were prepared by spin-casting 5 wt % solutions in dichloromethane on a silicon wafer at 500 rpm at room temperature. The film thickness was approximately 10 μm. Amorphous polylactides were prepared by annealing at 180 °C for 3 min and quenching in liquid nitrogen. Crystalline polylactides were prepared by isothermal crystallization at preset temperatures after annealing at 180 °C. For blending experiments, the PLLA or PDLA/ PEG ratio is three to one in weight. Solutions for drop casting were 2 wt % in dichloromethane. After drop casting on the glass slip, the sample is covered with another glass slip. Crystalline polylactides blends were prepared by isothermal crystallization at preset temperatures after annealing at 180 °C. Stereocomplex were produced by using 3 wt % tetrahydrofuran (THF) solutions of PLLA and PDLA mixed in different ratios at room temperature. After 1 h, precipitation takes place and the solution becomes opaque. The stereocomplex films used for VCD and PLM investigations are prepared by spin coating on a silicon wafer and quartz glass with spin rate of 500 rpm at room temperature and subsequently crystallized isothermally at different temperatures. The film thickness was approximately 10 μm. Characterization. DSC experiments were carried out in a PerkinElmer DSC 7 with temperature and heat flow scales at constant heating rates (10 °C/min) carefully calibrated with standards. The homopolymers and stereocomplex were first annealed for 3 min at Tmax = 180 and 230 °C, respectively. They were then rapidly cooled at 150 °C/min to room temperature and heated again to Tmax to determine Tg and Tm. Polarized light microscopy (PLM) was performed with either an Olympus BX-60 equipped with a hot stage controlled by an INSTEC 200 processor or a Carl-Zeiss Axiphot 2 equipped with a hot stage (Mettler FP82) and a FP80 processor. The sense of lamellar twist in the banded spherulites was determined by using a goniometric stage.20 UV−vis and CD spectra were performed using a JASCO J-815 spectrometer. Solution samples for CD measurement were placed in a cylindrical quartz cell with a light path of 1.0 mm. The concentration of the solution was 0.1 wt % for polylactide homopolymers in AcCN. FT-IR absorption and corresponding VCD spectra were acquired using a JASCO FVS-6000 spectrometer. Solution samples for VCD measurement were placed in a cylindrical CaF2 cell with a light path of 50 μm. Transmission electron microscopy (TEM) was carried out with a JEOL JEM-1200CXII. Scanning electron microscopy (SEM) observation was performed on Pt sputter-coated samples with a JEOL JSM-6700F working at 1.5 keV. Wide-angle X-ray diffraction (WAXD) experiments were conducted at the synchrotron X-ray beamline BL01C2 at the National Synchrotron Radiation Research Center (NSRRC) in Hsinchu, Taiwan, with an X-ray wavelength λ of 1.03 Å. Both the PLLA homopolymers and the equimolecular PLLA/ PDLA blends were heated to 230 °C for 3 min and cooled at 150 °C/ min to 150 °C where they stayed for 12 h. The WAXD data were collected at room temperature. Microbeam WAXD experiments were performed at two end-stations, namely (a) of beamline BL07A of Taiwan Light Source (TLS) with 12 keV X-rays (λ = 1.033 Å) and 20 μm beam size and (b) beamline BL25A of Taiwan Photo Source (TPS) with 15 keV X-rays (λ = 0.8265 Å) with 10 μm beam size. The WAXD data were collected with a Mar345 image plate detector with exposure time of 200 s for TLS BL07A and with a Pilatus 200K with an exposure time of 1 s for TPS BL25A. Films approximately 50 μm thick were deposited on a Kapton film for the microbeam WAXD experiments. Linear scans were performed along the radial direction of the spherulite.

blends of optically active enantiomeric polymers (i.e., between R- and S-configured (or L- and D-configured) polymer chains) may also crystallize differently from their enantiomeric chiral components.15 For instance, stereocomplexes between poly(Llactide) (PLLA) and poly(D-lactide) (PDLA) are formed both in the melts and in solution with crystalline structure, morphology, and physical properties significantly different from those of the individual chiral polylactides.16−18 In a previous communication,19 an original induced circular dichroism (ICD) has been observed in the chiral polylactides end-capped with pyrene moieties: the ICD of pyrene moieties is driven by the crystallization of the chiral polylactides. The ICD signals are attributed to the lamellar twisting with preferred handedness of the chiral polylactides that gives rise to the exclusive optical activity of the pyrene moieties. In the present report, we aim to study at the molecular level the helicity of the twisted lamellae in the polylactides banded spherulites. Isothermal crystallization of PLLA and PDLA yields banded spherulites. The helicity of the twisted lamellae is determined using polarized light microscopy (PLM) and is as expected, opposite for PLLA and PDLA.8,20 An exciton chirality method and couple-oscillator approach based on the through-space coupling of two (or more) chromophores with respect to their helical arrangement in space giving rise to a bisignate circular dichroism curve (i.e., split-type Cotton effect) are simple and extremely versatile methods to establish the absolute configuration and conformation of organic compounds.21,22 The split-type Cotton effect on the CO stretching motion of the vibrational circular dichroism (VCD) spectra of the chiral polylactides helps correlate the molecular chirality and the conformational chirality (helical sense of the chain) and ultimately investigate the role of intramolecular chiral interactions. The signatures of the split-type Cotton effect on the C−O−C vibration motion of the VCD spectra for the chiral polylactides are used to examine the correlation between the lamellar twist sense and the twisting direction of the intermolecular chiral interactions between polylactide helices chains in the crystal. The morphology and optical activities are compared with those of the blends and the crystalline stereocomplex of PLLA and PDLA. The morphologies can change by blending small amounts of compatible polymer diluents. The banding of spherulites is more regular than in neat homopolymer. Also, the formation of banded PLLA spherulites can be promoted by blending compatible poly(εcaprolactone) due to the diluent effect.23 For the crystalline stereocomplex, in view of the symmetric packing of L- and Dstems,24 a spherulitic texture without banded texture is expected. In contrast to the chiral polylactides,25 no significant VCD signal can be identified for the stereocomplex, which further demonstrates the interest of spectroscopic methods in analyzing the banded spherulites of chiral polylactides at a molecular level.

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EXPERIMENTAL SECTION

Synthesis of Polylactide Homopolymers. Typical ring-opening polymerization was used to prepare the chiral PLLA, PDLA, and poly(D,L-lactide) (PLA) homopolymers. The polymerization was carried out at 110 °C in oil bath under stirring by using tin(II) octoate [(tin(II) bis(2-ethyl hexanoate), Sn(Oct)2)] and benzyl alcohol as catalyst and initiator, respectively. First, the monomers were recrystallized twice by using toluene as solvent before polymerization. After polymerization, the polymers were purified by reprecipitation using methyl chloride as solvent and methanol as precipitant. The precipitates were filtered and kept in a vacuum at 70 B

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Figure 1. Observation of banded spherulites of (A) PLLA and (B) PDLA isothermally crystallized at 110 °C by PLM with the use of gypsum plate. Vertical sections (the red delimited rectangular areas in (A) and (B)) of (C) PLLA and (D) PDLA spherulites examined by PLM. The sample was rotated along the Y-axis in the right-handed positive sense during the PLM observation.

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RESULTS AND DISCUSSION Lamellar Twisting with Preferred Handedness. To identify the twist sense of lamellae in banded spherulites, a homemade goniometer was introduced in the optical microscope stage, and the sample was rotated along an axis parallel to the spherulite radial growth.19 Taking that radial growth as a vector, and by analogy between a twisted lamellae and a screw, the extinction rings move toward the vector’s origin, the spherulite center, when a twisted lamellae is rotated in the sense of its own twist. Conversely, the extinction rings move away from the spherulite center when the rotation is opposite to the lamellar twist. Figures 1A and 1B show the PLM images of the PLLA and PDLA isothermally crystallized at 110 °C. Figures 1C and 1D show the PLM images of the central vertical slices of the banded spherulites of the PLLA (Figure 1A) and PDLA (Figure 1B) applied a right-handed rotation along the Yaxis. For PLLA (Figure 1C), the extinction rings move “down” (i.e., toward the spherulite center), (cf. lines AA′ and BB′), indicating that the lamellar screw is left-handed. By contrast, for PDLA (Figure 1D), lines CC′ and DD′ move “up”, indicating that the twisted lamellae are right-handed. Recently, Xu and Guo et al. reported that the banded spherulites of poly[(R)-3-hydroxyvalerate] display two different types of growth sectors.26 Both are banded, but they differ both by the radial growth axis (a- or b-axes) and by the sense of lamellar twist. Growth along the b-axis is associated with righthanded lamellar twist while the growth along the a-axis is associated with left-handed lamellar twist. As illustrated by the poly[(R)-3-hydroxyvalerate] results therefore for a single chiral polymer and all conditions being identical, the growth axis is the factor determining the handedness of the twisted lamellae in banded spherulites. To determine the radial growth axis in

the present banded spherulites of PLLA and PDLA, microbeam WAXD experiments were carried out. As shown in Figure S1, the growth axis along the radial direction is the a-axis for both PLLA and PDLA isothermally crystallized at 130 °C from melt; the results are consistent with the data of Scandola and coworkers.27 Note that there is a possibility to have the variation on the growth axis along the radial direction while the isothermal crystallization is carried out at lower crystallization temperature. In order to check whether the radial growth axis does not change with crystallization temperature (as is observed e.g. in poly(ethylene adipate)), PLLA and PDLA were isothermally crystallized at 90 °C. At this low Tc, the radial growth direction could not be determined in view of the small size of the spherulites, and no banded texture was observed. Nevertheless, banded spherulites with larger size were obtained by blending PEG with the chiral polylactides for isothermal crystallization at 90 °C. As a result, the banded spherulites are able to be visualized under PLM. Consistently, the same radial growth a-axis was found for both PLLA/PEG (Figure S2A) and PDLA/PEG (Figure S2B). The link between the sense of lamellar twist and molecular and conformational chirality of the polymer had been established so far with couples of enantiomeric polymers, not only for PLLA and PDLA but also for the R- and S-poly(epichlorohydrin) and R- and Spoly(propylene oxide).28,29 As developed in the Introduction, this holds true when, as would be expected, the enantiomeric polymers have the same radial growth direction. The situation with poly(hydroxyvalerate) appears at first sight to break this dogma. Actually, it does not. As developed by Jun Xu and coworkers,30 a single scheme of unbalanced surface stresses in the different poly(hydroxyvalerate) growth sectors explains the observed different lamellar twists, simply because they are C

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Figure 2. (A) CD and corresponding UV−vis absorption spectra of polylactides in dilute AcCN solution. Concentration of the solution is 0.1 wt %. (B) VCD and corresponding FT-IR absorption spectra of polylactides in dilute CH2Cl2 solution. Concentration of the solution is 2 wt %. VCD and corresponding FTIR absorption spectra of (C) CO and (D) C−O−C vibrations of PLLA and PDLA in the amorphous state and the crystalline state isothermally crystallized at 110 °C for 6 h. The blue arrow indicates the characteristic absorption bands of α-form PLLA crystals.

is, at the characteristic absorption band attributed to the n → π* transition of the carboxyl group. By contrast, the solution of PLA shows CD silence. The spectroscopic results are similar to our previous studies.25 To clarify the origins of the CD signals by ruling out the effect of the anisotropic arrangement of polymer chains, linear dichroism (LD) measurements were performed. As shown in Figure S3, no significant LD signals in the range from 200 to 250 nm can be found, indicating that the carboxylic chromophore of the polylactide chain in solution is intrinsically isotropic. CD spectroscopy can be used to determine the handedness of the helical conformation in the chiral polymer with an achiral conjugated backbone and chiral centers located in the side chain (i.e., side-chain chirality). However, the n → π* transition is relatively delocalized; namely, the CD results might be significantly affected by the neighboring chiral centers on the corresponding absorptions.

exerted in two orthogonal growth directions. In other words, the conformational and crystallographic chirality dictates the lamellar twist, but it is mediated by the fold conformation/ structure or more precisely by the differences in fold conformation or structure on opposite fold surfaces. This analysis therefore requires to evaluate the possible chiral impact of the crystalline stems’ conformational chirality at two different levels: within the crystal lattice and as transferred to the folds. As developed now, such submolecular insights require a spectroscopic approach and, given the chirality issues, require circular dichroism investigations. Intramolecular and Intermolecular Chiral Interactions of Enantiomeric Polylactides. Figure 2A shows the CD and corresponding absorption spectra for the polylactides in dilute AcCN solution. A positive Cotton effect for PLLA and a negative Cotton effect for PDLA can be found at 210 nm, that D

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1222 cm−1 in the FTIR absorption spectra can be clearly identified.37,38 For Tc = 110 °C adopted in this experiment, the crystal structure of PLLA is the so-called α′ form.24 According to the exciton chirality method,21,22 the intensity of the splittype Cotton effect is inversely proportional to the square of the interchromophoric distance. Accordingly, the original optical activity of the VCD spectrum resulting from the coupling of the C−O−C vibrational absorption is an indicator of interactions between adjacent helical chains in the crystalline lamellae. Lamellar Twisting in Blends. In pure PLLA or PDLA, the extinction rings can only be identified at high crystallization temperatures that gives large enough spherulites for examination of the twisting sense of lamella. No recognizable extinction ring can be observed at low crystallization temperature (for instance, 90 °C) due to the small spherulite size. Note that it is feasible to acquire large spherulites with recognizable bands at low crystallization temperatures by introducing compatible poly(ε-caprolactone) (PCL) or PEG homopolymer for blending with crystalline polylactide to acquire large spherulite with enhanced twisting power due to the effect of diluents.23 Accordingly, blending of chiral polylactide and PEG was carried out to determine the handedness of twisting lamella and examine corresponding spectra at lower temperatures. Figures S5A and S5C show the PLLA and PDLA banded spherulites isothermally crystallized at 90 °C. Spherulitic texture is visible, but the extinction ring cannot be identified. We speculate that the decrease of the spherulite size is attributed to the increase in nucleation density at low crystallization temperatures. By contrast, with the introduction of 25 wt % of PEG, large spherulites with clear extinction rings can be found after crystallization of the blends for crystallization at 90 °C (Figures S5B and S5D). Consistently, the corresponding sense of lamellar twisting in the banded spherulites appears opposite handedness, indicating that the lamellar screws are left-handed for PLLA/PEG (75/25) and right-handed for PDLA/PEG (75/ 25) (Figure 3), respectively. As expected, the VCD spectrum of PDLA/PEG is the mirror image of the PLLA/PEG one (Figure 4A). Consistently, induced VCD signals appear in the

As a result, the handedness of helical conformation cannot be simply determined from the CD results. By contrast, the absorptions attributed to molecular vibrational motions are less sensitive to the neighboring vibrational modes. As a result, VCD becomes a powerful tool for determining the handedness of the helical conformation of chiral polymers with main-chain chirality.19,25,31 Figure 2B shows the VCD and corresponding absorption spectra of the polylactides in dilute dichloromethane (CH2Cl2) solution. For PLLA, a split-type Cotton effect with a negative VCD band at 1758 cm−1 and a positive band at 1769 cm−1 can be found at which the inflection point at 1765 cm−1 corresponds to the absorption of the CO stretching motion of the ester group in polylactide. Consistently, the VCD spectrum of the PDLA shows the mirror-image Cotton effect. By contrast, the VCD signals are silence for the racemic PLA solution. On the basis of the coupled oscillator method,21,22,32,33 the signature of the split-type Cotton effect in the VCD spectrum of PLLA indicates a negative chirality (the helical conformation of PLLA is left-handed) whereas the PDLA spectrum indicates a positive chirality (the helical conformation of PDLA helix is right-handed). The VCD results confirm that the chiral polylactides have a helical conformation with a preferred handedness that results from the transfer of molecular chirality to conformational chirality through the agency of intramolecular chiral interactions. The spectroscopic results are similar to our previous studies.25,31 Whereas the intramolecular chiral interactions can be identified by spectroscopic investigations of dilute solutions of the chiral polylactides, the intermolecular chiral interactions must be assessed on polymer aggregates. For amorphous PLLA, a split-type Cotton effect with a negative VCD band at 1753 cm−1 and a positive band at 1763 cm−1 is observed (Figure 2C). Also, the VCD spectrum of amorphous PDLA is the mirror image of the amorphous PLLA one. These results parallel to a significant extent the results obtained in dilute solution and help reach similar conclusions regarding the helical handedness and the presence of helical conformations in the amorphous state. Figure 2C shows the VCD and corresponding absorption spectra of PLLA and PDLA isothermally crystallized at 110 °C from the melt state. Quite noticeably, in contrast to the amorphous state, the intensity of the CO stretching motion is significantly amplified after crystallization. The absorption peak of CO stretching motion shifts to higher wavenumbers (so-called hypsochromic shift). Note that the VCD signals might result from anisotropic arrangement of the CO stretching motion or the anisotropic orientation of polymeric chains, resulting in artificial VCD signals, particularly in the bulk or thin-film states.34−36 To clarify the origins of the VCD signals, a VCD experiment followed by the approach suggested by Kuroda and Buffeteau was carried out at 0° and 90° at sample orientations.35,36 As shown in Figure S4, the VCD signals of the CO stretching motion at 0° and 90° at sample orientations appear similar split-type Cotton signals at the characteristic absorption bands of the CO stretching motion, indicating that the anisotropic effects on VCD measurement are insignificant.33,34 Most interestingly, induced VCD signals appear in the absorption bands of the C−O−C vibration in the range from 1100 to 1250 cm−1 after crystallization, whereas for amorphous chiral polylactides, no such VCD signals exist (Figure 2D). It is noted that the electron transition dipoles of the C−O−C vibration are almost parallel to the helical axis of chiral polylactide. Consistently, the effect of crystallization on the absorption variations of the C−O−C vibration at 1183 and

Figure 3. PLM images of (A) PLLA/PEG and (B) PDLA/PEG isothermally crystallized at 90 °C from melt with the use of gypsum plate. The delimited areas in (A) and (B) represent the observed slices during the rotation experiment. Vertical sections of (C) PLLA/PEG and (D) PDLA/PEG spherulites observed by PLM during the rotation. E

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Figure 4. VCD and corresponding FTIR absorption spectra of (A) CO and (B) C−O−C vibrations of PLLA/PEG and PDLA/PEG in the crystalline states for sample isothermally crystallized at 90 °C for 6 h.

observations. As shown in Figure S9, a periodic variation of height is evident in crystallized chiral polylactides. Both manifestations result from alternating flat and edge-on lamellae, i.e., lamellar twisting. 5 By contrast, the stereocomplex spherulites are made of straight crystalline lamellae that create a fibrillar texture, also observed in other samples with nonequivalent PLLA/PDLA ratios crystallized at temperature above 150 °C. The differences of lamellar textures between the chiral polylactides and their stereocomplexes have definitely a structural component linked with the chirality or its annihilation/compensation in the stereocomplex. At the same time, the lamellar morphology is very much dependent upon the unit-cell symmetry, with low cell symmetries (monoclinic, orthorhombic) promoting lamellar twistif only because of their frequently elongated lamellar shape. The higher symmetry unit cells tend to yield “flat” lamellae or with a long pitch because the radial and tangential growth rates are more similar. Tetragonal and hexagonal (or trigonal) unit cells yield mostly nonbanded spherulites, as illustrated with isotactic poly(4methylpentene-1) and isotactic polystyrene, respectively. The situation with the stereocomplex of PLLA and PDLA of the PLLA/PDLA stereocomplex is a further illustration of this high unit-cell symmetry. The crystal structure of the stereocomplex has first been described and still is indicated in many papers and reviews, as a triclinic unit cell that houses two chains, one of PLLA and one of PDLA. Such a triclinic cell geometry would call for or at least be consistent with lamellar twisting. However, this two-chain cell is actually a subcell of a larger trigonal cell that houses six chains, with space group R3c. The PLLA/PDLA unit cell is identical to the iPBu-1 Form I and isotactic polystyrene cells: same 3-fold helices, identical packing in the cell with any one helix surrounded by three antichiral helices. Isotactic polyolefins, being chemically achiral, can form conformational stereocomplexes from a single molecular species, whereas the PLLA/PDLA stereocomplex is of both chemical and conformational nature. At the same time, this

absorption bands of the C−O−C vibration in the range from 1100 to 1250 cm−1 (Figure 4B) for both PLLA/PEG and PDLA/PEG. Note that the handedness of lamellar twisting is opposite from that determined by Prud’homme and coworkers.20 These discrepancies are difficult to explain. Different crystallization conditions may be involved such as sample thickness, molecular weight of PEG, etc. Crystalline Spherulites of Stereocomplex. Different stereocomplexes investigated in this study were formed by precipitation of a THF mixed solution of PLLA and PDLA with a range of molar ratios. Figure S6A shows the DSC results of the precipitates from the mixed solutions as well as plain PLLA and PDLA. Both PLLA and PDLA melt in the expected 140− 155 °C range. By contrast, melting peaks above 210 °C in the blends indicate the formation of crystalline stereocomplex. The WAXD results (Figure S7) confirm the formation of the stereocomplex for equal PLLA and PDLA ratio and the progressive impact of homocrystallization for more unbalanced blends consistent with the early results of Tsuji and coworkers.16 Figure S6B shows the DSC thermograms of highly unbalanced PLLA/PDLA blends isothermally crystallized at different Tcs. When Tc < 150 °C, the melting of homopolymer crystals shows up as a sharp endothermic peak in addition to the broad high-temperature melting of the stereocomplex. For Tc > 150 °C, only the latter peaknow sharperis observed (Figure S6B, top curve). As shown in Figure S6C, only the stereocomplex high-temperature melting peak is observed for the equimolecular PLLA/PDLA blends; the conclusion is also supported by WAXD (cf. Figure S7). The observations are consistent with the results of Tsuji and co-workers.39 Figure S8 shows the spherulitic textures of the stereocomplex for different Tcs. They are optically negative, which corresponds to tangential orientation of the stems, as for the chiral polymers. They do not display any bandingthus lamellar twistat any Tc.40 The characteristics of and differences between crystalline lamellar morphologies of the homopolymers and stereocomplex crystallites are also revealed by SEM and TEM F

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Figure 5. Illustration of (A) the forming mechanism of twisted lamellae with a preferred handedness for isothermally crystallized PDLA and PLLA with a-axis spherulitic radial growth and (B) the crystalline growth of stereocomplex from the blends of the PDLA and PLLA.

cannot be isocline and isochiral or anticline and antichiral. Conversely, the stem tilt in the crystalline core of polyethylene lamellae determines the sense of lamellar twist: here, it is a structural feature that introduces structural asymmetry (but not structural chirality) that leads to a chiral manifestation (lamellar twist).42 In yet another example, the stems of PVDF gamma phase are all “isocline”. This polarity of the structure generates a difference in the chemical constitution of folds on opposite fold surfacesa fold volume imbalance (≈10 Å3/fold) that results in lamellar scrolling.43 As illustrated in Figure 5A, on the basis of the concepts proposed by Keith and Padden7,8 and Lotz and Cheng,5 the lamellar twisting is originated by the imbalanced stresses at the opposite folding surfaces. Owing to the mirror image of the chain configuration for the PLLA and the PDLA chains, it is intrinsic to give the formation of lamellar crystallites composed of the mirror image of helical chain dispositions in the crystalline core and also the folding surfaces. With the lamellar growth along the a-axis, on the basis of the folding model proposed by Iwata and Doi,44 the folding direction is along the [110] direction at the lamellar surface. In the lamellar core, the chiral polylactide stems adopt a 103 helical conformation and the α-form crystal lattice appears as a space group of orthorhombic P212121. As shown in Figure 5A, the surface stresses in sectors A and C are different to those in sectors B and D due to the discrepancies in the folding environments. Consequently, in each sector of a single lamella, the folds on the top and the bottom surface are of different encumbrances, resulting in different surface stresses.26,45 As a result, for the spherulitic growth with different axis, the imbalanced surface stresses lead to the bending of sectors A and C with growth front different to that on sectors B and D, resulting in the lamellar twisting in different handedness. By contrast, the absence of banded spherulite formation in the stereocomplex is

further complexityproper sorting out of the stem with the polylactide with the correct chirality at each stem sitemust affect the growth mechanisms and the growth rates, as examined in the next section. The origin of lamellar twisting depends upon many different factors and the cell symmetry discussed above is only one of them. Accordingly, it is essential to examine the other possible origins for the annihilation of chiral sense in the formation of stereocomplex, preferably at the submolecular level of the morphologies, related or not to their crystallography. As a result, the spectroscopic examination on the stereocomplex should provide original insights into the impact of molecular chirality on the hierarchical morphologies of the stereocomplex complex (see below for details). Radial Growth in Crystalline Chiral Polylactides and Corresponding Stereocomplex. As evidenced by the CD and VCD results, the chiral nature of polylactides results in intramolecular chiral interactions that determine the twist sense of helix. Upon crystallization, the chiral helices are packed essentially normal to the lamellar surface. Their intrinsic chirality generates asymmetric interactions within the crystalline core of the lamellae. These chirality-induced asymmetries are or must be transferred to the fold organization/ conformation. It is of interest to develop this argument at this stage. In polymer lamellae, the crystalline core and the folds are molecularly linked. As a result, analysis of the crystal core/ fold surface connections may provide molecular information that would not be accessible for any other system. Interestingly, the information may be deduced from either the fold or the crystalline core. A most classical example of the first kind deals with isotactic polypropylene, α phase for which the folds impose a specific stem organization. As elegantly demonstrated by Brückner and co-workers,41 because of restrictions on the accessible fold conformations, two stems linked by a fold must be either syncline and antichiral or anticline and isochiral, but G

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Figure 6. VCD and corresponding FTIR absorption of (A) CO and (B) C−O−C vibrations in stereocomplex from mixtures of enantiomeric chiral polylactides in THF solution.

Figure 7. VCD and corresponding FTIR absorption of (A) CO and (B) C−O−C vibrations in stereocomplex samples from the precipitates isothermally crystallized at 110 °C.

attributed to the releasing of the imbalanced stresses at the opposite folding surfaces of the crystalline lamellae in the PLLA/PDLA blends as illustrated in Figure 5B.24 Moreover, on the basis of the forming morphological texture with respect to the stereocomplex, the folding will not lead the formation of distinct sector as the ones in the inherent chiral homopolymers, giving the development of crystalline lamellae with elongated shape and eventually results in the formation of spherulitic texture. Also, the absence of banding spherulites could be attributed to the greater resistance to the twisting of stiffer, thicker, and/or wider crystals of the stereocomplex, resulting in the allowable twisting in a measurable range (i.e., lower twisting angle for crystalline lamella in stereocomplex). Note that the angle of twist, ϕ, is proportional to T/(bc2G) where T is the applied torque due to the imbalance of surface stresses in the present case, b is the width of the bar (here a lamella), c is the thickness of the bar, and G is the shear modulus. The thickness of lamella can be determined from the DSC thermal analysis methods by applying the Gibbs−Thomson equation to the DSC results of isothermal crystallization (see Supporting Information for details). Figures S10A and S10B show the

DSC results for PLLA and stereocomplex at PLLA/PDLA molar ratio of 5/5. On the basis of the Gibbs−Thomson equation calculation, the thickness of crystallized lamellae for stereocomplex is indeed larger than PLLA. As a result, the lower twisting angle for stereocomplex can be found due to its larger shear modulus and thinker lamella. To further examine the molecular packing of the stereocomplex, a spectroscopic analysis by VCD was carried out. The VCD spectra of blends at different PLLA/PDLA molar ratios in solution and of the precipitate from a THF solution and of isothermally crystallized samples were acquired. As shown in Figure 6A, the signature of split-type Cotton effect, corresponding to the absorptions of CO stretching motion for polylactides, is found in the solution state with nonequivalent PLLA/PDLA molar ratio. By contrast, the mixture with equivalent PLLA/PDLA molar ratio shows VCD silence. Also, the intensity of the split-type Cotton effect depends upon the molar ratio, indicating that the enantiomeric chiral polylactide possesses a preferred one-handed helical conformation at the initial state. As a result, the signals from CO stretching motion are contributed by the forming helical H

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conformation with a preferred sense due to intramolecular chiral interaction. By contrast, the VCD spectra show VCD silence in the absorptions of C−O−C vibration while the mixtures are in the solution or in the amorphous state from the precipitates (Figure 6B), reflecting that the VCD signals in the absorptions of C−O−C vibration are indeed attributed to the packing of helical polymer chains with preferred handedness due to intermolecular chiral interaction. Consequently, once the samples are isothermally crystallized at low temperatures (i.e., 110 and 130 °C), the mirror-imaged split-type Cotton effect in the absorptions of CO and C−O−C vibration for the blends at different PLLA/PDLA molar ratios, except for the blends with equivalent molar ratio, can be clearly identified (Figures 7 and Figure S11). It is noted that the crystallites of the blends with nonequivalent molar ratio obtained from low crystallization temperature (below the melting of the homopolymer) will possess the crystalline homopolymer and stereocomplex, as evidenced by the DSC results (Figure S6) and further verified by the dependence of the VCD intensity on the degree of nonequivalence in the PLLA/PDLA molar ratio. By contrast, in the case of the blends with equivalent molar ratio, there is no VCD signal in the crystallized blends at different crystallization temperatures. The blends with nonequivalent molar mixing ratio isothermally crystallized at high temperature (above the melting of crystalline homopolymer, at which the crystallization of chiral homopolylactides does not occur) are of particular interest. The VCD should be silent in the absorptions of C−O−C vibration due to the exclusive stereocomplex formation while the VCD signals in the absorptions of CO stretching motion always exist in the blends with nonequivalent molar ratios and remain after crystallization at different temperatures. Nevertheless, it is odd to observe the appearance of VCD signals in the absorptions of C−O−C vibrational motions once the crystallization is above the melting of crystalline homopolymer (e.g., at 180 °C) (Figure S12). This might be attributed to the occurrence of intermolecular chiral interaction (that is, the appearance of the VCD signals in the absorptions of C−O−C vibrational motions), resulting from the nucleation effect of the forming stereocomplex that enhances the aggregation capability of preferred handedness of helical polymer chains in the amorphous state. Detailed mechanisms with respect to the effects of stereocomplex on the nucleation of crystalline homopolymers have been extensively studied but the conclusive remarks are still in need of elucidation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00318. Figures S1−S12 and Table S1 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (R.-M.H.). *E-mail: [email protected] (M.-C. Li). ORCID

Tao Wen: 0000-0002-8376-0608 Wei-Tsung Chuang: 0000-0002-9000-2194 Rong-Ming Ho: 0000-0002-2429-7617 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Ministry of Science and Technology, Taiwan, for financially supporting this research under Contract MOST 103-2221-E-007-132-MY3 and MOST 105-2119-M007-011. Dr. Yu-Shan Huang, Jhih-Min Lin and Yu-Chun Chen are acknowledged for assistance during the microbeam X-ray diffraction at BL25A, Taiwan photon source. Dr. Bi-Hsuan Lin is acknowledged for microbeam X-ray diffraction at BL07A end station, Taiwan light source.

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REFERENCES

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CONCLUSIONS The helicity of the twisted lamellae in the banded spherulite of chiral polylactide was systematically studied. The conformational chirality of the chiral polylactide can be exclusively identified by VCD spectra with split-type Cotton effect whereas the molecular chirality of the constituted chiral entities can be determined using CD which yields a Cotton effect. The sense of the lamellar twisting in the banded spherulite was determined using PLM, and the determined sense is in line with the exclusive optical activity from VCD spectra. For a crystalline stereocomplex, the PLM results show spherulites without twisted lamellae due to the symmetric packing of chain conformations between L- and D-form sequences and also the formation of the folds with the same environment for chain packing on the lamellar surfaces, giving the development of crystalline lamellae with elongated shape and eventually results in the formation of spherulitic texture. I

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