Complex Crystal Formation of Poly(l-lactide) with ... - ACS Publications

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Complex Crystal Formation of Poly(L-lactide) with Solvent Molecules Hironori Marubayashi,† Shigeo Asai,* and Masao Sumita Department of Chemistry and Materials Science, Graduate School of Science and Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan S Supporting Information *

ABSTRACT: By screening examinations for a wide variety of organic solvents, we found that poly(L-lactide) (PLLA) forms the crystalline complex (ε-form) with the specific organic solvents such as tetrahydrofuran (THF) and N,N-dimethylformamide (DMF) below room temperature. It was revealed that PLLA has high selectivity for low molecular weight compounds to form the ε-crystals. By fiber diagram analyses for the ε-forms, it was found that PLLA chains take the 107 (left-handed 103) helical conformation and are packed in the orthorhombic lattice (a = 1.5−1.6 nm, b = 1.2−1.3 nm, c = 2.8−2.9 nm, and α = β = γ = 90°). Based on R-factor and packing energy calculations, the plausible crystal structure of PLLA−DMF complex was proposed, in which four PLLA chains and eight guest solvents are packed in the unit cell.

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INTRODUCTION The complexation of macromolecules with low molecular weight compounds via nonbonded interactions is categorized as the supramolecular chemistry composed of macromolecules and low molecular weight compounds. There have been many studies on crystalline complexes and clathrates composed of macromolecules and low molecular weight compounds: amylose/iodine,1 poly(vinyl alcohol)/iodine,2 cellulose/low molecular weight compounds,3 polymer/urea,4 polymer/cyclodextrins,5 poly(ethylene oxide) (PEO)/inorganic compounds,6 syndiotactic polystyrene (sPS) /solvents,7,8 polyethylenimine (PEI)/low molecular weight compounds,9 syndiotactic poly(methyl methacrylate) (st-PMMA)/solvents,10 st-PMMA/fullerenes,11 chitosan/low molecular weight compounds,12 etc. In particular, the complexation of amylose with iodine1 is wellknown phenomenon as the “iodo-starch reaction”. The complexation of PEO with metallic salts6 has a great effect on the ionic conductivity, which is of great importance in use of solid electrolytes. Petraccone et al. reported the nanoporous polymer crystals with pores or channels using sPS/solvents clathrates (δ- and ε-forms), which are suitable for applications such as chemical separation.8 Kawauchi et al. found that in aromatic solvents such as toluene st-PMMA encapsulates C60 molecules in its helical cavity to form a supramolecular inclusion complex.11 Uda et al. prepared the sPS clathrate containing organic dye (azulene) by the guest-exchange procedure and controlled the orientation of organic dye encapsulated in the sPS crystal.13 In addition, coordination polymers, which are constructed from transition metal ions and bridging organic ligands, attract much attention because these polymers can adsorb gases to a remarkable extent.14 Thus, studies on the complexation of macromolecules with low molecular weight compounds are of great importance in terms of both basic and applied researches. © 2012 American Chemical Society

Poly( L-lactide) (PLLA), one of the enantiomers of polylactide, can be synthesized from the plant-derived materials. PLLA shows the crystal polymorphism: the α-,15,16 α′ (δ)-,17,18 α″-,19 β-,20 and γ-21forms. Sasaki et al. clearly showed that PLLA helices in the α-form (orthorhombic unit cell of parameters a = 1.066 nm, b = 0.616 nm, and c = 2.888 nm) slightly deviate from the regular 107 helix by the linked-atom least-squares refinements coupled with the Rietveld whole-fitting method.16a Wasanasuk et al. performed a detailed analysis on the PLLA αform using 2-dimensional wide-angle X-ray diffraction (WAXD) and wide-angle neutron diffraction (WAND) for the ultradrawn sample.16b On the basis of X-ray fiber diagrams and polarized IR/Raman spectra, Zhang et al. proposed that the chain conformation and packing mode of the α′-form are slightly different from those of the α-form of PLLA.17b Marubayashi et al. revealed that the formation of disordered α (α″) crystals arises under high-pressure CO2 (0−20 °C and 3 MPa; 0−30 °C and 7−15 MPa), and the crystal structure transition from α″- to α-forms occurs with increasing CO2 temperature.19 Furthermore, it was indicated that CO 2 molecules are trapped in a PLLA unit cell during CO2-induced crystallization below room temperature. Puiggali et al. reported that stretching and stroking of PLLA give the frustrated packing of three chains with 32 helix in a trigonal unit cell of parameters a = b = 1.052 nm and c = 0.88 nm (β-form) using electron diffraction and conformational energy analysis.20b Cartier et al. showed that two antiparallel chains with 32 helical conformation are packed in an orthorhombic unit cell of parameters a = 0.995 nm, b = 0.625 nm, and c = 0.88 nm (γ-form) by epitaxial crystallization on hexamethylbenzene Received: October 17, 2011 Revised: December 22, 2011 Published: January 26, 2012 1384

dx.doi.org/10.1021/ma202324g | Macromolecules 2012, 45, 1384−1397

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around 140 °C using electron diffraction and packing energy analysis.21 There are a few studies on the complexation of PLLA, in which PLLA is a guest and low molecular weight compounds are hosts. Howe et al. succeeded in incorporating PLLA into the narrow channels of its inclusion compound with urea.22 Ohya et al. reported that the inclusion complex of αcyclodextrin with PLLA is preferentially formed compared to that with poly(D-lactide) (PDLA).23 As far as we know, there are no reports that conducted an exhaustive research on solvent-induced crystallization of PLLA and the complexation of PLLA, in which PLLA acts as a host. In this article, we describe the crystalline complex formation of PLLA with the specific solvent molecules by screening examinations for a wide variety of organic solvents using WAXD mainly. It is hoped that this study gives some insights into the molecular recognition ability of not only PLLA but also bio-based polymers having the structural similarity to PLLA such as poly(hydroxyalkanoate)s and poly(amino acid)s.

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Table 1. Organic Solvents Used for Solvent-Induced Crystallization of PLLA name

abbrev

acetone acetonitrile α-methyl-γbutyrolactone anisole β-butyrolactone chloroform cyclohexanone cyclopentanone diethyl ether 2,3-dihydrofuran 2,5-dihydrofuran 1,4-dioxane 1,3-dioxolane ethanol ethyl acetate ethylene carbonate formamide furan

EXPERIMENTAL SECTION

Samples. PLLA used in this study is Lacty #5408 with Mw of 107 000 and an optical purity of 98.6%, which was supplied from the Toyota Motor Corp. Amorphous films with a thickness of 0.45 mm were prepared by melt pressing of the pellets at 200 °C and 20 MPa for 5 min followed by quenching by ice water. The obtained films were confirmed to be completely amorphous by WAXD and differential scanning calorimetry (DSC). Oriented films were prepared by drawing the amorphous films in water at 50, 55, or 60 °C (around Tg of PLLA) by 2−5 times the original length. Drawing at 50−55 °C by 2−4 times the original length gave the oriented amorphous films (halo pattern around 2θ = 16.0° for Cu Kα), while drawing at 60 °C provided a small number of α′-crystals (e.g., strong diffraction peak at 2θ = 16.5° for Cu Kα). Organic solvents were used as received from commercial suppliers without further purification. Solvent Exposure. A PLLA amorphous film in the unoriented state (1 cm × 2 cm) was placed in a 20 mL vial container with 5 mL of organic solvents. In the case of highly volatile solvents such as acetone and tetrahydrofuran (THF), crystallization of PLLA was conducted by exposure to solvent vapor. On the contrary, in the case of solvents that are not highly volatile such as γ-butyrolactone (GBL), PLLA was crystallized by immersion into solvent in the liquid state. In principle, 25 (room temperature), 5, and −25 °C were selected as the crystallization temperature (Tc). In the case of organic compounds having the melting temperature (Tm) higher than 25, 5, or −25 °C, such as ethylene carbonate (EC) and 1,4-dioxane (DOX), Tc’s higher than Tm’s of these organic compounds were selected. Organic compounds having Tm higher than the glass transition temperature (Tg) of PLLA were omitted in this study. Namely, all Tc’s used in this study are lower than Tg of PLLA (ca. 60 °C) because our research interest is solvent-induced crystallization, not thermal crystallization. The periods for exposure of a film to a solvent atmosphere (i.e., crystallization periods) were changed from 1 to 60 days depending on solvent species to complete crystallization of PLLA. For the protic polar solvents such as ethanol, and hydrocarbons such as hexane, crystallization of PLLA was not finished in 60 days due to poor plasticizing effects of these solvents on PLLA. When a shape of a film was not kept due to dissolution of PLLA in organic solvents, solution casting was conducted at 25 and 40 °C for about 12 h. After taken out of a vial container, each PLLA film containing solvent molecules was weighed and sandwiched by polyimide tapes with a thickness of 50 μm to minimize volatilization of solvents. In the case of solvents that are not highly volatile, which was judged from small weight loss of the film containing solvents at room temperature, sealing by polyimide tapes was not conducted. Organic solvents used in this study are listed in Table 1. The details for solvent exposure conditions are shown in the Supporting Information. In principle, solvent desorption from each

AMGBL

BBL CHO CPO DEE 23DHF 25DHF DOX DOL EAc EC

γ-butyrolactone

GBL

name

abbrev

γ-valerolactone hexane isobutyraldehyde

GVL IBA

methyl acetate 2-methyl-1,3-dioxolane 2-methyltetrahydrofuran N-methylacetamide N-methylformamide N-methylpyrrolidone N,N-diethylformamide N,N-dimethylacetamide N,N-dimethylformamide 3-pentanone propylene carbonate 2-pyrrolidone tetrahydrofuran tetrahydrofurfuryl acrylate tetrahydrofurfuryl methacrylate toluene

MAc 2MDOL 2MTHF NMA NMF NMP DEF DMAc DMF 3PO PC 2Py THF THFA THFMA

PLLA film was carried out at room temperature under atmospheric pressure. When solvents were removed to some extent and desorption became relatively slow, surrounding temperature was set to 30−40 °C (300 °C). If one assumes that this weight loss is due to CPO, then the calculated CPO solubility is 0.28 g/gPLLA, which is in good agreement with the value obtained from the weighing (ca. 0.3 g/gPLLA). A similar result was obtained for the PLLA film containing ca. 0.3 g/gPLLA GBL. FTIR Spectra of PLLA ε-Forms. Figure 8 shows FTIR spectra of PLLA crystallized in a CPO, DMF, DOL, GBL, or THF atmosphere. The existence of solvents in PLLA films was confirmed by the solvent-derived IR bands,30 although it was hard to determine the location of solvents (i.e., PLLA amorphous or crystalline region). In particular for CPO and GBL, one would expect that the number of the extra solvents in the amorphous region was minimized by drying in air for the desired periods (just before the diffraction peak of α-form appears). First, we focus our attention on the spectral region of 1850−1600 cm−1 (Figure 8a). As for the carbonyl stretching band, there were almost no differences in the peak positions (1758 cm−1 for gt; 1777 cm−1 for gg31) and shape among the εforms and α-one. This result suggests that the interaction 1389


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Figure 4. Effect of solvent species on ε-crystal formation of PLLA.

solvents have a small number of α-crystals as well as the ε-ones (ε-rich state). Preparing conditions of the oriented ε- and ε-rich films (Table 2) were chosen to minimize the ratio of α-form. Roughly estimated magnitude relation of the ratio of ε- to αcrystals is as follows: THF ≈ GBL > DOL. Therefore, we first analyzed the fiber diagrams of εDMF- and εCPO-films (see Supporting Information), which are almost free from the αform. Based on results of fiber diagram analyses on εDMF-, εCPO, and α-films, the α-derived reflections were separated from the fiber diagrams for THF, GBL, and DOL. The fiber periods of εforms, which were calculated from the distance between neighboring layer lines, are in the range 2.8−2.9 nm, in good agreement with that of the α-form (2.888 nm: one period for a 107 helix of PLLA16a). Furthermore, the integrated intensity was relatively high on the layer lines with l = 0 (equatorial), 1, 3, 4, 6, 7, and 10, whereas relatively low on the layer lines with l = 2, 5, 8, and 9. Such dependence of diffraction intensity on the layer line order clearly shows that PLLA chains in the ε-form take the 107 helical conformation and the chain axis is perpendicular to ab plane (α = β = 90°), as in the α-form.34 As mentioned before, FTIR spectroscopic data suggest that the PLLA−solvent interaction is not enough to affect the CO stretching mode of PLLA. This infrared result would be linked with the diffraction result that there are almost no differences in a fiber period and helical sense among the ε- and α-forms. Namely, if solvent molecules have a great effect on the CO stretching mode of PLLA, then some conformational changes of PLLA should be induced, resulting in the changes in helical sense and a fiber period. In some cases, the complexation with low molecular weight compounds induces the conformational changes of host polymer. The chain conformation of PEI changes from the double-standard helix to the planar zigzag by the formation of crystalline hydrates.9 sPS takes the TTGG

Figure 5. Change of WAXD curve for εTHF-film with solvent desorption.

with solvent desorption. On the basis of these results, we move on to the next section, assuming that the ε-form is the PLLA− solvent complex crystal (cocrystal). Unit Cells of PLLA ε-Forms. Fiber diagrams for the ε- and ε-rich PLLA films are shown in Figure 9. The corresponding ξ−ζ converted patterns of the intensity-corrected fiber diagrams and Weissenberg photographs are shown in the Supporting Information. For comparison, the data of α-form are also shown. For THF, GBL, and DOL, unfortunately, we failed to prepare the oriented crystallized films having only the ε-crystals. Namely, the oriented crystallized films for these three 1390


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Figure 6. THF solubility dependence of (a) area, (b) intensity, and (c) position of ε-derived and α-derived diffraction peaks.

The density of PLLA is 1.248 g/cm3 in the amorphous state and 1.26−1.29 g/cm3 in the crystalline state (α-form).16,36 Therefore, the density of PLLA in the semicrystalline state (αform) is in the range between these values. In contrast, the observed density is 1.195 g/cm3 for the PLLA film having εcrystals with CPO (semicrystalline state). Thus, the observed density of εCPO-film was smaller than that of the amorphous PLLA (dobs = 1.254 g/cm3). For the εGBL-film, the observed density is 1.258 g/cm3. As mentioned in the Experimental Section, for the PLLA−solvent complexes having DMF, DOL, and THF, the density of each film was unable to be measured due to relatively fast desorption of these solvents. Since the contribution from the amorphous region needs to be considered, the observed densities were used as a rough indication, as shown later. Packing of PLLA Chains and Solvents in ε-Crystal Unit Cell. In this study, the crystal structure analyses on PLLA− DMF and PLLA−CPO complexes were performed because the oriented ε-films containing almost no α-crystals were able to be obtained only for these solvents. As a result, we succeeded in obtaining the plausible crystal structure of εDMF. In the case of PLLA−CPO complex, however, the packing model with R < 20% and a relatively low packing energy was unable to be obtained. Therefore, the crystal structure analysis process is shown below only for the PLLA−DMF complex. First, the number of PLLA chains in the unit cell was assumed to be four. If there are 2, 4, and 6 chains in the unit cell of PLLA−DMF complex, the calculated densities are 0.440, 0.880, and 1.321 g/ cm3, respectively. For the two-chain model, the calculated density is too small (dcal < 1.0), even if 20 solvent molecules are encapsulated in the unit cell (1:1 complex). In the case of the six-chain model without guests, the calculated density (1.321 g/ cm3 for the εDMF) is higher than that of the α-form (1.26−1.29 g/cm3). If one assumes that there are four chains and some solvents in the unit cell, the calculated densities become valid values in terms of the observed densities (1.195 g/cm3 for the εCPO-film and 1.258 g/cm3 for the εGBL-film). Second, values of two skeletal torsional anglesφ = (C′0, O″1, Cα1, C′1) and ψ = (O″1, Cα1, C′1, O″2)were optimized with the constraints of a constant fiber period (refined value), helical periodicity (ideal 107 helix), and a constant torsional

Figure 7. Thermogravimetric curve of PLLA crystallized in CPO vapor at 5 °C for 15 days and dried in air. Data of PLLA free from solvents are also shown for comparison.

helical conformation in the clathrate, whereas the planar zigzag conformation in the annealed samples (α or β).7,8 By the trial-and-error method taking into account the 21 helical symmetry of the 107 PLLA helix, indexing by the orthorhombic unit cell (a = 1.5−1.6 nm, b = 1.2−1.3 nm, c = 2.8−2.9 nm, and α = β = γ = 90°) was successful for the εforms, as shown in Table 3. For the ε-forms, the length of a-axis is about 1.5 times that for the α-form, and the length of b-axis is about twice that for the α-form. Namely, the area of ab plane of the ε-form is about 3 times that of the α-form. The space group of ε-forms was assumed to be P212121, which is same as that of the α-form, from the systematic absences of odd reflections for h00, 0k0, and 00l. It is unlikely that the chain folding direction changes during the ε-to-α transition with solvent desorption. Therefore, it should be reasonable to assume that the unit cell of each ε-form consists of parallel and antiparallel chains, as in the α-form. In the α-form, parallel chains are located in corners of the orthorhombic unit cell, and an antiparallel chain is in the center of the orthorhombic lattice. These parallel and antiparallel chains are related each other by 21 screw axes parallel to a- and b-axes, respectively (P212121).16 1391


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Figure 8. FTIR spectra of PLLA ε-films at (a) 1600−1850 cm−1 and (b) 600−1000 cm−1. Solvent-derived IR bands are marked with black triangles. Data of amorphous and α-films are also shown for comparison.

Third, energy and R-factor calculations without guest solvents were performed to save calculation time. With the conformation of PLLA fixed (rigid chain model), the chain packing style of PLLA was parametrized by four variables: (1) the translation to the a-axis direction from the center of chain (X), (2) the translation to the b-axis direction from the center of chain (Y), (3) the translation to the c-axis direction (Z), and (4) the rotation around the helix center (W). Z is a height of the ether oxygen atom in the first residue of a PLLA chain. The orientational angle (W) was determined as follows: W = W′ + 100.6°, where W′ is the angle (ab plane) between the unit vector parallel to the a-axis and the shortest vector from the center of chain to the ether oxygen in the first residue. If the position of one chain in the unit cell is determined by X, Y, Z, and W, then those of other three chains are determined automatically owing to the symmetry of P212121. Each chain packing parameter was changed, as follows: 0 ≤ X ≤ 0.25 (fractional coordinate), 0 ≤ Y ≤ 0.5 (fractional coordinate), 0 ≤ Z ≤ 0.1 (fractional coordinate), and 0 ≤ W ≤ 180°. A step of each packing parameter is as follows: ca. 0.02 nm for X, ca. 0.02 nm for Y, ca. 0.03 nm for Z, and 12° for W. Here, torsional energies and nonbonded interactions between atoms in the same chain (intramolecular interactions) were constant values because of the fixed conformation. Therefore, these energies were not calculated in this stage. The mapping of R-factor and host−host interaction energy for εDMF is shown in the Supporting Information. Since (X, Y) for the minimum Rfactor was not in agreement with that for the minimum energy, we selected some (X, Y) points, at which a relatively small Rfactor with an attractive packing energy was obtained. Here, the discrepancy between minima of R-factor and host−host interaction energy may be attributed to the instability of PLLA crystal lattice without solvents, which can be seen from a drastic change in the unit cell dimension by solvent desorption (ε-to-α transition), as mentioned above. Three packing models were selected for PLLA chains in the εDMF (models IDMF, IIDMF, and IIIDMF), in which guest solvents are not taken into consideration. The R-factor and the packing energy (EHH) for these three models are shown in Table 4. For

Figure 9. Observed fiber diagrams for oriented ε- and ε-rich films (a− e) and annealed one (f). Drawing direction equals the vertical direction.

Table 3. Lattice Constants of ε-Forms (α = β = γ = 90°) solvent

a (nm)

b (nm)

c (nm)

unique reflectionsa

CPO DMF DOL GBL THF airb

1.616 1.551 1.539 1.558 1.574 1.066

1.261 1.227 1.215 1.242 1.236 0.616

2.899 2.857 2.898 2.891 2.871 2.888

30 30 29 38 40 121

a

The number of unique reflections used for refinement of lattice constants. bLattice constants of the α-form (α = β = γ = 90°) reported by Sasaki et al.16a

angle (ω = (Cα1, C′1, O″2, Cα2) = 180°: ester bond) using the Miyazawa equation.37 These two torsional angles (φ and ψ) were changed around those of α-form (φ = −66.0° and ψ = 150.8°) with a range of ±10° and a step of 0.1°. The obtained results are as follows: φ = −63.7° and ψ = 148.5° for the εDMF (fiber period: 2.857 nm). 1392


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Table 4. Energy and R-Factor of Each Packing Model for PLLA−DMF Complex

a

model

EHHa

IDMF IIDMF IIIDMF IDMF′ IDMF′−R IDMF″ IDMF″−R

−37.2 −39.0 −20.7 −37.2 −32.7 −36.2 −32.6

EHGa

25.4 46.6 10.0 28.9

EGHa

51.1 79.3 18.0 55.6

EGGa

Eintraa

R (%)

−9.2 −10.4 −12.6 −14.5

−85.1 −85.1 −85.1 −85.1 −80.3 −85.1 −80.8

39.0 39.8 39.4 29.3 23.3 27.5 19.6

Information. By combining these three methods, the structural refinements were performed for the PLLA−DMF complex. PLLA has relatively small side groups (CH3 groups), so that relatively large intermolecular forces would act to the mainchain atoms, as mentioned by Takahashi and Tadokoro for PEO.35b As a result, helix distortion would be induced as in the α-form.16 One of the selected packing models having 8 DMF molecules per unit cell, which started from IDMF, gave a distinct decrease in the R-value by the constrained least-squares method, keeping the energetically favorable state. Here, the corresponding models before and after the refinement are named the models IDMF′ and IDMF′_R, respectively (Table 4 and Figure 10). In the

Energies are given in kcal/mol.

all the selected models, there are relatively large cavities (e.g., around (xf, yf) = (0.25, 0.5)), and each cavity is surrounded by four chains. Therefore, we assumed that guest solvents are encapsulated in these cavities. By the symmetry of P212121, if one puts a solvent in the cavity, another three solvents emerge automatically in the unit cell. Accordingly, the number of solvents in the unit cell is multiple of four (4, 8, 12, 16, 20, ...). For the models IDMF, IIDMF, and IIIDMF, R-factor calculations with guests were conducted by changing the packing of solvents in the symmetry of P212121 with PLLA chains fixed. To change the coordination style of guests, we used three parameters (r, z, μ) that represent the position of the selected atom in the solvent located at the lowest height (their zero point is the intersection of a 21 screw axis and Z = 0) and another three variables (φx, φy, φz) that determine rotation angles of the solvent around three axes in the internal coordinate system, in which a zero point is the center of mass for each solvent (see Supporting Information). For the εDMF-form, these positional parameters were changed in the following range: −0.20 nm ≤ r ≤ 0.30 nm (ca. 0.03 nm step), 0 nm ≤ z ≤ 1.40 nm (ca. 0.03 nm step), and 0° ≤ μ, φx, φy, φz ≤ 315° (45° step). We introduced solvents by four in the unit cell and calculated the R-factor of each system. Bond lengths, bond angles, and torsional angles of guests were set to constant values in the following calculations. Namely, only intermolecular interactions that act guest molecules in a period (EGH and EGG) were considered as a measure of the packing stability. By introducing four solvent molecules in the unit cell (a total of 4 guests), a decrease in R-factor was able to be obtained for some packing styles of guests. On the basis of a relatively small R-factor with the attractive packing energy, we selected some packing models (16 models). By introducing another four guest molecules (a total of 8 guests) to these selected models, the improvement of R-factor was seen for some packing models with the attractive packing energy (18 models). We tried to introduce another four guest molecules (a total of 12 guests) to these packing models, but valid models were unable to be obtained due to short contacts between the host and guest and those between guests. In the cases of 8-guest models, the calculated density is 1.059 g/cm3. Since the εDMF-film has the complex region (dcal = 1.059 g/cm3) and the amorphous one (dobs = 1.254 g/cm3), dobs of the εDMF-film (not measured) would be an intermediate value between 1.059 and 1.254 g/ cm3, which is comparable with the observed densities of ε-films (1.195 g/cm3 for CPO and 1.258 g/cm3 for GBL). For the structure refinement of PLLA εDMF-form, the linkedatom least-squares method29 was applied to the 8-guest models, which were obtained by the grid search algorithm, as mentioned above. We used three types of refinement methods (RM1, RM2, and RM3), as shown in detail in the Supporting

Figure 10. Flowchart of the crystal structure analysis process for PLLA−DMF complex. Gs: grid search algorithm; Rs: random search algorithm; Rs*: Rs around the optimized model (= IDMF′); and Rf: refinement by the constrained least-squares method.

model IDMF′_R, the R-factor was 23.3% for 30 unique reflections, with EHH = −32.7 kcal/mol, EHG = 46.6 kcal/mol, EGH = 79.3 kcal/mol, and EGG = −10.4 kcal/mol. In general, in the crystal structure analyses of polymers, the R-factor of 20% is a criterion. Therefore, our result (23.3%) is not enough in the current state. To reduce the R-factor, the positional parameters of PLLA chains and DMF molecules were changed randomly around those optimized by the grid search algorithm (IDMF′). The distance parameters (X, Y, Z, r, and z) were changed ±0.01 nm, and the angular ones (W, μ, φx, φy, φz) were changed ±20°. In addition, the following calculations were also done as the special cases: (i) changing only the host parameters randomly with the guest ones fixed and (ii) changing only the guest ones randomly with the host ones fixed. From the obtained packing styles, valid models were selected and refined, taking into account the R-factor and the packing energy (EHH + EHG + EGH + EGG). For one of them, the R-factor of 19.6% was obtained with EHH = −32.6 kcal/mol, EHG = 28.9 kcal/mol, EGH = 55.6 kcal/mol, and EGG = −14.5 kcal/mol. Here, this model is named IDMF″_R (see Table 4 and Figure 10). Before the refinement, the corresponding model is IDMF″. Here, the intramolecular interaction energy of PLLA chain (Eintra = EH + Etors) just exhibited a relatively small change by the structure refinement from the ideal 107 helix to the distorted helix with the 21 symmetry along the chain axis, as shown in Table 4. Figure 11 shows the ab- and ac-projections of IDMF″_R. Figure 1393


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12 is the ab-projection of IDMF″_R to show the positions of host and guest molecules clearly and compare with the α-form. Table 5 shows the comparison of the observed and calculated values of lattice spacings and structure factors for the εDMF (IDMF″_R). The atomic fractional coordinates of atoms in the PLLA chain (one period: 60 atoms) and DMF molecules (two molecules: 24 atoms) for IDMF″_R are shown in the Supporting Information. The skeletal torsional angles of PLLA in half a period (five residues) for IDMF″_R are also listed in the Supporting Information. Comments on Crystal Structures of PLLA−Solvent Complexes. For the PLLA−DMF complex, the valid model IDMF″_R (R < 20% with a relatively low packing energy) was able to be obtained. As shown in Figures 11 and 12, PLLA chains in the PLLA−DMF complex form the channel-shaped cavities along the chain direction (c-axis direction), in which DMF molecules are packed. As mentioned above, PLLA has relatively high selectivity of solvents in complexation, while sPS can form clathrates with both benzene and its substitutes. The channel-shaped cavity with a relatively short diameter for the PLLA−DMF complex should be responsible for such high selectivity of solvents. For the PLLA/PDLA stereocomplex, the existence of weak hydrogen bonding (Cα−H···O and CH3···O interactions) was reported by Sarasua et al.38 In the case of PLLA−DMF complex, it is considered that the same kind of interactions between the host (PLLA) and guest (DMF) have an important role in stabilizing the complex structure. Judging from FTIR data (CO stretching band), intermolecular interactions between PLLA and DMF molecules should be comparable with those between PLLA chains in the α-form. Namely, a large number of weak interactions between the host and guest should stabilize the PLLA−solvent complex. When PLLA was crystallized in the solvents that have the similar chemical structure to the ε-solvents (e.g., CH3-substituted), it should be difficult for PLLA chains to form the weak hydrogen

Figure 11. Packing model of PLLA chains and DMF molecules in the εDMF (IDMF″_R): (a) ab- and (b) ac-projections. Carbon atoms in PLLA chains and DMF molecules are shown by gray and blue, respectively. The unit cell is shown by a yellow rectangle.

Figure 12. Comparison of ab projection of PLLA−DMF complex with that of α-form: (a) PLLA−DMF complex with DMF shown, (b) PLLA− DMF complex with DMF hidden, and (c) α-form.16a Schematic illustrations of packing of PLLA chains and DMF molecules are also shown below the corresponding ab projections. White circle (P) and gray one (A) mean the parallel and antiparallel chains, respectively. Four parallel and one antiparallel helices are enclosed by a dotted line for comparison (b, c). The occupied regions by DMF molecules are represented by purple and green ellipsoids (a). 1394


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during solvent desorption. Namely, it should be assumed that solvent desorption proceeds gradually from the lamellar surface. By solvent desorption, there would remain many cavities that had been occupied by DMF molecules. In terms of the packing energy, this emptied form can be allowed (EHH < 0). However, the ε-to-α transition proceeds directly from the ε- to the αforms, not via the intermediate phase (e.g., emptied form). This result implies that the activation energy needed for the ε-to-α transition is relatively small. When solvent desorption proceeds to some extent (e.g., in a large part of a lamella), the chain rearrangement to the α-form should occur to fill cavities and take a more stable packing style, as shown in Figure 12. Also, other PLLA−solvent complexes (CPO, DOL, GBL, and THF) would have similar packing styles to the PLLA−DMF complex because of the similarity of these five complexes in unit cell dimensions and transition behavior to the α-form. In terms of the packing styles of hosts and guests (e.g., four chains in the P212121 orthorhombic lattice and guests encapsulated in the cavity surrounded by helices), the PLLA−DMF complex has the similarity to amylose cocrystallized with water and 2propanol.39

Table 5. Observed and Calculated Lattice Spacings and Structure Factors of ε-Form of PLLA with DMF (IDMF″_R) h

k

l

dobs (nm)

dcal (nm)

Fobs

Fcal

2 0 2 3 1 3 4 0 5 1 2 3 1 3 4 1 2 1 2 3 0 5 6 1 3 2 3 2 4 5 2 2 4 0 5 6 1 2 3 1 3 4 1 5 1 4 6 1 1 0 2 3 2 4

0 2 2 1 3 2 0 4 1 1 2 1 3 2 0 1 0 2 2 1 5 3 1 1 0 2 1 4 3 2 1 3 1 5 3 1 1 2 1 3 2 0 4 1 5 4 2 1 2 2 2 1 3 1

0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 5 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 6 7 7 10 10 10 10 10

0.762 0.625 0.487

0.777 0.614 0.482 0.478 0.396 0.396 0.389 0.307 0.301 0.913 0.475 0.471 0.392 0.392 0.385 0.677 0.602 0.490 0.430 0.427 0.238 0.240 0.245 0.574 0.419 0.400 0.397 0.265 0.262 0.259 0.431 0.306 0.311 0.226 0.227 0.232 0.427 0.339 0.337 0.304 0.305 0.301 0.255 0.255 0.216 0.215 0.213 0.376 0.332 0.259 0.246 0.245 0.224 0.226

285 211 420

230 208 399

609

575

108

150

27 586

81 462

385

333

46 63 288 363

70 112 339 370

110

80

0.383

0.314 0.912 0.471 0.398

0.679 0.587 0.478 0.434 0.242

0.570 0.412 0.395 0.258

0.429 0.309 0.233

0.427 0.339 0.306

0.254 0.219

0.379 0.335 0.261 0.246 0.226

49 251 193

53 203 101

224

191

151 180

113 157

223

130

54 211

133 315

187

228

387

371

398

362

65 153 174 275

146 203 138 306

236

233

■

CONCLUSIONS

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ASSOCIATED CONTENT

In this study, we investigated the crystalline complex formation of PLLA with low molecular weight compounds by screening examinations for a wide variety of organic solvents. As a result, we found that PLLA forms the crystalline complex (ε-form) with the specific five-membered ring compounds (CPO, DOL, GBL, and THF) or DMF below room temperature. In addition, the ε-to-α transition occurred with solvent desorption, in which solvent solubility of 0.3−0.4 g/gPLLA is a starting point. By fiber diagram analyses, it was revealed that PLLA chains in the εform take the 107 helical conformation as in the α-form. We succeeded in indexing by the orthorhombic unit cell (a = 1.5− 1.6 nm, b = 1.2−1.3 nm, c = 2.8−2.9 nm, and α = β = γ = 90°) for the PLLA ε-forms. Considering both R-factor and packing energy (host−host, host−guest, guest−host, and guest−guest interaction energies), the plausible crystal structure of εDMF (PLLA−DMF complex) was proposed (IDMF″_R), in which four PLLA chains and eight guest solvents are packed in the unit cell.

S Supporting Information *

Solvent exposure conditions (Table S1); the details on WAXD profile and fiber diagram analyses, details on nonbonded interaction and torsional energies, van der Waals parameters (Table S2); the calculated partial charges for PLLA, DMF, and CPO (Tables S3, S4, and S5, respectively); ξ−ζ conversion of intensity-corrected fiber diagrams (Figure S1); Weissenberg photographs (Figure S2); fiber diagram analysis processes for the oriented εDMF- and εCPO-films (Figures S3 and S4, respectively); the mapping of R-factor and host−host interaction energy for εDMF and εCPO (Figures S5 and S6, respectively); positional parameters of the guest molecule (Figure S7); refinement methods, atomic fractional coordinates for IDMF″_R (Table S6); and skeletal torsional angles of IDMF″_R (Table S7). This material is available free of charge via the Internet at http://pubs.acs.org.

bonding with these solvents in the channel-shaped cavities, resulting in the formation of noncomplex structure (α). Here, it should be reasonable to assume that each guest goes along the channel-shaped cavity and gets out of the PLLA crystal lattice 1395


Macromolecules

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AUTHOR INFORMATION

Corresponding Author

*Tel +81-3-5734-2432; Fax +81-3-5734-2431; e-mail asai.s.aa@ m.titech.ac.jp. Notes †

Research Fellow of the Japan Society for the Promotion of Science.

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ACKNOWLEDGMENTS H.M. thanks Prof. K. Tashiro (Toyota Technological Institute), Prof. T. Ozeki (Tokyo Institute of Technology), Prof. P. D. Hong (National Taiwan University of Science and Technology), Prof. O. Tarallo (Università degli Studi di Napoli Federico II), Prof. M. Wada (The University of Tokyo), and Dr. H. Shii for giving him beneficial suggestions. H.M. thanks Prof. H. Uekusa (Tokyo Institute of Technology) for letting him use a Weissenberg camera. H.M. also thanks Prof. T. Hayakawa and Dr. Y. Ishida (Tokyo Institute of Technology) for supplying some organic solvents. We thank the Toyota Motor Corporation, Japan, for supplying the PLLA pellets. Financial support from Tokyo Institute of Technology Global COE program “Education and Research Center for Emergence of New Molecular Chemistry” is gratefully acknowledged. This work was partly supported by Grant-in-Aid for JSPS Fellows (No. 22008912) from Japan Society for the Promotion of Science.

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