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Oct 9, 2015 - Poly(L‑lactic acid): Uniplanar Orientation in Cocrystalline Films and. Structure of the Cocrystalline Form with Cyclopentanone. Paola ...
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Poly(L‑lactic acid): Uniplanar Orientation in Cocrystalline Films and Structure of the Cocrystalline Form with Cyclopentanone Paola Rizzo,*,† Graziella Ianniello,† Vincenzo Venditto,† Oreste Tarallo,*,‡ and Gaetano Guerra† †

Dipartimento di Chimica e Biologia and INSTM Research Unit, Università degli Studi di Salerno via Giovanni Paolo II 132, 84084 Fisciano (SA), Italy ‡ Dipartimento di Scienze Chimiche, Università degli Studi di Napoli Federico II,Complesso di Monte S. Angelo, via Cintia, 80126 Napoli, Italy S Supporting Information *

ABSTRACT: Guest molecules, like cyclopentanone (CPO), N,Ndimethylformamide (DMF) tetrahydrofuran (THF) and 1,3−dioxolane (DOL), which are able to cocrystallize with poly(L-lactic acid) (PLLA), can also generate high degree of crystalline phase orientation, by simple sorption in amorphous unoriented PLLA films. This crystalline phase orientation is defined as a∥c∥ (a-parallel, c-parallel) because the ac plane of most crystallites is preferentially parallel to the film plane. This crystalline phase orientation, which is achieved without stretching even for high thickness films, is unprecedented for PLLA and can be maintained after suitable guest exchange procedures. The reported results confirm the recent hypothesis that cocrystallization with lowmolecular-mass guest molecules could be a common route for getting uniplanar orientations of polymer crystalline phases, even in the absence of stretching. Wide angle X- ray diffraction analyses of unoriented and oriented PLLA samples have allowed proposing the structure of the cocrystalline form of PLLA with CPO. In particular, four PLLA chains exhibiting the 10/7 helical conformation and 16 CPO guest molecules are packed according the space group P212121 in an orthorhombic lattice with a = 1.61 nm, b = 1.26 nm, and c = 2.90 nm. The presence in PLLA cocrystalline structures of dense bc layers with close-packed polymer helices allows an easy rationalization of the observed uniplanar orientation.

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INTRODUCTION Polymeric cocrystalline forms, i.e. structures were a polymeric host and a low-molecular-mass guest are cocrystallized, although early recognized in literature, have been ignored for many decades.1−15 Only in the last 2 decades have a few polymeric cocrystalline forms received attention in material science, mainly due to their ability to produce nanoporouscrystalline forms,16−21 as a consequence of guest removal.16,22,23 In the past decade, it has been also found that cocrystallization with suitable guest molecules of two commercial polymers: syndiotactic polystyrene (s-PS) and poly(2,6-dimethyl-1,4-phenylene ether) (PPO), can lead without any stretching procedure to high thickness f ilms (up to 200− 300 μm) with high degrees of different kinds of uniplanar orientations of the crystalline phases, i.e., with high degrees of parallelism of well-defined crystal planes with respect to the film plane.24−34 In particular, for s-PS films, three different kinds of uniplanar orientations can be achieved by simple procedures involving cocrystallization in the presence of suitable guest molecules (solution crystallization,24−26 solventinduced crystallization in amorphous samples,27−30 or solventinduced recrystallizations of suitable crystalline films31). The three observed uniplanar orientations correspond to the three © XXXX American Chemical Society

simplest orientations of a high planar-density layer (formed by close-packed alternated enantiomorphous s-PS helices) with respect to the film plane.32,33 As for PPO, two different kinds of uniplanar orientations have been recently obtained by cocrystallization, as induced by different guests in amorphous films.34 In general, for polymeric materials, the effects of orientation on physical properties are strongly relevant: for instance, the extent to which it is possible to increase stiffness and strength find no parallel with other materials.35 Moreover, crystalline phase orientation is particularly relevant for functional polymeric materials, when the functionality is mainly associated with crystalline phases, as for conductive36 or ferroelectric37 polymers. The recently achieved control of orientation of cocrystalline phases can also bring to control of orientation of active-guest molecules, not only in the crystalline phase but also in macroscopic films. In fact, several studies have shown that the kind and degree of orientation can be relevant for properties of cocrystalline polymer phases, whose functionality is associated with 3-dimensional ordered active guest molecules Received: April 30, 2015 Revised: September 24, 2015

A

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Macromolecules (which can be fluorescent,38−41 photoreactive,42−44 magnetic,45,46 ferroelectric,47,48 or chiral−optical49−56). The phenomenon of polymer crystalline phase orientation in the absence of mechanical stretching is quite common for thin films (with typical thickness lower than 100 nm)57−59 while for high thickness films has been mainly observed for polymer cocrystallization with low-molecular-mass guest molecules.24−34 Generally for high thickness polymers films, uniplanar orientations are generally achieved only as a consequence of (mainly biaxial) mechanical stretching. The easy achievement of uniplanar orientation, without mechanical stretching, by cocrystallization of s-PS and PPO with different low-molecularmass guest molecules, suggests that polymer cocrystallization could be a general route for getting uniplanar orientation. In this paper, we have explored the possibility to achieve uniplanar orientation, in the absence of mechanical stretching, for poly(L-lactic acid) (PLLA), i.e., another industrially relevant semicrystalline polymer,60 which is able to cocrystallize with many guest molecules as recently reported by the extended work of Asai and co-workers.61−63 The present study describes the achievement of uniplanar orientation by PLLA cocrystallization, as induced by sorption of different guest molecules in amorphous films (cyclopentanone, CPO; N,N-dimethylformamide, DMF; tetrahydrofuran, THF; 1,3-dioxolane DOL). The related collection of X-ray diffraction data for unoriented and oriented samples has also given the opportunity to define the crystal structure of the cocrystalline form of PLLA with CPO, which also allows an easy rationalization of the observed uniplanar orientation.

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Because the 020 plane is unique (i.e., there are no other equivalent directions in the crystal), f 020 is equal to +1 or −0.5 when the (020) crystal plane of all crystallites are perfectly parallel or perpendicular to the film plane, respectively. For the case of random orientation f 020 is equal to zero. The quantity cos2 x020 has been experimentally evaluated, by the above-described EDGE X-ray diffraction patterns, as 2

cos x020 = cos χ020 =

π /2

I(χ020 ) cos2 χ020 sin χ020 dχ020

∫0

π /2

I(χ020 ) sin χ020 dχ020

where I(χ020) is the intensity distribution of the 020 diffraction on the Debye ring at 2θCuKα = 14,0° and χ020 is the azimuthal angle measured from the horizontal line of EDGE patterns, like the one shown in Figure 2A. The azimuthal angles (χ020) calculated for the reflections of the PLLA/CPO cocrystalline form (reported in the sixth column of Table

Table 1. Comparison between Reflections of the PLLA/CPO Co-Crystalline Form, As Collected for a Powder and for an Amorphous Film Crystallized by CPO Sorption at −25 °Ca unoriented samples

EXPERIMENTAL SECTION

f020 = (3 cos x020 − 1)/2

∫0

(2)

Materials and Preparation Procedures. PLLA used in this study was purchased by Sigma-Aldrich and presents weight-averaged and number-averaged molecular masses Mw= 152000 and Mn= 99000, respectively. Solvents were purchased from Aldrich and used without further purification. Amorphous PLLA films with thickness in the range 70−200 μm were obtained by compression molding, after melting at 200 °C. Crystallization of amorphous films has been induced by vapor sorption procedures or immersion in pure liquids, at different temperatures. PLLA powders, with CPO content close to 23 wt % and exhibiting the PLLA/CPO cocrystalline form, were prepared from PLLA/CPO 10/90 wt % gels, after 1day of solvent desorption in air at room temperature. The amount of CPO, DMF, THF, and DOL guest molecules in cocrystalline films, evaluated by thermogravimetric measurements, is in the range 20−25 wt %. Characterization Techniques. Wide-angle X-ray diffraction (WAXD) patterns with nickel-filtered Cu Kα radiation were obtained, in reflection, by an automatic Bruker diffractometer. WAXD patterns were also obtained, in transmission, by using a Philips diffractometer with a cylindrical camera (radius = 57.3 mm). In the latter case the patterns were recorded on a BAS-MS imaging plate (FUJIFILM) and processed with a digital imaging reader (FUJIBAS 1800). In particular, photographic WAXD patterns were taken by having the X-ray beam parallel (EDGE) and perpendicular (THROUGH) to the film surface and by placing the film sample parallel to the axis of the cylindrical camera. The degree of uniplanar orientation with respect to the film plane of the 020 crystal plane ( f 020), has been formalized on a quantitative numerical basis using the Hermans’ orientation function,64 as extended to uniplanar orientations:32 2

2

(020) uniplanar oriented films

2θobs

Iobs

hkl

2θobs

χobs (deg)

χcalc (deg)

Iobs EDGE

Iobs. THROUGH

6.25 9.4 11.0 14.1 15.3 16.7 17.9

vw vw m m mw mw vs

101 111 200 020 121 301 220 123 221 310 311 223 321 130

6.3 9.4 11 14.0 15.4 16.7 17.9

− 43 90 0 21 90 36

− vw m vs w w s

mw − vs vw vw ms ms

18

66

20.1

43

90 42 90 0 23 90 37 37 38 67 67 45 49 13

21.9

12

20.1 22

28.3

m m

vw

131 400 401 040

w mw

vw s

m

22.2

90

15 90

28.3

0

0

m vw

−

The patterns of the films have been collected with beams perpendicular (THROUGH) and parallel (EDGE) to the film plane. As for the EDGE patterns, the observed azimuthal angles (χobs) are compared with those calculated (χcalc) in the assumption of complete parallelism of the (020) plane with respect to the film plane. a

1), for films presenting ideal 020 uniplanar orientation, were determined by using the equation:

⎛ |k| ⎞ χhkl = arccos⎜dhkl · ⎟ ⎝ b⎠

(3)

where hkl are the Miller indexes of the considered crystal plane and b the unit-cell axis. Calculated structure factors have been obtained as Fc = (∑| Fi|2Mi)1/2, where Fi is the structure factor and Mi the multiplicity factor of the reflection i (Miller indices (hkl)i) in powder diffraction profiles, and the summation is taken over all reflections included in the 2θ range of the corresponding diffraction peak observed in the X-ray

(1)

by assuming cos2 x020 as the average cosine squared values of the angle, x020, between the normal to the film surface and the normal to the (020) crystal plane. B

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Macromolecules powder diffraction profile. A thermal factor B = 8 Å2 and atomic scattering factors as in ref 65 have been assumed. The observed structure factors, F0, have been evaluated from the intensities I0 of the reflections observed in the powder diffraction profiles as F0 = (I0/LP)1/2, where LP is the Lorentz-polarization factor for X-ray powder diffraction.66 The experimental intensities I0 have been evaluated by measuring the area of the peaks in the X-ray powder diffraction profile, after subtraction of a straight baseline approximating the background and of the amorphous contribution. The discrepancy factor R has been evaluated as R = ∑|Fo − Fc|/∑Fo taking into account only the observed reflections. Calculated X-ray powder diffraction pattern were obtained with the Diff raction-Crystal module of the software package Cerius2 (version 4.2 by Accelrys Inc..) using an isotropic thermal factor (B = 8 Å2). A Gaussian profile function having a half-height width regulated by the average crystallite size along a, b, and c axes (La, Lb and Lc, respectively) was used. A good agreement with the half-height width of the peaks in the experimental profile has been obtained for La = Lb = 10 nm and Lc = 8 nm. Energy calculations were carried out by using the Compass force field67 within the Open Force Field module of Cerius2 by the smart minimizer method with standard convergence. The starting 10/7 conformation of the PLLA polymer chains was that undistorted reported in the literature for its α polymorph.68 Infrared spectra were obtained in wavenumber range 400−4000 cm−1 at a resolution of 2.0 cm−1 with a Vertex 70 Bruker spectrometer equipped with deuterated triglycine sulfate (DTGS) detector and a Ge/KBr beam splitter. The frequency scale was internally calibrated to 0.01 cm−1 using a He−Ne laser. 32 scans were signal averaged to reduce the noise. The content of the guest molecules in the films was determined by the intensity of FTIR guest peaks, as calibrated by thermogravimetric measurements. Thermogravimetric measurements (TGA) were performed with a TG 209 F1 equipment from Netzsch in a flowing nitrogen atmosphere at a heating rate of 10 °C/min.

Figure 1. X-ray diffraction patterns, as collected by an automatic powder diffractometer, of PLLA cocrystalline samples including CPO guest molecules: powder from gel (A); amorphous films as crystallized by immersion for 3 days in liquid CPO at +25 (B), + 2 (C), and −25 °C (D). Close to the main diffraction peaks, the Miller indexes of the PLLA cocrystalline form are indicated. Stars indicate reflections not shown by less crystalline literature samples. In part A, the amorphous and background contribution is shown by a dotted red line.

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intensities), indicating the formation of PLLA/CPO cocrystalline phase. A closer comparison with the pattern of Figures 1A shows that the pattern of Figure 1C exhibits increased and reduced intensity of the 020 and 200 reflections, respectively. In fact, the I020/I200 ratio increases from 1.2 up to 4.3 and suggests the presence of orientation of the 020 plane preferentially parallel to the film plane. The same crystallization procedure at −25 °C, leads to a WAXD pattern (Figure 1D) with only few peaks, which however can be again indexed by the abovedescribed PLLA/CPO cocrystalline form. In particular, the 020 reflection becomes the most intense, the weak 040 reflection is still clearly present, while the intense 200 reflection is completely disappeared. These results suggest the occurrence of (020) uniplanar orientation, i.e., an orientation of (020) crystal planes of most crystallites preferentially parallel to the film plane. A deeper understanding and a quantitative evaluation of the crystalline phase orientation for these films, can be achieved by WAXD patterns collected, in transmission, on a photographic cylindrical camera, as taken with beam parallel (EDGE) or perpendicular (THROUGH) to the film plane. In particular, EDGE and THROUGH patterns of the PLLA/CPO cocrystalline film obtained by immersion in liquid CPO at −25 °C (corresponding to Figure 1D), are shown in Figure 2, parts A and B, whereas their equatorial profiles are shown in Figure 2, parts A′ and B′, respectively. The presence of 020 and 040 arcs centered on the equator of the EDGE photographic pattern (Figure 2A) and the negligible intensity of these peaks in the THROUGH pattern (Figure 2B) and profile (Figure 2B′) clearly indicate the occurrence of 020 uniplanar orientation. Moreover, the THROUGH pattern (Figure 2B), only showing

RESULTS AND DISCUSSION Uniplanar Orientation in PLLA Films. To establish the possible occurrence of crystalline phase orientation it is generally relevant the availability of fully unoriented crystalline samples. Fully unoriented powders and aerogels exhibiting well formed cocrystalline phases have been obtained by drying procedures on polymer gels, both for s-PS69 and PPO.70 Analogous procedures are presently used to obtain highly crystalline and fully unoriented cocrystalline powders also for PLLA. In particular, the WAXD pattern of a PLLA/CPO cocrystalline powder, as obtained by drying (in air at room temperature) of a 10/90 by weight gel, is reported in Figure 1A. Close to the main diffraction peaks, the Miller indexes determined according to the orthorhombic unit cell proposed by Asai61(a = 1.61 nm, b = 1.26 nm, c = 2.90 nm) are indicated. The pattern of Figure 1A, compared with those reported in the literature,61−63 presents a higher crystallinity (xc ≈ 60%) as well as extra reflections at 2θ = 6.3°, 2θ = 9.4°, 2θ = 15.4°, and 16.7° (labeled by a star (∗) in Figure 1A). These new reflections can be indexed, on the basis of the above cited orthorhombic unit cell, as 101, 111, 121 and 301, respectively. WAXD patterns, collected by an automatic powder diffractometer, of amorphous PLLA films after crystallization procedures by immersion for 3 days in pure CPO at +25, +2, and −25 °C are shown in Figure 1, parts B−D, respectively. In agreement with previous reports,61 the guest-induced crystallization procedure at +25 °C leads to formation of unoriented α phase61,71 (Figure 1B). The same crystallization procedure at +2 °C leads to a pattern (Figure 1C) similar to that one of Figure 1A, (with only slight changes of the relative C

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Figure 2. Photographic WAXD patterns of a PLLA/CPO film as obtained by immersion in liquid CPO at −25 °C, as taken with X-ray beam parallel (EDGE, A) or perpendicular (THROUGH, B) to the film surface. Diffraction intensity profiles of these EDGE (A′) or THROUGH (B′) patterns, as taken along the equatorial line.

unpolarized Debye rings, indicates the absence of axial orientation. More quantitative information relative to the observed uniplanar orientation can be obtained by the data collected in Table 1. In particular, diffraction angles and relative intensities of the reflections of PLLA/CPO cocrystalline phases, for a powder (as collected by an automatic diffractometer, first and second columns) and for a film crystallized by CPO at −25 °C (photographic patterns, fourth, seventh, and eighth columns), are reported in Table 1. It is apparent that there is no change in the crystalline form, with only two broad peaks of the powder pattern (at 2θ ≈ 17.9° and 22°) being split in two components in the oriented film pattern. The azimuthal angles of maximum intensity (χobs) observed for the reflections of the EDGE pattern of Figure 2A and those calculated (χcalc), in the assumption of complete parallelism of the 020 planes with respect to the film plane, are also shown in fifth and sixth columns of Table 1. The good agreement between χobs and χcalc definitely proves the occurrence of 020 uniplanar orientation, for the obtained PLLA/CPO cocrystalline film. A quantitative evaluation of the degree of 020 uniplanar orientation, as obtained on the basis of azimuthal scan at 2θCuKα= 14.0° of the EDGE pattern of Figure 2A, indicates f 020 = 0.89. The observed 020 uniplanar orientation, by using a nomenclature analogous to that one used for δ form of sPS,32 can be also defined as a∥c∥ (a-parallel, c-parallel) uniplanar orientation, thus indicating that the ac plane of most crystallites is preferentially parallel to the film plane. The WAXD patterns of PLLA amorphous films, as crystallized by immersion for 3 days at −25 °C in different solvents, as collected by an automatic diffractometer immediately after cocrystallization, are shown in Figure 3. The pattern of the film crystallized by liquid DMF (Figure 3A) is similar to that one of the film crystallized by CPO (Figure 1D) with only small shifts of the diffraction angles, consistent with the orthorhombic unit cell with a = 1.55 nm, b = 1.23 nm, and c = 2.86 nm.61 Photographic patterns analogous to those of Figure 2A (see Supporting Information) clearly indicates that a high degree of a∥c∥ uniplanar orientation ( f 020 ≈ 0.9) can be achieved by cocrystallization with DMF.

Figure 3. WAXD patterns, as collected by an automatic powder diffractometer, of PLLA films presenting cocrystalline phases with different guest molecules: DMF (A), THF (B), and DOL (C).

The pattern of the film crystallized by liquid THF (Figure 3B) also presents peaks which can be indexed by an orthorhombic unit cell (a = 1.57 nm, b = 1.24 nm, c = 2.87 nm)61 with relative intensities intermediate between those of fully unoriented (shown in Figure 1A) or highly oriented samples (shown in Figure 1D). Photographic patterns analogous to those of Figure 2A allow establishing that the degree of a∥c∥ uniplanar orientation for the obtained PLLA/ THF cocrystalline phase is ≈0.7. The pattern of the film crystallized by liquid 1,3-dioxolane (DOL) (Figure 3C) shows the formation of the cocrystalline phase together with a significant amount of unoriented α phase (with typical reflections at 2θ = 16.7° and 19.1°).71 However, in this sample, the PLLA/DOL cocrystalline phase clearly exhibit uniplanar orientation, although with a low degree (f 020 < 0.5). It is worth adding that, by solution casting procedures (at room temperature as well as at +2 °C) with the same solvents (CPO, DMF and THF), only fully unoriented α-form films are obtained. The whole set of results of this section confirms that the occurrence of uniplanar orientation (in the absence of mechanical stretching) is a common phenomenon for polymer cocrystalline phases. Guest Exchange in PLLA Cocrystalline Phases. The relevance of crystalline phase orientations in polymer films is also related to their possible stability as a consequence of crystal phase transitions. Most of the achieved uniplanar orientations for cocrystalline phases of s-PS24−29 and PPO34 can be also maintained after guest-exchange procedures25,34 as well as after crystal to crystal transitions.24,25,28,29,31,72 The uniplanar orientation, as obtained for amorphous PLLA films by guest induced cocrystallization, can be fully maintained as a consequence of guest exchange procedures with suitable guest molecules. This is shown, for instance, for films exhibiting large PLLA/ CPO crystallites with high degree of a∥c∥ uniplanar orientation, whose WAXD patterns is shown in Figure 4A. The WAXD D

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Crystalline Structure of the PLLA/CPO Cocrystalline Form. The crystal structure determination of the PLLA-CPO cocrystalline form has been carried out on the basis of WAXD patterns of powders (Figure 1A) and oriented film samples (Figure 2 and Table1).The WAXD profile of powders after subtraction of the amorphous contribution (reported as a red dashed curve in Figure 1A) is shown in Figure 5A. Diffraction angles (2θobs), interplanar distances (dobs) and intensities (Iobs) of the reflections numbered in Figure 5A are reported in Table S1 of Supporting Information.

Figure 4. WAXD patterns of an amorphous PLLA film: (A) after crystallization induced by sorption of CPO at −25 °C, exhibiting the cocrystalline form with CPO; (B) after subsequent exchange of CPO with THF.

pattern of a cocrystalline PLLA/CPO film after exposure to THF vapor at −25 °C for 1 day and complete CPO replacement is shown in Figure 4B. The (020) diffraction peak, which is the most intense peak of the starting PLLA/ CPO cocrystalline film (Figure 4A), remain the most intense peak of the derived PLLA/THF cocrystalline film (Figure 4B). The shift of the 020 reflection from 2θ = 14,0° up to 2θ = 14.3° confirms the occurrence of the transition between the two cocrystalline phases. These results allow concluding that a∥c∥ uniplanar orientation is retained after guest exchange procedure. Quantitative evaluations of the degrees of a∥c∥ orientation as obtained on the basis of photographic EDGE patterns, indicate that the degree of orientation remains essentially unaltered (f 020 ≈ 0.85) as a consequence of guest exchange procedure and it is definitely higher than that one obtained by direct crystallization on amorphous film by THF ( f 020 ≈ 0.7). It is well-known that the exposure of PLLA cocrystalline films to solvents, which are not able to form cocrystals with PLLA, in most cases leads to the transition to the α-form.61 We have verified that exposure to toluene, acetonitrile, acetone and chloroform of PLLA cocrystalline films exhibiting high degrees of a∥c∥ uniplanar orientation not only produce transition toward the α-form but also leads to a complete loss of uniplanar orientation. This uniplanar orientation is also completely lost as a consequence of alternative routes leading to the α-form, such as long-term guest desorption at room temperature or guest extraction by using supercritical CO2. This complete loss of uniplanar orientation occurs although the starting cocrystalline forms (with CPO, DMF, THF, and DOL) and the derived α-form present the same 10/7 helical conformation.63 This behavior is different from that one observed for sPS films24,28,29,73 for which a substantial maintenance of the uniplanar orientations for solid−solid crystalline phase transitions is observed, as long as polymer chains retain their conformation.

Figure 5. (A) WAXD pattern of a PLLA/CPO cocrystalline powder after subtraction of amorphous contribution. The red upper curve represents the region of the diffraction profile with 2θ > 28°, plotted on an enlarged y-scale. Numbers refers to Table S1 of the Supporting Information. (B) Comparison between experimental WAXD pattern of the PLLA-CPO cocrystalline powder (black line) with the calculated one (red line) according to the structural model reported in Figure 6.

The experimental data reported in Table S1 can be indexed in terms of the orthorhombic unit cell proposed by Asai and coworkers for the PLLA-CPO cocrystalline form and determined on the basis of highly oriented fiber samples,61 with cell parameters equal to a = 1.61 nm, b = 1.26 nm, and c = 2.90 nm. Hence assuming this unit cell, we have considered for the PLLA/CPO clathrate a crystalline structure analogous to that one proposed by Asai and co-workers61 for the cocrystalline form with DMF, where four PLLA chains in a distorted 10/7 conformation and 8 guest molecules are packed according the P212121 space group.61 However, this packing brings to several short C−C and C−O nonbonded distances between PLLA chains and guest molecules as well as to a strong disagreement between calculated and observed structure factors. Consequently a complete reappraisal of the packing of the polymer chains and guest molecules has been carried out conducting to the model schematically reported in Figure 6 in which four PLLA chains in a regular 10/7 conformation are packed E

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Polymer Packing and Molecular Origin of the a∥c∥ Uniplanar Orientation. The ordered packing of the polymer helices in the cocrystalline PLLA/CPO structure of Figure 6, can allow finding the molecular origin of the a∥c∥ uniplanar orientation observed for guest induced cocrystallization of amorphous PLLA films. In fact, the induction of uniplanar orientation by guest molecules in polymer cocrystalline phases is generally associated with preferential orientation of the densest plane of polymer helices, being perpendicular or parallel to the film plane.32,33 For the crystalline structure of PLLA of Figure 6, the densest crystalline plane is the bc plane, with rows of parallel helices with minimum interchain (0.63 nm) and maximum interplanar (0.81 nm) distances (Figure 6). This densest plane, for the observed a∥c∥ uniplanar orientation is preferentially perpendicular to the film plane. It is worth citing that similar kinds of uniplanar orientation, with the densest plane of polymer helices prevailingly perpendicular to the film plane, have been also induced by cocrystallization with different guest molecules, both for δ and ε forms of s-PS. For instance, for δ monoclinic16 and triclinic cocrystalline phases33 as well as for the δ nanoporous-crystalline phase, two of the three observed uniplanar orientations, named a∥c⊥ and a⊥c∥, present the densest plane (ac, formed by rows of alternated enantiomorphous helices) preferentially perpendicular to the film plane.32 Analogously, for the orthorhombic ε form of s-PS, two of the three observed uniplanar orientations, named c⊥ and b⊥,31 present the densest crystalline plane (bc) preferentially perpendicular to the film plane. It is worth adding that in thick films uniplanar orientations with the densest plane of polymer helices prevailingly perpendicular to the film plane has been only observed for polymer cocrystallization with guest molecules. In fact, on the contrary, as a consequence of biaxial stretching of semicrystalline hydrocarbon polymers (with only van der Waals interchain interactions, like, e.g. polyethyleneterephthalate,74 isotactic polypropylene75,76 or syndiotactic polystyrene77,78) the densest plane of polymer helices is preferentially parallel to the film plane. It is not easy to understand for thick films the driving force associated with the attainment of uniplanar orientation, with densest planes of polymer helices prevailingly perpendicular to the film plane. However, it is clear that the phenomenon is associated with the dual role of the crystallization solvent, being solvent in the liquid phase and becoming guest in the solid phase. The achievement of the same kind of uniplanar orientation (a∥c∥) also for PLLA cocrystalline films with DMF, THF and DOL suggests that these cocrystalline forms could be characterized by a polymer packing with dense layers of helices, analogous to that described for the cocrystalline form with CPO (Figure 6). As a final comment, we add that the degree of orientation observed for the cocrystalline phases of PLLA with different guest molecules decreases in the sequence CPO ≈ DMF ( f 020 ≈ 0.9) > THF ( f 020 ≈ 0.7) > DOL (f 020 ≈ 0.5). This sequence corresponds to the sequence of stability at room temperature of these cocrystalline phases. In fact, cocrystals with CPO and DMF are stable for weeks, those with THF are stable only for few hours while films cocrystallized with DOL exhibit a large fraction of α-form (nearly 50%) already for measurements conducted immediately after crystallization (Figure 3C). This observation suggests that higher degrees of crystalline phase

Figure 6. Schematic representation of the packing model for the PLLA-CPO cocrystalline form in an orthorhombic unit cell with parameters a = 1.61 nm, b = 1.26 nm, and c (chain axis) = 2.90 nm, according to the symmetry of the space group P212121: projection along the c axis, a axis and b axis. PLLA helical chains are reported in magenta. The atomic fractional coordinates in the asymmetric unit for the packing model proposed are reported in Table S2 of the Supporting Information.

according the space group P212121 with their chain axis located at (a/4, b/4). In order to optimize the packing, polymer helices have been rotated around their chain axis and translated along the c axis direction by keeping fixed the chain conformation. In this way, an acceptable packing characterized by distances between non bonded atoms greater than 0.34 nm, has been obtained. Because the experimental density determined by flotation for the PLLA-CPO cocrystalline samples is 1.195 g/ cm361 (while those of PLLA α-form and of the amorphous phase are 1.26 g/cm3and 1.254 g/cm3, respectively), 16 CPO guest molecules have been arranged in the guest locations, leading to calculated density is 1.189 g/cm3. In the model of Figure 6, the positions and orientations of the guest molecules have been optimized by molecular mechanics calculations. Figure 5B compares the experimental WAXD pattern of an unoriented PLLA-CPO cocrystalline sample with the calculated pattern for the model of Figure 6. A quantitative comparison between the calculated structure factors for the model of Figure 6 and the experimental ones evaluated from the WAXD profile of Figure 5A, is also shown in Table S3 of the Supporting Information. A discrepancy factor R of 21% is obtained for the observed reflections only. This indicates that the structural model of Figure 6 is a good description for the crystalline structure of the PLLA/CPO cocrystalline form, even though some discrepancies are observed for the structure factors of the reflections with 2θ higher than 30° and for reflections at d ≈ 0.629, 0.530, and 0.403 nm. Although further improvement of the experimental and calculated diffraction data can be achieved by finely adjusting the relative arrangement of the chains and guest molecules in the unit cell, the agreement obtained for the model of Figure 6 with the experimental data may be considered satisfactory. Further refinements are in progress by using axially oriented cocrystalline samples. Finally, it is worth pointing out that a relevant feature of the model of Figure 6 is a definitely high degree of order in the packing of the polymer helices, with respect to that one reported in the literature for the PLLA/DMF cocrystalline form,61 characterized by the presence of dense bc layers of parallel helices with distance between adjacent chain axes equal to b/2 = 0.63 nm. This distance is very close to the shortest distance between chain axes (of PLLA helices in the same conformation) in the α-form (b = 0.62 nm).68,73 F

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the space group P212121 reported in Figure 6 (Table S2); comparison between observed (Fobs) and calculated (Fcalc) structure factors for PLLA/CPO cocrystalline form in the space group symmetry P212121 (Table S3); and photographic WAXD patterns of a PLLA/DMF film as obtained by immersion in liquid DMF at −25 °C, as taken with X-ray beam parallel (EDGE, A) or perpendicular (THROUGH, B) to the film surface (Figure S1) (PDF)

orientation, just as cocrystalline phase stability, is associated with stronger host−guest interactions.

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CONCLUSIONS Guest molecules like cyclopentanone (CPO), N,N-dimethylformamide (DMF), tetrahydrofuran (THF) and 1,3−dioxolane (DOL) that are able to cocrystallize with poly(L-lactic acid) (PLLA) can also generate uniplanar orientations of cocrystalline phases, by low-temperature sorption in amorphous unoriented polymer films. The observed uniplanar orientation has been defined as a∥c∥ (a-parallel, c-parallel), because the crystallites present orientation of their ac planes preferentially parallel to the film plane. The degree of a∥c∥ orientation is particularly high (f ≈ 0.9) for crystallization induced by CPO and DMF. The occurrence of this kind of orientation is revealed by Xray diffraction patterns as collected by standard automatic powder diffractometers, as well as by photographic patterns as collected by sending the X-ray beam perpendicular and parallel to the film plane (THROUGH and EDGE patterns). This kind of uniplanar orientation is unprecedented for PLLA and is obtained, without mechanical stretching, even for high thickness films (at least up to 200 μm). This uniplanar orientation can be also maintained after suitable guest exchange procedures, while it is completely lost by guest desorption also at room temperature or by guest extraction procedures with supercritical CO2. The reported results confirm the hypothesis that cocrystallization with low-molecular-mass guest molecules could be a common route for getting uniplanar orientation of polymer crystalline phases, even in the absence of stretching. X-ray diffraction analyses of unoriented and oriented PLLA samples and molecular mechanics calculations have allowed proposing the structure of the cocrystalline form of PLLA with CPO. In particular, four PLLA chains exhibiting a 10/7 helical conformation and 16 CPO guest molecules are packed according the space group P212121 in an orthorhombic lattice with a = 1.61 nm, b = 1.26 nm, and c = 2.90 nm. This structure presents a definitely high degree of order in the packing of the polymer helices, with respect to that one reported in the literature for the PLLA/DMF cocrystalline form. The presence in the cocrystalline structure of dense bc layers with close-packed polymer helices (absent in the literature model) allows an easy rationalization of the observed uniplanar orientation. In fact, as already found for two of the three uniplanar orientations observed for δ and ε forms of s-PS, the a∥c∥ uniplanar orientation observed for PLLA corresponds to a preferential orientation perpendicular to the film plane of the densest crystalline plane.

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AUTHOR INFORMATION

Corresponding Authors

*(P.R.) E-mail: [email protected]. Telephone: ++39-089-969582. Fax: ++39-089-969603. *(O.T.) E-mail: [email protected]. Telephone: +39-081674443. Fax: +39-081-674090. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. C. Daniel for useful discussions. Financial support of the “Ministero dell’Istruzione, dell’Università e della Ricerca” (PRIN) and of “Regione Campania” (CRdC and Legge 5) is gratefully acknowledged.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b00908. Observed diffraction angles (2θobs), interplanar distances (dobs) and experimental intensities (Iobs, in arbitraty units) of reflections of the PLLA-CPO cocrystals as obtained from the X-ray powder diffraction pattern reported in Figure 5A (Table S1); fractional coordinates of the atoms of the asymmetric unit of the structural model for the PLLA/CPO cocrystalline form according G

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