Molecular Structural Basis for Stereocomplex Formation of Polylactide

Oct 30, 2015 - (1-6) To solve this problem, our group recently employed an ... PLLA (l-PLLA) was calculated to be 367 nm using (Mw/M0) × (8.8 Å/3), ...
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Molecular Structural Basis for Stereocomplex Formation of Polylactide Enantiomers in Dilute Solution Wei Chen, Shijun Wang, Wei Zhang, Yutian Ke, You-lee Hong,* and Toshikazu Miyoshi* Department of Polymer Science, The University of Akron, Akron, Ohio 44325, United States S Supporting Information *

ABSTRACT: Poly( L -lactide) (PLLA) and poly( D -lactide) (PDLA) alternatively pack with each other and form stereocomplex crystals (SCs). The crystal habits of SCs formed in the dilute solution highly depend on the molecular weight (⟨Mw⟩). In this study, we investigated chain-folding (CF) structure for 13C labeled PLLA (l-PLLA) chains in SCs with PDLAs that have either high or low ⟨Mw⟩s by employing an advanced Double Quantum (DQ) NMR. It was found that the ensemble average of the successive adjacent re-entry number ⟨n⟩ for the l-PLLA chains drastically change depending on ⟨Mw⟩s of the counter PDLA chains in the SCs. It was concluded that the CF structures of lPLLA depending on ⟨Mw⟩s of PDLA determine the crystal habits of SCs.

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and 7 K g/mol, respectively, and proposed the spiral growth model where either PLLA or PDLA first folds along the (110), (12̅0), and (2̅10) planes, and afterward, the other follows the preceding growth.16 Cartier et al. reported the hexagonal morphology of PLAs SC crystals with the same number-average molecular weight ⟨Mn⟩ = 7 K g/mol. It was suggested that the imbalance of the molecular weights leads to the unique morphology differences.14 Tsuji et al. increased ⟨Mw⟩s up to 450 K g/mol and observed the circular morphology for the SCs.15 These unique molecular weight dependences of crystal habits are peculiar in the SCs. Thereby, it is hypothesized that the habits are related to the unique alternative packing of the PLA enantiomers: More specifically, the fold surface occupied by the PDLA and PLLA chains restrict folding events of themselves and affect self-arrangements inside the crystals as well as the macroscopic morphology. In this communication, we investigated the CF structures of the l-PLLA (13C 34% CH3-labeled PLLA) chains in different crystal habits by the DQ NMR approach. From the determined molecular basis, we envisage chain-level structures at the fold surface as well as the structural formation process of the triangular and circular habits. Nonlabeled PLLA (n-PLLA) (⟨Mw⟩ = 82 K g/mol), high(H: ⟨Mw⟩ = 120 K g/mol), and low molecular weight (L) PDLA (2.9 K g/mol), and l-PLLA (90 K g/mol) were synthesized through the ring-opening polymerization. The PLA SCs, that were obtained from the cocrystallization of l-PLLA and n-PLLA with H- or LPDLA, were prepared from 0.05 wt % acetonitrile solution at the crystallization temperature Tc = 56 °C over a period of 5 days to ensure complete crystallization

rystallization of polymers induces structural change from random coils to folded chains in the thin crystal lamellae. Over the last half century, however, the detailed CF structures (adjacent re-entry vs random re-entry) have been debated due to experimental limitations.1−6 To solve this problem, our group recently employed an advanced 13C−13C DQ NMR and selective isotope labeling to access CF structures of semicrystalline polymers.7−12 The re-entrance sites of the folded chains, ensemble average of the successive adjacent re-entry number ⟨n⟩, and the adjacent re-entry fraction ⟨F⟩ of 13Clabeled polymers, that is, isotactic-poly(1-butene) (iPB1)7−10 and isotactic-poly(propylene) (iPP),11,12 were successfully extracted from the 13C−13C DQ NMR probing the spatial proximity for labeled 13C nuclei. As results, it was demonstrated that the adjacent re-entry event is a natural consequence of the flexible chains in both solution and melt crystallization, even though the adjacent re-entry sequence in the former is longer than that in the latter.8 These NMR results indicated that crystallization mechanism of flexible polymers is dominated via intramolecular interaction within one polymer chain but between different stems. The chiral center in the polymer backbone induces configurationally different conformations. D- and L-Polylactides (PLAs) adopt 32 and 31 helical conformations, respectively, and make SCs where the enantiomers alternatively pack with each other in a trigonal unit cell with R3c or R-3c symmetry.13,14 The former means helical stems are aligned along one specific direction while the latter shows statistical up- (+) and downward (−) orientations. The SC formation is dominated via intermolecular interaction between different chains and thus is largely different from the crystallization mechanism of homopolymers. Racemic SCs lead to unique structural features14−16 and superior thermal property.17 Brizzolara et al. observed the triangular morphology for the PLLA/PDLA SC crystals with the weight-average molecular weight ⟨Mw⟩s of 16 © XXXX American Chemical Society

Received: September 20, 2015 Accepted: October 27, 2015

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DOI: 10.1021/acsmacrolett.5b00685 ACS Macro Lett. 2015, 4, 1264−1267

Letter

ACS Macro Letters (see sample preparation in SI). The habits for l-PLLA/n-PLLA/ LPDLA and HPDLA SCs were depicted in Figure 1a and b, respectively. The former shows triangular morphology while the latter represents circular one.

Figure 2. (a) Chain-packing structure of PLA SCs with R3c symmetry on the (001) plane and the five spin system including one reference (red), two overlapped interstem (green), and two intrastem methyl carbons (blue). (b) Two neighboring stems of PLLA along the c-axis. (c) 13C high-resolution SQ (black) and DQ (red) NMR spectra for lPLLA/LPDLA SCs with 1:1 ratio, and enlarged 13C methyl DQ signal with two Gaussian peaks corresponding to the crystalline (red) and amorphous signals (blue). (d) DQ buildup curves for l-PLLA/LPDLA (red circle) and l-PLLA/HPDLA (blue circle) SCs as a function of τex. The green and red curves represent the simulated DQ curves based on the original XRD results with T2 of 10.8 ms and on the 96% shrunk model with T2 of 9.5 ms, respectively.

Figure 1. AFM amplitude images of (a) the l-PLLA/n-PLLA/LPDLA SCs with the lamellar thickness of ∼10 nm and (b) the l-PLLA/nPLLA/HPDLA SCs with ∼400 nm obtained under the same condition (see text). The height images were obtained at the position marked by the dashed lines.

Based on the ⟨Mw⟩ and the assumption of adopting uniform 31 helices, the chain length ⟨l⟩ of the labeled PLLA (l-PLLA) was calculated to be 367 nm using (Mw/M0) × (8.8 Å/3), where M0 is the monomeric molecular weight and 8.8 Å is the unit cell parameter c of the stereocomplex. ⟨l⟩ = 334 nm for nPLLA (nonlabeled PLLA) is close to that of l-PLLA. Similarly, ⟨l⟩ = 11.8 nm for LPDLA and ⟨l⟩ = 497 nm for HPDLA were obtained. AFM height image determined the thickness of the lPLLA/n-PLLA/LPDLA single crystal to be ca. 10 nm. Due to stacking of multiple layers, we could not determine the single layer thickness for l-PLLA/n-PLLA/HPDLA SCs. It was assumed to be the same with that of the l-PLLA/n-PLLA/ LPDLA single crystal. Considering ⟨l⟩ and the crystal thickness, the maximum folding number nmax for l-PLLA, n-PLLA, LPDLA, and HPDLA were estimated to be 36, 32, 0, and 49, respectively. Direct polarization/magic angle spinning (DP/MAS) experiment was performed to measure the crystallinity (see NMR experiments in SI). The methyl group shows different lineshapes for the amorphous and crystalline signals at 16.3 and 15.4 ppm, respectively. Applying two Gaussian peaks to the observed signals provided the crystallinity for the l-PLLA/nPLLA/HPDLA and l-PLLA/n-PLLA/LPDLA SCs to be 75 and 80%, respectively (see Figure S1). Similar peak deconvolution was performed on the DQ analysis. Figure 2c illustrates the 13C single quantum (SQ, black line) and DQ (red line) NMR spectra for the l-PLLA SCs with LPDLA with a mixing ratio of 1:1 and the enlarged DQ methyl signal fitted by two Gaussian peaks. Figure 2d shows the DQ buildup curves for the l-PLLA/ LPDLA (red circle) and l-PLLA/HPDLA (blue) SCs as a function of excitation time (τex; see detailed DQ NMR conditions in SI), where the DQ efficiency (ξ) was calculated by integration ratios of the DQ methyl signal to the SQ one. The maximum DQ efficiency (ξmax) is 0.19 for both samples. The experimentally obtained DQ buildup curve is determined by the interacted 13C spin number, spin topology, and internuclear distances as well as relaxation process of T2. Thus, this curve can be described by ξ(τex) = a(τex)exp(−τex/ T2),18 where a(τex) is the pure DQ buildup curve reflecting on the 13C−13C spin network. T2 additionally may include

chemical shift anisotropy, rf imperfection, insufficient decoupling, and long-range interactions that were not involved in the simulation. In SCs, only the PLLA chains were randomly labeled by 13C. 13 C−13C dipolar interaction is inversely proportional to the third power of internuclear distance. A maximum five spin system including the reference (red) and four neighbor spins (intrastem, blue; interstem, green) at distances of less than 7 Å was statistically took into consideration (Figure 2a,b). On the basis of the original atomic coordinates determined by XRD, the DQ buildup curve with a T2 relaxation of 10.8 ms (shown as the green curve in Figure 2d) showed a slightly slower buildup curve compared to the experimental result (shown with the blue and pink circles). Finally, the 96% shrunk internuclear distances with a T2 relaxation of 9.5 ms could reproduce the DQ curve (red) that agreed well with the experimental data as shown in Figure 2d. Prior to the CF analysis, we investigated whether or not lPLLA in the SCs mixes with the n-PLLA chains at the stem level. The blending ratio dependence of the DQ curves for lPLLA in l-PLLA/n-PLLA/LPDLA was shown in Figure S2a. ξmaxs continuously decreased from 0.19 (blending ratio of 1:0:1) to 0.15 (1:1:2) and finally to 0.13 (1:4:5). The observed composition dependences of the DQ curves indicated that the l-PLLA chains are well mixed with n-PLLA chains at the stem level. Similar results were obtained for l-PLLA/n-PLLA/ HPDLA (see Figure S2b). The blending ratio of 1:4:5 was further used for CF analysis of l-PLLA in two SCs. The experimentally obtained DQ curve for the diluted lPLLA/n-PLLA/LPDLA SCs (ξmax = 0.13 at τex = 8.62 ms) was depicted as black circles in Figure 3a−c. First, we considered the isolated stem model, a major structure in the switchboard or random entry models where the internuclear dipolar couplings within the stem dominate the DQ curves in addition to the statistical interstem dipolar interactions. In the intrastem interaction, two neighboring spins at 7 Å colored by blue (Figure 3a) and two other spins at 8.8 Å colored by green were considered. As shown in Figure 3a, the calculated DQ buildup curve among spins within 7 Å (blue curve) was almost 1265

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component in one cluster corresponds to ⟨m⟩2 as schematically illustrated in Figure 3c. We changed ⟨m⟩ values from 2 to 6 and simulated the DQ buildup curves as a function of ⟨m⟩ values under the assumption that ⟨F⟩ = 100%. At ⟨m⟩ = 2, the simulated DQ curve showed a good consistency with the experimental data. The cluster with ⟨m⟩ = 2 and ⟨F⟩ = 100% means that four PLLA and four PDLA stems are included in one cluster, as shown in Figure 3c. The total stem number is similar to the stem number of 4−20 in the bundle of polyethylene proposed by Allegra.20 Notably, comparisons of the experimental and simulated DQ buildup curves cannot distinguish between the 2D conformation and the 3D conformation. In the bundle model,20,21 the 3D cluster corresponds to a building block in the secondary nucleation. Under the assumption that individual clusters have the same size and shape, depositions of the clusters cannot generate imbalances in diffusion as well as in nucleation rates. Consequently, it is suggested that the secondary nucleation and growth are only the possible route to generate the SC triangular morphology. In the case of SCs with HPDLA, the morphology shows a circular-shape as depicted in Figure 1b. Surprisingly, ξmax was lowered down to 0.11 for the l-PLLA/n-PLLA/HDPLA SCs with a blending ratio of 1:4:5 at τex = 8.62 ms (Figure 3d). This is a lower value compared to ξmax = 0.13, calculated on the basis of the cluster model using ⟨m⟩ = 2. Thus, the cluster model can be reasonably rejected but linear arrangement of the CF pattern was allowed to be compared with the experimental data. ⟨n⟩ dependence of the simulated DQ buildup curves was depicted in Figure 3d. The calculated curve with ⟨n⟩ = 1 was in agreement with the experimental data. This comparison indicates that the l-PLLA chains adopt only single folding to the next site as a mean structure in SCs with HPDLA. The experimental results for the CF structures in the two kinds of SCs with PDLA having different ⟨Mw⟩s clearly indicate that the l-PLLA chains with the same ⟨Mw⟩ adopts drastically different CF patterns by changing ⟨Mw⟩s of the counter chains. Such clear differences envisage us about the CF process of the PLA chains as well as their morphological differences. For the PLLA/LPDLA case, LPDLA (11.8 nm) chains would be preferentially deposited on the growth front due to the imbalance of ⟨Mw⟩s (Figure 4a(i)). The LPDLA chains with ⟨l⟩ of 11.8 nm do not fold on the growth front compared to crystal thickness of 10 nm. Afterward, the long PLLA chains are deposited on the growth front and then fold along the growth front (Figure 4a(ii−iii)). No interference of folding process of

Figure 3. Experimental DQ curves for l-PLLA/n-PLLA/LPDLA SCs with blending ratio of 1:4:5 and the simulated DQ buildup curve based on (a) isolated model, (b) CF pattern along the (110) plane, (c) 3D cluster with stem number ⟨m⟩ along one side, and one cluster with ⟨m⟩ = 2; (d) DQ buildup curves of l-PLLA/n-PLLA/HPDLA SCs with blending ratio of 1:4:5 and simulated DQ buildup curves as a function of ⟨n⟩ along the (110) plane.

consistent with that of the curve based on 8.8 Å (green). Therefore, two spins at 8.8 Å and longer distances do not contribute to the DQ curve. The simulated DQ curve based on the isolated model showed a much slower buildup curve containing a maximum of ξmax = 0.09 at τex = 10.0 ms, compared to the experimental results. This comparison of the experimental and simulated results rejects either the switchboard or random entry model as the major structure. Next, we considered the adjacent re-entry model of the lPLLA chains in the SCs. Under the assumption of the secondary nucleation, the growth front may induce linear arrangement of the folded chains (two-dimensional (2D) conformation) where the PLLA stem is deposited on the growth front and further grow along the (110), (12̅0), and (21̅ 0̅ ) planes via folding. For simplicity, the model along the (110) plane was only considered, and the simulated DQ buildup curves for the l-PLLA chains based on this model, as a function of ⟨n⟩ under the assumption of ⟨F⟩ = 100%, was depicted in Figure 3b. The calculated ξmax and the whole curves increased continuously with increasing ⟨n⟩ up to 20. The ⟨n⟩ dependence of the simulated curves no longer distinguishes individual curves for ⟨n⟩ ≥ 20. The simulated DQ buildup curve with ⟨n⟩ ≥ 20 and ⟨F⟩ = 100% along the (110) plane could reproduce the experimental data well. This result means that all l-PLLA chains adopt nearly perfect adjacent sequences along the growth front. Such CF pattern leads to successive upand downward selection of the PLLA stems within one chain (Figure 3b). The CF structure is consistent with the structure based on the hypothesis in regime I of the LH theory,4,19 as well as with the spiral16 and kinetics models.14 Bundle20,21 and aggregation models22 include 3D cluster where three-dimensional clusters, consisting of partial or whole chains, formed via self-folding in the prestage of crystallization.20−22 Our recent work elucidated that the iPB1 chains dominantly adopt 3D clusters in form III solution-grown crystals.7 Here, we tested this 3D model to the PLA SCs. It was assumed that the two types of the chains form cubic bundles under the assumption that each PLA adopts to the stem number of ⟨m⟩ in one row and the total stem number of one

Figure 4. Schematic illustration of the crystallization process of PLA SCs with L- (a) and HPDLA (b) at molecular levels. 1266

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(13) Cartier, L.; Okihara, T.; Ikada, Y.; Tsuji, H.; Puiggali, J.; Lotz, B. Polymer 2000, 41, 8909−8919. (14) Cartier, L.; Okihara, T.; Lotz, B. Macromolecules 1997, 30, 6313−6322. (15) Tsuji, H.; Hyon, S.; Ikada, Y. Macromolecules 1992, 25, 2940− 2946. (16) Brizzolara, D.; Cantow, H.-J.; Diederichs, K.; Keller, E.; Domb, A. J. Macromolecules 1996, 29, 191−197. (17) Tsuji, H.; Ikada, Y. Polymer 1999, 40, 6699−6708. (18) Karlsson, T.; Popham, J. M.; Long, J. R.; Oyler, N.; Drobny, G. P. J. Am. Chem. Soc. 2003, 125, 7394−7407. (19) Lauritzen, J. I.; Hoffman, J. D. J. Res. Natl. Bur. Stand., Sect. A 1960, 64A, 73. (20) Allegra, G.; Meille, S. V. Phys. Chem. Chem. Phys. 1999, 1, 5179−5188. (21) Allegra, G. J. Chem. Phys. 1977, 66, 5453. (22) Zhang, J.; Muthukumar, M. J. Chem. Phys. 2007, 126, 234904.

l-PLLA with LPDLA leads to the long folding sequence along the growth front. In contrast, for the PLLA/HPDLA system, the triangular morphology is no longer retained due to similarity of ⟨Mw⟩s for the enantiomers. The observed short adjacent re-entry sequence suggests that depositions of the two enantiomers sequentially occur side by side on the growth front and the two chains attempt to fold simultaneously (Figure 4b(iv)). As a result, available space at the fold surface restricts folding events for the two types of chains. To avoid surface crowds, the chains might fold along different directions from the growth front (Figure 4b(v)) or penetrate the upper and lower layers as shown in Figure 4b (vi). The latter would lead to multiple stacked layers with thickness of ∼400 nm (Figure 1b). It is concluded that disorder in CF patterns induced by the long chains is a structural origin to induce the circular morphology. In summary, advanced 13C−13C DQ NMR proving spatial proximity pattern of the labeled chains revealed that ⟨Mw⟩s of PDLA significantly influence the CF patterns for the counter PLLA chains in the SCs. The observed unique ⟨Mw⟩ dependence of morphologies was explained in terms of folding structures of the enantiomers, which alternatively pack with each other inside the crystals, at the surface.

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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/acsmacrolett.5b00685. Sample preparation, chemical characteristics of PLAs, solid-state NMR experiments, 13C DP/MAS spectra, and compositional dependence of DQ buildup curves (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Science Foundation (Grants DMR-1105829 and 1408855) and startup funds from the UA.

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REFERENCES

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DOI: 10.1021/acsmacrolett.5b00685 ACS Macro Lett. 2015, 4, 1264−1267