Stereocomplexation in Cyclic and Linear Polylactide Blends

Note pubs.acs.org/Macromolecules

Stereocomplexation in Cyclic and Linear Polylactide Blends Eun Ji Shin, Alexandra E. Jones, and Robert M. Waymouth* Department of Chemistry, Stanford University, Stanford, California 94305, United States S Supporting Information *

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INTRODUCTION Cyclic polymers differ from linear polymers by one bond linking the chain ends, yet the simple topological constraint1 of linking the ends of a linear chain influences macromolecular properties in ways that remain poorly understood.2−6 A cyclic topology might be expected to influence crystallization; several studies have recently compared the crystallization of cyclic and linear chains.7−15 Previous studies indicate that high molecular weight cyclic polymers crystallize into lamellar morphologies with similar lamellar dimensions as their linear isomers;7,12 for smaller cyclic chains, the lamellar thickness is constrained by the chain length.12,16,17 We have recently reported the synthesis of cyclic crystalline polylactides (PLAs)18 by zwitterionic ringopening polymerization of lactones.7,18−22 As polylactides are known to form stereocomplexes,23−26 we sought to investigate whether the combination of topological constraints and geometric constraints associated with the formation of stereocomplexes27−29 might influence the ability of cyclic polylactides to crystallize into stable stereocomplexes.30 Stereocomplexation between enantiomeric blends of linear poly(L-lactide) and poly(D-lactide)31,32 is proposed to be driven by weak CH3···OC hydrogen bond interactions between enantiomeric helical chains of L- and D-polylactide.27,33 The orientation of the PLLA and PDLA chains, either parallel or antiparallel, in the stereocomplex27,28,34,35 provides a constraint on crystallization. Stereocomplexation is a comparatively shortrange interaction−self-assembly on the order of several helix turns. Crystallization provides an additional constraint to the polymer chain at a slightly larger length scalethe dimension of a lamellar crystal. The topology (cyclic vs linear) is a structural constraint on the length scale of the molecular dimension (typically, but not always, larger than a lamellar dimension). Thus, for cyclic polymers to form crystalline stereocomplexes, several conformational constraints must be simultaneously accommodated; in this report we address whether cyclic chains are impeded from adopting the appropriate conformational orientations to crystallize into stereocomplexes.

Scheme 1. Polymerization of Lactide Using IMes

0.003 M) and IMes as the catalyst (I/C = 2) in THF for 30 s.36 The characteristics of the four polymer samples are shown in Table 1. The topology of the cyclic PLAs was established by comparing the intrinsic viscosities with the corresponding linear polymers (see Supporting Information). The ratio of the intrinsic viscosities, [η]cyclic/[η]linear, from the Mark−Houwink plots are ∼0.74 over the molecular weight range shown, similar to that predicted theoretically2 and observed experimentally for similar samples.18 The lower intrinsic viscosities are consistent with a cyclic topology but do not rule out the presence of small amounts of linear contaminants. The melting temperatures and optical rotations of the polylactides are lower than those reported in the literature for highly isotactic polylactides.37 Polylactides prepared under these conditions are not perfectly isotactic, as shown by the homonuclear decoupled 1H NMR spectra (see Supporting Information).38 In addition to resonances associated with the iii tetrad, iiiss hexads are evident, implicating competitive epimerization of stereogenic centers by the carbene, as previously reported.18,39 Blends of various combinations of the PLAs in Table 1 were prepared (Table 2) by solvent casting from dicholoromethane.40 For the DSC studies, PLLA/PDLA blends were melted at 220 °C for 15 min, cooled to 150 °C, and annealed for 24 h. The cyclic and linear PLLA samples were annealed at 90 °C for 24 h. All samples were then analyzed by differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS), and small-angle X-ray scattering (SAXS). As shown in Figure 1 and Table 2, both the linear and cyclic PLLA/PDLA blends exhibit melting temperatures ∼50 °C

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RESULTS AND DISCUSSION Linear and cyclic polylactides were synthesized by the Nheterocyclic carbene mediated ring-opening polymerization of either L- or D-lactide.18,20 Both L- and D-lactide were polymerized ([M]0 = 0.6 M) using the N-heterocyclic carbene, 1,3-dimesitylimidazol-2-ylidene (IMes, [I]0 = 0.006 M), in tetrahydrofuran (THF) for 30 s to give cyclic poly(L-lactide) (PLLA) and poly(D-lactide), respectively (Scheme 1). Linear poly(lactide)s were prepared by polymerizing L- or D-lactide ([M]0 = 0.6 M) using benzyl alcohol as the initiator ([I]0 = © 2011 American Chemical Society

Received: July 18, 2011 Revised: December 6, 2011 Published: December 15, 2011 595

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Table 1. Characterization of Polylactides Used in This Study entry 1 2 3 4

polymer type linear linear cyclic cyclic

PLLA PDLA PLLA PDLA

conva (%)

Mnb (kg/mol)

PDIc

f iiid

[α]De

Tmf (°C)

ΔHmg (J/g)

58 52 91 90

15 19 30 26

1.1 1.1 1.3 1.4

0.87 0.90 0.83 0.81

−131 +134 −110 +101

150 157 135 132

36 48 18 48

a

Percent conversion; determined by 1H NMR spectroscopy. bNumber-average molecular weight; determined by gel permeation chromatography (GPC) calibrated with polystyrene standards. cPolydispersity = Mw/Mn; determined by GPC calibrated with polystyrene standards. dFraction of isotactic (iii) tetrads. eOptical rotation; measured in chloroform with c = 0.9. fMelting temperature; measured using differential scanning calorimetry (DSC). gHeat of melting; measured using DSC.

stereocomplexes, whereas that of the cyclic PLLA shows peaks characteristic of those of linear PLLA. 31 These data unambiguously show that constraining polylactides into a cyclic topology does not impede their ability to form stereocomplexes. For the scattering patterns in Figure 2a, the samples were cooled from the melt (220 °C) at 5 deg/min to Ta = 150 °C, annealed at Ta for 24 h to allow stereocomplex formation, and then taken off the heater to cool to room temperature for the measurement (Figure 2a). The effect of thermal history27,46,47 is shown in Figure 2b; for samples that were not annealed, but simply cooled from the melt to room temperature, only the linear PLLA/PDLA blends showed evidence of stereocomplexation. While it is possible that the topology of the cyclic PLAs influences the rate at which they crystallize into stereocomplexes, it is more likely that this behavior is a consequence of the lower molecular weights and slightly higher tacticities of the linear PLLA and PDLA samples. Small-angle X-ray scattering was carried out to estimate the lamellar thickness (Lc) and long period spacing (Lp)48,49 of linear and cyclic PLLA and the linear and cyclic stereocomplexed blends. The lamellar thicknesses (Lc) and long periods (Lp) were estimated from the one-dimensional electron density autocorrelation function with the assumption of a lamellar two-phase morphology (see Supporting Information) and are shown in Table 3. The lamellar thickness and long period of the linear PLLA (8 and 16 nm, respectively) are smaller than those reported previously (Lc ∼ 16 nm and Lp ∼ 22 nm) for higher molecular weight highly isotactic linear PLLAs50,51 but comparable to PLLA copolymers containing between 6 and 12% meso-lactide.52 Notably, the lamellar thickness and long period of cyclic PLLA are ∼20% larger than those of linear PLLA. The long period (20 nm) is on the order of the extended chain length of this cyclic PLLA sample (if a 103 helix53 is assumed, then from the degree of polymerization a length of ∼37 nm can be estimated for an extended cyclic PLLA), implicating the lack of multiple chain-folding for the cyclic PLLA chains. The larger lamellar thickness and long period of the cyclic PLLA may be a consequence of a topological constraint on lamellar folding,12,50,52,54 but further studies are necessary to test the generality of this observation. In previous studies of high molecular weight cyclic and linear poly(ε-caprolactone)s, we observed no differences in lamellar or long period spacings between the cyclic and linear polyesters, but in the latter case the chain lengths were considerably longer than the lamellar thickness (∼700 nm extended chain length, Lc ∼ 9 nm).7 The lamellar thickness and long period of the cyclic PLLA/ PDLA blends are similar to those of the linear PLLA/PDLA blends (Table 3). Since stereocomplexation requires adjacent PLLA and PDLA helices, it is noteworthy that the chains are

Table 2. Blends of Polylactides Used for This Study entry

PLLA

PDLA

Tm (°C)

ΔHm (J/g)

B1 B2 B3 B4

linear linear cyclic cyclic

linear cyclic linear cyclic

189 178 185 179

52 20 42 15

Figure 1. Differential scanning calorimetry scans of (a) cyclic PLLA, (b) linear PLLA + linear PDLA, (c) cyclic PLLA + linear PDLA, and (d) cyclic PLLA + cyclic PDLA.

higher than the PLLA or PDLA homopolymers, indicative of the formation of stereocomplexes. The melting temperatures of the blends (178−189 °C) are lower than that reported for highly isotactic PLLA/PDLA stereocomplexes;41−45 this is likely due to the lower isotacticities of the PLLA and PDLA samples prepared with the carbenes.18 The heats of melting for blends involving cyclic polymers (entries B2, B3, and B4) are also lower than that of the linear PLLA/linear PDLA blend B1, but this is also likely a consequence of the slightly lower isotacticity of the cyclic PLAs relative to the linear PLAs.41,42 Analysis of the linear and cyclic PLLA/PDLA blends by wide-angle X-ray scattering provides further evidence for the formation of PLLA/PDLA stereocomplexes. The WAXS profiles of the blends B1, B3, and B4 exhibit peaks at 2θ = 12°, 21°, and 24° characteristic of that of PLLA/PDLA 596

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Figure 2. Wide-angle X-ray scattering patterns after (a) annealing at 150 °C (90 °C for cyclic PLLA) for 24 h and (b) cooling from the melt to room temperature.

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Table 3. Lamellar Thickness (Lc) and Long Period (Lp) composition linear cyclic linear cyclic cyclic

PLLA PLLA PLLA + linear PDLA PLLA + linear PDLA PLLA + cyclic PDLA

lamellar thickness (nm)

long period (nm)

8 10 7 8 8

16 20 14 16 15

Experimental procedures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

able to arrange themselves to satisfy the dual constraints of stereocomplexation and a cyclic topology. The lamellar morphology of the cyclic and linear PLLA/ PDLA blends were similar to that of the linear PLLA samples when samples were annealed at similar undercoolings ΔT (ΔT = Tm − Ta, for Tm is the measured melting temperature and Ta the annealing temperature). There have been only a few reports on the SAXS analysis of the PLA stereocomplexes.29,55,56 In Tsuji’s report,55 the estimated long period of the stereocomplex that was precipitated from acetonitrile solution at 80 °C and annealed at 216 °C was 12 nm, smaller than that reported for PLLA homopolymer films crystallized from the melt (22−35 nm). It was noted that this difference in lamellar thickness and long period could be partly due to the different thermal history.25

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ASSOCIATED CONTENT

S Supporting Information *

ACKNOWLEDGMENTS We gratefully acknowledge support from the NSF (NSF-DMR1001903; NSF-GOALI-CHE-0957386) A.J. was supported by a summer undergraduate research fellowship (CPIMA: NSFDMR-0213618), and E.S. is grateful for the Samsung Scholarship from the Samsung Foundation of Culture. We thank Purac for a generous donation of lactide. We thank Dr. Jihoon Kang and Professor Do Y. Yoon (Seoul National University, Korea) for the WAXS experiments. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a Directorate of SLAC National Accelerator Laboratory and an Office of Science User Facility operated for the U.S. Department of Energy Office of Science by Stanford University. We thank Dr. John Pople of SSRL for assistance with the scattering measurements.

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SUMMARY

REFERENCES

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