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Tailor-Made Onion-Like Stereocomplex Crystals in Incompatible Enantiomeric

Polylactide Containing Block Copolymer Blends

Lu Sun,† Lei Zhu,†,* Lixia Rong,‡ and Benjamin S. Hsiao‡

†Polymer Program, Institute of Materials Science and Department of Chemical, Materials, and Biomolecular Engineering, University of Connecticut, Storrs, CT 06269-3136

‡Department of Chemistry, State University of New York at Stony Brook, Stony Brook, NY 11794-3400

Estimation of Polylactide Stereocomplex Crystal Thickness Based on TEM Micrographs

An enlarged TEM micrograph of the SC5k-EO5k is shown as the inset in Figure S1. The dark and bright microdomains are placed perpendicular to the long direction of the micrograph. A profile scan along the horizontal direction was performed using Polar 2.6.9 software (Stonybrook Technology and Applied Research, Inc., Stony Brook, NY), and the result is shown in Figure S1. Each layer thickness is obtained by visual inspection of TEM micrograph and determination of the inflection points (see the vertical dashed lines). From this analysis, the average thicknesses for bright and dark layers are 15.88 ± 1.5 and 10.85 ± 0.7 nm, respectively. The bright layers correspond to PEB and crystalline polylactide (PLA) stereocomplex crystals, and the dark layers consist of PEO and amorphous PLA.

Since each PEB and PEO chain occupies the same cross-sectional area, each layer thickness is proportional to its volume:

V

V

l

l

PLAPEO

PEB

PLAPEO

PEB

aa ,, ++

= (1)

where lPEB is the PEB layer thickness, lPEO+PLA,a is the total thickness of PEO and amorphous PLA layer, VPEB is the PEB layer volume, and VPEO+PLA,a is the volume of PEO and amorphous PLA. Taking into account of PEB, amorphous PEO, and amorphous PLA densities (d) being 0.87, 1.124, and 1.250 g/cm3,[1] the PEB layer thickness can be thus estimated:

lPEO+PLA,a = 10.85 nm; VPEB = Mn

PEB/dPEB = 4200/0.87 = 4827 cm3/mol; VPEO+PLA,a = VPEO + VPLA,a = Mn

PEO/dPEO + MnPLLA,a/dPLA,a + Mn

PDLA,a/dPLA,a = 5000/1.124 + 1683/1.25 + 1782/1.25 = 7220 cm3/mol. Here, Mn

PLLA,a and MnPDLA,a are the

amorphous PLLA and PDLA molecular weights. Using the crystallinity of 67 wt.% determined by one-dimensional (1D) WAXD in Figure 1B for the SC5k-EO5k, Mn

PLLA,a = 5100×0.33 = 1683 g/mol. Mn

PDLA,a = 5400×0.33 = 1782 g/mol. From Eqn. (1), PEB layer thickness is calculated to be 7.22 nm. The total thickness of PLA stereocomplex crystal is thus 8.66 nm.

Since PEO is compatible with PLA and PEB is highly immiscible with PLA, we assume that the stereocomplex lamellar crystals are located closer to the PEB microdomains, as shown in schematics in Figure S2. Two possible structures are depicted in Figure S2; centrosymmetric

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(Figure S2A) and noncentrosymmetric (Figure S2B). In the centrosymmetric lamellar structure, the long period consists of two layers of PLA stereocomplex crystals. In the noncentrosymmetric lamellar structure, the long period consists of a single-layer PLA stereocomplex crystal. Note that in both lamellar structures each PEB and PEO chain has the same basal cross-sectional area. If a centrosymmetric lamellar structure is assumed, each PLA stereocomplex lamellar crystal has a thickness of 4.33 nm. If a noncentrosymmetric lamellar structure is assumed, the PLA stereocomplex lamellar crystal has a thickness of 8.66 nm. Considering the average molecular weight of PLLA and PDLA being 5120 g/mol, the extended chain length of the stereocomplex crystals is 13.82 nm. Note that the c-axis is 0.87 nm and the PLA chain adopts the 31 helical conformation in the trigonal stereocomplex crystals. Therefore, the PLA chains should be 3.19 times folded in the centrosymmetric structure and 1.60 times folded in the noncentrosymmetric structure.

0 50 100 150 200 2502k

4k

6k

8k

10k

12k

Inte

nsity

(a.

u.)

Distance (nm)

16.8 14.7 18.5 14.4 14.7 15.710.8 11.2 11.2 11.5 10.8 9.6

17.511.5

14.7 nm9.9 nm

0 50 100 150 200 2502k

4k

6k

8k

10k

12k

Inte

nsity

(a.

u.)

Distance (nm)

16.8 14.7 18.5 14.4 14.7 15.710.8 11.2 11.2 11.5 10.8 9.6

17.511.5

14.7 nm9.9 nm

Figure S1. Intensity profile as a function of distance obtained from the TEM micrograph of the SC5k-EO5k in Figure 2A. Each layer thickness is indicated below the intensity curve.

centrosymmetric

A

L

B

noncentrosymmetric

L

PEO-rich Phase

PEBPEOPLLAPDLA

PLA Phase

PEB Phase

centrosymmetric

A

L

B

noncentrosymmetric

L

centrosymmetric

A

L

centrosymmetric

A

L

A

L

B

noncentrosymmetric

LB

noncentrosymmetric

B

noncentrosymmetric

L

PEO-rich Phase

PEBPEOPLLAPDLA

PLA Phase

PEB PhasePEO-rich Phase

PEBPEOPLLAPDLA

PLA Phase

PEB PhasePEO-rich Phase

PEBPEOPLLAPDLA

PEBPEBPEOPEOPLLAPLLAPDLAPDLA

PLA Phase

PEB Phase

Figure S2. Schematics of (A) centrosymmetric (double-layer PLA stereocomplex crystals) and (B) noncentrosymmetric (single-layer PLA stereocomplex crystal) structures for the PEO-b-PLLA/PEB-b-PDLA stereocomplexes. The long period is defined as L.

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For SC5k-EO2k, an enlarged TEM micrograph is shown as the inset in Figure S3. A profile scan along the long direction is shown in Figure S3. Each layer thickness is estimated using the method described above; the average thicknesses for bright (PEB and crystalline PLA stereocomplex crystals) and dark (PEO and amorphous PLA) layers are 13.68 ± 1.4 and 7.02 ± 0.6 nm, respectively. The PEB layer thickness can thus be estimated using Eqn. (1):

lPEO+PLA,a = 7.02 nm; VPEB = Mn

PEB/dPEB = 4200/0.87 = 4827 cm3/mol; VPEO+PLA,a = VPEO + VPLA,a = Mn

PEO/dPEO + MnPLLA,a/dPLA,a + Mn

PDLA,a/dPLA,a = 2000/1.124 + 1188/1.25 + 1188/1.25 = 3680 cm3/mol. Using the crystallinity of 78 wt.% determined by 1D WAXD in Figure 1B, Mn

PLLA,a = MnPDLA,a = 5100×0.22 = 1188 g/mol.

The PEB layer thickness is calculated to be 9.18 nm. Therefore, the total thickness of

PLA stereocomplex crystal is 4.50 nm. If a centrosymmetric lamellar structure is assumed, each PLA stereocomplex crystal has a thickness of 2.25 nm. If a noncentrosymmetric lamellar structure is assumed, the PLA stereocomplex crystal has a thickness of 4.50 nm. Considering the average molecular weight of PLLA and PDLA being 5400 g/mol, the extended chain length of the stereocomplex crystals is 16.97 nm. Therefore, the PLA chains should be 7.54 times folded in the centrosymmetric structure and 3.77 times folded in the noncentrosymmetric structure.

0 20 40 60 80 100 1203k

4k

5k

6k

7k

8k

Inte

nsity

(a.

u.)

Distance (nm)

7.0 7.1 8.2 6.7 6.6 6.8 nm11.6 14.2 13.5 13.5 15.6 nm

0 20 40 60 80 100 1203k

4k

5k

6k

7k

8k

Inte

nsity

(a.

u.)

Distance (nm)

7.0 7.1 8.2 6.7 6.6 6.8 nm11.6 14.2 13.5 13.5 15.6 nm

Figure S3. Intensity profile as a function of distance obtained from the TEM micrograph of SC5k-EO2k in Figure 2B. Each layer thickness is indicated below the intensity curve.

Lamellar crystals having a thickness as thin as 2.25 nm are rarely seen for semicrystalline polymers. DSC results show that the PLA stereocomplex crystals in SC5k-EO2k and SC5k-EO5k have similar melting points; 193 °C for SC5k-EO2k and 199 °C for SC5k-EO5k. Similar melting points suggest similar PLA stereocomplex crystal thicknesses in the SC5k-EO2k and

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SC5k-EO5k. Based on the above calculation, the PLA stereocomplex crystals in both samples should have the same thickness around 4.3~4.5 nm. It is thus reasonable to assume a noncentrosymmetric structure for the SC5k-EO2k and a centrosymmetric structure for the SC5k-EO5k. We speculate that different structures, noncentrosymmetric versus centrosymmetric for the SC5k-EO2k and SC5k-EO5k, respectively, may result from the lamellar bending induced by unbalanced surface stresses due to dissimilar PEB and PEO molecular weights. This speculation, however, still needs experimental proof, and is currently underway. References: [1] The densities of amorphous and crystalline PEO are 1.124 and 1.239 g/cm3; amorphous

and crystalline PLA densities are 1.25 and 1.29 g/cm3; PEB density is 0.87 g/cm3. See: a) B. Wunderlich, Macromolecular Physics, Vol. 1, Crystal Structure, Morphology and Defects, Academic Press, New York, 1973, p. 388-389; b) D. Brizzolara, H.-J. Cantow, K. Diederichs, E. Keller, A. J. Domb, Macromolecules 1996, 29, 191-197.