Configurational Molecular Glue: One Optically Active Polymer Attracts Two Oppositely Configured Optically Active Polymers

Abstract

D-configured poly(D-lactic acid) (D-PLA) and poly(D-2-hydroxy-3-methylbutanoic acid) (D-P2H3MB) crystallized separately into their homo-crystallites when crystallized by precipitation or solvent evaporation, whereas incorporation of L-configured poly(L-2-hydroxybutanoic acid) (L-P2HB) in D-configured D-PLA and D-P2H3MB induced co-crystallization or ternary stereocomplex formation between D-configured D-PLA and D-P2H3MB and L-configured L-P2HB. However, incorporation of D-configured poly(D-2-hydroxybutanoic acid) (D-P2HB) in D-configured D-PLA and D-P2H3MB did not cause co-crystallization between D-configured D-PLA and D-P2H3MB and D-configured D-P2HB but separate crystallization of each polymer occurred. These findings strongly suggest that an optically active polymer (L-configured or D-configured polymer) like unsubstituted or substituted optically active poly(lactic acid)s can act as "a configurational or helical molecular glue" for two oppositely configured optically active polymers (two D-configured polymers or two L-configured polymers) to allow their co-crystallization. The increased degree of freedom in polymer combination is expected to assist to pave the way for designing polymeric composites having a wide variety of physical properties, biodegradation rate and behavior in the case of biodegradable polymers.

Figure 1. Molecular structures of unsubsitued and substituted PLAs.

Figure 2

Structural model of PLA SC (a), molecular arrangement (b) and helical direction of PLA chains (b) projected on the plane normal to the chain axis. The arrows indicate the relative directions of PLA helices. Panels (a) and (b) are Reprinted from ref. , T. Okihara, et al., J. Macomol Sci. Part B: Phys., vol. B30, 735-736, Crystal structure of stereocomplex of poly(L-lactide) and poly(D-lactide), pp. 119–140, Copyright (1991), with permission from Taylor & Francis. In panels (a) and (b), L-PLA and D-PLA are abbreviated as PLLA and PDLA, respectively. In panel (a), the arrows are added to original figure and in panel (b) a line between L-PLA and D-PLA is added.

Figure 3

WAXD profiles of blends crystallized by precipitation (a,c) and solvent evaporation (b,d). Panels (c) and (d) are magnified figures of panels (a) and (b), respectively, in the 2θ range of 8.5–12.5°. Shown ratios are those of D-PLA/L-P2HB/D-P2H3MB (mol/mol/mol). Dotted and broken lines indicate the crystalline diffraction angles for L-P2HB/D-P2H3MB and D-PLA/L-P2HB HTSC crystallites, respectively.

Figure 4

Interplanar distance (d) of SC crystallites in ternary polymer blends crystallized by precipitation (a) and solvent evaporation (b) for 2θ range of 8.5–12.5°, crystallinity (X_c) of 50/0/50 and ternary polymer blends crystallized by precipitation (c) and solvent evaporation (d). Dotted and broken lines in panels (a) and (b) indicate the d values for L-P2HB/D-P2H3MB HTSC crystallites in 0/50/50 blends and D-PLA/L-P2HB HTSC crystallites in 50/50/0 blends, respectively.

Figure 5

DSC thermograms of blends crystallized by precipitation (a) and solvent evaporation (b). Shown ratios are those of D-PLA/L-P2HB/D-P2H3MB (mol/mol/mol). Dotted and broken lines in panels (a) and (b) indicate the T_m values for L-P2HB/D-P2H3MB HTSC crystallites in 0/50/50 blends and D-PLA/L-P2HB HTSC crystallites in 50/50/0 blends, respectively.

Figure 6

WAXD profiles (a) and DSC thermograms (b) of D-PLA/D-P2HB/D-P2H3MB (25/50/25) (D/D/D) and D-PLA/L-P2HB/D-P2H3MB (25/50/25) (D/L/D) blends crystallized by precipitation and solvent evaporation. Dotted lines in panel (a) are representative diffraction angles of ternary stereocomplex crystallites and those in panel (b) are T_m values of ternary stereocomplex crystallite

Figure 7. Schematic representation of separate crystallization of D-configured D-PLA and D-P2H3MB and co-crystallization of D-PLA and D-P2H3MB by helical or configurational molecular glue of L-configured L-P2HB.