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Structural reorganization of cylindrical nanoparticles triggered by polylactide stereocomplexation.

Sun L, Pitto-Barry A, Kirby N, Schiller TL, Sanchez AM, Dyson MA, Sloan J, Wilson NR, O'Reilly RK, Dove AP - Nat Commun (2014)

Bottom Line: The stereocomplexation of isotactic poly(L-lactide) and poly(D-lactide) has led to improved properties compared with each homochiral material.During the formation of these stereocomplex micelles, an unexpected morphological transition results in the formation of dense crystalline spherical micelles rather than cylinders.This new mechanism for morphological reorganization, through competitive crystallization and stereocomplexation and without the requirement for an external stimulus, allows for new opportunities in controlled release and delivery applications.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK.

ABSTRACT
Co-crystallization of polymers with different configurations/tacticities provides access to materials with enhanced performance. The stereocomplexation of isotactic poly(L-lactide) and poly(D-lactide) has led to improved properties compared with each homochiral material. Herein, we report the preparation of stereocomplex micelles from a mixture of poly(L-lactide)-b-poly(acrylic acid) and poly(D-lactide)-b-poly(acrylic acid) diblock copolymers in water via crystallization-driven self-assembly. During the formation of these stereocomplex micelles, an unexpected morphological transition results in the formation of dense crystalline spherical micelles rather than cylinders. Furthermore, mixture of cylinders with opposite homochirality in either THF/H2O mixtures or in pure water at 65 °C leads to disassembly into stereocomplexed spherical micelles. Similarly, a transition is also observed in a related PEO-b-PLLA/PEO-b-PDLA system, demonstrating wider applicability. This new mechanism for morphological reorganization, through competitive crystallization and stereocomplexation and without the requirement for an external stimulus, allows for new opportunities in controlled release and delivery applications.

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Characterization of the formation of the stereocomplex spherical micelles from the homochiral polymers 1 and 2.Studies conducted in THF/H2O (20/80 v/v) at 65 °C as a function of time. (a) Cartoon illustration of the formation of stereocomplex spherical micelles from 1 and 2. (b) FT-IR spectra of dried nanoparticles revealed that the wavenumber of carbonyl vibrational stretch of polylactide shifted from 1,758 to 1,750 cm−1 over time. TEM images: (c) 3 h after starting the self-assembly; (d) after 5 h; (e) after 10 h; (f) after 20 h and (g) after 30 h. (h) WAXD diffractograms showed that the Bragg peak for stereocomplex formation between 1 and 2 increased over time. Cylindrical micelles obtained from the homochiral diblock copolymers 1 or 2 were found to decrease in length during the self-assembly (c–g). TEM samples were prepared by slow drying and negatively stained using PTA. Scale bars=500 nm.
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f2: Characterization of the formation of the stereocomplex spherical micelles from the homochiral polymers 1 and 2.Studies conducted in THF/H2O (20/80 v/v) at 65 °C as a function of time. (a) Cartoon illustration of the formation of stereocomplex spherical micelles from 1 and 2. (b) FT-IR spectra of dried nanoparticles revealed that the wavenumber of carbonyl vibrational stretch of polylactide shifted from 1,758 to 1,750 cm−1 over time. TEM images: (c) 3 h after starting the self-assembly; (d) after 5 h; (e) after 10 h; (f) after 20 h and (g) after 30 h. (h) WAXD diffractograms showed that the Bragg peak for stereocomplex formation between 1 and 2 increased over time. Cylindrical micelles obtained from the homochiral diblock copolymers 1 or 2 were found to decrease in length during the self-assembly (c–g). TEM samples were prepared by slow drying and negatively stained using PTA. Scale bars=500 nm.

Mentions: The self-assembly process for the formation of stereocomplex spheres was further investigated by fourier transfom-infrared (FT-IR) spectroscopy using dried nanoparticle samples (Fig. 2b). The vibrational stretch of the carbonyl group of polylactide was found to shift to a lower wavenumber (from 1,758 to 1,750 cm−1) after 3 h (Fig. 2b). Such a shift is attributed to the arrangement of the polylactide chains from a disordered state to an ordered one throughout the stereocomplex formation1017. By using WAXD (Fig. 2h), an obvious stereocomplex Bragg peak at a 2θ value of 12° is observed after 3 h, which indicates a fast formation of the stereocomplex micelles. Prolonged assembly under these conditions promoted significantly the stereocomplexation of 1 and 2 as well as the slight increase in diameter of these spheres (from 42±7 nm in Fig. 2d to 50±7 nm in Fig. 2g, respectively). A careful study of the TEM images during the self-assembly process (Fig. 2c–g) revealed an interesting phenomenon: a number of cylinders existed at the beginning of the self-assembly, but their lengths gradually decreased, and as such, the cylindrical morphology almost disappeared after 30 h. As the cylinders are believed to only self-assemble from the homochiral diblock copolymer 1 or 2 but not from the stereocomplex, we propose that the crystallization process is fast and leads to the resolution of the mixture to D and L homochiral cylinders, which over time undergo a morphological transition into stereocomplex spheres. Such a morphological transition is unusual and only one example has been previously reported26. Indeed, this observation is in contrast to that observed by Boutellier and coworkers1017 in which mixtures of poly(ε-caprolactone)-b-PLLA and poly(ε-caprolactone)-b-PDLA copolymers were self-assembled into cylinders in THF. These differences in behaviour are attributed to an absence of selective solvation and more similar block volume ratios than applied in this work.


Structural reorganization of cylindrical nanoparticles triggered by polylactide stereocomplexation.

Sun L, Pitto-Barry A, Kirby N, Schiller TL, Sanchez AM, Dyson MA, Sloan J, Wilson NR, O'Reilly RK, Dove AP - Nat Commun (2014)

Characterization of the formation of the stereocomplex spherical micelles from the homochiral polymers 1 and 2.Studies conducted in THF/H2O (20/80 v/v) at 65 °C as a function of time. (a) Cartoon illustration of the formation of stereocomplex spherical micelles from 1 and 2. (b) FT-IR spectra of dried nanoparticles revealed that the wavenumber of carbonyl vibrational stretch of polylactide shifted from 1,758 to 1,750 cm−1 over time. TEM images: (c) 3 h after starting the self-assembly; (d) after 5 h; (e) after 10 h; (f) after 20 h and (g) after 30 h. (h) WAXD diffractograms showed that the Bragg peak for stereocomplex formation between 1 and 2 increased over time. Cylindrical micelles obtained from the homochiral diblock copolymers 1 or 2 were found to decrease in length during the self-assembly (c–g). TEM samples were prepared by slow drying and negatively stained using PTA. Scale bars=500 nm.
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f2: Characterization of the formation of the stereocomplex spherical micelles from the homochiral polymers 1 and 2.Studies conducted in THF/H2O (20/80 v/v) at 65 °C as a function of time. (a) Cartoon illustration of the formation of stereocomplex spherical micelles from 1 and 2. (b) FT-IR spectra of dried nanoparticles revealed that the wavenumber of carbonyl vibrational stretch of polylactide shifted from 1,758 to 1,750 cm−1 over time. TEM images: (c) 3 h after starting the self-assembly; (d) after 5 h; (e) after 10 h; (f) after 20 h and (g) after 30 h. (h) WAXD diffractograms showed that the Bragg peak for stereocomplex formation between 1 and 2 increased over time. Cylindrical micelles obtained from the homochiral diblock copolymers 1 or 2 were found to decrease in length during the self-assembly (c–g). TEM samples were prepared by slow drying and negatively stained using PTA. Scale bars=500 nm.
Mentions: The self-assembly process for the formation of stereocomplex spheres was further investigated by fourier transfom-infrared (FT-IR) spectroscopy using dried nanoparticle samples (Fig. 2b). The vibrational stretch of the carbonyl group of polylactide was found to shift to a lower wavenumber (from 1,758 to 1,750 cm−1) after 3 h (Fig. 2b). Such a shift is attributed to the arrangement of the polylactide chains from a disordered state to an ordered one throughout the stereocomplex formation1017. By using WAXD (Fig. 2h), an obvious stereocomplex Bragg peak at a 2θ value of 12° is observed after 3 h, which indicates a fast formation of the stereocomplex micelles. Prolonged assembly under these conditions promoted significantly the stereocomplexation of 1 and 2 as well as the slight increase in diameter of these spheres (from 42±7 nm in Fig. 2d to 50±7 nm in Fig. 2g, respectively). A careful study of the TEM images during the self-assembly process (Fig. 2c–g) revealed an interesting phenomenon: a number of cylinders existed at the beginning of the self-assembly, but their lengths gradually decreased, and as such, the cylindrical morphology almost disappeared after 30 h. As the cylinders are believed to only self-assemble from the homochiral diblock copolymer 1 or 2 but not from the stereocomplex, we propose that the crystallization process is fast and leads to the resolution of the mixture to D and L homochiral cylinders, which over time undergo a morphological transition into stereocomplex spheres. Such a morphological transition is unusual and only one example has been previously reported26. Indeed, this observation is in contrast to that observed by Boutellier and coworkers1017 in which mixtures of poly(ε-caprolactone)-b-PLLA and poly(ε-caprolactone)-b-PDLA copolymers were self-assembled into cylinders in THF. These differences in behaviour are attributed to an absence of selective solvation and more similar block volume ratios than applied in this work.

Bottom Line: The stereocomplexation of isotactic poly(L-lactide) and poly(D-lactide) has led to improved properties compared with each homochiral material.During the formation of these stereocomplex micelles, an unexpected morphological transition results in the formation of dense crystalline spherical micelles rather than cylinders.This new mechanism for morphological reorganization, through competitive crystallization and stereocomplexation and without the requirement for an external stimulus, allows for new opportunities in controlled release and delivery applications.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK.

ABSTRACT
Co-crystallization of polymers with different configurations/tacticities provides access to materials with enhanced performance. The stereocomplexation of isotactic poly(L-lactide) and poly(D-lactide) has led to improved properties compared with each homochiral material. Herein, we report the preparation of stereocomplex micelles from a mixture of poly(L-lactide)-b-poly(acrylic acid) and poly(D-lactide)-b-poly(acrylic acid) diblock copolymers in water via crystallization-driven self-assembly. During the formation of these stereocomplex micelles, an unexpected morphological transition results in the formation of dense crystalline spherical micelles rather than cylinders. Furthermore, mixture of cylinders with opposite homochirality in either THF/H2O mixtures or in pure water at 65 °C leads to disassembly into stereocomplexed spherical micelles. Similarly, a transition is also observed in a related PEO-b-PLLA/PEO-b-PDLA system, demonstrating wider applicability. This new mechanism for morphological reorganization, through competitive crystallization and stereocomplexation and without the requirement for an external stimulus, allows for new opportunities in controlled release and delivery applications.

Show MeSH
Related in: MedlinePlus