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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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Morphological transition and the changes to the crystallinity on mixing of two homochiral cylinders 3 and 4.Conducted at 65 °C with the addition of fresh THF (final solvent composition THF/H2O 20/80 v/v). (a) Cartoon illustration showing the morphological transition from homochiral cylinders 3 and 4 to stereocomplex spheres. Scale bar=500 nm (b) FT-IR spectra of dried nanoparticles, which reveal the wavenumber of carbonyl group vibration of poly(lactide) shifted from 1,758 to 1,750 cm−1 over time. Scale bar=500 nm. (c–g) TEM images, which illustrate the length of the cylindrical micelles decreased while the population of spherical micelles increased over time. Scale bars=500 nm. (h) TEM expansion of 62 h time point. Scale bar=100 nm. (i) WAXD diffractograms illustrating that the intensity of stereocomplex Bragg peak at a 2θ value of 12° increased gradually, whereas the intensity of homochiral Bragg peak at a 2θ value of 16.6° decreased significantly over time. TEM samples were prepared by slow drying and negatively stained using PTA.
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f3: Morphological transition and the changes to the crystallinity on mixing of two homochiral cylinders 3 and 4.Conducted at 65 °C with the addition of fresh THF (final solvent composition THF/H2O 20/80 v/v). (a) Cartoon illustration showing the morphological transition from homochiral cylinders 3 and 4 to stereocomplex spheres. Scale bar=500 nm (b) FT-IR spectra of dried nanoparticles, which reveal the wavenumber of carbonyl group vibration of poly(lactide) shifted from 1,758 to 1,750 cm−1 over time. Scale bar=500 nm. (c–g) TEM images, which illustrate the length of the cylindrical micelles decreased while the population of spherical micelles increased over time. Scale bars=500 nm. (h) TEM expansion of 62 h time point. Scale bar=100 nm. (i) WAXD diffractograms illustrating that the intensity of stereocomplex Bragg peak at a 2θ value of 12° increased gradually, whereas the intensity of homochiral Bragg peak at a 2θ value of 16.6° decreased significantly over time. TEM samples were prepared by slow drying and negatively stained using PTA.

Mentions: To explore the morphological transition in more detail and demonstrate its potential utility, separate solutions of homochiral cylinders 3 (Ln=194 nm, Lw/Ln=1.22, Wn=53±6 nm) and 4 (Ln=188 nm, Lw/Ln=1.21, Wn=54±7 nm) were prepared from diblock copolymers 1 and 2, respectively, (Fig. 1a,b) under CDSA conditions for 30 h as described above. Subsequently, 1 ml (around 17 mg ml−1) of each cylinder solution (in pure water) was mixed together with an additional 0.5 ml of THF to the cylinder mixture to mimic the starting self-assembly conditions described above (vTHF/vH2O=20/80). The solution was then heated at 65 °C with a needle added through the cap to allow the THF to evaporate (Fig. 3a). By using such an approach, we observed a marked decrease in cylinder lengths and increase in the population of spherical micelles by TEM (Fig. 3c–g), which demonstrates the ability to trigger the morphological transition through the onset of stereocomplexation. Analysis by dynamic light scattering (DLS) also illustrated the decrease in the hydrodynamic diameters of the nanoparticles over time (Supplementary Fig. 8). In situ synchrotron small-angle X-ray scattering (SAXS) further confirmed the cylinder-to-sphere transition (Fig. 4). A comparison of the parameters obtained from the fitted SAXS curves (Supplementary Table 2 and Supplementary Fig. 9) revealed the presence of an increasing number of spheres along with a decreasing dimension of cylinders over time. These observations were in accordance with TEM and DLS results.


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)

Morphological transition and the changes to the crystallinity on mixing of two homochiral cylinders 3 and 4.Conducted at 65 °C with the addition of fresh THF (final solvent composition THF/H2O 20/80 v/v). (a) Cartoon illustration showing the morphological transition from homochiral cylinders 3 and 4 to stereocomplex spheres. Scale bar=500 nm (b) FT-IR spectra of dried nanoparticles, which reveal the wavenumber of carbonyl group vibration of poly(lactide) shifted from 1,758 to 1,750 cm−1 over time. Scale bar=500 nm. (c–g) TEM images, which illustrate the length of the cylindrical micelles decreased while the population of spherical micelles increased over time. Scale bars=500 nm. (h) TEM expansion of 62 h time point. Scale bar=100 nm. (i) WAXD diffractograms illustrating that the intensity of stereocomplex Bragg peak at a 2θ value of 12° increased gradually, whereas the intensity of homochiral Bragg peak at a 2θ value of 16.6° decreased significantly over time. TEM samples were prepared by slow drying and negatively stained using PTA.
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f3: Morphological transition and the changes to the crystallinity on mixing of two homochiral cylinders 3 and 4.Conducted at 65 °C with the addition of fresh THF (final solvent composition THF/H2O 20/80 v/v). (a) Cartoon illustration showing the morphological transition from homochiral cylinders 3 and 4 to stereocomplex spheres. Scale bar=500 nm (b) FT-IR spectra of dried nanoparticles, which reveal the wavenumber of carbonyl group vibration of poly(lactide) shifted from 1,758 to 1,750 cm−1 over time. Scale bar=500 nm. (c–g) TEM images, which illustrate the length of the cylindrical micelles decreased while the population of spherical micelles increased over time. Scale bars=500 nm. (h) TEM expansion of 62 h time point. Scale bar=100 nm. (i) WAXD diffractograms illustrating that the intensity of stereocomplex Bragg peak at a 2θ value of 12° increased gradually, whereas the intensity of homochiral Bragg peak at a 2θ value of 16.6° decreased significantly over time. TEM samples were prepared by slow drying and negatively stained using PTA.
Mentions: To explore the morphological transition in more detail and demonstrate its potential utility, separate solutions of homochiral cylinders 3 (Ln=194 nm, Lw/Ln=1.22, Wn=53±6 nm) and 4 (Ln=188 nm, Lw/Ln=1.21, Wn=54±7 nm) were prepared from diblock copolymers 1 and 2, respectively, (Fig. 1a,b) under CDSA conditions for 30 h as described above. Subsequently, 1 ml (around 17 mg ml−1) of each cylinder solution (in pure water) was mixed together with an additional 0.5 ml of THF to the cylinder mixture to mimic the starting self-assembly conditions described above (vTHF/vH2O=20/80). The solution was then heated at 65 °C with a needle added through the cap to allow the THF to evaporate (Fig. 3a). By using such an approach, we observed a marked decrease in cylinder lengths and increase in the population of spherical micelles by TEM (Fig. 3c–g), which demonstrates the ability to trigger the morphological transition through the onset of stereocomplexation. Analysis by dynamic light scattering (DLS) also illustrated the decrease in the hydrodynamic diameters of the nanoparticles over time (Supplementary Fig. 8). In situ synchrotron small-angle X-ray scattering (SAXS) further confirmed the cylinder-to-sphere transition (Fig. 4). A comparison of the parameters obtained from the fitted SAXS curves (Supplementary Table 2 and Supplementary Fig. 9) revealed the presence of an increasing number of spheres along with a decreasing dimension of cylinders over time. These observations were in accordance with TEM and DLS results.

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