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Toroidal structures from brush amphiphiles† Cite this: Chem. Commun., 2014, 50, 536
Hanying Luo, Jose ´ Luis Santos and Margarita Herrera-Alonso*
Received 6th September 2013, Accepted 27th October 2013 DOI: 10.1039/c3cc46834h www.rsc.org/chemcomm
The self-assembly of macromolecular brushes of varying amphiphilic character from homogeneous solution under rapid assembly conditions results in unique morphologies, distinct from equilibrium structures. Kinetic features of the assembly process enabled the formation of toroid structures, the sizes of which were readily tuned according to solvent quality.
The self-assembly of amphiphilic block copolymers is widely studied as an approach for engineering nano-objects. A variety of different features, including spherical micelles,1–8 cylindrical or worm-like micelles,1–3,9 toroids,8,10–12 and vesicles,2–4,6,9 result from the self-assembly of linear amphiphilic block copolymers by manipulating the ratio of dissimilar blocks, solvent quality, and coronal interactions.2,3,6,13,14 Aside from the possibilities enabled by chemical diversity, aggregate structure can also be manipulated through kinetic features of the assembly process.8,15–18 Elucidating the molecular and process parameters influencing assembly should enable the fabrication of increasingly complex morphologies, broadening their envisioned applicability. Unlike linear amphiphiles, morphological determinants of solution assemblies from more complex macromolecular building blocks, such as graft copolymers, are far less explored. In terms of morphological diversity and control over the characteristic sizes of self-assembled objects, there exists a lack of work showing the equivalence between graft copolymers and their linear analogs.9 Herein, we demonstrate that a decrease in the hydrophilic ratio of a family of amphiphilic asymmetric macromolecular brushes results in morphological transitions of self-assemblies from spherical micelles to bilayer structures (vesicles). Furthermore, we discuss process parameters leading to the formation of toroidal aggregates. To the best of our knowledge, this is the first time molecular brushes have been shown to offer access to the variety of morphologies shown here.
Department of Materials Science and Engineering, The Johns Hopkins University, Baltimore, Maryland 21218, USA. E-mail: [email protected] † Electronic supplementary information (ESI) available: Experimental procedures and additional figures. See DOI: 10.1039/c3cc46834h
536 | Chem. Commun., 2014, 50, 536--538
Molecular brushes are graft polymers wherein the distance between grafting points is smaller than the characteristic dimensions of the side-chains. Crowded grafting results in a uniquely extended backbone conformation and influences their physicochemical properties.19–21 Amphiphilic asymmetric macromolecular brushes, with hydrophobic and hydrophilic side-chains grafted onto the same backbone repeat unit,22–24 were constructed by a combination of ‘‘graft from’’ and ‘‘graft onto’’ strategies (Scheme 1). Poly(ethylene glycol) (PEG) and poly(D,L-lactide) (PLA) were chosen as hydrophilic and hydrophobic side chain components, respectively. The brush hydrophilic ratio, expressed in terms of the weight fraction of PEG (wPEG), was varied by changing the length of the PLA block (Table S3, ESI†). Grafted PEG chains had a fixed molecular weight of 750 Da (PEG16, subscript represents repeat units). Rapid self-assembly was triggered by a rapid change in solvent quality inside a multi-inlet vortex mixer (MIVM), which allows controllable changes in the magnitude and rate of solvent quality jump. In the MIVM, micromixing occurs in the millisecond range, providing a homogeneous environment
Scheme 1 Synthesis of molecular brushes with amphiphilic branchedside chains by a combination of ‘‘grafting from’’ and ‘‘grafting onto’’ routes.
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Fig. 1 Transmission electron micrographs of aggregate morphologies of amphiphilic branched brush copolymers, prepared by a rapid mixing process (top row) and by dialysis (bottom row). Differences in aggregate structure reflect the molecular characteristics of the block copolymers, with decreasing hydrophilic content from left to right, as well as the effect of processing conditions.
for hydrophobic association. Relevant process parameters include the final non-solvent : solvent ratio (H2O : THF, v/v), and mixing velocity, expressed in terms of the Reynolds number (Re),25 see ESI.† Fig. 1 (A through C) shows the resulting structures from the rapid self-assembly of molecular brushes with amphiphilic branched side chains as functions of wPEG. The system with the highest wPEG is characterized by forming homogeneous spherical micelles, with an average size of B20 nm. Decreasing wPEG results in the formation of a cornucopia of toroidal structures coexisting with others, such as long rods, ‘‘8’’ shaped, and lassos. Finally, vesicles are obtained at the lowest wPEG. Interestingly, samples B and C exhibit morphologies entirely different from the conventional spherical shapes generally produced by this method from linear diblock copolymer amphiphiles.25–27 Furthermore, as shown in Fig. 1, a few vesicles collapsed during the drying process for TEM sample preparation. To further investigate the influence of medium conditions on self-assembled structures from molecular brushes, we prepared samples by dialysis. This method differs from the rapid assembly process in the rate of solvent change and mixing energy. The resulting morphologies are shown in Fig. 1 (D through F). While spherical micelles are also present for the sample with highest wPEG, a few larger aggregates are also observed, leading to higher polydispersity than those obtained from rapid quenching. As wPEG drops, a network of cylindrical micelles form and, compared to its quenched equivalent, only a few toroidal structures are observed. The slow change in solvent quality results in the formation of a network-like structure and aggregates of rods, in contrast to the quenched sample wherein no large aggregates were found (Fig. 1, E vs. B). Lastly, giant vesicles were observed for the amphiphiles with the lowest PEG content, which were considerably larger than the equivalent bilayer aggregates formed by the rapid assembly method. Results from dynamic light scattering experiments of micellar and bilayer aggregates also confirm monodispersity
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and the smaller size of samples prepared via a rapid assembly (Fig. S7, ESI†). Morphological diversity in the self-assembled forms of linear amphiphiles is dictated by a balance of the interfacial curvature between unlike blocks, solvent selectivity, and coronal interactions.2,3,6,13,14,17 Prior work on linear amphiphilic polymers and rod shaped amphiphiles has shown that their morphological transitions occur with decreasing hydrophilic fraction.1–3 Interestingly, despite their difference in molecular structure, the morphological progression of brush amphiphiles parallels that of linear ones.22 Since the amphiphiles examined have identical hydrophilic components (PEG16) we exclude coronal repulsion effects as the determining factor. Instead, in analogy with linear amphiphiles, we attribute morphological diversity to the role of interfacial curvature or the interaction between dissimilar blocks. To understand brush packing, static light scattering experiments were performed on spherical aggregates from PGMA500g-(PEG16/PLA11) (wPEG = 0.28), by comparing their molecular weight (1.62 106 Da) to that of the molecular brush (1.44 106 Da). The similarity between both suggests that the structures shown in Fig. 1A consist of unimolecular aggregates. For unimolecular micelles, the hydrophobic backbone and PLA collapse to form the core in the presence of water while PEG chains stretch out and act as a steric stabilizing layer, preventing micelle fusion. As the PLA length increases, PEG stabilization becomes insufficient for unimolecular structures, requiring brushes to undergo intermolecular association into a cylindrical morphology. Simulation data of the self-assembly of asymmetric molecular brushes also show the formation of worm-like aggregates with decreasing hydrophilic content.22 Careful examination of the structures shown in Fig. 1B revealed that the average diameter of rods and toroids is similar to those of unimolecular micelles (B20–25 nm, Fig. 1A). Finally, a further decrease in wPEG results in the formation of vesicles, in which case PEG chains stretch out from inside and outside the membrane to stabilize the hydrophobic interlayer. PGMA500-g-(PEG16/PLA26) forms an unusual toroidal morphology when assembled by a rapid and large change in solvent quality (Fig. 1B). To the best of our knowledge, this is the first time toroid structures resulting from the self-assembly of molecular brushes have been observed. Morphological differences between this sample and that produced by dialysis suggest that the combined effect of shear forces and the rate and magnitude of solvent jump induce network separation and facilitate the end-to-end connection of cylinders into closed geometries. Shear flow is known to influence ring formation,12 and in our system the shear rate can achieve values of B9300 s 1, which we suspect is enough to influence structure formation particularly for worm-like aggregates. The sample prepared by dialysis was subject to vigorous stirring after assembly, however, no apparent differences were observed with respect to the non-stirred sample (Fig. S8, ESI†). Preservation of aggregate morphology despite energetic stirring suggests that shear effects and solvent quality change will conjunctly direct assembly morphology. Toroid formation via end-to-end connection of cylinders was recently shown for the co-assembled
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Fig. 2 TEMs of aggregate morphologies of PGMA500-g-(PEG16/PLA26), prepared by a rapid mixing process. The initial polymer concentration of samples A through C was 2.5 mg mL 1, while sample D was prepared at 0.5 mg mL 1. Reynolds number (Re) and final solvent quality are specified in each case.
structures of graft copolymers and hydrophobic homopolymers,28 and prior work on linear diblock copolymer self-assembly showed that external forces (e.g., shear flow) can direct structure formation.12,29 To further investigate the process parameters leading to this unique morphology, we examined different assembly conditions. For nanoparticles formed by linear diblock copolymers, differences in aggregate size with final solvent content are related to brush repulsion characteristics and core swelling.27 We examined the effect of final solvent quality (non-solvent : solvent ratio) on aggregates from PGMA500-g-(PEG16/PLA26) for three different ratios (3, 9 and 18) at relatively similar mixing velocities. The results are shown in Fig. 1 and 2 (1B, 2B and C). We observe that, while toroid width remained relatively constant (B20 nm), its average diameter decreases with an increase in solvent quality. For the lowest solvent ratio, the majority of ring diameters exceed 100 nm, with some exceptions in the range between 50–100 nm. At intermediate ratios we see a decrease in average ring diameter to values between 50–100 nm, while at the highest ratio ring diameters take their lowest value (o50 nm). Noting that a better solvent quality (lower solvent jump) also results in a higher local polymer concentration during mixing suggests that both concentration and solvent quality may facilitate cylinder growth over end-to-end connection. Assembly kinetics of linear diblock copolymers are also greatly influenced by mixing velocity.27 We compared aggregate structures from samples prepared using the same solvent jump but with a large difference in mixing velocity (Fig. 2A and B). While no apparent differences were observed in terms of feature diameters, we did notice mild aggregation for samples prepared with the lowest mixing velocity. Finally, we compared the effect of initial polymer concentration on aggregate morphology, as shown in Fig. 2B and Fig. S9 (ESI†). At lower initial concentration we observed more short rods and spheres, and fewer closed structures, suggesting that a certain local concentration is necessary for cylinders to form closed structures, as previously discussed. Based on our observations we hypothesize that a certain cylinder length is necessary for toroid formation, which will be mediated by process parameters during mixing. A variety of self-assembled structures were obtained by manipulation of the amphiphilic character of a series of
538 | Chem. Commun., 2014, 50, 536--538
Communication
PLA/PEG-grafted molecular brushes, the morphological transitions of which were shown to parallel those of linear diblock amphiphiles. The combination of shear effects and a rapid and large change in solvent quality resulted in self-assembled structures distinct from those achieved under equilibrium conditions. More importantly, we report on the first known examples of toroid formation from the self-assembly of molecular brush amphiphiles and the molecular and process parameters involved in their formation, demonstrating how features of the assembly process can be used to direct the formation of unique morphologies from architecturally complex macromolecules.
Notes and references 1 S. Jain and F. S. Bates, Science, 2003, 300, 460–464. 2 J. Bang, S. M. Jain, Z. B. Li, T. P. Lodge, J. S. Pedersen, E. Kesselman and Y. Talmon, Macromolecules, 2006, 39, 1199–1208. 3 T. Smart, H. Lomas, M. Massignani, M. V. Flores-Merino, L. R. Perez and G. Battaglia, Nano Today, 2008, 3, 38–46. 4 I. C. Reynhout, J. J. L. M. Cornelissen and R. J. M. Nolte, J. Am. Chem. Soc., 2007, 129, 2327–2332. 5 Y. Zheng, Y. Y. Won, F. S. Bates, H. T. Davis, L. E. Scriven and Y. Talmon, J. Phys. Chem. B, 1999, 103, 10331–10334. 6 Y. Y. He, Z. B. Li, P. Simone and T. P. Lodge, J. Am. Chem. Soc., 2006, 128, 2745–2750. 7 K. L. Wooley, Z. Li, J. Ma and N. S. Lee, J. Am. Chem. Soc., 2011, 133, 1228–1231. 8 D. J. Pochan, Z. Y. Chen, H. G. Cui, K. Hales, K. Qi and K. L. Wooley, Science, 2004, 306, 94–97. 9 C. Q. Wang, G. T. Li and R. R. Guo, Chem. Commun., 2005, 3591–3593. 10 H. G. Cui, Z. Y. Chen, K. L. Wooley and D. J. Pochan, Macromolecules, 2006, 39, 6599–6607. 11 Z. Y. Chen, H. G. Cui, K. Hales, Z. B. Li, K. Qi, D. J. Pochan and K. L. Wooley, J. Am. Chem. Soc., 2005, 127, 8592–8593. 12 H. Z. Yu and W. Jiang, Macromolecules, 2009, 42, 3399–3404. 13 S. J. Rowan, Nat. Mater., 2009, 8, 89–91. 14 S. Garnier and A. Laschewsky, Macromolecules, 2005, 38, 7580–7592. 15 R. C. Hayward and D. J. Pochan, Macromolecules, 2010, 43, 3577–3584. 16 C. W. Wang, D. Sinton and M. G. Moffitt, J. Am. Chem. Soc., 2011, 133, 18853–18864. 17 H. G. Cui, Z. Y. Chen, S. Zhong, K. L. Wooley and D. J. Pochan, Science, 2007, 317, 647–650. 18 J. Zhu, S. Zhang, K. Zhang, X. Wang, J. W. Mays, K. L. Wooley and D. J. Pochan, Nat. Commun., 2013, 4, 2297. 19 H. F. Gao and K. Matyjaszewski, J. Am. Chem. Soc., 2007, 129, 6633–6639. 20 Y. Xia, J. A. Kornfield and R. H. Grubbs, Macromolecules, 2009, 42, 3761–3766. 21 Y. Y. Yuan, Q. Du, Y. C. Wang and J. Wang, Macromolecules, 2010, 43, 1739–1746. 22 H. Y. Chang, Y. L. Lin and Y. J. Sheng, Macromolecules, 2012, 45, 4778–4789. 23 X. M. Lian, D. X. Wu, X. H. Song and H. Y. Zhao, Macromolecules, 2010, 43, 7434–7445. 24 L. N. Gu, Z. Shen, S. Zhang, G. L. Lu, X. H. Zhang and X. Y. Huang, Macromolecules, 2007, 40, 4486–4493. 25 Y. Liu, C. Y. Cheng, Y. Liu, R. K. Prud’homme and R. O. Fox, Chem. Eng. Sci., 2008, 63, 2829–2842. 26 J. R. Spaeth, I. G. Kevrekidis and A. Z. Panagiotopoulos, J. Chem. Phys., 2011, 135, 184903. 27 B. K. Johnson and R. K. Prud’homme, Phys. Rev. Lett., 2003, 91, 118302. 28 L. Chen, T. Jiang, J. P. Lin and C. H. Cai, Langmuir, 2013, 29, 8417–8426. 29 C. W. Wang, D. Sinton and M. G. Moffitt, ACS Nano, 2013, 7, 1424–1436.
This journal is © The Royal Society of Chemistry 2014
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Toroidal structures from brush amphiphiles† Cite this: Chem. Commun., 2014, 50, 536
Hanying Luo, Jose ´ Luis Santos and Margarita Herrera-Alonso*
Received 6th September 2013, Accepted 27th October 2013 DOI: 10.1039/c3cc46834h www.rsc.org/chemcomm
The self-assembly of macromolecular brushes of varying amphiphilic character from homogeneous solution under rapid assembly conditions results in unique morphologies, distinct from equilibrium structures. Kinetic features of the assembly process enabled the formation of toroid structures, the sizes of which were readily tuned according to solvent quality.
The self-assembly of amphiphilic block copolymers is widely studied as an approach for engineering nano-objects. A variety of different features, including spherical micelles,1–8 cylindrical or worm-like micelles,1–3,9 toroids,8,10–12 and vesicles,2–4,6,9 result from the self-assembly of linear amphiphilic block copolymers by manipulating the ratio of dissimilar blocks, solvent quality, and coronal interactions.2,3,6,13,14 Aside from the possibilities enabled by chemical diversity, aggregate structure can also be manipulated through kinetic features of the assembly process.8,15–18 Elucidating the molecular and process parameters influencing assembly should enable the fabrication of increasingly complex morphologies, broadening their envisioned applicability. Unlike linear amphiphiles, morphological determinants of solution assemblies from more complex macromolecular building blocks, such as graft copolymers, are far less explored. In terms of morphological diversity and control over the characteristic sizes of self-assembled objects, there exists a lack of work showing the equivalence between graft copolymers and their linear analogs.9 Herein, we demonstrate that a decrease in the hydrophilic ratio of a family of amphiphilic asymmetric macromolecular brushes results in morphological transitions of self-assemblies from spherical micelles to bilayer structures (vesicles). Furthermore, we discuss process parameters leading to the formation of toroidal aggregates. To the best of our knowledge, this is the first time molecular brushes have been shown to offer access to the variety of morphologies shown here.
Department of Materials Science and Engineering, The Johns Hopkins University, Baltimore, Maryland 21218, USA. E-mail: [email protected] † Electronic supplementary information (ESI) available: Experimental procedures and additional figures. See DOI: 10.1039/c3cc46834h
536 | Chem. Commun., 2014, 50, 536--538
Molecular brushes are graft polymers wherein the distance between grafting points is smaller than the characteristic dimensions of the side-chains. Crowded grafting results in a uniquely extended backbone conformation and influences their physicochemical properties.19–21 Amphiphilic asymmetric macromolecular brushes, with hydrophobic and hydrophilic side-chains grafted onto the same backbone repeat unit,22–24 were constructed by a combination of ‘‘graft from’’ and ‘‘graft onto’’ strategies (Scheme 1). Poly(ethylene glycol) (PEG) and poly(D,L-lactide) (PLA) were chosen as hydrophilic and hydrophobic side chain components, respectively. The brush hydrophilic ratio, expressed in terms of the weight fraction of PEG (wPEG), was varied by changing the length of the PLA block (Table S3, ESI†). Grafted PEG chains had a fixed molecular weight of 750 Da (PEG16, subscript represents repeat units). Rapid self-assembly was triggered by a rapid change in solvent quality inside a multi-inlet vortex mixer (MIVM), which allows controllable changes in the magnitude and rate of solvent quality jump. In the MIVM, micromixing occurs in the millisecond range, providing a homogeneous environment
Scheme 1 Synthesis of molecular brushes with amphiphilic branchedside chains by a combination of ‘‘grafting from’’ and ‘‘grafting onto’’ routes.
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Fig. 1 Transmission electron micrographs of aggregate morphologies of amphiphilic branched brush copolymers, prepared by a rapid mixing process (top row) and by dialysis (bottom row). Differences in aggregate structure reflect the molecular characteristics of the block copolymers, with decreasing hydrophilic content from left to right, as well as the effect of processing conditions.
for hydrophobic association. Relevant process parameters include the final non-solvent : solvent ratio (H2O : THF, v/v), and mixing velocity, expressed in terms of the Reynolds number (Re),25 see ESI.† Fig. 1 (A through C) shows the resulting structures from the rapid self-assembly of molecular brushes with amphiphilic branched side chains as functions of wPEG. The system with the highest wPEG is characterized by forming homogeneous spherical micelles, with an average size of B20 nm. Decreasing wPEG results in the formation of a cornucopia of toroidal structures coexisting with others, such as long rods, ‘‘8’’ shaped, and lassos. Finally, vesicles are obtained at the lowest wPEG. Interestingly, samples B and C exhibit morphologies entirely different from the conventional spherical shapes generally produced by this method from linear diblock copolymer amphiphiles.25–27 Furthermore, as shown in Fig. 1, a few vesicles collapsed during the drying process for TEM sample preparation. To further investigate the influence of medium conditions on self-assembled structures from molecular brushes, we prepared samples by dialysis. This method differs from the rapid assembly process in the rate of solvent change and mixing energy. The resulting morphologies are shown in Fig. 1 (D through F). While spherical micelles are also present for the sample with highest wPEG, a few larger aggregates are also observed, leading to higher polydispersity than those obtained from rapid quenching. As wPEG drops, a network of cylindrical micelles form and, compared to its quenched equivalent, only a few toroidal structures are observed. The slow change in solvent quality results in the formation of a network-like structure and aggregates of rods, in contrast to the quenched sample wherein no large aggregates were found (Fig. 1, E vs. B). Lastly, giant vesicles were observed for the amphiphiles with the lowest PEG content, which were considerably larger than the equivalent bilayer aggregates formed by the rapid assembly method. Results from dynamic light scattering experiments of micellar and bilayer aggregates also confirm monodispersity
This journal is © The Royal Society of Chemistry 2014
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and the smaller size of samples prepared via a rapid assembly (Fig. S7, ESI†). Morphological diversity in the self-assembled forms of linear amphiphiles is dictated by a balance of the interfacial curvature between unlike blocks, solvent selectivity, and coronal interactions.2,3,6,13,14,17 Prior work on linear amphiphilic polymers and rod shaped amphiphiles has shown that their morphological transitions occur with decreasing hydrophilic fraction.1–3 Interestingly, despite their difference in molecular structure, the morphological progression of brush amphiphiles parallels that of linear ones.22 Since the amphiphiles examined have identical hydrophilic components (PEG16) we exclude coronal repulsion effects as the determining factor. Instead, in analogy with linear amphiphiles, we attribute morphological diversity to the role of interfacial curvature or the interaction between dissimilar blocks. To understand brush packing, static light scattering experiments were performed on spherical aggregates from PGMA500g-(PEG16/PLA11) (wPEG = 0.28), by comparing their molecular weight (1.62 106 Da) to that of the molecular brush (1.44 106 Da). The similarity between both suggests that the structures shown in Fig. 1A consist of unimolecular aggregates. For unimolecular micelles, the hydrophobic backbone and PLA collapse to form the core in the presence of water while PEG chains stretch out and act as a steric stabilizing layer, preventing micelle fusion. As the PLA length increases, PEG stabilization becomes insufficient for unimolecular structures, requiring brushes to undergo intermolecular association into a cylindrical morphology. Simulation data of the self-assembly of asymmetric molecular brushes also show the formation of worm-like aggregates with decreasing hydrophilic content.22 Careful examination of the structures shown in Fig. 1B revealed that the average diameter of rods and toroids is similar to those of unimolecular micelles (B20–25 nm, Fig. 1A). Finally, a further decrease in wPEG results in the formation of vesicles, in which case PEG chains stretch out from inside and outside the membrane to stabilize the hydrophobic interlayer. PGMA500-g-(PEG16/PLA26) forms an unusual toroidal morphology when assembled by a rapid and large change in solvent quality (Fig. 1B). To the best of our knowledge, this is the first time toroid structures resulting from the self-assembly of molecular brushes have been observed. Morphological differences between this sample and that produced by dialysis suggest that the combined effect of shear forces and the rate and magnitude of solvent jump induce network separation and facilitate the end-to-end connection of cylinders into closed geometries. Shear flow is known to influence ring formation,12 and in our system the shear rate can achieve values of B9300 s 1, which we suspect is enough to influence structure formation particularly for worm-like aggregates. The sample prepared by dialysis was subject to vigorous stirring after assembly, however, no apparent differences were observed with respect to the non-stirred sample (Fig. S8, ESI†). Preservation of aggregate morphology despite energetic stirring suggests that shear effects and solvent quality change will conjunctly direct assembly morphology. Toroid formation via end-to-end connection of cylinders was recently shown for the co-assembled
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Fig. 2 TEMs of aggregate morphologies of PGMA500-g-(PEG16/PLA26), prepared by a rapid mixing process. The initial polymer concentration of samples A through C was 2.5 mg mL 1, while sample D was prepared at 0.5 mg mL 1. Reynolds number (Re) and final solvent quality are specified in each case.
structures of graft copolymers and hydrophobic homopolymers,28 and prior work on linear diblock copolymer self-assembly showed that external forces (e.g., shear flow) can direct structure formation.12,29 To further investigate the process parameters leading to this unique morphology, we examined different assembly conditions. For nanoparticles formed by linear diblock copolymers, differences in aggregate size with final solvent content are related to brush repulsion characteristics and core swelling.27 We examined the effect of final solvent quality (non-solvent : solvent ratio) on aggregates from PGMA500-g-(PEG16/PLA26) for three different ratios (3, 9 and 18) at relatively similar mixing velocities. The results are shown in Fig. 1 and 2 (1B, 2B and C). We observe that, while toroid width remained relatively constant (B20 nm), its average diameter decreases with an increase in solvent quality. For the lowest solvent ratio, the majority of ring diameters exceed 100 nm, with some exceptions in the range between 50–100 nm. At intermediate ratios we see a decrease in average ring diameter to values between 50–100 nm, while at the highest ratio ring diameters take their lowest value (o50 nm). Noting that a better solvent quality (lower solvent jump) also results in a higher local polymer concentration during mixing suggests that both concentration and solvent quality may facilitate cylinder growth over end-to-end connection. Assembly kinetics of linear diblock copolymers are also greatly influenced by mixing velocity.27 We compared aggregate structures from samples prepared using the same solvent jump but with a large difference in mixing velocity (Fig. 2A and B). While no apparent differences were observed in terms of feature diameters, we did notice mild aggregation for samples prepared with the lowest mixing velocity. Finally, we compared the effect of initial polymer concentration on aggregate morphology, as shown in Fig. 2B and Fig. S9 (ESI†). At lower initial concentration we observed more short rods and spheres, and fewer closed structures, suggesting that a certain local concentration is necessary for cylinders to form closed structures, as previously discussed. Based on our observations we hypothesize that a certain cylinder length is necessary for toroid formation, which will be mediated by process parameters during mixing. A variety of self-assembled structures were obtained by manipulation of the amphiphilic character of a series of
538 | Chem. Commun., 2014, 50, 536--538
Communication
PLA/PEG-grafted molecular brushes, the morphological transitions of which were shown to parallel those of linear diblock amphiphiles. The combination of shear effects and a rapid and large change in solvent quality resulted in self-assembled structures distinct from those achieved under equilibrium conditions. More importantly, we report on the first known examples of toroid formation from the self-assembly of molecular brush amphiphiles and the molecular and process parameters involved in their formation, demonstrating how features of the assembly process can be used to direct the formation of unique morphologies from architecturally complex macromolecules.
Notes and references 1 S. Jain and F. S. Bates, Science, 2003, 300, 460–464. 2 J. Bang, S. M. Jain, Z. B. Li, T. P. Lodge, J. S. Pedersen, E. Kesselman and Y. Talmon, Macromolecules, 2006, 39, 1199–1208. 3 T. Smart, H. Lomas, M. Massignani, M. V. Flores-Merino, L. R. Perez and G. Battaglia, Nano Today, 2008, 3, 38–46. 4 I. C. Reynhout, J. J. L. M. Cornelissen and R. J. M. Nolte, J. Am. Chem. Soc., 2007, 129, 2327–2332. 5 Y. Zheng, Y. Y. Won, F. S. Bates, H. T. Davis, L. E. Scriven and Y. Talmon, J. Phys. Chem. B, 1999, 103, 10331–10334. 6 Y. Y. He, Z. B. Li, P. Simone and T. P. Lodge, J. Am. Chem. Soc., 2006, 128, 2745–2750. 7 K. L. Wooley, Z. Li, J. Ma and N. S. Lee, J. Am. Chem. Soc., 2011, 133, 1228–1231. 8 D. J. Pochan, Z. Y. Chen, H. G. Cui, K. Hales, K. Qi and K. L. Wooley, Science, 2004, 306, 94–97. 9 C. Q. Wang, G. T. Li and R. R. Guo, Chem. Commun., 2005, 3591–3593. 10 H. G. Cui, Z. Y. Chen, K. L. Wooley and D. J. Pochan, Macromolecules, 2006, 39, 6599–6607. 11 Z. Y. Chen, H. G. Cui, K. Hales, Z. B. Li, K. Qi, D. J. Pochan and K. L. Wooley, J. Am. Chem. Soc., 2005, 127, 8592–8593. 12 H. Z. Yu and W. Jiang, Macromolecules, 2009, 42, 3399–3404. 13 S. J. Rowan, Nat. Mater., 2009, 8, 89–91. 14 S. Garnier and A. Laschewsky, Macromolecules, 2005, 38, 7580–7592. 15 R. C. Hayward and D. J. Pochan, Macromolecules, 2010, 43, 3577–3584. 16 C. W. Wang, D. Sinton and M. G. Moffitt, J. Am. Chem. Soc., 2011, 133, 18853–18864. 17 H. G. Cui, Z. Y. Chen, S. Zhong, K. L. Wooley and D. J. Pochan, Science, 2007, 317, 647–650. 18 J. Zhu, S. Zhang, K. Zhang, X. Wang, J. W. Mays, K. L. Wooley and D. J. Pochan, Nat. Commun., 2013, 4, 2297. 19 H. F. Gao and K. Matyjaszewski, J. Am. Chem. Soc., 2007, 129, 6633–6639. 20 Y. Xia, J. A. Kornfield and R. H. Grubbs, Macromolecules, 2009, 42, 3761–3766. 21 Y. Y. Yuan, Q. Du, Y. C. Wang and J. Wang, Macromolecules, 2010, 43, 1739–1746. 22 H. Y. Chang, Y. L. Lin and Y. J. Sheng, Macromolecules, 2012, 45, 4778–4789. 23 X. M. Lian, D. X. Wu, X. H. Song and H. Y. Zhao, Macromolecules, 2010, 43, 7434–7445. 24 L. N. Gu, Z. Shen, S. Zhang, G. L. Lu, X. H. Zhang and X. Y. Huang, Macromolecules, 2007, 40, 4486–4493. 25 Y. Liu, C. Y. Cheng, Y. Liu, R. K. Prud’homme and R. O. Fox, Chem. Eng. Sci., 2008, 63, 2829–2842. 26 J. R. Spaeth, I. G. Kevrekidis and A. Z. Panagiotopoulos, J. Chem. Phys., 2011, 135, 184903. 27 B. K. Johnson and R. K. Prud’homme, Phys. Rev. Lett., 2003, 91, 118302. 28 L. Chen, T. Jiang, J. P. Lin and C. H. Cai, Langmuir, 2013, 29, 8417–8426. 29 C. W. Wang, D. Sinton and M. G. Moffitt, ACS Nano, 2013, 7, 1424–1436.
This journal is © The Royal Society of Chemistry 2014
