Nanofibrillar Micelles and Entrapped Vesicles from Biodegradable

Article pubs.acs.org/Langmuir

Nanofibrillar Micelles and Entrapped Vesicles from Biodegradable Block Copolymer/Polyelectrolyte Complexes in Aqueous Media Nisa V. Salim,† Nishar Hameed,† Tracey L. Hanley,‡ Lynne J. Waddington,§ Patrick G. Hartley,∥ and Qipeng Guo*,† †

Polymers Research Group, Institute for Frontier Materials, Deakin University, Locked Bag 2000, Geelong, Victoria 3220, Australia Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia § CSIRO Materials Science and Engineering, 343 Royal Parade, Parkville, Victoria 3052, Australia ∥ CSIRO Materials Science and Engineering, Bag 10, Clayton South, Victoria, 3169, Australia ‡

S Supporting Information *

ABSTRACT: Here we report a viable route to fibrillar micelles and entrapped vesicles in aqueous solutions. Nanofibrillar micelles and entrapped vesicles were prepared from complexes of a biodegradable block copolymer poly(ethylene oxide)-block-poly(lactide) (PEO-b-PLA) and a polyelectrolyte poly(acrylic acid) (PAA) in aqueous media and directly visualized using cryogenic transmission electron microscopy (cryo-TEM). The self-assembly and the morphological changes in the complexes were induced by the addition of PAA/water solution into the PEO-b-PLA in tetrahydrofuran followed by dialysis against water. A variety of morphologies including spherical wormlike and fibrillar micelles, and both unilamellar and entrapped vesicles, were observed, depending on the composition, complementary binding sites of PAA and PEO, and the change in the interfacial energy. Increasing the water content in each [AA]/[EO] ratio led to a morphological transition from spheres to vesicles, displaying both the composition- and dilution-dependent micellar-to-vesicular morphological transitions.

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INTRODUCTION Most studies have established that when block copolymers are dissolved in a block selective solvent, they can spontaneously self-assemble to form well-defined micelles or aggregates by the association of the insoluble blocks.1,2 Several systems comprising block copolymers, such as AB,3,4 ABA,5,6 and ABC star-shaped7,8 molecules have been extensively investigated. By changing the molecular parameters (chemical structures, compositions and architectures of copolymers)9−11 or solution parameters (concentration, temperature, solubility, pH, ionic strength, etc.)11−14 of amphiphilic block copolymers, it has been possible to manipulate multicompartment micellar structures, including core−shell−corona spheres,15,16 cylinders,17 helices,18 segmented wormlike micelles,19 disks,20 plates,21 toroids,13 and “raspberry-like” micelles.22 As the morphology and structure of the core and the shell of the micelles have significant effects on their properties and practical applications, this considerable potential has been explored.23 The final morphology of the self-assembled complexes could be influenced by the interfacial energy of the soluble/insoluble interface, core-chain stretching, and entropy loss due to the insoluble blocks packed into aggregate micro domains. A comparatively easy way to control the nanoscale morphologies and create exceptional structures consists of the use of homopolymer and block copolymer mixtures. There is an interplay between two phase-transition phenomena, i.e., the © 2013 American Chemical Society

coexistence of homopolymer domains as a result of macrophase separation with micro structured domains rich in diblock copolymers which may induce morphological changes depending on the environmental conditions.24,25 Depending on the block copolymer composition and the molecular weight of the homopolymer, the aggregates of the complexes can be spherical, cylindrical, lamellar, or vesicular.26 A recent development has revealed that secondary interactions such as electrostatic, hydrogen bonding, metal− ligand coordination bonding, etc. can induce polymer selfassembly in solutions.27−29 By taking advantage of such interpolymer complexation, it is possible to manipulate novel and well-defined ordered nanostructures for diverse applications. The morphologies include spheres, rods, lamellae, vesicles, large compound micelles (LCMs), large compound vesicles (LCVs), and more.30−32 Hydrogen bonding plays a fundamental role in creating higher levels of hierarchy in structure formation of block copolymers by the self-assembly process.33−37 Comicellization of block-copolymers driven by hydrogen-bonding interactions will exhibit microphase separation even when their outer blocks are very short.21Most of the studies reported so far point out that the self-assembly of wellReceived: May 10, 2013 Revised: June 20, 2013 Published: June 21, 2013 9240

dx.doi.org/10.1021/la4017678 | Langmuir 2013, 29, 9240−9248

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Å2 for all imaging. Images were recorded using a Megaview III CCD camera and AnalySIS camera control software (Olympus) using magnifications in the range 60 000−110 000×. Dynamic Light Scattering (DLS). The hydrodynamic radius of the complex aggregates was measured on a Zetasizer Nano ZS apparatus equipped with He−Ne laser with a wavelength of 633 nm digital correlator. The temperature stability inside DLS sample holder was controlled at 25 °C, and the measurements were carried out at detection angle of 173°. Solutions of 0.5% (w/v) complex aggregates in water were used. The scattering intensity autocorrelation functions were analyzed by using the methods of CONTIN and Cumulant, which are based on an inverse-Laplace transformation of data. This gives access to a size distribution histogram for the analyzed complex solutions.43−45

defined block copolymers that associate intermolecularly via hydrogen bonding interactions are capable of fabricating hierarchical two-dimensional nanostructures.27,38−41 Recently, Salim et al. prepared some complicated aggregate structures such as multilamellar vesicles, interconnected compound vesicles, and LCVs via the interpolymer complexation of mixtures of block copolymers or block copolymer with homopolymer in aqueous solution.40,41 Relatively little work has been done to study the effect of addition of homopolymer to a diblock copolymer in a selective solvent. It was reported that the controlled assembly of diblock copolymers where one of the blocks is crystalline facilitates the growth of a series of complex nanoarchitectures in a controlled fashion.42 In this paper, on the basis of the structural measurements using cryogenic transmission electron microscopy (cryo-TEM), we observed the aggregate morphologies formed in water by the self-assembly of the PEO-b-PLA polymeric amphiphiles and also the packing properties of the molecules. Cryo-TEM allows the examination and direct visualization of particular micelles and vesicles, and thereby avoids many of the artifacts associated with conventional TEM. The morphology and size are expected to be as similar as possible to those as they exist in the aqueous environment. Moreover, the introduction of polyelectrolyte (PAA) homopolymer into PEO-b-PLA chains leads to changes in the size and structure of micellar aggregates, and various mixed micellar morphologies are formed due to the hydrogenbonding complexation between PAA and PEO. The interpolymer interaction between PAA and PEO-b-PLA leads to the formation of nearly spherical micelles, wormlike micellar aggregates, and vesicles when different stoichiometric ratios between the complexing blocks are used. The results reveal that the hydrogen-bonding complexation between PAA and PEO in the corona and the repulsion between PLA chains in the core as a function of the molar ratio of PEO to PAA manipulate the evolution. By changing the [EO]/[AA] ratios and also increasing water content, the sequence of morphologies follows the order of spherical, elongated, wormlike, fibrillar micelles, and vesicular morphologies such as unilamellar and entrapped vesicles. The formation of block copolymer vesicles in water is of particular interest because of their potential in various applications. In this two-directional study, how the addition of PAA to PEO-b-PLA influences the aggregate morphology of the resulting complexes was investigated. The effect of solvent composition on aggregate formation was also investigated. It was found that the effect of PAA was dependent on the block lengths, the width of the molecular weight distribution, and the stabilization of the morphologies, and, on the other hand, the water content has an influence on chain repulsion and aggregation number. Moreover, this work introduces a viable route to fibrillar micelles and entrapped vesicles in aqueous solutions.

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RESULTS AND DISCUSSION

Amphiphilic block copolymers in aqueous media can selfassemble to give regular micelles with a hydrophilic corona and a hydrophobic core. The structure and morphology of these systems can be examined by cryo-TEM as well as DLS. CryoTEM allows the visual rendering of compartmentalization of particular micelles in the aggregates thereby avoiding artifacts. Generally, to understand the morphological transformations of a block copolymer by the addition of a polyelectrolyte, both interfacial curvature and chain stretching within micelles can be considered.46,47 The increase in interfacial curvature tends to reconcile the increased asymmetry between the hydrophilic and hydrophobic segments with increasing hydrophilic composition. The interfacial curvature of the aggregates is directed by volume, conformational differences, and the interfacial energy between hydrophobic and charged hydrophilic blocks in the solution. The polymer blocks extend away from the interface as the interfacial energy increases, and the resultant aggregate structures choose to form flat interfaces to reduce interfacial contact.48,49 Therefore, various micellar structures can be tuned by adjusting the interfacial curvature. This study has featured a simple and effective way by addition of a polyelectrolyte into the corona volume of the block copolymer by exploiting intermolecular interactions between them. In addition, the dynamics of the morphological transitions and the structural rearrangement of micelles were studied by varying the [AA]/ [EO] ratio and also in response to a progressive change of the solvent quality of the solution. The hydrogen-bonding interactions between the ether oxygen of PEO blocks and the carboxylic acid groups of PAA were confirmed in previous studies.40,41 In this study, the aggregates were prepared by dissolving PEO-b-PLA first in THF followed by the addition of PAA in water. Because the block copolymer in the present study has a large weight fraction of PLA, it is impossible to prepare stable solutions by direct dissolution of the block copolymer molecules in water.50 THF is a common solvent for both blocks of the amphiphilic block copolymer (PEO-b-PLA) and water is a block-selective solvent for the PEO block but a precipitant for the PLA block. So, at particular water content, core−shell micelles with PLA chains as the core and PEO chains as the corona are formed in the polymer solution after dialysis. As the addition of water into the micelle solution continues, the structure of the core−shell micelles is kinetically frozen in the water solvent.51 Effect of Homopolymer Concentration. In the first part, changes in the morphology of PEO-b-PLA/PAA complexes by varying the [AA]/[EO] molar ratio of the complexes were studied. The water content at all these compositions was fixed

EXPERIMENTAL SECTION

Materials. The polymers used in the present study were poly(acrylic acid) (PAA) and poly(ethylene oxide)-block-poly(lactide) (PEO-b-PLA). The PEO-b-PLA block copolymer was purchased from Polymer Source, Inc., with Mn (PEO) = 5000, Mn (PLA) = 22 000, and Mw/Mn = 1.2. PAA samples with an average Mw = 1800 was obtained from Aldrich Chemical Co., Inc. Cryogenic Transmission Electron Microscopy (Cryo-TEM). The samples were examined using a Gatan 626 cryoholder and FEI Tecnai 12 transmission electron microscope (FEI, Eindhoven, The Netherlands) at an operating voltage of 120 KV. At all times low dose procedures were followed, using an electron dose of 8−10 electrons/ 9241


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Figure 1. Cryo-TEM images of (a) plain PEO-b-PLA block copolymer, and PEO-b-PLA/PAA complexes in aqueous solutions with [AA]/[EO] ratios of (b) 0.2, (c) 0.4, (d) 0.8, and (e) 1. The water content of all these compositions was fixed at 3 wt %. Holey carbon films were used for embedding of the vitrified aqueous solution of the complexes. (f) Hydrodynamic diameter (D h) distributions measured by DLS.

at 3 wt %. The addition of a PAA polyelectrolyte into the PEOb-PLA block copolymer induces major changes in the volume ratio of the corona compared to the core. Here, the intermolecular interactions between PAA and PEO chains can in turn reduce the electrostatic interactions within the PAA blocks. Figure 1 shows the cryo-TEM images of the change in the morphology of PEO-b-PLA/PAA complexes at various [AA]/ [EO] ratios. With increasing PAA content ([AA]/[EO] = 0.2 to 1), the morphology of the aggregates changes from spheres to vesicles with a number of intermediate morphologies. Figure 1a shows the micrograph of the spherical micelles of pure PEOb-PLA block copolymer, which yields an average diameter of 35 nm. Since water is a selective solvent for PEO, the insoluble PLA forms the core and PEO phase as the shell. The addition of PAA (at molar ratio [AA]/[EO] = 0.2), which selectively swells the PLA blocks of the block copolymer, enables the formation of a mixture of spherical and elongated spherical micelles with an average size of 50 nm (Figure 1b). It can be seen that in complexes at [AA]/[EO] = 0.4, only elongated micelles can be seen in Figure 1c. By increasing the [AA]/[EO] ratio to 0.8, a mixture of elongated micelles and vesicles is formed which is illustrated in Figure 1d. And finally at [AA]/

[EO] = 1, vesicles are the only morphology present (Figure 1e). The hydrodynamic diameter (Dh) and distribution of pure PEO-b-PLA and its complexes with PAA at various [AA]/[EO] ratios were determined by DLS experiments.40,41,43−45 For each PEO-b-PLA/PAA complex solution at 3 wt % water content, there is a single peak with a narrow distribution indicating the existence of low polydispersity within the aggregates in the solution. The complexes exhibit much larger Dh values ranging from 55 to 100 nm compared to the pure PEO-b-PLA block copolymer solution. The Dh values of the complex aggregates increase with increasing PAA concentration. As per DLS measurements, the PEO-b-PLA block copolymer forms spherical micelles with an average Dh of 53 nm. At low PAA concentrations, ([AA]/[EO] = 0.2), the complex solution contains spherical along with a few elongated micellar aggregates, which is shown by a single broad peak in DLS graphs. At [EO]/[AA] = 0.4, more elongated micelles were observed which can also be seen from the single DLS peak. With increasing [EO]/[AA] ratios from 0.8 to 1, the size of the aggregates and their distribution significantly increase, which can be identified by the broadening of the DLS peaks.The size 9242


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Figure 2. Cryo-TEM images of PEO-b-PLA with water content of (a) 9.1, (b) 14, and (c) 20 wt % along with hydrodynamic diameter (D h) distributions measured by DLS.

Figure 3. Cryo-TEM images of PEO-b-PLA/PAA complexes at [AA]/[EO] = 0.2 with water content of (a) 9.1, (b) 14, and (c) 20 wt % along with hydrodynamic diameter (D h) distributions measured by DLS.

exceeded by the thermodynamic penalty incurred by core-chain stretching and corona repulsion. As the content of PAA increases further ([AA]/[EO] = 0.8), the density of corona chains outside the core surface becomes high due to the high hydrophilic content in the complexes. This creates a strong repulsion among corona chains on the surface of the micelles, which will push the corona chains away from each other and thereby change the morphology to form vesicles with a minority of elongated spherical micelles (Figure 1d). When the molar ratio is ([AA]/[EO] = 1.0), vesicles are observed (Figure 1e). When hydrogen bonding between PEO and PAA takes place, a more compact corona is formed with neutral charge which leads to less corona chain repulsion and hence in effect volume of hydrophilic-to-hydrophobic has changed and favors vesicles (lamellar structure). Moreover, in hydrogen-bonding interactions, unlike other secondary interactions, the absorbed PAA chains can be penetrated into the corona of PEO-b-PLA micelles and form vesicles.41 Overall, the micelle−vesicle transformation is induced due to the increase in effective corona volume and consequent higher interfacial curvature between hydrophobic−hydrophilic blocks.53 Increasing the hydrophilic block length induces greater interfacial curvature and the increase in the aggregate size is due to the

of the micelles increases beyond 100 nm as the value of molar ratio closes to 1. Addition of PAA that interacts with only one of the blocks in the PEO-b-PLA block copolymer can significantly influence the equilibrium structure. At very low PAA content ([AA]/[EO] = 0.2), PAA forms strong hydrogen bonds with the PEO blocks in the corona and the effective size and radius of the corona chains increase dramatically. This preferential hydrogen bonding interaction between PEO and PAA, apparently, changes the balance of the three morphogenic contributions such as force balance between the core chains, the interfacial tension between the core and outside solvent, and repulsion between the corona chains due to reduction in configurational entropy.52,53 At this point, to reduce the total energy of the system, the spherical micelles start to transform into elongated micelles with a smaller diameter and higher radius of curvature. Upon the addition of more and more PAA ([AA]/[EO] = 0.2), which selectively swells the block copolymer, more elongated spherical micelles are observed along with very few spherical micelles depending on the composition. When the PAA content in the complexes increases to [AA]/[EO] = 0.4, the elongated spherical micelles continue to increase in size until the driving force to reduce the interfacial energy is 9243


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spheres and elongated spherical aggregates coexist with more elongated or wormlike structures. In PEO-b-PLA/PAA complexes, the increase in water content increases the solubility and volume of the complementary binding sites of PAA and PEO together with greater separation of ion pairs, while the solubility and volume of the hydrophobic PLA blocks was decreased. As the water content increases in complexes, the solvent quality becomes poorer for the core forming PLA blocks of the block copolymer. The TEM images and DLS curves of PEO-b-PLA/PAA complexes are detailed in Figures 3, 4, and 5. At a molar ratio of [AA]/[EO] = 0.2 and at low water content (9.1 wt %), the polymer chains self-assemble to form wormlike aggregates which are shown in Figure 3a with an average Dh of 52 nm. At 14 wt % of water content, the morphologies involving wormlike structures together with a few fibrillar micelles were observed (Figure 3b). With further increase in the water content, i.e., 20 wt %, long fibrillar micelles were the major morphology in the complexes with an average Dh of 75 nm (Figure 3c). These kinds of fibrillar micelles were previously reported with rod− coil block copolymers having a conjugated block.54

availability of more ether groups and the carboxylic units for hydrogen bonding complexation. Effect of Water Content in Complexes. The morphological changes of PEO-b-PLA/PAA complexes were further studied by changing the co-solvent, i.e., water composition at each molar ratio. Surprisingly, substantial morphology transformations were observed for the aggregates with an increase in the water content at different PAA ratios. In this section, three morphological changes were investigated which are triggered at various [AA]/[EO] ratios with different water compositions: 9.1, 14, and 20 wt %. Aggregates with different size distributions were formed in the solution at different complex compositions. Originally, pure PEO-b-PLA block copolymer showed spherical micelles at 3 wt % water content as shown in Figure 1a. Figure 2 shows the cryo-TEM images of pure PEO-b-PLA at various increasing water contents, and corresponding DLS curves are given below each image. Spherical and also minor elongated micelles are observed at 9.1 wt % of water (Figure 2a). From the DLS analysis, the average diameters are 40 nm for spheres and 35 nm for the elongated spheres, respectively. At 14 and 20 wt % of water (Figure 2b and c), mixtures of 9244


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stretching of the polymer chains, as well as the free energy of the components, increased. So, the aggregates adapt morphologically to minimize the total interfacial energy.56 Here the intermolecular interactions between PAA and PEO segments would restrict the intermolecular conformation and lead to a preferred parallel alignment, facilitating the formation of vesicles. A balance between interfacial tension and chain stretching while respecting the incompressibility of the core domain leads to vesicles. At molar ratio of [AA]/[EO] = 1, unilamellar vesicular structures are observed by changing the water content. Figure 6a shows the formation of unilamellar vesicles at 9.1 wt % water content. Figure 6b shows that only entrapped vesicles are observed at 14 wt % of water content. When the water content was increased further, the diameter of the entrapped vesicles increased as core chain stretching was building up again. As the water content increased to 20 wt %, vesicular structures appeared (Figure 6c) with bigger micelles entrapped within inside the vesicles. These vesicles are more inhomogeneous compared to the entrapped vesicles shown in Figure 6b. DLS curves show that the size of the vesicles increases beyond 300 nm as the value of molar ratio is 1. It is also speculated that the formation of entrapped vesicles is initiated by the formation of large vesicles from a solution of relatively high copolymer concentration in a water-rich condition. These entrapped vesicles look like vesicles with a spherical particle trapped inside them and, therefore, are called so. Such vesicles are able to selfassemble when the organic solvent in the core of the large vesicle is gradually replaced by water.57 Here the formation of entrapped vesicles is initiated by the new balance of interfacial tension, corona repulsion, and stretching, as well as the uneven distribution of polymer chains formed at the stoichiometric molar ratio of [AA]/[EO] = 1. Since water is a selective solvent for PAA and PEO, the aggregates comprise three different layers. That is to say, insoluble PLA forms the core and the hydrogen-bonded PAA/ PEO forms the corona, and the remaining EO units of the PEO block form hydrogen bonds with water, maintaining the solubility. Note at this concentration the molar ratio of hydrogen bonded components is stoichiometric, i.e., 1:1. For such systems, a long continuous linear succession of hydrogen bonds between the monomer units may result in a lamellar structure. This would rather resist the intermolecular

At [AA]/[EO] = 0.4, as the water content increases (9.1 wt %), the complexes show a mixture of spherical and elongated micellar morphologies (Figure 4a) with a broad DLS peak at 87 nm. At 14 wt % water content, only elongated micelles were observed and corresponding broad DLS peak was at 90 nm, shown in Figure 4b. It can be seen that the size of the micelles increases with increase in the water content. This is governed by a balance between two opposing factors: first, the increase in the interfacial tension, and second the increase in the electrostatic repulsion among PAA chains, which opposes an increase in the aggregation number and micellar size. At 20 wt % of water content, elongated micelles combine to form curled wormlike structures (Figure 4c). The two DLS peaks observed at this concentrations correspond to two distinct populations of aggregates:55 one at 43 nm (low volume micelles) and the other at 105 nm (high volume elongated structures). The bimodality in the distribution is an indication that there are two types of dispersing species in solution. Here, with an increase in the water content, the solvent quality for PLA blocks deteriorates and the interfacial tension increases. In response to this increase, the PEO-b-PLA/PAA complex system tends to decrease the total interfacial area by reducing the total number of aggregates and, therefore, a transition from spherical to wormlike aggregates was observed. It is suggested the formation of elongated structures is assisted by the polydispersity of the hydrophilic chains, provided the chains are able to stabilize the highly curved ends of the micelles. From DLS curves, the aggregates at 20 wt % complexes appear to have a broad, unimodal distribution of hydrodynamic radii around 85 nm (Figure 4c). In complexes with molar ratio [AA]/[EO] = 0.8, the aggregates form a mixture of spheres, elongated aggregates and a few vesicles at 9.1 wt % water (Figure 5a). DLS shows two distinct peaks at 45 and 130 nm, corresponding to the aggregates as shown in cryo-TEM. As the water content increases further, the relative number of spheres decreases and more elongated micelles appear, with a minority of vesicles seen at 14 wt % water (Figure 5b). TEM and CONTIN analysis of the DLS data (Figure 5b) suggest the presence of two diffusing species, one with Dh around 50 nm and another with Dh around 170 nm. Again when the water content increases to 20 wt %, the morphology changes to form vesicles (Figure 5c) with an average Dh of 200 nm. With increasing water content, the 9245


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the new morphologies are formed. Also, by increasing the cosolvent content in the PEO-b-PLA/PAA complexes, water becomes poorer for the core-forming PLA blocks, and the stretching of PLA chains in the core increases, which causes an increase in that component of the free energy and that reflects core chain stretching. When the stretching is too high and there is a change in the corona volume, complex aggregates have to adapt the geometry to relax the stretching, and thus the total free energy is minimized. The strong interaction between PEO and PAA leads to the change of the interaction among the corona building blocks (PEO) which could be the main reason for the morphological transitions from spherical micelles to vesicles and finally to unusual vesicles. It can be concluded that hydrogen-bonding interactions between the hydrophilic segments of amphiphilic block copolymers should be prerequisite to the transformation of aggregate morphology in these complexes. The combination of a block copolymer with a homopolymer aggregation furnishes an attractive means of constructing diverse self-assembled structures and allows further control of the morphology of the formed structures.

conformation and lead to preferred parallel alignment, which facilitates the formation of unusual bilayer structures. Overall, the aggregate morphology always changes in a direction that decreases the overall free energy. On the basis of all the above results, a schematic diagram of the two directional morphological transitions in PEO-b-PLA/PAA complexes is detailed in Scheme 1. The scheme shows the morphology changes from top to bottom with increase in molar ratio and left to right with increase in water content. Scheme 1. Schematic Representation of Various Morphologies Formed in PEO-b-PLA/PAA Complexes by Varying the Molar Ratio and Water Content

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CONCLUSIONS This work has explained the self-assembly and aggregation behavior of PEO-b-PLA block copolymer and a polyelectrolyte PAA in aqueous solution, mainly as a function of the PAA composition as well as increasing the water content. The interaction between PAA and PEO blocks led to the formation of spherical, rod like and worm like micelles, entrapped vesicles, and also unusual structures, depending on both the composition and the water content. Morphological transition from micelles to vesicles can be attributed to the varying strength of intermolecular hydrogen bonding between PAA and PEO blocks.

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ASSOCIATED CONTENT

S Supporting Information *

Sample preparation and cryo-TEM sample preparation. This material is available free of charge via the Internet at http:// pubs.acs.org.

The addition of the polyelectrolyte PAA consequently changes the volume ratio of corona to core, due to an increase in the hydrogen bonding interactions upon various aggregate formations. Additionally, when the water content varied, the aggregate morphology changed in a direction to decrease the overall free energy. The stretching of polymer chains in the core increased with rise of water content causes an increase in the free energy of PEO-b-PLA/PAA components. As the interfacial energy increases, blocks stretch away from the interface and resultant micellar structures prefer to form flat interfaces to minimize interfacial contact. In the present PEO-b-PLA/PAA block copolymer/polyelectrolyte system, morphology transition is a function of a number of factors such as lengths of block copolymer blocks and homopolymer, their composition, specific interactions, and also the cosolvent composition. Here, changes in the composition of the available hydrogen bonding sites help the morphological transitions in the present system. In addition, there is an entropic increase during the mixing of two polymers.58,59 The increase in chain stretching is due to the consequence of this change in entropy. If the kinetics of chain exchange and rearrangement of the aggregates is considerably slower than the change of solvent quality, some of the aggregates of the previous morphologies might survive while the aggregates with

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Australian Institute of Nuclear Science and Engineering (AINSE) Ltd for funding. N.V.S. was supported by a Deakin University Postgraduate Research Scholarship (DUPRS).

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REFERENCES

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