Elsevier

Polymer

Volume 171, 8 May 2019, Pages 1-7
Polymer

The important role of cosolvent in the amphiphilic diblock copolymer self-assembly process

https://doi.org/10.1016/j.polymer.2019.03.024Get rights and content

Highlights

  • The preferential adsorption of cosolvent in polymer aggregate influences self-assembly structures.

  • The preferential adsorption of cosolvent also affects vesicle formation kinetic pathways.

  • Explicitly considering solvent components in simulations is necessary.

Abstract

A common strategy to obtain self-assembly structures of amphiphilic block copolymers resorts to using solvent mixtures in which one solvent is a selective solvent while the other is a cosolvent for both components of the diblock copolymer. In this study, the noticeable influence of cosolvent on determining the block copolymer self-assembly structures as well as the corresponding kinetic pathways is studied via dissipative particle dynamics simulations. The preferential adsorption of cosolvent in polymer aggregate is found to alter the local solvent environment, which then results in various self-assembly structures in cases of different contents of cosolvent in the system. Based on Flory-Huggins theory, the preferential adsorption of cosolvent in different domains is analyzed to corroborate the influence of cosolvent on the self-assembly structures. Taking vesicles with similar size and shape as examples, we find that the preferential adsorption of cosolvent also affects their formation kinetic pathways. If there is less cosolvent in local environment of the polymer aggregate, the vesicle formation takes spherical micelle–wormlike micelle–membrane–vesicle pathway, in which the characteristic step is the bending and close of the membrane to form the vesicle. If there is more cosolvent in local environment of the polymer aggregate, the small micelles aggregate continuously till the vesicle forms. By comparing the simulations taking two solvent components explicitly with those using one solvent that has averaged solvation capability based on the two solvent components, we find the second simulation strategy may result in incorrect equilibrium self-assembly structures.

Introduction

Amphiphilic block copolymers can self-assemble into intriguing ordered structures, such as spherical micelles, worm-like micelles, and vesicles in selective solvents [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]]. These ordered structures have important applications in cosmetic materials, drug delivery, biotechnology, and so on [[17], [18], [19], [20], [21], [22]]. The formation mechanism of these self-assembly structures have been extensively studied both experimentally and theoretically [[23], [24], [25], [26], [27], [28], [29], [30], [31]] in recent years.

A common strategy to fabricate self-assembly structures of amphiphilic block copolymers starts from dissolving block copolymers in an organic cosolvent, which is normally a good solvent for both components of the block copolymers [32,33]. Then by slowly adding precipitant (typically water, miscible with the cosolvent), one block of the copolymer aggregates and forms the hydrophobic core of the self-assembly structure. In general, it is acknowledged that the cosolvent provides a solvent background to ensure homogeneous distribution of block copolymers in solution, and adding precipitant is the key driving force to induce the self-assembly.

In practice, the cosolvent should be easily mixable with the precipitant, and the ability of inducing the block copolymer self-assembly relies on the thermodynamically averaged property of the binary solvent mixture. Moreover, in theoretical and computational treatments, the solvent mixture is typically regarded as a “new one-component solvent” with physical property obtained from cosolvent and precipitant in a mean-field way. But as illustrated in a recent work by Kremer and coworkers [34], polymer chain is possible to collapse even in miscible good solvents due to preferential adsorption. It implies that for the solvent mixture of cosolvent and precipitant, preferential adsorption of cosolvent and/or precipitant may be strongly related to the collapse of solvophobic block in the process of block copolymer self-assembly. In experiments, we can also find the effect of different cosolvents on determining self-assembly structures of block copolymers. For example, Eisenberg and coworkers reported that polystyrene-block-poly (acrylic acid) (PS-b-PAA) could self-assemble into spherical micelles, rod-like micelles and vesicles when cosolvent was N,N-dimethylformamide (DMF), dioxane and tetrahydrofuran (THF) [35], respectively. They also found that PS-b-PAA could self-assemble into spherical micelles, cylindrical micelles or vesicles simply by changing THF concentration from 0% to 25%[35]. Recently, Denkova et al. put forward a “solvent nano-domain” concept after they observed different aggregation structures of polymers with different water-cosolvent ratios [36]. Actually, there are many similar experimental results [16,33,[37], [38], [39], [40]] implying the importance of cosolvent in the self-assembly process of block copolymers. However, a systematic study with clear conclusions on the role played by cosolvent during block copolymer self-assembly is still lacking.

The lack of clear conclusions on the cosolvent effect during block copolymer self-assembly is related to the limitation of experimental measurements. Currently two main methods, i.e., dynamic light scattering (DLS) and electron microscopy (EM), are frequently used to identify self-assembly structures of block copolymers in dilute solution. DLS can provide the information of aggregate size, while traditional EM can show the structures of aggregation after removing the solvents. However, none of them can clearly trace the distribution of cosolvent during the self-assembly process of block copolymers. Therefore, why different self-assembly structures can be obtained by simply changing cosolvent type or concentration? How does the cosolvent influence the self-assembly morphology and the corresponding pathway? Whether the cosolvent takes part in the whole self-assembly process or not? All these questions that should be answered to thoroughly understand the contribution of cosolvent on block copolymer self-assembly cannot be easily resolved.

Therefore in this study, we use dissipative particle dynamics (DPD) simulation method based on a simple block copolymer model to systematically investigate the influence of cosolvent in the process of block copolymer self-assembly. In simulations the distribution of cosolvent in the whole self-assembly process can be clearly identified, thus the role played by cosolvent can be clarified in detail. We find that the preferential adsorption of cosolvent in aggregates strongly correlates with the block copolymer self-assembly structures and kinetic pathways, even the cosolvent and the precipitant are miscible at any volume ratios. This result implies that cosolvent is more important than simply providing a solvent background and its influence on the self-assembly process of block copolymers cannot be simply treated in a mean-field way.

Section snippets

Dissipative particle dynamics and simulation model

DPD is a mesoscopic simulation method for studying soft matter systems over greater length and time scales. It was introduced by Hoogerbrugge and Koelman in 1992 [41], and was successfully used to study the self-assembly and phase separation processes of polymer systems [[42], [43], [44], [45], [46], [47], [48]]. In general, a DPD bead represents a group of atoms or a volume of fluid that is large on the atomistic scale but still macroscopically small. In this study, we have four types of DPD

Results and discussion

It is commonly acknowledged that W and G are miscible with each other in a typical self-assembly system, although some researchers have found that cosolvent G may form small micelles in water [54,55]. The miscibility between W and G is actually related to the solubility parameters of the binary solvents [56], which can be further mapped to Flory-Huggins χ parameters between them. In our simulations we can easily quantify the miscibility between W and G via calculating phase separation order

Conclusions

The influence of cosolvent on amphiphilic block copolymer self-assembly structures and the corresponding kinetic pathways is studied via dissipative particle dynamics simulations. The results show apparent preferential adsorption of cosolvent in copolymer aggregates, which is dependent on the compatibility difference between binary solvents and cosolvent with polymer. At a certain cosolvent concentration, increasing the preferential adsorption of cosolvent in polymer aggregate results in the

Acknowledgements

This work is supported by the National Science Foundation of China (21534004, 21833008), and JLU-STIRT program at Jilin University.

References (67)

  • G. Zhu et al.

    Tailoring interfacial nanoparticle organization through entropy

    Acc. Chem. Res.

    (2018)
  • S.G. Jang et al.

    Striped, ellipsoidal particles by controlled assembly of diblock copolymers

    J. Am. Chem. Soc.

    (2013)
  • E.L. Kynaston et al.

    Fiber-like micelles from the crystallization-driven self-assembly of poly(3-heptylselenophene)-block-polystyrene

    Macromol. Chem. Phys.

    (2015)
  • Z. Gao et al.

    Block copolymer ”crew-cut” micelles in water

    Macromolecules

    (1994)
  • A. Yousefi et al.

    Cosolvent effects on the spontaneous formation of nanorod vesicles in catanionic mixtures in the rich cationic region

    J. Phys. Chem. B

    (2011)
  • L. Zhang et al.

    Multiple morphologies of ”crew-cut” aggregates of polystyrene-b-poly(acrylic acid) block copolymers

    Science

    (1995)
  • Z.B. Li et al.

    Multicompartment micelles from abc miktoarm stars in water

    Science

    (2004)
  • L. Zhang et al.

    Ion-induced morphological changes in ”crew-cut” aggregates of amphiphilic block copolymers

    Science

    (1996)
  • T.P. Lodge et al.

    Access to the superstrong segregation regime with nonionic abc copolymers

    Macromolecules

    (2004)
  • H. Yu et al.

    Biomimetic block copolymer particles with gated nanopores and ultrahigh protein sorption capacity

    Nat. Commun.

    (2014)
  • Z. Liu et al.

    Two-dimensional assembly of giant molecules

    Sci. China Chem.

    (2018)
  • X. Li et al.

    Mapping coexistence phase diagrams of block copolymer micelles and free unimer chains

    Macromolecules

    (2018)
  • W. Ha et al.

    Prodrug-based cascade self-assembly strategy for precisely controlled combination drug therapy

    ACS Appl. Mater. Interfaces

    (2018)
  • R. Savić et al.

    Micellar nanocontainers distribute to defined cytoplasmic organelles

    Science

    (2003)
  • J.A. Massey et al.

    Fabrication of oriented nanoscopic ceramic lines from cylindrical micelles of an organometallic polyferrocene block copolymer

    J. Am. Chem. Soc.

    (2001)
  • R. Bleul et al.

    Techniques to control polymersome size

    Macromolecules

    (2015)
  • X.-f. Wen et al.

    Synthesis and dissipative particle dynamics simulation of cross-linkable fluorinated diblock copolymers: self-assembly aggregation behavior in different solvents

    Phys. Chem. Chem. Phys.

    (2011)
  • I. Hevus et al.

    Invertible micellar polymer sssemblies for delivery of poorly water-soluble drugs

    Biomacromolecules

    (2012)
  • C.H. Lee et al.

    Synthesis and characterization of solvent-invertible amphiphilic hollow particles

    Langmuir

    (2013)
  • Y. Zhou et al.

    Dissipative particle dynamics simulation on self-assembly behavior of rod-coil-rod triblock copolymer in solutions

    Macromol. Theory Simul.

    (2014)
  • S.E. Webber et al.
    (2012)
  • A. Das et al.

    Luminescent invertible polymersome by remarkably stable supramolecular assembly of naphthalene diimide (ndi) n-system

    Macromolecules

    (2013)
  • Y. Jin et al.

    Synthesis and self-assembly behavior of polyhedral oligomeric silsesquioxane-based triblock copolymers in selective solvents by dissipative particle dynamics simulation

    Phys. Chem. Chem. Phys.

    (2018)
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