Letter Cite This: ACS Macro Lett. 2018, 7, 956−961
pubs.acs.org/macroletters
Nonspherical Liquid Crystalline Assemblies with Programmable Shape Transformation Meng Huo,†,‡ Guangjie Song,§ Jun Zhang,§ Yen Wei,*,‡ and Jinying Yuan*,† Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education, and‡Key Lab of Bioorganic Phosphorus Chemistry and Chemical Biology of Ministry of Education, Department of Chemistry, Tsinghua University, 100084, Beijing, China § CAS Key Laboratory of Engineering Plastics and CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, China Downloaded via AUSTRALIAN NATL UNIV on July 24, 2018 at 03:50:30 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
†
S Supporting Information *
ABSTRACT: Liquid crystalline (LC) assemblies with tailored shape and programmable shape transformation were prepared via polymerization-induced self-assembly. The influence of polymerization temperature and solvent on the shape of the LC assemblies indicated that shape of the LC assemblies could be delicately regulated by the repulsive interaction among the solvophilic chains and LC ordering. Programmable shape transformation of ellipsoidal LC assemblies was achieved, taking advantage of the smectic-to-isotropic phase transition. The ellipsoidal assemblies could remain ellipsoids or transform to faceted spheres and spheres, depending on the temperature procedure used. Besides, the generated spheres could be reshaped to ellipsoids with high shape recovery ratio. Small angle X-ray scattering study indicated that the interplay of the reversible smectic-to-isotropic phase transition and kinetic trapping underpins the programmed shape transformation. As a general approach to LC assemblies with programmable shape transformation, our strategy would provide a reliable platform for nanoactuators, nanomotors, and adaptive colloidal devices.
T
belts, and lamellae were obtained by varying the composition of the copolymers. Upon UV irradiation, the cubic assemblies transformed to spheres because of the photoresponsive isomerization of the azobenzene mesogens. However, this strategy entails the design of mesogens with stimuli-responsive functional groups, which would increase the complexity in synthesis. According to de Gennes’ theory, LC polymers would lose their anisotropic chain conformation after the LC-to-isotropic phase transition.24,25 We envision that the universal LC-toisotropic phase transition may be exploited to construct nonspherical assemblies with dynamically tunable shape transformation. Therefore, in this contribution, we report the tailored preparation of the nonspherical LC assemblies, and their programmed shape transformation based on the LC-toisotropic phase transition (Scheme 1). We first investigated the influence of polymerization temperature and solvent on the shape of the LC assemblies. Then the programmed shape transformation of the LC assemblies under different temperature programs was studied in detail by transmission electron microscopy (TEM) and the underlying mechanism was revealed by in situ small-angle X-ray scattering (SAXS). Fabrication of LC polymer assemblies with controllable morphology and high concentration is the prerequisite for
he shape of polymer assemblies has a profound influence on their performance in drug delivery, nanomotors, nanoactuators, and hierarchical colloidal assemblies.1−5 Especially, to achieve optimal performance in complex circumstances, polymer assemblies with dynamically tunable shapes are highly needed.6−10 To fabricate these assemblies, several strategies were proposed, including self-assembly of stimuli-responsive polymers, incorporation of mesogens, and osmosis pressure-induced shape transformation, with selfassembly of stimuli-responsive polymers being the most studied.11−16 However, as the driving force for ordinary polymer assemblies lacks directionality, the shapes of stimuliresponsive polymer assemblies are mostly limited to spheres.17 Self-assembly of liquid crystalline (LC) block copolymers is an important method for fabricating nonspherical nanoparticles.18 Owing to the anisotropic packing of the mesogens, assemblies with a rich variety of anisotropic morphologies,19 including ellipsoidal micelles,20 faceted vesicles,21 ellipsoidal vesicles,22 and cuboids,23 could be prepared via self-assembly of LC block copolymers. Furthermore, incorporation of stimuli-responsive mesogens into the assemblies may endow these particles with shape-changing property. For example, Chen et al. prepared LC block copolymer assemblies poly(methacrylic acid)-b-poly[11-[4-(4-butylphenylazo)phenoxy]undecyl methacrylate)] (PMAA-b-PMAAz) via reversible addition−fragmentation chain transfer (RAFT) dispersion polymerization of azobenzene-containing methacrylate MAAz in ethanol.23 Ellipsoidal vesicles, cuboids, short © XXXX American Chemical Society
Received: May 26, 2018 Accepted: July 17, 2018
956
DOI: 10.1021/acsmacrolett.8b00409 ACS Macro Lett. 2018, 7, 956−961
Letter
ACS Macro Letters
Scheme 1. Tailored Preparation of Nonspherical LC Assemblies via Polymerization-Induced Self-Assembly (PISA) of PDMAb-PFOEMA
dynamic manipulation of the shape of LC polymer assemblies. However, the traditional self-assembly strategies, such as nanoprecipitation, have been suffering from low particle concentration and complex kinetic factors.26 Recently, polymerization-induced self-assembly (PISA) was developed as a new technique for fabrication of block copolymer assemblies with high concentration and excellent reproducibility.27−31 Taking advantage of the “living”/controlled dispersion and emulsion polymerization, PISA enables simultaneous synthesis and self-assembly of block copolymer in selective solvents.32−42 In our previous report, RAFT dispersion polymerization of 2-(perfluorooctyl ethyl methacrylate) (FOEMA) could afford cylindrical LC assemblies of tunable diameters.43 As a result, in this study, PISA of poly(N,N-dimethylaminoethyl methacrylate)-b-poly[2-(perfluorooctyl ethyl methacrylate)] (PDMA-b-PFOEMA) was used to fabricate nonspherical LC assemblies. To assess the feasibility of this strategy, alcoholic PISA of PDMA-b-PFOEMA was first investigated. PDMA67 (Mn = 9.3 kDa, Đ = 1.23, Figure S1a) macro-chain transfer agent (macroCTA) was synthesized to mediate the RAFT dispersion polymerization of FOEMA in ethanol, with the feed ratio of FOEMA/PDMA67 varying from 20 to 100 (Table S1). The solids content was set as 15 wt %, which is much higher than the nanoprecipitation method (usually
