Optically Active Conjugated Polymer Nanoparticles from Chiral

Note pubs.acs.org/Macromolecules

Optically Active Conjugated Polymer Nanoparticles from Chiral Solvent Annealing and Nanoprecipitation Hyojin Kim,†,‡ Young-Jae Jin,† Beomsu Shin-Il Kim,† Toshiki Aoki,*,§ and Giseop Kwak*,† †

School of Applied Chemical Engineering, Major in Polymer Science and Engineering, Kyungpook National University 1370 Sankyuk-dong, Buk-ku, Daegu 702−701, Korea ‡ Daegu Technopark Nano Convergence Practical Application Center, 891−5 Daecheon-dong, Dalseo-ku, Daegu 704−801, Korea § Department of Chemistry and Chemical Engineering, Graduate School of Science and Technology, and Center for Transdisciplinary Research, Niigata University, Ikarashi 2-8050, Nishi-ku, Niigata 950-2181, Japan

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In this study, we prepared optically active CPNs from CSA and subsequent NP using a PDPA derivative. The solvent chirality was readily transferred to the PDPA simply by annealing it in chiral limonene solution. During the NP process, the chirality-transferred polymer chains came to a standstill with the side phenyl rings being fully relaxed, thus providing coarsened nanoparticles with a highly porous network. The resulting CPNs were well dispersed in water for a long period with having a high zeta potential. The aqueous colloidal solution showed the same CD as the solution. Namely, the chirality information on PDPA in solution was transferred to the CPNs in the aqueous colloidal solution. Herein, we describe the preparation, structure, morphology, and chiroptical properties of PDPA-based CPNs in detail and suggest that the CSA and subsequent NP should be a very useful method for the preparation of highly porous, optically active nanoparticles. Poly[1-phenyl-2-(p-trimethylsilyl)phenylacetylene] (PDPAC1 in Chart 1) was chosen as the achiral polymer in the preparation of CPNs. The polymer used in this study had a high weight-average molecular weight (Mw) of 5.2 × 106 g mol−1 and a polydispersity index (PDI = Mw/Mn) of 3.2. Limonene was chosen as the chiral solvent to dissolve the PDPAC1 because limonene was expected to be a good solvent due to its solubility parameter (δ) of 16.5, which is very close to

ptically active polymers are very useful as chiral packing materials for optical resolution of various racemates.1 Circular dichroism (CD) is of critical importance for evaluation of their chirality. Although these types of polymers have been evaluated in solution state, their practical applications are commonly with them in solid state, such as powders and membranes. Evaluation in solid state may directly provide chiral information about the packing materials and make it more reliable. Nanoporous materials or small-sized particles would be especially useful for optical resolution because the extremely high surface area-to-volume ratio could lead to an amplified interaction between the chiral packing material and racemic mixture, thus enhancing the optical resolution.2 In this respect, optically active nanoporous nanoparticles should be one of the best candidates for highly advanced chiral packing materials. However, a facile preparation and evaluation of such elegant materials should be a great challenge. Among the many types of conjugated polymers, poly(diphenylacetylene) (PDPA) derivatives are particularly unusual for the following reasons. First, PDPAs are amorphous and exist in a glassy state, depending on the alkyl side chain length.3 Owing to their amorphous nature and low density, these polymers have relatively large fractional free volumes in the bulk solid state.4 Second, PDPA derivatives provide highly porous and extremely thin nanofibers from electrospinning or freeze-drying of the polymer solution.5 The nanoporous network structure is thought to be a result of abrupt phase separation or spinodal decomposition between the polymer and solvent. The porous structure is also very stable, both thermoand hydrodynamically, and is permanently maintained because of the stiff and rigid backbones of PDPAs. Third, the achiral PDPA derivative can be readily converted into optically active materials by heating it in a chiral solvent (chiral solvent annealing, CSA) such as limonene or pinene.6 Nanoprecipitation (NP) is a very useful and facile method to prepare nanoparticles from conjugated polymers.7 These polymer particles are referred to as conjugated polymer nanoparticles (CPNs). The principle mechanism for the formation of CPNs in the NP process is abrupt solvent quenching in water, which leads to phase separation between the polymer and solvent.8 The theoretical product yields of CPNs in the NP process are quantitative, although the process should be conducted in a considerably dilute solution, thus requiring large amounts of solvent and water. Moreover, this method provides much smaller-sized nanoparticles relative to other methods such as emulsion. © XXXX American Chemical Society

Chart 1. Chemical Structure of PDPAC1

Received: May 14, 2015 Revised: June 8, 2015

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DOI: 10.1021/acs.macromol.5b01034 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules the δ (15.0)9 of PDPAC1, as determined by the group contribution method. PDPAC1 was well dissolved in chiral limonene and then heated at 120 °C for various periods. Subsequently, the solution was slowly cooled to room temperature, precipitated in methanol, and then completely dried in air. Notably, when the dried polymer was redissolved in an achiral solvent, such as toluene or tetrahydrofuran (THF), the solutions showed a remarkable Cotton effect in the UV− visible absorption region (Figure 1). The 383 nm CD band is

Figure 1. CD and UV−vis spectra of CSA-treated PDPAC1 in toluene solution (c = 5.0 × 10−4 M).

ascribed to an asymmetric conformational change in the isotropic axial structure between the polyene backbone and phenyl side rings.10 This indicates that the chirality of limonene was transferred to the polymer by solvent annealing and retained even after redissolving in an achiral solvent. Moreover, the CD intensity at 383 nm significantly increases according to the annealing time. The CD band signs between the polymers treated with (+)- and (−)-limonenes are exactly opposite to each other. This indicates that the phenyl side rings underwent a significant asymmetric change during annealing, leading to an axial chirality in the side phenyl−phenyl stack structure. Consequently, an optically active PDPA derivative was obtained by a very simple CSA method. Next, the chirality-transferred PDPAC1 was dissolved in THF to be added quickly to an excess of water under vigorous stirring. Subsequently, the THF and excess water were evaporated to reach the desired concentration of the aqueous colloidal solution. This process affords nanoparticles with volume-average hydrodynamic diameters in the range of 50− 100 nm in the colloidal solution, depending on the concentration of the colloidal solution (Figure 2a). Moreover, the aqueous colloidal solutions are highly stable in dispersion over a long period (several months). In fact, the nanoparticles with an average diameter of 78 nm have a high negative zeta potential (ζ) value of −9.88 mV, confirming a high dispersion stability. Such colloidal stability has been commonly observed in CPNs obtained from NP in water. It is believed that the polymer chains are abruptly broken during the precipitation to afford chemical defects in order to form a charged hydrophilic surface of nanoparticle so as to inhibit the agglomeration in water.11 The scanning electron microscope (SEM) image clearly shows the nanoparticles in a dry state (Figure 2b). Notably, the particle size of the dried sample is almost the same as the hydrodynamic volume measured by dynamic light

Figure 2. (a) DLS size distribution profile for the aqueous PDPAC1 colloidal solution obtained by CSA in (+)-limonene and subsequent NP. (b) SEM image of the PDPAC1 nanoparticles dried in air. (c) DSC thermograms of the PDPAC1 nanoparticles on first cooling and second heating (heat flow rate ∼10 °C min−1, under nitrogen gas). (d) HR-TEM image of the PDPAC1 nanoparticle. (e) XRD pattern of the PDPAC1 nanoparticles.

scattering (DLS). This suggests that the nanoparticles barely swell in water and do not undergo a volume change. Consequently, the polymer chains within the nanoparticles should be hydrodynamically stable in water. In the differential scanning calorimetry (DSC) measurement, the nanoparticles show no significant thermodynamic transition over a wide temperature range of −20 to +180 °C, indicating neither phase transition nor local relaxation (Figure 2c). This suggests that the polymer chains therein exist in a glassy state at room temperature and are thermodynamically stable. The transmittance electron microscope (TEM) image and Xray diffraction (XRD) pattern of the nanoparticles show that the polymer chains therein exist exclusively in the amorphous domain. Very unusually, the nanoparticles appear in a randomly channeled pore structure, as shown in the TEM image, and no characteristic lattice fringe is observed (Figure 2d). An amorphous hollow peak is evident from the XRD pattern in the wide-angle region (Figure 2e). A sharp signal is usually seen at the small angle of 6.7° (interlayer distance ∼13.0 Å according to the Bragg equation),3 attributed to the lamellar layer in a bulk solid film, but the peak almost disappeared in the nanoparticle. A weak but explicit signal is observed at the small angle of 12.0°, corresponding to a distance of ∼6.59 Å. This small angle signal may be due to the coarsened structure, which B

DOI: 10.1021/acs.macromol.5b01034 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Chiral Solvent Annealing (CSA). The PDPAC1 was dissolved in (+)-limonene or (−)-limonene (c = 1.0 × 10−2 M), heated at 120 °C for 1, 3, 6, and 24 h, and then slowly cooled to room temperature. The solutions were precipitated in methanol and then completely dried in air. Nanoprecipitation (NP). The chirality-transferred PDPAC1 was dissolved in high-performance liquid chromatography (HPLC)-grade THF (c = 0.1 mM). Appropriate amounts of the THF solution were injected into distilled water (4.75 mL) with vigorous stirring. The THF and excess water were then slowly evaporated at 65 °C to reach the desired concentration of 5.0 × 10−4 M to provide aqueous colloidal solutions. Measurements. The weight-average molecular weight (Mw) and number-average molecular weight (Mn) of the polymer were evaluated by gel permeation chromatography (GPC, Shimadzu A10 Instruments, Polymer Laboratories, PLgel Mixed-B (300 mm in length) as a column, and HPLCgrade THF as the eluent at 40 °C) based on calibration with polystyrene standards. The CD/UV−vis spectra of the solution (SQ-grade cuvette with a path length of 10 mm) were measured at room temperature using a JASCO J-815 spectropolarimeter. XRD (PANalytical X̀ Pert PRO-MPD) was performed at room temperature at the Korea Basic Science Institute (Daegu). The samples were mounted directly in the diffractometer. The experiments were carried out using Cu Kα (1.54 Å) radiation at 40 kV and 25 mA. The size and zeta potential of the nanoparticles were determined using DLS (Zetasizer Nano-ZS90, Malvern). Field emission SEM imaging was performed using a Hitachi S-4800 instrument at an accelerating voltage of 5.0 kV. TEM imaging was performed using a Hitachi H-7600 instrument operated at 100 kV. DSC (TA Instruments Q2000) was performed in pure nitrogen gas at heating and cooling rates of 10 °C min −1.

approximately accords with the average pore channel spacing (∼6.6 Å) within the nanoparticle. The PDPA-based CPNs in the aqueous colloidal solution show almost the same CD as the molecularly dispersed polymer chains in the toluene solution (Figure 3). This

Figure 3. CD spectra of the PDPAC1 in toluene solution (5.0 × 10−4 M), aqueous colloidal solution (5.0 × 10−4 M), and cast film (film thickness ∼20 μm).

indicates that the chiral conformation of PDPAC1 does not change even after NP. Namely, the chiral information on the isolated polymer chains in the ideal dilute solution was transferred to the CPNs. This is because the PDPAC1 has a glassy-state, amorphous structure, and a nonplanar geometry due to the highly twisted backbone, resulting in the absence of an intermolecular stack structure during the NP process.12 It is probable that the polymer chains were abruptly quenched with the chiral structure being retained. It is very unusual that the polymer has exactly the same chiral information between the nanoparticle aggregate and the solution. On the other hand, the polymer film prepared by spin-casting shows quite a different CD in diffuse reflectance (DR) mode. The shape of the Cotton bands is different from that of the solution. Moreover, a very weak but apparent CD band newly appears at a longer wavelength of ∼440 nm, presumably due to the self-assembly aggregate formed during the slow evaporation of solvent.10 In summary, we successfully prepared optically active CPNs from CSA and subsequent NP using an achiral PDPA derivative. The chirality of the limonenes was transferred to the PDPA derivative effectively by annealing in solution state, and was retained even after redissolving in an achiral solvent. Subsequently, the NP of PDPA in water afforded thermo- and hydrodynamically stable nanoparticles in an aqueous colloidal solution. The CPN was found to be a randomly pore-channeled particle with a coarsened structure showing almost the same chiral information as that of the solution. The present CPNs are expected to be a highly advanced chiral packing material because of the high optical activity and porosity. The present combination of the CSA and NP techniques is applicable to other achiral polymers, which will be very useful for the novel development of optically active nanoparticles.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: (T.A.) [email protected]. *E-mail: (G.K.) [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) grants funded by the Korea government (MEST) (2014R1A2A1A11052446). This research was also financially supported by the “Sensitivity touch platform development and new industrialization support program” through the Ministry of Trade, Industry & Energy (MOTIE) and Korea Institute for Advancement of Technology (KIAT).

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EXPERIMENTAL SECTION Materials. The synthesis of PDPAC1 has been reported elsewhere.13 The polymers used in this study had a high Mw value of 5.20 × 106, and PDI of 3.2. The (+)-limonene (>99.0% ee grade: [α]20D +115.5 ± 1°, c = 10% in ethanol) and (−)-limonene (>99.0% ee grade: [α]20D −94 ± 4°, c = 10% in ethanol) were purchased from Sigma-Aldrich. C

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DOI: 10.1021/acs.macromol.5b01034 Macromolecules XXXX, XXX, XXX−XXX