Article pubs.acs.org/cm
Cite This: Chem. Mater. 2018, 30, 273−279
Semipermeable Microcapsules with a Block-Polymer-Templated Nanoporous Membrane Jaehoon Oh,†,§ Bomi Kim,‡,§ Sangmin Lee,‡ Shin-Hyun Kim,*,‡ and Myungeun Seo*,† †
Graduate School of Nanoscience and Technology, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Korea ‡ Department of Chemical and Biological Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Korea S Supporting Information *
ABSTRACT: Microcapsules with nanoporous membranes can regulate transmembrane transport in a size-dependent fashion while protecting active materials in the core from the surrounding, and are thereby useful as artificial cell models, carriers for cells and catalysts, and microsensors. In this work, we report a pragmatic microfluidic approach to producing such semipermeable microcapsules with precise control of the cutoff threshold of permeation. Using a homogeneous polymerization mixture for the polymerization-induced microphase separation (PIMS) process as the oil phase of water-in-oil-in-water (W/O/W) double emulsions, a densely cross-linked shell composed of a bicontinuous nanostructure that percolates through the entire thickness is prepared, which serves as a template for a monolithic nanoporous membrane of microcapsules with size-selective permeability. We demonstrate that the nanopores with precisely controlled size by the block polymer self-assembly govern molecular diffusion through the membrane and render manipulation of the cutoff threshold.
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and a liquid porogen16 yields mechanically stable microcapsules, precise control of pore size is elusive, especially on the sub-50 nm scale; this is because the characteristic length scales of separated domains are determined by interaction parameters and concentration or polymerization rate, both of which are very difficult to manipulate. Therefore, a robust microfluidic method to simultaneously achieve high mechanical stability of membrane and controllability over the cutoff threshold for permeation remains an important yet unmet demand. Microphase separation of block polymers has provided highly ordered porous nanostructures with the pore size of typically 5−50 nm.17 The ordered lattice morphology is predominantly determined by the relative fraction of blocks, and the feature size is dictated by molar mass. Although the microphase separation in the shell of double-emulsion drops can produce regular nanopores in the membrane of microcapsules through selective removal of one block, it is challenging to create the nanopores that percolate the entire thickness of membranes.18,19 This is because bicontinuous phases, such as gyroid, can be prepared only with a highly limited window of stability. We have recently developed a facile strategy for production of three-dimensionally (3D) continuous nanopores by employing polymerization-induced microphase separation (PIMS).20−22 PIMS offers a synthetically feasible route to producing a disordered bicontinuous phase via in situ synthesis
INTRODUCTION Microcapsules whose membranes possess uniform and welldefined pores can provide size-selective permeability.1,2 The membranes allow transport of materials smaller than the pores, while preventing the exchange of the larger. Such microcapsules can store large active materials without leakage and regulate transmembrane transport at the same time, which is beneficial for various chemical and biological applications, including catalytic reactions, microreactors, and immunoisolation of cells.3−6 For the production of semipermeable microcapsules, layer-by-layer deposition1,7 and interfacial colloidal assembly8 have been used on the particle or emulsion-drop templates. In addition, aerosol drops have also been used as templates to produce microcapsules with porous shells.9,10 However, the conventional methods have limited control over cutoff threshold, capsule uniformity, and encapsulation efficiency. Droplet-based microfluidics has enabled the production of monodisperse semipermeable microcapsules with high encapsulation efficiency by employing double-emulsion drops as templates.11−13 The shell membranes have been designed to have uniform pores that interconnect the cores and surrounding by employing either colloidal self-assembly or phase separation of polymers.14−16 Colloidal assembly in the shell produces regular pores at the interstitial voids, of which size is determined by that of templating colloids;14 this enables the formation of nanopores with sub-100 nm colloids. However, the colloidal assemblies are fragile against mechanical stress, only providing limited use. Although phase separation between two distinct polymers15 or between a growing polymer © 2017 American Chemical Society
Received: October 16, 2017 Revised: November 22, 2017 Published: December 8, 2017 273
DOI: 10.1021/acs.chemmater.7b04340 Chem. Mater. 2018, 30, 273−279
Article
Chemistry of Materials
conducting bulk copolymerizations of styrene and DVB in the presence of PLA-CTAs and AIBN at 70 °C. The condition of WPLA‑CTA = 30% and [styrene]:[DVB] = 4:1 was used for all three PLA-CTAs. The DVB content is known to affect onset of gelation that arrests the emergent microphase-separated morphology, as well as cross-linking density which is important in maintaining the mesopore structure against pore collapse.22 At the 4:1 ratio, the produced porous monolith has been reported to exhibit an ultimate tensile strength of 6.4 MPa, an ultimate elongation of ∼9%, and a Young’s modulus of ∼170 MPa.20 [PLA-CTA]:[AIBN] was varied to 1:1, 1:1.8, and 1:2.7 for PLA-CTA-13, PLA-CTA-24, and PLA-CTA-35, respectively. High AIBN concentration was used to conduct polymerization in air and achieve high cross-linking density by consuming more pendent double bonds of DVB repeating units in the P(S-co-DVB) matrix during the polymerization. The monolithic PLA-b-P(S-co-DVB) precursors were prepared, which were then treated with NaOH solution to remove PLA. The derived nanoporous monoliths were analyzed by Fourier transform infrared (FTIR) spectra, small-angle X-ray scattering (SAXS), scanning electron microscopy (SEM) images, and nitrogen sorption experiments, which manifest the formation of 3D continuous nanopores based on the PIMS process20 as shown in Figures S3−S6 and Table S2. In the SAXS data shown in Figure S3, all the samples showed a broad principal scattering peak (at q*) accompanied by a broad shoulder at 2q* which was consistent with the PIMS process that produced a disordered bicontinuous morphology consisting of PLA and P(S-co-DVB) microdomains with some degree of local order,20 similar to those formed by spinodal decomposition at the late stage.23 The position of q* exhibited a shift to smaller q with the increasing molar mass of PLA-CTA, indicating increases in the domain spacing d (=2π/q*) and also the size of the PLA microdomain as the WPLA‑CTA was fixed as 30%. Assuming densities of PLA and P(S-co-DVB) as 1.2524 and 1.0525, the average size of the PLA microdomains was roughly estimated as 5.8, 6.6, and 8.7 nm for PLA-CTA-13, PLA-CTA-24, and PLACTA-35, respectively. The shape of the SAXS pattern and the position of q* were virtually identical to those produced with low radical concentration,20 indicating that the effect of P(S-coDVB) homopolymer generated because of high AIBN loading26 in the early stage of polymerization would not significantly alter the microphase-separated morphology27 and eventually join the cross-linked P(S-co-DVB) matrix during the polymerization. Complete removal of PLA was confirmed by FTIR spectra (Figure S4), and visualization by SEM corroborated formation of the desired nanopore structure (Figure S5). Huge scattering intensity with virtually no change in the shape of the SAXS pattern supports formation of the nanopore templated by the PLA microdomain (Figure S3). The H2-type nitrogen sorption isotherms shown in Figure S6 were also consistent with the presence of 3D porous network28 with the increasing pore size as molar mass of PLA-CTA increased. The mean pore diameters were determined as 7.4, 8.8, and 10.9 nm for PLACTA-13, PLA-CTA-24, and PLA-CTA-35, respectively, from Barrett−Joyner−Halenda (BJH) analysis29 of nitrogen sorption isotherms. Porosity of the nanoporous monoliths was in the range 0.32−0.35 g mL−1, which matched well with the theoretical pore volume of 0.34 g mL−1 assuming that the porous space was entirely templated by PLA. The identical polymerization mixture was used in the microfluidic fabrication of double-emulsion drops as an oil phase to construct semipermeable microcapsules. To make the
and simultaneous cross-linking of block polymers that induces microphase separation during polymerization. Therefore, a 3D network of continuous nanopores can be easily created in the cross-linked polymeric matrix through the removal of sacrificial domains. Nanoporous monoliths and free-standing films (ca. 100 μm) have been successfully prepared by the PIMS process by the bulk polymerization and the polymerization of concentrated polymerization mixtures, respectively.20−22 We envisioned that if the polymerization mixture for the PIMS process can be segregated in confined space such as a thin middle layer (
