Photoinitiated Polymerization-Induced Microphase Separation for the Preparation of Nanoporous Polymer Films

We report on the use of photoinitiated reversible addition–fragmentation chain transfer (RAFT) polymerization for the facile fabrication of cross-linked nanoporous polymer films with three-dimensionally (3D) continuous pore structure. The photoinitiated polymerization of isobornyl acrylate (IBA) in the presence of 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (CTA) and 2,2-dimethoxy-2-phenylacetophenone as a photoinitiator proceeded in a controlled manner, yet more rapidly compared to thermally initiated polymerization. When polylactide-macroCTA (PLA-CTA) was used, PLA-<i>b</i>-PIBA with high molar mass was obtained after several minutes of irradiation at room temperature. We confirmed that microphase separation occurs in the PLA-<i>b</i>-PIBA and that nanoporous PIBA can be derived from the PLA-<i>b</i>-PIBA precursor by selective PLA etching. To fabricate the cross-linked nanoporous polymer, IBA was copolymerized with ethylene glycol diacrylate (EGDA) in the presence of PLA-CTA to produce a cross-linked block polymer precursor consisting of bicontinuous PLA and P­(IBA-<i>co</i>-EGDA) microdomains, via polymerization-induced microphase separation. We demonstrated that nanoporous P­(IBA-<i>co</i>-EGDA) monoliths and films with 3D continuous pores can be readily obtained via this approach

One-Step Synthesis of Cross-Linked Block Polymer Precursor to a Nanoporous Thermoset

Using a simultaneous block polymerization/in situ cross-linking from a heterofunctional initiator approach, we produced a nanostructured and cross-linked block polymer in a single step from a ternary mixture of monomers and used it as a precursor for a cross-linked nanoporous material. Using 2-(benzylsulfanylthiocarbonylsulfanyl)­ethanol as a heterofunctional initiator, simultaneous ring-opening transesterification polymerization of d,l-lactide in the presence of tin 2-ethylhexanoate as a catalyst and reversible addition–fragmentation chain transfer polymerization of styrene at 120 °C produced a polylactide-<i>b</i>-polystyrene (PLA-<i>b</i>-PS) block polymer. Incorporation of divinylbenzene in the polymerization mixture allowed in situ cross-linking during the simultaneous block polymerization to result in the cross-linked block polymer precursor in one step. This material was converted into cross-linked nanoporous polymer by etching PLA in a basic solution

Cross-Linked Nanoporous Materials from Reactive and Multifunctional Block Polymers

Polylactide-<i>b</i>-poly(styrene-<i>co</i>-2-hydroxyethylmethacrylate) (PLA-<i>b</i>-P(S-<i>co</i>-HEMA)) and polylactide-<i>b</i>-poly(styrene-<i>co</i>-2-hydroxyethylacrylate) (PLA-<i>b</i>-P(S-<i>co</i>-HEA)) were synthesized by combination of ring-opening polymerization and reversible addition–fragmentation chain transfer polymerization. ¹H nuclear magnetic resonance spectroscopy and size exclusion chromatography data indicated that the polymerizations were controlled and that hydroxyl groups were successfully incorporated into the block polymers. The polymers were reacted with 4,4′-methylenebis(phenyl isocyanate) (MDI) to form the corresponding cross-linked materials. The materials were annealed at 150 °C to complete the coupling reaction. Robust nanoporous materials were obtained from the cross-linked polymers by treatment with aqueous base to hydrolyze the PLA phase. Small-angle X-ray scattering study combined with scanning electron microscopy showed that MDI-cross-linked PLA-<i>b</i>-P(S-<i>co</i>-HEMA)/PLA-<i>b</i>-P(S-<i>co</i>-HEA) can adopt lamellar, hexagonally perforated lamellar, and hexagonally packed cylindrical morphologies after annealing. In particular, the HPL morphology was found to evolve from lamellae due to increase in volume fraction of PS phase as MDI reacted with hydroxyl groups. The reaction also kinetically trapped the morphology by cross-linking. Bicontinuous morphologies were also observed when dibutyltin dilaurate was added to accelerate reaction between the polymer and MDI

Semipermeable Microcapsules with a Block-Polymer-Templated Nanoporous Membrane

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

Magnetic Microrheology of Block Copolymer Solutions

The viscosity of poly­(styrene)-<i>b</i>-poly­(lactide) [PS-<i>b</i>-PLA] solutions in a neutral solvent was characterized by magnetic microrheology. The effect of polymer concentration on the viscosity of the block polymer solutions was compared with that of the PS and PLA homopolymers in the same solvent. The viscosity of PS-<i>b</i>-PLA solution, unlike the homopolymer solutions, showed a steep increase over a narrow concentration range. The steep rise was concomitant with microphase separation into an ordered cylindrical microstructure as determined by small-angle X-ray scattering. Hence microrheology proved effective as a means of characterizing the order–disorder transition concentration. During an in situ drying experiment, changes in local viscosity through the depth of a block copolymer solution were characterized as a function of drying time. Early in the drying process, the viscosity rose steadily and was uniform through the depth, a result consistent with steadily increasing and uniform polymer concentration. However, later in the drying process as the overall polymer concentration approached that required for microphase separation, the viscosity of the polymer solution near the free surface became an order of magnitude higher than that near the bottom of the container. The zone of high viscosity moved downward as drying proceeded, consistent with a microphase separation front

Hierarchically Porous Polymers from Hyper-cross-linked Block Polymer Precursors

We report synthesis of hierarchically porous polymers (HPPs) consisting of micropores and well-defined 3D continuous mesopores by combination of hyper-cross-linking and block polymer self-assembly. Copolymerization of 4-vinylbenzyl chloride (VBzCl) with divinylbenzene (DVB) in the presence of polylactide (PLA) macro-chain-transfer agent produced a cross-linked block polymer precursor PLA-<i>b</i>-P­(VBzCl-<i>co</i>-DVB) via reversible addition–fragmentation chain transfer polymerization. A nanoscopic bicontinuous morphology containing PLA and P­(VBzCl-<i>co</i>-DVB) microdomains was obtained as a result of polymerization-induced microphase separation. While a basic treatment of the precursor selectively removed PLA to yield a reticulated mesoporous polymer, hyper-cross-linking of the precursor by FeCl₃ generated micropores in the P­(VBzCl-<i>co</i>-DVB) microdomain via Friedel–Crafts alkylation and simultaneously degraded PLA to produce the HPP containing micropores in the mesoporous framework. The mesopore size of the HPP could be precisely controlled from 6 to 15 nm by controlling the molar mass of PLA. We demonstrate acceleration in adsorption rate in the HPP compared to a hyper-cross-linked microporous polymer

Interfacial Polymerization of Reactive Block Polymers for the Preparation of Composite Ultrafiltration Membranes

Interfacial polymerization of an acid chloride-containing block polymer and a multivalent amine in the presence of a macroporous support was explored as a means to generate a nanoporous thin film composite (TFC) membrane potentially useful for ultrafiltration. When polylactide-<i>b</i>-poly­(styrene-<i>co</i>-vinylbenzoyl chloride) (PLA-<i>b</i>-P­(S-<i>co</i>-VBC)) in an organic phase and <i>m</i>-phenylenediamine (MPD) in an aqueous phase were used as the reactive block polymer and the amine, respectively, a block polymer thin film was successfully formed on a polysulfone support. This nanostructured film could be converted into a nanoporous layer by subsequent PLA etching under mild basic conditions. While most organic solvents used to dissolve PLA-<i>b</i>-P­(S-<i>co</i>-VBC) damaged the support and decreased permeability of the resulting membrane, use of a mixture of methyl isobutyl ketone and acetonitrile produced a TFC membrane with high permeability

Optimization of Long-Range Order in Solvent Vapor Annealed Poly(styrene)-<i>block</i>-poly(lactide) Thin Films for Nanolithography

Detailed experiments designed to optimize and understand the solvent vapor annealing of cylinder-forming poly­(styrene)-<i>block</i>-poly­(lactide) thin films for nanolithographic applications are reported. By combining climate-controlled solvent vapor annealing (including in situ probes of solvent concentration) with comparative small-angle X-ray scattering studies of solvent-swollen bulk polymers of identical composition, it is concluded that a narrow window of optimal solvent concentration occurs just on the ordered side of the order–disorder transition. In this window, the lateral correlation length of the hexagonally close-packed ordering, the defect density, and the cylinder orientation are simultaneously optimized, resulting in single-crystal-like ordering over 10 μm scales. The influences of polymer synthesis method, composition, molar mass, solvent vapor pressure, evaporation rate, and film thickness have all been assessed, confirming the generality of this behavior. Analogies to thermal annealing of elemental solids, in combination with an understanding of the effects of process parameters on annealing conditions, enable qualitative understanding of many of the key results and underscore the likely generality of the main conclusions. Pattern transfer via a Damascene-type approach verified the applicability for high-fidelity nanolithography, yielding large-area metal nanodot arrays with center-to-center spacing of 38 nm (diameter 19 nm). Finally, the predictive power of our findings was demonstrated by using small-angle X-ray scattering to predict optimal solvent annealing conditions for poly­(styrene)-<i>block</i>-poly­(lactide) films of low molar mass (18 kg mol^–1). High-quality templates with cylinder center-to-center spacing of only 18 nm (diameter of 10 nm) were obtained. These comprehensive results have clear and important implications for optimization of pattern transfer templates and significantly advance the understanding of self-assembly in block copolymer thin films

Observing Phase Transition of a Temperature-Responsive Polymer Using Electrochemical Collisions on an Ultramicroelectrode

Herein, a study on a new lower critical solution temperature (LCST) polymer in an organic solvent by an electrochemical technique has been reported. The phase-transition behavior of poly­(arylene ether sulfone) (PAES) was examined on 1,2-dimethoxyethane (DME). At a temperature above the LCST point, polymer molecules aggregated to create polymer droplets. These droplets subsequently collided with an ultramicroelectrode (UME), resulting in a new form of staircase current decrease. The experimental collision frequency and collision signal were analyzed in relation to the concentration of the polymer. In addition, the degree of polymer aggregation associated with temperature change was also observed