Water-Assisted Microphase Separation of Cationic Random Copolymers into Sub-5 nm Lamellar Materials and Thin Films

Cell-guiding microporous hydrogels by photopolymerization-induced phase separation

Microporous scaffolds facilitate solute transport and cell-material interactions, but materials allowing for spatiotemporally controlled pore formation in aqueous solutions are lacking. Here, we propose cell-guiding microporous hydrogels by photopolymerization-induced phase separation (PIPS) as instructive scaffolding materials for 3D cell culture. We formulate a series of PIPS resins consisting of two ionic polymers (norbornene-functionalized polyvinyl alcohol, dextran sulfate), di-thiol linker and water-soluble photoinitiator. Before PIPS, the polymers are miscible. Upon photocrosslinking, they demix due to the increasing molecular weight and form a microporous hydrogel. The pore size is tunable in the range of 2-40 μm as a function of light intensity, polymer composition and molecular charge. Unlike conventional methods to fabricate porous hydrogels, our PIPS approach allows for in situ light-controlled pore formation in the presence of living cells. We demonstrate that RGD-functionalized microporous hydrogels support high cell viability (>95%), fast cell spreading and 3D morphogenesis. As a proof-of-concept, these hydrogels also enhance the osteogenic differentiation of human mesenchymal stromal cells, matrix mineralization and collagen secretion. Collectively, this study presents a class of cell-guiding microporous hydrogels by PIPS which may find applications in complex tissue engineering.

Water-Intercalated and Humidity-Responsive Lamellar Materials by Self-Assembly of Sodium Acrylate Random Copolymers

Herein, we report water-intercalated and humidity-responsive lamellar materials obtained from the self-assembly of sodium acrylate (ANa)/alkyl or oleyl acrylate (RA) random copolymers. The random copolymers efficiently absorbed water into the hydrophilic ANa/main chain phase from the outer environment to form lamellar structures consisting of the water-intercalated hydrophilic segments and the hydrophobic side chains. The lamellar formation involves controlling the weight fraction of hydrophilic segments containing water to 40-70 wt % by the RA content, hydrophobic side chains, and the amount of absorbed water. The domain spacing can be controlled in the range of 2-6 nm. More interestingly, the lamellar materials reversibly afford expansion and contraction of the domain spacing in the sub-1 nm level via the absorption and release of water, in response to relative humidity. The multilayered lamellar formation process via the intercalation of water was analyzed in situ by neutron reflectometry and atomic force microscopy measurements under humid conditions. The polymer film further served as a moisture-sensitive actuator that macroscopically induces deformation responsive to humidity.

Universal Access to Water-Compatible and Nanostructured Materials via the Self-Assembly of Cationic Alternating Copolymers

Herein, we report the water-assisted self-assembly of alternating copolymers bearing imidazolium cations and hydrophobic groups to create water-compatible and nanostructured materials. The copolymers efficiently absorbed water into the cationic segments from the outer environments, depending on the relative humidity. The absorbed water serves as hydrophilic molecules to modulate the weight fraction of hydrophilic/hydrophobic units in the samples. Thus, the morphologies and domain spacing of the nanostructures can be controlled by not only the side chains, but also the amount of absorbed water. The self-assembly of the cationic copolymers, developed herein, afforded universal access to various morphologies, including lamella, gyroid, and cylinder, in addition to the precision control of the domain spacing at the 0.01 nm level.

Lamellar Microphase Separation and Phase Transition of Hydrogen-Bonding/Crystalline Statistical Copolymers: Amide Functionalization at the Interface

Microphase separation of random copolymers, as well as that of high χ-low N block copolymers, is promising to construct sub-10-nm structures into materials. Herein, we designed statistical copolymers consisting of 2-hydroxyethyl acrylate (HEA) and N-octadecylacrylamide (ODAAm) to produce crystallization and hydrogen bond-assisted lamellar structure materials. The copolymers not only formed a crystalline lamellar structure with 3-4 nm domain spacing but also maintained an amorphous lamellar structure via phase transition above the melting temperature up to approximately 100 °C. The key is to introduce hydrogen-bonding amide junctions between the octadecyl groups and the polymer backbones, by which the polymer chains are physically fixed at the interface of lamellar structures even above the melting temperature. The stabilization of the lamellar structure by the amide units is also supported by the fact that the lamellar structure of all-acrylate random copolymers bearing hydroxyethyl and crystalline octadecyl groups is disordered above the melting temperature. By spin-coating on a silicon substrate, the HEA/ODAAm copolymer formed a multilayered lamellar thin film consisting of a hydrophilic hydroxyethyl/main chain phase and a hydrophobic octadecyl phase. The structure and order-disorder transition were analyzed by neutron reflectivity.

Morphologies of Comb-like Polyacrylic Acid/Polyacrylate Copolymers as Functions of the Degree of Derivatization with n-C22H45 Side Chains

Polymers with crystallizable side chains have numerous applications, and their properties depend on their crystal morphologies and phase separation. Structural analysis on a wide spatial scale plays an important role in controlling the thermal properties and higher-order structures of these polymers. In this study, we elucidated the melting and crystallization processes of copolymers with varying crystallizable side-chain fractions over a wide spatial range. Differential scanning calorimetry revealed that the enthalpies of melting and crystallization increased linearly with increasing crystallizable side-chain fraction. The results of wide-angle X-ray scattering indicated that the crystal lattice was hexagonal. Conversely, spherulite-like higher-order architectures with linear structures and radial spreading were observed in the highly crystallizable components, but no micrometer-scale structures were observed in the less crystallizable components. In situ small-angle X-ray scattering was used to elucidate the phase separation and mixing processes. Lamellar crystallites were observed at crystallizable side-chain fractions of >55 wt.%, whereas small crystallites were observed at fractions of <45 wt.%. At temperatures above the order-disorder transition temperature, density fluctuations caused by correlation holes were observed. These properties have a strong effect on the crystallizable side-chain fraction.