Spatial Heterogeneity Accompanying Gel Formation of Poly(N-isopropylacrylamide) Aqueous Solution at a Temperature below Cloud Point

Electrospinning of Aqueous Solutions of Atactic Poly(N-isopropylacrylamide) with Physical Gelation

The phase diagram of a given polymer solution is used to determine the solution’s electrospinnability. We constructed a phase diagram of an aqueous solution of atactic poly(N-isopropylacrylamide) (a-PNIPAM) based on turbidity measurements and the rheological properties derived from linear viscoelasticity. Several important transition temperatures were obtained and discussed, including the onset temperature for concentration fluctuations T1, gel temperature Tgel, and binodal temperature Tb. On heating from 15 °C, the one-phase a-PNIPAM solution underwent pronounced concentration fluctuations at temperatures above T1. At higher temperatures, the thermal concentration fluctuations subsequently triggered the physical gelation process to develop a macroscopic-scale gel network at Tgel before the phase separation at Tb. Thus, the temperature sequence for the transition is: T1 < Tgel < Tb~31 °C for a given a-PNIPAM aqueous solution. Based on the phase diagram, a low-temperature electrospinning process was designed to successfully obtain uniform a-PNIPAM nanofibers by controlling the solution temperature below T1. In addition, the electrospinning of an a-PNIPAM hydrogel at Tgel < T < Tb was found to be feasible considering that the elastic modulus of the gel was shown to be very low (ca. 10−20 Pa); however, at the jet end, jet whipping was not seen, though the spitting out of the internal structures was observed with high-speed video. In this case, not only dried nanofibers but also some by-products were produced. At T > Tb, electrospinning became problematic for the phase-separated gel because the enhanced gel elasticity dramatically resisted the stretching forces induced by the electric field.

Network Formation and Physical Properties of Epoxy Resins for Future Practical Applications

Epoxy resins are used in various fields in a wide range of applications such as coatings, adhesives, modeling compounds, impregnation materials, high-performance composites, insulating materials, and encapsulating and packaging materials for electronic devices. To achieve the desired properties, it is necessary to obtain a better understanding of how the network formation and physical state change involved in the curing reaction affect the resultant network architecture and physical properties. However, this is not necessarily easy because of their infusibility at higher temperatures and insolubility in organic solvents. In this paper, we summarize the knowledge related to these issues which has been gathered using various experimental techniques in conjunction with molecular dynamics simulations. This should provide useful ideas for researchers who aim to design and construct various thermosetting polymer systems including currently popular materials such as vitrimers over epoxy resins.

Spatially uniform dynamics in equilibrium colloidal gels

Gels of DNA nanostars, besides providing a compatible scaffold for biomedical applications, are ideal model systems for testing the physics of equilibrium colloidal gels. Here, using dynamic light scattering and photon correlation imaging (a recent technique that, by blending light scattering and imaging, provides space-resolved quantification of the dynamics), we follow the process of gel formation over 10 orders of magnitude in time in a model system of tetravalent DNA nanostars in solution, a realization of limited-valence colloids. Such a system, depending on the nanostar concentration, can form either equilibrium or phase separation gels. In stark contrast to the heterogeneity of concentration and dynamics displayed by the phase separation gel, the equilibrium gel shows absence of aging and a remarkable spatially uniform dynamics.

Thin Thermoresponsive Polymer Films for Cell Culture: Elucidating an Unexpected Thermal Phase Behavior by Atomic Force Microscopy

Application of poly-N-isopropylacrylamide (PNIPAM) and its more hydrophobic copolymers with N-tert-butylacrylamide (NtBA) as supports for cell sheets has been validated in numerous studies. The binary systems of these polymers with water are characterized by a lower critical solution temperature (LCST) in a physiologically favorable region. Upon lowering the temperature below the LCST, PNIPAM chains undergo a globule-to-coil transition, causing the film dissolution and cell sheet detachment. The character of the PNIPAM-water miscibility behavior is rather complex and not completely understood. Here, we applied atomic force microscopy to track the phase transition in thin films of linear thermoresponsive (co)polymers (PNIPAM and PNIPAM-co-NtBA) prepared by spin-coating. We studied the films' Young's modulus, roughness, and thickness in air and in distilled water in a full thermal cycle. In dry films, in the absence of water, all the measured parameters remained invariant. The swollen films in water above the LCST were softer by 2-3 orders of magnitude and about 10 times rougher than the corresponding dry films. Upon lowering the temperature to the LCST, the films passed through the phase transition observed as a drastic drop of Young's modulus (about an order of magnitude) and decrease in roughness in both polymers in a narrow temperature range. However, the films did not lose their integrity and demonstrated almost fully reversible changes in the mechanical properties and roughness. The thermal dependence of the films' thickness confirmed that they dissolved only partially and required an external force to induce the complete destruction. The reversible thermal behavior which is generally not expected from non-cross-linked polymers is a key finding, especially with respect to their practical application in cell culture. Both the thermodynamic and kinetic factors, as well as the confinement effect, may be responsible for this peculiar film robustness, which requires overcooling and the aid of an external force to destroy the film.