Generality of anomalous expansion of polymer chains in supercritical fluids

REFLEX: a program for the analysis of specular X-ray and neutron reflectivity data

The use of X-ray and neutron reflectivity has been generalized worldwide for scientists who want to determine specific physical properties (such as electron-density profile, scattering-length density, roughness and thickness) of films less than 200 nm thick deposited on a substrate. This paper describes a freeware program named REFLEX, which is a standalone program dedicated to the simulation and analysis of X-ray and neutron reflectivity from multilayers. This program was first written two decades ago and has been constantly improved since, but never published until now. The latest version of REFLEX covers generalized types of calculation of reflectivity curves including both neutron and X-ray reflectivity. In the case of X-rays, the program can deal with both s and p polarization, which is quite important in the soft X-ray region where the two polarizations can yield different results. Neutron reflectivity is calculated within the framework of non-spin-polarized neutrons. REFLEX has also been designed to include any type of fluid (such as supercritical CO2) on top of the analysed film and includes corrections of the footprint effect for analysis on an absolute scale.

Flory-type theories of polymer chains under different external stimuli

In this Review, we present a critical analysis of various applications of the Flory-type theories to a theoretical description of the conformational behavior of single polymer chains in dilute polymer solutions under a few external stimuli. Different theoretical models of flexible polymer chains in the supercritical fluid are discussed and analysed. Different points of view on the conformational behavior of the polymer chain near the liquid-gas transition critical point of the solvent are presented. A theoretical description of the co-solvent-induced coil-globule transitions within the implicit-solvent-explicit-co-solvent models is discussed. Several explicit-solvent-explicit-co-solvent theoretical models of the coil-to-globule-to-coil transition of the polymer chain in a mixture of good solvents (co-nonsolvency) are analysed and compared with each other. Finally, a new theoretical model of the conformational behavior of the dielectric polymer chain under the external constant electric field in the dilute polymer solution with an explicit account for the many-body dipole correlations is discussed. The polymer chain collapse induced by many-body dipole correlations of monomers in the context of statistical thermodynamics of dielectric polymers is analysed.

Novel Effects of Compressed CO2 Molecules on Structural Ordering and Charge Transport in Conjugated Poly(3-hexylthiophene) Thin Films

We report the effects of compressed CO₂ molecules as a novel plasticization agent for poly(3-hexylthiophene) (P3HT)-conjugated polymer thin films. In situ neutron reflectivity experiments demonstrated the excess sorption of CO₂ molecules in the P3HT thin films (about 40 nm in thickness) at low pressure (P = 8.2 MPa) under the isothermal condition of T = 36 °C, which is far below the polymer bulk melting point. The results proved that these CO₂ molecules accelerated the crystallization process of the polymer on the basis of ex situ grazing incidence X-ray diffraction measurements after drying the films via rapid depressurization to atmospheric pressure: both the out-of-plane lamellar ordering of the backbone chains and the intraplane π-π stacking of the side chains were significantly improved, when compared with those in the control P3HT films subjected to conventional thermal annealing (at T = 170 °C). Electrical measurements elucidated that the CO₂-annealed P3HT thin films exhibited enhanced charge carrier mobility along with decreased background charge carrier concentration and trap density compared with those in the thermally annealed counterpart. This is attributed to the CO₂-induced increase in polymer chain mobility that can drive the detrapping of molecular oxygen and healing of conformational defects in the polymer thin film. Given the universality of the excess sorption of CO₂ regardless of the type of polymers, the present findings suggest that CO₂ annealing near the critical point can be useful as a robust processing strategy for improving the structural and electrical characteristics of other semiconducting conjugated polymers and related systems such as polymer:fullerene bulk heterojunction films.

Adsorption of Surface-Modified Silica Nanoparticles to the Interface of Melt Poly(lactic acid) and Supercritical Carbon Dioxide

With the purpose of fabricating polymer nanocomposite foams and preventing coalescence in foaming processes, the interfacial tension of poly(lactic acid) (PLA)-silica composites is investigated in this work. Synthesized silica nanoparticles (SNs) with a CO2-philic surface modification are used as the dispersed nanoparticles. Interfacial tension is a key parameter in processing of polymer foams since it directly affects the final foam properties, such as cell size and cell density. Interfacial tension of silica-containing PLA and supercritical carbon dioxide (CO2) is measured using axisymmetric drop shape analysis profile (ADSA-P) pendant drop method at high pressures and high temperatures. The interfacial tension between PLA and supercritical CO2 is observed to decrease as a result of the nanoparticles' adsorption to the interface. These results indicate that the reduction in interfacial tension with increasing silica content significantly deviates from a linear trend; there is a minimum at 2 wt % loading of the SNs and then the interfacial tension curve reaches a plateau. Contact angle measurements show an affinity of the SNs for the polymer-supercritical CO2 interface, and these obtained results are used to calculate the binding energy of the nanoparticles to the PLA/CO2 interface. In addition to interfacial properties, the adsorption of silica nanoparticles at the interface is also studied in detail with scanning electron microscopy.

Melt crystallization/dewetting of ultrathin PEO films via carbon dioxide annealing: the effects of polymer adsorbed layers

The effects of CO2 annealing on the melting and subsequent melt crystallization processes of spin-cast poly(ethylene oxide) (PEO) ultrathin films (20-100 nm in thickness) prepared on Si substrates were investigated. By using in situ neutron reflectivity, we found that all the PEO thin films show melting at a pressure as low as P = 2.9 MPa and at T = 48 °C which is below the bulk melting temperature (Tm). The films were then subjected to quick depressurization to atmospheric pressure, resulting in the non-equilibrium swollen state, and the melt crystallization (and/or dewetting) process was carried out in air via subsequent annealing at given temperatures below Tm. Detailed structural characterization using grazing incidence X-ray diffraction, atomic force microscopy, and polarized optical microscopy revealed two unique aspects of the CO2-treated PEO films: (i) a flat-on lamellar orientation, where the molecular chains stand normal to the film surface, is formed within the entire film regardless of the original film thickness and the annealing temperature; and (ii) the dewetting kinetics for the 20 nm thick film is much slower than that for the thicker films. The key to these phenomena is the formation of irreversibly adsorbed layers on the substrates during the CO2 annealing: the limited plasticization effect of CO2 at the polymer-substrate interface promotes polymer adsorption rather than melting. Here we explain the mechanisms of the melt crystallization and dewetting processes where the adsorbed layers play vital roles.