CO₂ Selective PolyActive Membrane: Thermal Transitions and Gas Permeance as a Function of Thickness

It is generally accepted that the melting point of a semicrystalline polymer is associated with the thickness of the crystalline lamellae (Gibbs–Thomson equation). In this study, a commercially available multiblock copolymer PolyActive composed of 77 wt % of poly­(ethylene glycol terephthalate) and 23 wt % of poly­(butylene terephthalate) was dip-coated on top of a multilayer microporous support. The thickness was changed between 0.2 and 8 μm using coating solutions containing 0.75–7.5 wt % PolyActive. The surface temperature of the membrane during dip-coating was monitored using an infrared camera. Single gas permeances of N₂, H₂, CH₄, and CO₂ were measured between 20 and 80 °C at temperature steps of 2 °C. Spherulitic superstructures composed of radially directed lamellae were observed in the polarized light microscope in the prepared membranes. Atomic force microscopy studies showed that the thickness of the crystalline lamellae was in the order of 10 nm or 0.01 μm at the surface of the membrane. Therefore, according to the Gibbs–Thomson equation, the melting point should not change in the thickness range 0.2–8 μm. However, the gas permeance data showed that the melting point of the polyether domains of the 0.2 μm PolyActive layer was 10 °C lower compared to that of the 8 μm layer. The results can be explained by considering that the width of many crystalline lamellae significantly reduces as a function of film thickness, thereby reducing the average fold surface free energy/lateral surface free energy ratio

Conjugated Copolymers of Vinylene Flanked Naphthalene Diimide

We report the synthesis of a novel naphthalene diimide (NDI) monomer containing two (tributyl­stannyl)­vinyl groups. The utility of this building block is demonstrated by its copolymerization with five different electron-rich comonomers under Stille conditions. The resulting high molecular weight polymers show red-shifted optical absorptions in comparison to the analogous polymers without the vinylene spacer and a significant increase in the intensity of the low-energy intramolecular charge transfer band. The polymers all exhibit ambipolar behavior in bottom-gate, top-contact organic thin-film transistors. The insertion of a solution-processed barium hydroxide layer between the polymer and the gold electrode led to unipolar behavior with improved electron mobilities

Temperature-Dependence of Persistence Length Affects Phenomenological Descriptions of Aligning Interactions in Nematic Semiconducting Polymers

Electronic and optical properties of conjugated polymers are strongly affected by their solid-state microstructure. In nematic polymers, mesoscopic order and structure can be theoretically understood using Maier–Saupe (MS) models, motivating us to apply them to conjugated macromolecular systems and consider the problem of their material-specific parametrization. MS models represent polymers by worm-like chains (WLC) and can describe collective polymer alignment through anisotropic MS interactions. Their strength is controlled by a phenomenological temperature-dependent parameter, υ­(<i>T</i>). We undertake the challenging task of estimating material-specific υ­(<i>T</i>), combining experiments and Self-Consistent Field theory (SCFT). Considering three different materials and a spectrum of molecular weights, we cover the cases of rod-like, semiflexible, and flexible conjugated polymers. The temperature of the isotropic–nematic transition, <i>T</i>_IN, is identified via polarized optical microscopy and spectroscopy. The polymers are mapped on WLC with temperature-dependent persistence length. Fixed persistence lengths are also considered, reproducing situations addressed in earlier studies. We estimate υ­(<i>T</i>) by matching <i>T</i>_IN in experiments and SCFT treatment of the MS model. An important conclusion is that accounting explicitly for changes of persistence length with temperature has significant qualitative effects on υ­(<i>T</i>). We moreover correlate our findings with earlier discussions on the thermodynamic nature of phenomenological MS interactions