3 research outputs found
CO<sub>2</sub> 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 polyÂ(ethylene
glycol terephthalate) and 23 wt % of polyÂ(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 N<sub>2</sub>, H<sub>2</sub>, CH<sub>4</sub>, and
CO<sub>2</sub> 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 (tributylÂstannyl)Âvinyl 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, Ï…Â(<i>T</i>). We undertake
the challenging task of estimating material-specific Ï…Â(<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><sub>IN</sub>, 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 Ï…Â(<i>T</i>) by matching <i>T</i><sub>IN</sub> 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 Ï…Â(<i>T</i>). We moreover correlate our
findings with earlier discussions on the thermodynamic nature of phenomenological
MS interactions