Thermal, Structural, and Viscoelastic Characterization of cis-Poly(phenylene vinylene) Related to its Photo-Isomerization

Abstract

ABSTRACT:A cis-poly(phenylene vinylene) (cis-PPV) recently synthesized by Katayama, Ozawa, and coworkers undergoes photo-isomerization into trans configuration. This isomerization occurred rather quickly within 45 min in solvent-cast films at room temperature. The cis-PPV chains exhibiting this quick isomerization were expected to be highly mobile in the bulk state, and this expectation was examined from thermal, structural, and viscoelastic tests for a cis-PPV sample with M n ¼ 4300 and M w =M n ¼ 2:16. It turned out that the as-polymerized cis-PPV was partially crystalline and fully melted at T m ¼ $ 109C. The samples quenched from a molten state to lower temperatures ( 70 C < T m ) as well as cast from a solution were amorphous for few hours, enabling the viscoelastic test in the fully amorphous state. The WLF-type time-temperature superposition was valid for these quenched and/or cast sample, confirming the amorphous character of the cis-PPV chains therein. The terminal viscoelastic relaxation time of these chains, equivalent to a time required for thermal changes in a large-scale conformation, was ¼ $ 10 s at room temperature. This result suggested that the conformational changes of the amorphous chains (the major component in the cast films) were sufficiently fast to allow the efficient cis-to-trans photo-isomerization. [DOI 10.1295/polymj.38.184] KEY WORDS Poly(phenylene vinylene) / Semicrystalline / Viscoelastic Characterization / Recently, Katayama, Ozawa, and coworkers conducted a stereo-controlled synthesis of poly[(2,5-dioctyloxy-1,4-phenylene vinylene)-co-(1,4-phenylene vinylene)]s (PPV) using the Hiyama-and SuzukiMiyaura-type polycondensation methods.1,2 The PPV, having the structure shown in Scheme 1, is a -conjugated polymer offering applications to light-emitting diodes, 3 solar cells, 4 and so on. In these applications, it is very useful if we can make arbitrary patterning of thin PPV films cast on appropriate substrates. Katayama, et al.1 showed that the cis-to-trans photoisomerization by UV light occurred rather quickly within 45 min in the cis-PPV film prepared by solventcasting and that the resulting trans-PPV film, being partly crosslinked on the UV radiation, was insoluble in solvents while the cis-PPV film was soluble (in chloroform, for example). This photo-isomerization, followed by rinsing with solvents, provides us with an interesting route of pattern-writing in the residual trans-PPV film by use of appropriate UV masks, as already demonstrated previously. 1Because the photo-isomerization occurs rather quickly, the cis-PPV chains in the solvent-cast film are expected to be essentially in the amorphous state thereby exhibiting fast changes of their large-scale conformation. (It is quite unlikely that the isomerization occurs quickly for the chains in a rigid crystalline state.) However, no details have been elucidated for the thermal motion of the amorphous cis-PPV chains. Thus, we have conducted thermal, structural, and viscoelastic tests for a previously synthesized cis-PPV sample...

Formation of trans-Poly(thienylenevinylene) Thin Films by Solid-State Thermal Isomerization

“…The charge carrier mobility of polymers with trans -vinylene and thiophene-derived repeating units ( trans -poly­(thienylenevinylene)­s; PTVs) has been reported to be the highest among π-conjugated polymers. − However, trans -PTVs have strong interchain π–π interactions due to their planar structure, which limits their solubility. , The more twisted configuration of the cis -vinylene units tends to result in less optimized interchain stacking and thus higher solubility . We have demonstrated that thin films of cis -poly­( p -phenylenevinylene)­s undergo cis-to-trans isomerization under UV irradiation (Figure e). − A similar isomerization of cis -vinylene may provide a direct and efficient route to the fabrication of trans -PTVs thin films (Figure f).…”

“…Instead, thin films are fabricated from soluble precursor materials and directly converted to the desired compounds via heating and/or light exposure in the solid state. − While precursor reactions often form volatile byproducts that may disrupt the film morphology (Figure a–d), isomerization is an ideally “clean” pathway with no chemical byproducts. Our group has previously reported a precursor method using cis-to-trans isomerization of vinylene units integrated into a polymer backbone (Figure e). − …”