Showing papers by "Wen-Chang Chen published in 1995"

Small-Bandgap Conducting Polymers Based on Conjugated Poly(heteroarylene methines). 2. Synthesis, Structure, and Properties

Abstract: A series of soluble conjugated poly(heteroarylene methines) with varying number of 2,5-thienylene rings have been synthesized, characterized, and shown to have small intrinsic optical and electrochemical bandgaps. The conjugated polymers containing alternating aromatic and quinoid heteroarylene moieties in the main chain were prepared by oxidative dehydrogenation of precursor poly(heteroarylene methylenes) with 2,3-dicyano-5,6-dichloro-1,4-benzoquinone (DDQ). The optical and electrochemical properties of the conjugated poly(heteroarylene methines), such as bandgap, redox potentials, ionization potential, and electron affinity, were found to be significantly modified by the size of the quinoid moieties in the main chain and the type of donor or acceptor side group at the methine carbon. Some of the poly(heteroarylene methines) have intrinsic bandgaps (E g ) that are significantly smaller than found for the parent polythiophene (E g ∼ 2 eV) : PBTHBQ (E g = 1.14 eV) ; PBTBQ (E g = 1.27 eV) ; PBTTBQ (E g = 1.31 eV) ; and PBTNBQ (E g = 1.45 eV). These same four polymers have peak oxidation potentials in the range 0.29-0.54 V (SCE) and peak reduction potentials in the range -1.82 to -1.49 V (SCE). The electrical conductivity of iodine-doped thin films of PBTBQ, PBTNBQ, and PBTHBQ was 6 x 10 -3 , 2.2 x 10 -3 , and 1.6 x 10 -1 S/cm, respectively, compared to a conductivity of less than 10 -7 S/cm for the undoped materials. These results confirm that conjugated poly(heteroarylene methines) are small-bandgap conducting polymers which can be prepared by oxidative dehydrogenation of precursors.

Small-Bandgap Conducting Polymers Based on Conjugated Poly(heteroarylene methines). 1. Precursor Poly(heteroarylene methylenes)

Abstract: A series of 20 new poly(heteroarylene methylenes) containing one to four α-thiophene rings alternating with an sp 3 -carbon atom in the main chain were synthesized and characterized. The 11 as-synthesized poly[(2,5-thienylene)methylenes] were found to be highly dehydrogenated, thus resulting in red materials with conjugated segments larger than a single thiophene ring. However, poly[(α-bithienylene)methylenes], poly[(α-terthienylene)methylenes], and poly[(α-quaterthienylene)methylenes] were obtained with optical absorption spectra and π-conjugation lengths characteristic of the isolated thiophene oligomers. The chemical and physical properties of the poly(heteroarylene methylenes) such as the glass transition temperature (T g ) and solubility were significantly modified by the variation of the size of the oligomer in the main chain and by the side group substitution at the bridge carbon. The new polymers are useful precursors to small-bandgap polymers based on poly(heteroarylene methines) and are also useful model systems for the investigation of the effects of π-conjugation length on the electronic and optical properties of conjugated polymers.

Gradient-Index Polymer Fibers Prepared by Extrusion

Abstract: A closed extrusion process was developed to prepare gradient index polymer fibers by mutual diffusion of monomers (benzyl and methyl methacrylate) with different refractive indices in a host polymer (poly(methyl methacrylate)). The compositions of BzMa and MMA inside the fiber, imaging radius and distribution constant are estimated from the refractive index distribution

Acetone absorption in irradiated polycarbonate

Abstract: Acetone absorption in irradiated polycarbonate was investigated. The molecular weight and glass transition temperature of polycarbonate decreased with increasing the γ-ray dose. Experimental data obtained from acetone transport in polycarbonate with various doses correlated sufficiently well with Harmon's model which was accounted for case I, case II, and anomalous transport. The diffusion coefficient for case I, velocity for case II, and diffusion coefficient for diffusion front satisfied the Arrhenius plot for all γ-ray doses. The equilibrium solubility satisfied the van't Hoff plot regardless of the γ-ray dose. Additionally, in the low-temperature range activation energies of case I, case II and the diffusion front, and the heat of mixing were found to have the same value for doses greater than 100 kGy which was different from those for unirradiated polycarbonate. This observation implied that acetone transport in irradiated polycarbonate occurs via the same mechanism for doses greater than 1000 kGy. The equilibrium solubility and diffusion coefficient for diffusion front at a given temperature were the same for all doses in the high-temperature range.

Studies on the preparation of gradient‐index polymeric rods by interfacial‐gel copolymerization

Abstract: The effects of reaction conditions on the formation of defects and the refractive index profiles of gradient index poly(methyl methacrylate-co-benzyl methacrylate) rods were investigated

A novel method for preparing gradient index (GI) plastic rods

Abstract: The initiator diffusion technique was used in this study to prepare the gradient index(GI) plastic rods, in which two kinds of initiators were used, i.e., thermal initiator and redox initiator. Experimental results showed that a GI plastic rod with a parabolic profile of refractive index was obtained by the initiator diffusion technique. The difference of the refractive index from the center to the periphery (Δn) of the plastic rods was in the range of 0.004–0.014. Δn increased with increasing temperature due to the increase of the gel effect and diffusion rates of monomers at a higher temperature. Furthermore, the GI plastic rod had substantially less defects, e.g., bubbles and opaqueness, than that prepared from interfacial-gel copolymerization.

The electrochemical properties of polyaniline derivatives : poly(4,4'-diphenylamine methylenes) and poly(4,4'-diphenylimine methines)

Abstract: The electrochemical properties of poly(4,4′-diphenylamine methylenes) and poly(4,4′-diphenylimine methines) were investigated by cyclic voltammetry. A dehydrogenation reaction occured when poly(4,4′-diphenylamine methylenes) underwent a electrochemical reaction and transformed to poly(4,4′-diphenylimine methines). The fully oxidized poly(4,4′-diphenylimine methines) had the electrochemical band gaps of 1.60–1.72 eV, which were found to significantly smaller than those of the fully reduced poly(4,4′-diphenylamine methylenes).