Studies have been conducted at NASA Langley establishing structure-property relationships in polyimides that provide a means of lowering dielectric constants by reducing chain-chain electronic interactions and by incorporation of fluorine into the polymer molecular structure. The resulting polyimides exhibit excellent thermal stability and mechanical strength, improved resistance to moisture and enhanced solubility. Most polyimides containing fluorine in only one monomer are at least partially soluble in common organic solvents, and polyimides containing fluorine in both the dianhydride and the diamine are fully soluble. Although this enhanced solubility aides processability and is desirable for many applications, it is a severe disadvantage for aircraft applications where exposure to hydraulic fluids and other chemicals is likely to occur. This work involves the development of insoluble, low dielectric polymers prepared from 2, 2-bis[4-(3, 4-dicarboxyphenoxy)phenyl]hexafluoropropane dianhydride (BFDA) and selected fluorine-containing diamines.

BFDA-containing polyimide films had dielectric constants that ranged from 2.44 to 2.53 at 10 GHz and exhibited insolubility in solvents including N, N-dimethylacetamide, chloroform, diglyme and N-methylpyrrolidinone.

Onium salts and amine triflate salts decompose upon exposure to radiation or thermal treatment respectively to generate protonic acids. These salts can be blended with polyaniline and upon decomposition the resulting acids act as insitu dopants for the polymer. These novel doping techniques eliminate the need for external dopant solutions and thereby simplify the processing of conducting polyaniline.

Thermotropic chiral nematic copolymers containing (S)–(–)–1–phenylethyl alcohol and (R)–(–)–methyl mandelate with methacrylate backbone and those containing (S)–(–)–1–phenylethylamine and cholesterol with acrylate/methacrylate mixed backbone structures have been synthesized and characterized. It is found that the helical sense is dictated by chiral/nematic molecular interactions of a steric nature, and that the helical twisting power (HTP) is favored by the structural similarity between the chiral and nematic components. Furthermore, it is shown that HTP can be optimized with a suitable nematogenic comonomer by regulating the backbone flexibility within the acrylate/methacrylate series, which facilitates supramolecular arrangement without affecting the HTP.

Photoinduced absorption spectra of poly(1-N-carbazolyl-penta-1,3.diyne-5-acetoxy) show absorption peaks which depend on the sample preparation and on the temperature. The low temperature spectra show two well resolved absorptions centered at ∼0.5 and ∼1.2 eV respectively. In view of other experimental results on these systems, we have interpreted our data as evidence of charged bipolarons stabilized by interchain interactions.

Birefringence measurements of nematic solutions of the poly(p-phenylene benzobisthiazole), PBT, using interference between the ordinary and extraordinary waves created when a plane polarized wave enters the crystal, are described. The measured birefringence is high, e.g., ne – no varies from 0.05 to 0.09 at 633 nm for polymer concentrations 4×10−2 < c < 10×10−2(g/cm3). These data are discussed with the implications they have on the order parameter in these solutions.

Rheological and optical properties of polystyrene are modified by 200 keV He irradiation because of new molecular bonds induced by ion energy deposition. Total amount of bonds, as calculated by optical measurements, increases linearly with ion fluence without any dependence on the initial molecular weight. Density of interchain bonds or crosslinks, as calculated by rheological measurements, increases linearly at low fluence (∼1013 ions/cm2), while for high values shows a saturation behaviour.

Correlation between optical and rheological measurements allows us to calculate the fractions of interchain and intrachain bonds of ion irradiated 9000, 35000 and 100000 initial molecular weight samples.

Spectroscopic behavior of poly-3-methylthiophene (P3MT) has been studied employing derivative cyclic volt-absorptometric (DCVA) techniques. In the DCVA technique, the derivative absorption signal (dA/dt) is recorded as a function of the applied potential. The dA/dt signals, the spectroscopic analog of electrochemical currents in cyclic voltammetry, are capable of monitoring the potential dependency for the absorption band effectively discriminating against nonfaradaic signals. The DCVA studies on the P3MT system show that the neutral form of P3MT, absorbing at 490 nm (at less than 0.3 V vs. Ag), changes to the radical cation form, which absorbs at 760 nm. Initially, the formation of the radical cation goes through an isosbestic point, indicating that the conversion of the neutral to radical (polaron) form is chemically reversible. However, upon increasing the electrode potential, the rate of the radical formation at 760 nm starts to decrease, with the formation of another band at about 1250 nm, attributable to a quinoid (bipolaron) form. This trend begins above about 0.6 V, shifting to a more positive voltage as the thickness of the film grows. This observation indicates that the electrochemical conversion of the neutral to radical form, followed by the quinoid form, is a slow process controlled by the diffusion of counter ions through the film. In-situ conductivity measurements as a function of applied potentials support the observed spectroscopic behavior.

Charge carrier generation and transport mechanisms in polydiacetylene thin film single crystals., poly-PTS (2,4-hexadiyne-1,6-diol bis(p-toluenesulfonate)) and poly- BTFP (bis-(4-n-butyl-2,3,5,6-tetra-fluorophenyl) butadiyne), are studied by using steady state and transient photoconductivity techniques. The electric field dependence of the steady state photocurrent is superlinear for both samples. Dependence of photocurrent on incident light polarization has been investigated. The polarization dependence of photocurrent has completely different behavior for the polydiacetylene PTS and BTFP. Single-gap transmission line experiment has been designed to directly measure the drift velocity of PTS single crystals. A drift velocity of the order of 106cm/s was measured.

In this paper we have computed electronic density of states for several Q1D graphites: polyacene (PA), polyacenacene (PAA), polyphenanthrene (PP), polyphenanthrophenanthrene (PPhP), and polyperinaphthalene (PPN). The modified extended Hiickel method for finite Q1D chains has been adopted. The change of the electronic properties due to the growth of the Q1D-graphites toward the two-dimensional direction, starting from trans-polyacetylene, cis-polyacetylene or poly(p-phenylene), is discussed. Our calculations show that PA, PAA, and PPN are intrinsic conductors, while PP and PPh are semiconductors with energy gaps of 1,4 eV and 0,8 eV, respectively. The comparison with other computational results is presented.

The fracture energy and the critical strength of glassy polymers with molecular weight larger than the critical value for the onset of chain entanglements are proportional to the number of chain segments entangled about a unit plane. A new molecular model is presented to calculate the crossing density of these chain segments when the segment length is a stochastic variable. The crossing density depends on the mesh size of the entanglement network and the number of entanglement network strands per unit volume. Theoretical predictions of the variation of the fracture energy and strength with the molecular weight are compared with experimental results for various glassy polymers.