Jostein Mårdalen

X-ray scattering from oriented poly(3-alkylthiophenes)

Abstract Solution-cast films of poly(3-hexylthiophene) (P3HT) and poly(3-octylthiophene) (P3OT) were oriented by stretching to obtain more detailed information about their crystalline structure. For the first time reflections associated with the stretch direction l = 1–7 were revealed using a combination of X-ray film exposures and detailed intensity mapping by various detector scans. The reflections were located at finite angles to the stretch axis indicating a monoclinic (or triclinic) unit cell. The equatorial diffraction patterns (l = 0) of P3HT and P3OT show that the two structures are equal only with one differing lattice dimension caused by the length of the alkyl side chains. The crystalline structure for the unstretched (isotropic) and the stretched sample is found to be the sa me. Existence of h00 and absence of hk0, h0l and hkl reflections, all with finite integers h, k and l, indicate layer stacking disorder between bc-planes. Intensity calculations based upon this model corroborate well with the measured diffraction pattern on the assumption of disordered alkyl side chains.

Chain configuration of poly(3-hexylthiophene) as revealed by detailed X-ray diffraction studies

Abstract Detailed diffractometer and Laue x-ray studies of streched poly (3-hexylthiophene) films enabled us to identify several 001 reflections associated with the polymer chain direction. The chain repetition distance could then be determined for the first time, giving c =2·7.4 A =14.8 A . 7.4 A is shorter than expected for a model involving “alternating updown” monomers along the chain (7.8A), whereas the observed c value is in agreement with a new “up-up-down-down” model (14.9A). Intensity calculations assuming side chain disorder corroborate well with the new model.

Unexpected orientational effects in spin-cast, sub-micron layers of poly(alkylthiophene)s: A diffraction study with synchrotron radiation

Abstract Sub-micron layers of poly(3-octylthiophene) and poly(3-hexylthiophene) deposited by spin-casting on glass substrates were for the first time investigated by X-ray diffraction using synchrotron radiation. The polymers have a partial crystalline structure and are, in contrast to bulk materials, oriented with their side chains parallel to the substrate surface. Optical absorption spectra obtained from the same samples show that the conjugation length is related to the crystalline order.

Molecular structure of stretch oriented poly(3-hexylthiophene) studied by an extended X-ray diffraction mapping

Abstract Detailed x-ray diffraction intensity mapping of the reciprocal space of stretched poly(3-hexylthiophene) films enabled us for the first time to reveal reflections associated with the stretch direction (l = 1−7). The reflections are located at finite angles to the stretch axis and can therefore only be indexed on a monoclinic (or triclinic) unit cell. The monoclinic cell parameters are a =16.90 A , b =4.85 A , c =7.84 A , α=50.6° , ( a =4.85 A , b =16.90 A , c =7.84 A and β=129.4° by crystallographic conventions) and with two monomer units per cell. The values of a and c are directly measured whereas b and α are refined from structure factor calculations. Existence of h00 but absence of hk0, h0l and hkl reflections indicate layer disorder between oblique bc planes, whereas structure factor calculations indicate side chain disorder.

Structural and thermochromic properties of mixtures of poly(3-hexylthiophene) and poly(3-octylthiophene)

Abstract For the first time, the structural and thermochromic properties of mixtures of poly(3-hexylthiophene) (PHT) and poly(3-octylthiophene) (POT) have been studied. X-ray diffraction and optical absorption spectroscopy were employed, the latter on spin-cast films in the temperature range 20–210 °C. The results show that the mixtures are single-phase materials that are structurally intermediate between PHT and POT. It was found that the longest interchain distance a in the crystalline part of the polymer and the thermochromic transition temperature Tt both vary nonlinearly with composition in such a way that Tt and a are approximately linear to each other. This indicates that the side chains act primarily as spacers between main chains, rather than being directly involved indicates that the side chains act primarily as spacers between main chains, rather than being directly involved in the thermochromic transition. This transition may instead be induced by a melting of the polymer backbone.

The nucleation process and the crystalline structure of poly(3-alkylthiophenes) precipitated from marginal solvents

Abstract Soluble poly(3-alkylthiophenes) with alkyl side chains longer than butyl precipitate from solution to suspension during cooling from solvents with strong temperature dependent solubility (marginal solvents). The precipitation is accompanied by a colour change from yellow to red, which is in accordance with the thermochromic- and solvachromic shifts previously reported. Optical absorption measurements were used to monitor the precipitation process. The measurements showed an energy shift for the absorption edge and a growth of some sharper (fine structure) peaks. These peaks can be interpreted as a vibronic combination of the π - π ∗ electronic transition and the 1460 cm −1 vibrational mode, the latter known to occur in well ordered poly(3-alkylthiophenes). The fine structure peaks were used to monitor the crystallization process, and indicated a high degree of polymer ordering in the precipitants. Mats of the precipitated materials were studied by x-ray diffraction. The amount of crystallinity were high compared to ordinary solution cast samples, whereas the crystal structure itself was the same, i.e. straight polymer chains with an alternating “up-down” conformation of the thiophene rings. The polymer mats were found to be anisotropic with the alkyl side chains oriented perpendicular to the mat surface.

Poly(3-hexylthiophenes) studied by neutron scattering

Abstract With the aaim of obtaining information on the state of the alkyl side chains in polyalkylthiophenes neutron scattering studies were undertaken on deuterated and hydrogenated polyhexythiphenes. Both materials were prepared from bromohexanes and magnesium thiophenes, using FeCl3 as the oxidizing agent for the polymerization. Pronounced isotope effects were observed in the neutron scattering, showing clear orientation effects with stretched film samples. The data corroborate qualitatively with a previous model involving partial side chain disorder. Indication was found of possible “side chain pairing”.

Structural studies of poly(dimethyl-tetrathiophene, an intermediate between poly(thiophene) and poly(3-methylthiophene)

Abstract Grignard coupling of 2-bromo-3-methylthiophene and 5,5′-dibromo-2,2′-dithiophene, and chemical polymerization of the resulting tetramer using FeCl3 as coupling agent gave a new type of substituted polythiophene, namely Poly(3,3‴-dimethyl-(2,2′:5′,2′:5′,2‴)tetrathiophene)) (PDMTT), with two new and interesting features compared to conventional polythiophenes. Firstly, we have a polythiophene with exact methyl side group positioning, eliminating the possibility of side group position disorder. Secondly, and especially interesting from a structural point of view, we obtained a polymer which is intermediate between the unsubstituted polythiophene (PT) and poly(3-methylthiophene) (P3MT). The polymer was obtained as a black insoluble powder which turned out to be amorphous. X-ray diffraction profiles were measured and compared to the profiles of PT and P3MT. We were able to follow the intensity and position variation of the diffraction peaks as a function of the methyl substitution.

Neutron diffraction studies of deuterated and hydrogenated polyhexylthiophenes

Abstract Neutron diffraction studies, which constitute the first neutron structure work on solid polythiophenes, were performed on as-prepared powders of polyhexylthiophene (PHT) with the hexyl side chains deuterated (PHT-d) and hydrogenated (PHT-h). Although still preliminary in character, the diffraction profiles revealed two important features: (1) the strongest diffraction peak is weaker in PHT-d than in PHT-h, and (2) the diffuse scattering component is considerably stronger in PHT-d, both facts pointing to a considerable degree of side-chain disorder.

Structure of 3,4-diphenyl-1,2,4-triazoles, possible bases for new conductive polymers

Crystal and molecular structures of two new 3,4-diphenyl-1,2,4-triazoles show that an appreciable degree of electron delocalization is present in the combined two phenyl-triazole ring systems. Phenyl-ring twisting relative to the triazole rings is provoked by steric effects of side groups of the triazole rings.