Electron delocalization contribution to single crystal thermal expansion of a polydiacetylene

The thermal expansion contribution due to temperature-dependent π-electron delocalization is evaluated from spectral measurements on a single crystal polydiacetylene (poly-2,4-hexadiyne-1,6-diol bisphenylurethane). The observed temperature independence of backbone associated vibrations (less than ±1 cm^?1 change in ν_C?C and ν_C?C between 25 and 90°C) implies that thermal conformational fluctuations and equilibrium defect formation (which produce a negative thermal expansion coefficient) do not measurably affect π-electron delocalization. The separation of equilibrium defects is either much longer than that of nonequilibrium defects or much longer than required to appreciably limit π-electron delocalization in an effectively defect-free polymer. Arguments presented indicate that, in the experimental temperature interval, the observed thermal expansion coefficient in the chain direction is over an order of magnitude larger than the delocalization-associated contribution.     

Thermal expansion of a polydiacetylene single crystal

Summary Fractural length changes l/l of a polydiacetylene (toluene sulfonate) single crystal and its monomer have been measured along the crystallographica-,b-, andc-direction in the temperature range 100 to 400 K. Small steps in the temperature dependence of l/l at 194 K for the polymer and 159 K for the monomer are assumed to be caused by a well known phase transition. From the volume coefficient of thermal expansion the bulk Grüneisenparameter _b has been calculated. Temperature dependence as well as absolute values of _b for the polymer crystal are found to be in accordance with current theories assuming that both inter and intrachain vibrations strongly influence the thermoelastic behaviour of polymer crystals.

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Zusammenfassung Die relative Längenänderung l/l eines Polydiacetylen (Toluolsulfonat) Einkristalls und des zugehörigen Monomerkristalls wurde entlang der kristallographischena-,b-undc-Richtung im Temperaturbereich 100 bis 400 K gemessen. Kleine Stufen im Temperaturverlauf von l/l bei 194 K für den Polymerkristall und bei 159 K für den Monomerkristall werden als Einfluß einer bekannten Phasenumwandlung gedeutet. Aus dem Volumenausdehnungskoeffizienten wurde der Bulk-Grüneisenparameter _b berechnet. Sowohl die Temperaturabhängigkeit als auch absolute Werte von _b für den Polymerkristall befinden sich in Einklang mit theoretischen Modellen, wenn man annimmt, daß sowohl Inter- als auch Intrakettenschwingungen das thermoelastische Verhalten von Polymerkristallen beeinflussen.

     

Thermal expansion and compressibility in the chain direction of a polydiacetylene single crystal

An x-ray back-reflection rotating camera has been used to measure the lattice constant, thermal expansion, and compressibility in the polymer chain direction of a polydiacetylene, poly[bis(p-toluene sulfonate) of 2,4-hexadiyne 1,6-diol]. The thermal expansion coefficient of the polymer chain is small and positive (0.9 ± 0.2 × 10^?6 °K^?) at 300°K, but negative below about 70°K. Application of 3.43 kbar hydrostatic pressure at 299°K changed the unit cell dimension in the polymer chain direction by less than 10 ppm.     

Transient photoconductivity of a polydiacetylene single crystal

Pulsed photoconductivity results are presented for single crystals of a fully-conjugated polymer, poly-2,4-hexadiyne-1,6-diol bis(p-toluene sulfonate). Carriers of both sign are observed and are found to have essentially identical action spectra with a photoconduction onset around 3 eV. Upper and lower limits could be placed on the charge carrier mobilities normal to the (011) crystallographic plain, which is roughly the polymer chain direction: 3 ? μ_e,h ? 10^?3 cm²/V s. The mobilities normal to the chain are about a factor of 10 less. The carrier lifetimes are about 0.5 μs for both holes and electrons but strongly dependent on the method of polymerization of the precursor monomer crystals.     

Anisotropy of electrical properties of a polydiacetylene single crystal

The electrical properties of PDA single crystals are found to be highly anisotropic. The ratio of the charge carrier mobility parallel and perpendicular to the chain axis measured in the (100) crystal-plane is (8 ± 3) × 10². The quantum yield for photoelectric charge carrier generation is by a factor of 10² higher when the field is parallel to the chain.     

Negative thermal expansion materials

Finite element analysis is used to explore composites of negative thermal expansion materials with positive thermal expansion materials (ZrW₂O₈ in Cu and ZrO₂ in ZrW₂O₈) and evaluate how thermal and mechanical properties, rates of cooling/heating, and geometry and packing fraction influence the overall expansion and thermal stress. During rapid temperature changes, the transient short-time thermal expansion can be considerably larger than the steady-state value. Furthermore, thermal stress in the composite can be large, especially at the interface between the materials, and can exceed the material strength.     

Motion and recombination of charge carriers in a polydiacetylene single crystal

From photocurrent transients excited by a ruby laser γ/μ = (6.3±1.5) × 10^–6 V cm is derived for the ratio of carrier recombination coefficient to mobility in polydiacetylene-bis (toluenesulfonate) single crystals. The sum of electron and hole mobility in chain direction is 2.8 cm²/V s with a uncertainly of a factor of 2 in both directions.     

Heat capacity of a polydiacetylene single crystal from 3 to 300 K

Heat capacities C_p of a polydiacetylene-bis(toluene sulfonate) single crystal and its monomer have been measured in the temperature range from 3 to 300 K. The temperature dependence of C_p for both monomer and polymer crystals differs from that for monoatomic solids. By applying a chain lattice model for a polymer crystal, the temperature dependence of the heat capacity can be described assuming a phonon density of states given by bending and stretching modes of the polymer backbone. With a combination of one-dimensional and three-dimensional elastic continuum approximations, the heat capacity has been calculated and a good fit to the data has been obtained. A small peak in C_p was detected at 161 K for the monomer and at 198 K for the polymer. This may be ascribed to a lower-temperature phase transition in the polydiacetylene crystals evidenced by previous x-ray and spectroscopic measurements.     

Negative thermal expansion in rare earth molybdates

Negative thermal expansion in rare earth molybdates of A₂Mo₃O₁₂ family (A=Y, Er, Yb and Lu) is measured by high temperature X-ray diffraction and dilatometry. Rare earth molybdates which are isostructural with the corresponding rare earth tungstates, also exhibit this phenomena attributed to transverse acoustic vibrations. The rare earth molybdates of A₂M₃O₁₂ family with an orthorhombic structure (A=Y, Er, Yb and Lu) are highly hygroscopic and exhibit negative thermal expansion after the complete removal of water molecules. Axial thermal expansion co-efficient calculated from high temperature X-ray diffraction (RT-1073K) shows rare earth size effect. As the ‘A’ cation decreases in size, the thermal expansion co-efficient along ‘b’ axis and the linear thermal expansion co-efficient become less negative. The thermal expansion behaviour of the tetragonal La₂Mo₃O₁₂ is also reported to demonstrate the effect of crystal structure.     

Negative thermal expansion in oriented crystalline polymers

Measurements on the thermal expansivity α_∥ and α_? (along and normal to the draw direction, respectively) have been carried out for a series of oriented polymers with widely different crystallinities (0.36–0.81) and draw ratios (1–20) and over large temperature ranges covering the major amorphous transitions in each case. While α_? increases with temperature, α_∥ tends to decrease sharply above the transition temperature. For highly crystalline polymers, α_∥ decreases to values typical of polymer crystals (?1 × 10^?5 K^?1) and this can be attributed to the constraining effect of the crystalline bridges connecting the crystalline blocks. However, for polymers of lower crystallinity, α_∥ may become an order of magnitude more negative and this remarkable phenomenon is attributed to the rubber–elastic contraction of taut tie-moleucles. Since taut tie-molecules and bridges have drastically different effects on α_∥ at high temperatures, this allows a rough determination of their relative fractions.     

Negative thermal expansion of water in hydrophobic nanospaces

The density and intermolecular structure of water in carbon micropores (w = 1.36 nm) are investigated by small-angle X-ray scattering (SAXS) and X-ray diffraction (XRD) measurements between 20 K and 298 K. The SAXS results suggest that the density of the water in the micropores increased with increasing temperature over a wide temperature range (20-277 K). The density changed by 10%, which is comparable to the density change of 7% between bulk ice (I(c)) at 20 K and water at 277 K. The results of XRD at low temperatures (less than 200 K) show that the water forms the cubic ice (I(c)) structure, although its peak shape and radial distribution functions changed continuously to those of a liquid-like structure with increasing temperature. The SAXS and XRD results both showed that the water in the hydrophobic nanospaces had no phase transition point. The continuous structural change from ice I(c) to liquid with increasing temperature suggests that water shows negative thermal expansion over a wide temperature range in hydrophobic nanospaces. The combination of XRD and SAXS measurements makes it possible to describe confined systems in nanospaces with intermolecular structure and density of adsorbed molecular assemblies.     

Negative thermal expansion of polymer crystals: Lattice model

The lateral thermal motion of atoms in an inextensible polymer chain would cause a shortening along the chain direction. This effect could explain the observed negative thermal expansivity α along the chain direction of a polymer crystal. Using this idea, the expansivity α is calculated for a lattice of parallel linear chains. A value of ?1.3 × 10^?5 K is obtained for chains with a carbon backbone, in good agreement with data for polyethylene.     

Sensitized photoconduction in a polydiacetylene crystal (DCHD)

The photoconductivity of DCHD displays a maximum near 3.6 eV coinciding with the maximum of the S_o → S₁ absorption of the carbazole group. It is attributed to a sensitization involving charge transfer from the excited chromophore to the chain. The rate constant for non-radiative decay of the carbazole singlet due to energy transfer to the chain is 1.6 × 10¹³ s^?1, and for charge transfer ≈ 3 × 10¹¹ s^?1.     

Negative thermal expansion in single-component systems with isotropic interactions

We have devised an isotropic interaction potential that gives rise to negative thermal expansion (NTE) behavior in equilibrium many-particle systems in both two and three dimensions over a wide temperature and pressure range (including zero pressure). An optimization procedure is used in order to find a potential that yields a strong NTE effect. A key feature of the potential that gives rise to this behavior is the softened interior of its basin of attraction. Although such anomalous behavior is well-known in material systems with directional interactions (e.g., zirconium tungstate), to our knowledge, this is the first time that NTE behavior has been established to occur in single-component many-particle systems for isotropic interactions. Using constant-pressure Monte Carlo simulations, we show that as the temperature is increased, the system exhibits negative, zero, and then positive thermal expansion before melting (for both two- and three-dimensional systems). The behavior is explicitly compared to that of a Lennard-Jones system, which exhibits typical expansion upon heating for all temperatures and pressures.     

Spectral and thermal characterization of grown organic single crystal

The growth of semicarbazone of p-hydroxy benzaldehyde (SPHB) single crystal by slow evaporation solution growth technique is reported in this article. The grown crystal was subjected to powder XRD study to identify the crystalline nature. Single crystal XRD study was done to measure unit cell parameters and to confirm the crystal structure. In the presence of various functional groups of SPHB was identified by FTIR spectrum. Its optical behavior was examined by UV?CVis?CNIR spectrum and the crystal was found to have transparency in the region between 245 and 1100?nm. Thermal properties of the crystal were investigated using thermogravimetric and differential thermal analysis (TG-DTA), which indicated that the melting of the material occurred before decomposition. The nonlinear optical (NLO) property was tested by Kurtz?CPerry powder technique for second harmonic generations.     

Phase transition of a single sheet of sashlike polydiacetylene atomic sash on a solid surface

The phase transition of sashlike polydiacetylene (PD) atomic sash in an extrathin (0.4 nm) vapor-deposited monolayer on a graphite (0001) surface has been observed by in-situ scanning tunneling microscopy (STM). The conformation of the atomic sash composed of a row of lying alkyl chains stitched up with a PD chain is different around the crossing points of the alkyl and PD chains in the two phases, and hence the contrast of the STM image is different in the middle of the sashes. The difference is associated with the color transition of bulk PDs.     

Reversible thermal transition of polydiacetylene based on KTTKS collagen sequence

Here we explore the physico-chemical properties of a peptide amphiphile obtained by chemical conjugation of the collagen-stimulating peptide KTTKS with 10,12-pentacosadiynoic acid which photopolymerizes as a stable and extended polydiacetylene. We investigate the self-assembly of this new polymer and rationalize its peculiar behavior in terms of a thermal conformational transition. Surprisingly, this polymer shows a thermal transition associated with a non-cooperative increase in β-sheet content at high temperature.     

Spectral, thermal and optical properties of L-tryptophanium picrate: a nonlinear optical single crystal

A new organic nonlinear optical material L-tryptophanium picrate was synthesized. From the powder XRD pattern the various planes of reflections were identified. The grown crystals were subjected to FT-IR and (1)H NMR spectral analyses to confirm the synthesized compound. Thermal properties of the crystals were investigated using thermo gravimetric (TG) and differential thermal analyses (DTA). The second harmonic generation (SHG) efficiency of the title compound was found using the modified experimental setup of Kurtz and Perry.     

Energy levels of ionized states in a polydiacetylene crystal (PTS) studied by photoemission

Electron emission from partially and fully polymerized PTS single crystals has been measured upon excitation in the photon energy range 7 <hv < 11 eV. The ionization thresholds are 5.5 ± 0.1 eV for PTS polymer and 7.1 ± 0.1 eV for TS monomer. The bottom of the conduction band in PTS is located at A^P_C = ?3.1 ± 0.1 eV.