In this work, we show the effects of nanoconfinement on the crystallization of poly(ethylene oxide) (PEO) nanotubes embedded in anodized aluminum oxide (AAO) templates. The morphological characteristics of the hollow 1D PEO nanostructures were evaluated by scanning electron microscopy (SEM). The crystallization of the PEO nanostructures and bulk was studied with differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). The crystallization of PEO nanotubes studied by DSC is strongly influenced by the confinement showing a strong reduction in the crystallization temperature of the polymer. X-ray diffraction (XRD) experiments confirmed the isothermal crystallization results obtained by DSC, and studies carried out at low temperatures showed the absence of crystallites oriented with the extended chains perpendicular to the pore wall within the PEO nanotubes, which has been shown to be the typical crystal orientation for one-dimensional polymer nanostructures. In contrast, only planes oriented 33, 45, and 90° with respect to the plane (120) are arranged parallel to the pore's main axis, indicating preferential crystal growth in the direction of the radial component. Calculations based on classical nucleation theory suggest that heterogeneous nucleation prevails in the bulk PEO whereas for the PEO nanotubes a surface nucleation mechanism is more consistent with the obtained results.

Poly(ethylene oxide) confined in an anodic aluminum oxide solid matrix has been studied by different neutron scattering techniques in the momentum transfer (Q⃗) range 0.2≤Q=|Q⃗|≤1.9 Å−1. The cylindrical pores of the matrix present a diameter (40 nm) much smaller than their length (150 μm) and are parallel and hexagonally ordered. In particular, we investigated the neutron intensity scattered for two orientations of the sample with respect to the incident beam, for which the Q⃗ direction was either parallel or perpendicular to the pores for a scattering angle of 90°. Diffuse neutron scattering at room temperature has shown that the aluminum oxide has amorphous structure and the polymer in the nanoporous matrix is partially crystallized. Concerning the dynamical behavior, for Q<1 Å−1, the spectra show Rouse-like motions indistinguishable from those in the bulk within the uncertainties. In the high-Q limit we observe a slowing down of the dynamics with respect to the bulk behavior that evidences an effect of confinement. This effect is more pronounced for molecular displacements perpendicular to the pore axis than for parallel displacements. Our results clearly rule out the strong corset effect proposed for this polymer from nuclear magnetic resonance (NMR) studies and can be rationalized by assuming that the interactions with the pore walls affect one to two adjacent monomer monolayers.

We have studied the change in the magnetic properties produced on highly oriented pyrolytic graphite samples by irradiation of H, C, and N ions in the mega-electron-volt energy range. The use of specially made sample holders for the magnetic measurements provided high reproducibility allowing us to obtain directly the irradiation effects without any corrections or subtractions. Our results show that three magnetic phenomena are triggered by the defects produced by the irradiation, namely, Curie-type paramagnetism, ferromagnetism and an anomalous paramagnetic state that appears as precursor of the magnetic ordered state. Using SRIM simulations to estimate the amount of vacancies produced by the irradiation, the Curie-type paramagnetic response indicates an effective Bohr magneton number per nominally produced vacancy p = 0.27Ϯ 0.02 B. Direct measurements of the surface sample temperature during irradiation and the decrease in the ͑as-received͒ paramagnetic as well as ferromagnetic contributions after irradiation indicate that self-heating is one of the causes for small yield of ferromagnetism. Taking into account the hydrogen distribution in the virgin samples, the obtained results indicate that the induced ferromagnetism appears when the average vacancy distance is ϳ2 nm in the near surface region.

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Organic solar cell blends comprised of an electron donating polymer and electron accepting fullerene typically form upon solution casting a thin-film structure made up of a complex mixture of phases.These phases can vary greatly in: composition, order and thermodynamic stability; and they are dramatically influenced by the processing history. Understanding the processes that govern the formation of these phases and their subsequent effect on the efficiency of photo-generating and extracting charge carriers is of utmost importance to enable rational design and processing of these Organic photovoltaics (OPVs) have seen a rapid increase in performance over recent years, with certain polymer:fullerene blends now reaching efficiencies of more than 10%. This improvement has mainly been due to intense materials development efforts. Despite these activities, however, key understanding of various relevant aspects that dictate the structural and optoelectronics landscape of many OPV materials, is still lacking. For instance, it is still unclear why specic acceptors work well only in combination with certain donors. Moreover, thorough knowledge has not been established why small variations in the chemical structure of the active materials in certain cases can lead to signicant differences in device performance while in other scenarios essentially identical device characteristics are obtained. 2One reason for these differences that are observed in devices even for structurally very similar materials is that the manipulation of the chemical structure oen results in alteration of the energy levels, 1,3 which, in the case of the LUMO of the acceptor, should either promote charge dissociation or increase the open circuit voltage (V oc ) with a given donor. Chemical changes lead, however, also to a different miscibility of the two components,

We report on a new cross-section measurement for the 3 He(α, γ ) 7 Be reaction at three medium energies of E c.m. between 1 and 3 MeV. The interest stems from the significant role played by the reaction in calculating an accurate solar neutrino flux and the primordial 7 Li abundance. The energy dependence of the astrophysical S 34 factor observed in the present work, especially above 1 MeV, highlights the need to constrain theories in order to obtain a precise extrapolated value for S 34 (0). In this context, a comparison with the recent theoretical work in a fully microscopic fermionic molecular dynamics approach and a few other representative calculations emphasize the need for further experimental as well as theoretical work to resolve the existing conflicts. The cross section of the 3 He(α, γ ) 7 Be direct capture reaction was first studied by Holmgren et al. [1], being followed by a posteriori measurements of solar fusion reactions of ever-increasing sophistication and accuracy. These measurements are driven by the need to obtain more precise information that is crucial for a critical evaluation of solar models, solar neutrino fluxes, and big-bang nucleosynthesis (BBN) [2][3][4]. Specifically, the 3 He(α, γ ) 7 Be reaction plays a key role in calculating the high-energy solar neutrino flux that probes the temperature as well as the metallicity of the solar interior [3] and in explaining the primordial 7 Li abundances [4]. In recent years, the so-called 7 Li problem has attracted a considerable amount of attention as the disagreement between the observations and the predictions of the primordial 7 Li abundances became worse resulting in a persistent discrepancy of a factor of 3. This open problem poses severe challenges to the cosmological models that have been used for predicting the primordial abundances of nuclei [4][5][6][7]. In Ref.[5], a change of ∼16% was calculated in the central value of 7 Li abundance at BBN energies by utilizing the results from an evaluation of the available data on the astrophysical S 34 factor. This work highlights the need for an accurate knowledge of this reaction rate for reliable predictions of the 7 Li abundance and therefore for any progress towards a solution.There have been a number of efforts on both the experimental and the theoretical fronts that followed Ref. [1], in particular at low energies [8][9][10][11][12][13]. A "best" result of S 34 (0) = 0.56(3) keV b was reported in Ref. [3] from an evaluation of the modern data that became available after 1998 (Ref. [14]) for the energies below 1 MeV. This improved value can be compared with the previous recommendation [14] of 0.53 (5) keV b. Such * snarasingh@gmail.com efforts also suggested that the precision could be improved if the theoretical spread in the extrapolated values is minimized. In addition, recent work of the ERNA (European recoil mass separator for nuclear astrophysics) Collaboration [15] is in conflict with the data from Ref. [8] and highlights the importance of precise measurements at medium energies, ...

Electronic and optical properties of conjugated polymers are strongly affected by their solid-state microstructure. In nematic polymers, mesoscopic order and structure can be theoretically understood using Maier–Saupe (MS) models, motivating us to apply them to conjugated macromolecular systems and consider the problem of their material-specific parametrization. MS models represent polymers by worm-like chains (WLC) and can describe collective polymer alignment through anisotropic MS interactions. Their strength is controlled by a phenomenological temperature-dependent parameter, υ­(T). We undertake the challenging task of estimating material-specific υ­(T), combining experiments and Self-Consistent Field theory (SCFT). Considering three different materials and a spectrum of molecular weights, we cover the cases of rod-like, semiflexible, and flexible conjugated polymers. The temperature of the isotropic–nematic transition, T IN, is identified via polarized optical microscopy and spectroscopy. The polymers are mapped on WLC with temperature-dependent persistence length. Fixed persistence lengths are also considered, reproducing situations addressed in earlier studies. We estimate υ­(T) by matching T IN in experiments and SCFT treatment of the MS model. An important conclusion is that accounting explicitly for changes of persistence length with temperature has significant qualitative effects on υ­(T). We moreover correlate our findings with earlier discussions on the thermodynamic nature of phenomenological MS interactions.

It is generally accepted that the melting point of a semicrystalline polymer is associated with the thickness of the crystalline lamellae (Gibbs-Thomson equation). In this study, a commercially available multiblock copolymer PolyActive composed of 77 wt % of poly(ethylene glycol terephthalate) and 23 wt % of poly(butylene terephthalate) was dip-coated on top of a multilayer microporous support. The thickness was changed between 0.2 and 8 μm using coating solutions containing 0.75-7.5 wt % PolyActive. The surface temperature of the membrane during dip-coating was monitored using an infrared camera. Single gas permeances of N, H, CH, and CO were measured between 20 and 80 °C at temperature steps of 2 °C. Spherulitic superstructures composed of radially directed lamellae were observed in the polarized light microscope in the prepared membranes. Atomic force microscopy studies showed that the thickness of the crystalline lamellae was in the order of 10 nm or 0.01 μm at the surface of the membrane. Therefore, according to the Gibbs-Thomson equation, the melting point should not change in the thickness range 0.2-8 μm. However, the gas permeance data showed that the melting point of the polyether domains of the 0.2 μm PolyActive layer was 10 °C lower compared to that of the 8 μm layer. The results can be explained by considering that the width of many crystalline lamellae significantly reduces as a function of film thickness, thereby reducing the average fold surface free energy/lateral surface free energy ratio.