Valeria Arrighi

The glass transition and interfacial layer in styrene-butadiene rubber containing silica nanofiller

The mechanical behaviour of a styrene-butadiene rubber, containing 23.5% styrene and filled with up to 55% by weight with silica nanoparticles, has been examined. The spectra of these composites show a second relaxation of relatively low intensity located about 40 °C above the main α relaxation, the glass transition Tg, which occurs at −34.5 °C in the unfilled polymer. This is attributed to an interfacial layer of polymer molecules whose chain relaxation dynamics have been altered by interaction with the filler surface. This is accompanied by a shift of the Tg itself to lower temperature when the filler surface is organophilic, but not when the filler surface is hydrophilic, and possible mechanisms for this are discussed. The amount of interfacial polymer can be quantified by resolving the glass transition and the second relaxation using a curve-fitting approach. A first-order treatment is presented which estimates the layer thickness to be of the order of a few nanometers.

Quasielastic neutron scattering characterization of the relaxation processes in a room temperature ionic liquid

We report the first quasielastic neutron scattering measurements on a room temperature ionic liquid: 1-n-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6]. Data were collected using a medium resolution spectrometer as a function of temperature in the range 250–320 K. The data unequivocally indicate the existence of two different relaxation processes: a fast, localized motion occurring in the subpicosecond range and a slower process spanning the subnanosecond regime. These results provide experimental support to recently published molecular dynamics simulations. Evidence for slower, unresolved dynamics (under the present experimental conditions) is also obtained. Both temperature and momentum transfer dependence of the intermediate incoherent dynamic structure factor were investigated, after Fourier transformation into the temporal domain. The fast process shows no appreciable Q- and T-dependence. On the other hand the slow process shows evidence of a complex, non-Debye and non-Arrhenius behavior.

Temperature Dependence of the Primary Relaxation in 1-Hexyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}imide

We present results from complementary characterizations of the primary relaxation rate of a room temperature ionic liquid (RTIL), 1-hexyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl} imide, [C6mim][Tf2N], over a wide temperature range. This extensive data set is successfully merged with existing literature data for conductivity, viscosity, and NMR diffusion coefficients thus providing, for the case of RTILs, a unique description of the primary process relaxation map over more than 12 decades in relaxation rate and between 185 and 430 K. This unique data set allows a detailed characterization of the VTF parameters for the primary process, that are: B=890 K, T0=155.2 K, leading to a fragility index m=71, corresponding to an intermediate fragility. For the first time neutron spin echo data from a fully deuteriated sample of RTIL at the two main interference peaks, Q=0.76 and 1.4 A(-1) are presented. At high temperature (T>250 K), the collective structural relaxation rate follows the viscosity behavior; however at lower temperatures it deviates from the viscosity behavior, indicating the existence of a faster process.

Restricted dynamics in polymer-filler systems

We have investigated the dynamic properties of poly(dimethylsiloxane) (PDMS) and poly(vinyl acetate) (PVAc) filled with hydrophilic Aerosil (nano-particles of fumed silica) using the backscattering spectrometer IRIS at ISIS. Through the analysis of the incoherent scattering, the quasi-elastic neutron scattering (QENS) technique probes the motion of the hydrogen atoms and it therefore provides detailed information on the reduced mobility of chain segments in polymer filler systems. QENS measurements reveal a progressive slowing down of the motion with increasing particle surface area which is in agreement with the DMTA data of Tsagaropoulos and Eisenberg. In particular, the QENS spectra of the polymer-filler composites confirmed the existence of two dynamic processes. Therefore, data analysis was carried out by taking into account two contributions: (a) a quasielastic component due to chains not affected by the presence of fillers and (b) an elastic component from chains whose dynamics is strongly restricted. The latter depends on the specific surface area of the particles and their weight fraction in the composites.

On the difference in scattering behavior of cyclic and linear polymers in bulk

It has been suggested that, due to topological constraints, rings in the melt may assume a more compact shape than Gaussian chains. In this paper, we exploit the availability of narrow fractions of perdeuterated linear and cyclic polydimethylsiloxane (PDMS) and, through the analysis of the small angle neutron scattering (SANS) profiles, demonstrate the difference in scattering properties of linear and cyclic PDMS molecules. As expected for Gaussian chains, for the H/D linear PDMS samples, log-log plots of the scattered intensity versus scattering vector Q display a Q((-2)) dependence. However, for H/D cyclic blends, the scaling exponent is higher than 2, as predicted by computer simulations reported in the literature. We show that cyclic molecules in bulk display the characteristic maximum in plots of scattered intensity versus Q((-2)) that is expected on the basis of Monte Carlo calculations and from the Casassa equation [E. F. Casassa, J. Polym. Sci. A 3, 605 (1965)]. It is also shown that, for rings, the Debye equation [P. Debye, J. Appl. Phys. 15, 338 (1944)] is no longer appropriate to describe the SANS profiles of H/D cyclic blends, at least up to M(w) approximately 10 000. For these samples, the Casassa form factor gives a better representation of the SANS data and we show that this function which was developed for monodisperse cyclics is still adequate to describe our slightly polydisperse samples. Deviations from all above observations are noted for M(w)>11 000 and are attributed to partial contamination of cyclic samples with linear chains. The failure of both the Debye and the Casassa form factors could be due to contamination of the cyclic fractions by linear polymers or to a real conformational change.

Dynamic heterogeneity in polymer electrolytes. Comparison between QENS data and MD simulations

Abstract We have investigated the dynamics of poly(ethylene oxide) (PEO) lithium-based salt electrolytes (PEO–LiBETI) using quasi-elastic neutron scattering (QENS). Measurements were carried out on the spectrometer NEAT (HMI, Berlin) above the melting temperature of PEO ( T m ≈65°C). The experimental data fully support the Molecular Dynamics (MD)-derived model of a heterogeneous dynamics in dilute PEO-salt electrolytes. In agreement with MD simulations carried out on PEO–LiPF 6 , we find evidences for the existence of two dynamic processes: (a) a faster process that is described in terms of the pure PEO dynamics and (b) a second component which we identify with the slower motion of the PEO chains involved in PEO–Li + complexes. At this stage, due to the differences in PEO molecular weight and the anion chemical nature, the agreement with MD simulations is qualitative.

Effect of tacticity on the local dynamics of polypropylene melts

We present a quasielastic neutron scattering (QENS) study of the effect of tacticity on the local dynamics of polypropylene (PP). QENS measurements were carried out on different spectrometers. On IN10 (ILL, France) we have measured the decrease of the elastic intensity as a function of temperature for atactic (a-PP), isotactic (i-PP), and syndiotactic (s-PP) PP. The results show that the polyproylene sub-Tg dynamics is independent of tacticity. Measurements of the dynamic incoherent structure factor were carried out on the spectrometers IRIS and OSIRIS (ISIS, UK) and, after Fourier transform, the intermediate scattering functions were computed and analyzed. For all samples investigated, the intermediate scattering functions I(Q,t) show good overlap using shift factors that are close to those reported in the literature. Detailed analysis of the incoherent dynamic structure factor in terms of fast and slow decay processes indicates that in the subpicoseconds regime molecular motion is independent of tactici...

BODIPY-based conjugated microporous polymers as reusable heterogeneous photosensitisers in a photochemical flow reactor

BODIPY-based conjugated microporous polymers (BDP_CMP and PHTT_BDP) have been synthesised via two distinct methods of assembly: high-yielding Suzuki–Miyaura cross-coupling of BODIPY-containing building blocks, and post-synthetic conversion of an aldehyde-equipped CMP host that was synthesised in the absence of metal-based catalysts. Both approaches yielded BODIPY-based materials featuring a high BET surface area (484–769 m2 g−1) and a bathochromic shift in the maximum light absorbance (520–550 nm). Singlet oxygen production employing the BODIPY-based materials at 530 nm was carried out heterogeneously in a commercial photochemical flow reactor.

Miscibility of polymer blends of poly(styrene-co-4-hydroxystyrene) with bisphenol-A polycarbonate

The miscibility of blends of bisphenol-A polycarbonate (BAPC) and tetramethyl bisphenol-A polycarbonate (TMPC) with copolymers of poly(styrene-co-4-hydroxystyrene) (PSHS) was studied in this work. It has been demonstrated that BAPC is miscible with PSHS over a region of approximately 45–75 mol % hydroxyl groups in the copolymer. TMPC has a wider miscible window than BAPC when blended with PSHS. The blend miscibility was considered to be driven by the intermolecular attractive interactions between the hydroxyl groups of the PSHS and the π electrons of the aromatic rings of both polycarbonates (PCs). As the FTIR measurements showed, after blending of BAPC with PSHS, there is no visible shift of the carbonyl band of BAPC at 1774 cm−1, whereas the stretching frequency of the free hydroxyl groups of the copoly- mers at 3523 cm−1 disappeared. The large positive values of the segment interaction energy density parameter Bst-HS calculated from the group contribution approach indicated that the intramolecular repulsive interaction may also have played a role in the promotion of the blend miscibility.

Phase behaviour and orientational order of a main-chain nematic polyester : a combined SANS and NMR study

A main-chain semiflexible copolyester, containing 4,4′-dihydroxy-α,α′-dimethylbenzalazine as a mesogenic unit was studied by SANS and solid state NMR. SANS scattering patterns from mixtures of labelled and unlabelled LCP indicated the presence of an additional coherent signal superimposed on the single chain scattering function. The scattering from the pure polymers (hydrogenous, partially deuterated and fully deuterated LCP) was investigated to assess the origin of this extra signal. The results suggest that the disclination structure associated with a domain structure, typical of liquid crystalline systems, may be observable by SANS. In addition NMR experiments excluded the possibility of the existence of a biphasic region over a wide temperature range. NMR spectra evidenced a pure nematic phase for measurements performed on heating above the solid to nematic phase transition. The orientational order of the mesogenic units was investigated by proton and deuterium NMR. The order parameters were found to be very high: at the nematic to solid transition Szz was estimated to be 0.965 and it was approaching 0.67 at the nematic to isotropic phase transition.