Simona Gagliardi

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.

An Improved Algorithm for the Fourier Integral of the KWW Function and Its Application to Neutron Scattering and Dielectric Data

The Kohlrausch–Williams–Watts (KWW) function is widely used to describe relaxation behavior in glass‐forming polymers and other condensed systems. The application of this time domain function to frequency domain spectroscopy is, in principle, possible through its Fourier transform. Unfortunately analytical forms for the transform exist only in limited cases, and a number of methods, which circumvent this limitation, are described in the literature. Here we have revisited the problem of evaluating the Fourier integral in the general case, but with the specific aim of producing a fast and accurate algorithm suitable for use within common spreadsheet applications available on personal computers. Two methods were examined, a corrected discrete Fourier transform and, more successfully, a two‐series expansion representation. A previous problem associated with the use of complementary series, that they may not adequately cover the whole range of evaluation of the transform, was overcome by the substitution of a Padé approximant for one of the series. This was found to provide the basis for a robust and effective algorithm whose accuracy was tested against both symbolic mathematical expressions and standard tabulated values. The use of this algorithm in iterative nonlinear least squares curve fitting routines is illustrated by reference to quasi‐elastic neutron scattering data and dielectric relaxation spectra. The former involves a convolution of the real part of the integral with the instrumental resolution function; very satisfactory fits to scattering spectra for poly(vinyl acetate) and poly(isobutylene), at temperatures above their glass transitions, was achieved and the KWW characteristic relaxation parameters established. The dielectric spectrum due to the α‐relaxation of poly(vinyl acetate) is similarly well‐described, here using both the real and the imaginary parts of the integral. The KWW description of the PVAc relaxation is compared with that using the Havriliak–Negami equation and was found to provide an equally acceptable description of the data, and by using one less adjustable parameter.

Short-range order in blends of polycarbonates with polystyrenes

Abstract The local structure of a series of blends of bisphenol-A polycarbonate (BPA-PC) and tetramethyl BPA-PC (TMPC) with various styrene-based polymers (poly(para-chlorostyrene), polystyrene, poly(meta-chlorostyrene) and poly(ortho-chlorostyrene)) has been investigated. Measurements on the D7 diffractometer at ILL were carried out at room temperature. We show that while the amorphous halo of TMPC is only slightly perturbed by blending, major differences are observed in all BPA-PC blends. We conclude that inter-chain correlations in BPA-PC blends are considerably altered compared to the pure component.