The interface between polystyrene and poly(methy1 methacrylate) homopolymers was investigated as a function of the concentration of added symmetric diblock copolymer of PS and PMMA, denoted P(S-b-MMA), using neutron reflectivity. It was found that as the number of P(S-b-MMA) chains added to the interface between PS and PMMA was increased, the width of the gradient between the PS and PMMA segments broadened. In particular, the width of the interface between PS and PMMA homopolymers is 50 A and, with the addition of P(S-b-MMA), increases to -85 A. At an amount corresponding to approximately half the thickness of the lamellar microdomain periodicity in the bulk, the interface becomes saturated with the copolymer. Further addition of copolymer chains to the interface results in a marked increase in the off-specular scattering, which is associated with either an ordering of the copolymer chains at the interface or an increase in the curvature of the interface. Studies were also performed on mixtures of homopolymers and copolymers. Here, the interface between the lamellar microdomains of the P(S-b-MMA) is found to increase from 50 A for the pure copolymer to 75 8, as PS and PMMA homopolymer is added to the copolymer. The broadening of the interface seen in both sets of experiments is due to a significant penetration of the homopolymer into the interfacial region. These results are consistent with the reduction in the interfacial tension between the homopolymers with the addition of the diblock copolymer.
Recent studies of molecular orientation in sheared
liquid crystalline polymers have often
yielded contradictory results. To check the self-consistency of
methods for quantitative measurements
of molecular orientation, liquid crystalline solutions of
(hydroxypropyl)cellulose [HPC] and poly(benzyl
glutamate) [PBG] have been studied using flow birefringence, X-ray
scattering, and neutron scattering.
HPC X-ray scattering patterns show an arclike pattern with a
distinct peak as a function of scattering
vector, while PBG patterns show a more diffuse equitorial streak.
These differences are attributed to
more strongly correlated lateral packing in HPC solutions due to their
higher concentration. Measurements of orientation in steady shear flow agree well among the three
techniques. Lyotropic HPC and
PBG solutions differ in orientation at low shear rates. HPC
solutions exhibit near zero orientation at
low rates, while X-ray and neutron scattering measurements confirm
previous birefringence data showing
a low shear rate plateau of moderate orientation in PBG.
Differences with recent neutron scattering
measurements on PBG solutions that show low orientation at low shear
rate are attributed to choice of
solvent, rather than choice of technique. X-ray and optical data
are consistent in showing decreasing
orientation in HPC solutions during relaxation, but discrepancies are
found in relaxation of PBG solutions.
Large increases in flow birefringence suggest substantial
orientation enhancement. X-ray data on one
PBG solution confirm increasing orientation, but X-ray and neutron
scattering data on a more concentrated
solution show only modest changes in orientation. It is suggested
that flow birefringence fails in this
case due to texture coarsening to the point where there is no longer
effective averaging over the distribution
of director orientations along the light path.
of the oscillations in real space is somewhat longer on the trailing edge than on the leading edge. Therefore, interference effects are only observable when the two wave packets are nearly coincident as discussed by Klein, Opat, and Hamilton. 10 To our knowledge, this is the first experiment in which the detailed longitudinal shape of a neutron wave packet has been observed, and the uncertainty relation for neutrons in the longitudinal direction explicitly verified.
Stöber silica particles are used in a diverse range of applications. Despite their widespread industrial and scientific uses, information on the internal structure of the particles is non-trivial to obtain and is not often reported. In this work we have used spin-echo small angle neutron scattering (SESANS) in conjunction with ultra small angle X-ray scattering (USAXS) and pycnometry to study an aqueous dispersion of Stöber particles. Our results are in agreement with models which propose that Stöber particles have a porous core, with a significant fraction of the pores inaccessible to solvent. For samples prepared from the same master sample in a range of H2O : D2O ratio solutions we were able to model the SESANS results for the solution series assuming monodisperse, smooth surfaced spheres of radius 83 nm with an internal open pore volume fraction of 32% and a closed pore fraction of 10%. Our results are consistent with USAXS measurements. The protocol developed and discussed here shows that the SESANS technique is a powerful way to investigate particles much larger than those studied using conventional small angle scattering methods.
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