G.P. Felcher

The Form of the Enriched Surface Layer in Polymer Blends

The concentration profile at the surface in blends of deuterated and protonated polystyrene (d-PS and PS) is inferred from measurements of neutron reflectivity and secondary-ion mass spectrometry, using constraints provided by forward recoil spectrometry and X-ray reflectometry results on the same samples. The surface is enriched in d-PS, the volume fraction and the decay length of which are in good agreement with the predictions of mean-field theory but the form of the profile shows small, but statistically significant, deviations from that predicted by the theory.

Ordering in asymmetric poly (ethylene–propylene)–poly (ethylethylene) diblock copolymer thin films

We have used neutron reflection and phase contrast microscopy to investigate the morphology and surface topology of thin films of nearly symmetric (f=0.55) and asymmetric (f=0.77), poly (ethylene–propylene)–poly (ethylethylene) (PEP–PEE) diblock copolymers (f being the PEP volume fraction) and have identified three important differences in their ordering properties. First, annealed films of the asymmetric diblocks do not form the lamellar microstructure found in symmetric diblocks; their structure can instead be modeled in terms of the hexagonal packing of PEE cylinders observed in bulk small‐angle neutron scattering measurements. However, the cylinders show in‐plane distortions, which we interpret in the context of nonintegral layering. These distortions are amplified at the surfaces where the PEE assumes lamellarlike form. Second, as‐cast films of the asymmetric diblock are characterized by a microstructure lacking long‐range order, pinned between strongly segregated PEE at both surfaces. These films ca...

Interdiffusion of polymers across interfaces.

Neutron Reflection (NR) and Dynamic Secondary Ion Mass Spectroscopy (DSIMS) experiments were conducted on symmetrically deuterated polystyrene triblock bilayers (HDH/DHD) which directly probed the interdiffusion dynamics of the chains during welding. The HDH chains had their centers deuterated 50%, the DHD chains had their ends deuterated (25% at each end) such that each chain contained approximately 50% D. During welding, anisotropic motion of the chains produces a time-dependent oscillation (ripple) in the H and D concentration at the interface, which bears the characteristic signature of the polymer dynamics. These oscillations were compared with those predicted by Rouse, polymer mode coupling (PMC), and reptation dynamics. The following conclusions can be made from this study. (a) During the interdiffusion of high molecular weight HDH/DHD pairs, higher mobility of the chain ends caused a concentration oscillation which increased to a maximum amplitude, and eventually vanished at times, t > τD. The amplitude, or excess enrichment found, was appreciably more than that predicted by Rouse and PMC simulations, and was only slightly less than that predicted from reptation simulations. (b) The oscillations were completely missing in the 30 and 50K HDH/DHD polymers, which are only weakly entangled. The lack of oscillations for the 30 and 50K pairs may be due to a combination of surface roughness and fluctuations of order 30 A. (c) It was found that the position of the maximum in this ripple stayed at the interface during its growth. This is also consistent with reptation and has not been explained by other theories. (d) All dynamics models for linear polymers produce ripples, many of which are qualitatively similar to that predicted for reptation. However, each ripple bears the fingerprint of the dynamics in terms of its time-dependent shape, position, and magnitude, and the models are clearly distinguishable. Our results, in summary, support reptation as a candidate mechanism of interdiffusion at polymer(SINGLEBOND) polymer interfaces and its uniqueness is being further pursued.

Surface enrichment in polymer blends — A neutron reflection test

Abstract In polymer melts of protonated and deuterated polystyrene (PS and d-PS) surface segregation of d-PS occurs at temperatures and compositions in the one phase region close to the coexistence curve for phase separation. Neutron reflection is capable of obtaining detailed information on surface segregation, as corroborated by a test on a polymer blend containing 10% volume fraction of d-PS where a thermal treatment caused a surface enrichment of d-PS up to 28%.

An automated neutron reflectometer (POSY II) at the intense pulsed neutron source

Abstract A brief description is given of the time-of-flight neutron reflectometer POSY II at the Intense Pulsed Neutron Source (IPNS) at Argonne National Laboratory. Both data collection and refinement of the experimental data are nearly automatic. The neutron reflectivity is measured as a function of the neutron momentum perpendicular to the surface over a range from 0 to 0.1 A -1 . The instrument is capable of measuring reflectivities as low as 10 -6 . The neutron reflectivity is used to determine the composition profile of the layers close to the surface of the sample, with a depth resolution of ∽10 A.

Interdiffusion at the interface of polymeric bilayers: evidence for reptation?

Abstract Neutron reflection is used to study the interdiffusion in equal molecular weight polystyrene bilayer melts with a spatial resolution of 10 A. Interfacial widths and concentration profiles at the bilayer interface are obtained for annealing times up to and beyond the reptation time, τ d . For t τ d , the reptation model predicts a mean square displacement of monomers whose time dependence is a power law, the exponent of which changes with time. For the relatively lightweight polymers of M ∼ 233 000, the mean square displacements of monomers is in general agreement with predictions; the permanence of a discontinuity at the interface — which also follows from the reptation model — is observed only for molecular weights M ∼ 1 000 000.

Of butterflies and terraces

Abstract Ten years ago at a modest spallation-neutron source at Argonne National Laboratory, a “gizmo” was installed which was later christened a reflectometer. The popularity of the “gizmo” spread like wildfire: now virtually all neutron sources possess at least one of them. Some are graced by splendid names: CRISP and SURF at Rutherford, TOREMA at Jiilich, DESIR in Saclay and EVA in Grenoble. With the exception of SPEAR of Los Alamos, that touch is not followed in the United States; the beautiful data of Majkrzak and Satija at NIST (and Larry Passel1 at Brookhaven) come from “beam hole #8” or something of that sort. Why such a popularity? Because its function, the determination of the depth profile in thin films and at surfaces, turns out to be useful in a number of scientific fields. The notion itself is easy to understand. On top of that, the instrument is relatively cheap and friendly to the users. Perhaps the time has come to make an assessment of the success of reflectometry. Did this come out to b...

A neutron reflection test of the ‘reptation time’

Two chemically similar polymers in contact at a temperature above the glass transition interdiffuse with a concentration profile which, for times shorter than a reptation time, is thought to remain discontinuous at the interface. It is shown how the size of the concentration gap can be systematically tested by neutron reflection from bilayers of deuterated/protonated polymers. 6 refs., 2 figs.

Closed-form expressions for neutron and X-ray reflection and transmission coefficients of a one-dimensional profile

Abstract As the first part of an effort to systematically study the inversion problem in X-ray and neutron reflectivity experiments, closed-form expressions are derived for the reflection and transmission coefficients as functionals of the sample profile. The assumption used is that the reflection is mainly due to the first and the second order derivatives of the profile and thus the third and higher order derivatives are negligible. One of the two major characteristics of the formulas is that the reflection and transmission coefficients are explicitly expressed in terms of the profile; the other is that the formulas are valid over the entire range of momentum transfer Q. This procedure allows the straightforward calculation of the real space profile using the reflectivity data as the computer input, with an accuracy that still remains to be evaluated through both analytical and numerical analyses with the aid of model profiles.

Magnetic field penetration into the high temperature superconductor YBa2Cu3O7−x

The penetration depth of a magnetic field into a superconducting YBa2Cu3O7−x film was measured by polarized neutron reflection. The sample comprised an epitaxial film with the c-axis of its orthorhombic structure perpendicular to the films surface. Measurements at 14 K showed that a magnetic field (parallel to the surface) penetrates into the surface over a depth of 1400 A.