We have explored the use of chain end segregation as a means of controlling the properties of a polymer surface. Thin film blends of homopolystyrene (PS) and PS synthesized with low-energy oligotetrafluoroethylene chain ends (PS-TFE) were studied using neutron reflectivity. The fraction of PS-TFE that localizes near the surface was found to increase as a function of its concentration in the blend. Contact angle measurements indicate corresponding reductions in the surface tension due to the surface localization of the TFE chain ends. For a 10% blend of 6000 mol wt PS-TFE in 3 × 105 mol wt PS, the surface coverage of fluorocarbon ends was found to be >20%. A free energy model of the blends gives good qualitative agreement with the experimental results.

The strengthening of the interface between polystyrene (PS) and poly(methyl methacrylate) (PMMA) using a random copolymer P(S f -r-MMA1 - f ), where f is the fraction of styrene in the copolymer, was investigated. The maximal fracture toughness, measured by crack propagation, was found when f = 0.68. Neutron reflectivity measurements showed that this value of f corresponded to the point where the interfacial broadening on the PS and PMMA sides of the interface was symmetric. The symmetry of broadening and the optimization of the toughness at f = 0.68 are attributable to a composition-dependent segmental interaction parameter.

The added EC term in CVSP was determined to be adequate for both open and IMRT fields. Due to the dependence of calculation accuracy on (1) EC modeling, (2) internal convolution and density grid sizes, (3) implementation details in the algorithm, and (4) the accuracy of measurements used for treatment planning system commissioning, the authors recommend an evaluation of the accuracy of near-surface dose calculations as a part of treatment planning commissioning.

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The purpose of this study was to investigate whether helical tomotherapy would better dose-limit growing vertebral ring apophyses during craniospinal radiation as compared to conventional techniques. Four pediatric patients with M0 medulloblastoma received tomotherapy craniospinal radiation (23.4 Gy, 1.8 Gy/fx) by continuous helical delivery of 6 MV photons. Weekly blood counts were monitored. For comparison, conventional craniospinal radiation plans were generated. To assist in tomotherapy planning, a cross-sectional growth study of 52 children and young adults was completed to evaluate spine growth and maturation. Vertebral ring apophyses first fused along the posterolateral body-pedicle synostosis, proceeding circumferentially toward the anterior vertebral body such that the cervical and lumbar vertebrae fused early and mid-thoracic vertebrae fused late. For the four pediatric patients, tomotherapy resulted between 2% and 14% vertebral volume exceeding 23 Gy.Conventional craniospinal radiation predicted between 33% and 44% exceeding 23 Gy. Cumulative body radiation doses exceeding 4 Gy were between 50% and 57% for tomotherapy and between 25% and 37% for conventional craniospinal radiation. Tomotherapy radiation reduced neutrophil, platelet, and erythrocyte hemoglobin levels during treatment. Tomotherapy provides improved dose avoidance to growing vertebrae as compared to conventional craniospinal radiation. However, the long-term effects of tomotherapy dose avoidance on spine growth and large volume low dose radiation in children are not yet known.

Neutron specular reflectivity data obtained with a new Grazing Angle Neutron Spectrometer (CANS) from a N1C/Ti-multilayer sample, were analyzed and modelled for reconstructing the scattering length density profile as a periodic step potential for the layered material. There is some ambiguity in the results due to the uniqueness problem with missing phase information. For more complex layered materials, there is often insufficient knowledge about the layers to use modelling reconstruction without phase information. In the second part, we present a method in which this problem is solved for diffraction data from lipid multilayers: due to changes in chemistry (isomorphous heavy atom method), the phases are determined directly, and therefore the density profile of the lipid bilayer can be uniquely determined.

No abstract

The distribution of a random copolymer P(A f -r-B1-f ) at the interface between immiscible A and B phases as a function of the copolymer composition, denoted by the fraction f of A monomers, and as a function of the monomer−monomer interactions, or χ values, has been studied via lattice Monte Carlo simulations. A symmetric distribution of the copolymer was found for compositionally symmetric chains (f = 0.5) when the χ values were independent of the copolymer composition. In contrast, an asymmetric distribution of the copolymer was found for a compositionally asymmetric copolymer (f = 2/3) with composition-independent χ values. It was found that a symmetric distribution of the compositionally asymmetric copolymer can result only when the χ values are allowed to depend upon the copolymer composition. Such a symmetric distribution of a compositionally asymmetric copolymer is in qualitative agreement with miscibility, neutron reflectivity, and adhesion studies of polystyrene/poly(methyl methacrylate) systems.