Dislocation-controlled perforated layer phase in a PEO- b-PS diblock copolymer.

Small angle x-ray analyses show that the shear-induced hexagonal perforated layer phase in a poly(ethylene oxide)- b-polystyrene diblock copolymer consists of trigonal (R3;m) twins and a hexagonal (P6(3)/mmc) structure, with trigonal twins being majority components. Transmission electron microscopy reveals that the hexagonal structure is generated through sequential intrinsic stacking faults on the second layer from a previous edge dislocation line, while the trigonal twins are formed by successive intrinsic stacking faults on neighboring layers due to the plastic deformation under mechanical shear.

Guidelines of the Commission of the European Communities: a challenge for the control of packaging.

Questions arising from the Commission of the European Communities Directives and guidelines regulating packaging materials are discussed in relation to whether compliance ensures safety in use and the consequent analytical problems. Difficulties may arise from interactions between food contact materials and food involving mass transfer (migration, off-odours, 'scalping', loss of aroma) or mass transfer and chemical interactions and the implications for safety assurance and regulation are addressed. The criteria for suitable low molecular weight fatty food simulants and conditions for migration testing are presented. In food surveillance, the usefulness of various methods of analysis differs for monomers and for additives. For monomers, IR spectroscopy can identify the polymer type and which specific monomers need controlling; for unknown mixtures of additives, preliminary functional group identification by techniques such as 1H-NMR is useful.

Why do polyethylene crystals have sectors?

High-resolution (3.7 A in optical diffraction) electron microscope images have been obtained from a series of n-paraffin monolamellar crystals with chain lengths from n-C36H74 to n-C82H166. The higher molecular weight specimens, which do not undergo chain folding, form sectorized crystals and the molecular packing is found to include alternate bands of untilted and tilted chains along <130>. Their widths are consistent with those of Bragg fringe widths in bright-field images obtained at lower magnification. The chain tilt axis is near d*110. Lower molecular weight paraffins form nonsectorized crystals where the chains are generally untilted with occasional small inclinations around nonspecific axes. Surface decoration of the longer alkanes with polyethylene crystallites, first of all, reveals three preferred polyethylene crystal rod orientations ([100] plus two perpendicular to [110]) instead of the two commonly found for the lower alkane. Control studies on solid-solution crystals reveal that the third [100] orientation is a result of slight surface roughness due to unequal chain lengths or surface protrusions of chains; the new decoration is also randomly distributed. For pure n-C60H122 lamellae, however, suggestions of regular bands containing rods along [100], due to surface discontinuities along <130>, can also be seen. In contrast with polyethylene, these data suggest that crystal sectorization may be a function of chain-stem packing alone and that chain folds may play merely a secondary role in the polymer--e.g., by directing the collapse of pyramidal crystals on a flat surface.

The pre-melt phase of n-alkanes: Crystallographic evidence for a kinked chain structure.

Electron diffraction measurements on epitaxially grown crystals of orthorhombic n-hexatriacontane, n-C(36)H(74), give evidence for the chain-defect mechanism for linear chain melting. The derived structural model is also in accord with recent spectroscopic studies of odd-chain n-alkanes, and the diffraction data specifically exclude models based on helices or rigid chain rotors. Stability of a kinked chain structure, moreover, is indicated by an observed hysteresis effect that gives different pretransition temperatures for solution-grown and annealed crystals.