This article is the first of a series devoted to various aspects of exchange reactions in molten bisphenol‐A polycarbonate–poly(butylene terephthalate) mixtures. The statistical aspects of exchange reactions between two polycondensates of different chemical structure are considered. This theoretical approach enables us to calculate the mean lengths of the various sequences as well as several expressions for the degree of randomness in four‐component copolycondensates from triad or dyad concentrations as determined by NMR spectroscopy.
Hard X-ray lens-less microscopy raises hopes for a non-invasive quantitative imaging, capable of achieving the extreme resolving power demands of nanoscience. However, a limit imposed by the partial coherence of third generation synchrotron sources restricts the sample size to the micrometer range. Recently, X-ray ptychography has been demonstrated as a solution for arbitrarily extending the fi eld of view without degrading the resolution. Here we show that ptychography, applied in the Bragg geometry, opens new perspectives for crystalline imaging. The spatial dependence of the three-dimensional Bragg peak intensity is mapped and the entire data subsequently inverted with a Bragg-adapted phase retrieval ptychographical algorithm. We report on the image obtained from an extended crystalline sample, nanostructured from a silicon-on-insulator substrate. The possibility to retrieve, without transverse size restriction, the highly resolved three-dimensional density and displacement fi eld will allow for the unprecedented investigation of a wide variety of crystalline materials, ranging from life science to microelectronics.
Coherent diffraction imaging (CDI) is a lens-less microscopy method that extracts the complex-valued exit field from intensity measurements alone. It is of particular importance for microscopy imaging with diffraction set-ups where high quality lenses are not available. The inversion scheme allowing the phase retrieval is based on the use of an iterative algorithm. In this work, we address the question of the choice of the iterative process in the case of data corrupted by photon or electron shot noise. Several noise models are presented and further used within two inversion strategies, the ordered subset and the scaled gradient. Based on analytical and numerical analysis together with Monte-Carlo studies, we show that any physical interpretations drawn from a CDI iterative technique require a detailed understanding of the relationship between the noise model and the used inversion method. We observe that iterative algorithms often assume implicitly a noise model. For low counting rates, each noise model behaves differently. Moreover, the used optimization strategy introduces its own artefacts. Based on this analysis, we develop a hybrid strategy which works efficiently in the absence of an informed initial guess. Our work emphasises issues which should be considered carefully when inverting experimental data.
In this work, the exchange reaction taking place in molten blends of bisphenol-A polycarbonate and polybutylene terephthalate was studied. A direct transesterification mechanism catalyzed by titanium residues, present in commercial PBTP, was deduced. The transesterification reaction can be stopped at various levels by additives capable of complexing the titanium catalyst.This work enhances the possibility of a new approach in macromolecular engineering by directly combining polycondensates in a processing machine such as an extruder.
Exchange reactions in molten bisphenol‐A polycarbonate–poly(butylene terephthalate) mixtures are investigated by means of model reactions. Transesterification can result either from direct ester‐ester interchange or via alcoholysis or acidolysis. Among the various reactions investigated, only the PBTP alcoholysis by phenol is not found to occur. Taking into account that bisphenol‐A terephthalate units (A2B1) and butylene carbonate units (A1B2) are formed in equimolecular amounts, it is concluded that direct ester‐ester interchange is the most likely mechanism for PC ‐PBTP transesterification.
The structure of the four‐component copolyester resulting from the exchange reaction between molten bisphenol‐A polycarbonate and poly(butylene terephthalate) is analyzed as a function of the reaction time by infrared and nuclear magnetic resonance spectroscopy. By applying a statistical method developed earlier, the mean chain length of the various sequences as well as the degree of randomness is computed. The exchange reaction leads initially to the formation of a block copolyester with reduced solubility. As the reaction proceeds, a soluble random copolycondensate is progressively formed.
SynopsisThis paper is devoted to the study of the reactions taking place in molten bisphenol-A polycarbonate-poly(ethy1ene terephthalate) mixtures. The analysis of the reaction products by infrared, proton and nuclear magnetic resonance spectroscopy, and by thermogravimetric analysis. shows that the main reection is an exchange reaction identical to that occurring in bisphenol-A plycarbonate-poly(buty1ene terephthalate) mixtures. However, some other reactions consecutive to this exchange reaction also take place, probably resulting from the instability of the ethylene carbonates produced by transesterification.
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