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The critical fluctuations of a polymer blend (PVME/d-PS) were investigated by neutron small-angle scattering. A mean-field behavior was observed, except in a region very close to the critical temperature where a transition to an Ising-type behavior occurs. The width of this region is T -T C -2A K for a mixture with average molecular weights of 89000 (PVME) and 232000 (d-PS).PACS numbers: 64.60. Fr, 05.70.Jk, 61.12.Ex, Polymer blends are supposed to behave in a mean-field manner as long as the number of chain segments is sufficiently large. l ' 2 However, a transition to non-meanfield (Ising) critical behavior is expected close to the critical temperature T c , in a region whose width is estimated on the basis of the Ginzburg-Landau relation 1 ' 2 ; this leads towhere X=H mix /kT is the Flory-Huggins interaction parameter, //mix is the enthalpy of mixing, and N\,N 2 are the numbers of statistical segments for the components 1 and 2, respectively. A typical segment consists of about ten monomers. 2 X c refers to the critical temperature T c .The system under consideration has a compensation temperature r c0 mp where X passes through zero. 3 Consequently, we write near T comp , which is close to T c , In the case of our investigation, the right-hand side of (3) is about 0.1 K.However, recent Monte Carlo studies 4 indicate a nonmean-field behavior for a chain length as high as 32 statistical segments. From these calculations it can be stated that experiments on polymer blends of high molecular weight should show a mean-field to non-mean-field crossover "without need of unrealistic small values of T c -rr 4The first investigation of critical scattering of neutrons on the system polyvinylmethylether (PVME)-deutero polystyrene (d-PS) by Herkt-Maetzky and Schelten 5 and by Yang et at. 6 clearly showed a mean-field behavior, in particular the critical exponent 7=1. Our paper deals with recent experimental work on the same system in the critical region, in order to identify a crossover as expected by theory.The scattering function for the critical fluctuations according to the mean-field random-phase approximation 1 can be written as S(Q)-l =S?(Q)-{ +SHQ)~l-2X(Q),where 5*?,2 * s the Debye function of a single chain multiplied by (f>V ((p is the composition and V the molecular volume, 1 and 2 correspond to the two components). An expansion of (4) to fourth order of the scattering vector Q leads toS(0) _I , the reciprocal cross section in the forward direction, is related to the reduced temperature e = (T c -T)/T c by S(0)-l oc^ (6) with y = l. This form of the scattering function is not affected by polydispersity, i.e., by a distribution of the molecular weights. 7,8 In the case of Ising behavior, we may approximately maintain the validity of Eq. (6) with 7=1.26. This exponent holds for ordinary liquid mixtures (e.g., see Schwahn, Belkoura, and Woermann 9 ). The scattering function Eq. (4) holds for the homogeneous region of the phase diagram. On crossing of the binodal, thus entering the metastable region, a heterogeneous ...

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Highlights Significant differences in characterization of starches were observed among them. Imilla negra starch exhibited the highest apparent amylose and phosphorous content. Imilla negra starch presented higher resistant starch content in cooked starch. Loćka starch had the lowest apparent amylose content and higher crystallinity.

The system PVME/d-PS has been investigated by neutron small angle scattering. By extrapolating the critical scattering, the spinodal was determined and used to calculate the Flory–Huggins parameter χ(φ,T) in the frame of mean field theory. Separating the Gibbs potential of mixing ΔG in a Flory–Huggins term χ(φ,T) and a configurational term for the entropy ΔS, it turns out that χ is independent of the molecular weight, and of the weight distribution, i.e., it is a local or ‘‘segmental’’ quantity. The critical scattering was carefully investigated at the critical concentration. At temperatures Tc−T>2.4 K(Tc=141.9 °C), a mean field behavior of the susceptibility S(Q=0) was clearly observed, with a critical exponent γ=1. However, in a region of Tc−2.4 K≤T<Tc, a crossover occurs to a critical exponent γ≂1.26; it resembles the Ising behavior which is well established for normal liquid mixtures. The function S(Q) also describes the characteristic segmental length σ. Both σ and the range parameter of χ, r0, are independent of molecular weight and weight distribution within experimental errors. The binodal, i.e., the line between the homogeneous and the two-phase region was identified by a change of critical scattering caused by the precipitated domains. The spinodal approaches the binodal near Tc in a characteristic way as predicted for polydisperse systems.

Within porous Prussian blue to copper the highest H2 storage capacity is observed. Such behavior finds explanation in the crystal structure for Cu3[M(CN)6]2 with M = Fe, Co, Ir. The crystal structure of Prussian blue analogues is usually solved and refined with a cubic unit cell in the Fm3̅m space group, which corresponds to a random vacancy distribution. However, a careful evaluation of X-ray diffraction powder patterns of copper-containing compositions reveals a deviation from that structural model. The crystal structure for the considered series of copper hexacyanometallates(III) was found to be also cubic but in the Pm3̅m space group related to a nonrandom vacancy distribution. To this model 50% of vacancies for the building block, [M(CN)6], corresponds, which is quite different from the value of 33.3% in the Fm3̅m structural model. Mössbauer spectra and high pressure H2 adsorption isotherms support the assignment of the Pm3̅m space group for the studied series of copper Prussian blue analogues. The implications of a nonrandom vacancy distribution on the physical properties of these materials are discussed.

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Storage of maize products such as tortillas may cause starch retrogradation and lead to resistant starch (RS) formation. This study was carried out to determine if storage of maize tortillas under refrigerated conditions enhanced RS content and/or modified RS structure. Improved Costeño variety maize grain was nixtamalized and processed into tortillas which were stored for five and ten days at 5°C. Total resistant starch (TRS) and retrograded resistant starch (RRS or RS3) contents were determined on raw and nixtamalized maize grain and tortillas stored for zero, five and ten days. Differential scanning calorimetry (DSC), X‐ray diffraction (XRD) and near‐infrared (NIR) spectroscopy were use to evaluate structural changes in retrograded resistant starch isolated from each sample type. Total starch content was 67 ± 1.5% for all samples, TRS ranged from 3.3% in the raw grain at 7.2% in tortillas stored for ten days, while RRS starch content ranged from 0% in the raw grain to 3.2% in tortillas stored for ten days. DSC showed endothermic transitions corresponding to amylopectin and amylose retrogradation, at 31.9 and 139.7°C in RRS from tortillas stored for five days, and at 47.9 and 146°C in RRS from tortillas stored for ten days. These values agreed with the higher total RS content recorded after prolonged storage. XRD revealed a starch crystallinity of 13.7% in tortillas stored for five days and 15.3% in those stored for ten days. NIR spectroscopy analysis showed evidence of structural changes in polymeric order that were more pronounced in RRS of tortillas stored for ten days, due to increase in crystalline region.