Influence of pressure and crystallization rate on the surface microhardness of high-density polyethylene

The indentation microhardnessMH of high-density polyethylene crystallized at different pressuresp and crystallization rates _c has been investigated. The results confirm thatMH is an increasing function of lamellar thickness and therefore of density. The rate of increase depends on the crystallization conditionsp and _c. Crystallization at high pressure leads toMH values considerably higher than those of samples prepared under atmospheric pressure. The results are discussed in terms of compressed amorphous regions contributing to an elastic release after removal of the indenter. For samples crystallized under different pressures the hardness value turns out to be nearly independent of the elastic modulusE. For samples crystallized at different rates _c there is a steady decrease of microhardness with increasing _c (decrease of lamellar thickness) but no simple relationship withE. With higher rates _c the appearance of strained tie molecules provokes a rapid increase ofE whileMH decreases further because the crystals become smaller.     

Correlation of hardness and microstructure in unoriented lamellar polyethylene

The local deformation of the lamellar microstructure of isothermally melted crystallized unoriented polyethylene has been investigated using microindentation hardness (MH). The polymer can be visualized as a composite material consisting of hard and weak elements. The former, the lamellae, are considered to consist of mosaic blocks with liquid-like lattice distortions (paracrystallites). The latter are the interlamellar amorphous regions and the mosaic block lateral grain boundaries. The deformation mechanisms beneath the indenter are discussed in the light of current models of plastic deformation. MH is shown to depend on the packing density of the macromolecules in both phases and, as a result, it can be clearly correlated with the macroscopic density of the material. The unit cell expansion and lattice distortions increase in parallel as a consequence of increasing incorporation of chain defects within the lattice. This provokes a conspicuous decrease in the microhardness of the crystals. The increase in lattice distortions is consistent with the concurrent decrease of lamellar thickness and, hence, of the coherently diffracting lattice volume. These results unambiguously emphasize the physical significance of the mosaic block character of the lamellae in determining the micromechanical properties of the material. Finally it is shown that the strain boundary which defines the zone of crystal destruction under the indenter also depends on the average volume of the paracrystallites and on the volume fraction of crystalline material.     

Structural changes of injection molded starch during heat treatment in water atmosphere: Simultaneous wide and small‐angle X‐ray scattering study

Native starches with wide varying amylose content were processed by injection molding. The injection‐molded materials were conditioned in water for 20 days and sealed in glass capillaries. Simultaneous wide‐ and small‐angle X‐ray scattering (WAXS and SAXS, respectively) were recorded during thermal heating using a synchrotron source. Crystallinity, SAXS invariant, Q, and long period, L, were measured as a function of heating temperature. The injection‐molding process provokes a destruction of the crystal forms A (cereal starch) and B (tubercle starch) but favors a development of the crystal form V_h. After wet conditioning, WAXS of the injection‐molded samples shows again the appearance of the crystal forms A or B, and crystallinity reaches values similar or larger than those of native starch. A constant heating rate (5°C/min) was particularly used for a comparison of potato and corn starch with a similar amylose content. While the crystallinity associated to forms A and B slowly decreases below 55°C and then rapidly decreases until its disappearance at 85–90°C, the invariant shows a maximum around 40°C and rapidly decreases thereafter. The total nanostructure disappearance occurs at temperatures about 10°C higher for the case of potato starch. In addition, a recovery of the WAXS and SAXS maxima during the subsequent cooling process before reaching room temperature was observed only for potato starch. Analysis of WAXS and SAXS for the rest of the starch materials reveals clear differences in the structural parameters of the samples that cannot be easily explained solely on the basis of the amylose content. Thus, for Cerestar and Roquette, it is noteworthy that there was a continuous decrease of L until its total disappearance as well as the persistence of crystallinity (form B), presumably stabilized by the presence of the V_h structure (12–15%). Real‐time crystallization experiments on two amorphous injection molded samples, waxy maize (free amylose starch) and potato starch, are also discussed. It is shown that the absence of amylose delays the recrystallization of amylopectine during the experiment. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 301–309, 2004     

Effect of the cooling rate in the formation of glass on the α and β relaxations of some amorphous polymers

It has been observed in dynamic mechanical measures that the range of temperatures in which relaxation properties are affected by the diminishing of the free volume has, as a lower limit the temperature at which the maximum of G″ or t_gδ, corresponding to the β relaxation, occurs. In dielectric measures on polymers in which the β relaxation is not predominant over the α relaxation, this temperature range is scarcely affected by changes in frequency, whereas the peak shifts according to its activation energy. This fact leads to the conclusion that the decrease in free volume due to a slow cooling from a temperature above T_g mainly affects mobility of the main chains under T_g, without disturbing the groups that give rise to the secondary relaxations.     

Mechanical and dielectric properties of bulky side chain poly(methacrylates). Analysis of the low frequency phenomena. 1: Poly(5-indanyl methacrylate)

This paper composes the dynamic mechanical and dielectric relaxation properties for Poly (5-indanyl methacrylate) (P51M), a polymer with a bulky side chain. Measurements were carried out from about ?100°C to near 250°C. A secondary loss peak was observed in dielectric measurements at room temperature (20°C at 0.1 Hz.), whereas nothing was resolved in the mechanical spectrum. A prominent α relaxation associated with the glass transition was also observed near 107°C at 0.1 Hz. in the dielectric and the mechanical spectra. Dipolar dielectric loss overlapped with conductivity at high temperatures and low frequencies. A new method to split the conductive, interfacial, and dipolar contributions to the spectrum is proposed.     

Highly sensitive polymer-based cantilever-sensors for DNA detection

We present a technology for the fabrication of cantilever arrays aimed to develop an integrated biosensor microsystem. The fabrication process is based on spin coating of the photosensitive polymer and near-ultraviolet exposure. Arrays of up to 33 microcantilevers are fabricated in the novel polymer material SU-8. The low Young's modulus of the polymer, 40 times lower than that of silicon, enables to improve the sensitivity of the sensor device for target detection. The mechanical properties of SU-8 cantilevers, such as spring constant, resonant frequency and quality factor are characterized as a function of the dimensions and the medium. The devices have been tested for measurement of the adsorption of single stranded DNA and subsequent interstitial adsorption of lateral spacer molecules. We demonstrate that sensitivity is enhanced by a factor of six compared to that of commercial silicon nitride cantilevers.     

No Adverse Programming by Post‐Weaning Dietary Fructose of Body Weight,Adiposity, Glucose Tolerance,or Metabolic Flexibility

1 Scope

Metabolic programming can occur not only in the perinatal period, but also post‐weaning. This study aims to assess whether fructose, in comparison to glucose, in the post‐weaning diet programs body weight, adiposity, glucose tolerance, metabolic flexibility, and health at adult age.

2 Methods and results

Three‐week‐old male and female C57BL6/JRccHsd mice are given an intervention diet with 32 energy percent (en%) glucose or fructose for only 3 weeks. Next, all animals are switched to the same 40 en% high fat diet for 9 weeks. Neither body weight nor adiposity differs significantly between the animals fed with glucose or fructose diets at any point during the study in both sexes. Glucose tolerance in adulthood is not affected by the post‐weaning diet, nor are activity, energy expenditure, and metabolic flexibility, as measured by indirect calorimetry. At the end of the study, only in females fasting serum insulin levels and HOMA‐IR index are lower in post‐weaning fructose versus glucose diet (p = 0.02), without differences in pancreatic β‐cell mass.

3 Conclusions

Our present findings indicate no adverse programming of body weight, adiposity, glucose tolerance, and metabolic flexibility by dietary (solid) fructose in comparison to glucose in the post‐weaning diet in mice.     

Fast AlGaN metal-semiconductor-metal photodetectors grown onSi(111)

Metal-semiconductor-metal photodetectors have been fabricated on AlGaN grown on Si(111) by molecular beam epitaxy for solar-blind applications. A cutoff wavelength of 290 nm and an ultraviolet/visible contrast of more than three orders of magnitude are achieved. Time response measurements show fast exponential decays. With a minimum decay time of 150 ns. The photodetectors present responsivities that increase with bias, reaching 15 mA/W at 4 V bias     

Preparation of GdBa2Cu3O6+x (GdBCO) precursor powder by spray-drying a nitrate solution containing PEG

Several precursor powders, obtained after precipitation from metal nitrate solution containing polyethylene glycol (PEG) (inside a Pyrex glass reactor or by spray-drying), and their thermal evolution to GdBa₂Cu₃O_6+x (GdBCO) were analyzed by thermogravimetry, differential thermal analysis, Fourier-transform infrared spectroscopy, and X-ray diffraction. The amount of PEG had a crucial role in the BaCO₃ content of the “Kjeldahl precursors,” but a minor effect on the degree of transformation to GdBCO at 900°C, which did not reach completion after 1 hour. In contrast, a low-PEG spray-dried powder led to almost 100% GdBCO in only 5 minutes. The high degree of cation dispersion reached by spray-drying and the coexistence with a liquid phase can explain this short reaction time. The spray-dried powder compares favorably with the mechanical mix of metal oxides and Ba carbonate that is commonly used as precursor powder for the synthesis following a solid-state reaction.