Effect of DBPH and vacuum gamma radiation on metallocenic ethylene-1-hexene and ethylene-1-octadecene copolymers

Two different post reactor processes were compared, ⁶⁰Co vacuum gamma irradiation and chemical modification with 2,5-dimethyl-2,5-di(tert-butylperoxy)-hexane (DBPH), on two metallocenic copolymers. These copolymers have a similar molecular weight and crystallinity, but different side chain lengths and concentration of end vinyl groups. The influence of the crosslinking agents on the structure of the samples was studied using gel extraction, size exclusion chromatography (SEC), FTIR spectroscopy and differential scanning calorimetry (DSC). The analysis of the data indicates that crosslinking reactions predominated over scission reactions in all cases. At the same conversion, peroxide modified samples show higher crosslinking levels than irradiated samples. The modified polymers show a complex rheological behavior and an increment in their rheological properties due to crosslinking. FTIR data demonstrated a depletion of vinyl terminal groups with the increment of the absorbed dose and the peroxide concentrations applied. This depletion was more significant in the peroxide crosslinked samples. A mathematical model that accounts for scission and crosslinking reactions fitted well the experimental data.     

Energy dependence of multiplicity in proton-nucleus collisions and models of multiparticle production

This is a continuation of our earlier investigation (Gurtuet al 1974Phys. Lett. 50 B 391) on multiparticle production in proton-nucleus collisions based on an exposure of emulsion stack to 200 GeV/c beam at the NAL. It is found that the ratioR _em = 〈n _s〉/〈n _ch〉, where 〈n _ch〉 is the charged particle multiplicity in pp-collisions, increases slowly from about 1 at 10 GeV/c to 1·6 at 68 GeV/c and attains a constant value of 1·71 ± 0·04 in the region 200 to 8000 GeV/c. Furthermore,R _em = 1·71 implies an effectiveA-dependence ofR _A =A ^0.18,i.e., a very weak dependence. Predictions ofR _em on various models are discussed and compared with the emulsion data. Data seem to favour models of hadron-nucleon collisions in which production of particles takes place through adouble step mechanism,e.g., diffractive excitation, hydrodynamical and energy flux cascade as opposed to models which envisage instantaneous production.     

6α,7α-epoxy-5α,17α,dihydroxy-1-oxo-22R-witha-2,24-dienolide in leaves of Withania somnifera: Isolation and its crystal structure

6α,7α-epoxy-5α,17α,dihydroxy-1-oxo-22R-witha-2,24-dienolide (C₂₈H₃₈O₆) was isolated from Withania somnifera leaves. The structure of the withanolide was established by spectral analysis and X-ray diffraction studies as withanone. The compound crystallizes in the orthorhombic space group P2₁2₁2₁ with unit cell parameters: a = 9.191(10) Å, b = 12.858(6) Å, c = 21.400(16) Å, Z = 4. The crystal structure was solved by direct methods and refined to R = 0.0603 for 1742 observed reflections. There is positional disorder of the H atom in a hydroxy group (O5), resulting in two possible hydrogen-bond linkages. All the rings of the steroid skeleton are trans connected. Ring A exists in a half-chair conformation, ring B is intermediate between a half-chair and a sofa, ring C a distorted chair, and five-membered ring D is intermediate between a half-chair and an envelope. The δ-lactone ring E adopts a sofa conformation. The twist along the length of the steroid nucleus is negligible [C19–C10…C13–C18=1.8°]. Both the hydroxy groups are involved in hydrogen bonding.     

Computerized ultrasonic tomography for testing solid propellant rocket motors

Certain types of defects, such as inhomogeneities in, for example, a plastic-like propellant, are not easily detected by pulse-echo techniques, so a through transmission technique is required. However, the shape of the test object may not permit transmission through the object in any required direction as is normally necessary for computerized ultrasonic tomography.A solid propellant rocket motor is such an object and in the course of this work a scanning technique is employed together with a suitable reconstruction algorithm to obtain a reconstructed image of the cross-section under investigation, overcoming the problem of not being able to transmit pulses through the object along every line normally required. Both a computer simulation and a real system were realized for the test object and images were produced from both. The time of flight of an ultrasonic beam through the specimen is the parameter used in the reconstruction (sound velocity measurement).     

Synthesis and morphology of model PS‐b‐PDMS copolymers

True model linear poly(styrene‐b‐dimethylsiloxane) PS‐b‐PDMS copolymers were synthesized by using sequential addition of monomers and anionic polymerization (high‐vacuum techniques), employing the most recent experimental procedures that allow the controlled polymerization of each monomer to obtain blocks with controlled molar masses. The model diblock copolymers obtained were analyzed by using different techniques, such as size‐exclusion chromatography, ¹H NMR, Fourier transform infrared spectroscopy, small angle X‐rays scattering (SAXS), and wide angle X‐rays scattering (WAXS). The PS‐b‐PDMS copolymers obtained showed narrow molar mass distribution and variable PDMS content, ranging from 2 up to 55 wt %. Compacted powder samples were investigated by SAXS to reveal their structure and morphology changes on thermal treatment in the interval from 30 to 200 °C. The sample with the highest PDMS content exhibits a lamellar morphology, whereas two other samples show hexagonally packed cylinders of PDMS in a PS matrix. For the lowest PDMS content samples, the SAXS pattern corresponds to a disordered morphology and did not show any changes on thermal treatment. Detailed information about the morphology of scattering domains was obtained by fitting the SAXS scattering curves. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3119–3127, 2010     

Separation, identification, and quantification of selected withanolides in plant extracts of Withania somnifera by HPLC-UV(DAD)--positive ion electrospray ionisation-mass spectrometry

This paper describes a method for separation, identification, and quantification of selected withanolides in Withania somnifera plant extracts by HPLC-UV(DAD)-Mass Spectrometry (HPLC-MS). Withaferin-A (WS-3), 12-deoxywithastramonolide (WS-12DS), Withanolide A (WS-1), and Withanone (WS-2) were used as external standards. The compounds were isolated from Withania somnifera by repeated column chromatography of the root extract and their identity was established by 1H- and 13C-NMR and mass spectral data. The compounds were chromatographed on a Merck (250 x 4.6 mm ID, 5 microm) column and analyzed by Electrospray Ionization on a mass spectrometer in Selected Ion Mode (SIM). For quantification, [M + Na]+ ions were monitored. Linear calibration curves were obtained in the concentration range of 1.50 microg/mL to 6.5 microg/mL. The method was applied successfully to the detection and quantification of the said withanolides in a number of samples.     

Influence of amorphous block on the thermal behavior of well-defined block copolymers based on ε-caprolactone

In this work, we studied the thermal characterization of block copolymers based on ε-caprolactone. The copolymers were obtained by anionic polymerization techniques, using different co-monomers such as styrene (S) and dimethylsiloxane (DMS). Synthesized copolymers were characterized by H-nuclear magnetic resonance, size exclusion chromatography, and Fourier transform infrared spectroscopy. Isothermal crystallization was performed by differential scanning calorimetry (DSC), and Avrami’s theory was employed in order to obtain kinetics parameters of interest, such as the half-life for the crystallization process (t _1/2), the bulk crystallization constant (k), and the Avrami’s exponent (n). The spherulitic growth was measured by polarized optical microscopy in order to determine the crystallization behavior. Poly(ε-caprolactone) block (PCL) crystallization was analyzed by considering the physico-chemical characteristics of the neighboring block, PS or PDMS. The chemical nature of the neighbor block in the PCL-based copolymer affects the kinetics parameters of Avrami’s equation, as can be deduced by comparing the values obtained for pure PCL and the studied block copolymers. On the other hand, the apparent thermal degradation activation energies E _ad for PCL and block copolymers were determined by Ozawa’s method. The incorporation of PDMS instead of PS improves the stability of the resulting copolymer, as it was observed by thermogravimetric analysis.     

Molecular weight changes induced in an anionic polydimethylsiloxane by gamma irradiation in vacuum

An anionic almost monodisperse linear polydimethylsiloxane (PDMS) was subjected to gamma irradiation under vacuum at room temperature. The molecular weight changes induced by the radiation process have been investigated using size exclusion chromatography (SEC) with refraction index (RI) and multi angle laser light scattering (MALLS) detectors, to obtain the number and weight average molecular weights of the irradiated samples.The analysis of the data indicates that crosslinking reactions predominated over scission reactions. The results obtained by an SEC-RI have confirmed the presence of small, but measurable amounts of scission.A previously developed mathematical model of the irradiation process that accounts for simultaneous scission and crosslinking and allows for both H- and Y-crosslinks, fitted well the measured molecular weight data. This prediction is in accordance with the experimental data obtained by ²⁹Si-Nuclear Magnetic Resonance spectroscopy (NMR) and previously reported data for commercial linear PDMS (Satti et al., 2008).