Effect of nanoclay on relaxation of poly(vinylidene fluoride) nanocomposites

The effect of Lucentite™ STN nanoclay on the relaxation behavior of poly(vinylidene fluoride) (PVDF) nanocomposites was investigated using dielectric relaxation spectroscopy (DRS) and wide- and small-angle X-ray scattering. Lucentite™ STN is a synthetic nanoclay based on hectorite structure containing an organic modifier between the hectorite layers. The addition of this nanoclay to PVDF results in preferential formation of the beta-crystallographic phase. When the STN content increased to 5% and 10%, only the beta-phase was observed. Bragg long period and lamellar thickness both decrease with STN addition. The relaxation rates for processes termed α_a (glass transition, related to polymer chain motions in the amorphous regions) and α_c (related to polymer chain motions in the crystalline regions and fold surfaces) can be described either with the Vogel-Fulcher-Tamman equation or with Arrhenius behavior, respectively. DRS shows that the α_a relaxation rate increases with the concentration of STN because of the reduction of intermolecular correlations between the polymer chains, caused by the presence of layered silicate nanoclay particles, which serve to segregate polymer chains in the amorphous regions. Comparing samples with beta-crystal phase dominant, the relaxation rate for the α_c relaxation also increases with concentration of STN in all nanocomposite samples. Dielectric properties at low frequencies are dominated by the dc conductivity, and as more STN is added, the conductivity increases rapidly. The addition of 10% STN makes the dc conductivity increase by almost four decades when compared with neat PVDF. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2520–2532, 2009     

Structure elucidation of antiproliferative bisbenzylisoquinoline alkaloids from Anisocycla grandidieri from the Madagascar dry forest

Antiproliferative bioassay‐guided fractionation of the ethanol extract of the stems of Anisocycla grandidieri led to the isolation of the known alkaloids stebisimine (1), (+)‐1,2‐dehydrotelobine (2), (+)‐2'‐norcocsuline (3) and puetogaline B (4). Herein, we report the full NMR assignments of all compounds and the X‐ray crystallography of single crystals of compounds 1 and 3. Compounds 2 and 3 showed moderate antiproliferative activity against the A2780 human ovarian cancer cell line with IC₅₀ values of 4.1 ± 0.3 and 2.7 ± 0.3 μM, respectively, and they also displayed selective activity toward the H460 (large cell lung cancer), MCF‐7 (breast ductal carcinoma), and UACC‐257 (melanoma) cell lines. Copyright © 2013 John Wiley & Sons, Ltd.     

Mechanisms of silk fibroin sol-gel transitions

Silk fibroin sol-gel transitions were studied by monitoring the process under various physicochemical conditions with optical spectroscopy at 550 nm. The secondary structural change of the fibroin from a disordered state in solution to a beta-sheet-rich conformation in the gel state was assessed by FTIR and CD over a range of fibroin concentrations, temperatures, and pH values. The structural changes were correlated to the degree of gelation based on changes in optical density at 550 nm. No detectable changes in the protein secondary structure (FTIR, CD) were found up to about 15% gelation (at 550 nm), indicating that these early stages of gelation are not accompanied by the formation of beta-sheets. Above 15%, the fraction of beta-sheet linearly increased with the degree of gelation. A pH dependency of gelation time was found with correlation to the predominant acidic side chains in the silk. Electrostatic interactions were related to the rate of gelation above neutral pH. The overall independencies of processing parameters including concentration, temperature, and pH on gel formation and protein structure can be related to primary sequence-specific features in the molecular organization of the fibroin protein. These findings clarify aspects of the self-assembly of this unique family of proteins as a route to gain control of material properties, as well as for new insight into the design of synthetic silk-biomimetic polymers with predictable solution and assembly properties.     

Thermal analysis and X‐ray scattering study of metallocene isotactic polypropylene prepared by partial melting

In this study, we examine the effects of heating, nucleation, cooling, and reheating on the thermal properties and structure of metallocene isotactic polypropylene (m‐iPP) that had been prepared initially in a standard state containing nearly equal amounts of the crystallographic α and γ phases. Heat treatment was achieved through partial melting and annealing by the heating of samples to self‐nucleation temperatures (T_n's) that spanned and exceeded the entire range of melting of the standard state, from 122 to 160 °C. The relative amounts of α and γ crystals are determined from the area under the unique wide‐angle X‐ray reflections. The lower and upper endotherms are caused by the melting of γ and α crystals, respectively. Four distinct regions of T_n were identified on the basis of the thermal and structural parameters of m‐iPP. In region I, T_n is below the peak melting temperature of the γ phase. Here, γ crystals are annealed and α crystals are barely affected by T_n. In region II, T_n is above the peak of the lower endotherm but below the peak of the upper endotherm. γ crystals melt, and α crystals anneal. In both regions I and II, the portion of the sample melted at T_n recrystallizes epitaxially with existing parent α lamellae as the substrates, and the amount of α always exceeds the amount of γ. In region III, T_n is above the peak of the upper endotherm, and all γ crystals and some or all α crystals are melted at T_n. The number of α‐crystal nuclei steadily decreases as T_n increases, causing systematic depression of the crystallization and melting temperatures seen during cooling. Finally, in region IV, T_n exceeds the upper endotherm, and only small self‐nuclei or heterogeneous nuclei remain. Recrystallization is now suppressed to lower temperatures. For regions III and IV, a crossover behavior in the relative amounts of α and γ is observed during cooling from T_n. Because of the effective nucleating ability of α toward γ, as the temperature drops, the amount of γ increases and then exceeds the amount of α. With subsequent reheating, the reverse crossover occurs because of the lower melting point of γ. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1644–1660, 2002     

In vivo boron-11 MRI and MRS using (B24H22S2)4- in the rat.

In vivo boron-11 magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) were performed on a rat that had been infused with a potential boron neutron capture therapy agent, Na₄B₂₄H₂₂S₂, using methods for detecting nuclei with a short T₂ relaxation time. MRI and MRS were also performed on a euthanized rat that had been similarly infused in vivo. Boron-11 spectral intensities decreased in the living rat over a 25-h period. The results demonstrate the capability of MRI and MRS to noninvasively monitor the distribution and excretion of boron agents in vivo.     

Silk Fibroin Degradation Related to Rheological and Mechanical Properties

Regenerated silk fibroin has been proposed as a material substrate for biomedical, optical, and electronic applications. Preparation of the silk fibroin solution requires extraction (degumming) to remove contaminants, but results in the degradation of the fibroin protein. Here, a mechanism of fibroin degradation is proposed and the molecular weight and polydispersity is characterized as a function of extraction time. Rheological analysis reveals significant changes in the viscosity of samples while mechanical characterization of cast and drawn films shows increased moduli, extensibility, and strength upon drawing. Fifteen minutes extraction time results in degraded fibroin that generates the strongest films. Structural analysis by wide angle X‐ray scattering (WAXS) and Fourier transform infrared spectroscopy (FTIR) indicates molecular alignment in the drawn films and shows that the drawing process converts amorphous films into the crystalline, β‐sheet, secondary structure. Most interesting, by using selected extraction times, films with near‐native crystallinity, alignment, and molecular weight can be achieved; yet maximal mechanical properties for the films from regenerated silk fibroin solutions are found with solutions subjected to some degree of degradation. These results suggest that the regenerated solutions and the film casting and drawing processes introduce more complexity than native spinning processes.