Fiber average size and distribution dependence on the electrospinning parameters of poly(vinylidene fluoride trifluoroethylene)) membranes for biomedical applications

Poly(vinylidene fluoride-trifluoethylene) electrospun membranes were obtained from a blend of dimethylformamide (DMF) and methylethylketone (MEK) solvents. The inclusion of the MEK to the solvent system promotes a faster solvent evaporation allowing complete polymer crystallization during the jet travelling between the tip and the grounded collector. Several processing parameters were systematically changed to study their influence on fiber dimensions. Applied voltage and inner needle diameter do not have large influence on the electrospun fiber average diameter but in the fiber diameter distribution. On the other hand, the increase of the distance between the needle tip to collector results in fibers with larger average diameter. Independently on the processing conditions, all mats are produced in the electroactive phase of the polymer. Further, MC-3T3-E1cell adhesion was not inhibited by the fiber mats preparation, indicating their potential use for biomedical applications.

Influence of electrospinning parameters on Poly(hydroxybutyrate) electrospun membranes fiber size and distribution

Poly(hydroxybutyrate) (PHB) obtained from sugar cane was dissolved in a blend of chloroform and dimethylformamide (DMF) and electrospun at 40 ºC. By adding DMF to the solution, the electrospinning process for the PHB polymer becomes more stable, allowing complete polymer crystallization during the jet travelling between the tip and the grounded collector. The influence of processing parameters on fiber size and distribution was systematically studied. It was observed that an increase of tip inner diameter promotes a decrease of the fiber average size and a broader distribution. On the other hand, an increase of the electric field and flow rate produces an increase of fiber diameter until a maximum of ~2.0 m, but for electric fields higher than 1.5 kV.cm-1, a decrease of the fiber diameter was observed. Polymer crystalline phase seems to be independent of the processing conditions and a crystallinity degree of 53 % was found. Moreover, thermal degradation of the as-spun membrane occurs in single step degradation with activation energy of 91 kJ/mol. Furthermore, MC-3T3-E1 cell adhesion was not inhibited by the fiber mats preparation, indicating their potential use for biomedical applications.

The determinants of international equity and bond investment

The purpose of this research is fourfold. First, to investigate whether the determinants of international equity investment differ between investors with different degrees of information, experience and sophistication. For this purpose, the determinants of international equity investment of institutional and noninstitutional investors from 20 OECD countries, in the period 2001-2009, were analysed and compared. The results show that there are significant differences in the determinants of international equity investment between institutional and noninstitutional investors. Particularly, noninstitutional investors exhibit a more pronounced preference for equities of geographical nearby, contiguous and more transparent countries than institutional investors, suggesting that the effect of information costs and familiarity on international equity investment is stronger for less informed, experienced and sophisticated investors. Moreover, the preference for more developed equity markets and the contrarian behaviour are more severe for noninstitutional investors. Hence, the heterogeneity of institutional and noninstitutional investors in international equity investment is not negligible and therefore should be taken into account. Second, to investigate whether the determinants of international bond investment differ between investors with different degrees of information, experience and sophistication. For this purpose, the determinants of international bond investment of institutional and noninstitutional investors from 20 OECD countries, in the period 2001-2009, were analysed and compared. The results show that there are few significant differences in the determinants of international bond investment between institutional and noninstitutional investors. Particularly, the preference for bonds of more transparent countries and the return chasing behaviour are more pronounced for noninstitutional investors, whereas the preference for bonds with lower risk diversification potential is more pronounced for institutional investors. Hence, not only the results for international bond investment do not allow to support (or reject) the argument that information costs and familiarity are more important for less informed, experienced and sophisticated investors, but also they are contrary to the idea that financial variables, namely return and risk diversification, are more important for more informed, experienced and sophisticated investors. Third, to investigate whether the determinants of international equity investment differ from the determinants of international bond investment. For this purpose, the determinants of both international equity and bond investment of institutional and noninstitutional investors from 20 OECD countries, in the period 2001-2009, were analysed and compared. The results show that, although the effect of information costs on international equity investment tends to be stronger than on international bond investment, the differences between assets are not usually statistically significant, especially when the influence of financial variables is taken into account. Hence, it is not possible to conclude that international equity investment is much more information intensive than international bond investment, as suggested by Gehrig (1993) and Portes, Rey and Oh (2001), among others. Fourth, to investigate whether the flight to quality phenomenon is also observable in international investment and whether the flight to quality phenomenon is more pronounced for more sophisticated than for less sophisticated investors. For this purpose, a two-factor and three-factor ANOVA models, respectively, were applied to the international equity and bond investment of institutional and noninstitutional investors from 20 OECD countries in the period 2001-2009. The results suggest that the flight to quality phenomenon is also observable in international investment, as a change from business cycle of expansion to recession causes investors to significantly decrease the average weight invested in more risky assets (equities) and increase the average weight invested in less risky assets (bonds). The results also show that the variation on the average weight assigned to each type of asset, due to changes in business cycles, is significantly stronger for institutional investors than for noninstitutional investors, thereby suggesting that the flight to quality phenomenon is more pronounced for more sophisticated than for less sophisticated investors.