Crystallization and Stereocomplexation of PLA-mb-PBS Multi-Block Copolymers.

The crystallization and morphology of PLA-mb-PBS copolymers and their corresponding stereocomplexes were studied. The effect of flexible blocks (i.e., polybutylene succinate, PBS) on the crystallization of the copolymers and stereocomplex formation were investigated using polarized light optical microscopy (PLOM), differential scanning calorimetry (DSC), infrared spectroscopy (FTIR), and carbon-13 nuclear magnetic resonance spectroscopy (13C-NMR). The PLA and PBS multiple blocks were miscible in the melt and in the glassy state. When the PLA-mb-PBS copolymers are cooled from the melt, the PLA component crystallizes first creating superstructures, such as spherulites or axialites, which constitute a template within which the PBS component has to crystallize when the sample is further cooled down. The Avrami theory was able to fit the overall crystallization kinetics of both semi-crystalline components, and the n values for both blocks in all the samples had a correspondence with the superstructural morphology observed by PLOM. Solution mixtures of PLLA-mb-PBS and PLDA-mb-PBS copolymers were prepared, as well as copolymer/homopolymer blends with the aim to study the stereocomplexation of PLLA and PDLA chain segments. A lower amount of stereocomplex formation was observed in copolymer mixtures as compared to neat L100/D100 stereocomplexes. The results show that PBS chain segments perturb the formation of stereocomplexes and this perturbation increases with the amount of PBS in the samples. However, when relatively low amounts of PBS in the copolymer blends are present, the rate of stereocomplex formation is enhanced. This effect dissappears when higher amounts of PBS are present. The stereocomplexation was confirmed by FTIR and solid state 13C-NMR analyses.

Increased Surface Roughness in Polydimethylsiloxane Films by Physical and Chemical Methods.

Two methods, the first physical and the other chemical, were investigated to modify the surface roughness of polydimethylsiloxane (PDMS) films. The physical method consisted of dispersing multi-walled carbon nanotubes (MWCNTs) and magnetic cobalt ferrites (CoFe2O4) prior to thermal cross-linking, and curing the composite system in the presence of a uniform magnetic field H. The chemical method was based on exposing the films to bromine vapours and then UV-irradiating. The characterizing techniques included scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), Fourier transform infrared (FTIR) spectroscopy, optical microscopy, atomic force microscopy (AFM) and magnetic force microscopy (MFM). The surface roughness was quantitatively analyzed by AFM. In the physical method, the random dispersion of MWCNTs (1% w/w) and magnetic nanoparticles (2% w/w) generated a roughness increase of about 200% (with respect to PDMS films without any treatment), but that change was 400% for films cured in the presence of H perpendicular to the surface. SEM, AFM and MFM showed that the magnetic particles always remained attached to the carbon nanotubes, and the effect on the roughness was interpreted as being due to a rupture of dispersion randomness and a possible induction of structuring in the direction of H. In the chemical method, the increase in roughness was even greater (1000%). Wells were generated with surface areas that were close to 100 µm² and depths of up to 500 nm. The observations of AFM images and FTIR spectra were in agreement with the hypothesis of etching by Br radicals generated by UV on the polymer chains. Both methods induced important changes in the surface roughness (the chemical method generated the greatest changes due to the formation of surface wells), which are of great importance in superficial technological processes.

Euechinoidea and Cidaroidea respond differently to ocean acidification.

The impact of the chemical changes in the ocean waters due to the increasing atmospheric CO2 depends on the ability of an organism to control extracellular pH. Among sea urchins, this seems specific to the Euechinoidea, sea urchins except Cidaroidea. However, Cidaroidea survived two ocean acidification periods: the Permian-Trias and the Cretaceous-Tertiary crises. We investigated the response of these two sea urchin groups to reduced seawater pH with the tropical cidaroid Eucidaris tribuloides, the sympatric euechinoid Tripneustes ventricosus and the temperate euechinoid Paracentrotus lividus. Both euechinoid showed a compensation of the coelomic fluid pH due to increased buffer capacity. This was linked to an increased concentration of DIC in the coelomic fluid and thus of bicarbonate ions (most probably originating from the surrounding seawater as isotopic signature of the carbon - δ¹³C - was similar). On the other hand, the cidaroid showed no changes within the coelomic fluid. Moreover, the δ¹³C of the coelomic fluid did not match that of the seawater and was not significantly different between the urchins from the different treatments. Feeding rate was not affected in any species. While euechinoids are able to regulate their extracellular acid-base balance, many questions are still unanswered on the costs of this capacity. On the contrary, cidaroids do not seem affected by a reduced seawater pH. Further investigations need to be undertaken to cover more species and physiological and metabolic parameters in order to determine if energy trade-offs occur and how this mechanism of compensation is distributed among sea urchins.

Evaluation of cell affinity on poly(L-lactide) and poly(epsilon-caprolactone) blends and on PLLA-b-PCL diblock copolymer surfaces.

An evaluation of cell proliferation and adhesion on biocompatible film supports was performed. A series of films were compression molded from commercially available poly (L-lactide), PLLA, and poly(epsilon-caprolactone), PCL, and from their melt mixed blends (PLLA/PCL blends). These were compared with compression molded films of PLLA-b-PCL model diblock copolymers. The samples were analyzed by differential scanning calorimetry (DSC), contact angle measurements, and scanning force microscopy (SFM). Cell adhesion and proliferation were performed with monkey derived fibroblasts (VERO) and with osteoblastic cells obtained either enzymatically or from explants cultures of Sprague-Dawley rat calvaria. Migration studies were performed with bone explants of the same origin. The results obtained indicate that although all materials tested were suitable for the support of cellular growth, a PLLA-b-PCL diblock copolymer sample with 93% PLLA was significantly more efficient. This sample exhibited a unique surface morphology with long range ordered domains (of the order of 2-3 mum) of edge-on PLLA lamellae that can promote "cell contact guidance." The influence of other factors such as chemical composition, degree of crystallinity, and surface roughness did not play a major role in determining cell preference toward a specific surface for the materials employed in this work.

Self-nucleation and crystallization kinetics of double crystalline poly(p-dioxanone)-b-poly(epsilon-caprolactone) diblock copolymers.

The crystallization kinetics of each constituent of poly(p-dioxanone)-b-poly(epsilon-caprolactone) diblock copolymers (PPDX-b-PCL) has been determined in a wide composition range by differential scanning calorimetry and compared to that of the equivalent homopolymers. Spherulitic growth rates were also measured by polarized optical microscopy while atomic force microscopy was employed to reveal the morphology of one selected diblock copolymer. It was found that crystallization drives structure formation and both components form lamellae within mixed spherulitic superstructures. The overall isothermal crystallization kinetics of the PPDX block at high temperatures, where the PCL is molten, was determined by accelerating the kinetics through a previous self-nucleation procedure. The application of the Lauritzen and Hoffman theory to overall growth rate data yielded successful results for PPDX and the diblock copolymers. The theory was applied to isothermal overall crystallization of previously self-nucleated PPDX (where growth should be the dominant factor if self-nucleation was effective) and the energetic parameters obtained were perfectly matched with those obtained from spherulitic growth rate data of neat PPDX. A quantitative estimate of the increase in the energy barrier for crystallization of the PPDX block, caused by the covalently bonded molten PCL as compared to homo-PPDX, was thus determined. This energy increase can dramatically reduce the crystallization rate of the PPDX block as compared to homo-PPDX. In the case of the PCL block, both the crystallization kinetics and the self-nucleation results indicate that the PPDX is able to nucleate the PCL within the copolymers and heterogeneous nucleation is always present regardless of composition. Finally, preliminary results on hydrolytic degradation showed that the presence of relatively small amounts of PCL within PPDX-b-PCL copolymers substantially retards hydrolytic degradation of the material in comparison to homo-PPDX. This increased resistance to hydrolysis is a complex function of composition and its knowledge may allow future prediction of the lifetime of the material for biomedical applications.