Jone M. Ugartemendia

Effects of chain microstructures on mechanical behavior and aging of a poly(L-lactide-co-ε-caprolactone) biomedical thermoplastic-elastomer

Three statistical poly(L-lactide-co-ε-caprolactone) (PLCL) copolymers of 70% L-lactide content having different chain microstructures ranging from moderate blocky to random (R=0.47,0.69 and 0.92, respectively) were characterized by DSC, GPC and (1)H and (13)C NMR. The results demonstrate that higher randomness character (R→1) limits the capability of crystallization of LA-unit sequences shifting the melting temperature of the copolymers to lower values and reducing the crystallinity fraction substantially. The effect of different distributions of sequences of PLCL on crystallization and phase behavior was also studied for different storage times at room temperature (21±2°C) by DSC. The mechanical properties were evaluated by tensile tests during aging. The PLCL showing a random character closest to the Bernoullian distribution of sequences (l(LA)=1/CL) was found to exhibit higher strain capability and strain recovery values and is less prone to supramolecular arrangements. However, as a result of aging, L-lactide sequence blocks in the other PLCLs of smaller randomness character tend to crystallize prompting to a double T(g) behavior indicative of the existence of phase separation into two compositionally different amorphous phases. Physical aging leads also to dramatic changes in tensile behavior of the moderate blocky PLCLs that evolved from being an elastomeric to be partly a glassy semicrystalline thermoplastic, and, thus, can eventually condition its potential uses for medical devices.

Supramolecular structure, phase behavior and thermo-rheological properties of a poly (L-lactide-co-ε-caprolactone) statistical copolymer.

PLAcoCL samples, both unaged, termed PLAcoCLu, and aged over time, PLAcoCLa, were prepared and analyzed to study the phase structure, morphology, and their evolution under non-quiescent conditions. X- ray diffraction, Differential Scanning Calorimetry and Atomic Force Microscopy were complemented with thermo-rheological measurements to reveal that PLAcoCL evolves over time from a single amorphous metastable state to a 3 phase system, made up of two compositionally different amorphous phases and a crystalline phase. The supramolecular arrangements developed during aging lead to a rheological complex behavior in the PLAcoCLa copolymer: Around Tt=131 °C thermo-rheological complexity and a peculiar chain mobility reduction were observed, but at T>Tt the thermo-rheological response of a homogeneous system was recorded. In comparison with the latter, the PLLA/PCL 70:30 physical blend counterpart showed double amorphous phase behavior at all temperatures, supporting the hypothesis that phase separation in the PLAcoCLa copolymer is caused by the crystallization of polylactide segment blocks during aging.

Luminescence Study of Polymer Optical Fibers Doped With Conjugated Polymers

The characterization of the spectral emission and of the photostability of polymer optical fibers (POFs) doped with conjugated polymers has been carried out taking into account the distribution of the dopant in the fiber core. Four different conjugated polymers embedded in the matrix of the typical POF material have been analyzed. Measurements include, among others, evolutions of the emission with excitation wavelength and time, spectral changes for different excitation irradiances, and the influence of the propagation distance along the fiber.