The
effects of the crystallinity and polymorphism of PLLA and PDLA–PLLA
50:50 blends on the free volume and transport properties have been
studied. In the case of PDLA–PLLA 50:50 the increase on crystallinity
promotes a process of densification and a reduction in free volume
fraction and rigid amorphous fraction (RAF), contrary to PLLA in which
the enlargement of a rigid amorphous fraction led to a dedensification.
This result offers a unique opportunity to analyze separately the
opposite influence in the transport properties of the crystallization
and free volume associated with RAF. Overall, these findings provide
a better understanding on the relationship between crystallinity and
transport properties and would explain also the controversial data
reported in the literature.
Positron annihilation lifetime spectroscopy (PALS) was conducted to follow the evolution of free volume during crystallization of PLLA at 100°C. A finite lifetime distribution of three components was used to fit the PALS spectra. The analysis of the longest lifetime component (τ 3 ) indicates that the free volume distribution evolves during crystallization by increasing the number of holes yet decreasing their size; moreover, the free volume fraction increased during crystallization. Following the evolution of the shortest components a correlation was found with crystalline and amorphous phase contents present in PLLA. The first component (τ 1 ) was assigned to positron annihilation in occupied zones of the crystalline phase whereas the second component (τ 2 ) was attributed to annihilation by different amorphous arrangements. A model for the supramolecular arrangement of PLLA chains was devised in terms of free volume enlargement for annealed samples. According to this model transformations occur in mobile amorphous phase (MAP) and rigid amorphous phase (RAP) with PLLA chains evolving from folded or coil conformations in the asquenched samples containing uniquely MAP to opener (more extended) conformations in samples containing larger RAP and crystalline fractions. The proposed model provides a rational for the understanding of some unexpected effects associated with free volume that have been observed in several semicrystalline polymer systems, i.e., the lowering of density during crystallization (dedensification), the acceleration of polymer chains dynamics around the T g (dynamic fragility) due to a rigid amorphous phase confined by crystallites, and also the gas permeability behavior in terms of solubility and diffusion coefficients.
MBPIP seems to be less effective than FFP in the treatment of TTP. It is therefore prudent to avoid MBPIP until therapeutic equivalency to FFP has been established by randomized, controlled trials.
A 51-year-old woman came to our clinic 6 months after a right total hip arthroplasty. She had noticed a slowly growing mass in the proximal thigh and referred progressive pain in the right groin. Plain radiography revealed premature acetabular cup aseptic loosening, and in the computed tomography study, a 14-cm-diameter mass was observed. Revision surgery was performed, showing a metal-on-ceramic bearing surface. The histologic analysis of surrounding tissues was reported as massive metallosis. Although occasionally chosen for primary or revision hip arthroplasty, there is little information available about the in vivo wear behavior of this combination. This important fact should be taken into account before considering such a surface alternative.
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