Optically pure polylactides, poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA), were blended across the range of compositions with poly(-caprolactone) (PCL) to study their crystallization, morphology, and mechanical behavior. Differential scanning calorimetry and dynamic mechanical analysis (DMA) of the PLA/PCL blends showed two T g s at positions close to the pure components revealing phase separation. However, a shift in the tan ␦ peak position by DMA from 64 to 57°C suggests a partial solubility of PCL in the PLA-rich phase. Scanning electron microscopy reveals phase separation and a transition in the phase morphology from spherical to interconnected domains as the equimolar blend approaches from the outermost compositions. The spherulitic growth of both PLA and PCL in the blends was followed by polarized optical microscopy at 140 and 37°C. From tensile tests at speed of 50 mm/min Young's modulus values between 5.2 and 0.4 GPa, strength values between 56 and 12 MPa, and strain at break values between 1 and 400% were obtained varying the blend composition. The viscoelastic properties (E and tan ␦) obtained at frequency of 1 Hz by DMA are discussed and are found consistent with composition, phase separation, and crystallization behavior of the blends. POLYM. ENG. SCI., 46:1299 -1308, 2006.

The spectrum of PLLA is analyzed in order to investigate its crystallinity and crystalline morphologies. The carbonyl and ester bands of PLLA have been analyzed, and individual components have been successfully assigned. Nucleation always proceeds through curved lamellar crystals, this crystalline morphology being exclusive for low crystallization temperatures. At higher crystallization temperatures, a transition from curved crystals to flat lozenge-shaped lamellae is observed. Curved crystals with edge-on orientation and flat crystals with flat-on orientation affect the intensity of spectral bands. The total crystallinity has been obtained from a skeletal band at 955 cm -1 . In addition, intensity changes observed in the CdO stretching region during crystallization provide a simple procedure to obtain the relative population of the two crystalline morphologies. As crystallization temperature increases, the relative population of curved edge-on crystals is observed to decrease, but their population remains important even at the higher crystallization temperatures. The CdO stretching region shows a complex profile that can be fully explained assuming intramolecular through bond coupling and factor group splitting. The latter is also affected by crystalline perfection; hence, the observed crystalline components strongly depend on the crystallization temperature. In the CdO stretching region, perfectly flat crystals give two narrow components at 1767 and 1758 cm -1 . Curved crystals obtained at low crystallization temperatures give a broader band located at 1760 cm -1 attributed to factor group splitting averaged over the different curvatures shown by this crystalline morphology. This contribution is expected to depend on crystallization temperature according to theoretical considerations (larger nuclei sizes). DSC melting shows a shoulder at lower temperature attributed to the presence of the less stable edge-on crystalline morphology. Finally, the ester C-O stretching region also shows factor group splitting in both the perpendicular (split ∼ 10 cm -1 ) and parallel (split ∼ 18 cm -1 ) components.

This paper reports the analysis of the C=O stretching region of poly(L-lactide). This spectral band splits into up to four components, a phenomenon that a priori can be explained in terms of carbonyl-carbonyl coupling or specific interactions (such as C-H...O hydrogen bonding or dipole-dipole). Hydrogen bonding can be discarded from the analysis of the C-H stretching spectral region. In addition, low molecular weight dicarbonyl compounds of chemical structure similar to that of PLLA, such as diacyl peroxides, show a remarkable splitting of the carbonyl band attributed to intramolecular carbonyl-carbonyl coupling. Several mechanisms can be responsible for this behavior, such as mechanical coupling, electronic effects, or through-space intramolecular TDC (transition dipole coupling) interactions. Intermolecular dipole-dipole interactions (possible in the form of interchain TDC interactions) are proven to be of minor relevance taking into account the spatial structure of the PLLA conformers. The Simply Coupled Oscilator (SCO) model, which only accounts for mechanical coupling, has been found to predict adequately the relative intensity of the symmetric and asymmetric bands of dicarbonyl compounds. The dispersion curves predicted for PLLA by the SCO model also match those given by more general treatments, such as Miyazawa's first-order perturbation theory. Hence, the SCO model is adopted here as an adequate yet simple tool for the interpretation of band splitting caused by intramolecular coupling of polylactide. The four components observed in the C=O stretching band of semicrystalline PLLA are attributed to the four possible conformers: gt, gg, tt, and tg. The narrow bands observed for the interlamellar material are attributed to highly ordered chains, indicating the absence of a truly amorphous phase in the crystalline polymer. The interphase seems to extend over the whole interlamellar region, showing the features of a semiordered metastable phase. In amorphous PLLA, bands corresponding to gt, gg, and tt conformers also can be resolved by second derivative techniques, and curve-fitting results provide information about the conformational population at different temperatures.

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.

Bioresorbable polylactides are one of the most important materials for tissue engineering applications. In this work we have prepared scaffolds based on the two optically pure stereoisomers: poly(L: -lactide) (PLLA) and poly(D: -lactide) (PDLA). The crystalline structure and morphology were evaluated by DSC, AFM and X-ray diffraction. PLLA and PDLA crystallized in the α form and the equimolar PLLA/PDLA blend, crystallized in the stereocomplex form, were analyzed by a proliferation assay in contact with mouse L-929 and human fibroblasts and neonatal keratinocytes for in vitro cytotoxicity evaluation. SEM analysis was conducted to determine the cell morphology, spreading and adhesion when in contact with the different polymer surfaces. The preserved proliferation rate showed in MTT tests and the high colonization on the surface of polylactides observed by SEM denote that PLLA, PDLA and the equimolar PLLA/PDLA are useful biodegradable materials in which the crystalline characteristics can be tuned for specific biomedical applications.