Six polylactides, polymerized with Salen−Al−OCH3 initiator and having optical purities between 43% and 100%, were analyzed by differential scanning calorimetry, X-ray diffraction, and optical microscopy, following various crystallization conditions. It was found that each of those polylactides can crystallize, even those with low optical purities; their crystallization rate is, however, slower than those for high optical purity polyesters. Moreover, the low optical purity polymers tend to form stereocomplexes between the l and d sequences of the same polylactide, which behavior is ascribed to their multiblock microstructure. A correlation was found between the measured melting temperature of optically active polylactides and their average sequence length.

Monodisperse stereocomplex block copolymer micelles were obtained through the self-assembly of equimolar mixtures of poly(ethylene glycol)-block-poly(l-lactide) and poly(ethylene glycol)-block-poly(d-lactide) in water. These micelles possessed partially crystallized cores and mean hydrodynamic diameters ranging from 31 to 56 nm, depending on the lactide content. They exhibited kinetic stability and redispersion properties superior to micelles prepared with isotactic or racemic polymers alone. This study demonstrates the advantages of stereocomplex formation in the design of stabilized water-soluble nanoparticles.

Morphologies, growth rates, and melting of isothermally crystallized ultrathin (200-1 nm) poly( -caprolactone) (PCL) films have been investigated in real time by atomic force microscopy. The flat-on orientation of the lamellar crystals relative to the substrate was determined by electron diffraction. The truncated lozenge-shaped PCL crystals observed at low undercooling become distorted for films of thicknesses equal to or thinner than the lamellar thickness, which depends on the crystallization temperature but not on the initial film thickness. The melting behavior of distorted crystals differs from that of undistorted ones, and their growth is slower and nonlinear. The crystal growth rate decreases greatly with the film thickness. All these observations are discussed in terms of the diffusion of the polymer chains from the melt to the crystal growth front.

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The polarization dependences of several Raman bands of roll-drawn high-density polyethylene of uniaxial symmetry were investigated at different draw ratios between 7.0 and 11.7. When the right-angle scattering geometry was used, it was possible to determine quantitatively the principal components of the Raman tensors of the 1080-, 1130-, and 1170-cm-' vibrations. The orientation coefficients (P*(cos e)) and (P4(cos e)) of the C-C bonds with trans conformation and gauche defects were also calculated by using these bands. The results obtained from the 1080-cm-' band show that the orientation of gauche structures does not vary much with the draw ratio (between 7.0 and 11.7). On the other hand, the polarization behavior of the 1130-cm-' band indicates that trans conformers in the amorphous phase orient more readily in the draw direction. When the components of the Raman tensors obtained from right-angle scattering were used, it was found that backscattering measurements could yield good estimates of (P&os e) ) and (P~(cos e)) orientation averages. Finally, a method is proposed for the rapid evaluation of these parameters from calibration curves obtained from backscattering spectra measured with the polarization direction of both the incident and scattered light parallel to the draw direction. This method should be particularly useful for the determination of the distribution of the orientation of trans bonds in thick processed samples of polyethylene. IntroductionIn order to take full advantage of the intrinsic stiffness

The crystallization of poly(D-lactide) (PDLA) and poly(L-lactide) (PLLA) in ultrathin films (15 nm) has been followed between 125 and 160 °C using in situ atomic force microscopy. Using a forced nucleation technique, edge-on lamellae were observed, showing a curvature which is related to the polymer chirality. In the case of PLLA, the lamellae are S-shaped, contrary to the PDLA lamellae which are Z-shaped. This behavior was also observed on TEM pictures of PLLA and PDLA films crystallized in the same conditions, without any external nucleation. As shown by electron diffraction patterns, the crystalline unit cells of the two enantiomers are identical. For the first time, a relationship has been established between the molecular chirality of poly(lactide)s and their macroscopic behavior. Moreover, the direction of curvature of the lamellae can be linked with the sense of twisting of the poly(lactide) lamellae in banded spherulites, and the temperature dependence of the radius of curvature can be correlated with the distance between the extinction rings. Those observations are consistent with Keith and Padden's model since the curved crystals in ultrathin films can be considered as "half-lamellae", which give, when associated together, twisted complete lamellae.

The ring-opening polymerization and copolymerization of e-caprolactone, e-methyl-e-caprolactone, ß, -methyl-e-caprolactone (mixed isomers), and L-lactide using aluminum isopropoxide as initiator were investigated. The chain propagation proceeds through a living anionic type coordinated insertion mechanism. The kinetic features of this process are reported. The experimental monomer reactivity ratios indicate that e-caprolactone and its methyl derivatives yield random copolyesters. However, the e-caprolactone/L-lactide pair exhibits a departure from randomness with the preferred incorporation of L-lactide units. The thermal properties of the copolymers were investigated by differential scanning calorimetry. It was found that the crystallization of e-caprolactone units is limited, in all cases, to copolymers which are rich in this sort of unit. At the same time, the crystallization of L-lactide units was observed in copolymers with high concentrations of this comonomer.Analysis of the melting point depression data of the copolymers indicates that the L-lactide units are almost completely rejected from the caprolactone crystals, whereas about 50% of the e-methyl-ecaprolactone and , -methyl-e-caprolactone comonomer units are incorporated into the e-caprolactone crystals due to an obvious structural similarity. Finally, poly(c-caprolactone-co-e-methyl-e-caprolactone) samples are miscible with poly (vinyl chloride) (PVC) whatever the composition of the copolymer and the composition of the blend, whereas poly(e-caprolactone-co-L-lactide) samples are miscible with PVC uniquely for copolymer L-lactide contents equal to or smaller than 40 wt %. In all cases where miscibility was found, a negative thermodynamic interaction parameter was computed.

Recent literature has shown that the thickness of dip-coated films has a V-shaped dependence on dip-coating rate when very slow rates are included. For supramolecular block copolymer films, small molecule (SM) uptake and film morphology are also rate-dependent, as shown previously for a poly(styrene-b-4-vinylpyridine) (PS−P4VP) block copolymer in THF solutions containing naphthol (NOH) and naphthoic acid (NCOOH). Here, these investigations are extended to p-dioxane, toluene, and CHCl 3 solutions. The V-shaped thickness dependence is validated for each solvent, but with the V minimum displaced to lower dip-coating rates and thicknesses for the solvents with lower vapor pressures (p-dioxane, toluene), thereby decreasing the dip-coating rate range of the "capillarity regime" (slow side of the V) and consequently extending that of the "draining regime" (fast side of the V). The SM/VP uptake ratio varies with the nature of the solvent, particularly in the capillarity regime, where it is higher for solvents that are weak SM-VP hydrogen-bond competitors (toluene, CHCl 3 ). The draining regime generally shows greater SM uptake than the capillarity regime, in some cases reaching the solution ratio, with higher uptake observed for the SM with greater hydrogen-bond strength (NCOOH > NOH). The variation in film morphology with solvent and dip-coating rate (spherical for toluene; spherical and cylindrical for p-dioxane; spherical, cylindrical, and lamellar for THF; and lamellar only for CHCl 3 ; for a block copolymer whose equilibrium morphology in the bulk is near the cylindrical/lamellar phase boundary) depends on the initial solution state (whether micellar or not and hardness of micelles) and the SM uptake ratio. These factors, along with solvent evaporation rate and film thickness, influence the kinetics of morphology development in the drying films, the point at which the kinetics are frozen in, the effective block ratio, and the orientation of the morphological structures.