In this paper, ring closure click chemistry methods have been used to produce cyclic c-PLLA and c-PDLA of a number average molecular weight close to 10 kg/mol. The effects of stereochemistry of the polymer chains and their topology on their structure, nucleation and crystallization were studied in detail employing Wide Angle X-ray Scattering (WAXS), Small Angle X-ray Scattering (SAXS), Polarized Light Optical Microscopy (PLOM) and standard and advanced Differential Scanning Calorimetry (DSC). The crystal structures of linear and cyclic PLAs are identical to each other and no differences in superstructural morphology could be detected. Cyclic PLA chains are able to nucleate much faster and to produce a higher number of nuclei in comparison to linear analogues, either upon cooling from the melt or upon heating from the glassy state. In the samples prepared in this work, a small fraction of linear or higher molecular weight cycles was detected (according to SEC analyses). The presence of such "impurities" retards spherulitic growth rates of c-PLAs making them nearly the same as those of l-PLAs. On the other hand, the overall crystallization rate determined by DSC was much larger for c-PLAs, as a consequence of the enhanced nucleation that occurs in cyclic chains. The equilibrium melting temperatures of cyclic chains were determined and found to be 5 ºC higher in comparison with values for l-PLAs. This result is a consequence of the lower entropy of cyclic chains in the melt. Self-nucleation studies demonstrated that c-PLAs have a shorter crystalline memory than linear analogues, as a result of their lower entanglement density. Successive selfnucleation and annealing (SSA) experiments reveal the remarkable ability of cyclic molecules to thicken, even to the point of crystallization with extended collapsed ring conformations. In general terms, stereochemistry had less influence on the results obtained in comparison with the dominating effect of chain topology.

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The synthesis of symmetric cyclo poly(ε-caprolactone)-block-poly(l(d)-lactide) (c(PCL-b-PL(D)LA)) by combining ring-opening polymerization of ε-caprolactone and lactides and subsequent click chemistry reaction of the linear precursors containing antagonist functionalities is presented. The two blocks can sequentially crystallize and self-assemble into double crystalline spherulitic superstructures. The cyclic chain topology significantly affects both the nucleation and the crystallization of each constituent, as gathered from a comparison of the behavior of linear precursors and cyclic block copolymers. The stereochemistry of the PLA block does not have a significant effect on the nonisothermal crystallization of both linear and cyclo PCL-b-PDLA and PCL-b-PLLA copolymers.

Thermoplastic resin transfer molding (T-RTM) is attracting much attention due to the need for recyclable alternatives to thermoset materials. In this work, we have prepared polyamide-6 (PA6) and PA6/fiber composites by T-RTM of caprolactam. Glass and carbon fibers were employed in a fixed amount of 60 and 47 wt.%, respectively. Neat PA6 and PA6 matrices (of PA6-GF and PA6-CF) of approximately 200 kg/mol were obtained with conversion ratios exceeding 95%. Both carbon fibers (CF) and glass fibers (GF) were able to nucleate PA6, with efficiencies of 44% and 26%, respectively. The α crystal polymorph of PA6 was present in all samples. The lamellar spacing, lamellar thickness and crystallinity degree did not show significant variations in the samples with or without fibers as result of the slow cooling process applied during T-RTM. The overall isothermal crystallization rate decreased in the order: PA6-CF > PA6-GF > neat PA6, as a consequence of the different nucleation efficiencies. The overall crystallization kinetics data were successfully described by the Avrami equation. The lamellar stack morphology observed by atomic force microscopy (AFM) is consistent with 2D superstructural aggregates (n = 2) for all samples. Finally, the reinforcement effect of fibers was larger than one order of magnitude in the values of elastic modulus and tensile strength.

Employing the hydroxyl groups on the surface of Cloisite® 30B montmorillonite (Cl30B), the ring-opening polymerization of L-lactide was performed with a metal-free catalyst to yield a PLLA-g-Cl30B nanohybrid with low M n grafted PLLA chains (i.e., 9 kg mol −1 ). This nanohybrid was then melt mixed with PLA 4032D from NatureWorks, which is a slow-crystallizing PLA as it contains 2% D-isomers and has a high M n value (i.e., 123 kg mol −1 ). The samples were characterized by TEM, WAXS, SAXS, DSC and Polarized Light Optical Microscopy (PLOM) in order to study their crystallization kinetics in depth. The dispersion of the nanoclay was excellent and much better in the PLA/PLLA-g-Cl30B nanocomposites in comparison to PLA/Cl30B blends prepared as reference. In order to ascertain the role of the nanoclay, analogue PLA/PLLA blends without Cl30B were also prepared. The spherulitic crystallization kinetics from the melt was determined for all samples. The growth rate of neat PLA was accelerated approximately 3 times by incorporating the PLLA-g-Cl30B nanohybrid with an inorganic content of 5%. The overall crystallization kinetics from the glassy state of PLA was also accelerated in a similar way by the nanohybrid addition. Nevertheless, the PLA/ PLLA blends crystallized even faster indicating that the dominant effect that causes the acceleration of the crystallization of PLA is the plasticization of PLA by the low M n PLLA molecules. The changes in T g of PLA also support this explanation. In the case of the PLA/PLLA-g-Cl30B nanocomposites, even though the plasticizing effect of the PLLA chains still dominates, their action is counterbalanced by their tethering on one end, as they are grafted to the surface of the exfoliated clay nanoplatelets.