Agurtzane Mugica

Self-Nucleation of Crystalline Phases Within Homopolymers, Polymer Blends, Copolymers, and Nanocomposites

Self-nucleation (SN) is a special nucleation process triggered by self-seeds or self-nuclei that are generated in a given polymeric material by specific thermal protocols or by inducing chain orientation in the molten or partially molten state. SN increases the nucleation density of polymers by several orders of magnitude, producing significant modifications to their morphology and overall crystallization kinetics. In fact, SN can be used as a tool for investigating the overall isothermal crystallization kinetics of slow-crystallizing materials by accelerating the primary nucleation stage in a previous SN step. Additionally, SN can facilitate the formation of one particular crystalline phase in polymorphic materials. The SN behavior of a given polymer is influenced by its molecular weight, molecular topology, and chemical structure, among other intrinsic and extrinsic characteristics. This review paper focuses on the applications of DSC-based SN techniques to study the nucleation, crystallization, and morphology of different types of polymers, blends, copolymers, and nanocomposites.

Sequential crystallization and morphology of triple crystalline biodegradable PEO-b-PCL-b-PLLA triblock terpolymers

The sequential crystallization of poly(ethylene oxide)-b-poly(e-caprolactone)-b-poly(L-lactide) (PEO-b-PCL-b-PLLA) triblock terpolymers, in which the three blocks are able to crystallize separately and sequentially from the melt, is presented. Two terpolymers with identical PEO and PCL block lengths and two different PLLA block lengths were prepared, thus the effect of increasing PLLA content on the crystallization behavior and morphology was evaluated. Wide angle X-ray scattering (WAXS) experiments performed on cooling from the melt confirmed the triple crystalline nature of these terpolymers and revealed that they crystallize in sequence: the PLLA block crystallizes first, then the PCL block, and finally the PEO block. Differential scanning calorimetry (DSC) analysis further demonstrated that the three blocks can crystallize from the melt when a low cooling rate is employed. The crystallization process takes place from a homogenous melt as indicated by small angle X-ray scattering (SAXS) experiments. The crystallization and melting enthalpies and temperatures of both PEO and PCL blocks decrease as PLLA content in the terpolymer increases. Polarized light optical microscopy (PLOM) demonstrated that the PLLA block templates the morphology of the terpolymer, as it forms spherulites upon cooling from the melt. The subsequent crystallization of PCL and PEO blocks occurs inside the interlamellar regions of the previously formed PLLA block spherulites. In this way, unique triple crystalline mixed spherulitic superstructures have been observed for the first time. As the PLLA content in the terpolymer is reduced the superstructural morphology changes from spherulites to a more axialitic-like structure.

Plasticization and anti-plasticization effects caused by poly(lactide-ran-caprolactone) addition to double crystalline poly(L-lactide)/poly(ε-caprolactone) blends

The effect of adding poly(lactide-ran-caprolactone), P(LA-ran-CL), random copolymers to melt mixed poly(lactide)(PLA)/poly(e-caprolactone)(PCL) 80/20 blends is investigated. The concentration of P(LA-ran-CL) copolymer added to the blends was 2 wt% and two different types of P(LA-ran-CL) copolymers were employed. They varied in composition and molecular weight. Compression molded sheets were evaluated by SEM (Scanning Electron Microscopy), PLOM (Polarized Light Optical Microscopy), DSC (Differential Scanning Calorimetry) and tensile tests. SEM micrographs show that 80/20 PLA/PCL blends exhibit the typical sea-island morphology characteristic of immiscible blends with PCL finely dispersed in droplets on a PLA matrix. The PLA phase crystallizes during cooling from the melt only in blends containing P(LA-ran-CL) copolymers. The copolymer with the higher amount of e-caprolactone, and with a lower Tg, produces a larger plasticization effect in comparison with the other copolymer used (characterized by a higher amount of PLA) and can significantly increase the crystallization rate of PLA up to an order of magnitude. On the other hand, part of the P(LA-ran-CL) chains is dissolved in the PCL phase of the blends and induces an antiplasticizing effect that reduces the crystallization rate of the PCL dispersed phase.

Crystallization of Cyclic Polymers

The effect of chain topology (ring versus linear polymer chains) on polymer crystallization is reviewed. Recent advances in ring closure and ring expansion synthetic techniques have made available a range of well-characterized samples with higher levels of purity than available decades ago. Cyclic molecules are fascinating because the structural difference between them and linear chains is relatively small, yet their behavior can be completely different from that of their linear analogs of identical chain length. The effect of having no chain ends can dramatically change the polymer coil conformation and diffusion rate, as well as the chain entanglement density in the melt. These changes are reflected in different nucleation and crystallization kinetics for cyclic and linear polymeric chains. However, the results published so far seem to be dependent on the type of polymer employed. Therefore, a careful look at the literature and evidence reported for each group of materials has been assembled and compared. The possible reasons for some of the contradictions in the evidence are also discussed.

Can poly(ε-caprolactone) crystals nucleate glassy polylactide?

Accelerating the crystallization rate of polylactide (PLA) to extend its application range is an interesting challenge. In this work, the crystallization of PLA from the glassy state within 80/20 PLA/PCL blends, with or without the addition of three different compositions of poly(L-lactide-block-carbonate) (PLA-b-PC) diblock copolymers, is investigated. The samples were evaluated by SEM (Scanning Electron Microscopy), PLOM (Polarized Light Optical Microscopy) and DSC (Differential Scanning Calorimetry). The blends exhibit a sea-island morphology with PCL droplets finely dispersed in a PLA matrix. Adding PLA-b-PC copolymers reduces PCL droplet size. Upon using a 50/50 PLA-b-PC copolymer, a threefold particle size decrease was observed. At the same, the glass transition temperature of the PLA phase is depressed, as PLA-b-PC addition induces partial miscibility between PLA and PCL. Non-isothermal and isothermal DSC experiments demonstrate that PCL addition accelerates cold crystallization of PLA, even in neat PCL/PLA blends. However, the PLA spherulitic growth rate is insensitive to PCL addition; therefore, this acceleration is due to a remarkable nucleation effect that PCL droplets provoke on glassy PLA. By aging the blends below the Tg of PLA, the DSC results demonstrate that the crystallization of PCL droplets is responsible for the enhanced nucleation of the surrounding glassy PLA matrix. Such nuclei are activated at higher temperatures (at which PCL droplets are molten), when the sample is heated from the glassy state, and therefore PLA cold crystallization is accelerated. On the other hand, no significant nucleation effects for neat PLA/PCL blends are detected when PLA crystallization starts from the melt state.

Synthesis and Characterization of Double Crystalline Cyclic Diblock Copolymers of Poly(ε-caprolactone) and Poly(l(d)-lactide) (c(PCL-b- PL(D)LA))

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.

The role of PLLA-g-montmorillonite nanohybrids in the acceleration of the crystallization rate of a commercial PLA

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 Mn 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 Mn 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 Mn PLLA molecules. The changes in Tg 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.

Crystallization and Morphology of Block Copolymers and Terpolymers With More Than One Crystallizable Block

Morphology and crystallization of block copolymers are influenced by their chemical nature, microstructure, composition, and segregation strength between blocks. The crystallization conditions play an important role in the final structure and physical properties of the material. This chapter reviews recent literature on the morphology and crystallization behavior of miscible and phase segregated block copolymers and terpolymers with more than one crystallizable block. Depending on the level of segregation, the final morphology is a consequence of the microphase separation driven by the crystallization event or by the immiscibility between the blocks. Strongly segregated systems exhibit nanostructures of different geometries, while miscible block copolymers form mixed spherulitic-type structures with a distinctive birefringence alteration. Different crystallization phenomena, such as homogeneous nucleation, retarded or first-order crystallization kinetics, confined and fractionated crystallization, fractioned melting, reduced crystallinity, and thickening of the microdomains are discussed. Finally, novel ABC triblock terpolymers with a remarkable triple crystalline nature are first introduced.