Rapid Crystallization of Poly(lactic acid) by Using Tailor-Made Oxalamide Derivatives as Novel Soluble-Type Nucleating Agents

Lignin: A multi-faceted role/function in 3D printing inks

3D printing technology demonstrates significant potential for the rapid fabrication of tailored geometric structures. Nevertheless, the prevalent use of fossil-derived compositions in printable inks within the realm of 3D printing results in considerable environmental pollution and ecological consequences. Lignin, the second most abundant biomass source on earth, possesses attributes such as cost-effectiveness, renewability, biodegradability, and non-toxicity. Enriched with active functional groups including hydroxyl, carbonyl, carboxyl, and methyl, coupled with its rigid aromatic ring structure and inherent anti-oxidative and thermoplastic properties, lignin emerges as a promising candidate for formulating printable inks. This comprehensive review presents the utilization of lignin, either in conjunction with functional materials or through the modification of lignin derivatives, as the primary constituent (≥50 wt%) for formulating printable inks across photo-curing-based (SLA/DLP) and extrusion-based (DIW/FDM) printing technologies. Furthermore, lignin as an additive with multi-faceted roles/functions in 3D printing inks is explored. The effects of lignin on the properties of printing inks and printed objects are evaluated. Finally, this review outlines future perspectives, emphasizing key obstacles and potential opportunities for facilitating the high-value utilization of lignin in the realm of 3D printing.

Enhanced crystallization, heat resistance and transparency of poly(lactic acid) with self-assembling bis-amide nucleator

The application of poly(lactic acid) (PLA) is limited by its low crystallization rate. Conventional methods to increase crystallization rate usually result in a significant loss of transparency. In this work, a bundled bis-amide organic compound N'-(3-(hydrazinyloxy)benzoyl)-1-naphthohydrazide (HBNA) was used as a nucleator to prepare PLA/HBNA blends with enhanced crystallization, heat resistance and transparency. HBNA dissolves in PLA matrix at high temperature and self-assembles into bundle microcrystals by intermolecular hydrogen bonding at a lower temperature, which induces PLA to form ample spherulites and "shish-kebab-like" structure rapidly. The effects of HBNA assembling behavior and nucleation activity on the PLA properties and the corresponding mechanism are systematically investigated. As a result, the crystallization temperature of PLA increased from 90 °C to 123 °C by adding as low as 0.75 wt% of HBNA, and the half-crystallization time (t_1/2) at 135 °C decreased from 31.0 min to 1.5 min. More importantly, the PLA/HBNA maintains good transparency (transmittance > 75 % and haze is ca. 27 %) due to the decreased crystal size, even though the crystallinity of PLA is increased to 40 %, which also led to good heat resistance. The present work is expected to expand the application of PLA in packaging and other fields.

Synthesis of symmetric bis-α-ketoamides from renewable starting materials and comparative study of their nucleating efficiency in PLLA

An efficient and smart synthesis of bis-α-ketoamides has been disclosed. The desired products have been obtained through a Passerini multicomponent reaction using biobased aldehydes, acetic acid and bis-isocyanides (prepared from the corresponding biobased diamides), followed by a deprotection/oxidation step. The effect of the synthesized compounds on the crystallization behavior of poly(l-lactide) (PLLA) has been investigated by differential scanning calorimetry (DSC) in non-isothermal conditions. Among all the synthesized compounds, only a few are able to meaningfully enhance the nucleation of PLLA, as confirmed by a shift of the polymer crystallization peak temperature towards higher values. With the research of active polymer nucleating agents being mostly empirical, the combinatorial synthetic approach proposed herein, coupled with the possibility of a small scale mixing procedure, can potentially represent a useful strategy for the discovery of new efficient biobased polymer additives.

A new class of nucleating agents for poly(L-lactic acid): Environmentally-friendly metal salts with biomass-derived ligands and advanced nucleation ability

Adding nucleating agents has been a successful strategy to boost the heat resistance of poly(L-lactic acid) (PLLA) by increasing the crystallinity. In this study, a new series of bio-based complexes as nucleating agents for PLLA, including twelve combinations of three eco-friendly metal ions (Zn, Mg, Ca) and four biomass-derived α-hydroxy acids, were successfully synthesized to respectively investigate the effects of metal ions as well as ligands on nucleation capacity of complexes. By investigating the non-isothermal and isothermal crystallization at 135 °C of PLLA with 0.3 wt% loading of complexes, both zinc and magnesium salts of L-mandelic acid showed excellent nucleation capacities. And magnesium L-mandelate performed better, raising the crystallinity of PLLA to 44.4 % as well as minimizing its crystallization half-time from 73 min to 2.7 min. The growth and denser distribution of PLLA spherulites on the salt surface were also observed by POM, reflecting epitaxial nucleation as the possible mechanism. A novel inspiration, utilizing VESTA software to simulate the crystal structure of zinc L-mandelate (Zn(L-MA)₂), was proposed to determine the nucleation mechanism. Also, using polyethylene terephthalate (PET) as a test protocol, the rationality of the model could be approved by checking the fitness of nucleating prediction and experiment results.

Effect of Colorants and Process Parameters on the Properties of Dope-Dyed Polylactic Acid Multifilament Yarns

The color of textile fibers is typically imparted by submersion in a high-temperature dye bath. However, the treatment of the effluent is challenging and the textile industry is therefore a major source of water pollution. Current fashion trends favor biobased polymers such as polylactic acid (PLA) but exhaust dyeing at high temperatures causes hydrolytic degradation, reducing the crystallinity and tenacity of the yarn. To preserve the mechanical properties of PLA-based textiles, an alternative to exhaust dyeing called dope dyeing can be used, wherein colorants are incorporated into the polymer matrix during melt spinning. We evaluated this process by dope dyeing PLA with several colorants, then testing the thermal, physical, and mechanical properties of the yarn and the physical properties of circular-knitted fabrics. Although the colorants affected the crystallization behavior at lower cooling rates, during the melt-spinning process, the drawing speed had a greater effect on the crystallinity and mechanical properties of the dyed yarn. Scanning electron microscopy revealed that the colorants were well dispersed in the PLA matrix. We found that the colorants did not affect the physical properties of the knitted fabric. Our results can be used to develop more environmentally beneficial dope-dyed PLA yarn with improved mechanical properties.

Crystallization of microbial polyhydroxyalkanoates: A review

Polyhydroxyalkanoates (PHAs), produced by the microbial fermentation, is a promising green polymer and has attracted much attention due to its excellent biocompatibility, complete biodegradability, and non-cytotoxicity. The physical properties of PHAs are closely related to their chemical and crystalline structure. Therefore, deep understanding and regulating the structure and crystallization of PHAs are the key factors to improve the performance of PHAs. This review first provides a brief overview of the development history, chemical structure, and basic properties of PHAs. Then, the crystal structure, crystal morphology, kinetics theories and crystallization behavior of nucleation-induced PHAs are systematically summarized to provide a theoretical foundation for improving PHAs crystallization rate and physical properties. In the end, the outlook on the crystallization and application prospects of PHAs is also addressed.

Nucleating Agents to Enhance Poly(l-Lactide) Fiber Crystallization during Industrial-Scale Melt Spinning

The nucleating agent N,N′-bis(2-hydroxyethyl)-terephthalamide (BHET) has promising effects on poly(l-lactide) (PLA) under quiescent conditions and for injection molding applications, but its suitability for industrial-scale fiber melt spinning is unclear. We therefore determined the effects of 1% and 2% (w/w) BHET on the crystallinity, tenacity, and elongation at break of PLA fibers compared to pure PLA and PLA plus talc as a reference nucleating agent. Fibers were spun at take-up velocities of 800, 1400 and 2000 m/min and at drawing at ratios of 1.1–4.0, reaching a final winding speed of 3600 m/min. The fibers were analyzed by differential scanning calorimetry, wide-angle X-ray diffraction, gel permeation chromatography and tensile testing. Statistical analysis of variance was used to determine the combined effects of the spin-line parameters on the material properties. We found that the fiber draw ratio and take-up velocity were the most important factors affecting tenacity and elongation, but the addition of BHET reduced the mechanical performance of the fibers. The self-organizing properties of BHET were not expressed due to the rapid quenching of the fibers, leading to the formation of α′-crystals. Understanding the behavior of BHET in the PLA matrix provides information on the performance of nucleation agents during high-speed processing that will allow processing improvements in the future.

Tailoring the crystallization of poly(l-lactide) via structural optimization of hydrogen-bonding segments with different aliphatic spacer lengths

A series of hydroxy-terminated oxalamide segments (OXA-n, HO-(CH2)n-NHCOCONH-(CH2)n-OH, n = 2, 4 and 6) were designed as initiators for ring-opening polymerization and then poly(l-lactide) with OXA-n in the middle (PLLAOXA-n) were synthesized.

Enhanced Nonisothermal Crystallization and Heat Resistance of Poly(l-lactic acid) by d-Sorbitol as a Homogeneous Nucleating Agent

We report on enhancing crystallization and heat resistance of poly(l-lactic acid) (PLLA) by d-sorbitol as a small molecule nucleating agent via melt blending. During the reheating process, the cold crystallization disappeared and the crystallinity of nucleated PLLA exceeded 50%. The heat deflection temperature of PLLA was elevated from 56 to 132 °C by simply increasing the mold temperature (90 °C) without an additional annealing treatment. We also observed the polymorphic crystals of PLLA during melt crystallization, i.e., the coexistence of hexagonal and lenticular crystals, along with their various geometrical aggregates in addition to plenty of conventional spherulites. On the basis of the fact that the nonisothermal crystallization temperature of PLLA (110 °C at a cooling rate of 10 °C/min) was higher than the melting point of d-sorbitol (about 93 °C), we speculated that d-sorbitol promoted the crystallization of PLLA through a homogeneous nucleation mechanism.

Supernucleation, crystalline structure and thermal stability of bacterially synthesized poly(3-hydroxybutyrate) polyester tailored by thymine as a biocompatible nucleating agent

Naturally occurring thymine (TM) was incorporated into bacterial poly(3-hydroxybutyrate) (PHB) polyester to fabricate a novel and green biocomposite. Both 0.5% and 1% TM exhibit supernucleation effect on PHB, and crystallization kinetics suggests TM significantly increased T_c and X_c, and substantially shortened t_1/2 of PHB. Epitaxial nucleation caused by a perfect crystal lattice matching between PHB and TM, was proposed to elucidate nucleation mechanism of PHB. Hydrogen bond interaction exists between CO, C-O-C groups of PHB and -CH₃ (or -CH)/-NH- group of TM. TM interacted with CO group of PHB crystalline phase rather than that of amorphous one. In addition, two new IR crystalline bands assigned to C-O-C group of PHB appeared in the presence of TM, which arises from shift of two amorphous ones, respectively. TM enhanced onset thermal degradation temperature of PHB, mainly attributed to increased degree of crystallinity of PHB and flame retardance effect of TM.

Effect of Thermal History and Shear on the Viscoelastic Response of iPP Containing an Oxalamide-Based Organic Compound

We report on the role of temperature and shear on the melt behavior of iPP in the presence of the organic compound N1,N1′-(propane-1,3-diyl)bis(N2-hexyloxalamide) (OXA3,6). It is demonstrated that OXA3,6 facilitates a viscosity suppression when it resides in the molten state. The viscosity suppression is attributed to the interaction of iPP chains/subchains with molten OXA3,6 nanoclusters. The exact molecular mechanism has not been identified; nevertheless, a tentative explanation is proposed. The observed viscosity suppression appears similar to that encountered in polymer melts filled with solid nanoparticles, with the difference that the OXA3,6 compound reported in this study facilitates the viscosity suppression in the molten state. Upon cooling, as crystal growth of OXA3,6 progresses, the decrease in viscosity is suppressed. Retrospectively, segmental absorption of iPP chains on the surface of micrometer-sized OXA3,6 crystallites favors the formation of dangling arms, yielding OXA3,6 crystallites decorated with partially absorbed iPP chains. In other words, the resulting OXA3,6 particle morphology resembles that of a hairy particle or a starlike polymer chain. Such hairy particles effectively facilitate a viscosity enhancement, similar to branched polymer chains. This hypothesis and its implications for the shear behavior of iPP are discussed and supported using plate–plate rheometry and slit-flow experiments combined with small-angle X-ray scattering analysis.

Photodegradation and Biodegradation of Poly(Lactic) Acid Containing Orotic Acid as a Nucleation Agent

Orotic acid is a natural heterocyclic compound that acts as a nucleation agent in poly(lactic acid) (PLA). PLA materials with increasing orotic acid content were prepared and characterized. It was found that crystallinity of about 28% was reached with 0.3% content of the agent. Further enhancement in the content of the agent did not provoke any additional significant increase of crystallinity. Subsequently, it was investigated whether the orotic acid content affected photodegradation of PLA and, in the next phase, its biodegradation. The results of rheological measurements showed that the compound slightly accelerates photodegradation of the material, which was accompanied by the cleavage of PLA chains. Previous photodegradation was shown to accelerate the subsequent biodegradation by shortening the lag phase of the process, where the explanation is probably in the reduction of the polymer molecular weight during the photodegradation. Moreover, the presence of orotic acid in both initial and photodegraded samples was found to influence biodegradation positively by shortening the lag phase and increasing the observed maximal rate of the biodegradation.

Effect of Self-Assembly of Oxalamide Based Organic Compounds on Melt Behavior, Nucleation, and Crystallization of Isotactic Polypropylene

We report on the effect of an aliphatic oxalamide based nucleating agent (OXA3,6) on the melt and crystallization behavior of isotactic polypropylene (iPP) under defined shear conditions. Through polarized optical microscopy, we demonstrate that OXA3,6 self-assembles from the iPP melt into rhombic crystals whereas their size and distribution proved highly dependent on the employed cooling rates. The presence of 0.5 wt % of OXA3,6 in iPP results in a significant suppression in iPP melt viscosity, which could not be explained via molecular modeling. A possible cause for the drop in viscosity in the presence of OXA3,6 is attributed to the interaction (absorption) of high molecular weight iPP chains with the nucleating agent, thereby suppressing their contribution to the viscoelastic response of the melt. This proposed mechanism for the suppression in melt viscosity appears similar to that encountered by the homogeneous distribution of nanoparticles such as CNTs, graphene, and silica. Shear experiments, performed using a slit flow device combined with small-angle X-ray diffraction measurements, indicate that crystallization is significantly enhanced in the presence of OXA3,6 at relatively low shear rates despite its lowered sensitivity to shear. This enhancement in crystallization is attributed to the shear alignment of the rhombic OXA3,6 crystals that provide surface for iPP kebab growth upon cooling. Overall, the suppression in melt viscosity in combination with enhanced nucleation efficiency at low as well as high shear rates makes this self-assembling oxalamide based nucleating agent a promising candidate for fast processing.

The Effect of Thermal History on the Fast Crystallization of Poly(l-Lactide) with Soluble-Type Nucleators and Shear Flow

The N₁,N₁'-(ethane-1,2-diyl)bis(N₂-phenyloxalamide) (OXA) is a soluble-type nucleator with a dissolving temperature of 230 °C in poly(l-lactic acid) (PLLA) matrix. The effect of thermal history and shear flow on the crystallization behavior of the PLLA/OXA samples was investigated by rheometry, polarized optical microscopy (POM), differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), and scanning electron microscopy (SEM). The crystallization process of the PLLA/OXA-240 sample (i.e., pre-melted at 240 °C) was significantly promoted by applying a shear flow, e.g., the onset crystallization time (tonset) of the PLLA at 155 °C was reduced from 1600 to 200 s after shearing at 0.4 rad/s for even as short as 1.0 s, while the crystallinity (Xc) was increased to 40%. Moreover, the tonset of the PLLA/OXA-240 sample is 60%⁻80% lower than that of the PLLA/OXA-200 sample (i.e., pre-melted at 200 °C) with a total shear angle of 2 rad, indicating a much higher crystallization rate of the PLLA/OXA-240 sample. A better organization and uniformity of OXA fibrils can be obtained due to a complete pre-dissolution in the PLLA matrix followed by shear and oscillation treatments. The well dispersed OXA fibrils and flow-induced chain orientation are mainly responsible for the fast crystallization of the PLLA/OXA-240 samples. In addition, the shear flow created some disordered α'-form crystals in the PLLA/OXA samples regardless of the thermal history (200 or 240 °C).

Enhanced crystallization kinetics of poly(lactide) with oxalamide compounds as nucleators: effect of spacer length between the oxalamide moieties

Crystallization kinetics of poly (lactide) were enhanced by using oxalamide compounds as nucleators and the nucleation efficiency can be well tailored by tuning the aliphatic spacer length between the oxalamide moieties.

High-performance poly(lactide) composites by construction of network-like shish-kebab crystals

Novel high-performance PLA composites were prepared by solid-state uniaxial stretching with the OXA as needle-like nucleation templates leading to a unique network-like shish-kebab crystal structures with a tensile strength above 120 MPa.

Self-assembling process of Oxalamide compounds and their nucleation efficiency in bio-degradable Poly(hydroxyalkanoate)s

One of the key requirements in semi-crystalline polyesters, synthetic or bio-based, is the control on crystallization rate and crystallinity. One of the limiting factors in the commercialization of the bio-based polyesters, for example polyhydroxyalkanoates synthesized by bacteria for energy storage purposes, is the slow crystallization rate. In this study, we show that by tailoring the molecular structure of oxalamide compounds, it is possible to dissolve these compounds in molten poly(hydroxybutyrate) (PHB), having a hydroxyvalerate co-monomer content of less than 2 mol%. Upon cooling the polymer melt, the homogeneously dispersed oxalamide compound crystallizes just below the melting temperature of the polymer. The phase-separated compound reduces the nucleation barrier of the polymer, thus enhancing the crystallization rate, nucleation density and crystallinity. The findings reported in this study provide a generic route for the molecular design of oxalamide-based compounds that can be used for enhancing nucleation efficiency of semi-crystalline bio-based polyesters.