Control of Crystal Morphology in Poly(l-lactide) by Adding Nucleating Agent

The Effects of Nucleating Agents and Processing on the Crystallization and Mechanical Properties of Polylactic Acid: A Review

Polylactic acid (PLA) is a biobased, biodegradable, non-toxic polymer widely considered for replacing traditional petroleum-based polymer materials. Being a semi-crystalline material, PLA has great potential in many fields, such as medical implants, drug delivery systems, etc. However, the slow crystallization rate of PLA limited the application and efficient fabrication of highly crystallized PLA products. This review paper investigated and summarized the influence of formulation, compounding, and processing on PLA's crystallization behaviors and mechanical performances. The paper reviewed the literature from different studies regarding the impact of these factors on critical crystallization parameters, such as the degree of crystallinity, crystallization rate, crystalline morphology, and mechanical properties, such as tensile strength, modulus, elongation, and impact resistance. Understanding the impact of the factors on crystallization and mechanical properties is critical for PLA processing technology innovations to meet the requirements of various applications of PLA.

Synthesis of renewable furan-based phosphate and the superior flame retardancy in biodegradable polylactide

Currently, it has long been considered a challenge to provide sustainable additives for polylactide (PLA) in green way to endow it excellent comprehensive properties. Given the flammability and unsatisfactory crystallization performance of PLA, a furan-based phosphate furfurylamine trimethylphosphate (FATMP) was synthesized from 2-furfurylamine and amino trimethylphosphonic acid by a simple hydration reaction, and the PLA/FATMP composites were prepared by melting blending process. The tensile performance, crystallization behaviors, flame retardancy, and flame-retardant mechanism received special attention. Results showed that the incorporation of only 3 wt% FATMP could indeed increase the LOI value of PLA from 19.8 to 27.3 %, and simultaneously acquired V-0 rating in the vertical burning test owing to the favorable synergistic effect between the vapor phase and the condensed phase. Additionally, the half-crystallization time of PLA was decreased from 12.4 to 5.1 mins with the addition of FATMP, which acted as a nucleating agent. More appealingly, the tensile performance of PLA/FATMP composites was also well maintained. In general, the PLA/FATMP composites we proposed could be promising candidates in application fields where favorable flame retardancy and crystallization ability are required.

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.

Crystallization behaviors regulations and mechanical performances enhancement approaches of polylactic acid (PLA) biodegradable materials modified by organic nucleating agents

Polylactic acid (PLA) has attracted much attention because of its good biocompatibility, biodegradability, and mechanical properties. However, the slow crystallization rate of PLA during molding leads to its poor heat resistance, which limit its diffusion for many industrial applications. In this review, the relationship between PLA crystallization and its molecular structure and processing conditions is summarized. From the perspective of the regulation of PLA crystallization by organic nucleating agents, the research progress of organic micromolecule (e.g., esters, amides, and hydrazides), organic salt, supramolecular, and macromolecule nucleating agents on the crystallization behavior of PLA is mainly introduced. The nucleation mechanism of PLA is expounded by organic nucleating agents, and the effect of the interaction force between organic nucleating agents and PLA molecular chains on the crystallization behavior of PLA is analyzed. The effects of the crystallization behavior of PLA on its mechanical properties and heat resistance are discussed. It will provide a theoretical reference for the development and application of high-efficiency nucleating agents.

Enhanced heat resistance and expansion ratio of biodegradable poly (lactic acid)/poly (butylene adipate-co-terephthalate) composite foams via synergistic effect of nucleating agent and chain extension

Environmentally friendly foams have been studied extensively to handle the plastic pollution caused by disposable tableware. Herein, biodegradable poly (lactic acid) (PLA) – poly (butylene adipate-co-terephthalate) (PBAT) composites were first fabricated by melt blending, in which a reactive chain extender (ADR) was incorporated to enhance the interfacial compatibility and melt strength, a nucleating agent (TMC) was applied to improve the crystallinity. Subsequently, the foaming behavior of the composites was investigated via supercritical CO2 foaming. Specifically, a spherulite and shish-kebab-like nanoporous structure was observed from PLA and PLA/TMC foams, respectively, when the soaking temperature was far below melting temperature. With an increase of foaming temperature, the unmelted completely crystals served as physical cross-linking points, also contributing to a high melt strength. Combined with chain extension reaction of ADR, the PLA–PBAT/ADR/TMC foams showed the largest cell diameter and expansion ratio, which were 19 μm and 8.9, respectively. Furthermore, the heat resistance of PLA–PBAT/ADR/TMC foams was also significantly improved, owing to the high crystallinity induced by TMC. Consequently, the composite foams with superior heat resistance and toughness were obtained to broaden its application as biodegradable disposable tableware for hot food.

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.

The synergistic effect of heterogeneous nucleation and stress-induced crystallization on supramolecular structure and performances of poly(lactic acid) melt-spun fibers

As a kind of bio-based polymer, poly (lactic acid) has potential application in fibers fields. Due to the weak nucleation ability, PLA crystallizes slowly and forms large spherulites during the forming process, which deteriorates the properties of PLA fibers. In this work, melt-spun method is employed for the fabrication of PLA/T composite fibers using succinate diphenyl dihydrazide (TMC-306) as the nucleating agent, and then the hot-drawing and heat setting is performed to the as-spun fibers. Compared with pure PLA fibers, PLA/T fibers show faster crystallization rate and improved performance due to the synergistic effect of heterogeneous nucleation and stress-induced crystallization. The characterization of non-isothermal crystallization behavior indicates that the peak crystallization temperature as well as crystallinity of PLA composites is increased to 121.5 °C and 36.78 % respectively by blending 0.3 wt% TMC-306. Meanwhile, the obtained PLA/0.3T composite fibers are highly crystallized and oriented at hot-drawing ratio of 2.4 folds and heat setting temperature of 100 °C, and the conformational stability is noticeably enhanced. Further, the tensile strength and storage modulus of PLA/0.3T composite fiber are 3.46 cN/dtex and 46,953 MPa respectively, which are increased by 42 % and 41 % compared with neat PLA fibers.

An Investigation on the Efficacy of Orotic Acid as a Bio-Nucleating Agent for Poly-Lactic Acid under Quiescent Condition and Injection Molding

Polylactic acid (PLA) is a bio-based biodegradable polymer and is considered to be an environmentally friendly alternative to petroleum-based polymers for various applications. Neat PLA requires an extended period at elevated temperatures to attain its maximum crystallinity, which can be mitigated by the addition of nucleating agents. Orotic acid is a natural heterocyclic nucleating agent in PLA. The effect of orotic acid on the crystallization behavior of a commercial, high-purity PLA was studied in detail. A differential scanning calorimetry (DSC) technique was utilized for this purpose. A new protocol for the quantitative characterization of crystallization kinetics from DSC data was developed. It was found that the total crystallinity increased from 26% to 63% at 80 °C with 1% content of orotic acid. Meanwhile, the crystallization rate of PLA-OA blends increased by ~10 times as compared to neat PLA. The addition of orotic acid also reduced the incubation time by >17% under quiescent conditions. Injection molding experiments showed that highly crystallized (>50%) PLA samples could be fabricated with a 1% addition of orotic acid. The required mold temperature was reduced from the 120 °C recommended by the supplier to 80 °C.

The synergistic effect of polytetrafluoroethylene in-situ fibrillation and dibenzoyl sebacate hydrazide on the crystallization and foaming behavior of poly (lactic acid)

The fully degradable poly (lactic acid) foam with green environmental protection characteristics can alleviate the shortage of petroleum resources caused by the application of plastics. However, due to the inherent low melt strength and slow crystallization rate of linear PLA. It is difficult to obtain PLA microcellular foam with good morphology. In order to obtain PLA microcellular foam with ultra-high expansion ratio and small cell size, PTFE (polytetrafluoroethylene) nanofibers with excellent CO₂ adsorption rate were introduced. Self-assembled nucleator TMC-300(dibenzoyl sebacate hydrazide) was also introduced to blend with PLA to obtain small-sized cells. The results show that the PTFE entanglement network as a self-assembled template can effectively improve the early crystallization nucleation efficiency and increase the crystallinity of branched PLA (CBPLA)/TMC by 7 %. The microcellular foam with PTFE content of 0.5 wt% (CBPLA/TMC/PTFE 0.5) was successfully prepared by physical foaming agent, which had the lowest cell size (8.7 μm) And high expansion ratio (1200 %).

Tailoring the Barrier Properties of PLA: A State-of-the-Art Review for Food Packaging Applications

It is now well recognized that the production of petroleum-based packaging materials has created serious ecological problems for the environment due to their resistance to biodegradation. In this context, substantial research efforts have been made to promote the use of biodegradable films as sustainable alternatives to conventionally used packaging materials. Among several biopolymers, poly(lactide) (PLA) has found early application in the food industry thanks to its promising properties and is currently one of the most industrially produced bioplastics. However, more efforts are needed to enhance its performance and expand its applicability in this field, as packaging materials need to meet precise functional requirements such as suitable thermal, mechanical, and gas barrier properties. In particular, improving the mass transfer properties of materials to water vapor, oxygen, and/or carbon dioxide plays a very important role in maintaining food quality and safety, as the rate of typical food degradation reactions (i.e., oxidation, microbial development, and physical reactions) can be greatly reduced. Since most reviews dealing with the properties of PLA have mainly focused on strategies to improve its thermal and mechanical properties, this work aims to review relevant strategies to tailor the barrier properties of PLA-based materials, with the ultimate goal of providing a general guide for the design of PLA-based packaging materials with the desired mass transfer properties.

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.

Toughening Wet-Spun Silk Fibers by Silk Nanofiber Templating

Regenerated silk fibers typically fall short of silkworm cocoon fibers in mechanical properties due to reduced fiber crystal structure and alignment. One approach to address this has been to employ inorganic materials as reinforcing agents. The present study avoids the need for synthetic additives, demonstrating the first use of exfoliated silk nanofibers to control silk solution crystallization, resulting in all-silk pseudocomposite fibers with remarkable mechanical properties. Incorporating only 0.06 wt. % silk nanofibers led to a ∼44% increase in tensile strength (over 600 MPa) and ∼33% increase in toughness (over 200 kJ/kg) compared with fibers without silk nanofibers. These remarkable properties can be attributed to nanofiber crystal seeding in conjunction with fiber draw. The crystallinity nearly doubled from ∼17% for fiber spun from pure silk solution to ∼30% for the silk nanofiber reinforced sample. The latter fiber also shows a high degree of crystal orientation with a Herman's orientation factor of 0.93, a value which approaches that of natural degummed B. mori silk cocoon fiber (0.96). This study provides a strong foundation to guide the development of simple, eco-friendly methods to spin regenerated silk with excellent properties and a hierarchical structure that mimics natural silk. This article is protected by copyright. All rights reserved.

Modulation of the PLLA Morphology through Racemic Nucleation to Reach Functional Properties Required by 3D Printed Durable Applications

This paper presents an alternative for enhancing the durability of poly (L-lactide) (PLLA) by racemic nucleation following stereo-complexation with a selected poly (D-lactide) (PLDA). The compounds are obtained by melt blending of a PLLA grade, previously designed for 3D printing but with a low heat deflection temperature and impact resistance, with grades of PLDA differing in their molecular weight (Mw), D-lactide content (DS) and concentration. Our method considered how to reveal the racemic nucleation caused by stereo-complexation and its influence on functional properties. The FTIR study we performed showed that, depending on Mw, DS and concentration of the stereo-complexer (PDLA) used, bigger or smaller spectral changes can occur. The stereo-complexation was confirmed by the DSC analysis and, for the selected compound, by the POM, SEM, AFM microscopies, functional property and shapeability as 3D printing filaments. All the obtained results sustain the idea that, if a PLLA with Mw of 4.5 × 104 g·mol-1 is modified with PDLA with a medium Mw of 11.6 × 104 g·mol-1, medium DS of 4% and 1% concentration, a racemic nucleation is possible. It produces a racemic polylactic acid (PDLLA) with improved durability and good shapeability as 3D printing filaments. These results are explicable if the dependence of the intermolecular interactions appears between the PLLA and stereo-complexer PDLA. To enlarge the durable applicability of racemic polylactic acid (PDLLA), future research should identify other parameters controling the PLA stereo-complexing as the intensifying the mobility of the macromolecules, the finding of the optimal recemic cristalization window.

Crystallization of a Self-Assembling Nucleator in Poly(l-lactide) Melt

In the present work, crystallization of a soluble nucleator N, N', N″-tricyclohexyl-1,3,5-benzenetricarboxylamide (TMC-328) in a poly(l-lactic acid) (PLLA) matrix has been studied at different temperatures. Based on the change in solubility with temperature, different levels of supersaturation of TMC-328 in a PLLA matrix can be obtained. This nucleator presents a fibrous structure produced via self-assembling and develops into an interconnected network when the temperature is lowered. The TMC-328 crystal nuclei density is quantified via optical microscopy, using the average distance of the adjacent fibrillar structure, which shows a steady decrease with the decrease in temperature. The crystallization rates of TMC-328 were assessed through rheological measurements of network formation. Both fibrils' density and crystallization kinetics display a power law dependence on supersaturation. For the first time, the solid-melt interfacial energy, the size of the critical nucleus, and the number of molecules making up the critical nucleus of the nucleator TMC-328 in the PLLA matrix have been determined by adopting the classical nucleation theory. The subsequent crystallization of PLLA induced by this nucleator was investigated as a function of the fibrils' spatial density. The crystallization rate of PLLA is enhanced with the increase in the TMC-328 fibrils' density because of the availability of a larger nucleating surface. The self-assembled fibril of TMC-328 can serve as shish to form a hybrid shish-kebab structure after the crystallization of PLLA, regardless of the number of nucleation sites.

Manipulating Crystallization for Simultaneous Improvement of Impact Strength and Heat Resistance of Plasticized Poly(l-lactic acid) and Poly(butylene succinate) Blends

Crystalline morphology and phase structure play a decisive role in determining the properties of polymer blends. In this research, biodegradable blends of poly(l-lactic acid) (PLLA) and poly(butylene succinate) (PBS) have been prepared by melt-extrusion and molded into specimens with rapid cooling. The crystalline morphology (e.g., crystallinity, crystal type and perfection) is manipulated by annealing the molded products from solid-state within a short time. This work emphasizes on the effects of annealing conditions on crystallization and properties of the blends, especially impact toughness and thermal stability. Phase-separation morphology with PBS dispersed particles smaller than 1 μm is created in the blends. The blend properties are successfully dictated by controlling the crystalline morphology. Increasing crystallinity alone does not ensure the enhancement of impact toughness. A great improvement of impact strength and heat resistance is achieved when the PLLA/PBS (80/20) blends are plasticized with 5% medium molecular-weight poly(ethylene glycol), and simultaneously heat-treated at a temperature close to the cold-crystallization of PLLA. The plasticized blend annealed at 92 °C for only 10 min exhibits ten-fold impact strength over the starting PLLA and slightly higher heat distortion temperature. The microscopic study demonstrates the fracture mechanism changes from crazing to shear yielding in this annealed sample.

Polylactic Acid/Cerium Fluoride Films: Effects of Cerium Fluoride on Mechanical Properties, Crystallinity, Thermal Behavior, and Transparency

PLA is widely used in the field of disposable products for its good transparency, high strength, high modulus, and good processing performance. However, the crystallization rate and crystallinity of PLA are weak. In actual production, the PLA products that are typically obtained are amorphous with poor heat resistance, which greatly limits the application range of PLA products. Finding an effective nucleating agent to improve the transparency of PLA has been a hot topic in research. This study found that Cerium fluoride (CeF3) can effectively improve the crystallinity of PLA/CeF3 (P/F) films. When the content of CeF3 in PLA was 1 wt %, the retention ratio of visible light transmittance was 82.36%, the crystallinity was 29.8%, and the tensile strength was 59.92 MPa. Compared to pure PLA, the crystallinity of P/F1 increased by 56% and tensile strength increased by 8.76%. This study provided an alternative scheme that maintained the PLA film's transparency and improved the crystallinity of PLA, which significantly expanded the application of PLA.

Morphological and Chemical Analysis of Low-Density Polyethylene Crystallized on Carbon and Clay Nanofillers

Interest in carbon and clay-based nanofillers has grown in recent years. The crystallization behavior of low-density polyethylene (LDPE) was studied using a variety of notable nanofillers used in engineering applications and prepared using a solution crystallization method. Carbon nanotubes (CNTs), graphene oxide nano-platelets, clay (montmorillonite), and modified clay (surface-modified with trimethyl stearyl ammonium) were used to induce heterogeneous crystallization of LDPE. The crystallized LDPE samples, imaged using scanning and transmission electron microscopy, revealed different microstructures for each nanohybrid system, indicating these various nanofillers induce LDPE lamellae ordering. The underlying interactions between polymer and nanofiller were investigated using FTIR spectroscopy. X-ray diffraction (XRD) was used to determine crystallinity. This work examines how the differences in morphology and chemical structure of the nanofillers induce changes in the nucleation and growth of polymer crystals. These results will provide guidance on functional design of nano-devices with controlled properties.

Ternary blends from biological poly(3-hydroxybutyrate-co-4-hydroxyvalerate), poly(L-lactic acid), and poly(vinyl acetate) with balanced properties

Herein, poly(3-hydroxybutyrate-co-4-hydroxyvalerate) (P34HB), poly (L-lactic acid) (PLA), and poly(vinyl acetate) (PVAc) were initially melt compounded to prepare a ternary blend with balanced properties. Further, the miscibility, phase morphology, thermal and crystallization behaviors, and rheological and mechanical properties of the blends were studied. The dynamic mechanical analysis (DMA) results indicated that P34HB and PLA were partially miscible; however, PVAc showed full miscibility with PLA and P34HB. PVAc would selectively disperse in the PLA phase when considering low content, whereas it would gradually diffuse into the P34HB phase with the increasing PVAc concentration. A phase-separated morphology was observed for all the blends using scanning electron microscopy (SEM), and the diameters of the dispersed phases increased with the increasing PVAc concentration. The crystallization of P34HB was enhanced by the presence of PLA alone and was restrained by the simultaneous incorporation of PVAc and PLA. The rheological properties of P34HB were significantly improved because of the PVAc phase. Unexpectedly, the toughness and stiffness of the P34HB in ternary blends clearly improved because of the incorporation of PLA and PVAc.

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.

Supramolecular Interactions in Hybrid Polylactide Blends-The Structures, Mechanisms and Properties

The conformation of polylactide (PLA) chains can be adjusted by supramolecular interactions (the formation of hydrogen bonds or host-guest complexes) with appropriate organic molecules. The structures formed due to those intermolecular interactions may act as crystal nuclei in the PLA matrix ("soft templating"). In this review, the properties of several supramolecular nucleating systems based on synthetic organic nucleators (arylamides, hydrazides, and 1,3:2,4-dibenzylidene-d-sorbitol) are compared to those achieved with biobased nucleating agents (orotic acid, humic acids, fulvic acids, nanocellulose, and cyclodextrins) that can also improve the mechanical properties of PLA. The PLA nanocomposites containing both types of nucleating agents/additives are discussed and evaluated in the context of their biomedical applicability.

Supramolecular microtubes based on 1,3,5-benzenetricarboxamides prepared by self-assembly upon heating

A series of three 1,3,5-benzenetricarboxamides with peripheral tertiary N,N-dialkyl-ethylamino substituents with different length of the alkyl groups is reported. In particular, the N¹,N³,N⁵-tris[2-(diethylamino)-ethyl]-1,3,5-benzenetricarboxamide exhibits phase separation followed by self-assembly upon heating from aqueous solution into well-defined supramolecular fiber-like structures in the form of microtubes.