Crystallization Rate Minima of Poly(ethylene brassylate) at Temperatures Transitioning between Quantized Crystal Thicknesses

Organo-Mediated Ring-Opening Polymerization of Ethylene Brassylate from Cellulose Nanofibrils in Reactive Extrusion

Ethylene brassylate is a renewable macrolactone from castor oil that can be polymerized via ring-opening polymerization (ROP) to obtain a fully biosourced biodegradable polyester. ROP mediated by organometallic catalysts leads to high molar mass poly(ethylene brassylate) (PEB). However, the use of metal-free organocatalysis has several advantages, such as the reduction of toxic and expensive metals. In this work, a novel cellulose nanofibril (CNF)/PEB nanocomposite fabrication process by organocatalysis and reactive extrusion (REx) is disclosed. Here, ROP was carried out via solvent-free REx in the presence of CNFs using organic 1,5,7-triazabicyclo[4.4.0]dec-5-ene as a catalyst. Neat or lactate-esterified CNFs (LACNF) were used as initiators to investigate the effect of surface topochemistry on the in situ polymerization and the properties of the nanocomposites. A molar mass of 9 kDa was achieved in the presence of both unmodified and LACNFs with high monomer conversion (>98%) after 30 min reaction in a microcompounder at 130 °C. Tensile analysis showed that both nanofibril types reinforce the matrix and increase its elasticity due to the efficient dispersion obtained through the grafting from polymerization achieved during the REx. Mechanical recycling of the neat polymer and the nanocomposites was proven as a circular solution for the materials' end-of-life and showed that lactate moieties induced some degradation.

Quantitative Model of Multiple Crystal Growth Rate Minima in Polymers with Regularly Spaced Substituent Groups

A simple theory has been developed to explain quantitatively the multiple crystal growth rate minima observed experimentally in polyethylene brassylates (PEBs), polymers with regularly spaced "chemical defects", in this case, diester groups separated by 11 methylenes. The minima occur at the transitions where the fold length drops from 4 to 3 repeat units and from 3 to 2 units. An analytical rate-equation model was developed with elementary attachment and detachment steps of individual monomer repeat units, also including postattachment stem lengthening (stem conversion). The model produced a good fit to experimental crystallization rate curves for PEBs of three different molecular weights. The fits confirm in a quantitative way that the anomalies are caused by the self-poisoning effect, as proposed in the original experimental report on PEBs, based on the ideas developed in previous studies on long-chain n-alkanes. It is concluded that the rate minima in PEBs are the result of temporary attachment to the growth surface of stems that are too short to be stable yet long enough and close to stability to obstruct productive growth by stems of sufficient length. The results confirm the ubiquitous presence of self-poisoning at the growth front of polymer crystals in general and will help to achieve a better understanding of the complex process of crystallization of polymers. It will also allow the determination of more realistic parameters controlling their lamellar growth kinetics.

Polymorphism and Stretch-Induced Transformations of Sustainable Polyethylene-Like Materials

Herein we demonstrate that polyethylene-like bioderived, biodegradable, and fully recyclable unbranched aliphatic polyesters, such as PE-2,18, develop hexagonal crystal structures upon quenching from the melt to temperatures <∼50 °C and orthorhombic-like packing at higher quenching temperatures or after isothermal crystallization. Both crystal types are layered. While all-trans CH2 packing characterizes the structure of the orthorhombic-like form, there is significant conformational disorder in the staggered long CH2 sequences of the hexagonal crystals. On heating, the hexagonal crystals transform to the orthorhombic type at ∼60 °C via melt recrystallization, but no change is apparent during heating samples with the orthorhombic form up to the melting point (∼95 °C). The hexagonal structure is of interest not only because it develops under very rapid quenching from the melt but also because under uniaxial tensile deformation it undergoes a stretch-induced transformation to the orthorhombic structure. Compared to deformation of orthorhombic specimens that maintain the same crystal type, such transformation results in larger strains and enhanced strain hardening, thus representing a desired toughening mechanism for this type of polyethylene-like materials.

Efficient and well-controlled ring opening polymerization of biobased ethylene brassylate by α-diimine FeCl3 catalysts via a coordination-insertion mechanism

A highly efficient late-transition metal based catalytic system of α-diimine FeCl3 for well-controlled ring opening polymerization of a cheap and biobased macrolactone, ethylene brassylate (EB), is described herein. Proceeding via a coordination-insertion mechanism, such a catalytic system is capable of demonstrating unprecedented higher activities than previously reported organocatalysts or main-group metal based catalysts. Moreover, benefiting from the bulky nature of the α-diimine ligands, transesterification side reactions can be greatly suppressed, allowing the polymerization to proceed in a well-controlled living manner, as revealed from detailed kinetic studies. Additionally, such a catalytic system was also workable for ring opening copolymerization of EB and ε-caprolactone (ε-CL), giving the desired random copolymers with various compositions.

Poisoning by Purity: What Stops Stereocomplex Crystallization in Polylactide Racemate?

Formation of stereocomplex crystals (SC) is an effective way to improve the heat resistance and mechanical performance of poly(lactic acid) products. However, at all but the slowest cooling rates, SC crystallization of a high-molecular-weight poly(l-lactic acid)/poly(d-lactic acid) (PLLA/PDLA) racemate stops at a high temperature or does not even start, leaving the remaining melt to crystallize into homochiral crystals (HC) or an SC-HC mixture on continuous cooling. To understand this intriguing phenomenon, we revisit the SC crystallization of both high- and low-molecular-weight PLLA/PDLA racemates. Based on differential scanning calorimetry (DSC), supplemented by optical microscopy and X-ray scattering, we concluded that what stops the growth of SC is the accumulation of the nearly pure enantiomer, either PDLA or PLLA, that is rejected from the SC ahead of its growth front. The excess enantiomer is a result of random compositional fluctuation present in the melt even if the average composition is 1:1. The situation is more favorable if the initial polymer is not fully molten or is brought up to just above the melting point where SC seeds remain, as proven by DSC and X-ray scattering. Moreover, we find that not only is SC growth poisoned by the locally pure enantiomer but also that at lower temperatures, the HC growth can be poisoned by the blend. This explains why SC growth, arrested at high temperatures, can resume at lower temperatures, along with the growth of HC. Furthermore, while some previous works attributed the incomplete SC crystallization to a problem of primary nucleation, we find that adding a specific SC-promoting nucleating agent does not help alleviate the problem of cessation of SC crystallization. This reinforces the conclusion that the main problem is in growth rather than in nucleation.

Structural Organizations of a Mesogen-Terminated Semicrystalline Polymer: Chain Termini Ordering between Polymer Crystal Lamellae

In polymer crystallization, the chain end groups are generally excluded into the nanoscaled amorphous regions confined between crystal lamellae. Understanding the structural characteristic and evolution of interlamellar end groups is of great importance to control the macroscopic properties of polymers. However, the structural evolution of those confined end groups and related physical evidence remain unclear. Herein, we synthesized the end-functionalized poly(lactic acid)s with a self-assemblable mesogenic termini (4-hexyloxy-4'-cyanobiphenyl) and investigated the structural evolution of mesogenic termini between crystal lamellae. Intriguingly, the mesogenic termini can organize into an ordered layer structure between polymer crystal lamellae; such a process strongly depends upon the interlamellar spacing. A higher crystallization temperature (T_c) of the polymer allows for a larger interlamellar region, favoring the formation of an ordered mesogenic layer. However, a lower T_c results in a restricted interlamellar region, in which the end groups are strongly confined without sufficient mobility to undergo structural ordering. This study provides evidence for the structural ordering of chain termini confined between polymer crystal lamellae.

Development of Biodegradable Polyesters: Study of Variations in Their Morphological and Thermal Properties through Changes in Composition of Alkyl-Substituted (ε-DL) and Non-Substituted (ε-CL, EB, L-LA) Monomers

Three series of polyesters based on monomer combinations of ε-caprolactone (ε-CL), ethylene brassylate (EB), and l-Lactide (LLA) with the alkyl substituted lactone ε-decalactone (ε-DL) were synthesized at different molar ratios. Copolymers were obtained via ring opening polymerization (ROP) employing TBD (1,5,7-triazabicyclo-[4.4.0]-dec-5-ene), an organic catalyst which can be handled under normal conditions, avoiding the use of glove box equipment. The molar monomer composition of resulting copolymers differed from theoretical values due to lower ε-DL reactivity; their M_n and M_w values were up to 14 kDa and 22.8 kDa, respectively, and distributions were (Ɖ) ≤ 2.57. The thermal stability of these materials suffered due to variations in their ε-DL molar content. Thermal transitions such as melting (T_m) and crystallization (T_c) showed a decreasing tendency as ε-DL molar content increased, while glass transition (T_g) exhibited minor changes. It is worth mentioning that changes in monomer composition in these polyesters have a strong impact on their thermal performance, as well as in their crystallization degree. Consequently, variations in their chemical structure may have an effect on hydrolyic degradation rates. It should be noted that, in future research, some of these copolymers will be exposed to hydrolytic degradation experiments, including characterizations of their mechanical properties, to determine their adequacy in potential use in the development of soft medical devices.