The origin of memory effects in the crystallization of polyamides: Role of hydrogen bonding

Investigation of Recycled Expanded Polyamide Beads through Artificial Ageing and Mechanical Recycling as a Proof of Concept for Circular Economy

Car manufacturers are currently challenged with increasing the sustainability of their products and production to comply with sustainability requirements and legislation. One way to enhance product sustainability is by reducing the carbon footprint of fossil-based plastic parts. Particle foams are a promising solution to achieve the goal of using lightweight parts with minimal material input. Ongoing developments involve the use of expanded particle foam beads made from engineering plastics such as polyamide (EPA). To achieve this, a simulated life cycle was carried out on virgin EPA, including mechanical recycling. The virgin material was processed into specimens using a steam-free method. One series was artificially aged to replicate automotive life cycle stresses, while the other series was not. The mechanical recycling and re-foaming of the minipellets were then carried out, resulting in an EPA particle foam with 100% recycled content. Finally, the thermal and chemical material properties were comparatively analysed. The study shows that the recycled EPA beads underwent polymer degradation during the simulated life cycle, as evidenced by their material properties. For instance, the recycled beads showed a more heterogeneous molecular weight distribution (an increase in PDI from two to three), contained carbonyl groups, and exhibited an increase in the degree of crystallization from approximately 24% to 36%.

The Impact of Regiodefects on the Melt-Memory of Isotactic Polypropylene

The memory of crystalline phase in the melt of isotactic polypropylene (iPP) in regiodefective samples of iPP characterized by different concentrations regiodefects, constituted by secondary 2,1 propene units, is studied. The self-nucleation (SN) experiments have demonstrated that the presence of 2,1 regiodefects produces a strong memory of the crystalline phase in the melt that persists up to temperatures much higher than the melting temperature. The extension of the heterogeneous melt (domain II) containing self-nuclei increases with increasing the concentration of regiodefects. The higher the concentration of regiodefects the higher the temperature at which the self-nuclei are dissolved and the homogeneous melt is achieved. This demonstrates that a strong memory of the crystalline phase of iPP in the melt exists not only in copolymers with noncrystallizable bulky comonomeric units rejected from the crystals but even when small defects are largely included in the crystals.

Pseudo-Eutectic of Isodimorphism to Design Biaxially-Oriented Bio-Based PA56/512 with High Strength, Toughness and Barrier Performances

The biaxially-oriented PA56/512 has excellent mechanical strength, extensibility and water-oxygen barrier properties and has broad application prospects in green packaging, lithium battery diaphragm and medical equipment materials. The correlation between the aggregation structure evolution and macroscopic comprehensive properties of copolymer PA56/512 under biaxial stretching has been demonstrated in this work. The structure of the random copolymerization sequence was characterized by 13C Nuclear magnetic resonance (NMR). The typical isodimorphism behavior of the co-crystallization system of PA56/512 and its BOPA-56/512 films was revealed by differential scanning calorimetry (DSC) and X-ray diffraction (XRD) tests. And the aggregation structure, including the hydrogen bond arrangement, crystal structure and crystal morphology of PA56/512 before and after biaxial stretching, was investigated by XRD, Fourier-transform infrared spectroscopy (FTIR) and polarized optical microscopy (POM) tests. Furthermore, the effect of the biaxially-oriented stretching process on the mechanical properties of PA56/512 has been demonstrated. In addition, a deep insight into the influence of the structure on the crystallization process and physical-mechanical performance has been presented. The lowest melting point at a 512 content of 60 mol% is regarded as a "eutectic" point of the isodimorphism system. Due to the high disorder of the structural units in the polymer chain, the transition degree of the folded chain (gauche conformation) is relatively lowest when it is straightened to form an extended chain (trans conformation) during biaxially-oriented stretching, and part of the folded chain can be retained. This explains why biaxially stretched PA56/512 has high strength, outstanding toughness and excellent barrier properties at the pseudo-eutectic point. In this study, using the unique multi-scale aggregation structure characteristics of a heterohomodymite polyamide at the pseudo-eutectic point, combined with the new material design scheme and the idea of biaxial-stretching processing, a new idea for customized design of high-performance multifunctional polyamide synthetic materials is provided.

The Influence of Thermal Parameters on the Self-Nucleation Behavior of Polyphenylene Sulfide (PPS) during Secondary Thermoforming

During the secondary thermoforming of carbon fiber-reinforced polyphenylene sulfide (CF/PPS) composites, a vital material for the aerospace field, varied thermal parameters profoundly influence the crystallization behavior of the PPS matrix. Notably, PPS exhibits a distinctive self-nucleation (SN) behavior during repeated thermal cycles. This behavior not only affects its crystallization but also impacts the processing and mechanical properties of PPS and CF/PPS composites. In this article, the effects of various parameters on the SN and non-isothermal crystallization behavior of PPS during two thermal cycles were systematically investigated by differential scanning calorimetry. It was found that the SN behavior was not affected by the cooling rate in the second thermal cycle. Furthermore, the lamellar annealing resulting from the heating process in both thermal cycles affected the temperature range for forming the special SN domain, because of the refined lamellar structure, and expelled various defects. Finally, this study indicated that to control the strong melt memory effect in the first thermal cycle, both the heating rate and processing melt temperature need to be controlled simultaneously. This work reveals that through collaborative control of these parameters, the crystalline morphology, crystallization temperature and crystallization rate in two thermal cycles are controlled. Furthermore, it presents a new perspective for controlling the crystallization behavior of the thermoplastic composite matrix during the secondary thermoforming process.

Disappearance of Melt Memory Effect with Comonomer Incorporation in Isodimorphic Random Copolyesters

Melt memory effects in polymer crystallization have attracted much attention in the past few years. Although progress has been made in understanding how the chemical structure of polymers can affect melt memory, there are still some knowledge gaps. In this work, we study how incorporating a second comonomer unit that is partially included in the crystalline unit cell affects the melt memory effect of the major component in a random isodimorphic copolymer for the first time. This second comonomer unit depresses the melting temperature of the homopolymer, reduces the crystallinity, and distorts the crystalline unit cell. However, its effect on the stability of self-nuclei and the production of melt memory has not been studied so far. To this aim, we have selected poly[(butylene succinate)-ran-(ε-caprolactone)] random copolyesters PBS-ran-PCL that are isodimorphic, i.e., they exhibit a pseudoeutectic point. This point separates the formation of BS-rich crystals from CL-rich crystals as a function of composition. The results reveal that the melt memory effect of these isodimorphic copolymers is strongly reduced with the incorporation of even very small amounts of comonomer unit (i.e., 1 molar %). This indicates that the incorporation of a second comonomer unit in the polymer chain disrupts the intermolecular interactions present between the chain segments in the crystal lattice of the major component and reduces the capacity of the material to produce self-nuclei. This reduction is more drastic for copolymers in which the second comonomer unit is mostly rejected from the crystalline phase. Contrary to olefin-based copolymers, for copolyesters, the second comonomer unit eases the process to reach an isotropic melt state upon melting. This work reveals the impact of introducing comonomer units on the melt memory effect in isodimorphic random copolyesters.

Reorganization of Hydrogen Bonding in Biobased Polyamide 5,13 under the Thermo-Mechanical Field: Hierarchical Microstructure Evolution and Achieving Excellent Mechanical Performance

The hierarchical microstructure evolution of an emerging biobased odd-odd polyamide 5,13 (PA5,13) films under the thermo-mechanical field, stepping from hydrogen bond (H-bond) arrangement to the crystalline morphology, has been investigated systematically. It is found that the reorganization of H-bonds under the thermo-mechanical field plays a crucial role in the crystallization of PA5,13. Especially, it is revealed that the crystallization process under the thermo-mechanical field develops along the chain axis direction, while lamellar fragmentation occurs perpendicular to the chain axis. Consequently, a stable and well-organized H-bond arrangement and lengthened lamellae with significant orientation have been constructed. Laudably, an impressive tensile strength of about 500 MPa and modulus of about 4.7 GPa are thus achieved. The present study could provide important guidance for the industrial-scale manufacture of high-performance biobased odd-odd PAs with long polymethylene segment in the dicarboxylic unit combined with a large difference between the polymethylene segments in the dicarboxylic and diamine units.

The role of intermolecular interactions on melt memory and thermal fractionation of semicrystalline polymers

The origin of melt memory effects associated with semicrystalline polymers and the physical parameters involved in this process have been widely studied in the literature. However, a comprehensive understanding of the role of intermolecular interactions on melt memory is still being developed. For this purpose, we have considered aliphatic polyesters and we have incorporated amide and additional ester groups. Inserting these additional functional groups, the strength of the intermolecular interactions increases widening the melt memory effect. Not only the presence of the functional groups but also the position of these groups in the repeating unit plays a role in the melt memory effect as it impacts the strength of the intermolecular interactions in the crystals. The study of the effect of intermolecular interactions has been extended to successive self-nucleation and annealing thermal fractionation experiments to explore for the first time the role of intermolecular forces on the fractionation capacity of linear polymers. We demonstrated that intermolecular interactions act as intrinsic defects interrupting the crystallizable chain length, thus facilitating thermal fractionation. Overall, this work sheds light on the role of intermolecular interactions on the crystallization behavior of a series of aliphatic polyesters.

Pressure-induced phase transition in Glycinium maleate crystal

The multicomponent glycinium maleate single crystal was grown by the slow evaporation method. The crystal was submitted to pressures ranging from 1 atm to 5.6 GPa and Raman spectroscopy was used as a spectroscopic probe. The modifications of relative intensity bands related to the lattice modes at 0.3 GPa were associated with rearrangements of hydrogen bonds. Moreover, between 1.7 and 4.8 GPa the Raman results indicate that the crystal experience a long structural phase transition, which was confirmed by PCA analysis. DFT calculations gave us more precision in the assignments of modes. The behavior of the internal modes under pressure showed that the maleic acid molecule undergoes greater modifications than glycine amino acid. All observed modifications were reversible when the pressure was released.

Strain-Induced Form Transition and Crystallization Behavior of the Transparent Polyamide

A transparent polyamide, poly(4,4'-aminocyclohexyl methylene dodecanedicarboxylamide) (PAPACM12), was studied and characterized by in situ wide-angle X-ray diffraction (WAXD) to establish the relationship between its crystallization behavior, crystalline form transition under external fields, and macroscopic properties. During the heating process, cold crystallization occurred and increased, and there was no form transition below the melting point. During the isothermal process, PAPACM12 exhibited the same crystalline structure as that during the heating process. The crystalline structure of PAPACM12 was attributed to α-form crystal, which is the stable form, according to the WAXD diffraction peaks of the conventional AABB-type polyamides. During stretching deformation, the crystal transition from α-form to γ-form and strain-induced crystallization were observed to contribute to the PAPACM12 with higher breaking strength and elongation. This study firstly determine the crystalline structure of transparent polyamides, and then the controlled strain-induced crystallization and transformation are demonstrated to be effective preparation methods for polyamides with high properties.

Molecular dynamics and crystallization in polymers based on ethylene glycol methacrylates (EGMAs) with melt memory characteristics: from linear oligomers to comb-like polymers

In this article we present results on the glass transition, crystallization and molecular dynamics in relatively novel oligomers, oligo-ethylene glycol methacrylate (OEGMA), with short and long chains, as well as in the corresponding nanostructured comb-like polymers (POEGMA, short and long), the latter being prepared via the RAFT polymerization process. For the investigation we employed conventional and temperature modulated differential scanning calorimetry in combination with high resolving power dielectric spectroscopy techniques, broadband dielectric relaxation spectroscopy (BDS) and thermally stimulated depolarization currents (TSDC). Under ambient conditions short OEGMA (475 g mol-1, ∼4 nm in length) exhibits a remarkable low glass transition temperature, Tg, of -91 °C, crystallization temperature Tc = -24 °C and a significant crystalline fraction, CF, of ∼30%. When doubling the number of monomers (OEGMA-long, 950 g mol-1, chain length ∼8 nm) the Tg increases by about 20 K and CF increases to ∼53%, whereas, the Tc migrates to a room-like temperature of 19 °C. Upon formation of comb-like POEGMA structures the grafted OEGMA short chains, strikingly, are not able to crystallize, while in POEGMA-long the crystallization behaviour changes significantly as compared to OEGMA. Our results indicate that in the comb-like architecture the chain diffusion of the amorphous fractions is also strongly affected. The semicrystalline systems exhibit significant melt memory effects, this being stronger in the comb-like architecture. It is shown that these effects are related to the inter- and intra-chain interactions of the crystallizable chains. The dielectric techniques allowed the molecular dynamics mapping of these new systems from the linear oligomers to POEGMAs for the first time. BDS and TSDC detected various dynamics processes, in particular, the local polymer dynamics (γ process) to be sensitive to the Tg, local dynamics triggered in the hydrophilic chain segments by water traces (β), as well as the segmental dynamics (α) related to glass transition. Interestingly, both the short and long linear OEGMAs exhibit an additional relaxation process that resembles the Normal-Mode process appearing in polyethers. In the corresponding POEGMAs this process could not be resolved, this being an effect of the one-side grafted chain on the comb backbone. The revealed variations in molecular mobility and crystallization behavior suggest the potentially manipulable diffusion of small molecules throughout the polymer volume, via both the molecular architecture as well as the thermal treatment. This ability is extremely useful for these novel materials, envisaging their future applications in biomedicine (drug encapsulation).

A Facile Strategy to Fabricate Antistatic Polyamide 1012/Multi-Walled Carbon Nanotube Pipes for Fuel Delivery Applications

Developing antistatic long chain polyamide (LCPA) resins and fabricating the corresponding fuel pipes are challenges but necessary. Herein, a facile but effective strategy was put forward to fabricate LCPA resins with a superior conductivity, meeting the requirements of electrostatic sub-conductors. The strategy was based on, first, the incorporation of a large amount (15 wt%) of multi-walled carbon nanotubes (MWCNTs) into a polyamide 1012 (PA1012) matrix as a master batch, which formed a dense conductive network. Subsequently, it was diluted with PA1012 granules to produce base resins, and the reprocessed nanocomposites with a critical content of MWCNTs (3 wt%) could generate an effectively interconnected conductive network, with sparse and thinning features. Using the base resins, fuel pipes for automobiles, petrol stations and high pressure applications were successfully fabricated, where the thin conductive network was transformed into a thick one due to external field-induced re-agglomeration of MWCNTs. In this way, the obtained fuel pipes combined excellent conductive and barrier properties, and mechanical properties at high and low temperatures. These comprehensive properties also arose from the uniform dispersion of MWCNTs in an LCPA matrix, even without coupling agents; the attractive interaction between MWCNTs and the polyamide chains contributed to their strong interface adhesion. Thus, this research provides a versatile approach to fabricating antistatic LCPA resins, which will certainly extend their application to vehicle fuel systems.