Crystallization Of Polyamide Research Articles

Facile Characterization of Isothermal Crystallization and Microphase Separation Kinetics of Polyamide 6 (PA6)‐Based Thermoplastic Elastomers

AbstractThe facile characterization of isothermal microphase separation kinetics in polyamide 6 (PA6)‐based thermoplastic elastomers (TPAE‐6) has long posed a challenge for the development of suitable melt spinning processes. In this study, this challenge is addressed through differential scanning calorimetry (DSC) measurements. It is assumed that the enthalpy changes of TPAE‐6 during the isothermal process are a linear superposition of enthalpy changes associated with microphase separation and crystallization of PA6 in hard phases, resembling that of TPAE‐6 without soft segments (TPAE‐6‐0). The study reveals that, as the concentration of soft segments in TPAE‐6 increases, the accelerated dynamics of phase separation become stronger than the dilution of soft segments to PA6 segments during isothermal process, resulting in an increase in the microphase separation rate of TPAE‐6. Furthermore, despite microphase separation, the overall crystallization rate of TPAE‐6 decreases with rising isothermal temperature and varies with increasing soft segment content at different temperatures. Additionally, the crystallization mode of TPAE‐6 follows two‐dimensional, three‐dimensional, or a combination of both crystal growth mechanisms, accompanied by a heterogeneous nucleation mechanism.

Binary blends of polyamide 6 with aliphatic polyketone: the compatibility analysis of polymer components

The compatibility and interactions of polyamide 6 (PA6) and aliphatic polyketone (APK) were studied when they were mixed in a melt by extrusion. Using the relaxation spectrometry (RS) method, it was shown that the studied PA6/ APK blends had one glass transition temperature (Тg) located on the temperature scale between Тg of initial PA6 and APK. It was also found that in these mixtures, the PA6 b-relaxation peak, responsible for the local movement of small-scale structural units of the main chain and side branches, was socialized with the b-peak of APK. On this basis, conclusions were drawn about the compatibility of PA6 and APK at the segmental and small-scale level. The mutual influence of polymer components of PA6/APK blends on the dynamic shear modulus and the zG3 criterion is shown. The DSC analysis has revealed a slowdown in the crystallization of PA6 and APK in their blends, which is probably due to the formation of extensive interfacial (transitional) layers in the PA6/APK blend, in which crystallization of polymer components is hindered due to intermolecular interactions. The analysis of the IR-spectra in the range of 3000–3800 cm–1 has made it possible to establish the formation of hydrogen bonds between the polyamide –NH-groups and carbonyls –C=O, being part of APK, which contributes to an increase in the compatibility of the polymer components of the blend under study.

Exploring the relationship between interfacial adhesion, molecular dynamics, and the brill transition in fully bio-based polyamide 1010 nanocomposites reinforced by two-dimensional materials

One of the greatest goals of modern polymer science can be considered the production of high-performance polymer nanocomposites from renewable sources due to the growing interest in environmental and sustainability issues. Within that scope, a good strategy is to pair the processability of thermoplastic materials with the outstanding characteristics of 2D nanomaterials. However, the final performance of the produced nanocomposite is mainly attributed to the interphase, i.e., the volume fraction of the bulk polymer that has its molecular dynamics affected by interactions with the surface of the filler. Therefore, we aim to investigate how different 2D nanomaterials, i.e., graphene oxide (GO), hexagonal-boron nitride (h-BN), and molybdenum disulfide (MoS2) affect the molecular dynamics of a fully bio-based polyamide 1010 (PA 1010) by a new correlation between experimental results from differential scanning calorimetry, broadband dielectric spectroscopy, and dynamic mechanical analysis. For that, nanocomposites based on single and hybrid-fillers were produced through melt mixing. It was observed that, although all nanofillers seem to hinder the molecular relaxation of the polymer chains, GO and h-BN were more effective than MoS2. This remained true when the fillers were applied individually and in the hybrid form. This effect has been attributed to the following aspects: (I) higher enthalpy related to the melting of the polyamide crystals after the Brill transition, which indicates lower thermal motion being gained before the transition takes place; (II) higher activation energy of the glass transition, which was estimated from the fitting of the dielectric spectra to the Havriliak-Negami function and then to the Vogel-Fulcher-Tammann model; (III) lower values of the “adhesion factor”, which is calculated from the decrease in intensity of the mechanical tan(δ) in the nanocomposites with respect to the neat polymer, indicating higher adhesion for the composites filled with GO and h-BN. To confirm the trends observed experimentally for each nanomaterial, molecular dynamics simulations, which enabled the investigation of the adhesion force as a function of nanosheet displacement, were also carried out. The simulations corroborated with the experimental observations, in which h-BN leads to a more intense force of separation, followed by GO, and lastly by MoS2. Since the level of interfacial interactions is what mainly dictates the performance of polymer nanocomposites, it is suggested that GO, and especially h-BN, might present greater promise for novel bionanocomposites based on PA 1010.

Enhancement of flexural properties of carbon fiber-reinforced polyamide 6 via oriented crystallization of polyamide 6 among carbon fibers

The effect of the crystalline structure of polyamide 6 (PA6) among carbon fiber (CF) on the mechanical properties of PA6/CF composites was explored using two types of PA6 with different spherulite sizes. Infrared (IR) spectroscopy showed that intra/intermolecular interactions were nearly equivalent for the two types of PA6. The viscoelasticities in the molten state were almost identical; that is, the molecular weights of the PA6s were comparable. The mechanical properties of the PA6 matrix with smaller spherulites (PA6a) in PA6/CF were superior to those of the PA6 matrix with larger spherulites (PA6b). Polarized light microscopy, wide-angle X-ray scattering, and polarized Raman spectroscopy revealed oriented crystals of PA6a in the PA6a/CF composite and random crystals of PA6b in the PA6b/CF composite. Therefore, the superior mechanical properties of PA6/CF are due to the oriented crystal chains of PA6, which support the overall stress transfer in composites.

Promotion of crystallization of polyamide 6 at an interphase and a mechanical interlocking effect at an interface due to surface irregularities of carbon fibers

The effect of surface irregularities in the carbon fiber (CF) axial direction on the interfacial properties of polyamide 6 (PA6) was investigated using CFs with different amounts of oxygen functional groups on the surface, that is, chemical reactivity. The interfacial shear strength between PA6 and CF was measured using the microdroplet method. The crystal structure of PA6 at the interphase was characterized using unpolarized and polarized micro-Raman spectroscopy and polarized light microscopy. Not only the chemical interactions at the interface but also the surface irregularities of the CF were found to contribute to the formation of oriented crystals of PA6 in the CF axial direction at the interphase, and these oriented crystals of PA6 at the interphase enhanced the interfacial shear strength. The surface irregularities of the CF were found to cause mechanical interlocking effects. This study demonstrates the importance of CF surface irregularities in controlling interfacial properties.

Roles of specific hydrogen-bonding interactions in the crystallization kinetics of polymers

Polyamides hold specific hydrogen-bonding interactions as the hierarchical thermodynamic driving forces for crystallization. Their roles in crystallization kinetics are worthy of further investigation. We performed dynamic Monte Carlo simulations to compare crystallization kinetics among 16-mer melts holding hydrogen-bonding interactions with variable strengths specifically assigned to 2, 4 and 8 monomers on the chain, respectively. The results demonstrate higher densities and strengths of hydrogen-bonding interactions generally accelerating the cooling and isothermal crystallization as well as the lateral growth of lamellar crystals, consistent with the experimental observations of polyamide crystallization. Too strong hydrogen-bonding interactions will slow down polymer diffusion and thus bring retardation to crystallization kinetics. Furthermore, hydrogen-bonding interactions at the central 8–9 monomers make higher crystal growth rates than those at the evenly distributed 5–12 monomers, demonstrating the dominant roles of specific interactions in the hierarchical parallel packing of polymer chains at the lateral growth front of lamellar crystals.

Influence of silane interfacial chemistry on the curing process of anionic Polyamide 6 in glass reinforced composites

The manufacturing of composites based on reactive thermoplastic polyamide 6 (PA6), reinforced with glass particles or fibers for structural applications, is quite sensitive to the state of the interface. Surface hydroxyls present at the glass surface can completely inhibit both the PA6 polymerization and crystallization processes. Care must then be exerted when tuning the interfacial chemistry in this system. This work demonstrates how an adequate choice of silane coupling agent, combined with a precise surface grafting methodology and tight control of hydroxyl groups density on glass particulates, yields kinetics and degrees of polymerization and crystallization comparable to the pure PA6 resin. Various silanes were grafted onto the microparticles and their surfaces were characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, thermogravimetric analysis, confocal laser scanning microscopy and contact angle measurements. The activator was also grafted onto the glass surface to initiate polymerization directly from the surface. The results showed that all treated glass particles surfaces allowed obtaining a high degree of conversion but with varying polymerization and crystallization kinetics. The results provide fundamental information on the role of the surface chemistry of glass reinforcements on the polymerization and crystallization of PA6, which are essential for the proper control of composite manufacturing.

Properties and Structure of Thermoplastic Polyvinyl Alcohol/Polyamide Sea-Island Fibers.

Ultra-fine fibers derived from sea-island fibers have attracted great attention due to their excellent overall performance. However, green and efficient splitting of sea-island fibers is still a challenging task. In this work, thermoplastic polyvinyl alcohol (TPVA) was prepared by the physical blending of plasticizer. The modified TPVA showed a high decomposition temperature (285 °C) and a wide thermoplastic processing window. This made TPVA match well with polyamide 6 (PA6) to form conjugated melts at 250 °C. Corresponding PVA/PA6 sea-island fibers were first reported to realize water-splitting instead of alkali-extraction of "sea" polymers. The effects of sea/island mass ratios and different spinning speeds on the properties of PVA/PA6 sea-island pre-oriented yarn (POY) were investigated. A higher spinning speed enhanced the orientation-induced crystalline behavior of fiber, therefore increasing the tensile strength of fibers. As the increase of spinning speed from 1000 to 1500 m/min, the crystalline degree of corresponding POYs increased from 9.9 to 14.3%. The plasticizer in PVA did not diffuse to the PA matrix during spinning. However, PVA could induce the crystallization of PA6 via interfacial hydrogen bonding. When the spinning speed was 1500 m/min, and PVA/PA6 was 7:3, the tensile strength reached the highest value of 1.67 cN/dtex. The uniform diameters of ultra-fine PA6 fibers (2-5 μm) were obtained by an environment-friendly water-splitting process. The "sea" phase (TPVA) in sea-island fiber could be removed quickly by boiling water treatment in 3 min. This green and energy-saving sea-island fiber splitting technique is of great significance in reducing CO2 emissions during the preparation of super-fine fibers.

Crystal structures of polyamide 6 at the interphase and around carbon fiber and mechanical properties of their composites

This study reports the effect of the polyamide 6 (PA6) crystal structure at the interphase and around the carbon fiber (CF) on the mechanical properties of CF-reinforced PA6 composites. The transverse tensile properties of the unidirectional composites and the four-point bending properties of the quasi-isotropic composites were investigated using three types of CFs. Raman spectra of PA6 were measured to characterize the crystal structures of PA6 at the interphase and around the CFs. The results showed that higher α-crystal (the most thermodynamically stable crystal) content of PA6 at the interphase compared to β-crystal (mesophase) and γ-crystal (thermodynamically metastable) contributed to greater transverse tensile strength of unidirectional composites. Furthermore, it was demonstrated that the flexural strength of the quasi-isotropic composites increased in the case of small variations in the distribution of each crystal structure (α-, β-, and γ-crystal structures) around the CFs. This study provides a framework for the PA6 crystals around CFs to reflect the superior properties of CFs in PA6/CF composites.

Memory Effects in the Quiescent Crystallization of Polyamide 12: Self-Seeding, Post-Condensation, Disentangling, and Self-Nucleation beyond the Equilibrium Melting Temperature

This paper addresses the role of memory effects from earlier thermal protocols on the non-isothermal crystallization behavior of polyamide 12 (PA12). It turns out that memory effects can induce differences in crystallization temperatures as large as 30 °C when cooled from the melt at 10 °C/min. DSC experiments reveal that memory effects in PA12 result from (1) self-nucleation by the action of crystalline residues (self-seeding), (2) chemical post-condensation, and (3) polymer chain disentangling and re-entangling. Self-nucleation and disentangling facilitate crystallization, while melt post-condensation and re-entangling hamper it. It was demonstrated that repeated crystallization stimulates disentangling and that re-entangling in the liquid state proceeds more rapidly the higher the temperature. Effects due to solid-state post-condensation are very intriguing and particularly relevant to laser sintering based additive manufacturing. It was observed that solid-state post-condensation at 175 °C for 1 h negatively affects crystallization in a subsequent heating/cooling cycle but that solid-state annealing at 175 °C for 20 h leads to an enhanced crystal perfection and chain rearrangements whose non-crystalline self-nucleating remnants in the melt at 240 °C need more than 1 h to relax and no longer stimulate crystallization during cooling. Importantly, 240 °C is higher than the equilibrium melting temperature of PA12.

Mesoscale simulations of spherulite growth during isothermal crystallization of polymer melts via an enhanced 3D phase-field model

A finite difference-based 3D phase-field model is developed to investigate the spherulite growth at the mesoscopic scale during the isothermal crystallization of polyamide (PA) 12. The model introduces a phase-field variable to distinguish the crystalline and amorphous phases of polymers. The phase-field evolution equation is coupled with the heat conduction equation that considers the latent heat of crystallization. The evolution equations introduce both the dimensionless diffusivity and latent heat that are dependent on the crystallization temperature. A high-order finite difference-based numerical framework is applied to the phase-field model. Both the qualitative simulation results of the phase-field model such as the crystal morphologies and the quantitative results including the radial crystal growth rate, degree of crystallinity, and lamellar thickness are validated against experiments. The simulation for single-crystal growth shows that a high crystallization temperature results in a large crystal with a slow radial growth rate. The simulation for multi-crystal growth shows that the crystals impinge on each other and finally fill the whole domain during crystallization, which further demonstrates the capability of the model in simulating the spherulite growth during isothermal crystallization of polymer melts.

Competition between Heterogeneous Nucleation and Flow‐Induced Crystallization of Polyamide 66 and Its Carbon Nanotube Composites

Competition between Heterogeneous Nucleation and Flow‐Induced Crystallization of Polyamide 66 and Its Carbon Nanotube Composites

An Injecting Molding Method for Forming the HFRP/PA6 Composite Parts.

Carbon/glass fiber-reinforced polymer hybrid composite (HFRP) has the advantages of a light weight and high strength. For the lightweight design of automobile parts, composite parts made of HFRP and polymer materials are increasingly in demand. The method of the injection molding is usually adopted to fabricate composite part with HFRP and polymer materials. The connecting strength between the two materials has an important influence on the service life of the composite part. In this paper, HFRP and polyamide-6 (PA6) were used to fabricate a composite part by the injection molding method. In order to improve the connecting strength between HFRP and PA6, a kind of micro-grooves was fabricated on the HFRP surface. The micro-grooves on the surface of the HFRP provided sufficient adhesion and infiltrating space of molten PA6 material into the mold. In addition, the glass fiber in HFRP can also be used as nucleating agent to facilitate the rapid crystallization of PA6. The micro-grooves on the surface of HFRP were embedded into PA6 like nails, which could improve the connecting strength at the interface effectively. The paper investigated the effects of mold temperature, injection pressure, holding pressure and holding time on the injection quality and connecting strength of composite parts in detail. With a mold temperature of 240 °C, an injection pressure of 8 MPa, a holding pressure of 8 MPa and a holding time of 3 s, the maximum tensile strength of 10.68 MPa was obtained for the composite part. At the effect of micro-grooves, the tensile strength of the composite part could be increased by 126.27%.

Competition between Heterogeneous Nucleation and Flow-Induced Crystallization of Polyamide 66 and Its Carbon Nanotube Composites.

Both heterogeneous nucleation and flow-induced entropy reduction are the two well-known factors that accelerate polymer crystallization. However, the interplay of nucleation and flow-induced acceleration is still poorly understood. This work investigates the nucleating effect of carbon nanotubes (CNT) on both the quiescent and flow-induced crystallization kinetics of polyamide 66 (PA 66). The quiescent crystallization study indicates that CNT acts as a powerful nucleant, as suggested by the fact that the critical cooling rate to bypass crystallization and create the amorphous glassy state changes from 1000 K s-1 in PA 66 neat resin to a rate faster than 4000 K s-1 in the PA 66 nanocomposites. The flow-induced crystallization study indicates PA 66 onset crystallization time and morphology depend on the shear work introduced by rotational rheometry. A combined acceleration effect from CNT nucleants and flow-induced crystallization (FIC) persists when the CNT loading is under the saturation limit. However, if CNT loading meets the saturation limit, specific shear work shows no impact on the crystallization time, providing evidence that the role of the FIC acceleration effect no longer exists when nucleant acceleration dominates the crystallization of PA 66.

Effect of Aliphatic Chain Length on the Stress–Strain Response of Semiaromatic Polyamide Crystals

Effect of Aliphatic Chain Length on the Stress–Strain Response of Semiaromatic Polyamide Crystals

Axial Crystal Growth Evolution and Crystallization Characteristics of Bi-Continuous Polyamide 66 Membranes Prepared via the Cold Non-Solvent-Induced Phase Separation Technique.

Polyamide 66 microporous membranes were prepared by cold non-solvent-induced phase separation using polyamide 66-formic acid-propylene carbonate as a ternary membrane-forming system. The formed membranes exhibited a special bicontinuous structure consisting of interglued spherical crystals or interlocked bundles of microcrystalline aggregates. The variation of the microporous structure under the influence of preparation conditions, solvent, aging time, and polymer concentration affects the comprehensive performance of the membranes. For example, the cold-induced operation and the use of different membrane-forming solvents contributed to the crystallization of polyamide 66, resulting in an increased contact angle of polyamide 66 membranes, obtaining a high resistance to contamination of up to 73.5%. Moreover, the formed membranes still have high mechanical strength.

Efficient polymerization and crystallization kinetics coupling of polyamide 6 synthesis for liquid composite molding process modeling

AbstractVariabilities in polyamide 6 (PA6) composite manufacturing by liquid processes can occur due to polymerization, crystallization, and flow through a fibrous preform. Numerical simulations of the process predicting the kinetics can facilitate manufacturing optimization. This study proposes an efficient modeling approach that can be integrated in current simulation discretization methods such as the finite volume method (FVM) while considering the interaction between PA6 polymerization and crystallization. Using polymerization and crystallization models issued from the literature, a previous study determined Hillier coupling method to be able to predict PA6 kinetics. A simpler and more efficient coupling will be introduced and adapted to account for process variability. It was integrated into an FVM framework for process simulation of injection showcasing the capabilities of the model to predict potential crystallization discrepancies.

Nucleation and crystallization kinetics of polyamide 12 investigated by fast scanning calorimetry

AbstractNucleation and crystallization of polyamide 12 (PA 12) have been systematically investigated by fast scanning calorimetry at non‐isothermal and isothermal conditions. The critical cooling rates of crystallization and crystal nucleation were determined as 300 and 10,000 K/s, respectively. Moreover, the half‐times of nucleation (t1/2,nucl) and overall crystallization (t1/2,cry) show monomodal and bimodal dependencies on the crystallization temperature. t1/2,nucl has an approximate minimum value of about 0.0005 s at 333 K, which is about 10–20 K above the glass transition temperature, and t1/2,cry has two minima of about 0.05 and 0.8 s at about 333 and 383 K, respectively. Comparing the crystallization behavior of PA 12 with other polyamides, the activation energy for crystallization increases and the energy barrier of short‐range diffusion decreases with the increase of the amide‐group density in the chains.

Fracture properties of quasi-isotropic carbon-fiber-reinforced polyamide 6 laminates with different crystal structure of polyamide 6 due to surface profiles of carbon fibers

In this study, the mechanical properties of quasi-isotropic composites were investigated. These composites comprised polyamide 6 (PA6) and two types of carbon fibers (CFs) (CFsmooth and CFrough), and showed comparable mechanical properties but different surface irregularities. The flexural strength and modulus of the two types of PA6/CF laminates were comparable, whereas their fracture patterns differed significantly. The falling weight impact test showed that the impact absorbed energy of the PA6/CFsmooth laminate was higher than that of the PA6/CFrough laminate, which caused extensive delamination and a large number of fiber fractures. In contrast, the low impact absorption energy of the PA6/CFrough laminate resulted in the lack of internal microfractures observed. A polarized micro-Raman study of PA6 in the vicinity of CFs showed that the failure behavior of PA6/CF laminates was related to the distribution of PA6 crystals oriented between CFs in the interlayer.

The crystallization ofPA11,PA12, and their random copolymers at increasing supercooling: From eutectic segregation to mesomorphic solid solutions

The non-isothermal and isothermal crystallization of polyamide 11 (PA11), polyamide 12 (PA12), and their random copolymers was studied over the entire temperature range between the glass transition temperature and the high temperature melting point, using differential scanning calorimetry (DSC) and fast scanning calorimetry (FSC). The DSC and FSC thermal behavior was translated into structural evolutions, relying on earlier gathered X-ray based structural information and theoretical calculations. At low supercooling, the thermal behavior of the copolymers is typical for eutectic systems, involving monomeric segregation and crystallization into separate PA11 and PA12 crystals. Depending on the copolymer composition and the supercooling, the crystals are 4, 5, or 6 monomeric units thick. At very high supercooling, the PA11 and PA12 sequences cocrystallize into mesomorphic solid solutions with isodimorphic characteristics. At intermediate supercooling, the FSC heating traces following isothermal crystallization suggest that crystallization involves the simultaneous, isokinetic production of crystalline and mesomorphic patches. In this temperature range, lamellar crystals are 3 monomeric units thick throughout.