Structural studies of electrospun nylon 6 fibers from solution and melt

Nylon 6 (N6) fibers have been fabricated via two different electrospinning schemes, from solution of N6 and formic acid at room temperature as well as from N6 melt at elevated temperature. The crystal structures of electrospun N6 fibers from solution and melt, and the annealing effect on the structures were studied by using various techniques. Combined analysis of the differential scanning calorimetry (DSC) at various heating rates, temperature-dependent X-ray diffraction (XRD), and Fourier-transform infrared (FTIR) spectroscopy indicates that N6 fibers from melt predominantly exhibit the meta-stable γ-crystalline forms and low molecular orientation, while solution electrospun fibers from slowly evaporating solvent show both α- and γ-form crystals and higher degree of molecular orientation. At high annealing temperature, the meta-stable γ-crystals in melt electrospun fibers easily transform into thermodynamically stable α-form crystals, while crystals in solution electrospun fibers exhibit higher thermal stability. Nonisothermal modeling and in-situ measurements of jet temperature indicate that rapid quenching due to enhanced heat transfer by electrohydrodynamically driven air flow near the jet is responsible for the less stable γ-crystals and lower degree of molecular orientation in melt electrospun fibers.     

Fast-scanning calorimetry of electrospun polyamide nanofibres: Melting behaviour and crystal structure

This paper discusses the melting behaviour and crystal structure of electrospun (polymorphous) polyamide (PA) fibres and presents the importance of the heating rate used for their study by thermal analysis. A case study of PA6, PA46 and PA69 nanofibres is performed using a Rapid Heat-Cool DSC (RHC), a fast-scanning calorimeter capable of controlled heating rates above 1000 K min⁻¹. Thermal lag becomes significant at scan rates above 500 K min⁻¹ and the ideal scan rate for analysis of the crystal structure depends on the PA type. Measurements at the most suitable heating rate indicate that the thermal stability of the crystals is higher for nanofibres compared to bulk material. Additionally, more stable crystals are favoured in finer nanofibres. This is especially the case for PA6 nanofibres, having an increasing fraction of stable α-crystal phase with decreasing fibre diameter, as shown by both thermal analysis and X-ray diffraction.     

Microfibrillar composites based on polyamide/polyethylene blends. 1. Structure investigations in oriented and isotropic polyamide 6

The present paper discloses the structural changes caused by heating of polyamide 6 (PA6) samples with different thermal and mechanical histories in the 30-240 °C range. Wide and small-angle X-ray scattering (WAXS and SAXS) of synchrotron radiation, as well as solid-state nuclear magnetic resonance spectroscopy (NMR) measurements are performed. The NMR spectra show that in both isotropic and oriented samples there is a co-existence of α and γ-PA6 crystalline forms. Deconvolution of the WAXS patterns is performed to follow the temperature dependence of the unit cell parameters of the α and γ-forms and also of the equatorial (ECI) and total crystallinity indexes (CI), evaluating the contributions of the two crystalline phases. Estimates for the long spacing and for the average thicknesses of the crystalline (l_c) and amorphous (l_a) phases within the lamellae are calculated as a function of the heat treatment employing analysis of the linear correlation function calculated from the SAXS patterns. The X-ray results allowed the conclusion that upon heat treatment up to 160-200 °C, intensive transitions between the PA6 crystalline forms take place, whereby the content of the initial major crystalline phase decreases and that of the initial minor one increases reaching almost 1:1. Close to 200 °C a general trend toward increasing the content of the α-form is registered. The influence of annealing and quenching after melting on the PA6 crystalline structure is also studied.     

Compared structure and morphology of nylon-12 and 10-polyurethane lamellar crystals

A comparative study of the structure and morphology of nylon-12 and 10-polyurethane (10-PUR) lamellar crystals, was carried out. Lamellar crystals were obtained by isothermal crystallization from diluted solution. Electron diffraction of lamellae combined with WAXS data recorded from crystal sediments indicated that nylon-12 crystallized in either α-form or γ-form according to the solvent chosen for crystallization. The α-form was the crystal modification predominant in doubly oriented films of nylon-12 prepared by epitaxial crystallization. On the contrary, 10-PUR invariably crystallized in α-form regardless crystallization conditions. The α-form of nylon-12 and 10-PUR shared the same crystal structure with hydrogen-bonded sheets made of antiparallel chains and stacked with progressive shifting along both b and c directions. Lamellar crystals of nylon-12 in γ-form and 10-PUR in α-form displayed similar morphological features but only the former appeared to be sensitive to temperature. Upon heating the nylon-12 crystals near to melting, the real-time WAXS analysis evidenced the occurrence of a partial γ-to-α crystal transition, and in situ AFM observations revealed the appearing of more or less regular ridges on the crystal surface. None of these changes were observed in 10-PUR crystals when subjected to similar treatment.     

Microstructure and mechanical properties of microcellular injection molded polyamide-6 nanocomposites

The microstructure and mechanical properties of microcellular injection molded polyamide-6 (PA6) nanocomposites were studied. Cell wall structure and smoothness were determined by the size of the crystalline structure, which, in turn, were based on the material system and molding conditions. The correlation between cell density and cell size of the materials studied followed an exponential relationship. Supercritical fluid (SCF) facilitated the intercalation and exfoliation of nanoclays in the microcellular injection molding process. The orientation of nanoclays near the surface of microcells and between microcells was examined and a preferential orientation around the microcells was observed. Nanoclays in the microcellular injection molding process promoted the γ-form and suppressed the α-form crystalline structure of PA6. Both nanoclays and SCF lowered the crystallinity of the parts. Microcells improved the normalized toughness of the nanocomposites. Both microcells and nanoclay had a significant influence on the mechanical properties of parts depending on the molding conditions.     

PA6/稀土复合材料的结构和性能研究

通过熔融共混法制备了聚酰胺6(PA6)/稀土复合材料,采用红外光谱测试仪、差示扫描量热仪、X射线衍射仪等对其结构和性能进行了研究。结果表明,PA6与稀土没有发生化学相互作用,稀土对PA6起到异相成核作用,促进γ晶型的生成,但阻碍了α晶型的完美排列;复合材料的结晶度和结晶温度均高于纯PA6;随着稀土含量的增加,复合材料的拉伸强度略有降低,而断裂伸长率先增加后降低;稀土含量为6 %（质量分数,下同）时,断裂伸长率达到328.25 %;PA6/稀土复合材料的初始亮度随着稀土含量的增加而逐渐增强,而亮度呈指数规律衰减。     

Confined crystallization phenomena in immiscible polymer blends with dispersed micro-and nanometer sized PA6 droplets part 4: polymorphous structure and (meta)-stability of PA6 crystals formed in different temperature regions

The genesis and stability of different PA6 crystalline polymorphs, dispersed as micro- and submicrometer sized droplets inside an amorphous polymer matrix, are discussed over a very broad temperature range. Different PA6 droplet sizes lead to different PA6 crystallization events in a 100 °C wide temperature window that extends down to 85 °C. Static WAXD and DSC experiments on micrometer sized PA6 droplets indicate the formation of a stable γ-crystal phase in the region between 175 and 130 °C. Sub-micrometer sized PA6 droplets only crystallize at 85 °C in the β-phase. Upon heating above the PA6 glass transition, these crystals progressively increase their perfection and ultimately transform into the α-phase around 170 °C.     

原位聚合法制备PA6荧光材料及其性能

将荧光粉分散到己内酰胺（CL）水溶液中,除水以后加入引发剂促使CL阴离子开环聚合制备了一组不同荧光粉含量的聚酰胺6（PA6）荧光材料样品并对其性能进行表征。结果表明,少量荧光粉的加入对PA6晶型影响不大;采用CL阴离子开环聚合制备的PA6荧光材料光至荧光强度较高,其光至荧光强度受其中荧光粉含量影响较大;荧光粉含量为0.15 %（质量分数,下同）的PA6荧光材料荧光强度最强;荧光粉在PA6荧光材料样品中分布比较均匀,且随着荧光粉含量的提高,荧光粉容易团聚;荧光粉的加入未改变PA6的热分解规律,PA6荧光合材料样品的最大分解温度均低于PA6;荧光粉会抑制PA6 γ晶型的形成,促进PA6 α晶型形成。     

Deformation-induced phase transitions of polyamide 12 in its elastomer segmented copolymers

Deformation-induced phase transition behavior of polyamide 12 (PA12) in its segmented copolymers with polytetrahydrofuran (PTHF) was studied with in-situ wide angle X-ray scattering (WAXS) at different temperatures. In these segmented copolymers, which contain a high content of PTHF, a transformation from the stable γ phase to a metastable α″ phase is observed during tensile deformation at room temperature, which shows a similar diffraction behavior to that of the α phase but without an obvious melting point. The deformation-induced α″ phase is not a thermodynamic stable phase but arises from kinetic origins, which is in line with the condition for its formation. After the release of tensile force following deformation, the metastable α″ phase can partially transform back to the initial γ phase. The reversible phase nature may contribute somewhat to the elasticity of PTHF-PA12 systems as a result of this enthalpic contribution. Upon increasing the content of PTHF in the copolymer, the critical stress required to induce the new α″ phase increases. Upon increasing strain, the α″ phase will disappear in the samples that possess a particularly high content of PA12. Higher temperatures also prevent the γ phase from transforming to the α″ phase.     

Direct Observations on Structure Evolutions in Polyamide 6 during Deformation at High Temperatures with WAXS and SAXS

The deformation induced structure evolutions of polyamide 6 (PA6) during uniaxial tension at high tensile temperatures (60 °C and 90°C) were investigated with in situ wide- and small-angle X-ray scattering (WAXS and SAXS) technologies. The obtained data on structure evolutions revealed that they were different from the results measured at low temperature (30 °C). The α-phase got oriented once upon the beginning of deformation. After yielding the γ-phase started to be oriented following the α-phase. While, the breakdown of PA6 crystals along a and c axis overcame partial crystalline orientation at the high tensile temperatures (60 and 90 °C). The competition between stretch of amorphous phase and slippage of lamellae after yielding affected the deformation behavior of PA6. The collapse of lamellae was also confirmed from SAXS analysis and such disrupted lamellar structure resulted in the decrease of long spacing of PA6. The results showed that PA6 materials may show higher ductility at high temperatures. Therefore the crystals could be broken more easily and the formed lamellar fragments of PA6 could be preserved at larger strain at 90 °C. In addition, the yielding of PA6 and γ-phase orientation depended on the lamellar slippage during the deformation. POLYM. ENG. SCI., 60:581–586, 2020. © 2019 Society of Plastics Engineers     

Montmorillonite disperses in amorphous phase of RIM PA6 matrix via anionic ring opening polymerisation

The reaction injection moulding process was employed to prepare montmorillonite (O-MMT)/polyamide 6 (PA6) nanocomposites. O-MMT was functionalised by aminosilane in order to improve the compatibility and decrease the viscosity of caprolactam melts, which swell O-MMT. Small angle X-ray diffraction and transmission electron microscopy results demonstrated that PA6 was sufficiently polymerised via in situ anionic ring opening route at the interlayer of the premodified O-MMT that yields a disordered exfoliated structure. Wide angle X-ray diffraction, polarising optical microscopy and differential scanning calorimetry analysis indicated that the crystallinity of nanocomposites decreased as the loading of O-MMT increased. O-MMT can induce γ-phase crystals and eliminate perfect α-phase crystals into smaller imperfect crystals. The O-MMT primarily disperses in the amorphous phase of RIM PA6 matrix that improves the mechanical properties and heat distortion temperature (HDT) of nanocomposites. For the nanocomposite at 5 wt-% O-MMT, the HDT of neat RIM PA6 was improved from 78 to 163°C.     

Effects of crystallization temperature on the polymorphic behavior of syndiotactic polystyrene

The influence of crystallization temperature on formation of the α- and β-form crystals of syndiotactic polystyrene (sPS) was investigated by X-ray diffraction and non-isothermal differential scanning calorimetry analysis. For sPS samples without any thermal history, the crystallization temperature must be the intrinsic factor controlling the formation the α and β-form crystals. Being crystallized at different cooling rate from the melt, sPS forms the β-form crystal until the temperature cooled down to about 230 °C, and α-form crystal can only be obtained when the temperature was below about 230 °C.     

FT-IR spectroscopic study of hydrogen bonding in PA6/clay nanocomposites

Fourier transform infrared spectroscopy was used to investigate PA6/clay nanocomposites (PA6CN) with various cooling histories from the melt, including rapid cooling (water-quenched), middle-rate cooling (air-cooling) and slow cooling (mold-cooling). In contrast to pure PA6 dominated by the α-phase, the addition of clay silicate layers favor the formation of the γ-crystalline phase in PA6CN.We focus on the reason why silicate layers favor the formation of γ-phase in PA6. Vaia et al. suggested that the addition of clay layers forces the amide groups of PA6 out of the plane formed by the chains. This results in conformational changes of the chains, which limits the formation of H-bonded sheets so that the γ-phase is favored. If this assumption is correct, PA6CN is expected to show some differences as compared with PA6 with respect to hydrogen bonding.The silicate layers were indeed found to weaken the hydrogen bonding both in the α- and γ-phases. This was also confirmed by X-ray diffraction studies. The γ-phase is most likely concentrated in regions close to the silicate layers, whereas the α-phase is favored in the bulk matrix.     

Morphological alteration and strength of polyamide 6 subjected to high plane-strain compression

The present study attempts to find out the similarity and dissimilarity in structure and mechanical properties of PA6 deformed by channel die compression and by a new method combining the compression and rolling and which is known as rolling with side constraints. The resulting materials were characterized by texture investigation by WAXD and SAXS while mechanical properties by tensile tests and DMTA. The fractured surfaces of rolled samples at high deformation ratio was visualized by using SEM. The present study indicates that the crystalline texture and lamellar structure of PA6 samples after the deformation by the two methods are similar at the corresponding strains, some differences develop at a hardening stage at which the time allowed for creep under load plays an important role. Above the compression ratio of 1.8 an intense shear banding occurs at ±45°. With further deformation to compression ratio of 4.0 the shear band planes are tilted 20-26° from the flow direction only. In association with the appearance of shear bands a fraction of γ-form crystals is generated which are oriented with macromolecular chain axes along shear planes and in the direction of shear. Those γ-crystals are misaligned with α-crystals that are deformed and oriented along the flow direction. The bifurcation of orientation of α- and γ-crystals is a reason of a unique fracture behavior of highly oriented samples: fracture occurs along shear planes with γ-crystals and fracture surfaces are nearly flat.     

Free volume sizes in intercalated polyamide 6/clay nanocomposites

The effect of incorporating modified clay into a polyamide 6 (PA6) matrix, on the free volume cavity sizes and the thermal and viscoelastic properties of the resulting nanocomposite, was studied with positron annihilation lifetime spectroscopy, differential scanning calorimetry and dynamic mechanical analysis. At low concentrations of clay the fraction of PA6 crystals melting close to 212 °C was increased, while the fraction of the α-form PA6 crystals, melting close to 222 °C, was reduced. At higher concentrations of clay, a crystal phase with increased thermal stability emerged. Addition of more than 19 wt% clay caused a reduction of the heat of fusion of PA6. An unexpected reduction of the ΔC_p at the glass transition, contradicting the measured reduction of the heat of fusion, was detected, indicating an altered mobility in the non-crystalline regions. The viscoelastic response of PA6/clay nanocomposites, as compared to unfilled PA6, pointed towards a changed mobility in the non-crystalline regions. At high concentrations of clay (>19 wt%) an increase of the free volume cavity diameter was observed, indicating a lower chain packing efficiency in the PA6/clay nanocomposites. The increased free volume sizes were present both above and below the glass transition temperature of PA6.     

Thermal analysis, X-ray and electron diffraction studies on crystalline phase transitions in solvent-treated poly(hexamethylene terephthalate)

The crystal polymorphism, transformation, and morphologies in chloroform solvent-cast poly(hexamethylene terephthalate) (PHT) were examined by using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and temperature in situ transmission electron microscopy (TEM). Solvent-induced crystallization of PHT at room temperature yielded an initial crystal of γ-form, as confirmed by WAXD. Upon DSC scanning, the original γ-form in PHT exhibited three endothermic peaks, whose origins and association were carefully analyzed. The first peak, much smaller than the other two, is in the temperature range of ca. 100-120 °C. It was found that the solvent-induced γ-form was transformed to β-form at 125 °C via a solid-to-solid transformation mechanism. In addition, WAXD showed that γ- and β-forms co-existed in the temperature range of 100-125 °C. These mixed crystal forms were further identified using TEM, and the selected-area electron diffraction (ED) patterns revealed that both γ- and β-form crystals co-existed and were packed within the same spherulite. Solid-solid transformation from the solvent-induced γ-form to β-form in PHT upon heat scanning was presented with evidence and discussed.     

Isothermal crystallization of isotactic polypropylene blended with low molecular weight atactic polypropylene. Part I. Thermal properties and morphology development

The thermal properties and morphology development of isotactic polypropylene (iPP) homopolymer and blended with low molecules weigh atactic polypropylene (aPP) at different isothermal crystallization temperature were studied with differential scanning calorimeter and wide-angle X-ray scattering. The results of DSC show that aPP is local miscible with iPP in the amorphous region and presented a phase transition temperature at T_c=120 °C. However, below this transition temperature, imperfect α-form crystal were obtained and leading to two endotherms. While, above this transition temperature, more perfect α- and γ-form crystals were formed which only a single endotherm was observed. In addition, the results of WAXD indicate that the contents of the γ-form of iPP remarkably depend both on the aPP content and isothermal crystallization temperature. Pure iPP crystallized was characterized by the appearance of α- and γ-forms coexisting. Moreover, the highest intensity of second peak, i.e. the (0 0 8) of γ-form coexisting with (0 4 0) of α-form, and crystallinity were obtained for blended with 20% of aPP, the γ-form content almost disappeared for iPP/aPP blended with 50% aPP content. Therefore, detailed analysis of the WAXD patterns indicates that at small amount aPP lead to increasing the crystallinity of iPP blend, at larger amount aPP, while decreases crystallinity of iPP blends with increasing aPP content. On the other hand, the normalized crystallinity of iPP molecules increases with increasing aPP content. These results describe that the diluent aPP molecular promotes growth rate of iPP because the diluent aPP molecular increases the mobility of iPP and reduces the entanglement between iPP molecules during crystallization.     

Crystalline structure of propylene–ethylene copolymer fractions

Crystalline structure were studied on both block and random propylene–ethylene copolymer fractions, which were obtained by temperature rising elution fractionation. The peak characteristic of α-polypropylene (PP) was observed for all fractions, except the fraction eluted at room temperature. A characteristic peak of polyethylene crystals [i.e., (200) planes] was observed in some fractions, indicating the existence of long ethylene sequence in these fractions. This is in accordance with the analysis from Fourier transform infrared spectroscopy and ¹³C-NMR. The γ-form crystal of PP was observed in these copolymer fractions at atmospheric pressure. It is suggested that the insertion of comonomer into the isotactic PP chain makes the crystallizable sequences sufficiently short and produces the γ-form crystal. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68:381–386, 1998     

Effect of lactic acid on polymer crystallization chain conformation and fiber morphology in an electrospun nylon-6 mat

The role of lactic acid (LA) on the polymer crystallization chain conformation and the surface modification of the electrospun nylon-6 fibers were examined. The effect of different amounts of LA on the polymer crystallization chain conformation of nylon-6 mat was evaluated using XRD, FT-IR and Raman spectroscopy whereas the surface modification of the electrospun mats was examined by FE-SEM, contact angle and mechanical properties measurement. It was found that the transition of meta-stable γ-form into the thermodynamically stable α-form was achieved by increasing the amounts of LA in the blend mixture. The adhesive property of LA was found to be responsible for the transformation from non-bonded to the point-bonded structure of nanofibers in the electrospun nylon-6 mat. The resultant LA/nylon-6 hybrid mat with improved hydrophilicity and mechanical properties may be a potential candidate for tissue scaffold.     

A comparison of the temperature dependence of the modulus, yield stress and ductility of nanocomposites based on high and low MW PA6 and PA66

Nanocomposites with both organically modified and unmodified silicate have been prepared by an extrusion process using low and high molecular weight grades of PA6 and a low MW grade of PA66. Mechanical properties have been tested at temperatures ranging from 20 to 120 °C. The modulus increase in all nanocomposites with organically modified nanocomposites is similar: at room temperature an increase in the modulus of approximately 10% for each wt% of silicate is found. PA66 nanocomposites display an identical normalized modulus increase as PA6 nanocomposites, while unmodified silicate nanocomposites show a smaller increase in the modulus. The yield stress also increases with the addition of layered silicate. Low MW PA6 and PA66 nanocomposites show brittle fracture behaviour at room temperature, while high MW PA6 nanocomposites are ductile. With increasing temperature all nanocomposites become ductile at a certain temperature.