Real-time investigation of crystallization in nylon 6-clay nano-composite probed by infrared spectroscopy

Via time-resolved FTIR, we examined the real-time investigation of the structural change in molecular chain of nylon 6 during crystallization of neat nylon 6 and the corresponding nano-composite (N6C3.7) having fully exfoliated structure. The neat nylon 6 predominantly formed α-phase in the crystallization temperature (T_c) range of 155-195 °C. For N6C3.7 crystallization at low T_c range of 150-168 °C, where the network structure formed by the dispersed clay particles still affected chain folding of nylon 6, the formation of the γ-phase was dominant. The crystallization took place so rapidly (less than 1 s) without induction time of crystallization. At high T_c range (=177-191 °C), the stable growth of the α-phase crystal coexisting with γ-phase occurred in N6C3.7 crystallization. The growth mechanism in the subsequent crystallization processes (amides IIIα and IIIγ) was virtually the same in both N6C3.7 and neat nylon 6.     

Crystallization behavior of nano-composite based on poly(vinylidene fluoride) and organically modified layered titanate

To understand the effect of the nano-filler particles on the crystallization kinetics and crystalline structure of poly(vinylidene fluoride) (PVDF) upon nano-composite formation, we have prepared PVDF/organically modified layered titanate nano-composite via melt intercalation technique. The layer titanate (HTO) is a new nano-filler having highly surface charge density compared with conventional layered silicates. The detailed crystallization behavior and its kinetics including the conformational changes of the PVDF chain segment during crystallization of neat PVDF and HTO-based nano-composite (PVDF/HTO) have been investigated by using differential scanning calorimetric, wide-angle X-ray diffraction, light scattering, and infrared spectroscopic analyses. The neat PVDF predominantly formed α-phase in the crystallization temperature range of 110-150 °C. On the other hand, PVDF/HTO exhibited mainly α-phase crystal coexisting with γ- and β-phases at low T_c range (110-135 °C). A major γ-phase crystal coexists with β- and α-phases appeared at high T_c (=140-150 °C), owing to the dispersed layer titanate particles as a nucleating agent. The overall crystallization rate and crystalline structure of pure PVDF were strongly influenced in the presence of layered titanate particles.     

Crystallization of nylon 6–clay hybrid by annealing under elevated pressure

Nylon 6–clay hybrid (NCH) is a molecular composite of nylon 6 and uniformly dispersed silicate layers of montmorillonite. We found that the phase with the high melting temperature (HMT phase) in the NCH annealed under elevated pressure. The melting temperature of the HMT phase was 240°C. Nylon 6 annealed under elevated pressure did not have the HMT phase. Thus, the presence of the HMT phase was characteristic of the NCH. The relative heat of fusion of the HMT phase (heat of fusion of HMT phase/heat of fusion in the pressure annealed NCH) increased with increase in pressure. High-pressure differential thermal analysis (DTA) measurement revealed that the temperature, at which the relative heat of fusion showed a maximum value, was below about 20°C of the melting temperature of the original NCH under elevated presssure. It was considered that the nylon 6 crystallite near the melting temperature and the molecular mobility under elevated pressure were necessary to the appearance to the HMT phase. © 1994 John Wiley & Sons, Inc.     

Crystallization behavior of nylon 6 nanocomposites

The crystallization behavior of nylon 6 nanocomposites formed by melt processing was investigated. Nanocomposites were produced by extruding mixtures of organically modified montmorillonite and molten nylon 6 using a twin screw extruder. Isothermal and non-isothermal crystallization studies involving differential scanning calorimetry (DSC) were conducted on samples to understand how organoclay concentration and degree of clay platelet exfoliation influence the kinetics of polyamide crystallization. Very low levels of clay result in dramatic increases in crystallization kinetics relative to extruded pure polyamide. However, increasing the concentration of clay beyond these levels retards the rate of crystallization. For the pure nylon 6, the rate of crystallization decreases with increasing the molecular weight as expected; however, the largest enhancement in crystallization rate was observed for nanocomposites based on high molecular weight polyamides; this is believed to stem from a higher degree of platelet exfoliation in these nanocomposites. Wide angle X-ray diffraction (WAXD) and DSC were further used to characterize the polymer crystalline morphology of injection molded nanocomposites. The outer or skin layer of molded specimens was found to contain only γ-crystals; whereas, the central or core region contains both the α and γ-forms. The presence of clay enhanced the γ-structure in the skin; however, the clay has little effect on crystal structure in the core. Interestingly, higher levels of crystallinity were observed in the skin than in the core for the nanocomposites, while the opposite was true for the pure polyamides. In general, increasing the polymer matrix molecular weight resulted in a lower degree of crystallinity in molded samples as might be expected.     

Masterbatch production of polyamide 6‐clay compounds via continuous in situ polymerization from caprolactam and layered silicates

Mechanical properties of thermoplastic polymers can be improved by incorporation of nanoscaled layered silicates. To achieve a significant improvement, the silicates have to be well exfoliated within the polymer matrix. However, it is not always possible to produce exfoliated nanocompounds with the standard procedure of melt compounding. As an alternative to melt compounding, an in situ process for the production of polyamide 6‐nanocompounds is investigated. During the in situ production, the layered silicates are dispersed in the monomer caprolactam prior to the step of polymerization in a twin‐screw extruder, leading to an intercalation of the silicate filler. The production of a polyamide compound containing 0, 2, and 4 wt % nanoscaled silicates was successful. Young's modulus was increased by ～ 30–60%. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Surface Properties – A Key for Nucleation in Melt Crystallization Processes

Numerous organic products which are commercially refined by crystallization exhibit wide metastable zones, for example, xylene, bisphenol‐A, isocyanates, or pyridine derivatives. The practical meaning for layer crystallization processes is that a high degree of subcooling on crystallization surfaces is necessary to start nucleation at the beginning of a crystallization stage. The subsequent crystallization runs then uncontrolled, at much higher rates than designed until the subcooling has been dissipated. As a consequence dendritic crystal growth sets in, which is disadvantageous in terms of the separation efficiency of the crystallization process. A practicable countermeasure is seeding which, however, requires more complex equipment and generates additional process steps, resulting in additional costs. In this work an alternative way of reducing the negative impact of subcooling on crystallization, which is based on the reduction of the metastable zone itself rather than on the bypassing it, has been investigated. The width of the metastable zone depends on the activation energy for nucleation which in turn depends on the interfacial surface tension between the melt and the surface of the crystallization element. It has been shown in this work that the activation energy for nucleation and so the supercooling in a xylene isomer mixture can be considerably reduced when replacing stainless steel by PTFE as a material for the crystallization surface. In follow‐up trials it was found that the crystallization surfaces do not need to be wholly covered by PTFE but that just small PTFE nucleation zones on steel surfaces have the same positive effect on the separation by crystallization. Applied in industrial equipment such nucleation zones might contribute to the cost optimization of commercial layer crystallization processes.     

Crystallization kinetics and melting behavior of nylon 10,10 in nylon 10,10–montmorillonite nanocomposites

The crystallization kinetics and melting behavior of nylon 10,10 in neat nylon 10,10 and in nylon 10,10–montmorillonite (MMT) nanocomposites were systematically investigated by differential scanning calorimetry. The crystallization kinetics results show that the addition of MMT facilitated the crystallization of nylon 10,10 as a heterophase nucleating agent; however, when the content of MMT was high, the physical hindrance of MMT layers to the motion of nylon 10,10 chains retarded the crystallization of nylon 10,10, which was also confirmed by polarized optical microscopy. However, both nylon 10,10 and nylon 10,10–MMT nanocomposites exhibited multiple melting behavior under isothermal and nonisothermal crystallization conditions. The temperature of the lower melting peak (peak I) was independent of MMT content and almost remained constant; however, the temperature of the highest melting peak (peak II) decreased with increasing MMT content due to the physical hindrance of MMT layers to the motion of nylon 10,10 chains. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2181–2188, 2003     

Morphology and crystallization behavior of nylon 6‐clay/neat nylon 6 bicomponent nanocomposite fibers

Nylon 6‐clay hybrid/neat nylon 6, sheath/core bicomponent nanocomposite fibers containing 4 wt % of clay in sheath section, were melt spun at different take‐up speeds. Their molecular orientation and crystalline structure were compared to those of neat nylon 6 fibers. Moreover, the morphology of the bicomponent fibers and dispersion of clay within the fibers were analyzed using scanning electron microscopy and transmission electron microscopy (TEM), respectively. Birefringence measurements showed that the orientation development in sheath part was reasonably high while core part showed negligibly low birefringence. Results of differential scanning calorimetry showed that crystallinity of bicomponent fibers was lower than that of neat nylon 6 fibers. The peaks of γ‐crystalline form were observed in the wide‐angle X‐ray diffraction of bicomponent and neat nylon 6 fibers in the whole take‐up speed, while α‐crystalline form started to appear at high speeds in bicomponent fibers. TEM micrographs revealed that the clay platelets were individually and evenly dispersed in the nylon 6 matrix. The neat nylon 6 fibers had a smooth surface while striped pattern was observed on the surface of bicomponent fibers containing clay. This was speculated to be due to thermal shrinkage of the core part after solidification of the sheath part in the spin‐line. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 39996.     

Crystallization of polycaprolactone–clay nanocomposites

The isothermal crystallization process of polycaprolactone/clay nanocomposites was studied at several temperatures. The effects of the clay type (modified and unmodified) and clay content were analyzed. Bulk crystallization was studied with differential scanning calorimetry and modeled with Avrami's equation. The reinforcement phase lowered the time at which the first crystal nucleus appeared (i.e., the induction time) and fastened the global crystallization rate in comparison with that of neat polycaprolactone. The spherulitic growth was analyzed by optical microscopy with polarized light. The presence of the clay produced more and bigger spherulites in the same time and with the same undercooling degree. All these properties were strongly dependent on the polycaprolactone/clay compatibility and hence the dispersion degree achieved in the nanocomposites. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Toughening and phase separation behavior of nylon 6–PEG block copolymers and in situ nylon 6–PEG blend via in situ anionic polymerization

We have prepared in situ molded products of morphologically different nylon 6/polyethylene glycol (PEG) copolymers and their blends via anionic polymerization of ε-caprolactam in the presence of several kinds of PEG derivatives using sodium caprolactamate as a catalyst and carbamoyl caprolactam derivative as an initiator. Three carbamoyl caprolactams, such as tolylene dicarbamoyl dicaprolactam (TDC), hexamethylene dicarbamoyl dicaprolactam (HDC), and cyclohexyl carbamoyl caprolactam (CCC), with different functionalities and activities were used. Phase separation behavior was investigated by dynamic mechanical thermal analysis (DMTA) and DSC during in situ polymerization and melt crystallization. The mechanical properties of these molded products were evaluated. PEG segments in the block copolymers showed amorphous characteristics, whereas a large fraction of unreacted PEG segments was crystallized in as-polymerized samples, except for the products obtained using the CCC activator. The presence of PEG derivatives retarded the crystallization of nylon 6 part during in situ polymerization as well as melt crystallization. However, PEG segments did not alter the crystalline structure of nylon 6, showing α-crystalline modification. The nylon 6–PEG–nylon 6 triblock copolymers showed the highest impact strength, whereas the nylon 6–PEG diblock copolymers and in situ nylon 6–PEG blends showed no improved toughness. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1285–1303, 1999     

Poly(4‐methyl‐1‐pentene)/clay nanocomposites: effect of organically modified layered silicates

The preparation and properties of poly(4‐methyl‐1‐pentene) (PMP)/clay nanocomposites are reported. Melt intercalation of PMP is carried out with organoclays of different cation/charge exchange capacities and modifiers to facilitate intercalation of the polymer into the silicate layers. The effect of modifiers on the structure and properties of PMP/clay nanocomposites is explored. XRD patterns confirm the intercalation of polymer in the layered silicates as evidenced by the increase in the inter‐layer spacing which is dependent on the type of modifier used. Dynamic mechanical analysis shows increments in the storage modulus over the temperature range studied for all of the three clays, but the extent depends on the type of clay modifier used. The coefficient of thermal expansion is lower for all of the nanocomposites, as compared to the pristine polymer, indicating improved dimensional stability Copyright © 2003 Society of Chemical Industry     

Studies in the properties of nylon 6–glass fiber composites

Caprolactam has been anionically polymerized within the planar-random continuous glass mat reinforcement using a technique similar to reaction injection molding and up to 55% (w/w) [i.e., 33% (v/v)] glass fiber loading was achieved. The fiber volume fraction distribution across the diameter of the composite was observed to be reasonably uniform. The tensile stress–strain properties were determined. Composite modulus and strength appeared to be linearly dependent on the fiber volume fraction and increase with fiber volume content. The type of composite material studied has been used for compression molding of articles. Therefore, some tensile data were redetermined after compression molding and possible changes in degree of crystallinity resulting from the change in the thermal history monitored by differential scanning calorimetry. A 50% drop in the percent degree of crystallinity (monoclinic modification) of the as-polymerized composite and a deterioration in the tensile properties of the composite were observed after compression molding. On compression molding the mold surface needs to be completely covered with the composite sheet material; otherwise, matrix polymer flows out of the composite, and areas deficient in reinforcement result.     

Isothermal crystallization of copolyamides. I. Kinetics of crystallization of nylon 6–piperazine adipate and nylon 6–piperazine terephthalate random copolyamides

The kinetics of primary crystallization from the melt of nylon 6–piperazine adipate and nylon 6–piperazine terephthalate copolyamides were measured dilatometrically. It was found that the crystallization rate of the samples under investigation decreases with increase in the percentage comonomer content and rigidity of molecules over the entire temperature range investigated. The Avrami exponent n varied with temperature, values being from 2 to 4.     

Real-time investigation of crystallization in poly(vinylidene fluoride)-based nano-composites probed by infrared spectroscopy

Via time-resolved Fourier transform infrared spectroscopy (FTIR), we examined the real-time investigation of the conformational changes of poly(vinylidene fluoride) (PVDF) chain segment during crystallization of neat PVDF and the corresponding nano-composites having intercalated structure. It was shown that in the following crystallization processes the crystal growth was virtually the same in both nano-composites and neat PVDF. We have examined an annealing at an infinitely long time at 200 °C (∼20 min) to erase the thermal history in the nano-composites. The dispersed titanate nano-filler particles exhibited strong contribution to enhance the heterogeneous nucleation for the formation of both γ- and β-phase crystals.     

Studies on nylon 6/clay nanocomposites by melt‐intercalation process

The preparation of nylon 6/clay nanocomposites by a melt‐intercalation process is proposed. X‐ray diffraction and DSC results show that the crystal structure and crystallization behaviors of the nanocomposites are different from those of nylon 6. Mechanical and thermal testing shows that the properties of the nanocomposites are superior to nylon 6 in terms of the heat‐distortion temperature, strength, and modulus without sacrificing their impact strength. This is due to the nanoscale effects and the strong interaction between the nylon 6 matrix and the clay interface, as revealed by X‐ray diffraction, transmission electron microscopy, and Molau testing. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1133–1138, 1999     

Crystallization and orientation behaviors of poly(vinylidene fluoride) in the oriented blend with nylon 11

Oriented films of nylon 11/poly(vinylidene fluoride) (PVDF) blend were prepared by uniaxially stretching the melt-mixed blends. The drawn films of fixed length were heat-treated at 170 °C for 5 min to melt the PVDF component, followed by quenching in ice water or isothermal crystallization at various temperatures. The crystal forms and orientation textures of the obtained samples were studied using wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). It was found that PVDF can crystallize into both and β forms in the nylon 11/PVDF blends, and that the content of the β form increases with increasing crystallization temperature above 120 °C. The orientation behavior of the -form PVDF was observed to be dependent on the crystallization conditions: c-axis orientation to the stretching direction was produced for the sample crystallized below 50 °C; the a-axis of crystals was tilted from the stretching direction when PVDF was crystallized at about 75 °C; the parallel orientation of the a-axis to the stretching direction becomes dominant at higher crystallization temperatures (above 100 °C). In contrast, the β crystalline form maintains the c-axis orientation irrespective of crystallization temperature. It was shown by the confocal laser scanning microscopy that cylindrical domains of PVDF were dispersed in the oriented matrix of nylon 11. The mechanism for the formation of the unique orientation textures is discussed in detail. It was proposed that the a-axis orientation is a result of the trans-crystallization of PVDF in the cylindrical domains confined by the oriented matrix of nylon 11. The crystallization kinetics, WAXD analysis, and morphology studies preferred the trans-crystallization mechanism. The mechanical properties of the as-drawn and heat-treated samples were measured not only in the stretching direction but also in the direction perpendicular to it. It was found that the heat-treated samples show slightly lower tensile strength, but more elongation at the break in the two directions than the as-drawn samples.     

Fractionated crystallization in polyolefins–nylon 6 blends

The crystallization behavior of polyolefins–nylon 6 polymer blends was studied by differential scanning calorimetry (DSC) measurements. In these blends, the crystallization of the minor component often starts with distinctly deeper supercooling than that of the pure polymer, and proceeds in several separate steps. The origin of this phenomenon was studied and was related to the volume fraction of the dispersed phase and the compatibility between the dispersed phase and the matrix. © 1994 John Wiley & Sons, Inc.     

Thermal runaway of nylon 6–10 during drawing under constant load

Anhydrous nylon 6–10 filaments were cold drawn (by propagation of a preexisting neck) under constant load. The extension rate i, which is proportional to the neck velocity, was observed to be a continuous function of the load up to a certain critical extension rate i_c above which i increased discontinuously (“runaway”) by approximately two orders of magnitude. If the filaments is in N₂ gas, i_c ? 0.4 cm/min, whereas if it is in He gas, i_c ? 1 cm/min. The structure of the drawn filament produced by runaway is an opaque, microvoid structure which, after a suitable change in load, forms first in the center of a filament and spreads toward the surface. This instability is attributed to the heating of the shoulder of the neck during neck motion. An analysis based on the measured activation enthalpy for neck motion and the thermal properties of nylon and the gas is used to predict i_c values that are in rough agreement with experiment.     

Crystallization behavior of PA‐6 clay nanocomposite hybrid

Polyamide‐6 (PA‐6)/clay (modified montmorillonite) hybrid was synthesized by melt blending at high shear stress. ²⁷Al‐NMR of solid state shows that the clay is not modified after melt blending. Using wide‐line ¹H‐NMR and TEM, it is demonstrated that the nanocomposite exhibits mainly an exfoliated structure. It is shown that the modified montmorillonite induces the crystallization of PA‐6 predominantly in γ‐form. The presence of clay in PA‐6 increases the polymer crystallization temperature, and decreases its melting point. These phenomena show that a certain number of interactions develop near the reinforcing material, and that the latter plays a particular role of nucleating agent. However, the crystallization is not spherulitic and the assumption of macromolecular orientation in the vicinity of the clay is demonstrated by the observations carried out in DSC and AFM. These particular properties of orientation will have a particular importance on the mechanical behavior of the nanocomposite material. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2416–2423, 2002