Crystallization of a polyamide 11/organo-modified montmorillonite nanocomposite at rapid cooling

Fast scanning chip calorimetry (FSC) has been used for analysis of the crystallization behavior of a polyamide 11/organo-modified montmorillonite (PA 11/OMMT) nanocomposite. The addition of OMMT leads to a significant increase of the crystallization temperature of the polymer matrix only on cooling faster than about 10⁰ K s^–1. In case of slow cooling at rates typically used in standard differential scanning calorimetry (DSC), the nucleating effect of OMMT on crystallization of PA 11 is negligible. The critical cooling rate to suppress crystallization of PA 11 and to completely vitrify the relaxed melt increases at least by one order of magnitude due to the addition of OMMT. Furthermore, the enthalpy of crystallization is nearly independent on the cooling conditions in the analyzed cooling rate range from 10^–2 to 2?×?10³ K s^–1 in PA 11/OMMT nanocomposites. Isothermal crystallization experiments confirmed that the nucleating effect of OMMT on the crystallization of PA 11 increases with supercooling, being therefore of particular importance at cooling conditions relevant in polymer processing. The evaluation of the kinetics of crystallization of the PA 11/OMMT nanocomposite by FSC and DSC in a wide range of cooling rates/supercooling has been completed by analysis of the effect of OMMT on the α/δ’ polymorphism of PA 11 and the spherulitic superstructure.     

Crystallization kinetics of polyamide 6 containing pigments

In terms of the Avrami equation, the effects of selected organic pigments on the kinetic parameters of the isothermal crystallization of coloured polyamide 6 were examined in the temperature range 468–478 K. It was found that B Blue and BB Red are active nucleating agents, which is acribed to their crystalline structure. G Orange slightly accelerates the crystallization of PA 6 at lower temperatures and concentrations, whereas in other conditions its effect is the opposite.The isothermal data were confirmed by the results of non-isothermal crystallization. The degree of supercooling corresponding to the peak of the DSC curve was the measured feature in this case.It was found that B Blue and BB Red reduce the free energy of formation of criticalsize nuclei by 20 and 12%, respectively, which is reflected in the rates of crystallization.

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Zusammenfassung Mit Hilfe der Avrami Gleichung wurde der Einfluß einiger ausgewählter organischer Pigmente auf die kinetischen Parameter der isothermen Kristallisation gefärbter Polyamide 6 im Temperaturbereich 468–478 K untersucht. B Blue und BB Red erwiesen sich als aktive Keimbilder, was mit Hilfe ihrer kristallinen Struktur erklärt werden kann. Bei niedrigeren Temperaturen und geringeren Konzentrationen wird die Kristallisation von PA 6 durch G Orange leicht beschleunigt, unter anderen Bedingungen kann das Gegenteil beobachtet werden. Die isothermen Daten wurden durch die Ergebnisse bei nichtisothermer Kristallisation bekräftigt. In diesem Falle wurde das dem DSC Peak entsprechende Maß an Unterkühlung untersucht. Es wurde gefunden, daß B Blue und BB Red die freie Energie zur Bildung von Keimen kritischer Größe um 20 bzw. 12% herabsetzen, was sich in den Kristallisationsgeschwindigkeiten widerspiegelt.

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, -6 468–478 . , , . , . . . , , , 20 12%, .

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This work was supported by the Polish Academy of Science, goal 01.14.     

Isothermal and nonisothermal crystallization kinetics of novel odd-odd polyamide 9 11

A study on isothermal and nonisothermal crystallization kinetics of odd-odd polyamide 9 11 was carried out by differential scanning calorimetry (DSC). The equilibrium melting temperature of polyamide 9 11 was determined to be 199.1 °C. The Avrami equation was adopted to describe isothermal crystallization of polyamide 9 11. Nonisothermal crystallization was analyzed using both the Avrami relation modified by Jeziorny and the equation suggested by Mo. The isothermal and nonisothermal crystallization activation energies of polyamide 9 11 were determined to be −310.9 and −269.0 kJ/mol using the Arrhenius equation and the Kissinger method, respectively.     

Fire response of polyamide 6 with layered and fibrillar nanofillers

To improve the fire retardancy performance of polyamide 6 (PA 6), layered silicates are used in combination with multi-walled carbon nanotubes (CNTs) to address the issue of packing density and uniformity of the protective inorganic barrier formed at the burning surface of the nanocomposites during combustion. Benzimidazolium is used to modify the silicate layers instead of conventional alkyl ammonium surfactants to tackle the issue of thermal stability. However, the poor dispersion of the modified layered silicates in PA 6 has a significant negative effect on the fire performance of the materials and offsets the positive effect of CNTs. Also, the results point to the apparent contradictions with different fire exposure tests; that is, significant reductions in heat release and mass loss rates in the cone calorimetry test do not imply a higher rating in the UL 94 vertical burning test.     

Clay‐reinforced polyamide: Preferential orientation of the montmorillonite sheets and the polyamide crystalline lamellae

Intercalated and exfoliated nanocomposites were prepared by the extrusion and injection of polyamide‐6 and highly swollen or slightly swollen montmorillonite, respectively. The microstructure of pure compounds was characterized. The basal spacing was more homogeneous in swollen montmorillonite than in nontreated montmorillonite. This was attributed to the presence of a surfactant. Surfactant crystallites were observed in swollen montmorillonite and in the nanocomposites. Their distribution is discussed. The nanocomposites exhibited anisotropic properties attributed to the orientation of the montmorillonite sheets. The preferential orientation of the montmorillonite sheets was studied in detail by small‐angle X‐ray scattering (SAXS). It was found to be related to the injection direction. An average distance between two sheets was also measured from the SAXS spectra. The crystallographic state of the polyamide matrix was then analyzed. The orientation of the polymer lamellae relative to the montmorillonite sheets is discussed. The orientation of the montmorillonite sheets and the polyamide lamellae is expected to play a major role in the mechanical properties of nanocomposites. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1360–1370, 2001     

Flame retardant polypropylene through the joint action of sepiolite and polyamide 6

The influence of the incorporation of polyamide-6 (PA) and natural sepiolite nanoparticles on both the thermal degradation and fire behaviour of polypropylene (PP) matrix has been investigated by thermogravimetric analysis (TGA) and mass loss calorimetry. For that purpose, PP/PA blends and nanocomposites thereof were prepared by melt processing. TGA results evidenced that the use of maleic anhydride grafted-polypropylene (MA-g-PP) as compatibilizer led to a significant improvement in thermal stability under air. Such improvement was linked to the formation of a char layer preventing the thermo-oxidative degradation of PP. Interestingly, the thermal resistance of this char layer was further improved by adding 5 wt% of natural sepiolite leading to important increase of time to ignition and reduction of peak of heat release rate (pHRR) during mass loss calorimeter test.     

Oxygen barrier properties of organic montmorillonite modified polyamide 11/Poly (vinyl alcohol) films

Organic montmorillonite (OMMT) nano‐platelets were exfoliated and well dispersed in fully bio‐based polyamide 11/Poly(vinyl alcohol) (PA11/PVA) blends. Significantly lower oxygen permeation rates (OTR) were detected for the PA11_72.5PVA_27.5OMMT_x films than those of PA11 and PA11_72.5PVA_27.5 films. An extremely low OTR of 0.218 cm³/m²·day·atm was found for PA11_72.5PVA_27.5OMMT₁ film modified with 1 PHR optimum concentration of well dispersed OMMT nano‐platelets. Similarly, the free volume characteristics evaluated for PA11_72.5PVA_27.5OMMT_x film series reduced to a minimum as the OMMT concentration reached the optimum value. As revealed by dynamic mechanical and differential scanning calorimetric analyses of PA11_72.5PVA_27.5OMMT_x film series, all dynamic glass transition temperature (T_g), melting temperature (T_m) and percentage crystallinity (W_c) values of PA11_72.5PVA_27.5OMMT_x films were noticeably higher than those of PA11_72.5PVA_27.5 film without addition of OMMT. In fact, T_gs, T_ms and W_cs evaluated for PA11_72.5PVA_27.5OMMT_x films increased to a maximum, as their OMMT reached the optimum concentration. The considerably enhanced oxygen barrier resistance found for PA11_72.5PVA_27.5OMMT_x films was ascribed to the considerably reduced free volume characteristics and much longer permeation path caused by impermeably OMMT nano‐platelets well dispersed in PA11_72.5PVA_27.5OMMT_x films.     

Crystallization kinetics in relation to polymer processing

Phase distribution of quenched samples has been determined by a deconvolution procedure of WAXS spectra in a wide range of cooling rates. The informations collected together with isothermal and DSC results provide a very wide set of data on the crystallization kinetics of polymers relevant which covers conditions encountered in most polymer processing operations. They have been compared with predictions of a non-isothermal crystallization model assuming two independent and parallel crystallization processes competing during solidification.     

Composites of recycled nylon 11 and titanium based nanofillers

Nanocomposites of recycled nylon 11 and titanium based nanofillers were manufactured and their thermal and mechanical behavior analyzed. The nanofillers used vary in dimensions, being classified as 0-dimension (nanoparticles) and 1-dimension (nanofibers). The results show that dispersion of the fillers was the key variable governing the thermal and mechanical properties of the composites. The best performance was obtained for composites loaded with 1% and 5% of TiO₂ nanoparticles. For these composites, a 4 fold increase in the Young's modulus was obtained, although the deformation at rupture showed a decrease of almost 50%. The flow curve characteristics at the region of necking stabilization were correlated to a strain-hardening coefficient. It was postulated that the value of this strain hardening coefficient could be used as a measure of neck instability.     

Relaxation processes at the glass transition in polyamide 11: from rigidity to viscoelasticity

Relaxation processes associated with the glass transition in nonferroelectric and ferroelectric polyamide (PA) 11 are investigated by means of differential scanning calorimetry, dynamic mechanical analysis, and dielectric relaxation spectroscopy (DRS) in order to obtain information about the molecular mobility within the amorphous phase. In particular, the effects of melt quenching, cold drawing, and annealing just below the melting region are studied with respect to potential possibilities and limitations for improving the piezoelectric and pyroelectric properties of PA 11. A relaxation map is obtained from DRS that shows especially the crossover region where the cooperative alpha relaxation and the local beta relaxation merge into a single high-temperature process. No fundamental difference between quenched, cold-drawn, and annealed films is found, though in the cold-drawn (ferroelectric) film the alpha relaxation is suppressed and slowed down, but it is at least partly recovered by subsequent annealing. It is concluded that there exists an amorphous phase in all structures, even in the cold-drawn film. The amorphous phase can be more rigid or more viscoelastic depending on preparation. Cold drawing not only leads to crystallization in a ferroelectric form but also to higher rigidity of the remaining amorphous phase. Annealing just below the melting region after cold drawing causes a stronger phase separation between the crystalline phase and a more viscoelastic amorphous phase.     

Preparation and nonisothermal crystallization behavior of polyamide 6/montmorillonite nanocomposites

Polyamide 6 (PA6)/montmorillonite (MMT) nanocomposites were prepared via melt intercalation. The structure, mechanical properties, and nonisothermal crystallization kinetics of PA6/MMT nanocomposites were investigated by X‐ray diffraction (XRD), tensile and impact tests, and differential scanning calorimetry (DSC). Before melt compounding, MMT was treated with an organic surfactant agent. XRD traces showed that PA6 crystallizes exclusively in γ‐crystalline structure within the nanocomposites. Tensile measurements showed that the MMT additions are beneficial in improving the strength and the stiffness of PA6, at the expense of tensile ductility. Impact tests revealed that the impact strength of PA6/MMT nanocomposites tended to decrease with increasing MMT content. The nonisothermal crystallization DSC data were analyzed by Avrami, Ozawa, modified Avrami‐Ozawa, and Nedkov methods. The validity of these empirical equations on the nonisothermal crystallization process of PA6/MMT nanocomposites is discussed. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2878–2891, 2004     

Degradation of polyamide 11 in rotational moulding

The degradation of polyamide 11 (PA 11) was studied in samples rotationally moulded using a range of processing temperatures. The chemical changes occurring in the moulded material were assessed using FTIR, UV-vis spectroscopy, ¹H NMR spectroscopy, gas chromatography, rheometry and microscopy. The results showed that the degradation is complex and depends on the location of the material in the moulding. At the internal surface, the degradation is predominantly thermo-oxidative leading to the formation of carbon dioxide, carboxylic acids and conjugated unsaturated oligo-enimines (UV-vis-active chromophores). Away from the internal surface, the dominant process is through the recombination of broken chains and/or post-condensation reactions that increase the molecular weight without the formation of UV-vis-active chromophores. A degradation mechanism is proposed for PA 11 in rotational moulding.     

Thermal behavior and dehydroxylation kinetics of naturally occurring sepiolite and bentonite

Sepiolite and bentonite have a wide range of industrial applications based on their physicochemical properties such as surface area, thermal behavior, chemical composition, and mineralogic composition. The thermal behavior and kinetics of naturally occurring sepiolite and bentonite were determined in order to give an idea about the potential use of naturally occurring clay minerals in possible applications. Naturally occurring sepiolite and bentonite samples were heated to the temperature that was achieved at the end of the dehydroxylation process. Mineralogic and thermal characteristics of raw and heat treated samples were investigated by X-ray diffraction, Fourier transform infrared spectroscopy, and nitrogen adsorption/desorption analyses. Changes in the structure following heat treatment were used for the evaluation of the dehydroxylation properties of the samples. The dehydroxylation properties of the minerals are strongly affected by the crystal structure. Kinetic analyses, which were related to the dehydroxylation of naturally occurring sepiolite and bentonite, were conducted using dynamic thermogravimetry/derivative thermogravimetry analysis under nitrogen atmosphere. Flynn–Wall–Ozawa, Kissenger–Akahira–Sunose, and Friedman isoconversional methods were used to determine the activation energies of the dehydroxylation reactions of the samples. The results indicate that the activation energy of naturally occurring sepiolite showed a little variation at a particular conversion rate (0.3–0.7), while the activation energy of naturally occurring bentonite showed a significant variation within the range of variation of the conversion rate. The present study shows that the dehydroxylation reactions of naturally occurring sepiolite and bentonite were single mechanism reaction and complex mechanism reaction, respectively.     

Kinetics of styrene emulsion polymerization in the presence of montmorillonite

The effect of the pristine sodium montmorillonite (Na⁺-MMT) on the styrene emulsion polymerizations with different concentrations of SDS ([SDS]) was investigated. At constant [SDS], the polymerization rate is faster for the run with 1 wt.% Na⁺-MMT compared to the counterpart without Na⁺-MMT. Micelle nucleation predominates in the polymerizations with [SDS] ≧ 13 mM. On the other hand, the contribution of the polymerization associated with the Na⁺-MMT platelets increases significantly when [SDS] decreases from 13 to 9 mM. At [SDS] (e.g., 2 mM) < CMC, homogeneous nucleation controls the particle formation process and polymerization kinetics. Moreover, the contribution of the Na⁺-MMT platelets that act as extra reaction loci to the polymerization kinetics is even comparable to the run in the absence of Na⁺-MMT. The resultant polymer particle size, polymer molecular weight and zeta potential were characterized and a preliminary model was developed to qualitatively study the differences between the polymerizations in the presence and absence of 1 wt.% Na⁺-MMT.     

Crystallization specifics of carbonate-hydroxylapatite in the presence of strontium-containing agents

The effect of strontium-containing compounds on the crystallization of strontium-substituted carbonate- hydroxylapatite from a human synovial fluid prototype was studied. Synthesis products were investigated by FT-IR spectroscopy, X-ray diffraction, and differential thermal analysis. The amount of strontium in samples was determined by atomic emission analysis. For the synthesized phases, crystallite sizes were calculated by the Selyakov–Sherer formula, and the unit cell parameters were determined. The kinetically stable brushite phase was shown to transform into the more thermodynamically stable phase of strontium-containing carbonate-hydroxylapatite with increasing exposure time of precipitates under the mother solution. Some schemes were proposed for phase transformations under heat treatment.     

Effect of surface modification of montmorillonite on the properties of aromatic polyamide/clay nanocomposites

The surface modification of montmorillonite clay was carried out through ion‐ exchange reaction using p‐phenylenediamine as a modifier. This modified clay was employed to prepare aromatic polyamide/organoclay nanocomposite materials. The dispersion behavior of clay was examined in the polyamide matrix. Polyamide chains were synthesized from 4‐aminophenyl sulfone and isophthaloyl chloride (IPC) in dimethylacetamide. These amide chains were suitably end‐capped with carbonyl chloride end groups to interact chemically with modified montmorillonite clay. The resulting nanocomposite films containing 2–20 wt% of organoclay were characterized by TEM, X‐ray diffraction (XRD), thin‐film tensile testing; thermogravimetric analysis (TGA), differential scanning calorimetric (DSC) and water absorption measurements. Mechanical testing revealed that modulus and strength improved up to 6 wt% organoclay loading while elongation and toughness of nanocomposites decreased with the addition of clay content in the matrix. Thermal decomposition temperatures of the nanocomposites were in the range 225–450 °C. These nanocomposites expressed increase in the glass‐transition temperature values relative to pure polyamide describing interfacial interactions among the phases. The percent water uptake of these composites reduced upon the addition of modified layered silicate depicting improved barrier properties. Copyright © 2008 John Wiley & Sons, Ltd.     

Thermal transfer simulation regarding the rotational moulding of polyamide 11

Simulation of thermal phenomena in rotational moulding is very important to follow the evolution of the temperature in various zones of this process. It was a question of modelling heat gradients developing in rotational moulding part. Thermal model tested take into account the temperature change (thermal transfer mechanism) of melting and crystallization pseudo-stages (enthalpy method). Series of tests in polyamide 11 (PA11) were carried out by means of rotational moulding STP LAB, and non-isothermal crystallization kinetics of rotational moulding PA11 grade are measured and analysed by DSC technique type TAQ20. A result of non-isothermal crystallization of the studied polyamide was confronted with Ozawa model. In order to test the validity degree of enthalpy method (layer to layer), another approach based on Ozawa model has also been used in the case of cooling pseudo-stage. As results, the rotational moulding of PA11 was successfully carried out. The simulation of the fusion and crystallization stages, by application of Ozawa model coupled with enthalpy method gave a good representation of experimental data.     

Single-molecule enzyme kinetics in the presence of inhibitors

Recent studies in single-molecule enzyme kinetics reveal that the turnover statistics of a single enzyme is governed by the waiting time distribution that decays as mono-exponential at low substrate concentration and multi-exponential at high substrate concentration. The multi-exponentiality arises due to protein conformational fluctuations, which act on the time scale longer than or comparable to the catalytic reaction step, thereby inducing temporal fluctuations in the catalytic rate resulting in dynamic disorder. In this work, we study the turnover statistics of a single enzyme in the presence of inhibitors to show that the multi-exponentiality in the waiting time distribution can arise even when protein conformational fluctuations do not influence the catalytic rate. From the Michaelis-Menten mechanism of inhibited enzymes, we derive exact expressions for the waiting time distribution for competitive, uncompetitive, and mixed inhibitions to quantitatively show that the presence of inhibitors can induce dynamic disorder in all three modes of inhibitions resulting in temporal fluctuations in the reaction rate. In the presence of inhibitors, dynamic disorder arises due to transitions between active and inhibited states of enzymes, which occur on time scale longer than or comparable to the catalytic step. In this limit, the randomness parameter (dimensionless variance) is greater than unity indicating the presence of dynamic disorder in all three modes of inhibitions. In the opposite limit, when the time scale of the catalytic step is longer than the time scale of transitions between active and inhibited enzymatic states, the randomness parameter is unity, implying no dynamic disorder in the reaction pathway.