Crystallization of polyamides under elevated pressure: 4. Annealing of nylon-6 (polycapramide) under pressure

A study has been made on the annealing of nylon-6 under elevated pressure. Heat treatment of meltcrystallized nylon-6 at 6.5 kbar and 20°C below the beginning of melting for a period of 120 h yielded an increase in the heat of fusion from 14.2 to 41.2 cal/g and an increase in atmospheric melting temperature from 222° to 256°C (1 kbar = 100 MN/m²; 1 cal/g = 4.187 kJ/kg). Stepwise annealing by exposing nylon-6 to progressively higher temperatures at 6.5 kbar led to a heat fusion of 40.8 cal/g and a melting temperature of 269°C. Annealing was found to be particularly effective in improving the crystalline structure at pressures exceeding 4 kbar. The rate of annealing at 6.5 kbar increased with temperature in the range between 260° and 280°C. Electron microscopy of fracture surfaces disclosed that annealing could give rise to a marked increase in lamellar thickness. Wide-angle X-ray diffraction showed that crystal growth also occurred in the lateral direction and that the alpha-crystalline modification was preserved during annealing. From a comparison between the melting characteristics of nylon-6 obtained by pressure-induced crystallization from the melt and by annealing under pressure of folded-chain material, it is inferred that the folded-chain lamellar state may be an essential intermediate stage of the chain extension in polyamides under pressure.     

Crystallization of polyamides under elevated pressure: 5.Pressure-induced crystallization from the melt and annealing of folded-chain crystals of nylon-11, poly(aminoundecaneamide) under pressure

High pressure dilatometry, differential scanning calorimetry, electron microscopy, X-ray diffraction, and infra-red spectroscopy to study how the crystallization of nylon-11 from the melt, as well as annealing of the folded-chain crystals, are affected by pressure in the range from 1 to 10 kbar (1 kbar = 100 MN/m²) and temperature in the range from 200° to 320°C. Pressures exceeding 3 kbar and temperatures higher than 230°C are sufficient for growth of the chain-extended crystals of nylon-11 either by pressure-induced crystallization from the melt or by annealing of the folded-chain crystals. Crystallization from the melt or annealing at 320°C or higher, and 10 kbar, resulted in crosslinking of the polymer. The highest melting temperature and heat of melting found for the chain-extended crystals of nylon-11 were 226°C and 35 cal/g respectively, as compared to 190°C and 13.6 cal/g for the folded-chain material. The texture of the chain-extended crystals of nylon-11 was found to be spherulitic with well developed striations forming circle patterns, and polymer chains passing through several lamellae. No sharp boundaries were found between the chain-extended lamellae. The alpha-crystalline modification, found for the folded-chain crystals of nylon-11, was preserved in the high pressure crystallization and annealing experiments. Infra-red absorption bands at 1420 and 1225 cm^?1 seem to be associated with the presence of folds in the nylon-11 crystals. It is suggested that, during the initial stage of crystallization under pressure, folded-chain crystals are formed, with a crystalline order and long spacing larger than that of the starting nylon-11.     

Crystallization of polyamides under elevated pressure: 6.Pressure-induced crystallization from the melt and annealing of folded-chain crystals of nylon-12, polylaurolactam under pressure

The influence of pressure on the crystallization and annealing of polylaurolactam, nylon-12, has been investigated. The increase of the final melting temperature of this polyamide with pressure amounted to 20°C per kbar as determined by high pressure dilatometry. Crystallization as well as annealing under pressure led to a partial transformation of the pseudo-hexagonal or monoclinic crystal structure to an alpha modification. Samples crystallized at a pressure of 4.9 kbar (1 kbar = 100 MN/m²) displayed multiple melting behaviour, whereas annealing under pressure gave rise to one melting peak in the d.s.c. thermograms. The heat of fusion could be enhanced from 16 to 32 cal/g and the melting peak temperature could be increased from 179° to 209°C by annealing under 4.9 kbar and 260°C for 336 h. Small-angle X-ray scattering curves reveal that annealing brings about considerable broadening of the distribution of the crystal dimensions. The pressure treated nylon-12, consisting of well developed spherulites, could be fractured very easily along inter-spherulitic and trans-spherulitic planes. The striations in the fracture surfaces as observed in the electron microscope were arranged perpendicular to the radial arms of the spherulites. Annealing at 320°C and 10 kbar for 48 h caused efficient crosslinking of the polylaurolactam.     

Microstructure, nucleation and thermal properties of high-pressure crystallized MWCNT/nylon-6 composites

Multi-wall carbon nanotube (MWCNT)/nylon-6 composites made by in-situ polymerization and subsequently modified by treatment at 1.0 GPa (or 1.7 GPa) and 530 K have been studied by WAXD, DSC and NMR. The pressure treatment gives an amorphous to crystalline transformation where the crystallinity increases from ∼31% to as much as ∼58% concurrently as the nylon-6 crystals increase in size and attain a preferred orientation relative to the applied pressure. A composite of 2.1 wt% purified MWCNT in nylon-6 shows significantly higher melting temperature than neat nylon-6 after identical pressure treatments. The improved thermal stability of the composite is attributed to crystal growth in the presence of reinforcing MWCNTs. The NMR spectrum of a pressure treated composite is similar to that of nylon-6 single crystals, which suggests a reduction of crystal boundaries after treatment, but there is no indication of covalent bonds between the nylon-6 chains and the MWCNTs.     

Preparation of high-modulus nylon-6 fibre by an improved zone-annealing method

We have prepared high modulus, high strength nylon-6 fibres from crystalline polymers by an annealing method called ‘zone-annealing’. In this study the zone-drawing was repeated 4 times (heater temperature 80°C; heater moving speed 40 mm min^?1; under tension of 1.6 kg mm^?2). Zone-annealing conditions were decided after numerous preliminary experiments. It was thought that the amorphous molecular chains become selectively loose when the tension is removed after zone-annealing and relaxation leads to a decrease in macro-modulus. This was prevented by heat-setting on the zone-drawn and zone-annealed nylon-6 fibre.     

Crystallization of polyamides under elevated pressure: 2. Pressure-induced crystallization of nylon-6 (polycapramide) from the melt

A study has been made on the crystallization of nylon-6 from the melt under elevated pressures. Crystallization induced by pressures of up to 8 kbar at temperatures between 270° and 310°C did not lead to a significant increase of the melting temperature for nylon-6 containing 8% caprolactam. However, the melting peak temperature, as determined by differential scanning calorimetry was found to increase from 220° to 250°C for nylon-6 without caprolactam and crystallized under pressures exceeding 5 kbar for 50 h. The heat of melting of the nylon specimen crystallized under these conditions increased from 14 to 37 cal/g. Thermal decomposition of the polymer could be diminished by heating under pressure and extruding the nylon under vacuum prior to the high pressure crystallization experiments. The specific volume diminished gradually during isothermal crystallization and the melting temperature was found to increase with crystallization time. These observations point to a one stage process for the development of extended-chain crystals of nylon-6. The highest melting peak temperature of 256°C was recorded on nylon-6 which was crystallized at 315°C and 8 kbar for a period of 320 h.     

Thermal properties and transition studies of multi-wall carbon nanotube/nylon-6 composites

Transition behavior and thermal properties of a multi-wall carbon nanotube (MWCNT)/nylon-6 composite (P-composite) made by in situ polymerization and subsequently structurally modified by high-pressure–high-temperature treatment have been established. The thermal conductivity (κ) of nylon-6 improved ∼27% by the addition of 2.1 wt.% MWCNT filler simultaneously as the heat capacity per unit volume decreased ∼22% compared with that of nylon-6 at 1 atm and 298 K. Moreover, the MWCNT filler raises the glass transition temperature (T_g) of nylon-6, but the pressure dependence of T_g remains unchanged. A model for κ indicates that the interfacial thermal resistance between the MWCNT filler and the nylon-6 matrix decreases 20% up to 1 GPa and most significantly above 0.8 GPa. P-composite was structurally modified by a sluggish cold-crystallization transition at 1.0 GPa, 530 K, which further increased κ by as much as ∼37% as the crystallinity of nylon-6 improved from 31% to 58% with a preferred crystal orientation and increased crystal size.     

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.     

Direct radiation grafting of acrylic acid to nylon-6 fabric and the behaviour of the resulting graft copolymer

The role of certain cations as homopolymer inhibitors in the direct radiation grafting of acrylic acid to nylon-6 fabric was investigated. The grafting solution was methanol and water at a ratio of 30 : 70 vol%. The maximum graft yield was obtained at 0·08 wt% when using Fe²⁺ and at 0.7% for Fe³⁺. In addition, the graft yield obtained with the latter ion was higher than that for the former. Moreover, the presence of Na⁺ and K⁺ salts in concentrations as low as 0·1 wt% caused an increase in the graft yield. The hydrophilic properties investigated indicated that the water absorption of nylon-6 fabric after 9 days increased by 44 times after grafting with poly(acrylic acid) at 166% graft yield. This ratio became 123 times, when this graft copolymer was transformed to the sodium salt. These ratios became 105 and 490 times for the corresponding recrystallized forms, respectively. A study was made to gain a better understanding of the observed super water absorption using SEM and DSC analysis. SEM micrographs of the recrystallized copolymer indicated the formation of large pores with a dendritic structure. Moreover, DSC showed a decrease in both the heat of fusion and melting point. The grafted nylon-6 fabric showed a noticeable affinity for different dyestuffs.     

In situ Polymerization of Multi-Walled Carbon Nanotube/Nylon-6 Nanocomposites and Their Electrospun Nanofibers

Multiwalled carbon nanotube/nylon-6 nanocomposites (MWNT/nylon-6) were prepared by in situ polymerization, whereby functionalized MWNTs (F-MWNTs) and pristine MWNTs (P-MWNTs) were used as reinforcing materials. The F-MWNTs were functionalized by Friedel-Crafts acylation, which introduced aromatic amine (COC₆H₄-NH₂) groups onto the side wall. Scanning electron microscopy (SEM) images obtained from the fractured surfaces of the nanocomposites showed that the F-MWNTs in the nylon-6 matrix were well dispersed as compared to those of the P-MWNTs. Both nanocomposites could be electrospun into nanofibers in which the MWNTs were embedded and oriented along the nanofiber axis, as confirmed by transmission electron microscopy. The specific strength and modulus of the MWNTs-reinforced nanofibers increased as compared to those of the neat nylon-6 nanofibers. The crystal structure of the nylon-6 in the MWNT/nylon-6 nanofibers was mostly γ-phase, although that of the MWNT/nylon-6 films, which were prepared by hot-pressing the pellets between two aluminum plates and then quenching them in icy water, was mostly α-phase, indicating that the shear force during electrospinning might favor the γ-phase, similarly to the conventional fiber spinning.     

Thermal expansivity of oriented nylon-6 and nylon-6,6

Measurements on the thermal expansivities along (α_∥) and normal (α⊥) to the draw direction have been carried out from 120 to 380K for nylon-6 and nylon-6,6 with draw ratio λ between 1 and 3.6. The sharp drop in α∥ and the slight increase in α⊥ with increasing λ can be attributed to the gradual alignment of chain segments in both the crystalline and amorphous regions. Using the presently available expansivity data on nylon-6 crystals it is found that the observed orientation dependence below the glass transition can be quantitatively described by a two-phase aggregate model. Water acts as a plasticizer and so its absorption leads to a drop of about 80K in the glass transition temperature. At low temperature, however, water appears to form bridges between molecular chains. This gives rise to a stronger interchain interaction and hence a lowering of the thermal expansivity.     

Trifluoroethanol/chloroalkane mixtures: Excellent novel solvents for aliphatic polyamides

Certain mixtures of TFE/CHCl₃ were found to be excellent solvents for aliphatic polyamides such as nylon-6, nylon-66, nylon-11, nylon-12, nylon-69, nylon-610, and nylon-612. Intrinsic viscosities were measured and Mark–Houwink coefficients determined for nylon-6/TFE/CHCl₃, indicating TFE-rich mixtures to be better solvents than TFE alone. Similar results were obtained for TFE/CH₂Cl₂. Four ternary phase diagrams were constructed for nylon-6/TFE/chloroalkanes, of which the ones with CHCl₃ and CH₂Cl₂ are the most interesting. In the nylon-6/TFE/CHCl₃ diagram higher nylon-6 solubility in TFE-rich mixtures and a biphasic region in the CHCl₃-rich compositions are evident. In the TFE/CH₂Cl₂ system, the higher dissolution of nylon-6 is observed, but no biphasic regions are detected. In certain solvent compositions and/or polymer concentrations the polymer is incompletely soluble, making the phase diagrams rather complicated. Observed thermodynamic excess properties appear to relate to the quality of TFE/chloroalkanes as solvents for nylon-6. Studies on swelling of nylon-6 networks immersed in TFE/CHCl₃ show a behaviour previously described by Krigbaum and Carpenter for a general case where the formation of contracts between molecules of the two solvents is discouraged and their preferential solvation of the polymer is, therefore, encouraged. The phenomena observed in this work can be qualitatively explained as arising from antagonistic interaction of TFE molecules with chloroalkane ones. The presence of the polyamides in solution reduces such contacts, enchancing the dissolution of the polymer in the (TFE-rich) solvent mixtures.     

A possible mechanism of chain extension in nylon-6 during crystallization under pressure

The effect of pressure on crystallization from the melt and annealing of nylon-6 fractions has been investigated. Pressures exceeding 5 kbar and temperatures 40°C below the final melting temperature of the fractions under pressure, were sufficient to produce the extended-chain crystals by an isothermal crystallization from the melt as well as by annealing of the chain-folded material. Crystalline structure and the thermal characteristics of the extended-chain crystals formed from the fractionated nylo?n-6, were identical to those of unfractionated polymer. G.p.c. analysis of the extended-chain material formed from the fractions of narrow molecular weight distribution showed that this material most probably had a wide molecular weight distribution, similar to that of unfractionated nylon-6. Based on a change in the molecular weight distribution and decrease in the molecular weight, concurrent with the extended-chain crystallization it is suggested that on the thermal treatment of polyamides under pressure a transamidation reaction occurs between the CONH groups of the broken folds of adjacent lamellae, which may lead to the chain extension.     

Synthesis of Poly(∊-caprolactam) and Poly(ethyleneterephthalate) Star Polymers

Abstract

Two kinds of condensation-polymerization star polymers were prepared. One kind is star nylon-6 and the other is star PET. In both star polymers the arms are flexible and the cores are rigid aromatic fractal polyamides (FPs). The FPs are porous and of size comparable to the size of the flexible arms of the stars. The FPs are decorated with reactive sites appropriate for the grafting or growing of star arms. In the case of star nylon-6, two preparation methods are described: grafting of pre-existing nylon chains onto FPs, and growing nylon-6 arms from the FPs by polymerization of caprolactam in the presence of FPs. In the case of star PET, grafting of pre-existing PET chains was employed in order to create the star polymers. Various characterization techniques indicated that in the grafted star polymers up to 10 arms could be attached to each FP core. The results indicate, however, that fine control of the star formaton was not achieved yet. The required conditions to reach this target are spelled out.     

Polyaniline-Coated Short Nylon Fiber/Natural Rubber Conducting Composite

Natural rubber-Polyaniline (PANI)-Polyaniline coated short nylon-6 fiber (PANI-N6) composites were prepared by mechanical mixing and its cure characteristics, filler dispersion, mechanical properties, conductivity and thermal stability were evaluated. PANI was synthesized by chemical oxidative polymerization of aniline in presence of hydrochloric acid. PANI-N6 was prepared by in situ polymerization of aniline in the presence of short nylon-6 fiber. The composite showed higher tensile strength, tear strength and modulus values and lower elongation at break. The DC electrical conductivity and the thermal stability of the composites increased with PANI and PANI-N6 concentration. The highest conductivity obtained was 1.99 × 10^?6 S/cm.     

The setting angle of extended chain linear polyethylene

The unit cell parameters and the setting angle have been estimated for a series of linear polyethylene specimens crystallized at pressures from atmospheric to 6 kbar. Slight variations in unit cell parameters occur, however, significant differences can be discerned in the setting angles as a function of crystallization pressure. It is suggested that the setting angle of extended chain crystals is the equilibrium value for the crystal and that the setting angle is influenced by the presence of folded chains and the structure of the fold surface. Such a hypothesis would account for the spread in published values of setting angles for specimens crystallized both at atmospheric pressure and under strain.     

Morphology of undeformed and deformed polyethylene lamellar crystals

Morphology of undeformed polyethylene crystals obtained by high pressure crystallization was investigated by SEM. It was revealed by exposing the interior of the samples by microtoming followed by permanganic etching. The etching procedure was refined to reveal defected sites of lamellae in addition to differentiation of crystalline and amorphous phases. From 1 to 3 screw dislocations with large Burgers vector per 1 μm² of lamellae basal planes were detected. Lamellae, when viewed edge-on give an impression of a “blocky architecture”, while their real shape, as seen on SEM images of flat-on and oblique lamellae, resembles platelets with a few defects in the form of screw dislocations protruding a platelet.High pressure-crystallized polyethylene samples were deformed plastically by uniaxial compression and were studied by SEM and AFM. When the deformation is interrupted dislocations are arrested within the crystals. It was observed that in contrast to undeformed samples, the side faces of deformed lamellae were not any longer smooth and a large number of screw dislocations with low Burgers vectors crossing the lamellae thickness could be distinguished. These observations are in accordance with polymer crystal plasticity theory that relies on the rate controlled nucleation and propagation of screw dislocations across polymer crystals. An existence of numerous screw dislocations arrested in lamellae is a direct proof of action of fine crystallographic slips along the macromolecular chains in PE crystals during plastic deformation. The kinking of lamellae due to plastic deformation was also observed. Large sections of lamellae between kinks rotated towards the plane of compression while the chain stems in lamellae rotated in the opposite direction, away from the compression direction, which is a signature of the fine crystallographic slip.Plastically deformed polyethylene crystals are highly defected due to many dislocations incorporated within them - the density of dislocations was approximated as 10¹⁶ m⁻². However, deformed crystal melting temperature is nearly unaffected while the heat of melting is slightly reduced, yet only in thin crystals. It suggests that the arrested dislocations contribute more to the surface energy of lamellae basal planes rather than to a bulk energy of polyethylene crystals.     

Synthesis and characterization of aliphatic poly(amide urethane)s having different nylon 6 segments through non-isocyanate route

Two nylon-6 oligomers having polymerization degrees of 4.99 and 3.55 terminated with H₂N– and HO– groups (H₂N-PA-OHs) were prepared by the ring-opening polymerization of caprolactam with ethanolamine at different molar ratios. These oligomers were transformed into HO–terminated nylon-6 oligomers (HO-PA-OHs) through reaction with excessive caprolactone. The transurethane polycondensation of HO-PA-OHs with a diurethanediol, i.e., 1,6-bis(hydroxyethyloxy carbonyl amino)hexane (BHCH), was carried out at 170 °C under normal pressure for 4 h and at 180 °C under a reduced pressure of 3 mmHg for 6.5 h. A series of aliphatic thermoplastic poly(amide urethane)s having different short nylon-6 segments (s-PAUs) was prepared. The s-PAUs were characterized by viscometry, gel permeation chromatography, FT-IR, ¹H-NMR, solid ¹³C CP MAS NMR, differential scanning calorimetry, thermogravimetric analysis, wide angle X-ray scattering and tensile tests. Results showed Mn above 29,762, Mw above 36,725, Tm between 128.73 and 171.69 °C, and initial decomposition temperature over 266.25 °C. Tensile strength reached 31.50 MPa with strain at break up to 447.49 %. Some urea linkages were formed during transurethane polycondensation.     

Detection of glass transition in poly(ethylene terephthalate) and nylon-6 by positron annihilation

Positron annihilation lifetime spectroscopy (PALS) was used to investigate the phase transitions, mainly the glass transition, of poly(ethylene terephthalate) (PET) and nylon-6 during the thermal treatment of these polymers. The longest-lived component lifetime and intensity, indicative of ortho-positronium pick-off, exhibit thermal dependencies that can be attributed to the free-volume changes associated with structural transitions. Glass transition temperatures and the volume of intermolecular-space holes among polymer chains were obtained from the lifetime, τ₃ and intensity of formation, I₃, of the long-lived component of ortho-positronium. For PET, the free-volume fraction and thermal expansion coefficients related to the free-volume fraction were also obtained. Double glass transition behavior was noted in the analyzed polymers, which was consistent with their semicrystalline nature as revealed by differential scanning calorimetry. Increases in the slope of the lifetime-temperature plots for nylon-6 and PET were interpreted to suggest that glass transitions are followed by an increased free-volume cavity expansion as temperature is increased. The intensity response for PET was consistent with the association of glass transition with the reduction of crystalline consraint on segmental mobility in the amorphous phase. In contrast, the intensity behavior during the thermal treatment of nylon-6 seems to be governed more by the electronic effects occurring when the polymer chains acquire mobility than by free-volume changes. Since the sensitivity of PALS is in the order of nanometers, it is expected to give an alternative novel technique to estimate phase transitions and relaxations in polymers from the point of view of the free volume. © 1996 John Wiley & Sons, Inc.