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: 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.     

Crystallization of copolyamides under elevated pressure

The effect of pressure up to 10 kbar and temperature up to 320°C on melting and crystallization behaviour of various random copolyamides has been investigated. Samples used for the studies were obtained by condensation of caprolactam and respectively: piperazine adipate, piperazine terephthalate or hexamethylene terephthalate in various molar proportions. Samples were free from additives. Under comparable conditions of the thermal treatment, the melting temperature and the heat of melting of the crystals grown under pressure increased with increase in temperature and time of crystallization and decreased with increase in the comonomer content. The long periodicity of copolyamides crystallized under pressure increased from 50 to 600–900 Å, the melting temperature and the heat of melting increased from 208°C and 9 cal/g to 228°C and 24 cal/g respectively.     

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.     

Multiple melting endotherms in melt‐crystallized nylon 10,12

The melting behaviour of melt‐crystallized nylon 10,12 was investigated by differential scanning calorimetry (DSC) and wide‐angle X‐ray diffraction (WAXD). The results show that all nylon 10,12 crystals obtained under various conditions, including isothermal, non‐isothermal and stepwise crystallization, and also after partial melting or annealing, show multiple melting behaviour. It was found that each melting endotherm has a different origin. The highest melting peak corresponds to melting of the recrystallized material while the other melting endotherms are related to melting of lamellae with different thicknesses developing under different crystallization conditions. The equilibrium melting point of nylon 10,12 was also firstly estimated to be about 206 °C. © 2001 Society of Chemical Industry     

Influence of Temperature and Time on Isothermal Crystallization of Poly-L-lactide

The thermal behavior of poly-L-lactide (PLLA) isothermal crystallization upon cooling from the melt was investigated using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and polarizing microscope (POM) by changing the crystallization temperature and time. It was indicated that 110°C should be a critical temperature for PLLA melting crystallization. The melting point of crystallized PLLA discontinuously changed with crystallization temperature, increased with temperature, but decreased at about 110°C, and thereafter again increased with higher crystallization temperatures. At 110°C a multiple endothermic peak was observed. PLLA crystals of higher perfection form when crystallized under higher temperature, which reflects the effects of high chain mobility in higher temperatures. During isothermal crystallization, PLLA crystallites become increasingly perfect, and thicken with prolonged time, leading to an increasing melting point.     

Crystallization of polyamides under elevated pressure: 3. The morphology and structure of pressure-crystallized nylon-6 (polycapramide)

The morphology and structure of nylon-6, crystallized from the melt under elevated pressure, has been investigated. Scanning and transmission electron microscopy on replicas of the fracture surfaces reveal that crystallization under pressures exceeding 4 kbar (1 kbar = 100 MN/m²) results in rough lamellar structures with some step heights corresponding to the contour lengths of the chains. Infra-red spectroscopy on the pressure-crystallized nylon-6 shows a considerable improvement of crystalline order, a closer packing of the polymer chains within the crystal, and the presence of a large proportion of free N-H groups. An absorption band at 1170 cm^?1 was assigned to the presence of folds in the nylon-6 crystals. Wide-angle X-ray measurements indicate that the crystal modification of the pressure-crystallized nylon-6 is predominantly the alpha phase. Pressure gives rise to an increase in crystallite dimensions as well as to a decrease of the distance between the crystal planes bonded by the hydrogen bonds and by the Van der Waals forces.     

Isothermal crystallization and melting behavior of short carbon fiber reinforced poly(ether ether ketone) composites

Films of short carbon fiber reinforced poly(ether ether ketone) (PEEK) composite were formed by compression molding pellets for 10 min at 380 °C under air. A heating stage was used to prepare isothermally treated PEEK composites before DSC scan. The dependence of degree of crystallinity on the heating rate (10–80 °C/min) was investigated for specimens crystallized at different temperatures. The results indicated that 50 °C/min was an optimum heating rate to suppress the reorganization and to avoid the superheating of high crystallinity specimens with the sample weight of 10 mg. The upper peak temperature of double-melting peaks continued to increase with crystallization temperature. This peak temperature was related to the transition from regime II to III. The phenomenon of lower crystallinity and higher melting temperature supports the interpretation that the upper melting peak corresponded to crystals growing during the earlier stage of isothermal crystallization.     

Effect of pressure on the melting and crystallization behaviour of isotactic polybutene-1

The formation, melting and phase transition of isotactic polybutene-1 under high hydrostatic pressures were studied by high-pressure d.t.a. and X-ray diffraction up to 5 kbar. The d.t.a. thermogram of melting of form I shows a single endothermic peak up to 5 kbar. Form II crystallized directly from the melt at atmospheric pressure is metastable and it transforms to form I by the application of pressure. Above 900 bar, it transforms to form I completely and the endothermic peak of melting of form II is not observed. On crystallization from the melt under high pressure, the percentage content of form I' increases with crystallization pressure and at 1.6 kbar only form I' is crystallized. Above 2 kbar form II', which shows the same X-ray diffraction pattern as form II, is crystallized from the melt. The percentage content of form II' increases with pressure above 2 kbar, and that of form I' decreases up to 5 kbar. Upon heating under high pressure above 2 kbar, a solid-solid transition from form II' to form I' is observed in d.t.a. traces and the transition is confirmed by high-pressure X-ray diffraction. The melting temperature is expressed in the form of a quadratic equation as a function of pressure for four different forms in IPB-1.     

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.     

Effects of shearing and comonomer content on the crystallization behavior of poly(butylene succinate‐co‐butylene 2‐ethyl‐2‐methyl succinate)

Poly(butylene succinate‐co‐butylene 2‐ethyl‐2‐methyl succinate) (PBSEMS) random copolymers were prepared with different comonomer compositions. The effects of shearing and comonomer content on the crystallization behavior of these copolymers were investigated at 80 °C. The thermal and morphological properties of the resulting samples were also discussed. The copolymers showed a longer induction time and a slower crystallization rate with increasing comonomer content. The promoting effect of shear on the overall crystallization behavior was more notable for those copolymers containing more 2‐ethyl‐2‐methyl succinic acid (EMSA) units. The melting temperature of ‘as‐prepared’ poly(butylene succinate) (PBS) was ca. 115 °C, while that of the copolymers varied from 112 to 102 °C. Higher comonomer contents in the copolymers gave rise to lower melting temperatures and broader melting peaks. In addition, the isothermally crystallized samples showed multiple melting endothermic behavior, the extent of which depended on the comonomer content. The copolymers showed different wide‐angle X‐ray diffraction (WAXD) patterns from that of neat PBS, depending on the comonomer content and shear applied during crystallization. With increasing comonomer content, the copolymers crystallized without shearing, showing the shifting of a diffraction peak to a higher angle, while those crystallized under shear did not show any peak shift. Copyright © 2004 Society of Chemical Industry     

Influence of the method of mixing on homogeneity and crystallization kinetics of blends of linear and branched polyethylene

The influence of mixing method—solution and melt mixing—on the homogeneity and crystallization kinetics of a series of blends of single‐site materials of linear polyethylene and ethyl‐branched polyethylene was studied by differential scanning calorimetry. Data obtained for heats of melting and crystallization, melting and crystallization peak temperatures, and melting and crystallization temperature profiles were essentially the same for the samples obtained by the two mixing methods. The results obtained can be interpreted as indicating that melt mixing is capable of producing homogeneous melts of these relatively low molar mass polymers, given that solution mixing is considered to give perfectly homogeneous blends. The heat associated with the high temperature melting peak after crystallization at 125°C of the blend samples, obtained by the two preparation methods, was higher than that of the linear polyethylene included in the blends, suggesting that a part of the branched polyethylene crystallized at 125°C. The unblended branched polyethylene showed no crystallization at 125°C. Samples obtained by powder mixing showed independent crystallization and melting of the linear and branched polyethylene components. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1730–1736, 2004     

Preparation and characterization of polyamide‐6 with three‐branched chains

With trimesinic acid as a molecular weight regulator, the hydrolytic polymerization of ?‐caprolactam was carried out, and nylon‐6 or polyamide‐6 with three‐branched chains was obtained. Through a systematic study of the effects of conditions such as the reaction time and concentration of trimesinic acid on the polymerization, we found that the conversion of caprolactam was almost insensitive to the initial concentrations of the regulators, but the relative viscosity of the polymer decreased with increasing trimesinic acid. Characterization investigations showed that differential scanning calorimetry curves changed from a single peak for normal nylon‐6 to one main peak and one shoulder or one small peak for the branched polymer; the melting point of the star‐shaped nylon‐6 decreased with an increasing amount of trimesinic acid, whereas its crystallization temperature was higher than that of linear‐chain nylon‐6. A wide‐angle X‐ray diffraction study indicated that the crystal structure of the star‐shaped nylon‐6 still belonged to the α form, and the crystallizability of the branched polymer with an elevated amount of trimesinic acid during polymerization did not seem to be weakened; the characteristic absorption of infrared spectra provided indirect evidence for the existence of branched chains in the polymer. Moreover, the mechanical properties of star‐shaped nylon‐6 and linear‐chain nylon‐6 were compared. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 3184–3193, 2001     

Thermal properties and isothermal crystallization of syndiotactic polypropylenes: Differential scanning calorimetry and overall crystallization kinetics

Isothermal crystallization and subsequent melting behavior of five samples of syndiotactic polypropylene are presented. Crystallization studies were carried out in the temperature range of 60°C to 97.5°C using a differential scanning calorimeter (DSC). Subsequent DSC scans of isothermally crystallized samples exhibited double melting endotherms. The high melting peak was concluded to be the result of the melting of crystals formed by recrystallization during the reheating process. Overall crystallization kinetics was studied based on the traditional Avrami analysis. Analysis of crystallization times based on the modified growth rate theory suggested that, within the crystallization temperature range studied, the syndiotactic polypropylenes crystallize in regime III. Kinetic crystallizability parameters also were evaluated, and were found to be in the range of 0.41°C s⁻¹ to 2.14°C s⁻¹. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 44–59, 2000     

Formation of stable crystalline connecting bridges during the fabrication of polypropylene microporous membrane

The influence of hot stretching ratio and stretching temperature on the connecting bridge structure formed during the fabrication of polypropylene microporous membrane was studied. It was found that with the increase of stretching ratio to 200 % under temperature of 130 °C, bridge length was increased. Under the stretching ratio of 100 %, the main melting peak became wider, whereas similar phenomenon could not be seen under temperature of 110 °C. Under the ratio of 200 % and temperature of 130 °C, strong bridge structure lead to appearance of small shoulder on the right of main melting peak. Compared with that only through cold stretching, more connecting bridges were initiated during hot stretching. The unrestrained tie chains due to the chain disentanglement under high temperature, which were not involved in the annealing crystallization and cold stretching, could be stretched and crystallized into bridges. At the same time, some chains could also be pulled out from the initial lamellae. All these contributed to the increase of bridge number. During hot stretching, the broken bridges merged with other stable bridges and some bridges adhere to each other, resulting in the formation of stable bridges, which will be helpful to the stabilization of pore structure.     

Effects of silver nanoparticles on the dynamic crystallization and physical properties of syndiotactic polypropylene

The effects of silver (Ag) nanoparticles on the physical properties of syndiotactic PP (sPP) were investigated concentrating on the isothermal melt crystallization behavior under shear. sPP with 5 wt % Ag nanoparticles presented higher crystallization temperature (T_c) and heat of crystallization (ΔH_c) than pure sPP. At 90°C, the Ag nanoparticles had little effect on the induction time of crystallization but a little increased the half‐time (t_1/2) for the crystallization. At 100°C, however, the induction time was decreased with increasing the Ag content and the t_1/2 was decreased up to the Ag content of 0.5 wt %. DSC melting endotherms exhibited double melting peaks when crystallized at 90°C under shear but a single melting peak when crystallized at 100°C. The WAXD patterns exhibited that the presence of Ag nanoparticles did not produce any change in the crystal structure of sPP. The tensile strength of sPP is little changed up to the Ag content of 0.1 wt % but it was decreased with further addition. In addition, the introduction of less than 0.1 wt % Ag increased the elongation at break, but further addition decreased it abruptly. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Synthesis and crystallization behavior of nylon 12 14. II: Chain‐folded lamellar crystals and crystalline transformation behavior of nylon 12 14

Chain‐folded lamellar crystals of nylon 12 14 have been grown from a dilute 1,4‐butanediol solution with the “self‐seeding” technique. The morphology and structure of nylon 12 14 lamellar crystals were studied by both transmission electron microscopy (TEM) and wide‐angle X‐ray diffraction (WAXD). Two kinds of electron diffraction patterns were detected when different areas were selected for diffraction, which indicates that the α crystal phase and the β crystal phase coexist for nylon 12 14 under the present crystallization conditions. The WAXD diffractograms of the crystal mats confirm the results obtained from electron diffraction (ED). In addition, the changes of the crystal structure as a function of temperature for melt‐crystallized and dilute solution‐crystallized nylon 12 14 were monitored by variable‐temperature WAXD and variable‐temperature infrared spectroscopy (IR). It was found that the melt‐crystallized sample undergoes a Brill transformation at 80°C–90°C, but no Brill temperature can be observed for the dilute solution‐crystallized nylon 12 14.     

Self‐nucleation–induced nonisothermal crystallization of nylon 6 from the melt

The effect of thermal treatment over a wide range of temperature (130–280°C) on the crystallization behavior of nylon 6 was studied by using DSC, FTIR, and polarized light microscope equipped with a hot stage. The crystallization and the subsequent melting behavior of the nylon 6 samples treated at different temperatures (T_s) were classified into four types. When T_s was higher than 236°C or lower than 213°C, the crystallization behavior of nylon 6 was insensitive to the variation of T_s. When T_s was in the range of 213–235°C, the crystallization behavior was sensitive to the change of T_s. The polarized light microscopic experiments have demonstrated that a large amount of tiny ordered nylon 6 segments/cluster persisted when nylon 6 film are heated to 231°C. Consequently, the fastest crystallization speed was observed. As T_s was between 214 and 223°C, both the T_m and the ΔH_m were higher than those of the nylon 6 samples treated at other temperature. The polarized light microscopic investigations have also demonstrated that molten nylon 6 crystallizes by using the un‐molten nylon 6 crystals as nucleation center at 220°C. Crystallization at higher temperature produces nylon 6 with thicker crystalline lamella. The above results are helpful for rational design of thermal treatment procedure to obtain nylon 6 with different crystalline features. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42413.     

Effect of absorbed water on dynamic modulus for short fiber–CR composites

Short fiber–elastomer composites with 10 vol % fiber, nylon 6–CR and PET–CR composites, absorbed water either in the moisture atmosphere or in water. The effect of absorbed water on the viscoelastic properties for these composites was investigated. The temperature dependence of tan σ for the nylon–CR composite showed that the α-dispersion peak of nylon shifted to lower temperatures with increasing absorbed water content and that after displacement of the α-dispersion peak the additional small hump appeared at about 90°C. For the PET composite, the α-dispersion peak of PET shifted slightly to lower temperatures and the small shoulder at 90°C diminished with increasing absorbed water. The additional dispersion probably was caused by the interface between fiber and CR matrix and was independent of fiber orientation. The results suggested that nylon fiber absorbed a larger amount of water than CR matrix, while the water absorption for PET fiber was considerably less than for nylon fiber. The absorbed water in nylon fiber bonded stronger than that in CR matrix and was only slightly diminished by heat treatment under 100°C.     

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