Dissolution,Hydrolysis And Crystallization Behavior Of Polyamide 6 In Hydrothermal Conditions

Polyamide 6(PA 6) is a semi-crystalline polymer prepared by ring-opening polymerization of ?-caprolactam. Due to the hydrogen bonding between amide groups and dipole-dipole interaction, PA6 has a high melting point(Tm~220 °C) and dissolves only in strong polar solvents. The organic solvents were used in dissolving or processing PA6, not only has a high price, but also cause great pollution to the environment. Compared with strong polar organic solvents, water is an inexpensive, environment-friendly and naturally abundant solvent. So using water as a solvent for material processing is highly desirable. Furthermore, above the ambient boiling point and below the supercritical point(374 °C), water is in the superheated state that has low dielectric constant, weak hydrogen bonding and high mobility. So, under hydrothermal condition, the water molecules have a strong permeability in PA6, which not only induce the change of the internal structure, but also lead to the dissolution and hydrolysis.PA6 pellets and fiber were treated in different hydrothermal systems, and then the crystallization structure, melting and crystallization behavior, molecular interaction and molecular weight of PA6 were characterized by Wide Angle X-Ray Diffraction(WAXD), Small Angle X-Ray Scattering(SAXS), Differential Scanning Calorimetry(DSC), Fourier Transform Infrared Absorption Spectroscopy(FT-IR) and Two-Dimensional Infrared Correlation Spectroscopy(2D-IR) and Ubbelohde viscometer, respectively. The effects of different hydrothermal systems on the dissolution, hydrolysis and crystallization behavior of PA6 were studied by analyses of the results obtained from the various characterizations methodology mentioned above and the main results are summarized as follows:(1) The dissolution and recrystallization temperature of pure PA6 are about 160 °C and 120 °C respectively, which were determined by high pressure DSC. Comparing with the melting and crystallization temperature of dry PA6 pellets, the dissolution and crystallization temperature of PA6 in hydrothermal condition were decreased by 60 and 55 °C respectively.Between 140 °C and 200 °C, the dissolution, hydrolysis and crystallization behavior were studied. According to the changes of the structure and morphology of PA6 after heat treatment, the water heat treatment conditions were divided into four regions: the annealing region, the transition region, the dissolution region and the considerable hydrolysis region. When the hydrothermal temperature varied from 140 to 155 °C, hydrothermal system had the annealing effect on PA6. With the increase of hydrothermal temperature and time, the crystallinity, the thickness of lamellae and the degree of perfection of the crystalline region were increased, which caused the increase of melting temperature of PA6 samples. Melting temperature of PA6 increased to 234? after annealing, making a 13? increment as compared with the original PA6 pallets. When the hydrothermal temperature varied from 140 to 155 °C, the annealing effect of hydrothermal treatment was further enhanced. However, the surface of the PA6 was partially dissolved and then recrystallized. This indicated that the PA6 began to dissolve in the temperature region, and the temperature region is called the transition region. When the hydrothermal temperature was equal to and higher than 160 °C, the PA6 pallets completely dissolved in superheated water and then hydrolysis in different degree. According to the change of molecular weight, the temperature region above 160 ? could be divided into the dissolution region and the considerable hydrolysis region. Comparing with the crystallization behavior of PA6 from the melt, the crystallinity of the samples recrystallized from water increased, but the thickness of the lamellae and the melting temperature did not increase significantly.When the hydrothermal temperature is above 160 ?, the increase of heat treatment temperature and the prolonging of time led to the increase of the degree of hydrolysis of PA6, which result in the decrease of average molecular weight. The molecular weight of the hydrolysis products of PA6 goes down exponentially with the increase of hydrothermal time and temperature. If the molecular weight is 20,000 as the dividing line, the region above 160? can be divided into dissolution region and the considerable hydrolysis region. In the dissolution region, the maximum reduction of PA6 molecular weight is about 10%, but in the considerable hydrolysis region, the molecular weight of PA6 was decreased greatly,and the recrystallization and melting behavior also changed greatly. Finally, based on the TGA of the samples, it was found that the thermal stability of the samples decreased with the increase of the degree of hydrolysis of PA6. When the molecular weight of samples is greater than 10,000, the thermal stability is high. After the study of the annealing and crystallization behavior of PA6 under hydrothermal conditions, the time and temperature window of safe dissolution of PA6 under hydrothermal conditions was found, and it provided useful reference data for the hydrothermal degradation and recycling of PA6.(2) The effects of La Cl3 on the dissolution, crystallization and hydrolysis behavior of PA6 in the hydrothermal system of 160~250 ?, and the interaction between La(OH)2Cl and PA6 molecular chains were studied.When the hydrothermal temperature is between 160 and 200 °C, the La Cl3 has a certain role in promoting the hydrolysis of PA6. Comparing with the pure hydrothermal system, the PA6 recrystallized from solution have relatively lower mass and molecular weight. When the hydrothermal temperature reaches 200 °C, the La3+ ions will be greatly hydrolyzed in solution. Many H+ ions produced from the hydrolysis of La3+ have a catalytic effect on the hydrolysis of PA6, which leads to the cleavage of the C-N bond in the amide group and to form the amino and carboxyl groups, and result in the decrease of the molecular weight of PA6. When the system is cooled down, the hydrolysis products of PA6 will recrystallize from the solution. The hydrolysis products with three molecular weight distribution was formed. When the molecular weight is higher than 2000, the hydrolysis of PA6 will precipitate out from solution. When the molecular weight is in the range of 1000 to 2000, the hydrolysis of PA6 will form small particles which suspend in the solution. When molecular weight is lower than 1000, PA6 segments are dissolved in the solution in the form of linear and cyclic oligomers of PA6.The hydrolysis of La Cl3 not only release the H+ ions which promote the hydrolysis of PA6, but also the hydrolysis products of La3+ ions La(OH)2Cl has different effects on the crystallization behavior of PA6 with different molecular weight. If the crystal particle of La(OH)2Cl is mixed with the PA6 with higher molecular weight(?>2000), the La(OH)2Cl crystal particles had little effect on the crystallization behavior of PA6 and only ? phase can be formed in PA6. However, if the La(OH)2Cl exists in hydrothermal system in the form of one-dimensional amorphous structure, hydrogen bonds can be formed between hydroxyl groups on the surface of La(OH)2Cl and amide groups of PA6 chains. Hydrogen bond interaction will be beneficial in the aggregation on the La(OH)2Cl surface, and lead to the formation of ? and ? mixture crystal structure for PA6 with lower molecular weight(1000<?<2000). The PA6 near the La(OH)2Cl surface is primarily ? crystal structure, and far from the La(OH)2Cl surface is primarily ? crystal structure for the weak influence of amorphous La(OH)2Cl surface.The crystallization morphology of La(OH)2Cl particles also had different effects on the crystallization behavior of PA6 crystallized from the melt. Because La(OH)2Cl crystal particles have larger particle size and small specific surface area, so the interaction between La(OH)2Cl crystal particles and PA6 is negligible, and have little influence on the crystallization of PA6 from melt. However, the amorphous La(OH)2Cl with large specific surface area is able to make full contact with PA6 chains, and to form hydrogen bonds and coordination bonds with amide groups. So, when PA6 crystallizes from melt, La(OH)2Cl can inhibit the movement of PA6 molecule chains, which ultimately decreases the crystallization rate and the crystallinity of PA6, and even inhibit the crystallization of PA6 from the melt.Moreover, when the metal ions present in the hydrothermal system, the hydrolysis product of metal ions can form complex with PA6, so as to achieve a better dispersion. According to the special properties of the hydrolysis products of different metal ions, the composites can be used as filling material in order to modify and prepare polymer materials with specific properties, such as optical, magnetic and electrical properties.(3) Two dimensional wide angle X-ray diffraction, two dimensional small angle X-ray scattering and field emission scanning electron microscope were used to study the influence of hydrothermal temperature, hydrothermal time and the shrinkage stress on the structure of fiber.When PA6 fibers were treated in water, the transformation of the amorphous region to the crystal region began to happen even at low temperature. At high temperature, the crystal-crystal transformation from ? phase to ? phase happened. The increase of hydrothermal temperature of also led to the increase of crystallinity, the degree of crystalline perfection and crystallite size. In addition, the molecular chains in the amorphous region crystallized around the Shish structure of the raw fiber and grew along the radial direction of the fiber to form a Shish-Kebab structure during hydrothermal treatment. After higher temperature treatment, ellipsoidal micro-pores formed in fiber which could be monitored by two dimensional small angle X-ray scattering test. The increase of hydrothermal temperature also caused the change of fiber surface morphology making the smooth surface of the original fiber become roughness and a large number of macro-pores formed at 150??Under 150? hydrothermal condition, the hydrothermal time and shrinkage stress in fiber had no significant influence on the crystal structure and crystal packing structure, but had significant influence on the surface morphology and pore structure. For the relaxed fiber, the increase of hydrothermal time promoted the formation of macro-pores on fiber surface and the increase in the number of pores. For the fixed fiber, increase the hydrothermal time had less influence on the pore size, but led to the dissolution and recrystallization of fiber surface and the regular macro-pores structure was destroyed. Over all, the small and dense pores could be formed in the fixed state, while the bigger and sparse pores could be formed in the relaxed state for the fibers. From another perspective, the polymer materials with different pore sizes can be prepared by adjusting the hydrothermal time and other factors under hydrothermal condition, which provides a feasible way for the preparation of the micro-porous(or macro-porous) materials.(4) Hydrothermal treatment of the polyamide 6(PA6) fibers at 150 ? was studied. In situ two dimensional wide angle X-ray diffraction(2D-WAXD), in situ two dimensional small angle X-ray scattering(2D-SAXS), differential scanning calorimetry(DSC) and single fiber strength test were applied to investigate the structure transformation and mechanical properties after hydrothermal treatment. In the heating process, the hydrogen bonding layer spacing increases and then the layers separates, while the single hydrogen bond is hard to be destroyed because of the hydrogen bonding between the molecules chains. Comparing the raw fibers with ? phase, the PA6 fibers with ? phase have better structure stability and higher melting temperature. The increase of melting temperature is due to the increase of crystallinity and the degree of perfection of crystal region, and the micro- and macro-pores in fibers almost does not affect the melting temperature of the fiber. The crystal lamellae in fibers shows very weak orientation, but tensile can promote the lamellae orientation. When tensile strain increased from 0 to 50.7 %, the average long period of lamellae increased from 10.6 nm to 14.8 nm. The hydrothermal treatment induce amorphous region in fiber reduce, micro- and macro-pores form. Therefore the breaking strength and elongation of fiber shows decrease.