Study On Structure And Properties Of In-situ Nylon 6 Blends

In-situ nylon 6 blends were fabricated through the anionic ring-opening polymerization of ε-caprolactam in the presence of nylons with various chemical structures, taking full advantage of the low viscosity and good dissolving power of the melted nylon 6 monomer. The nylons selected included aliphatic nylon 66 and nylon 1212, semi-aromatic crystallizable PA6co6T and MXD6, and non-crystallizable semi-aromatic NSAP (Non-crystallizable Semi-aromatic Polyamide), PA6IcoT and 3Me6T as well. Melt-mixed blends between commercially available hydrolytic nylon 6 and other nylons were produced, for comparison, through co-extrusion and the subsequent injection molding. A combination of the characterization by using SEM, POM, DSC, DMA, WAXD, FT-IR and solution ¹³C NMR was carried out to investigate the changes in the chemical and supermolecular structures of nylon 6 as a result of blending. The blends can exhibit varied microstructures and mechanical properties through the selection of structures of the nylons added. Consequently, a reasonable explanation for the differences in the mechanical properties between the in-situ and melt-mixed blends was presented. The research enriches the in-situ blending of nylon 6 on the platform of the anionic ring-opening polymerization of ε-caprolactam. The main results are as follows:1. It is for the first time that the transreaction between the two nylons was verified for the in-situ blending. This achievement provides an alternative for the industrial production of copolyamide based on lactam. Moreover, the formation of copolymer resulted from transreaction offers an explanation for the homogeneous phase, the single glass transition, the exclusive crystallization of nylon 6 segments, the retarded crystallization and depressed crystallinity as well.2. The glass transition temperatures of the in-situ blends are determined by the competition between two factors having opposite influence. The formation of compolymers due to transreaction decreases the hydrogen-bonding content and crystallinity, which lower the intermolecular interaction and the glass transition temperature accordingly. On the other hand, the introduction of rigid segments originated from the added nylons would increase the glass transition temperatures owing to its existence in the amorphous region and larger free volume needed for mobility. The blends showing increased or decreased glass transition temperature can be obtained through the selection of nylons added.3. The in-situ and melt-mixed PA6/PA66 blends exhibit contrary trends of changes in mechanical properties with increasing PA66 content: the former shows a remarkable improvement in elongation at break but a drop in the strength and modulus; the latter exhibits a slight increase of strength and modulus but a decrease of elongation at break. Contrary to the in-situ blends, the melt-mixed ones show the PA66 crystallites, which promote the crystallization of nylon 6. The differences in the mechanical properties for the two blends canbe contributed to the presence or absence of transreaction and the different thermomechanical histories. The changes in the glass transition temperatures, spherulite size, crystallographic form and crystallite size and crystallinity are involved in the variation of mechanical properties.4. The in-situ PA6/HPN blends reach optimal balance between modulus and toughness: the elongation at break increases dramatically with the maintenance of modulus and strength. However, the melt-mixed blends show a slight increase in strength and modulus but a decline of toughness. Different from the PA6/PA66 blends, HPN fails to crystallize in the melt-mixed blends. The improvement of strength, modulus and elongation at break results from the slight drop of glass transition temperature, dramatically refined spherulites and the existence of nylon 6 α-form crystals. However, the melt-mixed blends exhibit higher glass transition temperatures and the only existence of nylon 6 γ-form crystals.5. The in-situ and melt-mixed PA6/NSAP blends behave similarly: an increase of strength and modulus and a reduction of elongation at break. A large number of rigid segments lead to an increase of glass transition temperature for both blends. Moreover, the non-linear rigid isophthalamide segments induce larger relative amount of nylon 6 γ-form crystals and depress the crystallization of nylon 6. Furthermore, PA6IcoT conducts similar effects as NSAP owing to the similar composition.6. If the added nylons contain only linear rigid segment-- terephthalamide and linear aliphatic segments, remarkable refinement of spherulites will occur. However, the coexistence of non-linear aromatic or aliphatic segments will erase this effect.7. If the added nylons contain non-linear aromatic segments or linear rigid segment--terephthalamide together with non-linear aliphatic segments, the nylon 6 molecules in the in-situ blends are prone to crystallize in γ-form, which endows the materials with higher drawability at elevated temperatures.8. It is found that the α-form nylon 6 crystals show higher toughness than the γ-form at room temperature. And the explanation has been put forward on the basis of the structures of the crystals.9. In view of the existence of interchange reaction between the two components in the in-situ blends, the conclusions concerning the changes of crystallographic form and crystalline morphology and the glass transition temperature with the introduction of certain segments also apply to the copolyamides produced hydrolytically.