The In Situ Polymerization and Characterization of PA6/LiCl Composites

PA6/LiCl composites were synthesized by in situ anionic polymerization based on the interaction between the inorganic salts and PA6. Sodium hydroxide as initiator and N-acetylcaprolactam as activator were used in the preparation of PA6/LiCl composites with variety of LiCl content. X-ray diffraction (XRD) and differential scanning calorimeter (DSC) testing results showed that both of degree of crystallinity and melting temperature of the composites were decreased under the influence of LiCl. And the γ crystal phase proportion increased with increasing the LiCl content to appropriate amount.


Introduction
Polyamide is general thermoplastic resin with repeat amide groups-[NHCO]-on the molecular main chain, and it is commonly known as nylon.PA6 is a very common and important polymer material used in different fields ranging from textile and carpet production to special technical applications.The intramolecular polar amide group of PA6 can be found in hydrogen bond, increasing the intermolecular forces, and exhibiting relatively high strength, good toughness, chemical and abrasion resistance, and so forth.Nowadays, superfine fibers have attracted more and more attention from people for occupying the special properties, and the research on the superfine fibers is developing quickly [1].However, the plenty of hydrogen bonding between chains in PA6, quickly crystal capacity, high crystallinity, and chain defects in the crystal lattice restrict the molecular arrangement and orientation, which hinders the achievement of higher DR during the PA6 spinning and the processing of the superfine fiber.
The studying emphasis of PA6 is always focusing on the modification in order to change the properties of polymer and be suitable for more applications [2][3][4][5].There are many modification methods, which in general can be divided into blending, in situ polymerization, and copolymerization.As for in situ polymerization of PA6 composite, the acquired composites are prepared by adding the modifier (polymer or inorganic particles) to the caprolactam molten monomer, then adding sodium hydroxide as initiator to open the ring, and introducing N-acetylcaprolactam as activators to a high-polymerization rate [6,7].This paper introduced the modification of PA6 and prepared a series of PA6/lithium chloride composite via in situ anionic polymerization.The characterization of the composites was carried out by FT-IR, NMR, DSC, and X-ray analysis methods.

The Preparation of N-Acetylcaprolactam (CCL)
. N-Acetylcaprolactam can increase the electronegativity of amido bond and improve the amide carbonyl activity of caprolactam anion.Toluene was used as solvent in the preparation; the ratio of acetic anhydride and caprolactam was 1.2 : 1, then heating reflux for 2 h and extracting the unreacted acetic anhydride and the acetic acid with distilled water.The synthesis of CCL was shown in Scheme 1.

The Characterization of N-Acetylcaprolactam (CCL).
The products were characterized by FT-IR and NMR, as shown in Figures 1 and 2. The XRD patterns in Figure 3 showed that the samples of PA6 and the composites displayed characteristic peaks at scattering angles of 20.0 ∘ and 23.60 ∘ [9], corresponding to the reflections of the crystalline planes (200) and combined (002)/(202), respectively, which was corresponding to monoclinic -phase morphology.The peak position of the composites had a slight displacement compared with PA6, which implied that the lattice dimension of PA6 changed with the addition of lithium chloride.It was interesting that the composite had obvious diffraction peak at about 21.4 ∘ , which corresponded to the -form of PA6.As can be seen from Table 1, compared with pure PA6, both the two diffraction angles in PA6/LiCl composites became larger, which could be due to the destruction effect of lithium chloride molecule on the hydrogen bonding between the PA6 chains.Comparing the proportion of -form of PA6 and the composites, we could find that the content of form reduced with increasing content of lithium chloride.Variation trend of the -crystal form also can be seen from Figure 3.With increasing content of lithium chloride, the intensity of -form crystal diffraction peak increased first and then decreased, which illustrated that the proportion of PA6 -form crystal phase increased with lithium chloride content increasing in a certain range.

DSC Analysis of PA6/LiCl
Composite.Figure 4 showed the heating curves after erasing thermal history of nylon and composites.In the process of constant heating rate, the melting temperature linear declined with the increasement   of lithium chloride content.When the content reached 2%, the melting temperature declined to nearly 20 ∘ C compared with that of PA6.Both the pure PA6 and the PA6/LiCl composites had multiple melting peaks, which may be due to the different degree of crystallinity and crystal perfection.Higher temperature crystallization peak corresponded to a perfect crystal melting, while the low temperature peak was the low degree of crystal perfection [10,11].With the addition of lithium chloride, lithium ion could form six membered ring-structure via complexing interaction with PA6 molecule, which reduced molecular chain migration rate and the degree of crystal perfection.

Conclusions
We prepared the PA6/LiCl composites via in situ anionic polymerization.IR and NMR result confirmed the successful synthesis of N-acetylcaprolactam which acted as activator.DSC and X-ray results showed that the -crystal phase proportion in composites increased with increasing the LiCl content to appropriate amount.While -form could improve the degree of deformation, which implied that the preparation of superfine fiber for PA6 became easily during the spinning.
LiCl/PA6 Composites.Lithium chloride which acted as modifier can interact with carbonyl group in caprolactam monomer and dissolve into the monomer, which means the well dispersed of modifier in the polymer composites.The experimental process was as follows: firstly, adding certain amount of caprolactam in a three-flask bottle and heating to 130 ∘ C under vacuum for 30 minutes to remove moisture and impurity in the system; secondly, quickly putting certain amount of lithium chloride into the bottle, keeping up the vacuum, and heating to 135 ∘ C at the same time until lithium chloride dissolved completely; thirdly, adding catalyzer sodium hydroxide into the melt mixture and heating to 140 ∘ C; fourthly, after catalyzing for 15 minutes, adding the activator (CCL) into the bottle fast; finally, pouring the melt mixture into a mold preheated at 180 ∘ C until the polymerization finished.

Table 1 :
The XRD parameters of the PA6/LiCl composites.