Study On Side-chain Liquid Crystal Polymers As Novel β-crystal Nucleator To Induce Crystallization Behavior Of Isotactic Polypropylene

Liquid crystal polymers (LCP) as novel functional polymer materials, which have developed quickly, and have been applied in the preparation of high modulus and strength fibers, and self-reinforcing liquid crystal materials. In spite of some studies on LCP as a novel polymer assistant to modify the thermoplastic materials have been reported, no research on side-chain LCP as new nucleators (LCP-NA) of isotactic polypropylene (iPP) is reported. Therefore, it is significant to study on LCP-NA not only because it has important theory and science values, but also it is hopeful to synthesize high efficient and multifunctional nucleators used in iPP. The need of high efficient nucleator of iPP is at an ever-increasing rate, so study on LCP-NA has great research meaning and wide application prospect.In this paper, two liquid crystal(LC) monomers were synthesized, which include 4-(undec-10-enoyloxy)biphenyl-4’-ethoxybenzoate(M1), cholesteryl undecenylenate (M2). The homopolymer LCP-NA1 was prepared by graft polymerization reaction with M1 and PMHS(DP=7), and the homopolymer LCP-NA2 was prepared by graft polymerization reaction with M1 and PMHS(DP=35). The homopolymer LCP-NA3 was prepared by graft polymerization reaction with M2 and PMHS(DP=35). The copolymers LCP-NA4 and LCP-NA5 were obtained by the copolymerization of M1,4-hydroxyphenyl-4’-undecylenate (N) and PMHS(DP=7) with different mole ratio of M1/N. The iPP was blended with different content of LCP-NA1-LCP-NA5 under specific processing conditions, and then 5 series of samples were obtained. The structures of N, M1, M2, and LCP-NA1-LCP-NA5 were characterized with fourier transform infrared spectrometer(FT-IR). The LC properties and thermodynamics properties of M1, M2, and LCP-NA1～LCP-NA5 were investigated with polarizing optical microscopy(POM), differential scanning calorimetry (DSC) and thermogravimetric analysis(TGA). Meanwhile, the influences of LCP-NA1～LCP-NA5 as new nucleators to the crystalline structure, morphology, melting behaviors and nonisothermal crystallization of iPP were investigated with wide angle X-ray diffraction (WAXD), POM and DSC. The effects of chemical structures, difference of LC phases, nucleator content, crystallization temperature and time on the crystalline structure and morphology were discussed.Both M1 and M2 are thermotropic LCs, M1 exhibited nematic threaded texture on heating process, nematic threaded texture, fan-shaped texture of smectic A(SA) phase, schlieren texture of smectic C(SC) phase, and mosaic texture of smectic B(SB) phase on cooling process. M2 exhibited typical cholesteric oily streak texture on heating process, broken focal conic texture of cholesteric, spiral texture of cholesteric, and fan-shaped texture of SA phase on cooling process. LCP-NA1, LCP-NA2, LCP-NA4 and LCP-NA5 exhibited nematic threaded textures, LCP-NA3 exhibited SA batonnet textures; POM results and DSC results show together that LCP-NA1～LCP-NA5 are all "liquid crystal state" in the range of temperature (90～140℃), which is the main crystallization temperature of iPP. And all the thermal decomposition temperatures (Td)>335.0℃, demonstrating that LCP-NA1～LCP-NA5 are thermally stable. Thus, LCP-NA1～LCP-NA5 accord with the design of LCP-NA.The test results of WAXD showed that:LCP-NA1～LCP-NA5 as LCP-NAs in iPP blends could play a role in heterogeneous nucleation effect, and the optimal processing conditions are as follows:the nucleator contents of LCP-NA1～LCP-NA5 are 1.0 wt%,0.4wt%,0.2wt%, 1.0wt% and 0.8wt% respectively. The crystallization temperatures (T) of LCP-NA1～LCP-NA5 are 125℃,125℃,130℃,130℃ and 130℃ respectively. All of the crystallization times (t) are 1h. Under the processing conditions above, the relative contents of the P-form{Kβ) are 70.2%,84.0%,71.0%,59.0% and 63.0% respectively. The result showed that LCP-NA2, which contains greater degree of polymerization of siloxane main-chain, has more ability of inducing β-form; nematic LCP-NA2 has more ability than smectic LCP-NA3; The hydroxyl-LC copolymers LCP-NA4 and LCP-NA5, which contain physical cross-link or hydrogen bond, have less ability than LC homopolymer LCP-NA1.The observation results of POM showed that:The adjunctions of LCP-NA1～LCP-NA5 have changed the crystalline morphologies, induced β-form, increased the number of crystal nucleuses, decreased the spherulite dimension and impeled spherulites to be evenly distributed. The a-spherulites were black and white, exhibited radial structures, had clear boundaries, and disappeared at 167.1～171.0℃; The P-spherulites were bright and colorful, exhibited sheaf-like structures, had obscure boundaries, and disappeared at 153.4～158.5℃.The study on the thermodynamics of crystallization showed that:The characteristic melting peaks of β-form appeared in all the DSC heating curves of iPP blends with LCP-NA1～LCP-NA5. The variation trends of Φβ* and Φβ values as functions of nucleator mass fraction and crystalline temperature, which were calculated on the basis of DSC heating curves of iPP blends, were similar to that of  values, which were calculated on the basis of WAXD curves of iPP blends. The study on crystallization kinetics showed that:Compared with the pure iPP, the crystallization peaks of all the DSC cooling curves of iPP blends with LCP-NA1-LCP-NAs shifted to high temperatures, the peaks became narrow, and the crystallization velocities have been increased. In addition, the non-isothermal crystallization kinetics was studied for LCP-NA4/iPP blends with 1.Owt% LCP-NA4, and Jeziorny method and the Mo Zhishen method had the same conclusion, and could describe well the non-isothermal crystallization kinetics for LCP-NA4/iPP blends with 1.0wt% LCP-NA4, but Ozawa method was not fit for the process. The crystallization activation energy (ΔE) of LCP-NA4/iPP blends with 1.0wt% LCP-NA4 is 198.4KJ/mol, which is less-than that of pure iPP(ΔE=261.0KJ/mol).