Studies On Synthesis Of Novel Bisamides Nucleating Agents And Effect On Crystallization Of Poly (L-lactic Acid)

Poly(L-lactic acid)(PLLA) is a kind of biodegradable material which is derived from renewable plant resources. It can be applied in automobile, electronics agriculture, food packing and medicine due to its characteristics of good mechanical properties, processing performance, biodegradable and biocompatibility. However, PLLA has many defects including slow crystallization rate, low crystalline degree and low-heating distortion temperature which affect the development of PLLA industry. Thus, developing new nucleating agents which can be used to improve the crystallization rate of PLLA is of great importance both in scientific research and practical appliance.In this work, five N, N'-bis(benzoyl) alkyl diacid dihydrazides were synthesized from benzoyl hydrazine and alkyl diacyl dichloride which was deprived from alkyl diacid via acylation. PLLA/N, N'-bis(benzoyl) alkyl diacid dihydrazide (NA) were prepared by melt blending and hot-press forming process. The nucleation effect of NA on crystallization of PLLA, crystallization behavior and melting behavior of PLLA/NA were investigated using differential scanning calorimetry (DSC), depolarized-light intensity measurement, polarized optical microscopy (POM) and wide angle X-ray diffraction (WAXD). Then, the interaction between PLLA and nucleating agent NA was discussed.The influence of the molar ratio, reaction temperature and reaction time to the yields of NA were investigated by orthogonal experiment, and the optimized reaction condition was obtained. We calculated the HOMO and LUMO energy levels for the alkyl diacyl dichloride and benzoyl hydrazine. Calculations were performed with the program VAMP using the semiempirical quantum chemical method PM3. The calculation results indicate the reaction difficulty of alkyl diacyl dichloride and benzoyl hydrazine is following: brassylic acid >dodecanedioic acid> azelaic acid>suberic acid>sebacic acid. Beside from the reaction of sebacoyl chloride with benzoic hydrazide, the remaining reaction of alkyl diacyl dichloride with benzoic hydrazide occur more and more difficulty with the increase of methylene number. The incomplete symmetry structure and differences of bond length and bond angle of optimized five N, N'-bis(benzoyl) alkyl diacid dihydrazide predict the difference of nucleation effect.Non-isothermal crystallization shows that the crystallization peak becomes sharper and shifts to higher temperature with incorporation of NA in the cooling process, this result indicates that NA can serve as a nucleating agent for the crystallization, and increase the overall crystallization rate of PLLA. Compared with other four NA, the nucleation activity of N, N'-bis(benzoyl) suberic acid dihydrazide NA(S) is higher than other N, N'-bis(benzoyl) alkyl diacid dihydrazides. Compared to the neat PLLA, with the addition of 0.8%NA(S), the onset temperature(T_o) increase from 105.88℃to 125.57℃, the degree of supercooling (ΔT_mc) decease from 55.32℃to 33.03℃, and the crystallization enthalpy(ΔH_c) increase from 1.379J·g-1 to 31.63J·g-1.Crystallization behavior of PLLA and PLLA/NA(S) are studied. The isothermal crystallization results indicate that crystallization rate of PLLA is affected by crystallization temperature and nucleating agent. NA(S) acts as heterogeneous nucleating agent. PLLA/NA(S) not only takes a heterogeneous nucleation process but also have other complicated nucleation mechanism. Upon the addition of 0.8% NA(S), the crystallization half-time of PLLA/NA(S) decreases from 26.5min to 1.4min at 115℃. Though nucleation density of PLLA is improved, the crystal perfection becomes worse, which results in a lower equilibrium melting point. The crystallization peak of PLLA and PLLA/NA(S) become broader with the increasing of cooling rates, and shift to lower temperature. At the same cooling rate, the onset temperature of PLLA/NA(S) and crystallization peak temperature are both higher than these of neat PLLA, which further verifies advanced nucleation ability of NA(S).The melting behavior of PLLA and PLLA/NA(S) were also investigated by DSC, the results show that double melting peaks of PLLA/NA(S) could been explained by melting-recrystallization model. The proportion of double melting is adjusted by addition of NA(S), the reason is that NA(S) increases the overall crystallization rate of PLLA. The melting enthalpy of PLLA/NA(S) through cold isothermal crystallization is higher than that through hot isothermal crystallization, and the gap of melting enthalpy of PLLA/NA(S) becomes small with the increasing of isothermal crystallization temperature, the gap decreases to minimum value 0.93 J·g~-1at 105℃.The interaction between PLLA and NA(S) was investigated by calculation and experiment. Calculation using PM3 indicate that the value of the HOMO–LUMO gap between HOMO of PLLA and LUMO of NA(S) is smaller than that of PLLA, which results in interaction between PLLA and NA(S) in the process of melt blending. The effects of hydrogen bonds between PLLA and NA(S) were also studied by IR,Raman and TGA.A theoretical model of interaction between PLLA and NA was established and the interaction was calculated by molecular dynamics method. The energy of interaction between PLLA and NA(S), N, N'-bis(benzoyl) azelaic acid dihydrazide (NA(A)), N, N'-bis(benzoyl) sebacic acid dihydrazide(NA(D)), N, N'-bis(benzoyl) dodecanedioic acid dihydrazide(NA(O)), N, N'-bis(benzoyl) brassylic acid dihydrazide(NA(B)) indicate that the energy of interaction between PLLA and NA is following:PLLA/NA(S)>PLLA/NA(D)>PLLA/NA(O)>PLLA/NA(A)>PLLA/NA(B).Molecular dynamics simulation results indicate that the nucleation effect of N, N'-bis(benzoyl) alkyl diacid dihydrazide with even methylene is better than that of N, N'-bis(benzoyl) alkyl diacid dihydrazide with odd methylene, and further confirm the nucleation effect of N, N'-bis(benzoyl) alkyl diacid dihydrazide on crystallization of PLLA.