Manipulation Of Poly (L-lactic Acid) Crystallization By Self-assembly Of P-tert-Butylcalix[8] Arene And Solvent Molecules

Poly(L-lactic acid), has attracted many interests as it possesses excellent biodegradability and good mechanical properties, and for the reason that many properties of poly(L-lactic acid) (PLLA) is similar to those of gernal plastics (polyethylene, polypropylene and polystyrene), PLLA has a broad market prospects. However, PLLA, which is hard but brittle, has a slow crystallization rate. Manipulation of PLLA crystallization can improve its mechanical property, and nucleating agents and crystallization promoters are often used in order to improve the crystallization of PLLA. Up to now, the PLLA used nucleating agents with high performance are few, and these agents are suffered by its complex synthesize process and the property that causes polymer degradation. Therefore, it's very significative to study the interaction mechanism between nucleating agent and PLLA and develop the high performance PLLA used nucleating agents.Firstly, a novel nucleating agent (TBC8-t), which is more powerful than the convertional PLLA used nucleating agent (talc), was prepared by recrystaling p-tert-butylcalix[8]arene (TBC8) from toluene. The structure was studied by WAXD, TGA and NMR. The results show that TBC8 and toluene are self-assembled into inclusion compounds (TBC8-t). The nucleation ability of nucleating agents were studied by DSC, and the results show that TBC8-t can greatly enhance the crystallization rate of PLLA, whereas TBC8 has little nucleation effect on the crystallization of PLLA. This phenomenon can be attributed to the polymorphous crystal structure of TBC8. The function of toluene molecules is to help TBC8 form a new crystal structure. Once the structure of the inclusion compounds has been formed, toluene molecules no longer have any influence with both the crystal structure of nucleating agent and the crystallization of PLLA. Secondly, the effect of a series of solvents as guest moleclulars on the nucleation ablility of TBC8 was investigated. The results show that the Tp is elevated dramatically with increasing number of alkyl carbon atoms in guest from zero to two, and reaches a maximum when ethylbenzene (the size of ethylbenzene is 7.23A) is used as guest (TBC8-eb, Tp=134.4℃), but decreases for longer alkyl chains. The decrease of Tp can be attributable to the fact that propylbenzene(8.16A) cannot act as an intercalating agent to increase the interplanar crystal spacing along (001) of TBC8. Not only the size but also the type of guest molecules affects the nucleation ablility of TBC8. When the terminal group is alkyl group, and the size of the guest is between 5.93A and 7.28.A, this kind of nucleating agent will have excellent nucleation ability for PLLA.The crystal morphology and form of PLLA/TBC8, PLLA/TBC8-t and PLLA/TBC8-eb were invesgated by POM,SEM,WAXD and the crystallization kinetics, and the results suggest that the geometry of crystal of PLLA and PLLA/TBC8 are three-dimension spherical growth, while TBC8-t and TBC8-eb can transform most of the original spherulite crystals of PLLA into sheaf-like crystals with the crystal form of PLLA unchanged.The the interaction between nucleating agent and PLLA and the conformational ordering of PLLA were studied by FTIR, and the results demonstrate that in the isothermal crystallization process at 140℃, the OH··O interaction between the C=O of PLLA and the-OH of TBC8 drives polymer chains to be adsorbed on the surface of nucleating agent, meanwhile the interchain interaction of-CH3 to make the conformational of nucleated PLLA become orderly, then a short helix emerges where a couple of-CH3 groups interact.Thirdly, the effect of composition of TBC8-eb and polyethylene glycol (PEG) on the crystallization of poly(L-lactic acid) (PLLA) was studied by DSC and POM, and the synergism between TBC8-eb and PEG on the crystallization of poly(L-lactic acid) was calculated through empolying Lauritzen-Hoffman model and Avrami equation. The results suggest that TBC8-eb acting as nuclei restrict the chain mobility of PLLA. To improve the mobility of chain segment, PEG which, though negative to nucleation, favors crystal growth, was added. The the transition temperature from regimeⅢto regimeⅡfor PLLA blended with TBC8 and PEG is at 130℃. Accordingly, the promoting effect of PEG on the crystal growth of PLLA is the controlling factor when the crystallization temperature is below 130℃, and the synergism between TBC8 and PEG can be observed below 130℃. On the contrary, the promoting effect of PEG does not play a leading role when the crystallization temperature is above 130℃, and the synergism cannot be observed obviously. And most of crystal in the PLLA with TBC8-eb and PEG is sheaf-like.Finally, the effect of nucleating agents on the mechanical properties was investigated, and the results show that with the addition of TBC8-eb (1wt%), the tensile strength of PLLA decresed by 4.27%, while the flexural modulus and impact strength increased by 1.23% and 32.65%, respectively. On the other hand, the addition of TBC8-eb (1wt%) and PEG (5wt%), the tensile strength and flexural modulus of PLLA decresed by 13.85%, and 5.93%, respectively, while the impact strength increased by 5.93%. The synergism between TBC8-eb and PEG on the impact strength of PLLA cannot be observed, which can be attributed to the fact that PEG enlarges the crystal size of PLLA.