Effect Of Some Nucleating And Antinucleating Agents On The Crystallization Behavior Of Poly(Lactic Acid)

Poly(lactic acid)(PLA) is a biodegradable polyester derived from renewableresources with increasing importance in the biomedical applications andcommodities. The requirements for the crystallization rate of PLA are different,depending on various applications. For injection molding articles, highercrystallization rate is preferred; while for blow-molding products, lowercrystallization rate is more suitable. The addition of nucleating agent andantinucleating agent can be efficient in controlling the crystallization behavior, andextending the applications of PLA. In this thesis, the effect of zincphenylphosphonate (PPZn) and multiamide(N,N’,N’’-tricyclohexyl-1,3,5-benzenetricarboxylamide, defined here as TMC) actedas nucleating agents, and cellulose acetate butyrate (CAB) acted as antinucleatingagent on the crystallization behavior of PLA has been investigated. The main resultsare as follows:(1) Time-resolved Fourier-transform infrared spectroscopy (FTIR) wasperformed to probe chain conformational changes and to determine thecrystallization kinetics during the isothermal crystallization of the PLA compositesand neat PLA, especially in the early stages. Both TMC and PPZn could serve aseffectively nucleating agents in accelerating the crystallization kinetics of PLA;however, the ability of TMC to induce crystallization was stronger than that of PPZn.In the case of neat PLA,─CH3interchain interactions preceded─(COC+CH3)interchain interactions during the crystallization. In the PLA/TMC composite, theconformational ordering began with─(COC+CH3) and─CH3interchain interactions,which resulted directly in a reduced induction period. In the PLA/PPZn composite,─CH3interchain interactions and intrachain interactions103helix formation werefollowed by─(COC+CH3) interchain interactions. The different sequence ofmolecular conformation peaks for PLA/PPZn and PLA/TMC compesites could beexplained in terms of surface-induced conformational order. Finally, effect of the morphology of the TMC and PPZn on the crystallization behavior of PLA wasdiscussed.(2) The effect of CAB on the crystallization behavior of PLA was investigatedby differential scanning calorimetry (DSC) and polarized optical microscopy (POM).Both the cold and melt crystallization of PLA was obviously suppressed in thepresence of CAB even at a CAB content as low as0.5wt%. The isothermalcrystallization kinetics was analyzed by the Avrami model. It was found that theAvrami exponent of the PLA crystallization was not significantly influenced by theaddition of CAB, indicating that the crystallization mechanism almost did notchange in the blends. The crystallization half-time of PLA/CAB blends increasedsignificantly with increase in CAB loading. The observation from opticalmicroscopy further showed that the presence of CAB decreased the number ofnucleation sites but had nearly no influence on the spherulite growth rate of PLA.The above observations indicate that CAB is an efficiently antinucleating agent ofPLA.