| Poly(lactic acid)(PLA)is an important bio-based biodegradable plastic,which has a wide range of applications in biomedical and general plastics.However,the poor crystallization ability of PLA limits its wider applications.The self-assembly nucleating agent can be dissolved into PLA melt and self-organized into various supermolecular frameworks during the cooling process,which can induce PLA to form interesting crystalline morphology and promote PLA crystallization.Therefore,it is a promising nucleating agent for PLA.It is notworthy that the crystalline morphology of PLA/self-assembly nucleating agent is related to the competition between the formation rate of the microfibers from the self-assembly nucleating agent and the growth rate of PLA crystals on the surface of the microfibers,while the optical purity of PLA and the addition of plasticizers have a significant effect on the crystallization rate of PLA.Therefore,in this thesis,the optical purity of PLA and the effect of plasticizer polyethylene oxide(PEO)on the crystalline morphology of PLA/self-assembly nucleator multiamide(TMC-328)were investigated.The main results are as follows:(1)The effect of PLA optical purity(containing 2%D and 4%D-lactic acid units,respectively)on the crystalline morphology of PLA/TMC-328 was investigated.Polarizing optical microscope(POM)results show that TMC-328 has similar solubility in PLA(2%D)and PLA(4%D)matrices.Differential scanning calorimeter(DSC)results show that under cooling at 2.5 ~oC/min,when the melting temperature T_f=220～240 ~oC,for the PLA(2%D)/0.1～0.15 wt%TMC-328,the crystallization peak temperature T_p of PLA(2%D)decreases with the increase of T_f,while the T_p of PLA(2%D)/0.2～0.3 wt%TMC-328 increases with the increase of T_f;for PLA(4%D)/0.15～0.3wt%TMC-328,a similar trend is observed,but PLA(4%D)/0.1 wt%TMC-328 has no obvious crystallization peak.The results of isothermal crystallization by using POM show that for PLA(2%D)/TMC-328(the crystallization temperature T_c was set as 140~oC),in the ranges of T_f=220～240 ~oC,spherulites and cone-like crystal coexist in the system containing 0.1 wt%TMC-328,while the system containing 0.15 wt%TMC-328 always forms cone-like crystals;the system containing 0.2 wt%TMC-328 can form shish-kebab crystals in the following isothermal crystallization process when T_f=220～230 ~oC,and short needle-like crystals can be formed at T_f=240 ~oC;in the system containing 0.3 wt%TMC-328,when T_f=220 ~oC,short rod-like crystals are formed during the isothermal crystallization process,shish-kebab crystals are formed when T_f=230～240 ~oC.For PLA(4%D)/TMC-328,the morphology after isothermal crystallization(T_c=130 ~oC)is similar to that of PLA(2%D)/TMC-328.The only difference is that PLA(4%D)/0.15 wt%TMC-328 forms dense crystallites in the subsequent isothermal crystallization process when T_f is 220,240 ~oC.Although cone-like crystals are formed when T_f is 230 ~oC,the size of kebab is significantly smaller than that of PLA(2%D)/0.15 wt%TMC-328.The reasons for the above differences can be attributed to the fact that the crystallization rate of PLA(4%D)is slower than that of PLA(2%D),and the formation of cone-like crystal is formed by the competition of TMC-328 crystals and PLA crystals.TMC-328 forms shish crystals,while PLA forms a kebab on the surface of the TMC-328 crystal.(2)The effect of the combination of TMC-328 and PEO on the crystalline morphology of PLA was investigated.(a)Firstly,the effect of PEO on the dissolution of TMC-328 in PLA melt was studied by POM.For PLA/TMC-328 without PEO,TMC-328 is completely dissolved in PLA melt when TMC-328 content is less than0.15 wt%at T_f=230 ~oC,while a small amount of TMC-328 remains in PLA melt when TMC-328 content is 0.2～0.3 wt%;for PLA/PEO/TMC-328,TMC-328 is completely dissolved in PLA/PEO melt even when TMC-328 content is 0.3 wt%at T_f=230 ~oC.This indicates that the introduction of PEO promotes the dissolution of TMC-328 in PLA melt.(b)Next,the effect of PEO on the morphology of PLA/TMC-328 under non-isothermal crystallization(T_f=230 ~oC,cooling rate was 5 ~oC/min)was studied.The results show that there is no obvious crystal in PLA/0.1 wt%TMC-328,a large number of tiny crystals appear in PLA/0.15 wt%TMC-328,and dendritic crystals with different length and thickness are formed in PLA/0.2～0.3 wt%TMC-328;while a large number of spherulites are observed in PLA/PEO/0.1～0.15 wt%TMC-328,and in PLA/PEO/0.2～0.3 wt%TMC-328,rod-shaped crystals and shish-kebab crystals with uniform length and thickness are observed.(c)Finally,the effect of melting temperature T_f(210～230 ~oC)on the morphology of PLA/PEO/TMC-328 under isothermal crystallization was studied.The results show that at the isothermal crystallization temperature of 130 ~oC,the cone-like crystals are observed in the PLA/PEO/0.15 wt%TMC-328 when T_f is 210～230 ~oC,but the cone-like crystals are only observed in the PLA/0.15 wt%TMC-328 without PEO when T_f=230 ~oC,while the microcrystalline morphology is observed at T_f=220 ~oC.For the PLA/PEO/0.2 wt%TMC-328,the cone-like crystals are formed under isothermal crystallized at 130 ~oC when T_f=210～220 ~oC,while the shish-kebab crystals are formed during the subsequent isothermal crystallization when T_f=230 ~oC.For PLA/PEO/0.3 wt%TMC-328,short needle like crystals are formed during the subsequent isothermal crystallization at T_f=230 ~oC.The above results can be attributed to the fact that the addition of PEO enhances the segment activity of PLA.Thus,the crystallization rate ratio of PLA to TMC-328 is increased. |
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