Studies On Properties And Plasticization Mechanism Of PLA/PEG Composites

Poly(lactic acid)(PLA) is a sustainable,biodegradable polymer derived from renewable resources.It has been widely used in medical,agriculture,textile and packaging fields due to its good mechanical properties and high thermal stability.With the increasingly serious problem of "white pollution" and the implementation of "Ban on Free Plastic Bags",PLA will be more widely used in all fields of our daily life.However,the inherent brittleness of PLA impedes its potential applications and attempts have been made to overcome this drawback.Poly(ethylene glycol)(PEG)is one of the plasticizers of PLA.Low molecular weight(MW)PEGs have good compatibility with PLA,and the addition of PEG can greatly improve the flexibility of PLA.However,low MW PEGs tend to migrate from the PLA matrix to the surface in use or storage,resulting in the aging of PLA.Increasing the MW of PEG could reduce the migration rate of PEG from the PLA matrix,but phase separation may happen due to the reduced miscibility between high MW PEG and PLA.In this dissertation,the PLA/PEG blends are systematically studied in the following three aspects:(1)We systematically studied the influence of the MW and amount of PEG on the mechanical and thermal properties of PLA/PEG blends.We observed a surprisingly big drop of elongation at break(?b=32%)in PLLA/PEG5-15%while?bS of PLLA/PEG-15%with other MW PEGs were?300%.In order to study the reason for this phenomenon and explain the plasticization mechanism of PLA/PEG blends with different MW PEGs,DSC,WAXD,SEM and FT-IR were used to study the crystallization behavior of PLA/PEG blends under different cooling or aging conditions.The results showed that PEG5 was easier to separate from PLA during the cooling process than other PEGs when the addition amount was 15%,resulting in the formation of PEG-rich and PLA-rich phases.PLA located in PEG-rich phase had lower Tcc and higher crystallization rate and crystalized into ?'-form crystal,while PLA located in PLA-rich phase had higher Tcc and lower crystallization rate and crystalized into ?-form crystal.SEM analysis revealed that PEG5 crystallized into nanocrystals with a diameter of?60 nm.For other PLA/PEG-15%blends,phase separation did not occur during the rapid cooling process,and PLA crystalized into mixed ?-form and ?'-form crystals.Phase separation may be the main reason for the decrease of ?b of PLA/PEG5-15%.(2)In order to reduce the aging of PLA/PEG blends caused by the migration of PEG during use or storage,we modified PEG1 and PEG2 by pyroglutamate to form PyroPEG1(Mn=1 kg/mol)and PyroPEG2(Mn=2 kg/mol)to plasticize PLA.After being aged at 50? for 96 h,PLA/PyroPEG1-20%and PLA/PyroPEG2-20%showed a reduction in the elongation at break(?b)from 243%and 376%to 120%and 270%,respectively,while the ?bs of PLA/PEG1 and PLA/PEG2 decreased from 424%and 404%to 37%and 110%,respectively.The reduction of ?b of the PLA/PEG blend was much greater than that of the corresponding PLA/PyroPEG blend,indicating that PLA/PyroPEG has excellent anti-aging properties,and modification by pyroglutamate can effectively reduce the migration of PEG This is mainly due to the formation of double hydrogen bonds between the pyroglutamate end groups of PyroPEG,which reduces the migration of PyroPEG.(3)In this part,we aimed to strengthen and toughen PLA using PEG2 as the plasticizer and sawdust(SD)or poly(N-acryloyl-L-histidine)modified sawdust(SD-g-P(A-His))as the nucleation agent.PLA/SD or PLA/SD-g-P(A-His)blends with 1-5%SD or SD-g-P(A-His)were prepared by melt blending.When the amount of SD(or SD-g-P(A-His))was 1%,Tg and the tensile modulus of the composite increased from 59.9? to 61.4?(or 61.6?)and from 1553 MPa to 2230 MPa(or 2143 MPa),respectively.DSC results showed that both SD and SD-g-P(A-His)efficiently promoted the crystallization of PLA as nucleation agents for PLA in PLA/SD and PLA/SD-g-P(A-His)blends,respectively,and SD-g-P(A-His)was better than SD.PLA/PEG2/SD or SD-g-P(A-His)with a fixed ratio of 84/15/1 was also prepared by melt mixing.The tensile test results showed that reinforced and toughened PLA/PEG2/SD and PLA/PEG2/SD-g-P(A-His)composites were successfully prepared.In addition,SD-g-P(A-His)was also proved to be an effective adsorbent for a variety of heavy metal ions.