Structural Design And Crystallization Behavior Of Acetamide Hybrid Poly(Lactic Acid)

Poly（lactic acid）（PLA）is a bio-based and degradable polymer material with the largest output and the most widely used.It has good biocompatibility,easy processing,good mechanical properties and other advantages.PLA can be widely used in medical,packaging,agriculture,textiles,daily necessities and other fields.However,PLA has some problems such as low nucleation rate,large spherulite size and low thermal deformation temperature.In this work,considering the defects in the molecular structure of PLA,an embedded amide nucleating unit（OXA）is designed by utilizing the acetamide chain segment（hydrogen bond group）with strong designability and effective in promoting PLA crystallization.The OXA is introduced into the PLA main chain to obtain acetamide hybrid PLA(PLA_OXA).The number,position and structure of hydrogen bond groups in PLA_OXA as well as the molecular chain compliance and the linkage between hydrogen bond units and PLA chain segments are systematically studied on the nucleation and crystallization of PLA.At the same time,an acetamide nucleating agent(HO-OXA_（m）-OH)with a structure similar to the embedded-OXA is designed and synthesized to enhance the crystallization,mechanical,and heat resistance properties of PLA through the combined action of the embedded and external amide groups.In addition,it is found that PLA_OXA designed through molecular structure can promote the formation of stereocomposites.This work will not only enrich the traditional nucleation and crystallization theory,but also provide theoretical guidance for the controllable construction of polymers with rapid nucleation crystallization.Moreover,it would provide important technical support for the preparation and property control of high-quality biodegradable materials.The main research contents are as follows:Firstly,the nucleating unit(OXA_（n）-OH)with different amide content is designed and it is introduced to the end of poly（L-lactide）（PLLA）through the ring-opening polymerization,forming the amide-terminated hybrid polylactic acid(PLLA_{OXA（n）-OH}).With the increase of the content of terminal amide（H-bond）,the nucleation efficiency,nucleation density,crystallization rate,crystallization temperature（T_c）and crystallinity（X_c）of PLLA_{OXA（n）-OH} are significantly improved.Molecular dynamics（MD）simulation is used to quantitatively analyze the types and quantities of H-bonds in PLLA and PLLA_{OXA（n）-OH}.Combined with in situ small angle X-ray scattering（SAXS）,the mechanism of multistage（HI-MS）nucleation induced by hydrogen bonds in PLLA_{OXA（2）-OH} is proposed.In this part,the process of PLLA crystallization induced by intermolecular forces（hydrogen bonds）is deeply discussed,and a new strategy is provided for the study of nucleation crystallization theory.On this basis,the effects of the structure of amide segments on H-bonds are further studied,and the effects of molecular structure on the formation of H-bonds and the crystallization performance of PLLA are deeply explored.A series of hydroxy-terminated oxalamide derivatives(HO-OXA_（n）-OH)are designed and synthesized,as initiators for L-lactide ring-opening polymerization（ROP）.PLLA with oxamide segments of different aliphatic spacer lengths in the middle of molecular chain(PLLA_{HO-OXA（n）-OH})is synthesized.MD simulation shows that the number of H-bonds increases with the increase of aliphatic spacer lengths,indicating that the increase of aliphatic spacer lengths increases the probability of H-bond formation.The results of the radius of gyration（R_g）and the glass transition temperature（T_g）show that the increase of the aliphatic spacer lengths improves the local flexibility of the molecular chain,and the increase of aliphatic spacer lengths will increase the conformation beneficial to PLLA chain crystallization.These improvements in the local flexibility and conformation of PLLA molecular chain increase the probability of H-bond formation,and then induce the thermodynamically stable local ordered structure with low entropy,and finally promote the nucleation and crystallization process.PLLA_{HO-OXA（6）-OH} can be rapidly crystallized during cooling,and the crystallization half-life time(t_1/2)of isothermal crystallization at 125℃is only 1.9 min which is 88.6%shorter than PLLA.The increase of the end chain length of amide can not only improve the crystallinity and crystallization rate of PLLA,shorten the crystallization time,but also reduce the spherulite size of PLLA,so that the material has good transparency.Based on the previous research,PLLA_OXA with high molecular weight is successfully synthesized by adjusting the synthesis process.The increase of molecular weight results in a slight decrease in the crystallization capacity of PLLA.In view of this,a small molecule nucleating agent(HO-OXA_（m）-OH)with similar structure to the embedded amide chain segment is designed and synthesized,and the blend of PLA（commercial,4032D）and nucleating agent HO-OXA_（m）-OH was prepared to screen the optimal nucleating agent structure.The results show that the nucleation efficiency of PLA non-isothermal crystallization is HO-OXA_（6）-OH>HO-OXA_（8）-OH>HO-OXA_（4）-OH>HO-OXA_（2）-OH when sufficient amount of HO-OXA_（m）-OH is added..The X_c of PLA/HO-OXA_（6）-OH is41.3%（T_c=117℃）,and the crystallization capacity of PLA/HO-OXA_（6）-OH is significantly improved.Moreover,the addition of HO-OXA_（6）-OH can also reduce the spheroid size of PLA.Therefore,the optimal nucleating agent structure is HO-OXA_（6）-OH（denoted as HO-OXA-OH）,and blends of PLLA and PLLA_OXA with nucleating agent HO-OXA-OH（a small amount added,one-third of the optimal amount added）are prepared..The crystallization,mechanical and thermal properties of PLLA are enhanced by the interaction of embedded and external amide groups.The research shows that the X_c and T_c of PLLA/HO-OXA-OH are38.8%and 113.4℃,while the X_c and T_c of PLLA_OXA/HO-OXA-OH are further increased to47.5%and 118℃,indicating that the combined effect of embedded and exogenous amides further enhances the crystallization ability of PLLA.In addition,the energy storage moduli of PLLA_OXA and PLLA_OXA/HO-OXA-OH at 90℃are 265 MPa and 795 MPa,respectively.Under the same conditions,the energy storage moduli of PLLA is only 3.7 MPa,indicating that the combined action of embedded amide and the embedded and applied amide groups can promote the crystallization of PLLA and make PLLA have excellent heat resistance.Another effective way to improve the crystallization and heat resistance of PLA is to form a stereocomposite（Sc）crystal with PLLA and dextral polylactic acid（PDLA）under certain conditions.Before the formation of Sc,PLLA and PDLA need to be associated and wrapped by themselves and then folded together for crystallization,in which the association ability of hydrogen bond is a key factor.Therefore,the hybrid polylactic acid complex is prepared by solution blending,and the ability of hybrid PLLA to form the composite crystal is investigated.It is found that the more the mixture ratio of left and right polylactic acid is close to the molar（or mass）ratio,the stronger the ability to form the composite crystal is.The higher the molecular weight of PLLA,the more difficult it is to form Sc.The molecular structure of PLLA_OXA has a significant impact on the formation of Sc,and the inter chain amide PLLA_HO-OXA-OH with high molecular weight can better promote the formation of Sc.Compared with PDLA/PLLA,the X_c of Sc crystal of PDLA/PLLA_HO-OXA-OH increases from8.1%to 27.6%,and the T_c increases from 123.5℃to 189.3℃.In addition,the tensile strength of PDLA/PLLA and PDLA/PLLA_HO-OXA-OH（63.1 MPa and 63.9 MPa）is slightly higher than that of PLLA（58.2 MPa）,and the Sc crystals give the materials better heat resistance.