Aliphatic Polyester Nanocomposites Based On Modified Layered Clay:Preparation,Structures And Properties

As one of the most promising polymer materials,aliphatic polyester not only comes from a variety of sources,but also has excellent biodegradability and good biocompatibility.And it is of great significance to energy conservation,emission reduction and green environmental protection,and is one of the development trends of degradable polymer packaging materials in the future.However,in the production practice,due to the poor mechanical properties,high crystallinity and low thermal stability of aliphatic polyester,this leads to their application in some field.Therefore,In order to expand the application of aliphatic polyester in the field of green packaging(such as barrier properties and antimicrobial properties),they need to be modified by copolymerization and nanoparticle filling to improve the performance.On the one hand,based on the ring opening polymerization of caprolactone(?-CL)initiated by macromolecule aliphatic polyester hydroxyl terminated hydroxyl group,the aliphatic polyester block copolymer was synthesized successfully,so as to design the aliphatic polyester with thermal and mechanical properties satisfying the design requirements.And the effect of macromolecule initiator content on the properties of the block copolymer was discussed.On the other hand,based on the idea of"ring opening polymerization"and"bionics principle",the surface modification of layered double hydroxides(LDHs)was successfully prepared.And the nanocomposites were prepared by blending the surface modified LDHs and the aliphatic polyester via solution casting method to obtain homogeneous films.The effects of different surface modified LDHs on the barrier properties,mechanical properties and other properties of nanocomposites were systematically investigated.Furthermore,we try to perform secondary functionalization on the surface modified LDHs.Therefore,we prepared Ag nanoparticles(AgNPs)loaded LDHs antibacterial particles(Ag-LDHs)by in-situ immobilization of AgNPs using the surface reducibility of modified LDHs itself.And the nanocomposites were prepared by blending Ag-LDHs and the aliphatic polyester via solution casting method to obtain homogeneous films.The antibacterial properties,barrier properties and other properties of Ag-LDHs and its nanocomposites were studied in detail.The main contents and results of the paper are as follows.1.Poly(?-caprolactone)-poly(butylenesuccinate)-poly(?-caprolactone)(PCL-PBS-PCL)was synthesized by the ring-opening polymerization of?-CL in the presence of the pre-polymerized PBS as the macro-initiator.It is revealed that the number averge molecular weights(M_n)of PCL-PBS-PCL can reach up to 12×10~4.Results show that with the increase of PCL blocks in the copolymers,the crystallization capacity of PBS blocks decrease,and the chain flexibility of PCL-PBS-PCL increase.Compared with the pre-polymerized PBS,the elongation at break of PCL-PBS-PCL reaches 571%(increased by over 500%)when n(?-CL):n(BS)is set as0.55:1.And tensile strength of PCL-PBS-PCL only drops by 15.4%.In consequence,PCL-PBS-PCL in appropriate proportion not only can be used as a biodegradable polymer material alone,but also be used as a compatibilizer of PBS/PCL composites.2.Owing to polyhydroxylic characteristic on the LDHs surface,LDHs-g-PCL was prepared by in-situ ring-opening polymerization of?-CL successfully.PCL-PBS-PCL and LDHs-g-PCL have the same PCL molecular chain segments,which can show good compatibility in the final nanocomposites.Therefore,LDHs-g-PCL/PCL-PBS-PCL nanocomposites were prepared via solution casting method.The results of thermal analysis confirm that the crystallization of PBS blocks is restrained markedly with the addition of LDHs-g-PCL.On the contrary,the crystallization of PCL chains is gradually increasing.The changes of crystallization for nanocomposites lead to the decrease in the high temperature melting peaks and the increase in the low temperature melting peaks.Compared with pure PCL-PBS-PCL,even though a slight decrease(6%)in tensile strength,a 35%increase in elongation at break and a 31%decrease in the oxygen permeability of nanocomposites are reached as LDHs-g-PCL content increase up to 20%(mass fraction).The key parameters to improve the barrier properties of nanocomposites are not only the barrier effect of the LDHs(a decrease in diffusion is expected because of a more tortuous path for diffusing molecules)but also the bulk effect that they develop in the polymer matrix.And this may be related to size and distribution of free volume holes.3.In view of the PCL molecular chains grafted on LDHs and PCL matrix have the same molecular chain structure,the effect of LDHs-g-PCL on the single aliphatic polyester was studied in an even better way.Therefore,based on the successful preparation of in-situ polymerized modified LDHs(LDHs-g-PCL),LDHs-g-PCL/PCL nanocomposites was prepared via solution casting method.LDHs-g-PCL/PCL nanocomposites with exhibited excellent mechanical performance and barrier properties can be obtained by employing adequate amount of LDHs-g-PCL.Compared with the pure PCL,tensile strength and elongation at break of nanocomposites are increased by 31%and 37%,respectively.And the oxygen permeability of nanocomposites decreased by nearly 78%as LDHs-g-PCL content increase up to 10%(mass fraction).It shows that the enhanced effect of LDHs-g-PCL on the mechanical and barrier properties of PCL is obviously better than that of PCL-PBS-PCL.Moreover,the addition of LDHs-g-PCL promotes the degradation of nanocomposites,and the degradation rate increases significantly with the increase of the LDHs-g-PCL content.4.In addition to the idea of"in-situ polymerization"to modify LDHs,inspired by the versatile adhesion capability of marine adhesive proteins,core-shell structured polydopamine-coated layered double hydroxide(LDHs@PDA)was prepared successfully by in-situ growing PDA on the surface of LDHs.And LDHs@PDA/PCL nanocomposites were prepared via solution casting method to obtain homogeneous films.It is revealed that the dispersion of LDHs@PDA has been significantly improved after the modification of PDA onto the surface of LDHs.The results of thermal analysis confirm that LDHs@PDA can act as a nucleating agent.And thermal stability of nanocomposites is improved due to core-shell structured LDHs@PDA.With incorporation of as low as 1 wt%of LDHs@PDA,a 20%increase in tensile strength,a 26%increase in elongation at break and a 26%reduction in the oxygen permeability are achieved.The improvement in the mechanical and gas barrier properties of nanocomposites at low filler loading is attributed to the strong interfacial interaction of LDH@PDA with the PCL matrix.5.Tannic acid(TA)and DA have the similar structure of catechol group.Compared with PDA,tannic acid is widely distributed,cheap and efficient.Inspired by versatile adhesive ability of mussel adhesion proteins,TA-Fe(III)functionalized LDHs(LDHs@TA-Fe(III))was firstly prepared via a facile and fast deposition of one-step assembled TA-Fe(III)coatings on the surface of layered clay.And LDHs@TA-Fe(III)/PCL nanocomposites were prepared via solution casting method to obtain homogeneous films.With incorporation of only 1 wt%LDHs@TA-Fe(III),a 47%increase in elongation at break and 21%reduction in the oxygen permeability are achieved.The reinforcement effect brought by LDHs@TA-Fe(III)is fairly noteworthy at low clay loadings potentially owing to the strong interfacial interactions between the clay and matrix.Moreover,the barrier mechanism analysis shows that the improvement of gas barrier performance depends not only on the high aspect ratio of layered clay,but also on the decrease of the impermeable phase,interface defects and the number of free volume holes in the matrix.6.Furthermore,we try to perform secondary functionalization on the surface modified LDHs.Therefore,we prepared Ag nanoparticles(AgNPs)loaded LDHs antibacterial particles(Ag-LDHs)by in-situ immobilization of AgNPs using the reducibility of PDA and TA on the LDHs surface.Meanwhile,we also use the common reductant polyvinylpyrrolidone(PVP)to directly immobilize Ag~+on the LDHs surface.These Ag-LDHs are named Ag-LDHs@PDA,Ag-LDHs@TA-Fe(III)and Ag-LDHs(PVP),respectively.And Ag-LDHs/PCL nanocomposites were prepared via solution casting method to obtain homogeneous antibacterial films.Ag-LDHs@PDA and Ag-LDHs@TA-Fe(III)exhibit better interfacial compatibility adhesion with PCL matrix,thus showing better mechanical properties and barrier properties in the final nanocomposites.Meanwhile,Ag-LDHs(PVP)exhibited the best antibacterial performance,followed by Ag-LDHs@TA-Fe(III)and Ag-LDHs@PDA,which is consistent with the antibacterial properties of Ag-LDHs in the final prepared nanocomposites.After giving the antibacterial properties of layered clay,the application of functional layered clay in the aliphatic polyester will be promoted.And the application of aliphatic polyester in the field of active antibacterial packaging will be promoted further.