Preparation And Properties Of Starch-based Degradable Plastics

For the environmental protection,the development of natural degradable polymer materials has become one of the major research focus in recent years.Among the natural polymers,starch（ST）is considered as the most promising candidate to develop biodegradable plastics owing to its low cost,renewability,biodegradability,high tensile strength,and biocompatible properties.Although plasticized ST has been widely used in packaging materials and disposable goods,it was far from meeting the demands because of its intrinsic characteristics,such as lack of thermoplastic properties,relatively low stretchability,and incompatibility with other hydrophobic polymers.To solve those problems,the works have developed two new kinds of ST-based composites by crosslinking methods.The contents are mainly divided into the following two parts:Ⅰ.Poly（ester-urethane）modified starch based on PCL（PUST）:The poly（ester-urethane）prepolymer（PCU）with terminal-NCO groups was synthesized by polycondensation between polycaprolactone diol（PCL）with different molecular weight and excessive isophorone diisocyanate（IPDI）,and the PCU were then crosslinked by ST to give the PUSTs with various ST content.The chemical structure of PUST films was characterized by FT-IR,and the influence of crosslinking degree on the physicochemical properties of PUST films.The crosslinked structure formed by carbamate bonds improved the compositional compatibility,resulting in a low crystallinity,hydrophilicity,water vapor transmittance（WVT）,high thermal stability and glass transition temperature（T_g）.Furthermore,the crosslined structure endowed the PUST films with outstanding mechanical properties,the ultimate tensile strength（UTS）,elongation at break（EAB）,and Young’s modulus UTS of the films were 4.09～13.60 MPa,53.2～592.8%and 13.94～60.59MPa respectively.The test of degradation showed that PUST films could be degraded in a relatively short time（about 140 days）,and the degradation rate decreased with the increase of crosslinking degree.In addition,the biological evaluation of PUST were performed by protein adsorption and cytotoxicity tests,and the results showed that PUST films possessed high anti-protein adsorption capacity（8.29μg/cm²）and good cell viability,demonstrating superior biocompatibility and cytocompatibility.Ⅱ.Dual-crosslinked modified starch based on oligochitosan and polyurethane（SPEO）:A dual-crosslinked strategy was adopted to design and prepare SPEO with different oligochitosan（OCS）content.The PCL-based polyurethane prepolymer（PPUP）with terminal-NCO groups and pendant aldehyde groups（-CHO）was firstly synthesized by PCL,dihydroxybenzaldehyde and excessive IPDI;then,the starch was modified by PPUP to form a cross-linked structure by condensation reaction between-NCO of PPUP and-OH of starch;and finally,the obtained poly（ester urethane）-modified starch（PEMS）was blended with OCS to produce dual-crosslinked structure via a chemical reaction between-CHO of PPUP and-NH₂ of OCS.The chemical structures of SPEO materials were characterized by FT-IR,which confirmed the expected SPEO composites were prepared successfully with dual-crosslinked structures linked by carbamate bonds and imine bonds.In addition,the in-depth characterizations of the SPEO films including crystallinity,thermal properties,mechanical properties,micromorphology,surface hydrophilicity,water absorption,WVTR capacity,in vitro degradation and bacteriostatic activities,were performed and reported.XRD analysis and surface/cross sectional micromorphologies indicated that SPEO possessed low crystallization capacity,and the formation of a dual-crosslinked structure could improve the compatibility of PCL and ST in SPEO.The SPEO composites exhibited good hydrophilicity in the surface hydrophilicity and water absorption tests,indicated there were still a large amount of-OH in the SPEO.The water vapor permeability results indicated that the dual-crosslinking strategy could provide a good barrier to water vapor,and the dual-crosslinked SPEO films possessing low WVTR（<128 g/（m²·24h））.TGA test showed that the SPEO composites possessed good the thermal stability,and the initial decomposition temperature was higher than210℃.There was no obvious T_g in DSC curves,which also proved that the SPEO composites had better compatibility.The SPEO-4 composite with the highest crosslinking density had the best mechanical properties in the curves(UTS:13.5 MPa,SAB:107%,YM:58.1 MPa,FT:12.4MJ/m^-3).Furthermore,the degradation rate increased with the increasing OCS content,which was similar to the regulation of water absorption.The inhibition zone assays of OCS-containing SPEO films against E.coli and S.aureus showed there were obvious bacteriostatic ability in SPEO–6,which contained more residual–NH₂ groups,exhibited an enhanced inhibitory effect with an inhibitory zone diameter of 13.5±0.2 mm for E.coli and 20.1±0.2 mm for S.aureus.Finally,biological evaluation of SPEO films were conducted by protein adsorption and cytotoxicity tests.The protein adsorption capacity of all SPEO films were lower than 8.44μg/cm² and cytotoxic evaluations were level 0 or level 1,indicating that the composites had excellent biocompatibility.In this thesis,the ST-based degradable plastics with high ST content prepared by crosslinking strategy had low cost,satified mechanical properties,biodegradability,low WVP,good biocompatibility,and certain bacteriostatic capacity,which not only hold great potential for broad application but also provided a design idea and theoretical basis for other ST-based plastics.