| The extensive use of petroleum-based plastic products has a growing impact on the human living environment,so the development of degradable materials has attracted more and more attention.Polylactic acid(PLA)produced from narural renewable resources,which has excellent biodegradability,biocompatibility,thermal stability and diversified processing methods(extrusion,injection molding,and spinning,etc.).Therefore,PLA is expected to gradually replace traditional petroleum-based materials and has a broad application market in agriculture,tableware,food packaging,and medicine.However,some shortcomings caused by the structural characteristics of PLA limit its further development and application.For instance,the crystallization rate of PLA melt is slow,resulting in poor heat resistance of its processed products,which limits its application in the field of high temperature resistance.It is well known that increasing the crystallinity of semi-crystalline plastics is one of the most direct and effective method to improve their heat resistance,and adding nucleating agents is considered to be one of the most effective ways to improve the crystallinity of materials.Therefore,in this paper,the biodegradable polyester material PLA was taken as the research object.While maintaining the inherent degradation advantages and biocompatibility of PLA,D-sorbitol(DS)and cellulose nanocrystals(CNC)were selected as nucleating agents of PLA to prepare new bio-based degradable composites.Finally,the effects of nucleating agent addition on the crystallization behavior and processing properties of PLA were studied.The main research contents are as follows:(1)The effect of D-Sorbitol(DS)on the crystallization and processibility of PLA was systematically explored.The results showed that,as expected,DS can act as nucleating agent for PLA and thus strikingly improve its crystallization properties.With 0.25%-0.75% DS in PLA/DS blends,the cole crystallization of PLA disappears accompanied by a concompanied by a concomitant elevation of crystallization temperature.Based on analyzing isothermal crystallization kinetics,it is clear that the incorporation of DS can significantly enhance the crystallization rate of all PLA samples.For instance,the semi-crystallization time of neat PLA is 14.2 min at 110 °C,while PLA/0.75% DS blend takes only 1.5 min.A rheological characterization demonstrated that,besides playing a role as a nucleating agent,DS also serves as a plasticizer which improves the melt processing performance as reducing the modulus and viscosity of PLA matrix.On the basis of FTIR data,we propose that hydrogen bonding interactions between PLA and DS are responsible for hampering a further improvement of both crystallization properties and rheological performance over adding a high content of DS.(2)The effect of cellulose nanocrystals(CNC)on the crystallization,mechanical and rheological properties of PLA was systematically explored.The results showed that CNC as a heterogeneous nucleating agent for PLA,inhibited cold crystallization and significantly increases the crystallization rate and crystallinity of PLA.According to the analysis of Avrami equation and Lauritzen-Hoffman theory,the overall crystallization rate of PLA was controlled by the nucleation rate.Moreover,an appropriate amount of CNC can be uniformly dispersed in the PLA matrix to improve its tensile strength.Finally,the rheological analysis showed that CNC exacerbated the nonNewtonian properties of PLA melt,and shortened the LVE.It can be seen from the frequency scanning that the dynamic modulus and complex viscosity of PLA in the low frequency region significant increased with increasing fraction of CNC.When the CNC loading exceeds 2.0%,due to the formation of a CNC infiltration network structure,PLA/CNC nanocomposites exhibit a “solid-like response” in the low frequency region.Furthermore,the addition of CNC increases the non-uniformity of the PLA matrix,and hinders the relaxation of molecular chains. |
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