Preparation Of In-situ Nucleating Agent And Recyclable Nucleating Agent And Their Regulation On The Structure And Performance Of Polylactic Acid

Improper use of polymer materials causes environmental problems,such as‘white pollution'.Biodegradable polymer materials are considered to be one of the best alternatives to non-degradable or non-recyclable polymers.In recent years,as a biodegradable polymer material,polylactic acid(PLLA)has excellent biocompatibility and processability,which has attracted widespread attention.However,the rigid molecular structure of PLLA leads to its slow crystallization speed,high brittleness,and poor heat resistance,which seriously affects its application and promotion.By directly adding nucleating agents to PLLA,its crystallinity can be effectively increased,thereby improve its properties,and obtain high-performance PLLA materials.It is worth noting that among many nucleating agents,organic amide nucleating agents have the characteristics of small addition amount and good nucleation effect,and are widely used in the actual production process.However,such nucleating agents often need to undergo complex organic synthesis and purification steps,and there are also problems of agglomeration in the process of mixing and dispersing with PLLA.In this study,a simple and efficient in-situ preparation of PLLA nucleating agent was conducted to complete the modification of PLLA during the processing.In addition,a magnetic nucleating agent was achieved to act as a recyclable nucleating agent.(1)4,4'-methane diisocyanate(M)and benzoyl hydrazide(P)were added into PLLA melt simultaneously to in-situ prepare PLLA nucleating agents.Nuclear magnetic resonance confirmed that M,P and PLLA in situ reacted during melt mixing.The crystallization behavior and crystal morphology were studied by differential scanning calorimetry,polarized optical microscope and SEM.The experimental results showed that the crystallization rate of PLLA was greatly improved by adding(M+P)and in-situ forming the nucleating agents.The half crystallization time of PLLA was greatly reduced from 42.0 min to 1.1 min at 135?.After reacting at 0.25%(M+P)for 5 min,the crystallinity increased from 10.3%to 42.1%.With crystallinity increasing,the size of PLLA crystal decreased significantly.The Vicat softening temperature of PLLA composites increased from 57.4?to 93.7?by adding 6.00%(M+P),indicating the heat resistance of PLLA has been significantly improved.This research provides a new strategy for designing high performance and high crystallinity PLLA materials through in-situ melt reaction.(2)A magnetic Fe₃O₄·SiO₂-PHB nucleating agent(MM)was formed by grafting the biodegradable polymer PHB to the surface of magnetic nanoparticles(Fe₃O₄·SiO₂).The SEM and TEM experiments verified the successful construction of the magnetic core-shell structure.The magnetic core realized the magnetic recycle of the nucleating agent,and the shell polymer realized the improvement of the crystallization performance of PLLA.The DSC experiment results showed that the addition of 10 wt%MM increased the crystallinity of PLLA from 10.2%to 37.9%.The half crystallization time of the PLLA-20-MM samples at 135?was nearly 10 times shorter than that of neat PLLA.With the increase of crystallinity,the barrier properties of the PLLA film have been greatly improved.Specifically,the unit oxygen permeability of the PLLA-20-MM sample is 62.5%lower than that of Neat PLLA.In addition,the nucleating agents could be easily recycled in a magnetic field and maintained high nucleation effect.Specifically,the non-isothermal and isothermal data indicated that crystallinity and half crystallization time almost the same with the nucleating agent using for the first time.This research provides a new idea for the strategy to prepare recyclable PLLA nucleating agents.