Influences Of Surface Characteristics Of SiO₂ Particles On Rheological Behaviors Of PLA/SiO₂ And Crystal Structures And Crystallization Behaviors Of PLA

Based on polylactic acid(PLA)matrix,the composites with different interfacial interactions were designed and prepared by melt blending PLA with two types of silica particles,respectively.The surface characteristics of silica particles are different,and their particle radius R are equivalent to each other but smaller than the gyration radius Rg of PLA chain.One type of silica particle is hydroxyl A200 and the other is R974 with 50% hydroxyls alkylated.The rheological behaviors of the obtained PLA/A200 and PLA/R974 composites were focused on and the crystallization behaviors and crystal structures of PLA in the composite samples were also examined.The results showed that the composites are distinct from each other in both linear viscoelastic and steady shear behaviors.PLA/A200 composite readily evolves from the much viscous state to a solid-like state.The viscosity of PLA/A200 composite under low shear rates increases exponentially with increasing A200 volume content.The viscosity of PLA/R974 composite decreases at low R974 volume contents but increases at high R974 volume contents according to Krieger-Dougherty model.The differences of rheological behaviors of the composite materials with different interfacial characteristics could be attributed to the different polymer-particle interaction and repulsive energy closely related to the surface modification.And this has been confirmed by the results of molecular dynamics simulation.Because of the weak interfacial interaction in the composite materials,the viscosity of PLA/R974 composite decreases at low volume fractions.The effects of addition of the two kinds of silica particles with different surface characteristics on PLA crystallization behaviors were further investigated.The WAXS results show that PLA crystal forms in the composites are influenced neither by the hydroxyl surface of A200 particle nor by the partially alkylated surface of R974 particle.The isothermal crystallization dynamics of PLA were investigated by DSC.The results show that,with R974 added,the half-time crystallization(t1/2)of PLA is longer than that of pure PLA within the experimental range of isothermal crystallization temperatures,indicating that the added R974 inhibited the PLA crystallization process.The A200 particle with stronger interfacial interaction with PLA inhibited PLA crystallization more obviouly.Furthermore,the Avrami parameters n and k demonstrated that there existed an imperfect early stage of the crystallization process at high isothermal crystallization temperatures.The long period L,lamellar thickness Lc,and the amorphous thickness La were obtained by SAXS.The lamellae parameters in PLA/A200 composite samples were much greater than those in pure PLA and PLA/R974 composite samples.