Theoretical Study of the Influence of Polymer Chains Structure on Nanoparticle Dispersion and Phase Transition of Rubber/plastic Blend
Author:Wei Chao Yang
Rubber/plastic blends have been used widely in industry,which are of great application value in fabrication of industrial products.It is a front challenge for scientists to study the evolution of particle dispersion and phase transition with atomic precision during the rubber/plastic blending process.Understanding the mechanisms and pathways of such evolution is important to prepare high performance rubber/plastic blends by controlling the phase structures and nucleation processes.In this paper,the Helmholtz free-energies of systems are constructed by a DFT approach,which incorporates the contributions of asymmetric particle—polymer interactions and polymer configuration to investigate the conformation variations of polymers,the dispersion of nanoparticles,the phase transition of rubber/plastic blends,and the interfacial properties by combining relevant theoretical models and molecular simulations.The main contents are summarized as follows:(1)To investigate the mechanisms in phase transitions,the microscopic conformations of polymer chains are firstly studied.A density functional approach by combing the PRISM equation,the test-particle method,and the modified interfacial statistical association fluid theory are developed to deal with the actual polymer conformations,with consideration of the effects of chain stiffness,polymer concentration,and short chain molecules.Based on the enthalpic contribution dominated by the interactions among polymers segments and the entropic contribution induced by polymer conformation,the form factor and structure factor of polystyrene(PS)chain with different values of polymerization degree are quantitatively evaluated.The radii of gyration and scaling exponents of PS,poly(methyl methacrylate)(PMMA),poly(ethylene oxide)(PEO),and polypropylene(PP)are calculated to analyze their expansion or contraction under different conditions.The calculation results are in qualitative agreement with the corresponding molecular simulations and experimental data.The theory is then employed to investigate the effects of chain stiffness,polymer concentrations,and polymer ratios on gyration radius and scaling exponent.This approach successfully related the microstructure to the macroscopic properties of polymers,which extends the application scope of classical density functional theory and lays a theoretical foundation for describing the polymer configurations in rubber/plastic blends.(2)The dispersion(or aggregation)behavior of nanoparticles in asymmetric polymer nanocomposites is investigated based on the basic characteristics of polymer chains.The actual packing fraction is corrected with special consideration of the asymmetric interactions between nanoparticles and polymer segments to improve the accuracy of theoretical predictions.The theoretical results are compared with the corresponding experimental data.Such comparison confirms the reliability of the theory for polymer nanocomposites with delicate microstructures.Take SiO2/PS and Octamethyloctasiloxane(POSS)/PP composites as the examples,the effects of particle size,concentration,and particle-polymer interactions on the radii of gyration of polymers to illustrate the different contributions to the conformation of polymer chains.It has proven that the particle dispersion and the radii of gyration of polymer show different characters due to variations in asymmetry and interactions.The results provide a deep understanding of chain conformation in polymer nanocomposites,and lay a theoretical foundation for the study of phase transitions in nanoparticle/rubber/plastic blends.(3)In this part,the phase transition of rubber/plastic blendsis studied simultaneously by the density functional theory and experiments,taking a kind of thermoplastic vulcanizate:the polyolefin elastomer(POE)/PP as an example.The results indicate that the phase transition is not a simple dynamic process influenced by shear,viscosity and other factors,but a complex dynamics evaluation which contains many thermodynamic behaviors.That is to say,the phase transitionis induced by thermodynamic driving force that is developed during the dynamic crosslinking process.The mechanisms in thermodynamic phase transition of POE/PP blends are explained at the molecular level.The effect of crosslinking is discussed and the evolution process from the sea—island to island-sea structure is displayed by AFM pictures.Then,the spatial density distribution and the corresponding free energy profiles of the interface are calculated,based on which the phase diagram for the POE/PP is obtained.This part also predicts the variations of interfacial tension as well as the critical nucleation size.The density functional theory predicted size of POE-rich nuclei agrees well with the experimental result.Thus,this work provides novel physical insights regarding the phase inversion in thermoplastic vulcanizates,provides a fundamental basis for further reseach.(4)The heterogeneous phase transition mechanism of POE/PP blends is studied based on the evolution of phase structure and the nucleation characteristics of rubber.In fact,POE/PP blends belong to the thermodynamic incomplete solubility system.POE/PP blends still exist in POE phase or PP phase.Therefore,once homogeneous phase transition occurs,it is inevitable that the heterogeneous phase transition takes place at the surface of PP or POE due to the lower nucleation free energy barrier in heterogeneous phase transition.For this reason,this part studies the effect of configuration of PP chains on POE/PP phase transition,and analyses the density distributions in heterogeneous phase transition.Therefore,the mechanism of the heterogeneous phase transition of POE/PP blends on PP and POE surface is explained from the micro-level by comparing the interfacial tension,the nucleation barrier,and the critical nucleation size in both homogeneous and heterogeneous phase transition in POE/PP blends.The morphologies in different stages of POE-rich and PP-rich droplet nucleation have shown in this part.(5)Based on the prior theoretical basis of particle dispersion in polymer nanocomposites,we study the effects of nanoparticles on heterogeneous phase transition process in POE/PP blends.Taking the POSS nanoparticles as an example,the influence of the rough structure on the heterogeneous phase transition is investigated by constructing a composite surface of POSS particles and PP and POE chains.Ignoring the influence of particles on the phase transition of POE/PP blend under low volume fraction.Based on this,the droplet nucleation energy barrier and critical nucleation size are predicted at low volume fraction.The quantitative characterization of the cantact angles of dropltes on the different composite surface.This work provides the theoretical guidance for design of high performance composite materials.