Study Of Molecular Dynamics And Application Of Poly(methyl Methacrylate)-based Ionomers

The ionic species in ionomers usually associate into ionic aggregates due to coulombic interaction,which behave as physical crosslinks dramatically changing the structure and properties of ionomers compared to their non-ionic counterparts.A great deal of research concentrates on the morphologies of ionic aggregates,yet the dynamics of ionomers that is important to the applications are poorly understood.Hence,it is of academic importance to investigate the morphology of ionic aggregates,molecular dynamics and their relationship under the influence of different structural factors and processing conditions.Besides,such an understanding contributes to reveal the unique properties of ionomers and provides a theoretical basis for optimizing the performances of these materials.In this dissertation,poly(methyl methacrylate)(PMMA)-based ionomers are chosen as model systems.The effects of neutralization level,carboxylation degree,counterion type and thermal treatment on the morphology of ionic aggregates,molecular dynamics and rheology behaviors were studied via a combination of small angle X ray scattering,modulated differential scanning calorimetry,dynamic rheological measurement and broadband dielectric spectroscopy.The role of ion associations on molecular relaxation behaviors is also discussed.Moreover,PMMA-based ionomers as processing aids were applied to rigid poly(vinyl chloride)to develop their application value.Ionic aggregates are clearly formed in matrix when neutralization level is above 80%.Increasing neutralization level causes notable retardation of the segmental motion,behaving as the increasement of relaxation time and elevation of glass transition temperature(Tg).Meanwhile the fragility index decreases due to the increasement of the efficiency of chain packing caused by intermolecular interaction.The coordination structure of counterion depends on the electron configuration of cation and differs from each other.Although the segmental relaxation time and heterogeneous is affected by counterion type,the glass transition temperature of ionomers is consistent.Dynamic rheology of the ionomer melts follow the time-temperature superposition principle and,at neutralization levels above 80%,shows a long-term relaxation process and nonterminal relaxation ascribed to ionic species and behaves like a critical gel.By analyzing the linear rheology in the framework of a "two phase"model,an interconnected multiplets network is identified as a mechanism being responsible for the fluid-to-solid transition of "cluster phase"-free ionomers with increasing neutralization level.Otherwise,lanthanum ionomer contains more crosslink structure of interconnected ionic aggregates and strongly restricts the mobility of nearby chain segments surrounding the ionic cores.Thermal treatment has a pronounced effect on the morphology of ionic aggregates and molecular dynamics of sodium ionomers.With the increase of annealing temperature and/or time,the multiplets of sodium are gradually agglomerating into cluster,resulting in the increased efficiency of chain packing and decreased segmental mobility.Therefore,the free volume of the system reduces,the Tg shifts to higher temperature,relaxation time increases and the material fragility decreases.In addition,thermal treatment diminishes the dynamic heterogeneity,presenting as narrowing distribution of a relaxation peak.However,excessive annealing temperature can cause dissociation of ionic aggregates leading to decreased relaxation time and increased dynamic heterogeneity.Furthermore,neither the dynamics nor the distribution width of the β relaxation process is affected by neutralization level,carboxylation degree,counterion type and thermal treatment,indicating that the focal environment of the segments is unchanged.The matrix glass transition of PMMA-based ionomers results from the cooperative rearranging motion of noncarboxylated MMA units between ionic aggregates.As carboxylation degree increasing,the Tg of ionomers shift to higher temperature,the concentration fluctuation of relaxation segments increases and the characteristic length of cooperative rearranging region decreases.And the fragility index increases due to the increasement of intermolecular interaction Counterion type does not affect the Tg and the characteristic length.However,fragility index strongly depends on the counterion type,which is attributed to the different intermolecular interaction governed by the nature of cation.PMMA-based lanthanum ionomers used as a kind of multifunctional aids for rigid poly(vinyl chloride)(PVC)resins can restrain the discoloration at the beginning of processing and benefit to the long-term stabilization of PVC in comparison with traditional stabilizer lanthanum stearate.Meantime,they can accelerate PVC plasticization more efficiently.The rigid PVC products stabilized with the ionomers present good transparency and enhanced tensile strength.