Glass Transition And Crystallization Behaviors Of Two-dimensional Confined Molecules

Upon reducing the dimension or altering the nanostructure,materials manifest abnormal physical behaviors compared to those of the bulk,which play a key role in the application of functional materials,nanoprinting,sensors,and electronic devices.Hence,the study of materials under confinement is essential for the design of nanostructured materials.The "size effect" and "interfacial effect" have mainly been suggested to account for the these variations.However,how does these two effects perform remain unclear,especially in high geometry dimension confined system.Here,focus on these two effects,we studied the glass transition,dynamics and crystallization behaviors of different materials confinde in AAO nanopores by traditional DSC method and broadband dielectric spectroscopy(BDS).First,the glass transition behaviors of poly(methyl methacrylate)(PMMA)nanofibers confined in pristine and surface-modified AAO templates are investigated.During an ultraslow cooling process(0.1 K/min)across the Tg,two glass transition temperatures(Tg,low and Tg,high)are clearly identified by DSC and BDS,which correspond to the core and shell respectively.The Tg,hihg originates from the transition of the adsorbed layer and is mainly dominated by the geometric curvature radius of the nanopores,rather than the chemical nature of the wall surface.A dramatic change in the glass transition behaviors is detected when the cooling rate is changed from 40 to 0.1 K/min,which reflects the inherent evolution between the shell and the core through a non-equilibrium interlayer.Furthermore,by studying the system before and after surface modification of the nanopores by silanization,we suggest that such evolution could be sped up through the benefit of the stronger interfacial interactions.Our findings provide insight into achieving stable glassy polymer structures confined in nanopores by balancing the geometric curvature,interfacial interactions and cooling rate.Then,the thermal and dynamics properties of confined indomethacin(IMC)and griseofulvin(GSF)in anodic aluminum oxide(AAO)nanopores were investigated by differential scanning calorimetry(DSC)and broadband dielectric spectroscopy(BDS).DSC results indicate the glass transition temperatures of both confined drugs show strong pore size-dependency.Compared with the case of IMC/AAO system which can be described by a typical two-layer model,GSF/AAO system presents an unusual scenery of three glass transition temperatures,indicating there exists a thermodynamic nonequilibrium interlayer between the bulklike core and interfacial layer.And by slow cooling,the GSF confined in AAO nanopores would turn into a more stable core-shell nanostructure via local rearrangement of interlayer molecules.BDS results demonstrate the significant change in temperature dependence of the structural relaxation time during cooling process due to the vitrification of the interfacial molecules and local density heterogeneity in isochoric condition.In addition,we also studied the variation of glass transition behavior for both drugs confined in surface-modified nanopores.Besides the pore size and the thermal treatment process,the interfacial and intermolecular interactions are suggested to be the dominated factors on the formation and stabilization of nanostructure for the supercooled liquids under two-dimensional confinement.At last,we studied the crystallization behavior of low molecular weight polyethylene glycol(PEG,Mn= 5260 g/mol)in AAO nanopores by DSC and BDS.Calorimetric results indicate the change of the nucleation mechanism,and the depressed Tc,Tm and degree of crystallinity for the confined system.And we trace the variation of dielectric relaxation spectroscopy of the confined PEG in the cooling process.We propose that PEO nanorods in AAO can be divided into two regions with quite different nucleation mechanisms:a bulk-like core and an interfacial layer.The heterogeneous nucleation mechanism prevails in the the core region.While in the interfacial layer,due to the interaction with the pore wall,the mobility of PEO chains are suppressed,and crystallize in the way of homogeneous nucleation or surface nucleation mechanism.In addtion,the heterogeneous nucleation will become more dominated in the surface modified AAO nanopores with reduced interfacial interaction,which confirms our hypothesis.