Glass Transition And Chain Conformation Of Confined Polymers

onfined at the nanoscale,polymers show peculiar behaviors characterized by dramatic changes in many physical properties with respect to bulk polymers.Such glassy materials find widespread applications in industry and in nanotechnology.Since Keddie et al.reported that the glass transition temperature(Tg)of polymer thin films spin-coated on silicon substrates decreased as the film thickness decreased in 1994,the confined polymers attract more and more attention.In recent decades,the results reported have shown disagreement about the behavior of ultrathin polymer films with decreasing film thickness and raised many fundamental questions.However,the physical theory of glass transition still remains a challenging and unsolved problem.In this thesis,we mainly detected the glass transition of confined polymers by calorimetries and investigated the chain conformation with the spectroscopic method.The studies were focused on planar thin films,nanotubes and polymer solutions.Firstly,we investigated the glass transition of polystyrene(PS)thin films by differential alternating current chip(AC-Chip)calorimetry.Previous reports claimed no thickness dependence of Tg for supported polymer thin films measured by AC-Chip method.However,for the first time we found the Tg dependence on thickness in two systems using this measurement.In the first system,we changed the chain architecture and investigated oligomers and star-shaped polymers.Comparing with general polymers,oligomers and dendrimers have more free chain ends which help to increase the interfacial free volume and contribute to the decreased Tg.The absence of entanglement in oligomers and increasing arm numbers of dendrimers can both enhance this effect.In the second system,we manipulated a controlled interface by spin-coating PS with immiscible surfactants such as tetraoctylammonium bromide(TOAB).The surfactant molecules assembled on the interface between thin film and substrate due to phase separation.The polar portion of TOAB adsorbed onto SiO2/Si substrate and the nonpolar portion contacted with PS,which released the stress and increased the interfacial mobility.Moreover,we detected the mobility of adsorbed TOAB on SiO2 nanoparticles to simulate the planar film state by the use of a NMR dipolar filter pulse sequence.The molecular mobility of TOAB at the interface was dramatically increased.Essentially,the increased interfacial mobility was the major reason for the decreased Tg in these two systems.Most of the existing studies were focused on planar thin films,so we also investigated PS nanotubes confined in nanoporous anodic aluminum oxide(AAO)templates.The geometric curvature was different from that of planar thin film.An increased Tg was detected for PS nanotubes in AAO with respect to the bulk polymer by traditional differential scanning calorimeter(DSC).And the Tg increased as the tube diameter decreased,which exhibited significant differences in vitrification behavior from the planar one.The effect was obvious for low molecular weight PS.Surprisingly,this dynamic property reverted back to the bulk value when the AAO template was chemically removed.We used the fluorescent non-radiative energy transfer(NRET)method to characterize the chain morphology.PS chains confined in AAO seemed to get closer comparing with the bulk,and converted back to the bulk state after removing the template,which agreed well with the Tg results.The study clearly showed that the hard wall imposed a crucial stress to the adsorbed thin film,and the geometric curvature contributed to the chain conformation and Tg.At last,we used the NRET method to preliminarily investigate the chain conformation in polymer solutions.NRET method has many advantages due to its high sensitivity and simplicity.Usually,fluorescent donor and acceptor molecules were labeled on different polymer chains to detect the inter-chain distance.In this thesis,we introduced the intra-chain NRET method,where the donor and acceptor were labeled on the same chain.Both inter and intra-chain distances got closer as the solution concentration increased.We could find an obvious transition point from the dilute to semi-dilute solutions,that was the critical chain overlap concentration C*.The C*obtained by inter and intra-chain NRET methods were the same,indicating that inter-chain proximity and intra-chain collapse took place synchronously.In addition,we investigated the effect of molecular size of solvent on chain conformation.The middle-sized solvent(such as dioctyl phthalate,DOP)depressed the extent of interpenetration of chains,so the C*value was higher than that in small-sized solvent.For the first time we simultaneously detected inter and intra-chain conformation in polymer solutions,and obtained a more distinct physical meaning.