Glass Transition Behavior And Chain Structure Of Confined Polymers

When polymer is confined at the nanoscale,their crystallization dynamics,viscosity,glass transition temperatures(Tg)and physical aging differ greatly from that of the bulk.The physical properties of confined polymer have attracted continuously attention ever since the first measurement of Tg for polymer thin film.Although numerous investigations have been published in this case,most of the work only focused on the apparent modified physical properties while very few of them paid attention to the microstructural information of polymer.This is mainly because of the lacked characterization techniques.When polymer is confined at the nanoscale,there only exists trace amount of sample,which makes it difficult to discriminate the sample signal itself from the background.In this thesis,we utilize fluorescence/Forster resonance energy transfer(FRET)to characterize the polymer structure.In addition,calorimetric method is used to study the glass transition behaviors.Based on the application of FRET in the field of polymer physics,we came to realize that it is hard to do quantitative FRET analysis when the polymer is random labeled with chromophores.Therefore,site-specific FRET dyes labeled polymer is prepared based on controlled/living radical polymerization methods.Firstly,we investigate the synthetic methodology for the preparation of heterotelechelic FRET dyes labeled polymers so as to determine the end-to-end distance of polymer coil.A new type of chain transfer agent of dithioester structure constituted of carbazolyl and anthryl was designed.Based on the Reverse-Addition Fragment Termination(RAFT)polymerization,PS,PMMA and PBMA were prepared with molecular weights ranging from several thousands to several tens of thousand Da and narrow polydispersity.PBMA was selected as an example to demonstrate that the polymerization kinetics follows one-order polymerization type.Secondly,heterotelechelic FRET dyes labeled polymers were prepared via the combination of site-specific ATRP and click chemistry.The ATRP initiators were chemically grafted with fluorescent acceptors.The acceptor could be dictated in the a chain terminal.The ω chain end was modified from bromide group into azide group.Then alkyne-modified carbazolyl was used to react with the azide ω end via a 2+3 cycloadditon reaction.On the basis of the aforementioned protocol,heterotelechelic FRET dyes labeled PS and PMMA were successfully prepared.By the elaborate control of the polymerization conversion,PS with the Mn of 5500 was prepared with quantitative labeling ratio at two terminals,both of which are higher than 90%.In terms of PMMA,the labeling ratio at the a end is nearly 100%whereas the labeling ratio at the ω end is only 58.2%because of the high conversion during the polymerization process.Based on the FRET analysis,we confirm that the chain end distribution function obeys Gaussian statistics,which in turn demonstrates that the synthetic structure is adequate enough for quantitative FRET analysis.This benefits from the synthetic methodology becacuse it is easy to be conducted irrespective of the ingenious control of the polymerization conversion or the loss of chain end functionalities.Thirdly,we push forward the quantitative FRET analysis into interchain distance measurements,this makes sense when there is one and only one dye located in a single polymer chain.Four types of di-functional ATRP initiators incorporated with carbazolyl and anthryl were prepared.PS,PMMA and PBMA with the molecular weight ranges from several thousands to several tens of thousands were prepared via bulk polymerization.From the UV-vis spectra analysis,the labeling ratio was verified to be all higher than 90%.This near quantitative labeling ratio guarantees the reliability for quantitative interchain FRET calculation.Based on the FRET spectra,the well-defined interchain distance for different polymer species and molecular weights agree with the theoretical ones,which demonstrates that the synthetic polymeric structure is robust enough for quantitative FRET analysis.Fourthly,we tried to utilize the middle-labeled polymer to provide unique perspective in thin-film-confined polymer structure.Based on the lifetime measurements,the dimensionality of FRET dye distribution in the spatial could be obtained from the fractal fitting.Because there is one and only one dye in a polymer coil,the spatial distribution of the dyes could reflect the polymer coil packing state.When the dyes are distributed in-plane,then we will have a monolayer packing of the polymer coils.Here,we treat PMMA as an example,we found that the polymer coils are distributed in a monolayer state when the film thickness goes down to 1.5Rg.The combination of the special polymer structure and fractal fitting help us to determine the packing morphology of polymer thin films.Based on the concentration analysis,we inferred a disentanglement behavior for ultrathin film.We also did the calorimetric measurements of the PMMA thin films by using AC-chip.The Tg increases with the decrease of film thickness.At last,we expand the confining geometries into higher degree by using the porous alumina templates(AAO).The glass transition behavior and chain structure of PBMA confined in AAO were investigated.Two separated Tgs were obtained after a slow cooling process of the confined melt,the Tg at higher temperature which is higher than bulk Tg represents the adsorption layer while the Tg at low temperature which is the same as bulk represents the core layer.The key factor to form adsorption layer is the occurrence of adsorption transition.During the sample preparation process,the infiltration temperature should be higher than the temperature threshold of adsorption transition.The interchain proximity of the adsorption layer is verified to be closer than bulk.For PBMA of high molecular weights,it is easier to get two Tgs when PBMA is confined in the nanopores with larger pore diameters while there would be only one Tg when PBMA is confined in small nanopores.Through surface modification,the formation of two Tgs for PBMA with high molecular weights in large nanopores could be hindered whereas it fails to prevent the formation of absorption layer for PBMA oligomers.This result indicates that the mechanism to affect the glass transition behavior for PBMA of different molecular weights might be different for interfacial modified confined space.