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The study of polymers in geometrically confined states by the thermal analysis, the spectroscopic study, and the morphological investigation

Authors Lu, Xiaolin
Issue Date 2004
Summary This thesis focuses on the study of the different geometrically confined states of polyacrylamide (PAL) in bulk film, single chain globules, and thin films. The thermal analysis, the spectroscopic study, and the morphological investigation were carried out. The main contribution of this thesis is that we have acquired a better understanding about the glass transition (Tg) behavior of polymers. Although the glass transition is a well known phenomenon for liquids with strong covalently bonded structures, and is especially noteworthy for amorphous polymers, understanding the glass transition still remains one of the most intriguing puzzles in condensed matter physics at present. The solution of the glass transition puzzle will ultimately influence different fields in polymer science, particularly biophysics and biochemistry. Our approach to this complicated assignment, the glass transition phenomenon, is to examine the glass transition behavior of polymer chains in 3 dimensional confinement for single molecular single chain globules, 1 dimensional confinement for polymer thin films, and 0 dimensional confinement for bulk state polymer. We found that the glass transition temperature of a polymer depends on several factors, such as the inter-chain interlock entanglement, the inter-chain molecular interactions, the intra-chain cohesional entanglement, and the local chain orientation and conformational entropy. These factors have been systematically investigated by carefully preparing the polymer samples in different confined states. The main conclusion is that, although the glass transition is a non-equilibrium dynamic property, the true glass transition can be reached when polymer chains are free of the inter-chain entanglement. A better example is illustrated, in this thesis, of the glass transition behavior for the well-annealed single chain globules. PAL single chain globules are prepared by spray drying from the dilute solution. The size and morphology of the single chain globules are characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). DSC characterization reveals that the Tg of the single chain globules, upon annealing, is higher than that of the bulk film. The PAL single chain globules are believed to be in a less entanglement state. Upon annealing, a single chain globule can collapse to a highly compact state, which will induce an increase in Tg. It is presented that there exists more cohesional entanglement in the single chain globules than that in the bulk film sample. For the bulk film, the inter-chain entanglement hinders the close packing of the PAL molecules. This structural difference results in the Tg increase for the single chain globules comparing to the bulk PAL films. The FTIR spectra reveal that a new band at 1225 cm-1 appears for both the bulk films and the single chain globules upon annealing, which is attributed to the molecular association effect. When the PAL thin film on the Si substrate is prepared by spin coating, the ellipsometric measurement shows that Tg decreases as the film thickness decreases. A simple model is derived to describe the relationship between Tg and the film thickness. The Tg reduction may be due to the increased free volume in the surface area (the interface between the polymer and the air). However, Tgs with different thickness detected by the ellipsometer are abnormally higher than those of the bulk film and the single chain globules. Since the spin coating can induce the orientation of polymer chains along the substrate surface, the cohesional entanglement is likely to form easily due to the parallel alignment of the local chain segments and Tg will be consequently higher. FTIR characterization reveals that the Si substrate, after treated by H2O2/H2SO4, is wetting to the PAL molecules. The hydrogen bonds are formed between the amino groups and the SiO2 layer on the top of the Si substrate. When the Au substrate is used for the spin-coated PAL films, the reflection/absorption FTIR (RA-FTIR) spectra indicate that a layer of PAL film of 70 nm is formed. Considering the film thickness and the value of an end-to-end distance, Ree, there is no orientation of the PAL molecules, taking into account also the dewetting nature of Au to PAL. The orientation of the PAL molecules appears when the thickness is higher than 70 nm on the Au substrate. This suggests that the wetting effect and the spin coating effect can help the polymer chains to form an extended state on the substrate surface. The grazing angle effect, the dispersion effect, and the interference of the electric fields on the RA-FTIR spectra were discussed. The grazing angle effect with the p-polarized light and a 85° incidence angle shows an average 2.6-times enhancement of the infrared signals compared to the unpolarized light for the films less than 100 nm. The dispersion effect shifts the carbonyl stretching vibration to the higher frequency/wavenumber for the thinner films and forms a splitting band when the film thickness increases. When the film thickness is of several hundred nanometers with a 85° incidence angle, the interference of the electric fields strengthens the infrared signals through a certain spectral range.
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2004
Language English
Format Thesis
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