FTIR And 2D Correlation Spectroscopy Study On Micro-Dynamics Mechanism Of Polymer Evolutions With External Perturbations

This thesis mainly investigates micro-dynamics mechanisms of polymer evolutions with external perturbations by FTIR (Fourier Transform Infrared Spectroscopy) and 2D (Two Dimensional) correlation spectroscopy methods. When the infrared light is injected onto the sample, different molecules of the sample absorb the infrared light with specific wavenumbers. By comparing the difference between injection and transmittance infrared lights, we are able to make sure the types of molecules inside the sample. Two dimensional infrared spectroscopy method is an advanced method which has higher resolution than the regular infrared spectroscopy, it's able to investigate the dynamic mechanism of samples on the functional group level. To obtain the two dimensional infrared spectra, we should give an external perturbation to the sample, this perturbation can be in many types, like changing temperatures, concentrations, pH values, etc. With the perturbation, we can obtain a series of dynamic spectra of the sample. And with correlation calculations we can finally obtain the 2DIR spectra, which can get the knowledge of micro-dynamics mechanism of various functional groups in the polymer. Based on previous researches, in this thesis, we mainly use FTIR,2D correlation spectroscopy, and PCMW2D (Perturbation Correlation Moving Window 2D) methods to understand more about the relationships between molecular structures and properties of the polymers.The thesis includes seven main chapters and two extra chapters. Chapter I introduces the basic and general knowledge of FTIR and 2D correlation spectroscopy methods. Meanwhile, we introduce the aim of our work and ideas of all the researches. In ChapterⅡ, we talk about the experimental details, like instruments, sample preparation methods, and experiment conditions, etc. In addition, the author also talks about her own experiences obtained in the actual infrared experiments. In Chapter III, we work on a "smart" polymer-Poly (N-isopropylacrylamide) (PNIPAM), which significantly responds to changing temperatures in the surroundings. By FTIR and 2D correlation spectroscopy methods, we understand the micro-dynamics mechanism of phase separation behavior of PNIPAM 20 wt% aqueous solution during heating, and also the inverse mechanism during cooling. Spectral data of the C-H vibration region and the Amide I region provide details about changes of the hydrophobic and hydrophilic groups of PNIPAM respectively during a heating-and-cooling cycle. The reversal of positions of the C-H bands upon cooling indicates the reversibility of temperature-induced dehydration of the hydrophobic groups. The change in hydrogen bonds of C=O…D-N constructed between C=O and N-D hydrophilic groups, as derived from the Amide I region, does not revert precisely in the cooling process due to the newly formed hydrogen bonds in the collapsed state, and a hysteresis phenomenon is observed. In our concentrated solution (20 wt%), the strength of those intra-/inter-chain hydrogen bonds even prevent the polymers from dissociating completely below the LCST during the cooling process. The micro-dynamics phase separation mechanism is obtained by application of the 2D correlation analysis to the spectral data. When temperature rises, the two-step dehydration of the CH3 groups occurs first, then the main-chain diffusion and aggregation takes place, and finally the hydrogen bond transition occurs. The dynamic sequence in the cooling process is also described.Chapter IV discusses a detailed study of the fundamental vibrations in the mid-infrared region and NH-proton deuteration result, presenting the assignment of bands in the near infrared spectrum of PNIPAM. Variable-temperature experiments and 2D correlation spectroscopy are used to find out the chemical mechanism and changing sequences of groups in PNIPAM, we conclude that bonded-NH groups turn into the free NH groups during the heating process, while the CH groups on the side-chain change prior to that on the main chains. The temperature-induced dissociation of the hydrogen-bonded NH groups is also found to proceed earlier than the conformational changes in the hydrocarbon chains.In Chapter V, the phase separation behavior of another "smart" polymer--Poly (vinyl methyl ether) (PVME) is studied by FTIR and 2D correlation spectroscopy. In FTIR analysis, the spectral data of O-H, CH3-O and C-H vibration regions provide details about changes of hydrophilic and hydrophobic groups in PVME respectively. The micro-dynamics phase separation mechanism of PVME aqueous solution is obtained by 2D correlation spectroscopy analysis. During heaitng the initially hydrated CH3 groups start to dehydrate as the first action of phase separation, and the initially hydrated CH2 follow to start their dehydration, interestingly, water molecules leave CH2 very fast, and the whole dehydration process of CH2 ends even earlier than that of CH3. After hydrophobic groups finish their dehydrations, hydrogen bonds between hydrophilic group and water (CH3-O…water) start to dissociate, the 1:2 adducts formed between PVME and water dissociate first, and transfer into the 1:1 adducts, while with further heating 1:1 adducts eventually dissociate and release free water molecules.To further understand the relationships between structures and temperature sensitive properties of PNIPAM and PVME, we compare these two "smart" polymers more directly and detailedly in ChapterⅥ. Finding that different numbers of hydrophilic groups induce different phase separation mechanisms of PNIPAM and PVME aqueous solution. PNIPAM has two hydrophilic groups, C=O and N-H, which interact with each other and form new hydrogen bonds after phase separation, and this kind of strong inter/intra-chain hydrogen bonds hinders PNIPAM aqueous solution reverse to its initial state during the cooling process; while PVME has only one hydrophilic group of CH3-O, so that in the cooling process PVME aqueous solution is able to reverse very easily. By comparisons of several other aspects, we get better understanding of differences between PNIPAM and PNIPAM.In chapterⅦ, one kind of dental adhesives has been investigated by FTIR and 2D correlation spectroscopy methods. Assignments of dental adhesives in the mid-infrared and near-infrared regions are ascertained detailedly. According to the high resolution of 2D correlation spectroscopy, the evolution mechanism of the curing process has been concluded as well, namely, double-bonds of adhesive functional monomers transfer into single-bonds during visible-light curing.In Extra Chapter I, we investigate the mixture of ionic liquid (1-butyl-3-methylimidazolium tetrafluoroborate, bmimBF4) and water (2.5%, molar fraction) under isothermal condition at 80℃. During heating, water is gradually evaporated, which produces influences on the interactions among cations, anions and water molecules. In FTIR analysis, we find an interesting "V" shaped changing trend in peak areas of the C-H (on the imidazole ring) stretching band and the B-F stretching band, the inflexion of the system is 913 s, gained by Moving Window method. A two-step variation is accordingly found during this process. Hydrogen bonds formed between water molecules and cations or water molecules and anions are destroyed by the reduction of water, making a fall in the former period of "V" shaped process; while electrostatic interactions newly formed between anions and cations leading to a rise during the latter period of this course.In Extra Chapter II, we further investigate the interactions between ionic liquids and water through their heating process by near-infrared spectroscopy. We find that the overtone band of v(O-H) has a blue shift during heating, and its peak area increases first and then decreases, which is very interesting. According to its different changes, we analyze the overtone region of v(O-H) in three temperature regions: (1)25-100℃, (2)105-160℃and (3)165-190℃by 2D correlation analysis. We find that there are lots types of hydrogen bonds in the ionic liquids/water mixture (concentration of water is 15 mol%). According to 2D correlation analysis, we detailedly assign bands in near-infrared region, and infer that during 25-100℃hydrogen bonds in BF4-…water…BF4-and BF4-…cyclic water dimer…BF4-structures partially dissociate; during 105-160℃, BF4-…water…BF4- continues to collapse and cation…water also begins to dissociate, and free water released from dissociations of the hydrogen bonds is evaporated; while during 165-190℃, few BF4-…water…BF4-still exists in the system and continues to break down. We have also studied the v(C-H) overtone region, finding that various amount of water affects interactions between cations and anions of ionic liquids.