Study On The Phase Transition Behavior Of Polymer Composite Systems Based On Ionic Liquids

Stimuli-responsive polymers can be defined as polymers which show obvious physical or chemical changes with the tiny simulation of environment temperature. These polymers are playing more and more important roles in various areas including chemical industry, manufacture and functional materials due to their sensitive responsiveness upon temperature. As a kind of "green solvent" with wide applications, ionic liquids (ILs) are usually melting organic salts with melting points lower than 100 ℃. ILs have been proposed in different kinds of applications, such as science research and industry. This thesis mainly concentrates on stimuli-responsive polymers and ionic liquids. The phase transition behavior and dynamic transition mechanism of several polymer composite systems were investigated by DSC, turbidity measurements, FTIR, NMR in combination with PCMW and 2Dcos analysis. In brief words, in consideration of the roles ILs played in the systems, our work can be divided into four systems where ILs were studied as solvent, additive and solute. While based on the intrinsic properties of ILs, the works can be seen as two kinds: systems where ILs have no phase transition behavior and those with phase separation type ILs.The thesis is devided into six chapters.Chapter Ⅰ is the introduction of the whole thesis, where we mainly introduced the concept, constitution and characteristic of stimuli-responsive polymers and ionic liquids. The detailed information about 2Dcos and PCMW is elaborated, acting as a background for the investigation methods in the following researches. Besides, the purposes, thoughts and contents of the researches were also briefly expounded.In Chapter Ⅱ, dynamic thermally phase transition behaviour of poly(ethylene oxide) (PEO) in 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) is investigated. A relatively high lower critical solution temperature (LCST) is observed which is distinguished from conventional thermo-sensitive polymers in aqueous solution. Four types of hydrogen bonds are found in this system, among which the type Ⅰ (hydrogen bonds between unsaturated C-H on the imidazole ring and oxygen atoms of PEO) and type Ⅱ (hydrogen bonds between saturated C-H on PEO backbone and fluorine atoms of IL anions) have more important influence on the phase transition. DSC revealed that the phase transition temperatures of PEO/[EMIM][BF4] solutions decrease with the increasing concentration of PEO, as well as with the decrease of scanning rate. PCMW determined the LCST to be ca.137℃ during heating and ca.131℃ during cooling. Finally,2Dcos was employed to elucidate the dynamic mechanism of PEO in [EMIM] [BF4] which experienced the process of hydrogen bonds disruption, chains aggregation and exclusion of IL molecules. Solutions with low PEO concentration tend to form into loosely aggregated globules, in comparison with more densely aggregated globules in samples with high PEO concentration.In Chapter Ⅲ, the influence of different Ionic Liquids (ILs) on the phase transition behavior of poly (N-Vinylcaprolactam) (PVCL) solution is discussed. DSC and Turbidity results reveal that hydrophilic ILs at a concentration of 0.25 mol/L only change the transition temperature slightly. The LCST experiences an increase while the phase separation behavior disappears gradually with the increasing concentration of 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]). As a contrast, addition of hydrophobic ILs, especially 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([EMIM][NTf2]) even at a low concentration, raises up the LCST greatly indicating the possibility of hydrogen bonds between [EMIM] [NTf2] and PVCL. Temperature-dependent FTIR spectra are employed to elucidate the dynamic mechanism of different influences on phase transition of PVCL. The hydrophilic ILs can change the state of PVCL chains indirectly by varying their interaction with water molecules. For comparison, the formation of hydrogen bonds between [EMIM] [NTf2] and PVCL results in higher temperature and more energy to complete the phase transition. However, with the continually increasing of concentration, extra [EMIM][NTf2] cannot be well dispersed in water due to its hydrophobicity, the LCST undergoes a decrease and reaches to equilibrium at last.Dynamic thermal phase transition behavior of a well-defined thermoresponsive ionic liquid (IL), [P4,4,4,4][SS] and its polymer, poly ionic liquid (PIL), P[P4,4,4,4][SS] synthesized by modified free radical polymerization is investigated in Chapter IV. Obvious distinction was observed between the LCST of monomer and polymer solutions indicating their largely different dynamic transition process. Further studies revealed that the aggregation of [P4,4,4,4][SS] is mainly driven by the synergetic variations of cations and anions while separation process of P[P4,4,4,4][SS] solution is found to be dominated by conformation changes of anions. Finally,2Dcos was employed to elucidate the sequence order of chemical group motions during heating. It is concluded that synergetic variations of cations and anions in [P4,4,4,4][SS] solution resulted in the randomly aggregated structure of globules during separation. As a contrast, in the P[P4,4,4,4][SS] solution, anions of repeated units tend to be wrapped into the aggregated globules while cations would distribute at the periphery of globules after transition. Additionally, several unusual phenomena were also interpreted, such as the different varying tendency in LCST of monomer and polymer solution with increasing concentration, and their rather different LCST at the same concentration.In Chapter V, influence of bovine serum albumin (BSA) on the phase transition behavior of the synthetic ionic liquid ([P4,4,4,4][SS]) together with the interactions between [P4,4,4,4][SS] and BSA were studied. Our results reveal that the addition of BSA would increase the phase transition temperature but weaken the transition behavior of [P4,4,4,4][SS] solution. DSC and turbidity data tell us the transition temperature of ternary system added with 20 wt% BSA is 3℃ higher than 20%(w/v) [P4,4,4,4][SS] solution. Interactions between [P4,4,4,4][SS] and BSA together with the phase transition behavior of [P4,4,4,4][SS] are responsible for the denaturation of BSA upon heating. PCMW determined the obvious distinctions in LCST a of different chemical groups manifesting their various response sequences in the phase separation and denaturation upon heating. Finally,2Dcos was employed to elucidate the sequence order of chemical group motions during heating. It’s worth noting that the appearance of isosbestic point in the C=O groups of FTIR spectra indicates the direct transformation of conformation of a-helix, random coil to β-sheet and β-turn without intermediate transition state. Additionally, the phase separation process of ionic liquid is able to recover to the original status before heating while the denaturation of BSA is irreversible after a cooling process.Chapter VI summarized the whole work of the thesis. Herein, several measurements and analysis especially the 2Dcos and PCMW were utilized to investigate the phase transition behavior of the above systems upon heating. Furthermore, the microdynamic mechanisms in the transition process were elucidated, which provides help for the deep understanding in the interactions between ILs, polymers and protein. The significance of this study is to provide theoretical and directive background for the application of ionic liquid/polymer composite systems in various areas.