Cononsolvency occurs if a mixture of two good solvents causes the collapse or demixing of polymers into a polymer-rich phase in a certain range of compositions of these two solvents. The better solvent is usually called cosolvent and another common solvent is called solvent. So far, the phase-transition mechanism behind cononsolvency is still rather controversially debated in literature. In this thesis, I experimentally investigated the cononsolvency effect of poly(N-isopropylacrylamide) (PNiPAAm) brushes with different grafting density in aqueous alcohol mixtures. I have used Vis-spectroscopic ellipsometry measurements and proved the hypothesis that the cononsolvency transition of PNiPAAm brushes consists of a volume phase-like equilibrium transition.
I found a strong collapse transition in PNiPAAm brushes followed by a reentry behavior as observed by ellipsometry measurements. Using a series of alcohols with increasing alkyl-chain length I have demonstrated that the cononsolvency effect is enhanced and shifted to smaller volume fractions of the alcohol. Particularly for the alcohol with increasing hydrophobic property this is correlated with an increasing tendency of demixing between the cosolvent and water. This is apparently in contrast to the hypothesis of strongly associative solvents being the origin of the cononsolvency effect. The hypothesis of preferential adsorption, on the other hand, can account for this case by assuming an increasing hydrophobically driven adsorption of the cosolvent on the polymer chains. The recently proposed adsorption-attraction model based on the concept of preferential adsorption, can be used to predict the corresponding phase-transition behavior. In particularly the model predictions for variation of the grafting density is in agreement with the experimental findings. However, to reflect the imperfect mixing of the longer alcohols in water as well as finite miscibility of the polymers in the common solvent, extensions of the model have to be considered. I have shown that the simplest extension of the model taking into account the Flory-Huggins parameter for polymer and water can account for the qualitative changes observed for temperature changes in my experiments.
Both a theoretical analysis and experimental observations show that the phase-transition mechanism of cononsolvency depends on the relative strengths of various interactions in the polymer solutions. A cononsolvency transition can be driven by a strong cosolvent-solvent attraction or by the preferential adsorption of cosolvent onto the polymer. By an extension of the adsorption-attraction model, I report on a comprehensive and quantitative theoretical study of the cononsolvency effect of neutral polymers such as PNiPAAm brushes, macro-gels and single long chains. The extended adsorption-attraction model is able to describe and predict the phase-transition behaviors of these systems in various aqueous alcohol solutions quantitatively. My analysis showed that besides the dominant role of polymer-cosolvent preferential adsorption and the monomer-cosolvent-monomer triple contacts (cosolvent-assisted temporary cross-linking effect) that define the strength of the collapse-transition in the cosolvent-poor region, other effects are shown to be of relevance: The non-ideal mixing between polymer and solvent plays a role in shifting the collapse transition to the lower-concentration region of cosolvent, and an increase of the demixing tendency between cosolvent and solvent on the polymer chains reduces the window width of the cononsolvency transition. Using data from my own experiments and literature I can show that the cononsolvency response of brushes, gels and single long polymer chain can be consistently described with the same model. The model parameters are consistent with their microscopic interpretation. In addition, weakening of the cononsolvency transition in cosolvent-poor aqueous solutions at high hydrostatic pressure can be explained by the suppression of demixing tendency between cosolvent and water, and between polymer and water in the case of PNiPAAm.
An investigation of the grafting-density effect in the cononsolvency transition of grafted PNiPAAm polymer, showed that a decrease of grafting density at the collapse state as well as the temperature is fixed, the swollen polymer chains can show various morphologies not limited to collapse brush. In addition, my experimental results clearly showed that the strongest collapse state can be only realized by polymer brushes with moderate grafting densities. My results display the universal character of the cononsolvency effect with respect to series of cosolvents and show that PNiPAAm brushes display a well-defined and sharp collapse transition. This is most pronounced for 1-propanol as cosolvent which is still fully miscible in water. Potential applications are switches built from implementation of brushes in pores and similar concave geometries can be realized by harnessing the cononsolvency effect of stimuli-responsive polymers such as PNiPAAm.
As an example of application of cononsolvency effect of grafted polymers, different molecular-weight PNiPAAm polymers are grafted around the rim of solid-state nanopores by using grafting-to method. I demonstrate that small amounts of ethanol admixed to an aqueous solution can trigger the translocation of fluorescence DNA through polymer-decorated nanopores. I can identify the cononsolvency effect as being responsible for this observation which causes an abrupt collapse of the brush by increasing the alcohol content of the aqueous solution followed by a reswelling at higher alcohol concentration. For the first time, I provide a quantitative method to estimate hydrodynamic thickness of a polymer layer which is grafted around the rim of nanopores. Regardless of the grafting density of a grafted PNiPAAm polymer layer around the rim of nanopores, in the alcohol-tris buffer mixtures, the polymer layer displays solvent-composition responsive behaviors in the range of metabolic pH values and room temperatures. Although in this study PNiPAAm was chosen as a model synthetic polymer, I believe in that the conclusions made for PNiPAAm can be also in general extended to other synthetic polymers as well as to biopolymers such as proteins. As a proof of concept of using synthetic polymers to mimic biological functions of cell-membrane channels, my study clearly transpired that cononsolvency effect of polymers can be used as a trigger to change the size of nanopores in analogy to the opening and closure of the gates of cell-membrane channels.:Chapter 1 Background and motivation 4
1.1 Liquid-liquid phase separation 4
1.2 Polymer phase separation in a pure solvent 5
1.3 Polymer phase separation in mixtures of two good solvents 10
1.4 Characterizing cononsolvency transition in experimental study 14
1.5 Research motivation 16
Chapter 2 Phase behaviors of PNiPAAm brushes in alcohol/water mixtures: A combined experimental and theoretical study 17
2.1 Introduction 17
2.2 Materials and Methods 17
2.2.1 Materials 17
2.2.2 Preparation of Polymer Brushes 18
2.2.3 VIS-Spectroscopic Ellipsometry Measurement 18
2.2.4 Determining a polymer brush’s overlap grafting density 19
2.2.5 Test of PNiPAAm solubility in short-chain polyols 20
2.3 The adsorption-attraction model 20
2.4 Equilibrium behavior of cononsolvency transition of PNiPAAm brushes 22
2.5 Role of volume of solvent molecules in the swelling of PNiPAAm brushes 24
2.6 Cononsolvency transition of PNiPAAm brushes in aqueous solutions of a series of alcohol 24
2.7 Isomer effect of alcohol in the cononsolvency transition of PNiPAAm brushes 27
2.8 Role of alcohol-water interaction in the cononsolvency transition of PNiPAAm polymers 28
2.9 Temperature effect in the cononsolvency transition of PNiPAAm brushes 30
2.10 Grafting-density effect in the cononsolvency transition of PNiPAAm brushes 33
2.11 Octopus-shape-micelle morphology of grafted PNiPAAm polymers 34
2.12 Chapter summary 35
2.13 Chapter appendix 37
2.13.1 Data extraction and reprocessing for the molar Gibbs free energy of mixing 37
2.13.2 Temperature effect in the cononsolvency transition of PNiPAAm gels 37
Chapter 3 The extended adsorption-attraction model 41
3.1 Introduction 41
3.2 An extension of the adsorption-attraction model 43
3.3 Numerical solution of the extended adsorption-attraction model 47
3.4 Validation of the extended adsorption-attraction model 50
3.4.1 Cononsolvency transition of polymer brushes and macro-gels in different alcohol-water mixtures 51
3.4.2 An analysis of the enthalpic interaction between cosolvent and solvent 57
3.4.3 The window width of the cononsolvency transition 60
3.4.4 Pressure effect in the cononsolvency transition of PNiPAAm polymers 61
3.4.5 Cononsolvency transition of a single long polymer 65
3.5 Chapter summary 66
3.6 Chapter appendix 67
3.6.1 Chemical potential change of mixing two components 67
3.6.2 The Enthalpic Wilson model 68
3.6.3 Estimation of effective Flory-interaction parameter 73
3.6.4 Crosslink-density effect in the cononsolvency transition of poly(N-isopropylacrylamide) micro-gel and macro-gel 74
3.6.5 Pressure effect on the dimensionless chemical potential change (μ) 75
3.6.6 Pressure effect on the cosolvent-solvent interaction (χcs) 76
3.6.7 Pressure effect on the polymer-solvent interaction (χps) 77
3.6.8 Chemical potential change of DMSO/water mixtures 78
Chapter 4 Gating the translocation of DNA through poly(N-isopropylacrylamide) decorated nanopores using the cononsolvency effect in aqueous environments 80
4.1 Introduction 80
4.2 Methods 80
4.2.1 Preparation of polymer-grafted gold membrane 80
4.2.2 Translocation experiments of fluorescence λ-DNA through nanopores 82
4.2.3 Method of identification and counting of DNA translocation events 84
4.3 Results and discussion 86
4.3.1 Grafting density effect on the swollen behaviors of PNiPAAm polymers around the rim of nanopores 86
4.3.2 Switching effect of polymer chains around the rim of nanopores in the tri-buffer/ethanol mixtures 88
4.3.3 Switching effect of polymer brushes on the flat surface in the tri-buffer/ethanol mixtures 92
4.3.4 An attempt of numerical fit of experimental data using the extended adsorption-attraction model 94
4.4 Chapter summary 95
4.5 Chapter appendix 96
4.5.1 An estimation of grafting density 96
4.5.2 The method of processing data 97
Chapter 5 Concluding remarks and outlooks 100
5.1 Concluding remarks 100
5.2 Outlooks: A preliminary discussion of the cononsolvency transition of polymer solutions 102
References and notes 108
List of figures 119
List of tables 128
Acknowledgements 130
List of publications 131
Erklärung 132
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:74795 |
| Date | 05 May 2021 |
| Creators | Yong, Huaisong |
| Contributors | Fery, Andreas, Sommer, Jens-Uwe, Technische Universität Dresden, Leibniz-Institut für Polymerforschung Dresden e. V. |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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