| Stimuli-responsive polymers can be defined as polymer structures that offer large changes in their physicochemical behavior with small changes in the environment they live in. Due to their unique characteristics, responsive polymers have potential in a broad range of applications, including drug delivery, diagnostics, tissue engineering and "smart" optical system, as well as biosensors, micro-electromechanical systems, coatings and textiles. Researches on stimuli-responsive polymers are lots of yet mainly focused on temperature-and/or pH-responsive polymers. Compared to linear polymers, hyperbranched polymer has large numbers of terminal functional groups for design, low solution viscosity, and excellent solubility, more easily prepared through one pot method than dendrimers. Introducing stimuli-responsive properties into hyperbranched polymer will make it much more valuable for applications. Our thesis is mainly about studies on phase transition behavior of temperature and pH responsive polymers in aqueous solution. Most of our paper focuses on phase transition mechanism of modified hyperbranched polymer during heating-cooling process.The thesis is divided into five parts.Chapter I is the introduction of the whole thesis, where we briefly introduce the basic concepts and related researches involving in the paper. It contains three parts that are introduction of stimuli-responsive polymers,2D correlation spectroscopy and research goals and mentality of the graduate thesis. The first part is the introductions of linear and dendritic polymers with stimuli-responsive properties. For linear polymers, progress in researches of typical LCST polymers such as PNIPAM and PDMAEMA accounts for most of the contents. And hyperbranched polymers especially hyperbranched polyethylenimine is the main part of dendritic polymers. The second part is about the rise, principle, spectral reading and application of2DCOS and PCMW.Tn chapter II, we perform researches on thermodynamics of hyperbranched poly(ethylenimine) with isobutyramide residues during phase transition. Through in-situ tracing by FT-IR measurement, we obtain a set of FT-IR spectra during heating process. The second derivative spectra and Gaussian fitting are adopted to separate three hydrogen bonds in amide I region. The difference of absorption coefficient of the three components results in a decrease of the total area of the amide I region. A better revival of hydrogen bonds between amide groups than that of linear PNIPAM may due to high specific area of the hyperbranched polymer, as well as tertiary amide I groups. By determination of isosbestic points and analysis of2Dcos, we find that the formation of C=O...D-N hydrogen bonds trigger the phase transition upon heating; during cooling, the driven force of the revival is the hydration of alkyl groups. Combining the results of DLS measurements, it reveals that thermosensitive hyperbranched polymer dissolves in water as small particles of3nm at room temperature. Upon heating, the small particles begin to shrink and then aggregate with each other for a relative stable state above LCST. The cooling process is just the opposite.In chapter Ⅲ, we synthesize a thermosensitive hyperbranched polymer by Michael addition reaction between NIPAM monomers and HPEI.1HNMR and calorimetric measurements reveal that with reaction, the degree of NIPAM increases which decreases the LCST. The concentration plays an important role on adjusting LCST especially at the lower concentration region. The increase of concentration leads to a decrease of LCST and there is weak hysteresis during cooling. Further study by conventional FT-IR reveals that the CH groups hydrate with water and C=O...D2O exist in the polymer solution. Comparing the spectra of amide I region to that of HPEI-IBAm and linear PNIPAM, we find that neither numbers of bands nor bands’shape before and after phase transition is the same, which should result from different molecular structures. By PCMW and2Dcos analysis, we determine the temperature range (33-41.5℃) and sequence order of groups during phase transition and its revival. An molecular mechanism is drawn out:upon heating, hydrogen bonds between amide groups are formed to help the polymer globule to collapse and expel water molecules from the system; after cooling, with diffusing of water molecules, C=O...D-N hydrogen bonds begin to break which in turn promotes hydration of CH groups and leads extension of the whole polymer system.In chapter IV, we successfully synthesize fluorescein (FITC) labeled thermosensitive hyperbranched polymer (FITC-PEI-NTPAM). Although no obvious heat flow observed in DSC curves during heating and cooling, we can still determine the phase transition behavior by turbidity measurement and HNMR spectra at various temperatures. A slightly more serious hysteresis than HPEI-NIPAM solutions is observed in the labeled polymer. Through fluorescence spectroscopy of FITC-PEI-NIPAM at different concentrations, it shows that fluorescence may quench at high concentration. Tracing fluorescence spectroscopy in the heating process reveals that the changes of fluorescence spectra of various concentrations are different: for relative diluted solution, fluorescences quench because of the aggregations of FITC on the periphery of globule structure, named Step I; at a higher concentration, upon heating, FITC on the periphery of globule structure aggregate first resulting in fluorescence quenching. When temperature arrives at LCST, FITC of different molecules aggregate after conformation adjustment and results in fluorescence enhancement, named Step II; continuing to increase polymer concentration, we find that the polymers experience both Step I and Step II at low temperature, but aggregate and precipitate when we continue to heat. Step III appears that fluorescence intensity decreases first followed by its increase. The application of the polymer in the detection of metal ions reveals a success. The UV absorption at282nm assigned to FITC is much more sensitive upon the addition of Cu2+ions compared to653nm assigned to absorbance of HPEI/Cu2+complex. In addition, the fluorescence intensity decreases obviously with increase of Cu2+concentration.Chapter V is the summary of the thesis. The main object of our work is about the thermo-and pH-responsive polymers and what we concern is their phase transition behaviors in aqueous solutions. Utilizing2D correlation analysis to trace changes at molecular level helps us understand more about microdynamics during phase transition process. Based on the modification of hyperbranched polyethylenimine, we prepare three kinds of responsive polymers. With discrimination in their structures, we find differences related to their structures during the phase transition. It is conducive for understanding the relationship between structure and performance better through study on phase transition behavior of hyperbranched polymers and their comparison with linear polymers. |
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