Synthesis And Self-Assembly Behavior Of Thermo-Responsive Polymers

The graduate thesis mainly concentrates on synthesis and self-assembly behavior of thermo-responsive polymers, which can be mainly divided into four parts: application of two-dimensional correlation spectroscopy (2DCOS) in investigating the evolving mechanisms of several thermo-responsive and reaction chemical systems, such as thermotropic liquid crystalline polymers and water-soluble LCST (lower critical solution temperature)-type polymers; synthesis of highly branched LCST-type polymers and their self-assembly behavior; graphene-based nanocomposites; surface-enhanced Raman scattering (SERS).Thermo-responsive polymers are the mostly investigated environment-sensitive intelligent or smart polymers up to the present. Wherein, thermotropic liquid crystalline polymers would self-assembly into long-range ordered structures under temperature stimulus, while LCST-type polymers experience a reversible coil-to-globule phase transition upon heating and cooling cycles with the relevance to the denaturation of proteins.2DCOS is a new strategy for interpreting various physical-chemical phenomena at molecular level. By spreading peaks along a second dimension,2DCOS can sort out complex or overlapped spectral features and get an enhanced spectral resolution. Due to the different response of different, species to external variable, additional useful information about molecular motions or conformational changes can be extracted which cannot be obtained straight from conventional1D spectra. Graphene, as a unique2D nanocarbon material, has stimulated great interest due to its extraordinary electronic, thermal and mechanical properties with intensive promising applications in nanoelectronic devices, sensors, and nanocomposites. SERS is an ultra-sensitive method for the detection of analytes even at single-molecule level, and plays a more and more important role in the realm of analysis. Recently, graphene has also been discovered to possess SERS activity based on chemical enhancement mechanism owing to its relatively smooth surface, high optical transmission, as well as surface plasmon range in terahertz.The thesis is devided into eight chapters.Chapter Ⅰ is the introduction of our entire research work, where we give a brief introduction in six fields:the concept, history, research progress and proposed questions in self-assembly mechanism of liquid crystalline polymers, especially mesojen-jecketed liquid crystalline polymers; the concept, classification, application and research progress of LCST-type polymers, especially poly(N-isopropyl acrylamide)(PNIPAM) and poly(N-vinylcaprolactam)(PVCL); the rise, principle, spectral reading and application of2DCOS and PCMW (perturbation correlation moving window); the fabrication methods and research progress of graphene and graphene-based nanocomposites; the concept, mechanism and research progress of SERS; research goals and mentality of the graduate thesis.In chapter Ⅱ, we will discuss our work on the application of2D correlation spectroscopy in investigating the precise supramolecular self-assembly nature of poly[di(butyl) vinylterephthalate](PDBVT), a novel thermotropic liquid crystalline polymer which can self-assembly into two-dimensional hexagonal columnar (2DΦH) phase without conventional mesogens. PCMW has found the weak phase transition temperature to be about85℃, and further devides the "S or anti-S" shaped spectral variations into three evolving stages.2DCOS results indicate that carbonyl firstly responds to temperature increasing, and then the adjacent flexible side chains and bulky phenylene start to adjust their conformations with the backbone a slowest response. PDBVT backbones would experience "extension-distortion-slight extension" consecutive motions in the formation of2D ΦH phase. Additionally, we use a combination method of spectral analysis and molecular simulation to interpret the carbonyl band splitting phenomenon of PDBVT.2DCOS asynchronous spectra of PDBVT during heating show four splitting bands at1707,1712,1731and1741cm-1. Accordingly, four PDBVT conformers are classified on the basis of carbonyls rotating in π-electron resonance system. Detailed spectral comparison and molecular dynamics simulation for the columnar phase of PDBVT are carried out to make a clear assignment of splitting bands to different conformers. Then the internal self-assembly nature of PDBVT can be concluded that the rotation of carbonyls at2-position (close to backbone) of phenylenes would take place along with the formation of2DD ΦH phase.In chapter Ⅲ, we present our investation of the self-assembly mechanism of several LCST-type polymer systems, which can be devided into four parts.(1) IR spectroscopy in comination with2DCOS and PCMW is employed to study the precise chain collapse and revival thermodynamic mechanism of PNIPAM hydrogel. Both Boltzmann fitting and PCMW have figured out the volume phase transition temperature for heating and cooling processes to be about35℃and33.5℃respectively, close to the results obtained from DSC. Furthermore, determination of the isosbestic points for v(CH3) and v(C=O) overlaid spectra show that the chain collapse of PNIPAM hydrogel take place along with some intermediate states while the chain revival occurs with only conversion between two single states. Finally,2DCOS discerns all the sequence of group motions of PNIPAM hydrogel, indicating that in the heating process, PNIPAM hydrogel occurs to collapse along the backbone before water molecules are expelled outside the network, while in the sequential cooling process, PNIPAM hydrogel has water molecules diffusing into the network first before the chain revival along the backbone occurs.(2) The role of water/methanol clustering dynamics on thermosensitivity of PNIPAM chains in concentrated solutions (10wt%) is investigated by turbidity, FT-IR and calorimetric measurements through point-by-point comparison. FT-IR spectral variations show that PNIPAM-methanol interactions are largely weakened and PNIPAM chains are more collapsed in water/methanol mixture (methanol volume fraction xm=0.17) than in pure water because of the formation of large water/methanol clusters, which, meanwhile, causes the decrease of hydration sites. On the other hand, weak hysteresis and excess recovery phenomena in the phase transition process can also be observed with the addition of methanol to PNIPAM aqueous solution due to the existence of water/methanol clustering dynamics.2DCOS and calorimetric analysis finally conclude that the role of water/methanol clustering dynamics is mainly embodied in the inhibition of the hydration process of PNIPAM chains which shows a faster thermal response than hydrogen bonding association.(3) IR spectroscopy in combination with2DCOS and PCMW is employed to illustrate the dynamic hydration behavior of PVCL in water, which exhibits a typical type I continuous LCST behavior. PCMW easily determines the transition temperature to be about43.5℃during heating and about42.5℃during cooling and the transition temperature range to be39.5-45℃. On the other hand,2DCOS is used to discern the sequence order of different species in PVCL and concludes that hydrogen bonding transformation predominates at the first stage below LCST while hydrophobic interaction predominates at the second stage above LCST. In combination with molecular dynamics simulation results, we find that there exists a distribution gradient of water molecules in PVCL mesoglobules ranging from a hydrophobic core to a hydrophilic surface. Due to the absence of self-associated hydrogen bonds and topological constraints, PVCL mesoglobules would form a "sponge-like’" structure which can further continuously expel water molecules upon increasing temperature; while PNIPAM with self-associated hydrogen bonds forms mesoglobules with a "cotton ball-like" structure without an apparent distribution gradient of water molecules and does not change much upon increasing temperature.(4) IR spectroscopy in combination with2DCOS and PCMW is employed to elucidate the dynamic self-aggregation behavior of a novel miktoarm star PNIPAM-based multihydrophilic block copolymer,(PNIPAM)2-(PVP-b-PAA)2. At pH=8,(PNIPAM)2-(PVP-b-PAA)2tends to self-assemble into micelles with PNIPAM in the core and ionized PAA segments in the shell during heating. IR investigation shows that PAA segments exhibits a similar "phase transition" behavior to PNIPAM segments, which can be ascribed to the indirect influence through the drastic content changes of water molecules along with the hydrophilic-to-hydrophobic transformation of PNIPAM segments. Boltzmann fitting and PCMW easily determine the transition temperature to be about33℃and the transition temperature range to be29.5-35℃. Moreover,2DCOS discerns a sequential group motion from PNIPAM to PVP and PAA segments. It is concluded that the three polymeric segments have relatively independent phase behavior during the formation of PNIPAM-core micelles, and the chain conformation adjustment induced by hydrophilic-to-hydrophobic transformation of PNIPAM segments should be driving force of the whole self-aggregation process. The dynamic self-aggregation process we proposed can be further confirmed by dynamic laser scattering (DLS) and zeta potential measurements.In chapter IV, we additionally employ2D correlation spectroscopy to study the microdyanmic and reaction mechanism of three small molecule systems.(1) Near IR (NIR) spectroscopy in combination with2DCOS and PCMW technique is employed to illustrate the gelation microdynamic mechanism of a hydrogelator N-octyl-D-Gluconamide (8-GA), which can rapidly self-agglomerate into helical bilayer micellar fibers upon cooling from spherical micelles. Boltzmann fitting and PCMW easily determine the gelation temperature to be about72℃and the transition temperature range to be70-75℃. Moreover, band shifting and splitting phenomena can be observed for CH-related overtones, indicating the formation of much ordered and tight hydrophobic core from octyl tails. On the other hand,2DCOS is used to discern the sequential orders during the gelation process and concludes that all the group motions have a continuous transfer from the octyl tail to the chiral carbohydrate head followed by the final immobilization of the solvent, which meanwhile, is actually a continuous dehydration process from the hydrophobic core to the outer hydrophilic chiral head. The driving force of the gelation process in microdynamics can only be the dehydration process of hydrophobic octyl chains, but with final helical superstructures being stabilized by amide-associated hydrogen bonding and the "chiral bilayer effect" of carbohydrate heads.(2) NIR spectroscopy in combination with2DCOS is employed to’study the dynamic self-disassociation behavior of hydrogen bond in neat liquid pyrrole in the heating process. Pyrrole can form various associates of different sizes via T-shaped hydrogen bonds, where π-electronic cloud serve as hydrogen bonding proton acceptor, and N-H as proton donor.2DCOS analysis concludes that less stable pyrrole dimers have the first response to the increasing temperature, and transform to monomers via an antiparallel intermediate geometry, then more stable cyclic polymers start to disaggregate, along with the content increasing of pyrrole monomers.(3) A designed ligand-accelerated Cu(Ⅰ)-catalyzed cycloaddition (CuAAC) reaction is monitored for the first time by real time infrared analysis technique based on ATR-FTIR principles. PCA (principal components analysis) and2DCOS results show that the consumption of alkyne and azide take place successively followed by the formation of the product1,2,3-triazole and a1:1complex of two reactants would be formed in the reaction process. The rate determining step of CuAAC reaction is also experimentally confirmed for the first time to be the transition of azide-alkyne1:1complex to the pre-product1,2,3-triazole. Our results are in good conformity with current catalytic mechanism proposed by K. B. Sharpless et al according to DFT calculation results.Chapter V mainly focuses on the synthesis of highly branched PNIPAM and its self-assembly behavior. Highly branched PNIPAM is first synthesized by self-condensing atom transfer radical copolymerization (SCATRCP) catalyzed by CuBr/tris[2-(dimethylamino)ethyl]amine (Me6TREN) complex in isopropanol at room temperature. This method shows good control over chain morphology and LCST type phase transition behavior of PNIPAM by varying reaction time and monomer/inimer feed ratio. On the basis of GPC, FTIR,1HNMR and DSC results, four stages are discerned during the polymerization procedure, at which chain architectures experience "linear-comblike-slightly branched-highly branched" transformations. Additionally, the synthesized highly branched PNIPAM chains are found to be able to self-assemble into vesicles in alcohols such as methanol and ethanol, which may be arising from the presence of large amounts of hydrophobic branched points inside polymer chains.Chapter VI concentrates on our research work on graphene-based polymer nanocomposites. It includes two aspects as follows.(1) We develop a facile methodology to immobilize well-defined polystyrene (PS) onto graphene sheets using "click" chemistry. The reaction is successful with high grafting yield of20wt%, evidenced by FTIR. Raman. XRD. TGA and AFM characterizations. Upon PS coupling, the resulting sheets can be well dispersed and fully exfoliated in DMF, THF, CH2CI2and toluene, which are all good solvents to PS.(2) We develop a facile one-step strategy for graphene oxide/PNIPAM interpenetrating polymer hydrogel networks (GO/PNIPAM IPN hydrogel) by covalently bonding GO sheets and PNIPAM-co-AA microgels directly in water, which exhibit dual thermal and pH response with good reversibility. The cross-linking reaction is highly efficient, resulting in a hydrogel network with better mechanical strength and a two-level structural hierachy. GO sheets are proved to be uniformly dispersed in PNIPAM hydrogel network with a disordered alignment. Due to the restoring froces provided by the network elasticity, the IPN hydrogel can rapidly exhibit volume phase transition upon temperature and pH changes. Our synthesized GO/PNIPAM IPN hydrogel may gain potential biological applications as carriers for controlled drug delivery.In chapter VII, we mainly investigate the competitive SERS effect and its internal mechanism in the application of noble metal nanoparticle (Au or Ag NP) decorated graphene sheets in enhancing the Raman signals of probe molecules in aqueous solution. Au/Ag NP decorated graphene sheets are fabricated according to a facile one-pot environmentally friendly method. Both the electromagnetic mechanism and chemical mechansim effects are coexisting among Au or Ag NPs, graphene sheets and absorbed analytes. Our results show that in aqueous solution the SERS effect of both Au and Ag NPs on absorbed probe molecules and on graphene is competitive, which varies dependent on the species and concentration of the absorbed probe molecules. By detailed comparison among three probe molecules (rhodamine6G, nile blue A, and4-aminobenzenethiol) with different coupling ability to graphene sheets, we finally attribute this phenomenon to be the result of the strong suppressing effect of macrocyclic probe molecules on the SERS of graphene induced by charge transfer as the probe molecules are coupled to graphene sheets.Chapter VIII is the summary and prospect of our research work. Our research mainly focuses on thermo-responsive polymers, including the investigation of responsive mechanism of different thermo-sensitive systems by2DCOS (as well as molecular dynamics simulation), preparation of highly branched PNIPAM through new methods, and introducing temperature and pH sensitivity into graphene sheets to fabricate GO/PNIPAM IPN hydrogel. We additionally discuss our results of other related research projects, e.g. application of2DCOS in gelation, hydrogen bond and reaction systems, preparation of graphene-based polymer nanocoposites and SERS. We do our researches referring both to structure-property relationship and to new materials synthesis and design, which may be helpful for other researchers who want to deepen their understanding of the self-assembly mechanism of thermo-responsive materials and guide rational design of experiments.