Study Of Internal Relaxation And Average Conformation Of Flexible Comb-Like Polymers In Dilute Solutions

The main focus in this dissertation is the study of internal dynamics of flexible comb-like chains in dilute solutions under good and theta(?)solvent conditions.The dynamic and static light scattering(DLS and SLS),as well as triple-detection size exclusion chromatography(TD-SEC),are the main techniques used in this thesis.Organization of this dissertation is as follow:Firstly,internal relaxation behavior of linear contour-based comb-like polystyrene(PS)chains in good solvent is discussed;secondly,we move to the discussion of internal relaxation behavior of comb-like PS chains in its ideal state i.e.,near ? state;lastly,conformational behavior of comb-like poly-2-ethyl-2-oxazoline(PEtOx)system is discussed.Specifically,the main contents of the three sections are as follows:1)In polymer physics,one of the most important and fundamental problem is the internal relaxation of polymer chains.In the first part of the thesis,internal relaxation behavior of linear contour-based comb-like PS samples in good solvent is discussed.Briefly,by using DLS,I have studied the internal dynamics of well-characterized(Mw/Mn-1.30 and Mw?107 g/mol)where Mw and Mn is the weight average and number average molar mass respectively,comb-like system in dilute solutions.The DLS results disclose that i)at very high q-regime the value of reduced first cumulant(?*,?*=[?0T(q)/kBTq3])is not sensitive to structural details(local rigidity and sidechain length);ii)the asymptotic power law(Dapp/D0)is also found to be a better indicator in the regime of 3.0<qRg<5.5,which could reflect the structural details for comb-like polymers similar to hyperbranched polymers,where Rg is the radius of gyration;iii)the intrachain relaxation of comb-like chains is satisfactorily describe by the classical theory of linear chains based on the comparison of experimentally calculated and theoretically computed values of the longest relaxation time(?1);iv)the grafting-induced backbone rigidity results in a greater separation of translational diffusion and internal motions peaks through the q-regime,on the other hand,significant suppression of the strength of overall internal motions is observed as compared to linear chain.Clearly,these results speculate that for a given system the modes of internal motions mainly determined by the relaxation of the main contour of a chain structure,rather than the local chain rigidity,my experimental data can be useful for further theoretical modeling of internal relaxation behavior of various topological polymers.2)In the second part of the thesis,internal relaxation behavior of comb-like PS in its ideal state i.e.,near theta point is considered in dilute solutions under ? solvent conditions.Precisely,the DLS study reveals that ?)for theta solvent the value of ?*at very high q-regime did not approaches to a plateau state but found a steadily decreasing trend;?)as observed previously,in intermediate regime 2.0<qRg<4.0,the asymptotic power law is still found a better indicator to observed the structural details with decreasing solvent quality for comb-like polymers;?)as move towards theta state from good solvent side the comb-like chains becomes much softer,in results,internal motions contribute much near theta point,but a significant suppression of overall internal motions is observed as compared to comb-like chains in good solvent.The current work also strengthens our previous observation that the main contour of a given chain structure defines the modes of internal motions,my experimental data in ?-regime provided more useful data for further theoretical modeling in ideal state of comb-like chains.3)In the third part of the thesis,we move to more flexible side-chain PEtOx to investigate the influence of grafting density on the comb-like chains conformation PEtOx.PEtOx in aqueous solution and PS in organic solution are two different systems to systematically count the possible factor affected the internal motions.The hydrogen bonding between PEtOx and water molecules may change their internal relaxation behavior.I have synthesized comb-like PEtOx by using azide-alkyne click chemistry,where grafting density varied systematically from 0 to 0.68.A low molar mass linear PEtOx(4K)with azide terminal was synthesized by the living cationic ring opening polymerization(LCROP)using methyl-tosylate as an initiator.Comb-like PEtOx was prepared by grafting of PEtOx-N3 on a poly(propargyl acrylate)backbone having alkyne moiety on each repeating unit and the unreacted alkynes were protected by adding excess of benzyl azide to avoid cross-linking/gel formation.TD-SEC was used to extract the different molecular parameters;weight-average molar mass,the radius of gyration,hydrodynamic radius(Rh)and intrinsic viscosity([?])respectively.Static combine with the multi-angle laser light scattering(MALLS)discloses that the whole radius of the comb-like PEtOx chain increases in a systematic manner with grafting density.When the contour length(Lc)is unchanged,it is obvious that the grafting density(?)leads to an enhancement in the persistence length(Lp)of the semi-flexible system.These results give the first experimental verification of the excluded volume theory for comb-like PEtOx at a whole grafting density regime for a soft and water-soluble chain,and validate a promising synthetic stage for steadily increasing the persistence length of a model flexible,water-soluble polymer.The synthesis and characterization pave the way for further investigating the mechanism behind Glasser-hay coupling,correlation between low grafting and longer sidechain length,use of reducing agent and importantly the internal dynamics of ultra-long comb-like PEtOx in aqueous solution in future.The above three parts of work has greatly deepened our understanding of chain internal dynamics and conformation of linear contour-based comb-like branched polymer.The characteristic relaxation time ?1 of comb-like chains in good solvent has been found to be satisfactorily described by the classic theory,and our result clarify that the main contour of a system describes the mode of internal motions,not the local chain rigidity.Simply,grafting will not always affect the relaxation modes of internal dynamics,whereas,the persistence length(backbone stiffness)of the main contour is also a dominant factor determining the number of relaxation modes for a given polymer system.Overall,based on the accomplished work in this thesis,it is safe to conclude that q-dependent relative strength of internal relaxation can be summarized as follows:linear chains(good solvent)>comb-like chains(good solvent)>comb-like chains(theta solvent)>hyperbranched chains(good solvent)>hyperbranched chains(theta solvent)?...