Conformational dynamics of polymer

A further application of time-resolved fluorescence measurements is in the study of conformational dynamics of polymer chains in solution. Fluorescence anisotropy measurements of macromolecules incorporating suitable fluorescent probes can give details of chain mobility and polymer conformation (2,14). A particular example studied in this laboratory is the conformational changes which occur in aqueous solutions of polyelectrolytes as the solution pH is varied (15,16). Poly(methacrylic acid) (PMA) is known to exist in a compact hypercoiled conformation at low pH but undergoes a transition to a more extended conformation at a degree of neutralization (a) of 0.2 to 0.3 (1 6). Similar conformational transitions are known to occur in biopolymer systems and consequently there is considerable interest in understanding the nature of the structures present in model synthetic polyelectrolyte solutions. 

This review has illustrated the important insights into the conformational dynamics of polymer chains which have been obtained from stochastic simulations. Stochastic simulations occupy an interesting position relative to com-... 

Polymers are large molecules and the degree of polymerisation, i.e. the number of monomers incorporated in the chain, wUl typically range from 100 to greater than 1000. The number of possible conformations for such a molecule would be around 2.0 X 10, a very large number indeed. Yet, despite these complications, we can stiU use this analysis to obtain reasonable estimates of the potential energy sur-frces and thence predictions of the conformational dynamics of polymer chains. 

Chong, S.-H. and Fuchs, M. (2002) Modecoupling theory for structural and conformational dynamics of polymer melts. Phys. Rev. Lett., 88, 185702. 

Another largely unexplored area is the change of dynamics due to the influence of the surface. The dynamic behavior of a latex suspension as a model system for Brownian particles is determined by photon correlation spectroscopy in evanescent wave geometry [130] and reported to differ strongly from the bulk. Little information is available on surface motion and relaxation phenomena of polymers [10, 131]. The softening at the surface of polymer thin films is measured by a mechanical nano-indentation technique [132], where the applied force and the path during the penetration of a thin needle into the surface is carefully determined. Thus the structure, conformation and dynamics of polymer molecules at the free surface is still very much unexplored and only few specific examples have been reported in the literature. 

For the investigation of polymer systems under spatial confinement, fluorescence microscopy is a powerful method providing valuable information with high sensitivity. A fluorescence microscopy technique with nanometric spatial resolution and nanosecond temporal resolution has been developed, and was used to study the structure and dynamics of polymer chains under spatial confinement a polymer chain in an ultra-thin film and a chain grafted on a solid substrate. Studies on the conformation of the single polymer chain in a thin film and the local segmental motion of the graft polymer chain are described herein. 

In spite of the highly simplified structure, this model retains essential characteristics for studying stable structural conformations and the concerted dynamics of polymer side chains, water, and protons at polymeric aggregates in PEMs. The approach implies that the effect of polymer dynamics on processes inside pores is primarily due to variations in chemical architecture, packing density, and vibrational flexibility of SGs. 

Nuclear magnetic resonance (NMR) spectroscopy is a most effective and significant method for observing the structure and dynamics of polymer chains both in solution and in the solid state [1]. Undoubtedly the widest application of NMR spectroscopy is in the field of structure determination. The identification of certain atoms or groups in a molecule as well as their position relative to each other can be obtained by one-, two-, and three-dimensional NMR. Of importance to polymerization of vinyl monomers is the orientation of each vinyl monomer unit to the growing chain tacticity. The time scale involved in NMR measurements makes it possible to study certain rate processes, including chemical reaction rates. Other applications are isomerism, internal relaxation, conformational analysis, and tautomerism. 

The dynamics of polymers may be interpreted by assuming the existence of domains. At high temperature (T Tg), each domain is occupied by a single conformer and the interactions between neighboring segments or conformers are considered to be negligible. Once the potential barrier is crossed, conformers relax independently of their neighbors. Each domain... 

Time-resolved fluorescence spectroscopy and fluorescence anisotropy measurements have been applied to study (i) excimer formation and energy transfer in solutions of poly(acenaphthalene) (PACE) and poly(2-naphthyl methacrylate) (P2NMA) and (ii) the conformational dynamics of poly(methacrylic acid) (PMA) and poly (acrylic acid) as a function of solution pH. For PACE and P2NMA, analysis of projections in which the spectral, temporal and intensity information are simultaneously displayed have been used to re-examine kinetic models proposed to account for the complex fluorescence decay behaviour that is observed. Time-resolved fluorescence anisotropy measuranents of fluorescent probes incorporated in PMA have led to the proposal of a "connected cluster" model for the hypercoiled conformation of this polymer existing at low pH. 

Segmental motions of P(3HB) and P(3HB-co-27%3HV) in chloroform-d solution have been studied by measuring NMR relaxation times and NOE factors as a function of temperature [72, 73]. Analysis of the relaxation data on the basis of the Dejean-Laupretre-Monnerie (DLM) model, which describes the dynamics of polymer chains [74], indicates that the local dynamics of a comonomer unit, e.g., 3HB, are independent of the presence of a nearby 3HV unit and vice versa that segmental motion of the P(3HB-co-27%3HV) copolymer described by cooperative conformational transitions [73] is similar to that for the P(3HB) homopolymer [72]. These motional characteristics of the P(3HB-co-3HV) copolymer chain are consistent with the conformational characteristics derived by the analysis of spin coupling as shown in Section 21.2.2.3 [63] and are consistent with the occurence of cocrystallization in this copolymer system. 

The conformational dynamics of PDMS cyclics of various sizes have been studied by ultrasound. The magnitudes of the dispersions obtained were used to estimate energy differences between stable and less-stable conformations. Analogous information has been obtained using excimer emission from small probes placed into a cyclic PDMS. Chemical shifts and relaxation times in the Si NMR spectra of cyclic PDMS have also been used for this purpose. There has been work with regard to the structure and dynamics of cyclics in general, including percolation of linear polymers in melts of cyclic polymers. ... 

Orrah, D. J. Semiyen, J. A. Dodgson, K. Pethrick, R. A., Studies of Cyclic and Linear Poly(dimethylsiloxanes) 29. An Investigation of the Conformational Dynamics of Cyclic Poly(dimethylsiloxane). Polymer 1990,29, 25-29. 

The study of photofragmentation and of conformational dynamics of complex polyatomic molecules, such as proteins and polymers, in the gas phase. 

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