Molecular dynamics, broadband dielectric spectroscopy

Additional information on ferroelectric and electroclinic switching can be obtained with broadband dielectric spectroscopy. It appears that the molecular dynamics of FLCPs are comparable to those of low molecular weight compounds [67]. However, the experimental observations are made more difficult for FLCPs than for low molecular weight SmC liquid crystals due to the conductivity contribution which takes place at frequencies below 10" Hz and to the difficulty to get a macroscopically well-aligned sample. 

Schonhals, A. 2002. Molecular dynamics in polymer model systems. In Broadband Dielectric Spectroscopy, ed. F. Kremer, A. Schonhals, p. 225. Berlin, Germany Springer. 

Broadband Dielectric Spectroscopy to Study the Molecular Dynamics of Polymers Having Different Molecular Architectures... 

Broadband dielectric spectroscopy (BDS) is a versatile experimental tool to study the dynamics of polymeric systems. In its modem form it covers the extraordinary frequency range from 10 Hz to 10 Hz with the option to extend both limits to lower and higher values, respectively. This enables one to analyse the molecular d3mamics on a large time scale especially if the temperature of the sample is varied as well. In the present review article examples will be discussed for polymers of widely varying molecular architectures (linear and cyclic chains, star-branched systems, and liquid crystalline polymers). 

Broadband dielectric spectroscopy enables one to analyse the dynamics of polar groups in polymeric systems. Due to its broad frequency range of more than 10 decades a manifold of different molecular fluctuations can be studied from the dynamic glass transition (spanning already more than 10 decades in times) to secondary relaxations. Additionally one finds in chiral liquid crystals cooperative processes like soft-and Goldstone modes. 

Abiad MG, CampaneUa OH, Carvajal MT (2010) Assessment of thermal transitions by dynamic mechanical analysis (DMA) using a novel disposable powder holder. Pharmaceutics 2 78-90 Adijanowicz K, WojnarowskaZ, WlodarczykP, Kaminski K, Paluch M, Mazgalski J (2009) Molecular mobUity in liquid and glassy states of Telmisartan (TEL) smdied by broadband dielectric spectroscopy. Eur J Pharm Sci 38 395-404... 

BordaUo HN, Zakharov BA, Boldyreva EV, Johnson MR, Koza MM, Seydel T, Fischer J (2012) Application of incoherent inelastic neutron scattering in pharmaceutical analysis relaxation dynamics in phenacetin. Mol Pharm 9 2434-2441 Bptker JP, Karmwar P, Strachan CJ, Cornett C, Tian F, Zujovic Z, Rantanen J, Rades T (2011) Assessment of crystalline disorder in cryo-milled samples of indomethacin using atomic pairwise distribution functions. Int J Pharm 417 112-119 Boutonnet-Fagegaltier N, Menegotto J, Lamure A, Duplaa H, Caron A, Lacabanne C, Bauer M (2002) Molecular mobihty study of amorphous and crystalline phases of a pharmaceutical product by thermally stimulated current spectrometry. J Pharm Sci 91 1548-1560 Bras AR, Noronha JP, Antunes AMM, Cardoso MM, Schdnhals A, Affouard Fdr, Dionfsio M, Correia NIT (2008) Molecular motions in amorphous ibuprofen as studied by broadband dielectric spectroscopy. J Phys Chem B 112 11087-11099... 

The information on molecular order and dynamics of LC polymers is usually obtained by a combination of methods which complement each other, such as Nuclear Magnetic Resonance (NMR) and broadband dielectric spectroscopy.Proton Nuclear Magnetic Resonance ( H-NMR) is especially suitable in this field, because after selective labeling the different fragments of the molecule (backbone, spacer, and mesogenic group) can be monitored separately. 

Kremer, E. (1997), Broadband dielectric spectroscopy on collective and molecular dynamics in ferroelectric liquid crystals, in Dielectric Spectroscopy of Polymeric Materials—Eundamental and Applications, Runt, J. P. and Eitzgerald, J. J., eds., ACS, Washington, DC, pp. 423 44. 

Modem broadband dielectric spectroscopy is a very useful tool to interrogate the molecular dynamics of polymers because response over a broad frequency range from the milli- to giga-Hertz region is possible (10). Therefore, motional processes which take place in polymers on extremely different time scales can be investigated vs. temperature. Moreover, the motional process depends on the morphology of the system. 

Muller, M., Eischer, E.W., Kremer, E. et al. (1995) The molecular dynamics of thermoreversible networks as studied by broadband dielectric spectroscopy. Colloid Polymer Science, 273,38-46. 

Natural rubber (NR) is a well studied elastomer. Of particular interest is the ability of NR to crystallize, specifically the strain-induced crystallization that takes place whilst the material is stretched. Moreover, in many elastomer applications, network chain dynamics under external stress/strain are critical for determining ultimate performance. Thus, a study on how the strain-induced crystallization affects the dynamics of a rubbery material is of outmost importance. Lee et al [1] reported their initial findings on the role of uniaxial extension on the relaxation behavior of cross-linked polyisoprene by means of dielectric spectroscopy. Nonetheless, to our best knowledge no in-depth study of the effects of strain induced crystallization on the molecular dynamics of NR has been undertaken, analyzing the relaxation spectra and correlating the molecular motion of chains with its structure. Broadband dielectric spectroscopy (BDS) has been chosen in order to study the dynamic features of segmental dynamics, because it is a comparatively simple technique for the analysis of the relaxation behaviour over a suitable frequency interval. This study is important from a basic and practical point of view, since an elongated crosslinked polymer at equilibrium may be considered as a new anisotropic material whose distribution of relaxation times could be affected by the orientation of the chains. 

Kessairi, K., Napolitano, S., Cappacioli, S., Rolla, P., and Wiibbenhorst, M. (2007) Molecular dynamics of atactic poly (propylene) investigated by broadband dielectric spectroscopy. Macromolecules, 40, 1786. 

Shenhar R, Norsten TB, Rotello VM (2005) Polymer-mediated nanoparticle assembly structural control and applications. Adv Mater 17(6) 657-669 Sinha G, Glorieux C, Thoen J (2004) Broadband dielectric spectroscopy study of molecular dynamics in the glass-forming liquid crystal isopentylcyanobiphenyl dispersed with aerosils. Phys Rev E 69 031707-031712... 

The organization of the present chapter is as follows. Dielectric techniques for molecular dynamics studies, in particular broadband dielectric spectroscopy (DS) and thermally stimulated depolarization currents (TSDC) techniques are shortly presented in the next section. Section 3, devoted to ionic conductivity measurements and analysis, focuses mostly on analysis, as the measuring techniques and equipment are often similar to those used for DS. The microphase separation and morphology of segmented PUs is discussed in the following Section 4, which completes the first introductory part of the chapter. Results obtained with selected PTE are presented in Section 5, followed by a larger Section 6 devoted to PU ionomers with ionic moieties in either of the HS and SS. PU ionomers of the latter type are often based on poly(ethylene oxide) (PEO) as the SS component and, for this reason. Section 6 includes a discussion of telechelics based on PEO, which may serve as model systems for PU ionomers. In Section 7, we discuss recent results obtained with nanocomposites based on PTE, a topic attracting much current interest, before we conclude with Section 8. 

Mijovic, J. Zhang, H. (2003). Local dynamics and molecular origin of polymer network-water interactions as studied by broadband dielectric relaxation spectroscopy, FTIR, and molecular simulations. Macromolecules, Vol.36, No.4, (February 2003), pp. 1279-1288, ISSN 0024-9297. 

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