Molecular dynamics in fluorinated side-chain maleimide copolymers as studied by broadband dielectric spectroscopy (original) (raw)
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The complex dynamics of a nearly alternating copolymer of vinylidene cyanide (1,1-dicyanoethylene, VCN) with 2,2,2-trifluoroethyl methacrylate (TFEMA), including two α-relaxations with diverging time scale in the glass transition temperature range, was thoroughly characterized by dielectric spectroscopy over wide temperature and frequency ranges and analyzed in the frame of the Ngai's coupling model. The dielectric relaxation strength as well as the glass transition temperature, the temperature dependence of the α-relaxation time, and the corresponding distribution of relaxation times were all larger than those of a reference TFEMA homopol-ymer, as expected from the introduction of the stiffening VCN units all along the macromolecular chain. The effect of casting solvent and applied poling electric field on the copolymer dielectric strength suggests the onset of local orientational order involving the strong dipoles in the VCN units, a requirement for piezo-and pyroelectricity in amorphous polymers.
The European Physical Journal E, 2011
The effect of the structure of copolymers (random, alternate or diblock) on their dynamics has been studied by dielectric spectroscopy. Six copolymers of styrene and methyl methacrylate (three diblocks, one alternate and two random) have been studied. The results show that the sub-T g transitions of the diblock samples can be described by one asymmetric Havriliak-Negami (HN) function, while two are necessary for the rest of the copolymers ( and relaxations). The characteristic times of the sub-T g relaxations show an Arrhenius temperature dependence and there is a strong coupling of the and relaxations at high temperatures. The deconvolution of the merging relaxations has been made in the framework of the Williams Ansatz set out in terms of Havriliak-Negami distributions. Because the 2D 2 H-NMR results excluded any significant contribution from the rotation of the methoxy group of the methacrylate group around the C-OCH 3 bond, the relaxation may be assigned to the rotation of the methyl methacrylate group in a styrene-rich environment. The Molecular Dynamics simulations of a poly(methyl methacrylate) homopolymer and of the alternate copolymer are in qualitative agreement with the experimental results, although they predict smaller values for the activation energy of the sub-T g relaxations.
European Polymer Journal, 2011
Aiming to develop new dielectric polymers containing CN and F groups with strong dipole moments, a novel copolymer of acrylonitrile (AN) and 2,2,2-trifluoroethyl acrylate (ATRIF) was synthesized in acetonitrile by free radical process as well as the respective homopolymer (poly(ATRIF)). The copolymer's composition and microstructure were analyzed by FTIR, 1 H and 13 C NMR spectroscopy and SEC. The molar incorporation of AN determined in the copolymer by NMR was 58 mol%. Thermogravimetric analysis of poly(AN-co-ATRIF) copolymer showed good thermal stability comparatively to the fluorinated homopolymer.
Macromolecules, 2004
Broadband dielectric spectroscopy has been used to analyze the molecular dynamics in a set of nine poly(ethene-alt-N-alkylmaleimide)s. The polymers were studied in the frequency range from 0.1 Hz to 10 MHz and at temperatures between 120 and 500 K. The alternating maleimide copolymers possess alkyl side chains varying in length from methyl to octadecyl. Four relaxation processes are observed: (i) a (secondary) -relaxation corresponding to librational fluctuations of the terminal end group of the alkyl side chain, (ii) the R′-relaxation being assigned to a relaxation of the side chain, (iii) the dynamic glass transition (R-relaxation), being designated to motions of the succinimide ring, and (iv) the R s-relaxation reflecting cooperative fluctuations of an ensemble of about 2-3 maleimide rings in an underlying helical superstructure. This model is supported by calorimetric measurements and published nuclear magnetic resonance data. † University of Leipzig.
Dipolar and Ionic Relaxations of Polymers Containing Polar Conformationally Versatile Side Chains
Macromolecules, 2010
This work reports a comparative study of the response of poly(2,3-dimethoxybenzyl methacrylate), poly(2,5-dimethoxybenzyl methacrylate), and poly(3,4-dimethoxybenzyl methacrylate) to electrical perturbation fields over wide frequency and temperature windows with the aim of investigating the influence of the location of the dimethoxy substituents in the phenyl moieties on the relaxation behavior of the polymers. The dielectric loss isotherms above T g exhibit a blurred relaxation resulting from the overlapping of secondary relaxations with the glass-rubber or R relaxation. At high temperatures and low frequencies, the R relaxation is hidden by the ionic conductive contribution to the dielectric loss. As usual, the real component of the complex dielectric permittivity in the frequency domain increases with decreasing frequency until a plateau is reached corresponding to the glass-rubber (R) relaxation. However, at high temperatures, the real permittivity starts to increase again with decreasing frequency until a second plateau is reached, a process that presumably reflects a distributed Maxwell-Wagner-Sillars relaxation or R 0 absorption. The R and R 0 processes appear respectively as asymmetric and symmetric relaxations in the loss electrical modulus isotherms in the frequency domain. To facilitate the deconvolution of the overlapping absorptions, the time retardation spectra of the polymers were computed from the complex dielectric permittivity in the frequency domain using linear programming regularization parameter techniques. The spectra exhibit three secondary absorptions named, in increasing order of time γ 0 , γ, and β followed by the R relaxation. At long times and well separated from the R absorption the R 0 relaxation appears. The replacement of the hydrogen of the phenyl group in position 2 by the oxymethyl moiety enhances the dielectric activity of the poly-(dimethoxybenzyl methacrylate)s. The temperature dependence of the relaxation times associated with the different relaxations is studied, and the molecular origin of the secondary relaxations is qualitatively discussed.
Polymer, 2004
The changes in the phase structures and molecular mobility caused by the ferroelectric -paraelectric phase transition of vinylidene fluoride (VDF) and trifluoroethylene (TrFE) copolymer, P(VDF 75 /TrFE 25 ), were analyzed using variable temperature (VT) solid-state 19 F MAS and 1 H ! 19 F CP/MAS NMR spectroscopy. The CF 2 signal of the VDF chain sequence and the CHF signal at the head-to-head linkage of VDF-TrFE sequence showed higher frequency shift in the temperature range 43 -92 8C, whereas no change was found for the CHF signal at the head-to-tail linkage of VDF -TrFE up to 92 8C. Hence, VT 19 F MAS spectra revealed that the VDF -TrFE head-to-tail sequence is the most stable part in polymer chains against trans-gauche conformational exchange motions below the phase transition temperature (Curie temperature, T c ) on heating. However, all chain sequences including TrFE units undergo conformational exchange at around T c : The phase transition behavior is clearly recognized in the 19 F spectral shapes, in which the broad signals of the ferroelectric immobile phase disappeared between 115 and 119 8C. In addition, T F 1r for all peaks decreased to a unique value (ca. 20 ms) at 119 8C, indicating that uniform molecular motion accompanied by a full chain rotation occurred at the temperature. The significantly longer T F 1r for all peaks (ca. 20 ms) in the paraelectric phase (119 8C) than that in the amorphous domain (,4 ms) at ambient temperature supports the conclusion that there is restricted rotational motion of polymer chains around the chain axis in the paraelectric phase. On cooling from 119 to 85 8C, a gradual decrease in gauche conformers in the paraelectric phase was confirmed by the low-frequency displacement of CF 2 signals in VDF sequences accompanied by slight decreases in T F 1 and T F 1r : The phase transition was observed between 85 and 77 8C on cooling, in which the characteristic signals of the paraelectric phase disappeared, the T F 1r values of all peaks quickly increased, and the broad crystalline signals abruptly appeared at 77 8C. q
The European Physical Journal E, 2012
The aim of this study is to analyze the mobility of polymer chains in semicrystalline poly(vinylidene fluoride) (PVDF). PVDF crystallizes from the melt in the α crystalline phase. The transformation from the α phase to the electroactive β phase can be induced by stretching at temperatures in the range between 80 and 140 • C. The spherulitic structure of the crystalline phase is deformed during stretching to form fibrils oriented in the direction of the strain. The amorphous phase confined among the crystalline lamellae is distorted as well and some degree of orientation of the polymer chains is expected. Dynamicmechanical and dielectric spectroscopy measurements were performed in PVDF films stretched to strain ratios up to 5 at temperatures between 80 and 140 • C. Dynamic-mechanical measurements were conducted between −60 • C and melting and in this temperature range the relaxation spectra show the main relaxation of the amorphous phase (called β-relaxation) and at higher temperatures a relaxation related to crystallites motions (αc-relaxation). Although the mean relaxation times of the β-relaxation are nearly equal in PVDF before and after crystal phase transformation, a significant change of shape of the relaxation spectrum proves the effect of chain distortion due to crystal reorganization. In stretched PVDF the elastic modulus of the polymer in the direction of deformation is significantly higher than in the transversal one, as expected by chain and crystals fibril orientation. The recovery of the deformation when the sample is heated is related with the appearance of the α c-relaxation. Dielectric spectroscopy spectrum shows the main relaxation of the amorphous phase and a secondary process (γ-relaxation) at lower temperatures. Stretching produces significant changes in the relaxation processes, mainly in the strength and shape of the main relaxation β. The Havriliak-Negami function has been applied to analyze the dielectric response.