Comparative study of the mechanical relaxation behavior of polymethacrylates containing different bulky side groups

The syntheses of poly(1,3‐dioxan‐5‐yl methacrylate), poly(cis‐2‐phenyl‐1,3‐dioxan‐5‐yl methacrylate), poly(trans‐2‐phenyl‐1,3‐dioxan‐5‐yl methacrylate), poly(cis‐2‐cyclohexyl‐1,3‐dioxan‐5‐yl methacrylate), and poly(trans‐2‐cyclohexyl‐1,3‐dioxan‐5‐yl methacrylate) are reported. The mechanical relaxation spectrum of the simplest polymer, poly(1,3‐dioxan‐5‐yl methacrylate), exhibits a prominent β relaxation centered at ?98 °C, at 1 Hz, followed in increasing order of temperature by an ostensible glass–rubber relaxation process. In addition to the β relaxation, the loss curves of poly(trans‐2‐phenyl‐1,3‐dioxan‐5‐yl methacrylate) and poly(trans‐2‐cyclohexyl‐1,3‐dioxan‐5‐yl methacrylate) display in the glassy state a high activation energy relaxation, named the β* process, that seems to be a precursor of the glass–rubber relaxation of these polymers. The mechanical spectra of poly(trans‐2‐cyclohexyl‐1,3‐dioxan‐5‐yl methacrylate) and poly(cis‐2‐cyclohexyl‐1,3‐dioxan‐5‐yl methacrylate) exhibit a low activation energy process in the low‐temperature side of the spectra, which is absent in the other polymers. The molecular origin of the mechanical activity of these polymers in the glassy state is discussed in qualitative terms. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1154–1162, 2002     

Dynamic mechanical and dielectric relaxations of poly(difluorobenzyl methacrylates)

This work reports the mechanical and dielectric relaxation spectra of three difluorinated phenyl isomers of poly(benzyl methacrylate), specifically, poly(2,4‐difluorobenzyl methacrylate), poly(2,5‐difluorobenzyl methacrylate) and poly(2,6‐difluorobenzyl methacrylate). The strength of the dielectric glass–rubber relaxation of the 2,6 difluorinated phenyl isomer is, respectively, nearly three and two times larger than the strengths of the 2,5 and 2,4 isomers. The 2,4 isomer presents a mechanical α peak the intensity of which is nearly two times that of the other two isomers. Both the mechanical and dielectric relaxation spectra display a subglass process, called γ relaxation, centered in the vicinity of −50 °C at 1 Hz and, in some cases, a subglass β absorption is detected at higher temperature partially masked by the glass–rubber relaxation. The mean‐square dipole moments per repeating unit, 〈μ2〉/x, measured at 25 °C in benzene solutions, are 2.5 D², 1.9 D², and 5.0 D² for poly(2,4‐difluorobenzyl methacrylate), poly(2,5‐difluorobenzyl methacrylate) and poly(2,6‐difluorobenzyl methacrylate), respectively. These results, in conjunction with Onsager type equations, permit to conclude that auto and cross‐correlation contributions to the dipolar correlation coefficient may have the same time‐dependence. On the other hand, dipole intermolecular interactions, rather than differences in the flexibility of the chains, seem to be responsible for the relatively high calorimetric glass‐transition temperature of the 2,6 diphenyl isomer, which is, respectively, nearly 36 °C and 32 °C above the T_g's of the 2,4 and 2,5 isomers. Molecular Mechanics calculations give a good account of the differences observed in the polarity of the polymers. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2179–2188, 2000     

Relaxation behavior of acrylate and methacrylate polymers containing dioxacyclopentane rings in the side chains

The synthesis of poly[(2,2‐dimethyl‐1,3‐dioxolan‐4‐yl) methyl acrylate)] (PACGA) and poly[(2,2‐dimethyl‐1,3‐dioxolan‐4‐yl) methyl methacrylate] (PMCGA) is reported. Both polymers present dielectric and mechanical β subglass absorptions at −128 and −115 °C, respectively, at 1 Hz, followed by ostensible glass–rubber or α relaxations centered in the vicinity of 0 and 67 °C, respectively, at the same frequency. The values of the activation energy of both the mechanical and dielectric β absorptions lie in the vicinity of 10 kcal mol⁻¹. The critical interpretation of the relaxation behavior of PMCGA suggests that dipolar intramolecular correlations play a dominant role in the response of the polymer to an electric field. The subglass relaxations of PACGA and PMCGA are further compared with the relaxation behavior of poly(1,3‐dioxane acrylate), poly(1,3‐dioxane methacrylate), and other polymers in the glassy state. The strong conductive processes observed in PMCGA at low frequencies and high temperatures were studied under the assumption that that these processes arise from Maxwell–Wagner–Sillars effects occurring in the bulk combined with Nernst–Planckian electrodynamic effects caused by interfacial polarization in the films. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 286–299, 2001     

Relaxation behavior of methacrylic polymers with bulky hydrophilic groups in their structures

The isochrones showing the temperature dependence of the loss relaxation modulus of poly(neopentyl glycol methacrylate) present an ostensible subglass absorption called β relaxation that roughly has the same intensity as the glass–rubber relaxation, or α process. The dielectric relaxation spectrum of this polymer also exhibits a well-developed β process followed at higher temperatures by the glass–rubber, or α relaxation, which strong conductive effects only permit to be detected at high frequencies. A detailed study of the conductive contributions to the dielectric loss above T_g was carried out using a theory that assumes that the dispersion observed in tan δ in the frequency domain arises from the Maxwell–Wagner–Sillars effect combined with Nernst–Planck electrodynamic effects caused by interfacial polarizations in the interface polymer electrodes. Attempts were made to evaluate the equivalent salt concentration that would produce the conductive effects experimentally observed. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3027–3037, 1999     

Direct signal analysis applied to the determination of the relaxation parameters from TSDC spectra of polymers

A new method to evaluate the thermally stimulated depolarization current curves based on the direct signal analysis is first applied here to the fitting of computer-generated curves to test its validity and accuracy. The method consists in finding the N elementary curves that best fit the experimental spectrum. The adjustment of the τ₀ values associated with each energy bin is also included in the fitting. The results are presented for a monoenergetic peak and a peak generated with a Gaussian broadened energy distribution both with a single τ₀. Good agreement is also found when a random noise of width σ₀ ≤ 0.01 J_max is added to the data to simulate experimental errors. Experimental complex curves corresponding to the γ and β overlapping relaxations in DGEBA-EDA epoxy resin system are separated into their fine structure components. The complex band corresponding to water dipoles physisorbed in different sites of the microporosity of bituminous coal is analyzed and different processes are identified. The results are compared to those performed on cleaned peaks. Finally, the TSDC peak corresponding to the glass transition temperature in bisphenol-A polycarbonate is analyzed and could only be fitted by using a Vogel-Fulcher expression for the temperature dependence of the relaxation times. © 1994 John Wiley & Sons, Inc.     

Electrical relaxation in in situ dried acid-form nafion

Electrical relaxation studies have been carried out on dried, acid-form Nafion 117 (copolymer of tetrafluoroethylene and perfluoro 3,6-dioxa-4-methyl-7-octenesulfonic acid and which has an equivalent weight of 1100). Significant electrical conductivity is found and the data are found to be non-Arrhenius. The results are converted to the electrical modulus representation and a conductivity relaxation is observed. A stretched exponential is best-fit to the conductivity relaxation and again, non-Arrhenius behavior is observed for the peak position. Finally, at low temperatures, a new dielectric relaxation peak is found which has an activation enthalpy of about 0.57 eV (13 kcal/mol). This relaxation is analogous to the γ-relaxation observed in mechanical relaxation studies. © 1994 John Wiley & Sons, Inc.     

Dielectric relaxation studies on a three component fully aromatic copolyester

The family of aromatic copolyesters based on the hydroxybenzoic acid (HBA) unit has been studied extensively by a number of groups. In particular the copolyesters of HBA and 2,6‐hydroxynaphthoic acid (HNA) have received much of attention due to their superior physical properties. This paper, however, describes some detailed dielectric studies on a related fully aromatic copolyester, poly(p‐oxybenzoate‐co‐p‐phenylene isophthalate), known simply as HIQ. This polymer is of particular interest because it can be solvent cast in an amorphous form, with no apparent crystallinity or frozen liquid crystalline texture and subsequently annealed, to increase the amount of crystalline and frozen liquid crystalline material. Dielectric measurements were therefore made on tape and film samples with differing morphologies produced by different processing histories. Measurements were made from 1 Hz to 10 kHz over the temperature range ?100 °C to 150 °C. A low temperature γ relaxation is observed which appears to be similar in nature to that which is observed in copolymers of HBA and HNA. An intermediate temperature β process is seen in isotropic samples which has a much higher activation energy than that found in HBA/HNA polyesters for example. The intensity of this process, which is not seen in mechanical measurements on highly oriented samples, increases on annealing and it is therefore suggested that the process is the glass transition of the liquid crystalline phase, though why the process is not seen in mechanical measurements on oriented samples is unclear. Copyright © 2001 John Wiley & Sons, Ltd.     

Analysis of the cure-dependent dielectric relaxation behavior of an epoxy resin

The dielectric relaxation behavior of an epoxy-amine resin was investigated using the Williams-Watts relaxation function. Phenomenologically, the dielectric features of the resin during cure are similar to those of stable materials. The distribution parameter of the dipole relaxation decreases from the onset of cure to a conversion near the gel point and then maintains a constant value. Based on the experimental observations and theoretical considerations, a single-frequency approach has been proposed for extracting the relaxation time of maximum loss. The relaxation data so obtained are independent of the measurement frequency and are in agreement with those acquired directly from the dipole loss peaks. © 1994 John Wiley & Sons, Inc.     

Dielectric relaxation spectroscopy and alignment behavior of a polymer‐dispersed liquid crystal and its component materials

The dielectric properties of a polymer‐dispersed liquid crystal (PDLC), a liquid‐crystal (LC) mixture (BL036), and three polymer matrices of PN314 containing different amounts of BLO36 were determined over a range of frequencies and temperatures and, for the LC and PDLC, over a range of voltages leading to homeotropic alignment of the LC. The overall dielectric relaxation process was a weighted sum of contributions from (1) the primary (δ) process in the LC arising from the motions of the dipoles about the short molecular axis and (2) dipole motions in the polymer matrix. The dielectric spectra were determined as a function of frequency, temperature, and, when appropriate, applied voltage. An equivalent electrical circuit was used as a working model to describe the dielectric behavior of the PDLC in the absence and presence of applied voltages. Agreement between the dielectric data and this model was achieved if a portion of the LC phase at the interface was assumed to be immobile. The director order parameter for the LC component in the PDLC was determined from dielectric measurements as the material was aligned homeotropically in an applied electric field. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 1173–1194, 2001     

A rigorous comparison of theoretical autocorrelation functions with dielectric relaxation parameters for the alpha process of polymers

Recently, tables of parameters used to represent experimental dielectric relaxation data as well as autocorrelation functions have become available. The experimental and autocorrelation function data were represented with the Havriliak–Negami function using rigorous statistical techniques. These tables include not only parameters and their temperature dependencies, but also the confidence intervals for all of the parameters. The important parameters for this work are the two shape parameters, α and β, which represent the width and skewness of the relaxation process, respectively. A comparison is made between the experimental data and several autocorrelation functions by calculating the minimum distance (in units of standard deviations) between the experimental values of αβ parameters for a specific polymer and the αβ parameters corresponding to the autocorrelation functions reported in these tables. Quantities derived from these minimum distances (a distribution function and an error function) are reported for each of the autocorrelation functions. These results are discussed in terms of the basic assumptions of the mode coupling theory. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1887–1897, 1997     

Dielectric relaxational behavior of poly(diitaconate)s containing cyclic rings in the side chain

Dielectric relaxations of several poly(diitaconate)s with cyclobutyl, cycloheptyl, and cyclooctyl groups in the side chain were investigated. The study was performed by determining the dielectric permittivity and loss, depending on the frequency and temperature. Dynamic dielectric measurements indicated several relaxations according to the chemical structure of the polymers. The dielectric behavior of these polymers was compared with those of poly(dicyclohexyl itaconate), previously reported. The α relaxations were analyzed with the Havriliak–Negami equation. Significant differences in the subglass relaxations were observed. A tentative explanation of the molecular origin of each absorption was proposed in terms of the number of carbon atoms of the ring and their conformational versatility. Strong conductive processes were observed in these polymers at low frequencies and high temperatures. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1059–1069, 2003     

Electric,dielectric, and dynamic mechanical behavior of carbon black/styrene‐butadiene‐styrene composites

The conductivity of styrene‐butadiene‐styrene block copolymers containing different amounts of extraconductive carbon black (CB) was investigated as a function of the mold temperature. The composites exhibited reduced percolation thresholds (between 1.0 and 2.0 vol % CB). The dynamic mechanical analysis characterization revealed that the glass‐rubber‐transition temperatures of both segments were not affected by the CB addition, although the damping of the polybutadiene phase displayed a progressive drop with an increase in the CB concentration. The normalized curves of tan δ/tan δ_max (where tan δ represents the value of the loss tangent at any measurement temperature and tan δ_max represents the loss tangent peak value at the corresponding temperature T_max) versus T/T_max (where T is the temperature and T_max is the maximum temperature), corresponding to both polystyrene and polybutadiene phases as well as the activation energy related to the glass‐rubber‐transition process, did not present any significant change with the addition of CB. The dielectric analysis revealed the presence of two relaxation peaks in the composite containing 1.5 vol % CB, the magnitude of which was strongly influenced by the frequency, being attributed to interfacial Maxwell‐Wagner‐Sillars relaxations caused by the presence of different interfaces in the composite. The mechanical properties were not affected by the presence of CB at concentrations of up to 2.5 vol %. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2983–2997, 2003     

Mechanical relaxation spectroscopy of three triepoxide-based polymers

Three network structure polymers formed by the chemical reactions of a triepoxide with aniline, 3-chloroaniline,and 4-chloroaniline were prepared and their shear modulus relaxation spectra studied over the 10⁻³- to 1-Hz range and temperatures up to their rubber modulus region. The decrease in the unrelaxed modulus with increase in temperature is found to be a reflection of both an increase in volume, and a decrease in the relaxed modulus of the sub-T_g relaxations process. It is quantitatively shown that the increase in the rubber modulus with increase in temperature above T_g is predominantly due to an increase in the entropy and not to a decrease in the number of cross-links density on thermal expansion. The unrelaxed modulus remained unaffected by the change in the overall size of the phenyl groups of the amines and of the steric hindrance to their rotations caused by the proximity of the chlorine atom to the cross-linking N-atom in the network structure, but the rubber modulus was effected. The shear modulus spectra could be fitted to a stretched exponential decay function with a temperature-independent stretch parameter of 0.25 for two polymers and 0.22 for one. The time–temperature superposition of the spectra did not yield a master curve, and a vertical displacement of the data also failed to produce it. This was more clearly demonstrated by the spectra of the mechanical loss tangent. After considering the various contributions to the shear modulus, it was concluded that deviations from the time–temperature superposition of the spectra are intrinsic to these polymers and arise from the change in the viscoelastic functions for segmental dynamics on change in the temperature such that the overall distribution of relaxation times remains unaffected. The mechanical loss tangent of the three polymers is found to be higher than that of polycarbonate at ambient temperature, implying a higher loss of mechanical energy before these polymers may fracture. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3071–3083, 1999     

Enhancing the molecular cooperativity of polyvinyl butyral using liquid additives

The specific role of acetonitrile and methoxypropionitrile, as accelerators of the relaxation dynamics of polyvinyl butyral (PVB), was investigated in polymer/additive mixtures with a saturation liquid content. The aim was to improve the ionic mobility of PVB‐based solid electrolytes to be used in solid dye‐sensitized solar cells. Mechanical and dielectric relaxation measurements between 120 K and 380 K revealed that the α‐relaxation observed above 330 K in dry‐PVB is shifted quite below room temperature in PVB/additives. Both the additives cause a growing intermolecular cooperativity, the sub‐glass β‐relaxation exhibiting a strength enhanced by a factor 3 and a frequency factor which increases from 10¹⁵ s⁻¹ to 10²¹ s⁻¹. This discloses an activation entropy as high as 165.7 J/K mol in comparison to 40.8 J/K mol in dry‐PVB. It is suggested the existence of cooperative transitions, mainly driven by bridges formed through additive molecules, which influence both short‐ and long‐scale segmental motions and also favor the ion dynamics in PVB/additive/electrolyte systems. The room temperature ionic conductivity σ_rt exhibits large changes from 6.4*10⁻¹⁴ S/m in dry PVB, through 1.5*10⁻⁸ S/m in PVB/LiI, to 2.45*10⁻⁵ S/m in PVB/MPN/LiI. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 340–346     

Dynamic mechanical and dielectric properties of ORMOCER® incorporating functionalized poly(styrene) latexes

Organically modified aluminosilicate hybrid materials incorporating polystyrene and poly(styrene‐co‐hydroxypropyl acrylate) latexes, (3‐glycidyloxypropyl) trimethoxysilane, and aluminum sec‐butoxide [Al(O^sBu)₃] were synthesized by a sol–gel process. The bulk materials obtained were macroscopically homogeneous dispersions with good mechanical properties. Dynamic mechanical and dielectric analyses of these new hybrid materials as a function of the Al(O^sBu)₃ concentration and copolymer composition revealed a series of transitions that represented relaxation processes of the incorporated polymer (glass transition), ?Al? O? Si?, the ?Si? O? Si? part of the network, and segmental motion of unreacted ?Si? (CH₂)₃OCH₂CHCH₂O chains. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 860–867, 2001     

Dielectric relaxation study of a H shaped liquid crystal dimer

The present article reports dielectric relaxation study of an unusual shaped liquid crystal dimer (H shape) in the temperature range 40–85°C. The study of this liquid crystal dimer is important due to the presence of azo central linkage, which is photosensitive. The dielectric relaxation study indicates coupling between the electric field and the liquid crystal molecule of H shaped dimer. Various dielectric properties for this liquid crystal dimer are quite different from the conventional liquid crystals. The dielectric relaxation observed in the smectic phase is due to the reorientation of molecule along the director in the presence of an applied electric field. The temperature dependence of the dielectric parameters, such as relaxation frequency, relaxation strength and distribution parameter have been evaluated for this liquid crystal dimer for both planar as well as homeotropically aligned cell and then compared. The Cole–Cole equation has been used to evaluate the aforementioned parameters.     

Polyazomethine with m-tolylazo side groups: thermal,dielectric and conductive behaviour

Using solution polycondensation, a new polyazomethine with m-tolylazo side groups (PAz) exhibiting thermotropic liquid crystalline phase was synthesised and its chemical structure was characterised with generally accepted methods. Its phase transition temperatures were detected with both polarising optical microscopy and differential scanning calorimetry. Using dielectric spectroscopy method, both real and imaginary parts of the permittivity were investigated in wide regions of temperature (from ?100°C to 170°C) and frequency (from 1 Hz to 1 MHz). Analysis of frequency dependent permittivity allowed finding three relaxations (α, β₁ and β₂) in PAz. β-relaxations were described with the Arrhenius equation, whereas α-relaxation was described with the Vogel–Fulcher–Tammann equation. The alternating current conductivity (ACC) of PAz was studied in the same regions of temperature and frequency. The frequency dependent ACC was described with an exponent power equation. Presentation of ACC as a function of inverse temperature allowed us to describe ACC with the Arrhenius equation.     

Dielectric relaxation analysis of cellulose oligomers and polymers in dependency on their chain length

For all cellulose‐like oligo‐ and polyglucans, beginning with the dimer cellobiose, a broad relaxation process at low temperatures is observed using the dielectric relaxation spectroscopy method. This relaxation has its molecular origin in orientational motions of the sugar rings via the glucosidic linkages. We investigated the dynamics of this main chain motion for β(1‐4) oligoglucans with 2, 3, 4, or 5 anhydroglucose units (AGUs), as well as for β(1‐4) polyglucans having a degree of polymerization molecular weight averages (DP_w) of 23, 37, 50, and 140 up to 3000. As a result we found that the activation energy (E_a) of the segmental chain motion has the lowest value (32 ± 1 kJ/mol) for cellobiose, followed by passing through a maximum for a DP between 7 and 15 with E_a = 51 ± 1 kJ/mol. Subsequently, the activation energy is decreased at a value around 44.8 ± 1.2 kJ/mol for chains containing more than 100 AGUs. Obviously, from a distinctly molecular dimension (DP_w ～ 100) the mean number of AGUs that take part in the local chain motions and cross‐correlation between the motions of neighboring AGUs are nearly the same and the chain length has no influence on the segmental motion. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2491–2500, 2001     

Low‐temperature relaxation of polymers around doped dyes studied by persistent spectral hole burning

Persistent spectral hole burning spectroscopy is applied to evaluate the low‐temperature relaxation around the dye molecules doped in several types of polymers. The doped dye is tetraphenylporphine, and the measured polymers are vinyl polymers and main chain aromatic polymers. The changes of microscopic environments around the dye are evaluated from the changes in the hole profiles during temperature cycling experiments. The relaxation behavior of the polymers is discussed in relation to their chemical structures. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 585–592, 1999