Dynamic mechanical and dielectric relaxations in poly(monoethylphenyl itaconate)

Dynamic mechanical and dielectric relaxational behavior of poly(monoethylphenyl itaconate) at different frequencies and temperatures was studied. Three relaxation zones are found. The dynamic mechanical response is dominated by a relaxation peak at room temperature, labeled β relaxation. Two prominent shoulders labelled as γ and α relaxations are observed. Because of the overlapping of the α and γ with the β relaxation, a deconvolution method to improve the understanding of these phenomena is proposed. In spite of the complexity of the experimental spectra, the proposed deconvolution method seems to be a convenient approach to interpret the relaxational behavior of this polymer. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2749–2756, 1997     

Influence of oxycycloaliphatic groups in the relaxation behavior of acrylic polymers

A comparative study on the mechanical and dielectric relaxation behavior of poly(5‐acryloxymethyl‐5‐methyl‐1,3‐dioxacyclohexane) (PAMMD), poly(5‐acryloxymethyl‐5‐ethyl‐1,3‐dioxacyclohexane) (PAMED), and poly(5‐methacryloxymethyl‐5‐ethyl‐1,3‐dioxacyclohexane) (PMAMED) is reported. The isochrones representing the mechanical and dielectric losses present prominent mechanical and dielectric β relaxations located at nearly the same temperature, approximately −80°C at 1 Hz, followed by ostensible glass–rubber or α relaxations centered in the neighborhood of 27, 30, and 125°C for PAMMD, PAMED, and PMAMED, respectively, at the same frequency. The values of the activation energy of the β dielectric relaxations of these polymers lie in the vicinity of 10 kcal mol⁻¹, ∼ 2 kcal mol⁻¹ lower than those corresponding to the mechanical relaxations. As usual, the temperature dependence of the mean‐relaxation times associated with both the dielectric and mechanical α relaxations is described by the Vogel–Fulcher–Tammann–Hesse (VFTH) equation. The dielectric relaxation spectra of PAMED and PAMMD present in the frequency domain, at temperatures slightly higher than T_g, the α and β relaxations at low and high frequencies, respectively. The high conductive contributions to the α relaxation of PMAMED preclude the possibility of isolating the dipolar component of this relaxation in this polymer. Attempts are made to estimate the temperature at which the α and β absorptions merge together to form the αβ relaxation in PAMMD and PAMED. Molecular Dynamics (MD) results, together with a comparative analysis of the spectra of several polymers, lead to the conclusion that flipping motions of the 1,3‐dioxacyclohexane ring may not be exclusively responsible for the β‐prominent relaxations that polymers containing dioxane and cyclohexane pendant groups in their structure present, as it is often assumed. The diffusion coefficient of ionic species, responsible for the high conductivity exhibited by these polymers in the α relaxation, is semiquantitatively calculated using a theory that assumes that this process arises from MWS effects, taking place in the bulk, combined with Nernst–Planckian electrodynamic effects, due to interfacial polarization in the films. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2486–2498, 1999     

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     

Dielectric relaxations in polymers containing dioxacyclohexane rings by thermostimulated depolarization currents

The dielectric activity of poly{5-[(methacryloxy)methyl]-5-ethyl-1,3-dioxacyclohexane} (PMAMED) and poly[(5-methacryloxy)-1,3-dioxacyclohexane] (PMAD) in the glassy region and in the glass-rubber transition is investigated by using global and partial thermostimulated discharge current (TSDC) techniques. The global TSDC curve for each polymer displays an ostensible β absorption in the glassy region followed in increasing order of temperature for a prominent α glass rubber relaxation. Partial depolarisation curves show in detail the regions of the glassy state in which more dielectric activity occurs. The TSDC curves for PMAMED are compared with those of its acrylate homologous, poly{5-[(acryloxy)methyl]-5-ethyl-1,3-dioxacyclohexane} (PAMED), finding that the methyl group in the former polymer only hinders long range micro-Brownian motions in the chains, thus shifting the glass-rubber relaxation to higher temperatures, without affecting in a significant way molecular motions in the glassy region. Small changes in the neighbourhood of the 1,3-dioxacyclohexane ring, such as suppression of a methylene group or replacement of the equatorial hydrogen in position 2 of the ring for a phenyl group, depresses the dielectric activity and shifts the β absorptions to lower temperatures. The interconversion between TSDC and a.c. dielectric results in the glassy region is discussed.     

Internal motions in an alternating copolymer of ethylene and tetrafluoroethylene

Internal motions in an alternating copolymer of ethylene and tetrafluoroethylene were investigated by dynamic mechanical and dielectric measurements and by nuclear magnetic resonance. At 1 Hz the α, β, and γ relaxations were observed at 110, ?25, and ?120°C in a quenched sample. The activation energy was 76 kcal/mole for the α relaxation and 10.6 kcal/mole for the γ relaxation. These relaxations are attributed to the motion of long and short segments in the amorphous regions, respectively. The β relaxation, which was observed only in the dynamic mechanical experiments, appears to occur in the crystalline regions. The copolymer is isomeric with poly(vinylidene fluoride), but it has a higher melting point and a much lower dielectric loss.     

Theory of the local mode relaxation in the glassy state of polymers

The local modes refer to vibrational motions of the main chain in the glassy state which are thermally excited with a relatively large amplitude and are consequently strongly damped into relaxational motions by the intermolecular viscous force. This paper describes a theory of strengths of the dynamic mechanical and dielectric local mode relaxations, in the latter of which the correlation of dipole arrangement along the main chain is considered. The results are compared with the observed strength and its dependence on temperature and pressure, in particular for the dielectric β relaxation of poly(vinyl chloride). Satisfactory agreement is obtained between theory and experiment.     

Influence of microstructure and semi-crystalline morphology on the β and γ mechanical relaxations of the metallocene isotactic polypropylene

In this work, the characteristics of the β and γ mechanical relaxations, i.e., temperature and relative intensity, of a series of metallocene iPP samples (MPP) are analysed. The hypothesis that the temperature and the intensity of the glass transition (β relaxation) and local sub-T_g motions (γ relaxation) are related mainly to chain parameters and morphology has been corroborated. On the one hand, it has been found a critical average isotactic length (n₁) around 30 propylene units, under which the β and γ dynamics are promoted with respect to the α relaxation. On the other hand, it is apparent that the features which determine the degree of constraint within the inter-lamellar region, i.e., the fraction of low-T_m crystals, drive the relative intensities of the α, β and γ relaxation processes.     

Molecular mechanics analysis of polymer conformation, 4. Local chain motions and molecular relaxations of poly(parabanic acids)

The structure of the repeating units of poly(parabanic acid) (PPA), a polyimide-like glassy heat resistant polymer, was analyzed by MM2 using various model compounds of PPA-M and PPA-T consisting of M (1,3-diazolidine-2,4,5-trione-1,3-diyl-1,4-phenylenemethylene-1,4-phenylene) and T (1,3-diazolidine-2,4,5-trione-1,3-diyl-3,3′-dimethyl-4,4′-biphenylylene) units, respectively. Based on the most stable structures obtained, the subunit rotations of the methylene, phenylene, and diazolidine groups were evaluated. The potential mobilities estimated for these subunits can reasonably be correlated with the dynamic mechanical and dielectric properties of PPA. The γ relaxation mode of both PPA-M and a copolymer PPA-TM is reasonably explained by the rotation of the methylene group. The major mode of the β relaxations of both PPAs is ascribed to the rotation of the phenylene ring of the M unit, while the β^* dielectric relaxation is attributed to the torsional vibration of the parabanic ring involving the adjacent aromatic moieties. These data suggest that PPA is similar to polycarbonate rather than polyimide in terms of the local chain motion. It is shown that the conformational analysis by MM2 is quite informative to understand the structure property relationships of polymers.     

Piezoelectric properties and molecular motion of poly(β-hydroxybutyrate) films

Piezoelectric, elastic, and dielectric properties of films of poly(β-hydroxybutyrate) (PHB), an optically active natural polymer, were measured as functions of frequency and temperature. In mechanical properties, three relaxation processes were observed at 10 Hz: the α dispersion at 130°C, the β dispersion at room temperature, and the γ dispersion at ?120°C. It was concluded from x-ray diffraction and the thermal expansion coefficient that the α dispersion can be ascribed to thermal molecular motions in the crystalline phase, that the β dispersion is the primary dispersion due to the glass transition, and that the γ dispersion is related to local molecular motion of the main chains in the amorphous phase. Piezoelectric relaxations were also observed in these relaxation regions. It is proposed that the high-temperature process is due to ionic dc conduction. The piezoelectric relaxation at room temperature is ascribed to the increase of piezoelectric activity in the oriented noncrystalline phase, in which the sign of the piezoelectric modulus is opposite to that in the oriented crystalline phase.     

Dynamic mechanical relaxations of polyterephthalates based on trimethylene glycols

The dynamic mechanical relaxations of poly(trimethylene glycol terephthalate) (PTMT), poly(ditrimethylene glycol terephthalate) (PDTMT) and two copolymers obtained from them have been studied between ? 150 and 200°C with a dynamic viscoelastometer. The four polymers show three relaxations that are designated α, β, and γ in order of decreasing temperature. The α relaxation is considered to be the glass transition of the polymers. The β relaxation is wider and weaker than the α, as normally occurs in the polyester series. The γ relaxation takes place at temperatures below ? 100°C and is usually overlapped by the β relaxation. The influence of thermal and mechanical histories on the nature, location, and intensity of the three relaxations is studied and discussed.     

A dielectric study of chain motions in poly(vinyl methyl ether)

The dielectric permittivity and loss of poly(vinyl methyl ether) (mol. wt. 30,000) have been measured from 12 Hz to 100 kHz at temperatures from 77 K to 320 K. Two relaxation processes, γ and β, are observed at T < T_g (245 K), and one above T_g. The Arrhenius plots of the γ and β processes have activation energies of 20 and 41 kJ mole^?1 respectively. The relaxation rate of the α process is described by the Vogel-Fulcher-Tamman equation or the William-Landel-Ferry equation. The relaxation rates of γ and β processes evaluated from the isochrones differ from those evaluated from the isothermal spectrum. The features of chain motions observed are similar to those in other polymer and rigid molecular glasses.     

Dielectric relaxations in PEEK by combined dynamic dielectric spectroscopy and thermally stimulated current

The molecular dynamics of a quenched poly(ether ether ketone) (PEEK) was studied over a broad frequency range from 10^?3 to 10⁶ Hz by combining dynamic dielectric spectroscopy (DDS) and thermo-stimulated current (TSC) analysis. The dielectric relaxation losses ε′′_KK has been determined from the real part ε′_T(ω) thanks to Kramers–Kronig transform. In this way, conduction and relaxation processes can be analyzed independently. Two secondary dipolar relaxations, the γ and the β modes, corresponding to non-cooperative localized molecular mobility have been pointed out. The main α relaxation appeared close to the glass transition temperature as determined by DSC; it has been attributed to the delocalized cooperative mobility of the free amorphous phase. The relaxation times of dielectric relaxations determined with TSC at low frequency converge with relaxation times extracted from DDS at high frequency. This correlation emphasized continuity of mobility kinetics between vitreous and liquid state. The dielectric spectroscopy exhibits the α_c relaxation, near 443 K, which has been associated with the rigid amorphous phase confined by crystallites. This present experiment demonstrates coherence of the dynamics of the PEEK heterogeneous amorphous phase between glassy and liquid state and significantly improve the knowledge of molecular/dynamic structure relationships.     

Segmental and secondary dynamics in hydrogen‐bonded poly(4‐vinylphenol)/poly(methyl methacrylate) blends

Broadband dielectric spectroscopy was used to study the segmental (α) and secondary (β) relaxations in hydrogen‐bonded poly(4‐vinylphenol)/poly(methyl methacrylate) (PVPh/PMMA) blends with PVPh concentrations of 20–80% and at temperatures from ?30 to approximately glass‐transition temperature (T_g) + 80 °C. Miscible blends were obtained by solution casting from methyl ethyl ketone solution, as confirmed by single differential scanning calorimetry T_g and single segmental relaxation process for each blend. The β relaxation of PMMA maintains similar characteristics in blends with PVPh, compared with neat PMMA. Its relaxation time and activation energy are nearly the same in all blends. Furthermore, the dielectric relaxation strength of PMMA β process in the blends is proportional to the concentration of PMMA, suggesting that blending and intermolecular hydrogen bonding do not modify the local intramolecular motion. The α process, however, represents the segmental motions of both components and becomes slower with increasing PVPh concentration because of the higher T_g. This leads to well‐defined α and β relaxations in the blends above the corresponding T_g, which cannot be reliably resolved in neat PMMA without ambiguous curve deconvolution. The PMMA β process still follows an Arrhenius temperature dependence above T_g, but with an activation energy larger than that observed below T_g because of increased relaxation amplitude. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3405–3415, 2004     

Nuclear magnetic relaxation and molecular motion in poly(vinylidine fluoride)

Pulsed NMR T₁, T₂, and T_1ρ measurements are reported for poly(vinylidine fluoride) (PVF₂). The results demonstrate clearly the presence of four relaxation processes, three amorphous and one crystalline. The α relaxation is undoubtedly a crystalline one, while β and γ are both amorphous, in agreement with earlier conclusions from dielectric and dynamic mechanical measurements. The fourth relaxation (β′) observed initially in the mechanical measurements of Kakutani, but undetected in dielectric experiments, has been confirmed in our results and the process is described by an activation energy of 15.1 kcl/mole. Motion of folds on the surface of crystal lamellae is deemed to be the responsible mechanism for the β′ relaxation. Two models have been considered in the interpretation of the α process; rotation of crystalline chains in the vicinity of defects and rotational oscillation of restricted amplitude of all crystalline chains about the main chain axes. Rotation of amorphous chains is a possible mechanism for the γ process while motions of a general nature are responsible for the β relaxation. Our experimental results again indicate that spin diffusion plays an important role in the overall NMR response of the polymer.     

Glass transition and polymer dynamics in silver/poly(methyl methacrylate) nanocomposites

Dynamic mechanical–thermal analysis (DMTA), differential scanning calorimetry (DSC), thermally stimulated depolarization currents (TSDC) and, mainly, broadband dielectric relaxation spectroscopy (DRS) were employed to investigate in detail glass transition and polymer dynamics in silver/poly(methyl methacrylate) (Ag/PMMA) nanocomposites. The nanocomposites were prepared by radical polymerization of MMA in the presence of surface modified Ag nanoparticles with a mean diameter of 5.6 nm dispersed in chloroform. The fraction of Ag nanoparticles in the final materials was varied between 0 and 0.5 wt%, the latter corresponding to 0.055 vol%. The results show that the nanoparticles have practically no effect on the time scale of the secondary β and γ relaxations, whereas the magnitude of both increases slightly but systematically with increasing filler content. The segmental α relaxation, associated with the glass transition, becomes systematically faster and stronger in the nanocomposites. The glass transition temperature T_g decreases with increasing filler content of the nanocomposites up to about 10 °C, in good correlation by the four techniques employed. Finally, the elastic modulus decreases slightly but systematically in the nanocomposites, both in the glassy and in the rubbery state. The results are explained in terms of plasticization of the PMMA matrix, due to constraints imposed to packing of the chains by the Ag nanoparticles, and at the same time, of the absence of strong polymer–filler interactions, due to the surface modification of the Ag nanoparticles by oleylamine at the stage of preparation.     

Effects of orientation on the mechanical and dielectric relaxation behavior of cycloaliphatic polyester networks

The mechanical and dielectric relaxation behavior of strained and unstrained networks, prepared from hydroxyl-terminated poly(diethylene glycol-trans-1,4-cyclohexane dicarboxylate) (PDGC), is studied over a wide interval of frequencies and temperatures. The mechanical relaxation spectrum exhibits a glass-rubber absorption, designated β, located in the vicinity of 0°C at 0.1 Hz, followed by a β relaxation which appears to be the result of two overlapping peaks centered at ?80°C (β₁) and ?110°C(β₂). These two peaks coalesce into a single peak in the case of strained networks. The dielectric relaxation spectrum also exhibits an α absorption followed by a subglass β relaxation whose width decreases as the elongation ratio λ increases. The activation energy associated with the mechanical β₁ appears to increase as λ increases. However, the activation energy of the dielectric β process does not show a clear dependence on the elongation ratio. The analysis of the conformational characteristics of PDGC chains indicates that rotational transitions through the C_cy? C* bonds of the acid residue would give rise to high dielectric activity. Conformational changes about the CH₂? CH₂ bonds of the glycol residue would produce significant mechanical activity but, comparatively, low dielectric activity. The glass-rubber absorption is slightly displaced toward the high-temperature side as the elongation ration increases, suggesting that the entropic effects overcome the volume effects. The glass-rubber transition is interpreted in terms of the free volume theory.     

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     

Relations between dynamic mechanical properties and melting behavior of nylon 66 and poly(ethylene terephthalate)

Nylon 66 films exhibiting form I melting behavior show the γ mechanical relaxation at ?140°C. Samples which have form II melting behavior do not show this relaxation. The γ relaxation disappears when material having form I behavior is converted to material having form II behavior by annealing or by cold drawing. The form I and form II types of melting behavior are also found in poly(ethylene terephthalate); the interconversions and thermal behavior of the forms are analogous to the nylon 66 case. In poly(ethylene terephthalate), the β relaxation at ?40 to ?60°C is present only when form I melting behavior is found. Conversion to form II melting behavior by annealing or drawing (80°C) again causes the relaxation to disappear. No β relaxation was found in amorphous polymer. The γ dispersion in nylon 66 and the β dispersion in poly(ethylene terephthalate) can therefore be associated with the crystalline structure responsible for form I melting behavior. Form I melting behavior has been associated with foldedchain crystals based on previous work. It is therefore postulated that the γ dispersion in nylon 66 and the β dispersion in poly(ethylene terephthalate) are associated with motions in the chain folds. This assignment is not inconsistent with the change in the γ dispersion of nylon 66 with the number of backbone CH₂ units, since these will affect the fold structure.     

Effects of thermal history,crystallinity, and solvent on the transitions and relaxations in poly(bisphenol-A carbonate)

The following system of nomenclature for the transitions and relaxations in polycarbonate has been proposed: α = T_g = 150, β = 70, γ = ?100, and δ = ?220°C (frequency range of 10–50 Hz). The three component peaks of the γ relaxation are denoted by γ₁, γ₂, and γ₃ relaxations correspond to phenylene, coupled phenylene-carbonate, and carbonate motions, respectively. Dynamic mechanical analysis of poly(bisphenol-A carbonate) using the DuPont 981–990 DMA system shows that the magnitude of the β relaxation depends upon the thermal history of the polycarbonate; annealing greatly reduces the intensity of the β relaxation. A relaxation map constructed for the β relaxation gives an activation energy of 46 kcal/mol. Exposure of polycarbonate to methylene chloride vapor for various times shows that after an induction period of about 5 min the intensity of the γ₃ relaxation at ?78°C decreases whereas the intensity of the γ₁ relaxation of ?30°C is unaffected and the ratio E″(γ₁)/E″(γ₃) increases linearly with the square root of time. This has been ascribed to the interaction of methylene chloride on the carbonate group in polycarbonate. Thermal crystallization of polycarbonate does not affect the positions of the γ relaxation and the glass transition peaks, but merely reduces their intensity. The glass transition peak intensity falls off sharply in comparison to the γ relaxation intensity. Both the γ₃ and γ₁ peaks in polycarbonate have been observed simultaneously for the first time by dynamic mechanical analysis. Impact strength measurements show that methylene chloride treatment of polycarbonate results in a change in mode of failure from ductile to brittle with a resultant 40-fold reduction in impact energy for fracture. Thermally crystallized polycarbonate exhibits brittle fracture with very low force and energy at break.     

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