Comparison of dynamic mechanical relaxation processes at cryogenic temperatures in polyesters

The relaxation behavior of poly (1,4-cyclohexylenedimethylene terephthalate) over the temperature range 4.2[ddot]-300[ddot]K is reported. The trans polymer shows a strong γ relaxation at 175[ddot]K (1 Hz) and a Δ relaxation at 45[ddot]K, whereas the γ and Δ peaks of the cis polymer are at 200[ddot] and 48[ddot]K, respectively. A copolymer of the two isomers shows a composite γ peak which is resolved into two components associated with the trans and cis glycols. A mechanism for the γ peak involving motions of short chain segments is proposed and discussed with reference to the analogous relaxation process in polyethylene terephthalate. The effects of crystallinity and orientation on the γ and Δ relaxations are also considered.     

Thermally stimulated current and creep of solid amylose

Dielectric and viscoelastic properties of solid amylose were investigated by thermally stimulated current and creep techniques in order to clarify the relation between its structure and properties, and also the different binding modes of water molecules. Several current peaks were observed, and these were decomposed into elementary processes by the fractional polarization technique. A peak at about —110[ddot]C was attributed to the rotational motion around the C5-C6 axis of methylol groups attached to the C5 atom in the glucose residue. A peak at about — 50dGC was attributed to bound water: This peak was eliminated by drying the specimen. A peak at about —230[ddot]C was attributed to another kind of bound water. A peak at about 30[ddot]C was attributed to dehydration during the experiment. Thus, three different relaxation modes of bound water were separated by the present work.     

Gamma relaxation in nitric acid-treated polyethylene single crystals

There has recently been considerable interest in the gamma relaxation of polyethylene (PE). Determined by dynamic mechanical measurements at 1 cps, this relaxation would be located near 150°K. The gamma loss peak has been observed both in bulk PE [1-5] and in mats of solution-grown crystals [5-8] by a variety of techniques. The origin of this relaxation has variably been attributed to the amorphous component [1-6,9,10], the crystalline component [8,11], or both [12,13]. According to the last view, the gamma relaxation consists of two superposed peaks (γ_a and γ_c) attributable, respectively, to the two components.     

A comparison of effect of PEG and EC plasticizers on relaxation dynamics of PEO–PMMA–AgNO3 polymer blends

In the present study, Ag⁺-ion conducting polyethylene oxide–polymethyl methacrylate based polymer blend electrolyte systems plasticized using different plasticizers, i.e., ethylene carbonate and polyethylene glycol, are reported. The polymer films were synthesized using the solution cast technique and characterized by differential scanning calorimetry technique. Electrical properties of polymer films were investigated by impedance spectroscopy in the frequency range of 2 MHz–10 Hz at temperature range from 303 to 373 K. Discussion of electrical properties of polymer films in the terms of conductivity and relaxation times is made.     

Dielectric relaxation in poly(ethylene oxide) complexed with alkali metal perchlorates

Audio frequency complex admittance measurements over the temperature range 5.5–350 K have been performed on poly(ethylene oxide) (PEO) complexed with lithium and sodium perchlorates. No evidence of an α_a relaxation was seen in any sample. In the Li-complexed material, the γ relaxation occurs at about 174 K and 1000 Hz which is the same as for pure PEO. In the Na-complexed materials, there appear to be more than one peak in this region, and the relative peak heights are concentration dependent. The result is that at the 8:1 doping level, there is a broad, skewed relaxation whose peak occurs at about 203 K and 1000 Hz, while at 4.5:1 NaClO₄ the relaxation appears much closer to a normal peak with a maximum at 185 K for the 1000 Hz data. Combined with previous results for thiocyanate complexed PEO, these results show that distortions in the polymer chain depend on both the cation and anion. Next, two new relaxations are observed at low temperatures. The first, occuring at 60–100 K is very weak and is common to all PEO samples. The second occuring at very low temperatures, is only found for perchlorate complexed PEO and is attributed to the localized motion of perchlorate-induced defects on the exterior of the chains. The high temperature electrical conductivity for PEO₈:NaClO₄ is found to be larger than PEO_4.5:NaClO₄ in agreement with the trend observed by other workers for PEO complexed with different alkali metal salts.     

Effect of electron beam irradiation on the structural,thermal and optical properties of poly(vinyl alcohol) thin film

Poly(vinyl alcohol) (PVA) polymer was prepared using the casting technique. The obtained PVA thin films have been irradiated with electron beam doses ranging from 20 to 300 kGy. The resultant effect of electron beam irradiation on the structural properties of PVA has been investigated using X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR), while the thermal properties have been investigated using thermo-gravimetric analysis and differential thermal analysis (DTA). The onset temperature of decomposition T ₀ and activation energy of thermal decomposition E _a were calculated, results indicate that the PVA thin film decomposes in one main weight loss stage. Also, the electron beam irradiation in dose range 95–210 kGy led to a more compact structure of the PVA polymer, which resulted in an improvement in its thermal stability with an increase in the activation energy of thermal decomposition. The variation of transition temperatures with electron beam dose has been determined using DTA. The PVA thermograms were characterized by the appearance of an endothermic peak due to melting. In addition, the transmission of the PVA samples and any color changes were studied. The color intensity Δ E was greatly increased with increasing electron beam dose, and was accompanied by a significant increase in the blue color component.     

Thermally stimulated currents in poly(vinyl chloride): Tacticity and molecular weight influence

Thermally stimulated currents (TSC) have been measured in several samples of poly(vinyl chloride) differing in tacticity and molecular weight as a result of polymerizing them at different temperatures. This has allowed us to characterize the relaxation behavior of PVC. No dielectric relaxation can be observed by this experimental technique at temperatures between liquid helium and liquid nitrogen. The β relaxation is observed around 173°K, with similar parameters in all samples studied. Around the glass transition the relaxation times isolated in the α peak follow a compensation law. Molecular weight and tacticity have a strong influence on the temperature of the maximum and the intensity of this relaxation, respectively.     

Amorphous linear polyethylene: Annealing effects

The effect of annealing amorphous linear polyethylene films prepared by an improved ultraquenching technique at temperatures just below and above a dynamic mechanical relaxation peak (torsion braid) observed at ∽190K has been characterized by electron microscopy and torsion braid analysis. Based on the results described, this peak is believed related to the lower glass transition temperature T_g(L), the T_g of wholly amorphous linear polyethylene, whereas the β peak at 260K is T_g (upper). Annealing just below T_g (L) results in a growth in size of the nodules observed in the as-quenched samples, whereas annealing above T_g (L) can result in the growth of single crystal-like structures, spherulites, and shish-kebobs. Storage of the crystallized samples at room temperatures results in a decrease in size of the relaxation peak during subsequent torsion braid spectroscopy measurements. The results indicate significant amounts of molecular motion can occur during crystallization even at T_g.     

Broad band dielectric behaviour of plasticized PEO-based solid polymer electrolytes

The conductivity and dielectric response of poly(ethylene oxide) (PEO) based plasticized polymer electrolyte systems were studied in the broad frequency range from 5 Hz to 1.8 GHz and in the temperature range from 248 K to 353 K. Propylene carbonate (PC) and ethylene carbonate (EC) were used as conventional plasticizers while poly(perfluorinated ethylene methylene oxide) (M03) was used as a new type of plasticizer. PEO-LiN(CF₃SO₂)₂ plasticized with M03 shows high enough conductivity values to be used as electrolyte in rechargeable lithium polymer batteries. At high frequency a dielectric relaxation is observed for pure PEO as well as for the salt containing systems in the GHz region that is assumed to be due to segmental motion of the polymer chains. In the salt containing systems, this relaxation is shifted to lower frequencies relative to that of pure PEO, this is attributed to transient cross-linking. However, at lower frequencies another dielectric response peak was detected in all samples containing salts. The effect of the plasticizer on this relaxation is complex. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Effect of different plasticizers on the secondary relaxation of poly(vinyl chloride)

Samples of poly(vinyl chloride) plasticized with variable amounts of either dibutyl phthalate (DPB) or dicyclohexyl phthalate (DCHP) were investigated by dynamic-mechanical measurements in the β relaxation temperature range. In this range of temperature, a superposition of the relaxation due to the cyclohexyl group with the PVC β peak was found for the samples plasticized with DCHP. By studying the dependence of the activation energy and of the peak broadness on the DCHP concentration it was possible to show that the PVC β relaxation is reduced to zero at the critical plasticizer weight fraction W₁ = 0.2. For the PVC-DBP series the β peak disappears at the same plasticizer content. These results strengthen the hypothesis that the β peak of PVC is due to a kind of cooperative motion since 1 mole of plasticizer for every 20 repeating units of the polymer is sufficient to suppress the PVC β relaxation.     

Persistent internal polarizations in poly(ethylene terephthalate) films. I. stress-induced polarizations

When heated under zero voltage, commercial polyester film exhibits current peaks manifesting glass transition and glycol linkage motions, as well as a broad, low background. The peaks are due to the depolarization of dipoles oriented by local stresses during manufacture. Their variations with sample thickness and heating rate permit the calculation of “thermodynamically reversible” values of T_g (58.7[ddot] ± 0.5[ddot] C) and T_glycol (87.3[ddot] ± 0.2[ddot]C). The broad background current is a manifestation of the alignment of paracrystalline order in a thermal gradient.     

Dynamic mechanical analysis of poly-α-amino acids. Models for collagen

A series of copolymers of L-leucine and L-glutamic acid was prepared as a model system for collagen, and the dynamic mechanical behavior was investigated as a function of water content. The results suggest a spectrum of waterpolymer interactions with the two predominant types represented by “tightly bound” and “loosely bound” water. Each gives rise to a characteristic relaxation peak: the former at 180–200°K (1 Hz) and the latter at about 260°K (1 Hz). One of the poly-α-amino acids, a 50:50 copolymer of L-leucine and L-glutamic acid, sorbed an amount of water comparable to that of native collagen. Below physiological temperature the relaxation spectrum of this polymer closely resembles that reported for collagen in both temperature location and intensity of relaxation peaks. This suggests that the use of model polymers may be of significant value in elucidating the structure and function of water in structural proteins.     

Investigation of the effect of gamma rays on electrical properties of chlorinated poly (vinyl chloride)

Dielectric constant, dielectric loss and AC conductivity were measured, in the frequency range 100 Hz to 5 MHz in chlorinated poly (vinyl chloride) (CPVC) before and after exposure to gamma irradiation at doses between 5.0 KGy and 50.0 KGy. The frequency dependencies of ε′, ε″ and σ_AC at 30 °C were investigated. A relaxation peak in the dielectric loss and a corresponding step in the dielectric constant have been observed, in the frequency ranges 10³ Hz to 10⁴ Hz. The dielectric constant ε′, dielectric loss ε″ and AC conductivity σ_AC are also found to increase at heating up to 100 °C. In addition the effect of gamma irradiation on the frequency dependencies of ε′, ε″ and σ_AC was measured at room temperature. The gamma irradiation leads to an increase in the efficiency of soft segments. Furthermore, the DC electrical conductivity of both the irradiated and non-irradiated samples was investigated. The induced electrical conductivity and the activation energy were measured, at various temperatures, as a function of gamma dose. It was found that the gamma radiation has a definite effect on the DC conductivity of the CPVC polymer.     

聚乙烯薄膜材料的红外光谱研究

由于红外吸收光谱法具有许多突出的优点,因此它在许多领域有广泛的应用。在薄膜、合成纤维、橡胶、塑料等高聚物的研究方面,用于单体、聚合物、添加剂的定性、定量和结构分析。一般高聚物的红外光谱中谱带的数目很多,而且不同种类的物质其光谱很不相同,特征性很强。此外红外光谱法的制样和实验技术相对比较简单,它适用于各种物理状态的样品。本实验研究以高聚物薄膜材料做样品,对样品高聚物进行红外光谱分析,分析表明,本实验所用样品高聚物成分为聚乙烯材料,这个实验结果也表明,用红外光谱法鉴定高聚物的组成非常有效。红外光谱法用于定量组分分析,与其它测量方法相比,具有制样简单方便、重复性好和测量精度高的特点。     

Dielectric relaxation and dc conductivity on the PVOH-CF3COONH4 polymer system

In the present paper, the ionic conductivity and the dielectric relaxation properties on the poly(vinyl alcohol)-CF₃COONH₄ polymer system have been investigated by means of impedance spectroscopy measurements over wide ranges of frequencies and temperatures. The electrolyte samples were prepared by solution casting technique. The temperature dependence of the sample’s conductivity was modeled by Arrhenius and Vogel-Tammann-Fulcher (VTF) equations. The highest conductivity of the electrolyte of 3.41×10^− 3 (Ωcm)^− 1 was obtained at 423 K. For these polymer system two relaxation processes are revealed in the frequency range and temperature interval of the measurements. One is the glass transition relaxation (α-relaxation) of the amorphous region at about 353 K and the other is the relaxation associated with the crystalline region at about 423 K. Dielectric relaxation has been studied using the complex electric modulus formalism. It has been observed that the conductivity relaxation in this polymer system is highly non-exponential. From the electric modulus formalism, it is concluded that the electrical relaxation mechanism is independent of temperature for the two relaxation processes, but is dependent on composition.     

Dielectric response of PVC polymer loaded with Ba0.3Na0.7Ti0.3Nb0.7O3 ceramic powder

Dielectric response of poly(vinyl chloride), (PVC), loaded with different amount of Ba_0.3Na_0.7Ti_0.3Nb_0.7O₃ (BNTN) ceramic powders was investigated in frequency range 100 Hz–1 MHz and temperature range 100–450 K. Ceramic solid solution of barium titanate and sodium niobate with composition Ba_0.3Na_0.7Ti_0.3Nb_0.7O₃ was obtained from BaCO₃, TiO₂, Na₂CO₃ and Nb₂O₅ by conventional method. Powders were prepared by grinding of ceramics. The obtained ceramics, used to produce BNTN–PVC composites, are characterized by the relaxor behaviour with a broad peak of dielectric permittivity ε′ at T _m ≈ 230 K. The microstructure of the powders was observed and the grain size was estimated using scanning electron microscope Hitachi S-4700. The EDS analysis confirms the qualitative and quantitative chemical composition of powders and ceramics. The BNTN–PVC composite samples of 0-3 connectivity were prepared from ceramic and polymer powders by hot-pressing method. The dielectric response of the composites displays features originated from the PVC polymer modified by those of BNTN ceramics. The relaxation time of the α-process of PVC obeys the Vogel–Fulcher law and decreases with increasing volume fraction of the ceramics.     

Persistent internal polarizations in poly(ethylene terephthalate) films. III. Cold crystallization

A polarization process, which may be induced in commercial poly(ethylene terephthalate) films by annealing, has been found to exhibit a current peak under zero bias near 120[ddot]C. This process is identified as the “cold crystallization” phenomenon, and involves the simultaneous gauche-to-trans isomerization and paracrystalline ordering of the glycol linkages.     

The decay of low-frequency elastic oscillations in a Ba2NaNb5O15 crystal

The internal friction and shear modulus of a barium sodium niobate crystal were studied using the torsion pendulum technique at a frequency of ～25 Hz in the temperature range from 80 to 800 K. An internal friction relaxation peak of the domain nature was observed at 228 K. A relaxation process responsible for this internal friction peak is explained through compensation of the electric charge (induced by a piezoelectric effect at the 180° domain walls) by charged point defects diffusing toward the domain walls.     

Effect of crosslinking on the low-temperature relaxation behavior of polyethylene

The dynamic mechanical properties of polyethylene crosslinked by chemical methods and by electron irradiation were determined from 77°K to 250°K on an acoustic spectrometer at a frequency of approximately 50 Hz. Parallel stress-strain, swelling, and x-ray scattering experiments were also carried out to determine the degree of crosslinking and crystallinity. It is found that the effect of chemical crosslinking is to suppress the loss intensity of the γ relaxation in polyethylene while preserving the symmetrical shape of the loss curve without changing the temperature position. The effect of radiation crosslinking, however, also changes the symmetrical nature of the curves. Possible mechanisms of these observed changes are discussed.     

Anelastic behavior of H2O ice single crystals doped with KOH

The internal friction of KOH-doped ice single crystals has been measured as a function of temperature in the frequency range from 0.1 to 20 Hz and compared with data on dielectric relaxation. There are two damping peaks, namely the α peak at around 85 K and the β peak at around 170 K. The Arrhenius relation of the α peak is almost equivalent to that for dielectric relaxation at the same temperatures. However, a relaxation corresponding to the β peak is not found in the dielectric measurements. The α peak is attributed to the rotation of water molecules by OH⁻ ions, which occupy H₂O sites. The β peak arises from the presence of the KOH, but no mechanism is offered at this stage.