Dynamic mechanical properties of methacrylic-acid-grafted polyethylene films

A dynamical mechanical relaxation study has been made of low density polyethylene films to which methacrylic acid has been grafted by γ irradiation. The grafted films retain the original degree of crystallinity and show only slight changes in melting points and melt viscosities. This indicates that the grafted methacrylic acid side chains are long, few in number, and completely phase separated from the polyethylene matrix. Three dispersion regions are observed in plots of the loss modulus, E″ vs. temperature at constant frequency and these are labeled γ, β, α′, in order of increasing temperature. The α′ peak, above 215°C was assigned to microbrownian segmental motions accompanying the T_g of polymethacrylic acid. The β peak, at ?20°C, was assigned to motions accompanying the T_g of branched polyethylene, and the γ peak, at ?120°C, was assigned to local motions of a few CH₂ sequences in polyethylene.     

Multiple transitions in atactic 1,2-polybutadienes observed from dynamic mechanical and dielectric relaxation data

Detailed investigation by dynamic mechanical, dielectric and ¹³C-NMR relaxation spectroscopies over a temperature range from ?180 to 100°C reveals the multiple transitions in atactic 1,2-polybutadienes, which are denoted as T₁₁, α, β, γ, and δ in conformity with reducing temperature. The spectroscopic data obtained show that α transition is the glass transition of the samples, β-and γ-relaxations are likely to be related to the flexibility of the main chain and the side chain motion, respectively, and δ loss process seems to be associated with the defects and inhomogeneity in structure. T₁₁ transition observed in the dielectric relaxation spectra may be manifested by the abrupt broadening of the resonance peaks in solid-state ¹³C-NMR spectra.     

Viscoelastic behaviour of vinylic copolymers of cellulose and their glass transition temperature by DSC

The viscoelastic response of some vinylic copolymers of cellulose prepared with vinyl acetatemethyl acrylate mixtures and with Ce(IV) ion as initiator, and native cellulose, were studied at 110 Hz in a range of temperatures from ?120–100°C. The viscoelastic spectrum of cellulose shows the β-relaxation that is not shown in its vinylic copolymers. We observed the same effect in the dielectric β-relaxation. For the vinylic copolymers of cellulose, one viscoelastic relaxation attributed to the α-relaxation of the grafted vinylic chains is observed. Some differences in the characteristics of this relaxation may be related to the composition of PVA/PMA vinylic side chains and to the ratio of cellulose in the copolymer. The plots of the Argand diagrams give us a better understanding of the viscoelastic behaviour of these materials. The results seem to indicate that the cellulose hinders the large-scale motions of the vinylic chains grafted onto it. The glass transition temperature (T_g) determined by differential scanning calorimetry (DSC) also shows the same fact: the T_g of the vinylic copolymers of cellulose are higher than both the T_g of polyvinyl acetate–polymethyl acrylate copolymers (PVA–PMA) without cellulose and the T_g of some blends of cellulose and the PVA–PMA whose composition was as similar as possible to the cellulosic copolymers. The importance of the covalent bonds between cellulose and the vinylic side chains in the structural transitions are revealed. The present results are compared with the dielectric α-relaxation that we described elsewhere. © 1993 John Wiley & Sons, Inc.     

Characterization of structural heterogeneity of polyurethane coatings

The thermal analysis techniques—Differential Scanning Calorimetry and ThermoStimulated Current—have been used to characterize a polyurethane high solid coating. The glass transition temperature, as determined by DSC, is 60°C. Below this glass transition temperature, an α_sub dielectric relaxation mode has been observed; it corresponds to cooperative movements precursor of the glass transition. The α_ss dielectric relaxation mode, located at ≈ T_g has been attributed to movements of soft sequences of the amorphous phase liberated at the glass transition temperature. The analysis of the fine structure shows that they are constituted of elementary processes characterized by relaxation times following a compensation law. Above T_g, the α_hs dielectric relaxation of hard sequences has been shown. It corresponds to hard sequences hydrogen bonded in polyurethane. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2786–2790, 2001     

Dynamic Mechanical Properties of Carbon Black Filled Ethylene Vinyl Acetate Rubber

Abstract

The dynamic mechanical properties of ethylene vinyl acetate (EVA) rubber filled with different loadings of carbon black and at different degrees of crosslinking were studied over a wide range of temperatures (-150° to +200°C). The loss tangent (tan δ) versus temperature plots indicated presence of different transitions. The α-transition (or the glass-rubber transition) corresponding to the maximum in tan δ value, occurred at ?17°C, which is the principal glass-transition temperature (abbreviated as T _g) of EVA rubber. The γ-transition occurred in the temperature region of ?125° to ?135°C, while the β-transition appeared as a shoulder in the temperature region of ?65° to ?75°C. Besides, there was also a high tempeature transition around +62°C which is known as liquid to liquid transition (T _1.1). Incorporation of carbon black filler did not cause any shift of T _g, while the tan δ peak values at T _g decreased sequentially with increase in filler loading. The γ- and β-relaxations were found to be insensitive to filler loading. The T _1.1 transition, however, was found to be suppressed by incorporation of carbon black filler particularly at high loading. Extent of crosslinking did not influence the T _g But, the T _1.1 transition, which was prominent with the lightly crosslinked system was found to be suppressed at high level of crosslinking. Strain dependent dynamic mechanical properties under isothermal conditions showed that the secondary structure breakdown of carbon black filler under the effect of strain amplitude is influenced by the degree of crosslinking of EVA rubber.     

Preparation and characterization of the copolymer containing N‐pyridyl bi(methacryl)imide unit

The copolymer of methacrylic acid anhydride and N‐2‐pyridyl bi(methacryl)imide was prepared based on the reaction of polymethacrylic acid with 2‐pyridylamine. The molecular structure was characterized by ¹H‐NMR, FTIR, UV–Vis, and circular dichroism techniques. The physical properties of polymethacrylic acid change significantly after an introduction of 6 mol % N‐2‐pyridyl bi(methacryl)imide unit. In particular, the thermal degradation of the polymer was systematically studied in flowing nitrogen and air from room temperature to 800°C by thermogravimetry at a constant heating rate of 10°C/min. In both atmospheres, a four‐stage degradation process of the copolymer of methacrylic acid anhydride and N‐2‐pyridyl bi(methacryl)imide was revealed. The initial thermal degradation temperature T_d, and the first, second, and third temperatures at the maximum weight‐loss rate T_dm1, T_dm2, and T_dm3 all decrease with decreasing sample size or changing testing atmosphere from nitrogen to air, but the fourth temperature at the maximum weight‐loss rate T_dm4 increases. The maximum weight‐loss rate, char yield at elevated temperature, four‐stage decomposition process, and three kinetic parameters of the thermal degradation were discussed in detail. It is suggested that the copolymer of methacrylic acid anhydride and N‐2‐pyridyl bi(methacryl)imide exhibits low thermal stability and multistage degradation characteristics. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1673–1678, 2002     

Physical and relaxation properties of flame-sprayed ethylene-methacrylic acid copolymer

A range of physical and chemical properties of flame-sprayed ethylenemethacrylic acid copolymer (EMAA) were assessed, following different processing conditions. Coatings were produced at a range of specific temperatures by varying the propane flow rate and gun traverse rate. The flame spraying process oxidizes the EMAA copolymer during processing, the extent of oxidation increasing with greater deposition temperatures. Coatings were scanned using dielectric relaxation spectroscopy at a frequency range from 10² to 10⁶ Hz over a temperature interval of -20 to 85°C. The glass transition temperature (usually denoted as the β′ relaxation in this system), is attributed to the microBrownian motion of long chain segments in the amorphous phase and is found to decrease with increasing deposit temperature. The oxidation process appears to reduce the position of the β relaxation due to chain scission. The molecular weight for the EMAA powder was reduced from 22,693 g/mol to 9302 g/mol when deposited at 271°C as shown by gel permeation chromatography. Despite the decrease in chain length, the activation energies for β′ relaxation increased with increasing coating temperatures. This is attributed to the increased polarity of the oxidized coatings resulting in greater intermolecular association, which outweighs the decreased chain length.     

Structures and Binary Mixing Characteristics of Enantiomers of 1-Oleoyl-2,3-dipalmitoyl-sn-glycerol (S-OPP) and 1,2-Dipalmitoyl-3-oleoyl-sn-glycerol (R-PPO)

We performed differential scanning calorimetry (DSC) and synchrotron radiation X-ray diffraction (SR-XRD) experiments of polymorphic structures and binary mixing characteristics of the enantiomers of 1-oleoyl-2,3-dipalmitoyl-sn-glycerol (S-OPP) and 1,2-dipalmitoyl-3-oleoyl-sn-glycerol (R-PPO). In the two enantiomers, oleic and palmitic acid moieties are asymmetrically connected at the sn-1 and sn-3 positions of the glycerol groups, with palmitic acid moiety at the sn-2 position. Pure enantiomer samples (>99 %) were synthesized and employed throughout this study. The following results were obtained. (1) A basic feature of the mixture systems of S-OPP and R-PPO is of a eutectic nature due to different polymorphic structures of two enantiomers and the racemic compound of PPO (rac-PPO). (2) Polymorphic behavior of S-OPP and R-PPO was quite similar, both having α-2 and β′-3, whereas rac-PPO contained α _rac-3, β′_rac-2, and β′_rac-3. The DSC measurements showed that the melting points of β′-3 (S-OPP = 35.3 °C and R-PPO = 34.9 °C) were higher than that of β′_rac-3 (31.0 °C). β was not crystallized in the pure enantiomers, and rac-PPO. (3) α_rac-3 was crystallized at low cooling rates (~2 °C/min), whereas α-2 of the two enantiomers was crystallized only with very rapid cooling (~10 °C/min). (4) Triple-chain-length structures were formed in α_rac-3, β′_S-3 (=β′_R-3), and β′_rac-3; α-2 with a double-chain-length structure was formed in both enantiomers. These results indicate the importance of the relationship between subcell packing and glycerol conformation in the polymorphism and mixing characteristics of asymmetric unsaturated–saturated-saturated mixed-acid triacylglycerols.     

Structure and properties of poly(ethylene terephthalate) crystallized by annealing in the highly oriented state. 3. Dynamic- and static-mechanical properties

Commercial undrawn and cold drawn (5 × ) poly(ethylene terephthalate) (PETP) fibers and bristles have been annealed with fixed ends for 6 h in vacuum at different temperatures between 60 and 260°C. With these samples static- and dynamicmechanical measurements have been carried out. It has been found that the α-and β-processes as well as the moduli depend on the annealing temperature (T_a) in different way, for undrawn and drawn material. The temperature position of the β-peak evaluated from tg δ and loss modulus as well as the step height of α- and β-processes are unsensitive to the T_a for the undrawn material in contrast to the drawn one for which maxima are observed. The appearance of these maxima is explained by the dominating role at the corresponding crystallization temperature of one of the two concurrent processes - crystallization and disorientation, reflected in the change of the effective density of amorphous regions. The dynamic and static measured moduli as well as the stress at break for drawn PETP decrease with the increase of annealing temperature as generally observed. The predominating significance of orientation and the state of amorphous phase in comparison with crystallinity is demonstrated. An extremely high deformation ability at room temperature (up to 200%) of previously drawn and annealed at 255 or 260°C bristles is observed. This originates from the solid state postcondensation and premelting phenomena taking place during annealing in vacuum.     

Environmental effects and relaxation in natural rubber

The change in attenuation of longitudinal ultrasonic waves has been measured as a function of frequency and temperature for natural rubber samples loaded with intermediate super abrasion furnace (ISAF) in increasing quantities and vulcanized with N-oxidiethylene benzotriazol sulfunamid (OBTS) after being subjected to natural aging for 5 years. Three relaxation processes, namely α, β, and γ, were noticed. The activation energy for each process was calculated and the results are discussed. Also, the dielectric constant ε′ and dielectric loss ε″ have been investigated for those samples after being immersed in various water media. The measurements were carried out in the frequency range from 100 Hz up to 100 kHz and at room temperature (～ 28°C). The results are compared with each other and the differences are interpreted.     

Thermal properties and dielectric relaxation of a multi‐component poly(ether‐co‐amide) based on polyamide‐12

DSC, dielectric relaxation and dynamic mechanical thermal analysis (DMTA) were carried out on two multi‐component poly(ether‐co‐amide) samples having different weight ratios of polyamide prepared by condensation polymerization with 12‐aminododecanoic acid, adipic acid and polyetherdiamine consisting of poly(tetramethylene oxide) and poly(propylene oxide). The melting temperature was lowered by an increase in the weight ratio of the polyamide segment. Three relaxation modes, α′, α_s and β, were found from dielectric relaxation measurements in different temperature ranges. The high temperature relaxation mode, α′, has a large dielectric constant, which disappears at the melting temperature of the polyamide crystal in the sample. The relaxation times for the segmental motion, α_s, were different for the samples, which is attributed to the difference in the composition of the uncrystallized polyamide segments in the amorphous domain. The glass transition temperature estimated from DMTA is located between those of constituting polymers. On the other hand, the activation energy of β‐mode observed at low temperatures is the same for samples with different polyamide ratios, which is attributed to the local motion of the polyether segments. The uncrystallized polyamide segments are miscible with the polyether segments, which results in a lowering of the glass transition temperature of the amorphous domain and enlarges the temperature range of the rubber state of the copolymer due to the high melting temperature of the polyamide segments. © 2016 Society of Chemical Industry     

Glas- und tieftemperaturrelaxation von copolyestern der terephthalsäure

The relaxation mechanisms of linear saturated copolyesters of terephthalic acid are influenced by the chemical structure of the used aliphatic, aromatic, and cyclic diols or dicarbonic acids. The glass transition temperatures of the copolyesters are in the range from -30 to 90°C. The numerical value of T_G can be calculated by the increment method with a mean error of ±5°C. Since this method can be applied also to other polymer systems or some other physical parameters a general procedure for optimizing the increments is given. With reference to known relaxation criterions the molecular motions of the copolyesters above and below T_G are identified. The two relaxation mechanisms below T_G show a statistical distribution of the different components of acids and diols along the chain. The γ′-relaxation is caused by orientational motions of carbonyl groups and depends on the chemical structure of the neighbouring segments. On the contrary the γ″-process is not influenced by the molecular environment and is caused by hindered rotation of CH₂-groups.     

Dielectric properties of halar,an alternating copolymer of ethylene and chlorotrifluoroethylene

Dielectric properties of Halar, a predominantly alternating 1 : 1 copolymer of ethylene (E) and chlorotrifluoroethylene (CTFE), have been obtained as a function of temperature (?100 to +175°C), frequency (10²–10⁶ Hz), and thermal history. The dielectric loss index (ε″), related to heat dissipation in electrical applications, shows no major change between 25 and 175°C. However, a significant increase in (ε″) between +25°C and ?100°C is not considered detrimental for the applications. Activation energies (δE) for the three major relaxations (α-, β-, and γ-) in both mechanical and dielectric experiments are similar, thus suggesting a similar phase origin of the molecular relaxations in the two techniques. In dielectric analysis, the lowest temperature γ-relaxation is the strongest while it is the weakest in mechanical technique. It is proposed that, in dielectric experiments, only the CTFE groups participate and due to restricted mobility at low temperatures (i.e., γ-relaxation), the dissipation factor or relaxation strength is maximum.     

Influence of the Strain on Dielectric and Ferroelectric Properties of 0.5BaZr0.2Ti0.8O3–0.5Ba0.7Ca0.3TiO3

0.5BaZr_0.2Ti_0.8O₃‐0.5Ba_0.7Ca_0.3TiO₃ ceramic and its epitaxial films on (0 0 1) SrTiO₃ substrate were prepared to compare their dielectric and ferroelectric properties. The ceramic has a high dielectric permittivity, a weak dielectric relaxation, a low ferroelectric Curie temperature (T_C) of 60°C and a fast polarization relaxation. The films show much lower dielectric permittivities and mild dielectric relaxations. Furthermore, the T_C of film with 40, 100, and 200 nm thickness is 155°C, 110°C, and 60°C, respectively, because the epitaxial strain decreases with the film thickness increasing. The higher the T_C is, the more stable the room‐temperature polarization is.     

A nonlinear regression approach to model the physical aging of poly(ether imide)

Differential scanning calorimetry (DSC) was used to measure the relaxation enthalpy of glassy amorphous poly(ether imide) (PEI) at three high isothermal aging temperatures between 190 and 200°C. A three-parameter model based on the Williams-Watts relaxation function was utilized to describe the aging process of PEI, and nonlinear regression approach was used to estimate the model parameters. The prediction according to this model was found to be more accurate in comparison with the two-parameter model based on the same Williams-Watts relaxation function. Furthermore, the peculiar aging behavior at the highest aging temperature of 200°C was explained from the molecular view point of transition from sub-T_g β-relaxation to α-relaxation near T_g.     

Effect of crystallization temperature on crystal modifications and crystallization kinetics of poly(L‐lactide)

The crystalline structure of poly(L ‐lactide) (PLLA) have been found to quite depend on the crystallization temperatures (T_cs), especially in the range of 100?120°C, which is usually used as the crystallization temperature for the industrial process of PLLA. The analysis of wide‐angle X‐ray diffraction and Fourier transformed infrared spectroscopy revealed that 110°C is a critical temperature for PLLA crystallization. At T_c < 110°C and T_c ≥ 110°C, the α′ and α crystals were mainly produced, respectively. Besides, the structural feature of the α′‐form was illustrated, and it was found that the α′‐form has the larger unit cell dimension than that of the α‐form. Moreover, the crystallization kinetics of the α′ and α crystals are different, resulting in the discontinuousness of the curves of spherulite radius growth rate (G) versus T_c and the half time in the melt‐crystallization (t_1/2) versus T_c investigated by Polarized optical microscope and Differential scanning calorimetry, respectively. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Study of dielectric relaxation of an epoxide oligomer by a direct current transient method

Dielectric α-relaxation of a bisphenol-A type epoxide oligomer has been investigated in the vicinity of the glass transition temperature (T_g) by the direct current (DC) transient method. The logarithm of the DC transient current for the oligomer was well approximated by the third order function of the logarithm of time. The complex dielectric constant was calculated through the Fourier transformation of that approximation function according to Simpson's integration rule in a frequency range of 10^?5 ? 1 Hz. At the temperature around the T_g (45°C), the dielectric α-relaxation process of the oligomer was found to be governed by the Havriliak-Negami equation. The relationship between the DC conductivity (σ) and the dielectric relaxation time (τ), σ·τ^m = const, is valid near and above the T_g of the oligomer. The DC transient current method combined with the DC conduction and the dielectric bridge measurements is considered to be a practical tool for analyzing the dielectric α-relaxation process of the epoxide oligomer over a wide frequency and temperature range.     

Molecular mobility in styrene-co-methacrylic acid random copolymers from 100 to 450 K

Molecular mobility of both the polystyrene homopolymer and a series of styrene-co-meth-acrylic acid copolymers with various amounts of methacrylic acid is analyzed in the tem-perature range from 100 to 450 K, by high-resolution dynamic mechanical spectrometry, IR, and differential scanning calorimetry. Isochronal spectrometry exhibits for polystyrene homopolymer two relaxations, the β and α relaxations, with increasing temperature. Styrene-co-methacrylic acid copolymers exhibit three mechanical relaxations, the γ, β, and α re-laxations, from 100 to 450 K. These relaxations could be related to the progressive occurring of motions of the side groups and of the backbone chains when increasing the temperature. This could be due to the progressive breakdown of hydrogen bonds, which could show a somewhat wide range of magnitude. Furthermore, The decreasing of the magnitude of the α relaxation with increasing methacrylic acid content could suggest the presence of a stable network of chemical cross-links induced by the presence of anydride so that molecular mobility could be partially inhibited above T_g. The remaining of a fluctuation network of hydrogen bonds (labile contacts) above T_g could also emphasize the decrease in molecular mobility. © 1993 John Wiley & Sons, Inc.     

Dielectric relaxations in poly [2,2-propane-bis-(4-phenyl thiocarbonate)]

The dielectric relaxation properties of poly[2,2-propane-bis-(4-phenyl thiocarbonate)] (PTC) have been studied. The existence of crystallinity, which can be eliminated by quenching, is detected. The degree of crystallinity of polymer samples was determined by differential scanning calorimetry in order to investigate the effect of this factor on the dielectric behaviour of this polymer. The thermal degradation of the samples was studied by thermogravimetry. The degradation of the polymer begins before the glass transition temperature T_g. The dielectric spectrum is complex showing several relaxation phenomena. With increasing temperature a γ relaxation can be observed at - 100°C (5 kHz). The activation energy obtained from an Arrhenius plot (lnfvs T^?1) is 6 kcal mol^?1. At 160°C the α relaxation which is associated with the glass transition temperature T_g is detected. The dielectric behaviour of this poly(thiocarbonate) is compared with the corresponding poly(carbonate).     

Molecular mobility in comb‐like copolymethacrylates with chalcone‐containing side‐chains

The molecular mobility of comb‐like copolymers of amyl methacrylate with N‐methacryloyl‐(4‐amino‐4′‐bromochalcone) with various concentrations (20, 30, 40, 50 and 60 mol%) of chalcone‐containing comonomer was studied using dielectric spectroscopy. It was found that chalcone chromophores participate in two forms of molecular mobility: cooperative motion in the rubbery state (δ‐relaxation) and local motion in the glassy state (β‐relaxation). In addition, α‐, β₁‐ and γ‐processes, being related to cooperative segmental mobility, to local motion of ester groups adjacent to the backbone and to local motion of terminal side‐groups, respectively, were evident. The molecular mobility of the β₁‐, β‐ and γ‐processes changed slightly with the concentration of chromophore groups. For the δ‐ and α‐processes, the glass transition temperatures, T_δ and T_g, increased with the molar fraction of chalcone groups, the difference between them remaining nearly constant (ca 55 °C). The T_δ values obtained should be considered as optimal temperatures for the corona‐poling of the chromophore‐containing polymers for the preparation of second‐order nonlinear optical polymer films with non‐centrosymmetric arrangement of chromophore groups. Copyright © 2011 Society of Chemical Industry