Thermally stimulated discharge current studies on the effect of thermal treatment on the strength of polycarbonate

It is well known that polycarbonate annealed at 80–130°C undergoes gradual changes in mechanical properties. Annealing below T_g (ca. 150°C) results in a decrease in impact resistance and an increase in strength. Polycarbonate has three single relaxation processes and some distributed relaxation processes in the temperature range between 100 and 250°K (the β transition region). The effect of thermal pretreatment on the relaxation has been investigated by the thermally stimulated discharge current technique. Partial heating, peak cleaning, and theoretical fitting have also been performed and the activation parameters associated with the relaxation processes have also been calculated to assist in the analysis of the relationship between effects of annealing and structural motions in polycarbonate.     

Relaxation Processes in Polymer Surface Layers

A recently built and patented device Nanoluminograph is used for the investigation of the relaxation processes in the surface layers of HDPE films, produced at various conditions. The work of the device is based on the phenomenon of radiothermoluminescence. The use of high-frequency low-power, low-temperature plasma as an ionizing source, and a reduction of the consumed energy down to several orders of magnitude (as compared to that used in similar devices) provides excitation of a surface layer as thin as 100–200 nm. The high sensitivity of the device enables one to reduce the excitation time to 1–2 seconds for obtaining a sufficient intensity of glow curves even from ultrathin layers 20–30 nm thick. All of that minimizes the modifying plasma action on the samples under investigation and provides well reproducible and reliable data. It is found that the intensity, number and positions of peaks on the glow curves are strongly influenced by the preparation conditions of polymer films. The complicated profile of glow curve peaks allows one to assume the overlapping of multiple relaxation processes. Decomposing and fitting peak profiles with the help of a PEAKFIT computer program result in revealing at least 4 relaxation transitions in the temperature region from 109 to 213 K. The temperatures and activation energies of relaxation transitions in surface layers appeared to be lower than those inferred from the DSC data for the bulk polymer. The activation energies of trap depletion upon heating are calculated. The nature of traps is discussed in terms of molecular conformations, morphology and structural defects, as well as the attribution of the observed relaxation transitions to defreezing mobility of different kinetic units.     

Effect of doping on thermoluminescence in polymers. II. Polycarbonate doped with triphenyl methane and xanthene dyes

The thermoluminescence (TL) of polycarbonate doped with triphenyl methane and xanthene dyes was measured following irradiation with ultraviolet light at 77 K and the effect exercised by systematically varying excitation wavelength, irradiation dose, and dopant concentration were investigated. The TL glow curves are characterized by two distinct peaks that can be attributed to genuine emissions of the dopant-polymer systems and involve continuous distributions of apparent activation energies as determined by partial heating techniques. The low-temperature peak extends over the temperature interval corresponding approximately to the complex β relaxation of the polymeric units (short-range chain motions), but no obvious correlation between the two phenomena can be evidenced because the TL peak is never structured and the values and type of evolution of apparent activation energies as a function of temperature are clearly different from those observed for the β relaxation. In this temperature range the TL properties are consistent with an ionization of the dye followed by a recombination process involving electron tunneling from trap to luminescence center rather than thermal excitation. The second TL peak appears in a unusually high temperature region (> 250 K) where no intrinsic relaxation of the polymeric matrix is known to occur. This is not to say that the chain motions cannot contribute to some extent to the detrapping processes. Generally speaking, however, the TL results show that interpreting glow curves only in terms of relaxation processes should be done with great caution and that in certain cases an observed emission must be considered as specific to the dopant-polymer system rather than to the polymer itself.     

The effect of annealing on the structure and relaxation processes of vinyl alcohol–ethylene copolymers

An annealing process has been applied to three samples of vinyl alcohol–ethylene (VAE) copolymers, richer in the former comonomer. The effect of such a process on the structure and on the relaxation mechanisms is studied. The structure of the three VAE copolymers has changed slightly. Nevertheless, the viscoelastic relaxation processes have been significantly affected for the thermal treatment. Two additional relaxations have appeared: one of them at temperatures above the relaxation associated to the glass transition, and the other at temperatures below the β mechanism of these copolymers. The temperature location, intensity, and apparent activation energy of the distinct relaxations found are discussed and compared with those in the original copolymers and the homopolymers, poly(vinyl alcohol) and polyethylene. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 1–12, 2001     

Pressure‐temperature study of dielectric relaxation of a polyurethane elastomer

The effect of hydrostatic pressure up to 1,361 atms on the dielectric properties of a segmented polyurethane elastomer (Dow 2103‐80AE) is studied at temperatures from 0°C to 80°C. The experimental results show that the relaxation time for both the I–process, associated with the molecular motions in the hard segments, and the α–process, associated with the glass transition, increases with pressure, and this shift is more pronounced for the I–process. Besides the glass transition, it is found that the I–process can be described by the Vogel–Fulcher (V–F) and Williams–Landel–Ferry (WLF) relations. At atmospheric pressure, T_g and T₀ for the I–process are 235.9 K and 4.2 × 10³ K, respectively. Based on the V–F and WLF relations and experimental results, it is found that a parameter, C₁, in the WLF relation is independent of the pressure. Thus, a method is introduced to determine the values of both the characteristic transition temperature (T_g) and activation energy (T₀) for the processes at different pressures. As the pressure increases from atmospheric to 1,361 atms, the increase of T_g for the I–process is about 30°C. The results also show that, for both the I– and the α–processes, T₀ decreases with increasing pressure. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 983–990, 1999     

Investigations on electric properties of polymers—VI: Thermally stimulated depolarization currents from poly(1-vinylnaphthalene)

The thermally stimulated depolarization currents (TSDC) from poly(1-vinyl naphthalene) were studied over the temperature range 220–420°K. Four relaxation peaks were observed. The first three peaks (β₁, β₂ and β₃) appeared below the glass transition temperature of the polymer. The β₁ peaks seems to arise from a single dipolar relaxation process; β₂, and probably β₃, arise from a dipolar relaxation distributed in activation energy. The α peak could be regarded as a result of simultaneous contributions of the dipolar relaxation and conduction processes. On the basis of published work, the molecular origins of the β peaks are suggested.     

Thermoluminescence from photoexcited polyethylene terephthalate

Thermoluminescence from polyethylene terephthalate (PET) has been investigated. A correlation was found between thermoluminescence (TL) and thermally stimulated current (TSC). The apparent activation energy was estimated at 0.23–0.50 eV for both TSC and TL from ?170 to 0°C. This activation energy presumably indicates the trap depth, which is decreased by molecular motions, since both TSC and TL are quenched efficiently with visible light, but not with infrared light of energy of the magnitude of thermal activation energy. The spectrum of TL glow curves agrees with the photoluminescence spectrum at ?185°C, which is composed of an excimer and a monomer fluorescence and also a structured phosphorescence at wavelengths longer than 400 nm.     

Thermoluminescence of irradiated polystyrene

Thermoluminescence of irradiated polystyrene has been studied in the temperature range 100 to 440°K. Three glow peaks with maximum at 160, 221, and 378°K have been observed. These peaks are analyzed by different methods and the activation energies which were obtained are compared. The activation energies are found to be 0.22, 0.48, and 1.45 eV for the peaks with maxima at 160, 221, and 378°K, respectively. Second-order kinetics is appropriate to all these cases. The glow peaks are attributed to the decay of the free radicals formed on irradiation and subsequent thermal stimulation. The peak with the maximum at 160°K is attributed to electron trapping by the carbonyl groups or peroxy radicals formed on irradiation. The curve with the peak at 221°K is attributed to the cyclohexadienyl radical, and the curve with the peak at 378°K is attributed to the chain radical ? CH₂? C (C₆H₅)? CH₂? . The centers responsible for the observed thermoluminescence are identified by correlation with electron spin resonance (ESR) data obtained on the same samples.     

Solid-state relaxations in poly(ethylene oxide). II. Study using the spin-labeling technique

Poly(ethylene oxide) (PEO) was spin-labeled with 3-chlorocarbonyl-2,2,5,5-tetramethylpyrroline-1-oxyl. Correlation times were calculated from 173 to 375°K. For estimating slow-motion rotational correlation times three models were used: Brownian diffusion, “moderate jump” diffusion, and “large jump” diffusion. Analysis of the changes in the extrema separation and spectra shape with temperature strongly suggests the presence of β and γ processes in solid poly(ethylene oxide). The Brownian diffusion model gives, for the γ relaxation, an activation energy of 8 kcal mole^?1 which is in good agreement with that found using dielectric relaxation measurements.     

The effect of pressure on the volume and the dielectric relaxation of linear polyethylene

The effects of pressure on the α (ca. 70°C, 1 kHz) and γ (ca. ?100°C, 1 kHz) relaxations of linear polyethylene were studied dielectrically between 0 and 4 kbar. Equation of state (PVT) data were also determined in the range of interest of these relaxations. The sample was rendered dielectrically active through oxidation (0.8 C?0 per 1000 CH₂). The α process (which occurs in the crystalline fraction) could be studied over a much wider temperature range than heretofore possible due to the effect of pressure in increasing the melting point. Examination of relaxation strength from 50 to 150°C showed that there must be two crystalline relaxation processes: the well-known α relaxation plus a competing one. The α relaxation is believed to be due to a chain twist–rotation–translation mechanism that results in rotation–translation of an entire chain in the crystal. The relaxation strength of the α process decreases and therefore indicates the presence of a second (faster and not directly observed) process that increases in intensity with increasing temperature. It is postulated that the second process is due to motion of defects that become more numerous through thermal injection at higher temperatures. Analysis of the relaxation data along with the PVT data allowed the constant volume activation energy for the α relaxation to be determined. It was found to be 19.4 ± 0.5 kcal/mole. The constant volume activation energy is important in modeling calculations of the crystal motions and is significantly smaller than the atmospheric constant pressure activation energy of 24.9 kcal/mole. The effect of pressure on the activation parameters and shape of the γ process was also determined. There has been controversy over whether the γ process occurs only in the amorphous fraction or in both the amorphous and crystalline phases. Since the two phases have quite different compressibilities, increasing the pressure should change the shape of the loss curves (versus frequency and temperature) if the process occurs in both phases. The shape (but not location) of the loss curves was found to be remarkably independent of pressure. This finding strengthens the view that the γ process is entirely amorphous in origin.     

Dielectric and thermal characterization of low density ethylene/10‐undecen‐1‐ol copolymers prepared with nickel catalysts

Ethylene and 10‐undecen‐1‐ol copolymers, prepared using a nickel complex as catalyst, were studied using differential scanning calorimetry (DSC), X‐ray diffraction, and dielectric relaxation spectroscopy. The behavior exhibited by copolymers containing incorporated 10‐undecen‐1‐ol amounts within 0.5 and 4.6 mol % was compared with neat polyethylene. DSC revealed that a new crystalline region with lower thickness lamellae emerges in copolymers due to the side‐chains crystallization. Nevertheless, the global crystallization degree decreases due to the loss of crystallinity that occurs in a greater extent in PE‐like regions. Dielectric relaxation spectroscopy detected two processes, a low activation energy process below ?20 °C related with localized mobility increasing in intensity and deviating to higher temperatures with the increase in 10‐undecen‐1‐ol amount, and a high activation energy process ascribed to the glass transition, located at higher temperatures for the different copolymers relatively to neat polyethylene. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2802–2812, 2007     

Mechanical loss processes in polysaccharides

Dynamic mechanical properties of cellophane, amylose, and dextran have been obtained over the temperature range 100–520°K and frequency range 10^?2 to 10⁺² Hz on specimens containing various amounts of water. Four mechanical transitions have been characterized. At about 180°K, there is a γ transition that has been assigned to rotation of methylol groups; no comparable transition was found to exist in dextran. At about 240°K, there is a β transition that has been assigned to rotation of methylol–water complexes, but the β transition in dextran appears to be due to some other kind of motion. In cellophane at about 450°K there is an α₂ transition which appears to have contributions from motion of chain segments in disordered regions. The α₁ transition for cellophane occurs at temperatures too high to measure and may be due to segmental motions in chains within crystalline regions. Dextran and amylose were found to have at these same temperatures α loss processes that probably correspond to glass–rubber transitions in amorphous material. The changes in these mechanical loss mechanisms due to moisture uptake suggest that sorbed water associates with glucose repeat units in ways ranging from those which stiffen molecular structure to those which allow greater freedom for other types of motion to occur.     

Simulated glass transition in free‐standing thin polystyrene films

In this article, we investigate the glass transition in polystyrene melts and free‐standing ultra‐thin films by means of large‐scale computer simulations. The transition temperatures are obtained from static (density) and dynamic (diffusion and orientational relaxation) measurements. As it turns out, the glass transition temperature of a 3 nm thin film is ～60 °K lower than that of the bulk. Local orientational mobility of the phenyl bonds is studied with the help of Legendre polynomials of the second‐order P₂(t). The α and β relaxation times are obtained from the spectral density of P₂(t). Our simulations reveal that interfaces affect α and β‐relaxation processes differently. The β relaxation rate is faster in the center of the film than near a free surface; for the α relaxation rate, an opposite trend is observed. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1160–1167, 2010     

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     

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.     

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 loss of polyethylene below 4.2°K arising from proton tunneling

The dielectric loss of high-density polyethylene (HDPE: melt-crystallized films and single-crystal mats), low-density polyethylene (LDPE), and copolymers of ethylene and vinyl alcohol (PEVA) was measured at 1.5 to 4.2°K in the frequency range from 10 Hz to 10 kHz. Results for HDPE show dispersion curves almost corresponding to a single relaxation and are interpreted in terms of phonon-assisted tunneling of the protons of hydroxyl groups which are accidentally attached to tertiary carbons (carbons with a short branch). Dispersion curves for LDPE and PEVA are quite broad, indicating a wide distribution of relaxation times. The loss level of PEVA passes through a maximum at 7.5% vinyl alcohol, suggesting that interaction between a couple of neighboring hydroxyl groups depresses the loss. A potential calculation for the rotation of a hydroxyl group in the orthorhombic lattice of polyethylene yielded a double-minimum potential for the tunneling when the lattice is assumed to have a distortion which is acceptable from x-ray analysis. The potential quantitatively explains the observed values of relaxation time and relaxation strength of HDPE. The concentration of hydroxyl groups in HDPE, which varies among the samples, is estimated to be of the order of 10¹⁶ cm^?3.     

β Relaxations in polyethylenes and their anisotropy

Measurements have been made of the anisotropy of viscoelastic behavior in specially oriented sheets of low-density polyethylene. The results for the cold-drawn sheets show a β relaxation process of very characteristic anisotropy. The annealed sheets show two relaxations in this region of temperature. The lower relaxation (about ?10°C) is identified as an interlamellar shear process. The higher relaxation (about 70°C) has a very similar anisotropy to the β relaxation in cold-drawn samples. Isotropic sheets of low-density polyethylene have been also investigated. Two β relaxations are found in these materials.     

The crystalline character of atactic poly(p-biphenyl acrylate) and poly(p-cyclohexylphenyl acrylate) in their dynamic mechanical response

Dynamic mechanical properties have been determined in atactic poly(p-biphenyl acrylate) (PPBA) and poly(p-cyclohexylphenyl acrylate) (PPCPA) in the temperature range from 80 to 540°K at frequencies in the range 10³–10⁴ Hz. The general behavior of the dynamic elastic modulus as a function of temperature shows a transition region from the glassy state at about 390°K for both polymers, a plastic region extending over a temperature interval of about 100°K, and another transition to the melt situated at 540 and 480°K for PPBA and PPCPA, respectively. The experimental data show that the mechanical behavior of both polymers strongly resembles that of crystalline polymers. The loss spectrum of PPBA shows the presence of several important maxima: one corresponding to the melting point, characterized by a very rapid increase of losses with increasing temperature (α′ relaxation), one in the glass-temperature range, characterized by a rather broad peak (α′ relaxation), and others below T_g, associated with secondary relaxation effects. The analysis of the different transitions and relaxations indicates that some of these processes can be ascribed to motions taking place in the ordered regions of the polymer. PPCPA shows a similar loss pattern; however, owing to the lower melting point the α maximum is partially submerged in the α′ relaxation associated with the melting process. Of particular interest is the γ process in the glassy state of this polymer, caused by the chair–chair transition of the cyclohexyl rings. The limited intensity of this relaxation as compared with that of most polymers containing cyclohexyl side groups, has been interpreted as due to the high ΔF associated with such a transition for cyclohexyl rings linked to phenylene groups. This leads to some interesting conclusions about the conformation of the side groups in PPCPA.     

Low-temperature mechanical relaxations in polymers containing aromatic groups

Studies have been made of the secondary relaxation processes in the solid state of a number of polymers containing aromatic groups in the polymer chain. The polymers investigated include one, polystyrene, with the aromatic group in side-chain positions, and six high polymers in which phenylene rings lie in the main backbone chain. In polystyrene, wagging and torsional motions of the side chain phenyl rings give rise to a low-temperature δ-relaxation which is centered at 33°K (1.7 Hz) and which has an activation energy of about 2.3 kcal/mol. Most of the polymers with phenylene rings in the main chain exhibit a low-temperature relaxation in the temperature region from 100°–200°K. This relaxation process is centered at 159°K (0.54 Hz) in poly-p-xylylene, at 162°K (0.67 Hz) in polysulfone, and at 165°K (1.24 Hz) in poly(diancarbonate). In poly(2,6-dimethyl-p-phenylene oxide), two overlapping low-temperature relaxations are found, one in the range 125–140°K and the other near 277°K (ca. 1 Hz). The low-temperature secondary relaxation process in all of these polymers is believed to be associated with local reorientational motion of the phenylene, or substituted phenylene, rings or with combined motion of the phenylene rings and nearby chain units. For these low temperature relaxation processes, the activation energy is about 10 kcal/-mole. The temperature location of the relaxation appears to depend on the specific units to which the phenylene rings are attached and on steric and polar effects caused by substituents on the ring. In the poly-p-xylylenes the relaxation is shifted to much higher temperatures by a single Cl substitution on the ring but remains at essentially the same temperature position when dichlorosubstitution is made. The effects of water on the magnitude and temperature location of the observed low temperature relaxations, as well as the implications of the study for other polymers containing aromatic groups in their backbone chains, are discussed.