Thermal Properties of Polymers at Low Temperatures

Abstract

The existing measurements and theories of the low-temperature thermal properties, heat capacity, and thermal conductivity of polymers are reviewed with particular attention paid to the differences between partly crystalline and amorphous polymers. The most striking feature of the low-temperature heat capacity of polymers is that in the liquid helium temperature range the heat capacity does not depend upon the cube of the temperature as for other solids. Further, only well below 1°K does the heat capacity approach the value predicted on the basis of the sound velocity. This behavior indicates the presence of a small number of low-frequency modes of vibration in the frequency spectrum. The fact that such anomalous behavior seems linearly related to the crystallinity implies that this behavior is associated with the amorphous structure, perhaps with motions of pendent groups within cavities formed in the amorphous structure. The thermal conductivity of semicrystalline and amorphous polymers differs considerably. Semicrystalline polymers display a temperature dependence of the thermal conductivity similar to that obtained from highly imperfect crystals, the thermal conductivity having a maximum in the temperature range near 100°K which moves to lower temperatures and higher thermal conductivities as the crystallinity is increased. Amorphous polymers display a temperature dependence similar to that obtained for glasses with no maximum but a significant plateau region in the range between 5 and 15°K. The theoretical interpretation of the thermal conductivity of these materials is considered.     

DSC studies of phase transitions in poly(diethylsiloxane)

The differential scanning calorimetry studies have shown that high-molecular linear poly(diethylsiloxane) can exist in two high-temperature polymorphs which melt at 280 and 290 K. The heats of fusion of the high-temperature polymorphs are 17 and 21 J/g, respectively. Each of the high-temperature forms arises from the corresponding low-temperature form the corresponding low-temperature form when the polymer is heated: the first at 214 K (transition heat is 28 J/g) and the second at 206 K (transition heat is 26 J/g). The mesophase formed from the molten high-temperature crystalline phases melts in a rather broad temperature range of 290 to 327 K, and the heat of this transition is 2.7 J/g. Crystallization of poly(diethylsiloxane) from the mesomorphic and the supercooled amorphous state is different. In the first case, apparently, the whole mesophase is converted to the crystalline phase and the samples have a crystallinity near 1. In the second case the crystallinity is only ca. 0.3. The temperature range in which the mesophase melts depends on the molecular weight of the polymer, presence of crosslinks and the conditions under which it has been formed, e.g., temperature.     

Nuclear spin relaxation in poly(diethylsiloxane)

Earlier work showed that heating causes poly(diethylsiloxane) to undergo a first-order transition from a semicrystalline solid to a more mobile viscous—crystalline material. The latter is composed of two phases and analogies between polymer and liquid crystal morphology and behavior have been made. The viscous—crystalline phase in PDES appears to be unique since the literature is devoid of other documented examples. In this study, spin—lattice and spin—spin relaxation times were measured over a wide temperature range. They show a glass transition at 138°K, a crystal—crystal transition at 206°K, and a transition around 250°K which results from translational motion of the polymer chains with respect to each other. This motion is observed in the amorphous phase at a lower temperature than in the crystalline phase. Translational motion in the crystalline phase is observed on melting of the crystallites. The spin—spin data permitted monitoring of the molecular motions in each phase and the data suggest that these phases exert some influence on the molecular motions of each other. The viscous—crystalline phase in PDES may represent a unique model for studying and understanding “precrystalline” behavior and structure in amorphous solids.     

Wide-line <Superscript>1</Superscript>H NMR study of 4′-(pentyloxy)-4-biphenylcarbonitrile in different phase states

A sample of 4′-(pentyloxy)-4-biphenylcarbonitrile was studied in the temperature range 20–70°C by wide-line ¹H NMR. The line shapes, half-widths, and second moments were determined. At 20°C, the spectrum is a broad structureless line, which was split into three components at temperatures above 47°C. This pattern persisted up to 65°C, with the components becoming narrower. Above 68°C, the spectrum rapidly collapses to give a structureless narrow line with a width δH < 0.01 G. Analysis of the spectra and comparison with theoretical calculations made it possible to elucidate the transition of the sample from the crystalline to the nematic and isotropic phases, as well as to determine the temperature range of the stability of the liquid-crystalline phase and the role of rotational mobility of protons of different functional groups and sometimes of these groups as a whole in the mechanisms of phase transitions.     

Effect of the Degree of Crystallinity on Charge Relaxation in a Polyimide

The method of thermoactivation spectroscopy is used to study effect of the degree of crystallinity on the mechanisms of homocharge relaxation in poly((4,4'-bis(4-aminophenoxy)biphenyl)imido-1,3- bis(3,3'-4,4'-dicarboxyphenoxy)benzene films. Two regions of charge relaxation are found. At a temperature of 177°С (450 K), there is a maximum of the thermally stimulated depolarization current related to the intrinsic conductivity of the dielectric. In samples with degrees of crystallinity of 20 and 40%, a low-temperature maximum is observed at 50°С (323 K), which is related to the release of charge carriers from traps formed at the interface of amorphous and crystalline phases.     

Numerical analysis of EPR spectra of coal, macerals and extraction products

The multi-component structure of the EPR spectra of Polish medium metamorphosed coal, its macerals and their pyridine extractrion products were analyzed. The bes numerical approximation of the EPR spectra for exinite, vitrinite and their extraction residues was obtained when two Lorentz and one Gauss curve were chosen for calculation. The EPR spectra of inertinite and its residue are superposition of two Lorentz lines. The EPR spectra of the extracts are fitted by single Lorentz lines. Four groups of paramagnetic centres which give the EPR signals with different ranges of linewidths are present in the macerals and the extraction products. Paramagnetic centres with the narrow EPR lines are present not only in inertinites, but also in exinites and vitrinites. Each of the four types of paramagnetic centres is present in macromolecular and molecular part of the macerals. As was expected, four component lines were detected for the EPR spectra of the coal and its residue. Paramagnetic centres responsible for the broad lines are located on structures consisting of a few aromatic rings. Paramagnetic centres of large aromatic structures are responsible for the narrow lines.     

Structural origin of the cryogenic relaxations in poly(ethylene terephthalate)

The effect of molecular organization (crystallinity, orientation) on the internal friction of poly(ethylene terephthalate) was studied by means of dynamic mechanical measurements at temperatures from 300 to 4.2°K, with a free-oscillating torsion pendulum at 1 Hz. It was found that crystallinity decreases the intensity of the composite γ relaxation at 210°K and gives rise to an additional loss maximum ε at 26°K. Uniaxial orientation broadens the γ relaxation and gives rise to an additional loss peak δ, at 46°K. The δ and ε losses are dependent on molecular organization, occurring only in samples containing aligned, taut chain segments and crystalline structures, respectively. They have a common activation energy of 4 kcal/mole. All three low-temperature relaxations in oriented specimens show pronounced directional anisotropy, which, in the γ loss, may be due to the preferred orientation of noncrystalline chain segments, while in the δ and ε losses, may be associated with the direction of defect structures. On the basis of the observed behavior of the δ and ε relaxations it is suggested that they may involve motions of defect structures and may thus participate in stress-transfer mechanisms at large deformations.     

Microwave saturation of EPR spectra of oxidised coal

Microwave saturation of multi-component EPR spectra of oxidized lignite Mequinenza (Spain) with a carbon content of 65.1 wt % and with a high sulphur content of 10.3 wt % was studied. The coal was oxidized with nitric acid (NHO₃), peroxyacetic acid (PAA), and in O₂/Na₂CO₃ system. Three different groups of paramagnetic centres exist in the coal samples analyzed. The EPR spectrum of the demineralised coal was a superposition of broad Gauss (ΔB _pp = 0.75 mT), broad Lorentz 1 (ΔB _pp = 0.42 mT) and narrow Lorentz 3 lines (ΔB _pp = 0.08 mT). The three EPR components with linewidths: 0.58–0.77 mT (Gauss line), 0.30–0.39 mT (Lorentz 1 line) and 0.05–0.06 mT (Lorentz 3 line) were recorded for the oxidized coal. The g-values were obtained for the samples studied in the ranges 2.0043–2.0046 (Gauss lines), 2.0035–2.0038 (Lorentz 1 lines) and 2.0032–2.0034 (Lorentz 3 lines). The broad Gauss and Lorentz 1 lines saturate at low microwave powers. The narrow Lorentz 3 lines of demineralised coal were not saturated at microwave power from the range considered. After the coal oxidation with HNO₃, PAA and in O₂/Na₂CO₃ system, the microwave saturation of the narrow Lorentz 3 lines was also observed, which indicated a degradation of the multi-ring aromatic structures upon oxidation.      

Amorphous polyacrylonitrile: Synthesis and characterization

Amorphous polyacrylonitrile was successfully synthesized with bis(pentamethyleneimino)magnesium in heptane at 70°C. The amorphousness of the polymer increased with rising polymerization temperature and was favored by the nonpolar solvent. The polymer showed regular head-to-tail sequences which were confirmed by converting the polymer into polyacrylic acid and polymethylacrylate. The amorphous PAN produced a broad x-ray diagram with a maximum at 2θ = 16.1° and a less intense halo at 2θ = 27.5°. This pattern did not change after heat treatment. The synthesis of amorphous PAN strongly supports Imai's hypothesis that polyacrylonitrile consists of paracrystalline and amorphous phases. The amorphous PAN also tends to support Minami's assignment of the two absorptions in the temperature dependence of the dynamic loss tangent; the absorption at the lower temperature (110°C) is due to molecular motions in the paracrystalline phase and the absorption at the higher temperature (160°C) is attributed to the molecular motions in the amorphous region.     

Dielectric relaxation and molecular motion in poly(vinylidene fluoride)

The dielectric properties of poly(vinylidene fluoride) have been studied in the frequency range 10 Hz to 100 kHz at temperatures between ?196 and 150°C. Three dielectric relaxations were observed: the α relaxation occurred near 130°C, the β near 0°C, and the γ near ?30°C at 100 kHz. In the α relaxation the magnitude of loss peak and the relaxation times increased not only with increasing lamellar thickness, but also with decrease of crystal defects in the crystalline regions. In the light of the above results, the α relaxation was attributed to the molecular motion in the crystalline regions which was related to the lamellar thickness and crystal defects in the crystalline phase. In the β relaxation, the magnitude of the loss peak increased with the amount of amorphous material. The relaxation times were independent of the crystal structure and the degree of crystallinity, but increased slightly with orientation of the molecular chains by drawing. The β relaxation was ascribed to the micro-Brownian motions of main chains in the amorphous regions. The Arrhenius plots were of the so-called WLF type, and the “freezing point” of the molecular motion was about ?80°C. The Cole-Cole distribution parameter of the relaxation time α increased almost linearly with decreasing temperature in the temperature range of the experiment. The γ relaxation was attributed to local molecular motions in the amorphous regions.     

Lactone polymers. I. Glass transition temperature of poly-ε-caprolactone by means on compatible polymer mixtures

Dynamic mechanical properties determined with a torsion pendulum were used to ascertain the glass transition temperature T_g of poly-ε-caprolactone. By measurements on compatible blends of poly-ε-caprolactone and poly(vinyl chloride), the T_g of amorphous poly-ε-caprolactone was shown to be 202°K at about 1 cps. This is 16°K lower than the T_g of annealed, crystalline polymer. The blend transition data were well fitted by both the Fox and the Gordon-Taylor expressions. The Fox expression was also used to describe the decrease from 233°K of the secondary low-temperature relaxation due to poly(vinyl chloride) by assuming the low temperature relaxation of poly-ε-caprolactone, 138°K, was responsible for the decrease in the blends. The 138°K relaxation due to poly-ε-caprolactone was decreased when more than 50% poly(vinyl chloride) was present.     

A 13C NMR study of the effect of γ-irradiation on the chain dynamics of poly(ethylene oxide)

A comparison of solid-state ¹³C nuclear magnetic resonance (NMR) spectra of virgin and vacuum γ-irradiated poly (ethylene oxide) (PEO) evidences marked differences. The unirradiated PEO shows a well-resolved amorphous resonance and a weak, broad envelope of crystalline resonances, while the irradiated PEO presents well-resolved resonances for both the crystalline and amorphous carbons. Upon recrystallization from the melt both PEO samples yield solid-state ¹³C NMR spectra that are closely similar to that of the virgin, unheated sample. Observation of both melt-recrystallized samples at ?60°C yields similar spectra with well-resolved crystalline resonances. Crosslinking is the predominant chemical change occurring during the γ-irradiation of PEO under vacuum and produces a change in the motional character of the crystalline phase. This change is not the result of a reduction in crystallinity as evidenced by differential scanning calorimetry (DSC) observations. The most probable explanation is that the crosslinks are concentrated at the surface of the crystalline lamellae with a resultant change in the low frequency molecular motions of the crystalline chains. This motional change shifts the T_1p^H such that the crystalline carbon nuclei can now be cross-polarized at room temperature and the resonance linewidth is reduced. Following melting and recrystallization the motional characteristics of the irradiated PEO are nearly identical to those of the unirradiated sample, probably as a result of a redistribution of the crosslinks throughout the amorphous phase during recrystallization.     

A 15N nuclear magnetic resonance study of the base-catalyzed NH2 exchange reactions of acetamide and thioacetamide

The base-catalyzed ? NH₂ exchange reactions of acetamide and thioacetamide were studied by ¹⁵N nuclear magnetic resonance spectroscopy by the use of line-shape analysis. The ¹⁵N NMR spectra of these primary amides at intermediate exchange rates were broad doublets, which indicated that the two amide protons were exchanging at different rates. The line-shape analysis indicated that the ratio of exchange rates was 6±1 for acetamide and 3±1 for thioacetamide.     

用X-射线衍射法测定聚对苯二甲酸乙二醇酯/聚对苯二甲酸丁二醇酯共混体系的结晶度

Based on Hermans and Weidinger postulate that the crystalline fraction is proportional to crystalline intensity Ic and that the amorphous fraction is proportional to amorphous intensity Ia, it is. Suggseteet that for PBT/PET blends we assume that the intensity at 18. 2° is completely originated from amorphous scattering. The separation between the peaks at 17. 2° and 20. 8° is 3. 6°m, which is regarded as large enough to justify this assumption. Thus , it is possible to draw an acceptable demaraction line between the crystalline and amorphous scattering. The degree of crystallinity Xc is given by X_c-I_c/(I_c+KI_a),coefficient K is available by the values of Ic plotted against the values of Ia.For PBT/PET blends,we obtain that K is 0.95 and that correlation coefficient is 0.92 According to the method revieved above,we investi-gated the crystallinity of PBT/PET blends prepared by different ways.     

Effect of orientation,anisotropy, and water on the relaxation behavior of nylon 6 from 4.2 to 300°K

The relaxation behavior of nylon 6 from 4.2 to 300°K was investigated as a function of orientation, anisotropy and moisture content by using an inverted free-oscillating torsion pendulum. Three new relaxations, δ at 53°K, ? below 4.2°K, and ζ at 20°K, were discovered. The characteristics of these new relaxations strongly depend on the orientation anisotropy, and concentration of adsorbed water in the specimens. The results suggest that the mechanism of the γ process is associated with the motions of both the polar and methylene units. The mechanism of the β relaxation is postulated to originate with motions of both non-hydrogen-bonded polar groups and polymer—water complex units. The behavior of the α peak is consistent with the hypothesis that it originates with the rupture of interchain hydrogen bonding due to the motions of long-chain segments in the amorphous regions. Finally, the data strongly support the proposition that two types of water, tightly bound and loosely bound, exist in nylon 6.     

Effects of swelling and annealing on the viscoelastic behavior of solution-crystallized and melt-crystallized samples of fractionated polypropylene

Dynamic viscoelastic measurements, E′ and E″, were carried out on solution-crystallized and melt-crystallized samples of fractionated isotactic polypropylene over the temperature range of ?100°C to 150°C. The molecular weight ranged from 1.26 × 10⁴ to 1.77 × 10⁵. The effects of swelling and annealing on the α and β peaks were more pronounced for the lower molecular weight fraction than for the higher one. It was found that both the untreated solution-crystallized and quenched melt-crystallized samples contain a fair amounts of a constrained amorphous phase in which the molecular motions were so depressed that the corresponding peak could be observed as the low-temperature component of the α peak. These constraints on the molecular motions are considered to originate from the spatial restrictions imposed by the presence of the surrounding crystallities.     

Crosslinking of polyaromatic ether-ketones with biphenylene derivatives

Polyaromatic ether-ketones prepared from isophthaloyl dichloride, diphenyl ether, and various amounts of biphenylene and its derivative, were compressed to films under various conditions, and their crosslinking behaviors were studied. Although strong and flexible films could be obtained from low melting crosslinkable polymers, the low molecular weight polymers, which melt below 300°C, decompose at the biphenylene ring opening temperature of over 290°C, giving films full of gas bubbles. The addition of metal complexes to decrease the biphenylene ring opening temperature did not improve the film quality, although gas bubbles could be avoided. The strong and flexible films were amorphous, and the brittle films were mixtures of crystalline and amorphous polymers.     

Effect of amorphous sequences on the longitudinal acoustic modes in partially crystalline polymers. I. Transfer matrix method

A novel theoretical scheme is developed which enables the determination of the LAM-like vibrations of polymer chains made up of crystalline and amorphous parts as they occur in partially crystalline structures. The boundary conditions effective at the junction points are formulated in terms of the compliances of the associated amorphous sequences. These compliances can be derived from their eigenfrequencies and eigenvectors in a disconnected state. The treatment uses a matrix formalism which can be extended to include bending and torsional motions in a general state of vibration of the crystalline stem. A first numerical example demonstrates that the LA mode of a crystalline stem can be strongly perturbed by the coupling to the adjacent amorphous sequences. Interpretation of frequencies and line shapes of observed LA modes should always include these coupling effects; their neglect can lead to considerable errors.     

Structure of native and regenerated cellulose as revealed by proton NMR analysis

Differences in fiber structure between cotton and cuprammonium rayon are studied by a refined broad-line proton NMR analysis of samples swollen with deuterated dimethyl sulfoxide, which has no effect on the spectra but enhances differences in molecular mobility between crystalline and noncrystalline regions. The spectra obtained are decomposed into four components: broad, medium, narrow, and extremely narrow. These components are identified as contributions, respectively, from crystalline and rigid noncrystalline (frozen glassy) material, a noncrystalline glassy component exhibiting local segmental motion, a noncrystalline rubbery component exhibiting liquidlike molecular motion, and protons included in DMSO-d₆ as an impurity. The mass fraction of the narrow component in cotton was about 0.01, whereas it was as high as 0.18 in cuprammonium rayon. It is concluded that even in the swollen state, native cellulose is devoid of a liquidlike mobile component, but regenerated cellulose contains a considerable amount of a noncrystalline component involving liquidlike segmental motion of molecules.     

Effects of swelling and annealing on the viscoelastic behavior of the melt-crystallized and solution-crystallized polypropylene

The effects of swelling and annealing treatments on viscoelastic behavior were studied in melt-crystallized and solution-crystallized samples of isotactic polypropylene (iso-PP) over the temperature range ?150 to 150°C. The log E″ versus T curves exhibited α, β, and γ peaks in order of decreasing temperature. The β peak of the melt-crystallized sample shifted to higher temperatures after annealing, but was not affected by swelling. The α peak of melt-crystallized polymer was affected by swelling treatments. It increased in height and shifted to lower temperatures almost linearly with the volume fraction of absorbed solvent. The magnitude of the shift was independent of the solvent species—toluene, p-xylene, tetralin, carbon tetrachloride—however, it depended significantly on the temperature at which the sample had been heat treated. For solution-crystallized polymer, no peaks in log E″ were observed in the temperature range of the β peak of melt-crystallized material, but the α peak appeared larger and broader, and at higher temperature than the corresponding peak in the melt-crystallized polymer. After swelling or annealing, the low-temperature component of the α peak of the solution-crystallized sample decreased in height and at the same time a new loss peak appeared at ?55 and 0°C, respectively, is swollen and annealed samples. In particular, in the case of annealing treatments, the high-temperature component of the α peak shifted to still higher temperatures. From these results on the solution-crystallized sample it can be deduced that the segmental motions in the amorphous phase are very strongly constrained by surrounding crystalline phases as compared with those in the amorphous phase of the melt-crystallized sample, and the constraints imposed on the segmental motions are released to a great extent by both treatments. Finally, swelling effects on the γ peak were examined. The γ peak of the melt-crystallized sample decreased in height after swelling. On the other hand, the γ peak of the solution-crystallized sample separated into two peaks, which might be attributed to the mechanical relaxations in the crystalline and amorphous phases.