Методом свободно затухающих крутильных колебаний получены и проанализированы спектры внутреннего трения в системе хитозан поливиниловый спирт в широком температурном интервале. По температурно-частотным зависимостям, определены времена релаксации в областях релаксационных переходов. Установлено, что полученный в виде пленки композитный материал хитозан поливиниловый спирт, имеющий соотношение компонентов 1:1, представляет собой однородную (дисперсную) систему, в которой выявлено заметное влияние поливинилового спирта на релаксационные характеристики хитозана. By the method of freely damping torsional vibrations, the internal friction spectra were obtained and analyzed in the chitosan polyvinyl alcohol system over a wide temperature range. From the temperaturefrequency dependences, relaxation times in the regions of relaxation transitions are determined. It was found that the composite material obtained in the form of a fi lm, chitosan polyvinyl alcohol, having a component ratio of 1:1, is a homogeneous (dispersed) system, in which a noticeable effect of polyvinyl alcohol on the relaxation characteristics of chitosan was revealed.

A procedure for the quantitative characterization of complex stressed states and their effect on the relaxation properties of polymer materials has been proposed. Throughout the temperature interval where the thermal degradation of polymers can be ignored, three measurements runs for recording internal friction spectra Δ(T) are performed. As has been shown for cellulose diacetate and poly(vinylformals), a difference between attenuation decrements Δ1(T)–Δ2(T) for the initial samples and the same samples in the second measurement cycle reflects the presence of cryptoheterogeneity. This difference achieves its maximum values at temperatures close to the glass transition temperature T α but becomes noticeable even at temperatures several tens of degrees lower than T α.

The relaxation transitions in plasticized PMMA (T g = 75°C) were studied by the method of mechanical spectroscopy in the temperature range from -200 to +300°C. At the equivalent frequency ν eq , the mechanical loss maxima correspond to the strength maxima which were estimated in tensile experiments performed on a tensile machine at the loading rate w. This fact agrees with the concepts concerning the effect of mechanical (hysteresis) losses on the strength of polymers. Upon transition from the brittle to quasi-brittle stale with the corresponding relaxation γ e -transition related to the mobility of a side ester group, one may observe an abrupt decrease in strength from 150 to 30 MPa at the brittleness temperature T br . In this case, the stress concentration coefficient at microcrack tip decreases by a factor of three. The main reason responsible for the strength decrease is provided by transition from the grouped to individual elementary event of chain rupture at the microcrack tip. At temperatures below the dynamic glass transition temperature Ta coinciding with the strength α-maximum, one may observe the region of the quasi-brittle state confined by the brittleness temperature T br and the quasi-brittleness temperature T qbr . At temperatures above Ta in the region of rubber elasticity, the strength maxima coincide with the high-temperature relaxation λ-transitions related to the breakdown of a molecular network, and fracture is provided by overcoming the intermolecular forces rather than by the rupture of chemical bonds in chains. Hence, in the fracture at temperatures above T α , a key role belongs to the relaxation processes and mechanical losses. The α-relaxation maximum is related not to mechanical losses but to molecular orientation, that is, the tensile drawing to 250% and a transition of the polymer to a high-strength oriented state. The effect of plasticizing agent on the strength and relaxation properties of PMMA is limited by three relaxation transitions, namely, γ e -, β, and a which correspond to the strength maxima at the brittleness temperature T br , the quasi-brittleness temperature T qbr . and the dynamic glass transition temperature T α which depends on the loading frequency.

According to the data of dynamic mechanical spectroscopy, a glassy PVC experiences six short-scale relaxation transitions which are related to the torsional mobility of chain CH2 groups (beta (CH2) - and beta'(CH2)-transitions) and polar CHCl groups (beta (Cl)- and beta'(Cl)-transitions) and to the dissociation of dipole-dipole CCl . . . CCl bonds between chains which serve as the pointlike junctions of a molecular network (pi (Cl)- and pi'(Cl)- transitions). As was assumed, the splitting of the transitions into two close transitions is provided by the presence of two types of microstructures in the PVC chains, isotactic and syndiotactic fragments which affect both the dipole moments of CHCl groups and intermolecular interactions. According to the data of dielectric relaxation, one may observe a single well-pronounced beta (Cl)-transition instead of the six short-scale relaxation transitions. In this case, the data of mechanical and dielectric relaxation fit a single frequency-temperature curve for the beta (Cl)-relaxation. The other transitions are less pronounced due either to the absence of electric dipole moments in the groups (a CH2 group) or to a low concentration of the corresponding polar groups (CHCl groups in the beta'(Cl) -transition) or to a low dielectric activity of the junctions of a molecular network (pi (Cl)- and pi'(Cl) -transitions). For the alpha -relaxation and high-temperature large-scale lambda -transitions, the data of mechanical and dielectric relaxation coincide and are well described by similar frequency-temperature dependences. The dielectric activity of the alpha- and lambda -processes may be explained by the fact that the polar CHCl groups are always involved in both free segments participating in the alpha -relaxation and bound segments participating in the lambda -relaxation. The microtacticity of the polymer chains of PVC is shown to exert a marked effect on alpha -relaxation and glass transition temperature and, probably, on the lambda -relaxation processes.

Three types of inorganic glasses with different polymeric structures-three-dimensional network structure (silica glass), two-dimensional branched structure (vitreous B2O3), and linear structure of macromolecules (vitreous selenium)-exhibit three different atomic-molecular mechanisms of the glass transition. These mechanisms are matched by different numerical values of the preexponential factor B-alpha in the Boltzmann-Arrhenius-type equation for the relaxation time tau(alpha). The B-alpha factor is invariant for glasses of the same structural type, for example, silica and alkali silicate glasses with the silicon-oxygen network structure. The difference in the B-alpha value between three types of glasses is associated with the difference in the kinetic units participating in the alpha-relaxation processes. The more complex and bulky the kinetic unit, the larger the B-alpha factor.

A notion of two mechanisms of the stress influence on the fracture and durability of thin polymeric films and fibers with discrete structures, free of microcracks prior to loading, is formulated. The loading leads to the appearance of microcracks in the weak (amorphous) microscopic regions of the discrete structure. The microcracks have various dimensions, multiple of the size of the microscopic regions in the discrete structure. This results in the formation of a discrete spectrum of strength and durability. The stress does not affect the discrete dimensions of the microcracks, but determines the distribution of samples over the strength levels and, hence, on the average strength and durability values. An increase in the tensile stress leads to a decrease in the average durability as a result of the redistribution of samples between the strength levels. Redistribution of the microcracks with various lengths over the various strength levels under the action of stress and, accordingly, a decrease in the strength and durability, is related to an increase in the number of low-strength samples under the action of stress, rather than to a change in the levels of strength and durability. The thermofluctuation mechanism dominates in massive polymers and thick films, in which the discrete spectrum of strength levels is represented by a single lowest level.

A high-frequency (10(8)-10(10) Hz) relaxation transition was studied by the method of Brillouin scattering in polyurethane elastomers with various chemical compositions (characterized by the ratio r = = [NCO] : [OH]). This transition can be reliably interpreted as the beta(CH2)-relaxation related to the rotational mobility of CH2 groups in the polymer backbone. Both transitions are described by the same straight line on a frequency versus temperature diagram, corresponding to an activation energy of 32-38 kJ/mol. There are other weak transitions in polyurethane, among which the most intense is that related to the rotational mobility of C6H4 phenylene groups in the backbone. This process is characteristic of the aromatic linear polymers with phenylene groups in the backbone, and has an activation energy of 50-60 kJ/mole. As the ratio r varies from 0 to 1, the frequency of the network of hydrogen bonds increases. This frequency growth, while significantly affecting the alpha-relaxation (glass transition), only weakly influences the small-scale beta(CH2)- and beta(C6H4)-relaxation processes.

Linear polymers rarely have repeating units with only one fragment with inner rotation of the C-C bond in the main chain and, correspondingly, only one process of [beta]-relaxation associated with inner rotation, When the repeating units have two or more different fragments with inner rotation, several beta-relaxation transitions of this type are, observed. A marked contribution to segmental mobility and chain flexibility is provided by a low-temperature process for which, in the high-frequency region, two situations are observed, The first situation takes place when the alpha- and beta-relaxation transitions approach each other but do not coincide, In the second case, at a certain high frequency, the beta-relaxation lime becomes equal to the alpha-relaxation time, and both processes do not exist separately, With decreasing temperature, at a certain point D, this coupled alpha beta-process splits, and the beta-relaxation temperature is tower than the alpha-relaxation temperature. Other types of the beta-relaxation transitions observed at lower frequencies behave in different way, Depending on the frequency, they occur at temperatures either below or above the alpha-relaxation temperature, and the corresponding frequency-temperature diagrams show the intersection points. This behavior is observed independently of the presence of side groups in polymer chains. When a linear polymer has side groups, they affect the rate of both alpha- and beta-relaxation transitions of the above-mentioned type.

Rotational mobility of two fragments (CH 2 and C(CH 3 )COOCH 3 ) about the main-valence chain axis in PMMA leads to the appearance of the β CH2 - and β-relaxation transitions, the segmental mobility, and the α-relaxation process. The low-temperature β CH2 -relaxation produces the main effect on the segmental mobility and α-relaxation in PMMA. A comparatively high-temperature process of β-relaxation is observed both below and above the α-transition temperature. The frequency-temperature curves for the β- and α-relaxations have a point of intersection that separates the frequency range into two intervals. In the high-frequency region, β-relaxation takes place above T α and does not affect the α-relaxation. In the low-frequency region, β-relaxation precedes the α-relaxation and influences the α-transition (the glass transition temperature T g decreases by 10 K). In low-molecular-mass glasses with rigid molecules, the α-relaxation is not preceded by the β-relaxation, since the latter process is absent.

Rotational mobility of two fragments (CH2 and C(CH3)COOH3) about the main-valence chain axis in PMMA leads to the appearance of the beta CH2-and beta-relaxation transitions, the segmental mobility, and the alpha-relaxation process. The low-temperature beta CH2-relaxation produces the main effect on the segmental mobility and alpha-relaxation in PMMA. A comparatively high-temperature process of beta-relaxation is observed both below and above the alpha-transition temperature. The frequency-temperature curves for the beta- and alpha-relaxations have a point of intersection that separates the frequency range into two intervals. In the high-frequency region, beta-relaxation takes place above T and does not affect the alpha-relaxation. In the low-frequency region, beta-relaxation precedes the alpha-relaxation and influences the alpha-transition (the glass transition temperature T-g decreases by 10 K). In low-molecular glasses with rigid molecules, the alpha-relaxation is not preceded by the beta-relaxation, since the latter process is absent.

Linear polymers (PAN, PMMA, polychloroprene, PS, etc.) with polar side groups exhibit a system of side groups, which can be divided into two subsystems. One of these comprises free side groups participating in the process of gamma-relaxation related to the rotational motion of the groups about the axis perpendicular to the polymeric chain. The other subsystem represents the side groups linked with local dipole-dipole bonds forming physical junctions of the molecular network. The latter polymers exhibit, in addition to the gamma-relaxation, a pi-type relaxation related to the destruction of the physical junctions. The activation energy for the pi-relaxation depends on the dipole moment. The greater the dipole moment of the monomer unit, the higher is the activation energy. The extrapolation of the activation energy to the zero dipole moment yields the threshold activation energy U-0 = 53.5 kJ/mol. The interaction of side groups in nonpolar polymers also leads to the formation of local physical junctions of the van der Waals type, and gives rise to the mu-relaxation with an activation energy below the above threshold value. In a branched nonpolar PB with CH=CH2 side groups, the mu(PB)-relaxation has an activation energy of U-mu = 51.5 kJ/mol, which is close to the threshold.

Acta PolymericaVolume 42, Issue 1 p. 48-49 Short Communication The effect of molecular orientation on relaxation transitions in polyamide 6 S. V. Baglyuk, S. V. Baglyuk Kiev State Pedagogienl Institute „A. M. Gorki”︁, Department of General Physics, Pirogova 9, Kiev-30, USSRSearch for more papers by this authorA. G. Barteneva, A. G. Barteneva Kiev State Pedagogienl Institute „A. M. Gorki”︁, Department of General Physics, Pirogova 9, Kiev-30, USSRSearch for more papers by this author S. V. Baglyuk, S. V. Baglyuk Kiev State Pedagogienl Institute „A. M. Gorki”︁, Department of General Physics, Pirogova 9, Kiev-30, USSRSearch for more papers by this authorA. G. Barteneva, A. G. Barteneva Kiev State Pedagogienl Institute „A. M. Gorki”︁, Department of General Physics, Pirogova 9, Kiev-30, USSRSearch for more papers by this author First published: January 1991 https://doi.org/10.1002/actp.1991.010420115AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume42, Issue1January 1991Pages 48-49 RelatedInformation

In spite of differences in the chemical structure of crystalline polymers containing methylene groups in their chain backbone, the molecular mobility of CH2 groups determines the appearance of the basic discrete spectrum of relaxation times (groups of β-, α- and λ-transitions). With the exception of PA-6, the activation energies and temperatures of transitions of these groups of relaxation processes in PE, PP, POM and PEO are practically identical. Some relaxation transitions in these polymers are specific because of the presence of oxygen and nitrogen atoms in their chains, leading to polar or hydrogen bonds connecting the polymer chains.

The nature of the β- and α-groups of the relaxational transitions in crystalline polymers of the caprone, PE and PP type is similar and associated with the molecular motions of the CH2 groups in the amorphous phase in the glassy state (β- and β1-transitions), segmental motion (α-, α1- and α2-transitions) and also with the mobility of the CH2 groups in the crystalline phase (β2-transition). Specific to caprone are the μ- and π-transitions associated with the breakdown of the local intermolecular bonds formed by the hydrogen and dipole-dipole bonds. Molecular orientation weakly influences the low temperature processes of relaxation (β, β1, μ and π) but significantly the segmental relaxation processes (α, α1 and α2). Dissolved water strongly influences the intensity of the maximum of the μ transition and also the temperature position of the α group transitions.

It has been shown from a comparison of the temperature dependence of the tensile strength and the internal friction spectrum of Capron fibre over a wide temperature range (−200 to +225°C) that the maxima on the temperature dependence of the strength correspond to various relaxation transitions in the Capron fibre but the strength maxima are displaced towards lower temperatures relative to the mechanical-loss maxima. The reasons for this lie in the different time scales of the tests and in the non-linearity of the relaxation processes at the high stresses characteristic of tests to measure the strength of a material.