CC BY
13Physics of Complex Systems, 2022, vol. 3, no. 2 _www.physcomsys.ru
N Check for updates
UDC 538.9 EDN EAWOSX
https://www.doi.org/10.33910/2687-153X-2022-3-2-55-59
Molecular mobility research in polyethylene composite films
Yu. A. Gorokhovatskiy1, E. Ä. Volgina1, A. N. Ivanova1, D. E. Temnov™
1 Herzen State Pedagogical University of Russia, 48 Moika Emb., Saint Petersburg 191186, Russia
Condensed Matter Physics. Dielectrics
Authors
Yuriy A. Gorokhovatskiy, ORCID: 0000-0001-5085-2525, e-mail: gorokh-yu@yandex.ru Elena A. Volgina, e-mail: Volgina.elena.1999@mail.ru Anna N. Ivanova, e-mail: ivannnik@inbox.ru
Dmitry E. Temnov, ORCID: 0000-0002-9560-4346, e-mail: detem@yandex.ru
For citation: Gorokhovatskiy, Yu. A., Volgina, E. A., Ivanova, A. N., Temnov, D. E. (2022) Molecular mobility research in polyethylene composite films. Physics of Complex Systems, 3 (2), 55-59. https://www.doi.org/10.33910/2687-153X-2022-3-2-55-59. EDN EAWOSX. "
Received 18 March 2022; reviewed 15 April 2022; accepted 20 April 2022.
Funding: This study is part of the State-Commissioned Assignment of the Ministry of Education of Russia, project No. FSZN-2020-0026.
Copyright: © Yu. A. Gorokhovatskiy, E. A. Volgina, A. N. Ivanova, D. E. Temnov (2022). Published by Herzen State Pedagogical University of Russia. Open access under CC BY-NC License 4.0.
Abstract. a-relaxation in composite films of low-density polyethylene with soot particles was investigated through thermostimulated depolarisation and differential scanning calorimetry. It is shown that the joint application of these methods can be used to determine the number of relaxers involved in the relaxation processes of a polymer above its glass transition temperature. The parameters and the number of electrically active defects in composite polyethylene films filled with different percentages of technical carbon are calculated.
Keywords: polyethylene composite films, thermal activation spectroscopy, differential scanning calorimetry, relaxation, electrically active defects
Introduction
High-pressure polyethylene (LDPE) plates of 11503-070 (GOST 16337-77) with a density of 0.92 g/cm3 were used as research objects. The polymer was mixed with the filler on laboratory microrolls at a temperature of 135 ± 5 °C with a mixing time of 3 min. The 0.2 mm thickness plates were prepared according to GOST 12019-66 at a temperature of 170 ± 5 °C with pressure hold time of 5 min.
Technical carbon (soot) was used as a filler. Radiographic investigation of soot particles has shown that they are composed of separate, small-sized, graphite-like crystalline cells. Carbon atom location in the layer is the same as in graphite, while the distance between the layers is greater than in graphite crystals. The locations of the ends of parallel layers on the surface show an increase in energy and, consequently, greater adsorption capacity (Shevchenko 2010).
As shown in (Alekhina et al. 2019; Boriev et al. 2019; Gorokhovatsky et al. 2020), polyethylene films demonstrate multiple relaxations, referred to as a-, p- and y-in order of decreasing temperature of their observation. There is an a relaxation in the area of 60 °C, which is considered the most complex relaxation and involves at least two mechanisms of molecular mobility. One of them is the beginning of rotational movement inside the crystal, and the other is the sliding of grains' borders between layers, i. e., the shift of chains to half of the primitive cell along the molecular axis (Bharadwaj, Boyd 2001).
In the area -60 °C ^ -30 °C, p-relaxation is observed in polyethylene, which is associated with glass transition of the polymer. y-relaxation at a temperature of about -120 °C is usually attributed to
a sufficiently localised molecular motion in the polymer chains (Beatty, Karasz 1979). It should be noted that the intensity of the a transition in polyethylene is usually much lower than the intensity of the transition in the field of glass transition. It was recorded in the temperature range of 10-120 °C, depending on sample preparation and measurement techniques. The temperature Ta, at which the a-transition occurs, correlates with the degree of crystallinity of the sample (Ashcraft, Boyd 1976). However, it was shown that this temperature primarily depends on average crystallite thickness (Alberola et al. 1990; Popli et al. 1984). The dependence of Ta on crystallite thickness is expressed particularly strongly in the range from 50 to 200 °C; with thicknesses exceeding the given one changes in the transition temperature are relatively small (Popli et al. 1984). This relationship is illustrated in Fig. 1.
Fig. 1. The dependence of the temperature of a-relaxation in polyethylene from the average crystallite thickness
(Popli et al. 1984)
In this paper, a-relaxation in low density polyethylene was investigated with thermostimulated depolarisation and differential scanning calorimetry.
Experimental results and discussion
For the study of electrical relaxation processes in polymeric materials, the methods of thermoactivational current or charge spectroscopy are widely used. This choice is explained by high data yield (high sensitivity and resolution) and ease of technical implementation and experimental data processing.
However, in many cases, e. g., near glass transition temperatures and mechanical relaxations of polymers, the use of traditional methods of experimental results processing for thermal activation spectroscopy leads to abnormally high values of activation energy of Ea electrically active defects (EAD) participating in the relaxation process and unrealistic relaxation time values less than 10-20 seconds (Shabanova, Temnov 2021). This is due to the fact that in this case the relaxation process involves the cooperative movement of molecular chain segments, and the precise determination of the activation energy of one relaxer requires taking into account the change in the free volume of the polymer during the relaxation transition.
Yu. A. Gorokhovatskiy, E. Â. Volgina, A. N. Ivanova, D. E. Temnov
There is an empirical analysis method, based on an equation of activated Eyring states, that is commonly used for such situations and that may give more physically realistic EAD parameters (Bucci et al. 1966). In this case, the following equation in used:
( AS^l
[RT ) exp — R J
V
(1)
where k is Boltzmann's constant, h is Plank's constant, and AH, AS are the enthalpy of the activated states end the entropy and free energy, respectively. It follows that the construction of a graph corresponding to equation (1) in Arrhenius coordinates ln(1/rT) ~ 1/T makes it possible to calculate the enthalpy and the entropy, and only then the activation energy:
E„ = AH-TAS
(2)
and t0. Relaxation timesr(T) can be obtained from experimental spectra of TSC-TS (method of fractional purification) using the standard Bucci-Fieschi-Guidi method (Bucci et al. 1966).
Figs. 2-3 show the curves of the TSD in the temperature range of a-relaxation obtained for LDPE samples with 5% and 20% of technical carbon obtained with fractional purification (Shabanova, Temnov 2021).
Fig. 2. TSC-TS curves for LDPE samples with 5% technical carbon
Fig. 3. TSC-TS curves for LDPE samples with 20% technical carbon
The graphs show that as the filler percentage increases, the maximum of the relaxation process shifts to the high temperature area by about 13-15 °C. According to (Popli et al. 1984), this may indicate some increase in the crystallite thickness while increasing the filler concentration.
The activation energy per EAD calculated on the basis of formula (2), taking into account the measurement error, was almost the same and equal to 0.94 ± 0.02 eV. However, the transition enthalpy ДИ for different filler concentrations differs significantly: it was 1.82 eV for 5% filler concentration and 0.87 eV for 20% concentration per relaxer.
Another quite popular method for testing a-transitions in polymers is differential scanning calorimetry. Many researchers investigate phenomena occurring in polymers above the glass transition temperature within the framework of kinetic representations, thermodynamics, fluctuation theory or free volume concept, which are not necessarily mutually exclusive (Berstein, Egorov 1990). With the DSC method, it is possible to determine the glass transition temperature, the width of the glass transition interval, as well as the activation energy of the transitions lying above the glass transition temperature calculated for a mass unit of the polymer.
The comparison of the data obtained for the a-relaxation of polyethylene by TSD and DSC methods makes it possible to determine the number of relaxers involved in this relaxation process.
Fig. 4 shows the DSC heating curves for LDPE samples with different technical carbon content.
In the range of 50-60 °C, a relaxation process is clearly observed, attributed to the a-relaxation of the LDPE. Experimental DSC data processing with dedicated Callisto software makes it possible to determine the change of enthalpy of this transition, indicated in Table 1.
-io H-1-1-1-1-1-1-1-1-1-1-1-1-1-1
0 20 40 50 80 100 120 140
Temperature, °C
Fig. 4. DSC curves for LDPE samples with 5% and 20% of technical carbon Table 1. Values of enthalpy changes for PEVD with different technical carbon content
Soot content (%) Enthalpy change AH (J/g) Number of relaxers (pc/g)
5% -1.713 5.8 x 1018
20% -1.601 1.1 x 1019
Taking into account the data obtained with TSD, the number of relaxers involved in relaxation was also determined. As the filler content increases, the number of relaxers increases significantly, which can be explained by the increase in the polymer's degree of crystallinity when the filler is applied and filler particles become the centres of crystallisation of the polymer matrix. Further research by independent methods is needed to accurately interpret the results.
Yu. A. Gorokhovatskiy, E. Ä. Volgina, A. N. Ivanova, D. E. Temnov
Conclusion
The combined use of thermal activation spectroscopy and thermal analysis methods in polymer research makes it possible to obtain additional information on relaxation processes observed above the glass transition temperature. This paper shows that the increase in the filler content in polyethylene affects the parameters of the relaxation process: as the filler content increases, the crystallite thickness increases and the number of kinetic units involved in the relaxation process increases as well.
Conflict of Interest
The authors declare that there is no conflict of interest, either existing or potential.
References
Alberola, N., Cavaille, J. Y., Perez, J. (1990) Mechanical spectrometry of alpha relaxations of high-density polyethylene. Journal of Polymer Science: Polymer Physics Edition, 28 (4), 569-586. https://doi.org/10.1002/polb.1990.090280410 (In English)
Alekhina, R. A., Lomovskaya, V. A., Simonov-Emelyanov, I. D. et al. (2019) Relaxation and physicomechanical characteristics of polyethylenes with different molecular weights. Fine Chemical Technologies, 14 (6), 104-114. https://doi.org/10.32362/2410-6593-2019-14-6-104-114 (In English) Ashcraft, C. R., Boyd, R. H. (1976) A dielectric study of molecular relaxation in oxidized and chlorinated polyethylenes. Journal of Polymer Science: Polymer Physics Edition, 14 (12), 2153-2193. https://doi.org/10.1002/pol.1976.180141204 (In English)
Beatty, C. L., Karasz, F. E. (1979) The glass transition of linear polyethylene. Journal of Macromolecular Science.
Part C. Polymer Reviews, 17 (1), 37-60. https://doi.org/10.1080/00222357908080904 (In English) Berstein, V. A., Egorov, V. M. (1990) Differentsial'naya skaniruyushchaya kalorimetriya v fizikokhimii polimerov [Differential scanning calorimetry in physicoche of polymers]. Leningrad: "Khimiya" Publ., 256 p. (In Russian) Bharadwaj, R. K., Boyd, R. H. (2001) Conformational dynamics in polyethylene under isochoric conditions: A molecular dynamics simulation study. Journal of Chemical Physics, 114 (11), 5061-5068. https://doi.org/10.1063/L1345878 (In English) Boriev, A. A., Pshikachev, A. G., Thakov, R. B. (2019) Osobennosti dinamicheskikh i mekhanicheskikh svojstv modifitsirovannogo polietilena nizkoj plotnosti [Features of dynamic and mechanical properties of modified low density polyethylene]. Izvestiya Kabardino-Balkarskogogosudarstvennogo universiteta — Proceedings of the Kabardino-Balkarian State University, 9 (4), 34-38. (In Russian) Bucci, C., Fieschi, R., Guidi, G. (1966) Ionic thermocurrents in dielectrics. Physical Review, 148 (2), article 816.
https://doi.org/10.1103/PhysRev.148.816 (In English) Gorokhovatsky, Yu. A., Demidova, N. S., Temnov, D. E. (2020) Electric charge relaxation in the polyethylene with mineral inclusions of diatomite. St. Petersburg Polytechnic University Journal — Physics and Mathematics, 13 (2), 9-16. https://doi.org/10.18721/JPM.13201 (In English) Popli, R., Glotin, M., Mandelkern, L., Benson, R. S. (1984) Dynamic mechanical studies of a and p relaxations of polyethylenes. Journal of Polymer Science: Polymer Physics Edition, 22 (3), 407-448. https://doi.org/10.1002/ pol.1984.180220306 (In English) Shabanova, N. S., Temnov, D. E. (2021) Analysis of the TSD spectra of polymers near the glass transition temperature using the fractional purification method. Physics of Complex Systems, 2 (4), 157-164. https://doi.org/10.33910/2687-153X-2021-2-4-157-164 (In English) Shevchenko, V. G. (2010) Osnovy fiziki polimernykh kompozitsionnykh materialov [Basics of physics of polymer composite materials]. Moscow: Moscow State University Publ.,98 p. (In Russian)
