Phase diagram of the low-density polyethylene – dimethyl terephthalate system: A new topology
Introduction
An interest in phase equilibria and thermodynamics in mixtures of semicrystalline polymers with low molecular mass (LMM) components arose in the middle of XX century. As a result, a number of papers addressing this topic were published, just a few to mention [[1], [2], [3], [4], [5]]. It was concluded [6] that such systems obey the Gibbs phase rule so that there is no fundamental difference between such systems and mixtures of two crystallizable LMM components. Regarding topology, all phase diagrams were reduced to two types that represent semicrystalline polymer mixtures with thermodynamically “good” and “poor” solvents, respectively, as schematically shown in Fig. 1a and Fig. 1b.
Later Smith and Pennings [7] studied high density polyethylene (HDPE) mixtures with 1,2,4,5-tetrachlorobenzene and hexamethylbenzene and supplemented the above classification with one more diagram type plotted in Fig. 1c. It describes phase equilibria between a semicrystalline polymer and a crystalline LMM component with close melting temperatures.
As seen from Fig. 1a, the phase diagram semicrystalline polymer – good solvent contains only one boundary curve AB, which is called the polymer liquidus line. In the case of poor solvent (Fig. 1b), the diagram contains the polymer liquidus line, CD, and a binodal of the liquid-liquid equilibrium, ABC. The diagram in Fig. 1c includes two liquidus lines of the polymeric and LMM components, AB and BC, intersecting at the eutectic point D, and the solidus line, DBE.
In 1985 Hagström [8] investigated phase equilibria in HDPE – dimethyl terephthalate (DMT) mixtures by differential scanning calorimetry (DSC). His diagram contains the polymer liquidus line, the solidus line with an eutectic point at the polymer mass fraction w2 ≈ 0.85, and the LMM component liquidus line with a horizontal segment in the range 0 < w2 < 0.7 corresponding to the DMT melting temperature. Above this segment, there should be two coexisting liquid phases that can become miscible at a higher temperature, but this domain of the diagram was not explored. Note that the possibility of a liquid-liquid binodal above the LMM component liquidus line was predicted in ref. [9] in terms of the Flory theory. Therefore, an existence of binary systems with phase diagrams combining features of the second and third diagram types is quite expectable.
In this study, we use an original optical method to construct such diagram for a system never before studied, low-density polyethylene (LDPE) – DMT. Aside from the liquid-liquid coexistence curve and component liquidus lines, it contains a solubility curve of DMT in the amorphous regions of LDPE. The importance of the latter curve was revealed about ten years ago, when we elaborated a new concept [10] that treats semicrystalline polymers as single-phase microheterogeneous liquids, in which crystallites act as crosslinks of an intermacromolecular network. With this approach, we demonstrated [[11], [12], [13], [14], [15], [16], [17], [18], [19], [20]] that the solubility curve is essential for all three types of the phase diagrams depicted in Fig. 1, since it reflects the transformation of an initial two-phase system semicrystalline polymer – LMM component into a single-phase solution of the LMM component in the polymer in the course of slow heating. With the solubility curve at the phase diagram, one can also develop a physically clear scenario of phase structure evolution in the course of cooling down the homogenized mixtures of different compositions undergoing thermally induced phase separation (TIPS). We interpret the experimental data by optical and scanning electron microscopy, DSC, and nitrogen adsorption-desorption method in order to predict the main features of capillary-porous bodies formed via TIPS in the LDPE – DMT mixtures of various compositions.
Section snippets
Experimental
Low-density polyethylene (PE 15803-020, Tomskneftekhim, Russia) with a molar mass Mn = 26 kg⋅ mol-1 and dispersity Đ = 4.4 determined by GPC, flow index of 1.32 ± 0.05 g⋅(10 min)-1 measured at 190 °C under a load of 2.16 kg (DIN EN ISO 1133:2005) and crystallinity degree of 42% found by X-ray diffraction was used [17]. Its density at 25 °C, ρ = 0.915 ± 0.003 g⋅ cm-3, and melting (full amorphization) temperature, Tm = 111.6 ± 0.4 °C, were obtained from the kinetic experiments on hydrostatic
Stepwise heating of heterogeneous systems
Phase diagram constructed by the optical method is shown in Fig. 2 along with optical images illustrating the events that take place when LDPE – DMT systems of different compositions undergo stepwise heating.
The mixture with w2 = 0.95 is two-phase at room temperature (point K). It consists of an LDPE pellet and DMT crystals (image 1 in Fig. 2). Heating of this mixture results in an apparent decrease in the visible amount of DMT, growth of the polymer pellet and lowering its opalescence
Conclusions
In this study, we constructed an experimental phase diagram of the LDPE – DMT mixture using the original optical method and DSC. Compared to the HDPE – DMT diagram described in the literature [8], our diagram contains fragments of the liquid-liquid equilibrium binodal, curves EG and HF in Fig. 2, and the solubility curve BD, which describes the dependence of the DMT solubility in the amorphous regions of LDPE. The new phase diagram combines the features of two previously known diagram types
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
CRediT authorship contribution statement
Andrey V. Basko: Investigation, Visualization, Writing - original draft. Konstantin V. Pochivalov: Conceptualization, Methodology, Supervision. Alexander V. Bazanov: Investigation, Formal analysis. Georgiy A. Shandryuk: Investigation, Formal analysis. Alexander A. Ezhov: Visualization, Validation, Writing - original draft. Vladimir V. Artemov: Investigation, Formal analysis. Yaroslav V. Kudryavtsev: Project administration, Funding acquisition, Writing - review & editing.
Acknowledgements
This study was supported by RFBR (project 19-33-50015). V.V.A. acknowledges support from the State Program of FSRC “Crystallography and Photonics”. DSC measurements were performed using the equipment of Shared Research Center of TIPS RAS. SEM measurements were performed using the equipment of Shared Research Center of FSRC “Crystallography and Photonics” RAS supported by the Ministry of Science and Higher Education of Russia (project RFMEFI62119X0035).
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