The Thermal Processes within Polyethylene in Mixtures with a High Content of Inorganic Components after Plastic Deformation under High Pressure

Abstract

Mixtures of polyethylene and 80% germanium dioxide, magnesium, magnesium oxide, and sodium chloride were subjected to plastic deformation under a pressure of 0.5–4.0 GPa, and were then investigated by differential scanning calorimetry. The enthalpy of melting of the polymer in certain mixtures reached 300 J/g. On thermograms of deformed mixtures, exothermic processes were observed. The observed thermal effects are possibly due to interphase interaction at the phase boundary.

Keywords

polyethylene heterogeneous mixtures high pressure plastic strains melting crystallisation

As a result of plastic deformation under high pressure, carried out on apparatus of the Bridgman anvil type, the crystal structure of polyethylene (PE), according to X-ray diffraction analysis, passes into a near X-ray amorphous state. However, on the diffraction patterns of specimens there is no amorphous halo. This indicates that the structure of the polymer is in an ultradispersed state, where the size of the ordered regions does not exceed a few nanometres.

The heating of such specimens at 30°C for 15 min leads to the appearance on the X-ray patterns of lines of monoclinic structure, while after heating to 100°C the crystal structure of the polymer is fully recovered.

Zhorin et al. [1] established by differential scanning calorimetry (DSC) that on thermograms of PE specimens deformed under a pressure of 2 GPa at temperatures ranging from room temperature to 90–100°C there were weak (8–10 J/g) endothermic peaks which were related to the melting of superfine crystallites formed during plastic deformation under pressure.

Plastic deformation leads to the breakdown of polymer chains, which becomes more intense with increasing degree of deformation and pressure of working.

Compression of polymers on metal anvils leads to intensification of the process of electron injection from the anvil metal into the polymer being worked, while breakdown of the polymer chains during deformation is accompanied with electron emission. Electrons in the polymer can be entrapped by structural defects. Thus, in the process of plastic deformation under high pressure, a system of trapped electrons is formed in the polymer. When the pressure is removed, some of the electrons escape from the specimens, but some remain in the structural traps [2].

In this way, recovery of the structure of the polymer after plastic deformation occurs in the presence of charges, which can polarise fragments of the macromolecules and thus affect the relaxation processes during heating.

The high-pressure cell in which deformation of the specimens is carried out comprises a capacitor – a dielectric layer is placed between two metal anvils. The charge states arising in this capacitor should depend on the state of the plates – whether they are in the closed state (in the present case the anvils are grounded) or in the open state (in the present case the anvils are insulated from the mass of the press). By varying in this way the electrophysical conditions during the working of specimens under pressure, it is possible to influence charge state formation in the specimens.

It is well known that electrical double layers are formed at the boundary of different phases. It can be expected that the different charges arising on opposite surfaces will also be able to have an influence both on the formation of the supermolecular structure of the polymer during deformation and on the relaxation processes during heating.

It was of interest to carry out a DSC investigation of deformed PE in mixtures with different components, and to investigate the influence of the electrophysical conditions under which the pressure working is carried out on the thermal processes taking place in the polymer during heating.

Experimental

Low-density polyethylene (LDPE) (GOST 16303–80), high-density polyethylene (HDPE), and nascent supermolecular polyethylene (NSMPE) were subjected to plastic deformation under a pressure of 0.5–4.0 GPa. The deformation of specimens was conducted on anvils of hardened steel with a diameter of the working surfaces of 15 and 20 mm at room temperature. The edge zone of specimens was chosen for analysis. DSC analysis was conducted on a DSM-2M calorimeter at a heating rate of 16 deg/min. PE mixtures with inorganic components (germanium dioxide GeO₂, magnesium Mg, magnesium oxide MgO, sodium chloride NaCl) were prepared in a mortar. To insulate the anvils from the mass of the press, the bearing surfaces were lined with oiled paper of 0.5–1 mm thickness.

Results and Discussion

On the thermograms of the initial polymers there were solitary endothermic peaks of melting with maxima at 108, 132, and 142°C and with enthalpies of melting of 110, 180, and 160 J/g, respectively, for LDPE, HDPE, and NSMPE.

The pressure working of all selected polymers on grounded anvils did not alter the appearance of the thermograms but led to a reduction in the temperature of the melting peak maximum by 1–2 deg. The pressure working of LDPE and HDPE on insulated anvils did not alter the overall appearance of the thermograms; in the case of NSMPE the deformation of specimens under a pressure of 1.5 GPa led to the appearance on the thermograms of a shoulder on the low-temperature side of the melting peak, which in specimens worked at pressures of 2 GPa and above was transformed into a peak with a maximum 11–12 deg lower than the main melting peak ( Figure 1 ). On the thermograms of cooling of such specimens there was a solitary exothermic peak corresponding to crystallisation of the polymer and coinciding in characteristics to the corresponding peak in the initial polymer. When NSMPE specimens worked at pressures of 2 GPa and above were reheated, only a solitary melting peak with a maximum at 140°C was present on the thermograms.

Figure 1

Thermograms of specimens of NSMPE: 1 – initial; 2 – deformed under a pressure of 1.5 GPa; 3 – deformed under a pressure of 2.3 and 4 GPa. Beneath upward arrow to left of figure: endo; Horizontal axis: T, °C

The enthalpies of melting were calculated on the basis of the thermograms of polymers deformed under different pressures. The pressure dependences of the enthalpies of melting presented in Figures 2a, b, and c for different polymers are extremal in form. The maximum values of the enthalpies of melting for specimens worked on insulated anvils always lie at a higher pressure than those for specimens worked on grounded anvils. The maximum enthalpies for the grounded and insulated variants of pressure working for each of the chosen polymers differed little. The enthalpies of melting of LDPE and HDPE worked under a pressure of 2 GPa and above practically coincided with the enthalpies of the initial polymers, while the enthalpy of NSMPE was 18–20% higher than in the initial polymer.

The thermograms of specimens worked under pressure both on grounded and on insulated anvils in the case of reheating coincided with the thermograms of the initial polymers.

The maximum increase in the enthalpy of melting for LDPE amounted to 60%, for HDPE to 35%, and for NSMPE to 80%. Elsewhere [3–5], in investigations of specimens of oriented PE, increase in T_m by 4–8 deg was established, while the enthalpy of melting was 10–20% higher than in the initial polymer. Such changes were attributed to the emergence of stressed macromolecules in the amorphous phase, which increased the heat content of the amorphous regions.

For PE with 100% crystallinity, the enthalpy of melting would amount to 290 J/g. Calculation of the degree of crystallinity in deformed polymers from the maximum values of the enthalpies of melting gives a degree of crystallinity of 57% for LDPE, a degree of crystallinity of 86% for HDPE, and a degree of crystallinity close to 100% for NSMPE.

The enthalpy of melting reflects the energy of intermolecular interaction. In the initial polymer specimens the intermolecular interaction is determined by the energy of van der Waals interaction. In specimens deformed under high pressure, the charges trapped by structural defects can polarise fragments of macromolecules, creating conditions for the emergence of polarisation bonds, which will lead to an increase in the overall energy of intermolecular interaction. During heating, structural defects with trapped charges escape from the polymer, and intermolecular interaction will again be determined by the energy of van der Waals interaction.

It is known that T_g and T_m increase in polymers under high pressure at a rate of 130–170 deg/GPa. In the case of PE, this means that the deformation of specimens under a pressure of 0.5 GPa occurs in the temperature range within which the process of devitrification of the polymer takes place, while at pressures of 1 GPa and above the glassy polymer undergoes plastic deformation. It is possible that the supermolecular structure of polymers that is formed during deformation in the temperature range of glass transition differs from the structure formed under a pressure at which the amorphous phase of the polymer finds itself in the glassy state. It is possible that the extremal pressure dependences of the enthalpy of melting are related to the different conditions of formation of the supermolecular structure of the polymers.

In this way, after deformation under a certain pressure, an increase in the enthalpy of melting by 30–80% was recorded in the polymers. Working on insulated anvils hardly affected the enthalpy of melting, but it increased the pressure at which the enthalpy reached its maximum values. In NSMPE specimens deformed on insulated anvils, melting was described by a bimodal peak, which indicates the formation of a two-phase structure. From the results given in Figure 2 it can be seen that the pressures at which the enthalpies of melting in LDPE reach maximum values after deformation in grounded and insulated variants amount to 1.0 and 1.5 GPa respectively; in NSMPE both these pressures are lower and amount to 0.5 and 1.0 GPa; at the same time, in HDPE the pressure of deformation in the grounded variant is the same as in NSMPE (0.5 GPa), and in the insulated variant the same as in LDPE (1.5 GPa).

Figure 2

The enthalpies of melting of (a) LDPE, (b) HDPE, and (c) NSMPE as a function of the pressure in the grounded (1) and insulated (2) variants of deformation. Vertical axes: ΔH, J/g; Horizontal axes: P, GPa

Elsewhere [6–8], in order to change the electrophysical conditions under which polymers find themselves during plastic deformation, 80 wt% metals, oxides, and metal salts was introduced into HDPE specimens. In most cases, after working under a pressure of 2 GPa, melting in such mixtures was described by bimodal peaks, while in the temperature region below T_m it was possible to observe broad endo- and exothermic peaks.

It was of interest to increase the content of the inorganic component in the mixtures in order to increase the phase boundary. It was to be expected that in mixtures with a branched phase boundary a large proportion of the fragments of macromolecules would turn out to be at the phase boundary, which is the point of concentration of the charge states.

Mixtures of NSMPE with 90 wt% GeO₂, MgO, Mg, and NaCl were then subjected to working at different pressures.

GeO

After treatment under a pressure of 0.5 GPa in the grounded variant, a bimodal peak was present on the thermogram of the NSMPE + 90% GeO₂ mixture in the temperature range of melting ( Figure 3 ). As the pressure of working increased, there was an increase in the intensity of the low-temperature peak, but even after working under a pressure of 4 GPa a high-temperature shoulder was present on the thermogram. After deformation under a pressure of 1 GPa, a weak (4–6 J/g) exothermic peak appeared on the thermogram in the temperature range 40–90°C (calculation was carried out for the total mass of the specimen). With increase in the pressure of deformation to 2 GPa, the enthalpy of this peak increased to 20–25 J/g (calculation for the total mass of the specimen); with increase in the pressure of deformation to 2 GPa, the enthalpy of this peak increased to 20–25 J/g (calculation for the total mass of the specimen); with increase in the pressure of deformation to 3 and 4 GPa, the enthalpy of the exothermic process did not change.

Figure 3

Thermograms of NSMPE + 90% GeO₂ mixtures after deformation under pressures of 0.5 GPa (1) and 2 GPa (2). Beneath upward arrow to left of figure: endo; Horizontal axis: T, °C

The exothermic peak on the thermograms of the specimen mixtures may be attributed both to chemical interaction of the polymer with the oxide and to the process of crystallisation of the polymer. If the exothermic peak were related to the chemical process, there would be a reduction in the amount of polymer in the mixture, which would tell on the enthalpy of crystallisation. However, on the thermogram of cooling there was an exothermic peak of crystallisation with an enthalpy of 190–200 J/g. In this case, the enthalpy of crystallisation of the polymer in deformed mixtures may reach 200–250 J/g.

The general appearance and the changes on the thermograms of mixtures worked in the insulated variant with increase in the pressure of deformation coincided with that described for the grounded variant of working.

Figure 4 gives the pressure dependences of the enthalpy of melting of NSMPE in mixtures with 90% GeO₂, from which it can be seen that the dependences are extremal in form; the maximum value of the enthalpy of melting amounted to 215–225 J/g (in the initial polymer 160 J/g, and in the deformed polymer 290 J/g). In specimen mixtures, the pressure at which the maximum value of the enthalpy of melting was recorded proved to be higher with the grounded variant of deformation than with the insulated variant.

Figure 4

The enthalpies of melting of NSMPE in a mixture with 90% GeO₂ as a function of pressure in the insulated (1) and grounded (2) variants of deformation. Vertical axes: ΔH, J/g; Horizontal axes: P, GPa

Thus, the plastic deformation of mixtures of NSMPE + 90% GeO₂ led to an increase in the enthalpy of melting of the polymer phase. Along with this, in specimen mixtures the process of cold crystallisation of the polymer takes place.

MgO

In the formation of the phase boundary, it is important for the volume fraction of the polymer to be as small as possible. To achieve this ratio, the molecular weight of the oxide must be minimal. In this connection, mixtures of NSMPE with 90 wt% MgO (the molecular weight of MgO is 2.6 times lower than the molecular weight of GeO₂) were subjected to pressure working.

Thermograms of NSMPE + 90% MgO mixtures after deformation under a pressure of 1 GPa outwardly did not differ from the thermograms of mixtures of NSMPE with GeO₂ shown in Figure 3 : there was a melting peak and also an exothermic peak. The enthalpy of melting in the grounded variant of deformation amounted to 250 J/g, and the enthalpy of the exothermic process was 200 J/g; in the insulated variant of working, the enthalpy of melting did not change, while the enthalpy of the exothermic process increased to 500 J/g (calculation for the content of polymer in the specimens).

It was of interest to investigate the thermograms of mixtures after deformation under a pressure of 1 GPa, obtained with different heating rates in the calorimeter. Such measurements were conducted on NSMPE + 90% MgO mixtures deformed under a pressure of 1 GPa in the insulated variant. It turned out that neither the shape of the melting peaks nor the melting temperatures nor the enthalpies of melting differed at heating rates of 4, 8, and 16 deg/min. Thus, the parameters of the melting process did not change with a reduction in the heating rate by a factor of 4 – this is typical of phase transitions.

The temperature of the exothermic peak on thermograms obtained during heating at a rate of 8 deg/min decreased by 12 deg, while its enthalpy amounted to 150 J/g (calculation for the content of polymer in the mixtures). This is typical behaviour of substances in relaxation processes. Thus, on thermograms of deformed NSMPE + 90% MgO there is an endothermic peak describing the phase transition (the melting of the polymer) and an exothermic peak relating to the relaxation process.

Specimens of mixtures after deformation under a pressure of 1 GPa were heated to different temperatures, cooled, and reheated.

After initial heating to 70°C, there was an exothermic peak on the thermogram with an enthalpy of 100–120 J/g, and also a bimodal peak of melting with an enthalpy of 250 J/g.

After initial heating to 95°C, on the thermogram there was only a peak of melting with an enthalpy of 250 J/g.

After heating to a temperature above T_m, on the thermogram of reheating there was a solitary peak of melting with a maximum at 140°C and an enthalpy of 250 J/g.

In this way, the results of measurements of heat effects in specimen mixtures indicate that the enthalpy of melting of the polymer depends on the heating regime. The fact that during reheating the enthalpy of melting of the polymer amounts to 250 J/g, and not to 160 J/g as in the initial polymer, indicates the strong influence of the oxide phase on the process of crystallisation of the polymer. This is possibly due to the formation of intermolecular bonds at the phase boundaries.

The chemical bonds in metal oxides can change from ionic to covalent bonds. In this connection it was of interest to investigate the thermal processes in the polymer in mixtures with components with another type of chemical bond – metal or ionic. The most suitable materials in this regard are magnesium, which possesses a small atomic weight, and sodium chloride, a typical compound with an ionic bond.

Mg

On the thermograms of NSMPE + 90% Mg, after deformation under a pressure of 0.5 GPa, along with a bimodal peak of melting there was an exothermic peak in the temperature range 30–110°C with a maximum at 75–80°C. The thermograms of specimens worked under pressure in the grounded and insulated variants hardly differed. The endo- and exothermic peaks on the thermograms overlapped slightly, which made it possible to calculate their enthalpy.

Figure 5 presents pressure dependences of the enthalpies of thermal processes for NSMPE + 90% Mg mixtures. The pressure dependence of the enthalpy of melting passes through a minimum, while the dependence for the exothermic process passes through a maximum. Here, the pressures at which the extrema appear differ little. Thus, the presence of metal in the mixtures did not influence the general nature of the thermograms, but led to the maximum value of the enthalpy of melting of the polymer reaching 350–360 J/g, and the enthalpy of the exothermic process reaching 400 J/g (calculation for the content of polymer in the mixture).

Figure 5

The dependences of the enthalpies of the exothermic (1) and endothermic (2) processes on the pressure of deformation in NSMPE + 90% Mg mixtures. Vertical axes: ΔH, J/g; Horizontal axes: P, GPa

NaCl

On the thermograms of NSMPE + 90% NaCl mixtures, after deformation in the grounded variant under a pressure of 0.5–1.5 GPa, there was a bimodal peak of melting, and below the temperature range of melting a broad endothermic peak ( Figure 6 ). After deformation under a pressure of 2 GPa and above, the form of the thermogram changed, depending on the degree of strain – the anvil turning angle. Thus, after deformation by an angle of 300 deg, on the themogram there was a peak of melting and a low-temperature endothermic peak – the thermogram outwardly repeated the thermogram of the specimen deformed under a pressure of 0.5–1.5 GPa with a turning angle of 600 deg. Increase in the anvil turning angle to 600 deg led to the appearance on the thermogram of an exothermic peak with a maximum at 55–60°C. In the insulated variant of working, the thermograms had a form of this type after working under a pressure of only 1.5 GPa.

Figure 6

Thermograms of NSMPE + 90% NaCl mixtures: 1 – pressure 0.5–1.5 GPa, turning angle 600 deg; 2 – pressure 2.0 GPa, turning angle 300 deg; 3 – pressure 2.0 GPa, turning angle 600 deg. Beneath upward arrow to left of figure: endo; Horizontal axis: T, °C

On the basis of the obtained thermograms, the enthalpies of the thermal processes – exothermic, low-temperature endothermic, and melting processes – were calculated. On thermograms with a melting peak and with a low-temperature endo peak, a common heat flux was calculated in the temperature range 30–150°C, while melting of the polymer was calculated from the intensity of the heat flux above line AB. The enthalpy of the exothermic peak was calculated from the heat flux intensity below line CD, while the enthalpy of the overall endothermic effect on these thermograms was calculated from the heat flux intensity above line DB.

The magnitudes of the enthalpies of the exothermic process for NSMPE + 90% NaCl mixtures differed greatly for the grounded and insulated variants of pressure working ( Figure 7 ). The limiting magnitude of enthalpy for the grounded variant was 50 J/g, and for the insulated variant 85 J/g (calculation for the content of polymer in the specimen).

Figure 7

The dependences of the enthalpy of the exothermic process on pressure in NSMPE + 90% NaCl mixtures in the grounded variant (1) and in the insulated variant (2) of pressure working. Vertical axes: ΔH_exo, J/g; Horizontal axes: P, GPa

The pressure dependences of the enthalpy of melting and the overall endothermic process pass through a maximum ( Figure 8 ). The maximum values of the enthalpies differ little, while the pressure at which the maximum value of enthalpy is reached is higher in the case of working in the insulated variant.

Figure 8

The dependences of the enthalpy of the overall endo process (1) and the enthalpy of melting of NSMPE (2) for the grounded variant of deformation (a) and for the insulated variant of pressure working (b). Vertical axes: ΔH, J/g; Horizontal axes: P, GPa

The difference in enthalpies of the overall endothermic process and melting can be regarded as the enthalpy of melting of superfine crystallites. The enthalpy of melting of such crystallites formed during working in the grounded variant amounted to 190 J/g, and in the insulated variant to 160 J/g. Taking into account the error in graphic separation of the peaks, we can say that the enthalpy of melting of fine crystallites is not dependent on the conditions under which deformation under pressure has been carried out – whether in the grounded or in the insulated variant.

It was of interest to investigate mixtures with a lower NaCl content in the specimens. Figure 9 presents thermograms of NSMPE with 40 and 80 wt% NaCl, worked under pressure in the grounded and insulated variants, which indicate that bimodal peaks of melting appear only in NSMPE + 80% NaCl mixtures. A low-temperature endothermic peak appears in both mixtures, but only if working has been conducted in the insulated variant.

Figure 9

Thermograms of NSMPE + 40% NaCl mixtures (1, 2) and NSMPE + 80% NaCl mixtures (3, 4) after deformation under a pressure of 2 GPa: 1, 3 – grounded variant; 2, 4 – insulated variant. Beneath upward arrow to left of figure: endo; Horizontal axis: T, °C

NSMPE is an extremely specific polymer: firstly, it is a nascent polymer; secondly, it is supermolecular; thirdly, it has a high degree of crystallinity. In this regard, it was of interest to investigate the thermal processes in mixtures of LDPE with a high GeO₂ and NaCl content, i.e. with the components in the mixtures that enabled the most interesting results to be obained for the thermal processes in NSMPE.

The thermograms of LDPE + 90% GeO₂ presented in Figure 10 indicate that in appearance they match the thermograms of analogous mixtures based on NSMPE in the grounded variant of high-pressure working. These show that the thermal effects in mixtures of PE with a high content of inorganic components are not dependent on the supermolecular structure of the polymer.

Figure 10

Thermograms of an LDPE + 90% GeO₂ mixture (1) and an LDPE + 90% NaCl mixture (2) after deformation under a pressure of 2 GPa. Beneath upward arrow to left of figure: endo; Above downward arrow to left of figure: exo; Horizontal axis: T, °C

Thus, deformation under high pressure leads to an increase in the enthalpy of melting in specimens of different polyethylenes by a factor of 1.3–1.8. The pressure of deformation at which maximum values of the enthalpies for each polymer were reached depended on the electrophysical conditions under which deformation was carried out. In mixtures with inorganic components, the enthalpy of melting could reach 350 J/g (NSMPE + 90% Mg) and 400 J/g (NSMPE + 90% NaCl).

On the thermogams of deformed mixtures at temperatures below the T_m of the polymer, endothermic and exothermic peaks were recorded. The endothermic process can be attributed to the melting of fine crystallites, and the exothermic peak either to the chemical process at the phase boundary or to the process of cold crystallisation of the polymer.

References

Zhorin

V.A.

, Vys. Soed., A24(5): 953 (1982).

Zhorin

V.A.

, Vys. Soed., 43(12): 2163 (2001).

Peterlin

, and Meinel

, J. Polym. Sci., B5(8): 613 (1967).

Sumita

, J. Polym. Sci., Polym. Phys. Ed., 15(5): 837 (1977).

Godovskii

Yu.K.

The Thermophysics of Polymers. Khimiya, Moscow (1972).

Zhorin

V.A.

, Dokl. Akad. Nauk, 378(1): 64 (2001).

Zhorin

V.A.

, Vys. Soed., B43(7): 1262 (2001).

Zhorin

V.A.

, Zh. Fiz. Khim., 76(1): 54 (2002).