2019 Volume 2 Issue 3

INEOS OPEN, 2019, 2 (3), 68–77  Journal of Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences Download PDF DOI: 10.32931/io1911r

Effect of Chemical Structures of Heat-Resistant Tribostable Thermoplastics on Their Tribological Properties: Establishing a General Relationship
 

The present review summarizes the results of investigations on the relationship between chemical structures and tribological properties of heat-resistant tribostable thermoplastics: amorphous linear, crystalline linear and cardo polymers and copolymers. A solution to this problem is provided by the introduction of a notion of polymer antifrictionality, which represents the ratio of a dispersion component to the total energy of intermolecular interaction. It is also demonstrated that, besides the value of antifrictionality which can be calculated by a computational method, the tribological properties are determined by a polymer molar mass.

Key words: heat-resistant thermoplastics, antifrictionality, cardo copoly(arylene ether ketones), tribochemical processes.

 

1. Terms and definitions

Tribostable heat-resistant antifriction polymers represent linear high-molecular compounds with the reduced friction coefficients and the improved wear resistance [1].

Chains of tribostable polymers do not contain reactive functional groups. During friction, these polymers undergo mainly destruction processes. An important feature of linear tribostable polymers is the manifestation of a self-lubricating effect by some of them.

Energy of intermolecular interaction (Е_imi) of linear polymers is the sum of energies of dispersion interaction (Е_disp), dipole–dipole interaction (Е_dip-dip), and hydrogen bonds (Н_b). Note that the polymers considered in the review do not contain hydrogen bonds.

Two-term friction law is expressed as a dependence of the friction force (F_fr) on F_mol and F_mech, where F_mol characterizes the force of adhesion resistance to deformation. The mechanical component (F_mech) of the friction force in materials characterizes the resistance of a surface layer to shear deformation under applied load. The expression "forming a positive gradient of mechanical properties" is frequently used [2].

Antifrictionality of a polymer characterizes the molecular component of the friction force and is defined by the chemical structure of a polymer unit [3].

Antifriction properties of a polymer result from its antifrictionality owing to the polymer structure (chemical structure of monomers, molar mass, and crystallinity degree). In the review, the role of this parameter is considered only for unfilled polymers.

2. Tribochemical processes in tribostable polymers

The goal of this section is to summarize the results of investigations on tribostable amorphous poly(arylene ether ketones) (PAEKs) to prove the destructive character of friction of this type of polymers.

The investigations on development of amorphous PAEKs have been successfully carried out in USSR since 1967. An alternative to PAEK is an industrial partially crystalline poly(ether ether ketone) (PEEK), which, despite the high heat-resistance and melting point, has serious drawbacks, including the relatively low glass-transition temperature (143 °С) and, concurrently, the high processing temperature (380–390 °С) [4]. Since a fraction of a crystalline phase in this industrial cast polymer composes only ~30%, it demonstrates noticeable deformation changes in the temperature range of ~130–150 °С. In some cases, this prevents from the use of PEEK as a construction material of critical components in modern machine engineering.

Amorphous poly(arylene ether ketones) [5, 6], represented, for example, by a polymer obtained from 4,4'-difluorobenzophenone and bisphenol A, are tribostable heat-resistant amorphous thermoplastics. They also serve as ideal models for investigation of tribochemical processes in tribostable polymers, since they readily dissolve in common organic solvents. The latter enables a detailed study of tribodestruction processes in these polymers by gel permeation chromatography (GPC).

The tribological behavior of PAEK (Scheme 1) was analyzed by the changes in its molar mass characteristics during friction using sleeves obtained by injection molding at 260, 330, and 350 °С. The molar masses of these samples were determined by GPC using a polystyrene standard. The results obtained are presented in Table 1.

poly(arylene ether ketone) (PAEK)

 

poly(ether ether ketone) (PEEK)

Scheme 1. Structures of PAEK and PEEK polymers.

Table 1. Changes in the average molar masses of PAEK samples during processing and tribological tests according to GPC data

Sample	Mw · 103	Mn · 103	Mw /Mn
Initial (powder)	53.6	6.9	7.76
Processing at 260 °С
Molded	52.6	8.1	6.49
Surface layera (after friction)	49.8	7.3	6.82
Wear products	22.7	6.3	3.60
Processing at 330 °С
Molded	54.1	7.3	7.41
Surface layera (after friction)	57.2	7.5	7.63
Wear products	20.5	5.6	3.66
Processing at 350 °С
Molded	250.3	6.7	37.36
Surface layera (after friction)	29.0	5.7	5.09
Wear products	22.2	4.4	5.05

During friction of the samples molded at 260 and 330 °С, the molar-mass distributions (MMD) of their surface layers almost did not differ from the initial one and can be satisfactorily described by an empirical relation between М_w and M_w/M_n [7]. The processing of PAEK at 350 °С led to an increase in M_w to 250·10³ Da and to a drastic broadening of MMD (M_w/M_n = 37.36), which was associated with the formation of a branched structure. Unlike the processes that took place during treatment, friction of this sample led to a decrease of the molar masses both in the wear products and in the surface layer, which indicates a destructive character of the tribochemical process.

It is interesting to note that in all the performed experiments, tribodestruction proceeded to almost the same average molar masses (М_w = 20.5·10³–22.7·10³ Da, Table 1). Similar results were observed earlier upon investigation of mechanical destruction [8], but in this case, they were demonstrated for the first time under conditions of the more complex tribological impact and low rates and load.

The wear products did not contain a gel fraction; the friction was characterized only by a reduction of their molar masses [7].

Hence, the tribostable heat-resistant linear polymers undergo destruction upon friction that leads to a reduction of M_w. Earlier it was shown [9] that the destruction processes are caused by the cleavage of the less strong thermally unstable bonds in a bisphenol A moiety.

Relationship between the energy of intermolecular interaction of PAEK with the friction force. A computational method

A semiempirical method for computer modeling of polymers at the atomic–molecular level, which is used in this work, is based on the application of increments of different atoms and their basic groups. In the course of development of the method, a calibration was carried out using the physical characteristics of polymer standards which properties are well studied. The computational method was successfully used in investigations on thermophysical and other properties of polymers [10].

A hypothesis was suggested that the friction of antifriction thermoplastics is affected by a level of tangential shear force required to overcome the resistance of a surface layer, which is determined by the energy of intermolecular interaction. This allowed us to pass from a purely descriptive concept about the relationship between chemical structures of antifriction thermoplastics and their friction to a mathematical expression using the computational method for this purpose [10].

As it was already mentioned, the energy of intermolecular interaction is the sum of the energies of dipole–dipole interaction, Н-bonds, and dispersion interaction. Since the polymers under consideration do not contain Н-bonds, they are not taken into account in the equation.

Table 2 shows that the total energy of intermolecular interaction of PAEK includes mainly the energy of dispersion interaction (Е_disp > Е_dip-dip), which implies the reduced resistance of a polymeric body under tangential shear forces in antifriction polymers, where the friction force must tend to a minimum value (F_fr → min). This is possible only in the case if the resistance of a surface layer is low, since the energy of dispersion interaction Е_disp significantly exceeds Е_dip-dip. In this case, the friction resistance is minimal owing to the molecular component F_mol, i.e., F_mol → min. Since both parameters F_mol and F_mech are interconnected, to fulfill this dependence (F_mol → min), it is necessary to reach the minimal volume of the deformed surface layer during friction, i.e., to realize friction in the finest surface layer. This is possible upon implementation of the positive gradient rule of mechanical properties, i.e., at the maximum allowable value of the mechanical component. Hence, in a formula of the two-term friction low F_fr = F_mol + F_mech at F_fr → min, the friction force components must be as follows: F_mol → min, F_mech → max.

Table 2. Energy of intermolecular interaction in PAEK

Total energy of intermolecular interaction, kJ/mol	421
Energy of dipole–dipole interaction, kJ/mol	75.8
Energy of dispersion interaction, kJ/mol	345.2
Ratio of the energy of dipole–dipole interaction to the total energy, %	18
Ratio of the energy of dispersion interaction to the total energy, %	82

The calculations demonstrated that the chemical structure of PAEK with the low energy of intermolecular interaction, which stemmed from the weak dispersion forces, predetermines the possibility of production of an antifriction material with the reduced friction coefficient on its base.

3. Role of the chemical structure and the molar mass of a tribostable poly(arylene ether ketone) (PAEK) in the formation of an antifriction wear-resistant material

The most studied heat-resistant amorphous thermoplastics are poly(arylene ether ketones), for example, the simplest poly(arylene ether ketone) synthesized from 4,4'-difluorobenzophenone and bisphenol A (Scheme 1).

The effect of a high molar mass on friction of thermoplastics is ambiguous. In tribostable aliphatic polymers, such as ultrahigh-molecular-weight polyethylene and polytetrafluoroethylene, a growth in the molar mass positively affects their friction [11].

The first tribological tests with PAEK were carried with a ball counterbody that afforded weak friction heating: Ø 5 mm at v = 0.5 m/s, Р = 10 MPa. The samples from low-molecular (η_intr = 0.4 dL/g) to high-molecular (η_intr = 2.5 dL/g) PAEKs were selected. The use of a friction joint with the low coefficient of mutual overlapping [12] allowed for a significant increase in the pressure and improvement of heat removal, which provided the more reliable results.

On the resulting dependence of the friction coefficient and wear on the molar mass of PAEK (Fig. 1), one can outline three zones of changes in the tribological parameters.

Figure 1. Dependence of the friction coefficient and wear on the intrinsic viscosity of PAEK samples based on 4,4'- difluorobenzophenone and bisphenol A: (1) 0.41, (2) 0.53, (3) 0.75, (4) 1.00, (5) 1.16, (6) 2.49 dL/g; 1 h per each point.

– In the first zone (0.40–0.53 dL/g), a relatively low friction coefficient and continuing wear reduction may evidence that the molar mass of the polymer is within the ranges of low physicomechanical parameters, which were determined during investigation of the film samples [13].

– In the second zone (0.53–1.00 dL/g), a growth of the molar mass leads to a gradual reduction in the wear and a simultaneous increase in the friction coefficient.

– In the third zone (1.00–2.50 dL/g), the samples display a reduction of the wear and the friction coefficient, which is especially pronounced for the high-molecular sample with η_intr ≈ 2.49 dL/g (Table 1).

Therefore, we established that PAEK sample with η_intr ≈ 2.49 dL/g and M_w ~ 120·10³ Da has significantly improved tribological properties. To understand the processes that took place in the second and third zones, the results of investigation of the wear products by GPC are discussed below.

Changes in MMD during friction. To find out the reasons for improvement of tribological properties in the case of high-molecular PAEK (Table 3) and to determine the processes that take place in the second zone, we studied the changes in MMD during molding and in the wear products of the following PAEK samples: sample 1 with η_intr ≈ 0.75 dL/g and M_w = 46·10³ Da and sample 2 with η_intr ≈ 2.49 dL/g and M_w = 201·10³ Da.

Table 3. Average molar masses and intrinsic viscosities of the initial samples and their wear products

Sample			Mn	Mw		Mz	ηintr, dL/ga
Initial sample 1			12.8·103	45.8·103		80.3·103	0.75
Wear product of sample 1			11.6·103	32.8·103		47.4·103	–
Initial sample 2			14.1·103	201·103		770·103	2.49
Wear product of sample 2			11.5·103	120.6·103		741·103	–

Molding of the samples at 260 °С almost did not change MMD. This can be seen from Figure 2, where curves a and b almost merge to a single line. These data are not included in Table 3.

The wear products of both of the polymers demonstrated a shift of GPC peak c (Fig. 2) to the lower mass-average molar masses (Table 3). It is important to mention the differences in MMD of two initial samples. Low-molecular sample 1 had a distribution typical for polycondensation products that consist of linear chains and cyclic oligomers. As a result, the total ratio of mass-average and number-average masses (M_n) M_w/M_n appeared to be significantly higher than 2.

 

Figure 2. Changes in MMD of samples 1 and 2 during friction: (a) initial PAEK powder, (b) molded sample, (c) wear products. Note that the wear products of the high-molecular sample include low-molecular fraction В1 and high-molecular fraction B2.

At the same time, the ratio of the sedimentation-average mass (M_z) to the mass-average mass M_z/M_w, which characterizes a high-molecular part of the polymer, was close to the theoretical one. A peculiarity of MMD of high-molecular sample 2 is the high value of M_z/M_w, which is likely to be explained by the presence of a branched high-molecular fraction. The wear products of sample 1 retained the type of initial MMD with the expected change of M_z/M_w, but the friction of the high-molecular sample led to the retention of M_z and an essential reduction of M_w in the wear products (Fig. 2). This means that the most high-molecular (branched) part of the sample converts to the wear products almost without any change, which is reflected in an essential bimodal character of MMD of the wear products. Presumably, a combination in the polymer both of a high-molecular fraction, which possesses the improved mechanical properties [13], and a low-molecular one leads to a positive gradient of mechanical properties in a surface layer during friction. This predetermines the improvement of tribological properties of the polymers: both the friction coefficient and the wear resistance.

XPS studies of PAEK. Analysis of the O 1s photoelectron spectra revealed the presence of only ether and ketone groups on the surface. This allowed us to carry out a quantitative estimation of the changes of each of the bonds during molding and friction. It was found that, independent from the molar mass, the surface of PAEK is particularly sensitive to the heat impact. This led to a considerable reduction in the concentration of the less heat-resistant ether groups and, consequently, an increase in the relative content of the keto groups (Fig. 3).

Figure 3. O 1s photoelectron spectra of PAEK: (a) sample 1, (b) sample 2 (initial powders; samples after 1 min and 30 min of friction; molded samples).

After friction, the surface compositions of the low-molecular and high-molecular samples considerably differed. In sample 1, friction for only 1 min already led to essential changes in the chemical composition, which was associated with the wear and the appearance of slightly changed subsurface products on the surface. In the case of the high-molecular polymer, there was observed a more expected gradual process of enrichment of the surface layers with the lower lying slightly changed products for 30 min of friction, i.e., the rate of this process depended on M_w of the sample.

Hence, the high wear resistance of high-molecular sample 2 is determined by the character of tribodestruction, which leads to the formation of a surface layer that includes decomposed and initial macromolecules with the higher molar masses. These peculiarities of the tribostable high-molecular polymer facilitate the formation of a positive gradient of mechanical properties. One of the reasons that favor the wear of high-molecular PAEK can be oxidative processes revealed by XPS, which rapidly develop in the finest surface layers.

The main conclusions from the above-described investigation are that the antifriction characteristics of the polymer under consideration are affected not only by the level of intermolecular interaction, but also by the second factor, namely, the molar mass of the polymer. In the case of PAEK, one should take into account the presence of cyclic moieties in its structure, which contents increase for the high-molecular polymers [14]. The best antifriction properties and the high wear resistance can be achieved at the maximum molar mass (M_w ~ 201·10³ Da).

4. Friction of heat-resistant crystalline and amorphous thermoplastics

To date a series of novel heat-resistant amorphous and crystalline thermoplastics of various structures have been synthesized for the application in the initial state or as binding agents for filled composites in friction joints. This refers to crystalline polymers poly(ether ether ketone) (PEEK) [15–17] and poly(phenylene sulfide) (PPS) [18, 19] and amorphous polysulfones (PSF) (Scheme 2) [20].

Amorphous polymers	Crystalline polymers
PAEK	PEEK
PSF	PPS

Scheme 2. Heat-resistant amorphous and crystalline thermoplastics.

The prominent materials studies devoted to tribology of industrially valuable thermoplastics concern the correlation of their tribological properties with physicomechanical characteristics of the polymers. Zhang et al. showed the advantages of the use of amorphous PAEK with the higher molar mass in friction joints [15].

This allowed us to study crystalline polymers from the same viewpoint as amorphous PAEK.

Using the computational method, we calculated the energies of intermolecular interaction and one of its components, namely, the energies of dispersion interaction for a series of amorphous and crystalline heat-resistant thermoplastics (Table 4). In definition of Е_imi, the presence of crystalline phases in PPS and PEEK was neglected due to their relatively low contents (30%).

Table 4. Energies of intermolecular interaction and Е_disp/Е_imi ratios of the heat-resistant polymers

Parameter	Heat-resistant thermoplastics
	Amorphous		Crystalline
	PAEK	PSF	PEEK	PPS
Еimi, kJ/mol	421	461	258	73
Ratio of Еdisp to the total energy, %	82	68	91	91

Of particular interest was to check whether the developed concept has predictive power. For this purpose, we compared the results of evaluation of Е_imi for PSF and PAEK. From the data presented in Table 4, it is obvious that PSF has the same value of intrinsic viscosity as PAEK, but differs by the reduced value of Е_disp compared to that of PAEK. The experimental data on friction of PAEK and PSF, which had the same intrinsic viscosities (η_intr = 0.5 dL/g), confirmed this assumption (Fig. 4, curves 1 and 2). The friction curve of PSF displayed a sharp increase of the oscillation amplitude and instability of the friction coefficient. This character of friction is defined as seizure. This experiment revealed that the suggested concept has predictive power for a general regularity in the relationship between a chemical structure and tribological properties.

Figure 4. Dependence of the friction coefficient on the test duration: (1) PSF, η_intr = 0.5 dL/g; (2) PAEK, η_intr = 0.5 dL/g; (3) PAEK, η_intr = 2.49 dL/g.

Partially crystalline polymers

The group of tribostable heat-resistant thermoplastics under consideration includes not only amorphous, but also partially crystalline polymers: PEEK and PPS. To understand the nature of friction in crystalline thermoplastics, we applied the same approaches which were used for the amorphous polymers (Table 4). The ratio of the energy of dispersion interaction to the total energy of intermolecular interaction in these crystalline polymers is higher (>90%) than in the case of the amorphous polymers (70–80%). This shows the potential for creation of antifriction materials with the reduced friction coefficients.

However, the experiments revealed (Fig. 5) that the friction coefficients of both of the crystalline polymers were slightly higher than that of high-molecular amorphous PAEK and were close to the friction coefficient of PAEK with the lower intrinsic viscosity (~0.5–0.75 dL/g). Such a tribological behavior of partially crystalline (~30%) thermoplastics (PEEK, PPS) is likely to be connected with the predominance of amorphous structures with the reduced molar masses in their compositions (up to ~70%). This assumption was confirmed by the thermomechanical tests: both of the crystalline polymers exhibited clearly defined glass-transition points: 140–143 °C (PEEK) and 90–100 °C (PPS). The structures of partially crystalline thermoplastics allow one to assume that the friction of these polymers as well as that of the amorphous ones depends mainly on the value of dispersion component Е_disp as well as on the molar mass of amorphous-crystalline structures of the thermoplastics. An increase in the molar masses of PPS and PEEK can afford tribostable crystalline thermoplastics with the improved tribological characteristics. Investigations with extrusion PEEK confirmed our assumptions. The friction coefficient of this polymer appeared to be close to the friction coefficient of high-molecular PAEK (Fig. 5).

Figure 5. Dependences of the friction coefficient on the test duration for the partially crystalline polymers: (1) cast PEEK, (2) PPS, (3) extrusion PEEK.

The results of investigations allowed us to distinguish between two tribological notions, antifrictionality and antifriction properties, in the case of heat-resistant thermoplastics. For heat-resistant tribochemically stable thermoplastics, the antifrictionality is defined only by the chemical structure of a polymer unit and is characterized by the predominance of the energy of dispersion interaction over that of the dipole–dipole forces. The antifrictionality can be realized in polymers which feature antifriction properties: reduced friction coefficient (f) and low wear intensity (J). Most frequently, the antifrictionality of polymers is manifested in antifriction composite materials, where these polymers serve as binding components. To control the value of a mechanical component of the friction force (F_mech), the material must contain different functional additives. It was shown [9] that the polymer material where PAEK has the high molar mass can exhibit not only a low friction coefficient, but also a high wear resistance [15].

5. Cardo poly(arylene ether ketones). Effect of cardo moieties in chains on the tribological properties of these polymers

The name of cardo polymers originates from the latin word "cardo", which means a loop, since a unit of a cardo polymer has an element which is included in a side cyclic group. Cardo groups can be considered as loops of a main macromolecular chain.

Intensive studies on cardo polymers started in 1961, when S. N. Salazkin et al. synthesized at INEOS RAS a heat-resistant cardo polyarylate using phenolphthalein and isophthalic acid [21].

Friction studies with PAEK in the 1990–2000s were mainly concerned with the only one representative of cardo polymers that was synthesized based on analogous PAEK (PEK-C). It was found that this polymer is characterized by the small temperature ranges (50–90 °С) for the application in friction. However, the reasons for such a behavior as well as the effect of a molar mass and occurring tribochemical processes were not established.

The main goal of this section is to summarize the results obtained in investigations with cardo copolymer poly(arylene ether ketones) (co-PAEK) during friction for elucidation of a common dependence between their chemical structures and friction.

Plastometric dependences. For a better understanding of the character of friction of polymers (including cardo ones), of particular importance is the preliminary investigation of their thermomechanical or plastometric dependences (Fig. 6).

Figure 6. Plastometric curves for cardo co-PAEK-5, co-PAEK-7,
and co-PAEK-8 (curves 1, 2, 3, respectively) (Р = 1 MPa).

Table 5 lists the data for PAEK and cardo copolymers differing in chemical structures. We chose the samples that have close values of molar masses or intrinsic viscosities (η_intr) (Scheme 3).

Table 5. Calculated parameters of the structures of diane homo-PAEK and cardo co-PAEK

 

By the structures of chain units, the flexible polymers from the first group [22] represent random copolymers of phenolphthalein anilide and bisphenol A (co-PAEK-5). In the second group of rigid copolymers, the structural units represent combinations of the fragments of two cardo bisphenols, namely, phenolphthalein anilide with phenolphthalein (co-PAEK-7) or with phenolfluorene (co-PAEK-8) (Scheme 3).

Friction of cardo copolymers

Earlier the correlation between a chemical structure and friction for diane homo-PAEK was established based on the contribution of a dispersion component to the total energy of intermolecular interaction (Scheme 3, Table 5) [9]. Almost all the cardo copolymers explored (co-PAEK-5, co-PAEK-8), including rigid copolymers, demonstrated the high ratio of the dispersion component to E_imi, which ranged from 79.7% to 84%. Theoretically, these copolymers can afford materials with good antifriction properties.

Table 5 lists the values of the components of Е_imi at the ratio of fragments p/q = 0.5/0.5 and the molar masses of the polymers which ensure the appearance of a high-elastic (HE) state.

The preliminary investigations on friction of the cardo polymers showed that good tribological parameters cannot be attained only owing to two essential components, namely, antifrictionality and molar mass. To elucidate the conditions for application of cardo polymers as antifriction materials, we suggested that the consideration of flexibility of a cardo polymer chain is necessary.

The most typical structure among flexible co-PAEKs is characteristic of co-PAEK-5 (Scheme 3). This polymer combines the earlier studied effect of the molar mass of a diane polymer [23] with the presence of a cardo phthalimidine fragment. The viscosity parameter (η_intr = 0.68 dL/g) allows one to compare its friction characteristics with those of more rigid co-PAEK-8 which features a close value of viscosity (η_intr = 0.62 dL/g).

At the pressure of 10 MPa, co-PAEK-5 has a relatively low friction coefficient (~0.4) (Fig. 7). Of note is insignificant oscillation amplitude and rapid (5–6 min) wearing-in, when the oscillation amplitude of f_fr composes ~0.1. An increase in the pressure during friction to 15 MPa leads to a considerable decrease in the friction coefficient up to 0.2, which is retained further during the whole experiment. An ultimate pressure on co-PAEK-5 is 25 MPa, when the wear arises and the friction coefficient starts to increase relative to the pressure of 15 MPa (Fig. 7, Table 6).

Figure 7. Friction of cardo co-PAEK-5 (1, 1', 1") and co-PAEK-8 (2, 2') at the load of 10 MPa (1 and 2), 15 MPa (1' and 2'), and 25 MPa (1").

Table 6. Wear of the cardo co-PAEK samples at different pressures

Sample	Pressure, P, MPa	Wear, J·10–3 ga
co-PAEK-5	10	0
	15	0
	25	0.6
co-PAEK-8	10	2.2
	15	11.8
co-PAEK-7	10	0.8
	15	17

In the structures of rigid co-PAEKs, the dispersion component Е_disp prevails. This indicates the antifrictionality of these polymers (Table 5). Despite this fact, friction of rigid co-PAEK-8 (η_intr = 0.62 dL/g), which contains bulky fluorene and phthalimidine moieties, is characterized by a sustainably high friction coefficient (0.7). Among the samples explored, this copolymer demonstrated the maximum wear during friction: J = 22×10^–4 g (Table 6, Fig. 7).

In the analysis of the friction dependences of co-PAEK-7 (η_intr = 0.42 dL/g), of particular interest was the effect of the reduced molar mass of this polymer on its tribological properties. Based on the earlier developed concept about the relation of the energy of intermolecular interaction and the molar mass of PAEK with friction, this polymer must feature the reduced friction coefficient and high wear (Fig. 1). The performed experiment (Fig. 8) confirmed this assumption. The friction coefficient at the pressure of 10 MPa composed 0.3, but the sample can stand this load for 27 min and the pressure of 15 MPa only for 17 min. Presumably, as well as in the case of the low-molecular amorphous polymers, this is connected with the deteriorated physicomechanical parameters compared to those of the high-molecular analogs. Thus, co-PAEK-7 with the reduced viscosity η_intr = 0.42 dL/g has the tensile strength equal to 72 MPa, and co-PAEK-5 with η_intr = 0.68 dL/g has the rupture strength of 84 MPa.

Figure 8. Friction of cardo co-PAEK-7 at 10 MPa (1) and 15 MPa (2).

To find out the reasons for the high wear in the rigid cardo polymers, they were studied by X-ray photoelectron spectroscopy (XPS). The investigations were concerned with the molded samples and the samples obtained after friction for 30 min. It was found that friction of flexible co-PAEK-5 in the first 30 min is strongly affected by the residual solvent, N,N-dimethylacetamide. The solvent still remains in the polymer despite the high temperature of processing (300 °С), although the boiling point of DMAA is ~165 °С.

Friction of rigid co-PAEK-8 for 30 min (Fig. 9) resulted in a reduction of the intensity of the peak with the binding energy of 532.17 eV, which characterizes the content of N–C=O groups in DMAA and cardo groups of co-PAEK-8.

Figure 9. O 1s photoelectron spectra of co-PAEK-8 surface: (а) before friction; (b) in 30 min of friction.

At the same time, the amount of the remaining N–C=O phthalimidine groups does not give an accurate answer to the question about the nature of the groups observed in the spectrum: only those corresponding to the main chain or a mixture with the reduced amount of the phthalimidine fragments instead of the reduced amount of the solvent molecules. To address this issue, N 1s spectra were scrutinized.

Figure 10 shows the N 1s spectrum of co-PAEK-8 which was measured in 30 min of friction. A peak at the binding energy of 400.09 eV refers to the phthalimidine moiety, and a weak peak at 401.12 eV corresponds to the solvent group (relative concentration ~8%). This implies that the most probable mechanism of a tribochemical process is the destruction by the weakest C–N bond (~308 kJ/mol), which is not connected with the main polymer chain, compared to the energy of the C–C bond (348 kJ/mol).

Figure 10. N 1s photoelectron spectra of co-PAEK-8 surface.

Thus, the XPS studies showed that for cardo co-PAEK polymers the main destruction processes during friction involve rigid N-phenylphthalimidine cardo groups.

6. Conclusions

The main outcome of the work performed consists in the creation of a unified scientific concept about the relationship between the chemical and physical structures of heat-resistant thermoplastics and their tribological properties.

The results of investigations of the following three groups of antifriction heat-resistant thermoplastics were summarized:

1) amorphous linear heat-resistant thermoplastics—poly(arylene ether ketones) and polysulfone;

2) crystalline linear polymers—poly(ether ether ketone) and poly(phenylene sulfide);

3) cardo antifriction polymers and copolymers of poly(arylene ether ketones).

An important factor for understanding the nature of friction in these polymers is the concept about their antifrictionality as a direct manifestation of the chemical structure and potential possibility of production of antifriction materials on their base. Using the computational method, the antifrictionality can be presented through a mathematical expression as the ratio of a contribution of the dispersion component (predominantly 60–90%) to the total energy of intermolecular interaction, which allows one to estimate preliminary the opportunities of application of a polymer as an antifriction material.

Each group of the polymers explored has its own optimal ranges of the maximum possible molar masses. Each group of the polymers which possess antifriction properties has the peculiarities that restrict the possibilities of their use as antifriction materials. Besides the molar mass of the polymers, which must correspond to the most reasonable level, for each group of the polymers one should take into account the specific factors, such as macromolecule flexibility in the case of cardo polymers, molar mass in the case of diane polymers, and crystallinity degree in the case of crystalline polymers.

MMD analysis and XPS studies allowed us to establish that the main advantage of the high molar mass (intrinsic viscosity) in diane polymers consists in the creation of a discrete surface, which ensures the realization of a positive gradient of mechanical properties during friction.

In the case of the crystalline polymers, the discrete surface is created owing to the supporting supramolecular formations, polymer crystals. During friction of the cardo copolymers, the high tribological parameters can be achieved upon formation of a molecular discrete surface where the role of supporting groups are played by the bulky cardo moieties.

Acknowledgements

This work was supported by the Ministry of Science and Higher Education of the Russian Federation.

The authors are grateful to Prof. O. A. Serenko for discussion and recommendations.

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