2023 Volume 64 Issue 7 Pages 1325-1330
The aim of the article is to discuss the characteristics of structural transformations occurring under ultra-high plastic deformation in polymeric materials of various types: semi-crystalline and amorphous polymers, polymer blends and composites, polymer powders, and their influence on properties. Methods of obtaining ultra-high plastic deformations such as the equal channel angular extrusion (ECAE) process and its modified version - the equal channel multi-angular extrusion (ECMAE) process - are considered.
Plastic deformation is one of the most commonly used methods for the physical (structural) modification of polymers in order to improve their physical and mechanical properties. Under its influence, specific changes occur in the supramolecular structure or phase transformation of the polymers, although the chemical structure of the macromolecules does not change. The extent of such rearrangements depends on the amount of accumulated plastic deformation that does not lead to mechanical fracture, polymer morphology (linear chains, branched, cross-linked polymers), stress-strain state, strain rate and temperature, etc.1) Conventional methods of plastic deformation of polymers, which involve a change in the shape of the original workpiece, are drawing, rolling, conical extrusion, forging.2) They make it possible to achieve the degree of plastic deformation up to 1000% of the relative strain or up to 2.4 of the true logarithmic strain (e).1,3) Alternative methods are methods of severe plastic deformation, in particular, the equal-channel angular extrusion process (ECAE) and its modified version - the equal-channel multi-angle extrusion process (ECMAE)4–6) (Fig. 1). These processes are characterized by a large component of uniform compressive stress, which prevents the formation and growth of cleavage cracks, and by maintaining the shape of the workpiece, which allows the process to be repeated repeatedly to accumulate ultra-high strains. They are also characterized by the possibility to realize different deformation routes. By varying the orientation of the specimen between successive passes, very different deformation textures can be generated.7)
Schematics of the ECAE (a) and ECMAE (b) processes.
The author of Ref. 8) formulated a hypothesis according to which additional channels of elastic energy dissipation open up during ultra-high megaplastic deformation of metallic materials, such as dynamic recrystallization, phase transitions, dislocation rearrangements and the release of latent heat of deformation origin. Their activation shifts the fracture stage towards higher values of strain. These processes can be cyclic, i.e. after dynamic recrystallization or amorphization, plastic deformation in a newly formed recrystallized lamella or amorphous phase region starts as if anew. Then defects are accumulated again in the considered microvolume under yield stress and the process repeats. As a result, deeper structural and phase transformations may occur in the material before it is fractured. This idea has been further developed in the concept of ultra-SPD, which promises the use of large shear strains that can lead to the mixing of elements in multiphase materials at the atomic level and the formation of new phases with specific functional properties.9–11)
In the case of polymers, the diagram of the main types of plastic deformation proposed in Ref. 8) can take the form shown in Fig. 2. The boundary between the microplastic and macroplastic deformations is clearly determined through the strain corresponding to the macroscopic yield strength (relative strain ε ≈ 10%). At the same time, the boundary between the macroplastic and megaplastic deformations remains uncertain. Arbitrarily, we will assume the relative strain ε ≈ 1000% (or the true strain e ≈ 2.4) to be this boundary. So far, this hypothesis has not been tested for polymeric materials, and the boundary of transition from the region of macroplastic deformation to the region of megaplastic (ultrahigh) deformation has not been determined. It is obvious that the activation of the above-mentioned additional channels of elastic energy dissipation would serve as a sign of the beginning of such a transition.
Schematic diagram of the main types of plastic deformation of polymers.
Most studies on ECAE of polymers are limited to the case of 1–2 strain cycles, corresponding to a true strain of 0.8–2.4 (depending on the channel intersection angle). In contrast, ECMAE was used to reach the range of plastic deformations - e = 6.7–8.5. It can be assumed that these values of e correspond to the range of ultra-high plastic deformations. Previous reviews4–6) have discussed the structure-property relationship as a function of the severe plastic deformation scheme and the type of polymer. In this review, the results of studies on structural changes of polymers using ECAE and ECMAE methods and their analysis within the framework of the concept are presented. In particular, the fact that the activation of the processes of dynamic recrystallization, amorphization and phase transformation can only occur at sufficiently high values of accumulated plastic deformation and does not occur at low values of accumulated plastic deformation is highlighted.
Semi-crystalline polymers: In ECAE of linear low-density polyethylene, high-density polyethylene, isotactic polypropylene, and poly(ethylene terephthalate), a transition from the initial spherulitic structure to an oriented fibrillar structure was observed in the region of accumulated deformations e = 0.8–2.4.12–28) Such transition was accompanied by processes of fragmentation of the initial crystal structure and formation of small defective crystals. Two preferred orientations of crystalline lamellae with planes originally parallel or perpendicular to the flow direction were found.17,18) The position of the tie molecules, which may be parallel or perpendicular to the flow direction, was a possible reason why they did not remain aligned along the basic axis of stretching and compression. Bimodal orientation of crystalline lamellae was also observed in the range of plastic deformations e = 6.7–8.5 obtained with ECMAE.29,30) However, in this range of deformations, the authors31–39) observed an increase in the degree of crystallinity of high-density polyethylene, polyamide 6, and polyoxymethylene compared to the starting polymers, indicating the onset of a dynamic recrystallization process (deformation-induced crystallization). An increase in the degree of crystallinity with an increase in accumulated strain was also accompanied by an increase in the degree of perfection of the crystallites and the proportion of stretched tie molecules.40) It should be noted that in this deformation range (e = 6.7–8.5) a plateau was observed in the dependences of the thickness of crystalline lamellae and the degree of crystallinity as a function of strain, which may indirectly indicate the possibility of the beginning of activation of the reverse process - amorphization of the crystalline phase.
Although the authors did not perform detailed structural investigations using SAXS and WAXS tests in this work, the available DSC results nevertheless show no activation of the process of fragmentation of the crystalline lamellae in the strain range e = 6.7–8.5. In the case of polylactide, a phase transition from the α- to the β-form was also detected in the strain range e = 8.5.41) A phase transition (conversion of α-form crystals to γ-form crystals) was also observed in polyamide 6 at e = 4.0, but in this case the initial structure was pre-oriented with a draw ratio of 2.42) Interesting results were obtained in Ref. 43) when a bidirectional invar effect was obtained in rod-shaped billets of high-density polyethylene, polyoxymethylene, and polytetrafluoroethylene by processing by the ECMAE method at e = 8.5 (Table 1). The invar effect achieves an extremely low coefficient of thermal expansion on the order of (1–10) × 10−6 K−1 as in invar alloys. In polymers, the invar effect is due to the small linear coefficient of thermal expansion parallel to the molecular orientation. The closeness of the obtained values of the coefficient of thermal expansion to the values of the coefficient of thermal expansion of the crystalline domains along the c-axis according to the X-ray diffraction data (−12 × 10−6 K−1 44)) shows that the oriented polymer structure should be crystal structures with a high degree of continuity when the crystalline blocks are separated by a small amount of amorphous phase and a large amount of stretched tie molecules. Unfortunately, the range of deformations e = 2.4–6.7 is still unexplored, so the mechanisms of activation of the processes of dynamic recrystallization, and amorphization remain unclear.
The results of Refs. 29, 30, 34, 37) suggest that the boundary strain corresponding to the transition from the range of macroplastic deformation to the range of megaplastic (ultrahigh) deformation may depend on the following factors: the intensity of deformation ΔΓ (determined by the angle of intersection of the channels Θ according to the formula ΔΓ = ctgΘ) and the selected deformation route. In particular, it was shown in Ref. 34) on the example of two intensity of deformation of 0.54 and 0.83 that the increase in the crystallinity degree and thickness of crystalline lamellae is faster at a higher intensity of deformation. At the same time, the invar effect in semi-crystalline polymers was not achieved at a strain rate of 0.54 regardless of the magnitude of the accumulated strain.43) It should be noted that when the angle of channel intersection is extremely small (extremely low strain intensity), the deformation may degenerate by simple shear and the material will bend at the channel intersection instead. In this case, repeating cycles multiple times with the goal of accumulating ultra-high plastic deformations does not result in the required deformation of the structure.
The influence of the deformation route on the course of the processes of dynamic recrystallization, amorphization, etc. has not yet been studied. The most important research concerns the influence of the deformation route on the heterogeneity of deformation and the evolution of crystalline textures. For example, in Ref. 27) it was found that the homogeneous simple shear that the ECAE-deformed piece with e = 0.8–2.4 should experience was a heterogeneous deformation manifested by periodic shear bands and warps. The results obtained with Route A showed significant warping. Flow localization persisted with Route C, but significantly lower warping was observed. Textural changes also occurred in this region of accumulated strain, but the crystalline textures differed from each other depending on their localization in the sample volume, as there was a deformation gradient in the transverse direction and the local plastic strain decreased from the top to the bottom of the sample.20,27) At the same time, there was a steady flow region where the plastic strain was quite uniform along the longitudinal direction except for the edges. It was observed that routes A and C reached their saturation levels quickly (after about four passes, e = 3.3), compared to routes BA and BC, which require a high number of passes to reach their steady state.20,45) In the strain range e = 6.7–8.5, the plastic deformation was uniformly distributed throughout the volume of the polymer material and a change in the deformation route resulted in a uniform change in the crystalline microstructure, in particular a change in the angle of the preferred orientations of the crystalline lamellae was observed.30)
The possibility of forming biaxially oriented structures in the strain range e = 6.7–8.5, consisting of larger and more perfect crystallites connected by a large number of stretched tie chains, the possibility of activating phase transformations (occurrence of crystal polymorphism and the formation of crystalline phases with higher plasticity or thermal stability), makes it possible to obtain a unique combination of multiple increased values of stiffness, strength (with low anisotropy of the latter) and wear resistance, while maintaining plasticity at a level close to that of the original samples.27,30,46) Compared to the latter, the density and melting point increase. Figure 3 shows an example of the dependence of tensile strength and strain at break on accumulated strain for high density polyethylene subjected to ECMAE.
Influence of the value of accumulated strain on tensile strength and strain at break of high density polyethylene exposed to ECMAE.
Amorphous polymers: As with semi-crystalline polymers, the nature of the structural changes in amorphous polymers, in particular the occurrence of processes of molecular orientation and intermolecular packing, depends on the value of the true strain. For polycarbonate and poly(methyl methacrylate) it was found that the onset temperature for the glass transition is reduced due to ECAE at e = 1–2.47–52) At this stage, the exact causes for this change are still unclear. However, in the case of ECMAE, the occurrence of two glass transition temperatures Tg for poly(methyl methacrylate) and polycarbonate was observed in the range of e = 4.4–8.5.53) The additional peak complemented the main endothermic peak at Tg, which was shifted towards higher temperatures (Fig. 4). Although the formation of a crystalline phase is energetically unfavorable for amorphous polymers, the density of the formed amorphous phase nevertheless increased in the range of deformation e = 4.4–8.5. Compared with the original material, polycarbonate and poly(methyl methacrylate) exhibit an increase in stiffness, strength and, at the same time, high plasticity values. This severe plastic deformation method also improves the quasi-static crack resistance and impact fracture toughness of these materials. As for other characteristics, the situation here is more complicated and apparently requires additional studies on a larger number of materials.
DSC traces for specimens of (a) poly(methyl methacrylate) and (b) polycarbonate: (1) e = 0, (2) e = 4.4, and (3) e = 8.5. Reproduced with permission,53) 2015, Wiley.
Polymer composites. Systematic studies of the structural transformations and that occur in polymer composites over a wide range of plastic deformations, including the range of ultrahigh plastic deformations, have not yet been performed. Nevertheless, it was found, for example, in Ref. 54) that in polyolefin/graphite nanoplate composites at e = 8.5 there were processes of orientation of transition layers and polymer matrix, healing of pores in the polymer matrix and at the interface, and an increase in the degree of crystallinity due to induced crystallization deformation.
In the case of nanocomposites polypropylene/organic montmorillonite, polyamide-6/clay, the processes of exfoliation of the filler and intercalation of the polymer into the layers of the filler, typical for the process of plastic deformation, took place in the range of e = 1.2–2.3.55–62) The thickness of the primary particle layer of up to ten micrometers was reduced by these processes to hundreds of nanometers.
The absence of a destruction stage in the ultra-high plastic deformation range, as in polyolefin/graphite nanoplate composites,54) allowed the thickness of the filler aggregates to be reduced to the thickness of individual layers at e = 8.5. The latter allowed the use of sufficiently high filler concentrations (up to 10 mass%), which remained uniformly distributed in the polymer matrix. This resulted in a greater reinforcing effect of the filler63–65) as well as better electrical properties.65)
Polymer blends. For polymer blends, there is also practically no information about the characteristics of the processes of structural changes in the range of small, medium and ultra-high plastic deformations. Plastic deformation improved the interfacial adhesion between the components of polymer blends.1) For example, in a blend of polypropylene and high density polyethylene, as the accumulated strain increased, the area between polypropylene and high density polyethylene increased so that they were in closer contact.66) In the range of e = 1.2–2.4, the shape of minor high-density polyethylene phase inclusions changed from particles to long strips, significantly increasing the contact area between polypropylene and high-density polypropylene. The increased bond strength between high density polyethylene and polypropylene led to an improvement in the compatibility of high density polyethylene and polypropylene.
In Ref. 67), processing a blend of acrylonitrile-butadiene-styrene copolymer/poly(ethylene terephthalate) by the ECMAE method with e = 8.5 resulted in thinning of the polymer layers to nanometer sizes. The ultra-high deformation ensured the formation of polymer layers with a thickness equal to the thickness of the interphase. The final blend consisted entirely of the interphase. Interestingly, the degree of crystallinity of poly(ethylene terephthalate) in the acrylonitrile-butadiene-styrene copolymer/poly(ethylene terephthalate) blend increased in the ultrahigh deformation region, while in the initial blend the degree of crystallinity decreased compared to the original poly(ethylene terephthalate), which can be attributed to the inhibitory effect of the acrylonitrile-butadiene-styrene copolymer on the crystallization of poly(ethylene terephthalate). Although the processes of dissipation of elastic energy have not been studied, the possibility of such profound transformation of the structure without its destruction allowed the preparation of miscible blends based on immiscible polymers. This can be very useful for mechanical recycling of mixed plastic waste as well as for obtaining high impact strength of polymer blends.
Polymer powders. Strain-induced diffusion resulted in effective consolidation of polymer powders (nylon 12, ultra-high molecular weight polyethylene, starch, wood flour, maple hardwood) in the solid phase, in some cases eliminating the need for the addition of a plasticizer or binder.68–75) The consolidated materials exhibited significantly higher hardness and stiffness compared to a reference material produced by injection molding. The consolidated nylon 12 materials were even better than or comparable to some nylon 12 clay nanocomposites. In the range of e = 1.2–4.6, amorphization of the original crystal structure occurred. For polypropylene, for example, the degree of crystallinity at e = 8.5 exceeded that of the original powder. The latter proves the difference between the processes taking place in the range of ultra-high plastic deformations. Figure 5 shows SEM images of polypropylene before and after strain-induced consolidation.
Morphologies of compacted polypropylene powders before and after ECMAE. From left to right: e = 0; e = 8.5. Reproduced with permission,75) 2021, BME-PT Hungary.
These results indicate the need for further research to identify the features of structural and phase transformations during the transition from macroplastic to megaplastic (ultra-high) deformation and in the region of megaplastic (ultra-high) deformation. According to Glezer’s concepts, the ultra-high strain region is characterized by a shift of the fracture stage towards higher strain values due to the processes of dynamic recrystallization, amorphization and phase transformation. The available data for polymeric materials do not contradict these ideas. Nevertheless, the physics of the processes occurring in the ultra-high strain region of polymers has not been adequately studied. It is also necessary to clarify the magnitude of strain corresponding to the transition from the region of macroscopic deformation to the region of ultra-high plastic deformation. It can be assumed that the boundary of such a transition is determined not only by the type of polymer, but also by the intensity and deformation route. These circumstances are important for a number of practical applications, such as mechanical recycling of mixed plastic waste and solid-state consolidation of powder polymers.
