Effect of Initial Polypropylene Structure on Its Deformation Via Crazing

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EuropeanEuropean Polymer Polymer Journal 100 (2018) Journal 233–240

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Eﬀect of initial polypropylene structure on its deformation via crazing mechanism in a liquid medium ⁎ Alena Yu. Yaryshevaa, , Dmitry V. Bagrovb, Artem V. Bakirovc,d, Larisa M. Yaryshevaa, Sergey N. Chvalunc,d, Aleksandr L. Volynskiia T a Department of Chemistry, Lomonosov Moscow State University, Leninskie Gory 1-3, 119991 Moscow, Russia b Department of Biology, Lomonosov Moscow State University, Leninskie Gory 1-12, 119991 Moscow, Russia c National Research Center Kurchatov Institute, Akademika Kurchatova pl. 1, 123182 Moscow, Russia d Institute of Synthetic Polymer Materials RAS, Profsoyuznaya st. 70, 117393 Moscow, Russia

ARTICLE INFO ABSTRACT

Keywords: Atomic force microscopy has been employed to study the structure of isotactic polypropylene (PP) deformed in a Semicrystalline polymers physically active liquid medium (PALM) by the crazing mechanism. The investigations have been performed Crazing directly in the liquid, in which deformation is carried out, and under conditions excluding PP contraction. In Atomic force microscopy order to study the effect of the initial PP structure on the crazing mechanism, the polymer structure was varied Polymer structure by annealing. The crazing mechanism, as well as the parameters and morphology of the polymer, has been Polymer deformation investigated at different tensile strain values as depending on the initial PP structure. It has been shown that the Polypropylene development of deformation via the classical or intercrystallite crazing mechanisms is predetermined by the degree of crystallinity and the thickness of lamellae in initial PP. Before annealing, PP is deformed via the classical mechanism, i.e. as a uniform one-phase material, with the formation of crazes and zones of the bulk polymer located between them. Because the lamellae are «weak», the lamellar structure is transformed into a fibrillar structure, with porosity developing in crazes between fibrillar strands. After annealing of PP, the thickness of lamellae, degree of crystallinity, and, accordingly, the drag of crystallites increase, cavitations arise in the amorphous phase, and the fibrillar-porous structure develops in the space between crystallites moving apart from each other.

1. Introduction The theory of the joint action of a medium and stress on the formation of crazes and cracks in polymers had, initially, been developed In view of the diversity and complexity of the structures of crys- for glassy polymers and was, then, used to advantage for crystalline talline polymers, many models and mechanisms of their deformation polymers. In [12,13], a mechanism was proposed that involved stress- have been considered in the literature (reviews [1–7]). The deformation induced swelling and relevant plasticization of polymer amorphous of crystalline polymers is accompanied by various processes, such as the phase. The authors of [14] established the key role of the closeness separation, slip, bending, reorientation, fragmentation, microfibrilla- between the solubility parameters of a polymer and a liquid: the higher tion, local melting, martensitic transformations, stress-induced crys- the affinity of a liquid for a polymer the higher the stress-induced tallization of lamellae, development of cavitations, etc. swelling and plasticization of the polymer. Stretching of polymers in physically active liquid media (PALMs) by Liquids that do not cause swelling of polymers may also efficiently the crazing mechanism is a peculiar type of deformation. It is char- affect their crazing. The influence of such media is associated with the acterized by the development of nanosized porosity in a polymer adsorption-induced decrease in the polymer strength (Rehbinder’s ef- [8–11]. The formed nanopores are filled with the liquid medium, in fect). Crazing is accompanied by the total development of an interfacial which the stretching is performed. Morphologically, such porosity is surface in a polymer due to its dispersion into fibrils – nanosized ag- realized via the nucleation and growth of peculiar zones of a plastically gregates separated by voids. Since the specific surface area of a crazed deformed polymer. These zones (crazes) have a fibrillar-porous struc- polymer amounts to several hundred square meters per gram [10], ture composed of oriented polymer fibrils 5–20 nm in diameter sepa- adsorption of liquid molecules on the surface of such a polymer may rated by microvoids of nearly the same sizes. dramatically reduce the interfacial surface energy.

⁎ Corresponding author. E-mail address: [email protected] (A.Y. Yarysheva). https://doi.org/10.1016/j.eurpolymj.2018.01.040 Received 3 November 2017; Received in revised form 16 January 2018; Accepted 30 January 2018 Available online 01 February 2018 0014-3057/ © 2018 Elsevier Ltd. All rights reserved. A.Y. Yarysheva et al.

European Polymer Journal 100 (2018) 233–240

Fig. 1. SEM micrographs taken from cleavages of polypropylene deformed by 100% in (a) ethanol via the classical and (b) heptane via the intercrystallite crazing mechanisms [16]. Panel (a): (1) craze boundary, (2) bulk polymer, and (3) craze with ﬁbrils. The stretching axis is denoted by arrow.

At present, two types of crazing are considered, which are realized in the medium used for crazing. upon deformation of polymers in PALMs, namely, the classical crazing Isotactic polypropylene (PP) is of special interest for the study and (accompanying stretching of glassy and some crystalline polymers) comparison of the two types of crazing, because this polymer can be [8–10] and the intercrystallite or delocalized crazing (observed only for deformed via both the classical and intercrystallite crazing mechanisms crystalline polymers) [10,15]. The main difference between these two depending on the nature of a liquid medium (Fig. 1). However, there is types of crazing consists in the sizes of local zones of deformation. For one more possibility of the passage from the classical to the inter- polymers deformed by the classical crazing mechanism, the craze width crystallite crazing, namely, the modification of an initial PP structure may be as large as several micron. In this case, crazes develop due to the by annealing followed by deformation in a liquid medium in which the gradual consumption of a nonoriented bulk polymer, which con- polymer is unswellable. For example, before annealing PP is deformed tinuously passes into a highly dispersed oriented state in crazes. The in aliphatic alcohols via the classical crazing mechanisms, and after fibrillar-porous structure is thermodynamically unstable, and, after a annealing PP is deformed in aliphatic alcohols via the intercrystallite medium is removed from the pores and the stress is eliminated, coa- crazing mechanisms. The goal of this work was the visualization of the gulation of fibrils, contraction of a sample, and collapse (healing) of the structures of initial and annealed PP, as well as the structures the porous structure occur. polymer crazed in PALMs before and after annealing. Thus, we in- In the case of classical crazing, the large width of crazes and the vestigated the influence of the initial PP structure on the mechanism of presence of undeformed polymer zones enable one to observe the re- its crazing, i.e., the classical or intercrystallite one. sidual fibrillar-porous structure of crazes with the help of scanning electron microscopy (SEM) even after removal of the medium (Fig. 1a). 2. Materials and methods However, the «native» structure, i.e., the structure formed directly during the classical crazing of a polymer in a liquid medium, has not yet 2.1. Materials been visualized and studied.

The case of the intercrystallite crazing is still more complex for Isotactic PP films with a thickness of 90 µm (Mw 250 kDa, Mw/ observation of a polymer structure. Previous attempts to observe in- Mn = 3.5) produced by melt extrusion and the same films annealed dividual crazes with the use of SEM failed because of the collapse of the under laboratory conditions at 140 °C for 180 min were used in the porous structure after stress elimination and removal of a liquid experiments. Ethanol (85%) was applied as a PALM. medium (Fig. 1b) [16]. The majority of the data on the structure of crystalline polyolefins 2.2. Procedure for investigating the structure of PP deformed in the medium deformed in liquid media were previously obtained by the small-angle X-ray scattering (SAXS) and pressure-driven liquid permeability Variations in the structure of polymers being stretched is, as a rule, methods, which made it possible to estimate the effective diameters of studied by AFM using a microscope installed directly on a tensile- pores, the specific surface areas and diameters of fibrils, and the long testing machine. The scanning is performed with a cantilever actually periods for diverse polymers and liquid media [16]. However, the free of limitations on sample sizes and masses. The measurements with calculation of the structural parameters by the aforementioned methods the use this system, which combines an atomic force microscope and a requires the use of model concepts of the fibrillar-porous structure. stretching unit, are usually carried out in the following way: the load on Thus, although the notions of the structure of crazed polymers were a film being stretched is increased stepwise, and an AFM image is taken formulated rather long ago, the visualization of these notions and the from a chosen fragment of the film surface at each strain value. The direct determination of structural parameters have remained to be native structure of PP was studied as described below. First, a sample unsolved problems so far. was stretched in the liquid medium to some tensile strain value. Then, Atomic force microscopy (AFM) is successfully used to study the the sample remaining in the liquid medium and fixed in the clamps of surface structure of polymers [17]. This method was, for the first time, the stretching unit was fastened over its perimeter to a circular frame to used in [15,18] to investigate the «native» structure of high-density retain its sizes and prevent it from contraction. The sample fastened to polyethylene (HDPE) deformed by the intercrystallite crazing me- the frame was placed into a Petri dish containing the medium (ethanol), chanism, because AFM enables one to carry out investigations directly which had been used as a PALM during stretching. A support was

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samples had sizes of 40 × 20 mm) at a constant rate of 5.4 mm/min with an Instron 4301 universal dynamometer. Polymer films to be deformed in the medium were fixed in the clamps of the dynamometer in a special bag filledEuropean with the Polymer liquid Journal medium. 100 (2018) Three 233–240 or four samples were subjected to mechanical tests in each series of experiments. The root- mean-square error in the measured values was no larger than 5%.

2.6. Small-angle X-ray diﬀraction analysis

The analysis was performed with Nanostar (Bruker AXS) and Hecus S3-Micropix setups equipped with generators of Cu Kα radiation (λ = 0.154 nm). The long period was calculated from SAXS data as L = (k∗λ/2sin θ where angle θ was determined from the position of the maximum in a scattered light intensity curve. Correction coefficients k for Nanostar (Bruker AXS) and Hecus S3-Micropix were 0.94 and 1, Fig. 2. Scheme of sample preparation: (A) initial sample fastened in the clamps, (B) stretching of the sample in the PALM (stretching direction is denoted by the arrow), (C) respectively. fixation of the stretched sample in the circular frame, and (D) AFM measurement [18]. 2.7. Differential scanning calorimetry placed at the center of the film to avoid vibration. The samples to be examined by AFM were prepared in accordance with the scheme de- The thermophysical properties of PP were investigated with a TA picted in Fig. 2 [18]. The used scheme allowed us to perform the AFM 4000 thermoanalyzer (Mettler) equipped with a DSC-30 low-tempera- examination of the surface of a deformed film directly in the medium ture cell. The heating rate and sample mass were 10 K/min and avoiding the contraction of the samples. Thus, we investigated the 0.5–1 mg, respectively. The degree of PP crystallinity was calculated as native structure of the samples. follows:

ΔH 2.3. AFM experiments χ = ·100% ΔHχ=100%

A Solver ProM atomic force microscope (NT-MDT, Zelenograd, where ΔH is the melting heat experimentally determined for PP and Russia) in the «scanning by probe» configuration equipped with a ΔHχ=100% = 190 J/g is the melting heat of an ideal PP crystal. Lamella Smena scanning head and closed-loop feedback sensors (scanner range thickness was calculated by the following equation: d = Lsaxs (ρρ/)χ χ of 100 × 100 × 7 μm) was used to perform the AFM experiments by where Lsaxs is the long period determined from SAXS data; ρ and ρχ are scanning in the semicontact and contact modes. Scanning in the PALM the densities of the polymer and its crystalline phase, respectively; and was performed with the help MSCT-AUHW cantilevers (Bruker) oper- χ the degree of crystallinity determined by DSC. ating in semicontact and contact modes. The Femtoscan online software package (Advanced Technologies Center, Russia) was employed to process the AFM images. Each scan 3. Results and discussion was processed with the help of the «Fit Lines» and «Plane Fit» functions to avoid the shift of rows and the general slope, respectively. The noise 3.1. PP structure before and after annealing was, when necessary, suppressed by median filtering or averaging. The fi – macroscopic height variations were, sometimes, eliminated by spline Isotactic PP lms with spherulite sizes of 1.5 4 µm were chosen for filtering. To plot and analyze the sections, each section profile was the experiments Fig. 3a. Unannealed isotactic PP had a smectic struc- fl constructed by averaging the profiles over three adjacent section lines. ture with relatively irregularly packed chains. Annealing in uences the To determine the parameters of the deformed PP structure, at least 100 microscopic structure and mechanical properties of the polymer [19]. sections in the direction of the stretching axis and at least 100 sections Annealing of a semicrystalline polymer at temperatures well above its perpendicular to the stretching axis were analyzed. The mean values glass-transition temperature increases the mobility of polymer chains, and the standard deviations of the long period L, the distance between thereby inducing secondary crystallization. This process leads to an the adjacent fibrils l and the lamella width d were calculated. increase in the degree of crystallinity, healing of defects in the crystal packing, and thickening and lateral growth of initial lamellae [20–27]. Upon annealing, the smectic phase of PP is transformed into the 2.4. Determination of PALM-crazed polymer porosity monoclinic crystalline modification [28] (WAXS on Fig. 3b). The spherulitic α-PP structure consists of «parent» lamellae radially growing The porosity of the polymer deformed in the PALM was determined from a center and «daughter» lamellae growing transversally with re- by measuring the increment of a sample volume after stretching. The spect to the parent lamellae (Fig. 3c). Such structure is referred to as fi geometric sizes of PP lms stretched to preset strain values and fastened «cross-hatched» [29,30]. The β-modification of PP is a metastable phase in the clamps were measured using an IZV-2 optimeter and a projector with a low packing density. Lamellae of β-spherulites grow radially with a tenfold multiplication. The total porosity was calculated by the from a center. Being a more stable modification, α-spherulites prevail in − following equation: W =(Vt V0)/V0, where V0 and Vt are the volumes commercial samples; however, this circumstance does not exclude the of the initial and stretched samples, respectively. The measurements presence of spherulites of other types and the smectic phase in them. fi were performed for, at least, ve samples. The root-mean-square error The SAXS study of PP films has shown that the long period for was no larger than 3%. unannealed PP (una-PP) is 12 nm, while that for annealed PP (a-PP) is two times larger (24 nm) (Fig. 3d), and, according to the DSC data, the 2.5. Mechanical properties of PP deformed in air and PALM degree of crystallinity increases from 49 to 57% after annealing. The calculations of the lamella thickness from the SAXS and DSC data have Mechanical properties of PP were investigated in air and ethanol shown that it increased from 5.4 to 12.6 nm. Thus, annealing increases under the conditions of planar stretching (the working part of the the thickness of lamellae and the degree of crystallinity of PP.

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Fig. 3. (a) AFM images of initial PP surface. (b) WAXS patterns of (1) una-PP and (2) a-PP. (c) Structure of α-spherulite: a, b, and c are the crystallographic axes [29,30]. (d) SAXS patterns of (1) una-PP and (2) a-PP.

3.2. Mechanical behavior of PP deformed via the classical and crazing is a result of two factors: a physically active medium and a intercrystallite crazing mechanisms stress. The medium is the same with respect to the polymer samples of the same nature (a-PP and una-PP). However, for a-PP, which has a Fig. 4 presents the stress-strain curves for PP deformed in air and higher crystallinity and a larger thickness of lamellae, as compared with PALM before (una-PP) and after (a-PP) annealing. una-PP, the stress developing in the deformed polymer is, as a whole, The comparison between the figures shows that una-PP is stretched higher, and its stress–strain curve of deformation in air lies markedly at lower stresses than a-PP is (Fig. 4a, curves 1,3). Moreover, it can be higher than that of una-PP (Fig. 4a, curves 1,3). Hence, the plasticizing seen that the yield point of una-PP in PALM is actually no lower than and adsorption-active effects of PALM are strongly pronounced upon that for the polymer stretched in air (Fig. 4a, curves 1,2). However, the the development of deformation by the crazing mechanism; therefore, curves describing an increase in the specific volume show a rise in the the yield point substantially decreases upon the deformation in PALM porosity with strain, which is typical of deformation by the crazing (Fig. 4a, curves 3,4). In una-PP, which has weak lamellae and lower mechanism (Fig. 4b). Unannealed PP can be deformed only by 150% crystallinity, the stress arising in the course of deformation is much (Fig. 4b, curve 1), with the porosity being no higher than 50% with lower than that in a-PP; therefore, the joint action of the medium and respect to the initial volume. The stretching of a-PP in the liquid stress is much less pronounced, and a substantial decrease in the yield medium exhibits a lower yield point than the stretching in air (Fig. 4a, point relative to that for stretching in air is not observed (Fig. 4a, curves curve 3,4). The porosity of a-PP increases with a rise in the tensile strain 1,2). (Fig. 4b, curve 2). The observed fact that a decrease in the yield point upon deformation in PALM exists for a-PP and is absent for una-PP is related to different initial structures of PP before and after annealing. From the point of view of the mechanical response of a polymer,

Fig. 4. Panel (a): engineering stress-strain curves for una-PP deformed in (1) air and (2) PALM and a-PP deformed in (3) air and (4) PALM. Panel (b): volume porosities as functions of tensile strain for (1) una-PP and (2) a-PP deformed in PALM.

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Fig. 5. AFM images of a-PP deformed in PALM via the delocalized mechanism by (a) 100, (b) 300%; proﬁle sections for measuring the long period (L), the thickness of lamellae (d), the sum of the ﬁbril thickness and the pore width (l). The stretching direction is denoted by the arrows.

3.3. AFM study of the structural evolution of PP deformed via the classical L and intercrystallite crazing mechanisms 90 The difference between the structures of PP deformed by 100% via the two crazing mechanisms in different PALMs has been considered in 75 the Introduction (Fig. 1). It can be seen that the structure of PP crazed 60 by the classical mechanism is nonuniform and consists of bulk polymer zones crossed by crazes (Fig. 1a). The structure of PP deformed by the 45 intercrystallite crazing mechanism looks like a uniform one (Fig. 1b). Annealing of PP changes the classical mechanism to the intercrystallite 30 mechanism of crazing in the same PALM. AFM enables one to study in greater detail the structure of crazes directly in a medium used for 15 sample stretching, i.e., under the conditions that exclude fibril coagu- 0 lation, which accompanies the removal of the medium, and prevent 0 50 100 150 200 250 300 ε,% samples from contraction. In the AFM images obtained after a-PP deformation in the PALM via Fig. 6. Dependences of the average long period values along the stretching axis on the tensile strain for a-PP deformed in PALM. the intercrystallite crazing mechanism by 100 (Fig. 5a) and 200%, the fibrillar-porous structure is not seen clearly, although the volumetric data indicate that the porosity of the samples relative to the initial Table 1 polymer sample is as high as 80% (Fig. 4b). Individual finely crumbled Parameters (mean ± standard deviation) of the structure of a-PP as depending on tensile fragments of lamellae are observed as granules or knots with sizes (d is strain (ε) during crazing. denoted on Fig. 5b) of 18 ± 3 nm. As the tensile strain is increased to ε,% L, nm d, nm l, nm 300%, the lamellae are further separated, and the fibrillar-porous structure of the deformed polymer and the lamella fragments oriented 100 36 ± 8 18 ± 3 – – along the stretching axis become detectable. The size of lamella frag- 200 47 ± 12 17 ± 3 300 55 ± 15 18 ± 4 23 ± 5 ments (granules) remains almost unchanged at all of the studied strain values, while the distance (letter l is denoted on Fig. 5b) between the tops of fibrils (the sum of the fibril thickness and the pore width), which region 2 in Fig. 1 a plotted on the basis of the SEM data. During de- can be determined only at a tensile strain of 300%, is nearly formation, large wide (50–100 nm) flakes of the polymer material (3) 23 ± 5 nm. are «splintered off» from this zone, with a network of fibrils (4) oriented The long period (L is denoted on Fig. 5b, distance between the tops in the direction of the stretching axis being observed between them. of lamella fragments in the direction of the stretching axis) increases The internal structure of a craze is presented in Fig. 7b. It can be almost linearly with tensile strain (Fig. 6). It is obvious that the seen that the fibrillar strands consist of lamella fragments oriented stretching is accompanied by separation of lamellae, rotations of la- along the stretching axis. Pores are observed between the strands. The mellae oriented in other directions (tangential or radial), and shear thickness of the strands and the size of pores are comparable with the deformation and slip of lamella fragments. A linear increase in PP long size of the cantilever; therefore, the only parameter that may be reliably period with a rise in the strain value was also observed using SAXS in measured is the distance between the tops of the fibrillar strands, i.e., [31]. the sum of the strand thickness and the pore width (l). According to the The structural parameters of a-PP deformed via the intercrystallite results of the measurements, this parameter is equal to nearly crazing mechanism are presented in Table 1. 20–40 nm; i.e., the strand thickness and the pore size may be estimated The AFM image of una-PP deformed in the PALM via the classical to be no larger than 10–20 nm. crazing mechanism by 100% (Fig. 7a) distinctly shows the craze The analysis of spherulite deformation is complicated by the fact boundary (it is denoted by numeral 1 in the image and corresponds to that lamellae in different parts of a spherulite are oriented at different boundary 1 in Fig. 1a) and undeformed zone 2, which corresponds to angles to the direction of the applied force and are deformed in

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Fig. 7. (a) Craze of una-PP deformed in PALM by 100% via the classical crazing mechanism along the vertical stretching axis, (1) craze boundary, (2) bulk polymer, (3) breakaway blocs, and (4) fibril network; (b) AFM images of the internal structure of una-PP deformed in PALM by 100% via the classical crazing mechanism. The stretching axis is denoted by the arrow. different manners [32,33]. The general pattern of spherulite deforma- structures are observed in both cases. However, in the case of the tion comprises the initial transformation of its shape into an ellipse classical crazing, the fibrillar strands are formed by lamella fragments elongated in the direction of the stretching axis followed by the dis- arranged along the stretching axis, with the porous structure being ruption of the spherulites with increasing strain. In geometrically dif- observed between them. Upon the intercrystallite crazing of PP, the ferent regions of a spherulite (equatorial, meridional, and diagonal), lamellae, which have become thicker after annealing, are separated and deformation processes occurring in the crystalline and amorphous fragmented; therefore the fibrils and porous structure are located be- phases develop altogether and mutually influence each other [3,30,34]. tween the lamellae. It can be seen that, in the case of the classical The analysis of the deformation-induced transformations of PP spher- crazing, fibrils are much thicker than those observed upon the inter- ulites is a still more complex problem. The initial complex structure of crystallite crazing. For comparison, Fig. 8c schematically represents PP α-spherulites, which consist of radial and tangential lamellae, leads crazes of PP deformed by different crazing mechanisms. to discrepant results and difficulties in the interpretation of the data on So, why an increase in the thickness of lamellae and the degree of the structural rearrangements in PP. For example, the IR study of the crystallinity as a result of annealing lead to the change in the de- deformation of an individual α -spherulite of PP [35] has led to the formation mechanism? conclusion that transverse lamellae are present not only in the initial, A necessary condition for the realization of the intercrystallite but also in the deformed polymer. (delocalized) crazing is a high structural microheterogeneity of a The SAXS study [36] of the structure of isotactic PP being deformed polymer. Such microheterogeneity is inherent in crystalline polymers, in carbon dioxide under supercritical conditions has resulted in the which represent two-phase systems consisting of crystallites uniformly suggestion of a scheme of deformation with the formation of pores distributed in an amorphous matrix. In the case of PP, the maximum between stacks of lamellae. The authors of the cited work explained the structural heterogeneity is reached only after annealing, which in- formation of the porous structure in PP by the plasticizing effect of creases the thickness of lamellae and the degree of crystallinity. carbon dioxide dissolved in amorphous zones of PP; i.e., they actually It is obvious that the formation of a fibrillar-porous structure upon considered the crazing of PP, although this term as such was not used. crazing of crystalline polymers is associated with the appearance of X-ray diffraction and SEM studies [33] have shown that the de- cavitations. Beginning from Peterlin's works [1,39], many deformation formation of PP subjected to cavitation is accompanied by the frag- models have been used to consider the role of cavitations in the plas- mentation of crystallites with a substantial decrease in their long- ticity of crystalline polymers. Cavitation is the phenomenon of a local itudinal sizes (by 50–55% of the initial values). The fragmentation of instability associated with the appearance of microvoids in the struc- lamellae seems to be the most intense process among those occurring in ture of polymers subjected to stretching or shearing. The problems the range of local tensile strains equal to 0.3–1.5. At local tensile strains concerning the nucleation and development of cavitations during de- higher than 1.5, an analyzed material is, probably, deformed via rota- formation of crystalline polymers have been considered in greatest tion of lamella fragments, variations in the shapes of cavities, and detail elsewhere [40–44]. It has been found that cavitations are nu- further crystallographic slips with no marked variations in the length of cleated before the yield point, and their appearance is associated with crystallites; i.e., the lamella fragmentation process seems to be com- the elimination of the confinement in the deformation of the amor- pleted. PP deformation in air, which is accompanied by the appearance phous component upon the onset of the deformation of the crystalline of cavitations, fragmentation of lamellae, and orientation of lamella component. A new fibrillar morphology arises at the stage of strain fragments along the stretching axis, was considered in [37]. softening in the vicinity of the yield point. The related processes of AFM images show that PP deformation in the PALM via the delo- micropore formation in the amorphous phase and fragmentation of calized crazing mechanism, as well as in air, is accompanied by dis- lamellae weaken the mutual confinement of the phases and increase the ruption of spherulites, fragmentation of lamellae, and separation of the molecular mobility of polymer chains in both of them. In [40], it was fragments of lamellae. The orientation of PP macromolecules in the shown that an increase in the free volume of a polymer enhanced the course of crazing in a PALM has been confirmed by the data of work intensity of cavitation processes. The authors have assumed that the [38], in which FTIR studies have shown that the degree of PP or- nucleation of cavitations is related to stress-induced fluctuations in the ientation increases linearly with tensile strain. free volume of macromolecules upon stretching. At the macrolevel, the Upon deformation via the classical mechanism, at a total tensile appearance of cavitations can be seen as whitening of samples at tensile strain of 100%, a craze of una-PP contains a structure, which, at first strain values close to the yield point [41]. sight, is very similar to the structure of a-PP deformed by 300% via the The deformation mechanism of crystalline polymers is affected by intercrystallite crazing mechanism (Fig. 8a,b). Fibrils and porous many factors, such as the strain rate, temperature, and the molecular

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Fig. 8. AFM images and schematic representation of crazes in (a) una-PP deformed in PALM by 100% via the classical crazing mechanism and (b) a-PP deformed in PALM by 300% via the intercrystallite crazing mechanism. (c) The scheme constructed based on the images a,b. mass and structure of a polymer. Being a component of the deformation normal to the stretching axis. In a simplified form, it may be supposed process, cavitations depend on the factors related to the deformation that two forces are applied to a stretched body; i.e., an expanding conditions and structural characteristics. In particular, an important mechanical stress and the surface tension force, which «strives» to di- factor determining the possibility of the appearance of cavitations is minish the interfacial surface area. Cavitations arise due to the ex- lamella thickness [41,45,46], which must be larger than some critical panding stress in the sites of craze nucleation (the sites of the local value. Otherwise, intralamellar slip and shift rather than the appear- increase in the free volume). The subsequent fate of the local sites of ance of cavitations will be prevailing processes. In materials with thin deformation and cavitations depends on a medium in which deforma- and compliant lamellae, the deformation of crystallites begins before tion is realized. When deformation is implemented in a PALM, the the stress overcomes the drag of the amorphous phase. The authors of highly developed surface of growing fibrils is stabilized, and the process [41] have assumed that a high absolute value of the negative pressure occurs via the crazing mechanism; the widening of the crazes is ac- necessary for the nucleation of cavitations in polymers may be caused companied by the fragmentation of lamellae, transformation of the la- by entanglements of macromolecular chains, therefore, deformation mellar structure into the fibrillar one, and the development of pores may induce cavitations only in polymers, for which the critical shear from the cavitations. stress necessary for a slightest slip of crystals is higher than the stress Thus, before annealing, PP with thin lamellae is, as well as glassy required for the nucleation of cavitations in the amorphous phase. polymers, deformed via the classical mechanism, i.e., as a uniform one- However, the data of our work have shown that deformation of phase material, with the formation of crazes and zones of the bulk polymers with thin lamellae (e.g., una-PP) in a PALM is also accom- polymer located between them. Because the lamellae are «weak», the panied by the nucleation and development of cavitations. According to lamellar structure is transformed into a fibrillar structure, with cavi- the published data [10], the nucleation of classical crazes is caused by tations and porosity developing in crazes between fibrillar strands. the imperfectness of an initial polymer surface, which contains a large After annealing, the thickness of lamellae and, accordingly, the drag number of microdefects with different «risk levels». The nucleation of of crystallites increase, cavitations arise in the amorphous phase, and each individual craze is of a local-critical character, with the density of the fibrillar-porous structure develops in the space between crystallites the crazes being dependent on the deformation conditions and the moving apart from each other rather than in the locally instable zones perfectness of a polymer surface. As soon as a craze is nucleated at a containing surface defects as it does in the case of the classical crazing. structural defect of a polymer surface, it begins to grow in the direction The formation of the cavitations and the growth of the pores are

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