Structural–Mechanical AFM Study of Surface Defects in Natural

Article pubs.acs.org/Macromolecules

Structural−Mechanical AFM Study of Surface Defects in Natural Rubber Vulcanizates Ilya A. Morozov*,†,‡ †

Institute of Continuous Media Mechanics UB RAS, Academika Koroleva st. 1, 614013, Perm, Russia Perm State University, Bukireva st. 15, 614990, Perm, Russia

‡

S Supporting Information *

ABSTRACT: Structural−mechanical properties of surface defects in NR vulcanizates in undeformed and stretched states including static cracks are studied by atomic force microscopy. Elastoplastic microdefects (cracks and strands) are observed on the undeformed surface at certain sample cutting conditions. In the stretched materials, matrix detachment is found at the poles of inclusions but the propagation of such longitudinal nanocracks is hampered by inclusions and cross-linking heterogeneities. The elastic modulus within these detachments is 2 times higher the matrix. The properties of other defects− transverse cracks in the stretched rubber depend on filler concentration and distance from the crack tip. Such cracks are covered by polymer strands (thickness ∼20 nm) and a network of nanofibrils (∼1 nm). The measured modulus near the crack tip reaches 1 GPa and reduces to 200 MPa at the end. The observed features are related to the strain induced crystallization and high local extension of NR.

■

INTRODUCTION Natural rubber vulcanizates are one the most widely used materials in the rubber industry. NR has the unique ability to undergo strain-induced crystallization, which provides essential material strengthening and results in increased crack propagation and fracture resistance. Influence of loading conditions on morphology of surface of fractured materials is extensively studied for many years: pancake samples subjected to hydrostatic loading,1 fatigue defects with material relaxation2,3 and under different loading conditions,4,5 fractal analysis of wear surface and debris,6 and so on. However, it is important to consider what predetermines rupture and to reveal factors that could hamper it. The analysis of the polymer state in the vicinity of defects before sample rupture at micro- and nanoscales enhances the understanding of strengthening mechanisms and fracture resistance. Gent and Park7 investigated nucleation and propagation of adhesive (on the boundary of an individual inclusion) and cohesive (in the space between inclusions) defects in silicone elastomer filled with glass beads. SEM investigation of stretched NR has indicated that the centers of nucleation of elliptic microcracks are the micron-sized zinc oxide inclusions, which are surrounded by oriented polymer strands.8 Huneau et al.9,10 explored the cavitation in the stretched filled NR. It was shown that the crack propagation in carbon black agglomerates (size ≥5 μm due to SEM limitations) is accompanied by the formation of vacuoles around the entire agglomerate surface due to fracture of agglomerate at the matrix boundary.10 In general, the fracture behavior of the material (adhesive/ © XXXX American Chemical Society

Figure 1. Illustration of AFM experiment.

cohesive) on the boundary with filler agglomerate depends on the activity of filler surface. A network of oriented microstrands was revealed in transverse notches of stretched filled NR by SEM analysis.11 These strands had different cross-section shape and connected crack edges. Watanabe et al.12 studied inhomogeneous strandlike stiff oriented structures on the surface of stretched NR by Received: June 18, 2016 Revised: August 2, 2016

A

DOI: 10.1021/acs.macromol.6b01309 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

nanoscale but also to measure the elastic modulus of defects which is higher than of surrounding matrix due to crystallization and orientation of NR.

Table 1. Composition of the Materials amounts (phr) ingredients

NR0

NR5

NR30

NR50

NR silica seosil 115Gr stearic acid zinc oxide antiaging (6C) silane Si 266 sulfur NS DPG

100 0 2 3 0.5 0 1.58 1 1

100 5 2 3 0.5 0.4 1.58 1 1

100 30 2 3 0.5 2.4 1.58 1 1

100 50 2 3 0.5 4 1.58 1 1

■

MATERIALS AND METHODS

Preparation of Samples. The results of investigation of NR vulcanizates filled with silane-activated silica Seosil 115 Gr are presented in this work. The filler fraction was: 0, 5, 30, and 50 phr (phr = weight of filler per 100 g of polymer); these rubbers will be abbreviated as NR0, NR5, NR30, and NR50 in further discussion. The materials were manufactured by Sumitomo Rubber Industries and their compositions are given in Table 1. In addition, carbon black N220 filled NR vulcanizates were examined in the same way and no significant differences with silicafilled rubbers were found in the frame of this study. Therefore, only silica-filled rubbers are discussed further. A clean surface of the samples required for AFM imaging was prepared in two ways. In the first case, the rubber was just cut by a scalpel. This approach produces many scratches, contaminations and other surface defects associated with strain-induced crystallization of NR. The structural−mechanical properties of such imperfections are of interest in the framework of this paper. In the second method, a scalpel was pressed against prestretched (by 50%) sample. Retraction of the rubber reduces the knife−surface contact, so the number of accompanying defects is much smaller. Such samples were used to study stretched materials. For study stretched material, rubber strips (2 mm thick × 3 mm wide) were fastened in a miniature tensile device. Small notches (∼0.5 mm depth) were made on the edges of samples. After the stretching, the notches are open and propagate deeper into the sample, i.e. the rupture crack occurs (first mode of deformation). The rubber in the crack tip experiences extreme loads−any increase in tensile stress could lead to further crack propagation. The surface near crack tip and end (Figure 1) was investigated in this work. At first sight, the crack tip might be recognized as its end, but the obtained results show that the maximum elastic modulus and, consequently, extension of NR is reached in the region denoted as a tip in Figure 1. AFM Measurements. In the experiments, the AFM probe was lowered directly into the open notch (Figure 1) and the region in the vicinity of the crack axis was scanned. The obtained scanning profiles had the parabolic shape with upward branches.

AFM. Apparently, appearance of such features was related with cross-linking inhomogeneities and surface preparation conditions. X-ray microtomography (μCT) enabled visualization of the nucleation and propagation of large defects (size ≥ 13 μm) in macroscopically representative samples.13 The combination of X-ray μCT and mechanical testing machine was used to establish relationship between mechanical behavior and changes of rubber sample microstructure.14 Crack branches15,16 in rubber and secondary defects appear17 during rupture propagation. In particular, crack branching could be enhanced by cyclic loading with partial material unloading.3 X-ray diffraction was employed by Trabelsi et al.18 to study different transition zones of NR crystallinity at the tip of transverse notch in the statically stretched material. The results of wide-angle X-ray diffraction16 showed that the degree of NR crystallinity in crack tip at static loading is about two times higher than that observed at dynamic loading. SEM is the most common technique for direct examination of cracks and defects in polymers, but it does not provide high resolution images (at the scale of individual filler nanoinclusions) of nonconducting materials. In the present work atomic force microscopy (AFM) is used in a structural− mechanical study of surface defects in filled and unfilled NR vulcanizates. The use of such technique made possible to investigate not just the surface structure at micro- and

Figure 2. Surface height and corresponding elastic modulus of undeformed NR0: the area without defects (a) and the surface crack covered with a network of nanofibrills (b). Hereinafter dark colors correspond to low, bright−high values. B

DOI: 10.1021/acs.macromol.6b01309 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 3. Surface relief and elastic modulus of the strand on the surface of undeformed NR0 (a) and NR5 (b). where F is the applied indentation force and z is the vertical displacement of cantilever base. High indentation rate removes undesirable inelastic effects and provides high-resolution mapping of the surface mechanical properties.

Figure 5. Examples of longitudinal crack stop in NR5 stretched three times. Arrows indicate cracks. Tip−sample adhesion plays a significant role in nanoindentation of polymeric materials. The Derjaguin−Muller−Toporov (DMT) and Johnson−Kendall−Roberts (JKR) theories are commonly used models of elastic interactions that take into account adhesion. The DMT model assumes that adhesive interactions act outside the region of contact and the JKR model−in region of contact only. According to Maugis, the JKR and DMT theories represent two opposite extremes of a general model20 called the Maugis−Dugdale (MD) model. The use of the MD model to get elastic modulus of a material implies a solution of seven nonlinear parametric equations, which is not exactly trivial task when processing AFM measurements. In21 one can find convenient expressions that approximate MD model with an error 3. The smallest observed size of inclusions with the detached polymer is 130 nm (C in Figure 4). Judging by the shape of inclusions, these are zinc oxide particles. The stiffness of the material at the crack tip is 1.6−2 times higher than that of the surrounding polymer (see the inset in Figure 4). No detachments occur near cross-linking inhomogeneities (nanoinclusions with higher modulus than the surrounding matrix as shown before in Figure 2). However, they play a role of stress concentrators−an increase in matrix modulus is observed at the poles of these inclusions. It must be noted, that measured stiffness of deformed samples is not strictly the elastic modulus of the material because it is affected by the elongation (the higher elongation, the higher measured indentation stiffness). Detachments at the poles of inclusions are asymmetric−a crack near the inclusion A propagates upward much more readily, and a crack near the B propagates only downward. The longitudinal nanocrack stops if it meets other inclusions. It would be nearly impossible to detect extended detachments (near small-sized inclusions) in vulcanizates with even 5 phr of filler (silica or carbon black). Figure 5 illustrates the cases of longitudinal crack stop in the 5 phr filled NR stretched three times. Silicon oxide inclusions are not explicitly (as in Figures 2a and 4) observed in the filled material. This means that the filler surrounds ZnO, forming mixed agglomerates: ZnO + silica (or carbon black). We now turn our attention to a study of transverse cracks (open notches) in the stretched NR. The structural− mechanical properties of the material in cracks depend on its distance to the crack tip. Figure 6 presents AFM-images obtained near the tip and end of the transverse crack in the unfilled stretched NR (image location in crack is shown schematically by the insets in the right). The surface near end of crack (Figure 6a) is relatively smooth; the extended polymer strands spread from the crack edges to its axis and bent in the direction of its end (shown by “A” in Figure 6a). On approaching the tip (and in the case when the crack has the form of a trench crossing the entire sample), the longitudinal ruptures (Figure 6b) and parallel strands are formed on the surface. Modulus of unfilled NR near the crack end is 300 MPa and at the tip 600 MPa, respectively. Such high modulus points out that the material is in the oriented (i.e., high local extension) (semi)crystalline state. The modulus of the unfilled NR in a transverse crack reaches a maximum at a distance ±3−4 μm away from the rupture axis and then it reduces sharply. The elastic modulus profiles along one of the cross sections (shown by a dotted line) are given in Figure 6. A close-up view of strands is shown in Figure 7a. Their thickness is 8−30 nm. The height of a strand above the surface level is 6−8 nm. Examination of rupture surface with AFM-

Figure 8. Surface near crack end in filled NR. Dotted lines indicate boundaries of crack. “A” shows the nodes of branching and “B” shows thick and lengthy strands.

cannot be measured in the given experiments; thus the highest shown value of modulus corresponds to the polymer. The cutting of nonstretched samples gives rise to numerous permanent surface microdefects. Some defects had a shape of cracks (Figure 2b) with the edges connected by nanofibrils of thickness ∼3 nm.27 The modulus in the area of fibrils slightly exceeds the surrounding polymer (Figure 2b). Another type of defects on the cut surface is strands−the result of contamination of the NR after the contact with a blade. The thickness of strands can reach 500 nm (Figure 3a). The polymer in such strands has an oriented structure, and its modulus is 10−16 MPa. Strands oriented in the direction of the cut are also observed on the surface of filled NR (Figure 3b). Small strands are formed around filler inclusions; separate thick strands appear in the regions where the cut crosses the agglomerate−all are oriented in the direction of cutting (here, vertically). As the filler concentration increase, the number of small strands around inclusions increases as well. The elastic modulus of a thick strand is 30−40 MPa (Figure 3b). Indentation traces (depth of ∼9 nm) are remained on the strand surface (insert on the height image in Figure 3b) after the indentation with significant load (750 nN, indentation depth 280 nm). Thus, the material in these strands is elastoplastic. Indentation outside of these semicrystalline E

DOI: 10.1021/acs.macromol.6b01309 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 9. Surface and modulus of the strands in cracks of NR5 and NR50. Modulus values at some points are given in GPa. The arrow indicates filler cluster.

Figure 11. Crack in NR50 near its very end.

Figure 10. Filler clusters in the crack of NR5.

fibril has a circle cross-section and extends from the surface on a half, the true thickness will be 1.0−1.4 nm. Areas with lower, by an order of magnitude, modulus than the surrounding polymer are observed on the crack surface (marked by “B” in Figure 6, close-up view in Figure 7b). Their elastic modulus is close to the strand-like defects on the undeformed surface (Figure 3). Apparently, they are partially crystallized matrix fragments broken as the crack propagates. The addition of filler increases the number of strands. Surface near crack end of silica-filled NR are presented in Figure 8

probe of small radius (4 nm) provides some more information about the structure at the crack tip (unfortunately, cantilevers of these probes has low spring constant and are not suitable for stiffness measurements). The insert in Figure 7a shows a region in-between the strands; the image was obtained with a sharp probe. As one can see, the structure of smooth regions in highresolution has the form of a nanofibril network. The measured thickness of a fibril is 4 nm (a confidence level is limited by the sharpness of an AFM-probe tip). The height of the fibril part extended from the surface is 0.5−0.7 nm. If it is assumed that a F

DOI: 10.1021/acs.macromol.6b01309 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

On approaching the end of crack, the width of the stiff region decreases, and the crack begins to weave its way between clusters and disappears gradually. The modulus of a polymer also reduces. Figure 11 shows the crack in NR50 near its very end. Separate wide polymer strands that connect the crack edges are visible; the modulus of strands is ∼350 MPa. The structure of the crack surface changes as it approaches the tip. Instead of a large number of short strands, straight and lengthy fibers and many ruptures orthogonal to the crack axis were observed (Figure 12). The polymer modulus reaches 1 GPa on the area of few tens of microns wide. At the same scale, the overstressed stiff zone deviates from the straight axis−the crack branches into the regions without filler. Filler clusters (NR50 in Figure 12) are oriented along the strands but do not split into separate inclusions (or aggregates in the case of carbon black). The matrix in the regions free of filler (shown by the arrows in Figure 12) withstands a main load−the modulus in these regions has maximum in comparison with the surrounding polymer and reaches 1 GPa.

■

Figure 12. Structure of the crack near tip in NR5 and NR50.

CONCLUSION

The structural−mechanical properties of surface defects in NR vulcanizates in undeformed, stretched state and in the vicinity of open cracks in stretched materials were investigated. It has been found that depending on cutting conditions, elastoplastic inhomogeneities can be formed on the surface of undeformed NR: cracks connected by the network of nanofibrils and strands of 5−10 times greater modulus than the surrounding polymer. In the stretched materials the polymer detaches from the poles of zinc oxide particles (in unfilled material) or from the zinc oxide/filler clusters (in filled rubber). The minimum size of an inclusion at which detachment occurs is ∼130 nm. Polymer modulus in the vicinity of such longitudinal crack in the stretched material is 1.5−2 times higher the surrounding matrix. Filler inclusions, as well as stiffer segments of the polymer hamper the propagation of such nanocracks. Polymer strands are formed (thickness ∼25 nm, rarely up to 100 nm) in static transverse cracks in the stretched materials near the axis of rupture. Such strands connect filler inclusions and cross-linking inhomogeneities. The elastomer in-between the strands have the structure of nanofibril network (fibril thickness ∼1 nm). The structure of cracks and strands depends on filler concentration−at 5 and 30 phr the crack propagates into the regions without filler. Apart from the filler content, the number of strands in a crack is also increases. Near the tip of a transverse crack the elastic modulus reaches 1 GPa on the area of a few tens of microns wide, and ruptures and lengthy fibers are formed on the surface. Such high measured stiffness related to strain-induced crystallization of NR and high local extension of the material. On approaching the end of crack, the matrix modulus reduces to 200−300 MPa, the stiff region has sharp edges and its width decreases to a few microns. The number of strands increases, they begin to branch and bend toward the crack tip. Clusters retain their structure in cracks, and no splitting into separate inclusions is observed. According to observed modulus, the matrix in the regions free of filler (but not inbetween filler) withstands maximum load.

(similar results were obtained for carbon black). At low filler fraction (5 phr) the crack edges are connected by lengthy strands that occasionally branch at different angles (shown by “A”). The nodes of strand network are filler inclusions or rigid features of the polymer matrix (see Figure 2a). As the filler concentration increase, the length of strands decreases, and the width of the area with high modulus becomes smaller. The boundaries of the overstressed crack region are shown by dotted lines in Figure 8. In NR5 and NR30 the crack deviates from the straight direction into the areas free of filler. No such branching is observed for NR50−high filler concentration and its relatively uniform distribution suppresses the crack deviation. Thick and lengthy strands were observed (arrows “B” in Figure 8) in NR30 and NR50. Their appearance is attributed to partial ruptures of the surface layer during crack propagation. Some part of the material shrinks into such thick strands which look like laying above the crack surface and filler inclusions. The strands in details are shown in Figure 9. The thickness of most strands is 20−30 nm, which is comparable with the sizes of silica inclusions; separate thick strands (shown by “B” in Figure 8)up to 100 nm. The elastic modulus of polymer of the crack surface in filled NR reaches ∼1 GPa. The modulus of the strands is a little lower than surrounding smooth matrix. It is important to note that the neighboring filler inclusions in clusters preserve their relative position. Earlier, this was demonstrated by the analysis of the microstructure of stretched rubber.28 One of the clusters is shown by the arrow in Figure 9, and other two are given in more detail in Figure 10. No separation of clusters into small groups caused by stretching occurs. The matrix in the regions free of filler withstands a main portion of load. The modulus of the polymer between two clusters (Figure 10) is lower than that of the polymer above and below the clusters. In this case, the so-called occluded rubber is observed−matrix partially shielded from deformation by the close proximity of filler inclusions. The insert in Figure 10 indicates two dense silica clusters. The detachment of the polymer occurred, but clusters themselves did not split into separate inclusions. G

DOI: 10.1021/acs.macromol.6b01309 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

■

(17) Gabrielle, B.; Guy, L.; Albouy, P.-A.; Vanel, L.; Long, D. R.; Sotta, P. Effect of tear rotation on ultimate strength in reinforced natural rubber. Macromolecules 2011, 44, 7006−7015. (18) Trabelsi, S.; Albouy, P. A.; Rault, J. Stress-induced crystallization around a crack tip in natural rubber. Macromolecules 2002, 35, 10054− 10061. (19) Pittenger, B.; Erina, N.; Su, C. Quantitative Mechanical Property Mapping at the Nanoscale with PeakForce QNM. Bruker Corporation Application Note 2012, 128, 1−12. (20) Maugis, D. Adhesion of Spheres: The JKR - DMT transition using a Dugdale model. J. Colloid Interface Sci. 1992, 150, 243−269. (21) Pietrement, O.; Troyon, M. General equations describing elastic indentation depth and normal contact stiffness versus load. J. Colloid Interface Sci. 2000, 226, 166−171. (22) Shi, X.; Zhao, Y.-P. Comparison of various adhesion contact theories and the influence of dimensionless load parameter. J. Adhes. Sci. Technol. 2004, 18, 55−68. (23) Villarrubia, J. S. Algorithms for scanned probe microscope image simulation, surface reconstruction and tip estimation. J. Res. Natl. Inst. Stand. Technol. 1997, 102, 425−454. (24) Bykov, V.; Gologanov, A.; Shevyakov, V. Test structure for SPM tip shape deconvolution. Appl. Phys. A: Mater. Sci. Process. 1998, 66, 499−502. (25) Sui, Y. C.; Saniger, J. M. Characterization of anodic porous alumina by AFM. Mater. Lett. 2001, 48, 127−136. (26) Przybyszewska, M.; Zaborski, M. The effect of zinc oxide nanoparticle morphology on activity in crosslinking of carboxylated nitrile elastomer. eXPRESS Polym. Lett. 2009, 3, 542−552. (27) Morozov, I. A.; Lauke, B.; Heinrich, G. AFM investigation of structure and mechanical properties of crack zones in CB filled/ unfilled vulcanizates. Kaut. Gummi Kunstst. 2013, 66, 71−76. (28) Morozov, I. A.; Lauke, B.; Heinrich, G. Quantitative microstructural investigation of carbon-black-filled rubbers by AFM. Rubber Chem. Technol. 2012, 85, 244−263.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01309. Details of processing of AFM force curves (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*(I.A..) E-mail: [email protected]. Notes

The author declares no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work is supported by RFBR: Grants 15-08-03881 and 1401-96002. REFERENCES

(1) Gent, A. N.; Lindley, P. B. Internal rupture of bonded rubber cylinders in tension. Proc. R. Soc. London, Ser. A 1959, 249, 195−205. (2) Le Cam, J. B.; Huneau, B.; Verron, E. Description of fatigue damage in carbon black filled natural rubber. Fatigue Fract. Eng. Mater. Struct. 2008, 31, 1031−1038. (3) Saintier, N.; Cailletaud, G.; Piques, R. Cyclic loadings and crystallization of natural rubber: An explanation of fatigue crack propagation reinforcement under a positive loading ratio. Mater. Sci. Eng., A 2011, 528, 1078−1086. (4) Saintier, N.; Cailletaud, G.; Piques, R. Crack initiation and propagation under multiaxial fatigue in a natural rubber. Int. J. Fatigue 2006, 28, 61−72. (5) Le Cam, J. B.; Huneau, B.; Verron, E. Fatigue damage in carbon black filled natural rubber under uni- and multiaxial loading conditions. Int. J. Fatigue 2013, 52, 82−94. (6) Stupak, P. R.; Donovan, J. A. Fractal analysis of rubber wear surfaces and debris. J. Mater. Sci. 1988, 23, 2230−2242. (7) Gent, A. N.; Park, B. Failure processes in elastomers at or near a rigid spherical inclusion. J. Mater. Sci. 1984, 19, 1947−1956. (8) Le Cam, J. B.; Huneau, B.; Verron, E.; Gornet, L. Mechanism of Fatigue Crack Growth in Carbon Black Filled Natural Rubber. Macromolecules 2004, 37, 5011−5017. (9) Huneau, B.; Masquelier, I.; Marco, Y.; Brzokewicz, O.; Charrier, P. Fatigue damage in carbon black filled natural rubber investigated by X-ray microtomography and scanning electron microscopy. Constitutive Models for Rubber VIII 2013, 393−398. (10) Huneau, B.; Masquelier, I.; Marco, Y.; Le Saux, V.; Noizet, S.; Schiel, C.; Charrier, P. Fatigue crack initiation in a carbon black−filled natural rubber. Rubber Chem. Technol. 2016, 89, 126−141. (11) Beurrot, S.; Huneau, B.; Verron, E. In-situ SEM study of fatigue crack growth mechanism in carbon-black filled natural rubber. J. Appl. Polym. Sci. 2010, 117, 1260−1269. (12) Watabe, H.; Komura, M.; Nakajima, K.; Nishi, T. Atomic force microscopy of mechanical property of natural rubber. Jpn. J. Appl. Phys. 2005, 44, 5393−5396. (13) Le Saux, V.; Marco, Y.; Calloch, S.; Charrier, P. Evaluation of the fatigue defect population in an elastomer using X-ray computed micro-tomography. Polym. Eng. Sci. 2011, 51, 1253−1263. (14) Tada, T.; Schneider, K.; Heinrich, G.; Ishikawa, M. Effect of Constrained Strain on Failure Behavior of Rubber Vulcanizates. Constitutive Models for Rubbers IX 2015, 439−443. (15) Hainsworth, S. V. An environmental scanning electron microscopy investigation of fatigue crack initiation and propagation in elastomers. Polym. Test. 2007, 26, 60−70. (16) Bruning, K.; Schneider, K.; Roth, S. V.; Heinrich, G. Straininduced crystallization around a crack tip in natural rubber under dynamic load. Polymer 2013, 54, 6200−6205. H

DOI: 10.1021/acs.macromol.6b01309 Macromolecules XXXX, XXX, XXX−XXX