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Thus, this peculiar stress whitening behavior under large strain deformation ... Pingdong WeiJunchao HuangYing LuYi ZhongYongfeng MenLina ZhangJie Cai...
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Cavitation in Isotactic Polypropylene at Large Strains during Tensile Deformation at Elevated Temperatures Ying Lu, Yaotao Wang, Ran Chen, Jiayi Zhao, Zhiyong Jiang, and Yongfeng Men* State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, University of Chinese Academy of Sciences, Renmin Street 5625, 130022 Changchun, P. R. China S Supporting Information *

ABSTRACT: Two quenched isotactic polypropylene samples with different molecular weight were used to explore the sudden stress whitening activated at large strains during stretching at elevated temperatures via the ultrasmall-angle Xray scattering technique. This kind of whitening was confirmed to be caused by the creation of cavities within the highly oriented samples. The critical strain for initiating the stress whitening increases with the increase of stretching temperature and molecular weight whereas the critical stress for the stress whitening depends only on molecular weight irrespective of stretching temperature. Thus, this peculiar stress whitening behavior under large strain deformation can be understood as a consequence of disentanglement of the highly oriented amorphous network initiated by the breaking of interfibrillar tie chains. During large strain deformation, two independent processes can occur. Besides the disentanglement induced cavitation, the stacked lamellar structure within microfibrils can be destroyed in the high molecular weight sample. In such case, the critical stress for lamellae destruction is lower than that of breaking of interfibrillar tie chains. stretching temperature lower than 70 °C. Bao et al.16 also pointed out that the cavitation in iPP diminished at high deformation temperature. Furthermore, different types of cavitation proceeding in P1B during deformation processes at different temperatures were observed where more intensive cavitation in P1B stretched at 100 °C than at 75 and 90 °C was shown. This kind of cavitation is named as the type of “cavitation without reorientation” with cavities orienting along the deformation direction as soon as they appear. Such a cavitation process at high stretching temperature has been attributed to the crystalline form transition from more loosely packed form II to dense packed form I which takes place at the strain not far away from the yield point.5 Cavitation around the yield point normally does not affect the drawability of the samples, meaning that such cavities can be effectively stabilized after generation. Structurally, it is still not certain on how such cavities change their shapes upon further elongation of the material and which role they play upon ultimate failure of the material. Such uncertainty is due to the complex structural transformation of isotropic semicrystalline polymer with spherulitic structure upon stretching. In general, the materials end up with a highly oriented hierarchical fibrillar structure.17−19 Peterlin20 and Ward et al.21 proposed a

1. INTRODUCTION Polyolefin, e.g., isotactic polypropylene (iPP),1 polyethylene (PE),2,3 and poly(1-butene) (P1B),4,5 stretched at low temperatures is always accompanied by a macroscopic view of whitening. The appearance of this phenomenon is due to the production of cavities in the materials around the yield point.6−11 In general, the process of cavitation depends on factors such as preparation method of the material and testing conditions. Both higher crystallization temperature12 and lower stretching temperature can promote this process.13 In addition, the strain whitening was found also heavily depending on microstructures such as crystallinity and lamellar thickness in polyethylene.14 Early work by Peterlin suggested that such cavitation process occurred during the transformation of crystalline lamella into microfibrils (a process termed as microfibrillation) based on electron microscopy investigations of single crystals during stretching.15 However, in bulk samples microfibrillation can proceed without cavitation evidenced by many tensile experiments of semicrystalline polymers without strain whitening. Phenomenologically, such results of whitening or not during stretching can be understood as a result of competition between yielding and cavitation. Only if the intrinsic cavitation stress of the sample is smaller than its yield stress can cavitation be promoted before large-scale plastic flow sets in. Pawlak and Galeski13 investigated the influence of deformation temperature on the activation of cavitation in iPP. They found that the stress whitening is only present at the © XXXX American Chemical Society

Received: April 19, 2015 Revised: August 10, 2015

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Macromolecules model of the fibrillar structure where the sample is composed of fibrils being made up of microfibrils. The microfibrils contain stacks of crystalline lamellae with normal parallel to the tensile axis and the crystalline lamellae are embedded in an amorphous network. Meanwhile, Peterlin20 considered that the stretching of the fibrillar structure led first to slippage between adjacent fibrils followed by the slippage of microfibrils. It was considered that it is easier to overcome the opposing forces imposed by chains connecting adjacent fibrils than separating more strongly coupled microfibrils or pulling apart lamellae that are tied by a dense network of amorphous chains. Nevertheless, Tang et al.22 demonstrated experimental results showing a contrary model that slippage between microfibrils occurs earlier than that between fibrils in uniaxial deformation of overstretched polyethylene. They suggested the following deformation mechanism for the fibrillar structured samples: The first step is a slight stretching of the amorphous layers between the crystalline lamellae within microfibrils followed by the slippage of microfibrils resulting in an increase of the length of the fibrils. This inter-microfibrillar slip is limited as the adjacent microfibrils are linked by inter-microfibrillar amorphous chains that are increasingly stretched during slippage creating a network of taut polymer chains. Hence, the next step of deformation is accommodated by the slippage of the fibrils. Practically, both slip processes mentioned above are limited and can be terminated by either breakage of the taut network or disentanglement of the polymer chains of finite length. The last step of deformation is related to the ultimate failure behavior of a semicrystalline polymer which is therefore industrially highly relevant. For example, the essential step of production of high strength packaging tapes from iPP is the orientation of macromolecules by tensile deformation at elevated temperature to high draw ratios up to 15. The final products are smooth and flawless when iPP with melt flow index around 3 or lower was drawn at sufficiently high temperature yet below the melting point. If either of these conditions is not fulfilled, the resulting tape is whitened. Besides whitening, there is an effect of peeling of tiny microfibrils which is intensified under similar conditions as the tendency to whitening although strong peeling can be present without apparent whitening. These problems are common and known for plastics engineers and can be overcome based on empirical knowledge without an in-depth understanding of the microscopic mechanism. In this work, we focused on the structural changes during stretching of quenched iPP samples of different molecular weight at late stage aiming to provide new insights into the creation and development of microscopic failure in connection with basic molecular parameters of the system. The choice of quenched iPP as a model system is due to the absence of cavitation before fibrillation process in the samples. Macroscopically, we observed a strong stress whitening at the late stage of stretching where only fibrillar structure presents in the system. This stress whitening was caused by the generation of cavities of very different refractive index in the fibrillar structure as evidenced by in situ ultrasmall-angle X-ray scattering (USAXS) experiments. Such cavitation process at the late stage of deformation can be linked to the amorphous network rapture due to breaking of interfibrillar tie molecules followed by disentanglement creating interfibrillar voiding. Molecular weight dependency of the critical stress for initiating the stress whitening supports such an idea that breaking of interfibrillar tie molecules followed by a disentanglement process at the late stage of deformation is prior to macroscopic failure.

2. EXPERIMENTAL SECTION Two iPPs were purchased from Aldrich Polymer Products, being the iPP250K (product no. 427888) with a molecular weight of Mw = 250 000 g/mol and a polydispersity (Mw/Mn) of 3.7, and the iPP580K (product no. 427853) with a molecular weight of Mw = 580 000 g/mol and a polydispersity (Mw/Mn) of 3.5. The pellets were molded in a hot press at 200 °C, developing films of 0.5 mm in thickness, which were quickly quenched into the ice water for half an hour. After that tensile bars with dimensions of 10 × 5 × 0.5 mm3 were obtained with the aid of a punch. Uniaxial tensile deformation was carried out using a portable tensile testing machine (TST350, Linkam, UK) with a clamping distance of 15 mm. In order to measure the strain of the deformed area, optical photoimages of the samples were employed during stretching processes. The deformation of the materials is not homogeneous ascribed to the appearance of necking so that the engineering strain is no longer suitable for describing such deformation process. Therefore, the true strain is needed to express the local strain of materials during stretching process. Thus, the Hencky strain εH is used as a basic quantity of the true strain, which is defined as εH = 2 ln

b0 b

(1)

where b0 and b are the initial and instantaneous widths of sample during stretching process, respectively. In situ USAXS experiments were conducted with a modified Xeuss system of Xenocs, France, at a sample-to-detector distance of 6558 mm providing effective scattering vector q (q = (4π sin θ)/λ, where 2θ is the scattering angle and λ the wavelength of the X-rays) range from 0.022 to 0.24 nm−1. A multilayer focused Cu Kα X-ray source (GeniX3D Cu ULD, Xenocs SA, France, λ = 0.154 nm) and scatterless collimating slits were used during the experiments. The size of the primary X-ray beam at the sample position was 0.8 × 0.8 mm2. USAXS images were recorded with a Pilatus 100K detector of Dectris, Swiss (487 pixels × 197 pixels, pixel size = 172 μm). We used a stepwise tension at a constant cross-head speed of 20 μm s−1 (equal to an initial strain rate of 0.0013 s−1) at deformation temperatures of 77, 86, 95, 105, 115, 125, and 135 °C. The collection times for USAXS patterns were set as 300 s after each step. In order to describe the lamellae thicknesses of oriented iPP samples, SAXS experiments were also performed with the abovementioned modified Xeuss of Xencos but at a sample-to-detector distance of 1064 mm providing q range from 0.113 to 1.63 nm−1. Samples were stretched to reach an engineer strain around 240% before whitening at different deformation temperatures and kept in tension during SAXS tests. Each SAXS pattern was collected within 30 min, which was then background corrected and normalized using the standard procedure. The one-dimensional radial scattering intensity distributions were first integrated within ±10° along horizontal direction (deformation direction) of 2D SAXS patterns, and then the correlation function K(z) can be directly obtained by applying the inverse Fourier transformation ∞

K (z) =

∫0 I(q) cos(qz) dq ∞

∫0 I(q) dq

(2)

which was used for computing the thicknesses of crystalline lamellae (dc), amorphous layers (da), and the long period (dac) of the samples composed of two-phase layer-like systems.23 The normally applied Lorentz correction of multiplying q2 in calculating correlation function of layer-like systems was not followed in the current case due to the highly orientated nature of the system.24 The detail description of determination of the values of dc, da, and dac according to the correlation function is provided in Figure S1 of the Supporting Information. Attributing to the weight crystallinities of both iPPs measured by DSC being more than 50% at any stretching temperature, the smaller value obtained by correlation function curves was defined as da, and dc was obtained via the function of dc = dac − da for all stretched iPP samples. The crystallinity value within the stacked B

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Macromolecules lamellae can be obtained as ϕl = dc/dac which is also termed as linear crystallinity as it measures the fraction of crystalline layer along the normal direction only. Obviously, iPP developed a crystal form of α phase at all selected stretching temperatures. And therefore the application of correlation function is reliable here. Differential scanning calorimeter (DSC) measurements were carried out with a DSC1 Stare System (Mettler Toledo Swiss) under a N2 atmosphere with a heating rate of 10 K/min. The deformed samples before whitening and after broken were both scanned from 25 to 200 °C. The ideal values of heat of fusion for 100% crystallinity of ΔHid = 207 J/g for iPP25 was chosen to calculate the weight crystallinity ϕw. For studying the fracture surface of stretched iPP, a scanning electron micrograph (SEM) was implemented on a XL30 ESEM FEG field emission scanning electron microscope.

from transparent to opaque simultaneously in the whole tensile spline. However, the whitening seemed to be presented at some local place near the edges and then propagated to the center area of the test bar here. Meanwhile, the initial strains and stresses of whitening activated in iPP are highlighted by open squares in Figure 1. Both of these critical strains and stresses showed a dependence on the molecular weight of iPP. The higher the molecular weight, the larger the strains and stresses for inducing the whitening were observed. But the two parameters gave different dependencies on the deformation temperature. It is visible that the critical strains increased with elevating the deformation temperature while the critical stresses nearly kept at a constant value under all stretching conditions. For iPP250K, the initial stresses were almost around 50 MPa, whereas the values near to 90 MPa were presented in iPP580K. As the appearance of stress whitening relied on a certain stress, one can attribute that inhomogeneous whitening phenomenon (starting from the edge) to the possible inhomogeneous distribution of the stress field across the sample being higher at the edge than the interior. In order to understand the inner structure evolution of the highly oriented iPP during stress whitening, USAXS measurements were performed. Figure 2 collects the selected USAXS patterns of iPP during stretching. More USAXS patterns of iPP drawn at different temperatures are provided in Figure S4 of the Supporting Information. Several features can be identified in those USAXS patterns. First, the most significant feature of the results is the appearance of very intense scattering streaks perpendicular to the stretching direction abruptly at a certain strain. Because of the fact that the scattering vectors qx and qy of the USAXS techniques are limited, the scattering from crystalline lamellar stacks in the iPP exceeds these ranges so that such scattering cannot be observed. As a consequence, the scattering streaks detected here could only be ascribed to some elongated structures with much larger dimension than the lamellar stacks and with very low density. Indeed, such large scale low density structures differ with the iPP matrix in refractive index, causing obvious whitening of the deformed iPP as was shown by the photographs in Figure 1. Furthermore, the strong stress whitening can also be caused by assemblies of objects with size smaller than wavelength of visible light.26 Second, one observes additional two intense scattering streaks distributed at much smaller q range along stretching direction in iPP250K stretched at 77 °C. Similar scattering along stretching direction can also be observed for iPP580K sample under the same deformation condition but to a much less extent. These scattering streaks along stretching direction eventually disappeared for both iPPs stretched at higher temperatures. A closer inspection on the scattering streaks along different directions provides more insight into their microstructure origins. The scattering streaks perpendicular to the stretching direction appeared at that direction followed by a gradual increase in intensity without changing direction whereas the ones along stretching direction showed different trend. The scattering streaks along stretching direction occurred first at somewhat slightly oblique angle with respect to the stretching direction followed by a further tilting away from the stretching direction. Moreover, the much smaller q range for scattering streaks along stretching direction indicates a larger dimension of the scattering object than those responsible for the perpendicular scattering streaks. Therefore, it is safe to conclude that the scattering patterns with the form

3. RESULTS AND DISCUSSION Figure 1 registers the true stress−strain curves of quenched iPP250K and iPP580K stretched at different temperatures,

Figure 1. True stress−strain curves of iPP250K (top) and iPP580K (bottom) measured at the temperatures indicated in the plot. The open squares represent the positions of initial whitening of the samples on the stress−strain curves. The selected photographs of the two iPPs deformed at 77 °C were also included.

including the corresponding optical photos of the materials deformed at 70 °C taken at the selected strains. Similar photos of iPPs deformed at different temperatures can be found in Figures S2 and S3 of the Supporting Information. Clearly, the most attracting point upon inspecting these photos is the whitening phenomena which took place at large strains. This behavior can be clearly distinguished from the general whitening reported in polyolefin1,4,5 at least in two aspects. The first one is the onset strain for trigging the stress whitening. The ordinary stress whitening widely analyzed in deformed semicrystalline polymers almost appears close to the yield point1,4,5 while the stress whitening observed in current work occurred at a strain much larger than the strain of yield point. The second difference between them is the proceeding of whitening within the material. The ordinary one just transforms C

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Figure 2. Selected USAXS patterns of iPP250K (left) and iPP580K (right) deformed at 77, 105, and 135 °C taken at different strains as indicated on the graph (deformation direction: horizontal).

of two elongated streaks perpendicular to each other are originated from different object of low density in the system. When deformed at relatively low temperature, a mechanism generating plate-like scattering objects with low density in the system is activated. Nevertheless, the major effect here is still the occurrence of the elongated low density objects along the stretching direction causing strong scattering perpendicular to the stretching direction and strong stress whitening shown macroscopically. One may notice difference in critical strains denoting the beginning of stress whitening shown in Figure 1 by the naked eye and Figure 2 by considering the scattering intensity changes. Such discrepancies are due to the fact that macroscopically one observes always the occurrence of stress whitening starting from the edges of the samples while the USAXS patterns were collected by focusing the X-ray beam in the middle of the samples. Therefore, the critical strains read out from the USAXS patterns are larger than those denoted in the true stress−strain curves in Figure 1. The following discussions will be based on X-ray scattering results representing the bulk properties of the materials in the middle of the samples. By integrating the USAXS patterns, the corresponding integrated scattering intensities of iPPs during deformation processes at different temperatures are depicted in Figure 3. The integrated intensity Q is defined as27 Q=

qmax

∫q ∫q min

qmax

min

I(qx , qy) dqx dqy

Figure 3. Overall USAXS patterns integrated intensity as a function of strain for iPP250K (top) and iPP580K (bottom) samples stretched at different temperatures.

stress whitening started to be detected by USAXS measurements in both systems. In both iPP250K and iPP580K, the critical strain where the total scattering intensity starts to increase shifts to larger strain values when the samples were deformed at higher temperatures. The strong increase in integrated scattering intensity clearly indicates the occurrence of new structures with very low density. We denote such low density object as cavities keeping also in mind that these cavities may not be completely empty but rather filled with polymeric chain segments and fragmental crystallites. Importantly, the corresponding critical strains for iPP580K are larger than those for iPP250K at the same temperature. This result indicates that the large strain stress whitening in iPP depends heavily on the molecular weight. To further evaluate the dimension of the scattering object, the Ruland formula was chosen to calculated the length of the long axis of these cavities which is described as follows:28

(3)

Obviously, Q used in this work is different from theoretical total scattering intensity of the system as we used relative scattering distribution in the evaluation. Nevertheless, Q calculated in this way represent relative value of volume fraction and electron density difference between phases in the system. Clearly, before considerable deformations (about a true strain of 1.6), no obvious change in total scattering intensity was observed, indicating a kind of rearrangement of the crystalline lamellar structure without creating new phases. The integrated scattering intensity began to increase at certain strains where D

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Macromolecules Bobs =

1 2π + BΦ lc q

increasing the molecular weight of iPP and elevating stretching temperature. In an effort to elucidate the microscopic origin of the observed stress whitening in highly oriented iPPs at elevated temperatures, we investigated several structural parameters in the system. First, crystallinity values of samples before and after stress whitening stretched at different temperatures have been given in Figure 5. One observes first of all an increase in

(4)

where Bobs, lc, and BΦ respectively denote the integral breadth, the length, and the misorientation of cavities. The method for calculating such parameters mentioned above was provided in Figures S5 and S6 of the Supporting Information. Scattering patterns with vertical streaks were used to compute the size of voids oriented along the deformation direction. The results obtained are summarized in Figure 4. As it appeared, the

Figure 5. Weight crystallinities measured by DSC of iPP samples as a function of deformation temperature in the state before stress whitening (bottom points) and in the finally deformed state after stress whitening (top points).

crystallinity for all samples stretched at all temperatures after stress whitening. On average, iPP580K gained more crystallinity after stress whitening than iPP250K. The amount of crystallinity increased due to stress whitening is larger than thermal effect at higher temperature. The results indicate a change in global entanglement state upon stress whitening promoting more freed polymeric chain segments being able to crystallize further. Apparently, either a significant amount of chains has been broken or disentanglements occur extensively can produce such an effect. To further clarify the actual mechanism of the observed stress whitening, microstructural parameters at the lamellar level have been determined by SAXS measurements. Figure 6 presents the final lamellar thicknesses and linear crytallinities developed at all stretching temperatures of both iPPs before stress whitening. The weight crystallinity of each deformed iPP sample measured by DSC was above 50%, meaning that the lamellar thickness dc is larger than the amorphous thickness da deduced from the correlation functions. As it presented, the dc and dac increased gradually with the increase of the deformation temperature, which was similar to previous reports.29,30 Meanwhile, these parameters also gave a negligible increase due to increase in molecular weight. This trend is similar to the crystallization behavior recently reported in the investigation of crystallization in iPP31 and P1B32 from the quiescent melt. Comparing the results of linear crystallinites in Figure 6 and weight crystallinites presented in Figure 5, one finds that much larger linear crystallinities than the weight ones was exhibited in iPP580K while the two kinds of crystallinities were almost equal in iPP250K. As was defined, the linear crystallinity represented the volume crystallinity, describing the relative volume fraction of

Figure 4. Length of long axis and misorientation of cavities obtained via Ruland method as a function of strain for iPP250K (top) and iPP580K (bottom) stretched at different temperatures.

lengths of the voids along its long axis are mostly in the range of few hundred nanometers which are within the range of visible light so that strong stress whitening was observed. Moreover, the lengths of these voids measured by SEM provided in Figure S7 of the Supporting Information were close to the value of 500 nm, which agrees well with the values obtained from Ruland method. In addition, the misorientation decreased gradually with increasing the strain which indicates the cavities aligned more and more along deformation direction. According to Figures 1−4, four points can be concluded: (i) The stress whitening found in highly oriented iPP was induced by cavitation. Such cavitation clearly differs from the ordinary cavitation treated in previous cases. (ii) The initial stresses of this cavitation depended only on the molecular weight of iPP but not on the stretching temperature. The higher the molecular weight, the larger the critical stresses will be required for triggering the cavitation. (iii) The initial strains of this cavitation relied on molecular weight as well as the deformation temperature. Both the higher molecular weight and higher stretching temperature resulted in larger strains for activating this behavior. (iv) Cavities oriented perpendicular and parallel to the deformation direction were both generated during stretching process at relatively low temperature. The perpendicular cavities were significantly restrained with E

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ment process is activated, resulting in the observed further crystallization of the freed polymeric chain segments. Such chain scission and disentanglements provided nucleation and growth of cavities in between fibrils of the system as they cause eventually local amorphous network rapture. Clearly, disentanglement process alone cannot explain the critical stress behavior of the stress whitening phenomenon as stress for disentanglement process should be related to actual temperature which is not observed. Breaking of interfibrillar tie molecules, on the other hand, depends rather only on number of tie molecules per unit cross section in the system. Experimental evidence of chain scission has been reported long before where radical formation was observed after stretching of semicrystalline polymeric fibers via the electron spin resonance technique.35,36 Further evidence can be found in Figure 7 which displays the selected SAXS patterns of the two iPP samples taken at the strains from the states before whitening to the states of whitening. As can be seen, a significant decrease of scattering intensity along stretching direction is observed in patterns of iPP580K from top to bottom, whereas this phenomenon is evidently not presented in iPP250K samples. One still can observe the intensive scattering along stretching direction in iPP250K in spite of the appearance of scattering from elongated objects along stretching direction of large electron density contrast to the matrix (the cavities) in the perpendicular direction. Clearly, the disappearance of scattering from lamellae in iPP580K before macroscopic whitening indicated that the structure of stacked crystalline lamellae within microfibrils must be reorganized in such a way that the crystallites lost their positional ordering along their normal. Indeed, under high stress, lamellae within microfibrils can be broken as having been reported in stretching oriented high density polyethylene shishkebab films.34 Our results for the two iPP samples with different molecular weight suggest that the destruction of lamellae and the occurrence of stress whitening are two processes independently associated with the stress. The initiation of both processes needs a certain critical stress. The critical stress for the destruction of lamellae can be considered

Figure 6. Thicknesses of crystalline lamellae (dc), amorphous layers (da), the long spacing (dac), and the linear crystallinity (ϕl) of iPP samples as a function of deformation temperature.

crystalline phase in the stacks composed crystalline lamellae and amorphous layers. Clearly, the inter-microfibrillar and interfibrillar amorphous phases were not considered in the linear crystallinities according to the SAXS data processing whereas such phases contribute to the total sample weight in DSC measurements. Therefore, the observed different differences in linear and weight crystallinites in the two iPPs with different molecular weights suggest that there were much less amounts of amorphous phase between fibrils and microfibrils in iPP250K than in iPP580K. With above structural information in mind, we can now speculate possible underlying mechanism of cavitation in the current systems. Obviously, much less interfibrillar tie molecules present in the iPP250K samples than in the iPP580K samples.33 This leads to a much higher stress per interfibrillar tie molecule during stretching in iPP250K samples so that an apparent lower macroscopic critical stress for breaking such tie molecules. After chain scission a disentangle-

Figure 7. Selected SAXS patterns of iPP250K (left) and iPP580K (right) deformed at 77, 105, and 135 °C taken at different strains as indicated on the graph. From top to the bottom: from the states before whitening to the states of whitening (deformation direction: horizontal). F

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Figure 8. Schematic presentation of the formation of cavities in fibrillar network of iPP (size not to scale).

head cavitation weakened or even completely disappeared at high stretching temperature. The last stage is the growth and orientation of the two kinds of cavities. With further stretching, the cavities generated between adjacent side surfaces of fibrils continue to elongate along the tensile axis and the cavities produced between head-to-head fibrils change their orientation toward the stretching direction. In the case of the high molecular weight sample, destruction of stacked lamellar structure can occur, but the global stress whitening mechanism remains the same as a consequence of the tie molecule breaking followed by the disentangling process of the highly oriented network.

the same for both samples, as they possess lamellae of similar thickness. However, the critical stress for whitening differs from sample to sample as is also shown in Figure 1. The results presented in Figure 7 that stacked lamellae structure was still persistent after stress whitening in iPP250K indicate that the critical stress for destruction of lamellae structure is larger than the critical stress for stress whitening in this sample. With the increase of molecular weight from iPP250K to iPP580K, critical stress for introducing tie chain breaking and disentanglement increases significantly, exceeding the critical stress for lamellae destruction. Therefore, one observed a disappearance of SAXS scattering intensity distribution of stacked lamellae within microfibrils in iPP580K after stress whitening. With the structural information above, we can finally propose a deformation model as shown in Figure 8 for describing the process of cavitation in highly oriented iPP upon stretching. The microstructure of the oriented iPPs can be simplified by considering several structural elements. Globally, the system can be viewed as a highly stretched entangled network embedded by highly oriented fibrils being composed of microfibrils. Amorphous chains in between crystalline lamellae within microfibrils do not contribute directly to the cavitation process as discussed above. Upon stretching, slippage of fibrils past each other led to further elongation of the already highly stretched entangled network. Such slippage is limited by the interfibril connections mainly entangled chains and tie molecules connecting adjacent fibrils. The entangled chains and tie chains can be disentangled upon further stretching due to breaking of the tie molecules creating nucleation sites for the development of cavities. In such case, cavities with their long axis oriented along stretching direction show up as was measured by the USAXS scattering intensity distribution perpendicular to the stretching direction. The general occurrence of such USAXS intensity distribution perpendicular to stretching direction at all stretching conditions suggests that this mode of cavitation induced by interfibrillar tie molecule breaking followed by disentanglement of amorphous network is the major mode of stress whitening in oriented iPP. Nevertheless, one still observes USAXS intensity distribution along the stretching direction under certain conditions, suggesting the occurrence of cavities with their long axis perpendicular to the stretching direction. These perpendicular cavities can be attributed to the separation of the head to head connected fibrils. Indeed, such separation is much more difficult than the interfibrillar tie molecule breaking and disentanglements due to the preferential orientation of the chains. One finds supports of this idea from the USAXS results. First, the vertical scattering streak denoting interfibrillar side cavitation occurred earlier than the horizontal streak representing headto-head cavitation between fibrils in iPP250K at 105 °C and iPP580K at 77 °C shown in Figure 2. Second, such head-to-

4. CONCLUSIONS In summary, we have investigated the stress whitening triggered in quenched isotactic polypropylenes stretched to a highly oriented state at elevated temperatures by the in situ USAXS technique. Unlike in the case of cavitation activated around the yield point, the cavitation observed in the current work was proven to be related to the failure of the highly oriented amorphous network induced by interfibrillar tie molecule breaking followed by disentanglements of the highly oriented amorphous network connecting adjacent fibrils. The critical strains for activating such cavitation depend on both the molecular weight of iPP and the deformation temperature while the stresses for this behavior depend only on the molecular weight. A larger stain was needed for obtaining the critical stress for the breaking of the taut interfibrillar tie molecules and disentangling the network at elevated temperatures due to progressive decoupling between microfibrils and fibrils so that a weakening of the global network. However, the critical stress for such taut tie chain scission followed by disentanglement process depends only on molecular weight.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b00818. The method for determining the values of dc, da, and dac according to the correlation function, the macroscopic photos of iPP samples during stretching processes at all temperatures and the USAXS patterns of iPPs deformed at some temperatures, the process of computing Bobs and lc, and the results of SEM (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.M.). G

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(31) Lu, Y.; Wang, Y. T.; Jiang, Z. Y.; Men, Y. F. ACS Macro Lett. 2014, 3, 1101−1105. (32) Wang, Y. T.; Lu, Y.; Jiang, Z. Y.; Men, Y. F. Macromolecules 2014, 47, 6401−6407. (33) Seguela, R. J. Polym. Sci., Part B: Polym. Phys. 2005, 43, 1729− 1748. (34) Brady, J. M.; Thomas, E. L. J. Mater. Sci. 1989, 24, 3311−3318. (35) Verma, G. S. P.; Peterlin, A. Colloid Polym. Sci. 1970, 236, 111− 115. (36) Crist, B.; Peterlin, A. Makromol. Chem. 1973, 171, 211−227.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21134006).

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DOI: 10.1021/acs.macromol.5b00818 Macromolecules XXXX, XXX, XXX−XXX