Subsequent but Independent Cavitation Processes in Isotactic

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Subsequent but Independent Cavitation Processes in Isotactic Polypropylene during Stretching at Small and Large Strain Regimes Dong Lyu, Ran Chen, Ying Lu, and Yongfeng Men Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01650 • Publication Date (Web): 14 Jun 2018 Downloaded from http://pubs.acs.org on June 15, 2018

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Industrial & Engineering Chemistry Research

Subsequent but Independent Cavitation Processes in Isotactic Polypropylene during Stretching at Small and Large Strain Regimes Dong Lyu, †,‡ Ran Chen,† Ying Lu, †,* Yongfeng Men †,‡,* †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied

Chemistry, Chinese Academy of Sciences, Renmin Street 5625, 130022 Changchun, P.R. China ‡

University of Science and Technology of China, Hefei 230026, P.R. China

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT Isotactic polypropylene (iPP) samples with different molecular weights were used to investigate the relationship between cavitation processes triggered at small and large strain regimes. The cavitation triggered at large strains was observed in all deformation cases regardless of the occurrence of cavitation activated at small strain. The initiation of large strain cavitation process required a critical stress which was mainly determined by the molecular weight of iPP, since it was a result of the disentanglement of the highly oriented amorphous network initiated by the breakage of interfibrillar load bearing tie chains. In general, iPP with higher molecular weight required a higher critical stress to trigger this cavitation and presented more intensive whitening. However, the intensity of cavitation activated at small strains decreased with increasing iPP molecular weight. The cavities generated at small strain are therefore supposed to have no contribution to the cavitation initiated at large strain.


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1. INTRODUCTION Isotactic polypropylene (iPP) has been variously used as commercial products such as automobile, packaging, and electric appliance due to its excellent solvent resistance, unique mechanical properties, and low cost.1-3 To understand the mechanical properties of iPP therefore deeply attracts attentions from both industry and academia. One of the most important subjects is the mechanical fracture of iPP during deformation, which normally depends on the stretching temperature,4-8 load rate,9,

10

and material properties including crystallinity,11-15 molecular

weight,16-20 and some other parameters.21 In general, two principal fractures classified as brittle fracture and ductile one have been proposed for a solid polymer under load.21 Brittle fracture of polymers is considered as a consequence of the propagation of cracks through the material,22 which is usually observed in glassy polymers during stretching,23-25 such as polyethylene terephthalate (PET) and polystyrene (PS).26 However, typical feature of ductile behavior is always found in semi-crystalline polymers, which is reflected by the appearance of plastic deformation.27 Upon stretching, a semi-crystalline polymer transforms from an original isotropic spherulitic morphology into a highly oriented fibrillar one,28 and finally ends up with the fragmentation of materials with further stretching the fibrillar structure.29 The mechanism of this process can be described as block slippage within the crystalline lamellae taking place first at small deformation, followed by the stress-induced fragmentation and recrystallization starting at certain strain,30 and finished with the disentanglement of the highly oriented amorphous network.31 Aside from the processes mentioned above, the cavitation process32-34 under some conditions, e.g., low stretching temperature, can be triggered as well. Clearly, the production of cavities is one of the crucial factors affecting the ultimate life of polymers. Two different cavitation processes activated in iPP around yield point35 and at large strains31 have been revealed separately. Studies emphasizing the



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occurrence of cavitation around yield point never included the statements of cavitation activated at larger strain.36-38 Meanwhile, the work addressed the phenomenon of cavitation at large strain also excludes the cavitation founded around yield point.31 Rare report revealed whether the cavitation around yield point can enhance the nucleation and growth of cavitation at large strain or not since the former one cannot be removed with further stretching as long as it took place. In most cases, the preparation methods of material and testing conditions can influence the process of cavitation around yield point.39-44 Pawlak and Galeski45 observed the core layer in injection moulded iPP showing an earlier growth of cavitation than the skin region at which the crystalline structure was poorly developed and the crystallinity was lower. Bao et al. noticed46 a reducing intensity of cavitation appeared in deformed iPP at elevated temperatures. Although many efforts have been paid to consider the potential influential factors over the occurrence of cavitation around yield point, the attention focusing on the cavitation process at large strain is rare. The parameter which can influence the cavitation activated at large strain is currently referred to only the molecular weight of iPP31 if the iPP are featured with full phase47 and without preorientation.48 The higher the molecular weight of iPP, the larger critical stress is required for initiating the cavitation at large strain. More importantly, a fracture of materials often follows this cavitation.31 Therefore, the connections between these two cavitation behaviors in iPP are vital to figure out the final fracture mechanism of iPP during stretching process. Because both of these two cavitation performances31, 49 can create cavities in the system31, 49 and these cavities cannot be healed during the whole deformation process.50 The purpose of this work is to investigate the relationship between these two cavitation processes in iPPs of different molecular weights through wide, small, and ultra-small angle X-ray scattering (WAXS, SAXS, and USAXS) techniques. The annealing and stretching temperature are the two


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major parameters to control the presentation of cavitation at small and large strains. As being found, the behavior of cavitation at small strain depended on the molecular weight of iPP, annealing temperature, and deformation temperature. A lower molecular weight iPP or a higher temperature annealed sample always presented more intensive cavitation phenomenon around yield point at low deformation temperature. With increasing stretching temperature, this cavitation process could be completely suppressed. Besides, the cavitation triggered at large strain took place in all iPP samples irrespective of the parameters of molecular weight, annealing temperature, and deformation temperature. The critical stress for initiating such cavitation at large strains was only determined by the molecular weight of iPP and the appearance of cavitation at small strain did not affect this critical stress. In other words, the large strain cavitation can be observed no matter of the occurrence of small strain one. Hence, the cavities generated at small strain are regarded as having no effect on strengthening the cavitation initiated at large strains. 2. EXPERIMENTAL SECTION Table 1 lists the detailed information of four iPP samples with different molecular weights purchased from Aldrich Polymer Products. Films of 0.5 mm in thickness were firstly prepared using iPP pellets through a hot press machine at 200 oC, which were then quenched into the ice water and stabilized for 30 minutes. After that, quenched films were annealed at 30, 50, 70, 90, 110, and 130 oC in a vacuum oven for 20 hours, respectively. These annealed films for measurements were cut into dumbbell-shaped strips with dimensions of 26 × 5 × 0.5 mm3 with the aid of a punch. Uniaxial tensile deformation was performed by using a portable tensile testing machine (TST350, Linkam, UK) with a clamping distance of 15 mm. We employed 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 30, 40, 50, 60, 77, 86, 90, 105, 115, and 125 oC. Each tensile case



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had been carried out at least two times. Optical photo images of the samples were captured during stretching processes in order to measure the strain of the deformed area. The Hencky strain H was used as a basic quantity of the true strain, which is defined as  H  2ln

b0 b

(1)

where b 0 and b are the initial and instantaneous widths of sample during deformation process, respectively. Table 1. The Basic Physical Properties of iPPs Used in Experiments.

a

Catalog No.

Name

Mw / g mol-1

Mn /g mol-1

Tm / oC a

427896

iPP190K

190,000

50,000

165

427888

iPP250K

250,000

67,000

165

427861

iPP340K

340,000

97,000

165

427853

iPP580K

580,000

166,000

165

Detected by DSC with a heating rate of 10 K min-1.

DSC measurements were carried out with a DSC1 Stare System (Mettler Toledo Swiss) under N2 atmosphere. Samples (5-10 mg) were heated up from 25 to 200 oC at a rate of 10 K min -1 after annealed under different conditions. The ideal values of heat of fusion for 100 % crystallinity of Hid= 207 J g-1 for iPP51 was chosen to calculate the weight crystallinity w. WAXS experiments were conducted with a custom-designed micro-focus X-ray diffraction setup of Xenocs, France. The system is equipped with a Cu Kα X-ray generator (GeniX3D Cu, Xenocs SA, France, λ= 0.154 nm) and a strong focusing mirror focuses the X-ray to a spot size of 40 × 60 m2 at the sample position. Each WAXS diagram obtained in the center of the sample was collected within 30 seconds at a sample-to-detector distance of 39.8 mm through a Pilatus 100K detector of Dectris, Swiss (487 pixels × 197 pixels, pixel size= 172 m).


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SAXS experiments were performed at a modified Xeuss system of Xenocs, France, at a sample-

to-detector distance of 2437 mm providing effective scattering vector q (

q 

4 sin 



, where 2

is the scattering angle and  the wavelength of the X-ray.) range from 0.05 to 1.15 nm-1. A multilayer focused Cu Kα X-ray source as described above 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. One SAXS image was recorded within the exposure time of 30 minutes with the help of a Pilatus 100K detector of Dectris, Swiss. The one dimensional radial scattering intensity distributions were integrated within ± 10o along horizontal direction (deformation direction) of 2D SAXS patterns, and the value of long spacing d ac was calculated by using the Bragg equation, d ac 

2

q max

(2)

the qmax represents the maximum q point in the SAXS patterns for the periodic lamellar structure when scanning along certain directions. USAXS experiments were carried out using the same equipment employed for SAXS experiments but with a sample-to-detector distance of 6558 mm providing effective scattering vector q range from 0.022 to 0.24 nm-1. The size of the primary X-ray beam at the sample position was 0.6 × 0.6 mm2 and the exposure time of each USAXS image was set as 300 seconds. In terms of describing the relative value of volume fraction and electron density difference between phases in the iPP during stretching, the total scattering intensity Q of one USAXS pattern (integrated from 0o to 180o) is expressed as25

Q  (ce ,c  ce ,m )2 c(1  c ) 

q max

q max

min

min

q q

I(q x ,q y )dq xdq y

(3)

where C e,c and Ce,m denote the average electron density distribution of cavities and the iPP matrix



in the system, and c refers to the volume fraction of cavities in iPP sample, respectively. 3. RESULTS AND DISCUSSION 500

o

iPP190K

400

Ta / C

(110)

30 50 70 90 110 130

(111)+(131) (040) (130) 

300 200 100 0 120

(110)

iPP250K

100

(040)

80

(111)

(131)

(130)

60 40 20

I / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 120

iPP340K

100

(110)

80 60

(040)

(111) (131) (130)

(040)

(111) (131) (130)

40 20 0 120

iPP580K

100

(110)

80 60 40 20 0 8

10

12

14

16

18

2 /

o

20

22

24

26

Figure 1. WAXS profiles of iPP samples with different molecular weights measured at room temperature after annealed at different temperatures. One dimensional WAXS curves of iPP samples after annealing at different temperatures are given in Figure 1. Nearly all the characteristic scattering peaks belonged to modification, except iPP340K and iPP580K annealed at 30 oC in which an extremely weak diffuse peak around 15o was


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found. The fraction of the latter peak, which originated from mesophase52, was tiny small and thus could be ignored. Since the mesophase can be generated by quenching iPP into the ice water with a cooling rate faster than 80 K s-1,53, 54 such high cooling rate was not achieved in our experimental process resulting in a major formation of  phase. For the lowest molecular weight iPP190K, no significant difference were observed in its WAXS profiles, indicating that the annealing temperature (Ta) has a slight influence on modifying the crystalline structure of quenched iPP190K. However, some distinct behaviors were observed in annealed iPP samples with higher molecular weight. For example, the intensity of crystallographic peak of (110) in iPP250K gradually increased at elevated Ta. This behavior became more evidently in the other two higher molecular weight iPP samples. Clearly, such phenomenon manifests that the crystalline structure reorganized to more perfect crystallites caused by annealing temperatures. The different influences of Ta on the rearrangement in crystalline structure of iPP reveal iPP with different molecular weights possessing different crystallization rates during quenching. Figure 2 collects the DSC melting curves of all annealed iPP samples. The melting behaviors in iPP190K samples were almost independent of the annealing temperature, except for the sample annealed at 130 oC which showed a weak melting peak just above 130 oC. This peak is commonly addressed as “annealing peak” arisen from a melting-recrystallization process including a large part of crystallites.55, 56 The DSC results of iPP190K were virtually agreed with its WAXD results as exhibited in Figure 1. In the case of iPP250K, the “annealing peak” was also observed in the samples annealed above 110 oC, meaning a reorganization of crystallites occurred when iPP250K was annealed above 110 oC. As for the other two molecular weight iPP samples, the “annealing peak” also occurred when annealing temperature was above 70 oC. Such performances are consistent with the WAXD results in Figure 1 as well.



25 20 15 10 5

o

Ta / C 30 50 70 90 110 130

iPP190K

0 -5 o

iPP250K

Ta / C 30 50 12 70 90 8 110 130 4 16

HF / mW

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 -4 9 6 3 0

o

Ta / C 30 50 70 90 110 130

iPP340K

-3 -6 9 6 3 0

o

Ta / C 30 50 70 90 110 130

iPP580K

-3 -6 40

60

80

100

o

120

140

160

180

T/ C

Figure 2. DSC melting curves of iPP with different molecular weights after annealed at different temperatures. (Heating rate: 10 K min -1) In order to understand more detailed crystallization behaviors of different iPP samples during quenching and annealing processes, the crystallinities obtained using DSC curves and the long spacing dac derived from SAXS profiles are depicted in Figure 3. As it turned out, the two lower molecular weight iPP samples apparently displayed a larger value of crystallinity than the other


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two higher molecular weight iPP samples under the same conditions. Especially, the crystallinities of the former two iPP samples changed slightly while the latter ones exhibited an obvious increase as the annealing temperature was elevated. Moreover, the long spacing dac of iPP190K was larger than the other three iPP samples when the annealing temperature was below 70 oC. The difference of dac among iPP samples with different molecular weights began to reduce when the Ta arrived above 90 oC. Because the annealing process promoting a melting and recrystallization of crystallites in the three higher molecular weight iPP samples. 70 iPP190K

iPP250k

iPP340K

iPP580K

w / %

60 50 40 30 20 18

dac / nm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


16 14 12 10 8 30

40

50

60

70

80 o 90 100 110 120 130

Ta / C

Figure 3. The weight crystallinity w (top) and the long spacing dac (bottom) of annealed iPP samples obtained from DSC and SAXS profiles, respectively. Therefore, two conclusions can be deduced combing the DSC, WAXS, and SAXS findings. Firstly, the crystallization temperature of iPP190K and iPP250K was higher than the one of the other two iPP samples during cooling process, leading to a larger fraction of crystallinity presented in the former two samples. That is ascribed to the lower molecular weight iPP composed of shorter



molecular chains exhibiting an advantage of chains mobility, resulting in a faster crystallization rate during quenching. This phenomenon had been confirmed in our previous report.57 Secondly, the higher molecular weight iPP samples of iPP340K and iPP580K crystallized almost below 70 o

C, thus, the important variations of crystalline structure and crystallinity were observed as the Ta

increased to 70 oC. Here, we can formulate that the higher the molecular weight iPP, the lower crystallinity and smaller dac would be achieved for a certain low annealing temperature. 120 100

o

o

Td = 30 C

Ta / C 30 50 70 90 110 130

a= 130 oC

80 60

H=0

H=0.08

H=1.09

a= 30 oC

40 20

 / MPa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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H=0

H=0.30

H=1.45

0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 120 o

100

a= 130 oC

Td = 95 C

80 H=0

60

a= 30

H=0.11

H=1.54

H=0.15

H=1.75

oC

40 20

H=0

0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

H

Figure 4. Selected true stress-strain curves of annealed iPP190K samples measured at the temperatures indicated in the plot. (The selected photographs of iPP190K samples annealed at 30 and 130 °C during deformation were also included.)


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As mentioned above, the preparation methods of materials and test conditions can affect the cavitation around yield point39-43 but the cavitation triggered at large strain was mainly determined by the molecular weight.31 Stretching annealed iPP samples at different temperatures were therefore used to explore the cavitation activated at different regions. Figure 4 exhibits the selected true stress-strain curves of iPP190K deformed at different temperatures. Similar curves of other three iPP samples are provided in Figure S1, S2, and S3 in the Supporting Information. In general, the sample annealed at high temperature showed a poor ductility compared to the ones annealed at low temperature when the deformation temperature (Td) was low. This is because the sample annealed at higher temperature held a higher crystallinity leading to a higher concentration of defects that eventually initiate final failure of the material. Consequently, the higher molecular weight iPP with a lower crystallinity usually presented excellent plasticity at low Td as proven by the results given in Figure S2 and S3. Meanwhile, the ductility of annealed iPP190K samples could be improved at high Td as evidenced by the curves in the bottom of Figure 4 where larger breaking strains were observed. That is attributed to the enhanced chains mobility at high Td. As is shown by the photographs in Figure 4, a whitening started to appear in 130 oC annealed iPP190K at the strain of 0.08 when being stretched at 30 oC. This whitening could rapidly cover the whole sample as it first appeared. Different from this sample, the iPP190K annealed at 30 oC had a much weaker whitening at small strain while a much stronger whitening on some localized areas at large strain under the same Td. These difference disappeared based on these photographs captured at Td of 95 oC at which only localized whitening at large strain could be observed. Such observations clearly demonstrate that the occurrence of cavitation at small strain depends on the annealing and stretching temperature but the activation of cavitation at large strain is irrespective of these parameters. The critical stress required for initiating the latter cavitation process was



marked out on the curves using open circles. All of them located around the value of 40 MPa regardless of the Ta and Td, excluding the result of 130 oC annealed sample stretched at 30 oC. Because the initiation of cavitation at large strain was masked by the cavitation triggered at small strain under this deformation condition. The near constant stress demanded for triggering cavitation at large strain is in accordance with our previous work.31 130 iPP190K 120

iPP250K o

Ta / C

30

iPP340K 50

iPP580K

70

90

110

130

110 100 90

/ MPa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80 70 60 50 40 30 20 30

40

50

60

70

80

o

90

100

110

120

130

140

Td / C

Figure 5. The critical macroscopic stresses triggering stress whitening at large strains in annealed iPP samples with different molecular weights as a function of deformation temperature. Figure 5 provides all the critical stresses activating the cavitation at large strain in iPP samples under different conditions, except for the values in iPP190K and iPP250K annealed at 130 oC stretched at 30 oC. Although these stress fluctuated visibility for a certain molecular weight iPP, they almost lay down in the same range. The range of the corresponding stress of iPP190K, iPP250K, iPP340K, and iPP580K located at 41 ± 5 MPa, 50 ± 5 MPa, 69 ± 5 MPa, 90 ± 5 MPa, respectively. That means the molecular weight of iPP is the primary factor controlling the stress whitening at large strain. One thing should be pointed out that these stresses for whitening in


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samples annealed at 130 oC in both of iPP340K and iPP580K exceeded their own range which have been marked using black ellipses. The reason will be discussed later in this work. The cavitation at large strain was proposed to be caused by the disentanglement of the highly oriented amorphous network initiated by the breaking of inter-fibrillar load bearing tie chains according to our previous findings.31 However, the cavitation process triggered at small strains is much earlier than the disentanglement of the highly oriented amorphous network since this cavitation often takes place at the process of block slippage within the crystalline lamellae. In order to figure out the relationship between these two cavitation processes, further illustrations will be given through USAXS results owing to the strong difference in electron density between the polymer matrix and the voids in the relevant angular range accessible by this technique.58, 59 Selected USAXS patterns of selected iPP samples stretched at different Tds are shown in Figure 6. The iPP190K annealed at 30 oC stretched at 30 oC is chosen to give an illustration. The cavities first occurred around the yield point can be assumed to have a disc like shape with the normal oriented parallel to the deformation direction as indicated by the shape of USAXS patterns. This phenomenon is caused by the fact that the cavities first generated within the lamellar stacks with normal perpendicular to the deformation direction.60 Then, these cavities began to change their shapes and sizes and eventually becoming rod-like with long axis aligned along the deformation direction with further stretching. Because the original lamellar stacks can be broken and finally form the new lamellae within fibrillar structures with normal parallel to the deformation direction due to stress induced melting and recrystallization. Moreover, intensive vertical and horizontal scattering streaks could be observed at the strain of 1.2. Both of them resulted from the cavities generated at large strain. The vertical ones are considered as the cavities born between two adjacent micro-fibrils/fibrils initiated by breaking inter-fibrillar load bearing tie chains. The horizontal ones



Page 16 of 38

are regarded as the cavities formed between the heads of micro-fibrils/fibrils.31 This case is commonly observed in the sample stretched at low temperatures but this kind of cavities becomes negligible with increasing Td. iPP190K-Ta30 Td / oC

30 H

0

0.21

0.37

0.55

0.92

1.20

1.37

H

0

0.11

0.53

0.97

1.31

1.55

1.76

95

iPP190K-Ta130 Td / oC 30 H

0

0.08

0.36

0.61

1.13

1.37

1.43

H

0

0.19

0.67

1.37

2.01

2.19

2.28

H

0

0.19

0.31

0.77

1.36

1.69

1.75

0.14

0.67

1.81

2.24

2.30

2.45

95

iPP580K-Ta130 Td / oC 30

95 H

0

Figure 6. Selected USAXS patterns of iPP190K annealed at 30 oC (top) and 130 oC (middle) as well as iPP580K annealed at 130 oC (bottom) deformed at 30 and 95 oC taken at different strains as indicated on the graph. (Deformation direction: horizontal) In the case of iPP190K annealed at 130 oC stretched at 30 oC, the orientation of cavities showed a slight difference as compared with the one in the iPP190K annealed at 30 oC before these cavities totally oriented along the deformation direction. The USAXS pattern in the former sample gradually shaped to “butterfly” one from the strain of 0.08 to 0.36 while the one in the latter sample just extended along both of vertical and horizontal directions at the same strain range. It indicates that tilted cavities were developed in the former system33 and the cavities grew in both directions


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of perpendicular and parallel to the deformation direction61 in the latter case with further stretching. Such difference can be ascribed to the movement of original lamellae in different iPP190K samples. The stress for destroying the thicker lamellae in the one annealed at higher temperature is larger than the one in the sample annealed at lower temperature with thinner lamellar. These thicker lamellae cannot be broken at the same time limiting the growth of cavities along different directions. But they can vary their orientation gradually to the deformation direction with increasing strain before being destroyed. Thus, the orientation of cavities changed with the orientation of these lamellae bringing out tilted cavities. In the case of thinner lamellae, they can be easier destroyed providing the place for cavities growing in both of perpendicular and parallel directions with respect to the stretching direction. Besides, the cavitation triggered at large strain in iPP190K annealed at 130 oC was observed at the strain of 1.37 as evidenced by the appearance of strong horizontal and vertical streaks on the USAXS pattern. For iPP580K annealed at 130 oC stretched at 30 oC, the apparent streaks along vertical direction on USAXS patterns can be observed until at the strain of 0.31. It means that the cavities with long axis oriented along the deformation direction formed in the system. This cavitation was also related to the crystalline level. The retardation of cavitation in iPP580K annealed at 130 oC is due to the low resistance to plastic deformation in this system which then results in the plastic deformation taking place first followed by the cavitation process.45 Furthermore, the large strain cavitation in this sample was found at the strain of 1.67.



50 40

iPP190K-Ta130

o

Td / C

30

40

50

60

70

77

86

95

30 20 10 0 50

0.0

0.2

0.4

0.6

iPP190K-Td30 40

Q / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 38

0.8

1.0

1.2

o

Ta / C

1.4

1.6

70

30

1.8

2.0

110

2.2

2.4

130

30 20 10 0 1.6 1.4

0.0

0.2

0.4

0.6

iPP190K-Td95

0.8

1.0

1.2

o

Ta / C

1.4

30

1.6

1.8

2.0

110

70

2.2

2.4

130

1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

H

1.4

1.6

1.8

2.0

2.2

2.4

Figure 7. The overall USAXS patterns integrated intensity as a function of strain for iPP190K samples stretched at different conditions: iPP190K annealed at 130 oC stretched at different temperatures (top), iPP190K annealed at different temperatures stretched at 30 oC (middle) and 95 o

C (bottom). The evolution of cavities generated at small strains changed at elevated stretching temperature.

For example, the case in iPP190K annealed at 130 oC stretched at 95 oC was similar to the case discussed in iPP190K annealed at 30 oC stretched at 30 oC. Because the SAXS patterns in both samples showed two directions extended streaks at small strain range. The USAXS patterns in iPP190K annealed at 30 oC stretched at 95 oC were similar to the ones in iPP580K annealed at 130


Page 19 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


o

C stretched at 30 oC, indicating the cavities with long axis along the stretching direction were

generated first in the system. Especially, the cavitation at small strain disappeared in iPP580K annealed at 130 oC stretched at 95 oC according to the corresponding USAXS patterns. Such variations mentioned above can be ascribed to the easier plastic deformation in samples caused by higher chains mobility at higher deformation temperature. Inspecting the USAXS patterns representing cavitation initiated at large strains, the cavities occurred still with long axis along the deformation direction as reflected the strong vertical streaks on the patterns. Here, one can observe that the most intensive scattering streaks at small strains were found in iPP190K annealed at 130 o

C drawn at 30 oC which were contributed by the enhanced cavitation at small strain. But the

phenomena in the highest molecular weight iPP580K followed another way. The sample annealed at 130 oC stretched at 30 oC presented much weak scattering intensity while it showed strong scattering streaks at large strains. It indicates that the two cavitation processes are governed by different parameters. A further comparison of cavitation processes in iPP samples stretched at different conditions is registered in Figure 7 and Figure 8. Figure 7 lists the total scattering intensity Q of iPP190K samples stretched at different temperatures. As indicated in the top of Figure 7, iPP190K-Ta130 sample stretched at 30 oC showed an increase of Q below the strain of 0.2. This value gradually reduced with further increasing H attributing to some cavities developing to large sizes which exceeded the detectable range of USAXS and thinning of tensile bars. However, a re-increased Q was presented around the strain of 1.4 caused by the occurrence of cavitation triggered at large strain. As Td increased to 40 oC, the value of Q sharply decreased in the small strain region while there was only a minor change in the high strain range. The Q mostly kept constant and then increased after activating the cavitation at large strain where was highlighted by the oblique line



when the Td was above 50 oC. For samples stretched at a certain temperature, the whole variation tendency of Q was the same with the performances mentioned above. Besides, the annealing temperature strongly affected the values of Q at low strain range and had no influence on the ones at large strain region which was dominated by the cavities produced at large strain. In Figure 8, one finds that the higher molecular weight of iPP the lower values of Q were achieved at small strains, whereas the larger values of Q were obtained at high strains regardless of the Td. This indicated that the molecular weight had an opposite influence on the two cavitation processes initiated at different regions. 50

o

iPP190K iPP250K iPP340K iPP580K

Td = 30 C 40 30 20 10 0

Q / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 38

30

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 o

Td = 95 C 25

1.0 0.8

20

0.6 0.4

15 10

0.2 0.0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

5 0 -5 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

H

Figure 8. The overall USAXS patterns integrated intensity as a function of strain for iPP with different molecular weights annealed at 130 oC stretched at 30 oC (top) and 95 oC (bottom).


Page 21 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In order to estimate the cavity dimensions produced at different strains, we employed a model fitting procedure proposed by Fischer et al.62 and its modified version by our group33. In the case of cavities without tilting, the former method is appropriate and the cavities are morphemically considered as cylinder-shaped objects with length in tensile direction and radius in transverse direction. The scattering intensity is thus given by the superposition of all scattering objects in the sample which is expressed as62 2



I(q R ,q L ) 

2

 2J 1(q R R )  sin(q L L / 2)    D(R ,L )dRdL  qR R   qLL / 2 

4 2 R L  0

(4)

where D(R, L) is the size distribution function of cavities having radius R and length L. R and L are two non-correlated parameters, thus, the D(R,L) is equal to D1(R)D2(L). The equation 4 then can be described as 

I(q R ,q L ) 

 0

2

 2J 1(q R R )  D 1(R )dR   qR R 

R 4

 2

L 0

 sin(q L L / 2)  D 2(L )dL  qLL / 2 

(5)

Hence, we can take two rectangular slices parallel to the axes, one perpendicular to the stretching direction for the radius scattering and the other parallel to the stretching direction for the length scattering. For the tilted cavities, the modified cavitation model33 is used which is defined as  / 2 2

I(q ,  , )  4 V 2  02

  0

0

J 12(qR sin ( , )) sin 2(qL / 2 cos (  , )) D ( L ) dL 1  (qR sin (  , ))2 D 2(R )dR h(  )sin(  )dd (qL / 2 cos (  , ))2

(6)

where  is the angle between the normal direction of the broad cylinder and the stretching direction (the x axis).is the angle between the projection of the normal direction of the broad cylinder on the yz plane (perpendicular to the stretching direction) and the y axis. is the angle between q and the normal direction of the broad cylinder. The fitting data obtained using USAXS patterns in Figure 6 through equations of 5 and 6 are summarized in Figure 9.



500 450 400

L / nm

350

iPP190K Ta30-Td30

iPP580K Ta130-Td30

Ta30-Td95

Ta130-Td95

Ta130-Td30

300

Ta130-Td95

250 200 150 100 50 0

R / nm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 38

280 260 240 220 200 180 160 140 120 100 80 60 40 20 0

0.0

0.0

0.2

0.2

0.4

0.4

0.6

0.6

0.8

0.8

1.0

1.0

1.2

1.2

H

1.4

1.6

1.8

2.0

2.2

2.4

2.6

1.4

1.6

1.8

2.0

2.2

2.4

2.6

Figure 9. Plots of the average cavity length (top) and radius (bottom) against true strain of iPP samples stretched at different temperatures. For iPP stretched at a higher temperature, only the fitting data at large strains are successfully achieved due to the improper fitting of the USAXS patterns with weak scattering at small strains. The average cavity length during stretching increased with increasing strain in all iPP samples. The final lengths of cavities in different deformation cases were stabilized in the range of 350-500 nm which falls into the range of visible right, thus, a phenomenon of whitening was observed in these samples with cavitation. For the development of the average cavity radius, it displayed a different tendency which was related to the stretching temperature as compared to the cavity length. When iPP samples stretched at 30 oC, the cavities first appeared with larger radius and then decreased with increasing strain to a certain value. After that, this parameter increased again with further increasing strain. The first reduction of cavity average radius can be ascribed to that the


Page 23 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


cavities gradually oriented along the deformation direction and the growth of radius was hindered due to the radius growth direction perpendicular to the stretching direction. The followed increase after first decrease was arisen from the appearance of cavities with long axis perpendicular to the deformation direction generated at large strains. This kind of cavities is considered to form between the heads of micro-fibrils/fibrils in iPP at low stretching temperature.31 However, these cavities become negligible with increasing Td. It can be a reason for explaining the smaller average cavity radius found in iPP stretched at high temperatures. In a word, the final cavity length can be fixed in the range between 350-500 nm irrespective of the cavities generated at different regions. Here, the influence of stretching temperature, annealing temperature, and molecular weight of iPP on the formation of cavitation at different strains can be demonstrated as following. For one certain molecular weight iPP stretched at a preset temperature, the higher the annealing temperature is, the more intensive cavitation is at small strain due to the higher crystallinity and larger lamellar thickness. For one molecular weight iPP annealed at a certain temperature, the lower the deformation temperature is, the more intensive cavitation around yield point is, caused by the fact that the poor chains mobility impeded the plastic deformation.35 The cavitation triggered at large strain is not influenced by the variation parameters referred in these two situations. For a certain annealing and stretching temperature, the lower molecular weight iPP displayed a more intensive cavitation at small strain but a weaker cavitation at large strain. Therefore, we can conclude that higher annealing temperature, lower stretching temperature, and lower molecular weight of iPP promoted the cavities around yield point while cavitation activated at large strain can only be affected by molecular weight. Dating back to the general-reported cavitation at different regimes, the one initiated around yield point normally starts from a sample with spherulitic state33, 50 while the one activated at large strain



Page 24 of 38

always evolves from fibrilliar structure.63 Under the condition of same lamellar thickness, the size of spherulites affects strongly the development of cavitation at small strain in PB-1 as has been shown during stretching samples of different thermal histories.64 The PB-1 sample with larger spherulites showed larger dimension of cavities than the one with smaller spherulites at the same strain. Furthermore, as being pointed out by Ran et al. during stretching a semi-crystalline poly(4methyl-1-pentene) (P4M1P) sample below glass transition temperature of the amorphous phase,33 different spherulitic morphologies may result into different types of cavitation around yield point. A two-step cavitation process was observed in case loosely packed spherulitic structure present in P4M1P sample with inter-spherulitc cavitation occurring first followed by intra-spherulitic cavitation. After the generation of the cavities at small strains, these cavities can be gradually stabilized due to isotropic distribution of the lamellae in the spherulites together with fibrils formed by the irregular aggregated amorphous chains and fragmented crystalline blocks. This can be verified by the absence of catastrophic failure of the samples during further stretching.64 The stabilized cavities further oriented along the deformation direction and the newly-born micro-fibrils with stacked crystalline lamellae inside finally formed fibrils. At large strain, the system can thus be regarded as a highly stretched amorphous network built up by load bearing inter-microfibrillar/fibrillar tie chains embedded with microfibrils/-fibrils. Clearly, the pre-existed cavities generated at small strains were not involved in this fibrillar network. Furthermore, the breakage of intermicrofibrillar/-fibrillar tie chains could lead to disentanglements of this highly oriented amorphous network at certain macroscopic critical stress, bringing out cavitation at large strain.31 This apparent macroscopic critical stress can be affected by morphological details of fibrils which could induce the different effective number of load bearing inter-microfibrillar/-fibrillar tie chains at the


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same cross-section area when the polymers were prepared via different routes.48 For example, the critical stress triggering large strain cavitation in iPP was found to be affected by the initial crystal forms when the samples were stretched below certain temperature which influenced detailed fibrillar structural features.47 In consideration of these phenomena, the independent processes of cavitation at small and large strains in iPP could be understood. For iPP with a certain molecular weight, the initial state of iPP relied on the crystallization and stretching conditions. Thus, the cavitation taking place at small strain was influenced by the annealing and stretching temperature. Nevertheless, the formation of highly oriented amorphous network in iPP during stretching is somehow determined by the intrinsic properties of iPP according to the previous study.31 It means that the stretching or annealing temperature cannot vary the final fibrillar network induced in the stretching process for iPP with a certain crystal form, unless the initial amorphous network of iPP was changed at the stretching or annealing temperature before deformation.47 The latter case could result in a variation of the highly oriented amorphous network formed during stretching, followed by a change in the critical stress for initiating large-strain-cavitation.47 In current work, no phase transition occurred in iPP during annealing or before stretching process. As a consequence, the cavitation found at large strain was independent of annealing or stretching process for iPP with a certain molecular weight. In other words, the appearance of large-strain-cavitation was unrelated to the occurrence of small-strain-cavitation. Additionally, Ran et al.63 addressed that the cavitation trigged at large strain was not influenced by the occurrence of cavitation at small strain in a recent work on tensile stretching of P4M1P. The rough schematic describing the appearance of two cavitation processes under different conditions are thus depicted in Figure 10. The lowest molecular weight iPP190K presented a



Page 26 of 38

significant vertical shift away from the main boundary line shared by the other three molecular weight iPPs. This shift means that the small strain cavitation is difficult to avoid in iPP190K, which is supposed to be a consequence of the molecular weight. We proposed that the small strain cavitation is triggered in competition with the crystalline block slips.32, 60 The relative number of entangled chains and tie molecules in unit volume of amorphous phase in different molecular weight iPP can be deduced from the weight crystallinity and lamellar thickness of each sample. At low annealing temperatures, the lowest molecular weight iPP190K always possessed the largest crystallinity, indicating that lowest fraction of amorphous phase in the iPP190K sample. In other words, the smallest number of entangled chains was presented in this sample. Although the similar crystallinity and lamellae thickness can be observed in these four different molecular weight iPP samples at high annealing temperatures, the chances for one chain passing through the adjacent lamellae in higher molecular weight samples composed of longer chains should be larger than the one in the lower molecular weight samples containing shorter chains. It means that the number of tie molecules in the former sample is always larger than the one in the latter iPP at any case. Therefore, a low molecular weight iPP possesses less number of entangled chains and tie molecules in unit volume of amorphous phase weakening the coupling of crystalline lamellae so that promoting easy cavitation in such system. However, these four iPP samples still shared one common behavior which is that the cavitation triggered at large strain is presented at any case and independent of the occurrence of cavitation activated at small strain.


Page 27 of 38

130

iPP190K iPP250K iPP340K iPP580K

120

110

Only cavitation at large strain

100

Cavitation at small and large strains

o

Td / C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


70

Only cavitation at large strain

60

Cavitation at small and large strains

50 40 36

38

40

42

44

46

48

50

52

54

w / %

Figure 10. Dividing lines of two cavitation processes dependency of iPP molecular weight, weight crystallinity w, and deformation temperature. The appearance of cavitation at large strain was considered as a consequence of the disentanglement of the highly oriented amorphous network initiated by the breaking of interfibrillar load bearing tie chains.31 Cavities produced at small strain neither bear load nor affect the load-bearing amorphous network at large strain. For example, iPP190K annealed at 110 oC generally showed a more intensive cavitation at small strains than iPP190K annealed at 30 oC while similar intensities of cavitation initiated at large strains were presented in the two samples under the same Td as shown in Figure 7. Especially, one can notice that the critical stresses for activating the latter cavitation process were the same in these two samples as was given in Figure 5. We therefore speculate that the cavitation occurred at small strain could not strengthen the cavitation initiated at large strain of iPP in terms of results provided in the current work. The reason is that cavities originally grown at small strain are well-developed before creating a highly oriented amorphous network of taut polymer chains (mainly referring to the limited extensibility of inter-



Page 28 of 38

microfibrillar/ -fibrillar tie chains).65 Even though these cavities exist in the system during the whole deformation process, they are not the essential elements constructing the oriented amorphous network. Such cavities do not share the load which is borne by the oriented amorphous network, bringing out negligible influence on the cavitation activated at large strain of iPP. Furthermore, we observed larger critical stresses needed to trigger the cavitation at large strain in iPP340K and iPP580K annealed at 130 oC stretched at 30 and 40 oC as compared to their common critical stresses. Recently, we investigated the influence of annealing temperature on the fibrillation process during deformation using iPP340K.7 It was found that the samples annealed at higher temperatures (110 and 130 oC) produced some content of microfibrils composed of small crystallites with mesophase and preserved some small crystallites originated from the breaking of initial block crystallites. The latter small crystallites still having the correlation in the long period, possessed much larger thicknesses than these newly-born small crystallites with mesophase and survived even to a strain of 1.65 at low Td. Such behaviors were caused by the limited chain mobility at low Td which cannot provide sufficient stress (mechanical melting).66 A higher annealing temperature could enhance these behaviors while such performances disappeared at high Td because of the improved chain mobility. Therefore, we suggested that the load is potentially allocated to both of the oriented amorphous network and small crystallites originated from the breaking of initial block crystallites at low Td. It can be envisaged that a larger critical stress is required for providing efficient stress activating cavitation at large strain in iPP340K and iPP580K samples annealed at 130 oC stretched at low Td since a portion of stress is borne by some un-melted original crystallites. 4. CONCLUSIONS Two cavitation processes separately triggered at small and large strains in iPP samples with


Page 29 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


different molecular weights during stretching were explored by means of ultra-small angle X-ray scattering (USAXS) technique. A lower molecular weight iPP, a higher annealing temperature, and a lower stretching temperature could enhance the cavitation initiated at small strain. However, this cavitation had no contribution to the cavitation activated at large strain since the latter one is caused by the disentanglement of the highly oriented amorphous network initiated by the breaking of inter-microfibrillar/-fibrillar load bearing tie chains. The large strain cavitation showed up in all deformation cases irrespective of the existence of the cavitation initiated at small strains or not. Because the cavities generated at small strains are excluded from the oriented amorphous network and cannot share the load which is borne by this fibrillar network. Therefore, only molecular weight can affect the latter cavitation process. A higher molecular weight iPP usually requires a larger critical stress for this behavior. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. True stress-strain curves of iPP250K, iPP340K, and iPP580K samples. (PDF) AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENT



Page 30 of 38

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Subsequent but Independent Cavitation Processes in Isotactic

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