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Stretching Temperature Dependency of Fibrillation Process in Isotactic Polypropylene Ying Lu, Ran Chen, Jiayi Zhao, Zhiyong Jiang, and Yongfeng Men J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b05071 • Publication Date (Web): 29 Jun 2017 Downloaded from http://pubs.acs.org on June 30, 2017

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The Journal of Physical Chemistry B is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Stretching Temperature Dependency of Fibrillation Process in Isotactic Polypropylene Ying Lu,* 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

ACS Paragon Plus Environment


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ABSTRACT： Isotactic polypropylene samples annealed at three temperatures were used to explore their fibrillation behaviors during tensile deformation at elevated temperatures via in-situ synchrotron small and wide angle X-ray scattering techniques. Fibrillation denotes the transition of the isotropic spherulitic morphology into a highly oriented one during tensile stretching a semi-crystalline polymer. It was found that the fibrillation was accomplished by a stress-induced melting and recrystallization process. Three regions named as fibrillation with the formation of only mesophase, of both mesophase and α crystallites, as well as of complete α modification were identified in a map of deformation and annealing temperature. Such results are tightly linked to the different molecular mobility of the samples prepared at different annealing temperatures and deformed at different temperatures. Lower annealing temperature and higher deformation temperature facilitate formation of α modification after stress-induced mechanical melting of the original crystallites. However, thicker lamellar crystallites deformation at lower temperatures presenting limited chain mobility end up with large amount of orientated mesophase structure.


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1. INTRODUCTION Typical polyolefin polymers, e.g., polyethylene (PE), polypropylene (PP) and polybutene-1 (PB-1), act as an important class of materials having a wide range of application in our daily life. Understanding of the basic mechanism of plastic deformation at the scale of crystalline lamellae, of these materials has been a subject of particular interest for the past several decades.1-5 Apparently, intensive researches have been published discussing some distinct opinions according to the experimental results. Such inconsistent findings always keep attracting researchers to dig the potential deformation mechanism in semi-crystalline polymers. In terms of the stretching process, the arguments essentially exist in whether the deformation mechanism is controlled by slips within the lamellae including crystallographic fine slips and intralamellar mosaic block slips6-9 or guaranteed by melting and recrystallization.10-14 Both arguments found supports from experimental results which include microscopic and X-ray diﬀraction investigations favouring the slip mechanisms7, 15 and small-angle X-ray and neutron scattering (SAXS and SANS) experiments supporting the melting and recrystallization mechanism.16-19 Based on the results of true stress-strain experiments for measuring the tensile deformation behavior of semi-crystalline polymers, both arguments discussed above are recently supposed to be activated at different strains during stretching.20, 21 Upon stretching, block slippage within the crystalline lamellae takes place first, followed by the stress-induced fragmentation and recrystallization starting at certain strain determined by the stability of crystalline blocks and the state of entangled amorphous network.22 Besides, the long spacing of new crystalline lamellae within fibrils developed during the deformation process depends only on the stretching temperature irrespective of the initial crystallization conditions as well as drawing rate.21, 23 Actually, the deformation behavior of materials on the microscopic and macroscopic scale is


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expected to be strongly affected by the drawing temperature as pointed out in literatures.24, 25 Both crystallographic deformation via rearrangement of the crystalline blocks and stress-induced fragmentation and recrystallization have been proven to be enhanced when raising the tensile deformation temperature.16 It is therefore indispensable to understand the variations of microstructure occurring in the deformational process upon stretching at different temperatures. Isotactic polypropylene (iPP) as a complex polymorphism polymer is well known for its four main crystalline forms: (α, β, γ, ε)26-28 and one smectic phase.29-31 Except for the α modification, the rest phases could only be induced under special conditions.32,

33

Due to the convenient

approach for preparing the α phase and the unique characteristic of α phase noted as cross-hatch structure which is composed of parent and daughter lamellae,34, 35 numerous investigations on the deformation process of iPP with this kind of modification has already been conducted. In general, the α phase can transform into the mesophase at low temperature attributing to the breaking of crystallites within the cross-hatch structure into small cluster caused by massive chain slips.34 In the case of elevated deformation temperatures, the phase transition will be prevented ascribed to the fact that mesophase is not stable enough at high temperatures29 and enhanced chain mobility can promote the folded-chain crystallization to develop new crystallites.36 Another common phenomenon of iPP stretched at low temperature is about the appearance of cavitation around the yield point,37, 38 including the sample prepared by the methods of isothermal crystallization, slow-cooled down in the air, and annealed at high temperatures. However, the quenched iPP is usually deformed without stress whitening around yield point whereas presents stress whitening at large strains at high temperature as a consequence of breaking the inter-fibrillar tie chains of the highly oriented amorphous network.39 Thus, it is available to explore the mechanism of


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plastic deformation at the scale of crystalline lamellae of iPP by taking advantage of quenched polypropylene before the fragmentation process of fibrils. In current work, experiments were carried out on quenched and annealed iPP samples to consider the deformation behaviors at elevated deformation temperatures by utilizing the synchrotron SAXS and wide-angle X-ray diffraction (WAXD) techniques. The deformation mechanism for fibrillation was confirmed to be in accordance with the stress-induced melting and recrystallization theory. Meanwhile, the deformation temperature played a dominant role in controlling the breaking of the original crystallites and the formation of new crystallites during stretching process. Either the low annealing temperature or high deformation temperature generally promoted the production of new crystallites during fibrillation. 2. EXPERIMENTAL SECTION iPP was purchased from Aldrich Polymer Products, 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 oC, developing films of 0.5 mm in thickness, which were quickly quenched into the ice water for half an hour. After that, quenched films were annealed at 30, 100 and 130 oC in a vacuum oven for 20 hours, respectively. Then, “dog bone” tensile bars with dimensions of 26×5×0.5 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. 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 30, 40, 50, 60, 77, 90, 115 and 135 oC. In order to measure the strain of the deformed area, optical photo images of the samples were employed during stretching processes. The true strain is needed to express the local strain of materials


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during stretching process because the deformation of the materials is not homogenous ascribed to the appearance of necking. Therefore, the Hencky strain ε H was used as a basic quantity of the true strain, which is defined as

ε H = 2 ln

b0 b

(1)

where b0 and b are the initial and instantaneous widths of sample during stretching process, respectively. In- situ synchrotron SAXS and WAXD experiments were both conducted at beamline 4B9A, BSRF, Beijing, China, with the wavelength of X-ray being 0.154 nm. Each SAXS and WAXD diagram obtained in the center of the sample was collected within one minute at a sample to detector distance of 3070 mm and 137 mm, respectively. The one dimensional WAXD curves were integrated from 0 to 180o of 2D WAXD patterns. For calculating the fraction of the α phase, the mesophase, and the amorphous phase, Gaussian peaks fitting of WAXS curve was first carried out by using Matlab, and then followed the below formulas40

fme =

Ame Ame + Aα + Aam

(2)

Aα Ame + Aα + Aam

(3)

fam = 1 − fme − fα

(4)

fα =

where Ame, Aα, and Aam represent the integrated area of diffraction peaks corresponding to mesophase, α phase, and amorphous phase, respectively. The characteristic reflections of the α phase located at 2θ = 14.1o, 16.9o, 18.5o, 21.2o, 21.8o, and 25.5o, of the mesophase positioned at 2θ = 15o and 21.4 o, and of the amorphous halo spotted on 2θ = 17 o. In the case of SAXS


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experiments, it provides the effective scattering vector q range from 0.084 to 0.98 nm-1. 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 dac was calculated by using the Bragg equation,

d ac =

2π q max

(5)

the qmax represents the maximum q in the SAXS patterns for the periodic lamellar structure when scanning along certain directions. 3. RESULTS AND DISCUSSION The true stress-strain curves are registered in Figure 1, including selected in-situ two-dimensional SAXS patterns of the iPP samples at different strains. Clearly, the annealed samples stretched at 30 oC showed different stress at the same strain. The larger the annealing temperature was, the higher the true stress was observed. Such differences can be reduced in these annealed samples stretched at 77 oC. Inspecting with these SAXS patterns, they first showed an isotropic feature for the samples at the un-deformed state while they then gradually became anisotropic ones with increasing deformation. A pattern with two straight streaks perpendicular to the stretching direction at high draw ratios can finally develop, indicating a highly oriented lamellar structure after deformation.21 Nevertheless, the change tendency of scattering patterns during deformation process seemed to rely on the annealing and stretching temperatures. During deformation, the two scattering spots shifted closer to the beam center as increasing the annealing temperature for the iPP samples drawn at 30 oC. Whereas these differences disappeared upon inspecting the SAXS patterns obtained at stretching temperature of 77 oC. Such behaviors were clearly different from the previous findings that the formation of


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new crystallites is only related to the deformation temperature but independency of original crystallization conditions.21 This law is not suitable for the current case of annealed iPP samples stretched at 30 oC whereas it holds for samples drawn at 77 oC. In addition, one can observe the equatorial streaks with a tilt angle along vertical direction in all annealed samples stretched at 77 o

C. But for samples deformed at 30 oC, these patterns only showed up in the one annealed at 30

o

C. This special equatorial streaks can be ascribed to the scattering from micro-fibrils.41 Clearly,

deformation temperature plays an important role during the tensile deformation of annealed iPPs.

Figure 1. True stress-strain curves of annealed iPP samples drawn at 30 (top) and 77 oC (bottom). (Selected SAXS patterns taken at different strains were also indicated on the graph (stretching direction: horizontal))


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0.000008 o

Ts= 30 C

εΗ

o

Ta= 30 C

Iq / a.u.

0.000006

2

0.000004

0.000002

0.000000

εΗ

o

0.000012

Ta= 100 C

2

Iq / a.u.

0.000009

0.000006

0.000003

0.000000 0.000020

o

Ta= 130 C εΗ

0.000016

0.000012

2

Iq / 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


0.000008

0.000004

0.000000 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

-1

q / nm

Figure 2. One-dimensional scattering intensity distribution profiles as a function of strain of annealed iPP samples stretched at 30 oC.


9


o

0.000008

Ts= 77 C

εΗ

o

Ta= 30 C

2

Iq / a.u.

0.000006

0.000004

0.000002

0.000000 0.000010

εΗ

o

Ta= 100 C

0.000006

2

Iq / a.u.

0.000008

0.000004

0.000002

0.000000 0.000015

o

Ta= 130 C

εΗ

0.000012

0.000009

2

Iq / 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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0.000006

0.000003

0.000000 0.0

0.1

0.2

0.3

0.4

0.5

q / nm

0.6

0.7

0.8

0.9

1.0

-1

Figure 3. One-dimensional scattering intensity distribution profiles as a function of strain of annealed iPP samples stretched at 77 oC.


10

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In an effort to give a detailed description of the evolution of the lamellar long spacing during deformation process, the one dimensional scattering intensity distributions along the stretching direction are collected in Figure 2 and Figure 3. The red arrows in the figures mark out the directions of increasing strain. The decrease of scattering intensity with further stretching may be associated with the thinning of the sample and the decrease of the density difference between crystalline and amorphous phase formed during stretching. According to these one-dimensional profiles showed in Figure 2, one finds obvious variation of qmax with strain among annealed samples. These qmax values of all annealed iPPs obtained during deformation process moved to the low q range compared with the initial qmax. Moreover, these values of qmax apparently increased with increasing the annealing temperature. For the annealed samples drawn at 77 oC (Figure 3), the difference of qmax can be found during the early deformation process but the qmax of samples in the late deformed state were almost the same. Figure 4 summarizes the values of long spacing of iPP samples as a function of strain by using Bragg equation. Here, the 30 oC annealed iPP drawn at 30 oC is chosen to give an example. At the small strains from 0 to 0.2, the dac presented a slight increase which is assigned to block slippage within the crystalline lamellae accompanied by slight extension of the amorphous phase along the stretching direction. With further increasing of strain, the value of dac almost arrived at a plateau value. For the rest two annealed iPP samples drawn at 30 oC, both of them displayed the similar tendency as the trend observed in the 30 oC annealed iPP. But the difference can be found in the late deformation stage that the values of dac of annealed samples depend on the annealing temperature. As the drawing temperature elevated to 77 oC, the development of long spacing among annealed samples was nearly the same to the samples stretched at 30 oC at the early deformation stage. However, the difference appeared in the late deformation stage as


11


reflected by the fact that the values of dac of annealed samples were the same after certain strains. Such deformation behaviors were in line with the typical deformation mechanism in semi-crystalline polymers that block slips within the crystalline lamellae take place first, followed by the stress-induced fragmentation and recrystallization starting at certain strain.22 One thing should be noted that the dac in the final deformed stage increased or decreased comparing to the un-deformed dac which is decided by the original crystallization condition and stretching temperature.21 The dac increases if the deformation temperature is higher than annealing temperature, and vice versa. Thus, we achieved an increased dac of 30 oC annealed iPP but a decreased one in 130 oC annealed sample for stretching at 77 oC. Because of the inconsistent values of the final dac showed in different stretching temperature for three temperatures annealed iPPs, it is too rough to demonstrate that the deformation mechanism is guaranteed by slips or controlled by stress-induced melting and recrystallization.22

24

dac/ nm

22

o

Ta / C

o

Ts= 30 C

30

100

130

20 18 16 14 12 10 22 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

o

Ts= 77 C

20

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

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18 16 14 12 0.0

0.2

0.4

0.6

0.8

1.0

εΗ 1.2

1.4

1.6

1.8

2.0

2.2

Figure 4. The long spacing dac as a function of strains of annealed iPP samples deformed at 30 (top) and 77 oC (bottom).


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12000

o

unde formed iPP sam ples

Ta / C 30 100 130

(110) α

I/ a.u.

10000

8000

(04 0)α (130 ) α 6000

(131) α

(111 )α

4000

2000

5000

(110) α

o

T s=30 C

(111 )α +(131)α

4500

(040)α (130 )α

I/ a.u.

4000

3500

mesoph ase 3000

2500

(1 10) α

o

1500

T s=50 C

(11 1)α +(1 31) α

(040) α (130) α I/ a.u .

1000

mesoph ase 500

0

o

T s=7 7 C

(110)α

1500

(040) α

(111 )α +(131)α

(13 0) α

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


1000

500

0 10

12

14

16

2θ /

18

o

20

22

24

Figure 5. WAXD profiles of isotropic iPPs and of annealed iPPs after deformed at the indicated temperatures. The indices of crystallographic planes are also marked.


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To clarify possible influence of crystalline forms on the deformation behaviour of iPP samples, WAXD data of isotropic and deformed samples are collected in Figure 5. Samples annealed at the lowest temperature crystallized into the α form completely and no trace of diffraction peak of mesophase can be found. This is ascribed to the fact that the mesophase can only be obtained under the fast cooling rate higher than 80 K/s42 whereas the quenched process for our sample visibly cannot achieve such high cooling rate. The fractions of α phase, amorphous phase, and mesophase of samples displayed in Figure 5 are summarised in Table 1 using the equations from 2 to 4. Comparing the fractions of amorphous phase between annealed samples and their corresponding stretched ones at Ts of 30 oC, a larger fraction of amorphous phase could be observed in their stretched samples. This behaviour can be understood as a result of lower stress-induced crystallization temperature than the crystallization temperature of annealed samples. The higher the crystallization temperature was, the lower fraction of amorphous phase could be obtained. Even though these samples were firstly quenched into the ice water, they could finish the crystallization during quenching before the temperature of the system cooled down to the same temperature of ice water. Otherwise, the mesosphase could be observed after quenching. Focusing on the sample stretched at 30 oC, the characteristic diffraction peak of mesophase around 15o can be observed in all annealed samples. The only difference is the fraction of mesophase produced in the annealed samples. After stretching, the 30 oC annealed iPP possessed a larger amount of mesophase as compared to the ones annealed at high temperatures. As pointed out in the reference, the α phase can transform into the mesophase at low deformation temperature attributing to the breaking of the cross-hatch


14

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structure caused by massive chain slip.34 Evidently, it is more difficult to induce fragmentation of initial crystallites at low drawing temperature when they are thicker. In Table 1, one can find larger fractions of α phase in samples annealed at higher temperature after they were stretched at 30 oC. This result clearly provided a further evidence that much more initial crystallites in higher temperature annealed samples had been preserved during stretching. Moreover, some part of initial crystallites in samples annealed at higher temperature transformed into amorphous phase after stretching. The low mobility of molecular chains at 30 oC constrained the movement of amorphous chains converting back to crystalline state.43, 44 Table 1. The fractions of α phase, of mesophase, and of amorphous phase of curves displayed in Figure 5.

o

Ts = 30 oC

initial state

Ta /

Ts = 50 oC

Ts = 77 oC

C

fα

fme

fam

fα

fme

fam

fα

fme

fam

fα

fme

fam

30

0.52

0

0.48

0.17

0.30

0.53

0.28

0.21

0.51

0.50

0

0.50

100

0.57

0

0.43

0.28

0.23

0.49

0.30

0.19

0.51

0.51

0

0.49

130

0.60

0

0.40

0.30

0.22

0.48

0.32

0.18

0.50

0.51

0

0.49

In the case of elevated deformation temperature (50 oC), a distinct diffraction peak of mesophase can still exist whereas the difference in the fraction of mesophase among annealed samples can be ignored. When the deformation temperature was 77 oC, there was only α form presented in all annealed iPPs attributing to the fact that the mesophase cannot be stable at such high temperature.29 Clearly, different variations of crystalline form can be found in annealed samples drawn at 30 oC while such difference became


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much weaker as elevating the deformation temperature. Here, one may consider that the disagreement at the scale of lamellae among annealed samples drawn at 30 oC was essentially caused by the formation of mesophase. This supposition was not fully convincing with respect to the change of crystalline form as comparing the iPP annealed at 100 and 130 oC drawn at 30 oC. For this two annealed samples, the formation of mesophase was almost the same during stretching whereas the difference of dac still existed. Thus, more deformation experiments were carried out on annealed samples at the stretching temperature ranged from 30 to 77 oC. The corresponding results of the one-dimensional scattering intensity profiles of annealed iPP samples in the final deformed state are given in Figure 6. Illustration will be first started from the sample annealed at 30 oC. The qmax of the final deformed state of this kind of iPP evidently decreased with elevating the deformation temperature excluding the result obtained at the stretching temperature of 30 oC. This variation tendency exactly followed the deformation mechanism of stress-induced melting and recrystallization.21 The change of qmax for the samples annealed at high temperature showed a quite unusual behaviour. For the sample annealed at 130 oC, the position of qmax almost kept constant below the stretching temperature of 40 oC while two separate qmax peaks can be observed at the stretching temperature ranging from 40 to 70 oC. The peak 2 apparently cannot be assigned to the second-order scattering of the peak 1 for two reasons. Firstly, the relative position of these two peaks is not doubled as required for lamellar stacking systems. Secondly, the intensity of peak 1 became weaker but that of peak 2 increased intensively step by step at elevated deformation temperatures. In addition, the position of peak 1 was almost unvaried but that of peak 2 gradually shifted


16

to the lower range of q when the stretching temperature was increased. Clearly, the position of peak 2 was connected with the deformation temperature. The sample annealed at 100 oC also presented the similar tendency like sample annealed at 130 oC with increasing deformation temperature, but which displayed a smaller temperature range of the existence of two peaks than that of sample annealed at 130 oC.

undeformed iPP samples 0.0000025

o

0.37 nm

0.0000020

T a/ C

-1

0.46 nm

30 100 130

-1

2

Iq / a.u.

0.0000015

0.62 nm

-1

0.0000010

0.0000005

0.0000000 o

0.0000016

T s/ C

o

T a=30 C

30 40 50 60 70 77

2

Iq / a.u.

0.0000012

0.0000008

qo =0.62 nm

-1

0.0000004

0.0000000

o

2

Iq / a.u.

0.0000009

T a =100 C

qo=0.46 n m

-1

0.0000006

0.0000003

0.0000000

0.0000008

o

T a=130 C qo =0.37 nm

0.0000006

-1

peak 2

peak 1

2

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


Iq / a.u.

Page 17 of 35

0.0000004

0.0000002

0.0000000 0.0

0.1

0.2

0.3

0.4

0.5-1

q / nm

0.6

0.7

0.8

0.9

Figure 6. One-dimensional scattering intensity distribution profiles of undeformed iPPs and of annealed iPPs after drawn at the indicated temperatures.


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Based on the results shown in Figure 6, the further analysis of stretching and annealing temperature dependency of deformation mechanism should be focused on the stretching temperature ranging from 50 to 70 oC. Thus, the changes of qmax during deformation process in the above-mentioned temperature range are displayed in Figure S1 to Figure S3 in the supporting information. Figure S1 exhibits the one-dimensional scattering profiles of 30 oC annealed samples during deformation at different temperatures. For this iPP sample, the change of these scattering curves was the same to the results showed in Figure 2. Only one scattering peak showed up during the whole stretching process. But for the sample annealed at high temperatures, such scattering curves exhibited some special features. In Figure S2, for the 100 oC annealed sample drawn at 50 oC, only one peak existed in the scattering curves at the early deformation stage whereas a newly formed peak showed up at larger strains. Especially, the original one did not change its position while the position of the new one varied with strain. Similar behaviours were also found in 100 oC annealed sample deformed at 60 oC. The slight difference is that only the new peak displayed at final strain state, meaning the original scattering peak was consumed and replaced by the new-born scattering peak eventually. As the stretching temperature elevated to 70 oC, only one scattering peak could be observed during the whole deformation process. Which indicates the deformation mechanism in 100 oC annealed iPP stretched at that temperature became the same as the case observed in 30 oC annealed iPP. In Figure S3, one can observe more distinct phenomena in 130 oC annealed sample which were resemble to the performance mentioned-above in 100 oC annealed sample.


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In the cause of evaluating the relationship between peak 1 and peak 2 observed in high temperature annealed samples during deformation process, Figure 7 registers the integrated intensity of two peaks changed with strain via curve fitting method. Here, 100 o

C annealed sample were chosen to give a detailed explanation. The fraction of peak 1

was constant at the early deformation stage and started to decrease at the strain of the occurrence of peak 2. With further stretching, the increase of the fraction of peak 2 relied on the basis of spending the fraction of peak 1. Meanwhile, the final fraction of peak 2 displayed a relationship to the deformation temperature that it can completely replace peak 1 as the deformation temperature increased to a certain value. Such analogous trend can be further manifested by the results showed in 130 oC annealed sample. 1.2

o

Ts / C

50

o

60

Ta= 100 C

1.0

Fraction

0.8 0.6

solid points: peak 1 open points: peak 2

0.4 0.2 0.0 -0.2 1.2

0.0

0.2 o

Ts / C

0.4

0.6

50

0.8

60

1.0

70

1.2

1.4

1.6

1.8

o

Ta= 130 C

1.0 0.8

Fraction

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



0.6 0.4 0.2 0.0 -0.2 0.0

0.2

0.4

0.6

0.8

εΗ 1.0

1.2

1.4

1.6

1.8

2.0

Figure 7. The fraction of integration area of two peaks as a function of strain of annealed iPP samples deformed at different temperatures.


19


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Figure 8 presents values of the long spacing according to Figure 2 and Figure 3, and Figure S7 to S9, including the dac of peak 1 and peak 2 presented in high temperature annealed samples during deformation process. Clearly, in the final deformed state dac of sample annealed at 30 oC increased with elevating the deformation temperature. In the case of samples annealed at high temperature which exhibit two peaks situation, the value of dac of peak 1 almost stayed unchanged as varying the stretching temperature while the one of peak 2 increased with increasing deformation temperature. As for samples annealed at high temperature but stretched at high temperature, there were only one peak and the value of dac also increased with the increase of deformation temperature. Putting the results in Figure 7 and Figure 8 together, we can speculate that the breaking of original crystallites and the formation of new crystallites depended on both of crystallization condition and deformation temperature. Actually, the deformation temperature dependency of stress-induced structure in iPP has been confirmed by Hsiao et al. via WAXD technique.36 In their work, it was found that the poor chain mobility would restrict the motion of chain slipping as well as chain disentanglement at low deformation temperature, which results in a strong local stress concentration of the entanglement and thus leads to the fragmentation of neighbouring crystal lamellae by chain pull-out (equal to mechanical melting). Besides, these stretched-out chains transform into the oriented mesophase being featured with extended-chain conformation but without the regular registration of helical hands as in the crystalline form. Whereas, at high stretching temperature, the tensile deformation will induce chain disentanglement, reducing the constraints for folded-chain crystallization from inter lamellar coiled


20

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segments attributing to the enhanced chain mobility weaken the amorphous entanglement network at high temperature. 18 17 16

dac/ nm

15

o

Ts / C

30

50

60

70

77

o

Ta= 30 C

14 13 12 11 10 9 0.0 20 18

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

o

Ta= 100 C

dac/ nm

16 14 12


10 8 0.0 26 24

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

o

T a= 130 C

22

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


20 18 16 14 12 10 0.0

0.2

0.4

0.6

0.8

1.0

εΗ 1.2

1.4

1.6

1.8

2.0

Figure 8. The long spacing dac as a function of strain of annealed iPP samples deformed at different temperatures. According to Hsiao’s work,36 it becomes easy to understand the different deformation behaviors presented in annealed iPP samples in our work. At low deformation temperature of 30 oC, the thinner lamellae were easy to break and to develop into


21


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Page 22 of 35

mesophase. However, the thicker lamellae needed much higher stress to be destroyed considering the similar amorphous entanglement network. This might be the reason the higher annealing temperature was, the lower fraction of mesophase was, as the WAXD results shown in Figure 5. The 30 oC annealed sample displayed a larger fraction of mesomorphic structure while the higher temperature annealed samples still had an obvious amount of α phase when they deformed to the similar strain at 30 oC. Such results indicate that a large number of crystallites had been stretched to mesophase in 30 o

C annealed sample.36 One thing should be emphasized here is that iPP samples annealed

at higher temperatures can be still completely transformed to mesophase by providing sufficient stress via further stretching at 30 oC. However, a further stretching will induce stress whitening at large strains39 which disturbs the measurement of lamellar structure via SAXS technique due to strong scattering from the cavities. Thus, further results of iPP samples annealed at higher temperatures stretched to larger strains at low deformation temperature were not analyzed. Furthermore, the long spacing of the crystallites developed at 30 oC in the deformation process relied apparently on the initial annealed temperature of iPP. The long spacing derived from peak 1 also showed the similar behaviour. They seemed to have a memory of the original crystallites. This results can be associated with the breaking of the block crystallites which were damaged into small crystallites but still have the correlation in long period. Obviously, such small cluster in the iPP samples annealed at higher temperatures with a larger initial long spacing can be stretched to a larger long spacing. By providing sufficient local stress, these small crystallites can transform into mesophase or recrystallize to α phase. That is the reason that similar fraction of mesophase can be


22

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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


observed in annealed samples with elevating the deformation temperature. Increasing the deformation temperature, the enhanced stress can promote the breaking of original crystallites into small crystallites. More importantly, even these small crystallites with larger long spacing can also develop into mesophase or recrystallized to α modification crystallites due to enhanced chain mobility under such high temperature condition.36, 45 This is the reason for decrease of the fraction of original crystallites with decreasing of annealing temperature and increasing of stretching temperature in Figure 7 as well. Clearly, the dac originated from peak 2 belonged to the new crystallites with α phase and mesophase formed during the stretching process, showed a relationship with the deformation temperature. If the molecular chain mobility is not high enough for carrying the rearrangement of chain conformation, it was rational to obtain the mesophase with a similar dac of recrystallized α phase. One can notice a special feature that the new-born thinner lamellae were more stable than the original thicker lamellae since the latter ones were consumed gradually during stretching in Figure 8. Such result was in agreement with our previous findings46 that lamellae formed through stress-induced crystallization generally showed a smaller size than lamellae isothermally crystallized from random melt under the same temperature. Because the crystallization process of polymer chains was guaranteed by these thermal dynamic parameters of the enthalpy H, the entropy S, and the Gibbs free energy G in the melt.47 A stress-induced localized melting led to the freed polymeric chains preferentially aligned along stretching direction which were different from the polymer chains randomly distributed in the quiescent melt. Thus, different values of these thermal dynamic parameters in different molten states caused the different lamellae thicknesses obtained


23


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Page 24 of 35

through stress-induced crystallization and crystallization from quiescent melt routes. The size of new-born lamellae through stress-induced crystallization was therefore more stable than original thicker lamellae during stretching. Here, we can figure out the annealing and stretching temperature dependency of dac formed during stretching according to Figure 9 which summarizes the change of dac with annealing and deformation temperature. In this diagram, three regions can be identified with respect to the deformation behaviours of annealed samples obtained at different conditions. The variation of dac of 100 oC annealed sample was still selected as an example: (1) The region I (below 40 oC). The block crystallites in iPP samples can be broken into small crystallites, such crystallites can completely transform into mesophase under sufficient stress (mechanical melting).36 The long spacing of small cluster only relied on the initial lamellar thickness so that we observed annealing temperature dependency of dac formed at deformation temperature of 30 oC. Moreover, the fibrillation process produced micro-fibrils composed of these small crystallites with mesophase characteristic due to that the poor chain mobility constrained the rearrangement motion of chains conformation.48 Visibly, this region showed up in all temperature annealed iPP samples. (2) The region II (between 40 and 50oC), it is defined as fibrilltion with the formation of both mesophase and α crystallites. Some original crystallites were destroyed to small cluster which can develop into mesophae under sufficient stress, while the rest of the crystalline blocks were broken and recrystallized into the α phase due to the increasing chain mobility as elevating the deformation temperature. Clearly, the long spacing of small crystallites (peak 1) was only related to the initial lamellar thickness. So that we observed the constant dac obtained from peak 1 relied on the annealing


24

Page 25 of 35

temperature but independent of the deformation temperature. However, the dac obtained from peak 2 belonged to the recrystallized α phase and mesophase during fibrillation, which only depended on the deformation temperature. (3) In region III (above 50 oC), it is named region of fibrillation with the formation of only α crystallites. In this region, the significantly enhanced chain mobility can improve the crystallization ability of chain segments fragmented from the original crystallites so that they recrystallize into α phase. The dac of newly developed crystallites was only linked to the deformation temperature. 26

o

Ta / C

24

30 100 solid points: peak 1 open points: peak 2

130

22

I

20

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


II o

40 C

III o

50 C

18 16 14 12 10 30

40

50

60

70

80 o 90

Τs / C

100

110

120

130

Figure 9. Phase diagram of the long spacing dac of annealed iPP samples developed after stretching at different temperatures. Furthermore, the 130 oC annealed iPP sample possessed a larger temperature region II (40 to 70 oC) than the 100 oC annealed iPP, ascribed to higher chain mobility required for the fragmentation and recrystallization of original crystallites. In order to acquire high enough chain mobility for supporting these movements, high local deformation temperature is required. This high local temperature can be provided by high local stress


25


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Page 26 of 35

via stretching sample to a large strain.16 In our work, all samples deformed to the similar strain in region II in order to prevent the stress whitening at large strain.39 It can be expected that a larger amount of breaking thicker lamellae with poor chain mobility were preserved at such strain, meaning that the complete recrystallization of thicker lamellar would be improved only by elevating the stretching temperature. Hence, we found the largest temperature range of region II in 130 oC annealed iPP. Details on 30 oC annealed iPP deformed at region II should be discussed. This low temperature annealed sample only showed a single scattering peak of SAXS curves while both mesophase and α phase were captured after deformation. This behaviour can be interpreted for two reasons. First, the long spacing of oriented mesophase was close to the long spacing of α phase formed during deformation process. This view is supported by comparing the dac of 30 oC deformed sample and 40 to 60 oC deformed samples. The slight decrease of dac in the 40 oC deformed sample can be attributed to the dac of newly formed α phase being smaller than the one of mesophase, resulting in a decrease of the average dac. The dac of newly formed α phase increased with elevating deformation temperature, then, the average dac started to increase again at the deformation temperature above 50 oC. Second, the newly formed α phase at low deformation temperature may be composed of imperfect crystallites, leading to a similar density to the highly oriented mesophase. Accordingly, it is difficult to separate the scattering peaks from mesophase and recrystallized α phase. Therefore, only one scattering peak can be observed in 30 oC annealed iPP samples deformed at the temperature of region II. We also cannot point out the exact range of region II for 30 oC annealed iPP but this range must be the smallest one as this kind of iPP sample possesses the thinnest lamellae.


26

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4. CONCLUSIONS In conclusion, stretching temperature dependency of fibrillation process was observed in annealed iPP samples by means of in-situ synchrotron SAXS and WAXD techniques. The deformation temperature was revealed to be an important factor in guiding the breaking of original crystallites and the production of new crystallites. Three deformation regions were found according to the annealing and stretching temperature dependency of deformation behaviours. The region I defined as the formation of fibrils composed of only mesophase crystallites was located at low deformation temperature. The region II which consisted of fibrils formed by mesophase and α crystallites was found in moderate stretching temperature range. The last region performing fibrils constructed of complete α crystallites was presented in high drawing temperature. Such cases proved that the deformation mechanism was guaranteed by the stress-induced melting and recrystallization. During the deformation process, the development of new crystallites was under the precondition of consuming the original crystallites. Because of melting and recrystallization of original crystallites restricted by poor chain mobility at low stretching temperature, thus, the lamellae broke into small crystallites and then developed to mesophase with enough stress at low temperature. The most efficient way for the growth of new crystallites was to increase the chain mobility via elevating the deformation temperature, and thus we observed an increase of new crystallites with increasing deformation temperature for all annealed iPP samples. The breaking and recrystallization of thicker lamellae apparently needed much stronger chain mobility, leading to a larger temperature range of coexisted mesophase and new crystallized α phase in higher temperature annealed iPP after deformation. Moreover, the dac of new crystallites induced


27


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Page 28 of 35

in stretching process was only relied on stretching temperature, which is the typical result from the stress-induced melting and recrystallization process. ASSOCIATED CONTENT Supporting Information. One-dimensional scattering intensity distribution profiles as a function of strain of annealed iPP samples stretched at 50, 60, and 70 oC. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (21134006, 51525305). We thank Prof. Zhonghua Wu, Dr. Guang Mo and Prof. Zhihong Li of BSRF for assistance during SAXS and WAXD experiments. REFERENCES (1) Lin, L.; Argon, A. S. Structure and Plastic-Deformation of Polyethylene. J. Mater. Sci. 1994, 29, 294-323. (2) Chen, X. D.; Xu, R. J.; Xie, J. Y.; Lin, Y. F.; Lei, C. H.; Li, L. B. The Study of Room-Temperature Stretching of Annealed Polypropylene Cast Film with Row-Nucleated Crystalline Structure. Polymer 2016, 94, 31-42. (3) Chen, W.; Li, X. Y.; Liu, Y. P.; Li, J.; Zhou, W. M.; Chen, L.; Li, L. B. The Spatial Correlation Between Crystalline and Amorphous Orientations of Isotactic Polypropylene during Plastic Deformation: An in Situ Observation with FTIR Imaging. Chin. J. Polym. Sci. 2015, 33, 613-620.


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Stretching Temperature Dependency of Fibrillation ... - ACS Publications

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