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1-octene random and block copolymers synthesized from living coordination polymerization
Accepted Manuscript Elastomeric Properties of Ethylene/1-Octene Random and Block Copolymers Synthesized from Living Coordination Polymerization Weifeng Liu, Xiao Zhang, Zhiyang Bu, Wen-Jun Wang, Hong Fan, Bo-Geng Li, Shiping Zhu PII:
S0032-3861(15)30097-5
DOI:
10.1016/j.polymer.2015.07.019
Reference:
JPOL 17979
To appear in:
Polymer
Received Date: 3 May 2015 Revised Date:
16 June 2015
Accepted Date: 11 July 2015
Please cite this article as: Liu W, Zhang X, Bu Z, Wang W-J, Fan H, Li B-G, Zhu S, Elastomeric Properties of Ethylene/1-Octene Random and Block Copolymers Synthesized from Living Coordination Polymerization, Polymer (2015), doi: 10.1016/j.polymer.2015.07.019. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT Elastomeric Properties of Ethylene/1-Octene Random and Block Copolymers
Synthesized from Living Coordination Polymerization Weifeng Liu,1 Xiao Zhang,1 Zhiyang Bu,1 Wen-Jun Wang,1 Hong Fan,1 Bo-Geng Li1* and Shiping Zhu2* State Key Laboratory of Chemical Engineering, College of Chemical & Biological Engineering,
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1
Zhejiang University, Hangzhou P.R. China 310027; 2
Department of Chemical Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4L7
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* Corresponding authors, E-mail: [email protected] & [email protected]
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Abstract: The elastomeric performance of a series of ethylene/1-octene random and block copolymers prepared from living coordination polymerization was studied in this work. All the polymer samples exhibited good toughness. The living random copolymers had better elasticity than commercial metallocene random copolymers. The strain recovery of the living random copolymers was about 90% with an applied strain up to 800%. It was demonstrated that the
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copolymer elasticity was mainly determined by their block structure, with little influence by their molecular weight. It was also found that either too high or too low content of hard-block content deteriorated the copolymer elastomeric behavior. The diblock copolymer having about 42 wt% hard
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block content (LBC1) gave the best elasticity among the studied block copolymers and retained the
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strain recovery about 80% under the applied stain of 600%.
Key Words: living coordination polymerization, olefin block copolymer, thermoplastic elastomer
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Introduction Ethylene/α-olefin copolymer with high comonomer incorporation is one of the most widely-used high performance thermoplastic elastomers (TPEs) [1]. Compared with other TPE types, polyolefin-based thermoplastic elastomers have received considerable attention due to their better
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chemical resistance, lower density, excellent weatherability and lower cost for elasticity formation [2]. For good elastomeric property of low modulus and high recovery from large deformation, crystallinity must be lower than 20 wt% [3], which is usually realized by incorporating α-olefins, such as propylene, 1-butene, 1-hexene and 1-octene into polyethylene main chain to disrupt
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crystallization of the methylene sequences. Thermal and mechanical properties of the ethylene/α-olefin random copolymer elastomers (POE) have been well studied since their
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commercialization by DOW and ExxonMobil [4~7]. Most of these commercially available POE products are made from metallocene catalysts and were usually compared with those copolymers from Z-N catalysts. It has been demonstrated that metallocene-catalyzed ethylene/α-olefin copolymers have narrower molecular weight and composition distributions than Z-N’s, and thus
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have better mechanical properties [4]. However, little work reported on the comparison of mechanical properties between the metallocene-catalyzed copolymers and those from living coordination copolymerization with post-metallocene catalysts. In 2006, a novel type of ethylene/1-octene olefin block copolymer (OBC) was commercialized
EP
by DOW through a chain shuttling polymerization (CSP) mechanism [8,9]. In CSP, two post-metallocene catalysts are employed, with one catalyst good at ethylene/α-olefin
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copolymerization that generates soft amorphous copolymer segments and the other catalyst poor in the copolymerization that produces hard semi-crystalline segments. The hard and soft segments are shuttled between the two metallic active centers through an appropriate chain shuttling agent. The multiblock copolymers (hard-soft-hard…) are thus formed. It was found that OBC exhibited better elasticity at high temperature than POE [10]. Although great attention has been paid to OBC mechanical properties and melt phase behaviors [11~17], their structure/property relationships are still lacking. In OBC, the soft and hard segments are statistical numbers and lengths. The comonomer incorporation in the hard and soft segments is not clear. There is usually certain amount of random copolymer impurities in the commercial OBC products [18,19]. In researching OBC
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ACCEPTED MANUSCRIPT performance, it is difficult to focus on a single factor in establishing a clear relationship between chain microstructure and property. In contrast, living olefin copolymerization can provide well defined and controlled polyolefin chain microstructures. Polyolefins with an explicit chain microstructure can be obtained from living coordination polymerization. In the recent years, numerous ethylene/α-olefin block copolymers were synthesized through
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living olefin polymerization [20~22]. For example, PE-b-PEP-b-sPP and PE-b-PEP-b-PE triblock copolymers were first synthesized by Fujita group using a fluorinated FI-Ti catalyst, which is a well-known
living
coordination
polymerization
catalyst
system
[23].
Coates
group
reported
the
synthesis
pentablock
iPP-b-PEP-b-iPP
triblock
copolymers,
copolymers, and
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iPP-b-PEP-b-iPP-b-PEP-b-iPP
of
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PE-b-Poly(ethylene-co-1-hexene) diblock copolymers were also synthesized by Fujita group [24].
iPP-b-PEP-b-iPP-b-PEP-b-iPP-b-PEP-b-iPP heptablock copolymers using another fluorinated FI-Ti catalyst [25]. The advent of living olefin polymerization technique allows precise design of the polyolefin chain microstructure, so as to tailor-make polyolefin materials [22]. Some authors noted that ethylene/α-olefin block copolymers synthesized from living polymerization technique exhibited
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good elastomeric behavior and excellent toughness [26]. However, detailed mechanical studies on the living block copolymers were very scattered.
In our previous work, we synthesized a series of ethylene/1-octene random and block copolymers
EP
by living coordination polymerization [27~29] and demonstrated that the copolymer samples possessed narrower intra-chain composition distributions than those prepared from conventional
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metallocene catalysts [19]. The objective of this work is to investigate their elastomeric behaviors.
Materials and methods:
The ethylene/1-octene copolymer samples used in this work were synthesized from bis[N-(3-methylsalicylidene)-2,3,4,5,6-pentafluoroanilinato] titanium(IV) dichloride/dMAO [30]. The synthesis procedures for copolymer samples and the characterization of molecular weight, composition and DSC properties have been described in our previous papers [27~29]. The characteristics of the random copolymer samples are listed in Table 1 and those of the block copolymers are in Table 2. LRC1 and LRC2 in Table 1 are living random copolymers made from
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ACCEPTED MANUSCRIPT the living coordination polymerization. MRC is a commercial ethylene/1-octene random copolymer sample (Engage 8150) produced from CGC catalyst by DOW Chemical Company. LBC in Table 2 is living block copolymers from the living coordination polymerization. The samples that submitted to mechanical tests were solution cast into uniform films with thickness between 70 and 100 µm. Xylene was used as the solvent. The film thickness was
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measured by a digital electron thickness gauge. Dumbbell-shaped tensile specimens for uniaxial elongation tests and hysteresis experiments were cut from the solution cast films. The tensile stress-strain behavior was performed on the universal material tester Zwick/Roell Z020 at room
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temperature with the cross-head speed of 800%/min. The test method is GB1040.2-2006. Each test was repeated at least four times. The initial length of the specimens between the clamps was 50 mm,
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having width of 4 mm. For the elastic strain recovery test, the hysteresis experiments were performed on the MTS tensile instrument (MTS Systems China Co. Ltd) at room temperature. The test method is GB1040.3-2006. The fixed strain of 300% or a gradually increased strain between 50 and 800% was used in the hysteresis experiments. The specimens were cyclically loaded to the targeted strain and unloaded to stress zero with the strain rate of 125%/min. The initial clamp
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distance was 80 mm, with the width of 6 mm, the force transducer was 2.5 KN. The strain recovery in the hysteresis test was calculated as: Strain Recovery = (1- ε plastic / ε applied )×100%, where ε applied
each cycle.
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is the applied strain, ε plastic is the residual plastic strain, after the stress was unloaded to zero in
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Results and discussion Random copolymers
As shown in Table 1, LRC1 and LRC2 possessed almost the same average 1-octene incorporation, the same molecular weight distribution polydispersity index, the same melting temperature and crystallinity. However, the weight-average molecular weight of LRC2 (608.5 kg/mol) is almost doubled than LRC1 (317.9 kg/mol). The stress-train curves of these random copolymers are shown in Figure 1 and the tensile properties are listed in Table 1. All these copolymers possessed very good toughness, as the samples were not broken at the strain of 1200% with the strain speed of
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LRC1 LRC2 MRC
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stress (MPa)
20
15
10
0 200
400
600 800 strain (%)
1000
1200
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0
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5
Figure 1. Engineering stress-strain curves of the ethylene/1-octene random copolymers.
To study elasticity of the elastomer samples, the hysteresis experiments were first performed under the fixed strain of 300%. Figure 2 shows their hysteresis curves with ten recycles. All the
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samples suffered prominent deformation after the first load cycle (with about 30% residual strain). The solid state morphology structure obviously changed in the first cycle. However, no further change was observed in the subsequent load cycles, as the hysteresis loop after the second cycle
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showed little difference. Figure 3 compares the evolution of the strain recovery with cycle times. The strain recovery of LRC1 and LRC2 were stable over 85%, almost constant as the load cycle
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times increased. While the strain recovery of MRC showed a slight decrease trend with the load cycle times. The order of the strain recovery of these random copolymers followed: MRC < LRC1 LRC2.
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Table 1. Characteristics of the ethylene/1-octene random copolymers
Tcd
Tmd
△Hmd
XC,DSCe
5% secant
ultimate tensile
ultimate
( )
( )
( )
(J/g)
(wt%)
modulus (MPa)
stress (MPa)
elongation (%)
1.14
-61.2
8.3
37.4
25.4
9
2.8 ± 0.2
> 15
> 1200
60.85
1.15
-58.0
14.3
36.9
26.5
9
6.6 ± 0.3
> 22
> 1200
12.13
2.22
-55.2
35.2
50.9
40.0
14
11.3 ± 0.3
> 7.9
> 1200
(mol%)
(104g/mol)
LRC1
14.7
31.79
LRC2
14.5
MRCf
13.3
PDI
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Noa
c
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Mwc
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Tgd
F2 b
Sample
a: LRC: living random copolymer (random copolymer made from living coordination polymerization), MRC: metallocene-catalyzed random copolymer; b: average 1-octene molar incorporation in the copolymers, determined by the high temperature
13
C NMR; c: determined by high temperature GPC; d: determined by DSC; e:
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Crystallinity from heat of melting, XC,DSC = ∆Hm/∆Hm0, ∆Hm0 = 293 J/g; f: Engage 8150, from DOW chemical.
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3
LRC2
LRC1
Stress (MPa)
Stress (MPa)
1st cycle 1.0
0.5
1st cycle
2
Subsequent cycles 1
0.0 0
50
100
150 Strain (%)
200
250
0
300
0
50
(a)
3
1
150 Strain (%)
200
250
300
(b)
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1st cycle
2
100
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Stress (MPa)
MRC
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Subsequent cycles
Subsequent cycles
0 0
50
100
150
200
250
Strain (%)
300
(c)
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Figure 2. Hysteresis curves of the ethylene/1-octene random copolymers at the applied strain of 300%, (a) LRC1, (b) LRC2, (c) MRC.
EP
100
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Strain Recovery (%)
80
LRC1 LRC2 MRC
60
40
20
0 0
2
4 6 Cycle Number
8
10
Figure 3. Evolution of the strain recovery of the ethylene/1-octene random copolymers with cycle times at the applied strain of 300%.
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ACCEPTED MANUSCRIPT The hysteresis experiment was then performed under the gradually increased strains between 50 and 800%. The results are shown in Figure 4. All samples gave a typical elastomeric performance. The hysteresis loop enlarged gradually as the applied strain increased. For LRC1 and LRC2, the residual permanent strain (unrecovered plastic deformation) after each cycle remained less than 100%. Figure 5 compares the strain recoveries. The elasticity of LRC1 was similar to LRC2, both
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giving stable strain recovery about 90% with little influence by the applied strain less than 800%. For the commercial sample MRC, the strain recovery of MRC was stable at about 90%, similar to LRC1 and LRC2, only when the strain was less than 200%. It decreased significantly as the applied
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strain increased to higher than 200%. In our previous work, it was demonstrated that the living random copolymer possessed narrower intra-chain composition distribution than the commercially
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metallocene-catalyzed random copolymer [19]. The elasticity difference between LRC and MRC found in this work could thus be attributed to that MRC had a broader intra-chain composition distribution than LRC’s, resulting in a broader distribution of crystal thickness in the MRC sample [19]. The larger crystals in MRC were more vulnerable to break and deform at high strains, leading to significant decrease in the strain recovery with increased applied strain. The hysteresis results
LRC2.
MRC < LRC1
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between 50 and 800% also showed that the elasticity of the random copolymers was in an order of:
Figure 3 and Figure 5 demonstrated that the living ethylene/1-octene random copolymers had
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better elasticity than the metallocene-catalyzed random copolymer counterpart due to a narrower intra-chain composition distribution. The negligible difference of elasticity between LRC1 and
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LRC2 demonstrated that the elasticity was little influenced by their molecular weights. 2.5
LRC1
1.5
4.5 Stress (MPa)
Stress (MPa)
2.0
LRC2
6.0
1.0
3.0
1.5
0.5
0.0
0.0
0
2
4 Strain
6
8
0
(a)
2
4 Strain
6
8
(b)
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Stress (MPa)
4
3
2
0 0
2
4 Strain
6
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1
8
(c)
Figure 4. Hysteresis curves of the ethylene/1-octene random copolymers with the applied strain between
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50~800%, (a) LRC1, (b) LRC2, (c) MRC.
80
60
40
20
0
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Strain Recovery (%)
100
2
4 Strain
6
8
EP
0
LRC1 LRC2 MRC
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Figure 5. Evolution of the strain recovery with applied strain for the ethylene/1-octene random copolymers.
Uniaxial tensile properties of block copolymers The living block copolymers used in this work are schematically illustrated in Scheme 1. The structure data was listed in Table 2. The detailed synthesis and structure analysis of these living block copolymers could be found in our previous work [29]. The soft block in all the block copolymers had about 19 mol% 1-octene while the hard block contained only 5 mol% 1-octene. LBC1 and LBC2 possessed similar hard block length, LBC1 and LBC3 had similar soft block length, LBC4 and LBC5 possessed ultrahigh molecular weight and similar hard block content, but LBC4 was diblock and LBC5 was triblock.
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Table 2. Characteristics of the ethylene/1-octene block copolymers. hard block
F2
b
Mw c
Tm d
Tg d
Tc d
△H d Xc e
( )
( )
( )
(J/g) (%) modulus (MPa) stress (MPa) strain (%) fracture (%)
PDI
5% secant
fracture
fracture recovery after
content (wt%)
LBC1
42%
12.3
42.45
1.16
78.7, 24.3
-61.6
60.6, 2.9 47.0
16
7.8 ± 0.3
18.5 ± 2.0
670 ± 20
91
LBC2
23%
11.6
57.04
1.58
85.8, 22.4
-61.3
67.3, 7.6 43.0
15
3.5 ± 0.2
13.2 ± 2.0
647 ± 20
94
LBC3
63%
9.5
65.85
1.48
88.3
-60.4
74.0
LBC4
74%
6.3
115.45
1.30
98.9
-59.6
81.2
LBC5
78%
8.0
111.01
1.63
83.9
-53.5
67.1
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No. a
58.6
20
33.0 ± 4.0
21.9 ± 1.4
760 ± 30
67
58.9
20
73.8 ± 3.0
18.9 ± 2.2 1076 ± 56
59
45.2
15
35.2 ± 3.6
21.5 ± 2.5
77
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(mol%) (104g/mol)
c
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Sample
917 ± 36
a: LBC: living block copolymer, structure data of these block copolymers was cited from Ref. [29]; b: average 1-octene molar incorporation in the copolymer,
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∆Hm0 = 293 J/g.
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determined by high temperature 13C NMR, c: determined by high temperature GPC; d: determined by DSC; e: crystallinity from heat of melting, XC,DSC = ∆Hm/∆Hm0,
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Scheme 1. Schematic structures of the living block copolymers.[29]
Figure 6 shows the stress-strain curves of the block copolymers. All the samples gave uniform
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deformation, without typical yielding and necking during the stretch. Although the yielding phenomenon of LBC4 sample was more obvious than the others, its deformation was still uniform. All these samples exhibited good toughness. The stress upswings were strong, due to their high molecular weight. Table 2 summarizes the secant modulus at 5% strain, the ultimate stress and
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strain, and the strain recovery data after fracture. For the diblock copolymers (LBC1~4), the hard block content was in an order of: LBC2 < LBC1 < LBC3 < LBC4. The order of secant modulus at 5%
LBC3 LBC5 LBC1
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EP
strain followed the same trend.
stress (MPa)
20
LBC4
LBC2
15
10
5
0 0
200
400
600 800 strain (%)
1000
1200
Figure 6. Engineering stress-strain curves of the ethylene/1-octene block copolymers.
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ACCEPTED MANUSCRIPT It was demonstrated by Hiltner et al. [10] and Wang et al. [31] that the slip-link theory was effective in describing the entire stress-strain curve of ethylene/1-octene block copolymer elastomers. According to the slip-link theory, the uniaxial stress-strain behavior of ethylene/1-octene copolymer elastomer is contributed from two parts: the slipping links (σs) and rigid cross-links (σc). The slip-links mainly depend on crystallinity but are not sensitive to
describe the slip-link model: σ = σs + σc మ మ ቂଵିమ ቀమ ା ቁቃ ಓ
ቀଵାηమ + ାηቁ మ
ଶ
ۗ ۖ
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భ
ሺ1+ηሻ൫ଵିమ ൯మ ቀି మ ቁ ಓ
ηቀି మ ቁ ቀି మ ቁమ ۘ ۔൫ଵିమ ൯ሺଵାηሻ ଵ ಓ ಓ ቂ − ቃ + − + ۖ ۖ మ మ ሻሺାηሻ మ ቀమ ାమቁ ሺଵାηమ ሻమ మ ቀమ ାమቁ ሺଵାη ሺାηሻ ଵି ଵି ە ಓ ಓ ۙ
σc =Nc kT ቀλ − మ ቁ ቊ ଵ
൫ଵିమ ൯
మ ಓ
ቂଵିమ ቀమ ା ቁቃ
మ
−
భ
భ
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σs =Ns kT
ۓ ۖ
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molecular weight, while the cross-links are determined by the molecular weight [7]. Equations 1~3
మ
మ ಓ
ଵିమ ቀమ ା ቁ
ቋ
(1)
(2)
(3)
where Ns is the density of sliplinks, Nc is the density of crosslinked chains, k is Boltzman constant, T is temperature in K, and λ is the draw ratio, η is the slippage parameter, and α is the
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inextensibility parameter. Figure 7 shows the correlation results of the slip-link model with the experimental stress-strain data. Table 3 lists the fitted model parameters with η = 1.1. Except for
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LBC4, the model fit the data well.
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LBC3
LBC4
LBC1
20
stress (MPa)
LBC5
LBC2
15
10
5
0 2
4
6
8
extension ratio
10
12
(a)
13
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(c)
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(b)
Figure 7. Slip-link fitting of the engineering stress-strain curves of ethylene/1-octene block copolymers; (a) Model correlation (lines) with the experimental data (points); (b) Simulated sliplink contribution; (c)
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Simulated crosslink contribution.
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Table 3. Slip-link model parameters for the block copolymers.
Sample
NskT (MPa)
NckT (MPa)
α
LBC1
10.9
0.02
0.102
LBC2
16.0
0.02
0.100
LBC3
17.2
0.08
0.090
LBC4
14.4
0.56
0.051
LBC5
9.2
0.27
0.076
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ACCEPTED MANUSCRIPT A careful examination of the relative contribution from slip-link in Figure 7b revealed some discrepancy in the slip-link theory explanation. The crystallinity of the block copolymers did not differ much from each other (see Table 2). However, the slip-link contributions in LBC1, LBC2 & LBC3 were much higher than those in LBC4 & LBC5 (Figure 7b). This might be induced by the ultrahigh molecular weight of LBC4 & LBC5. The ultrahigh molecular weight with extremely
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strong chain entanglement in LBC4 & LBC5 restricted crystallization process and inhibited lateral attachment and detachment of the crystallizable chain segments, giving less slip-link contributions, as shown in Figure 7b. The cross-link contribution in Figure 7c increased with the molecular weight,
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which is consistent with the slip-link theory.
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Elatomeric behaviors of block copolymers
From the tensile test results given in Table 2, LBC1 & LBC2 possessed strain recovery higher than 90% after fracture. Both samples had the hard block content lower than 50%. The strain recovery decreased substantially when the hard block content was higher than 50%. To further study their elastomeric behavior, the hysteresis experiments were performed. Figure 8 shows the
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hysteresis curves at various strains from 50 to 800%. Only LBC1~3 results are shown in the figure. LBC4 &LBC5 are very similar to LBC3. It was found that, with an increase of the applied strain, the permanent strain of LBC1 increased slightly, exhibiting a typical hysteresis behavior of
EP
elastomers. In comparison, the permanent strain of LBC3 increased significantly as the applied
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strain increased, exhibiting a typical behavior of plastics. LBC2 fell in between LBC1 and LBC3.
3.0
LBC2
2.0 Stress (MPa)
2.5 stress (MPa)
2.5
LBC1
2.0 1.5 1.0
1.5
1.0
0.5
0.5
0.0 0.0
0.0 0.0
1.5
3.0 Strain
4.5
6.0
(a)
1.5
3.0 Strain
4.5
6.0
(b)
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LBC3
Stress (MPa)
2.8
2.1
1.4
0.0 0.0
1.5
3.0
4.5
6.0
Strain
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0.7
7.5
(c)
SC
Figure 8. Hysteresis curves of the ethylene/1-octene block copolymers: (a) LBC1, (b) LBC2, (c) LBC3.
Figure 9 shows the strain recovery estimated from the hysteresis curves. With the hard block
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content <50% (LBC1 & LBC2), the strain recovery decreased gradually with the applied strain in a similar trend. When the hard block content was over 60% (LBC3, LBC4 & LBC5), the strain recovery decreased much more quickly, particularly when the applied strain was below 400%. LBC1 gave the best elastomeric behavior among the studied block copolymers, of which the strain recovery retained about 80% under the applied strain of 600%. Their order of elastomeric recovery
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was: LBC4 < LBC3 < LBC5 < LBC2 < LBC1, confirming that there existed a range of the hard block content for the optimal elasticity, that is, neither too high nor too low hard block content would benefit elastomeric behavior of the ethylene/1-octene block copolymers. In this work, LBC1
EP
with about 42 wt% hard block content exhibited the best elasticity.
100
Strain Recovery (%)
AC C
LBC1 LBC2 LBC3 LBC4 LBC5
80
60
40
20
0 0
2
4 Strain
6
8
Figure 9. Evolution of the strain recovery with applied strain for the ethylene/1-octene block copolymers.
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ACCEPTED MANUSCRIPT In Figure 9, the strain recoveries of LBC3 and LBC5 under the low applied strains (≤ 150%) were comparable with LBC2, but the former dropped rapidly as the applied strain increased. Too high hard block content led to crystals vulnerable to plastic deformation at the large strains. It was evident, in comparing LBC4 and LBC5, that with similar molecular weights, similar hard-to-soft block ratios, similar hard and soft block segment compositions, the hard-soft-hard triblock structure
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gave better elasticity than the hard-soft diblock structure.
Conclusion
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The elastomeric performance of a series of ethylene/1-octene random and block copolymers were studied in this work. All the copolymers exhibited very good toughness. The living random were
found
to
have
better
elasticity
than
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copolymers
a
commercially
available
metallocene-catalyzed random copolymer counterpart due to their narrower intra-chain composition distributions. The strain recovery of the living random copolymers was stable at about 90% even at the applied strain of 800%. The hysteresis experiments demonstrated that elasticity of the ethylene/1-octene copolymer elastomers was mainly determined by their block structure and was
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little influenced by their molecular weight. There existed a range of the hard-block content for an optimal elastomeric behavior. Either too high or too low hard-block content deteriorated the elasticity. The diblock copolymer having about 42 wt% hard block content (LBC1) exhibited the
EP
best elasticity among the studied block copolymers. It retained the strain recovery about 80% under the applied stain of 600%. For the block copolymers with more than 60 wt% hard block content, the
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strain recovery decreased quickly as the applied strain increased.
Acknowledgement
We would like to thank the financial supports from the National Basic Research Program of China (2011CB606001) and the National Natural Science Foundation of China (U1462115). We would also like to thank Dr. Hanguang Wu and Prof. Ming Tian of Beijing University of Chemical Technology for their kind assistance in this work.
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References and Notes 1.
Drobny JG. Handbook of Thermoplastic Elastomers. Norwich: William Andrew Publishing, 2007, [chapters 7 and 15].
2.
Kresge EN, In Thermoplastic Elastomers, 3rd ed.; Holden G, Kricheldorf HR, Quirk RP, Eds.;
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Hanser Publishers: Munich, 2004; Chapter 5, p93. Wang HP, Chum S, Hiltner A, Baer E. J Polym Sci Part B: Polym Phys, 2009; 47: 1313-1330.
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1996; 34: 1301-1315.
Bensason S, Stepanov EV, Chum S, Hiltner A, Baer E. Macromolecules 1997; 30: 2436-2444.
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Arriola DJ, Carnahan EM, Hustad PD, Kuhlman RL, Wenzel TT. Science 2006; 312: 714-719.
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Arriola DJ, Carnahan EM, Cheung YW, Devore DV, Graf DD, Hustad PD, WO Int. Pat.
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7.
090427, 2005.
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10. Wang HP, Chum SP, Hiltner A, Baer E. J Appl Polym Sci 2009; 113: 3236-3244. 11. Wang HP, Khariwala DU, Cheung W, Chum S, Hiltner A, Baer E. Macromolecules 2007; 40: 2852-2862.
12. Khariwala DU, Taha A, Chum S, Hiltner A, Baer E. Polymer 2008; 49: 1365-1375.
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13. Tong ZZ, Huang J, Zhou B, Xu J, Fan Z. Macromol Chem Phy 2013; 214: 605-616. 14. He P, Shen W, Yu W, Zhou C. Macromolecules 2014; 47: 807-820.
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15. Jin J, Du J, Xia Q, Liang Y, Han C. Macromolecules 2010; 43: 10554-10559. 16. Li S, Register RA, Weinhold JD, Landes BG. Macromolecules 2012; 45: 5773-5781. 17. Park HE, Dealy JM, Marchand GR, Wang J, Li S, Register RA. Macromolecules 2010; 43: 6789-6799.
18. Li S, Register RA, Landes BG, Hustad PD, Weinhold JD. Macromolecules 2010; 43: 4761-4770. 19. Liu W, Wang WJ, Fan H, Yu L, Li BG, Zhu S. Eur Polym J 2014; 54: 160-171. 20. Domski GJ, Rose JM, Coates GW, Bolig AD, Brookhart M. Prog Polym Sci 2007; 32: 30-92. 21. Makio H, Terao H, Iwashita A, Fujita T. Chem Rev 2011; 111: 2363-2449.
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ACCEPTED MANUSCRIPT 22. Ye Z, Xu L, Dong Z, Xiang P. Chem Commun 2013; 49: 6235-6255. 23. Mitani M, Mohri J, Yoshida Y, Saito J, Ishii S, Tsuru K, Matsui S, Furuyama R, Nakano T, Tanaka H, Kojoh S, Matsugi T, Kashiwa N, Fujita T. J Am Chem Soc 2002; 124: 3327-3336. 24. Furuyama R, Mitani M, Mohri J, Mori R, Tanaka H, Fujita T. Macromolecules 2005; 38: 1546-1552.
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25. Edson JB, Wang Z, Kramer EJ, Coates GW. J Am Chem Soc 2008; 130: 4968-4977.
26. Hotta A, Cochran E, Ruokolainen J, Khanna V, Fredrickson GH, Kramer EJ, Shin YW, Shimizu F, Cherian AE, Hustad PD, Rose JM, Coates GW. PANS 2006; 103: 15327-15332.
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27. Liu W, Zhang K, Fan H, Wang WJ, Li BG, Zhu S. J Polym Sci Part A: Polym Chem 2013; 51: 405-414. W, Guo S, Fan
H, Wang WJ, Li BG, Zhu S. AIChE J 2013; 59: 4686-4695.
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28. Liu
29. Liu W, Andrew TO, Wang WJ, Fan H, Li BG, Zhu S. Polym Chem 2015; 6: 3800-3806. 30. Mitani M, Furuyama R, Mohri JI, Saito J, Ishii S, Terao H, Nakano T, Tanaka H, Fujita T. J Am Chem Soc 2003; 125: 4293-4305.
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31. Kuang X, Liu G, Wen T, Zhang X, Dong X, Wang D, Acta Polymerica Sinica 2013; 5: 679-687.
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Figure Captions Figure 1. Engineering stress-strain curves of the ethylene/1-octene random copolymers. Figure 2. Hysteresis curves of the ethylene/1-octene random copolymers at the applied strain of
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300%, (a) LRC1, (b) LRC2, (c) MRC. Figure 3. Evolution of strain recovery of the ethylene/1-octene random copolymers with cycle times at the applied strain of 300%.
between 50~800%, (a) LRC1, (b) LRC2, (c) MRC.
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Figure 4. Hysteresis curves of the ethylene/1-octene random copolymers with the applied strain
Figure 5. Evolution of the strain recovery with applied strain for the ethylene/1-octene random
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copolymers.
Scheme 1. Schematic structures of the living block copolymers.
Figure 6. Engineering stress-strain curves of the ethylene/1-octene block copolymers. Figure 7. Slip-link fitting of the engineering stress-strain curves of ethylene/1-octene block copolymers; (a) Model correlation (lines) with the experimental data (points); (b)
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Simulated sliplink contribution; (c) Simulated crosslink contribution. Figure 8. Hysteresis curves of the ethylene/1-octene block copolymers: (a) LBC1, (b) LBC2, (c) LBC3.
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copolymers.
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Figure 9. Evolution of the strain recovery with applied strain for the ethylene/1-octene block
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Highlights
LRC had better elasticity than MRC.
The strain recovery of LRC kept 90% with an applied strain up to 800%.
LBC1 retained the strain recovery about 80% under the applied stain of 600%.
The elasticity of block copolymers was mainly determined by their block
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structure.
ACCEPTED MANUSCRIPT Supporting Information
Elastomeric Properties of Ethylene/1-Octene Random and Block Copolymers Synthesized from Living Coordination Polymerization Weifeng Liu,1 Xiao Zhang,1 Zhiyang Bu,1 Wen-Jun Wang,1 Hong Fan,1 Bo-Geng Li1* and Shiping
1
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Zhu2*
State Key Laboratory of Chemical Engineering, College of Chemical & Biological Engineering, Zhejiang University, Hangzhou P.R. China 310027;
Department of Chemical Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4L7
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2
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* Corresponding authors, E-mail: [email protected] & [email protected]
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Endotherm
S1. DSC curves of ethylene/1-octene random copolymers
0
50 Temperature (°C)
LRC1
MRC
100
150
(a)
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-50
LRC2
Exotherm
LRC2
LRC1 MRC
-50
0 50 Temperature (°C)
100
150
(b)
ACCEPTED MANUSCRIPT Figure S1. DSC melting (a) and cooling (b) curves of ethylene/1-octene random copolymers studied in this work.
S2. DSC curves of ethylene/1-octene block copolymers
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LBC5
Endotherm
LBC4
LBC3
LBC2
50 Temperature (°C)
100
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0
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Exotherm
-50
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LBC1
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-50
0 50 Temperature (°C)
150
(a)
LBC5 LBC4 LBC3 LBC2 LBC1
100
150
(b)
Figure S2. DSC melting (a) and cooling (b) curves of ethylene/1-octene block copolymers studied in this work.
S0032-3861(15)30097-5
DOI:
10.1016/j.polymer.2015.07.019
Reference:
JPOL 17979
To appear in:
Polymer
Received Date: 3 May 2015 Revised Date:
16 June 2015
Accepted Date: 11 July 2015
Please cite this article as: Liu W, Zhang X, Bu Z, Wang W-J, Fan H, Li B-G, Zhu S, Elastomeric Properties of Ethylene/1-Octene Random and Block Copolymers Synthesized from Living Coordination Polymerization, Polymer (2015), doi: 10.1016/j.polymer.2015.07.019. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT Elastomeric Properties of Ethylene/1-Octene Random and Block Copolymers
Synthesized from Living Coordination Polymerization Weifeng Liu,1 Xiao Zhang,1 Zhiyang Bu,1 Wen-Jun Wang,1 Hong Fan,1 Bo-Geng Li1* and Shiping Zhu2* State Key Laboratory of Chemical Engineering, College of Chemical & Biological Engineering,
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1
Zhejiang University, Hangzhou P.R. China 310027; 2
Department of Chemical Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4L7
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* Corresponding authors, E-mail: [email protected] & [email protected]
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Abstract: The elastomeric performance of a series of ethylene/1-octene random and block copolymers prepared from living coordination polymerization was studied in this work. All the polymer samples exhibited good toughness. The living random copolymers had better elasticity than commercial metallocene random copolymers. The strain recovery of the living random copolymers was about 90% with an applied strain up to 800%. It was demonstrated that the
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copolymer elasticity was mainly determined by their block structure, with little influence by their molecular weight. It was also found that either too high or too low content of hard-block content deteriorated the copolymer elastomeric behavior. The diblock copolymer having about 42 wt% hard
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block content (LBC1) gave the best elasticity among the studied block copolymers and retained the
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strain recovery about 80% under the applied stain of 600%.
Key Words: living coordination polymerization, olefin block copolymer, thermoplastic elastomer
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Introduction Ethylene/α-olefin copolymer with high comonomer incorporation is one of the most widely-used high performance thermoplastic elastomers (TPEs) [1]. Compared with other TPE types, polyolefin-based thermoplastic elastomers have received considerable attention due to their better
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chemical resistance, lower density, excellent weatherability and lower cost for elasticity formation [2]. For good elastomeric property of low modulus and high recovery from large deformation, crystallinity must be lower than 20 wt% [3], which is usually realized by incorporating α-olefins, such as propylene, 1-butene, 1-hexene and 1-octene into polyethylene main chain to disrupt
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crystallization of the methylene sequences. Thermal and mechanical properties of the ethylene/α-olefin random copolymer elastomers (POE) have been well studied since their
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commercialization by DOW and ExxonMobil [4~7]. Most of these commercially available POE products are made from metallocene catalysts and were usually compared with those copolymers from Z-N catalysts. It has been demonstrated that metallocene-catalyzed ethylene/α-olefin copolymers have narrower molecular weight and composition distributions than Z-N’s, and thus
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have better mechanical properties [4]. However, little work reported on the comparison of mechanical properties between the metallocene-catalyzed copolymers and those from living coordination copolymerization with post-metallocene catalysts. In 2006, a novel type of ethylene/1-octene olefin block copolymer (OBC) was commercialized
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by DOW through a chain shuttling polymerization (CSP) mechanism [8,9]. In CSP, two post-metallocene catalysts are employed, with one catalyst good at ethylene/α-olefin
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copolymerization that generates soft amorphous copolymer segments and the other catalyst poor in the copolymerization that produces hard semi-crystalline segments. The hard and soft segments are shuttled between the two metallic active centers through an appropriate chain shuttling agent. The multiblock copolymers (hard-soft-hard…) are thus formed. It was found that OBC exhibited better elasticity at high temperature than POE [10]. Although great attention has been paid to OBC mechanical properties and melt phase behaviors [11~17], their structure/property relationships are still lacking. In OBC, the soft and hard segments are statistical numbers and lengths. The comonomer incorporation in the hard and soft segments is not clear. There is usually certain amount of random copolymer impurities in the commercial OBC products [18,19]. In researching OBC
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ACCEPTED MANUSCRIPT performance, it is difficult to focus on a single factor in establishing a clear relationship between chain microstructure and property. In contrast, living olefin copolymerization can provide well defined and controlled polyolefin chain microstructures. Polyolefins with an explicit chain microstructure can be obtained from living coordination polymerization. In the recent years, numerous ethylene/α-olefin block copolymers were synthesized through
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living olefin polymerization [20~22]. For example, PE-b-PEP-b-sPP and PE-b-PEP-b-PE triblock copolymers were first synthesized by Fujita group using a fluorinated FI-Ti catalyst, which is a well-known
living
coordination
polymerization
catalyst
system
[23].
Coates
group
reported
the
synthesis
pentablock
iPP-b-PEP-b-iPP
triblock
copolymers,
copolymers, and
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iPP-b-PEP-b-iPP-b-PEP-b-iPP
of
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PE-b-Poly(ethylene-co-1-hexene) diblock copolymers were also synthesized by Fujita group [24].
iPP-b-PEP-b-iPP-b-PEP-b-iPP-b-PEP-b-iPP heptablock copolymers using another fluorinated FI-Ti catalyst [25]. The advent of living olefin polymerization technique allows precise design of the polyolefin chain microstructure, so as to tailor-make polyolefin materials [22]. Some authors noted that ethylene/α-olefin block copolymers synthesized from living polymerization technique exhibited
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good elastomeric behavior and excellent toughness [26]. However, detailed mechanical studies on the living block copolymers were very scattered.
In our previous work, we synthesized a series of ethylene/1-octene random and block copolymers
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by living coordination polymerization [27~29] and demonstrated that the copolymer samples possessed narrower intra-chain composition distributions than those prepared from conventional
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metallocene catalysts [19]. The objective of this work is to investigate their elastomeric behaviors.
Materials and methods:
The ethylene/1-octene copolymer samples used in this work were synthesized from bis[N-(3-methylsalicylidene)-2,3,4,5,6-pentafluoroanilinato] titanium(IV) dichloride/dMAO [30]. The synthesis procedures for copolymer samples and the characterization of molecular weight, composition and DSC properties have been described in our previous papers [27~29]. The characteristics of the random copolymer samples are listed in Table 1 and those of the block copolymers are in Table 2. LRC1 and LRC2 in Table 1 are living random copolymers made from
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ACCEPTED MANUSCRIPT the living coordination polymerization. MRC is a commercial ethylene/1-octene random copolymer sample (Engage 8150) produced from CGC catalyst by DOW Chemical Company. LBC in Table 2 is living block copolymers from the living coordination polymerization. The samples that submitted to mechanical tests were solution cast into uniform films with thickness between 70 and 100 µm. Xylene was used as the solvent. The film thickness was
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measured by a digital electron thickness gauge. Dumbbell-shaped tensile specimens for uniaxial elongation tests and hysteresis experiments were cut from the solution cast films. The tensile stress-strain behavior was performed on the universal material tester Zwick/Roell Z020 at room
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temperature with the cross-head speed of 800%/min. The test method is GB1040.2-2006. Each test was repeated at least four times. The initial length of the specimens between the clamps was 50 mm,
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having width of 4 mm. For the elastic strain recovery test, the hysteresis experiments were performed on the MTS tensile instrument (MTS Systems China Co. Ltd) at room temperature. The test method is GB1040.3-2006. The fixed strain of 300% or a gradually increased strain between 50 and 800% was used in the hysteresis experiments. The specimens were cyclically loaded to the targeted strain and unloaded to stress zero with the strain rate of 125%/min. The initial clamp
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distance was 80 mm, with the width of 6 mm, the force transducer was 2.5 KN. The strain recovery in the hysteresis test was calculated as: Strain Recovery = (1- ε plastic / ε applied )×100%, where ε applied
each cycle.
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is the applied strain, ε plastic is the residual plastic strain, after the stress was unloaded to zero in
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Results and discussion Random copolymers
As shown in Table 1, LRC1 and LRC2 possessed almost the same average 1-octene incorporation, the same molecular weight distribution polydispersity index, the same melting temperature and crystallinity. However, the weight-average molecular weight of LRC2 (608.5 kg/mol) is almost doubled than LRC1 (317.9 kg/mol). The stress-train curves of these random copolymers are shown in Figure 1 and the tensile properties are listed in Table 1. All these copolymers possessed very good toughness, as the samples were not broken at the strain of 1200% with the strain speed of
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25
LRC1 LRC2 MRC
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stress (MPa)
20
15
10
0 200
400
600 800 strain (%)
1000
1200
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0
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5
Figure 1. Engineering stress-strain curves of the ethylene/1-octene random copolymers.
To study elasticity of the elastomer samples, the hysteresis experiments were first performed under the fixed strain of 300%. Figure 2 shows their hysteresis curves with ten recycles. All the
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samples suffered prominent deformation after the first load cycle (with about 30% residual strain). The solid state morphology structure obviously changed in the first cycle. However, no further change was observed in the subsequent load cycles, as the hysteresis loop after the second cycle
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showed little difference. Figure 3 compares the evolution of the strain recovery with cycle times. The strain recovery of LRC1 and LRC2 were stable over 85%, almost constant as the load cycle
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times increased. While the strain recovery of MRC showed a slight decrease trend with the load cycle times. The order of the strain recovery of these random copolymers followed: MRC < LRC1 LRC2.
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Table 1. Characteristics of the ethylene/1-octene random copolymers
Tcd
Tmd
△Hmd
XC,DSCe
5% secant
ultimate tensile
ultimate
( )
( )
( )
(J/g)
(wt%)
modulus (MPa)
stress (MPa)
elongation (%)
1.14
-61.2
8.3
37.4
25.4
9
2.8 ± 0.2
> 15
> 1200
60.85
1.15
-58.0
14.3
36.9
26.5
9
6.6 ± 0.3
> 22
> 1200
12.13
2.22
-55.2
35.2
50.9
40.0
14
11.3 ± 0.3
> 7.9
> 1200
(mol%)
(104g/mol)
LRC1
14.7
31.79
LRC2
14.5
MRCf
13.3
PDI
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Noa
c
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Mwc
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Tgd
F2 b
Sample
a: LRC: living random copolymer (random copolymer made from living coordination polymerization), MRC: metallocene-catalyzed random copolymer; b: average 1-octene molar incorporation in the copolymers, determined by the high temperature
13
C NMR; c: determined by high temperature GPC; d: determined by DSC; e:
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Crystallinity from heat of melting, XC,DSC = ∆Hm/∆Hm0, ∆Hm0 = 293 J/g; f: Engage 8150, from DOW chemical.
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3
LRC2
LRC1
Stress (MPa)
Stress (MPa)
1st cycle 1.0
0.5
1st cycle
2
Subsequent cycles 1
0.0 0
50
100
150 Strain (%)
200
250
0
300
0
50
(a)
3
1
150 Strain (%)
200
250
300
(b)
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1st cycle
2
100
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Stress (MPa)
MRC
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Subsequent cycles
Subsequent cycles
0 0
50
100
150
200
250
Strain (%)
300
(c)
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Figure 2. Hysteresis curves of the ethylene/1-octene random copolymers at the applied strain of 300%, (a) LRC1, (b) LRC2, (c) MRC.
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100
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Strain Recovery (%)
80
LRC1 LRC2 MRC
60
40
20
0 0
2
4 6 Cycle Number
8
10
Figure 3. Evolution of the strain recovery of the ethylene/1-octene random copolymers with cycle times at the applied strain of 300%.
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ACCEPTED MANUSCRIPT The hysteresis experiment was then performed under the gradually increased strains between 50 and 800%. The results are shown in Figure 4. All samples gave a typical elastomeric performance. The hysteresis loop enlarged gradually as the applied strain increased. For LRC1 and LRC2, the residual permanent strain (unrecovered plastic deformation) after each cycle remained less than 100%. Figure 5 compares the strain recoveries. The elasticity of LRC1 was similar to LRC2, both
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giving stable strain recovery about 90% with little influence by the applied strain less than 800%. For the commercial sample MRC, the strain recovery of MRC was stable at about 90%, similar to LRC1 and LRC2, only when the strain was less than 200%. It decreased significantly as the applied
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strain increased to higher than 200%. In our previous work, it was demonstrated that the living random copolymer possessed narrower intra-chain composition distribution than the commercially
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metallocene-catalyzed random copolymer [19]. The elasticity difference between LRC and MRC found in this work could thus be attributed to that MRC had a broader intra-chain composition distribution than LRC’s, resulting in a broader distribution of crystal thickness in the MRC sample [19]. The larger crystals in MRC were more vulnerable to break and deform at high strains, leading to significant decrease in the strain recovery with increased applied strain. The hysteresis results
LRC2.
MRC < LRC1
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between 50 and 800% also showed that the elasticity of the random copolymers was in an order of:
Figure 3 and Figure 5 demonstrated that the living ethylene/1-octene random copolymers had
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better elasticity than the metallocene-catalyzed random copolymer counterpart due to a narrower intra-chain composition distribution. The negligible difference of elasticity between LRC1 and
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LRC2 demonstrated that the elasticity was little influenced by their molecular weights. 2.5
LRC1
1.5
4.5 Stress (MPa)
Stress (MPa)
2.0
LRC2
6.0
1.0
3.0
1.5
0.5
0.0
0.0
0
2
4 Strain
6
8
0
(a)
2
4 Strain
6
8
(b)
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Stress (MPa)
4
3
2
0 0
2
4 Strain
6
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1
8
(c)
Figure 4. Hysteresis curves of the ethylene/1-octene random copolymers with the applied strain between
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50~800%, (a) LRC1, (b) LRC2, (c) MRC.
80
60
40
20
0
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Strain Recovery (%)
100
2
4 Strain
6
8
EP
0
LRC1 LRC2 MRC
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Figure 5. Evolution of the strain recovery with applied strain for the ethylene/1-octene random copolymers.
Uniaxial tensile properties of block copolymers The living block copolymers used in this work are schematically illustrated in Scheme 1. The structure data was listed in Table 2. The detailed synthesis and structure analysis of these living block copolymers could be found in our previous work [29]. The soft block in all the block copolymers had about 19 mol% 1-octene while the hard block contained only 5 mol% 1-octene. LBC1 and LBC2 possessed similar hard block length, LBC1 and LBC3 had similar soft block length, LBC4 and LBC5 possessed ultrahigh molecular weight and similar hard block content, but LBC4 was diblock and LBC5 was triblock.
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Table 2. Characteristics of the ethylene/1-octene block copolymers. hard block
F2
b
Mw c
Tm d
Tg d
Tc d
△H d Xc e
( )
( )
( )
(J/g) (%) modulus (MPa) stress (MPa) strain (%) fracture (%)
PDI
5% secant
fracture
fracture recovery after
content (wt%)
LBC1
42%
12.3
42.45
1.16
78.7, 24.3
-61.6
60.6, 2.9 47.0
16
7.8 ± 0.3
18.5 ± 2.0
670 ± 20
91
LBC2
23%
11.6
57.04
1.58
85.8, 22.4
-61.3
67.3, 7.6 43.0
15
3.5 ± 0.2
13.2 ± 2.0
647 ± 20
94
LBC3
63%
9.5
65.85
1.48
88.3
-60.4
74.0
LBC4
74%
6.3
115.45
1.30
98.9
-59.6
81.2
LBC5
78%
8.0
111.01
1.63
83.9
-53.5
67.1
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No. a
58.6
20
33.0 ± 4.0
21.9 ± 1.4
760 ± 30
67
58.9
20
73.8 ± 3.0
18.9 ± 2.2 1076 ± 56
59
45.2
15
35.2 ± 3.6
21.5 ± 2.5
77
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(mol%) (104g/mol)
c
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Sample
917 ± 36
a: LBC: living block copolymer, structure data of these block copolymers was cited from Ref. [29]; b: average 1-octene molar incorporation in the copolymer,
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∆Hm0 = 293 J/g.
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determined by high temperature 13C NMR, c: determined by high temperature GPC; d: determined by DSC; e: crystallinity from heat of melting, XC,DSC = ∆Hm/∆Hm0,
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Scheme 1. Schematic structures of the living block copolymers.[29]
Figure 6 shows the stress-strain curves of the block copolymers. All the samples gave uniform
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deformation, without typical yielding and necking during the stretch. Although the yielding phenomenon of LBC4 sample was more obvious than the others, its deformation was still uniform. All these samples exhibited good toughness. The stress upswings were strong, due to their high molecular weight. Table 2 summarizes the secant modulus at 5% strain, the ultimate stress and
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strain, and the strain recovery data after fracture. For the diblock copolymers (LBC1~4), the hard block content was in an order of: LBC2 < LBC1 < LBC3 < LBC4. The order of secant modulus at 5%
LBC3 LBC5 LBC1
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25
EP
strain followed the same trend.
stress (MPa)
20
LBC4
LBC2
15
10
5
0 0
200
400
600 800 strain (%)
1000
1200
Figure 6. Engineering stress-strain curves of the ethylene/1-octene block copolymers.
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ACCEPTED MANUSCRIPT It was demonstrated by Hiltner et al. [10] and Wang et al. [31] that the slip-link theory was effective in describing the entire stress-strain curve of ethylene/1-octene block copolymer elastomers. According to the slip-link theory, the uniaxial stress-strain behavior of ethylene/1-octene copolymer elastomer is contributed from two parts: the slipping links (σs) and rigid cross-links (σc). The slip-links mainly depend on crystallinity but are not sensitive to
describe the slip-link model: σ = σs + σc మ మ ቂଵିమ ቀమ ା ቁቃ ಓ
ቀଵାηమ + ାηቁ మ
ଶ
ۗ ۖ
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భ
ሺ1+ηሻ൫ଵିమ ൯మ ቀି మ ቁ ಓ
ηቀି మ ቁ ቀି మ ቁమ ۘ ۔൫ଵିమ ൯ሺଵାηሻ ଵ ಓ ಓ ቂ − ቃ + − + ۖ ۖ మ మ ሻሺାηሻ మ ቀమ ାమቁ ሺଵାηమ ሻమ మ ቀమ ାమቁ ሺଵାη ሺାηሻ ଵି ଵି ە ಓ ಓ ۙ
σc =Nc kT ቀλ − మ ቁ ቊ ଵ
൫ଵିమ ൯
మ ಓ
ቂଵିమ ቀమ ା ቁቃ
మ
−
భ
భ
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σs =Ns kT
ۓ ۖ
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molecular weight, while the cross-links are determined by the molecular weight [7]. Equations 1~3
మ
మ ಓ
ଵିమ ቀమ ା ቁ
ቋ
(1)
(2)
(3)
where Ns is the density of sliplinks, Nc is the density of crosslinked chains, k is Boltzman constant, T is temperature in K, and λ is the draw ratio, η is the slippage parameter, and α is the
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inextensibility parameter. Figure 7 shows the correlation results of the slip-link model with the experimental stress-strain data. Table 3 lists the fitted model parameters with η = 1.1. Except for
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LBC4, the model fit the data well.
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LBC3
LBC4
LBC1
20
stress (MPa)
LBC5
LBC2
15
10
5
0 2
4
6
8
extension ratio
10
12
(a)
13
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(c)
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(b)
Figure 7. Slip-link fitting of the engineering stress-strain curves of ethylene/1-octene block copolymers; (a) Model correlation (lines) with the experimental data (points); (b) Simulated sliplink contribution; (c)
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Simulated crosslink contribution.
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Table 3. Slip-link model parameters for the block copolymers.
Sample
NskT (MPa)
NckT (MPa)
α
LBC1
10.9
0.02
0.102
LBC2
16.0
0.02
0.100
LBC3
17.2
0.08
0.090
LBC4
14.4
0.56
0.051
LBC5
9.2
0.27
0.076
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ACCEPTED MANUSCRIPT A careful examination of the relative contribution from slip-link in Figure 7b revealed some discrepancy in the slip-link theory explanation. The crystallinity of the block copolymers did not differ much from each other (see Table 2). However, the slip-link contributions in LBC1, LBC2 & LBC3 were much higher than those in LBC4 & LBC5 (Figure 7b). This might be induced by the ultrahigh molecular weight of LBC4 & LBC5. The ultrahigh molecular weight with extremely
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strong chain entanglement in LBC4 & LBC5 restricted crystallization process and inhibited lateral attachment and detachment of the crystallizable chain segments, giving less slip-link contributions, as shown in Figure 7b. The cross-link contribution in Figure 7c increased with the molecular weight,
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which is consistent with the slip-link theory.
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Elatomeric behaviors of block copolymers
From the tensile test results given in Table 2, LBC1 & LBC2 possessed strain recovery higher than 90% after fracture. Both samples had the hard block content lower than 50%. The strain recovery decreased substantially when the hard block content was higher than 50%. To further study their elastomeric behavior, the hysteresis experiments were performed. Figure 8 shows the
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hysteresis curves at various strains from 50 to 800%. Only LBC1~3 results are shown in the figure. LBC4 &LBC5 are very similar to LBC3. It was found that, with an increase of the applied strain, the permanent strain of LBC1 increased slightly, exhibiting a typical hysteresis behavior of
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elastomers. In comparison, the permanent strain of LBC3 increased significantly as the applied
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strain increased, exhibiting a typical behavior of plastics. LBC2 fell in between LBC1 and LBC3.
3.0
LBC2
2.0 Stress (MPa)
2.5 stress (MPa)
2.5
LBC1
2.0 1.5 1.0
1.5
1.0
0.5
0.5
0.0 0.0
0.0 0.0
1.5
3.0 Strain
4.5
6.0
(a)
1.5
3.0 Strain
4.5
6.0
(b)
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LBC3
Stress (MPa)
2.8
2.1
1.4
0.0 0.0
1.5
3.0
4.5
6.0
Strain
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0.7
7.5
(c)
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Figure 8. Hysteresis curves of the ethylene/1-octene block copolymers: (a) LBC1, (b) LBC2, (c) LBC3.
Figure 9 shows the strain recovery estimated from the hysteresis curves. With the hard block
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content <50% (LBC1 & LBC2), the strain recovery decreased gradually with the applied strain in a similar trend. When the hard block content was over 60% (LBC3, LBC4 & LBC5), the strain recovery decreased much more quickly, particularly when the applied strain was below 400%. LBC1 gave the best elastomeric behavior among the studied block copolymers, of which the strain recovery retained about 80% under the applied strain of 600%. Their order of elastomeric recovery
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was: LBC4 < LBC3 < LBC5 < LBC2 < LBC1, confirming that there existed a range of the hard block content for the optimal elasticity, that is, neither too high nor too low hard block content would benefit elastomeric behavior of the ethylene/1-octene block copolymers. In this work, LBC1
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with about 42 wt% hard block content exhibited the best elasticity.
100
Strain Recovery (%)
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LBC1 LBC2 LBC3 LBC4 LBC5
80
60
40
20
0 0
2
4 Strain
6
8
Figure 9. Evolution of the strain recovery with applied strain for the ethylene/1-octene block copolymers.
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ACCEPTED MANUSCRIPT In Figure 9, the strain recoveries of LBC3 and LBC5 under the low applied strains (≤ 150%) were comparable with LBC2, but the former dropped rapidly as the applied strain increased. Too high hard block content led to crystals vulnerable to plastic deformation at the large strains. It was evident, in comparing LBC4 and LBC5, that with similar molecular weights, similar hard-to-soft block ratios, similar hard and soft block segment compositions, the hard-soft-hard triblock structure
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gave better elasticity than the hard-soft diblock structure.
Conclusion
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The elastomeric performance of a series of ethylene/1-octene random and block copolymers were studied in this work. All the copolymers exhibited very good toughness. The living random were
found
to
have
better
elasticity
than
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copolymers
a
commercially
available
metallocene-catalyzed random copolymer counterpart due to their narrower intra-chain composition distributions. The strain recovery of the living random copolymers was stable at about 90% even at the applied strain of 800%. The hysteresis experiments demonstrated that elasticity of the ethylene/1-octene copolymer elastomers was mainly determined by their block structure and was
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little influenced by their molecular weight. There existed a range of the hard-block content for an optimal elastomeric behavior. Either too high or too low hard-block content deteriorated the elasticity. The diblock copolymer having about 42 wt% hard block content (LBC1) exhibited the
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best elasticity among the studied block copolymers. It retained the strain recovery about 80% under the applied stain of 600%. For the block copolymers with more than 60 wt% hard block content, the
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strain recovery decreased quickly as the applied strain increased.
Acknowledgement
We would like to thank the financial supports from the National Basic Research Program of China (2011CB606001) and the National Natural Science Foundation of China (U1462115). We would also like to thank Dr. Hanguang Wu and Prof. Ming Tian of Beijing University of Chemical Technology for their kind assistance in this work.
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References and Notes 1.
Drobny JG. Handbook of Thermoplastic Elastomers. Norwich: William Andrew Publishing, 2007, [chapters 7 and 15].
2.
Kresge EN, In Thermoplastic Elastomers, 3rd ed.; Holden G, Kricheldorf HR, Quirk RP, Eds.;
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Hanser Publishers: Munich, 2004; Chapter 5, p93. Wang HP, Chum S, Hiltner A, Baer E. J Polym Sci Part B: Polym Phys, 2009; 47: 1313-1330.
4.
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Bensason S, Minick J, Moet A, Chum S, Hiltner A, Baer E. J Polym Sci Part B: Polym Phys.
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1996; 34: 1301-1315.
Bensason S, Stepanov EV, Chum S, Hiltner A, Baer E. Macromolecules 1997; 30: 2436-2444.
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Arriola DJ, Carnahan EM, Hustad PD, Kuhlman RL, Wenzel TT. Science 2006; 312: 714-719.
9.
Arriola DJ, Carnahan EM, Cheung YW, Devore DV, Graf DD, Hustad PD, WO Int. Pat.
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7.
090427, 2005.
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10. Wang HP, Chum SP, Hiltner A, Baer E. J Appl Polym Sci 2009; 113: 3236-3244. 11. Wang HP, Khariwala DU, Cheung W, Chum S, Hiltner A, Baer E. Macromolecules 2007; 40: 2852-2862.
12. Khariwala DU, Taha A, Chum S, Hiltner A, Baer E. Polymer 2008; 49: 1365-1375.
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13. Tong ZZ, Huang J, Zhou B, Xu J, Fan Z. Macromol Chem Phy 2013; 214: 605-616. 14. He P, Shen W, Yu W, Zhou C. Macromolecules 2014; 47: 807-820.
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15. Jin J, Du J, Xia Q, Liang Y, Han C. Macromolecules 2010; 43: 10554-10559. 16. Li S, Register RA, Weinhold JD, Landes BG. Macromolecules 2012; 45: 5773-5781. 17. Park HE, Dealy JM, Marchand GR, Wang J, Li S, Register RA. Macromolecules 2010; 43: 6789-6799.
18. Li S, Register RA, Landes BG, Hustad PD, Weinhold JD. Macromolecules 2010; 43: 4761-4770. 19. Liu W, Wang WJ, Fan H, Yu L, Li BG, Zhu S. Eur Polym J 2014; 54: 160-171. 20. Domski GJ, Rose JM, Coates GW, Bolig AD, Brookhart M. Prog Polym Sci 2007; 32: 30-92. 21. Makio H, Terao H, Iwashita A, Fujita T. Chem Rev 2011; 111: 2363-2449.
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ACCEPTED MANUSCRIPT 22. Ye Z, Xu L, Dong Z, Xiang P. Chem Commun 2013; 49: 6235-6255. 23. Mitani M, Mohri J, Yoshida Y, Saito J, Ishii S, Tsuru K, Matsui S, Furuyama R, Nakano T, Tanaka H, Kojoh S, Matsugi T, Kashiwa N, Fujita T. J Am Chem Soc 2002; 124: 3327-3336. 24. Furuyama R, Mitani M, Mohri J, Mori R, Tanaka H, Fujita T. Macromolecules 2005; 38: 1546-1552.
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25. Edson JB, Wang Z, Kramer EJ, Coates GW. J Am Chem Soc 2008; 130: 4968-4977.
26. Hotta A, Cochran E, Ruokolainen J, Khanna V, Fredrickson GH, Kramer EJ, Shin YW, Shimizu F, Cherian AE, Hustad PD, Rose JM, Coates GW. PANS 2006; 103: 15327-15332.
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27. Liu W, Zhang K, Fan H, Wang WJ, Li BG, Zhu S. J Polym Sci Part A: Polym Chem 2013; 51: 405-414. W, Guo S, Fan
H, Wang WJ, Li BG, Zhu S. AIChE J 2013; 59: 4686-4695.
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28. Liu
29. Liu W, Andrew TO, Wang WJ, Fan H, Li BG, Zhu S. Polym Chem 2015; 6: 3800-3806. 30. Mitani M, Furuyama R, Mohri JI, Saito J, Ishii S, Terao H, Nakano T, Tanaka H, Fujita T. J Am Chem Soc 2003; 125: 4293-4305.
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31. Kuang X, Liu G, Wen T, Zhang X, Dong X, Wang D, Acta Polymerica Sinica 2013; 5: 679-687.
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Figure Captions Figure 1. Engineering stress-strain curves of the ethylene/1-octene random copolymers. Figure 2. Hysteresis curves of the ethylene/1-octene random copolymers at the applied strain of
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300%, (a) LRC1, (b) LRC2, (c) MRC. Figure 3. Evolution of strain recovery of the ethylene/1-octene random copolymers with cycle times at the applied strain of 300%.
between 50~800%, (a) LRC1, (b) LRC2, (c) MRC.
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Figure 4. Hysteresis curves of the ethylene/1-octene random copolymers with the applied strain
Figure 5. Evolution of the strain recovery with applied strain for the ethylene/1-octene random
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copolymers.
Scheme 1. Schematic structures of the living block copolymers.
Figure 6. Engineering stress-strain curves of the ethylene/1-octene block copolymers. Figure 7. Slip-link fitting of the engineering stress-strain curves of ethylene/1-octene block copolymers; (a) Model correlation (lines) with the experimental data (points); (b)
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Simulated sliplink contribution; (c) Simulated crosslink contribution. Figure 8. Hysteresis curves of the ethylene/1-octene block copolymers: (a) LBC1, (b) LBC2, (c) LBC3.
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copolymers.
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Figure 9. Evolution of the strain recovery with applied strain for the ethylene/1-octene block
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Highlights
LRC had better elasticity than MRC.
The strain recovery of LRC kept 90% with an applied strain up to 800%.
LBC1 retained the strain recovery about 80% under the applied stain of 600%.
The elasticity of block copolymers was mainly determined by their block
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structure.
ACCEPTED MANUSCRIPT Supporting Information
Elastomeric Properties of Ethylene/1-Octene Random and Block Copolymers Synthesized from Living Coordination Polymerization Weifeng Liu,1 Xiao Zhang,1 Zhiyang Bu,1 Wen-Jun Wang,1 Hong Fan,1 Bo-Geng Li1* and Shiping
1
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Zhu2*
State Key Laboratory of Chemical Engineering, College of Chemical & Biological Engineering, Zhejiang University, Hangzhou P.R. China 310027;
Department of Chemical Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4L7
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2
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* Corresponding authors, E-mail: [email protected] & [email protected]
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Endotherm
S1. DSC curves of ethylene/1-octene random copolymers
0
50 Temperature (°C)
LRC1
MRC
100
150
(a)
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-50
LRC2
Exotherm
LRC2
LRC1 MRC
-50
0 50 Temperature (°C)
100
150
(b)
ACCEPTED MANUSCRIPT Figure S1. DSC melting (a) and cooling (b) curves of ethylene/1-octene random copolymers studied in this work.
S2. DSC curves of ethylene/1-octene block copolymers
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LBC5
Endotherm
LBC4
LBC3
LBC2
50 Temperature (°C)
100
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0
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Exotherm
-50
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LBC1
AC C
-50
0 50 Temperature (°C)
150
(a)
LBC5 LBC4 LBC3 LBC2 LBC1
100
150
(b)
Figure S2. DSC melting (a) and cooling (b) curves of ethylene/1-octene block copolymers studied in this work.