Optically Transparent Functional Polyolefin Elastomer with Excellent

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Letter Cite This: ACS Macro Lett. 2019, 8, 299−303

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Optically Transparent Functional Polyolefin Elastomer with Excellent Mechanical and Thermal Properties Xiangyang Song,† Lixin Cao,† Ryo Tanaka,‡ Takeshi Shiono,*,‡ and Zhengguo Cai*,† †

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Material Science and Engineering, Donghua University, Shanghai 201620, People’s Republic of China ‡ Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan

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S Supporting Information *

ABSTRACT: Synthesis of a variety of optically transparent polyolefin elastomers consisting of hard segment of polynorbornene and several kinds of soft segments such as atactic polypropylene, poly(ethylene-co-propylene), and poly(ethylene-co-1-hexene) was achieved. The block copolymers exhibited excellent toughness and thermal property with efficient elastic recovery. Most importantly, the introduction of hydroxyl group into polynorbornene segment not only modulated surface property of block copolymers, but also improved their mechanical properties.

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based elastomers with keeping their excellent transparence and heat resistance. We have previously synthesized A−B−A block copolymers, consisting of poly(norbornene-co-1-octene) and aPP segments using a fluorenylamidodimethyltitanium complex 1,27 which is effective for the fast living (co)polymerization of ethylene, propylene, higher α-olefin, and norbornene.28,29 The block copolymer showed high transparency and stress at break value of 19.3 MPa at maximum strain of 384%. Clearly, the introduction of amorphous segment can significantly improve the toughness of COC. However, the block copolymer did not exhibit elastomeric property. Guided by this knowledge, we speculated that novel optically transparent elastomers would be obtained by changing the kinds of soft segments and their block length. Herein, we report the synthesis of a variety of new class of A−B−A block copolymers consisting of hard PNB or hydroxy functionalized PNB, which shows much higher Tg value than those of PMCH and COCs, and several kinds of soft segments such as aPP, poly(E-co-P), and poly(E-co-1-hexene) using 1. The block copolymers show excellent toughness with efficient elastic recovery. The introduction of hydroxyl group can improve their surface and mechanical properties. To investigate the influence of the length of soft segment on the mechanical properties, we synthesized a series of PNBbased A-B-A triblock copolymers with varied weight fraction of hard segment incorporated (f hard) using 1a activated by modified methylaluminoxane (MMAO)/2,6-di-tert-butyl-4-

ince the discovery of polyolefin elastomers (POE) obtained by the copolymerization of ethylene and higher α-olefins with metallocene catalysts, several kinds of POE have been developed based on block or graft copolymers.1−15 Most of these copolymers composed of crystalline polyethylene or polypropylene as “hard” segments and amorphous atactic polypropylene or ethylene-higher α-olefin copolymer as the “soft” segments with low Tg value to create physical cross-links needed for elastic recovery. Amorphous polymer with high Tg value (≈72 °C) was also used as the “hard” segment to promote new kind of POE. Sita et al. reported that a structurally well-defined A−B−A block copolymer of PMCH (poly(1,3-methylenecyclohexane)) and atactic PP (aPP) obtained by using living polymerization with hafnocene catalyst exhibited good mechanical properties with stress at break values ranging from 8.9 to 20.3 MPa, strain at break values ranging from 1390% to 2773%, and elastic recovery value reaching up to 94 ± 1%.16 Polynorbornene (PNB) obtained via coordination−insertion polymerization with transition metal catalysts is a class of attractive polymers with great optical and thermal properties, including high transparency, low birefringence, high glass transition temperature (Tg), nonhygroscopicity, and so on.17 NB-based cycloolefin copolymers (COCs) synthesized by the copolymerization of NB with olefin can control these properties by comonomer content, comonomer sequence distributions, and the structure of comonomer.18−26 Despite these achievements, another concern about NB-based polymers is their poor toughness and hydrophilicity, which limits further application of polymers. Therefore, it is important to investigate strategies for enhancing these properties, even to the synthesis of polar functionalized NB© XXXX American Chemical Society

Received: January 4, 2019 Accepted: February 28, 2019

299

DOI: 10.1021/acsmacrolett.9b00005 ACS Macro Lett. 2019, 8, 299−303

Letter

ACS Macro Letters Table 1. Synthesis of A−B−A Block Copolymers and Their Mechanical Properties entrya 1 2 3 4 5 6 7 8 9 10 11 12 13

monomers (mmol) NB (0.64) NB (0.64)-P (15.59) NB (0.64)-P (15.59)-NB (1.91) NB (0.64)-P (29.52) NB (0.64)-P (29.52)-NB (1.91) NB (0.64)-E (6.13)/P (15.40) NB (0.64)-E (6.13)/P (15.40)-NB (1.91) NB (0.64)-E (6.13)/H (2.42) NB (0.64)-E (6.13)/H (2.42)-NB (1.91) NB (0.64)-H (4.52) NB (0.64)-H (4.52)-NB (1.91) aPP PNB/aPP blend

yield (g) 0.06 0.71 0.88 1.29 1.47 0.88 1.05 0.43 0.60 0.44 0.60

Mnb (104) 1.20 17.7 26.0 23.1 30.2 16.8 20.8 16.5 24.3 16.3 23.2 19.3 1.2/19.3

PDIb 1.15 1.19 1.12 1.14 1.11 1.36 1.27 1.28 1.35 1.09 1.14 1.20

f hardc 1.00 0.07 0.37 0.05 0.27 0.07 0.25 0.07 0.37 0.07 0.35 0 0.06

Tgd (°C) e

-e -e/−3.1 -e -e/−2.1 -e -e/−29.7 -e -e/−59.5 -e -e/−53.3 −1.6 -e

σf (MPa) h

nd 4.29 16.37 4.16 8.19 5.82 12.16 6.63 11.35 6.08 7.74 0.09 0.44

εf (%) h

nd 2009 1037 3328 1300 2044 1002 2659 911 5 5 543 600

rec.g (%)

Ef (MPa)

h

ndh 0.27 2.00 0.16 0.70 2.65 4.12 21.10 160.00 ndh ndh 1.57 2.44

nd ndh 79 ± ndh 90 ± 78 ± 74 ± 72 ± 40 ± ndh ndh ndh ndh

1 1 1 1 1 1

a Polymerization conditions: Ti = 20 μmol, Al/Ti = 400, solvent = toluene (50 mL), 30 °C. bNumber-average molecular weight and molecular weight distribution determined by GPC using polystyrene standard. cFraction of hard segment calculated from the Mn value of each segment and the overall Mn value. dDetermined by DSC. eNot determined under 400 °C. fStress at break (σ), strain at break (ε), and Young’s modulus (E) determined by stress−strain tensile testing. gElastic recovery measured after a 300% strain step cycle test (10 cycles) for each sample. hNot detected.

methylphenol (BHT), where BHT was employed as the modifying reagent of trialkylaluminum in MMAO.30 The polymerization results are shown in Table 1. This parameter, f hard, could be easily regulated by the relative amount of the monomers added. Each-step polymerization was performed quantitatively regardless of the monomers used. The conversion for each monomer reached almost 100%. The yields of the block terpolymers obtained were increased in all the cases accompanied by the increase of Mn values with narrowing of the MWD values (1.11−1.12) in each polymerization. The unimodel GPC curves of the copolymers displayed a clear shift to a higher molecular weight region, which was indicative of successful block copolymer growth. The PNB-b-aPP-b-PNB copolymers showed the Tg at approximately 0 °C by differential scanning calorimetry (DSC, −30−250 °C), as expected for the high molecular weight aPP block. Given much higher Tg value of PNB obtained by coordination−insertion polymerization (approximately 400 °C), the Tg value for it was not observed within the temperature ranges from −30 to 250 °C. The 13C NMR spectrum of PNB-b-aPP-b-PNB obtained showed a set of mixed signals of the homopolymers of PNB and PP obtained, rather than that of PNB−PP random copolymer, indicating the formation of expected triblock copolymers (Figure S2). Tensile test indicated that PNB-b-aPP-b-PNB copolymer with f hard value of 0.37 (Table 1, entry 3) gave higher stress at break value (σ = 16.37 MPa) and Young’s modulus (E = 2.00 MPa) than that with f hard value of 0.27 (Table 1, entry 5), suggesting that the increasing of the hard segment content could increase ultimate tensile strength and tensile toughness of the material (Figure 1). Although the stain at break value showed a slight reduction in the former, both copolymers showed good elastic property of strain at break above 1000%. Hysteresis testing were conducted by extending the polymer samples to 300% strain over 10 cycles to determine their elastic recovery, which could be easily tuned by adjusting the f PNB value of the triblock copolymer. After subsequent stress−strain cycles, the copolymer with f hard = 0.27 displayed an excellent elastic recovery of 90 ± 1% compared to that with f hard = 0.37 (elastic recovery of 79 ± 1%; Figure S4). These results showed

Figure 1. Stress−strain curves for PNB-based A−B diblock copolymers and A−B−A triblock copolymers (Table 1, entries 2, 3; entries 5, 6; entries 7, 8; and entries 9, 10).

that the mechanical properties of the PNB-based copolymers could be controlled by changing the block length. To avoid the loss of elasticity of PNB-based block copolymers with aPP as the soft segment below their Tg (≈0 °C), we selected random ethylene copolymers such as poly(Eco-P) and poly(E-co-1-hexene) as the soft segments because of their lower Tg values. DSC analysis showed the Tg of approximately −30 °C for PNB-b-P(E-co-P)-b-PNB and approximately −60 °C for PNB-b-P(E-co-H)-b-PNB, respectively, indicating that the material had more excellent low temperature resistance. Furthermore, compared with PNB-baPP-b-PNB copolymer with similar f hard value, PNB-b-P(E-coP)-b-PNB exhibited better tensile strength with competitive elastic recovery of 74 ± 1% (Table 1, entries 5 and 7), while PNB-b-P(E-co-H)-b-PNB exhibited slightly lower tensile strength with clearly lower elastic recovery of 40 ± 1% (Table 1, entries 3 and 9). Poly(1-hexene) (PH) with longer branched chains was also used as the soft segment. 300

DOI: 10.1021/acsmacrolett.9b00005 ACS Macro Lett. 2019, 8, 299−303

Letter

ACS Macro Letters Table 2. Synthesis of A−B−C Block Copolymers and Their Mechanical Properties

entrya 1 2 3 4 5 6 7 8 9

monomers (mmol) NB NB NB NB NB NB NB NB NB

(0.64)-P (0.64)-P (0.64)-P (0.64)-P (0.64)-P (0.64)-P (0.64)-P (0.64)-P (0.64)-P

(16.63) (16.63)-(NB-U−OH) (16.63)-(NB-U−OH) (29.10) (29.10)-(NB-U−OH) (29.10)-(NB-U−OH) (41.57) (41.57)-(NB-U−OH) (41.57)-(NB-U−OH)

NB/U−OHb 1:1 3:1 1:1 3:1 1:1 3:1

yield (g)

Mnc (104)

0.75 0.98 0.95 1.25 1.58 1.51 1.80 2.11 2.06

17.3 19.9 22.3 23.1 26.1 30.1 28.2 29.6 32.1

PDIc 1.21 1.19 1.17 1.28 1.27 1.25 1.30 1.32 1.28

f hardd g

nd 0.19 0.28 ndg 0.17 0.26 ndg 0.09 0.16

σe (MPa) g

nd 20.18 11.27 ndg 13.72 10.27 ndg 7.48 6.54

εe (%) g

nd 1164 1816 ndg 1538 2124 ndg 2112 2413

rec.f (%)

Ee (MPa)

g

ndg 3.81 1.10 ndg 2.28 1.37 ndg 0.87 0.70

nd 88 ± 86 ± ndg 90 ± 92 ± ndg 91 ± 92 ±

1 1 1 1 1 1

Polymerization conditions: Ti = 20 μmol, Al/Ti = 400, solvent = toluene (50 mL), second additive amount of NB = 1.91 mmol, 30 °C. bThe mole ratio of NB to U−OH added for the second time. cNumber-average molecular weight and molecular weight distribution determined by GPC using polystyrene standard. dFraction of hard segment calculated from the Mn value of each segment and the overall Mn value. eStress at break (σ), strain at break (ε), and Young’s modulus (E) determined by stress−strain tensile testing. fElastic recovery, measured by after a 300% strain step cycle test (10 cycles) for each sample. gNot detected. a

Unfortunately, PNB-b-PH-b-PNB with low Tg value (−53.3 °C) did not exhibit any elasticity (Table 1, entry 11). For comparison, aPP and PNB/aPP blend having similar Mn values with those of the block copolymers were also prepared by the same catalytic system (Table 1, entries 12 and 13). The results confirmed that the triblock copolymers had better mechanical properties than PP homopolymer and PP/PNB blends. To optimize the mechanical and surface properties for the material, a series of PNB-b-aPP-b-P(NB-co-U−OH) triblock copolymer with varied f hard values (0.09−0.28) and hydroxyl content was synthesized by incorporation of 10-undecen-1-ol (U−OH) into copolymer backbone (Table 2). This parameter, f hard, was evaluated based on both NB and NB/U−OH glassy segments. The hydroxyl content of this material with narrow MWD values (1.17−1.32) was controlled by changing the comonomer composition in feed. The Tg of poly(NB-co-U−OH) copolymer segment was not observed by DSC analysis due to its relatively low content in the copolymer, which could be found in our previous work.31 The 13C NMR spectrum (see Supporting Information) indicated that the resonances corresponding to PNB and random PP sequences were observed together with those of the sequences of the NB/U−OH copolymer, respectively, testifying the formation of expected triblock copolymers. The softness and toughness of the materials could be easily tuned by changing the content of the hard segments and hydroxyl groups. Compared to the previous synthesis of the triblock copolymers shown in Table 1, the introduction of polar monomer dramatically improved the mechanical properties even at lower f hard values. The maximum stress at break value of the material reached up to 20.18 MPa, while the strain at break value (ε = 1164%; Figure 2) and elastic recovery (rec. = 88 ± 1%) of it still be kept at considerable high values. All block copolymers exhibited excellent elastic recovery above 85% (Figure 3). Interestingly, the block copolymer with lower f hard value but more content of hydroxyl groups, however,

Figure 2. Stress−strain curves for PNB-b-aPP-b-P(NB-co-U−OH) triblock copolymers (Table 2: entries 2, 3, 5, 6, 8, and 9).

showed higher tensile strength and elastic recovery (Table 2, entries 2 and 3). These results benefited from the hydrogen bonding interaction produced by hydroxyl groups in the copolymer chains which made the physical cross-links formed in the elastomer much stronger, giving materials more excellent mechanical properties. We determined the water contact angle (WCA) of PNB-baPP-b-P(NB-co-U−OH) triblock copolymer films to evaluate their surface properties (Figure S7). As compare to the WCA (109.0°) of nonfunctional PNB-b-aPP-b-PNB copolymer, the WCA values of PNB-b-aPP-b-P(NB-co-U−OH) copolymers were determined to be 90.0°, 94.5°, 96.1°, 97.1°, 101.4°, and 103.2°, respectively, indicating the successful incorporation of hydroxyl groups could dramatically alter the surface properties. The decrease of WCA values also demonstrated the increase of hydroxy group content in PNB-b-aPP-b-P(NB-co-U−OH) triblock copolymers. 301

DOI: 10.1021/acsmacrolett.9b00005 ACS Macro Lett. 2019, 8, 299−303

Letter

ACS Macro Letters

In conclusion, we have synthesized a series of novel norbornene based A−B−A and A−B−C block copolymers with different kinds of soft segments such as aPP, poly(E-co-P), and poly(E-co-1-hexene) using the 1-MMAO/BHT system for the first time. Their excellent toughness with efficient elastic recovery left a deep impression on us. The triblock copolymer (PNB-b-aPP-b-P(NB-co-U−OH)) synthesized by introducing hydroxyl group into PNB segment gave excellent thermal, mechanical properties and surface behavior for optically transparent material, which is a potential and versatile new material. The mechanical properties of all triblock copolymers could be optimized by changing f hard values and the hydroxyl content in the copolymer chains, respectively. Most importantly, it also provides a guideline for exploiting a new family of elastomers with high performance and more applications in the future.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.9b00005. Synthesis and characterization of the block copolymers (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

Figure 3. Repetitive stress−strain recovery tests for PNB-b-aPP-bP(NB-co-U−OH) triblock copolymers (Table 2: entries 2, 3, 5, 6, 8, and 9). A total of 10 cycles at 300% strain were performed.

ORCID

Ryo Tanaka: 0000-0002-6085-074X Takeshi Shiono: 0000-0002-1118-9991 Zhengguo Cai: 0000-0001-5784-3920

All triblock copolymers obtained were processed to thin films by solution casting, and the UV−vis spectra of the prepared films showed that the transmittance of PNB-b-aPP-bPNB, PNB-b-P(P-co-E)-b-PNB, and PNB-b-aPP-b-P(NB-coU−OH) copolymer films were above 90% in the visible light region (350−750 nm), regardless of the hard segment content of the samples and the incorporation of the long-chained polar comonomer (U−OH), which could be served as an optically transparent material (Figure 4).

Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (NSFC, 21174026), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, Development Foundation and the Fundamental Research Funds for the Central Universities (No. CUSF-DH-D-2017041).

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REFERENCES

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Figure 4. UV−vis spectra of PNB-b-aPP-b-PNB, PNB-b-P(E-co-P)-bPNB, and PNB-b-aPP-b-P(NB-co-U−OH) triblock copolymers thin films (Table 1: entries 3, 5, and 7; Table 2: entries 2, 3, 5, 6, 8, and 9). 302

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ACS Macro Letters

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DOI: 10.1021/acsmacrolett.9b00005 ACS Macro Lett. 2019, 8, 299−303