Preparation of Comb-Shaped Polyolefin Elastomers Having Ethylene

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Preparation of Comb-Shaped Polyolefin Elastomers Having Ethylene/1-Octene Copolymer Backbone and Long Chain Polyethylene Branches via a Tandem Metallocene Catalyst System Kailun Zhang,† Pingwei Liu,† Wen-Jun Wang,*,† Bo-Geng Li,† Weifeng Liu,*,‡ and Shiping Zhu*,§,∥

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†

State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou, P. R. China 310027 ‡ School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, P. R. China 510640 § School of Science and Engineering, The Chinese University of Hong Kong, Shenzhen, P. R. China 518172 ∥ Department of Chemical Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4L7 S Supporting Information *

ABSTRACT: A novel type of comb-branched ethylene/1-octene copolymer elastomers (CPOEs) was synthesized, and their properties were systematically investigated in this work. The polymers had crystalline polyethylene (PE) long chain branches attached to amorphous ethylene/ 1-octene random copolymer backbones. The unique structure was generated through a tandem catalyst system consisting of a Zr ligated with phenoxycycloalkylimine (FI-Zr) and a constrained geometry catalyst (CGC-Ti). Linear PE macromonomers, with over 88% chains terminally unsaturated, were synthesized with the FI-Zr catalyst, while the CGC-Ti catalyst was employed in the copolymerization of ethylene, 1-octene, and the PE macromonomer. The resulting CPOEs possessed high melting temperature (>120 °C) and low glass transition temperature (1200%. Some samples performed better strain recovery than the commercial POE Engage 8150.

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INTRODUCTION Polymeric materials properties are mainly determined by their chain microstructures. Polymers are the products of processes. Polymer samples prepared from the same recipe and having similar average molecular weights and comonomer contents could be structurally tailored to demonstrate a variety of different behaviors because of richness in their chain microstructures.1−4 The engineering of long chain branching (LCB) in the polyolefin industry represents an outstanding example in this respect. It is well-known that LCB can induce a remarkable effect on the rheological properties of polyolefin melts and thus improve the processability of the materials, such as in blow molding operations and extrusion processes.5−7 The molecular weight of LCB must exceed the entanglement molecular weight (Me), which is about 1300 g/mol for polyethylene.8,9 Long chain branching can be formed through various approaches, such as intermacromolecular chain transfer in LDPE,10 postpolymerization modification,11 single-site metallocene catalyst,12−15 binary catalyst system,16,17 and so forth. An effective approach to synthesize long chain branched (LCBed) polyolefins is to employ two tandem catalysts in a single reactor.16 In such a system, one catalyst produces macromonomers and the other copolymerizes the macro© XXXX American Chemical Society

monomers with a main monomer to form LCBs. Soares et al. combined rac-Et(Ind)2ZrCl2 and [Me2Si(NtBu)(Me4Cp)]TiMe2 catalysts, where the former polymerized ethylene to generate vinyl-ended polyethylene (PE) macromonomer and the latter copolymerized ethylene with PE macromonomer.18,19 They found that degree of LCB could be controlled by adjusting the ratio of the two catalysts. They also used racEt(Ind)2ZrCl2 to prepare macromonomers having different numbers of pendant vinyl groups by copolymerization of ethylene with different diene monomer concentrations.20 The macromonomers were isolated and introduced in a second step to copolymerize with ethylene and α-olefin using [Me2Si(NtBu)(Me4Cp)]TiCl2 catalyst. Increasing pendant vinyl groups and decreasing α-olefin concentration increased the long chain branching density. Soares and co-workers have also developed a mathematical and simulated molecular properties with different polymerization conditions in both semibatch reactors and continuous stirred-tank reactor (CSTRs).21,22 The results showed that CSTRs were more efficient than semibatch reactors for the preparation of copolymers with higher long Received: August 8, 2018 Revised: October 18, 2018

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DOI: 10.1021/acs.macromol.8b01711 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Schematic Preparation of Poly(ethylene/1-octene)-graf t-polyethylene Comb-Branched Block Copolymers Using the FI-Zr/CGC-Ti Catalyst Systema

a

The cyan color domains represent crystallization of long chain branches; the orange color domains represent cocrystallization between the unreacted macromonomers and long chain branches.

which is highly desired for the sake of environmental-friendly recyclability. Among TPEs, polyolefin elastomer (POE) represents one of the most widely used thermoplastic elastomers in many application fields. Commercial POE was first developed by Dow Chemical using a constrained geometry metallocene catalyst, named as the INSITE technique.27 The polymers possess many excellent properties, such as low density (95 mol %, and high activity under atmospheric pressure.36 The catalyst bearing a tert-butyl group at the ortho-position of oxygen (named as PTFI in this paper) gave very poor copolymerization ability for ethylene and 1-octene, as similar catalysts were used in chain shuttling polymerization for preparation of hard blocks of olefin block copolymers (OBC) with little 1-octene incorporation.28 This catalyst was hypothesized to be a good candidate for the tandem system. Therefore, the copolymerization behavior of PTFI was first investigated. The results of ethylene homo- and copolymerization with 1octene at 90 °C are listed in Table 1. The DSC curves are shown in Figure S1. By comparison of M1 and M2, the melting temperature and melting enthalpy of the products decreased only slightly with 1-octene addition, indicating very poor 1octene incorporation of PTFI. The crystallinity was at a relatively high level. The macromonomers synthesized with PTFI showed good potential as hard segments in the tandem system. The molecular weight of the macromonomers decreased slightly with 1-octene addition, as shown in Figure S2. The small peak in the GPC curves may be attributed to isomerization of PTFI at the high temperature. Even in the presence of 1-octene, the molecular weights of the synthesized macromonomers were still much higher than the Me of polyethylene (1300 g/mol), enabling the formation of chain entanglement as long chain branches. The vinyl-end content of the macromonomers was fully characterized. The introduction of high concentration 1-octene had little influence on the vinyl-end content (>88 mol %) according to 1H NMR spectra shown in Figure 1a.42 In an ethylene copolymerization with 1-octene, there can be four D

DOI: 10.1021/acs.macromol.8b01711 Macromolecules XXXX, XXX, XXX−XXX

0.5 1 1.5 2 0 1

P1 P2 P3 P4 P5 P6c P7d P8e 8150

10/30 10/30 10/30 1/30 0/30 10/30

tb (min)

32.0 39.9 40.0 39.8 30.9 32.9

yield (g)

F̅ 3̅f (mol %) 0.19 0.47 0.64 0.37 0 0.23 0.13 0

F̅ 2̅f (mol %) 20.5 20.0 21.5 19.5 23.1 14.8 20.0 20.2 12.0

123.7 124.4 125.1 124.3 39.2 123.5 124.3 126.6 57.6

Tmg (°C)

Tcg (°C) 107.6 108.0 110.3 108.0 6.0 105.4 109.5 114.6 40.7

Tgg (°C) −63.7 −63.4 −63.5 −62.8 −68.3 −59.8 −65.6 −67.0 −59.6 28.0 58.9 80.7 44.4 2.6 43.2 38.3 25.0 26.4

ΔHmg (J/g) 100.4 94.7 88.0 94.6 94.9 97.5 97.3 86.0 115.8

Mwh (kDa)

Đci 2.26 2.15 2.21 2.36 2.11

Mwci (kDa) 104.9 104.0 104.4 103.5 107.0

Đh 4.27 6.13 8.66 6.13 3.20 6.10 6.64 7.42 2.26 9.2

4.50 9.40 16.5 9.0

Wt1j (%)

18.6

10.6 25.3 34.7 21.0

Wt2j (%)

50.5

62.6 62.9 52.5 59.0

X3k (%)

2.5

1.8 4.1 5.0 3.0

Nl

a

Polymerization conditions: stirring rate = 600 rpm, CGC-Ti = 10 μmol, Al/(Zr + Ti) = 1000; T = 90 °C, P = 11 bar, 1-octene = 31 mL. bt1/t2 represents reaction time before and after adding CGC and 1-octene; P5 did not add PTFI catalyst. c1-Octene = 25 mL. dSolution blending of P1 and M4 to have the same residual macromonomers as in P2. eSolution blending of P5 and M4 to have the same residual macromonomers as in P2. fF̅ 2 represents mole fraction of 1-octene in CPOEs; F̅ 3 represents mole fraction of macromonomers in CPOEs. gDetermined by DSC. hDetermined by GPC. iWeightaverage molecular weight and polydispersity index of the synthesized CPOEs without including unreacted macromonomer residues. jWt1 represents weight ratio of unreacted macromonomer residues, estimated from GPC diagrams; Wt2 represents weight ratio of the synthesized macromonomers estimated from DSC. kThe conversion of macromonomers. lAverage number of grafted long chain branches per backbone chain.

PTFI (μmol)

runa

Table 2. Preparation and Characterization Results of the Synthesized CPOEs

Macromolecules Article

Figure 2. GPC curves of the synthesized CPOE samples.

run P5. The 1-octene concentration in run P5 was the same as in P1−P3, so that sample P5 could be used as a control for the polymer backbone of CPOE samples in P1−P3. Compared to P5, the high molecular weight part in the bimodal distributions of P1−P3 slightly shifted toward the right side as the macromonomer concentration increased, indicating more macromonomers inserted into the copolymers. The DSC curve of P5 in Figure 3 also suggested that the backbones of CPOE samples P1−P3 had little crystallization

Figure 3. Thermal behavior of the synthesized CPOE samples.

(or just fringed micelles) with a weak and broad melting peak near room temperature. The melting peaks of P1−P3 between 100 and 130 °C mainly resulted from cocrystallization of the residual free macromonomers and the inserted macromonomers (that is, long chain branches in CPOEs). Compared with M4, the slightly lower melting temperature of P1−P3 was ascribed to insertion of the long chain branches and cocrystallization of PE macromonomers with the backbones. However, compared with the commercial POE sample ENGAGE 8150, the crystalline long chain branches imparted CPOE samples synthesized in this work a higher melting temperature and a higher crystallization temperature, while maintaining a low glass transition temperature. With the increase in PTFI catalyst amount, the melting enthalpy at

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DOI: 10.1021/acs.macromol.8b01711 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules around 120 °C increased. The prepolymerization time with PTFI in P4 was much shorter than in P1−P3, but their yields were comparable, indicating that the pre-homopolymerization had little effect on the activity of CGC-Ti catalyst, but a shorter pre-homopolymerization time would produce fewer macromonomers, resulting in a lower melting enthalpy, as shown in Table 2. Analysis of Chain Microstructures. The final product was a complex mixture consisting of unreacted macromonomers (including those polyethylene chains bearing no vinyl end), ethylene/1-octene copolymer chains without incorporated macromonomer, and comb-branched copolymer chains. We tried to use Soxhlet extraction to separate the final product (see the Supporting Information). We used n-hexane, cyclohexane, n-heptane, and toluene to extract the final mixture. The results showed that unreacted macromonomers existed in all the extracted components. The ethylene/1-octene copolymer component was extracted with n-hexane because of its high 1-octene incorporation. The weight percentage of nhexane soluble component showed that the content of ethylene/1-octene copolymer decreased as more macromonomer participated in the polymerization. Only 0.6 wt % of the n-hexane-soluble component extracted from sample P3 showed that nearly all the backbones in P3 were incorporated with at least one macromonomer. It is well-known that the resolution of 13C NMR is currently limited to six carbons. However, we can still use 13C NMR in combination with GPC and DSC to extract the information about long chain branching, based on the following assumptions: (1) no crystallization of ethylene−1-octene copolymer backbones and negligible variation of the melting enthalpy of macromonomers, whether inserted or free; (2) the same molecular weight distribution for the free and inserted macromonomers; and (3) no 1-octene insertion into the macromonomers. The mass fraction of the generated macromonomers in total was obtained from the melting enthalpy measurement. The mass fraction of the residual macromonomers was obtained from the GPC bimodal curves. On the basis of these results, we could estimate the following parameters, such as the molecular weight of CPOEs, the number of macromonomers grafted into a backbone, and the mole incorporation of 1-octene and macromonomers. The data are listed in Table 2, with the detailed estimation provided in the Supporting Information (P2 as an example). After removing residual macromonomers, the weightaverage molecular weights of the synthesized CPOEs were similar. As the amount of macromonomers increased, the macromonomer incorporation gradually increased, which was confirmed by both data of the average mole incorporation data and the average number of macromonomers per chain. The average number of long chain branches per chain in sample P3 was close five. These long chain branches could crystallize and act as the physical cross-linking points. Samples P1−P4 had similar 1-octene mole incorporation as their 1-octene feeding concentration was the same. More than 20 mol % 1-octene incorporation yielded amorphous backbones of CPOEs, in agreement with the previous DSC data. Figure 4 shows the typical 13C NMR spectra of CPOEs, with their substructure nomenclatures given in Figure 5. The estimates of the triad and diad sequence distributions are listed in Table 3. The sequence distributions of CPOE backbones were estimated by subtracting all the macromonomers from the total CPOEs, assuming that the macromonomers only

Figure 4. 13C NMR spectra of the synthesized CPOEs.

provided the EEE sequence (see the Supporting Information). When the macromonomers were excluded, samples P1−P4 were found to have similar triad sequence compositions, implying that the macromonomers had little effect on the reactivity ratio of ethylene/1-octene. The copolymer products were the mixture of CPOEs and macromonomers. To further elucidate their chemical composition distribution (CCD), the successive self-nucleation and annealing (SSA) was performed. The DSC melting curves after SSA treating are shown in Figure 6. All the samples showed several separate melting peaks. The pure macromonomer sample M4 exhibited good crystallization ability with the maximum melting peak centered at 133 °C. However, after solution blending with the copolymer products (samples P7 and P8), the melting peak at 133 °C disappeared, and the highest melting peak shifted to 130 °C. For the copolymerization samples P1−P3, the SSA melting curves showed more obvious multiple melting peaks, and the melting peak at 133 °C disappeared. As the macromonomer concentration increased, the melting peak at 130 °C became stronger, indicating that the unreacted macromonomers could form larger crystals. The separate multiple melting peaks above 100 °C were ascribed to the inserted long chain branches having different chain lengths and the cocrystallization effect between the unreacted macromonomers and those long chain branches or backbones. The average lamellar thickness and crystalline methylene sequence length (CMSL) values were calculated according to the procedures reported in the literature.43,44 These values of sample P4 were only slightly smaller than those of samples P1 and P2 due to the shorter time of prehomopolymerization with PTFI catalyst. These results also confirmed poor 1-octene incorporation ability of the PTFI catalyst, regardless of pre-homopolymerization time. The prehomopolymerization had little effect on the enthalpy of the synthesized macromonomers. The lamellar thickness and CMSL values of the blended samples P7 and P8 were larger than the copolymer products P1−P3 due to the insertion of macromonomers. The average length of CMSL was larger than the average lamellar thickness, which was caused by the fact that some polymer chains were adequately long to fold and some had defects excluded from the crystals. F

DOI: 10.1021/acs.macromol.8b01711 Macromolecules XXXX, XXX, XXX−XXX

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Figure 5. Nomenclature of CPOE substructures.

Table 3. Triad Distributions of the CPOE Samplesa P1 P2 P3 P4 P5 P6

Table 4. Lamellar Parameters of the CPOE Samples

R(EOE)

R(EOO)

R(OOO)

R(OEO)

R(OEE)

R(EEE)

9.80 8.66 8.34 9.15 11.5 7.91

8.51 8.27 8.71 7.70 10.8 5.78

1.98 2.08 2.36 1.20 3.35 0.70

2.97 2.79 2.90 2.66 3.84 1.53

21.3 18.7 17.4 20.9 24.3 19.1

55.5 59.5 60.2 58.4 46.3 65.0

CMSLa

lamellar thickness sample

L̅ n (nm)

L̅ w (nm)

I

L̅ n (nm)

L̅ w (nm)

I

P1 P2 P3 P4 P6 P7 P8 M4

8.41 8.41 9.27 8.05 8.82 9.57 11.5 15.6

9.75 9.72 10.9 9.48 10.3 11.0 13.0 17.2

1.16 1.16 1.18 1.18 1.17 1.15 1.13 1.10

15.3 15.3 18.6 14.5 16.8 19.1 26.9 53.7

21.3 21.2 27.5 20.9 23.6 25.8 36.2 72.2

1.39 1.39 1.48 1.45 1.41 1.36 1.35 1.35

a

Triad distribution excluding all the synthesized macromonomers.

Mechanical Properties of CPOEs. The mechanical properties of the synthesized samples were evaluated by uniaxial tension tests at a tensile rate of 50 mm/min with a corresponding strain rate of 200%/min. The engineering stress−strain curves are shown in Figure 7a, and the test results are summarized in Table 5. Compared with the pure random copolymer sample P5, the elastic modulus and ultimate tensile stress of the CPOE samples (P1−P4, P6) were significantly improved. In samples P1−P3, the increase in PTFI catalyst amount led to higher macromonomer concentration in the polymerization system. Thus, more long chain branches were formed in the CPOEs, resulting in improved elastic modulus and ultimate tensile stress. The long chain branches were crystallizable and functioned as efficient physical cross-linking points under external loading. The tensile strength of P6 was higher than those of P1−P3 due to the combined effect of backbone crystallization and side chain hard segment, as P6 had less 1-octene incorporated into the backbone. All these tandem polymerized CPOE samples (P1−P4) performed obviously better elongation than the commercial 8150. This benefited from the nearly amorphous backbone structure of CPOE, which could be stretched much longer. The improved

a

Crystalline methylene sequence length.

elongation at break offered this special CPOE structure an advantage over the commercial POE sample. There were some interesting observations by comparison of the tensile properties between CPOEs and the solution blending samples, as shown in Figure 7b. P7 was prepared by solution blending of P1 and M4 to have the same macromonomer residues as P2. However, the toughness and tensile stress of P7 were much poorer than P1, and there was even no comparison between P7 and P2. As the difference between P2 and P7 was that the number of long chain branches in P2 was much higher than P7, it could be further concluded that the crystalline long chain branches in CPOEs could form efficient physical cross-linking, which improved the modulus, strength, and toughness. P8 was prepared by solution blending of common random copolymer P5 and M4 to have the same macromonomer residues as P2. The strength and modulus of P8 were slightly higher than those of P5, indicating that the crystalline macromonomers could reinforce the rubbery random copoly-

Figure 6. DSC heating scans after applying successive self-nucleation/annealing (SSA). G

DOI: 10.1021/acs.macromol.8b01711 Macromolecules XXXX, XXX, XXX−XXX

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Figure 7. Engineering strain−stress curves of (a) different CPOEs samples and (b) comparison of the tensile properties between CPOEs and the solution blending samples.

increased, more long chain branches formed in CPOEs, and the crystallinity of the tandem polymerization products increased, as verified by the AFM phase images in Figures 8b and 8c, resulting in stronger interfacial interactions between the crystals and the amorphous matrix. But for the solution blending sample P7, severe phase separation occurred with unevenly distributed crystal phase domains, as shown in Figure 8d. There were fewer long chain branches in P7 than in P2 and P3, resulting in much poorer interfacial interactions. This uneven phase separation and weak interfacial interactions could also explain the poor mechanical performance of the solution blending sample P7 compared with the in situ tandem polymerization products P2 and P3. The mechanical performance study revealed that the separation of residual macromonomers was not necessary in this work, for the purpose to improve modulus, strength, and toughness for CPOEs with crystalline long chain branches synthesized by the tandem polymerization. Elasticity of CPOEs. To examine the elastic recovery behavior of the samples, a hysteresis experiment was performed at 300% strain. All the samples suffered prominent deformation after the first load cycle. However, no obvious change was observed in the subsequent load cycles, as the hysteresis loop after the second cycle showed little difference. The typical hysteresis curve of P2 is shown in Figure 9. The evolution of the strain recovery versus load cycle number was compared. All the samples showed stable elasticity as the load cycle increased, except the commercial POE 8150, of which

Table 5. Mechanical Properties of the Synthesized CPOEs sample P1 P2 P3 P4 P5 P6 P7 P8 8150

5% secant modulus (MPa) 7.9 26.0 43.1 16.0 0.57 19.2 12.2 2.64 5.8

± ± ± ± ± ± ± ± ±

0.2 0.5 1.2 0.2 0.6 0.3 0.2 0.1 0.3

ultimate elongation (%) 1330 1100 1020 1290 1200 990 620 400 775

± ± ± ± ± ± ± ± ±

30 12 40 15 50 20 100 80 20

ultimate tensile stress (MPa)

recovery after fracturea (%)

± ± ± ± ± ± ± ± ±

83.3 74.8 72.6 80.0

4.3 6.7 8.0 5.3 0.59 11.3 2.5 0.65 16.5

0.1 0.1 0.2 0.1 0.1 0.6 0.3 0.5 0.8

80.4

85.6

a

Test after 24 h.

mer. In spite of this, the tensile property of P8 had no comparison with P2 because of the lack of crystalline long chain branches in P8. The microscopic phase images in Figure 8 revealed that phase separation occurred in all the samples P1−P3 as well as in the solution blending sample P7. For the tandem polymerization products, the in situ generated macromonomers could cocrystallize with the long chain branches in CPOEs, forming finely distributed lamellar crystals in the amorphous rubbery matrix of ethylene/1-octene random copolymer backbones. The cocrystallization could significantly enhance interfacial interactions, as illustrated in Scheme 1. As the concentration of the in situ generated macromonomers

Figure 8. AFM phase images of sample (a) P1, (b) P2, (c) P3, and (d) P7. H

DOI: 10.1021/acs.macromol.8b01711 Macromolecules XXXX, XXX, XXX−XXX

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negative effect on the elastic recovery. The sample with the least content of macromonomer residues possessed the best elasticity.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01711. Characterization of macromonomers and blending samples, estimate of the structural composition of CPOEs, calculation of triad sequence composition, SSA calculation, Soxhlet extraction results (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Figure 9. Evolution of the strain recovery at the applied strain of 300%.

ORCID

the strain recovery exhibited more obvious decline from about 75% after the first cycle to 65% after ten cycles. As the amount of macromonomers increased (as in P1−P3), the elastic recovery reduced. This result could match with the fracture recovery data as in Table 5. The fracture recovery of the CPOE samples (P1−P4 and P6) was comparable with 8150. But by comparison of samples P1, P2, and P3, it could be found that as the macromonomer concentration increased, the fracture recovery experienced some decline. Obviously, the unreacted macromonomer residues had a significant effect on the elastic recovery. Sample P1 having the least residual macromonomers possessed the best elasticity.

Wen-Jun Wang: 0000-0002-9740-2924 Shiping Zhu: 0000-0001-8551-0859 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the financial support from the National Natural Science Foundation of China (21536011, 21420102008, and U1462115). The authors also thank Ms. Li Xu in the analysis center of the State Key Laboratory of Chemical Engineering at Zhejiang university for performing DSC and SSA measurements.

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CONCLUSIONS In conclusion, a novel type of comb-shaped ethylene/1-octene copolymer elastomers (CPOE) having crystalline long chain branches has been synthesized through a tandem polymerization catalyst system of FI-Zr and CGC-Ti. The designed CPOE was composed of crystalline polyethylene (PE) side chains grafted onto an amorphous ethylene/1-octene random copolymer backbone. Linear PE macromonomers, with over 88% chains bearing terminal double bonds, were synthesized with the FI-Zr, while the CGC-Ti was used for the copolymerization of ethylene, 1-octene, and PE macromonomers. More than 20 mol % 1-octene incorporation ensured the amorphous backbone of CPOEs. The average number of long chain branches per backbone chain obtained by this tandem catalyst system could reached nearly five. The resulting CPOEs possessed high melting temperature (>120 °C) and low glass transition temperature (1200%, and some samples had better strain recovery than commercial 8150. The improved elongation at break offers this special CPOE structure a clear advantage over the commercial POE sample. The crystalline long chain branches in CPOEs could form efficient physical cross-linking, which improved the modulus, strength, and toughness. It was demonstrated that the separation of unreacted macromonomer residues was not necessary in achieving the improved modulus, strength, and toughness for CPOEs synthesized by the tandem polymerization in this work. However, the unreacted macromonomers had some

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DOI: 10.1021/acs.macromol.8b01711 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b01711 Macromolecules XXXX, XXX, XXX−XXX