Semicrystalline Polymer Binary-Phase Structure Templated Quasi

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Semicrystalline Polymer Binary-Phase Structure Templated Quasi-block Graft Copolymers Jipeng Guan, Yanyuan Wang, Chenyang Xing, Lijun Ye, Yongjin Li, and Jingye Li J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b05069 • Publication Date (Web): 13 Jul 2017 Downloaded from http://pubs.acs.org on July 15, 2017

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The Journal of Physical Chemistry

Semicrystalline Polymer Binary-Phase Structure Templated Quasi-block Graft Copolymers Jipeng Guan,†,‡, § Yanyuan Wang,† Chenyang Xing,† Lijun Ye,† Yongjin Li*,† Jingye Li‡ † College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, No. 16 Xuelin Rd., Hangzhou 310036, People’s Republic of China ‡ Shanghai Institute of Applied Physics, Chinese Academy of Sciences, No.2019, Jialuo Road, Jiading District, Shanghai 201800, People’s Republic of China § University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China Supporting Information

*e-mail: [email protected] (Yongjin Li) 1

ACS Paragon Plus Environment


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ABSTRACT Herein, we report a simple strategy to synthesize quasi-block graft copolymers using the binary phase structure of semicrystalline polymers as the template. An unsaturated ionic liquid, 1-vinyl-3-butylimidazolium

bis

(trifluoromethylsulfonyl)

imide

([VBIm]

[TFSI]),

is

thermodynamically miscible with poly (vinylidene fluoride-co-hexafluoropropylene) (P(VDFco-HFP)) in solution. The solidification of P(VDF-co-HFP)/[VBIm] [TFSI] blend leads to the expelling of ILs from the crystalline region and the ILs are only located in the amorphous region. The electron-beam irradiation (EBI) at the solid state of the blends results in the locally grafting of the ILs onto the polymer blocks in the amorphous region, while the EBI does not affect the chemical structure of the crystalline region. Therefore, the quasi-block graft copolymers were achieved with IL-grafted blocks segregated by the unmodified blocks. The achieved block copolymers can be microphase separated into the various nanostructures, as the block copolymers with well-defined structure, upon varying the grafting ratios. The microphase separated quasi-block grafted copolymers exhibit excellent mechanical properties and good electrical properties. The elongation at break is 480% and the stress at break is as high as 30 MPa for the sample with the lamellar-like structure having the grafting ratio of 45.4 wt%.

2


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1.INTRODUCTION 1. Block copolymers comprise two or more distinct chemical blocks (homopolymer subunits) that are linked by covalent bonds. Block copolymers have attracted significant attentions due to the diverse properties and potential applications in various fields.1-9 Moreover, block copolymers can microphase separate to form periodic nanostructures, depending on the composition of the blocks and the interaction parameters of the blocks.10-11 Therefore, the microphase separated block copolymers provide a diversity template for the nano science and technology.1-4, 8-9 Block copolymers have been mainly synthesized by the living polymerization technique,12-22 such as anionic,15, 19 cationic,18-19 free-radical13-14, 20 and metal-catalyzed16, 21 polymerizations. Obviously, all these strategies are still not easy and only limited block copolymers are commercially available. The simple and massive synthesis of block copolymers is still a big challenge in both academia and industry. Semicrystalline polymers are generally considered to have binary phase structure in solid state with the crystal lamellae connected by the amorphous regions.23-29 The crystal long period of semicrystalline polymers ranges usually from several to tens nanometers. This length locates just the similar range of microphase structure of block copolymers. In the crystal lamellae region, the polymer molecular chains are either folded or stretched to form ordered structure, while the molecular chains in the amorphous region are basically disordered or random.24-25, 27 A single polymer chain may be partly in a crystalline lamella and partly in the amorphous region.29 Some tie molecules have the molecular chains that start in one lamella, cross the amorphous region, and then join another lamella.23, 28 Obviously, one can define the molecular chains crossing the neighboring lamella and amorphous region to be the crystalline blocks and amorphous blocks, 3



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but with the same chemical structure. However, it is theoretically feasible to prepare block like copolymers by using this template if either crystalline blocks or amorphous blocks were chemically modified (as shown in Scheme 1).

Scheme 1. Schematic illustration of binary-phase structure of semicrystalline polymers templated quasi-block graft copolymers.

Small

molecules

that

have

specific

interactions

with

polymers

are

usually

thermodynamically miscible with the polymers in melt or solution.30-33 The homogenous melt or solution (Scheme 1A) with small molecules could crystallize with the temperature decreasing or the solvent evaporation. If the small molecules cannot cocrystallize with the polymers, the small molecules will be fully expelled out from the crystal lamella and only locate in the amorphous regions (Scheme 1B).34 It is clear that the locally grafting small molecules onto the polymer chains in the amorphous region in the solid state results in the chemically modified blocks covalent bonded with the non-modified polymer blocks originally formed the crystal lamella 4


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(Scheme 1C). Such quasi-block copolymers may exhibit similar microphase behaviors as the well-defined block copolymers if the grafted blocks are immiscible with the neat blocks (Scheme 1D, E). We have previously grafted ionic liquid, 1-vinyl-3-butylimidazolium chloride, onto the polyvinylidene fluoride (PVDF) segments in the amorphous region by electron irradiation.35-37 The block like graft copolymers form spherical nanodomains in the melt due to the microphase separation. Unfortunately, the further investigation about the synthesis of block like copolymer via binary-phase template was terminated for the limited grafting ratios in the PVDF/IL blend systems. We consider that the limited compatibility between the polymer and the ionic liquid restricted the high grafting ratio and only spherical morphology was obtained after the microphase separation. In this work, we improved the basic idea of binary-phase structured template in the purpose of fabricating the quasi-block graft copolymers with various architectures and the microphase separation behaviors of the synthesized block graft copolymers were investigated. Through controlling the electron beam (EB) absorbed doses, various PIL grafting ratios have been obtained in the P(VDF-co-HFP)/[VBIm] [TFSI] blends and various nanostructures were achieved in the block graft copolymer films after microphase separation. These obtained quasiblock graft ionic copolymers exhibited excellent mechanical properties and good electrical properties which have potential applications in energy conversion devices and/or flexible electronic devices.

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2. EXPERIMENTAL SECTIONS 2.1 Materials. The copolymer P (VDF-co-HFP) (Sigma-Aldrich) and an unsaturated ionic liquid (IL), 1vinyl-3-butylimidazolium bis (trifluoromethylsulfonyl) imide ([VBIm] [TFSI]) were used as received. P (VDF-co-HFP) has Mw of 455, 000 and Mn of 110, 000, respectively. The solvents such as dimethylformamide (DMF), acetone and methanol (CH3OH) were commercially available and used without any further puriﬁcation. 2.2 Synthesis of P-b-P(g) Block Graft Copolymers. Firstly, 1.0 g of P(VDF-co-HFP), 4.0 g of IL ([VBIm] [TFSI]) were mixed with 10 g of acetone in a 20 mL reagent bottle at 50 °C for 2h. When the polymer was fully dissolved in acetone and mixed with IL completely, the transparent solution was casted on the aluminum foil and evaporated the solvent at room temperature for 24 h. The P(VDF-co-HFP)/IL (P/IL) blend was obtained after removing the residual solvent in vacuum oven at 40 °C for 12 h. Secondly, the P/IL blend was irradiated with electron beam (EBI), denoted as EB-P/IL blend. Then the irradiated gels were extracted with CH3OH for 48 h at 90 °C to remove the unreacted IL and homopolymerized IL (PIL) which was not anchored onto the P(VDF-co-HFP) chains, denoted as Ex-P-g-PIL blend. The samples were dried in blowing oven at 60 °C for 6 h following. Finally, the grafted copolymer films were obtained with melting the samples at 200 °C and holding for 30 min. In the synthetic process, in order to produce a series of block graft copolymers with different grafting ratios, absorbed doses of 20, 45, 75 and 100 kGy were used. Here, the block graft copolymers were denoted as P-b-P(g)(X) below, where P(g) represents the grafted amorphous block, P corresponds to the unmodified crystalline block and X is the corresponding 6


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grafting ratios (wt %) in the block graft copolymers. The corresponding data were listed in Table 1. Table 1. The grafting reaction of [VBIm] [TFSI] (IL) onto P (VDF-co-HFP) copolymer.

P(VDF-co-

Absorbed

Grafting

Dose

Ratio

(kGy)

(wt%)

[VBIm][TFSI] Samples

HFP) /g /g

P(VDF-co-HFP)

1

0

0

---

P-b-P(10.4)

1

4

20

10.4±1.4

P-b-P(18.3)

1

4

45

18.3±2.1

P-b-P(28.3)

1

4

75

28.3±1.8

P-b-P(45.4)

1

4

100

45.4±3.0

2.3 Characterization. The reaction in the irradiation process was determined by Fourier Transform Infrared Spectroscopy (FTIR, Bruker Tensor) and Nuclear Magnetic Resonance Spectroscopy ( NMR). In FTIR spectra, the transmittance mode was used with a resolution of 4 cm-1. In 1H NMR analysis, a Varian 500 MHz spectrometer at 23 °C was applied with DMF-d7 as the solvent while the chemical shifts were reference to tetramethylsilane (TMS). Gel permeation chromatography (GPC) is performed via two MZ-Gel SDplus 10.0 µm bead-size columns (10E5 and 10E3 Å) and Optilab T-rEX detector. The mobile phase is DMF with chromatographically pure at a flow rate

7



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of 1 mL/min at 35 °C. The system is calibrated with PS standards from 2×103 to 106 g mol-1. The elution diagrams are analyzed via the ASTRA 6 software from Wyatt Technology. The crystallization and melting behaviors of P-b-P(g) block graft copolymers were determined by differential scanning calorimeter (DSC; TA Instruments, Q2000) over a temperature range of -20 to 200 °C of 10 °C/min under a N2 atmosphere. Thermal Gravimetric Analyzer (TGA, TA Instrument, Q500) was applied to investigate the thermostability of samples. The temperature range was from 30 to 800 °C with 20 °C/min under N2 environment. Dynamic thermomechanical analysis (DMA, TA Instrument, Q800) was used to characterize the samples in the tensile mode. All the measurements were performed in the linear region with 0.03% strain. Dynamic loss (tan δ) was determined at a frequency of 5 Hz and a heating rate of 3 °C/min, as a function of temperature from −80 to +200 °C. The crystal forms were determined by wide-angle X-ray diffraction (WAXD, Bruker D8) in the range of 2θ from 5° to 45° with scanning speed of 2 °/min. The electric properties of samples were evaluated by measuring their surface resistivity (Rsurface). An ultrahigh-resistivity meter (MCP-HT450) with a ring URS electrode was used. The nanostructure of the samples was observed directly using a transmission electron microscope (TEM) (Hitachi HT7700) operating at an acceleration voltage of 100 kV. The samples were ultramicrotomed at −120 °C to a section with a thickness of about 80~90nm. The sections were then stained by ruthenium tetroxide (RuO4) for 3 h. Tensile tests were carried out using a tensile testing machine (Instron, Model 5966) at a crosshead speed of 5 mm/min at room temperature and 60~70% relative humidity. The samples were punched out in dumbbell shape. At least three specimens were tested for each sample. The mass grafting ratio (GR, wt%) was calculated with the formula as follows:

8


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GR(wt%) =

(݉ଵ − ݉଴ ) × 100% ݉ଵ

where ݉ଵ is the weight of P-b-P(g) block graft copolymer after extraction with CH3OH; ݉଴ is the weight of P-b-P(g) block graft copolymer. The grafting ratio was also measured by 1HNMR. The GR obtained by the two methods are almost the same, as shown in Table S1.

3. RESULTS and DISCUSSION 3.1 Synthesis of P-b-P(g) Block Graft Copolymers. Poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-co-HFP))

copolymer was

thermodynamically miscible with the unsaturated ionic liquid, 1-vinyl-3-butylimidazolium bis(trifluoromethylsulfonyl) imide ([VBIm] [TFSI]), over the whole composition ratio investigated in this work (as shown in Figure 1A, B and C with the full transparency of the blends). 38-42 The polymer blends were obtained with solution blending of P(VDF-co-HFP) and IL ([VBIm] [TFSI]) with a mass ratio of 1:2 to 1:4. P(VDF-co-HFP) is crystallizable in the blend, as convinced by the melting and crystallization peaks in the DSC curves in Figure 1D. The large amount of IL loadings leads to the very large crystal long period, which is not observable from the small angle X-ray scattering (SAXS) profile (Figure 1E). Clearly, the reactive monomer, IL, locates at the amorphous regions of the copolymer selectively (Scheme 1B).34 Due to the large loading of ILs in the systems, the amorphous region swelled by ILs is dominant in the blends. When the polymer blends are irradiated with electron beam (EBI), the reactive monomers are grafted onto the polymer chains in the amorphous regions (Scheme 1C). Note that the EBI was carried out at room temperature, so the grafting occurs locally and only in 9



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the amorphous region. On the other hand, the EBI dosage was well controlled and EBI shows few damage or influence on the crystalline region. The unmodified blocks in the block graft copolymer are still able to crystallize (as will be discussed in section 3.3). The grafting ratios of ILs onto the P(VDF-co-HFP) can be easily controlled by varying the absorbed doses, ranging from 10.4 wt% to 45.4 wt%, as shown in Table 1.

Figure 1. The photographs of P(VDF-co-HFP)/IL([VBIm][TFSI]) blends with different mass ratios: (A) 1:2, (B) 1:3 and (C) 1:4; (D) DSC curves of P(VDF-co-HFP)/IL([VBIm][TFSI]) blend with 1:4 mass ratios; (E) the SAXS profiles of P(VDF-co-HFP)/IL([VBIm][TFSI]) blend with 1:4 mass ratio.

The 1H NMR spectrum was used to confirm the grafting reaction as shown in Figure 2A. Compared with neat copolymer, some peaks of ILs became broaden after grafted onto the copolymer, such as the peaks at 0.70-0.85 ppm (peak i), 1.09-1.29 ppm (peak h), 1.53-1.75 ppm (peak (a+g)), 3.85-4.06 ppm (peak (b+f)), 7.05-9.68 ppm (peak (d+c)) and 8.75-9.46 ppm (peak e) in 1H NMR spectrum in Figure 2A.43-46 The broaden of IL signals may be ascribed to the 10


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retardation of mobility after grafting reaction. The detail chemical shift values were listed in Table S2. On the other hand, the

19

F NMR spectrum was also applied to study the grafting

reaction in Figure 2B. The signals assigned to TFSI anion are centered at -78.0 to -78.9 ppm for CF3 group (-SO2CF3). The results verified that ILs have been successfully grafted onto the P(VDF-co-HFP) segments in the amorphous regions and block graft copolymers were achieved. Figure 2C shows the GPC curves of the precursor polymer and the final block graft copolymers. After EBI, the GPC traces of block graft copolymers shift to the lower retention time which may be induced by the decrease of elution volume after grafting of ILs in the precursor polymer. However, the fact that the value of Mw/Mn did not change obviously indicates that the electron beam has no obvious influence on the structure of polymer main chains (Table S3). Besides, some random chain cleavage may exist in the block graft copolymers. Note that the value of Mw/Mn of neat copolymer measured by out GPC analysis is different from the value provided by the supplier. This can be attributed to the different test method in the GPC measurement. In the FTIR analysis of block graft copolymers, the characteristic peaks of imidazole group were also observed in Figure S1.

11



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(A) a j' j, j'

j

b a

P-b-P(g)(18.3)

c

e

bm

c

N

d

N

e [TFSI -] j'

d+c g

i h

9.5 9.0 8.5 8.0 7.5 7.0

i

10

8

j

f

6

a+g

b+f

4

h

2

δH ppm

(B)

P-b-P(g)(28.3) P(VDF-co-HFP) [VBIm][TFSI]

-70

-80

-90

-100

-110

-120

δF ppm

(C)

P(VDF-co-HFP) P-b-P(g)(18.3) P-b-P(g)(28.3)

16

18

20

22

24

26

28

Retention Time(min) Figure 2. (A) 1H NMR spectra of neat P (VDF-co-HFP), IL ([VBIm] [TFSI]) and P-b-P(g)(18.3) block graft copolymers; (B)

19

F NMR spectra of ionic liquid ([VBIm][TFSI]), neat P(VDF-co-HFP) and P-b-P(g)(18.3)

block graft copolymers; (C) the GPC traces of P(VDF-co-HFP), P-b-P(g)(18.3) and P-b-P(g)(28.3) block graft copolymers.

12


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3.2 Nanostructured Morphologies of Quasi-Block Graft Copolymers after the Microphase Separation Structure well-defined block copolymers can usually microphase separate into various types of nanostructures, depending on the interaction parameters of the blocks and the volume fractions of each block. 9-11 The microphase separation behaviors of the EBI synthesized block graft copolymers were also investigated. After extraction of all the unreacted IL monomers and the homopolymerized ILs (PIL), the quasi-block graft copolymers were heated to above melting temperature (200oC) and hold at the temperature for 30 min. The samples were then cooled down to room temperature. The diverse nanoscale morphologies were observed depending on the grafting ratios in Figure 3. All samples were stained with RuO4 and the dark regions in the TEM images correspond to the IL-rich regions where the ILs pendant group of grafted segments can be easily stained. Typical spherical nanodomains are well dispersed in the polymer matrix at the grafting ratio of 10.4 wt% (Figure 3A). With increasing the absorbed doses, more ILs were grafted onto the polymer chain segments in the amorphous regions and the grafting density of ILs increased gradually as shown in Scheme 2B. Thus, the volume fraction of P(g) block was enhanced and more grafted segments would aggregate together to form the nanodomains. As shown in Figure 3B, more nanodomains are formed with 18.3 wt% grafting ratios. Continuing to increase the grafting ratios, more nanodomains can be formed which may leads to the connection of neighboring nanodomains and formation of larger domains (Scheme 2D). When the grafting ratio rises to 28.3 wt%, the size of nanodomains increased from 15.2±2.5 nm to 26.3±3.2 nm and a gyroid like structure was obtained in Figure 3C. The sample microphase separates into the lamellar-like morphology for the grafting ratios of 45.4 wt% (Figure 3D). The lamellar long period is about 40-45 nm. These morphological results are basically consistent with the 13



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morphological phase diagram reported in the diblock copolymers from spherical to the lamellar structures with increasing the volume fraction of one block.10-11 The all microphase separated samples are transparent due to the nanostructured morphology, as shown in Figure S3. In order to study the effect of block-like structure further in phase separation process, the fully random graft copolymer was prepared through irradiating P(VDF-co-HFP)/IL blend with electron beam at melting state. The same post treatment was carried out and the morphologies were investigated by TEM measurement. In Figure 3E, the random graft copolymer with 22.5 wt% grafting ratio phase-separated into domains with wide size distribution from 20 nm to 200 nm. In contrast, the block graft copolymers with similar grafting ratios can separate into homogenous nanodomains with 26.3±3.2 nm in Figure 3B. This means that only the quasi-block structure leads to the formation of uniform nanostructured morphology. As depicted in Scheme 2, the high EBI dosage leads to the large grafting ratio, which in turn results in the interconnection of the IL-grafted molecular chains. Therefore, the various morphologies from the spherical-like structure to the lamellar-like structure were achieved after the melting of the irradiated sample in the melt.

14


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Figure 3. TEM images of P-b-P(g) with various grafting ratios (wt%): (A) P-b-P(g)(10.4); (B) P-b-P(g)(18.3); (C) P-b-P(g)(28.3); (D) P-b-P(g)(45.4); (E) TEM image of random graft copolymer obtained at melt state irradiation ( the IL grafting ratio is 22.5 wt%).

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Scheme 2. The schematic illustration of formation of various nanostructure morphologies with various grafting ratios.

To provide deeper insight into the nanostructure of the synthesized block graft copolymers after microphase separation, the morphologies were further examined by small-angle X-ray scattering (SAXS) at room temperature, as shown in Figure 4. The left column in Figure 4 shows the 2-D SAXS pattern with the large scattering vector range, the middle column enlarges the inner scattering at the low scattering vector range and the right column shows the 1-D SAXS profiles of (intensity) Iq2–q curves of samples along the direction of arches (4D and 4E). As expected, the 2-D SAXS patterns of neat copolymer is featureless and only one isotropic circular ring is observed which corresponds to the crystal lamellar morphology without any orientation 16


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(Figure 4A and A1). However, the microphase separated block graft copolymers show two distinct scattering rings. The scattering at the low q value corresponds to the correlation between the microphase separated phase in the samples (Figure 4B1-C1), while the scattering at the higher q value corresponds to the correlation of the crystal lamellar structure of the polymer matrix. In P-b-P(g) (10.4) sample with low grafting ratio, IL-grafted segments forms spherical nanodomains dispersed homogenously in the polymer matrix without any orientations (Figure 4B). The fact that both the inner ring and outer ring are isotropic indicates the random orientation of both spherical nanodomains and the crystal lamellae of P(VDF-co-HFP) matrix. However, for the sample having the lamellar structure with high grafting ratio, both the inner scattering and outer scattering show the sharp arches, indicating the alignment of both the microphase separated layer structures and the crystal lamella structure (Figure 4C and C1). Moreover, it is clear that the arches from the two types of structure are vertical to each other, which means that crystals in the matrix are aligned vertical to the ion-rich layers. We consider that such orientation of the lamellar structure originates from the thin film nature during sample preparation. The thin films were used for the EBI and subsequently melt phase separation, so the layer oriented structure was obtained. The detail analysis is still underway and will report in the future. It should also be noted that the peak position at the q=0.15nm-1 for the P-b-P(g) (45.4) corresponds to the layer structure of 41.9 nm, which is consistent with the TEM results (Figure 3D). Moreover, different from the most SAXS patterns of block copolymers with well-defined structure, only one strong scattering peak was observed for the block graft copolymer. This is attributed to the irregularity of the synthesized block graft copolymers. It should be again noted that the quasi-block graft copolymers is different from the welldefined block-copolymers in terms of the chemical structure and the microphase separation 17



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behaviors. Semicrystalline polymers itself showed very broad SAXS peak (Fig. 4A) due to variations of lamellae and amorphous layers thickness variations. The grafting of ILs occurs selectively in the amorphous regions. Complex chain trajectory of polymer chains in semicrystalline polymers induces obviously heterogeneous grafting densities along individual chains and the achieved block copolymers show only irregular molecular chain structure. Graft length and grafted block number are varied dependent on the chain trajectory of the crystallized polymers, as shown in Scheme 1. Therefore, well defined nanostructured morphologies have not been achieved.

Figure 4. The 2-dimensional (2-D) SAXS patterns of samples: (A) and (A1) neat P(VDF-co-HFP), (B) and (B1) P-b-P(g)(10.4), (C) and (C1) P-b-P(g)(45.4); the 1-D SAXS profiles of (intensity) Iq2–q curves of samples along the direction of arches in Figure 4D and 4E.

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3.3 Crystallization Behaviors of Quasi-Block Graft Copolymers after Microphase Separation P(VDF-co-HFP) copolymer is a polymorphism copolymer and it exhibits several crystal phases, such as α-phase, β-phase and γ-phase.34-37 Figure 5A shows the XRD of neat P(VDF-coHFP) and block graft copolymers with various grafting ratios after microphase separation. In the neat copolymer, three characteristic diffraction peaks were observed at 18.4°, 19.9°, 26.6°, which are assigned to the (020), (110) and (021) reflections of nonpolar α crystal phase (Fig. 5A).34-37 When ILs were grafted onto the polymer chains, the characteristic peaks of nonpolar α crystal phase did not change obviously. This mean that the grafting of ILs on the polymer chains does not influence on the crystal forms in the crystalline region. Note that a very broad weak halo was observed in the XRD patterns at 11.9° for the sample with large IL grafting ratio. This is corresponded to the ionic liquids. The same characteristic peak was also observed in the XRD patterns of ionic liquid (Figure S2). In order to analysis crystal forms in detail, the FTIR measurements were carried out for the samples in Figure 5B. The characteristic peaks at 764 cm1

and 976 cm-1 are the characteristic absorptions of nonpolar α crystal phase which were

observed in all the samples. After grafting of ILs, the intensity of absorption peak at 840 cm-1 became stronger compared with neat copolymer which corresponds to the polar γ (or β) phase. It is ascribed to the inductive effect of ILs in the polymer matrix.34 However, the inductive effect to polar crystal forms is very limited for the microphase separated samples due to the all grafted ILs was restricted in the ionic domains. Therefore, in all the block graft copolymers, the nonpolar α crystal phase was the dominant crystal form, as shown in both XRD and FTIR results. 19



(020)a, (020)γ

(A)

(110)α, (110)γ

Intensity(a.u.)

IL

(021)α (e) (d) (c) (b) (a)

(100)α

8

16

24

32

2Theta/degree

(B) Absorbance(a.u.)

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

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764

796

840

IL (e) (d) (c) (b) (a)

700 720 740 760 780 800 820 840 860 880 900

wavenumber/cm

-1

Figure 5. XRD and FTIR patterns: (a) neat P(VDF-co-HFP), (b) P-b-P(g)(10.4), (c) P-b-P(g)(18.3), (d) P-bP(g)(28.3) and (e) P-b-P(g)(45.4).

The melting and crystallization behaviors of neat copolymer and block graft copolymers after microphase separation were investigated by DSC analysis in Figure 6. In neat copolymer, the melt-crystallization temperature (Tc) is 111.8 °C. For the P-b-P(g) (10.4) copolymer, the Tc decreased from 111.8 °C to 106.2 °C. With increasing the ILs grafting ratios, the Tc become 20


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lower and the crystalline peaks become wider. This can be ascribed to the grafted ionic liquids which restricted the movement of polymer chains for the electrostatic interaction and suppressed the crystallization in the crystalline process. Meanwhile, two peaks are observed in the cooling curves of grafting copolymers. The different crystalline behaviors may be related to the selective grafting of ILs in the semicrystalline polymer under electron beam irradiation. In the cooling process, the crystallization of IL-grafted segments was suppressed for the electrostatic interaction between ILs pendant groups. In contrast to segments unmodified with ILs, they would exhibit lower Tc as shown in Figure 6A. The melting behaviors of samples were also displayed in Figure 6B. The melting temperature (Tm) of the neat copolymer is 139.5 °C. After grafting of ILs, the Tm decreased slightly from 139.5 to 137.2 °C in P-b-P(g) (45.4). The melting peaks of block graft copolymers also become wider. It can also be attributed to the existence of ILs pendant group which suppressed the crystallization of P(VDF-co-HFP). Note that the melting enthalpy of samples did not change obviously as shown in Table S4. It means that although the ILs grafted on the polymer backbone suppressed the crystallization of P(VDF-co-HFP), the grafting density of block graft copolymers has no influence on the degree of crystallinity distinctly.46,47

21


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Cooling

(A)

Exo

Neat P(VDF-co-HFP) P-b-P(g) (10.4) P-b-P(g) (18.3) P-b-P(g) (28.3) P-b-P(g) (45.4)

64

80

96

112

128

144

Temperature/ °C

(B)

Heating


Exo

Heating flow(a.u.)

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

Heating flow(a.u.)


0

25

50

75

100 125 150 175

Temperature/ °C Figure 6. Differential scanning calorimeter (DSC) curves of neat P(VDF-co-HFP) and P-b-P(g) block graft copolymers: (A) first cooling, (B) second heating.

3.4 Thermal Stability of P-b-P(g) Block Graft Copolymers after Microphase Separation The thermal stability of block graft copolymers were examined by TGA analysis in Figure 7. In neat copolymer, the initial degradation temperature is 407.8 °C. After grafting of ILs, the initial degradation temperature (Td) induced to 330.2 °C in P-b-P(g) (10.4) copolymer. With increasing ILs grafting ratios, the initial degradation temperature (Td) become lower. When the 22


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grafting ratio is 45.4 wt%, the Td is 301.9 °C which still exhibited excellent thermal stability. In the DTG curves (Figure 7B), only one degradation peak is observed in the neat copolymer. However, after grafting of ILs, two distinct degradation peaks are observed. The peak at lower temperature corresponded to the degradation of grafted ILs. The peak at higher temperature can be attributed to the degradation of polymer backbone which is adjacent to the Td of neat copolymer. In addition, the degradation temperature related to grafting ILs become lower with increasing the grafting ratios and the intensity of corresponding peak was strengthened.

Weight/%

100

P(VDF-co-HFP) P-b-P(g) (10.4) P-b-P(g) (18.3) P-b-P(g) (28.3) P-b-P(g) (45.4)

(A)

80

60

40

20 100

200

300

400

500

600

700

800

Temperature/°C Deriv.Weight (%/°C)

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


200

(B)

150 P(VDF-co-HFP) P-b-P(g) (10.4) P-b-P(g) (18.3) P-b-P(g) (28.3) P-b-P(g) (45.4)

100

50

0 250

300

350

400

450

500

550

Temperature/°C Figure 7. TGA and DTG analysis of P-g-PIL graft copolymers with various grafting ratios.

23



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3.5 Mechanical and Electrical Properties of Block Graft Copolymers We have successfully fabricated the irregular block graft copolymers containing ions by using the binary-phase structures of semicrystalline polymers as the template and ionic liquid as the grafting monomer. The nanostructures from spherical-like to lamellar-like morphologies were also realized through varying the grafting ratios because of the ionic liquid grafted block segregating from the unmodified block. This is very important for designing the ion containing copolymers as the organic electronic devices, gas permeation membranes and solid-state polymer electrolyte in batteries and fuel cells.44, 46-57 To achieve both excellent mechanical properties and high ionic conductivities, significant efforts

46, 51, 52, 55, 58

have been paid to synthesize the ion

containing copolymers and various nanostructured morphologies have also been obtained with strong microphase separation between neutral and ionic compositions of block or graft copolymers. However, all of these strategies were extremely complicate. In the present work, the block graft copolymers with ions can be easily and massively prepared. We are therefore very interested in the physical performance of the synthesized block graft copolymers. Figure 8A shows the strain-stress curves of the samples. It is clear that all the samples have excellent mechanical properties with not only high strength, but also excellent ductility. The yield strength is higher than 15 MPa and the broken strength is higher than 20 MPa for the all samples. With increasing grafting ratios, the elongation at break decreases due to the relatively high glass transition temperature of the ion-rich phases, but the elongation at break for the P-b-P(g)(45.4) sample is still as high as 480%. Figure 8B shows the dc electric resistivity of neat P(VDF-coHFP) and microphase separated P-b-P(g) block graft copolymers. Although the microphase separation confined the ions in the nanophases, the electrical conductivity still increases with the gradually connection of the nanoconductive phases. The Rsurface of neat copolymer films is higher 24


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than 1012 Ω/□. The P-b-P(g)(45.4) sample with the lamellar nanostructure have the surface resistivity of about 1.2×1010 Ω/□. Obviously the lamellar-like structure may form the ionic movement channels in the sample, so the higher conductivity was achieved. It should be mentioned that such conductivity could be further enhanced if the free ions were incorporated into the lamellar channels. Considered the stretchability and the conductivity of the microphase separated materials, the materials might be used as the flexible solid electrolytes, which will be our future research work. Figures 8C and D show the storage modulus and the tan delta of all the samples as a function of temperature. Obviously, all the samples show almost the same storage modulus at each temperature over the whole temperature range. On the other hand, all the microphase separated samples show two relaxation peaks. The relaxation at -32.5 oC corresponds to the glass transition temperature of P(VDF-co-HFP) phase and the relaxation at about 60 oC corresponds to the chain relaxation of the grafted chains in the ion rich phase.44 With increasing the grafting ratios, the intensity of the relaxation peak at about 60 °C enhances due to the increased phase volume. Additionally, the P-b-P(g) block graft copolymers maintained the decent transparency in comparison with neat copolymers (Figure S2). The relaxation peaks indicates again the heterogeneous structure (microphase separated) of the quasi-block graft copolymers.

25




30

Ω/ ) Rsurface(Ω/

Stress/MPa

(B)

(A)

40

20

10

12

10

11

10

0 0

200

400

600

800

1000

0

10

Strain(%) 1000

0.30

(C)

30

40

50

(D) P(VDF-co-HFP) P-b-P(g) (10.4) P-b-P(g) (18.3) P-b-P(g) (28.3) P-b-P(g) (45.4)

Tanδ

100 0.20

10

20

PIL(wt%)

0.25

E'/MPa

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

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0.15 1

P(VDF-co-HFP) P-b-P(g) (10.4) P-b-P(g) (18.3) P-b-P(g) (28.3) P-b-P(g) (45.4)

0.10

0.1

0.05 -60 -40 -20

0

20 40

60 80 100 120 140

-60

-30

Temperature/ °C

0

30

60

90

Temperature/ °C

Figure 8. (A) Tensile test and (B) dc conductivity of P-b-P(g) block graft copolymers; Dynamic thermomechanical analysis (DMA): (C) the storage modulus and (D) the loss tangent as a function of temperature with frequency of 5Hz for the block graft copolymers.

4. CONCLUSIONS In summary, the quasi-block graft copolymers have been successfully synthesized by using binary-phase structured template and various nanostructure morphologies from spherical-like to lamellar-like structures were realized by altering the grafting ratios for the first time. The fabrication strategy is very simple and can be used for massive production of the block graft 26


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copolymers. Meanwhile, we succeeded in preparing the nanostructured ion containing block graft copolymers and they exhibited excellent mechanical property and high conductivity in comparison with neat copolymer. These materials have a great potential application for organic electronic devices, gas permeation membranes and solid-state polymer electrolyte in batteries and fuel cells. This facile strategy paves a novel and effective way to synthesize quasi-block graft copolymers and the same strategy would be also applied for other polymer/monomer blend systems.

ASSOCIATED CONTENT Supporting Information. Figure S1, S2 and S3, Table S1, S2, S3 and S4. This material is available free of charge via the Internet at http://pubs.acs.org. All experimental procedures and additional spectral data (PDF). AUTHOR INFORMATION Corresponding Author * E-mail: (Y.L.) [email protected]. Fax: +86 57128867899. Telephone: +86 57128867026. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (21674033, 21374027) and Program for New Century Excellent Talents in University (NCET27



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13-0762). NMR measurements were supported by professor Yingfeng Tu at Soochow University of China.

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(57) Hoarfrost, M. L.; Tyagi, M. S.; Segalman, R. A.; Reimer, J. A. Effect of Confinement on Proton Transport Mechanisms in Block Copolymer/Ionic Liquid Membranes. Macromolecules 2012, 45, 3112-3120. (58) Ciftci, M.; Arslan, M.; Buchmeiser, M.; Yagci, Y. Polyethylene-g-Polystyrene Copolymers by Combination of ROMP, Mn2(CO)10-Assisted TEMPO Substitution and NMRP. ACS Macro Lett. 2016, 5, 946-949.

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Semicrystalline Polymer Binary-Phase Structure Templated Quasi

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