Nanostructured thermosets containing π-conjugated polymer nanophases: Morphology, dielectric and thermal conductive properties

Accepted Manuscript Nanostructured Thermosets Containing π-Conjugated Polymer Nanophases: Morphology, Dielectric and Thermal Conductive Properties Jingang Li, Houluo Cong, Lei Li, Sixun Zheng PII:

S0032-3861(15)30021-5

DOI:

10.1016/j.polymer.2015.05.057

Reference:

JPOL 17895

To appear in:

Polymer

Received Date: 8 April 2015 Revised Date:

22 May 2015

Accepted Date: 31 May 2015

Please cite this article as: Li J, Cong H, Li L, Zheng S, Nanostructured Thermosets Containing πConjugated Polymer Nanophases: Morphology, Dielectric and Thermal Conductive Properties, Polymer (2015), doi: 10.1016/j.polymer.2015.05.057. 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.

ACCEPTED MANUSCRIPT Graphical Abstract Nanostructured Thermosets Containing π-Conjugated Polymer Nanophases: Morphology, Dielectric and Thermal Conductive Properties

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Jingang Li, Houluo Cong, Lei Li and Sixun Zheng*

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ACCEPTED MANUSCRIPT Nanostructured Thermosets Containing π-Conjugated Polymer Nanophases: Morphology, Dielectric and Thermal Conductive Properties Jingang Li, Houluo Cong, Lei Li and Sixun Zheng
Department of Polymer Science and Engineering and the State Key Laboratory of

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Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, P. R.

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China




Corresponding author Email: [email protected] (S. Zheng); Tel: 86-21-54743278; Fax: 86-21-54741297. 1

ACCEPTED MANUSCRIPT ABSTRACT The nanostructured thermosets containing poly(3-hexylthiophene) (P3HT) nanophases were prepared by incorporating poly(ε-caprolactone)-block-poly(3-hexyl thiophene)-block-poly(ε-caprolactone) (PCL-b-P3HT-b-PCL) triblock copolymer into epoxy. The PCL-b-P3HT-b-PCL triblock copolymer was synthesized via the

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combination of the polycondensation of 2-bromo-3-hexyl-5-iodothiophene and the ring-opening polymerization of ε-caprolactone; it was characterized by means of 1H nuclear magnetic resonance spectroscopy (1H NMR), gel permeation chromatography (GPC) and differential scanning calorimetry (DSC). The morphologies of the

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nanostructured thermosets were investigated by means of dynamic mechanical thermal analysis (DMTA), transmission electron microscopy (TEM) and small angle

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X-ray scattering (SAXS). The results of small angle X-ray scattering (SAXS) showed that the P3HT nanophases were formed via self-assembly mechanism of the triblock copolymer in epoxy thermosets. Compared to control epoxy, the nanostructured thermosets containing the conjugated nanophases significantly displayed the enhanced dielectric constants. In the meantime, the thermal conductivity of the nanostructured

nanophases.

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thermosets was also enhanced and increased with increasing the content of P3HT

(Keywords: epoxy; block copolymer; π-conjugated nanophases; dielectric constant;

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thermal conductivity)

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ACCEPTED MANUSCRIPT INTRODUCTION Nanostructured thermosetting polymers have provoked a considerable attention since this class of materials can exhibit versatile properties depending on the morphologies of the thermosets and the properties of the nanophase components. By the use of block copolymers, the nanostructured thermosets can be prepared via either

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self-assembly or reaction-induced microphase separation approach [1-4]. In 1997, Hillmyer and Bates et al [1,2] first reported the preparation of nanostructured epoxy thermosets by the use of poly(ethylene-co-ethyl ethylene)-block-poly(ethylene oxide) (PEEE-b-PEO) diblock copolymer via a self-assembly approach. In their protocol, the

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precursors of epoxy used as the selective solvent of PEEE-b-PEO diblock copolymer and the self-assembly nanophases of PEEE were formed in the mixture of the epoxy

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precursors and the diblock copolymer. The self-assembled morphologies were then locked in via a subsequent curing reaction. Recently, it is found that ordered or disordered

nanostructures

in

thermosets

can

be alternatively formed

via

reaction-induced microphase separation (RIMPS) approach [3,4]. In RIMPS approach, it is not required that the block copolymers are self-assembled into the nanophases before curing reaction. The nanophases are not formed until the curing reaction occurs

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with the sufficiently high conversion of the precursors. In the past decade, there have been a great number of reports on the preparation of the nanostructured thermosets by using a variety of block copolymers via self-assembly or RIMPS approach [5-21]. It is realized that thermosets can be significantly toughened via the formation of the

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nanostructures; the mechanisms of toughness improvement are attributable to the debonding of micelles (or vesicles) from thermosetting matrix, crack deflection or

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frictional interlocking in the thermosets displaying the terraced morphology [22]. In the previous reports, the studies have mainly focused the impact of the nanostructures on the mechanical properties of the thermosets. In fact, the properties of the nanostructured thermosets are additionally influenced by the nature of the functional nanophases dispersed in the thermosetting matrix; the properties of thermosets can be modified and tailored by controlling the functional properties of the component nanophases. For instance, it was reported that dielectric properties of epoxy thermosets can be enhanced by incorporating polyelectrolyte nanophases [23]. However, such an investigation remained largely unexplored. Epoxy polymers have been used as structural materials such as adhesive, 3

ACCEPTED MANUSCRIPT coatings and matrix of composites due to their excellent chemical resistance and mechanical strength [24-26]. In addition, epoxy polymers are a class of major electric encapsulation materials for embedded passives such as resistors, capacitors, and inductors due to their low temperature processability, mechanical flexibility, and compatibility with integrated circuit boards [27-28]. Recently, it has been realized that

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the mechanical properties (i.e., fracture toughness) of epoxy thermosets can be significantly improved via the formation of nanostructures by the use of block copolymers [25,29-33]. However, the investigations on the modulation of other properties of epoxy thermosets via the formation of nanophases by using block

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copolymers remain largely unexplored; there have been few reports on the investigation of the modification of dielectric and thermal conductive properties of epoxy thermosets [23]. Poly(3-alkylthiophene)s (P3AT) are a class of important

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π-conjugated conductive polymers, which can be used as organic field-effect transistors (OFET) [34], organic photovoltaics (OPV) [35] and chemical sensors [36,37] due to their small band gap, high electrical conductivity [38], light-emitting ability [39], high field effect mobility [24,40] and solution processability. The semi-conductive nature of P3AT inspired of exploring to utilize the π-conjugated

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polymers to modify the dielectric and thermal conductive properties of epoxy thermosets. Compared to control epoxy, the dielectric constants of the epoxy thermosets containing P3AT could be enhanced due to: i) the formation of electronic polarization from the P3AT microdomains and ii) the formation of polarization at the

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interface between epoxy and P3AT microdomains. Owing to the rapid polarization process, the electronic polarization is particularly important for the enhancement of

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dielectric constant of the materials at high frequency of the applied electrical field. In addition, the thermal conductivity of epoxy thermosets could be improved with the introduction of P3AT nanophases. It has been found that the π-conjugated P3AT microdomains with specific morphologies could be constituted as the carriers of heat current to facilitate the heat transport in the materials [41,42]. To the best of our knowledge, there has been no previous report in this aspect. In this work, we explored to introduce poly(3-hexylthiophene) into epoxy thermosets via the formation of nanophases, to improve the dielectric and thermal conductive properties of the materials. Toward this end, a poly(ε-caprolactone)-blockpoly(3-hexylthiophene)-block-poly(ε-caprolactone) (PCL-b-P3HT-b-PCL) triblock 4

ACCEPTED MANUSCRIPT copolymer was synthesized and incorporated into epoxy thermosets. The purpose of this work is twofold: i) to explore to incorporate π-conjugated nanophases into epoxy thermosets with an amphiphilic triblock copolymer and ii) to investigate the dielectric behavior and thermal conductivity of the thermosets containing the π-conjugated nanophases. Toward this end, a PCL-b-P3HT-b-PCL triblock copolymer was via

the

combination

2-bromo-3-hexyl-5-iodothiophene

and

of the

the

polycondensation

of

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synthesized

ring-opening

polymerization

of

ε-caprolactone and then the formation of P3HT nanophases in the thermosets was investigated by means of transmission electron microscopy (TEM), small angle X-ray

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scattering (SAXS) and dynamic mechanical thermal analysis (DMTA). The dielectric properties were measured by means of a broad-band dielectric spectroscopy (BDS) and thermal conductivity of the thermosets containing P3HT nanophases was

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addressed on the basis of the nanostructured morphologies of the materials.

EXPERIMENTAL Materials

185-210

was

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Diglycidyl ether of bisphenol A (DGEBA) with epoxide equivalent weight of purchased

from

Shanghai

Resin

Co.,

China.

4,4’-Methylenebis(2-chloroaniline) (MOCA) and N-bromosuccinimide (NBS) were

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purchased from Shanghai Reagent Co., China. ε-Caprolactone (99%) (CL) and the Grignard reagent (i.e., i-PrMgCl) were purchased from Acros Co., Shanghai, China. Before use, CL was dried over calcium hydride at room temperature for 72 hours and

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then distilled under reduced pressure. Stannous octanoate [Sn(Oct)2] was purchased from Aldrich Co. Shanghai, China. Lithium aluminum hydride (LiAlH4), Ni(dppp)Cl2 [dppp=propane-1,3-diylbis(diphenylphosphane)]

were

purchased

from

Energy

Chemical Co., China and stored in inert gases. 3-Hexylthiophene was synthesized in this laboratory by following the method of literature reported by Chaloner et al. [43]. All other reagents and solvents used in this work were purchased from Shanghai Reagent Co., China. Before use, toluene and tetrahydrofuran were refluxed over sodium and then distilled. Synthesis of 2-Bromo-3-hexyl-5-iodothiophene 5

ACCEPTED MANUSCRIPT First, 2-bromo-3-hexylthiophene was synthesized via the reaction of 3-hexylthiophene with N-bromosuccinimide (NBS). To a flask equipped with magnetic stirrer, 3-hexylthiophene (10.000 g, 59.4 mmol) and THF (100 mL) were charged with vigorous stirring and then NBS (10.580 g, 59.4 mmol) was added at 0 oC. The reaction was carried out at 0 oC for 2 hours and then 100 mL of deionized water

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was added. The organic layer was extracted out with diethyl ether and the organic layer was successively washed with 10 % aqueous solutions of Na2S2O3, KOH and deionized water and dried over anhydrous magnesium sulfate (MgSO4). The filtrate was distillated under reduced pressure and the colorless oil liquid (12.730 g) was

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obtained with the yield of 86%. 1H NMR (ppm, CDCl3): 7.18 (d, 1H, -CHCHS-), 6.79 (d, 1H,-CCHCH-), 2.56 (t, 2H,-CCH2CH2-), 1.57 [quint, 2H,-CH2CH2(CH2)3-], 1.31

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[m, 6H, -CH2(CH2)3CH3] and 0.89 [t, 3H,-(CH2)3CH3].

Second, the reaction of 2-bromo-3-hexylthiophene with iodine was used to afford

2-bromo-3-hexyl-5-iodothiophene.

Typically,

to

a

flame-dried

flask,

2-bromo-3-hexylthiophene (10.000 g, 40.4 mmol) dissolved in 90 mL of dichloromethane were charged with vigorous stirring. At 0 oC, iodine (5.680 g, 22.4 mmol) and iodobenzene diacetate (7.860 g, 24.4 mmol) were successively added to

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this flask; the reaction was performed at 0 °C for 5 hours. Thereafter, 30 mL of 10% aqueous solution of sodium thiosulfate (Na2S2O3) was added; the mixture was extracted with 200 mL of diethyl ether. The organic layer was collected and washed with 10% aqueous solution of Na2S2O3 (40 mL×3) and then dried over anhydrous

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MgSO4. After filtration, the solvent and a small amount iodobezene were removed via rotary evaporation under reduced pressure. The residue was purified by passing silica

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gel column with heptane eluent to afford 2-bromo-3-hexyl-5-iodothiophene (12.750 g) with the yield of 85%. 1H NMR (ppm, CDCl3): 6.97 [s, 1H, -CCHCH-], 2.54 [t, 2H, -CCH2CH2-], 1.56 [quint, 2H, -CH2CH2(CH2)3-], 1.32 [m, 6H, -CH2(CH2)3CH3], 0.89 [t, 3H, -(CH2)3CH3].

Synthesis of α-Bromo-terminated Poly(3-hexylthiophene) To

a

flask

containing

120

mL

of

anhydrous

tetrahydrofuran,

2-bromo-3-hexyl-5-iodothiophene (10.000 g, 26.80 mmol) was charged with vigorous stirring. The flask was purged with argon for 30 min at 0 °C and then i-PrMgCl (2.0 6

ACCEPTED MANUSCRIPT M solution in THF, 14.00 mL, 28.00 mmol) was added with a syringe. The mixture was maintained at 0 oC for 30 min and then a suspension of Ni(dppp)Cl2 (436.61 mg, 0.80 mmol) in THF (30.0 mL) was added. The reaction was carried out at 25 oC for 24 hours; 20 mL of HCl (5.0 M) was quickly added to terminate the reaction. The mixture was further stirred for 30 min and then precipitated into a great amount of

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cold methanol. The purple solids (2.850 g) were obtained with the yield of 64%. 1H NMR (ppm, CDCl3): 6.98 (s, 1H, -CCHCH-), 2.80 (t, 2H,-CCH2CH2-), 1.71 [m, 2H, -CH2CH2(CH2)3-], 1.34 [m, 6H, -CH2(CH2)3CH3], 0.91[t, 3H,-(CH2)3CH3]. GPC (PS

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standard, THF): Mn = 4,300 Da with Mw/Mn=1.12.

Synthesis of α,ω-Dihydroxyl-terminated Poly(3-hexylthiophene) the

above

α-bromo-terminated

poly(3-hexylthiophene)

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First,

(denoted

Br-P3HT-H) was reduced into α,ω-dihydro poly(3-hexylthiophene) (denoted H-P3HT-H). Typically, the above Br-P3HT-H (2.200 g, 0.51 mmol) was dissolved in anhydrous 300 mL of THF under N2 atmosphere and then 10 mL of 1.0 M LiAlH4 solution in THF was added. The reaction was performed at room temperature for 2

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hours; the reaction was terminated by adding 2.0 mL of 5.0 M aqueous solution of hydrochloric acid. The mixture was then dropwise added into a great amount of cold methanol to afford the precipitates. The precipitates were re-dissolved with tetrahydrofuran and the as-obtained solution was then dropped into cold methanol to

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afford the precipitates. This procedure was repeated three times to purify this polymer; the polymer (2.000 g) was obtained with the yield of 91%. 1H NMR (ppm, CDCl3):

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6.98 (s, 1H, -CCHCH-), 2.80 (t, 2H,-CCH2CH2-), 1.71 [m, 2H, -CH2CH2(CH2)3-], 1.34 [m, 6H, -CH2(CH2)3CH3],

0.91 [t, 3H,-(CH2)3CH3].

Second, α,ω-diformyl poly(3-hexylthiophene) (denoted HOC-P3HT-COH)

was synthesized from α,ω-dihydro poly(3-hexylthiophene). Typically, H-P3HT-H (1.200 g, 0.28 mmol) was dissolved in anhydrous toluene (300 mL) under nitrogen atmosphere and then N,N-dimethylformamide (5.000 mL, 65 mmol) and phosphorus oxychloride (POCl3) (4.00 mL, 43.40 mmol) were added. The reaction was carried out at 75 °C for 50 hours and the solution was cooled down to room temperature, followed by adding 140 mL of the saturated aqueous solution of sodium acetate. The solution was maintained at this temperature with vigorous stirring for additional 2 7

ACCEPTED MANUSCRIPT hours. After concentration via rotary evaporation, the mixture was dropped into a great amount of cold methanol to afford the precipitates. The solids were dissolved with 150 mL of chloroform and the solution was washed with deionized water three times. Concentrated via rotary evaporation, the solution was then dropped into cold methanol to afford the precipitates. This procedure was repeated three times to purify

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the product. After dried in vacuo at 30 oC for 24 hours, the product (1.060 g) was obtained with the yield of 88.3%. 1H NMR (ppm, CDCl3): 10.01 (s, 1H, -CCHO), 6.98 (s, 1H, -CCHCH-), 2.80 (t, 2H,-CCH2CH2-), 1.71 [m, 2H, -CH2CH2(CH2)3-], 1.34 [m, 6H, -CH2(CH2)3CH3], 0.91 [t, 3H,-(CH2)3CH3].

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Finally, the above α,ω-diformyl poly(3-hexylthiophene) was reduced into α,ω-dihydroxymethyl poly(3-hexylthiophene) (denoted HO-P3HT-OH) with lithium

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aluminum hydride (LiAlH4). To a flask, HOC-P3HT-COH (1.000 g, 0.23 mmol) and 150 mL of anhydrous tetrahydrofuran were charged with vigorous stirring under a dry nitrogen atmosphere and then 4.64 mL of 1.0 M LiAlH4 solution in THF was then added. The reaction was performed at room temperature for 2 hours and then 1.0 mL of 5.0 M aqueous solution of hydrochloric acid was added with vigorous stirring for 2 hours. The mixture was dropped into a great amount of cold methanol to afford the

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precipitates. The precipitates were re-dissolved in tetrahydrofuran and the solution was re-dropped into cold methanol to obtain the solid polymer. This procedure was repeated three times. After dried in vacuo at room temperature for 24 hours, the polymer (0.860 g) (i.e., HO-P3TH-OH) was obtained with the yield of 86 %. 1H NMR

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(ppm, CDCl3): 6.98 (s, 1H, -CCHCH-), 4.78 (d, 2H, -CH2OH), 2.80 (t, 2H,-CCH2CH2-), 1.71 [m, 2H, -CH2CH2(CH2)3-], 1.34 [m, 6H, -CH2(CH2)3CH3], 0.91

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[t, 3H,-(CH2)3CH3].

Synthesis of PCL-b-P3HT-b-PCL Triblock Copolymer Poly(ε-caprolactone)-block-poly(3-hexylthiophene)-block-poly(ε-caprolactone) (denoted PCL-b-P3HT-b-PCL) triblock copolymer was synthesized via the ring-opening

polymerization

α,ω-dihydroxyl-terminated

ε-caprolactone

of

poly(3-hexylthiophene)

(CL)

with

(HO-P3HT-OH)

as

the the

macromolecular initiator. To a flask equipped with a magnetic stirrer, HO-P3HT-OH (0.500 g, 0.116 mmol), CL (2.000 g, 17.54 mmol) and 10 mL of anhydrous toluene 8

ACCEPTED MANUSCRIPT were charged with vigorous stirring. The flask was connected to a Schlenk line and the system was degassed via three pump-freeze-thaw cycles and then Sn(Oct)2 dissolved in toluene [1/1000 (wt) with respect to CL] was added using a syringe. The polymerization was carried out at 110 oC for 24 hours. Cooled to room temperature, the reacted mixture was dropped into cold methanol to afford the precipitates. The

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polymer was re-dissolved with tetrahydrofuran and the solution was re-dropped into cold methanol to obtain the precipitates. This procedure was repeated three times to purify the product. After dried in vacuo at 30 oC for 48 hours, the polymer (2.200 g) was obtained with the yield of 88%. 1H NMR (ppm, CDCl3): 6.98 (s, 1H, -CCHCH-),

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5.21 (d, 2H, -CH2OH), 4.02 ~ 4.10 (m, 2H, -OCCH2CH2CH2CH2CH2O-), 2.80 (t, 2H, -CCH2CH2-), 2.22 ~ 2.32 (m, 2H, -OCCH2CH2CH2CH2CH2O-), 1.50 ~ 1.72 [m, 9H,-OCCH2CH2CH2CH2CH2O-,

-CH2CH2(CH2)3-],

1.20

~

1.50

[m,

8H,

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-OCCH2CH2CH2CH2CH2O-, -CH2CH2(CH2)3-] and 0.91 [t, 3H,-(CH2)3CH3]. GPC (PS standard, THF): Mn = 21,130 with Mw/Mn = 1.20.

Preparation of Nanostructured Thermosets

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Desired amount of PCL-b-P3HT-b-PCL triblock copolymer was added to pre-weighted DGEBA at 100 oC with continuous stirring until the mixtures became homogenous

and

transparent

and

then

the

curing

agent,

4,4’-methylenebis(2-chloroaniline) was added with continuous stirring until the full

hours

plus

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dissolution. The mixtures were poured into Teflon molds and cured at 150 oC for 3 180

o

C

for

2

hours.

The

epoxy

thermosets

containing

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PCL-b-P3HT-b-PCL up to 40 wt % were obtained.

Measurement and Techniques Nuclear Magnetic Resonance Spectroscopy (NMR) The 1H NMR measurement was carried out on a Varian Mercury Plus 400 MHz NMR spectrometer at 25 oC. The samples were dissolved with deuterium chloroform (CDCl3) and the solutions were measured with tetramethylsilane (TMS) as an internal reference.

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ACCEPTED MANUSCRIPT Gel Permeation Chromatography (GPC) The molecular weights of polymers were measured on a Waters 717 Plus autosampler gel permeation chromatography apparatus equipped with Waters RH columns and a Dawn Eos (Wyatt Technology) multiangle laser light scattering detector. The measurements were carried out at 25 oC with tetrahydrofuran as the

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eluent at the rate of 1.0 mL/min.

Small-Angle X-ray Scattering (SAXS)

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The SAXS measurements were taken on a small angle X-ray scattering station (BL16B1) with a long-slit collimation system in Shanghai Synchrotron Radiation

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Facility (SSRF), Shanghai, China, in which the third generation of synchrotron radiation light sources was employed. Two dimensional diffraction patterns were recorded using an image intensified CCD detector. The experiments were carried out with the radiation of X-ray with the wavelength of λ = 1.24 Å at room temperature (25 o

C). For the measurements at elevated temperatures, a TH MS600 Linkam hot stage

with the precision of 0.1 oC was used as an insertion device. Two dimensional

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diffraction patterns were recorded by using an image intensified CCD detector. The intensity profiles were output as the plot of scattering intensity (I) versus scattering

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vector, q = (4π/λ) sin(θ/2) (θ = scattering angle).

Transmission Electron Microscopy (TEM)

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The TEM experiments were conducted on a JEOL JEM-2010 high-resolution

transmission electron microscope at an acceleration voltage of 120 kV. The samples were trimmed using a microtome machine and the sections of the samples with the thickness of c.a. 70 nm were stained with RuO4 to increase the contrast. The stained specimen sections were placed in 200 mesh copper grids for observations.

Differential Scanning Calorimetry (DSC) DSC measurements were performed on a Perkin Elmer Pyris-1 differential scanning calorimeter in a dry nitrogen atmosphere. The instrument was calibrated 10

ACCEPTED MANUSCRIPT with a standard Indium. The samples (about 10.0 mg in weight) were first heated up to 250 °C and held at this temperature for 3 min to eliminate thermal history, followed by quenching to -70 °C. In all the cases, the heating rate of 20 °C/min was used to record the heating thermograms and the cooling rate of -10 °C/min to record the cooling thermograms. Melting temperature (Tm) and crystallization temperature (Tc) minima of exothermic transitions, respectively.

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Thermogravimetric Analysis (TGA)

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were taken as the temperatures at the maxima of endothermic transitions and the

A TA thermogravimetric analyzer (Q-5000) was used to investigate the thermal stability of the triblock copolymer. The samples (about 5.0 mg) were heated

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in nitrogen atmosphere from room temperature to 800 oC at the heating rate of 20 oC × min-1.

Dynamic Mechanical Thermal Analysis (DMTA)

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Dynamic mechanical tests were carried out on a TA Instruments DMA Q800 dynamic mechanical thermal analyzer (DMTA) equipped with a liquid nitrogen apparatus in a single cantilever mode. The frequency used was 1.0 Hz, and the heating rate of 3.0 °C/min was used. The specimen dimension was 25 × 5.0 × 1.75 mm3. The

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experiments were carried out from -80 to 250 oC.

Dielectric Measurements The thermosets were machined into a cylinder with the height of c.a. 1.0 mm

and the diameter of 10.0 mm. Before the dielectric measurements, the surfaces were highly polished and then aluminum foils were pasted on the two surfaces of the cylinders and used as the working electrodes. Permittivity and dielectric loss measurements were performed using an impedance analyzer (Aglient 4294A) with 16451B dielectric test fixture in the frequency range of 103 ~ 107 Hz, applying 0.5V of alternating current voltage across two sides of the cylinder-shaped specimens.

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ACCEPTED MANUSCRIPT Thermal Conductivity Measurement The specimens for thermal conductivity measurements were machined into cylindrical shape with the diameter of 10.3 mm and the thickness of 1.0 mm. First, the thermal diffusivity (α) was measured. The thermal diffusivity coefficient (α) was measured on the LFA 427 (NETZSCH, Germany) according to ASTM E1461 by

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flash method with 0.4 ms of laser pulse width at room temperature. Second, the thermal capacity (Cp) was measured by using differential scanning calorimetry (DSC) (TA Q2000, USA) [44,45]. The absolute values of thermal conductivities (λ) were

(1)

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λ =α ρ Cp

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calculated according to the following equation:

where ρ is the density; Cp the heat capacity of the specimen and α the thermal diffusivity

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RESULTS AND DISCUSSION

Synthesis of PCL-b-P3HT-b-PCL Triblock Copolymer The route of synthesis for PCL-b-P3HT-b-PCL triblock copolymer was shown in Scheme 1. First, an α,ω-dihydroxyl-terminated poly(3-hexylthiophene)

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(HO-P3HT-OH) was synthesized and it was then used as a macromolecular initiator for the ring-opening polymerization of ε-caprolactone (CL) to obtain the ABA

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triblock copolymer. The starting compound for the syntheses was 3-hexylthiophene; it was

brominated

with

N-bromosuccinimide

(NBS)

to

afford

2-bromo-3-hexylthiophene and further iodinated to obtain a polymerizable monomer, i.e., 2-bromo-3-hexyl-5-iodothiophene. In the presence of i-PrMgCl and Ni(dppp)Cl2, the

polycondensation

of

2-bromo-3-hexyl-5-iodothiophene

afforded

α-bromo-terminated poly(3-hexylthiophene) (denoted Br-P3HT-H). The Br-P3HT-H was reduced into α,ω-dihydro poly(3-hexylthiophene) (denoted H-P3HT-H) with lithium aluminum hydride (LiAlH4). The terminal groups of H-P3HT-H were transformed into aldehyde groups by the use of phosphorus oxychloride and sodium acetate, i.e., α,ω-diformyl poly(3-hexylthiophene) (denoted OHC-P3HT-CHO) was 12

ACCEPTED MANUSCRIPT obtained. The formal groups of OHC-P3HT-CHO were finally reduced into hydroxymethyl

groups

with

LiAlH4,

i.e.,

the

macromolecular

initiator,

α,ω-dihydroxymethyl poly(3-hexylthiophene) (HOCH2-P3HT-CH2OH) was obtained. Shown in Figure 1 are the 1H NMR spectra of α-bromo-, α,ω-dihydro, α,ω-diformyl-, α,ω-dihydroxymethyl-terminated poly(3-hexylthiophene)s. For α-bromo-terminated

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poly(3-hexylthiophene), the signals of resonance at 0.91, 1.0~1.5, 1.70 and 2.80 ppm are assignable to the protons of methyl and methylene groups and the peak at 6.98 ppm is attributable to the proton of thiophene ring. Notably, the 1H NMR spectrum of α,ω-dihydro poly(3-hexylthiophene) was almost the same as that of α-bromo-

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poly(3-hexylthiophene). For α,ω-diformyl poly(3-hexylthiophene), there appeared a new peak of resonance at 10.02 ppm assignable to the proton of the terminal formyl

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groups. After the reduction of the formyl groups, the signal of resonance at 10.02 ppm completely disappeared; concurrently, there appeared a new signal of resonance at 4.78 ppm. This signal is assignable to the protons of methylene groups in the terminal hydroxymethyl groups. According to the ratio of the integral intensity of the resonance peak at 4.48 ppm to that at 0.91 ppm, the molecular weight of the α,ω-dihydroxymethyl-terminated poly(3-hexylthiophene) was estimated to be Mn = The

molecular

weight

of

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7,300.

the

α,ω-dihydroxymethyl-terminated

poly(3-hexylthiophene) was measured by means of gel permeation chromatography (GPC) to be Mn = 4,300 with Mw/Mn=1.12. Notably, the value of molecular weight from 1H NMR spectroscopy was much higher than that from GPC. There could be an

EP

underestimation of molecular weight measured with GPC since a polymer with rigid chain (e.g., P3HT) would display a hydrodynamic volume much lower than the

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corresponding PS standard with the identical molecular weight. Also shown in Figure 1 is the 1H NMR spectrum of the PCL-b-P3HT-b-PCL triblock copolymer. It is seen that apart from the signals of resonance assignable to poly(3-hexylthiophene), the peaks of resonance at 1.64, 2.30, 3.64 and 4.06 ppm are assignable to the protons of methylene groups of PCL block. With the occurrence of the ROP, notably, the signal of resonance at 4.78 ppm assignable to the methylene protons of hydroxymethyl groups of α,ω-dihydroxymethyl poly(3-hexylthiophene) was observed to shift to 5.21 ppm. It should be pointed out that the signal of resonance at 3.64 ppm is attributable to the methylene protons of the hydroxymethyl groups of PCL block. According to the ratio of the integral intensity of this peak to other methylene protons of PCL block, 13

ACCEPTED MANUSCRIPT the length of PCL blocks was calculated to be LPCL = 8,600 Da. Therefore, the overall molecular weight of the triblock copolymer was estimated to be Mn = 24,500 Da. This value was comparable to the value measured by means of gel permeation chromatography

(GPC).

Shown

in

Figure

2

are

the

GPC

curves

of

α,ω-dihydroxymethyl poly(3-hexylthiophene) and PCL-b-P3HT-b-PCL triblock

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copolymer. It is seen that the unimodal distribution of molecular weights were exhibited in both of the cases. For PCL-b-P3HT-b-PCL triblock copolymer, the molecular weight was measured to be Mn = 21,200 with Mw/Mn = 1.20. According to the values of molecular weights with 1H NMR spectroscopy, the mass fraction of 1

SC

P3HT block in the triblock copolymer was calculated to be fP3HT ≈ 0.30. The results of H NMR spectroscopy and GPC indicate that the PCL-b-P3HT-b-PCL triblock

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copolymer has been successfully obtained.

The PCL-b-P3HT-b-PCL triblock copolymer was subjected to differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) and the DSC and TGA curves are presented in Figure 3. In the DSC measurement, there were a glass transition at c.a., 178 oC and an endothermic transition at 56.6 oC in the heating scan; there appeared an exothermic peak at 18.5 oC in the cooling scan (See Figure 3A).

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The glass transition was responsible for P3HT block whereas the exothermic (or endothermic) transition in the heating (or cooling) curve resulted from the PCL block. In the heating scan, the endothermic peak is attributable to the melting transition whereas in the cooling scan the exothermic peak is ascribed to the crystallization

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transition of PCL blocks. The fact that in the heating scan the melting transition of PCL block appeared ahead of the glass transition of P3HT block indicates that the

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triblock copolymer was microphase-separated. It is proposed that the P3HT microdomains were dispersed in the matrix of PCL in the triblock copolymer in terms of the mass fraction of the triblock copolymer. In the TGA measurement, the triblock copolymer displayed a two-step degradation profile. The first degradation occurred at c.a. 300 oC whereas the second at c.a. 460 oC (See Figure 3B). The former is attributable to PCL block whereas the latter to P3HT block.

Formation of P3HT Nanophases in Thermosets The PCL-b-P3HT-b-PCL triblock copolymer was incorporated into epoxy to 14

ACCEPTED MANUSCRIPT prepare the nanostructured thermosets. The triblock copolymer was easily dispersed into the precursors of epoxy; all the mixtures composed of DGEBA, MOCA and the triblock copolymer were homogeneous and transparent at room and elevated temperature. The clarity indicates that no macroscopic phase separation occurred. After cured at 150 oC for 3 hours plus 180 oC for 2 hours, the epoxy thermosets with

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the PCL-b-P3HT-b-PCL were obtained. In this work, the content of the triblock copolymer in the thermosets was controlled to be up to 40 wt%. Notably, all the cured products were also homogenous and transparent, suggesting that no macroscopic phase separation occurred after the curing reaction.

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The morphologies of the epoxy thermosets containing PCL-b-P3HT-b-PCL triblock copolymer were investigated by means of transmission electron microscopy

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(TEM). To increase the contrast, the sections of the thermosets were stained with RuO4. In this case, P3HT could be stained whereas epoxy matrix remained almost unaffected. Shown in Figure 4 are the TEM micrographs of the thermosets containing 10, 20, 30 and 40 wt% of PCL-b-P3HT-b-PCL triblock copolymer. Notably, all these thermosets displayed the microphase-separated morphologies. In terms of the difference in electron density, the dark region was attributable to P3HT microdomains

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whereas the matrix (i.e., shallow region) to epoxy matrix. For the thermoset containing 10 wt% of PCL-b-P3HT-b-PCL, the spherical P3HT microdomains with the diameter of 20 ~ 50 nm were formed and dispersed in the continuous matrix (Figure 4A). With increasing the content of PCL-b-P3HT-b-PCL triblock copolymer,

EP

some cylindrical P3HT microdomains with the diameter of 10 ~ 20 nm appeared and their number increased with increasing the content of the triblock copolymer (Figure

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4B through 4D). The TEM result indicates that the nanostructured thermosets involving epoxy and P3HT were obtained, in which the π-conjugated nanophases with spherical and cylindrical shapes were dispersed into the continuous epoxy matrix. The epoxy thermosets containing P3HT nanophases were subjected to small

angle X-ray scattering (SAXS) and the SAXS profiles are shown in Figure 5. Notably, the scattering phenomena were exhibited in all the cases. The intensity of the scattering peaks was significantly increased with increasing the content of PCL-b-P3HT-b-PCL triblock copolymer. While the content of PCL-b-P3HT-b-PCL was 30 wt% or higher, the scattering peaks became increasingly discernible. With increasing the content of PCL-b-P3HT-b-PCL triblock copolymer, the scattering 15

ACCEPTED MANUSCRIPT peaks shifted to the positions with the higher q values. According to Bragg equation, the long periods in the epoxy thermosets containing 30 and 40 wt% of PCL-b-P3HT-b-PCL triblock copolymer were calculated to be L = 48.3 and 41.9 nm, respectively. The decreased long periods implies that the average distance between adjacent

P3HT

microdomains

decreased

with

increasing

the

content

of

with the morphological observation by means of TEM.

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PCL-b-P3HT-b-PCL triblock copolymer. The results of SAXS are in good agreement

By the use of block copolymer, the formation of nanophases in thermosets could follow either self-assembly [1,2] or reaction-induced microphase separation

SC

(RIMPS) [3,4] mechanism depending on the miscibility of the copolymer blocks with the thermosets after and before curing reaction. For self-assembly approach, it is

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required that the block copolymer is self-organized into nanophases in the precursors of the thermosets; the pre-formed self-assembly nanophases were then fixed with the subsequent curing reaction [1,2]. In RIMPS approach, all the copolymer blocks are miscible with the precursors of the thermosets and no self-organized nanophases are formed before the curing reaction. The nanophases are not formed until the curing reaction occurs with the sufficiently high conversion. In the present case, it is of

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interest to investigate the formation mechanism of the P3HT nanophases in epoxy thermosets. Toward this end, the mixtures of the precursors of epoxy (i.e., DGEBA + MOCA) with the triblock copolymer were subjected to small angle X-ray scattering (SAXS) after and before the curing reaction. Representatively shown in Figure 6 are

EP

the SAXS profiles of the epoxy mixture containing 40 wt% of PCL-b-P3HT-b-PCL triblock copolymer at 25 and 150 oC and the thermoset cured at 150 oC for 3 hours. At

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25 oC, a broad scattering peak displayed at c.a. q = 0.18 nm-1, indicating that the mixture was microphase-separated. It is proposed that the mixture of DGEBA and MOCA behaved as the selective solvent of PCL-b-P3HT-b-PCL triblock copolymer and the triblock copolymer self-assembled into the nanophases in the mixture. While the mixture was rapidly heated up to 150 oC (i.e., the curing temperature), nonetheless, the scattering peak did not disappear. This observation suggests that at the beginning of the curing reaction, the mixture still remained microphase-separated. After cured at 150 oC for 3 hours, the scattering peak still existed and its intensity was significantly increased. The increased scattering intensity indicates that the difference in electron density between the micelle and the solvent (i.e., thermosetting matrix) was increased. 16

ACCEPTED MANUSCRIPT In addition, it is noted that the scattering peak shifted to the position with a lower q value (q=0.15 nm-1). This observation could be explained on the basis of the demixing of the epoxy precursors from the swollen P3HT micelles with the occurrence of the curing reaction. The SAXS result indicates that the formation of the nanophases in the epoxy thermosets containing PCL-b-P3HT-b-PCL triblock copolymer followed the

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self-assembly mechanism other than reaction-induced microphase separation. According to the results of TEM and SAXS, it is proposed that in the nanostructured thermosets, the P3HT subchains of the triblock copolymer were demixed out of the epoxy network as the dispersed microdomains whereas the PCL

SC

subchains remained mixed with epoxy networks through inter-component miscibility. This scenario can be further demonstrated by means of dynamic mechanical thermal analysis (DMTA). Show in Figure 7 are the DMTA curves of the nanostructured

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thermosets in the range of -80 ~ 250 oC. For control epoxy, a well-defined relaxation peak (i.e., α transition) was displayed at c.a. 160 oC, which is responsible for the glass-rubber transition of the crosslinked polymer. Apart from the α transition, the thermoset exhibited the secondary transitions (viz. β-relaxation) at -56 oC and 70 oC, respectively. The former is attributed predominantly to the motion of hydroxyl ether

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structural units [-CH2CH(OH)CH2O-] whereas the latter to that of diphenyl groups in amine-crosslinked epoxy [46,47]. Upon adding the PCL-b-P3HT-b-PCL triblock copolymer, the α transitions increasingly shifted to the lower temperatures; the values

EP

of the α transition temperatures (viz. Tg’s) decreased with increasing the content of PCL-b-P3HT-b-PCL triblock copolymer. The decreased Tg’s are accounted for the miscibility of epoxy with PCL block, i.e., the epoxy matrix was plasticized by PCL

AC C

that possessed the Tg as low as -65 oC. Apart from the α peaks assignable to the thermosetting matrices composed of epoxy and PCL, another α peak attributable to P3HT microdomains was also discernable in the DMTA spectra (See Figure 7). For the nanostructured thermosets containing 20 and 30 wt% of PCL-b-P3HT-b-PCL, the α peaks of the P3HT microdomains were detected at c.a. 185 oC. The Tg value was quite close to those reported in literature [48]. While the content of PCL-b-P3HT-b-PCL triblock copolymer was 40 wt%, the α peak was measured to be c.a. 141 oC. The decreased Tg could be associated with the incomplete microphase separation of the epoxy precursors from the P3HT microdomains. The results of SAXS, TEM and DMTA demonstrated that the nanostructured epoxy thermosets 17

ACCEPTED MANUSCRIPT containing nanophases were successfully obtained.

Dielectric Properties It is of interest to investigate the dielectric behavior of the epoxy thermosets

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containing π-conjugated polymer nanophases. The nanostructured thermosets were composed of epoxy, P3HT and PCL. To isolate the effect of P3HT nanophases on the dielectric properties of epoxy thermosets, it is necessary to examine the effect of PCL subchains. In terms of DMTA results, it is known that PCL subchains of the triblock

SC

copolymer were miscible with epoxy at the segmental level. Therefore, we prepared the binary thermosetting blends of epoxy with PCL by controlling the contents of PCL to be identical with the nanostructured thermosets. The binary thermosetting blends

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were subjected to the dielectric measurements. Shown in Figure 8 are the plots of dielectric constant and dielectric loss at 25 oC as functions of AC frequency, respectively. With inclusion of the miscible PCL, notably, both dielectric constants and dielectric loss were slightly decreased. The decreased dielectric constant and loss could be explained on the basis of the inclusion of the diluent (viz. PCL). In other

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words, the concentrations of the dipoles in the binary thermosetting blends were lower than that in the control epoxy and that PCL could act as the inert diluent of the dipoles. In addition, the inclusion of PCL with the concentrations investigated (viz. 8, 16, 24 and 32 wt%) would not significantly change the mobility of the dipoles in the

EP

thermosets at the present experimental temperature (i.e., 25 oC). Shown in Figure 9 are the plots of dielectric constant and loss of the

AC C

nanostructured thermosets at 25 oC as functions of AC frequency. For the control epoxy, the dielectric constant was measured to be 4.61 and 3.89 at the frequencies of 103 and 107 Hz, respectively. The dielectric constant decreased with increasing the frequencies of the applied electric field, indicating that the establishment of effective polar polarization failed to come up with the frequencies of AC field. Therefore, the dipolar polarization dominated the dielectric behavior of this polymer. Compared to the control epoxy, the dielectric constants of the nanostructured thermosets were significantly enhanced and the dielectric constants increased with increasing the content of PCL-b-P3HT-b-PCL triblock copolymer. At the lower frequency of AC field (e.g., 103Hz), the dielectric constant of the thermoset containing 40 wt% of 18

ACCEPTED MANUSCRIPT PCL-b-P3HT-b-PCL was increased to 6.25. The increased dielectric constants were attributable to the incorporation of the π-conjugated conductive P3HT nanophases. It is proposed that there are two types of major polarizations in the thermosets containing conjugated polymer (i.e., P3HT) nanophases: i) the dipolar polarization from all the dipoles in the crosslinked networks and ii) the electronic polarizations

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from the π-conjugated P3HT nanophases. The former resulted from all the polar bonds in the thermosets whereas the latter from the π electrons of the conjugated structures of polythiophene chains. In addition, the formation of P3HT nanophases resulted in the creation of a great amount of the interface between epoxy matrix and

SC

P3HT nanophases. According to Maxwell-Wagner polarization mechanism [45,46], there was a great amount of charge accumulation at the interface between P3HT nanophases and epoxy matrix under an external electrical field. Therefore, the

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interfacial polarization also significantly contributed the enhancement of dielectric constants. At lower frequencies, the charges easily build up at the P3HT-epoxy interfaces and opposed the external electrical field. As a consequence, the effective field acting on the epoxy matrices and P3HT microdomains would reduce its strength. Both dipolar and electronic polarizations all contributed the decrease in the strength of

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the effective field acting on the epoxy matrices. As a consequence, the dielectric constants increased with inclusion of P3HT nanophases. Nonetheless, the times required for the establishment of both of the above polarizations are quite different. Generally, the former requires about 10-9 s whereas the latter about 10-15 ~ 10-13 s

EP

[49-51]. For dipolar polarization, the movement of the charges resulting from dipolar polarization failed to come up with the high frequencies and the development of

AC C

depolarization fields was interrupted and the depolarization field effects of dipolar polarization was significantly reduced at higher frequency (e.g., 107 Hz). As the consequence, the contribution of dipolar polarization to the dielectric constant was decreased. In contrast, the contribution of electronic polarization to the dielectric contribution remained almost unchanged owing to its rapid response rate to AC field, which can be accounted for the observation that at the high AC frequency (e.g., 107 Hz), the nanostructured thermosets still remained the enhanced dielectric constants (Figure 9). In addition, it is seen that at the high frequency (viz. 107Hz) the thermoset containing 20 wt% of PCL-b-P3HT-b-PCL displayed the highest dielectric constant among all the nanostructured thermosets investigated. In the meantime, the dielectric 19

ACCEPTED MANUSCRIPT constant of the thermoset containing 40 wt% of PCL-b-P3HT-b-PCL was lower than that of the thermoset containing 30 wt% of PCL-b-P3HT-b-PCL at this frequency. This observation could be associated with the formation of the cylindrical P3HT nanophases in the thermosets containing 30 and 40 wt% of PCL-b-P3HT-b-PCL (Figures 4C and 4D). On the one hand, the formation of cylindrical nanophases was

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not favorable to promote the interface polarization between epoxy and P3HT since cylindrical nanophases would have much lower specific surface area than spherical nanophases. On the other hand, the decrease in dielectric constant is attributable to the effect of conductance owing to the formation of the cylindrical nanophases. In the

nanowires to decrease the degree of polarization.

SC

nanostructured thermosets, the cylindrical P3HT microdomains could constitute the

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Also from Figure 9, it is seen that the dielectric loss of the control epoxy remained almost invariant regardless of the AC frequency. This observation suggests that the mobility of the dipoles in the crosslinked network was not sufficiently strong to cause the dissipation of energy with increasing the frequency of applied electric field. Upon adding PCL-b-P3HT-b-PCL triblock copolymer to the control epoxy, the dielectric loss was slightly increased at the low frequency (viz. 103 Hz) of the applied

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electric field. The increased dielectric loss is attributable to the increase in the quantity of the mobile dipoles with the incorporation of the triblock copolymer. Besides, the slightly enhanced dielectric loss at the low frequency of AC field was also related to the interfacial polarization relaxation between P3HT microdomains and

EP

epoxy matrices. With increasing the frequency of AC electric field, however, the motion of the incorporated dipoles and the interfacial polarization relaxation

AC C

increasingly failed to keep up with the frequency of the AC electric field. As a result, the dielectric loss was decreased to the level of the control epoxy thermoset.

Thermal Conductivity

It is of interest to investigate the thermal conductivity of the epoxy thermosets containing π-conjugated P3HT nanophases. In this work, the thermal diffusion coefficient (α) and thermal capacity (Cp) were respectively measured and then the thermal conductivity (λ) was calculated according to eq. 1. Shown in Figure 10 are the plots of thermal conductivity (λ) and thermal diffusion (α) as functions of 20

ACCEPTED MANUSCRIPT the content of PCL-b-P3HT-b-PCL triblock copolymer for the nanostructured thermosets. Notably, the values of λ and α for the nanostructured thermosets were significantly higher than those of the control epoxy, respectively; both λ and α values increased with increasing the content of the triblock copolymer. While the content of the PCL-b-P3HT-b-PCL triblock copolymer was 40 wt%, the thermal conductivity

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value was obtained to be 0.27 K/mW, which was much higher than that of the control epoxy by 42%. This result showed that the heat transfer in the nanostructured thermosets was significantly improved with the formation of P3HT nanophases. It has been proposed that the important contribution to heat transfer by the diffusion of

SC

energy could be follow non-propagating vibrational modes [51,52], the anharmonic coupling of localized modes [53] and the ballistic propagation of delocalized modes [54]. For amorphous polymers, the thermal conductivity of is quite dependent on the

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heat carrier propagation length, which can be expressed in the mean free path of thermal photons [41]. In multi-component polymer systems, the thermal conductivity was additionally affected by the morphologies of the materials [55-58]. In the present case, the π-conjugated P3HT nanophases in place of epoxy network will improve the thermal conductance due to the relatively high thermal conductance of P3HT [59]. In

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addition, the thermal conductance of the nanostructured thermosets was quite dependent on the morphologies of P3HT nanophases. It is proposed that the continuous P3HT nanophases such as cylindrical, lamellar and gyroid microdomains will further contribute the improvement of thermal conductance owing to their

EP

continuity. Cola et al [42] have shown that pure polythiophene nanofibers can have a thermal conductivity up to ~ 4.4W/mK, which was more than 20 times higher than the

AC C

bulk polymer value while the polymer still remained amorphous [60,61]. In this case, cylindrical P3HT nanophases were formed for the thermosets containing 30 and 40 wt% of PCL-b-P3HT-b-PCL triblock copolymer. The migration of electrons along with the cylindrical microdomains could facilitate the transport of heat and thus the thermal conductivity of the materials was enhanced. It is proposed that the significant increase in thermal conductivity for the thermoset containing 40 wt% of PCL-b-P3HT-b-PCL triblock copolymer is accounted for the formation of a great number of cylindrical P3HT nanoobjects (See Figure 4).

CONCLUSIONS 21

ACCEPTED MANUSCRIPT Poly(ε-caprolactone)-block-poly(3-hexylthiophene)-block-poly(ε-caprolacto ne) (PCL-b-P3HT-b-PCL) triblock copolymer was successfully synthesized via the combination of the polycondensation of 2-bromo-3-hexyl-5-iodothiophene and the ring-opening polymerization of ε-caprolactone. This triblock copolymer was incorporated into epoxy and the nanostructures of thermosets containing π-conjugated

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P3HT nanophases were investigated by means of transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS) and dynamic mechanical thermal analysis (DMTA). It was found that spherical and cylindrical nanophases of P3HT were formed and dispersed in the continuous epoxy matrix. Compared to control

SC

epoxy, the dielectric constants of the nanostructured epoxy thermosets containing the conjugated nanophases were significantly enhanced. With the incorporation of P3HT nanophases, the thermal conductivity of the epoxy thermosets was also increased with

ACKNOWLEDGMENT

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increasing the content of P3HT nanophases.

The financial supports from Natural Science Foundation of China (No.

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51133003 and 21274091) were gratefully acknowledged. The authors thank the Shanghai Synchrotron Radiation Facility for the support under the projects of Nos. 10sr0260 & 10sr0126. The authors would like to express their gratitude to Professor Jianhua Yang in Shanghai Institute of Ceramics, Chinese Academy of Sciences for the

AC C

EP

help in the measurement of thermal conductivity.

22

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37. Li B, Sauvé G, Iovu MC, Jeffries-El M, Zhang R, Cooper J, Santhanam S, Schultz L, Revelli JC, Kusne AG, Kowalewski T, Snyder JL, Weiss LE, Redder GK, McCullough RD, Lambeth DN. Volatile Organic Compound Detection using

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2’’-terthiophene, 3, 3’’-dimethyl-2, 2’:5’, 2’’-terthiophene and 5’-(2-Thienyl)-2, 2’:3’, 2’’-terthiophene. J Chem Soc Perkin Trans 2 1997;1597.

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Oligosilsesquioxane: 2002;35:8378.

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Semiconducting Polymers. Appl Phys Lett 2013;102:251912.

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Scheme 1 Synthesis of PCL-b-P3HT-b-PCL Triblock Copolymer

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ACCEPTED MANUSCRIPT FIGURE CAPTIONS Figure 1

1

H

NMR

spectra

α,ω-bromo-,

of

α,ω-dihydroxyemthyl-terminated

α,ω-diformyl-

poly(3-hexylthiophene)s

and and

PCL-b-P3HT-b-PCL triblock copolymer; GPC curves of α,ω-dihydroxyemthyl poly(3-hexylthiophene) and PCL-b-P3HT-b-PCL triblock copolymer; Figure 3

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Figure 2

The thermal properties of PCL-b-P3HT-b-PCL triblock copolymer: A) DSC curves of the PCL-b-P3HT-b-PCL triblock copolymer. Up: the

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heating scan at the rate of 20 °C/min after quenching from the melt at 250 °C. Down: the cooling scan at the rate of 10 °C/min; B) TGA curve

Figure 4

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of PCL-b-P3HT-b-PCL triblock copolymer at the rate of 20 °C/min. TEM images of the epoxy thermosets containing: 10 (A), 20 (B), 30 (C) and 40 wt % (D) of PCL-b-P3HT-b-PCL triblock copolymer; Figure 5

SAXS profiles of the epoxy thermosets containing PCL-b-P3HT-b-PCL;

Figure 6

SAXS

profiles

of

the

epoxy

thermosets

containing

40%

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PCL-b-P3HT-b-PCL triblock copolymers: A) at 25 oC; B) at 150 oC; C) after cured at 150 oC for 3 hours plus 180 oC for 2 hours; DMA curves of the epoxy thermosets containing PCL-b-P3HT-b-PCL;

Figure 8

Dielectric constants and dielectric loss tangent of control epoxy and the

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Figure 7

binary thermosetting blends of epoxy and PCL at 25 oC as functions of

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AC frequency; Figure 9

Dielectric constants and dielectric loss tangent of control epoxy and the binary thermosetting blends of epoxy and PCL-b-P3HT-b-PCL at 25 oC as functions of AC frequency;

Figure 10 Thermal diffusivity coefficient and thermal conductivity control epoxy and the binary thermosetting blends of epoxy and PCL-b-P3HT-b-PCL.

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H

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b c O

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Figure 6

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Figure 8

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Figure 9

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PCL-b-P3HT-b-PCL triblock copolymer was synthesized via sequential polymerization Epoxy thermosets containing P3HT nanophases were prepared with the copolymer The thermosets displayed enhanced dielectric constant and thermal conductivity

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