Photoluminescence Control of Cellulose via Surface Functionalization

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Article

Photoluminescence control of cellulose via surface functionalization using oxidative polymerization Thien An Phung Hai, and Ryuichi Sugimoto Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.7b01067 • Publication Date (Web): 28 Sep 2017 Downloaded from http://pubs.acs.org on October 4, 2017

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Photoluminescence control of cellulose via surface functionalization using oxidative polymerization

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Thien An Phung Hai, Ryuichi Sugimoto*

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School of Environmental Science and Engineering, Kochi University of Technology,

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Miyanokuchi, Tosayamada, Kami, Kochi 782-8502, Japan

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Correspondence to: Ryuichi Sugimoto, School of Environmental Science and Engineering,

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Kochi University of Technology, Miyanokuchi, Tosayamada, Kami, Kochi 782-8502, Japan

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TEL: +81-877-57-2516, Fax: +81-887-57-2520, E-mail: [email protected];

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[email protected] or [email protected]

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Abstract.

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Control of the photoluminescence properties of cellulose is conducted by introduction of

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conducting polymers including fluorene (F) and 3-hexylthiophene (3HT) on cellulose surface

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through FeCl3 oxidative polymerization. The UV-Vis absorption peak of cellulose grafted with

14

the 3-hexylthiophene and fluorene copolymer was increasingly blue-shifted with increasing

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fluorene content and the shift in the peak position in photoluminescence spectra depend on the

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initial 3HT:F ratio of the copolymer. The crystallinity and thermal stability of cellulose

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decreased slightly upon graft polymerization with PF and P3HT, while the quantum yield,

18

determined using absolute methods, increased from 3.1 to 9.7% with increasing fluorene content.

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The roles of the 3HT and F copolymers in improving the properties of cellulose were thoroughly

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studied by FT-IR, UV-Vis, fluorescence, X-ray diffraction (XRD), thermo gravimetric (TG),

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transmission electron microscopy - energy dispersive X-ray (TEM-EDX), and quantum yield

22

measurements. Mechanistic insight into the grafting reaction is also provided.

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1. Introduction.

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Cellulose is a biopolymer and a polysaccharide consisting of long chains with many thousands of

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β-1,4- glucose connections.1,2 Since it is a sustainable and biodegradable resource, cellulose and

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its derivative have great potential in many applications such as pharmaceuticals, fibers, paper,

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paints and textiles.3,4 To enhance the physical and chemical properties of cellulose and improve

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its functionality, cellulose surface modifications have been researched for more than two * Corresponding author. Email: [email protected] (Ryuichi Sugimoto) ACS Paragon Plus Environment

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decades. Graft copolymerization is a typical technique for cellulose modification. This technique

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has been classified into three approaches: the grafting to, grafting from, and grafting through

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processes.2,3 Moreover, there are three main methods for the grafting of vinyl monomers onto

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cellulose and its derivatives3: (1) free radical polymerization5-8 , (2) ionic and ring opening

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polymerization9-14, and (3) living radical polymerization15-21. Polymer-grafted celluloses have

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attracted considerable attention in the chemical field due to their potential applications in

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antimicrobial surfaces,22 smart membrane materials,4,23 targeted drug delivery,24 DNA

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hybridization detection,25 heavy metal absorption,26 and so forth.

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Conjugated polymers are of growing importance due to their properties that can be applied in a

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wide range of areas such as organic solar cells,27-31 organic transistors,32,33 polymer light-emitting

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diodes,34 chemical sensors,35 non-linear optics, and energy storage.36

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Recently, advances in cooperation between the conjugated polymer and biomaterials have driven

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new interest in engendering materials with functionalities, further opening the scope of

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applications. A polythiophene-cellulose composite has been synthesized by a two-step reaction

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in which the thiophene monomer was oxidatively copolymerized with oligothiophene-substituted

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cellulose derivatives.37 The fabrication of a thin oriented film of n-butylcinnamoylcellulose and

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polythiophene was shown to concurrently improve conductivity and mechanical properties.38

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Composites of cellulose and different conducting polymers (polypyrrole and polyaniline) that

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utilize the interplay of the two materials have been produced.39,40 However, the main purpose of

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this work was the synthesis of the composite materials. The goal of our research is to graft

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conjugated polymers such as poly(3-hexylthiophene) directly onto cellulose.

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The oxidative polymerization of thiophene was discovered three decades ago and is now widely

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used due to its mild reaction conditions.41-47 Herein, we report new methodology for surface

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modification of cellulose by directly grafting conjugated polymers to cellulose by oxidative

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polymerization. In this report, P3HT and PF are used as the conjugated polymers due to their

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unique properties such as high solubilities, high emission efficiencies over a wide variety of

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colors, low operating voltages,48 enhancement of organic light-emitting diode (OLED)

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performance,49 high photoluminescence quantum efficiencies, and thermal stabilities.50,51 The

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resulting grafted celluloses were characterized by Fourier-transform infra-red (FT-IR), UV-Vis,

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Fluorescence, 1H NMR spectroscopies, quantum yield as well as X-ray diffraction (XRD), 2 ACS Paragon Plus Environment

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transmission electron microscopy energy dispersive X-ray (TEM-EDX),

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chromatography (GPC) and thermogravimetric (TG) analyses.

gel permeation

61 62

2. Experimental

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2.1 Materials

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Cellulose powder was purchased from Wako Pure Chemical Industries Ltd. Cellulose paper was

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purchased from Toyo Roshi Kaisha, Ltd. 3-Hexylthiophene (3HT), fluorene (F) monomer and

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anhydrous FeCl3 were obtained from Tokyo Chemical Industry Co. Ltd. and used without further

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purification. Analytical grade solvents such as chloroform (containing 150 ppm amylene as a

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stabilizer) and methanol were purchased from Wako Pure Chemical Industries Ltd and used as

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received. Chloroform was dried over a 4 Å molecular sieves for 8 h, and purged with argon gas

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for 20 min before use.

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2.2 Equipment

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UV-Vis spectra were obtained by diffuse reflectance measurements with a Jasco V-650 UV-Vis

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spectrometer, set in absorbance photometric mode, with a UV-Vis bandwidth of 2.0 nm, data

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interval of 1.0 nm, and a scan rate of 400 nm min-1. Thermogravimetric (TG) analyses were

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conducted in air on a Hitachi STA 7200 RV thermal analysis system, from 20 to 700 °C at a flow

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rate of 25 ml min-1, and a heating rate of 10 °C min-1. 1H NMR spectra (400 MHz) and IR

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spectra were recorded on a Bruker Ascend 400 NMR spectrometer and a Jasco 480 Plus FT-IR

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spectrometer, respectively. X-ray diffraction patterns were recorded with Cu-Kα radiation (X-ray

79

wavelength: 1.5418 Å) in steps of 0.02° over the 10–70° 2θ range on a Rigaku Smartlab

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diffractometer equipped with a D-tex detector. Transmission electron microscopy (TEM) images

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were acquired with a JEOL JEM-2100F microscope. Energy dispersive X-ray (EDX) maps and

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line scan spectra were recorded on an Oxford INCA Energy TEM 250. Gel permeation

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chromatography (GPC) was performed on a system equipped with a Jasco PU-2080 Plus pump

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and a Jasco RI-2031 plus intelligent RI detector. Fluorescence spectra were recorded at room

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temperature on a Jasco spectrofluorometer FP-8300. Quantum yields were measured on

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Hamamatsu UV-NIR absolute PL quantum yield spectrometer.

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2.3 Grafting and sample preparation 3 ACS Paragon Plus Environment

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All reactions in this work were carried out in an oven-dried Schlenk flask with a stopcock under

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an argon atmosphere. Cellulose powder (0.4 g) and FeCl3 (0.4 g) were dispersed in chloroform (7

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ml) with a magnetic stirrer. The mixture was ultrasonicated for 20 min and cooled to 0 °C in an

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ice bath. A solution of 3HT (100 mg, 0.6 mmol) in chloroform (3 ml) was dropped into the

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suspension of cellulose and FeCl3, with stirring, and the reaction mixture was stirred under argon

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for 2 h at 0 °C. The stoichiometric ratio of 3HT to FeCl3 was 1:4. The reaction was terminated by

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the addition of methanol. The product was washed with methanol to remove any residual FeCl3,

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followed by extraction with chloroform to eliminate free poly(3-hexylthiophene) (P3HT)

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homopolymer using a Soxhlet extractor. Finally, the cellulose grafted with 3-hexylthiophene was

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dried under vacuum for 12 h. Grafting of fluorene and the copolymer of fluorene and 3-

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hexylthiophene to cellulose powder was conducted using a similar procedure to that described

99

above. The grafted celluloses are hereinafter referred as (3HT/F-a/b)-g-cellulose, where a/b is the

100

ratio of 3HT and F used during the polymerization conditions. The grafting of the copolymer of

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fluorene and 3-hexylthiophene to cellulose paper was conducted using a similar procedure to that

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described above. The difference between cellulose paper and cellulose powder is the shape of the

103

material; cellulose powder is composed of fine dry particles, while cellulose paper is a thin

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material.

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3. Results and discussion

106

During the grafting reactions of poly(3-hexylthiophene) (P3HT), polyfluorene (PF), and their

107

copolymers to cellulose, the self-polymerizations of 3-hexylthiophene and fluorene occurred

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concurrently with graft polymerization to cellulose. The ungrafted 3-hexylthiophene (3HT) and

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fluorene(F) polymers were extracted from the grafted products with chloroform.

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Graft ratio (Gr) was calculated by the following formula: (A - B - C) / B × 100; where A is the

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total weight of cellulose and crude graft product, B is the weight of original cellulose, and C is

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the weight of extracted homopolymer or copolymer of fluorene and 3-hexylthiophene.

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this paper, the molecular weight of the extracted copolymer was used as the molecular weight of

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the copolymer grafted to cellulose. The characterization data for the copolymers extracted from

115

the grafted cellulose powder is similar to that extracted from the grafted cellulose paper. As

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shown in Table 1, (3HT/F-0/100)-g-cellulose (entry 1) had the lowest molecular weight while

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(3HT/F-100/0)-g-cellulose (entry 5) had the highest molecular weight. In addition, decreasing

52-54

In

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fluorene (F) monomer content, the molecular weight of the grafted (3HT/F)-cellulose copolymer

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increased ( from entry 2 to entry 4). These results can be explained on the basis of the difference

120

in oxidation potential between 3-hexylthiophene (3HT) and fluorene (F).55 Table 1: Characterization of grafted and extracted polymers

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(3HT/F) Entry -gcellulose

Molecular weight of graft copolymer (*) Mw, kDa PDI 0.9 1.4 2.5 1.9 4.1 2.6 4.9 2.7 115 2.8

Fluorene content in copolymer (**)

Gr %

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1 0/100 6.2 100 2 20/80 6.7 75 3 34/66 5.8 60 4 58/42 6.1 35 5 100/0 7.2 0 (*) Determined by GPC; (**) Determined by 1H NMR

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Figure 1 displays the 1H NMR spectra of the (3HT/F-100/0), (3HT/F-0/100), and (3HT/F-34/66)

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copolymers extracted with chloroform. The two peaks between δ 2.0 and 3.0 in Figure 1a

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correspond to the α-methylene protons of poly(3-hexylthiophene) (P3HT), while the signal at δ

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4.0–4.2 in Figure 1b is assigned to the methylene groups of polyfluorene (PF). The 3HT/F

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copolymer (Figure 1c) is identified by the two groups of signal at δ 2.0–3.0 (α-methylenes of the

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3HT unit) and δ 4.0–4.2 (methylene groups of the F unit). The F content in the copolymer can be

129

calculated from the areas of the peaks in these two ranges.42 The 1H NMR spectra of the (3HT/F-

130

20/80) and (3HT/F-58/42) copolymers extracted with chloroform are similar to Figure 1c,

131

although the F contents of these copolymers depend on the 3HT/F molar feed ratio. CDCl3

CDCl3 (1a) - extracted (3HT/F-100/0) copolymer g

Hd

Hb

Hd

Hb

c-f m

g

f

Hc

e

a,b,c

d

Ha

Hc

Ha

{

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

Biomacromolecules

c b

d

a

a

7.10

7.08

7.06

7.04

7.02

7.00

6.98

6.96

S

*

n

*

(1b) - extracted (3HT/F-0/100) copolymer

b 7

132

6

5

4

3

2

1

8

7

6

5

4

3

2

ppm

ppm


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CDCl3

(1c) - extracted (3HT/F-34/66) g copolymer

c-f f

e d c

Hb'

Hd'

Hd'

b

a

Hb'

S

n

m Hc'

a',b',c'

Ha'

{

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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g

Hc'

Ha'

d' a

8

133

7

b 6

5

4

3

2

1

ppm

134 135 136 137

Figure 1. 1H NMR spectra of: (1a) extracted (3HT/F-100/0) copolymer (entry 5, Table 1); (1b) extracted (3HT/F-0/100) copolymer (entry 1, Table 1); and (1c) extracted (3HT/F34/66) copolymer (entry 3, Table 1)

138 139

As shown in Figures 2 and 3, the color of the (3-hexylthiophene and fluorene) copolymers

140

grafted to cellulose paper and cellulose powder depends on the fluorene content of copolymer.

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These color differences can be clearly identified with the naked eye.

142 143 144

Figure 2. Images of the (3-hexylthiophene and fluorene) copolymers grafted to cellulose paper

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146 147

Figure 3. Images of the (3-hexylthiophene and fluorene) copolymers grafted to cellulose

148

powder

149 150

The FT-IR spectra of cellulose and the grafted (3HT/F)-g-celluloses are shown in Figure 4. The

151

IR spectrum of cellulose has been reported on many previous occasions.4,21,40,56-58 A broad peak

152

in the 3250–3500 cm-1 region is associated with O-H stretching vibrations. A sharp peak at 2899

153

cm-1 corresponds to C-H stretching vibrations.56,57 A number of peaks assigned to C-H and C-O

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bending vibrations of cellulose are observed in the 1314–1372 cm-1 range. A small additional

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peak at 748 cm-1 is observed in the IR spectrum of the grafted cellulose powder that is attributed

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to the presence of out-of-plane deformation C-H vibration of the aromatic groups.

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aromatic C-H out-of-plane vibration is associated with the aromatic rings of 3-hexylthiophene

158

and fluorene.

59

This


Biomacromolecules

(3HT/F-0/100)-g-cellulose

Transmittance Intensity

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cellulose 4000

3500

3000

2500

2000

1500

1000

500

cm-1

159 160 161

Figure 4. FT-IR spectra of the (3-hexylthiophene and fluorene) copolymers grafted to cellulose powder

162 163

Figure 5a depicts the UV-Vis spectra of cellulose paper grafted with the (3HT/F) copolymers.

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The UV-Vis spectra of (3HT/F)-g-cellulose powder samples are similar to those the (3HT/F)-g-

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cellulose paper samples. As can be seen in Figure 5a, the ungrafted cellulose paper exhibits no

166

absorption peak; however, all of the (3HT/F)-g-cellulose materials show strong absorption bands

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between 250 and 650 nm, which are assigned to the π-π* transitions of P3HT, PF, and the

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P3HT/PF copolymer. The absorption maximum of (3HT/F-0/100)-g-cellulose is located at 371

169

nm, which is attributed to PF, while the absorption maximum of (3HT/F-100/0)-g-cellulose is

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seen at 509 nm, which is associated with P3HT.42 When the F content in the grafted cellulose

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copolymer was increased from 42 to 80%, the absorption maximum shifted to shorter

172

wavelengths, as shown in Figure 5a and Table S1 (Supporting Information). The emission

173

maximum of (3HT/F)-g-cellulose are also shown in fluorescence spectra (Figure 5b) and


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summarized in Table S1. The decrease in F content in the grafted cellulose lead to the redshift of

175

(3HT/F)-g-cellulose in the fluorescence spectra. The optical band gaps of the grafted cellulose

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copolymers were determined from the onset of absorption by determining the point of

177

intersection between the tangent to the curve and the x-axis, as shown in Figure 5a. The

178

maximum absorption wavelength (λmax), the emission peaks, the onset absorption wavelength

179

(λonset), and optical band gap (Egop) are summarized in Table S1. The optical band gap of

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cellulose is 4.03 eV, while the band gap energy of the cellulose grafted with the copolymer of PF

181

and P3HT, varied between 1.86 and 2.59 eV. The change in the band gap results from the F unit

182

in the copolymer that, in conjunction with the 3HT unit, extends the conjugation length of the

183

copolymer backbone of cellulose.

100

Normalized Intensity

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Biomacromolecules

(3HT/F-0/100)-g-cellulose (3HT/F-20/80)-g-cellulose (3HT/F-34/66)-g-cellulose (3HT/F-58/42)-g-cellulose (3HT/F-100/0)-g-cellulose cellulose paper

50

0 200

(5a)

400

600

800

Wavelength / nm

184


Biomacromolecules

100


(3HT/F-0/100)-g-cellulose (3HT/F-20/80)-g-cellulose (3HT/F-34/66)-g-cellulose (3HT/F-58/42)-g-cellulose (3HT/F-100/0)-g-cellulose

50

0 400

185

600

800

Wavelength / nm

(5b)

186 187

Figure 5. UV-Vis spectra (5a) and Fluorescence spectra (5b) of cellulose paper and grafted cellulose paper

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The quantum yields of cellulose paper and the (3HT/F)-g-cellulose paper samples were

189

determined by absolute method. The quantum yields of cellulose powder and the (3HT/F)-g-

190

cellulose powder samples were similar to those of the corresponding cellulose paper samples. As

191

shown in Figure 6, the quantum yields of cellulose paper and (3HT/F-100/0)-g-cellulose sample

192

were the lowest, while (3HT/F-0/100)-g-cellulose exhibited the highest quantum yield. The

193

quantum yield increased with increasing F content. The presence of the F unit helps to tune the

194

emission, resulting in a high quantum yield. 9.7 10

8

Quantum Yield, %

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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5.8 6

4.3 4

3.1

2

0

0 cellulose

0 100/0

58/42

(3HT/F) -gcellulose

(3HT/F) -gcellulose

34/66

20/80

(3HT/F) (3HT/F) -g-gcellulose cellulose

0/100 (3HT/F) -gcellulose

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Figure 6. Quantum yields of cellulose paper and (3HT/F)-g-cellulose paper samples

197 198

Figure 7a shows the TG curves of cellulose paper and (3HT/F) copolymers grafted on cellulose

199

paper. As shown in the Figure 7a, the TG curve of cellulose shows two stages of degradation in

200

air. The first stage of degradation occurs from 270 to 345 °C, with a weight loss from 6 to 72%.

201

The second stage of degradation ranges from 347 to 525 °C, with a weight loss between 72 and

202

98%. The decompositions of the (3HT/F)-g-cellulose paper samples also proceeded in two steps;

203

however, the degradation of the grafted celluloses began at lower temperatures to that of

204

cellulose. For instance, the (3HT/F-20/80)-g-cellulose sample started to degrade at 230 °C, while

205

(3HT/F-58/42)-g-cellulose began to decompose at 247 °C. Table S2 summarizes the

206

decomposition temperatures and corresponding weight losses for cellulose paper and the grafted

207

samples. (3HT/F-0/100)-g-cellulose (3HT/F-20/80)-g-cellulose (3HT/F-34/66)-g-cellulose (3HT/F-58/42)-g-cellulose (3HT/F-100/0)-g-cellulose cellulose paper

50

0

209 210

50

0 100

208

(3HT/F-0/100)-g-cellulose (3HT/F-20/80)-g-cellulose (3HT/F-34/66)-g-cellulose (3HT/F-58/42)-g-cellulose (3HT/F-100/0)-g-cellulose cellulose powder

100

TG, %

100

TG, %

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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(7a)

200

300

400

500

Temperature oC

600

700

100

(7b)

200

300

400

500

600

700

Temperature oC

Figure 7. Thermogravimetric (TG) curves for: (7a) (3HT/F)-g-cellulose paper, and (7b) (3HT/F)-g-cellulose powder

211 212

The results listed in Table S2 indicate that the decomposition temperatures of cellulose paper at

213

different weight losses are higher than the corresponding temperatures for the (3HT/F)-g-

214

cellulose paper samples. The decomposition behavior of the cellulose powder and (3HT/F)-g-

215

cellulose powder samples (Figure 7b) are similar to that of the ungrafted and grafted cellulose

216

paper samples.

217

The differences in the thermal decomposition properties of these samples can be observed clearly

218

from the first-derivative thermogravimetric (DTG) curves shown in Figure 8, where the 11 ACS Paragon Plus Environment

Biomacromolecules

219

temperature peak is used as a measure of thermal stability; the results are summarized in Table

220

S3. The temperature peak of cellulose paper is observed at 344 °C, indicating that the

221

decomposition rate of the cellulose paper is highest at 344 °C. The DTG curves for the (3HT/F)-

222

g-cellulose paper samples show sharp peaks at lower temperatures than cellulose paper (Table

223

S3). The decomposition temperatures of the (3HT/F)-g-cellulose paper samples are generally

224

lower, by about 25 °C, than that of cellulose paper. 100

100



0 100

0 100



0 100

0 100

(3HT/F-34/66)-g-cellulose 0 100

DTG (% / min)

DTG (% / min)

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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0 100



0 100

cellulose paper

225

(8a)

cellulose powder 0

0

100

200

300

400

Temperature ( oC)

500

600

700

100

200

(8b)

300

400

500

600

700

Temperature ( oC)

226 227

Figure 8. First-derivative thermogravimetric (DTG) curves of: (8a) (3HT/F)-g-cellulose paper samples, and (8b) (3HT/F)-g-cellulose powder samples

228

The DTG curves of the cellulose powder samples are similar to those of the corresponding

229

cellulose paper samples; the peak temperature for cellulose powder is about 337 °C, while the

230

peak temperatures of the (3HT/F)-g-cellulose powder samples are lower (Figure 8b and Table

231

S3).

232 233

Figure 9 displays the differential thermal analysis (DTA) curves of all samples. The DTA peak

234

temperatures also provide a measure of thermal stability and are listed in Table S3. 12 ACS Paragon Plus Environment

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235 100

100



0 100

0 100



0 100

0 100



DTA (µV)

0 100

(3HT/F-58/42)-g-cellulose DTA (µV)

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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0 100


0 100



0 100

cellulose paper cellulose powder

0

0

100

200

300

400

500

600

700

(9b)

100

200

300

400

500

600

700

236

(9a)

237 238

Figure 9. Differential thermal analysis (DTA) curves of: (9a) (3HT/F)-g-cellulose paper samples, and (9b) (3HT/F)-g-cellulose powder samples

239

The DTA curve of cellulose paper displays two peaks, a sharp peak at around 359 °C, and a

240

small broad peak at around 514 °C. In case of the (3HT/F)-g-cellulose paper samples, the DTA

241

peaks are shifted toward lower temperatures (Figure 9a and Table S3) when compared to

242

cellulose paper. The shifts in the DTA peak temperatures are similar to those observed for the

243

DTG peak temperatures.

244

In summary, the TG, DTG, and DTA curves indicate that the grafting of (3HT/F) copolymers to

245

cellulose results in a less thermally stable material. A similar tendency has been reported for

246

cellulose grafted with vinyl monomers such as methyl acrylate, methyl methacrylate, and 2-

247

hydroxyethylmethacrylate, among others; a decrease in the thermal stability of the grafted

248

cellulose was found.52,53,60-62 The crystallinity of cellulose was also found to be affected

249

significantly by the thermal stability of the cellulose.63-65 The grafting of the (3HT/F) copolymers

250

to cellulose increases the amorphous regions of cellulose resulting in altered crystallinity; as a

Temperature ( oC)

Temperature ( oC)


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251

consequence, this change in crystallinity leads to a decrease in the thermal stability of the

252

material.

253

The thermal stabilities of (3HT/F-58/42)-g-cellulose and a simple mixture of the (3HT/F-58/42)

254

copolymer and cellulose ((3HT/F-58/42)/cellulose mixture) were compared. The DTG and DTA

255

curves and the data obtained from those curves are shown in Figures 10 and 11, and Table S4.

256

The thermal stabilities of P3HT and PF are higher than that of cellulose itself. The TG curve of

257

the (3HT/F-58/42)/cellulose mixture (Figure 10) shows a two stage decomposition processes.

258

The first stage corresponds to the thermal decomposition of cellulose and occurs between 250

259

and 350 °C, while the second stage occurs between 355 and 590 °C and is due to the

260

decomposition of the (3HT/F-58/42) copolymer chain. The DTG peak temperatures of P3HT and

261

PF are 504 and 629 °C, respectively. These values are considerably higher than that of cellulose

262

(337 °C). The thermal decomposition behavior of the (3HT/F-58/42)/cellulose mixture also

263

shows two DTG peak temperatures, the first at 337 °C is attributable to the degradation of

264

cellulose, and the second at 560 °C corresponds to the decomposition of the (3HT/F-58/42)

265

copolymer chain. The thermal decomposition behavior of the (3HT/F-58/42)/cellulose mixture is

266

different from that of the (3HT/F)-g-cellulose samples. The decomposition temperatures of the

267

(3HT/F)-g-celluloses were slightly lower than that of cellulose itself, while the thermal

268

decomposition temperatures of the simple mixture of the 3HT/F copolymer and cellulose was

269

higher than that of cellulose itself. The incompatibility of the constituent polymers causes a

270

phase-separated structure in the polymer blend.

271

changes the crystallinity of the cellulose chain resulting in a decrease in thermal stability.67 The

272

(3HT/F-58/42)/cellulose mixture is simply mixed between the 3HT or F monomer units and the

273

cellulose structure. Therefore, the (3HT/F-58/42)/cellulose mixture shows the respective

274

temperature peaks of each of the constituents of the mixture.

66

The grafting conjugated polymer on cellulose


Page 15 of 25

TG, %

100

50

(3HT/F-58/42)/cellulose mixture homopolymer PF cellulose powder homopolymer P3HT

0 100

200

276

300

400

500

600

700

Temperature oC

275

Figure 10. TG curves of PF, P3HT, cellulose powder, and a (3HT/F-58/42)/cellulose mixture

277 (3HT/F-58/42)/cellulose mixture hompolymer PF homopolymer P3HT cellulose powder

100

DTA (µV)

100

DTG (% / min)

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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50

100

279 280

50

0

0

278

(3HT/F-58/42)/cellulose mixture homopolymer PF homopolymer P3HT cellulose powder

200

(11a)

300

400

500 o

Temperature( C)

600

700

(11b)

100

200

300

400

Temperature oC

500

600

700

Figure 11. DTG (11a) and DTA (11b) curves of PF, P3HT homopolymer, cellulose powder and a (3HT/F-58/42)/cellulose mixture

281 282

The crystallinity of cellulose is expected to change by the direct grafting of 3HT or F to the

283

cellulose chain, which may result in reduced thermal stability.67

284

Cellulose exists in two crystalline forms, cellulose I and II.

285

percent crystallinity (% Cr) of cellulose and the grafted cellulose products were calculated from

286

the XRD patterns using the 2θ peak intensities between 18 and 19° (the diffraction intensity of

287

the amorphous portion) and between 22 to 23° (the intensities of both amorphous and crystalline

288

phases) 2 as follows:

68

The crystallinity index (Ic) and


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=

−

% =

+

289 290

As shown in Figures 12 and 13, all diffraction patterns exhibited peaks at 2θ = 17 and 22.5°,

291

which correspond to the amorphous and crystalline phases of cellulose. The data in Tables S5

292

and S6 reveal that the crystallinity indexes and percentage crystallinities of the various grafted

293

materials decrease slightly upon grafting. After grafting, the crystallinity index varied from 0.66

294

to 0.69 in the case of cellulose paper, while it ranged from 0.53 to 0.56 for cellulose powder. The

295

percent crystallinities of cellulose paper and cellulose powder are 76.91 and 70.05, respectively.

296

These values also decrease upon grafting, to 74.89% for the paper, and 68.24% for the powder.

297

The grafting of the (3HT/F) copolymer to the cellulose backbone increases the amorphous region

298

because of the incorporation of (3HT/F) copolymer chains that hinder the crystallization of the

299

cellulose chain. The thermal stability of cellulose depends mainly on its crystallinity.67 These

300

XRD results are in agreement with the TG results; after grafting cellulose crystallinity decreases

301

resulting in a decrease in thermal stability 69,70.

(3HT/F-0/100)-g-cellulose (3HT/F-20/80)-g-cellulose (3HT/F-34/66)-g-cellulose (3HT/F-58/42)-g-cellulose (3HT/F-100/0)-g-cellulose cellulose paper

Intensity (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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10

302 303

15

20

25

30

35

40

45

50

Diffraction angle, 2θ

Figure 12. XRD patterns of cellulose paper and (3HT/F)-g-cellulose paper samples

304 305


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(3HT/F-0/100)-g-cellulose (3HT/F-20/80)-g-cellulose (3HT/F-34/66)-g-cellulose (3HT/F-58/42)-g-cellulose (3HT/F-100/0)-g-cellulose cellulose powder

Intensity (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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10

20

25

30

35

40

45

50

Diffraction angle, 2θ

306 307

15

Figure 13. XRD patterns of cellulose powder and (3HT/F)-g-cellulose powder samples

308 309

The TEM image of the (3HT/F-58/42)-g-cellulose sample is shown in Figure 14a and the

310

corresponding EDX distribution maps of the elements are shown in Figures 14b–e. As shown in

311

Figures 14 and 15, carbon, oxygen, chlorine, and sulfur are present in the (3HT/F-58/42)-g-

312

cellulose sample. These results confirm the presence of sulfur, which originates from the

313

heterocyclic thiophene (3HT) unit grafted to the cellulose.

314

O

C (a) (c)

(b)

S 315 316

(d)

Cl (e)

Figure 14. EDX maps of (3HT/F-58/42)-g-cellulose


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317 318

Figure 15. The EDX spectrum of (3HT/F-58/42)-g-cellulose

319

Figures 16–18 provide overviews of the grafting of poly(3-hexylthiophene) (P3HT),

320

polyfluorene (PF), and a copolymer of 3-hexylthiophene (3HT) and fluorene (F) to cellulose.

321

Although celluloses grafted with (3HT/F) copolymers were successfully synthesized and

322

confirmed by FT-IR, XRD, TG, UV-vis, fluorescence and TEM EDX techniques, the

323

mechanisms of the grafting reactions involving cellulose and fluorene or 3-hexylthiophene in the

324

presence of anhydrous FeCl3 remains unclear. The Fe2+_H2O2 system (Fenton reagent) has been

325

used to graft vinyl monomers to cellulose for over two decades.2 Most previous research

326

supposed that the presence of Fe3+ had a negative effect on grafting and led to termination of the

327

growing grafted chain.7,8 However, some complex reagents composed of Fe3+, ascorbic acid,

328

potassium fluoride, and ethylenediaminetetraacetic acid have been used in grafting reactions.7,71

329

In our method, anhydrous FeCl3 was well dispersed in chloroform by ultrasonic treatment; the

330

reaction of FeCl3 take place heterogeneously because of its insolubility in this solvent. In the

331

solid state, the Fe3+ ion at the surface of the crystal has one unshared chloride ion and one empty

332

orbital, leading to strong Lewis acidity and high hygroscopicity.72 Furthermore, in the solid state

333

the oxidant activity of FeCl3 is high.41 The grafting mechanism of the (3HT/F) copolymer to

334

cellulose may occur in a similar fashion to the grafting of P3HT to cellulose. The following

335

reaction steps are proposed for the grafting reaction of 3HT to cellulose. In the first step, radical

336

formation on the cellulose backbone may occur on the oxygen atom of the (-CH2OH) group

337

(Figure 16). At the same time, 3HT monomers are oxidized to the corresponding radical cations

338

by FeCl3.72 Initially, the oxidized 3HT monomer may couple with the free radical on the oxygen

339

atom of the (-CH2OH) group. In the propagation step, FeCl3 continues to oxidize the 3HT

340

molecules on the surface of cellulose to the radical cations; this surface radical cation is then

341

thought to couple with another 3HT radical cation in solution. Repetition of this process is 18 ACS Paragon Plus Environment

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342

thought to bind the P3HT to the surface of the cellulose. Termination may arise from the

343

substitution of the hydrogen atom of a 3HT molecule with a chlorine end group (Figure 16).73,74

344

This also explains why the chlorine atom was detected in the (3HT/F)-g-cellulose by TEM EDX

345

(Figures 14 and 15).

346

347

348 349

Figure 16. Grafting of P3HT to cellulose cellulose

cellulose

OH

anhydrous FeCl3

+

351

Cl

n

O

OH

350

O

2 h, 0o C, CHCl3

H

m

Figure 17. Grafting of PF to cellulose Hexyl

cellulose

cellulose Hexyl

OH

+

352 353

OH

m

+

O m''

anhydrous FeCl3

n

S

S

H

n''

Hexyl

2 h, 0o C, CHCl3

O m'

S

n'

Cl

Figure 18. Grafting of a random copolymer of PF and P3HT to cellulose 19 ACS Paragon Plus Environment

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354 355 356

4. Conclusions

357

Surface-modified cellulose was successfully synthesized by directly grafting conjugated

358

polymers through oxidative polymerization promoted by FeCl3. The UV-Vis absorption peak of

359

the grafted cellulose was increasing blue-shifted, and the fluorescence quantum yield were

360

remarkably enhanced, with increasing fluorene content. Cellulose crystallinity decreased slightly

361

after graft polymerization with PF and P3HT. The sulfur atoms of the grafted P3HT units on the

362

cellulose were also detected by TEM-EDX. The thermal stabilities of the (3HT/F)-g-celluloses

363

were slightly lower than that of cellulose itself, while the thermal decomposition behavior of a

364

simple mixture of a 3HT/F copolymer with cellulose was superior to that of cellulose alone.

365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380


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381 382

References

383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425

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(58) Higgins, H. G.; Stewart, C. M.; Harrington, K. J.: Infrared spectra of cellulose and related polysaccharides. J. Polym. Sci. Pol. Chem. 1961, 51, 59-84. (59) Socrates, G.: Infrared and Raman Characteristic Group Frequencies. John Wiley & Sons, Ltd 2001, West Sussex, England, 268-269. (60) Nishioka, N.; Funakoshi, M.; Inamoto, K.; Uno, M.; Ueda, A.: Thermal Decomposition of Cellulose/Synthetic Polymer Blends Containing Grafted Products. V. Cellulose/Polystyrene Blends. J. Appl. Polym. Sci. 2010, 118, 2482–2487. (61) Fernández, M. J.; Fernández, M. D.; Casinos, I.; Guzmán, G. M.: Thermal behavior of cellulosic graft copolymers. I. Cotton grafted with vinyl acetate and methyl acrylate. J. Appl. Polym. Sci. 1990, 39, 2219-2235. (62) Fernández, M. J.; Fernández, M. D.; Casinos, I.; Guzmán, G. M.: Thermal behavior of cellulosic graft copolymers. II. Cotton grafted with binary mixtures of vinyl acetate and methyl acrylate. J. Appl. Polym. Sci. 1990, 39, 1101-1119. (63) Basch, A.; Lewin, M.: The influence of fine structure on the pyrolysis of cellulose. I. Vacuum pyrolysis. J. Polym. Sci., Polym. Chem. Ed. 1973, 11, 3071–3093. (64) Basch, A.; Lewin, M.: The influence of fine structure on the pyrolysis of cellulose. II. Pyrolysis in air. J. Polym. Sci., Polym. Chem. Ed. 1973, 11, 3095–3101. (65) M. E. Calahorra; M. Cortázar; J. I. Eguiazábal; Guzmán, G. M.: Thermogravimetric analysis of cellulose: Effect of the molecular weight on thermal decomposition. J. Appl. Polym. Sci. 1989, 37, 3305–3314. (66) Isayev, A. I.: Encyclopedia of Polymer Blends - Structure V3. Wiley VCH 2016. (67) Reich, L.; Stivala, S. S.: Elements of Polymer Degradation. McGraw-Hill 1971, New York. (68) Kamide, K.: Cellulose and cellulose derivatives molecular characterization and its applications. Elsevier Science 2005. (69) Liu, Z.-T.; Yang, Y.; Zhang, L.; Liu, Z.-W.; Xiong, H.: Study on the cationic modification and dyeing of ramie fiber. Cellulose 2007, 14, 337–345. (70) Siqueira, G.; Bras, J.; Dufresne, A.: Cellulose Whiskers versus Microfibrils: Influence of the Nature of the Nanoparticle and its Surface Functionalization on the Thermal and Mechanical Properties of Nanocomposites. Biomacromolecules 2009, 10, 425–432. (71) Misra, B. N.; Dogra, R.; Mehta, I. K.: Grafting onto cellulose. V. Effect of complexing agents on Fenton's reagent (Fe2+ - H2O2)- initiated grafting of poly(ethyl acrylate). J. Polym. Sci., Polym. Chem. Ed. 1980, 18, 749-752. (72) Niemi, V. M.; Knuuttila, P.; 0sterholm, J.-E.; Korvola, J.: Polymerization of 3alkylthiophenes with FeCl3. Polym. Rep. 1992, 33, 1559-1562. (73) Liu, Y.; Nishiwaki, N.; Saigo, K.; Sugimoto, R.: Kinetics Study on 3-Hexylthiophene Polymerization with Iron(III) Chloride. Bull. Chem. Soc. Jpn. 2013, 86, 1076-1078. (74) McCarley, T. D.; Noble, C. O.; IV; DuBois, C. J.; Jr.; McCarley, R. L.: MALDI-MS Evaluation of Poly(3-hexylthiophene) Synthesized by Chemical Oxidation with FeCl3. Macromolecules 2001, 34, 7999-8004.

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Biomacromolecules

100

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9.7

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Quantum Yield, %

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


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Photoluminescence Control of Cellulose via Surface Functionalization

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