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In-Situ Synthesis and Characterization of Silver/Polymer Nanocomposites by Thermal Cationic Polymerization Processes at Room Temperature: Initiating Systems Based on Organosilanes and Starch Nanocrystals. Mohamad-Ali Tehfe, Romain Jamois, Patrice Cousin, Said Elkoun, and Mathieu Robert Langmuir, Just Accepted Manuscript • DOI: 10.1021/la504518c • Publication Date (Web): 20 Mar 2015 Downloaded from http://pubs.acs.org on April 1, 2015

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Langmuir

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In-Situ Synthesis and Characterization of Silver/Polymer

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Nanocomposites by Thermal Cationic Polymerization Processes

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at Room Temperature: Initiating Systems Based on

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Organosilanes and Starch Nanocrystals.

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*Mohamad-Ali Tehfe, Romain Jamois, Patrice Cousin, *Saïd Elkoun, *Mathieu Robert

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Carrefour of Innovative Technology and Ecodesign (CITE), Faculty of Engineering, University of Sherbrooke; 2500 blvd Université, Sherbrooke, Canada.

Authors E-mail addresses: *[email protected]; *[email protected] ; *[email protected];

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Abstract:

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New methods for the preparation of silver nanoparticles/polymer nanocomposite

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materials by thermal cationic polymerization of ɛ-caprolactone (ɛ-CL) or α-pinene oxide (α-

19

PO) at room temperature (RT) and under air were developed. The new initiating systems

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were based on silanes (Si), starch nanocrystals (StN) and metal salts. Excellent

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polymerization profiles were revealed. It was shown that silver nanoparticles (Ag(0) NPs)

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were in-situ formed and that the addition of StN improves the polymerization efficiency. The

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as-synthetized nanocomposite materials contained spherical nanoparticles homogenously

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dispersed in the polymer matrices. Polymers and nanoparticles were characterized by gel

25

permeation chromatography (GPC), X-ray diffraction (XRD), transmission electron

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microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and UV-vis spectroscopy. A

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coherent picture of the involved chemical mechanisms is presented.

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Keywords: Silanes, Silver Salts, Starch Nanocrystals, Thermal Cationic Polymerization, ɛ-

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caprolactone, α-pinene oxide.

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ACS Paragon Plus Environment

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Introduction:

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Cationic polymerization (CP) reactions are very attractive.1-5 CP has been successfully

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applied to a large variety of monomers such as cyclic ethers or esters and related derivatives,

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epoxides and vinyl ethers. It was shown that mechanical properties of CP based material are

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generally different from those of well known acrylic polymers prepared through free radical

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polymerization (FRP). The use of monomers less toxic than acrylates is also highly

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worthwhile. Interestingly, the oxygen inhibition, usually observed in FRP, does not occur.

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The cationic ring opening polymerization (ROP) of ɛ-caprolactone (ɛ-CL) or α-pinene oxide

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(α-PO) has received less attention than the other modes of activation .6-10 In addition the use

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of HCl, trifluoromethanesulfonic acids, organomagnesium complexes, yttrium triflate as

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initiators (see [6-10] and references therein) has also drawn attention. It was reported that the

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addition of a little amount of water or the use of an alcohol in the reaction mixture enhances

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the polymerization rate.6-10 The development of new and efficient cationic initiating systems

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for the polymerization of ɛ-CL or α-PO at room temperature (RT) and under air is still

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valuable due to the wide range of applications of biodegradable polycaprolactone (PCL)

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and/or biodegradable and bio-renewable polypinene oxide (PPO) in packaging, medicine and

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tissue engeenering. For instance, PCL can be used as an additive to improve the processing of

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resins and/or their end use properties (e.g. impact resistance). Incorporating nanoparticles

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(NP) to PCL could give an extra properties, such as bactericide property due to the presence

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of Ag.11-15 The incorporation of Ag(0) through photopolymerization processes has been

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elegantly presented in [13,14].

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In general, nanocomposite materials containing noble metal and dispersed nanoparticles

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in a polymer matrix may exhibit peculiar physical and chemical properties that are of high

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scientific and technological importance.16 These new composites are used for optical17,

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electrical18 and medical19 applications as well as data storage.20 In particular, silver

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nanoparticles are very important for their excellent electrical conductivity21, anti-microbial

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effect22 and optical properties.23

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Typical examples of silver-based polymer nanocomposites24 are polyvinyl alcohol,25

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polyimide,26 polystyrene,27 epoxy and acrylate based systems.11,12,14,27-31 The commonly

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encountered drawback of these nanocomposites is the tendency of nanoparticles to

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agglomerate due to their high surface reactivity, which leads to poor mechanical properties.

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Most of the useful approaches/strategies to avoid particle agglomeration and obtain

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homogeneous dispersion of thermodynamically unstable nanoparticles in a polymer matrix


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are based on nanoparticles stabilization by inert additives.13,15 Alternative thermal and

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photochemical methods involving similtanous reduction and polymerization processes have

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been recently investigated.13,15,30,31 Despite potential applications, there are still limitations in

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applying direct excitation of metal salts or complexes for thermal or photochemical

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preparation of nanocomposites.

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Among various initiation systems for CP of epoxides, redox couples are interesting32,33

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as exemplified by a recent new tri-component combination33,34 based on onium salt and

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organosilane (R3SiH) as reducing agent and platinum, palladium or rhodium as metal

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catalyst. On the other side, we recently developed a two-component redox system based on a

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tris(trimethylsilyl)silane TTMSS/silver salt combination which allows to efficiently initiate

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the ROP of epoxides at RT and incorporate metal M(0) nanoparticles in the film.30

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In the present paper, we propose to use such a redox system based on an organosilane,

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such as diphenylsilane (DPSi) or 1,1,3,3-tetramethyldisiloxane (TMDSi)/starch nanocrystals

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(StN)/silver salts couple for thermal cationic polymerization of ɛ-CL and/or α-PO (Scheme

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1). Our system should open up a new way for i) the cationic ROP of ɛ-CL or α-PO initiated

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by a true redox process at RT and under air, ii) the in-situ incorporation of nanoparticles into

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PCL or PPO and iii) using a vegetal source, such as starch nanocrystals (StN) to improve the

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polymerization efficiency. On the other hand, starch is a cheap, abundant, renewable and

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biodegradable semi-crystalline raw material from which nano crystalline particles can be

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extracted. These starch nanoparticles/nanocrystals have many potential applications, such as

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plastic fillers, food additives, drug carriers, implant materials, fillers in biodegradable

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composites, coating binders, adhesives, etc.35 Starch nanoparticles also have a great potential

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for use in papermaking wet end, surface sizing, coating and paperboard as part of

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biodegradable adhesives for substitution of petroleum based ones. In previous works, the

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production and characterization of starch nanoparticles have been reported in relation to their

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interesting performance as reinforcing agents.35,36 Alternatively, the purpose of using starch

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nanocrystals as additive is i) to improve the efficiency of polymerization, ii) to reinforce

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agent of the polymer (improvement of mechanical property). Finally, the overall

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polymerization process has been investigated and the cured films characterized.

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Experimental part:

100 101

1) Compounds:

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The compounds with the highest purity available used are presented in Scheme 1.

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Diphenylsilane (DPSi), 1,1,3,3-tetramethyldisiloxane (TMDSi), Starch from corn, Silver

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hexafluoroantimonate (AgSbF6), Silver sulfate (Ag2SO4), ɛ-caprolactone (ɛ-CL) and α-Pinene

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oxide (α-PO) were all purchased from Sigma-Aldrich and used as received.

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DPSi

TMDSi

ε-CL

α-PO

HO HO

HO HO

RCH2-OH

107

Scheme 1.

108 109 110

2) Extraction of Starch Nanocrystals (StN):

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Waxy maize starch nanocrystals were obtained according to the following method.35,36

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Briefly, acidic hydrolysis of 36.725 g of waxy maize starch granules was performed in a 250

113

ml 3.16 M H2SO4 solution, at 40 oC and 100 rpm. The mixture was subjected to an orbital

114

shaking action during 5 days. Subsequently, the ensuing insoluble residue was washed with

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distilled water and separated by successive centrifugations at 10,000 rpm and 10 oC until

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neutrality. The aqueous suspensions of starch nanoparticles were stored at 4 oC after adding

117

several drops of chloroform.

118 119

3) Cationic Polymerization (CP):

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Silver salts (0.1 to 1% w/w) and starch nanocrystals StN (2 to 10% w/w) were dissolved

121

in the selected monomer (ɛ-CL or α-PO) and the organosilanes (DPSi or TMDSi) were

122

introduced into the formulation using a syringe at RT. The polymerization of ɛ-CL or α-PO


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was carried out in pill-box to get a sample of 1g. A magnetic stirrer in contact with the pill-

124

box was used to ensure a good homogeneity. The addition of the silane into the formulation

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corresponds to time t = 0 s.

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For ɛ-CL, the progress of the exothermic polymerization is followed by monitoring the

127

sample temperature using a thermocouple connected to a DaqPro-5300 (resolution 0.1°C).

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The reaction is too slow with α-PO to use a similar procedure. It has been shown in [33-34]

129

that the increase of the temperature is directly related to the monomer conversion. The

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present set-up used here being not adiabatic, the relationship between the conversion and the

131

temperature will be valid only for the first steps of the polymerization process.

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4) Characterization:

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UV measurements were carried out with a Spectra max plus 384 spectrometer, whereas

135

Fourier-transform infrared spectra (FTIR) were performed on an ABB Bomen Spectrometer.

136

The number average molecular weight (Mn), weight average molecular weight (Mw),

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and polydispersity index (PDI=Mw/Mn) of polymer samples were determined by gel

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permeation chromatography (GPC). THF was used as the solvent at a flow rate of 1.0

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mL/min. Samples were prepared at 1 mg/ml in THF and were injected by Waters 717

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autosampler. GPC system (Waters 1515 high performance liquid chromatography pump,

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1mL/min, 30oC) was equipped with Waters Styragel HR0.5, HR4E, HR5 and a Waters model

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2414 refractive index detector. The molecular weight and PDI of each polymer sample was

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calibrated using polystyrene standards.

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X-ray diffraction (XRD) of powder was performed using a Philips X’Pert

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diffractometer equipped with a general area detector diffraction system with Cu Kα radiation

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(k = 0.1542 nm) operating at 50 kV and 40 mA. The experiments were performed in the

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diffraction angle range 2θ = 30–90o.

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Transmission electron microscopy (TEM) analyses of composite materials were carried out at an accelerated voltage of 80 kV using a Hitachi H-7500 TEM. X-ray photoelectron spectroscopy (XPS) measurements were performed on a KRATOS Axis Ultra electron energy analyzer operating with an Al Kα monochromatic source.

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Raman spectrum was collected with a double stage Dilor XY spectrometer equipped

153

with a Princeton Instruments Model LN/CCD-1024 detector. A Spectra-Physics model 164

154

argon ion laser was used for excitation at 785 nm. The laser was focused at an angle of 35◦

155

onto the sample with a spot size of approximately 10 µm. The Raman spectrum was collected

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for 300 s at a laser power of 25 mW. Data acquisition was conducted under computer control.


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Results and discussion:

159 160

1) Redox Initiating Systems at RT:

161

Using the DPSi or TMDSi/AgSbF6 initiating systems, the polymerization of ɛ-CL is

162

quite slow under air and at RT for the different selected concentrations of AgSbF6 (gel time

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about 8-10 hours) and tack free is obtained after e.g., 17h, 15h, 13h and 9h for 0.1, 0.2, 0.5

164

and 1% w/w AgSbF6, respectively (Figure 1). 32

2

31

30

O

T ( C)

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28

1

27

26 0

20

40

60

80

100

Time (s)

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Figure 1. Comparison of the polymerization initiating ability under air and at RT of sample containing 0.5% (wt) of AgSbF6 and 1% (wt) of DPSi in ɛ-CL: (1) without StN and (2) with 10% (wt) of StN.

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The polymerization does not occur using organosilanes or AgSbF6 independently. As a

171

result, a combination of both compounds is necessary to initiate the process. For DPSi or

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TMDSi/Ag2SO4, a similar process occurs but much more slowly (e.g. tack free time ~22

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hours at RT for 0.5% w/w of Ag2SO4 vs ~13 hours for 0.5% of AgSbF6). This is likely

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ascribed, as usually, to the less nucleophilic character of SbF6− compared to SO42− which

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ensures a higher reactivity of the cationic monomer species in the propagation of the ɛ-CL

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polymerization.30 The formation of PCL is well evidenced by IR spectra of the obtained

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polymer (Figure 2A) which are fully consistent with the spectra reported which show

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polyester bands at 1065 and 1108 cm-1 (Figure 2B).37,38 Remarkably, the polymerization is

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initiated by the proposed system in glass vials, under air and at RT.

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1

A

1108

B

1065

Abs.

1

Abs.

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2

0 1000

1

1 1200

1400

1600

1800

2000

0 1000

1050

υ (cm )

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1100 -1

1150

1200

υ (cm )

-1

Figure 2. (A) Real-time FT-IR spectra of sample containing 0.5% (wt) of AgSbF6 and 1% (wt) of DPSi in ɛ-CL (1- before and 2- after polymerization). (B) Zoom for the Figure 1A specta in the 1000 – 1200 cm-1 wavenumber range

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Interestingly, the addition of starch nanocrystals (StN) strongly enhances the

187

polymerization rate (Figure 1). A decrease of the tack free time is also noted i.e. ~ 130, 60

188

and 30 min for 2, 5 and 10 % w/w of StN, respectively. Remarkably, using the StN/AgSbF6

189

(10%/0.5% w/w) initiating system, the polymerization is quite slow under air and at RT (tack

190

free time about 13 hours). Consequently, the different three-component systems

191

(StN/Silane/Silver salts) can be considered as excellent initiating systems for thermal cationic

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polymerization under air and at RT (Table 1).

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However, the great interest of the proposed approach lies on the fact that the initiating

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system is efficient using simple glass vials and unpurified ɛ-CL, even in the presence of water

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or oxygen, which cana ct as polymerization inhibitors with other synthetic approaches.39 This

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can open new opportunities for mass production and applications of PCL.

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Table 1. Approximative tack free time (hour) for thermal cationic polymerization using silane/silver salt with and without StN.

ɛ-CL AgSbF6

DPSi

TMDSi

StN

DPSi/StN

DPSi/StN

DPSi/StN

(1% w/w)

(1% w/w)

(10% w/w)

(1%/2% w/w)

(1%/5% w/w)

(1%/10% w/w)

17

17

15

15

(0.1% w/w) AgSbF6 (0.2% w/w)


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AgSbF6

13

13

9

9

22

22

13

2

1

0.5

(0.5% w/w) AgSbF6 (1% w/w) Ag2SO4 (0.5% w/w) 200 201

Moreover, the proposed new systems are quite versatile as they are also able to initiate

202

the polymerization of epoxy monomers, such as α-pinene oxide (α-PO) (Figure 3), which

203

indicates that the formation of the cationic species is also efficient in this case.

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IR spectroscopy of the product formed by the thermal cationic polymerization of α-PO

205

shows the presence of a prominent aldehydic carbonyl band at 1726–1727 cm-1 (Figure 3).

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This result consistent with rearrangement to campholenic aldehyde as reported in the

207

literature and depicted in ref [40]. 0,05

A bs .

0,3

2 1

0,2 0,00 1600

Abs.

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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1700

1800

1900

2000

-1

υ (cm )

0,1

2 1 0,0 600

800

1000

1200

1400

1600

1800

2000

-1

υ (cm )

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Figure 3. Real-time FT-IR spectra of sample containing 0.5% (wt) of Ag2SO4 and 1% (wt) of DPSi in α-PO (1- before and 2- after polymerization). Insert : Zoom for the spectra in the range between 1800 – 2000 cm-1.

212 213

2) In-situ Incorporation of Silver and Starch based Nanoparticles:

214

The in-situ formation of silver nanoparticles during the polymerization process is

215

demonstrated by UV-visible spectroscopy (Figure 4A). The addition of organosilanes to

216

silver salts in ɛ-CL [Ag2SO4 was used instead of AgSbF6 to avoid the formation of polymer


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217

and facilitates the characterization of Ag(0)] leads to the formation of Ag(0) nanoparticles

218

clearly evidenced by UV-visible spectroscopy [i.e. surface plasmon resonance (SPR) band

219

typical of silver nanoparticles observed at about 400 nm (Figure 4B)]. Unprotected Ag

220

nanoparticles characterized by the size domain < 15 nm display a SPR band at a similar

221

wavelength.41,42 Remarkably, the addition of StN to organosilanes and silver salts in ɛ-CL or

222

α-PO leads to a faster and higher formation of Ag(0) nanoparticles as compared to sample

223

without StN (Figure 4C-D and Figure 1A-C in Supporting Information).

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3

0,4

A

Ag(0)

B

60 sec

Ag(0)

30 sec

2

10 min

O.D.

O.D.

20 sec

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6 min

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3 min

5 sec

0 sec

1 400

450

1 min

500

550

600

650

700

0,0 350

0 sec 400

450

500

λ (nm)

2

550

600

650

700

λ (nm)

2

Ag(0)

2

D

C

10 min

O.D.

6 min

O.D.

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3 min

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0 sec 400

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0

4

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8

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Time (min)

λ (nm)

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Figure 4. (A) UV-visible absorption spectra of AgSbF6 in ɛ-caprolactone just before (1) and after addition of TMDSi (2); Insert : Evolution of Ag(0) NPs colors during the synthesis in ɛcaprolactone at RT and under air. (B) UV-visible absorption spectra of Ag2SO4 with DPSi in ɛ-caprolactone at various times under air and at RT. (C) UV-visible absorption spectra of


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Ag2SO4 with DPSi and starch nanocristals (StN) in ɛ-caprolactone at various times under air and at RT. (D) Kinetic of the reaction followed by an increase of the maximum absorption of silver nanoparticles during the synthesis at 404 nm.

234

3) Reaction Mechanisms :

235

Based on other redox reactions involving silanes,43,44 processes (r1) and (r2) have

236

already been proposed for the R3SiH/AgSbF6 interaction leading to the formation of a

237

silylium cation alone or silylium cation and a Brönsted acid, (r1) being assumed as the main

238

reaction. On the other side, the formation of silylium cations when using chlorosilanes

239

(R3SiCl) instead of R3SiH with silver salts (r3) was well characterized.44 These reactions

240

might proceed by an attack of the electrophile on a Si-H bond.43,44 Free radicals were not

241

involved and an ionic mechanism was assumed. 2 R3SiH + 2 AgSbF6 →→ 2 R3Si+ SbF6− + 2Ag(0) + H2

(r1)

R3SiH + 2 AgSbF6 →→ R3Si+SbF6− + 2Ag(0) + H+SbF6−

(r2)

R3SiCl + AgSbF6 →→→ R3Si+SbF6− + Ag(0) + products

(r3)

242

In this work, the role of reactions (r1 and r2) is clearly outlined: silane/AgSbF6 (1%/1%

243

w/w) leads to an efficient polymerization process (tack free time about 9 hours at RT) (Table

244

1). The good polymerization ability of silane/AgSbF6 systems appears here in full agreement

245

with the formation of R3Si+SbF6− and H+SbF6− which are largely known2,3,45-47 as efficient

246

initiating species for both thermal and photochemical cationic polymerization of ɛ-CL and α-

247

PO.

248

The formation of Ag(0) nanoparticles is also evidenced by the presence of SPR band at

249

404 nm. By analogy, we consider that reaction (r1) corresponds to the major pathway of the

250

R3Si+ production during the polymerization initiation process using R3Si-H/AgSbF6.

251

Upon addition of StN, StN can also be oxidized by silver salt leading to a Ag(0) NPs

252

synthesis and Brönsted acid (H+) (r4a) 48,49 or by R3Si-M+ leading to a Brönsted acid (H+)

253

(r6)50,51 which can further participate to the initiation process. Remarkably, the combination

254

of StN/AgSbF6 (10%/0.5% w/w) acts as a very powerful initiating system (tack free time

255

about 13 hours at RT) (Table 1). On the other hand, StN can be oxidized by R3Si+ and

256

converted into an alkyl cation (RCH2+) (r7) known to easily initiate a ring opening

257

polymerization (ROP) process.2,4,52-54 This is also well supported by the observation of

258

R3SiOH bands in the 800-1100 cm-1 region of the Raman spectrum (Figure 5A) and an IR

259

band at 1088 cm-1 (Figure 5B).55-57


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In these reactions, cations are probably generated to initiate the ring opening

260

polymerization of the epoxy monomer (r8).

261

RCH2OH + 2Ag+ → Ag(0) + 2H+ + RCHO +

(r4a)

+

RCH2OH + H → RCH2 + H2O +

−

+

R3Si SbF6 + M → R3Si-M SbF6 +

(r4b)

−

(r5)

−

R3Si-M SbF6 + RCH2OH → RCH2O-M-SiR3 + H

+

(r6)

R3Si+ SbF6− + RCH2OH → R3SiOH + RCH2+

(r7)

R3Si+ SbF6−, H+ and RCH2+ + M → Polymer

(r8)

262 0,4

A

B

1

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R3Si-OH (1088 cm )

-

3

0,2

2

2

|

1100

Abs.

Intensity (a.u.)

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|

900

800

1 0,0 1000

1020

1040

1060

1080

1100

1120

-1

υ (cm )

Wavelenght (cm-1) 263 264 265 266 267 268 269 270 271

Figure 5. (A) Raman spectra of sample containing (1) 0.5% (wt) of AgSbF6 and 1% (wt) of DPSi; (2) 0.5% (wt) of AgSbF6, 1% (wt) of DPSi and 5% (wt) StN; (3) 0.5% (wt) of AgSbF6, 1% (wt) of DPSi and 10% (wt) StN in ɛ-CL after polymerization in the 800-1100-cm-1 region, showing the SiOH stretching peaks. (B) Real-time FT-IR spectra of sample containing 0.5% (wt) of AgSbF6 and 1% (wt) of DPSi in ɛ-CL after polymerization (1) without StN and (2) with 10% (wt) of StN.

272

A series of GPC measurements was carried out on the products of the thermal cationic

273

polymerization of ɛ-CL and α-PO used to evaluate StN as initiator. Molecular weights (Mn

274

and Mw) and polydispersity indexes (PDI=Mw/Mn) are presented in Table 2. The formed

275

PCL and PPO are characterized by a polydispersity index of 1.68-1.92 and 1.07, respectively,

276

which is in agreement with previous results on cationic processes.39,40 For initiating systems

277

without additional StN, the theoretical Mns (calculated from the degree of polymerization as

278

DP = [monomer]/[initiator]) are in good agreement with the calculated one (entry 2: 15029

4) Characterization of PCL and PPO polymers:


11

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Page 12 of 22

279

g/mol; on the assumption that each Si-H function participate to the initiation of

280

polymerization). This is in agreement with a decrease of [DPSi], which leads to higher Mn

281

values. More interestingly, the addition of StN affects the kinetic (see above) but also Mn

282

value, which is slightly lower (entry 1 vs. entry 2). For entry 1, the theoretical initiator

283

concentrations deducted from experimental DPs are much higher than [DPSi], which

284

demonstrates that StN also plays a role in the initiating step. This property is quite useful

285

since the final Mn can be tuned by adding StN (Table 2; entry 1). For PPO, a low molecular

286

mass was obtained by thermal cationic polymerization (entry 3: 535 g/mol). The amount of

287

polymer obtained varied depending on the monomer and concentration of initiator used (i.e.

288

for ɛ-CL, 79–84% polymer was obtained with the highest yield obtained using 1 wt % DPSi

289

as the initiator vs. the polymerization of α-PO using the same concentration of DPSi gave

290

60% yield). The overall yields and molecular weights for the polymerization of α-PO were

291

lower than those for ɛ-CL.

292 293 294

Table 2. Characterization of PCL and PPO polymers formed in the presence of different initiation systems. Entry

Monomer

AgSbF6

Ag2SO4

DPSi

StN

(% w/w)

(% w/w)

(% w/w)

(% w/w) 10

Mn

Mw

PDI

11543

22138

1.92

1

ɛ-CL

0.5

1

2

ɛ-CL

0.5

1

15029

25309

1.68

3

α-PO

1

535

575

1.07

0.5

295 296 297

5) Characterization of Ag(0) nanoparticles (NPs) :

298

Functionalized Ag(0) NPs were characterized by XRD, TEM, and XPS. The XRD

299

analysis confirms the presence of silver nanoparticles with a high degree of crystallinity as

300

shown in Figure 6, where the presence of diffraction peaks corresponding to (111), (200),

301

(220), (311), and (222) planes indicates the presence of the face-centered cubic (fcc)

302

crystalline structure of silver nanoparticles (JCPDS 00-004-0783). This result is consistent

303

with reported one in the literature and depicted in ref [58-60].

304 305

Remarkably, the addition of StN to organosilanes and silver salts in ɛ-CL or α-PO increases the formation of Ag(0) NPs, which is consistent with r6.

306


12

Page 13 of 22

60000 55000 50000

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

Langmuir

2

45000

(111)

40000

(200)

1

35000 30000 25000

(220)

20000

(311) (222)

15000 10000 30

40

50

60

70

80

2θθ (°)

307 308 309

Figure 6. XRD patterns of as-synthesized Ag(0) nanoparticles.1- without starch nanocrystals (StN) and 2- with 5% of StN.

310 311

Ag(0) NPs exhibit spherical shape as senne by TEM analysis (Figure 7). The bright-

312

field TEM micrographs of films cured using different concentrations of silver salts (0.1 and

313

0.5% w/w) and different organosilanes (DPSi or TMDSi) dissolved in different monomers (ɛ-

314

CL or α-PO) are reported in Figure 7. It is clear that metallic particles are well dispersed with

315

no significant agglomeration. Remarkably, by increasing the concentration of silver salt or

316

after the addition of StN, the nanoparticles size is not affected and higher silver nanoparticle

317

content is evidenced (Figure 7A vs. 7B and Figure 7C vs. 7D). These morphological results

318

are in agreement with r1, r6 and XRD measurements.

319 320

A

20 nm

B

20 nm


13

Langmuir

D

C

100 nm

100 nm

321 322 323 324 325

Figure 7. TEM analysis of nanoparticles obtained using different concentrations of AgSbF6 (% w/w) with 1% (w/w) DPSi in ɛ-CL : (A) 0.1% and (B) 0.5%. For (C) and (D) TEM analysis of obtained nanoparticles using 0.5% (w/w) of Ag2SO4 (% w/w) with 1% (w/w) DPSi in α-PO: (C) wihout StN and (D) with 5% StN.

326 327

XPS analysis was performed to investigate the surface of Ag(0) NPs. The survey

328

spectrum of Ag(0) NPs identifies the presence of Ag, C, O, F and Si (Figure 8A). The high-

329

resolution spectrum of Ag 3d presented in Figure 8B shows the Ag 3d splitting into Ag 3d3/2

330

(374.4 eV) and Ag 3d5/2 (368.4 eV) peaks with a peak separation of 6.0 eV and confirms that

331

the valence state of silver element is Ag(0). Remarkably, by increasing the concentration of

332

silver salt, the formation of Ag(0) NPs at the surface increased. The mass concentration of

333

silver nanoparticles formed on the surface is about 0.52 and 1.03% for 0.5 and 1% of

334

AgSbF6, respectively (Figure 8C-D). These results are in agreement with r1, TEM and XRD

335

measurements.

336 1,4x10

5

1,2x10

5

1,0x10

5

8,0x10

4

6,0x10

4

A

700

F 1s O 1s

650

F KLL a C KLL

4,0x10

4

2,0x10

4

O KLL a F KLL b

Ag 3d

Si 2s

0,0 1400

B

1200

1000

800

600

Ag 3d 5/2

C 1s

Intensity (CPS)

Intensity (CPS)

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

Page 14 of 22

400

200

Si 2p

Ag 3d 3/2

600

550

500

O 2s

0

450 380

Energy (eV)


376

372

368

364

Energy (eV)

14

360

Page 15 of 22

6,0x10

4,0x10

4

C

4

4

F 1s C KLL

2,0x10

C 1s

O 1s

O KLL a F KLL a F KLL b

4

0,0 1400

Intensity (CPS)

8,0x10

Intensity (CPS)

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

Langmuir

Ag 3d

Si 2s Si 2p

1200

1000

800

600

400

200

5x10

4

4x10

4

3x10

4

2x10

4

1x10

C 1s

D

C KLL

O KLL a

F 1s

F KLL a F KLL b

4

Ag 3d

O 2s

0

0 1400

Si 2s

1200

Energy (eV)

337 338 339 340 341

O 1s

1000

800

600

400

200

Energy (eV)

Figure 8. (A) XPS survey spectrum of Ag(0) NPs in ɛ-CL with 0.5% (w/w) AgSbF6, 1% (w/w) DPSi and 10% (w/w) StN. (B) high-resolution spectrum of Ag 3d. (C) and (D) XPS survey spectra of Ag(0) NPs for different concentrations of AgSbF6 (% w/w) with DPSi (1% w/w) in ɛ-CL : (C) 0.5% and (D) 1%.

342 343 344

Conclusion:

345

In the present study, an efficient and straightforward method of preparation of silver epoxy

346

nanocomposites by thermal cationic polymerization using a silane/silver salt couple under air

347

has been developed. Remarkably and unlike published works so far, heating is not required to

348

activate the reaction. The thermal polymerization process at RT has been demonstrated to be

349

highly efficient in the concomitant cationic ring-opening polymerization (ROP) of epoxy ring

350

and silver ion reduction. Interestingly, the addition of starch nanocrystals (StN) to the

351

silane/silver salt systems leads a drastic increase of polymerization reactivity and efficiency

352

respectively. Highly crosslinked network were achieved and the nanocomposite obtained

353

contains spherical nanoparticles homogenously dispersed in the polymer network without

354

significant agglomeration as a result of a surrounding effect of polymer chains formed by the

355

rapid cationic chain growth. The mechanical and optical properties of the obtained new

356

organic polymer/silver nanoparticles will be investigated in near future.

357 358 359


15

Si 2p O 2s

0

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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

Page 16 of 22

360

Supporting Information Available:

361

The supporting information contains a figure that shows:

362

A- UV-visible absorption spectra of Ag2SO4 with TMDSi in α-pinene oxide at various times

363

under air and at RT.

364

B- UV-visible absorption spectra of Ag2SO4 with TMDSi and starch nanocristals (StN) in ɛ-

365

caprolactone at various times under air and at RT.

366

It also shows the Kinetic of the reaction followed by increase of the maximum absorption of

367

silver nanoparticles during the synthesis at 404 nm.

368

This information is available free of charge via the Internet at http://pubs.acs.org/

369 370


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(60) Carotenuto, G.; Nicolas, F. Reversible Thermochromic Nanocomposites Based on

529

Thiolate-Capped Silver Nanoparticles Embedded in Amorphous Polystyrene. Materials 2009,

530

2, 1323-1340.

531 532 533 534 535


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Langmuir

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TOC Graphic:

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In-Situ Synthesis and Characterization of Silver/Polymer

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Nanocomposites by Thermal Cationic Polymerization Processes

539

at Room Temperature: Initiating Systems Based on

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Organosilanes and Starch Nanocrystals.

541 542

Mohamad-Ali Tehfe, Romain Jamois, Patrice Cousin, Saïd Elkoun, Mathieu Robert

543

544 545 546 547 548


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Polymer

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