Improvement of the Thermal and Optical Performances of Protective

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Improvement of the thermal and optical performances of protective polydimethylsiloxane space coatings with cellulose nanocrystal additives Mikael Planes, Jérémie Brand, Simon Lewandowski, Stephanie Remaury, Stéphane Solé, Cédric Le Coz, Stephane Carlotti, and Gilles S#be ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09043 • Publication Date (Web): 27 Sep 2016 Downloaded from http://pubs.acs.org on September 28, 2016

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ACS Applied Materials & Interfaces

Improvement

of

the

thermal

and

optical

performances

of

protective

polydimethylsiloxane space coatings with cellulose nanocrystal additives Mikael Planes,a,b,d Jérémie Brand,a,b Simon Lewandowski,c Stéphanie Remaury,d Stéphane Solé,e Cédric Le Coz,b Stéphane Carlotti,a,b,* Gilles Sèbe,a,b,* a

Univ. Bordeaux, Bordeaux INP, LCPO, UMR 5629, F-33600 Pessac, France

b

CNRS, LCPO, UMR 5629, F-33600 Pessac, France

c

ONERA-The French Aerospace LAB F-31055 Toulouse France

d

CNES-French Aerospace Agency, 18 avenue Edouard Belin F-31401 Toulouse cedex 9, France

e

MAP-ZI- 2 rue Clément Ader 09100 Pamiers, France

KEYWORDS Polydimethylsiloxane, cellulose nanocrystals, functionalization, thermal stability, UV stability ABSTRACT This work investigates the possibility of using cellulose nanocrystals (CNCs) as bio-based nanoadditives in protective polydimethylsiloxane (PDMS) space coatings, to improve the thermal and optical performances of the material. CNCs produced from wood pulp were functionalized in different conditions with the objective to improve their dispersibility in the PDMS matrix, increase their thermal stability and provide photoactive functions. Polysiloxane, cinnamate, chloroacetate and trifluoroacetate moieties were accordingly anchored at the CNCs surface by silylation – using two different approaches – or acylation with different functional vinyl esters. The modified CNCs were thoroughly characterized by FT-IR spectroscopy, solid-state NMR spectroscopy and thermogravimetric analysis, before being incorporated into a PDMS space coating formulation in low concentration (0.5 to 4 wt.%). The crosslinked PDMS films were subsequently investigated with regards to their mechanical behavior, thermal

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stability and optical properties after photo-ageing. Results revealed that the CNC additives could significantly improve the thermal stability of the PDMS coating – up to 140 °C, depending on the treatment and CNC concentration – without affecting the mechanical properties and transparency of the material. In addition, the PDMS films loaded with as low as 1 wt.% halogenated nanoparticles, exhibited an improved UV-stability after irradiation in geostationary conditions.

1. Introduction Over the past decades, polydimethylsiloxane (PDMS) has been widely investigated due to its unique properties such as a high flexibility, optical transparency in the Vis-NIR spectral regions, good thermal stability and resistance to oxidation or hydrolysis.1,2,3 PDMS networks have been mentioned in a wide variety of common applications such as adhesives, fabric finishing agents and coatings,4 but are also increasingly considered in high-value aerospace applications, in particular as encapsulating agent or protective space coating.5 In the latter case, the silicon material needs to meet specific requirements with regards to its thermal and UV stability, which condition its protective and optical properties.6 In particular, the space coating should limit heat transfer to the equipment and retain a high transparency in the 250-1000 nm wavelength range (transmittance should be > 90 %). The polydimethylsiloxane employed is generally produced by cross-linking a high molar mass hydride-terminated PDMS by hydrosilylation in the presence of a platinum catalyst (Karstedt's catalyst).7,8,9 Its utilization is however often limited by its insufficient thermal stability and sensitivity to photo-degradation under specific UV irradiation.10,11 Indeed, the homolytic photo-cleavage of siloxane bonds by solar radiations generally results in an increased absorption (coloration of the material), a loss in transparency, and loss in thermomechanical properties. To protect the PDMS films, some researchers have investigated the deposition of ZnO or ZnO-dopped quantum dots in the form of multilayers as UV-blocking system.12,13,14 The


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utilization of organic UV stabilizers such as Hindered Amine Light Stabilizers (HALS)15 or UV Absorbers16 (UVA) have been also envisaged, leading to the absorption-dissipation of part of the UV irradiation. Improvements in the PDMS thermal stability have been noted with ferric oxide or mica fillers,17 but such inorganic particles generally provokes a concomitant loss in transparency owing to light scattering.18 In view of their nanoscale structure, high aspect ratio and excellent mechanical properties, cellulose nanocrystals (CNCs) have the potential to serve as alternative fillers to improve further the properties of PDMS coating, while maintaining the transparency of the material. CNCs are needle-shaped nanoparticles, which are typically produced by submitting microcrystalline cellulose, paper or pulp to concentrate sulfuric acid combined with sonication. The treatment provokes the hydrolysis of the amorphous regions, leading to the release of nano-sized cellulose crystallites bearing sulfate ester groups at their surface.19,20,21 It is now well established that CNCs can mechanically reinforce polymers without modification of transparency, provided that the CNCs are well dispersed in the matrix and enable a good interfacial adhesion.22 This imply that the CNCs surface should be previously tailored by appropriate functions through chemical modification to prevent the self-aggregation of the hydrophilic particles in hydrophobic matrices.23,24 In particular, we have recently shown that the controlled surface silylation of nanofibrillated cellulose could significantly improve its dispersion and reinforcing properties in PDMS networks.25 Using a similar approach, we believe that a concomitant improvement in thermal and/or photo-stability could be additionaly imparted if the grafted moieties displayed thermo-resistant and/or UV absorbing properties.26 In this work, CNCs with various surface functionalities were incorporated into a PDMS space coating formulation and the cross-linked nanocomposites were investigated with regards to their mechanical behavior, thermal stability and optical properties after photo-ageing. CNCs produced from wood pulp were chemically modified by i) silylation with two different methods and ii) acylation with three


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functional vinyl esters, with the objective to introduce functions that can favorably impact the properties of the PDMS coating. Silylation has proven to be an efficient method to improve the thermal stability of CNCs and promote a good interface with PDMS,25 while acylation can serve as a tool to introduce photoactive cinnamate or halogen moieties.27 Cinnamates are interesting as they can absorb UV irradiation through reversible dimerization and/or trans-cis isomerization processes,28 while halogenated groups have been reported to catch the radicals form during the UV exposure of polymers.26 The modified CNCs were characterized by FT-IR, solid state NMR spectroscopy and TGA analysis. Various PDMS nanocomposite samples loaded with 0.5, 1 or 4 wt.% of CNCs were subsequently prepared and compared with respect to their mechanical behavior, thermal stability and optical properties after photoageing.

2. Experimental 2.1. Materials Vinyl cinnamate (VCin, 95 %), methyl trimethoxysilane (MTMS, 99%), Karstedt’s catalyst (platinum divinyltetramethylsiloxane complex in xylene, 2.1-2.4 wt.% in Pt), maleic anhydride (MA, 99 %) were purchased from Sigma Aldrich. Vinyl chloroacetate (VClAc) and vinyl trifluoroacetate (VtFAc) were purchased from TCI (Europe). The hydride terminated poly(dimethylsiloxane) PDMS6-8 (viscosity 2-3 10-2 cm2/s, Mw = 500-600 g.mol-1) and allyl glycidyl ether (AGE, 97 %) were purchased from ABCR. All solvents (DMF, DMSO, THF, acetone, DCM, heptane, chloroform) were purchased from Sigma Aldrich and used with no further purification, except DMSO and DMF, which were dried on molecular sieve (4 Å). The cellulose nanocrystals (CNCs) were purchased from the university of Maine (USA) (they contain 1.05 wt.% of sulfur). The polydimethylsiloxane resin was provided by MAP as a two-components


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material composed of a base (MAPSIL QS1123) and curing agent (MAPSIL QS1123). The base contains predominantly a vinyl-terminated PDMS with Pt catalyst (Karstedt). The curing agent is a mixture of vinyl-terminated PDMS and hydride-terminated PDMS. The cross-linked reference PDMS sample was prepared by mixing the base with the curing agent in a 10:1 weight ratio. All chemicals were used as received. Suprasil quartz windows for UV ageing experiments were purchased from Eurolabo (France). 2.2. Silylation of cellulose nanocrystals Hydrolysis/condensation of MTMS (Silylation 1): The first method used was a silylation process in water, based on sol-gel chemistry. CNCs (0.5 g) were dispersed in water (20 mL) by sonication (Bandelin 3200, power 25 %, 30s). One drop of acetic acid was added to the dispersion and stirred during 20 min. MTMS (0.5 g) was introduced in water (20 mL) with one drop of acetic acid and stirred until complete dissolution of the MTMS sol. The MTMS solution was added dropwise to the CNCs dispersion (in 5 min.) and stirred for 3 h at 20 °C. The silylated CNCs (CNC-MTMS) were finally freeze-dried with no further washing step. Hydrosilylation with PDMS6-8 (Silylation 2): The second method used involved the grafting of short PDMS chains (PDMS6-8) in three steps. CNCs (1 g) were first esterified with maleic anhydride (MA) (0.3 g) in DMF (25 mL), at 95 °C and for 90 min. The modified material was subsequently Soxhlet extracted overnight, in acetone. In the second step, the MA-treated material was reacted with 10 ml allyl glycidyl ether (AGE), for 7 h at 80 °C. After reaction, the material was again Soxhlet extracted overnight in acetone. In the third step, the MA-AGE-treated CNCs were hydrosilylated in 10 mL hydride-terminated PDMS, with 0.1 mL of Karstedt’s catalyst. The reaction was carried out at 80 °C, for 3 h. The silylated material (CNC-PDMS6-8) was finally washed by centrifugation with dichloromethane and heptane, and dried at 50°C.


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2.3. Acylation of cellulose nanocrystals with vinyl esters CNCs (0.5 g) were dispersed in DMSO (20 mL) with potassium carbonate (200 mg) and vinyl ester (0.5 mL), in a round-bottomed flask. The dispersion was homogenized by sonication (Bandelin 3200, power at 25 %, 30 s). The reaction was carried out for 45 min at 80 °C, under stirring. The modified CNCs were washed twice with THF then acetone, and re-dispersed in water before freeze-drying. This protocol was used for the modification with vinyl cinnamate (VCin), vinyl trifluoroacetate (VtFAc) and vinyl chloroacetate (VClAc). The obtained modified CNCs are named CNC-Cin, CNC-tFAc and CNC-ClAc, respectively. 2.4. Preparation of nanocomposites Unmodified/modified CNCs (50, 100 or 400 mg) were dispersed in 2 to 4 ml chloroform by sonication (Bandelin 3200, 0.5 kJ), and immediately introduced in 10 g of PDMS base (MAPSIL QS1123) under mechanical stirring. After 2h, the solvent was removed under vacuum, and 1g of curing agent (MAPSIL QS1123) was added under stirring. The mixture was then degassed under vacuum. For the thermomechanical analysis, the formulation was poured into a Teflon mold to obtain studs of 1 cm in diameter approximately. The PDMS films used in the photo-ageing experiments were prepared by blade casting the PDMS formulation on SUPRASIL Quartz (20x20x2 mm), using a blade adjusted to obtain a thickness of 100 µm. The films were cross-linked by curing the samples at 70 °C for 15h. 2.5. Infrared spectroscopy (FT-IR) Fourier transform infrared spectra were recorded using a Nicolet FT-IR spectrometer (AVATAR 370) in transmission mode. For each analysis, 2 mg of sample were mixed with 180 mg of KBr. All spectra were recorded between 400 to 4000 cm-1, with a resolution of 4 cm-1 and 64 scans. To compare samples, the same baseline correction was used and the spectra were normalized to the C-O stretching vibration of glucopyranose ring at 1060 cm-1, which was not affected by the chemical modifications.


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2.6. Solid state CP-MAS NMR spectroscopy Solid-state 13C and 29Si CP-MAS (cross-polarization magic angle spinning) NMR measurements were performed on Bruker Avance II 500 MHz and 400 MHz spectrometers, respectively. The

13

C and

29

Si

spectra were recorded at 100.61 MHz (2000 scans) and 79.49 MHz (12000 scans), respectively, using 4 mm ZrO2 MAS rotors. The spinning rate was fixed at 10 kHz with an acquisition delay of 5 s. Solidstate 35Cl and 19F static NMR measurements were performed on Bruker Avance II 500 MHz and Bruker Avance I 300 MHz spectrometers, respectively. The 35Cl and 19F spectra were recorded at 49 MHz (2500 scans) and 282.4 MHz (2000 scans), respectively, in static mode.

35

Cl NMR spectra were acquired by

means of a quadrupolar echo pulse sequence. The acquisition parameters were as follows: spectral windows of 132 kHz, π/2 pulse width of 2.10 µs, recycle delay of 2 s, echo delay that separates pulses of 40 µs. Lorentzian noise filtering with a width of 100 Hz was applied prior to Fourier transformation from the top of the echo signal.

19

F NMR spectra were recorded using a single pulse sequence. The

acquisition parameters were: π/2 pulse width of 3.75 µs, a recycling delay of 5 s and spectral width of 114 kHz (405 ppm). A Lorentzian filtering function of 2 Hz was applied. Chemical shifts are reported relative to external references: glycine for 13C (C=O set at 176.03 ppm), tetramethylsilane for 29Si (Si set at 0 ppm), NaCl for 35Cl (Cl set at 0 ppm) and trifluoro acetic acid for 19F (F set at –77 ppm). 2.7. Thermal and dynamic mechanical analyses Thermo-gravimetric analyses (TGA) were performed on two TA Instrument devices. The unmodified/modified CNCs were analyzed on a TA-Q500 instrument, under nitrogen flow. Samples (15 mg) were heated from 20 to 100 °C at a rate of 15 °C.min-1 (to dry the samples), held at 100 °C for 20 min, and then heated again from 100 °C to 600 °C at the same speed. The weight measured after the drying step was considered as the initial weight of the sample. The PDMS films were analyzed on TA-


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Q50 instrument, under nitrogen flow. Samples (10 mg) were heated from 20 to 600 °C at a rate of 10 °C.min-1. Differential scanning calorimetry analyses (DSC) were carried out on a DSC Q100 LN2 TA Instrument. PDMS samples (5 mg) were heated from -150 °C to 150 °C at a rate of 5 °C.min-1, with a t/T modulation of ± 0.64°C every 60 s. The instrument was calibrated with indium sample. Dynamic mechanical analyses (DMA) were performed on a TA Instrument RSA3. PDMS samples (studs of 6 mm in height and 15 mm in diameter) were heated from 0 °C to 200 °C under nitrogen atmosphere, at a rate of 5 °C.min-1. The measurements were performed in compression mode at a frequency of 1 Hz, with an initial static force of 0.5 N and strain sweep of 0.1 %. 2.8. Optical properties before and after photo-ageing Photo-ageing experiments were performed in a vacuum chamber coupled to a short arc xenon lamp (Oriel solar simulator, 1 000 W). This equipment allows reproducing the solar irradiation distribution close to the geostationary solar conditions. During irradiation, the pressure was maintained under 10-5 mbar and a water cooling system was used to control the temperature of the sample holder (25-30 °C). The vacuum chamber was coated with black paint to inhibit the reflection of UV radiations. The experiment time was in a range 500-650 of equivalent solar hours (EHS) corresponding to six month of geostationary flight (1112 ESH by year on N/S faces).29 Two irradiation sessions were performed: i) with PDMS containing 0.5 wt.% of CNCs, CNC-PDMS6-8 or CNC-Cin (ESH = 515); ii) with PDMS containing 0.5 or 1 wt.% of CNC-ClAc or CNC-tFAc (ESH = 625). The optical performances of the PDMS films were evaluated by comparing the transmittance of the films in the 250-1000 nm region, before and after photo-ageing. The transmittance spectra were recorded on a Perkin Elmer Lambda 900 double beam UV-Vis-NIR spectrometer coupled with an integrating sphere of 150 mm in diameter. The scanning speed was set to 400 nm.min-1 and the resolution was 1 nm. The gain in UV stability (∆g in %)


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was estimated by comparing the mean transmittances of the different materials in the selected wavelength range, before and after irradiation. The mean spectral transmittance (Ts) of the material in a selected wavelength range (λ1- λ2) can be calculated using the following equation (Eq 1):30,31

=

. .

.

(Eq 1)

where Is (W.µm-1.m-2) is the spectral irradiance of the sun. The level of degradation in the selected range (∆Ts) is given by: ∆ % = − (Eq 2) The gain in UV stability (∆g in %) is then deduced from ∆Ts: ∆g % = ∆T #$%& − ∆T '()

(Eq 3)

3. Results and discussion 3.1. Synthesis and characterization of functional CNCs The CNCs used in this study were isolated by sulfuric acid hydrolysis of wood pulp, according to a general procedure widely described in the literature.32,33 They consist in rod-like particles with estimated dimensions of 110 ± 48 nm in length and 5 ± 1 nm in thickness, based on AFM topography images (Figure 1a). The CNCs surface was tailored with various functions using different reaction pathways inspired from the literature (Figure 1b-d).19,20,21 The silylation of CNCs was performed according to two different approaches. The first method involved the freeze-drying of a water suspension of CNCs in the presence of methyltrimethoxysilane (MTMS), leading to the condensation of a polysiloxane layer at the nanocellulose surface (Figure 1b).25


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c

CNC-PDMS6-8

b

d

CNC-MTMS

CNC-Cin

CNC-tFAc

a (MTMS)

CNC 0

CNC-ClAc 2 µm

Figure 1. (a) AFM topography image of pristine CNCs. (b) General reaction scheme presenting the functionalization of CNCs by silylation with MTMS, (c) by hydrosilylation with PDMS6-8 and (d) by acylation with V-Cin, VtFAc or VClAc. In the second approach, a multi-step procedure was employed (Figure 1c):34 vinyl groups were first introduced to the CNCs surface by stepwise addition of maleic anhydride (MA) and allyl glycidyl ether (AGE). Subsequently, a low molecular mass hydride-terminated PDMS (PDMS6-8) was anchored at the terminal alkene via hydrosilylation. Acylation reactions were performed according to the general scheme presented in Figure 1d, with potassium carbonate as catalyst and under microwave activation.27 The vinyl alcohol formed during the process tautomerizes to acetaldehyde and the equilibrium is naturally shifted towards the ester formation. Cinnamate, chloroacetate and trifluoroacetate moieties were accordingly grafted at the CNCs surface, by reacting the accessible hydroxyl groups with VCin, VClAc and VtFAc, respectively. All samples were characterized by FT-IR spectroscopy to identify the specific absorption bands of the grafted functions (Figure 2).


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a

b

ν(C-F) n. a.

DMSO traces

n. a. δ(CH2)

ɣ(CH2Cl)

CNC-tFAc

ν(C-O) ν(C=O)ester

C=O

CH2-Cl

CNC-ClAc

ν(C=C)exo ɣ(=C-C)ring

* * * ν(C=C)ring δ(CH)

C=O

CNC-Cin

c

c

g

d-g

d f

b

e

b a

δ(CH)Silane H3C-Si

CNC-PDMS6-8

ν(Si-OH)

H3C-Si

CNC-MTMS C1

CNCs 1800 1600 1400 1200 1000 800

Wavenumbers

600

C4

C2-3-5

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


180 160 140 120 100 80

(cm-1)

C6

60

40

20

0

Chemical shifts (ppm)

Figure 2. (a) FT-IR absorbance spectra and (b) 13C CP-MAS NMR spectra of CNCs after modification with MTMS (CNC-MTMS), PDMS6-8 (CNC-PDMS6-8), VCin (CNC-Cin), VClAc (CNC-ClAc) and VtFAc (CNC-tFAc). ν: stretching vibration; δ, in-plane bending; γ, out-of-plane bending; n.a., not assigned.

On the spectrum of unmodified CNCs, the characteristic vibrations of cellulose appear at 3300 cm-1 (νOH), 2870 cm-1 (νCH) 1426 cm-1 (δCH), 1367cm-1 (δCH), 1332 cm-1 (δOH), 1317 cm-1 (δCH), 1200 cm-1 (δCH and δOH), 1157-893 cm-1 (νC-O and νC-C) and 651 cm-1 (νO-H).35 After silylation with both MTMS and PDMS6-8, the vibrations of the silane moieties were identified between 1250 and 1300 cm-1 (δCH of the methyl groups) and between 830 cm-1 and 730 cm-1 (νSi-C and/or νSi-O).36,37,38,39 Due to the high number


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of methyl and methylene groups in the PDMS6-8-treated sample, additional C-H stretching vibrations were observed at 2970 cm-1 in that case. The vibrations of the grafted cinnamate groups emerged at 1715 cm-1 (νC=O), 1632 cm-1 (νC=C exocyclic) and 767 cm-1 (γ=C-H aromatic) after esterification with VCin. The small signals at 1577 cm-1, 1493 cm-1 and 1449 cm-1 (see asterisk in Figure 2a) were assigned to the conjugated double bonds of the aromatic ring (νC=C). The absorption of the exocyclic C=C bond at 1632 cm-1 is relatively strong due to the conjugation with the aromatic ring.39 Two vibrations assigned to the chloroacetate group were detected after the treatment with VClA at 1750 (νC=O) and 1255 cm-1 (γCH2Cl), but the ester function was not clearly identified when VtFAc was used (CNC-tFAc in Figure 2a). In the latter case, a new unexpected band was detected at 1680 cm-1, which was not categorically assigned at this stage of the study. The functionalized CNCs were further characterized by spectroscopy and, when relevant, by 29Si,

35

Cl and

19

13

C CP-MAS NMR

F CP-MAS NMR spectroscopy. In Figure 2b, the

13

C NMR signals of cellulose were easily identified at 105 ppm (C1), 89 ppm (C4 crystalline), 85 ppm

(C4 amorphous), 75, 73, 72 ppm (C2/C3/C5), 65 ppm (C6 crystalline) and 63 ppm (C6 amorphous). Regardless of the functionalization performed, no significant change in the shape or sharpness of the cellulose signals (in particular the crystalline contributions) was detected, indicating that the grafting was essentially located at the surface of the nanocrystals. In the spectra of silylated samples, the carbons of the methyl groups bound directly to the silicon atom emerged at -3 ppm (CNC-MTMS) or 1 ppm (CNC- PDMS6-8). The complete hydrolysis of MTMS was supported by the absence of signal in the region of methoxy carbons at about 50 ppm. With CNC-Cin, the cinnamate group was observed in the form of weak signals at 160 ppm (C=O) and in the 120-180 ppm region (sp2 carbons of the exocyclic C=C and aromatic ring). The carbon shifts of the chloroacetate moiety were detected at 170 (C=O) and 40 ppm (CH2Cl) in CNC-ClAc, although the intensity of the CH2Cl signal could be overestimated by the presence of residual DMSO (expected in the same area). The additional signal observed at 160 ppm in that case, was not identified at this stage of the study, but could result from side reactions related to the


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electron withdrawing effect of Cl (nucleophilic substitutions). The presence of chloride was confirmed by the 35Cl NMR spectrum, in the form of a single signal at 48 ppm (Figure 3c). In consistency with the FT-IR results, the carbon shift of the trifluoroacetate moiety could not be detected in CNC-tFAc, although the presence of fluoride was attested by the 19F CP-MAS NMR spectrum (signal at –74 ppm in Figure 3d). Here, the electron withdrawing effect of the trifluoromethyl group might have a negative impact on the stability of the grafted ester moieties, in particular during the washing step with water. Indeed, it has been reported that poly(vinyltrifluoroacetate) polymers can release trifluoroacetic acid in contact with air moisture.40 The unidentifed infrared band at 1680 cm-1 in the FT-IR spectrum (Figure 2a), could then arise from the carbonyl stretching vibration of some residual trifluoroacetate ions.

2

1

a

b

6 4

2 1

3

3

4 5 5

100

50

0 29

-50

-100

-150

-200

-250

100

50

0 29

Si Chemical shift (ppm)

c

-50

-100

-150

6

-200

-250

Si Chemical shift (ppm)

d

1500

1000

500

35

0

-500

-1000

200

100 19

Cl Chemical shift (ppm)

0

-100

-200

F Chemical shift (ppm)

Figure 3. 29Si, 35Cl and 19F solid state NMR spectra of CNCs treated with (a) MTMS, (b) PDMS6-8, (c) VClAc and (d) VtFAc.


13


The structure of the MTMS condensed at the CNCs surface was deduced from the three chemical shifts observed at –65, –56 and –45 ppm in the 29Si NMR spectrum, which correspond to Si atoms with three, two or one siloxane bridge, respectively (Figure 3a).25 The condensation and/or hydrogen bonding between the hydrolyzed MTMS and cellulose surface is expected to occur at the Si[OX]2 or Si[OX] sites. In the case of the CNC-PDMS6-8 sample, three characteristic signals were detected at -20, -10 and 10 ppm, corresponding to the three silicon environments found in the grafted PDMS (Figure 3b). The relative ratios of

29

Si peak areas are in agreement with the degree of polymerization of the grafted

PDMS6-8. The thermograms of the unmodified, silylated and esterified cellulosic samples are shown in Figure 4 and the degradation data are summarized in Table 1.

100

CNC CNC-MTMS CNC-PDMS6-8

80

Weight (%)

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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CNC-Cin CNC-ClAc CNC-tFAc

60

40

20

0 0

100

200

300

400

500

600

Temperature ( °C) Figure 4. TGA thermograms of CNCs before and after modification with MTMS, PDMS6-8, VCin, VClAc and VtFAc.


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Table 1. TGA data obtained after analysis of unmodified, silylated and esterified CNCs. Sample

CNCs

CNC-MTMS

CNC-PDMS6-8

CNC-Cin

CNC-ClAc

CNC-tFAc

T5% [°C] Tmax [°C] Yc [%]

270 296 20

264 289 29

210 308 11

256 317 18

248 346 22

225 324 12

T5%: 5 % weight loss temperature; Tmax: temperature of maximum rate of degradation, Yc: char yield at 600 °C.

The thermogram of unmodified CNCs is consistent with the thermal behavior generally observed for cellulose nanocrystals bearing sulfate ester groups at their surface.41,42,43 Thermal degradation of cellulose takes place in a multi-step process that involves i) evaporation of adsorbed water at 25-100 °C, ii) decomposition of cellulose at 250-400 °C into tarry volatiles (levoglucosan, anhydrosugars) and char through concurrent depolymerization and dehydration reactions and iii) conversion of aliphatic char into polycyclic aromatics at 400-600 °C.44 Even in small concentration, the sulfate ester groups at the CNCs surface are known to negatively impact the thermal stability by significantly decreasing the degradation temperature of cellulose.41 Although an improvement in thermal stability could be expected after silylation with MTMS,25 no significant modification of the thermogram was noted (only a marginal decrease in T5% and Tmax), probably because the CNCs surface was not sufficiently covered by the hydrolysed MTMS in our experimental conditions. The increase in char yield (Yc) measured could be related to the formation of inorganic species containing silicon. In all other cases the degradation started earlier (T5% decreased) but the temperature of maximum rate of degradation (Tmax) increased significantly, indicating that the grafted moieties retarded cellulose degradation to some extent. The samples treated with PDMS6-8, VClAc and VtFAc displayed a similar weight loss between 100 and 250 °C (∼7 %), which was associated with the progressive cleavage of the grafted moieties at the CNCs surface. The initial degradation was much higher with CNC-Cin (weight loss ∼13 % between 100 and 250 °C), indicating that the photoactive cinnamate moieties accelerated the degradation of cellulose, most probably by promoting chain scissions through free-radical mechanisms (cinnamate derivatives can


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undergo radical trans-cis isomerization under thermal activation28). At high temperature, the best improvements were obtained with CNC-ClAc and CNC-tFAc, which displayed an increase in Tmax of 50 °C and 28 °C, respectively. The higher thermal stability was assigned to the presence of chlorine and fluorine within the material, which might act as radical scavengers during cellulose degradation when radical scissions are involved. The better results obtained with chlorine are consistent with the better thermal stabilization expected with such compounds.26,45

3.2. Properties of PDMS networks loaded with CNCs Various nanocomposite films were prepared by incorporating 0.5, 1 or 4 wt.% of unmodified/modified CNCs into a PDMS space coating formulation. The cross-linking of the material was performed by curing the material at 70 °C for 15h, in the presence of Karstedt’s catalyst. The analysis of the uncured PDMS by DSC revealed a unique exothermic event at 80 °C, corresponding to the formation of the 3D network. No exothermic peak was detected when the neat or filled PDMS was cured at 70 °C for 15h and subsequently analyzed by DSC, indicating that the material was totally cross-linked in these conditions. The glass transition temperature was recorded at around -120 °C for both the neat and filled PDMS. The composites were tentatively characterized by TEM, to verify the dispersibility of the fillers in the matrix. Unfortunately, the preparation of the samples by cryo-cutting turned out to be tricky (due to the low Tg of PDMS) and useful micrographs could not be obtained. Nevertheless, all PDMS films were transparent, indicating that the multiple light scattering effects that could arise from possible CNCs aggregates were negligible.


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3.2.1. Influence of the CNCs on the dynamic mechanical properties of PDMS The most important criteria for the space coating application are the initial transparancy, the thermal stability and the optical stability under UV irradiation. Accordingly, the improvement of the mechanical properties is not critical for this study, although the mechanical performances should not be significantly deteriorated by the addition of the CNCs filler. To verify this point, the dynamic mechanical properties of the PDMS composites were investigated by dynamic mechanical analysis (DMA), in the temperature range between 30 and 250 °C. The unfilled PDMS material displayed a storage modulus of about 3.106 Pa, in consistency with the E’ value expected with this type of elastomeric materials.46,47 The mechanical properties of the films did not change significantly after the incorporation of 0.5 or 4 wt.% of unmodified/modified CNCs (Figure 5), indicating that the network formation was not compromised by the presence of the cellulose nanoparticles. Indeed, the length the PDMS chains used in the formulation (length ~ 80-120 nm since Mn ~ 20000-30000 g.mol-1) is high enough to tolerate the incorporation of 5 nm-wide nanorods inside the 3D network, without changing significantly the mechanical properties of the material. The improvement of the mechanical performances of a polymer matrix filled with CNCs fillers is conditioned by i) a good adhesion at the filler/matrix interface and ii) the formation of a percolation network within the matrix, which is effective above a minimum particle concentration (percolation threshold).48 In this context, the slight decrease in storage modulus noted with unmodified CNCs can be assigned to i) the low compatibility with the hydrophobic PDMS matrix and ii) the low filler concentrations used in our study. After functionalization of the CNCs surface, a stronger filler/matrix interface is expected, but the filler content remains low and the possibility of forming a percolation network by hydrogen bonding is reduced after substitution of the OH groups. Consequently, the positive effect imparted by the surface modification is probably counterbalanced by the negative impact on the percolation network, leading to the marginal modification of E’ noted in Figure 5.


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Figure 5. DMA curves (30-250 °C) of neat and filled PDMS films with (a) 0.5 wt.% and (b) 4.0 wt.% of unmodified/modified CNCs.

3.2.2. Influence of the CNCs on the thermal degradation of PDMS The TGA thermograms of the different PDMS composites are presented in Figure 6 and the degradation data are summarized in Table 2. The main degradation temperature of neat PDMS recorded at Tmax = 405 °C is associated with the depolymerization of PDMS backbone through random scission reactions and formation of low molar mass cyclic siloxane species.48,49 The incorporation of unmodified, silylated or esterified CNCs into the PDMS matrix improved significantly the thermal stability of PDMS, as indicated by the high increase in Tmax recorded in the presence of fillers. The degradation of cellulose within the composites was detected in the temperature range between 250 and 360 °C (depending on the surface treatment), with samples containing 4 wt.% filler (Table 2). The corresponding weight loss measured varied between 1.8 and 3 wt.%, indicating that part of the degradation products remained trapped within the PDMS matrix. These products could act as thermal stabilizers inside the material, by disturbing the random scission reactions. Once dispersed in the PDMS matrix, these tarry of char


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residues could indeed delay the formation of cyclic siloxane volatiles by rigidifying the PDMS network or interacting with the PDMS fragments formed during depolymerization.

Figure 6. TGA thermograms of neat PDMS and PDMS loaded with (a) 0.5 wt.% and (b) 4.0 wt.% of unmodified/modified CNCs.

Regardless of the type of CNCs used, the thermal stability of the composites was found to increase with increasing CNCs content, but differences were noted depending on the surface treatment. These differences did not correlate with the intrinsic thermal stability of the particles previously measured by TGA (Figure 4 and Table 1). The highest enhancements were obtained with CNC-Cin, CNC-PDMS6-8 and CNC-MTMS, for which Tmax increased by 65-105 °C with 0.5 wt.% filler, and by 120-140 °C with 4 wt.% filler. In comparison, the initial thermal degradation under nitrogen was retarded by 20 °C when 5 wt% ferric oxide particles were incorporated into PDMS matrices, but the temperature of maximum rate of degradation decreased.17 Moreover, no improvements were noted when glass frit or mica were tested in inert atmosphere (T5% and Tmax decreased). The better performances obtained with some modified CNCs compared with others could be related to their surface chemistry: the CNCs treated VCin, PDMS6-8 and


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MTMS could be more compatible with the PDMS matrix and promote a better dispersion (silylation in particular, has been shown to improve the dispersion of nanocellulose in PDMS networks25). The better dispersed fillers could then have a stronger influence on the thermal degradation processes, by increasing the probability of disturbing the random scission reactions of the PDMS. Nevertheless, the nature of the chemical grafts at the CNCs surface could also be at stake and additional experiments are required to clarify this point.

Table 2. TGA data obtained after analysis of neat PDMS and PDMS loaded with the various CNC fillers. Cellulose degradation within the composite Samples

T5% (°C)

Neat PDMS PDMS with 0.5 wt.% of CNCs PDMSCNC PDMSCNC-MTMS PDMSCNC-PDMS6-8 PDMSCNC-Cin PDMSCNC-ClAc PDMSCNC-tFAc PDMS with 4 wt.% of CNCs PDMSCNC PDMSCNC-MTMS PDMDCNC-PDMS6-8 PDMSCNC-Cin PDMSCNC-ClAc PDMSCNC-tFAc

Tmax(°C)

Yc (%)

Temperature range (°C)

Weight loss (wt.%)

360

405

24

-

-

420 400 400 400 390 370

460 470 490 510 460 430

19 23 27 23 21 22

-

-

410 405 420 425 425 415

500 550 535 525 475 500

22 33 23 23 20 21

240-290 250-300 280-310 250-300 260-350 290-360

2.7 2.0 3.0 2.3 2.8 1.8

T5%: 5 % weight loss temperature; Tmax temperature of maximum rate of degradation; Yc: char yield at 600 °C

3.2.3. Evolution of the optical properties after photo-ageing Optical performance is one of the key parameters when formulating a good aerospace PDMS coating, which should display a high initial transmittance (> 90 %) in the 250-1000 nm wavelength range.


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Unexposed PDMS films (100 µm in thickness) were accordingly compared with samples irradiated for 500 to 650 equivalent solar hours (ESH), corresponding to geostationary conditions. The UV stability of the composites was evaluated by measuring the transmittance of the films in the 250-1000 nm region before and after photo-ageing, using a procedure especially designed for the characterization of aerospace coatings. The evolution of transmittance in 250-400 nm region was particularly regarded, as this wavelength range represents 90 % of the UV dose emitted by the sun. These experiments were limited to composite films meeting the transparency requirements in our experimental conditions, i.e. the composites filled with CNCs, CNC-PDMS6-8, CNC-Cin, CNC-ClAc and CNC-tFAc. Two irradiation sessions were performed, i) with 0.5 wt.% of CNCs, CNC-PDMS6-8 or CNC-Cin (ESH = 515), and ii) with 0.5 or 1 wt.% of CNC-ClAc or CNC-tFAc (ESH = 625). The UV transmittance of PDMS films before and after photo-ageing, is presented in Figure 7. Before irradiation, all the films displayed a constant transmittance of 90 % or more in the 250-1000 nm region, except when the PDMS was loaded with CNC-Cin (Figure 7a). In that particular case, an absorption band was measured at about 270 nm, which was assigned to the inherent absorption of the cinnamate moiety grafted at the CNC surface (expected at 254 nm). Whatever the sample, a significant decrease in transmittance is observed between 250 and 400 nm after photo-ageing, due to the homolytic photocleavage of siloxane bonds,10,50 but differences were noted depending on the CNC filler. The unmodified nano-filler had quasi no impact on the optical properties of the films after photo-ageing, while the photoactive CNC-Cin particles accentuated further the photo-degradation. Some improvement was noted with CNC-PDMS6-8, above 300 nm, but the best results were obtained CNC-ClAc and CNC-tFAc. The loss in transmittance was significantly attenuated with these two fillers (Figure 7b), most probably because the chlorine and fluorine atoms dispersed in the PDMS matrix acted as radical scavengers within the material.26 During photo-aging, the bonds in PDMS (Si-H, Si-C, Si-O and C-H) are subjected to homolytic cleavages, leading to the formation of highly reactive free radicals, which can undergo


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various inter- and/or intra-molecular rearrangements.10,51,52 When CNC-ClAc and CNC-tFAc are incorporated in the PDMS matrix, additional radicals can be formed from the labile halogens, which can lead to the entrapment of some of the PDMS radicals like in the case of Hindered Amine Light Stabilizers.53,54 The gain in UV stability (∆g in %) was estimated by comparing the mean transmittances of the different materials in the selected wavelength range, before and after irradiation (Eq.3 in the experimental section30,31) and reported in Table 3. The gain in UV stability in the 250 to 450 nm region was quite significant with 0.5 wt.% CNC-ClAc or CNC-tFAc (~ 2-3 %), and increased further when the filler content was doubled (~ 6-7 %). The better results were obtained with CNC-ClAc, which allowed improving the UV stability by 6.9 %. In comparison, PDMS coatings prepared with spin-coated ZnO particles displayed UV-blocking properties between 250 and 350 nm, but had a detrimental effect in the 350-550 nm region, where up to 20% loss in initial transparency was noted.12


22

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a

b

100

100

Before Ageing

Before Ageing

80

Transmittance

Transmittance

80

60

PDMS (neat) PDMSCNC (0.5 wt.%)

40

PDMSCNC-PDMS6-8 (0.5 wt.%)

20

0 200

300

400

500

600

700

800

900

60

PDMS (neat) PDMSCNC ClAc(0.5 wt.%)

40

PDMSCNC ClAc(1 wt.% ) PDMSCNC tFAc(0.5 wt.%)

20

PDMSCNC-Cin (0.5 wt.%)

0

1000

PDMSCNC tFAc(1 wt.%) 300

400

500

λ (nm)

700

800

900

1000

10 0

After Ageing

80 60 40 20

Transmittance

Transmittance

600

λ (nm)

1 00

0

80

After Ageing

60 40 20 0

30 0

400

5 00

600

7 00

λ (nm)

8 00

90 0

1 0 00

300

400

500

600

700

λ (nm)

800

900

1000

60 50

70

Transmittance

Transmittance

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


60

50

515 ESH

40

40 30 20

625 ESH

10

2 60

2 80

300

32 0

λ (nm)

34 0

36 0

3 80

260

280

300

320 340 λ (nm)

360

380

Figure 7. UV-spectra (250-1000 nm range) of PDMS nanocomposite films prepared with 0.5 or 1 wt.% CNCs fillers, before and after photo-ageing. (a) PDMS loaded with PDMSCNC, PDMSCNC-PDMS6-8 or PDMSCNC-Cin. (b) PDMS loaded with PDMSCNC-ClAc or PDMSCNC-tFAc. ESH = equivalent solar hours.


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Table 3. Gains in UV stability (∆g) in the 250-1000 nm and 250-450 nm regions, for PDMS films loaded with various CNCs fillers (0.5 wt.% or 1 wt.%).

Type of filler

Filler content (wt.%)

Wavelengths range (nm)

∆g (%) (Gain in UV stability

CNC

0.5

CNC-PDMS6-8

0.5

250-1000 250-400

0.5 0.7

250-400

1.5

CNC-tFAc

0.5

250-1000

0.3

250-400

1.7

1.0

250-1000

2.0

CNC-tFAc

250-400

6.4

CNC-ClAc

0.5

250-1000

0.8

250-400

3.3

1.0

250-1000

1.9

CNC-ClAc

250-400

6.9

4. Conclusion We have presented a novel approach to improve the thermal and optical performances of protective polysimethylsiloxane (PDMS) space coatings, using cellulose nanocrystals (CNCs) as bio-based nanoadditives. Before investigating the composites, the CNCs surface was silylated or acylated in various conditions, to provide functions that can promote the dispersion of the hydrophilic nanoparticles in the hydrophobic PDMS matrix, while improving the thermal and/or optical performances of the material. The grafting of polysiloxane, cinnamate, chloroacetate and trifluoroacetate moieties was accordingly envisaged, using different synthetic strategies. The success of the different reactions was confirmed by FT-IR and solid-state NMR spectroscopy, except in the case of the vinyl trifluoroacetate treated particles, which contained fluoride but did not display the expected ester function. Various PDMS nanocomposite films were subsequently prepared by cross-linking a PDMS space coating formulation in the presence of 0.5, 1 or 4 wt.% modified/unmodified CNCs. The mechanical properties and


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transparency of the films did not change notably after the incorporation of the fillers, indicating that the nanoparticles had a marginal impact on the network formation in the concentration range studied. The thermal stability on the other hand was significantly improved, even when the CNCs surface was not modified. This positive impact was assigned to the release of stabilizing species during cellulose decomposition, which may disturb the scission reactions occurring during PDMS depolymerization. The best results were obtained with the silylated and cinnamate treated particles, which may display the best dispersive properties in the PDMS matrix. The thermal stability of the silicone polymer could be increased by up to 105 °C with only 0.5 wt.% filler in that case, and up to 140 °C with 4 wt.%. The optical performances on the other hand, could be improved only when the halogenated nanoparticles were used. The loss in transmittance after irradiation in geostationary conditions was significantly attenuated with these two fillers, most probably because of the radical scavenging properties of the chlorine and fluorine atoms. The best results were obtained with the chlorinated nanocrystals, which allowed improving the UV stability by 6.9 % in the 250-450 nm region, with as low as 1 wt. % filler. This additive could present the best compromise between thermal and optical properties for an application in geostationary conditions, but the chloroacetate content at the CNCs surface, and the loading level in the PDMS matrix have yet to be optimized. Moreover, the coating properties could be further improved by investigating the impact of synergetic combinations of halogenated and silylated CNC fillers. The current results illustrate the potential of functionalized cellulose nanocrystals as active fillers in polysiloxane matrices, which compete favorably with mineral fillers and can find applications in area where the thermal and photo-stability of the material is critical. In optoelectrics for instance, there is an increasing demand for PDMS coatings withstanding high temperature in working conditions, due to the increasing miniaturization and multifunctionalization of the electrical devices. The benefits of the CNCs fillers was demonstrated in our study, but further breakthrough could be made by investigating other surface functionalizations, or combining CNCs with different surface properties.


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Acknowledgments This work was supported by a co-funding, CNES and ONERA for the UV studies. The part on chemical functionalization of cellulose nanocrystals was supported by ADEME and the Aquitaine region. The authors would also thank MAP for their participation in this project and the IECB laboratory for the solid state NMR analysis.

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Table of Contents Graphic

Geostationary conditions

UV

Protective space coating PDMS Cellulose nanocrystals

Chemical functionalization


Improvement of the Thermal and Optical Performances of Protective

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