Impact of Silica Nanoclusters on Furfuryl Alcohol Polymerization and

Article pubs.acs.org/JPCC

Impact of Silica Nanoclusters on Furfuryl Alcohol Polymerization and Molecular Mobility Nicolas Bosq,‡ Nathanael Guigo,*,‡ Guillaume Falco,‡ Jacques Persello,† and Nicolas Sbirrazzuoli*,‡ ‡

Institut de Chimie de Nice, UMR CNRS 7272, Université Côte d’Azur, 06100 Nice, France Institut de Physique de Nice, UMR CNRS 7010, Université Côte d’Azur, 06100 Nice, France

†

S Supporting Information *

ABSTRACT: Nanocomposite materials present attractive properties and are widely employed in various applications. Most of the time, the insertion of nanoparticles in a polymer matrix induces an enhancement of its performances, yet the effect of the filler on the polymerization mechanisms and the glass transition is less often investigated. In the present study, the PFA/silica nanocomposite was studied to highlight the variation of its polymerization behavior, thermomechanical properties and glass transition induced by the presence of a clustered silica nanoparticles network. The structure of nanosilica clusters was studied by thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR). The furfuryl alcohol (FA) polymerization was studied via its activation energy variation in the presence of nanosilica clusters and anhydride maleic (MA) that led to some modifications of the polymerization mechanism. An enhancement of thermal stability and an increase of glass transition temperature have been put in relief by dynamic mechanical analysis (DMA) and were correlated to the presence of silica. Finally, the activation energy associated with the glass transition highlighted a change of the polymer chain motion process in the presence of silica. combination of organic and inorganic polymeric systems.12 The tetrafurfuryloxysilane (TFOS) which was synthesized from FA was the first example of twin monomer that has generated hybrid interpenetrated network of PFA and SiO2.12,13 More recently, PFA/SiO2 hybrid network was prepared neither from in situ sol−gel process nor twin polymerization but from polymerization of FA with pre-existing silica spherical nanoparticles. These latter were decorated with furan entities onto surface which resulted in enhanced interactions with the furanic polymer as exemplified by the higher thermal performance of the nanocomposite.14 Pre-existing silica particles can present different architectures. Colloidal aggregates can be formed through assembly of silica nanoparticles held together by surface−surface forces.15 These aggregates can create some bridges with each other which can tune the mechanical or the flowing properties. For instance, they can have different resistance to compressive stress depending on the their dispersion in aqueous media.16 The surface chemistry and the morphology appear to be the key step in the interplay between the matrix and the filler. Usually the silica surface is rather hydrophilic due to the presence of hydroxyl groups which thus limits the filler compatibility to hydrophilic matrices. The characteristic of the interphase between the filler and the matrix at molecular level governs

1. INTRODUCTION In order to reduce the environmental footprint, alternative resources to oil-based feedstocks must be considered. Biomass is the unique sustainable source of carbons for the preparation of organic polymers and materials. As an instance, large quantities of naturally existing polymers (cellulose, hemicellulose, lignin, tannins) or deconstructed building blocks such as sugars (glucose, xylose) can be obtained from the lignocellulosic biomass.1 The furfural is a platform chemical obtained from the dehydration of C5 sugars (xylose) which is mostly converted into furfuryl alcohol (FA).2 The FA monomer can polymerize through cationic condensation reaction and finally leads to the polyfufuryl alcohol (PFA).2 This thermosetting polymer can be used for many applications since it displays a good chemical inertness and thermal stability with high carbon content. The PFA is employed for the preparation of binders and fire resistant materials,3,4 and as well for the foundry molds.5,6 It is well-known that the elaboration of organic−inorganic hybrids is valuable since it allows the combination of organic polymer properties with those of inorganic fillers. The pioneering studies on PFA/SiO2 hybrids was presented in the work of Spange et al.7−9 and Kawashima et al. 10 via simultaneous polymerization. In these latter, the silica network was synthesized via a sol−gel process and the simultaneous polymerization with FA resulted in hybrid network.11 Following an alternative strategy, the twin polymerization was introduced which allows preparing, in one single procedure, a nanometric © XXXX American Chemical Society

Received: December 22, 2016 Revised: March 7, 2017 Published: March 17, 2017 A

DOI: 10.1021/acs.jpcc.6b12882 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

concentrated until a weight fraction of ∼0.05. The final pH was ∼9, and the final ionic strength was ∼5 × 10−3 M. 2.2. Surface Modification. In order to transform the sodium alkoxide functions into hydroxyl functions onto the silica surface, the silica was blend and stirred with sulfuric acid solution ([H2SO4] = 80 g·L−1) until the pH reaches a value of ∼0.5. Several washes were then performed with deionized water until the pH reaches a value of ∼3 in order to set the equilibrium below:

the major changes in properties. The interfacial (covalent or noncovalent) bonding can on one hand modify the course of polymerization as shown for epoxy/amine curing.17 On the other hand, the presence of nanohererogeneities within a polymeric matrix alters the molecular mobility of the polymer chains. These modifications are peculiarly highlighted in the glass transition behavior of the interphase regions.18 However, very few studies put in parallel the role played by nanofillers both on the polymerization pathway and the relaxation behavior of polymer chains. Therefore, this paper aims to understand how the introduction of pre-existing nanofillers before the polymerization modifies its kinetic as well as the chain motional processes after polymerization. In such a perspective, the FA/ SiO2 system is particuliarly interesting since stable dispersion of silica could be obtained before polymerization14 and the inclusion of silica network within the PFA matrix is known to modify its molecular mobility.11,14 The originality of this work is that the silica nanoparticles employed here were clustered with each other prior dispersion with the monomer. In consequences, the clustered silica was synthesized in a specific way to obtain a high specific surface bearing a high density of hydroxyl groups. The hydroxyl groups present in the surface of nanosilica clusters were investigated with thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR). The dispersion and the morphology of silica clusters in the monomer were studied with diffractometry and transmission electronic microscopy (TEM). First, the influence of clustered silica on FA polymerization was studied by means of differential scanning calorimetry (DSC) and the apparent activation energy of polymerization was calculated from advanced isoconversionnal methods.19−26 Then, the observed final macroscopic properties and glass transition behavior were tightly linked to the dispersion of clustered nanoparticles into the matrix and to the surface morphology of silica. The elaborated PFA and PFA/ silica cross-linked materials were characterized by TGA and dynamic mechanical analysis (DMA) in order to highlight the variations of thermomechanical properties. The frequency analysis of glass transition was performed from DMA and the associated apparent activation energy at the glass transition was estimated from the Arrhenius equation.27

SiO− + H+ = SiOH

The value of pH located between 0.5 and 3 allows the transformation of SiO− in SiOH functions. 2.3. Elaboration of the PFA/MA Matrix. MA was mixed in FA solution at a weight ratio of FA/MA = 100/2 under vigorous mechanical stirring. The PFA/MA was prepared by heating ∼200 mL of the FA/MA mixture in a round flask at 110 °C during 30 min. The mixture was vigorously stirred during the overall process to ensure its homogeneous heating. The obtained pasty PFA/MA resin was extracted from the round flask and was placed in a Teflon mold. The PFA/MA resin was cured during 2 h at 160 °C under pressing (∼1 MPa). The rectangular samples of cured PFA/MA matrix were extracted from the mold and were heated at 200 °C during 1 h. The ensuing material presents no residual exothermic heat flow on DSC thermogram, suggesting the completion of cure. 2.4. Elaboration of PFA/MA/SiO2(c) Nanocomposite. The nanosilica clusters (SiO2(c)) have been added to FA solution at a weight ratio of FA/SiO2(c) = 95/5. The nanosilica clusters where then dispersed into the solution by ultrasonic probe. In order to avoid any cross-linking reaction during the dispersion step, the MA was added to the mixture by sonication with a weight ratio of FA/MA/SiO2(c) = 95/2/5. The blend was heated in a Teflon flask at ∼100 °C until the apparition of a pasty resin. The resin is finally placed in a Teflon mold and cured during 2 h at 160 °C under pressing (∼1 MPa). The cured PFA/MA/SiO2(c) nanocomposite was extracted from the mold to perform a postcuring at 200 °C during 1 h. As for the PFA/MA matrix, the nanocomposite material did not present any residual exothermic peak suggesting the completion of cure. 2.5. Experimental Techniques. Fourier transform infrared (FTIR) spectra were performed on a PerkinElmer Spectrum BX II spectrometer. The analysis was conducted in reflective diffusion mode. A total of 64 scans were recorded for each spectrum from 4000 to 400 cm−1 using a resolution of 4 cm−1. Prior to FTIR measurement, the SiO2(c) sample was dried at 60 °C under vacuum and was dispersed into KBr powder. The obtained mixture was finally disposed on a frame to be analyzed. The morphology of SiO2(c) and its dispersion into the PFA/ MA matrix were observed by transmission electronic microscopy (TEM) using a JEOL JEM-1400 with an accelerator voltage of 120 kV. In order to perform the analysis of the nanocomposite, an ultramicrotome was employed to cut ultrathin sections (∼80 nm) of the materials. The obtained micrographs display the inorganic components black/gray colored. The differential scanning calorimetry (DSC) was performed on DSC 1 from Mettler Toledo. The different liquid samples of ∼15 mg were placed into 30 μL high pressure steel pans. Each sample was heated from −30 to +300 °C at various heating

2. EXPERIMENTAL SECTION 2.1. Nanosilica Clusters Synthesis. The synthesis has been conducted according to the nucleation-and-growth process described by Iler15 and Parneix et al.16 An aqueous sodium silicate solution was neutralized with sulfuric acid, as described in the equation below: 3.4SiO2 /Na 2O + H 2SO4 3.4SiO2 + Na 2SO4 + H 2O

(2)

(1)

−1

First, sulfuric acid ([H2SO4] = 17 g kg ) was added to a dilute sodium silicate solution ([SiO2] = 2.5 g kg−1) until a pH of ∼9 to allows the nucleation of silica particles. The nucleation rate was controlled by temperature in the range 60−90 °C with a constant stirring at 250 rpm. The simultaneous addition of a sodium silicate solution ([SiO2] = 39 g kg−1) and of a sulfuric acid solution ([H2SO4] = 17 g kg−1) allows the particle growth from the existing nuclei. The control of addition rate leads to a pH maintained at 9 and to a temperature kept at 90 °C. After cooling to room temperature, the mixture was washed with deionized water in order to remove sodium sulfate and other ions. In the end, the dispersion of nanosilica (SiO2) was B


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The Journal of Physical Chemistry C rates (1, 2, 4, and 6 K min−1) under nitrogen flow (50 mL min−1). The thermogravimetric analysis (TGA) was performed on a TGA 851e from Mettler-Toledo. The microbalance has a precision of ±0.1 μg and is kept at constant temperature (∼22 °C) during analyses in order to avoid its weight variation with temperature. Samples of 10 ± 3 mg were placed into 70 μL alumina pans. The samples were heated from 25 to 900 °C at 10 °C min−1 under nitrogen flow (50 mL min−1). Dynamic mechanical analysis (DMA) experiments were conducted on a Triton Technology Ltd. instrument from Mettler-Toledo. The samples were submitted to a longitudinal deformation in nonisothermal mode from 25 to 180 °C using a heating rate of 1 °C min−1, a frequency of 1 Hz and a strain of 0.01%. Prior to DMA analyses, the samples were precisely sized into a rectangular shape. The viscoelastic parameters are tightly linked to the sample size so their dimensions were measured precisely (±0.01 mm) and the different samples present dimensions that were typically ∼30 mm long, ∼4 mm wide, and ∼2 mm thick. To take into account the potential discrepancies between the samples, five different samples per materials were prepared and were subjected to DMA.

Figure 1. Diffractometry measurements of SiO2(c)/FA submitted to different ultrasonic treatment. The value of the energy is indicated by each curve. For an energy of 90 kJ the measurement was performed at an initial time t (full line) and at the time t + 10 min (dot).

Table 1. Estimation of Clusters Size with the Energy Transmitted by the Ultrasonic Probe

3. RESULTS AND DISCUSSION 3.1. Characterization of Silica Surface. The morphology of nanosilica clusters (SiO2(c)) has been investigated by TEM analysis and the results are depicted in Supporting Information. The synthesized nanoparticles are clustered with each other. According to Parneix et al.,16 a fine layer of silicate (∼1 nm) envelops each unit in order to create a silica network. Besides, their study shows that this silica displays a specific surface of ∼180 m2/g and a OH groups surface density of 5 OH nm−2. The FTIR analysis is known to be well adapted for the characterization of nanosilica surface and the chemical structure of the samples can be investigated via this technique.28−30 The FTIR spectra of SiO2(c) is thus shown in Supporting Information. A sharp and strong band appears at ∼1100 cm−1 and corresponds to the Si−O−Si bending vibrations from the nanoparticle core. The broad band at ∼3200 cm−1 corresponds to the O−H stretching and the band at ∼970 cm−1 corresponds to the Si−OH bending vibrations. It confirms that the presence of silanol groups at the surface of the silica clusters. The TGA is a technique widely used in many studies for the characterization of silica surface.31−33 The thermal degradation of nanosilica clusters was studied here and the TGA thermogram of SiO2(c) is displayed in Supporting Informations. Weight loss between 25 and 150 °C corresponds to the evaporation of the residual water trapped into the silica powder. On the other hand, weight loss between 200 and 900 °C is associated with the reorganization mechanisms onto the silica surface and to the thermal degradation of the nanoparticles. This weight loss is due to the hydroxyl groups of the silica that react with each other in this temperature range.34 Consequently, the TGA thermogram attests to the presence of hydroxyl groups on the nanosilica surface and corroborates the information obtained from FTIR analysis. 3.2. Dispersion of Silica into the FA. The data presented on Figure 1 and Table 1 shows the size distribution of nanosilica clusters dispersed into the FA as a function of the energy provided via sonication. The results of the analysis display a downward shift of the nanosilica cluster size distribution with increasing the energy. The nanosilica clusters

energy/kJ

clusters size/μm

0 45 90

10−140 1−90 0.07−0.57

size and their dispersion into the FA can be consequently optimized via the energy transmited by the ultrasonic probe to the blend. This result can be explained by the wavelength value of ultrasonic waves in the FA liquid monomer that is similar to the initial silica clusters size. An homogeneous dispersion of the nanosilica clusters into the FA is then obtained. Besides, it must be stressed that the distribution of nanosilica clusters size after receiving an energy of 90 kJ is similar at the time t and at a the time t + 10 min. As no additional energy is provided to the blend during these 10 min, this result shows that the dispersion of SiO2(c) into the FA is then stable during a consequent period of time and does not reaggregate before proceeding to the acid-induced polymerization. 3.3. Polymerization Kinetics. Normalized nonisothermal DSC curves of FA/MA/SiO2(c), FA/MA, FA/SiO2(c), and FA polymerization are presented in Figure 2. The nonisothermal DSC curves were used to analyze the heat released during the polymerization of the different systems. The values of the temperature at the maximum of polymerization peak (Tp) and the enthalpy of polymerization or total heat release of reaction (Q) are listed in Table 2. The polymerization reaction of each system is characterized on the nonisothermal DSC curves by an exothermic peak, as it was highlighted by Milkovik et al.35 for the polymerization of FA with paratoluenesulfonic acid. As shown in Figure 2, the FA without any acidic initiator does not polymerize. According to the Tp value of FA/SiO2(c) and FA/MA/SiO2(c), the presence of MA in the blend leads to an early initiated polymerization. Yet, the plot of FA/SiO2(c) curve displays an exothermic peak at Tp ∼ 187 °C. This result puts in relief that the polymerization can be performed without the presence of the initiator if silica is added to FA. This behavior is justified by the apparition of a polymerization peak in the presence of the C


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Figure 3. DSC data (left axis) of the heat released during nonisothermal polymerization and the corresponding variation of extent of conversion with temperature (right axis). Blue: FA/MA/ SiO2(c). Green: FA/SiO2(c). The heating rate of each experiment (in °C min−1) is indicated on each curve.

Figure 2. DSC data of the heat released during nonisothermal polymerization at a heating rate of 6 °C min−1.

Table 2. Thermodynamic Parameters of the Polymerization of the Three Systems

a

sample

Tp/°C

Qa/J·g−1

FA/SiO2(c) FA/MA FA/MA/SiO2(c)

187 ± 1 154 ± 1 117 ± 1

354 ± 20 507 ± 25 465 ± 25

much higher compared to the first acidity of the maleic acid (pKa = 1.83). The Figure 4 shows the FA polymerization pathway in the presence of MA and silica clusters. As presented in part 1 of Figure 4, the hydroxyls in surface of silica clusters help to open the MA thus initiating the formation furfuryl carbenium centers. As shown in part 2 of Figure 4, the carbenium latter will lead to the condensation into furanic oligomer and their cross-linking through Diels−Alder cycloaddition. The dependence of Eα on extent of conversion (α) has been evaluated from the DSC data and is plotted in the inset of Figure 5. This Eα-dependence provides information on the polymerization mechanisms of the different systems. The observed variations of Eα values put in relief the presence of multistep kinetics that appear during this nonisothermal acid-catalyzed polymerization.38 The comparison of FA/MA and FA/MA/ SiO2(c) polymerization data allows to analyze specifically the effect induced by the nanosilica clusters. In order to study this effect, the Eα values were compared on Figure 5 with the Eα values of FA/MA from a previous study.38 For α < 0.6, the Eα values are lower for FA/MA compared to FA/MA/SiO2(c). On the other hand, for α > 0.6, the opposite trend is observed due to the diffusion of molecules that is limited by the high viscosity of the medium at this stage of the reaction. In this configuration the silica induces an effect of confinement and promotes the reaction between the unreacted small chain segments as shown in part 3 of Figure 4. This effect is characterized by lower values of effective activation energy for FA/MA/SiO2(c) compared to FA/MA. The lower Eα values reported for FA/MA for α < 0.6 seems to be in contradiction with the higher reaction temperature found for this system (Figure 2 and Table 2) in comparison with FA/MA/SiO2(c). A similar effect has been reported in the study of Alzina et al.37 and was explained by an increase in the frequency of diffusion jumps in the presence of nanoparticles. On the other hand, analysis of the Eαdependence on temperature of Figure 5 permit a comparison of the Eα values for a same reaction temperature and show that similar values are obtained at the early stages of the reaction, i.e. between 100 and 110 °C, while the values are lower for FA/

Value reported to the mass of FA.

nanosilica clusters only. The influence of clustered silica on the polymerization can be estimated from the comparison of Tp and Q values from FA/MA and FA/MA/SiO2(c) blends. The value of Tp for FA/MA appears to be higher compared to FA/ MA/SiO2(c). This result is explained by the catalytic effect of the clustered nanosilica on the FA polymerization process. This effect is consistent with the results of previous studies with the insertion of montmorillonite in PFA36 and DGEBA37 that led to the modification of polymerization mechanisms. The data displayed in Table 2 shows that the integration of DSC peaks lead to different values of Q for each system. The polymerization enthalpies of FA/MA and FA/MA/SiO2(c) are similar. In opposition, the value of Q for FA/MA/SiO2(c) is higher compared to FA/SiO2(c). This result is justified by the presence of MA which initiates the formation of carbenium centers and thus the polycondensation.38 Figure 3 shows the normalized nonisothermal DSC curves and extent of conversion during the polymerization of FA/MA/ SiO2(c) and FA/SiO2(c) blends at respective heating rates of 1, 2, 4, and 6 °C min−1. The results highlight the presence of a single exothermic peak with a shoulder appearing at each heating rate. The polymerization peak is shifted to higher temperatures when the heating rate increases. This behavior highlights a dependence of the polymerization on heating rate and has been observed in a previous study.38 In the case of FA/ SiO2(c) the shoulder appears at a temperature below Tp and corresponds to the slow initiation of polymerization process by the silanol groups from the silica cluster. On the other hand, for the FA/MA/SiO2(c) the shoulder is located at a temperature above Tp. This latter result is explained by the presence of MA that promotes the initiating reactions of the polymerization process, such as reactions of condensation. Indeed the pKa is about 7.1 for the surface hydroxyl groups of silica39 which is D


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Figure 4. Sketch of FA polymerization in the presence of silica clusters. Part 1: initiation and formation of furfuryl carbenium centers. Part 2: acid catalyzed condensation of FA; formation of conjugated sequences leading to cross-links via Diels−Alder cycloaddition. Part 3: interaction of silica cluster with the PFA network.

the reaction, because a higher frequency of diffusion jumps correspond to a higher value of the pre-exponential factor A. Indeed, higher E values shift the reaction to higher temperature, while higher A values shift the reaction to lower temperature. Recently, Zavaglia et al.36 have followed the same kinetic approach to determine the influence of organophilic montmorillonite on the FA polymerization rate. They found that in the diffusion controlled part of reaction (i.e., α > 0.6), the presence of exfoliated clay layers lead to higher energetic barrier (Eα) when longer polymer chains need to move cooperatively to allow the continuation of cross-linking. The opposite behavior (i.e., diminution of the energetic barrier) observed in the presence of silica clusters shows that the cooperativity is not rate determining as in the presence of MA or organophilic montmorillonite. It is rather the confinent of polymer chains which dominates and accelerate the reaction rate. Indeed, the effect of confinement on the FA polymerization was also studied by Bertarione et al.40 in the restricted spaces of zeolites channels and faster formation of oligomeric intermediates was observed. The data of Figure 5 displays also the highest values of Eα for the FA/SiO2(c) blend on the whole α range. The polymerization rate is much slower when it is calatyzed by silanol groups only compared to MA. This comment is correlated with the DSC curves on Figure 2 where the exothermic peak of FA/ MA/SiO2(c) polymerization is located at lower temperatures compared to FA/SiO2(c). Yet it must be stressed that the Eα variations of FA/SiO2(c) are similar to the Eα variations of FA/ MA and FA/MA/SiO2(c). These variations display however a

Figure 5. Variation in the effective activation energy (Eα) with temperature and extent of conversion (α) (inset) during the polymerization of the different blends under nonisothermal conditions β = 1−6 °C min−1. Red triangles: FA/MA. Green lozenges: FA/ SiO2(c). Blue circles: FA/MA/SiO2(c).

MA/SiO2(c) for T > 110 °C. This means that, at the early stages of the reaction, the energetic barrier (E) is similar for the two systems. Because the system with silica react at lower temperature (Figure 2 and Table 2), this imply a higher value of the pre-exponential factor, i.e. of the term A of the arrhenius equation. Thus, it can be deduced a higher frequency of diffusion jumps in the presence of silica for the early stages of E


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Figure 6. TEM pictures of PFA/MA/SiO2(c) nanocomposite at (left) 10 μm scale and (right) 5 μm scale.

thermomechanical beavior of the PFA/MA/SiO2(c) nanocomposite in comparison with the PFA/MA material. 3.4. Thermomechanical Behavior and Glass Transition of PFA/MA in the Presence of Silica Clusters. In order to verify the dispersion of the silica nanoparticles into the crosslinked PFA/MA the observation of the microstructure was performed by TEM on PFA/MSiO2(c) nanocomposite (5 wt % nanosilica) and the micrographs are displayed in Figure 6. The TEM observation shows that the silica adopts a conformational network into the solid matrix, and that the cluster size is uniformly about a few micrometers. It is clearly revealed that the silica clusters are homogeneously dispersed and it appears that the organic matrix is free of any massive aggregated blocks. The good dispersion of the silica in the solid medium is then consistent with the evaluation of its dispersion into the liquid monomer, as shown in Figure 1. It indicates that the polymerization has not induced a reagglomerration of the clusters to their initial 10−100 μm size due to stable interactions with the furanic matrix. The effect of nanosilica clusters on thermal stability has been considered from the TG curves of PFA/MA and PFA/MA/ SiO2(c) samples as displayed in Figure 7. The results are presented in Table 3 where Td90% corresponds to the temperature measured at 10% loss of initial mass. The curves put in relief a difference of ∼32 °C for Td90% between the PFA/ MA and the nanocomposite. This result indicates that the addition of nanosilica clusters highly improves the thermal stability of the PFA/MA matrix. The increase can be explained

lower intensity. It can be then concluded that the overall polymerization mechanisms for the three systems remain identified as described by Guigo et al.38 for the FA/MA blend. The Eα values on temperature plotted in Figure 5 corresponding to the polymerization of FA/MA/SiO2(c) are shifted to lower temperatures compared to the Eα values of FA/ SiO2(c). The first Eα values of FA/MA/SiO2(c) are located at ∼80 °C whereas the first values of FA/SiO2(c) appear at ∼125 °C. This shift of temperature corroborates the results from the DSC curves of Figure 2 and from the data of Table 2 with the assumption that the catalyst (MA) promotes the polymerization process. Besides it must be stressed that the effective activation energies of FA/SiO2(c) are higher than those of FA/ MA/SiO2(c). This observation is consistent with the promotion of the polymerization process by the catalyst that leads to a decrease of Eα values. Despite of the temperature shift, the intensity of Eα variations are similar for both materials. The overall shape of the Eα-dependency is very similar, leading to the conclusion that both polymerizations involve similar rate limiting steps, excepted at the very beginning of the process. Nevertheless, these limiting steps do not occur at the same temperatures. The variation of curve slopes represents the change in the rate limiting step of the mechanism that control the overall polymerization rate. The presence of an autocatalytic step is also highlighted on these Eα-dependence curves. This step is characterized by a decrease of Eα values from 100 to 80 kJ mol−1 at ∼80 °C for FA/MA/SiO2(c) and from 160 to 120 kJ mol−1 between 120 and 140 °C for FA/ SiO2(c). This autocatalytic step is limited to the very beginning of the reaction (α ∼ 0.02, T ∼ 80 °C) for the FA/MA/SiO2(c) system, while it is expanded until α = 0.15 (T ∼ 120−140 °C) for FA/SiO2(c) system. Because of the presence of MA in the blend that acts as an initiator, this step is less rate determining. Indeed, the interaction between SiO2(c) and MA could lead to the ring-opening of the MA in its diacidic form as presented in part 1 of Figure 4. The catalyst is then more effective to promote the polymerization process, so that the control by the autocatalytic step is less pronounced in this case. Then, for both systems the Eα-dependency shows a constant value attributed to a single rate limiting step dominated by condensations reactions. This step ranges between 80 and 100 °C for FA/ MA/SiO2(c) system, while it occurs between 140 and 170 °C for FA/SiO2(c) system. This step is followed by a well-marked decrease attributed to a shift between chemical control to diffusion control of small unreacted monomers that reach the glassy state. These results clearly show that the polymerization of FA is highly promoted with the simulatenous presence of MA and SiO2(c). It is consequently necessary to investigate the

Figure 7. Thermogravimetric (TG) (left-hand axis) and differential thermogravimetric (DTG) curves (right-hand axis) vs temperature for the nonisothermal degradation under N2 atmosphere, at a heating rate of 10 °C min−1. Black: PFA/MA. Purple: PFA/MA/SiO2(c). F


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The Journal of Physical Chemistry C Table 3. Thermomechanical Data of PFA/MA and PFA/MA/SiO2(c)a

a

sample

Td90%/°C

Tg (max. of E″)/°C

Tg (max. of tan δ)/°C

peak height (max. of tan δ)

PFA/MA PFA/MA/SiO2(c)

361 ± 1 393 ± 1

40 ± 2 59 ± 1

84 ± 8 104 ± 2

0.30 ± 0.08 0.25 ± 0.02

Key: Td90%, temperature measured at 10% loss of initial mass; Tg, glass transition temperature (α-relaxation); E″, loss modulus.

MA. This result can be justified by the presence of silica nanoparticles that lead to an increase of the material stiffness. The constraint imposed by the DMA apparatus is then transferred from the soft matrix to the rigid nanoparticles. This effect corroborates thus the presence of strong interactions between the filler and the matrix. This behavior is also present in the rubbery state of the two materials. The E′ modulus at the rubbery plateau (i.e., for T > 160 °C) can be also correlated with the cross-link density. Higher is the rubbery modulus and higher is the cross-link density. It indicates that the furanic chains have developed slightly higher cross-link density in the presence of silica clusters. On the other hand, E″ is proportional to the energy dissipated in the material in the form of heat. Figure 8 shows that E″ values are also influenced by the presence of silica clusters. At low temperature (i.e., T < 50 °C), E″ values are lower for PFA/MA/SiO2 compared to PFA/MA which indicate that less energy is dissipated in the material due to presence of very stiff silica clusters. This observation is consistent with the higher E′ values obtained for the nanocomposite. Then at T > 50 °C, the E″ values are higher for the nanocomposite. It indicates that the silica modify not only the overall stiffness of the material (i.e., shown in E′ values) and that the molecular mobility of the PFA chains (i.e., instead shown in the E″ values) is also impacted. In such a perspective, the well-known cooperative process is a good indicator of molecular dynamics. The temperature of αrelaxation, which can be associated with the glass transition temperature as a first approximation, is then estimated either from the maximum of the E″ peak or from the maximum tan δ (i.e., E″/E′) peak. However, these two peaks correspond to various types of molecular motions. The E″ peak correspond to motions of local segments while larger motions of Gaussian submolecules, known as Rouse modes, are rather associated with the maximum of the tan δ peak.45 The thermomechanical data gathered in Table 3 correspond to the mean values (and standard deviations) obtained on five different DMA samples per material. The glass transition temperature was estimated at the maximum of the E″ peak (Figure 8) and at the maximum of the tan δ peak (not shown here). For the PFA/MA, the relaxation spectrum is very broad since there is a large difference (∼45 °C) between the Tg(E″) and the Tg(tan δ). This underlines a substantial gap between the different modes of molecular motions (i.e., segmental mobility and the Rouse modes). The cross-links due to Diels−Alder cycloadditions (part 2 of Figure 4) are likely to generate small free volume holes in PFA/MA which will restrict more the larger motions such as the Rouse mode (i.e., highlighted in Tg(tan δ)) compared to pure segmental motions (i.e., highlighted in Tg(E″)). As shown in Table 3, the PFA/MA/SiO2 exhibits much higher values of Tg(E″) and Tg(tan δ) compared to PFA/MA. In both cases, the increase is about +20 °C. It indicates that the presence of silica clusters will restrict both the local segmental motions and the larger motions. It suggests that the silica clusters have then intimate interactions with the PFA network (part 3 of Figure 4) as the different levels of PFA molecular motions are impacted by their presence. The interfacial

by the presence of the homogeneously dispersed silica network into the matrix as observed from TEM pictures on Figure 6. The nanosilica network is created from the controlled dispersion of the nanosilica clusters and from the good adhesion between silica and matrix surfaces. The same behavior has been observed in a previous study that involves a silica synthesized ex situ.14 The study of Grund et al.13 mentions a significant weight loss at 320 °C for the PFA/MA/SiO2(c) nanocomposite elaborated from the twin polymerization procedure. In the present investigation, the first significant weight loss of PFA/MA/SiO2(c) sample is observed at T ∼ 335 °C showing that its thermal stability is slightly higher. Yet the benefice on PFA/MA thermal stability with silica clusters is slightly lower compared to our previous study14 in which the PFA nanocomposite displays a first significant weight loss at T ∼ 340 °C and a Td90% at ∼420 °C. This effect is due to the different morphology between these two silica fillers and shows that the heat is more able to diffuse through clustered silica nanoparticles. However, the present procedure is easier to handle since the silica surface does not require chemical modification. The influence of nanosilica clusters on PFA/MA glass transition can be highlighted via DMA. This technique provides reliable informations on the polymer viscoelastic behavior, related to structure morphology and intrinsic properties.41−44 The variations of the loss modulus E″ and the elastic modulus E′ with temperature (or frequency) correspond to the mechanical spectra of the material. The variations of E″ and E′ with temperature are presented in Figure 8. The continuous decrease of the E′ modulus on heating can be explained by the transition from the glassy state to the rubbery state. Yet the values of PFA/MA/SiO2(c) are higher on the whole temperature range compared to the values of PFA/

Figure 8. Loss modulus and elastic modulus (inset) vs temperature measured on heating by DMA at a heating rate of 1 °C min−1 and a frequency of 1 Hz. Black: PFA/MA. Purple: PFA/MA/SiO2(c). G


Article

The Journal of Physical Chemistry C cohesion between the filler and the matrix is enhanced due to the high specific surface and OH density of the nanosilica clusters. The peak height of PFA/MA/SiO2(c) from Table 3 displays a similar value compared to that of PFA/MA. This observation highlights a diffusion of vibration energy that is similar in the presence or in the absence of the nanoparticles. 3.5. Frequency Analysis of Glass Transition. Multiples frequencies were used for the measurement on heating of PFA/ MA and PFA/MA/SiO2(c) glass transition by DMA. This procedure allows to estimate the apparent activation energy Ea (J mol−1) at the glass transition from the following Arrhenius equation: f = A exp( −Ea /RT )

magnitude as the values obtained for the polystyrene (605 kJ mol−1),48 for the polyethylene terephthalate (432 kJ mol−1),47 and for the epoxy-amine thermoset (715 kJ mol−1).49 It is found that the effective activation energy is higher for the nanocomposite. This increase in Eα reveals that the nanosilica clusters induce an additional constraint on the chain motion at the glass transition and a nanoconfinement of PFA polymer chains. This nanoconfinement leads to enhanced intermolecular interactions that induce a larger energy barrier encountered by the polymer chains during their cooperative motions. The effects of this confinement are finally consistent with both an increased reaction rate and thermal stability.

(3)

4. CONCLUSION In the present work, the nanosilica was synthesized via precipitated silica process and displayed a high density of hydroxyl groups. A thin layer of silica was created in periphery of nanoparticles to create the clusters. In consequence, the morphology of the employed silica is strictly different compared to the conventional isolated nanoparticles. The effect of these nanosilica clusters clearly promotes the FA polymerization leading to initiation at lower temperatures (catalytic effect) thus avoiding the autocatalytic activiation of MA. Moreover, lower energetic barriers are obtained for the final stage due to nanoconfinenement effects. According to microscopy observations, it has been shown that the silica is homogeneously dispersed into the PFA after complete polymerization and creates a nanocluster network into the matrix, without requiring any major surface modification. Results were obtained hereby from thermomechanical analysis with comparing PFA and PFA/silica nanocomposites. They allow to correlate the presence of this network with the increase of the nanocomposite elastic modulus, glass transition temperature (both Tg(E″) and Tg(tan δ)) and the enhancement of thermal stability. The multifrequency analysis of PFA and PFA/silica highlighted the frequency dependence of Tg for both materials. It was concluded that the presence of silica nanoclusters leads to higher energetic barrier for the PFA cooperative chains motions.

where the logarithm of the frequency is ploted with the reciprocal temperature of transition: ln f = ln A − (Ea /RT )

(4)

In the eqs 3 and 4, f is the frequency (Hz), A is the preexponential factor, R is the gaz constant (8.314 J·mol−1.K−1) and T is the temperature of the glass transition (K), taken as the maximum of tan δ.46 The results of DMA multifrequency measurement are displayed in Figure 9. The plot of ln( f) vs Tg−1 is presented

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ASSOCIATED CONTENT

S Supporting Information *

Figure 9. tan δ vs temperature measured by DMA on heating at 1 °C min−1 for PFA/MA/SiO2(c) at different frequencies set at 0.1 Hz (dot), 1 Hz (full line), and 10 Hz (dash). Inset: Dependence of the logarithm of frequency on reciprocal Tg for multifrequency DMA experiments. Black triangles: PFA/MA. Purple circles: PFA/MA/ SiO2(c). The linear regression to each set of data is represented by a line.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b12882. TEM images, FTIR spectrum, and TGA thermogram of the silica nanoparticles (PDF)

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AUTHOR INFORMATION

Corresponding Authors

in the inset of Figure 9. The applicability of the Arrhenius equation is justified by the use of a narrow range of frequencies from 0.1 to 10 Hz for the calculation of the apparent activation energies.47 For both materials, the observation of Figure 9 allows to notice that the increase of the perturbation frequency induces an upward shift of the tan δ peak. According to these results, the glass transition appears to be frequency dependent and can be assimilated to a cooperative α-relaxation. The slope of each ln( f) vs Tg−1 lines in Figure 9 yields to an average value of apparent activation energy that is found to be Eα = ∼ 246 ± 1 kJ mol−1 for the PFA/MA and Eα = ∼340 ± 10 kJ mol−1 for the PFA/MA/SiO2(c). These values of Eα present the same

*(N.S.) E-mail: [email protected]. *(N.G.) E-mail: [email protected]. ORCID

Nicolas Sbirrazzuoli: 0000-0002-6031-5448 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the French Ministry of Higher Education for the financial support of this research. The authors would like to gratefully thank Mettler-Toledo Inc. for scientific H


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Impact of Silica Nanoclusters on Furfuryl Alcohol Polymerization and

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