Effect of Nanoparticle Functionalization on the ... - ACS Publications

James A. Throckmorton , Greg Feldman , Giuseppe R. Palmese , Andrew J. Guenthner , Kevin R. Lamison , Neil D. Redeker , Patrick N. Ruth. Polymer ...
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Effect of Nanoparticle Functionalization on the Performance of Polycyanurate/Silica Nanocomposites Andrew J. Guenthner,*,† Christopher M. Sahagun,‡ Kevin R. Lamison,§ Josiah T. Reams,§ Timothy S. Haddad,§ and Joseph M. Mabry† †

Aerospace Systems Directorate, Air Force Research Laboratory, Edwards AFB, California 93524, United States National Research Council/Air Force Research Laboratory, Edwards AFB, California 93524, United States § ERC Incorporated, Air Force Research Laboratory, Edwards AFB, California 93524, United States ‡

S Supporting Information *

ABSTRACT: The impact of silica functionalization in determining the performance of polycyanurate networks polymerized from 1,1-bis(4cyanatophenyl)ethane, known commercially as Primaset LECy, reinforced with modified fumed silica, was elucidated through systematic comparison of the properties of nanocomposite networks in which the silica surface treatment was altered. Three types of surfaces were investigated: moderately acidic (unmodified silanol), neutral (alkylsilane modified), and slightly basic (3-aminopropylsilane modified). In terms of cyanate ester cure, the acidic surface proved to be moderately catalytic, the neutral surface mildly catalytic due to slight residual silanol content, and the basic amino-functional surface mildly inhibitory. In terms of network performance, the amino-functional surface led to significant degradation of the network at elevated temperatures, while the silanolfunctional surface outperformed the alkyl-functional surface in terms of protection against hydrolytic degradation. In agreement with expectations, the addition of 2−5 wt% of relatively well-dispersed silica nanoparticles had negligible impact on the fracture toughness of the cyanurate networks. Overall, these results demonstrate that the functionalization of nanoparticle additives for polycyanurate networks is an important determinant of performance and must be taken into consideration in the development of polycyanurate nanocomposites, even at levels that are too low to strongly affect mechanical properties.

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INTRODUCTION Networks derived from cyanate ester monomers, known as polycyanurate networks,1−4 are utilized in several of the most demanding high-performance applications currently known. These include thermonuclear fusion reactors such as ITER,5−7 where, in blends with epoxy resins, they impart critical resistance to neutron irradiation. Also included are interplanetary space probes such as the MESSENGER mission to Mercury8 and the Mars Curiosity rover,9 where mechanical strength and toughness across a broad temperature range and low off-gassing are important considerations. In addition, cyanate esters have been utilized in space structures closer to Earth, such as the James Webb Space Telescope10 and the Orion Earth re-entry heat shield,11 where dimensional stability and flame resistance, respectively, drive materials requirements. Numerous other applications such as filament wound aerospace structures12 and high-temperature microelectronic circuit boards13 also make use of the outstanding thermal, mechanical, and electrical properties of polycyanurate networks. In most practical applications, polycyanurate networks (polycyanurates), even when serving as composite resins, are not utilized as single-component materials systems. Rather, they are formulated with a variety of additives and reinforcements in order to maximize performance in their intended applications. © XXXX American Chemical Society

One of the most common, and most important, of these reinforcements is silica. The properties of polycyanurate/silica composites have been investigated and reported in the literature on limited occasions.14−27 One very important practical question has received relatively little attention (the work of Wooster et al.28 being a notable exception): How does the nature of the silica/polycyanurate interface affect the kinetics of cyanate ester cure and the resulting polucyanurate physical properties? Untreated silica particles have an interface rich in silanol groups. Although this type of interface is often reported for polycyanurate/silica nanocomposites, other types of surface chemistry may provide significant benefits. For instance, Yan et al.29 recently reported significant improvements in cyanate ester nanocomposite mechanical performance when nanosilicas were modified with resin-compatible terpolymers. Devaraju et al.30 reported the beneficial effects of epoxy group functionalization on the dielectric properties of silicareinforced cyanate ester nanocomposites, while Jothibasu et Received: February 4, 2016 Revised: June 9, 2016 Accepted: June 13, 2016

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Industrial & Engineering Chemistry Research al.31 examined nanocomposites in which 3-aminopropylsilane was used as a coupling agent. As indicated by the aforementioned reports, one of the key advantages of the silica surface is that it may be modified to provide a wide variety of different chemical functionalities at the silica/polycyanurate interface. In addition to altering the interface itself, surface functionalities may also affect the properties of the bulk resin through changes in the availability of protons (i.e., by altering the resin pH). Many of the chemical reactions important for understanding the performance of polycyanurates, including cure and hydrolytic degradation, are affected by pH. Such effects are illustrated in Figure 1.

result, variables such as pH that are important to catalysis become significant factors in determining the overall performance of these networks. In fact, we find that, at the nanoparticle loading levels studied, performance characteristics are significantly altered by effects arising from catalyzed chemical reactions, whereas the more traditional effects of nanoscale reinforcement such as improvements in thermal and mechanical performance are small. The use of nanocomposite interfaces to control variables such as pH thus represents a technique that is particularly well-suited to enhancing the performance characteristics of polycyanurate networks.

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EXPERIMENTAL SECTION Functionalization of Silica Nanoparticles. Aerosil 200 and Aerosil R805 fumed silicas were obtained from Evonik and dried at 150 °C for at least 24 h under vacuum, and then stored under a dry nitrogen atmosphere in a glovebox prior to use. Aerosil 200 has a reported nominal average primary particle size of about 12 nm.33 The Aerosil R805 is produced by treatment of fumed silica with an aliphatic silane containing an octyl group, resulting in a surface comprised of saturated aliphatic chains. The 13−19 nm reported nominal average particle size of Aerosil R80534 is slightly larger than that of Aerosil 200, in accordance with expectations for a coated particle. To prepare the reactive amine-functional silica, the following procedure, adapted from previous silica surface modification work performed in our laboratory,35 was undertaken. Aerosil 200 (10 g) was placed in a 500 mL round-bottomed flask affixed to a Schlenk line and dried for 48 h at 200 °C under vacuum. The sample was allowed to cool to room temperature under vacuum, and then a dry nitrogen atmosphere was introduced, followed by a 400 mL dry chloroform solution of 3aminopropyltrimethoxysilane (11 g, used as received from Gelest, Inc.). Care was taken to avoid the introduction of environmental water into the flask. Based on the reported density of silanol groups on the Aerosil 200 surface,36,37 a large stoichiometric excess of the silane was present in the reaction vessel. Grafting of aminopropyl groups was accomplished by allowing the mixture to stir for 9 days at room temperature in a dry nitrogen environment. The resulting surface-modified nanoparticles were separated via centrifugation, followed by Soxhlet extraction for 3 days in dry chloroform. The modified nanoparticles were then dried under vacuum at 130 °C for 72 h, and stored in a dry environment prior to use. Preparation of the Silica/Cyanurate Nanocomposite. Primaset LECy was obtained from Novoset, Inc. (Peapack, NJ) and stored at 5 °C prior to use. The catalyst consisted of a 30:1 weight mixture of copper(II) acetylacetonate (Roc-Ric, used asreceived) dissolved in 4-nonylphenol (97%, mixture of isomers, Aldrich, used as-received), added to monomers at 2 parts per hundred by weight. Batches of catalyst were periodically prepared on a scale of a few grams and stored under ambient conditions for up to 30 days while being used as needed. Prior to mixing, LECy monomer was prepared by de-gassing for a few minutes under vacuum at 50 °C, and used immediately afterword. To fabricate nanocomposites, silica particles (if used) were added to around 10 g of prepared LECy monomer in a scintillation vial at loadings of up to about 5 parts per hundred. The loading levels were selected so as to give a predetermined amount of theoretical surface area per unit weight of nanocomposite. A conversion table relating surface area to weight fraction is provided in the Supporting Information,

Figure 1. Introduction of trace hydroxide ions into the cyanurate network. These ions can accelerate hydrolysis of the cyanurate bond.

Modifying the chemical functionalization at the interface, in order to affect the pH characteristics of the resin, therefore represents a relatively unexplored pathway toward controlling and improving the performance of polycyanurate networks. In addition to affecting variables such as chemical bonding and diffusion, the presence of nanoparticles can also impact the bulk characteristics of resin matrices by altering the supply of trace ions, and thereby changing the rates of key chemical reactions within the bulk resin. Figure 1 shows how amine groups on the surface of a silica particle can interact with adventitious water to produce hydroxide ions in trace quantities, which can then diffuse into the bulk, where, for instance, they may accelerate hydrolysis of cyanurate bonds. In this way, “surface” effects may alter bulk network properties, even when the nanoparticles are not completely dispersed and the effective surface area is small. In this work, we focus on developing a practical understanding of the effect of interface interactions on the performance of silica/polycyanurate nanocomposites. In order to generate comparative data on the performance of nanocomposites, we specifically utilize three different types of interfaces: a relatively acidic silanol-rich interface, a relatively neutral non-polar alkylsilane interface, and a more basic aminoalkylsilane interface. We show that the acid/base characteristics of the interface have a very substantial impact on performance, with hydrolytic stability playing a key role. Indeed, one of the distinguishing characteristics of cyanate esters and polycyanurate networks is that these systems, which are generally inert as pure compounds, exhibit chemistry that is highly dependent on catalytic reaction mechanisms.32 As a B

DOI: 10.1021/acs.iecr.6b00498 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article 290 °C, 60 min 25−290 °C @ 2.5 °C/min

290 °C, 60 min 25−290 °C @ 2.5 °C/min

175−210 °C @ 0.5 °C/min

210 °C, 360 min 210 °C, 360 min 175−210 °C @ 0.5 °C/min

290 °C, 60 min 25−290 °C @ 2.5 °C/min not used

150−175 °C @ 0.5 °C/min

150−175 °C @ 0.5 °C/min

150−175 °C @ 0.5 °C/min 100−150 °C @ 1 °C/min

100−150 °C @ 1 °C/min

100−150 °C @ 1 °C/min

LECy/silanol

LECy/octyl

LECy/ 3-amino

hold ramp

25−290 °C @ 2.5 °C/min

210 °C, 360 min not used

hold ramp

175−210 °C @ 0.5 °C/min

175 °C, 600 min 175 °C, 600 min 175 °C, 600 min 175 °C, 600 min

hold ramp

150−175 °C @ 0.5 °C/min

150 °C, 240 min 150 °C, 240 min 150 °C, 240 min 150 °C, 240 min

hold ramp sample type

100−150 °C @ 1 °C/min

post-cure step (if used) step 3 step 2 step 1

Table 1. Cure Schedules for Nanocomposite Samples C

LECy

Section S1. Because average particle diameters are typically based on measurements of effective surface area, the technique is robust toward differences in particle polydispersity or shape. Each mixture was stirred by hand for a few seconds, and then each vial was tightly sealed and placed in water at 50 °C in an ultrasonic bath. Each vial was then subjected to low-power sonication for 5 min (“de-gas” setting), followed by full-power (“sonicate” setting) sonication for 60 min. The resultant samples appeared homogeneously dispersed by visual inspection. The appropriate amount of catalyst was then added, and the resultant liquid was stirred by hand for a few seconds to achieve homogeneity. The formulated monomer was then either placed directly into a hermetically sealed differential scanning calorimetry (DSC) pan or poured into one or more silicone rubber molds having a bar-shaped cavity approximately 75 mm long, 12 mm wide, and 3 mm thick, and then placed into an oven under flowing nitrogen for cure. Some DSC samples were also prepared without the addition of catalyst. Table 1 lists schedules employed for curing the samples. Both regular cure (low hold temperatures) and “post-cure” (high hold temperatures) began by placing samples under nitrogen in an oven pre-heated to 100 °C. A single cure schedule could not be used for all sample types. In particular, additional catalysis induced by the presence of the Aerosil 200 led to exothermic runaway reactions when the cure schedule utilized for the other samples was tried with this system. After curing, samples were allowed to cool to room temperature and then de-molded. Selected samples of all types were also subjected to a “post-cure” to achieve near 100% conversion of cyanate ester groups to cyanurate. Infrared (IR) Spectroscopy. The as-prepared or asreceived nanoparticles were thoroughly rinsed in chloroform and then dried briefly using a flowing air stream. The particles were then diluted to roughly 1% by volume with KBr powder and pressed into pellets. The pellets were then placed in a Thermo Corporation Nicolet 6700 FT-IR spectrometer in attenuated total reflectance mode and measured by averaging 128 scans at a wavelength resolution of 2 cm−1. Differential Scanning Calorimetry (DSC). For investigations of cure, 3−5 mg samples of uncured nanocomposites (liquid form, without added catalyst) were placed in hermetically sealed DSC pans. The samples were equilibrated at 50 °C and then heated to 375 °C at 10 °C/min. The instrument was then equilibrated at 100 °C, following which a second heating scan to 375 °C at 10 °C/min was completed. For cured nanocomposite samples, which did contain catalyst, 5−10 mg chunks of cured specimens were broken off using a razor blade and placed in hermetically sealed DSC pans. These samples were scanned using a procedure identical to that for uncured samples, except that the initial equilibration was at 100 °C. All scans were performed under 50 mL/min of flowing nitrogen using a TA Instruments Q200 differential scanning calorimeter. Thermogravimetric Analysis (TGA). For analysis of the nanoparticles, specimens weighing around 5 mg were placed in ceramic TGA pans and then heated at 2.5 °C/min to 290 °C, followed by a 1 h hold at 290 °C, under nitrogen. This procedure emulates the post-cure cycle and was used to assess the thermal stability of the nanoparticles during post-cure of nanocomposites. For analysis of cured nanocomposites, pieces weighing approximately 5 mg were removed and placed in ceramic TGA pans. These samples were equilibrated at 50 °C and then heated to 850 °C under nitrogen or, in separate experiments, in air, at 10 °C/min. All TGA experiments were

290 °C, 60 min

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particle diameter of Aerosil R805 is slightly larger, in the range of 13−19 nm, with a less acidic surface. The Aerosil R805 is formed via treatment of fumed silica based on Aerosil 200 with an octylsilane.34 Although dispersion pH measurements indicate that some acidic silanol groups remain, for the purposes of the following discussion this surface will be classified as “neutral”. In contrast, treatment of Aerosil 200 with an aminosilane as carried out in our laboratory produces a comparatively basic surface. The chemical nature of each of the three interface types was examined via transmission FT-IR of the purified and washed nanoparticles. Figure 2 shows a comparison of the spectra of

performed using a TA Instruments Q5000 thermogravimetric analyzer, with sample gas purge rates of 25 mL/min and balance gas (always nitrogen) purge rates of 10 mL/min. Oscillatory Thermomechanical Analysis (TMA). Cured discs were also tested via oscillatory TMA with a TA Instruments Q400 series analyzer under 50 mL/min of nitrogen flow. The discs were initially held in place with a compressive force of 0.2 N using the standard ∼5 mm diameter flat cylindrical probe. The force was then modulated at 0.05 Hz over an amplitude of 0.1 N (with a mean force of 0.1 N). The sample scanning procedures included initial heating of the sample to 380 °C at 50 °C/min, two cycles of heating and cooling between −50 and 150 °C, used to estimate the thermal lag,38 and a final heating to 380 °C at 50 °C/min. These procedures applied to both dry and wet samples. Other Physical Characterization. Water uptake was determined by immersing samples in water held at 85 °C for 96 h, with complete drying in a desiccator followed by weighing before immersion, and a quick towel drying and re-weighing after immersion. The weight gains are reported on a dry resin basis. Atomic force microscopy (AFM) images were obtained using a Digital Instruments Nanoscope III instrument in tapping mode. Transmission electron microscopy (TEM) imaging was performed by the University of Dayton Research Institute (Dayton, OH) on a Hitachi H7600 instrument run at ∼100 kV. Samples for TEM imaging were prepared on a Leica EM UC 6 ultramicrotome. The microtomed samples were then placed on holey carbon coating on 400-mesh copper grids. Specimens for mechanical property analysis were prepared by casting 76 mm × 76 mm × 3 mm sheets of nanocomposites resin in a vertical mold. The mold was formed by clamping two Teflon-coated aluminum plates with a U-shaped silicone gasket prepared using the mold-making procedures described earlier. Uncured nanocomposites were then injected through the opening on the top side of the gasket, filling the mold. The entire assembly was then cured under nitrogen in an oven at 150 °C for 1 h, followed by 210 °C for 24 h. The specimens were then cut into rectangular pieces measuring 76 mm × 9 mm × 3 mm for testing using a Struers rotary saw with a diamond-coated wheel. Fracture toughness testing was carried out using general methodology similar to ASTM D 5045. A 3kip MTS testing machine with a 110 N load cell was utilized in three-point bend mode with a span width of 36.3 mm between 6.35 mm diameter supports. Samples were notched and provided with an initial crack perpendicular to the 3 mm × 76 mm face at the center of the specimen using diamond saw blades of 0.51 mm diameter for the notch and 0.20 mm diameter for the crack. A microscope was utilized to determine the actual notch and crack dimensions formed. The samples were loaded to failure at 10 mm/min, using a calibrated linear variable displacement transducer to record displacement.

Figure 2. FT-IR spectra of various silica nanoparticles; the curves have been offset slightly for clarity.

each nanoparticle type after dispersion in KBr. The hydroxylated nature of the surface of Aerosil 200 is apparent from the broad band near 3500 cm−1. In the alkylsilane-treated Aerosil R805, this band is greatly reduced in intensity, but not eliminated entirely. In the amino-functionalized silica, the broad band at about 3500 cm−1 merges with another broad band near 3200 cm−1, which is indicative of amine functionality on the surface. The characteristic triple peak at 2840−2960 cm−1, indicating CH3-terminated alkyl functionality, is also clearly visible in the Aerosil R805 spectrum, while in the spectrum of the aminopropylsilane-treated surface, a smaller and less distinct peak appears, consistent with the much shorter alkyl chain length of the modified surface. Elemental analysis of the nanoparticles (details provided in Supporting Information, Section S1) also indicates clearly that hydrocarbon functionalities are present in the Aerosil R805 surface, while both hydrocarbon and nitrogen-containing functionalities are present on the aminosilane-treated nanoparticles. For both types of grafted surfaces, a theoretical grafting density near 1.5 chains/nm2 was indicated (see Section S1 of the Supporting Information); these grafting densities are close to those previously reported for modified silica nanoparticles35 and provide coverage levels close to an equivalent monolayer. (Particularly with the aminosilane, the actual coverage may be far from uniform.) Residual contamination on all surfaces appears to be no more than about 0.5% of total weight. Taken together, these results indicate that the surface functionalization proceeded as expected, with distinct indications found for each functional group expected to be present, and no other functional groups indicated. Another important aspect of the nanoparticles themselves is the chemical stability of the interface during cyanate ester cure.

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RESULTS AND DISCUSSION Nanoparticle Characteristics and Nanocomposite Morphology. The manufacturer’s data for Aerosil 200 indicates that it has a nominal primary particle diameter near 12 nm, with an acidic surface.33 The silanol groups on the surface are capable of chemical reactions with cyanate ester groups;23 however, the density of silanol groups36,37 is significantly higher than any reasonably achievable density of grafting groups (see Supporting Information, Section S1). Thus, even if chemical reactions take place, a significant density of acidic silanol groups will remain. The reported primary D

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Industrial & Engineering Chemistry Research These cures involve temperatures of up to 290 °C for periods of up to several hours. At these temperatures, a number of chemical reactions are possible, including a limited amount of silanol dehydration either through self-condensation or through reaction with amine groups. To check these possibilities, highresolution TGA of the nanoparticles with no added cyanate ester was carried out. A sample of pure LECy was also subjected to the same protocol in order to compare the magnitude of weight loss from chemical reactions at the nanoparticle interface with the magnitude of weight loss from other sources of volatile generation inherent in the cyanate ester resin. The results are shown in Figure 3 for a slow ramp to

Figure 4. AFM image of cryo-fractured neat LECy (a) and LECy with 8 m2/g Aerosil 200 (b), Aerosil 805 (c), and 3-amino-treated Aerosil 200 (d). (a), (c), and (d) were fractured after cure at 210 °C for 360 min, while (b) was fractured after cure at 175 °C for 600 min.

Additional information on the morphology of the aggregates at smaller length scales was provided by TEM imaging. A gallery of representative TEM images, along with additional AFM images and commentary, is shown in the Supporting Information, Section S3. TEM revealed that the 100−500 nm aggregates visible in AFM are composed, in all cases, of primary particles ranging from 5 to 50 nm in diameter, grouped mainly in chain-like configurations. This arrangement is reported to be typical for Aerosil.33 Significant differences among the nanocomposite samples at the aggregate scale were not apparent. The nanocomposite morphology thus appears to be mainly influenced by the nanocomposite fabrication process rather than than intrinsic nanoparticle compatibility. In all cases, the nanoparticle aggregates were sufficiently dispersed and compatible with the monomeric resin that large-scale reagglomeration did not take place. Detailed studies of the compatibility of LECy, and of dense polymer networks in general, with differing types of non-reactive silica nanoparticles, have not been widely reported. Based on our previous dispersion studies of graphitic surfaces of widely varying polarity in LECy,40 in which non-polar surfaces were highly incompatible, we had a weak expectation for the non-polar octyl-functional silica to be least compatible with LECy. Yet the only difference observed in the octyl-functional silica nanocomposite was a smoother cryo-fractured surface. Such a difference could result simply from weaker molecular interactions between the aggregates and the matrix rather than a difference in morphology. A detailed investigation of these issues is beyond the scope of the present work. For the purposes of this work, it is sufficient to note that no major differences in morphology among nanocomposites were found; thus, differences in performance are not likely to be due to differences in nanoscale morphology. Effect of Surface Type on Cyclotrimerization. The main effects of nanoparticle surfaces on the properties of polycyanurate nanocomposites involve the alteration of key chemical reactions that take place near these surfaces. The most significant chemical reaction is the thermal cure of cyanate ester groups to form the polycyanurate network. Numerous previous investigations of the cure mechanisms of cyanate esters have consistently shown that the cyanurate ring is the favored

Figure 3. TGA (in nitrogen) of silica nanoparticles with various surface functionalities, compared to neat LECy.

290 °C. Results for the isothermal portion of the 290 °C cure, during which no major changes in solid sample mass were observed, are shown in the Supporting Information, Section S2. Figure 3 indicates that the Aerosil 200 particles are the most thermally stable, being indistinguishable from LECy in terms of weight loss. The Aerosil R805 nanoparticles exhibit a very small but detectable weight loss, while the amino-functional nanoparticles lose significantly more weight during heating. Even so, the weight loss amounts to a small fraction of the silane present. The basic character of the amine groups grafted to the silica surface would likely result in an acceleration of silane condensation, thereby leading to some weight loss. This weight loss would produce a small amount of water, equal to roughly 0.01−0.04% of the system mass in the nanocomposites under study. Based on previous investigations of the effect of moisture content in cyanate esters,39 these levels of added moisture are about 1 order of magnitude too small to have a significant effect on the chemistry of the nanocomposites. With the nature of the nanoparticles themselves established, the next logical area of investigation is the geometric arrangement of the nanoparticles within the polycyanurate nanocomposites. Figure 4 shows representative AFM images of the neat LECy, Aerosil 200, Aerosil R805, and amine-functional nanoparticles in the cryo-fractured surfaces of polycyanurate nanocomposites after cure (but without post-cure). In each case, the nanoparticles are observed as aggregate features of 100−500 nm size, consistent with their as-manufactured state.23 Such aggregates are entirely absent from the neat resin. The aggregates appear least prominent in the alkyl-functional Aerosil R805 nanocomposite, and most prominent in the silanol-functional Aerosil 200 nanocomposite. The surface roughness characteristics show the same trend, with the surface of the Aerosil 200 nanocomposite being roughest, that of Aerosil R805 smoothest, and all nanoparticle-containing surfaces being far rougher than the neat LECy. E

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surface, however, has a slight inhibitory effect on the onset of cure. These results are readily understood in terms of the catalysis of cyanate esters by free protons (as happens when phenols are utilized as catalysts). The acidic silanol groups provide a source for these protons, as has been noted in previous studies.17−19,22 The comparatively basic amino groups, however, provide a proton sink that reduces the “background” level of protons available for catalysis, thus raising the cure onset temperature. Figures 5 and 6 show that both the temperature at the onset of reaction and the temperature at peak heat flow show similar behaviors as a function of nanoparticle loading. Interestingly, a non-linear inhibitory effect on cure is observed at loading levels greater than about 5 m2/g for all of the surfaces studied, with the effect being strongest for hydroxyl and weakest for 3-amino-functional surfaces. Higher concentrations of nanoparticles may inhibit the diffusion of protons via increased tortuosity, though this effect should be small due to the aggregated nature of the nanoparticles. On a more speculative note, diffusion of protons could be inhibited by the particle surfaces in more subtle ways, such as by altering the molecular topology of the networks near the particle interfaces. Such effects might be responsible for inhibition of cure. In order to verify that the introduction of acidic and basic functionalities at the nanoparticle surfaces does not alter the mechanism of cyclotrimerization, the thermodynamic characteristics of cure as a function of nanoparticle loading were measured. Figure 7 shows the enthalpy of cyclotrimerization,

product, due to the relatively large (negative) heat of formation of the cyanurate ring.41,42 One would therefore not expect to see alterations of the product of cross-linking due to these surfaces. On the other hand, the chemistry of cyanate ester cure is highly dependent on catalysis, thus one would expect the chemical characteristics of nanoparticle surfaces to provide a range of catalytic effects. Because these effects are driven by the nanoparticle surfaces, and because the nanoparticle specific surface area varies from one product type to another, comparisons among nanoparticle types were conducted using the amount of nanoparticle nominal surface area (rather than weight fraction) as the independent variable. In this case, the “nominal” surface area is that calculated on the basis of the diameter of primary particles, assuming all such particles are fully wet by the matrix, even if they are present as aggregates. Quantification of the catalytic effects of these surfaces is readily obtained from non-isothermal DSC analysis of uncured nanocomposites mixtures with none of the traditional cure catalyst added. Figures 5 and 6 show the onset and peak

Figure 5. Cure onset temperatures as a function of silica nanoparticle surface type and loading level.

Figure 7. Enthalpy of cyclotrimerization (per −OCN equivalent) as a function of silica nanoparticle loading and surface type. The lines indicate the correction for volume occupied by nanoparticles.

Figure 6. Cure peak temperatures as a function of silica nanoparticle surface type and loading level.

which for cyanate esters is 110 kJ/equiv for cyclotrimerization of all common cyanate ester monomers, as a function of nanoparticle surface type and nominal surface area.41 As indicated by the lines, the apparent enthalpy of cyclotrimerization should decrease slightly, due to replacement of some of the matrix material by the bulk nanoparticles. Given that no adjustable parameters are needed to compute this correction, the experimentally observed trend matches the expected values well. Although some systematic deviations on the order of 2 kJ/equiv are apparent, these are at least as likely to result from systematic integration errors associated with the choice of baseline in the analysis of DSC data than from identifiable side reactions. Effect of Surface Type on Network Properties. Figure 8 shows the conversions (measured by DSC) on networks cured at both low (175−210 °C) and high (290 °C) temperature as a function of nanosilica surface type, loading, and cure condition. For the silanol (hydroxyl) and octyl surfaces, the conversions at low temperature cure are controlled by vitrification. The need to avoid runaway cure required a lower cure temperature (175 °C) for the more catalytically active nanocomposite featuring

exotherm temperatures, respectively, observed for each type of nanoparticle surface examined, as a function of surface area. For the cyclotrimerization of cyanate esters, there is always a “background” level of catalytic activity due to trace impurities. The introduction of silica nanoparticles decreases the concentration of these impurities in the immediate vicinity of the nanoparticle surface, and effectively replaces them with the set of impurities and reactive functional groups present at the particle surface. Thus, a decrease in onset and peak temperature indicates that the surface (including any impurities present on it) is more catalytically active than the “background”, while an increase in these temperatures indicates that it is less catalytically active than the “background”. According to the data in Figures 5 and 6, the acidic untreated “hydroxyl” silanol surface (Aerosil 200) provides the greatest catalytic effect, as expected. The treated “octyl” Aerosil R805 surface provides a similar catalytic effect that is about one-third as large, likely due to the presence of residual silanol groups on the surface. The comparatively basic 3-amino-functional F

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Figure 8. Conversion to cyanurate as a function of cure temperature, silica nanoparticle loading level, and surface type. The “low hold” cure temperature was 175 °C for unmodified silica, and 210 °C for all modified silicas. The “high hold” cure temperature was 290 °C for all samples.

high levels of silanol groups, resulting in a lower “as-cured” conversion compared to the corresponding octyl-functional nanocomposite. When cured at 290 °C, however, both silanol and octyl systems showed complete conversion at all loadings. The conversion increased modestly at nanosilica loading increased in the silanol-functional nanocomposite, likely due to increasing catalytic activity. For both the silanol- and octylfunctional nanocomposites, the conversions achieved are typical for LECy at the given final cure temperatures. For the 3-amino-functional nanocomposites, however, there are several discordant features seen in Figure 8. For lower temperature cures, the 3-amino system showed a higher apparent conversion than that of the octyl system. Moreover, 95% conversion of cyanate ester groups to cyanurate typically requires cure temperatures at or above 250 °C in uncatalyzed LECy. Furthermore, increasing the cure temperature from 210 to 290 °C resulted in only slight increases in apparent conversion. The only expected feature seen in Figure 8 for the 3-amino-functional nanocomposites is the slight decrease in conversion with increasing nanosilica content, in line with the inhibitory effect of these nanoparticles on cure as noted previously. An analysis of the λ parameter in the diBenedetto equation43 (shown in the Supporting Information, Section S4) indicates that, while the glass transition temperature (TG)− conversion relationships are typical for the silanol and octylfunctional nanocomposites, the 3-amino-functional nanocomposites exhibit TG−conversion relationships characterized by loss of cyanate ester groups without the corresponding formation of a network. The apparent “residual cure” of these systems may therefore actually be due to side reactions that do not form a network. The observed TG values for the 3-aminofunctional nanocomposites suggest conversion of about 90% of cyanate ester groups to cyanurate (see Supporting Information, Section S6). Therefore, even at 210 °C, side reactions may have consumed a small portion of the cyanate ester groups without creating network junctions. Further insight into the nature of the polycyanurate networks formed in these nanocomposites may be found by examining the TG of “fully cured” networks (that is, networks subjected to heating to 350 °C), as shown in Figure 9. Ideally, polycyanurate networks exhibit TG values at full conversion that are independent of the cure protocol. Moreover, the presence of nanoparticles should have only a modest effect on this

Figure 9. TG at “full cure” (second heating to 350 °C at 10 °C/min in the DSC) for polycyanurate nanocomposites as a function of nanoparticle loading and cure protocol for silanol (a), octyl (b), and 3-amino (c) surface treatments. The “low hold” cure temperature was 175 °C for unmodified silica, and 210 °C for all modified silicas. The “high hold” cure temperature was 290 °C for all samples. The “no hold” case represents samples heated directly to 350 °C at 10 °C/min in the DSC prior to measurement.

value.21,23 Thus, the data series in Figure 9 ideally should collapse into a single, flat line at about 300 °C. For the silanol- (Figure 9a) and octyl-functionalized (Figure 9b) nanosilicas, the observed behavior is reasonably close to ideal, with only a modest dependence on cure protocol, a slight decrease in TG as loading increases, and a TG at full cure of 280−295 °C. Comparisons with TMA data using more rapid heating rates (see Supporting Information, Section S5) demonstrate that these small departures from ideality are most likely due to bond scission in ∼2% of network segments. Because some degradation of LECy polycyanurate networks is known to take place on heating to 350 °C, the nanoparticles likely do not have a major impact on network structure for the silanol- and octyl-functionalized nanocomposites. In contrast, the 3-amino-functionalized nanocomposites (Figure 9c) show a strong dependence on cure protocol, with a “fully cured” TG as G

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Industrial & Engineering Chemistry Research low as 210 °C when cured under the low hold conditions. The most likely explanation is that side reactions (including both consumption of cyanate ester groups without network formation and subsequent scission of network segments on heating to 350 °C) affect as many as ∼10% of the network segments. The much higher TG values seen in the “no hold” cure protocols indicate that exposure to elevated temperatures for extended periods of time favors these side reactions. Perhaps the most common side reactions in polycyanurate networks involve water. Hydrolysis of the cross-linked network along with the conversion residual uncured cyanate ester groups to carbamate may take place. Network hydrolysis may lead to phenolic products44 or may involve a scission of the cyanurate ring,45 leading in both cases to a decrease in crosslink density and therefore a decrease in TG. Carbamate formation leads to subsequent decomposition, forming groups that do not participate in cross-linking reactions.46 The hydrolysis of polycyanurate networks is known to accelerate in a basic environment;47 thus, one explanation for the data in Figures 8 and 9 is that the 3-amino-functional particle surfaces provide a higher pH environment that greatly increases hydrolysis of the networks over time. In order to investigate this phenomenon, the moisture uptake was measured after immersion in deionized water for 96 h at 85 °C (Figure 10). As in previously reported work,48 the moisture uptake is highly sensitive to conversion. The majority of the variation in measured water uptake can be understood in terms of variation in “network conversion”, which is based upon the glass transition temperature rather than residual heat of cure, in effect measuring the actual extent of network formation rather than simply the disappearance of monomer (see Supporting Information, Section S6). As has been noted by many researchers, including detailed studies by Georjon and Galy,49 continued formation of the network at high conversions leads to excess free volume, perhaps due to a greater propensity to form small cyclic structures,50 facilitating water uptake. Beyond this main effect, there is an apparent decrease in water uptake for the nanocomposites containing the relatively hydrophobic octyl-functional silica, at least at less than complete conversion. This effect is more apparent when data for unmodified LECy from recently published work48 are included, as in Figure 10. Statistical analysis via multiple regression (see Supporting Information, Section S10) shows this effect to be significant only if probable outliers are excluded. The “knock down” in TG due to moisture exposure was determined by dynamic TMA of samples before and after exposure to water at 85 °C for 96 h, with the results shown in Figure 11. One weakness of this technique is that, despite the use of a very rapid heating rate, some of the measured dry glass transition temperatures may be artificially high due to residual cure of the samples during the measurement itself.51 A comparison of TG values measured by DSC and TMA (see Supporting Information, Section S5) indicates that such in situ cure was an issue only for the silanol- and octyl-functional nanocomposites at the highest loading level, as indicated by the partly filled data points in Figure 11a,b. In Figure 11, the difference between the filled and unfilled data points indicates the extent of hydrolysis of cyanurate groups. It can clearly be seen that the silanol-functional surfaces experience the least hydrolysis, followed by the octyl-functional nanocomposites with greater hydrolysis, and the 3-aminofunctional nanocomposites with the greatest amount of

Figure 10. Moisture uptake (on a dry weight basis) after exposure to 85 °C water for 96 h for LECy/silica with silanol-functional (a), octylfunctional (b), and 3-amino-functional (c) nanoparticles, including recently published data47 on unmodified LECy in which the published glass transition temperature data are used to compute the “network conversion” in accordance with the methods described in the Supporting Information, Section S6, and the equivalence of DSC and TMA for glass transition temperature measurements, as demonstrated under these conditions in the Supporting Information, Section S5. The numbers beside each data point indicate the silica loading in m2/g nanocomposite.

hydrolysis. More quantitative estimates of the extent of hydrolysis of both cyanurate and residual cyanate ester groups (see Supporting Information, Sections S6 and S10) provide a rough estimate of 1−2%, 4−6%, and 8−12% loss of cyanurate groups (as a fraction of the initial amount of uncured cyanate ester groups) during water exposure and subsequent heating to 350 °C for the hydroxyl-, octyl-, and 3-amino-functional nanocomposites, respectively. A similar level of loss of uncured cyanate ester groups (presumably through carbamate formation) was also observed. Analysis of post-cured samples (also provided in the Supporting Information, Sections S6 and S10) shows similar trends. In general, the silanol-functional nanocomposites show the best resistance to hydrolysis, with octylfunctional nanocomposites having similar performance, and H

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Figure 12. (a) Comparative mass loss rates and (b) total mass remaining measured by TGA in air for neat LECy and LECy nanocomposites at loadings of 8 m2/g.

mass loss onset temperature was seen for the 3-aminofunctional nanocomposites, followed by the silanol-functional nanocomposites, and, least of all, the octyl-functional nanocomposites. The behavior of the secondary mass loss was highly variable, as is typical for TGA analysis of polycyanurates,51 but the presence of the silica consistently resulted in a shift to higher decomposition temperatures. Note that under the conditions of the TGA test, there is little time for hydrolysis in the 3-amino-functional nanocomposites; thus, the somewhat decreased onset of mass loss may reflect acceleration of other types of degradation reactions (such as chain scission by disproportionation) rather than merely an increased rates of hydrolysis-induced decomposition. In general, it should be noted that in every test of stability performed, the 3-amino-functional nanocomposites performed the worst, ranging from just slightly worse than the other types for mass loss via rapid thermal oxidation to an order of magnitude worse for overall loss of cross-link density during extended periods at elevated temperatures under nitrogen. Although increased rates of hydrolysis due to the higher pH associated with the amino-functional surfaces clearly play a significant role, and may account for all of the observed differences, accelerated degradation by other mechanisms cannot be ruled out. From an applications perspective, the surface characteristics of the nanocomposites clearly play a major role in determining the stability of the networks. One final area worthy of investigation is mechanical properties. The addition of fumed silica without significant chemical bonding to the matrix at the loading levels (up to about 5% by weight) used in this study would not be expected to alter the stiffness characteristics of the networks by much. However, the use of this particle type could have a positive influence on fracture toughness,28 although in this case also the loading levels are lower than those typically employed when

Figure 11. TG before and after exposure to 85 °C water for 96 h as determined by dynamic TMA for silanol (a), octyl (b), and 3-amino (c) surface types. Note that comparisons with DSC indicate no significant in situ cure that would render the measurements unreliable, except for some samples at the highest loading level (partly filled “gray” data points).

significantly greater hydrolysis in the 3-amino-functional nanocomposites. These trends are in line with the expected effect of pH on the hydrolysis of polycyanurates, as mentioned previously. Another source of network degradation is by decomposition and volatilization, a phenomenon that can be monitored via TGA analysis. Figure 12 provides a representative summary of the results by showing the rate of mass loss (Figure 12a) and total mass remaining (Figure 12b) in air for the different nanocomposite types at a surface area of 8 m2/g. A full analysis of all conditions and all characteristics of the TGA curves, including activation energies of degradation, is provided in Supporting Information, Section S8. Figure 12 shows that, for all nanocomposite types, the presence of silica shifts the secondary mass loss to higher temperatures, slightly decreases the total amount of the initial mass loss, and slightly reduces the onset temperature of initial mass loss. The greatest decrease in I

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Industrial & Engineering Chemistry Research toughening is the primary goal.27 To investigate this aspect, we selected three promising candidate formulations along with a LECy control, prepared large samples using a special casting technique, cut and notched five specimens of each, and then tested these for fracture toughness. We selected two loading levels of the unmodified (silanol-functional) silica due to the superior properties of these nanocomposites, one loading level of the octyl-functional silica, and no loading levels of the 3amino-functional silica due to the observed degradation of the polymer network, which would greatly complicate interpretation of the data. The results are shown as a box plot in Figure 13, with statistical analysis shown in Table 2. (More detailed

useful improvements in fracture toughness, higher loadings than those utilized for this study are likely required. There is thus a range of nanoparticle loading for which significant effects on performance can be achieved through modification of network chemistry without significantly altering physical properties.

■

CONCLUSIONS Using nanocomposites made from Primaset LECy and functionalized fumed silicas with a variety of surface treatments, it has been shown that many aspects of application-relevant performance are strongly affected by surface chemistry. In particular, the manner in which the surface chemistry of the fumed silica regulates the pH of the nanocomposite appears to play a significant role in determining the stability of the polycyanurate networks. With respect to the cure chemistry of the cyanate ester groups, acidic silanol surfaces proved to be moderately catalytic, the octyl surface mildly catalytic due to slight residual silanol content, and the basic amino-functional surface mildly inhibitory. With respect to network degradation during cure, the silanol and octyl surfaces had little effect; however, the amino-functional surface led to significant degradation of the network at elevated temperatures. Hydrolytic stability testing showed that hydrolysis of cyanurate linkages was considerably greater in the amino-functional networks compared to the others, with the silanol-functional networks performing best. These results are in agreement with the known dependence of hydrolytic stability on environmental pH in polycyanurate networks. In contrast, TGA testing revealed only slight changes in the onset of thermal degradation, and fracture toughness testing on the silanol and octyl-functional nanocomposites at 2−5 wt% silica showed no definitive effects. Overall, however, these results demonstrate that the surface chemistry of nanoparticle additives for polycyanurate networks is an important determinant of performance and must be taken into consideration in the development of polycyanurate nanocomposites.

Figure 13. Box plot (no outliers by definition, whiskers for maximum and minimum values, interquartile calculation includes median in upper and lower sub-ranges) of fracture toughness of individual LECy/silica nanocomposite samples. Note that there are five data points for Dry LECy and LECy + 5 phr Aerosil 200, and four data points for LECy + 2 phr Aerosil 200 (hydroxyl-functional) and LECy + 2 phr Aerosil R805 (octyl-functional).

Table 2. Statistical Analysis of Fracture Toughness Measurements Summary Statistics

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fracture toughness sample LECy LECy/ silanol LECy/ octyl LECy/ 3-amino

sum count (MPa m1/2)

mean (MPa m1/2)

variance sample std dev (MPa2 m) (MPa m1/2)

5 5

15.45 14.25

3.09 2.85

0.62 0.67

0.91 0.82

4

10.41

2.60

0.09

0.29

5

14.89

2.98

0.21

0.45

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b00498. Elemental analysis of nanoparticles (S1); isothermal TGA analysis of nanoparticles (S2); additional AFM and TEM images (S3); diBenedetto equation analysis (S4); comparison of TG by DSC and TMA measurements (S5); quantitative estimates of extent of hydrolysis (S6); additional DSC data (S7); additional TGA data (S8); additional dry and wet TMA data (S9); additional analysis of moisture uptake and hydrolysis (S10); and additional fracture toughness data (S11) (PDF)

Analysis of Variance source of variation

sum of squares (MPa2 m)

degrees of freedom

mean square (MPa2 m)

F

p-value

between samples within sample total

0.58

3

0.19

0.46

0.71

6.23

15

0.42

6.81

18

ASSOCIATED CONTENT

S Supporting Information *

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AUTHOR INFORMATION

Corresponding Author

data are provided in the Supporting Information, Section S11.) Note that, per the ASTM specification, the values obtained are conditional fracture toughness values (KQ), because the yield stress for these samples is not known. Although the data are not normally distributed, tests of differences in means such as the analysis of variance shown in Table 2 are robust for the level of non-normality present in the data. In this case, no statistically significant differences among the sample types could be identified with a high level of certainty. In order to achieve

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The support from the Air Force Office of Scientific Research is gratefully acknowledged. C.M.S. thanks the National Research Council Research Associateship Program, under which a J

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portion of this work was completed. The authors wish to thank Mr. Garry Abfalter of the University of Dayton Research Institute, Structures and Materials Evaluation Group, for performing the fracture toughness assessment, Ms. Barbara Miller of the University of Dayton Research Institute for providing the TEM images, Mr. Neil Redeker of ERC Incorporated (Air Force Research Laboratory) for thermal analysis testing, and Mr. Michael Ford of ERC Incorporated (Air Force Research Laboratory) for help with data compilation.

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