Influence of Nanomaterial Compatibilization Strategies on Polyamide

Feb 1, 2016 - The incorporation of small amounts of nanofillers in polymeric matrices has enabled new applications in several industrial sectors...
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Influence of Nanomaterial Compatibilization Strategies on Polyamide Nanocomposites Properties and Nanomaterial Release during the Use Phase Elisabet Fernández-Rosas,†,‡,∥ Gemma Vilar,†,‡,∥ Gemma Janer,† David González-Gálvez,† Victor Puntes,‡,§,⊥ Vincent Jamier,‡,§ Laurent Aubouy,†,‡ and Socorro Vázquez-Campos*,†,‡ †

LEITAT Technological Center, C/de la Innovació 2, 08225 Terrassa (Barcelona), Spain Centre for NanoBioSafety and Sustainability (CNBSS), 08193 Bellaterra (Barcelona), Spain § Catalan Institute of Nanoscience and Nanotechnology (ICN2), Campus de la UAB, Edifici CM3, 08193 Bellaterra (Barcelona), Spain ‡

S Supporting Information *

ABSTRACT: The incorporation of small amounts of nanofillers in polymeric matrices has enabled new applications in several industrial sectors. The nanofiller dispersion can be improved by modifying the nanomaterial (NM) surface or predispersing the NMs to enhance compatibility. This study evaluates the effect of these compatibilization strategies on migration/release of the nanofiller and transformation of polyamide-6 (PA6), a thermoplastic polymer widely used in industry during simulated outdoors use. Two nanocomposites (NCs) containing SiO2 nanoparticles (NPs) with different surface properties and two multiwalled carbon nanotube (MWCNT) NCs obtained by different addition methods were produced and characterized, before and after accelerated wet aging conditions. Octyl-modified SiO2 NPs, though initially more aggregated than uncoated SiO2 NPs, reduced PA6 hydrolysis and, consequently, NM release. Although no clear differences in dispersion were observed between the two types of MWCNT NCs (masterbatch vs direct addition) after manufacture, the use of the MWCNT masterbatch reduced PA6 degradation during aging, preventing MWCNT accumulation on the surface and further release or potential exposure by direct contact. The amounts of NM released were lower for MWCNTs (36 and 108 mg/ m2) than for SiO2 NPs (167 and 730 mg/m2), being lower in those samples where the NC was designed to improve the nanofiller−matrix interaction. Hence, this study shows that optimal compatibilization between NM and matrix can improve NC performance, reducing polymer degradation and exposure and/or release of the nanofiller.

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INTRODUCTION The incorporation of small amounts of nanomaterials (NMs) to polymers leads to new and/or improved features such as mechanical, physical, electrical, magnetic, thermal, or optical properties. As a consequence, production of plastics and thermoplastics doped with nanofillers increases every year, it reached thousands of tons for carbon nanotube nanocomposites (NCs) just in Europe during 2011, and a big increase of their production is envisaged in the near future.1 The enhancement of NC properties with small size particulate additives is partly due to the higher contact surface area between NM and polymer. Thus, a homogeneous distribution of the nanofiller in the polymeric matrix is essential to reach maximal interaction and achieve the desired properties.2−4 The effectiveness in the nanofiller dispersion is directly related with the NM surface properties,5,6 affecting the organization of the polymeric structure, and consequently the physical, chemical, and mechanical characteristics of the NC.7,8 © XXXX American Chemical Society

Polyamides are highly used engineered thermoplastic materials due to their good performance/cost balance, and due to some of their chemical and mechanical properties such as rigidity, chemical resistance, abrasion resistance, and barrier properties. Polyamide-6 (PA6) is one of the most extendedly used thermoplastics globally, with applications as NC in packaging (doped with nanoclays), electroconductive materials (carbon nanotubes), or structural components (silica). Silica nanoparticles (NPs) in polyamide act as flame retardant, give mechanical resistance (antiscratching, antiabrasive, and anticorrosive), and display improved electrical and mechanical properties.9,10 Multiwalled carbon nanotubes (MWCNTs) in PA6 act as nucleating agents,11 leading to new crystallographic Received: November 23, 2015 Revised: January 29, 2016 Accepted: February 1, 2016

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DOI: 10.1021/acs.est.5b05727 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

environmental rain is a critical parameter in aging. Indeed, the combination of both factors could accelerate photochemical degradation of thermoplastics, such as PA6. Existing studies commonly use the norm ISO4892-250 for aging, to study the degradation of epoxy and polyamide NC materials. Only a few of these studies explored the effect of accelerated (dry or wet) aging in NCs containing SiO2 NPs or MWCNTs. Generally, UV light radiation changes the physicochemical properties of the composites and damages mainly the sample surface,51,52 sometimes causing the appearance of protrusions of the nanofiller.53−56 In some cases, direct evidence was provided of NCs (silica NPs in epoxy and PA, and MWCNT in epoxy) in which NMs were found dissociated from the matrix among released debris.51,55,57 Other authors reported measurable, but insignificant, release of dissociated titania NPs from paints and MWCNTs from thermoplastic polymers.10,58 The reduced number of reports available and their changing variables (broad range of nanofillers and matrices and different methods to simulate aging and evaluate release of NMs) hampers the comparison among studies and identification of factors determining release. The goal of the present study was to evaluate how different degrees of NM−matrix interaction affect physical and chemical properties of NCs during simulated outdoor use, including polymer degradation and consequent exposure and/or release of the nanofillers. In this first study we proposed an indirect method to quantify the release when the released material is not collected. With this aim, four different PA6 NCs with 3% of MWCNTs or silica NPs were generated. Carbon nanotube polymeric composites were produced by extruding multiwalled carbon nanotubes in powder form (MWCNT) or in a masterbatch form (MWCNTMB) to produce the pellets for NC injection, while silica NPs used for this study were the following: uncoated SiO2 NPs and SiO2 NPs with hydrophobic octylsilane groups covalently attached to its surface (SiO2− octyl NPs). The physical properties of the four PA6 NCs were characterized before and after an accelerated wet aging process. In addition, the release of NMs during the aging process was evaluated with the aim of showing the relevance of different addition and compatibilization strategies for the NCs safety, in addition to their better performances.

morphologies of the polymer which enhance mechanical, thermal, and electrical properties compared to the raw (undoped) polymer; carbon nanotubes also change viscosity, affecting their own dispersion and orientation.12−15 Chemical modification of the NM surface is commonly used to enhance the compatibility between polymer and NM.16 In the case of silica NPs, the most common surface modification is the use of silane coupling agents, with a general structure that can be represented as RSiX3 (X represents hydrolyzable groups and R represents a nonhydrolyzable organic group). This surface modification improves the interaction between silica NPs and the polymer.17,18 Different silane coupling agents have been developed, such as aminopropyl methydiethoxysilane (APMDES), vinyltriethoxysilane (VTES), and dimethyldichlorosilane (DDS), with the aim of conferring to the silica different degrees of hydrophilicity/hydrophobicity or acting as coupling agents.19−24 In the case of carbon nanotubes, physical and chemical methods have been reported to satisfactorily disperse these nanofillers in polymers.25 Physical methods comprise the direct mixing of carbon nanotubes with the matrix using mechanical forces or the use of masterbatches, while chemical methods are based on carbon nanotubes surface modifications such as the use of surfactants or polymer wrapping technology.26,27 Masterbatches, i.e., thermoplastic polymers containing high loading of nanotubes (15−20%), are becoming widely used in industry for the NCs preparation, particularly for melt mixing extrusion to improve carbon nanotubes dispersibility in the polymeric matrix.28−30 Another advantage of the masterbatches is that they offer a dust-free environment, minimizing safety concerns. The increasing use of nanotechnology worldwide, together with the limited data available on NM safety when they are included in conventional products, raised concerns about the safety of such products.31−34 Most of the research in exposure to NMs has been focused in occupational environments as (i) workers are considered the most vulnerable point in the value chain and (ii) the release is usually localized and, therefore, easier to detect and quantify the exposure.35−38 But NMs may be released in any step of a nanoenabled product life cycle, and the critical step would depend on the type of product (e.g., for a cosmetic product, the consumer exposure would be expected to be very high).39 Once a nanoproduct reaches the consumer, NMs can be released from the matrix to the environment during different use processes, which could have a negative impact on human health and the ecosystem.40−45 NM release from nanoenabled products during its use depends on several factors, mainly on the intended use and the stresses that the product will suffer, such as the environmental conditions the product will be exposed to, but it will also depend on the compatibility of the NM and the matrix, and the distribution of NMs within the nanoenabled product. Hence, understanding and quantifying the release of NMs from solid matrices during their use-phase is crucial. At present, no more than 80 studies have been published investigating the release of NMs from solid matrices.46−49 Only half of the published studies had control raw samples and/or replicate testing. Weathering scenarios have not been widely explored (less than 20 papers), though long-term photochemical exposure is one of the most relevant processes leading to polymer degradation. The most frequently used approach is dry aging (UV light exposure). Only a reduced set of these studies used wet aging, including the presence of water combined with UV light irradiation exposure, although

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MATERIAL AND METHODS Nanomaterials. Two types of nanosized silica (7−14 nm primary size) were obtained from PlasmaChem: uncoated SiO2 (hydrophilic; PL-SiOF) and octylsilane modified (SiO2−octyl, hydrophobic; PL-SiOF-OS, 99.8% pure). Two types of carbon nanotubes were acquired from Nanocyl: unmodified MWCNTs (NC7000, 9.5 nm × 1.5 μm, 90% pure), produced via a catalytic carbon vapor deposition process, and a MWCNTs masterbatch that consists in a PA6 matrix with a 15% content of the same MWCNTs (PLASTICYL PA1503). Extrusion and Injection of the Composites. Before the extrusion/injection processes, PA6 (Ultramid B24, BASF) was dried in a dehumidifier (Portable Drying Conveyor, PDC Wittmann) to prevent hydrolytic degradation and alteration of its properties, as the thermoplastic polymer PA6 is an aliphatic hygroscopic homopolymer. To enhance dispersion of the NMs within the polymeric matrix, the silica NPs or MWCNTs were mixed with the dried PA6 polymer and extruded in a corotating twin screw extruder (TSE20, Brabender) to obtain a pellet with a 3% of nanofiller content. Afterward, NC pellets (3% nanofiller content) were injected to generate standard test specimens as B

DOI: 10.1021/acs.est.5b05727 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology described by the UNE-EN ISO 527 norm, in a 100 tons clamping force injection machine (TM110, Battenfeld) (see details in Figure S1). Accelerated Aging Process. To simulate outdoors use of the NC specimens, accelerated aging was performed in a climatic chamber (Suntest XXL+, Atlas). Conditions chosen were based on ISO 4892/06: irradiation 60 W/m2 (at 300−400 nm), internal and external borosilicate filters, 65 °C (±3 °C) black standard temperature or BST, 50% (±5%) relative humidity, 1/29 min (±0.5 min) wetting/drying cycles, and continuous irradiation. The total exposure time was 1000 h, comparable to 1 yr outdoor exposure in a low mountain Mediterranean climate with an average annual temperature of 15 °C: temperate winters, rainy springs and autumns, and hot and dry summers. The entire surface of the specimens (94.30 cm2) was homogeneously irradiated, flipping, rotating, and changing their position within the chamber after each weathering cycle (4 cycles of 250 h; Figure S1). The mass of each specimen was registered before and after the aging process. Composites Characterization. To evaluate the changes that occurred during aging, all the NCs were characterized before and after the aging process using different techniques: thermogravimetry (TGA), Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), transmission electron microscopy (TEM) with energy-dispersive Xray spectroscopy (EDX), and scanning electron microscopy (SEM). TGA (Q500, TA Instruments, USA) was used to evaluate the percentage of organic/inorganic matter of the PA6 and PA6 NC samples before and after the aging process and of the raw NMs used in NC preparation (and subsequently the organic/ inorganic matter weight loss of the samples during the aging process). The analysis was performed applying a temperature increase of 20 °C/min from 50 to 300 °C, 10 °C/min from 300 to 600 °C, and 20 °C/min from 600 to 800 °C, with an air or N2 flow rate of 10 mL/min. The analyses were done using 7− 10 mg of sample and maintaining inner−outer ratio of the specimens (as changes in the surfaces are more important and results might be biased). Organic/inorganic matter determination was done under air, as organic matter is better removed by calcination. However, MWCNT quantification was done under N2 to avoid being calcined together to PA6. Weight loss under 250 °C was attributed to the presence of water in the polyamide. Decomposition of the organic matter, including PA6 and SiO2 functionalization, was considered to occur from 250 to 700 °C in the experiments under air, and from 250 to 550 °C in the experiments under N2, where MWCNT started degrading at about 600 °C. Hence, the inorganic material was determined with the residue remaining at 700 °C for experiments done under air and at 550 °C for the experiments under N2. NM concentrations (CNM) in the different PA6 NCs were calculated assuming, first, that the difference between the residue in the NC (rNC) and in plain “nonaged” PA6 (rPA6) would be the residue due to NM additivation; and, second, that NMs in the NC would burn or decompose equally to raw NM (see eq 1). All the samples were analyzed by triplicate, standard deviations of the residues determined for each sample and error propagation for NM content determination. C NM =

rNC − rPA6 × 100 rNM

DSC was used to determine melting and crystallization temperatures (Tam and Tac, respectively), glass transitions and the estimation of the crystallization kinetics in each NC injected. A mDSC (Q20, TA Instruments, USA) with a refrigerated cooling system module was used for these analyses. The conditions selected were a temperature increase of 20 °C/ min from 30 to 320 °C for the first heating cycle, followed by a crystallization step at 10 °C/min down to 25 °C, and a second heat up cycle to 320 °C, with an air flow rate of 60 mL/min. The first heating cycle was used to impose a standard heat history on the material. The controlled cooling allowed determining the enthalpy of crystallization and also determining the enthalpy of melting comparing the materials directly to each other on the second heating cycle. Three samples were analyzed in all cases. Functional groups present in the samples were identified using a FT-IR spectrophotometer (IR Affinity-1 8400, Shimadzu). Attenuated total reflection (ATR) correction was done at 2000 cm−1, and for the baseline correction the points considered were 520, 1800, and 4000 cm−1. Spectra were normalized against the least changed band at 1800 cm−1 to avoid any concentration effect due to sampling efficiency. Finally, distribution of NM on the surface and inside of the polymeric matrix were determined using SEM-Merlin imaging (MERLIN FE-SEM, Zeiss) and high resolution TEM (HRTEM, Jeol 2010, JEOL Ltd.), respectively. For TEM microscopy, 2 mm3 cubes of nonaged and aged specimen tests were cut and embedded in Epon resin. After resin polymerization (60 °C, 48 h), sections with a thickness of 50 nm were cut with an ultramicrotome and placed on TEM grids (Formvar carbon-coated Cu grids, EMS). These grids were also analyzed by EDX to determine the atomic composition of the sample. Quantification of Released Materials. The total amount of material that was released from the samples (RelNC) was determined indirectly, and was estimated to be equivalent to the mass loss of the specimens studied (n = 5) after undergoing the aging process (see eq 2). Samples were weighted using an analytical balance with a mass resolution of 10−4 g (A 120S, Sartorius). For better comparison, the released material is shown as mass of released NC per mass of NC (mgNC/gNC) and per surface of NC (gNC/mNC2). RelNC =

mNCnonaged − mNCaged mNCnonaged

(2)

The amount of NM released is assumed to be equivalent to the difference between the NM content present in the nonaged and in the aged NC. The mass of NM (mNM) in each NC was determined using the NM concentrations that had been previously calculated using the TGA profile (See eq 3). Note that if the NM concentration in a NC is not significantly different between the nonaged and the aged NCs, the NM concentration in the released NC is equivalent to the NM concentration in the NC. In this case, eq 3 can be simplified to eq 4. RelNM = =

(1) C

mNMnonaged − mNMaged mNCnonaged mNCnonagedC NMnonaged − mNCagedC NMaged mNCnonaged

(3)

DOI: 10.1021/acs.est.5b05727 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology If C NMaged = C NMnonaged ,

RelNM = RelNCC NMnonaged (4)

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RESULTS AND DISCUSSION Effects of Nanofiller Nature and Interaction with PA6 Based on the Composites Properties. Differences in the composite additivation process or in the NM surface properties can change the nanofiller distribution and consequently NC properties. Within this study, uncoated SiO2 NPs and MWCNTMB (premixed to obtain a high-loaded and homogeneous blend of MWCNTs) were expected to perform better with PA6. Contrarily, SiO2−octyl NPs and pristine MWCNTs were expected to form aggregates and distribute more heterogeneously in PA6, consequently resulting in worse NC properties. Effects on SiO2 Nanoadditived Composites. A series of techniques were used to evaluate up to which extent the different NMs and additivation methods led to differences in nanofiller−matrix interactions, NM dispersion, and consequently NC properties. Qualitative observation of ultrafine TEM sections of the inner regions of PA6 NCs indicated that a uniform distribution was achieved with uncoated SiO2 NPs, with NPs both isolated and clustered. In contrast, a dominance of aggregates, but homogeneously distributed in the matrix, was observed in PA6/SiO2−octyl NC (Figure 1 and Figures S2− S5). These structural differences were reflected on the thermal properties of the NC, as shown by the DSC heating curves (Table 1) and TGA isotherms (Figures S6−S9). It is known that PA6 exhibits three crystalline forms, α, β, γ and, typically, two melting peaks, the higher corresponding to α and β crystalline forms, and a smaller corresponding to γ form. These three crystalline forms were present at the same ratio than in the non-nanoadditived PA6 in both SiO2 NCs. Crystallinity increased for NC with SiO2−octyl NPs (from 41.4% to 45.1%) while decreased for NC with uncoated SiO2 NPs (from 41.4% to 38.2%). This suggests that the octylsilane groups favor the reorganization of the polymeric chains compared to −OH groups on the uncoated SiO2 NPs. The changes in crystallinity had been observed previously by other authors.59,60 Nevertheless, as it has been reported before,60 both SiO2 NPs acted as nucleation sites modifying the crystalline structure of PA6 and the binding strength between NPs and matrix. These conclusions are supported by TGA thermograms as in PA6/ SiO2−octyl thermal degradation started before (especially after aging) and finished at higher temperatures (see Figure S9). Depending on the intended application of the NC, this change in crystallinity would be desirable or not. Effects on MWCNT Nanoadditived Composites. The differences in the level of dispersion observed between the two types of SiO2 NCs contrast with the rather homogeneous dispersion achieved for the two types of MWCNT NCs (Figure 1). Nevertheless, their physical properties differed. Premixed MWCNTMB delayed considerably the melting point (melting point: 260.5 °C compared to ∼220 °C in PA6 and other NCs) and the thermal degradation (see Supporting Information, Figures S10−12). The addition of MWCNT in both ways increased PA6 crystallinity and crystallization temperature due to their nucleating effect, as the melting PA6 interacts easily with the surface of MWCNTs.61 However, these changes are much evident in the NC using MWCNTMB, as crystallization shifted more than 50 °C compared to raw PA6 (from 192 °C in the

Figure 1. TEM images of inner sections corresponding to the different NC studied. Bar = 200 nm.

raw polymer to 245 °C), and crystallinity and enthalpy of crystallization considerably increased (from 41.4% to 49.6% and from 75.2 to 81.7 J/g, respectively) (Table 1). A double crystallization peak appeared in the nonpremixed PA6/ MWCNT NC. This indicates the presence of two different types of crystal morphologies, which according to recent reports can be explained by different hypotheses. Transcrystalline structures could be grown first in the closest regions of the carbon nanotubes, parallel to its surface,62,63 and spherulite crystalswhich is the most common supramolecular structure of PA6would grow, in the regions with no direct contact with MWCNT.64 On the other hand, Phang and coworkers65 interpreted the double peak as two-step crystallization. Laird and Li66 suggested that the addition of MWCNT on PA66 has a double effect: providing heterogeneous nucleation sites for polyamide crystallization, and hindering D

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Environmental Science & Technology Table 1. Values of DSC and Percentage of Crystallinity NC PA6 PA6 aged PA6/SiO2−octyl PA6/SiO2−octyl aged PA6/SiO2−uncoated PA6/SiO2−uncoated aged PA6/MWCNT PA6/MWCNT aged PA6/MWCNTMB PA6/MWCNTMB aged

midpoint Tac (deg C)a 191.5 191.3 191.3 191.6 189.6 190.9 196.2 196.4 245.2 244.5

(0.1) (0.4) (0.5) (0.4) (0.3) (0.7) (0.4)/208.0 (1.4) (0.7)/207.4 (0.6) (0.1) (0.3)

ΔHc (J/g)a 75.2 78.4 83.0 75.4 64.3 70.4 71.5 77.5 81.7 92.9

(1.4) (0.6) (1.0) (0.9) (1.5) (2.0) (0.2) (0.6) (0.5) (0.8)

ΔHcc (J/g)b

85.1 77.3 66.0 72.3 73.8 79.9 85.3 97.0

(1.1) (1.0) (1.7) (2.1) (0.3) (0.8) (0.5) (1.1)

midpoint Tam (deg C)a 218.8 216.9 217.7 217.0 216.9 216.0 219.8 220.1 260.5 261.4

(0.2)/191.7c (3.4)/191.7c (1.9)/192.6c (2.6)/192.9c (2.3)/192.1c (1.9)/190.8c (0.4) (0.4) (0.3) (0.5)

(1.5) (3.8) (4.0) (1.6) (0.9) (0.1)

ΔHm (J/g)a 78.8 81.2 83.8 81.7 70.9 73.7 82.9 82.3 90.5 88.0

(0.8) (0.9) (0.1) (0.3) (1.7) (1.5) (2.7) (0.7) (0.6) (0.7)

ΔHcm (J/g)b

85.9 83.7 72.8 75.6 85.5 84.8 94.5 92.0

(0.1) (0.3) (1.7) (1.5) (2.8) (0.7) (0.7) (0.2)

crystallinity (%)b 41.4 42.6 45.1 43.9 38.2 39.7 44.8 44.5 49.6 48.3

(0.5) (0.5) (0.1) (0.2) (0.9) (0.8) (1.4) (0.4) (0.4) (0.1)

* a Tc/Tam = crystallization/melting temperatures; ΔHc/ΔHm = change in enthalpy during crystallization/melting directly calculated from DSC; ΔHcc/ ΔHcm = change in enthalpy during crystallization/melting corrected with PA6-NM content. aStandard deviation in brackets (n = 3 in all cases). b Error in brackets, calculated through propagation of error with error of calculated enthalpy and error of NM content (Table 2). cTemperature of γ form isomer.

Figure 2. SEM-Merlin surface images of the different NC studied. Bar = 500 nm.

proportion of isolated NPs before aging, afterward both SiO2 NPs were distributed mainly in aggregates within PA6 (Figures S2 and S3). Conversely, MWCNTs showed always a homogeneous distribution (Figures S4 and S5). In summary, the doping of PA6 with silica NPs and MWCNTs affected almost all the structural and physical parameters evaluated. The dispersion of the nanofillers observed at the microscale suggests

large crystal growth by the tube network. Altogether, these data indicate that, despite no structural differences could be observed by TEM, the premixing of PA6/MWCNTMB led to a higher thermal resistance and to an increase in the crystallinity of the polymer. Independently of the compatibilization strategy used, we observed that though PA6/SiO2 NC showed a much higher E

DOI: 10.1021/acs.est.5b05727 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology a stronger interaction NM-polymer for the SiO2−octyl NPs and MWCNTMB. In all cases, the nanofillers improved thermal stability and increased PA6 crystallinity (particularly the MWCNT NCs). Influence of Nanofiller Nature and Interaction with PA6 on Composite Aging. Several processes during the use phase can result on the degradation of the matrix. Indeed, it is known that moisture, UV irradiation, thermal and mechanical stresses can dramatically change the characteristics of PA6, and UV light combined with oxygen cause oxidation of this polymer and chain scission.67 These changes could result on the alteration of the physical and functional properties of the NC, degradation or even cracking of the polymer in the surface and, consequently, NM and NC release to the environment. Several NCs are intended to be used in outdoor conditions, where climatic conditions (such as UV light and rain) can degrade them. A stronger interaction nanofiller−matrix leads to a better dispersion of the NM, which could result in a more robust and endurable NC and less NM release. In this study the outdoors usage of NCs was simulated by using accelerated aging in controlled UV and rain conditions to evaluate their effect on the degradation of NCs. Influence of SiO2 NPs on PA6 Composite Aging. TEM images of NC cuts indicate that the initial homogeneous distribution of nonaggregated silica NPs (or very small aggregates of 4−10 NPs) in the PA6/SiO2-uncoated NC changed during accelerated aging (Figure 1 and S2, and compare differences with Figure S3). As a consequence, homogeneously distributed aggregates/small clusters were formed during aging. NP migration observed is larger than predicted by previous calculations, which shows very slow diffusion of NPs in polymers.68−70 However, this migration might be explainable in PA6 due to the aging process, which induces both hydration of the composite and PA chains reorganization, and these phenomena might allow small migration necessary for the NPs clustering observed. On the surface, where the opacity of the SiO2 NPs does not protect the polymer from degradation, the matrix of both silica NCs cracked and became more roughened after aging (Figure 2, top). Accumulated SiO2 NPs (partially or totally exposed) were clearly observed on the surface of the PA6/SiO2− uncoated and −octyl NCs after aging. In an interlaboratory study published recently by our group,55 where similar siloxanecoated SiO2 NPs (8−20 nm) in a PA NC were aged under two different dry and wet procedures (60 W/m2, 300−400 nm) and one dry procedure (140 W/m2, 295−400 nm), the hydrophobic silica nanofiller (4 wt % content) was also initially homogeneously distributed in the matrix in form of micrometric aggregates, and NM aggregation could only be observed in the surface due to polymer degradation and release. Additionally, FT-IR did show chemical degradation or decomposition of PA6 functional groups in all the aged samples (Figure 3, top panel). During aging processes the chain scission in raw PA6 was evidenced by the onset of bands at 1680 and 1728 cm−1, attributed to oxidized species (i.e., CO− NH−CO imides and the CO of peracids) and a decrease in the emission of several bands attributed to amide groups (1551, 1630, 3256, 3308 cm−1) and hydrogen carbon chains (2853 and 2916 cm−1), which are part of the PA matrix.71 In the PA6/ SiO2−uncoated NC, the degradation pattern observed by FTIR was the same, though changes in the bands specific for NPs were more intense. This is probably caused by the acid Si−OH groups in the surface of the silica NPs, which accelerate the

Figure 3. Difference before and after aging of the attenuated total reflection FT-IR spectra, in the raw PA6 and the different NC studied. Peaks were spectra were normalized against the least-changed band at 1800 cm−1.

degradation of PA6 (hydrolysis of amides) especially around the nanofiller, causing a heterogeneous degradation. When the polymer was doped with the less reactive SiO2−octyl NPs the chemical degradation of the surface was reduced compared to the raw polymer and to the PA6/SiO2−uncoated NC. In PA6/ SiO2−octyl NC, an increase of absorbance at ∼1630 and ∼3300 cm−1 (attributed to CO and to the stretching of labile hydrogens respectively) indicates the formation of other degradation products of PA6 (i.e., oxidation products). Regarding the NM, the signal attributed to SiO2 (1088 cm−1) increased in the PA6/SiO2−uncoated NC, pointing again to an increase of exposure of the nanofiller on the surface due to the degradation of the matrix accelerated by the Si−OH groups (for extra information see the Supporting Information, Figures S13 and S14). In fact, other authors had described this phenomenon: Nguyen and co-workers observed a substantial accumulation of silica NPs on the surface of an epoxy/SiO2 NC (5 wt %) after exposure to very intense dry aging (480 W/m2, 295−400 nm).72 Wohlleben et al. observed accumulations of silica NPs on the surface of PA6/SiO2−siloxane under both wet−dry aging (60 W/m2, 300−400 nm) and dry aging (140 W/m2, 295−400 nm).55 On a different study, Dahiya and colleagues observed that the presence of −CH2OH groups in the surface of the nanofiller can cause sensitization of the overall degradation reactions.73 Contrasting with the mentioned results and the present work, Wohlleben and co-workers reported that dry aging (60 W/m2, 300−400 nm) only caused minimal PA degradation and no surface accumulation of SiO2 F

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Table 2. Thermal Degradation of PA6 as Raw Polymer and the NC and Loss of Material Calculated after Aging, Comparable to One Year of Outdoor Exposure TGA nonaged

sample

gas used

PA6/SiO2−unc PA6/SiO2−octyl PA6/MWCNT PA6/MWCNTMB

air N2 air air N2 N2

PA6

residue (%)a 0.20 0.60 2.75 2.50 3.66 4.82

(0.07) (0.01) (0.03) (0.06) (0.04) (0.05)

aged

NM concentration (%)b

residue (%)a

total weight loss

NM concentration (%)b

0.31 (0.02) 2.58 2.50 3.06 4.22

(0.11) (0.12) (0.04) (0.06)

2.73 2.48 3.61 4.86

(0.13) (0.14) (0.10) (0.07)

2.57 2.48 3.01 4.27

(0.17) (0.15) (0.10) (0.07)

(mgNC/gNC)c

(gNC/m2NC)e

10.31 (0.01)

11.8 (0.3)

24.90 5.86 3.09 0.75

28.4 (0.8) 6.7 (0.2) 3.53 (0.09) 0.86 (0.03)

(0.01) (0.01) (0.01) (0.01)

NM weight lossd

(mgNM/gNC)

(gNM/m2NC)e

0.64 (0.02) 0.146 (0.007) 0.095 (0.001) 0.032 (0.001)

0.73 (0.03) 0.167 (0.009) 0.108 (0.003) 0.036 (0.001)

a Determined at 700 °C in TGA under air and at 550 °C in samples under N2. Standard deviation in brackets (n = 3 in all cases). bDetermined considering “nonaged PA6” residue as non-NM and that the residues of raw SiO2−unc, SiO2−octyl, and MWCNT are 98.65% (±0.07), 92.10% (±0.07), and 100% (±0.01). Errors in brackets were calculated by propagation of error. cStandard deviation in brackets (n = 5 in all cases). dAs there are not any significant difference in NM concentrations in nonaged vs aged NC, NM concentration of the nonaged NC is considered the same as in the exposed and/or released material. Errors in brackets calculated by propagation of error. eCalculated considering that the material has a mass/ surface ratio of 8.8 × 10−4 m2/g (±0.2 × 10−4). Errors in brackets calculated by propagation of error.

∼3300 cm−1) in the FT-IR spectrum (Figures 3, S13−14). In the nonpremixed PA6/MWCNT NC, different degradation (oxidation) products of the polymeric chain were observed: FTIR spectra showed increase in the absorbance intensity of bands at ∼1630 and ∼3300 cm−1, indicating the formation of amines (chain scission). Hence, a global analysis of the results indicate that the premixing of MWCNTMB in PA6 protected the polymeric matrix from degradation (as shown in SEM and TEM images and mass loss analyses), and resulted in an aged polymer with higher crystallinity and thermally more stable than the nonpremixed PA6/MWCNT NC or the raw polymer (Tables 1 and 2, Figures S2 and S13−14). The PA6/MWCNTMB NC showed a more intense surface degradation than raw PA6, but neither physical degradation nor aggregation of MWCNTs could be observed (as in the case of nonpremixed PA6/ MWCNT NC). Furthermore, our results prove that the way of addition of MWCNTs in the polymeric matrix modulates the effectiveness of their protective effect. Contrarily, SiO2 NPs did not show protective effect on PA6 photooxidation, and degradation of the polymer was not dependent on the degree of dispersion of the nanofiller in the matrix. However, SiO2 NPs functionalized with octyl groups reduced the hydrolysis of PA6 amides, which shows the importance of surface functionalization in the degradation process of the polymer composite. The different degree of polyamide degradation observed during aging, caused by SiO2 NPs and MWCNT, is consistent with a previous report, in which Nguyen and co-workers51 observed a lower photodegradation after dry aging of epoxy NCs containing carbon nanotubes compared to silica nanofillers or the raw polymer. The protrusion of both SiO2 NPs and MWCNTs after wet aging on the surface of the degraded polymer had also previously been reported by our group in similar NCs (PA6/ SiO2-hydroxysilane NPs and PA6/MWCNT-NH2).74 Contrarily to what has been stated by other authors, surface degradations of MWCNT NCs were more intense than in raw PA6 (Figures 3, bottom, and S13−14). This could be explained by the local heating due to the energy absorption of carbon nanotubes on the NCs surface. Previous studies on this subject are not very conclusive: Some reported a MWCNTs enhancement of surface degradation of different NCs,74,75 while others

on PA/SiO2 NC (4 wt % nanofiller), which shows the importance of PA hydrolysis in NCs degradation and, so, their higher deterioration under dry-wet aging protocols.10 To our knowledge, no other studies exist on the release of silica NPs from solid matrices under accelerated aging conditions. Thermal degradation of the two types of silica NCs were very similar to raw PA6, but they presented higher degradation temperatures than raw PA6 after the aging process (Figures S6−9). Besides, no differences in thermal stability (crystallization/melting points) could be observed nor before neither after aging for the two silica NCs compared to the raw polymer, and the crystallinity degree of the NCs did not significantly change (Table 1). In summary, silica NPs in polyamide tended to aggregate (TEM images in Figures S2−3) and did not seem to reduce PA6 degradation (FT-IR in Figure S14), rather the contrary. The aging process reduced (sometimes eliminated) the differences on structural and physical properties between the two types of silica NCs. On the polymer surface, uncoated SiO2 catalyzed the hydrolysis of polyamide. In contrast, the lower reactivity of octylsilane groups had a positive effect, reducing PA6 chain scission and specially protecting the amide groups. Influence of MWCNTs on PA6 Composite Aging. The aging process did not show significant alterations in the thermal properties of the inner regions in the PA6/MWCNT NCs, which preserved the improved properties observed before aging (Table 1 and Figures S10−12). However, the integrity of the polymer was maintained only in the case of PA6/MWCNTMB NC; in the nonpremixed PA6/MWCNT NC aggregation of the carbon nanotubes and degradation of the matrix (cracking, increase of roughness) could be observed (Figures 1 and 2, bottom). In both NCs, carbon nanotubes protruded from the surface of the degraded PA6, which was not observed before the NCs were exposed to wet aging. Surface cracking and carbon nanotubes accumulation on the PA6 surface had also been observed by other authors in epoxy/MWCNT NCs.51,53 Similar to these studies, the aggregation of MWCNTs was mostly observed at the surface of the sample, where UV light combined with oxygen caused oxidation and chain scission and consequently amide degradation in the case of PA6/ MWCNTMB NC. This was evidenced by a reduction of amide CO (around 1630) and N−H groups (1551 and G

DOI: 10.1021/acs.est.5b05727 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Article

Environmental Science & Technology reported that MWCNTs protect NCs from degradation.48,51 Therefore, it is important to highlight that (i) a high absorption might make the damage to be concentrated in the outer layer, protecting most of the sample from degradation; (ii) polyamides are not only affected by photochemical degradation, chain hydrolysis plays a very important role as well; and (iii) apart from chemical degradation, cracks or bad compatibilization could cause deeper damages and enhance the global degradation of the sample. PA6−Nanofiller Interactions and Their Influence on NM Release. SiO2 NP Release from PA6 Composites. The total amount of material released during the aging of PA6/ SiO2−uncoated (28.4 g/m2) NC was four times higher than in PA6/SiO2−octyl NC (6.7 g/m2), and even higher than in raw PA6 (11.8 g/m2) (Table 2). This could be explained by the reactivity of the acidic Si−OH groups of the NM, as it can be observed in the FT-IR spectra (see previous section). TGA results showed that there was no statistical difference in NM concentration between aged and nonaged silica NCs, so we can assume that NM release during aging of PA6/SiO2uncoated (0.73 g/m2) was also four times higher than in PA6/ SiO2−octyl samples (0.167 g/m2). These results indicate that, although octyl functionalization of silica did not seem appropriate for its compatibilization with PA6, it is important to avoid direct contact between silica acidic surface and PA6 matrix. In previous works, the release of silica NPs from solid matrices has been studied, without quantification of the released material.51,57Other studies have focused on protocols for direct quantification of released NM. In the specific case of PA6/SiO2 NC, Wohlleben and co-workers observed the release of material after dry and wet aging of PA/SiO2−siloxane NC.55 MWCNT Release from PA6 Composites. The amount of material released by both NCs with carbon nanotubes (3.53 and 0.86 g/m2, for PA6/MWCNT and PA6/MWCNTMB, respectively) was lower than for raw PA6 and the two silica NCs, indicating that MWCNTs, premixed or not, better protected PA6 from degradation during aging (Table 2). Moreover, the NCs produced with premixed MWCNTs presented a very low release, 1 order of magnitude lower than raw PA6 and one-fourth compared to the other NC produced with direct MWCNT−PA6 mixing. As the nanofiller concentration did not change significantly during aging, the amounts of MWCNTs released also were four times higher in the nonpremixed NC (0.108 g/m2) compared to the masterbatch (0.036 g/m2). This is consistent with the good preservation of the NC observed in the properties of the specimens, where almost no evidence of degradation were found in the masterbatch NC. Previous studies evaluating the release of MWCNTs from NCs during dry or wet aging processes had reported diverse results. Nguyen and colleagues51 reported that after 11 days of very intense UV exposure of epoxy/MWCNT NC (480 W/m2, 295−400 nm), MWCNTs formed a dense entangled network on the degraded NC surface, with no evidence of release even after prolonged exposure times; suggesting that the MWCNTs network formed on the composite blocked the MWCNTs release. Similarly, almost no release of MWCNTs was observed from degraded POM/MWCNT NC, even with ultrasound treatment.10 In contrast, in another study, Hirth and co-workers observed that photochemically degraded polyurethane/ MWCNT composites released micron-sized and submicrometric (