Novel Nanocomposites of Poly(lauryl methacrylate) - ACS Publications

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Novel Nanocomposites of Poly(lauryl methacrylate)-Grafted Al2O3 Nanoparticles in LDPE Carmen Cobo Sánchez,† Martin Wåhlander,† Nathaniel Taylor,‡ Linda Fogelström,† and Eva Malmström*,† †

School of Chemical Science and Engineering, Department of Fibre and Polymer Technology and ‡School of Electrical Engineering, Department of Electromagnetic Engineering, KTH Royal Institute of Technology, SE−100 44 Stockholm, Sweden S Supporting Information *

ABSTRACT: Aluminum oxide nanoparticles (NPs) were surface-modified by poly(lauryl methacrylate) (PLMA) using surface-initiated atom-transfer radical polymerization (SI-ATRP) of lauryl methacrylate. Nanocomposites were obtained by mixing the grafted NPs in a low-density polyethylene (LDPE) matrix in different ratios. First, the NPs were silanized with different aminosilanes, (3-aminopropyl)triethoxysilane, and 3-aminopropyl(diethoxy)methylsilane (APDMS). Subsequently, α-BiB, an initiator for SIATRP, was attached to the amino groups, showing higher immobilization ratios for APDMS and confirming that fewer self-condensation reactions between silanes took place. In a third step SI-ATRP of LMA at different times was performed to render PLMA-grafted NPs (NP-PLMAs), showing good control of the polymerization. Reactions were conducted for 20 to 60 min, obtaining a range of molecular weights between 23 000 and 83 000 g/mol, as confirmed by sizeexclusion chromoatography of the cleaved grafts. Nanocomposites of NP-PLMAs at low loadings in LDPE were prepared by extrusion. At low loadings, 0.5 wt % of inorganic content, the second yield point, storage, and loss moduli increased significantly, suggesting an improved interphase as an effect of the PLMA grafts. These observations were also confirmed by an increase in transparency of the nanocomposite films. At higher loadings, 1 wt % of inorganics, the increasing amount of PLMA gave rise to the formation of small aggregates, which may explain the loss of mechanical properties. Finally, dielectric measurements were performed, showing a decrease in tan δ values for LDPE-NP-PLMAs, as compared to the nanocomposites containing unmodified NP, thus indicating an improved interphase between the NPs and LDPE. KEYWORDS: SI-ATRP, grafting-from, nanocomposite, LDPE, PLMA grafted nanoparticles, Al2O3



INTRODUCTION Nanocomposites have attracted significant interest during the last decades, and today a vast number of publications and reviews are available that all address the importance of the interphase between inorganic nanofillers and a polymer matrix.1−15 A range of inorganic nanoparticles (NPs) of varying size and shape has been synthesized for various purposes. As an example, enhancement of different mechanical properties upon addition of surface-modified nanofillers compared with the neat matrices and unmodified nanofiller materials is expected due to the improved dispersion of NPs as an effect of the modification.4,9,10,16−20 However, to obtain the benefits from both compounds of a nanocomposite, the interphase between the nanoparticle filler and the matrix is of utmost importance. Often, surface modification must be employed to achieve good dispersion and an improved interphase between NPs and matrix.5,6,11,16−20 Enthalpic stabilization of NPs can be achieved when small molecules of appropriate polarity are attached to the NP surface. Likewise, polymer grafts are known to favor wetting and formation of entanglements with the matrix, thus stabilizing the fillers of a nanocomposite, either enthalpically and/or entropically.2,3,7−9,14,15,19,21−28 Thus, the nature of the NPs, the chemistry of the grafted polymer, the graft length, and © 2015 American Chemical Society

grafting density, are all important factors to control, to tune the NPs’ interactions with the polymer matrix.8,12,14,19,29 Alkoxysilanes have been extensively used for the surface modification of glass, ceramics, and metal oxides. However, depending on the metal oxide, the stability of the Me−O−Si bond may be substantially lower than that of the Si−O−Si bond. Because the Si−O−Al bond is known to be less stable compared to the Si−O−Si bond,33 multifunctional silanes could be used to obtain a more robust silane coupling. (3Aminopropyl) dimethylmethoxysilane (APDMMS), a monofunctional silane, gives rise to a self-assembled monolayer30−32,35−40,42−46 but is more sensitive toward hydrolysis than multifunctional counterparts. The rapid developments in the field of reversible-deactivation radical polymerization (RDRP) has brought about unprecedented tools for surface-initiated radical polymerization with sufficient control, resulting in polymer grafts with controlled molecular weight and narrow molecular-weight distribution.1−4,7,8,10,14,15,28,40,41 As a consequence, there is a plethora Received: July 16, 2015 Accepted: October 15, 2015 Published: October 15, 2015 25669

DOI: 10.1021/acsami.5b06427 ACS Appl. Mater. Interfaces 2015, 7, 25669−25678

Research Article

ACS Applied Materials & Interfaces Scheme 1. Overall Reaction Scheme to Accomplish the NP-PLMAs

bottom flask and introduced in an oil bath heated to 150 °C and kept there for 22 h under vacuum to remove impurities and bound water. Irganox 1076 (octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate)) was provided by Ciba Specialty Chemicals Inc., Switzerland. Chemicals were used as received from Sigma-Aldrich unless stated otherwise; (3-aminopropyl)triethoxysilane (APTES, 98%), 3-aminopropyl (diethoxy)methylsilane (APDMS, 97%), 3-aminopropyl(dimethylmethoxy)silane (APDMMS), 2-bromoisobutyryl bromide (α-BiB, 98%), copper(I) bromide (Cu(I)Br, 98%), copper(II) bromide (Cu(II)Br2, 99%), 4-(dimethylamino)pyridine (DMAP, 99%), triethylamine (TEA, 99%), 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA, 97%), tetrabutylammonium fluoride in tetrahydrofuran (TBAF, 1 M). Lauryl methacrylate (LMA, 96%) was destabilized prior to use by passing it through a column of Al2O3 (neutral). Deionized water, ethanol (EtOH, 96%), tetrahydrofuran (THF, analytical grade), dichloromethane (DCM, analytical grade), methanol (HPLC-grade), diethyl ether (HPLC-grade), and toluene (HPLC-grade) were used without further purification. Silanization of Nanoparticles Using APDMS or APTES (Scheme 1, Reaction 1). NPs (8.05 g) were placed in a 500 mL round-bottom flask containing distilled water and EtOH (400 mL, 1:1). The flask was placed in an ice/water bath and ultrasonicated three times using an ultrasonic rod (30% amplitude, 90 s). A magnetic stirrer was inserted in the flask, whereafter APDMS/APTES (1.80 g/ 1.55 g, 8.10 mmol) was added and left to stir at room temperature (RT) for 18 h. NP-APDMS/NP-APTES were ultrasonicated (30% amplitude, 60 s) in EtOH/THF (300 mL, 1:1) and centrifuged (10 °C, 5 min, 10 000 rpm). This process was repeated twice. NPAPDMS/NP-APTES were then redispersed in THF (300 mL) and stored in a sealed flask. TGA, Fourier transform infrared (FT-IR), and transmission electron microscopy (TEM) were used to characterize these materials. Immobilization of ATRP Initiator on NP-APDMS and APTES to Form NP-APDMS-I and NP-APTES-I (Scheme 1, Reaction 2). α-BiB (1.92 g, 8.35 mmol), TEA (1.04 mL, 7.50 mmol), and DMAP (15 grains) were added to NP-APDMS/NP-APTES (8.05 g) and dispersed in THF (100 mL). The flask was sealed and placed on a shaking table for 24 h. The flask was opened, and the reaction was quenched by the addition of EtOH (2 mL). The milky-white dispersion of NP-APDMS-I/NP-APTES-I was purified following the same methodology as described for NP-APDMS/NP-APTES with the exception of using THF, DCM, and toluene (80 mL), two cycles per solvent. Finally, NP-APDMS-I/NP-APTES-I were dispersed in toluene (80 mL) and kept in sealed flasks. TGA and FT-IR were used to characterize the NP-APDMS-I/NP-APTES-I. Synthesis of Poly(lauryl methacrylate)-Grafted Nanoparticles through Surface-Initiated Atom-Transfer Radical Polymerization from NP-APDMS-I to form NP-PLMA (Scheme 1, Reaction 3). An aliquot of the NP-APDMS-I dispersion, corresponding to the desired amount of initiating moieties (∼0.80 g, 0.04 mmol), and toluene (12.50 g) were added to a 25 mL round-bottom flask. The flask, equipped with a magnetic stirrer, was put in the ultrasonicating bath (5 min), whereafter it was placed in an ice/water bath. LMA (7.51 g, 29.5 mmol) and HMTETA (16.98 mg, 73.71 μmol) were added to the flask. The flask was sealed with a septum and degassed, applying a cycle of vacuum (5 min) and argon (5 min). The flask was quickly opened to add Cu(I)Br (9.51 mg, 6.62 μmol) and Cu(II)Br2

of polymerization systems that has been utilized for grafting from NPs. Atom transfer is probably the most widely used method for surface-initiated atom-transfer radical polymerization (SI-ATRP; i.e., grafting-from procedures). Lauryl methacrylate is a monomer that can be polymerized by SI-ATRP and is interesting to include in polyethylene (PE) as poly(lauryl methacrylate) (PLMA) has been demonstrated to have PE-like segmental mobility and can be considered as a copolymer with a backbone of poly(methyl methacrylate) and PE-like side-chains.49 Further, the chain length of PLMA can be better controlled, in contrast to polymerization of ethylene. TiO2 NPs have been modified by a two-step process, silanization51 and radical polymerization of LMA,53 showing an addition of PLMA of ∼2.5% weight loss by thermogravimetric analysis (TGA) in the nanohybrids. Also, Daugaard et al.52 grafted lauryl acrylate (LA) and stearyl acrylate (SA) from carbon nanotubes by ATRP (15 to 40% weight loss by TGA after 24 h of reaction), and nanocomposites with polypropylene were fabricated. However, in these studies, neither molecular weights nor grafting densities were characterized. One of the most challenging applications of NPs nowadays is their addition to cross-linked polyethylene (XLPE) for improved insulation cable materials.42,43 The incorporation of nanosized fillers in the insulating part of high-voltage cables may be a promising method to increase the electrical breakdown strength and to alter the charge distribution in the material.18,42−48 In this study, we investigate PLMA as a suitable compatibilizer for LDPE nanocomposites, with the purpose of investigating how the addition of PLMA-grafted nanoparticles to a matrix of LDPE affects the material properties. Previously, we conducted SI-ATRP of butyl acrylate from a robust multilayer of APTESmodified Al2O3 NPs and subsequently prepared nanocomposites by solvent mixing with poly(ethylene-co-butyl acrylate).15 The final grafting density was successfully correlated to the number of available amine groups. However, because of entrapment of amine groups in the formed multilayer, in this study it was decided to use the bifunctional APDMS as we aim to form a more robust anchoring to the NPs than obtained for the corresponding monofunctional silane, APDMMS. In the final step, the PLMA-grafted NPs are compounded with LDPE by extrusion where after the nanocomposite materials will be characterized with respect to physical and mechanical as well as dielectric properties.



EXPERIMENTAL SECTION

Materials. Low-density polyethylene (LDPE) (Mn = 14 kDa and Mw = 75 kDa (size-exclusion chromatography (SEC)), 1.4 branches (ethyl: 0.3; butyl: 0.7; pentyl: 0.1; hexyl and longer: 0.3) per 100 backbone carbon atoms as specified by Borealis. Aluminum oxide nanoparticles (NPs) with an average diameter of 40−50 nm (NPs, NanoDur 99.5%) were purchased from Nanophase Inc. USA with 70 wt % δ-phase and 30 wt % χ-phase. NPs were placed in a round25670

DOI: 10.1021/acsami.5b06427 ACS Appl. Mater. Interfaces 2015, 7, 25669−25678

Research Article

ACS Applied Materials & Interfaces Table 1 TGAa sample

silane wt %

NP-APDMS NP-APDMS-I NP-APTES NP-APTES-I

0.65 ± 0.1

equation 1b ρsilane

initiator wt %

ρinitiator

ratio ρsilane/ρinitiator

1.33 ± 0.2 1.08 ± 0.1

0.98 ± 0.1

1.33 ± 0.1

1

0.85 ± 0.1

0.43

1.96 ± 0.2 0.85 ± 0.1

a

Weight losses as determined by TGA for silanized NPs (NP-APDMS and NP-APTES) and for the corresponding samples after immobilization of the ATRP initiator. bGraft densities ρ (molecules/nm2) calculated according to equation Figure S1, Supporting Information.

(1.65 mg, 0.702 μmol), whereafter it was sealed and subjected to two additional vacuum−argon cycles. The flask was immersed in an oil bath preheated to 100 °C to start the polymerization. Reactions were conducted for 20 min (NP-PLMA-23K), 40 min (NP-PLMA-42K), and 60 min (NP-PLMA-83K). To purify the crude products, THF (20 mL) was added to the NPPLMAs, and the solution was passed through a column of Al2O3 (neutral) for copper removal. Subsequently, the NP-PLMAs were centrifuged (RT, 10 min, 6000 rpm). The supernatant was removed, and NP-PLMAs were redispersed in THF (10 mL) using the ultrasonicator bath and centrifuged (10 min, 6000 rpm). This process was repeated twice. NP-PLMA-23K (23 000 g/mol), NP-PLMA-42K (42 000 g/mol), and NP-PLMA-83K (83 000 g/mol) were obtained by this procedure. NP-PLMAs were dispersed in THF (16 mL) and stored. TGA, FT-IR, and TEM were used to characterize the material. General Procedure for the Cleavage of PLMA from NPPLMAs. NP-PLMAs (100 mg) were placed in a 30 mL vial containing TBAF (5 mL of THF, 0.1 M in THF). The mixture was left on the shaking table for 72 h. The resulting product was centrifuged (10 min, 6 000 rpm), and the supernatant containing the cleaved polymer was recovered. This solution was rotoevaporated, and diethyl ether (5 mL) was added to precipitate TBAF from the mixture. The diethyl ether with the remaining polymer was recovered and rotoevaporated. The cleaved polymer was characterized by SEC in chloroform. Nanocomposite Formation through Extrusion and Film Formation of Naked NPs and NP-PLMAs with LDPE (LDPE-NPs and LDPE-NP-PLMAs). LDPE was frozen with liquid nitrogen for 1 h and ground (0.5 mesh). Irgacure 1072 (200 ppm, 1.0 mg) was dissolved in heptane (2 mL) in a 60 mL vial. NPs/NP-PLMAs (50− 100 mg of inorganic content) were added, and the flask immersed in the ultrasonicating bath (10 min) for better dispersion. LDPE (∼5.0 g) was added, and the mixture was placed in a vortex for further mixing (30 min). The premixed samples were placed in a vacuum oven (50 °C) overnight to remove the solvent. The dried materials were finally placed in the vortex for further mixing (30 min). Extrusions were performed in a twin miniextruder (100 rpm, 150 °C, 6 min), obtaining ∼4.5 g of nanocomposite. Films (75 μm, 6.5 cm in diameter) were hot-pressed (130 °C, 10 min, 200 kN). Dynamic scanning calorimetry (DSC), contact-angle measurements (CAM), tensile testing, dielectrical measurements, and scanning electron microscopy (SEM) were used to characterize the films. Thicker films (1 mm, 15 × 11 mm2) were hot-pressed under identical conditions. Instrumentation and Characterization Methods. Molecular weights of the cleaved polymers were determined with a Verotech PLGPC 50 Plus system with a PL-RI Detector and two Mixed-D (300 × 7.5 mm) columns from Varian. Polystyrene standards with a range of 580−400 000 g/mol were used for calibration, and the solvent used was chloroform at 30 °C at a 1 mL/min flow rate. Ultraviolet−visible (UV−vis) spectroscopy: Shimadzu UV-2550 UV−vis Spectrophotometer (Kyoto, Japan), software UVProbe 2.0 (contains spectrum, photometric, and kinetic modules). Fourier transformation infrared spectroscopy: PerkinElmer Spectrum 2000 FT-IR equipped with a MKII Golden Gate, single-reflection ATR System (from Specac Ltd., London, U.K.). The ATR-crystal was a MKII heated Diamond 45° ATR Top Plate. Thermogravimetric analysis was performed with a TA Instruments Hi-Res TGA 2950 analyzer under nitrogen flow at 50 mL/min.

Heating rate was 10 °C/min and heating was performed from 40 to 700 °C. Dynamic scanning calorimetry was performed with a DSC 1 from Mettler-Toledo. Samples of 5−10 mg were measured through a cycle of heating−cooling−heating, with a starting temperature of −60 °C and up to 160 °C at a heating/cooling rate of 10 °C/min. Crystallization and melting behavior of the nanocomposites were assessed with this method using a ΔHf value of 293 g/mol,54 taking into account the LDPE matrix as the sole component, due to the small loadings of material. An Instron 5944 tensile tester was used to characterize the 75 μm films in at least quadruplicates, with a 50 N load cell with a gauge length of 30 mm and 5 mm width. The strain rate was 50%/min. The conditions for testing were 23 ± 1 °C and 50% relative humidity. Holey carbon 400 mesh copper grids (Ted Pella, Inc., USA) were used to characterize the nanoparticles. A Hitachi HT7700 transmission electron microscope at 100 kV was used. A Hitachi S-4800 field emission scanning electron microscope was used to characterize the fractured film surfaces. Samples were sputtered with a 5 nm layer of Pd−Au (sputter model 208RH). Dynamical mechanical thermal analysis with a film fixture for tensile testing (DMTA, Q800 from TA Instruments) was used to characterize the mechanical properties of the 1 mm thick films. The tests were run from 25 to 95 °C, using a heating rate of 5 °C min−1. They were performed in strain mode with oscillating amplitude of 10 μm, frequency of 1 Hz and force track of 125%. Contact-angle measurements with a CAM200 contact-angle meter (KSV Instruments Ltd.) for measuring static water contact angles was used. A drop of Milli-Q water (5 μL) was placed on the 75 μm thick films, and 5 pictures per second were acquired during 30 s. The associated CAM200 software with a Young−Laplace fitting method was used to process the images taken by the CCD camera. Atomic force microscopy (AFM) with a Multimode 8 (Bruker, USA) with ScanAsyst in Air mode was utilized to characterize the nanocomposite films, using a cantilever with tip radius 2 nm, 70 kHz resonance frequency and spring constant 0.4 N m−1 (ScanAsyst-Air, Bruker, USA). A ZM1 grinder from Retsch GmbH, Germany equipped with a 0.5 mesh, was used to grind LDPE before extrusion. An X-Plore twinscrew extruder (Xplore Instruments) was used to prepare the nanocomposites. A TP400 laboratory press (Fontijne Grotnes B.V.) was used to hot-press films. A USC300T ultrasonic cleaner from VWR was used for quick dispersion of NPs and NP-PLMAs in different solvents. Dielectric loss-tangent was measured using a Megger IDAX300. Measurements were made using sinusoidal voltage of peak value 100 V, at frequencies from 150 to 0.05 Hz. Graft density ρ was calculated using the equation in the Supporting Information (Figure S1) where the specific surface area of the NPs is 41 m2/g, the molecular weight of volatile for APDMS 73.28 g/mol, 74.28 g/mol for APTES,29 and 149.9 g/mol for the α-BiB initiating species. For the NP-PLMAs these values were their corresponding molecular weights.



RESULTS AND DISCUSSION Silanization of Nanoparticles and Immobilization of α-BiB. Initially, three different amine-functional silanes, 25671

DOI: 10.1021/acsami.5b06427 ACS Appl. Mater. Interfaces 2015, 7, 25669−25678

Research Article

ACS Applied Materials & Interfaces APDMMS, APDMS, and APTES, were used to modify the NPs in the first step, Scheme 1. As previously reported in literature, the triethoxy-functional APTES gives rise to a rather thick, gooey-type surface modification, not rendering all amine groups available for further modification.15,20,31,32,34,35 In an attempt to form a thinner and more defined silane layer, while still rendering a larger fraction of amine groups accessible, the use of the dimethoxy-functional counterpart (APDMS) as well as the monoethoxy-functional counterpart (APDMMS) were also explored in this work. The silanization of the NPs with APDMS and APTES to give NP-APDMS or NP-APTES was performed in a 1:1 EtOH/H2O mixture for 24 h at RT. Thermal gravimetric analysis was used to assess the amount of silanes that had been attached to the NPs, and the recorded weight losses for NP-APDMS and NP-APTES were found to be 0.65 ± 0.1 and 0.98 ± 0.1 wt %, respectively, Table 1. Using the equation in Figure S1 (Supporting Information) the grafting densities can be calculated from these data and found to be 1.33 ± 0.2 and 1.96 ± 0.2 silanes/nm2, respectively. This corroborated our hypothesis that APDMS gives rise to a slightly thinner silane layer, reducing the probability of selfcondensation. Inspired by the success of the use of APDMS, the monomethoxy-functional amine silane, APDMMS, was also evaluated; however, it gave rise to a substantially lower modification (0.47 ± 0.1 wt %) and was not further explored. The amine groups on NP-APDMS and NP-APTES were used for immobilization of α-BiB, a widely used species for SIATRP. By TGA it was found that the weight losses were 1.08 ± 0.1 and 0.85 ± 0.1 wt % for NP-APDMS-I and AP-APTES-I, Table 1, with the corresponding grafting densities of 1.33 ± 0.1 and 0.85 ± 0.1 molecules/nm2, respectively, as calculated by the equation in Figure S1, Supporting Information. This observation can be explained by the amine groups in NPAPTES being less available for further modification than amine groups in NP-APDMS. The results for NP-APTES and NPAPTES-I are in agreement with published results.15 From grafting density data it can de deduced that the fraction of amine groups being converted to ATRP-initiating sites is much higher for NP-APDMS than for NP-APTES, Table 1. Thus, it is evident that the amine groups in APDMS-residues are more accessible than in APTES. The presence of polar groups could be detrimental for cable insulation applications; therefore, it was decided to discard NP-APTES and only use NP-APDMS for the further work. FT-IR spectroscopy (samples mounted in KBr pellets) proved useful to monitor the success of the surface modification of the NPs, Figure 1. After reacting the NP with APDMS a peak appeared at 1550 cm−1, emanating from primary amine groups (N−H, bend). After immobilization of α-BiB, the peak at 1550 cm−1 disappeared, and a new peak at 1640 cm−1 appeared, which can be attributed to the formation of amide bonds (CO, amide stretch) corroborating that the immobilization of the ATRP initiator was successful. Grafting of Lauryl Methacrylate from NP-APDMS-I to give NP-LMAs. NP-PLMAs were obtained by SI-ATRP of LMA from NP-APDMS-I, using HMTETA and Cu(I)Br/ Cu(II)Br2 in toluene at 100 °C. The degree of polymerization was controlled by varying the reaction time; 20, 40, and 60 min were used. A fraction of the NP-PLMAs was subjected to TBAF in THF for 72 h to cleave the polymer grafts through selective cleavage of the silicon−oxygen bonds.55 The cleaved polymer was collected and analyzed by SEC, Table 2. The molecular weight

Figure 1. FT-IR spectra (KBr pellets) of NPs, NP-APDMS, and NPAPDMS-I.

Table 2. Characterization of NP-PLMAsa SEC (cleaved PLMA) sample NP-PLMA23K NP-PLMA42K NP-PLMA83K

reaction time (min)

Mn

20

TGA NP-PLMA

PDI

weight loss (%)

ρPLMA

22 800

1.4

7.7

0.05

40

41 700

1.3

13.5

0.05

60

82 850

1.2

24.4

0.05

a

Molecular weights, PLMA content, and grafting densities (ρ, PLMA chains/nm2) of NP-PLMAs calculated according to the equation in Supporting Information Figure S1.

increased proportionally to reaction time, and polydispersities were acceptable (