1 A Novel Bio-Residue to Compatibilize Sodium Montmorillonite and

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A Novel Bio-Residue to Compatibilize Sodium Montmorillonite and Linear Low Density Polyethylene Bjarke Høgsaa, Ellie H. Fini, Jesper de Claville Christiansen, Albert M Hung, Masoumeh Mousavi, Erik Appel Jensen, Farideh Pahlavan, Thomas H. Pedersen, and Catalina-Gabriela Sanporean Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04178 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 5, 2018

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Industrial & Engineering Chemistry Research

A Novel Bio-Residue to Compatibilize Sodium Montmorillonite and Linear Low Density Polyethylene Bjarke Høgsaa 1, Ellie H. Fini1*, Jesper de Claville Christiansen,2 Albert Hung,1 Masoumeh Mousavi,1 Erik Appel Jensen,2 Farideh Pahlavan,1 Thomas H. Pedersen,3 Catalina-Gabriela Sanporean2 1

North Carolina A&T State University, Sustainable Infrastructure Materials Lab, 1601 E. Market St., Greensboro, NC 27411, Phone: (336) 336-28536, Fax: (336) 334-7126 2

Aalborg University of Denmark, Department of Mechanical and Manufacturing Engineering, Materials Science and Engineering Research group, Fibigerstraede 16, 9220 Aalborg East, Aalborg, Denmark

3

Aalborg University of Denmark, Department of Energy Technology, Pontoppidanstraede 101, 9220 Aalborg East, Aalborg, Denmark * Corresponding author. Tel.: 336-285-3676; fax: 336-334-7126; e-mail: [email protected] (E. H. Fini)

ABSTRACT Despite the improved thermal stability and mechanical properties of polymer-clay nanocomposites compared to pure polymeric materials, creating nanocomposites with enhanced thermo-mechanical properties requires a good compatibility and dispersion of the clay within the polymeric matrix. This paper introduces a bio-residue extracted from waste bio-mass to modify montmorillonite clay to compatibilize it with linear low density polyethylene (LLDPE). The bio-modified clay was compounded, melt blended and injection molded with LLDPE, and the thermomechanical properties of the resulting nanocomposites were investigated with oscillatory rheometry, TG, XRD, ATR-FTIR, and TEM to assess the compatibility of the bio-modified clay and the polymer. The structure of the bio-modified clay ranged from partially intercalated to fully exfoliated. Hansen solubility parameters indicate that almost all of the identified compounds in the bio-residue are soluble with polyethylene. Density functional-based modeling showed a trade-off between electrostatic screening and dispersion interactions affecting the overall interlayer spacing in polymer-clay nanocomposites.

Keywords: Nanocomposites, sodium montmorillonite, bio-residue, intercalant, surfactant 1 ACS Paragon Plus Environment

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INTRODUCTION Polymer nanocomposites have gained increased attention in recent decades due to improved properties compared to micro-composite or pure polymeric materials. There has been extensive investigation of the enhancement effects of introducing nanofillers and compatibilizers to the polymeric matrix using various manufacturing processes such as melt blending with shearing energy or solution and in situ polymerization.1-5 An emerging trend in the development of new nanocomposites with improved properties is the introduction of bio-renewable materials as matrix, filler, or compatibilizer.6 The selection of compatibilizer is critical for improving dispersion of the filler in the matrix and realizing enhanced nanocomposite properties. One conventional system with incompatible materials is polyolefins with layered nano-clay, including montmorillonite (MMT).6-8 This incompatibility is due to the different chemical structure of the two materials; the polymer is hydrophobic, and the clay is hydrophilic. This difference in chemical structure and compatibility can be overcome with organic modification of the clay to an organo-clay or grafting of more polar compounds to the polymer material.9,10 A secondary alternative is utilization of a compatibilizer, which acts as a bridge between the polymer and the filler.11-16 The degree of compatibility can be assessed by characterizing the clay dispersion in the polymer matrix, where poor compatibility results in poorly dispersed, agglomerated nano-clay and reduced mechanical and thermal properties of the nanocomposite. To evaluate the resulting properties in any given nanocomposite, it is vital to have a deep understanding of the interaction mechanisms between compatibilizer molecules and the clay structures within a polymeric matrix, from the molecular level to the macro level.1,6,9 Bio-residue (BR) produced from animal waste is one of a new group of renewable bio-adhesives showing promise as inexpensive, environmentally friendly materials and chemical alternatives in a variety of applications. A thermochemical conversion process transforms liquefied swine manure to a bio-oil, and a fraction of the oil is distilled into BR. This BR has shown great potential as a surfactant for montmorillonite clay,17 and analysis of the chemical composition of bio-binder, a post-production product

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of the BR, supports BR's use as a surfactant for the organic modification of montmorillonite.18,19 GC-MS has confirmed that the BR composition is dominated by amphiphilic compounds, long fatty acids, and amide compounds.17 The interaction between the BR and MMT was believed to be hydrogen bonding and dipole–dipole interaction between the carboxylic acid of the long fatty acids or the amide groups to silicon oxide groups of the MMT. Experimental and theoretical results showed that in a 1:1 ratio of this animal waste-BR and Na-MMT, their interaction leads to intercalated Na-MMT structures.17 The long aliphatic chains of the surfactant compounds are believed to be sites compatible with polyolefin, which should result in a connection between the organo-clay and the polymer matrix material. In this work, the targeted BR was used as a new type of compatibilizer for Na-MMT and linear low density polyethylene (LLDPE) polymer. NaMMT was modified with different amounts of BR to produce an organo-clay that was extruded with LLDPE to produce polymer-clay nanocomposites.

MATERIALS AND METHODS Materials The experiments used Flexirene MS 20A, a commercial grade of LLDPE polymer with a melt flow index (MFI) of 26 g/10min measured at 190 ºC / 2.16 kg, a specific density of 0.921 g/cm3, and a melting point of 117 ºC. Cloisite Na+, a natural, commercial montmorillonite (MMT) enriched with sodium, was acquired from Southern Clay Products (part of BYK Additives). Bio-residue (BR) is a product of a conversion process of swine manure to bio-adhesives. The process to obtain BR is not the focus of this work and is reported elsewhere.17-19 Organic Modification of MMT For each organo-clay, the specified amount of BR and MMT were mixed in a 1:1 water:ethanol, with 1 gram of MMT per 50 ml of deionized water. The blend was mixed with magnetic stirring for 25 min at 80 ºC. After mixing, the suspension was left to swell for 24 hours, then reheated for 5 min and mixed with



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a second solution of 1 gram of BR in 10 ml of ethanol, for 15 minutes. The mixture was ultrasonicated with an ultrasonic horn for 2 minutes and left to settle for 24 hours. After a centrifugation procedure at 500 RPM, the sediment, organo-clay, was collected and air dried at ambient temperature. The collected organo-clay was ground by hand into a fine powder for further processing. Three organo-clays with different initial ratios of MMT to BR (2:1, 1:1, and 1:2 by weight) were produced and denoted based on the weight percentage of BR: OMMTBR 33, OMMTBR 50, and OMMTBR 66. Preparation of the Nanocomposite Nanocomposites were processed by melt blending with two-step processing: initial compounding to obtain a master batch, followed by an extrusion process to obtain nanocomposites with a 2% filler concentration. The initial compounding of the pure polymer LLDPE with a specified amount of organoclay was performed on an Xplore MC 15, Micro compounder with a temperature profile of 180 ºC in all heating zones and screw speed of 100 RPM. The melt temperature was registered to be approximately 175 ºC and a residence time of 180 sec. The modified granules were extruded and diluted with LLDPE to the specified clay concentration of 2 %, in each type of nanocomposite, in a Thermo Scientific Prism Euro-lab 16 twin screw extruder with heating profile of 180 °C in all heating zones of the barrel. The granules were extruded one time to evaluate the initial effect of intercalation/exfoliation of the unmodified and BR-modified organo-clays in the nanocomposite structure. The granules were then extruded three additional times with an approximate residence time of 30 sec per extrusion to assess the effect of increased residence time. Nanocomposite granulate was injection molded in a HAAKE MiniJet Pro Piston Injection Molding System, with a temperature setting of 180 °C, a pressure of 600 bars for 20 sec, and a post-pressure of 600 bars for 20 sec, with a tensile bar mold for XRD measurements. The mold was specified after ISO 527-25A specifications and a mold temperature of 30 °C. Four types of nanocomposites were denoted, based on the type of unmodified or organo-clay in the LLDPE: LLDPE MMT, LLDPE OMMTBR 33, LLDPE


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OMMTBR 50 and LLDPE OMMTBR 66. The injection molded samples were prepared and used for XRD analysis to insure standardization of the samples.

Materials Characterization Methods Thermogravimetric Analysis (TG) Thermal analysis was performed on a TA Discovery TGA using a platinum pan in nitrogen atmosphere with a flow of 10 ml/min with a heating rate of 10 ºC/min. Measurements were taken from 40 to 1000 ºC for the organo-clays and from 40 to 600 ºC for the nanocomposite granulate. X-ray Diffraction (XRD) In order to study the effect of the addition of BR as a modifier on the extent of clay intercalation, Xray diffraction (XRD) was performed on a PANalytical Empyrean diffractometer with CuKα radiation, running at 45 kV and 40 mA in the interval of 3-40°. The nanocomposite samples were secured in a conventional sample holder with steel spacer discs to ensure appropriate measuring height, and the heads of the tensile bar were cut to size.

The degrees of intercalation/exfoliation of the obtained

nanocomposites were investigated to evaluate the interactions between the organo-clay and the polymer matrix in each of the materials systems. Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR) ATR-FTIR measurements were performed on a Spectrum One spectrometer from PerkinElmer using a zinc selenide crystal in absorbance mode with a resolution of 4 cm-1 and averaged over 4 scans. The spectra of the unmodified and modified materials were acquired from 500 to 4000 cm-1 to assess interactions between the nano-clay and the BR. Dynamic Light Scattering (DLS) Unmodified and organo-clays were examined with a Zetasizer Nano ZS DLS from Malvern to determine the average zeta potential and particle size. 0.1 mg samples of the different clays were diluted in 15 ml ethanol, and the suspension was ultrasonicated for 20 minutes. A few milliliters of the 5 ACS Paragon Plus Environment


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suspension were inserted into a drip cell, and the analyses were conducted at approximately 25°C. Transmission Electron Microscopy (TEM) Unmodified and organo-clay powders were embedded in Epofix™ epoxy resin (Electron Microscopy Sciences) with no additional grinding or mixing other than very brief stirring to extract air bubbles. The resins were cured at room temperature for at least 24 h. An RMC Boeckeler PT-PC PowerTome ultramicrotome was used to cut the samples into thin slices of 40 nm that were picked up on 200 mesh copper TEM grids. A CR-X cryosectioning system was used to obtain thin sections of 60-90 nm of LLPDE-organo-clay composites for preliminary TEM imaging. The thin sections were coated with 5 nm of carbon using a Leica EM ACE200 coating system and imaged in a Zeiss Libra 120 TEM operating at 120 kV. Oscillatory Rheometry Rheological tests were made using a Paar Physica MCR 500 rheometer in a cone-and-plate configuration with a 25-mm diameter 2° cone and a gap of 0.105 mm. The cone-and-plate geometry was chosen due to the limited amount of material produced, a test with a 25 mm 2°cone can be made with as little as 0.3 g of material ensuring a filled gap. Loading procedure The material was carefully loaded onto the middle of the plate and slightly pressed down; the oven hood was then lowered down to the plate and left for exactly 5 minutes. After 5 minutes, the oven hood was lifted and the material visually inspected to ensure it was molten and no bubbles present. The gap was then set (gap setting is normal force controlled by the instrument) this is a two-step procedure; the instrument stops the cone 50 micrometer above the gap, the excess material is then trimmed away and the cone is lowered to the measuring gap position; the oven hood was lowered and the test was started with an initially programmed delay of 5 minutes to ensure thermal stability. The measurements were performed at 170°C using a fresh sample in each test. The test was in the frequency range from 600 rad/s to 0.06 rad/s using a 5% strain.


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Hansen Solubility Parameters Hansen solubility parameters (HSP) describe the cohesive forces of a given material from three different contributions: dispersive, polar, and hydrogen bond cohesive forces, δD, δP, and δH, respectively. Compatibility between two materials is evaluated based on the relative energy difference (RED) expressed in Equation 1. = /

(1)

Ro is the interaction radius of the polymer and is set to 8.1 for LPDE in this study. is the “Hansen distance” between the BR compounds and the polymer given by Equation 2. = 4(∆δD ) + (∆δP ) + (∆δH )

(2)

where ∆δX is the difference between the individual HSP of any given two compounds. Values of RED < 1 suggest that the compounds are compatible. An alternate way to visualize compatibility is to calculate the fractional HSP (f X) of the compounds and plot them in a ternary diagram (Teas chart).

=

δD δD + δP + δH

=

δP δD + δP + δH

=

δH δD + δP + δH

Determining the HSP of a specific compound requires extensive laboratory solvolysis tests and is beyond the scope of this work. Instead, the HSP of the BR compounds are estimated by the Yamamoto-Molecular Break (Y-MB) method using the commercial software HSPiP. The chemical compounds identified in BR via GC-MC were evaluated based on HSP.17 Computational Methods Periodic density functional theory (DFT) calculations were performed using a CASTEP module implemented in the Accelrys Materials Studio program package (version 6.0).20-22 This widely used code performs electronic structure calculations within the density functional formalism for a wide range of materials that can be thought of as an assembly of nuclei and electrons, including crystalline solids. In the plane-wave pseudopotential approach, Kohn-Sham (wavefunction) equations are solved in terms of plane wave basis sets by imposing periodic boundary conditions through Bloch’s Theorem. As density 7 ACS Paragon Plus Environment


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functional, the Perdew, Burke, and Ernzerhof (PBE)23 functional based on the generalized gradient approximation (GGA) was employed to describe the exchange-correlation energy. Dispersion interactions are dominant in the stacking of MMT layers and the corresponding lattice parameters. Adding the dispersion correction to the DFT functional (DFT-D) lead to a more realistic prediction of cell geometry and lattice parameters. Accordingly, the study was performed using PBE in combination with Grimme’s long-range dispersion correction24 to treat van der Waals dispersion interactions. Calculations were done with a 3 × 1 × 1 k-point mesh for the Brillouin zone sampling and a plane wave cutoff of 450 eV. The overall quality for the numerical integrations was set to “Fine” grid, and the convergence criteria for full geometry relaxations of the periodic system, including atomic positions, cell volume, and lattice parameters, are 1.0×10-5 eV/atom, 3×10-2 eV/Å, 5×10-2 GPa, and 1×10-3 Å for energy, maximum force, stress, and displacement, respectively. The electron-ion interaction is described by ultrasoft pseudopotential,25 allowing for a low kinetic energy cutoff and fewer plane waves required for the expansion of the periodicity part of electron wavefunctions. In our simulations, the fast-Fourier transform (FFT) grid for the electron density was set at 32 × 120 × 120 “Fine” mesh. A combination of pseudopotential and FFT reduces the number of plane waves representing electronic wavefunction accurately. The “real space” dialog was specified for transformation of pseudopotentials to a real space representation. DFT-Based Molecular Modeling of Na-MMT Montmorillonite (MMT) is a smectite clay with a multilayered structure. The MMT layers consist of sheets of octahedral (O) aluminum oxide and tetrahedral (T) silica that are crystalized in a TOT (2:1) or TO (1:1) arrangement. MMT naturally occurs with a variety of chemical compositions and structural features, due to random isomorphic substitutions of Al3+ and Si4+ by lower-valence metal cations, such as Mg2+ and Al3+, respectively. This substitution leads to negatively charged MMT platelets, which are compensated for by the presence of cations located in the interlayer gallery. The MMTs are named based on exchangeable cations residing between sheets, which are commonly alkali or alkaline-earth metal ions. 8 ACS Paragon Plus Environment

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There is no transparent information for MMT in the crystallographic database, mainly due to the isomorphous substitutions of octahedral sheets leading to imperfect crystals of MMT clay. So, a unit cell of pyrophyllite is commonly used to set up a MMT cell model. The primitive unit cell of pyrophyllite used in this study was derived from the structural and crystal-chemical features presented by Lee˗Guggenheim,26 reported in AMCSD (American Mineralogist Crystal Structure Database). This unit cell with a chemical formula of Al4Si8O24 has the geometrical structure of a=5.16, b=8.97, c=9.35 Å, and α=91.2˚, β=100.5˚, γ=89.6˚. The pyrophyllite unit cell was employed to calibrate the measuring system used and to properly define some parameters like plane wave cutoff. In this respect, Ecut-off values of 350, 400, 450, and 500 eV were used to optimize the ions and cell parameters, and the results were compared to their counterparts in the Lee˗Guggenheim reference cell. On this basis, at Ecut-off value of 450 eV, the best consistency is achieved between the optimized parameters (a=5.25, b=9.11, c=9.37 Å, and α=91.3˚, β=100.5˚, γ=89.9˚) and reference cell parameters. Setting up the Na-MMT unit cell started from a 1 × 2 × 1 ( × × !) expansion of the primitive unit cell of pyrophyllite, followed by a single substitution of Al → Mg in the octahedral layer, and then insertion of one Na atom in the interlayer space to rebalance the charge, resulting in a chemical formula of Na˗MgAl7Si16O48. To insert the BR and LLDPE˗BR into the interlayer space, a vacuum gap about 10 Å was introduced to the structured unit cell of Na-MMT. To provide a proper coverage between these additives and the silicate layers of Na˗MMT, the cell was replicated in a dimension (2 × 1 × 1), with chemical formula Na2˗Mg2Al14Si32O96. The geometry of the final supercell was optimized (all cell parameters and ions) under the same conditions used for the initial unit cells of pyrophyllite and NaMMT. The optimized supercell was characterized by the following parameters; a= 10.53 Å, b=18.23 Å, c=18.69 Å, and α=88.3˚, β=100.5˚, γ=90.2˚.



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RESULTS AND DISCUSSION Characterization of BR-Modified MMT Organo-Clays The structures of the organo-clays were studied with XRD as shown in Figure 1, and the interlayer spacing was calculated using Bragg’s law. XRD diffractograms indicated characteristic peaks of Namontmorillonite at 2θ 7.57°, 20 ° and 28°, respectively 27,28 . The peak at 2θ =20°, characteristic for nonbasal reflections (02, 11), was present for all nanocomposites, indicating that the crystalline layered structure was not affected by the modification process

29, 30

. The 2 θ value at 28° was attributed to (005)

reflection. After organic modification, it was observed that the (005) reflection peak of the MMT pattern disappeared in nanocomposites. Thus, taking into account these findings, it is inferred that the MMT layered structure became disordered

31,32

. However, to further clarify the change in structure of MMT

d(001) reflection was investigated. The intensity and position of the d001 peak of the organo-clays decreased with an increasing amount of BR, which showed increased interlayer spacing of the organoclays. The interlayer spacing increased by 12.8 %, 25.6%, and 26.5% for the OMMTBR 33, OMMTBR 50 and OMMTBR 66, respectively, when compared to unmodified MMT. With an increasing amount of BR, the structure of the clays changed from an intact structure of the unmodified MMT to a nearly exfoliated structure of OMMTBR 66.


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Figure 1. XRD analysis of the d001 peaks of the unmodified MMT and the organo-clays with different amounts of BR.

TG results also showed that the dosage of BR used during organic modification of MMT had a clear effect on the thermal stability of the organo-clay compared to the unmodified nano-clay. Thermal decomposition of the organo-clays can be divided into four regions. First is the evolution and evaporation of absorbed water and gaseous species that occurs below 180 °C (region I). Organic substances evolve from 200 to 500 °C (region II). Dehydroxylation of the aluminosilicate of the MMT occurs from 500 to 700 °C (region III), and evolution of products associated with residual organic carbonaceous residue occurs between 700 and 1000 °C (region IV).33-35 The BR undergoes one primary thermal decomposition stage at 200-500 ºC.17 The organo-clays had increased mass losses in both region III (0.5 – 1.6% wt. mass lost) and region IV (3.8 – 8.7% wt.) compared to unmodified MMT (Table S1 and Figure S1 in Supporting Information). These increased mass losses must come from BR compounds trapped inside the clay galleries or bounded on the clay surfaces. OMMTBR 50 had the highest mass losses in the third and fourth regions, but also the lowest onset temperature of the three organo-clays in region II. This could indicate that an optimum amount of modifier is needed for full penetration of the clay galleries. The lower thermal stability observed for OMMTBR 50 could be attributed to the faster decomposition of surfactant components (especially long-chain fatty acids) with the addition of a clay catalyst. 36 ATR-FTIR analysis of the nano-clays supported the TG results; peaks associated with both the MMT (primarily 1120-1025 cm-1) and the BR were present in the organo-clays (Figure S2 in Supporting Information).17,37 The BR peaks increased in intensity relative to the MMT band with higher BR dosage, indicating a greater amount of organic material content. Compared to the FTIR spectra of pure BR, the carbonyl C=O stretching band (~1700 cm-1) and the OH/NH stretching band (3600-3000 cm-1) were suppressed in the organo-clay, possibly due to binding of specific chemical groups to the silicate or an artifact of the modification process. 11 ACS Paragon Plus Environment


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DLS measurements given in Figure 2 showed a non-monotonic variation of particle size and electrostatic charge with BR dosage. The OMMTBR50 had the second highest zeta potential magnitude with -18.4 mV, surpassed only by the unmodified MMT. Colloidal suspensions with a high zeta potential (negative or positive) are electrically stabilized, while colloids with low zeta potentials tend to aggregate 38

.Thus, the OMMTBR 50 produced the most stable suspension of the organo-clays and showed the

smallest average particle size, suggesting better dispersion and exfoliation of the nano-clay platelets. OMMTBR 33 and 66 had low zeta potentials and large average particle sizes, representative of highly agglomerated clay structures.

Figure 2. Particle size and zeta potential of the unmodified MMT and the organo-clays measured by DLS.

Based on the previous results, only pure MMT and OMMTBR50 were selected for further examination with TEM. TEM images of ultramicrotome sections of untreated MMT and OMMTBR 50 embedded in epoxy resin are shown in Figure 3. The unmodified MMT exhibited minimal dispersion and an interlayer spacing of 1.2 nm as expected (Supporting Information Figure S9), whereas the OMMTBR 50 showed a wide range of structures including a significant proportion of intercalated or fully exfoliated clay sheets. Particles of non-exfoliated clay and excess BR were also observed in the same sample (data 12 ACS Paragon Plus Environment

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not shown). The partial exfoliation and variation in the interlayer spacing observed by TEM corroborate the XRD results that show a shift and broadening of the MMT d001 peak.

Figure 3. TEM images of epoxy-embedded (a) untreated MMT and (b, c) OMMTBR 50 (both images).

HSP Analysis of the Interaction between BR Compounds and Polyethylene Several studies have shown that Hansen solubility parameters (HSP) could be used as a preliminary compatibility assessment tool to test if a polymer is soluble and compatible with intercalant compounds 3942

.Exfoliation and dispersion is quite complex and can be driven by mechanical and chemical interactions.

The solubility hypothesis assumes that a surface-treated nanoparticle will be compatible with a polymer if the backbone of the molecules on the nanoparticle surface is a solvent for the polymer. Organic modification of the strongly charged clay surface is achievable using appropriate functional groups that can substitute for the native counterions. However, after surface modification and introduction into a polymer system, the stability of the nanoparticle dispersion depends on minimizing the enthalpy of mixing because the entropic contribution for large molecules or particles is very small. Therefore, a surfactant should be chemically similar to the chosen polymer matrix. In order to predict if BR compounds could ensure interaction between LLDPE and the organo-clay, the HSP of a low density polyethylene sample was compared to the estimated HSP of several BR compounds (Table S2 in Supporting Information) to determine mutual solubility properties 43. Calculated values of the relative 13 ACS Paragon Plus Environment


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energy difference (RED) suggest that all of the BR compounds considered except decane are compatible (RED < 1) with LDPE. The fractional HSP values plotted in the Teas chart in Figure 4 show that some classes of BR compounds may be more effective as compatibilizers than others. For example, the amides identified in the BR are more compatible with LDPE than the free fatty acids (FFAs), although the FFAs are also potential compatibilizers. However, these compounds identified by GC-MS represent only the volatile and lightweight compounds of the BR, which are only about 14% wt. of the BR based on TG results.17 Further studies into the remaining compounds of the BR are needed, but these results are a good indication that BR could be a strong compatibilizer for polyolefin-clay nanocomposites. The results for LDPE are based on three reported results in the Hansen software. To get a more accurate result, measured Hansen parameters of the LLDPE are needed.

Figure 4. Teas chart showing the fractional HSP values of LDPE and BR compounds. BR compounds are divided into free fatty acids (FFA), amides, and others.

Characterization of LLDPE/Organo-clay Nanocomposites It has been demonstrated that small-strain oscillatory shear measurements can be used to determine the state of dispersion and exfoliation, as well as the effect of compatibilizing agents, in clay-filled nanocomposites 44-46. If the clay is dispersed, intercalated, and exfoliated, the low-frequency plateau in the complex viscosity should be replaced by an increasing viscosity as the frequency becomes lower. This 14 ACS Paragon Plus Environment

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trend indicates a yield stress usually associated with soft solids. The rheological analyses confirmed interactions between the organo-clay and polymer, as evidenced by deviation from the low frequency plateau indicating the formation of a soft solid; the rest of the replicates exhibit the same behavior as the neat material, indicating inhomogeneity in samples (Figure 5a).

Figure 5. The complex viscosity as a function of angular frequency of LLDPE OMMTBR 50, which has been extruded (a) once and (b) four times.

The significant differences observed in complex viscosity at low frequency indicate that samples contain both agglomerates and well-dispersed exfoliated clay. This is probably due to the fact that one extrusion does not give the necessary mechanical energy to disperse the clay. 46 Samples that underwent four extrusions in an effort to enhance dispersion were found to be more homogeneous, and all replicates except one (run no. 4) showed at least a little evidence of forming a low frequency “tail” (Figure 5b). Accordingly, the increased residence time and mechanical shearing energy in the extruder improved the dispersion of the modified clay in the nanocomposites, although there remains significant variation between samples. The averaged data for the complex viscosity of pure LLDPE and the nanocomposites are shown in Figure 6. The modified nanocomposites all have nominally higher viscosity than the pure polymer, increasing slightly with increasing dosage of BR. However, the experiments did not show signs of formation of a soft solid in LLDPE OMMTBR 33 or 66. LLDPE OMMTBR 50 was clearly exceptional, showing the highest viscosity among all samples with a pseudo-solid response at low frequency. The low complex viscosity of LLDPE MMT is because of an agglomerated structure due to incompatibility 15 ACS Paragon Plus Environment


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between the base polymer and the clay. It should be noted that the standard deviations of the complex modulus for nanocomposites at the 33 and 66 dosages of BR were lower than for the 50 dosage (Figures S3 and S4 in Supporting Information).

Figure 6. Average measurements of complex viscosity versus angular frequency for pure LLPDE and the nanocomposites with unmodified and modified MMT with different dosages of BR, after 4 extrusions.

The rheology data supported the HSP analysis, and it should be possible to use the BR as a compatibilizer between clay and LLPDE. However, it also shows that further work has to be done in order to find the optimal extrusion procedure. TG analysis was performed to examine the variation in clay concentration among the nanocomposite samples (Figure S1 and Table S3 in Supporting Information). The nanocomposites showed residual mass at 600 °C that increased with an increasing amount of BR. The designated 2% filler concentration was not achieved, but a clay concentration was present in all nanocomposites, which was confirmed by FTIR results. The reason for the reduced residue content, which was lower than 2% in samples, could be attributed to poor dispersion . Due to agglomerations of the nano or organoclay in the nanocomposites, it is unlikely to cut samples with the same amount of clay. 16 ACS Paragon Plus Environment

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The nanocomposites undergo one primary thermal decomposition stage at about 450-460 °C (Figure S1 and Table S3 in Supporting Information). The onset temperature of the nanocomposites increased with the addition of MMT, OMMT33, and OMMT66, which indicates that the MMT increases the thermal stability. However, LLDPE OMMTBR 50 had the lowest onset temperature of 451.1°C, which is lower than the onset temperature of 455°C for LLDPE. This result is in contradiction with the idea that good dispersion and compatibility increases thermal stability. XRD analysis of the nanocomposites indicated several possible interactions between the polymer and the modified organo-clays (Figure 7). All the nanocomposites with organo-clay have intercalated structures with a 31-34% increased interlayer spacing, when compared to LLDPE MMT. The LLDPE nanocomposites present 2 signals, one around 6.5° characteristic for MMT and the other one close to 9° characteristic for LLDPE. Taking into consideration the first signal, the LLDPE OMMTBR50 showed a broader peak with a decrease in intensity as compared to the others nanocomposites. Thus, it can be considered that LLDPE OMMTBR50 presented the highest interlamellar distance among the other nanocomposites.

Figure 7. XRD analysis of the d001 peaks of the pure polymer and the nanocomposites with unmodified MMT and organo-clays with different amounts of BR.

Chemical group analysis of the nanocomposites was done within three distinct sections of the FTIR 17 ACS Paragon Plus Environment


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spectra (Figure 8a and Supporting Information Figures S5 and S6). The first section (3000-3600 cm-1) of the LLDPE OMMTBR 50 has a wide peak that spans the whole section. This peak is associated with OH groups.47,48 Because the excess water was removed with the centrifugation process, the presence of OH groups could be due to bounded hydroxyl groups on the clay surface. The second section (1500-1750 cm-1

) shows that the nanocomposites have peaks from the polymer and peaks from the organo-clays. The

modified nanocomposites showed peaks associated with C=O from esters and carboxylic acids. 49 The most significant change is in the LLDPE OMMTBR 50, with a shift from 1595 cm-1 to smaller peaks at 1575 and 1581 cm-1, which are associated with –NO2 stretching and amides or C=C stretching. 48 The degree of clay dispersion in nanocomposites can be evaluated in the third section (950-1150 cm-1), where four distinct peaks corresponding to four Si-O stretching modes in the MMT can be identified.50-53. The position and intensity of these peaks is associated with the intercalation/exfoliation in MMT nano-clays (Fig. 8a). 53 These four peaks originate from three in-plane bands (1120, 1048, and 1025 cm-1) and one out-of-plane band (1080 cm-1).

50

Based on the position and sizes of the peaks,

intercalated/exfoliated structures were present in the organo-clays. With increasing dosage of BR, the structure of the obtained nanocomposites became more exfoliated compared to the nanocomposite with MMT. The results of the FTIR analysis differ from the conclusions of the XRD analysis, where the most exfoliated structure occurs with the LLDPE OMMTBR 50, not the LLDPE OMMTBR 66. However, the FTIR data may be partly confounded by absorptions by BR functional groups (especially ethers and alcohols) that arise in the same region.


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Figure 8. (a) Section III of the absorbance spectra of the pure LLDPE and montmorillonite (MMT) that is unmodified or a nanocomposite modified with different amounts of BR (OMMTBRxx) and (b) TEM of LLDPE OMMTBR 50.

Preliminary TEM results (Fig. 8b) of cryo-ultramicrotomed thin sections of LLPDE OMMTBR 50 indicate that some of the organo-clay is fully exfoliated and well dispersed as individual sheets, but much of it remains as large aggregates. Figure 3 showed a large variation in the degree of exfoliation of OMMTBR 50, so it is likely that only the portion of organo-clay that was most exfoliated was properly dispersed in the polymer.

Modeling of Molecular Interactions in Nanocomposites Bio-Modification of Na-MMT Chemical analysis of the swine-waste-based bio-oil18,54 shows that among the wide range of compounds identified, a significant portion of this material consists of alkyl-amides such as hexadecanamide and n-butyl octadecanamide. Considering the distinctive performance of the amide groups in this bio-product, a truncated form of hexadecanamide (hexanamide, C5H11CONH2) was chosen as a representative member of the amide compounds in BR that may act as compatibilizers between MMT and LLDPE. The structure of the Na-MMT supercell and the intercalation of hexanamide into the Na-MMT interlayer space are shown in Figure 9. As reported in Table 1, compared to unmodified Na-MMT, this intercalation is accompanied by a 4.0% increase in cell height of MMT (from 18.69 Å to 19.44 Å). In our recent study,17 we showed how dispersive dipole-dipole interactions between carboxylic acid or amide functional groups and the silicate layer of Na-MMT reduce the positive charge of the gallery space and lead to an increase in basal d-spacing of Na-MMT and better intercalation. This change of basal d-spacing might also be attributed to H-bonding of the amide to oxygens of the silicate layer, leading to partial



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shielding of attractive electrostatic interactions between the MMT layers and increased separation of the layers. Another point worth noting is that a strong interaction between a surfactant head-group and the silicate layers of MMT does not necessarily result in expansion of clay layers; the DFT-based results confirm that the expansion of the gallery space due to intercalation of hexanamide is not very significant. In this respect, Heinz 55 believes that the nature and strength of the interactions between the MMT surface and a surfactant head-group affect parameters such as cleavage energy and diffusion of surfactants more than structural parameters like gallery space.

a: MMT supercell

b: MMT–Amide

c: MMT˗Amide/LLDPE 20 ACS Paragon Plus Environment

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Figure 9. DFT-based molecular modeling of (a) Na-MMT, (b) hexanamide-modified Na-MMT, and (c) intercalation of a simplified LLDPE molecule into hexadecanamide-modified Na-MMT

Intercalation of a Simplified LLDPE Molecule into Bio-modified MMT The role of alkyl surfactants in organically modified clay is not limited to intercalation and expansion of the clay layers; alkyl surfactants also create an organophilic environment in the interlayer space that can trap or adsorb the chains and side branches of polymers like LLDPE. LLDPE is produced from copolymerization of ethylene and higher α-olefin comonomers to yield a linear ethylene polymer with randomly distributed short branches that are typically 2-6 carbons long, depending on the specific comonomers used.56,57 The branched structure hinders chain packing, resulting in low densities. In this study, a molecular model for LLDPE was built consisting of a linear 10-carbon backbone with 2- and 4carbon side branches (Figure S7 in Supporting Information). Simulating the interaction between the bio-modified clay (MMT˗BR) and LLDPE is the most important part of modeling the clay nanocomposite. As the first step, the interaction between the LLDPE model molecule and hexanamide outside of the MMT cell was examined. To this end, different reasonable conformations of the two compounds were modeled to estimate binding energies (Figure S8 in Supporting Information). DFT-D calculations were carried out using the DMol3 module58,59 of the Materials Studio program from Accelrys, version 6.0. The DFT-D results predict a range of binding energy from 10 to 13 kcal/mol (without BSSE correction) for the lowest-energy structures of LLDPE– amide. This stabilizing energy could be attributed to the overall attractive dispersion forces arising from induced-dipole contacts of alky chains, and also from dipole-dipole forces of H...O contacts. Note that, in real samples, the main contribution of this stabilizing energy is related to the dispersion interactions between the long chains of amides or acids in bio-residue and the side branches of LLDPE, because the heteroatoms of a head-group are either involved with siloxane cavities of the clay surface or far-reaching from the LLDPE side branches.



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To quantify the effect of MMT bio-modification on its interaction with LLDPE, an intercalated MMT˗amide/LLDPE model (Figure 9c) was designed and optimized. Remarkably, the introduction of LLDPE into the bio-functionalized MMT gallery reduces the basal d-spacing compared to MMT–amide (11.5% decrease), as reported in Table 1. This behavior is in accordance with the XRD results for LLDPE-OMMTBR nanocomposites, where the interlayer spacings of OMMTBR50 and OMMTBR66 decrease by 1.0-1.4 Å when mixed into LLDPE. Similar observations have also been reported for silylation of Na-MMT, in which silane modifiers with longer chains lead to modified MMT with smaller d-spacing. 60 A possible explanation for this decrease of the basal d-spacing is that the insertion of organic compounds between the MMT layers introduces van der Waals interactions that effectively pull surfactant, polymer, and clay all closer together. Models of alkylammonium-modified MMT show that once electrostatic interactions are screened by adsorbed surfactant, van der Waals interactions determine the cleavage energy of the clay layers, depending on alkyl chain length and packing. 55,61 The influence of van der Waals interactions is highlighted in the model of MMT–amide/LLDPE (Figure 9c), in which the bulky LLDPE molecule lies almost horizontally upon the silicate layer. Based on our models, the interactions involved in the intercalation of MMT by amide/LLDPE are schematically illustrated in Figure 10. In the natural clay minerals, the primary cohesive force between MMT layers is the electrostatic attraction between the negatively charged silicate layers and alkali cations (Figure 10a). Separation of the layers reduces this electrostatic force and is accompanied by partitioning of the alkali cations (Figure 10b). Surface modification of the separated layers by short chain amphiphilic surfactants partially shields the remaining electrostatic interactions, slightly increasing the basal d-spacing (Figure 10c). Finally, the addition of a polymer further screens any electrostatic interactions but also bridges the silicate layers and surfactant by dispersive van der Waals interactions, decreasing the dspacing.


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The decrease in d-spacing for LLDPE-OMMTBR nanocomposites could be partly but not entirely explained by this phenomenon, because the average d-spacing measured by XRD (1.34-1.48nm) is much smaller than the model and only wide enough to fit one or two layers of packed alkyl chains. Perhaps more importantly, the computational models demonstrate that dispersion interactions play a critical role in determining the structure of organo-clays and can increase cleavage energy. While this study focused on aliphatic surfactants as possible compatibilizers, there are many more unidentified compounds in BR. Some compounds may be more polarizable and generate stronger dispersion interactions than alkyl chains do, and they might bind the silicate layers more tightly together when intercalated. While speculative, this hypothesis may explain why OMMTBR50 nanocomposites seemed to show better thermal properties than OMMTBR66; a higher BR dosage increases the chances that more polar residues bind to MMT and effectively “glue” the nano-clay together.

Table 1: Modeled MMT cell height during amide intercalation and amide/LLDPE intercalation. Cell height c (Å) MMT Supercell

18.69

MMT–Amide

19.44

MMT–Amide/LLDPE

17.20

A b C D Figure 10. Schematic illustrations of different interlayer environments in Na-MMT: (a) untreated clay, (b) separation of layers and partitioning of cations between the layers, (c) shielding effect of amide and partial reduction of electrostatic forces, (d) shielding effect of amide-LLDPE chain, noticeable decrease in electrostatic forces, and increase of van der Waals interactions. 23 ACS Paragon Plus Environment


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CONCLUSIONS The use of renewable, environmentally friendly materials in novel applications or to replace existing constituents in material systems is a strong emerging trend in the development of new nanocomposites with tailored or improved properties. The bio-modifier used in this study has proven itself as a strong modifier/intercalant for montmorillonite nano-clay, as reported in earlier work.17 The Hansen solubility parameters showed that the identified compounds in the bio-residue were highly compatible with low density polyethylene, especially the amide compounds. Upon modification of clay with bio-residue, the resulting organo-clays showed a range of partially intercalated to fully exfoliated structures which were reflected in alteration of clay’s gallery spacing as measured by XRD and TEM. The results are interesting, however to achieve the best results, the system needs to be further investigated in order to get a better homogenous state of organo-clay. Imperfection of creating a homogenous nano-composite is attributed to inadequacy of processing. The homogeneity of the LLPDEclay nanocomposites can be further improved via optimizing processing parameters and enhanced extrusion techniques; which were not within the scope of the present study. DFT models showed that alkyl-amide compounds similar to those found in bio-residue can effectively bind to MMT layers and promote expansion by screening electrostatic interactions. The models also showed that the addition of a polymer could reverse expansion, and also demonstrated the importance of van der Waals interactions in determining the structure of organo-clays. Further studies are needed to obtain homogeneous organo-clays and nanocomposites, however the target bio-residue was proven to be a strong, environmentally friendly and renewable intercalant for MMT and a compatibilizer in LLDPE/clay nanocomposites.

SUPPORTING INFORMATION Additional information and plots from TG, FTIR, oscillatory tests, DFT-based molecular structures as well as a full list of descriptor for Hansen solubility parameters.


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ACKNOWLEDGMENTS This research is sponsored by Aalborg University, the Denmark-America Fulbright Commission, the National Science Foundation (Award No: 1546921) as well as MIT’s Materials Research and Engineering Center. The content of this paper reflects the view of the authors, who are responsible for the facts and the accuracy of the data presented. The authors would like to acknowledge Dr. Shiahn Chen with MIT and Dr. Dong-hong Yu, with Aalborg University for their support and guidance with TEM and X-ray diffraction measurements, respectively. The authors also thank Dr. Greg Becker of Boeckeler Instruments, Inc. and for his assistance and advice regarding ultramicrotome sample preparation.

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Table of Content

TEM images of ultramicrotome sections of a) untreated MMT, and b) bio-modified MMT embedded in epoxy resin


1 A Novel Bio-Residue to Compatibilize Sodium Montmorillonite and

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