Exploring Copper-Amino Acid Complexes in Cross-linking of Maleated

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Exploring Copper-Amino Acid Complexes in Crosslinking of Maleated Ethylene Propylene Rubber Sanjay Pal, Mithun Das, and Kinsuk Naskar Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b03254 • Publication Date (Web): 29 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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Industrial & Engineering Chemistry Research

Exploring Copper-Amino Acid Complexes in Cross-linking of Maleated Ethylene Propylene Rubber Sanjay Pal#, Mithun Das# and Kinsuk Naskar#*

# Rubber Technology Centre, Indian Institute of Technology Kharagpur- 721302, West Bengal, India

KEYWORDS: Lysine; Tryptophan Copper; Elastomer; Recyclability

ABSTRACT

Herein, we report a network of maleic anhydride grafted ethylene propylene rubber (M-EPM), crosslinked by the coordination complexes that combines high tensile strength, high stretchability, and reprocessability aspects. At appropriate temperature, coordination cross-linked M-EPM rubber can be easily reprocessed without compromising physical properties. At first, we demonstrate the synthesis and characterization of coordination complexes of two type of amino acids, i.e., L-Lysine and LTryptophan, which form complex with Cu(II) ion through their respective amino acid group. As followed by various analytical analysis, these complexes of L-Lysine and L-Tryptophan exhibit distinct behavior after incorporation into the M-EPM rubber. The copper-ligand bonds can easily break and reform, while ACS Paragon Plus Environment

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the copper center remains attached to the amino acid ligands through the stronger interaction with the carboxylic group, which enables highly dynamic stress-induced ion exchange process. This notion is firmly endorsed by the enhanced physicomechanical attributes of M-EPM rubber and the recyclability aspect.

1. INTRODUCTION Since the invention of vulcanization in the early 19th century, and until now, overwhelming improvements towards the technology and resulting vulcanized rubber products have been witnessed. The supply/demand of raw rubbers, economic contribution from the rubber and automobile industries, and consumptions of various other rubber based goods are still growing at a steady rate. The market of industrial rubber is estimated to grow from USD 26.99 Billion in 2017 to USD 33.82 Billion by 2022, at a compound annual growth rate (CAGR) of 4.6 %. The increasing demand from the automotive, building & construction industries in the Asia Pacific drives the industrial rubber business forward, as the region has the largest number of automotive production plants globally, and it is witnessing significant infrastructural developments.1, 2 However, concerns such as recyclability of cross-linked elastomers have also grown to an alarming level over the past few decades. Various conventionally accepted cross-linking methods, otherwise effective at enhancing the elasticity of rubber, yield irreversible covalent bonds in three-dimensional ACS Paragon Plus Environment

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network structure.2, 3 Moreover, the excellent properties of such cross-linked rubber compounds are associated with the practical impossibility of reprocessing.4

-6

Therefore, scientists and

environmentalist are making a relentless effort to curb the rapidly growing rubber related wastes. An apparent avenue of reutilizing the cross-linked elastomers would be finding new alternatives to the existing cross-linking methods due to some or all of the following reasons: a large volume of rubbers are used for tire building applications, billions of rubber tires are scrapped each year after service life. Approximately 50 % of which is incinerated for a less efficient and undesirable form of energy which cannot be achieved without air pollution.5 As a matter further, these waste tires are used for other purposes such as crumb rubber (12 %), civil applications (16 %) and landfill dump purposes (14 %).7,

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The compliance with legal requirements in some countries can be a serious

issue. For example, as per new European legislation as from January 2015 onward, the automotive industries are bound to recycle 95 wt % of all materials in used cars.8 The practical impossibility of reprocessing of conventionally cross-linked rubbers has resulted in the heavy mass of rubber wastes, being dumped at the outskirts of our city and posing a serious environmental threat. An approach that involves grafting of functional groups along the chains of elastomer, which has long been explored for various purposes, could be the key to problems as mentioned above.9 Many of the grafting processes have been commercially established and practiced for decades.10 These ACS Paragon Plus Environment

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grafting processes are often carried out to cater the requirements of special applications.11–13 For instances,

chlorosulphonated

polyethylene

(CSM

or

Hypalon

rubber)

is

produced

by

chlorosulfonation of polyethylene in a solution of chlorinated hydrocarbons, chlorine and sulfur oxide. CSM rubber is known for its resistance towards chemicals, UV, and high temperature.14 Similarly, epoxidation of Natural Rubber is a modern and efficient method of chemical modification. Maleic anhydride-grafted ethylene-propylene (M-EPM) elastomer is commercially available under the brand name Keltan 1519R by ARLANXEO, Germany. The peroxide-initiated free radical grafting of maleic anhydride (MA) onto saturated polymers is a commercially established process. M-EPM rubber is a commonly used compatibilizer for rubber/thermoplastic blend system which improves miscibility and many physical properties. M-EPM rubber causes a physical interlocking between the blend components and imparts a compatibilizing effect to the blends.15 Functional groups present along the chain of M-EPM or carboxylated acrylonitrile butadiene rubber (XNBR) have also been explored in quest of developing a recyclable cross-linked elastomer, which involves uniformly distributed ionic clusters in a soft elastomeric matrix. Such cross-linked elastomers are often called as ‘ionomers’; ionomers are typically formed as a result of neutralization of anhydride ring or carboxylic group present in the elastomer backbone with metal ions, such as Zn2+ of ZnO.16 The network of ionic aggregates behaves as elastic-responsive junctions within the elastomer matrix. ACS Paragon Plus Environment

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Hence, ionomers exhibit substantially enhanced physicomechanical properties relative to initial physical attributes of precursor elastomer. Several workable routes have been proposed over the issues such as reversible cross-linking and recyclability of elastomers; these routes commonly involves either or combination of interactions like hydrogen bonding, ionic interactions, thermo-reversible covalent bonding, etc., 17–21 A wide selection of such interactions has been exploited. For example, Ying-Li et al. have demonstrated the preparation of self-healable elastomer cross-linked via metal-ligand coordination for dielectric applications.22 Burnworth et al. reported photo-stimulated healable polymers using coordination chemistry between transition metal ions (La3+, Zn2+) and 2,6-bis(1-methylbenzimidazolyl)pyridine.23 Zhenan Bao, and coworkers have successfully introduced a Fe(III)-2,6-pyridinedicarboxamide coordination complex into a linear poly(dimethylsiloxane polymer, which served as a cross-linking unit and exhibited autonomous self-healing ability.24 Although works of literature on the development of recyclable rubbers are available, there are very few reports for this matter that could highlight the application of industrially viable biologically sourced materials. Herein, we describe a novel approach that takes advantage of the versatility in preparing the reactive metal-amino acid complex,26- 30 which can create a network structure formation within the elastomeric matrix. Metal-ligand coordination bonding sites can be easily introduced via fairly ACS Paragon Plus Environment

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simple chemical reactions onto functional groups along the chains of M-EPM rubber. Highly dynamic metal-ligand interactions accompanied by ionic interactions in such coordination complex system should give high physical properties and greater stretchability. Furthermore, we demonstrate the recyclability of coordination cross-linked M-EPM rubber.

2. EXPERIMENTAL 2.1 Materials. The maleic anhydride-g-ethylene-propylene (M-EPM) elastomer used in this study is a commercially available Keltan 1519R product (Arlanxeo, Germany). The maleic anhydride and ethylene content was reported to be 1.9 and 49 wt % respectively. The number average molecular weight and molecular weight distribution index (PDI) of M-EPM elastomer was determined to be 70 kDa and 1.9, respectively. The M-EPM rubber was dried for 1 h at 170 °C under vacuum to convert all carboxylic acid groups, formed upon hydrolysis, back to anhydride. L- Lysine (molar mass 146.19 g/mol, ≥98% TLC, SigmaAldrich) and L-tryptophan (molar mass 204.23 g/mol, ≥98% HPLC, Sigma-Aldrich) amino acids were used in the experiment without further purification. The chemical structure of the two amino acid is shown in Figure 1. Anhydrous copper(II) sulfate (159.6 g/mol) was prepared in the laboratory by heating the copper sulfate pentahydrate (249.6 g/mol, Merck) at 400 °C temperature in the muffle furnace. ACS Paragon Plus Environment

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Thermogravimetric profile of copper sulfate pentahydrate is shown in Figure S1 of the supporting information. A reference cross-linked M-EPM rubber sample was prepared by using 2 phr of Perkadox 14-40B pd peroxide (PO, Akzo Nobel Polymer Chemicals) and heating at 170 °C.

Figure 1. Chemical structure of the two amino acids; (a) L- Lysine and (b) L- Tryptophan.

2.2(a). Preparation of copper amino acid complexes. Coordination complexes of amino acids were prepared by mixing the two separate solutions of given amino acid and copper sulfate. First, a pale blue aqueous solution of CuSO4 was prepared by dissolving the 1.59 g (10 mmol) of CuSO4 powder in the 20 ml distilled water. Meanwhile, a separate solution was prepared by dissolving the L-Lysine (2.92 g, 20 mmol) and NaOH (0.8 g, 20mmol) in 20 ml of distilled water. The aqueous CuSO4 solution was then slowly poured into the solution of L-Lysine and NaOH with continuous stirring. Mixing of these two solutions immediately resulted in the formation of a deep blue solution. The resulting solution was stirred at room temperature for the next six hours. Similarly, the copper-tryptophan (Cu.Try) complex was prepared by slowly adding the aqueous solution of CuSO4 to the mixture of L-Tryptophan (4.08 g, 20 mmol) and NaOH (0.4 g, 20 mmol) in 20 ml distilled water.

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A week later, crystals of copper-amino acids (Cu.Aa) complexes were obtained by slow evaporation, under vacuum, at 40 °C temperature. The total amount, 4.3 g (82 % yield) of copper-lysine and 5.8 g (90 % yield) copper-tryptophan complex, was recovered. The Cu.Aa were characterized by techniques such as FTIR, SEM, MALDI/TOF, and XRD. The schematic pathway of Cu.Aa complexes preparation is shown in Scheme 1. 2.2(b). Incorporation of copper amino acid complexes in M-EPM elastomer. Different samples were prepared using an internal mixer (Thermo Haake PolyLab oS RheoDrive4) at 130 °C for 20 min with 60 rpm rotor speed. The ingredients were mixed according to the formulation as described in Table 1. Next, the compounds were homogenized by a laboratory size two-roll mixing mill (Polymix 110L, size 203 × 102 mm, Servitech GmbH, Wustermark, Germany) for two minutes. The compounds were eventually vulcanized under pressure at 10 MPa and 170 °C temperature for 30 min. For a comparison, peroxide cross-linking was also performed at 170 °C temperature following a conventional recipe for M-EPM. In this case, M-EPM elastomer was cross-linked with 2 phr of Perkadox 14-40B pd peroxide. The peroxide cured M-EPM elastomer served as the reference sample.

Table 1. Formulation for various rubber compounds a


8

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Sample name

M-EPM

Perkadox 14-

Cu.Lyb

Cu.Tryc

40B pd

Stearic acid (St)

peroxide (PO)

M-EPM

100

-

-

-

-

M-EPM-2-PO

100

2

-

-

-

M-EPM-2.5-Cu.Ly

100

-

2.5

-

-

M-EPM-2.5-Cu.Ly-2.5-

100

-

2.5

-

2.5

M-EPM-5-Cu.Ly-2.5-St

100

-

5

-

2.5

M-EPM-5-Cu.Ly-5-St

100

-

5

-

5

M-EPM-2.5-Cu.Try

100

-

-

2.5

-

M-EPM-5-Cu.Try

100

-

-

5

-

M-EPM-5-Cu.Try-2.5-St

100

-

-

5

2.5

M-EPM-5-Cu.Try-5-St

100

-

-

5

5

St

aAll

weights are taken in parts per hundred parts of rubber (phr). b c

copper-Lysine complex

copper-Tryptophan complex

2.3 Characterization.

Curing study:


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The cure characteristics of compounds were monitored using a Monsanto R-100, USA, oscillating disc rheometer (ODR) at different temperature. The measurements were carried out at 11 kN die pressure, 1.7 Hz (100 cpm) frequency and an oscillatory rotation amplitude of ±1.00 with 3° arc. The degree of cure at time t can be computed through the following expression:

𝐞𝐱𝐭𝐞𝐧𝐭 𝐨𝐟 𝐜𝐮𝐫𝐞 𝐚𝐭 𝐭𝐢𝐦𝐞 (𝐭), 𝛂 =

𝐌𝐭 ― 𝐌𝐦𝐢𝐧 𝐌𝐞𝐧𝐝 ― 𝐌𝐦𝐢𝐧

(1)

Where Mt, Mmin, and Mend are torque values at a given time of cross-linking, minimum torque, and at the end of the ODR experiment, respectively. For rubber curing process, the nth-order and the autocatalytic model equations are generally used for the characterization of isothermal vulcanization kinetics. The nth-order kinetics model is a simple model equation to describe the overall curing process (Equation 2).

𝐝𝛂 𝐝𝐭

= 𝐤.(𝟏 ― 𝛂)𝐧

(2)

Here n and k denote the order of cross-linking reaction, and the temperature-dependent rate constant, respectively. The nth-order kinetic equation is based upon the assumption that the maximum reaction rate occurs at the very initial stage of the reaction, whereas the autocatalytic kinetic model equation ACS Paragon Plus Environment

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expresses that the maximum reaction rate takes place at moments after the minimum torque. In this study, the autocatalytic kinetic model has been employed. The autocatalytic model equation can be expressed as follows:

𝐝𝛂 𝐝𝐭

= 𝐤.𝛂𝐦.(𝟏 ― 𝛂)𝐧

(3)

Here m and n are the temperature-dependent orders of reactions. The values of k, m, and n are calculated through nonlinear multiple regression analysis of the experimental data. The function k(T)in both eq 2 and 3 is related to the activation energy, as expressed by the Arrhenius equation.

𝐥𝐧 𝐤 = 𝐥𝐧 𝐀 ―

𝐄𝐚 𝐑𝐓

(4)

Where, Ea, R and A denotes the cross-linking activation energy, universal gas constant and empirical constant, respectively.From an Arrhenius plot of ln k versus 1/T, the activation energy of vulcanization (Ea) can be easily computed.

Mechanical properties and reprocessability analysis: Stress-strain properties of samples were measured using a Universal Testing Machine (UTM maximum capacity 48 kN) from Tinius Olsen (United Kingdom). Complying with ASTM D-412 standard, 20 mm gauge-length samples were elongated at a strain rate of 500 ± 50 mm/min. For each ACS Paragon Plus Environment

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measurement, five samples were tested, and one median plot was selected as the characteristic of each sample type. The reprocessability of samples were studied under repeated cycles of remolding and testing. The variation in tensile strength of different compounds at three successive molding cycle were monitored. Tear strength and hardness of the cross-linked samples were measured according to the standard ASTM D-624 and ASTM D-2240 respectively.

Field Emission Scanning Electron Microscopy (FESEM): Surface images of amino acid and corresponding copper complex particles were taken using MERLIN- ZEISS instrument at 5 kV electron beam accelerating voltage with Signal A InLens setup.

FT-IR analysis: Infrared spectroscopic analysis of compounds was performed on the Perkin Elmer Spectrum-Two instrument over the mid-IR 400-4000 cm-1 range with eight repetitive scans and 2 cm-1 spectral resolution using an ATR setup.

MALDI-TOF/MS analysis: Bruker, The New Ultraflextreme TM MALDI TOF/TOF system, equipped with linear TOF mode, 20 KV ion acceleration voltage, and maximum data acquisition speed of 2 KHz was used to determine the mass spectra of amino acid and corresponding copper complexes. 2,5-Dihydroxybenzoic acid as the matrix (98% Sigma Aldrich) was used for the mass spectra analysis of L-Lysine and L-Tryptophan. ACS Paragon Plus Environment

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Sample aliquot was prepared from DI water, by mixing the amino acid (10 mg/ml), salt (10 mg/ml) and matrix (30 mg/ml) in a 1:1:10 ratio. Due to the strong affinity between copper and the molecules of DHB matrix, which could otherwise appear in the mass spectra, Cu.Ly and Cu.Try complexes were characterized without DHB matrix. Cu.Ly complex was directly studied by placing the sample aliquot on the target plate. Cu.Try complex was analyzed in the presence of an excess of copper sulfate. An aqueous aliquot of a mixture of Cu.Try complex (10 mg/ml) and copper sulfate (10 mg/ml) in 1:10 ratio was prepared and analyzed.

X-ray diffraction analysis: X-ray diffraction (XRD) experiments were executed in the wide-angle range. The measurements were carried out with Bruker diffractometer (XRD). The wavelength of the X-ray was 1.54 Å (Cu Kα radiation). Samples were scanned in between 2θ range of 5 and 25°

Stress relaxation and multi-strain sweep: Stress relaxation and multi-strain sweep analysis was carried out on the Rubber Process Analyzer (ALPHA TECHNOLOGIES, RPA 2000, made in USA). The test program for this set of analysis is described as follows: first, about 5 g of the sample was placed between the platins of the RPA machine. The temperature of the platins was constantly maintained at 170 °C temperature in each part of the analysis. In order to avoid unnecessary internal chemical reactions, the conditioning of compound was ACS Paragon Plus Environment

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done for 30 min by applying the angular strain of 0.05 deg and 0.1 Hz frequency. After a further 60 s of pre-heat time, an angular strain of 3 deg was applied, and then, the decay of stress throughout next 600 s was measured, with threshold torque value fixed at 0.05 dN-m. The time required for stress to decay to 36.7 % of its initial value was reported as the relaxation time. After that, a program consisting of two successive strain sweep was executed at 0.5 Hz frequency and strain ranging from 0.1 - 1000 %.

3. RESULTS AND DISCUSSION

PART A: Synthesis and characterization of copper-amino acid complexes: The complexes of copper-amino acid (Cu.Aa) was prepared deploying a fairly simple process. Anhydrous copper sulfate (CuSO4) was preferred over copper sulfate pentahydrate (CuSO4 .5H2O) since 5 moles of H2O per mole of CuSO4 .5H2O molecules remain inert in the Cu.Aa aqueous solution. Since the formation of metal complexes is often highly dependent on the pH of the solution, suitable alkalinity of the aqueous medium is essential for the coordination complex, and this is because there is a competition for the ligand between the metal ion and the proton as they both bind to the same atoms of the ligand. Therefore, a strong base such as NaOH was used during the synthesis of coordination complexes. ACS Paragon Plus Environment

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Unlike Cu.Ly complex, the Cu.Try complex is insoluble in water, and therefore precipitate is formed immediately. The insoluble nature of copper-Tryptophan complex is due to the bulky aromatic moiety and due to chelation via formation of coordination linkage between the amine and copper atom. Although the initial color of copper-Lysine and copper-Tryptophan complexes appears deep blue, the color of dried coordination complex of Cu.Ly and Cu.Try are different. Ample studies suggest that one mole of copper atom can combine with two moles of amino acid via coordination bond formation (Scheme 1).

Scheme 1. Schematic pathway of preparing coordination complexes; (a) Cu-Lysine, and (b) CuTryptophan complex.

MALDI-TOF/MS analysis:


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A principle proof for the successful formation of coordination bond between the amino acid and copper atom was brought by comparative MS analysis of L-Lysine, L-Tryptophan, Cu.Ly and Cu.Try complex (Figure 2). The difference between the mass spectra of amino acid and the corresponding complex is absolutely clear. Principle peak at 147.57 and 205.59 m/z value can be ascribed to the monoisotopic mass of L-Lysine (146.10) and L-Tryptophan (204.08) respectively. Mass spectra of Cu.Ly complex, consisting of three higher principal peaks relative to that of L-Lysine, suggest disproportionate fragmentation of Cu.Ly complex during the time of flight(TOF). The peak at 165.35 m/z value can be ascribed to the hydrated-Lysine adduct (18.0+147.57 = 165.57), and peaks at 191.50 and 231.45 m/z are due to dehydrated, and sodium-Cu.Ly complex, respectively. Disproportionate fragmentation of Cu.Try complex also resulted in the multiple peaks, that appear at 211.65 and 268.75 m/z value. These peaks can be ascribed to the CuSO4.3H2O crystals and Cu.Try complex, respectively. In order to determine the stability of the two amino acid and corresponding complexes under the rubber processing conditions, thermal stability was investigated by the thermogravimetric analysis (Figure S2 of the Supporting Information). L-Lysine and L-Tryptophan decompose at temperature 250 and 320 °C, respectively. Although corresponding complexes of the two amino acid decomposes at a relatively lower temperature, the decomposition temperature is still higher than the temperature at


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which the complexes have been incorporated into the M-EPM rubber. Cu.Ly and Cu.Try complex compound decomposes at 200 and 280 °C temperature, respectively.

Figure 2. MALDI-TOF/MS spectra of (a) L-Lysine and Cu-Lysine complex; and (b) L-Tryptophan and Cu-Tryptophan complex. ACS Paragon Plus Environment

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FTIR analysis: Another illustrative result, that confirms the successful synthesis of Cu.Aa complexes of L-Lysine and L-Tryptophan, as ligands, was followed by transmission FT-IR spectroscopic analysis (Figure 3). The characteristic peaks and corresponding assignment to various functional groups of pure L-Lysine and L-Tryptophan are furnished in Table 2.30 Some broad characteristic peaks that describe the presence of intermolecular H-bond between the carboxylic acid (~ 2588 cm-1) and amine moiety (~2110 cm-1), in both the cases of L-Lysine and L-Tryptophan amino acid, disappear almost completely from the spectra of Cu.Ly and Cu.Try complexes. L-Lysine and L-Tryptophan can associate with H-bond in two different ways. One H-bond is formed by the head groups of the two independent amino acid, with the ammonium groups acting as hydrogen bond donors and the carboxylate groups acting as hydrogen bond acceptors. The second H-bond involves the terminal -NH2 groups of L-Lysine and the –NH group of L-Tryptophan, with hydrogen bonding occurring solely between them. These network of H-bonds is ruptured completely as the coordination bond is formed between the amino acid and copper atom. Additionally, the shifting of carbonyl stretching peak of pure L-Lysine, from 1577 cm-1 to 1611 cm-1, is due to the formation of coordination bond and deprotonation of 𝛼– amino group. The X-ray diffraction analysis (Figure S3 of the Supporting Information) illustrates the after-effect of coordination bond formation between the amino acid and copper atom. The crystal structure of L-Lysine and L-Tryptophan ACS Paragon Plus Environment

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may be described as a layered arrangement of amino acid molecules, in common with the crystal structures of many other amino acids.31–35 The inter-layer distance in corresponding complex is increased by 1 Å.

Figure 3. FT-IR analysis of amino acids and corresponding complexes with copper atom.


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Figure 4. Scanning electron microscopic images: (a) wax like surface of L-Lysine amino acid; (b) rodlike structure of Cu-Lysine coordination complex particles; (c) wax like surface of L-Tryptophan amino acid; (d) layered-flake structure of Cu-Tryptophan coordination complex.

Table 2. Summary of characteristic peaks and corresponding assignments to the functional groups of L-Lysine and L-Tryptophan amino acid.

Amino acid

Characteristic

Functional group

peaks (cm-1)

Lysine

Tryptophan

3287 + 3357

–O–H and –N–H overlapping stretching vibration

2848 + 2922

–C–H stretching vibration

1577

–COO asymmetric stretching due to protonated 𝛼– amino group

1508

–N–H bending vibration

1441 + 1458

–C–H bending vibration

3401

–OH and –NH overlapping stretching vibration

3014

aromatic –C–H stretching vibration

2976

–C–H stretching vibration

1663

>C=O carbonyl stretching vibration

1584

Aromatic C=C stretching vibration


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1457

–C–H bending vibration

Morphology analysis: A detailed morphological analysis was accomplished by means of scanning and transmission electron microscopy. Figure 4 compares the surface morphology through the scanning electron micrograph of amino acid and corresponding complex. The difference could be easily seen in the image of L-Lysine and Cu.Ly complex. The wax-like surface of L-Lysine and the rod-like architecture of Cu.Ly complex indicates the existence of a strong copper-amino acid interaction, which brought about such drastic morphological changes. Similarly, the flake-like structure of Cu.Try complex is produced from a completely different waxy nature of L-Tryptophan amino acid. The ImageJ software, a Java-based image processing program, was used for computing the average dimension of 25 particles. The average particle size of Cu.Ly and Cu.Try complexes was found to be 3.4 μm × 1 μm and 2.5 μm × 1.5 μm respectively.

PART B: Incorporation of Copper-Amino Acid Complex in M-EPM Elastomer: Typical commercially available M-EPM elastomer is a random copolymer of ethylene and propylene monomer, and additionally grafted with small amount of maleic anhydride, usually upto 2 wt%. CopperACS Paragon Plus Environment

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amino acid complexes of L-Tryptophan and L-Lysine were incorporated in M-EPM rubber using an internal mixer at 130 °C temperature and 60 rpm rotor speed. Mixing-torque vs time plot that obtained from the Haake internal mixer was analyzed during the mixing of M-EPM with other ingredients (Figure 5). At 130 °C, shortly after the addition of M-EPM rubber into the mixing chamber, the torque begins to fall due to the softening of the M-EPM rubber. The addition of stearic acid, which acts as a plasticizer at elevated temperature, brought about a further fall in the mixing torque. However, at the later stage of mixing, the addition of Cu.Ly complex caused a significant rise in the mixing torque, which persisted till the end of the mixing. In contrary to the compounding of Cu.Ly complex, the addition of CuTry complex could not bring about a similar rise in the torque for the same dose of the coordination complex. The significant rise in the mixing torque instinctively suggests about some in-situ chemical reactions that presumably occurs between the M-EPM and CuLy complex.


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Figure 5. Real time mixing parameters of compounding of M-EPM rubber (100 phr), Cu.Ly complex (2.5 phr ), and stearic acid (2.5 phr).

Curing Study: At elevated temperature, the viscosity of rubber compound changes to the degree as controlled by factors such as type of cross-linking complexes, the dosage of cross-linking complexes and stearic acid. The reaction kinetics and cross-linking behavior of Cu.Aa complexes with M-EPM rubber was studied by the oscillating disc rheometer (ODR). For maximum possible interactions with the maleic groups of M-EPM rubber, a sufficient amount of Cu.Aa complex was incorporated into the M-EPM rubber and subjected to the ODR study. Rheometric study reveals (Figure 6a) that the rheo-torque of Cu.Ly containing compounds increases over time, which could be explained through the chemical ACS Paragon Plus Environment

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interactions between the Cu.Ly complex and maleic groups of the M-EPM rubber, eventually resulting in the formation of cross-linked network structure.

Figure 6. ODR torque curve for various samples exhibiting the development of cross-linked network in M-EPM rubber at specified temperature. Image part (a) exhibits the ODR cure characteristics of various Cu.Ly containing M-EPM rubber, (b) ODR cure characteristics of Cu.Try containing M-EPM rubber, (c) ODR analysis of the sample M-EPM-5-Cu.Ly-5-St at different temperature, and (d) linear arrhenius plot (R-Square = 0.96) for different samples .


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The result implies higher cross-linking efficiency of Cu.Ly/M-EPM rubber system in combination with stearic acid. Stearic acid plays an important role in this system; in addition to plasticization. Stearic acid also offers some additional reactive carboxylic groups that readily react with compounds such as zinc salts36–38 and several other amine-based compounds. The large availability of carboxylic groups, due to the addition of stearic acid in M-EPM rubber, brought about an extra cross-linked network structure. On the other hand, the torque-time curve of Cu.Try complex containing M-EPM rubber is throughout lower relative to the torque-time curve of Cu.Ly compounded M-EPM rubber within 30 min of experimental time (Figure 6b). Unlike the Cu.Ly/St. acid/M-EPM system, incorporation of stearic acid into Cu.Try based M-EPM rubber did not lead to any significant development in the rheo-torque. This behavior could be explained through the differences in the chemical structure of the two complexes, i.e., Cu.Ly and Cu.Try (addressed in the FT-IR analysis of complex containing M-EPM samples). The ODR study suggests stronger cross-linking efficiency of Cu.Ly complex as compared with Cu.Try. Based on the curing kinetics study, a comparison was made about the efficiency of curing systems, offered by Cu.Ly complex and Perkadox 14-40B pd peroxide. The extent of cure (α) was determined from the ODR plots at three different temperatures, 150, 170, and 190 °C (Figure 6c). The cross-linking kinetic parameters (Table 3) of various samples were determined from the rheometer data. Based on the activation energies (Ea) evaluation, M-EPM-5-Cu.Ly-2.5-St and M-EPM-5-Cu.Ly-5-St sample ACS Paragon Plus Environment

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exhibit 94.6 and 76.2 kJ/mol activation energy, respectively. Whereas, M-EPM-PO system exhibit relatively high activation energy, i.e. 120.5 kJ/mol. The cross-linking rate for the peroxide cured M-EPM rubber is slow, but ultimately enhances the degree of cross-linking. The lower values of activation energy is an indicative of the effective cross-linking process.39 The slope of the straight line in ln k versus 1/T plot (Figure 6d) gives the activation energies for these different rubber composites.

Table 3. Kinetic Parameters and Activation Energies of M-EPM-PO and M-EPM-5-Cu.Ly-st compound.

Sample

M-EPM-PO

M-EPM-5-Cu.Ly-2.5St

M-EPM-5-Cu.Ly-5-St

T (°C)

k

m+n

Ea (kJ/mol)

150

0.0567

0.42

120.5

170

0.1897

1.41

190

1.1031

2.23

150

0.0541

0.31

170

0.1384

1.22

190

0.5574

2.55

150

0.04

0.13

170

0.0862

0.78

94.6

76.2


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190

0.2616

1.67

Table 4. Tensile strength (T.S), elongation at break (E.B) and hardness (Shore A) data summary.

Sample name

E.B

T.S

100 %

200 %

300 %

Hardnes

(%)

(MPa)

modulu

modulu

modulu

s (shore

s

s

s

A)

(MPa)

(MPa)

(MPa)

M-EPM

539

0.9

0.8

0.9

0.9

47

M-EPM-PO

700

2.3

1.0

1.1

1.3

52

M-EPM-2.5-Cu.Ly

969

2.4

0.9

1.1

1.3

52

M-EPM-2.5-Cu.Ly-2.5-

839

2.8

1.0

1.3

1.5

47


472

3.2

1.2

1.9

2.4

52

M-EPM-5-Cu.Ly-5-St

405

3.4

1.3

2.1

2.7

52

M-EPM-2.5-Cu.Try

631

1.1

1.0

1.0

1.1

48

M-EPM-5-Cu.Try

725

1.3

0.9

1.1

1.2

50


618

1.8

0.8

1.1

1.5

46

M-EPM-5-Cu.Try-5-St

568

2.1

0.9

1.3

1.7

48

St

Mechanical properties analysis: ACS Paragon Plus Environment

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As a result of coordination cross-linking, particularly in the Cu.Ly based M-EPM system, distinct improvements were observed in the mechanical properties relative to the raw M-EPM rubber such as elongation at break tensile strength and hardness. On the basis of data obtained from the stress-strain analysis and as furnished in Table 4, it is apparent that 2.5 phr of Cu.Ly complex brought about 168 % improvement in the tensile strength. The tensile strength of Cu.Ly based M-EPM rubber increased even further after fine-tuning the Cu.Ly to stearic acid ratio. A combination of 5 phr of Cu.Ly complex and 5 phr of stearic acid yields a ~275% improvement in the tensile strength. A fairly large elongation at break has been observed for the Cu.Ly containing M-EPM rubber compounds. Unlike Cu.Ly complex, the Cu.Try compounded M-EPM rubber does not show similar development in the mechanical properties. A mere 13 and 46% improvements were observed in the tensile strength after addition of 2.5 and 5 phr of Cu.Try complex, respectively. Addition of 5 phr of stearic acid to the M-EPM-5-Cu.Try compound could improve the T.S only by 133 %. The stress-strain analysis is in good agreement with the torque-time study made in the ODR test. The significant rise in the rheological and initial physicomechanical attributes of M-EPM rubber suggests the formation of cross-linked network structure, particularly occurring within Cu.Ly compounded M-EPM rubber.

FTIR analysis:


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FT-IR studies of Cu.Aa compounded M-EPM rubber along with the raw M-EPM rubber was conducted to gain insight about the formation of different types of coordination complexes (Figure 7). In the IR spectra of raw M-EPM rubber, the peak at 1711 cm-1 can be ascribed to the stretching vibration of the carboxylic groups. M-EPM exhibits strong band at wavenumber 1460 cm-1 and 1380 cm-1, which can be attributed to the overlapping asymmetric CH3-bending and CH2-scissoring, and the symmetric CH3bending vibrations of the elastomer backbone, respectively. The absorption band at 723 cm-1 can be assigned to the methyl rocking vibration of the elastomer backbone. It is apparent from the spectra of M-EPM rubber compounded with 2.5 phr of Cu.Ly that carboxyl peak intensity at 1711 cm-1 is reduced due to its reaction with the primary amine group of Cu.Ly complex. The new broad and diffused peaks at 1865 cm-1 and 1780 cm-1 can be observed, and which are believed to be due to the cyclic anhydride ring and asymmetric stretching of carboxyl group respectively. This refers to the formation of a maleimide structure. Addition of 2.5 phr of stearic acid results in a strong band at wavenumber 1700 cm-1, attributable to the stretching vibration of amide groups that formed after the reaction between stearic acid and the primary amine group of Cu.Ly complex. On the other hand, no shifts in the characteristics band of raw M-EPM rubber is observed after incorporation of Cu.Try complex. Unlike Cu.Ly/M-EPM rubber system, it is inferred from the IR analysis that Cu.Try complex remain inert whatsoever and present merely as an inert filler in the rubber matrix, ACS Paragon Plus Environment

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Figure 7. FT-IR spectra of raw M-EPM elastomer and other Cu.Aa containing M-EPM rubber samples.

The plausible interactions between M-EPM and Cu.Aa is shown in Scheme 2. Undeniably, as compared to Cu.Try, the coordination complex of Cu.Ly interacts with M-EPM rubber in a completely different manner. In all likelihood, interactions between M-EPM rubber and Cu.Ly complex follow the covalent reaction that results in the formation of maleimide structure. Seemingly, Cu.Ly complex connects two chains of M-EPM rubber, which leads to a cross-linked network structure (exhibited through an insoluble piece of cross-linked M-EPM rubber with 2.5 phr of Cu.Ly complex; glass vial A). However, and as corroborated with ODR analysis, Cu.Try complex remain idle whatsoever. Week Hbond that presumably exist between the maleic group and the indole substituent disappears in solvents ACS Paragon Plus Environment

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such as chloroform; and consequently, no illustrative sign of cross-link network is observed (exhibited through the dissolved piece of Cu.Try containing M-EPM rubber; glass vial B).

Scheme 2. Schematic representation of probable structure formation between the M-EPM rubber and copper-amino acid complexes. After 24 hrs of dipping time in CHCl3-THF solvent mixture, the pictorial condition of M-EPM rubber compounded with copper-amino acid complexes is shown in this figure; (a) M-EPM rubber compounded with 2.5 phr of Cu.Ly complex swells in the CHCl3-THFsolvent mixture, whereas (b) M-EPM rubber compounded with 2.5 phr of Cu.Try complex, gets completely dissolve in the CHCl3-THF solvent mixture. ACS Paragon Plus Environment

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AFM analysis: The dispersion of complex particles accompanying stearic acid in the matrix of M-EPM rubber was investigated by the comparative study of AFM phase images. Coordination cross-linked samples that comprise fairly large dosage of ingredients were chosen for investigation in order to make unbiased inference based on the accessible molecular interaction. A 5 μm x 5 μm wide AFM phase images of three different samples i.e., raw M-EPM rubber, M-EPM-5-Cu.Ly-5-St, and M-EPM-5-Cu.Try-5-St, (Figure 8) exhibit different phase morphology. Raw M-EPM rubber, which does not comprise any of the base ingredients as mentioned in Table 1, obviously would exhibit single-phase morphology. However, incorporation of the two Cu-amino acid complex in combination with stearic acid seems to have a prominent effect on the phase and topography of the samples. AFM phase image of M-EPM-5-Cu.Ly5-St exhibit uniform distribution of Cu.Ly complex particles along with stearic acid. However, incorporation of 5 phr of Cu.Try complex in combination with 5 phr of stearic acid yields a distinct layered surface topography, as may be seen along the green line drawn on Figure 8(c). The observed layered topography of M-EPM-5-Cu.Try-5-St sample is presumably due to the flake-like structure of the Cu.Try complex, as revealed from Figure 4.


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Figure 8. AFM phase images: (a) raw M-EPM rubber; (b) M-EPM-5-Cu.Ly-5-St; (c) M-EPM-5-Cu.Try5-St.

Stress Relaxation: Stress relaxation test, which monitors the time-dependent decay of stress under a steady strain, is strongly dependent on the temperature and other factors that affect the mobility of macromolecular chains within the elastomeric sample. The Maxwell spring-dashpot solid model quite commonly illustrates the time-dependent viscoelastic response of a rubbery material. In the Maxwell model expression for the time-dependent stress decay, under a steady strain and at a constant temperature, the term ‘stress relaxation time (𝜏)’ is quite frequently computed; stress relaxation is a ratio of viscosity ACS Paragon Plus Environment

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over stiffness. The SI unit of τ is second (s), and this has an important significance towards the quantification of material’s viscoelastic nature. τ is physically a time needed for the initial stress to fall to 1/e (0.367) of its value. A longer stress relaxation time (𝜏) certainly would be the indication towards the development of a cross-linked network structure, which inhibits the mobility of the chains in the elastomeric matrix. The stress relaxation data of various samples (Figure 9. and Table 5.) reveal that the M-EPM rubber compounded with Cu.Try complex yields the shortest relaxation time (0.11 s). However, Cu.Ly complex containing M-EPM rubber exhibit a longer 𝜏 value, which suggests the formation of cross-link network structure resulting from the reactions between the Cu.Ly complex and the maleic anhydride group of M-EPM rubber. Addition of stearic acid plasticizes the rubber compound and bring extra sites that readily react with Cu.Ly complex. M-EPM rubber compounded with 5 phr of Cu.Ly complex and 5 phr of stearic acid yield the highest 𝜏 value (537.3).


34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Normalized Relaxation Modulus

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M-EPM-2.5-Cu.Try M-EPM-5-Cu.Try M-EPM-5-Cu.Try-2.5-St M-EPM-5-Cu.Try-5-St M-EPM-2.5-Cu.Ly M-EPM-2.5-Cu.Ly-2.5-St M-EPM-5-Cu.Ly-2.5-St M-EPM-5-Cu.Ly-5-St

1.0

0.5

1/e = 0.367

0.0 0.01

0.1

1

10

100

1000

Time (s)

Figure 9. Normalized stress relaxation plot of various samples

Table 5. Stress relaxation time for various samples Sample description

𝝉 (s)

With Cu-Lysine complex M-EPM-2.5-Cu.Ly

0.7

M-EPM-2.5-Cu.Ly-2.5-St

0.9


27.8

M-EPM-5-Cu.Ly-5-St

537.3

With Cu-Tryptophan complex M-EPM-2.5-Cu.Try

0.2

M-EPM-5-Cu.Try

0.2


35



0.1

M-EPM-5-Cu.Try-5-St

0.1

Page 36 of 49

Reprocessability analysis: Multi-strain sweep analysis of various samples was followed at typical rubber processing temperature, i.e., 170 °C. An unambiguous pattern could be easily seen in Figure 10, as the sigmoidal drop of shear storage modulus (G’) over strain on a log10 scale is commonly experienced from the typical pseudoplastic materials. The decay of G’ over strain could be attributed to the breakdown of network structure. The two domain of samples, which differs in the use of types of Cu.Aa complex also differs on the ground that the two successive cycles of strain sweep results in a distinct trail of G’ values. For example, the second strain sweep cycle, of all samples of M-EPM rubber cross-linked via Cu.Ly complex, brought about a higher trail of G’ values relative to that of the first strain cycle, but Cu.try containing M-EPM rubber samples exhibit quite a similar trail of G’ values in both strain cycle. A summary of shear storage modulus (G’) at 10 % strain is furnished in Table S1 of the supporting information. Especially in the lower strain amplitude region, samples such as M-EPM-2.5-Cu.Ly, exhibit distinct differences in the G’ value of first and second strain cycle (95.04 kPa and 112.94 kPa at 10 % strain, respectively). Under the influence of stearic acid as a plasticizer at 170 °C temperature, the likelihood of ACS Paragon Plus Environment

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redistribution/rearrangement of Cu.Ly particles and stress-induced ion exchange process could be taken as an explanation for this behavior. The samples of other domain, i.e., M-EPM-2.5-Cu.Try and M-EPM-5-Cu.Try, differs from that of first domain. The trail of G’ values of both strain cycle is lower as compared to those of Cu.Ly sample counterpart, possibly due to the unreactive indole substituent that present in the Cu.Try complex. No distinct differences in the trail of G’ values could be seen by comparing the two strain cycle, which therefore clearly endorse the results of various analysis that reveal distinct behavior of Cu.Try-M-EPM rubber system from the Cu.Ly cross-linked M-EPM rubber.

Figure 10. The trail of shear storage modulus (G’) over two successive strain cycle for various samples: (a) M-EPM-2.5-Cu.Ly; (b) M-EPM-2.5-Cu.Ly-2.5-St; (c) M-EPM-5-Cu.Ly-2.5-St; (d) M-EPM-5-Cu.Ly-5St; (e) M-EPM-2.5-Cu.Try; and (f) M-EPM-5-Cu.Try. ACS Paragon Plus Environment

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Apparently, all compounds are reprocessable at typical rubber processing temperature. As studied by the multi-strain sweep analysis, significant mechanical chain scission could be ruled out on the ground that none of the samples exhibit lower G’ trail in the second strain cycle, which could have otherwise indicated the polymer chain scission. The variation in the tensile strength of different compounds at two successive molding cycle is shown in Figure 11. All compounds show fairly similar tensile strength values. M-EPM rubber compounded with Cu.Ly or Cu.Try complex can be reprocessed using general processing and compression molding machines.

M-EPM-5-Cu.Try

M-EPM-2.5-Cu.Try

M-EPM-5-Cu.Ly-5-St


M-EPM-2.5-Cu.Ly-2.5-St

M-EPM-2.5-Cu.Ly 0.0

2nd recycle 1st recycle 0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Tensile Strength (MPa)

Figure 11. Tensile strength of various samples at successive molding cycles.

4. CONCLUSIONS


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Complexes of biologically sourced and industrially viable L-Lysine and L-Tryptophan amino acids were prepared using anhydrous copper sulfate. The chemical structure and morphological analysis of Cu.Ly and Cu.Try complexes were followed by several characterization techniques. FT-IR and mass spectroscopic analysis, and corroborated with X-ray diffraction pattern, suggest a successful formation of copper-amino acid complexes. The effect of Cu.Ly and Cu.Try complexes on the physio-mechanical properties of raw M-EPM rubber was studied through various rubber testing equipment such as ODR, UTM, and RPA instruments. Apparently, due to the different chemical structure of L-Lysine and L-Tryptophan, distinct differences in the behavior of rubber compounds were observed in terms of mechanical and rheological properties. Cu.Ly complex forms crosslinked network structure, which resulted in the improvement of initial physio-mechanical properties of M-EPM rubber. However, due to the unreactive indole substituent of L-Tryptophan amino acid, Cu.Try complex rather remain inert in the M-EPM rummer matrix. Stress relaxation time, which could be perceived as the measure for the degree of cross-linking or physical interactions, was observed higher for Cu.Ly containing M-EPM rubber as compared to the Cu.Try compounded MEPM rubber. Addition of an appropriate amount of stearic acid in Cu.Ly based M-EPM rubber enhances several properties, but could also act as a plasticizer at typical rubber processing temperature. Multi-strain sweep analysis suggests that the M-EPM rubber compounded with the ACS Paragon Plus Environment

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complexes of copper-amino acid is recyclable. We conclude that the complex of L-Lysine amino acid with copper could offer potential usefulness in the cross-linking of M-EPM rubber at commercial scale, whereas, Cu.Try complex, in combination with Cu.Ly, may be used to optimize some of the properties. ASSOCIATED CONTENT

Supporting Information.

1. Preparation of copper sulfate (CuSO4) from copper sulfate pentahydrate (CuSO4. 5H2O).

2. Supplementary figure of thermogravimetric analysis of amino acids and corresponding complex compound with Cu(II) ion.

3. X-ray diffraction analysis. 4. Supplementary table of storage shear modulus (G’) at 10 % strain from the multi-strain sweep analysis.

ACKNOWLEDGMENT

Authors cordially acknowledge the financial support from the Indian Institute of Technology (IIT), Kharagpur (India). The state-of-the-art research facility at Rubber Technology Centre at IIT Kharagpur ACS Paragon Plus Environment

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is also greatly acknowledged. We are grateful to our research co-workers, Aswathy T. Raju and Asit Baran Bhattacharya for their technical support in this work.

AUTHOR INFORMATION

*Corresponding Author:

Kinsuk Naskar Tel: +91-3222-281748.

ORCID Sanjay Pal: https://orcid.org/0000-0002-7558-4392 Mithun Das: https://orcid.org/0000-0002-8208-648X Kinsuk Naskar: https://orcid.org/0000-0002-8536-4983


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