Terpene Based Sustainable Elastomer for Low Rolling Resistance

Subscriber access provided by Northern Illinois University

Article

Terpene Based Sustainable Elastomer for Low Rolling Resistance and Improved Wet-grip Application: Synthesis, Characterization and Properties of Poly(styrene-co-myrcene) Preetom Sarkar, and Anil K. Bhowmick ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01038 • Publication Date (Web): 17 Aug 2016 Downloaded from http://pubs.acs.org on August 19, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Sustainable Chemistry & Engineering is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 42

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

ACS Sustainable Chemistry & Engineering

Terpene Based Sustainable Elastomer for Low Rolling Resistance and Improved Wet-grip Application: Synthesis, Characterization and Properties of Poly(styrene-comyrcene)

Preetom Sarkar and Anil K. Bhowmick* Rubber Technology Centre, Indian Institute of Technology Kharagpur Kharagpur- 721302, West Bengal, India

*Corresponding author: Email: [email protected] (AKB) Tel.: +91 (3222) 283180; Fax: +91 (3222) 220312

ACS Paragon Plus Environment


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

ABSTRACT: Incited by the unprecedented surge of developing sustainable polymer, we demonstrate herein a green emulsion polymerization route towards the development of sustainable rubbery materials based on β-myrcene (MY) and styrene (ST) for low rolling resistance and improved wet grip applications. The microstructure of the synthesized copolymers was found to be governed by the ST weight percent. For example, the copolymers having less than 40 weight percent of ST had 1,2 vinyl and 3,4 addition product along with 1,4 -cis and –trans microstructure of the polymyrcene unit, whereas the copolymers having higher weight percent of ST had only 1,4 -cis and –trans microstructure of the polymyrcene unit. The copolymers displayed improved onset degradation temperature and sub ambient glass transition temperatures. The copolymer having a 70/30 weight/weight ratio of MY/ST displayed a molecular weight of 51,500 Da and a glass transition temperature of -35.2 °C. The 70/30 rubber vulcanizate exhibited satisfactory mechanical properties (tensile strength of 6.4 MPa and elongation at break of 395 %). Dynamic mechanical analysis of the vulcanizate reveals improved traction and low rolling loss over a standard tyre tread compound, thereby making it a promising material for tyre.

KEYWORDS: Bio-based elastomer, Emulsion polymerization, β-myrcene, Styrene, Rubber 2 ACS Paragon Plus Environment

Page 2 of 42

Page 3 of 42

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


INTRODUCTION The quest for developing sustainable polymeric materials has been unanimously the aspiration of polymer science and technology fraternity in recent years. With pressing environmental concerns and incessant depletion of fossil reserves, the topic itself has become self-propelling. Of late, an increased number of published monographs on this topic reveal a glaring graphic of growing interest in this domain - in both academic and industrial sectors.1-8 Two overarching goals in this blooming discipline are - to explore renewable resources - instead of fossil reserves - and to adopt robust and green methodology – in place of specialized techniques. Among the wide variety of renewable supplements, terpenes, a forestry feedstock (obtained mainly from conifers), are particularly interesting as they carry a carbon skeleton of isoprene unit (building block of natural rubber).9,10 Terpenes are highly abundant class of passive natural products - available in turpentine, citrus, and pine oils - and could be easily turned into platform chemicals using existing petrochemical technologies.11,12 Furthermore, a new industrial biotechnological production pathway, developed by Amyris, promises large-scale supplies of terpenes via fermentation of plant sugars and cellulose waste.13-15 Lately, the acyclic monoterpene: β-myrcene, possessing a conjugated double bond, has been exploited to fabricate various macromolecular materials. β-myrcene is obtained on a large scale by pyrolysis of β-pinene, which is one of the key compounds of turpentine.16 Recently, Kim et al. reported metabolic engineering of Escherichia coli for the production of myrcene.17 On the polymerization front, highly active lutetium18 and lanthanide19,20 based catalysts were used to polymerize β-myrcene. Ring closing metathesis polymerization21 and reversible addition-fragmentation chain transfer polymerization22 methods were also adopted. Georges et al. reported a coordinative chain transfer co- and ter-polymerization of βmyrcene.23 Bolton et al. prepared a thermoplastic elastomer based on α-methyl-p-

3 ACS Paragon Plus Environment


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

methylstyrene and β-myrcene by living anionic polymerization method.24 Although the preceding investigations showcase a state-of-the art affair in developing β-myrcene based polymer, from a manufacturing and sustainability point of view, only a few of them could be translated into an industrial production frame. Emulsion polymerization, being a green chemical process, is used extensively (if not solely) to manufacture a wide spectrum of polymeric materials (synthetic rubbers, adhesives, latex paints etc.).25 It is noteworthy that, in spite of being a preferred manufacturing process, meagre number of research articles has featured in recent times, which adopts this methodology to prepare sustainable polymers.26 We did a preliminary study on the synthesis of polymyrcene elastomer by emulsion polymerization method and examined its structureproperty relationship.27 To impart polarity into the otherwise hydrocarbon polymyrcene, and to improve the interactions with various functional fillers, we have also copolymerized a biobased functional synthone with β-myrcene.28 Styrene, on the other hand, is one of the major building blocks for numerous polymeric materials. Commercially available styrene is mainly produced from petroleum and the production process involves energy intensive steps. In recent times, a number of reports have appeared in the literature describing the preparation of styrene by biotechnological route viz. by a de novo designed path employing Escherichia coli29 and from forestry waste using a strain of Penicillium expansum.30 In coherence with the academia, various biotechnological companies like – Genencor and Amyris also have launched biobased isoprene as an alternative to its petro based analogue.31 Although these developments are certainly a big leap towards the sustainable growth of polymer industry, the scaling up of these processes in an economical way is challenging. In perpetuation with our attempt to make sustainable rubbery materials, we aimed at exploring the use of β-myrcene further as a renewable constituent in place of petro based 1,3


Page 4 of 42

Page 5 of 42

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


butadiene. In spite of the growing awareness regarding sustainability, replacement of petroderived 1,3 butadiene by sustainable alternatives still remains an obscure area in elastomer science and technology. Apart from being a benign supplement, the liquid physical state of βmyrcene offers additional edge over gaseous 1,3 butadiene during storage and handling, thereby making the entire polymerization operation less energy intensive. Thus, in our present work, we report a series of copolymers based on β-myrcene (MY) and styrene (ST) [poly(ST-co-MY)] by emulsion polymerization method. The synthesized materials are similar to conventional petro derived styrene butadiene rubber (SBR). In the context of sustainability, this kind of terpene-based rubber could be an ancillary material with improved properties in various applications. The chemical structure of the synthesized polymers was ascertained by spectroscopic measurements. Reactivity ratio of the monomers was also determined. A probable reaction mechanism has been proposed. The as fabricated biobased elastomer could be readily processed and cross-linked by the conventional rubber processing method and equipments. Various physico-mechanical properties like hardness, chemical crosslink density, tensile strength etc. of the vulcanized rubber were determined. The wet grip and rolling resistance properties of the vulcanizate was measured indirectly from dynamic mechanical analyzer.

EXPERIMENTAL SECTION Materials. The monomers - β-myrcene (MY, 98%) and styrene (ST, ≥ 99%) were received from Sigma Aldrich chemical company and the inhibitor was removed by shaking with 2(M) NaOH solution. Sodium bicarbonate (NaHCO3, 99%) and ammonium persulphate (APS, 98%) were procured from E. Merck India. Sodium dodecyl sulphate (SDS, 99%) was obtained from Loba Chemie, India. All these chemicals were reagent-grade and used as received. Deionized water (DI) was utilized for all the experiments.



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

Page 6 of 42

Emulsion co-polymerization of β-myrcene and styrene. The copolymers of β-myrcene and styrene were prepared by persulfate initiated emulsion co-polymerization method. For comparison, β-myrcene and styrene were also polymerized individually following the same reaction condition. A representative procedure for the preparation of poly(MY50ST50) copolymer is described as an example, where the suffix indicates the weight percentage of each component. At first, 0.125 g of the emulsifier (SDS), 0.075 g of buffer (NaHCO3) and 12.5 g of DI water (see Table 1) were mixed under magnetic stirring into a round bottom flask at 300 rpm for 20 min. Subsequently, β-myrcene monomer (2.5 g, 3.16 ml) was added into the reaction flask slowly over a period of 10 min followed by the addition of styrene monomer (2.5 g, 2.750 ml). To stabilize the system, the reaction mixture was left as such for a further 20 min. Thereafter, the reactor was sealed and an inert blanket was created by flushing nitrogen. The reaction temperature was set to 70 ºC, followed by the injection of thermal initiator (aqueous solution of APS) in two successions. Table 1. Recipe for Poly(MY50ST50) Copolymer Synthesis.

#

materials

amount (g)#

β-myrcene

2.5

styrene

2.5

deionized water

12.5

sodium dodecyl sulphate

0.125

ammonium persulfate

0.0175

sodium bicarbonate

0.075

Amounts are based on 100 phr (parts per hundred parts of rubber) of monomer

Batch size: 5g (based on total amount of monomers).

The polymerization reaction was continued for 20 h. After the specified time, poly(ST-co-MY) copolymers were precipitated by pouring into large volume of acidified ethanol with vigorous stirring. The coagulum was then washed thoroughly several times with DI water and dried at 40 ºC in vacuum for 30 h. Following the same experimental protocol, 6 ACS Paragon Plus Environment

Page 7 of 42

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


other copolymers with varying amounts of β-myrcene and styrene were prepared. To study the kinetics of the polymerization, small amount of aliquots was taken out during the initial stage of the reaction at 10 min interval and the conversion was determined gravimetrically. The synthetic route for the polymerization reaction is presented in Scheme 1.

Scheme 1. Emulsion co-polymerization of β-myrcene and styrene.

Compounding and vulcanization of poly(ST-co-MY) copolymer. Conventional sulphur cure system was employed to vulcanize the synthesized elastomer. Amongst the series of synthesized copolymers, poly(MY70ST30) was chosen for compounding because its chemical composition has comparable styrene content with that of commercial SBR. For comparison purpose, pristine polymyrcene was also vulcanized and tested following the same experimental protocol. The compounding formulation is given in Table 2. The rubber and the compounding ingredients were mixed using a 6''/12'' two-roll mill (Schwabenthan, Berlin) having a friction ratio of 1:1.2 (front roll:back roll). The optimum cure time of the compounded mix was then determined by a Monsanto Rheometer R100S machine, USA at 150 °C. The compound was then cured in a 12''/12'' hydraulic compression moulding machine (David Bridge Company, England) at 150 °C for the optimum cure time, as determined from the rheometer studies above.



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

Page 8 of 42

Table 2. Compounding Formulation.

#

ingredients

loading (phr)#

rubber

100

zinc oxide

4

stearic acid

2

N 330 carbon black

50

process oil

5

i-PPD$

1.5

accelerator CBS@

0.8

sulphur

2.0

phr stands for parts per hundred parts of rubber

$

p-phenylenediamine

@

N-cyclohexyl-2-benzothiazole sulfenamide

Measurements and Characterization. Characterization of the unvulcanized gum rubber. The Fourier transform infrared spectroscopy (FT-IR) of the synthesized copolymers was recorded in a Perkin Elmer Spectrum 400 machine (resolution 4 cm-1) in an universal attenuated total reflectance (UATR) mode with a total of twelve scans per sample. 1

H and

13

C nuclear magnetic resonance (NMR) spectra were recorded in an

AVANCE III 400 Ascend Bruker instrument operating at 400 MHz. Tetramethylsilane (TMS) was used as an internal standard. The samples were dissolved in deuterated chloroform (CDCl3) and the chemical shift values were reported in δ(ppm) relative to the internal standard. The molecular weight of the synthesized copolymers was measured using an Agilent PLGPC 50 (gel permeation chromatography) instrument having a refractive index detector. The instrument was equipped with PLgel 5 mm Mixed-D column. Tetrahydrofuran (THF) was used as the eluent (sample concentration 1 mg/ml and flow rate 1 ml/min) and polystyrene standard was used for calibration. This is a standard practice in the absence of model polymer 8 ACS Paragon Plus Environment

Page 9 of 42

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


sample of different molecular weights. The gel percentage of the copolymers was calculated as the ratio of dried polymer weight to its original value after extraction with THF for 8 h. Gaussian-09 software32 was used to perform the density functional theory (DFT) calculation. The geometry (optimised structure) and frequency of β-myrcene were calculated with Becke's three-parameter Lee-Yang-Parr exchange-correlation function (B3LYP) method by using 6-311G (d, p) as basis set. The room temperature solubility of the synthesized polymer was checked (as 0.1 % (w/v)) in various common organic solvents such as tetrahydrofuran (THF), chloroform (CHCl3), dimethyl sulphoxide (DMSO), methyl ethyl ketone (MEK) and toluene. The theoretical solubility parameter of the synthesized copolymer was also calculated. The particle size of the diluted emulsion latex was measured by using Malvern Nano ZS dynamic light scattering (DLS) instrument employing a 4 mW He-Ne laser (λ = 632.8 nm) at a scattering angle of 90 degree. The X-ray diffraction (XRD) analysis was carried out using X'Pert PRO machine from PANalytical company in the range of 10° - 50°. CuKα (0.154 nm) was used as a radiation source with an accelerating voltage and current of 40 kV and 30 mA respectively. The scan rate was kept at 3° per min. The thermal transition of the copolymers was measured in a NETZSCH DSC 200F3 Maia® differential scanning calorimetry (DSC) machine under nitrogen atmosphere. The samples were heated from -100 °C to +100 °C at a heating rate of 10 °C/min. In order to erase thermal history, the glass transition temperature (Tg) was determined from the second heating run. Tg was reported as the temperature of the midpoint of the heat flow change, as determined from the baseline tangents using NETZSCH Proteus Thermal Analysis software.



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

TA Instrument SDT Q600 equipment was used to perform the Thermogravimetric analysis (TGA) of the synthesized copolymers. The experiment was carried out under nitrogen purging at a heating rate of 10 °C/min. Morphology of the synthesized copolymers was observed by atomic force microscope (AFM, Agilent Technologies USA, model 5500) under tapping mode of operation. The cantilever tip (radius of curvature of 10 nm) was made of etched silicone. The force constant and resonance frequency of the tip were 42 N/m and 150 kHz respectively. Dilute solution (0.1% (w/v)) of the copolymer was prepared in chloroform followed by drop casting the copolymer solution onto small glass wafer. The sample was then dried at room temperature for 3 days. The rheological measurements of the unfilled gum copolymers was performed using a MCR102 (Anton Paar, Austria) modular compact rheometer . Parallel plate attachment (PP25) was used throughout the experiments. Room temperature strain sweep experiments were performed at a frequency of 1 Hz within a strain range of 0.01-100 %. Frequency sweep for the copolymer samples was conducted at 25 °C, at a constant strain of 0.1 % (within the linear viscoelastic region of the strain sweep experiments). Temperature sweep experiments were carried out at 0.1 % constant strain, 1 Hz frequency and in between -80 to 100 °C with a ramp rate of 5 °C/min. Temperature control was done by using CTD450L measuring cells. The normal force was kept at 1 N for all the cases. Molecular dynamics (MD) simulation was performed using Dassault Systèmes BIOVIA Material Studio v16.1.0.21 software.33 At first, the polymer chains were built with respective repeat units. Thereafter, the geometry and energy of these chains were optimized and three configurations with minimum energy were chosen to generate cubic simulation boxes (by Amorphous Cell module). The Amorphous Cell (AC) thus constructed, was subjected to geometry and energy optimization like individual chains. Then the AC structure was


Page 10 of 42

Page 11 of 42

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


annealed in five cycles with initial temperature of 300 K and mid cycle temperature of 550 K. The annealing was performed by NVT ensemble (where N is number of particles, V is volume and T is temperature). Thereafter, equilibrium MD simulation was carried out by NPT dynamics (where N is number of particles, P is pressure and T is temperature) and the system was cooled down to 80 K at 20 K interval (for details see Supporting Information). Characterization of the rubber vulcanizate. The tensile strength of the vulcanized rubber samples was measured according to ASTM D 412 method. Dumb-bell shaped specimens were cut from the prepared sheets and tensile strength was measured in a Zwick/Roell Z010 Universal Testing Machine at room temperature at a separation rate of 500 mm/min. The data were analyzed by testXpert II software of the Zwick/Roell machine and the results presented are average of three measurements. Dynamic mechanical analysis (DMA) of the vulcanizates was performed using a METRAVIB 50N (France) dynamic mechanical analyzer in tension mode. Temperature sweep experiments were carried out at 1 Hz frequency and 0.1% dynamic strain over a temperature range of −120 to +120 °C, at a heating rate of 3 °C/min. The chemical crosslink density of the cured samples was measured by equilibrium solvent swelling method. After recording the initial weight of the samples, these were immersed for 72 hours at room temperature into a swelling tube containing toluene. After the stipulated period, the specimens were taken out of the solvent and the surfaces of the samples were smoothly blotted with a tissue paper to remove the excess solvent. The samples were then immediately weighed in a weighing balance to obtain the swollen weight of the samples after extraction of the soluble materials. Thereafter, the samples were dried at 60 °C for 24 h to get the de-swollen weight of the samples. The overall crosslink density was determined by using the well known Flory-Rehner equation:34,35



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

-[(1- ) + + χ ] = ( ) (

- )

Page 12 of 42

(1)

where, = molecular weight between the crosslinks, = density of the vulcanized sample

= molar volume of the swelling liquid, = Flory-Huggins solvent rubber interaction parameter, which is determined by Hildebrand equation as:

χ=

( − )2

(2)

where, R = universal gas constant = 1.987 cal K-1 mol-1, T = experimental temperature in Kelvin, = molar volume of the swelling liquid, = solubility parameter of solvent and = solubility parameter of rubber. The volume fraction of rubber( ), as calculated from equilibrium solvent swelling data is given by the following equation:

=

()/ ! ()/

(3)

where, " = de-swollen weight of the test specimen, $ = the weight fraction of the insoluble fraction, H = weight of the test specimen, = density of the rubber, = density of the solvent and % = weight of the absorbed solvent.

RESULTS AND DISCUSSION Synthesis of Poly(ST-co-MY) by persulfate initiated emulsion polymerization. Persulfate catalyzed emulsion co-polymerization of β-myrcene and styrene was performed at 70 °C for 20 h. Sodium dodecyl sulphate (SDS) was used as an anionic surfactant, and sodium bicarbonate was used to stabilize the pH of the reaction mixture. High temperature ensures complete thermal decomposition of the water-soluble APS initiator. A series of poly(ST-coMY) with different ST and MY contents were synthesized. Table 3 collates the data on the molecular weights, percentage yield, gel content and Z-average diameter of the latex particles for various co-polymers. It is evident from the table 12 ACS Paragon Plus Environment

Page 13 of 42

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


that among the series of copolymers, poly(MY90ST10) showed a molecular weight of 18,300 Da. With increase in styrene content, the molecular weight increased up to 1,11,800 Da for poly(MY50ST50) copolymer. Highest molecular weight (1,24,600 Da) was obtained in the case of poly(MY0ST100) . This change in molecular weight is also supported by Z-average diameter of the latex particles (Table 3). The polydispersity index (PDI) of the polymers lies in the range of 1.55 - 4.01 with a reasonable yield percent varying in between 79-92 %. With increasing amount of MY unit, gel percent increases (up to 11%), as it favours micro crosslinking in between the macromolecular chains. Table 3. Molecular Weight, PDI, Yield, Gel Content and Latex Particle Size of Poly(ST-

co-MY) Samples with Varying Styrene Content. a

polymer samples

Mn (Da)

Poly(MY100ST0)

b

PDI

% Yield

% Gel

Z-average diameter (nm)

93,200 (±150)

4.01

92

11

95.0 (±4)

Poly(MY90ST10)

18,300 (±195)

1.55

79

9

55.2 (±3)

Poly(MY80ST20)

36,000 (±210)

1.68

80

6

62.0 (±3)

Poly(MY70ST30)

51,500 (±180 )

1.66

82

5

65.7 (±2)

Poly(MY60ST40)

70,300 (±185)

1.62

80

4

75.0 (±4)

Poly(MY50ST50)

1,11,800 (±200)

1.78

81

3

115.8 (±2)

Poly(MY30ST70)

67,780 (±220)

1.85

82

1

59.0 (±3)

a

1,24,600 116.0 2.0 86 0 (±175) (±4) Subscripts in the polymer sample designation denote the weight percent of the respective comonomer in the feed. # Values in the parenthesis indicate the standard deviation based on three measurements. a Values obtained from the GPC measurement. bValues obtained from the DLS measurement. Poly(MY0ST100)



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

The mechanistic events of emulsion co-polymerization are governed by various factors like solubility behaviour, polarity and relative reactivity of the monomers and stability of the emulsion. These factors can influence the characteristics of the emulsion co-polymers to a significant extent. According to the recipe furnished in our work, the presence of the following species may be envisaged within the polymerization system: emulsified monomer reservoirs of MY and ST, monomer-swollen micelles, free surfactant molecules, and watersoluble initiator.36 The presence of various species and different intervals of emulsion copolymerization are pictorially presented in Figure 1. Figure 2a-b represents the kinetic plot of the polymerization and variation of molecular weight with particle size respectively. The plot of logarithmic conversion vs polymerization time reveals a first order kinetics. During interval I, nucleation takes place. Due to polar nature, ST monomer has relatively higher water solubility (300 mg/lit vs. 4.09 mg/lit for MY at 25 °C)37 as well as higher reactivity (-309.7 a.u. vs. -390.6 a.u. for MY as calculated from the density functional theory, see Figure S1, Supporting Information for optimized structures) than the MY monomer. It is thus presumed that the formation of active radicals (with water-soluble initiator fragments) is initiated in the aqueous phase with the ST monomers. As the surfactant concentration in the recipe (34.7 × 10-3 mol/L) is higher than the critical micelle concentration of SDS (8.0 × 10-3 mol/L),38 micellar nucleation is the principle mechanism of particle formation in this case.39 According to Maxwell-Morison model, after attaining a critical z value, ( where z is the critical degree of polymerization for entry into micelle; for styrene/persulfate system z = 2-3),40 the active radicals become water insoluble and enter into the monomer swollen micelles and subsequently convert them into individual particle nuclei. Further propagation of the polymer chains at this point requires uninterrupted supply of monomers from the emulsified monomer reservoirs of MY and ST (interval II). This transport of monomers not only involves aqueous phase diffusion, but also the collision of the


Page 14 of 42

Page 15 of 42

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


monomer droplets with the particle nuclei.41 The interaction parameter of MY and ST was calculated and found to be 0.6691. This large positive value indicates that the two liquids would not be miscible and would thus form their own reservoirs. Moreover, MY is a hydrocarbon, whereas ST is an aromatic compound, which would further prevent their association. Apart from this, the two monomers also differ widely in water solubility, which makes the formation of mixed reservoirs less probable. The kinetic rate for poly(MY0ST100) is highest (kapp = 3.54 × 10-2 min-1) because of the above factors.

Figure 1. Pictorial illustration of various steps in emulsion copolymerization of β-myrcene and styrene. 15 ACS Paragon Plus Environment


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

Figure 2. (a) Kinetic plots of emulsion co-polymerization, X = % conversion, kapp = apparent rate constant (b) Variation of Mn with Z-average diameter of latex particles for the copolymers.

For 90/10 MY/ST copolymer, most of the STs are exhausted in making the active radicals at the nucleation stage. Because of the hydrocarbon nature, the MY monomers would tend to form larger monomer reservoirs by self-association. Larger size of the monomer reservoirs would thus have less momentum during agitation and hence less collision with the particle nuclei. This phenomena impedes the transport of the MY monomers from the monomer reservoirs to the particle nuclei. Since MY constitutes the major share in this composition, it affects the chain growth significantly. Thus, although the presence of ST unit increases the polymerization rate (kapp = 2.16 × 10-2 min-1) considerably unlike poly(MY100ST0) (kapp = 6.87 × 10-3 min-1, taken from reference 28, Figure 2a), the disruption of continuous supply of MY monomer reduces the molecular weight of the poly(MY90ST10) copolymer, which is further reflected in its lower Z-average diameter of the latex particles (Table 3 and Figure 2b). With higher amount of ST [poly(MY50ST50)], an increased flux of active radicals are formed during the nucleation step, but due to higher stability of the MY radical (Figure S1, Supporting Information), the initial rate of the co-polymerization decreases (kapp = 1.02 × 10-2 16 ACS Paragon Plus Environment

Page 16 of 42

Page 17 of 42

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


min-1, Figure 1a). However, in the later stage, the supply of MY remains uninterrupted (as lower reservoir size enhances monomer transport by collision with the particle nuclei). As the percentage of MY and ST monomers are equally weighted and the polymerization is governed by both kinetics and thermodynamics of the system, both the aforementioned phenomena contribute to the polymerization, leading to a substantially higher molecular weight (1,11,800 Da, Table 3 and Figure 2b) product. Once the monomer reservoirs are depleted completely, the remaining monomers within the growing particles are polymerized (Interval III).

Solubility behaviour. The solubility of the 70/30 copolymer was checked in different common solvents at room temperature. The solubility behaviour and the calculated theoretical solubility parameter (Hoftyzer and van Krevelen method)42 are presented in Table 4. The copolymer showed improved solubility over polymyrcene in MEK and toluene.

Table 4. Solubility Behavior and Theoretical Solubility Parameter. solvents

polymer samples (theoretical solubility parameter, MPa1/2)a

THF

CHCl3

DMSO

MEK

Toluene

Poly(MY100ST0) (16.8)

±

±

-

-

s

Poly(MY0ST100) (20.61)

+

+

-

+

+

Poly(MY70ST30) (17.57)

±

±

-

s, ±

±

+: completely soluble, ±: partially soluble, -: insoluble, s: swells but not soluble a

Values are calculated by van Krevelen method, reference 42. For detailed calculation, see

Supporting Information section and Table S1. 17 ACS Paragon Plus Environment


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

Chemical structure of the synthesized Poly(ST-co-MY) copolymers. The chemical structure of the synthesized copolymers was characterized by FTIR spectroscopy as illustrated in Figure 3. The characteristic peaks at 696 and 773 cm-1 for mono-substituted aromatic sp2 C-H bending frequencies, are retained both in poly(MY0ST100) and poly(MY50ST50), indicating successful inclusion of the styrene moiety into the copolymer structure. The absorption frequencies for sp2 C-H bending in the region around 907 and 990 cm-1 disappear upon polymerization for both MY and ST monomer, indicating successful copolymerization. The conjugated C=C stretching peak at 1595 cm-1 in MY disappears upon polymerization.

Figure 3. FTIR spectra of various homo and copolymers.


Page 18 of 42

Page 19 of 42

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


The broad absorption peaks in the region of 2850, 2925 and 2965 cm-1 indicate the stretching vibration of –CH, -CH2 and –CH3 groups respectively in the copolymer. This also includes the contribution of –CH2 and –CH group from the ST unit. Figure S2 (Supporting Information) represents the overlay FTIR spectra of various copolymers having different ST contents. The peaks for mono-substituted aromatic sp2 C-H bending frequencies at 699 and 755 cm-1 increases with the increase in the ST content. The structure of poly(ST-co-MY) copolymers was additionally confirmed by 1H and 13

C NMR spectra, as presented in Figure 4a-b. For understanding, the NMR spectra of the ST

monomer and polystyrene were also recorded (Figures S3-S5, Supporting Information). The chemical shift values of magnetically different protons and carbons are well assigned. The relevant chemical shift values (δ, ppm) are listed below: Poly(MY50ST50): 1H NMR (CDCl3, 400 MHz): δ- 7.18 (5H, A, B, D), 5.05 (4H, 1, 2, 1', 2'), 2.50 (1H, E), 1.87 (18H, 3-6, 3'-6' and F, G), 1.70 (12H, 7, 8, 7', 8'). Poly(MY50ST50): 13C NMR (CDCl3, 400 MHz): δ- 145.1 (e), 131.2 (ii, vi, ii', vi'), 128.0 (a, b), 125.7 (i, v, i', v'), 124.5 (d), 42.5 (g), 37.5 (f), 27.0 (iii, iv, iii', iv'), 25.7 (ix, x, ix', x'), 17.6 (vii, viii, vii', viii').



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

Figure 4. (a)

1

H NMR spectrum of Poly(MY50ST50) (b)

Poly(MY50ST50) 20 ACS Paragon Plus Environment

13

Page 20 of 42

C NMR spectrum of

Page 21 of 42

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


1

H NMR spectrum of poly(MY50ST50) copolymer is presented in Figure 4a. After co-

polymerization, the peak for three protons at δ = 7.00, 6.04 and 5.52 ppm of the ST unit (Figure S3 and S5, Supporting Information) fade away. The characteristic signals for aromatic protons appear as a conglomerate peak at δ = 7.18 ppm in the copolymer. These observations confirm the copolymerization of styrene with β-myrcene. The signature peak at δ = 5.05 ppm is attributed to the olefinic protons of 1,4 cis and 1,4 trans microstructures of the polymyrcene unit. Absence of signature peaks at around δ = 4.5 and 5.5 ppm indicates that 1,2 vinyl and 3,4 addition products were not formed. Thus, unlike the polymyrcene microstructure, which contains 1,2 vinyl and 3,4 addition units along with 1,4 cis and 1,4 trans microstructures27, the poly(MY50ST50) copolymer predominantly accommodates 1,4 cis and 1,4 trans units in its copolymer backbone. The methine group (>CH-) of the ST unit in the copolymer appears as a shoulder peak at δ = 2.50 ppm. Methylene and methyl protons of the

copolymer are located at δ = 1.87 and 1.70 ppm respectively. The olefinic carbon centres of ST monomer at δ = 138.0 and 114.0 ppm disappear after the polymerization (Figure S4 and Figure 4b). Presence of aromatic carbon centres at δ = 128.0 and 124.5 ppm further confirms the inclusion of ST unit into the copolymer (Figure 4b). The characteristics chemical shift values for 3,4 addition unit and 1,2 vinyl structure at δ ~155 and 116 ppm were absent. The presence of bulky phenyl group prevents the formation of such adducts in the poly(MY50ST50) copolymer. This finding is similar to our previous investigation of dibutyl itaconate and β-myrcene based copolymers.28 The methylene and methyl carbons of MY and ST units were assigned in between δ = 27.0 - 17.6 ppm in the 13C NMR spectrum. The composition (Table S2, Supporting Information) of all the other copolymers were analyzed by 1H NMR spectra. It was found that the final co-polymer composition matches with the monomer charged. For example, for a 50/50 MY/ST charge, the final co-polymer



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

composition was found to be MY/ST = 51/49. The analysis of the copolymer microstructure (Table S2 and Figures S6-S10, Supporting Information) from 1H NMR reveals an interesting finding. It was found that, neither 1,2 vinyl nor 3,4 addition product with respect to myrcene was present for the copolymers having 40 or higher weight % ST content. However, for copolymers with lower amount of ST content (10, 20 and 30 weight %), the presence of 1,2 vinyl and 3,4 addition product with respect to myrcene was evident. It was anticipated that, the copolymers having higher than 30% ST content cannot accommodate such structures, whereas lower amount of ST does not hinder the formation of such kind of microstructural defects. Monomer reactivity ratios. The reactivity ratio of each monomer in a pair is one of the most important factors in governing the composition of the resulting copolymer. The reactivity ratio of the monomers, β-myrcene (MY) and styrene (ST) in the persulfate initiated emulsion copolymerization was determined by two classical linear methods: Fineman-Ross (FR) and Kelen-Tüdös (KT). In order to determine the copolymer composition, the area under the respective peaks in the 1H NMR spectrum was calculated. The relevant parameters and calculations for FR and KT methods are presented in Table S3 (see Supporting Information). The FR and KT plots are shown in Figure 5. The results collated in Table 5 show that for the MY/ST emulsion copolymerization system, the reactivity ratio of MY is slightly greater than unity and that of ST is less than unity. The values obtained from both the methods are in good agreement with each other.


Page 22 of 42

Page 23 of 42

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


Figure 5. Determination of reactivity ratios of β-myrcene and styrene by (a) Fineman - Ross method and (b) Kelen - Tüdös method.

Table 5. Reactivity Ratios of β-myrcene (MY) and Styrene (ST). Methods

r'(

r)*

r'( + r)*

Fineman-Ross

1.200

0.873

1.048

Kelen-Tüdös

1.191

0.858

1.022

The reactivity ratios in the present study were quantified in emulsion polymerization system and found to be: rMY = 1.2 and rST = 0.8. This indicates that MY monomer is more reactive than the ST monomer toward both the propagating species. Thus, the copolymer will contain a larger proportion of the more reactive monomer in random placement in the initial stage. Apart from the ‘azeotropic’ co-polymerizations, where copolymerization occurs without a change in the feed composition, this kind of observation is common in many other cases of copolymerization. Due to the inherited ‘composition drift’, co-monomers are not added uniformly along the growing copolymer chains.43 For example, reactivity ratio for butadiene and styrene in emulsion system also lies in between 1.4-1.6 and 0.5-0.7 respectively.44,45 Our estimated values for rMY and rST are similar to this system. The product of the reactivity ratios dictates the copolymerization behaviour. As the values of the reactivity ratio are not too different from each other and the product of the reactivity ratios is 1.04, the 23 ACS Paragon Plus Environment


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

present case fall under the category of ‘moderate ideal behavior’.43 The corresponding plot is given in Figure S11 (Supporting Information section). Nevertheless, regardless of the reactivity ratios, the copolymers registered single inflection point in the DSC studies, as discussed later. Wide-angle X-ray diffraction of MY-ST copolymers. One of the criteria of a rubber is that it should be completely amorphous in nature. X-ray diffractrograms (Figure S12, Supporting Information) confirms the amorphous nature of various synthesized MY-ST copolymers. A broad amorphous halo at 2θ = 19° is reminiscence of the diffraction pattern on natural rubber.46 Presence of branched structure and pendent phenyl side group prohibits close packing of the macromolecular chains and thereby renders the copolymers amorphous. Glass transition temperature Glass transition temperature (Tg) dictates the properties of a polymer to a great extent. For a polymer to be used in rubbery applications, a sub-ambient glass transition temperature and elastic nature is desirable. By employing compositional information, the Tg of a copolymer can be predicted. The weight fraction of different monomers and the glass transition temperature of the individual homopolymer can be empirically correlated to the Tg of the resulting copolymer by Fox equation47 as:

,

= ∑

./ , /

(4)

where, Tg (in K) is the computed glass transition temperature of the copolymer, Tgi (in K) is the glass transition temperature of homopolymer composed of monomer ‘i’ and wi is the weight fraction of monomer ‘i’ in the copolymer. This equation was employed to calculate the theoretical Tg of the synthesized copolymers and the experimental values were obtained from the DSC measurements (Table S4, Supporting Information). The thermogram of the copolymers (Figure S13, Supporting Information) indicates complete amorphous and random nature of the synthesized materials. Polymyrcene registered a glass transition temperature at 69.0 °C and polystyrene displayed a glass transition temperature at 103.8 °C. Depending 24 ACS Paragon Plus Environment

Page 24 of 42

Page 25 of 42

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


upon the weight percent of β-myrcene, the Tg values of the synthesized copolymers varies in the temperature range of -60.9 to 37.2 °C.

Figure 6. Variation of glass transition temperature of copolymers with β-myrcene content

Figure 6 represents the plot of Tg vs. composition from the theory and the experiments. With increase in β-myrcene content, the inflexion point shifts toward more sub ambient temperature indicating rubbery nature of the synthesized copolymers. Increased macromolecular chain flexibility due to the incorporation of β-myrcene unit causes the decrease in glass transition temperature. It is to be noted that the Fox equation and other similar theoretical relationships do not take into consideration the effects of adjoining dissimilar monomer units on steric and energy factors of copolymer backbone. Also the freedom of rotation and the contribution towards free volume of a given co-monomer would not be the same as in the homopolymers.48 Hence, there is some discrepancy.



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

Molecular dynamics simulation. Molecular dynamics (MD) simulation (see Supporting Information for detail steps) was performed with the poly(MY70ST30) copolymer to have an idea about the spatial disposition of the macromolecular chains. The amorphous cell (Figure 7a) module illustrates highly coiled copolymer chains of poly(MY70ST30). The glass transition temperature of the copolymer was evaluated from the MD simulation by plotting specific volume against absolute temperature (as obtained from NPT ensemble) and determining the inflexion point from the graph (Figure 7b). The theoretical value from the MD simulation was found to be -21 °C. The highly entangled macromolecular chains coupled with substantially high molecular weight and sub ambient glass transition temperature ensures rubbery behaviour of the copolymer.

Figure 7. (a) Amorphous unit cell of poly(MY70ST30) at 300 K, displayed by macromolecular backbone (magenta colour); carbon: grey, hydrogen: white, cell parameters: a: 27.43 Å, b: 27.29 Å, c: 27.56 Å. (b) Plot of specific volume with temperature for poly(MY70ST30) (points were obtained during NPT dynamics at 20 K temperature interval).


Page 26 of 42

Page 27 of 42

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


Thermal degradation characteristics. The thermal stability of the synthesized polymers was assessed by thermogravimetric analysis (TGA). Figure 8 represents the thermal degradation curve of the copolymers and the homo-polymers. The characteristic degradation temperatures, such as, temperature of 20% weight loss (T20), temperature corresponding to

Figure 8. TGA thermograms of various homo and co-polymers.

maximum rate of degradation (Tmax), and rate of degradation at Tmax and percent residue at 750 °C are collated in Table S5 (Supporting Information). It is evident from Figure 8 that the degradation pattern of the synthesized copolymers lies in between the two homo-polymers. A single step degradation pattern was observed for polystyrene sample, whereas all the copolymers and polymyrcene showed distinct two-step degradation behaviour. It is reported in the literature that the decomposition of 1,4 –cis; 1,4 –trans and random polybutadiene occurs in two stages, whereas single stage degradation is observed for 1,2 vinyl polybutadiene.49 Thus, it is evident that the microstructure of a polymer significantly affects 27 ACS Paragon Plus Environment


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

its thermal degradation behavior. Unlike natural rubber which is ~100% 1,4 –cis in nature, the synthesized polymyrcene has a mixed microstructures of 1,4 –cis; 1,4 –trans; 1,2 vinyl and 3,4 addition product as analyzed from the NMR spectra. The two-step degradation of polymyrcene also could be related to its different microstructures. The copolymers also showed improved T20 than the pristine polymyrcene elastomer. This initial improvement could be explained due to the enhanced interchain interaction of the benzene ring of the styrene unit. Among the series of synthesized polymers, polystyrene displayed highest Tmax value. A complete saturated backbone structure and the presence of aromatic group could be attributed to this finding. The decrease in Tmax values of the copolymers can be attributed to the presence of unsaturation in the polymer backbone. The existence of pendent unsaturation leads to the formation of crosslinked structure amongst the β-myrcene units of the copolymers. Presence of 3,4 structure and 1,2 vinyl defect in the pristine polymyrcene additionally aids in such phenomena. This is also reflected in the relatively lower degradation rate of polymyrcene compared to the copolymers. As polystyrene does not contain unsaturation/micro structural defects, such kind of crosslinking is absent and thus higher degradation rate is evident. The percent residue of the copolymers remained in the range of 0.01-1.90 %. Morphological analysis of the copolymer. The morphology of the copolymer was visualized by atomic force microscopy (AFM). Figure S14 (Supporting Information) presents a representative AFM topography (Figure S14a) and phase trace (Figure S14b) of poly(MY50ST50) copolymer. The brighter region in Figure S14a represents the rubbery phase, which protrudes out from the surface due to its elastic nature. The corresponding areas have appeared as softer regions (brown spots) in the phase image. This observation of morphology is in line with our earlier findings with β-myrcene/dibutyl itaconate copolymer.28


Page 28 of 42

Page 29 of 42

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


Rheological analysis of the unfilled gum copolymers. In order to understand the viscoelastic nature of the synthesized materials, rheological measurements were performed. The physical state of the neat polymyrcene was soft and rubberlike, whereas the polystyrene had a white powdery appearance. Copolymers having more than 50 weight percent of βmyrcene had rubbery feeling upon physical handling, whereas poly(MY30ST70) appeared as hard crumb like material. Figure S15a-b (Supporting Information) depicts the room temperature strain and frequency sweep plots of various synthesized polymers. Amongst the series of polymers, the storage modulus of the neat polymyrcene registers lower value than the copolymers. Inclusion of styrene unit increases the storage modulus value and the order follows the same trend as observed in the case of molecular weight, i.e. it increases to a maximum for poly(MY50ST50). Higher molecular weight leads to greater entanglement among the polymer chains and hence such an increase in storage modulus for the copolymers. The linear viscoelastic region (LVR) of the polymers was found to be until 100 % strain, thereafter chain disentanglement starts. The frequency dependence of complex viscosity for various synthesized copolymers (Figure S15b, Supporting Information) shows pseudoplastic nature with shear thinning behaviour. As polystyrene and poly(MY30ST70) appeared as hard crumb like materials, their rheological analysis could not be performed at room temperature. Rheological analysis of these samples was conducted at 110 °C (above their Tg) and the plots are presented in Figure S16 (Supporting Information). Similar to other polymers, polystyrene and poly(MY30ST70) also show shear thinning nature in the entire frequency range studied. As the experimental temperature was higher than the glass transition temperature, the value of the complex viscosity of poly(MY30ST70) appears at a lower level than the 50/50 MY/ST copolymer. On the other hand, due to high molecular weight, polystyrene registered highest complex viscosity amongst the synthesized polymers. The plot of temperature at Tanδmax and G'max against the styrene content is presented in Figure S17.



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

The glass transition temperature of all the polymers is presented in Table S4 (Supporting Information). Pristine polymyrcene registered a glass transition around -57.2 °C and the value lies between -39.8 [poly(MY90ST10)] to 24.3 °C [poly(MY50ST50)] for the synthesized copolymers. These values of Tg follow the same trend as in the DSC measurement. The higher glass transition temperatures are due to the dynamic mode of the experiment. Incorporation of styrene moiety into the copolymer reduces the elastic nature, thereby reducing the damping factor compared to the pristine polymyrcene elastomer. Vulcanization characteristics and physico mechanical properties. From an application point of view, rubbers are only useful after its transformation into a crosslinked state. Most of the rubbers will not be able to offer good properties until they are vulcanized. The characteristic rheometer curve of the compounded poly(MY70ST30) is presented in Figure 9a and the relevant curing parameters like minimum torque (ML), maximum torque (MH), scorch time (Ts2), optimum cure time (Ts90), delta torque (∆M)) are given in Table 6. Various physical and mechanical properties of the rubber vulcanizates are collated in Table 7. Polymyrcene showed higher values of both ML and MH due to higher molecular weight (93,200 Da vs. 51,500 Da). However, the ∆M was higher for poly(MY100ST0). This is also reflected in the crosslink density of the two systems. It is also evident from the rheometer curve that the copolymer had better scorch safely (Ts2) over the pristine polymyrcene. Presence of higher percentage of 1,2 vinyl and 3,4 addition microstructure makes polymyrcene more susceptible towards sulphur vulcanization and hence lower scorch safety. Due to the lower degree of such microstructural defects, the poly(MY70ST30) copolymer showed lower state of cure as well as crosslink density (Table 7). Also, the lower amount of double bonds available for crosslinking in the copolymer reduced the crosslink density and maximum torque.


Page 30 of 42

Page 31 of 42

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


Figure 9. (a) Rheometer curve of compounded poly(MY70ST30) (b) Tensile stress-strain plot of poly(MY70ST30) vulcanizate.

Table 6. Curing Parameters of the Compounded Rubbers. curing parameters sample

ML (dN.m)

MH (dN.m)

∆M (dN.m)

Ts2 (min:s)

Ts90 (min:s)

Poly(MY100ST0)

13.8

46.1

32.3

2.9

13.9

Poly(MY70ST30)

1.6

31.1

29.5

6.6

15.8

Table 7. Physical and Mechanical Properties of the Rubber Vulcanizates. sample properties Poly(MY100ST0)

Poly(MY70ST30)

density (g/cm3)

1.105

1.098

hardness (Shore A)

57

53

crosslink density (mol/cm3)

2.50 × 10-4

1.18 × 10-4

tensile strength (MPa)

4.5

6.4

elongation at break (%)

100

395



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

The hardness of 50 phr N330 filled system was low (53-57 Shore A) as compared to natural or synthetic rubbers, thus making them interesting as a promising material for soft rubber application. The calculated crosslink densities are presented in Table 7. The calculated values are in the same order as that of other synthetic rubbers. As expected, the crosslink density of polymyrcene vulcanizate was higher than the copolymer. These findings are also in line with rheometer data. For polymyrcene sample, presence of additional 1,2 vinyl and 3,4 microstructure facilitates the formation of additional crosslinks amongst the macromolecular chains. The tensile stress-strain plot of the poly(MY70ST30) rubber vulcanizate is depicted in Figure 9b and the mechanical properties are presented in Table 7. The copolymer exhibited reasonably high tensile strength of 6.4 MPa and an elongation of 395 %. The tensile strength of the polymyrcene vulcanizate was found to be ~ 4.5 MPa with an elongation at break at 100 %. The lower elongation is attributed to the presence of additional crosslink points (higher crosslink density) and also the pendent -CH=C(CH3)2 unit within the polymyrcene elastomer network which hinders the elongation. Dynamic mechanical properties of the rubber vulcanizates. The dynamic mechanical properties of the rubber vulcanizates were examined and compared with a standard tyre tread compound. The tan δ value at 0 °C (indirect measure of wet skid resistance) and 60 °C (indirect measure of rolling resistance) for all the samples were noted. The overlay plots for the tan δ are presented in Figure 10. An ideal passenger car tyre tread material should offer good wet skid resistance or road grip as well as good rolling resistance. The poly(MY70ST30) vulcanizate displayed good wet skid resistance (higher tan δ at 0 °C) as well as low rolling loss ((lower tan δ at 60 °C) over the standard tyre tread vulcanizate, making it a preferred elastomer of choice for automotive application.


Page 32 of 42

Page 33 of 42

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


Figure 10. Tan δ overlay plot of various rubber vulcanizates. The inset shows the enlarged temperature region of 0-70 °C.

CONCLUSIONS In this present work, a facile method for the preparation of a biobased elastomer from βmyrcene and styrene is demonstrated. The reactivity ratios of the monomer pair, as determined by Fineman-Ross and Kelen-Tüdös method, indicated moderate ideal copolymerization behaviour. The synthesized copolymers displayed pseudoplastic flow behaviour, improved onset degradation temperature and subzero glass transition temperature. It was observed that neither 1,2 vinyl nor 3,4 addition product with respect to myrcene was present for the copolymers having 40 or higher weight % ST content. However, for the copolymers with lower amount of ST, the presence of 1,2 vinyl and 3,4 addition product along with 1,4 –cis and 1,4 –trans microstructure with respect to myrcene was evident. The synthesized 70/30 copolymer was compounded and vulcanized by conventional rubber 33 ACS Paragon Plus Environment


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

processing methods and equipments. The carbon black filled vulcanizate exhibited a tensile strength of 6.4 MPa with an elongation at break of 395 %, indicating decent performance behaviour. The rubber vulcanizate also exhibited good wet skid resistance as well as lower rolling loss when compared with a standard tyre tread compound, making it an appealing material for automotive sectors. Thus, instead of modifying the compounding recipes or using functionalized fillers, the synthesized material could be an independent solution for the problem of magic triangle, encountered in tyre industries. In addition, the microstructure of these sustainable rubbers could be tailored easily by judicious choice of copolymer composition. The present study therefore constitutes a green ramp for the development of sustainable and inexpensive rubbery materials of the future for engineering applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS publication website at http://pubs.acs.org The ground state optimized structures of styrene and β-myrcene, Calculation of theoretical solubility parameter, Group contributions for theoretical solubility parameter, FTIR spectra of Poly(ST-co-MY) copolymers with varying ST content, 1H NMR spectrum of styrene (ST) monomer,

13

C NMR spectrum of styrene (ST) monomer, 1H NMR spectrum of polystyrene,

Composition and microstructures of various copolymers as analyzed by 1H NMR spectra, 1H NMR spectrum of Poly(MY60ST40), 1H NMR spectrum of Poly(MY70ST30), 1H NMR spectrum of Poly(MY80ST20), 1H NMR spectrum of Poly(MY90ST10), 1H NMR spectrum of Poly(MY30ST70), Calculation of monomer reactivity ratios, Parameters for calculation of reactivity ratio of MY and ST, Plot of ST mole fractions in feed and copolymer, X-ray diffractrograms of various synthesized polymers, Glass transition temperature of various


Page 34 of 42

Page 35 of 42

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


polymers, DSC traces of various homo and co-polymers, Molecular dynamics simulation, Characteristic degradation temperatures and percent residue at 750 °C, AFM image of the poly(MY50ST50) sample, Room temperature plot of storage modulus as a function of percent strain and complex viscosity versus angular frequency for various copolymers, Plot of complex viscosity versus angular frequency for Poly(MY0ST100) and Poly(MY30ST70) samples at 110 °C, Variation of G'max and temperature at Tanδmax with styrene content. AUTHOR INFORMATION Corresponding Author *Email: [email protected], [email protected] (AKB) Tel.: +91 (3222) 283180; Fax: +91 (3222) 220312. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors would like to thank IIT Kharagpur for providing the facilities. P.S. would like to acknowledge IIT Kharagpur for awarding a research fellowship to carry out this work. P.S. gratefully acknowledge Mr. Ayan Dey of University College of Science & Technology, University of Calcutta, for performing the MCR experiments. REFERENCES (1) Gandini, A.; Lacerda, T. M. From Monomers to Polymers from Renewable Resources: Recent Advances. Prog. Polym. Sci. 2015, 48, 1–39. (2) Eissen, M.; Metzger, J. O.; Schmidt, E.; Schneidewind, U. 10 Years after Rio-Concepts on the Contribution of Chemistry to a Sustainable Development. Angew Chem. Int. Ed. 2002, 41, 414-436. (3) Belgacem, M. N.; Gandini, A. Monomers, Polymers and Composites from Renewable Resources; 1st ed.; Elsevier: Amsterdam, 2008.



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

(4) Gandini, A. Polymers from Renewable Resources: A Challenge for the Future of Macromolecular Materials. Macromolecules 2008, 41, 9491-9504. (5) Han, Y.; Yuan, L.; Li, G.; Huang, L.; Qin, T.; Chu, F.; Tang, C. Renewable Polymers from Lignin Via Copper-free Thermal Click Chemistry. Polymer 2016, 83, 92-100. (6) Singh, R. Facts, Growth and Opportunities in Industrial Biotechnology. Org. Process Res. Dev. 2011, 15, 175-179. (7) Miller, S. A. Sustainable Polymers: Opportunities for the Next Decade. ACS Macro Lett. 2013, 2, 550-554. (8) Mathers, R. T. How Well Can Renewable Resources Mimic Commodity Monomers and Polymers? J. Polym. Sci. A Polym. Chem. 2012, 50, 1–15. (9) Wilbon, P. A.; Chu, F.; Tang, C. Progress in Renewable Polymers from Natural Terpenes, Terpenoids and Rosin. Macromol. Rapid Commun. 2013, 34, 8-37. (10) Grau, E.; Mecking, S. Polyterpenes by Ring Opening Metathesis Polymerization of Caryophyllene and Humulene. Green Chem. 2013, 15, 1112-1115. (11) Winnacker, M.; Rieger, B. Recent Progress in Sustainable Polymers Obtained from Cyclic Terpenes: Synthesis, Properties and Application Potential. ChemSusChem 2015, 8, 2455-2471. (12) Gershenzon, J.; Dudareva, N. The Function of Terpene Natural Products in the Natural World. Nat. Chem. Biol. 2007, 3, 408 – 414. (13) Carroll, A. L.; Desai, S. H.; Atsumi, S. Microbial Production of Scent and Flavor Compounds. Curr. Opin. Biotech. 2016, 37, 8-15. (14) Leavell, M. D.; McPhee, D. J.; Paddon C. J. Developing Fermentative Terpenoid Production for Commercial Usage. Curr. Opin. Biotech. 2016, 37, 114-119. (15) https://amyris.com/collaborations/%CE%BCpharm. (Accessed July 2016).


Page 36 of 42

Page 37 of 42

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


(16) Behr, A.; Johnen L. Myrcene as a Natural Base Chemical in Sustainable Chemistry: A Critical Review. ChemSusChem. 2009, 2, 1072 – 1095. (17) Kim, E. M.; Eom, J. H.; Um, Y.; Kim, Y.; Woo, H. M. Microbial Synthesis of Myrcene by Metabolically Engineered Escherichia coli. J. Agric. Food Chem. 2015, 63, 4606-4612. (18) Liu, B.; Li, L.; Sun, G.; Liu, D.; Li, S.; Cui, D. Isoselective 3,4-(co)polymerization of Bio-renewable Myrcene Using NSN-ligated Rare-earth Metal Precursor: An Approach to a New Elastomer. Chem. Commun. 2015, 51, 1039-1041. (19) Loughmari, S.; Hafid, A.; Bouazza, A.; Bouadili, A. E.; Zinck, P.; Visseaux, M. Highly Stereoselective Co-ordination Polymerization of β-Myrcene from a Lanthanide-Based Catalyst: Access to Bio-Sourced Elastomers. J. Polym. Sci. Part A: Polym. Chem. 2012, 50, 2898-2905. (20) Georges, S.; Bria, M.; Zinck, P.; Visseaux, M. Polymyrcene Microstructure Revisited from Precise High-Field Nuclear Magnetic Resonance Analysis. Polymer 2014, 55, 38693878. (21) Kobayashi, S.; Lu, C.; Hoye, T. R.; Hillmyer, M. A. Controlled Polymerization of a Cyclic Diene Prepared from the Ring-Closing Metathesis of a Naturally Occurring Monoterpene. J. Am. Chem. Soc. 2009, 131, 7960-7961. (22) Hilschmann, J.; Kali, G. Bio-based Polymyrcene with Highly Ordered Structure via Solvent Free Controlled Radical Polymerization. Eur. Polym. J. 2015, 73, 363-373. (23) Georges, S.; Toure, A. O.; Visseaux, M.; Zinck, P. Coordinative Chain Transfer Copolymerization and Terpolymerization of Conjugated Dienes. Macromolecules 2014, 47, 4538-4547. (24) Bolton, J. M.; Hillmyer, M. A.; Hoye, T. R. Sustainable Thermoplastic Elastomers from Terpene-Derived Monomers. ACS Macro Lett. 2014, 3, 717-720.



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

Page 38 of 42

(25) Chern, C.S. Principles and Applications of Emulsion Polymerization, 1st ed., John Wiley & Sons, New Jersey, USA, 2008. (26) Wang, R.; Ma, J.; Zhou, X.; Wang, Z.; Kang, H.; Zhang, L.; Hua, K. C.; Kulig, J. Design and Preparation of a Novel Cross-Linkable, High Molecular Weight, and Bio-Based Elastomer by Emulsion Polymerization. Macromolecules 2012, 45, 6830-6839. (27) Sarkar, P.; Bhowmick, A. K. Synthesis, Characterization and Properties of a Bio-based Elastomer: Polymyrcene. RSC Adv. 2014, 4, 61343-61354. (28) Sarkar, P.; Bhowmick, A. K. Green Approach Toward Sustainable Polymer: Synthesis and Characterization of Poly(myrcene-co-dibutyl itaconate). ACS Sustainable Chem. Eng., 2016, 4, 2129-2141. (29) McKenna, R.; Nielsen, D. R. Styrene Biosynthesis from Glucose by Engineered E. Coli. Metab. Eng. 2011, 13, 544-554. (30) Azeem, M.; Borg-Karlson, A. K.; Rajarao, G. K. Sustainable Bio-production of Styrene from Forest Waste. Bioresource Technol. 2013, 144, 684-688. (31)

Yang,

J.;

Zhao,

G.;

Sun,

Y.;

Zheng,

Y.; Jiang,

X.; Liu,

W.;

Xian,

M. Bio-isoprene Production Using Exogenous MVA Pathway and Isoprene Synthase in Escherichia coli. Bioresource Technol. 2012, 104, 642-647. (32) Gaussian 09 software, Gaussian Inc., Wallingford, CT, 2010 (33) Biovia Materials Studio software, version 16.1.0.21, Dassault Systèmes, Japan: 2016. (34) Flory, P. J.; Rehner, J. Jr. Statistical Mechanics of Cross‐Linked Polymer Networks II. Swelling. J. Chem. Phys. 1943, 11, 521-526. (35) Deuri, A. S.; Bhowmick, A. K. Aging of EPDM Rubber. J. Appl. Polym. Sci. 1987, 34, 2205-2222. (36) Chern, C.S. Emulsion Polymerization Mechanisms and Kinetics. Prog. Polym. Sci. 2006, 31, 443-486.


Page 39 of 42

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


(37) Yalkowsky, S. H., He, Y.; Jain, P. Handbook of Aqueous Solubility Data. 2nd ed.; CRC Press, Boca Raton, Florida, USA, 2010. (38) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentration of Aqueous Surfactant Systems. NSRDS-NBS 36, Department of Commerce, U. S. Government Printing Office, Washington D.C., 1971. (39) Thickett, S. C.; Gilbert, R. G. Emulsion Polymerization: State of the Art in Kinetics and Mechanisms. Polymer 2007,48, 6965-6991. (40) Maxwell, I. A.; Morrison, B. R.; Napper, D. H.; Gilbert, R. G. Entry of Free Radicals into Latex Particles in Emulsion Polymerization. Macromolecules 1991, 24, 1629-1640. (41) Ni, H.; Ma, G.; Nagai, M.; Omi, S. Effects of Ethyl Acetate on the Soap-Free Emulsion Polymerization of 4 Vinylpyridine and Styrene. II. Aspects of the Mechanism. J. Appl. Polym. Sci. 2001, 82, 2692–2708. (42) Krevelen, D. W. V.; Nijenhuis, K. T. Properties of Polymers: Their Correlation With Chemical Structure; Their Numerical Estimation And Prediction From Additive Group Contributions, 4th ed.; Elsevier: Amsterdam, 2009, ch. 7. (43) Odian, G. Principles of Polymerizations, 4th ed.; Wiely Interscience: USA, 2004, ch. 6. (44) Quirk, R. P.; Pickel, D. L. G. Polymerization: Elastomer Synthesis. In Science and Technology of Rubber, Mark, J. E.; Erman, B.; Eirich, F. R. Academic Press, 3rd edition, New York, USA, 2005. (45) Cowie, J. M. G.; Arrighi, V. Polymers: Chemistry and Physics of Modern Materials, 3rd ed.; CRC Press: USA, 2007, ch. 5. (46) Huneau, B. Strain-Induced Crystallization of Natural Rubber: A Review of X-Ray Diffraction Investigations. Rubber Chem. Technol. 2011, 84, 425-452. (47) Fox, T. G.; Flory, P.J. Second-order Transition Temperatures and Related Properties of Polystyrene. J. Appl. Phys. 1950, 21, 581-591.



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

(48) Johnston, N. W. Sequence Distribution-Glass Transition Effects. III. α-MethylstyreneAcrylonitrile Copolymers. Macromolecules 1973, 6, 453-456. (49) Luda, M. P.; Guaita, M.; Chiantore, O. Thermal Drgradation of Polybutadiene, 2 Overall Thermal Behaviour of Polymers with Different Microstructure. Macromol. Chem. Phys. 1992, 193, 113-121.


Page 40 of 42

Page 41 of 42

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


for Table of Contents use only

Terpene Based Sustainable Elastomer for Low Rolling Resistance and Improved Wet-grip Application: Synthesis, Characterization and Properties of Poly(styrene-comyrcene)

Preetom Sarkar and Anil K. Bhowmick*

Green fabrication of sustainable elastomer for low rolling resistance and improved wet grip application



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

Page 42 of 42

for Table of Contents use only

Terpene Based Sustainable Elastomer for Low Rolling Resistance and Improved Wet-grip Application: Synthesis, Characterization and Properties of

Poly(styrene-co-

myrcene)

Preetom Sarkar and Anil K. Bhowmick*

Green fabrication of sustainable elastomer for low rolling resistance and improved wet grip application

ACS Paragon Plus Environment

Terpene Based Sustainable Elastomer for Low Rolling Resistance

Recommend Documents