Facile noncovalent assembly of MWCNT-LDH and CNF-LDH as reinforcing

hybrid fillers in thermoplastic polyurethane/nitrile butadiene rubber blends

Saheli Roy,1 Suneel Kumar Srivastava,1* and Vikas Mittal2

1Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India

2Department of Chemical Engineering, The Petroleum Institute, Abu Dhabi, United Arab Emirates

Abstract

Zinc-Aluminium layered double hydroxides (LDH) were assembled on pristine

multiwalled carbon nanotubes (MWCNT) and carbon nanofiber (CNF) through noncovalent

assembly and characterized. Further these hybrids were used as reinforcing filler in TPU/NBR

(1:1 w/w) blend. Our investigations showed TPU/NBR blend containing 0.50 wt % MWCNT-

LDH hybrid exhibit superior mechanical properties (Tensile strength: 126% and storage

modulus: 321% at -60 0C) compared to neat TPU/NBR. In contrast, 0.50 wt % CNF-LDH hybrid

filled TPU/NBR blend nanocomposites showed enhanced thermal stability (25 0C) and

crystallization temperature (36 0C) with respect to neat blend. Such improvements in mechanical

and thermal properties could be attributed to better homogeneous dispersion, stronger interfacial

interaction and synergistic effect.

Keywords: Hybrid filler, Noncovalent assembly, TPU/NBR blends, Mechanical properties, Thermal properties

……………………………………………………………………………………………………………*For correspondence: sunit@chem.iitkgp.ernet.in Tel: 0091-32222-283334 Fax: 0091-3222-25303

1. Introduction

In last few decades, numerous research works have been carried out on polymer nanocomposites

based on single polymer matrix. However, thermoplastic nanocomposites containing inorganic

nanofiller and soft elastomer blends seem to be the new approach in the nanocomposites studies.

[1-3] Polymer blends have attracted much attention in developing new polymeric materials by

simple and cost-effective methods exhibiting superior physical properties, e.g., chemical,

mechanical and thermal properties often compared to individual homopolymers or copolymers.[4-

5] In this regard, considerable attention has been focused on polymer blend nanocomposites [6-11]

and they are found to exhibit properties of polymer blends as well as merits of polymer

nanocomposites.[6-12]

According to available literature, blending of thermoplastic polyurethane (TPU) with

acrylonitrile butadiene rubber (NBR) find appreciations in the field of automotive gaskets,

gaskets/co-extrusion, protective covers, tubing pipes, and grips etc.[10-13] The presence of TPU in

the blend accounts for the improvement in tensile strength, fuel/oil, weather, ozone and oxygen

resistance, and NBR promotes the solvent resistance and thermal stability.[11,12] Desai et al.[10]

prepared TPU:NBR blends of variable composition (30:70, 50:50, 70:30 by wt%) and observed

co-continuous phase in TPU:NBR (50:50) blend owing to uniformly dispersed phases. Further,

there exist limited work on formation of TPU/NBR blend nanocomposites with carbon black,

layered double hydroxide (LDH), MWCNT-MgAlLDH and silica. [10-13]

Multi-walled carbon nanotubes (MWCNTs) are one of the most important 1D material

exhibiting high aspect ratio (<1000), excellent mechanical, thermal, electrical and magnetic

properties.[14-18] Carbon nanofibers (CNFs) are other form of carbon with an aspect ratio greater

than 100, large surface area, excellent and electrical properties, high temperature resistance. [19,20]

Both MWCNT and CNF find application in Li-ion batteries, biosensor, energy storage device,

electrochemical substance and nanofiller for polymer nanocomposites.[14-22] In addition, two

dimensional materials, MoS2,[15] clay [11,12] etc have also been receiving considerable attention. In

this perceptive, layered double hydroxide (LDH) belongs to family of anionic clay and finds

application in the field of catalysts,[23] drug delivery,[24] environmental protection [25] and polymer

nanocomposites,[11,12] etc. In recent years, 3D hybrids, constituting unique conjugates of two or

more nanomaterials have achieved considerable recognition as fillers in polymer. [12, 26-28]

MWCNT/LDH and MWCNT/CNF are two such examples of 3D hybrid fillers, which exhibit

wide range of applications including as nanofillers in polymer nanocomposites.[12,28-39]

MWCNT/LDH and CNF/LDH hybrids can be prepared by in-situ growth, [29,30,36] hydrothermal,

[31,32] co-precipitation,[33,34] wet mixing,[35] noncovalent assembly[12] and dry grinding,[28] pore

precipitation [37-39] methods.

In this present work, we report a simple and successful pathway to prepare MWCNT-

LDH and CNF-LDH hybrids via noncovalent assembly using sodium dodecyl sulphate as a

linker between one dimensional carbonaceous material (MWCNT and CNF) and Zn-Al LDH.

These hybrids have subsequently been used as nanofillers in fabricating TPU/NBR (TN) blends

followed by characterization and investigation of their mechanical and thermal properties.

2. Experimental

2.1. Materials

Thermoplastic elastomer, TPU (Desmopan-9385®) used was a polyether type with melt

flow rate of 4 cm3 (10 min)-1 and a density of 1.12 g cm-3 and provided by Bayer, Germany.

Nitrile butadiene elastomer, NBR (Krynac 2865F) was supplied by Lanxess, Germany.

Multiwalled carbon nanotubes (carbon > 95%, O.D × L 10-20 nm × 5-15 µm) and carbon

nanofiber (carbon> 99.9%, O.D × L 100 nm × 20-200 µm) were purchased from Shenzhen

Nanotech Prot. Co Limited, China. and Sigma-Aldrich respectively. Sodium dodecyl sulfate,

SDS (SRL Pvt., Mumbai, India), Zn(NO3)2.6H2O (Sigma aldrich), Al(NO3)3.9H2O (E.Merck,

India), NaOH (S.D.Fine chemicals, Boisar), dicumyl peroxide, DiCUP-98 (Hercules, U.S) and

dry tetrahydrofuran (Merck, India) were used as received.

2.2. Preparation of LDH

Zinc-aluminium layered double hydroxide (Zn-Al LDH) was prepared through standard

coprecipitation method followed by thermal crystallization.[11] In this procedure, Zn(NO3)2.6H2O

(14.20 g/0.075 mol) and Al(NO3)3.9H2O (9.25 g/(0.025 mol) were dissolved in 100 mL distilled

water placed earlier in a 250 ml round bottom flask followed by dropwise addition to 100 mL

aqueous NaOH (8 g/0.2 mol) under vigorous stirring. The pH of the solution was maintained to

10±0.1 with the help of 1M NaOH solution and kept at room temperature for 30 minutes. The

round bottom flask consisting white slurry was placed on an oil bath with a water condenser and

subjected to aging at 70–75 oC for 15 h. Finally, product was filtered, washed and kept for drying

at room temperature for 24 h.

2.3. Preparation of surfactant modified CNT (SFCNT), surfactant modified CNF (SFCNF)

In this procedure, 1 mg/ml pristine carbon nanotube (PCNT) was added to the aqueous

solution of 1 % SDS (greater than the critical micellar concentration).[40] The solution so obtained

was sonicated for 30 mins followed by centrifugation. The resulting product (referred as SFCNT)

was finally dried at room temperature for 24 hour. Identical procedure was adopted for

preparation of surfactant modified CNF (referred as SFCNF)

2.4. Preparation of SFCNT-LDH and SFCNF-LDH hybrids

The aqueous suspension of 0.1 g of Zn-Al-LDH was poured to a round bottom flask

containing 0.2 g of SFCNT, placed in 50 mL of deionized water. Subsequently, the entire

solution was subjected to vigorous stirring at 70 oC for 15 h followed by refluxing at 100 oC for 6

h. The product (SFCNT-LDH hybrid) obtained in this manner was filtered and vacuum dried at

60 oC for 24 h. The similar methodology was also adopted for the preparation of SFCNF-LDH

hybrid from Zn-Al-LDH and SFCNF.

2.5. Fabrication of TN nanocomposites containing hybrids

SFCNT-LDH and SFCNF-LDH hybrid reinforced TN nanocomposites were fabricated

by solution blending method.[12] Accordingly, a desired amount (0.25, 0.50, 0.75 and 1 wt%) of

hybrid was sonicated in 15 mL of dry THF for half an hour followed by its slow addition to a

solution of 5 g of TPU and NBR (50:50 wt%), already dissolved in 50 mL THF. Thereafter, for

another 3h stirring was extended to ensure good dispersion of the hybrid in the polymer matrix.

Finally, 2 phr of DCP was added into the solution and the stirring was continued for another 30

mins. Then the solution was casted in a Teflon petridish at room temperature and left for

evaporation of THF. The dried films of the samples prepared in this manner were roll-milled at

room temperature and finally subjected to compression moulding at 165 oC for 6 mins to obtain

the sheets of TN nanocomposites. The entire procedure exhibiting fabrication of TN

nanocomposites with hybrid fillers has been schematically depicted in scheme 1.

Scheme 1. Fabrication of TN nanocomposites with hybrid fillers.

2.6. Characterization

Room temperature wide angle X-ray diffraction (WAXD) analysis of hybrids in powder

form and its TPU/NBR nanocomposites were carried out in PANalytical (PW3040/60), model

‘X’ pert pro with Cu Kα radiation (λ=0.1542 nm) in the range of diffraction angle 2θ = 5-70 at a

scanning rate of 2 min-1. Fourier transform infrared (FTIR) of the samples were recorded on a

PerkinElmer RXI FTIR spectrometer in a wave number range; 400–4000 cm-1. High resolution

transmission electron microscopic (HRTEM) images of the hybrid filler and its corresponding

TN blend nanocomposites were taken on a JEOL 2100 TEM (acceleration voltage:200 keV). In

case of hybrid powder, it was sonicated in THF solution followed by drop casting on the copper

grid, whereas, in its TN blend, the films were subjected to ultramicrotomy with a Leica ultracut

UCT. Following this, the cryosections of 50-70 nm thickness was obtained using freshly

sharpened glass knives with cutting edges of 45º at -50 ºC (sample) and -60 ºC (knife). The

cryosections were collected individually on a copper grid of 300 mesh. The tensile analysis was

performed according to ASTMD 412-98 standard method using Tinius Olsen h10KS universal

testing machine at 25 C with a crosshead speed of 200 mm/min. The nine dumbbell shaped

specimens (total length of the dumbbell 70 mm) consisting of working length 30 mm, width 4

mm, thickness 0.53–0.56 mm were punched from the respective polymer films. Dynamic

mechanical analysis (DMA) was carried with the help of Dynamic Mechanical Analyzer DMA

Q800, (TA Instruments, Lukens Drive, Newcastle, Delaware) on 10 x 6 x 1.5 mm films at a

frequency 1 Hz and heating rate of 3 °C min-1 over a temperature range of -80 to 60 oC. Thermal

stability measurements were analyzed in range of 50-600 C at a heating rate of 10 C min-1 under

nitrogen atmosphere in a Discovery TGA (TA instruments). Differential scanning calorimetry

(DSC) data of these samples were determined by means of a 204 F1 Phoenix differential

scanning calorimetric instrument from Netzsch in the temperature range of -70 to +240 C (scan

rate:10 C min-1) under nitrogen atmosphere with a heating-cooling-heating cycle.

3. Results and Discussion

3.1. Establishment of hybrid nanostructures

X-ray diffraction patterns of LDH, PCNT, SFCNT, SFCNT-LDH hybrid, PCNF, SFCNF, and

SFCNF-LDH hybrid are displayed in Figure 1. LDH exhibits characteristic diffraction peaks

corresponding to (003), (006), (012), (110) and (113) planes.[41] The diffraction patterns of PCNT

and PCNF show the presence of (002) plane at 2~26 and 26.3o respectively.[12,19] Further, the

peak position of SFCNT and SFCNF remain more or less unaltered, but their corresponding peak

intensity has reduced compared to the pristine carbonaceous material (MWCNT and CNF). Such

observations are manifestation of easy compromise in crytallinity, defects and amorphous phase

formation.[12,42] Diffractograms of SFCNT-LDH and SFCNF-LDH hybrids also show the

presence of characteristic peaks of individual components of the hybrids, appeared at same

positions. However, in both the hybrids, the overall intensity of peaks has considerably reduced

compared to their individual components. This is in all probability due to the presence of

electrostatic interaction between the counter parts of the hybrids.[12]

Figure 1. XRD spectra of (a) LDH (b) PCNT, (c)

SFCNT (d) SFCNT-LDH hybrid, (e) PCNF, (f) SFCNF and (g) SFCNF-LDH hybrid.

FTIR spectra of LDH, PCNT, SFCNT, SFCNT-LDH hybrid, PCNF, SFCNF and

SFCNF-LDH hybrid are displayed in Figure 2. LDH exhibits the presence of peaks at ~3452,

1634, 1384, 786 and 549 cm-1 corresponding to stretching vibration of hydroxyl groups,

deformation mode of water molecules at the inorganic layers, vibration mode of NO3- anion and

metal–oxygen stretching and bending modes respectively. [43] PCNT shows the appearance of

peaks due to –OH stretching vibrations of surface groups (~3430 cm -1) and conjugated –C=C–

bonds (~1600 cm-1).[12] The corresponding peaks appear at 3378 and 1641 cm-1 in the FTIR

spectrum of PCNF.[20] Despite of these peaks, two additional peaks appear in the spectrum of

PCNF at 1569 and 1380 cm-1 corresponding to –CH2 bending mode and C-O bonding

respectively.[20] The peaks at 3415 (3327) , 2925(2926) , 1675 (1637), 1575 (1564), 1210(1206)

and 1098 (1058) cm-1 corresponding to respective –OH stretching vibration, -CH2 stretching

mode, –C=C– bond vibration, -CH2 bending mode, skeletal vibration involving the bridge S–O

stretch and C–C band stretching, confirms the presence of SDS in SFCNT (and SFCNF).[44,45]

Further, it is evident from the spectra of both nanohybrids that all peak positions are shifted more

or less from their individual counterparts, specially, the peak positions correspond to –OH

stretching vibration, -CH2 stretching and bending modes, skeletal vibration involving the bridge

S–O stretch and metal–oxygen stretching and bending modes are significantly shifted with

respect to their individual counterparts. All these observations confirm the presence of

electrostatic interaction between SDS coated carbon allotropes and LDH in both the

nanohybrids.

Figure 2. FT-IR spectra of (a) LDH (b) PCNT, (c) SFCNT (d) SFCNT-LDH hybrid, (e) PCNF,

(f) SFCNF and (g) SFCNF-LDH hybrid.

Figure 3 shows HRTEM images of LDH, PCNT, SFCNT, SFCNT-LDH hybrid, PCNF,

SFCNF and SFCNF-LDH hybrid. HRTEM image shows that LDHs are stacked orderly and

strongly adhere to each other.[46] It is also distinctly clear that PCNTs remain in the bundled

form,[27] whereas PCNFs exist as tangled hollow cylinder.[22] SDS-modified forms of PCNT and

PCNF show presence of patches on the smooth surfaces of PCNTs and PCNFs. Further, HRTEM

images of the hybrids clearly indicate that plate-like LDHs are attached to the surface of

SFCNTs and SFCNFs due to high affinity of the positively-charged layers of LDH towards the

negatively-charged sulfonate groups (–SO3−) of SDS modified carbon allotropes.

Figure 3. HRTEM

images of (a) LDH (b) PCNT, (c) PCNF, (d) SFCNT (e) SFCNF and (f) SFCNT-LDH hybrid,

(g) SFCNF-LDH hybrid (attached LDH platelets are shown by arrow).

3.2. Nanostructure of TN Blend

The properties of the polymer blends are controlled by the miscibility and phase behavior of the

matrix. The phase morphology of TPU/NBR blend with varying TPU/NBR ratio of 30:70, 50:50

and 70:30 (by weight) was investigated earlier.[10-12] These studies showed that only 50:50

TPU/NBR blend form co-continuous phase of both components running through one another,

forming interpenetrating network. Therefore, in this work nanostructure and properties of

TPU/NBR (50:50) blends have been investigated in presence of hybrid fillers.

Wide angle X-ray diffraction patterns of TN nanocomposites containing 0, 0.25, 0.50,

0.75, 1 wt% of both SFCNT-LDH and SFCNF-LDH hybrids are displayed in Figures S1 and S2

respectively (under supporting information). TN exhibits a broad peak at 2θ ~20o which is

attributed to characteristic peaks of TPU and NBR.[12] It is also noted that 002 and 003 peaks

corresponding to hybrid fillers are completely disappeared in all TN nanocomposites, which

signifies the probable partial exfoliation of the hybrid fillers into TN matrix. However,

disappearance/appearance of diffraction peaks corresponds to hybrid fillers is not a necessary

indicator for the formation of partially exfoliated/intercalated nanocomposites.[11,12] In addition,

the peak position of neat TN at 2θ ~20o remains almost same in its hybrid filled nanocomposites.

The full width at half maximum (FWHM) values of neat TN and its nanocomposites are

summarized in Table S1. The reduction in FWHM of nanocomposites compared to neat TN

signifies the formation of relatively ordered structure. This could be attributed to the strong

interfacial interaction between the polymer matrix and the hybrid nanofillers. [12,26,27] FWHM

values in Table S1 depict that at lower filler loading SFCNT-LDH hybrid loaded TN

nanocomposites are more ordering compared to that of SFCNF-LDH hybrid loaded TN

nanocomposites, which is likely to be reflected in their mechanical properties.

Figure 4 displays the HRTEM images of TN nanocomposites containing 0.50 and 1 wt%

of both the SFCNT-LDH and SFCNF-LDH hybrid fillers. It is evident that 0.50 wt% loaded

SFCNT-LDH and SFCNF-LDH hybrids form interconnected network, uniformly spread

throughout the TN matrix. At 1.0 wt% filler loadings, hybrids are found to be aggregated in TN.

All these findings clearly suggest that enhanced properties could be achieved by incorporating

0.50 wt% hybrid filler in TN matrix.

Figure 4. HRTEM images of TN

nanocomposites containing (a) 0.50, (b) 1 wt% of SFCNT-LDH hybrid and (c) 0.50, (d) 1 wt%

of SFCNF-LDH hybrid.

3.3. Properties of the TN nanocomposites

3.3.1. Mechanical measurements

The variation of tensile strength (TS) and elongation at break (EB) of TN nanocomposites with

respect to SFCNT-LDH and SFCNF-LDH hybrid filler content are displayed in Figures 5 (a) and

(b) respectively. Figures S3 and S4 (under supporting information) show the corresponding

representative stress strain plots of TN nanocomposites with different amount (0 to 1wt%) of

SFCNT-LDH and SFCNF-LDH hybrid filler respectively. It is also noted that 0.50 wt% SFCNT-

LDH loaded TN nanocomposites exhibit improvement in tensile strength (126%) and EB (1.50

times) compared to neat TN. On the other hand, with respect to neat TN, 0.50 wt% SFCNF-LDH

hybrid filled TN nanocomposites also show 122% and 1.43 times improvements in tensile

strength and EB respectively. The enhanced mechanical properties of TN nanocomposites clearly

suggest the reinforcing effect of both the SFCNT-LDH and SFCNF-LDH hybrid fillers in TN.

Most likely, optimum homogeneous dispersion of hybrid fillers into the TN matrix, leading to

strong interfacial interaction between hybrid filler and polymer matrix, resistance exerted by the

sterically hindered hybrid surface itself [12,26,27] and synergistic effect of the hybrid filler could

account for such significant increments in mechanical properties of TN nanocomposites.

Further, at higher filler loadings tensile strength and elongation at break slightly decrease due to

the tendency of the hybrid fillers to agglomerate, giving rise to initiating sites for crack

propagation. TEM images of TN

nanocomposites also support this

statement.

Figure 5(a). Variation of tensile strength and elongation at break of TN nanocomposites with

SFCNT-LDH hybrid content.

Figure 5(b). Variation of tensile strength and elongation at break of TN nanocomposites with

SFCNF-LDH hybrid content.

Further, tensile measurements on TN nanocomposites containing 0.25 wt% LDH, 0.25 wt%

SFCNT and 0.25 wt% SFCNF have been carried out and compared with neat TN, 0.50 wt%

SFCNT-LDH and 0.50 wt% SFCNF-LDH hybrid loaded TN nanocomposites. The synergistic

effect of SFCNT (or SFCNF) and LDH on the mechanical properties of TN is evident from the

findings, shown in Figures 6 (a) and (b). The observations show improvement in tensile strength

of 20, 41.4 (24.7) and 126 (122) % and enhancement in EB upto 257.3, 301 (286.3) and 440

(420) % for TN nanocomposites containing 0.25 wt% LDH, 0.25 wt% SFCNT (0.25 wt%

SFCNF) and 0.50 wt% SFCNT-LDH (0.50 wt% SFCNF-LDH) hybrid, respectively, compared

to neat TN. These findings clearly confirm the synergistic effect of SFCNT (SFCNF) and LDH

in reinforcing of TN nanocomposites.

Figure 6 (a). Representative stress-strain plots of (a) neat TN and its nanocomposites containing

(b) 0.25 wt% LDH, (c) 0.25 wt% SFCNT and (d) 0.50 wt% SFCNT-LDH hybrid.

Figure 6 (b). Representative stress-strain plots of (a) neat TN and its nanocomposites containing

(b) 0.25 wt% LDH, (c) 0.25 wt% SFCNF and (d) 0.50 wt% SFCNF-LDH hybrid.

In dynamic mechanical analysis (DMA), storage modulus (E΄) is the elastic modulus of

the materials whereas the loss modulus (E") is related to the energy loss during friction, which is

generated due to the polymer chain movement. Figures 7, 8 and S5, S6 (under supporting

information) and Table 1 present the findings of neat TN and its SFCNT-LDH and SFCNF-LDH

hybrid filled nanocomposites from DMA. It is also reflected from the Figures that storage

modulus and loss modulus of hybrid filled TN nanocomposites are always higher compared to

neat TN, where TN nanocomposites with 0.50 wt% hybrid loading exhibit maximum

improvements. Such increment in modulus is attributed to the excellent dispersion of filler into

the matrix, which causes strong interaction of filler with the polymer chains, resulting load

transfer by the hybrid filler to the polymer matrix.[12,26-28] Further increment in filler loads starts a

tendency of filler agglomeration into the matrix which causes decrease in moduli. This

phenomenon is supported by the tensile data and TEM images. It is found that loss modulus (E")

value (at -30 oC) of pure TN (47.22 MPa) has been improved by 237, 339, 219, and 200%, with

0.25, 0.50, 0.75 and 1wt% SFCNT-LDH hybrid loading respectively. The corresponding

improvements in loss modulus of SFCNF-LDH loaded TN blends are found to be 165, 262, 209,

and 202%. The findings suggest that both the hybrid fillers not only influence the elastic

properties strongly but also increase the friction between filler and polymer. The maxima of E"

signify the respective glass transition temperatures (Tg) of neat TN and its hybrid filled

nanocomposites. It is noted that all the TN nanocomposites exhibits slight positive shifting in Tg

compared to neat TN. This is in all probability due to restricted mobility of the polymer chains in

presence of hybrid filler. SFCNT-LDH hybrid shows relative increment in Tg compared to

SFCNF-LDH hybrid in TN matrix, which signifies that SFCNT-LDH hybrid exerts

comparatively better restriction than the other hybrid filler.

Figure 7(a). Storage modulus of (a) Neat TN and its nanocomposites containing (b) 0.25, (c) 0.50, (d) 0.75 and (e) 1wt% SFCNT-LDH hybrid.

Figure 7(b). Storage modulus of (a) Neat TN and its nanocomposites containing (b) 0.25, (c) 0.50, (d) 0.75 and (e) 1wt% SFCNF-LDH hybrid.

Figure 8(a). Temperature dependence curve of tan delta of (a) Neat TN and its nanocomposites containing (b) 0.25, (c) 0.50, (d) 0.75 and (e) 1wt% SFCNT-LDH hybrid.

Figure 8(b). Temperature dependence curve of tan delta of (a) Neat TN and its nanocomposites containing (b) 0.25, (c) 0.50, (d) 0.75 and (e) 1wt% SFCNF-LDH hybrid.

Figures 8 (a) and (b) display the effect of hybrid fillers on dissipation factor (tan δ) of

neat TN, as a function of temperature. The height reduction and shifting of tan δ peaks are

evident in TN nanocomposites compared to neat TN, which could be attributed to the internal

friction among the nanofiller-nanofiller, nanofiller-polymer matrix and polymer matrix-matrix

under some external stresses.[12,26,27] The tan δ maxima in these plots also signify the Tg of neat TN

and its hybrid filled nanocomposites. All the hybrid filled nanocomposites have comparatively

higher Tg than neat as the hybrids impose restriction on the mobility of the polymer chains at the

closer vicinity to glass transition temperature.[12,26,27]

Table 1. Storage modulus at different temperatures, glass transition temperature (Tg) and height

of tan δ value of pure TN and its composites.

Sample E΄ (MPa) at -60○C

% improve

ment

E΄ (MPa) at 25○C

% improve

ment

Tg(○C) from E´´

tan δ height

Pure TN

TN/ SFCNT-LDH (0.25 wt %)

TN/ SFCNT-LDH (0.50 wt %)

TN/ SFCNT-LDH (0.75 wt %)

TN/ SFCNT-LDH (1.0 wt %)

TN/ SFCNF-LDH (0.25 wt %)

TN/ SFCNF-LDH (0.50 wt %)

TN/ SFCNF-LDH (0.75 wt %)

TN/ SFCNF-LDH (1.0 wt %)

571

1920

2402

1950

1840

1702

2158

1960

1820

-

236

321

241

222

198

278

243

219

2.70

7.78

10.30

9.92

7.77

5.50

10.15

9.13

5.86

-

188

281

267

187

104

276

238

117

-36

-33

-30

-34

-35

-36

-32

-34

-35

0.79

0.75

0.66

0.74

0.76

0.76

0.71

0.72

0.76

3.3.2. Thermal analysis

Thermal behaviour of neat TN and its hybrid (SFCNT-LDH and SFCNF-LDH) filled

nanocomposites have been investigated in temperature range of 50-650 oC under nitrogen

atmosphere and findings are displayed in Figures 9 (a), (b) and Table 2. It is observed that TN

and its nanocomposites undergo three step degradation processes. The first step of degradation

(~230-300 oC) involves the rapid rapture of urethane linkages of TPU, evolving isocyanate and

polyol.[11,12] The second degradation step in the range of 330-480 oC corresponds to thermal

degradation of isocyanate to produce urea and degradation of butadiene segments of NBR. [11,12]

The third and final degradation (>500 0C) is associated with decomposition of earlier formed

urea and acrylonitrile segment of NBR.[11,12] Subsequently, this final step yields a small but

different amount of carbonaceous char for each nanocomposite. It is also clearly evident from

thermograms that TN nanocomposites of both the SFCNT-LDH and SFCNF-LDH hybrids show

better thermal stability compared to neat TN. Considering 10 and 50% weight loss, thermal

stability of 0.50 wt% SFCNT-LDH (SFCNF-LDH) hybrid loaded TN nanocomposites is

maximum improved by 16 and 23 oC (20 and 25 0C) respectively. Such enhanced thermal

stability of neat TN in presence of hybrid filler is in all probability due to strong interfacial

interaction of polymer/filler and the combined stabilizing effect of SFCNT (SFCNF) and LDH .

[12,26,27] The comparatively better thermal stability, achieved by TN nanocomposites of SFCNF-

LDH hybrid is attributed to the

preparatory method (thermal

treatment) of CNF, which

makes it highly inert.[47] It is

also observed that TN

nanocomposites exhibit

relatively higher char residues

compared to neat TN due to

the presence of hybrid filler.

Figure 9(a). TGA curves of neat TN and its nanocomposites containing 0.25, 0.50, 0.75 and 1wt

% SFCNT-LDH hybrid.

Figure 9(b). TGA curves of neat TN and its nanocomposites containing 0.25, 0.50, 0.75 and 1wt

% SFCNF-LDH hybrid.

Table 2. TGA data for neat TN and its nanocomposites.

Sample Td(10%)a (oC) Td(50%)

c (oC) Residue (wt%)

at 650 oC

Pure TN

TN/ SFCNT-LDH (0.25 wt %)

293

304

415

433

1.2

4.4

TN/ SFCNT-LDH (0.50 wt %)

TN/ SFCNT-LDH (0.75 wt %)

TN/ SFCNT-LDH (1.0 wt %)

TN/ SFCNF-LDH (0.25 wt %)

TN/ SFCNF-LDH (0.50 wt %)

TN/ SFCNF-LDH (0.75 wt %)

TN/ SFCNF-LDH (1.0 wt %)

309

305

302

300

313

307

306

438

437

436

434

440

438

436

5.4

5.8

6.4

5.9

6.6

7.7

8.3

Footnote : a) Td(10%) –Temperature at 10% weight loss.b) Td(50%) – Temperature at 50% weight loss.

DSC measurements were performed with neat TPU and neat NBR. The results showed

that TPU had two melting temperatures (Tm) at 75 and 171 0C due to soft segment and hard

segments of TPU respectively,[26,27] whereas, neat NBR had its Tm at 176 0C.[12] Figures S7 and S8

(under supporting information) show the second heating cycle of TN (50:50) nanocomposites of

SFCNT-LDH and SFCNF-LDH hybrid respectively and the corresponding findings are

summarized in Table 3. According to this, second heating curves show improvements in Tg of all

TN nanocomposites with respect to neat TN. It is anticipated that such improvements in T g of

nanocomposites could be ascribed to the restricted mobility of polymer chains in presence hybrid

fillers. DSC data in Table 3 show that Tm of all TN nanocomposites have improved compared to

neat TN (153 ○C) in all probability due to nucleating effect of hybrid filler, which forms more

ordered crystallites.[12,26,27] It is also noted that 0.50 wt% SFCNT-LDH (0.25 wt% SFCNF-LDH)

hybrid loading in TN matrix exhibit maximum improvements in Tm by 3 oC (2 oC) compared to

neat TN. Further, at higher hybrid loading Tm value of TN nanocomposites reduce at all possibly

due to aggregated hybrid filler in TN matrix, which reduce the contact area between filler and

matrix, resulting in comparatively lower melting temperature.

The cooling (first) DSC curves of TN nanocomposites, represented by Figures 10 (a) and

(b) show that incorporation of hybrid fillers in TN matrix enhance the crystallization temperature

(Tc) of the resulting nanocomposites compared to neat TN (~ 80 ○C). Our study also reveal that

0.50 wt % SFCNT-LDH

(SFCNF-LDH) filled TN

nanocomposites exhibit maximum

increment in crystallization

temperature (Tc) by 18 ○C (36 oC).

This is in all likelihood due to

positive heterogeneous

nucleating effect of hybrid filler,

which could improve the

intrinsic crystallization tendency of the TN matrix significantly.[12,26,27] It is to be noted that

compared to other filler SFCNF-LDH hybrid filler has pronounced effect on crystallization

temperature of TN matrix, which is ascribed to the preparatory method (thermal treatment) of

CNF, which made it highly crystalline and inert.[47] At higher filler loading, decrease in Tc could

be attributed to the limited nucleating proficiency of the hybrid filler, causing slower

crystallization process.[12,26,27]

Figure 10(a). First cooling curves of (a) neat TN and its nanocomposites containing (b) 0.25, (c)

0.50, (d) 0.75 and (e) 1wt% SFCNT-LDH hybrid.

Figure 10(b). First cooling curves of (a) neat TN and its nanocomposites containing (b) 0.25, (c)

0.50, (d) 0.75 and (e) 1wt% SFCNF-LDH hybrid.

Table 3. Glass transition temperature (Tg), Melting temperature (Tm), crystallization temperature

(Tc) of TN and its nanocomposites.

Sample Tg (○C) T(m,hard) (○C) T(c,hard)(○C)

Pure TN

TN/ SFCNT-LDH (0.25 wt %)

TN/ SFCNT-LDH (0.50 wt %)

TN/ SFCNT-LDH (0.75 wt %)

-45

-43

-41

-42

153

155

156

154

80

89

98

88

TN/ SFCNT-LDH (1.0 wt %)

TN/ SFCNF-LDH (0.25 wt %)

TN/ SFCNF-LDH (0.50 wt %)

TN/ SFCNF-LDH (0.75 wt %)

TN/ SFCNF-LDH (1.0 wt %)

-44

-44

-42

-43

-44

153

155

153

152

150

86

114

116

111

108

4. CONCLUSIONS

SFCNT-LDH and SFCNF-LDH hybrid have been successfully developed through

noncovalent assembly and further they are used as nanofiller in TN (50:50) blend. Subsequently,

their mechanical and thermal properties have been investigated through tensile testing, DMA,

TGA and DSC measurements. Mechanical behaviour shows that SFCNT-LDH hybrid filled

blend nanocomposites have better mechanical strength (storage modulus 321% and tensile

strength 126%) compared to SFCNF-LDH hybrid loaded blend nanocomposites (storage

modulus 278% and tensile strength 122%). In thermal analysis TN nanocomposites of SFCNF-

LDH hybrid exhibit better thermal stability (20 and 25 oC at 10 and 50% weight loss

respectively) than SFCNT-LDH loaded TN blend (16 and 23 oC at 10 and 50% weight loss

respectively). DSC results reveal that blend nanocomposites with SFCNF-LDH hybrid had

enhanced crystallization temperature (36 oC) rather than SFCNT-LDH hybrid filled blend

nanocomposites (18 oC). The observed improvements in mechanical and thermal properties could

be attributed to homogeneous dispersion and strong interfacial interaction between TN matrix

and the fillers.

Acknowledgements

The authors are grateful to CSIR, New Delhi, India, for financial support. We express

our gratitude to Bayer Germany and Lanxess (Germany) for providing us TPU (Desmopan-9385)

and NBR (Krynac 2865F) respectively. Author expresses thanks to Rubber Technology Centre,

Indian Institute of Technology, Kharagpur for providing compression moulding and DMA.

References

[1] M. Mehrabzadeh, M. R. Kamal, Polym. Eng. Sci. 2004, 44, 1152.

[2] B. B. Khatua, D. J. Lee, H. Y. Kim, J. K. Kim, Macromolecules 2004, 37, 2454.

[3] M. Y. Gelfer, H. H. Song, L. Liu, B. S. Hsiao, B. Chu, M. Y. Rafailovich, M. Si, V. Zaitsev,

J. Polym. Sci. Part B:Polym. Phys. 2003, 41, 44.

[4] X. Li, J. K. Mishra, S. D. Seul, I. L. Kim, C. S. Ha, Compos. Interfaces 2004, 11, 335.

[5] Y. Wang, Q. Zhang, Q. Fu, Macromol. Rapid Commun. 2003, 24, 231.

[6] M. C. Mistretta, M. Morreale, F. P. La Mantia, Polym. Degrad. Stab. 2014, 99, 61.

[7] Y. Li, H. Shimizu, Polymer 2004, 45, 7381.

[8] T. Kuila, S. K. Srivastava, A. K. Bhowmick, A. K. Saxena, Compos. Sci. Technol. 2008, 68,

3234.

[9] S. Agarwal, V. K. Saraswat, Opt. Mater. 2015, 42, 335.

[10] S. Desai, I. M. Thakore, A. Brennan, S. Devi, J. Macromol. Sci., Part A 2001, 38, 711.

[11] M. Kotal, S. K. Srivastava, A. K. Bhowmick, Polym. Int. 2010, 59, 2.

[12] S. Roy, S. K. Srivastava, J. Pionteck, V. Mittal, Polym. Int. 2015, (Accepted).

[13] J. H. Tan, X. P. Wang, J. J. Tai, Y. F. Luo, D. M. Jia, Express Polym. Lett. 2012, 6, 588.

[14] C. Wei, L. Dai, A. Roy, T.B. Tolle, J. Am. Chem. Soc. 2006, 128, 1412.

[15] K. Bindumadhavan, S. K. Srivastava, S. Mahanty, Chem. Commun. 2013, 49, 1823.

[16] N. Li, Y. Huang, F. Du, X. He, X. Lin, H. Gao, Y. Ma, F. Li, Y. Chen, P.C. Eklund, Nano

Lett. 2006, 6, 1141.

[17] T. P. Chua, M. Mariatti, A. Azizan, A. A. Rashid, Compos. Sci. Technol. 2010, 70, 671.

[18] M. J. Jiang, Z. M. Dang, S. H. Yao, J. Bai, Chem. Phys. Lett. 2008, 457, 352.

[19] J. Fu, H. Qiao, D. Li, L. Luo, K. Chen, Q. Wei, Sensors 2014, 14, 3543.

[20] Y. Shi, X. Feng, H. Wang, X. Lu, Wear 2008, 264, 934.

[21] Q. Si, K. Hanai, T. Ichikawa, A. Hirano, N. Imanishi, O. Yamamoto, Y. Takeda, J. Power

Sources 2011, 196, 6982.

[22] R. Vellacheri, V. K. Pillai, S. Kurungot, Nanoscale 2012, 4, 890.

[23] J. Wang, G. Fan, H. Wang, F. Li, Ind. Eng. Chem. Res. 2011, 50, 13717.

[24] B. Sels, D. De Vos, M. Buntinx, F. Pierard, A. Kirsch-De Mesmaeker, P. Jacobs, Nature

1999, 400, 855.

[25] Y. Zhao, S. He, M. Wei, D. G. Evans, X. Duan, Chem. Commun. 2010, 46, 3031.

[26] S. Roy, S. K. Srivastava, J. Pionteck, V. Mittal, Polym. Compos. 2014, DOI

10.1002/pc.23350

[27] S. Roy, S. K. Srivastava, J. Pionteck, V. Mittal, Macromol. Mater. Eng. 2015, 300, 346.

[28] B. Pradhan, S. K. Srivastava, Compos. Part A 2014, 56, 290.

[29] Y. Cao, Y. Zhao, Q. Li, Q. Jiao, J. Chem. Sci. 2009, 121, 225.

[30] M. Q. Zhao, Q. Zhang, X. L. Jia, J. Q. Huang, Y. H. Zhang, F. Wei, Adv. Funct. Mater.

2010, 20, 677.

[31] X. Xiang, L. Bai, F. Li, AIChE J. 2010, 56, 2934.

[32] B. Du, Z. Fang, Nanotechnology 2010, 21, 315603/1.

[33] H. Wang, X. Xiang, F. Li, J. Mater. Chem. 2010, 20, 3944.

[34] H. Wang, X. Xiang, F. Li, AIChE J. 2010, 56, 768.

[35] S. Huang, H. Peng, W. W. Tjiu, Z. Yang, H. Zhu, T. Tang, T. Liu, J. Phys. Chem. B 2010,

114, 16766.

[36] F. He, Z. Hu, K. Liu, S. Zhang, H. Liu, S. Sang, J. Power Sources 2014, 267, 188.

[37] N. N. A. H. Meis, J. H. Bitter, K. P. De Jong, Ind. Eng. Chem. Res. 2010, 49, 8086.

[38] M. G. Álvarez, A. M. Frey, J. H. Bitter, A. M. Segarra, K. P. de Jong, F. Medina, Appl.

Catal. B Environ. 2013, 134-135, 231.

[39] F. Winter, A J. van Dillen, K. P. de Jong, Chem. Commun. 2005, 3977.

[40] E. F. Cruz, Y. Zheng, E. Torres, W. Li, W. Song, K.Burugapalli, Int. J. Electrochem. Sci.

2012, 7, 3577.

[41] D. Li, Z. Tuo, D. G. Evans, X. Duan, J. Solid State Chem. 2006, 179, 3114.

[42] J. Liu, G. Chen, J. Yang, L. Ding, Mater. Chem. Phys. 2009, 118, 405.

[43] A. A. A. Ahmed, Z. A. Talib, M. Z. B. Hussein, Appl. Clay Sci. 2012, 56, 68.

[44] S. Yesil, C. Winkelmann, G. Bayrama, V. L. Saponara, Mater. Sci. Eng. A 2010, 527, 7340.

[45] M. K. Singh, A. Agarwal, R. Gopal, R. K. Swarnkar, R. K. Kotnala, J. Mater. Chem. 2011,

21, 11074.

[46]. Y. X. Zhanga, X. D. Haoa, M. Kuanga, H. Zhaoc, Z. Q. Wen, Appl. Surf. Sci. 2013, 283,

505.

[47] S. M. Andersen, M. Borghei , P. Lund , Y.-R. Elina, A. Pasanen, E. Kauppinen, V. Ruiz, P.

Kauranen, E. M. Skou, Solid State Ionics 2013, 231, 94.