SURFACE CHARACTERISTICS, SPECTROSCOPIC...

Chapter 3

SURFACE CHARACTERISTICS, SPECTROSCOPIC INVESTIGATIONS AND

THERMAL BEHAVIOUR OF BANANA FIBRES- MACRO TO NANO SCALE

Abstract

Surface chemistry and surface morphology of the reinforcement play an important role for the development of a strong interface between reinforcement and the matrix. In the present study, the surface of chemically modified and unmodified banana fibres with varying fibre diameters were investigated by atomic force microscopy, transmission electron microscopy, and scanning electron microscopy and environmental scanning electron microscopy. The crystallinity and diameter of the micro and nanofibres were compared using X-ray diffraction studies. In addition to the surface morphology, the acid-base property of a fibre surface determines its interaction with other materials with which they are in contact. The surface composition and surface polarity of the fibres with diameters in the macro and nano range were determined using solvatochromic measurements involving various probe dyes and also by electrokinetic studies. Zeta potential measurements carried out on the fibres confirmed the results obtained from solvatochromic measurements. The thermal behaviour of macro, micro and nanofibres were also compared. Substantial increase in thermal stability was observed from macro to nano fibres which proved the high thermal stability of nanofibres to processing conditions of biocomposite preparation. The composition of the fibres before, after steam explosion and acid hydrolysis were also analysed using FT-IR. The results of solvatochromism and zetapotential corroborate FTIR results.

The results of this paper have been submitted for publication in Colloid and Interface Science

134 Chapter 3

3.1 Introduction

Fibre reinforced thermosetting composites are highly beneficial because

the reinforced materials improve the strength and toughness of the plastics

(1-3). Natural fibres which are rich in cellulose, the most abundant

biopolymer, is sustainable, biodegradable and have low density.

Additionally these materials have low toxicity and abrasiveness. Natural

fibres find application in the production of automotive parts (4).

Nanofibrils separated from the natural fibres have also been used for

processing nanocomposites so that the mechanical properties may be

improved (5). The reinforcement is regarded as a nanoparticle when

atleast one of the dimensions is lower than 100nm. The nanocomposites

assume exemplorary and novel properties, unseen in conventional

macrocomposites, when this particular feature is attained (7). Gandini and

Belgacem (7, 8) illustrated that the use of cellulose nano crystals as a

reinforcing phase in nanocomposites has numerous well known

advantages. Recently Cherian et al. synthesized nanofibril whiskers from

banana fibres (9).

Electrokinetic phenomena can be observed by contacting a solid surface

with a polar liquid medium, because of the existence of an electrical

double layer at the solid-liquid interface, dispersion or acid-base interaction

(10). The surface polarity of grafted carbon fibres was determined by contact

angle measurements and confirmed by zeta potential measurements.

Bismarck et al. (11) reported on the characterization of modified jute fibres

using zeta potential measurements. Pothen et al. (12) studied the influence of

chemical treatments on the electrokinetic properties of cellulose fibres.

Bellmann et al (13) investigated the electrokinetic properties of natural fibres

Surface characteristics, spectral studies…

135

and concluded that this is suitable to analyse the swelling characteristics of

fibres. Measurements such as scanning probe microscopy and scanning

electron microscopy were employed to assist with the interpretation of

results.

The present work highlights the investigation results of the morphological

and surface properties of banana fibres in microfibrillated and

nanostructured forms. Careful analysis of the literature indicates that no

study has been reported on the systematic comparison between these

properties of microfibrilated and nanostructured cellulose fibres. Plant

based cellulose nanofibres have generated a great deal of interest as a

source of nanometer sized fillers because of their sustainability, easy

availability, and the related characteristics such as a very large surface to

volume ratio, high tensile strength, high stiffness, high flexibility, good

dynamic, mechanical, electrical and thermal properties as compared with

other commercial fibres (14-17). The use of nano reinforcements in the

polymer matrix has been predicted to give improved properties compared

to the neat polymer and micro composites based on the same fibres.

Therefore it is of great interest to examine the possibilities of cellulose

based nanofibres as reinforcing elements. Treating various biomass

resources by steam explosion has been studied by many researchers,

(18-20). During steam explosion process the raw material is exposed to

pressurised steam followed by rapid reduction in pressure resulting in

substantial break down of the lignocellulosic structure, hydrolysis of the

hemicelluloses fraction, depolymerisation of the lignin components and

defibrillisation (21). The effect of the difference in non-cellulosic

136 Chapter 3

composition and degree of structural disruption on the thermal stability is

an important issue to be investigated (22).

3.2 RESULTS AND DISCUSSION

3.2.1 Characterization of banana fibre

Electron Microscopical Analysis:

Banana fibres obtained from local sources was subjected to steam

treatment to obtain micro and nano fibres. The diameters of these three

were compared usig SEM. The fibre diameter of the raw sample was

observed to be in the range of 80µm while that for micro fibres are in the

range 10-15µm. The average fibre diameter was found to be much lower

for nanofibres. (5-15nm in diameter and 200-250nm in length). The fibre

diameter distribution curves are shown in Fig 3.1 (a, b and c). From the

distribution curves it is seen that there is a decrease in fibre diameter as

we move from macro to nano scale. In the case of macro, maximum

number of fibres have 80µm diameter. For micro, maximum number of

fibres have 10µm diameter and for nano fibres the number of fibres with

minimum fibre diameter (5nm) was found to increase.


137

20 40 60 80 1000

5

10

15

20

25

30

35

Num

ber

of fi

bres

Fibre diameter (µm)

macro fibre

0 5 10 15 200

5

10

15

20

25

30

35

Num

ber

of fi

bres


micro fibre

(a) (b)

5 10 15 20 25 300

5

10

15

20

25

30

35

Num

ber

of fi

bres


nano fibre

(c)

Figure 3.1 Fibre diameter distribution curves of banana fibre (a) macro, (b) micro and (c) nano fibres

The structure and appearance of banana fibres in micro to nano-scale by

SEM is shown in Fig 3.2 (a, b and c). Steam treatment of macrofibre at high

pressure reduces the fibre diameter. It is clear from the SEM micrographs

that high pressure steam treatment helps in fibre separation and fibrillation

138 Chapter 3

(Figure 3.2 b). The tendency for fibre defibrillation was found to increase

during the transformation to nanoform. These conclusions were further

supported by the ESEM images shown in Fig 3.2 (c).

(a) (b)

(c)

Figure 3.2 SEM images of (a) macro banana fibre, (b) micro banana fibre, (c) ESEM image of banana nanofibre

It is evident from the ESEM images that the tendency for fibrillation

increased with chemical treatment and high pressure drop. The drop in

pressure facilitates the increase in the fibrillation process of the banana fibres

whose size range is in nanometer scale. It can be seen from ESEM image

shown in Fig 3.2 (c) that the cellulose nano fibres obtained from the banana

fibres are in the entangled fibril form and the length to diameter are in the


139

range of 250 to 5 nm, having a wide range of aspect ratio (length/diameter),

the average value being 50.

Transmission electron microscopic analysis

The TEM investigation of the micro fibres (Fig 3.3 a) shows that the fibres

have a length of 250nm and a diameter of 10-15µm. The TEM of the

synthesised nanocellulose fibres (Fig 3.3 b) produced were in the form of

interconnected web like structure. The fibres were also found to have a

decrease in fibre diameter as well as a change in the composition (Table 3.1).

Most of the nanofibrils are agglomerates of hundreds of individual cellulose

nanocrystals.

(a) (b)

Figure 3.3 TEM of (a) micro and (b) nano banana fibres

Table 3.1 Chemical composition of macro, micro and nano fibre

Material α cellulose (%) hemicellulose (%) Lignin (%) moisture ( %)

Macro fibre 64.00 ± 2.82 18.60 ± 1.60 4.90 ± 0.70 12.50 ± 0.47

Micro fibre 82.40 ± 2.51 12.01 ± 0.38 3.64 ± 0.53 1.96 ± 0.36

Nano fibre 95.80 ± 0.58 0.40 ± 0.01 1.86 ± 0.39 1.94 ± 0.42

140 Chapter 3

Scanning probe microscopic studies

The surface roughness averages of the fibre samples were measured using

AFM and it was found to be decreased based on the fibre diameter from

macro fibre to nano fibre. Both micro and nano fibres resulted in the most

significant removal of noncellulosic components (Table 3.1). These results

(Fig 3.4) truly indicated that steam correlated acid treatment helped to

develop fibres of higher cellulosic component, and thus suggest a more

effective removal of the middle lamella and the primary cell wall and

therefore a more cellulose rich surface as supported by and effective

reduction of fibre dimension to nano range. Environmental scanning electron

and transmission microscopic studies corroborate the above results.

(a) (b)

Figure 3.4 SPM image of (a) macro banana fibre, (b) micro banana fibre

and (c) nano banana fibre

(a)

(b)

(a) (b)


141

AFM and TEM suggested that only few lateral associations occur between

adjacent nanofibres. Nanofibres are much more clearly defined probably because

of the removal of amorphous zones and they seem to be more interwoven.

Optical analysis

The ocular polarized luminosity manifestation of the macro, micro and nano

banana fibres are shown in Fig 3.5 (a), (b) and (c). As it is seen from these

Figures, macro fibres are definitely not a monolithic and homogeneous single

fibre with a circular cross section but rather a bundle or a composite with an

elliptic or polygonal cross section consisting of several fibrous plant cells

(elementary fibrous cell). The high pressure chemical treatments results in

the structural changes as well as chemical changes on the fibre surfaces,

causing destruction of plant cell wall and helps in the isolation of the white

shining cellulose fibres.

Figure 3.5 Optical microscopic images of (a) macro (b) micro (c) and nano banana fibres

(a)

50 µm 10 µm

100 nm

142 Chapter 3

X-ray diffraction studies

XRD studies of the banana fibres were done to investigate the crystallinty

and the diameter of the fibres. From the XRD data, it is clear that the nano

fibres show a highly crystalline structure. Nanofibres exhibit a higher

crystallinity due to the efficient removal of noncellulosic polysaccharides

and dissolution of the amorphous zones by acid hydrolysis combined with

steam explosion (23, 24). The increase in the % crystallinity index of

microfibrils and nanofibrils occurs because of the removal of cementing

materals which leads to better packing of cellulose chains. Fig 3.6 shows the

XRD pattern of macro, micro and nano fibrils. The crystallinity is increased

for nano and gives a relatively intense peak at 2θ = 22.7o. For macro fibre the

crystallinity is very low and shows an amorphous nature. The sharp peak

observed in the case of nano fibres point to increased crystallinity. The

broadening of the peak at maximum 2θ proves the decrease in diameter. The

diameter is calculated using Scherrer formula (25)

D = K λ / B cos θ 3.1

K is constant (0.89), λ = X-ray wavelength, B = full width at half max and

θ = Bragg angle.

(a)

(b)


143

Figure 3.6 XRD pattern of different stages of banana fibre (1) macro fibre, (2) micro fibre and (3) nanofibre

Table 3.2 Ratio of intensities of cellulose I and cellulose II crystallites

Material I 22.7o Crystallinity (%) I 22.7

o/I20.4o

Macro fibre I22.7o = Iamor=10.5 - -

Micro fibre 22.9 54.18 0.57

Nano fibre 39.8 73.62 0.93

By contrast, the crystalline part of nano fibre corresponds to cellulose I (26)

and shows highest scattering intensity at 22.7o. A comparison of θ/2θ scans

obtained for macro to nano fibres reveals differences in crystallinity and

cellulose I/cellulose II content. Fig 3.6 illustrates that macro fibre exhibits a

small shoulder at a scattering angle of 22.7o indicating the presence of

cellulose I. This shoulder becomes more prominent in micro, further more

144 Chapter 3

intensified and develops into an obvious intense peak in nano fibre. The ratio

of scattering intensity at 22.7o vs. intensity at 20.4o (I22.7o/I20.4

o) is indicative

of cellulose I vs cellulose II content. On going from macro to nano fibres,

this ratio increases from 0.43 to 0.57 and finally to 0.93 (Table 3.2).

Increasing amounts of cellulose I facilitate the elevated increase in crystallinity

of the overall material. When the value of cellulose I is more enhanced than

cellulose II, the overall crystallinity is amplified.

Fourier transform infrared spectrometry

The IR spectrum of the banana fibres is shown in Fig 3.7. The peaks in the

area 3420cm-1 arise due to –OH stretching vibrations of hydrogen bonded

hydroxyl group. The hydrophilic tendency of macro, micro and nano are

reflected in the broad absorption band at 3700-3100cm-1 region due to the

presence of –OH groups present as main component. The peak at 2921cm-1 is

due to aliphatic saturated C-H stretching vibration in hemicellulose and

cellulose. The peak at 1731cm-1 in macro fibre is due to acetyl (–C=O

stretching) and ester groups of hemicellulose, pectin and lignin (27). This

peak is absent in the micro and nano fibrils due to the removal of carboxylic

groups and ester groups due to the alkali treatment and sodium carboxylate

may be formed which decreases the intermolecular hydrogen bonds and

solubility of pectins. The peak at 1621cm-1 indicates the presence of lignin

by C=C vibration (28). The bands in the region 1250-1050cm-1 involve the

C-O stretching of primary and secondary alcohols in cellulose, hemicellulose

and lignin (29). The peak at 1430cm-1 is due to lignin components (30). The

intensity is decreased for micro and nano fibrils due the dissolution of

hemicellulose and lignin. The 1050cm-1 peak is assigned to the ether linkage

(C-O-C) in lignin and hemicellulose. A peak at 1430cm-1 is seen for


145

microfibril showing that the removal of lignin is partial. The peak area is

decreased for nanofibrils. The narrowing of the peak at 3421cm-1 is due to

the formation of free hydroxyl groups by breaking up of hydrogen bonds.

The sharpening of the peak at 2921cm-1 reveals the increase of crystallinity

and thereby the increase of cellulose in the fibres (9). Table 3.3 also gives the

assignment of IR absorption peaks of macro, micro and nano fibres.

4000 3500 3000 2500 2000 1500 1000 500

0

50

321

10501370

1029

1621

1430

2911 1247

1750

3420

Tra

nsm

ittan

ce %

Wave number (cm -1)

1 Macro fibre2 Micro fibre3 Nano fibre

Figure 3.7 FTIR spectra of macro, micro and nano fibres

Table 3.3 Assignment of IR absorption peaks of macro, micro and nano fibres

Material ―OH

stretching (cm-1)

C―H stretching

(cm-1)

C = O stretching

(cm-1)

Absorbed water (cm-1)

C―H Stretching

(cm-1)

Aromatic ring

vibration of lignin

(cm-1)

C―O stretching

(cm-1)

Macro fibre 3420 2911 1750 1621 1370 1247 1050

Micro fibre 3429 2922 - 1627 1370 - 1050

Nano fibre 3430 2923 - - 1328 - 1050

146 Chapter 3

Solvatochromic measurements

Solvatochromic methods have been proved to be effective in characterising

lignocellulosic fibres. The correspondence of the empirical polarity

parameters determined has been discussed in relation to results from zeta

potential measurements and FTIR measurements.

400 500 600 7000.0

0.1

3

2

1

Ab

sorp

tion

(a.

u)

Wavelength (nm)

1.Macro fibre2.Micro fibre3.Nano fibre

Figure 3.8 UV/vis absorption spectra of furan dye loaded banana fibre (1) macro fibre (2) micro fibre and (3) nano fibre

Table 3.4 UV/Vis absorption maxima and values of the Kamlet-Taft polarity parameters for the three probe dyes used on the banana macro, micro and nano fibres

Samples ννννmax (1)

(10-3cm-1)

ννννmax (2)

(10-3cm-1)

ννννmax (3)

(10-3cm-1) α β ππππ* AN ET (30)

Macro fibre 19.65 26.6 19.2 1.54 0.47 0.46 60.7 59.8

Micro fibre 20.2 26.5 18.0 1.62 0.51 0.34 69.9 63.76

Nano fibre 20.6 27.1 18.6 2.0 0.57 0.27 74.2 64.42


147

Fig 3.8 shows the UV/vis absorption spectra of furan dye loaded cellulose

fibres. The spectra show a definite change in the absorption peaks. The

hydrogen bond donating acidity and basicity of the micro and nanofibres

were determined using solvatochromic measurements using different probe

dyes. The α value (Table 3.4), which shows the surface acidity, has been

found to increase with reduction in fibre diameter. Chemical treatments

carried out on the macro fibre to reduce the fibre diameter, dissolve out

hemicelluloses and lignin, making available more hydroxyl groups on the

surface. In addition, it exposes the different acidic groups associated with the

natural fibre on the fibre surface. This gives rise to increased α value in the

case of nanofibres compared to the microfibres. The probe dyes used in

solvatochromic measurements detect those acidic hydrogen atoms and hence

the increased acidity in the case of nano fibres compared to microfibres. The

π* term for specific interaction in the case of cellulose fibre batches have

been reported by other researchers (31) because the HBD attack upon one of

the two lone pairs of the dimethyl amino group of the probe dye can take

place. In the present case also the nanofibres have the highest acidity value

compared to the untreated fibres and microfibrils. The interaction of the lone

pairs of electrons on the highly acidic sites of the nanofibres can be attributed

to the lower π* value in the case of the fibres. The overall polarity of the

environment given by ET (30) is also found to increase when the fibre

diameter is decreased. This can very well be explained based on the surface

groups which become exposed when the fibres are subjected to alkali

treatment and steam explosion.

148 Chapter 3

Zeta potential measurements

Zeta potential measurements were carried out to investigate the surface

properties and the possible interactions. Nature of the surface of banana

fibres and banana fibril can be understood through the studies of the

influence of pH on zeta potential. The pH that agrees with the zero of the

zeta potential (Iso electric point, IEP) goes to decide the acidity or the

basicity of the solid surfaces qualitatively. Thus at this pH the number of

negative charges equals the number of positive ones (37). It is the IEP that

characterises the acidity of the surface. When the IEP values are low there is

dominance for the number of acidic groups. Banana macro fibre holds the

IEP value 2.5, microfibre 2.1 and that of nanofibre is 1.6, thus indicating an

acidic surface (Fig 3.9). The results obtained from zeta potential

measurements are consistent with the solvatochromic measurement values.

The presence of carboxyl and OH groups which go to charge the natural

cellulose fibres –vely attribute to this acidic surface. Fig 3.9 portrays the pH

dependence on the zeta potential of macro, micro and nano cellulose fibres.

Macro fibre shows a zetapotential of -7.8mV, microfibre -21.3 mV and

nanofibre -27.5 mV. The chemical constitutions, polarity of the fibre surface,

porosity of the fibre and swelling behaviour in water happen to be the factors

for reckoning the electro kinetic potential of fibres if the liquid phase stands

constant. As per the findings of Kanamaru (33), the zeta potential of any

fibre comes down if there is more adsorption of water. The inner surface of

fibres get expanded due to the swelling processes. While the electrochemical

double layer is anticipated to shift in the swelling layer, the slipping plain is

seen to migrate towards the bulk electrolyte. There is a decrease of zeta

potential with increasing swelling time due to the potential drop in the


149

electrochemical double layer. But the layer is influenced by the adsorption of

electrolyte ions. During the swelling process the amount of adsorbed

potential determining ions is seen to come down because of the competitive

adsorption of water. Correspondingly the zeta potential also records a

simultaneous decrease. The decrease in IEP value of the nanofibril depicts an

increase in surface acidity which leads to better adhesion properties with

resin matrix during composite formation.

1 2 3 4 5 6 7 8 9 10 11 12

-30

-25

-20

-15

-10

-5

0

5

Zet

a po

tent

ial (

mV

)

pH-Wert (gemessen in 10-3M KCl)

macro fibre micro fibre nano fibre

Figure 3.9 pH dependence of zeta potential of macro, micro and nano

cellulose fibres

Thermal properties

The natural fibres present three main weight loss regions (Fig 3.10). The

initial weight loss in the region 50–100oC is mainly due to moisture

evaporation. The temperature region ranging from 220-300oC is mainly

attributed to thermal depolymerisation of hemicellulose and the cleavage of

glycosidic linkages of cellulose (34). The degradation of cellulose take place

150 Chapter 3

between 275 and 400oC (35). The TGA and DTG curves of banana macro,

micro and nanofibrils are illustrated in Fig 3.10 and Fig 3.11 respectively.

The thermal degradation of all samples takes place between 275-480oC. The

fibres show a very small weight loss below 100oC as a result of evaporation

of moisture. Between 230-350oC the main degradation occurs. In the case of

macro fibre dehydration and degradation of lignin, hemicellulose and

cellulose occurs between 263 and 280oC. About 70% of degradation

occurred in this temperature range. At 263oC hemicellulose undergoes

degradation (36). The DTG curve of macro banana fibre shows a peak at

347oC (mass loss 51%) which is due to the thermal decomposition of α-

cellulose (37). The weight of the charred residue left was about 2.2%. During

the formation of microfibrils, hemicellulose, lignin, and pectin get dissolved

out partially in alkali and results in a fibrillated structure. The increase in the

% crystallinity index of microfibrils and nanofibrils reduces the moisture

absorption. The DTG curve of banana microfibril (Fig 3.11) exhibits two

peaks. The initial shoulder peak at about 60oC corresponds to a mass loss of

absorbed moisture and the major decomposition peak at about 356oC (mass

loss 51%) is attributed to α-cellulose decomposition (36,38). The differential

curve of microfibrils (Fig 3.11) shows a slight increase in the degradation

temperature (268oC) of hemicellulose which indicates the presence of trace

quantity of the hemicellulose. Fig 3.11 also represents the DTG curve of

banana nanofibrils in which we can see a major decomposition peak at 385oC

(mass loss 52%) due to α- cellulose decomposition. The main degradation

temperature gets shifted towards a higher temperature region. The shift has

been found to be higher for nanofibrils. From the above Figures it is clear

that there is a shift in the major decomposition temperature from 347oC to

385oC as we go from macro fibre to nanofibrils. About 85% decomposition


151

occurred at 480oC. Dissolution of the various components leaves α-cellulose

as the residual material which has been reported to be crystalline (39). A

greater crystalline structure required a higher degradation temperature. The

increase in the degradation temperature in the nanofibrils occurs due to the

high crystallinity of the fibre structure. Therefore it can be concluded from

these results that the developed nanofibres exhibits enhanced thermal

properties compared to the macro fibre and micro fibre so that it can act as a

suitable reinforcing element in biocomposite preparation.

0 100 200 300 400 500 6000

20

40

60

80

100

nano fibremicro fibre

macro fibre

Wei

ght (

%)

Temperature (οC)

Figure 3.10 Thermograms of macro, micro and nano cellulose fibres

152 Chapter 3

0 100 200 300 400 500 600

1.0

0.5

0.0

micro fibre

nano fibre

macro fibre

DT

G (

%/ o

C)

Temperature (oC)

Figure 3.11: DTG curves of macro, micro and nano cellulose fibres

Table 3.5 shows the decomposition temperatures as well as the residual mass

of the banana fibres. From the Table, it is clear that the banana fibres become

more hydrophilic when converted to micro and nanofibrils. This is due to

the increase in fibre fineness, surface area and the increase in cellulosic

components obtained as a result of steam explosion followed by bleaching

which facilitates moisture evaporation at a higher temperature. It can be seen

from the Table 3.5, that the degradation temperature of macro fibre is lower

compared to that of micro and nano fibre. This is because in the raw banana

fibre cellulose is organized into fibrils, which are surrounded by a matrix of

lignin, hemicelluloses and pectins. Hemicelluloses are intimately integrated

into the structure of the cellulose, and located within and between the

cellulose fibrils. This strong association between the hemicellulose and

cellulose fibrils is believed to decrease the average crystallinity of the

cellulose fibrils (40). These impurities may initiate more active sites and

accelerate the beginning of thermal degradation. The fibre residue remaining


153

after heating to 600oC in both micro and nano cellulosic fibres indicates the

presence of the carbonaceous materials in the banana fibre. Results show that

at 600oC, the highest residue was obtained for macro banana fibres and the

lesser residue was obtained for nano fibrils (1.8%). The relatively low

amount of residue in nanofibrils may be due to the removal of hemicelluloses

and lignin from the fibres. These results are very consistent with results

obtained from the chemical estimation, SPM and FTIR measurements.

Table 3.5 Residual mass and decomposition temperatures of macro, micro and nano banana fibres

Fibre Initial decomposition temperature (oC) Final decomposition

temperature (oC) Residue mass

(wt%)

Macro 250 337 2.2

Micro 290 356 2.0

Nano 320 385 1.8

3.3 Conclusion

Cellulose micro and nano fibrils of banana fibres were isolated using high

pressure hydrothermal process. Characterisation of the synthesised micro and

nanofibrils were done using AFM, TEM, SEM, ESEM, XRD, optical

microscopy, solvatocromism, electrokinetic studies and TGA. The chemical

composition of macro,micro and nano fibres were determined using ASTM

standards. From the chemical examination, major constituents of these fibres

were found to be cellulose. The percentage of cellulose components were

found to be increased during steam explosion and acid hydrolysis. The lignin

and hemicellulose components were found to be decreased from macro to the

nano fibres. The IR studies give evidence for the dissolution and chemical

154 Chapter 3

modification that occurred during steam explosion and further treatment of

the fibres for steam explosion in acidic medium.

The morphological structures of the macro to nano fibres were compared.

The observed fibre diameter of macro to nano fibres were respectively 80µm,

10µm and 50nm. XRD studies were done to investigate the fibre size and

percentage crystallinity of the modified fibres. The XRD studies also

revealed that there is a reduction in the size of fibres during steam explosion

in alkaline medium and reduction in size to the nanometer range during

repeated steam explosion in acidic medium. The percentage crystallinity of

the fibres was also found to increase from steam exploded fibres to repeated

steam explosion in acidic conditions. It was observed that the nano fibres

show a highly crystalline structure. The crystallinity was found to be

increased from micro to nano structure. This higher crystallinity was due to

the more efficient removal of noncellulosic polysaccharides and dissolution

of amorphous zones by acid hydrolysis combined with steam explosion. The

SPM analysis also showed that there was reduction in the size of banana

fibres to the nanometer range (below 40 nm). The TEM analysis also

supported the evidence for the formation of nanofibrils of banana fibres by

repeated steam explosion in acidic conditions. The average length and

diameter of the developed nanofibrils were found to be between 200-250nm

and 4-5nm respectively. Overall polarity (ET30) of the fibres was found to

increase when the fibre diameter was decreased. The surface acidity was

proved to be increased from macro to nano scale as the IEP value decreased.

The thermal stability of the nanofibres were found to be much higher than

that of the micro fibres. The increased thermal stability can be attributed to

the dissolution of less thermally stable components from the fibre surface


155

when subjected to alkali and steam treatments. Substantial increase in

thermal stability was observed from macro to nano fibres which proved the

high thermal stability of nanofibres to processing conditions of biocomposite

preparation. The results of solvatochromism and zetapotential corroborate

FTIR results.

156 Chapter 3

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