Morphology and Rheological Behavior of PolylacticAcid/Clay Nanocomposites

Satpal Singh,1 Anup K. Ghosh,1 Saurindra N. Maiti,1 Sumanta Raha,2 Rahul K. Gupta,2

Satinath Bhattacharya21 Centre for Polymer Science & Engineering, Indian Institute of Technology, Delhi, New Delhi, India

2 Department of Chemical and Metallurgical Engineering, Rheology and Materials Processing Centre,RMIT University, Melbourne, Vic., Australia

The investigated rheological properties of polylacticacid (PLA)/clay nanocomposite are important to under-stand the effect of organically modified layered sili-cates (OMLS) (clay) on processing as well as thechange in viscoelastic properties due to polymer fillerinteraction. The time sweep result revealed that thethermal stability improved with addition of nanoclaydue to the formation of percolating network structure.It was also supported by multi wave ramp test. The fre-quency sweep analysis showed that the dynamic mod-uli increased with addition of nanoclay. Viscoelasticspectra (DMTA) showed an increase of the storage andloss moduli with the increase in the clay content. Wideangle X-ray diffraction (WAXD) and transmission elec-tron microscopy (TEM) were used to determinethe degree of intercalation, or exfoliation and nano-structure level of clay dispersion on PLA nanocompo-sites. XRD data demonstrated complete exfoliation atlower nanoclay content. On increasing the nanoclaycontent, exfoliated and partially intercalated structureswere obtained. POLYM. ENG. SCI., 52:225–232, 2012. ª 2011Society of Plastics Engineers

INTRODUCTION

The need to develop new polymeric system based on

renewable resources results from the environmental and

economic concerns of today. These renewable resources

based system have been playing an important role as mate-

rials for humanity through their exploitation in a progres-

sively more elaborated fashion. Poly(lactic acid) (PLA) is

at present one of the most promising renewable resources

based polymer. PLA has various other advantages also

including (i) its excellent mechanical properties and ther-

moplastic processibility for different molding application,

(ii) its production consumes quantities of carbon dioxides,

(iii) it provides significant energy savings, (iv) it is recycla-

ble and compostable, and (v) when burned it produces no

nitrogen oxide gases and only one-third of the combustible

heat generated by polyolefins [1–4]. Unfortunately, some

properties, such as limited thermal stability during the melt

processing, heat deflection temperature, gas barrier prop-

erty, and melt viscosity of PLA, are not good enough for a

wide range of application [5]. To improve these properties,

the preparation of nanocomposites with clay such as organ-

ically modified layered silicates (OMLS) has been adopted

[6]. The method has already proven to be effective for

PLA/layered silicates preparation [7–9] and developing

structures [8–11], as well as rheology [6, 12–15] and ther-

mal properties [10, 11, 16–18].

PLA is commercially and largely available in a wide

range of grades. So for effective processing, fundamental

understanding of its rheological properties must be

obtained. On the other hand, the rheology offers a mean

to assess the state of dispersion of OMLS into PLA

matrix, the degree of polymer filler interaction, the struc-

ture–properties relationship, and effective change of its

viscoelastic properties. Dispersion of OMLS in the PLA

matrix leads to intercalation and exfoliation of organo-

clay. Stress relaxation tests which apply an instantaneous

deformation (strain) to the sample and record the time

dependent decay of stress offer effective measure to

understand the same. The rate of decay of the stress

depends on the viscoelasticity of the polymer at the test

temperature. This is useful to interpret the viscoelastic

behavior observed at low frequencies region in the

dynamic measurements for the PLA nanocomposites.

Rheology of nanocomposites is of interest for several

reasons. The morphology in a polymer clay nanocompo-

site is typically intercalated or exfoliated. An exfoliated

structure is desirable as it tends to produce excellent ther-

mal and mechanical properties at very low filler levels.

The viscosity is sensitive to the structure and so can be

used to evaluate the morphology. The proper characteriza-

tion of a polymer’s viscosity is also critical to making the

Correspondence to: A.K. Ghosh; e-mail: anupkghosh@gmail.com

DOI 10.1002/pen.22074

Published online in Wiley Online Library (wileyonlinelibrary.com).

VVC 2011 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—-2012

correct choice of grade, so that a high quality product can

be manufactured successfully.

A wide range of tests are generally carried out to char-

acterize the rheological properties of polymers. One suited

to nanocomposites is dynamic oscillatory shear. This

measures elastic and viscous properties simultaneously.

Oscillatory tests are run at small amplitudes at various

frequencies in the linear elastic range of the material so

that any networks are not disrupted. This is important for

materials such as nanocomposites, which contain stiff ani-

sometric clay particles in the matrix. The low frequency

range is of particular interest as it is most sensitive to

melt elasticity and to the network formation. Storage

modulus (G0) and loss modulus (G00) are reported, which

represent the relative degrees of the material to recover

(elastic response) or flow (viscous response) respectively,

as the rate of deformation (frequency) changes. Changes

in the ratios of the moduli (tan d) can be used to evaluate

the extent of delamination of the organoclay tactoids (tac-

toid size effects the physical and mechanical properties of

nanocomposites). The onset of solid-like behavior can be

used to determine the geometrical percolation of the

nanoparticles as well as to predict the optimum concentra-

tion for mechanical and physical properties.

The importance of extensional rheological measure-

ment is to understand extensional flows of molten poly-

mer. Solidification of polymer melts in industrial opera-

tions, such as film blowing involving stretching both in

machine and transverse directions have significant effects

on the properties of the final products. Liviu et al.

reported the high L-content PLA exhibit considerable

strain hardening during the extensional rheology. The

strain induced hardening was also observed in PLA nano-

composite system [14, 19]. The strain hardening generally

results due to the presence of a high molecular weight tail

or due to the perpendicular alignment of the OMLS layers

towards the stretching direction. Thus, the extensional

rheological data become very critical for processing oper-

ations like film blowing, film casting, and fiber spinning.

In this article, PLA nanocomposites were prepared by

using the melt blending technique. The detailed melt

rheological studies of a series of PLA nanocomposites are

discussed in terms of dynamic sweep (strain, frequency,

and time), steady shear viscosity, stress relaxation, and

extensional viscosity. The observation from rheology has

been corrected with the interpretation of XRD and TEM.

EXPERIMENTAL

Materials

Poly(L-lactic acid), PLA (Nature works1 4032D), was

procured from Cargill/Dow LLC (Minnetonka, MN).

It contains more than 98% of L-lactide and less than 2%

D-lactide. Organically treated montmorillonite layered sili-

cate, Cloisite1 30B containing methyl-bis (2-hydroxy-

ethyl) tallow alkyl ammonium cations was obtained from

Southern Clay Ltd (TX). The interlayer distance (d-spac-ing) of the nanoclay was 1.84 nm.

Compounding and Film Blowing of PLA/Nanoclay

PLA and Cloisite1 30B were dried at 808C for 4 h under

reduced pressure. PLA and Cloisite1 30B were com-

pounded on a Prism EUROLAB-16 co-rotating, completely

intermeshing, twin screw extruder (16 mm diameter and an

L/D ratio of 40:1), with an attached strand die. Melt process-

ing of polylactic acid and nanoclay containing 1 phr

(PNC1), 3 phr (PNC3), and 5 phr (PNC5) of nanoclay was

carried out at the screw rotation speed of 100 rpm. The barrel

temperature of the extruder ranged from 180 to 2008C from

the feed to the die zone. Unfilled PLA (reference material)

was also melt-processed under identical shear and thermal

condition. The PLA and PLA nanocomposites were dried

for 4 h at 808C and then film blown using a single-screw ex-

truder (Thermo-Haake Rheomex 252) equipped with a spiral

flow blown film die with a 25 mm diameter and blown film

take-off (‘‘Postex’’) unit. The temperature of the heating and

die zone was 2008C and the screw speed was 60 rpm. The

take-off speed of 700 cm/min was used.

Characterization

Wide angle X-ray diffraction (WAXD) was used to

determine the degree of intercalation, or exfoliation and

nanostructure level of nanocomposites. Blown film sam-

ples were investigated in the transmission mode (coupled

y/2y) to check the effect of the nanoparticle orientation.

A wide angle goniometer was coupled to a sealed-tube

source of filtered Cu Ka radiation operating at 40 kV and

30 mA (PW 3040/60 X’ Pert PRO).

The dispersion of the silicate layers was also investi-

gated by transmission electron microscopy (TEM). For

TEM samples of PLA nanocomposite pellets, prepared by

freshly cut glass knives with cutting edge of 458, were usedto get the cryosections of 50 nm thickness by using a Leica

ultra cut UCT microtome. JEOL-2100 electron microscope

(Tokyo, Japan) with LaB6 filament and operating at an

accelerating voltage of 200 kV was used to obtain the

bright field images of the cryomicrotomed samples.

Melt rheological measurements were studied with an

advanced research grade rheometer (ARES; Rheometric

scientific). The measurements were performed with a

parallel plate geometry using 25 mm diameter plates and

the sample thickness was �1.75 mm. The compression

molded samples were investigated at temperature 1808C.The following tests were performed: (i) dynamic strain

sweep to assess the limits of linear viscoelastic property

at 1 rad/sec; (ii) dynamic frequency time sweep to

assess the improvement in the stability of sample during

testing, with addition of clay content; (iii) dynamic fre-

quency sweep over an angular frequency range starting

from high 100 rad/sec down to 0.1 rad/sec; and (iv)

226 POLYMER ENGINEERING AND SCIENCE—-2012 DOI 10.1002/pen

multi wave temperature ramp test: In this test at a time

three different frequency (high, medium and low) data

can be obtained, with increasing temperature and time.

During this test set, the temperature change was negligi-

ble (only 0.58C) in 1 h time.

Steady shear viscosity measurements were conducted

by using Parallel plate (Low steady shear viscosity) and

Malvern Capillary Rheometer-RH7 (High steady shear vis-

cosity). Uniaxial elongational measurements were con-

ducted at a constant Hencky strain rate by using ARES-

EVF (Extensional Viscosity Fixture) rheometer. Tests were

performed at temperatures of 1808C at an extension rate of

e ¼ 0.05, 0.1, and 0.5 sec21 using rectangular shaped sam-

ples. At every extension rate three tests were carried out to

ensure the reproducibility of the experiments.

Dynamic mechanical properties were measured with a

TA instrument, DMA Q 800, in a single cantilever bend-

ing mode. The dynamic storage and loss moduli were

determined at a constant frequency (x) of 1 Hz or as a

function of temperature from 2908C to 1508C at a heat-

ing rate of 2 8C/min.

RESULTS AND DISCUSSION

XRD Characterization

WAXD pattern (see Fig. 1) illustrates the differences in

layer spacing of the nanocomposites. The diffractogram of

unfilled PLA film and Closite 30B powder were also

shown as a comparison. The diffractogram for Closite 30B

showed the primary silicate reflection at 2y ¼ 4.838 whichcorresponds to a layer spacing of 1.83 nm. This maximum

was not observed for the PLA nanocomposites. The

unfilled PLA film did not show any reflection peak (below

2y ¼ 108). The diffraction patterns of the nanocomposites

PNC1, PNC3, and PNC5 showed shifting of the character-

istic peak towards lower 2y value of 2.58. This feature is

characteristic of a good dispersion of the organoclay,

achieved by an intercalation followed by tactoids forma-

tion and then exfoliation of the nano platelets in the PLA

matrix [20]. This indicates that polymer chains diffused

into the silicate galleries expanding the clay interlayer

spacing. As reported in literature, Cloisite 30B organoclay

has favorable enthalpic interaction between diols present in

the organic modifier with the C¼¼O bonds present in the

PLA backbone, which is expected to play a significant role

in observing exfoliated morphology [21].

The nanocomposite film containing 1 phr clay did not

show any primary silicate reflection peak but the intensity

of background scattering in between 2y ¼ 2–78 was

increased compared to that in case of unfilled PLA film.

It indicates that at lower nanoclay content, the clay layers

can disperse better and as a result more PLA chains can

enter into the clay galleries. This is typically the case for

nanocomposites having lower clay content which is re-

sponsible for the exfoliation mechanism.

The diffractograms of PNC3 and PNC5 showed only

very small bulge around 2y ¼ 2.58 (d001 ¼ 3.5 nm). The

PNC5 diffractogram showed that the peak sharpness was

higher than that of the PNC3. The intensity of the charac-

teristic peak of the clay increases as the clay content

becomes higher. This was because the number density of

the intercalated clay particles in the polymer matrix

increases, as the clay content rises, and many more X-

rays diffracted [22]. Increase of the clay loading impedes

exfoliation of clay layers due to the restricted area

remaining available in the polymer matrix and hence, the

degree of intercalation enhances [7].

TEM Characterization

The TEM images (Fig. 2a–c) corresponding to the

samples corroborates the XRD measurements. In all these,

the exfoliated structure was found to be present in PNC1,

whereas for both PNC3 and PNC5 exfoliated and partially

intercalated structure were mostly observed.

Shear Rheology

Dynamic strain sweep was used to determine the linear

viscoelastic region (LVR) of pure PLA and its nanocom-

posites. The LVR was found to decrease with the addition

of nanoclay. The pure PLA showed a linear region up to

50% strain applied at 1808C, whereas it decreased to

about 10% strain for PNC5 with 5 phr clay loading (see

Fig. 3). Hence the strain amplitude was taken as 5% for

all subsequent linear viscoelastic tests [20].

The thermal stability of the melt samples during the

dynamic measurements, dynamic frequency time sweep

tests were employed to look for any changes with time in

their storage modulus (G0) over time at a constant fre-

quency (1 rad/sec). As it can be seen in Fig. 4, all melt

samples had nearly constant G0 up to 100 sec and thus

thermal stability for up to that time [20].

Figure 5a and b show the storage modulus (G0) and

loss modulus (G00) as a function of frequency for PLA

nanocomposite samples. The G0 and G00 of PNCs were

higher than those of pure PLA at all frequencies and their

FIG. 1. WAXD patterns of PLA nanocomposites.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2012 227

values increased with increasing clay content especially at

low frequencies (\10 rad/sec). In the case of homopoly-

mer samples, it is expected that they would exhibit the

characteristic low-frequency terminal behavior expressed

by the power laws G0 ! x2 and G00 ! x, as was seen

for pure PLA (see Fig. 5). The PLA nanocomposite sam-

ples, however, showed a gradual deviation from the termi-

nal liquid-like behavior to a solid-like behavior with

increasing clay concentration, as evidenced by the

decrease in the slope of low frequency G0 and G00 with

respect to frequency, plotted on log–log scale. For PNC

5, the solid-like behavior was most pronounced. PNC5

FIG. 2. TEM images of various PLA nanocomposites (a) PNC1, (b)

PNC3, and (c) PNC5.

FIG. 3. Dynamic strain sweep of PLA nanocomposites.

FIG. 4. Dynamic time sweep of PLA nanocomposites.

FIG. 5. (a) G0 and (b) G00 of PLA nanocomposites.

228 POLYMER ENGINEERING AND SCIENCE—-2012 DOI 10.1002/pen

also showed largest increase in both G0 and G00 at low

frequencies. At low frequency (�0.1 rad/sec), G0

exceeded G00 showing a plateau which is generally

‘‘Pseudo-solid-like behavior’’ of the materials at low

deformation frequencies [23, 24].

Figure 6 shows the frequency dependency of the com-

plex viscosity (g*) for PLA nanocomposite sample of

PLA. The g* of the PNC0 showed a Newtonian plateau

at low and mid frequency ranges followed by shear-thin-

ning behavior at the highest frequencies. The Newtonian

plateau viscosity was also observed for the nanocomposite

with the lowest loading, PNC1. However, at higher clay

loadings such as for PNC3 and PNC5, no Newtonian pla-

teau could be observed within the frequency range and

the viscosity continued to increase with decreasing fre-

quency indicating a pseudo-yield-stress behavior at low

frequencies. Such pseudo-yield-stress behavior was most

prominent for the nanocomposite with the highest clay

loading, PNC5.

The stress relaxation data of PLA nanocomposites are

shown in Fig. 7. The slopes of G(t) curves decreased with

increase of clay content, which indicated that the relaxa-

tion time increased with the increase in clay content [6].

The stress relaxation data of PNC5 showed a solid-like

behavior. This behavior is due to the presence of interca-

lated silicate layers that are randomly oriented in PLA

matrix, forming a three dimension network structure.

Another explanation could be due to the physical jam-

ming of the dispersed intercalated silicate layers owing to

their highly anisotropic nature. On the basis of this meso-

scopic structure and at low silicate loadings beyond a crit-

ical volume fraction, the tactoids and the individual layers

were incapable of freely rotating and when subjected to

shear they were prevented from relaxing completely. This

incomplete relaxation due to the physical jamming or per-

colation of the Closite 30B lead to the pseudo-solid-like

behavior observed in both the intercalated and exfoliated

hybrids [25].

The multi wave ramp data, presented in Fig. 8, show

the time dependencies of the storage modulus at three dif-

ferent frequencies which were measured simultaneously.

Figure 8 shows that while the moduli decreased with time

for all the samples, the slopes of G0 curves decreased with

increasing the clay content. Only PNC5 showed the

decrease in G0 with time at high frequency (100 rad/sec),

G0 was constant with time at medium frequency (10 rad/

sec) and G0 was slightly improved with time at low fre-

quency (1 rad/sec). This has been brought out by confirm-

ing the data from nanocomposites samples at 1 rad/sec as

shown in the inset in Fig. 8. It is well known that inter-

connected structures with anisometric fillers result in an

apparent yield stress which is visible in dynamic measure-

ment by a plateau of G0 or G00 versus frequency at low

frequencies. This effect is more pronounced in G0 than G00

[15, 26]. Therefore, an increase in the moduli and viscos-

ity, at low frequency range, reflects an interconnected

structure and reinforcement of the molten PLA by cloisite

30B. The reinforcement effect results from the interac-

tions between the components due to hydrogen bonding

of hydroxyl groups in the organic ‘‘surfactant’’ in the

organoclay and carbonyl groups of PLA chain segments.

The interactions are stronger for system having the larger

interface area; thus being related to higher dispersion of

the organoclay in the PLA matrix [15, 23].

Extensional Rheology

The extensional viscosity for pure PLA and its nano-

composites at different extensional strain rates (0.05, 0.1,

and 0.5 sec21) at 1808C are presented in Fig. 9. The

FIG. 6. Complex viscosity (g*) of PLA nanocomposites.

FIG. 7. Stress relaxation data of PLA nanocomposites.

FIG. 8. Multi wave temperature ramp test of PLA nanocomposites at

different frequency.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2012 229

results for pure PLA showed no strong strain hardening

characteristics very similar to PNC1, etc., nanocompo-

sites. Similar results were shown by Pasanovic-Zujo et al.

and Gupta et al. While working with different EVA nano-

composites [27, 28].

The extensional viscosity, gE(t), in the linear viscoelastic

range can be determined using the following equation [29].

ZEðtÞ ¼ 3Xi

gi ti 1� exp�t

ti

� �� �(1)

where ti is the characteristic relaxation time (sec) and gi isthe relaxation modulus (Pa).

As reported earlier [30] for PP/clay nanocomposites, the

Trouton ratio, (TR), defined as the ratio of uniaxial exten-

sional viscosity to shear viscosity was higher than 3, which

is typical for Newtonian fluids. In polyisobutylene systems

filled with alumina powder (diameter \1 lm), Trouton’s

law was shown not to be obeyed for higher filler content in

uniaxial flow [28]. In this study, PLA-nanocomposites with

high clay content also showed a similar response.

Figure 9a–d shows the transient extensional viscosity

as a function of time for the PLA nanocomposites.

McKinley and Hassager suggested that beyond the maxi-

mum Hencky strain, the elongating polymeric strip

becomes unstable due to free surface perturbations, which

grow and result in necking of the sample followed by

complete rupture [31]. It can be seen in Fig. 9 that the

elongational viscosity did not change with increasing the

strain rate, only samples ruptured earlier at faster rates.

Figure 10 shows that the extensional viscosity increased

with increasing clay loading at two different strain rate. The

extensional viscosity of PLA nanocomposite up to PNC3

showed small strain hardening behavior before the rupture.

But strain hardening phenomena was slightly decreased for

PNC5. This phenomenon could be explained by the forma-

tion of exfoliated and intercalated structure of PLA nano-

composites. XRD and TEM results showed the PNC1 with

the exfoliated structure. PNC3 showed exfoliated and par-

tially intercalated structure and PNC5 showed intercalated

structure. Intercalated structure or aggregated parts act as

weak points, so before the strain hardening behavior was

observed, samples were broken [31].

FIG. 9. Transient extensional viscosity at various extension rates (a) PNC0, (b) PNC1, (c) PNC3, and

(d) PNC5.

FIG. 10. Transient extensional viscosity of PLA nanocomposites at the

strain rate of (a) 0.05 sec21 and (b) 0.5 sec21.

230 POLYMER ENGINEERING AND SCIENCE—-2012 DOI 10.1002/pen

Dynamic-Mechanical and Thermal Analysis

Dynamic-mechanical and thermal analysis (DMTA)

studies were performed on solid nanocomposite samples

to see the effect of temperature on their storage modulus,

loss modulus, and tan d. For all the samples the following

characteristics were observed for the storage modulus

with rising temperature (Fig. 11a): a gradual decrease in

the region—908C to 508C, a rapid drop below 60–708Cdue to the glass transition temperature (Tg), and an

increase in the cold crystallization range (around 1008C).With increased nanoclay loading, an increase in the stor-

age modulus in the whole temperature range was

observed. The loss modulus (Fig. 11b) changed with

increasing temperature showing a maximum at about

658C, which corresponded to segmental relaxations

related to glass transition of polylactic acid. The maxima

of the loss modulus around 1008C reflected an increase in

the mechanical loss due to the cold crystallization. How-

ever, additions of nanoclay particle in PLA matrix did not

show significant shift and broadening on the maxima of

the tan d curves.

CONCLUSIONS

This article presents a comprehensive melt rheologi-

cal study of PLA nanocomposite. The frequency sweep

data showed a monotonic increase in storage moduli,

loss moduli, and dynamic viscosities with nanoclay

content. In the case of PNC5, a pseudo-solid-like

behavior was observed. The PNC5 multi wave ramp

test showed that at low frequency (1 rad/sec) G0 was

slightly improved with time due to formation of perco-

lating network structure. The transient extensional vis-

cosity increased with increase in the clay content.

Viscoelastic spectra (DMTA) showed an increase of

the storage and loss moduli with the increase of the

organoclay content and improved dispersion. WAXD

and TEM results indicated that PNC1 showed the

exfoliated structure, PNC3 showed exfoliated and par-

tially intercalated structure, and PNC5 showed exfoli-

ated and intercalated structure.

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