structure and dynamics of polyethylene/clay films

Structure and Dynamics of Polyethylene/Clay Films

F. P. La Mantia,1 N. T. Dintcheva,1 G. Filippone,2 D. Acierno2

1Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universita di Palermo, Viale delle Scienze,90128 Palermo, Italy2Dipartimento di Ingegneria dei Materiali e della Produzione, Universita di Napoli, Federico II,Piazzale Tecchio 80, 80125 Napoli, Italy

Received 4 April 2006; accepted 25 June 2006DOI 10.1002/app.25009Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: The relationships between structure and rhe-ology of polyethylene/clay hybrid composite blown filmswere investigated through rheological tests both in shear andelongational flow. Two polymer matrices (low density poly-ethylene, LDPE and linear low density polyethylene, LLDPE)with different relaxation kinetics were used. Independentlyfrom the matrix, morphological analyses (TEM, XRD, andSEM) indicate that the hybrid structures are similarly consti-tuted of delaminated platelets or tactoids having a relevantdegree of orientation along the draw direction. This stronglyaffects the rheological behavior of materials. However, de-spite the similarities emerged from morphological analyses,both shear (steady shear and oscillatory) and elongationmeasurements show a negligible effect upon the rheology ofLDPE-based nanohybrids. Conversely, relevant increasesof shear viscosity, dynamic moduli and melt strength ofLLDPE-based nanohybrids have been detected. The effects

of homopolymer relaxation kinetics have been investigatedby means of stress relaxation tests. The results obtained seemto be consistent with the existence of a roughly bimodal pop-ulation of dynamical species: a matrix component behavinglike the homopolymer, and a fraction interacting with the fil-ler, whose rheological behavior is controlled by the particlesand their interactions with the polymer. Mechanical proper-ties of hybrid films were also investigated. Differently fromwhat happens in the melt state, the solid-state propertiesmainly depend on the filler amount. The relative increases oftensile modulus and melt strength are of the same order ofmagnitude for both the matrices used, indirectly confirmingthe similarities in hybrids structures. � 2006 Wiley Periodicals,Inc. J Appl Polym Sci 102: 4749–4758, 2006

Key words: structure-rheology relationships; films; poly-ethylene (PE); rheology; nanolayers

INTRODUCTION

Polyethylene (PE) films are used for flexible foodpackaging, garbage bags, liners, and agricultural films.Improvement in macroscopic polyethylene film prop-erties, such as enhancement of mechanical and barrierproperties and reductions in thermal expansion coeffi-cient, can be obtained by adding small quantity oforganoclay.1,2 In recent years, most studies haveshown that organically modified clays can be well dis-persed in polar polymers like polyamides usingappropriate processing techniques and conditions.3,4

Conversely, for the more commonly used polyolefins,like polyethylene and polypropylene, the obtaining ofnanocomposites characterized by an enhancement oftheir final properties appears much more difficult,because of the lack of suitable interactions of hydro-phobic polyolefins with the polar surface of theclays.2,5 The fillers should not affect the film clarity,dispersion and orientation of filler and the process-

ability of materials. Therefore, the study of rheologicalproperties of polymer melt layered-silicate compositesbecome more and more a critical issue to gain a funda-mental understanding of processability and morphol-ogy for these materials. Rheology represents a sensi-tive tool to provide information about the structure ofmaterials. Linear oscillatory low-frequency behavior isrepresentative for the unperturbed structure in thehybrid. Often, it has been proposed that the formationof a superstructure of the dispersed layers in the poly-mer matrix governs the linear viscoelastic propertiesof layered-silicate-based nanocomposites.6,7 On theother hand, the steady shear flow behavior allowsinferring important remarks related with the process-ability of the materials. At high shear rates or pro-longed action of slow shear forces, the silicate layersdo increasingly align in parallel, leading to the obser-vation of pronounced shear-thinning.8,9 Often, theextent of this shear-thinning has been correlated to theconcentration of clay platelets.10 Finally, the responseto the nonisothermal elongational flow is a key issuefor the preparation of film by film-blowing.

In this work, the response of PE/silicate-layerednanocomposite films to external flow and the result-ing mechanical properties have been investigated. Theoverall objective of this article is to map out the mor-

Correspondence to: G. Filippone ([email protected]).Contract grant sponsors: MIUR (PRIN) and University

of Palermo.

Journal of Applied Polymer Science, Vol. 102, 4749–4758 (2006)VVC 2006 Wiley Periodicals, Inc.

phology-rheology and the morphology-mechanicalproperties behaviors of two commercial grade poly-ethylene/clay composite films. The effect of clay ontwo different types of polyethylene, low density poly-ethylene (LDPE) and linear low density polyethylene(LLDPE), was investigated.

EXPERIMENTAL

Materials

The materials used in this work were two samples offilm-grade polyethylenes, a linear low density poly-ethylene (Clearflex FG166, Mw ¼ 130,000 g mol�1, Mw/Mn ¼ 3.8; MFI190/2.16 ¼ 0.27 g 10 min�1, and r ¼ 0.918g cm�3 at room temperature) and a low density poly-ethylene (Riblene FC30, Mw ¼ 175,000 g mol�1, Mw/Mn ¼ 5.76, MFI190/2.16 ¼ 0.28 g 10 min�1 and r ¼ 0.922g cm�3 at room temperature), both from PolimeriEuropa. To produce polyethylene/clay hybrid films,two commercial PE-based nanocomposite master-batches, named Nanoblend 21011 and Nanoblend20011 and supplied by PolyOne, were blended withthe homopolymers. Nanoblend 21011 and Nanoblend20011 contain 40% wt/wt of MMT clays dispersedinto LLDPE and LDPE, respectively. The clays are sur-face-modified with polyethylene-graft-maleic anhy-dride to help their dispersion into the polymer matri-ces. The masterbatches, defined as intercalated bysuppliers, were compounded with FG166 and FC30 toproduce LLDPE- and LDPE-based nanohybrid filmsat 2, 4, and 8% wt/wt of clays.

Melt compounding

The compounding and the production of the filmswere carried out in a Brabender single screw extruder(D ¼ 19 mm, L/D ¼ 25) attached to a Brabender Plasti-corder PLE 651 and equipped with a die for film anda Brabender film blowing unit. The thermal profilewas 120–1408C, 140–1508C, and 150–1608C and thescrew speed was 80 rpm. The drawing speed of thefilm was kept constant at about 3 m min�1. The finalthickness of the films was in the range 100–110 mm.

Mechanical characterization

Mechanical tests were carried out with a universalInstron machine mod 4443, according to ASTM D882.Average values for tensile strength (TS) and elonga-tion at break (EB) have been calculated. The reprodu-cibility of the results was 65%.

The ball drop tests were carried out in CEAST(Italy) machine mod 6205/000, according to ASTMD1709. At least 10 samples were tested and the repro-ducibility was about 63%.

Rheological characterization

The rheological behavior of materials was investi-gated both in steady shear and oscillatory flow. Astrain-controlled rotational rheometer (ARES L.S.,Rheometric ScientificTM) was used to measure the dy-namic moduli and steady shear viscosities of materi-als. All tests were carried out using 25 mm diameterparallel plates fixture in inert atmosphere at 1608C.Samples were obtained by superposing five blownfilms, and inserting them between the rheometer plates.The samples were heated up to the testing temperatureand then they were squeezed to obtain� 0.6-mm thick-ness disks.

To evaluate the steady shear viscosities, the mini-mum time required to reach a steady state at a givenshear rate was established during preliminary tests.Because of the onset of melt fracture phenomenaappearing at high shear rates, the viscosity data werecollected from 0.01 to a maximum of 10 s�1. Followingto the results of linearity check tests, linear viscoelasticmoduli were evaluated through frequency scans from0.01 to 100 rad s�1 with amplitude of 1%. To evaluatethe effects of clay alignment on hybrid flow proper-ties, rheological tests were also carried out upon sam-ples mixed for about 3 min at 1608C (Rheocord 600p,Haake), with the purpose to obtain a random distribu-tion of silicate layers or their tactoids. Linear stressrelaxation measurements were performed submitt-ing the samples to a single step strain g0 ¼ 1% at timet ¼ 0, and the shear stress evolution during time s(t)was measured to obtain the modulus G(t). Finally, adouble-bore capillary rheometer (RH7, Bohlin Instru-ments) was used to extent the range of investigatedsteady shear rate up to � 105 s�1. Both the Bagley andMooney–Rabinowitsch corrections were performedfor the capillary data by using a zero-length capillary.

A capillary viscometer, Rheologic 1000, (CEAST,Italy) equipped with a drawing system was used toinvestigate the rheological behavior in nonisothermalelongational flow. The capillary diameter was 1 mmand the length-to-diameter ratio was 40. The force inthe molten filament at breaking is measured directlyand is known as melt strength, MS. The breakingstretching ratio, BSR, was calculated as the ratiobetween the drawing speed at breaking and the extru-sion velocity at the die. The data were collected at1608C.

Morphological analyses

To obtain morphological information about the micro-structure of composites, transmission electron micro-graphs (TEM) were taken from 60 to 100 nm thick,microtomed sections of polymer-clay composite blownfilms by using a Philips EM 208 TEM with 100 keVaccelerating voltage. WAXD analyses were performed

4750 LA MANTIA ET AL.

using a diffractometer (Siemens D-500 Krystalloflex810) in the reflection mode with an incident X-raywavelength of 0.1542 nm (Cu Ka) and at a scan rate of1.08 min�1. The specimens for WAXD were preparedby compression molding (D ¼ 20 mm; thickness2 mm) using a laboratory Carver press preheated to atemperature of 1908C. Furthermore, scanning electronmicroscope (SEM, Leica 420) was used to detect the

presence of clay stacks, recognized through energydispersive X-ray spectroscopy (EDS EDAX, Oxfordmod. INCA 200). The samples for SEM investigationswere obtained using the rheometric apparatus, bymelting five superposed blown films at 1608C for3 min and squeezing them between parallel plates,previously covered with antiadhesive PTFE sheets tomake the sample removal easier after the treatment.

Figure 1 TEM micrographs of LLDPE (a, b, c) and LDPE (d, e, f) based hybrids at 8 wt % for various magnifications.

STRUCTURE AND DYNAMICS OF POLYETHYLENE/CLAY FILMS 4751

Then, the specimens were cryo-fractured and SEMobservations were carried out on the samples, previ-ously coated with gold to generate electric current ontheir surfaces.

RESULTS AND DISCUSSION

Composite morphology

The internal structure of hybrids was examined viathe transmission electron microscopy (TEM), allowingto the direct visualization of the hybrid structure inand the dispersion quality for silicate layers into thepolymer matrix. A typical morphology of polymer-layered silicate hybrids, characterized by well distrib-uted and partially exfoliated (dispersed as individualplatelets) clay platelets, emerges from the analysis ofFigure 1, in which the TEM micrographs of 8 wt %hybrids are shown for various magnifications. Similarpictures have been detected for the other composi-tions. Independently from the polymer matrix and theclay loading, it is important to note that the presenceof clay layers in the form of flexible delaminated pla-telets have been always identified, together with inter-calated zones plus some agglomerated tactoid.

In addition to TEM investigations, WAXD analyseswere performed to better inspect the hybrid structure,and the X-rays traces obtained are reported in Figure 2for all the samples, together with those of two nano-clay concentrates. A slight increase of the interlayerdistances respect to the masterbatches can be noticedonly for two samples at the lower nanoclay content,but in its complex, the effect of the compoundingappears rather negligible. A peak in the range of2y ¼ 2.3–2.68 (corresponding to d-spacing of 3.8 and3.4 nm, respectively) and a second peak around2y ¼ 6.48 have been detected for all the samples justlike the values obtained for the two masters Nano-blend 20011 and 21011. Moreover, the compoundingdoes not causes any broadening of the X-ray traces,indicating small effects also on interlayer distancedistribution.

Though the lack of information about the clays doesnot allow inferring direct indications concerning theeffective increase of interlayer d-spacing respect to thevirgin clays, in agreement with TEM results, WAXDanalyses indicate again that different concentrationsand polymer matrices do not significantly modify thestructure of the hybrids. The degree of intercalation/exfoliation is similar to that of commercial master-batches and in its complex, the structure of the hybridblown films is characterized by the simultaneous pres-ence of intercalated and delaminated layers, in addi-tion to coexisting agglomerated tactoids.

Rheological behavior

The steady shear flow curves of the 4 and 8 wt %LLDPE-based hybrids are shown in Figure 3 in com-parison with that of pure polyolefin. The viscosity ofcomposites in the low shear rate region depends onthe amount of filler, and the addition of nanoclay

Figure 2 X-ray traces of the two masters and of all thenanocomposites.

Figure 3 Effect of the clay content on the shear viscosityof LLDPE-based nanocomposites at 1608C.


causes the progressive increase of the zero-shear-rateviscosity with filler loading and the shift toward lowershear rate of the Newtonian plateau. Wagener et al.10

used a power law expression to fit the low shear rateviscosity data of polymer-clay nanocomposites, pro-posing the shear-thinning exponent n as a semiquanti-tative measure of nanodispersion of the samples. Therationale for fitting only the low shear rate data is that,in this range, the rheological response is most repre-sentative for the unperturbed platelet structure in thecomposite.

As shear rate increases, the matrix becomes shear-thinning and its chains start to align with the flow.This accelerates the clay orientation, causing a shear-thinning behavior for filled systems more pronouncedthan that of the matrix, and the crossing of the viscos-ity curves occurs at shear rate � 1 s�1, independentlyfrom the filler loading. Different from what generallyreported in literature for polymer-based nanocompo-sites,5,6,11 exhibiting viscosities progressively similarto that of the matrices with increasing the shear rate,the curves of nanocomposites become lower than thatof the unfilled LLDPE, and an overlapping onlyoccurs at very high shear rates. Generally, a reductionof shear viscosity of a polymer layered silicate nano-composite is attributed to a slipping between polymerand silicate layers or their tactoids.12 This phenom-enon, enhanced by clay flow-induced orientation,becomes more pronounced with increasing the shearrate. The steady shear viscosity curves of the 4 and 8wt % LDPE-based nanocomposites are reported inFigure 4 together with that of the neat LDPE. In spiteof the identical chemical nature of polymer matrices,only differing in the molecular architecture, and thesimilar microstructure emerged from TEM andWAXD investigations, the improvement of viscositydetected for LLDPE-based hybrids at low shear rates

is absent for LDPE-based hybrids. Several authorsobserved a small effect of clays for nanohybrids withpolymer matrix characterized by a markedly shear-thinning behavior.2,13 They concluded that the lowshear rate viscosity of nanocomposites is mainlydominated by the clay loading, instead of by the ma-trix viscosity. The negligible effect of nanoparticlesobserved for the LDPE-based nanocomposite could berelated to the long relaxation time of polymer matrix,as will be discussed in the following.

The comparison of dynamic moduli of nanohybridsand neat LLDPE is reported in Figure 5. Consistentwith the addition of solid particles into a polymermatrix, the dynamic moduli of LLDPE-based nano-hybrids increase at all frequencies with increasing thefiller loading.14 Nevertheless, alterations in the low-frequency behavior emerge, and the low frequencystorage modulus exhibits the development of a plateau.Furthermore, the frequency dependence of the high-frequency relaxation behavior of the LLDPE-basedhybrids is essentially unaffected by the addition of thefiller, suggesting that the faster polymer chain relaxa-tion modes are unaltered by the presence of silicate.

To explain this pseudosolid-like feature, generallydetected for several other intercalated polymer-basedlayered-silicate nanocomposites, two different hypoth-eses are generally proposed.7,15–17 The first arises fromfrictional interactions between clay particles, causinga physical jamming of the dispersed silicate layersbecause of their highly anisotropic nature and simplegeometric constraints. The local correlation betweensilica layers (filler-filler networks) causes the presenceof domains, and this would cause the enhanced low-frequency modulus and the related low power-lawdependence. The second mechanism arises from theconfinement of polymer chains between the silicate

Figure 4 Effect of the clay content on the shear viscosityof LDPE-based nanocomposites at 1608C.

Figure 5 Linear storage (full symbols) and loss (emptysymbols) moduli for the LLDPE-based nanocompositesand unfilled polymer at 1608C.


layers or tactoids and from the physical interactionsbetween these two species. A sort of polymer-fillernetwork form, characterized by different relaxationtimes respect to the bulk, and it would be responsiblefor the alterations in low-frequency rheology of thenanocomposites.

The dynamic moduli of LDPE-based hybrids shownin Figure 6 confirm the negligible effect of the clayemerged in steady shear flow, at least within the fre-quency range investigated. Independently from theclay loading, the frequency dependencies of both stor-age and loss moduli are, within the experimentalerror, identical to that of unfilled LDPE, indicatingthat the relaxation modes are unaltered by the pres-ence of the silicate.

The SEM images of 8 wt % nanocomposites arereported in Figure 7. The samples obtained by super-position of blown films were cryo-fractured perpen-dicularly to the flow direction, indicated by thearrows. The images of the fractured surfaces show theexistence of a preferential orientation along withthe film biaxial drawing direction, at least for claytactoids singled out by EDS analysis. Okamoto et al.observed biaxial flow-induced clay alignment inpolypropylene/clay nanocomposite foams.18 Severalauthors have investigated the effect of clay orientationon viscoelasticity of polymer layered-silicate nano-composites over a wide range of matrices, layered sili-cates, flow, and techniques. A decrease of linear visco-elastic moduli is generally expected following theusage of large amplitude shear flow able to induce theorientation of filler.7,11,19 The differences detectablebetween aligned and unaligned polymer layered-sili-cate nanocomposites have been explained in termsof a filler-filler networking mechanism. The high ani-sotropy of either layers or their tactoids leads to the

observation of pseudosolid-like phenomena for ex-tremely low silicate loading, while considerably highfiller loading are expected for some filler network thatwould form when nanoparticles are preferentially ori-ented in the shear direction.7

The comparisons of storage and loss moduli forfilms and mixed (unoriented) 8 wt % hybrids areshown in Figures 8 and 9. Dynamic oscillatory testswere also performed on mixed samples previouslysubjected to a prolonged preshear (600 s at 1 s�1). Inde-pendently from the polymer matrix, the overlay ofdynamic moduli of shear-aligned and filmed samples,besides confirming the clay alignment in blown films,indicates the reversibility of the flow-induced orienta-tion of layered silicates in both the nanohybrid films.The effect of clay orientation appears more pro-nounced for the LLDPE-based nanohybrids, whereinthe unoriented sample shows a significant enhance-ment of the pseudosolid-like feature. Nevertheless, aslight increase of dynamic moduli can be observed also

Figure 6 Linear storage (full symbols) and loss (emptysymbols) moduli for the LDPE-based nanocomposites andunfilled polymer at 1608C.

Figure 7 SEM images of 8 wt % LLDPE (above) and LDPE(below) based nanocomposites. Circles indicate silicate tac-toids, as emerged through EDS analysis. The arrows indicatethe film drawing direction.


for the LDPE-based unoriented hybrid in the wholerange of frequencies investigated. Moreover, the fre-quency dependence of mixed LDPE hybrids is differ-ent from that of homopolymer, indicating an effect offiller on composite rheology, which is undetectablewhen the clays are aligned in the flow direction.

To probe into the unusual viscoelastic behaviorobserved in the dynamic measurements for thehybrids, linear stress relaxation tests have been car-ried out upon films and mixed (presheared or not) 8wt % composites, and the results are shown in Figures10 and 11. First of all, as expected on the basis of thedifferences in molecular architecture, we observe thatthe homopolymers show different relaxation kinetics.

The LLDPE relaxes quickly than the LDPE in whichmore branched chains employ longer times to recovertheir relaxed configuration after the imposing of thestrain. For the hybrid systems, the moduli of both thecomposites are higher than those of correspondingunfilled polymer at any fixed time. Moreover, accord-ing to the results of dynamic viscoelastic measure-ments, the hybrid moduli are higher when layered sil-icates are unoriented. We further observe that therelaxation kinetic of the LLDPE-based hybrids is simi-lar to that of the homopolymer at short time (i.e., fort < 0.1 s). At longer time, however, the modulus G(t)of the unfilled polymer drop down � 30 s after theimposition of strain, while the hybrids behave like apseudosolid-like material for times as long as � 2000 s,

Figure 9 Effect of clay alignment on linear storage (fullsymbols) and loss (empty symbols) moduli for the 8 wt %LDPE-based nanocomposites and unfilled polymer at 1608C.

Figure 8 Effect of clay alignment on linear storage (fullsymbols) and loss (empty symbols) moduli for the 8 wt %LLDPE-based nanocomposites and unfilled polymer at1608C.

Figure 10 Linear stress relaxation moduli for 8 wt %LLDPE-based nanocomposites and unfilled polymer at1608C.

Figure 11 Linear stress relaxation moduli for 8 wt %LDPE-based nanocomposites and unfilled polymer at 1608C.


the particle orientation only affecting the plateauvalue of G(t). The relaxation behavior of LDPE-basedhybrids is similar to that of unfilled polymer up to� 100 s. For higher times, the homopolymer relaxeslike a liquid at t ¼ 200 s, while the addition of fillerhas a profound influence only on the long time relaxa-tion of the hybrids.

The linear viscoelastic behavior of the hybridsdetected through dynamic and stress relaxation mea-surements could be roughly explained with the notionof two distinct populations of dynamic species withinthe hybrids, consistent with the hypotheses proposedby Ren et al.:7 a matrix portion, substantially unaf-fected by the presence of layered silicate, whose relax-ation process are homopolymer-like, governs thehybrid rheology at short times or, alternatively, athigh frequencies. The second dynamical class, whosebehavior is instead governed by the filler network dy-namics, is characterized by slower relaxation kinetic,responsible for the pseudosolid-like feature observedat long times. Therefore, the presence of filler emergeswhen the homopolymer-like fraction relaxes fasterthan pseudosolid-like one. Alternatively, the homo-polymer-like fraction governs the rheology of thehybrid, and the filler network dynamics could be hin-dered within the time scale of experiments.

The relaxation modulus is related to the dynamicoscillatory shear G0 and G00 via the relaxation spectrumH(t) as in Ref. 20.

G0ðoÞ � GðtÞ��t¼1=o ¼

Z þ1

�1

o2t2

1þ o2t2� e�ð1=otÞ

� �Hdðln tÞ

(1)

Therefore, the frequencies investigated during dy-namic measurements correspond to the range from

10�2 up to 102 s of stress relaxation experiments. In thisexperimental frequency window, the rheology ofLDPE-based nanocomposites is governed by thehomopolymer-like fraction, because of the slow relaxa-tion kinetic of unfilled polymer. This explain the ab-sence of alterations in the low-frequency behavior ofthe hybrids, while pseudosolid-like feature is emergedfor the LLDPE-based nanocomposites, characterizedby a faster relaxation kinetic of polymer matrix.

The overlay of shear and complex viscosities for 8wt % hybrids are shown in Figure 12. Similar resultshave been observed for 4 wt % hybrids. The empiricalCox-Merz rule requires that

Z�ðoÞ ¼ Zð_gÞ for o ¼ _g (2)

The eq. (2), generally found to be applicable for homo-polymers, fails for filled polymer systems and meso-structured materials.21 Ren and Krishnamoorti foundthat the quiescent state linear dynamic oscillatoryZ*(o) always exceeds Zð _gÞ, the discrepancy being larg-est at low o and _g, and they attributed the failure ofCox–Merz rule for polymer layered-silicate nanocom-posites to the ability of steady shear flow to alter thequiescent random arrangement of the silicate layers ortheir tactoids probed during small amplitude oscilla-tory tests.11 Nevertheless, the Cox–Merz rule worksquite well for LDPE-based nanocomposite, as demon-strated in Figure 12. We think that, because of the longrelaxation time of LDPE matrix causing a homopoly-mer-like behavior of LDPE-based hybrids within theexperimental frequency range, the alterations of sili-cate layers/tactoids network produced by the steadyshear flow are hidden and, as a consequence of this,the Cox–Merz rule keep working despite of the meso-structured nature of the nanocomposite.

The ability of the melt to form film when subjectedto a nonisothermal biaxial elongational flow can becharacterized by the value of the melt strength (MS)and breaking stretching ratio (BSR), reported for allthe samples in Table I. For both systems, the increaseof MS values with increasing the nanofiller content isnoticed, coupled with slight reductions in BSR (lessthan 10%). However, once again the increase of MS

Figure 12 Complex and shear viscosity curves for 8 wt %LLDPE (full symbols) and LDPE (empty symbols) basedhybrids at 1608C.

TABLE IMelt Strength and Breaking Stretching Ratio (BSR)

of Homopolymers and Hybrids

MS (cN) BSR

LLDPE 2.4 180LLDPE þ 2 wt % 2.9 175LLDPE þ 4 wt % 3.1 167LLDPE þ 8 wt % 4.2 151

LDPE 11.9 21LDPE þ 2 wt % 12.3 21LDPE þ 4 wt % 12.6 19LDPE þ 8 wt % 13.1 17


results significant only for the LLDPE-based samples(increase up to 75%), while negligible effects (notmore than 10%) can be noticed for the samples havingLDPE as matrix. Several authors suggested that theeffects upon draw properties of nanohybrids wererelated to flow-induced clay alignment along with thestretching direction,5,19 although the existence of atleast some fraction of the silicate layers exhibitingdrawing flow-perpendicular orientation has also beenreported.18

The trend of the results summarized in Table I, inagreement with the shear flow results, clearly suggestthat the presence of oriented clay particles stronglyimproves the values of the melt strength and the con-sequent scarce ability to produce film of LLDPE.However, despite of the similarity in the hybrid meso-strutures emerged from morphological analyses (TEM,WAXD, and SEM), the effect of clay on the elonga-tional rheology of LDPE-based nanohybrids probablycannot be observed because of the homopolymer-likebehavior of hybrids in the melt state, so that negligibleeffects on the shear and elongation rheology of LDPE-based nanocomposites can be noticed.

Mechanical properties

The percentage increase of mechanical properties(elastic modulus, E, tensile strength, TS, elongation atbreak, EB, and ball drop, BD) of the polyolefin/clayhybrid systems respect to the homopolymers are sum-marized in Table II. The hybrids are more rigid thenhomopolymers without remarkable loss of ductility.In particular, elastic modulus and tensile strengthincrease with increasing the nanoblend concentrationfor all the samples, while negligible variations of theelongation at break are detected. A slight increase ofthe EB emerges in the transverse direction, probablydue to a better orientation in this direction. The balldrop values, representing a measure of the impactstrength of the film, increase too for the hybrid sys-

tems. The increase of elastic modulus and tensilestrength is roughly monotonic with filler content,while negligible effects are detectable upon the valuesof elongation at break within the experimental error.

It is important to observe that the effect of fillerdoes not show a clear trend with respect to the natureof the matrix, and the relative enhancement of me-chanical properties respect to the neat matrices arecomparable for both the LLDPE- and LDPE-basedhybrids. Though this could appear in disagreementwith rheological results, this is not surprising consid-ering that both WAXD and TEM analyses have indi-cated comparable mesostructure for the hybrids, inde-pendent from the nature of polymer matrix. The slightdiscrepancies of the dimensionless mechanical prop-erties of LLDPE- and LDPE- based hybrids could beprobably attributed to some difference in the clay ori-entation of tested samples rather than to the effect ofeither the adhesion between the matrix and the claysor the different degree of intercalation/exfoliationstate. The results shown suggest that the increase ofmechanical properties of the films as a consequence ofadding the clay has to be mainly related to the rein-forcement effect promoted by the filler, the polymerhost playing a minor role in determining of the solidstate properties of the hybrids. Conversely, the influ-ence of matrix is resulted relevant in the melt state,the relaxation kinetics of homopolymer being able toshield the effect of clays within the investigated ex-perimental range.

SUMMARY

The rheology, the ability to produce films and the me-chanical properties of two different layered-silicate-based polyethylene hybrids for film manufacturinghave been investigated. Despite the identical chemicalnature of two polymer matrices, only differing in themolecular architecture, and the similar degrees ofexfoliation/intercalation deduced from morphologi-

TABLE IIMain Mechanical Properties (E, TS, EB, and BD) of the Two Homopolymers and Their Relative Increases for

Nanohybrid Blown films With Respect to the Matrices in the Two Drawn Directions

Machine direction Transverse direction

E (MPa) TS (MPa) EB (%) E (MPa) TS (MPa) EB (%) BD (g mm�1)

LLDPE 109 34.3 688 151 28.6 604 3.9LDPE 96 33.5 665 133 24.2 485 3.2

Increases of mechanical properties respect to the homopolymers (%)LLDPE þ 2 wt % þ23.8 þ2.3 �0.44 þ20.5 þ1.75 þ0.16 þ5.13LLDPE þ 4 wt % þ40.4 þ3.5 þ1.74 þ47.7 þ5.60 þ1.82 þ10.2LLDPE þ 8 wt % þ75.2 þ18.6 þ0.58 þ64.2 þ20.6 þ10.1 þ28.2

LDPE þ 2 wt % þ26.0 þ0.60 �0.75 þ18.8 þ2.10 �1.03 þ6.25LDPE þ 4 wt % þ44.8 þ3.00 �0.75 þ37.6 þ3.72 þ1.03 þ9.37LDPE þ 8 wt % þ64.6 þ15.2 �3.76 þ69.2 þ14.9 þ5.15 þ15.6

E, elastic modulus; TS, tensile strength; EB, elongation break; BD, ball drop.


cal analyses, the presence of filler clearly affects onlythe rheological behavior of LLDPE-based nanocompo-sites, while the LDPE-based hybrid rheology appearscomparable to that of unfilled LDPE. SEM observa-tions and linear dynamic measurements indicate aneffect of layered silicate or tactoids orientation uponflow behavior of both the hybrids. The results ofdynamic tests carried out on mixed samples, andstress relaxation measurements are consistent withthe existence of a roughly bimodal population of dy-namical species: a matrix component behaving likethe homopolymer matrix, and a fraction interactingwith the filler, whose rheological behavior is con-trolled by the clays and their interactions with thepolymer. The rheology of the hybrids depend on thewhole of relaxation kinetics of these two dynamicalspecies. The long relaxation times of LDPE producesthat the flow behavior of the hybrids within the timescale of the experiments is governed by the homo-polymer. Conversely, the presence of filler standoutfor LLDPE-based hybrids at low frequencies andshear rates, and a pseudosolid-like feature related tothe filler become noticeable. Moreover, the Cox–Merzrule, generally inapplicable to mesostructured materi-als like those investigated in this study, still works forthe LDPE-based hybrids because of its homopolymer-like rheological behavior. Even in elongational flow,the effect of layered silicate strongly improves theability to make LLDPE-based films without signifi-cantly modifying the draw properties of LDPE-basedhybrids.

The picture is different for the mechanical proper-ties in the solid state. The increase of the elastic mod-uli and tensile strengths have been observed for allthe filled films, together with a slight reduction of theelongation at break, but no significant differencebetween LDPE- and LLDPE-based films has beendetected. The similarities of hybrid microstructuresresulted from morphological analyses suggest that theincreases of mechanical properties of the hybrids

respect to the neat polymers is a consequence of theirmicrostructure. Different from what happened in themelt state, the polymer host seems to play a minorrole in the determining of the solid state properties ofmaterials.

The authors thank Polimeri Europa for supplying the poly-ethylene samples and also Prof. P.L. Magagnini (Universityof Pisa) for X-ray spectra.

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structure and dynamics of polyethylene/clay films

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