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On surface deformation of melt-intercalated polyethylene-clay nanocomposites during scratching.

INTRODUCTION

In the last decade, polymer nanocomposites have received significant attention, both in the industry and in the academia. A number of experimental investigations have indicated that polymer nanocomposites exhibit new and sometimes improved properties that are not displayed by the individual phases or by their conventional composite counterparts [1-8]. In addition to improved gas barrier properties, polymer nanocomposites are generally characterized by superior strength, stiffness, dimensional stability, and heat resistance. These additional benefits can be utilized for automotive and packaging applications.

The study of scratch is important to understand some mechanical processes such as grinding, polishing, and other fabrication methods involving wear. Therefore, the susceptibility of polymeric materials to scratch is a serious concern [9-12]. Scratch introduces visual damage in materials and may also act as a macroscopic stress raiser reducing the mechanical strength [13-18]. Plastically deformed polymeric materials exhibit a whiter appearance called 'stress whitening' that is detrimental to optical clarity and aesthetic perception. It is also detrimental to tensile and fatigue loading because it implies that the energy absorbing mechanisms under stress has been exhausted; therefore, further stress application will lead to early failure. Thus, the susceptibility of polymeric materials to scratch damage is detrimental to maximizing their applications. In the last decade, a number of mineral reinforcements have been examined and their utility in enhancing scratch resistance established. Important among those are wollastonite [19-28], talc [26, 27], clay [2, 29, 30], and calcium carbonate [31]. More recently inorganic nanoparticles are being increasingly utilized as additives to enhance polymer performance, providing numerous commercial opportunities, ranging from components of advanced aerospace systems to commodity plastics.

Bermudez et al. examined the influence of scratch velocity on several thermoplastics, including polystyrene (PS), styrene-acrylonitrile (SAN), polyamide 6 (PA6), polyethersulfone (PES), and polysulfone (PSU), and showed that the scratch damage is related to the toughness of polymers [32]. Also, Briscoe et al. examined the influence of applied normal load, sliding velocity, angle of indenter, and lubrication [12, 33]. A number of factors have been observed to affect the scratch resistance in polymeric materials including modulus, crystallinity [17, 34], yield or brittle strain, yield or brittle stress, dynamic friction coefficient, and toughness [28]. Scratch deformation processes of diverse nature including fully elastic, elastic-plastic, ironing, wedging, crazing-tearing, grooving, edge-cracking, and chipping were observed depending on the applied normal load, cone angle, and scratch velocity.

Based on the study of micromechanisms in relation to tensile deformation behavior of polypropylene (PP) [19, 20], the deformation processes responsible for stress whitening during scratch damage was explained using strain-strain rate deformation maps. The relationship between deformation mechanisms and scratch visibility was examined using the phenomena of light scattering and reflection from scratches [10, 35-37]. This approach involved the quantification of polarized light scattered from the scratch by considering the difference in intensities between bright and dark regions when compared with scratch visibility of different materials. Later on, the quantification of stress whitening sensitivity method was determined with the help of optically scanned images (1, 17, 23,).

Recently, a wide range of inorganic mineral reinforcements have been investigated from the viewpoint of improving scratch resistance [19-31]. In general, the increase of scratch resistance in mineral-filled polymers is accompanied by a significant loss in ductility and toughness in relation to their neat counterparts. In contrast to wollastonite and talc, our previous results on high density polyethylene (PE) reinforced with calcium carbonate implied that both impact strength and scratch resistance was significantly enhanced in relation to neat PE. Until now, a number of studies discussing the effect of reinforcement of polymers with nanoclay on the improvement of mechanical properties have been reported in the literature [1, 2, 29]. Misra and coworkers studied the near surface scratch morphology in terms of plastic deformation, crystallinity, and stress-whitening in PP clay nanocomposites characterized by higher toughness over neat polymer. The objective of the present work is to describe a microstructural characterization approach to study the plastic deformation process and associated stress-whitening that is induced during scratching in PE-clay nanocomposites. The difference in the surface deformation and stress-whitening behavior of neat PE and PE-clay nanocomposites is discussed in terms of modulus, crystallinity, ductility, and clay-induced structural change. It is of interest to examine PE-clay nanocomposite system because in contrast to PP-clay nanocomposites, the impact toughness of PE-clay was significantly reduced in the temperature range of -50 to +70[degrees]C [29].

EXPERIMENTAL PROCEDURE

Materials and Physical Properties

Commercially available grade of high density PE copolymer produced by Solvay (formal product name: ethane-hexene-1 copolymer), and developed for blow molding automotive fuel tanks and other large parts, where the finished part demands environmental stress crack resistance (ESCR), excellent processability, and superior impact properties was used to process PE-4 wt% clay nanocomposites. This grade has a melt flow rate of 9 g/10 min at 190[degrees]C/2.16 kg. A natural montmorillonite clay surface modified with dimethyl dialkyl ammonium (Nanomer I.44P, Nanocor) was used as the reinforcement filler. The nanocomposites were prepared by mixing the appropriate amount in twin counterrotating screw extruder at 100 rpm followed by injection molding of bars. The storage modulus of neat PE and PE-clay nanocomposites was studied by dynamic mechanical analysis (DMA). The DMA was carried out using TA instruments 2980 in single cantilever mode from -100 to 150[degrees]C. The testing frequency was 1 Hz and the heating rate was 3[degrees]C/min.

Crystallization Behavior, Structural Characteristics, and Dispersibility

The study of degree of crystallinity assumes particular significance because higher crystallinity, in general, increases modulus and yield stress, and reduces toughness. The change in percentage crystallinity, and structural characteristics induced by clay is important in understanding the deformation behavior. The crystallization behavior of neat PE and clay-reinforced PE nanocomposites was studied by differential scanning calorimetry (DSC). The PE and PE-clay nanocomposites were heated from room temperature (~20[degrees]C) to 200[degrees]C and held at the high temperature for about 3 min to erase the previous thermomechanical history and to obtain a completely relaxed melt. Then, the melt was cooled to 30[degrees]C at rate of 10[degrees]C/min, and a second scan was carried out at rate of 10[degrees]C/min.

The dispersibility and intercalation of PE into the clay layers was studied by transmission electron microscopy (TEM). The staining was carried out in the vapor phase. The trimmed specimen was stained with solid ruthenium tetroxide (Ru[O.sub.4]) for 10 h in a vial. Sections of 100-200 nm were cut using a Leica ultramicrotome equipped with a diamond knife and collected in a trough filled with water and placed directly on 400-mesh copper grids. Transmission electron micrographs were taken with Hitachi H-7600 at an acceleration voltage of 100 kV.

Mechanical Properties

The tensile bars (ASTM D-638 type I 3-mm-thick dog-bones) of neat and PE-clay nanocomposites were tested in uniaxial tension at 20[degrees]C using a computerized MTS 210 tensile testing machine at selected displacement rate of ~5 mm/min to determine tensile properties (modulus, yield strength). The Izod impact tests were carried out using an instrumented falling weight Tinius Olsen impact tester (Model 899) with an impact velocity of 1 m/s. The notched specimens were subjected to the impact test at room temperature.

Scratch Tests

Surface damage was introduced by producing a well-defined scratch on the surface. The scratch equipment consisted of a balance beam scrape adhesion tool that utilized different types of stylus namely, Hoffmann, needle, and loop. In this study, a Hoffmann-type stylus having a hardened stainless steel cylinder and a contact arc of diameter 6.945 mm was used. The samples were fixed on a leveling platform attached to a displacement stage and normal load was applied by placing dead-weights on the indenter holder. Mechanically-induced surface damage in the form of a scratch was introduced on the surface of the samples using loads of 2 and 6 kgf. The scratch test conditions were identical for all the three polymeric materials to enable a direct comparative evaluation. The scratch direction was the longitudinal axis of the injection-molded specimen.

Surface Deformation

The microstructural evolution associated with the scratch process was studied using scanning electron microscope (Hitachi 7600). Sections were cut from the scratched area and the characteristics of deformation process and micromechanisms involved were examined.

Quantification of Surface Damage

The effect of clay on the resistance to surface deformation was evaluated by atomic force microscope (AFM, Nanoscope (R) IIIa, Digital Instruments, Santa Barbara, CA) in terms of average scratch roughness. The AFM provides excellent Z-resolution to quantify the surface topography. Atomic force microscopy was carried out using tapping mode to prevent possible surface damage because of continuous contact. All the scans were made in air and the tip used for the study of samples was 'tapping mode etched silicon probe' (TESP). The length of the tip was 125 [micro]m. The scan parameters and AFM scan leveling procedures were identical for all the investigated polymeric materials to enable direct comparison of images. A number of measurements were made at different positions along the scratch length for each sample to ensure that the data was a true representative of the characteristics of the scratched surface.

To quantify the stress whitening or visual damage associated with the scratch deformation, stress whitened samples of both neat and reinforced polymeric materials were optically examined in the dark field image mode using a Sony CCD camera and the images were saved as TIFF (tagged image file format). Subsequently, the recorded images were processed in the gray mode using Image J software (based on NIH software). The image analysis functions available in the software include gray value against x-y coordinates of all the pixels [14-16].

RESULTS AND DISCUSSION

Physical and Mechanical Characteristics

Crystallinity and Macromolecular Structure. The crystallization data (percentage crystallinity, melting, and crystallization temperatures) obtained from DSC experiments for neat and PE-clay are presented in Table 1. The percentage crystallinity was estimated using a value of heat of fusion of 293 J/g for 100% crystalline PE [29]. The results indicate that the addition of nanoclay particles to PE increases crystallinity and this is attributed to the nucleating effect of clay, and the nucleating effect is influenced by the content of the particles. The decrease of crystallinity on increasing the clay content from 4 to 8 wt% may be ascribed to the increased suppression effect of clay in PE-8 wt% clay nanocomposite. It has been suggested by Homminga et al. [38] and Fornes and Paul [39] that clay has both nucleation and suppression effect on the crystallization of polymer matrix. The suppression effect increases with increase in clay content, thereby reducing the mobility of polymer chains in the PE matrix. The presence of too many dispersed particles makes large crystalline domains difficult to form in the restricted and confined space. While the crystallization temperature of PE-clay nanocomposites remains similar when compared with neat PE and this is related to the particle-matrix interaction, as discussed by us recently [29]. The interfacial interaction plays a critical role in the free energy of cluster formation and the rate of nucleation, the weak interaction lowers the rate of nucleation. The DMA results indicated that the glass transition temperature, [T.sub.g] shifts only slightly to lower temperature on reinforcement with clay (Table 1). This observation, further confirms the weak interaction between PE matrix and nanoclay. In contrast to PE-clay system, in our recent study of PP-clay nanocomposites, both the crystallinity and crystallization temperature of polymer nanocomposite was increased, and was attributed to strong nucleating role of clay and interfacial interaction [40].

The lamellar thickness (l) is listed in Table 1 and was examined using Thomson-Gibbs equation [41]:

l = 2[gamma][T.sub.m.sup.0]/([DELTA][H.sub.[rho]]([T.sub.m.sup.0] - [T.sub.m]).) (1)

where [T.sub.m.sup.0] is the equilibrium melting temperature, [T.sub.m] is the detected melting temperature by DSC, [gamma] the surface free energy, [DELTA]H the heat of fusion for 100% crystalline PE, and [rho] the density. A small but consistent increase of lamellae thickness with increase in clay content implies the perfection of crystals improved with the addition of clay particles.

Representative TEM micrograph of PE-4 wt% clay nanocomposite is presented in Fig. 1. Figure la and lb suggest intercalated structure in PE-clay nanocomposites, and that large agglomerates of clay are not observed (i.e. small clay tactoids are present).

Tensile and Impact Properties. The tensile modulus and yield stress data are listed in Table 1. In addition to the increase of the elastic modulus from 606 MPa in neat PE to about 925 MPa in 8 wt% clay nanocomposite, there was a small consistent increase in yield stress on reinforcement of PE with nanoclay particles. This is attributed to the reinforcement effect of clay. A similar behavior has been observed for other polymer nanocomposite systems including PP-clay [40], polystylene-carbon nanotube [42], PP-Si[O.sub.2] [43], and Nylon 66-clay [44].

Room temperature Izod impact strength for PE and PE-4 wt% clay is summarized in Table 1. It can be clearly seen that impact strength decreases with increase in content of clay in the PE matrix. Such behavior was attributed to reduced particle-matrix interaction and crystal structure of the nanocomposite [29].

In general, an increase in crystallinity or increase in spherulite size increases the modulus because large spherulites are considered to have a significantly higher load-bearing capability. In the present case, there is increase in crystallinity (Table 1) but the spherulite size decreases [29] on reinforcement of PE with clay. To rationalize these observations, it is reasonable to say that two mutually opposing forces influence the properties--the reinforcement influence of clay and the nucleating effect. The reinforcement resulting from clay nanofillers increases crystallinity and has a positive effect on modulus and yield stress, while the nucleating effect of nanoclay particles yielding smaller spherulite has a negative effect. The reinforcement effect easily overwhelms any effect due to smaller spherulite size induced by the clay. The crystallinity increases even though the spherulite size becomes smaller because of the significantly higher nucleation density induced by the nanoclay particles [45].

Ouderni and Philips [46] studied the effect of crystallinity and spherulite size individually in PP and observed that an increase in crystallinity or spherulite size decreased the toughness, consistent with Friedrich's conclusion [47]. These observations of Friedrich [47] and Ouderni and Philips [46] seems to be applicable for neat polymers but not for the composites. In our case, there is a decrease in spherulite size [44] and increase in crystallinity both of which according to the observations of Friedrich [47] and Ouderni and Philips [46] should have mutually opposite effects as discussed above. The observation suggests that the behavior of the nanocomposite is not a simple function of spherulite size and crystallinity, but is a complex function of other factors and includes lamellar thickness and crystalline long period [48]. These aspects have been discussed by us in detail elsewhere [29].

[FIGURE 1 OMITTED]

Micromechanism of Scratch Deformation in Neat and Clay Reinforced Polyethylene

Representative Scanning electron micrographs (SEM) of the scratch morphology of neat PE, 4 and 8 wt% nanoclay-PE under identical conditions of scratch test are presented in Figs. 2-4. The scratch morphology was similar at the investigated loads of 2-6 kgf, except that the aggressiveness and severity of deformation was greater at higher loads. Thus, the SEM micrographs are presented for representative load of 6kgf. In all cases, periodic ripples having the resemblance of parabolic geometry with cracks initiating at both the extreme ends of the scratch tracks and extending towards the center of the scratch. The periodic nature of the scratch tracks can be discussed in terms of stick-slip motion between the tip of the indenter and the surface of the material [18]. During the stick stage, there is no relative motion between the indenter tip and the surface of the material, but the indenter continues to apply stress on the sample surface resulting in deformation of the material underneath the indenter. The tangential or horizontal stress acting during the stick stage is less than the critical stress, but increases with time. Once the stress applied by the indenter on the sample surface exceeds the required critical stress, the slip stage initiates, resulting in relative motion between the tip of the indenter and the sample surface. The slip stage terminates once the applied stress falls below the critical stress, resulting in subsequent onset of the stick stage when the indenter and material surface come into contact again. The material plastically deforms and accumulates in front of the indenter during the slip stage. Thus, the periodically rippled scratch tracks reflect the sequential accumulation and release of tangential force as depicted in Fig. 5 and is discussed in detail in recent work [30].

[FIGURE 2 OMITTED]

In Fig. 2 for neat PE, it may be noted that besides periodic, parabolic ripple-like scratch tracks, there are some faint straight lines (Fig. 2b) running parallel to the scratch direction together with wrinkles/ridges that are perpendicular to the scratch direction (Fig. 2c). The horizontal lines are characteristic of ironing mode of deformation and ridges are caused due to stress relaxation (stress release). If we follow the traces of ridges at higher magnification, the vertical ridges are accompanied by tearing (Fig. 2b and 2c). At lower loads, the severity of deformation (e.g. degree of tearing) was less pronounced. A close look at Fig. 2a and 2b and on the basis of our previous work on PE-calcium carbonate system, we believe that the continuity and intensive nature of the parabolic tracks is disrupted by ironing mode of deformation as is more apparent in the nanocomposites.

[FIGURE 3 OMITTED]

In comparison to neat PE, the scratch behavior of PE-4 and 8 wt% clay nanocomposite under identical test condition of 6 kgf exhibited significantly reduced surface deformation (superior scratch resistance). While parabolic tracks continued to be observed, however, the severity of parabolic tracks (cracking associated with tracks) was significantly reduced (Figs. 3 and 4). Also, pronounced wrinkles/ridges and tearing that significantly contributes stress whitening is greatly reduced on reinforcement with 4 and 8 wt% nanoclay. Ironing seems to be the dominant mode of deformation. The significantly reduced surface damage behavior can be attributed to the reinforcement of PE with nanoclay.

The effects of nanoclay particles on surface deformation can be summarized as follows:

a. the cracking at the end of parabolic cracks decreases with increase in percentage of nanoclay (Figs. 2-4),

b. the width of the scratch decreases with increase in nanoclay particles but increases with applied load (Table 2),

c. the severity of deformation (density of wrinkles/ridges and tearing) decreases with clay content involving lower volume of the material and shallow scratch (Figs. 2-4, Table 2).

In comparison to PP-clay nanocomposites, the addition of nanoclay particles decreases the toughness of PE and also the crystallization temperature. Based on the results presented here, it is not inappropriate to suggest that the nanoclay has significantly stronger impact on transformation of scratch deformation mechanism (from periodic cracked ripples accompanied with tearing in neat PE to ironing-dominated behavior in nanocomposites) compared to the reinforcement with calcium carbonate [31]. Analyzing the above comparative behavior of clay in PE and PP [30] and calcium carbonate in PE [31], in terms of mechanical properties, a greater resistance to scratch deformation is offered by nanoclay in comparison to calcium carbonate. Reinforcement of PP [30] or PE with nanoclay increased modulus and yield strength, while the addition of calcium carbonate increased modulus, but yield strength remained unaffected [31]. An increase of both modulus and yield strength increases the elastic regime in PP or PE-clay nanocomposite system in comparison to PE-calcium carbonate system [31], where only modulus increased, implying greater elastic recovery in PP [30] or PE-clay nanocomposite system.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

It has been previously [10, 49] suggested that along the scratch length, a significantly higher tensile stress is generated at the tail end of the scratch (behind the indenter) during scratch deformation than that caused by the indentation. The magnitude of maximum tensile stress is higher for high modulus materials and is applicable for the nanocomposite system suggested here. During scratch deformation, when the tensile stress behind the scratch is greater than the tensile strength of the polymeric material, plastic deformation occurs. From the above discussion, PP or PE-clay nanocomposite systems should experience higher tensile stress because of higher modulus. But the reduced susceptibility to scratch deformation of PP or PE-clay nanocomposites suggests the favorable effect of higher modulus and yield strength of polymer nanocomposites. It is speculated that the positive influence of nanoclay is related to a shift in the von Mises stress from the surface to the subsurface region leading to reduction in the maximum tensile stress induced by the scratch and, consequently, reduced surface damage. In fact, TEM observations of the near surface microstructure in scratched PP blends have clearly indicated the extension of deformation fields into the bulk and are characterized by dilation of material inside the shear bands [50].

Comparing with PE-clay nanocomposites, in PP reinforced with 4-8 wt% nanoclay particles, a similar but more drastic effect of nanoclay particles was observed under identical test conditions [30]. In neat PP, the application of higher loads introduces severe ironing deformation, disrupting the periodic parabolic nature of scratch tracks such that the material plastically flows along the ironing zone. Under identical test conditions, the severity of surface deformation in PP-clay nanocomposites which leads to stress-whitening is significantly reduced. At lower loads, the periodic nature of tracks is retained together with ironing, while at higher loads, ploughing occurred [30]. The different effect of nanoclay particles in PP and PE systems may be ascribed to the effect of toughening. Our present results indicate that PP was significantly toughened by nanoclay particles, while the toughness of PE decreased. The decreased toughness in PE-clay nanocomposite leads to reduced effect of nanoclay particles on the scratch behavior when compared with PP-clay nanocomposite.

Quantitative Evaluation of Scratch Damage

The magnitude of scratch damage can be quantified in terms of percentage elastic recovery and scratch hardness using the maximum depth of the scratch and width of the scratch. In addition, average surface roughness is also a measure of the magnitude of surface deformation. The width and maximum depth of the scratch was determined by light microscopy and scanning electron microscopy, respectively. The maximum depth was measured by examining the sample in cross-section. The surface roughness was determined by AFM averaging three different regions of 50 X 50 [micro][m.sup.2] along the length of the scratch. From the quantitative determination of the above parameters, the scratch resistance of the material can be predicted. The elastic recovery, [h.sub.e], was calculated using:

[h.sub.e] = [[F.sub.n](1 - [v.sup.2])]/2EW (2)

where [F.sub.n] is the normal applied load, v is Poisson's ratio, E is modulus of elasticity, and W is the scratch width.

Scratch hardness, which is also a measure of the scratch resistance, was calculated using:

H = [F.sub.n]/[pi](2rd - [d.sup.2]) (3)

where [F.sub.n] is the scratch load in kgf, d is the depth of the scratch in mm, r is the radius of the stylus in mm for the Hoffman stylus, and [pi] = 3.14. Equation 3 involving depth of the scratch is sensitive to the nature of the material [2.

From Table 2, we note that average scratch roughness, depth of the scratch, and scratch width decrease with increase in percentage of nanoclay particles, implying improvement in scratch resistance of PE-clay nanocomposites compared to neat PE. Also, the lower depth of the scratch is consistent with greater elastic recovery (Table 2). The scratch hardness predicted using Eq. 3 is presented in Table 2. There is a direct relationship between scratch depth and scratch hardness (Table 2) i.e. materials with higher scratch hardness are expected to exhibit greater resistance to scratch damage. A similar effect was observed when PE was reinforced with calcium carbonate, however, the scratch hardness at identical scratch test conditions was significantly lower [31].

[FIGURE 6 OMITTED]

Quantification of Stress Whitening During Scratch Damage

Optical imaging of scratched and unscratched region and difference in gray level between them has proved to be an appropriate method to quantify the stress whitening. A quantitative analysis of stress whitening in terms of the change in gray level between scratched and unscratched region is presented in Fig. 6. The relative change in gray level decreased with increase in percentage clay suggesting that the investigated polymer nanocomposite system exhibits superior resistance to stress whitening (minimum scratch visibility) consistent with the above SEM and AFM results.

The formation of voids, micro-crazing, cracks, and wrinkles/ridges are generally believed to be responsible for the origin of stress whitening during scratch damage in polymeric materials. The relationship between light scattering and deformation is complex because it is influenced by the size of deformation features and also by the change in the refractive index induced by molecular level changes that concern entanglement of polymer chains and reorientation of polymer molecules, or difference in refractive index between matrix and reinforcement minerals. Deformation features of size similar to the wavelength of visible light or entities that have refractive index different from that of air and are of size corresponding to wavelength of light generally promote scattering of visible light giving the appearance of a whitened zone. Surface roughness is another factor that contributes to light scattering. In the present study, the reinforcement of PE with nanoclay significantly reduces the cracking associated with parabolic tracks, and almost complete reduces the formation of wrinkles/ridges and tearing that contributes to stress-whitening. Thus, the high scratch visibility of PE can be attributed to the high density of wrinkles/ridges and tearing of the material.

CONCLUSIONS

The reinforcement of PE with 4-8 wt% nanoclay has an overriding influence on modulus and yield strength of polymer nanocomposites. This is accompanied by increase in crystallinity, elastic recovery, and resistance to mechanically induced scratch damage. With the addition of nanoclay particles, the severity of the scratch deformed region is significantly reduced, involving a lower volume of the nanofiller and a consequently more shallow scratch. Scratch of neat PE is characterized by periodic tracks (with extensive cracking at ends), wrinkles/ridges, and tearing of material. Reinforcement with clay significantly reduces the severity of plastic deformation. A quantitative evaluation of scratch damage parameters including maximum depth of the scratch, average scratch roughness, and scratch hardness indicates reduced susceptibility to scratch deformation of PE-clay nanocomposites. The severity of surface deformation in polymer nanocomposites system is significantly lower on comparison with conventional reinforcement fillers such as talc, wollastonite, or calcium carbonate consistent with scratch hardness results. The scratch hardness is a relevant parameter that can be used to determine resistance to scratch deformation. Microcracks and other surface features such as wrinkles/ridges are the primary source of light scattering resulting in stress whitening.

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A. Mudaliar, Q. Yuan, R.D.K. Misra

Center for Structural and Functional Materials, University of Louisiana at Lafayette, Lafayette, Louisiana 70504-4130

Department of Chemical Engineering, University of Louisiana, Lafayetta, Louisiana 70504-4130

Correspondence to: R.D.K. Misra; e-mail: dmisra@louisiana.edu
TABLE 1. Physical and mechanical properties of neat polyethylene and
polyethylene-clay nanocomposites.

 Heat of % Crystallization Melting
 fusion Crystallinity temperature temperature
Material (J/g) DSC ([degrees]C) ([degrees]C)

Neat-HDPE 116.2 39.8 115.0 133.3
4 wt% clay-HDPE 147.4 (a) 56.1 (a) 114.4 133.7
8 wt% clay-HDPE 140.6 (a) 48.0 (a) 113.9 134.6

 Glass Modulus
 Lamellar transition at 5 Yield stress
 thickness temperature mm/min at 5 mm/
Material (nm) ([degrees]C) (MPa) min (MPa)

Neat-HDPE 7.72 -114.1 606.3 23.9
4 wt% clay-HDPE 7.79 -116.2 767.0 24.6
8 wt% clay-HDPE 8.49 -116.6 924.9 28.3

 Room
 temperature
 Elongation Izod impact
 at break strength
Material (%) (kJ/[m.sup.2])

Neat-HDPE 800 53.7
4 wt% clay-HDPE 362 13.4
8 wt% clay-HDPE 101 16.2

(a) For 100% polyethylene.

TABLE 2. Comparison of scratch deformation parameters of neat
polyethylene and polyethylene-clay nanocomposites.

 Scratch deformation parameters
 Maximum depth Average width RMS scratch roughness
 of the scratch of the scratch (nm) (measured from
 ([micro]m) (mm) AFM)

Neat-PE
2kgf 45 1.51 276
6kgf 67 2.14 470
4 wt% clay-PE
2 kgf 41 1.25 250
6 kgf 54 1.95 333
8 wt% clay-PE
2 kgf 27 1.18 127
6 kgf 44 1.88 167

 Scratch deformation parameters
 Average scratch
 roughness (nm) Elastic
 (measured from recovery Scratch hardness
 AFM) (%) (kgf/[mm.sup.2])

Neat-PE
2 kgf 216 0.087 2.05
6 kgf 386 0.184 4.14
4 wt% clay-PE
2 kgf 153 0.083 2.25
6 kgf 275 0.160 5.13
8 wt% clay-PE
2 kgf 98 0.069 3.41
6 kgf 127 0.129 6.29
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Author:Mudaliar, A.; Yuan, Q.; Misra, R.D.K.
Publication:Polymer Engineering and Science
Date:Nov 1, 2006
Words:5665
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