粘度的对性能的影响

In?uence of Melt Viscosity on the Structure and Properties of Electrically Conductive Nanocomposites Produced by Masterbatch Process

Stefano Besco,Alessandra Lorenzetti,Denis Hrelja,Carlo Boaretti,Martina Roso,Dino Ferri,Michele Modesti*

Electrically conductive polypropylene nanocomposites containing multi-walled carbon nano-tubes were produced by melt blending by dilution of a concentrate masterbatch containing 10wt.%of ?ller.In order to promote polymer in?ltration within primary agglomerates a ultra-low melt viscosity polypropylene grade has been used for masterbatch production.Moreover,three polypropylenes with different melt viscosities have been used for successive dilutions.Filler dispersion has been assessed by both indirect macroscale analyses and direct morphological investigations.Characterization showed percolation thresholds oc-curring for very low ?ller contents.During dilution,lower polymer viscosities lead to more effective residue agglomerates dispersion,hence better electri-cal

properties.

1.Introduction

Due to their structure and outstanding properties,carbon nanotubes (CNT)have attracted a lot of attention for their applications in the production of electrically conductive polymer https://www.360docs.net/doc/269310362.html,T exhibit high mechanical strength,Young’s modulus,and strain to failure,as well as remarkable electric properties.Furthermore,even ?re

behavior performances and thermal conductivity of polymers can be improved by the addition of such nanostructured ?llers depending on their level of disper-sion within the host matrix.Many issues must be faced in order to meet these expectations due to the colloidal nature of CNTs,which makes them very dif?cult to disperse in polymer media and in particular in non-polar systems,such as polyole?https://www.360docs.net/doc/269310362.html,Ts,even during their synthesis,have a remarkable tendency to aggregate and form microsized bundles that can be broken down only by means of high shear stresses,but with the risk to reduce particles aspect ratio due to shortening phenomena.[1]

The studies concerning the preparation of polymer/CNT nanocomposites are mainly based on three different approaches:i)mixing in polymer solutions,ii)in situ polymerization,and iii)melt blending.By far,even analyzing

S.Besco,A.Lorenzetti,D.Hrelja,C.Boaretti,M.Roso,M.Modesti University of Padova,Department of Industrial Engineering,via Marzolo 9,35131Padova,Italy E-mail:michele.modesti@unipd.it D.Ferri

Basic Chemical and Plastics Research Centre,Versalis S.p.A.,via Taliercio 14,46100Mantova,

Italy

Full

Paper

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DOI:10.1002/mame.201300336

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scienti?c literature,one of the most used techniques to produce thermoplastic-based polymer/CNT nanocomposites is the melt-compounding method due to its ?exibility,cost-effectiveness,safety,and scalability up to industrial level.[2]

The investigation of polypropylene (PP)/CNT-based sys-tems obtained by melt blending has been considered a focus for the present work.PP is a typical non-polar and partially crystalline polymer.Hence,due to the low interactions between PP and CNT,dispersion is very dif?cult,and usually leads to high percolation concentrations.

In their work,Tjong et al.[3]prepared several PP/multi-wall carbon nanotubes (MWCNT)nanocomposites by melt-compounding using two different mixing devices;they found that the best dispersion can be achieved in the presence of intensive shearing.PP/MWCNT nanocompo-sites prepared at high shear rates in fact exhibited remarkably lower percolation thresholds than the ones produced using low shear rates.

Pan et al.[4]investigated instead the improvement of the dispersion of MWCNT in a PP matrix by means of chemical modi?cation of the ?ller together with the addition of a masterbatch as a compatibilizer.The relationship between the dispersion of MWCNTs and the electrical conductivity of the composites has been investigated by controlling several parameters,such as CNTs modi?cation,compatibilization,melt mixing conditions,and post-heat treatments.The authors reported that the enhanced interfa-cial adhesion between the polymer and the ?ller could improve the dispersion of CNT.On the other hand,the electrical conductivity of the composites results reduced.

M €u

ller et al.[5]reported about the in?uence of feeding conditions of different MWCNT grades on ?ller dispersion,electrical conductivity,and mechanical properties of PP-based composites.MWCNTs were fed at selected concen-trations either from the main hopper of a twin-screw extruder (TSE;Berstorff ZE 25)or by using a side-feeding unit under the same processing conditions (rotation speed,throughput,and temperature pro?le).The results indicate that,depending on the packing of pristine MWCNT agglomerates,the more compact MWCNT grade should be added from the main hopper,whereas for the more loosely packed MWCNT grade the addition using the side feeder leads to better dispersion,lower electrical resistivity,and higher improvements for mechanical properties.

Several others examples can be reported about papers that were focused on the effects of processing route and parameters on the properties of polymer/CNT composites,mainly prepared by masterbatch dilution technique,[6–13]but only few works investigated the speci?c effects of the melt viscosity during the melt-compounding process.

In order to study the in?uence of melt viscosity and molecular weight on nanotube dispersion and electrical resistivity,Kasaliwal et al.[14]used three different poly carbonate grades (PCs)varying in molecular weights.PCs were melt-compounded with MWCNT using a small-scale compounder and working at constant temperature but varying mixing speed,thereby applying different magnitudes of shear stress.The composites based on the lowest molecular weight matrix showed smaller sized un-dispersed primary agglomerates compared with com-posites that have higher molecular weight matrices,highlighting the role of matrix in?ltration into primary nanotube agglomerates as the ?rst step of dispersion.In a study recently reported by Socher et al.,[15]polymer/MWCNT composites were produced using ?ve different matrices each having three different levels of viscosity,in order to identify the general in?uence of matrix polarity and viscosity on the electrical properties and the state of MWCNT dispersion.The results shown as the lowest percolation thresholds were always found for the compo-sites based on the lowest viscosity grades.The effect of nanotube shortening was quanti?ed as well,showing that the phenomenon became more pronounced with high viscosity matrices,hence partially explaining their higher percolation thresholds.Together with dispersion,in fact,CNT length shortening is also affected by melt viscosity and can remarkably in?uence electric properties,as investigat-ed,for example,by Krause et al.[16]and Castillo et al.[17]

A recent study by Mi c u 9s ?k et al.[18]is particularly interesting,considering the aim of the present https://www.360docs.net/doc/269310362.html,parative experiments regarding the electrical and mechanical behavior of composites prepared using a commercially available PP/MWCNT masterbatch contain-ing 20wt%MWCNT are reported.The concentrate was diluted with various grades of PP characterized by different melt ?ow index values (MFI)and maleic anhydride (MA)grafting levels.The results demonstrate that the grade and viscosity of the diluting polymer plays an important role on the dispersion of CNT.In the presence of MA grafting,the best results were achieved in terms of interaction between polymer and ?ller but improvement of the dispersion was prevented by the incompatibility between modi?ed PP used for dilution and the PP/MWCNT pristine masterbatch.The authors also concluded that lower-molecular-weight chains can better penetrate inside the masterbatch and lead to better dispersion of the masterbatch particles as compared to longer chains.The increase of polydispersity index could be a further reason for inappropriate dispersion of CNT.

Regarding the CNT dispersion mechanism,during melt compounding several progressive steps are involved that also relate to processing conditions and material properties.According with Alig et al.,[19]the initial wetting of the micron-sized agglomerates depends mainly on the interfa-cial energy between CNT and polymer.Then the polymer in?ltration into the primary agglomerates occurs,and it represents a key phenomenon for the successive dispersion and distribution steps by means of erosion mechanism.

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In?ltration depends both on interface properties and on polymer viscosity.

The formation of secondary agglomerates,mostly during forming processing,has also been recognized as a further important step in?uencing composites ?nal properties in terms of electric conductivity.

In this paper,a comparative study between different PP-based CNT composites prepared by melt blending with a masterbatch route is reported.Considering in particular the discussion reported by Mi c u 9s ?k et al.[18]about the effects of melt viscosity on MWCNT dispersion in PP,the idea of the authors was to further investigate the possibility to maximize the polymer in?ltration mecha-nism at masterbatch level by using a particular ultra-low viscosity PP and without using chemical modi?cations or compatibilizers to improve polymer–?ller interactions.Moreover,the in?uence of polymer melt viscosity on the dispersion and properties of PP-CNT nanocomposites has been evaluated by using different standard PP grades (characterized by different MFI values)for masterbatch dilution in order to produce nanocomposites with compo-sitions varying between 0.25and 4wt%MWCNT.An extensive characterization has been done by means of electric,mechanical,thermal,and morphological analyses,in order to correlate materials structures and properties.

2.Experimental Section

2.1.Raw Materials

For the preparation of MWCNT masterbatch a high-?uidity PP from Basell Polyole?ns Company N.V.,Metocene MF650Y (MFI 1800g/10min at 2308C and 2.16kg),was used and labeled PP 1800.Three grades of PP homopolymer were considered for masterbatch dilution and labeled PP 450(MFI 450g/10min at 2308C and 2.16kg,Bor?ow HL504FB from Borealis Polyole?ne GmbH),PP 35(MFI 35g/10min at 2308C and 2.16kg,from Sigma–Aldrich Co.LLC)and PP 2(MFI 2g/10min at 2308C and 2.16kg,Hostalen PP N2122from Basell Polyole?ns Company N.V.).Commercial MWCNT (NC7000)produced by CCVD technique has been supplied by Nanocyl S.A.(Belgium).According to previous investigations,[16]NC7000nanotubes have an average diameter and length,respectively of 10.0and about 1300nm,and a purity of about

90%measured by means of thermal gravimetric analysis as declared by the producer.[20]

An estimation of the average molecular weights of the different PP grades has been done on the basis of the results reported by Bremner and Rudin.[21]The following equation has been purposed to correlate the MFI values of several commercial linear PPs with their average molecular weights (M w ):

k

MFI

?eM w Tx e1T

where k is a constant value depending on physical parameters such as polymer viscosity,and the mass used for MFI test and x varies between 3.4and 3.7.In Table 1,a comparison between the MFI values and the calculated average molecular weights for the different PP used in the study has been reported.

https://www.360docs.net/doc/269310362.html,posites Preparation

A co-rotating TSE (Dr.Collin Gmbh.Mod.ZK25)with a screw diameter of 25mm and a L /D ratio of 27has been used for masterbatch production and dilution.The extruder was equipped with one main volumetric feeding hopper and with a vacuum venting system in order to remove the vapors and gases during melt mixing.Experimental set-up and screw design are shown in Figure 1.

A ?xed masterbatch composition was chosen (MWCNT 10wt%and PP 180090wt%).The polymer was ?rst grounded to ?ne powder and then dry mixed with MWCNT (both dried overnight at 808C with a SGW Industry Owen by Crase s.r.l.,Italy).A temperature pro?le ranging from 150to 1608C was used together with 50rpm screws speed (25%of maximum value)and with a feeding throughput of about 2kg h à1.

Polymer-MWCNT composites containing 0.25–0.5–1–2–4wt%MWCNT have been then obtained by dilution of the masterbatch with three different grades of PP (PP 2,PP 35,and PP 450,respectively).Polymer and masterbatch in pellet form have been dried overnight at 808C (SGW Industry Owen by Crase s.r.l.)and dry premixed before being fed to TSE.The temperature pro?les ranged from 150to 1758C and the screw speed has been set up to 50rpm (25%of maximum screws speed).The amount of shear rate during dilution compounding was evaluated by means of the following equation:[14]

_g

?p eD à2d TN

e2

T

Polymer MFI (2308C/2.16kg)

[g/600s]

M w

[g mol à1]eh ?M =h ?

PP Tg _?10eh ?M =h ?

PP Tg _?100eh ?M =h ?

PP Tg

_?250PP 1800180072552–––PP 4504501054891903014PP 35352102203062PP 2

2

455297

14

4

2

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where D is the diameter of the screw (25mm),d is the screw-barrel clearance (ranging from 200to 300m m),and N is the screws speed in revolutions per second.Considering these data together with the screw speed conditions reported above,theoretical shear rate values ranging from 200to 300s à1have been calculated.This estimation will be useful for the evaluation of melt viscosities values during compounding conditions.

A compression molding device (Dr.Collin Gmbh.Mod.P200)has been used for the production of samples (mainly for electrical and mechanical characterization)working at a temperature of 2008C and with a maximum pressure of 15bar and a cycle time of 840s (240s for heating/molding,600s for cooling to room temperature at about 158C min à1).

https://www.360docs.net/doc/269310362.html,posites Characterization

Sinusoidal measurements were performed in shear with a Rheometrics Mechanical Spectrometer (RMS)model 800.The linear viscoelastic functions were measured using the 25mm parallel plate geometry (gaps ranging between 0.8and 0.9mm)at 1808C in a frequency range between 0.1and 300rad s à1.Round disks with a diameter of 20mm were cut from compression molded samples.The temperature was stable within 0.28C in the range of measure.Strain sweeps were previously performed to ensure a linear viscoelastic response and strain values between 3and 150%were chosen.All tests were done under a N 2atmosphere in order to prevent polymer degradation.

According to the method recently reported by Socher et al.,[15]the state of CNT dispersion at micron scale in the composites was studied using light transmission microscopy (LM)technique.Thin sections of a few microns in thickness were cut from compression-molded samples using a LKB 2088Ultrotome V.The samples were then observed with a Leica DMR microscope combined with a Leica DFC480R2camera.The software LAS Version 3.8.0has then been used for image acquisition.

The agglomerate area ratio A /A 0was determined from the light micrographs by calculating the ratio of the total area A of remaining agglomerates to the total area of the micrograph A 0using the software ImageJ Version 1.46r.Agglomerates larger than 5m m were taken into account for the calculation.[15]Several cuts were investigated for each sample and an average value together with the experimental standard deviation was considered.

The morphology of the composites at nanoscale has been assessed by transmission electron microscopy (TEM).Thin slices with a thickness of about 100nm have been obtained with a LKB Ultratome V from compression molded samples and observed with a TEM model FEI Tecnai 12.In order to estimate MWCNT average lengths,the software ImageJ Version 1.46r has been used to measure CNT lengths from TEM images using the full visible length of each separate nanotube.The collected data were then analyzed in order to obtain the average values together with the standard deviations.

The electric volume resistivity was measured on compression-molded samples (thin disks with diameter 60mm and thickness 0.4–0.6mm).For low-conductivity samples (i.e.,with volume resistivity higher than 108V cm),a Keithley 8009Resistivity Test Fixture combined with a Keithley electrometer Model 6517were used.For conductive samples (i.e.,with volume resistivity lower than 107V cm),strips 50mm long and 10mm wide were connected with silver contacts to a DMM 2000Keithley multimeter.

Polymer crystallization properties were investigated by means of standard differential scanning calorimetry (DSC Q200;TA Instruments,USA).Specimens were cyclically heated and cooled from 40to 2208C in hermetic pans,using heating/cooling ramps and working at 108C min à1in nitrogen atmosphere.

Dynamic-mechanical measurements (DMA Q800;TA Instru-ments)were carried out on compression molded samples (width 10mm,thickness 4mm,and length 20mm)in order to analyze the amorphous phase behavior of the composites in dynamic and non-isothermal conditions.Temperature ramp measurements were performed using a single cantilever ?xture at a frequency

of

Figure https://www.360docs.net/doc/269310362.html,yout of the melt processing apparatus with the details of screws con?guration.

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1Hz (amplitude 15m m)and at an heating rate of 38C min à1from à100to 1208C.

The real MWCNT contents in polymer/MWCNT composites were assessed with a Q600analyzer (TA Instruments)operating in nitrogen from ambient temperature to 10008C at a heating rate of 208C min à1,and the residue was analyzed after thermal degrada-tion at 8008C.The average weight of the analyzed samples was about 10mg.

Flexural modulus and deformation under maximum load were measured using a universal testing machine (Sun 2500,Galdabini,Italy)operating with a crosshead speed of 2mm min à1according to UNI ISO 128,with specimens having dimensions of 10,4,and 80mm for width,thickness,and length,respectively.All test specimens were conditioned in air at 238C and 40%R.H.for at least 48h before being analyzed.

3.Results and Discussion

3.1.Rheological Characterization

Viscosity curves have been obtained for all the pristine PP and for the concentrate masterbatch by means of a rotational rheometer in plate-plate geometry operating at 1808C.The results in terms of complex viscosity (Eta ?)versus frequency are reported in Figure 2.Remarkable differences can be observed between the three PP grades,and as expected,the complex viscosity of PP 2is higher than the one of PP 35(and both are higher than the one of PP 450)in the whole analysis range.On the other hand,the peculiar trend of the masterbatch,with a non-Newtonian behavior even at low frequencies,is typical of percolated systems containing high amounts of CNT.

According to the results obtained with Equation (2)and considering the frequency-shear rate equivalence stated by Cox–Mertz rule,the complex viscosity values (Eta ?)at the

higher frequencies (200–300rad s à1)have to be considered in order to estimate the relative melt behavior of the different matrices during melt mixing.

The viscosity ratios have been calculated as the ratios between masterbatch and pristine PP complex viscosities at speci?c shear rates considering three different frequencies below (10and 100rad s à1)and close (250rad s à1)to processing conditions.The results have been reported in Table 1.While for the lowest frequency value complex viscosities ratios higher than 1000have been be obtained,considering the frequencies in the range of melt compounding (250rad s à1)these values drop down in a range of between 2and 14.As a simple indication,in a shear ?ow and considering a Newtonian behavior of the ?uid,the critical capillary number indicating particle breakup is typically reached with viscosity ratios ranging from 0.1and 2.[22]Hence,based on this criteria and assuming the Cox–Merz law is applicable at these high shear rates,PP 2and PP 35likely can theoretically show droplet breakup.

The issue is in fact much more complex and the differences in melt viscosities for pristine PP 1800and diluting PP grades have to be considered as well.

The high difference in average molecular weights (Table 1),in particular between PP 1800and PP 2,could remarkably in?uence polymers miscibility in the molten state and therefore the morphology and properties of ?nal compounds.As previously reported by Mi c u 9s ?k [18]an excessive difference for masterbatch and diluting PP M w ,leading to an increase of polydispersity index,can be identi?ed a reason for inappropriate dispersion of the ?ller.3.2.Evaluation of MWCNT Content

The real amount of MWCNT with respect to the 10wt%MWCNT nominal value has been measured by means of TGA using Equation (3)and considering the residues of pure components after thermal degradation in nitrogen at 8008C.Considering the residue of the composite (R C ),of pristine MWCNT (R MWCNT ,96.6wt%),and of pristine polymer (R PP1800,0.69wt%),the weight fraction of MWCNT (v )can be calculated as follows:R C ?v R MWCNT te1àv TR PP1800

e3T

An experimental value of (10.3?0.16)wt%has been found,with a low standard deviation value that indicates a good homogeneity of the masterbatch.The comparison between nominal and real CNTs contents (calculated by Equation 3)for the different composites indicates that the real values are in good according with the nominal ones (see Figure 3),within experimental error.Regarding MWCNT 0.25wt%composites,due to the low amount of residue produced after thermal degradation,a reliable value

could

Figure https://www.360docs.net/doc/269310362.html,plex melt viscosity versus frequency plot for the different pristine polypropylenes and for the PP 1800/MWCNT 10wt%

masterbatch.

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not be obtained due to the in?uence of instrument sensitivity on experimental https://www.360docs.net/doc/269310362.html,posites Morphology

A simple but effective approach to evaluate composites morphology is light microscopy.In order to quantify the dispersion of primary agglomerates,the agglomerate area ratio A /A 0has been evaluated analyzing thin cuts from the composites at different MWCNT contents above and below percolation (see Figure 7about percolation behavior).

In Figure 4,a comparison between the composites obtained by dilution with PP 2,PP 35,and PP 450at different CNT contents has been reported.In a qualitative way,for PP 35-and PP 450-based composites,the particles dispersions are very similar and homogeneous.Moreover,considering A /A 0ratio,the calculated values are very low,hence indicating an effective dispersion during melt mixing with a very small amount of residue agglomerates with dimensions higher than the set up threshold.On the other hand,for PP 2-based composites,a generalized presence of large residue aggregates with dimensions varying from 20up to 100m m (not reported here)can be observed.In this case,the agglomerate area ratio can only con?rm the qualitative observation,with average values that are about one order greater than the ones obtained for PP 35and PP 450composites.Due to their shape and dimensions these agglomerates,apparently due to masterbatch drop-lets not in?ltrated by the polymer,can be only related to the uneffective mixing during dilution with PP 2.Hence,even if in presence of a favorable masterbatch/diluting PP viscosity ratio value (see Table 1),the worst results in terms of dispersion have been obtained.This may indicate that,even if pristine PP 1800rheological properties are remarkably in?uenced by 10wt%CNT content,when dealing with highly ?lled systems a prevision based on the comparison between the melt rheology of masterbatch and diluting polymer (Figure 4)may bring to improper hypothesis.

Beside dispersion,also MWCNT shortening must be considered due to its potential effects on the physical properties of the composites.This issue has been recently investigated by Socher et al.[15]and by Krause et al.[16,23]In order to assess the effects of the different melt viscosities of PP 2,PP 35,and PP 450on CNT lengths reduction,the determination of an average CNT length has been done.Several methods to compare length distribution of as-grown tubes and of CNT recovered from the composites by dissolution has been recently described.[24]

A statistic estimation of the average CNT length of the composites based on PP 35(low percolation threshold,see Figure 7)and on PP 2(high percolation threshold,see Figure 5)has been done by analyzing their

TEM

Figure 3.TGA estimation of the real MWCNT content for the composites obtained varying dilution PP grade (a:PP 450,b:PP 35,and c:PP 2)and ?ller

amount.

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micrographs,with the only aim to compare the average lengths of the particles in the two situations (hence without consider the obtained values as absolute values).The analysis of 2D TEM images of 3D particles,together with shortening effects due to nanotubes cutting during samples

microtoming,leads in fact to estimation errors that,even being the same in the two cases,on the other hand cannot allow an accurate and absolute value of particles length.The results show an average length of 431?131nm for PP 35base composites and of 425?120nm for PP 2-based ones.Considering the standard deviation,indicate a measured average length value that is almost the same for both composites (both containing 0.5wt%MWCNT for compari-son),hence with no peculiar effects on MWCNT lengths due to different polymer viscosities.3.4.Electrical Properties

The data regarding electrical characterization of the composites obtained diluting PP 1800-based masterbatch with several different PP grades are reported in Figure 5.The best results in terms of percolation performances were achieved with PP 35-and PP 450-based composites,as indicated by the sharp drop of electrical resistivity between 0.25and 0.5wt%MWCNT corresponding approximately to 0.13–0.25vol%if a density of 1.8g cm à3is considered for the ?ller.[18]PP 2-based composites show instead a different behavior with a remarkable shift of percolation threshold up to above MWCNT 1wt%.

Considering the evidences from the discussion of the morphological characterization (evaluation of

CNT

Figure 4.Light microscopy images of composites containing different amounts of MWCNT (below and above percolation,see also Figure 7)and obtained by dilution using different PP grades.Agglomerate area ratios A /A 0are indicated for each

sample.

Figure 5.Electrical properties measured for PP-MWCNT nanocomposites based on different PPs and with different MWCNT

contents.

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dispersion and shortening),the low percolation threshold obtained for PP 35and PP 450composites is de?nitely due to an effective dispersion during melt mixing,with a very low fraction of residue primary agglomerates in the ?nal composites (Figure 4).When using low viscosity PP for masterbatch production and dilution,effective in?ltration and consequently an effective CNT dispersion can be achieved,hence remarkably low percolation thresholds have been found with respect to the results reported in previous studies about similar systems.[18]The higher value of percolation achieved with PP 2-based composites,as already indicated when discussing compounds morpholo-gy,is probably due to an excessive difference of the molecular weights (hence of the rheological properties)of PP 2and PP 1800.This is also in accordance with what was reported by other authors about similar systems,[18]indicating that mixing polymers with the same chemical structure but remarkably different macromolecular lengths leads to worse dispersion of the ?ller in the ?nal composites and hence to less effective improvements of electrical conductivity.Moreover,considering the similar magnitude of CNT shortening for PP 35-and PP 2-based composites,the phenomenon of length reduction cannot be addressed to investigate whether it is responsible for the worse electrical behavior of the latter.3.5.Thermal Properties

Observing Figure 6,the temperature of crystallization of pristine PP 1800is intermediate between the ones of pristine PP 2and PP 450and very close to that of pure PP 35.An increase of the crystallization temperature for all the analyzed composites can be highlighted regardless of the type of PP used for dilution.Regarding PP 1800possible effects on the crystallization temperatures of the composites,it has recently been reported that,for similar systems,the lowest is the molecular weight of the polymer,the highest is the number of free chain ends increasing the nucleation density.[18]Moreover,regarding MWCNT effects,other authors already emphasized their action in promoting PP heterogeneous crystallization together with a nucleation effect regarding a -crystalline form.[18,24]

As the degree of crystallinity can in?uence the mechani-cal properties,the melting enthalpies of the composites have been compared.In Figure 7,the normalized speci?c melting enthalpies (with respect to pristine PPs values)have been evaluated by integration of melting peaks during second heating step and have been plotted as a function of MWCNT content and of the different PP grades.Moreover,the speci?c enthalpies have been evaluated considering the real amount of crystallizable polymer in the composites,hence rescaling the weight of the samples with the real MWCNT contents evaluated by TGA using Equation 3(see also Figure 3).

Two opposite experimental trends can be observed for PP 35composites at one side and for PP 450and PP 2composites on the other.In the ?rst case,a maximum increase up to 40%(4wt%MWCNT)for the normalized melting enthalpy value can be observed while for PP 450-and PP 2-based composites a maximum reduction of about 20%(4wt%MWCNT)can be evidenced.

A resonable explanation for this behavior can be related to a melt plasticizing effect of PP 1800for PP 35due to the effective melt mixing of the two PP grades.On the other hand,due to the remarkable difference of their

molecular

Figure 6.Crystallization temperatures measured by DSC for pristine PPs and relative MWCNT-based composites at different

compositions.

Figure 7.Melting enthalpies measured by DSC for pristine PPs and relative MWCNT-based composites at different compositions.Values normalized with pristine PPs

ones.

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weights,only little mixing probably occurred between PP1800and PP2melts,hence preventing melt plasticization phenomena.

Analyzing separately the three situations,for PP450-based composites the crystallization process can be hypothesized to be thermodynamically controlled,with the defects of PP1800leading to a decrease of the PP450 crystallinity due to the mixing occurring between the melts.Regarding PP35-based composites and assuming a mixing between the melts as well,the reduction of crystallinity induced by PP1800defects is probably compen-sated and overcome by a kinetic control,hence with the result of a global increase of PP35crystallinity that is proportional with PP1800content.

Following this hypothesis,the decrease of PP2crystallin-ity can only be explained with an absence of melt mixing (or a very limited degree of melt mixing)between PP2and PP1800,hence with the two PP crystallizing separately and causing a reduction of PP2crystallinity.

The amorphous phase of PP composites has been investigated by means of dynamic-mechanical tests evaluating the behavior of glass transition temperatures (T g),which are directly related to the cohesive energy and packing density of the polymer.In Figure8,T g variation versus MWCNT content has been reported(measured as the maximum of loss modulus function)as a function of dilution PP grades.Regarding pristine PPs,it has been found that their T g vary from3to58C above08C for PP450, PP35,and PP1800,whereas only PP2has a glass transition below08C.

Even if in every case a single and well-de?ned loss modulus peak has been identi?ed,due to the slight differences between the different PP grades T g values,no considerations can be made about their miscibility. However,an effect of PP1800can be clearly evidenced on the decrease of the T g for PP450-and PP35-based composites and on the increase of the same for PP2-based ones.

3.6.Mechanical Properties

The reinforcing effect of nanoparticles can be considered as a further indirect index of the magnitude of?ller dispersion: the higher is polymer–?ller contact interface area,the higher is the reinforcing effect without remarkable losses in ductility,due to effective MWCNT–macromolecules interactions at nanoscale.On the other hand,it has been reported that the addition of MWCNT can lead to an increase of elastic moduli followed by decreases in elongation and ductility due to ductile–brittle behavior transitions,[24,25]together with the presence of residue MWCNT agglomerates acting as stress concentrators. The behavior of?exural elastic modulus has been investigated as a function of MWCNT content for the nanocomposites obtained by dilution with the three different PP grades.The resulting values,normalized with respect to pristine matrices,are reported in Figure9.Even considering the experimental error,remarkable increases have been observed as a function of MWCNT content, independently from the polymer used for dilution and with similar trends for all the three cases considered. The considered systems have been obtained by dilution of a masterbatch containing about10wt%of MWCNT and 90wt%PP1800.Therefore,a considerable amount of PP1800 has been melt mixed with PP2,PP35,and PP450

during Figure8.Glass transition temperatures measured by DMA(peak

of loss modulus function)for pristine polypropylenes and relative

MWCNT-based nanocomposites at different compositions and

compared with theoretical behavior described by Fox

equation.

Figure9.Flexural elastic moduli for the different composites

obtained by dilution with PP450,PP35,and PP2.Values normalized

on respective pristine PPs

ones.

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dilution step.Hence,for a correct evaluation of the net MWCNT effects on the ?exural modulus of the composites,the contribution of PP 1800must be considered as well.Considering that PP 1800modulus is remarkably higher than the one of PP 450(1.2times),PP 35(1.4times),and PP 2(1.3times),its contribution to the increase of the overall ?exural moduli has been evaluated by means of a classic linear rule of mixing (even considering the partial miscibility of the polymers,see thermal characterization for details).A reinforcing effect due to PP 1800has been estimated,with an average contribution to the overall moduli increases ranging from 10%for PP 450-based composites up to about 25%for PP 2-based ones.

A further factor partially affecting the mechanical behavior is the amount of PP crystalline phase.The degree of crystallinity in?uences Young’s modulus due to the high stiffness of crystalline phase if compared to amorphous one.Considering the results of thermal properties characteriza-tion by DSC (Figure 7),the amount of crystalline phase increases only for PP 35-based composites,with a possible contribution to the overall moduli increase (not estimated here)while for PP 450-and PP 2-based composites the decrease of melting enthalpies seems to indicate that the increase of ?exural elastic moduli is completely due to CNT and PP 1800reinforcing effects.

In Figure 10,the values of ?exural deformation under maximum load (coinciding with deformation at break for brittle samples,indicated with a cross)for the different composites have been compared with respect to pristine PPs properties (normalized data).It is important to observe that,whereas for pristine PP 450the deformation at

break and deformation at maximum load coincide,for PP 35-based composites the fragile failure is reached only for MWCNT contents above 1wt%(MWCNT 4wt%for PP 2-based ones).

Hence PP 450composites,because of the intrinsic brittle-ness of pristine PP matrix,are only partially affected by the negative effects on ductility caused by MWCNT,with a maximum reduction of deformation at break (correspond-ing with deformation at maximum load)of 20%at 4wt%MWCNT content.On the other hand,PP 2-based composites maintain a ductility similar the one of pristine PP 2until 4wt %MWCNT content,where a drop of 50%can be observed.PP 35-based composites have an intermediate behavior,with a remarkable reduction of deformation under maxi-mum load that can be observed already for 1wt%MWCNT content and decreasing progressively way with the increase of ?ller content until a drop of 75%at 4wt%MWCNT.A complementary effect is of course due to the characteristic brittleness of PP 1800.

4.Conclusions

In this paper,PP/MWCNT nanocomposites have been produced by melt compounding using a 2-step masterbatch route.In order to maximize polymer in?ltration and hence to facilitate the dispersion of the ?ller during dilution step,an ultra-low viscosity PP grade has been used as polymer matrix for masterbatch,without adding compatibilizers or chemical modi?cations for CNT.Moreover three different PP grades have been considered for masterbatch dilution,in order to assess the in?uence of melt viscosity during this step as well.

The possibility to obtain very low percolation thresholds by taking advantage from the promoted polymer in?ltra-tion at masterbatch level has been con?rmed by the results obtained with morphological and electrical characteriza-tion.It can be concluded that,when ultra-low molecular weight polymer is used for masterbatch production,then to get the optimal dispersion in the ?nal compound the molecular weight of the diluting polymer cannot be too high as to not mix effectively with the lower M w master-batch resin.

Even if masterbatch and diluting PP grades can show a certain matching level regarding their rheological proper-ties,due to the complexity of melt mixing process the properties of pristine masterbatch carrier polymer must be considered as well,possibly avoiding excessive polydisper-sity indices (hence possible miscibility gaps)in the ?nal compounds.

The low degree of mixing between PP 2and PP 1800has been deduced also from melting enthalpy data analysis.In particular,it has been observed that when melt mixing occurs a plasticization effect due to PP 1800

ultra-low

Figure 10.Deformation under maximum load for the different composites obtained by dilution with PP 450,PP 35,and PP 2.Values normalized on respective pristine PPs ones.The ‘x’symbols indicate the samples with brittle behavior (deformation under maximum load coincides with deformation at

break).

Influence of Melt Viscosity on the Structure and Properties ...

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molecular weight can induce an increase of the degree of crystallinity due to a kinetically driven crystallization mechanism.In the absence of melt mixing,plasticization does not occur,with the result of a decrease in diluting PP crystalline content as in the case of PP2-based https://www.360docs.net/doc/269310362.html,T shortening has also been evaluated in order to investigate possible correlations with diluting PPs melt viscosities.It has been observed that the CNT shortening phenomena,often worsening electrical properties,in this case is independent from matrix viscosity but mainly affected by processing conditions.

Acknowledgements:The authors acknowledge Dr.Federico Caicci and Dr.Francesco Boldrin of Electron Microscopy Service of Department of Biology at University of Padova (Italy).This study was ?nanced by University of Padova under the PRAT project CPDA115982/11.

Received:August 28,2013;Revised:November 5,2013;Published online:January 20,2014;DOI:10.1002/mame.201300336Keywords:carbon nanotubes;melt mixing;nanocomposites;poly(propylene);processing

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