Material Behaviour
Multiwalled carbon nanotube promotes crystallisation while
preserving co-continuous phase morphology of polycarbonate/
polypropylene blend
Mohammed Arif P.
a, Sarathchandran C.
b, Aravinda Narayanan
c, Allisson Saiter
d,
Roberto Terzano
e, Ignazio Allegretta
e, Carlo Por
fido
e, Nandakkumar Kalarikkal
a,f,**,
Sabu Thomas
a,b,*aInternational and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, Kerala 686560, India bSchool of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala 686560, India
cDepartment of Physics, Birla Institute of Technology& Science, Pilani, Hyderabad 500078, India
dNormandie Univ, UNIROUEN Normandie, INSA Rouen, CNRS, Groupe de Physique des Materiaux, 76000 Rouen, France eDepartment of Soil, Plant and Food Sciences, University of Bari“Aldo Moro”, Via Amendola 165/A, 70126 Bari, Italy fSchool of Pure and Applied Physics, Mahatma Gandhi University, Kottayam, Kerala 686560, India
a r t i c l e i n f o
Article history: Received 9 August 2017 Received in revised form 16 September 2017 Accepted 18 September 2017 Available online 22 September 2017
Keywords:
Blend nanocomposites Co-continuous morphology SAXS
Modulated temperature DSC X-ray micro-computed tomography
a b s t r a c t
The influence of multiwalled carbon nanotubes (MWCNTs) on phase morphology, lamellar structure, thermal stability, melting behaviour and isothermal crystallisation kinetics of polycarbonate/poly-propylene (PC/PP) blend nanocomposites has been investigated. Both neat blends and PC/PP (60/40)/ MWCNT nanocomposites were prepared by melt mixing method. Morphological analyses were per-formed by high-resolution X-ray micro-computed tomography and scanning electron microscopy. The co-continuous morphology of the blend was retained irrespective of MWCNT loading. In addition, a substantial refinement in the co-continuous structure was observed. Wide angle and small angle X-ray scattering studies were used to analyse the structural properties of the blend nanocomposites. The addition of MWCNT increases the long period of polypropylene. The influence of addition of MWCNT on the crystallisation temperature and equilibrium melting temperature (Tm) of polypropylene was fol-lowed. The MWCNTs promote crystallisation rate of polypropylene in the blend nanocomposites.
© 2017 Elsevier Ltd. All rights reserved.
1. Introduction
The applications of polymer blend nanocomposites in diverse fields are strongly controlled by their microstructure, compatibility, crystallinity, thermal properties, and viscoelastic behaviour[1e4]. The compatibility of the components in a polymer blend plays a crucial role in deciding the various end uses. The lack of compati-bility between blend components leads to phase separation, thus resulting in poor mechanical properties. However, this is not always
true since, in some cases, a synergistic improvement in properties can be obtained. The morphology controls the physical properties of polymer blends and then ultimately determines the usefulness of the polymer blends. Polypropylene (PP) and polycarbonate (PC) are thermoplastics having good mechanical properties and process-ability. However, the low heat resistance and poor low-temperature impact performance of PP restrict its application to a limited range of service temperature. In order to overcome these drawbacks, blending of PP with engineering thermoplastics having higher heat resistance and better impact properties is practised. On the other hand, PC is an expensive engineering thermoplastic having high heat and impact resistance so that blending of PP with PC could provide a new blend system which could be used in many appli-cations in an economical way. However, PC/PP blends are immis-cible. Depending on blend composition, the morphology may vary from the dispersed phase of PC in PP to co-continuous phase of PP and PC which in turn affects the mechanical properties[5]. PP is a * Corresponding author. International and Inter University Centre for
Nano-science and Nanotechnology, Mahatma Gandhi University, Kottayam, Kerala 686560, India.
** Corresponding author. International and Inter University Centre for Nano-science and Nanotechnology, Mahatma Gandhi University, Kottayam, Kerala 686560, India.
E-mail address:sabupolymer@yahoo.com(S. Thomas).
Contents lists available atScienceDirect
Polymer Testing
j o u rn a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / p o l y t e s t
https://doi.org/10.1016/j.polymertesting.2017.09.026
semi-crystalline polymer and PC is an amorphous polymer. In blends, if one component is a semi-crystalline, the existence of the second component has a substantial effect on the crystallisation process[6]. This complex process depends on many factors such as the second component, the miscibility, the thermal history, the processing conditions, the degree of dispersion, etc. The nanofillers have a strong ability to enhance the crystallisation of polymer matrices [7]. Carbon nanotubes (CNTs) have been widely estab-lished as efficient nucleating agents that facilitate the crystal-lisation of polymers [5]. The integration of CNTs into a polymer matrix leads to enhancement of mechanical properties and to a superior morphological stability[8]. Sharma et al. found that long period (L) of PVDF increases in poly (vinylidene fluoride)/poly (methyl methacrylate) (PVDF/PMMA) blend as compared to the neat PVDF[9]. The increase of long period (L) with the addition of PMMA establishes the presence of PMMA in the interlamellar re-gions enabling in amorphous miscibility. Xiang et al. studied that crystallisation and melting properties of MWCNTs based high-density polyethylene/polyamide 6 (HDPE/PA6) blend nano-composites[10]. Lee et al. reported that the thermal properties of PLA/PP/MWCNT blend nanocomposites had improved with the increase in MWCNT content[11]. Li et al. reported the composition and morphology effects on the crystallisation properties of PP in the PC/PP blend composites[12]. Laoutid et al. studied the effects of nanosilica loading on the crystallisation properties of PP in the PP/ PC blends[13].
The key questions are the following: Does the addition of MWCNT alter the phase morphology of the PP/PC blend? Does MWCNT reinforce the blend and thereby increase its thermal sta-bility? Does MWCNT effect the crystallisation behaviour of PP in the PC/PP blend? In this work, X-ray micro computed tomography and scanning electron microscopy were used as the testing methods to investigate the phase morphology of blend nanocomposites con-taining MWCNTs. The thermal stability of the blend nano-composites were characterised using thermo-gravimetric method. The effect of MWCNT loading on structural and crystallisation ki-netics of the developed PC/PP/MWCNT blend nanocomposites were studied using WAXS/SAXS and differential scanning calorimetry techniques.
2. Experimental and characterisations 2.1. Materials
Polycarbonate (Makrolon® 6557- density¼ 1200 kg/m3, melt flow index (300C/1.2 kg)¼ 10 g/10min) was purchased from Bayer Material Science, Germany. The isotactic polypropylene (H350 FG-density ¼ 946 kg/m3, melt flow index (230C/2.16 kg) ¼ 38 g/ 10min) was supplied by Reliance India Ltd, India. The multiwalled carbon nanotubes (90% purity) was provided by Nanocyl (NC7000), Belgium. The average length of MWCNTs is 1.5
m
m and the diameter varies from 10 to 20 nm.2.2. Sample preparation
All the composites were prepared by Brabender-33 internal melt mixer with a cavity size of 55 cm3and a chamber temperature of 230C. Mixing was conducted at 60 rpm for 10 min. Prior to mixing, PC was dried at 120C in an oven for 12 h to remove all moisture. The prepared samples were compression molded (100 kg cm2) at a temperature of 230C and were then cooled to room temperature at the same pressure.
2.3. Morphological studies
The morphology of PC/PP blends and MWCNT based nano-composites were studied using Scanning Electron Microscopy (SEM) (JEOL- ESEM). Samples were cryogenically fractured and the cut surface was coated with gold in order to make it conductive.
High-resolution X-ray micro-Computed Tomography (micro-CT) was also used to study the morphology of PC/PP blend and MWCNT based nanocomposites and to evaluate the effect of MWCNTs addition on the blend co-continuous morphology. Micro-CT ana-lyses were performed using a Sky Scan 1272 high resolution micro-tomograph (Bruker Gmbh, Berlin) equipped with a W micro-focus source (<5
m
m spot size) and a 16 MP CCD detector. For each polymer sample, a small specimen of about 1.5 1.5 7 mm was cut with a stainless steel cutter and analysed with a source voltage of 40 kV and a current of 250m
A. The scanning conditions were: pixel size 0.70m
m, rotation step 0.10 deg (0e180 deg), exposure time of 1150 min andflat field correction on. After scanning and projection images acquisition, reconstruction was performed by using the NRecon software (version 1.6.10.4, InstaRecon®). A 500m
m cubical volume of interest (VOI) was selected for each blend (being representative of the whole scanned volume) by using the software DataViewer (version 1.5.2.4, Bruker microCT®). The 3D rendering and the morphology were obtained by means of the software CTvox (version 3.1.1 r1191, Bruker microCT®). Morpho-metric analysis and 3D modelling and visualization were carried out respectively with the software CT Analyser (version 1.15.4.0þ 4, Bruker microCT®) and CTvol (version 2.3.1.0 4, Bruker microCT®). 2.4. WAXS/SAXS measurementsWide- and small-angle X-ray scattering (WAXS/SAXS) mea-surements were performed using a XEUSS SAXS/WAXS system by Xenocs, operated at 50 kV and 0.60 mA. The X-ray radiation was collimated with a FOX2D mirror and two pairs of scatter less slits from Xenocs. The data were collected in the transmission mode geometry using Cu K
a
radiation (wavelengthl
¼ 1.54 Å).2.5. Calorimetric investigations
The calorimetric investigations were studied with a Q100 Calorimeter (TA instrument®). Using standard Tzerotechnology, the instrument was calibrated for baseline, heatflow and temperature. The nitrogen gasflow was 50 mL min1for all the experiments. The calibration of the temperature was execute using indium and zinc as standards, and the calibration in energy was carried out using indium. The samples were subjected to afirst heating from 40C up to 200C, cooled down as quickly as possible for quenching and a second heating was applied by using a modulated temperature program (MT-DSC measurements): a heat-only mode (oscillation amplitude of 0.318C, oscillation period of 60 s and heating rate of 2C min1from40C up to 200C), which is advised to study semi-crystalline systems [13e15]. The corresponding reversing heatflow signals are presented here for all the samples.
The melting and crystallisation kinetics of PP in PC/PP blend and PC/PP/MWCNT nanocomposites were also investigated in detail using PerkinElmer Diamond DSC. PC/PP blend and PC/PP/MWCNT nanocomposites were heated to a temperature well above the Tmof PP followed by rapid cooling (40C min1) to the selected Tcof 132C and held at that temperature for 60 min. The sample was then heated at 10C$min1to 200C. The endotherm obtained was used to calculate the crystallisation kinetics. Samples were then cooled down to room temperature at 10C$min1. The crystal-lisation temperature (Tc) selected was 132C, as the crystallisation proceeded at a reasonable rate at this temperature.
2.6. Thermo-gravimetric analysis
Thermal stability of all the sample was investigated by a Simultaneous Thermal Analyser (Perkin Elmer STA 6000, USA) under nitrogen atmosphere from ambient up to 800 C at 20C$min1.
2.7. Fourier transform infrared spectroscopy
FTIR analysis was performed by Shimadzu IR prestige 21 FTIR spectrometer. The absorption spectra were recorded from 650 to 4000 cm1using a ZnSe ATR attachment.
3. Result and discussions 3.1. Morphological studies
The effects of different PC/PP ratios on the phase morphology of PC/PP blends are shown inFig. 1. The phase separation between PP and PC is seen for all the blend composites, because of PP/PC blends are immiscible. For PC/PP (20/80), PC is a minor phase and dispersed in the PP matrix marked as red arrows (Fig. 1a). For PC/PP (40/60) blend, the dispersed phase is not regular, having an ellip-tical or sphere-shaped shape (Fig. 1b). Co-continuous morphology observed in PC/PP (60/40) blends, is shown inFig. 1c. When the PC content is more than 60%, PP became the dispersed phase with small droplets marked as yellow arrows (Fig. 1d).
The number-average domain diameter (Dn) and the weighteaverage domain diameter (Dw) values of PC/PP are given in
Fig. 2. The Dnvalue of dispersed polycarbonate phase is 9.6
m
m for the PC/PP (10/90) blend and 25.19m
m for the PC/PP (30/70) blend, respectively. The Dnand Dwvalues were found to increase with PC content and this is due to the coalescence of the PC phase. In the PC/ PP (40/60) and PC/PP (50/50) blend the PC phase has no regular shape and the domain diameter cannot be estimated. For the PC/PP (60/40) blend, both PC and PP phases form a continuous structure.A careful examination of the PC/PP(60/40) SEM images (Fig. 1c), revealed a continuous PC phase in which PP domains are dispersed and a continuous PP phase in which PC particles are dispersed (marked as red circles in Fig. 1c). This complex co-continuous morphology may be due to viscoelastic phase separation, viscos-ity mismatch and the large difference in the glass transition tem-perature (Tg) of the blend components (Tgof PC around 145C and Tgof PP around 0C). A similar behaviour was reported by Shi et al. for PP/PMMA blends[16].
Fig. 3shows the SEM micrographs of the cryo-fractured blend surface of PC/PP (60/40)/MWCNT nanocomposites. Interestingly, the blend with MWCNT also displays co-continuous morphology. A refinement of the co-continuous structure can be seen with the addition of MWCNTs. The compatibilising action of MWCNT re-duces the interfacial tension and interfacial thickness. The blend
Fig. 1. SEM images of PC/PP blends: (a) PC/PP (20/80); (b) PC/PP (40/60); (c) PC/PP (60/40) (d) PC/PP (80/20).
system with MWCNT also showed viscoelastic phase separation process as indicated by the phase in phase morphology. Due to the polarity of the PC phase, most of the MWCNTs could be localised in the PC phase and thus the viscosity of PC phase was increased. This leads to increase the viscosity mismatch between the PC and PP phases. Therefore, viscoelastic phase separation process is promi-nent in the system.
Fig. 4shows the phase tomograms of the PC/PP blend nano-composites with and without MWCNTs. The bright and dark phases correspond to the PC and PP rich phases, respectively. The phases in the nanocomposites are clearly visible and show co-continuous morphology. From the tomography images, the PC/PP (60/40) blend and PC/PP (60/40)/MWCNT nanocomposites show a co-continuous morphology. This aspect can be confirmed from the 3D models of PC/PP blends with 3% and 5% of MWCNT loading (Fig. 5). In the images, the orange colour corresponds to the PC phase and violet corresponds to the PP phase. The extent of the interpenetration of both the phases in the co-continuous micro-structure improves dramatically with the addition of MWCNTs. FromFig. 4, as the loading of MWCNTs increases, a refinement in the co-continuous morphology can be seen. The phase thicknesses of the both phases reduced upon addition of MWCNT into the blend. This may be due to the reduction of the interfacial tension and increased interfacial thickness through the compatibilising action of MWCNTs.
From X-ray phase tomography-volume rendering image of nanocomposites, clearly showed that phase in phase morphology is due to the viscoelastic phase separation process. InFig. 5c and f, a continuous PC phase in which PP droplets are dispersed and a continuous PP phase in which PC particles are dispersed can be observed. Similar structures are observed in the SEM images also. Such a complex phase structure is associated with the viscoelastic phase separation process which is associated with the dynamic asymmetry arising from the differences in Tgand viscosity disparity between PP and PC. The less viscous PP encapsulates the more
viscous PC particles to minimise the energy of mixing.
From SEM and X-ray tomography studies, we could confirm that the co-continues morphology of PC/PP (60/40) remains unchanged with the addition of MWCNTs, and MWCNTs plays a major role in compatibilising the immiscible PC/PP blend system, by reducing interfacial tension between the two polymers and suppressing coalescence.
3.2. Wide angle X-ray diffraction studies
Wide-angle X-ray scattering (WAXS) patterns of PP/PC blends and blend nanocomposites are presented inFig. 6. The following crystallographic planes of the monoclinic
a
-phase of PP: (110), (040), (130), (111) and (130), correspond to 2q
¼ 14.1, 16.9, 18.5, 21.2 and 22, respectively [17]. After blending with PC, the in-tensity of the PP peaks decreases. The variations of peak height could be due to the variation of the spherulite size, deformation of boundaries or long range order induced in the PP matrix by the dispersion of PC domains. For blends containing more 60% of PC more, the spectra show an extra peak at 2q
¼ 16.2. This peak corresponds to theb
crystal form PP ((300) diffraction plane). This proposed that a mix ofa
andb
crystal forms of the PP is formed in the blend. The observedb
form in the PC/PP blends is due to the crystallisation of PP in a spatially confined state[18]. WAXS pat-terns of the nanocomposites are shown inFig. 6b. The characteristic peaks of the pure blend were also observed for the nanocomposites and the position of the peaks remained almost unchanged with the introduction of MWCNTs.3.3. Small angle X-ray diffraction studies
The Lorentz-corrected SAXS profiles of blend and MWCNT based nanocomposites are shown in Fig. 7. The PP shows the first maximum at 0.33 nm1, correlated to the alternating crystalline and amorphous structures of the lamellae. The long period L was Fig. 3. SEM images of PC/PP(60/40) blend with (a) 3 wt % MWCNT (b) 5 wt% MWCNT and (c) PC/PP(60/40) 3 wt % MWCNT blend with PC phase etched (d) PC/PP(60/40) 5 wt % MWCNT blends with PC phase etched.
calculated using Bragg's law (L¼2
p
/q). An additional peak is seen for PP at higher q values indicating the regular stacking of lamellar structure[19,20].The SAXS datafit best to a model that combined power-law scaling at low wave-vectors and Teubner-Strey model for small-angle diffraction at higher wave-vectors [21]. This model was originally developed for bi-continuous emulsions[22]. Lodge and co-workers also have used this model for polymer blends[23e25]. This model uses threefitting parameters to define the observed scattering peaks. The combined form factor from power-law scaling and structure factor from Teubner- Strey model is as follows:
IðQÞ ¼QApþ 1
a2þ c1Q2þ c2Q4þ background
(1)
In Equation(1), A, a2, c1, and c2are variables which can get by nonlinear least-squaresfitting of the experimental data. The pa-rameters L and
x
, correspond to the long period of PP and the average distance between the PP-rich domains; they are related to the constants in Equation.(1)as follows:L¼ 2
p
" 1 2 a2 c2 1=2 c1 4c2 #1=2 (2)x
¼ " 1 2 a2 c2 1=2 þ c1 4c2 #1=2 (3)Fig. 8shows the long period L and correlation length
x
plotted for different blends and MWCNT loaded nanocomposites, respec-tively. FromFigs. 7 and 8a, it can be understood that by the addition of PC, thefirst maximum of PP is shifted to higher q values, i.e. the long period of PP is decreased with the addition of PC from 18.7 nm down to 15.2 nm for the 50/50 blend system. The observed decrease of the long period may be due to the increase of the micro-crystallinity region in the blend system[18]. For the 60/40 blend system, the long period is slightly increased. It may be due to the co-continuous structure of the 60/40 blend system. The position of thefirst maximum of PP cannot be clearly detected in the blend containing 80% of PC. At higher concentration of PC, the long period again decreases, due to the abundant concentration of amorphous regions of PC[26]. The addition of MWCNT into the 60/40 blend system, shows a weak peak corresponding to the PP lamellar morphology. Upon adding the MWCNT, the long period increases. The scattering intensity of the MWCNT based nanocomposite increased over all values of q, due to the scattering from dispersed MWCNTs[20]. The Teubner-Strey model structure factors were well matched with experimental data.3.4. Fourier transform infrared spectroscopic analysis
Fig. 9illustrates the FTIR spectra of the PC/PP blend and MWCNT based nanocomposites. In the IR spectra of pure PP, the peak at 973 cm1corresponds to the rocking vibration ofeCH2- and the absorption peak at 997 cm1corresponds to the rocking vibration Fig. 4. Phase tomography image of (a) PC/PP (60/40) (b) PC/PP (60/40)/MWCNT 1% (c) PC/PP (60/40)/MWCNT 3% (d) PC/PP (60/40)/MWCNT 5%.
ofeCH2. The peak at 1167 cm1is attributed to the antisymmetric deformation ofeCH3- and the peak at 1455 cm1corresponds to the symmetric deformation of -CH2. The peak at 1376 cm1 is attributed to the bending vibration of -CH3. The peaks at 2980 cm1, 2918 cm1, 2868 cm1and 2839 cm1correspond to the stretching vibration of -CH [27]. As shown in Fig. 9, the characteristic ab-sorption peaks of PC at 1004 cm1and 1189 cm1, correspond to the C-O stretching vibrations, and 1501 cm1, correspond to the C¼ C stretching vibration. The peak at 1780 cm1is attributed to the C¼O stretching vibration and peak at 2972 cm1 corresponds to the eCH- stretching vibration[28]. After a detailed analysis of the IR spectra for these blends and nanocomposites, it has been noticed
that there is no shift in the peaks of functional groups in the PC/PP blend and nanocomposites spectra. This confirmed that there is no chemical interaction between the component polymers and nanotubes[29].
3.5. Thermal gravimetric (TG) analysis
The TGA thermograms of PC/PP blends and PC/PP/MWCNT blend nanocomposites are shown inFig. 10. All the samples dis-played single step degradation process [30,31]. Polycarbonate shows a higher thermal stability than pure polypropylene. The initial decomposition temperature (Ti) of the PC is 480C and the Fig. 5. 3D models of PC/PP/MWCNT blend composites by X-ray phase tomography: Volume rendering image of (a) PC, (b) PP, (c) PC/PP of the blend PC/PP (60/40)/3% MWCNT nanocomposite, and (d) PC, (e) PP, (f) PC/PP of the blend PC/PP (60/40)/5% MWCNT nanocomposite.
maximum decomposition temperature (Tmax) is 540C. The initial decomposition temperature of PP is 410 C and the maximum decomposition temperature 490C. The addition of PP component reduced the thermal stability of the PC/PP blend composites, compared with neat PC. From the thermograms (Fig. 10a) it is clear that Tiand Tmaxincrease with the PC content in the blend.Fig. 11 shows the effect of blend ratio on the weight percentage of res-idue remaining at different temperatures indicating an increase in the amount of residue with PC content in the blend.
Thermogravimetric analysis of PC/PP/MWCNT nanocomposites is shown in Fig. 10b. Many researchers have reported that the incorporation of MWCNT improved the thermal stability of poly-mer matrix [32,33]. The thermal stability was slightly enhanced with the addition of MWCNT. The improving of thermal degrada-tion temperatures of the nanocomposites is related with the rein-forcing effect of the MWCNT dispersed in the polymer matrix of the polymer composites.
Fig. 7. Lorentz-corrected SAXS profile of (a) PC/PP blend composites and (b) PC/PP/MWCNT nanocomposites.
Fig. 8. Structural parameters of blends and nanocomposites from Teubner- Strey model.
3.6. Calorimetric investigations
The thermograms of the PC/PP blends and PC/PP/MWCNT nanocomposites obtained from MT-DSC measurements corre-sponding to the second heating after quenching from the melt (in order to avoid the thermal history of the samples) are shown in
Fig. 12. The glass transition temperature of PC is clearly visible at 145C inFig. 12a. When PP is added, a clear melting peak of PP is observed at 145e160C. The melting peak superimposed with Tgof PC. The double peak shape observed for all the melting peaks is related to the overlapping of melting and crystal reorganisation in the same temperature range as classically observed for different semi-crystalline systems[34,35]. The thermograms corresponding to the PC/PP/MWCNTs nanocomposites are shown in Fig. 12b. Incorporation of MWCNTs to the PC/PP (60/40) blend is found to increase the melting temperature indicating that MWCNTs restrict the mobility of polymer chains[36].
The MT-DSC cooling thermograms of PC/PP blends are shown in
Fig. 13a. Pure polypropylene shows a clear crystallisation peak at 104C. For PC/PP blends having more than 50% of PP content, only one single crystallisation peak can be seen. For PC/PP (60/40) and
PC/PP (80/20) blends shown double crystalline peaks can be observed [12]. For blends with single crystallisation peak, the crystallisation temperature is higher than of crystallisation tem-perature of pure polypropylene. This could be the PC phase acted as nucleating agent. A Similar behaviour was described by Li et al.[12]
and Karger et al.[37]for PC/PP and PP/EPDM blend, respectively. For blend having multiple crystallisation peaks, thefirst crystal-lisation peak is lower than of pure PP. This could be the heteroge-neous nucleation in the smaller PP droplets[12].
MWCNTs can act as a heterogeneous nucleating agent and in-fluence the crystallisation behaviour of polymers[38,39].Fig. 13b shows cooling thermograms of PC/PP/MWCNT blend nano-composites. The addition of MWCNTs increases the crystallisation temperature of PP from 111C up to 117C. It is due to the MWCNTs acting as heterogeneous nucleating agent in the polymer matrix.
Semi-crystalline polymers represent a highly heterogeneous system consisting of amorphous fractions, rigid amorphous frac-tions and crystalline fracfrac-tions. The influence of the addition of PC on the crystalline fraction of PP can be investigated by following the melting properties and crystallisation kinetics. The melting endo-therms obtained for PC/PP/MWCNT nanocomposites were used to calculate the equilibrium melting temperature (Tm) using Hofmann-Weeks equation(4) [40], Tm¼2Tc
b
þ Tmo 1 1 2b
(4)where
b
is the thickening factor and is expected to have a value higher than 1. The results obtained are shown inFig. 14.The Tm remains almost the same up to 5 wt% of MWCNT loadings and thereafter a sharp decrease is observed. This indicates that MWCNT is incompatible with the crystalline fractions of PP and the decrease in Tmobserved for nanocomposite with 7.5 wt% of MWCNT points to crystal imperfections arising from the hin-drance of segmental dynamics of PP by MWCNT.
Isothermal crystallisation kinetics of PP in PP/PC blend and PP/ PC/MWCNT nanocomposites were investigated using Avrami equation(5),
XðtÞ ¼ 1 exphK1n=
A t
in
(5)
where X(t) is the progress of crystallisation, KAis the rate constant Fig. 10. TGA curves of (a) PC/PP blends with different compositions (b) PC/PP/MWCNT nanocomposites.
and“n” is the Avrami exponent. The results obtained are given in
Table 1. Isothermal crystallisation studies were conducted at a Tcof 132C, as the crystallisation was observed to proceed at a reason-able rate for the blends and nanocomposites. In the case of PC/PP blends and PC/PP/MWCNT nanocomposites, the crystallinity range between 3 and 20% was strictly used for Avrami studies, keeping in mind the fact that the error in the value of n and KAwill be mini-mum at this range[41].
It can be observed fromTable 1that the Avrami exponent‘n’ values are in between 2 and 3, suggesting a thermal nucleation with the three-dimensional growth of PP crystals in the PC/PP blends and nanocomposites. The crystallisation half time t0.5is defined as the time to attain 50% of crystallinity. The shorter the half time, the faster the crystallisation rate. The t0.5 can be estimated using equation(6). t0:5¼ ln 2 KA 1 n (6)
The t0.5 values of PC/PP blend and MWCNT based nano-composites are shown inTable 1. The t0.5values of PC/PP blend are nanocomposites.
Fig. 13. Shows the thermograms obtained during the cooling from the melt of PC/PP blends and PC/PP/MWCNT nanocomposites.
found to decreasing with increasing PC content in the blend. Pure PP has the maximum t0.5value. In PC/PP/MWCNT nanocomposites t0.5values were found to decrease with MWCNT loading.
Furthermore, the crystallisation rate of PP in the blend and nanocomposites has also been estimated and could describe by the reciprocal of t0.5.The rate of crystallisation versus percentage of PP in pure blends as well as for nanocomposites is shown inFig. 15. The rate of crystallisation decreases with increasing % of PP because concomitantly the amount of glassy phase (PC) has decreased in the pure blend [42] In the nanocomposites, with the addition of MWCNT, the rate of crystallisation initially increases rapidly but approaches a constant value. It could be due to the fact that MWCNTs act as nucleating agents promoting the crystallisation rates of PP[43]. The rate of crystallisation is increased by a factor of nearly 4 (0.42 min1for 7.5% of MWCNT and ~0.13 min1for the pure blend) with the addition of MWCNT to the blend. At a higher loading of MWCNTs, the rate approaches to a constant value due to the agglomerations of MWCNTs.
4. Conclusions
In this study, a thorough investigation has been made on the morphological, structural and thermal properties of PC/PP blends and MWCNT based nanocomposites. For PC/PP (60/40), the blend has a co-continuous morphology which was in confirmed by SEM and X-ray micro-CT studies. The refinement in the co-continuous structure indicated that MWCNT plays an important role in com-patibilising immiscible PC/PP blends. A viscoelastic phase separa-tion process was observed in the neat blends and in MWCNT
nanocomposites. The structural parameters were calculated using Teubner- Strey model from SAXS data. The thermal stability of the blends increased with the addition of PC into the blend system. The thermal stability of PC/PP blend nanocomposites was improved with the addition of MWCNT. The isothermal crystallisation ki-netics of PC/PP blend MWCNT based nanocomposites have been examined. Avrami model was used to determine isothermal crys-tallisation parameters of blends and nanocomposites. Avrami an-alyses suggested that crystallisation of PP in blend and nanocomposite samples assumes three-dimensional crystal growth. The crystallisation of PP has increased by the addition of PC into the blend. In the nanocomposites, MWCNTs promote the crystallisation of PP.
Acknowledgments
This project is funded by Department of Electronics and Infor-mation Technology (DeitY), Govt. Of India, New Delhi (grant num-ber: 1(02)/2012 EMCD dated September 26, 2012). The authors would like to acknowledge the Dr. Bhoje Gowd. E, Senior Scientist, National Institute for Interdisciplinary Science and Technology (NIIST), Trivandrum, India for the WAXS and SAXS measurements. The authors also would like to acknowledge thefinancial support from DST Nanomision, DST-PURSE, UGC-SAPe Govt. Of India. References
[1] A. Blaise, C. Baravian, S. Andre, J. Dillet, L.J. Michot, R. Mokso, Investigation of the mesostructure of a mechanically deformed HDPE by synchrotron
Table 1
Avrami parameters of PC/PP blends and PC/PP/MWCNT nanocomposites.
Amount of PP t0.5 n KA r2 100 24.4 2.43 2.90 104 0.9999 90 20.72 2.20 8.73 104 0.9999 80 16.11 2.50 6.64 104 0.9998 70 15.02 2.92 2.56 104 0.9998 60 10.14 2.33 3.17 103 0.9997 50 9.17 2.53 2.53 103 0.9996 40 6.5 2.48 1.15 102 0.999 PP/PC/MWCNT nanocomposites 1 3.41 2.13 5.06 102 0.9999 3 2.57 2.16 9.06 102 0.9986 5 2.45 2.36 8.33 102 0.9999 7.5 2.41 2.39 6.83 102 0.9985
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