2/6/2016
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`Rheological and thermal properties of blends of a long—chain branched polypropylene and different linear polypropylenes
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`Article outline
`
`Abstract
`
`Keywords
`1. Introduction
`2. Experimental
`3. Results and discussion
`4. Conclusions
`Notation
`Acknowledgement
`References
`
`Figures and tables
`
`Table 1
`
`
`
`Chemical Engineering Science
`
`'
`
`Volume 64, Issue 22, 16 November 2009, Pages 4719-4731
`
`Morton Denn Festschrift
`
`Rheological and thermal properties of blends of a long—chain
`
`branched polypropylene and different linear polypropylenes
`
`Seyed H. Tabatabaei, Pierre J. Carreau
`
`, Abdellah Ajji
`
`CREPEC, Chemical Engineering Department, Ecole Polytechnique, C.P. 6079, Succ. Centre ville,
`Montreal, QC, H30 3A7 Canada
`
`Received 28 August 2008, Revised 2 April 2009, Accepted 3 April 2009, Available online 14 April 2009
`Show less
`
`doi:10.1016/j.ces.2009.04.009
`
`Get rights and content
`
`Abstract
`
`Blends ofa long—chain branched polypropylene (LCB—PP) and four linear polypropylenes
`(L-PP) having different molecular weights were prepared using a twin screw extruder.
`The linear viscoelastic properties suggested the immiscibility of the high molecular
`weight L—PP based blends, and the miscibility ofthe low molecular weight L—PP based
`blends. in addition, the Palierne emulsion model showed good predictions ofthe linear
`viscoelastic properties for both miscible and immiscible PP blends. However, as
`expected, the low—frequency results showed a clear effect of the interfacial tension on the
`elastic modulus ofthe blends forthe high molecularweight L—PP based blends. A
`successful application of time—temperature superposition (TTS) was found forthe blends
`and neat components. Uniaxial elongational properties were obtained using a SER unit
`mounted on an ARES rheometer. A significant strain hardening was observed for the
`neat LCB—PP as well as for all the blends. The influence of adding LCB—PP on the
`crystallinity, crystallization temperature, melting point, and rate of crystallization were
`studied using differential scanning calorimetry (DSC). ltwas found that the melting point
`and degree of crystallinity ofthe blends first increased by adding up to 20 wt% of the
`branched component but decreased by further addition. Adding a small amount of LCB-
`PP caused significant increase of the crystallization temperature while no dramatic
`changes were observed for blends containing 10 wt% LCB—PP and more. Furthermore,
`the crystalline morphology during and after crystallization of the various samples was
`monitored using polarized optical microscopy (POM). Compared to the neat linear
`polymers, finer and numerous spherulites were observed forthe blends and LCB—PP.
`Dynamic mechanical (DMA) data ofthe blends and pure components were also analyzed
`
`and positive deviations from the Fox equation forthe glass transition temperature, T9,
`were observed forthe blends.
`
`Keywords
`
`Polymers; Polypropylenes; Linear and branched polymer blends; Rheological
`properties; Thermal properties; Miscibility; immiscibility
`
`1. Introduction
`
`Due to the higher melting point, lower density, higher chemical resistance, and better
`mechanical properties of polypropylene (PP) in comparison to polyethylene (PE), it is
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`Rheological and thermal properties of blends of a long-chain branched polypropylene and different linear polypropylenes
`
`widely employed for many industrial applications. However, the linear structure of L-PP
`limits its applications for processes where good extensional properties and melt strength
`are required such as thermoforming, film blowing, foaming and fiber spinning. On the
`other hand, it is well known that branched polymers have enhanced extensional
`properties and their blending with linear counterparts can improve their elongational
`behavior, particularly for PE (Ajji et al., 2003; Lohse et al., 2002; Steffi, 2004). With the
`recent development of branched PP, it is expected that the elongational properties of L-
`PP can be effectively enhanced when blended with a long-chain branched polypropylene
`(LCB—PP).
`
`Long-chain branches are commonly introduced to linear PP via electron beam irradiation
`and post reactor chemical modification (Auhi et al., 2004; Tian et al., 2006a). Their effects
`on the processability have been reported in the literature (Gotsis et al., 2004; Stange and
`Miinstedt, 2006). Gotsis et al. (2004) showed that branching up to an optimum level
`improved the processability in foaming and thermoforming processes, while further
`branching did not have a dramatic effect. Stange and Mtlnstedt (2006) found that the
`strain hardening of branched polypropylenes caused not only a high melt strength, but
`also showed a significant homogeneity of deformation in the elongational experiments,
`which allowed forming foams with higher expansion ratios than L-PP.
`
`in several cases (Ajji et al., 2003; Stange et al., 2005; McCallum et al., 2007; Fang et al.,
`2008), the rheological behavior of blends of linear and branched polypropylene and
`polyethylene has been investigated. it has been reported that branched polymers exhibit
`higher shearthinning, elasticity, and strain hardening compared to linear ones. Stange et
`al. (2005) showed that adding a small amount of LCB-PP significantly influences the
`rheological properties, especially the elongational behavior of linear PP blends. In
`addition, they found that the strain hardening of PP blends decreased as the strain rate
`increased while for the neat LCB-PP, enhancement of strain hardening was observed.
`McCallum and coworkers (2007) realized that the blending of branched and linear PP not
`only promoted the melt strength, butthe mechanical properties increased as well. Ajji et
`al. (2003) showed that adding a small amount of low density polyethylene (LDPE)
`increased the strain hardening of linear low density polyethylene (LLDPE) resins. They
`also found that 10-20 wt% of LDPE is sufficientfor improving the extensional property of
`LLDPE. Fang et al. (2008) concluded that an increase in the length of short branches
`and, possibly, the presence ofa few long branches in metallocene LLDPEs and
`comparable molecular weights with LDPE could improve the miscibility of LLDPE/LDPE
`blends.
`
`To our knowledge no work has been published regarding the effect of molecular weight of
`linear polypropylene on the rheological behavior of blends of linear and long-chain
`branched polypropylene. Using different rheological characterization methods, it will be
`shown that molecular weight has a crucial role on the miscibility of L-PP and LCB-PP. in
`addition, the influence of adding LCB-PP on the thermal properties, crystallization, and
`solid state behavior ofthe blends will be explored.
`
`2. Experimental
`2.1. Materials
`
`Four commercial linear polypropylenes (PP40, PP28, PP08, and PP04) and a
`commercial branched polypropylene (LCB-PP) were selected. The PP28 and PP08 were
`supplied by ExxonMobil Company and had a melt flow rate (MFR) of2.8 g/10 min (under
`ASTM conditions of
`and 2.16 kg) and 0.8 g/10 min, respectively, while the PP40,
`PP04, and LCB-PP were obtained from Baseil Company and had a MFR of4 g/10 min,
`0.4 g/10 min, and 2.5 g/10 min, respectively. The main characteristics ofthe resins are
`shown in Table 1. The molecular weights of the L—PPs were evaluated from the relation
`between the zero—shear viscosity and the molecular weight (Fujiyama and lnata, 2002).
`The molecular weight distribution (MWD) was measured using a GPC (Viscotek model
`350) with 1,2,4—Trichlorobenzene (TCB) as a solvent at a column temperature of 140 °C.
`
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`The melting point, Tm, and the crystallizationtemperature, Tc, ofthe resins were
`obtained using differential scanning calorimetry. Blends containing 20, 40, and 60 wt%
`
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`Rheological and thermal properties of blends of a long-chain branched polypropylene and different linear polypropylenes
`
`LCB-PP were prepared using a twin screw extruder (Leistritz Model ZSE 18HP co-
`rotating twin screw extruder) followed by water cooling and pelletizing. The temperature
`profile along the barrel (from hopperto die) was set at 160/ 1 80/ 1 90/200/200/200/200 °C.
`The extrusion was carried out at 80 rpm. During blending, 3000 ppm ofa stabilizer,
`lrganox B225, was added to avoid thermal degradation ofthe polymers. To make sure
`that all samples have the same thermal and mechanical history, unblended components
`were extruded underthe same conditions.
`
`Table 1.
`
`Main characteristics ofthe neat polymers.
`
`MFR
`(g/10 min) Nomencl.
`4.0
`PP40
`
`no“
`(kPa s)
`9.8
`
`nob
`(kPa s)
`9.8
`
`Company
`Basell
`
`Resin
`code
`Pro-fax
`6523
`PP4612 Exx0nM0bi|
`PP5341
`ExxonM0bi|
`Pro—fax
`Basell
`6823
`Pro—fax
`814
`
`Basell
`
`2.8
`0.8
`0.4
`
`2.5
`
`501
`
`543
`772
`812
`
`PP28
`PP08
`PPO4
`
`14.4
`43.6
`58.3
`
`16.5
`49.5
`67.5
`
`LCB-PP
`
`18.6
`
`19.5
`
`N/A
`
`MW/Mn
`2.8
`
`3.9
`2.7
`4.3
`
`2.3
`
`159.8
`
`161.0
`160.0
`159.6
`
`158.4
`
` l r
`Zero—shear viscosity values obtained from the Carreau—Yasuda model, T=190 °C.
`
`a
`
`b Zero-shear viscosity values obtained from the area under the weighted relaxation spectrum curves,
`T=190 “C.
`
`Table options
`
`2.2. Rheological measurements
`
`Dynamic melt rheological measurements were carried out using a Rheometric Scientific
`SR5000 stress controlled rheometer with parallel plate geometry (diameter of 25 mm and
`a gap of 1 .5 mm). All measurements were carried out at
`under nitrogen
`atmosphere to avoid thermal degradation. Molded discs of2 mm thick and 25 mm in
`diameter were prepared using a hydraulic press at
`. Time sweep tests were first
`performed at a frequency of 0.628 rad/s for 2 h. Material functions such as complex
`viscosity, elastic modulus, and weighted relaxation spectrum in the linearviscoelastic
`regime were determined in the frequency range from 0.01 to 500 rad/s. In orderto obtain
`more accurate data, the frequency sweep test was carried out in four sequences while
`the amount of applied stress in each sequence was determined by a stress sweep test.
`
`To measure the uniaxial elongational viscosity, an ARES rheometer equipped with a
`SER universal testing platform from Xpansion Instruments was used. The model used
`was SER—HV—AO1, which is a dual windup extensional rheometer. It is capable of
`generating elongational rates up to
`. Measurements were performed at
`nitrogen atmosphere.
`
`under
`
`2.3. Thermal analysis
`
`Thermal properties of the various specimens were determined using a TA instrument
`
`differential scanning calorimeter (DSC) Q 1000. The samples were heated from 50 to
`at a heating rate of
`to eliminate initial thermal history, and then cooled to
`at the same rate. The melting point and the degree of crystallinity were determined
`from the second heating ramp, also performed at a rate of
`.Crystallinity values
`are reported, using a heat offusion of209 J/g forfully crystalline polypropylene (PP)
`(Arroyo and Lopez-Manchado, 1997).
`
`2.4. Polarized optical microscopy (POM)
`
`Crystallization monitoring was performed using polarized optical microscopy (OPTlHOT-
`2) to follow spherulites growth. For the non isothermal crystallization tests, films with a
`thickness of
`were prepared using a twin screw extruder equipped with a slit die.
`The films were first heated on a programmable hot stage (Mettler FP82HT) from room
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`Rheological and thermal properties of blends of a long-chain branched polypropylene and different linear polypropylenes
`
`and were kept at that temperature for
`at a heating rate of
`temperature to
`1 min to eliminate initial thermomechanical history, and then cooled to room temperature
`at the same rate.
`
`2.5. Dynamic mechanical analysis (DMA)
`
`The solid state behavior of different samples was characterized using a TA instrument
`dynamic mechanical analyzer (DMA) 2980 inside an environmental test chamber (ETC).
`
`and frequency of 1 Hz
`at a rate of
`The temperature ranged from -40 to
`was applied to the rectangular shape samples. To generate low temperatures and to
`control temperature during heating, liquid nitrogen was used. The glass transition
`
`temperature was determined from the maximum of the G " curves.
`
`3. Results and discussion
`
`3.1. Rheological characterization of the neat PPs
`
`The complex shear viscosities normalized by the zero shear viscosities, obtained using
`the Carreau—Yasuda model (see Table 1), are plotted as a function offrequency for the
`neat PPs in Fig. 1. The LCB-PP exhibits a pronounced shear-thinning behavior due to the
`presence of long-chain branches. As the molecular weight ofthe L-PPs increases the
`behavior becomes more shear-thinning and the transition from the Newtonian plateau to
`the power-law region occurs at lowerfrequencies.
`
`Fig. 1.
`Normalized complex viscosity as a function of frequency for neat PPs;
`
`Figure options
`
`, as a function offrequency is shown in Fig. 2. A
`The plot ofthe loss angle,
`monotonic decrease in the loss angle is observed forthe L-PPs while the LCB-PP shows
`an inflection in the curve with a tendency towards a plateau at high frequencies. Wood-
`Adams et al. (2000) related the magnitude and breath of the plateau to the weight fraction
`of branched chains. The larger elasticity ofthe LCB-PP compared to L-PPs at low
`frequencies is attributed to more entanglements due to the presence of long-chain
`branches. However, as the frequency increases, the number of entanglements
`decreases due to the more shear-thinning character ofthe branched PP (see Fig. 1).
`
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`Rheological and thermal properties of blends of a long-chain branched polypropylene and different linear polypropylenes
`
`Fig. 2.
`Loss angle versus frequency for heat PPs;
`
`Figure options
`
`To compare the relaxation behavior of the L—PPs and LCB-PP, the weighted relaxation
`
`spectra evaluated from dynamic moduli (G I, G " , 0.)) using the NLREG (non linear
`regularization) software (Honerkamp and Weese, 1993) are plotted in Fig. 3 (the vertical
`dash lines represent the range of frequencies covered during the experiments). The area
`underthe spectrum curve represents the zero-shear viscosity ofthe melt and it is
`reported in Table 1. A good agreement between these values and those obtained using
`the Carreau—Yasuda model is observed, suggesting that the relaxation spectra are
`accurate. It is obvious that the LCB-PP shows a longer relaxation time than the L—PPs,
`indicating that long-chain branches affect more the relaxation time than the larger
`molecules present in PP08 and PP04. The larger relaxation time for LCB-PP is attributed
`to changes in stress relaxation mechanisms. The simple reptation, as expected for linear
`polymers, no longer suffices to relieve stress when there are enough branches present
`and slower events such as arm retraction must occur.
`
`Fig. 3.
`Weighted relaxation spectra for neat PPs;
`frequencies covered during the experiments).
`
`(the vertical dash lines represent the range of
`
`Figure options
`
`3.2. Rheological characterization of the blends
`
`The complex shear viscosities as a function of frequency for the PP40/LCB-PP and
`PP04/LCB-PP blends (two extreme cases) are shown in Figs. 4(a) and (b), respectively.
`It is clear that adding the LCB-PP causes a pronounced shear-thinning behavior due to
`the presence of long-chain branches (the power—index calculated for frequencies ranging
`from 0.1 to 10 rad/s is reported in the legend). In addition, it is obvious that the viscosities
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`Rheological and thermal properties of blends of a long-chain branched polypropylene and different linear polypropylenes
`
`of the PP4O based blends are intermediate between those of the neat components while
`those of the PPO4 based blends are closerto that of the PP04. The behavior is typical of
`linear polymer melts and the complex viscosity of the blends follows the log additivity rule
`for low molecular weight based blends. This is shown in Figs. 5(a) and (b), where the
`complex viscosities at differentfrequencies are plotted as a function of the LCB-PP
`content. The logarithmic additivity rule is expressed as (Utracki and Schlund, 1987):
`
`log .I}"‘(m1 : (fry |og{q"(m,1,1i + (I — (,r'Jfi] ]og(i,I’(c.-L1):
`
`Tu m
`
`(1)
`on
`
`where (pig is the branched PP content in weight percent and r]* is the complex shear
`viscosity. The PP40 based blends obey the log additivity rule, as depicted in Fig. 5(a),
`suggesting miscibility of both PP components. However, significant deviations from this
`empirical relation are observed in Fig. 5(b) for the PP04 based blends in the entire
`composition range, suggesting that these two PP parts are immiscible. Deviations from
`the log mixing rule for blends of linear and branched PPs and PEs have been reported
`in the literature (Fang et al., 2008; Ho et al., 2002; Liu et al., 2002). It is believed that
`not only the amount of LCB-PP influences the miscibility of the blends, but also the
`molecular weights of both components as well as the branching structure (e.g. star-like
`or tree—|ike) of the LCB-PP are important factors affecting the miscibility.
`
`mo
`2on¢s_Lcn-pp
`aoms_Lcn-pp
`aoms_Lcn-pp
`Lcnw
`
`3
`
`!—‘L
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`cl._«
`i
`1:-
`
`'3‘
`out9.~_.
`‘
`I-'
`
`In‘
`
`Ill’
`
`
`
`I0"
`
`Io-=
`
`IO"
`
`10*‘
`
`10'
`
`10*
`
`103
`
`g!!gg:fl'0.
`oogifi E
`*1"
`090
`1*‘!
`an
`*1
`
`0 PM
`on
`v
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`M:
`
`'0]
`
`I
`
`I
`
`I|_I.I.I.|I
`10')
`
`IIII|.|.|]
`I0"
`
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`
`104
`
`I_l1I].||I
`I[l'
`
`I
`
`|_|I|.|.L|]
`I00
`{D (radfs)
`
`I
`
`|_|IIIIII
`II)?
`
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`
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`W3
`
`Fig. 4.
`Complex viscosity as a function of frequency for: (a) PP40/LCB-PP blends and (b) PPO4/LCB-PP blends;
`T: 1903C.
`
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`Rheological and thermal properties of blends of a long-chain branched polypropylene and different linear polypropylenes
`
`El
`
`:1‘(Pas)
`
`n‘(Pa.s]
`
`ins
`
`Fig. 5.
`Complex shear viscosity at different angularfrequencies as a function of branched PP content for: (a)
`PP40/LCB—PP blends and (b) PP04/LCB-PP blends; T = 190 3C (the dash lines showthe additivity rule).
`Figure options
`
`The zero-shear viscosity obtained from the Carreau—Yasuda model is shown as a
`function ofthe LCB-PP content in Fig. 6. Good agreement with the log additivity rule for
`the blends having components with close melt flow indexes (i.e. PP40/LCB—PP and
`PP28/LCB-PP blends) is found, suggesting miscibility ofthe PP components. However,
`large deviations from the empirical rule are observed forthe PP04/LCB-PP and
`PPO8/LCB-PP blends, suggesting that the two components are immiscible. It should be
`noted that at low frequencies (Newtonian region), a large increase in the number of
`entanglements due to the LCB-PP addition is expected for all the samples. Therefore, it
`is unlikely that the deviations from the log mixing rule could be explained forthe
`PP04/LCB-PP and PPO8/LCB-PP blends only.
`
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`Rheological and thermal properties of blends of a long-chain branched polypropylene and different linear polypropylenes
`
`
`
`Fig. 6.
`Zero-shear viscosity as a function of LCB-PP content;‘ (the dash lines show the additivity
`mle).
`
`Figure options
`
`To see the effect of molecular weight of L-PP on the relaxation behavior of the blends, the
`weighted relaxation spectra of the PP40/LCB-PP and PP04/LCB-PP blends are
`illustrated in Figs. 7(a) and (b), respectively. The addition of LCB-PP changes the
`relaxation mechanism from simple reptation to arm retraction, which retards the
`movement of chains along their backbone; hence, the maxima in the curves shift to
`longertimes and the spectrum shape becomes broader. Note that forthe PP40/LCB-PP
`blends, the positions of the peaks are intermediate to those of the neat components,
`indicating again miscibility, while forthe PP04/LCB-PP blends non uniform changes in
`the peaks are observed.
`
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`Fig. 7.
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`Rheological and thermal properties of blends of a long-chain branched polypropylene and different linear polypropylenes
`
`Weighted relaxation spectrum for: (a) PP40/LCB-PP blends and (b) PPO4/LCB-PP blends;-
`(the vertical dash lines represent the range of frequencies covered during the experiments).
`
`Figure options
`
`The behavior can also be analyzed using Co|e—Co|e plots of n " versus n ', as illustrated
`in Fig. 8. The semicircular shape for the PP40/LCB-PP blends (Fig. 8(a)) is another
`evidence of miscibility (Kwang et al., 2000; Utracki, 1991), while some synergistic effects
`forthe PPO4/LCB-PP blends (Fig. 8(b)) are observed. It should be noted that the curves
`
`of the high MW based blends are closerto that of the neat L-PP component, which
`confirms the results demonstrated in Fig. 4.
`
`Fig. 3.
`Co|e—Co|e plots for: (a) PP40/LCB-PP blends and (b) PPO4/LCB-PP blends; -.
`Figure options
`
`Figs. 9(a) and (b) illustrate the storage modulus of the PP40/LCB-PP and PPO4/LCB-PP
`blends, respectively. Forthe PP4O based blends, at low frequencies, the storage
`modulus of the LCB-PP is larger while the effect becomes inversed at high frequencies.
`Forthe blends containing a high molecular weight component (Fig. 9(b)) some
`synergistic effects at low frequencies are seen for all compositions, which is possibly due
`to the immiscibility of these blends. The increase of elasticity at low frequencies is
`common in immiscible blends and has been interpreted in the context of emulsion
`models (Chun et al., 2000; Palierne, 1990; Utracki, 1991).
`
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`Rheological and thermal properties of blends of a long-chain branched polypropylene and different linear polypropylenes
`
`Fig. 9.
`Storage modulus as a function of frequency for: (a) PP40/LCB-PP blends and (b) PP04/LCB-PP blends;
`2.
`
`Figure options
`
`Palierne (1990) developed a model to predict the linear viscoelastic properties of
`immiscible emulsion-type blends. For a narrow distribution of droplet diameters
`(Bousmina and Muller, 1993) and constant interfacial tension, the complex modulus of
`the blend is expressed by:
`
`-
`where H* is defined as:
`
`
`
`‘2’
`
`(3)
`
`where (p is the volume fraction of the droplets of volume average radius RV, a is the
`
`interfacial tension, and 1 and I are the complex moduli of the matrix and droplets,
`respectively.
`
`Due to the low optical contrast between the PP components it was impossible to obtain
`
`the dimensions of drops in the PP blends and hence RV. Following Fang et al. (2005)
`
`and Hussein and Williams, 2001 and Hussein and Williams, 2004, (1/RV was used as a
`single parameter to find the best fits of the experimental data for the blends containing
`20 wt% LCB-PP with low and high molecular weight L-PP components.
`
`Figs. 10(a) and (b) demonstrate the influence of (XI RV (0, 100, and 500) on the storage
`and loss moduli predicted by the Palierne model forthe (80/20) PP40/LCB-PP and
`(80/20) PP04/LCB-PP, respectively. It is obvious that the G "values predicted by the
`
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`Rheological and thermal properties of blends of a long—chain branched polypropylene and different linear polypropylenes
`
`model are not sensitive to the value ofa/RV and are in good agreement with the
`
`experimental data. In contrast, the G’ values are dramatically influenced by the value of
`
`G/RV at low frequencies. Forthe (80/20) PP40/LCB-PP blend the best fit ofthe model
`
`with the experimental data was achieved for cx/R,,=0 while for (80/20) PP04/LCB—PP
`
`blend, the best fit was obtained for a/R,,=’l O 0 . A value ofthe interfacial tension equal to
`0 forthe (80/20) PP40/LCB-PP blend is indicative of miscibility. However, the non zero
`
`value ofa/RV forthe (80/20) PP04/LCB—PP blend is indicative ofthe presence ofa
`dispersed phase. Assuming an interfacial tension ofO.1 mN/m forthe PP pair
`(PP04/LCB—PP), the corresponding droplet radius is estimated to be around
`the hypothesis of immiscibility forthese blend components is reasonable.
`
`. Hence,
`
`Fig. 10.
`Sensitivity ofthe Palierne model predictions of G’ and G" to different values of cx/RV for: (a) (80/20)
`PP40/LCB-PP blend and (b) (80/20) PP04/LCB—PP blend;
`
`The role of adding the long—chain branched component on the temperature sensitivity of
`complex viscosity ofthe blends is examined via the time—temperature superposition
`(TTS) principle. The results for the unblended polymers as well as for two blends are
`depicted in Figs. 11(a) and (b), respectively (to facilitate the comparison between data,
`
`the curves have been shifted by a multiplication factor as indicated). The shift factor, aT,
`was obtained from the temperature dependency of the zero-shear viscosity and was
`larger forthe branched polymer compared to the linear one. From Fig. 11, it is clearthat
`TTS holds for all the samples. van Gurp and Palmen (1998) proposed a refined analysis
`to check the validity ofthe TTS principle. The TTS principle is respected when the plot of
`
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`the loss angle,
`
`, as a function of complex modulus, G *= ( G '2+ G "2)1/2,
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`Rheological and thermal properties of blends of a long-chain branched polypropylene and different linear polypropylenes
`
`superimpose in a single curve for all temperatures. The analysis of van Gurp and Palmen
`(1 998) was examined for (60/40) PP40/LCB-PP and (60/40) PP04/LCB-PP blends as
`well as their neat components (the results are not shown). A single curve for each sample
`for various temperatures was observed, indicating that the TTS principle holds for all the
`samples. Macaubas and Demarquette (2002) investigated the applicability of TTS for a
`blend containing 10vvt% PP in PS and a blend containing 10 wt% PP in HDPE using the
`van Gurp—Palmen analysis. The immiscible 10 wt% PP/HDPE blend was observed to
`respect the principle, but not the immiscible 10 wt% PP/PS blend. This was explained by
`large differences of the flow activation energy, horizontal shift factor, and of the interfacial
`tension for PP and PS compared to those of PP and HDPE. In our case, although
`different rheological characterization methods suggested immiscibility of the PPO4/LCB—
`PP blend systems, no significant differences forthe flow activation energy and the shift
`factor were found forthe blend components. In addition, a small value of interfacial
`tension between the components is expected. Hence, these observations can explain
`the validity of TTS for the (60/40) PP04/LCB-PP blend.
`
`Fig. 11.
`Time—temperature superposition for: (a) PP40, (60/40) PP40/LCB-PP blend, LCB—PP and (b) (60/40)
`PP04/LCB-PP blend (the reference temperature is
`).
`
`In contrast to oscillatory shear data, uniaxial extension is very sensitive to molecular and
`microstructural parameters (Milnstedt et al., 1998; Wagner et al., 2000). The transient
`elongational viscosity
`of the resins and blends at different strain rates and
`is
`illustrated in Fig. 12 (as in Fig. 11, the curves have been shifted by a multiplication). As
`expected, the linear polypropylenes respect the linear viscoelastic behavior over a large
`strain range where the transient elongational viscosity is equal to three times that the
`transient viscosity in simple shear determined using the relaxation spectrum according to
`the following equation:
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`Rheological and thermal properties of blends of a long-chain branched polypropylene and different linear polypropylenes
`
`(4)
`
`To obtain the transient elongational viscosity from the above equation 100 modes (N)
`were used.
`
`Fig. 12.
`, for: (a)
`Transient elongational viscosities as a function of time at different Hencky strain rates,
`PP40/LCB-PP blends and (b) PP04/LCB—PP blends;
`(the solid lines represent the linear
`behavior calculated from the relaxation time spectrum).
`
`Figure options
`
`A significant strain hardening is observed forthe LCB-PP as well as for all the blends. A
`sharper increase in the extensional viscosity curves and deviations from the linear
`viscoelasticity at shorter times were found as the weight fraction of the LCB-PP increases
`from 0.20 to 1.Stange et al. (2005) found that adding a small amount ofthe same long-
`chain branched PP to a linear PP (different than those used in this work) caused strain-
`hardening and the effect was attributed to |ong—chain branching. Similar results for
`blends of LDPE and LLDPE were reported by Ajji et al. (2003) and Wagner et al. (2004).
`Note that for the PP04/LCB—PP blends at all strain rates the shape ofthe curves are
`similarto that ofthe PP04, in accordance with the shear data (see Fig. 4 and Fig. 8),
`which is probably due to immiscibility of these blends. As for the shear properties, it is
`clearthat the elongational properties are dominated by the high molecular weight
`component.
`
`The behavior ofthe PP28/LCB-PP blends was similarto that ofthe PP40/LCB-PP blends
`
`and that ofthe PP08/LCB-PP was close to the PP04/LCB—PP. Forthe sake ofsimplicity
`and brevity, the results forthe PP28/LCB and PP08/LCB blends are not shown.
`
`3.3. Thermal characterization
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`Rheological and thermal properties of blends of a long-chain branched polypropylene and different linear polypropylenes
`
`In the melting curve ofthe neat L-PPs, a small peak around
`
`was observed and
`
`attributed to the presence ofa small amount of beta ([3) or hexagonal crystalline form of
`PP (Jang etal., 2001). The magnitude ofthe peak was reduced when up to 5 wt% ofthe
`branched PP was added and disappeared by further addition ofthe LCB-PP. As the peak
`was not seen forthe neat LCB-PP, it could be concluded that the presence of long-chain
`branches prevented the formation of beta crystals. Tian et al. (2006b) studied the
`crystalline structures of linear and long chain branched PPs using wide—angle X—ray
`
`diffraction (WAXD). They found that linear PPs could crystallize in the C1 and 8 forms,
`
`while branched PPs crystallized only in the ct crystalline form, in agreement with our DSC
`results.
`
`Melting and crystallization temperatures obtained from the peak positions as well as the
`degree of crystallinity ofthe various materials and blends are presented in Table 2. The
`melting point ofthe blends first increases by adding 20 wt% LCB-PP to the linear PPs
`while further addition reduces it. The reason forthis behavior is unclear at present and
`different trends for the melting point of blends of linear and branched PPs have been
`reported in the literature (McCal|um et al., 2007). Generally, a single melting peak was
`observed for all the blends. However, as the melting points ofthe components are very
`close, this cannot be considered as an indication of miscibility.
`
`Table 2.
`
`Melting point (Tm ), crystallization temperature (TC), and degree of crystallinity (XC) ofthe neat PPs as
`well as the blends (1 indicates the standard deviation of the crystallinity).
`
`Sample
`PPO4
`
`(20/80)
`PP04/LCB-PP
`
`(40/60)
`PP04/LCB-PP
`
`(60/40)
`PP04/LCB-PP
`LCB-PP
`PP08
`
`(20/80)
`PP08/LCB-PP
`
`(40/60)
`PP08/LCB-PP
`
`(60/40)
`PP08/LCB-PP
`LCB-PP
`
`159.6
`
`161.9
`
`116.9
`
`125.5
`
`XE
`36.811.1
`
`Sample
`PP28
`
`41 .610.7
`
`161.7
`
`126.4
`
`37.510.5
`
`(20/80)
`PP28/LCB-PP
`
`(40/60)
`PP28/LCB-PP
`
`160.5
`
`127.1
`
`158.6
`160.0
`
`161.5
`
`128.4
`117.3
`
`126.0
`
`36.911.2
`
`(60/40)
`PP28/LCB-PP
`35.011.0 LCB-PP
`37.811.5 PP40
`
`40210.8
`
`(20/80)
`PP40/LCB-PP
`
`(40/60)
`PP40/LCB-PP
`
`160.5
`
`127.3
`
`39.610.8
`
`161.0
`
`162.1
`
`114.4
`
`126.5
`
`XC
`40010.9
`
`42.211.4
`
`161.6
`
`127.5
`
`41.310.7
`
`160.6
`
`127.7
`
`35.611.1
`
`158.6
`159.8
`
`161.3
`
`128.4
`119.2
`
`126.9
`
`35.010.9
`38.8119
`
`40.811.1
`
`160.9
`
`127.5
`
`40.210.7
`
`160.2
`
`127.6
`
`158.6
`
`128.4
`
`39.110.3
`
`(60/40)
`PP40/LCB-PP
`35.011.0 LCB-PP
`
`160.0
`
`127.8
`
`37.311.2
`
`158.6
`
`128.4
`
`35.010.9
`
`Table options
`
`Adding LCB-PP to the linear PPs causes a significant increase in the crystallization
`
`temperature, To, for all LCB contents, with the maximum change occurring at 20 wt%
`LCB-PP (see Table 2).