Viscoelastic properties of PLA/PCL blends compatibilized ... · Semba et al. (2006). DCP decomposes to produce radi-cals, which abstract hydrogen from both PLA and PCL to yield free

© 2017 The Korean Society of Rheology and Springer 295

Korea-Australia Rheology Journal, 29(4), 295-302 (November 2017)DOI: 10.1007/s13367-017-0029-8

www.springer.com/13367

pISSN 1226-119X eISSN 2093-7660

Viscoelastic properties of PLA/PCL blends compatibilized with different methods

Boo Young Shin* and Do Hung Han

School of Chemical Engineering, Yeungnam University, 280 Daehak-ro, Gyeongsan 38541, Republic of Korea

(Received May 22, 2017; final revision received August 16, 2017; accepted September 1, 2017)

The aim of this study was to observe changes in the viscoelastic properties of PLA/PCL (80/20) blends pro-duced using different compatibilization methods. Reactive extrusion and high-energy radiation methodswere used for blend compatibilization. Storage and loss moduli, complex viscosity, transient stress relax-ation modulus, and tan δ of blends were analyzed and blend morphologies were examined. All compati-bilized PLA/PCL blends had smaller dispersed particle sizes than the non-compatibilized blend, and wellcompatibilized blends had finer morphologies than poorly compatibilized blends. Viscoelastic propertiesdifferentiated well compatibilized and poorly compatibilized blends. Well compatibilized blends had higherstorage and loss moduli and complex viscosities than those calculated by the log-additive mixing rule dueto strong interfacial adhesion, whereas poorly compatibilized blends showed negative deviations due toweak interfacial adhesion. Moreover, well compatibilized blends had much slower stress relaxation thanpoorly compatibilized blends and didn’t show tan δ plateau region caused by slippage at the interfacebetween continuous and dispersed phases.

Keywords: viscoelastic properties, compatibilization, PLA/PCL blend, reactive extrusion, high-energy radi-ation

1. Introduction

Blending of polymers provides an excellent means ofdeveloping materials with enhanced properties, andrequires much less effort than that required to synthesizea completely new polymer (Silva et al., 2010a). Unfortu-nately, most polymer pairs are thermodynamically immis-cible and exhibit phase separation, poor interfacialadhesion, and coarse and unstable morphologies, and con-sequently, have poor mechanical properties. Therefore, thecompatibilization of polymer blends is important in termsof producing materials with enhanced properties (Asthanaand Jayaraman, 1999; Baker et al., 2001; Clapper et al.,2006; Deanin et al., 1999; Silva et al., 2010a). Methods ofcompatibilizing immiscible polymer blends have beendescribed in the literatures, and include the addition of apremade polymeric compatibilizer, reactive blending, andexposure to high-energy radiation (Cleland et al., 2003;Dong et al., 2001; Khan et al., 2012; Komada et al., 2007;Koning et al., 1998; Singh, 2001; Singh and Bahari, 2003;Spardo et al., 1996; Utracki, 2002; Woods and Pikaev,1994; Żenkiewicz et al., 2008). The advantages, disad-vantages, and commercial applications of different poly-mer blend compatibilization methods have been presentedby Konig et al. (1998) and Utracki (2002).

The results of compatibilization processes differ consid-erably in terms of mechanical, rheological, and morpho-logical properties. In particular, the melt viscoelasticproperties of immiscible blends system are strongly

dependent on interfacial adhesion. The viscoelastic prop-erties of non-compatibilized blends are relatively wellunderstood both experimentally and theoretically (Gu et

al., 2008; Ibar, 2009; Kwon and Cho, 2016; Silva et al.,2010b; Wu et al., 2008; Yee et al., 2007). For compati-bilized blends, several researchers have also tried to estab-lish, both theoretically and empirically, relationshipsbetween the viscoelastic properties and interfacial phe-nomena of non-reactive compatibilized blends producedby addition of premade copolymer (Yee et al., 2007), aphysical interaction agent (Silva et al., 2010a; 2010b), oran organoclay (Salehiyan et al., 2014a) and those of reac-tive compatibilized blends (Al-Itry et al., 2014; Asthanaand Jayaraman, 1999; Corre et al., 2011; Semba et al.,2006; Silva et al., 2010b; Yee et al., 2007). In addition,efforts have been made to quantify interfacial adhesionusing measures of interfacial tension to understand theeffect of interfacial adhesion on viscoelastic properties andblend morphologies (Al-Itry et al., 2014; Silva et al.,2010b; Velankar et al., 2004; Yee et al., 2007). However,it is difficult to derive theoretical equations that predictviscoelastic properties of reactive compatibilized blends.

To the best of our knowledge, no experimental study hasbeen undertaken to compare the viscoelastic properties ofblends compatibilized using different techniques, althoughmany studies have compared the viscoelastic behaviors ofblends using the same method at different levels of com-patibility (Al-Itry et al., 2014; Asthana and Jayaraman,1999; Bhatia et al., 2007; Salehiyan et al., 2014a; Silva et

al., 2010b; Yee et al., 2007). In this study, we measured the viscoelastic properties of*Corresponding author; E-mail: [email protected]

Boo Young Shin and Do Hung Han

296 Korea-Australia Rheology J., 29(4), 2017

PLA/PCL blends compatibilized using four different reac-tive compatibilization methods, that is, two in situ reactivecompatibilization (reactive extrusion) methods and twohigh-energy radiation methods. The first method involvedthe addition of dicumyl peroxide (DCP) as described bySemba et al. (2006). DCP decomposes to produce radi-cals, which abstract hydrogen from both PLA and PCL toyield free radicals. These free radicals react to form cross-links at interfaces between continuous and dispersedphases. The second method involved the addition of reac-tive compatibilizer, which can induce interfacial reactionbetween the continuous and dispersed phases. In thisstudy, we used a chain extension/branching agent contain-ing nine glycidyl methacrylate as the reactive compatibi-lizer (Al-Itry et al., 2012). The epoxide groups of thiscompatibilizer can react with both hydroxyl and carboxylgroups, and thus, produce ester linkages at the interfacebetween PLA and PCL. The third and the fourth methodsboth involved exposing PLA/PCL mixtures with or with-out glycidyl acrylate (GMA) to high-energy radiation toenhance compatibility (Kodama et al., 2007; Shin andHan, 2013; Singh, 2001). To prepare high-energy radiationcompatibilized blends, an accelerated electron-beam wasused as the high-energy radiation source. When a PLA/PCL mixture without GMA is exposed to an electron-beam, free macroradicals of PLA and PCL can be formedby hydrogen abstraction at quaternary carbon atom sites inPLA (Nugroho et al., 2001) and at arbitrary carbon atomsites in PCL (Kondyurin et al., 2008). These free mac-roradicals then react to form crosslinks at interface betweenthe two phases. On the other hand, when PLA/PCL mix-ture containing GMA is exposed to electron-beam, radi-ation initiated cross-copolymerization occurred at theinterface between PLA and PCL facilitated by GMA(Shin and Han, 2013).

Empirical melt viscoelastic properties, that is, complexviscosities, storage and loss moduli, tan δ, normalizedcomplex viscosities, transient stress relaxation moduli,and complex moduli of compatibilized PLA/PCL blends,a non-compatibilized blend, and of pure components weremeasured. Changes in morphologies of blends were alsoobserved.

Viscoelastic properties are, we believe, of high reliabilitybecause they were obtained by a single operator, using thesame instruments and standard experimental and moldingprotocol for all samples (Ibar, 2009). Furthermore, meltmixing for preparing compatibilized and non-compati-bilized blends was performed using the same twin-screwextruder and operation conditions.

2. Experimental

2.1. MaterialsPLA (NatureWorks® PLA Polymer 4032D) of density

1.24 g/cm3 and melt flow index 6.5 g/10 min (measured at190°C at a load of 2.16 kg) was purchased from Nature-Works LLC and PCL (TONE-787) with specific gravity of1.145 and an average molecular weight of 80000 g/molwas obtained from Dow/Union Carbide. Glycidyl meth-acrylate (GMA) and dicumyl peroxide (DCP) were sup-plied by Sigma-Aldrich (WI, USA). Joncry® ADR 4368(functionality: f = 9, glass transition temperature: 94°C,EEW (epoxy equivalent weight): 285 g/mol, molecularweight (Mw): ~ 6800 g/mol) was supplied by BASF.

2.2. Preparations non-compatibilized and compatibi-lized blends

Sample codes and compositions of the studied blendsare listed in Table 1. All blends were produced at a PLA/PCL ratio of 80/20 (weight percent). Reactive compatibi-lizing agent and initiator are added at phr (parts per hun-dred parts of resin) based on the total mass of PLA andPCL. Blend-0 was defined as the non-compatibilizedPLA/PCL (80/20) blend; Blend-1 as the reactive compat-ibilized blend produced by adding initiator (DCP 0.2 phr),as described by Semba et al. (2006); and Blend-2 as thecompatibilized blend obtained by adding Joncry®, asdescribed by Al-Itry et al. (2012). These blends wereproduced by mixing constituents in a plastic bag beforebeing extruded in a twin-screw co-rotating extruder (SMPLATEK Co. Ltd., TEK 30, Korea). The screw had adiameter of 30 mm and an L/D ratio of 36:1. The extruderwas operated at 150 rpm at a constant feed rate of 15 kg/hr. Barrel and die temperatures were set at 160-190°C and185°C, respectively. Extrudates were cooled in chilledwater (~20°C) and cut into pellets of diameter ≤ 1 mm.Pellets were then dried for 24 hours at 60°C. For preparingBlend-3 and Blend-4, melt mixtures composed of PLA/PCL (for Blend-3) and PLA/PCL/GMA (for Blend-4),were produced using the same extruder and operationconditions as described for Blend-1 and Blend-2 beforeradiation treatment. And then, the pellets so obtained wereirradiated using a commercial electron-beam accelerator(ELV-0.5, BINP, Russia, at a maximum beam current of40 mA and beam energy of 0.5-0.7 MeV) under a nitrogenatmosphere. The irradiation dose was controlled by vary-ing beam currents from 0.5 to 10 mA and conveyor speedsfrom 1 to 2 m/min. Radiation doses were measured bydosimetry (GENESYS 20, Thermo SCIENTIFIC Co.)using dosimetry film (B3 WINdose Dosimetry, GEX Co.).The acceleration energy used was 0.7 MeV and the effec-tive penetration depth was ~2 mm for a substrate ofdensity 1 g/cm3 (Han et al., 2006; Woods et al., 1994).The compatibilized blends produced were then dried in anoven at 60ºC for 12 hours to eliminate residual radicals.

2.3. CharacterizationMorphologies of blends were studied by observing cryo-


Korea-Australia Rheology J., 29(4), 2017 297

genic fractured surfaces using a scanning electron micro-scope (SEM, Hitachi model s-4200, Japan). Viscoelasticproperties were measured using a rotational rheometer(Physica MCR 301; Anton Paar GmbH, Germany). Theequipment was run in parallel plate geometry with a plateof diameter 25 mm and a gap of 1.5 mm at 190°C. Toobserve linear viscoelastic limits, strain sweep tests ofpure PLA and PCL, the non-compatibilized blend (Blend-0), and the compatibilized blend (Blend-2) were perform-ed at a fixed frequency of 1 rad/s at 190°C and strainsvarying from 0.01 to 500%. Dynamic frequency sweeptesting was performed at a strain of 2% in the angular fre-quency range 0.5 to 500 rad/s. Dynamic time sweeptesting was also performed at an angular frequency of 1.0rad/s and a strain of 2% to examine the thermal stabilitiesof blends. Transient stress relaxation moduli of all blendsand pure PLA and PCL were observed in linear visco-elastic regime after 2% single step strain at 190°C.

3. Results and Discussion

3.1. Morphology

The shapes, sizes, and spatial distributions of phases arethe results of complex interplay between components vis-cosities, interfacial adhesion, blend composition, and pro-cessing conditions (Ibar, 2009). In this study, we reducedcomplexity associated with compatibilization by fixing theblend ratio and by preparing blends using the same pro-cessing conditions. Compatibilization reduces dispersedphase particle sizes by reducing interfacial tension andsuppressing dispersed phase coalescence. Figure 1 showsSEM images of cryofractured surfaces of compatibilizedand non-compatibilized blends. The SEM image of Blend-0 (non-compatibilized) exhibited bare interface, which istypical of immiscible blends, and a PCL particle size ofabout 10 μm. On the other hand, compatibilized blendshad smaller dispersed phase particle sizes and more occu-pied interface than non-compatibilized blend. However,the particle sizes of compatibilized blends differ some-what. The radii of dispersed phase particles of blends fol-lowed the sequence: Blend-0 (non-compatibilized blend)> Blend-3 > Blend-1 > Blend-2 ~ Blend-4. Generally, moreeffective compatibilization resulted in a finer blend mor-

phology, and thus, it was expected Blend-2 and Blend-4would be better compatibilized than Blend-1 and Blend-3.

3.2. Viscoelastic propertiesDynamic strain sweep test was performed to determine

the linear viscoelastic limits of PLA, PCL, Blend-0, andBlend-2. As shown in Fig. 2, Blend-2 showed the shortestlinear viscoelastic regime, which extended to a strain ofabout 80%. Therefore, all the dynamic tests were per-formed at 2% of strain within the linear viscoelasticregime.

Thermal degradation occurs during the melt processingof polymers, and this affects their final properties andthose of their blends (Al-Itry et al., 2012; Corre et al.,2011; Gu et al., 2008). Thus, the thermal stability of poly-mers is important for long term melt processing, such as,that required for extrusion processes. To investigate ther-mal stabilities, oscillatory complex viscosities were mea-sured as a function of time at a strain of 2% and an

Table 1. Sample codes of PLA/PCL blends.

Blendcode

PLA(wt%)

PCL(wt%)

DCP(phr)

Joncryl(phr)

GMA(phr)

Irradiation dose (kGy)

Blend-0 80 20 - - - -

Blend-1 80 20 0.2 - - -

Blend-2 80 20 - 1 - -

Blend-3 80 20 - - - 20

Blend-4 80 20 - - 3 20

Fig. 1. SEM images of cryofractured surfaces of (a) Blend-0, (b)Blend-1, (c) Blend-2, (d) Blend-3, and (e) Blend-4.



angular frequency of 1 rad/s at 190°C. Complex viscosi-ties (η*(t)), normalized with respect to their initial com-plex viscosities at t = 0 (η*0), are presented as a functionof time for PLA, PCL, and five PLA/PCL blends in Fig.3. The normalized complex viscosity (η*(t)/η*0) of PLAdecreased to 0.61, whereas that of PCL remained constantfor 30 min, indicating PCL is more thermally stable thanPLA during melt processing. As shown in Fig. 3, Blend-0 (non-compatibilized) had a higher η*(t)/η*0 than PLAdue to blending with the more thermally stable PCL. Inter-estingly, the normalized complex viscosities of Blend-2and Blend-4 increased to 1.85 and 1.21, respectively. Thisincrease in normalized complex viscosities might be dueto the formation of high molecular weight copolymercaused by interfacial reactions (Semba et al., 2006; Correet al., 2011; Shin and Han, 2013). In addition, it is pos-sible that chain extension or grafting occurred within con-tinuous or dispersed phases. A similar result for thenormalized complex viscosity of a compatibilized blendwas reported by Al-Itry et al. (2012). However, Blend-1

and Blend-3 had lower η*(t)/η*0 values than Blend-0,which was possibly attributed to chain scission duringcompatibilization process (Dong et al., 2001; Semba et

al., 2006; Ismail et al., 2010). Figures 4 and 5 show storage (G') and loss (G'') moduli

of PLA, PCL, and their blends as a function of angularfrequency (ω) and G' and G'' values calculated using thelog-additive mixing rule. In the terminal region, the curvesof PLA and PCL melts were expected to be follow thepower law of G' ∝ ω2 and G'' ∝ ω (Liu et al., 2002; Guet al., 2008). In the lower frequency region, G' and G''

slopes of pure PLA and PCL approached approximately 2and 1, respectively (Figs. 4 and 5). On the other hand, theG' and G'' slopes of all PLA/PCL blends were lower thanthose of PLA and PCL. Non-compatibilized blends havebeen reported to have higher storage moduli than theircomponents and to exhibit storage modulus plateau at low

Fig. 2. Dynamic strain sweep test results (performed at 190°C)for PLA, PCL, and compatibilized and non-compatibilized PLA/PCL (80/20) blends.

Fig. 3. Normalized complex viscosities, η*(t)/η*0 (t = 0), of PLA,PCL, and compatibilized and non-compatibilized PLA/PCL (80/20) blends measured at 190°C.

Fig. 4. Storage moduli as a function of angular frequency (mea-sured at 190°C) for PLA, PCL, and compatibilized and non-com-patibilized PLA/PCL (80/20) blends.

Fig. 5. Loss moduli as a function of angular frequency (mea-sured at 190°C) for PLA, PCL, and compatibilized and non-compatibilized PLA/PCL (80/20) blends.



frequency caused by shape relaxation of the dispersedphase (Al-Itry et al., 2014; Gu et al., 2008; Kwon andCho, 2016; Salehiyan et al., 2014a; Silva et al., 2010b;Wu et al., 2008; Yee et al., 2007). Interestingly, the stor-age moduli of Blend-1 and Blend-3, like those of Blend-0, also exhibited shape relaxation at a frequency of ~5 rad/s, which suggested Blend-1 and Blend-3 were poorlycompatibilized. However, storage moduli of Blend-2 andBlend-4 were much higher than that of PLA and did notexhibit storage modulus plateau at low frequency due tolarge shape relaxation responses with longer relaxationtimes. This result means that Blend-2 and Blend-4 arewell compatibilized blends because well compatibilizedblends should have uniform droplet distribution withsmaller sizes which indicates large shape relaxationresponses with longer relaxation times (Salehiyan et al.,2014b).

On the other hand, at higher frequencies, the G' curvesof Blend-0, Blend-1, and Blend-3 converged to the sameplateau modulus, which was similar to that of pure PCL,but lower than that predicted by the log-additive mixingrule. This negative deviation was possibly caused by slip-page at the interface between the PLA continuous andPLA dispersed phases due to insufficient interfacial adhe-sion (Silva et al., 2010b). Meanwhile, the G' curve ofBlend-2 exhibited positive deviation behavior from thelog-additive mixing rule and converged to the plateaumodulus of PLA at higher frequency, and that of Blend-4followed the log-additive mixing rule. These results ofstorage modulus behaviors represent increases in interfa-cial adhesion enough to overcome the slippage at theinterface. Furthermore, these storage modulus results sug-gest that the extent of interfacial interactions of blends fol-lowed the order Blend-2 > Blend-4 > Blend-1 > Blend 3~ Blend-0.

The loss moduli of blends exhibited behaviors similar tothose of storage moduli, but those of Blend-0, Blend-1,and Blend-3 were less affected by shape the relaxation andthey showed negative deviations from log-additive mixingrule even at low frequency, as shown in Fig. 5.

Figure 6 shows complex viscosities (η*) of PLA, PCL,and five blends as a function of angular frequency (ω) andη* values calculated using the log-additive mixing rule.Complex viscosities (η*) of PLA and PCL exhibited New-tonian flow behavior at lower frequencies, but weak shearthinning behavior at high frequencies (Bhatia et al., 2007).The flow behaviors of Blend-0, Blend-1, and Blend-3showed weak frequency dependent complex viscositiesand negative deviations from the log-additive mixing ruleat low frequencies, whereas those of Blend-2 and Blend-4 exhibited strong shear thinning and positive deviations.At a frequency of 0.5 rad/s, complex viscosities of blendsfollowed the order: Blend-2 > Blend-4 > Blend-1 > Blend-0 > Blend-3. Interestingly, the η* of Blend-3 was lower

than that of Blend-0 despite compatibilization, which waspossibly caused by radiation induced PLA degradationand poor interfacial adhesion. It has been well establishedPLA is prone to degradation when exposed to high-energyradiation (Dong et al., 2001; Ismail et al., 2010; Shin et

al., 2010). Observation of stress relaxation provides an excellent

means of determining the effect of compatibilization onrheological properties of polymer blend (Al-Itry et al.,2014; Hernández-Jiménez et al., 2002; Silva et al., 2010a).Transient stress relaxation moduli (G*(t)), normalized withrespect to initial relaxation moduli at t = 0 (G*0), are pre-sented as a function of time for PLA, PCL, and the fivePLA/PCL blends in Fig. 7. PLA and PCL relaxed in a sin-gle step, whereas all blends exhibited two-step relaxationbehavior. The relaxation patterns of Blend-0, Blend-1, andBlend-3 were very similar. For these blends, a first rapidrelaxation, which lay between those of PLA and PCL,

Fig. 6. Complex viscosities as a function of angular frequency(measured at 190°C) for PLA, PCL and compatibilized and non-compatibilized PLA/PCL (80/20) blends.

Fig. 7. Normalized transient stress relaxation moduli, G*(t)/G*0 (t= 0), (measured at 190°C) of PLA, PCL, compatibilized andnon-compatibilized PLA/PCL (80/20) blends.



probably represented the molecular relaxation of PLAmatrix, while the second relaxation was due to the relax-ation of PCL droplets elongated by shear force (Al-Itry et

al., 2014; Silva et al., 2010a). The relaxation curves ofBlend-2 and Blend-4 also showed two-step relaxationbehavior; however had much slower relaxations of firstand second stress relaxation than Blend-0, Blend-1, andBlend-3. The first step of the molecular relaxation of PLAwas slower than those of PLA and PCL, which might bedue to formation of high molecular weight molecules ofcross-links or grafts resulting from compatibilization. Inaddition, the second step relaxations caused by PCL drop-let relaxation of Blend-2 and Blend-4 were much longerthan Blend-0 (non-compatibilized), Blend-1, and Blend-3.In particular, the stress relaxation curve of Blend-4 resem-bled that of a pseudo-solid material with a very long relax-ation time (Abdel-Goad and Pötschke, 2005; Incarnato et

al., 2004). Although all blends exhibited the second stagerelaxation due to PCL droplet relaxation, the second stagerelaxation processes of Blend-0, Blend-1, and Blend-3 weremuch faster than those of Blend-2 and Blend-4, whichmight have been caused by different extents of elongationof PCL droplets when the shear strain was applied. It hasbeen shown when blends are deformed by shear stress,dispersed droplets in a well compatibilized blend are elon-gated longer due to strong interfacial adhesion, whilethose of poorly compatibilized blend are less elongateddue to slippage between the continuous and dispersedphases (Silva et al., 2010a). Furthermore, relaxation timesare longer for the blends having more elongated droplets.Therefore, if the extent of compatibility were determinedusing transient stress relaxation times, interfacial adhesionwould follow the order, Blend-4 > Blend-2 > Blend-3 ~Blend-1 ~ Blend-0.

Loss tangent, tanδ (G''/G'), is a dimensionless variable

that provides a measure of the ratio of energy loss toenergy stored during cyclic deformation (Abdel-Goad andPötschke, 2005; Ferry, 1980). Figure 8 shows tanδ curvesas a function of angular frequency for PLA, PCL, and thefive blends. Three different types of tanδ curves wereobserved. First, the curves of PLA and PCL showed agradually decrease in tanδ, which exhibited strong fre-quency dependency at low frequency but weak at highfrequency, indicating liquid-like fluid behavior becamesolid-like fluid behavior on increasing frequency, which istypical of linear polymer melts. Second, tanδ curves ofBlend-0, Blend-1, and Blend-3 showed a plateau region inthe frequency range 5-50 rad/s, indicating an additionalenergy loss, and then converged to similar tanδ values athigh frequency. We suppose this additional energy losswas caused by slippage at the interface between the twophases caused by weak interfacial adhesion. Third, Blend-2 and Blend-4 showed no plateau region within the testedfrequency range, indicating no additional energy lossduring dynamic deformation and no slippage between thetwo phases due to strong interfacial adhesion. In addition,Blend-2 and Blend-4 had very low tanδ values and weakfrequency dependent behavior as would be expected ofsolid-like materials or a partially cross-linked polymermelts (Incarnato et al., 2004). Therefore, we suggest thatthe existence of a plateau region in tanδ curves providesa qualitative means of assessing interfacial adhesion.

4. Conclusions

In this research, we studied the effect of the compati-bility of PLA/PCL (80/20) blends compatibilized by var-ious methods and non-compatibilized on the viscoelasticproperties measured within linear viscoelastic regime.Storage and loss moduli, complex viscosities, stress relax-ation moduli, and tanδ values of blends were compared toinvestigate the effects of compatibility. In addition, weexamined the blend morphology.

All compatibilized PLA/PCL blends were found to havefiner morphologies than the non-compatibilized blend,and of compatibilized blends, Blend-2 and Blend-4 hadthe smallest PCL particle sizes.

Normalized complex viscosity results revealed thatBlend-2 and Blend-4 had better thermal stabilities thanBlend-1 and Blend-3.

Storage moduli (G') of Blend-0, Blend-1, and Blend-3exhibited shape relaxation, and thus, showed positivedeviations of storage moduli form the log-additive mixingrule at low frequency, however, at higher frequency,showed negative deviations. Storage moduli of Blend-2and Blend-4 showed positive deviation or similar to thatpredicted by the log-additive mixing rule over tested fre-quency range and converged to that of PLA at high fre-quency.

Fig. 8. tanδ curves as a function of angular frequency (measuredat 190°C) for PLA, PCL, and compatibilized and non-compat-ibilized PLA/PCL (80/20) blends.



Complex viscosity (η*) curves of all compatibilizedblends exhibited more frequency dependent flow behaviorthan non-compatibilized blend (Blend-0) and pure constit-uents. The complex viscosities of Blend-2 and Blend-4showed positive deviations, whereas those of Blend-0,Blend-1, and Blend-3 showed negative deviations fromthe log-additive mixing rule.

Transient stress relaxation modulus and tanδ curves ofcompatibilized blends showed two types of behaviors.Blend-1 and Blend-3 showed fast relaxation behavior andexhibited tanδ plateau due to additional loss energy causedby slippage at the interface between the PLA matrix andPCL dispersion, which were similar to that observed forthe non-compatibilized PLA/PCL blend, whereas Blend-2and Blend-4 exhibited slower relaxation behavior andshowed no tanδ plateau region.

Based on the morphology, storage modulus, complexviscosity analysis, transient stress relaxation modulus, andtanδ results obtained, we divided the compatibilized blendsinto two groups, that is, a well compatibilized blend groupconsisting of Blend-2 and Blend-4 and a poorly compat-ibilized group consisting of Blend-1 and Blend-3.

Finally, we conclude that well compatibilized blendscould be distinguished using their viscoelastic properties.These blends had similar or higher G', G'', and η* valuesthan those calculated using the log-additive mixing rule,whereas poorly compatibilized blends showed negativedeviations of G' (except at low frequency), G'', and η*. Inaddition, poorly compatibilized blends exhibited a plateauregion in tanδ curve and fast stress relaxation process.

Acknowledgement

This research was supported by Yeungnam Universityresearch grants in 2016.

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Documents

Viscoelastic properties of PLA/PCL blends compatibilized ... · Semba et al. (2006). DCP decomposes to produce radi-cals, which abstract hydrogen from both PLA and PCL to yield free