Rheol Acta DOI 10.1007/s00397-016-0941-8

ORIGINAL CONTRIBUTION

Dynamics of partially miscible polylactide-poly(ε-caprolactone) blends in the presence of cold crystallization Maziar Derakhshandeh 1 & Nazbanoo Noroozi 1 & Laurel L. Schafer 2 & Dimitris Vlassopoulos 3 & Savvas G. Hatzikiriakos 1

Received: 12 December 2015 / Revised: 3 May 2016 / Accepted: 5 May 2016 # Springer-Verlag Berlin Heidelberg 2016

Abstract Previous studies on polylactide (PLA)/poly(εcaprolactone) (PCL) biodegradable blends revealed enhanced nucleation of PLA. The mechanism by which this enhancement of nuclei density occurs attracted significant interest. In this study, precursors’ transportation from PCL into PLA phase is invoked in order to interpret the experimental findings related to phase separation and crystallization. This mechanism is supported by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The DSC data revealed that PCL crystallinity within the blends is decreased with PLA content. In addition, POM data showed more nuclei within PLA as the PCL weight fraction is increased. The linear viscoelastic properties of these blends in the vicinity of the phase separation and cold crystallization boundary are examined. It is shown that the combination of POM and rheometry allows determining the phase diagram of the blend and identifying changes occurring due to phase separation and crystallization. Thus, the complete phase diagram of such systems can be fully determined, essential in process design and optimization.

Keywords Blend . Crystallization . Phase separation . Spinodal decomposition . Cold crystallization

* Savvas G. Hatzikiriakos [email protected] 1

Department of Chemical and Biological Engineering, The University of British Columbia, Vancouver, BC, Canada

2

Department of Chemistry, The University of British Columbia, Vancouver, BC, Canada

3

FORTH, Institute of Electronic Structure and Laser, Department of Materials Science and Technology, Heraklion 70013, Crete, Greece

Introduction Biodegradable polymers such as poly(ε-caprolactone) (PCL) and polylactide (PLA) are considered as highpotential substitutes for conventional plastics. However, drawbacks such as brittleness of PLA and low modulus of PCL are prohibiting factors for wide-ranging applications and processing of these materials (Nair and Laurencin 2007; Ray and Bousmina 2005). Blending has been widely implemented to keep the best attributes of these polymers (Broz et al. 2003; Correlo et al. 2005; Gaona et al. 2012; Na et al. 2002). Many research groups have reported on PLA/ PCL blends addressing improvement of mechanical properties compared to individual components, morphology of blends (Biresaw and Carriere 2004; Broz et al. 2003; Chen et al. 2003; Correlo et al. 2005; López‐Rodríguez et al. 2006; Na et al. 2002), and viscoelastic properties of these systems (Noroozi et al. 2012b; Ugartemendia et al. 2014; Wu et al. 2008, 2010; Zhang et al. 2009b). In multicomponent systems, the final properties are highly influenced by the properties and miscibility of the individual constituents. Previous studies on the PLA/PCL blends revealed immiscibility of these blends over a certain range of temperatures (Noroozi et al. 2012a; Patrício and Bártolo 2013). The close solubility parameters of PCL and PLA, 9.2 and 10.1 (cal/cm3)1/2, respectively (Coleman et al. 1990), and the lack of specific interactions between PCL and PLA leads to partial miscibility of their blends (López‐Rodríguez et al. 2006). The miscibility of PLA/PCL blends can be enhanced by using compatibilizers, which modify the interfacial properties either chemically or physically (Castillo et al. 2010; Gardella et al. 2014; Harada et al. 2008; Salehiyan and Hyun 2013; Shin 2013; Tuba et al. 2011). The compatibilized blends have shown better mechanical properties such as elongation at break and impact resilient compared to virgin PLA.

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Amorphous PLA is known to undergo cold crystallization during heating at low heating rate due to increase in its chain mobility and reorganization of amorphous chain into a crystalline structure (Salmerón Sánchez et al. 2007). Interestingly, blends of PLA/PCL have shown enhanced cold crystallization kinetics within the PLA phase, which cannot be explained trivially unless the blends are considered miscible (Castillo et al. 2010; Dell’Erba et al. 2001; Gardella et al. 2014; Harada et al. 2008; López‐Rodríguez et al. 2006; Wu et al. 2011). In the compatibilized blend of PLA/PCL, the enhancement in cold crystallization within PLA phase is attributed to the increased PLA chain mobility imposed by miscible PCL chains at temperatures higher than the PCL melting temperature (Castillo et al. 2010; Gardella et al. 2014; Harada et al. 2008; Tuba et al. 2011; Wu et al. 2011). Surprisingly, an increase in PLA nuclei density was observed during cold crystallization for immiscible/noncompatibilized blends of PLA/PCL. In previous studies, the influence of PCL phase on the degree and kinetics of PLA crystallization was emphasized (Sakai et al. 2009; Zhang et al. 2013). In particular, PLA/PCL double-layer films were prepared by solvent casting, and the isothermal crystallization of PLA film was monitored at a desired temperature well above the melting point of PCL. The PCL layer induced small increase in growth rate of PLA crystals confirming the immiscibility of the layers. Limited miscibility at the interface was proposed to explain the extensive increase in nuclei density observed within the PLA film (Zhang et al. 2013). In another study, the nucleation density of PLA within immiscible PLA/ PCL blend after different aging time was examined (Sakai et al. 2009). Since all aging temperatures were well below the glass transition temperature of PLA, it is expected that the nuclei formation within PLA diminishes. However, the nucleation density was increased with the increase of PCL weight fraction, aging time, and aging temperature. The enhancement in nuclei density was explained by the limited miscibility of PLA and PCL at the interface (Sakai et al. 2009). From the above, it is evident that, despite the substantial progress in the field, the interplay of viscoelasticity, miscibility, and cold crystallization in this type of blends remains an open issue in part because of the lack of systematic data. In other words, design guidelines that allow a priori selection of the appropriate blend system with desired performance are missing. In this work, we address this point in part. We probe the crystallization behavior of PLA in various PLA/PCL blends with different composition using differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The data presented below complement previous studies and explain the observed

enhanced nuclei density of PLA within the blend in detail. Since the cold crystallization of PLA often happens at temperatures comparable to phase separation temperature in PLA/ PCL blends (Buddhiranon et al. 2011; Meredith and J Amis 2000), it complicates their phase behavior (Inaba et al. 1986, 1988). The lower critical solution temperature (LCST) phase behavior at a temperature range above the melting point of PCL has been previously reported for a PLA/PCL system using optical technique (Buddhiranon et al. 2011; Meredith and J Amis 2000). However, to the best of our knowledge, the rheological behavior of these blends in the transitional region has never been reported. Addressing the points above will provide a means to tailor the thermodynamic and rheological behavior in these systems. This is much needed in view of their potential for extending the range of their application via design of new composites. The rheological response in the pre-transitional area of binary blend systems has been employed before in many reports in order to determine their phase boundary (Ajji and Choplin 1991; Ajji et al. 1991; Chopra et al. 1998; Kapnistos et al. 1996b; Vlassopoulos et al. 1997; Zhang et al. 2001). The enhanced concentration fluctuations near the transition have been shown to be the origin of the complex rheological response in the vicinity of the critical region, giving rise to enhanced viscoelastic moduli at long times (Bousmina et al. 2002; Kapnistos et al. 1996b; Zhang et al. 2008). Despite the significant advances so far, there remain open questions especially with respect to the role of heating rate and frequency, as well as the synergistic effect of cold crystallization, material, and thermodynamic parameters on the boundary and kinetics of phase separation in complex polymer blends such as PLA/PCL (Niu and Wang 2006; Zou et al. 2012). From a practical point of view and due to several practical applications of this system, knowing the rheology and phase separation is important. The Fredrickson-Larson model (Fredrickson and Larson 1987) for block copolymers was adopted by several investigators for polymer blends to study rheologically their phase behavior (Ajji and Choplin 1991; Ajji et al. 1991; Kapnistos et al. 1996b). This approach is also used in the present work. The heating rate used is known to be important factor in these studies, and it has not been studied systematically in conjunction with the rheological approach (Kapnistos et al. 1996a). In the current study, we examine the phase separation behavior of PLA/PCL blend rheologically. The importance of various parameters such as heating rate and frequency is examined. DSC and POM image data is used along with the rheological measurements to reveal the interrelation between cold crystallization and phase separation, which occur at comparable temperatures. This study

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provides (i) new information on the mechanism by which nuclei enhancement of PLA occurs within the blend system and (ii) information on the phase behavior of the PCL/PLA blends with emphasis on the importance of molecular parameters, cold crystallization, and heating/cooling rates affecting the phase diagram.

Experimental section Materials A commercial grade of poly(ε-caprolactone) (PCL) polymer with monomer molar mass of 114.14 g/mol was supplied by Perstorp (Capa® technology). In addition, three polylactide (PLA) polymers with monomer molar mass of 144.13 g/mol were used in this study and supplied by NatureWorks Co. Ltd., USA. The molecular characteristics, namely weight average molar mass Mw and the polydispersity index (PDI ≡ Mw/Mn, where Mn is the number molar mass), of these polymers were determined by gel permeation chromatography (GPC-LS) using Waters liquid chromatography. Table 1 lists these properties. Blend preparation The PCL/PLA blends of various composition ratios were prepared by continuous solution mixing and casting of 8 wt.% solutions in chloroform for 2 h to ensure mixing at the molecular level. The solutions were kept in Petri dishes in a fume hood overnight and then gradually dried under vacuum for 4 days at 45 °C to allow complete evaporation of the solvent. Considering the sensitivity of PCL and PLA to moisture and hydrolytic degradation, the samples were additionally dried in a vacuum oven at 35 °C for 24 h, before the rheological measurements. Samples for rheological measurements were compression-molded at 180 °C for 3 min before cooling, resulting in sheets with thickness of about 1 mm. The compression molding

Table 1 Molecular and thermal characteristics of studied PLAs and PCL samples

temperature was above the melting temperature of the polymers obtained from DSC (SHIMADZU DSC-60), see Table 1. Thermal characterization A SHIMADZU TGA-50 was employed to determine the thermal stability limit of the polymers (see Table 1). The analysis was performed in nitrogen with the flow rate of 50 mL/min up to 600 °C. The melting peak and glass transition temperatures were obtained by using a SHIMADZU DSC-60 calibrated by indium. Calorimetry was performed in nitrogen atmosphere with approximately 1.8–2.5 mg of sample. Samples were heated to 200 °C with heating rates of 2 °C/min. Rheological measurements The rheological measurements for the blends were performed by a stress-controlled rheometer (Anton Paar MCR 501) equipped with a cone-and-plate geometry (diameter of 25 mm and cone angle of 4°) using nitrogen gas to prevent thermal degradation of PCL and PLA components. As discussed above, the samples were kept in the vacuum oven at 35 °C for 24 h to remove any solvents and humidity and to have the same thermal history. The rheological properties of the pure components of several PCLs and PLAs as functions of molecular weight characteristics have been studied and reported before (Noroozi et al. 2012b; Othman et al. 2011). The following types of rheological testing were performed: First, the thermal stability of the blends was confirmed by dynamic time sweep tests at the angular frequency of 0.1 Hz rad/s at 170 °C. Stress amplitude sweep tests were performed at the constant frequency of 1 Hz to determine the limit of linear viscoelasticity. Isochronal dynamic temperature ramp tests were carried out at different frequencies of 0.05, 0.1, 1, and 10 Hz and different heating rates of 0.2, 0.5, and 1 °C/min to investigate the effects of frequency and heating rate. The temperature range in this study was 70–130 °C covering the miscible and phase-separated region in these blends.

Sample

Mw (105 g mol−1)

PDI≡Mw/Mn

Me (g mol−1)

Tg (°C)

Tm (°C)

Td (°C)c

Capa®6800 PLA 2002D PLA 3051D PLA 3251D

0.8840 1.0690 0.9247 0.5535

1.22 1.82 1.82 1.62

3900a 4400b 4400b 4400b

−60.0 56.5 55.0 58.8

57.0 – – 165.1

347.5 262.8 216.1 178.5

a

Ref. (Fredrickson and Larson 1987)

b

Ref. (Kapnistos et al. 1996a)

c

Td: thermal degradation temperature measured by TGA

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Measurements were all performed in oscillatory shear mode and constant shear stress of 20 Pa in the linear viscoelastic regime. The measurements were repeated three times for each type of experiment. POM A Mitutoyo VMU-V polarized microscope was used in conjunction with the Anton Paar MCR-501 in order to reveal the evolution of any formed crystalline structure and distinguish this from the partial miscibility of the blends. In a typical experiment, the test specimen is placed in the space between two glass parallel plates (43 mm in diameter). After reaching the desired gap size of 0.1 mm, the test specimens were equilibrated at 200 °C for 5 min in order to eliminate the thermal histories before cooling to 30 °C at the rate of 2 °C/min. The samples were reheated at the same rate to 200 °C after soaking for 10 min at 30 °C. The crystalline microstructure, which is formed by cold crystallization within the blend matrix over time, is observed in situ with the polarized microscope.

Results and discussion Thermal analysis The thermal characteristics of the homopolymers of PCL and PLAs and their blends were determined by a SHIMADZU DSC-60 differential scanning calorimetry (DSC). Figure 1 depicts the endothermic heat flows of the second heating scan

corresponding to the melting characteristics of the polymers and their blends using the heating rate of 2 °C/min. A third heating scan was also performed in order to check for reversibility/reproducibility of the process. The third and second heating scans were found to be nearly identical. PLA 3051D and PLA 2002D are considered amorphous polymers as they did not show any crystallization peak under cooling mode. However, as can be seen from Fig. 1a, b, they revealed a small endothermic peak due to cold crystallization of amorphous PLA chains (Fortunati et al. 2012; Liao et al. 2007; Mano et al. 2004; Righetti and Tombari 2011; Salmerón Sánchez et al. 2007; Wu et al. 2007; Zhang et al. 2004). Cold crystallization occurs due to rearrangement of the amorphous region of PLA into a crystalline phase during heating at low heating rates. Cold crystallization is accompanied with an exothermic peak in DSC heat signal (in the range of 100– 120 °C in our case). These exothermic peaks for virgin PLAs are found to be broader compared with peaks occurring within blends. It is worth noting that the heat of fusion during melting of formed PLA crystals are almost equal to the heat of crystallization which is released during the cold crystallization. Therefore, crystals existing within these PLAs (PLA 2002D and PLA3051D) are formed by cold crystallization. In blends with small amount of PCL (i.e., <40 %), cold crystallization is enhanced and much sharper peaks at lower temperatures are observed in DSC heat signals compared to neat PLAs. This is attributed to conformational rearrangements via cooperative segmental movements typically in the case of nucleation sites and in the present case the PCL molecules (Ferry 1980). The onset of cold crystallization obtained from

Fig. 1 DSC thermographs (second heating cycle) of the polymers used in this study and their blends at a rate of 2 °C/min a PLA 3051D and PCL Capa®6800, b PLA 2002D and PCL Capa®6800, and c PLA 3251D and PCL Capa®6800

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DSC for various compositions is reported in Table 2. The double peaks in the melting of the blends (around 150– 170 °C depending on the blend’s composition) reveal that two different types of PLA crystal with different perfection exist within the system. The first endothermic peak shows PLA crystals with less perfection probably due to the presence of limited amount of miscible PCL chains within the formed structure (Castillo et al. 2010). For the crystalline polymer PLA 3251D, neither cold crystallization occurs nor does a double peak appear during the fusion of the formed crystals indicating that cold crystallization is a characteristic of amorphous PLA. To calculate the degree of crystallinity, the theoretical heat of fusion of 100 % crystalline PLA (93 J/g) and PCL (139.5 J/ g) is used (Fischer et al. 1973; Guo and Groeninckx 2001): Xf ¼

ΔH f ΔH f ð100%Þ  wi

ð1Þ

where Xf is the degree of crystallinity of PLA or PCL, ΔHf is the heat of fusion obtained by integrating the area under DSC thermograph, ΔHf(100 %) is the heat of fusion of 100 % crystalline PLA or PCL, and wi is the weight fraction of PLA or PCL within the blend. Integrating the areas under the peaks results a PLA crystallinity of maximum ∼25 % (discussed below with reference to Fig. 5) for the first two sets of blends that involve amorphous PLAs, plotted in Fig. 1. In the third set of blends that involves the crystalline PLA3251D, the degree of crystallinity formed is significantly higher (>40 %). To further examine the origin of cold crystallization, we studied the thermal behavior of several blends by POM at identical conditions with those used in DSC, i.e., soaking the sample at 200 °C before cooling and reheating at the rate of 2 °C/min. Figure 2a shows the thermal history, and Fig. 2b shows the corresponding crossed POM images obtained at various times along the thermal experimental protocol for the PLA3051D/PCL6800 80/20 blend. At 200 °C, both PLA and PCL are in the melt state, and thus, no anisotropy is expected. Therefore, the image appears completely dark (image “a” in Fig. 2b). PCL crystals are observed at around 30 °C. After saturating at 30 °C for 10 min, the blend was heated at the rate of 2 °C/min. PCL crystals are expected to start melting at around 50 °C and the melting process to be completed at 60 °C (see DSC in Fig. 1). A little shift (lag) in these temperatures is expected under the polarized microscopy technique due to larger (thicker) sample that delays the heat transfer into

Table 2

Onset of cold crystallization for different blend’s composition

Sample

Virgin PLA

80/20

70/30

60/40

PLA 2002D/PCL PLA 3051D/PCL

110 110

94 94

91 90

97 97

and/or from the test specimen (Derakhshandeh et al. 2014). Nevertheless, all PCL crystals are melted at about 66 °C as shown in Fig. 2b (image “e”). Surprisingly, anisotropy is observed within the matrix at 105 °C (image “g” in Fig. 2b) due to cold crystallization discussed above. This anisotropy increases in strength first due to crystal perfection and subsequently decreases with temperature increase. At temperatures higher than ∼159 °C, the blend becomes completely isotropic due to the complete melting of these crystals. Since this temperature is similar and close to the melting point of virgin PLAs (∼165 °C), it is fair to state that the cold crystals formed in the range of ∼90–120 °C are PLA crystals with infinitesimal PCL molecules within their structures. As mentioned, cold crystallization kinetics of PLA molecules is enhanced in blends as shown by Fig. 1, which is also in agreement with previous studies (Castillo et al. 2010; Dell’Erba et al. 2001; Gardella et al. 2014; Harada et al. 2008; López‐Rodríguez et al. 2006). In order to explain this kinetic enhancement, the limited miscibility of PCL and PLA chains was suggested as the underlying mechanism (Sakai et al. 2009; Zhang et al. 2013). This mechanism fails in explaining the increase of nuclei density within the PLA phase in the blends. Therefore, in the current study, we propose the encapsulation of precursors (which consists of nuclei/ mesophase with a tendency to form either melt or crystals) by the aligned PLA chains during demixing to be responsible for the observed behavior. Of note, the decrease of PCL crystallinity from 30 % (for the pure PCL) to lower than 10 % (for the PLA/PCL 80/20 blend) as will be shown later in Fig. 5 cannot be explained by the limited chain miscibility between PCL and PLA. Therefore, an enhanced miscibility of PLA and PCL chains shall be considered in order to explain the behavior in accord with previous works (Newman et al. 2009; Patrício and Bártolo 2013; Tuba et al. 2011). This enhanced mixing can be induced by the migration of precursors (mesophase consisting of highly aligned PCL chains) for crystallization from the PCL phase to the PLA phase, which is discussed in detail below. It is well-understood that the equilibrium melting temperature of a given semi-crystalline polymer differs from the peak temperature obtained during melting within DSC (Petraccone et al. 1985; Marand et al. 1998). To put this into a perspective, different crystallization conditions such as different quiescent isothermal crystallization temperatures yield crystals with different degree of perfection. The crystals with larger degree of perfection show more resilience toward melting. Therefore, the melting peak in the DSC signal is postponed to a higher temperature as the degree of crystal perfection increases. Hoffman-Weeks extrapolation is often applied upon the quiescent isothermal crystallization data to infer the equilibrium melting temperature which corresponds to nuclei-free state (Marand et al. 1998; Petraccone et al. 1985). The equilibrium

Rheol Acta Fig. 2 a The experimental thermal protocol used in POM for the PLA3051D/PCL 80/20 blend. The corresponding POM images are taken at the following temperatures (shown in Fig. 2b) a 200 °C, b 33 °C, c 30 °C, d 60 °C, e 66 °C, f 105 °C, g 120 °C, h 150 °C, and i 159 °C. b. Crosspolarized microscopy images of the blend PLA3051D/PCL 80/20 using thermal history shown in Fig. 2a

melting temperature has molecular weight dependency, and it can be ∼20 °C larger than the obtained value for peak temperature observed during heating (Chen et al. 1997). Based on the previous works, the PCL in the current study is expected to have equilibrium melting temperature of ∼80 °C (Chen et al. 1997) which is comparable to the onset of cold crystallization. Therefore, precursors (within PCL domain) which are nuclei/ mesophase can exist at temperatures comparable to binodal phase separation. The occurrence of binodal decomposition triggers the PLA chain mobility contributing to more PLA lamella. According to the classical theory of nucleation, the nucleation rate, R,

depends on the activation barrier of the surface free energy for the crystal nuclei formation, ΔGS, and the barrier related to chain mobility, ΔGD (James 1981).   ΔGS þ ΔGD ð2Þ R ¼ A exp − kT As demixing occurs, the PLA lamella are deposited preferably on the remaining precursors (which consist of PCL chains) which results in the reduction of the surface free energy for the crystal nuclei formation, ΔGS to zero (Bartczak et al. 1987; Wenig and Asresahegn 1993). Therefore, the

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PCL chains which are either in mesophase or solid (nuclei) are encapsulated within PLA-rich domain explaining the enhanced nuclei formation and improvement in the onset of cold crystallization for blends compared with virgin PLA (see Table 2). Since most of these PCL chains should be in the center of the crystals, one expects small changes in the growth kinetics in agreement with previously published data (Zhang et al. 2013). To support our explanation further, image data at four different temperatures is shown in Fig. 3 for PLA2002D/ PCL 80/20 blend. As the temperature is increased, the PLA crystals start to melt. Surprisingly, the melting phenomenon starts from the center of the crystals indicating less crystal perfection. This phenomenon is shown for crystals labeled as 1, 2, 3, and 4 in Fig. 3. Hence, it is likely that PCL chains coexist within the crystals with a larger concentration at the center (where nuclei was formed) contributing to less crystal perfection. These POM image data and the proposed mechanism are in agreement with the double peak observed within DSC during melting (see Fig. 1). Sakai et al. (Sakai et al. 2009) studied the nucleation behavior of PLA in immiscible PLA/PCL blends after aging at different aging time/temperatures below glass transition of PLA (but above Tg of PCL). PLA chain mobility was infinitesimal due to wise selection of aging temperatures. Nucleation enhancement with aging time or PCL content was observed. Also, the saturation value of nuclei within PLA domain increased as the aging temperature is increased (all aging temperature were below and above Tg of PLA and PCL, respectively). As aging temperature/time is increased, the amorphous part of PCL chain becomes more mobile, and

therefore, the probability of PCL crystals to be at the interface is increased. Once the temperature is increased above the melting temperature of PCL, the surviving precursors can migrate into PLA phase due to interlocking of PLA lamella upon the precursors’ surface. These precursors can diffuse into PLA domain possibly by the PLA chain movement. Therefore, as aging time or temperature is increased, an enhanced PLA nucleation is expected. Our POM image data show the formation of tiny crystals within PLA right after the phase separation which is supported by the explanation given above (see also Fig. 4 and the related discussion below). To further support this idea, we have studied a blend with a high PCL concentration, i.e., PLA 3051D/PCL 30/70 by optical microscopy. The results are shown in Fig. 4. At 30 °C, PCL crystals dominate the matrix, which melt completely at temperatures higher than about 60 °C. As shown in Fig. 4b, at the temperature of 86 °C (which is above the binodal phase separation temperature, see Fig. 15), the blend matrix consists of PLA-rich and PCLrich domains. Precursors for crystallization, which are within the PCL-rich phase, transfer into the PLA-rich domain which is confirmed by the larger number of crystals compared to that of 70/30 blend (not shown here). This scenario facilitates cold crystallization compared to that of virgin PLA by providing additional sites for nucleation and thus decreasing the activation barrier of the surface free energy for the crystal nuclei formation, ΔGS. It can be inferred from the POM images (Fig. 4) that nuclei density is enhanced not only close to PCL interface but also within the PLA-rich phase. Therefore, we suspect that the increased chain mobility of PCL chains at the interface is not sufficient to describe the dynamics of our system. Similar results indicating larger PLA nucleation density were obtained by various researchers (Sakai et al. 2009; Zhang et al. 2013). It

Fig. 3 Cross-polarized microscopy images of the blend PLA2002D/ PCL6800 80/20 at various temperatures during heating at 2 °C/min. a PLA crystals at 130 °C, b at 145 °C, c at 150 °C, and d at 163 °C

Fig. 4 Cross-polarized microscopy images of the blend PLA3051D/PCL 30/70 at various temperatures during heating at 2 °C/min. a PCL crystals at 30 °C, b PLA crystals at 86 °C, c at 122 °C, and d at 129 °C

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is worth noting that the nuclei transfer into another domain was also observed and reported for iPP/LDPE blends (Gałeski et al. 1984) and was explained in terms of interfacial energy difference between the nuclei and the molten components of the immiscible blend. The glowing boundary in Fig. 4b reveals oriented PLA chains, which are migrated to the interface of PLA/PCL phase during demixing. Figure 4c shows another viewing area for the same sample at 122 °C. As seen in this image, PLA-rich phase is nuclei dominated (at this stage, the cold crystallization process is completed). PLA crystals which are formed in the PCL-rich domain can move slowly in the PCL domain due to movement of PCL chains. Figure 4c, d reveals the chain mobility of PCL molecules. As seen from these images, crystals labeled as 1, 2, and 3 in Fig. 4c have moved into new locations within the PCL-rich matrix in Fig. 4d. The time difference between images in Fig. 4c, d is about 210 s, and the distance traveled by each crystal is ∼91 μm; thus, these crystals are estimated to move at the speed of 0.43 μm/s. The discussion above is further supported by the POM micrographs of other blends we studied, i.e., the PLA3051D/PCL6800 blend with different composition (not shown here). Blends with higher PCL content have shown higher nucleation activity in the PLA-rich domain. POM data for the phase separation and the subsequent cold crystallization show the precursors migration during liquidliquid phase separation (LLPS) as discussed above. This has also reported in previous studies for other blend systems (Tsuburaya and Saito 2004; Wang et al. 2002; Zhang et al. 2005, 2006). If nuclei/precursors encapsulation happens according to the proposed theory, it is reasonable to expect the PCL crystallinity to decrease as its concentration decreases within the various blends since the probability of PLA chain deposition upon the precursors will be increased. To test this, the relative crystallinity of PLA and PCL as a function of blend composition is obtained using Eq. 1 and the results plotted in Fig. 1. Figure 5 shows the relative crystallinity

Fig. 5 Degree of crystallinity obtained from DSC using Eq. 1 for the two sets of blends using PLA3051D and 2002D. The PCL crystallinity decreases with decrease of its concentration, while PLA exhibits a maximum at PLA concentrations of about 70– 80 %

within the blends as a function of PLA concentration. It is clearly shown that the PCL crystallinity decreases with its concentration which is in agreement with published literature (Newman et al. 2009; Patrício and Bártolo 2013; Tuba et al. 2011). The crystallinity of PLA shows a maximum at PLA content of 70–80 %, which suggests an optimum concentration for enhanced cold crystallization kinetics. In blends with low PLA content, it is possible that large nuclei density induced by PCL molecules freezes the PLA chains limiting their ability to rearrange, which in turn yields slower kinetics as indicated by DSC results. On the other hand, in blends with large PLA contents, a limited number of nuclei/precursors are available within PCL phase which also results in slower kinetics. Therefore, it is reasonable to consider an optimum in cold crystallization kinetics with respect to PLA concentration. Our earlier work on these blends revealed that there is no significant change in the melting and glass transitions of PCL and PLA in the blends, which suggests the immiscibility of PLA and PCL. However, our previous study emphasized their behavior at high temperatures where these blends are already phase-separated (Noroozi et al. 2012a). The focus on the rest of this study is to examine their thermodynamic behavior over the whole range of temperatures with focus on the range between the melting points of the pure components where cold crystallization and phase separation occur. As this work demonstrates, these blends are partially miscible and form phases of specific PLA/PCL compositions (partial miscibility/immiscibility). This was also discussed above in the context of some polarized images. Binodal decomposition from the viscoelastic response in oscillatory shear mode As mentioned earlier, temperature sweep tests at constant frequency have been performed to identify the phase separation temperature (binodal). Figure 6a demonstrates the typical evo-

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lution of elastic (storage) modulus, G′ in a temperature range of 70–130 °C, for a PLA3051/PCL blend. G′ decreases with increasing temperature in the homogenous region due to the enhance mobility of the polymer chains (moving away from Tg and Tm). At some threshold temperature, there is a drastic variation in the elastic modulus as a result of concentration fluctuations (thermodynamic forces), which overcome the mobility of the chains and induce additional stresses (Kapnistos et al. 1996b; Zhang et al. 2008). The slope change from negative to positive of the dynamic moduli indicates the onset of binodal temperature (Chopra et al. 1998). The binodal temperature in our study is defined as the point of intersection of lines passing through the descending and ascending parts of storage modulus as clearly shown in Fig. 6b. The elastic part of the complex viscosity (η″) of PLA 3051D/PCL (60/40) is reported at different temperatures in Fig. 7. All the data plotted correspond to phase-separated region as will be shown below when the complete phase diagram will be presented. Failure of time-temperature superposition at low frequencies in this temperature range is an indication of the phase-separated region. From the application of the Fredrickson-Larson model, one expects no frequency dependence of the order-disorder transition temperature in the terminal regime. This is examined here since in some cases the terminal region was not reached. In principle, the phase separation temperature, being a material parameter, should not depend on frequency. For the PLArich blend with 60 wt.% PLA (Fig. 8a), at the highest frequency of this study (10 Hz), there is a slight shift in the critical temperature. For the PCL-rich blend, a relatively higher shift in transition temperature was observed over the same frequency range compared to the blend with 60 wt.% PLA (Fig. 8b) apparently promoting shear-induced mixing (Easwar 1992; Zou et al. 2012) although this effect appears to be small. Figure 9 summarizes the dependence of the binodal separation temperatures on frequency for the near-critical (60/40) and the off-critical (35/65) blend. The data should exhibit a plateau at low frequencies (typically below 0.1 Hz) to match data in the

Fig. 6 a Temperature evolution of elastic modulus (G′) under small amplitude oscillatory shear at a fixed frequency of 0.1 Hz and heating rate of 0.5 °C/min for different compositions of the PLA3051D/PCL blends. b The intersection of two lines passing through the descending and ascending part of G′ defines the binodal temperature

absence of flow. In this study, the phase separation behavior was investigated at the frequency of 0.1 Hz. In the remaining part of this section, we demonstrate the effect of heating rate on the PLA/PCL blends. It can be clearly observed from Fig. 10a, b that the heating rate strongly affects the binodal temperature. The lower heating rate shifts the binodal temperature, Tb, to the lower region for all compositions. The results also show that the higher heating rates result into lower elastic modulus, which is in agreement with previous reports on PS/PVME blends (Madbouly and Ougizawa 2004). The binodal temperatures follow a linear relation with heating rate for all blends (Fig. 11 plots the data for two blends), although a nonlinear dependency has been reported before for PS/PVME blends (Madbouly and Ougizawa 2004). It should be noted that the difference of the value of binodal temperature obtained from linear extrapolation of the data to zero heating rate (Fig. 11) from the value that corresponds to the small heating rate of 0.2 °C/min is less than 2–3 °C. Cold crystallization onsets shortly after binodal phase separation, and thus, one may expect the storage modulus to increase since crystals form within the blend matrix. This may interfere with the obtained binodal temperature at the first glance. However, since cold crystallization happens simultaneously or shortly after binodal phase separation temperature, its effect on the temperature at which the change in the slope of storage modulus from negative to positive happens (binodal phase separation temperature) is minimal given the linear scale of temperature. Figure 12 shows the phase diagram (binodal decomposition) for the PLA/PCL blends determined by using different heating rates. The results confirmed that this is a lower critical solution temperature (LCST) system, which is in agreement with the previous report of Meredith et al. (Meredith and J Amis 2000). As can be observed, a faster heating rate shifts the critical temperature by about 15 °C, where the effect is slightly lower for blends with higher/lower PCL content. However, the critical concentration region remained unchanged independent of the heating rate. The continuous lines

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Fig. 7 Time-temperature superposition of the elastic part of the complex viscosity (η″) of PLA3051D/PCL (60/40) as a function of the frequency at the reference temperature of 120 °C

in Fig. 12 represent polynomial fits of the experimentally determined binodal temperatures to guide the eye. The lowest curve is the calculated binodal curve by linear extrapolation of the binodal temperatures to zero heating rate (see Fig. 11). As discussed above, the difference between this curve (zero heating rate) and the one corresponding to the lowest experimentally used heating rate of 0.2 °C (taken as the binodal curve for heating rates below a minimum) is within 2–3 °C. Spinodal decomposition The spinodal temperatures can be determined by employing the Fredrickson-Larson approach by extrapolation of the linear range of the curve [G″(ω)2/(TG′(ω))]2/3 versus 1/T (Ajji and Choplin 1991; Ajji et al. 1991; Kapnistos et al. 1996a). At low and high temperatures, the [G″(ω)2/(TG′(ω))]2/3 versus 1/T shows deviation from linearity. The originally proposed theory has been developed for block copolymers for the onephase region close to the critical point. The spinodal determination in this modified theory for blends is based on the assumption that both near-critical and off-critical compositions follow the critical fluctuations in the same way (Kapnistos

Fig. 8 Dependence of the dynamic moduli (G′) versus temperature on frequency used in small amplitude oscillatory shear at different frequencies from 0.05 to 10 Hz and heating rate of 0.5 °C/min for a the (60/40) PLA3051 D/PCL blend and b the (35/65) PLA3051D/PCL blend

Fig. 9 Dependence of the binodal temperature on frequencies ranging from 0.05 to 10 Hz at a constant heating rate of 0.5 °C/min for the (35/65) and (60/40) PLA3051D/PCL blends. The continuous lines have been drawn to guide the eye

et al. 1996a; Zhang et al. 2009a). Since the linear range is determined qualitatively in this method, an error of ±2 °C is repeatedly reported in this approach (Kapnistos et al. 1996a; Zhang et al. 2009a). Figure 13 displays typical results of [G″(ω)2/(TG′(ω))]2/3 versus 1/T for the 35/65 PLA3051 D/PCL blend and the extrapolated spinodal temperatures of 112.5, 106.9, and 103.5 °C for the heating rates of 0.2, 0.5, and 1 °C/min, respectively. These values indicate a linear dependence, which can be extrapolated to obtain the spinodal temperature at zero heating rates. As shown in Fig. 13, a linear region which can be used to estimate the spinodal decomposition temperature cannot be well-defined. Nevertheless, it is noted that only the part of data corresponding to temperatures lower than the binodal temperature in the homogenous region is considered. In addition, linear extrapolation was carried on the portion of data that results the maximum correlation factor. The spinodal points calculated from the Fredrickson-Larson theoretical approach (Fredrickson and Larson 1987) are plotted in Fig. 14. The rheologically determined phase diagram of PLA3051D/PCL blend from the isochronal temperature ramp is illustrated in Fig. 15. The binodal and spinodal points are

Rheol Acta Fig. 10 a Dependence of dynamic moduli (G′) on temperature under oscillatory shear at different heating rates from 0.2 to 1 °C/min and constant frequency of 0.1 Hz for the (70/ 30) PLA3051D/PCL blend. b Dependence of dynamic moduli (G′) on the temperature under oscillatory shear at different heating rates ranging from 0.2 to 1 °C/min and constant frequency of 0.1 Hz for the PLA3051 D/PCL blend

calculated by extrapolation at zero heating rate (lower curves in Figs. 12 and 14). It should be noted that below the melting point of PCL (dashed line), at high PCL concentrations, the structure is mainly single phase crystals. However, at higher temperatures (before PLA cold crystallization), a two phase system coexists (combination of crystals dispersed in a PCLrich homogeneous liquid phase). By increasing the temperature further, the homogeneous liquid and crystal mixture, transforms into a liquid-liquid phase-separated system as has been demonstrated above. When the binodal temperature is reached, phase separation takes place via nucleation and growth. This mechanism needs an activation energy which is reflected in the endothermic peak usually observed in DSC (Arnauts et al. 1994; Dreezen et al. 2001). However, in our system no endothermic peak is observed around the binodal temperature (Fig. 1). This is attributed to the combination of small enthalpy of demixing (Arnauts et al. 1994) and cold crystallization which is an exothermic phenomenon and takes place right after phase separation. Hence, the exothermic behavior seen may serve as an effective measure of the activation energy for phase separation (suppressing the endothermic peak). When cold crystallization of PLA occurs, the FredricksonLarson method cannot be used unambiguously to determine

Fig. 11 The heating rate dependency of binodal temperatures for blends (70/30) and (35/65) PLA3051D/PCL blends that seems to be the same

a

b

the spinodal temperature since a PLA crystalline phase coexists with two amorphous liquid phases. If one considers the Gibbs phase rule (Gibbs 1961), F = C + 2 − P where F is the degree of freedom, C the number of components, and P the number of phases which exist in the system, a single degree of freedom is expected. Therefore, the PLA/PCL system becomes indeterminate, i.e., if one starts from a low temperature at a certain composition, as cold crystallization kicks-in, PLA crystals are formed changing the initial composition. Thus, when spinodal separation occurs, this would correspond to an indeterminate composition, although the associated error will be small due to the small degree of cold crystallinity formed. Nevertheless, the so-extracted ill-defined spinodal curve is also plotted in Fig. 15 to show that this type of phase separation does occur above the cold crystallization temperature with a caution to a small degree of error in the horizontal direction.

The effect of Mw of PLA on the phase diagram The effect of molecular weight of PLA on the phase diagram was also studied. Figure 16 depicts the binodal phase

Fig. 12 Binodal phase diagrams for the PLA3051D/PCL blend at heating rates of 0.2, 0.5, and 1 °C/min and that calculated by extrapolating to zero heating rate. The continuous lines are the polynomial fits to the data to guide the eye

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Fig. 15 Phase diagrams of PLA3051D/PCL blend at the constant frequency of 0.1 Hz. The dotted lines are the extrapolation fit of the binodal and spinodal points Fig. 13 Plot of (G′′/G′T)2/3 versus 1/T for the (35/65) PLA3051D/PCL blend at the fixed frequency of 0.1 Hz and different heating rates of 0.2, 0.5, and 1 °C/min. The dashed lines display the extrapolation of the linear portion of each curve in order to determine the temperature for the spinodal decomposition

separation points of PLA/PCL blends with different molecular weights of PLA. The results indicate that the phase boundary shifts to the lower temperatures with increase of the molecular weight of PLA for the nearly amorphous PLAs (PLA 3051D and PLA 2002D). This is a consequence of the fact that the critical Flory-Huggins interaction parameter χc is reduced, and hence, the critical temperature drops for this LCST blend (where the coefficient B of the entropic contribution to the interaction parameter χ = A + B/T is negative) (Rubinstein and Colby 2003). This observation is consistent with the phase separation behavior of PLA/ PCL presented before (Meredith and J Amis 2000; Yang et al. 1997). According to the differential scanning calorimetry (DSC) analysis, both of these PLAs have low degree of crystallinity (see Fig. 5). However, their level of

Fig. 14 Spinodal phase diagrams of PLA3051D/PCL blends at the constant frequency of 0.1 Hz and heating rates of 0.2, 0.5, and 1 °C/min. The lines have been drawn to guide the eye

crystallinity shows increase in the presence of PCL in the blends (see Fig. 5) (Noroozi et al. 2012a). This increase in degree of crystallinity can be attributed to transfer of precursors from PCL-rich phase to PLA-rich phase as discussed in detail above. In contrast to the morphology for the nearly amorphous PLAs, crystalline PLA 3251D, which has the lowest molecular weight, shows the phase boundary shift to the lowest temperature level. The DSC thermogram shows the high degree of crystallinity for PLA 3251D which can be the reason for the occurrence of the metastable region at lower temperature. Figure 16 reveals that although the phase boundary temperatures are altered by the molecular weight of the blend component, the critical composition shows no significant dependency on the molecular weight for these PLA/PCL blends within experimental error. The estimated critical volume fractions for PLA2002D/PCL and PLA3251D/PCL are ϕc = 0.5 and ϕc = 0.59, respectively.

Fig. 16 Binodal phase diagrams of PLA/PCL blends for different molecular weight PLAs at the constant frequency of 0.1 Hz and heating rate of 0.5 °C/min. The continuous lines have been drawn to guide the eye. The estimated critical volume fractions for PLA2002D/PCL and PLA3251D/PCL blends are ϕc = 0.5 and ϕc = 0.59, respectively

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Conclusions Rheological techniques have been used to investigate the thermo-rheological behavior of the PLA/PCL blends. The effects of frequency in small amplitude oscillatory shear and heating rate on the phase separation have been addressed depending on the composition of the blend in the near-critical and off-critical region. The frequency has been found to have an effect on the binodal region with the binodal temperatures to increase at higher frequencies; this behavior is related to shear-induced mixing. Change in the heating rate also has shown a significant effect on the binodal region by shifting it to the higher temperatures for all blends studied. The Fredrickson-Larson theory could not be employed to study the spinodal decomposition of these blends in whole blend’s composition range due to occurrence of cold crystallization. In addition, it was found that an increase of the molecular weight of amorphous PLA shifted the phase diagram to lower temperatures. Enhanced nucleation density and the kinetics of cold crystallization of PLA chains in the PLA/PCL blends, as obtained by DSC and POM, was attributed to precursors’ migration from PCL-rich phase into PLA-rich phase. The precursors’ transfer theory was supported by a series of microscopy images and DSC results in agreement with previous studies. Decreasing the PCL concentration within the blend lowers the PCL crystallinity and thus supporting the precursors’ transfer phenomenon. It was concluded that cold crystallization of amorphous PLA blends with PCL is enhanced by binodal decomposition and occurs simultaneously and/or right after. Acknowledgments Financial assistance from the Natural Sciences and Engineering Research Council (NSERC) of Canada with the support from the NOVA Chemicals is gratefully acknowledged. Many thanks to Perstorp and Natureworks for kindly providing the materials for this project.

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