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Shear-induced nonisothermal crystallization of two grades of PLA
Polymer Testing 50 (2016) 172e181
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Material behaviour
Shear-induced nonisothermal crystallization of two grades of PLA Joanna Bojda, Ewa Piorkowska* Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90 363 Lodz, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 November 2015 Accepted 8 January 2016 Available online 12 January 2016
Shear-induced nonisothermal crystallization of two commercial polylactides (PLAs) differing in optical purity was studied. The molten polymers were sheared at selected temperatures (Ts) and subsequently cooled. The crystallization was followed by a light depolarization method, whereas the specimens were analysed ex-situ by DSC, 2D-WAXS and SEM after etching. It was found that the effect of shear, especially on the crystallinity developed during post-shearing cooling, intensified with a decrease of Ts from 160 to 146 C, and with increasing shear rate and strain. Moreover, the effect of shear on PLA1.5 with D-lactide content of 1.5% was stronger than PLA2.8 with 2.8% of D-lactide, although maximum crystallinity of both polymers was practically the same. A decrease of cooling rate from 30 to 10 C/min increased crystallinity of both PLAs, except for those shearing conditions which induced high crystallinity even during faster cooling. Although SEM examination revealed some row-nucleated forms, no significant crystal orientation was detected by 2D-WAXS, indicating that, under the experimental conditions, the shear induced predominantly point-like nuclei. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Biodegradable polymers Polylactide Shear-induced crystallization Structure Melting behavior
1. Introduction An upper temperature limit of applicability of an amorphous polymer is determined by its glass transition temperature (Tg) while that of a semicrystalline polymer by its melting temperature (Tm), usually much higher. Polylactide (PLA), a biodegradable and bio-based polyester, which recently gained enormous attention [1e3], has Tg in the range of 55e60 C. Optically pure poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) are highly crystalline, with Tm up to 180 C [4,5]. PLA usually crystallizes in the orthorhombic a-form, although two other forms, b and g, are also known and the disorder a0 phase forms below 100 C [4,6]. Generally, commercial PLAs are copolymers of L-lactide and a small amount of D-lactide. Crystallizability of PLA decreases with decreasing of its optical purity [4,5,7,8]; slowly crystallizing PLAs can be quenched to the glassy state and cold-crystallized during heating. It is worth noting that amorphous PLA is rigid and brittle at room conditions. Crystallinity slightly increases its modulus of elasticity, decreases the drawability [9] and the rate of biodegradation [10,11] but improves the barrier properties [12] and the thermal stability. During industrial processing of polymers, flow plays a vital role. Flow-induced molecular orientation can strongly influence the
* Corresponding author. E-mail address: [email protected] (E. Piorkowska). http://dx.doi.org/10.1016/j.polymertesting.2016.01.006 0142-9418/© 2016 Elsevier Ltd. All rights reserved.
crystallization and the resulting structure; both are controlled by the interplay between crystallization and chain relaxation. Shearinduced crystallization strongly depends on shear temperature _ and total strain. The longest polymer chains have a (Ts), rate ðgÞ large effect on the flow-induced crystallization, hence polymer molecular characteristics, especially the high molar mass tail of mass distribution, is crucial [13e15]. Shear flow can increase the nucleation rate by several orders of magnitude. Depending on the molecular characteristics and shear conditions, different structures can emerge varying from spherulites to row-nucleated and shishkebab morphologies [14,15]. It was postulated [16] that, to generate point-like nuclei and fibrillar nuclei, g_ has to exceed the inverse reptation time and the inverse Rouse relaxation time of the high molar mass tail, respectively, although when the flow is strong enough but too short, intermediate regimes were also defined. Others believe that the controlling parameter is the mechanical work [13,17,18]. Despite the need, few studies of the influence of shear flow on PLA crystallization have been described, and mainly concern the crystalline morphologies [19e23]. In optically pure PLLAs, sheared at 5/s and then crystallized isothermally at Ts, fibrils were found [20] in temperature ranges dependent on molar mass of PLLA; most fibrils were preferentially formed on contaminants and on a surface of the rotating plate. PLA shear-induced cylindritic forms and row nucleated spherulites, the latter below 120 C, were also found in [21], and the fibrillar nuclei were accompanied by point-like ones.
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Contrary to that, only point-like nuclei were observed in PLA _ up to 10/s [23]. Studies sheared at 135 C, despite slightly higher g, of the shear-induced nonisothermal crystallization of PLA are also very limited. Crystallization of PLAs during cooling at a rate (v) < 5 C/min was examined after shearing at 150 C, with g_ 6/s for 60 s; the amount of row-nucleated forms and crystallization onset temperature increased with increasing g_ [22]. Others [23] cooled sheared PLA even slower, at 1 C/min; temperature of crystallization completion increased with increasing shear time and leveled off at about 115 C. Differences in molar masses and optical purity of PLAs studied previously [19e23], and also different shearing and crystallization conditions, make a comparison of the results difficult. Moreover, during processing, polymer melts are subjected to very high g_ s and quenched. Hence, understanding the effect of higher g_ and v is crucial and has a significant practical importance. The v is even more critical for optically impure PLAs than for PLLA; a range of commercial PLAs do not crystallize during cooling, even at 10 C/ min. Our studies focused on the effect of shear on nonisothermal crystallization in two such commercial PLAs with different D-lactide contents, especially on the crystallinity and orientation. The molten PLAs were sheared at 146e160 C, at g_ of 10e300/s, for 5 or 30 min and, subsequently, cooled at 10 or 30 C/min. The crystallization was followed in-situ by polarized light microscopy (PLM), and then the specimens were studied ex-situ by different techniques. 2. Experimental The study utilized two commercial PLAs purchased from NatureWorks LLC: PLA 2002D (PLA2.8) and PLA 4042D (PLA1.5), both with density of 1.24 g/cm3, described in Table 1. The PLAs will be referred to as PLA2.8 and PLA1.5, where the number stands for Dlactide content. The molar mass and D-lactide content of PLA2.8 were larger than those of PLA1.5. Both PLAs were vacuum dried at 100 C for 4 h and then homogenized in a Brabender mixer (Duisburg, Germany) at 60 rpm for 5 min at 190 C. For further studies, 200 mm thick films were compression molded at 180 C for 3 min and quenched to room temperature (RT). Although the molar masses of both PLAs decreased during processing, the molar mass of PLA2.8 remained slightly higher than that of PLA1.5. The detailed analysis of the high molar mass tails of the molar mass distributions (not shown) revealed only a small difference; PLA2.8 contained slightly more of the longest macromolecules than PLA1.5. A Linkam CSS-450 optical shearing system (Waterfield, UK) was used to precisely control shear flow field and thermal history of the films. It was mounted in a polarizing light microscope (PLM) Nikon Eclipse 80i equipped with a Nikon DS-Fi1 video camera for moni toring the crystallization. The specimens were: 1. heated to 180 C
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at 30 C/min, 2. held at 180 C for 3 min to erase thermal history, 3. cooled at 30 C/min to Ts from the range of 146e160 C, 4. held at Ts and sheared at g_ ranging from 10 to 300/s for time (ts) of 5 or 30 min, 5. after cessation of shear, cooled at v of 10 or 30 C/min to RT. The need for relatively long shearing time originated from the weak effect of shearing at Ts close to 150 C on crystallinity development in both PLAs. Control specimens were subjected to the same thermal treatment in quiescent conditions. Increase of melt annealing temperature to 190 C had no effect on the crystallization and crystallinity. To examine the effect of the thermal treatment on PLAs molar masses, films held at 150 C for 5 and 30 min were analysed. The results, which are presented in Table 1, show that the average molar mass decreased, especially after longer dwell time, but still the difference between PLAs persisted. To follow the conversion of melt into the crystalline phase, the intensity of transmitted depolarized light was measured. The relative volume conversion degree, avr(T), was calculated according to the expression:
avr ðTÞ ¼ ½IðTÞ IðT0 Þ=½IðTe Þ IðT0 Þ
(1)
where I(T) is the intensity of transmitted depolarized light at temperature T, whereas To and Te denote the initial temperature and the final temperature, to which the specimen was cooled. After cooling to RT, the specimens were removed from the device and examined ex-situ. In the plateeplate geometry, g_ varies along a radius, hence only pieces located at an appropriate distance from the centre were analysed. Crystallinity and thermal properties of the specimens were analysed by differential scanning calorimetry (DSC) using a TA Instrument 2920 DSC (New Castle, USA) during heating at 10 C/min. Some specimens were kept at 180 C for 3 min, cooled to RT and heated again to confirm that holding for several minutes at 180 C, as in the Linkam hot stage, was sufficient to erase their thermal history. Crystal structure was examined in the transmission mode using a WAXS camera coupled to the X-ray generator (sealed-tube, fine point CuKa filtered source operating at 50 kV and 35 mA), Philips (Eindhoven, The Netherlands). The incident beam was normal to the film plane. Imaging plates, used for recording the scattering patterns, were analysed with a PhosphorImager SI system (Molecular Dynamics). To have insight into the structure, the films were studied under a scanning electron microscope (SEM) Jeol JSM-5500 LV (Tokyo, Japan), after etching according to a modified method of He et al. [24], at 37 C, in a solution of 61 mg of Trizma base, 2 mg of sodium azide and 4 mg of Proteinase K (all from SigmaeAldrich) in 5 ml of distilled water. Proteinase K is known to degrade L-lactide units,
Table 1 Characteristics of PLAs studied. Melt flow rate (MFR) according to data provided by the producer. D-lactide content was determined by polarimetry in dichloromethane. Molar masses: number, weight and z-average, Mn, Mw, and Mz, respectively, were measured by a SEC method with MALLS detector in dichloromethane (data were averaged on two runs). 150/5 and 150/30 denote films held at 150 C for 5 and 30 min, respectively. Sample code PLA2.8 as obtained quenched film 150/5 150/30 PLA1.5 as obtained quenched film 150/5 150/30
MFR at 210 C [g/10min]
Mn [kg/mol]
Mw [kg/mol]
Mz [kg/mol]
D-lactide
15.0 e e e
80.5 73 63 64
128 114 108.5 99.5
190.5 166 160.5 146
2.8 e e e
18.8 e e e
71.5 65.5 61.5 52
109.5 108.5 99 82.5
159.5 156.5 144 121
1.5 e e e
content [mol%]
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preferentially in the amorphous phase. After etching for 2e8 h, depending on crystallinity, the samples were washed with distilled water in an ultrasonic bath for 30 min, dried and sputtered with gold. The rheological properties of PLAs were also studied, as described in the Appendix.
3. Results and discussion 3.1. Shear-induced crystallization Fig. 1 compares heating thermograms of PLA2.8 and PLA1.5 sheared at 150 C for 30 min at 50/s and then cooled to RT with those of control specimens with the same thermal history. Above the glass transition, with Tg at about 57e58 C, some specimens cold-crystallized prior to melting. A difference between the melting enthalpy (DHm) and the cold-crystallization enthalpy (DHcc) is equal to the enthalpy of crystallization (DHc) during the entire thermo-mechanical treatment prior to heating in DSC. The control specimens were amorphous prior to heating, as can be judged from DHc ¼ 0, although cold crystallization of PLA1.5 was more intense than that of PLA2.8, primarily due to the smaller D-lactide content. The thermograms of sheared PLAs cooled at 30 C/min also exhibited cold-crystallization exotherms but DHc was about 22 J/g for PLA2.8 and 33 J/g for PLA1.5, clearly evidencing significant crystallinity reached during post-shearing cooling. The sheared specimens cooled at 10 C/min did not exhibit cold-crystallization (DHcc ¼ 0), only melting with DHm of 35 J/g for PLA2.8 and 38 J/g for PLA1.5, because crystallization was completed during postshearing cooling. It is obvious that shear caused crystallization in both PLAs, although v exceeded those employed in Ref. [22] and that D-lactide content played an important role in the shearinduced crystallization of PLAs. Fig. 2 compares the effect of shearing and cooling conditions on DHc and mass crystallinity (cc) of PLAs; cc was calculated assuming the heat of fusion of 100%-crystalline PLA of 106 J/g [5]. In general,
Fig. 1. DSC heating thermograms of PLA2.8 and PLA1.5 specimens sheared at 50/s for 30 min at 150 C, and next cooled at 10 and at 30 C/min, and control unsheared specimens with the same thermal history.
longer ts as well as lower Ts and v enhanced cc. cc enlarged also with _ although increase of g_ beyond 50e100/s resulted in a increasing g, decrease of cc due to loss of control over the experimental conditions, probably caused by melt fracture and viscous heating (see Appendix). It appears that the effect of Ts was critical; shearing PLA2.8 or PLA1.5 at 160 C even for 30 min did not induce any crystallinity, _ However, cc increased with decreasing Ts although regardless of g. it depended on g_ and was also augmented by longer ts as well as by slower cooling; a decrease of v to 10 C/min resulted in a marked increase of cc except for those shearing conditions for which high cc was already reached during faster cooling. During faster cooling, high cc, slightly below 40 wt%, developed only in PLAs sheared at 148 C for 30 min, when crystallization was already detected by PLM during shearing. At 146 C crystallization was observed in PLA2.8 even during 5 min of shearing; for g_ of at least 50/s cc z 30 wt% developed during cooling at 30 C/min. To develop high cc during slower cooling, shearing at 146 and also at 148 C for 5 min with at least 25/s was sufficient. Shearing of PLA2.8 for 30 min at 146 C led to cc close to that attained after shearing at 148 C (not shown), whereas in PLA1.5 it was not possible because of too strong shear-induced crystallization. To induce cc z 40 wt% during faster cooling PLA1.5 had to be sheared at 148 C for 30 min but also shearing at 146 C for 5 min with a sufficiently high g_ led to cc z 30 wt%. A similar effect was also reached after shearing at 150 C for 30 min. To develop high cc in this polymer during slower cooling, shearing at 146e148 C for 5 min was sufficient, but relatively high cc above 20 wt% or even above 30 wt% was also reached for higher Ts of 150e152 C, depending, however, on g_ and ts. In most cases, cc of PLA1.5 exceeded that of PLA2.8 subjected the same thermo-mechanical treatment, but the difference diminished with decreasing Ts, and practically vanished for shearing at 146e148 C, which is clearly shown in Fig. 3. Moreover, practically the same maximum ccs of 38 and 39 wt% were found for PLA2.8 and PLA1.5, respectively, after shearing at 148 C with g_ of 10e50/s for 30 min (when crystallization already started during shearing), followed by cooling at 10 C/min. Shearing at 50e100/s for 5 min and at 10/s for 30 min results in similar strains. During shearing of PLA2.8 at 150 C and PLA1.5 at and 152 C under these conditions, no crystallization was detected by PLM and significant cc developed during post-shearing cooling. It appears that smaller cc developed in the specimens sheared at 50e100/s for 5 min than in those sheared at 10/s for 30 min, which suggests that g_ was not decisive. It must be noted that, even if no crystallization was detected by PLM, shear-induced precursors or even nuclei were already formed after the first 5 min of shearing at 10/s (as can be judged from cc of the respective specimens, espe cially those cooled at 10 C/min) and were present during further shearing. It is well known that shear-induced nucleation, which can be considered as physical crosslinking, and also growth of crystallites, changes the rheological behavior of the melt in a complex way, and affects the flow [13], therefore the interpretation is not straightforward. Moreover, at 50e100/s, the melt fracture and viscous heating can affect the obtained results to some extent decreasing the effect of shear. DSC measurements allowed us to determine only the final cc developed during shearing and post-shearing cooling. The light depolarisation method enables following the increase of avr. To compare increase of crystallinity in samples with different cc, volume crystallinity av(T) was calculated as avr(T) cv, where cv denotes the final volume crystallinity evaluated based on cc and the densities of the amorphous and crystalline phases of PLA [25]. It must be mentioned, however, that cc-cv < 1% because of the small difference in the densities [25,26]. Fig. 4a shows that av practically did not increase below 90 C,
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Fig. 2. Crystallization enthalpy and mass crystallinity of PLA2.8 and PLA1.5 sheared at various temperatures for 5 and 30 min and next cooled at 10 and 30 C/min. Open symbols indicate conditions, in which crystallization was detected by PLM already during shearing.
Fig. 3. Effect of shearing temperature on crystallization enthalpy and mass crystallinity of PLA2.8 and PLA1.5 sheared at 50/s for 5 and 30 min. The meaning of open symbols as in Fig. 2.
even when it was low, because of a very slow crystal growth caused by increased resistance of the molten polymer to the pulling of macromolecules by crystallization forces. Only early onset of
crystallization enabled sufficient time for development of high cc. Differentiation of av(T) with respect to T permits calculating crystallization rate, as shown in Fig. 4b. The influence of Ts, g_ and ts
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Fig. 4. Increase of crystallinity degree during cooling at 10 C/min in PLA2.8 and PLA1.5 after shearing at 150 C for 5 and 30 min at 25 and 50/s (a), derivative of crystallinity degree with respect to temperature (b), and the effect of shear rate on crystallization peak temperature (Tc) of PLA2.8 and PLA1.5 during post-shearing cooling at 10 C/min (c). Tc values are shown only for those shearing conditions, in which no marked crystallization was detected by PLM during shearing and which led to significant cc, above 10 wt%, developed during cooling.
on the peak temperature (Tc) during cooling at 10 C/min is illustrated in Fig. 4c. In general, Tc increased with decreasing Ts, _ although for high g_ it decreased similarly to cc, increasing ts and g, due to loss of control over the experimental conditions, as mentioned above. Tc of PLA1.5 sheared at 150 C for 5 min was higher than that of PLA2.8 with the same thermo-mechanical history. Comparison of Fig. 4c with Fig. 2 shows that the high Tc correlates to the high cc; the shear-induced nucleation elevated crystallization temperature of PLA, and thus enabled reaching relatively large cc before temperature decreased to the range in which crystallization could not proceed. The results show the crucial role of Ts on the shear-induced nonisothermal crystallization of both PLAs, as lower temperature increases relaxation times of polymer chains and augments the _ ts, v and PLA nucleation, although the effect depended also on g, optical purity. Slower cooling enabled longer time for crystallization in the temperature range in which crystals could grow, hence increased cc. This indicates the significance of the post-shearing crystallization of both PLAs from the relaxed melt, during which both the nucleation density and the crystal growth rate play a crucial role. These results also show the role of D-lactide content in shear-induced crystallization of PLA. The crystal growth rate of PLA decreases with increasing D-lactide content [4,8], and even relatively small changes in the optical purity can result in a significant difference. In general, a decrease of molar mass of PLA also
accelerates the crystal growth, but in the range of molar masses typical of commercially available PLAs, the effect is not as dramatic as that of the optical impurity [4]. Moreover, a smaller D-lactide content can facilitate nucleation during the quiescent as well as shear-induced crystallization. It can be noted that, in PLA1.5 sheared at 155 C for 30 min with g_ of 10e50/s and then cooled at 10 C/min, cc of about 15 wt% developed, evidencing the shearinduced nucleation. On the contrary, PLA2.8 with the same thermo-mechanical history was amorphous, which clearly shows the absence of nuclei. This indicates that the smaller D-lactide content also facilitated shear-induced nucleation. It is worth noting that the differences between maximum shearinduced ccs of both PLAs decreased with decreasing Ts and v, and the maximum cc values of both PLAs were similar. Hence, the main reason of differences in cc was incomplete conversion of melt into polycrystalline aggregates. When the shear induced sufficiently intense nucleation and cooling time was long enough to complete crystallization in PLAs, similar cc developed in both polymers. 3.2. Melting The crystallization, if not accomplished during the postshearing cooling, continued during heating in DSC as illustrated in Fig. 1. Thus, the melting enthalpy DHm measured during heating was proportional to crystallinity developed, not only
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during the post-shearing cooling, but also during subsequent heating in DSC. Heating thermograms of PLA control specimens with c c ¼ 0, which exclusively cold-crystallized in DSC at about 125 C, exhibited melting endotherms with peak temperature (T m ) at about 150e154 C. In general, the sheared specimens with low c c , which predominantly cold-crystallized during heating in DSC, had similar T m s. The other specimens exhibited more complex melting behavior, reflecting various crystallization temperature ranges and c m (calculated from D H m ) corresponding to crystallinity developed during shearing, postshearing cooling and subsequent heating. Frequently, double or even triple melting peaks, or peaks with low or high temperature shoulders, were observed. The highest and the lowest T m s were in the range 157e161 C and 140e150 C, respectively. The examples of plots of D H m and c m against g_ are shown in Fig. 5. The specimens with high c c , above 30 wt%, exhibited only weak or even zero exothermic effect prior to melting like, for instance, those sheared at 148 C for 30 min and next cooled at 10 C/min, and their c m and c c values were close. The specimens with low c c exhibited a stronger exothermic effect, hence their c m significantly exceeded c c , although it was usually below 30e35 wt%. Except for a few cases, for instance specimens sheared at 148 C for 30 min, c m of PLA1.5 exceeded that of PLA2.8 with the same thermomechanical history. Moreover, the exothermic peak interpreted as a 0 to a recrystallization [6] was observed neither in PLA2.8 nor in PLA1.5.
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3.3. Structure Examples of 2D-WAXS patterns of the sheared specimens are shown in Fig. 6. In general, the intensity of reflections correlated with cc determined from DSC; the larger the cc the stronger the reflections. Moreover, the 2D-WAXS patterns did not evidence any significant crystal orientation with respect to the shearing direction (SD), which is suggestive of shear-induced point-nucleation rather than the row-nucleation. All the patterns of crystalline specimens show strong (200)/ (110) reflection typical of the orthorhombic a-form. The pronounced (200)/(110) reflection, indicating significant cc, is always accompanied by (203) as well as by (010) and (210) reflections, the two latter typical of the order a-form [6]. As was mentioned above, high cc correlates with high Tc of the PLAs, and it is known that in PLLA the order a-phase forms during quiescent crystallization above 100 C [6], whereas shear causes formation of this phase even below 100 C [19]. Examples of SEM micrographs of etched surfaces of PLA films in Fig. 7 show cylindritic, row-nucleated structures parallel to SD, despite the lack of crystal orientation evidenced by 2D-WAXS. However, these forms were not visible in specimens subjected to longer etching. SEM micrographs in Fig. 8 a and b show the structure of PLA1.5 with high cc sheared at 148 C with 25/s for 30 min, and next cooled at 10 C/min. After 3 h of etching, the surface was smooth (not shown), with poorly discernible rownucleated forms. 4 h of etching exposed rather isotropic structure with some very visible row-nucleated forms, which
Fig. 5. Effect of shear rate on melting enthalpy of PLA2.8 and PLA1.5 sheared at 148 and 152 C various conditions and next cooled at 10 and 30 C/min. The meaning of open symbols as in Fig. 2.
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Fig. 6. 2D-WAXS patterns of PLA2.8 and PLA1.5 sheared at 150 C for 30 min and next cooled at 10 C/min. Arrow indicates the horizontal shearing direction.
Fig. 7. SEM micrographs of etched specimens of PLA2.8 and PLA1.5 sheared at various conditions and next cooled at 30 C/min.
disappeared after 6 h of etching. Thus, the row-nuclei formed in the surface layers, adhering to quartz-glass slides, rather than in the bulk. Yamazaki et al. [20] found the fibrils in optically pure PLLAs sheared at 5/s and next crystallized isothermally at Ts, but only for weight average molar mass MPS w (calibrated by polystyrene standards) of at least 420 kg/mol, which corresponds to Mw of about 300 kg/mol, much higher than used in the present study. Interestingly, for this Mw, the fibrils were observed only around 150 C. In PLLAs with larger Mw, the fibrils formed in broader temperature ranges but never above 160 C. It must be added that, during isothermal crystallization of PLA at Ts, after
shearing at 1/s for 8 min, the cylindritic forms were observed only above 120 C, whereas for lower Tc only spherulites, although aligned in rows, were found [21]. Contrary to that, no row nuclei were reported in PLA sheared at 135 C, even at 10/s for up to 65 s [23]. According to [20], with decreasing temperature, the advantage of bundle type nucleation of fibrils over folded nuclei of spherulites decreases, which results in the lower limit of temperature range of fibril formation, despite the increase of relaxation times of polymer chains. Moreover, most fibrils reported in Ref. [20] were preferentially formed from contaminants such as catalyst residue, dust and surface of the rotating plate, the latter
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Fig. 8. SEM micrographs of PLA1.5 sheared at 25/s for 30 min at 148 C and cooled at 10 C/min, after etching for 4 h (a) and 6 h (b). (c) SEM micrograph of etched cross-section of PLA1.5 specimen sheared at 10/s for 5 min at 146 C and cooled at 30 C/min. Shearing direction horizontal.
similar to our findings. It is worth noting that Shen et al. [27] reported formation of shish-kebabs on isotactic polypropylenesubstrate interface and explained this phenomenon as a result of wall slip, occurring via disentanglement of chains adsorbed on the wall from those in the bulk, and orientation of the adsorbed disentangled chains that promoted the formation of row-nuclei. Jay et al. [28] suggested that, in a polymer undergoing a shear flow on a macroscopic scale, the deposit of a chain segment on the surface of the solid layer at a microscopic level can change the flow to a local elongational flow, which affects crystallization more than a shear flow. In our study, the row-nucleated forms were observed by SEM predominantly on surfaces of PLA specimens and no crystal orientation was detected by 2D-WAXS. Those observations suggest that crystallization in the bulk was mostly nucleated on the point-like nuclei, rather than on the oriented fibrils, despite the strong shear flow. This could be caused either by inadequate fraction of sufficiently long PLA chain promoting the orientation and intense formation of fibrillar nuclei, or the presence of D-lactide, which impedes formation of ordered crystalline regions within oriented precursors. However, the chains on surfaces adhering to quartz glass slides formed the fibrillar nuclei, hence D-lactide did not prevent crystallization of oriented fibrils under more favourable conditions promoting orientation and nucleation. To have better insight into the shear-induced morphology, PLA1.5 sheared at 146 C at 10/s for 5 min and cooled at 30 C/ min, was analysed by SEM. This specimen was selected because its shear-induced crystallization was not completed during postshearing fast cooling, leaving regions not fully filled with crystalline aggregates where the morphology can be better seen. On the other hand, the shear-induced nucleation was strong enough to reach cc ¼ 36 wt% during slower cooling. To expose the internal structure, the specimen was cut in a plane parallel to the SD and to the velocity gradient, and etched. The micrograph in Fig. 8c shows rather isotropic structure filled with densely nucleated crystalline aggregates. One can distinguish many short fibrillar forms, parallel to the SD, with length up to 2 mm, which can be fibrils but also edges of flat lamellae stacks. Rarely elongated structures, like the one in the inset in Fig. 8c, suggestive of the presence of a fibrillar nucleus several micrometres long, could be found. The length of short fibrillar structures seen in Fig. 8c, equal to the diameter of the polycrystalline aggregates visible on the same micrograph, suggests that both forms were nucleated at the same time; the alignment of short fibrils parallel to the SD can be possibly related to the formation of the shear-induced nuclei in the flow field. The above findings corroborate the conclusion concerning the predominant shearinduced point-like nucleation in the bulk of PLAs studied
under the experimental conditions. 4. Conclusions Shearing intensified nonisothermal crystallization of both slowly crystallizing PLAs, studied by enhancing nucleation, predominantly in the form of point-like nuclei, although row-nuclei were also observed. This led to increase of Tc during postshearing cooling and cc. The effect was temperature dependent, and strongly intensified with Ts decreasing below 160 C in a relatively narrow temperature range. Both, Tc and cc increased with increasing g_ (and strain), although for g_ beyond 50e100/s a decrease was observed due to loss of control over the experimental conditions. The decrease of v from 30 to 10 C/min markedly elevated cc, which indicates the importance of post-shearing crystallization from the relaxed melt during cooling. In general, no crystal orientation with respect to the shearing direction was detected by 2D-WAXS. Row-nucleated structures were revealed in sheared specimens after etching but predominantly in the surface layers adhering to microscope quartz glasses; longer etching exposed rather isotropic structure. This indicates that, in the bulk, the shear-induced nucleation was in the form of point-like nuclei rather than row-nuclei. Although shearing intensified nonisothermal crystallization of both commercial PLAs, the effect of shear on PLA1.5 with smaller D-lactide content was stronger than on PLA2.8 with larger D-lactide content; higher cc developed in the former. Nevertheless, the maximum cc of PLA1.5, 39 wt%, was close to that of PLA2.8, although such crystallinity was reached only when crystallization started during shearing and completed during cooling. Crystallization, if not accomplished during the postshearing cooling, continued during heating in DSC. The melting behavior of specimens reflected their thermo-mechanical history and crystallinity levels developed during shearing, post-shearing cooling and subsequent heating, which led to single melting peaks with low or high temperature shoulders or to multiple melting peaks. Acknowledgement This research project has been supported by the European Union European Regional Development Fund, Contract No.POIG.01.03.0100-007/08-00, project Biodegradable Fibrous ProductsBIOGRATEX. Appendix We interpret the decrease of the effect of shear on crystallization with g_ increasing beyond 50e100/s as a result of loss of control over
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the experimental conditions. Othman et al. [29] reported fracture of PLA melt in capillary rheometry beginning to occur at shear stress of 0.2e0.3 MPa. Steady shear viscosity h can be calculated from the modulus of complex viscosity measured in oscillatory shear, jh*(u)j, where u is frequency expressed in [rad/s], according to the empirical Cox-Merz law: jh*ðuÞj ¼ hðg_ ¼ uÞ, which is obeyed in PLA in a broad frequency range [30]. Rheological properties of both PLAs were studied using a rheometer ARES LS2 (TA Instruments). The experiments were performed in a cone-plate geometry at 150 and 160 C. One mm thick disks with the diameter of 25 mm were prepared by compression molding at 190 C. The disks were placed in the measuring cell and heated to 180 C. Next, the gap was set and temperature was decreased. Low-amplitude oscillatory shear experiments were carried within the linear viscoelastic regime, over a frequency range 1e512 Hz beginning from the highest fre_ calculated based on the measured h*(u) quency. The plots of hðgÞ are shown in Fig. A1. It follows that, in g_ range of 50e100/s, the shear stress is in the range of 0.2e0.3 MPa. Indeed, at g_ 50/s fracture of PLA melts was observed during shearing, as shown in micrograph in Fig. A2. Another reason for the decrease of cc can be viscous heating. The amount of heat per volume unit and time unit, E, generated during simple shear flow between two parallel plates, _ g_ 2 . The increase of temperature can be estimated by is equal to hðgÞ solving the heat conduction equation taking into account the heat generation in a flowing polymer [31]. The steady state solution of the equation, assuming that the walls are kept at constant temperature, yields the expression for the increase of temperature, DT, in equal distances from the walls, 0.5h, in the form: E h2/(8k) where k is the heat conduction coefficient, usually about 0.2 W/(m oC) for polymer melts. Although such a simple approach neglects g_ gradient along a radius of rotating plates, convection, thermal resistance at polymer-wall interfaces and temperature dependence of h, it can still provide information about at least the order of DT. Moreover, as seen in Fig. A1, 10 C increase of temperature causes a decrease of h in the g_ range from 50 to 300/s only by 25e15%. In Fig. A1, DT at 150 C is plotted as a function of g_ for 200 mm thick films of both PLAs. It can be seen that DT reaches 1e3 C for g_ ranging from 100 to 300/s. Even such small temperature increase can contribute to a decrease of cc and Tc in that range, in which the effect of shear strongly depends on Ts. This conclusion is supported by the observation that more pronounced decrease of cc was found
Fig. A1. Shear rate dependence of steady shear viscosity of PLA2.8 and PLA1.5 at 150 and 160 C, and temperature increase, DT, inside specimens sheared at 150 C caused by viscous heating.
Fig. A2. LM micrograph of PLA2.8 during shearing at 150 C with 50 /s.
for Ts range from 150 to 155 C in which the effect of shear was strongly Ts-dependent. References [1] E.S. Inkinen, M. Hakkarainen, A.Ch. Albertsson, A. Sodergard, From lactic acid to poly(lactic acid) (PLA): characterization and analysis of PLA and its precursors, Biomacromolecules 12 (2011) 523e532. [2] A.P. Gupta, V. Kumar, New emerging trends in synthetic biodegradable polymers- polylactide: a critique, Eur. Polym. J. 43 (2007) 4053e4074. [3] T. Lim, R. Auras, M. Rubino, Processing technologies for poly(lactic acid), Prog. Polym. Sci. 33 (2008) 820e852. [4] S. Saeidlou, M.A. Huneault, H. Li, C.B. Park, Poly(lactic acid) crystallization, Prog. Polym. Sci. 37 (2012) 1657e1677. [5] J.R. Sarasua, R.E. Prudhomme, M. Wisnieski, A. Le Borgne, N. Spassky, Crystallization and melting behavior of polylactides, Macromolecules 31 (1998) 3895e3905. [6] J. Zhang, K. Tashiro, H. Tsuji, A.J. Domb, Disorder-to-order phase transition and multiple melting behavior of poly(L-lactide) investigated by simultaneous measurements of WAXD and DSC, Macromolecules 41 (2008) 1352e1357. [7] H. Tsuji, Y. Ikada, Crystallization from the melt of poly(lactide)s with different optical purities and their blends, Macromol. Chem. Phys. 197 (1996) 3483e3499. [8] J. Huang, M.S. Lisowski, J. Runt, E.S. Hall, R.T. Kean, N. Buehler, J.S. Lin, Crystallization and microstructure of poly(L-lactide-co-meso-lactide) copolymers, Macromolecules 31 (1998) 2593e2599. [9] G. Perego, G.D. Cella, C. Bastioli, Effect of molecular weight and crystallinity on poly(lactic acid) mechanical properties, J. Appl. Polym. Sci. 59 (1996) 37e43. [10] T. Iwata, Y. Doi, Morphology and enzymatic degradation of poly(L-lactic acid) single crystals, Macromolecules 31 (1998) 2461e2467. [11] H. Tsuji, S. Miyauchi, Poly(L-lactide): VI effects of crystallinity on enzymatic hydrolysis of poly(L-lactide) without free amorphous region, Polym. Degrad. Stab. 71 (2001) 415e424. [12] D.G. Hristova-Bogaerds, M. Drieskens, R. Peeters, J. Mullens, D. Franco, P.J. Lemstra, Structure versus properties relationship of poly(lactid acid). I. Effect of crystallinity on barrier properties, J. Polym. Sci. Part B Polym. Phys. 47 (2009) 2247e2258. [13] G.W.M. Peters, L. Balzano, R.J.A. Steenbakkers, Flow-induced crystallization, in: E. Piorkowska, G.C. Rutledge (Eds.), Handbook of Polymer Crystallization, John Wiley &Sons, Inc., Hoboken, NJ, 2013, pp. 399e431. [14] B.S. Hsiao, Role of chain entanglement network on formation of flow-induced crystallization precursor structure, in: G. Reiter, G.R. Strobl (Eds.), Progress in Understanding of Polymer Crystallization, Springer, Berlin, 2007, pp. 133e 149. [15] R.H. Somani, L. Yang, L. Zhu, B.S. Hsiao, Shis-kebab precursor structures in entangled polymer melts, Polymer 46 (2005) 8587e8623. [16] J. van Meerveld, G.W.M. Peters, M. Hutter, Towards a rheological classification of flow induced crystallization experiments of polymer melts, Rheol. Acta 44 (2004) 119e134. [17] H. Janeschitz-Kriegl, E. Ratajski, M. Stadlbauer, Flow as an effective promotor of nucleation in polymer melts: a quantitative evaluation, Rheol. Acta 42 (2003) 355e364.
J. Bojda, E. Piorkowska / Polymer Testing 50 (2016) 172e181 [18] O.O. Mykhaylyk, P. Chambon, R.S. Graham, J.P.A. Fairclough, P.D. Olmsted, A.J. Ryan, The specific work of flow as a criterion for orientation in polymer crystallization, Macromolecules 41 (2008) 1901e1904. [19] S. Huang, H. Li, S. Jiang, X. Chen, L. An, Crystal structure and morphology influenced by shear effect of poly(L-lactide) and its melting behavior revealed by WAXD, DSC and in-situ POM, Polymer 52 (2011) 3478e3487. [20] S. Yamazaki, M. Itoh, T. Oka, K. Kimura, Formation and morphology of “shishlike” fibril crystals of aliphatic polyesters from the sheared melt, Eur. Polym. J. 46 (2010) 58e68. [21] X.J. Li, Z.M. Li, G.J. Zhong, L.B. Li, Steady- shear- induced isothermal crystallization of poly(L-lactide) (PLLA), J. Macrom. Sci. Part B Polym. Phys. 47 (2008) 511e522. [22] X. J.Li, G.J. Zhong, Z.M. Li, Non-isothermal crystallization of poly(L-lactide) (PLLA) under quiescent and steady shear conditions, Chin. J. Polym. Sci. 28 (2010) 357e366. [23] Y. Zhong, H. Fang, Y. Zhang, Z. Wang, J. Yang, Z. Wang, Rheologically determined critical shear rates for shear-induced nucleation rate enhancements of poly(lactic acid), ACS Sustain. Chem. Eng. 1 (2013) 663e672.
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Material behaviour
Shear-induced nonisothermal crystallization of two grades of PLA Joanna Bojda, Ewa Piorkowska* Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90 363 Lodz, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 November 2015 Accepted 8 January 2016 Available online 12 January 2016
Shear-induced nonisothermal crystallization of two commercial polylactides (PLAs) differing in optical purity was studied. The molten polymers were sheared at selected temperatures (Ts) and subsequently cooled. The crystallization was followed by a light depolarization method, whereas the specimens were analysed ex-situ by DSC, 2D-WAXS and SEM after etching. It was found that the effect of shear, especially on the crystallinity developed during post-shearing cooling, intensified with a decrease of Ts from 160 to 146 C, and with increasing shear rate and strain. Moreover, the effect of shear on PLA1.5 with D-lactide content of 1.5% was stronger than PLA2.8 with 2.8% of D-lactide, although maximum crystallinity of both polymers was practically the same. A decrease of cooling rate from 30 to 10 C/min increased crystallinity of both PLAs, except for those shearing conditions which induced high crystallinity even during faster cooling. Although SEM examination revealed some row-nucleated forms, no significant crystal orientation was detected by 2D-WAXS, indicating that, under the experimental conditions, the shear induced predominantly point-like nuclei. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Biodegradable polymers Polylactide Shear-induced crystallization Structure Melting behavior
1. Introduction An upper temperature limit of applicability of an amorphous polymer is determined by its glass transition temperature (Tg) while that of a semicrystalline polymer by its melting temperature (Tm), usually much higher. Polylactide (PLA), a biodegradable and bio-based polyester, which recently gained enormous attention [1e3], has Tg in the range of 55e60 C. Optically pure poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) are highly crystalline, with Tm up to 180 C [4,5]. PLA usually crystallizes in the orthorhombic a-form, although two other forms, b and g, are also known and the disorder a0 phase forms below 100 C [4,6]. Generally, commercial PLAs are copolymers of L-lactide and a small amount of D-lactide. Crystallizability of PLA decreases with decreasing of its optical purity [4,5,7,8]; slowly crystallizing PLAs can be quenched to the glassy state and cold-crystallized during heating. It is worth noting that amorphous PLA is rigid and brittle at room conditions. Crystallinity slightly increases its modulus of elasticity, decreases the drawability [9] and the rate of biodegradation [10,11] but improves the barrier properties [12] and the thermal stability. During industrial processing of polymers, flow plays a vital role. Flow-induced molecular orientation can strongly influence the
* Corresponding author. E-mail address: [email protected] (E. Piorkowska). http://dx.doi.org/10.1016/j.polymertesting.2016.01.006 0142-9418/© 2016 Elsevier Ltd. All rights reserved.
crystallization and the resulting structure; both are controlled by the interplay between crystallization and chain relaxation. Shearinduced crystallization strongly depends on shear temperature _ and total strain. The longest polymer chains have a (Ts), rate ðgÞ large effect on the flow-induced crystallization, hence polymer molecular characteristics, especially the high molar mass tail of mass distribution, is crucial [13e15]. Shear flow can increase the nucleation rate by several orders of magnitude. Depending on the molecular characteristics and shear conditions, different structures can emerge varying from spherulites to row-nucleated and shishkebab morphologies [14,15]. It was postulated [16] that, to generate point-like nuclei and fibrillar nuclei, g_ has to exceed the inverse reptation time and the inverse Rouse relaxation time of the high molar mass tail, respectively, although when the flow is strong enough but too short, intermediate regimes were also defined. Others believe that the controlling parameter is the mechanical work [13,17,18]. Despite the need, few studies of the influence of shear flow on PLA crystallization have been described, and mainly concern the crystalline morphologies [19e23]. In optically pure PLLAs, sheared at 5/s and then crystallized isothermally at Ts, fibrils were found [20] in temperature ranges dependent on molar mass of PLLA; most fibrils were preferentially formed on contaminants and on a surface of the rotating plate. PLA shear-induced cylindritic forms and row nucleated spherulites, the latter below 120 C, were also found in [21], and the fibrillar nuclei were accompanied by point-like ones.
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Contrary to that, only point-like nuclei were observed in PLA _ up to 10/s [23]. Studies sheared at 135 C, despite slightly higher g, of the shear-induced nonisothermal crystallization of PLA are also very limited. Crystallization of PLAs during cooling at a rate (v) < 5 C/min was examined after shearing at 150 C, with g_ 6/s for 60 s; the amount of row-nucleated forms and crystallization onset temperature increased with increasing g_ [22]. Others [23] cooled sheared PLA even slower, at 1 C/min; temperature of crystallization completion increased with increasing shear time and leveled off at about 115 C. Differences in molar masses and optical purity of PLAs studied previously [19e23], and also different shearing and crystallization conditions, make a comparison of the results difficult. Moreover, during processing, polymer melts are subjected to very high g_ s and quenched. Hence, understanding the effect of higher g_ and v is crucial and has a significant practical importance. The v is even more critical for optically impure PLAs than for PLLA; a range of commercial PLAs do not crystallize during cooling, even at 10 C/ min. Our studies focused on the effect of shear on nonisothermal crystallization in two such commercial PLAs with different D-lactide contents, especially on the crystallinity and orientation. The molten PLAs were sheared at 146e160 C, at g_ of 10e300/s, for 5 or 30 min and, subsequently, cooled at 10 or 30 C/min. The crystallization was followed in-situ by polarized light microscopy (PLM), and then the specimens were studied ex-situ by different techniques. 2. Experimental The study utilized two commercial PLAs purchased from NatureWorks LLC: PLA 2002D (PLA2.8) and PLA 4042D (PLA1.5), both with density of 1.24 g/cm3, described in Table 1. The PLAs will be referred to as PLA2.8 and PLA1.5, where the number stands for Dlactide content. The molar mass and D-lactide content of PLA2.8 were larger than those of PLA1.5. Both PLAs were vacuum dried at 100 C for 4 h and then homogenized in a Brabender mixer (Duisburg, Germany) at 60 rpm for 5 min at 190 C. For further studies, 200 mm thick films were compression molded at 180 C for 3 min and quenched to room temperature (RT). Although the molar masses of both PLAs decreased during processing, the molar mass of PLA2.8 remained slightly higher than that of PLA1.5. The detailed analysis of the high molar mass tails of the molar mass distributions (not shown) revealed only a small difference; PLA2.8 contained slightly more of the longest macromolecules than PLA1.5. A Linkam CSS-450 optical shearing system (Waterfield, UK) was used to precisely control shear flow field and thermal history of the films. It was mounted in a polarizing light microscope (PLM) Nikon Eclipse 80i equipped with a Nikon DS-Fi1 video camera for moni toring the crystallization. The specimens were: 1. heated to 180 C
173
at 30 C/min, 2. held at 180 C for 3 min to erase thermal history, 3. cooled at 30 C/min to Ts from the range of 146e160 C, 4. held at Ts and sheared at g_ ranging from 10 to 300/s for time (ts) of 5 or 30 min, 5. after cessation of shear, cooled at v of 10 or 30 C/min to RT. The need for relatively long shearing time originated from the weak effect of shearing at Ts close to 150 C on crystallinity development in both PLAs. Control specimens were subjected to the same thermal treatment in quiescent conditions. Increase of melt annealing temperature to 190 C had no effect on the crystallization and crystallinity. To examine the effect of the thermal treatment on PLAs molar masses, films held at 150 C for 5 and 30 min were analysed. The results, which are presented in Table 1, show that the average molar mass decreased, especially after longer dwell time, but still the difference between PLAs persisted. To follow the conversion of melt into the crystalline phase, the intensity of transmitted depolarized light was measured. The relative volume conversion degree, avr(T), was calculated according to the expression:
avr ðTÞ ¼ ½IðTÞ IðT0 Þ=½IðTe Þ IðT0 Þ
(1)
where I(T) is the intensity of transmitted depolarized light at temperature T, whereas To and Te denote the initial temperature and the final temperature, to which the specimen was cooled. After cooling to RT, the specimens were removed from the device and examined ex-situ. In the plateeplate geometry, g_ varies along a radius, hence only pieces located at an appropriate distance from the centre were analysed. Crystallinity and thermal properties of the specimens were analysed by differential scanning calorimetry (DSC) using a TA Instrument 2920 DSC (New Castle, USA) during heating at 10 C/min. Some specimens were kept at 180 C for 3 min, cooled to RT and heated again to confirm that holding for several minutes at 180 C, as in the Linkam hot stage, was sufficient to erase their thermal history. Crystal structure was examined in the transmission mode using a WAXS camera coupled to the X-ray generator (sealed-tube, fine point CuKa filtered source operating at 50 kV and 35 mA), Philips (Eindhoven, The Netherlands). The incident beam was normal to the film plane. Imaging plates, used for recording the scattering patterns, were analysed with a PhosphorImager SI system (Molecular Dynamics). To have insight into the structure, the films were studied under a scanning electron microscope (SEM) Jeol JSM-5500 LV (Tokyo, Japan), after etching according to a modified method of He et al. [24], at 37 C, in a solution of 61 mg of Trizma base, 2 mg of sodium azide and 4 mg of Proteinase K (all from SigmaeAldrich) in 5 ml of distilled water. Proteinase K is known to degrade L-lactide units,
Table 1 Characteristics of PLAs studied. Melt flow rate (MFR) according to data provided by the producer. D-lactide content was determined by polarimetry in dichloromethane. Molar masses: number, weight and z-average, Mn, Mw, and Mz, respectively, were measured by a SEC method with MALLS detector in dichloromethane (data were averaged on two runs). 150/5 and 150/30 denote films held at 150 C for 5 and 30 min, respectively. Sample code PLA2.8 as obtained quenched film 150/5 150/30 PLA1.5 as obtained quenched film 150/5 150/30
MFR at 210 C [g/10min]
Mn [kg/mol]
Mw [kg/mol]
Mz [kg/mol]
D-lactide
15.0 e e e
80.5 73 63 64
128 114 108.5 99.5
190.5 166 160.5 146
2.8 e e e
18.8 e e e
71.5 65.5 61.5 52
109.5 108.5 99 82.5
159.5 156.5 144 121
1.5 e e e
content [mol%]
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preferentially in the amorphous phase. After etching for 2e8 h, depending on crystallinity, the samples were washed with distilled water in an ultrasonic bath for 30 min, dried and sputtered with gold. The rheological properties of PLAs were also studied, as described in the Appendix.
3. Results and discussion 3.1. Shear-induced crystallization Fig. 1 compares heating thermograms of PLA2.8 and PLA1.5 sheared at 150 C for 30 min at 50/s and then cooled to RT with those of control specimens with the same thermal history. Above the glass transition, with Tg at about 57e58 C, some specimens cold-crystallized prior to melting. A difference between the melting enthalpy (DHm) and the cold-crystallization enthalpy (DHcc) is equal to the enthalpy of crystallization (DHc) during the entire thermo-mechanical treatment prior to heating in DSC. The control specimens were amorphous prior to heating, as can be judged from DHc ¼ 0, although cold crystallization of PLA1.5 was more intense than that of PLA2.8, primarily due to the smaller D-lactide content. The thermograms of sheared PLAs cooled at 30 C/min also exhibited cold-crystallization exotherms but DHc was about 22 J/g for PLA2.8 and 33 J/g for PLA1.5, clearly evidencing significant crystallinity reached during post-shearing cooling. The sheared specimens cooled at 10 C/min did not exhibit cold-crystallization (DHcc ¼ 0), only melting with DHm of 35 J/g for PLA2.8 and 38 J/g for PLA1.5, because crystallization was completed during postshearing cooling. It is obvious that shear caused crystallization in both PLAs, although v exceeded those employed in Ref. [22] and that D-lactide content played an important role in the shearinduced crystallization of PLAs. Fig. 2 compares the effect of shearing and cooling conditions on DHc and mass crystallinity (cc) of PLAs; cc was calculated assuming the heat of fusion of 100%-crystalline PLA of 106 J/g [5]. In general,
Fig. 1. DSC heating thermograms of PLA2.8 and PLA1.5 specimens sheared at 50/s for 30 min at 150 C, and next cooled at 10 and at 30 C/min, and control unsheared specimens with the same thermal history.
longer ts as well as lower Ts and v enhanced cc. cc enlarged also with _ although increase of g_ beyond 50e100/s resulted in a increasing g, decrease of cc due to loss of control over the experimental conditions, probably caused by melt fracture and viscous heating (see Appendix). It appears that the effect of Ts was critical; shearing PLA2.8 or PLA1.5 at 160 C even for 30 min did not induce any crystallinity, _ However, cc increased with decreasing Ts although regardless of g. it depended on g_ and was also augmented by longer ts as well as by slower cooling; a decrease of v to 10 C/min resulted in a marked increase of cc except for those shearing conditions for which high cc was already reached during faster cooling. During faster cooling, high cc, slightly below 40 wt%, developed only in PLAs sheared at 148 C for 30 min, when crystallization was already detected by PLM during shearing. At 146 C crystallization was observed in PLA2.8 even during 5 min of shearing; for g_ of at least 50/s cc z 30 wt% developed during cooling at 30 C/min. To develop high cc during slower cooling, shearing at 146 and also at 148 C for 5 min with at least 25/s was sufficient. Shearing of PLA2.8 for 30 min at 146 C led to cc close to that attained after shearing at 148 C (not shown), whereas in PLA1.5 it was not possible because of too strong shear-induced crystallization. To induce cc z 40 wt% during faster cooling PLA1.5 had to be sheared at 148 C for 30 min but also shearing at 146 C for 5 min with a sufficiently high g_ led to cc z 30 wt%. A similar effect was also reached after shearing at 150 C for 30 min. To develop high cc in this polymer during slower cooling, shearing at 146e148 C for 5 min was sufficient, but relatively high cc above 20 wt% or even above 30 wt% was also reached for higher Ts of 150e152 C, depending, however, on g_ and ts. In most cases, cc of PLA1.5 exceeded that of PLA2.8 subjected the same thermo-mechanical treatment, but the difference diminished with decreasing Ts, and practically vanished for shearing at 146e148 C, which is clearly shown in Fig. 3. Moreover, practically the same maximum ccs of 38 and 39 wt% were found for PLA2.8 and PLA1.5, respectively, after shearing at 148 C with g_ of 10e50/s for 30 min (when crystallization already started during shearing), followed by cooling at 10 C/min. Shearing at 50e100/s for 5 min and at 10/s for 30 min results in similar strains. During shearing of PLA2.8 at 150 C and PLA1.5 at and 152 C under these conditions, no crystallization was detected by PLM and significant cc developed during post-shearing cooling. It appears that smaller cc developed in the specimens sheared at 50e100/s for 5 min than in those sheared at 10/s for 30 min, which suggests that g_ was not decisive. It must be noted that, even if no crystallization was detected by PLM, shear-induced precursors or even nuclei were already formed after the first 5 min of shearing at 10/s (as can be judged from cc of the respective specimens, espe cially those cooled at 10 C/min) and were present during further shearing. It is well known that shear-induced nucleation, which can be considered as physical crosslinking, and also growth of crystallites, changes the rheological behavior of the melt in a complex way, and affects the flow [13], therefore the interpretation is not straightforward. Moreover, at 50e100/s, the melt fracture and viscous heating can affect the obtained results to some extent decreasing the effect of shear. DSC measurements allowed us to determine only the final cc developed during shearing and post-shearing cooling. The light depolarisation method enables following the increase of avr. To compare increase of crystallinity in samples with different cc, volume crystallinity av(T) was calculated as avr(T) cv, where cv denotes the final volume crystallinity evaluated based on cc and the densities of the amorphous and crystalline phases of PLA [25]. It must be mentioned, however, that cc-cv < 1% because of the small difference in the densities [25,26]. Fig. 4a shows that av practically did not increase below 90 C,
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175
Fig. 2. Crystallization enthalpy and mass crystallinity of PLA2.8 and PLA1.5 sheared at various temperatures for 5 and 30 min and next cooled at 10 and 30 C/min. Open symbols indicate conditions, in which crystallization was detected by PLM already during shearing.
Fig. 3. Effect of shearing temperature on crystallization enthalpy and mass crystallinity of PLA2.8 and PLA1.5 sheared at 50/s for 5 and 30 min. The meaning of open symbols as in Fig. 2.
even when it was low, because of a very slow crystal growth caused by increased resistance of the molten polymer to the pulling of macromolecules by crystallization forces. Only early onset of
crystallization enabled sufficient time for development of high cc. Differentiation of av(T) with respect to T permits calculating crystallization rate, as shown in Fig. 4b. The influence of Ts, g_ and ts
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Fig. 4. Increase of crystallinity degree during cooling at 10 C/min in PLA2.8 and PLA1.5 after shearing at 150 C for 5 and 30 min at 25 and 50/s (a), derivative of crystallinity degree with respect to temperature (b), and the effect of shear rate on crystallization peak temperature (Tc) of PLA2.8 and PLA1.5 during post-shearing cooling at 10 C/min (c). Tc values are shown only for those shearing conditions, in which no marked crystallization was detected by PLM during shearing and which led to significant cc, above 10 wt%, developed during cooling.
on the peak temperature (Tc) during cooling at 10 C/min is illustrated in Fig. 4c. In general, Tc increased with decreasing Ts, _ although for high g_ it decreased similarly to cc, increasing ts and g, due to loss of control over the experimental conditions, as mentioned above. Tc of PLA1.5 sheared at 150 C for 5 min was higher than that of PLA2.8 with the same thermo-mechanical history. Comparison of Fig. 4c with Fig. 2 shows that the high Tc correlates to the high cc; the shear-induced nucleation elevated crystallization temperature of PLA, and thus enabled reaching relatively large cc before temperature decreased to the range in which crystallization could not proceed. The results show the crucial role of Ts on the shear-induced nonisothermal crystallization of both PLAs, as lower temperature increases relaxation times of polymer chains and augments the _ ts, v and PLA nucleation, although the effect depended also on g, optical purity. Slower cooling enabled longer time for crystallization in the temperature range in which crystals could grow, hence increased cc. This indicates the significance of the post-shearing crystallization of both PLAs from the relaxed melt, during which both the nucleation density and the crystal growth rate play a crucial role. These results also show the role of D-lactide content in shear-induced crystallization of PLA. The crystal growth rate of PLA decreases with increasing D-lactide content [4,8], and even relatively small changes in the optical purity can result in a significant difference. In general, a decrease of molar mass of PLA also
accelerates the crystal growth, but in the range of molar masses typical of commercially available PLAs, the effect is not as dramatic as that of the optical impurity [4]. Moreover, a smaller D-lactide content can facilitate nucleation during the quiescent as well as shear-induced crystallization. It can be noted that, in PLA1.5 sheared at 155 C for 30 min with g_ of 10e50/s and then cooled at 10 C/min, cc of about 15 wt% developed, evidencing the shearinduced nucleation. On the contrary, PLA2.8 with the same thermo-mechanical history was amorphous, which clearly shows the absence of nuclei. This indicates that the smaller D-lactide content also facilitated shear-induced nucleation. It is worth noting that the differences between maximum shearinduced ccs of both PLAs decreased with decreasing Ts and v, and the maximum cc values of both PLAs were similar. Hence, the main reason of differences in cc was incomplete conversion of melt into polycrystalline aggregates. When the shear induced sufficiently intense nucleation and cooling time was long enough to complete crystallization in PLAs, similar cc developed in both polymers. 3.2. Melting The crystallization, if not accomplished during the postshearing cooling, continued during heating in DSC as illustrated in Fig. 1. Thus, the melting enthalpy DHm measured during heating was proportional to crystallinity developed, not only
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during the post-shearing cooling, but also during subsequent heating in DSC. Heating thermograms of PLA control specimens with c c ¼ 0, which exclusively cold-crystallized in DSC at about 125 C, exhibited melting endotherms with peak temperature (T m ) at about 150e154 C. In general, the sheared specimens with low c c , which predominantly cold-crystallized during heating in DSC, had similar T m s. The other specimens exhibited more complex melting behavior, reflecting various crystallization temperature ranges and c m (calculated from D H m ) corresponding to crystallinity developed during shearing, postshearing cooling and subsequent heating. Frequently, double or even triple melting peaks, or peaks with low or high temperature shoulders, were observed. The highest and the lowest T m s were in the range 157e161 C and 140e150 C, respectively. The examples of plots of D H m and c m against g_ are shown in Fig. 5. The specimens with high c c , above 30 wt%, exhibited only weak or even zero exothermic effect prior to melting like, for instance, those sheared at 148 C for 30 min and next cooled at 10 C/min, and their c m and c c values were close. The specimens with low c c exhibited a stronger exothermic effect, hence their c m significantly exceeded c c , although it was usually below 30e35 wt%. Except for a few cases, for instance specimens sheared at 148 C for 30 min, c m of PLA1.5 exceeded that of PLA2.8 with the same thermomechanical history. Moreover, the exothermic peak interpreted as a 0 to a recrystallization [6] was observed neither in PLA2.8 nor in PLA1.5.
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3.3. Structure Examples of 2D-WAXS patterns of the sheared specimens are shown in Fig. 6. In general, the intensity of reflections correlated with cc determined from DSC; the larger the cc the stronger the reflections. Moreover, the 2D-WAXS patterns did not evidence any significant crystal orientation with respect to the shearing direction (SD), which is suggestive of shear-induced point-nucleation rather than the row-nucleation. All the patterns of crystalline specimens show strong (200)/ (110) reflection typical of the orthorhombic a-form. The pronounced (200)/(110) reflection, indicating significant cc, is always accompanied by (203) as well as by (010) and (210) reflections, the two latter typical of the order a-form [6]. As was mentioned above, high cc correlates with high Tc of the PLAs, and it is known that in PLLA the order a-phase forms during quiescent crystallization above 100 C [6], whereas shear causes formation of this phase even below 100 C [19]. Examples of SEM micrographs of etched surfaces of PLA films in Fig. 7 show cylindritic, row-nucleated structures parallel to SD, despite the lack of crystal orientation evidenced by 2D-WAXS. However, these forms were not visible in specimens subjected to longer etching. SEM micrographs in Fig. 8 a and b show the structure of PLA1.5 with high cc sheared at 148 C with 25/s for 30 min, and next cooled at 10 C/min. After 3 h of etching, the surface was smooth (not shown), with poorly discernible rownucleated forms. 4 h of etching exposed rather isotropic structure with some very visible row-nucleated forms, which
Fig. 5. Effect of shear rate on melting enthalpy of PLA2.8 and PLA1.5 sheared at 148 and 152 C various conditions and next cooled at 10 and 30 C/min. The meaning of open symbols as in Fig. 2.
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Fig. 6. 2D-WAXS patterns of PLA2.8 and PLA1.5 sheared at 150 C for 30 min and next cooled at 10 C/min. Arrow indicates the horizontal shearing direction.
Fig. 7. SEM micrographs of etched specimens of PLA2.8 and PLA1.5 sheared at various conditions and next cooled at 30 C/min.
disappeared after 6 h of etching. Thus, the row-nuclei formed in the surface layers, adhering to quartz-glass slides, rather than in the bulk. Yamazaki et al. [20] found the fibrils in optically pure PLLAs sheared at 5/s and next crystallized isothermally at Ts, but only for weight average molar mass MPS w (calibrated by polystyrene standards) of at least 420 kg/mol, which corresponds to Mw of about 300 kg/mol, much higher than used in the present study. Interestingly, for this Mw, the fibrils were observed only around 150 C. In PLLAs with larger Mw, the fibrils formed in broader temperature ranges but never above 160 C. It must be added that, during isothermal crystallization of PLA at Ts, after
shearing at 1/s for 8 min, the cylindritic forms were observed only above 120 C, whereas for lower Tc only spherulites, although aligned in rows, were found [21]. Contrary to that, no row nuclei were reported in PLA sheared at 135 C, even at 10/s for up to 65 s [23]. According to [20], with decreasing temperature, the advantage of bundle type nucleation of fibrils over folded nuclei of spherulites decreases, which results in the lower limit of temperature range of fibril formation, despite the increase of relaxation times of polymer chains. Moreover, most fibrils reported in Ref. [20] were preferentially formed from contaminants such as catalyst residue, dust and surface of the rotating plate, the latter
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Fig. 8. SEM micrographs of PLA1.5 sheared at 25/s for 30 min at 148 C and cooled at 10 C/min, after etching for 4 h (a) and 6 h (b). (c) SEM micrograph of etched cross-section of PLA1.5 specimen sheared at 10/s for 5 min at 146 C and cooled at 30 C/min. Shearing direction horizontal.
similar to our findings. It is worth noting that Shen et al. [27] reported formation of shish-kebabs on isotactic polypropylenesubstrate interface and explained this phenomenon as a result of wall slip, occurring via disentanglement of chains adsorbed on the wall from those in the bulk, and orientation of the adsorbed disentangled chains that promoted the formation of row-nuclei. Jay et al. [28] suggested that, in a polymer undergoing a shear flow on a macroscopic scale, the deposit of a chain segment on the surface of the solid layer at a microscopic level can change the flow to a local elongational flow, which affects crystallization more than a shear flow. In our study, the row-nucleated forms were observed by SEM predominantly on surfaces of PLA specimens and no crystal orientation was detected by 2D-WAXS. Those observations suggest that crystallization in the bulk was mostly nucleated on the point-like nuclei, rather than on the oriented fibrils, despite the strong shear flow. This could be caused either by inadequate fraction of sufficiently long PLA chain promoting the orientation and intense formation of fibrillar nuclei, or the presence of D-lactide, which impedes formation of ordered crystalline regions within oriented precursors. However, the chains on surfaces adhering to quartz glass slides formed the fibrillar nuclei, hence D-lactide did not prevent crystallization of oriented fibrils under more favourable conditions promoting orientation and nucleation. To have better insight into the shear-induced morphology, PLA1.5 sheared at 146 C at 10/s for 5 min and cooled at 30 C/ min, was analysed by SEM. This specimen was selected because its shear-induced crystallization was not completed during postshearing fast cooling, leaving regions not fully filled with crystalline aggregates where the morphology can be better seen. On the other hand, the shear-induced nucleation was strong enough to reach cc ¼ 36 wt% during slower cooling. To expose the internal structure, the specimen was cut in a plane parallel to the SD and to the velocity gradient, and etched. The micrograph in Fig. 8c shows rather isotropic structure filled with densely nucleated crystalline aggregates. One can distinguish many short fibrillar forms, parallel to the SD, with length up to 2 mm, which can be fibrils but also edges of flat lamellae stacks. Rarely elongated structures, like the one in the inset in Fig. 8c, suggestive of the presence of a fibrillar nucleus several micrometres long, could be found. The length of short fibrillar structures seen in Fig. 8c, equal to the diameter of the polycrystalline aggregates visible on the same micrograph, suggests that both forms were nucleated at the same time; the alignment of short fibrils parallel to the SD can be possibly related to the formation of the shear-induced nuclei in the flow field. The above findings corroborate the conclusion concerning the predominant shearinduced point-like nucleation in the bulk of PLAs studied
under the experimental conditions. 4. Conclusions Shearing intensified nonisothermal crystallization of both slowly crystallizing PLAs, studied by enhancing nucleation, predominantly in the form of point-like nuclei, although row-nuclei were also observed. This led to increase of Tc during postshearing cooling and cc. The effect was temperature dependent, and strongly intensified with Ts decreasing below 160 C in a relatively narrow temperature range. Both, Tc and cc increased with increasing g_ (and strain), although for g_ beyond 50e100/s a decrease was observed due to loss of control over the experimental conditions. The decrease of v from 30 to 10 C/min markedly elevated cc, which indicates the importance of post-shearing crystallization from the relaxed melt during cooling. In general, no crystal orientation with respect to the shearing direction was detected by 2D-WAXS. Row-nucleated structures were revealed in sheared specimens after etching but predominantly in the surface layers adhering to microscope quartz glasses; longer etching exposed rather isotropic structure. This indicates that, in the bulk, the shear-induced nucleation was in the form of point-like nuclei rather than row-nuclei. Although shearing intensified nonisothermal crystallization of both commercial PLAs, the effect of shear on PLA1.5 with smaller D-lactide content was stronger than on PLA2.8 with larger D-lactide content; higher cc developed in the former. Nevertheless, the maximum cc of PLA1.5, 39 wt%, was close to that of PLA2.8, although such crystallinity was reached only when crystallization started during shearing and completed during cooling. Crystallization, if not accomplished during the postshearing cooling, continued during heating in DSC. The melting behavior of specimens reflected their thermo-mechanical history and crystallinity levels developed during shearing, post-shearing cooling and subsequent heating, which led to single melting peaks with low or high temperature shoulders or to multiple melting peaks. Acknowledgement This research project has been supported by the European Union European Regional Development Fund, Contract No.POIG.01.03.0100-007/08-00, project Biodegradable Fibrous ProductsBIOGRATEX. Appendix We interpret the decrease of the effect of shear on crystallization with g_ increasing beyond 50e100/s as a result of loss of control over
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the experimental conditions. Othman et al. [29] reported fracture of PLA melt in capillary rheometry beginning to occur at shear stress of 0.2e0.3 MPa. Steady shear viscosity h can be calculated from the modulus of complex viscosity measured in oscillatory shear, jh*(u)j, where u is frequency expressed in [rad/s], according to the empirical Cox-Merz law: jh*ðuÞj ¼ hðg_ ¼ uÞ, which is obeyed in PLA in a broad frequency range [30]. Rheological properties of both PLAs were studied using a rheometer ARES LS2 (TA Instruments). The experiments were performed in a cone-plate geometry at 150 and 160 C. One mm thick disks with the diameter of 25 mm were prepared by compression molding at 190 C. The disks were placed in the measuring cell and heated to 180 C. Next, the gap was set and temperature was decreased. Low-amplitude oscillatory shear experiments were carried within the linear viscoelastic regime, over a frequency range 1e512 Hz beginning from the highest fre_ calculated based on the measured h*(u) quency. The plots of hðgÞ are shown in Fig. A1. It follows that, in g_ range of 50e100/s, the shear stress is in the range of 0.2e0.3 MPa. Indeed, at g_ 50/s fracture of PLA melts was observed during shearing, as shown in micrograph in Fig. A2. Another reason for the decrease of cc can be viscous heating. The amount of heat per volume unit and time unit, E, generated during simple shear flow between two parallel plates, _ g_ 2 . The increase of temperature can be estimated by is equal to hðgÞ solving the heat conduction equation taking into account the heat generation in a flowing polymer [31]. The steady state solution of the equation, assuming that the walls are kept at constant temperature, yields the expression for the increase of temperature, DT, in equal distances from the walls, 0.5h, in the form: E h2/(8k) where k is the heat conduction coefficient, usually about 0.2 W/(m oC) for polymer melts. Although such a simple approach neglects g_ gradient along a radius of rotating plates, convection, thermal resistance at polymer-wall interfaces and temperature dependence of h, it can still provide information about at least the order of DT. Moreover, as seen in Fig. A1, 10 C increase of temperature causes a decrease of h in the g_ range from 50 to 300/s only by 25e15%. In Fig. A1, DT at 150 C is plotted as a function of g_ for 200 mm thick films of both PLAs. It can be seen that DT reaches 1e3 C for g_ ranging from 100 to 300/s. Even such small temperature increase can contribute to a decrease of cc and Tc in that range, in which the effect of shear strongly depends on Ts. This conclusion is supported by the observation that more pronounced decrease of cc was found
Fig. A1. Shear rate dependence of steady shear viscosity of PLA2.8 and PLA1.5 at 150 and 160 C, and temperature increase, DT, inside specimens sheared at 150 C caused by viscous heating.
Fig. A2. LM micrograph of PLA2.8 during shearing at 150 C with 50 /s.
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