Poly(lactic acid)-Based in Situ Microfibrillar Composites with

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Poly(lactic acid)-Based in Situ Microfibrillar Composites with Enhanced Crystallization Kinetics, Mechanical Properties, Rheological Behavior, and Foaming Ability Adel Ramezani Kakroodi,† Yasamin Kazemi,† WeiDan Ding,† Aboutaleb Ameli,‡ and Chul B. Park*,† †

Microcellular Plastics Manufacturing Laboratory, Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, Ontario M5S 3G8, Canada ‡ Advanced Composites Laboratory, School of Mechanical and Materials Engineering, Washington State University Tri-Cities, 2710 Crimson Way, Richland, Washington 99354, United States ABSTRACT: Melt blending is one of the most promising techniques for eliminating poly(lactic acid)’s (PLA) numerous drawbacks. However, success in a typical melt blending process is usually achieved through the inclusion of high concentrations of a second polymeric phase which can compromise PLA’s green nature. In a pioneering study, we introduce the production of in situ microfibrillar PLA/polyamide-6 (PA6) blends as a cost-effective and efficient technique for improving PLA’s properties while minimizing the required PA6 content. Predominantly biobased products, with only 3 wt % of in situ generated PA6 microfibrils (diameter ≈200 nm), were shown to have dramatically improved crystallization kinetics, mechanical properties, melt elasticity and strength, and foaming-ability compared with PLA. Crucially, the microfibrillar blends were produced using an environmentally friendly and cost-effective process. Both of these qualities are essential in guarantying the viability of the proposed technique for overcoming the obstacles associated with the vast commercialization of PLA.

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ration of different particles and fibers, such as talc, carbon nanotubes (CNTs), and cellulose nanofibers (CNFs), have been considered for improving the crystallization behavior and melt strength of PLA with varying degrees of success.8,14−17 In a study by Nofar et al.,8 talc proved to be effective in enhancing PLA’s crystallization kinetics under both atmospheric and high pressures. Inclusion of nanoclay and nanosilica, however, led to slower crystallization kinetics due to the reduced mobility of the PLA chains in the presence of nanoparticles. Inclusion of small amounts of CNT has been shown to improve the crystallinity of PLA at a low cooling rate of 5 °C/min.18 However, it is worth noting that the cooling rates during the actual processing of PLA are much faster. In addition, the incorporation of nanoparticles also presents several challenges such as poor level of dispersion and increase in the price of product. Several environmental and health concerns are also associated with the production, use, and disposal of nanoparticles which further reduce their attractiveness.19 Third, the addition of plasticizers, aimed at improving PLA’s toughness, is known to reduce its stiffness and strength significantly.20,21 Further, these plasticized products have unstable morphologies due to migration of plasticizers (leaching) from the PLA matrix.7,22 Fourth, blending of PLA with other polymers is considered as the

INTRODUCTION Environmental concerns associated with petroleum based polymers have generated an immense interest in the development of cost-effective and practical technologies for production of biobased polymeric products.1−3 Poly(lactic acid) (PLA) is one of the most important biobased and biodegradable plastics that can be produced from such renewable resources as corn starch and sugar cane.4−6 PLA also has promising processability and mechanical properties, especially stiffness.7 However, the majority of the efforts dedicated to vast usage of PLA as a commodity thermoplastic have had limited success due to PLA’s many limitations. PLA has slow crystallization kinetics and poor melt strength, which restrict its application in film blowing and foaming.8,9 Intrinsic brittleness of PLA, due to having short and semirigid chains, and its low thermal stability also prevent the large scale adoption of PLA in the packaging industry.10,11 Several methods have been employed in order to improve PLA’s properties. However, each of these approaches has been reported to have several drawbacks. First, postannealing is usually performed in order to improve the crystallinity of otherwise amorphous PLA products.12 However, this method increases the product’s cost and production time. Furthermore, postannealing increases the crystallinity of PLA by enlarging its already existing crystals. The resultant product has a small number of large crystals which deteriorate its toughness, increase its brittleness, even further.7,13 Second, the incorpo© XXXX American Chemical Society

Received: September 16, 2015 Revised: November 2, 2015

A

DOI: 10.1021/acs.biomac.5b01253 Biomacromolecules XXXX, XXX, XXX−XXX

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with the desired shapes. This significantly limits the commercialization of their product. In this work, we present the first true PLA-based in situ MFC containing stable PA6 domains with microfibrillar morphologies. Generation of PA6 microfibrils with submicronic diameters were successfully achieved using an environmentally friendly, cost-effective, and fast melt mixing-hot stretching process. Due to PA6’s higher melting point, it was possible to further process the generated MFCs into compression-molded bars. These were used for a comprehensive characterization of their morphological, mechanical, and rheological properties as well as their foaming ability and crystallization behavior.

most cost-effective and practical way to enhance its properties.7,23,24 However, the ability to control the morphology of such blends as a means to tailor their properties, e.g., their mechanical properties, crystallization kinetics, and processability, has been an ongoing challenge.25,26 Furthermore, it is essential to keep the concentration of the second polymeric phase (in the blends) to a minimum to make sure that the ecofriendly nature of PLA would not be compromised. As noted above, the ability to control the morphology of a polymeric blend is vital. Among all blend morphologies, in situ microfibrillar composites (MFCs) have shown great potential in improving the crystallization behavior, mechanical properties, and melt strength of thermoplastic hosts such as polypropylene (PP),27−29 and polyethylene (PE).30 In this technique, a homogeneous blend of matrix (A) and dispersed phase (B) polymers (the melting point (Tm) of B must be at least 40 °C higher than A) is first produced at Tprocessing> TmA & TmB using a conventional melt blending equipment. The blend is subsequently stretched, either in molten (hot stretching) or frozen (cold drawing) states, which transforms B from spherical domains into highly oriented microfibrils with high aspect ratios. Then, the resulting fibrillated blend is further processed at TmA< Tprocessing< TmB to produce isotropic products with desired shapes.28,31,32 During this step, the matrix melts (and its molecules relax), and it is reshaped while the dispersed B microfibrils maintain their microfibrillar shapes. The success of this process is controlled by such characteristics of A and B polymers as their viscosity ratio, compatibility, melting points, and the processing conditions.33 The unique procedure by which the MFCs are produced has several advantages over the traditional composite production methods. First, in traditional composites, especially nanocomposites, the strength and stiffness is usually restricted by a poor dispersion of fillers in the matrix.34 In MFCs, on the other hand, the microfibrils are made by the elongation of already well-dispersed spherical B domains, making the microfibrils perfectly dispersed.35 Naturally, a high level of dispersion of the microfibrils increases their specific surface area, and this substantially benefits their properties. Second, the B microfibrils’ high aspect ratio, in conjunction with their flexible nature, causes them to bend and form an entangled network. Such a network has been reported to significantly improve the matrix’s melt strength, and consequently, its processability.28,30 Third, the MFCs eliminate the environmental concerns and health hazards that are associated with the production and use of traditional fibers. Fourth, MFCs are less expensive, lighter, and easier to produce than traditional composites. Being an extremely simple, cost-effective, and fast process, in situ microfibrillation could potentially be the ideal technique to simultaneously alleviate PLA’s brittleness, poor melt strength, and slow crystallinity. However, we were surprised by the immense lack of research in this area. To the best of our knowledge, the only notable article regarding enhancement of PLA by in situ microfibrillation was published by Xie et al.35 They reported that the in situ microfibrillation of PLA/ poly(butylene succinate) (PBS) improved the mechanical properties of the PLA matrix. However, there was a flaw in their product that arose because they neglected an essential prerequisite for MFCs: melting point of the dispersed phase must be at least 40 °C higher than that of the matrix (PLA). The fact that the melting point of PBS (≈114 °C) is much lower than that of PLA (150−160 °C) means that their stretched MFCs could not be further processed into products

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EXPERIMENTAL SECTION

Materials. Commercially available grades of PLA and PA6 were chosen for production of MFCs. Fiber grade PLA, Ingeo biopolymer 6201D, was purchased from NatureWorks LLC (USA) and was used as received. This product had a weight-average molecular weight (Mw) of 160 000−170 000 g/mol, a crystalline melt temperature of 155−170 °C, and 1.2−1.6 mol % of D-isomer units. PA6, Ultramid B27 E, with a melting point of 220 °C, was kindly donated by BASF SE (Germany) and used as the dispersed phase. Sample Preparation. Prior to mixing, PA6 was dried at 100 °C for at least 8 h to avoid hydrolysis. PLA was also dried at 70 °C for 12 h in a vacuum oven. PLA/PA6 blends with 3, 7, 15, and 25 wt % of PA6 were produced using a Leistritz corotating twin screw extruder with a screw diameter of 27 mm and an L/D ratio of 40. The temperature profile in the extruder’s zones was 160 °C, 180 °C, 205 °C, 210 °C, 220 °C, 220 °C, 230 °C, 230 °C, 230 °C, and 230 °C. The compounds were processed at a screw speed of 100 rpm and a feeding rate of 4 kg/ h. The pure PLA was also processed using the same conditions. The above processing conditions were chosen to provide a high level of mixing while minimizing PLA’s degradation. In order to produce MFCs, the extrudates with a melt temperature of around 235 °C were hot-stretched at the die exit using a take-up roller at a high draw ratio (that is, the ratio of area of the transverse section of unstretched extrudate to that of stretched extrudate36) of nearly 2000 and were collected in the form of fibers with diameters of 80 μm. The drawn extrudates froze before reaching the take-up roller which was located 3 m away from the die exit. The produced fibers were then chopped into 1 mm long pieces. For blends with spherical domains of PA6, the extrudates were quenched in a water bath and collected without stretching. All of the produced samples, whether pure PLA or compounds with spherical or microfibrillar PA6 domains, were compression-molded using a laboratory hydraulic press, Carver SC7620, at 180 °C for 5 min. This was followed by cooling at a rate of 40 °C/min, using cooling air, to form isotropic MFCs. This temperature was chosen to maintain the microfibrillar structure of PA6 domains in the MFCs. Please note that henceforth the term “fiber” refers to stretched PLA/PA6 compounds, while “microfibril” refers to elongated PA6 domains that are dispersed within the PLA matrix. Scanning Electron Microscopy (SEM). The morphologies of the compression-molded PLA/PA6 blends, with spherical and microfibrillar PA6 domains, were observed using a Quanta FEG 250 scanning electron microscope after the surfaces were sputter coated with platinum. Isothermal Crystallization Kinetics Investigations. Differential scanning calorimetry (DSC) experiments were performed under the atmospheric pressure, in a nitrogen atmosphere, using a Q100 DSC from TA Instruments. High pressure DSC (HP-DSC) studies were also conducted under 45 bar, in a CO2 atmosphere, using a NETZSCH DSC 204 HP. In both the DSC and HP-DSC studies, peak crystallization temperatures (Tpeak) (that is, the temperature where each sample’s crystallization rate was at its highest) were first determined using nonisothermal DSC experiments with a cooling rate of 5 °C/min. The Tpeak approximately equaled 97 °C for the atmospheric pressure and 87 °C for the high pressure experiments. B


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Biomacromolecules The lower Tpeak under high pressure resulted from the facilitation of PLA chain movement caused by the plasticizing effect of the dissolved CO2. Subsequently, the samples’ isothermal crystallization kinetics were studied as follows: (i) in both the DSC and HP-DSC studies, the samples were first heated to 180 °C at a heating rate of 20 °C/min, and the temperature was maintained for 5 min to remove the thermal histories. The samples were then rapidly cooled, at 20 °C/min, to the intended testing temperatures (that is, 90 °C, 97 °C, and 110 °C for the DSC experiments and 80 °C, 87 °C, and 100 °C for the HP-DSC experiments). These temperatures were maintained until full crystallization occurred. The Avrami Equation37 was employed to analyze the kinetics of isothermal crystallization of samples at their Tpeak as follows:

ln[− ln(1 − X(t ))] = n ln t + ln k

SEM micrographs of the cryogenically fractured surfaces of the foamed samples were used for their characterizations. The average cell size for each foamed sample was measured using ImageJ software. The cell population density (Nc) (that is, the number of cells in a unit volume of unfoamed polymer) for each sample was calculated using the following equation:38

Nc =

⎛ nb ⎞3/2 ⎜ ⎟ ×ϕ ⎝ l2 ⎠

(2)

where φ was the volume expansion ratio (φ = ρ/ρf), and ρ and ρf were the densities of samples before and after foaming, respectively. The densities were measured using the water displacement method according to ASTM D792. nb was the number of cells in an SEM micrograph that covered an area of l2.

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(1)

In Equation 1, X(t) is the relative crystallinity (the ratio of crystallinity at time t to the final crystallinity), k is the constant for the crystal nucleation and growth rates, and n is the Avrami exponent that provides insight into the mechanisms of crystal nucleation and growth. The values of n were determined by plotting ln[−ln(1 − X(t))] versus ln(t). Wide Angle X-ray Scattering (WAXS). The WAXS patterns of the PLA and the PLA/PA6 blend with 3 wt % of PA6 microfibrils, that were compression-molded and cooled at 40 °C/min, were determined using a Philip PW3710 X-ray Diffractometer. To study the effects of isothermal crystallization, patterns of the same samples were also determined after a short annealing process for 7 min at 97 °C. The tests were performed under ambient conditions, using a Ni filtered Cu−Kα radiation with a wavelength (λ) of 1.5406 Å. A generator voltage of 40 kV and a current of 40 mA were used. The scan angle ranged from 5° to 30° at a speed of 0.02 (2θ/s). Tensile Tests. The tensile properties of the compression-molded samples, both before and after isothermal crystallization at 97 °C for 7 min, were measured according to ASTM D638 using an Instron model 5848 microtester. Five rectangular-shaped specimens with dimensions of 1 × 6 × 120 mm3 were characterized for each compound. The tests were performed at a crosshead speed of 5 mm/min using a gauge length of 50 mm. Shear Rheological Measurements. The linear viscoelastic behaviors of pure PLA and PLA/PA6 blends with spherical and microfibrillar PA6 domains were studied using the parallel-plate geometry, with a diameter of 25 mm and a 1 mm gap, in an Advanced Rheometric Expansion System (ARES) from TA Instruments. All of the tests were performed at 180 °C, which was above PLA’s melting point and yet low enough to prevent PA6 microfibrils from melting and recoiling. A controlled strain, within the linear viscoelastic range of the samples, was first obtained using the strain sweep tests. Then, frequency sweep tests, at 0.1−400 rad/s, were performed. Uniaxial Elongational Viscosity Measurements. Uniaxial elongational viscosities were also measured using an Elongational Viscosity Fixture (EVF) attached to the ARES rheometer. The tests were conducted at 170 °C at constant strain rates (ε) of 0.01, 0.1, and 1 s−1. It was empirically found that this temperature provided the most consistent results especially for pure PLA. At higher temperatures, pure PLA samples sagged severely. However, it was not possible to measure the elongational viscosities of the MFCs with PA6 contents of 7 wt % and higher. These melts were too stiff, and the clips, from EVFs, were unable to hold the samples during the rotation of the drums. Foaming Experiments. A batch foaming process was employed to study the effects of the in situ microfibrillation process on PLA’s foaming ability. Compression-molded bars with dimensions of 1.3 × 5 × 10 mm3 were placed in a temperature regulated chamber, which was connected to a syringe pump filled with CO2 as the physical blowing agent. After placing each sample inside the chamber, it was purged with compressed CO2. Subsequently, the chamber was pressurized to 13.8 MPa and heated to 140 °C for an exposure time of 1 h, after which the pressure of the chamber was released quickly, and the samples were removed. These conditions were chosen because they provided the best foam-morphologies for pure PLA.

RESULTS AND DISCUSSION Figure 1 shows the phase morphologies of the as-extruded blends containing different PA6 concentrations. It is shown that

Figure 1. SEM micrographs of the cryo-fractured surfaces of PLA/PA6 blends with 3 wt % (A), 7 wt % (B), 15 wt % (C), and 25 wt % (D) of PA6 before hot stretching (scale bars = 2 μm).

the diameters of the spherical PA6 domains in these unstretched compounds fell within the submicronic (in blends with 3 and 7 wt % of PA6) to micronic (in blends with 15 and 25 wt % of PA6) ranges. The small dimensions of these spheres can be ascribed to the good compatibility (low interfacial free energy) between PLA and PA6. The self-compatibilization behavior of PLA/PA compounds has been previously reported by Stoclet et al.25 as a result of possible hydrogen bonding between the amino groups (NH) of polyamide and the carbonyl groups (CO) of PLA. They explained that, in the case of low compatibility between the phases, submicronic morphology would be highly unstable. Xie et al.35 also suggested that, in the case of a high interfacial tension between the phases, the spherical domains of the dispersed phase are expected to be as large as several microns in diameter, which would create challenges in generating fine microfibrils during the hot-stretching step. Thus, our submicron sized PA6 domains show great potential for the production of MFCs with fine microfibrils. Figure 2 shows the morphologies of the cryogenically fractured surfaces of in situ microfibrillar structures along with a quantitative analysis of the distribution of diameters of PA6 microfibrils in each blend. After hot stretching, the PA6 phase was fully extended into fine microfibrillar structures. The average diameter of the PA6 microfibrils in the PLA/PA6:97/3 wt % was as low as 198 nm. Increasing the concentration of C


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water. Figure 3 shows an SEM micrograph of a cryogenically fractured surface of PLA/PA6:97/3 wt % after etching. A

Figure 3. SEM micrograph of a fractured surface of PLA/PA6:97/3 wt % in situ MFC after the surface etching of PLA matrix.

network of highly entangled PA6 microfibrils was observed, which was a result of their high aspect ratios and flexibility. However, it was impossible to measure the aspect ratios of individual PA6 microfibrils. As noted earlier, the extruded PLA/PA6 compounds were stretched into fibers with diameters of around 80 μm to form in situ MFCs. These fibers were then chopped and compressionmolded at 180 °C (the isotropization stage) in order to produce composites with randomly oriented PA6 microfibrils in the PLA matrix. It is shown in Figure 2A1−C1 that, after compression molding of the stretched PLA/PA6 fibers with 3, 7, and 15 wt % of PA6, the PA6 microfibrils were randomly oriented in PLA matrix. As a result, the produced MFCs were isotropic. However, Figure 2D1 shows that in the case of the PLA/PA6:75/25 wt % MFC, the PA6 microfibrils were highly oriented even after the isotropization step. Such behavior is a result of a significant increase in the viscosity of the blends due to the reduction in the mobility of PLA molecules at high concentrations of the dispersed microfibrils. During compression molding of PLA/PA6:75/25 wt % MFCs, the extremely high viscosity of the blend, caused by the presence of high concentration of PA6 microfibrils, prevented the PA6 microfibers from reorienting. Further, PLA molecule’s low mobility is expected to have deteriorated the ability to create strong connections between chopped PLA/PA6 fibers due to the poor interpenetration of PLA molecules from adjacent fibers. Consequently, the produced compression-molded sample merely contained poorly sintered chopped PLA/PA6 fibers. As shown in the schematic demonstration in Figure 4, after the MFC with a high PA6 content was compression molded, the chopped PLA/PA6 fibers were randomly dispersed, while the PA6 microfibrils inside each fiber still maintained their original orientation. Figure 5 shows the final crystallinities and the crystallization half-times (t1/2) (the time required to achieve 50% of the final crystallinity) of each sample under the atmospheric pressure and under 45 bar CO2 pressure at different temperatures. Figure 6 also presents the DSC (at 97 °C) and the HP-DSC (at 87 °C) thermograms of the samples. Addition of the PA6 phase improved the crystallization kinetics (reduced the t1/2) of the PLA in both the spherical and microfibrillar blends. This was ascribed to the heterogeneous crystal nucleation effect of the PA6 phase. Atmospheric DSC results at 97 °C, for instance, showed that the crystallization half-time of the PLA decreased from 1412 to 760 s and to 221 s after inclusion of 3 wt % PA6 phase in the form of spherical and microfibrillar domains,

Figure 2. SEM micrographs of as-broken surfaces of PLA/PA6 in situ MFCs with 3 wt % (A1), 7 wt % (B1), 15 wt % (C1), and 25 wt % (D1) of PA6 microfibrils (all scale bars = 2 μm). Quantitative analyses of the distributions of diameters of PA6 microfibrils in blends with 3, 7, 15, and 25 wt % PA6 are also shown in (A2), (B2), (C2), and (D2), respectively. D stands for the average diameter.

PA6 phase to 25 wt % increased the average diameter of microfibrils to 233 nm. From the SEM micrographs, no debonding was observed between the PLA and PA6 phases after the hot stretching stage. Such behavior is essential in providing efficient load transfer from PLA to PA6 domains during the stretching process. Further, the thin PA6 microfibrils were fully dispersed in the PLA matrix with no trace of agglomeration of the fibers, even at high concentrations. The microfibrils also seem to have very high aspect ratios. It is also worth mentioning that the generated microfibrils had much larger sizes (volumes) than their precursor spheres. This was especially so in blends with 3 and 7 wt % of PA6. Fakirov et al.39 proved that such observations were due to the simultaneous extension and coalescence of the dispersed spheres during the hot stretching stage. In other words, each PA6 microfibril was formed through a connection of several elongated spheres. Selective surface etching of the PLA matrix was performed to provide insight into the aspect ratio of PA6 microfibrils in the blends. Samples were immersed in a water−methanol (1:2 by volume) solution containing 0.025 mol/L of sodium hydroxide for 96 h at 23 °C, and then they were rinsed with distilled D


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Figure 4. Schematic demonstration of the differences between the microstructures of compression-molded in situ MFCs with low and high contents of PA6 microfibrils. The poor level of sintering between the chopped PLA/PA6 fibers with a high content of PA6 microfibrils is emphasized by the use of dashed lines.

Figure 6. Isothermal DSC thermograms of PLA and PLA/PA6 blends under atmospheric pressure (at 97°°C) (A) and 45 bar CO2 pressure (at 87 °C) (B).

(increased the crystallization rate) in polymeric blends. Figure 5 shows that although the kinetics of crystallization of PLA was improved after the inclusion of the PA6 phase, the final crystallinities were generally lower for these blends compared with pure PLA. For instance, the final crystallinity of PLA at 97 °C was reduced from 34.8% to 28.2% and to 24.9% after the inclusion of 25% of the spherical and microfibrillar PA6 domains, respectively. Nofar et al.8 observed similar behaviors in PLA-based composites containing talc, nanosilica, and nanoclay. They ascribed it to the reductions in the mobility of PLA molecules in the composites due to the presence of a higher number of crystals. They suggested that the presence of fewer and larger crystals in pure PLA, due to the lack of a heterogeneous nucleation effect, facilitated the crystal growth by causing less physical hindrance. Further, Figure 5B shows that, at atmospheric pressure, the final crystallinities of the MFCs were lower than the blends with spherical PA6 domains. This can also be ascribed to microfibrils’ greater ability to reduce the mobility of PLA chains. However, it must be noted that although pure PLA and PLA/PA6 blends with spherical domains had slightly higher final crystallinities compared with the MFCs, their crystals took much longer to form, which is highly undesirable from a processing point of view. A study of PLA’s crystallization behavior under high pressures is also of great importance due to the role of crystals

Figure 5. Crystallization half-times and final crystallinities of PLA and PLA/PA6 blends under atmospheric pressure (A and B) and 45 bar CO2 pressure (C and D). (S) and (F) stand for blends with spherical and microfibrillar PA6 domains, respectively.

respectively. Similar trends were also observed for compounds with other PA6 concentrations at all of the isothermal crystallization temperatures. Improved crystallization kinetics of blends with microfibrillar PA6 domains compared to the unstretched blends was due to the larger specific surface area (ratio of surface to volume), and the higher aspect ratio (ratio of length to diameter), of the PA6 phase in these blends. In other words, the in situ generated microfibrils, which had a larger specific surface area than the spherical domains, provided more heterogeneous crystal nucleation sites. Rizvi and Park28 and Li et al.40 also reported that microfibrillation increased the heterogeneous nucleation efficiency of the dispersed phase E


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Biomacromolecules in the foaming process during both the cell nucleation and cell growth steps.41−46 The plasticization effects of dissolved CO2 on the crystallization behaviors of the samples are shown in Figure 5C,D. Under a 45 bar CO 2 atmosphere, the crystallization kinetics of all the samples was improved. Comparisons between Figure 5A and Figure 5C show that the crystallization half-times of pure PLA and PLA/PA6:97/3 wt % with spherical and microfibrillar PA6 domains decreased by 54%, 78%, and 69%, respectively, when the pressure increased from atmospheric to 45 bar. Such an increase in the crystallization kinetics was due to the enhanced mobility (reduced dissipation energy) of the PLA chains in the presence of dissolved CO2, which increased their ability to reach the crystallization sites.42 Similar to the atmospheric pressure experiments, the presence of the PA6 phase enhanced the crystallization kinetics of PLA at 45 bar, while the improvements were more significant in the MFCs. An interesting behavior was observed after comparing the final crystallinities of the samples under atmospheric and high pressure conditions. Figure 5D shows that despite the increase in the crystallization rates, the pure PLA’s final crystallinity was significantly lower under 45 bar CO2 pressure compared to the atmospheric pressure case. Further, contrary to the atmospheric pressure experiments, the inclusion of PA6 phase, especially in the microfibrillar form, improved the final crystallinity of PLA under high pressure conditions. These observations suggest that the confinement of the PLA chains’ mobility (caused by the presence of the microfibrils), which had a negative effect on the crystallinity of samples under atmospheric pressure, was beneficial under 45 bar CO2 pressure. Figure 7 shows the corresponding Avrami double-log plots for the isothermal crystallization of the samples at their Tpeak. The values of the Avrami exponent (n) are also presented in Table 1. The inclusion of the PA6 microfibrils at atmospheric conditions increased the value of the Avrami exponent (n) from 2 to nearly 3. For instance, n increased from 2.02 to 2.68 and

Table 1. Values of the Avrami Exponent (n) for the Isothermal Crystallization of Pure PLA and the PLA/PA6 Compounds under Atmospheric Pressure and 45 Bar CO2 Pressure 1 bar

45 bar

PA6 Content (wt %)

Spherical

Fibrillar

Spherical

Fibrillar

0 3 7 15 25

2.02 2.11 2.12 1.98 2.03

2.02 2.68 2.75 2.60 2.63

1.95 2.06 2.00 1.96 2.03

1.95 2.42 2.46 2.48 2.58

2.75 after the addition of 3 and 7 wt % PA6 microfibrils, respectively. This is ascribed to the strong heterogeneous nucleation ability of the microfibrils.8 High pressure experiments were shown to reduce the value of n. Nofar et al.8 ascribed this behavior to the plasticization effects of dissolved CO2, which could alter the crystallization mechanism by reorienting less closely packed crystals into planar structures. Wide angle X-ray scattering (WAXS) provides useful information on the crystallinity and the crystalline structure of polymers. Figure 8 shows the WAXS patterns of pure PLA

Figure 8. WAXS patterns of pure PLA and PLA/PA6:97/3 wt % with microfibrillar PA6 domains before and after isothermal crystallization (at 97 °C for 7 min). Isothermally crystallized samples are denoted by asterisks. (F) stands for blends with microfibrillar PA6 domains.

and the microfibrillar PLA/PA6:97/3 wt % blend before and after the samples were exposed to an isothermal crystallization process at 97 °C for 7 min. These conditions were chosen based on the results from the DSC characterizations under atmospheric pressure. The pure PLA exhibited no peaks while a broad hump was observed in its pattern. These observations indicate that PLA was completely amorphous, which was a result of the high cooling rate (40 °C/min) that was applied during the compression molding process. The isothermal heat treatment of pure PLA (for 7 min at 97 °C) caused only a slight crystallization which was evidenced by the emergence of a small peak at 2θ = 16.4° (α form crystals47). However, the complete crystallization of pure PLA would require a lengthy exposure to isothermal crystallization (about 50 min), as suggested by Figure 6. Interestingly, the WAXS pattern of the microfibrillar PLA/PA6:97/3 wt % blend, on the other hand, demonstrated its semicrystalline state even before performing the isothermal crystallization process. Figure 8 shows the presence of a prominent peak at 2θ = 16.4° and the emergence of a smaller

Figure 7. Avrami double-log plots of PLA and PLA/PA6 blends with spherical and microfibrillar PA6 domains under atmospheric pressure (A and B) and 45 bar CO2 pressure (C and D). (S) and (F) stand for blends with spherical and microfibrillar PA6 domains, respectively. F


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Biomacromolecules peak at 2θ = 18.7° (both indicative of α form crystals) for this sample. Significant improvement in the PLA’s crystallization kinetics after in situ generation of PA6 microfibrils enabled the PLA phase to crystallize even under a high cooling rate of 40 °C/min. In addition, it was found that a very short isothermal crystallization of this compound led to further increase in the intensity of the peaks in its WAXS pattern. The absence of the peak at 2θ = 18.7° in WAXS patterns of PLA is ascribed to PLA’s low crystallinity. In an interesting study on the properties of PLA/organoclay nanocomposites, Di et al.48 reported that crystallinity contributed more to PLA’s mechanical properties than the addition of nanoparticles did. Thus, improvements in the crystallization kinetics and the crystallinity of PLA after the creation of PA6 microfibrils are expected to dramatically benefit PLA’s mechanical properties. The tensile properties of pure PLA and the PLA/PA6 blends with spherical and microfibrillar PA6 domains, both before and after isothermal crystallization at 97 °C for 7 min, are presented in Figure 9. It is shown in Figure

described in Figures 2 and 4. Figure 9A shows that in the isothermally crystallized samples, this reduction in modulus was much less noticeable. This could be ascribed to the presence of crystals or even to possible improvements in the sintering of the PLA/PA6 fibers. Figure 9B also shows that the inclusion of small amounts of PA6 in the form of in situ generated microfibrils increased the tensile strain at break of the PLA significantly. The strain at break of the compound with 3 wt % microfibrillar PA6 was 82% higher than that of pure PLA. This was a result of the high flexibility of long PA6 microfibrils and also of their crystal nucleation effect which led to the formation of a large number of small crystals. Isothermal crystallization of the blends, however, reduced their deformability. The reduction was more noticeable after crystallization of MFCs which was due to their higher crystallinities. Figure 9C shows that the tensile strength of microfibrillar PLA/PA6:97/3 wt % compound was 55% (before the isothermal crystallization) and 80% (after the isothermal crystallization) higher than that of the pure PLA. Such a remarkable improvement in PLA’s strength was a result of the simultaneous increase in its modulus and deformability after the formation of the PA6 microfibrils. We emphasize that although the improvement of PLA’s tensile modulus using different reinforcements has been frequently reported in the past, increasing PLA’s tensile strength had remained a challenge.49−51 This was due to a severe reduction in PLA’s deformability after the addition of most reinforcements. However, the current study shows that the inclusion of 3 wt % PA6 microfibrils increased the modulus, the strain at break, and the strength of PLA simultaneously. In pure polymers, the rheological properties are controlled by the degree of topological interactions (entanglement) of the polymer chains as well as by the friction between the molecules (especially in low molecular weight polymers).52 The addition of fillers in particle or fiber forms can alter the rheological properties of polymers significantly. Such effects are related to the filler’s concentration, size, shape, deformability, level of dispersion, and the degree of interaction between the polymer and the fillers.52 Thus, the study of the viscoelastic behavior of polymer composites can provide insight into their microstructure. Figure 10 shows the shear responses of molten PLA and the PLA/PA6 compounds with spherical and microfibrillar PA6 domains. The pure PLA had a classic viscoelastic response over the studied frequency range. At low frequencies (long time scales), PLA’s storage modulus (G′) was highly dependent on the frequency while the loss tangent (the ratio of loss modulus (G″) to G′) was greater than 1. These behaviors suggest a viscous response at low frequencies. A relatively elastic response was observed for PLA at high frequencies (short time scales), where G′ was much less dependent on the frequency and the loss tangent was nearly 1.53 Figure 10A1−A3 show that the inclusion of up to 7 wt % of spherical PA6 domains did not noticeably improve the PLA melt’s elastic response. Only high concentrations (that is, 15 and 25 wt %) of PA6 spheres caused a significant increase in the storage modulus and a decrease in the loss tangent of the PLA melt. The semilogarithmic plots of the phase angle (δ) versus the complex shear modulus (G*) (van Gurp plots) of these compounds are also presented in Figure 10A3. At low G*s, phase angles of about 90° were calculated for the compounds with spherical PA6 contents of lower than 15 wt %, implying the dominance of the viscous flow.53 In the compounds with a

Figure 9. Tensile modulus (A), strain at break (B), and strength (C) of pure PLA and blends with spherical and microfibrillar PA6 domains. Isothermally crystallized samples are denoted by asterisks. (S) and (F) stand for blends with spherical and microfibrillar PA6 domains, respectively.

9A that the inclusion of PA6 in the form of spherical domains did not improve PLA’s tensile modulus significantly even after the isothermal crystallization process. These results are in good agreement with the slow crystallization kinetics of these blends from the DSC and the WAXS studies. On the other hand, the tensile moduli of the MFCs were shown to be significantly higher than that of PLA. Remarkably, the presence of only 3 wt % of microfibrillar PA6 was sufficient to improve PLA’s tensile modulus by 38% after isothermal crystallization for 7 min. The highest tensile modulus was observed from the sample with 15 wt % PA6 microfibrils. However, a further increase in the concentration of PA6 to 25 wt % resulted in a sharp drop in the compound’s modulus. Such behavior was clearly the result of the flawed microstructure of this compound, which was earlier G


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Biomacromolecules

collective influence of the physical entanglements of the PA6 microfibrils and the restrictions in the PLA chains’ mobility (caused by the physical hindrance effect of the solid PA6 microfibrils) made the microfibrils much more effective in enhancing the elastic behavior (increasing the relaxation time) of PLA melt in comparison with the spherical domains. It is noteworthy that a high level of microfibril dispersion is essential in production of the network at such a low concentration. Similar gelation phenomena have also been discussed by other researchers,54,55 as the result of the entanglement of long and flexible polymer chains. Winter and Chambon56 suggested that at the gel point, where physical or chemical gelation occurs, G′ and G″ have similar dependencies on the frequency. As a result, the loss tangent is independent of frequency at the gel point. In Figure 11, the loss tangent is plotted as a function of the microfibril

Figure 10. Linear viscoelastic behavior of pure PLA compared with PLA/PA6 blends with spherical (A1−A3) and microfibrillar (B1−B3) domains of PA6. (S) and (F) stand for blends with spherical and microfibrillar PA6 domains, respectively.

Figure 11. Variation of the loss tangent with the microfibril content at different frequencies ranging from 0.1 to 125 rad/s. Different lines represent different frequencies. The gel point appears as the point where loss tangent is independent of the frequency (that is, where lines intersect).

high PA6 content, an increase in the melt’s elastic behavior was confirmed by a dramatic reduction in the values of phase angles. The increase in the PLA melt’s elastic behavior at high concentrations of PA6 spheres can be attributed to the restriction of the PLA chains’ mobility (slower relaxation) in the vicinity of the solid PA6 spheres. Figure 10B1, on the other hand, shows that the addition of a very small amount (only 3 wt %) of in situ generated PA6 microfibrils was sufficient to increase the PLA melt’s storage modulus by an impressive 3 orders of magnitude. Further, the frequency-dependence of the PLA melt’s storage shear modulus, especially at low frequencies, was substantially weakened after the addition of PA6 microfibrils. This was indicative of the transition in the melt’s viscoelastic behavior from a liquid-like to a solid-like state.30 The addition of 3 wt % PA6 microfibrils also reduced the loss tangent (see Figure 10B2) and the phase angles (see Figure 10B3) of the PLA melt, indicating the elastic behavior’s dominance. Such an increase in the melt’s elastic behavior suggests the formation of a physically entangled network, which would have been created by the topological interactions of the PA6 microfibrils. This phenomenon is also known as the physical gelation or rheological percolation.54 The creation of such a network is a typical characteristic of the composites with deformable and long (that is, high aspect ratio) fibers.28,30 The entangled network of microfibrils was capable of storing the deformation energy over extended periods of time. The

content at varying frequencies (0.1−125 rad/s). Additional samples with microfibril contents of 1 and 2 wt % have been added. Based on Winter and Chambon’s criterion, the gel point can be depicted at a microfibril content of nearly 2.5 wt % at which the lines of loss tangents of different frequencies intersect. The response of a polymeric melt during an extensional flow is mainly controlled by the physical entanglement of their macromolecules. These entanglements, in turn, depend on the molecular structure such as length, deformability, and the degree of branching.30,57 For instance, PLA’s linear, short, and semirigid chains are known to cause poor performance during the elongational flow.8,48 Figure 12 shows the responses of pure PLA and the PLA/PA6:97/3 wt % MFC melts under elongational deformations at the strain rates of 0.01, 0.1, and 1 s−1. We have also provided the linear viscoelastic behavior envelope of each sample given by η+E = 3η+. η+ was the sample’s linear viscoelastic shear viscosity and was obtained from the start-up of a steady shear flow at a strain rate of 0.003 s−1.58 The deviation of the experimental values of the elongational viscosities from this envelope is a measure of the strain hardening behavior.28,30 As expected, the PLA had a very low elongational viscosity and a negligible strain hardening behavior at all strain rates. The PLA’s deformability was also small due to H


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Biomacromolecules

Figure 12. Uniaxial elongational responses of PLA and PLA/PA6:97/3 wt % MFC melts at strain rates of 0.01, 0.1, and 1 s−1.

the premature breakage of its melt, which was caused by the quick disentanglement of its chains during the flow. It is shown in Figure 12 that the inclusion of 3 wt % of PA6 microfibrils led to significant improvements in the PLA melt’s elongational characteristics. Strong strain hardening effects (nearly 1 order of magnitude increase in the elongational viscosity) were observed in the case of the microfibrillar compound, which suggested increase in the relaxation times of the PLA molecules.57 The PLA melt’s deformability was also significantly improved, especially at a strain rate of 0.01 s−1. This finding is of great importance in the processing of linear polymers. The formation of a three-dimensional network of deformable and well-dispersed PA6 microfibrils, which was also confirmed by the shear viscosity measurements, was responsible for the improved elongational behavior of PLA melt.27,30 As noted earlier, the high melt viscosities of the MFCs with PA6 contents of higher than 3 wt % made it impossible to perform the elongational viscosity measurement experiments. The changes in the PLA melt’s rheological properties (that is, improved elasticity (Figure 10) and strain hardening (Figure 12)) after the in situ microfibrillation process, promise improvements in its foaming-ability.4,43,59 The environmental and commercial significance of using PLA foams in packaging and insulation applications encouraged us to provide the first report on the foaming-ability of PLA-based MFCs. Batch foaming experiments were performed at 140 °C under a CO2 atmosphere with a pressure of 13.8 MPa for a 1-h exposure time. Figure 13A,B1 shows that the inclusion of 3 wt % microfibrillar PA6 phase significantly improved the microstructure of the produced foams. The addition of the 3 wt % PA6 microfibrils reduced the PLA foam’s average cell size from 70 to 23 μm, while its cell density increased by a remarkable 2 orders of magnitude (from 2.4 × 106 cells/cm3 to 1.8 × 108 cells/cm3). Such dramatic enhancements in the microstructure of the foams can be primarily ascribed to the effects of the previously noted three-dimensional network of the physically entangled PA6 microfibrils on the elasticity and strength of the PLA melt. The MFC melt’s strain hardening behavior (as shown in Figure 12) prevented the cells from collapsing during the extensional flow which had been induced in the cell growth stage of the foaming process.30,48 Figure 13B2, on the other hand, shows a very poor microstructure for the foamed samples of PLA/PA6:97/3 wt % with spherical PA6 domains. A lack of the capability of the spherical PA6 domains to enhance the

Figure 13. SEM micrographs of cryogenically fractured surfaces of foamed samples of pure PLA (A), PLA/PA6:97/3 wt % with microfibrillar (B1) and spherical (B2) PA6 domains, and PLA/ PA6:93/7 wt % with microfibrillar (C1) and spherical (C2) PA6 domains. Note that magnifications of (A) and (B1) are different from the magnifications of (B2), (C1), and (C2).

PLA’s rheological properties was responsible for the undesirable foam microstructure. Figure 13C1 shows that foaming of the compound with 7 wt % PA6 microfibrils was impossible. No cells were generated in the sample, while the escaping gas formed huge pockets just beneath the skin layers. This behavior was the result of an excessive increase in the melt viscosity of the blend. We mentioned earlier that it was impossible to even measure the elongational viscosity of this sample due to its high melt strength. Samples with higher concentrations of PA6 microfibrils showed similar behaviors and were not included in Figure 13.

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CONCLUSIONS This research represents the first study made on the production of true in situ MFCs based on PLA. PLA/PA6MFCs were produced using a cost-effective and environmentally friendly extrusion and hot stretching process. Morphological observations of unstretched PLA/PA6 compounds revealed a desirable submicronic to micronic range for the diameters of the PA6 spheres. Microfibrillation of the compounds produced fine PA6 fibrils with diameters as low as 200 nm. DSC studies at isothermal conditions proved that the inclusion of PA6 increased the PLA’s crystallization kinetics, while the improvements were much more significant for the blends with microfibrillar PA6 domains. The in situ generation of only 3 wt % PA6 microfibrils reduced the PLA’s crystallization halftime from 1412 to 221 s. High pressure DSC experiments (under 45 bar CO2 pressure) showed an increase in the crystallization kinetics of the PLA and the compounds with spherical and microfibrillar PA6 domains because of the plasticization effects of the dissolved CO2. The WAXS studies showed that after compression molding using a relatively high cooling rate of 40 °C/min, the PLA was almost completely amorphous while the compound with 3 wt % PA6 microfibrils showed a remarkable content of α crystals. A short isothermal I


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Biomacromolecules crystallization process, 7 min at 97 °C, was enough to significantly improve the crystallinity of the compound with 3 wt % microfibrillar PA6, while crystallinity of the pure PLA increased only marginally. Mechanical characterizations showed that the enhanced crystallinity of PLA, after inclusion of the microfibrillar PA6 domains, caused substantial improvements in its tensile properties. Crucially, the samples showed improvements in both their stiffness and deformability. The impact of adding 3 wt % microfibrillar PA6 on the PLA melt’s rheological properties was also studied. It was shown that the PLA melt’s storage modulus increased by over 3 orders of magnitude. The dependence of G′ on the frequency decreased, which implied that the elasticity of the melt had improved. It was also shown that the inclusion of 3 wt % of PA6 microfibrils caused significant improvements in the PLA’s melt strength and strain hardening behavior. Improvements in the rheological properties were attributed to the formation of a three-dimensional network of the entangled PA6 microfibrils. A similar concentration of the spherical PA6 domains had negligible effects on the PLA’s rheological behavior. The improvements in the rheological properties of the microfibrillar compounds greatly benefitted the foaming-ability of PLA at such a low microfibril concentration (3 wt %). However, the inclusion of higher concentrations of the microfibrils prevented the cell formation due to the excessive increase in the PLA’s melt strength. We emphasize that microfibrillation is a practical, costeffective, and environmentally friendly method, and that it guaranties melt blending’s success as a technique to improve PLA’s characteristics. After the inclusion of only 3 wt % of PA6 phase, significant improvements in PLA’s characteristics occurred. Thus, we believe that this is an ideal approach to PLA’s commercialization, and that it makes PLA a competitive candidate in the commodity thermoplastics’ market (in areas such as packaging, textile products, automotive parts, and construction products), without sacrificing its biodegradability.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Address: 5 King’s College Road, Toronto, Ontario M5S 3G8, Canada. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Consortium for Cellular and Microcellular Plastics (CCMCP) for their financial support. We also appreciate the donation of PA6 polymer from BASF SE.

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