Elsevier

European Polymer Journal

Volume 71, October 2015, Pages 231-247
European Polymer Journal

Macromolecular Nanotechnology
Non-isothermal crystallization behaviors of poly(lactic acid)/cellulose nanofiber composites in the presence of CO2

https://doi.org/10.1016/j.eurpolymj.2015.07.054Get rights and content

Highlights

  • Crystal nucleation density was significantly enhanced by cellulose nanofibers.

  • The crystallization rate increased with the cooling rates.

  • Dissolved CO2 increased PLA’s degree of crystallinity.

  • Tc, Tm, and Tg decreased with an increase in CO2 pressure.

  • Cellulose nanofibers and CO2 pressure did not induce a new crystalline structure.

Abstract

The effects of cellulose nanofiber (CNF) on the non-isothermal crystallization behaviors of poly(lactic acid) (PLA) at atmospheric pressure and at various CO2 pressures were investigated using a regular differential scanning calorimeter (DSC) and a high-pressure DSC at different cooling rates of 1, 2, 3, and 5 °C/min. The POM images revealed that the CNFs acted as crystal nucleating agents, increasing the number of crystals and decreasing the crystal sizes. The non-isothermal crystallization showed that PLA’s crystallization rate increased with cooling rates and the incorporation of CNFs accelerated the overall crystallization kinetics by providing more nuclei, thereby decreasing the crystallization half-time. The degree of crystallinity was not proportional to the CO2 pressure. The highest crystallinity was obtained at a higher pressure with increased cooling rates. The activation energy analysis showed that the incorporation of CNFs restricted the movement of PLA molecular chains, thereby hindering crystallization. By comparing the Avrami analysis, Mo analysis, and activation energy results, it was speculated that heterogeneous crystal nucleation with the presence of CNFs might be the dominant factor in determining the overall non-isothermal crystallization rate of the PLA/CNF composites. Wide angle X-ray diffraction (WAXD) diffractograms showed that CNFs and CO2 pressure had no influence on the crystalline structure of PLA. The effects of CNF content, CO2 pressure, and cooling rate on Tc, Tm, and Tg were also investigated.

Introduction

Each year, large amounts of petroleum-based plastic waste are produced worldwide, however, waste disposal has caused serious environmental problems. Poly(lactic acid) (PLA) is a bio-based, linear aliphatic polyester made entirely from annually renewable resources (e.g., corn and sugar beets) [1]. It consumes 50% less non-renewable energy during production than traditional petroleum-based plastics (e.g., polypropylene, low-density polyethylene, polystyrene, nylon, etc.) and is readily biodegradable for waste disposal purposes [2], [3]. PLA has good physical and mechanical properties which are comparable to many petroleum-based plastics [1], [2], [4]. In addition, recent improvements in PLA manufacturing processes have made it a commercially available and large-volume plastic at competitive market prices [1], [2], [5]. Consequently, PLA is becoming one of the most promising sustainable alternatives to many petroleum-based counterparts. PLA is suitable for a wide-range of products including food packaging, nonwoven fabrics, and electronics [2], [4], [5]. It has also been widely used in biomedical applications due to its biodegradability and biocompatibility [4], [6]. Nevertheless, the brittleness, slow crystallization rate, and low glass transition temperature of PLA limit its usages [1], [2], [7], [8]. Enhancing the crystallization rate and developing higher crystallinity in PLA can improve its mechanical strength and service temperature, thereby broadening its applications [2], [7], [8], [9], [10], [11].

Different strategies have been employed to enhance the crystallization kinetics of PLA including chain branching [12], [13], [14], [15], iso-/non-iso- thermal treatment under plasticizing gas environments at elevated pressures [8], [12], [16], [17], polymer blending [18], [19], [20], [21], [22], incorporation of inorganic/organic fillers or nucleating agents [8], [21], [23], [24], [25], [26], [27], [28], [29], [30], and strain-induced crystallization [31], [32], [33]. Among these, adding inorganic/organic fillers or nucleating agents has been considered one of the most common and effective approaches. Inorganic particulates such as nanoclay [8], nanosilica [8], [34], graphene [35], and carbon nanotubes [26], [36] have been extensively studied. However, these nanoparticulates are inorganic and pose considerable health risks from their manufacturing process to their final disposal [37], [38]. In contrast, organic natural cellulosic fillers have attracted great interest in recent years due to their sustainability and natural abundance [39], [40], [41]. Natural cellulosic fillers, produced from annually renewable resources, are lightweight, biodegradable, and biocompostable. The combination of cellulosic fillers and PLA offers the possibility of generating a new class of fully biorenewable resource-based and biodegradable composites. Several kinds of cellulosic fillers have been used as crystal nucleating agents for PLA and to tailor the thermal and mechanical properties of PLA for different end uses. These include wood flour (WF) [39], cellulose fiber (CF) [24], [39], microcrystalline cellulose (MCC) [39], microfibrillated cellulose (MFC) [9], [41], cellulose nanofiber (CNF) [40], and cellulose nanocrystal (CNC) [42], [43].

Among these, CNFs have been highlighted recently as additives or reinforcement in various polymer systems to tailor the performance of the polymer matrix [9], [11], [44], [45], [46], [47], [48], [49]. CNFs are long, flexible, and entangled cellulose fibrils of 20–90 nm in diameter and several micrometers in length [50], [51]. CNFs possess a Young’s modulus of 115–150 GPa in the longitudinal direction and tensile strength of up to 2 GPa [47], [52], [53], [54]. These mechanical properties are comparable to or even higher than those of high strength glass fibers. The reinforcing ability of CNFs in PLA has been theoretically modeled [55], [56]. The incorporation of CNFs could increase the tensile strength of PLA by over 2 times at 20 wt% [55] and increase the tensile modulus around 3 times at 5 wt% fiber content [56]. The predicted values could be even higher if a 3D CNF percolated network structure was taken into consideration [55]. CNF also has a very low coefficient of thermal expansion at 1 × 10−7 K−1 along the longitudinal direction [57]. Additionally, CNFs have huge potential in biomedical applications due to their low cytotoxicity and genotoxicity [37], [58], [59]. These intriguing properties make CNFs an attractive component for high performance polymer nanocomposites.

The effect of CNFs on PLA’s crystallization behavior has been recently investigated [40], [41], [60], [61]. However, the dependence of crystallization behaviors on the CNFs was not studied systematically. A few of these studies concluded that CNFs, acting as heterogeneous crystal nucleating agents, increased PLA’s nucleus density, its crystallization rate, and its degree of crystallinity [40], [41], [60]. Conversely, another study found well-dispersed CNFs decreased PLA’s chain mobility, and hindered the crystallization process [61]. Thus, our work to understand the effect of CNFs on PLA crystallization kinetics is greatly needed.

Although CNFs have excellent reinforcing ability, it is practically challenging to achieve an acceptable dispersion level because of the hydrophilic nature of CNFs and the hydrophobic characteristic of PLA [51], [56], [62], [63], [64]. CNFs tend to aggregate rather than disperse in PLA. Also, upon drying, the agglomeration of CNFs is irreversible due to the strong hydrogen bonds established through a large number of hydroxyl groups on the fiber surfaces [62], [63], [64]. During composite processing, especially during melt compounding, a sufficient shearing force is required to distribute and disperse the CNFs. However, the shear stress exerted on the fiber could lead to significant fiber breakage and would reduce the fiber length and the aspect ratio. As a result, the experimental values from mechanical tests were much lower than the theoretical ones. Many researchers have reported that the addition of 5–20 wt% CNFs improved PLA’s tensile strength and modulus by only 8.6–33% and 10–73%, respectively [9], [56], [65], [66]. The low percentage gain was attributed to the non-uniform fiber dispersion and to the decreased fiber aspect ratio [9], [56], [65], [66].

To overcome some of the aforementioned issues during composite preparation, one option is to use CO2-assisted polymer processing [67], [68], [69], [70], [71], [72], [73], [74], [75]. There are several advantages to this. First, dissolved CO2 can significantly prevent particulate aggregation and improve dispersion [67], [69], [70], [71], [72], [73], [75]. At a high pressure, supercritical CO2 enters the space between the particulates [67], [69], [76], [77]. When the pressure decreases below a critical point, CO2 changes from its supercritical to gaseous state. The volume expansion due to CO2 phase change forces the stacked particulates to separate, which improves the particulate dispersion [67], [69]. At the same time, this also facilitates the polymer chain intercalation into the inter-particulate space [67], [69]. In addition, fiber breakage can be reduced by the decreased melt viscosity from the plasticizing effect of dissolved CO2 [71], [74]. The use of CO2 also allows for polymer processing, especially for biodegradable polymers, at lower temperatures, due to the depressed melting temperature [8], [12], [68], [78]. This minimizes their thermal degradation and maximizes the processing window. Moreover, the presence of CO2 can also enhance the polymer’s crystallization [8], [12], [79], [80], [81]. All these benefits from a CO2-assisted polymer process would enable to produce PLA composite with superior mechanical properties [9], [67], [69], [70].

The crystallization kinetics of PLA influences most regular polymer processes, especially for injection molding and extrusion. During these processes, it is a challenge to achieve high PLA crystallinity within a very short residence time for obtaining final products with desirable properties. In injection molding process, the slow crystallization rate of PLA leads to a longer molding cycle in order to develop a crystalline molded part possessing high mechanical strength and modulus [10]. The crystallization kinetics of PLA also influences foam processes. Formed crystals during foam processing can affect cell nucleation through local stress variation and gas supersaturation [82], [83], [84], [85]. In addition, entanglement of polymer molecules through these crystals could increase PLA’s melt strength, thus improving its ability to expand and suppress cell coalescence and coarsening [15], [17], [81], [82].

In this context, we primarily focus on the crystallization kinetics of PLA/CNF composites in the presence of CO2. In CO2 environment, the PLA crystallization behaviors can be significantly affected by the dissolved CO2 due to the plasticization effect. The dissolved CO2 can depress the glass transition temperature (Tg) [8], [12], [79], [86], [87] and the crystallization temperature (Tc) [8], [12], [79] of PLA by facilitating the molecular chain mobility. Nofar et al. reported a decrease of about 40 °C and 15 °C in the Tg and Tc, respectively, for a linear PLA when it was exposed to 60 bar CO2 pressure at a cooling rate of 2 °C/min [8], [12]. Some researchers concluded that during isothermal crystallization, the dissolved CO2 enhanced PLA’s crystallization rate in the crystal-growth controlled region, whereas it reduced the crystallization rate in the nucleation controlled region [87], [88]. In contrast, Nofar et al. found that the crystallization rate was enhanced in presence of CO2 and the rate was enhanced to various degrees depending on the gas pressure [8], [12], [79]. However, the final crystallinity did not always increase with the gas pressure because PLA’s crystallization kinetics (that is, crystal nucleation and growth) is pressure dependent [8], [12], [79].

To the best of our knowledge, the effect of CO2 on the crystallization behaviors of PLA/CNF composites has never been studied. Furthermore, the practical study of the non-isothermal PLA crystallization behaviors is important because the materials used are subjected to non-isothermal conditions in most of the continuous processing equipment. Hence, it is crucial to establish the relationship between processing and the material properties. Therefore, the objective of our study is to examine the effect of CNFs on the non-isothermal crystallization kinetics of PLA at atmospheric pressure and at various pressures in the presence of CO2 using a regular differential scanning calorimeter (DSC) and a high-pressure DSC. PLA composites with various CNF contents were prepared and different CO2 pressures were used. An array of methods including Avrami, Ozawa, and Mo analyses were used to examine the non-isothermal crystallization kinetics. The crystal morphologies of the neat PLA and the PLA/CNF composites were also investigated using polarized optical microscopy (POM) and by wide angle X-ray diffraction (WAXD).

Section snippets

Materials

Semi-crystalline Ingeo™ 8052D linear PLA (4.5 mol% D-content) was supplied by NatureWorks® LLC. The density and melt flow index of this PLA are 1.24 g/cm3 and 14 g/10 min (210 °C/2.16 kg), respectively. The CNFs were extracted from wheat straw and obtained in aqueous suspension from the Centre for Biocomposites and Biomaterials Processing, University of Toronto. The manufacturing process of CNFs was detailed in a previous study [50]. Carbon dioxide (CO2) (99% pure, Linde Gas LLC) was used as the

Effect of CNFs on the crystallization of PLA

To examine the effect of the CNF content on the crystallization of PLA, POM was conducted with four different CNF contents (0, 0.1, 0.5, and 1.0 wt%) under the same isothermal heat treatment conditions of 120 °C, as shown in Fig. 1. It was noted that the crystallite structure and crystallization kinetics between the neat PLA and PLA/CNF composites were notably different. Without the presence of CNFs in the matrix, the crystal nucleation rate of neat PLA was slow and only few large crystals were

Conclusions

In this study, the non-isothermal crystallization behavior of a linear PLA and its composites with various CNF contents was investigated at the atmospheric pressure in a regular DSC and various CO2 pressures in a high-pressure DSC. The presence of CNFs had a significant influence on non-isothermal crystallization of PLA. On the one hand, CNFs, acted as an effective nucleating agent, enhancing the crystal nucleation density and decreasing the crystal sizes. On the other hand, the incorporation

Acknowledgements

We greatly appreciate the financial support provided by AUTO21, the National Sciences and Engineering Research Council of Canada (NSERC), the Ontario Graduate Scholarship (OGS), and the Consortium for Cellular and Micro-Cellular Plastics (CCMCP) and the Centre for Biocomposites and Biomaterials Processing (CBBP), University of Toronto.

References (106)

  • C.C. Liao et al.

    Stretching-induced crystallinity and orientation of polylactic acid nanofibers with improved mechanical properties using an electrically charged rotating viscoelastic jet

    Polymer

    (2011)
  • S. Huang et al.

    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

    (2011)
  • Z. Xu et al.

    Morphology, rheology and crystallization behavior of polylactide composites prepared through addition of five-armed star polylactide grafted multiwalled carbon nanotubes

    Polymer

    (2010)
  • Y. Song et al.

    Crystallization behavior of poly(lactic acid)/microfibrillated cellulose composite

    Polymer

    (2013)
  • E. Espino-Pérez et al.

    Influence of chemical surface modification of cellulose nanowhiskers on thermal, mechanical, and barrier properties of poly(lactide) based bionanocomposites

    Eur. Polym. J.

    (2013)
  • A. Pei et al.

    Functionalized cellulose nanocrystals as biobased nucleation agents in poly(l-lactide) (PLLA) - Crystallization and mechanical property effects

    Compos. Sci. Technol.

    (2010)
  • J. Dlouhá et al.

    Cellulose nanofibre–poly(lactic acid) microcellular foams exhibiting high tensile toughness

    React. Funct. Polym.

    (2014)
  • A.N. Nakagaito et al.

    Production of microfibrillated cellulose (MFC)-reinforced polylactic acid (PLA) nanocomposites from sheets obtained by a papermaking-like process

    Compos. Sci. Technol.

    (2009)
  • A. Alemdar et al.

    Biocomposites from wheat straw nanofibers: morphology, thermal and mechanical properties

    Compos. Sci. Technol.

    (2008)
  • B. Wang et al.

    Isolation of nanofibers from soybean source and their reinforcing capability on synthetic polymers

    Compos. Sci. Technol.

    (2007)
  • K.-Y. Lee et al.

    On the use of nanocellulose as reinforcement in polymer matrix composites

    Compos. Sci. Technol.

    (2014)
  • L. Fu et al.

    Present status and applications of bacterial cellulose-based materials for skin tissue repair

    Carbohydr. Polym.

    (2013)
  • A.N. Frone et al.

    Morphology and thermal properties of PLA-cellulose nanofibers composites

    Carbohydr. Polym.

    (2013)
  • J.-M. Raquez et al.

    Polylactide (PLA)-based nanocomposites

    Prog. Polym. Sci.

    (2013)
  • N. Herrera et al.

    Plasticized polylactic acid/cellulose nanocomposites prepared using melt-extrusion and liquid feeding: Mechanical, thermal and optical properties

    Compos. Sci. Technol.

    (2015)
  • A. Iwatake et al.

    Cellulose nanofiber-reinforced polylactic acid

    Compos. Sci. Technol.

    (2008)
  • M. Jonoobi et al.

    Mechanical properties of cellulose nanofiber (CNF) reinforced polylactic acid (PLA) prepared by twin screw extrusion

    Compos. Sci. Technol.

    (2010)
  • Q.T. Nguyen et al.

    An improved technique for exfoliating and dispersing nanoclay particles into polymer matrices using supercritical carbon dioxide

    Polymer

    (2007)
  • S.P. Nalawade et al.

    Supercritical carbon dioxide as a green solvent for processing polymer melts: processing aspects and applications

    Prog. Polym. Sci. (Oxford)

    (2006)
  • A. Ameli et al.

    Electrical properties and electromagnetic interference shielding effectiveness of polypropylene/carbon fiber composite foams

    Carbon

    (2013)
  • A. Ameli et al.

    Through-plane electrical conductivity of injection-molded polypropylene/carbon-fiber composite foams

    Compos. Sci. Technol.

    (2013)
  • A. Ameli et al.

    Polypropylene/carbon nanotube nano/microcellular structures with high dielectric permittivity, low dielectric loss, and low percolation threshold

    Carbon

    (2014)
  • G. Zhang et al.

    Reduced fibre breakage in a glass-fibre reinforced thermoplastic through foaming

    Compos. Sci. Technol.

    (2005)
  • Y.G. Li et al.

    Measurement of the PVT property of PP/CO2 solution

    Fluid Phase Equilib.

    (2008)
  • S.H. Mahmood et al.

    Determination of carbon dioxide solubility in polylactide acid with accurate PVT properties

    J. Chem. Thermodyn.

    (2014)
  • M. Nofar et al.

    Poly (lactic acid) foaming

    Prog. Polym. Sci.

    (2014)
  • M. Nofar et al.

    Comparison of melting and crystallization behaviors of polylactide under high-pressure CO2, N2, and He

    Polymer

    (2013)
  • S.N. Leung et al.

    Mechanism of extensional stress-induced cell formation in polymeric foaming processes with the presence of nucleating agents

    J. Supercrit. Fluids.

    (2012)
  • A. Wong et al.

    Fundamental mechanisms of cell nucleation in polypropylene foaming with supercritical carbon dioxide – effects of extensional stresses and crystals

    J. Supercrit. Fluids

    (2013)
  • Y. Li et al.

    Effects of molten poly(d, l-lactide) on nonisothermal crystallization in stereocomplex of poly(l-lactide) with poly(d-lactide)

    Thermochim. Acta

    (2013)
  • G.Z. Papageorgiou et al.

    Crystallization kinetics and nucleation activity of filler in polypropylene/surface-treated SiO2 nanocomposites

    Thermochim. Acta

    (2005)
  • A. Jeziorny

    Parameters characterizing the kinetics of the non-isothermal crystallization of poly(ethylene terephthalate) determined by d.s.c

    Polymer

    (1978)
  • R.E. Drumright et al.

    Polylactic acid technology

    Adv. Mater.

    (2000)
  • M. Jamshidian et al.

    Poly-lactic acid: production, applications, nanocomposites, and release studies

    Compr. Rev. Food Sci. F

    (2010)
  • E.T.H. Vink et al.

    The eco-profile for current Ingeo® polylactide production

    Ind. Biotechnol.

    (2010)
  • R. Auras et al.

    An overview of polylactides as packaging materials

    Macromol. Biosci.

    (2004)
  • E.T.H. Vink et al.

    The sustainability of natureworks™ polylactide polymers and Ingeo™ polylactide fibers: an update of the future

    Macromol. Biosci.

    (2004)
  • L. Suryanegara et al.

    The synergetic effect of phenylphosphonic acid zinc and microfibrillated cellulose on the injection molding cycle time of PLA composites

    Cellulose

    (2011)
  • L. Suryanegara et al.

    Thermo-mechanical properties of microfibrillated cellulose-reinforced partially crystallized PLA composites

    Cellulose

    (2010)
  • M. Nofar et al.

    Crystallization kinetics of linear and long-chain-branched polylactide

    Ind. Eng. Chem. Res.

    (2011)
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