Polylactide/organically modified montmorillonite composites; effects of organic modifier on thermal characteristics

https://doi.org/10.1016/j.polymdegradstab.2016.09.028Get rights and content

Abstract

The effects of interspace distance and the possible chemical interactions between PLA and the organic modifier of montmorillonites, Cloisite 15A, 20A and 30B on thermal degradation of PLA in the absence and presence of water vapor were investigated by direct pyrolysis mass spectrometry, (DP-MS) in addition to XRD, TEM, DSC, TGA analyses. The DP-MS results clearly showed that the way in which the polymer was incorporated into the nanocomposite strongly depends on the mixing technique, the interspacing between the clay layers and mostly on the type of the organic modifier used. Strong evidences for interactions of the organic modifier of Cloisite 30B with PLA chains resulting in exfoliated structure were detected. In the presence of water vapor, the relative yields of products due to trans-esterification and cis-elimination reactions were diminished. Thermal degradation of PLA nanocomposites was shifted to low temperatures as the partial pressure of H2O was increased.

Introduction

Polylactide, (PLA) is a promising alternative to petroleum based polymers, and can be produced from renewable sources. However, some PLA properties such as low elongation at break, poor flexibility and toughness, low thermal stability, and slow nucleation and crystallization limit its commercial uses. In order to enhance the mechanical and thermal properties needed for several applications, nanofillers such as layered silicates or carbon nanotubes are incorporated into polymer matrixes. One of the common applications to overcome these problems is the incorporation of clay minerals in PLA matrix [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. As natural nanoclay is hydrophilic, and by exchanging ions present in between layers with various organic cations, clay can be compatibilized with a wide variety of hydrophobic polymers. It has been determined that thermal degradation of polylactide (PLA) and PLA/organo-modified clay, Cloisite 30B, methyl tallow bis-2-hydroxyethyl ammonium modified montmorillonite, nanocomposites took place during processing via internal mixer and twin-screw extruder and the organic modifier was believed to accelerate the thermal degradation of the PLA matrix [1], [2]. It has been proposed that addition of organically modified clays into the PLA matrix although improves the mechanical, and barrier characteristics and biodegradability, yet, promotes the degradation rates [4], [5]. The influence of organic modifier of the clay (Cloisite 30B, Cloisite 15A and Dellite 43B) on thermal behavior of PLA based nanocomposites was studied by Araju and coworkers [6]. Incorporating Cloisite 30B (C30B) in PLA matrix was investigated as a function of the content and the hydration state of the nanoclays [7], [8]. The nanocomposites containing undried C30B exhibited better degrees of dispersion and exfoliation [7], [8]. On the other hand, it has been determined that the decrease in molecular weight is smaller for PLA nanocomposites compared to pure PLA submitted to thermo-oxidative degradation. Polymer–organoclay interaction, morphology and rheological response, under shear, and elongational flow, of different melt compounded PLA nanocomposites involving montmorillonites organically modified by Cloisite 30B and Nanofil SE3010 were investigated [15]. Higher clay dispersion and exfoliation levels were obtained for polylactide nanocomposites prepared involving C30B. It has been proposed that the stronger polar interactions between the phases in polylactide samples filled with C30B, determine increments in elongational viscosity, but inhibit the strain hardening behavior. In a recent study, chemical compatibility between PLA and Na-montmorillonite (MMT), was improved by organically modifying the surface of Na-MMT first by cetyl trimethyl ammonium bromide and resorcinol bis(diphenyl phosphate) using ion-exchange and adsorption technique [16]. The homogenous dispersion of co-modified MMT in PLA matrix resulted in not only remarkable enhancements in the storage modulus but also improvements in both thermal stability and fire resistance [16]. These improvements were attributed to both the physical barrier effect of the MMT nanosheets and charring effect of the co-modified MMT. Pochan and Krikorian investigated the effect of organic modifiers derived from Cloisite 30B, Cloisite 25A and Cloisite 15A on the extent of clay modification during the formation of PLA nanocomposite [17], [18]. TEM and XRD studies revealed that Cloisite 30B was the best layered silicate among them, leading to a significant incorporation of PLA chains into layered silicates. This behavior was associated with the interaction between diols present in the organic modifier with Cdouble bondO bonds present in PLA backbone [17]. The influences of geometry of nanoparticles, spherical SiO2, rod-like halloysite, (HNT) and plate-like organically modified montmorillonite, (OMMT) on fire behavior, mechanical and thermal properties of PLA containing aluminium diethylphosphinate (PLA/AlPi) were also investigated [19], [20]. It has been determined that, among these nanoparticles, OMMT was most effective not only in reduction in fire risks suppressing both heat release and mass loss rates but also in enhancement of thermal stability.

We studied morphological and thermal characteristics of PLA/montmorillonite nanocomposites, namely PLA/Cloisite 15A (C15A), PLA/Cloisite 20A, (C20A) and PLA/Cloisite 30B (C30B) systematically by applying direct pyrolysis mass spectrometry in addition to classical techniques such as XRD, TEM, SEM, DSC and TGA. Furthermore, decomposition of PLA composites in the presence of water vapor was evaluated. Although, several studies have been reported PLA/organically modified montmorillonite composites, to our knowledge, the effects of clay and organic modifiers on the thermal degradation product distribution in the PLA/OMMT composites in the presence and absence of water vapor has not been investigated. The application of direct pyrolysis mass spectrometry (DP-MS) allowed investigation of product distribution based on the type of the organic modifier and the interspace between the clay platelets and give strong evidences for the type of chemical interactions between Cloisite 30B and PLA chains.

Section snippets

Materials

Polylactide, PLA, (Mn ∼ 190000), was purchased from Cargill Dow. Organically modified montmorillonites, dimethyl ditallow ammonium modified montmorillonites, Cloisite 15 A and 20A (C15A and C20A) and methyl tallow bis-2-hydroxyethyl ammonium modified montmorillonite Cloisite 30B (C30B), were provided by Southern Clay Products Inc. The characteristics of montmorillonites are summarized in Table 1. PLA/clay composites (1, 3 or 5 wt% of inorganic content) were either melt-compounded using a DSM

Morphology of PLA/montmorillonite composites

TEM and XRD measurements were used to determine the dispersion states of the Cloisites 15A, 20A and 30 B in the PLA matrix. The increase of the interlayer distance defined as the difference between interlayer spacing in the nanocomposite and in the corresponding organo-modified clay-depends on the clay type and on its content, The diffractogram of Cloisite 15A shows three peaks at around 2θ = 3.34, 5.18 and 7.72° corresponding to inter-reticular distances of d001 = 2.64, 1.70 and 1.14 nm,

Conclusion

The morphological analyses of PLA/montmorillonite nanocomposites indicated presence of intercalated and exfoliated structures. Both XRD and TEM results indicated better dispersion of C30B in PLA matrix. The organic modifiers were lost at lower temperature regions during the pyrolysis of PLA composites involving C15A and C20A confirming the increase in interlayer spacing. For PLA/C30B, the loss of organic modifier was detected in the temperature range where PLA decomposition took place and the

Acknowledgments

This work is partially supported by TUBITAK Research Fund 112T493.

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