Promoting Interfacial Interactions with the Addition of Lignin in Poly(Lactic Acid) Hybrid Nanocomposites.

Bindu Patanair,Martin George Thomas,Hanna J. Maria,Sabu Thomas,Poornima Parathukkamparambil Pundarikashan,Nicolas Delpouve,Varsha Krishna Kumar,Kalaprasad Gopalan Nair,Allisson Saiter-Fourcin

doi:10.3390/polym13020272

Abstract

In this paper, the calorimetric response of the amorphous phase was examined in hybrid nanocomposites which were prepared thanks to a facile synthetic route, by adding reduced graphene oxide (rGO), Cloisite 30B (C30B), or multiwalled carbon nanotubes (MWCNT) to lignin-filled poly(lactic acid) (PLA). The dispersion of both lignin and nanofillers was successful, according to a field-emission scanning-electron microscopy (FESEM) analysis. Lignin alone essentially acted as a crystallization retardant for PLA, and the nanocomposites shared this feature, except when MWCNT was used as nanofiller. All systems exhibiting a curtailed crystallization also showed better thermal stability than neat PLA, as assessed from thermogravimetric measurements. As a consequence of favorable interactions between the PLA matrix, lignin, and the nanofillers, homogeneous dispersion or exfoliation was assumed in amorphous samples from the increase of the cooperative rearranging region (CRR) size, being even more remarkable when increasing the lignin content. The amorphous nanocomposites showed a signature of successful filler inclusion, since no rigid amorphous fraction (RAF) was reported at the filler/matrix interface. Finally, the nanocomposites were crystallized up to their maximum extent from the glassy state in nonisothermal conditions. Despite similar degrees of crystallinity and RAF, significant variations in the CRR size were observed among samples, revealing different levels of mobility constraining in the amorphous phase, probably linked to a filler-dimension dependence of space filling.

Highlights

The bio-renewability, ease of processing, and suitability for mechanical recycling [1], as well as decent mechanical properties, make poly(lactic acid) (PLA) among the best substitutes to polymers derived from petroleum sources [2]
In a previous study on PLA/clay nanocomposites [34], we reported that exfoliated systems exhibit an increase in the cooperative rearranging region (CRR) size in comparison to neat PLA, in agreement with the observations of Chen et al on polystyrene/clay nanocomposites [23], whereas intercalation is characterized by a decrease in the CRR size
Our results show accelerated degradation with the addition of Cloisite 30B (C30B) in contrast to the reported stabilizing effects [47]

Summary

Introduction

The bio-renewability, ease of processing, and suitability for mechanical recycling [1], as well as decent mechanical properties, make poly(lactic acid) (PLA) among the best substitutes to polymers derived from petroleum sources [2]. Nanocomposites, including nucleophilic nanofillers like nanoclay, graphene oxide (GO), and MWCNT, in PLA exhibit better protection against UV radiation [6], improved Young’s modulus, antibacterial activity, potential use in biomedical applications [7], and increased thermal conductivity, as well as mechanical and electrical properties [8]. We used MT-DSC in this study to characterize PLA/lignin nanocomposites containing nanosheets (rGO), layer-like nanoclays (C30B), or nanotubes (MWCNT). This set of nanofillers was chosen in order to highlight possible shape and size effects influencing both the morphological and thermal properties of the hybrid nanocomposites. Since a number of the properties of PLA depend on its degree of crystallinity, the impact of the nanofiller choice on the crystallization kinetic was reported

Materials and Methods

Conclusions