For several decades, polylactic acid (PLA) has been extensively investigated as a potential alternative to commercial petroleum-based polymers and plastics [1]. Nevertheless, PLA suffers from a set of drawbacks that include poor thermal stability, low heat distortion temperature, low crystallization ability, high cost, and high brittleness, among others factors that limit its widespread application [2]. It has been reported that blending PLA with other rubbery polymers may not only benefit its ductility and toughness but also widen its processing window [3]. Moreover, it is hypothesized that the addition of nanoparticles to PLA may produce synergistic effects that improve thermal stability, heat distortion, solvent resistance, and increase its rate of biodegradation [3].

In this study, PLA (PLE 005-1, NaturePlast, France) is enhanced by a) blending it with polybutylene adipate terephthalate (PBAT), a biodegradable rubbery biopolyester (PBE-006 NaturePlast, France), at selected ratios, b) adding a selected coupling agent (Lotader AX-8900, Terpolymer: ethylene-methyl acrylate-glycidyl methacrylate), and c) dosing a combination of cellulose nanofibrils (Freeze-dried experimental grade powder, University of Maine, CNF) and graphitic carbon nitride (in-house synthesized g-C₃N₄) [4] at chosen concentrations determined by a detailed design of experiment (DOE). The blends and compounding were conducted by high-speed extrusion using a PA11 Thermofisher Scientific Twin-screw Extruder (11-mm in diameter and L/D ratio of 40) at optimized processing conditions and screw configuration (i.e., maximum specific energy of mixing).

The neat PLA (our control, tested dried as molded, ASTM D638 Type V) exhibited a Young’s modulus (YM), an ultimate tensile strength (UTS), an elongation-at-break (EB), and a toughness value of 3.955±0.398 GPa, 75.97±5.38 MPa, 2.93±0.26%, and 1.380±0.19×10⁶ J/m³, respectively. Interestingly, at an 80/20 PLA/PBAT ratio, the blends displayed an EB and toughness value of 125.80±16.08%, and 65.05±9.13×10⁶ J/m³, respectively. These values correspond to about a 40-fold and 50-fold increase with respect to the control’s properties, respectively. In addition, this blend ratio retained virtually all the stiffness and strength of the pure PLA. Significant phase separation occurred between the PLA and PBAT domains as observed by SEM, which may be responsible for the effective toughening effect attained ^[5].

The next stage of this investigation will include the addition of the coupling agent to engineer the polymeric domains’ interphase and that of the nanoparticles to achieve compatibility and functionality in the blends. The experimental methodology for this study will be carried out following a detailed DOE that will first analyze the responses using an optimal custom response surface methodology (RSM) with a point exchange algorithm [6] and I-optimality criterion (I-OC) [6, 7, 8]. Secondly, the influence of g-C₃N₄ and CNF nanoparticles will be studied by DOE optimal blend point using a combined optimal custom design of mixture and process to predict synergia as well as the individual impact of the nanoparticles. Thus, by using I-OC [6, 7] with point exchange algorithm, the optimal g-C₃N₄/CNF ratios and concentration levels for the mechanical, thermal, microstructural and biodegradability responses will be predicted. In this opportunity, we intend to present the latest work on this processing approach for the enhancement of PLA biopolymer.

References

(1) Sharma, A. A. Singh, A. Majumdar and B. S. Butola, "Harnessing the ductility of polylactic acid/ halloysite nanocomposites by synergistic effects of impact modifier and plasticiser," Composites Part B, pp. 1-10, 2020.

(2) Russo, D. Acierno and G. Filippone, "Mechanical performance of polylactic based formulations," in Biocomposites: Design and Mechanical Performance, Woodhead Publishing Series in Composites Science and Engineering: Number 61, 2015, pp. 19-20.

(3) Misra, J. Pandey and A. Mohanty, Biocomposites: Design and Mechanical Performance, Woodhead Publishing Series in Composites Science and Engineering: Number 61, 2015.

(4) Majdoub, A. Amedlous, Z. Anfar and A. Jada, "Engineering of H-Bonding Interactions in PVA/g-C3N4 Hybrids for Enhanced Structural, Thermal, and Mechanical Properties: Toward Water-Responsive Shape Memory Nanocomposites," Advanced Materials Interfaces, pp. 1-13, 2022.

(5) Lai, J. Li, P. Liu, L. Wu, S. J. Severtson and W.-J. Wang, "Mechanically reinforced biodegradable Poly(butylene adipate-co-terephthalate) with interactive nanoinclusions," Polymer, pp. 1-7, 2020.

(6) C. MONTGOMERY, Design and Analysis of Experiments, 9 ed., John Wiley & Sons, 2007, pp. 550-554.

(7) H. Myers, D. C. Montgomery and C. M. Anderson-Cook, Response Surface Methodology: Process and Product Optimization Using Designed Experiments. THIRD EDITION, WILEY SERIES IN PROBABILITY AND STATISTICS, 2009, pp. 369-371.

(8) J. Anderson and P. J. Whitcomb, RSM Simplified: Optimizing Processes Using Response Surface Methods for Design of Experiments, Second Edition 2nd Edition, Second ed., Productivity Press, 2016, pp. 137-152.