Processing of polylactide nanofibrous and film structures: Effects of polymer blending and bio-additives on structure-property relationships

Abstract

The shift from fossil-based polymers to biopolymers is crucial for achieving sustainability and reducing environmental pollution caused by non-biodegradable thermoplastic polymers. Among biopolymers, polylactide (PLA) stands out as the most commercially available and widely studied since it is derived from renewable resources, i.e., cornstarch and sugarcane, is biodegradable, and has good mechanical and barrier properties. PLA offers a promising alternative to conventional thermoplastic such as polystyrene and polyethylene terephthalate in various engineering applications, such as food packaging, biomedical devices, filtration, and technical textiles. However, PLA also has certain limitations, including its brittleness, low melt strength, and slow crystallization rate. These drawbacks can be addressed by blending PLA with soft polymers, incorporating plasticizers, or reinforcing it with nanoparticles or additives. The aim of this thesis is to tailor the properties of PLA-based nanofibrous and polymer film structures by incorporating bio-based additives into the polymer matrix and to comprehensively analyze and interpret the structure-property relationships of the developed materials. In this context, it is also aimed to establish know-how on the development of polymer solution formulations containing bio-based additives, and the investigation of the suitable organic solvent systems, and the concentrations of the incorporated bio-based additives. In order to develop materials by processing the polymer solution formulations with and without the bio-based additives, electrospinning, solution casting, and melt processing techniques were selected due to several factors: Electrospinning is commonly employed in laboratory-scale studies, offers pilot-scale production capabilities, and holds promise for industrial application; meanwhile, solution casting and melt processing are widely used techniques for industrial-scale processing of PLA. This PhD thesis presents a comprehensive investigation into the design, processing, and functionalization of PLA-based materials, aiming to enhance their performance, versatility, and applicability in sustainable material systems. By integrating four interconnected studies, the thesis systematically explored how both intrinsic factors— such as D-lactide content and molecular weight—and extrinsic parameters—such as solvent systems, blend composition, and nanoparticle incorporation—govern the processability and final properties of PLA-based materials. The optimization of binary solvent systems for improved CNC dispersion reveals key strategies for enhancing the quality of PLA/CNC nanocomposites via solution casting method. The development of electrospun PLA/PBAT nanocomposite webs with controlled CNC loadings further elucidates the impact of multicomponent formulations on the structural performance of biodegradable fibrous webs. Finally, the incorporation of natural anthocyanins into PLA nanofibers introduces a novel approach to stimuli-responsive material design, enabling the fabrication of colorimetric pH sensors with potential use in smart packaging. Collectively, the thesis provides new scientific and technical insights into the design of sustainable polymer systems tailored for various material applications. In the first research paper, the main objective was to explore the influence of D-lactide content and the molecular weight of PLA on its electrospinning behavior since these are critical parameters that significantly impact its crystallizability, processability, and final properties. Especially, while the effect of D-lactide content on PLA processability has been extensively investigated in extrusion, thermoforming, foaming, and melt spinning, its role in electrospinning remained unexplored until this study. Accordingly, the electrospinnability of three different grades of PLA: two amorphous grades with high (aPLA-H) or low molecular weight (aPLA-L) and a semicrystalline grade (cPLAH) with a high molecular weight were assessed. Due to its high crystallizability and molecular weight, cPLA-H produced coarser nanofibers, particularly in solvent systems with high CHL content (≥75%). aPLA-H yielded coarser nanofibers compared to aPLA-L, attributed to its higher molecular entanglement. The electrospun cPLA-H mats exhibited the highest storage modulus (~15 MPa), which can be ascribed to their elevated crystallinity (~37%), whereas aPLA-L displayed the lowest storage modulus (~10 MPa) due to its amorphous nature and reduced molecular entanglement. One of the most significant findings obtained from this fundamental study is that the nanofibrous webs developed using semicrystalline PLA exhibit better mechanical properties compared to those developed using amorphous PLAs. Another key finding is that the use of semicrystalline PLA caused several challenges both during the preparation of the polymer solution, such as the slow dissolution of the polymer in the solvent, and during the electrospinning process, including gelation of the polymer solution inside the syringe and rapid solidification at the needle tip, leading to clogging issues. Based on these findings, the following approach has been adopted for the rest of the study: If the objective is to achieve superior mechanical properties or further enhance them, semicrystalline PLA grades will be preferred. On the other hand, for the development of materials where mechanical properties are not the primary concern, amorphous PLA grades will be selected due to their ease of processing. A cost-effective and efficient strategy to enhance the processability of PLA involves incorporating nanoparticles as reinforcing fillers, leading to the development of PLAbased nanocomposites. CNCs, bio-based and biodegradable polysaccharide nanoparticles, offer advantages such as low density, high surface area, and excellent mechanical properties, making them promising for eco-friendly nanocomposites. Due to their unique properties, CNCs have potential applications in various fields, including reinforcement, barrier films, biomedical implants, drug delivery, nanofibers, and technical textiles. In the second research paper, semicrystalline PLA grades, having different molecular weight, were employed. The effects of PLA molecular weight, categorized as high (HPLA), medium (MPLA), and low (LPLA), and the blend ratio of dichloromethane (DCM)/dimethyl sulfoxide (DMSO) on the dispersion quality of cellulose nanocrystals (CNCs) in solution-cast PLA/CNC nanocomposite films were analyzed via small amplitude oscillatory shear rheology. Poor CNC dispersion was observed in nanocomposites prepared with 100% DCM due to its low dielectric constant, whereas increasing the DMSO content (50% v/v) improved CNC dispersion. LPLA exhibited superior CNC dispersion, while HPLA hindered nanoparticle diffusion, resulting in poorer dispersion in HPLA. The influence of CNC dispersion on fiber formation was also explored, revealing that incorporating 1 wt.% CNCs in LPLA led to finer (~1200 nm) and more uniform fibers. In the third research paper, LPLA grade was chosen to be employed on purpose since our previous study revealed that CNCs were dispersed better in this PLA grade, which has a high melt flow rate (MFR) and a low molecular weight, compared to its high molecular weight counterparts. Accordingly, this study presents, for the first time, the electrospinning of CNC-loaded nanofibers from blends of LPLA and poly(butylene adipate-co-terephthalate) (PBAT) polymers. The polymer blend ratio and the solvent composition of DCM/ DMSO were optimized to ensure smooth electrospinnability and the successful formation of uniform, bead-free nanofibers. The findings revealed that well-dispersed CNCs significantly enhanced mechanical, thermal, and wettability properties, even at a low concentration of 1 wt.%. Further explore the use of PLAs in novel applications such as colorimetric nanofibrous pH sensors for food packaging, in the fourth research paper, unlike previous studies, the primary objective was to functionalize PLA-based nanofibrous webs through the incorporation of a bio-additive. This study focused on developing bio-based pH indicators using nanofibrous webs composed of amorphous PLA as the host polymer and anthocyanin, extracted from black carrot, as a natural pH-sensitive colorant. The high surface area of the electrospun nanofibrous webs enhanced the interaction between dye molecules and the surrounding medium. The colorimetric sensing was evaluated by treating the webs with buffer solutions, with colorimetric changes observed both visually and via spectrophotometric measurements. Nanofibrous webs containing 3 wt.% anthocyanin exhibited distinct color transitions from reddish pink/dark purple to pinkish gray upon pH exposure. Notably, 50% of the samples had a total color difference (ΔE) exceeding 5, while 30% displayed ΔE values greater than 12. These findings suggest that anthocyanin-loaded nanofibrous webs have potential for pH-sensitive applications, particularly in food packaging, where monitoring pH variations is crucial. In light of the findings from the published studies as summarized above, new research studies have been launched. The experimental phases of these ongoing studies are still in progress; however, the scope of the works and preliminary results obtained so far were shared in the last chapter of this thesis as not yet unpublished studies. The first study, given under this chapter, aims to focus on examining how adding CNCs affects especially the mechanical strength of PLA/PBAT nanofibrous webs. In this context, a broader and more systematic experimental framework was implemented with the objective of enhancing the mechanical properties of the fabricated nanofibrous webs, thereby assessing their suitability for diverse applications including biomedical scaffolds, active and intelligent food packaging, etc. The second study aims to focus on the development of biopolymer-based nanocomposites through melt processing. Accordingly, PLA/CNC masterbatches, formulated using the PLA grade that demonstrated superior CNC dispersion were blended with PBAT via melt mixing to develop bio-nanocomposites. The resulting compounds were then fabricated into compression-molded samples, targeting the food packaging applications, where mechanical performance, barrier properties and biodegradability are crucial. This study emphasizes the critical role of enhancing dispersion efficiency to ensure homogeneous distribution of nanofillers within the polymer matrix. Overall, these studies aim to advance the development of fully renewable polymeric systems, underscoring the crucial function of biopolymer-based nanocomposites in promoting the transition toward a sustainable and circular economy.