Development of novel biopolyamide compounds as a green and sustainable alternative to petroleum derived polymers and their applications in medical field

Abstract

Polyamides, widely known as nylons, represent a crucial category of polymers renowned for their versatile properties and extensive applications. With exceptional strength, durability, and heat resistance, polyamides like polyamide 6 (PA6) and polyamide 6.6 (PA66) have become highly preferred materials across various industries. However, their petroleum-derived nature poses significant environmental challenges, including the depletion of finite resources and the accumulation of plastic waste. Therefore, the global need for sustainable alternatives to petroleum-derived polymers has led to extensive research into polymer materials derived from renewable resources and those that are biodegradable. In response, the development of bio-based polyamides, such as polyamide 5.6 (PA56), and their biocomposites have gained great importance in polymer research. The primary objective of this thesis was to develop novel biopolyamide compounds as green and sustainable alternatives to petroleum-derived polymers. Specifically, the research aimed to achieve the enzymatic synthesis of bio-based polyamide 5.6 using monomers from renewable feedstocks, develop novel PA56 composites that can replace petroleum-derived composites currently used in various industrial applications, and introduce antimicrobial activity to bio-based PA56 for its potential applications in the medical field. The thesis focuses on finding sustainable alternatives to overcome the environmental issues caused by the extensive use of petroleum-derived polymers. It underlines the increasing global demand for polymers and the urgent need to prevent the environmental problems arising from this trend, such as finite resource depletion and plastic waste accumulation. Furthermore, the thesis highlighted the present scarcity of PA66 due to the limited production of its key monomer, hexamethylenediamine. This scarcity has driven researchers to explore innovative solutions to meet the growing demand for durable and high-performance polymers. The thesis discusses the potential of utilizing bio-based diamines derived from renewable biomass sources in polymer synthesis. It proposes the development and adoption of bio-based polyamides, particularly focusing on bio-based polyamide 5.6 synthesized from renewable sources like cadaverine, to address these concerns and fulfill the emerging material scarcity in the polymer industry. The first part of the study focused on the properties of bio-based polyamide 5.6 (PA56). A comprehensive investigation of the physical, mechanical, rheological, thermal, and flammability properties of bio-based polyamide 5.6 identified the unique properties of PA56 by highlighting its strong points compared to those of its petroleum-derived counterparts, such as polyamide 6 (PA6) and polyamide 6.6 (PA66). PA56 exhibited the highest water absorption among these polyamides. This high water absorption capacity of PA56 makes it advantageous for textile applications. Additionally, PA56 demonstrated similar mechanical properties to PA66, making it suitable as a high-performance engineering thermoplastic. Its rheological and thermal properties further suggest its potential in applications requiring high flowability and thermal stability. Overall, a detailed investigation of the properties of PA56 revealed its potential as a sustainable alternative to its petroleum-derived counterparts across various industrial applications. In the second part of the study, enzymatic synthesis reactions, catalyzed by enzymes like lipases, were presented as an efficient and environmentally friendly method for producing polyamides. The enzymatic synthesis of bio-based polyamide 5.6 from dimethyl adipate and cadaverine (pentane-1,5-diamine) monomers was achieved using lipase enzyme as a catalyst. This solvent-free, single-step process produced PA56 without intermediates or by-products. Successful enzymatic synthesis of bio-based polyamide 5.6 by utilizing immobilized lipase from Candida antarctica as a biocatalyst, along with a renewable monomer like cadaverine showed their potential for sustainability. The enzymatic synthesis reactions were conducted at 60 °C, 70 °C, and 80 °C for 8, 16, 24, and 48 hours, with lipase enzyme concentrations of 5%, 10%, and 20% (w/w) to evaluate the effects of temperature, time, and enzyme concentration on the synthesis process. The effects of these parameters were investigated through the monomer conversion rate. The study observed that higher monomer conversions were achieved with the higher enzyme concentration, suggesting a positive relationship between enzyme concentration and monomer conversion rate. However, polymerization periods exceeding 24 hours at temperatures above 60 °C adversely affected monomer conversion. The decrease in monomer conversion at high temperatures and prolonged reaction times is attributed to factors such as enzyme denaturation and product inhibition. The average molecular weights of the synthesized polyamide 5.6 were determined using gel permeation chromatography (GPC). To facilitate this analysis, the polyamide 5.6 was modified through N-trifluoroacetylation reaction, which involved reacting its amine groups with trifluoroacetic anhydride to form N-trifluoroacetylated polyamide 5.6 (N-TFA-PA56). This modification improved the solubility of PA56 in tetrahydrofuran (THF), the eluent used for GPC analysis. The highest number average molecular weight (Mn) and weight average molecular weight (Mw) were obtained as 11,900 Da and 20,300 Da, respectively for the reaction with enzyme concentration of 20% (w/w), a temperature of 70 °C, and a reaction time of 24 hours. Therefore, these reaction conditions were identified as optimal for lipase catalyzed synthesis reaction of bio-based polyamide 5.6. Instrumental analysis techniques including Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) were applied to the enzymatically synthesized polyamide 5.6 products for further characterization. These analytical methods verified the successful enzymatic synthesis of PA56 and highlighted its potential as a bio-based alternative to petroleum-derived polyamides. The third part of the study involved developing novel PA56 and PA66 composites with improved properties. Polymer compounding by extrusion method was performed to tailor the properties of polyamide 5.6. Formulation studies employing glass fibers and flame retardants led to the development of novel bio-based composites with improved properties desired for specific applications in various industries. In these formulations, glass fibers and flame retardants enhanced the mechanical and flammability properties of polyamide 5.6, respectively.