Degradation kinetics of starch-g-poly(phenyl methacrylate) copolymers

Highlights

• Degradation kinetics of starch-g-poly(phenyl methacrylate) copolymers were studied.

• Pyrolysis of copolymers is complex and takes place in two large stages.

• The initial degradation at ≤300 °C involves two-step consecutive reaction.

• The structural graft depolymerization at temperatures higher than 320 °C is occurred.

• The E(α) values are dependent on α, the amount and grafting efficiency.

Abstract

Both the thermal behavior and kinetic decomposition of starch-g-poly(phenyl methacrylate) copolymers were studied by the TG/DSC/FTIR/QMS coupled technique under inert conditions. In order to compare the results, potato starch was used as a reference material. The results indicate that the thermal degradation takes place in two large stages: (i) destruction of the molecular polysaccharide blocks at temperatures ≤300 °C and (ii) structural graft depolymerization from 320 to 440 °C. The kinetic analysis in terms of E(α) on α indicates that during the initial stage, with activation energies ranging from 140 to 205 kJ mol⁻¹, a two-step reaction mechanism involving two consecutive reactions takes place, an initial endothermic reversible reaction followed by an irreversible one, which is related to the cracking of the starch glycosidic bonds. Meanwhile the degradation analysis of the graft polymer reveals that the stage involves several overlapping steps with average activation energies between 120 and 180 kJ mol⁻¹.

Introduction

Nowadays, one of the most intensively studied trends in polymer chemistry is searching for new materials based on either natural or renewable resources with improved structural, physical and chemical properties, which can practically replace the petrochemical polymers used in many fields such as medicine, pharmaceutics, food, cosmetic, packing, textile, paper, plastic industries, etc. The development of novel environmentally friendly materials is of increasing interest for both academic and industrial research. Such materials can be an alternative to non-degradable synthetic polymers since their use reduces the preparation cost of final products and limits the accumulation of plastic wastes in the environment, thus reducing the environment pollution [1], [2], [3], [4], [5], [6]. Among natural, carbohydrate polymers, which can be modified in order to produce novel materials with improved properties, polysaccharides such as starch and its derivatives coming from different botanical sources have currently gained increased attention over other natural and synthetic biodegradable polymers due to superior characteristics such as especially very low costs of the raw materials, biodegradability and availability [3], [7]. In order to enhance inert properties of starch such as high hydrophilic nature, low stability in acidic environment, low moisture resistance, poor processing, etc., which drastically limit its utilization, both physical and chemical modification methods have been developed. The chemical modification of starch is generally achieved through hydrolysis, oxidation, esterification, etherification, phosphorylation, dextrination, cross-linking or grafting [3], [8], [9], [10], [11], [12]. The modification of natural-based-polymer backbones enables the researchers to obtain many compatible and biodegradable materials with either completely modified or new physical and chemical properties that are different from those featured by raw starches, which can find their place in practical applications [13], [14], [15], [16], [17], [18], [19].

As it is already known, the chemical structure of carbohydrates is dramatically altered by thermal treatments. According to a literature survey, many studies have been carried out in order to evaluate the thermal decomposition pathways and degradation kinetics of different botanical-origin starches and starch-based materials by means of different thermal methods, e.g. DSC, TG/FTIR, TG/MS and other analytical techniques such as NMR, MS, and GC. In this sense, it should be pointed out that the thermal behavior of starch depends on many factors such as amylose/amylopectin content, crystallinity, botanical source, atmosphere, etc. The decomposition mechanism of starch is complex and as temperature is increased, it occurs in two or three main stages depending on the established conditions and it is directly connected with the emission of some gaseous decomposition products and tar or char residues [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]. Regardless of the numerous studies on the thermal behavior of starch, it could be concluded that the thermal decomposition of starch under inert conditions includes the thermal condensation between hydroxyl groups of starch chains, dehydration of neighboring hydroxyl groups in the glucose ring, cleavage of glycosidic bonds and breakdown of the glucose ring resulting in the liberation of water molecules and some low molecular mass volatile decomposition products. As the temperature is raised (up to ca. 400 °C), a highly cross-linked system is formed, which at high temperatures, undergoes further carbonization reactions, generating amorphous carbon structures [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34].

However, during the modification of starch, both the physical blending and covalent bonding with other compounds exert a considerable influence on the thermal decomposition profile and, as a consequence, on the degradation mechanism and kinetics of the obtained novel starch-based materials [24], [32], [35], [36], [37], [38], [39]. As a result, the synthesis of novel polymeric materials requires not only the evaluation of their structure and basic properties but also the knowledge of the chemical structure changes that occur out of increased temperatures, the thermal reaction pathways and degradation kinetics of the novel materials, which are essential factors that play a fundamental role in the understanding of the thermal behavior and physical properties of the materials. In order to evaluate the possible final and practical applications of novel materials, it would be helpful to control both their influence on the environment pollution when manufactured at high temperatures and the quality of the final products as well as the optimization of their processing. According to the aforesaid, the main objective of the present paper is to evaluate the thermal behavior, decomposition mechanism and decomposition kinetics of novel starch-g-poly(phenyl methacrylate) copolymers which can find their place as more environmentally friendly polymeric materials in the food, paper, textile or plastic industries. The detailed studies on the thermal decomposition profile, decomposition mechanism and activation energy E(α) data were performed under inert atmosphere by means of the simultaneous TG-DSC-FTIR-QMS analysis. As a reference material, raw potato starch was chosen in order to compare and discuss the results.