MACROMOLECULES RHEOLOGICAL MODELLING

MACROMOLECULES RHEOLOGICAL MODELLING Athan Labropoulos. S. Anestis and Shorena Bulbulashvili anestis@essolutions.eu, athanlab@teiath.gr ABSTRACT Food macromolecules are commonly used as ingredients in several food product formulations changing their viscosity and consequently changing the flow properties of the food systems and giving the products distinct functional properties, i.e. desirable texture and sensorial characteristics. Their use in the food and pharmaceutical areas has been in an increased rate the last years. Hydrocolloids play an important role in microencapsulation of bioactive compounds and in controlling rheological properties of the food. Searching and knowing the relation between the individual components of these systems, the resulting microscopic structure and ultimately their rheological properties can determine their functional activity and find better applications. Therefore, it is a challenge to link macroscopic properties of materials with their structural conformation. However, molecular modelling offers the opportunity to allow the exploration of new applications for food polymeric systems. Studying molecular dynamics simulations, we investigate how the stiffness of the chains affects the formation of a percolated space-filling network. Experimental rheological data for polymeric solutions with different persistence length, in practice describes the polymer flexibility. Keywords: Rheology, Viscosity, modelling, macromolecules; modelling 1. INTRODUCTION Macromolecules, such as proteins and polysaccharides, are commonly used as ingredients in several food product formulations. Their use has increased in the recent years and their applications in the food technology area can be considered countless. Their ability of building up viscosity and consequently changing the flow properties of systems containing water as solvent make them unique ingredients with distinct functional properties to provide desirable texture and sensorial characteristics [1]. In the pharmaceutical and biological areas, hydrocolloids play an important role in microencapsulation of bioactive compounds [2] and in controlling drug delivery [3]. Therefore, knowing the relation between the individual components of these systems, the resulting microscopic structure and ultimately their rheological properties can determine their functional activity and better address their applications. Several works studied long chain polymers in foods; however most of them have only focused on experimental characterization. In addition to that, it is still a challenge to link macroscopic properties of materials with their structural conformation and types of linkages. Molecular modelling offers the opportunity to bridge this gap and allow the exploration of new applications for food polymeric systems. The objective of the present work was to perform computational and experimental studies of polymeric chains that would form an entangled network with enhanced rheological properties caused by physical interactions among the chains. Using molecular dynamics simulations, we investigate how the stiffness of the chains affects the formation of a percolated space-filling network. We also provide experimental rheological data for two polymeric solutions with different persistence length, which in practice describes the polymer flexibility. 2. MATERIALS & METHODS To study how the stiffness of large chain molecules affects the rheology of polymeric systems, food grade Xantan Gum (XG, Keltrol, CPKelco) and Hydroxypropyl cellulose (HPC, Klucel HF Pharma, Hercules Inc.) were used. They both are high molecular weight polysaccharides with cellulosic backbones. However, the persistence length of XG is 120nm whereas the persistence length of HPC is 5-10nm, which makes XG a much stiffer polymeric chain. The polymeric solutions were characterized by using a rotational AR-G2 Rheometer (TA Instruments, Delaware, USA). Molecular dynamics (MD) simulations were performed to study the aggregation process of large polymeric chains. The force on each particle of the model polymer is calculated by adding bonding and non-bonding contributions. The polymer chains are modelled as a string of N beads, connected via a FENE potential. The chain flexibility is controlled by an angular harmonic potential acting on the angle defined by three neighbouring beads. 3. RESULTS AND DISCUSSIONS Macromolecules were studied in 1920 by Nobel laureate Hermann Staudinger where he only mentions high molecular compounds meaning excess of 1,000 atoms [4]. The phrase polymer, introduced by Berzelius in 1833, had a different meaning from that of today; it simply was another form of isomerism and had little to do with size [5]. The four types of macromolecules (or very large molecules), are nucleic acids, proteins, lipids and carbohydrates The term to describe large molecules varies among the disciplines. For example, in biology refers to macromolecules as the four large molecules comprising living things and in chemistry the term refer to aggregates of molecules held together by intermolecular forces rather than covalent bonds but which do not readily dissociate [6]. According to the standard IUPAC definition, the term macromolecule as used in polymer science refers only to a single molecule. For example, a single polymeric molecule is appropriately described as a "macromolecule" or "polymer molecule" rather than a "polymer", which suggests a substance composed of macromolecules [7]. The structure of simple macromolecules, such as homo-polymers, may be described in terms of the individual monomer subunit and total molecular mass. Bio-macromolecules, require multifaceted structural description such as to describe proteins. Thus, "macromolecules" tends to be nanoparticles called "high polymer" [8] Different behaviour is observed for Xanthan Gum and HPC systems (Figure 1). For Xanthan Gum, independent on the concentration, the storage modulus G’ is higher than the loss modulus G”. This indicates that this system may exhibit a solid-like behaviour in the investigated frequency range. In addition, it also shows that both storage and loss moduli are not significantly affected by frequency. On the other hand, HPC solutions did exhibit a typical behaviour of viscoelastic liquids, except for 1% HPC system. In order to further investigate the effect of chain flexibility on their rheology, molecular dynamics simulations were performed. Figure 1 & 2 shows a snapshot of the two entangled networks formed by systems with different angular potential. Small amplitude oscillatory shear (SAOS) tests were also performed to obtain the rheological properties of the systems (Figure 3). Frequency (Hz) Figure 1. Different behaviour is observed for Xanthan Gum Figure 2. Different behaviour is observed for HPC systems Figure 3.Small amplitude oscillatory shear (SAOS) tests REFERENCES 1. 12.Gullapalli, S.; Wong, M.S. (2011). "Nanotechnology: A Guide to Nano-Objects" (PDF). Chemical Engineering Progress. 107 (5): 28–32. 2. Borgogna, M., et al. 2010. Food microencapsulation of bioactive compounds: Rheological and thermal characterisation of non-conventional gelling system. Food Chemistry. 122(2): p. 416-423. 3. Foegeding, E.A. 2007. Rheology and sensory texture of biopolymer gels. Current Opinion in Colloid & Interface Science. 12(4-5): p. 242-250. 4. Jenkins, A. D.; Kratochvíl, P.; Stepto, R. F. T.; Suter, U. W. (1996). 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