Testing Classical Approach to Polymer Solutions on SAXS Data of λ-Carrageenan, κ-Carrageenan and Methylcellulose Systems.

Jure Cerar

doi:10.17344/acsi.2015.1383

Abstract

In this paper we present the performance of the classical approach to polymer solutions in evaluation and interpretation of the experimental SAXS data obtained for aqueous solutions of two gelling polysaccharides κ-carrageenan and methylcellulose and a non-gelling polysaccharide λ-carrageenan. In a systematic structural SAXS study of various types of polymer solutions we pointed out and discussed the issues encountered and connected to the fact that the studied gels are obviously not "homogeneous gels" in terms of the structural details that the SAXS technique can resolve and the issues connected to the limited experimental resolution of the SAXS technique for such systems. In parallel the necessary modifications of the classical approach equations for the evaluation of the SAXS data are discussed. Furthermore, the detailed structural results of the studied aqueous polymeric systems in liquid state, during the onset of the gelation, and even in the gel state are presented.

Highlights

Methylcellulose (MC) and κ-carrageenan (KC) are well known food additives (E407 and E461, respectively) that are commonly used as gelling, thickening and stabilizing agents
Even though there are some similar studies available on the small-angle X-ray scattering (SAXS) data nowadays, we still miss thorough systematic reports on such analysis of experimental SAXS data. The lack of such studies could be reasoned by the necessity of some additional considerations that must be dealt with the SAXS data, i.e. the experimental broadening effect, and by expectedly lower experimental resolution of the SAXS technique in respect to SANS
We interpret this drop in the forward scattering intensity as the consequence of the decreasing intramolecular polymer entanglements with increasing thermal energy of the polymer molecules. These LC solutions are namely known to be non-gelling and stay viscous throughout the studied temperature regime – these effects are considered to be mainly of intramolecular origin. It is worth mentioning already at this point that in the case of SAXS data of viscous polymer solutions it has already been observed on a couple of occasions that the fits based on Equation (1) or better Equation (10) were not satisfactory

Summary

Introduction

Methylcellulose (MC) and κ-carrageenan (KC) are well known food additives (E407 and E461, respectively) that are commonly used as gelling, thickening and stabilizing agents. The transition temperatures of aqueous solutions of both polymers have already been thoroughly studied previously utilizing a variety of different techniques.[1,2] KC and MC are very similar in structure, their gelling mechanisms differ significantly and have been extensively studied in the case of both polymers.[3,4,5,6,7] It is namely generally accepted that the gelation of KC is a two-step process, where initially the transition from coils to double helices occurs with cooling and with further reduction of the temperature the double helices start to crosslink and form a three dimensional network This results in the formation of an elastic gel. The phase separation on a micro scale is obvious, because in the gel state the samples are highly turbid and show considerable syneresis.[1,3]

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