Abstract
The formation of a rigid porous biopolymer scaffold from aqueous samples of 1% w/v (suspension) and 5% w/v (gel) corn starch was studied using optical and nuclear magnetic resonance (NMR) techniques. The drying process of these systems was observed using a single-sided NMR scanner by application of the Carr–Purcell–Meiboom–Gill pulse sequence at different layer positions. The echo decays were analyzed and spin–spin relaxation times (T2) were obtained for each layer. From the depth dependent T2 relaxation time study, it was found that the molecular mobility of water within the forming porous matrix of these two samples varied notably at different stages of film formation. At an intermediate stage, a gradual decrease in mobility of the emulsion sample towards the air–sample interface was observed, while the gel sample remained homogeneous all along the sample height. At a later stage of drying, heterogeneity in the molecular dynamics was observed in both samples showing low mobility at the bottom part of the sample. A wide-angle X-ray diffraction study confirmed that the structural heterogeneity persisted in the final film obtained from the 5% corn starch aqueous sample, whereas the film obtained from the 1% corn starch in water was structurally homogeneous.
Highlights
Starch is the second most abundant natural biopolymer after cellulose from plant origin.The glucose (D-glucopyranose) unit is the only monomer present in starch with two main constituents, namely, linear amylose and highly branched amylopectin
T2 was uniform within comparison, in an earlier study on gelatin biopolymer film formation [16], it was shown that a fast decaying component could be distinguished in the dried film sample which decayed below 100 μs
A number of studies were conducted to follow the evaporation of the solvent from two different starch samples containing 1% and 5% of starch in water (w/v) during the process of film formation
Summary
Starch is the second most abundant natural biopolymer after cellulose from plant origin. This involves interactions that occur mainly by hydrogen bonding among starch chains, a process known as retrogradation In this process, amylose molecules start to be prone to double helix formation with the same type of molecules as well as with the long branches of amylopectin [7]. Low-field [16,17,18,19,20,21] and high-field [22] NMR techniques have been employed successfully to study the film formation of different polymers and to characterize the prepared films [23] In this contribution, a study of starch film formation is presented which follows the evolution from the initial polymer suspension (starting from a dilute system) or gel (starting from a concentrated system) to the final film using low-field NMR. The evolution of the molecular mobility of starch until the final stages of film drying was investigated with spatial resolution for the first time using NMR relaxometry, providing insight into the development of resolved polymer concentration
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