Grafting of Starch Nanoparticles with Polymers

Abstract

As a biocompatible and biodegradable polysaccharide, starch has sparked significant interest for various industrial applications, but its poor mechanical properties limit its uses without chemical or physical modification. The work reported herein concerns the development of synthetic techniques to modify starch by graft polymerization via cerium (IV) activation.

Starch nanoparticles (SNPs) were modified with acrylic acid (AA) in water under acidic conditions via activation with cerium (IV) in combination with potassium persulfate (KPS). The reactions were conducted with either the as-supplied SNPs containing glyoxal, or after purification (without glyoxal), for different target molar substitution (MS) values. A novel purification protocol using methanol extraction and centrifugation was implemented to purify the samples. This method proved to be selective to isolate the poly(acrylic acid) (PAA) homopolymer contaminant from the starch-g-PAA copolymer, and more reliable than the gravimetric analysis methods reported in the literature. The starch-g-PAA copolymers were characterized by dynamic light scattering (DLS), and degradation of the starch substrate allowed the determination of the molar mass of the PAA side chains via gel permeation chromatography (GPC) analysis.

In the presence of aldehydes the rate of polymerization of AA increased significantly (by > 37 %), and the highest grafting efficiencies were obtained for glyoxal and butyraldehyde. The combination of cerium (IV) with glyoxal and KPS resulted in the highest polymerization rate and grafting efficiency. Increasing the glyoxal concentration also increased the rate of monomer conversion and the grafting efficiency. The increased rate of polymerization provided further insight into the grafting mechanism, as it was discovered that esterification reactions between starch and PAA also contributed significantly to the grafting process, particularly at longer reaction times. In the presence of aldehydes, the production of large amounts of PAA homopolymer resulted in esterification dominating the grafting process. Model reactions involving direct coupling of linear PAA samples with starch were investigated. All the reactions were characterized by high coupling efficiencies for a target MS = 3, and higher molar mass PAA samples (30 and 250 kDa) coupled faster than a lower molar mass sample (1.8 kDa), as expected in terms of reaction probabilities. The importance of esterification was also confirmed with model reactions using 2-hydroxyethyl acrylate, a monomer not containing a free carboxylic acid functional group, which yielded notably lower grafting efficiencies.

Overall, the grafting mechanism for starch and acrylic acid promoted by cerium (IV) therefore appears more complex than described previously, particularly in the presence of aldehydes: The high overall grafting efficiencies observed result from two distinct reactions occurring concurrently, namely grafting via cerium (IV) activation, as well as the esterification of free PAA homopolymer. The additional insight gained for these reactions was possible due to the newly developed purification protocol, used in combination with NMR spectroscopy analysis, which provided detailed composition data for the different sample fractions and a better understanding of the grafting mechanism.

Furthermore, preliminary results were obtained for starch modified with acrylonitrile and cerium (IV) in water under acidic conditions. Extraction of the polyacrylonitrile (PAN) homopolymer component was more difficult due to its solubility characteristics, but mixtures of dimethylacetamide with water (up to 10 % by volume) provided consistent results. High grafting efficiencies (> 67 %) were obtained for the starch-g-PAN copolymers, and characterization of the products was performed by Fourier transform-infrared spectroscopy, DLS, GPC, and atomic force spectroscopy. Hydrolysis of the starch substrate yielded hollow PAN shells or spheres, depending on the MS level of the copolymer, with potential applications in nanoencapsulation.