Abstract
In this work, the transformation of chitosan-grafted-polyacrylamide (GPAM) aggregates in aqueous solution upon heating was explored by cryo-electron microscope (cryo-TEM) and dynamic light scattering (DLS), and larger aggregates were formed in GPAM aqueous solution upon heating, which were responsible for the thermo-thickening behavior of GPAM aqueous solution during the heating process. The heating initiates a transformation from H-bonding aggregates to a large-sized cluster formed by self-assembled hydrophobic chitosan backbones. The acetic acid (HAc) concentration has a significant effect on the thermo-thickening behavior of GPAM aqueous solution; there is a critical value of the concentration (>0.005 M) for the thermo-thickening of 10 mg/mL GPAM solution. The concentration of HAc will affect the protonation degree of GPAM, and affect the strength of the electrostatic repulsion between GPAM molecular segments, which will have a significant effect on the state of the aggregates in solution. Other factors that have an influence on the thermo-thickening behavior of GPAM aqueous solution upon heating were investigated and discussed in detail, including the heating rate and shear rate.
Highlights
Thermo-responsive polymers have been extensively studied over the past few decades owing to their industrial and biomedical applications [1,2,3,4,5,6,7]
The microstructures of GPAM aqueous solution observed by transmission electron microscope (TEM), which were obtained from dried solution samples, could not be the same as those in water
This may be attributed to stronger hydrophilicity and electrostatic repulsion induced by more protonation of –NH2, whichs inhibit the hydrophobic aggregation
Summary
Thermo-responsive polymers have been extensively studied over the past few decades owing to their industrial and biomedical applications [1,2,3,4,5,6,7]. Chitosan (CS) has received tremendous interest owing to its intrinsic properties, biocompatibility, nontoxicity, amphipathic, accessibility, and abundance in nature [15,16] It is the precursor for other applications such as drug delivery [17,18,19], emulsifier [20], water treatment [21,22,23,24], and so on. We observed the thermo-thickening behavior of GPAM aqueous solution and proposed a preliminary outline of the molecular mechanism based on nuclear magnetic resonance (NMR) and transmission electron microscope (TEM) results. It was found the transformation from a hydrogen bonding (H-bonding) aggregate to a hydrophobic aggregate upon heating was responsible for the thermo-thickening [12]. The influences of acid concentration, ramp rate, and shear rate on thermo-thickening of GPAM aqueous solutions are addressed
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