Abstract
The temperature-induced formation of nanoparticles of two poly(methacrylic acid) (PMA) isomers, atactic (aPMA) and isotactic (iPMA), displays an opposite thermoresponsiveness. The formation of nanoparticles is investigated in dilute aqueous solutions in the presence of cations of different charge (Na+, Mg2+, and La3+) first by UV spectroscopy and visual observations. In the aPMA case, aggregation and phase separation take place with increasing temperature, whereas with iPMA intermolecular associates gradually disintegrate upon heating the solutions; however, no macroscopic phase separation is observed, and the process is completely reversible. Stable associates exist in iPMA solutions at all temperatures if higher valency ions (Mg2+ and La3+) are present, whereas with aPMA, only La3+ ions induce irreversible (stable) aggregation. Detailed structural characteristics of iPMA and aPMA aggregates are determined by dynamic and static light scattering and additionally characterized by pyrene fluorescence measur...
Highlights
Stimuli-responsive polymers or so-called smart polymers are of great importance in various fields, especially in the pharmaceutical field, because they are able to respond to a variety of environmental changes that affect their microstructure and through this their physiological and chemical properties.[1,2]
Various types of interactions play a role in structuring of polymeric materials, for example, electrostatic, hydrophobic and dipolar interactions, hydrogen bonding, van der Waals interactions, which result in excluded volume effects and in chain connectivity, and others.[5,8,9]
The UV absorbance of Atactic poly(methacrylic acid) (aPMA) solution during heating in the presence of Na+ and Mg2+ ions is at first close to zero (Figure 1b) and steeply increases at temperatures above approximately 80 °C. This increase is ascribed to the formation of large particles, which were observed visually as strong turbidity of the samples. This UV result resembles the lower critical solution temperature (LCST) behavior of polymer solutions and is in agreement with some previous literature reports on aPMA, which were performed either at considerably higher polymer concentrations[6,7] or for polymer chains with a considerably higher molar mass[10] in comparison to our study
Summary
Stimuli-responsive polymers or so-called smart polymers are of great importance in various fields, especially in the pharmaceutical field, because they are able to respond to a variety of environmental changes that affect their microstructure and through this their physiological and chemical properties.[1,2] As such, they can be used in controlled drug delivery, because they are able to release the drug at a specific site in the body, reducing adverse side effects.[1,3,4] Under the concept of “environmental changes”, variations in temperature, pH, ionic strength, light and magnetic field, ionic factors, etc., are considered.[1,2,5] When the starting conditions in the surrounding environment are re-established, smart polymers usually recover to their initial state. Apart from drug delivery, smart polymers appear as promising materials in tissue engineering, as biosensors for different molecules, as bioseparation devices, in contact lenses, breast implants, diapers, wound compresses, etc.[1−3,6,7]. Various types of interactions play a role in structuring of polymeric materials, for example, electrostatic, hydrophobic and dipolar interactions, hydrogen bonding, van der Waals interactions, which result in excluded volume effects and in chain connectivity, and others.[5,8,9] Attractive interactions, which are established between charged units of polyelectrolyte molecules and oppositely charged components in the solution, often lead to association/aggregation. The driving forces for these processes are entropic release of counterions and water, while Coulombic interactions present the enthalpic part.[8−10]
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