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Deswelling induced morphological changes in dual pH- and temperature-responsive ultra-low cross-linked poly(N-isopropyl acrylamide)-co-acrylic acid microgels

Authors
  • Islam, Molla R.1
  • Tumbarello, Maddie1
  • Lyon, L. Andrew1
  • 1 Chapman University, Department of Chemistry, Schmid College of Science and Technology, Orange, CA, 92866, USA , Orange (United States)
Type
Published Article
Journal
Colloid & Polymer Science
Publisher
Springer-Verlag
Publication Date
Mar 27, 2019
Volume
297
Issue
5
Pages
667–676
Identifiers
DOI: 10.1007/s00396-019-04492-8
Source
Springer Nature
Keywords
License
Yellow

Abstract

Poly(N-isopropylacrylamide) microgels prepared without exogenous cross-linker are extremely “soft” as a result of their very low cross-linking density, with network connectivity arising only from the self-crosslinking of pNIPAm chains. As a result of this extreme softness, our group and others have taken interest in using these materials in a variety of bioengineering applications, while also pursuing studies of their fundamental properties. Here, we report deswelling triggered structural changes in poly(N-isopropylacrylamide-co-acrylic acid) (ULC10AAc) microgels prepared by precipitation polymerization. Dynamic light scattering suggests that the deswelling of these particles not only depends on the collapse of the pNIPAm chains but is also influenced by the ionization state of the acrylic acid moieties present in the copolymer. The ULC10AAc microgel behaves like a traditional cross-linked pNIPAm microgel at pH 3.5, showing a sharp decrease in the hydrodynamic diameter around the lower critical solution temperature (LCST) of pNIPAm. As the pH is increased to 4.5, we observe multiple transitions in the deswelling curve, suggesting inhomogeneity in the structure and/or composition of the microgels. At pH 6.5, the microgels cease to be thermoresponsive over the studied temperature range due to increased charge repulsion between the fully deprotonated AAc groups and an increase in gel osmotic pressure due to solvated counterion ingress. Atomic force microscopy images of particles deposited at different temperatures reveal a temperature-induced morphological change, with punctate structures forming inside microgels at pH 4.5 and 6.5 and temperature above the gel volume phase transition temperature (VPTT). Graphical abstract.

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