Volume transition and internal structures of small poly(N‐isopropylacrylamide) microgels

• Published in: Journal of polymer science. Part B, Polymer physics Vol. 43; no. 7; pp. 849 - 860
• Main Authors: Arleth, Lise, Xia, Xiaohu, Hjelm, Rex P., Wu, Jianzhong, Hu, Zhibing
• Format: Journal Article
• Language: English
• Published: Hoboken Wiley Subscription Services, Inc., A Wiley Company 01.04.2005; Wiley
• Subjects: Applied sciences; Exact sciences and technology; light scattering; nanoparticles; neutron scattering; N‐isopropylacrylamide; Organic polymers; Physicochemistry of polymers; Properties and characterization; Solution and gel properties; Hydrodynamic radius; Nanoparticle; Second virial coefficient; Temperature effect; Radius of gyration; light scattering; Experimental study; Phase transitions; Acrylamide derivative polymer; neutron scattering; Interparticle interaction; nanoparticles; Concentration effect; Internal structure; Microgel; Monodispersed particle; N-isopropylacrylamide; Aqueous dispersion
• ISSN: 0887-6266; 1099-0488
• DOI: 10.1002/polb.20375

Monodispersed poly(N‐isopropylacrylamide) (PNIPAM) nanoparticles, with hydrodynamic radius less than 50 nm at room temperature, have been synthesized in the presence of a large amount of emulsifiers. These microgel particles undergo a swollen–collapsed volume transition in an aqueous solution when the temperature is raised to around 34 °C. The volume transition and structure changes of the microgel particles as a function of temperature are probed using laser light scattering and small angle neutron scattering (SANS) with the objective of determining the small particle internal structure and particle–particle interactions. Apart from random fluctuations in the crosslinker density below the transition temperature, we find that, within the resolution of the experiments, these particles have a uniform radial crosslinker density on either side of the transition temperature. This result is in contrast to previous reports on the heterogeneous structures of larger PNIPAM microgel particles, but in good agreement with recent reports based on computer simulations of smaller microgels. The particle interactions change across the transition temperature. At temperatures below the transition, the interactions are described by a repulsive interaction far larger than that expected for a hard sphere contact potential. Above the volume transition temperature, the potential is best described by a small, attractive interaction. Comparison of the osmotic second virial coefficient from static laser light scattering at low concentrations with similar values determined from SANS at 250‐time greater concentration suggests a strong concentration dependence of the interaction potential. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 849–860, 2005