Effects of Temperature and pH on the Contraction and Aggregation of Microgels in Aqueous Suspensions

• Chemically cross-linked poly(N-isopropylacrylamide) (PNIPAM) microgels   and PNIPAM with different amounts of acrylic acid groups   (PNIPAM-co-PAA) were synthesized and the temperature-induced   aggregation behaviors of aqueous suspensions of these microgels were   investigated mainly with the aid of dynamic light scattering (DLS) and   turbidimetry. The DLS results show that the particles at all conditions   shrink at temperatures up to approximately the lower critical solution temperature (LCST), but the relative contraction effect is larger for   the microgels without acid groups or for microgels with added anionic   surfactant (SDS). A significant depression of the cloud point is found   in suspensions of PNIPAM with very low concentrations of SDS. The   compression of the microgels cannot be traced from the turbidity   results, but rather the values of the turbidity increase in this   temperature interval. This phenomenon is discussed in the framework of   a theoretical model. At temperatures above LCST, the size of the   microgels without attached charged groups in a very dilute suspension   is unaffected by temperature, while the charged particles (pH 7 and 11)   continue to collapse with increasing temperature over the entire   domain. In this temperature range, low-charged particles of higher   concentration and particles containing acrylic acid groups at low pH   (pH 2) aggregate, and macroscopic phase separation is approached at   higher temperatures. This study demonstrates how the stabilization of   microgels can be affected by factors such as polymer concentration,   addition of ionic surfactant to particles without charged acid groups, amount of charged groups in the polymer, and pH.

Subject headings

NATURVETENSKAP  -- Kemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences (hsv//eng)

Keyword

Chemistry

Kemi

Fysikalisk kemi

Physical Chemistry

Publication and Content Type

ref (subject category)

art (subject category)