Small-angle x-ray and neutron scattering studies of the volume phase transition in thermosensitive core-shell colloids

The volume transition in thermosensitive colloidal core-shell particles is investigated by small-angle x-ray scattering (SAXS), small-angle Neutron sc attering (SANS), and dynamic light scattering (DLS). The latex particles ar e dispersed in water and consist of a solid poly(styrene) core with a diame ter of 100 nm. The thermosensitive shell is made up of poly(N-isopropylacry lamide) (PNIPA) chains crosslinked by 2.5 mol% N,N'-methylenbisacrylamide ( BIS). Water is a good solvent for PNIPA at room temperature but becomes a p oor solvent above 32 degreesC. The PNIPA network of the shell undergoes a v olume transition at this temperature. As a result the diameter of the parti cle shrinks. The scattering intensities of the particles measured by SAXS a nd SANS as a function of temperature may be decomposed into a part deriving from the overall structure and a part originating from the fluctuations wi thin the network. The analysis of the overall structure leads to the volume fraction of the swollen network at different temperatures. SANS in conjunc tion with contrast variation demonstrates that the network is confined in a well-defined shell. SAXS and SANS data therefore allow the phase diagram o f the network in the shell of the particles to be derived, i.e., the averag e volume fraction of the network in: the shell can be determined as a funct ion of temperature. DLS corroborates this result but demonstrates that ther e is a small fraction of chains exceeding the outer radius derived from SAX S and SANS. The static intensity caused by the fluctuations of the network becomes the leading contribution at high scattering angles. SAXS data show that this part can be described by a Lorentzian both below and above the vo lume transition. The analysis demonstrates that critical fluctuations of th e network around the transition temperature are fully suppressed. This find ing is explained by the strong steric constraint of the network by its conf inement within a shell of colloidal dimension.; The swelling and shrinking can only take place; along the radial direction and the chains are bound to the solid surface of the cores which remains constant during the transitio n. (C) 2002 American Institute of Physics.