Two high molecular weight copolymers of poly(N-isopropylacrylamide) (PNIPAM) densely grafted with short poly(ethylene oxide) (PEO) chains (PNIPAM-g-PEO) were studied by NMR and laser light scattering. The long PNIPAM chains with densely grafted PEO branches had a random coil conformation at very dilute concentrations and low temperatures (i.e., T less than or equal to 30 degrees C). When the temperature was increased above 31 degrees C, the copolymers could undergo a broad "coil-to-globule" transition. The collapsed copolymer chains had a [R-g][R-h] value of about 1.0 with PNIPAM chains inside the core and the hydrophilic PEO chains on the surface. This kind of PNIPAM-g-PEO copolymers was studied as a DNA separation medium in capillary electrophoresis. Several advantages of the copolymers as a separation medium for DNA fragments were achieved, such as an automatic coating ability for the capillary inner wall, an easier injection into the capillary channel due to the slightly adjustable viscosity with temperature (up to 31 degrees C), a high resolution (i.e., one base pair resolution), and fast separation time. In contrast, the homo-PNIPAM or PEO showed worse DNA separation efficiency under similar conditions. The high DNA separation efficiency of the PNIPAM-g-PEO copolymers is related to the polymer chain conformation. The long copolymer chains in a random coil conformation with densely grafted PEO branches could form a physical network with a relatively stable and uniform pore size at high concentrations (i.e., greater than or equal to 10 wt %). The separation medium has a high sieving ability for DNA separation in terms of DNA migration mechanisms. The collapsed copolymer chains in the globule state could destroy the chain network and thus lose the DNA separation ability.