Copolymers composed of various amounts of 4'-(11-acryloyloxyundecyloxy)biphenyl-4-carboxylic acid (11ABA) and acrylic acid (AA), poly(11ABA-co-AA)s, were synthesized, and their liquid crystalline structures were investigated both in dry and in water-swollen states. Wide- and small-angle X-ray studies revealed that both dry and swollen poly(11ABA-co-AA)s show a liquid crystalline structure, which transforms from smectic C with the bilayer (SmC2) to smectic C with the monolayer (SmC1), when the 11ABA composition, F([11ABA]/([11ABA] + [AA])), decreases from 0.72 to 0.44. The phase diagram of these copolymers was established by changing F and the temperature. It shows that the samples can build up a liquid crystalline structure for F as low as 0.07 at the elevated temperature of 155 degreesC. The enhanced ordering of the liquid crystalline structure and the thermostability of the copalymers were explained in terms of hydrogen bonding formed between carboxyls of mesogenic side chains and AA.