In this study, the roles of molecular parameters and solution properties on the formation of various nanoscale aggregates of conjugated polymers, dialkyl-poly(p-phenyleneethynylene)s (PPEs), on mica are reported. Characteristics of the aggregates probed by atomic force microscopy are found to depend on the dimensions of side chain and backbone. The short chain didodecyl-PPEs with average degrees of polymerization (DP) of 16 preferably assemble into networks of curvy nanoribbons with thickness less than 10 angstrom, while the longer PPEs (DP = 96, 500) form micrometer-long nanoribbons with diameter ranging from 15 to 32 nm. When replacing the dodecyl side chain by shorter alkyl groups, 2-(ethyl)hexyl and 3,5,5-trimethylhexyl, micrometer-size sheets and clusters of nanoribbons are obtained. Interchain association of PPEs in solution is also found to be an important factor for the formation of nanoribbons. (c) 2006 Elsevier B.V. All rights reserved.