Three polyetherimides (PEIs) with the same backbone of Ultem 100 but different lengths of the alkyl side chains were simulated by using molecular dynamics and molecular mechanics techniques to investigate the effect of side chain length on their properties and physical mechanism behind. Simulation results, which are consistent to the experimental data, show that PEI-5 with four methylene units in each alkyl side chain has higher T-g (glass transition temperature) and higher tensile strength, but lower tensile elongation at break than those of PEI-6 with five and PEI-8 with seven methylene units in each alkyl side chain. However, unlike the traditional phenomena, conformational analysis provides that PEI-5 with the highest T-g gives the highest flexibility to the polymer chain, whereas PEI-8 with the lowest T-g imparts the lowest flexibility resulting from attachment of longer alkyl side chain increase the rigidity of backbone. From the calculated ratio of the accessible volume to the total volume for each system, the highest ratio of PEI-8 indicates that long alkyl side chains generate more free volume than short side chains, acting as an internal plasticizer in bulk structure. It is the internal plasticizing effect that is predominantly responsible for the abnormal properties, instead of the rigidity from side chains. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 595-599, 2010