ORDERED STRUCTURAL EVOLUTION AND RELAXATION BEHAVIORS OF A SERIES MICROPHASE SEPARATED “HAIRY-ROD” POLYIMIDES WITH MULTIPLE ALKYL SIDE CHAINS

A series of “hairy-rod” polyimides, BBPA(n), with multiple alkyl side chains was prepared from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and 4,4′-biphenyldiamine substituted in the 2,2′-positions with benzoate, which was substituted in the 3,4,5-positions with ether side chains of varying lengths. The number of the methylene units, n, in these alkyl side chains were in even numbers ranging from 8 to 18. Combining techniques of one-dimensional (1D) and 2D wide angle x-ray diffraction, 1D small angle X-ray scattering, differential scanning calorimetry experiments, it was found that this series of “hairy-rod” polyimides possess a micro-phase separation between the backbones and side chains. This led to the formation of ordered structures in two different length scales, of which both are hexagonal packing: one is attributed to the alkyl side chains on the sub-nanometer scale, and another is for the whole polymer chains on the nanometer scale. The development of the hexagonal structure on the sub-nanometer scale was critically dependent upon the lengths of the alkylside chains. Three relaxation processes were captured by dynamic mechanical analysis, i.e., segmental motion of the backbones, α the melting of the side chain crystals, β1, which exits only for the materials with longer side chains(n=18,16); and the subglass relaxation of side chains, β2- The peak relaxation temperature of the α process decreased with increasing the length of side chains, while the one of the β2 process increased. The activation energy of the α relaxation was relatively independent on the length of side chain, whereas, β2 process showed the increasing of activation energy with increasing the length of side chains.

ORDERED STRUCTURAL EVOLUTION AND RELAXATION BEHAVIORS OF A SERIES MICROPHASE SEPARATED “HAIRY-ROD” POLYIMIDES WITH MULTIPLE ALKYL SIDE CHAINS

 A series of “hairy-rod” polyimides, BBPA(n), with multiple alkyl side chains was prepared from 3,3¢,4,4¢-biphenyltetracarboxylic dianhydride (BPDA) and 4,4¢-biphenyldiamine substituted in the 2,2¢-positions with benzoate, which was substituted in the 3,4,5-positions with ether side chains of varying lengths. The number of the methylene units, n, in these alkyl side chains were in even numbers ranging from 8 to 18. Combining techniques of one-dimensional (1D) and 2D wide angle x-ray diffraction, 1D small angle X-ray scattering, differential scanning calorimetry experiments, it was found that this series of “hairy-rod” polyimides possess a micro-phase separation between the backbones and side chains. This led to the formation of ordered structures in two different length scales, of which both are hexagonal packing: one is attributed to the alkyl side chains on the sub-nanometer scale, and another is for the whole polymer chains on the nanometer scale. The development of the hexagonal structure on the sub-nanometer scale was critically dependent upon the lengths of the alkyl side chains. Three relaxation processes were captured by dynamic mechanical analysis, i.e., segmental motion of the backbones, a; the melting of the side chain crystals, b1, which exits only for the materials with longer side chains (n = 18,16); and the subglass relaxation of side chains, b2. The peak relaxation temperature of the a process decreased with increasing the length of side chains, while the one of the b2 process increased. The activation energy of the a relaxation was relatively independent on the length of side chain, whereas, b2 process showed the increasing of activation energy with increasing the length of side chains.