Persistence Length of PEGMA Bottle Brushes Determined by Pyrene Excimer Fluorescence

Seven pyrene-labeled poly(oligo(ethylene glycol) methyl ether methacrylate)s (PyEG₅-PEG_nMAs) were prepared with n = 0, 3, 4, 5, 7, 9, and 19 ethylene glycol units by copolymerizing a small amount of penta(ethylene glycol) 1-pyrenemethyl ether methacrylate with an EG_nMA monomer. The conformation of the PyEG₅-PEG_nMA polymers evolved from a random coil for PyEG₅-PEG₀MA or poly(methyl methacrylate) to a polymeric bottle brush (PBB) architecture with increasing side chain length. The fluorescence decays of the PyEG₅-PEG_nMA samples were fitted according to the fluorescence blob model (FBM) whose parameters were used, in combination with the Kratky-Porod equation, to calculate the persistence length of these polymers. The persistence lengths obtained from the PEF experiments were found to increase with the square of the number (N_S) of non-hydrogen atoms in the side chain as expected theoretically. The persistence lengths found with the PyEG₅-PEG_nMA samples in DMF also matched those found earlier for another series of PEG_nMA samples labeled with 1-pyrenebutanol. The good agreement found between the persistence lengths obtained with the PEG_nMA samples labeled with two different pyrene derivatives illustrates the robustness of the method and its applicability for measuring the unknown persistence length of polydisperse polymer samples.