RESUMEN
We have systematically mapped the phase behavior of a series of symmetric CE/C/E ternary copolymer/homopolymer mixtures, where C is poly(cyclohexylethylene) and E is poly(ethylene), identifying the location in composition of the technologically important bicontinuous microemulsion (BµE) channel as a function of diblock molecular weight. The lamellar-to-disorder transition, characterized by dynamic mechanical spectroscopy, small-angle X-ray scattering, and optical transmission measurements, exhibits increasingly second-order behavior as the BµE state is approached with increasing homopolymer content. Real-space transmission electron microscopy images obtained from rapidly frozen specimens evidence the development of large-scale fluctuating smectic correlations in the disordered state as the order-disorder transition is approached. This discovery provides fresh insights into the unexplained role of fluctuations in the formation of the BµE in ternary mixtures formed from binary blends of homopolymers that display an Ising-like critical point and a symmetric diblock copolymer governed by a weak, fluctuation-induced, first-order phase transition.
RESUMEN
We have engineered a new class of pH-responsive polymer films on gold surfaces by first developing a controlled, surface-catalyzed polymerization to prepare a copolymer film consistent with poly(methylene-co-ethyl acetate) and subsequently hydrolyzing the ester side chains to varying extents to yield carboxylic acids (denoted as PM-CO2H). When pH is increased, the acid groups become deprotonated or charged, dramatically increasing their water solubility and greatly altering the film properties. The carboxylic acid content within the copolymer film can be adjusted by changing the monomer concentration ratio used in the polymerization process or the length of time for the hydrolysis. We have designed PM-CO2H films to consist predominately (>95%) of polymethylene (PM) so that the film is hydrophobic in the uncharged state and, thereby, exhibits an extremely large pH-induced response in barrier properties once ionized. The effect of polymer composition on pH response was investigated by electrochemical impedance spectroscopy (EIS), reflectance-absorption infrared spectroscopy (RAIRS), and contact angle measurements. At a 1%-4% molar acid content, the copolymer film exhibits a 5 orders of magnitude change in its resistance to ion transport over 2-3 pH units. The pH at which this response begins can be tailored from pH 5 to pH 10 by decreasing the acid content in the film from 4% to 1%.