No abstract

The isotherm thickness and hydrogen-bonding interactions of water layers adsorbed on hydrophilic and hydrophobic surfaces were quantified and compared. The hydrophilic and hydrophobic surfaces were modeled with an OH-terminated native oxide layer on silicon and a HF-etched silicon terminated with hydrogen, respectively. The silicon substrate allows the use of attenuated total reflection infrared (ATR-IR) spectroscopy for quantitative measurement of adsorbed water without interferences from the gas phase water. On the hydrophilic Si–OH surface, the average thickness of the strongly hydrogen-bonded water layer increases up to ∼2 molecular layers as relative humidity (RH) increases, beyond which the weakly hydrogen-bonded structure is dominant. On the hydrophobic Si–H surface, the adsorbed water layer consists predominantly of the weakly hydrogen-bonded structure and its average thickness remains less than a monolayer even at RH = 90%. The differences in the thickness and structure of adsorbed water layers on hydrophilic versus hydrophobic surfaces found from ATR-IR measurements provide critical insights needed for better understanding of various physical processes affected by water adsorption in ambient conditions.

No abstract

Two types of polylactic acid (PLA) films (one amorphous and one semi-crystalline) were produced by sheet extrusion. Talc was used as a nucleation agent for the semi-crystalline PLA. The films were annealed above their T g or were uniaxially orientated in two ways: (1) via a drawing system in front of the extruder and die or (2) via a three-roller stretching system. The slower crystallization rate and lower melting stress of the PLA resulted in amorphous film using the drawing system. Annealing above T g increased crystallinity and polymer chain relaxation, which resulted in increases in both strength and toughness. Stretching above T g also produced simultaneous crystallization and chain relaxation, which resulted in increases in both modulus and toughness. Both modulus and tensile strength in the stretching direction were higher than in the crosswise direction. Talc acted not only as a rigid filler to reinforce the PLA, but also as a nucleation agent for the PLA, especially during annealing. POLYM. ENG. SCI., 48:634-641,