The behavior of polymer blend surface is important to many technologies. In practice, the surface composition and morphology are critical to their performance. Therefore, the surface properties and the surface characterization techniques of polymer blends have become more important....[ Read more ]

The behavior of polymer blend surface is important to many technologies. In practice, the surface composition and morphology are critical to their performance. Therefore, the surface properties and the surface characterization techniques of polymer blends have become more important.

The surface properties of polymer blends are highly affected by the intermolecular interaction between the two polymers. The intermolecular interaction may be used to increase the miscibility of polymer blends. In this thesis, two systems --- miscible and immiscible polymer blends --- were studied.

Part I focuses on the effects of hydrogen bonding, casting solvents, and molecular weight on the surface composition. Systematic studies were performed on the blends of Poly (4-Vinyl phenol) (PVPh) and Poly (4-Vinyl pyridine) (PVPy) in different solvents, including ethanol and N, N-dimethylformamide (DMF). The surface properties of PVPhIPVPy films were characterized by time-of-flight secondary ion mass spectrometry (ToF-SIMS), X-ray photoelectron spectroscopy (XPS), and contact angle measurements. The bulk behavior of the PVPh/PVPy samples was determined by elemental analysis and differential scanning calorimetry (DSC) study.

Positive derivation of glass transition temperature (T_g) was obtained for both the blends and the complexes, indicating a strong interaction between the PVPh and PVPy for the blends and complexes. The characteristic positive ions were identified and used in demonstrating the intermolecular hydrogen bonding between PVPh and PVPy in the SIMS spectra obtained. The possible processes for the formation of the hydrogen bonding were proposed. The peak intensity of the characteristic pyridyl ions of the blends, and especially of the PVPh/PVPy complexes, such as the peak at m/z=106, was greatly enhanced by the hydrogen bonding. Both X-ray photoelectron spectroscopy and contact angle results showed no surface segregation of any component for the blends and complexes for a low molecular weight PVPh and PVPy. Excess of PVPy was found at the surface of the blends when a high molecular weight PVPh was used. However, after annealing at 90°C in a vacuum oven for five days, the surface and bulk compositions were the same. These findings revealed that the surface of blends of high molecular weight polymers may not be in the thermodynamic equilibrium state. The quantitative relationship between the ion intensity ratio and the surface composition was established for the hydrogen-bonded polymer blends and complexes. Negative ion ratios were shown to be very useful in SIMS quantitative study. A linear relationship was established between the relative intensity of the characteristic negative ions and the surface concentration obtained from XPS data.

In part II, the influence of poly(styrene-co-4-vinylphenol) (STVPh-x, x represents the vinylphenol content, mole %) copolymers with various vinylphenol contents on the surface morphology of immiscible blends of polystyrene (PS) and poly( N-vinyl-2-pyrrolidone) (PVP) was investigated by time-of-flight secondary ion mass spectrometry. The images obtained from using ToF-SIMS positive total ion and negative characteristic ions clearly showed that the sizes of the dispersed phase (PVP) decreased with the addition of STVPh-5 (5 mole % of vinylphenol) random copolymer. The spectra obtained retrospectively from the matrix and the dispersed phase indicated that the matrix phase was very similar to pure PS and the dispersed phase was very similar to pure PVP. The addition of the compatibilizer, STVPh-9 (9 mole % vinylphenol), to the blend of PS and PVP resulted in a different morphology; the PVP became a semi-continuous phase and the PS changed to a partial continuous phase. The morphological changes were caused by the formation of a single phase between PVP and STVPh-9 due to an increase in the density of the hydrogen bond formed between PVP and STVPh-9. When the vinylphenol content was lower than 2 mole %, the particle size of the PVP-rich phase stayed the same. The compatibilizing effect was much weakened as the concentration of STVPh-5 decreased to 1 wt %.