Sphere Formation and Characterization

The formation and size of the colloidal spheres depend on the molecular structures and preparation conditions such as the initial concentration of the polymer in THF, the water content in the medium, and the water-dropping rate. As the polymers studied are of different types and have polydispersity in the molecular weight and DF, it is difficult to give a quantitative relationship to correlate the polymer structures with the colloid size and formation details. As a general tendency, CWC decreases as the molecular weight and hydrophobicity increase, and the colloid size increases as the hydrophobicity increases. The influence of the polymer structure can be better understood after discussing the colloid formation mechanism in Section 5.3.3. The effect of the preparation conditions on the colloid formation and size is discussed herein by using PEAPE (DF = 49.9%) as a typical example. 

The water content in the medium plays a key role to control the colloid formation. The influence of the water content can be studied by using dynamic light scattering (DLS) and UV-vis spectroscopy. DLS indicates that when the water content increases above CWC, the hydrodynamic radius (i b) gradually increases as the water content increases. When the water content is above a certain value, i h starts to decrease as the water content further increases and then stabilizes at the final value. The structure evolution in the process can be better understood from the UV-vis spectroscopic investigation. It is well known that the photoisomerization rate and isomerization degree at the photostationary state are related to the free volume surrounding the azo chromophores (Kumar and Neckers, 1989). For azobenzene-type molecules, the isomerization behavior can be monitored by UV-vis spectroscopy and used as a molecular probe to detect the environmental variation in the systems. 

The trans-to-cis isomerization degree at the photostationary state indicates a similar tendency (Fig. 5.6). The isomerization degree at the photostationary state is estimated from (Victor and Torkelson, 1987) 

When the water content further increases, the isomerization degree starts to decrease. When the water content increases from 37.5% to 80% (vol%), the isomerization degree decreases from 80% to ca. 55%. For the spheres in the water suspension, obtained by quenching with an excessive amount of water and dialysis, the isomerization degree decreases to even 33%. The preceding results obtained from the UV-vis spectroscopic measurements indicate that in the water addition process, the aggregates undergo a collapse process after the water content is above 40% (vol%). 

In the water addition process, the water-adding rate shows a critical influence on the colloid formation. The slow addition rate is necessary to obtain uniform colloids. In a broad water-dropping rate, the sizes of the colloids obtained at the final stage are almost the same. However, when the water-dropping rate is extremely slow, the colloid size will abruptly increase as the dropping rate further decreases and even the aggregates will precipitate from the suspension. 

---