Abstract
It is well known that viscosity, self-diffusion coefficient and glass transition temperature are a strong function of molecular weight (MW). Here we report the film formation characteristics of an acrylic (BMA/MMA/MAA) latex as a function of its average MW, considering four molecular weights ranging from 7,500 to 705,000 Daltons. As a means of taking into account the lower glass transition temperature and viscosity expected in the lower MW latices, we compare the film formation behaviours at the same temperature increment above their respective minimum film formation temperatures (MFT). We find that the lower MW latices form a film with a lower void concentration and lower surface roughness in comparison to the higher MW latices at the same temperature relative to the MFT. Non-invasive analysis of the film formation using ellipsometry finds evidence for void formation in the high MW latex (Mw = 705, 000 Daltons) up to 30°C above MFT. In contrast, the lowest MW latex (Mw = 7,500 Daltons) forms a dense film with few, if any, voids immediately upon the evaporation of water and at temperatures as low as 5°C above MFT. These differences can be partly attributed to varying degrees of plasticization by water. In addition, the film morphology at the polymer/air interface was investigated using atomic force microscopy. Surface features were attributed to the presence of surfactant. A low MW latex shows surfactant features at a lower film formation temperature (relative to the MFT) and/or a shorter film-formation time in comparison to the high MW latex.