Oligomer/polymer mixtures as model adhesives:
Impact of compatibility on surface segregation behaviour

Abstract

Molecular migration, segregation and self-organisation in polymers are phenomena that govern the processing, performance and life-time of many materials and formulations. Despite the widespread use of oligomer/polymer mixtures for industrial purposes, such as in paints, coating, packaging and adhesives, relatively little can be found in literature regarding the characterisation of such asymmetric systems. Hence, the aim of this thesis is to determine the causes and consequences of the surface segregation of low molecular weight components in simplified adhesive formulations. In addition, it focuses on producing data for the validation of a new model that bridges the gap evidenced in the existing theories.
First, the key-parameters impacting such surface segregation were hypothesised by extrapolating the conclusions from the research conducted on polymer blends. Then, model systems were defined in order to mimic the molecular migration in hot-melt adhesives. Experimental evidence was generated which showed that the glassy polystyrene domains can be neglected in the model formulations. Hence, binary mixtures of oligomer and rubbery polymer were chosen to evaluate the impact of the oligomer molecular weight and volume fraction, interaction parameter as well as temperature. A systematic evaluation of the compatibility was conducted and correlated with similarity in saturation and solubility parameter. The results were compared with the predictions of the Flory-Huggins theory, for polymer blends. A clear relationship between surface segregation and bulk compatibility was evidenced by comparing the compatibility study with results obtained by ion beam analysis and neutron reflectometry. In some systems, a nearly pure surface wetting layer of well-defined few nm thickness was observed, which was surprisingly independent of sample thickness. In others, lateral segregation on scale of few µm was characterised by atomic force microscopy. While surface segregation is driven by disparity in molecular weight in highly compatible systems, this trend reverses as the critical point is approached. Oligomers of higher molecular weight can phase-separate from the bulk and form a wetting layer at the surface. The impact of surface segregation on mechanical surface and bulk properties was also assessed by atomic force microscopy and oscillatory rheology. It was found that the mechanical bulk properties of the polymer were greatly affected by the presence of oligomer, due to plasticisation effects.