Wetting of brushes by polymer melts

The scientific and practical importance of thin polymer films is evident and in many applications polymer films are required. Hence, studying properties of polymer films is relevant. Adsorption of polymer at liquid/solid interfaces can stabilise particles in a matrix. Homopolymers are often used for steric stabilisation of particles although these polymers can also destabilise the system by bridging. Diblock copolymers with one adsorbing (anchor) block and one non-adsorbing (buoy) block may be a much better choice. Bridging between particles is then avoided because the protruding buoy blocks do not adsorb on the opposite surface. The use of polymers as stabilisers can also improve the wettability of the pigment particles. The wetting behavior of a liquid onto a surface can be modified by grafting suitable polymers to the substrate. However, grafting polymers very densely onto a surface may have the opposite effect. This thesis deals with the wetting behavior of brushes by polymer melts. These brushes are obtained either by adsorbing block copolymers or by a chemical grafting procedure.

In chapter 2 we describe the adsorption of PVP/PS block copolymers consisting of a poly-4-vinylpyridine anchor block and a polystyrene buoy block onto silicon oxide surfaces. We investigated the adsorption kinetics and the adsorbed amounts as function of the composition of the block copolymer. Adsorption from solution is very fast and the surface is covered with the polymer within ten seconds. Adsorption from the melt generates denser brushes due to the fact that the chains are less swollen in the melt than in a good solvent. The properties of brushes obtained by both preparation techniques are compared to theoretical scaling laws. Good agreement for both data sets is found, indicating that with both preparation methods brushes are formed which are not too far from equilibrium.

Chemical grafting of brushes is described in chapter 3. We used a polystyrene with a vinyl end group which can react both with hydrogen-terminated silicon and with silicon oxide. Grafting performed directly from the melt results in very dense brushes. Reaction of the vinyl-terminated polystyrene with passivated silicon forms a silicon-carbon bond, whereas reaction of the same polymer with a silicon oxide surface gives a silicon-oxygen-carbon bond. The latter bond can be hydrolysed by boiling water or basic solutions. The chemical grafting technique allows us to prepare mixed brushes. Also, a reaction of functionalised polymer with a hydrogen-terminated silicon surface which is oxidised selectively at UV-illuminated patches (thus giving silicon oxide patches) can be performed. After hydrolysis, a brush pattern remains on the silicon parts, whereas the silicon oxide parts are free of polymer. Repeating the reaction with a polymer with a different chain length enables patterns long and short chains in a polymeric brush to be prepared.

Chapter 4 describes the wetting behavior of a polystyrene melt on a polystyrene brush as a function of the grafting density and the chain length of the brush and the melt. Dewetting was observed for low grafting densities (allophobic region). Increasing the grafting density resulted in complete wetting. For high molar mass of the melt dewetting was again observed at very high grafting densities (autophobic regime). Low molar mass polymers showed complete wetting at high grafting densities. Numerical self-consistent-field calculations applied to this system shows that wetting can be controlled by changing the grafting density or by changing the length of the polymer melt. These calculations support our experimental observations.

The wetting and dewetting transitions are characterised by two intermediate regimes present at both the allophobic and the autophobic transition. In chapter 5 we focus on these intermediate regimes. The first intermediate regime consist of droplets in equilibrium with a mesoscopic thin film and a nearly dry surface. This equilibrium situation can occur if the free energy curve as function of the film thickness has a special shape with a double minimum. This was confirmed by numerical self-consistent-field calculations. The second intermediate regime is polymer droplets in equilibrium with a mesoscopic thin film. In this situation no dry regions are present.

Chapter 6 describes the temperature effect of a polymer droplet on top of a brush. At a grafting density of 0.55 nm ^-2 , the droplet shows a decreased contact area at increasing temperatures. A brush consisting of chains with a grafting density of 1 nm ^-2 showed no effect of temperature. The former brush can be considered as a 'soft' brush in which the melt chains can partially mix with the brush. At elevated temperatures the chains are expelled from the brush and the droplet reduces its contact area. A 'hard' brush does not show this behavior. In this chapter, also the wetting behavior of a bimodal brush by a melt is descibed. Incorporating very few long chains into a soft brush shifts the wetting behavior from dewetting to complete wetting. However, at high amounts of long chains (~60%) autophobic dewetting is again observed.

The deformation of the adsorbed PVP/PS polymer films by the low tip-to-sample forces in the AFM is described in chapter 7. The contact mode AFM experiments showed that the AFM tip produces rims oriented perpendicularly to the scanning direction. A wide range of molar masses of both blocks was investigated to check if there is a dependence of the rim distance of the block copolymers on the size of the PVP block copolymer. A linear relation between the composition of the polymer at a constant length of the polymer was found. A tentative explanationis proposed.

In chapter 8 we present some observations using three different systems for adsorption and wetting experiments. The first is a polystyrene layer on a brush of a triblock copolymer of vinylpyridine (end blocks) and styrene (middle block). The triblock copolymer shows complete wetting behavior when the triblock copolymer layer is annealed after its deposition, whereas there is partial wetting on a not annealed film. In the former case, peculiar needle like structures became visable when the sample with its overlayer of free chains, was annealed.

The second system investigated in this chapter is film formation and wettability of PS on top of a layer of a polypropylene imine dendrimer with a attached chain of PS. In all cases dewetting was observed, probably due to the relatively low amounts of hairs onto the surface. Finally, the adsorption of poly-2-vinylpyridine-polycaprolactone (PVP/PCL) block-copolymers onto silicon oxide surfaces is described, as well as the wettability of the homopolymer polycaprolactone onto such adsorbed layers. In this case the partial crystallinity of the polymer is a complicating factor.