Molecular structure and interfacial behaviour of polymers

The aim of this study was to investigate the influence of the molecular structure on the interfacial behaviour of polymers. Theoretical models were developed for three different systems. All these models are based on the self-consistent field theory of Scheutjens and Fleer for the adsorption of homopolymers.

This self-consistent field theory is a lattice model. All possible polymer conformations on the lattice are taken into account. The potential of a conformation is sum of the local potentials of the segments of the molecule. In each layer a mean field approximation is used to calculate the mixing energy. The volume fraction profile is determined by the segmental potentials and vice versa. A numerical method is used to solve the obtained set of equations.

In chapter 2 the influence of association of block copolymers on adsorption is considered. In order to model spherical aggregates (micelles), the planar lattice, as used for modelling planar aggregates (membranes) and adsorption on flat surfaces, is replaced by a spherical lattice. The equilibrium solution concentration in a micellar solution is determined by a small system thermodynamics argument. The adsorption of diblock copolymers with long lyophobic and short lyophilic blocks shows strongly cooperative effects. A single molecular layer is present if the lyophobic block adsorbs. The adsorption isotherm shows an S-shape at the onset of adsorption. A strong increase of the adsorbed amount occurs near the cmc and above the cmc the adsorbed amount is almost constant. A bilayer at the surface can be formed if the lyophilic block adsorbs. Adsorption of the lyophilic blocks would expose the insoluble blocks to the solvent. Therefore, a second layer of molecules adsorbs with their lyophobic block towards the molecules attached to the surface. The influence of the interaction energies and the block sizes on these trends is described. The results obtained show good qualitative agreement with experimental results on surfactant adsorption.

The adsorption of random copolymers from solution is described in chapter 3. Experimentally, random copolymers are usually very polydisperse, both in chain length and in primary structure. Random copolymers which are only polydisperse in primary structure are considered here. They can be prepared experimentally by random chemical modification of monomer units of monodisperse homopolymers. The sequence distribution of random copolymers is determined by the fractions of the segment types in the polymer and the correlation factors between them. For random copolymers consisting of two different segment types, a blockiness parameter B is defined. The extremes of this parameter are -1 and 1, where the lower limit depends on the fractions of the different segment types. A value of B = -1 represents an alternating copolymer, whereas B = 1 stands for a mixture of two homopolymers. The complete statistical sequence distribution is implemented into the theory. In the results section random copolymers with two different segment types are studied. Chains with a higher than average content of adsorbing segments are preferentially adsorbed from the bulk solution. Only in the first few layers near the surface this preferential effect plays a role. In the remainder of the profile the segment types are more randomly mixed. The adsorption behaviour of these random copolymers is remarkably different from the adsorption of diblock copolymers. In the latter case, the chains have their adsorbing segments mainly in the layers near the surface, whereas further away from the surface long dangling tails of nonadsorbing segments are found. Random copolymers cannot spacially separate their segments so easily. Much higher adsorbed amounts are found for diblock copolymers than for random copolymers with the same fraction of adsorbing segments. The adsorption of random copolymers is less than that of homopolymer of equal length and consisting of the same type of adsorbing segments. Only for very high adsorption energies the adsorbed amounts are essentially the same. An increase in the blockiness parameter of the chains gives an higher adsorbed amount, but it is always below the adsorbed amount of the homopolymer. Analytical expressions have been derived which relate the interaction parameters of purely random copolymer and homopolymer.

In chapter 4 the interactions between surfaces coated with grafted polymer (also called hairy plates or soft surfaces) in the presence of nonadsorbing polymer is studied. The interaction free energy between the surfaces is obtained from the partition function. which is rederived for this more general case. For hard plates the interaction is fully determined by the osmotic pressure of the bulk solution and the depletion layer thickness. However. It turns out that In the case of soft surfaces the hairs have an attractive contribution to the free energy of interaction at a plate separation just below twice the hydrodynamic layer thickness of the grafted layer. The hairs mix mutually more easily than with free polymer. At a larger overlap of hairs the interaction becomes repulsive. In contrast with bare planar surfaces, the free energy of interaction between hairy surfaces shows a minimum as a function of the concentration of free polymer in the bulk solution. At a certain (very low) surface coverage the attraction is minimal. For even lower and for larger grafting densities the plates become more attractive. Increasing the repulsion between the hairs and free polymer makes the attraction stronger. The solvencies of grafted and free polymer have a less pronounced effect. Without free polymer, the interaction between the hairy surfaces becomes attractive if the solvency becomes worse than theta conditions.

It can be concluded that the self-consistent field theory has been successfully extended to three rather complex but technologically relevant systems. In this way a better understanding of the behaviour of polymers near interfaces has been obtained.