Phase behavior of thin film blends of block copolymers and homopolymers: Changes in domain dimensions

We used atomic force microscopy to examine the phase behavior of thin film mixtures of a symmetric A-B diblock copolymer of degree of polymerization NA-B = 650 with (i) two homopolymers (N-A = 20 and 250) and (ii) a symmetric A-B diblock copolymer (NA-B = 200). Thin film symmetric diblock copolymers are known to undergo a transition from a disordered phase to an ordered lamellar phase in which the lamellae orient parallel to the plane of the substrate. We examined films sufficiently thin in order to determine the effect of the substrate on-the phase behavior. Homopolymer volume fractions of up to 50% were studied. The interlamellar spacing increased with the addition of both low- and high-molecular-weight homopolymers, although it was less extensive for the lower molecular weight homopolymer. Both situations could be well-described by theory relying on assumptions about the homopolymer distribution in the domains and the areal densities occupied by the copolymer junctions. In contrast to the homopolymers, the lamellar spacing of blends formed with the second diblock copolymer (NA-B = 200) decreased with increasing volume fraction of this shorter chain copolymer. This behavior could be described by assuming that L proportional to N-w(2/3), where N-w is the weight-average degree of polymerization of the mixture. We also show that the degree of swelling of the domains depends on the proximity of the domain to the substrate.