Interface thickness, block copolymers

Small-angle scattering intensity in the high angle (q) region can be analysed to provide information on interface thickness (e.g. the lamellar interface thickness block in copolymer melts or core-corona interface widths in micelles). For a perfectly sharp interface, the scattered intensity in the POrod regime falls as q A (Porod 1951). For an interface of finite width this is modified to... 

Comparing Eqs (3.23) and (3.25), it can be shown that the interface becomes broader in systems with low-molecular-weight components [45]. The experimentally estimated interfacial thicknesses [45-48] were found to be in reasonable agreement with the Helfand-Tagami and Broseta predictions. The distribution of the chain segments for systems containing at the interface a block copolymer as a compatibilizer, was derived by Noolandi [49]. Typical values of the interfacial thickness in different polymer blends are summarized in Table 3.1. 

Estimates of interfacial thicknesses have been made by analysis of electron micrographs or by scanning analytical electron microscopy. X-Ray and neutron scattering methods have also been used but most extensively in the study of the interface in block copolymers. It is generally necessary to fit the scattering profile to a complete model of the morphology which contains a diffuse interface. 

Neagu, C., Puskas, J.E., Singh, M.A., and Natansohn, A. Domain sizes and interface thickness determination for styrene-isobutylene block copolymer systems using solid-state NMR spectroscopy. Macromolecules, 33, 5976-5981, 2000. 

In addition to the previously mentioned driving forces that determine the bulk state phase behavior of block copolymers, two additional factors play a role in block copolymer thin films the surface/interface energies as well as the interplay between the film thickness t and the natural period, Lo, of the bulk microphase-separated structures [14,41,42], Due to these two additional factors, a very sophisticated picture has emerged from the various theoretical and experimental efforts that have been made in order to describe... 

In block copolymer thin films, the perpendicular orientation of microdomains relative to the substrate cannot be achieved by the shear methods developed in the bulk case. Based on the additional variables (film thickness and surface/interface interactions) in block copolymer thin films, as described in Sect. 2.1.2, three different strategies are generally applied for orienting block copolymer thin films ... 

Fig. 4a—d. Lamellar structures in thin films that are not considered further in detail in the present article a Thin film confined between inequivalent walls, where the lower one favors the B-rich domains and the upper one the A-rich domains. Then an arrangement where the interfaces run parallel to the walls requires that thickness D and wavelength X are related as D=(n+1/2)A, n=0,l, 2... b Thin film on a substrate that favors B-rich domains undergo at the order-disorder transition (ODT) of the block copolymer melt a phase separation into a fraction x of thickness nXh and a fraction 1-x of thickness (n+1) Xh, such that D=[xn+(l-x) (n+l)] K if the air also favors B-rich domains, c If the air favors A-rich domains instead, the phase separation happens in a fraction x of thickness (n-l/2)A and a fraction 1-x of thickness (n+ 1/2)X with n= 1,2,3... d If the block copolymer film undergoes dewetting at the substrate, droplets form with a step-pyramide like structure ( Tower of Babel [30]). 

Instead of observing the change of the morphology as a function of the film thickness, surface boundaries could also be used to control the wetting layer morphology at interfaces, the surface topographies, and the microdomain period [148]. In the case of symmetric or asymmetric wetting of the block copolymer at... 

Extensive neutron reflectivity studies on surfactant adsorption at the air-water interface show that a surfactant monolayer is formed at the interface. Even for concentration cmc, where complex sub-surface ordering of micelles may exist,the interfacial layer remains a monolayer. This is in marked contrast to the situation for amphiphilic block copolymers, where recent measurements by Richards et al. on polystyrene polyethylene oxide block copolymers (PS-b-PEO) and by Thomas et al. on poly(2-(dimethyl-amino)ethylmethacrylamide-b-methyl methacrylate) (DMAEMA-b-MMA) show the formation of surface micelles at a concentration block copolymer, where an abrupt change in thickness is observed at a finite concentration, and signals the onset of surface micellisation. 

For this type of test, the two homopolymers are typically compression molded into 1-2 mm thick sheets. A solution of the block copolymer is then spun-cast on one of the two polymers. The solvent and the homopolymer substrate to receive the block copolymer must be chosen so that the surface of the homopolymer substrate is not dissolved during the spin-casting operation. After a drying step, the two slabs are welded with the block copolymer in between to form an interface. The welding temperature is chosen to be above the respective glass transitions of the two homopolymers and the two slabs are held under a moderate pressure for a time sufficient to allow the local organization of the block copolymer at the interface to reach a metastable equilibrium. 

A second, seemingly less artificial, method would be to add a certain amount of block copolymer to one of the homopolymers and let it diffuse to the interface. This method has not been used to prepare fracture samples with deuterium-labeled block copolymers for several practical reasons Dissolving 5% of copolymer in a sheet of 50x100x2 mm dimensions requires 500 mg of deuterated polymer while a similar interface can be obtained by spin-casting with approximately 5 mg. The time involved to achieve true diffusional equilibrium over millimeter-scale thicknesses is prohibitively long at typical welding temperatures and the presence of a background concentration of deuterium-labeled polymer... 

The situation for diblock copolymers has been examined in detail by Washiyama et al. in the PS-PVP system [33]. As shown in Fig. 33, they found that for symmetric copolymers of 510-540, the fracture toughness passed through a maximum at 2 = 0.2 chains/nm2 corresponding to a nominal areal density of chains a little higher than what is contained in a pure block copolymer lamella of thickness L. At higher values of 2, TEM observations showed that a multilamellar structure formed at the interface, as illustrated in Fig. 34 for the 510-540 copolymer. The fracture toughness of such an interface dropped sharply to stabilize then at a plateau value. The areal density at which Cjc reached this plateau value corresponded closely to what would be expected for the thickness of 3/21. The... 

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