The stability of polystyrene (PS) films on silicon oxide is improved by blending with poly(styrene-block-methyl methacrylate) (PS-b-PMMA) having a short, adsorbing MMA block and long, dangling PS block (degree of polymerization N). Relative to PS (degree of polymerization P), hole growth velocity decreases by 5 and 17 times upon adding 0.05 volume fraction of PS-b-PMMA (φ) for N ∼ P and N ∼ 4P, respectively. In contrast to PS, holes approach a constant value and do not coalesce. The N ∼ 4P system provides better stabilization because of its broader interfacial width. For N ∼ P, hole velocity decreases as φ increases and then becomes constant for φ > 0.03. Relative to blends, holes grow faster in bilayers and eventually coalesce. AFM analysis shows that the PS hole floor is smooth whereas the blend floor contains patches. The film stabilizing effect of block copolymers can be attributed to a decrease in capillary driving force, entanglements at the matrix/copolymer interface, and brush grafting density.

The adsorption of block copolymers from a homopolymer melt is studied as a function of matrix molecular weight using neutron reflectivity and low energy forward recoil spectrometry. The block copolymer is poly͑deuterated styrene-block-methylmethacrylate͒ (dPS-b-PMMA), which contains short MMA and long dS blocks. The MMA block adsorbs to the silicon oxide surface, whereas the dPS extends into the matrix chains. The dPS-b-PMMA is blended with a polystyrene matrix of molecular weight P. Volume fraction profiles and copolymer coverage (z*) are investigated as a function of P. We find that z* initially increases rapidly with P and remains almost constant for P larger than 2N B (N B is the number of dS segments͒. We also observe that the thickness of the adsorbed layer (L) as well as the interfacial width between the brush and the matrix (w) initially decrease rapidly with increasing P and become weakly dependent on P for PϾ2N B . By measuring z*, L, and w as a function of P we observe a crossover from a stretched to collapsed brush at Pϳ2N B . For PϾ2N B the matrix chains are driven from the adsorbed layer by entropy. Self-consistent mean field predictions are in qualitative agreement with experimental results and provide an estimate for the MMA-wall interaction energy, Ϫ8kT/block. ͓S1063-651X͑97͒50609-X͔ PACS number͑s͒: 61.25. Hq, 36.20.Ϫr, 68.35.Gy

Neutron reflectivity (NR) and low-energy forward recoil spectrometry (LE-FRES) were used to study surface enrichment in miscible blends of deuterated polystyrene, d-PS, and poly(styrene-co-4-bromostyrene), PBr0.049S, having a 0.049 mole fraction of 4-bromostyrene units. The d-PS component was found to segregate preferentially to the polymer blend/air interface, whereas no enrichment of either component was detected at the polymer blend/silicon interface. The experimental values of the surface concentration, φ 1, and the surface excess, z*, of d-PS were interpreted using both the theory of Schmidt and Binder (SB) (J. Phys. II (Paris) 1985, 46, 1631) and the self-consistent field (SCF) approach of Genzer et al. (Phys. Rev. E 1994, 50, 2373). Although both SB and SCF models were found to be in good qualitative agreement with the experimentally measured values of φ 1 and z*, we demonstrate that the latter proves to be in better quantitative agreement with the experimental results. Moreover, a comparison of the SB and SCF volume fraction profiles of d-PS revealed that the SCF model described more accurately the experimental profile. We also demonstrated that adding long-range interactions to the surface potential in the SCF model produced just minor changes in the shape of the d-PS profile near the surface.

Using low-energy forward recoil spectrometry (LE-FRES) and neutron reflectivity (NR), the interfacial excess, z*, of an asymmetric poly(deuterated styrene-block-methyl methacrylate) (dPS-b-PMMA) at the polymer matrix/silicon oxide interface was found to decrease as the bromostyrene mole fraction, x, in a poly(styrene-ran-4-bromostyrene) (PBr x S) matrix systematically increased. For matrix degrees of polymerization, P = 480 and 3846, z* decreased by 15% and 33%, respectively, as x increased. Neglecting the matrix−substrate interaction energy, , self-consistent mean-field (SCMF) calculations predicted an increase in z* with x, consistent with an increase in unfavorable matrix−dPS interactions, χ. By including a small attractive interaction ( = −0.01k B T) between the matrix and substrate, the SCMF z* decreased by ca. 50%, in qualitative agreement with experiments. Thus, as x increased (and therefore ), matrix chain competition for silicon oxide counteracts the expected increased adsorption due to χ. Furthermore, the dPS volume fraction profile in PBr0.136S had a lower z* and was thinner than for the neutral matrix case.

The variation of entanglement density with interface width at an interface between two polymers is calculated using the relationships between chain packing and entanglement. The chain packing is obtained by the use of self-consistent mean-field techniques to calculate the average chain conformations within the interface region. The interface width is controlled by an assumed value of Flory−Huggins interaction parameter χ between the two polymers. As the value of χ is increased from 0 (completely miscible) to 0.1 (immiscible with a sharp interface), the calculated entanglement density is found to decrease by about a factor of 2. These modified entanglement densities are used within an existing model of interface coupling to estimate the effect of entanglement changes on the variation of interface toughness with interface width.