Surface Enrichment in a Miscible Polymer Blend: An Experimental Test of Self-Consistent Field and Long-Wavelength Approximation Models J. Genzer,* ,² A. Faldi, ‡ R. Oslanec, and R. J. Composto Department of Materials Science and Engineering and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6272 Received July 28, 1995; Revised Manuscript Received May 9, 1996 X ABSTRACT: 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, φ1, 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 φ1 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. 1. Introduction Wetting, lubrication, and weatherability are just a few examples of materials properties that are controlled by the near-surface composition of polymer coatings. 1,2 These coatings may contain several components, one of which usually enriches the surface. The macroscopic characteristics can be routinely measured by traditional techniques such as contact angle measurements. How- ever, the engineering of surface properties of polymer coatings requires control over microscopic parameters such as polymer-surface interactions, molecular weight, etc. With the emergence of new depth-profiling tech- niques, such as neutron reflectivity, the surface com- position of polymers can be measured with unprec- edented resolution. Combined with theoretical models, these new experimental tools have helped polymer scientists develop a better understanding of the driving forces that control the behavior of polymers at surfaces. 3 Using low-energy forward recoil spectrometry (LE- FRES) and neutron reflectivity (NR), we have measured both the surface excess, z*, and the surface volume fraction, φ 1 , of the segregating species in a binary miscible polymer blend. These measurements are the first rigorous test of the widely used model of Schmidt and Binder (SB) and the self-consistent field (SCF) approach. We will show that the SCF model is in better agreement with experimental results, mainly because it does not use the long-wavelength approximation made in the SB approach. Systematic experimental studies of the surface en- richment in miscible binary polymer blends have been in progress since the first quantitative study by Bhatia and co-workers for blends of polystyrene and poly(vinyl methyl ether). 4 To date, the most complete study was carried out by Jones and co-workers, who used FRES, NR, time-of-flight FRES (TOF-FRES), and dynamic secondary ion mass spectrometry (DSIMS) to investigate the surface enrichment in isotopic blends of high mo- lecular weight polystyrenes, d-PS:PS. 5-8 It was dem- onstrated that in this system the surface is enriched by the d-PS component, which has a lower surface energy than its hydrogenated counterpart. The effect of molecular weight on the surface enrichment in d-PS: PS blends was studied by Composto et al. using NR 9 and Hariharan et al. by NR and DSIMS. 10 Hariharan et al. 11 as well as Budkowski et al. 12 also studied surface segregation as a function of film thickness. Surface enrichment was also examined in mixtures of statistical copolymers. Norton et al. used NR to investigate the surface enrichment of deuterated poly- (ethylenepropylene), d-PEP, in isotopic mixtures of d-PEP with its hydrogenated counterpart, PEP. 13 In addition to an isotope effect, surface segregation in statistical copolymer blends can be driven by a differ- ence in copolymer composition. For example, Steiner and co-workers used nuclear reaction analysis (NRA) to measure surface segregation of a deuterated poly- ethylene and poly(ethylethylene), d-PE y PEE 1-y , statisti- cal copolymer blended with PE z PEE 1-z having a differ- ent copolymer composition (y * z). 14 In this system, the component with the larger PEE content was found to segregate preferentially at the surface. NR, 15,16 DSIMS, 15 TOF-FRES, 17 and LE-FRES 16 were used to investigate surface segregation in binary mixtures of poly(styrene- co-acrylonitrile)’s, d-S y AN 1-y and S z AN 1-z where y * z. It was found that the SAN with the lower AN content segregates to the surface. Recently, mixtures of a homopolymer and a statistical copolymer were used as model systems to investigate the phenomenon of surface enrichment. Bruder and Brenn 18,19 and Gluckenbiehl et al. 20 studied mixtures of d-PS and poly(styrene-co-4- bromostyrene), PBr x S, where x is the mole fraction of the 4-bromostyrene units, and detected that the surface region was enriched with d-PS, the lower surface energy component. ² Current address: Department of Materials Science and En- gineering, Cornell University, Ithaca, NY 14853. ‡ Current address: Exxon Chemical Company, P.O. Box 5200, 5200 Bayway Drive, Baytown, TX 77522-5200. X Abstract published in Advance ACS Abstracts, June 15, 1996. 5438 Macromolecules 1996, 29, 5438-5445 S0024 9297(95)01108 9 CCC $12 00 © 1996 A i Ch i lS it + +