Appl. Phys. A 74 [Suppl.], S342–S344 (2002) / Digital Object Identiﬁer (DOI) 10.1007/s003390201857 Applied Physics A Materials Science & Processing Phase separation of weakly incompatible polymer blends conﬁned in isolated droplets P. Müller-Buschbaum 1, ∗ , R. Cubitt 2 , W. Petry 1 1 TU München, Physik-Department, LS E13, James-Franck-Str.1, 85747 Garching, Germany 2 ILL, BP 156, F-38042 Grenoble, France Received: 13 July 2001/Accepted: 24 October 2001 –  Springer-Verlag 2002 Abstract. Phase separation of the weakly incompatible blend system polystyrene and polyparamethylstyrene conﬁned in isolated droplets is investigated. The droplet geometry im- poses a two-dimensional spatial restriction. With specular, diffuse and grazing incidence small-angle neutron scattering the surface topography as well as the chemical morphology inside the droplets is determined. Due to the differences in surface tension, a core–shell-like structure characterized by one most prominent length scale inside the droplets is in- stalled. PACS: 68.45.Gd; 68.55.Jk; 83.10.Nn Phase separation phenomena are widely observed in bulk polymer blend systems. Due to the large degree of polymer- ization the entropic part of the free energy is extremely small and thus two different polymers are frequently not miscible. In bulk systems the phase separation of polymer blends was intensively investigated. As a function of blend composition and polymer-n-polymer interaction different phase separation morphologies have been observed. The symmetry breaking at an interface forces a phase separation morphology aligned parallel to the interface [1]. As a consequence, in thin poly- mer blend ﬁlms a surface directed spinodal decomposition is observed. With decreasing ﬁlm thickness of the blend ﬁlm, below a critical ﬁlm thickness, this process is suppressed [2] and phase separation perpendicular to the interface results [3]. One way to introduce an additional constraint is the fur- ther decrease of the ﬁlm thickness. Due to the internal size of the individual polymer molecules which is described by the radius of gyration R g , a further change in the phase separation is observed below a characteristic thickness of 2 R g [4]. A sec- ond way is the investigation of very small droplets containing the polymer mixture. In general, these droplets have the geo- metrical shape of a spherical cap. The height of the droplets is small compared to their diameter. Consequently, the polymer molecules are restricted into a two dimensional circular area. ∗ Corresponding author. (Fax: +49-89/2891-2473; E-mail: muellerb@ph.tum.de) Of course such droplets need to be prepared on top of a solid support like glass surfaces. The detailed preparation proced- ure based on a destabilization mechanism of an initially ho- mogeneous blend ﬁlm is explained elsewhere [5]. In Fig. 1 an example of small droplets as measured with atomic force mi- croscopy (AFM) is shown. These droplets are well separated and nearly monodisperse in their size. While atomic force microscopy pictures very nicely the real space surface of the sample, it delivers neither infor- mation about the chemical composition of the surface nor of the internal morphology. One way to detect the internal arrangement of the two polymer species of a blend is the use of neutron scattering. Due to deuteration of one kind of polymer even chemically very similar polymers like poly- styrene and polyparamethylstyrene, which differ only by one methyl group, are distinguishable. Other surface characteri- zation methods like friction and stiffness measurements yield not enough contrast to distinguish between the components and a selective dissolution of one component is not possible. In the presented investigation specular, diffuse and grazing in- cidence small angle neutron scattering were applied to detect the surface topography as well as the chemical morphology inside the droplets. The used polymers, deuterated polystyrene (dPS) and polyparamethylstyrene (PpMS), have a narrow molecular weight distribution of M w / M N (dPS) = 1.09 and M w / M N (PpMS) = 1.06. For the used molecular weight M w = 157 000 g/mol the blend system dPS:PpMS is weakly incom- patible. The polymer-polymer interaction parameter χ = A + B/T with A =-0.011 and B = 6.8K -1 was determined previously [6]. Both polymers were blended in a symmet- ric blend ratio of dPS and PpMS in a toluene solution. By varying the polymer concentration different ﬁlm thicknesses were installed. Prior to spin-coating (1950 rpm for 30 s) the substrates, ﬂoat glass plates, were cleaned in an acid bath. The cleaning bath consists of 100 ml of 80% H 2 SO 4 , 35 ml H 2 O 2 and 15 ml deionized water. After 15 minutes at 80 ◦ C in the acid bath the substrates were taken out, rinsed in deion- ized water and dried with compressed nitrogen. Immediately before coating the dry substrates were ﬂushed with fresh toluene.