This thesis is organized as follows:
• Chapter 2 includes the presentation of the model system used in this study and the analytical methods used to characterize the system.
• The processes involved when soft polymeric materials are imaged with tap-ping mode scanning force microscopy (TM-SFM) are discussed in detail in chapter 3. A way to quantify the indentation of a SFM tip into a soft sam-ple is presented. The procedure enables the reconstruction of a true surface image and a clear assignment of the phase contrast to material properties in our two component system.
• A strong segregation theory (SST) for cylinder forming block copolymers is introduced in chapter 4. The theory is an extension of the advanced SST of Olmsted and Milner [Olm94] to thin films. In particular the princi-pal microdomain spacings of a cylinder forming structure in a thin film is discussed.
• Chapter 5 covers the phase behavior of a cylinder-forming ABA triblock copolymer/chloroform system in thin films. The use of the solvent allows us to tune the interactions governing the system. The stable phases are mapped as a function of film thickness and polymer concentration.
• The principal microdomain spacings of the thin film microdomain struc-ture are investigated in detail in chapter 6 and compared to the SST re-sults presented in chapter 4. A new image analysis algorithm provides the recognition and the localization of the different structures in the SFM phase contrast images. It also allows the determination of local director and, if applicable, the curvature of the microdomain structure. The mi-crodomain spacings are discussed as a function of various parameters like the film thickness, the polymer concentration and the local curvature of the structure.
• In chapter 7 the first in-situ observation of phase transitions in thin block copolymer films is presented. The measurements capture the fluctuations
of the microdomain structure, the nucleation and growth of two new phases and the healing of a phase by defect motion and annihilation.
• A summary of the results presented in this thesis is given in chapter 8.
2.1 Polymers
2.1.1 Polystyrene-b-polybutadiene-b-polystyrene (SBS) triblock copolymer
The model block copolymer used in this study is a polystyrene-b-polybutadiene-b-polystyrene (SBS) triblock copolymer. SBS was obtained from Polymer Source Inc. with molecular weights Mw,P S = 14.0 kg/mol, Mw,P B = 73.0 kg/mol, and Mw,P S = 15.0 kg/mol (PS is polystyrene, PB is 1,4-polybutadiene). The poly-dispersity of the polymer is given by the manufacturer asMw/Mn = 1.05.
The bulk behavior of the polymer is determined by the volume fraction fP S
of PS and the interaction χP S,P BN. Taking into account the densities of the homopolymers (1.05 g/cm3 for PS and 0.93 g/cm3 for PB) [Bra89] the volume fraction of PS isfP S = 0.26. The degree of polymerizationN may be calculated by the molecular weight of the monomers and the molecular weight of the blocks.
The result isN = 1628.
The interaction parameter of PS-PB block copolymers was studied by several groups [Hew86, Owe89, Sak92, Ada98]. The determination of the χP S,P B pa-rameter relies on a comparison of the experimentally found ODT temperature to a theoretically expected one. Note that there are different methods to obtain χP S,P B, which are not consistent [Mau98]. Here we would like to approximately determine the degree of segregation of our block copolymer and therefore average over the data forχP S,P B found in literature.
It should be noted that the interaction parameter depends on the microstruc-ture of the polymers, i.e. the percentage of 1,2 linkage of PB. The temperamicrostruc-ture applied for the measurements always exceeded 100◦C. The resulting χ parame-ters agree well for temperatures around 150◦C, so we get a rather large scatter for the χparameter at 25◦C.
Owens et al. reported for a SB diblock copolymer with Mn = 1.86 104g/mol and fP S = 0.49 and 95% 1,2 linkage of BP
χS−B =−0.021 + 25/T. (2.1) Hewel et al. found for a series of SB diblock copolymers with fP S = 0.3 and 1.80 g/mol≤Mn ≤unit[4.00]g/mol and undefined microstructure
χS−B =−0.027 + 28/T. (2.2) Sakurai et al. examined commercially available triblock SBS copolymers from Shell, the TR-1102 and the Kraton D-1102. They have a volume fraction of PS of fP S = 0.31 and fP S = 0.28, 7.4% and 9.8% 1,2 linkage of BP and Mw = 5.9 104g/mol and Mw = 5.7 104g/mol, respectively. A neutral and theta solvent dioctyl phthalate (DOP) was added to the polymers with concentrations of the polymer of φP = 0.61...1. Their result is
χS−B = 6.5910−3 + 13.6/T. (2.3)
For T = 25◦C averaging yields a value of χP S,P B = 0.061 ± 0.008. The total interactions between the blocks is calculated by multiplying χP S,P B by the degree of polymerization N in the case of diblock copolymers. For ABA triblock copolymers an intuitive calculation would be χN/2, since the triblock may be considered as two linked diblocks with degree of polymerization N/2.
Calculating χN/2 at T = 298 K yields 51.2, 54.5 and 42.5 for equations 2.1, 2.2 and 2.3, respectively. The average value is 49 ±6. Therefore our block copolymer is in the intermediate segregation regime. Dilution with the nonselec-tive solvent chloroform as used in this study should lead to a weaker segregated system according to the ”dilution approximation” as discussed in chapter 1.
The phase behavior is also influenced by the asymmetry of the monomers.
Although the segment length of the two monomers are almost identical (bP S = 0.70; bP B = 0.65), there exists a large asymmetry in the monomer volumes
vk =Mk/rhok: vP S= 107.4 cm3/mol; vP B = 58.3 cm3/mol.
The polymer is ideally suited for SFM studies, since the two components ex-hibit a large mechanical contrast and the majority component PB is soft at room temperature. The glass transition temperature of the homopolymer PB is Tg,P B = −83..−107◦C and the one of PS is Tg,P B = 80..100◦C [Bra89]. Since both ends of the copolymer are anchored in glassy domains a pickup of single molecules by the SFM tip is highly unlikely.
The surface tensions to air and the interfacial tensions to the substrate, i.e.
SiOxare important for the thin film behavior of the polymer. The surface tension of PB ofγP B = 31 mN/m is considerably smaller than the surface tension of PS, γP S = 41 mN/m [Sto96]. The interactions of the blocks to the substrate are unknown quantitatively. Harrison et al. [Har98a] found the PB block of PS-PB diblocks covering the native oxide of the SiOx substrate by SIMS.
2.1.2 Homopolymers
Homopolymers have been used to study the swelling behavior of PS and PB in chloroform. Polystyrene (PS) with and Polybutadiene (PB) homopolymers were obtained from Polymer Standard Service (Mainz, Germany). The PS ho-mopolymer had a molecular weight of Mw,P S = 520 kg/mol and a polydis-persity of Mw/Mn = 1.03. The PB homopolymer had a molecular weight of Mw,P B = 47 kg/mol and a polydispersity of Mw/Mn= 1.04.