Article pubs.acs.org/Macromolecules
Molecular Order and Dynamics of Nanometric Thin Layers of Poly(styrene‑b‑1,4-isoprene) Diblock Copolymers Wycliffe K. Kipnusu,†,* Mahdy M. Elmahdy,†,‡ Martin Tress,† Markus Fuchs,† Emmanuel U. Mapesa,† Detlef-M. Smilgies,§ Jianqi Zhang,⊥ Christine M. Papadakis,⊥ and Friedrich Kremer† †
Institute of Experimental Physics I, Leipzig University, Linnéstraße 5, 04103, Leipzig, Germany Department of Physics, Mansoura University, Mansoura 35516, Egypt § Cornell High Energy Synchrotron Source (CHESS), Wilson Laboratory, Cornell University, Ithaca, New York 14853, United States ⊥ Technische Universität München, Physik-Department, Physik weicher Materie, James-Franck-Straße 1, 85748 Garching, Germany ‡
ABSTRACT: Order and dynamics of poly(styrene-b-1,4-isoprene), P(S-b-I), diblock copolymers in nanometer thin layers with different isoprene volume fractions ( f PI) and identical molecular weight of the styrene blocks are studied by a combination of grazing-incidence small-angle X-ray scattering (GISAXS), atomic force microscopy (AFM), and broadband dielectric spectroscopy (BDS). GISAXS and AFM reveal randomly oriented lamellar structures in the films and a parallel orientation at the top surface, respectively. Using BDS, three well separated relaxation processes are detected: (i and ii) the dynamic glass transitions (segmental mode) in the styrene and isoprene blocks, respectively, and (iii) the normal mode relaxation representing fluctuations of the isoprene chain as a whole or parts of it. While the first two do not show any thickness dependence in their spectral positions, the latter becomes faster with decreasing sample thickness. This reflects the difference in the length-scale on which the molecular fluctuations take place. al.24 combined different experimental techniques to investigate the effect of film thickness (150−2410 nm) on the phase behavior of poly(styrene-b-isoprene) (P(S-b-I)) and found that the order-to-disorder transition temperature (TODT) increases with decreasing film thickness. Symmetrical diblock copolymers often form a lamellar morphology with flat lamellar interfaces.25 In thin films, the orientation of these structures depends on the details of the polymer/substrate interactions, on the commensurability between the film and the lamellar thickness as well as on the interplay of surface tension and surface entropy at the polymer/ air and the polymer/substrate interface.26 Systematic studies on lamellae-forming poly(styrene-b-butadiene) P(S-b-B) have been carried out using grazing-incidence small-angle X-ray scattering (GISAXS) and atomic force microscopy (AFM)27,28 where it was found that in low molar mass P(S-b-B) the lamellae are parallel, whereas for high molar masses, the lamellae are perpendicular to the substrate. The impact of molar mass, and chain topology on the molecular dynamics of bulk P(S-b-I) has been studied by broadband dielectric spectroscopy (BDS).29−31 It was found that the variation of
1. INTRODUCTION Investigation of structure and dynamics of diblock copolymers in thin films is of special interest both for fundamental understanding and for their applications in emerging nanotechnologies including nanotemplating,1 organic optoelectronics,2 and antireflection coatings.3 Diblock copolymers are attractive for the aforementioned applications because they form periodic “microdomain” structures of various geometries on nanometric length scales. For this reason, they are also ideal candidate materials to probe the effect of the internal selfordering on the molecular dynamics especially under external confinement, for instance in thin films. Dynamics of thin films of homopolymers have been studied extensively,4−12 and still remain a hot topic, partly because of the contrasting results regarding the impact of geometrical constraints on the glass transition temperature (Tg).13−15 Internal ordering in the form of mesophase structures introduces additional constraints to the molecular dynamics of thin films of copolymers, and it is interesting to find how this alters the overall dynamics. The mesophase morphology is controlled by a number of parameters, such as the Flory−Huggins interaction parameter (χ), degree of polymerization (N), block volume fraction (f), chain architecture (e.g., linear, star, etc.),16 the roughness of the substrate,17 interactions at the air/polymer and polymer/ substrate interfaces,18−21 and the film thickness.22,23 Jung et © 2013 American Chemical Society
Received: September 17, 2013 Revised: November 4, 2013 Published: December 11, 2013 9729
dx.doi.org/10.1021/ma4019334 | Macromolecules 2013, 46, 9729−9737
Macromolecules
Article
Table 1. Molecular Characteristics of P(S-b-I) (SI) Diblock Copolymers and Their Corresponding Homopolymers (PI and PS) Used in This Studya sample
MnPS (g/mol)
MnPI (g/mol)
Mn(total)
Mw(total)
Mw/Mn
f PI
χN at 298 K
SI-65 SI-81 PI-23 PI-42 PS-58
39 800 39 000 -
25 600 42 000 -
65 400 81 000 23 000 42 000 58 317
68 670 87 480 24 380 44 520 58 900
1.05 1.08 1.06 1.06 1.01
0.43 0.55 1.0 1.0 0
69.5 93.5 -
a The Flory−Huggins interaction parameter (χ), degree of polymerization (N) were obtained as discussed in refs 32 and 33, respectively. f PI is the volume fraction of isoprene blocks while Mw/Mn is the polydispersity.
2.2. Film Preparation. Nanostructured and blank silicon substrates were cleaned in an ultrasound bath of pure acetone and then dried with compressed dry nitrogen flow. As a second step, the substrates were inserted into a plasma cleaner (Plasma Cleaner Femto Timer) for ∼5 min and thereafter cleaned with a jet of supercritical CO2 while dried at 150 °C. Solutions of different concentrations of P(S-b-I) in chloroform were spin-cast on cleaned blank wafers at 3000 rpm for 20 s to form thin films of various thicknesses. To remove residual solvent and eliminate stress imposed by the spin-casting, all polymer films were annealed for 48 h at 423 K (i.e., Tg(PS) + 50 K) in oil-free high vacuum (10−6 mbar) before the GISAXS, AFM and BDS measurements. This annealing temperature is below the order-todisorder transition temperature (TODT) of P(S-b-I). Using the experimental value of (χN)ODT = 17.5 for both f(PI) = 0.43 and 0.55,32 we estimate the TODT of the present samples to be 603 and 661 K, respectively. 2.3. Atomic Force Microscopy (AFM). The absolute film thickness, and the surface topography were measured with a Veeco Dimension 3000 Metrology AFM (Digital Instruments, Veeco Metrology Group) operated in tapping mode by using silicon tips (nominal tip radius of curvature
