Thickness-Dependent Thermal Transition Temperatures in Thin Conjugated Polymer Films ² M. Campoy-Quiles, ‡ M. Sims, ‡ P. G. Etchegoin, § and D. D. C. Bradley* ,‡ Ultrafast Photonics Collaboration, Carbon Based Electronics Consortium and Experimental Solid State Physics Group, Blackett Laboratory, Imperial College London, South Kensington, London SW7 2BZ, U.K., and The MacDiarmid Institute for AdVanced Materials and Nanotechnology, and School of Chemical and Physical Sciences, Victoria UniVersity of Wellington, PO Box 600, Wellington, New Zealand ReceiVed March 14, 2006; ReVised Manuscript ReceiVed July 17, 2006 ABSTRACT: We report the effects that geometrical confinement has on both the glass transition temperature, T g , and the crystalline phase transition temperature, T c , for two widely studied conjugated polymers, namely poly(9,9-dioctylfluorene) [PFO] and poly(9,9-dioctylfluorene-co-benzothiadiazole) [F8BT]. The T g and T c values were determined for thin film samples via temperature-dependent ellipsometry measurements. The thickness- dependent T g (T c ) behavior is characterized by three regimes, namely, (i) large d or bulk samples for which T g ) T g bulk (T c ) T c bulk ), (ii) intermediate d samples for which T g > T g bulk (likewise for T c ), and (iii) ultrathin samples for which T g drops again (likewise for T c ). The intermediate regimes occur for 160 nm > d > 60 nm and 300 nm > d > 80 nm for PFO and F8BT, respectively. The higher-than-bulk T g and T c values offer the potential to design more robust and thermally stable polymer optoelectronic devices, including light-emitting diodes, lasers, and solar cells. Introduction The condensed phase behavior of polymer glasses is depend- ent upon the ability of the constituent molecules to undergo large-scale cooperative motions (R-relaxation). The glass transi- tion temperature, T g , marks, in simplest terms, the temperature at which the elastic modulus (and viscosity) of the polymer glass falls and hence at which R-relaxation becomes more probable. A polymer’s ability to become rubberlike can, however, be affected by local variations in the interaction with its surround- ings, e.g., near to a substrate interface. The wetting properties of a particular substrate can then play a central role in determining a local T g . In situations where interfacial forces are attractive they can act to inhibit cooperative dynamics and lead to a rise in T g . On the other hand, the extra mobility afforded to polymer chains by a relaxation of constraints at a “free” surface may reduce T g , especially, for instance, if there is a segregation of chain ends to the surface and a consequent reduction in packing density. Deviations from bulk thermody- namic properties should not then be surprising when the polymer film thickness acts to confine motions on characteristic molec- ular length scales. For example, the dependence of T g on film thickness has been extensively investigated 1-4 in nonconjugated polymer films such as poly(methyl methacrylate) (PMMA) and polystyrene (PS) (for a review see ref 5). Low molecular weight glass formers 6-8 have been similarly studied. In most cases, there is an apparent depression in polymer T g with decreasing film thickness. 1-5,9 Confinement effects typically onset at d < 40- 60 nm in PS, although this value is reported to depend on parameters such as polymer molecular weight 4 and chain stiffness. 10 Such behavior has been related to an enhanced free volume (lower T g ) afforded by a relaxation in chain mobility constraints at the polymer-air interface 3 coupled with relatively weak polymer-substrate interactions. This is especially clear in studies of free-standing PS films, which show a much more pronounced reduction in T g than for films supported on SiO x substrates. 2 Interestingly, the T g depression in PS shows little dependence on the type of substrate used. 5 On the other hand, the thickness dependence of T g in other polymers has been shown to be very sensitive to the type of substrate used. For example, T g in poly(2-vinylpyridine) (P(2)PV) increases with decreasing film thickness when films are spin-coated onto the oxide surface of a Si wafer. 11 It has also been shown that the T g of isotactic-PMMA films can either increase or decrease as a function of decreasing film thickness depending on whether the polymer is deposited on an aluminum or a silicon surface. 12 Very recently, nonmonotonic variations with thickness (where both T g > T g bulk and T g < T g bulk are found for the same polymer over different thickness ranges) have been reported for thin films of PS and PS derivatives when probed with shear-modulated scanning force microscopy. 13 Ellison et al. 10 have correspond- ingly encouraged the study of polymer systems with physico- chemical properties distinct from typical PS and PMMA test bed samples in order that a better understanding of the range of behaviors and the underlying physical processes can be developed. These authors have also focused attention on the need to consider nonuniformity in T g across the thickness of a film: Any suppression or enhancement in T g is not expected to be a uniform effect since the interactions that control it are clearly nonuniform. 10,13,14 Many of the studies to date have been motivated by the importance to advances in technology of an understanding of the behavior of polymers on ever decreasing length scales. The as yet largely unpredictable behavior of confined polymer systems has implications for a wide range of applications. In addition to the already mentioned variations in T g , there are documented variations in melting point, 14 chain diffusion, 9 intrinsic viscosity, 9 electrical insulation, 15 optical anisotropy, 16 and film morphology and moisture uptake. 17 Here, we are particularly interested in understanding confinement effects ² Based in part on a presentation to the American Physical Society March Meeting, 2005. ‡ Imperial College London. § Victoria University of Wellington. * Author for correspondence: e-mail D.Bradley@imperial.ac.uk. 7673 Macromolecules 2006, 39, 7673-7680 10.1021/ma0605752 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/03/2006