Detection of surface mobility of poly (2, 3, 4, 5, 6-pentafluorostyrene) films by in situ variable-temperature ToF-SIMS and contact angle measurements Yi Fu a , Yiu-Ting R. Lau b , Lu-Tao Weng a,c , Kai-Mo Ng a,d , Chi-Ming Chan a,e,⇑ a Department of Chemical and Biomolecular Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong b Nano and Advanced Materials Institute, Hong Kong Science Park, Shatin, New Territories, Hong Kong c Materials Characterization and Preparation Facility, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong d Advanced Engineering Materials Facility, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong e Division of Environment, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong article info Article history: Received 25 April 2014 Accepted 26 May 2014 Available online 14 June 2014 Keywords: Poly (2, 3, 4, 5, 6-pentafluorostyrene) ToF-SIMS Principal component analysis Contact angles Thin films Surface reorientation abstract Poly (2, 3, 4, 5, 6-pentafluorostyrene) (5FPS) was prepared by bulk radical polymerization. The spin-cast films of this polymer were analyzed using time-of-flight secondary ion mass spectrometry (ToF-SIMS) at various temperatures ranging from room temperature to 120 °C. Principal component analysis (PCA) of the ToF-SIMS data revealed a transition temperature (T T ) at which the surface structure of 5FPS was rear- ranged. A comparison between the results of the PCA of ToF-SIMS spectra obtained on 5FPS and polysty- rene (PS) indicate that the pendant groups of 5FPS and PS moved in exactly opposite directions as the temperature increased. More pendant groups of 5FPS and PS migrated from the bulk to the surface and verse versa, respectively, as the temperature increased. These results clearly support the view that the abrupt changes in the normalized principal component 1 value was caused by the surface reorienta- tion of the polymers and not by a change in the ion fragmentation mechanism at temperatures above the T T . Contact angle measurement, which is another extremely surface sensitive technique, was used to monitor the change in the surface tension as a function of temperature. A clear T T was determined by the contact angle measurements. The T T values determined by contact angle measurements and ToF-SIMS were very similar. Ó 2014 Elsevier Inc. All rights reserved. 1. Introduction The mobility of polymer chains at the free surface relative to the mobility of those in the bulk is a subject of great interest in poly- mer science [1–13]. Studies of the glass transition temperature (T g ) of thin films of polystyrene (PS) and its derivatives by several research groups using ellipsometry [9], Brillouin light scattering [14], local thermal analysis [15], and X-ray reflectivity [16,17] found enhanced chain mobility at the surface at temperatures below its bulk glass transition temperature (T b g ). However, these characterization techniques may not be the most suitable ones to measure the T g of the mobile surface layer (T s g ) because they mea- sure an average T g across the thickness of the whole film especially when the mobile surface layer may be much thinner than the film thickness. Mansfield and Theodorou performed a molecular dynamics computer simulation to investigate the free surface of a glassy atactic polypropylene exposed to vacuum at a temperature of 22 °C below its T b g [10]. The local mass-density distribution at the polypropylene/vacuum interface suggests the existence of a lower-density surface layer of roughly 1 nm thick. This lower-den- sity surface layer can be interpreted as a layer of polymer chains with higher mobility than those of the bulk. By combining an equa- tion-of-state theory and molecular simulation methods, Jo and Choi developed a new method to analyze the polymer surface properties of an amorphous polyethylene with two hundred mono- meric units [18]. The density profiles of the polymer at various temperatures indicate the existence of a one- to two-nm-thick lower-density surface layer. Based on these results, the appropriate techniques that should be used in determining T s g s must have sam- pling depths less than 1–2 nm and sensitive to changes induced by molecular motion as temperature increases. Keddie et al. first reported the measurements of T g values of PS thin films using ellipsometry and found the average T g of the films http://dx.doi.org/10.1016/j.jcis.2014.05.058 0021-9797/Ó 2014 Elsevier Inc. All rights reserved. ⇑ Corresponding author at: Department of Chemical and Biomolecular Engineer- ing, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong. E-mail address: kecmchan@ust.hk (C.-M. Chan). Journal of Colloid and Interface Science 431 (2014) 180–186 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis