340 SIMS Proceedings Papers Received: 4 September 2009 Revised: 1 March 2010 Accepted: 12 March 2010 Published online in Wiley Online Library: 11 May 2010 (wileyonlinelibrary.com) DOI 10.1002/sia.3404 Development of chain-folding of semicrystalline polymers in thin films: a combined ToF-SIMS and PCA analysis Yiu-Ting R. Lau, a Lu-Tao Weng, b Kai-Mo Ng a,c and Chi-Ming Chan a,d* Poly(bisphenol-A-etheralkane), BA-Cn(n, which is the number of aliphatic CH 2 units, = 8 and 10), composed of rigid and flexible segments, is a semicrystalline polymer. In 50-nm-thick films, the polymers tend to develop flat-on lamellae at the surfaces. Principal component analyses (PCAs) of the ion images were used to discriminate the conformation between the amorphous and flat-on lamellar surfaces of the polymers. The changes of the surface structure were directly detected by time-of-flight secondary ion mass spectrometry chemical imaging when flat-on lamellae were developed from the polymer melts. These structural changes were related to the lengths of the folded segments, which were strongly influenced by the crystallization temperature. The PCA loading analyses suggested that the length of the folded segments, which contained flexible and the rigid segments with an ether linkage, increased when the crystallization temperature decreased, suggesting the kinetic origin of polymer crystallization. Copyright c  2010 John Wiley & Sons, Ltd. Keywords: time-of-flight secondary ion mass spectrometry (ToF-SIMS); principal component analysis(PCA); fold nature; flat-on lamellae Introduction Methods to control the surface properties and wetting behavior of a polymer include the use of polymer blends, block copolymers, as well as grafted or end-functionalized architectures that contain surface-active components. [1,2] The surface energy difference between the components and the composition of the lower energy components are the two major factors controlling the surface segregation of amorphous polymers. [1–4] Surface segregation is much more complex in semicrystalline polymers than in amorphous polymers because of the interactions of polymer chains in the crystal phases. The component with lower surface energy did not necessarily segregate on the crystal surfaces when it was incorporated into a crystallizable polymer backbone because the enthalpic decrease due to crystallization can compensate for the increase of the surface free energy due to the migration of the component with lower surface energy to the bulk plus the decrease in the entropy of mixing. [5] Recently, we demonstrated a viable physical method to control the surface structure of a monodisperse oligomer. [6] The fold conformations of the lamellae could be controlled by the crystallization temperature. All these studies have clearly demonstrated the significance of crystallization in controlling the surface chemical properties of semicrystalline polymers. Up to now, very few reports were found in the literature on the investigation of the chemical changes of the polymer surfaces when the polymers crystallized from their melts. [5–8] The nature of chain-folding in polymer crystals is still not completely resolved [9] because of the absence of a definitive experimental probe. [10] In our previous work, [11] we used time-of-flight secondary ion mass spectrometry (ToF-SIMS) and atomic force microscopy (AFM) as complementary tools to differentiate edge-on and flat-on lamellar surfaces of a semiflexible polymer. The results, which were based on a univariate analysis of selected ions, indicated that ToF-SIMS can be potentially used as a direct probe to determine the fold conformations of polymer crystals. A number of fold structures are expected for high-molecular-weight polymers. Therefore, quantification based on univariate analyses of selected ions with known molecular specificities would be difficult and many potentially useful ions may be overlooked. Since the last decade, principal component analysis (PCA) and other multivariate statistical methods have often been used to facilitate the data interpretation of ToF-SIMS spectra and ion images. [12] Apart from the fact that PCA is a data-reduction method, it removes users’ bias toward the ion selection. In this paper, we investigate the potential applicability of ToF- SIMS and PCA to studying the development of chain-folding of high-molecular-weight polymers. PCA analyses of mass-resolved chemical images of amorphous and flat-on lamellar surfaces allowed direct discrimination of the surface conformation of the chain molecules before and after crystallization. One key advantage of ToF-SIMS is that chemical labeling for detection is not needed. The chemical information gives insights into the kinetic pathway of polymer crystallization, which remains one of the unsettled problems in polymer physics. [13] Furthermore, as ∗ Correspondence to: Chi-Ming Chan, Department of Chemical and Biomolecular Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong. E-mail: kecmchan@ust.hk a Department of Chemical and Biomolecular Engineering, Hong Kong University of Science and Technology, Hong Kong b Materials Characterization and Preparation Facility, Hong Kong University of Science and Technology, Hong Kong c Advanced Engineering Materials Facility, Hong Kong University of Science and Technology, Hong Kong d Division of Environment, Hong Kong University of Science and Technology, Hong Kong Surf. Interface Anal. 2011, 43, 340–343 Copyright c  2010 John Wiley & Sons, Ltd.