Polymorphic Behavior of Syndiotactic Polystyrene Crystallized in Cylindrical Nanopores Hui Wu, † Wei Wang, † Yan Huang, † Cheng Wang, ‡ and Zhaohui Su* ,† State Key Laboratory of Polymer Physics and Chemistry and State Key Laboratory of Rare Earth Resources Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, P. R. China ReceiVed July 6, 2008 ReVised Manuscript ReceiVed August 18, 2008 Introduction Structure and morphology of polymers under nanoscale confinement have attracted considerable interest recently. For semicrystalline polymer materials, the degree of crystallinity and the orientation of the crystalline domains are important factors in controlling their physical properties, and many efforts have been devoted to the understanding of the crystallization and orientation behavior of polymers in one-, 1-4 two-, 5-14 or three-dimensional 15-18 confinements. For example, in 1D confinement, an extensive reduction in crystallinity and a reduction in the rate of crystallization were reported for ultrathin films; 1 the backbone of poly(di-n-hexylsilane) lied extended with the polymer axis parallel to the plane of the film, while the side chains were extended with their carbon plane mostly perpendicular to the substrate. 2 In 2D confinements, i.e. in nanocylindrical geometry, polymers inside organic 5-8 or inor- ganic 9-13 templates exhibit preferred orientation. Studies on the crystallization of poly(ethylene oxide) (PEO) confined in nanocylinders indicate that the orientation of the PEO crystals is dependent on the crystallization temperature. 6,7 The crystals in the nanorods crystallized at low supercooling exhibit a perpendicular orientation, 8-12 and the crystallinity is reduced in contrast to the bulk. 11,12 In nanotrenches PVDF crystals were found to orient with the chain axis parallel to the walls. 14 For polymer under 3D confinements, nanoscale polyethylene spheres 15 and PEO droplets 16 were investigated, and in both cases the crystallization was initiated by homogeneous nucle- ation, which is much different from that for typical polymers in the bulk where heterogeneous nucleation dominates. Homo- geneous nucleation within each nanosphere led to isothermal crystallization which followed first-order kinetics. 15 In addition, it has been reported that for the crystallization of semiflexible homopolymers confined in isolated nanodomains nucleation occurs predominantly at the domain interface, and the domain interface accelerates the process. 17 In addition to experimental results, dynamic Monte Carlo simulation has been applied to study polymer crystallization under cylindrical confinement. 19-21 It is well-known that anodic aluminum oxide (AAO) mem- branes consisting of ordered straight separated cylindrical pores are ideal templates for the fabrication of polymer nanorods and for morphological study of polymers under cylindrical confine- ment. 9-13 The thermal stability and mechanical rigidity of the alumina wall provide a strictly constrained environment and avoid the breakdown of the cylindrical confinement. The characteristic feature size and shape of the nanomaterial are easily controlled by the distribution of the template pores, and thus the mechanical, optical, and electrical properties of the polymer are largely determined by the internal morphology. Syndiotactic polystyrene (sPS) is a semicrystalline polymer that has received considerable academic and industrial attention owing to its desirable physical properties such as high stiffness both above and below the glass transition temperature, good thermal and chemical stability, and low dielectric constant. 22,23 It exhibits rapid crystallization rate, high crystallinity, and high melting temperature due to the high stereoregularity of the polymer chain. 23 Of particular interest is that sPS exhibits complicated polymorphic behavior involving four identified crystalline forms: R, , γ, and δ. It has been shown that the γ- and δ-forms possess s(2/1)2 helical chain conformation of regular repetition of TTGG, while the R- and -forms both contain planar zigzag chains (TTTT conformation). 24 The polymorphism of this polymer is strongly dependent on the thermal and solvent treatment. 25 Typically, R-form is favored under the conditions of fast cooling from the melt to room temperature, 24 or melt crystallization at low temperatures (<230 °C), 26 or cold crystallization from quenched glass. 24 On the other hand, the -form can be obtained by either slow cooling from the melt, 27 or growth from solution, 27 or transformation from other crystal forms (δ, γ, or R) with the plasticization of the solvents. 25,28-30 In our previous work, we studied the crystallization of sPS in the nanopores of 200 and 80 nm using FTIR and TEM, and it was found that at low supercooling only -crystals formed in the nanorods with the c-axis aligning perpendicular to the axial direction of the nanorod, and the degree of crystallinity was significantly lower than that in the bulk. 11 In order to explore the morphology and phase behavior of the polymer under stronger confinement conditions, in the present work we prepared sPS nanorods with diameter of 32 nm using AAO templates and investigated the crystallization and polymorphic behavior of the polymer in the cylindrical nanopores. Experimental Section SPS pellets (Dow Questra F-2250, M w ) 2.6 × 10 5 , M w /M n ) 2.0) were used as received. A transparent amorphous sPS film with thickness of ∼180 µm was obtained by compression molding several sPS pellets at 300 °C and quickly quenching the film in ice water. The AAO templates with 32 nm pore diameter and 90 µm pore depth were prepared via a two-step anodization process using oxalic acid as the electrolyte. Details of the anodization process can be found elsewhere. 31 Prior to use, the templates were ultrasonicated in solvents of different polarity, such as deionized water, ethanol, chloroform, and acetone, for 2 min each. In order to study the polymorphic and crystallization behavior of the polymer confined within the 32 nm nanopores, nanorods with different thermal histories were prepared. An alumina membrane was placed on top of the sPS film supported by a glass slide, and the assembly was heated at 300 °C for 1.5 h under a nitrogen atmosphere so that the sPS melt was drawn into the membrane pores by capillary force. The assembly was cooled down from molten state to 255 °C quickly and crystallized at this temperature for 2 h and then cooled to room temperature slowly in a homemade temperature controller (cooling rate less than 1 °C/min) (sample A). Sample B was crystallized from the molten state at 255 °C for 2 h and then quenched in ice water. Sample C was obtained by annealing sample B at 255 °C for 2 h again and then cooled to room temperature in the temperature controller slowly. Sample D was crystallized from the * To whom all correspondence should be addressed: Tel (86)431- 85262854; Fax (86)431-85262126; e-mail zhsu@ciac.jl.cn. † State key Laboratory of Polymer Physics and Chemistry. ‡ State Key Laboratory of Rare Earth Resources Utilization. 7755 Macromolecules 2008, 41, 7755-7758 10.1021/ma801498b CCC: $40.75  2008 American Chemical Society Published on Web 09/27/2008