Comparison of poly(ethylene oxide) crystal orientations and crystallization behaviors in nano-confined cylinders constructed by a poly(ethylene oxide)-b-polystyrene diblock copolymer and a blend of poly(ethylene oxide)-b-polystyrene and polystyrene☆
Introduction
In recent years, interests have been attracted to the crystallization behaviors and crystal orientations in nano-confined environments [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. To generate a nano-confined environment, one usually uses crystalline–amorphous diblock copolymers [such as poly(ethylene oxide)-b-polystyrene (PEO-b-PS) diblock copolymers] or copolymer blends (such as PEO-b-PS with PS or PEO homo-polymer blends). These systems are chosen because when the two components are in a strong phase separation, which is much below their order-disorder transition temperature (TODT), phase structure can be controlled to be lamellae, double gyroids, hexagonal cylinders (HC), or face-centered cubic spheres based on diblock copolymer or blend volume fractions [11]. If the glass transition temperature (Tg) of amorphous blocks (or blends) is higher than the melting temperature (Tm) of crystalline blocks (or blends), a hard nano-confinement can be created [12].
Among those extensive studies reported about confined crystallization and crystal orientation using block copolymers as templates, the cylinder phase structure is particularly interesting to us because it constructs a two-dimensional (2D) confined environment [21], [22], [23]. It was reported that a blend of PEO-b-PS (with number average molecular weight of the PEO blocks =8.7 kg/mol and number average molecular weight of PS blocks, =9.2 kg/mol, a copolymer that forms a lamellar phase structure) and homo-PS (with =4.6 kg/mol), having a PEO volume fraction of 0.32, formed an HC phase structure [21]. Since, the of the homo-PS was lower than the of the PS blocks in the PEO-b-PS, the homo-PS was preferentially mixed with the PS blocks to construct this HC structure [24]. The cylinders were formed by the PEO blocks within the PS matrix, and the cylinder diameter was determined to be 13.7 nm. The TODT of this blend system was 175 °C. The glass transition of the PS () is at 64 °C, which was higher than the melting temperature of PEO blocks (=50 °C) [21], [24]. This meets the criteria of a hard confinement, TODT≫> [12]. Using simultaneous 2D small angle X-ray scattering (SAXS) and wide angle X-ray diffraction (WAXD) techniques, it was found that the crystal orientation changes (the c-axes of the PEO crystals) within the cylinders were dependent on crystallization temperatures (Tc). At very low Tc (<−30 °C), PEO crystals were randomly oriented within the confined cylinders. Starting at Tc=−30 °C, the crystal orientation changed to be inclined with respect to the long cylinder axis, . The tilt angle between the c-axis of the PEO crystals and continuously increased with increasing Tc and finally reached 90° when Tc≥2 °C [21]. Namely, the c-axes of the PEO crystals were now perpendicular to the of the cylinders.
Recently, we also investigated the effect of 1D confinement of various sizes (dPEO) on crystal orientation changes in the lamellar phase structure using a series of PEO-b-PS samples with different and [25]. It was found that the crystal orientation, in particular, the Tc region where the c-axes of the PEO crystals was inclined with respect to lamellar surface normal, became narrowed with the releasing of the confined dPEO.
The question becomes whether this blend system truly represents the 2D confinement for the PEO block crystal orientation changes and crystallization behaviors. We have, therefore, synthesized a PEO-b-PS diblock copolymer with =8.8 kg/mol and =24.5 kg/mol and a PEO volume fraction of 0.26. In order to avoid the effect of the confined size on the crystal orientation changes, the of the diblock copolymer was selected to be 8.8 kg/mol, which is very close to the in the blend sample. Therefore, the diblock copolymer provides the HC phase structure with an almost identical PEO cylinder diameter of 13.3 nm compared to that in the blend (13.7 nm). For the diblock copolymer, no order–disorder transition is observed by SAXS even at 190 °C; the TODT of this sample is thus higher than 190 °C. The is at 77 °C, which is higher than the melting temperature of PEO blocks (=49 °C). The criteria for hard confinement are thus met [12]. By comparing the Tc dependence of the PEO block crystal orientation changes and crystallization behaviors in this diblock copolymer with those in the blend, we expect to achieve understandings of how the 2D confined spaces and their structural regularity affect the PEO block crystal nucleation and growth.
Section snippets
Materials and sample preparation
The PEO-b-PS diblock copolymer was sequentially synthesized using anionic block copolymerization of styrene and ethylene oxide monomers. The synthetic procedures can be found elsewhere [26], [27]. The PS precursor was characterized by size exclusion chromatography (SEC) using a polystyrene standard and had a of 24.5 kg/mol and a polydispersity of 1.02. The was determined by proton nuclear magnetic resonance to be 8.8 kg/mol. Note that this in the copolymer is very similar to that
Comparing phase structures in the copolymer and the blend
Although the PEO volume fractions of these two systems are different (0.32 versus 0.26), both the diblock copolymer and the blend exhibit HC phase structures with a 2D hexagonal lateral packing. Two bright-field TEM micrographs of thin sections of the copolymer samples after RuO4 staining are shown in Fig. 2(a) and (b). Since, PEO blocks are easier to be stained by RuO4 than PS blocks, the PEO cylinders appear darker than the PS matrix. Fig. 2(a) roughly represents a head-on view (along the x
Conclusion
In summary, the PEO crystal orientations within the 2D confined HC phase structure are studied by using the 2D SAXS and WAXD measurements. The samples used to generate the HC phase structures are either a diblock copolymer or a blend of the diblock copolymer with a PS homopolymer. Since, TODT≫>, the PEO block crystallization takes place in a 2D confined PS glassy environment. In these two samples, we have kept the two to be almost identical. The confined space size (the diameter
Acknowledgements
This work was supported by NSF (DMR-0516602). The 2D SAXS and WAXD research was carried out at the National Synchrotron Light Source in Brookhaven National Laboratory supported by the Department of Energy. We appreciate that the PerkinElmer Company set up a Pyris Diamond DSC in SZDC's laboratory.
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On the occasion of the 65th birthday of Prof. David C. Bassett for his pioneer contributions in polymer crystal physics.