Elsevier

Polymer

Volume 47, Issue 15, 12 July 2006, Pages 5457-5466
Polymer

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

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Abstract

A poly(ethylene oxide)-b-polystyrene (PEO-b-PS) diblock copolymer with a number average molecular weight of PEO blocks, MnPEO=8.8 kg/mol, and a number average molecular weight of PS blocks, MnPS=24.5 kg/mol, (volume fraction of the PEO blocks, fPEO, was 0.26) exhibited a hexagonal cylinder (HC) phase structure. Small angle X-ray scattering results showed that the PEO cylinder diameter was 13.3 nm, and the hexagonal lattice was a=25.1 nm. The cylinder diameter of this HC phase structure was virtually the same as that in the blend system constructed by a PEO-b-PS diblock copolymer (MnPEO=8.7 kg/mol and MnPS=9.2 kg/mol) and a PS homo-polymer (MnPS=4.6 kg/mol) in which the fPEO was 0.32. The cylinder diameter in this blend sample was 13.7 nm and the hexagonal lattice was a=23.1 nm. Comparing crystal orientation and crystallization behaviors of this PEO-b-PS copolymer with the blend, it was found that the crystal orientation change with respect to crystallization temperature was almost identical. This is attributed to the fact that in both cases the PEO block tethering densities and confinement sizes are very similar. This indicates that when the MnPS of PS homo-polymer is lower than the PS blocks, the PS homo-polymer is located inside of the PS matrix rather than at the interface between the PEO and PS in the HC phase structure. On the other hand, a substantial difference of crystallization behaviors was observed between these two samples. The PEO-b-PS copolymer exhibited much more retarded crystallization kinetics than that of the blend. Based on the small angle X-ray scattering results, it was found that in the blend sample, the HC phase structure was not as regularly ordered as that in the PEO-b-PS copolymer, and thus, the HC phase structure contained more defects in the blend. This led to a suggestion that the primary nucleation process in the confined crystallization is a defect-controlled process. The mean crystallite sizes were estimated by the Scherer equation, and the PEO crystal sizes are on the scale of the confined size.

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 MnPEO=8.7 kg/mol and number average molecular weight of PS blocks, MnPS=9.2 kg/mol, a copolymer that forms a lamellar phase structure) and homo-PS (with MnPS=4.6 kg/mol), having a PEO volume fraction of 0.32, formed an HC phase structure [21]. Since, the MnPS of the homo-PS was lower than the MnPS 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 (TgPS) is at 64 °C, which was higher than the melting temperature of PEO blocks (TmPEO=50 °C) [21], [24]. This meets the criteria of a hard confinement, TODTTgPS>TmPEO [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, aˆ. The tilt angle between the c-axis of the PEO crystals and aˆ 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 aˆ 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 MnPEO and MnPS [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 MnPEO=8.8 kg/mol and MnPS=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 MnPEO of the diblock copolymer was selected to be 8.8 kg/mol, which is very close to the MnPEO 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 TgPS is at 77 °C, which is higher than the melting temperature of PEO blocks (TmPEO=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 MnPS of 24.5 kg/mol and a polydispersity of 1.02. The MnPEO was determined by proton nuclear magnetic resonance to be 8.8 kg/mol. Note that this MnPEO 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, TODTTgPS>TmPEO, the PEO block crystallization takes place in a 2D confined PS glassy environment. In these two samples, we have kept the two MnPEO 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.

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