Self-organization of stack-up block copolymers into polymeric supramolecules

Polyethylene oxide –b– polypropylene oxide -b- polyethylene oxide (EO106PO70EO106) block copolymer self-organizes into polymeric supramolecules, characterized by NMR as phase transition from the isotropic stack-up block structure to the ordered cubic polymeric supramolecular structure. Its dependence on both temperature and copolymer concentration is clearly shown by the changes in line shape and chemical shift of the PO70blockβ,γresonances.

copolymers [4] was proposed to produce very longrange linear nanostructures, due to extension more or less indefinitely in both directions. The synthesized conical molecules, which are shaped like a badminton shuttlecock, were reported to stack together in a directed manner [6]. The specific shapes open up the huge potential for directionalities of alignment, causing by hydrogen-bonding and/or weak van der Waals interactions.
Pluronics Ò F127 is the subject of interest for this study. It has the formula of EO 106 PO 70 EO 106 . As illustrated in Scheme 1, this triblock compound consists of a hydrophobic PO 70 block sandwiched by two hydrophilic EO 106 blocks. For simplicity, there are two different modes of interaction for self-assembled block copolymer, namely hydrophobic PO 70 and hydrophilic EO 106 packing segments. In both cases, the packing of large molecules, i.e., EO 106 PO 70 EO 106 , means that only a fraction of molecules will be in direct contact due to hydrogen-bonding, polar or van der Waals forces. Because of the unique amphiphilic property, the material self-assembles into stacking structures. Hydrogen-bonding among the PO 70 units are expected to drive the triblock molecules to assemble into linearrotating cylinder structures [4]. Its phase behavior is temperature and concentration dependent, which relies on the level of dehydration of EO 106 and PO 70 block. An additional self-assembly process pushes the corona-surrounded domains into unusual anisotropic interactions, which was suggested to be a cubic phase [7]. NMR (nuclear magnetic resonance) for studying liquid crystalline systems was discussed, [8] to elucidate thermotropic and lyotropic phase transitions. The studies of the 13 C NMR of EO 61 PO 41 EO 61 (F87) at low concentration less than 1% (w/w) have been documented previously, [9,10] even the self-assembly behavior in water of a mixture of EO 13 PO 30 EO 13 (L64) and EO 37 PO 58 EO 37 (P105), was explored [11] by 2 H NMR at 25°C. However, the experimental application of these techniques and the interpretation of their results are more complicated than in homogeneous systems [7,12]. To date, no complete NMR study of F127 polymer has been published. This study is focused on the 1 H NMR analysis of F127 in D 2 O. All spectra were recorded on samples dissolved in D 2 O contained in a 5 mm o.d. NMR tube, on a Varian Unity 500 MHz NMR Spectrometer equipped with a 5 mm inverse probe. Excitation pulse width was approximately 81°(10 ls), data acquisition time 4.096 s, relaxation delay time 6 s, pulse repeat time approximately 10 s. The residual HDO peak was used as a secondary reference as a function of temperature [13] to calibrate the chemical shifts. Although not ideal, this should remove the gross effects of temperature dependence of the chemical shift.
As shown in Fig. 1, the chemical shifts of both PO 70 and EO 106 blocks appear to be temperature-dependent. There is a fine structure (b CH2 or c CH ) at 20°C, and partial overlap with b' CH2 units of EO 106 block. The spectra at 40 and 60°C are similar; the resonances of 1 H (a CH3 , c CH and b CH2 ) of PO 70 block decrease as temperature increases. At temperatures above the phase transition, [7] the signal is increased, due to an increased relaxation rate of the interacting PO 70 blocks, with the decrease of segmental mobility [14]. The attachment is proposed through a block of segments (PO 70 ) where the block may be considered to be stacked by hydrogen-bonding among the PO 70 units as illustrated in Scheme 2.
The EO 106 PO 70 EO 106 self-assembly system is envisaged as a series of central-stacked linear units with a cubic phase. As shown in Fig. 2, the resonances of 1 H (c CH and b CH2 ) of PO 70 block are also dependent on concentration. Fig. 2 clearly illustrates the phase transition from the isotropic stack-up block structure to the ordered cubic polymeric supramolecular structure. Its dependence on both temperature and copolymer concentration is clearly shown by the changes in line shape and chemical shift of the PO 70 block b, c resonances.
Under aqueous conditions, the PO 70 block is expected to display more hydrophobic interaction over range of 35 to 80°C, [15] thus increasing the tendency for mesoscopic ordering to occur. The -CH 2 -resonace of EO 106 blocks is also dependent on temperature and concentration. With increasing temperature, the signal is broadened, indicating a transition, which causes a decrease of the amount of mobile polymer segments. In comparison to low concentration, 20 and 30% polymer solutions were placed in an oven at 80°C over night to homogenize the solutions. As indicated in Fig. 2, the resonance is broadened as concentration increases. The increased line width of 1 H (b ' CH2 ) can be attributed to an increased relaxation rate of the interacting EO 106 blocks, and thus reflects a reduced mobility of the segments observed. Also, chemical shifts of 1 H (c CH and b CH2 ) towards high field indicate electron density increased as molecules closely attach due to PO 70 units assembly, while the chemical shift of 1 H (b ' CH2 ) from the -CH 2 -resonace of EO 106 blocks remains 3.670 ppm. As shown in Fig. 3, the use of the EO 106 PO 70 EO 106 amphiphile as a template to form silica-based nanostructured materials [4] extends more or less indefinitely in both directions to produce very long-range linear nanostructures.
NMR can be an important source of information on the behavior of self-assembly of block copolymers. The hydrophobic PO 70 domains self-associate into a core to escape contact with water, pushing the hydrophilic EO 106 domains into a corona surrounding the core. It can help elucidate the mechanisms of interactions with the building blocks. The concept of stacking interactions has become increasingly important, with isotropic, anisotropic, or hierarchical structures being obtained, depending on the type of template selforganization mechanism employed.