Block copolymers cross-linking effects

Hayward, R.C., Chmelka, B.E., Kramer, E.J. Template cross-linking effects on morphologies of swellable block copolymer and mesostructured silica thin films. Macromolecules 38(18), 7768-7783 (2005)... 

With a typical of 25 000-30000 the molecular size is low compared wjth most conventional covalently cross-linked elastomers. With such rubbers values of about 100000 are desirable so that the effects of a significant amount of non-load-bearing chain ends do not occur. Such a problem does not arise in block copolymers terminated by hard segments. 

The formation of cellular products also requires surfactants to facilitate the formation of small bubbles necessary for a fine cel] structure. The most effective surfactants are polyoxyalkylene-polysiloxane copolymers. The physical properties of polyurethanes are derived from their molecular structure and determined by the choice of building blocks as well as the suprainolecular structures caused by atomic interaction between chains. The ability to crystallize, the flexibility of the chains, and spacing of polar groups are of considerable importance, especially in linear thermoplastic materials. In rigid cross-linked systems, e.g., polyurethane foains, other factors such as density determine the final properties. 

It Is apparent the crystalline cross-links In the block-graft copolymer are as effective as the chemical cross-links In rubber. 

Pluronic formulations below their CMC with anticancer drugs have been effective in treating MDR tumors [222], whereby these positive results are linked to the functions of Pluronic (PEO/PPO/PEO) block copolymers in crossing the plasma membrane, suppressing ATP production, and to gene modulation. The formulation seems to be effective with MDR tumors but less so with sensitive tumors. This approach lacks tumor specificity, and little is known about its influence on normal cells that express Pgp. On the other hand, Pluronic micelles under ultrasound stimulation have been effective in treating MDR cells [219]. 

The outstanding behavior of these rubbers arises from the natural tendency of two polymer species to separate. However, this separation is restrained in these polymers since the blocks are covalently linked to each other. In a typical commercial SBS triblock copolymer with about 30% styrene content, the styrene blocks congregate into rigid, glassy domains which act effectively to link the butadiene segments into a network (Figure 4.7) analogous to that of cross-linked rubber. 

SBS block copolymers degrade primarily via cross-linking. The thermal stabihty of SBS copolymers can be improved with the addition of a more effective stabilizer system. Figures 7 and 8 show gel formation and discoloration of a SBS copolymer stabilized... 

FIGURE 54.5 Preparation of disulfide cross-linked PIC micelles containing siRNA. (Reprinted with permission from Biomacromolecules, 12(9), Christie, R.J., Miyata, K., Matsumoto, Y., Nomoto, T., Menasco, D., Lai, T.ch., Pennisi, M. et al. Effect of polymer structure on micelles formed between siRNA and cationic block copolymer comprising thiols and amidines, 3174. Copyright 2011 American Chemical Society.)... 

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