Triblock copolymers molecular architecture

In the last few years there have been new creative methods of preparation of novel hydrophilic polymers and hydrogels that may represent the future in drug delivery applications. The focus in these studies has been the development of polymeric structures with precise molecular architectures. Stupp et al. (1997) synthesized self-assembled triblock copolymer, nanostructures that may have very promising applications in controlled drug delivery. Novel biodegradable polymers, such as polyrotaxanes, have been developed that have particularly exciting molecular assemblies for drug delivery (Ooya and Yui, 1997). 

We use polystyrene-Z>-polybutadiene block copolymers as the starting material with preformed polymer architecture. These polymers are comparatively cheap and easily accessible.1 For the present problems a series of narrowly distributed polystyrene-6-polybutadiene block copolymers with rather different molecular weights were synthesized via anionic polymerization (Figure 10.4, Table 10.1). As a test for the modification of technological products, a commercial triblock copolymer was also used. 

The study of both star and linear PS-fr-PEO-fr-PCL triblock copolymers demonstrates the complexity of the crystallization behavior of ABC triblock copolymers and also the multiple possibilities of modifying the crystallization behavior of the block components by changing composition and/or molecular architecture. 

Since the relaxation mechanisms characteristic of the constituent blocks will be associated with separate distributions of relaxation times, the simple time-temperature (or frequency-temperature) superposition applicable to most amorphous homopolymers and random copolymers cannot apply to block copolymers, even if each block separately shows thermorheologically simple behavior. Block copolymers, in contrast to the polymethacrylates studied by Ferry and co-workers, are not singlephase systems. They form, however, felicitous models for studying materials with multiple transitions because their molecular architecture can be shaped with considerable freedom. We report here on a study of time—temperature superposition in a commercially available triblock copolymer rubber determined in tensile relaxation and creep. 

Figure 21.5 Molecular architectures and models of (a) AB diblock copolymer, (b) extended amphiphilic dendron, and (c) ABC triblock copolymer 28 (Reprinted with permission from B. K. Cho et al., Chem. Mater. 2007, 19, 3611 -3614. Copyright 2007 American Chemical Society.)...

Diblock and triblock copolymers are usually synthesized anionically, which inherently limits the possible blocks and homopolymers. However, the anionic route gives very good control over molecular weight, molecular architecture and polydispersity and is ideally suited for a systematic study of the effect of connector molecules. Furthermore, the problems frequently encountered during the anionic synthesis have been ironed out for many common polymers and the synthesis, although tedious, can be considered a routine procedure. 

Firestone et al. investigated the relationship between the molecular architecture of a series ofpoly(ethyleneoxide)-b-poly(propylene oxide) (PEO—PPO) di- and triblock copolymers and the nature of their interactions with lipid bilayers [213], The number of repeat units in the hydrophobic PPO block has been found to be a critical determinant for the polymer-lipid bilayer association. Further studies showed that temperature, polymer architecture and concentration also control the mode of interaction of PEO—PPO—PEO copolymers with lipid bilayers. Increasing either the number of repeat units in the PEO block or the polymer concentration promotes a greater degree of structural ordering [197],... 

It is possible to prepare diblock, triblock, multiblock, random block, star and graft copolymers simply by controlling their synthesis [7]. Such diversity of chemical architecture can be exploited for the design of different polymersomes membranes with diverse degree of entanglements and sub-structures. Figure 2 emphasizes the potential bilayer assembly depending on the molecular architecture of the copolymer utilized. 

Living polymerizations in which initiation is fast and quantitative and which have irreversible growth offer several advantages over conventional polymerizations. In addition to the ability to obtain polymers with controlled molecular weights and narrow molecular weight distributions, it is also possible to control the polymer architecture and chain end functionality. For example, diblock and triblock copolymers containing liquid crystalline blocks have been prepared by living polymerizations. 

Fewer studies have been devoted to the behavior of more complex macro-molecular structures. The architectures that have attracted the interest of investigators lately are those of hnear triblock copolymers (ABA) and terpoly-mers (ABC). 

Various reports [19-25, 32-41] on the effects of the copolymer molecular weight and molecular architecture on its compatibilization efficiency for polymer blends rank in the order tapered diblock > conventional diblock > triblock and smaller molecular weight > higher molecular weight... 

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