Structure formation in semicrystalline diblocks

In this chapter, structure formation in semicrystalline diblocks containing PE, PEO and other crystalline blocks is discussed in Section 5.2. Section 5.3 is concerned with theories for the equilibrium crystallization of block copolymers, whilst Section 5.4 summarizes recent experimental work on the kinetics of crystallization. There have been few studies of crystallization in thin block copolymer films, and consequently Section 5.5 is correspondingly short. Finally, structure formation in glassy diblocks is considered in Section 5,6. 

The mechanical and thermal properties of a range of poly(ethylene)/ poly(ethylene-propylene) (PE/PEP) copolymers have been examined by Mohajer et al. (1982). They studied the effect of variation of composition and copolymer architecture on the polymer properties by synthesizing a range of PE-PEP-PE and PEP-PE-PEP triblocks and PE-PF.P diblocks with high molecular weights (M 200 kg mol (.The crystallinity, density and melting enthalpy for all copolymers were found to be linearly dependent on the PE content, indicating microphase separation of PE and rubbery PEP in the solid state. The 

PE-PEP diblock were similar to each other at high PE content (50-90%). This was because the mechanical properties were determined predominantly by the behaviour of the more continuous PE phase. For lower PE contents (7-29%) there were major differences in the mechanical properties of polymers with different architectures, all of which formed a cubic-packed sphere phase. PE-PEP-PE triblocks were found to be thermoplastic elastomers, whereas PEP-PE-PEP triblocks behaved like particulate filled rubber.The difference was proposed to result from bridging of PE domains across spheres in PE-PEP-PE triblocks, which acted as physical cross-links due to anchorage of the PE blocks in the semicrystalline domains. No such arrangement is possible for the PEP-PE-PEP or PE-PEP copolymers (Mohajer et al. 1982). 

Seguela and Prud homme (1989) investigated a PE-PEP-PE triblock copolymer containing 27wt% poly(ethylene) cast from a neutral solvent close to the Tm of PE and well below it. The samples cast above Tm crystallized within the assumed hexagonal-packed cylinder microphase-separated structure. However, SAXS experiments performed on the samples cast at room temperature suggested that crystallization occurred without prior microphase separation in the melt. This path dependence is a general feature of crystallization in block copolymers. 

Cohen et al. (1990) studied a poly(styrene)-poly(ethylene) (PS-PE) diblock that was solvent cast from toluene. Crystallization within microphase-separated PE spheres occurred when solvent-casting was done above the PE block melting temperature, Tm (see Fig. 5.2). When solvent was removed below Tm crystallization did not occur within spherical microdomains, instead TEM and SANS experiments suggested an irregular structure. Nojima et al. (1994) suggest that crystallization from the melt in this sample occurred within the microphase-separated block in the former case due to the high molecular weight of the 

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Theoretical and experimental investigations dealing with the polyelectrolyte adsorption and the structure of polyelectrolyte liquid films have been carried out for more than twenty years in order to give a better understanding of emulsion stabilization mechanisms by polyelectrolytes on a microscopic scale. In this section, we describe the structure of liquid films composed of three kinds of polyelectrolytes the diblock amphiphilic polyelectrolytes, which lead to the formation of brushes, the homopolyelectrolytes, which form a semicrystalline structure within the films, and finally the amphiphilic random polyelectrolytes. Special attention is given to the charged monomer layer thickness at interfaces. 

Investigations on the self-assembly in thin films of an amorphous P2VP-PS-P2VP triblock copolymer, forming cylindrical microdomains in bulk, showed that the orientation of microdomains due to the P2VP/substrate interactions persisted in the entire film in contrast to the diblock case [14], This was viewed as a result of the formation of an interconnected structure in the triblock coming from the formation of loops within the microdomains. More recently, AFM and SAKS measurements on a PEO-PBO-PEO amorphous-semicrystalline triblock thin film revealed the presence of a semicrystaUine PEO monolayer at the substrate, comprised of unfolded chains, and PBO blocks at the air/polymer siuface in a looped conformation (15]. 

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