Disordered Microdomains

Microdomain stmcture is a consequence of microphase separation. It is associated with processability and performance of block copolymer as TPE, pressure sensitive adhesive, etc. The size of the domain decreases as temperature increases [184,185]. At processing temperature they are in a disordered state, melt viscosity becomes low with great advantage in processability. At service temperamre, they are in ordered state and the dispersed domain of plastic blocks acts as reinforcing filler for the matrix polymer [186]. This transition is a thermodynamic transition and is controlled by counterbalanced physical factors, e.g., energetics and entropy. 

AFM Atomic force microscopy aPP Atactic polypropylene DSC Differential scanning calorimetry HDPE High-density polyethylene iPP Isotactic polypropylene LLDPE Linear low-density polyethylene MD Microdomain ODT Order-disorder transition PB Poly(butadiene)... 

Hashimoto T et al (1999) The effect of temperature gradient on the microdomain orientation of diblock copolymers undergoing an order-disorder transition. Macromolecules 32(3) 952-954... 

Heteropolymers can self-assemble into highly ordered patterns of microstructures, both in solution and in bulk. This subject has been reviewed extensively [1,123-127]. The driving force for structure formation in such systems is competing interactions, i.e., the attraction between one of the monomer species and the repulsion between the others, on the one hand, and covalent bonding of units within the same macromolecule, on the other hand. The latter factor prevents the separation of the system into homogeneous macroscopic phases, which can, under specific conditions, stabilize some types of microdomain structures. Usually, such a phenomenon is treated as microphase separation transition, MIST, or order-disorder transition, ODT. 

Much experimental work has appeared in the literature concerning the microphase separation of miktoarm star polymers. The issue of interest is the influence of the branched architectures on the microdomain morphology and on the static and dynamic characteristics of the order-disorder transition, the ultimate goal being the understanding of the structure-properties relation for these complex materials in order to design polymers for special applications. 

Floudas and coworkers [88] investigated the static and kinetic aspects of the order-disorder transition in SI2 and SIB miktoarm stars using SAXS and rheology. At temperatures above the order-disorder transition (ODT) the mean field theory describes the experimental results quite well. Near the ODT, SAXS profiles gave evidence for the existence of fluctuations. Both samples separated into cylindrical microdomains below the ODT. The ODT was determined on shear oriented samples and found, by SAXS, to be 379 K in both cases. This was confirmed by rheology. The discontinuities in SAXS peak intensity and in the storage modulus near the ODT were more pronounced for the miktoarm stars than for the diblocks. The %N values, where % is the interaction parameter and N the... 

PU elastomers based on a polyester diol as the soft block and isocyanate groups coupled via a chain extender as the rigid block, have been investigated. The role of the chain extender is to induce some mobility and disorder within the hard microdomains. The elastomers are selectively deuterated in the rigid block. 2H NMR spectra show the coexistence of a solid and a mobile fraction [89]. The solid fraction shows no onset of fast molecular motions at all, while the mobile fraction increases with temperature. These results suggest that some of the hard microdomains (perhaps the most disordered ones) melt successively when temperature is raised. 

Of course the modulus of a block copolymer with ordered spherical microdomains is much lower than that of a crystalline solid. Near the disordering transition, the potential energy holding each domain or atom in place is of order ksT, and the modulus is roughly vksT, where v is the number of domains or atoms. This gives an elastic modulus 10 -10 dyn/cm for typical block copolymers with spherical domains, as opposed to 10 -10 dyn/cm for atomic crystals. Ordered spherical diblock copolymers are therefore soft solids. They deflect under an imposed shear stress, but do not flow continuously unless that stress exceeds a critical value, the yield stress (Watanabe and Kotaka 1984). 

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