Segmented polyurethanes microphase separation

One alternative is to select precursors which form a gas as a reaction product in situ during the network formation of thermosets. However this approach is restricted to a very limited number of precursors reacting via a polycondensation mechanism to split off a gas. For example, flexible polyurethane foams are commercially produced using CO2 that is liberated as a reaction product of the isocyanate monomer with water [5]. Very recently, Macosko and coworkers studied the macroscopic cell opening mechanism in polyurethane foams and unraveled a microphase separation occurring in the cell walls. This leads to nanosized domains, which are considered as hard segments and responsible for a rise in modulus after the cell opening [6]. 

The variation of Tg of the soft matrix in segmented polyurethanes as a function of composition or segmental chemical structure has been monitored and used as an indicator of the degree of microphase separation. Factors influencing the phase-separation process in these MDI-based polyurethanes have been summarized by Aitken and Jeffs (69) as follows (a) crystallization of either component, (b) the steric hin-... 

Microdomain size in phase-separated copolymers plays a fundamental role in determining various macroscopic physical properties in the solid state. The difference in segmental mobility between the hard and soft domains governs the physical properties of microphase-separated polyurethane elastomers [7]. In this respect, the development of structure-property relations at the molecular level which relate directly to macroscopic behavior is the focus of this sub-section. One can exploit the well-documented difference between domain mobility [7-10] and the i3C NMR chemical shift distinction between the 0 .H2 resonances in the hard and soft segments to probe the microdomain morphology of polyether-... 

Proton spin-temperature equilibration between the hard- and soft-segment-rich domains of the polyurethane elastomer on the order of 10-100 ms might be considered fast relative to a macroscopically phase-separated blend [26] or copolymer, but slow relative to a strongly interacting mixture [25]. This is reasonable for a microphase-separated material whose solid state morphology has been the subject of considerable theoretical and experimental research. Under fortuitous circumstances, intimate (near-neighbor) contact between dissimilar molecules in a mixture can be studied via direct measurement of proton spin diffusion in a two-dimensional application of the 1H-CRAM PS experiment (Combined Rotation And Multiple Pulse Spectroscopy). Belfiore et al. [17,25,31] have detected intermolecular dipolar communication in a hydrogen-bonded cocrystallized solid solution of poly(ethylene oxide) and resorcinol on the f00-/xs time scale, whereas Ernst and coworkers [26] report the absence of proton spin diffusion on the 100-ms time scale for an immiscible blend of polystyrene and poly(vinyl methyl ether), cast from chloroform. 

Chu B, et al. Microphase separation kinetics in segmented polyurethanes effects of soft segment length and structure. Macromolecules 1992 25(21) 5724—9. 

Nakamae K, Nishino T, Asaoka S. Microphase separation and surface properties of segmented polyurethane—Effect of hard segment content. Int J Adhes Adhes 1996 16(4) 233-9. 

Huang SL, Chao MS, Ruaan RC, Lai JY. Microphase separated structure and protein adsorption of polyurethanes with butadiene soft segment Eur Polym J 2000 36(2) 285-94. 

Based on various experimental studies, one can schematically represent the morphology of segmented polyurethane (elastomer or flexible foam polymer) on the nano- and micro-scale as shown in Figm-e 2.1. For the range of hard segments volume fraction less than 50%, much of the space is occupied by the soft phase matrix. Microphase-separated nano-domains of the hard phase are dispersed in this matrix they can be individual islands or can form percolated networks. Finally, there could also be some larger (micron-sized) macrophase-separated domains of hard phase, where hard phase domains are ordered at the macro-scale (this is especially true in the case of flexible foams). The relative amounts of all these elements depend on the formulation and processing history. 

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