Hard segment phase

Note Hard-segment phase domains are typically of 2-15 nm linear size. 

Note Soft-segment phase domains are often larger than hard-segment phase domains and are often continuous. 

Sheth JP, Wilkes GL, Fornof AR, Long TE, Yilgor I. Probing the hard segment phase coimec-tivity and percolation in model segmented poly(urethane urea) copolymers. Mactomolecules 2005 38 5681-5685. 

The electron diffraction results support the occurrence of soft-segment crystallinity and a more disordered hard-segment phase in the solvent-cast samples. Moreover, electron diffraction indicates isolated crystalline soft-segment regions persist even in samples of up to 43% hard segment. Electron diffraction is very sensitive to small local fluctuations in the overall structure because the diffraction patterns can be obtained from regions less than 1 /un in diameter and less than. l-/un thick, whereas DSC and WAXS, of course, measure bulk polymer which yields averages over the whole sample. 

This strong dependence of the order of both the soft- and hard-segment phases on the sample preparation technique may account for some of the previous disagreement on the extent of phase separation in a given polyurethane. 

Figure 1. Molecular schematics of some possible segmented polyurethane morphologies. For convenience, good phase separation is assumed, (a) Hard-segment domains dispersed within a soft-segment matrix (b) interconnected hard-segment phase giving rise to continuous phases of both hard and soft segments (c) soft-segment phase is dispersed within a continuous matrix of hard segments.

Since the linear relationship between H and Tg (see Chapter 3) seems to be valid over a rather wide temperamre range (in the present case it is proven for Tg values between —50 and 250 °C at least), eq. (5.16) can be rewritten in such a way that it also accounts for the amorphous hard-segment phase ... 

Polymer 2 based on MDI/DPG/PPG-2025tm/l,4-BDO is also two phase however, the crystalline hard segment phase has been transformed to a glassy-amorphous phase as shown by the Tg at 105°C and the soft segment glass transition is not as well-defined as in Polymer 1. The major softening observed at 99C by IMA very closely corresponds to the 105°C Tg measured by DSC. 

Segmented polyether polyurethanes are widely studied biomaterials that can be conceptualized (1-3) as a virtual network in which the continuous phase contains the polyether chain molecules. These chain molecules are tied at their ends to the molecular, species (diisocyanate, diamine) which serve as junctions by associating into clusters. These clusters of high melting point form the hard-segment phase and are dispersed in the continuous polyether phase. 

Molecular models (2) of the hard-segment phase of segmented polyether polyurethanes tested in the in vitro bead column (4) show a high platelet retention index (p = 0.8) regardless of molecular composition. The variable p is defined as the fraction of platelets in whole citrated human blood entering a test column that are retained on the bead surfaces, averaged for... 

Key PEi 2, polyether sequence (from a,a)—dihydroxy precursor) PTMO, PPO, or PEO, HSDI, diisocyanate residue in hard-segment phase (TDI, MDI, or IPDI) CDI, connector diisocyanate residue (TDI only) and ED, ethylenediamine residue. 

Figure 5.2 AFM image of the microphase segregated SMPF. The phase image indicates the microphase separation of the polymer with lighter hard segment phase regions and darker soft segment phase regions. Source [10] Reproduced with permission from the Royal Society...

Figure 7.13 shows the DSC test results of the as-spun SMPF and the same SMPF cold-drawn to 100%. From Figure 7.13, the DSC curve of the original SMPF shows a melt transition peak at about 167.9 °C, which can be ascribed to the melting transition of the hard segment phase. After 100% cold drawing, the DSC curve of the SMPF shows dual peaks at about 168.4 °C and... 

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