Crystallinity hard segment

Crystalline Hard Segment Amorphous Soft Segment... 

Chain extenders Low-molecular-weight molecules that usually react with isocyanates. They form rigid, crystalline, hard segments in the polyurethane and lengthen the main urethane chain by end-to-end attachment. 

This major morphological difference between 1,4-BDO extended polymers (crystalline hard segment) and those extended with any of the diols containing pendant groups (non-crystalline hard segment) also yields significant differences in the shear moduli and loss factors of these polymer systems. 

An example of a crystalline hard segment is MDI/1,4-BD0 which has a Tin or melt point by DSC. Table 1 shews that MDI/1,4-BD0 hag a small Tg at 110°C, a Tc or crystallization temperature at 182°C and a series of three successive crystallite melting points at 202°C, 220°C, and 236°C the upper temperature being the np of the pure equilibrium crystal. 

The sane compatibility/incompatibility rules as discussed for amorphous hard segments and soft segments do not apply with crystalline hard segments. Extenders which form crystalline hard segments with MDI aggregate into bundles or form a hard segment domain within the amorphous soft segment phase (3). 

Thus, crystalline hard segment based polymer systems will generally have separate hard and soft segment glass transitions. 

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. 

Polymer 3 based on MDI/Poly G 55-56 /1,4-800 exhibits an incompatible two phase polymer consisting of an amorphous soft segment Tg at -38°C and crystalline hard segment Tn s at 222°C and 241°C. IMA shews a high 160°C onset softening temperature and a major softening at 205°C, which is very close to the melt temperature of the hard segment. 

The process involves transesterification (catalyst, 200 °C) followed by polycondensation (250 °C, second stage. Morphological studies show the presence of crystalline (mp 190-200 °C) polyester lamellae in a continuous amorphous phase. In contrast to the A-B-A thermoplastic elastomers where the domains are formed from amorphous polystyrene segments, the domains here are formed from crystalline hard segments containing the 1,4-glycol polyester moiety. 

Figure 5.1 Schematic molecular structure of an SMPF. The soft segment and hard segment form the amorphous soft phase and crystalline hard segment domain, respectively. Abundant hydrogen bonds form between the amino group and carbonyl group in the crystalline hard domain. The amorphous phase and crystalline phase are partially oriented. Source [10] Reproduced with permission, from the Royal Society...

VIBRATHANE B 625 is a polyether prepolymer based on polytetramethylene glycol (PTMEG) and MDI raw materials. Based on the 6.5% NCO content, it can be assumed that this was prepared using 1 mole of PTMEG-1000 polyol and 2.2 moles of MDI. It can also be assumed that the HER chain extender produces crystalline hard segments... 

The Durometer hardness was measured after the cast elastomers were aged for at least 7 days at 23 °C and 50% relative humidity. The details are given in Table 8.3. From the results, it is evident that as the crystalline hard segment content of the elastomer is decreased, the elastomer hardness also decreases. By analysing the soft and hard segments and their relation to hardness, it appears that the amorphous hard segment behaves like the polyether soft segment. 

Crystalline Hard Segment —I Segment I I— Amorphous (Hard) Segment —I... 

In order to understand the effect of high MW diols on the formation of amorphous hard segments and their interaction with the crystalline hard segments, FT-IR analysis of the cast polyurethanes was performed. Samples were cast as films on a NaCl plate from a hot dimethyl sulphoxide (DMSO) solution. 

The crystalline hard segment melting, crystallisation and heats of transition observed during the DSC analysis are given in Table 8.14. 

Higher thermal stability due to high crystalline hard segment melting. High flexibility due to low Tg. 

---