Crystalline Hardness

Laboratory tests and experience during use have demonstrated that the nylons have extremely good abrasion resistance. This may be further improved by addition of external lubricants and by processing under conditions which develop a highly crystalline hard surface e.g. by use of hot injection moulds and by annealing in a non-oxidising fluid at an elevated temperature (150-200°C for nylon 66). 

The most recent converts are in the health care industry. Pharmaceutical and biological applications have become myriad since the early 1980s. The first widespread application was for the identification/ qualification of incoming raw materials. Since then, applications have appeared for moisture (bound and free), blend uniformity of powders, tablet and capsule assays, counterfeiting, polymorphism, degree of crystallinity, hardness (of tablets), dissolution prediction, isomerism, as... 

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. 

The primary variable in each of these families is the ratio of rubbery soft block to glassy or crystalline hard block, thus offering a wide range in balance of soft flexible properties vs. strength, heat, and chemical resistance. 

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. 

Fig. 9 Multiblock copolymers consisting of a poly(ethylene glycol) soft block and a tetrapeptide Ala-Gly-Ala-Gly, crystalline hard block in two variants a Templated system in which an aromatic hairpin turn is used to force parallel jS-sheet formation, b Non-templated system in which peptide segments are free to form parallel and/or antiparallel /1-sheets. Reprinted with permission from [43]. Copyright 2001 American Chemical Society...

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. 

Table 4 shows the data from different liquid compositions using EDA as template. It can be seen that the crystallinity and mechanical strength of the sample arrive to a maximum when the ratio of EDA to H2O was 3.3. According to table 3, it should be specially pointed out that the inorganic ammonia in liquid phase could remarkably improve the mechanical strength while the crystallinity hardly changes. 

Although the dynamic mechanical properties and the stress-strain behavior iV of block copolymers have been studied extensively, very little creep data are available on these materials (1-17). A number of block copolymers are now commercially available as thermoplastic elastomers to replace crosslinked rubber formulations and other plastics (16). For applications in which the finished object must bear loads for extended periods of time, it is important to know how these new materials compare with conventional crosslinked rubbers and more rigid plastics in dimensional stability or creep behavior. The creep of five commercial block polymers was measured as a function of temperature and molding conditions. Four of the polymers had crystalline hard blocks, and one had a glassy polystyrene hard block. The soft blocks were various kinds of elastomeric materials. The creep of the block polymers was also compared with that of a normal, crosslinked natural rubber and crystalline poly(tetra-methylene terephthalate) (PTMT). 

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