Hard phase crystallinity

Fig. 4.12 Graph showing the effects of varying hard phase crystallinity on two measures of inelasticity — (a) and...

O Amorphous hard phase Hard domain -Q Crystalline hard phase (crystalline)... 

The pseudocross-links, generated by the hard-segment interactions, are reversed by heating or dissolution. Without the domain crystallinity, thermoplastic polyurethanes would lack elastic character and be more gum-like in nature. In view of the outlined morphology, it is not surprising that many products develop their ultimate properties only on curing at elevated temperature, which allows the soft- and hard-phase segments to separate. 

The segments derived from the condensation reaction of the butanediol and the diisocyanate agglomerate into separate phases, which are hard and crystalline. The elastomeric chains are thus cross-linked to form a network similar in many ways to that given by the simple... 

The density of the polymer will clearly depend on the density of the soft phase (usually low), and the density of the hard phase (generally higher with crystallisable polar blocks) and the ratio of the soft and hard phases present. It will also clearly depend on the additives present and to some extent on the processing conditions, which may affect the crystalline morphology. 

Coran and Patel [33] selected a series of TPEs based on different rubbers and thermoplastics. Three types of rubbers EPDM, ethylene vinyl acetate (EVA), and nitrile (NBR) were selected and the plastics include PP, PS, styrene acrylonitrile (SAN), and PA. It was shown that the ultimate mechanical properties such as stress at break, elongation, and the elastic recovery of these dynamically cured blends increased with the similarity of the rubber and plastic in respect to the critical surface tension for wetting and with the crystallinity of the plastic phase. Critical chain length of the rubber molecule, crystallinity of the hard phase (plastic), and the surface energy are a few of the parameters used in the analysis. Better results are obtained with a crystalline plastic material when the entanglement molecular length of the... 

Aluminum oxide, A1203, is known almost universally as alumina. It exists with a variety of crystal structures, many of which form important ceramic materials (see Section 14.22). As a-alumina, it is the very hard, stable, crystalline substance corundum impure microcrystalline corundum is the purple-black abrasive known as emery. Some impure forms of alumina are beautiful, rare, and highly prized (Fig. 14.25). A less dense and more reactive form of the oxide is y-alumina. This form absorbs water and is used as the stationary phase in chromatography. 

Tor instance crystalline phase, amorphous phase, hard phase, soft phase, phases formed by different polymeric components in blends or block copolymers. 

Again, crystallinity may be replaced by hard phase fraction , soft phase fraction , or whatever designation applies better to die material that is studied. 

How should such rigid domain coupling work In principle domains can only be rigidly coupled by a bridge of hard-phase material which has a different density. We know that the polyester hard-phase is semicrystalline. So the observation is indicative for a structure in which the hard domains are subdivided into crystalline and amorphous zones. 

Block copolymers of the A—B—A type where A is a thermoplast and B an elastomer can have properties at ambient temperatures which would normally be expected from a crosslinked rubber. The cause of this phenomenon are the physical crosslinks produced by the thermoplastic blocks which may be either crystalline or amorphous (glassy). Above the melting temperature of the hard phase such materials flow and can be processed by the usual thermoplastic processing techniques. 

The substantial work on polystyrene/polybutadiene and polystyrene/ polyisoprene blends and diblock and triblock copolymer systems has lead to a general understanding of the nature of phase separation in regular block copolymer systems (5,6). The additional complexities of multiblocks with variable block length as well as possible hard- and/or soft-phase crystallinity makes the morphological characterization of polyurethane systems a challenge. 

The data shows the shift in soft segment T from -48 C (100 percent 1,4-BDO) to 6 C (100 percent dMPD) as expected due to phase mixing. As anticipated, hard segment crystallization occurs when the DMPD content is less than 50 percent. The shear modulus data (Figure 9) show a steady decrease in the rubbery modulus as the DMPD content of the blend increases because the degree of hard segment crystallinity is decreasing, and these results... 

There are certain advantages and disadvantages to this type of material compared to conventional elastomers. The advantages concern the existence of reversible physical cross-links. When a TPE is heated, the cross-links disappear if the temperature increases beyond the Tg of the hard phase or above the of the hard domain if it is crystalline. In contrast, conventional elastomers display thermostable structure. The physical nature of... 

Crystalline Systems. Lower Tqs, flatter rubbery plateau modulus, higher hard segment crystallinity, all support the view that phase separation is more complete MDI-BDO and MDI-ED0 based polyurethanes as catalyst concentration decreases. 

The contribution of the crystalline high melting nylon 6 blocks and soft-block hard-phase separation are also reflected in the resistance to heat sag exhibited by NBC. Table IV shows the heat sag at 163°C as these values are related to the flexural modulus. Even at the very low end of the modulus spectrum, sag values were quite low. 

The fact that in these bulk polymer samples no photoisomerization could be detected has to be attributed only to the severe restrictions of the local chain segmental mobility around the chromophore, i. e., it is due to the predominant incorporation of the azochromophoric units in the hard phase the mobility of chain segments within the hard domain is widely suppressed in this system, cousing a nearly complete immobilization of the chromophore in the hard phase. This behaviour is comparable to systems where the photochrome was incorporated in highly crystalline polyamide or polyimide (2,20,21). 

Hot-melt thermoplastic elastomer systems (23. 24) are also effective coating materials. These materials are generally based on copolymers that are comprised of hard (crystalline or glassy) and rubbery (amorphous) segments contained in separate phases. The hard-phase regions form physical cross-links below their crystallization or vitrification temperature, and the system therefore has elastomeric properties. The moduli and low-temperature characteristics of these materials can be tailored to compare reasonably well with silicone rubbers at -40 C. However, they are limited in high-temperature applicability because of enhanced creep or flow due to softening. 

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