Amorphous hardness

The melt temperature of a polyurethane is important for processibiUty. Melting should occur well below the decomposition temperature. Below the glass-transition temperature the molecular motion is frozen, and the material is only able to undergo small-scale elastic deformations. For amorphous polyurethane elastomers, the T of the soft segment is ca —50 to —60 " C, whereas for the amorphous hard segment, T is in the 20—100°C range. The T and T of the mote common macrodiols used in the manufacture of TPU are Hsted in Table 2. 

Block copolymers can contain crystalline or amorphous hard blocks. Examples of crystalline block copolymers are polyurethanes (e.g. B.F. Goodrich s Estane line), polyether esters (e.g. Dupont s Hytrel polymers), polyether amides (e.g. Atofina s Pebax grades). Polyurethanes have enjoyed limited utility due to their relatively low thermal stability use temperatures must be kept below 275°F, due to the reversibility of the urethane linkage. Recently, polyurethanes with stability at 350°F for nearly 100 h have been claimed [2]. Polyether esters and polyether amides have been explored for PSA applications where their heat and plasticizer resistance is a benefit [3]. However, the high price of these materials and their multiblock architecture have limited their use. All of these crystalline block copolymers consist of multiblocks with relatively short, amorphous, polyether or polyester mid-blocks. Consequently they can not be diluted as extensively with tackifiers and diluents as styrenic triblock copolymers. Thereby it is more difficult to obtain strong, yet soft adhesives — the primary goals of adding rubber to hot melts. 

FIG. 16 SPFM image of a droplet formed as a result of dewetting of Zdol on an amorphous hard carbon substrate film. No layering around the drop was observed. (From Ref. 70.)... 

In ternary systems, amorphous hard clusters can be formed. As explained above, at certain fraction of hard units and a certain conversion of functional groups, percolation threshold of the hard structure is reached. It has been found experimentally by analyzing the ultimate behavior of three- and four-component... 

This equation represents a generalization of the additivity of microhardnesses for high-crystallinity polymers, see eq. (4.3). However, Hsph and Ha, instead of describing the crystal and amorphous hardness, now represent the microhardnesses of the spherulites Hsph — 200 MPa) and interspherulitic regions Ha — 120 MPa), respectively. 

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 ... 

A characteristic feature of the material under investigation is its very low microhardness - between 25 and 35 MPa depending on the crystalline modification present. These values are up to 5-6 times lower than those for semicrystalline homo-PBT regardless of the crystalline modification. Moreover, the obtained values for H of PEE (Table 6.3, Fig. 6.5) are about half the amorphous hardness, // , of PBT, being 54 MPa as reported by Giri ef a/. (1997). This means that there should be other factors responsible for the very low H values of the copolymer. 

The use of monomers that produce amorphous hard segments produces highly compatible systems with poorly defined rubbery plateau and elastomeric behavior. Because no heterogenous conditions develop during the reaction, molecular weight in amorphous polyurethane systems tends to be less affected by reaction rates or polymerization conditions. 

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). 

First, it is of interest to compare (Fig. 15) the radial distribution functions for an amorphous close-packed assembly of macroscopic (steel) spheres and for the amorphous hard-sphere solid obtained by gradual densification by MD of a 500-sphere system with the usual periodic boundary conditions. Although there are some minor differences for the third peak in g(r), there is little doubt that the mechanical shaking, and the more thermodynamic shaking in the MD experiment, are leading the system toward the same structure. The mechanical shaking experiment can give... 

Fig. IS. Comparison of radial distribution functions for an MD-generated amorphous hard-sphere soUd at a density (po 1.2) very slightly less than the amorphous close-packed density (circles), and a random closed-packed assembly of macroscopic spheres at po —1.22 (histogram). From Refs. 10 and 78.

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 compare the Tg observed from the peak temperatures of loss modulus (G ), tan 8 and loss compliance (J ) curves, the peak temperatures are plotted as a function of the amorphous hard segment and shown in Figure 8.9. As can be seen in the figure, the amorphous hard segment produced by the high MW aromatic diol extenders has a pronounced effect on the soft segment Tg which, in turn, ultimately dictate the low temperature properties of the elastomers made from them. 

Figure 8.9 Amorphous hard segment versus peak temperatures...

In addition to storage modulus reduction, a continuous reduction in the Tg, an increase in tan 8 and increase in the loss compliance values are also noticed when the amorphous hard segment content of the elastomers is increased. 

Since the cast urethane elastomer properties are greatly influenced by the soft and hard segment contents and their phase separation behaviour, they are theoretically calculated from the molar ratios of the prepolymer and chain extenders and are reported in Table 8.12. Table 8.12 shows the hard segment content of the elastomer is 45.5% for HER-HP and is reduced to 38.2% for TG-275. This reduction in the hard segment content is due to the formation of 8.8% amorphous hard segment from TG-275 chain extender. 

On the other hand, a gradual increase in tensile strength and compression set values is observed for cast elastomers containing more amorphous hard segment contents. 

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. 

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