Prepolymer procedure

PMMA) were prepared by both a one-shot procedure and a prepolymer procedure. 

While the products have single and broad glass transitions, the SINs prepared by the prepolymer procedure show slightly broader transitions than those by the one-shot procedure. The percent energy absorption determined from dynamic properties and pendulum impact tests increases directly with PU content for both series. However, the values from the prepolymer procedure are always larger than those obtained from the one-shot procedure. 

Figure 3. Complex modulus and tan 6 at 110 Hz for PU/PMMA 75/25 SINs (a) one-shot procedure (b) prepolymer procedure.

Properties of PU/PMMA SINs made by Prepolymer Procedure... 

FATIGUE CRACK PROPAGATION. As shown in Figures 7 and 8, the SINs from both the one-shot and the prepolymer procedure were significantly superior in FCP behavior to the crosslinked-PMMA control. The FCP resistance was improved with increasing PU content. Unfortunately, when the amount of PU was larger than 50... 

PREPOLYMER PROCEDURE. The cavitation around the PU domains is seen in Figure 14(a), but the extent is relatively small. The dominant factor for the energy dissipating deformations that occur in the vicinity of the crack tip of a propagating crack is presumably the localized plastic deformation (Figure 14). 

Figure 14. Scanning electron micrograph of fractured surfaces for PU/PMMA made by prepolymer procedure. (a) 25/75 SINs, AK = 0.70...

Finally, the difference in the fractography between the one-shot (Figure 11) and the prepolymer procedure (Figure 14) may be due the different morphology. The nearly miscible one-shot SINs behaved like a homopolymer, since the size of the PU domain was too small (several tens of nm). On the other hand, the more separated... 

FCP resistance for the SINs increases with PU content up to 50% and is better in the prepolymer material than in the one-shot material, since the former always has larger values of percent energy absorption. With respect to micromechanisms of failure, the generation of discontinuous growth bands associated with shear yielding is involved in the SINs from the one-shot procedure. On the other hand, the fracture surfaces of the SINs from the prepolymer procedure show extensive stresswhitening phenomenon which is associated with the cavitation around PU domains and localized shear deformation. 

A series of studies on the PUs segmental orientation by using IR dichroic measurements were also reported by us [67,127]. We investigated a series ofpolyesteric PUs based on PEA, and the isocyanate of variable geometry DBDI (ca 40% HS fraction). To avoid HS crystallinity given by diisocyanate couples such as DBDI/EG or DBDI/BG, the materials were chain extended with DEG. PUs were synthesized by a standard prepolymer procedure as reported elsewhere [67]. 

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