We use rheological measurements to examine the yielding behavior of a microgel system spanning the range from soft jammed glassy suspensions dominated by inter-particle repulsion to colloidal gels produced by attractive interactions. Under repulsive conditions, the suspensions display a prototypical soft glassy yielding response in which the shear loss modulus exhibits a single peak on increasing strain during the crossover from elastic to viscous behavior. By contrast, under fully attractive conditions the colloidal gel displays a more complex yielding, with two distinct peaks in the loss modulus in the vicinity of the yield strain. It is apparent that the gels yield initially by network rupture, followed by shear induced densification which leads to the formation of compact clusters. We show that the second peak in the loss modulus is consistent with the subsequent breakup of these dense clusters. We quantitatively map the steady progression from simple glassy yielding to the more complex gel response on increasing attraction strength by the evolution of peak locations, magnitudes and frequency dependencies.Notably, the peak locations diverge as the network becomes more fragile and spatially heterogeneous with increasing attraction strength. There is little frequency dependence in the peak positions, but the amplitude of the second yielding peak shows a non-monotonic dependence with a maximum near 5 rad s À1 . Time-resolved measurements and decreasing strain sweeps highlight pronounced differences in the reversibility of the network rupture and cluster breakup processes. Correspondingly, the linear viscoelastic properties of the gel are strongly dependent on mechanical history whereas the glass exhibits no such dependence.

This paper discusses an effective method to prepare cross-linked isotactic polypropylene (PP-X) with high purity (almost 100% gel content and no contamination) and high melting temperature and crystallinity. The reaction scheme involves a linear poly(propylene-co-p-(3-butenylstyrene)) copolymer (PP-BSt) containing few pendent styrene groups and the subsequent thermal treatment without any external reagent. The intermediate PP-BSt copolymers were prepared by a specific rac-CH2(3-tert-butyl-Ind)2ZrCl2/MAO catalyst that not only performs iso-specific propylene insertion but also incorporates BSt units with a preferred α-olefin insertion over styrene moiety. The combined features permit the preparation of linear PP-BSt copolymers with high molecular weight and high catalyst activity, without the presence of hydrogen. The resulting PP-BSt copolymers having some flexible pendent styrene moieties are completely soluble in xylene at elevated temperatures, and the solution case films show active cross-linking activity at temperatures >160 °C by engaging in a Diels−Alder [2 + 4] interchain cycloaddition reaction between the pendent styrene units. Evidently, the flexibility of styrene units is important, which enhances the interchain cyclization to form a complete 3-D network, even with a very small amount of BSt units. In contrast, the corresponding poly(propylene-co-p-divinylbenzene) (PP-DVB) prepared by the same metallocene catalyst only shows moderate cross-linking efficiency under similar conditions.

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