Role of tie molecules in the yielding deformation of isotactic polypropylene

Abstract

We examined the effects of the tie-molecule fraction on the yielding behavior of two isotactic polypropylenes, one having little ethylene content and the other as the homopolymer with no ethylene. The tie-molecule fraction of the samples used in this study was controlled by blending ethylene-␣-olefin of an ␣-olefin content above 50 mol % in the blend of which the copolymers were incorporated into the amorphous regions of polypropylene (PP). An excellent linear relationship was observed between the measured yield stress and the tie-molecule fraction estimated from the Huang-Brown model, suggesting that the tie-molecule fraction and lamellar stiffness determine whether the lamellar fragmentation is easily activated or not, depending on the PP composition. Furthermore, an extended Huang-Brown model predicts a lamellar cluster connecting about five lamellae, which has a potential to account for morphological transformation of the spherulitic structure into a fibrillar one. Comparing the immiscible blends showing a phase-separated morphology with the partially miscible blends mentioned above, the yield stress was lowered by the presence of rubber phase, apparently in a similar manner; but the yielding processes were clearly discriminated between both cases when the yield stress was plotted against the tie-molecule fraction.

The effect of annealing on the elastoplastic response of isotactic polypropylene

“…The strains ε and e * that characterize transitions to steady plastic flows at active loading and unloading, respectively, are rather close to each other and are weakly affected by thermal treatment. This result confirms the hypothesis [4,34] that elastoplastic deformation of a semicrystalline polymer in a sub-yield region is mainly associated with transformations in the amorphous phase. q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q qq q q qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qqq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq qq q…”

“…Two explanations may be provided for our findings. The first is based on the Nitta-Takayanagi [4] and Meyer-Pruitt [34] concepts that attribute plastic deformation of semicrystalline polymers to slippage of tie chains along lamellae and pulling out of chains from disintegrated lamellar blocks (both processes are associated with transformations in the amorphous phase whose state is not influenced by thermal treatment). The other explanation is based on the assumption that an increase in perfectness of crystallites at annealing (which makes lamellae stronger) is compensated by disappearance of tangential lamellae (which results in weakening of spherulites).…”

Yielding Behavior of Tough Semicrystalline Hydrogels

“…The average l / L is 2.09 ± 0.03, indicating that the distance between adjacent active tie links is about twice the thickness of the lamellar clusters (Figure b). l / L ≅ 2 was also reported for semicrystalline polymers such as polyethylene and isotactic polypropylene ,, and reveals that the minimum size of fragmented clusters L ( l /2) 2 is close to L 3 . The thickness L of lamellar clusters was estimated as almost equal to the end-to-end distance of unperturbed polymer chains in the melt .…”

“…We can explain the significant toughness improvement in semicrystalline hydrogels upon addition of C12M with the formation of layered lamellar crystals as revealed by SAXS measurements. Previous works conducted on semicrystalline polymers indicate that the existence of so-called active tie molecules interconnecting the lamellar clusters play a significant role in their mechanical strength and yielding behavior. − For the present hydrogels, the applied external force during tensile testing results in the unfolding of the amorphous layers between lamellar crystals, leading to the appearance of lamellar clusters interconnected by tie molecules, as schematically illustrated in Scheme a. Lamellar clusters consist of several lamellar crystals separated by amorphous domains, and tie molecules bridging them are part of the amorphous region but they are able to unfold under the external force.…”

“…According to the lamellar cluster model of Nitta et al, − the bending force acting on the lamellar clusters through the tie molecules results in disintegration of the clusters into their fragments, and this occurs at the yield point. Assuming that the onset of disintegration of the lamellar cluster occurs when the elastic energy reaches a critical value U y , the yield stress σ y of semicrystalline polymers is given by − where L is the thickness of lamellar clusters interconnected by tie molecules with a support span length l ( l > L , Scheme a). We estimated the yield energy U y of the hydrogels from the area under the stress–strain curve up to the yield point.…”