The Deformability of Polymers: The Role of Disordered Mesomorphic Crystals and Stress‐Induced Phase Transformations

Abstract

Isotactic polypropylene (iPP) is of enormous and still increasing commercial importance because of its favored combinations of properties, such as good rigidity, high thermal resistance, low density, and ease of processability, which can be achieved at relatively low costs.[1] The wide range of applications of iPP results from its versatility and the variety of possible modifications of the basic material, which already starts in the polymerization reactor [1] and can be further achieved during crystallization and processing. In particular, the full understanding of the crystallization and complex polymorphic behavior made it possible to tailor the properties through control of the crystallization of the different crystalline forms (a, b, and g forms) and of the mesophase.As examples for these concepts, many properties are changed and enhanced by modification of the neat polymer, such as the mechanical strength by incorporation of glass fibers, optical clarification by addition of nucleation agents, [1] or enhancement of the ductility and flexibility by crystallization in the mesophase.[2] While highly crystalline samples of iPP crystallized in the stable a form results in a rigid material, the same sample crystallized in the mesophase by rapid quenching of the melt at low temperature results in a ductile and flexible material with greatly enhanced deformability. This effect has been related to the different morphology of the mesomorphic crystals. Mesophases are highly disordered crystals, characterized by a degree of order intermediate between the disorder of the amorphous phase and the order of crystalline phases, and are generally easily obtained by quenching the melt at low temperatures. In the cases of iPP the crystallization of solid mesophases by quenching the melt produces defective crystals of size and morphology which are very different from the spherulites obtained at low cooling rates or high crystallization temperatures. In general, with the transformation from ordered crystals to mesomorphic entities, a change of the crystallite morphology from almost perfect spherulites to bundlelike or nodular crystals is observed. [3][4][5] All these variations in number, size, and perfection of the crystallites and morphological details significantly influence the mechanical behavior of the materials, that is, the rigidity, impact strength, and ductility.[2]The crystallization of iPP in the mesophase may offer additional options for tailoring the properties.[2] In fact, optically transparent but highly crystalline films of iPP can be prepared by nonspherulitic crystallization, without employing optical clarifiers, [5b] through crystallization of the nonspherulitic mesophase by rapid cooling of the melt, and then in a second step through tailoring the crystallinity and mechanical characteristics by annealing at elevated temperature, to induce the transformation from the mesophase to crystals of the a form, and maintaining the absence of higher-order organization in a spherulitic superstructure. [4] Here, we ...

Fast cooling of (non)-nucleated virgin and recycled poly(propylenes): Effect of processing conditions on structural and mechanical properties

“…For all materials, similar trends are observed for similar cooling conditions. It is seen that the modulus of the g-crystal phase and the mesomorphic phase materials is lower than for the a-crystal phase material, which was also observed in other studies [30]. The reason for this is the reduced crystallinity.…”

“…Another study showed that for i-PP's with similar crystallinities the a-crystal phase was ductile with an elongation-at-break of 300%, an elongation-at-break of over 600% for the mesomorphic phase and an elongation-at-break of over 1000% for the g-crystal phase, which was explained in terms of a phase transition from the g-crystal phase to the mesomorphic phase during deformation [30]. A study on the effect of stereoregularity of i-PP on the crystal phase formation showed that the mechanical properties of i-PP can be tuned using metallocene catalysts [31].…”

Tailoring Mechanical Properties of Isotactic Polypropylene Via Crystallization of the Mesophase and Control of Stereodefects Concentration

“…The plastic deformation of unoriented films of Figure 3 and the elastic recovery observed in the stress–strain curves of Figure 5 are associated with polymorphic transformations that occur during stretching and relaxation 29, 36, 37. For the more stereoregular non‐elastic samples iPP( 1 )–iPP( 4 ), the α or γ forms of slowly crystallized samples transform by stretching at high deformations in the mesomorphic form,36, 37 whereas in quenched mesomorphic samples, no structural transformations generally occurs during stretching and only orientation of the mesomorphic crystals occurs 39. An example is shown in Figure 6 for the sample iPP( 3 ) with 3.7 mol% of rr defects.…”

“…More stereoirregular samples iPP( 4 ) and iPP( 5 ) with concentrations of rr defects of 5.9% and 11%, respectively, show instead unexpected better ductility when slowly crystallized from the melt in the γ form than when crystallized in the nodular solid mesophase (Figure 3). 39…”

“…The occurrence of the phase transformations of the γ form during stretching (Figure 7A‐C) is in part responsible of the high ductility and flexibility of the stereoirregular samples iPP( 4 ) and iPP( 5 ) (Figure 3A),29, 36, 37 even higher than those observed in the same samples crystallized by quenching in the mesophase (Figure 3B). In fact, in addition to the morphological effects related to the thin and elongated crystals of γ form with needle‐like morphology40 embedded in the amorphous matrix and the flexibility of the intracrystalline amorphous stereoirregular iPP chains,39 the formation of the mesomorphic form during stretching (Figure 7A–C) facilitates the further successive deformation up to very high deformation, resulting in high ductility and flexibility (Figure 3A) 29, 36, 37, 39. Moreover, the occurrence of this phase transformation produces an effective mechanism able to gradually dissipate some amount of mechanical energy and produces a neat increase of the values of the strain at break.…”