Mesomorphic form of isotactic polypropylene in stereodefective polypropylene: Solid mesophase or liquid-crystal like structure

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

“…All samples crystallize by quenching in the mesophase, regardless of the isotacticity, as indicated by the presence of the two broad reflections at 2 θ = 15° and 21°, typical of the mesophase, in the diffraction profiles of Figure 1B 35. For the more stereoregular samples with concentration of rr defects lower than 6–7 mol%, only few seconds at 0 °C is enough to stabilize the mesomorphic bundle and inhibit the crystallization of the common α form at room temperature.…”

“…It is worth remarking that the most irregular sample iPP( 5 ) with 11% of rr defects crystallizes in the mesophase only keeping the sample for long time (10 days) at 0 °C (Figure 1B) 35. For shorter permanence time at 0 °C the sample crystallizes as mixtures of α form and mesomorphic form at room temperature, when the sample is removed from the bath at 0 °C and heated to room temperature 35. As during deformation of this sample, the mesomorphic form transforms into α form (see Figure 7), the mechanical properties (Figure 4B) are only slightly affected by the permanence time at 0 °C.…”

“…The different behavior of the sample iPP( 5 ) in the mesomophic form at room temperature and at 4 °C may be due to the intrinsic thermal properties of this sample. Indeed, we recall, that the crystallization temperature of the sample iPP( 5 ) is below room temperature (≈20 °C),35 and this makes the stress‐induced crystallization of the α form from the mesophase a monotropic transition at room temperature. When stretching and successive relaxation are performed at 4 °C, instead, the mesophase is more stable so that its crystals orient upon stretching and transform into the α form by releasing the tension.…”

“…For the more stereoregular samples with concentration of rr defects lower than 6–7 mol%, only few seconds at 0 °C is enough to stabilize the mesomorphic bundles and inhibit the crystallization of the common α form at room temperature. For the most stereoirregular sample iPP( 5 ) with concentration of rr defects of 11%, prepared with the catalyst 5 of Scheme , the formation of the mesophase becomes more difficult and can be obtained only if the sample is kept at 0 °C for long time (about 10 days) 35. Special care has been taken to obtain films with uniform thickness (0.3 mm) and minimal surface roughness, according to the recommendation of the standard ASTM D‐2292–85.…”

Designing polymer crystallinity: An industrial perspective

“…The mechanism of mesophase formation has been studied extensively, however its nature has been debated for long time. De Rosa et al showed that solidification into mesomorphic phase, as bundles of chains in 3/1 helical conformation that need to achieve a large enough size to grow, follows a nucleation and growth process thereby supporting the view of a solid mesophase . It was found that ethylene comonomers delay the kinetics of mesophase formation, however to a lower extent compared to the effect on the crystallization in a phase .…”