Equilibrium Melting Temperature of Syndiotactic Polypropylene

Abstract

Isothermal crystallizations from the melt of samples of syndiotactic polypropylene having different degrees of syndiotacticity have been performed. The thermal behavior and the structural features of the melt-crystallized samples have been analyzed by differential scanning calorimetry and X-ray diffraction. According to this analysis, less stereoregular samples, crystallized in disordered modifications of form I with defects implying local arrangements of the chains such as in form II, present recrystallization phenomena during heating. For most stereoregular samples relevant recrystallization phenomena are absent. Extrapolation to the equilibrium melting temperature of syndiotactic polypropylene was attempted by using the Hoffmann−Weeks method, taking into account the occurrence of recrystallization and superheating phenomena. A value of nearly 182 °C has been obtained for the equilibrium melting temperature of the fully syndiotactic polypropylene.

Mechanically and thermally induced chain conformational transformations between helical form I and trans-planar form III in syndiotactic polypropylene using FT-IR and Raman spectroscopic techniques

“…However, the existence of absorption bands assigned to trans-planar conformations is also present. On the other hand, the occurrence of the bands which are related with the trans-planar conformation of sPP in the strained specimen is in agreement with the abovementioned studies, which report that crystalline phase transformation from the helical form I to the trans-planar form III take place with the application of strain [7,17,18,11,30,[31][32][33][34][35].…”

“…After cooling at room temperature, a film of 0.4 mm thick was obtained. According to several studies, the above-described preparation leads to the formation of a disordered modification of form I [33,58]. The temperature that was used in these experiments has been found that is high enough to obtain samples with high degree of crystallinity [59,60].…”

“…It corresponds to the C-centered structure in which the helices have the same chirality. Both forms are characterized by chains in (T 2 G 2 ) n helical conformation [14,17,18]. In addition, form III can be obtained through a cold drawing process of samples quenched from the melt at low temperatures [17 -22] and it is a metastable polymorph of sPP.…”

“…For instance, stretching the stable crystalline helical conformation results in a situation where chains in the trans-planar configuration dominate. Upon release of the applied tension on sPP samples, the presence of a 'memory effect' is observed where the complete return to the situation before the application of stress takes place [18,30]. De Rosa et al [11] observed that when a sPP specimen in trans-planar form III is released from the applied tension, the crystalline moieties remain nearly oriented while the transition from the metastable trans-planar form III into the helical form II occurs.…”

Calorimetric analysis of the multiple melting behavior of melt-crystallized poly(l-lactic acid) with a low optical purity

“…Especially, the melting of semicrystalline thermoplastic is a very complex process, which is influenced significantly by the crystallization conditions. Double or multiple melting behavior has been observed in many semicrystalline polymers such as polypropylene (PP) [23,24], polystyrene (PS) [25], poly(ether ether ketone) (PEEK) [26][27][28], poly(ethylene terephthalate) (PET) [29,30], poly(butylene-2,6-naphthalate) (PBN) [31], poly(trimethylene terephthalate) (PTT) [32], and some biodegradable aliphatic polyesters, e.g., poly(butylene succinate) and copolymers [33][34][35]. It is an important research subject in the field of polymer science to gain a basic understanding of the structural evolution related to thermal history (i.e., crystallization conditions) and to provide a deep insight into the crystallization and melting processes of semicrystalline polymers.…”