Polypropylene equilibrium melting points

Samuels also made DSC measurements on a large number of samples of polypropylene resins that were isothermally crystallized in the temperature range of 130-160°C. Two endotherms were also observed for these samples, and when the endotherms were extrapolated to the equilibrium melting point, the high-temperature melting point was predicted to be 220°C, while the low-temperature melting point was predicted to be 185°C. The dual endotherm behavior of the isothermally crystallized polymer seemed to be a manifestation of the same crystal melting process as that observed for restrained fibers. 

Juhasz, R, Varga, J., Belina, K., and Marand, H. 2003. Determination of the equilibrium melting point of the P-form of polypropylene. Journal of Thermal Analysis and Calorimetry 69 561-574. 

Time- and temperature-dependent small angle X-ray scattering (SAXS) experiments, at first carried out by us for syndiotactic polypropylene and related copolymers, contradicted the basic assumption of a control of the lamellar thickness by the supercooling below the equilibrium melting point [7]. As it turned out, lamellar thicknesses are determined by the supercooUng below another temperature which is always located above the equilibrium melting point. In addition, the thicknesses are not affected by the presence of co-units. 

However, the advent of superfast DSC shows that at lower crystallization temperatures, the melting difference between the two populations becomes less significant, leading to a one-peak situation (Toda et al. 2014). This technique is ideal for investigating the equilibrium melting point of isotactic polypropylene, but careful work even with traditional DSC (Yamada et al. 2(X)3a) can be used to deduce reorganizational effect and thereby determine this temperature. 

Isotactic polypropylene shows polymorphism. X-ray diffraction shows the presence of ot and y crystals in 50/50 proportions in poly(i-propylene-stat-ethylene) (6.6 mol%) crystallized at 393 K. The measured enthalpy change associated with melting was A/121 — A/ia2i = 44 J g and the onset of occurred at 395 K. The heat of fusion at the equilibrium melting point (460.7 K) is 206] g for the ot phase and 165 J g for the y phase. The specific heats of the crystalline and amorphous components were given by Gaur and Wunderlich (1981) as follows ... 

This paper describes the theory which permits us to characterize adequately the stereosequence length in stereoregular polymers from the equilibrium percent crystallinity at room temperature and from the melting points of the polymers. Results based on this theory are given on the characterization of the isotactic stereosequence length in the crystalline fractions of polypropylene oxide polymers made from the following catalyst systems (a) ferric chloride (17, 19) (b) diethyl zinc-water (10) (c) diethyl zinc-water-isopropylamine (d) diethyl zinc-water-cyclohexylamine (14). 

All isotactic vinyl polymers can be arranged in four ways in the usual crystals. Polypropylene has as stable conformation a 2 3/1 helix that consists of successive tg(—) or tg rotational isomeric states (left-handed or right-handed helices). In addition, it was possible for the helices to have the —CHj groups point up or down , relative to the crystallographic c-axis. In the ideal, monoclinic crystal form I (P2j /c) all four types of helices occur only at specific, symmetry-related positions The equilibrium melting temperature and entropy are 460.7 K and 15.1 J/(K mol), respectively. 

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