Polypropylene equilibrium melting temperature

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. 

The equilibrium melting temperature of isotactic polypropylene is still debated between values ranging from 458 to 493 K. See for example refs [200, 201,202],... 

Li and co-workers [63] studied the crystallisation and melting behaviour of poly(]3-hydroxybutyrate (P-HB)-co-P-hydroxyvalerate (P-HV)) and a blend of poly(P-HB-co-P-HV)/polypropylene carbonate (30/70 w/w) using DSC and FT-IR spectroscopy. Transesterification occurred between poly(P-HB-co-P-HV) and polypropylene carbonate during the melt blending process. During crystallisation from the melt, the crystallisation temperature of the blend decreased by 8 °C compared with that of neat poly(P-HB-co-P-HV) and the melting temperature decreased by 4 °C. This indicated that the presence of polypropylene carbonate reduced the perfection of the poly(P-HB-co-P-HV) crystals, inhibited by the crystallisation of poly(p-HB-co-P-HV) and weakened its crystallisation ability. The equilibrium melting temperatures of... 

The main effect of pressure on crystallization is to shift the equilibrium melting temperature to higher values. It can be explained by Equation (15.1) and Equation (15.2), but here the decrease of the entropy of fusion is due to the reduction of free volume. It means that during cooling, crystallization occurs at higher temperatures, which is observed experimentally (Fig. 15.3). For polypropylene, the increase of the equilibrium melting temperature can be described by the following equation [28] ... 

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. 

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. 

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). 

PDMS) with different viseosities (from 50 mPas to 5 Pas) and found that equilibrium state is established in less than 200 sec between libers with diameters of df < 155 o,m and a high molecular weight PDMS with a viscosity of 5 Pas. In the case of polymeric melts, the time to reach the equilibrium can take up to several minutes for a polypropylene melt on approx. 60 tm glass fibers at temperatures above T > 160°C [93]. 

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