Kinetics of crystallization of isotactic polypropylene between 120 and 160^°C.

Abstract

Physical changes which accompany the crystallization of isotactic polypropylenes have been observed by dilatometry and optical microscopy at temperatures between 120 and 160°C. Samples were fused at 220°C. and then held at a predetermined crystallization temperature for times ranging up to many hours. Rate constants for the crystallization were calculated from volume changes and from measurements of radial growth of spherulites. Polypropylenes from different sources exhibited quite different concentrations of spherulites and growth rates. Crystallization appears to proceed, at least at the higher temperatures, by spherical growth from a fixed number of nuclei, apparently arising from foreign matter in the polymer. With decreasing temperature, the number of nuclei (hence also the number of spherulites) increases, since apparently the conditions for nucleation become less critical. The crystallization follows the kinetics of a nucleation‐controlled process according to an Avrami equation, ‐log (1 ‐ ΔV / ΔV ∞) = kt3. Deviations from the third power of time at the lower temperatures may be the result of a secondary ordering process, slower than the primary crystallization.

Double crystallization of biaxially oriented polypropylene

“…K and n values are listed in Table 2. The re- Table 2 Kinetic characteristics determined on the basis of isothermal crystallization performed after erasure of "memory" (TF = 523 K, tF = 5 min) sults obtained agree well with literature data for unoriented polymers [2][3][4]10] and with data obtained for unoriented samples in our laboratory [20]. The dependence of the crystallization rate on temperature can also be satisfactorily described by the model proposed by Godowsky and Slonimsky [21], and results obtained are close to those found for unoriented PP [20,21].…”

“…The characteristics of the crystallization of isotactic polypropylene (PP) have been studied by several research groups [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. It has been established on the basis of the dilatometric [1 -4, 6-8], optical polarization [5,10] and calorimetric [9, 10] investigations of isothermal crystallization that the crystallization isotherms can be described by the Avrami equation.…”

Crystallization kinetics of metallocene type polypropylenes

“…2(a) are calculated for Tin= 220~ while in Fig. The crystallization rates of the metallocene polymers studied here have been mea-sured in a temperature range for which the spherulite growth rates of unfractionated [1,8,12] and fractions of Ziegler-Natta type poly(propylenes) [18] follow Regime II. In both cases the data are well represented by a straight line.…”

“…In spite of the large number of papers related to this subject [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] very few works where direct efforts were made to analyze these variables independently from one to another [17,18]. Parameters such as molecular mass, molecular mass distribution and the stereoirregularity of the chains are known to influence the crystallization behavior as does the formation of the different known crystallographic forms and lamellar morphologies.…”

Crystallization and melting behavior of iPP studied by DSC

“…26 The values of the Avrami exponents are preThe diffraction pattern recorded on i-PP film (Universal X-ray diffractometer, KCua, 40 kV, 20 sented in Table I. For the studied interval of crystallization temperatures (388-400 K), different valmA) is represented in Figure 1, and the percent of crystallinity calculated by the Hermans-Weiues for n, ranging from 1.93 to 4.39, were found.…”