P 65 ISSRNS 2012: Abstracts / Synchrotron Radiation in Natural Science Vol. 11, No 1 – 2 (2012)
STRUCTURE DEVELOPMENT DURING ISOTHERMAL CRYSTALLIZATION OF HIGH-DENSITY POLYETHYLENE:
SYNCHROTRON SMALL-ANGLE X-RAY STUDY
Cz. ´Slusarczyk∗
Institute of Textile Engineering and Polymer Materials, University of Bielsko-Bia la, 2, Willowa Str, 43–309 Bielsko-Bia la, Poland
Keywords: SAXS, high-density polyethylene, crystallization
∗e-mail : cslusarczyk@ath.bielsko.pl
Crystalline polymers show ordering at different size scales, namely, the arrangement of molecules in the unit cell, lamellar crystals and the aggre- gation of these lamellae into superstructures such as spherulites [1]. The classical picture of polymer crystallization involves the creation of a stable nu- cleus from the supercooled polymer melt and then the growth of the crystalline region into the lamel- lar structures and beyond. For most polymers, two stages of crystallization are acknowledged. A fast primary stage during which spherulites grow lin- early until they impinge on each other is denoted as primary crystallization. A slow secondary crys- tallization process is observed when the sample is completely composed of spherulites. The secondary crystallization involves the thickening of the crys- tals, growth of new lamellae within or between exist- ing lamellae stacks, and growth of entire new lamel- lae stacks from remaining amorphous regions within the spherulites.
In the present paper isothermal melt crystalliza- tion in high density polyethylene (HDPE) was stud- ied by time-resolved synchrotron small-angle X-ray scattering (SAXS) over a wide-range of supercool- ings. The SAXS profile was analyzed both by cor- relation, γ(r), and interface distribution, IDF, func- tions [2, 3]. Assuming an ideal two-phase system with sharp phase boundary the IDF is a superposi- tion of three contributions associated with the size distributions of crystalline (lc), amorphous (la) lay- ers and the distribution of the long period (LP ).
Curve fits may be performed to deconvolute these
contributions. In this study, curve fit was evaluated by assuming Gaussian distributions of lc, la, and LP. The relative standard deviation σC/lc is an ad- ditional parameter which is varied during crystal- lization and can be used for analysis of this process.
Results obtained indicate the following: (1) At large supercooling (40◦C) the thickening of the crys- talline layer does not occur. At lower supercool- ing all thicknesses increase with time. These re- sults confirm the predictions that crystallization of HDPE involves the direct transformation of melt into orthorombic phase. (2) At large supercooling lc presents a broad distribution which the relative standard deviation increases with time. At lower supercooling TC shows much more sharper distri- bution. In this case the relative standard deviation decreases with time.
References
[1] G. Reiter, G.R. Strobl, Progress in Understanding of Polymer Crystallization (Springer, Berlin Heidel- berg 2007).
[2] C. ´Slusarczyk, “Time-resolved SAXS investigations of morphological changes in a blend of linear and branched polyethylenes during crystallization and subsequent melting,” J. Alloy. Comp. 382 (2004) 68 – 74.
[3] C. ´Slusarczyk Interface distribution function from SAXS data for heat treated polyethylene in Applied Crystallography (H. Morawiec, D. Str´o˙z, Editors, World Scientific 2004).
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