Determining Polymer Crystallinity

Table of Contents

Introduction
Measurements and Results
Simultaneous SAXS and WAXS Measurements

Introduction

To convert polymers from raw materials into the final product, a processing stage is required that involves a molten state, commonly known as melt. The final product will normally be in a semi-crystalline state after cooling.

The nanostructure of this semi-crystalline state, which develops at the point of solidification, greatly influences the mechanical and physical properties of the polymer product. It is therefore vital to study and define the crystallinity for industrial R&D and process optimization as well as fundamental studies.

In the following experiment, wide-angle X-ray scattering (WAXS) measurements were carried out on isotactic polypropylene (i-PP), employing a temperature control stage from Linkam.

Measurements and Results

A thin aluminum foil is used to enclose an i-PP pellet (from Innovia Films, U.K.) so as to prevent direct air exposure and guarantee good thermal contact. Subsequently, the polymer sample was mounted and fixed into the temperature control stage (Linkam HFSX350). The WAXS configuration of the Xeuss 2.0 SAXS/WAXS system measured the crystallization of the i-PP pellet in real-time.

A quenching experiment at 148 °C from the fully molten state was carried out, with an exposure time of 200 seconds. Shown in Figure 1 is the single-dimensional scattering curves, and corresponding analysis of the crystalline and amorphous fraction of the sample 16 minutes after the quench. To calculate the crystallinity, the ratio of the integrals of the crystallized and amorphous curves is used, as elaborated by Panine et al1.

As the Xeuss 2.0 SAXS/WAXS system has a negligible signal-to-noise ratio, it is possible to study low levels of crystallinity even during real-time crystallization. Therefore, crystallinity levels well below 5%2 can be determined, as depicted in Figure 1. Here, the crystallinity is estimated at 1.1%.

1D scattering curves and estimation of crystalline fraction.

Figure 1. 1D scattering curves and estimation of crystalline fraction.

Simultaneous SAXS and WAXS Measurements

The mechanical and physical properties of the solid polymer result from its crystallization kinetics, which influences both the nanoscale structure and the level of crystallinity.

Characterization at long length scales (SAXS investigation) gives information about the morphology of the crystalline phase; while the crystallographic order can be described by short length scale measurements (WAXS).

It is essential to study the evolution as a function of temperature at both length scales simultaneously, in order to gain a better understanding about the underlying processes. Such simultaneous SAXS/WAXS measurement capability is provided by the Xeuss 2.0 SAXS/WAXS system.

References

  1. Panine et al, Polymer, 2008, 49, 676-680
  2. Wang et al, Macromolecules, 2000, 33, 978-989

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