Elsevier

Polymer Testing

Volume 28, Issue 1, February 2009, Pages 36-41
Polymer Testing

Test Method
Following phase transitions by depolarized light intensity. The experimental setup

https://doi.org/10.1016/j.polymertesting.2008.09.012Get rights and content

Abstract

A new setup for light depolarization measurements was designed. Two innovative elements have been introduced. The first is an electronic system which enables depolarization ratio to be registered directly. The second is a system of temperature control allowing effective implementation of a temperature–time program according to the particular requirements. Direct registration of depolarization ratio instead of intensity of depolarized light for individual components (parallel and perpendicular), as is performed in the usual apparatus, allows elimination of light scattering effects because of the insensitivity of depolarization ratio to the scattering level. Application of the new setup was shown for crystallization and melting of isotactic polypropylene (i-PP). Comparison of phase transitions in i-PP, as registered by light depolarization and DSC, indicates some differences. Possible sources of the observed differences are discussed.

Introduction

Crystallization of polymers can be followed by light depolarization measurements. Depending on the material and external conditions (temperature, rate of temperature changes, etc.), crystallization can be rapid or slow. In the case of isotactic polypropylene, forming various polymorphs, phase content strongly depends on crystallization conditions [1]. The light depolarization technique (LDT) is sensitive to structural changes occurring in polymorphic systems.

The measurement of light depolarized by birefringent polymer crystals nucleated and grown in an isotropic matrix was first applied by Fischer and Schram [2], and later by Magill [3], [4], [5], [6]. The method was further developed by Ding and Spruiell [7], Supaphol and Spruiell [8], Brucato et al. [9], De Santis [10], and others.

In the early LDT studies, it was assumed that the degree of light depolarization was a direct measure of the degree of crystallinity. Ziabicki and Misztal-Faraj [11], [12], [13] showed that this was not true, and a more exact relationship was derived.

The present work is devoted to a new setup for experimental measurement of phase transitions via intensity of depolarized light. Some preliminary experimental data are shown.

Section snippets

Theoretical fundamentals

Early papers describing the light depolarization technique (LDT) were based on the assumption that the degree of crystallinity, x, was proportional to the intensity of depolarized light [3], [4], [5].xII0where I0 is incident light intensity, and I transmitted light intensity at crossed polarizers.

Ding and Spruiell [7] took into account the scattering contribution, IS and modified the formula for relative crystallinityxII0IS

More recently, Ziabicki and Misztal-Faraj [11], [12] demonstrated

Setup with two detectors (Fig. 1)

In contrast to earlier experiments [2], [14] (in which intensities transmitted through crossed (I) and parallel polarizers (III) were measured separately), our setup allows registering the depolarization ratio directly. The experimental setup is shown schematically in Fig. 1, Fig. 2.

Polarized light with λ = 633 nm is emitted by a He–Ne laser (5 mW). The Galileo beam expander expands the original beam to 5 mm in diameter, while the use of a diaphragm allows reduction of the beam according to the

Material

Isotactic polypropylene (i-PP) (Himont), Mw = 473 × 103 Da, Mn = 79 × 103 Da, with isotacticity index 0.96 was tested. The film, with thickness 240 μm, was prepared by compression-molding using a laboratory press at 190 °C. The sample was held without pressure for 3 min, to allow complete melting, then with a load for a further 7 min. Cooling to the room temperature was performed by cold water circulating in the press. Then, samples for light depolarization measurements were prepared according to Section 3.1.

Conclusions

A new setup for light depolarization measurements was projected and constructed in such a way to fulfill specific demands of the new theoretical model [11], [12]. Two innovative elements have been introduced: an electronic system which enables direct registration of depolarization ratio, and a temperature control system assuring effective implementation of a temperature–time program according to the actual requirements. Direct recording of depolarization ratio instead of intensity of separate

Acknowledgments

This work has been supported by Ministry of Science and Higher Education under Grant N507 019 31/0563. The authors would also like to thank for additional support under the agreement on scientific cooperation between Polish Academy of Sciences and the National Academy of Sciences of Ukraine – research project for years 2006–2008.

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