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Área de trabajo: Ciencia de los Materiales.

Título: Observando la Orientación Molecular en Películas de polipropileno isotáctico con Espectroscopia Raman Polarizada.

Título original: Observing Molecular Orientation in Isotactic Polypropylene Films with Polarized Raman Spectroscopy.

Autor: Robert Heintz, Mark Wall, Dick Wieboldt, Thermo Fisher Scientific, Madison, Wisconsin, USA.

Introduction

Engineering the properties of polymers and polymer composites requires the ability to control molecular orientation through the processing steps and in the final product. Molecular orientation can affect mechanical properties such as strength and flexibility as well as physical characteristics such as optical and thermal properties. Raman spectroscopy provides details on the chemical makeup of a polymer. Polarized Raman spectroscopy is ideally suited for characterizing molecular orientation.

Isotactic polypropylene (iPP) is a good example to illustrate the analysis of molecular orientation in polymers. It is a classic example of a stereo-regular polymer. iPP has a simple hydrocarbon backbone consisting of carbon-carbon single bonds with pendant methyl groups (-CH3) arranged along one side of the polymer chain. This regular repeating structure leads to a high degree of crystallinity. iPP can be found in a variety of different polymorphic forms. Several different crystalline phases have been identified (a, b, l) as well as a mesophase (smectic) and an amorphous state.

The crystalline structures have a common helical conformation of the polymer chains but differ in the relative ordering and positioning of the helical chains.1 It is interesting to note that a seemingly simple repeating unit can display such diversity in molecular structure which, in turn, determines the properties of the material.

 

Experimental

Raman measurements for this report were made using two different Raman microscopes. The single point data was obtained using a Thermo Scientific™ DXR™2 Raman microscope and the imaging data was collected using a Thermo Scientific™ DXR™2xi Raman imaging microscope. While both of these Raman microscopes can collect single point data and both can collect Raman images, each is particularly suited to different types of applications. The data was collected using a 532 nm laser but the polarization option supports other laser excitation wavelengths (455 nm, 633 nm, and 785 nm) as well.

 

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