Determining the Percentage Crystallinity of Polymers using X-ray Diffraction
The crystallinity of polymers influences several macroscopic material properties such as mechanical strength and chemical resistance. X-ray diffraction (XRD) is a non-destructive experimental method that allows the crystallinity of different sample types, including semi-crystalline samples such as polymers, to be determined.
Polymers are materials made up of large molecules composed of many repeating subunits, called monomers.1 While there are naturally occurring polymers, such as rubber and proteins, the majority of modern polymers are man-made. As they can be engineered to display a diverse range of physical properties, they have become an essential part of our daily life; whether it is polyethylene packaging materials, or polyester in clothing, polymers are everywhere. One of the reasons that polymers are so widely used is that they are light-weight, chemically resistant, and most importantly, very cheap to produce.
Depending on the chemical structure, polymers are typically either amorphous or semi-crystalline. Semi-crystalline means that both crystalline and amorphous domains are found in the same sample.2 This behavior originates from the fact that polymers are made up of long molecular chains, which can entangle with each other in a disordered manner, giving rise to amorphous domains. At the same time, the chains can align themselves in a lamellar form, forming crystalline domains.3 The amount of the crystalline components present in the whole sample is called the percentage crystallinity, or simply crystallinity.
X-ray diffraction (XRD) is the ideal tool to investigate the crystallinity of polymers as the crystalline fraction results in relatively sharp diffraction peaks, while the amorphous component leads to either a single or multiple broad intensity halo(s) in the diffraction pattern. Moreover, there can also be different crystalline forms of the same material within a sample (polymorphs), which results in additional peaks in the diffraction pattern. This means that the crystalline phases can be identified, quantified and compared to the amount of the amorphous phase present.
1. M. M. Coleman et al. (1997), Fundamentals of polymer science: an introductory text
2. G. W. Ehrenstein et al. (2001), Polymeric materials: structure, properties, applications. DOI: doi.org/10.3139/9783446434134.004
3. A. Keller (1952), Morphology of crystallizing polymers. Nature 169: 193-914. DOI: doi.org/10.1038/169913a0
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