Due to the structure characteristics of huge macromolecular size and the very low motion feature of the polymer chain which is very difficult to arrange regularly their long chain into the three dimensional space, thus the polymer will incompletely crystallize except for the macroscopic single crystal of polydiacetylene polymerized by means of solid state crystalline polymerization. Therefore, there is the concept of degree of crystallinity in polymer science which is defined by the weight fraction of crystal component in the entire polymer sample [1, 2]. In general, the degree of crystallinity in normal crystalline polymer is below 50%, even though the degree of crystallinity of high density polyethlene with the simplest structure is a little more than 90%.
The degree of crystallinity has great influence on the physical properties of polymer materials. Many physical properties such as mechanical properties, optical properties, thermal properties and electronic properties of polymers are related to their degree of crystallinity. So that several measurements, including infrared spectrometry, density method, X-ray diffraction, differential scanning calorimetry and nuclear magnetic resonance etc., have been developed for measuring the degree of crystallinity in polymers.
Here raise two issues. One is the data of degree of crystallinity in same polymer measured by different methods are not comparable, which is beyond the experimental error. The explanation that different measurements reflect different microscopic sizes in polymers has been proposed to address this issue very well. Another question is whether the degree of crystallinity within polymer uniform? It is well known that crystallization of polymers and the resulted degree of crystallinity are greatly affected by the molecular weight (MW) of polymers . With the increase of MW, the degree of crystallinity decreases. Interestingly, the MW of synthetic polymers is not a constant but has a distribution due to the non-uniform lengths of polymer chains. Accordingly, the degree of crystallinity within polymer should be non-uniform. The result obtained by the existing methods is an average value of the degree of crystallinity only. Strangely enough, such an obvious question has not been considered until Prof. Li's work .
Utilizing Raman spectrum they measured the degree of crystallinity of spin-coated poly(ε-caprolactone) (PCL) film. Prof. Li's group took the PCL lamellar single-crystalline and molten state as the standard samples of fully crystalline and amorphous PCL. For a semi-crystalline PCL sample, Prof.Li's groupdecomposed its Raman spectrum into the single-crystalline band and the amorphous band byfitting with the Voigt function. By means of measuring their peak position, full-width at half maximum and integral intensity, the degree of crystallinity of PCL can be related to a parameter fc = Ac/(Ac + Aa) (where Ac, and Aa are the integral intensity of singlecrystalline band and amorphous band, respectively).
The fc values have been measured at position 2 (center of the spherulite) and position 1,3 (symmetrically located on two sides of the center). The resulting fc is not uniform but presents a distribution along the radial direction of spherulite, with lower degree of crystallinity in the spherulite center as shown in Fig. 1. Moreover their temperature dependences are different as well.
|Fig. 1. (A) An image of PCL individual spherulite and (B) the fc as a function of temperature at different positions marked in the image.|
An important result is from measurement along the radial direction of PCL spherulites. It is found that the fc gradually increases and plateaus from the spherulite center to the periphery within ~40 μm range as shown in Fig. 2.
|Fig. 2. The non-uniform degree of crystallinity fc along the radial direction of PCL spherulites with a lower fc region of ~40μm.|
Based the results from Raman spectrum of PCL spherulites a new concept of the distribution of degree of crystallinity should be, for the first time, introduced in polymer science. This concept will certainly have some influence on study of physical properties of polymer, and will also be worth to investigate more crystallization process of polymers.
P.S. He, Structure and Properties of Polymers, Alpha Science International Ltd., London, 2014, p.115.
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