Electron microscopy polymer spherulites

In this chapter we investigate the morphology of a series of polyurethanes based on polycaprolactone polyol (PCP), diphenylmethane diisocyanate (MDI), and butanediol (BDO). Samples of as-batch-reacted and solution-cast polymers were examined by optical microscopy, transmission electron microscopy, electron and x-ray diffraction, and differential scanning calorimetry. Our interest is to provide a mapping of the size and shape of the domains (and any superstructure such as spherulites) and the degree of order as a function of the fraction of each phase present. 

Much effort has been devoted to investigating the detailed architectures and the construction of spherulites. Early investigations of the crystallization of polymers through optical microscopy (OM) [7,8] posited that polymer spherulites consisted of radiating fibrous crystals with dense branches to fill space. Later, when electron microscopy (EM) became available, spherulites were shown to be comprised of layer-like crystallites [9,10], which were named lamellae. The lamellae are separated by disordered materials. In the center of the spherulites, the lamellae are stacked almost in parallel [5,6,11-15]. Away from the center, the stacked lamellae splay apart and branch, forming a sheaf-like structure [11,13-15]. It was also found that the thicknesses of lamellae are different [5,6,11,12]. The thicker ones are believed to be dominant lamellae while the thinner ones are subsidiary lamellae. 

Microscopy is the study of the fine structure and morphology of objects with the use of a microscope. Microscopes range from optical microscopes, which resolve details on the micrometer level, to transmission electron microscopes that can resolve details less than one nanometer across. The size and visibility of the polymer structure to be characterized generally determines which instrument is to be used. For example, the size and distribution of spherulites can be observed by optical techniques, but a study of their internal structure requires electron microscopy. Combinations of the various microscopy techniques generally provide the best insight into the morphology of polymer materials [1]. Table 2.1 shows the basic properties of the different microscopes, for the purpose of comparison. 

Natural and synthetic textile fibers were among the earliest materials studied by electron microscopy. Guthrie [1] and Stoves [2] described the techniques and applications of fiber microscopy to industrial practice. Somewhat later, evidence was provided for an oriented microfibrillar texture in polymer fibers [3]. Evidence suggested there is an arrangement of fine structures about 50 nm long and 5 nm wide in semicrystalline fibers [4,5]. Peterlin [6,7] observed the formation of fibrils and microfibrils by the deformation and transformation of spherulites using various microscopy techniques. 

To some polymer scientists the word morphology means the assembly and relative arrangement of crystals or of second phase particles, while others use supermolecular structure to describe the same thing. Whatever the name, optical and electron microscopy and the complementary techniques of light and x-ray scattering are used to determine such structure. Common arrangements in crystalline polymers solidified frpm the melt are spherulites, row structures, stacks or bundles of lamellae, rods and fibrils. Poorly ordered materials may have... 

Compatibility of additives with polymer, and their effect on polymer crystallinity, were evaluated using differential scanning calorimetry. Scanning electron microscopy was used to follow additive dispersion. WAXS and optical microscopy were used to determine effect of additives on crystallization kinetics and spherulite formation. ... 

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