Spherulitic polymer

Figure 5.16 Images of a polymer spherulite plotted as in Fig. 5.15 except the material experienced faster quenching to below the crystallization temperature. Reproduced from figure 2 of Ref. 31, with permission.

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. 

HMS can crystallize in either banded or nonbanded spherulitic morphologies as illustrated in Figures 2a and 2b respectively (14). Banding is typical of HMS crystallized below 56°C whereas nonbanded spherulites are formed by crystallization above 56°C (14). Banding of polymer spherulites is thought to be related to the periodic twisting of... 

The growth of polymer spherulites involves the segregation of noncrystal-lizable material into the regions between the lamellar ribbons. The components that arc not incorporated into the crystallites include additives like oxidation stabilizers, catalyst residues, and so on. as well as comonomer units or branches. The spherulite structures and interspherulitic boundaries are held together primarily by polymer molecules which run between the twisted lamellar subunits and the spherulites themselves. Slow crystallization at low degrees of supercooling... 

Figure 45. Optical micrographs, between crossed polarizers, of alkane C162H326 to which 10 wt % was added of (a) C122H2461 and (b) C246H494. Tc = 120 °C. The mixture with the longer-chain guest molecules (b) shows a texture resembling that of polymer spherulites (from ref 204 by permission of American Chemical Society).

Ultraviolet and fluorescent microscopy has been applied to a variety of polymer systems to investigate changes of morphology and composition on the scale of 0.25 ym upwards. Studies are briefly described on the behaviour of stabilisers in polypropylene, diffusion of additives in polymers, spherulite morphology, polyolefin oxidation, inhomogeneities in epoxy resins and polymer blends. 

Fig. 3 AFM image of Pd/PPX nanocomposite a Phase-contrast image dark regions are polymer spherulites and light spots are Pd nanoparticles located at the boundaries between polymer globules, b Cross-section A-A, Profile maximums correspond to Pd particles embedded into the boundary surfaces between the polymeric globules...

An ordered packing of macromolecules may also cause an optical anisotropy and birefringence, which are characteristic, for instance, of polymer spherulites. Because of the radial anisotropy of a spheruHte and the convergence of beams in the spherical structure, the interference picture represents the so-caUed Maltese cross, the center of which is located in the center of spherulite. No calculations are performed using such a picture but the photoelasticity method is very efficient in revealing qualitatively the presence of any spherulites, or a mesomorphic or ordered sate of polymeric chains. 

JS A polymer spherulite growing into the melt. In polyethylene the ciystalline fibrils are thin lamellae. The molecules crystallize most rapidly on to the (010) plane the b axis is therefore the direction of most rapid growth and is p lel to the spherulite radius R. The a and c axes are randomly distributed around R. If the solidification is isothermal, the lamellae are all of the same thickness. In order to fill space the raoiating lamellae must branch and give birth to daughter lamellae as they grow out into the melt. Amorphous polymer is left trapped between the crystals. 

K " and n can be extracted from the intercept and the slope of Avrami plot, lg[-ln(l-.A0] versus lg(f-f ), respectively. The prime requirement of Avrami model is the ability of spherulites of a polymer to grow in a free space. Besides, Avrami equation is usually only valid at low degree of conversion, where impingement of polymer spherulites is yet to take place. The rate of crystallization of polymer can also be characterized by reciprocal half-time (/ 5). The use of Avrami model permits the understanding on the kinetics of isothermal crystallization as well as non-isothermal crystallizatioa However, in this chapter the discussion of the kinetics of crystallization is limited to isothermal conditions. 

Bassett DC (2003) Polymer Spherulites A Modem Assessment. JMacromolSci, Physics B42 227-256. 

Fig. 10.18 Illustration of (a) the lamellar crystal, (b) the optical indicatrix and (c) the Maltese-cross extinction in polymer spherulites...

Matsuo, M. 1980. Deformation mechanism of polymer spherulite by linear isothermal viscoelastic theory. J. Chem. Phys. 72 899-910. 

Growth Rate of Polymer Spherulites Crystallized Isothermally from the Melt by Polarizing Optical Microscopy... 

The growth rate of polymer spherulites crystallized isothermally from the melt, G, can be measured by a polarizing optical microscope with a hot-stage. The crystallization temperature, T., was controlled by the hot-stage. The following procedure is recommended for the measurement of the spherulitic growth rate of polymers crystallized from the melt ... 

---