Spherulites radii

This unfavorable aging process is a major drawback for the commercial use of the PHB homopolymer. Reducing the spherulite radius by means of a nucleating agent did result in a minor increase of the maximum elongation by only a few percent, which is still insufficient for tough applications [18]. 

Video microscopy with crossed polarizers permits the direct and non-invasive observahon of the nucleahon and growth process for many substances, and thus the study of the hme evoluhon of the spherulite radius R t). When the growth is controlled by diffusion the radius of the spherulites increases as R t) a while when the growth is determined by a nucleation-controlled process (incorporahon of atoms or molecules to the surface of the crystalline part) the radius increases linearly with hme, R t) a t. 

The existence of crystal lamellae in melt-crystallised polyethylene was independently shown by Fischer [28] and Kobayashi [39]. They observed stacks of almost parallel crystal lamellae with amorphous material sandwiched between adjacent crystals. At the time, another structure was well known, the spherulite (from Greek meaning small sphere ). Spherulites are readily observed by polarised light microscopy and they were first recognised for polymers in the study of Bunn and Alcock [40] on branched polyethylene. They found that the polyethylene spherulites had a lower refractive index along the spherulite radius than along the tangential direction. Polyethylene also shows other superstructures, e.g. structures which lack the full spherical symmetry referred to as axialites, a term coined by Basset et al. [41]. 

Morphology changes were observed by optical microscopy and small-angle light scattering. The pure components exhibit spherulitic structures, each with different orientation of the optic axis with respect to the spherulite radius. Spherulites become disordered and larger with the introduction of small amounts of the second component. Larger amounts of the second component result in a loss of spherulitic order. 

Figure 17. Change of the average spherulite radius R as a function of degree of swelling for the carboxylic acid and sodium carboxylated membranes having 1100 EW. L and L0 are the bulk linear dimensions under wet and room-temperature dry...

The average spherulite size depends (among factors including the extent of approach to equilibrium during crystallization) on the extent of nucleation. When more nuclei are present, more spherulites will form, but the typical spherulite will be smaller. For ideal spherulitic crystallization where spherulites of identical radius pack as efficiently as possible into the crystalline fraction, Equation 6.36, based on geometrical considerations, can be used to relate the maximum possible spherulite radius (Rmax) to the density (number/cc) of nuclei (pn). Since many fabrication processes will result in a distribution of spherulite sizes which can be rather broad if different parts of a specimen experience significantly different thermal histories (as usually happens, for example, in injection molding), Equation 6.36 is obviously an idealization. 

Figure C2.1.12. Schematic drawing of the cross section through a spherulite. The lines indicate the eonneetivity of the crystalline lamellae. The inner strueture of a lamella is also shown and eonsists of parallel polymer chains with their axes perpendicular to the spherulite radius.

By means of several optical techniques, viz. small angle laser light scattering (SALLS), optical microscopy, etc, the spherulite structure can be studied. From the photographic scattering pattern the spherulitic radius, R, can be calculated as a function of the crystallization time and/or blend composition [Stein, 1964] ... 

A general observation is a decrease of the spherulitic radius with increasing content of the amorphous polymer when a same crystallization time is used (see Figure 3.5 and Table 3.5 PCL/PVC). 

Table 3.5. Maximum spherulite radius, R, as a function of crystallization time (t ) and blend composition...

PP/SBS (melt-mix) 95/5 Spherulite radius -1 with addition of SBS SBS is a weak nucleating agent for PP [Karger-Kocsis, 1979]... 

For pure iPP and PB-1 homopolymers and their respective blends, the spherulite radius increases linearly with time t for all T, investigated. For all samples, the isothermal radial growth rate G was calculated at different as G = dR /dt. Generally, the G values decrease an increase in the values and with increase in the amount of noncrystallizable component in the blend. As shown in Fig. 6.1, where the relative G values of the iPP-based blends are reported for = 125°C, the depression of the G values was more pronounced for the blends prepared with HOCP as the second component. 

The spheruhte dimension, at constant T, increases with increasing concentration of noncrystallizable component. The spherulite radius R increases linearly with crystalhzation time for pure iPP and iPP/PB-l/HOCP blends for all investigated. For all samples, the isothermal radial growth rate, G = dR/dt, calculated at different Tc, is reported in Table 6.11. With the increase in the T, the G values appear to decrease for all investigated compositions. The blends prepared with the same fraction of iPP show G values that decrease with increasing of HOCP fraction at constant Tc value. 

Figure 9.7 Growth curves of spherulite radius R at 403K for (a) iPP/EHR33 and (b) iPP/EHR51. (From Reference 24 with permission from John Wiley Sons, Inc.)...

Graft copolymer between polypropylene and LCP Angle of the incident and scattered beams corresponding to the maximum pattern intensity Spherulite radius... 

FIGURE 11.8 Fully-developed spherulite grown from the melt, comprising chain-folded lamellae (magnified section) and branching points that help to impart a spherical shape to the structure. Most rapid growth occurs in the direction of the spherulite radius R. (Adapted from McCrum, N.B., Buckley, C.P., and Bucknall, C.B., Principles of Polymer Engineering, Oxford University Press, 1988. With permission.)... 

For each T the spherulite radius R increases linearly with time t and no decrease of the growth rate G = dR/dt is observed over long time indicating that during the growth the concentration of... 

Fig. 42. The angles P and

Figure 4. Variation of spherulite radius growth rate with temperature. Notice that the data can be fitted to equation (5) using tlie appropriate characteristic constants.

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. 

To illustrate In drawn poly ethylene) fibers, the speed of light is less in the direction of the fibers than in the direction perpendicular to this. Here, light parallel to the fiber direction shows a higher refractive index. In drawn poly(ethylene) fibers, the molecular axes are largely parallel to the fiber axis. Since poly(ethylene) forms negative spherulites, the molecular axes must be at right angles to the spherulite radius. 

---