Spherulites size distribution

In the case of heterogeneous nudeation with strong interactions, on the other hand, the number of spherulites remains constant with time. In addition, all the spherulites are of equal size. In purely homogeneous nudeation, the number of nuclei is constantly increasing, and a spherulite size distribution is observed. 

Wilfong et al. (1986) reported on the effects of blending low concentrations (1-10 wt%) polyolefin with PET on the crystallization and toughening behavior of the latter. The authors studied blends of PET with LLDPE, HDPE, PP, and poly (4-methylpentene-l), all of them having a lower melting point than PET (Table 3.15). Polyolefin melts did not enhance the nucleation of PET, although the spherulite size of the PET matrix was found to be 2.5-3 times larger than for the homopolymer, with a broader spherulite size distribution. Both the crystallization... 

Fignre 10.18 Spherulite size distribution for (a) iPP and (b) iPP/LDPE (80/20) blend, measured from thin sections of iso-thermally crystallized bulk samples (T = 131°C). Reprinted from Galeski et al. [77], Copyright 1984, with permission from... 

Figure 8. Spherulite size distribution (radius vs. depth) in 2 mm thick injection molded plates of PEBA Samples G, H, and I (see Table 1)...

Under defined conditions, the toughness is also driven by the content and spatial distribution of the -nucleating agent. The increase in fracture resistance is more pronounced in PP homopolymers than in random or rubber-modified copolymers. In the case of sequential copolymers, the molecular architecture inhibits a maximization of the amount of the /1-phase in heterophasic systems, the rubber phase mainly controls the fracture behavior. The performance of -nucleated grades has been explained in terms of smaller spherulitic size, lower packing density and favorable lamellar arrangement of the /3-modification (towards the cross-hatched structure of the non-nucleated resin) which induce a higher mobility of both crystalline and amorphous phases. 

In the case of semi-crystalline PET, comparing the TEM photographs and the measured spherulite sizes, it can be assumed that the individual reactive particles should be distributed in within the spherulitic structure. Concerning the non-reactive one it is highly probable, knowing the small size of the semi-crystalline microstructure, that the modifier clusters remain outside the spherulites. 

In semicrystalline polymers, fillers may act as reinforcement, as well as nucle-ation agents. For example in PP, nanoscale silica fillers may nucleate the crystallization resulting in spherulites that show enrichment in particles in the center of the spherulite (Fig. 3.64). For a quantitative analysis of, e.g., filler sizes and filler size distributions, high resolution imaging is necessary and tip convolution effects [137-140] must be corrected for. The particles shown below are likely aggregates of filler particles considering the mean filler size of 7 nm [136]. 

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. 

An analysis of the effect of the size distribution of spherulites demonstrates that the size obtained from the position of the maximum is highly weighted in favour of the large sizes. An effect of the distribution of sizes is to wash out the maxima corresponding to higher order interference. 

Polymer structure n. (1) A general term referring to the relative positions, arrangements in space, freedom of motion of atoms in a polymer molecule, and orientation of chains. Such structural details have important effects on polymer properties such as the second-order-transition temperature, flexibility, and tensile strength. (2) The microstructure of a polymer, as observed by light- or electro-microscopic techniques, and including crystalline structure, birefringence, distribution of sizes of filler particles and spherulites, and distribution of reinforcement directions. These, too, have important influences on macroscopic properties and behavior. 

Ti02 particles in HDPE matrix aid the polymer chains in crystalhzing into more perfect and thermally stable lamella. The number of nucleation sites is dramatically increased and the crystal size decreased. The solar reflectance was related to the degree of crystallinity, the spherulite size of HDPE, refractive index, and distribution of Ti02 particles in HDPE matrix. The ratile Ti02 increased the total solar reflectance from 28.2 to 51.1%. ... 

Knowledge of nucleation data is essential for controlling physical properties of a polymer which depend to a great extent on the spherulite average size, size distribution and the size of so-called weak spots -defects of spherulitic structure including cavities and frozen stresses which resulted from voliune contraction during crystallization, all determined by the primary nucleation process. 

This texture is related to the crystallinity of the polymer film, and a range and distribution of spherulite sizes can be related to both process variables and applications. 

Discussing the effect of the peroxide content, the structural parameters (MW, MWD) and the supramolecular characteristics such as the spherulite size and distribution on the fracture toughness parameters and the failure mechanisms. The analysis is carried out on real injected CRPPs in which neither thermal treatment was applied nor nucleating agents were used. 

Ten scanning views were chosen randomly under enlargement at lOOOx for quantitative determination. The image analysis software Image Pro-Plus 4.5 was used to obtain the distribution and spherulite size, taken as the maximum length inside the spherulite. Percentage of porosity was also determined from the microstructural images. 

Table 5. Porosity and normal distribution parameters of the spherulite size of ptropylene homopolymer, PPO, and the controlled-rheology-polyptropylenes, PP-CRs mean spherulitic size, D, and variance, o. ...

Fig. 2. Normal distributions of the spherulite size for polypropylene homopolymer, PPO, and controlled-rheology polypropylenes, CRPP154, CRPP402 and CRPP546.

Regarding the evolution of the supramolecular properties of the propylenes homopolymers, the average spherulite size seems to enhance and the distribution width seems to reduce, attaing a more unifrom distribution, as the DTBP content increases. The reduction of Mw/ promoted by peroxide addition, leads to the maintenace of the crystalline degree, with the only growth of the spherulite size at the expense of the reduction in the density of tie molecules. However, for very high peroxide content, this trend is not followed because of the presence of pores. 

The filler also influences the size distribution of the spherulites. Small additions of filler lead to a spherulite distribution which depends on the shape and size of the filler particles. With the presence of anisodiametric particles, the distribution is non-unifonn and exhibits two maxima. For any particular average size of spherulites, the character of the size distribution may vary. At the same time, high and low concentrations of fillers influence the supermolecular structure in different ways. 

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