Spherulite size

The greater the percentage crystallinity the higher the yield point and tensile modulus. It has also been shown that by raising the quench temperature the spherulite size is increased and that this greatly decreases the impact toughness. 

The polymer is liable to depolymerisation at temperatures just above T. In the case of pure polymer there is a tendency for the few spherulites to grow to sizes up to 1mm diameter. Spherulite size may be reduced by the use of nucleating agents and by fast cooling. 

We can nucleate crystallization from the melt by incorporating finely ground inorganic crystalline compounds such as silica. Nucleation of injection molded nylons has three primary effects it raises the crystallization temperature, increases the crystallization rate, and reduces the average spherulite size. The net effect on morphology is increased crystallinity. This translates into improved abrasion resistance and hardness, at the expense of lower impact resistance and reduced elongation at break,... 

Both the rate of nuclei formation and the crystal growth rate can also be expected to influence the spherulite size. It has been reported (hat, in the temperature range 130-180 °C, the spherulite size increases with increasing temperature [74], This trend can be expected to extend to higher temperatures as the nucleation rate decreases. On the other hand, the presence of nucleating... 

Haze is generally caused by the scattering of light in crystalline polymers. Optical inhomogenities with dimensions in the wavelength range of visible light cause haze. The latter often corresponds to the spherulite volume fraction, spherulite size and crystallinity. An increased size of spherulites results in... 

The morphology of the spherulites was in the form of a Maltese Cross , which was confirmed by the Avrami exponent value in the DSC study. The spherulite size of the binary blends was smaller than that of pure PET and PEN. 

The effect of heterogeneous nucleation on the crystallization of isotactic polypropylene from the melt can be easily established as follows. A small amount of powdered polypropylene is well mixed with about 0.1 wt% of sodium benzoate in a mortar or by means of an analytical mill. Some of the mixture is transferred with a spatula to a microscope slide and melted at about 250 °C on a hot block. A cover slip is pressed on to the melt with a cork to obtain as thin a film as possible.The sample is held at 200-250 °C for some minutes and then allowed to crystallize at about 130 °C on the hot stage of the microscope an unadulterated polypropylene sample is crystallized in the same way. Both samples are observed under a polarizing microscope during crystallization,the difference in spherulite size between nucleated and untreated polypropylene can be seen very clearly. An ordinary microscope can also be used by placing polarizers on the condenser and eyepiece, and adjusting these to give maximum darkness. 

From vv = 15 to iv = 19. spherulites remain in equilibrium with isooctane. The spherulite size differs markedly (from 100 nm to 8000 nm). Syntheses in this phase region (15 < vv < 20) show formation of particles having a higher polydispersity in size and in shape than those observed at low water content. As matter of fact, triangles, squares, cylinders, and spheres are observed. 

The structure of crystalline polymers may be significantly modified by the introduction of fillers. All aspects of the structure change on filling, crystallite and spherulite size, as well as crystallinity, are altered as an effect of nucleation [9]. A typical example is the extremely strong nucleation effect of talc in polypropylene [10,11], which is demonstrated also in Fig. 2. Nucleating effect is characterized by the peak temperature of crystallization, which increases significantly on the addition of the filler. Elastomer modified PP blends are shown as a comparison crystallization temperature decreases in this case. Talc also nucleates polyamides. Increasing crystallization temperature leads to an increase in lamella thickness and crystallinity, while the size of the spherulites decreases on... 

Andrews76 gave results of the work of Reed and Martin on cis-polyisoprene specimens crystallized from a strained cross linked melt and on solid state polymerized poly-oxymethylene respectively, explaining the results by simple two phase models. He also summarized the studies of Patel and Philips775 on spherulitic polyethylene which showed that the Young s modulus increased as a function of crystallite radius by a factor of 3 up to a radius of about 13 n and then decreased on further increasing spherulite size. 

It nevertheless remains difficult to discriminate morphological effects at the lamellar level from other factors, such as crystallinity and spherulite size [128], on the basis of the available evidence [21, 23, 24, 129, 130, 131, 132] (cf. Sect. 3.4.1). This also makes it difficult to discount alternative explanations for the improved ductility of the ft phase above Tg, based on its intrinsically higher molecular mobility, for example [133, 134, 135, 136]. One is therefore forced to conclude that any or all of the factors referred to here may play a significant role in the observed behaviour. 

Copolymerization also affects morphology under other crystallization conditions. Copolymers in the form of cast or molded sheets are much more transparent because of the small spherulite size. In extreme cases, crystallinity cannot be detected optically, but its effect on mechanical properties is pronounced. Before crystallization, films are soft and rubbery, with low modulus and high elongation. After crystallization, they are leathery and tough, with higher modulus and lower elongation. 

Figure 7. Transmission electron micrographs of solution-cast films of (upper left) 1/312, (lower left) 1/4/3, and (above) 1/6/5. Spherulite size decreases with decreasing hard-segment content.

Spherulite size shows a general increase in going from the shear region towards the core. 

Increasing EPDM content results in irregular spherulitic texture, smaller spherulite size, and loss of sharpness in the spherulite boundaries. 

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. 

The effects of morphology (i.e., crystallization rate) (6,7, 8) on the mechanical properties of semicrystalline polymers has been studied without observation of a transition from ductile to brittle failure behavior in unoriented samples of similar crystallinity. Often variations in ductlity are observed as spherulite size is varied, but this is normally confounded with sizable changes in percent crystallinity. This report demonstrates that a semicrystalline polymer, poly(hexamethylene sebacate) (HMS) may exhibit either ductile or brittle behavior dependent upon thermal history in a manner not directly related to volume relaxation or percent crystallinity. 

Both banded (Tc = 52°C) and unbanded (Tc = 60°C) spherulitic morphologies had essentially identical stress-strain curves despite a difference in crystallinity of 8% and variations in spherulite size for these two crystallization conditions. These changes in crystallinity and spherulite size might compensate sufficiently to allow similar bulk deformation behavior. However, the sample crystallized at 52 °C should have smaller spherulites and thinner lamellae than the sample crystallized at 60 °C because of a greater probability of tie molecules. This, combined with its lower crystallinity, should allow more ductile behavior for the 52° C crystallized sample. The fact that both specimens deform similarly indi-... 

The PP microstructure modifications obtained in presence of the different fillers were analysed degree of crystallisation X. (from DSC analysis), and spherulite size Dj measured from... 

The content of amorphous phase and the small size of spherulites lead to an improvement of the fracture toughness of Polypropylene [16]. In presence of mineral filler, the particle surface chemistry can induce some specific microstructural characteristics of the PP matrix parameters such as degree of crystallisation, spherulite size, and p phase content (a/p ratio) [16]. 

With CaC03, the spherulite size is significantly reduced (Ds = 10-15 pm) and the particle surface chemistry induces some specific microstructural characteristics of the PP matrix small size surface treated CaC03 particles promote formation of the p phase. Without surface treatment, CaC03 has a nucleating effect the degree of crystallisation is increased by about 20%(X(. = 65%). 

Ultrafine Si02 particles have no significant effect on the PP microstructure the degree of crystallisation is constant, and the spherulite size is slightly reduced. This could be explained by the amorphous structure of Si02, and the size of the particles (10 to 10 times smaller than Ds). 

The effect of crystallinity on the PP fracture behaviour was observed from tests on the neat polymer, by using different crystallisation temperatures and annealing treatment spherulite sizes range from 20 pm to 80 pm, and crystallinity X. from 64% to 75% [20 -21]. As the crystallinity is increased, the elastic modulus is enhanced and the toughness (both critical energy Jq 2 and propagation energy) is considerably reduced a ductile to brittle transition is observed at Xg > 70% This is coherent with results from Ouedemi [22]. 

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. 

---