Resins spherulites

Some PBT resins are sold in pelletized form directly from the resin polymerization reactor. These grades are produced as white, opaque pellets due to the presence of spherulites. Since all of the commercial methods for resin polymerization involve melt processes, PBT powder is only available by grinding the pellets. 

Because of the capacity to tailor select polymer properties by varying the ratio of two or more components, copolymers have found significant commercial application in several product areas. In fiber-spinning, ie, with copolymers such as nylon-6 in nylon-6,6 or the reverse, where the second component is present in low (<10%) concentration, as well as in other comonomers with nylon-6,6 or nylon-6, the copolymers are often used to control the effect of spherulites by decreasing their number and probably their size and the rate of crystallization (190). At higher ratios, the semicrystalline polyamides become optically clear, amorphous polymers which find applications in packaging and barrier resins markets (191). 

Nucleants. Although nylons crystallize quickly, it is often an advantage, particularly for small parts, to accelerate this process to reduce cycle time and increase productivity. Nylon-6, which crystallizes more slowly than nylon-6,6, also benefits from nudeation in unreinforced formulations. Nucleants are generally fine-particle-size solids or materials that crystallize as fine particles before the nylon. The materials, eg, finely dispersed silicas or talc, seed the molten nylon and result in a higher density of small uniformly sized spherulites in nylon-6 the crystalline form is also changed. Nudeation increases tensile strength and stiffness but makes the material more britde. Mold shrinkage is lower for nudeated resins. 

Most polymers fall in the class of translucent resins. These include acetal, polyamide, polybutylene terephthalate (PBT), polyethylene, and polypropylene as examples. There are very few neat polymers that are truly opaque (this depends on thickness as well). Liquid crystal polymer (LCP) is an example of a typically opaque polymer. It is theorized that these semicrystalline and crystalline resins will scatter some portion of incident light due to spherulitic crystal structure and the amorphous-crystalline region interfaces themselves. 

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 injection molded composites of polypropylene containing short glass fibers, the fiber orientation depended on the flow pattern (which, in turn, is related to mold thickness, the position of the gate, and flow rate)." Substantial variation was detected along the thickness of the sample. Crystallites followed a pattern of fiber distribution but they grew in a direction perpendicular to the direction of the fiber and specimen surface. The direction of spherulite growth was different in neat resin where crystallites grew parallel to the surface of the mold (specimen). 

The spherulites of polypropylene crystals (80 pm) were larger than those of polypropylene/epoxy blends. This is because epoxy resin particles acted as nucleating sites, increased the number and decreased the size of PP spherulites in the blends. The normalized dynamic DSC cooling curves of polypropylene/epoxy blends are shown in Fig. 21.7 and the data from the DSC studies are summarized in Table 21.5. 

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. 

Nudeating agents thermodynamically encourage the initiation of spherical, radiating crystalline regions (spherulites) at multiple, well-distributed sites throughout a polymer matrix. Added to the resin usually right after the polymerization... 

The properties of PLA, as indeed those of other polymers, depend on its molecular characteristics, as well as on the presence of ordered structures, such as crystalline thickness, crystallinity, spherulite size, morphology and degree of chain orientation. The physical properties of polylactide are related to the enantiomeric purity of the lactic acid stereo-copolymers. Homo-PLA is a linear macromolecule with a molecular architecture that is determined by its stereochemical composition. PLA can be produced in a totally amorphous or with up to 40 per cent crystalline. PLA resins containing more than 93 per cent of L-lactic acid are semi-crystalline, but, when it contains 50-93 per cent of it, it is entirely amorphous. Both meso- and D-lactides induce twists in the very regular PLLA architecture. Macromolecular imperfections are responsible for the decrease in both the rate and the extent of PLLA crystallization. In practise, most PLAs are made up of L-and D,L-lactide copolymers, since the reaction media often contain some meso-lactide iir turities. 

In reality, the morphology of a polycrystalline thermoplastic consists of spherulites that holds for common polymer types (e.g. PP, PE, PA6, PA6,6, and polyether-ether ketone (PEEK) crystalhzed under common conditions). Some semicrystalline polymers and moderately filled composites of the above resins may exhibit lamellar crystalline morphology without any spherulitic order. As a result of random orientation of individual crystallites in spheruhtes and the manner of their connectivity, the elastic modulus of about 10 GPa has been extrapolated for a hypothetical ideal polycrystalhne PE containing no amorphous phase from the dependence of the elastic modulus of PE on the degree of crystallinity. Presence of amorphous phase reducing the content of crystalhne phase results in a further reduction of the overall elastic modulus of the semicrystalline polymers compared with ideal monocrystals (109). 

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