Poly crystallite size

Kinoshita has also shown that ORR data for supported catalysts in hot, concentrated H3PO4 (180 °C, 97-98% acid) reported in three different studies were also fit by this model. Since the physical basis for the crystallite size effect in sulfuric acid is anion adsorption, it would be a considerable reach to suggest that the same physical basis applies to this size effect, i.e., structure-sensitive anion adsorption. There are, nonetheless, indications that this is the case. Anion adsorption in dilute phosphoric [43] has a very similar structure sensitivity as sulfate adsorption, i.e., strongest adsorption on the (111) face, and on poly-Pt anion adsorption and/or neutral molecule adsorption in dilute phosphoric has a strongly inhibiting effect on the kinetics of the ORR [43]. Sattler and Ross [16] report a similar crystallite size dependence of the ORR on supported Pt in dilute phosphoric acid at ambient temperature as that found in hot, concentrated acid with the same catalysts. But it is unclear whether similar adsorption chemistry would exist in the extreme conditions of hot, concentrated phosphoric acid. 

In LCP blends with PBT, the increase in crystallinity is associated with the decrease in crystallite size (nucleation favored) when LCP concentration is low (20%). At high LCP content the increase in crystallinity is associated with the increasing crystallite size suggesting the favored growth of crystal domain. Poly(ethylene 2,6-naphthalate), PEN, spherulite dimensions decrease in the presence of LCP. This is caused by a heterogeneous nucleation by the crystallized LCP particles on the molten PEN matrix. Also presence of LCP nucleates poly(ether imide) increasing its crystallization rate. ... 

O. Yoda, "Crystallite size and lattice distortions in the chain direction of irradiated poly(aryl-ether-ketone)." Polymer Communications Guildford, pp. 16-19,... 

Pristine poly(acetylene) in either cis- or trans-form exhibits optical spectra typical for a semiconductor. The absorption peak of the cis-polymer is structured with maxima at 2.1, 2.3 and 2.4 eV, the absorption coefficient is about 4 10 cm" The structureless and very broad absorption of the trans-polymer peaks at 1.9 eV with an absorption coefficient of 3 10 cm . The large halfwidth of the peak is generally explained as being due to the rather broad chain length distribution in addition, the small crystallite size and crystal defects may contribute to the broadening. A discussion is still ungoing whether the peak is linked to a direct interband transition or whether it is of an excitonic character.2-4, 7,32... 

The crystallinity and average crystallite size of undrawn and cold-drawn films of poly(ethylene-co-carbon monoxide) increased upon UV irradiation... 

The methods used for the crystallinity determination can also be used to determine the ratios of the different polymorphs that are often present in some polymers. In the case of PA6 shown in Figure 2.6a, the scan is resolved into the contribution from its two polymorphs, a and y, along with the amorphous halo [47]. Relative areas of the various peaks are used to calculate the relative amounts of the a and y components as well as the total crystallinity. The method can also be extended to determine when more than one polymer is present in the sample, such as polymer blends [48,53], Figure 2.6b shows an example of a mixture of amorphous poly(2,6-dimethyl-p-phenylene ether) (PPE) and PA6. The amorphous templates of PPE and PA6 were obtained from the scans of the homopolymers as discussed in Section 2.5.1. These templates were used as constraints in least squares fitting the data from the blend. Such analyses were useful in demonstrating that crystallinity and crystallite sizes of the PA6 were smaller in an alloy of the two polymers than in a blend [48]. Similar analyses have been carried out in a blend of two crystalline polymers, polyethylene and polypropylene [53]. 

The first three materials listed in Table 11.6 are amorphous thermoplastics. CR-39 is a cross-linked, amorphous network (see Section 17.2). Most highly crystalline polymers are hazy because the crystals and the amorphous phases do not have the same index of refraction and light is scattered at the interfaces. Poly(4-methyl-l-pentene) is unusual in that the two phases have nearly the same index of refraction. The haze in crystalline polymers can be reduced if the crystallite size is very small. A sorbitol-based clarifier for polypropylene is bis(3,4-dimethyldibenzylidene) [25]. It acts as a nucleating agent and makes it possible to produce a water bottle with PET-like clarity. 

Porat et al. performed TEM (zero-loss bright field) studies of very thin Nation films that were cast from ethanol/water solutions, and some of the conclusions are as follows. It was suggested that the backbone had a planar zigzag conformation in large orthorhombic crystallites as in polyethylene, in contrast with the helical conformation found in poly(tetra-fluoroethylene). This is an interesting result, although there are no other studies that support this view. Sulfur imaging indicated the presence of sulfonate clusters that are 5 nm in size. 

Crystalline polymers exhibit the following basic properties They are opaque as long as the size of the crystallites or spherulites, respectively, lies above the wavelength of light. Their solubility is restricted to few organic solvents at elevated temperature. The following crystalline polymers have attained technical importance as thermoplastic materials polyethylene, polypropylene, aliphatic polyamides, aliphatic/aromatic polyamides, aliphatic/aromatic polyesters, poly-oxymethylene, polytetrafluoroethylene, poly(phenylene sulfide), poly(arylene ether ketone)s. 

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