Crystalline structures chain reactions

For most polymers, the yield of hydroperoxides is relatively low even in the presence of oxygen excess. The relatively high values were, e.g., obtained during oxidation of atactic polypropylene [79], In the initial phases of oxidation, the yield of hydroperoxide related to 1 mol of oxygen absorbed is 0.6 at 130 °C when passing the maximum concentration it decreases considerably. In isotactic polypropylene, the maximum yield of hydroperoxides attains the value 0.2, only [80]. This may be probably related with a local accumulation of hydroperoxides in domains of defects in the crystalline structure which leads to an increased ratio of participation of hydroperoxide groups in the chain reaction of an oxidation process (induced decomposition of hydroperoxides) and finally to a lower yield of hydroperoxides... 

If a similar situation were always to exist with all crystalline carbohydrates, i.e. major amounts of hydrogen, a large number of minor products and a considerable unresolved dimer fraction, the attention that this field has found would perhaps have been undeserved. Interestingly, however, radiation-induced chain reactions were observed in the case of several crystalline carbohydrates (see Tables 4 and 5 note also the dominance of the chain product (6-deoxy-D-t/rreo-2,5-hexodiulose) compared to the fragmentation products, in Table 4). The chain reaction which often leads to an isomer of the substrate molecule or its dehydration product is governed by the crystal structure, as it is no longer observed in crystals of the ssune compound, but with a different structure. 

Unlike the influence of morphology on mechanical behavior, the effect on biological behavior is a less mature area of endeavor. For absorbable polymers, the effect of crystalline structure on diffusion and reaction rates provides insight. The relative amount of crystalline phase influences the rate of diffusion of water into a hydrolytically unstable polymer. Furthermore, the rate of hydrolysis of a given ester group in the polymeric chain will depend on whether the group resides in a self-protecting crystal or whether it exists in an unprotected, easily accessed, amorphous phase. 

The role chitin as a material of highly ordered crystalline structure has been reported in the study [96]. X-ray diffraction analysis was carried out in order to find the changes of the crystalline structure upon the substitution reaction with NCO terminated prepolymer. The X-ray diffraction studies showed that crystallinity mainly depends on the concentration of chitin in the polyurethane backbone, crystallinity increased as the concentration of chitin into the final PU increased (Fig. 3.22). The crystallinity of some polymers was clearly observed by optical microscopic studies [114]. The results of X-ray diffraction experiments correlate with optical microscopy findings. A crystalline polymer is distinguished from an amorphous polymer by the presence of sharp X-ray Unes superimposed on an amorphous halo. Under an optical microscope, the presence of polycrystalline aggregates appear as spherulites [114]. The spheruhtes are made of small crystallites and grow Irom a nucleus at their centre. They consist of narrow chain folded lamellae growing radially. Since the fibrous crystals are radial, the chains folded with the lamellae are circumferentially oriented. From the evaluation of the X-ray and optical microscopic studies, it has been observed that the involvement of chitin in the PU formulation and have improved crystallinity of the final polyurethane. 

Zia et al. [115] presented that nanostructure and morphological pattern of chitin/bentonite clay based polyurethane bionanocomposites. The clay dispersion within chitin was characterized by both XRD and optical microscopy (OM), which is the most frequently, used and approachable methods to study the structure of nanocomposites. There are one acetamide (-NHCOCH3) group at C-2 position and two (two hydroxy (-OH)) groups at C-3 (C3-OH) and C-6 (C6-OH) positions on chitin chains which can serve as the coordination and reaction sites [95], The crystalline structure of chitin has been reported by many researchers [96],... 

Compared to starch, natural cellulose is much more crystalline and therefore more difficult to breakdown by any means. Some enzymes can catalyse oxidation reactions of either cellulose itself or the lower molecular weight oligomers produced from the enzymatic hydrolysis of cellulose (Aubert et al., 1988). By incorporating esters groups in cellulose molecular chains, the crystalline structure of cellulose is disrupted as a result, cellulose esters show much lower crystallinity in comparison with the original cellulose and can be more readily degraded in active biological environments. 

The aromatic semiflexible polyesters can be formed by varying the dicarboxylic multi phenyl acid structure (naphthalene and diphenol) and non-linear aromatic diol (3,4 -dihydroxybenzophenone). 3,4 -dihydroxybenzophenone can be synthesized from 4-methoxy bromobenzene and m-methoxybenzonitrile via multistep reaction, which can be condensed with respective diacid chloride in o-dichlorobenzene at reflux temperature [43]. It is well known that dimethyl siloxane spacer can make a flexible/semiflexible LC polymer. Thermotropic flexible liquid-crystalline main chain polyesters... 

The restricted delocalization of the p electron is essentially due to the energy difference of 2e = 0.4 eV per unit cell between the butatriene and acetylene chain structure. The chain length dependencies of the fine structure and hyperfine structure constants of the ESR spectra as well as the chain length dependencies of the absorption series have been described by theoretical model calculations. The complexity of the polymerzation reaction in diacetylene crystals has been demonstrated by the manifold of different reaction products observed in the experiments. The large amount of detailed information concerning the electronic structures and concerning the reaction mechanisms is essentially due to the crystalline structure of this model system and to the thermal stability of the reaction intermediates at low temperatures. 

---