Increasing the crystallinity

The strength of semi-crystalline polymers is larger than that of amorphous polymers. If the crystallinity of a polymer can be increased, the mechanical properties are improved accordingly. 

The crystallinity can also be increased by orienting the chain molecules under mechanical loads. This was already discussed in section 8.4.1 for the plastic deformation of an amorphous thermoplastic close to the glass temperature. By applying a tensile load, the fibres are drawn and straightened, forming crystalline fibres bundles. 

The size of the side chains and thus the mobility of the chain molecules also influences the crystallinity. The more immobile the molecules are, the more difficult it is to arrange them in a closely packed and regular manner. 

Polymers with stiff chain molecules usually have a lower crystallinity than those made of mobile chains. Exceptions from this rule do occur, however Due to its very stiff and straight chain molecules, ptfe can reach crystallinity values of up to 90%. This is only possible if the cooling speed is very low in technical applications, the crystallinity is therefore usually less. 

Aramid is one example for a polymer that can be manufactured with high crystallinity in this way. This is mainly used to produce aramid fibres. Due to the aromatic rings on the backbone (see figure 1.23), the molecule is extremely stiff. It thus does not fold up, but usually exists in rod-like form (similar to ptfe). Well above the melting temperature, these rods are disordered in the 

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The acetal polymers exhibit a high crystallinity." The percentage crystallinity will depend on the quench temperature and will range from about 77%, when quenched at 0°C, to about 80% when quenched at 160°C. Annealing will increase the crystallinity, this being most marked at 150°C Figure 19.5). 

We can manipulate the properties of nylon products by changing the conditions under which we crystallize them. The degree of crystallinity is increased by slow cooling, annealing, and by crystallization from highly oriented melts. As we increase the crystallinity level, stiffness and yield strength increase at the expense of impact strength. 

The introduction of cyano groups in the 3-position of diketimiminate ligands greatly increased the crystallinity of their complexes (Figure 49, 95), due to the formation of one-dimensional coordination polymers. A new f3-oxo-<5-diimine ligand, in turn, allowed the synthesis of a bimetallic organozinc complex 96, by the same methods that had been used for the syntheses of the mononuclear species 92-94.154... 

This method requires the addition of a mixed M(II)/M(III) salt solution to an alkaline solution containing the desired interlayer anion. Preparations under conditions of high supersaturation generally give rise to less crystalline materials, because of the high number of crystallization nuclei. Because this method leads to a continuous change in solution pH, the formation of impurity M(0H)2 and/or M(OH)3 phases, and consequently an LDH product with an undesired M(II)/M(III) ratio, often results. Thermal treatment performed following coprecipitation may help increase the crystallinity of amorphous or badly crystallized materials. 

This procedure causes reduced fusion defects and increases the crystallinity, density, stiffness, hardness, and yield strength. Further, it results in an increased resistance to creep, oxidation and fatigue. 

It has recently been shown (26) that rayon can be subjected to an after treatment, consisting of a very brief exposure to steam, which greatly increases the crystallinity. The water acts as a typical plasticizer by reducing the interaction forces-i.e., the hydrogen bonds-between the cellulose chains this allows them to rearrange themselves so that greater order (crystallinity) is achieved. Table XV shows the increase in crystallinity that can be obtained in this manner. (Similar effects are also known with synthetic fibers-e.g., 6,6-Nylon). 

The influence of surface active agents on the crystallinity of lyophilized diltiazem hydrochloride was examined by Seitakari et al. (38). As the concentration of either polysorbate 80 or poloxamer 188 was increased, the crystallinity of freeze-dried diltizem hydrochloride was found to increase. 

If the preheated preform is stretched and blown into a hot mold, for example, 100-110°C and held there for a short period, the resulting Tg is in the range of 88°C, allowing for hot fill capabilities of such bottles, increasing the crystallinity to the 28-30% level and further decreasing the O2 permeability to one-third that of an nonoriented amorphous... 

Slow crystallization promotes disentangling since the process of reeling-in of chains from the entangled melt onto the crystal surface is promoted and this in turn enhances the maximum drawability, despite the fact that this increases the crystallinity. However, there is a critical lower limit for the number of entanglements between crystals in order to achieve high drawability and in the extreme limit of very slow isothermal crystallization, linear standard polyethylenes (not the UHMW-PE grades) can lose their drawability completely and become brittle materials. 

The concentration and adding speed of acid solution should be properly controlled during the acid extraction of USY zeolite in order to enhance the framework Si/Al ratio and micropore volume simultaneously retain or slightly increase the crystallinity for DUSY zeolite. The selected optimum conditions of acid treatment for USY zeolite have been used to make the catalyst which applied to small scale FCC unit with the excellent catalytic cracking performances ... 

Catalyst Preparation. Previous results obtained in our laboratories showed that the catalyst system used in this study yields polymer of high crystallinity and low percentages of heptane solubility. We have found that preforming and aging the catalyst components further increases the crystallinity and improves reproducibility of kinetic data. 

Another key difference is the need to cofeed water to achieve good selectivities for acrolein. Water is proposed to increase the crystallinity and the number of active sites for propane oxidation while at the same time decreasing the number of acid sites on the surface of the vanadium phosphate catalyst that are thought to be responsible for overoxidation of the products 258). 

Crystallinity indexes calculated according to the method described by Segal et al. (32) showed that the old cotton has a crystallinity of about 38 . Aqueous treatments increased the crystallinity of the historic cotton sample to about 45 . However, the crystallinity of contemporary cotton, which is about 70 , was not reached (30). This increase suggests that water acts as an internal plasticizer and allows a segmental reorientation which leads to an increase in crystallinity. Water-induced crystallization of amorphous cellulose fibers has been reported (17). Kalyanaraman (33) investigated orientation factors of cotton fibers from historic samples and found that the orientation values of the museums samples are smaller than the values of present-day cottons. He opined that cotton may have lost its orientation over time. In view of this... 

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