Crystallinity level

In this article we have reviewed the results of a joint spectroscopic and morphological investigation of -sexilhienyl (T(J. The lowest singlet electronic level, which is assigned to I B , splits in the single crystal into four crystalline levels. The structure of the exciton band is investigated by the combined absorption and... 

Stereodefects in syndiotactic polymers have similar effects to those in their isotactic counterparts, reducing crystallinity levels and changing the associated physical characteristics. 

Due to the low level of branching, there is little to hinder the crystallization of high density polyethylene. We routinely observe crystallinity levels in excess of 60%, which translate into densities ranging from approximately 0.94 to 0.97 g/cm3. High density polyethylene is the stiffest of all the polyethylene types. In some cases we incorporate small amounts of a comonomer, such as 1 -hexene, which reduce the crystallinity level. This improves toughness at the expense of stiffness. 

The crystallinity levels in ethylene ionomers are generally low due to their high levels of branching and the clustering of the metal salts. At high temperatures, the clusters dissociate and the individual chains can move independently in the molten state, permitting them to be molded. When the ionomer is cooled dusters reform, crosslinking the chains... 

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. 

Whereas several techniques may thus be used to study a certain characteristic of a polymer sample, for instance IR and Raman spectroscopy and X-ray diffraction as well as NMR may be used to determine or infer the crystallinity level of a sample, different techniques work differently and therefore usually do not measure the same. What this means is that crystallinity levels obtained from the same sample may differ when a different technique is applied, see, for example, ref. [23] and chapter 7 and references therein. However, these differences do not necessarily imply one technique being better than another. In fact these differences may contain useful information on the sample (see, for example, ref. [25]). 

These methods, and the most simple one with K — 1, were comparatively analyzed in a series of iPPs with crystallinity levels ranging from 0.50 to 0.80. The results are shown in Table 1. For this system, the corrections proposed by Natta and the most simple subtraction method (K — 1) gave similar results. The values obtained by the Hermans-Weidinger method are only slightly lower however, the method developed by Mo and Zhang yield consistently higher values. From this comparison, it appears that the most used simple method with K — 1 gives reasonable crystallinity values. 

It was the objective of this work to investigate the effect of variation in block architecture (number and the order of the blocks) on the crystallinity level, morphology, the stress-strain and hysteresis behavior of this series of polymers. In addition, the composition ratio of the two block types is expected to play a crucial role in determining the bulk material properties of the block copolymers. This is related to the fact that the mechanical properties of block copolymer are typically influenced more substantially by the behavior of the continuous phase, as will be demonstrated.(1,22)... 

Knitted nets are produced from blown film or specialized extmsion lines with corotating dies. The film rolls are slit into small tapes of 2-5 mm width. Then the tapes are stretched on heated rolls (galettes) in several steps to achieve and maintain a high level of orientation. Typical stretching factors are three to five times. Thus, the materiel undergoes a strain-induced crystallization, which maximizes the crystallinity level in the tape. 

Information about the crystallinity level and crystal size in a food can be obtained by submitting the food matrix to X-ray diffraction. For example,... 

Crystallization and shrinkage were influenced by cooling rate (part thickness and mold temperature). While the oil-heated mold maximized crystallization, cooler (water-heatable) molds produced crystallinity levels of 25% or better (Chapter 1). Parts molded with the high mold temperature did exhibit better surface finishes. Shrinkage was relatively low for all processing conditions and design variables. 

The highest yield of levoglucosenonc (29 %) has been obtained by pyrolysis of microcrystalline Munktell cellulose, which is characterized by the highest index of crystallinity, levelling-off DP and the lowest hydrophiliiy. 

This term describes improved properties of energetic materials. In the 1990s it was shown that with careful recrystallization techniques the sensitivity RDX can be reduced on a crystalline level. Nowadays, reduced sensitivity variants of RDX, assigned as RS-RDX or l-RDX (for insensitive), are provided by different manufacturer and tested, particularly in plastic bonded explosives PBXfor Insensitive Munitions IM. Besides, RS-variants of HMX and other high explosives, and characterization and quality assessment techniques for Reduced Sensitivity are in the scope of actual research. 

In principle, many factors affect the rate of degradation of PHAs. They include the properties of the polymers, such as the molecular composition, the resulting intermo-lecular interactions, hydrophilic and hydrophobic behavior, the structural parameters, like degree of crystallinity, level of orientation, surface structure, and molecular weight. Concerning biopolymers, the related factors are the growth of microorganisms, especially the pH-value, temperature, aerobic or anaerobic conditions etc. 

Nearly all properties mentioned under group A are influenced by the temperature locations of the Tg/Tm values and the crystallinity level (Hf-value). The Tm-value and the thermal stability determine together the polymer s processing window. The moisture sensitivity, finally, is important in connection with the barrier properties of a polymeric system. Furthermore moisture absorption influences several physical properties considerably (see chapter 5.2). 

Minkova and Nikolova [1989a,b] investigated the effect of electron irradiation on the thermodynamic and mechanical properties of thin films (100 pm) of LDPE/iPP blends (Table 11.9). Melt blending of the polymers was done at 180-225°C. The tubular films were extraded (145-190°C) and cooled slowly to room temperature (this would keep the crystallinity level high). The samples were irradiated (50-120 kGy) with electrons from an E-250 accelerator (energy not specified probably 250 keV). 

Crystallinity and Crystallization Rate With crystalline materials, the level of erystallinily is an important factor for determining polymer properties. Degrees of crystallinity can be determined by IR spectroscopy, X-ray diffraction, density measurements, and thermal melhod.s. In most cases DSC is one of the easiest methods for determining levels of crystallinity. The crystallinity level is obtained by measuring the enthalpy of fusion for a sample (A/f )simpit nd comparing it to the enthalpy of fusion for the fully crystalline material The fractional crys-... 

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