Crystalline polymers, structural refinement

The whole-pattern-fitting structure-refinement method, which was first introduced by Rietveld and used for neutron diffraction powder patterns, does yield from x-ray diffraction patterns correct, refined structural information for linear polymers. Remarkably precise lattice parameters are obtained incidentally in the use of the method. The method lends itself to improved estimations of the fraction of amorphous and crystalline materials, or of two polymorphic forms, present. As improved profile functions come in to use, the method promises to provide crystallite size information, almost as a spin-off benefit. 

The lack of crystalline order in polymers such as cellulose or silk significantly reduces the number of interference spots in diffraction investigations hence the patterns often cannot be analysed unambiguously. In these cases a crystal structure refinement using NMR chemical shifts will be of great value, because NMR spectroscopy does not require long-range crystalline order. In our second example of cellulose, the situation occurred that the C4 values in BPT C chemical shift calculations deviated seriously from... 

Through the use of X-ray analysis, electron diffraction, and the supporting experiments of infrared absorption and Raman spectroscopy, much information has been collected on crystalline polymers. Today, the data are analyzed through the use of computer techniques. Somewhere along the line, however, the investigator is required to use intuition to propose models of crystal structure. The proposed models are then compared to experiment. TThe models are gradually refined to produce the structures given above. It must be... 

The unit cell dimensions are important because they are used to calculate crystalline density which can then be used to determine percentage crystallinity using a simple two-phase model of crystalline and amorphous regions. X-ray-based unit cell determination is usually of considerable academic interest and tbe structures of all the common PAEK can be found in the literature [2, 3]. In fact PEEK and PEK were the first polymers to have their structures determined by pattern-fitting structure refinement (PFSR) [2, 3]. This technique produces more accurate unit cells by virtue of fitting X-ray peaks... 

An extremely versatile computer program, POLYMIN, which is designed specifically for the structural refinement of crystalline polymers has been developed. It incorporates several features not found in standard refinement programs and thereby eliminates many of the difficulties encountered in their use. The main advantages of the program are that it ... 

The recent studies of the phase structure of linear polyethylene by refined NMR analyses are reviewed. The phase structure of the polymer in various crystalline forms, including bulk-crystals, solution-crystals and drawn fibers, is discussed in terms of different modes of molecular mobilities in a wide range of temperature. 

X-ray diffraction techniques are the only way of determining the crystal structure of natural and synthetic polymers, although the x-ray data itself obtained from a crystalline polymeric fiber or film is not sufficient to allow complete refinement of the structure. Conformational analysis and electron diffraction represent complementary methods which will facilitate the determination of the structure. The necessary requirements for the x-ray approach are crystallinity and orientation. X-ray data cannot be Obtained from an amorphous sample which means that a noncrystalline polymeric material must be treated in order to induce or improve crystallinity. Some polymers, such as cellulose andchitin, are crystalline and oriented in the native state.(1 )... 

Zeolites. Zeolites are crystalline inorganic polymers made of aluminosilicates and have open framework structures. Natural zeolites (faujasites) have pores of sufficient size to be useful in petroleum refining. Synthetic faujasite-type zeolites are now available in large commercial quantities and have become an important catalyst in the petroleum industry. Comprehensive reviews on the application of zeolite catalysis are available (ll lS). 

Now we report the results of a conformational analysis which was performed on isotactic and syndiotactic polypropylene many years ago (more refined calculations on isotactic polymers performed by us lately, will be reported in the lecture The crystalline structure of addition polymers. Research problems ). 

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