Polymer crystallinity, determination

Using proton NMR of solutions, the composition of polymers can be analyzed.47 Carbon-13 NMR spectroscopy is a useful tool for studying the sequence length of segments in copolymers and thereby determining the blockiness of the copolymer. With solid-state NMR, the mobility of chain segments can be studied and the crystallinity determined. 

Both vibrational spectroscopies are valuable tools in the characterization of crystalline polymers. The degree of crystallinity is calculated from the ratio of isolated vibrational modes, specific to the crystalline regions, and a mode whose intensity is not influenced by degree of crystallinity and serves as internal standard. A significant number of studies have used both types of spectroscopy for quantitative crystallinity determination in the polyethylenes [38,74-82] and other semi-crystalline polymers such as polyfethylene terephthalate) [83-85], isotactic poly(propylene) [86,87], polyfaryl ether ether ketone) [88], polyftetra-fluoroethylene) [89,90] and bisphenol A polycarbonate [91]. 

The most relevant property of stereoregular polymers is their ability to crystallize. This fact became evident through the work of Natta and his school, as the result of the simultaneous development of new synthetic methods and of extensive stractural investigations. Previously, the presence of crystalline order had been ascertained only in a few natural polymers (cellulose, natural rubber, bal-ata, etc.) and in synthetic polymers devoid of stereogenic centers (polyethylene, polytetrafluoroethylene, polyamids, polyesters, etc.). After the pioneering work of Meyer and Mark (70), important theoretical and experimental contributions to the study of crystalline polymers were made by Bunn (159-161), who predicted the most probable chain conformation of linear polymers and determined the crystalline structure of several macromolecular compounds. 

Effect of Internal Mobility (Flexibility). It seems that the internal mobility or flexibility of the plasticizer molecule plays an important, if not the most important, role in determining plasticizer efficiency. This appears to be true irrespective of the polymer which is being plasticized, unless there are overriding physical factors involved, such as polymer crystallinity. In general, the lowering of T0 will be proportional to the temperature difference between (Tg)polymer and (Tg)plasticizer- This is illustrated in Table XIV. This table also shows that if the polymer itself is quite flexible, such as polychloroprene (Neoprene), the plasticizer efficiency is quite small, and may even result in negative AT values. 

The fibrillar structure of crystalline polymers is determined by molecular characteristics, the initial morphology and orientation conditions. Recently, a complex investigation of the effect of molecular parameters (MW, MWD and degree of branching) and orientation parameters (temperature and draw ratio) on the morphology of PE and its thermomechanical behaviour has been reported 181 185). 

Polymerization of propylene oxide-a-d was carried out by the EtZnNBu ZnEt catalyst in benzene solution in the presence of varying amounts of added water at 70° C, and was terminated after 7 days. The microstructure of the crude polymer was determined by the 1H-NMR method and the yields of amorphous and crystalline polymers were determined by a fractionation method (Fig. 16). When the amount of added water was increased up to 0.3 mole per mole of catalyst, the yield of crystalline polymer increased remarkably, whereas that of amorphous one remained nearly constant, and the isotactic dyad content (I) increased remarkably while syndiotactic one (S) remained almost constant. Thus, the striking parallel was observed between the yield of crystalline polymer and the isotactic dyad content, and between the yield of amorphous polymer and the syndiotactic dyad content. It is therefore concluded that water contributes more remarkably to the formation... 

MHz, a it/2 pulse was 2.2 ysec and the data were acquired with the carrier frequency below resonance (the rhs of the spectrum in each figure). These spectra may be used to determine polymer crystallinity and to determine various kinds of macromole-cular motion. Also spin-lattice relaxation times in the rf interaction frame (Ti ) addition to conventional T- and T p relaxation times have been measured to help elucidate the various mechanisms responsible for the observed chemical shift line shapes. 

Each of the methods cited yields a measure of average crystallinity, which is really only defined operationally and in which the polymer is assumed artificially to consist of a mixture of perfectly ordered and completely disordered segments. In reality, there will be a continuous spectrum of structures with various degrees of order in the solid material. Average crystallinities determined by the different techniques cannot always be expected to agree very closely, because each method measures a different manifestation of the structural regularities in the solid polymer. 

In comparing the shear fracture surfaces of amorphous and semi-crystalline polymers, it appears that the features in both cases are quite similar (Fig. 39a -c ). This indicates that, under comparable conditions, the local stress field rather than details of the crystalline-amorphous microstructure of the polymers tested determines the operating deformation mechanism. Only secondary effects arise from the morphology of the cry stalline material. 

In this chapter we study the characteristics that determine the crystallinity of polymers, crystalline morphology, and the factors affecting the crystallization and melting of polymers. We describe the amorphous state, focusing on the glass transition, a fundamental property for defining the mechanical behavior of polymers. The entire description refers exclusively to synthetic polymers. 

The use of GC to determine polymer crystallinity is of particular interest, since it is one of the few methods which do not depend on X-ray determinations of crystal structure for calibration. Presumably the method will be used more generally when suitable instruments are commercially available. 

The development of modern microcomputers and associated instrumentation enables the automatical of a nunber of IGC techniques. Automation is desirable because often 50 to 100 separate injections of very small volumes of probes are required over a period of time as the temperature of the GC is slowly increased, for example in the determination of transition temperatures or crystallinity. This paper will discuss the determinations of polymer crystallinity and the surface area of polymer-coated particles using automated instrumentation. 

The density of a polymer sample can be readily determined by allowing it to float in a density-gradient column, which is a vertical column containing a mixture of liquids with different (known) densities. The density of a small piece of polymer is determined from the position it adopts when it is dropped into the column. The density of the crystalline regions pc can be calculated from a knowledge of the crystal structure [19]. The amorphous density pa can sometimes be measured directly if the polymer can be obtained in a completely amorphous form, for example by rapid cooling of a polymer melt. Otherwise it can be determined by extrapolating either the density of the melt to the temperature of interest or that of a series of semicrystalline samples to zero crystallinity. 

J. P. Runt, article titled " Crystallinity Determination", Encyclopedia of Polymer Science and Engineering, 4, Wiley-Interscience, New York (1986). 

A common application of DSC is the determination of the weight fraction of crystalline material in semicrystalline polymers. The method is based on the measurement of the polymer sample s heat of fusion, AHf, and the plausible assumption that this quantity is proportional to the crystalline content. If by some process of extrapolation the heat of fusion, AHf, of a hypothetical 100% crystalline sample is known, then the weight fraction of crystallinity is AHf/AHJ (155). The determination of polymer crystallinity has been reviewed by Gray (156) and Dole (157, 158). 

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