MOLECULAR STRUCTURE OF POLYMERS

In this chapter we examine the flow behavior of bulk polymers in the liquid state. Such substances are characterized by very high viscosities, a property which is directly traceable to the chain structure of the molecules. All substances are viscous, even low molecular weight gases. The enhancement of this property due to the molecular structure of polymers is one of the most striking features of these materials. 

C 1-NMR spectroscopy is the method of choice for determining the molecular structure of polymers in solution [230]. Polyolefin 13C NMR is mainly quantitative ID 1-NMR multiple pulse techniques are used for spectral interpretation. The resolution obtained in 13C NMR spectra of LDPE is an order of magnitude larger than in the corresponding 1H-NMR spectra... 

In this chapter we have explored the various methods by which polymer scientists characterize the molecular structure of polymers. Given the complex molecular distribution found in most polymers, the best that we can do in many cases is to measure some average value or distribution of values that represents the polymer. Armed with these values polymer scientists and engineers can design or select resins suitable for a myriad of practical applications. 

As indicated in Fig. 2.4 all of these atoms have at least one unpaired electron in the valence or outer electron shell. A covalent bond, as suggested by the word covalent, is a bond which shares at least one pair of valence electrons between two atoms. When examining the molecular structure of polymers, it is found that all commercial polymer molecules are formed from covalent bonds. Examination of these... 

The principal interest of the rubber hydrochloride structure (apart from its bearing on the theory of the relation between the physical properties and the molecular structure of polymers) is that it formed the first test of validity and usefulness of the principle of staggered bonds. Abnormal structures f In all the structures considered so far two... 

Figure 4. The strategy of determining the crystal and molecular structure of polymers based on model refinement...

Polymer Surface Analysis. The major technique used for the surface analysis of polymers has been X-ray photoelectron spectroscopy (XPS or ESCA). However, this technique is often not adequate to determine the molecular structure of polymers. This has prompted many workers to explore the potential of SIMS for this work (11-16). Significant problems encountered with ion beam bombardment in conjunction with electron beam charge neutralization have been drift in the polymer surface potential and thermal damage from the combined effects of the electron and ion beams. These problems do not exist when utilizing FAB in conjunction with photoelectron charge neutralization. 

A number of reviews can be consulted for an introduction to the fundamentals both theoretical and practical covering XPS. These include Riggs and Parker (2) and the book by Carlson (3). Electron spectroscopy is reviewed in alternate years in the Fundamental Reviews issue of Analytical Chemistry. The last literature review was published in 1980 (4) and this and previous reviews can be consulted for a coverage of all aspects of the literature of XPS. A number of recent symposia have been held on applications of surface analytical methods in various aspects of materials science such as the symposium on characterization of molecular structures of polymers by photon, electron, and ion probes at the March 1980 American Chemical Society meetings in Houston ( 5) and the International Symposium on Physiochemical Aspects of Polymer Surfaces at this meeting as well as the symposium on industrial applications of surface analysis of which this article is a part. Review articles on various applications of XPS in materials science are listed in Table I. 

Raman spectroscopy has been widely used to study the composition and molecular structure of polymers [100, 101, 102, 103, 104]. Assessment of conformation, tacticity, orientation, chain bonds and crystallinity bands are quite well established. However, some difficulties have been found when analysing Raman data since the band intensities depend upon several factors, such as laser power and sample and instrument alignment, which are not dependent on the sample chemical properties. Raman spectra may show a non-linear base line to fluorescence (or incandescence in near infrared excited Raman spectra). Fluorescence is a strong light emission, which interferes with or totally swaps the weak Raman signal. It is therefore necessary to remove the effects of these variables. Several methods and mathematical artefacts have been used in order to remove the effects of fluorescence on the spectra [105, 106, 107]. 

The material behavior of polymers is totally controlled by their molecular structure. In fact, this is true for all polymers synthetically generated polymers as well as polymers found in nature (bio-polymers), such as natural rubber, ivory, amber, protein-based polymers or cellulose-based materials. To understand the basic aspects of material behavior and its relation to the molecular structure of polymers, in this chapter we attempt to introduce the fundamental concepts in a compact and simple way. 

There are three important phenomena seen is polymeric liquids that make them different from simple fluids a non-Newtonian viscosity, normal stresses in shear flow, and elastic effects. All these effect are a result of the complex molecular structure of polymer macromolecules. 

Fig. 16 A Optical micrograph of two-dimensional photopattern generated by photolysis of a metallized hb-PY (81) through a copper-negative mask. B Image with a higher magnification and C molecular structure of polymer complex 81...

In molecular structure of polymers (as opposed to Linear), refers to side chains attached to the main chain. Side chains may be long or short. 

A new rheo-photoacoustic Fourier transform infrared cell has been developed to perform stress-strain studies on polymeric materials. The rheo-photoacoustic measurements lead to the enhancement of the photoacoustic signal and allow one to monitor the effect of elongational forces on the molecular structure of polymers. Propagating acoustic waves are detected as a result of infrared reabsorption and the deformational and thermal property changes upon the applied stress. 

Comprehensive texts on dynamic mechanical spectroscopy in relation to the molecular structure of polymers are given in Refs 1-3. 

Several theoretical works predict the onset of mesophases (1-5) and take into account the molecular structure of polymers. There are two kinds of structures that are often cited the rigid or semi-rigid linear polymers and the connected ones or those containing mesogenic side chains ( ). 

In theory, it is not difficult to associate this elastic behavior with the molecular structure of polymers. The coiled conformation of polymers is responsible for the anomalous macroscopic deformation observed in... 

In conclusion this paper has tried to focus on some of the analytical techniques which ceui be utilized In the study and correlation of molecular phenomena with material failure. An understanding which Is Important because of the Inherent complexity of the molecular structure of polymers and the effect It has on material properties. 

The same thermodynamic approach can be extended to the control of the molecular structure of polymers. With regard to molecular structure,.the formation of a... 

The proposed thermodynamic method thus makes it possible to control both the supramolecular and molecular structures of polymers. Potential uses for this method seem to be far from exhausted. In particular, it can be used in thermodynamic analyses of the supramolecular structures of polymers, and in the synthesis of stereo-regular polymers and regular cross-linked polymers. 

The details of the molecular structure of polymers profoundly influence the observed Tg s, as illustrated by Table 5-2, where we may contrast the Tg of polydimethyl siloxane, -123 °C, with that of poly(calcium phosphate), + 525 °C. At least approximately, we may separate the observed effects into intermolecular and intramolecular parts. The latter refer to structural parameters affecting the stiffness of the chain backbone we shall examine these first. 

A study of the solid state behavior of polymeric systems is important because of the engineering applications of polymeric materials. These applications stem from their physical properties in the solid phase, which in turn are a natural consequence of the unique molecular structure of polymer molecules. 

Viscoelastic properties of biopolymeric materials such as carbohydrates and protein can be used to characterize their three-dimensional conflgura-tion in solutions. This configuration affects their functionality in many food products. An understanding of how the molecular structure of polymers affects their rheological properties can make it possible to predict and improve the flow behavior of newly developed food products that have such... 

It is well known that the morphological and molecular structures of polymers play an important role in their wear behavior. It seems that the degree of crystallinity is also a structural factor of semicrystalline polymers important to their wear. Lontz et al. ( ) reported that the wear of poly(tetrafluoroethylene),(PTFE) decreased with the increase in crystallinity. Tanaka et al. (2 ) studied the wear of heat-treated PTFE specimens and concluded that the wear rate was affected by the width of the band in the fine structure rather than crystallinity. Recently, Hu et al. ( 3) have studied the effect of crystallinity on wear of PTFE using various heat-treated specimens. They have shown that the wear decreases with the increase in crystallinity, when molecular weight is constant. Eiss et al. ( ) reported that poly(chlorotrifluoroethylene) of a crystallinity of 65% exhib-ted higher wear than that of 45%. The results obtained by the authors mentioned above indicate that the effect of crystallinity on the wear of polymers is somewhat complicated and further investigation is needed to clarify the effect of crystallinity on polymer wear. 

In the previous sections the diaractnization of the molecular structure of polymers prepared by emulsion polymerization has been discussed. The eventual aim of making emulsion polymers is invariably the preparation of polymeric materials with desired properties. The present section deals briefly with exanqrles of the thermal and mechanical properties of emulsion polymers. Also special attention will be givoi K> flie important relation between molecular microstructure and properties. 

In this chapter we focus on how molecular structures of polymer and surfactant additives affect DR behavior, the rheology and nanostructures of their dilute solutions, and possible mechanisms of polymer and surfactant drag reduction. 

Rheology is widely applied for two aspects to get some insight into the processing behavior of polymer and to have information on the molecular structure of polymers [66]. 

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