Polymer structure history

Polymer structure and formulation. As an example, Woo et al. [7] measured OIT values for series of commercial PVC resins and polyester thermoplastic elastomers (TPEs). The researchers used the ASTM D3895-80 procedure, but substituted air as the oxidising gas instead of pure oxygen. A dependency on thermal processing history of the TPE film samples appeared to influence the measured OIT in the PVC study, chemically different chain ends affected polymer stability and hence OIT values. 

The synthesis of peptidyl polymers has a long and elegant history, beginning with the construction of homopolypeptides and random copolypeptides. More recently, biomolecules that incorporate peptide entities into classic polymer structures have been created. Sequential peptide polymers have been constructed that model the three-dimensional structure of connective tissue proteins. This article describes methods for the assembly of a variety of peptidyl polymers. 

This symposium presented an unusual opportunity in that we discussed the methods used to determine polymer structures from fiber diffraction data, rather than concentrating on the actual structures derived and their possible implications. At Case Western Reserve University we have been involved in determination of the structures of cellulose and chitin. This paper describes our analyses (1-6) of the structures of cellulose I and II and a- and 6-chitin, emphasizing the manner in which structural decisions were taken in each case. Efforts to determine these structures have a history of over 60 years, and it has only been with the advent of least squares techniques for the refinement of polymer structures (7) [notably the LALS method (8)], and the development of our present knowledge of polysaccharide stereochemistry, that solutions have become possible. In what follows we will look first at our methods for measuring intensities and thereafter will review the work on each of the four structures. 

Davis EM, Benetatos NM, Regnault WE, Winey KI, Elabd YA. The influence of thermal history on structure and water transport in parylene c coatings. Polymer 2011 52(23) 5378-86. 

This brief review of the field of rigid-rod poisoners does not exhaustively cover the whole area, since many new rigid-rod polymers have been synthesized and studied recently. We have tried to use several typical rigid-rod polymers, such as PBZT, PBZO, and PPP, as model compoimds to describe the history, polymerization, structure, properties, applications, and the developing directions of the rigid-rod polymers. A few areas of present or potential novel levels of performance have been described to interest the readers in the great scope of the polymers. 

Induced stress is always a factor in structural applications and can result from processing conditions, thermal history, phase transitions, surface degradation, and variations in the expansion coefficient of components in a composite. Since the modulus of a material is not only temperature dependent but time dependent as well, the stress relaxation behaviour of polymers and composites is of great importance to the structural engineer. Stress relaxation is also important to the polymer chemist developing new engineering plastics because relaxation times and moduli are affected by polymer structures and transition temperatures. Therefore, it is essential that polymeric materials that will be subjected to loading stress be characterised for stress relaxation and creep behaviour. 

Equation 2 is an analytical statement of the solution-diffusion mqdel of penetrant transport in polymers, which is the most widely accepted explanation of the mechanism of gas permeation in nonporous polymers (75). According to this model, penetrants first dissolve into the upstream (i.e, high pressure) face of Ae film, diffuse through the film, and desorb at the downstream (Le. low pressure) face of the film. Diffusion, the second step, is the rate limiting process in penetrant permeation. As a result, much of the fundamental research related to the development of polymers with improved gas separation properties focuses on manipulation of penetrant diffusion coefficients via systematic modification of polymer chemical structure or superstructure and either chemical or thermal post-treatment of polymer membranes. Many of the fundamental studies recorded in this book describe the results of research projects to explore the linkage between polymer structure, processing history, and small molecule transport properties. 

Solubility characteristics of high molecular weight polymers can be problematic. Long dissolution times and fish eyes in the final solution are often observed. Associating polymers have additional complexity. The solution history of the polymer can affect inter- and intramolecular associations which influence dissolution rates. Preparing solutions above the polymer overlap concentration, C, requires patience and care. The design of the polymer structure can greatly affect solubility and dissolution characteristics. 

The similarity of all the -Run-Tg s suggests that die polymer structures are strictly comparable in all die cases considered here when the same level of cure is reached and the thermal history is removed. In addition because 2 -Run-Tg s for all the polymerization cycles are approximately equal to l -Rm-Tg s for HIGH cycle for bodi materials it is manifest that the postcure originates the maximum degree of cure. 

Though the factors that govern Tg have been known for some years, there is still a wide variation in values for particular polymers. Polymer Tg s are sensitive to parameters which may or may not have been evaluated by the authors. Published values should be reviewed considering all the factors which affect T,. The main factors affecting Tg values are polymer structure, sample crystallinity, diluent types and concentrations, molecular weight distributions, previous thermal history of the sample, and system pressure. More detailed treatments are given in reviews (6,48,49,1241-1249). 

To provide a rational framework in terms of which the student can become familiar with these concepts, we shall organize our discussion of the crystal-liquid transition in terms of thermodynamic, kinetic, and structural perspectives. Likewise, we shall discuss the glass-liquid transition in terms of thermodynamic and mechanistic principles. Every now and then, however, to impart a little flavor of the real world, we shall make reference to such complications as the prior history of the sample, which can also play a role in the solid behavior of a polymer. 

The discovery and development of polypropylene, the one genuinely new large tonnage thermoplastics material developed since World War II, forms part of what is arguably the most important episode in the history of polymer science. For many years it had been recognised that natural polymers were far more regular in their structure than synthetic polymers. Whilst there had been some improvement in controlling molecular architecture, the man-made materials, relative to the natural materials, were structurally crude. 

Lipson (1943, 1944), who had examined a copper-nickeMron ternary alloy. A few years ago, on an occasion in honour of Mats Hillert, Cahn (1991) mapped out in masterly fashion the history of the spinodal concept and its establishment as a widespread alternative mechanism to classical nucleation in phase transformations, specially of the solid-solid variety. An excellent, up-to-date account of the present status of the theory of spinodal decomposition and its relation to experiment and to other branches of physics is by Binder (1991). The Hillert/Cahn/Hilliard theory has also proved particularly useful to modern polymer physicists concerned with structure control in polymer blends, since that theory was first applied to these materials in 1979 (see outline by Kyu 1993). 

The current-voltage and luminance-voltage characteristics of a state of the art polymer LED [3] are shown in Figure 11-2. The luminance of this device is roughly 650 cd/m2 at 4 V and the luminous efficiency can reach 2 lm/W. This luminance is more than adequate for display purposes. For comparison, the luminance of the white display on a color cathode ray tube is about 500 cd/m2l5J. The luminous efficiency, 2 lm/W, is comparable to other emissive electronic display technologies [5], The device structure of this state of the art LED is similar to the first device although a modified polymer and different metallic contacts are used to improve the efficiency and stability of the diode. Reference [2] provides a review of the history of the development of polymer LEDs. 

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