Natural rubber description

The first good description of the permeation process in polymers dates back to 1831 when Mitchell (22) noted that natural rubber membranes allowed the passage of carbon dioxide faster than hydrogen under equivalent conditions. Mathematically, one can describe the permeation process in terms of Eq(l) using a permeability coefficient of component i, P, ... 

Graham, who was one of the first to consider the permeabilities of natural rubber films to a wide range of gases, found responses such as that seen in Fig. 2a. The description he formulated in 1866 of the so-called "solution-diffusion" mechanism still prevails today (30). He postulated that a penetrant leaves the external phase in contact with the membrane by dissolving in the upstream face of the film and then undergoes molecular diffusion to the downstream face where it evaporates into the external phase again. Mathematically, one can state the solution-diffusion model in terms of permeability, solubility and diffusivity coefficients, as shown in Eq(2). 

Presently, the amount of data on transport in uniaxially oriented amorphous polymers is small in comparison with that of semicrystalline materials. The transport properties of oriented natural rubber (22), polystyrene (i3.,ii), polycarbonate (22.), and polyvinyl chloride (22,22) among others have been reported. One of the more complete descriptions of the effects of uniaxial orientation on gas transport properties of an amorphous polymer is that by Wang and Porter (34) for polystyrene. 

The most well-studied polysiloxane viz., [Mc2SiO]n has a Tg of -123 °C [1]. This is one of the lowest values for any polymer. As mentioned earlier in Chapter 2 the glass-transition temperature of a polymer corresponds to a description of its amorphous state. The Tg of a polymer can be taken as a measure of the torsional freedom of polymer chain segments. Above its Tg a polymer has reorientational freedom of motion of its chain segments, while below its Tg this is frozen. Usually elastomeric materials have low glass-transition temperatures. For example, natural rubber has a glass-transition temperature of -72 °C. Similarly, polyisobutylene has a ass-transition temperature of -70 °C. Thus, poly(dimethylsiloxane) has consid-... 

Fig. 2. Family tree of polymer dispersions. The time axis in this graph denotes the time when the different kinds of pol3mier dispersions came to awareness of mankind starting with natural latex and ending with recent developments in polymeric colloidal complexes. Some important historic milestones are the following the biosynthesis of natural rubber in plants takes place on earth since millions of years, the first heterophase polymerization was mentioned in a patent filed in 1912 (21), the first process description to make an artificial latex was published in 1923 (40), and the first block copolymer dispersion made by heterophase polymerization was described in 1952 (41).

Until about 1910, the term rubber was sufficiently descriptive for most purposes. It typified natural products derived from various trees and plants that could be formed into solids of various shapes which could be bent, flexed rapidly, or stretched with the amazing ability to return to essentially the initial form. As synthetic materials emerged, particularly synthetics that were directed toward capabilities different from those of natural rubber, considerable confusion resulted as to descriptive terminology. Hence the literature was rife with such terms as rubber, rubbery, rubberlike, and similar inept descriptions. Eventually H. L. Fisher struck a major blow to this confusion and coined the term elastomer to embrace natural as well as synthetic products with those mechanical properties generally associated with natural rubber. 

Eight basic natural rubber types are recognized internationally by appearance and description only. These are ... 

The inherent properties of polymers of the poly isobutylene family, particularly the chemical inertness, age and heat resistance, long-lasting tack, flexibility at low temperatures, and the favorable FDA position on selected grades, make these products commercially attractive in a variety of pressure-sensitive and other adhesives, in automotive and architectural sealants, and in coatings. An added dimension is achieved in the blendability of the polyisobutylene polymers with each other and with other adhesive polymers such as natural rubber, styrene-butadiene rubber, EVA, low molecular weight polyethylene, and amorphous polypropylene to achieve specific properties. They can, for example, be blended with the highly unsaturated elastomers to enhance age and chemical resistance. A description of poly isobutylene polymer family use in adhesive and sealant applications follows. 

Based on the procedures described in the previous sections, one can obtain nanomechanical maps of a wide variety of polymeric and biological materials, including carbon black (CB)-reinforced natural rubber (NR) [40], carbon nanotube (CNT)-reinforced NR [41,42], reactive polymer blend [43], block copolymers [9,21,44,45], deformed plastics [46,47], human hair [48,49], honeycomb-patterned polymer films [50-52], CNT-reinforced hydrogel [53], and diffusion front of polymer [54,55]. The detailed descriptions are also found in other literatures [56-59]. Hereafter, several example studies are reviewed. 

In the very beginning we must recognise the fact that there are two different definitions of rubber . One is based on the chemical make-up, such as natural rubber and polybutadiene. The other is based on its physical behaviour. Sometimes a statement is made, after a long time (gum) rubber flows . In this statement the rubber is chemical by definition. The physical description is after a long time the material is no longer a rubber because it flows . 

If we exclude most of the biopolymers such as wood, cotton, silk, and others, one of the first polymers in use was natural rubber (NR). The Indians of South America were playing with rubber balls made from latex when the Spanish arrived in the Americas in the 1400s. The first description of the rubber ball dates back to 1496. 

The first alkene polymer to be used in society was polyisoprene, a natural product extracted from the sap of rubber trees. See our Box for a description of the history of rubber. The monomer from which this polymer is constructed... 

During the first decade extensive descriptive studies were carried out on natural polymers of all kinds proteins (wool, silk, and leather), carbohydrates (cellulose, starch, and gums), and other resinous products (shellac, rubber, and gutta-percha). Three large domains of scientific and technical interest came into being ... 

Due to the dual filler and crosslinking nature of the hard domains in TPEs, the molecular deformation process is entirely different than the Gaussian network theories used in the description of conventional rubbers. Chain entanglements, which serve as effective crosslinks, play an important role in governing TPE behavior. The stress-strain results of most TPEs have been described by the empirical Mooney-Rivlin equation ... 

This chapter lays the groundwork for the various topics discussed in subsequent chapters. The mathematics associated with the statistics of the isolated chain are developed starting with the bonding and structure found in small molecules. Several models of chain structure are presented. Finally, the size distribution of polymer chains is introduced and their description in terms of mathematical equations derived, origin of rubber elasticity, the nature of polymer crystalline and polymeric heat capacities and the miscibility of polyblends. 

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