Homopolymer defined

High density polyethylene (HDPE) is defined by ASTM D1248-84 as a product of ethylene polymerisation with a density of 0.940 g/cm or higher. This range includes both homopolymers of ethylene and its copolymers with small amounts of a-olefins. The first commercial processes for HDPE manufacture were developed in the early 1950s and utilised a variety of transition-metal polymerisation catalysts based on molybdenum (1), chromium (2,3), and titanium (4). Commercial production of HDPE was started in 1956 in the United States by Phillips Petroleum Company and in Europe by Hoechst (5). HDPE is one of the largest volume commodity plastics produced in the world, with a worldwide capacity in 1994 of over 14 x 10 t/yr and a 32% share of the total polyethylene production. 

The molecular weight and the distribution of multiple molecular weights normally found within a commercial polymer influence both the processibiUty of the material and its mechanical properties. Eor a few well-defined homopolymers, an analysis of composition and molecular weight is sufficient to define the likely mechanical properties of the polymer. 

The glass transition temperatures of the nylons appear to be below room temperature so that the materials have a measure of flexibility in spite of their high crystallinity under general conditions of service. The polymers have fairly sharply defined melting points and above this temperature the homopolymers have low melt viscosities. Some thermal properties of the nylons are given in Table 18.4. 

In a somewhat wider sense, one can define amphiphiles as molecules in which chemically very different units are linked together. For example, the structures formed by A B block copolymers in demixed A and/or B homopolymer melts and their phase behavior are very similar to those of classical amphiphiles in water and/or oil [13,14]. Copolymers are used not only to disperse immiscible homopolymer phases in one another, but also to create new, mesoscopically structured materials with unusual and interesting properties [15]. 

Before analyzing in detail the conformational behaviour of y9-peptides, it is instructive to look back into the origins and the context of this discovery. The possi-bihty that a peptide chain consisting exclusively of y9-amino acid residues may adopt a defined secondary structure was raised in a long series of studies which began some 40 years ago, on y9-amino acid homopolymers (nylon-3 type polymers), such as poly(/9-alanine) 3 [14, 15], poly(y9-aminobutanoic acid) 4 [16-18], poly(a-dialkyl-/9-aminopropanoic acid) 5 ]19], poly(y9-L-aspartic acid) 6 ]20, 21], and poly-(a-alkyl-/9-L-aspartate) 7 [22-36] (Fig. 2.1). 

The simplest, from the viewpoint of topological structure, are the linear polymers. Depending on the number m of the types of monomeric units they differentiate homopolymers (m=1) and copolymers (m>2). In the most trivial case molecules in a homopolymer are merely identified by the number l of monomeric units involved, whereas the composition of a copolymer macromolecule is defined by vector 1 with components equal to the numbers of mono-... 

It turns out that in solutions of c < 0.1 gL 1 thermosensitive homopolymers, such as PNIPAM, PVCL, and PVME, themselves, form stable colloids in water at elevated temperature in the absence of additives or chemical modification [141-147]. The colloids remain stable upon prolonged heat treatment, without detectable aggregation or precipitation. Also, core-shell particles consisting of PNIPAM and a hydrophobic block are stable not only below but also above the LCST up to 50 °C, when the PNIPAM block is expected to be insoluble [185]. Factors that determine the colloidal stability as defined in Sect. 1.1 do not explain, it seems, their stability. In this review we have compiled a fist of all the reported instances where the formation of stable particles was detected in aqueous solutions of neutral thermosensitive neutral polymers at elevated temperature. We present studies of homopolymers, as well as their copolymers consisting of thermosensitive fragments and ei-... 

The theoretical and (model) experimental work referred to above has largely been concerned with linear homopolymers adsorbed on regular surfaces. However, there is a vast literature of experimental studies on more complex systems. Unfortunately, in many cases the systems are either ill-defined and/or only adsorption isotherms have been established for drawing general conclusions or comparison with theory such studies are of little use. On the theoretical side, clearly the work needs to be extended towards these more complex systems. In particular, developments are required in the following areas (starts have already been made in some cases) ... 

Similarly to homopolymers, source-based nomenclature has been applied to copolymers [4]. The principal problem is to define the kind of arrangement in which various types of monomeric units are related to each other. Seven types of separate arrangements have been defined, which are shown in Table 1, where A, B and C represent the names of monomers. The monomer names are linked either through an italicized qualifier or connective (infix), such as -CO- , to form the name of the copolymer, as in poly(styrene-co-acrylonitrile). The order of citation of the monomers is arbitrary. 

PVC materials are often defined to contain 50% or more vinyl chloride units by weight. PVC is generally a mixture of a number of additives and often other units, such as ethylene, propylene, vinylidene chloride, and vinyl acetate. Structurally similar products, but with differing properties, are made from the chlorination of PE but almost all PVC is made from the polymerization of vinyl chloride. Typical homopolymers are about 400-1000 units long. 

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