Condensation polymers crystallinity

When X = Y, as in polyethylene, poly-(tetrafluoroethylene), polyisobutylene, and poly -(vinylidene chloride), the polymers are highly crystalline products with sharply definable melting points (except for polyisobutylene, which crystallizes readily on stretching but with difficulty on cooling). Oriented specimens of high strength may be obtained, exactly as in the crystalline condensation polymers. 

Although polymers in-service are required to be resistant toward hydrolysis and solar degradation, for polymer deformulation purposes hydrolysis is an asset. Highly crystalline materials such as compounded polyamides are difficult to extract. For such materials hydrolysis or other forms of chemolysis render additives accessible for analysis. Polymers, which may profitably be depolymerised into their monomers by hydrolysis include PET, PBT, PC, PU, PES, POM, PA and others. Hydrolysis occurs when moisture causes chain scissions to occur within the molecule. In polyesters, chain scissions take place at the ester linkages (R-CO-O-R ), which causes a reduction in molecular weight as well as in mechanical properties. Polyesters show their susceptibility to hydrolysis with dramatic shifts in molecular weight distribution. Apart from access to the additives fraction, hydrolysis also facilitates molecular characterisation of the polymer. In this context, it is noticed that condensation polymers (polyesters, -amides, -ethers, -carbonates, -urethanes) have also been studied much... 

The solubility, water and oil repellency, thermal and thermooxidative stability, and Tg are enhanced and crystallinity and water absorption are decreased by introducing hexafluoroisopropylidene units, rather than units ofisopropylidene, into polymer backbone of aromatic condensation polymers. 

When Paul Flory wrote his famous book Principles of Polymer Chemistry in 1952, he indicated an alternative scheme for polymer synthesis [1]. He theorized about synthesizing condensation polymers from multifunctional monomers. These polymers were predicted to have a broad molecular weight distribution and to be non-entangled and non-crystalline due to their highly branched structure. However, they were considered to be less interesting since they would provide materials with poor mechanical strength, and at that time Flory did not feel it was worthwhile pursuing this line of research. 

Much attention has been paid to the synthesis of fluorine-containing condensation polymers because of their unique properties (43) and different classes of polymers including polyethers, polyesters, polycarbonates, polyamides, polyurethanes, polyimides, polybenzimidazoles, and epoxy prepolymers containing pendent or backbone-incorporated bis-trifluoromethyl groups have been developed. These polymers exhibit promise as film formers, gas separation membranes, seals, soluble polymers, coatings, adhesives, and in other high temperature applications (103,104). Such polymers show increased solubility, glass-transition temperature, flame resistance, thermal stability, oxidation and environmental stability, decreased color, crystallinity, dielectric constant, and water absorption. 

Preston, J. Synthesis and properties of rodlike condensation polymers. A. Blumstein, ed. Liquid Crystalline Order in Polymers . Academic Press, New York 1978, pg. 141... 

The development is reviewed of liquid-crystalline polymers whose mesophase formation derives from the nature of the chemical units in the main chain. The emphasis lies primarily on highly aromatic condensation polymers and their applications. The general properties of nematic phases formed by such polymers are surveyed and some chemical structures capable of producing nematic phases are classified in relation to their ability to form lyotropic and thermotropic systems. The synthesis, properties, physical structure and applications of two of the most important lyotropic systems and of a range of potentially important thermotropic polymers are discussed with particular reference to the production and use of fibres, films and anisotropic mouldings. 

In addition to the physical state of reactants, it should be remembered that the ideal behavior is encountered only in the gaseous state. As the polymerization processes involve liquid (solution or bulk) and/or solid (condensed or crystalline) states, the interactions between monomer and monomer, monomer and solvent, or monomer and polymer may introduce sometimes significant deviations from the equations derived for ideal systems. The quantitative treatment of thermodynamics of nonideal reversible polymerizations is given in Ref. 54. 

PPE is a key component of several blends. It is totally miscible with sPS and can interact with polar polymeric components, for instance polyamides (through hydrogen bonding interactions between PPE oxygens and amidic NH) and other condensation polymers moreover, other important properties are improved by PPE, such as mechanical properties (entry 1), and a better control of crystallinity is obtained (see Section 4.1.1 for discussion). PPE also acts as a processing aid, improving the melt flowability [15]. 

Nylon-6, [-(CH2)5-NH-C(=0)-]n belongs to the important class of polyamide condensation polymers. Polyamides are characterised by the presence of secondary amides —NHCO— in the backbone so hydrogen bonds are fonned between neighbouring chains. These strongly influence the mechanical properties of the polymer. Nylon-6 has two crystalline forms, a and y, which differ in the conformation of the backbone in the a form it is planar and in the y form it is helical. INS studies [27] of both forms of nylon-6, including oriented samples, have been made. The amide V, VI and VII modes that involve out of plane deformations of the -NHCO- group were shown to depend on the crystal form. The assignments were supported by DFT calculations on model compounds. 

Several examples are known, in which one or even both monomers in a liquid crystalline [A-B]n condensation polymer contain cyclo-aliphatic rings [50]. The trans chair form makes a larger contribution than the cis boat form. 

Typical condensation polymers, such as polyester and nylon, often exhibit these properties. If the fiber is to be ironed, its Tg should be above 200 °C if it is to be drawn from the melt, its Tg should be below 300 °C. Branching and cross-linking are undesirable because they disrupt crystalline formation even though a small amount of cross-linking may increase some physical properties if effected after the material is suitably drawn and processed. 

C for crystalline condensation polymers such as polyamides and polyesters. For more polar polymers, dimethylformamide and aqueous eluents may be employed, but care is required so as to avoid solute-gel interaction effects. Adsorption and partition effects are always likely to occur when polymor-solvent interactions are not favourable, when polar polymers are separated with less polar eluents and when packings have surface active sites. If the solvent has considerable affinity for the surface, then no polymer is adsorbed. Also, since adsorption is more prevalent with poor solvents, good solvents must be used to prevent occurrence of preferential solvent—adsorbent interactions. The choice of eluent may be restricted because of sample solubility considerations and small quantities of an adsorption active substance may be added to the eluent in order to suppress sample adsorption. 

When a polymerization is accompanied by phase transition, the overall thermodynamic parameters are the sum of parameters of the chemical reaction and phase transition (cf. p. 11). Thus, for instance, thermodynamics of polymerization in the crystalline state from a liquid monomer will be given by the thermodynamics of formation of the amorphous (condensed) polymer and polymer crystallization, provided, that polymerization proceeds in the solid state with monomer packing in the crystalline state simultaneously with propagation. 

If the fiber is to be ironed, its Tg should be above 200°C. Branching and crosslinking are nndesirable since they inhibit crystalline formation. Even so, some crossfinkmg may be present to maintain a given orientation, snch as desired in permanent press clothing. While most fibers are made from condensation polymers, new treatments allow some fibers to be made from olefinic materials snch as polypropylene (Table 3). 

Livolant F, Leforestier A (1996) Progress in Polymer Science 21 1115 (a) Economy J, Goranov K (1994) Advances in Polymer Science 117 221 (b) Preston J (1978) Synthesis and properties of rod-like condensation polymers. In Blumstein A (ed) Liquid crystalline order in polymers. Academic Press, New York... 

A large number of commercially important condensation polymers are employed as homopolymers. These include those polymers that depend on crystallinity for their major applications, such as rylons and fiber-forming polyesters, and the bulk of such important thermosetting materials like phenolics and urea-formaldehyde resins. In many applications, condensation polymers are used as copolymers. For example, fast-setting phenolic adhesives are resorcinol-modified, while melamine has sometimes been incorporated into the urea-formaldehyde resin structure to enhance its stability. Copolyesters find application in a fairly broad spectrum of end uses. 

Most polymers that function properly at ambient temperature quite frequently have limited performance at sustained elevated temperatures. This invariably limits the utility of polymeric materials. The low thermal stability is generally due to decreased crystallinity and/or thermal decomposition. Polymer chemists have, through some ingenious ways, synthesized polymer — such as aromatic polyimides and the so-called ladder polymers — specifically designed for high-temperature applications. However, it has also been possible to modify polymers to improve their thermal stability and hence extend then-range of utility. A few examples of condensation polymers illustrate this point. 

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