Irreversibility, polymer chain

Under compression or shear most polymers show qualitatively similar behaviour. However, under the application of tensile stress, two different defonnation processes after the yield point are known. Ductile polymers elongate in an irreversible process similar to flow, while brittle systems whiten due the fonnation of microvoids. These voids rapidly grow and lead to sample failure [50, 51]- The reason for these conspicuously different defonnation mechanisms are thought to be related to the local dynamics of the polymer chains and to the entanglement network density. 

Polymer-matrix materials include a wide range of specific materials. Perhaps the most commonly used polymer is epoxy. Other polymers include vinyl ester and polyester. Polymers can be either of the thermoset type, where cross-linking of polymer chains is irreversible, or of the thermoplastic type, where cross-linking does not take place but the matrix only hardens and can be softened and hardened repeatedly. For example, thermoplastics can be heated and reheated, as is essential to any injection-molding process. In contrast, thermosets do not melt upon reheating, so they cannot be injection molded. Polyimides have a higher temperature limit than epoxies (650°F versus 250°F or 350°F) (343°C versus 121°C or 177°C), but are much more brittle and considerably harder to process. 

The radical initiators are not, strictly speaking, catalysts—though often referred to as such—for each radical that initiates a polymer chain becomes irreversibly attached to it and, if of suitable composition, may be detected in the molecules of product. The efficiency of some initiators may be so great that, after any induction period, every radical generated leads to a polymer chain. 

Thermoplastic polymers can be heated and cooled reversibly with no change to their chemical structure. Thermosets are processed or cured by a chemical reaction which is irreversible they can be softened by heating but do not return to their uncured state. The polymer type will dictate whether the compound is completely amorphous or partly crystalline at the operating temperature, and its intrinsic resistance to chemicals, mechanical stress and electrical stress. Degradation of the basic polymer, and, in particular, rupture of the main polymer chain or backbone, is the principal cause of reduction of tensile strength. 

Pyda and co-workers [49, 60] measured the reversible and irreversible PTT heat capacity, Cp, using adiabatic calorimetry, DSC and temperature-modulated DSC (TMDSC), and compared the experimental Cp values to those calculated from the Tarasov equation by using polymer chain skeletal vibration contributions (Figure 11.7). The measured and calculated heat capacities agreed with each other to within < 3 % standard deviation. The A Cp values for fully crystalline and amorphous PTT are 88.8 and 94J/Kmol, respectively. 

In general, the behavior of all classes of polymer behavior is Hookean before the yield point. The reversible recoverable elongation before the yield point, called the elastic range, is primarily the result of bending and stretching of covalent bonds in the polymer backbone. This useful portion of the stress-strain curve may also include some recoverable uncoiling of polymer chains. Irreversible slippage of polymer chains is the predominant mechanism after the yield point. 

Polyurethane multiblock copolymers of the type described by Eqs. 2-197 and 2-198 constitute an important segment of the commercial polyurethane market. The annual global production is about 250 million pounds. These polyurethanes are referred to as thermoplastic polyurethanes (TPUs) (trade names Estane, Texin). They are among a broader group of elastomeric block copolymers referred to as thermoplastic elastomers (TPEs). Crosslinking is a requirement to obtain the resilience associated with a rubber. The presence of a crosslinked network prevents polymer chains from irreversibly slipping past one another on deformation and allows for rapid and complete recovery from deformation. 

Yamaguchi, I. Osakada, K. and Yamamoto, T. (2000) Pseudopolyrotaxane composed of an azobenzene polymer and y-cyclodextrin. Reversible and irreversible photoisomerization of the azobenzene groups in the polymer chain, Chem. Commun. 1335-1336. 

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