Crosslinked Amorphous Materials

In these materials, chains are linked together by covalent bonds. This forbids any transition to a fluid state. Examples range from elastomers (e.g., silicones, natural and synthetic rubbers) to thermosetting resins like the epoxides (e.g., Araldite). The relevant quantity here is the molecular weight between crosslink points. 

Elastic (rubbery) above Tg. Here the rubbery aspect is permanent, unlike in the case above, and with respect to temperature, only chemical degradation will limit it. 

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Another PET copolymer system that was recently demonstrated to be sufficiently stable to standard synthesis conditions, yet photochemically reactive, is that of PET-/ -phenylene bisacrylic acid (PBA) (Figure 6.9) [74], Upon UV irradiation, PET copolymers containing up to 15mol% of PBA were shown to irreversibly undergo [2 + 2] cycloaddition reactions to produce lightly crosslinked, amorphous materials. 

At temperatures above their melting points, semi-crystalline polymers resemble amorphous polymers. We find the same states as described for crosslinked amorphous materials, provided that there are covalent bonds between the chains. 

Amorphous material often produces tie chains that connect two or more different crystals. These tie chains increase the properties of the solid resin by forming a temporary three-dimensional crosslinked system. As the resin is melted in an extruder, the crystals and the tie chains are destroyed, and the polymer acts like a... 

Various a-methylenemacrolides were enzymatically polymerized to polyesters having polymerizable methacrylic methylene groups in the main chain (Fig. 3, left). The free-radical polymerization of these materials produced crosslinked polymer gels [10, 12]. A different chemoenzymatic approach to crosslinked polymers was recently introduced by van der Meulen et al. for novel biomedical materials [11]. Unsaturated macrolactones like globalide and ambrettolide were polymerized by enzymatic ROP. The clear advantage of the enzymatic process is that polymerizations of macrolactones occur very fast as compared to the chemically catalyzed reactions [13]. Thermal crosslinking of the unsaturated polymers in the melt yielded insoluble and fully amorphous materials (Fig. 3, right). 

In most polymeric as well as non-polymeric amorphous materials, the ability to undergo large-scale molecular motions implies the freedom to flow, so that the material becomes a fluid above Tg. However, in the special class of polymers commonly described as thermosets , covalent crosslinks limit the ability to undergo large-scale deformation. Consequently, above Tg, thennosets become elastomers (also known as crosslinked rubbers ). 

Polymers II a-f were found by X-ray diffraction to be noncrystalline amorphous materials. Similar structured polymers were prepared for free radical vulcanization by the introduction of vinyl crosslinking sites. The polymers were formulated Into high consistency elastomers reinforced with silica and were free radical vulcanized. The properties for only lib and Ild are shown In Table I with a commercial elastomer prepared from polymethyl (3,3,3-tr1flu-oropropyl)s11oxane (LS) shown for comparison. Also included Is an elastomer prepared from the following copolymer (III),... 

Linear or branched, that is, not crosslinked, thermoplastic materials, usually first begin to soften on heating and then on further heating (amorphous polymers) begin to flow over a rather ill-defined temperature range (see Figure 3.1). Partially crystalline plastics in general have narrow melt-... 

A new approach was developed to describe deformation behavior of crosslinked amorphous elastomers. - Stress relaxation arrd change of network properties during deformation of an elastomer in the whole range of strain up to the rapture of material is taken into accormt. 

This chapter has covered the molecular structure of the high polymers which are the materials from which plastics are made. They are basically simple chemical entities which are connected chemically to form long chain molecules and in the case of thermosets, crosslinked from chain to chain. The structure of the polymeric materials has been compared to other types of materials such as metals, ceramics, amorphous materials such as glass, and nonmetallic crystals. 

Monnerie, L., Lauprgtre, F.O. and Halary, J.L. (2005) Investigation of solid-state transitions in linear and crosslinked amorphous polymers. Adv. Polym. Sci., 187, 35-213 (2011) Polymer Materials Macroscopic Properties and Molecular Interpretations, John Wiley Sons, Hoboken, New Jersey. 

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