Molecular weight brittleness

Crosslinking results in an increase in molecular weight, brittleness and stiffness. Chain scission results in reduced molecular weight, softness and loss in tensile strength. 

The intrinsic viscosity of the polymer prepared using the phosphorylation reaction is 0.2 dl/g in NMP at 30°C. The physical properties of the materials prepared thus far are those expected for rigid chains of moderate molecular weight. Brittle films can be cast from aqueous ammonium hydroxide solution or from polar aprotic solvents. The films which form are brittle. 

The synthesis of PLA is a multistep process which starts from the production of lactic acid and ends with its polymerization [2-4, 6-7]. An intermediate step is often the formation of the lactide. Figure 21.2 shows that the synthesis of PLA can follow three main routes. Lactic acid is condensation polymerized to yield a low molecular weight, brittle polymer, which, for the most part, is unusable, unless external coupling agents are employed to increase its chains length. Second route is the azeotropic dehydrative condensation of lactic acid. It can yield high... 

Poly(lactic acid) (PLA) is produced from the monomer of lactic acid (LA). PLA can be produced by two well-known processes — the direct polycondensation (DP) route and the ring-opening polymerization (ROP) route. Although DP is simpler than ROP for the production of PLA, ROP can produce a low-molecular-weight brittle form of PLA. Generally, several substances are involved in the production of PLA, and these relationships have been summarized in Figure 2.1. The lactic acid for the process is obtained from the fermentation of sugar. Lactic acid is converted to lactide and eventually to PLA. It should be noted that there are two different terms, poly(lactic acid) and polylactide , for the polymer of lactic acid. Both terms are used... 

PLA has had an interesting history. Bischoff (7) and Carothers (8) noted the ease with which the polymer and cyclic dimer (dilac-tide) were interconverted. However until techniques were developed for purifying the monomer, the cyclic dimer in this case, only low molecular-weight, brittle polymers could be obtained (9). The purification of dilactide is facilitated by the formation of a... 

In methacrylic ester polymers, the glass-transition temperature, is influenced primarily by the nature of the alcohol group as can be seen in Table 1. Below the the polymers are hard, brittle, and glass-like above the they are relatively soft, flexible, and mbbery. At even higher temperatures, depending on molecular weight, they flow and are tacky. Table 1 also contains typical values for the density, solubiHty parameter, and refractive index for various methacrylic homopolymers. 

Low Temperature Brittleness. Brittleness temperature is the temperature at which polyethylene becomes sufficiently brittle to break when subjected to a sudden blow. Because some polyethylene end products are used under particularly cold climates, they must be made of a polymer that has good impact resistance at low temperatures namely, polymers with high viscosity, lower density, and narrow molecular weight distribution. ASTM D746 is used for this test. 

Because of the high cross-link density of polyisocyanurates as prepared above, the resultant foams are brittle, so that there has been a move towards polyisocyanurate-polyurethane combinations. For example, isocyanurate-con-taining polyurethane foams have been prepared by trimerisation isocyanate-tipped TDI-based prepolymers. The isocyanurate trimerising reaction has also been carried out in the presence of polyols of molecular weight less than 300 to give foams by both one-shot and prepolymer methods. 

Polyurethane adhesives are formed by the reaction of various types of isoeyanates with polyols. The polar urethane group enables adhesion to various surfaees. Depending on the raw materials, glue lines with rubber-like elastic to brittle-hard behavior ean be aehieved. The presence of reactive terminal groups provides a ehemieally hardened adhesive. When polymerized to a high enough molecular weight, the adhesive ean be physically rather than chemically hardened, i.e. a hot melt. 

This luminous brittle star has been briefly studied recently (Mallefet and Shimomura, 2004, unpublished). The animal contained a high level of coelenterazine luciferase activity (4 x 1012 photons s-1g 1), which is comparable to those in the luminous antho-zoans such as the sea pansy Renilla and sea pen Ptilosarcus (Shimomura and Johnson, 1979b). There is no evidence for the presence of a photoprotein in this brittle star. Thus, the luminescence system of Amphiura filiformis is considered to be a coelenterazine-luciferase system, differing from that of Ophiopsila californica. The luciferase has a molecular weight of 23,000 on the basis of gel filtration on Superdex 200 Prep, and catalyzes the luminescence reaction of coelenterazine in the presence of oxygen the light emission (A.max 475 nm) is optimum at pH 7.2. 

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