Weak links


Physical Properties and Morphology. Elastomers require a system of cross-bonding which links the flexible molecular chains to each other to give a network stmcture. Without this network, the elastomer would not have the properties to make it commercially useflil. Thermoset elastomers accomplish this cross-bonding by forming molecular cross-links, irreversible chemical bonds formed during the vulcanisation step. Thermoplastic elastomers do not undergo a vulcanisation step and the network is produced by reversible physical bonds between the polymer chains (181). Copolyesterether elastomers are cross-bonded through a crystallisation process. These block copolymers contain a crystalline phase which physically locks the flexible soft segments into an elastic network below the crystalline melting point of this phase.  [c.302]

Typically, PS loses about 10% of its when it is fabricated. A significant amount of research has been carried out to determine the nature of the weak links in PS (92—95). Various modes of initiation of degradation mechanisms have been proposed (/) chain-end initiation, (2) random scission initiation, and (3) scission of weak links in the polymer backbone. It has been suggested that chain-end initiation is the predominant mechanism at 310°C, whereas random scission produces stable molecules. Evidence for weak-link scissions comes mainly from studies showing loss of molecular weight vs degradation time. These plots usually show a rapid initial drop in molecular weight indicating initial rapid weak-link scission. However, the picture is also comphcated by the fact that the mechanism of degradation is temperature-dependent. It appears that weak-link scissions taking place at high temperatures initiate depolymerization whereas at lower temperatures scissions simply cause a decrease in molecular weight. In any case, a clear difference in thermal stabihty has been shown between PS produced using free-radical (FRPS) and anionic (APS) polymerization (Fig. 7). This difference is due mainly to the initiator-derived fragments that remain in the polymer after isolation.  [c.510]

Effect of Fiber Variability on Stress-Strain Behavior. Textile fibers are not uniform, varyiag ia both composition and fineness within a fiber and between fibers. The length of the fibers is also different from fiber to fiber. Because the fibers vary, their tensile properties also vary. The variation ia properties from one fiber to another influences the distribution of loads on fibers ia a textile stmcture (3). The mean measured strength of a sample decreases as the sample length iacreases. The probabiUty of finding more weak links along the fiber iacreases as the fiber length iacreases. Because weak links can not share equally ia the distribution of the load, the maximum load beariag capacity of the fiber is thus reduced. The weak link effect (52—55) also affects breaking extension. If a fiber breaks under a light load owiag to the presence of a weak place, the rest of the specimen will have a comparatively small extension and the breaking extension will be low. The mean breaking extension decreases as the specimen length iacreases (3). The straia of a fiber is also affected by nonuniform diameter aloag the fiber length, ie, the thin places extead more than the thick places. Variations ia fiber morphology may cause the modulus to vary from place to place ia a fiber or betweea fibers ia a givea sample (3).  [c.455]

Effects in humans for which links with exposure to endocrine disrupters have been suggested include the following.  [c.5]

Polymer reactivity differs from the reactivity of simple molecules in two special respects. The first of these is due to the fact that a number of weak links  [c.95]

There is much evidence that weak links are present in the chains of most polymer species. These weak points may be at a terminal position and arise from the specific mechanism of chain termination or may be non-terminal and arise from a momentary aberration in the modus operandi of the polymerisation reaction. Because of these weak points it is found that polyethylene, polytetrafluoroethylene and poly(vinyl chloride), to take just three well-known examples, have a much lower resistance to thermal degradation than low molecular weight analogues. For similar reasons polyacrylonitrile and natural rubber may degrade whilst being dissolved in suitable solvents.  [c.96]

By preventing the initial formation of weak links. These will involve, amongst other things, the use of rigorously purified monomer.  [c.97]

The JHA allows you to identify weak links in the system. Once you discover the weak links, you can update the JHA to reflect the needed changes.  [c.43]

Design and development plans need to identify the activities to be performed, who will perform them, and when they should commence and be complete. One good technique is to use a network chart (often called a PERT chart), which links all the activities together. Alternatively a bar chart may be adequate. In addition there does need to be some narrative, as charts rarely convey everything required.  [c.239]

Applying Einstein s formula, which links with the diffusion coefficient  [c.611]

In summai y, the Wright brothers had gotten it right. All the components of their system worked properly and harmoniously—propulsion, aerodynamics, control, and structures. There were no fatal weak links. The reason for this was the natural inventiveness and engineering abilities of Orville and Wilbur Wright. The design of the Wright Flyer is a classic first study in good aeronautical engineering. There can be no doubt that the Wright brothers were the first true aeronautical engineers.  [c.36]

It is commonly found that polymers are less stable particularly to molecular breakdown at elevated temperatures than low molecular weight materials containing similar groupings. In part this may be due to the constant repetition of groups along a chain as discussed above, but more frequently it is due to the presence of weak links along the chain. These may be at the end of the chain (terminal) arising from specific mechanisms of chain initiation and/or termination, or non-terminal and due to such factors as impurities which becomes built into the chain, a momentary aberration in the modus operandi of the polymerisation process, or perhaps, to branch points.  [c.925]

The combination of weak links and unzipping can be catastrophic and has been a particular problem in the commercial development of some polymers, in particular polyacetals.  [c.925]

Thermal stability is largely concerned with chemical reactivity which may involve oxygen, u.v. radiation or depolymerisation reactions. The presence of weak links and the possibility of chain reactions involving polymer chains may lead to polymers having lower thermal stability than predicted from studies of low molecular weight analogues.  [c.935]

These consist of a chain of carbon atoms each carrying 0 to 3 hydrogen atoms except for the simplest molecule, methane CH. Each carbon atom is linked to four other atoms which can be either carbon or hydrogen. Their general formula is( 2  [c.3]

In this group certain carbon atoms are linked only to three atoms, which implies the existence of one or more double bonds between carbon atoms.  [c.8]

Asphaltenes are obtained in the laboratory by precipitation in normal heptane. Refer to the separation flow diagram in Figure 1.2. They comprise an accumulation of condensed polynuclear aromatic layers linked by saturated chains. A folding of the construction shows the aromatic layers to be in piles, whose cohesion is attributed to -it electrons from double bonds of the benzene ring. These are shiny black solids whose molecular weight can vary from 1000 to 100,000.  [c.13]

The determination of equilibria is done theoretically via the calculation of free energies. In practice, the concept of fugacity is used for which the unit of measurement is the bar. The equation linking the fugacity to the free energy is written as follows >  [c.149]

The properties required by jet engines are linked to the combustion process particular to aviation engines. They must have an excellent cold behavior down to -50°C, a chemical composition which results in a low radiation flame that avoids carbon deposition on the walls, a low level of contaminants such as sediment, water and gums, in order to avoid problems during the airport storage and handling phase.  [c.178]

The pour point is the lowest temperature at which an oil can still pour while it is cooled, without agitation, under standardized conditions. The pour point of paraffinic bases is linked to the crystallization of n-paraffins. The pour point of naphthenic bases is related to a significant viscosity increase at low temperatures. This property can be improved by additives.  [c.283]

Other efforts to use radical polymerization to synthesize biodegradable carboxylated polymers have been based on combining low molecular weight oligomers through degradable linkages and by introducing weak links into the polymer backbone. BASF (118) and NSKK (119) have patented acryUc oligomers chain-branched with degradable linkages designated X in (7). GdUo Werke has patented copolymers of acryUc acid and enol sugars (120). The degradabihty of these polymers has not been clearly estabbshed, but the branching is likely to be a problem.  [c.480]

MPEVAENFQQCLERWAI CLSVGGELAHMANQAAEAILKGGNEAQLKNAQALMHEAMKTRKYSEQLAQEFAHCAYI CARASQ (see Table 1). Felix also contains a successfliUy designed disulfide, which links the first and fourth heflces. Although Felix has no homology with any natural protein, its architecture was based on the four helix clusters of hemerythrin and cytochrome The designed protein has been shown to contain a very high a-helix content and is readdy soluble in water.  [c.202]

RNA (ribonucleic acids). Ribonucleic acids are like DNA except that the 2 -deoxy-D-ribose moiety is replaced by a D-ribose moiety and the fourth nucleotide thymidylic acid (T) is replaced by uridylic acid (U). RNA does not generally form complete douplex molecules like DNA, i.e. it is generally monomeric, except in certain viruses. The two main classes of RNA are messenger-RNA (mRNA) and transfer-RNA (tRNA). mRNA transcribed from the DNA gene followed by the splicing out of the non-coding nucleotides (of the introns) and codes for a specific gene. There are many different tRNAs, at least one of which links to a specific a-amino acid, that bind to the mRNA via the ribosome (a set of proteins) to the RNA triplets (three nucleotides) which code for the particular a-amino acids. An enzyme then joins the a-amino acids of two adjacent tRNA-a-amino acid ribosome complexes bound to the mRNA to form a peptide bond. Thus peptide bonds and consequently polypeptides and proteins coded by the DNA via the respective mRNA are produced.  [c.565]

Properties like Young s modulus may well vary with direction in the unit cell for this (and other) reasons we need a succinct description of crystal directions. Figure 5.6 shows the method and illustrates some typical directions. The indices of direction are the components of a vector (not reciprocals, because infinities do not crop up here), starting from the origin, along the desired direction, again reduced to the smallest integer set. A single direction (like the 111 direction which links the origin to the corner of the cube  [c.50]

Weak links, particularly terminal weak links, can be the site of initiation of a chain unzipping reaction. A monomer or other simple molecule may be abstracted from the end of the chain in such a way that the new chain end is also unstable. The reaction repeats itself and the polymer depolymerises or otherwise degrades. This phenomenon occurs to a serious extent with polyacetals, polyfmethyl methacrylate) and, it is believed, with PVC.  [c.97]

Much greater success has been achieved with organic polymers than with inorganic materials. In spite of the considerable heat resistance of graphite there is a common misconception that carbon-containing compounds must have a poor heat resistance. It has been calculated that, assuming in polyethylene the weakest bond is the C—C bond with a dissociation energy of 82.6 kcal/mole (347 kJ/ mole) the temperature at which the polymer decomposes at 1 %/ minute will be 806°C. In practice polymethylene (as a linear polyethylene model) loses weight at 1%/minute at 415°C. This emphasises the fact that in real polymers slight structural abeirations, and also the possibility of low-energy reaction path, particularly by chain reaction processes, will lead to less than the theoretical maximum service temperature. It also indicates that the few weak links which can cause catastrophic degradation in a single-backbone polymer will be far less serious where chains are cross-linked or linked together in a ladder-like structure.  [c.846]

The alwve approach, with visualization of air movements and particle challenge tests together with calculation of a risk factor, presents a method, the LR method (Limitation of Risks), to evaluate the risk of human interference in the critical zone of the bench. It also gives valuable information concerning potential weak links. A more thorough description of the LR method is given in Ljungqvist and Reinnuiller.  [c.933]

Fig. 33(a,b) shows a series of snapshot pietures as a result of a eomputer experiment probing the kineties of dewetting. The loeal darkness of eaeh snapshot indieates the loeal eoverage of the substrate surfaee. Coverage fluetuations (white spots) appear rather early and get rapidly amplified. The substrate regions, eovered with polymer, have very irregular surfaee initially and are eonneeted with many weak links later, these hnks disappear, and the droplets of adsorbed polymer eompaetify, a pattern similar to spinodal deeomposition.  [c.620]

These data show that in 1997 Canadian electric power customers experienced an average of 2.35 power failures and on average were without electricity for 3,70 hours. Their demands were satisfied 99.9578 percent of the time. The SAIFI, SAIDI, and lOR values shown are averages and vary widely across the country and for different segments of any given area. In general, urban customers experienced much higher levels of reliability than did rural customers. The CEA statistics also show that approximately 80 percent of the mternaptions seen by the average customer occurred due to failures within the low-voltage distribution system, where the supply configuration is basically radial in nature. The high-voltage network, which links the generating facilities with the major load points, is known as the bulk supply network and accounts for approximately 20 percent of the power failures experienced by an average customer. Distribution system failures are usually local in nature and tend to affect a relatively small  [c.422]


See pages that mention the term Weak links : [c.309]    [c.153]    [c.568]    [c.81]    [c.923]    [c.214]    [c.358]    [c.10]    [c.43]    [c.63]    [c.64]    [c.219]    [c.289]    [c.321]    [c.345]    [c.347]    [c.352]    [c.357]    [c.384]    [c.384]    [c.20]   
Plastics materials (1999) -- [ c.97 ]