Strain cyclic

Janzen, W., and Ehrenstein, G. W., Hysteresis Measurements for Characterizing the Cyclic Strain and Stress Sensitivity of Glass Fiber Reinforced PBT, SPEANTEC, May 1989. 

New kinds of living polymer systems result from the reactions of transition metals with cyclic, strained olefins 16). These polymerizations proceed through the intermediacy of metal carbenes and are exemplified by the polymerization of norbomene initiated by bis(cyclopentadienyl)-titane-cyclobutane described recently by Grubbs17>. 

Testing J y Percent cyclic strain Specimens failed Accumulated cycles... 

All of the above results were generated by tests in which the cyclic strain was started low and gradually increased until all of the specimens broke. Longer term fatigue tests at different, discrete, strain levels are also planned for the future. 

This comparison demonstrates that the exocyclic double bond length is little affected by the cyclic strain, which was also found for the radialenes see Section n.E. However, significant deviations were found for conjugated systems, and the same holds for the linear and branched dienes and polyenes. 

Enediynes amino, 127, 20, 217 antiaromatic region in, 14-15 Bergman cyclization, 3, 18, 25 C1-C5 cyclization of, 5f 25 cyclic, strain and antiaromaticity in, 11-16... 

Cyclic strain limit, of shape-memory alloys, 22 345... 

Besides aromatization, the energy resulting from relief of cyclic strain can be a driving force of the dienone-phenol rearrangement. Thus, it was reported that dienone 302 is... 

For cyclic 1,2-diol-Cr(V) complexes, the multiplicity of the room-temperature CW-EPR signal depends upon the cyclic strain in the ligand.50 For these systems, the observed EPR hyperfrne pattern of the Cr(V)-diolate2 formed by reduction of Cr(VI) with GSH in the presence of c/.v-1.2-cyclohexanediol (6) or trans-1,2-cv c I oh e x an ed i ol... 

The polymerization of cyclic, strained olefins by transition metal alkylidenes of general formula L M = CRR (L = ligand, R, R = H, alkyl, aryl) yields polymers formed via ring-opening that contain unsaturated double bonds within each repetitive unit. Since the mechanism is based on repetitive metathesis steps, this polymerization reaction is known as ring-opening metathesis polymerization (ROMP) (Scheme 1). 

Figure 3.1 The five most common mechanical tests (I) constant elongation for tensile strength determinations, (2) constant force for creep determinations, (3) fixed elongation for stress relaxation determinations, (4) cyclic strain for dynamic mechanical determinations, and (5) impact for impact determinations. (After J. Fried, Plastics Engineering, July 1982, with permission.)...

In contrast, a cyclic strained geometry influences the stereochemistry with a general shift to inversion for endocyclic leaving groups (37, 38). Relevant examples are summarized in Table XIX. 

We do not intend to discuss aspects of the use of cyclic strain conditions to obtain information on viscous, visco-elastic, and relaxation properties of polymers, although this, by itself, may present a significant interest for theoretical and experimental research. The aim of this publication is to review recent works which provide a basis for various technical applications, i.e., facilitate rearrangement of molding processes. 

G proteins are another family of membrane proteins believed to modulate mechanochemical transduction pathways. Mechanical stimulation changes the conformation of G protein that leads to growth-factorlike changes that initiate secondary messenger cascades leading to cell growth. It has been reported that cyclic strain of smooth muscle cells significantly decreased steady-state levels of G protein and adenylate cyclase activity. Muscular stimulation also appears to be coupled with G protein activation in small arteries. 

Isolated keratinocytes subjected to cyclic strain exhibit a significant increase in cell proliferation, DNA synthesis, and protein synthesis compared to stationary or constantly loaded cells, which appear to involve changes in cyclic AMP. Takei et al. (1997) reported a strain-induced reduction in the levels of cyclic adenosine monophosphate, protein kinase A (PKA), and prostaglandin E2 (PGE2) as compared to stationary controls. Takei et al. (1997) also studied the effects of cyclic strain on protein kinase C (PKC) activation and translocation in cultured keratinocytes. 

Airway smooth muscle cells isolated from canine tracheae and bronchi subjected to cyclic strain exhibit increased cell number and DNA synthesis in cell culture. The content of total cellular protein, especially contractile proteins including myosin, myosin light chain kinase, and desmin, was increased compared to cells cultured under static conditions. 

Du W, Mills I, Sumpio BE. Cyclic strain causes heterogeneous induction of transcription factors. AP-1, CRE binding protein and NF-kB, in endothelial cells Species and vascular bed diversity. J Biomech. 1995 28 1485-1491. 

Iba T, Sumpio B. Morphological response of human endothelial cells subjected to cyclic strain in vitro. Microvasc Res. 1991 3 841-848. 

Takei T, Han O, Ikeda M, Male P, Mills I, Sumpio BE. Cyclic strain stimulates isoform-specific PKC activation and translocation in cultured keratinocytes. J Cell Biochem. 1997a 67 327-337. 

Wiersbitzky M, Mills I, Sumpio BE, Gewirtz H. Chronic cyclic strain reduces adenylate cyclase activity and stimulatory G protein subunit levels in coronary smooth muscle cells. Exp Cell Res. 1994 210 52-55. 

Chain transfer to polymer in the polymerization of THF is discussed separately, because THF is the model mononKr for which the polymerization is best understood. This transfer leads to the transformation of cyclic, strained tertiary oxonium ions into non-strained oxonium ions, linear or macrocyclic, e.g. (for the intermolecular transfer resulting in a linear structure) ... 

Fig. 32. Cyclic strain recovery in an extensively crazed blend containing CSS particles...

Dynamic Mechanical (Low Strain Deformation). When a cyclic strain of small ampUtude is applied to a strip of material, a cyclic stress will be generated in response by the sample. If the material is ideal (Hookian) and stores all the input energy, the cyclic stress is in phase with the applied cyclic strain. Viscous components cause a finite phase lag or phase angle, 8, between the stress and strain. represents the elastic, real, or storage modulus while E" is the viscous, imaginary, or loss modulus. Tan 8 is equal to the ratio E /E" and is related to the molecular relaxations that occur in the sample. Transition temperatures and associated activation energy can be determined (72) by varying the frequency of oscillation at a fixed temperature or the temperature at a fixed frequency. 

---