Small main-chain motions

In this paper we discuss (1) small main-chain motions and their effect on the flow processes, (2) the embrittlement of polycarbonate, (3) the formation of microvoids from sample preparation and their effect on the brittleness of polymer glasses, and (4) the modification of the degree of brittleness of polymer glasses at the filler interface in polymer composites. 

Small Main-Chain Motions and Their Effect on the Flow Processes. 

We studied the effect of high surface area carbon black and fused silica fillers on the large, secondary glass transition of non-crystallizable SBR (31.6% styrene) copolymer which is associated with small main-chain motions of frans-polybutadiene units (25). Figure 4 shows plots... 

Higher pressures of CO2 cause correspondingly larger increases in . At 800 torr an increase of 19% to 183 sec-1 is observed. Based on standard relaxation rate theory (29), increased relaxation rates are indicative of a shift in the average cooperative main-chain motions to higher frequencies. Conversely, this means that even small amounts of CO2 increase the cooperative motions of the polymer chains. 

In polymer 1, the Kerr effect coincides in sign with the Maxwell effect, which means that the orientation of the side chains in an electric field correlates with the direction of the main chain. However, both the small values of K and [ig / Ql] and the absence of dispersion of the Kerr effect show that the chains of polymers 1 and 6 exhibit no high orientational axial and orientational polar orders characteristic of polymers with mesogenic side groups. In these polymers (1 and 6), the Kerr effect is due to small scale chain motion the kinetic elements are much smaller in size and their mobility is much higher than, for example, for polymer 7. 

Typically, activation energies for low-temperature (i.e., y) relaxations are small (ca. 10-80kJmoF ). Heijboer [30] has suggested that for sub-Tg relaxations other than local main-chain motions... 

Such an effect of small molecule antiplasticiser is not specific to BPA-PC. It seems to occur with most polymers undergoing a transition originating from motions in the main chain. In the present paper, the effects of antiplasticisers on the f3 transition of poly(ethylene tere-phthalate) and epoxy networks are analysed in Sects. 4 and 7, respectively. 

It is interesting to point out the change in the relative heights of the and a peaks between PMMA and CMIM20 (Fig. 129). In the latter polymer, only a small part of the dielectric relaxation happens through the j3 motional processes. The cooperative motions involved in the a transition are required for achieving an important relaxation, whereas it is the opposite for PMMA. Such a behaviour is consistent with the hindrance of main-chain cooperativity by the rigid CMI units. 

The results presented in Table II show that even small amounts of gas affect the cooperative main-chain molecular motions of glassy polymers. Evidence that the presence of gases in polymer cause structural and dynamic changes can be seen in the depression of the Tg (42, 43, 44), and in the increased viscoelatic relaxation rates (43, 44) of... 

At low temperatures (A zone) the polymer is found in the vitreous state. In this state the polymer behave as a rigid solid with low capacity of motions and then the strain is very low. To produce a small strain it is necessary a great stress. Therefore in this zone only specific and local motions take place and the polymer can be considered as undeformable. As the temperature increases (B zone) the glass transition temperature, Tg, is reached and the motions of the different parts of the polymers increases but is not enough to produce important strain. Under this conditions the polymers behave as a rubber. If the temperature remain increasing (C zone) the polymer behave as deformable and elastic rubber but the modulus is small. In this zone the motions of the side chains and also of the main chain increases due to the application of the strain. 

The experimental results show that the activation entalphy of the /3 relaxation is for times that of the y relaxation and the activation entropy is ten times greater. Starkweather has described the secondary relaxation a simple or complex according to the value of the activation entropy. In general there is low cooperativity in the former relative to the latter. Motions of small groups at cryogenic temperatures are examples of the first type of relaxation while some fi relaxations involving the participation of the main chain are representative of the second. 

Ubiquitin is a small (76 amino acids) extremely stable protein containing a broad collection of secondary structure elements including parallel and antiparallel beta strands assembled into a mixed beta sheet, alpha and 3io helices and a variety of turns (Vijay-Kumar et al., 1987 Di Stefano Wand, 1987). In previous work, we have examined the fast main chain dynamics of ubiquitin by use of 15n NMR relaxation methods (Schneider et al., 1992). These data were analyzed in terms of the so-called model free treatment of Lipari and Szabo (1982a,b). The amplitudes of motion of the backbone amide N-H vectors of the packed regions of the protein are generally highly restricted and show no apparent correlation with secondary stmcture context but do show a strong... 

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