Bibliothek

Notizen: Abstract The longitudinal momentum distribution of7Be was measured after the break-up reaction of8B in C, Al, and Pb targets at 1471 A·MeV. We observed a narrow distribution with a FWHM of (81±6) MeV/c in all targets. The experimental results indicate an extended spatial distribution of the loosely bound proton in8B, and agree with QRPA calculations.

Notizen: The effect of monomer sequence on physical properties was investigated for butadienestyrene solution copolymers made by organolithium initiation. The polymers varied from random copolymers of uniform composition along the polymer chain to ideal block polymers of specific block sequence arrangement and included rubbers of intermediate degrees of randomness. Uniform composition random copolymers exhibit a single glass transition temperature and a very narrow dynamic loss peak corresponding to this transition. The glass transition can be predicted from the styrene content and the microstructure of the butadiene portion of the rubber. Random copolymers in which composition varies along the polymer chain, and to some extent between molecules, exhibit a single glass transition, but the dynamic loss peak is broadened. The extent of this broadening is shown to be compatible with the sequence distribution, polymer segments of various compositions losing mobility at different temperatures. This indicates a tendency for association between segments of different temperatures. This indicates a tendency for association between segments of different chains which are similar in composition. Block copolymers display two transitions, corresponding to Tg for each type of block. The position and width of the dynamic loss peaks are related to block length and compositional purity of the blocks.

Notizen: Stress softening of carbon black-reinforced butadiene-styrene rubber was studied as a function of the rate and temperature of the original tensile deformation. To a good approximation, stress softening depends on the product of the extension rate and a temperature function which is analytically well represented by the familiar Williams-Landel-Ferry relationship. When the elongation of the original deformation is also varied, a good correlation is obtained between stress softening and the maximum stress attained in the original extension, irrespective of the particular combination of strain, strain rate and temperature used to achieve this stress. Variables which tend to increase the stiffness of the vulcanizate, such as increased degree of crosslinking or carbon black chain structure, also increase stress softening; dilution by plasticizers decreases it. Prestressing at high strain rates and low temperatures affects the stress-strain curve of the softened vulcanizates beyond the elongation of the original extension. Connections are established between stress softening and viscoelastic and failure behavior. The evidence presented favors the contribution of several mechanisms to the general phenomenon of stress softening. These are thixotropy of transient filler structures, network chain rupture, and breakage of “permanent” filler structure. The latter appears to be most important at high strain rates, low temperatures, and with highly reticulated “structure” blacks.

Notizen: Styrene-diene (butadiene or isoprene) block copolymers of the SDS or $ ({\rm SD\rlap{--} )} $ type exhibit a plateau in the dynamic storage modulus located between the glass transitions of the polydiene and polystyrene domains. When the polydiene is the continuous phase, the height of this plateau can be estimated with good success from the entanglement spacing molecular weight of the polydiene and the filler effect of the polystyrene domains. The effect of introduction of a center block-compatible diluent can also be calculated, although the simple procedure used here tends to underestimate the plasticizer effect, particularly at high diluent concentration. Nevertheless, the calculation furnishes a useful criterion of compatibility of the polydiene center blocks and low molecular weight resins used commonly as tackifiers in pressure-sensitive adhesives. Center block compatibility is essential for the development of tack in these compositions.

Notizen: Flow curves, log (rate of shear) versus log (shear stress), as functions of temperature were obtained for several butadiene-styrene copolymers of fixed (25%) styrene content, differing in monomer sequence distribution. A random copolymer of constant composition along the polymer chain and narrow molecular weight distribution (MWD) exhibited behavior similar to linear, narrow MWD polybutadienes; the flow was Newtonian at low shear stresses, and the flow curves for various temperatures were accurately superimposable by a shift along the log (shear rate) axis. In a random copolymer varying in composition along the polymer chain, non-Newtonian behavior was more pronounced, and temperature-shear rate superposition did not succeed, a trend further perpetuated in copolymer of a single long styrene block sequence. The latter resemble branched polymers, as would be expected from association of the styrene blocks. With two styrene blocks, association produces network structures below the glass transition of polystyrene with consequent loss of flow. Disruption of these associations above Tg (styrene) imparts the greatest thermoplasticity to these elastomers. There is evidence, however, that some of the associations persist at temperatures well in excess of Tg (styrene).

Notizen: The copolymerization of butadiene and styrene by lithium alkyls can be regulated to give either random or block copolymers. The block copolymers exhibit characteristic mechanical behavior which is attributable to their two-phase domain structure. In random copolymers free of long sequences of styrene there exists, nevertheless, the possibility of varying the sequence distribution by changing the manner in which composition varies along the polymer chain. Since copolymers of butadiene and styrene differing sufficiently in composition are likewise incompatible and will form multi-phase systems, it is likely that microheterogeneity can exist in certain “random” copolymers.Five copolymers of monomer ratio 70 : 30 butadiene/styrene, varying from a uniformly randomized sample, in which composition was very nearly independent of conversion, to a block polymer containing 22% block styrene chemical analysis, were prepared for the present investigation. Composition vs. conversion data indicated that all but the last polymer were free of long styrene sequences, with the composition distribution (along the chain) broadening systematically throughout the remainder of the series.The melt viscosity of the unvulcanized copolymers was distinctly affected by sequence distribution effects. Thus, the temperature coefficient of the apparent viscosity was independent of shear stress only for the uniformly randomized copolymer. In all others temperature superposition of the non-NEWTONian flow curves was impossible, the discrepancies becoming larger the broader the composition distribution. The results can be explained qualitatively by association effects attributable to a domain structure similar to that found in block polymers. When these copolymers were cross-linked with dicumyl peroxide at 153 °C and the dynamic properties of the networks examined, no clear evidence of a domain structure was found except in the block polymer. Only the latter exhibited more than a single loss maximum. Temperature-frequency reduction of the dynamic measurements was successful with all but the block polymer. Whereas the parameters C1 and C2 in the WILLIAMS-LANDEL-FERRY equation appear to change systematically with the degree of randomness, there is evidence that this is attributable to a slight systematic drift toward higher vinyl unsaturation with increasing randomization of the monomer sequence. Relaxation spectra calculated for 25 °C were very nearly the same for all four random copolymers. When the polymers were cross-linked by gamma radiation at room temperature, the resulting networks did show properties indicative of a domain structure in the compositionally more heterogeneous copolymers. It is proposed that compatibility of chain segments of varying composition at the temperature of cross-linking leads to a suppression of the domain structure in the peroxide-cured rubbers, as segments of different composition are joined together. Independent evidence from stress-optical measurements supports this interpretation.The present investigation permits the conclusion that differences in sequence distribution of butadiene-styrene copolymers have, at best, only very minor effects on the visco-elastic properties of conventional vulcanizates, provided the polymers contain no long sequences of styrene units, i.e., polystyrene blocks detectable by classical methods. This is not true of the low shear melt viscosity, which senses relatively small differences in the composition and/or sequence distributions of the uncured rubbers.

Notizen: Die quantitative Bestimmung von mono- und bifunktionellen organischen Fremdsubstanzen in Terephthalsäure und Dimethylterephthalat erfolgt nach Veresterung und Extraktion durch gaschromatographische Analyse. An flüssigkristallinen stationären Phasen gelingt eine vollständige Trennung aller interessierenden Spurenverunreinigungen, wobei die Matrixkomponente am Chromatogrammende erscheint. Eine Übertragung der Analysenmethode auf die Polymeranalytik ist nach Druckmethanolyse von Polyethylenterephthalat möglich.