Crystalline Sequence Research Articles

Conformation of isotactic polystyrene (IPS) in the bulk crystallized state as revealed by small-angle neutron scattering

Neutron scattering experiments have been performed on isotactic polystyrene (IPS) samples in the bulk crystallized state ( T crystallization = 185°C). The determination of the conformation of tagged chains ranging from 2.5 × 10 5 to 7 × 10 5 has been undertaken on two different hydrogenated IPS matrices. A matrix of usual molecular weight ( M w = 4 × 10 5) leads to results which do not agree with Flory's model. In this case, measurements on radius of gyration R g show on the one hand an important increase of this parameter (∼40%) with increasing crystallinity for the highest molecular weight tagged chains and on the other hand a variation with molecular weight like M 0.78. These results are interpreted with a schematic model involving a long crystalline sequence incorporated in the monocrystal along the 110 plane and two amorphous wings. Such an assumption is confirmed by the scattering behaviours in the intermediate range. On the other hand, by using an IPSH matrix of very high molecular weight ( M w = 1.75 × 10 6), and the same tagged chains previously considered, a very weak variation of R g with increasing crystallinity is observed. This leads to consider in this case Flory's conformation which is corroborated by data obtained in the intermediate range.

Sequence complexity and tissue distribution of chick lens crystallin mRNAs

Using hybridization reactions with a cDNA copy, the complexity of polysomal polyadenylated mRNA from the day-old chick lens was found to correspond to 5800–7200 sequences of average size, arranged in three abundance classes. Experiments with heterologous cDNAs suggest on a qualitative basis that many of the sequences expressed in 8-day embryonic neural retina and pigmented epithelium mRNAs are also present in lens mRNA. A cDNA fraction complementary to the most abundant lens mRNAs, representing an approximate minimum of four sequences, was used to assay the dosage of putative crystallin sequences in these and other embryonic tissues. Neural retina and pigmented epithelium cytoplasmic mRNAs have low concentrations of these sequences, which appear to be absent from mRNA prepared from headless bodies and muscle.

Liquid Industrial Waste Storage by Underground Injection: ABSTRACT

With a yearly discharge of liquid industrial waste of more than 14 trillion gallons before treatment, underground injection is a valuable tool to aid industry with the problems of storing and treating its generated waste. Disregarding mechanical factors, reservoir characteristics exert the greatest influence on operating safety and economics of injection systems. Ideally, a host reservoir should be a uniform salaquifer of large areal extent, substantial thickness, high porosity and permeability, and low pressure with adequate overlying and underlying aquicludes containing fluids compatible with injected wastes. There should be a minimum of faulting and abandoned wells near the injection site. Compressibility of water, rock, and gas in solution, in addition to any previously removed fluids, provide the space necessary for injection into the host reservoirs. The pressure effect at various distances from the wellbore, for given times and volumes of injected fluids, is important in predicting long-range reservoir performance, effect on inadequately plugged wells in the vicinity, effect of injection near potentially valuable mineral deposits, and unintentional formation fracturing. Potentiometric levels and gradients should be determined for host reservoirs to help analyze and anticipate fluid movement and monitoring methods needed. With over 300 waste-injection wells in operation today, the vast majority are injecting into reservoirs at depths of less than 6,000 ft. Approximately 70% are injecting into sandstone and unconsolidated sand and about 25% are injecting into limestone and dolomite, with the remaining percentage injecting into crystalline or evaporite sequences. Porosities in these wells range from under 10% to over 30%; permeabilities range from under 1 md to over 5 Darcys, giving a wide range of potential injection volumes and pressures and associated operating conditions. End_of_Article - Last_Page 1823------------

EFFECT OF CRYSTALLINE SEQUENCE LENGTH AND ITS DISTRIBUTION ON MELTING OF TWO COMPONENT COPOLYMERS

The behaviors of melting in copolymers may be affected by the copolymer compositions, the crystalline sequence length and its distribution, the stereochemical relations among the repeating structural units and the conditions of crystallization, etc.. Therefore, it may be difficult to deduce explicit expressions on melting in copolymers.In this work, the effect of crystalline sequence length and its distribution on the melting behaviors of copolymers composed of units of type A, capable of crystallizing and other units B incapable of crystallizing, is discussed in particular from a standpoint differing from that of our preceding paper.Derived equation relating to the crystallinity against the absolute temperature for random copolymer was in accord with the relation obtained in the preceding paper. If considered the Gibbs free energy difference between the crystalline regions and the amorphous regions as a variable quantity together with the amorphous fraction and the absolute temperature, the derived equation may be valid for fair explanations of the experimental relations between the crystallinity and the absolute temperature.

Side‐chain crystallinity. II. Heats of fusion and melting transitions on selected copolymers incorporating n‐octadecyl acrylate or vinyl stearate

AbstractThe heats of fusion and the melting transitions of the crystallinity present in the side chains were determined for selected copolymers incorporating n‐octadecyl acrylate or vinyl stearate. A major purpose of this investigation was to ascertain the effect of interrupting the long ordered 18‐carbon side chains by randomly interspersed amorphous side chains of various lengths. For this purpose the lower acrylate homologs (C1 through C8 and including oleyl, C8) were copolymerized over the composition range with n‐octadecyl acrylate. It was found that simple dilution of the crystalline component (from comonomer b) by the amorphous component (from comonomer a) governed the decline in the heats of fusion and the fraction of crystallinity present. High crystallization rates were encountered because equilibrium crystallinity was nearly achieved for most of the copolymers. Melting point depression was less than theory in copolymers having short amorphous comonomer side chain lengths, but approached the theoretical depression as these side chains became very long. Thus the outer methylene sequences (the crystalline sequences) of the fatty co‐units could bridge the smaller amorphous a units, giving rise to larger crystal sizes than theory specified. Main‐chain stiffness, when present in the melt, had a small effect on the distribution of crystallite sizes but exhibited a much larger influence in preventing the attainment of equilibrium crystallinity, especially at high amorphous comonomer compositions. However, crystallinity was still high compared with that of copolymers described in the literature crystallizing through their main‐chain units. When long blocks of crystalline segments were present (as in compositionally heterogeneous vinyl stearate copolymers), melting point depression was small and followed the theoretical probability sequence function.

Statistical-Mechanical Study of Stability of Folded-Chain Polymer Crystals with Irregular Fold Surfaces

A model of folded-chain crystals of polymers is considered, in which the length of crystalline sequences between folds is assumed to be uniform, while the number of segments comprising the folds is allowed to vary. The partition function is formulated by the generating function technique, with the fixed crystalline layer thickness introduced as an additional parameter of the system. The random-walk problem in the presence of a barrier, as the representation of folds, is solved for simple and body-centered cubic lattices. As variants of the basic model, the two ends of the fold are either assumed constrained in adjacent positions in the adjacent re-entry model, or are allowed at an arbitrary separation in the random re-entry model. The free energy of the system as a function of crystalline layer thickness exhibits no minimum, confirming the extend-chain crystals as the thermodynamically most stable. As the temperature is raised toward the melting point, the number of segments in folds increases beyond bound in the random re-entry model; while they remain about 10 in the adjacent re-entry model. In both models the size of the amorphous chain ends grows much faster than that of folds with increasing temperature. The frequency distribution of fold sizes is very broad. Even when the majority of folds consists of tight loops of a small number of segments, the fraction of very large folds is small but still significant. The entropy term of the surface free energy, calculated for the random and adjacent re-entry models, differs by as much as 50 erg/cm2.

Deformation and exhumation of the Higher and Lesser Himalayan Crystalline sequences in the Kumaon region, NW-Himalaya based on structural and fission-track analysis

DOI = 10.3126/hjs.v5i7.1294 Himalayan Journal of Sciences Vol.5(7) (Special Issue) 2008 p.101-102

Structural and metamorphic equivalence across theLHS-HHCS contact, Sikkim Himalaya – indicator of post-deformation metamorphism or the wrong MCT?

DOI = 10.3126/hjs.v5i7.1261 Himalayan Journal of Sciences Vol.5(7) (Special Issue) 2008 p.59