Evidence of distinct water species in cellulosic environments from broad-line NMR

Broad-line NMR measurements were made on water in cellulosic samples. The spectra for oriented rayon at 65% relative humidity and room temperature consisted of a visually apparent doublet (S_D) and a narrow singlet (S_N) which was shifted upfield about 7 ppm from the center of the doublet. The doublet separation varied as A(3 cos²θ—1), where θ is the angle between the fiber axis and the magnetic field; the maximum doublet separation was 350 mG at both 15.1 and 56.4 MHz, indicating the doublet is dipolar in origin. The peak-to-peak linewidth of the narrow singlet was orientation and field dependent. The upfield shift of the narrow singlet from the doublet center was field dependent. Spectra for vacuum-treated rayon consisted of only the narrow singlet, which was orientation dependent. The doublet separation decreases with increasing moisture content and is essentially zero for oriented rayon samples at 100% relative humidity; the resulting spectra due to the collapsed doublet and S_N singlet was an asymmetric line. Temperature-dependent measurements were made on oriented rayon at 65% relative humidity with the fiber axis parallel to the magnetic field; when the temperature was increased from about 0° the peak-to-peak linewidth of the doublet halves and the doublet separation decreased. As the temperature was increased from about 30 to about 65°C, a previously unobserved singlet (S_E) became visible. The relative amplitudes of the three lines varied with temperature as follows: The S_E singlet increased, the S_D doublet decreased, and the amplitude of the S_N singlet remained constant. Measurements using oriented cotton samples and randomly packed rayon samples indicated that the NMR line shape of the water spectra depends upon the physical properties of the macromolecular substrate. The doublet component of the spectra is attributed to a water species (S_D) which is highly bonded to the macromolecular substrate. The S_E singlet is attributed to an energetic water species (S_E) which is rapidly tumbling in the macromolecular environment. The S_N singlet is not due to free water.     

13C NMR studies of some hydroxycoumarins and related compounds

The ¹³C chemical shifts and one-bond carbon-hydrogen coupling constants have been obtained for some hydroxycoumarins and their corresponding acetoxy and methoxy derivatives. The changes in the one-bond carbon-hydrogen coupling constants resulting from the conversion of a hydroxy group to an acetoxy group represent a simple method of assignment of the ¹³C NMR signals in coumarins which contain one or more hydroxy groups in the benzenoid ring.     

Some further observations on the systems cellulose/trifluoroacetic acid/CH2Cl2 and cellulose triacetate/TFA/CH2Cl2

There is little or no trifluoroacetylation of cellulose dissolved in TFA-CH₂Cl₂ admixtures. Both cellulose and cellulose triacetate (CTA)are slowly degraded in the solvent. Cellulose forms a mesophase as low as 4%(w/w)concentration, but CTA has a much higher critical concentration, 20% (w/w), in TFA-CH₂Cl₂. The cellulose behaves as a rigid rod in TFA-CH₂Cl₂ (70/30v/v) and its persistence length calculated using the lattice model approximates its chain length, presumably due to extensive interaction with the solvent. As expected, due to low polymer-solvent interactions, the persistence length of CTA in TFA-CH₂Cl₂ is only one-fourth the chain length.     

Effect of ionizing radiation on the dynamic mechanical properties of an epoxy and a graphite fiber/epoxy composite

TGDDM/DDS epoxy and T300/NARMCO 5208 composite specimens were exposed to 0.5 MeV electrons to dose levels up to 10,000 Mrad, and the effects of radiation on the mechanical properties were characterized using dynamic mechanical analysis (DMA). In nominally cured specimens there remain unreacted epoxide groups because the epoxy system vitrifies during the cure, preventing additional reaction. DMA shows that ionizing radiation continues the reactions of epoxide groups. Also, the ultimate glass transition is shown to decrease monotonically with radiation dose. The room temperature elastic modulus of the epoxy increases slightly with radiation, but at temperatures exceeding 100°C there is a slight decrease with radiation. There is a dynamic loss phenomena associated with the composites, not seen in the epoxy, that is thought to be due to the interphase region between the fiber and the matrix.     

The liquid-crystalline properties of selected cellulose derivatives

The liquid-crystalline properties of three cellulose esters, phenylacetoxy cellulose (PAC), 4-methoxyphenylacetoxy cellulose (4MPAC), and p-tolylacetoxy cellulose (TAC) and two cellulose silyl ethers, trimethyl silyl cellulose (TMSC) and t-butyldimethylsilyl cellulose (TBDMSC), are reported. Hot-stage polarized light microscopy provided evidence regarding the formation of thermotropic mesophases in the PAC, 4MPAC, TAC, and TMSC in bulk form upon heating. The concomitant DSC data showed further evidence of the thermotropic nature of these materials. PAC, 4MPAC, TAC, and TMSC formed lyotropic mesophases at 44, 48, 50, and 27 wt%, respectively in CH₂Cl₂. The presence of fingerprint patterns in wholly anisotropic solutions in conjunction with optical rotation measurements confirmed the cholesteric nature of these liquid crystalline solutions. TBDMSC formed neither a lyotropic nor a thermotropic liquid-crystalline phase due to the low degree of substitution (DS 0.68) of this derivative. The hydroxyl substituents of PAC, 4MPAC, TAC, and TMSC may be readily removed under mild conditions to regenerate cellulose.     

Molecular orientation of spider silks in the natural and supercontracted states

Two theoretical models are presented to predict average molecular orientation properties of spider silk which has unknown molecular orientation and chemical structure. Birefringence and elongation (or shrinkage) data are required for two distinct lengths of a sample in order to fit the data. The first model is based on the assumptions that each molecule behaves as the average and that the deformations are affine. The second model is based on a random distribution of the molecules as the fiber is first formed and that the molecular deformations are affine when the fiber is elongated to any length. Data were taken from spider silks in the natural state and in a shrunken state (due to water). The second model gives a much better fit of the data. The models are applicable to other systems.     

Investigations of a series of PPDI-based polyurethane block copolymers. II. Annealing effects

In a previous study, the morphologies of a group of paraphenylene diisocyanate (PPDI)-based polyurethane block copolymers were examined. These polyurethanes exhibited a multiphase structure with an interfacial boundary thickness estimated to be on the order of 1 nm and crystallization of the polyoxytetramethylene (POTM) flexible segment. Further studies involving annealing of these polyurethanes are reported here. An annealing time of 4 h was used, and the annealing temperature varied from 125 to 200°C. The samples have been characterized using differential scanning calorimetry (DSC) and with wide- and small-angle x-ray scattering (WAXS, SAXS) in order to determine the effects of annealing on the microphase structure. Annealing increases the phase separation of the two phases as evidenced by sharper endotherms in DSC thermograms and increased intensities in WAXS diffractometer traces. Annealing also slightly increases the transition zone thickness and long-period spacing. At the highest annealing temperature in this study, the long-period spacing increases dramatically due to hard segment domain aggregation.     

Photophysical properties of pyrene in interaction with the surface of melanin particles

Melanins perform their biological activity (photoprotection and light enhanced chemical reactivity) under the form of porous aggregates on which ions and neutral molecules can be adsorbed. For this reason, the photochemistry of natural and synthetic melanins must be investigated in the framework of the physico-chemical theory of the heterogeneous reactions and a detailed knowledge of the surface properties, is therefore, necessary. In this work, some surface characteristics of melanin particles have been investigated taking advantage of the photophysical behaviour of pyrene, a dye widely used in studies of the interface properties of micelles and colloidal semiconductors. Our fluorescence study has allowed to obtain valuable informations regarding the micro-environmental polarity of the melanin surface (that influences the vibronic structure of the emission spectra), the excimer formation, the lifetimes of the emissions and the kinetics of quenching by Cu2+.