Lateral Dimensions Of Crystallites Research Articles

Acid-Catalyzed and Solvolytic Desulfation of H2SO4-Hydrolyzed Cellulose Nanocrystals

Cellulose nanocrystals (CNCs) prepared by H(2)SO(4) hydrolysis have sulfate groups on their surface, which have negative implications for some CNC applications. In this study, two desulfation methods were evaluated, and the properties of desulfated CNCs were compared to those of unsulfated CNCs, prepared by HCl hydrolysis. H(2)SO(4)-hydrolyzed CNCs from softwood sulfite pulp were subjected to either a mild acid hydrolytic desulfation or a solvolytic desulfation in dimethyl sulfoxide via the pyridinium salt. Removal of the sulfate groups was confirmed by conductometric titration and X-ray photoelectron spectroscopy. The effect of the desulfation procedure on the lateral crystallite dimensions was analyzed by X-ray diffraction. The extent of particle aggregation in the samples was assessed by atomic force microscopy and dynamic light scattering. The acid hydrolytic method achieved only partial desulfation and produced gradually decreasing sulfate contents upon successive repetition. The solvolytic method achieved nearly complete desulfation in a single step. The desulfated CNCs showed similar particle aggregation as the HCl-hydrolyzed CNCs, but the extent of aggregation was slightly less.

Double-crystal X-ray topography of freestanding HVPE grown n-type GaN

High-resolution X-ray topography (HRXT) and rocking curve measurements were performed on unintentionally doped, freestanding HVPE grown n-type Gallium Nitride (GaN). Based on the rocking curve widths, the dislocation density is estimated to be in the range of 10 5–10 7/cm 2, and the lower limit of average crystallite sizes to be 340–500 nm normal to the surface of the film. The lateral dimensions of crystallites and cavities were obtained from HRXT images, and are estimated to be in 200–500 nm, and 0.5–400 μm ranges, respectively. Although the GaN films are freestanding, they are warped with a radius of curvature of about 0.5 m, as determined from topographic measurements. The warpage is attributed to thermal mismatch between GaN and the substrate during growth.

Fibrous long-chain organic acid cellulose esters and their characterization by diffuse reflectance FTIR spectroscopy, solid-state CP/MAS13C-NMR, and X-ray diffraction

Unsaturated and saturated organic acids with 11 and 18 carbon atoms, respectively, were used in a heterogeneous esterification reaction in the pyridine/toluene sulfonyl chloride system to prepare fibrous cellulose esters with different degrees of substitution. Highly bleached sulfite cellulose fibers were esterified during a 1- or 2-h reaction time with the following organic acids: undecylenic acid, undecanoic acid, oleic acid, and stearic acid. In all cases, the heterogeneous esterification yielded partially substituted cellulose esters retaining their fibrous structure. The substitution reaction was confirmed by diffuse reflectance infrared spectroscopy and the chemical structures of cellulose esters were identified by solid-state CP/MAS 13C-NMR (75.3 MHz). X-ray diffraction analyses showed broadening of the diffraction peaks with a higher degree of substitution of cellulose esters, which suggests structural changes within the cellulose fibers. Because the broadening peaks of X-ray spectra or the unassigned C-4 region of substituted cellulose chains in NMR spectra do not allow the calculation of dimensional changes of cellulose crystallites in cellulose esters, the lateral dimensions of crystallites in only cellulose fibers were calculated. The value derived from NMR (4.6 nm) differs by about 11% when compared with the value calculated from X-ray diffraction data (4.1 nm). © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1354–1365, 2000

Estimation of the lateral dimensions of cellulose crystallites using [formula omitted] NMR signal strengths

Differences in proton rotating-frame spin relaxation rates were exploited to edit the 13 C NMR spectra of solid lignocellulosics, separating signals assigned to cellulose crystallites from signals assigned to amorphous material. Clusters of signals at 89 and 85 ppm were assigned to C-4 in the interiors and on the surfaces of cellulose crystallites, respectively. Relative signal areas were used to estimate the weight-averaged lateral dimensions of crystallites, using a model in which crystallites have approximately square cross sections. The same 10 samples were also characterized by wide-angle X-ray scattering (WAXS). There was a strong correlation ( r 2=0.988) between the two sets of lateral dimensions, but those estimated by WAXS were typically 10% lower than those estimated by NMR. The deviations were attributed to differences in molecular conformations between interior and surface chains, causing broadening of the WAXS peaks. In the case of an eleventh sample containing well-ordered xylan, the NMR and WAXS methods were in good agreement only after exclusion of a xylan signal at 82.6 ppm from the NMR data.

Dynamic structure and aqueous accessibility of poly( p-phenylene terephthalamide) crystallites

A more complete description of the morphology of poly( p-phenylene terephthalamide) (PPTA) in both as-polymerized polymer and various fibre forms is developed by the utilization of a number of methods (dark-field transmission electron microscopy, X-ray diffraction, solid state n.m.r. spectroscopy, and partial isotopic exchange of accessible amide moieties). This description focuses primarily on the effect of lateral crystallite dimensions on the microscopic dynamic structure and the concomitant macroscopic accessibility properties. The observation of a heterogeneous dynamic structure reflects the imperfect structural consolidation during the processing of PPTA in both as-polymerized polymer and fibre forms. The imperfect microcrystalline structure formed during processing is shown to be the origin of macroscopic accessibility for small molecules and also provides a plausible structural basis for biological accessibility.

Development of crystalline order during hot-drawing of poly(ethylene terephthalate) film: influence of strain rate

We have studied the evolution of crystalline order during drawing of poly(ethylene terephthalate) at 90°C and at strain rates in the range 0.01–2.1 s −1. The onset of crystallization always occurs at the inflection point in the stress-strain relationship, which shifts to higher strain levels and lower stress levels as strain rate decreases. Crystallinity develops in two regimes: in the low stress regime (regime 1) crystallinity increases more rapidly than in the high stress regime (regime 2). The onset of regime 2 occurs in the region of the sharp upturn in the stress-strain relationship, which shifts to higher strain levels as strain rate decreases, and at a characteristic level of crystallinity, which is independent of strain rate. We suggest that regime 1 involves the formation of a crystallite network which, at the characteristic level of crystallinity, has sufficiently crosslinked and reinforced the polymer that further deformation generates large stresses. WAXS determinations of lateral crystallite dimensions, normal to the 010 and 100 planes, reveal that crystallization in regime 2 involves modest growth of small crystallites (∼2.5−3.5 nm), with preferential growth occurring normal to 010. A preliminary investigation of the influence of molecular weight indicates that a lower rate of molecular relaxation at higher molecular weight reduces the strain-rate dependent shift in the onset of crystallization.

Supermolecular structure of native and isolated cellulose

The degree of crystallinity, crystalline dimensions and defects were determined in native and isolated celluloses by X-ray analysis. The degree of crystallinity and the length of crystallites in native cellulose from grassy plants is less than in native cellulose from wood, flux and the cotton plant. The lateral crystallite dimensions of native cellulose from grassy plants and wood are 3.5–4.0 nm, from flax and cotton linters 5–7 nm. The separation of cellulose from the plant material by various delignification methods is accompanied by lateral cocrystallization of the crystallites leading to aggregation of elementary fibrils and loss of their individuality. Cellulose crystallites show low defectivity.

X‐ray diffraction and infrared studies on annealed highly syndiotactic poly(vinyl chloride)

AbstractX‐ray diffraction scans and infrared absorption spectra for the C‐CI stretching region were obtained for a highly syndiotactic poly(vinyl chloride) (PVC) sample made by the urea clathrate method. When the polymer was annealed at a series of increasing temperatures up to 180°C, x‐ray diffraction measurements showed that the crystallinity increases steadily with annealing temperature. Even at 50°C an increase above the original value of 63% was detectable and by 180°C it had reached 70% with a further increase to 78% after cooling to ambient temperature. There is a concurrent significant increase in the lateral crystallite dimensions. However, the infrared spectrum did not change, in agreement with the recent prediction of Moore and Krimm that there is no observable band splitting from interchain interaction in crystalline regions, thus indicating that these C‐CI bands cannot be used as a measure of crystallinity. The results from computer curve fitting of the spectra suggest that at least 85% of the polymer consists of long planar syndiotactic sequences and there is therefore substantial order along the chain direction. A mechanism for the increase in crystallinity on annealing, involving the lateral ordering of these regular chains, is discussed. Furthermore, as the temperature is raised some amorphous material is converted to a nematic phase, and this may crystallize during the subsequent cooling.