Composites reinforced with cellulose based fibres

Small Angle X-Ray Scattering

The theory of phase-ordering dynamics that is the growth of order through domain coarsening when a system is quenched from the homogeneous phase into a broken-symmetry phase, is reviewed, with the emphasis on recent developments. Interest will focus on the scaling regime that develops at long times after the quench. How can one determine the growth laws that describe the time dependence of characteristic length scales, and what can be said about the form of the associated scaling functions? Particular attention will be paid to systems described by more complicated order parameters than the simple scalars usually considered, for example vector and tensor fields. The latter are needed, for example, to describe phase ordering in nematic liquid crystals, on which there have been a number of recent experiments. The study of topological defects (domain walls, vortices, strings and monopoles) provides a unifying framework for discussing coarsening in these different systems.

Theory of phase-ordering kinetics

In the X-ray scattering curve of chain-molecules in solution, essentially four regions may be distinguished as is shown by a more detailed mathematical discussion:

(1)The scattering at the smallest angles, showing an approximately Gaussean course, whose ordinate, when extrapolated to zero angle, is proportional to the square of the mol. weight. It is due to the diffraction effect of the whole coiled chain-molecule.
(2)This is followed by a region of v-values where the scattering falls off as 1/v2. It is due to parts of the chain molecule which themselves already occur in all possible orientations.
(3)After a transition region (which is also accounted for by the theory) there follows a scattering which is proportional to 1/v. It is to be considered as the scattering of mostly straight parts of the molecule.
(4)The scattering at large angles, caused by smaller parts of the molecule (monomer group and single atoms), which at the same time indicates the periodicity (= length of structural unit in the direction of the chain axis) by a step, at an angle corresponding to Bragg's law. Its sharpness, too, is a measure for the coiling of the molecule.


It can be shown that the abscissa of the intersecting point of the 1/v2 and 1/v branches, when extrapolated to the transition region, is connected in a characteristic way with the statistical chain element or the “length of persistance”, which represent two different measures for the stiffness of the chain molecule.

Measurements with polyvinyl bromide lead to a length of persistance of 11 ± 1 A, which agrees satisfactorily with the value of 13 A for polyvinyl chloride as derived from the birefringence of flow. At large angles two weak steps were found, one of which corresponds to the monomer length of 2.56 A.

Measurements on solutions of cellulose nitrate gave a curve ∼ 1/v without any steeper ascent up to the smallest angles, corresponding to 300 A. We have, therefore, to conclude that the chain-element of cellulose nitrate is in any case larger than 150 A, which again is in agreement with the results of other physical measurements.

Though only a small number of experimental results is available so far, it seems to be established that the procedure yields a direct evaluation of the shape of chain molecules in solution and, in this respect, appears superior to other methods. However, the experimental difficulties are considerable.

Röntgenuntersuchung gelöster Fadenmoleküle

Some pertinent results and views from the earlier history of lignin chemistry, pointing to the importance of the arylpropane skeleton, are outlined. Later development, beginning with the dehydrogenation theory and experimental studies on the dehydrogenative polymerization of p-hydroxycinnamyl alcohols, is then reviewed. Finally, recent degradative work resulting in a detailed picture of lignin structure is discussed.

Lignin chemistry—past, present and future

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Die Röntgenkleinwinkelstreuung von dichtgepackten kolloiden Systemen

Zusammenhang zwischen mittlerem Endpunktsabstand und Kettenlänge bei Fadenmolekülen

Die Chemie der Pflanzenzellwand

A short review is given of the development of the theory and of the new experimental technique of x-ray low-angle scattering. The study of human γ-globulin leads to a cylindrical particle of about elliptical cross section with axes of 19 and 57 A. and with a length of 230–240 A. The volume is found to be 1.98 × 105 A.3, which involves a molecular weight of 1.62 × 105 in satisfactory agreement with the value found by ultracentrifuge measurements.

Determination of the shape of gamma‐globulin by means of the x‐ray low‐angle method

Die Rontgenkleinwinkelstreuung ge stattet auch bei dichtgepackten kolloiden Systemen eine theoretisch einwandfreie Auswertung. Es ist moglich, die innere Oberflache sowie ein Mas fur die Grose der kolloiden Inhomogenitaten zu bestimmen. Eine Strukturkennziffer und der Streukurvenverlauf geben qualitativen Aufschlus uber die Struktur des Systems (strukturell homogen oder inhomogen; kornige, faserige oder blattrige Struktur). Die fur die Auswertung erforderliche Auflosung und Genauigkeit werden durch eine neue Aufnahmemethode auf Zahlrohrbasis gewahrleistet. Zahlreiche Beispiele aus verschiedenen Gebieten, Pulver, Gele und Mineralien, illustrieren die Anwendungsmoglichkeit der Kleinwinkelmethode.

G. Porod

Papers

Die Röntgenkleinwinkelstreuung von dichtgepackten kolloiden Systemen

Zusammenhang zwischen mittlerem Endpunktsabstand und Kettenlänge bei Fadenmolekülen

Die Chemie der Pflanzenzellwand

Determination of the shape of gamma‐globulin by means of the x‐ray low‐angle method

Röntgenkleinwinkeluntersuchungen an dichtgepackten kolloiden Systemen