K. te Nijenhuis

Investigation into the ageing process in gels of gelatin/water systems by the measurement of their dynamic moduli

SummaryDuring the ageing (renaturation) process of 1–5% aqueous gelatin solutions the dynamic moduli were measured in the temperature range from −1.2 °C to 25.2 °C. The renaturation process appears to be strongly dependent on concentration, temperature and thermal history. The process at 25.2 °C appears to be of 3rd order, whereas the rate determining step appears to be the aggregation of mono helices into triple helices.ZusammenfassungWährend der Alterung (Gelierung, Renaturierung) von 1–5% Lösungen von Gelatine in Wasser wurden deren Speicher- und Verlustmoduln in einem Temperaturbereich von −1,2 °C bis 25,2 °C gemessen. Der Alterungsprozeß hängt stark ab von der Konzentration, der Temperatur und der thermischen Vorgeschichte. Die Ordnung des Prozesses bei 25,2 °C ist drei und der geschwindigkeitsbestimmende Reaktionsschritt ist die Aggregation von monohelices in Tripelhelices.

Encapsulation by interfacial polycondensation. I. The capsule production and a model for wall growth

Abstract Capsules are produced by the interfacial reaction of terephthaloyldichloride, TDC, (dissolved in dibutylphthalate) and diethylenetriamine, DETA, (dissolved in the aqueous phase) at the interface of drops of the organic phase dispersed in the aqueous phase. The membrane produced exists of a top-layer and a sub-layer as in asymmetrical membranes. A model which relates the growth of the capsule wall and the concentration of TDC inside the capsule with time, is presented. For a spherical capsule the model shows both for wall thickness and TDC concentration a linear relationship with √t after the formation of a top-layer. The sub-layer is formed from cellular pores, whose walls are formed around droplets at the organic side of the membrane. The pore size increases from the initial interface into the capsule.

Elastic and Acoustic Properties of Vessel Mimicking Material for Elasticity Imaging

The mechanical and acoustic properties of agar-gelatin gels, used to construct vessel mimicking phantoms for ultrasonic elasticity studies, were investigated. Gels with varying compression moduli were made using a gelatin solution (8% by weight) with a variable amount of agar(1%-3% by weight). Carborundum particles were added as scattering material. The compression modulus was determined using a dynamic mechanical analyzer. The dependence of the compression modulus and the acoustic parameters on the agar concentration, as well as on the age and the temperature of the samples, was investigated. The results show that the compression modulus is strongly influenced by these factors, while the effect on the acoustic parameters is less. Compression moduli spanning a useful range for vascular phantom construction with realistic acoustic parameters can be achieved by varying the amount of agar. Phantoms constructed from these gels are well suited to serve as a model for plaque containing vessels.

Investigation into the ageing process in gels of gelatin/water systems by the measurement of their dynamic moduli: Part II: Mechanism of the ageing process

SummaryThis second paper on the ageing of aqueous gelatin gels (1–5%) deals with the mechanism of the renaturation process. The maximum gelation temperature, i. e. the gel temperature, increases with increasing concentration. For the heat of cross-linking a value of ΔH° = - 442 kJ/mol was found, corresponding with mono-helices of 24 windings and a length of 230 Å. The influence of the thermal history is considerable, and the heat of activation of the rate determining step is very small. Hence, the temperature dependence of the degree of ageing is not caused by temperature dependence of the reaction rate constant, but by the increase in stability of small structures which accompanies a decrease in temperature.ZusammenfassungDiese zweite Veröffentlichung betreffs der Alterung von 1–5% Lösungen von Gelatin in Wasser behandelt den Mechanismus des Renaturationprozesses. Die maximale Gelierungstemperatur, d. h. die Geltemperatur, nimmt zu mit zunehmender Konzentration. Für die Verknüpfungswärme wurde gefunden ΔH° = -442 kJ/mol. Dieser Wert korrespondiert mit Monohelices von 24 Windungen und einer Länge von 230 Å. Die Abhängigkeit von der thermischen Geschichte ist bedeutend und die Aktivierungsenergie des geschwindigkeitsbestimmenden Reaktionsschrittes ist sehr klein. Demnach wird die Temperaturabhängigkeit des Alterungsgrades nicht verursacht durch die Temperaturabhängigkeit der Reaktionsgeschwindigkeitskonstante, sondern durch die Zunahme der Stabilität kleiner Strukturen mit abnehmender Temperatur.

Encapsulation by interfacial polycondensation. II: The membrane wall structure and the rate of the wall growth

The encapsulation of dibutylphthalate by interfacial polycondensation of terephthaloyldichloride (TDC) and diethylenetriamine (DETA) was studied. The growth rate of the capsule wall is determined by diffusion of DETA through the wall into the oil phase. Due to the reaction of TDC with DETA the concentration of TDC inside the capsules decreases. Both the concentration decrease of TDC and the growth of the wall thickness are linear with √t. This verifies the model relations between the wall thickness and the TDC concentration. The rate of wall growth is a function of the polymer properties of the membrane. The concentration ratio of TDC and DETA in the oil and aqueous phase, respectively, determines the polymer properties and consequently, also the growth rate of the capsule wall. Mesurements of the rate of hydrolysis of encapsulated TDC show that no hydrolysis will take place during wall formation, as hydrolysis only takes place in the aqueous phase.

Encapsulation by interfacial polycondensation. III. Microencapsulation; the influence of process conditions on wall permeability

Abstract Microcapsules are produced by immersing uniform sized droplets of an organic phase into an aqueous solution. The process of polycondensation of terephthaloyldichloride (TDC), dissolved in the organic phase, and diethylenetriamine (DETA) in combination with ethylenediamine (EDA) or hexamethylenediamine (HMDA), dissolved in the aqueous phase, was studied. This process produced the capsule wall at the interface; the capsule diameter varied between 0.5 and 1 mm. The influence of the concentration ratio of TDC and DETA, the addition of EDA or HMDA, their concentration relative to the DETA concentration in the aqueous phase and the kind of the organic phase on the diffusion coefficient of DETA through the polymer produced was measured. The decrease of the TDC concentration in the capsules and the growth of the capsule wall could be described by the same relationship as derived from macrocapsules. Also the influence of the TDC/DETA concentration ratio was in agreement with the results obtained for macrocapsules. Addition of diamine to the aqueous phase reduces the permeability of the capsule wall; an increase of the ratio of the diamine and the triamine concentration also reduces the permeability.

Birefringence build-up and relaxation in a sheared polymer melt: Range of validity of Lodge's rubberlike liquid theory*)

SummaryA description is given of a coaxial cylinder apparatus, suitable for the measurement of the flow birefringence of polymer melts at extremely low shear rates (~10−3 s−1) and, in particular, under transient conditions.Transient stresses including normal stresses, which grow after a suddenly imposed constant rate of shear and relax after a sudden cessation of a steady shear flow, are investigated with the aid of the flow birefringence technique. For this purpose use is made of the stress-optical law, which is carefully checked. Lodges rubberlike liquid model is chosen for the description of the flow behaviour of polymer melts. One of the primary reasons for this choice lies in the fact thatLodges equations are suitable for the application of a method which was originally designed bySchwarzl andStruik for the practical calculation of interrelations between linear viscoelastic functions. In this way it was possible to find quantitative criteria for the limits of validity of the rubberlike liquid model. In particular, the dynamic storage and loss moduli were used as reference functions for the interpretation of the flow birefringence data.The rubberlike liquid model appears to be valid for polymer melts at extremely low rates of shear and, for any rate of shear, when the total shear applied, does not exceed unity.ZusammenfassungEin koaxialer Zylinderapparat für die Messung der Strömungsdoppelbrechung von polymeren Schmelzen wird beschrieben. Der Meßbereich des Apparates beschränkt sich auf extrem kleine Schergeschwindigkeiten (~10−3 s−1). Die Eignung für Messungen unter zeitlich veränderlichen Spannungszuständen ist von Bedeutung.Veränderliche Schub- und Normalspannungen, die beim Einschalten einer konstanten Schergeschwindigkeit wachsen und beim Ausschalten wieder relaxieren, werden mit Hilfe der Strömungsdoppelbrechung untersucht. Zu diesem Zweck wird die Gültigkeit der spannungsoptischen Regel einer genauen Prüfung unterzogen.Das Modell der „rubberlike liquid“, wie es vonLodge vorgeschlagen wurde, wird zur Beschreibung des Fließverhaltens polymerer Schmelzen herangezogen. Die Wahl dieses Modelles erfolgte in erster Linie deshalb, weilLodges Gleichungen sich für die Anwendung einer Methode eignen, die ursprünglich vonSchwarzl undStruik für die praktische Berechnung von Relationen zwischen linear-viskoelastischen Funktionen entwickelt wurde. Auf diese Weise konnten quantitative Kriterien für die Gültigkeit desLodgeschen Flüssigkeitsmodelles erstellt werden. Im speziellen Fall wurden die dynamischen Speicher- und Verlustmoduln als Referenzfunktionen für die Interpretation der Strömungsdoppelbrechung verwendet.Das „rubberlike-liquid“-Modell erweist für polymere Schmelzen Brauchbarkeit für extrem kleine Schergeschwindigkeiten und, bei beliebigen Schergeschwindigkeiten, für totale Schubdeformationen kleiner als eins.

Correlation between viscoelastic behaviour and small angle X-ray scattering of thermoreversible polyvinyl chloride gels

During the thermo-reversible ageing process of 10% solutions of polyvinyl chloride in di(2-ethylhexyl)-phthalate (DOP) as well as in Reomol ATM, the small angle X-ray scattering and the storage modulus were measured. The ageing of polyvinyl chloride proceeds in Reomol much faster than in DOP. A clear correlation between the storage modulus and the crystalline structure in the gels during the ageing process is shown. The distance between the crystallites appears to be independent of the degree of ageing, in DOP as well as in Reomol. In both cases this distance is estimated to be ∼300 A.

Investigation of the aging process of a polyvinyl chloride gel by the measurement of its dynamic moduli

SummaryDuring the aging process of a 10%-solution of polyvinyl chloride in di-(2-ethylhexyl)phthalate at temperatures between −20 and 110°C its storage and loss moduli were measured in a frequency range from 1.6×10−4 to 40 cycles/sec. The specially defined rate of aging plotted against the aging temperature closely resembles the crystallization curve of a normally crystallizable polymer. Although the internal structure of the system changes, when the temperature is changed, the time-temperature reduction could still be applied provided some precautions were taken.Bueches theory for networks, combined withFlorys post-gelation treatment, has been used to calculate some gel parameters: the molecular weight between cross-links, the sol fraction, the monomeric friction coefficient etc. It is shown that the aging process strongly depends on the thermal history. The cross-links formed by crystallites, differ very much in functionality. Continued aging, after previous aging at lower temperature, causes, after a first melting of unstable crystallites, a recrystallization process. During this process entanglements are formed. This results in a pseudo-equilibrium rubber plateau at higher frequencies, followed by a dispersion region and a lower equilibrium rubber plateau at lower frequencies.ZusammenfassungWährend der Alterung (Gelierung) einer zehnprozentigen Lösung von Polyvinylchlorid in Di-(2-Äthylhexyl)-Phthalat wurden deren Speicher- und Verlustmoduln in einem Frequenzbereich zwischen 1,6⋅10−4 und 40 Hertz gemessen. Die speziell definierte Alterungsgeschwindigkeit wurde gegen die Alterungstemperatur aufgetragen. Die so erhaltene Kurve ist der Kristallisationskurve eines normal kristallisierenden Polymeren ähnlich. Obwohl die innere Struktur des Systems sich mit der Temperatur ändert, konnte die Zeit-Temperatur-Reduktion doch ausgeführt werden, wenn geeignete Vorsorgmaßnahmen getroffen wurden.Bueches Netzwerktheorie wurde in Kombination mitFlorys Behandlung der Nachgelierung zur Berechnung einiger Parameter des Gels verwandt. Es handelt sich hierbei um das Molekulargewicht zwischen Verknüpfungsstellen, die Solfraktion, den Reibungskoeffizienten der monomeren Einheit u.a. Es wird gezeigt, daß der Alterungsprozeß stark von der thermischen Vorgeschichte abhängt. Die Verknüpfungsstellen, die durch Kristallite gebildet werden, zeigen starke Unterschiede in der Funktionalität. Fortgesetzte Alterung bei höherer Temperatur nach vorhergegangener Alterung bei tieferer Temperatur verursacht nach anfänglichem Schmelzen unstabiler Kristallite einen Rekristallisationsprozeß. Während dieses Prozesses werden Verhakungen (entanglements) gebildet. Diese bewirken die Entstehung eines zweiten (im Pseudo-Gleichgewicht befindlichen) Gummiplateaus bei höheren Frequenzen, auf das bei niedrigeren Frequenzen nach Passieren eines Dispersionsgebietes ein echtes (im Gleichgewicht befindliches) Gummiplateau folgt.

Turbidity study of the process of film formation of thin films of aqueous acrylic dispersions

Abstract This study was directed towards determining the factors that define the process of film formation of binder particles in drying aqueous dispersion coatings, based on acrylic polymers. The work described focuses on the infrastructure of drying and ageing thin films of acrylic latices. In concentrated latices the binder particles are arranged in closely packed structures which cause colored light patterns, the so-called Bragg diffractions. The light waves move within the latex film, where the waves are scattered by the internal structure composed of the spheres and water voids. The pattern of light transmission reveals the internal structure of the latex film. From the change in interference during the drying process of a thin latex film, it is possible to follow the internal movement and deformation of polymer spheres (coalescence process). Further coalescence results in a transparent film. When this film is immersed in water, the remaining internal interfaces between the adhered binder particles swell, thus regenerating the interference pattern. It is expected that during ageing of the film, the proportion of internal interfaces will decrease with time, so that when the aged film is immersed in water the remaining internal interfaces will swell. The resulting interference pattern reveals the decrease in the interfaces between the deformed polymer particles in the dried latex film (auto-adhesion process).