Simultaneous Small-angle X-ray Scattering Research Articles

Cold Crystallization of Narrow Molecular Weight Fractions of PEEK

Several amorphous narrow molecular weight fractions of poly(aryl ether ether ketone) or PEEK have been heated from below their glass transition temperature to above their final melting point. The semicrystalline morphology induced by cold crystallization and its thermal evolution are studied by time-resolved simultaneous small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction, and differential scanning calorimetry. The long period and the crystalline and the amorphous thicknesses are computed from the correlation function of the SAXS curves. The evolution of the structural parameters of the various semicrystalline samples can be reconciled with the existence of a single distribution of lamellae which undergoes reorganization during the continuous heating. The reorganization process as well as the semicrystalline morphology depends on the molecular weight of the fractions. The amorphous thickness strongly increases with the molecular weight while the thermal evolution of the thickness of the crystalline layers is similar for all fractions. These experimental results support the view that the larger degree of entanglements of the high molecular weight samples impedes the reorganization mechanism with the consequence that the apparent melting temperature decreases with increasing average molecular weight.

Orientational Ordering of a Poly(oxyethylene)−Poly(oxybutylene) Diblock Copolymer Gel under Steady Shear Flow

Orientational ordering was investigated in the hexagonally packed rod phase formed by a poly(oxyethylene)poly(oxybutylene) diblock copolymer in concentrated aqueous solution. The resulting soft gel was subjected to steady shear in a Couette cell, with simultaneous small-angle X-ray scattering (SAXS) experiments. Orientation function coefficients were extracted from SAXS patterns to quantify the degree of orientation. It is found that the averaged Legendre polynomial orientational order parameters increase logarithmically with a linear increase in shear rate. A limiting degree of orientation of P2 0.6 is reached for a shear rate of = 100 s-1, above which an increase in shear rate does not significantly improve the orientation. Time-resolved measurements reveal that the steady-state degree of orientation was developed within the gel over a time scale t 30 s and upon cessation of shear relaxed to an intermediate orientation level over a time scale of t 1 h.

Ordered Phases in Aqueous Solutions of Diblock Oxyethylene/Oxybutylene Copolymers Investigated by Simultaneous Small-Angle X-ray Scattering and Rheology

The phase behavior of gels of E40B10 in 0.2 mol dm-3 aqueous K2SO4 was studied as a function of temperature and concentration. E40B10 is a diblock copolymer of poly(oxyethylene) (E) and poly(oxybutylene) (B), where the subscripts denote the number of repeats. The phase of the material was characterized by both simultaneous rheology and small-angle X-ray scattering, (SAXS). Depending on polymer volume fraction in the range 23−38% a body-centered cubic (bcc) structure or a face-centered cubic (fcc) structure was observed at low temperature, and at high temperature a hexagonally packed rod structure was formed. The phase transitions were shown to be characterized by discontinuous changes in the values of the dynamic shear moduli. A bcc−fcc transition was observed at high concentration, the corresponding transition temperature increasing with increasing polymer concentration. The effects of reciprocating shear were shown to increase the degree of order, manifested as a sharpening of the diffraction peaks in the SAXS pattern. The dynamic moduli decreased rapidly on the application of oscillatory shear and recovered equally rapidly when the deformation ceased. The decrease in moduli was shown, via the SAXS patterns acquired simultaneously, to be correlated to structural changes within the gel.

Small-angle X-ray scattering and wide-angle X-ray scattering experiments combined with thermal and spectroscopic analysis techniques

Time resolved simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) experiments can provide structural information on the changes in different levels of organisation in a system when it is subjected to a perturbation. These perturbations can be chemical, thermodynamic or physical. However, often there is a desire to gain insights into the mechanism of phase transitions beyond the purely structural information. This can be achieved by combining SAXS and WAXS experiments with thermal analysis or spectroscopic methods. In order to be able to accurately correlate the results from these techniques with the evolution in the structure it is desirable to perform these experiments simultaneously and on the same sample. The higher the time resolution the greater the importance of technique combinations.

Real Time SAXS/WAXS/DTA Measurements of the Crystalline/Amorphous Transition of Low Molecular Weight Hydrogenated Polybutadiene

Abstract The behaviour of hydrogenated polybutadiene has been investigated during melting and recrystallization experiments. The techniques used to follow these processes were simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) and differential thermal analysis (DTA). The results show that there are two stages in the melt/recrystallization process. The major transition occurs at around 130°C on heating and at 100°C upon cooling, where the long range and short range order disappears and appears, respectively. On both heating and cooling, however, an additional change is seen in the long range order at a lower temperature than the main transition. This is characterised by the disappearance and appearance of a structure peak in the SAXS data. It is suggested that this may be due to micro-crystallites which are formed as a result of branching or from the free ends of polymer chains.

SAXS/WAXS studies of the devitrification of Cr-doped cordierite glass using synchrotron radiation

In situ simultaneous Small Angle X-ray Scattering (SAXS) and Wide Angle X-ray Scattering (WAXS) experiments have been carried out on Cr-doped cordierite (Mg2Al4Si5O18) glass. Samples were initially pre-treated at 875°C close to the glass transition to promote nucleation and this was followed by further treatments at 920°C and 1040°C to generate dispersed crystal growth. The initial treatment revealed no changes in the SAXS or the WAXS data, but the subsequent treatments at higher temperatures revealed large changes in microcrystallinity developing with time. In particular taking the SAXS data for q < 0.05Å−1 the average Guinier radius, Rg, was found to increase with heat treatment from initial values of ∼ 10Åto final values of ∼ 30Å. I(q) versus q−n plots (for q ≥1/Rg) revealed increasing values of n indicative of changes in mesostructure from rough to smooth fractal architecture. From the WAXS data that was obtained concurrently two separate crystalline phases were identified during devitrification-spinel (MgAl2O4) and a stuffed quartz (MgxAl2xSi1−xO2(1 + x))-each growing at different rates. The growth of spinel correlates well with the SAXS invariant, reflecting the major contrast in electron density between this phase and that of the residual glass. For the heat treatment conditions employed, WAXS reveals that the stuffed quartz phase appears later than the spinel phase and also grows less rapidly. For prolonged annealing, a SAXS feature develops at ∼ 0.022Å−1, corresponding to an average interparticle distance of ∼ 280Å. This is considerably longer than the statistical separation of Cr atoms in the glass (14Å) and suggests some clustering of Cr takes place as devitrfication progresses.

Real time SAXS/WAXS/DTA measurements of the crystalline/amorphous transition of low molecular weight hydrogenated polybutadiene

The behaviour of hydrogenated polybutadiene has been investigated during melting and recrystallization experiments. The techniques used to follow these processes were simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) and differential thermal analysis (DTA). The results show that there are two stages in the melt/recrystallization process. The major transition occurs at around 130°C on heating and at 100°C upon cooling, where the long range and short range order disappears and appears, respectively. On both heating and cooling, however, an additional change is seen in the long range order at a lower temperature than the main transition. This is characterised by the disappearance and appearance of a structure peak in the SAXS data. It is suggested that this may be due to micro-crystallites which are formed as a result of branching or from the free ends of polymer chains.

Simultaneous SAXS/WAXS and d.s.c. analysis of the melting and recrystallization behaviour of quenched polypropylene

The behaviour of a quenched ethylene-propylene block copolymer cast film has been investigated during melting and recrystallization experiments carried out at heating and cooling rates of 10°;C min −1. The experimental techniques used to follow the melting and recrystallization processes were differential scanning calorimetry (d.s.c.) and simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS). In particular, the WAXS results show that the melting of the material is preceded by a solid-state transition involving transformation of the quenched structure into the monoclinic α modification. This transition appears as an exotherm in the range 65–120°C on the d.s.c. melting trace. SAXS results show that an increase in the amount of long-range order occurs during this transition. The sample was then found to melt at 162°C. Upon cooling from the melt, recrystallization of the monoclinic α modification was found to occur in the range 123-105°C without any apparent prior formation of the quenched structure.

Time-resolved X-ray study of poly(aryl ether ether ketone) crystallization and melting behaviour: 1. Crystallization

Time-resolved simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) experiments using synchrotron radiation were made in order to study the isothermal crystallization behaviour of poly(aryl ether ether ketone) (PEEK). Two types of long period ( L B and L M C), the scattering invariant Q, and the linear degree of crystallinity within the lamellar stacks x CL were estimated from the Lorentz-corrected plot and the correlation function. As a result, the average lamellar thickness l C was calculated as the product of L M C and x CL during crystallization at temperatures of 230–310°C. The bulk degree of crystallinity (by weight), ψ C, was determined by WAXD. It was found that both long periods decreased, Q increased and l C remained approximately constant with time. The decrease in the long period was explained by the lamellar insertion within the lamellar stacks, whereas the increase in Q was attributed to the increase in the bulk-volume degree of crystallinity x C, rather than x CL. In addition, the values of the long period and of Q were found to increase with temperature as a result of the thicker lamellae. Finally, our results support the model of two populations in lamellar thickness, namely the primary lamellae which are developed first in bundle-like stacks, and the narrower, secondary lamellae (as a small fraction) that are inserted later between the primary lamellae under various spatial restrictions.