Harry Reynaers

Crystallization phenomena in bacterial poly[(R)-3-hydroxybutyrate]: 3. Toughening via texture changes

Poly[(R)-3-hydroxybutyrate] (PHB) is a bacterial storage polyester that is genuinely biodegradable, but its brittleness hampers its wide applicability as a biodegradable plastic. Most attempts to improve the mechanical properties of PHB have focused on incorporating comonomers, but at the expense of production costs. Strikingly, it appeared possible to toughen PHB homopolymer using a simple annealing treatment. The present study attributes this remarkable phenomenon to a change in the lamellar morphology, which reduces the amorphous-crystalline interface area and thus the constraint imposed on the amorphous chains by the crystals. This elevates the relaxation strength of the amorphous regions, giving rise to the favourable fracture behaviour.

Evaluation of the impact of annealing on gelatinisation at intermediate water content of wheat and potato starches: A differential scanning calorimetry and small angle X-ray scattering study

Abstract The DSC (differential scanning calorimetry) thermograms of wheat and potato starches at 50% (w/w) water content are characterised by two gelatinisation endotherms. Two separate phenomena coinciding with the two DSC endotherms can be distinguished in the SAXS patterns of 50% (w/w) suspensions of wheat and potato starches during heating from 25 to 95 °C at 2 °C/min: an increase in peak integral in the temperature domain of the first (G) endotherm and a marked decrease in peak integral in the temperature domain of the second (M1) endotherm. One- and two-step annealing affect only the G endotherm, leading to a shift to a higher temperature of up to 8 °C, sharpening of the peak and an increase in enthalpy, while the completion temperature of the M1 endotherm remains unchanged. Static SAXS measurements indicate that the repeat distances of crystalline and amorphous lamellae in wheat (105 A) and potato (99 A) starch granules are unaffected by annealing. One- and two-step annealing intensify the SAXS peaks. The most striking difference between the SAXS gelatinisation profiles of native and annealed starches is that there is no increase in peak integral at the onset of gelatinisation of annealed starches. The effects following annealing are interpreted as a decreased water absorption during gelatinisation. Annealing leads to a retardation of the initial swelling and cooperative melting of the granules, without altering the stability of the most perfect crystallites.

Use of SAXS and linear correlation functions for the determination of the crystallinity and morphology of semi-crystalline polymers. Application to linear polyethylene

The use of correlation functions to obtain the morphological parameters of crystalline-amorphous two-phase lamellar systems is critically reviewed and extended. It is shown that processing of the experimental SAXS-patterns only significantly affects the curvature of the autocorrelation triangle and that the parameters of the corresponding ideal two-phase structure can be determined independently of the data processing procedure. The methods to be used depend on the normalization of the correlation function. The validity of the formulation is illustrated for a sample of linear polyethylene, cooled and heated at 10°C per min. Crystallite thickening during crystallization and surface melting during heating are observed. The overall crystallinity and the fraction of semi-crystalline stacks during crystallization and melting are determined quantitatively as a function of temperature using the total scattering power of the corresponding ideal two-phase structure, correlation functions, and a scaling procedure. Absolute intensities are not required. The SAXS results are confirmed by independent techniques (DSC, WAXD, and SALLS). During crystallization, amorphous regions are present outside the semi-crystalline regions because growing spherulites do not fill space completely. During melting, larger amorphous regions develop in the spherulites because of the complete melting of stacks.

Miscibility and morphology of binary polymer blends of polycaprolactone with solution-chlorinated polyethylenes

Abstract Binary blends of solution-chlorinated polyethylenes (CPE) with polycaprolactone (PCL) were prepared by the coprecipitation technique. The phase behaviour of the mixtures was determined using optical microscopy, light transmission measurements, dynamic mechanical analysis and differential scanning calorimetry. The superstructure of the semicrystalline CPE/PCL blends was studied by small-angle X-ray diffraction, while the melting behaviour of PCL in the blends was investigated by differential scanning calorimetry. The miscibility was found to be dependent on the chlorine content of CPE and the temperature, while CPE is segregated interfibrillarly or interspherulitically during the crystallization of PCL in the blend. The double melting behaviour is attributed to a secondary crystallization process.

Miscibility, crystallization and melting behaviour, and semicrystalline morphology of binary blends of polycaprolactone with poly(hydroxy ether of bisphenol A)

Abstract Blends of poly(hydroxy ether of bisphenol A) (Phenoxy) with polycaprolactone (PCL) were prepared by the coprecipitation technique. The melt miscibility of the polymers was studied by optical microscopy, light transmission measurements and dynamic mechanical analysis. The crystallization kinetics of PCL in the miscible Phenoxy/PCL blends were studied using optical microscopy and the segregation behaviour of Phenoxy due to the crystallization of PCL was examined by means of optical microscopy and small-angle X-ray diffraction, while the melting behaviour of PCL in the blend was explored by differential scanning calorimetry. The polymers were found to be miscible over the entire composition and temperature range (up to 200°C), while Phenoxy is segregated interlamellarly as well as interfibrillarly and interspherulitically during the crystallization process of PCL.

Morphology of homogeneous copolymers of ethene and 1‐octene. I. Influence of thermal history on morphology

The morphology of homogeneous copolymers of ethene and 1-octene synthesized using a V-based Ziegler-Natta catalyst was studied as a function of the short chain branching content (SCBC) and the molar mass. Linear polyethylenes (LPE) were used as reference material. For the linear samples an increase in molar mass results in an increase of the long period and the crystalline lamella thickness. A decrease of cooling rate results in an increase of the melting temperature, the long period and the crystalline lamella thickness and an evolution from spherulitic structures to perfectly stacked lamellae. For the branched samples, increasing the SCBC results in a decrease of the melting and the crystallization temperature, crystallinity, spherulite radius, the long period, and the crystalline lamella thickness. The two latter tend to a limiting value on reaching a SCBC of 20CH3/1000C. On the other hand, an increase of the a axis and to a lesser extent the b axis of the unit cell is observed. Decreasing the cooling rate affects only the crystallinity of the least branched samples. Furthermore decreasing the cooling rate results in smaller spherulites, has a minor influence on the lamellar parameters and reduces the dimensions of the basal plane of the unit cell. Increasing the molar mass of the branched samples results in a drop of the crystallinity, a deterioration of the superstructure, enlarges the amorphous layer thickness and the dimensions of the basal plane. All these observations can be accounted for by the different crystallization regimes being applicable when different molar masses, SCBC and cooling rates are used.

Morphology and melting behaviour of semi-crystalline poly(ethylene terephthalate): 3. Quantification of crystal perfection and crystallinity

Abstract The semi-crystalline state of bulk-crystallized poly(ethylene terephthalate) and its relation to the melting behaviour of the polymer have been thoroughly investigated as a function of the thermal history by wide-angle X-ray diffraction. The experimental data, analysed according to the method of Ruland, allow estimation of the absolute degree of crystallinity and the diffuse disorder scattering. The results of this study give a better and more complete insight into the complex thermal behaviour of PET; moreover they corroborate the need for a broad experimental approach in studies related to the melting behaviour of polymers.

Miscibility, crystallization and melting behaviour, and morphology of binary blends of polycaprolactone with styrene-co-maleic anhydride copolymers

Abstract Blends of polycaprolactone (PCL) and random copolymers of styrene and maleic anhydride (SMA) with different maleic anhydride contents were prepared by the coprecipitation technique. The miscibility of both polymers in the melt and in the solid state was studied by means of optical microscopy, light transmission measurements and dynamic mechanical analysis. The crystallization behaviour of PCL in the miscible blends was examined using optical microscopy and the morphology of the semicrystalline PCL/SMA blends was investigated by means of small-angle X-ray diffraction measurements. Their melting behaviour was studied by differential scanning calorimetry. SMA containing 14 and 25 wt% MA was found to be miscible with PCL over the entire composition and temperature range (up to 200°C). SMA appears to segregate interlamellarly during the isothermal crystallization of PCL. The double melting behaviour of PCL in the blends was attributed to a secondary crystallization process and not to a partial melting-recrystallization-remelting process.

Complex melting of semi-crystalline chicory (Cichorium intybus L.) root inulin

When concentrated solutions (30-45% by weight) of inulin (degree of polymerization 2-66, number average degree of polymerization 12) are cooled at 1 degree C/min or 0.25 degree C/min from 96 degrees C to 20 degrees C, suspensions of semi-crystalline material in water are formed. A thermal nucleation process was detected by optical microscopy: the 8-like shaped crystallites resulting from primary nucleation at higher temperature are larger than those resulting from secondary nucleation at lower temperature. Differential scanning calorimetry (DSC) thermograms display melting profiles with three to four partly overlapping endotherms that vary as a function of concentration, cooling rate during crystallization and storage time at 25 degrees C of the crystallite suspension. Recrystallization during melting was observed. The wide-angle X-ray scattering patterns of the samples at 25 degrees C correspond to those of the hydrated crystal polymorph. The structural changes during melting indicated the existence of a single crystal polymorph throughout melting. A periodicity of 95 A, arising from alternating regions of different electron density, is detected in the small angle X-ray scattering patterns at 25 degrees C. The stepwise increase of the long period upon heating is related to the existence of two types of lamellar stacks: one with a higher long period, resulting from the primary nucleation and thus crystallized at high temperature, and a second one with a smaller long period, formed by crystallization at lower temperature. The lamellae formed at low temperature melt at a lower temperature than those formed at high temperature, explaining the existence of the two DSC-endotherms.

Dynamic light scattering by κ- and λ-carrageenan solutions

Abstract The intensity correlation functions of κ- and λ-carrageenan in various salt solutions and at different concentrations have been determined with the help of dynamic light scattering. From the first cumulant of these correlation functions the values of the translational diffusion coefficients D have been derived. They increase with macromolecular concentration. The extrapolated values to infinite dilution of the diffusion coefficients increase with increasing salt concentration as expected from the salt concentration dependence of the r.m.s. radii of gyration determined previously by static light scattering. The translational diffusion coefficient of λ-carrageenan in 0.1 m NaCl is smaller than the corresponding value for the κ species. This is consistent with the difference in contour length and linear charge density of the two samples used. No satisfactory interpretation for the concentration dependence of the diffusion coefficient seems to be possible at present. Although current theories for the macromolecular and salt concentration dependence of D, taking into account charge effects, seem to be applicable, they do not allow for a consistent interpretation of the data. No specific difference between the solution behavior of κ- and λ-carrageenan has been detected.