A. A. Apostolov

Crystallization of water in some crosslinked gelatins

Abstract Five gelatin samples crosslinked with 1,4-diisocyanatobutane, 1,6-diisocyanatohexane, 1,12-diisocyanatododecane, 1,3-butadiene diepoxide and 1,2,7,8-diepoxyoctane were used to investigate the behavior of the gelatin–water system. The crosslink densities were estimated quantitatively by the values of the molecular weight between crosslinks Mc. Differential scanning calorimetry indicated that the water was only partially crystallizable below 0°C, and the fraction of non-crystallizable water depends on the nature of the crosslinking agent. This fraction tends to zero for high overall water fraction, approaching unity. The overall water weight fraction at which crystallizable water forms was found to be of the order of 0.35 for four of the samples, whereas for the sample crosslinked with 1,3-butadiene diepoxide it is surprisingly high, specifically 0.68.

DSC and TGA studies of the behavior of water in native and crosslinked gelatin

Water molecules absorbed into gelatin are found to be only partially crys- tallizable. The fraction of noncrystallizable water depends on whether the gelatin is native or crosslinked, and on the crosslinking conditions as well. This dependence is explained by the Tg-regulation effect newly proposed by Rault and coworkers for water-swollen gelatin cooled below 0°C. According to this effect, a part of the frozen water cannot crystallize because during the cooling the amorphous gelatin-water phase becomes glassy before the water crystallization temperature is reached. During the heating of water-plasticized gelatin samples in a TGA cell, the crystallizable water separates from the gelatin, mainly in the temperature interval 50 -100°C, whereas the noncrystallizable water leaves the gelatin gradually over the entire temperature inter- val investigated, up to 300°C.

Mechanical properties and transition temperatures of cross-linked oriented gelatin. 1. Static and dynamic mechanical properties of cross-linked gelatin

This study is an extension of previous work on cellulosics [(1994)Colloid Polym Sci 272: 284, 393] that showed that unusually good mechanical properties can be obtained by drying a swollen network of semirigid chains in a state of strain. This novel approach is applied in this investigation to gelatin, because of its attractive environmental characteristics but poor mechanical properties in the unmodified form. Since drawing of non-crosslinked gelatin is not practical, crosslinking by formaldehyde was used, followed by swelling, drawing and drying at fixed length. Mechanical tests were performed in static and dynamic modes. In this way improvements of Youngs modulusE, and stress at breakσb were determined as a function of gelatin concentration during drying. An increase inE andσb up to 2–3 times, and in the dynamic modulusE′ up to 6 times, was obtained when the draw ratio λ reached 4–5, after whichE, E′, andσb were found to decrease. Such behavior is explained by the highest orientation being achieved at λ=4–5, as proved by x-ray analysis. At λ=10–20 the orientation is lost due to relaxation of chain segments, which is preceded by partial destroying of the network structure (chemical and physical), possibly via chain scission, but probably mostly by the pulling out of chains from crystallites. In any case, the mechanical properties become poor again.The improvements reported above were referred to the undrawn but crosslinked gelatin. Compared to the starting isotropic non-crosslinked material, the improvement is slightly higher. The observation that the improvements are less than those obtained for the cellulosics is explained by the coexistence of interpenetrating chemical and physical networks, which is typical of gelatin. This structural feature drastically reduces the orientability of the chains and the improvements that can be expected in the mechanical properties.

Structure of segmented poly (ether ester)s as revealed by synchrotron radiation

Abstract The structure and phase transitions of block copolymers of PBT and PEO having different composition were investigated by DSC, SAXS, and WAXS. Two different glass transitions and two melting points were observed. The increase of the integral SAXS intensity with temperature was much smaller than in the case of homopolymers, which is explained by a broad melting range. The long period strongly increases with temperature, which is attributed to growth of domains.

Mechanical properties of native and crosslinked gelatins in a bending deformation

Gelatin samples, native and chemically crosslinked with three different diisocyanates, were studied by bending-creep measurements. These samples were characterized by the number-average molecular weight of a chain segment between two points of crosslinkage Mc. The chemical network was found to contribute to a marked extent to the mechanical behavior of the samples. The dependence of the creep compliance on the time for different loads was determined. The experimental results were compared with calculated ones according to a model, comprising four parameters, to obtain a better understanding of structure–property relationships for these materials. A very good agreement between the model and experimental data was found. Two of the fitting parameters, however—the relaxation time and η (which is connected with the viscosity)—were found to strongly depend on the time of the experiment.

Biodegradable Laminates Based on Gelatin, 1

Compression-molded gelatin/starch (1:1 w/w) blend shows improved mechanical properties as compared to neat gelatin. Compression molded laminates reinforced by fabrics were prepared from this blend as well as from neat gelatin. The fabric is either linen or silk. Some of the laminates were additionally crosslinked with methylenedi-p-phenyl diisocyanate. Thus a total of ten different samples were obtained. They were characterized by means of mechanical testing, namely measuring of Youngs modulus, tensile strength, elongation at break, and impact strenght. Substantial increases of Youngs modulus and tensile strength in comparison to neat gelatin by a factor of 2-3 and 4-5, respectively, were found for both linen- and silk-reinforced, uncrosslinked and crosslinked laminates. The deformation at break increases much more for the linen-based than for the silk-based laminates (about 12 vs. 8 times in average) in comparison to both neat gelatin and gelatin/starch matrix. The improvement in the mechanical properties of the laminates is visible most drastically in the impact behavior. The impact strength is low and almost equal for the neat gelatin and the gelatin/starch samples, but increases by a factor of 10 to 30 when the silk- and linen-reinforcement is involved. This improvement can be partly explained by the good matrix-fabric adhesion due to their similar chemical composition (in the uncrosslinked samples) and also to the chemical links between them (after crosslinking). In addition, all materials prepared are biodegradable, i.e., environmentally friendly since they do not pollute the nature.

CRYSTALLIZATION IN PARTIALLY MOLTEN ORIENTED BLENDS OF POLYCONDENSATES AS REVEALED BY X-RAY STUDIES*

Oriented fibers or films of binary polymer blends from polycondensates were investigated by two-dimensional (2D) wide-angle X-ray scattering (WAXS) during the finishing process of microfibrillar reinforced composite (MFC) preparation, that is, heating to a temperature between the melting temperatures of the two components, isothermal annealing, and subsequent cooling. It is shown that the crystallization behavior in such MFC from polycondensates depends not only on the blend composition, but also on thermal treatment conditions. Poly(ethylene terephthalate)/polyamide 12 (PET/PA12), poly(butylene terephthalate)/poly(ether ester) (PBT/PEE), and PET/PA6 (polyamide 6) composites were prepared in various compositions from the components. Materials were investigated using rotating anode and synchrotron X-ray source facilities. The effect of the annealing time on the expected isotropization of the lower melting component was studied in the PET/PA6 blend. It was found that PA6 isotropization took place after 2 h; shorter (up to 30 min) and longer (up to 8 h) melt annealing results in oriented crystallization due to different reasons. In PET/PA12 composites, the effect of PA12 transcrystallization with reorientation was confirmed for various blend compositions. The relative strength of the effect decreases with progressing bulk crystallization. Earlier presumed coexistence of isotropic and highly oriented crystallites of the same kind with drawn PBT/PEE blend was confirmed by WAXS from a synchrotron source. *Dedicated to Prof. Francisco J. Baltá Calleja on the occasion of his 65th birthday.

Structures and mechanical properties of zone‐drawn–zone‐annealed blends of cocrystallizing poly(butylene terephthalate) and a poly(ether ester)

Poly(butylene terephthalate) (PBT) and a poly(ether ester) (PEE) based on PBT and poly(ethylene glycol) were melt-blended and extruded as films with quenching. They were then zone-drawn (ZD) and zone-annealed (ZA) at various stresses (between 10 and 50 MPa) at temperatures of 160 and 190° C. The goal was to improve their mechanical properties relative to those of the same blend, but cold-drawn (X = 5) and isothermally annealed with fixed ends at the same temperatures for 6 h. All samples were characterized by DSC, WAXS, SAXS, and static mechanical property measurements. In contrast to the isothermally annealed samples, the zone-drawn and zone-annealed ones exhibit one population of crystallites arising from the homo-PBT, as demonstrated by the DSC and SAXS measurements. In addition, however, the WAXS photographic patterns indicate that zone annealing at 190°C results in isotropization of crystallites originating from the PEE, resulting in the formation of a microfibrillar-reinforced composite. It is assumed that some of the isotropic crystallization occurs on preexisting homo-PBT crystallites, i.e., a partial cocrystallization occurs, improving the adhesion between the components of the blend. The structural features created in the zone-drawn-zone-annealed materials result in higher values of the Youngs modulus and tensile strength in comparison to the materials receiving the simple isothermal treatment (1,200 vs. 480 MPa and 213 vs. 113 MPa, respectively).

Morphological characterization during deformation of a poly(ether ester) thermoplastic elastomer by small-angle x-ray scattering*)

The scattering behavior of pre-drawn and annealed bristles of a highly deformable poly(ether ester) themoplastic elastomer based on poly(butylene terephthalate) as hard segments and poly(ethylene glycol) as soft segments in a ratio of 57/43 wt.-% is studied. Small-angle x-ray seattering measurements with an area detector are carried out on bristles with and without application of stress up to 195% relative deformation. Two-dimensional scattering patterns are used for morphological characterization of the sample.At small deformations one morphology peak is found, corresponding to a periodicity that changes affinely with deformation. The morphology of the smaple represents assemblies of mutually parallel crystalline lamellae, positioned perpendicular to the stretching direction both under and without stress. When macrodeformation increases a second peak appears, and a four-point pattern is observed in the relaxed state. In this intermediate deformation range coexisting morphologies contribute to the scattering. Additional contributions arise from lamellae, which are inclined to the stretching direction, as well as from lamellae, which are again perpendicular to the stretching direction, as a result of microfibril relaxation and loss of interfibrillar contacts. At large deformations the latter morphology dominates and the 2D-scattering pattern again shows a two-point character. A morphological model for this behaviour is discussed, where the break of interfibrillar contacts during deformation and the inhomogeneous stress field in the sample play an important role.

Characterization of layered silicate-reinforced blends of thermoplastic starch (TPS) and poly(butylene adipate-co-terephthalate)

A two-step melt blending procedure was used to produce binary systems composed of thermoplastic starch (TPS) and poly(butylene adipate-co-terephthalate) (PBAT). To improve the properties of the blends, two different layered silicates, viz. bentonite (BT) and organically modified montmorillonite (oMMT) were incorporated. First, TPS and its layered silicate nanocomposites were prepared via extrusion compounding during which starch was plasticized with glycerol and water. In the second step, PBAT was added to TPS/layered silicate to produce blends in a batch-type mixer. Mechanical and thermal properties were determined. The blends showed acceptable ductility over 50wt.% PBAT content, although at the cost of strength and stiffness. By contrast to oMMT the BT became intercalated in TPS and TPS/PBAT blends. The reinforcing effect of BT and oMMT was most prominent for the glassy states of both TPS and TPS/PBAT blends. Thermal, and thermooxidative properties were not significantly affected by the presence of layered silicates.