A. Kloczkowski

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.

A novel orientation technique for semi-rigid polymers. 1. Preparation of cross-linked cellulose acetate and hydroxypropylcellulose films having permanent anisotropy in the swollen state

It has been predicted that unusually good mechanical properties can be obtained by drying swollen networks of semi-rigid chains while they are in the deformed state, as described in several theoretical investigations [Macromolecules,23: 5335, 5341 (1990),24: 901 (1991)]. The present investigation involves the preparation of networks of this type from cellulose acetate (CA) and hydroxypropylcellulose (HPC), in order to test these concepts. The cross-linking required to maintain anisotropy during the drying process was obtained using formaldehyde, while the polymers were in either the anisotropic or isotropic state. Control of the cross-linking was obtained by studying the effects of the concentration of formaldehyde, temperature, and reaction time.The liquid-crystalline phase separations in CA and HPC, and in their networks, were studied with cross-polarized optical microscopy. CA and HPC showed anisotropic phases in trifluoroethanol and in methanol, respectively, and under shear the HPC systems exhibited the band textures associated with macroscopic orientation. In the case of the uncross-linked polymers, this band texture disappeared shortly after shearing was discontinued. The networks prepared by cross-linking the HPC in either liquid-crystalline solutions or in isotropic solutions also showed band textures, but these textures now persisted long after removal of the shearing stress. As shown in the following paper, the extensibility required in the proposed processing technique was highest for the networks prepared in the isotropic state, suggesting that these materials should have the greatest potential for dramatic improvements in mechanical properties.

Oriented Gelatin—A New Source for High-Performance Materials

Abstract Two types of gelatin with different gel-forming capabilities (“bloom values”) were used in a novel processing technique to improve their mechanical properties when in the form of films. Networks consisting of the water-soluble gelatin chains were prepared by diisocyanate crosslinking in 2,2,2-trifluoroethanol—in which gelatin forms no collagen folds (physical junctions) at room temperature. The liquid-crystalline behavior of both the uncrosslinked gelatin and its networks were studied with regard to the effects of molecular weight and nature of the solvent. Mechanical property measurements indicated dramatic increases in tensile strength, tensile modulus, and toughness after drying the swollen films under stain, both uniaxial and equi-biaxial. The improvements in mechanical properties were determined as a function of polymer concentration at stretching, elongation during drying, and molecular weight of the gelatin. In all cases the improvements increased monotonically with the increase of elongat...

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).

A novel orientation technique for semi-rigid polymers. 2. Mechanical properties of cellulose acetate and hydroxypropylcellulose films

The networks of cellulose acetate and hydroxypropylcellulose prepared in the first part of this investigation were studied with regard to their mechanical properties. The quantities of particular interest were increases in tensile modulus and tensile strength obtained by drying the swollen films under strain, both uniaxial and equi-biaxial. These increases or improvements in mechanical properties were determined as a function of polymer concentration during cross-linking, polymer molecular weight, degree of cross-linking, and elongation during drying. In all cases, the improvements increased with increase in elongation during drying, and the largest increases were obtained in the case of the highest molecular weight polymer which had been lightly cross-linked in dilute (isotropic) solutions. The extent of ordering in these systems was gauged approximately by measurements of birefringence, which were correlated with their tensile moduli and tensile strengths.

Structural and mechanical studies of a blend of poly(butylene terephthalate) and poly(ether ester) based on poly(butylene terephthalate) and poly(ethylene glycol)

Poly(butylene terephthalate) (PBT), poly(ether ester) (PEE) based on PBT and poly(ethylene glycol) (PEG) (PBTPEG = 4951wt%) and their blend are extruded with quenching, drawn ×5 and annealed together with undrawn samples at 170, 180, 190 and 200°C for 6 h in vacuum. All samples are characterized by differential scanning calorimetry (d.s.c.), small-angle and wide-angle X-ray scattering, dynamic mechanical thermal analysis and static mechanical measurements. D.s.c. and X-ray results show the absence of complete cocrystallization of homo-PBT and PBT from PEE in the blend in accordance with other reports. The drawn blend annealed at 200°C reveals much higher (by 30°C) Tm than the PEE component (4951). Also the mechanical properties of the blend are in between those of the components (homo-PBT and PEE 4951) but improved compared to those of PEE with the same chemical composition (PEE 7525). Both observations are explained by partial cocrystallization, i.e. formation of crystallites consisting of two populations of crystallites differing in their size, perfection, origin and time of appearance. The lack of complete cocrystallization and miscibility in this particular blend as well as in other cases with PEE containing less than 75 wt% PBT is explained by the insufficient length of PBT hard segments in PEE necessary for the formation of lamellar thickness typical of the crystallites of homo-PBT.

A closed form solution for the internal dynamics of polymer chains. I. Bonds with independent rotational potentials

An analytical solution to the master equation governing the conformational dynamics of linear polymer chains is formulated. Symmetric chains with N bonds subject to independent rotational potentials are considered. The eigenvalues of the transition rate matrix, which characterize the frequencies of the various relaxation modes, and the corresponding eigenvectors and eigenrows are obtained in closed form. A simple recurrence equation permits one to express the eigenvalues of the N‐bond motion in terms of the nonzero eigenvalues associated with the isomeric transitions of single bonds. This leads to a clear understanding of the increase in conformational mobility with N.

Novel orientation techniques for the preparation of high-performance materials from semi-flexible polymers such as the cellulosics

Abstract The materials being investigated are prepared by the following sequence of steps: (i) identifying polymer chains of sufficient stiffness to give liquid-crystalline, anisotropic phases (either homopolymers, or block copolymers consisting of stiff and flexible sequences), (ii) cross linking the chains, in the presence of solvent, thus conferring sufficient solidity for the polymer to remain in a deformed state for any length of time, with the solvent preventing the premature ordering of the stiff chains or sequences, (iii) deforming the swollen network uniaxially or biaxially to induce segmental orientation, and (iv) removing the solvent, at constant length or at constant force, causing a first-order transition, and thus yielding a single-phase , homogeneous, and highly-ordered material. Although the cellulosics, starch derivatives and poly(γ-benzyl-L-glutamate) are of particular interest because of their biodegradability, experiments are also being carried out on Kevlar®, poly( n -alkyl isocyanates), poly(benzobisoxazoles), and poly(benzobisthiazoles). In the case of high-temperature polymers, this method would represent an alternative to high-temperature heat treatments, which have some disadvantages. In the case of less stable polymers, such as the cellulosics, it would be the only way to achieve this uniform ordering, and would thus represent a uniquely new processing technique.

Theory of Strain-Induced Crystallization in Real Elastomeric Networks

It has frequently been observed that some elastomeric networks show an abrupt increase in the nominal stress, and consequently the modulus [f*], at high elongations.1 Molecular interpretation of this upturn has generally been attributed to strain-induced crystallization.

Networks with Semi-Flexible Chains

Classical theories of rubber elasticity1 are based on the flexible-chain model. A flexible chain may be classified as one with a characteristic ratio of the order of unity. The elasticity of the network is primarily of intramolecular origin arising from the entropy of the individual chains. Intermolecular contributions are of secondary importance. The phantom network model in which the chains do not experience any interaction with their neighbors seems to be a good firstorder approximation for real networks that consist of flexible chains. Recently, a large body of experimental work has been reported on networks made by crosslinking semi-flexible or semi-rigid chains. Stress-strain, swelling and birefringence measurements on these networks show significant deviations from the predictions of the classical network model. Among these networks are those prepared from aromatic polyamide chains2,3 from cellulose and amylose4–6 and from side-chain and main-chain liquid-crystalline systems.7–11 The chains constituting these networks have characteristic ratios which are several orders of magnitude larger than those of classical flexible chains. The networks are marked with very high degree of segmental orientability under macroscopic deformation and a discontinuous stress-strain behavior indicating a phase transition under external stress. These experimental observations can not be predicted by the classical network theories. Instead, a theory recognizing the reduced flexibility of these semi-rigid chains is required.