Elastic modulus and thermal conductivity of ultradrawn polyethylene

The axial and transverse Young's modulus and thermal conductivity of gel and single crystal mat polyethylene with draw ratios λ = 1–350 have been measured from 160 to 360 K. The axial Young's modulus increases sharply with increasing λ, whereas the transverse modulus shows a slight decrease. The thermal conductivity exhibits a similar behavior. At λ = 350, the axial Young's modulus and thermal conductivity are, respectively, 20% and three times higher than those of steel. For this ultradrawn material both the magnitude and the temperature dependence of the axial Young's modulus are close to those of polyethylene crystal. The high values of the axial Young's modulus and thermal conductivity arise from the presence of a large percentage (∼85%) of long needle crystals. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3359–3367, 1999     

Elastic modulus of the crystalline regions of cellulose polymorphs

The elastic modulus E_l of the crystalline regions of cellulose polymorphs in the direction parallel to the chain axis was measured by x-ray diffraction. The E_l values of cellulose I, II, III_I, III_II, and IV_I were 138, 88, 87, 58, 75 GPa, respectively. This indicates that the skeletons of these polymorphs are completely different from each other in the mechanical point of view. The crystal transition induces a skeletal contraction accompanied by a change in intramolecular hydrogen bonds, which is considered to result in a drastic change in the E_l value of the cellulose polymorphs. © 1995 John Wiley & Sons, Inc.     

Elastic modulus of the crystalline regions of chitin and chitosan

The elastic moduli E_l of the crystalline regions of α‐chitin and chitosan in the direction parallel to the chain axis were measured by X‐ray diffraction. The E_l values were 41 GPa for α‐chitin, and 65 GPa for chitosan, respectively, at 20°C. The contracted skeletons of α‐chitin and chitosan are the key factor for the low E_l values compared with that (138 GPa) of cellulose I. The E_l value of α‐chitin was constant at 41 GPa both at −190°C and 150°C, which indicates that the molecular chain of α‐chitin is stable against heat within the temperature and stress range studied. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1191–1196, 1999     

Unimodal and bimodal networks of poly(dimethyl-siloxane) in pure shear

Model networks of poly(dimethylsiloxane) (PDMS) were prepared by tetrafunctionally endlinking hydroxyl-terminated chains of various molecular weights. Some networks were prepared from mixtures of chains so as to yield a bimodal distribution of network chain lengths and, in some cases, these networks were prepared in solution. The stress–strain behavior of these unimodal and bimodal networks was studied in pure shear, which was imposed by stretching a sheet of the network having a large ratio of width to length in the direction perpendicular to the width. The pure-shear moduli of both types of networks generally were found to depend markedly on strain. Stress–strain isotherms for unimodal networks prepared from chains of one or the other of two molecular weights were well interpreted using the constrained-junction model of Flory and Erman. The bimodal networks showed large increases in the pure-shear modulus at high strains which were similar to those reported for uniaxial extension and compression. Endlinking in solution decreases the modulus in general and its upturn in particular, presumably because of diminished chain-junction entangling.     

Bulk and microscopic properties of copolymer networks in mixed aqueous solvents

Amphiphilic films and hydrogels have been prepared from ethanol/water solutions containing a hydrocarbon-grafted water-soluble cellulose ether. These materials are characterized by dispersed hydrophobic microdomains which form spontaneously in the solvent due to the inherent incompatibility of the side chains with water. At low applied shear stress, the microdomains behave as temporary linkages of finite lifetimes, imparting viscoelastic properties to the networks. The molecular weight between microdomains was found to be independent of the volume fraction of polymer in the gel, and the number of linkages per backbone ranges from 22.8 ± 1.3 to 26.2 ± 1.5 over the frequency range 30–50 rad/s. The behavior of the solutions and gels was characterized using fluorescence and dynamic rheological measurements. It was demonstrated that the microdomains are capable of sequestering water-insoluble solutes. © 1992 John Wiley & Sons, Inc.     

Dried gels from linear low-density polyethylene: Morphology,thermal behavior,and mechanical properties

Dried gels of a linear low-density polyethylene cast from decalin solutions are investigated with particular attention toward structural, thermal, and mechanical properties. The number-average and weight-average molecular weights are M_n = 32,000 and M_w = 160,000. In the concentration range 1.00–0.20, the swollen gels exhibit nearly isotropic shrinkage upon drying, which is relevant to an ideal crosslinked network behavior. For the concentrations below 0.20, a strong departure from the isotropic shrinkage indicates that the chains begin to disengage from the macromolecular network owing to the dilution effect. The melting behavior of the dried gels shows that crystallization from solution improves the crystal perfection notably as concerns the more defective crystals. The concomitant decrease of the crystal thickness judged from small-angle X-ray scattering is ascribed to a reduction of the surface free energy which is consistent with the build up of regular chain-folded macroconformations. The drawability of the dried gels is considerably improved with increasing dilution as a result of the gradual disentanglement of the coils prior to the crystallization in solution. But beyond concentration 0.20, the drawability drops because of the loss of intermolecular cohesion when the chains begin to disengage from the network. The drastic change of yield behavior between the melt-crystallized and solution-crystallized samples reveals a ductile-to-brittle transition in the mechanism of failure of the crystallites at low strain. This phenomenon is related to the improvement of regular chain-folding.     

Increase in dynamic modulus of nylon 6 fiber in repeated heating and cooling cycles under sinusoidal deformation

The repeated heating and cooling cycles under sinusoidal deformation have been investigated on nylon 6 fibers. The fibers zone-drawn twice at high temperatures were used, which have a crystallinity of 52.2% and a birefringence of 59.4×10^?3. The heating and cooling cycle was performed twice at a frequency of 110 Hz over a temperature range from 0°C to 180°C and 190°C. The crystallinity and birefringence of the treated fiber were 51.7% and 60.7×10^?3, respectively, indicating almost no changes in molecular orientation and crystallinity. However, the dynamic modulus, E′, increased steadily over whole temperature range measured. Finally, the E′ value reached 21 GPa at room temperature and 10 GPa ever at 180°C. The elongation of fiber after two cycles was only about 5%. © 1993 John Wiley & Sons, Inc.     

The complex shear modulus of polymeric and small-molecule liquids

The complex shear moduli of some ordinary liquids and their dependence on the shear deformation angle have been measured by the resonance method at a frequency of 73.5 kHz. From the results obtained, it is concluded that liquids exhibit a low-frequency shear elasticity—a property unknown before—which is associated with collective interactions of liquid molecules.     

Moduli of model elastomeric networks prepared from polymer chains having a high plateau modulus,and their interpretation in terms of the constrained-chain model

High-molecular weight polybutadiene chains having approximately 47% cis-1,4 units and 45% trans-1,4 units were crosslinked through their carbon-carbon double bonds using p-bis(dimethylsilyl) benzene as crosslinking agent and chloroplatinic acid as catalyst. This particular polymer was chosen because the high plateau modulus it exhibits in the un-crosslinked state is taken to indicate large numbers of chain entanglements, and stress–strain measurements on such networks have frequently been interpreted with the assumption that the trapping of such entanglements during crosslinking should contribute significantly to their modull. It is shown in the present investigation that such results are equally well interpreted in terms of the new constrained-chain theory of rubbery elasticity. © 1993 John Wiley & Sons, Inc.     

Elastic moduli and structure of low density polyethylene

Summary Low density polyethylene film is drawn at room temperature four times the original length and subjected to thermal annealing at 60, 80, and 100 °C keeping the film length constant. Long spacing measured by SAXS increased with increasing temperature of annealing; the increase of the long spacing is presumed to be due to the decrease of the number of micelles through relaxation during the annealing. Simultaneous measurement of the changes of the long spacing and the film length by stretching is carried out and stress-extension curves are obtained. The values of the initial moduli of the long spacingE ₁ and the film lengthY are very near to each other. Elastic modulus of the crystal latticeE _c is known to be 235 GN/m² and that of the amorphous regionE _a is found to be 0.15 GN/m². When higher stress is applied than in the case of the initial modulus, the percentage of extension of film is much greater than that of the long spacing. The discrepancy is explained by the increase of the number of micelles through stress crystallization.Dedicated to Professor Dr. K. Ueberreiter on the occasion of his 70th birthday.     

Rheological properties of filled gels. Influence of filler matrix interaction

The dynamic moduli of gels filled with particles have been studied as a function of the volume fraction of dispersed particles _f (0–0.4) and of the way in which they interact with the gel matrix. Two gels of different nature were studied, viz. polyvinyl alcohol (PVA) — Congo red gels (a so-called rubber gel) and casein gels made by acidification of skimmed milk. Emulsion droplets stabilized by different macromolecules have been used as dispersed particles. If there was no interaction between the macromolecules adsorbed on the particles and the gel matrix, both the filled PVA and the filled casein gels showed a small decrease in the elastic moduli with _f , approaching the behaviour theoretically predicted for foams. In the case of interaction, the results for filled PVA gels roughly fitted the theoretical predictions, if the deformability of the emulsion droplets and the formation of an intermediate layer between the dispersed particles and the gel matrix was taken into account. The increase in the elastic moduli of the acid milk gels with _f was much greater than expected and was probably due to aggregation of the dispersed particles during gelation.     

Conductivity and the electric modulus in polymers

At relatively high temperatures and low frequencies the electrical properties of many polymers are dominated by ionic conductivity. In cases where the glass transition is obscured by conductivity, it can be revealed by treating the data in terms of the electric modulus. The magnitude of the activation energy for conductivity (24–67 kcal/mol) for Nylons 6, 66, and 6I, polybutylene terephthalate, and polycarbonate indicates that the transport of charged particles requires the motion of polymer segments.     

Structure and permeability of gels

The textures of silica gels made by two-step acid/base and acid/acid catalysis of TEOS have been examined by thermoporometry (TPM) and NMR, and their permeabilities (D) have been measured by a thermal expansion technique. Using the pore size distribution given by TPM, which includes a large proportion of macropores (30 nm), calculated values of D are seriously overestimated. We conclude that, consistent with a theoretical prediction, compliant materials such as gels undergo contraction during freezing in the calorimeter, so that most of the macropore volume reported by TPM is actually extracted from mesopores. The mesopore radius reported by TPM is underestimated by only 20%, even if 50% of the pore liquid is drained during crystallization, assuming that the change in pore radius is related to the cube root of the volume change. NMR does not distinguish macropores, because of diffusional averaging, but provides an apparent distribution that permits an accurate estimate of the permeability.     

Transient behavior of entangled polymers at high shear rates

A new model is presented for describing the time-dependent flow of entangled polymer liquids at high shear rates. The results were obtained by extending the Doi and Edwards theory to include the effect of chain stretching. This nonlinear phenomenon is predicted to occur when the product of the shear rate and longitudinal relaxation time of the polymer exceeds one. If a constant-shear-rate flow is started under these conditions, it is shown that the shear stress and the normal stress are considerably larger than that predicted by the original reptation model. We also find that both of these stresses can pass through maxima before reaching a steady state and that the times required to reach these maxima are constants independent of the shear rate. In general the new model requires the numerical solution of coupled partial differential equations. However, at the highest shear rates where reptative relaxation is no longer important, an analytical solution for the stresses is found. The results obtained here are shown to agree well with experimental data and to be an improvement over a simpler model recently proposed.     

Minimization of spatial inhomogeneity in polystyrene gels formed by free-radical mechanism

The spatial inhomogeneity in polystyrene (PS) gels has been investigated with the static light scattering technique. PS gels were prepared starting from styrene monomer and ethylene glycol dimethacrylate crosslinker in a homogeneous solution. The gel synthesis parameters varied were the crosslinker concentration, the primary chains length and the quality of the polymerization solvent. The gels were characterized by elasticity tests as well as by light scattering measurements at a gel state just after their preparation. The degree of spatial gel inhomogeneity decreased with decreasing crosslinker content, with decreasing primary chain length or, with increasing quality of the polymerization solvent. It was shown that the gel synthesis parameters varied mainly affect the distance between the pendant vinyl groups locating on the same macromolecule during the gel formation process. Increasing the distance between the pendant vinyl groups reduces the rate of the multiple crosslinking reactions so that the resulting PS gels exhibit a lesser degree of inhomogeneity.     

Large amplitude oscillatory shear studies on the strain-stiffening behavior of gelatin gels

Linear and nonlinear viscoelasticity of gelatin solutions was investigated by rheology. The dynamic mechanical properties during the sol-gel transition of gelatin followed the time-cure superposition. The fractal dimension df of the critical gel was estimated as 1.76, which indicated a loose network. A high sol fraction ws = 0.61 was evaluated from the plateau modulus by semi-empirical models. Strain-stiffening behavior was observed under large amplitude oscillatory shear(LAOS) for the gelatin gel. The strain and frequency dependence of the minimum strain modulus GM, energy dissipation Ed, and nonlinear viscoelastic parameter NE was illustrated in Pipkin diagrams and explained by the strain induced helix formation reported previously by others. The BST model described the strain-stiffening behavior of gelatin gel quite well, whereas the Gent and worm-like chain network models overestimated the strain-stiffening at large strains.     

Aging effect on the mechanical properties of hybrid gels

Sol-gel derived unsupported films and thin rods have been obtained from co-hydrolysis of triethoxysilane and methyldiethoxysilane. The materials are flexible, dense and transparent. Films and rods have been aged for different periods of time in air at room temperature. The elastic modulus has been measured by means of tensile or flexural tests. Measurements showed an increase of elastic modulus with aging time and showed different values for films and rods. The observed evolution of mechanical properties has been related to a corresponding structural modification as highlighted mainly by MAS-NMR studies. Analyses pointed out the crucial role of condensation processes and showed that the stiffness increase arises from the formation of relatively few bonds which link and constrain pre-existing mobile network regions.