Changes In Crosslink Density Research Articles

Scanning acoustic microscopy: a tool for measuring crosslinking gradients in photopolymer materials

The crosslinking gradients in photopolymer material having a curved surface can be determined by using acoustic techniques. The reflection coefficient at the liquid/material interfaces measured by low frequency microechography is related to the mechanical properties of the material and also the profile of the surface. This method makes it possible to take away the effect of relief and local slope and easily to obtain a map of the mechanical properties, in particular of crosslinking density changes and a topography of the surface.

脂肪酸アミドのブルーム挙動に関する研究 （第３報） ステアリン酸アミドのブルームにおける自由エネルギー勾配論の適合性

Suitability of the free energy slope theory in the blooming of stearamide (SA) from NBR samples have been investigated for three important factors such as concentration, the pressure to SA particles (elastic modulus in NBR samples) and abusolute temperature (T). In this theory, a parabolic law must applies between those factors and blooming rate. The parabolic law did not apply between blooming rate and concentration factors such as the amount of SA particles condensed, the diffusion coefficient of SA, the solubility of SA and the volume fraction of SA in NBR samples. This shows that the concentration factors do not suited the free energy slope theory at all. The external and internal pressure to SA particles in NBR samples was given by two different methods, the elongation of samples and the change of crosslinking density. The former did not suit at all to this theory because the blooming rate decreased with the elongation ratios. The latter was concluded not to suit to this theory as a whole though the parabolic law apply partially in the relation between crosslinking density and blooming rate. In the relation between the blooming rate and 1/T, the parabolic law was not applied at all. The activation energies in the blooming of SA from NBR samples were 46.6kJ mol-1 for 18%AN-NBR and 48.3kJ mol-1 for 18%AN-NBR. From above results, the blooming of SA was concluded not to suit the free energy slope theory.

Low-frequency microechographic characterization of crosslinking gradients and surface profiles in photopolymer materials

The crosslinking gradients in a photopolymer material with a curved surface can be determined using acoustic techniques. The reflection coefficient at the liquid/material interface measured by low-frequency microechography is related to the mechanical properties of the material and also to the profile of the surface. This method makes it possible to remove the effect of relief and local slope and to draw up a map of mechanical properties, in particular of crosslinking density changes.

NMR Imaging of Elastomers: A Review

Abstract From curiosity driven investigations about 10 years ago NMR imaging of materials has developed into a useful tool for characterization of polymers, in particular of elastomer products. A clear indicator of this development is the increasing number of imaging spectrometers in use at industrial research and quality-control laboratories. Typical applications of NMR imaging to elastomers are investigations of the homogeneity of a compound or product components like gaskets or tire profiles, studies of the aging behavior under different loading conditions, the mapping of stress, strain and temperature distributions, and the analysis of material change after application of an overload. Elastomers constitute a class of materials particularly suitable to NMR imaging, because they are rich in protons, the most sensitive, stable NMR nucleus, and the material is soft, giving rise to small dipole-dipole couplings and thus to comparatively narrow lines and favorable imaging conditions. Yet the residual dipole-dipole interaction which remains unaveraged from the thermal motion of intercrosslink chains is a most valuable source of information. It determines the relaxation times so that measurement of various relaxation parameters provides information on cross-link density, stress, strain, and temperature in rubber materials. Spectroscopic investigations on the other hand are not very informative, because they are sensitive to chemical change, which is often too small to be detectable under reasonable conditions even for chain scission or additional cross-linking during aging. The measurement of relaxation times does not require homogeneous magnetic fields, so that new NMR devices based on inexpensive, permanent magnets can be developed, which are applied to the surface of arbitrarily large elastomer products. The NMR MOUSE is such a mobile NMR scanner, which has been shown to be sensitive within limits to changes in cross-link density, strain and temperature. Thus NMR imaging is progressing from the acquisition of entire images which show distributions of material properties to the monitoring of volume-selective NMR information, which can be expected to be used not only for quality control and failure analysis but also for process control and on-line monitoring in the future.

Changes in the Macromolecular Structure of a Type I Kerogen during Maturation

The solvent swelling of a maturation series consisting of 5 Type I kerogens from the Uinta basin has been studied. The swelling of the kerogens in 10 solvents has been used to determine the solubility parameter (δ) of the samples. It is 9.5−10.0 (cal/cm3)1/2. The same data have been used to calculate the number average molecular weights between cross-links (reciprocal of the cross-link density) for the samples. The cross-linking increases during early maturation, and sharply above 80% maturation, but remains about constant from 20% to 80% maturation. Oil formation involves a reconstruction of the kerogen during which bond cleavage to form bitumen is accompanied by bond making to increase the remaining kerogen cross-link density. The change in cross-link density closely parallels a model for maturation in this basin published previously by Sweeney et al. (Am. Assoc. Pet. Geol. Bull. 1987, 71, 967−985).

Force Modulation AFM of Elastomer Blends: Morphology, Fillers and Cross-linking

In order to determine and optimize structure–property relationships in polymer blends, a knowledge of microscopic morphology and physical properties is essential. However, much of this microscopic morphology and physical property information has not been readily available using conventional characterization techniques. In this work, the capability of force modulation atomic force microscopy (AFM) for determining the detailed microstructure and physical properties of industrial polymer blends is evaluated for the first time. When combined with cryogenic facing, force modulation mapping is shown to be a very valuable microcharacterization technique that has a wide range of applicability. The modulus contrast obtained in the analysis of blends and composites allows one to distinguish polymer phases and related morphology with ease and with good specificity. This capability is demonstrated on a variety of filled and unfilled elastomer/plastic and elastomer/elastomer blends. In addition, analyses are successfully performed on a variety of blends containing isobutylene-based polymer that cannot be unambiguously characterized by electron microscopy techniques (isobutylene-based polymers rapidly degrade under electron bombardment). Force modulation mapping is also shown to be quite useful in distinguishing different types of fillers and their distributions in different polymer domains. Moreover, the capability of force modulation mapping for determining microscopic changes in cross-link density is examined. Examples of cross-linking mapping are presented for NR/BIMS, PP/EPDM and BIMS/BR blends. Finally, a semiquantitative calibration of relative force modulation amplitude vs. NR cure state is presented. All of these types of analyses are obtained without a need for the topographical contrast required by conventional AFM techniques and without the need for the complicated and time-consuming etching, staining and cryosectioning (thin films) typically required by electron microscopy techniques. © 1997 John Wiley & Sons, Ltd.

Hidrogéis de poliHEMA para reparo de defeitos da cartilagem articular: 1 - síntese e caracterização mecânica

Este trabalho visa a obtenção de hidrogéis de poli(2 hidróxi etil metacrilato) - poliHEMA com propriedades mecânicas adequadas ao uso dos mesmos no reparo de defeitos da cartilagem articular. Para tanto, duas alternativas foram estudadas: a variação da densidade de reticulação e a obtenção de blendas do tipo redes semi interpenetrantes (sIPN) de poliHEMA reticulado e diferentes polímeros como reforço. Amostras de hidrogéis foram obtidas por polimerização térmica e caracterizadas quanto à capacidade de absorção de água e de solução aquosa de NaCl 0,15 M e quanto ao comportamento mecânico, através de ensaios de fluência a indentação. Os resultados mostraram que a obtenção de blendas sIPN usando copolímero de MMA-AA como reforço é uma alternativa interessante para melhorar as propriedades mecânicas sem diminuir muito a capacidade de absorção de água dos hidrogéis.

Influence of Polyfunctional Monomers and Fillers on Radiation Vulcanization of Ethylene-Propylene Diene Rubber

Radiation vulcanization of raw EPDM in the presence of polyfunctional monomers; diallylamine, diallylsulphide, ethylene glycol dimethacrylate, NN-methylene bis-acrylamide, triallyl cyanurate, trimethyloyl propane trimethacrylate and tetramethylol methane tetraacrylate has been carried out. The efficiency of the monomers studied in enhancing the crosslinking and the change in crosslinking density has been followed up by measurement of physical and mechanical properties of the vulcanized rubber. Also, the dependence of the properties of the loaded EPDM, with white and black fillers, on the type of filler has been examined.

Changes in Crosslink Density of Structural Silicone Sealants Due to Ozone and Moisture

Abstract The changes in the crosslink density, elastically effective chains and entanglements in the network, and the molecular weights between crosslinks of three different commercially available structural silicone sealants used in glazing applications were monitored as a function of specific aging factors. The aging variables included simultaneous exposure to ozone and moisture in which the pH of the moisture was maintained at discrete levels in the acid and the alkaline range. The simultaneous effect of external loading on the performance of these sealants was also studied. Unexposed and unstressed test samples kept at ambient laboratory conditions were used as the control. Results of these weathering studies indicate that the crosslink density of the sealants will decrease during the initial period as a result of their exposure, and some surface cracking was observed. However, following this initial exposure period, the sealants showed an increase in the crosslinking activity. All three formulations ...

Fourier transform infrared and 13C‐NMR spectroscopic characterization of model epoxy networks

AbstractModel epoxy networks based on a diglycidylether of bisphenol A (DGEBA) or of butanediol (DGEBU) and cured with different mixtures of monoamines and diamines were prepared to allow changes in cross‐link density. The choice of aliphatic or aromatic amines permitted assessment of the influence of network chain flexibility. Solid state 13C‐NMR spectra showed that no secondary reactions leading to the creation of ether linkage occur during the condensation reaction. Glass transition temperatures (Tg) and the temperatures of the maximum of the exothermic cross‐linking reaction (Texo) were determined. A rectilinear increase of Tg as a function of the density of cross‐links was observed for all the systems under study. Similarly, an increase in the stiffness of the backbone units resulted in an increase in Tg. Time‐temperature‐transformation (TTT) isothermal cure diagrams were constructed and infrared kinetics were performed at 90°C. Gelation and vitrification times were shown to be dependent on the nature of the amines used to create the network structure. © 1993 John Wiley & Sons, Inc.

Monodisperse porous polymer particles: Formation of the porous structure

AbstractMonodisperse porous styrene‐divinylbenzene copolymer particles were prepared via seeded emulsion polymerization using a mixture of linear polymer (polystyrene seed) and non‐solvent as inert diluent. Experimental evidence was presented to describe the mechanism of formation of porous polymer particles during the copolymerization and solvent extraction stages, in which porosity was a consequence of phase separation in the presence of diluents. Pore structure formation was investigated by changes in copolymerization kinetics, gel content, crosslinking density, particle morphology, surface area, pore volume, and pore size distribution. The process of copolymerization was presented, based on the concepts of production, agglomeration, and fixation of the interior gel microspheres of polymer particles. A portion of linear polymer used as diluent was found to participate in the network structure while the porous matrix was built‐up. The influence of the removal of the linear polymer from the matrix pores during the solvent extraction process on the porous structure was also discussed.

Network swelling of coals

In this paper the solvent swelling technique is applied to coal residues obtained by catalytic hydrogenation, to monitor the changes produced in crosslinking density during the hydrogenation process. An attempt is made to correlate the volumetric swelling ratio with coal rank, temperature and residence time as well as with gas evolution (CO 2 and H 2S). The results indicate that for the two coals studied, the crosslinking is strongly dependent on temperature and residence time. Even though the crosslinking density is lower for the raw subbituminous coal than for the bituminous one, the former crosslinks more readily and at a lower temperature. This temperature is more suitable for monitoring the changes produced in the subbituminous coal network. The main changes in crosslinking density are produced at short residence times for both coals. The same percentage of CO 2 is evolved in both cases, but it is less influential on crosslinking in the bituminous coal than in the subbituminous one. However, the opposite is true for the evolution of H 2S.

Bapn Dose Dependence of Mature Crosslinking in Bone Matrix Collagen of Rabbit Compact Bone: Corresponding Variation of Sonic Velocity and Equatorial Diffraction Spacing

Crosslinking density in demineralized bone matrix collagen was found to depend on the beta-aminopropionitrile (BAPN) dose level for compact bone from rabbit femurs. The dependence was demonstrated for the hydroxypyridinium (HP) concentration, a mature crosslink. A more consistent dependence on BAPN dosage was observed for the fraction of the demineralized bone matrix insoluble in 0.5 M acetic acid (AIF) corresponding to the remaining crosslinked collagen. The average HP concentration in 21 week old controls was 0.24 moles HP/mole collagen which decreased to 0.13 +/- 0.04 for the same age rabbits dosed with 1 gm BAPN/kg/day for 13 wks. The comparable mean AIF values were 0.91 for controls and 0.75 for maximum dose level. Most of the effect of BAPN on crosslinking was observed at the lower dosages below 0.2 g/kg/day. On the other hand, overt osteolathyritic symptoms are seen only for BAPN dosages greater than 0.2 g/kg/day. The mean sonic plesio-velocity was previously found to decrease from 3.4 to 3.03 km/sec as the BAPN dosage was increased. A similar close relationship was discovered for the equatorial diffraction spacing in fully mineralized bone which increased from 1.235 for normal rabbit bone to 1.28 nm for maximum dose. Most of the effect on these physical properties is exhibited at the highest BAPN dosage after much of the decrease in mature crosslinking density has been observed and when the further decrease in mature crosslinking density proceeds very slowly with increased drug dosage. These observations suggest that osteolathyrism does not become manifest until practically all mature crosslinking that can be affected has been inhibited. The mineralization process apparently can be maintained in the newly laid collagen even in the presence of severe osteolathyritic conditions. Intermolecular crosslinking in bone collagen appears to play an important role in the development of bone properties whether by direct or indirect processes. Much of the effects on bone properties occur at the higher BAPN dosages where overt osteolathyrism is observed and where there seem to be only small changes in crosslinking density.

Evaluation of coal conversion by swelling measurement

This paper makes an attempt to interrelate the volumetric swelling ratio of a coal with their yields in conversion products obtained by hydroliquefaction. This work analyzes the swelling behaviour in pyridine of the residues from mild, catalytic and dry hydroliquefaction at increasing severity conditions as a tool for knowing a little more about hydrogenation mechanisms. It is carried out with two different rank coals and their corresponding chloroform-extracted residues at two temperatures and for various hydrogenation times. Solvent swelling measurements have been used to follow changes in crosslinking density due to hydrogenation. The volumetric swelling ratio is a sensitive, fast, helpful and not complex measurement. However, complementary information is required for structural conclusions.

Elastic constants of the iron oxide doped yttria-stabilized zirconia

Ultrasonic compressional and shear wave velocity measurements have been made on the (ZrO2)1−A (Y2O3)A−X (Fe2O3) X system at 5 MHz. The amounts of Fe2O3 dopant ranging from 0 up to 10 mol% were mixed with the ceramic matrix. In this paper the ultrasonic compressional and shear wave velocity measurements made on this system at room temperature are discussed. The elastic moduli, Poisson's ratio and density are found to be sensitive to the additions of the Fe2O3 dopant and they show an anomalous behaviour. A qualitative explanation of the compositional dependence of the elastic moduli and Poisson's ratio is given in terms of changes in packing density, bond strength, co-ordination number and cross-link density.

In vivo degradation of polymers : II. Change of mechanical properties and cross-link density in silicone rubber pacemaker lead insulations during long-term implantation in the human body

The results of mechanical testing, microscopic examinations and swelling of the explanted clinically used silicone rubber pacemaker lead insulations are presented. An experimental set of 100 lead insulations implanted for times ranging from 3 d to 11 yr were evaluated for change of mechanical properties and crosslink density during clinical use. The results show that the silicone rubber tubing suffers a gradual structural change due to its exposure to the physiological environment of the human body. A spiral deformation pattern due to the permanent set of the silicone rubber tubing in contact with the electrode leading wire was observed.

The influence of irradiation temperature and oxygen on crosslink formation and segment mobility in gamma-irradiated polydimethylsiloxanes

By swelling to equilibrium and esr spin probe technique the changes in crosslinking density and polymer segment mobility in γ-irradiated polydimethylsiloxanes (PDMS) were determined in dependence of irradiation atmosphere and irradiation temperature. In comparison with the polymer samples irradiated in the presence of oxygen, the samples irradiated in vacuum showed in spite of a higher degree of crosslinking a higher mobility of the polymer segments. The discrepancy can be explained taking into account that irradiation in presence of oxygen generates polar side groups: -COOH, -CHO and -OH. Dipolar interactions and hydrogen donor acceptor interactions should contribute to the decreased mobility of polymer segments in presence of these groups.

Coal macromolecular structure and reactivity

The macromolecular structure of coals will affect their reactivity in liquefaction reactions through the number of bonds which must be broken to achieve a given molecular weight reduction and through mass transport processes. Two criticisms of the macromolecular structure model, one based on solvent swelling and the other based on alkylation reactions, are shown to be unfounded. Coals undergo ready reaction with maleic anhydride at rates which are diffusion controlled. The absence of any effect of solvent swelling on these diffusion rates in two coals is discussed without resolution and data are presented on the activation energy for diffusion and the effect of reagent size. Solvent swelling measurements have been used to follow changes in cross-link density due to pyrolysis. A lignite rapidly forms new cross links concomitant with pyrolytic decomposition while cross link formation in a bituminous coal only occurs after a significant amount of tar formation.

The influence of network topology on time-crosslink density reduction

The time-crosslink density reduction principle asserts that changes in crosslink density of polymer networks are reflected as multiplicative time-scale shifts of the distribution of retardation times in the terminal zone of viscoelastic response. Dramatically differing sensitivities of these shifts to crosslink density changes have been reported. We have proposed that differences in network topology associated with crosslink density changes may be responsible for such behavior. To test this hypothesis we have carried out creep experiments on polyurethane samples where the crosslink density was varied through photodimerization of anthracene molecules which were incorporated into the networks so that specific different topological changes would result. We have found that the coupling of dangling chain ends causes relatively slight shifts in the time scale of the distribution function while shifts associated with crosslink formation are much more substantial. From these results we have concluded that network topological changes accompanying crosslink density variations have strong influences on the quantitative aspects of time-crosslink density reduction.

Effect of Glycols as Chain Extenders on the Cross-Linking Density of Microcellular Urethane Elastomers

Results are presented from an investigation of the effect of the quantity of ethanediol, 1,6-hexanediol and 1,10-decanediol, applied as microcellular chain extenders, as well as the effect of the NCO-to-OH molar ratio, on changes in the cross-linking state caused by primary and secondary bonds in microcellular elastomers. The urethane elastomers tested were prepared by the two-stage method from the Mipolur-type quasiprepolymer and polyol mixtures having varying quantities of exchangeable glycols. Also demonstrated was the differential effect of the glycols tested on changes in cross-linking by primary and secondary bonds, and changes in cross-linking density as a function of their quantity were found for ethanediol and 1,6-hexanediol. A continuous increase in the quantity of primary and secondary bonds as a function of the quantity of 1,10-decanediol was found. A change of the NCO-to-OH molar ratio within 1,10 to 1,6 gave an extreme case of the change of the cross-linking state by primary and secondary bonds, and whatever the glycol tested, the maxima appear at an NCO-to-OH molar ratio of 12 to 13.