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  • oriented films  (3)
  • Model networks  (2)
  • Monte Carlo simulations  (2)
  • 1
    Electronic Resource
    Electronic Resource
    Polysiloxane elastomers with bimodal chain-length distributions (1994)
    Springer
    Journal of inorganic and organometallic polymers and materials 4 (1994), S. 31-44 
    Details
    ISSN: 1572-8870
    Keywords: Polysiloxane elastomers ; end-linking reactions ; bimodal networks ; ultimate strength ; maximum extensibility ; non-Gaussain elasticity ; Monte Carlo simulations ; strain-induced crystallization ; segmental orientation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Most elastomers are prepared by an adventitious, random cross-linking process and thus have a broad, unimodal distribution of network chain lengths. It is possible, however, to obtain materials of controlled chain-length distribution by restricting the reactivity of the chains to their ends, and then end linking these chains with a multi-functional reactant. The networks of this type that have proved to be of greatest interest consist of short chains end linked with long chains to yield abimodal distribution of network chain lengths. These bimodal networks have unusually high extensibility for their values of the modulus and ultimate strength, and thus considerable toughness. Most such elastometers have been prepared from chains of poly(dimethylsiloxane), by carrying out either a condensation reaction between hydroxyl-terminated chains and tetraethoxysilane, or an addition reaction between vinyl-terminated chains and a poly(methylhydrogen siloxane) oligomer. The present review discusses the preparation of such materials, the characterization of some of their properties, and the interpretation of these properties in terms of the molecular theories of rubberlike elasticity.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00684026
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  • 2
    Electronic Resource
    Electronic Resource
    Simulations on aspects of elastomer structure that provide unusual toughness (1996)
    Springer
    Journal of computer-aided materials design 3 (1996), S. 311-318 
    Details
    ISSN: 1573-4900
    Keywords: Elastomers ; Mechanical properties ; Monte Carlo simulations ; Crystallization ; Toughness ; Filler reinforcement
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Summary A variety of examples are given to illustrate how computer-modeling techniques are being used to elucidate the structures of elastomeric polymer networks, and to provide guidance on how to control this structure to maximize mechanical properties such as toughness. One topic involves generation of distribution functions of chain end-to-end distances to replace the Gaussian limit, since non-Gaussian behavior can correspond to substantial increases in modulus and ultimate strength. Attempts are also being made to simulate the structure and properties of networks having trimodal distributions of chain lengths, in hopes of getting additional improvements beyond those shown in experiments on bimodal networks. Strain-induced crystallization has always been important in the area of elastomers (since the crystallites thus generated provide substantial in situ reinforcement) and is being simulated using Monte Carlo methods to generate chains having representative sequence distributions for different polymerization conditions. The chains are then placed alongside one another to determine matched-sequence runs that could lead to the formation of crystallites. Finally, the long-standing problem of clarifying exactly how fillers reinforce elastomeric materials is being approached using Monte Carlo simulations of chains in the vicinity of impenetrable filler particles. In this way it is possible to determine how reinforcement depends on the perturbation of distributions caused by this excluded-volume effect. Important extensions would include physical adsorption and chemisorption of the chains onto the filler particles.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01185669
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  • 3
    Electronic Resource
    Electronic Resource
    A novel orientation technique for semi-rigid polymers. 2. Mechanical properties of cellulose acetate and hydroxypropylcellulose films (1994)
    Springer
    Colloid & polymer science 272 (1994), S. 393-399 
    Details
    ISSN: 1435-1536
    Keywords: Cellulose acetate ; hydroxypropylcellulose ; cross-linking ; liquid-crystalline state ; anisotropic state ; novel orientation technique ; oriented films ; mechanical properties ; maximum extensibility ; tensile modulus ; tensile strength ; birefringence
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00659450
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  • 4
    Electronic Resource
    Electronic Resource
    Elastomeric properties of end-linked networks of high cross-link functionality. Accounting for possible changes in effective functionality with extent of reaction and chain-length distribution (1994)
    Springer
    Colloid & polymer science 272 (1994), S. 504-515 
    Details
    ISSN: 1435-1536
    Keywords: Model networks ; bimodal networks ; poly(dimethylsiloxane) ; elastomers ; rubber elasticity ; mechanical properties ; elongation modulus ; stress-strain isotherms ; end-linking reactions ; Flory ; Erman theory
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract This study reanalyzes some elastomeric properties in elongation reported for poly(dimethylsiloxane) (PDMS) networks of high cross-link functionality which had been prepared by using multifunctional siloxane oligomers to end link vinyl-terminated PDMS chains. The extent of reaction of the vinyl end groupsP vi spanned the range of 0.40 to 0.95. These networks had elongation moduli that significantly exceeded the values predicted by the Flory-Erman theory, except at very low values ofP vi. Trends in their stress-strain isotherms, as characterized by the Mooney-Rivlin constants 2C 2 and the ratio 2C 2/C1, also appeared to be different from those predicted by theory. Neglected in such standard analyses, however, was the fact that the segments between cross-links along the junction precursor molecules can themselves act as short network chains, contributing to the modulus and giving a strongly bimodal distribution of both network chain lengths and cross-link functionalities. Of particular interest is the apparent change in functionality with extent of reaction and chain length distribution. The results thus obtained do suggest strong dependence of the observed values of the phantom modulus on the network chain-length distribution, particularly at very small values of the ratio of the length of the short chains to the long ones. Calculations based on recognition of these complications can be used to characterize more realistically the deformation of such networks. The results give much better agreement with experiment. Such behavior could be an important characteristic of elastomeric networks in general. Also, a preliminary attempt was made to bridge theory with experiment based on Kloczkowski, Mark, and Erman's recent theory of fluctuations of junctions in regular bimodal networks. The agreement between theory and experiment thus obtained is rather satisfactory and lends further support to assumptions that take into account the possibly bimodal nature of these high-functionality networks.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00653214
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  • 5
    Electronic Resource
    Electronic Resource
    Mechanical properties and transition temperatures of cross-linked oriented gelatin (1996)
    Springer
    Colloid & polymer science 274 (1996), S. 334-341 
    Details
    ISSN: 1435-1536
    Keywords: Gelatin ; crosslinking ; drawing ; oriented films ; dymeanie mechanical properties
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract 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 Young's modulusE, and stress at breakσ b were determined as a function of gelatin concentration during drying. An increase inE andσ b 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′, andσ b 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00654053
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  • 6
    Electronic Resource
    Electronic Resource
    Mechanical properties and transition temperatures of crosslinked-oriented gelatin (1997)
    Springer
    Colloid & polymer science 275 (1997), S. 307-314 
    Details
    ISSN: 1435-1536
    Keywords: Key words Gelatin ; crosslinking ; drawing ; oriented films ; dynamic mechanical properties ; glass transition
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract  This second part of a systematic study of the properties of crosslinked-oriented gelatin involves the effects of orientation and water content on the glass transition temperature T g and on the melting behavior. The samples were the same as those in the preceding study, and their transition temperatures were determined by both differential scanning calorimetry and dynamic mechanical thermal analysis. The crosslinked gelatin which had been room-conditioned showed two transition temperatures: the lower one was attributed to T g of the water-plasticized gelatin, and the higher one was interpreted as T g of dried gelatin superimposed by melting. A rather unusual situation arose because of the fact that the T g and melting temperatures T m (217 and 230 °C, respectively) are so similar. Using water as plasticizer not only decreases T g but produces imperfect crystallites which melt below the T g of the system. The presence of the amorphous phase in the glassy state would presumably make it essentially impossible to define a melting point or crystallization temperature in the normal manner, as an equilibrium between crystalline and amorphous phases.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s003960050087
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  • 7
    Electronic Resource
    Electronic Resource
    Some dynamic mechanical properties of unimodal and bimodal networks of poly(dimethylsiloxane) (1991)
    Springer
    Polymer bulletin 26 (1991), S. 357-362 
    Details
    ISSN: 1436-2449
    Keywords: Model networks ; bimodal networks ; dynamic mechanical properties ; poly(dimethylsiloxane)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Summary Elastomeric networks of polydimethylsiloxane prepared by end-linking chains having molecular weights in the range 18,500 to 220 g mol-1 were studied from -128 to 50°C using a Rheovibron DDV III Viscoelastometer. In the case of the unimodal networks, the glass transition temperature Tg was generally insensitive to degree of cross-linking. The intensity of the tan δ relaxation, however, increased by over an order of magnitude over the range of cross-link densities investigated. Bimodal networks prepared from mixtures of relatively long and very short PDMS chains also had values of Tg which were insensitive to degree of cross-linking. Finally, as expected, the intensities of the tan δ peak for the bimodal networks could not be explained on the basis of simple additivity of contributions from the relatively long and the very short network chains.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00587981
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