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- Lange, J, et al.
(författare)
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Gelation behaviour during chainwise crosslinking polymerisation of methacrylate resins
- 1999
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Ingår i: Polymer. - 0032-3861 .- 1873-2291. ; 40, s. 5699-5707
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Tidskriftsartikel (refereegranskat)abstract
- The gelation process in chainwise crosslinking polymerisation of methacrylate resins with average functionalities of 2.1-50 (1.05-25 methacrylate groups/molecule) was investigated using dynamic mechanical analysis. For all systems a crossover in tan delta (tan delta independent of frequency) was observed at gelation. The gel time was found to decrease with increasing functionality of the system. Within the measured frequency ranges, power law behaviour for the dynamic modulus was observed at the gel point in all systems. A value of the power law exponent n of 0.4 +/- 0.2 to 0.6 +/- 0.1 (increasing with increasing functionality of the system) was determined for gelation during reactions at T greater than or equal to T-g infinity. This trend suggests that the differences in screening between the systems dominate over the difference in fractal dimension. For reaction temperatures below T-g infinity a value of n = 0.3 +/- 01 was obtained, which was attributed to the influence of micro-vitrification. (C) 1999 Elsevier Science Ltd. All rights reserved.
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| 4. |
- Lange, J., et al.
(författare)
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Residual stress build-up in thermoset films cured above their ultimate glass transition temperature
- 1995
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Ingår i: Polymer. - 0032-3861 .- 1873-2291. ; 36:16, s. 3135-3141
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Tidskriftsartikel (refereegranskat)abstract
- The build-up of in-plane stress over time during curing and on cooling of thermoset films is investigated. The stress in a thin contracting film on a rigid substrate is analysed, and obtained as an integral of essentially the shear modulus and thickness of the curing film. Films are cured and cooled between parallel plates in a dynamic torsional rheometer, which allows dynamic shear modulus and film thickness to be monitored simultaneously. Stress predictions are compared with independent stress observations, obtained using a bilayer beam bending technique. A conventional epoxy system and a low molar mass difunctional acrylate are studied and compared. The epoxy, in agreement with the literature, exhibits no detectable stress during the curing reaction, nor during cooling to the glass transition temperature (Tg), but develops stress on cooling below Tg. The acrylate, by contrast, generates considerable stress throughout the reaction and cooling, with the major part of the stress originating above Tg. The observed stress build-up agrees well with the theoretical calculations based on the time-evolution of shear modulus and film thickness. Finally, approximate formulae for the estimation of residual stress are given. It is shown how the overall residual stress, as well as the contributions from the different parts of the cure process in a wide variety of systems, can be estimated from the mechanical and thermal properties of the polymer and the substrate.
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| 5. |
- Lange, J., et al.
(författare)
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Residual stress build-up in thermoset films cured below their ultimate glass transition temperature
- 1997
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Ingår i: Polymer. - 0032-3861. ; 38:4, s. 809-815
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Tidskriftsartikel (refereegranskat)abstract
- The stress build-up during isothermal cure below the ultimate glass transition temperature of epoxy and acrylate films is investigated in detail. Four systems are studied; two acrylates and two epoxies, with different crosslink densities. Relaxation modulus and film shrinkage are measured simultaneously during cure. The stress build-up is measured independently using a bi-layer beam bending technique. A model for the build-up of cure stresses is proposed, in which stresses are generated by the cure shrinkage and decay by viscoelastic relaxation. The relaxation is described by a simple, modified Maxwell model. Owing to the absence of memory in the Maxwell model, the resulting equation is simple and numerical stress computation straight-forward. The stress build-up over time is thus simulated for the four model systems based on the relaxation and shrinkage data, and the simulations compared with the experimentally observed stress build-up. The model successfully predicts the cure stresses where more standard elastic methods fail. It is found that the amount of stress build-up during cure varies greatly between the different systems. In general, a higher crosslink density results in higher stress build-up. The stress on cure ranged from less than 1% of the total stress on cure and cool-down in a lightly crosslinked epoxy to more than 30% of the total stress in densely crosslinked epoxies and acrylates. Finally simple approximations for estimating the stress levels after cure and cool-down from basic material properties, e.g. modulus and cure shrinkage, are proposed. © 1997 Elsevier Science Ltd. All rights reserved.
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- Marklund, S, et al.
(författare)
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Molecular basis for the dominant white phenotype in the domestic pig.
- 1998
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Ingår i: Genome research. - 1088-9051. ; 8:8, s. 826-33
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Tidskriftsartikel (refereegranskat)abstract
- The change of phenotypic traits in domestic animals and crops as a response to selective breeding mimics the much slower evolutionary change in natural populations. Here, we describe that the dominant white phenotype in domestic pigs is caused by two mutations in the KIT gene encoding the mast/stem cell growth factor receptor (MGF), one gene duplication associated with a partially dominant phenotype and a splice mutation in one of the copies leading to the fully dominant allele. The splice mutation is a G to A substitution in the first nucleotide of intron 17 and leads to skipping of exon 17. The duplication is most likely a regulatory mutation affecting KIT expression, whereas the splice mutation is expected to cause a receptor with impaired or absent tyrosine kinase activity. Immunocytochemistry showed that this variant form is expressed in 17- to 19-day-old pig embryos. Hundreds of millions of white pigs around the world are assumed to be heterozygous or homozygous for the two mutations. [The EMBL accession numbers for porcine KIT1*0101, KIT1*0202, KIT2*0202, and KIT2*0101 are AJ223228-AJ223231, respectively.]
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