Toughening by nanostructure

Block copolymer modified epoxy resins have generated significant interest since it was demonstrated that the combination could lead to nanostructured thermosets through self-assembly. Over moderate to high polymer concentration the system behaves as expected for a block copolymer in a solvent selective for one block. Two types of copolymers have been studied: non-reactive and reactive modifiers. Morphologies such as copolymer vesicle and spherical/wormlike micelles can be formed under the appropriate conditions. The enhancement of the modified thermosets' mechanical properties depends on the morphology adopted by the polymers. Besides improving mechanical properties, the morphology was found to also have an effect on the glass transition in the studied systems. In this review we collect the available data on the block copolymers used to fabricate nanostructured epoxy resins and critically appraise the properties reported

Structural Variation and Chemical Performance—A Study of the Effects of Chemical Structure upon Epoxy Network Chemical Performance

Epoxy resins are used widely as protective coatings, in a wide range of harsh chemical environments. This work explores the influence of subtle structural variation in both epoxy and amine monomers upon chemical performance of cured networks, whether changing molecular geometry, the nature of the chemistry, or the mass between cross-linking reactive groups. To achieve this, four industrially relevant epoxy resins (two based on bisphenol A—Epikote 828 (E828) and Dow Epoxy Resin 332 (DER 332)—and two based on bisphenol F—Dow Epoxy Resin 354 (DER 354) and Araldite PY306 (PY306)) and the isomerically pure para–para-diglycidyl ether of bisphenol F (ppDGEBF) were used to explore variation caused by epoxy monomer variation. Four similar amines (meta-xylylenediamine (MXDA), para-xylylenediamine (PXDA), 1,3-bis(aminomethyl)cyclohexane (1,3-BAC), 1,4-bis(aminomethyl)cyclohexane (1,4-BAC)) were used to explore any variations caused by regioisomerism and aromaticity. Bisphenol F-based resins were found to outperform bisphenol A-based analogues, and chain extension within the epoxy component was found to be detrimental to performance. For amines, 1,3-substitution (vs 1,4) and aromaticity were both found to be beneficial to chemical performance

Early stages of crystallization in isotactic polypropylene

An experimental study of the early stages of crystallization in iPP has shown a qualitative difference between the behavior at low supercooling with that observed with a deep quench. To address previous misgivings in the limits of resolution of crystallites by wide-angle scattering, a new detector has been used that has many orders of magnitude improvements in count rate. At low degrees of undercooling there is a substantial gap between the appearance of a peak in the small-angle scattering, associated with electron density modulations, and the resolution of crystallites. This early growth in electron density has been analyzed in terms of a spinodal decomposition process and the stability limit of isotactic polypropylene determined for three different samples of varying molecular weight. The underlying physics of the early stages of crystallization are discussed and a number of scenarios eliminated; at high temperature Avrami kinetics are not observed whereas at low temperatures the structure in both the small-angle and wide-angle regimes grow contemporaneously following secondary nucleation