No abstract

Block copolymers incorporating epoxy-reactive functionality in one block have been studied as modifiers for model epoxy resins. As observed previously with nonreactive poly(ethylethylene)-b-poly-(ethylene oxide) (OP) copolymers, reactive poly(epoxyisoprene)-b-polybutadiene (BIxn) copolymers and poly(methyl acrylate-co-glycidyl methacrylate)-b-polyisoprene (MG-I) copolymers form ordered microstructures in blends with epoxy precursors as evidenced by small-angle X-ray scattering and transmission electron microscopy. Like OP copolymers, both types of reactive copolymer remain well-dispersed within the epoxy matrix during cure. Additionally, DSC and solubility studies suggest that these copolymers can react with the curing epoxy resin to form covalent linkages between the copolymeric microstructures and the cross-linked epoxy resin. The capability of these types of copolymers to template epoxy resin is proposed to arise from a combination of (1) the miscibility of one block with the epoxy components well into the curing process and (2) the energetic barriers to nucleation and growth of copolymer-rich regions in the epoxy matrix as network formation occurs and the epoxy molecular weight diverges.

No abstract

Thermodynamic interactions between the two blocks of a block copolymer are generally understood in terms of three parameters: degree of polymerization (N), block volume fraction (f i ), and Flory-Huggins interaction parameter ( ). 1 The parameter represents the enthalpic interaction between the two dissimilar components of the copolymer and can be approximated in terms of the solubility parameters (δ i ) of the two blocks as where V is molar volume, k B is the Boltzmann constant, and δ i can be calculated from group contributions or determined experimentally. 2 The phase behaviors of many copolymer systems have been examined in detail and have been found to agree with those predicted by theory. 3-6 In these systems, N and f i are both adjustable through polymerization stoichiometry, while is predetermined by the choice of monomer pair. Recently, two examples of chemically modified polyisoprene-polystyrene (PI-PS) block copolymers with adjustable parameters have been described, where varies with the extent of PI modification. 7,8 To extend this concept to allow the use of a polydiene (PD) block as the constant block, the introduction of a chemoselective modification technique is necessary. As PDs have lower solubility parameters than PS, the large solubility difference between PD and modified PD blocks should allow access to a greater range of parameters.Living anionic polymerization of dienes permits control over repeat unit microstructure and therefore degree of double-bond substitution. 4,5,9 A two-step polymerization sequence allows the preparation of poly-(1,4-isoprene)-b-poly(1,2-butadiene) (1,4PI-1,2PB: BI). 5 As alkene reactivity depends on degree of substitution, differentiation between the triply substituted and singly substituted double bonds of BI should allow the preparation of functional block copolymers through selective transformation of one block. 10 Specifically, the chemoselectivity of epoxidation for more-substituted olefins should facilitate the preparation of reactive epoxidized copolymers from diene block copolymers. As 1,4PI and 1,2PB do not phase separate, 5 selective epoxidation of 1,4PI double bonds should allow the preparation of a series of block copolymers exhibiting a wide range of values.PDs have been epoxidized with most standard reagents for alkene epoxidation. 11-13 High selectivity for backbone double bonds in PI has been observed with all systems, but the peracid systems often lead to significant extents of undesirable side reactions. 11 Selective conversion of PB backbone double bonds has been achieved with tBuOOH/metal systems, though these methods are sensitive to oxygen and moisture. 13 As dimethyldioxirane (DMD) can be used as a mild alkene epoxidation reagent and has demonstrated chemoselectivity similar to that of peracids, 14 it was examined as a complementary means toward preparation of selectively epoxidized polydienes.Treatment of symmetric copolymer BI (I block: 94% 1,4-addition; B block: 83% 1,2-addition; M n ≈ 21 000) with DMD (generated in situ from acetone and O...

No abstract