Walter F. Schroeder

Evolution of morphologies of a PE-b-PEO block copolymer in an epoxy solvent induced by polymerization followed by crystallization-driven self-assembly of PE blocks during cooling

Polymerization-induced nanostructuration combined with crystallization-driven self-assembly was used to generate complex nanostructures in an epoxy network. A PE-b-PEO block copolymer (Mn = 1400; 50 wt% PEO), was dispersed in diglycidylether of bisphenol A (DGEBA) and homopolymerization initiated by a tertiary amine was carried out at 120 °C (above the melting temperature of PE). The plasticization produced by the miscible PEO blocks decreased the Tg of the cured matrix to values located below the crystallization temperature of PE. Therefore, crystallization-driven self-assembly of PE blocks took place during the cooling step through the rubbery region of the epoxy network. Depending on the initial amount of PE-b-PEO dispersed in DGEBA, a variety of nanostructures could be generated, such as a dispersion of disk-like micelles (6.7 nm in thickness), a concentrated dispersion of short nanoribbons (50–200 nm in length and 6.7 nm in thickness), partially stacked and oriented in space, and complex spherulitic structures composed of large stacked nanoribbons. The thickness of micellar objects was close to the theoretical value of fully extended PE chains of the block copolymer. IR spectroscopy confirmed the all-trans conformation of PE chains. Therefore, crystals were formed by interdigitated PE chains, with PEO blocks tethered at both planar interfaces in an alternating way. The way in which these complex nanostructures affect the fracture resistance or functional properties (such as shape memory) of the resulting epoxy networks has yet to be analyzed.

Unidirectional freezing as a tool for tailoring air permeability in macroporous poly(ethylene glycol)-based cross-linked networks

Unidirectional freezing followed by photopolymerization at subzero temperatures was used to obtain highly air-permeable monoliths with ordered porous structures. Scaffolds were obtained from aqueous solutions of a poly(ethylene glycol)dimethacrylate (PEGDMA) oligomer, a photosensitizer and a reducing agent. Solutions were vertically frozen in liquid nitrogen at a controlled rate to induce the oriented growth of ice crystals and then cryo-photopolymerized under blue-light irradiation. Ice crystals were finally removed under vacuum producing macroporous hydrophilic networks with aligned pores. Porosities ranged between 80 and 95%, depending on the initial concentration of PEGDMA. The influence of processing variables on the final properties of the materials was addressed, concerning particularly the effect of porosity and freezing directionality on air permeability. Compared to porous PEGDMA-based monoliths with non-aligned macropores, gas permeability was two to three times higher for oriented scaffolds at the same porosity level, a fact explained by the easier transport of gas molecules through the aligned structures. However, the role of pore orientation on gas permeability was shown to be less marked as porosity increased. The results demonstrate that the use of unidirectional freezing strongly increases the permeability of monolithic samples up to values usually required, for instance, in tissue engineering applications (higher than 2D). These findings provide new perspectives on pore design principles toward future scaffolding of polymeric cross-linked matrices.

Comparison of Lattice-Fluid Binary Parameters For Mixtures and Block Copolymers

The aim of this report is to discuss the method of determination of lattice-fluid binary interaction parameters by comparing well characterized immiscible blends and block copolymers of poly(methyl methacrylate) (PMMA) and poly(ϵ−caprolactone) (PCL). Experimental pressure-volume-temperature (PVT) data in the liquid state were correlated with the Sanchez—Lacombe (SL) equation of state with the scaling parameters for mixtures and copolymers obtained through combination rules of the characteristic parameters for the pure homopolymers. The lattice-fluid binary parameters for energy and volume were higher than those of block copolymers implying that the copolymers were more compatible due to the chemical links between the blocks. Therefore, a common parameter cannot account for both homopolymer blend and block copolymer phase behaviors based on current theory. As we were able to adjust all data of the mixtures with a single set of lattice-binary parameters and all data of the block copolymers with another single set we can conclude that both parameters did not depend on the composition for this system. This characteristic, plus the fact that the additivity law of specific volumes can be suitably applied for this system, allowed us to model the behavior of the immiscible blend with the SL equation of state. In addition, a discussion on the relationship between lattice-fluid binary parameters and the Flory–Huggins interaction parameter obtained from Leiblers theory is presented.