A discussion of the molecular mechanisms of moisture transport in epoxy resins

A typical epoxy formulation can absorb several weight percent of water, seriously degrading the physical properties of the resin. In two preceding publications (Soles, C. L.; Chang, F. T.; Bolan, B. A.; Hristov, H. A.; Gidley, D. W.; Yee, A. F. J Polym Sci Part B: Polym Phys 1998, 36, 3035; Soles, C. L.; Chang, F. T.; Gidley, D. W.; Yee, A. F. J Polym Sci Part B: Polym Phys 2000, 38, 776), the role of electron density heterogeneities, or nanovoids (as measured through positron annihilation lifetime spectroscopy), in the moisture-transport process is elucidated. In this article, the influence of these nanovoids is examined in light of both the specific epoxy–water interactions and the molecular motions of the glassy state to develop a plausible picture of the moisture-transport process in an amine-cured epoxy resin. In this description, the topology (nanopores), polarity, and molecular motions act in concert to control transport. Water traverses the epoxy through the network of nanopores, which are also coincident with the polar hydroxyls and amines. In this respect, the nanopores provide access to the polar interaction sites. Furthermore, the sub- T g (glass-transition temperature) molecular motions coincident with the onset of the Β-relaxation process incorporate these polar sites and, hence, regulate the association of water with the epoxy. In effect, the kinetics of the transport mirror the dynamics associated with the local-scale motions of the Β-relaxation process, and this appears to be the rate-limiting factor in transport. The volume fraction of the nanopores does not appear to be rate-limiting in the case of an amine-cured epoxy, contrary to popular theories of transport. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 792–802, 2000Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/35006/1/16_ftp.pd

Self-Assembly of ABC Bottlebrush Triblock Terpolymers with Evidence for Looped Backbone Conformations

Bottlebrush block copolymers offer rich opportunities for the design of complex hierarchical materials. As consequences of the densely grafted molecular architecture, bottlebrush polymers can adopt highly extended backbone conformations and exhibit unique physical properties. A recent report has described the unusual phase behavior of ABC bottlebrush triblock terpolymers bearing grafted poly(dl-lactide) (PLA), polystyrene (PS), and poly(ethylene oxide) (PEO) blocks (LSO). In this work, a combination of resonant soft X-ray reflectivity (RSoXR), near-edge X-ray absorption fine structure spectroscopy (NEXAFS), and self-consistent field theory (SCFT) was used to provide insight into the phase behavior of LSO and underlying backbone chain conformations. Consistent with SCFT calculations, RSoXR measurements confirm a unique mesoscopic ACBC domain connectivity and decreasing lamellar periods (d0) with increasing backbone length of the PEO block. RSoXR and NEXAFS demonstrate an additional unusual feature of brush LSO thin films: when the overall film thickness is ∼3.25d0, the film–air interface is majority PS (>80%). Because PS is the midblock, the triblocks must adopt looping configurations at the surface, despite the preference for the backbone to be extended. This result is supported by backbone concentrations calculated through SCFT, which suggest that looping midblocks are present throughout the film. Collectively, this work provides evidence for the flexibility of the bottlebrush backbone and the consequences of low-χ block copolymer design. We propose that PEO blocks localize at the PS/PLA domain interfaces to screen the highest χ contacts in the system, driving the formation of loops. These insights introduce a potential route to overcome the intrinsic penalties to interfacial curvature imposed by the bottlebrush architecture, enabling the design of unique self-assembled materials

Using Block Copolymer Self-Assembly to Imprint the Crystallization of Polymer Dendrites

We utilize the self-assembly of cylinder-forming block copolymer (BCP) films to create templates for dendritic polymercrystallization patterns. This templating was achieved by simply spin-casting thin films from a solution containing both the BCP [polystyrene-block-poly(ethylene oxide) (PS-b-PEO)] and a homopolymer (polyethylene oxide) under controlled vapor atmosphere conditions, without the need for any additional processing (e.g. solvent or thermal annealing). The BCP first organized into a hexagonal array of vertically oriented PEO cylinders that served to template dendritic PEOhomopolymer crystals on the surface of the BCP pattern. No surface defects such as dewetting holes or macroscopically phase-separated domains were observed on top of the BCP film. We find that the PEOdendrites crystallized on this BCP template exhibit a periodic height undulation pattern on their surface. The undulation pattern directly reflects the hexagonal pattern symmetry and associated height undulations of the BCP underneath these crystals. The formation of this hierarchically organized polymercrystallization morphology illustrates how one self-assembly can be used as a template to control the organization of another self-assembly process—a fabrication strategy of potentially great significance in the programming of complex structures using self-assembly