Session P10: Morphology Evolution and Structure-Property Relationship in Multicomponent Curing Systems

Functional bicontinuous nanostructures by in situ formation of block polymer modified thermosets

Block polymers are remarkable hybrid materials that can self-assemble on nanoscopic length scales. By controlling the composition, architecture, and molar mass, synthetic chemists can finely tune the morphologies adopted by these materials. Of the typical block polymer morphologies, bicontinuous phases such as the gyroid structure have been targeted due to their special utility in various applications that require interpenetrating domains structured on the nanoscale. While block polymers can be designed to self-assemble into such bicontinuous nanostructures, the window of thermodynamic stability is often quite narrow, and thus such structures can be difficult to experimentally access. In this presentation I will discuss the design, synthesis, self-assembly and applications of block polymer modified materials that adopt bicontinuous nanostructures through a polymerization induced microphase separation process. The enabling concept is the in situ formation of block polymers in a thermosetting system using controlled polymerizations such that their chemical synthesis and self-assembly occur in a single process. One of the important elements necessary for the adoption of bicontinuous structures is that the block polymer is crosslinked during synthesis to form a thermosetting material that results in chemical fixation of the final morphology. The central technique we utilize is controlled radical polymerization from a macromolecular chain transfer agent in the presence of a mixture of monomers that can undergo crosslinking reactions. I will discuss how we developed this approach, mechanistic consideration associated with the formation of bicontinuous structures, and applications of the resulting nanostructured materials in, for example, polymer electrolyte membranes for use in batteries and fuel cells.   

Anisotropic Bicontinuous Nanoporous Materials with Controlled Pore Sizes from Randomly End-linked Copolymer Networks.

Anisotropic nanoporous materials are of interest for a variety of applications including light-weight structural materials, filtration membranes, and photonic structures. We have studied the microphase separation of randomly end-linked copolymer networks (RECNs), as a robust method to generate anisotropic porous materials. Specifically, end-linking of telechelic acrylate terminated polystyrene (PS) and poly(D, L-lactide) (PLA) strands with a tetra-functional thiol crosslinker, followed by solvent removal, induces microphase separation into bicontinuous morphologies across a wide range of composition. Stretching these crosslinked networks above the glass transition temperatures of the constituent polymers provides a simple way to introduce anisotropy, yielding continuous but anisotropic pores following etching of the PLA phase in alkaline solution. Small angle X-ray scattering (SAXS) shows a progressive increase in anisotropy with applied strain, while both SAXS and transmission electron microscopy (TEM) suggest that the primary pore size remains unchanged, at a value determined by the molecular weight of the telechelic polymer strands.   

Structure and phase behavior of styrene-ethylene/propylene (SEP) diblock copolymer in crosslinkable acrylate monomers

Self-assembly of block copolymers in crosslinkable resins is of significance in applications such as toughening, rheology control, and encapsulation. Examples of block copolymers in acrylate resins however are limited despite their important industrial applications. In this presentation, we will describe self-assembly of a commercial block copolymer, a styrene-ethylene/propylene (SEP) diblock, in a mixture of two commonly-used commercial acrylic monomers, isobornyl acrylate (IBOA) and tricyclodecane dimethanol diacrylate (TDDA). The S block is compatible with both IBOA and TDDA, while the EP block is compatible with IBOA but not TDDA. This study demonstrated that the micellar morphology can be tuned by changing the monomer selectivity, leading to different transitions. It shows the effect of acrylate monomer selectivity on the micellar morphology of SEP diblock copolymer. With varying the monomer selectivity by increasing composition of TDDA, the predominant micellar shape changes from spheres to cylinders. The detailed micellar morphologies were characterized by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM).   

Enhanced Stress Relaxation and Reduced Cure Stress in Thermosets with Ferrocene-Based Crosslinkers

Organometallic sandwich compounds are characterized by facile isomerization among a variety of unique states. For example, ferrocene exhibits an extraordinarily low barrier to rotation of its cyclopentadienyl (Cp) ligands about the metal-Cp axis. We propose that this phenomenon can be exploited to enhance stress relaxation of polymers containing organometallic sandwich backbone moieties. Here, we describe the synthesis and characterization of several thermosets that employ ferrocene derivatives as crosslinkers. In particular, we compare a ferrocene diamine to several conventional diamines in the crosslinking of epoxy resin. Stress relaxation and dynamic mechanical analyses reveal that the ferrocene-based thermosets are distinguished from conventional thermosets by their capacity for physical relaxation. More importantly, these materials exhibit markedly different residual stress evolution during cure. For example, the cure stress in ferrocene-based thermosets drops precipitously with decreasing crosslink density. Our results highlight the unique role organometallic chemistry can play for stress management of thermosets and, more broadly, in manipulating their structure-property relationships.   

Reaction Induced Phase Separation in Multi-component Epoxy Thermosets for Large Thickness Casting

Residual stresses developed during curing of epoxy thermosets in thick section casting is a serious technical problem leading to cracks and other defects during the cure process. Residual stresses in cast thermosets can be managed by flexibilization and or toughening of the thermoset. Incorporation of disparate building blocks in an epoxy resin, composed of so-called soft-segment and hard-segments has been utilized to prepare such flexibilized thermosets that are resistant to residual stress related defects. Formulation selection can result in polymers that exhibit material behavior tunable from elastomeric behavior to a plastic-plastic behavior with high glass transition temperatures, which arises from a range of morphologies achieved during cure induced phase separations. The multi-phase morphology also results in higher glass transition temperatures and novel cure profiles. Modification of the soft-segment components in the resin has been used to tune the mechanical behavior of the thermosets.   

Understanding Ion Transport in Epoxy-based Polymer Electrolyte.

We prepare epoxy-based networked polymer electrolytes including Li salts with either ionic liquids or plastic crystals. The epoxy resins are particularly attractive as polymer matrices for solid polymer electrolytes due to their high mechanical performance combined with good adhesive properties. The selected electrolyte components are allowed to boost ionic conductivity owing to solvating the Li cation and plasticizing the epoxy matrix. As a result, the curing of a homogeneous mixture of epoxy and electrolyte can generate a two-phase system in which the epoxy phase is selected to provide mechanical strength and the electrolyte phase is selected to maximize ionic conductivity. Here, we conduct an investigation of the effect of electrolyte types and their concentration on the conductometric, dielectric and rheological properties of epoxy-based networked polymer electrolytes, using dielectric relaxation spectroscopy and oscillatory shear. These results are complemented by morphology studies in order to understand structure-property relations. Our study leads to insight regarding optimal design of multifunctional electrolytes for energy storage devices.   

Recyclable crosslinked polymer networks with full property recovery made via one-step controlled radical polymerization

Rubber tires illustrate well the issues ranging from economic loss to environmental problems and sustainability issues that arise with spent, covalently crosslinked polymers. A nitroxide-mediated polymerization (NMP) strategy has been developed (see Adv. Mater. 2016, 28. 6746) that allows for one-step synthesis of recyclable crosslinked polymers from monomers or polymers that contain carbon-carbon double bonds amenable to radical polymerization. Resulting materials possess dynamic alkoxyamine crosslinks that undergo reversible decrosslinking as a function of temperature. Using polybutadiene as starting material, and styrene, an appropriate nitroxide molecule and bifunctional initiator for initial crosslinking, a model for tire rubber can be produced by reaction at temperatures comparable to those employed in tire molding. Upon cooling, the crosslinks are made permanent due to the extraordinarily strong temperature dependence of the reverisible nitroxide capping and uncapping reaction. Based on thermomechanical property characterization, when the original crosslinked model rubber is chopped into bits and remolded in the melt state, a well-consolidated material is obtained which exhibits full recovery of properties reflecting crosslink density after multiple recycling steps.   

Impact of x-Linkable Polymer Blends on Phase Morphology and Adhesion

Adhesion to dissimilar substrate is highly important to multiple industrial applications such as automotive adhesives, food packaging, transportation etc. Adhesive design has to include components that are affinity to both substrates, e.g. high surface energy polar and low surface non-polar substrates. Typically, these adhesive components are thermodynamically incompatible with each other, leading to macrophase separation and thus adhesive failure. By using functional adhesive components plus some additives, the adhesive can be in-situ cross-linked to prevent the macrophase separation with controlled phase morphology. Herein, we present the study on a cross-linkable adhesive formulation consisting of acrylic emulsion and polyolefin aqueous dispersion with additives for enhancing cross-linking and controlled phase morphologies. Contact angle measurement and ATR-IR spectroscopy are used to characterize the properties of adhesive surface. DMA is used to study the mechanical property of adhesive before and after cross-linking. The detailed phase morphologies are revealed by AFM, SEM and TEM. The resulting adhesive morphologies are correlated with the adhesive performance to establish structure-property relationship.   

Synthesis, structure and properties of highly elastic poly(dimethylsiloxane)/graphene oxide composite elastomer membranes

This study illustrates that amine functional groups on the ends of telechelic poly(dimethylsiloxane) (PDMS) can undergo post-processing reactions with surface epoxy groups on graphene oxide (GO) to form a robust elastomer during simple heating. In these materials, GO acts as a nanofiller that reinforces the mechanical properties and participates as a multifunctional crosslinker that promotes elastic properties. Experiments indicate that the telechelic PDMS/GO elastomer is highly crosslinked (e.g., more than 75 wt \% is a non-dissolving crosslinked gel) but highly flexible such that it can be stretched up to 300\% of its original length. After processing these materials into membranes, the permeability for some common gases was studied as a function of GO concentration. Due to the macromolecular network and tortuous pathways formed during the curing reaction, factor of two enhancements in gas selectivities were observed for CO2/N2 and CO2/CH4 compared to neat PDMS membranes. Considering the expected thermal and chemical tolerance of the PDMS/GO composite membrane detailed in this work suggests these membranes could be useful in applications such as post-combustion CO2 capture, CO2 removal from natural gas and in other industries that use or process CO2.   

Polymeric Helical Microactuators: Achieving High Speed and High Extension.

We proposed novel concepts of actuation mechanism and fabrication method to induce fast and highly extensional motion in polymeric actuators. Exceptionally high extensional strain over 1400{\%} was realized by the geometric transformation from a helix to a stretched ribbon in a responsive and reversible manner. As a model system, sub-micron sized helices were fabricated from a self-assembled, UV-cured, thermo-responsive polymer. By controlling the size and the geometrical asymmetries in the system, a combined effect of surface tension and differential volumetric strain during the transition between swollen-deswollen state induced by phase transition of the material was successfully utilized to generate torsional stresses in the system. For the full understanding of the results, a finite element analysis and measurements on the poroelastic properties of the material were conducted as well as a demonstration on macroscopic system made with PDMS. Finally, remaining questions on the chirality of helices were presented.   

Effect of Cross-Linking on Free Volume Properties of PEG Based Thiol-Ene Networks

According to the Fox and Loshaek theory, in elastomeric networks, free volume decreases linearly with the cross-link density increase. The aim of this study is to show whether the poly(ethylene glycol) (PEG) based multicomponent thiol-ene elastomeric networks demonstrate this model behavior? Networks with a broad cross-link density range were prepared by changing the ratio of the trithiol crosslinker to PEG dithiol and then UV cured with PEG diene while maintaining 1:1 thiol:ene stoichiometry. Pressure-volume-temperature (PVT) data of the networks was generated from the high pressure dilatometry experiments which was fit using the Simha-Somcynsky Equation-of-State analysis to obtain the fractional free volume of the networks. Using Positron Annihilation Lifetime Spectroscopy (PALS) analysis, the average free volume hole size of the networks was also quantified. The fractional free volume and the average free volume hole size showed a linear change with the cross-link density confirming that the Fox and Loshaek theory can be applied to this multicomponent system. Gas diffusivities of the networks showed a good correlation with free volume. A free volume based model was developed to describe the gas diffusivity trends as a function of cross-link density.