61st Annual Report on Research 2016

The focus of our research is on understanding at the molecular level the relationship of the surface chemistry of graphene oxide (GO) to the intermolecular forces between GO and the polymer[HCS1]. The objective is a fundamental understanding at the molecular level of how modifying GO’s surface chemistry can achieve large improvements in desired properties at very low loadings[HCS2], one part per ten thousand. Initially the focus was on characterizing the increase of GO’s C:O ratio after exposure to heat and UV light, the resulting shift from hydrophilic to hydrophobic and on intermolecular forces between GO and the polymer. Realizing the limitations of a focus on the C:O ratio, research shifted to a broader approach: functionalizing the GO surface by reacting GO’s epoxy groups with molecules compatible with polyamides, polyimides acrylic and epoxy coatings and to characterize improvement in hydrolytic degradation and water vapor transmission. Our recent experimental results show that at the molecular level properly functionalized GO nanosheets inhibit polymer backbone mobility. Evidence of this effect is the observed decrease in the rates of nucleation and crystal growth, verifying GO’s effect on polymer chain mobility and thereby hydrolysis and water vapor diffusion. Characterizing the relationship of the functional groups’ chemistry to that of the polymer and understanding at the molecular level when and why this leads to improvements in macroscopic GO-polymer properties is our fundamental goal. This year the large effect of functionalized GO versus as-produced GO on the reduction in water vapor transmission in a thermoset polyimide was published in POLYMER. We are continuing work on comparing the effect of changes in GO’s surface chemistry with the objective of increasing intermolecular forces between the GO surface and a thermoplastic polyamide, acrylic coatings and epoxy marine coatings. Using a low 60 degree temperature, we are able to react the epoxy groups on as produced GO with small molecule functional groups which are similar to the chemistry of the polymer backbone and where possible the polymer monomer itself. In the latter case the GO may become chemically bound to the polymer chain. Our second major objective is to measure the resulting intermolecular attractive forces at the nanoparticle interface between the GO’s surface chemistry and the polymer. Previously we published a paper on using atomic force microscopy (AFM) in a peeling test to measure indirectly the intermolecular forces between individual GO sheets and the polymer. Last week a paper was accepted describing an expansion of this measurement technique to measure the strength of the attractive forces between a range of widely used polymers and GO compared to reduced GO which has fewer oxygen groups on the surface. Most important, we have now begun learning how to functionalize the AFM probe tip with GO nanosheets and to directly measure the attractive force between the surface chemistry of the GO nanosheet and the polymer. After much work to develop methods to coat an AFM probe with GO and functionalizing molecules, this year’s work has been extremely encouraging. The results show that we can directly measure the attractive snap down force and adhesive removal force between GO nanosheets on the AFM probe tip and a particular polymer. Thereby using different polymers and AFM probes with different GO functionalized groups we now can measure directly nanoparticle polymer attractive forces due to interactions between: nacent AFM probes, modified probes chemically coated, and GO-coated probes with a range of polymers. This suggests with further development of this AFM method to directly measure intermolecular GO-polymer forces, we will be able to understand at the molecular level how to design the GO surface chemistry to optimize intermolecular forces and thereby enhance macroscopic properties. This sharply contrasts with the current highly inefficient “educated Edisonian” approach of functionalizing GO with a variety of molecules to try to enhance performance properties. Particularly valuable is the ability to characterize and understand at the fundamental molecular level the role of the strength of the intermolecular force between the nanosheet and the polymer on changes in macroscopic properties. Five undergraduates during the academic year and three during the summer worked with four graduate students on this project. Below is the title and abstract of the 2016 paper published in POLYMER, a paper accepted and being published in September 2016 and titles of two papers to be submitted in the coming months. The Effect of Functionalized versus Unmodified Graphene Oxide on Polyimide at Low Concentrations Graphene oxide nanoparticles produced by Tour’s method (GO) and GO functionalized with 4-4’ oxydianiline (ODAGO) are incorporated at 0.01 to 0.10 weight percent (wt%) into a polyimide (PI) made from 3,3’-benzophenonetetracarboxylic dianhydride (BTDA) and 4-4’ oxydianiline (ODA). The performance properties of these two systems GO-PI and ODAGO-PI at extremely low GO concentrations are compared. The ODAGO-PI nanocomposites performance properties are comparable to previous results citing concentrations 10 times higher and displayed significantly greater improvement than unfunctionalized GO-PI films. The 0.01 wt% ODAGO-PI film demonstrated a factor of ten decrease in water vapor permeability. The 0.10 wt% ODAGO-PI film displayed the maximum increase of 82% in Young’s modulus. The water vapor permeability results were fit to the Nielson law. The resulting aspect ratio at the lowest 0.01 wt% ODAGO concentration was 100 times that of the AFM measured value and much  lower for the ODA-PI. The aspect ratio decreased in value with increasing concentration, approaching the AFM measured value at the highest concentration. ATR-FTIR, WAXS, Ramon and Tg measurements were made to help explain these results based on the tortuosity effect combined with the nanosheets making the PI chain more rigid, decreasing chain mobility Assessing Graphene Oxide/Polymer Interfacial Interactions via Peeling Test