Mark D. Dadmun

Acetylation of Cellulose Nanowhiskers with Vinyl Acetate under Moderate Conditions

A novel and straightforward method for the surface acetylation of cellulose nanowhiskers by transesterification of vinyl acetate is proposed. The reaction of vinyl acetate with the hydroxyl groups of cellulose nanowhiskers obtained from cotton linters was examined with potassium carbonate as catalyst. Results indicate that during the first stage of the reaction, only the surface of the nanowhiskers was modified, while their dimensions and crystallinity remained unchanged. With increasing reaction time, diffusion mechanisms controlled the rate, leading to nanowhiskers with higher levels of acetylation, smaller dimensions, and lower crystallinity. In THF, a solvent of low polarity, the suspensions from modified nanowhiskers showed improved stability with increased acetylation.

Understanding the chemistry of the development of latent fingerprints by superglue fuming.

Abstract:  Cyanoacrylate fuming is a widely used forensic tool for the development of latent fingerprints, however the mechanistic details of the reaction between the fingerprint residue and the cyanoacrylate vapor are not well understood. Here the polymerization of ethyl‐cyanoacrylate vapor by sodium lactate or alanine solutions, two of the major components in fingerprint residue, has been examined by monitoring the time dependence of the mass uptake and resultant polymer molecular weight characteristics. This data provides insight into the molecular level actions in the efficient development of latent fingerprints by superglue fuming. The results show that the carboxylate moiety is the primary initiator of the polymerization process and that a basic environment inhibits chain termination while an acidic environment promotes it. The results also indicate that water cannot be the primary initiator in this forensic technique.

The nematic to isotropic transition of a liquid crystal in porous media

An experimental study of the effect of disorder on the nematic to isotropic (N–I) transition of a liquid crystal is reported. Differential scanning calorimetry is used to monitor the N–I transition of p‐azoxyanisole in a series of controlled pore glasses. An increase and a decrease in the N–I transition temperature is observed with varying pore sizes. The direction of the temperature shift depends on the pore size. The observed increase in the transition temperature can be attributed to surface induced ordering, while the observed decrease can be explained as a result of the interruption of the orientational correlation of the liquid crystalline order due to confinement in pores. These results agree, qualitatively, with Monte Carlo results showing that quenched disorder can lower the transition temperature, round and lower the heat capacity peak, and change the order of the transition.

Ternary behavior and systematic nanoscale manipulation of domain structures in P3HT/PCBM/P3HT-b-PEO films

Nanophase separation plays a critical role in the performance of donor–acceptor based organic photovoltaic (OPV) devices. Although post-fabrication annealing is often used to enhance OPV efficiency, the ability to exert precise control over phase separated domains and connectivity remains elusive. In this work, we use a diblock copolymer to systematically manipulate the domain sizes of an organic solar cell active layer at the nanoscale. More specifically, a poly(3-hexylthiophene)-b-poly(ethylene oxide) (P3HT-b-PEO) diblock copolymer with a low polydispersity index (PDI = 1.3) is added to a binary blend of P3HT and 6,6-phenyl C61-butyric acid methyl ester (PCBM) at different concentrations (0–20 wt%). Energy-filtered TEM (EFTEM) results suggest systematic changes of P3HT distribution as a function of block copolymer compatibilizer concentration and thermal annealing. X-ray scattering and microscopy techniques are used to show that prior to annealing, active layer domain sizes do not change substantially as compatibilizer is added; however after thermal annealing, the domain sizes are significantly reduced as the amount of P3HT-b-PEO compatibilizer increases. The impact of compatibilizer is further rationalized through quantum density functional theory calculations. Overall, this work demonstrates the possibility of block copolymers to systematically manipulate the nanoscale domain-structure of blends used for organic photovoltaic devices. If coupled with efficient charge transport and collection (through judicious choice of block copolymer type and composition), this approach may contribute to further optimization of OPV devices.

Enhancing the Quality of Aged Latent Fingerprints Developed by Superglue Fuming: Loss and Replenishment of Initiator

Abstract:  The recovery and identification of latent fingerprints from a crime scene are crucial to many investigations. The cyanoacrylate (superglue) fuming method (CFM), which develops fingerprints by growing a polymer coating over the print residue, is a powerful method but encounters severe limitations when prints are aged or exposed to harsh environmental conditions. We examine the aging process and how the changes that occur to a fingerprint residue over time influence the growth of polymer during development. We identify loss of initiator by erosion and degradation that, when coupled with a loss of water from the print residue, result in a decreased ability to polymerize ethylcyanoacrylate. Then, we present a methodology by which the ability of aged latent fingerprints to polymerize ethylcyanoacrylate is recovered. Two print enhancement agents, acetic acid and ammonia, are demonstrated to improve the growth of polymer from the print ridges by over an order of magnitude, while retaining the integrity of the print structure. Comparison between the two enhancement agents indicate that the enhancement occurs due to ridge coating by the ammonia or acetic acid and pH control of the latent print.

Compatibilization of poly(vinyl chloride) and polyolefin elastomer blends with multiblock/blocky chlorinated polyethylenes

Asymmetric double cantilever beam and peel test experiments were completed to evaluate the ability of chlorinated polyethylenes to compatibilize poly(vinyl chloride) (PVC) and polyolefin elastomer (POE) blends. A series of chlorinated polyethylenes that are blocky in nature (bCPEs) with varying composition (% chlorine) and molecular weight (melt index) were compared to one chlorinated polyethylene, where the chlorine is randomly distributed throughout the chain (rCPE). Results indicate that improvement in the interfacial adhesion between the PVC and the POE is dramatically more pronounced with the bCPEs than with the rCPE. In addition, the optimum bCPE composition was determined to be 20% chlorine and the interfacial adhesion force was found to increase with increasing molecular weight. Finally, the POE/CPE interaction was found to govern the ability of the chlorinated polyethylene to compatibilize PVC and POE.

Big Effect of Small Nanoparticles: A Shift in Paradigm for Polymer Nanocomposites

Polymer nanocomposites (PNCs) are important materials that are widely used in many current technologies and potentially have broader applications in the future due to their excellent property tunability, light weight, and low cost. However, expanding the limits in property enhancement remains a fundamental scientific challenge. Here, we demonstrate that well-dispersed, small (diameter ∼1.8 nm) nanoparticles with attractive interactions lead to unexpectedly large and qualitatively different changes in PNC structural dynamics in comparison to conventional nanocomposites based on particles of diameters ∼10-50 nm. At the same time, the zero-shear viscosity at high temperatures remains comparable to that of the neat polymer, thereby retaining good processability and resolving a major challenge in PNC applications. Our results suggest that the nanoparticle mobility and relatively short lifetimes of nanoparticle-polymer associations open qualitatively different horizons in the tunability of macroscopic properties in nanocomposites with a high potential for the development of advanced functional materials.

Controlling Interfacial Dynamics: Covalent Bonding versus Physical Adsorption in Polymer Nanocomposites

It is generally believed that the strength of the polymer-nanoparticle interaction controls the modification of near-interface segmental mobility in polymer nanocomposites (PNCs). However, little is known about the effect of covalent bonding on the segmental dynamics and glass transition of matrix-free polymer-grafted nanoparticles (PGNs), especially when compared to PNCs. In this article, we directly compare the static and dynamic properties of poly(2-vinylpyridine)/silica-based nanocomposites with polymer chains either physically adsorbed (PNCs) or covalently bonded (PGNs) to identical silica nanoparticles (RNP = 12.5 nm) for three different molecular weight (MW) systems. Interestingly, when the MW of the matrix is as low as 6 kg/mol (RNP/Rg = 5.4) or as high as 140 kg/mol (RNP/Rg= 1.13), both small-angle X-ray scattering and broadband dielectric spectroscopy show similar static and dynamic properties for PNCs and PGNs. However, for the intermediate MW of 18 kg/mol (RNP/Rg = 3.16), the difference between physical adsorption and covalent bonding can be clearly identified in the static and dynamic properties of the interfacial layer. We ascribe the differences in the interfacial properties of PNCs and PGNs to changes in chain stretching, as quantified by self-consistent field theory calculations. These results demonstrate that the dynamic suppression at the interface is affected by the chain stretching; that is, it depends on the anisotropy of the segmental conformations, more so than the strength of the interaction, which suggests that the interfacial dynamics can be effectively tuned by the degree of stretching-a parameter accessible from the MW or grafting density.

Unexpected molecular weight effect in polymer nanocomposites

The properties of the interfacial layer between the polymer matrix and nanoparticles largely determine the macroscopic properties of polymer nanocomposites (PNCs). Although the static thickness of the interfacial layer was found to increase with the molecular weight (MW), the influence of MW on segmental relaxation and the glass transition in this layer remains to be explored. In this Letter, we show an unexpected MW dependence of the interfacial properties in PNC with attractive polymer-nanoparticle interactions: the thickness of the interfacial layer with hindered segmental relaxation decreases as MW increases, in sharp contrast to theoretical predictions. Further analyses reveal a reduction in mass density of the interfacial layer with increasing MW, which can elucidate these unexpected dynamic effects. Our observations call for a significant revision of the current understandings of PNCs and suggest interesting ways to tailor their properties.

Correlation of polymeric compatibilizer structure to its impact on the morphology and function of P3HT:PCBM bulk heterojunctions

The impact of various polymeric compatibilizers, including end-functionalized P3HTs and diblock copolymers containing P3HT, on the structure and function of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunctions is presented. Careful analyses of small angle neutron scattering curves provide a measure of the miscibility of PCBM in P3HT, the average PCBM domain size, and the interfacial area between PCBM and the P3HT-rich phase in the uncompatibilized and compatibilized systems. Differential scanning calorimetry (DSC) also provides information regarding the changes in the crystallinity of P3HT due to the presence of the compatibilizer. Results show that most compatibilizers cause the domain sizes to decrease and the P3HT crystallinity to increase; however, some cause an increase in domain size, suggesting that they are not effective interfacial modifiers. The correlation of morphology with photovoltaic activity shows that the decreased domain size, increased crystallinity and increased interfacial area do not always result in improved power conversion efficiency (PCE). It appears that the introduction of an insulating molecule at the PCBM:P3HT interface as a compatibilizer results in a decrease in PCE. Thus, the presence of the compatibilizer at this interface dominates the photovoltaic activity, rather than the morphological control.