David Uhrig

Synthesis of well-defined multigraft copolymers

This short review article focuses on significant developments made in the pursuit of well-defined graft copolymers over the past decade or so. The state-of-the-art for synthesis of the complex copolymers has been greatly advanced over this time period. Anionic polymerization techniques now allow for synthesis of narrow polydispersity multigraft copolymers with control over branch spacing, number of branch points, and branch point functionality. Other controlled/living polymerization techniques are being exploited to increase the chemical diversity of these materials. Future work in this area will focus on synthesis of more complex architectures and incorporation of three or more chemical building blocks into the materials.

Living anionic polymerization

For about 40 years living anionic polymerization has been the premier technique for the synthesis of model polymers of controlled architecture and narrow molecular weight distribution (MWD). Nowadays, despite the continuing development of new strategies for the synthesis of well-defined polymers and copolymers (e.g. group transfer polymerization, living radical polymerization, etc.), anionic polymerization continues to be the most reliable and versatile method for the synthesis of a wide variety of model polymers. The main reason for the broad utility of living anionic polymerization is that the conditions necessary for the efficient generation of polyanions that do not undergo termination or chain transfer reactions are well established for many monomers [Hsieh HL, Quirk RP. Anionic polymerization: principles and practical applications, New York: Marcel Dekker, 1996]. Despite these successes, significant challenges remain in the field of anionic polymerization. These challenges include extension of the strategies for anionic synthesis of polymers with controlled architectures (such as star and graft copolymers), further development of strategies for chain end functionalization, and mastering control over polymerization of (meth)acrylates. Recent advances in these areas are summarized in this article.

Morphological behavior of A2B2 star block copolymers

A series of A 2 B 2 four-arm, miktoarm stars of polystyrene and polyisoprene have been synthesized. The morphological behavior of these materials has been characterized using TEM, SAXS, and SANS, and was found to agree in general with the predictions of Milners theory for miktoarm star morphological behavior. One sample was found to exhibit a cylindrical morphology where lamellae were predicted; this behavior is similar to other discrepancies observed in previous studies of miktoarm star morphological behavior. A second sample, predicted to form a bicontinuous morphology, was found to exhibit hexagonally packed cylinders. The tetrafunctional junction point also results in an increase in spacing for the lamellar sample in this series over that of an AB diblock, but not as great as the increase previously observed for A 8 B 8 16-arm star 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.

Fluorinated bottlebrush polymers based on poly(trifluoroethyl methacrylate): synthesis and characterization

Bottlebrush polymers are densely grafted polymers with long side-chains attached to a linear polymeric backbone. Their unusual structures endow them with a number of unique and potentially useful properties in solution, in thin films, and in bulk. Despite the many studies of bottlebrushes that have been reported, the structure–property relationships for this class of materials are still poorly understood. In this contribution, we report the synthesis and characterization of fluorinated bottlebrush polymers based on poly(2,2,2-trifluoroethyl methacrylate). The synthesis was achieved by atom transfer radical polymerization (ATRP) using an α-bromoisobutyryl bromide functionalized norbornene initiator, followed by ring-opening metathesis polymerization (ROMP) using a third generation Grubbs’ catalyst (G3). Rheological characterization revealed that the bottlebrush polymer backbones remained unentangled as indicated by the lack of a rubbery plateau in the modulus. By tuning the size of the backbone of the bottlebrush polymers, near-spherical and elongated particles representing single brush molecular morphologies were observed in a good solvent as evidenced by TEM imaging, suggesting a semi-flexible nature of their backbones in dilute solutions. Thin films of bottlebrush polymers exhibited noticeably higher static water contact angles as compared to that of the macromonomer reaching the hydrophobic regime, where little differences were observed between each bottlebrush polymer. Further investigation by AFM revealed that the surface of the macromonomer film was relatively smooth; in contrast, the surface of bottlebrush polymers displayed certain degrees of nano-scale roughness (Rq = 0.8–2.4 nm). The enhanced hydrophobicity of these bottlebrushes likely results from the preferential enrichment of the fluorine containing end groups at the periphery of the molecules and the film surface due to the side chain crowding effect. Our results provide key information towards the design of architecturally tailored fluorinated polymers with desirable properties.

Hydrodynamics of polystyrene–polyisoprene miktoarm star copolymers in a selective and a non-selective solvent

The hydrodynamics of PSnPIn miktoarm (mixed arm) star copolymers made from polyisoprene (PI) and polystyrene (PS) arms are studied in a selective and a non-selective solvent, n-hexane and THF, respectively. It is found that in n-hexane, the number of arms affects the organization of the miktoarm copolymers: stars with 2 arms (a linear diblock for reference purposes) or 4 arms show aggregation in this selective solvent, whereas no aggregation is observed for stars with 8 and 16 arms in the concentration region studied. This behavior is due to shielding posed by the soluble blocks, which prevents the insoluble blocks from coming together. Interestingly, the contribution from aggregates observed for the two arm star (PS1PI1 diblock) at the highest concentration studied is rather small because the chains predominantly exist as single diblocks in n-hexane. This result may be due to the fact that low molecular weight PS is slightly soluble in linear hydrocarbon solvents. The hydrodynamic sizes found in THF are similar to those in n-hexane for the 2 and 4 arm stars but smaller for the 8 and 16 arms stars. We propose that this is a result of both the limited free space needed for motion of the chains and also because of an increased probability of heterocontacts between the collapsed PS blocks and the swollen PI arms near the stars core.

Insight into the interactions between pyrene and polystyrene for efficient quenching nitroaromatic explosives

A pyrene (Py) in polystyrene (PS) matrix shows rapid fluorescence quenching in the presence of 2,4-dinitrotoluene (2,4-DNT). The fluorescence quenching does not occur in the absence of the PS matrix, implying that the molecular architecture of the PS chain is critical. However, the function of the PS matrix is not well understood. Here, we investigate various Py/PS binary thin films containing PS of diverse molecular architecture (i.e., linear, centipede and 4-arm star) and molecular weight (i.e., 2.5, 35, 192, 350 and 900 kDa) to understand the effects of these molecular descriptors on the fluorescence quenching. The findings suggest that the electron-rich nature of Py/PS facilitates the photoinduced electron transfer from Py/PS to the electron-deficient 2,4-DNT, resulting in effective quenching of Py excimers. Moreover, cyclic voltammetry (CV) and UV-vis absorption verified that PS reduces the lowest unoccupied molecular orbital level of Py, promoting the Py excimer quenching efficiency in the presence of nitroaromatic molecules. The quenching process is found to be independent of molecular architecture and molecular weight, suggesting that energy migration along the PS backbone may not be the key mechanism. This simple and concise concept provide the insight into the selection for highly-efficient sensing materials.

Combatting ionic aggregation using dielectric forces—combining modeling/simulation and experimental results to explain end-capping of primary amine functionalized polystyrene

Chain-end functionalization of living poly(styryl)lithium using 1-(3-bromopropyl)-2,2,5,5-tetramethyl-1-aza-2,5-disilacyclo-pentane (BTDP) to generate primary amine end-functionalized polystyrene was investigated using high vacuum anionic polymerization techniques. 13C NMR spectroscopy and Matrix Assisted Laser Desorption Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF MS) were used to evaluate polymer end-groups and demonstrated that quantitative amine functionalized polymer was attained under appropriate reaction conditions. In general, the polymerization of styrene was conducted in benzene and the end-capping reaction was performed by adding tetrahydrofuran (THF) to the reaction prior to the addition of BTDP in THF at room temperature. Results indicated that approximately 20% THF by volume is required to obtain 100% end-capping free from side reactions. When too little or no THF was present, side reactions such as lithium halogen exchange followed by Wurtz coupling resulted in unfunctionalized head-to-head dimer as well as other byproducts. Modeling and simulation of the solvent effects using hybrid methods (the so-called QM/MM method) suggest that THF effectively dissociated the anionic chain-end aggregation, thereby resulting in the desired primary amine functionalized polymer. Molecular dynamics (MD) simulations were conducted to develop an understanding of the physics of counterions involved in the end-functionalization process.

Aligned Carbon Nanotube Polymer Composites

Carbon nanotube (CNT) polymer nanocomposites are promising new materials with a unique combination of mechanical and transport properties. As physical properties such as mechanical behavior, dielectric relaxation, thermal and electrical conductivities depend strongly on morphological structures of composites, we illustrate in this study the structure/property relationship in vertically aligned CNT (VACNT)/polymer nanocomposites. We have prepared VACNT/polystyrene composites and characterized their morphologies and properties. We have reported previously the continuous variation of alignment order along the height of CNT, which remain unaltered upon forming composites as revealed by small angle neutron scattering (SANS). Nanoindentation shows that both the elastic modulus and hardness vary along the CNT growth direction due to the varying tube density, alignment order and entanglement.Copyright