Effect of Plasticizer on PVAc-d3 Dynamics using 2H Solid-State NMR

Poly(vinyl acetate) (PVAc) is an important polymer for use in many applications due to its various bulk and surface characteristics. The structure of poly(vinyl acetate) results in a relatively low glass transition temperature (Tg) and makes it important for processing and applications such as paints, adhesives, thin films and surface coatings. Poly(vinyl acetate) can also be easily plasticized, widening its range of applications. The term plasticizer refers to a species that will decrease Tg of a polymer.1 Polymer-plasticizer systems have been studied for years and continue to be of current interest. The effects of polymer-plasticizer interactions2 and plasticizer structure3 on polymer chain motions have been studied extensively, but there is still much to be learned in this area. Deuterium NMR is an excellent tool for studying the mobility of polymer chains. In this work, we report the investigation of the effect of plasticizer content on the Tg of poly(vinyl acetate) using deuterium nuclear magnetic resonance (2H NMR) spectroscopy. The methyl groups of poly(vinyl acetate) were deuterated in order to probe the mobility of their segments using the 2H NMR technique. Random orientations of the methyl groups, with respect to a magnetic field, are reflected as a powder pattern in the 2H NMR spectrum for segments with little or no motion. Segmental mobility averages the quadrupole couplings and ultimately leads to the collapse of the powder pattern into a single resonance for rubbery polymers when the motion is on the scale of or faster than the interactions.

Segmental Dynamics of Poly(Vinyl Acetate)- D₃ Absorbed on Silica by Solid State 2HNMR: Effect of Small Molecule Plasticizer

There has been significant progress in understanding the behavior of polymeric thin films on surfaces. The change in the apparent glass transition temperature (Tg) of supported thin polymer films with thickness (Tg -nanoconfinement effect) has been widely studied over the last fifteen years. While many studies focused on the thickness dependence of Tg, there are other aspects which are important to understand in a nanoconfined polymer system. For example, polymeric photoresists contain small-molecule photoactive compounds, plasticizers, and processing aids. These components interact with the polymer chains and affect the segmental mobility, which, in turn, affects the Tg - nanoconfinement effect. Plasticizers or low molecular mass diluents are often blended with polymers to increase flexibility by effectively shifting the Tg to lower temperatures. 1 the mechanism of plasticization is not understood in great detail, but the conventional model envisions a dynamic interaction between the polymer and diluent, resulting in reduced chain-chain interactions, reduced local viscosity and increased chain mobility.2 in this paper we report the effect of plasticizer on the dynamics of poly(vinyl acetate) (PVAc) chains adsorbed on silica surfaces using solid-state deuterium nuclear magnetic resonance (NMR) and modulated differential scanning calorimeter (MDSC). The deuterium NMR technique is an excellent tool to probe interfacial phenomena and study the dynamics of polymer chains on surfaces.3,4 in our lab, we have used deuterium NMR extensively for characterizing different types of polymers like poly(methyl acrylate)5,6,7 and PVAc8 on silica surfaces. The methyl groups of PVAc have been deuterated in order to probe the mobility of their segments. Random orientations of the methyl groups, with respect to the static magnetic field, result in powder patterns in the 2H NMR spectrum for segments with little or no motion. Segmental mobility averages the quadrupole couplings and ultimately leads to the collapse of the powder pattern into a single resonance for rubbery polymer

Designing new architectures for controlling solid state properties of conjugated polymers

Conjugated polymers and oligomers are great materials for use in the next generation devices namely organic field effect transistors, light emitting diodes and polymeric solar cells. Apart from having the potential for developing power-efficient, flexible, robust and inexpensive devices, conjugated polymers can also be tuned by molecular design to optimize device characteristics. One key problem for the full commercial exploitation of conjugated polymers is that the charge carrier mobility of the state-of-the-art polymer semiconductors is much lower than required for many applications. The performance of the devices is strongly dependent on the molecular structure and supermolecular assembly of the conjugated polymer chains. This thesis covers our attempts to design molecular structure to control and improve the solid state properties of conjugated polymers. The relative placement of side chains along the backbone has a great influence on the solid state ordering of conjugated polymers. Poly(2,5-disubstituted-1,4-phenylene ethynylene)s (PPE)s, an important class of conjugated polymers, are generally synthesized by Pd-catalyzed coupling polymerizations of appropriately substituted diiodo and diethynyl benzenes (i.e., A-A and B-B type monomers). In asymmetrically substituted PPEs, this results in an irregular substitution pattern of the side chains along the polymer backbone. We report a new synthetic approach to prepare regioregular unsymmetrically substituted PPEs by polymerization of 4-iodophenylacetylenes (i.e., A-B type monomer). We provide a detailed discussion of various approaches to the synthesis of PPEs with different regioregularities and provide a description of the differences between regioregular and regiorandom analogs. The effect of regioregularity becomes even more important when the two side chains are very dissimilar or amphiphilic. We explore the effect of relative placement hydrophobic (dodecyloxy) / hydrophilic (tri(ethylene glycol) and hydrophobic (dodecyloxy)/fluorophilic (fluoroalkyl) side chains along the poly(1,4-phenylene ethynylene) backbone. We found that the regioregular substitution of the polymer backbone provides a structure in which the side chains segregate to afford a Janus-type structure. The regioregular polymer chains pack more densely in a monolayer at the air-water interface, and pack into a bilayer in the solid state to form a highly crystalline material. Pentacenes are very important organic molecules for use as semiconductor in oFETs due to their low band gap and high field effect mobility. One approach to reduce the bandgap of a polymeric system and improve performance is to include low bandgap small molecules into the conjugated backbone. A new copolymer system consisting of pentacene and terthiophene was developed and its optical and electronic properties along with its stability were evaluated. We report the use of ultrasonication of P3HT as a novel operationally-simple process to significantly improve the field effect mobility of P3HT-based FETs, thereby potentially eliminating the need for dielectric surface modifications or further processing of the device. Investigation of the sonicated polymer samples by number of characterization techniques indicates that ultrasonication leads to aggregation and ordering of the P3HT chains resulting in increase in the mobility.Ph.D.Committee Chair: Collard, David; Committee Member: Beckham, Haskell; Committee Member: Griffin, Anselm; Committee Member: Liotta, Charles; Committee Member: Tolbert, Lare

Effect of plasticizer on segmental dynamics of bulk and adsorbed poly(vinyl acetate)-d₃

Ts work consists of three topics. The first part deals with deuterium nuclear magnetic resonance (²H NMR) study of effect of small molecule plasticizers on deuterium labeled bulk poly(vinyl acetate) (PVAc-d₃). The segmental dynamics of the PVAc-plasticizer system was studied as a function of temperature at different plasticized amounts. The system was also studied with modulated differential scanning calorimetry (MDSC). The second part is concerned with the effect of plasticizers on PVAc-d₃ adsorbed on silica surfaces. The adsorbed polymers were studied using ²H NMR and MDSC for different adsorbed amounts and different plasticizer contents as a function of temperature. The spectra obtained from the ²H NMR for both the above parts i.e. for bulk and adsorbed samples was simulated and fitted to understand the segmental dynamics of PVAc in detail. The third part investigates the decomposition of 2,2\u27-azobis(isobutyronitrile) (AIBN) in emulsion-gels and silica emulsion-gels for room temperature polymerization. This study involved the use of UV- spectrometer for studying the decomposition of AIBN as a function of time --Introduction, lead 1

Segmental Dynamics of Bulk Poly(vinyl Acetate)-d₃ by Solid-state ²H NMR: Effect of Small Molecule Plasticizer

The effect of dipropyleneglycol dibenzoate, a plasticizer, on the glass-transition temperature (Tg) of poly(vinyl acetate) was studied using deuterium solid-state NMR and modulated differential scanning calorimetry (MDSC) from 0 to 20% plasticizer content. Quadrupole-echo 2H NMR spectra were obtained for methyl deuterated PVAc-d3 samples with different plasticized amounts. The Tgʼs of different plasticized samples were determined from NMR as the temperatures at which the deuterium powder patterns collapsed. It was found that the Tgʼs decreased by approximately 6 °C for every 5% increment in the plasticizer content and that the trends in the NMR-determined Tgʼs, that is, Tg(NMR), were consistent with those determined by modulated differential scanning calorimetry (MDSC). The Tg(NMR) values were about 36 °C above those of the Tg(DSC) values. This difference in the Tgʼs was due to the different time scales of the two experiments which could be accounted for on the basis of time−temperature superposition principles. The experimental NMR line shapes were fitted using a set of simulated spectra generated from the MXQET simulation program. The spectra were based on a model of nearest-neighbor jumps on a truncated icosahedron (soccer ball). The resulting average correlation times were also found to fit a time−temperature superposition with the same parameter. While the Tg was decreased by the amount of plasticizer, it was found that the breadth of the transitions from either the NMR line shapes or the MDSC thermograms did not seem to change much with the amount of added plasticizer

Plasticization of Adsorbed Poly(vinyl Acetate) on Silica by Deuterium Solid-state NMR

Deuterium nuclear magnetic resonance spectroscopy and temperature modulated differential scanning calorimetry (TMDSC) were used to probe the segmental dynamics of methyl-labeled polyvinyl acetate)-d3 (PVAc-d3) adsorbed on Cab-O-Sil silica in the presence and absence of a plasticizer, dipropyleneglycol dibenzoate. Unlike the effect of this plasticizer on the bulk polymer, where the reduction in the glass transition temperature (T g), was proportional to the amount of plasticizer added, the effectiveness of the plasticizer on the adsorbed polymer depended on the amount of polymer adsorbed. For samples with very small amounts of adsorbed polymer (i.e., 0.81 mg/m2), there was little or no effect of plasticizer on the dynamics of the adsorbed polymer. For samples with more adsorbed polymer (i.e., 1.42 or 1.81 mg/m2), the efficiency of the plasticizer was easily measurable on a fraction of the adsorbed polymer that can be considered loosely bound. The 2H NMR line shape changes obtained from the adsorbed samples were fitted to a small jump model (based on the vertices of a “soccer-ball” shaped polyhedron) to obtain the distributions of jump rates contributing to the spectra. The same samples were also studied using TMDSC, and the results were consistent with the NMR results in terms of the effects of the plasticizer on the amount of polymer adsorbed. On the basis of the TMDSC experiments, the plasticizer tends to have a greater effect on the more loosely-bound polymer and lesser effect on a fraction that is more-tightly bound. Compared to the bulk polymer, where the glass transition range was on the order of 10 K, the range for the adsorbed polymer was more around 60 K. © 2009 American Chemical Society

Room-temperature Decomposition of 2,2\u27-Azobis(isobutyronitrile) in Emulsion Gels with and Without Silica

The decomposition of 2,2\u27-azobis(isobutyronitrile) (AIBN) at room temperature was studied in emulsion gels, with and without silica, using UV/visible spectroscopy. The emulsion gels consisted of toluene, AIBN, an aqueous solution of a surfactant (either hexadecyltrimethylammonium bromide, CTAB, or sodium dodecyl sulfate, SDS), and, in some cases, fumed silica. The AIBN compositions were determined in the opaque emulsion gels by converting them to transparent microemulsions. The decomposition rate constants for AIBN were determined to be 3.7 x 10[-8] s[-1] (±0.6 × 10[-8]s[-1]) for the emulsion gels and 10.2 x 10[-8] s[-1](±1.3 x 10[-8] s[-1]) for the silica-containing emulsion gels. These rate constants were significantly greater than that in toluene solution, which, by our technique, is below our measurement threshold (effectively 0)