Gerard T. Caneba

Polymerization control through the free-radical retrograde-precipitation polymerization process

In this article, we present results of our work in a novel polymerization process [called the free-radical retrograde-precipitation polymerization (or FRRPP) process] that occurs at temperatures above the lower critical solution temperature. In this process, conversion-time plots for styrene polymerization in ether show autoacceleration at the beginning, followed by a relatively long period of reduced conversion rate starting at conversions as low as 30% and at operating temperatures way below the glass transition of the reacting system. Molecular weight and polydispersity index data also indicate early autoacceleration (in the form of overshoots in these values), whereas the latter period of slow conversion rate is accompanied by stable levels of molecular weight and polydispersity index. Polymer radical concentration measurements show an initial sharp rise, followed by an asymptotic value, even after almost all the initiator molecules have already decomposed into radicals. With end-group analyses of product polystyrene and polymer radical data, we calculate a proportion of live polymeric radicals to asymptote at levels of 80–84% of all polymeric species, even after almost all initiator molecules have already decomposed into radicals. All the data presented herein verify the postulate of a controlled polymerization mechanism for the FRRPP process. Our results have become the basis for an anti-gel effect phenomenon that is derived from prior theoretical and experimental observations, in which phenomenological diffusivities vanish at the spinodal curve of the phase envelope. The universality of this behavior in FRRPP systems is manifested from similar observations in styrene polymerization in acetone and methacrylic acid polymerization in water.

Transport phenomena aspects of the free-radical retrograde-precipitation polymerization (FRRPP) process

We have been studying free-radical polymerization that is accompanied by phase separation above the lower critical solution temperature. In the past, we have experimentally shown evidence of hot regions in the reactive system. We have also shown in the past that eventually the system exerts control over the rate of propagation as well as termination. In this work, we invoke a concept in polymer physics (the coil-to-globule transition) to help explain the mechanism of thermal trapping within the polymerization zones. The diffusivities of polymer chains at different stages in the reaction are calculated using appropriate methods. From the diffusivities, the propagation and termination rate coefficients are calculated using the Achilias-Kiparissides gel effect model. With experimental kinetic data, we then estimate rates of monomer consumption within polymer-rich particles. Using a pseudo-steady-state heat transfer model, we are able to show that interior temperatures of polymer-rich particle domains greater than about 1 mm can reach spinodal temperature values at the early stage of polymerization. Polymer-rich particle sizes are obtained from the same reactor system whereby a small amount of crosslinker is added to preserve particle morphology. This experiment indicates that even under turbulent flow conditions, relatively large particles can exist in the reactor fluid. This agrees with the physical implications of the coil-to-globule transition. However, since these particles were obtained during the period of slow conversion rate, our heat transfer calculations indicate that interior particle temperatures would be almost the same as surface temperatures. This points to an unknown radical-trapping mechanism at this stage of the polymerization process.

Studies of the polymerization of methacrylic acid via free‐radical retrograde precipitation polymerization process

In this paper, we present some new results of our work in a novel polymerization process (called the free-radical retrograde precipitation polymerization, or FRRPP, process) that occurs at temperatures above the lower critical solution temperature. Our polymerization experiments basically involve the methacrylic acid–poly(methacrylic acid)–water system. Experimental results indicate a gradual increase in conversion with time after what seemingly is the onset of phase separation. In an equivalent solution polymerization system, conversion of methacrylic acid reaches almost 100% at a much shorter time than in the FRRPP system. Molecular weights of poly(methacrylic acid) at different times for the FRRPP system are not dramatically different from those obtained in the solution system. However, the FRRPP system yields a relatively narrow molecular weight distribution at a wide range of conversion compared to that obtained in the equivalent solution system. The unique characteristics of the FRRPP process is shown in the asymptotic time behavior of the free-radical concentration compared to the decay behavior in other polymerization systems.

Surface properties of silicone-containing block-graft copolymer/polystyrene systems

Silicone-containing polymers have unique properties that make them commercially viable materials, in spite of their relatively high cost to manufacture. To expand their use profile, copolymerization with or incorporation into organic polymers has been an attractive approach. Block copolymers of organic blocks and silicone-containing blocks were shown to have specialized physical properties including low surface energies. The major drawback has been the prohibitive cost and difficulty of their manufacture. In this work, a low-cost synthesis process was used to produce the copolymers in a convenient way. The product is a copolymer of a polystyrene block with a silicone-containing block. To test the applicability of this block copolymer, we solvent-blended it with polystyrene and characterized the resulting surface of the dried solid from the blend. Contact angle measurements on the surfaces and subsequent calculations of surface free energies indicated concentration of the silicone groups on the surfaces of...

Characterization of PP/Mg(OH)2 and PP/Nanoclay Composites with Supercritical CO2 (scCO2)

In this article, supercritical carbon dioxide (scCO2) is used to form a high density microcellular foam structure to reduce the polymer use and facilitate dispersion of Mg(OH)2 and Nanoclay fillers. A twin-screw extruder system was used to predistribute the inorganic filler from the PP polymer, resulting composite PP/filler pellets. This followed by the use of a single-screw extruder wherein supercritical carbon dioxide is introduced in the formulation. Finally the resulting foam PP/filler/CO2 pellets are injection molded into test samples. The structure and properties of the composites are characterized using a scanning electron microscopy (SEM), Differential scanning calorimetry (DSC), and density measurements. Furthermore, PP/Clay/Mg(OH)2 polymer composites are subjected to examinations to obtain their yield and tensile strengths, elasticity modulus, % elongation, Izod impact strength, hardness, Heat deflection temperature (HDT), Vicat softening point and Melt flow index (MFI).

Extrusion with Carbon Dioxide (CO2) and Characterization of ABS/Mg (OH)2/Nanoclay Composites

In this paper, carbon dioxide (CO2) is used to form a high-density microcellular thermoplastic foam structure in order to reduce polymer consumption and facilitate dispersion of Mg (OH)2 and nanoclay fillers. A twin-screw extruder system was used to predistribute inorganic fillers into the ABS polymer, resulting in composite ABS/filler pellets. This is followed by the use of a single-screw extruder wherein supercritical carbon dioxide is introduced into the formulation. Finally, the resulting foam ABS/filler/CO2 pellets are injection- molded into test samples. The structure and properties of the composites are characterized using scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). Furthermore, ABS/Mg(OH)2/nanoclay polymer composite samples are tested to obtain their yield and tensile strengths, elastic moduli, yield and tensile elongations, izod impact strengths, hardness values, heat deflection temperatures (HDT), Vicat softening points, and melt flow indices (MFI). These tests reveal that for the overall reduction in the amount of polymer in the samples, material properties did not generally deteriorate and even showed improvements in some areas. Moreover, resulting injection-molded samples have been shown to possess dimensional integrity due to the continued expansion of CO2 during the molding operation.

FREE-RADICAL RETROGRADE-PRECIPITATION POLYMERIZATION: A MATHEMATICAL MODELING STUDY OF POLYMERIZATION OF STYRENE IN DIETHYL ETHER

Polymerizing a monomer above the lower critical solution temperature (LCST) of its polymer-monomer-(non)solvent mixture has demonstrated better control characteristics than conventional free-radical polymerization kinetics. Reaction kinetics of polymerization in a poor solvent are strongly influenced by heat and mass transfer properties, as understood from modeling the transport phenomena in our earlier work. The study has now been extended to model the reaction kinetics in a styrene-diethyl ether system. The model was based on the CCS model for free radical polymerization, with the modification proposed by Achilias-Kiparissides. Computer simulation results agree well with those obtained from experiments carried under similar conditions, with the onset of phase separation as the only adjustable parameter. Drawbacks of the model are lack of analysis for the effect of monomer concentration and the absence of an appropriate radical trapping mechanism.

In situ fabrication of thermoreversible microgels

We have developed a novel in situ fabrication procedure for stimuli-responsive microgels that can respond to an external trigger. Microgels were produced by devising a novel lithographically assisted, synchrotron-radiation-induced polymer synthesis strategy. Microgel structures based on N-isopropylacrylamide, a thermoreversible hydrogel were prepared using this technique. The morphology of these gels was found to be richly nanostructured and is reversibly responsive to a temperature change, as observed from small angle neutron scattering measurements. The pore size changes reversibly from 540 nm at room temperature (25/spl deg/C), to 390 nm at 45/spl deg/C. Due to their size and synthesis method, these stimuli-responsive microgels can be used as in situ sensors, actuators, micro drug delivery systems, or size-based separation units.

Synthesis of ultrafast response smart microgel structures

Hydrogels are “smart” polymers that respond to an external stimulus with a change in their physical characteristics. The response of gel microstructures, or microgels, to an external stimulus depends on their size and synthesis route. Smart microgels based on poly (N-isopropylacrylamide) and poly (methacrylic acid) were prepared by combining the aspects of x-ray lithography and a novel synchrotron-radiation-induced polymerization. The morphology of microgels prepared by this novel synthesis route was characterized by optical and atomic force microscopy to better understand their response properties. Microgels obtained from this method are in a hydrophobic state and are richly nanoporous in their morphology. Average pore size of these gel networks lies within a few hundreds of nanometers as observed from atomic force microscopy. Due to their ultrafast response, these microgel structures can be used as microtransducers that respond to a change in moisture concentration.

Low VOC Paints and Coatings

This section pertains to the implementation of emulsion polymerization of MMA in n-Heptane under conventional precipitation environment (as low as 60°C reactor operating temperature), although there is the likelihood that emulsion particles react at temperature above the lower critical solution temperature (LCST). Even though phase separation below the upper critical solution temperature (UCST) is believed to occur at the start of reaction within emulsion particles, the hour-glass phase envelope of the PMMA–MMA–n-Heptane system has been shown to provide a pathway system temperature to attain higher levels where they could well go above the LCST. The polymerization system is based on a redox initiation formulation, and the formation of block copolymer in emulsion particles is implemented in a two-stage polymerization procedure. Solvent/precipitant is removed through a stripping operation. In order to minimize residual monomer in the system at the end of the two-stage reaction run and solvent/precipitant removal step, a chase initiator solution is admixed into the reactor. Product emulsion is used as a binder for a latex paint formulation, which is applied as a film and tested for various coating properties.