Anthony F. Johnson

Ring strain and polymerizability of cyclic esters

Abstract Ring strain is one of the most important factors affecting the polymerizability of a cyclic monomer. In this work, the ring strains in γ-butyrolactone, δ-valerolactone and ϵ-caprolactone were inferred on the basis of infrared and Raman spectroscopy data. The thermodynamic factors influencing the ability to polymerize of these three lactones and other cyclic diesters in the glycolide series were studied using molecular modelling methods. The ring strain was observed to increase with increasing ring size for the three lactones studied increasing the thermodynamic scope for these monomers to polymerize. For the glycolide series, ring strain and polymerizability decreased with increasing substitution on the α-carbon.

Synthesis of well-defined graft copolymers via coupled living anionic polymerization and living ROMP

Abstract Well-characterized polystyrene macromonomers containing a norbornene unit at the chain end were prepared by capping living polystyrene with exo-5-norbornene-2-carbonyl chloride. The macromonomers were then ring opened polymerized using well-defined Schrock molybdenum initiators, Mo(N-2,6-i-Pr2C6H3)(OCMe3)2(CHR) where R is CMe3 or CMe2Ph, to produce well-defined graft copolymers. Well-characterized graft copolymers with polystyryl grafts of average degrees of polymerization 15, 22, 35, 50, 100 and 130 were successfully prepared. Ring opening metathesis polymerization (ROMP) of macromonomers with a molecular weight of 1550 ( DP =13 ) goes to completion at a macromonomer:initiator molar ratio of 200:1. Ring opening metathesis polymerization of macromonomers with molecular weights of 3500 and 4900 ( DP s of 32 and 46, respectively) also goes to completion even at comparatively high macromonomer:initiator molar ratios of 100:1. This produces relatively long polynorbornene backbone chains with relatively short polystyrene grafts. On the other hand, macromonomers with number average molecular weights of 10 200 and 13 200 ( DP s of 96 and 127) are completely polymerized only up to macromonomer:initiator molar ratios of 20 and 10:1, respectively. Beyond this stage polymerization stops without destruction of the living chain ends before complete consumption of macromonomers, giving short polynorbornene backbone chains with relatively long polystyrene grafts.

Mechanical Deformation of Polyurethanes

Abstract The mechanical properties of thermoplastic polyurethanes (TPU) depend upon their composition and the complex two‐phase morphologies, which originate from microphase separation of chains segments. In the present work, poly(ether‐urethanes) were prepared with hard segment contents from 36% to 71% by weight, by systematically varying the length of the soft‐segment macrodiol. Samples were prepared by hot pressing or solvent casting, and the resulting hard‐ and soft‐segment morphologies were characterized by using small‐angle x‐ray scattering (SAXS) and transmission electron microscopy (TEM). Environmental scanning electron microscopy (ESEM) was used to study the fracture surfaces of TPU samples. The deformation behavior of the morphology was studied in real time, by using 2‐dimensional SAXS (2D‐SAXS) at the Daresbury synchrotron radiation source. Two distinct mechanisms were identified, with the dominant mechanism in a given material dependent on the copolymer composition and the extent of microphase separation, which developed during processing.

Kinetics of polyesterification: modelling and simulation of unsaturated polyester synthesis involving 2-methyl-1,3-propanediol

Abstract The kinetics of the individual key reactions involved in the formation of unsaturated polyester resins have been studied using the very reactive glycol, 2-methyl-1,3-propanediol (MPD). This diol has been reacted in turn with maleic anhydride (MA), phthalic anhydride (PA) and isophthalic acid (IA) under isothermal conditions in the temperature range 180–210 °C and the kinetic constants for the following reactions have been obtained MA+MPD, PA+MPD, IA+MPD, MA+PA+MPD and MA+IA+MPD. The relative reactivities of MPD with MA and PA measured by monitoring the loss of carboxyl groups at 180 and 200 °C were found to be 2.26 and 1.70, respectively. At 200 °C PA is more reactive than IA (ratio approximately 1.31) in homopolyesterification. In the copolyesterifications involving PA and IA where cross catalysis can occur, the PA reacted approximately 1.25 times faster than IA at 200 °C. The differences in reactivity might be expected to have a significant effect on the coreactant sequence lengths in prepolymers formed by the concurrent reaction of PA, IA and MPD particularly at low conversions thus on the final properties of the cured resins in which they are employed. The properties of the products are not examined in this paper.

Free volume-based modelling of free radical crosslinking polymerisation of unsaturated polyesters

A new mechanistic kinetic model is presented for the cure behaviour of unsaturated polyester (UP) resins. The model is based on free radical polymerisation mechanism and the free volume concept. The quasi steady-state assumption for the free radical concentration is not used, and the decrease in initiator efficiency with conversion and radical trapping are modelled separately. The glass transition temperature of partially cured samples was measured employing differential scanning calorimetry (DSC) in conjunction with dynamic mechanical analysis (DMA), and the values obtained were incorporated into the model. DSC obtained conversion-time data for a standard commercially available UP resin under isothermal conditions. The kinetic parameters of the model were estimated using parameter optimisation procedures resulting in good agreement between model predictions and experimental data. Modelling in combination with experimental cure data showed that at higher isothermal cure temperatures a greater extent of physical trapping of radicals occurs rendering them inactive.

Improvement of dimensional stability of nylon-6 block copolymer using phenolic resin by reaction injection molding

In order to decrease moisture uptake and hence provide rigidity and dimensional stability in a nylon-6 block copolymer (NBC), powdered phenolic resin was incorporated into the formulation as a filler and processed by reaction injection molding. A novolac resin was cured with hexamethylenetetramine to produce the phenolic resin, which was modified with diethylamine in order to remplaced the OH groups by diethylamine groups. 5% by weight of modified and unmodified powdered phenolic resin was used as a filler in the nylon-6 block copolymers, and reinforced-nylon-6 block copolymer plaques were produced by reaction injection molding at 145°C. The materials were characterized by dynamic mechanical thermal analysis (DMTA), and their flexural modulus, impact test, and dimensional stability were evaluated. In unmodified phenolic-resin-reinforced nylon-6 block copolymer, water absorption was decreased by 90% compared with the NBC without filler. Two mechanisms of interaction between the NBC and the phenolic resin have been proposed.

Examination of Hard Segment and Soft Segment Phase Separation in Polyurethane Medical Materials by Electron Microscopy Techniques

A combination of transmission electron microscopy (TEM) and in situ tensile testing in an environmental scanning electron microscopy (ESEM) was used to evaluate the static bulk and dynamic surface morphologies of medical polyurethanes. TEM results showed phase-separated hard segment and soft segment structures. Surface morphology as a function of strain was studied using ESEM in conjunction with a tensometer.

The Effect of Polyurethane Composition and Processing History on Mechanical Properties

Uniaxial stress‐strain behavior of a wide range of polyurethane elastomers was compared with current models of rubber‐like elasticity. Although the data could be described well by a semi‐empirical model, a systematic discrepancy was observed with more theoretically based models. This took the form of an additional energy term during deformation, which depended on the polyurethane composition. Microdomain fragmentation may provide a possible explanation. Alternatively, it may be due to the effects of segmental interactions on the relaxation of polymer chain entanglements during deformation.

Modeling, Simulation and Control of Polymerization Processes: Some Aspects for Tailored Synthesis

Polymerization reactor control can sometimes be used to augment chemistry for the tailored synthesis of polymers. This theme is explored giving attention to generic methods for the control of molecular weight distribution for living polymerization processes when carried out in flow reactors. The role of modeling and simulation for achieving targets are considered.

Viscometric investigation of the aggregation and transfer reactions of polystyryllithium in ethylbenzene at elevated temperatures

Abstract A high-vacuum falling-ball viscometric method has been used to measure the equilibrium constant for the aggregation of free polystyrylanions in ethylbenzene. The rate of the transfer reaction between polystyryllithium and ethylbenzene has also been measured. All measurements have been made in the absence of monomer under conditions relevant to large-scale polymerization processes, i.e. with solutions containing a high weight fraction of polyanions (approximately 15–70% w/w) and at temperatures in the region of 70°C. The equilibrium constant ( K d ) for the aggregation of polystyrylanions was found to be in the range 2 × 10 −3 mol l −1 to 3 × 10 −2 for polystyryl anions of Mn 30 to 161 kg mol −1 at 70°C. The chain-transfer constant ( k tr ) determined at 70°C over a range of polymer molecular weights and concentrations is (2.21 ± 0.05) × 10 −5 l mol −1 s −1 . A number of assumptions have been made in order to arrive at the constants which are reported, including, (i) some concerning the viscosity behaviour of concentrated polymer solutions, (ii) that the polymers studied are essentially monodisperse, (iii) that polystyryl anions are two-fold associated in ethylbenzene, and (iv) that only the free polystyrylanions engage in the transfer reaction.