Wayne D. Cook

Photopolymerization kinetics of dimethacrylates using the camphorquinone/amine initiator system

Abstract The kinetics of photopolymerization of dimethacrylates with the camphorquinone (CQ)/amine pair has been studied by isothermal d.s.c. and u.v.-vis. spectroscopy. In accordance with the proposed initiation mechanism of electron and proton transfer from the amine to the triplet CQ ketone, only amines with an abstractable proton on the α-carbon undergo significant rates of polymerization. However, in contrast to other studies of exciplex formation from triplet state ketones and quenchers, only a poor correlation was observed between the rate of radical formation (measured in terms of the maximum polymerization rate) and ionization potential. In general, the efficiency of amines in enhancing initiation was: tertiary > secondary > primary amine. A kinetic scheme for CQ consumption and radical formation was in satisfactory agreement with the experimentally observed rate dependence on the radiation intensity and on the CQ and amine concentrations. For low amine concentrations, the polymerization rate had a half order dependence on concentration, but at intermediate levels, the rate was independent of amine concentration. The polymerization was slightly retarded at high amine concentrations. The influence of CQ concentration and radiation intensity on the polymerization rate generally fitted the theoretical half power relationship, however evidence for primary radical termination and pseudo first-order termination was found at high and low initiation rates, respectively.

Thermal aspects of the kinetics of dimethacrylate photopolymerization

Abstract The photopolymerization kinetics of four bisphenol-A based dimethacrylate resins were studied from −40 to 160°C by isothermal differential scanning calorimetry. The limiting conversion was low at temperatures near the glass transition temperature ( T g ) of the monomer but increased rapidly as the curing temperature was raised. These data were successfully fitted to a theoretical relationship between curing temperature, conversion and T g . At higher cure temperatures, the limiting conversion was topologically controlled, in agreement with the Loshaek-Fox theory. The photopolymerization rate increased rapidly to a maximum and then decreased slowly as the monomer was consumed. Using the encounter theory for reaction rates, the kinetics of the propagation and termination steps were expressed by combination of thermal (Arrhenius) and free volume (WLF) terms. These expressions were then combined with the theories of translation and reaction diffusion, allowing the prediction of the temperature dependence of the overall polymerization rate. Despite its simplicity, many of the observed kinetic features were reproduced by the model.

DSC studies of the curing mechanisms and kinetics of DGEBA using imidazole curing agents

Abstract The curing mechanisms and kinetics of diglycidyl ether of bisphenol A using 1-methylimidazole (1-MI), 2-methylimidazole (2-MI), 2-phenylimidazole (2-PhI) and 1,2-dimethylimidazole (1,2-DMI) as the curing agents were studied using scanning and isothermal differential scanning calorimetry (DSC). Both scanning and isothermal DSC studies indicated that only 1-MI was an effective curing agent, resulting in a high degree of conversion and high T g , at relatively low concentrations. In the scanning DSC studies, multiple peaks were observed for the 2-MI and 2-PhI curing systems whereas only a single peak was observed for the 1-MI curing system. These peaks were assigned to adduct formation, etherification (via the alkoxide anion) and to the process of imidazole regeneration. In the isothermal DSC studies, two peaks were observed for all curing systems being attributed to adduct formation and etherification. The differences in curing behaviour of the three imidazole curing agents was discussed in terms of steric versus inductive effects caused by the substituent attached to the imidazole ring located at the 2-position, and of differences in their initiation mechanism. The curing mechanisms and kinetics of the 1-MI curing system was also investigated in the presence of a salt, tetramethylammonium chloride, hydrochloric acid and water, and were discussed in terms of their effects on adduct formation and on the stability of the propagating alkoxide anion.

The mechanical characteristics and in vitro biocompatibility of poly(glycerol sebacate)-Bioglass® elastomeric composites

Biodegradable elastomeric materials have gained much recent attention in the field of soft tissue engineering. Poly(glycerol sebacate) (PGS) is one of a new family of elastomers which are promising candidates used for soft tissue engineering. However, PGS has a limited range of mechanical properties and has drawbacks, such as cytotoxicity caused by the acidic degradation products of very soft PGS and degradation kinetics that are too fast in vivo to provide sufficient mechanical support to the tissue. However, the development of PGS/based elastomeric composites containing alkaline bioactive fillers could be a method for addressing these drawbacks and thus may pave the way towards wide clinical applications. In this study, we synthesized a new PGS composite system consisting of a micron-sized Bioglass filler. In addition to much improved cytocompatibility, the PGS/Bioglass composites demonstrated three remarkable mechanical properties. First, contrary to previous reports, the addition of microsized Bioglass increases the elongation at break from 160 to 550%, while enhancing the Youngs modulus of the composites by up to a factor of four. Second, the modulus of the PGS/Bioglass composites drops abruptly in a physiological environment (culture medium), and the level of drop can be tuned such that the addition of Bioglass does not harden the composite in vivo and thus the desired compliance required for soft tissue engineering are maintained. Third, after the abrupt drop in modulus, the composites exhibited mechanical stability over an extended period. This latter observation is an important feature of the new composites, because they can provide reliable mechanical support to damaged tissues during the lag phase of the healing process. These mechanical properties, together with improved biocompatibility, make this family of composites better candidates than plastic and related composite biomaterials for the applications of tissue engineering.

A simple method for the measurement of polymerization shrinkage in dental composites

OBJECTIVE In this study a simple non-contact method was developed to measure the polymerization shrinkage of dental composites. METHODS A gas pycnometer was used to determine the volumes of specimens prior to and after photopolymerization and from which the total volumetric shrinkage could be determined. RESULTS Four commercial composites were studied and were found to have polymerization shrinkages varying from 1.6 to 2.5%. The method was found to be labour efficient and produced reproducible results with a standard deviation of approximately 10%. SIGNIFICANCE This method is appropriate for shrinkage measurements where only the total amount shrinkage is required and in particular for the measurement of shrinkage of photocured materials which are sensitive to water absorption.

Kinetics and network structure of thermally cured vinyl ester resins

Abstract Bisphenol-A diglycidyl ether dimethacrylate was blended with styrene at varying concentrations and this model vinyl ester resin (VER) was compared with two commercial VERs. The VERs were characterized using gravimetry, FTIR spectroscopy, NMR spectroscopy, differential scanning calorimetry (DSC) and DMTA. NMR spectroscopy differentiated between a novolac epoxy-based multimethacrylate oligomer and the two bisphenol-A epoxy-based dimethacrylate oligomers. Reaction kinetics were studied using scanning and isothermal DSC and isothermal FTIR spectroscopy using benzoyl peroxide as the thermal initiator. The presence of oxygen was found to inhibit significantly the polymerization. Increased initiator concentration raised the rate of isothermal polymerization, but did not affect the final conversion while increased styrene concentration reduced the polymerization rate constant and increased the total conversion. This was interpreted in terms of the variations in the termination rate and the stability of the styryl radical on the cure rate and the effect of vitrification on the extent of cure. From measurements of the dynamic mechanical properties as a function of temperature, the breadth of the glass transition tan δ curve and the magnitude of the rubbery modulus was found to increase while the tan δ maximum decreased with increased crosslink density. The T g , as measured by DSC, and the temperature of the tan δ maximum, as measured by DMTA, were not significantly affected by the styrene content in the resin per se , but were dependent on the combined effects of composition and crosslink density of the network.

Fracture toughness of water-aged resin composite restorative materials

OBJECTIVES The purpose of this study was to assess the effects of aging experimental dimethacrylate resin composites in water at 37 degrees C for periods up to 6 wk by measuring the variations in fracture toughness (K(c)), elastic modulus (E), fracture energy (G(c)), and water sorption. METHODS Six experimental resins were formulated from dimethacrylate resins, and were filled to 86 wt% (ca. 70 vol%) with treated inorganic filler to form six experimental composites. The fracture toughness was determined using a double torsion technique, the elastic modulus was measured in flexure, and the fracture energy was calculated from the fracture toughness and elastic modulus. RESULTS As a result of aging in water, K(c) and the G(c) increased, and the elastic modulus decreased, but all values approached a plateau near 6 wk. Water sorption also occurred during this period, mainly during the first 2 wk. SIGNIFICANCE Variations in the mechanical properties are interpreted as being due to plasticization of the resin matrix by water, which appears to lower the yield stress and increase in the size of the plastic zone ahead of the crack, thereby causing the observed increase in G(c) and K(c). After approximately 6 wk, no further changes in properties occurred.

Yielding behaviour in model epoxy thermosets — I. Effect of strain rate and composition

Abstract Yielding behaviour was investigated in two series of networks where the molecular architecture (crosslink density) was varied without any significant chemical changes in the structure. The DGEBA-based networks cured with aromatic amines had higher yield stresses than related networks cured with aliphatic amines, due to differences in the rigidity of the backbone. In each series, the yield stress was also found to rise with increased crosslinking. This may be caused by the increased constraint of the crosslinks on the molecular segments involved in the yielding process. The Eyring equation was found to satisfactorily describe the dependence of the yield stress on the strain rate. Surprisingly, the calculated volume of the flow unit was independent of both the crosslink density and the nature of the amine.

Curing behaviour of IPNs formed from model VERs and epoxy systems I amine cured epoxy

Abstract The curing behaviour for an Interpenetrating Polymer Network (IPN) formed from a model Vinyl ester resin (VER) and an amine cured epoxy resin has been studied by scanning DSC and isothermal mid-FTIR. The interactions between the VER initiating system (azobisisobutyronitrile or cumene hydroperoxide, benzoyl peroxide or methyl ethyl ketone peroxide/cobalt octoate) and the epoxy curative (aniline, diaminodiphenyl methane, butylamine or diamino-octane) have been examined. For most of the IPNs, there was evidence for a reduction in reaction rate due to the dilution of each reacting system by the other resin components. When cumene hydroperoxide, benzoyl peroxide or methyl ethyl ketone peroxide (with or without cobalt octoate) were used as the radical initiating systems, there was strong redox interaction between the peroxide and the amine, which caused acceleration of the peroxide decomposition in the early stages of reaction and also resulted in premature depletion of the initiator system. Evidence for a grafting reaction between the amines and the methacrylate groups by Michael addition was also found. In some systems cured isothermally at 70°C, the slow cure of the epoxy component allowed the unreacted DGEBA to plasticise the system and enhance the extent of cure of the VER components prior to vitrification of the IPN. As a consequence of the additional crosslinks introduced by the VER component, premature vitrification occurred during the slower cure of the DGEBA component, thus reducing the extent of cure of the epoxy groups in the IPN. Near full cure could be obtained for most of the IPN systems when post-cured at elevated temperature, indicating that the presence of each network imposed minimal topological restrictions on cure.

Yielding behaviour in model epoxy thermosets : II. Temperature dependence

The temperature and strain-rate dependence of yielding in compression has been investigated for a series of model thermosets in which crosslink density is varied without compositional change. The yield stress was only indirectly influenced by the crosslink density through its effect on the molecular mobility and the glass transition temperature. The Eyring activation volume was relatively insensitive to changes in crosslink density or composition but increased as the temperature was raised. The Robertson theory of yielding was found to give an approximate prediction for the temperature and strain rate dependence of the yield stress and the adjustable molecular parameter in this theory was close to the theoretical value. Neither the Argon nor the Bowden theories yielded significantly improved fits to the data. The use of these structurally related polymer systems appears to provide a very sensitive test of yielding theories.