P.E.M. Allen

Water in methacrylates—I. Sorption and desorption properties of poly(2-hydroxyethyl methacrylate-co-glycol dimethacrylate) networks

Sorption and desorption kinetics of water in copolymers of 2-hydroxyethyl methacrylate with ethylene glycol dimethacrylate and oligo (ethylene glycol) dimethacrylates are reported and analysed in terms of Ficks and Frischs laws. Sorption kinetics range from Fickian to Alfrey Case II with the Frisch parameter n ranging from 0.5–0.9 In desorption, n was less than 0.5 in non-Fickian cases. Diffusivities calculated according to Ficks law are reported. The results are consistent with the dual mode sorption mechanism, i.e. penetration into voids or low density domains followed by redistribution as the high density domains soften and relax.

Properties of dimethacrylate copolymers of varying crosslink density

Abstract Homopolymers and copolymers of tetrafunctional oligo-ethylene dimethacrylate monomers of widely varying chain lengths are investigated by thermal and fracture techniques. Differential scanning calorimetry of catalysed monomer shows that the polymerization process is inhomogeneous. Quantitative analysis of these results suggest that the mechanism of cure is complex and may be ascribed to complex copolymerization, partial phase separation or a form of interpenetrating network. Dynamic mechanical spectra of the copolymers reveals that their mechanical properties vary monotonically between those of the constituent homopolymers. In some systems, double-peak behaviour is noted in the glass transition region. This is ascribed to possible phase separation. Short rod fracture, a fracture toughness technique suitable for glassy polymers, is also used on a copolymer system to obtain information about the copolymer morphology. The transition from ‘stick-slip’ to a smoother fracture mode with higher concentrations of the longer, flexible monomer is accompanied by a concomitant decrease in fracture toughness. This behaviour is explained in terms of the inhomogeneity of both homo- and copolymer dimethacrylate systems and failure occurring through points of weakness in the system, the regions of lower crosslink density.

The radical polymerization of dimethacrylate monomers—the use of NMR spectrometry to follow the development of network structure

Abstract 13 C NMR spectrometry has been used to identify and assay residual unsaturation in the cure of ethylene glycol dimethacrylate, and distinguish between monomer molecules and pendant double-bonds. Direct evidence of heterogeneity is provided by the distinction between constrained and unconstrained monomer. The predictions of a simulation using a kinetic gelation model coincide with the experimental estimations. Cure estimated by differential scanning calorimetry does not correlate linearly with residual unsaturation.

Polymerization of methylmethacrylate by organometallic compounds—vii. The n-butyl magnesium bromide-di-n-butyl magnesium system in THF + toluene solution

Abstract The molar-mass distributions of the products at 223°K and below were multimodal having three distinct and separate components, viz. volatile side-products, methanol-soluble polymer ( DP n ≈ 50 ) and methanol-insoluble polymer ( DP n = 200–600 ). At low THF concentrations, with n-BuMgBr, small amounts of a third distinct and independent polymeric product appeared at higher molar mass. The distributions of configurational triads of the chains of the methanol-soluble and insoluble polymers were non-Bernoullian but different. It is proposed that the two components are formed at two distinct and independent active centres. This eneidic mechanism explains the very high values of DP w / DP n reported in previous studies of Grignard initiation of MMA. The configurational triad distribution of the methanol-soluble and insoluble components remained unchanged both when n-Bu2Mg replaced n-BuMgBr as initiator and when excess MgBr2 was present. It is proposed that the same active centres operated throughout. In contrast to t-BuMg systems, there is no evidence that MgBr2 or BrMg- groups play any other role in the polymerization than to reduce the initiator efficiency. The effects of temperature and the addition of strongly solvating solvents were tested. Tetraethylene glycol dimethyl ether caused the precipitation of MgBr2 from the initiator solution but did not otherwise affect the mechanism. However when hexamethylphosphoroamide was added at 223°K the polymerization took a different mechanism. Sixteen per cent of n-BuMg- groups remain unreacted during the polymerization. It is proposed that the presence of these groups at the active centre accounts for the influence of the Bu group on the stereospecificity of the polymerization.

Some physical and mechanical properties of isotactic-atactic poly(methyl methacrylate) blends and stereoblocks

Abstract Dynamic-mechanical, DSC, and fracture-toughness measurements are reported for blends of it-PMMA with at-PMMA and syndiotactic-rich PMMA, both in the amorphous state and after solvent and thermally induced crystallization. Crystalline stereocomplex forms even in the blends of 74% isolactic with commercial at-PMMA under SIC. Some stereoblock PMMA shows two melting points and two distinct crystalline forms; one corresponds to it-PMMA the other probably to stereocomplex. Microcrystallinity in the blends greatly enhances stiffness. The decline of the storage shear modulus with temperature involves several processes. Blending of it-PMMA (even low mol. wt) into at-PMMA greatly improves its fracture toughness.

Polymerization of methylmethacrylate by organometallic compounds—VIII. Initiation by n-, i-, s- or t-butylmagnesium compounds—evidence for an eneidic pseudoanionic mechanism

Abstract The molar-mass distributions of PMMA initiated by various n-, s-, i- and t-butylmagnesium compounds were investigated. In most cases the distributions were polymodal. In some cases the different peaks were shown to have different chain configurations. The position of the peaks of the distribution was independent of THF concentration in the solvent mixture, but the relative amount of polymer in each peak changed, as did the steric triad composition of the whole sample. It is postulated that the molar-mass distribution and stereospecificity of the polymer are controlled by an eneidic mechanism in which persistent, independent growth-centres of different reactivity and stereospecificity are formed in the initiation step. The character of the polymer produced is primarily determined by conditions prevailing in the initiation stage: structure of the butyl group; presence or absence of halide; solvent composition and temperature. The concentrations of the various growth sites (down to the μM range, with the more reactive) and their reactivities have been calculated, subject to certain assumptions concerning the mechanism. Some conclusions about the structures of some of them can be drawn, but, in general, no direct evidence is obtainable. In cases where the mechanism was examined in detail, the propagation reaction was most easily explained in terms of the formation of a monomer-complex at the growth site which proceeds through a four-centre orientation to insert in a covalent MgC bond. The mechanism is probably more accurately classified as pseudoanionic rather than anionic.

Changes of proton-enhanced, magic-angle-spinning 13C-NMR dynamic parameters during cure of tetra(ethylene glycol)dimethacrylate

Abstract Dynamic NMR and mechanical properties TSL, T1p(C) at 60 kHz, Tg at ≈1 Hz and its half width, log decrement (Δmax) and compression modulus (E) were measured for poly[tetra(ethylene glycol)dimethacrylate] (PTEGDMA) at different extents of cure. The β, γ and water-induced transitions were also noted. The spin-lock, cross-polarization time-constant, TSL, declined to its limiting value before the vitrification point. T1p(C), relaxation time in the rotating 13C field, increased sharply at or just after vitrification. The change is greatest at quaternary C, decreasing through CH2, CH2O and CH3, being barely significant in the latter. The observed changes affirm that T1p(C) is sufficiently influenced by spin-lattice relaxations to provide a monitor of the damping of mid kHz components during cure. The small change observed at CH3 shows that this group is not the origin of changes in relaxation rates at other groups. Correlations with E and Tg show the importance of mid kHz components of group motion to the cooperative motions determining the response to macroscopic strain at lower frequencies. MAS 13C-NMR showed that PTEGDMA contains residual unsaturation at limiting cure and that two types, constrained and unconstrained, can be distinguished.

Isotactic and stereoblock poly(methyl methacrylates) produced by butylmagnesium initiators

Abstract The use of tert-, sec-, iso- and normal-butylmagnesium bromides and chlorides in toluene, tetrahydrofuran and mixed solvents to prepare Bernoullian, stereoblock and isotactic poly(methyl methacrylate). PMMA, and the factors controlling the stereochemistry are described. By careful control of solvent composition, highly isotactic polymer can be prepared with all the butylmagnesium bromides, but none of the corresponding dibutylmagnesiums. The halide content does not influence stereospecificity when it is in excess of the Grignard stoichiometry but it influences the mol. wt distribution and retards the polymerization. The mol. wt distributions are usually polymodal and dependent on the same factors that control stereospecificity. The effect of THF concentration on stereospecificity operates in a manner quite distinct from its effect on complex concentration and rate. The concentration of residual THF in toluene-rich solution determines the type of initiation and propagating species that prevail. Structures are proposed for those responsible for stereospecific initiation and propagation. It is concluded that tert-butylmagnesium bromide is the most reliable and robust initiator for preparing isotactic PMMA.

Reactions and complex formation between triethyl-aluminium and methyl methacrylate

The reaction of triethylaluminium and methyl methacrylate (MMA) was followed at room temperature in sealed NMR tubes. Hydrolysed reaction products were also analysed. Ethylation of MMA occurs only when MMA/A1 < 1.0. At higher ratios a strong, unreactive 11 complex prevails. Evidence presented suggests that a second 12 complex is responsible for the reactions at lower ratios. The normal reaction is carbonyl addition: twofold carbonyl adducts of the type: CH2CMeCEt2OAl, forming rapidly. Subsequent changes appear to involve only slow exchange or association of these ethylaluminium alkoxides. Reduction of the carbonyl group occurs to a minor and irreproducible extent. There is no evidence for 14 attack on MMA.

Dynamic-mechanical properties and cross-polarized, proton-enhanced, magic angle spinning 13C-NMR time constants of urethane acrylates—1. Homopolymer networks

Abstract Dynamic-mechanical properties have been measured for a series of urethane acrylates containing polyether and polyester soft segments. Polymer glass transition temperatures predicted from a crosslinking relation showed better agreement with experimental T g values than those obtained from applying the Fox equation to hard and soft segments in the polymer. Proton-enhanced, magic angle spinning 13 C-NMR time constants were obtained for a polyether soft segment urethane acrylate series. Increasing soft segment molecular weight resulted in a release of both near static and high frequency (mid-kHz) components of motion in the soft segment.