Peter A. Lovell

Chain Transfer to Polymer and Branching in Controlled Radical Polymerizations of n-Butyl Acrylate

Chain transfer to polymer (CTP) in conventional free-radical polymerizations (FRPs) and controlled radical polymerizations (ATRP, RAFT and NMP) of n-butyl acrylate (BA) has been investigated using (13) C NMR measurements of branching in the poly(n-butyl acrylate) produced. The mol-% branches are reduced significantly in the controlled radical polymerizations as compared to conventional FRPs. Several possible explanations for this observation are discussed critically and all except one refuted. The observations are explained in terms of differences in the concentration of highly reactive short-chain radicals which can be expected to undergo both intra- and inter-molecular CTP at much higher rates than long-chain radicals. In conventional FRP, the distribution of radical concentrations is broad and there always is present a significant proportion of short-chain radicals, whereas in controlled radical polymerizations, the distribution is narrow with only a small proportion of short-chain radicals which diminishes as the living chains grow. Hence, irrespective of the type of control, controlled radical polymerizations give rise to lower levels of branching, when performed under otherwise similar conditions to conventional FRP. Similar observations are expected for other acrylates and monomers that undergo chain transfer to polymer during radical polymerization.

Toughened carbon/epoxy composites made by using core/shell particles

Abstract Toughened epoxy resin composites have been prepared by resin-transfer moulding by using a range of toughening agents. Two types of epoxy-functional preformed toughening particles were investigated and have a three-layer morphology in which the inner core is crosslinked poly(methyl methacrylate), the intermediate layer is crosslinked poly(butyl acrylate) rubber and the outer layer is a poly[(methyl methacrylate)-co-(ethyl acrylate)-co-(glycidyl methacrylate)]. The presence of glycidyl groups in the outer layer facilitates chemical reaction with the matrix epoxy resin during curing. Comparisons were made with acrylic toughening particles that have a similar structure, but which do not have the epoxy functionality in the outer shell, and with a conventional carboxy-terminated butadiene acrylonitrile (CTBN) liquid rubber toughening agent. The composites were characterised by using tensile, compression and impact testing. The fracture surfaces and sections through the moulded composites were examined by means of optical and scanning electron microscopy. Short-beam shear tests and fragmentation tests were used to investigate the interfacial properties of the composites. In general, use of the epoxy-functionalised toughening particles gave rise to superior properties compared with both the non-functionalised acrylic toughening particles and CTBN.

Studies of rubber-toughened poly(methyl methacrylate): 1. Preparation and thermal properties of blends of poly(methyl methacrylate) with multiple-layer toughening particles

Abstract The use of sequential emulsion polymerization to prepare toughening particles which comprise two, three and four radially alternating rubbery and glassy layers is described. The conditions which lead to control of particle size and morphology are discussed. The particles were crosslinked during their formation in order to ensure that they retained their size and morphology during blending with poly(methyl methacrylate) (PMMA). In this way, rubber-toughened (RT) PMMA materials with rubbery phases of predefined particle size and morphology were produced, as confirmed by transmission electron microscopy. The glass transitions of the three types of RTPMMA materials were essentially identical because the matrix PMMA was common to each blend and because the rubbery and glassy phases of each of the types of toughening particle were identical in composition.

NMR studies of free-radical polymerization and copolymerization of monomers and polymers containing allyl groups

Abstract Reactivity ratios for free-radical copolymerization of allyl acetate (A) with (i) methyl methacrylate (M), (ii) n-butyl acrylate (B) and (iii) styrene (S) have been evaluated from either copolymer compositions or dyad fractions determined by 1H and 13C-NMR spectroscopy; the values obtained are respectively: (i) rA = 0.024 ± 0.009 and rM = 41 ± 6, (ii) rA = 0.04 ± 0.02 and rB = 11.7 ± 1.0, and (iii) rA = 0.021 ± 0.001 and rs = 66 ± 4. These values indicate that in copolymerizations of allyl methacrylate with (meth)acrylates and styrene, conversion of the allylic CC bonds will only become significant when the extent of reaction of all other types of CC bonds has reached very high values. NMR studies of the structure of poly(allyl methacrylate) formed by both bulk and dilute solution polymerization have provided no evidence for the in-chain lactone rings which may result from intramolecular cyclopolymerization of allyl methacrylate; the signals in the NMR spectra of poly(allyl methacrylate) are entirely consistent with “normal” polymerization at the methacrylic and allylic CC bonds. Theoretical predictions based upon the measured reactivity ratios, and results from a model reaction, indicate that the very low concentrations of pendant methacrylic CC bonds which result from polymerization of the allylic CC bonds in molecules of allyl methacrylate, make a significant contribution to grafting and crosslinking.

Surface modification and its effect on the interfacial properties of model aramid-fibre/epoxy composites

Abstract The micromechanical behaviour of model Kevlar/epoxy composites made by using as-received fibres with a poly(vinyl alcohol) size was compared to that of fibres where the size was removed, and to the behaviour of model composites containing fibres modified by the introduction of amine groups which were able to take part in the reaction of the epoxy with the hardener. Raman spectroscopy was used to measure the fibre strains in the model composites as a function of distance along the fibre as the matrix load was increased, thus enabling the interfacial behaviour of the model composites to be deduced. The model composites containing the as-received fibres showed an elastic response up to 1% matrix strain with failure occurring through debonding at the interface near the ends of the fibres. Model composites containing fibres which had been washed to remove the size showed different behaviour with an elastic response to 1.5% matrix strain. In this case failure was through matrix yielding. Finally, the model composites containing fibres with pendant amine groups did not fail at the maximum matrix strain (2.25%) used in these experiments, this being interpreted in terms of an increase in the strength of the interface.

Click chemistry as a route to surface functionalization of polymer particles dispersed in aqueous media

Proof-of-principle studies are described which show unequivocally that Huisgen click coupling can be performed successfully at the surface of polymer nano/submicron-size particles dispersed in aqueous media, thereby defining a generic strategy for attachment of a wide variety of functional molecules and providing a powerful, versatile route for the synthesis of surface functional particles.

Toughening of epoxy resins using particles prepared by emulsion polymerization: effects of particle surface functionality, size and morphology on impact fracture properties

A range of sub-micron size three-layer toughening particles have been prepared by sequential emulsion polymerization, each type of particle comprising a glassy polymer core, a rubbery inter-layer and an outer layer of glassy polymer, which was functionalized using either glycidyl methacrylate or methacrylic acid. Each type of particle was dispersed at a range of levels in a commercial diglycidyl ether of bisphenol-A (DGEBA), which then was cured using either piperidine or commercial diamino-3,5-diethyltoluene. The materials obtained were characterized by transmission electron microscopy, dynamic mechanical analysis and instrumented impact testing. Inclusion of the toughening particles gave rise to a substantial increase in the toughness of piperidine-cured DGEBA, but only a marginal enhancement in the toughness of diamino-3,5-diethyltoluene-cured DGEBA because of the much higher crosslink density of this epoxy resin matrix. The presence of particle surface functionality was essential to optimizing toughness, but above a certain level (about 5mol%) the concentration of functional groups in the surface layer had no further effect on the toughness. Carboxylic acid functionality led to slightly inferior impact toughness compared with epoxide functionality and also gave rise to difficulties in processing the blends of particles and epoxy resin. The effects of particle size and morphology were investigated using particles with surface layers containing epoxide functionality. The size of the particles was shown to be important, with the results indicating that the particle diameter at the periphery of the rubbery layer needs to be more than 0·35μm. There also was an effect of particle morphology, with toughness decreasing as the size of glassy polymer core was increased. ©1997 SCI

Fragmentation analysis of glass fibres in model composites through the use of Raman spectroscopy

Raman spectroscopy has been used to monitor deformation micromechanics in a model discontinuous fibre composite comprising a single glass fibre in an epoxy resin. The glass fibre was coated with a diacetylene-containing urethane copolymer that was subsequently cross-polymerised thermally. During composite deformation, the stress-induced Raman band shifts of the polydiacetylene sequences in the cross-polymerised coating were used to map the distributions of strain along glass fibres inside the epoxy resin matrix. The fragmentation of the fibre has been followed in detail and the behaviour analysed using classical shear-lag analysis. Values of the interfacial shear stress along fibre fragments were determined from the measured fibre strain distributions and were shown to be limited by the shear yield stress of the matrix. The effect of adhesion between the coating and the fibre upon the strain distributions has been investigated in detail. The fibre strain distributions can only be determined accurately when the adhesion is good. However, in the case of poor adhesion, although the strain distribution in the coating follows that of the matrix, the fragmentation process can still be monitored.

Ab initio calculation of the rate coefficient for short-chain branching in free-radical polymerizations

Short-chain branching in polyethylene, which involves a six-centre transition state, is studied by ab initio quantum mechanics up to the QCISD(T) level. The calculation gives a (low-pressure) activation energy of 73 kJ mol(-1) and a frequency factor of 4.8 x 10(12) s(-1). The frequency factor for this six-centre transition state is expected to be of acceptable accuracy and also applicable to homologous systems, such as short-chain branching to polymer in acrylates. These results overestimate the amount of ethylene short-chain branching observed in experiment, but the discrepancy is within the uncertainties of both experiment and calculation

Dilute Solution Viscometry

One of the most characteristic features of a dilute polymer solution is that its viscosity is considerably higher than that of the pure solvent. This arises because of the large differences in size between polymer and solvent molecules, and can be significant even at very low polymer concentrations, especially for polyelectrolytes and polymers with high molecular weights. Dilute solution viscometry is concerned with accurate quantitative measurement of the increase in viscosity and allows determination of the intrinsic ability of a polymer to increase the viscosity of a particular solvent at a given temperature. This quantity provides a wealth information relating to the size of the polymer molecular shape, degree of polymerization and polymer–solvent interactions. Most commonly, however, it is used to estimate the molecular weight of a polymer. This involves the use of semi-empirical equations which have to be established for each polymer/solvent/temperature system by analysis of polymer samples whose molecular weights are known. Thus the estimates of molecular weight are not absolute. Nevertheless, in comparison to other methods for characterization of polymers in solution (e.g. membrane osmometry and light scattering), dilute solution viscometry is simple, fast and inexpensive. It also has the advantage that it is applicable over the complete range of attainable molecular weights. For these reasons, dilute solution viscometry has been the most widely used method of polymer characterization since the birth of polymer science and continues to be used today.