D.T. Turner

Polymethyl methacrylate plus water: sorption kinetics and volumetric changes

Abstract Sheets of polymethyl methacrylate were weighed periodically in water and, after surface mopping, in air. A case was made for dual mode sorption kinetics with 40–60% of the water accommodated in microvoids and the remainder taken up by swelling. The kinetics of sorption departed from Ficks laws in ways which appeared to be mutually inconsistent. For example, the value of the diffusion coefficient appeared to increase not only in the course of sorption but also in desorption. This inconsistency was interpreted as an artifact due to a retarded swelling component of dual mode kinetics.

Influence of the molecular weight of poly(methyl methacrylate) on fracture morphology in notched tension

Abstract Specimens of poly(methyl methacrylate) (PMMA) were prepared by radiolysis of a polymer from an initial viscosity-average molecular weight ( M v ) of 1.2 × 10 6 down to 2.6 × 10 3 . At a molecular weight of 1 × 10 5 , abrupt changes in fracture morphology were observed correlating with a similarly abrupt decrease in fracture surface energy (γ). As the molecular weight was decreased further, the fracture morphology resembled more that of very brittle materials such as silicate glasses. Evidence was obtained that Wallner lines can influence the disposition of ribs but not their spacing. An empirical relationship was established between functions of rib spacing ( r ) and fracture surface energy.

Influence of the molecular weight of poly(methyl methacrylate) on fracture surface energy in notched tension

Abstract Specimens of poly(methyl methacrylate) (PMMA) were prepared by the radiolysis of a polymer from an initial viscosity average molecular weight of M v = 1.1 × 10 6 down to 1.5 × 10 4 . Corresponding values of fracture surface energy, ranging from 3.5 × 10 5 erg/cm 2 to 4.5 × 10 2 erg/cm 2 , were calculated from tensile data using Griffiths equation. A theoretical dependence of fracture energy on molecular weight was derived on the assumption that only molecules exceeding a critical molecular weight can contribute to the work of plastic deformation. Comparison with experimental data indicates this molecular weight to be about 1 × 10 5 . Limitations of the theoretical treatment are discussed.

Water sorption of poly(methyl methacrylate): 2. Effects of crosslinks☆

Abstract Glassy crosslinked networks were made by copolymerization of methyl methacrylate both with ethylene glycol dimethacrylate and with triethylene glycol dimethacrylate, using a redox initiator. Residual monomer was largely or wholly removed by extraction with water, by heating at 100°C, or by γ-irradiation. Water sorption conformed approximately to Ficks laws but with a retarded swelling component which increased with crosslink density. Approximate values were obtained for apparent diffusion coefficients both in sorption and desorption. The pattern of results was similar for both dimethacrylate systems. Values of diffusion coefficients were little influenced by crosslink density up to a dimethacrylate feed of 50 wt%. The saturation value for water uptake increased with increasing feed of dimethacrylate to more than twice the value for the linear polymer, i.e. poly(methyl methacrylate).

The glass transition temperature of poly(N-vinyl pyrrolidone) by differential scanning calorimetry

Abstract Previously a wide range of values have been reported for the glass transition temperature, Tg, of poly(N-vinyl pyrrolidone), PVP, and it was suggested that lower values are due to variable uptakes of water caused by the hygroscopic nature of the polymer. Now it has been found that there are large variations in Tg, even in carefully dried specimens of PVP. Other factors found to influence Tg are residual monomer and the molecular weight of PVP. Polymers prepared by bulk polymerization, either by γ-irradiation or by heating with 2-azobisisobutyronitrile, have much lower values of Tg than dried ones prepared containing 30% water. The difference is mainly due to depression of Tg by residual monomer which, in the absence of water during polymerization, fails to react completely because of conversion to a glassy state. An unexplained observation is that even when all residual monomer has been removed, polymers prepared by bulk polymerization still have a lower Tg than would be expected from their molecular weight.

Glass transition elevation by polymer entanglements

Abstract For some polymers a plot of glass transition temperature, T g , versus reciprocal molecular weight can be taken to define two lines which intersect at a molecular weight which is designated as M g . The value of M g agrees, within a factor of two, with critical values of molecular weight reported for other properties which are generally attributed to incipient formation of a network of entanglements. Therefore, it is suggested that an increased elevation of T g at molecular weights greater than M g is due to an increasing concentration of entanglements. A more detailed analysis was made by extending the Fox-Flory theory of the glass transition to include negative contributions to the free volume from entanglements. This extension leads to revised estimates of the free volume per chain end which are much smaller (6–19 A 3 near T g ) than previous estimates which took no account of entanglements. These smaller values are interpreted to mean that the jump units are correspondingly small at T g , such as envisaged in the Gibbs-diMarzio theory of the glass transition.

Water sorption of poly(methyl methacrylate): 3. Effects of plasticizers

Abstract Plasticized poly(methyl methacrylate) was made by γ-irradiation of mixtures of monomer and various phthalates. Samples were immersed in water and uptake and diffusion coefficients determined. More reliance was placed on determinations made in desorption because these did not involve complications due to loss of components by dissolution. As expected, plasticizers reduced water uptake simply due to their more hydrophobic nature. An additional reduction was attributed to the filling, by plasticizer, of microvoids which otherwise would have been able to accommodate water. The diffusion coefficient of water increased with plasticizer content and there was an indication that this increase was greater above the glass transition temperature.

Water sorption of poly(methyl methacrylate): 1. Effects of molecular weight

Abstract A sample of poly(methyl methacrylate), PMMA, of low molecular weight (10 4 D) took up less water (1.2%) than samples of high molecular weight (10 6 D: 2.0%). In contrast, the uptake of water was only slightly dependent on molecular weight for samples made by radiolysis in the glassy state. It is concluded that uptake of water depends on the closer molecular packing possible in polymers of lower molecular weight. However during radiolysis in the glassy state, this potential is not fully realized because of limited mobility. In more detail, the small changes in the diffusion coefficient and uptake of water in irradiated samples were consistent with closer packing in samples with M n ⩽ 10 000. It is concluded that molecular packing proceeds more readily below the critical molecular weight for formation of an entangled network.

Glass transition elevation of polystyrene by crosslinks

Abstract Analysis of data by Ueberreiter and Kanig on the influence of crosslinking on the elevation of the glass transition temperature ( ΔT ) of polystyrene gives a plot which departs from a straight line in a way which is suggestive of an entanglement effect. Therefore this work was reinvestigated by differential scanning calorimetric studies of networks prepared by copolymerization of styrene and p -divinyl benzene (DVB). ΔT was found to be simply proportional to the feed of DVB. A case is made that the earlier results differ because of an artifact caused by residual monomer and it is concluded that in crosslinked polystyrene, there is no evidence that entanglements contribute to an elevation of T g . Similar conclusions are drawn from previously reported data on natural rubber and polymethyl methacrylate.

Tensile strength elevation of brittle polymers by entanglements

Abstract Plots of tensile strength ( T ) versus reciprocal number average molecular weight ( M −1 ) have been made using previously reported data for linear polymers tested in the glassy state. Over a wide range of molecular weights there is conformity to Florys empirical equation T = A - BM −1 , in which A and B are constants. Values of M obtained by extrapolation to T = 0 correlate with critical values of molecular weight which are diagnostic of incipient formation of an entangled network. The entanglement thesis is developed further by reference to a model in which brittle strength is attributed to the breaking of covalent backbone bonds. Theoretical values are calculated which exceed experimental values by a factor of only three. Such close agreement is attributed to the insensitivity of glassy linear polymers to flaws.