Randal W. Richards

Transesterification in poly(ethylene terephthalate) and poly(ethylene naphthalene 2, 6-dicarboxylate) blends; the influence of hydroxyl end groups

Two samples of a single composition blend of PET and PEN were prepared by solution blending using different solvents such that the hydroxyl end groups in one blend were modified. The rate of transesterification in each blend was studied using proton NMR, an established technique for this system. The results indicate that the rate of transesterification is influenced dramatically by end group modification, providing clear evidence that hydroxyl end groups participate in the reaction mechanism.

A 13C n.m.r. study of transesterification in mixtures of poly(ethylene terephthalate) and poly(butylene terephthalate)

Abstract Poly(ethylene terephthalate) and poly(butylene terephthalate) homopolymers and mixtures of these two polymers have been heated in the absence of oxygen at 573 K for 30 min and 476 K for 6 h. The resultant polymers have been analysed using solution viscometry, d.s.c., density determination and 13C n.m.r. Degradation has been observed when the polyesters are heated at 573 K and this is particularly evident for the poly(butylene terephthalate). Assignment of resonances due to homogeneous and heterogeneous dyads and degradation products in the n.m.r spectra have been made using model compounds and a random copolyester which have been synthesized by us. Using these assignments we have calculated sequence length distributions of ethylene and butylene sequences in the materials obtained. On heating at 573 K transesterification of the polyester present in the minority is complete and a single random copolymer is obtained. From the melting point data, it is clear that two copolyesters result when the mixtures are heated at 476 K for 6 h. The values for the number average sequence length suggest that block copolyesters are produced.

Hydroxyl accessibility in celluloses

Abstract The crystalline content of a range of celluloses has been evaluated by X-ray diffraction and infrared spectroscopy. Chemical derivatization of these materials with trifluoroacetic anhydride in conjunction with X-ray photoelectron spectroscopy has been used to measure hydroxyl accessibility. A linear relationship is found between cellulose crystallinity and the rate of hydroxyl labelling.

Differential scanning calorimetry and optical microscopy investigations of the isothermal crystallization of a poly(ethylene oxide)-poly(methyl methacrylate) block copolymer

The isothermal crystallization of the poly(ethylene oxide) block in a linear diblock copolymer of poly(methyl methacrylate) poly(ethylene oxide) with a poly(ethylene oxide) weight fraction of 0.76, has been evaluated using optical microscopy and differential scanning calorimetry. The copolymer was quenched from the melt to a range of crystallization temperatures between 289 K and 316 K and the crystallization monitored by observation of the increase in radius of spherulites (microscopy) or the enthalpy of fusion (calorimetry) as a function of time. Comparison experiments were also made on physical blends of the two homopolymers where the weight fraction of polyethylene oxide ranged from ∼0.6 to 0.9. The block copolymer has an observed melting point which is 2–3 K lower and the spherulite growth rate was reduced compared with the equivalent blend. The growth rates calculated from optical microscopy have been subjected to crystallization regime analysis. All three regimes are observable in the block copolymer for the supercooling conditions used here, only regimes I and II are evident for the pure poly(ethylene oxide), and for the blends regime I appears to be completely suppressed. From the regime analysis a fold surface free energy in the block copolymer of 16–20 erg cm−2 has been obtained, which is much less than that obtained for the pure poly(ethylene oxide) or the blends. An explanation based on the favourable enthalpy of mixing with poly(methyl methacrylate) is suggested. Enthalpy of fusion data from isothermal crystallization studies on all polymers in the d.s.c. have been analysed using Avrami theory. The Avrami exponent was obtained together with an effective rate constant of crystallization. The exponent suggests that crystallization takes place via homogeneous nucleation with a spherical growth morphology, growth being controlled by the rate of attachment of molecules to the interface. By comparison of the Avrami exponent with values obtained for blends differing only in the molecular weight, the influence of melt viscosity on growth control is evident.

The determination of segment density profiles of polyethylene oxide layers adsorbed at the air-water interface

The adsorption of polyethylene oxide of molecular weight 17 800 and 87 000 at the air/water interface has been studied by neutron reflection and surface tension measurements. Over the concentration range 10−4−10−1 wt% of polymer the surface excesses determined by neutron reflection were consistent with the results derived from the surface tension using the Gibbs equation. Reflectivity measurements were made on three samples of different isotopic composition at a fixed concentration of 0.1 wt%. The adsorbed polymer layers were found to be essentially completely immersed in the water, contrary to earlier suggestions that a significant fraction of the polymer protrudes into the vapour phase. The distribution profiles of the polymers with respect to the surface normal direction are composed of two major blocks, one constituting the layer at the top surface and the other forming a diffuse region extending into the bulk solution. Over 90% of the homopolymer is in the first region with a thickness of 18 ± 2 A, and the remaining diffuse region has a thickness of 35 ± 5 A. Alternatively, the volume fraction profile of the polymer can be described as a 1 : 1 sum of two half Gaussians of widths 10 and 35 A. Change of molecular weight and addition of salt do not change either the surface coverage or the segment density profiles significantly.

The generation of end group information from poly(styrene)s by means of matrix-assisted laser desorption/ionisation-collision induced dissociation

Information on the masses of the end groups of four poly(styrene) polymers, with relatively low average molecular weights and polydispersities, has been obtained from matrix-assisted laser desorption/ionisation-collision induced dissociation (MALDI-CID) data. The masses of the end groups were calculated, using the equations shown, from the mass-to-charge ratios of two series of intense ion peaks that were observed at low mass-to-charge ratios in the MALDI-CID spectra. Certain mechanisms are proposed here to account for the formation of these ions and others that are observed in the spectra.

Isotopic labelling and composition dependence of interaction parameters in polyethylene oxide/polymethyl methacrylate blends

Abstract Mixtures of deuterated polyethylene oxide with hydrogenous polymethyl methacrylate and hydrogenous polyethylene oxide with deuterated polymethyl methacrylate have been investigated using small angle neutron scattering. The range of temperatures used was 423–473 K and the volume fraction of polyethylene oxide in the mixtures varied from ca. 0.1 to ca. 0.27. Values of the effective interaction parameter ( χ eff ) were obtained using the incompressible random phase approximation. Both temperature dependence and composition dependence were noted in the values of χ eff . The temperature dependence observed was compared with that predicted by equation of state theory. A notable feature of the composition dependence of χ eff was the change in behaviour as temperature increased. Changes in the excess volume of mixing have been cited as a possible source of this behaviour, but the currently available data do not allow us to confirm this. Using the relation between χ eff and a composition independent interaction parameter proposed from Monte Carlo simulations, approximate values of the composition independent interaction parameter have been obtained.

Equilibrium concentration profiles of physically end tethered polystyrene molecules at the air-polymer interface

Abstract Deuteriopolystyrene of molecular weight ca 32 000 g mol −1 has been functionalized at one end with a fluorocarbon. On mixing this polymer with an unfunctionalized hydrogenous polystyrene a surface excess layer of the deuteriopolymer forms on annealing at 423 K. The surface excess as a function of deuteriopolymer content has been obtained using nuclear reaction analysis and neutron reflectometry, and the shape of the profile from neutron reflectometry alone. From previous secondary ion mass spectral data and by comparison with predictions of surface enrichment theory, it is concluded that the deuteriopolystyrene is tethered by the fluorine label to the air surface. The parameters of this polymer ‘brush’ obtained experimentally are the surface volume fraction of labelled polymer, the brush height, the surface excess and the layer thickness. The surface excess and surface volume fraction of the deuteriopolymer and the adsorption isotherm (surface excess at a function of equilibrium bulk volume fraction) have been compared to the predictions of a self-consistent field theory. Best agreement is obtained with a sticking energy of 1.6 k B T ; however, there appear to be some disparities when compared to the limiting ‘dry’ brush predictions. There is evidence from neutron reflectometry of brush formation before annealing of the polymer films.

Theory of surface light scattering from a fluid–fluid interface with adsorbed polymeric surfactants

We present a microscopic theory for the interfacial rheology of a fluid–fluid interface with adsorbed surfactant and calculate the effect of this on surface light scattering from the interface. We model the head and tail groups of the surfactant as polymer chains, a description that becomes increasingly accurate for large molecular weight surfactants, i.e., polymeric surfactants. Assuming high surface concentrations so that we have a double-sided polymer brush monolayer, we derive microscopic scaling expressions for the surface viscoelastic constants using the Alexander–deGennes model. Our results for the surface elastic constants agree with those in the literature, while the results for the viscous constants are new. We find that four elastic constants, i.e., γ (surface tension), e (dilational elasticity), κ (bending modulus), λ (coupling constant), and three viscous constants, i.e., e′,κ′,λ′ (the viscous counterparts of e, κ, and λ, respectively) are required for a general description of interfacial vis...

Rouse and Reptation Dynamics of Linear Polybutadiene Chains Studied by 2H NMR Transverse Relaxation

Deuterium NMR has been used to investigate two different types of dynamics of linear polybutadiene chains in the melt. The transverse relaxations of short Rouse chains of molecular weight 640- 3000 were biexponential, which has been attributed to separate decays of the methylene and methine deuterons. Interpretation of the transverse relaxation rates using a model for Rouse dynamics, combined with molecular simulations, gave the shortest Rouse unit as approximately 4.4 monomers and the shortest Rouse time as 8.3 × 10 -7 s. The reptation dynamics of higher molecular weight entangled chains were investigated using ABA isotopic triblock copolymers, of total molecular weight 14000-135000, where A is protonated polybutadiene of molecular weight greater than the entanglement molecular weight and B is a deuterated block. These polymers were specifically synthesized so that the fast motion of the Rouselike chain ends should not complicate the signal. The fundamental parameters found for the Rouse chain were used in the reptation model, assuming fast dynamics, and gave an entanglement molecular weight, M e , of 5380 or approximately 21 Rouse units. This M e is more than twice the conventional value, obtained from rheology, and is more suggestive of the critical molecular weight M c , consistent with previous NMR work. The theoretical analysis used in this work is based on the assumption that the chain dynamics are fast on the time scale set by the NMR deuterium quadrupolar interaction. The highest molecular weight samples were found to not satisfy this criterion and indicate the molecular weight at which a new theoretical approach is needed.