1993 рік

The dynamics of segmental and chain orientation in poly(styrene-b-isoprene) (P(S-b-I)) block copolymer with PI fractionfp I = 0.61, degree of polymerization N = 70 and zN ~- 9 at 25°C (Z is the interaction parameter) were investigated by depolarized Rayleigh scattering (DRS) and dielectric spectroscopy (d.s.) over a broad temperature range (240-370 K) in the disordered phase. Two distinct primary relaxation processes with vastly different timescales were associated with segmental orientation within PS- and PI-rich regions selectively probed by DRS and d.s. techniques. The width of the distribution relaxation function for the slow segmental process was found to increase with decreasing temperature, whereas the freezing of the slow segmental fluctuations was observed for the first time within the time window of photon correlation spectroscopy. Compared with an equivalent PI homopolymer chain, the first normal mode describing the end-to-end fluctuation of the copolymer chain displayed slower time ~1 but similar amplitude &Nffl, v The new experimental results were discussed in terms of the fluctuation picture of block copolymers in the disordered melt, the chain conformation and the friction coefficient ratio of the foreign blocks.

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Differential scanning calorimetry (d.s.c.) of composite samples made out of poly(ethyl methacrylate) (PEMA) and nylon fibres with a wide range of fibre volume fractions reveals several interesting features. A decrease in the glass transition temperature of the series as the fibre fraction increases, as well as a decrease of the specific heat increment at the transition are clearly detected; the glass transition (as determined by d.s.c.) even disappears in samples with high nylon content. The effect on the response thermograms of different ageing thermal histories is analysed with the help of a four-parameter model which predicts the qualitative features of the structural relaxation process. The parameters delivered by the fitting of the model to the observed behaviour suggest that the modified response of PEMA when acting as a matrix (when compared to its behaviour as a pure polymer) is caused by a shift of the equilibrium relaxation times with no accompanying widening of the relaxation time distribution. This supports an interpretation of the observed changes in terms of a bulk modification of PEMA in the composites, probably as a consequence of interactions of its molecules with nylon in the process of forming the composites

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The yielding behaviour of various thermoplastics and thermosets is reviewed in terms of a thermally activated dislocation propagation mechanism. The activation parameters are related to the local polymer structure (entanglement topology, chain flexibility) and to its molecular dynamics reflected in the secondary relaxation processes.

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Crystallization from the melt of trans-l,4-polyisoprene was followed using Fourier-transform infra-red spectroscopy. The spectral subtraction factor versus time was obtained at the crystallization temperature and after cooling to 25°C for fractions and for the parent material. Crystallization of the/? crystal form was followed at 36 and 43°C and of the ~ form at 43 and 51 °C. Samples crystallized at 25°C and subsequently annealed at 43°C were also monitored. The amorphous fraction for prenucleated melt-crystallized :~-form-containing samples increased with molecular weight at the crystallization temperature and after cooling to 25°C. Samples containing the /~ crystal form showed a molecular-weight dependence at a crystallization/annealing temperature of 43°C that disappears upon cooling the sample to 25°C. The results are discussed in terms of lamellar surface disorder and lamellar thickness distribution.

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The permeability of oxygen through membranes of poly(cyclohexyl acrylate) at temperatures in the vicinity of the glass transition temperature (Tg) is determined using an electrochemical experiment. The diffusion coefficient of the gas follows Arrhenius behaviour with an activation energy of 22 kcal mol- ', a surprisingly high value if it is considered that the range of activation energies observed in most systems is 1-20 kcal mol- 1. The interpretation of the free volume of the membrane at Tg suggests that the critical volume necessary for permeation of the gas to take place is nearly four times lower than the value of this parameter for the mechanical and dielectric processes. It is suggested that bulky side groups may aid the transport of gas in the glassy state but presumably hinder it in the rubbery state.

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2-Acetoxyethyl methacrylate (AcEMA) has been synthesized from 2-hydroxyethyl methacrylate (HEMA) and polymers and copolymers of the two monomers have been prepared. The effect of composition on the physical properties of the copolymers has been evaluated by d.s.c, and determination of tensile mechanical properties, both on dry and wet samples, flexural dynamic mechanical properties (d.m.t.a.) on dry samples, and water sorption and desorption kinetics. The addition of the more flexible component (AcEMA) significantly lowers the copolymer glass transition and affects its overall properties. These differences are explained by the d.m.t.a, measurements, with storage and loss moduli strongly depending on the copolymer composition. The mechanical behaviour of the copolymers ranges from glassy to rubbery, as a function of composition, with elastic moduli in the range of 100-2500 MPa for dry copolymers and 35-0.3 MPa for wet copolymers. Similarly, the equilibrium water content is affected by the balance of hydrophobic-hydrophilic monomer content, with a higher amount of water being absorbed in copolymers containing less AcEMA

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As basic research for the solution spinning of high T? oxide superconductor, the interaction between poly(vinyl alcohol) (PVA) and copper (Cu) (II) acetate in aqueous solution and in the solid state with and without propionic acid was studied. The interaction between PVA and Cu (II) ions in aqueous solution was extremely weak or non-existent. Complex formation of PVA with Cu (II) was observed in the solid state. The complex was changed from the co-ordination of the Cu (II) ion to the hydroxy groups on PVA to the Cu (II) ion complexing with two acetate ions and the hydroxy groups of PVA by adding propionic acid.

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In order to characterize the various residual fragments or those created during the use and polymerization of bismaleimide prepolymers, different compounds characteristic of the expected fragments were synthesized and physicochemically characterized by Fourier-transform infra-red spectroscopy, nuclear magnetic resonance spectroscopy and differential scanning calorimetry. Thus, depending on heating temperature, the bismaleimide telechelic group gives rise to chain extension at 140-150°C in the presence of aromatic diamines, either by addition or by maleimide ring opening. At higher temperatures (200°C) it causes a crosslinking reaction, which forms a three-dimensional system.

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The time-dependent deformation behaviour of a commercial grade of high-performance polyethylene fibres (Dyneema SK66), was studied using static and dynamic mechanical testing. A mathematical model is proposed, where the total deformation of the fibre is regarded as being composed of a stress-linear delayed elastic component and a non-linear plastic flow contribution. The thermo-rheological characteristics of the delayed elastic contribution are obtained using dynamic mechanical thermal analysis. The non-linear plastic flow component is characterized separately using long-term creep experiments. Model predictions of stress relaxation and tensile experiments are in good agreement with the experimental data.

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A linear low-density polyethylene (LLDPE) with hexyl branches has been blended with high-density polyethylene (HDPE) and a comparison has been made of the effects of three methods of blending on the quality of the blends produced. A twin-screw extruder, roll mills and solution precipitation have been compared for their effectiveness in producing a uniform blend. Compositional variations have been measured by Fourier-transform infra-red spectroscopy, crystallization and melting characteristics have been studied using differential scanning calorimetry, and the morphology has been examined by wide-angle X-ray scattering and scanning electron microscopy. In general, it is concluded that in the melt the two grades of PE are compatible and co-crystallize on quenching. The most uniform product is produced by the twin-screw extruder, since the blends have higher melting points, crystallinity, yield stress and elongation at break. The solution-blended product crystallizes at a lower temperature than the others, consistent with a reduction in heterogeneous nucleation

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