1999 рік

A water-insoluble dye was used to probe the particle nucleation loci in the semibatch emulsion polymerization of styrene (ST). Most of the dye molecules are present in the monomer droplets and some of them are solubilized in the monomer-swollen micelles. The extremely hydrophobic dye molecules cannot diffuse from the monomer droplets and micelles to the growing latex particles. Thus, determination of the amount of dye incorporated into the resultant latex particles provides valuable information on the particle nucleation mechanism. The mixed modes of particle nucleation (micellar and homogeneous nucleation) were proposed when the surfactant concentration ([S]) is above its critical micelle concentration (CMC). In the absence of micelles (i.e., [S] < CMC) most of the latex particles are produced via homogeneous nucleation. Monomer droplet nucleation cannot be ruled out because an appreciable amount of dye still can be detected in the resultant latex particles. The influence of the initiator concentration on the particle nucleation mechanisms was also investigated. The experimental data, again, support the proposed mixed modes of particle nucleation when [S] > CMC. Homogeneous nucleation plays an important role in the particle formation period when [S] < CMC

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Here we extend the investigation described in the preceding companion communication. To rectify its notch brittleness, high-density polyethylene (HDPE) was modified by rigid particulate fillers consisting of three different sizes of CaCO 3 particles of 3.50, 0.70 and 0.44 mm weight average diameter in various volume fractions. Mechanical properties including notched Izod impact energy of the extrusion-blended/ injection-moulded samples were examined as a function of filler particle size and filler volume fraction. In exactly the same manner as exhibited by the rubber particle-modified compositions described in the previous communication, the toughness of the CaCO 3-filled materials increased dramatically when the mean interparticle ligament thickness of the matrix polyethylene dropped to values below 0.6 mm. The stiff fillers used in this study provided the unusual additional benefit of substantially increasing the Young’s modulus of the compounds while also dramatically improving their impact energy.

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A force field suitable for modeling fluoropolymers and oligomers in the solid state has been derived from MOPAC semiempirical molecular orbital calculations on perfluorohexadecane. A conformational energy profile was generated using the PM3 Hamiltonian, and then valence parameters of a molecular mechanics (MM) energy expression, including a six-term cosine dihedral potential, were adjusted with a nonlinear least squares fitting algorithm to reproduce the profile. Minimum energy helical conformations of 48/22 and 13/6 were obtained when the geometries of C60F122 molecules in isolation and in a crystalline cluster, respectively, were optimized using the refined force field. The X-ray diffraction pattern calculated from the crystalline cluster indicated an equatorial d-spacing of 4.9685 Å. These intra and intermolecular structural characteristics for the cluster are in agreement with experimental X-ray diffraction data. Energy penalties of helix reversal defects in isolated chains and in a crystalline environment were also investigated.

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The formation of a graft polymer is observed when polypropylene as a film reacts with maleic anhydride in the vapour phase in the presence of peroxide. The activity of the employed peroxides decreases in the order: dicumylperoxide . bis(tert. butylperoxyisopropyl)benzene . benzoylperoxide. The grafting reaction is not limited to the surface, but also takes place inside the film. Besides this reaction takes place on the film surface the formation of groups which give in ATR spectra a diffuse absortion band in the range between 1664 and 1500 cm 21. The modified film is characterised by a wetting tension of 35 dynes/cm and exhibits the anhydride group characteristic properties.

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The relationship between the charge-recombination luminescence (CRL), weak light emitted after u.v.-irradiation, and the development of properties during isothermal cure of epoxy/amine thermosetting resins was investigated. The changes in CRL were compared with the transitions and changes in properties during cure as measured through dynamical mechanical analysis on separate samples cured in a rheometer. It was shown that gelation did not lead to any detectable change in CRL, whereas vitrification caused a pronounced increase in CRL. Measurements on fully cured systems at different temperatures confirmed that CIU only was observed when the system was in the glassy state. It was also found that the decay kinetics of the CRL signal were essentially independent of the temperature and of the degree of cure of the sample

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Studies of non-isothermal crystallization morphologies were carried out in thin films of HDPE ranging in thickness of 15 nm–1.2 mm, adjacent to either (104) surfaces of calcite crystals or between thin layers of ethylene–octene rubber, both in support of the two preceding investigations of the toughening mechanism of HDPE with rubber and CaCO 3 particles. Combined WAXS measurements and AFM imaging of the crystallization forms established that, for films of thickness less than 0.3 mm, lamellar crystallites preferentially grew 'edge-on’, in sheaf-like morphology, on the calcite or rubber interfaces with the (100) crystallographic planes of the lamellae lying parallel to the interfaces. In films thicker than 0.4 mm the characteristic banded spherulitic morphologies of twisted lamellae became dominant. These observations furnish strong support for the toughening mechanism discussed in the two companion studies (I) and (II).

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Molecular mechanics and dynamics simulations have been utilized to probe the nature of helix reversal activity in polytetrafluoroethylene (PTFE). The results of the simulations indicated that helix reversals do form and migrate in PTFE crystals. At low and intermediate temperatures, the most important defect structure was a helix reversal band: two helix reversals which bracket a small chain segment having the opposite helical sense from the parent molecule. The size of this reversal band defect was equal to approximately half of the helical repeat unit in the low and intermediate temperature phases. In the high temperature phase where intermolecular effects are diminished, a wider distribution of reversal band sizes was observed during the simulations. The impact of helix reversal activity on rotational disorder was also examined. In addition to the rotational disorder created by the presence and motion of helix reversals, a mechanism was identified by which significant reorientation of a chain segment (having the same helical sense as the host chain) about the molecular axis can occur when it is bracketed by two helix reversal bands.

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The layer-by-layer self-assembly of polyaniline (PAN) and polyacrylic acid (PAA) has been accomplished via the alternate spontaneous adsorption of base-type PAN from its dilute N-methylpyrrolidinone solution and PAA from its aqueous solution based on the acid–base reaction of PAN and PAA. The thickness of the films can be manipulated at nanometer scale by the concentration of PAN solution and recycling times. It is found that the PAN base film cannot be doped to emeraldine salt by PAA in the presence of ammonium persulfate. The UV–Vis spectroscopy was used to characterize the self-assembling process and the oxidation states of PAN. The electrochemical cyclic voltammetric and electrochromic properties of the self-assembled films were also studied.

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The equation for polymer concentration fluctuation derived from Onuki’s equations of motion is coupled with Onuki’s postulate for the partial stresses generated by concentration fluctuation to formulate the dynamic structure factor S(q,t) [t is time and q the magnitude of the scattering vector]. The actual calculation is made for systems in which the elastic relaxation modulus L(t) is given by a linear combination of n exponential functions. It is shown that the corresponding S(q,t) consists of n + 1 exponential functions of time, and that the relative strengths and decay rates of these functions are related by a set of algebraic equations to the diffusion coefficient and cooperative diffusion coefficient, as well as the parameters characterizing L(t). These equations for n = 2 are used to analyze dynamic light scattering data on semidilute solutions of a polyisobutylene fraction in isoamyl isovalerate (0 solvent) and n-heptane (good solvent). The results give the instantaneous moduli of the solutions which are well compared with the rubbery plateau moduli from viscoelastic measurements, and the friction coefficients which are identical for both solvents when compared at comparable polymer concentrations

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Sorption and desorption data for n-hexane–natural rubber and n-hexane–low-density polyethylene were analysed to reveal the cause of the s-shaped sorption curves frequently occurring in highly swollen polymers. The model permitted the influence of solute-concentrationdependent diffusivity, sample geometry, boundary concentrations and swelling-induced mechanical stresses on the transport data to be examined. The calculated solute diffusivity varied by several orders of magnitude, depending on the choice of parameters included in the model. The inclusion of direct mechanical stress relaxation parameters only gave a slight improvement of the fit to the experimental data. The inclusion of a time-dependent surface concentration was the only way to fit the s-shaped sorption curves for both natural rubber and lowdensity polyethylene. Although isotropic three-dimensional swelling of natural rubber occurred over the whole sorption transient period, this condition was unable to explain the swelling (thickness increase) of low-density polyethylene. In the latter system, a model consisting of two stages had to be adopted: stage I where the swelling was mainly one-dimensional, and stage II which occurred later and was characterized by three-dimensional swelling similar to that occurring in natural rubber. During the transient sorption period, the ratio between natural rubber and low-density polyethylene of the ratio of the thickness to cross-sectional area was close to their bulk modulus ratio, which suggests that it is the bulk modulus rather than the Young’s modulus which determines the sorption characteristics of polymers above T g.

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