摘要:

Intramolecular screening in hydrogen-bonded polymer blends has its genesis in chain connectivity. The covalent linkages that occur between polymer segments results in an accentuation of the number of same-chain contacts compared to that calculated on the basis of a simple random mixing of segments. Using experimental infrared spectroscopic data derived from blends of random copolymers ofn-butyl methacrylate-stat-4-vinylphenol with styrene-stat-2-vinyl pyridine, strong evidence for the presence of intramolecular screening has been obtained. This is a significant factor that must be taken into consideration in theories of the mixing of polymers.

ISSN:0022-2348    

DOI:10.1080/00222349808220473

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

In the present study, conductive polyaniline-p-toluene sulfonic acid (PANI-pTSA) blends with thermoplastic polymers were prepared by melt processing. The blends′ characterization focused on their morphology in light of the components′ interaction and the resulting electrical conductivity. The PANI-pTSA blends were compared with blends containing PANI-DBSA (dodecyl benzene sulfonic acid). Generally, the level of interaction between the doped polyaniline and the matrix polymer determines the blend morphology and the resulting electrical conductivity. Similar solubility parameters of the matrix polymer and doped PANI lead to high levels of PANI dispersion within the matrix and to formation of conducting paths at low PANI contents. The morphology of a conducting network is described by a primary structure of small dispersed PANI particles interconnected by a secondary, short-range, fine fibrillar structure. The doped PANI network locates within the amorphous regions of a semicrystalline matrix, leading to reduction of the percolation concentration.

ISSN:0022-2348    

DOI:10.1080/00222349808220474

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Single crystals of the uniformn-alkane C294H590were grown from dilute solution in toluene in the temperature range 75°C to 85°C; the subsequent dissolution behavior during reheating was determined using calorimetry. Two crystal forms were recorded corresponding to once-and twice-folded chains. During isothermal crystallization, when primary crystallization was in the twice-folded form, transformation from the twice-folded to the once-folded state was observed. The processes of growth and transformation were followed as a function of the concentration (as derived from endo-therm areas of material crystallized) of the alkane remaining in solution. These data were used, along with dissolution onset temperatures, to plot an equilibrium phase diagram for the dissolution temperature of the twice-folded crystal form as a function of solution concentration. The importance of supersaturation, as well as supercooling, in the behavior of longn-alkanes is highlighted, in contrast to the behavior of polyethylene. Crystallization and nucleation rates were measured using calorimetric methods. At the lower crystallization temperatures, it was possible to monitor crystallization directly from the crystallization exotherms; at the higher crystallization temperatures, the degree to which crystallization had proceeded as a function of time was determined from dissolution endotherms. A distinct minimum in the crystallization rate was observed at 80.5°, near the transition temperature from the primary growth of twice-folded crystals (found at lower temperatures) and once-folded crystals (found at higher temperatures). This is in continuity with the previously observed minimum associated with the transition between the once-folded and extended conformation in shortern-alkanes. Here, for the first time, the transition applies to two-folded conformations, but the same explanation for the rate minimum is invoked.

ISSN:0022-2348    

DOI:10.1080/00222349808220475

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Atomic force microscopy (AFM) has been applied to observe single-chain polymethylmethacrylate (PMMA) particles that were stored for six months at room temperature after being sprayed from very dilute solutions in good solvents, good/poor mixed solvents, and a theta solvent. Nearly monodisperse PMMA standards of molecular weights ranging from 7.90 × 104to 1.3 × 106were used to investigate the effect of molecular weight (MW) on the size of the single-chain PMMA particles. These single-chain particles showed close to spherical shapes. The morphology of single-chain PMMA particles of a given molecular weight has found to be identical in spite of different solvents used for solution spraying. Molecular weight dependence of the particle dimension was also found. The diameters of single-chain PMMA particles after correction of tip-geometry effect were compared with the values estimated from molecular weight and density.

ISSN:0022-2348    

DOI:10.1080/00222349808220476

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The sorption and diffusion of dichloromethane liquid in a wide range of activities and at different temperatures in poly(ether imide) (PEI) films was studied. They were prepared by solution casting at slow solvent evaporation conditions. The sorption behavior is characterized by two zones depending on temperature and penetrant activity. In zone I, at low activity, the isotherms follow Henry's law. The range of activity between 0.25 and 0.30 is characterized by a negative deviation from Henry's law and shows an accentuated dependence on previous thermal history of the films. In zone II, with a range of activity higher than 0.35, the isotherms follow Flory-Huggin's behavior. In both zones, the diffusional behavior, based on sorption measurements, shows a linear relationship between the diffusion coefficient and the equilibrium sorption concentration of dichloromethane. In the activity range between 0.25 and 0.35, an accentuated discontinuity indicates a possible conformational structural change in the original polymeric matrix of the PEI films induced by the plasticization effect of the dichloromethane. Correlations between transport properties and infrared analysis confirm this proposition. The study of diffusion at different temperatures makes possible the Arrhenius plot analysis of the thermodynamic diffusion coefficient. Two different linear relationships were obtained for zones I and II with activation energiesEa= 30803 J · mol−1andEa= 13613 J · mol−1, respectively. These results indicate that the conformational change (zone I and zone II) observed is typically exothermic and therefore permits the proposition that the structural PEI formed in zone II is more ordered than in the zone I, corroborating the negative deviation of Henry's law. The two Arrhenius plots intersect at 150°C. PEI films annealed at this temperature show a dichloromethane sorption without negative deviation. On the other hand, PEI exhibits aTBtransformation at 70°C related to the conformational changes of the benzimide groups. The results obtained for the PEI films annealed at 150°C confirm that the conformational change observed is related to the benzimide groups. This fact was also confirmed by infrared analysis. Finally, the study of diffusion at different temperatures allowed creation of a temperature-penetrant concentration diagram, which is very useful for visualizing the different behavior zones.

ISSN:0022-2348    

DOI:10.1080/00222349808220477

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Morphological changes during isothermal melt crystallization in poly(aryl ether ether ketone) (PEEK)/poly(ether imide) (PEI, Ultem® 1000) blends were monitored via real-time small-angle x-ray scattering (SAXS) using synchrotron radiation. SAXS data were analyzed using a novel combination of correlation and interface distribution functions to determine the lamellar crystal thicknesseslcand interlamellar noncrystalline or “amorphous” layer thicknessesla. The higher glass transition noncrystalline PEI component slows the PEEK crystallization substantially, butla(about 40 Å) andlc(about 85 Å) are independent of crystallization time and blend composition. This is consistent with the known independence of melting temperature with blend composition. These results indicate that PEEK crystallizes in densely crystalline lamellar stacks through all stages of primary crystallization, and that the noncrystalline PEI is almost entirely excluded from the stacks at all times during spherulitic growth. The rate of increase in scattering invariant is found to be reduced during spherulite impingement due to spatial limitations.

ISSN:0022-2348    

DOI:10.1080/00222349808220478

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

A new type of time-dependent and strain-history-dependent viscoelasticity was discovered in semidilute polymer solutions. Dynamic moduli G′ and G″ of 20% and 10% nitrile butadiene rubber (NBR) solutions were recorded as a function of time while oscillatory shear deformations were maintained. The moduli decrease with time was observed only at lower frequencies. The time dependence ofG′was more pronounced than that ofG″. At a higher temperature, the time dependence was extended toward higher frequencies also, and the time dependence became stronger. Lowering the concentration of solution gave a similar effect as increasing temperature. After the cessation of oscillations, a slow recovery was observed. The recovery was somewhat faster at the higher temperature. The time-dependent moduli and their recovery were explained by the change and recovery of structures associated with long branches and gels in the NBR. The structure change occurred at higher frequencies also, but it was not observed during the application of oscillation. Only in subsequent measurements at lower frequencies could the structure change be detected. Thus, the change may be regarded as strain history dependent. The mechanism of the structural change was explained with either the entanglement or osmotic pressure models, depending on concentration.

ISSN:0022-2348    

DOI:10.1080/00222349808220479

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Photochromic properties of copolyethers based on bisphenol-A 4, 4′-dihy-droxydiphenyl, 4, 4′-dihydroxy-azobenzene, and 3,3-bis(chloromethyl)oxetane (BCMO) both embedded in a polymer matrix (polyvinyl alcohol, PVA) and in the pure state, deposed on quartz disks, have been studied. The polarization spectra of the studied compounds have been obtained. From the analysis of the absorption spectra in polarized light, it has been established that the oscillator responsible for the appearance of the electronic absorption band executes vibrations with an average orientation along the stretching direction of the studied foils.

ISSN:0022-2348    

DOI:10.1080/00222349808220480

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor