摘要:

AbstractThe isothermal melting behavior of poly(L‐lactic acid) spherulites has been studied in order to understand the mechanism of melting using hot‐stage microscopy. When the periphery of the spherulite melts or shrinks, the central portion undergoes thickening as a result of which the melting or shrinking rate is not linear. Annealing studies at 448 K reveal that there is an increase in melting temperature and heat of fusion with annealing time, suggesting an increase in lamellar thickness. The high activation energy (828 kJ/mol) obtained in this investigation suggests the highly complex nature of the melting process wherein both melting and thickening oc

ISSN:0098-1273    

DOI:10.1002/pol.1984.180220401

出版商:John Wiley&Sons, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractThe tensile stress—strain curves of various types of polyethylene were compared from 77 to 298 K in nitrogen, isopentane, and the inert environment of helium at various strain rates. It was found that in general polyethylene crazes in a gas such as nitrogen at a temperature below 1.6 times its boiling point and in isopentane. Although the behavior of polyethylenes is similar to that of other polymers with regard to crazing in gases at low temperatures, they are in general less sensitive to the gas. The decrease in tensile strength of polyethylene in an environmental gas increases with crystallinity. The differences in the intrinsic low‐temperature brittle fracture stress are attributed to differences in the density of tie molecules. The intrinsic yield point at room temperature showed the usual increase with increasing crystallinity, but all the polyethylenes have the same yield point below the γ transition tempera

ISSN:0098-1273    

DOI:10.1002/pol.1984.180220402

出版商:John Wiley&Sons, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractThe creep behavior of ultrahigh‐modulus polyethylene monofilaments has been studied over the temperature range 20–70°C. A wide range of samples was examined in an attempt to determine the influence of draw ratio, molecular weight, copolymerization, and crosslinking by γ irradiation Prior to drawing. Results are also presented for a solution‐spun fiber. It is proposed that the permanent flow creep arises from a combination of two creep processes, one of which is associated with the crystalline regions of the oriented structure and the other with a molecular

ISSN:0098-1273    

DOI:10.1002/pol.1984.180220403

出版商:John Wiley&Sons, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractIn this study, three different polymers were blended: a racemic PMEPL (PMEPL‐RAC) which consists of a random distribution of repeat units of R or S configurations, an isotactic PMEPL made of repeat units of S configuration [PMEPL‐S(−)], and a second isotactic PMEPL made of repeat units of R configuration [PMEPL‐R(+)]. The acronym PMEPL stands for poly[α‐methyl‐α‐ethyl‐βpropiolactone]. The racemic PMEPL is optically inactive but the isotactic PMEPLs are optically active. In the first part of this study, an equimolar mixture of PMEPL‐S(−) and PMEPL‐R(+) was made. This racemate mixture leads to the formation of stereocomplex between the two polymers which has a melting point roughly 40°C higher than those of the isotactic polymers. The racemate morphology is composed of large spherulites. In the second part of this study, binary blends of PMEPL‐S(−) and PMEPL‐R(+) were prepared with an excess of one of the isotactic PMEPLs. The stereocomplex is preferentially formed between the isotactic polymers and this complex controls almost entirely the morphology of the sample. The enthalpy of fusion of the stereocomplex in the binary blends can be predicted. however, the excess isotactic polymer is trapped between the lamellae of the spherulites of the stereocomplex and its crystallization is hindered by the high viscosity of the partially crystallized medium. In the third part of this study, ternary blends were prepared. It is shown that PMEPL‐RAC does not from a complex with the isotactic polymers. The PMEPL‐S(−)/PMEPL‐R(+) stereocomplex is again preferentially formed and it controls the morphology of the sample. It decreases the degree of crystallinity of both the excess isotactic polymer and the PMEPL‐RAC. This control of the morphology is effective even in samples in which the sterocomplex accounts for

ISSN:0098-1273    

DOI:10.1002/pol.1984.180220404

出版商:John Wiley&Sons, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractA brief résumé is given of the role of structural heterogeneity, magnetic dipolar couplings, molecular structure, and molecular motion in determining the1H NMR spectra and relaxation properties of heterogeneous solids such as semicrystalline polymers. Measurements of1H spin‐lattice relaxation in laboratory (T1) and rotating frames (T 1ρθ) are reported for a number of solid polyolefin samples. These include solution‐crystallized and melt‐crystallized polyethylene, annealed and quenched isotactic polypropene, and isotactic polybut‐1‐ene. In addition, broad‐line1H spectra, both normal and partially (T 1ρ90) relaxed, are reported for these materials as well as a number of pulsed NMR experiments having the philosophy of the so‐called Goldman–Shen experiment. Spin‐lattice relaxation (T1) for all samples is a single exponential process, whereas rotating‐frame relaxation comprises three exponential processes both on‐resonance (θ = 90°) and off‐resonance at the magic angle (θ = 54.7°), with the latter generally being much slower. The spectra show clearly the existence of components having differing degrees of mobility and, with the exception of the solution‐crystallized polyethylene, the partially (T 1ρ90) relaxed spectra indicate a correlation between breadth of resonance line and magnitude ofT1ρ. The Goldman–Shen‐type experiments indicate a spin‐diffusional transport of magnetization between the different spectral and (T 1ρ90) components. A computer program has been used to simulate the NMR behavior of a three‐region system comprising repeating units of infinite lamellae of different widths, each region having different intrinsic relaxation times and spin diffusion coefficients. The results demonstrate that the observed1H NMR behavior of these samples can be interpreted in terms of this model and that,inter alia, the long‐timeT 1ρ90behavior reflects, qualitatively, the time taken for magnetization to diffuse a distance of the ord

ISSN:0098-1273    

DOI:10.1002/pol.1984.180220405

出版商:John Wiley&Sons, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractFrom analysis of infrared spectra it was found that in syndiotactic (s) poly(methyl methacrylate) (PMMA) in solution, longssequences contain an increased population of diads with a skeletal conformationtt(in the staggered approximation). Self‐aggregation ofs‐PMMA in solution leads to a further increase of the fraction of longssequences in the extended chain conformation, and to an ordering of easter groups. When solids‐PMMA is isolated from a solution in which it exists in the aggregated state, these characteristics are preserved in the solid. The polymer appears partially crystalline by x‐ray scattering, and it exhibits fibrillar morphology under the electron microscope. Ordered structures ofs‐PMMA melt at temperatures about 150°C, while the presence of residual solvent decreases the temperature of melting. Solids‐PMMA obtained from solutions in which aggregation of the polymer does not take place, likes‐PMMA which did not come into contact with solvent, contains a higher proportion of syndiotactic diads with a skeletal conformationtg; these samples are amorphous and morphologically structureless. Analogies between the structure of ordereds‐PMMA and the structure of the PMMA stereocomplex ar

ISSN:0098-1273    

DOI:10.1002/pol.1984.180220406

出版商:John Wiley&Sons, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractA simple modification of the viscometer used to determine the viscosity of concentrated solutions is described which permits the ready measurement of viscosity versus shear rate behavior above room temperature. Basic experimental viscometry was performed on the ethyl cellulose–chloroform system using a B‐type (Brookfield‐like) rotating‐cylinder viscometer. Data are compared with theoretical predictions. The flow curves exhibit yield stress at low shear rate. The yield stress is dependent on concentration and temperature. The viscosity versus concentration curves show typical mesomorphic behavior. The concentration dependences of viscosity and yield stress are similar. The critical concentration increases with temperature. In the single‐phase region (isotropic or anisotropic state) the viscosity decreases with temperature and the apparent activation energy (Ea) for flow is positive, but in the biphasic range the viscosity increases with temperature andEais negative. The experimental results for the critical concentration appear to agree with Flory's theory of rodlike molecules when the ratio of persistence length to diameter of the chain is taken as the asp

ISSN:0098-1273    

DOI:10.1002/pol.1984.180220407

出版商:John Wiley&Sons, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractCorrection factors defined as the ratio of the true size parameter and the size parameter deduced from Fraunhofer diffraction theory are presented for weakly anisotropic transparent fibers. These correction factors enable the Fraunhofer diffraction approximation to be used for sizing weakly anisotropic fibers from light‐scattering measurements. Fibers in the diameter range 4–50 μm and refractive index range 1.45–1.56 are considered. It is shown that for fibers having birefringence −0.005 ≤ Δm ≤ 0.01, the diameters may be obtained to within 9% accuracy if the fiber diameter lies betwee

ISSN:0098-1273    

DOI:10.1002/pol.1984.180220408

出版商:John Wiley&Sons, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractCrystallization of polyethylene under hydrostatic pressures of 1–4.5 kbar is directly observed using pulsed proton NMR. The rate of growth of extended‐chain polyethylene crystals is measured over this pressure range and to a maximum temperature of 227°C. The observed crystallization isotherms are superimposable on a log time scale; this implies a consistent mechanism for extended‐chain growth over this pressure range. Avrami coefficients for high‐pressure extended‐chain crystallization are determined to be 1.3–1.7. A decrease of crystal nucleus surface free energies with increasing pressure is indicated. Findings are consistent with Wunderlich's model of initial folded‐chain crystallization followed immediately by chain extension. Future applications of this NMR technique are brie

ISSN:0098-1273    

DOI:10.1002/pol.1984.180220409

出版商:John Wiley&Sons, Inc.    

年代:1984

数据来源: WILEY

摘要:

AbstractThe crystal structure of poly(p‐xylylene), as polymerized, is the α form. This transforms irreversibly to the β from by annealing or drawing. To clarify the mechanism of this transition, structural changes of the α and β crystals were examined with a high‐temperature stage in the electron microscope. Two high‐temperature phases, β1and β2, were found and their structures were analyzed. In these structures lattice distortions due to rotational and translational motions of chains are in troduced, especially in the β2form. The α → β transition is induced through such a disordered phase. The statistical arrangement of a molecule in the β‐form unit cell results from freezing the disorder in the high

ISSN:0098-1273    

DOI:10.1002/pol.1984.180220410

出版商:John Wiley&Sons, Inc.    

年代:1984

数据来源: WILEY