Proton spin–lattice and spin–spin relaxation times in isotactic polypropylene. II. Effects of stretching ratio and temperature

Proton spin–lattice, T₁, and spin–spin, T₂, relaxation times of uniaxially stretched polypropylene film were measured at 40°C using a wide line pulse spectrometer operating at 19.8 MHz. T_1l, the longer T₁, increases almost linearly with increasing stretching ratio, and T_2a, T₂ of the amorphous region, decreases gradually as the stretching ratio is increased. These results can be interpreted in terms of the increased constraints to molecular motion in the amorphous region. The fraction of the rigid protons in the sample, F_c, increases with increasing stretching ratio, while the crystallinity calculated from the density, X_d, does not change largely. The difference between F_c and X_d, therefore, increases as the stretching ratio is increased. This indicates that the physical structure of the highly stretched sample is far from the ideal two-phase model. The influence of the stretching temperature was also investigated. There are only slight increases in T_1l and in F_c for the samples stretched in a temperature range from 80°C to 150°C, whereas the considerable increase in T_2a occurs. The most notable change introduced at a high temperature stretching is the increase in the chain mobility in the amorphous region.     

Proton magnetic relaxation times of isotactic polypropylene films treated with tetrachloroethylene

Proton spin-lattice (T₁) and spin-spin (T₂) relaxation times of isotactic polypropylene films with different thermal histories, treated with tetrachloroethylene, were measured with pulsed NMR. For the melt-quenched sample T₁₁ (the longer T₁) increases and T_2a (the longest T₂) decreases with increasing treatment temperature. The mass fraction of mobile region (F_a) decreases and the mass fraction of rigid region (F_c) increases with increasing treatment temperature. T₁₁ increases almost linearly with increasing F_c, indicating that the removal of the mobile amorphous region is mainly responsible for the change in T₁₁. For the annealed sample T₁₁ also increases with increasing treatment temperature, although a slight decrease in T₁₁ for the low treatment temperature is observed. There is a large decrease in T_2a when the treatment temperature is raised, suggesting that a comparatively selective extraction occurs for the annealed sample.     

Thermal characteristics and crystallinity of Ziegler–Natta isotactic polypropylene/metallocene isotactic polypropylene polyblended fibers

Ziegler–Natta isotactic polypropylene (ZN‐iPP) and metallocene isotactic polypropylene (m‐iPP) were extruded (in ratios of 75/25, 50/50, and 25/75) from one melt twin‐screw extruder to produce three ZN‐iPP/m‐iPP polyblended polymers and, subsequently, spin fibers. In this study, we examined the rheology of the ZN‐iPP/m‐iPP polyblended polymers and the thermal characteristics and crystallinity of the ZN‐iPP/m‐iPP polyblended fibers using gel permeation chromatography, rheometry, differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction, density gradient analysis, and extension stress–strain measurement. The apparent melt viscosity of the ZN‐iPP/m‐iPP polyblended polymers revealed positive‐deviation blends. The 50/50 blend of ZN‐iPP/m‐iPP had the highest apparent melt viscosity. For five samples, the complex melt viscosity decreased with the angular frequency, which represented typical non‐Newtonian behavior. The Cole–Cole plot, which consisted of the imaginary part of the complex melt viscosity versus the real part of the complex melt viscosity plot, of the ZN‐iPP/m‐iPP polyblended polymers showed a semicircular relationship with the blend ratios. It indicated that the ZN‐iPP/m‐iPP polyblended polymers were miscible. We analyzed the shear modulus data (G′ vs G″) by plotting them on a log–log scale. The plot revealed almost the same slopes for the ZN‐iPP/m‐iPP polyblended polymers, which indicated a good miscibility between the ZN‐iPP and m‐iPP polymers. The experimental DSC results demonstrate that the ZN‐iPP and m‐iPP polymers constituted a miscible system. The crystallinity and tenacity of the ZN‐iPP/m‐iPP polyblended fibers initially increased and then fell as the m‐iPP content increased. Meanwhile, the 50/50 blend of ZN‐iPP/m‐iPP had the highest crystallinity and tenacity. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Enthalpy relaxation studies in isotactic polystyrene. Effects of crystallinity

Summary The enthalpy relaxation of the amorphus isotactic polystyrene is strongly affected from the crystalline phase induced by annealing at temperatures between Tg and Tm. All the parameters describing the relaxation process, H, T_max and T_ons depend also on the above T_g annealing conditions as the induced crystallinity alters the quantity and the quality (i.e.Tg, T_gons, T_g) of the remaining amorphous phase.     

Effects of thermal aging on isotactic polypropylene crystallinity

The effects of elevated temperature aging on the microstructural changes of isotactic polypropylene matrix in a composite have been studied using wide‐angle X‐ray scattering (WAXS) and Fourier‐transform infrared spectroscopy (FTIR). The objective was to quantify small and slow changes in crystallinity due to thermal aging. To minimize sample variability, polypropylene resin was extracted from the molded composite plaque. Changes in crystallinity level and crystalline form were detected using WAXS after prolonged aging at 90 and 140°C. FTIR was utilized to monitor in‐situ crystallinity changes and to detect oxidation products due to thermal decomposition. The level of crystallinity was monitored by changes in the absorbance ratio of A₉₉₇/A₉₇₃ and A₈₄₁/A₉₇₃; the former ratio was found to be more sensitive for detecting crystallinity changes. Aging at 140°C resulted in oxidation. The kinetics of secondary crystallization for the aging conditions studied was characterized using Avrami plots. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Directional solidification of isotactic and atactic polypropylene blends

The directional solidification of polypropylene (PP) films results in an oriented semicrystalline microstructure and may offer a method to improve the properties of a product. The directional solidification of isotactic PP samples blended with 0% to 50% atactic PP, by mass, was therefore studied. The effects of composition and processing conditions were monitored to determine how they affect the quality and microstructure of the directionally solidified films. Difficulty was encountered in reproducing testable samples with a unidirectional crystal microstructure. Tensile testing of directionally solidified films was used to quantify the yield strength and elastic modulus of the films. These properties were compared with those of other PP films. The tensile test results do not support the hypothesis that enhanced mechanical properties were produced by directional solidification of the PP films. Improving the sample fabrication method and optimizing the processes involved may, however, lead to directionally solidified PP films with enhanced mechanical properties. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1516–1528, 2000     

Thermal degradation kinetics of isotactic and atactic polypropylene

The thermal degradation of isotactic and atactic polypropylene was investigated in bulk and in solution. The degradation in bulk was studied with thermogravimetric analysis, and the degradation in solution was studied by the dissolution of the polymer in paraffin oil. The degradation in solution was investigated from 230 to 350°C. The effect of the hydrogen donor on the degradation of the polymer in solution was also studied at 350°C. Continuous distribution kinetics were employed to model the degradation kinetics for the degradation in solution. The rate coefficients were obtained, and the activation energy was calculated from an Arrhenius plot. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2206–2213, 2003     

Isothermal crystallization of isotactic polypropylene blended with low molecular weight atactic polypropylene. Part I. Thermal properties and morphology development

The thermal properties and morphology development of isotactic polypropylene (iPP) homopolymer and blended with low molecules weigh atactic polypropylene (aPP) at different isothermal crystallization temperature were studied with differential scanning calorimeter and wide-angle X-ray scattering. The results of DSC show that aPP is local miscible with iPP in the amorphous region and presented a phase transition temperature at T_c=120 °C. However, below this transition temperature, imperfect α-form crystal were obtained and leading to two endotherms. While, above this transition temperature, more perfect α- and γ-form crystals were formed which only a single endotherm was observed. In addition, the results of WAXD indicate that the contents of the γ-form of iPP remarkably depend both on the aPP content and isothermal crystallization temperature. Pure iPP crystallized was characterized by the appearance of α- and γ-forms coexisting. Moreover, the highest intensity of second peak, i.e. the (0 0 8) of γ-form coexisting with (0 4 0) of α-form, and crystallinity were obtained for blended with 20% of aPP, the γ-form content almost disappeared for iPP/aPP blended with 50% aPP content. Therefore, detailed analysis of the WAXD patterns indicates that at small amount aPP lead to increasing the crystallinity of iPP blend, at larger amount aPP, while decreases crystallinity of iPP blends with increasing aPP content. On the other hand, the normalized crystallinity of iPP molecules increases with increasing aPP content. These results describe that the diluent aPP molecular promotes growth rate of iPP because the diluent aPP molecular increases the mobility of iPP and reduces the entanglement between iPP molecules during crystallization.     

Changes of spin–spin relaxation time for EPR composites during mixing process

The changes of the mobility of rubber molecules for carbon black-filled EPR composites during mixing process in an internal mixer have been elucidated from T₂ measurement using pulsed nuclear magnetic resonance (NMR). The thickness of the tightly bound region which is formed by the direct interaction with carbon black was stabilized with mixing but the restriction of the molecular mobility increased with further mixing. The amount of the loosely bound region increased considerably throughout the mixing, indicating that this region is formed by entanglement and crosslinking of the molecules. T₂ of the mobile region which is a predominant part in the composites is independent of the mixing time. EPR has the thinner tightly bound shell and longer T_2S compared with EPDM, showing the effect of unsaturated bond in EPDM which can become a strong bond site with carbon black.     

Changes of spin–spin relaxation time for EPR composites during mixing process

The changes of the mobility of rubber molecules for carbon black-filled EPR composites during mixing process in an internal mixer have been elucidated from T₂ measurement using pulsed nuclear magnetic resonance (NMR). The thickness of the tightly bound region which is formed by the direct interaction with carbon black was stabilized with mixing but the restriction of the molecular mobility increased with further mixing. The amount of the loosely bound region increased considerably throughout the mixing, indicating that this region is formed by entanglement and crosslinking of the molecules. T₂ of the mobile region which is a predominant part in the composites is independent of the mixing time. EPR has the thinner tightly bound shell and longer T_2s compared with EPDM, showing the effect of unsaturated bond in EPDM which can become a strong bond site with carbon black.     

E.s.r. study of γ-irradiated isotactic and atactic polypropylene

An e.s.r. study of γ-irradiated isotactic (i-) and atactic (a-) polypropylene (PP) was carried out. Both the 17 line (or 9 line) spectrum for i-PP and the 6 line spectrum for a-PP are attributed to the tertiary carbon radical, CH₂C(CH₃)CH₂. The difference in the hyperfine structure between the spectrum of i-PP and that of a-PP can be explained by different radical conformations owing to the stereoregularity. Hyperfine coupling and the radical conformation for the tertiary carbon radical in i- and a-PP were determined from the cos² ? rule. It is shown that the δ methylene proton as well as the β methylene proton contribute to the slight anisotropy in the stretched sample. The spectral change from 9 lines to 17 lines with elevation of temperature, observed at low temperature in i-PP, could be explained by the hindered oscillation of the β methylene proton.     

Isothermal crystallization kinetics and morphology development of isotactic polypropylene blends with atactic polypropylene

The crystallization kinetics and morphology development of pure isotactic polypropylene (iPP) homopolymer and iPP blended with atactic polypropylene (aPP) at different aPP contents and the isothermal crystallization temperatures were studied with differential scanning calorimetry, wide‐angle X‐ray diffraction, and polarized optical microscopy. The spherulitic morphologies of pure iPP and larger amounts of aPP for iPP blends showed the negative spherulite, whereas that of smaller amounts of aPP for the iPP blends showed a combination of positive and negative spherulites. This indicated that the morphology transition of the spherulite may have been due to changes the crystal forms of iPP in the iPP blends during crystallization. Therefore, with smaller amounts of aPP, the spherulitic density and overall crystallinity of the iPP blends increased with increasing aPP and presented a lower degree of perfection of the γ form coexisting with the α form of iPP during crystallization. However, with larger amounts of aPP, the spherulitic density and overall crystallinity of the iPP blends decreased and reduced the γ‐form crystals with increasing aPP. These results indicate that the aPP molecules hindered the nucleation rate and promoted the molecular motion and growth rate of iPP with smaller amounts of aPP and hindered both the nucleation rate and growth rate of iPP with larger amounts of aPP during isothermal crystallization. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1093–1104, 2007     

1H magnetic relaxation times in isotactic polypropylene crystallized at high temperature. Effect of molecular weight

The temperature dependence of ¹H spin-lattice, T₁(H), and spin-spin, T₂(H), relaxation times of isotactic polypropylene with relatively low molecular weight, M?_w = 1.95 × 10⁵, and ultra-high molecular weight, M?_w = 1.78 × 10⁶, crystallized at high temperature (155° C) for a long time (200 h), was measured with a broad line pulse spectrometer. T₁(H) for the ultra-high molecular weight sample is shorter than that for the low molecular weight sample, while T₂(H) for the ultra-high molecular weight sample is longer than that for the low molecular weight sample. From the analysis of free induction decay (FID), three components of T₂(H), i.e. T_2c(H), T_2m(H) and T_2a(H), were obtained. These relaxation times are associated with the crystalline, intermediate and amorphous regions, respectively. Here the intermediate regions are the regions in which chain molecules have intermediate mobility between that of crystalline and amorphous regions. A decrease in signal intensity of methylene, methine and methyl carbons was measured as a function of delay time following a 90° pulse before cross-polarization. From the slope of a log (intensity) versus t² plot a relaxation time associated with rigid regions of proton nuclei, T_2r(H), was obtained, where the rigid regions are the regions in which cross-polarization occurs easily. The value of T_2r(H) lies between that of T_2c(H) and T_2m(H), indicating that the intermediate regions act as the rigid regions so far as cross-polarization is concerned.     

Effect of stretching temperature on proton spin-lattice and spin-spin relaxation times of isotactic polypropylene film

Structural properties of isotactic polypropylene film during stretching were investigated mainly by the measurements of proton spin-lattice, T₁ and spin-spin, T₂, relaxation times. Both T₁ and T_2a, T₂ of the most mobile amorphous regions, of the sample stretched at 150°C are longer than those of the sample stretched at 130°C. These results indicate that in the sample stretched at 150°C the proportion of mobile amorphous region decreases, while the amorphous region achieves enhanced molecular mobility.     

Static uniaxial compression of polyisoprene–montmorillonite nanocomposites monitored by 1H spin–lattice relaxation time constants

Previous work has focused on the use of microscopy to explore the mechanisms of deformation in polymer nanocomposites. That technique creates a qualitative representation that may not be statistically representative of the bulk properties. This paper illustrates the utility of solid‐state NMR, which inherently measures bulk behavior, to both identify and quantify mechanisms of deformation. Specifically, in this study, increases in the interfacial area of various modifications of Cloisite clay in 1,4‐cis polyisoprene nanocomposites were monitored during uniaxial compression. Interaction of the Fe⁺³ in the clay with the polymer decreases the polymer's ¹H spin–lattice relaxation time constant (T₁). In some of the nanocomposites, the increase in the interfacial area of the clay platelets was observed by a decrease in the polymer's T₁ with successive amounts of strain. The observation of these changes with static ¹H‐NMR is limited by the dispersion of the clay. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1806–1813, 2005     

Organic solvent treatment of isotactic polypropylene films, 1. Effects of organic solvent treatment on proton spin-lattice and spin-spin relaxation times

An isotactic polypropylene film was treated with cyclohexane, toluene, 1-chloronaphthalene, and glycerin. The morphological changes were characterized by measurements of weight loss, density, molecular weight, and spin-lattice and spin-spin relaxation times. There is a relatively narrow temperature range at which a sharp increase in weight loss occurs corresponding to the kind of the solvent. The change in density occurs almost linearly with treatment time for all solvents. Molecular weight also increases as the weight loss is increased. There is a considerable increase in T₁ with increasing treatment temperature. T₂ of the amorphous region, T_2a, also increases as the treatment temperature is increased, but some decrease in T_2a is found in the samples treated with toluene and cyclohexane at higher temperatures. The fraction of the rigid region, F_c, increases and that of the mobile one, F_a, decreases with proceeding solvent extraction. The value of F_c is considerably different from the crystallinity calculated from the density in the early stage of the treatment followed by the gradual approach to the value calculated from the density, although some different behaviour was found with the samples treated with cyclohexane.     

Warpage–crystallinity relations in rotational molding of polypropylene

The authors present a study of the crystalline properties of selected series of polypropylene (PP) materials in relation to their warpage at conditions relevant for rotational molding. The PP materials have different crystallization temperatures and kinetics. The authors study the crystalline features of the materials using hot‐press experiments and differential scanning calorimetry (DSC). In the DSC, the authors do constant cooling rate runs as well as isothermal crystallization and subsequent heating. A multimode crystallization kinetics model was fitted to the DSC cooling runs. Regression analysis was done to relate the results from the crystallization experiments to warpage. The derived empirical model shows that the crystallization temperature, crystallization half‐time, and heat of fusion are the most significant parameters influencing warpage. Materials with warpage deviating from the model average are discussed taking aspects of the multimode kinetics into account. The present work could represent a basic platform for understanding and predicting the warpage during rotational molding process. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Self‐interference flow of an isotactic polypropylene melt in a cavity during injection molding. II. Morphology and crystallinity

This article is principally concerned with the morphology and crystallinity of isotactic polypropylene (iPP) parts molded by injection molding, during which a self‐interference flow (SIF) occurs for the melt in the cavity. Scanning electron microscopy shows that a transverse flow takes place in SIF samples. Wide‐angle X‐ray diffraction and differential scanning calorimetry show that SIF moldings exhibit a γ phase, in addition to α and β phases, and high crystallinity. Meanwhile, the results for iPP moldings made by the conventional flow process, that is, conventional injection molding, are reported for comparison. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2791–2796, 2003     

Electrical properties of chlorinated atactic polypropylene (CAPP) and PVC–DOP–CAPP system

The electrical properties, such as dielectric properties and volume resistivity, of chlorinated atactic polypropylene(CAPP), obtained by thermal chlorination of APP, have been studied. The results show that dielectric constant, loss tangent, and volume resistivity of the chlorinated samples increase with the increase in degree of chlorination. The effect of CAPP as secondary plasticizer in the PVC–DOP system has also been studied. Dielectric study of PVC compounds shows a single relaxation peak, indicating no phase separation. In the case of PVC compounds, it has been observed that with the increase in CAPP content in PVC compounds dielectric constant and loss tangent decrease whereas volume resistivity increases. The increase in volume resistivity was further supported by the increase in activation energy value, obtained from volume resistivity vs. 1/T plot.     

Influence of γ-irradiation on proton spin–lattice relaxation of SBR used as antirads and its ESR investigation

Proton spin–lattice relaxation time t₁ was measured on SBR samples with carbon black or kaolin filler using modified linseed oil. The NMR pulse technique at 90MHz was used in the temperature range from 180 to 400 K. The temperature dependence of t₁ indicates that samples filled with carbon black have similar molecular dynamics to the standard unfilled SBR samples. The activation energy for the motion of the main chain for these samples amounts to 16.4kJ/mol. Samples containing linseed oil modified with para-toluidine showed an activation energy of about 14.6kJ/mol and were not affected by γ-irradiation. Values of the minimum relaxation time t^min₁ were increased by γ-irradiation in comparison with a standard SBR sample. ESR measurements carried out at room temperature by means of an X-band spectrometer indicated that unidentified radicals within the rubber were formed during its mastication with vulcanizing additives. The ESR spectra did not change during the vulcanization process. Samples filled with carbon black showed a broadening of the ESR line; this is consistent with the increase in the electrical conductivity.