Investigation of the rigid amorphous fraction in Nylon-6

A three-phase model, comprising crystalline, mobile amorphous, and rigid amorphous fractions (χ _c, χ _MA, χ _RA, respectively) has been applied in the study of semicrystalline Nylon-6. The samples studied were Nylon-6 alpha phase prepared by subsequent annealing of a parent sample slowly cooled from the melt. The treated samples were annealed at 110°C, then briefly heated to 136°C, then re-annealed at 110°C. Temperature-modulated differential scanning calorimetry (TMDSC) measurements allow the devitrification of the rigid amorphous fraction to be examined. We observe a lower endotherm, termed the ‘annealing’ peak in the non-reversing heat flow after annealing at 110°C. By brief heating above this lower endotherm and immediately quenching in LN₂-cooled glass beads, the glass transition temperature and χ _RA decrease substantially, χ _MA increases, and the annealing peak disappears. The annealing peak corresponds to the point at which partial de-vitrification of the rigid amorphous fraction (RAF) occurs. Re-annealing at 110°C causes the glass transition and χ _RA to increase, and χ _MA to decrease. None of these treatments affected the measured degree of crystallinity, but it cannot be excluded that crystal reorganization or recrystallization may also occur at the annealing peak, contributing to the de-vitrification of the rigid amorphous fraction. Using a combined approach of thermal analysis with wide and small angle X-ray scattering, we analyze the location of the rigid amorphous and mobile amorphous fractions within the context of the Heterogeneous and Homogeneous Stack Models. Results show the homogeneous stack model is the correct one for Nylon-6. The cooperativity length (ξ_A) increases with a decrease of rigid amorphous fraction, or, increase of the mobile amorphous fraction. Devitrification of some of the RAF leads to the broadening of the glass transition region and shift of T _g.     

Density fluctuations in amorphous and semicrystalline polymers

Summary The small-angle scattering of amorphous and semicrystalline polymers contains an intensity component due to density fluctuations within the crystalline and amorphous domains.For amorphous polymers, the density fluctuations aboveT _g correspond to the theoretical value for a fluid system in thermodynamic equilibrium. BelowT _g , a temperature dependence proportional to T is observed over a range of about 50°. At lower temperatures, a linear relationship with a smaller slope has been found which extrapolates to a non-zero value at 0 °K. This value corresponds to the frozen-in disorder, the slope at low temperatures is related to thermal vibrations and can be evaluated in terms of photon-phonon scattering.Semicrystalline polymers show a temperature dependence of the density fluctuation similar to that of the amorphous polymers. At constant temperature the density fluctuations vary linearly with crystallinity.Natural rubber shows an increase of the density fluctuations with increasing cross-linking densities from which information on the density changes in the vicinity of a cross-link and on the statistics of the distribution of cross-linking can be obtained.

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Zusammenfassung Die Kleinwinkelstreuung amorpher und teilkristalliner Polymere besitzt eine Intensitätskomponente, die von Dichtefluktuationen innerhalb der kristallinen und amorphen Bezirke herrührt. Für amorphe Polymere entspricht die Dichtefluktuation oberhalb vonT _g dem theoretischen Wert für ein fluides System im thermodynamischen Gleichgewicht. UnterhalbT _g wird eine Temperaturabhängigkeit proportional zuT über einen Bereich von etwa 50° beobachtet. Bei tieferen Temperaturen wird eine lineare Beziehung mit einer geringeren Steigung gefunden, welche zu einem endlichen Wert bei 0 °K extrapoliert werden kann. Dieser Wert bezieht sich auf die eingefrorene Fehlordnung, die Steigung bei tiefen Temperaturen ist auf thermische Schwingungen zurückzuführen und kann als Photon-Phonon-Streuung ausgewertet werden.Teilkristalline Polymere zeigen eine Temperaturabhängigkeit der Dichtefluktuation, die der von amorphen Polymeren ähnlich ist. Bei konstanter Temperatur ändert sich die Dichtefluktuation linear mit der Kristallinität.Naturkautschuk zeigt eine mit der Vernetzungsdichte ansteigende Dichtefluktuation, aus der man Information über die Dichteänderung in der Umgebung eines Netzpunktes und die Statistik der Netzpunktverteilung erhalten kann.

     

Correspondence between the rigid amorphous fraction and nanoconfinement effects on glass formation

Semicrystalline polymers' properties are strongly influenced by the presence of a “rigid amorphous fraction” (RAF) of material with enhanced glass transition temperature and suppressed mobility. Here, we review striking similarities between this domain and near‐interface regions of altered dynamics in other nanostructured polymers, including thin films, nanocomposites, layered films, and ionomers. Building on these observations, we present results of molecular simulations of model nanolayered polymers in which one layer is crystalline. The results of these simulations, which bear close similarities to semicrystalline homopolymers while providing a well‐defined equilibrium crystal size, suggest that the RAF shares the same phenomenology and mechanism as dynamic interphases in these other polymeric systems. Specifically, the fraction of rigid amorphous material is driven by the scale of cooperative rearrangements characteristic of dynamics in polymers and other supercooled liquids. These results situate the RAF as a specific instance of a general tendency towards broad dynamic interphases in polymers and other glass‐forming materials. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 907–918     

The low-temperature endotherm in poly(ethylene terephthalate): partial melting and rigid amorphous fraction mobilization

A detailed investigation of the low-temperature endotherm of poly(ethylene terephthalate) (PET) performed by temperature-modulated differential scanning calorimetry is presented. The origin of the small endotherm, generally observed a few degrees above the crystallization temperature in PET and in many other polymers, is a widely discussed matter. The most frequent interpretation considers it the result of partial fusion with superposition of a recrystallization process even if it has also been proposed that it can originate from enthalpic recovery connected to mobilization of the rigid amorphous fraction. In an attempt to resolve the question, a new method for the interpretation of the modulated heat-flow-rate curve resulting from a temperature modulation program is proposed. The procedure consists of the analysis of the initial points of the steady-state heat-flow-rate signals in the heating and cooling semiperiods with the temperature modulation being performed with a sawtooth profile. The study conducted in parallel on the reversing specific heat capacity and the heat-flow-rate curves, observed on heating after isothermal crystallization at various temperatures, showed that multiple processes, involving both the crystalline and the rigid amorphous fraction, overlap in the temperature range in which the low-temperature endotherm is observed. The origin of the endotherm under investigation is therefore connected with both partial fusion of the crystalline portions and enthalpy recovery subsequent to structural relaxation of the rigid amorphous fraction. An estimation of the relative percentages of the two different processes is presented and discussed.     

Development of the crystallinity and rigid amorphous fraction in cold‐crystallized isotactic polystyrene

The phase structure of crystalline isotactic polystyrene (iPS) has been investigated with temperature‐modulated differential scanning calorimetry (TMDSC), wide‐angle X‐ray scattering (WAXS), and Fourier transform infrared (FTIR) spectroscopy. Quenched amorphous samples have been cold‐crystallized at 140 or 170 °C for various crystallization times. The degree of crystallinity obtained from WAXS, with the ratio of the crystal peak intensity to the total peak intensity, shows excellent agreement with the crystallinity determined from TMDSC total heat flow endotherms. For the first time, FTIR results show that the absorbance peak ratio (I/I) has a linear correlation with the crystalline mass fraction (χ_c) for cold‐crystallized iPS according to the following relation: I/I = 0.54χ_c + 0.16. This relationship allows the crystallinity of iPS to be determined from infrared spectroscopy analyses in cases in which it is difficult to perform thermal or X‐ray measurements. On the basis of the measurements of the heat capacity increment at the glass transition, we find that a significant amount of the rigid amorphous fraction (RAF) coexists with the crystalline and mobile amorphous phases in cold‐crystallized iPS. The RAF increases systematically with the crystallization time, and a greater amount is formed at a lower crystallization temperature. A three‐phase model (crystalline phase, mobile amorphous phase, and rigid amorphous phase) is, therefore, appropriate for the interpretation of the structure of cold‐crystallized iPS. The origin of the low‐temperature endothermic peak (annealing peak) has been investigated with TMDSC and FTIR spectroscopy and has been shown to be due to irreversible relaxation of the RAF. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 3026–3036, 2003     

Calculation of small-angle neutron scattering by macromolecules in the semicrystalline state

Summary The conformation of macromolecules in the semicrystalline state has been studied by various authors with respect to the validity of the adjacent re-entry — or switchboard model by application of small-angle neutron scattering. Analytical as well as Monte-Carlo calculations show that the experimental results for melt crystallized polyethylene and isotactic polypropylene can be interpreted on the basis of the solidification model. In this model it is assumed that crystallization occurs by straightening of coil sequences without a long range diffusion process.

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Zusammenfassung Die Konformation von Makromolekülen im teilkristallinen Zustand wurde von verschiedenen Autoren im Hinblick auf die Gültigkeit des adjacent re-entry- oder switchboard-Modells mit Hilfe der Neutronenkleinwinkelstreuung untersucht. Sowohl analytische Rechnungen als auch Monte-Carlo Rechnungen zeigen, dag die an schmelzkristallisiertem Polyäthylen und isotaktischem Polypropylen gefundenen experimentellen Ergebnisse auf der Basis des Erstarrungsmodells interpretiert werden können. In diesem Modell wird angenommen, daß sich Knäuelsequenzen wahrend der Kristallisation ausrichten, ohne dag weitreichende Diffusionsprozesse erfolgen.

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With 8 figures and 3 tables     

Crystal-amorphous interphases in binary semicrystalline/amorphous polymer blends

Crystal-amorphous interphases in binary polymer blends that are miscible in the melts but phase separate due to crystallization of one polymer have been investigated theoretically by employing lattice models and experimentally by dielectric spectroscopy measurements. Theory predicts the extent of tight adjacent re-entry to depend strongly on the energy E_q disfavoring the tight-fold conformations and to increase slightly with favorable interaction energy - χ_AB in the blends. The interfacial region of varying composition is predicted to depend strongly on χ_AB, with the interfacial thickness varying with the reciprocal of |χ_AB|^1/2. Therefore, in the limit χ_AB → 0 the amorphous polymer, which is miscible in the melt, is predicted to be completely excluded from the interlamellar region, in agreement with experimental results. Dielectric relaxation experiments on semicrystalline blends of poly(vinylidene fluoride) (PVDF) with poly(methyl methacrylate) (PMMA) or poly(vinyl pyrrolidone) (PVP) show the existence of nearly pure PVDF interphase which is not penetrated by PMMA or PVP, despite their strongly favorable interactions with PVDF. These experimental results are discussed and compared with theoretical predictions.     

Dielectric relaxation of poly(phenylene sulfide) containing a fraction of rigid amorphous phase

The dielectric relaxation behavior of poly(phenylene sulfide), PPS, has been investigated from room temperature to 180°C. This study was undertaken to examine the mobility of the amorphous phase through the glass transition region, to determine the contribution that rigid amorphous phase material makes to the relaxation process. Semicrystalline samples contain a fraction of the rigid amorphous phase, which was determined from the heat capacity increment at the glass transition, using degree of crystallinity determined from x-ray scattering. In the dielectric experiment, we measured the temperature and frequency dependence of the real and imaginary parts of the dielectric function. ε″ vs. ε′ was used to determine the dielectric relaxation intensity, δε = ε_s–ε∞, at temperatures above the glass transition. For amorphous PPS, δε decreases as temperature increases, while for all semicrystalline PPS, δε increases with temperature. The ratio of semicrystalline intensity to amorphous intensity determines the total fraction of dipoles which are already relaxed at a given temperature. Results indicate that more and more rigid amorphous phase material relaxes as the temperature is increased. This provides the first evidence that rigid amorphous phase material in PPS contains chains that possess different levels of molecular mobility. Finally, to the temperature of the loss peak maximum, at a given frequency, we assign the value of the dielectric T_g. For both melt and cold crystallization, the dielectric T_g systematically decreases as the crystallization temperature increases, and as the fraction of rigid amorphous phase decreases.     

Flexibility of macromolecules and structure formation in amorphous polymers

The reflection indicating the presence of nanostructures with a coherence length of ~10 nm is found for the first time on the wide-angle X-ray diffraction patterns of amorphous polymers. It is shown that the structural organization in the amorphous polymers is formed much more rapidly and effectively if macromolecules contain regions with increased local flexibility or short spacings with another mechanism of hindered internal rotation around single bonds than that in the chain. Average intermolecular distances in the polymers with nanostructures are substantially smaller than those in the structureless ones. The temperature range of existence of the structural organization in the amorphous polymers above their glass-transition temperature becomes narrower with an increase in the kinetic rigidity of macromolecules.     

Loop entanglement of semicrystalline polyethylene in amorphous region: Diamond lattice approach

Linear polyethylenes in the amorphous region have been simulated as restricted random walks on a diamond lattice between two absorbing planes. The links among loops were studied by treating loops as oriented curves. The local conformations of polyethylene chains (i.e., trans and gauche energy differences) were considered in the simulation, thereby determining the effect of crystallization temperature on the loop entanglement. It was found that the total Gauss winding and link density of linked loops increased with the thickness of the amorphous region. This result agrees with that of the cubic lattice model. The link probability decreases very slowly with the thickness of the amorphous region. On the other hand, the results presented clearly indicate that all statistical measures of linked loops decrease with temperature. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 348–353, 1999     

Contribution of crazing to the toughness of amorphous and semicrystalline polymers

Crazes or craze-like deformation bands occur in many different polymers. Mechanisms of initiation and growth of the crazes have been studied using different techniques of electron microscopy. Examples of modifying amorphous and semi-crystalline polymers are given to demonstrate to which extent the macroscopic toughness of the polymers is affected by changes in the microscopic structure and by the amount of crazes.     

Small- and intermediate-angle neutron scattering from amorphous and semicrystalline polyethylene terephthalate

Small- and intermediate-angle (0.006 < Q < 0.25 Å) neutron scattering experiments have been performed on polyethylene terephthalate (PET). Amorphous and semicrystalline samples with deuterated PET molecular weights of 24,200 and 40,500 were prepared. The average value of R/M determined in the small-angle region for the amorphous samples is 0.406, in close agreement with the experimental θ-solvent (0.398–0.420) and theoretical (0.394) values for unperturbed dimensions. The intermediate-angle results for the amorphous sample are consistent with the random coil prediction to Q = 0.08 Å, but beyond this Q value there is some deviation from Gaussian behavior. The semicrystalline samples show a decrease in the radius of gyration as measured in the small-angle region and an increase in intensity (when compared to the amorphous data) in the intermediate-angle region, especially for the low-molecular-weight material. These results suggest that crystallization in PET is accompanied by greater molecular reorganization than has previously been observed in most bulk polymers.     

Temperature-Modulated Differential Scanning Calorimetry IX. Some comments on the rigid amorphous fraction in semi-crystalline poly(ethylene terephthalate)

Journal of Thermal Analysis and Calorimetry - The increment of heat capacity at the glass transition for semi-crystalline poly(ethylene terephthalate) (PET) observed by temperature-modulated...     

Intrinsic viscosity of rigid macromolecules in dilute solution in steady shear flow

A rigid linear array of beads in a Newtonian fluid is used to model a rod-like macromolecule in a dilute solution. Following the work of Kotaka, an expression is obtained relating the intrinsic viscosity to the velocity gradient. Computed results are compared with the experimental results of Strömberg.     

Electrochromism of indolinobenzospirans dissolved in rigid amorphous polymer matrices

The effect of a static electric field on the position of absorption bands of colored stereoisomers of two photochromic compounds of the indolinobenzospiropiran group dissolved in matrices of poly(methyl methacrylate), poly(n-butyl methacrylate), polystyrene, and polycarbonate at room temperature was studied. In the case of matrices with the polymethacrylates the effects observed show that applied electric field produces a shift of thermal equilibrium towards stablization of stereoisomers of larger dipole moment. On the basis of kinetic measurements of attainment of thermal equilibrium on switching the electric field off, the rate constants of their interconversion have been determined. In the case of matrices with polystyrene and polycarbonate, a small red shift only was observed.