Rigid Amorphous Phase Research Articles

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

AbstractThe 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 (χc) for cold‐crystallized iPS according to the following relation: I/I = 0.54χc + 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

Positron Annihilation Lifetime Spectroscopy of Poly(ethylene terephthalate): Contributions from Rigid and Mobile Amorphous Fractions

Systematic divergences in the orthopositronium (o-Ps) annihilation lifetimes, τ3, and intensities, I3, are observed, when comparing melt-crystallized and cold-crystallized poly(ethylene terephthalate) (PET) as a function of crystallinity. Following a previous analysis of corresponding deviations in oxygen permeability, the divergences in I3 and τ3 are traced to distinct characteristic values for the probability of o-Ps formation and o-Ps lifetime in the rigid amorphous phase (RAF) associated with the crystalline lamellae and the mobile amorphous regions (MAF) which are unperturbed by the presence of the crystal phase. Utilizing independent information on the volume fractions of RAF and MAF, a quantitative analysis of the o-Ps annihilation parameters is possible.

DSC and DMTA study of annealed cold-drawn PET: a three phase model interpretation

Cold-drawn poly(ethylene terephthalate) (PET) samples annealed at different undercoolings are studied by means of differential scanning calorimetry and dynamic mechanical thermal analysis. When heating from room temperature, the onset of the glass transition region in cold-drawn, un-annealed samples is found to be significantly lower than in the case of un-oriented PET. On the contrary, the presence of crystalline lamellae in oriented PET cause a shift (and spread out) of the glass transition region towards higher temperatures. The crystal thickening process caused by heating above the annealing temperature, is suggested to take place after a rigid amorphous phase linked to the basal surface of the lamellae has softened. It is found that the low-temperature (between 100 and 140 °C) annealed samples have a glass dispersion region ranging significantly above the annealing temperature itself. This circumstance leads to envisage vitrification as a possible mechanism able to limit lamellar thickening during the annealing process at these low temperatures.

Thermal properties relevant to the processing of PET fibers

AbstractThe thermal properties of poly(ethylene terephthalate) (PET) conventional fibers and microfibers are measured and compared to bulk samples. It is shown that the glass transition temperature (Tg) of the fibers can be monitored with modulated differential scanning calorimetry (MDSC). The Tg region is about 30°C wide and shifted to approximately 110°C for conventional as well as for micro‐PET fibers. The Tg of these fibers is compared to the Tg of cold‐crystallized bulk samples. Upon crystallization, a shift and even a split up of Tg is observed. The second Tg is much broader and is situated around 90°C. This Tg is related to the appearance of a rigid amorphous phase. In comparison, the mobility of the amorphous phase in fibers is even more restricted. The whole multiple melting profile observed on the fibers is the result of a continuous melting and recrystallization process, in contrast to bulk PET. The heat‐set temperature is shown to trigger the start of melting and recrystallization. It is seen in the MDSC as an exotherm in the nonreversing signal and an excess contribution in the heat‐capacity signal. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3840–3849, 2003

Effect of intercrystallite straight-chain segments on Young's modulus of gel-spun polyethylene fibers

The tensile stress and Young's modulus of a series of gel-spun polyethylene fibers were measured in parallel to measurements of transverse and longitudinal sizes of crystallites, molecular orientation and straight-chain-segment (SCS) length distribution in order to find the correlation of the mechanical and microstructural characteristics during drawing. The low-frequency-Raman data evidence the predominance of the chain-straining process on the ends of crystallites. As a result, there appears a rigid intermediate substance (rigid amorphous phase (RAP)) between crystal cores and true amorphous phase. The RAP is composed of various-lengths SCS oriented in the fiber axis direction and arranged without longitudinal ordering. The RAP transforms gradually to uniform-length taut-tie molecules (TTM). The Young's modulus correlates to the increase of the RAP in length, but there were no detectable response of the mechanical parameters to the formation of the TTM.

Dielectric and dynamic mechanical relaxation behaviour of poly(ethylene 2,6-naphthalene dicarboxylate). II. Semicrystalline oriented films

The dielectric and dynamic mechanical behaviour of bi-stretched non-treated and annealed semicrystalline poly(ethylene 2,6-naphthalene dicarboxylate) (PEN) films are studied as a function of different morphologies obtained by thermal treatments at temperatures close to the melting temperature of a semicrystalline film. Differential scanning calorimetry (DSC) shows that the glass transition temperatures do not change significantly with the thermal treatment for bi-stretched films. However, the melting temperatures and the degree of crystallinity increase with the value of annealing temperature. Both dielectric relaxation spectroscopy (DRS) and dynamic mechanical analysis (DMA) display three relaxation processes. In order of decreasing temperature, can be observed: the α-relaxation due to the glass transition, the β ∗-process assigned to cooperative molecular motions of the naphthalene groups which aggregate and the β-relaxation due to local fluctuations of the carbonyl groups. The α-relaxation process shifts to higher temperatures for the 250 and 260 °C treated bi-stretched semicrystalline samples compared to the sample thermally treated at 240 °C according to DRS data but shifts to lower temperatures according to the DMA measurements for the three annealed samples. This discrepency results from the different sensitivity of each methods with regards to the release of orientation. At a fixed frequency the temperature associated to β ∗-relaxation is lower for the non-treated bi-stretched semicrystalline samples than for the treated ones using DMA but no difference can be seen in DRS. The associated apparent activation energies are rather high which suggest cooperative motions. It is assumed that the orientation of the samples prevents coupling between the naphthalene groups due to the stretched chain configuration in the amorphous phase. The activation energy for the β-process given by DRS is independent of the thermal treatment and the value agrees with those found for poly(ethylene terephthalate) (PET) and amorphous PEN. Evidence of the decrease of orientation in the sample with thermal treatment can be seen via the onset of mobility, both by DRS and DMA. Thus, the orientation induces a greater change of properties compared to the crystalline samples obtained from the thermal treatment of an amorphous sample. Finally, a three phase model is proposed since there is evidence of a rigid amorphous phase present in PEN biaxially stretched samples which was favoured by the dependence of dielectric relaxation strengths on the degree of crystallinity for the β ∗- and α-relaxation.

The thermal properties of complex, nanophase-separated macromolecules as revealed by temperature-modulated calorimetry

Linear, flexible macromolecules are long recognized as phase structures limited to micrometer and nanometer dimensions with covalent bonds crossing the interfaces. This special, usually non-equilibrium structure leads to unique properties and a multitude of changes for different thermal and mechanical histories. Analyses that enable the study of these properties are temperature-modulated calorimetry and related techniques which allow the separation of equilibrium and non-equilibrium responses. Research on these topics is reviewed and combined to a model for the nanophases. The new approach to the complex nanophase systems yields a better understanding of the relationship between structure and thermodynamic properties. Special emphasis is placed on the size and surface effects on the glass and melting transitions, the development of rigid-amorphous phases, and the reversible melting within the globally metastable structure.

Influence of Semicrystalline Morphology on the Physical Aging Characteristics of Poly(phenylene sulfide)

We report on the influence of semicrystalline morphology on the physical aging characteristics of poly(phenylene sulfide) or PPS. Specifically, the semicrystalline morphology of PPS was described in terms of a three-phase system comprising a crystalline phase, a mobile-amorphous phase, and a rigid-amorphous phase. The physical aging kinetics were observed to depend on the relative amounts of the mobile-amorphous and rigid-amorphous phases, with accelerated aging rates measured in specimens with higher rigid-amorphous phase fraction. We suggest that the rigid-amorphous phase, which includes chain segments that are more tightly packed relative to the mobile-amorphous phase, is able to accelerate physical aging due to its relative proximity to a state of lower configurational entropy. It is also possible for localized cooperative relaxations within the rigid-amorphous phase to facilitate molecular rearrangements that accelerate the aging process. We also suggest that the configurational entropic state of the...

Thermal Aging of Heterophasic Propylene−Ethylene Copolymers: Morphological Aspects Based on ESR, FTIR, and DSC

Electron spin resonance (ESR) spectra of nitroxide radicals formed in thermally treated heterophasic propylene−ethylene copolymers (HPEC) containing bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate (Tinuvin 770) as a hindered amine stabilizer (HAS) were studied in the temperature range 100−433 K; the nitroxides are derived from the HAS and are termed HAS−NO. The results were compared with ESR spectra of the same radicals obtained first by oxidation of Tinuvin 770 and then doped in HPEC and related homopolymers polyethylene (PE) and polypropylene (PP); these nitroxides are termed spin probes. ESR spectra indicated that HAS−NO and the spin probes in HPEC and homopolymers reside in a range of amorphous sites differing in their dynamical properties. Evidence for the various sites was obtained from the ESR line widths, the temperature variation of the extreme separation in the ESR spectra, and the effect of stress on the ratio of the fast and slow components in the spectra of HAS−NO in thermally treated HPEC. The relative population of sites was explained by assuming that the crystalline domains exert a restraining effect on chains located in vicinal amorphous domains; in PE the restraining effect was more pronounced in the polymer with the higher crystallinity (HDPE vs LDPE). Additional support for this assumption was provided by Fourier transform infrared (FTIR) spectra of HPEC and related polymers and by differential scanning calorimetry (DSC). The dynamically restrained chains evidenced by the spin probes are thought to be located in a rigid amorphous phase, which was described in the literature. In HPEC the fast spectral component represents unrestrained ethylene−propylene rubber (EPR) chains, and the slow spectral component represents amorphous PP and EPR chains in the rigid amorphous phase. This study has demonstrated the exceptional sensitivity of ESR spectra from nitroxide radicals to polymer morphology and degree of crystallinity.

Dielectric Relaxations in Structurally Disordered Materials

Chain mobility in multiphase polymeric materials is studied by thermally stimulated depolarization currents and dielectric spectroscopy in poly(styrene)-b-poly(butadiene)-b-poly(c-caprolactone) triblock copolymers and poly(carbonate)/poly(c-caprolactone) blends. The variation in the relaxation time distribution of the dielectrically active components and in the number of orientable dipoles in the amorphous phases of these materials is interpreted as the result of the existence of a rigid amorphous phase constrained by the crystalline regions or in the case of the blends by a phase segregation which takes place when the crystallization process of the blend components advances. The mean relaxation times for the secondary and segmental motions are not affected when the phases are segregated.

Shrinkage behavior after the heat setting of biaxially stretched poly(ethylene 2,6‐naphthalate) films and bottles

AbstractAmorphous preforms of poly(ethylene 2,6‐naphthalate) (PEN) were biaxially drawn into bottles up to the desired volume under industrial conditions. These bottles were used to characterize the shrinkage behavior of the drawn bottles with or without heat treatment and to study structural variations during heat setting. During drawing, a rigid phase structure was induced, and the amount of the induced rigid phase structure was linearly related to the square root of the extra first strain invariant under equilibrium conditions. During the production of these bottles, this equilibrium was not attained because of high stretching conditions and rapid cooling after stretching. The structure after orientation contained a rigid amorphous phase and an oriented amorphous phase. The shrinkage behavior was a function of the temperature and time of heat setting. Long heat‐setting times, around 30 min, were used to characterize the possible structural variations of the oriented PEN after heat setting at equilibrium. Under the equilibrium conditions of heat setting, the start temperature of the shrinkage was directly related to the heat‐setting temperature and moved from 60°C without heat treatment up to a temperature of 255°C by a heat‐setting temperature of 255°C; this contrasted with poly(ethylene terephthalate) (PET), for which the start temperature of shrinkage was always around 80°C. For heat‐setting temperatures higher than 220°C, the structural variations changed rapidly as a function of the heat‐setting time, and the corresponding shrinkage of the heat‐set samples sank below 1% in a timescale of 30–60 s for a film thickness of 500 μm. The heat treatment of the oriented films taken out of the bottle walls with fixed ends stabilized the induced structures, and the shrinkage of these heat‐set films was zero for temperatures up to the heat‐setting temperature, between 220 and 265°C, if the heat‐setting time was sufficient. According to the results obtained, a heat‐setting time of 30 s, for a film thickness of 500 μm, was sufficient at a heat‐setting temperature of 255°C to stabilize the produced biaxially oriented PEN bottles and to take them out the mold without further shrinkage. During the drawing of PEN, two different types of rigid amorphous phases seemed to be induced, one with a mean shrinkage temperature of 151°C and another rigid amorphous phase, more temperature‐stable than the first one, that shrank in the temperature range of 200–310°C. During heat setting at high temperatures, a continuous transformation of the less stable phase into the very stable phase took place. The heat‐set method after blow molding is industrially possible with PEN, without the complicated process of subsequent cooling before the molds are opened, in contrast to PET. This constitutes a big advantage for the blow molding of PEN bottles and the production of oriented PEN films. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1462–1473, 2003

High-Resolution Solid-State 13C NMR Studies of Poly[(R)-3-hydroxybutyric] Acid

Solid poly[(R)-3-hydroxybutyric] acid was examined by high-resolution 13C MAS NMR, differential scanning calorimetry and infrared spectroscopy. The 13C methyl group resonance consists of three components: the rigid amorphous phase, the crystalline phase and the mobile amorphous phase. Spectral deconvolution, using the Lorentz function, reveals the relative amounts as 62% crystalline and 38% amorphous at 333 K, and 42% crystalline, 40% pure amorphous and 18% β-orthorhombic at 413 K. NMR indicates a large difference in molecular mobility between the crystalline and amorphous regions of the sample. Infrared spectroscopy shows that the stretching at 1725 cm−1 (characteristic of the α-form) comes from the crystalline region, and the bands at 1744 cm−1 (characteristic of the β-form) and 1800 cm−1 come from the amorphous region.

Local and segmental dynamics in homopolymer and triblock copolymers with one semicrystalline block.

Thermally stimulated depolarization currents, TSDC, experiments have been performed on a series of poly(styrene)-b-poly(butadiene)-b-poly(epsilon-caprolactone) triblock copolymers SBC with different proportions of the poly(epsilon-caprolactone) crystallizable block, PCL. The morphology of the segregated microphases varies with the PCL content and has been observed by transmission electron microscopy. The crystallinity of the PCL block is estimated by wide angle x-ray scattering, WAXS. The relaxation times distribution is extracted by a numerical decomposition of the TSDC spectra and it is shown that this distribution is not significantly changed on going from the homopolymer to the triblock copolymer with 16 wt % to 77 wt % of PCL in the original samples. Better segregation of the mesophase structure is reached when the samples are annealed at 413 K and important variations in the TSDC and WAXS spectra are observed as a result of the thermal treatment. For the S09B14C77 triblock copolymer the results obtained can be explained by postulating the existence of a rigid amorphous phase in the PCL block. Such rigid amorphous phase is located between the core-shell cylinders formed by the other blocks [with poly(styrene)(PS) as core and poly(butadiene)(PB) as shell] and is constrained by undulated lamellae of crystalline PCL material. In the case of S35B15C50 triblock copolymer, an important amount of diffuse PS-PCL interphase where the homopolymers are mixed must be present before annealing. The results for the material with the less abundant PCL block are explained as a result of the confinement in nanotubes of PCL surrounded by PB embedded in a vitreous PS matrix. Broadband dielectric experiments on these same materials confirm the results obtained by TSDC spectroscopy.

Effect of rigid amorphous phase on glass transition behavior of poly(trimethylene terephthalate)

In this work, a three-phase model consisting of crystalline, mobile amorphous and rigid amorphous phases (RAP) was used to describe the structural formation of poly(trimethylene terephthalate) (PTT) at various crystallization conditions. The variation in thickness and fraction of each phase was studied on both crystallization temperature T c and crystallization time t c effects from differential scanning calorimetry and small angle X-ray scattering analyzes. The results showed that the rigid amorphous fraction and thickness of PTT increased with increasing T c. Meanwhile, there is no remarkable change in crystalline fraction and thickness, and long period. A characteristic length ξ a for cooperative motion of polymer chains in amorphous phase was determined from the variation in glass transition interval using dynamic mechanical analysis. The change in this length scale of glass transition was considered to be correlated with the variation in the thickness of rigid and mobile amorphous phases. The T g of PTT increases and the relaxation peak becomes broader at high temperature flank with T c. These facts are considered due to that the RAP formation leads to the suppression of cooperative motion for amorphous chains.

The Influence of Drawing in Hot Water on The Morphological Properties of Pet Films as Measured by DSC and Modulated DSC

PET films uniaxially drawn in hot water are studied by means of conventional DSC and modulated DSC (MDSC).Glass transition is studied by MDSC which allows to access the glass transition temperature T g and the variations of ΔC p=C p1−C pg (difference between thermal capacity in the liquid-like and glassy states at T=T g). Variations of T g with the water content (which act as plasticizer) and with the drawing (which rigidifies the amorphous phase) are discussed with regard to the structure engaged in these materials. The increments of ΔC p at T g are also interpreted using a three phases model and the 'strong-fragile’ glass former liquid concept. We show that the ‘fragility’ of the medium increases due to the conjugated effects of deformation and water sorption as soon as a strain induced crystalline phase is obtained. Then, ‘fragility’ decreases drastically with the occurring rigid amorphous phase.

Three phase model in drawn thermoplastic polyesters: comparison of differential scanning calorimetry and thermally stimulated depolarisation current experiments

Differential scanning calorimetry and thermally stimulated depolarisation current measurements are performed to quantify various phases present in amorphous and semi-crystalline polyester samples uniaxially drawn above their respective glass transition temperature. Results show the appearance of a crystalline phase induced by stretching and of a part of the amorphous phase which does not participate in glass transition. The existence of this phase—called rigid amorphous phase—is enhanced by the presence of crystallites rather than by the drawing.

Gas Solubility in and Foamability of Various Forms of Syndiotactic Polystyrene

A high pressure microbalance was used to measure solubility of CO2 at 35°C and pressures to 55 atm in amorphous, meso, and α-crystalline forms of syndiotactic polystyrene, sPS. Diffusion coefficient of CO2 in the various samples were derived from sorption kinetics. When normalized with respect to 100% amorphous content, the solubility and diffusivity of CO2 was found to be highest in the semi-crystalline α-form, followed by the meso and the amorphous forms. This rather unusual observation was attributed to the existence of microvoids in α-sPS and meso-sPS, and due to the finite solubility of gas in the rigid amorphous phase associated with these two forms. All three forms of sPS were foamed at various temperatures in the range 50 to 295°C to give microcellular structures. The cell walls in foams obtained from amorphous sPS showed unusual, starburst type structure. The density of meso-sPS foams first decreased and then increased with foaming temperature due to the transformation of the mesophase to the α-phase with increasing temperature. The microvoids in α-sPS and meso-sPS are conjectured to serve as the nucleating sites rather than the often suggested phenomenon of heterogeneous nucleation induced by the crystallites.

Rigid amorphous phase and low temperature melting endotherm of poly(ethylene terephthalate) studied by modulated differential scanning calorimetry

AbstractThe rigid amorphous phase, the low temperature melting endotherm, and their development with thermal treatment in poly(ethylene terephthalate) (PET) were investigated by means of modulated differential scanning calorimetry. The differential of the reversing heat capacity and nonreversing heat flow signals were used to analyze the behavior of the glass transition and the low temperature melting endotherm. With increasing annealing time, the increment of the heat capacity at the glass‐transition temperature decreased and the increment of heat capacity at the annealing temperature increased. It was suggested that the origin of the low temperature melting endotherm mainly resulted from the transition of the rigid amorphous fraction for the PET used. The glasslike transition of the rigid amorphous fraction occurred between the glass transition and melting. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2779–2785, 2001

Study of Dielectric Relaxation Processes in Poly(p-phenylene sulfide) Using a Thermally Stimulated Discharge Current Technique

The various dielectric relaxations in poly(p-phenylene sulfide) have been investigated using a thermally stimulated discharge current (TSDC) technique. The TSD current spectra in the temperature range 30 to 250°C was analyzed as a function of poling temperature (100–250°C), poling field (5–18 kV/cm), poling time (1–4 h), specimen thickness (0.8–1.6 mm), heating rate (1–4°C/min.) and storage time (2–720 h). From the variation of the properties of peaks appearing in the TSDC spectra, it is concluded that the peak appearing at around 110°C (β-relaxation) is a dipolar transition due to C–S linkage. Samples poled at higher (Tp > Tg) show a peak at around 170°C (β ′-relaxation), which has been associated with the formation of cross-linked ether linkages. The β ′-relaxation gives all the character of dipolar polarization. The peak at around 210°C (α-peak) has been ascribed to a space charge transition. A rigid amorphous phase (RAP) and unsaturated phenylene groups provide the deep charge trapping centers. The activation energy obtained from the Bucci plot method confirms the respective nature of these peaks. The observed dependence of peak temperature on poling temperature and poling time indicates the presence of continuous distributed relaxation.

The influence of semicrystalline morphology on the dielectric relaxation properties of poly(3-hydroxybutyrate)

The dielectric relaxation characteristics of poly(3-hydroxybutyrate) (PHB) in the glass–rubber (α) relaxation region has been investigated. A series of cold-crystallized samples were examined, with emphasis on the influence of semicrystalline morphology on relaxation properties. The presence of crystallinity had a marked impact on the α-relaxation characteristics of the various cold-crystallized specimens as compared to the wholly amorphous material. The constraining influence of the crystallites produced a progressive relaxation broadening and a positive offset in relaxation temperature. With regard to the dielectric relaxation strength, Δ ε, we found that the amorphous phase relaxation in the semicrystalline sample had a completely different temperature dependence compared to the wholly amorphous sample, leading to an increase in relaxation strength as the temperature increases above the glass transition temperature ( T g). This was explained by the existence of a rigid amorphous phase interface which relaxes gradually above the T g of the mobile amorphous material. We suggest that the mobile material is essentially located in the amorphous gaps between lamellar stacks.