Three statistical mechanical arguments that favour chain folding in polymer systems of lamellar morphology

Three arguments are given that imply substantial chain folding in crystalline-amorphous polymer systems of lamellar morphology. These arguments assume random coil behaviour in the amorphous region. (1) The random walk character of the polymer chain portions comprizing the amorphous regions topologically constrains the number of crystalline stems that connect covalently with the amorphous regions to be small. Quantitative estimates are made and lower bounds on the amount of chain folding are given. Estimates of (2) the tightness of loops and (3) the average spatial separation between consecutive crystal stems (along a given chain) are made. The results favour substantial amounts of adjacent and near-adjacent re-entry. Existing experimental measurements on diblock copolymers consisting of crystallizable and non-crystallizable blocks provide a measure of the random walk character of the chains in the amorphous region. It is argued that such systems form lamellar structures with large amounts of chain folding in the crystalline regions as a condition of thermodynamic equilibrium.     

Time-resolved X-ray scattering and calorimetric studies on the crystallization behaviors of poly(ethylene terephthalate) (PET) and its copolymers containing isophthalate units

Time-resolved small-angle X-ray scattering (SAXS) measurements were carried out for PET and its copolymers undergoing isothermal crystallization. Wide-angle X-ray diffraction and differential scanning calorimetric measurements were also performed. Our data analysis of the SAXS results for PET and the copolymers clearly demonstrate that the one layer thickness l₁ (derived directly from the correlation functions of the measured SAXS profiles) is the lamellar crystal thickness d_c, not the amorphous layer thickness d_a. The observed d_c values are found to be always smaller than d_a, regardless of polymer composition. d_c is highly dependent on the crystallization temperature, showing that the degree of supercooling is the major factor determining the thickness of lamellar crystals. No thickening, however, occurs in isothermal crystallizations. The kinked isophthalate units in the copolymer are found to be mostly excluded from the lamellar crystals during the crystallization process, leading to an increase of the amorphous layer thickness. Moreover, the kinked, rigid nature of the isophthalate unit was found to restrict crystal growth along the chain axis of the copolymers and also to lower their crystallinity. Unlike d_c, d_a decreases with crystallization time, causing a reduction of the long period in the lamellar stack. This drop in d_a is interpreted in this paper by taking into account several factors that could influence crystallization behavior: the d_a distribution in the lamellar stacks and its variation with time, the number of lamellae in the lamellar stacks and their effect on the SAXS profile, and the relaxation of polymer chains in the amorphous layers.     

Structure and properties relationship of melt reacted polyamide 6/malenized soybean oil

In this work, a maleinized soybean oil (SOMA) was melt reacted with polyamide 6 and the thermal, rheological, and morphological properties were evaluated. It was observed that the maleinized soybean oil reacted with polyamide chains, increasing the molecular weight of the polymer. Addition of SOMA also promoted an increase in the amount of α crystalline phase as well as in the crystallinity index. The average amorphous layer thickness (L_a) was enhanced with the addition of 1 wt % of SOMA, while the average crystalline layer thickness (L_c) were significantly enlarged with the increase in SOMA content, indicating that SOMA structures were located at the interfacial region between amorphous and crystalline. The addition of 5 wt % of SOMA plasticized the PA6, reducing its glass transition temperature. However, the sample containing 5 wt % of SOMA showed an accentuated pseudoplastic behavior as compared to other samples. Addition of SOMA also reduced the tensile strength and increased the elongation at break. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43050.     

Change of molecular aggregation state in amorphous region with uniaxial extension of preoriented polypropylene

Preoriented isotactic polypropylene was uniaxially drawn at various testing directions and testing temperatures. Change of the molecular aggregation state in amorphous region with extension was elucidated by measurements of melting temperature, enthalpy of fusion, and birefringence at each stage of extension. Melting temperature depends on both crystallite thickness and orientation function of amorphous chains. It is assumed that the enthalpy change of amorphous region takes place when oriented amorphous chains are transformed into random state by heating. The ratio of the enthalpy change of amorphous region in the sample after extension to that in the sample before extension monotonously increased with increasing orientation function of amorphous chains, f_a, independent of testing direction and testing temperature. Increase of true stress with drawing led to increase of f_a. Increase of f_a with extension depended on the testing angle θ between the testing direction and the direction of the crystal c-axis of the preoriented sample, and f_a most remarkably increased in extension at θ = 45°.     

Effect of supercritical CO2 on phase structure of PEO/PVAc blends evaluated from SAXS absolute intensity measurement

The effect of supercritical CO₂ on the morphological structure of crystalline/amorphous PEO/PVAc blends was investigated by means of SAXS with the measurement of absolute scattering intensity. The morphological structure of PEO/PVAc exhibited a considerable change upon CO₂ treatment as demonstrated by the drastic increase of scattering intensity, or the enhancement of electron density contrast between the crystalline and amorphous layers in the lamellar stacks, resulting from the swelling of amorphous PEO via the incorporation of CO₂ into the interlamellar (IL) regions and/or the expulsion of PVAc from the IL regions. Upon CO₂ treatment, the crystal and amorphous layer thickness (l_c and l_a, respectively) were both increased. Compared with the increase of l_a, the increase of l_c was relatively significant and was attributed to the occurrence of melting and recrystallization during CO₂ treatment leading to thicker PEO crystals via a depression of equilibrium melting temperature and/or an increase of crystal fold surface free energy. The measured electron density contrast revealed that the distance of segregation in PEO/PVAc blends involved the extralamellar segregation before CO₂ treatments and the swelling of interlamellar region dominated the drastic increase of scattering intensity after CO₂ treatments. The finding of extralamellar morphology was consistent with the magnitude of volume fraction of lamellar stacks in the blends. The lamellar size distribution appeared to be broader and the lamellar stacks more disorganized for the blends after CO₂ treatments according to SAXS one-dimensional correlation function profiles.     

Stress induced lamellar thickening in poly(ethylene succinate)

The crystalline structure evolution of poly(ethylene succinate) during tensile deformation was investigated by in-situ synchrotron X-ray scattering. Crystal phase transition from α to β form was confirmed to be fully reversible upon stress loading and unloading. An increase of long period was observed during the α–β crystal transition, which was attributed to the increase of both amorphous layer thickness and crystalline layer thickness (lamellar thickness). The crystalline layer thickness was scaled with the fraction of β crystal, and the change of which was in well agreement with the difference in the repeating length along the crystallographic c axis. Both the crystalline layer thickness and the amorphous layer thickness were fully recoverable. A possible molecular model was proposed to visualize the mechanism for the crystal transition and lamellar thickening.     

Effect of supercritical CO2 on morphology of compatible crystalline/amorphous PEO/PMMA blends

The effect of supercritical CO₂ on the morphological structure of compatible crystalline/amorphous poly(ethylene oxide) (PEO)/poly(methyl methacrylate)(PMMA) blends was investigated by means of small angle X-ray scattering (SAXS) with the measurement of absolute scattering intensity. The morphological structure of PEO/PMMA exhibited a considerable change upon CO₂ treatment as demonstrated by the drastic increase of scattering intensity, or the enhancement of electron density contrast between the crystalline and amorphous layers in the lamellar stacks, resulting from the swelling of amorphous PEO via the incorporation of CO₂ into the interlamellar (IL) regions. Upon CO₂ treatment, both the crystal and amorphous layer thickness (l_c and l_a, respectively) were increased with the extents depending on the blend composition. The increasing of l_a upon CO₂ treatment was decreased with increasing PMMA content, suggesting that PEO was much easier to be swollen by CO₂ than PMMA. Compared with the increase of l_a, the increase of l_c was much more significant and was attributed to the occurrence of melting and recrystallization during CO₂ treatment that led to thicker PEO crystals caused by an increased crystal surface free energy. The measured electron density contrast revealed that the distance of segregation in the PEO/PMMA blends involved the extralamellar segregation before CO₂ treatments and the swelling of IL region dominated the drastic increase of scattering intensity after CO₂ treatments. The finding of extralamellar morphology was consistent with the magnitude of volume fraction of lamellar stacks in the blends. The CO₂ treatment could increase the distance of segregation for neat PEO and PEO-rich blends. The lamellar size distribution appeared to be broader and the lamellar stacks more disorganized for the blends after CO₂ treatments according to SAXS one-dimensional correlation function profiles.     

Copolymer crystallization: Approaching equilibrium

Melt crystallization of random copolymers leads to solids with crystalline fraction w_c and final melting temperature that are substantially below the predictions of Flory's equilibrium crystallization theory. Model ethylene/butene random copolymers, when crystallized as multilayer films by rapid solvent evaporation, exhibit increased w_c (50% relative) and (4 K) compared to melt crystallized values. For a copolymer with 0.92 mol fraction ethylene, the density-derived crystallinity w_c=0.6 is the same as that from Flory's theory, although the maximum observable crystal thickness from remains about 25% of the theory value. These effects are seen because crystallization from solution occurs without many of the constraints to segment dynamics that limit crystalline fraction during melt crystallization. Crystal thickness is dominated by secondary nucleation barriers in both melt and solution. Chain or sequence folding is much more regular in the solution crystallized material, and amorphous layer thickness is reduced from about 8 nm to 3 nm.     

Rotating frame 1H n.m.r. spin-lattice relaxation studies of modified isotactic polypropylene

Rotating frame ¹H n.m.r. spin-lattice relaxation times T_1ϱ have been used to study relaxation processes in partially crystalline polypropylene and polypropylene modified by a copolymer of ethylene and alkylaminoacrylate. Measurements were carried out within the temperature range of 250–420 K. Three relaxation times T_1ϱ were detected over the whole temperature range. α_c relaxation in the crystalline regions of the polymer, relaxation processes β(U) and β(L), associated with an apparent double glass transition, and α_a relaxation process, which was ascribed to a free reorientation of the whole chains in the amorphous regions, were observed by temperature dependences of these relaxation times. In addition to the α_c relaxation, another relaxation process with relatively low molecular mobility was found in crystalline regions. The influence of the polymer modifier on the observed relaxation processes greatly depends on its amount.     

Isotactic polypropylene/polymethylmethacrylate blends: Effects of addition of propylene-graft-methylmethacrylate copolymer

A novel graft copolymer of unsaturated propylene with methyl methacrylate (uPP-g-PMMA) was added to binary blends of isotactic polypropylene (iPP) and atactic poly(methyl methacrylate) (aPMMA) with a view to using such a copolymer as a compatibilizer for iPP/aPMMA materials. Optical microscopy (OM), scanning electron microscopy, wide angle X-ray scattering (WAXS), and small angle X-ray scattering (SAXS) techniques showed that, contrary to expectation, the uPP-g-PMMA addition does not provide iPP/aPMMA compatibilized materials, irrespective of composition. As a matter of fact the degree of dispersion of the minor component achieved following the addition of uPP-g-PMMA copolymer remained quite comparable to that exhibited by binary blends of iPP and aPMMA with no relevant evidence of adhesion or interconnection between the phases. On the other hand the crystalline texture was deeply modified by the copolymer presence. With increasing uPP-g-PMMA content (w/w) the iPP spherulites were found to become more open and coarse and the dimensions and number per unit area of the amorphous interspherulitic contact regions were found to increase. According to such OM results the copolymer uncrystallizable sequences were assumed to be mainly located in interfibrillar and interspherulitic amorphous contact regions. SAXS analysis demonstrated that the phase structure developed in the iPP/aPMMA/uPP-g-PMMA blends is characterized by values of the long period increasing linearly with increasing copolymer content (w/w). Assuming a two phase model for the iPP spherulite fibrillae, constituted of alternating parallel crystalline lamellae and amorphous layers, the lamellar structure of the iPP phase in the ternary blends is characterized by crystalline lamellar thickness (L_c) and an interlamellar amorphous layer (L_a) higher than that shown by plain iPP and L_c and L_a values both increased with increasing uPP-g-PMMA content (w/w). Such SAXS results have been accounted for by assuming that a cocrystallization phenomenon between propylenic sequences of the uPP-g-PMMA copolymer and iPP occurs. The development of the iPP lamellar structure in the iPP/aPMMA/uPP-g-PMMA blends was thus modeled hypothesizing that during such a cocrystallization process copolymer PMMA chains with comparatively lower molecular mass remain entrapped into the iPP interlamellar amorphous layer forming their own domains. Moreover, evidence of strong correlations between the crystallization process of the uPP-g-PMMA copolymer and the iPP crystallization process was shown also by differential scanning calorimetry and WAXS experiments. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 2377–2393, 1997     

Isotactic polypropylene/polystyrene blends: Effects of the addition of a graft copolymer of propylene with styrene

A novel graft copolymer of unsaturated propylene with styrene (uPP-g-PS) was added to binary blends of isotactic polypropylene (iPP) and atactic polystyrene (aPS) with a view to using such a copolymer as compatibilizer for iPP/aPS materials. Differential scanning calorimetry, optical microscopy, scanning electron microscopy (SEM), wide angle X-ray scattering, and small angle X-ray scattering (SAXS) techniques have been carried out to investigate the phase morphology and structure developed in solution-cast samples of iPP/aPS/uPP-g-PS ternary blends. It was found that the uPP-g-PS addition can provide iPP/aPS-compatibilized materials and that the extent of the achieved compatibilization is composition-dependent. Blends of iPP and aPS exhibited a coarse domain morphology that is characteristic of immiscible polymer systems. By adding 2% (wt/wt) of uPP-g-PS copolymer a very broad particle-size distribution was obtained, even though the particles appeared coated by a smooth interfacial layer, as expected according to a core–shell interfacial model. With increasing uPP-g-PS content (5% wt/wt), a finer dispersion degree of particles, together with morphological evidence of interfacial adhesion, was found. With further increase of uPP-g-PS amount (10% wt/wt) the material showed such a homogeneous texture that neither domains of dispersed phase nor holes could be clearly detected by SEM. The type of interface developed in such iPP/aPS/uPP-g-PS blends was accounted for by an interfacial interpenetration model. The iPP crystalline texture, size, neatness, and regularity of iPP spherulites crystallized from iPP/aPS/uPP-g-PS blends were found to decrease when the copolymer content was slightly increased. Assuming, for the iPP spherulite fibrillae, a two-phase model constituted by alternating parallel crystalline lamellae and amorphous layers, it was shown by SAXS that the phase structure generated in iPP/aPS/uPP-g-PS blends is characterized by crystalline lamellar thickness (L_c) and interlamellar amorphous layer thickness (L_a) higher than that shown by plain iPP; the higher the copolymer content, the higher the L_c and L_a. It should be remarked that considerably larger increases have been found in L_a values. Such SAXS results have been accounted for by assuming that a cocrystallization phenomenon between propylenic sequences of the uPP-g-PS copolymer and iPP occurs and that during such a process PS chains grafted into copolymer sequences remain entrapped in iPP interlamellar amorphous layers, where they form their own separate domains. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65:1539–1553, 1997     

X-ray investigations on annealed fibers of poly(p-phenylene-1,3,4-oxadiazole)

The influence of annealing on the supermolecular structure of commercial, thermostable fibers, spun from solutions of poly(p-phenylene-1,3,4-oxadiazole) (POD) in H₂SO₄, is examined. The crystalline α-modification of thermally treated POD fibers has an orthorhombic unit–cell probably of space group P2₁2₁2₁. The symmetry of the single POD chain in these crystallites is 2₁. The unit–cell dimensions are a = 1.235 nm, b = 0.655 nm, c = 1.40···1.47 nm, where c depends on the annealing temperature T_a. The unit cell contains 4 chains of two monomers each. Annealing up to T_a of about 755 K causes increases in crystallite size, crystalline orientation, and linear degree of order, combined with an improved axial Young's modulus E. Thermal degradation at higher temperatures leads to the breaking of tie molecules in general, while UV-radiation selectively damages tie molecules that are not taut.     

Influence of crystallinity and chain interactions on the electrical properties of polyamides/carbon nanotubes nanocomposites

This work evaluates the effects of the crystallinity degree and π–π interactions between nanoparticles and a polymeric matrix on the electrical properties of polyamides and carbon nanotubes (CNT) nanocomposites. Two polymeric matrices were chosen; polyamide (PA) 6.6, a semi-crystalline polymer, and PA 6I-6T (here called aPA), a semi-aromatic and amorphous polyamide. The PA 6.6 crystallinity degree did not significantly change. Both lamellar thicknesses, amorphous (L_a) and crystalline (L_c), were estimated through Small-angle X-ray scattering. L_a increased significantly when CNTs were added. Both nanocomposites presented almost the same percolation threshold. In aPA nanocomposites, the π–π interaction between aromatic groups of CNTs and aPA is not only responsible for a homogeneous CNT dispersion, but also creates a direct path, parallel to the electrodes, for electron conduction after the percolation limit. In the PA 6.6 nanocomposites, the CNTs preferably disperse in the amorphous regions, forming a conductive network.     

Plastic deformation of crystalline polymers

Under uniaxial tensile load, the plastic deformation of unoriented crystalline polymers first transforms the lamellae into a fibrous structure. Usually the drawing is inhomogeneous with a neck propagating through the sample. The higher the draw ratio, the higher the axial elastic modulus as a consequence of the larger fraction of taut tie molecules in amorphous layers connecting the crystalline blocks of each microfibril. As a consequence of the almost 1/(1 ? α) times higher strain of amorphous layers under tensile load, the taut tie molecules are much more strained than the chains in crystal blocks. Hence, their contribution to elastic modulus is substantially higher than one would guess from their fraction β. This is more so in polyethylene with higher crystallinity (α = 0.8) than in nylon 6 with low crystallinity (α = 0.5). Even for the highest modulus polyethylene E = 70 GPa ～ 0.3 × E_c, one needs less than 7.5 percent of taut tie molecules. The plastic deformation of the fibrous structure markedly enhances the number of interfibrillar tie molecules in nylon 6 and to a lesser extent in polyethylene and polypropylene. Homogeneous drawing without a neck transforms the whole sample into a fibrous structure rather uniformly so that for a long while one has the lamellar and fibrillar morphology side by side. The end effect on the structure obtained does not differ appreciably from inhomogeneous drawing with neck propagation. The drawing of polymers with a liquid crystal structure yields a highly aligned fibrous structure with very few chain folds and an exceptionally high elastic modulus and strength. But the axial connection of individual highly oriented and ordered domains is affected by a relatively small fiaction of tie molecules, and this is responsible for reduction of the elastic modulus below the value of the ideal crystal lattice.     

Raman spectroscopic study of the microstructure of carbon films developed from cobalt chloride‐modified polyacrylonitrile

Polyacrylonitrile (PAN) was modified with cobalt chloride at 90°C for 5 min. The carbon films prepared from original and modified PAN films were carbonized up to 1300°C. The structure of the resulting carbon film was studied using X‐ray diffraction and Raman spectroscopy. The stacking size obtained from X‐ray diffraction approaches the L_c value of the resulting carbon films as the heat treatment temperature increased. The mean average carbon basal planes in crystalline (Lc/d) also increased with increasing pyrolysis temperature. Raman spectra confirmed the progressive structural ordering as treatment temperature increased. During pyrolysis, a substantial decrease in the intensity of the band near the 1350 cm⁻¹ region was observed, indicating a decrease in the disordered structure. The crystal size (L_a) of the resulting carbon films also showed a remarkable increase with increased heat treatment temperature. The resulting carbon films developed from the modified PAN films had higher L_c and L_a than those developed from the original PAN film. It was established that cobalt catalyzes graphitization of amorphous carbon during pyrolysis. This modification not only promoted the growth of crystal size but also increased the close packing of the carbon basal planes. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 2219–2225, 1999     

Relationship between orientation of amorphous chains and modulus in highly oriented polypropylene

By using oriented polypropylene prepared by forced quenching in a zone-drawing-type apparatus, the effect of taut tie molecules on the modulus is studied by measuring the changes of superstructure with an increasing draw ratio and the temperature at which the oriented polypropylene was annealed. Superstructure is analyzed by means of an x-ray method, differential scanning calorimetry, thermal shrinkage, birefringence, and infrared spectrum. Modulus increases with an increasing orientation function of amorphous chains, ?_a, and is decided only by the value of ?_a, so long as the higher value of orientation function of the crystal c axis does not change with the draw ratio or annealing. The taut tie molecules in ultrahigh-modulus polypropylene are loosened by annealing at temperatures below 420 K, but would be incorporated into folded lamellar crystals above the annealing temperature of 420 K. The taut tie molecules does not always have a 3₁ helix conformation.     

The suppression of strain induced crystallization in PET through sub micron TiO2 particle incorporation

The effects of TiO₂ particles on the crystallization and uniaxial stress-strain behavior of poly(ethyleneterephthalate) (PET) films from amorphous precursors were investigated. The addition of small fraction of sub micron sized TiO₂ particles were found to suppress the mechanical relaxation processes associated with high temperature side of the β relaxation while enhancing the low temperature relaxation. When crystallized from unoriented precursors, TiO₂ particles act as a nucleation agent and enhance the thermally induced crystallization of the PET chains. However, when stretched from the amorphous precursors in rubbery temperature range (T_g<T_p<T_cc), the TiO₂ concentration levels as low as 0.35 wt% was found to reduce the overall stress and retard strain hardening and accompanying stress induced crystallization. As a result, under the same stretching conditions, the films containing TiO₂ were found to possess lower crystallinity and orientation levels. This was attributed to suppression of stress induced crystallization as these particles act to disrupt the formation of crystalline lattice by their physical presence. This may be as a result of the reduction of chain entanglements in the presence of these sub micron sized TiO₂ particles in the structure of the polymers that retard the formation of physical network whose nodes are made up of entanglements and small crystalline domains. The development of this long range ‘connected’ network is primarily responsible for the rapid upturn in the stresses at the onset of strain hardening observed in stress strain curves.This method represent a unique way to apply ‘anti-nucleating agent’ effect to control the development of stress induced crystallization that will help control the film and fiber forming processes.     

Distribution of chain defects and microstructure of melt crystallized polyethylene

A combined study using wide-angle X-ray diffraction (WAXD) and small-angle X-ray diffraction (SAXD) of a series of mostly low density polyethylenes with a wide range of chain defect concentrations (0.1–7%) crystallized from the melt is reported. The data presented here complement the earlier results obtained by Swan⁷ and Holdsworth and Keller⁸ for copolymers. The concurrent unit cell expansion and long period decrease with increasing chain defect concentration leads to a picture of chain defects (branches, unsaturations) being distributed between the crystalline lamellae and the surface layer. Based on a model which assumes inclusion of defects within the lattice by means of a generation of 2gl kinks, (supported by the parallel increase of paracrystallinity) an estimation of the concentration of chain defects, ?_c, incorporated into the crystal lattice (<1%) is attempted. The density of defects in the non-crystalline regions, ?_a, turns out to be substantially larger than ?_c and supports the view of a clustering of defects, ?_c and ?_a are both increasing functions of ? with a tendency to level off for ? > 6. According to this model, the fraction of defects incorporated into the lattice does not exceed 20% of the total number of defects in any of the samples investigated. The fraction of defects excluded from the lattice (>80%), on the other hand, sets a higher permissable limit to the crystal thickness value achieved.     

Experimental evidence for trapped chain entanglements: their influence on macroscopic behaviour of networks

The swelling and elastic properties of poly(dimethyl siloxane) networks prepared by end-linking and subsequently swollen in heptane and toluene have been investigated as a function of the volume fraction, v_c, at which networks are generated. Increases in both swelling degree and shear modulus with v_c demonstrate the increase in number of trapped entanglements with v_c. The results may be described by a simple scaling law approach.     

Structure formation in poly(ethylene terephthalate) upon annealing as revealed by microindentation hardness and X-ray scattering

The micromechanical properties (microindentation hardness, H, elastic modulus, E) of poly(ethylene terephthalate) (PET), isothermally crystallized at various temperatures (T_a) from the glassy state are determined to establish correlations with thermal properties and nanostructure. Analysis of melting temperature and crystal thickness derived from the interface distribution function analysis of SAXS data reveals that for T_a<190 °C the occurrence of two lamellar stack populations prevails whereas for samples annealed at T_a>190 °C a population of lamellar stacks with a unimodal thickness distribution emerges. The H and E-values exhibit a tendency to increase with the degree of crystallinity. The results support a correlation E/H∼20 in accordance with other previously reported data. The changes of microhardness with annealing temperature are discussed in terms of the crystallinity and crystalline lamellar thickness variation. Unusually high hardness values obtained for PET samples crystallized at T_a=190 °C are discussed in terms of the role of the rigid amorphous phase which offers for the hardness of amorphous layers constrained between lamellar stacks a value of H_a∼150 MPa. On the other hand, for T_a=240 °C the decreasing H-tendency could be connected with the chemical degradation of the material at high temperature.