Effect of high‐temperature annealing on the microstructure and mechanical properties of polypropylene with shish kebab or spherulite structure

Various annealing temperatures below, near, or above the melting temperature were used to anneal polypropylene with oriented shish kebab and isolated spherulite structures in this work. The results showed that a high annealing temperature decreases the time needed to achieve the ideal material property. When the annealing temperature is near or above the melting temperature, the impact strength would be 1.6 times improved by partial melting and recrystallization. The crystal structure of the oriented shish kebab or isolated spherulite structures was improved when annealed at 150 °C, whereas annealing at 165 or 170 °C recombined the crystal lamellae of the structure. Moreover, the high crystallinity and thick lamellae improved the impact and yield strength values of the spherulite structure. However, excessively high crystallinity and thick lamellae in the oriented shish kebab structure did not result in good mechanical performance. Therefore, the prediction of mechanical properties for the shish kebab structure based on crystallinity and lamellar thickness is not feasible. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46465.     

Crystallization and morphology of nylon-6,6 crystals: 1. Solution crystallization and solution annealing behaviour

In the course of crystallizing nylon-6,6 from solution, previous studies on the relation between lamellar thickness (/) as assessed by low-angle X-ray scattering and crystallization temperature (T_c) were extended further. This includes the extension of the T_c range and the use of solvents also other than butane-1,4-diol employed hitherto. Upwards extension of the temperature range was achieved by the annealing of crystals, formed at a lower T_c′ in their own concentrated mother liquor at higher temperatures (T_A). Most surprisingly, the/versusT_c curve joined up smoothly and with considerable overlap with the/versusT_A curve, a behaviour which if to be proved general may have weighty implications for existing ideas on the origin of chain-folding. The use of different solvents confirmed that/is determined by the supercooling (and not by T_c or T_A alone) also in the present polymer. The/versus$\text{T}{}_{\mathrm{c}}\text{T}{}_{\mathrm{A}}$ relation consists of a long horizontal plateau followed by an upswing at higher $\text{T}{}_{\mathrm{c}}\text{T}{}_{\mathrm{A}}$ values. As in previous works, along the horizontal plateau and at the beginning of the upswing the X-ray diffraction pattern displayed two subsidiary (non-Bragg) maxima. However, at the newly achieved higher end of the upswing three such maxima could be obtained, this being the first instance of a variation in the number of such maxima in a given polymer. The number of subsidiary maxima provides a direct measure of the length of the fold stems, and their intensities gives information on the nature of the fold. The newly achieved 3-maxima case reaffirms and strengthens previous evidence that there is a strong preference for acid units within the fold portions and that along certain positions of the/versus$\text{T}{}_{\mathrm{c}}\text{T}{}_{\mathrm{A}}$ curve the fold is tight (one acid unit). Heating in a non-solvent provided new examples of distinct doubling of the initial fold length in the course of conventional annealing treatment. Calorimetric (d.s.c.) studies were also carried out on many of the nylon-6,6 preparations. D.s.c. melting curves indicate that changes in the internal morphology of nylon-6,6 crystal occur if scanning speeds less than 80 K min^?1 are used. Measurements on single crystals prepared at different solution and/or annealing temperatures yield a heat of fusion value of 61 cal g^?1 for the ideal nylon-6,6 crystal. The value of 69 erg cm^?2 is obtained for the fold surface energy of the single crystals. The thermodynamic melting temperature, T⁰_m′ for this polymer is found to be the surprisingly high value of 301°C.     

Influence of annealing treatment on the structure and properties of poly(4‐methyl‐1‐pentene)‐based films and membranes

The poly(4‐methyl‐1‐pentene) casting films were prepared by melt extrusion and annealed below the melting temperature. The effect of annealing conditions on the structure and properties of casting films and stretched membranes was discussed. In this work, a new peak around annealing temperature, as shown in melting curves, revealed the increase in thickness of lamellar structure. Annealing treatment led to improvements of amorphous thickness and crystal orientation. And the thickness of crystal phase correlated with the logarithm of annealing time. The increase in annealing temperature or time led to the improvements of the hard elasticity of samples. Additionally, the larger porosity of stretched membranes was observed as the annealing time and temperature increased. An optimum annealing condition to prepare microporous membranes was 30 min, 200 °C. This work also discussed the importance of annealing treatment in the preparation of microporous membranes. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46491.     

Variations in polymeric structure of ferroelectric poly(vinylidene fluoride) films during annealing at various temperatures

To clarify the thermal degradation mechanisms of uniaxially drawn poly(vinylidene fluoride) (PVDF) films, variations due to annealing in the polymeric structures of the films were investigated using the small‐angle X‐ray scattering (SAXS) and Fourier transform infrared (FTIR) spectroscopy. The films were composed of lamellar crystals that were stacked perpendicular to the stretch direction. Although the crystallinity of the films decreased during annealing in the temperature range above the preannealing temperature, the lamellar structure was maintained even after the annealing process. There are two kinds of irreversible relaxation mechanisms during the annealing process of the films, including both a decrease in crystallinity within the lamellae and also thickening of the lamellae. A significant lamella thickening effect was observed when the films were annealed above ～ 100°C. FTIR spectra suggested some disordered structures are developed during thickening of the lamellae. Furthermore, a long‐range periodic structure was formed in the films that were annealed above the melting temperature of PVDF. The polymeric structures formed during the fabrication process (including high‐order structures and disorders in molecular conformation) were clarified as having a significant influence on the annealing behavior of ferroelectric PVDF films. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Annealing of polyethylene crystals in the hexagonal phase

The kinetics of thickening of polyethylene crystals in the hexagonal phase is studied. Distributions of thickness, L, of bands and/or lamellae are obtained by electron microscopy for a specimen annealed at p=6.1 kbar, T=242°C for 30, 300, 3600 and 86400 seconds. The dependence of the average band thickness $\text{L}\text{?}\text{(t)}$ on the annealing time t can be expressed as $\text{L}\text{?}\text{(t)=46·}\text{log}\text{(}\text{t}\text{t}{}_0\text{)+}\text{L}\text{?}\text{(t}{}_0\text{) [}\text{nm}\text{]}$. Moreover, these distributions can be superposed on each other if we scale L by $\text{L}\text{?}\text{(t)}$. A model for the thickening process of a lamellae is proposed; coalescence of two bands occurs by disappearance of the boundary with the rate proportional to exp(?αL). The value of α depends on the mobility of defects in the boundary and is discussed in detail.     

The effects of annealing (solid and melt) on the time evolution of the polymorphic structure of polyamide 6

To understand whether and how the thermal history, especially the melting annealing, affects the polymorphism and thermal property of polyamide 6 (PA6), the temperature‐modulated differential scanning calorimetry technology was used to investigate the effects of thermal histories, including annealing temperature, annealing time, and cooling rate, on the polymorphic behavior and thermal property of PA6. It was found that longer annealing time and faster cooling rate favored the formation of α crystal when PA6 samples were annealed in the solid‐state at 175°C. As the annealing temperature was elevated to 195°C, faster cooling rate also favored the formation of α crystal, whereas longer annealing time was more favorable for the formation of γ‐form crystals. When PA6 samples were annealed in the melt‐state (245°C), however, although the α crystal was dominating crystalline phase, the formation of γ crystal was greatly enhanced with increasing annealing time and cooling rate. Moreover, a small endothermic peak was observed in the low‐temperature region in PA6 samples annealed at 175°C and 195°C, which might be related to the melting of microcrystals formed in the amorphous regions during annealing. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Influence of annealing temperature on the lamellar and connecting bridge structure of stretched polypropylene microporous membrane

The influence of annealing temperature on the lamellar and connecting bridge structure of stretched polypropylene microporous membrane was investigated using small‐angle X‐ray scattering, temperature‐modulated differential scanning calorimetry and scanning electron microscopy. It is found that with increasing annealing temperature from 105 to 145 °C, the main lamella melting peak combines with that from connecting bridges and a uniform pore arrangement is obtained in the microporous membrane. The annealed lamella thickness is increased and lamellar structure is improved, due to the occurrence of melting and recrystallization during annealing. At the same time, more secondary crystals are formed. The melting and recrystallization and secondary crystals contribute to the appearance of an annealing peak in the differential scanning calorimetry curve of annealed film. During the following cold and hot stretching, the secondary crystals disappear and convert to initial connecting bridges. The improved lamellar structure can support the scaffold of pore structure, resulting in a uniform connecting bridge arrangement. But further increasing the temperature to 150 °C degrades the initial lamellar structure, leading to a decrease of pore arrangement in the stretched microporous membrane. Annealing leads to the difference of lamellar structure: the initial lamellar structure is improved and some weak secondary crystals are formed in the amorphous region. © 2014 Society of Chemical Industry     

Crystallization of polyamides under elevated pressure: 4. Annealing of nylon-6 (polycapramide) under pressure

A study has been made on the annealing of nylon-6 under elevated pressure. Heat treatment of meltcrystallized nylon-6 at 6.5 kbar and 20°C below the beginning of melting for a period of 120 h yielded an increase in the heat of fusion from 14.2 to 41.2 cal/g and an increase in atmospheric melting temperature from 222° to 256°C (1 kbar = 100 MN/m²; 1 cal/g = 4.187 kJ/kg). Stepwise annealing by exposing nylon-6 to progressively higher temperatures at 6.5 kbar led to a heat fusion of 40.8 cal/g and a melting temperature of 269°C. Annealing was found to be particularly effective in improving the crystalline structure at pressures exceeding 4 kbar. The rate of annealing at 6.5 kbar increased with temperature in the range between 260° and 280°C. Electron microscopy of fracture surfaces disclosed that annealing could give rise to a marked increase in lamellar thickness. Wide-angle X-ray diffraction showed that crystal growth also occurred in the lateral direction and that the alpha-crystalline modification was preserved during annealing. From a comparison between the melting characteristics of nylon-6 obtained by pressure-induced crystallization from the melt and by annealing under pressure of folded-chain material, it is inferred that the folded-chain lamellar state may be an essential intermediate stage of the chain extension in polyamides under pressure.     

Annealing effect on the microstructure of poly(vinyl chloride)

The effect of annealing on the microstructure of commercial grade poly(vinyl chloride) was investigated by calorimetric, X-ray and viscoelastic measurements. The degree of crystallinity increases with increasing annealing temperature from above the glass transition temperature up to 130°C, at which point the degree of crystallinity takes on a maximum value. Also, the crystal melting temperature increases with increasing annealing temperature. Thermal analysis and X-ray study suggest that the crystallite of poly (vinyl chloride) decomposes by thermal degradation when annealed, above 170°C. The isothermal crystallization process is analyzed using Avrami's equation employing the degree of crystallinity as a function of annealing time at various annealing temperatures. The crystallization rate has a maximum value at around 140°C. It is expected that the crystalline texture grows in the shape of a lineal-like habit, judging from the magnitude of Avrami's constant and from a study of the X-ray intensity distribution. The α_f-transition was observed to occur at temperatures 5 to 10°C lower than the crystalline melting temperatures for annealed specimens of poly(vinyl chloride) using a dynamic spring analysis. The α_f-transition may be attributed to thermal molecular motions with a long time scale, resulting from the cross-link points introduced by the small crystallites.     

Pitch coke structure and its transition into graphite: Influence of the separation process of quinoline-insoluble matter

It has been established that the texture of pitch coke depends on the pre-carbonization treatment of the pitch and the separation of the primary quinoline-insoluble matter (OI). Scanning electron microscopy was used to examine the texture of fracture surfaces of pitch cokes prepared from the raw pitch and from pitches deprived of their QI fractions by quinoline extraction or filtration, three basic structures, lamellar, globular and composite, being determined. The arrangement of the lamellar structure is affected by the method of separation of QI and results in different graphitizable products. In pitch, filtered at 350 °C, the first symptoms of lamellar arrangement were already observed at heat treatment $\text{1 > 450}\text{°C}$.     

Effect of prior sample history on n-hexane sorption in glassy polystyrene microspheres

The effect of prior swelling or thermal annealing on the kinetics and apparent equilibria of n-hexane vapour sorption at 30°C in submicron diameter polystyrene microspheres was studied at penetrant activities of 0.75 and 0.10. Preswelling with n-hexane at an activity of 0.90 increased both the rate and the apparent equilibrium of subsequent n-hexane sorption at both the lower activities. Conversely, thermal annealing reduced the sorption rate and apparent equilibrium sorption relative to the measured sorption in the ‘as-received’ sample. Cycling experiments, at the high activity ($\text{p}\text{p}\text{° = 0.75}$), revealed that the effects of the earlier preswelling or thermal history are effectively erased by the first sorption cycle. In contrast, at the lower activity ($\text{p}\text{p}\text{° = 0.10}$), the effects of preswelling or thermal annealing decay more gradually consequent to repeated cycling. The observed convergence of the apparent equilibrium penetrant concentration, in all samples, to a single value upon repeated cycling suggests that a true glassy state equilibrium sorption can be approached by conditioning microsphere samples by the particular cycling histories studied in this work.     

X‐ray diffraction studies on reverse‐annealed polyethylenes

Linear low and high density polyethylene sheets were compression molded and crystallized at a 5–10°C/min cooling rate. Parts of the sheets were annealed at different temperatures up to 2°C below the melting temperature. The small angle X‐ray scattering (SAXS) and the wide angle X‐ray scattering intensities of the annealed samples were studied. SAXS intensities showed particle scattering with a bimodal size distribution. The estimated radii of gyration were 15–17 nm and 5–7 nm, respectively. The crystallinity and the radius of gyration increased slightly with increasing annealing temperature for some samples; others did not show any change. No peaks characteristic of intercorrelated lamellar crystallinity in the SAXS intensities developed during the annealing. The original broad peak of high density polyethylene disappeared from the SAXS recordings on annealing. The length of the perfect chain versus melting temperature was calculated by the Thomson‐Gibbs formula and Flory's concept of melting temperature depression where methyl groups and tertiary carbon atoms at the branches were regarded as second components (solvent). Linear relationships were found for both cases. Experimental data for a linear low density polyethylene obtained from the literature were in between the two functions. A lamellar model of crystallization corresponding to the data is proposed. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 340–349, 2001     

Secondary crystallization and premelting endo‐ and exotherms in oriented polymers

Crystallization in polyamide 6 (nylon 6) fibers during annealing was studied in detail by following the changes that occurred in the neighborhood of crystalline relaxation temperatures, by using wide‐ and small‐angle X‐ray scattering and differential scanning calorimetry (DSC). Two distinct crystallization regimes were observed depending on whether annealing was carried out below or above onset of crystalline relaxation at ～190°C. In fibers annealed below 190°C, minor melting peaks were followed by exothermic transitions. These were attributed to ～1.5% (by weight) of microcrystals formed during annealing that melt and recrystallize during the DSC scan. These microcrystals are nucleated from unoriented amorphous chains between the lamellar stack within a fibril, and are shown to account for the observed increase in the crystalline orientation and decrease in permeability. Fibers annealed above 190°C did not show the exotherm and had significantly fewer microcrystals. The crystallization in this regime was attributed to the growth of existing lamellae, as evidenced by the increase in crystallite size, crystalline density, crystalline orientation, lamellar spacing, and lamellar intensity. The changes at annealing temperatures >190°C are accompanied by increased dyeability, indicative of more open amorphous regions. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 447–454, 2006     

Thermal behaviour and annealing of poly(vinylidene fluoride)

In this study, we report the melting behavior of poly(vinylidene fluoride) (PVF₂) annealed in a differential scanning calorimeter. PVF₂ annealed under isothermal conditions often shows double or triple melting endotherms depending on the annealing temperature (T_a) and the heating rate. The lower melting peak temperature increases as T_a increases. When the annealing time is varied, there is a systematic increase in the size of the lower endotherms. This suggests that a portion of the main endothermic response is due to reorganization during the scan. Annealing PVF₂ not only increases the degree of crystallinity, but also improves the crystal perfection. The ability of an annealing sample to reorganize decreases as the annealing time increases. However, an additional third melting peak appears when PVF₂ is annealed at 140°C for a sufficiently long time. The existence of this peak suggests that more than one kind of distribution of crystal perfection may occur when PVF₂ is quenched from the melt into liquid nitrogen and subsequently annealed.     

Mechanical anisotropy in oriented polychlorotrifluoroethylene

The dynamic tensile modulus (E) and loss (tan δ) of various oriented samples of polychlorotrifluoroethylene have been measured at 0°, 45° and 90° to the draw direction over the range of ? 100 to 160°C. For the cold-drawn samples the mechanical anisotropy at the lowest temperature is determined by the overall chain orientation resulting in E₀ >E₄₅ >E₉₀. Above the γ relaxation (20°C) a shear process is activated leading to a change in the anisotropy pattern to E₀ >E₉₀ >E₄₅. Cold-drawing followed by annealing gives rise to further changes so that E₉₀ >E₀ >E₄₅ above the β relaxation (120°C). The effect of annealing has been attributed to the relaxation of the amorphous regions and the development of lamellar texture, and the anisotropy in tan $\text{δ}\text{E}$ at the β relaxation is found to be consistent with the interlamellar shear model.     

Untersuchung der ursachen für das auftreten mehrerer schmelzpeaks in den DSC-thermogrammen von hochverstreckten und getemperten folien aus isotaktischem polypropylen

Isotactic polypropylen films were highly stretched and annealed during 20 s under stretching tension at temperatures up to 206°C at maximum. The melting behaviour of these films was investigated by using a differential scanning calorimeter (DSC — 1 B). Two melting peaks at about 165 and 174°C respectively, were observed. The heat of fusion of the former peak predominates, if the film temperature during annealing exceeds 190°C. Its position is independent of the stretching ratio. This melting peak is assigned to crystalline domains, which show a long identity period of 140 to 160 Å and corresponds to an amorphous orientation factor of 0 to ?0.2. The higher melting peak appears during stretching. This peak is assigned to crystalline domains, which show a long identity period longer than 200 Å and corresponds to an amorphous orientation factor of 0.4 to 0.5. This fraction refers obviously to the paracrystalline building blocks of the microfibrils, which are transformed at a film temperature higher than 190°C to a greater part into smaller lamellaforming paracrystallites. The transformation of the fibrillar into a lamellar texture was visualised by transmission electronmicroscopy.     

Lamellar thickening and chain-extended growth of polyethylene

The lamellar thickness of chain-extended polyethylene produced by annealing at high pressure increases logarithmically with time. The rate of increase is faster than for the same material annealed at 1 bar (≡10⁵N/m²), but the large crystal thickness owes more to the initial increase than to enhanced thickening. The similarity of high and low pressure behaviour on annealing supports the hypothesis that chain-folded and chain-extended growth of polyethylene are models, one of the other.     

Thermal and mechanical properties of tetramethyl-p-silphenylenesiloxanedimethylsiloxane block copolymer

Several kinds of $\text{tetramethyl}\text{-p-}\text{silphenylenesiloxane}\text{dimethylsiloxane}$ (TMPS/DMS) block copolymers having various compositions and segment lengths were synthesized by the polycondensation of p-bis(dimethylhydroxysilyl)benzene and silanol-terminated DMS oligomers of different degrees of polymerization, which were 19, 43, 300, 380 and 540 DMS monomer units. The compositions ranged from TMPS/DMS wt% ratio of 100/0 to 24/76. For these copolymers, differential scanning calorimetry was carried out to determine the melting temperatures, the heat of fusion and the crystallinities. The melting temperatures and the crystallinities of the block copolymers were found to decrease as DMS contents were increased from 11 to 76 wt% and as DMS segment lengths were decreased from 540 to 19. The crystalline parts of TMPS segment would be increased according to the long TMPS sequences which were obtained from the copolymerizations by using DMS oligomers with high degrees of polymerization such as 300, 380 and 540. The stress-strain behaviour and the dynamic mechanical behaviour were also investigated for these copolymers. The tensile strength was decreased and the percentage elongation was increased with increasing DMS content and segment length. In the case of the copolymers for which the DMS contents remained constant at 26 wt%, two major transitions were observed at around ?120° and ?10°C for the copolymers having DMS block sizes of 300, 380 and 540. But for the copolymers having those of 19 and 43 the two transitions merged together at ?50°C. The relaxations at ?120°C corresponding to the glass transition of DMS component and those at ?10°C are due to the amorphous TMPS phase which is separated from the DMS phase owing to the longer sequence length. The relaxation observed around ?50°C is due to the shorter sequence length of TMPS in the main chain plus the presence of more flexible DMS component. It may be suggested that the long sequence length causes large domains of hard and soft phases which consist of TMPS and DMS blocks respectively.     

Thermodynamics of HCl in water—ethylene carbonate mixtures

The emf of the cell: Pt; H₂ (1 atm.)/HCl (m), Ethylene carbonate (x), H₂O (100?x)/AgCl, Ag is measured at 5° interval in the temperature range 25 to 45°C. The standard potential of the silver—silver chloride electrode, E^o_m, is determined in each solvent mixture covering the range 0–78% EC. The temperature coefficient is calculated using the method of least squares. The mean activity coefficient of HCl, γ_±, is determined for molalities between 0.01 and 0.05 m in each case. The primary, secondary and total medium effects are calculated. From the variation of the mean activity coefficient with temperature, the relative partial molal heat content $\text{L}$₂ of HCl is computed. $\text{L}$₂ passes through a minimum at 30% for 0.05 m HCl at all temperatures. The effect of temperature on $\text{L}$₂ permits the evaluation of the corresponding relative partial molal specific heat $\text{J}$₂.     

Effects of annealing on the hierarchical crystalline structures and mechanical properties of injection‐molded bars of high‐density polyethylene

The effect of annealing on the microstructural evolution and mechanical properties of high‐density polyethylene parts molded via gas‐assisted injection molding was investigated using scanning electron microscopy, differential scanning calorimetry, two‐dimensional wide‐angle X‐ray diffraction and tensile testing. The results indicated that a variety of annealing temperatures could induce considerable variations in the hierarchical structures, crystallinity, lamellar thickness and yield stress of the molded bars. According to these results, the annealing temperatures could be divided into three regions. In the low‐temperature region of annealing at 80 °C, the spatial variation of the superstructure developed along the thickness direction and mechanical properties of the annealed sample were mainly unchanged and similar to those of the original specimen. At 100 and 120 °C, the intermediate temperature region of annealing, the thickness of the crystals, degree of orientation and yield stress of annealed samples were greatly improved. Finally, at 127 °C, the degree of orientation decreased and yield stress slightly improved, an indication of the high‐temperature annealing region being characterized by increasing melting/recrystallization and causing relaxation of oriented molecular chains. A model is proposed to interpret the mechanism of the annealing treatment of the samples at various temperatures. © 2013 Society of Chemical Industry