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.     

The rigid amorphous fraction of cold-crystallized polyamide 6

The rigid amorphous fraction (RAF) of polyamide 6 ordered/crystallized on heating initially fully amorphous glassy samples has been analyzed. Variation of the maximum annealing temperature allowed generation of partially ordered samples with different amount, perfection and morphology of mesophase or crystals. In samples with low fraction of mesophase between 0 and 20 %, the RAF increases with increasing mesophase fraction to reach a maximum value of 50 %. Further increase of the fraction and perfection of the ordered phase achieved by annealing at high temperature leads to a decrease of the RAF. The ratio between mobile and rigid amorphous fractions increases with increasing crystallinity, suggesting increasing decoupling of crystals and amorphous phase in samples of high crystallinity, and confirming results obtained earlier on poly(ethylene terephthalate) and isotactic polypropylene. The study contains a comparison of the RAF estimated from calorimetric analysis of the heat-capacity increment and dynamic-mechanical analysis of the area of the loss-factor peak on devitrification the mobile amorphous fraction, and a discussion of the effect of the phase composition of cold-ordered/crystallized PA 6 on the stiffness.     

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.     

四氟乙烯-全氟丙基乙烯基醚共聚物结构和性能

采用广角X射线衍射和差示扫描量热法研究了不同组成的四氟乙烯-全氟丙基乙烯基醚（TFE-PPVE)共聚物的结晶结构、结晶度和结晶熔融温度,结果发现,当共聚物中γ(PPVE)由0．48％增大到4．02％时,晶粒尺寸从87．2nm减小到22．0nm,同时结晶度和结晶熔融温度明显降低,熔体流动速率增大,尤其当γ(PPVE)大于2．5％时,增大显著。     

退火处理对成核聚丙烯性能的影响

采用退火处理对磷酸酯钠盐成核的聚丙烯（PP）体系进行了性能调控,研究了退火时间和退火温度对成核PP力学性能、耐热性能和结晶性能的影响。结果表明,随着退火温度的提高和退火时间的延长,PP的力学性能、耐热性能和结晶能力均得到了明显提升,特别是在120 ℃下,退火0.5 h,PP的拉伸强度从38.0 MPa提高到39.4 MPa,弯曲模量从1227 MPa提高到1882 MPa,热变形温度从112.2 ℃提高到122.7 ℃,结晶度从36 %提高到44 %。     

Oxygen solubility and specific volume of rigid amorphous fraction in semicrystalline poly(ethylene terephthalate)

The existence of rigid amorphous fraction (RAF) in semicrystalline poly(ethylene terephthalate) (PET) is associated with the lamellar stack crystalline morphology of this polymer, the regions where several crystalline lamellas are separated by very thin (20-40 Å) amorphous layers. In contrast, regular or mobile amorphous fraction is associated with much thicker interstack regions. The oxygen transport properties of PET isothermally crystallized from the melt (melt-crystallization) or quenched to the glassy state and then isothermally crystallized by heating above T_g (cold-crystallization) were examined at 25 °C. Explanation of unexpectedly high solubility of crystalline PET was attributed to the formation of RAF, which in comparison with mobile amorphous phase is constrained and vitrifies at much higher than T_g temperature thus developing an additional excess-hole free volume upon cooling. Measurements of crystallinity and jump in the heat capacity at T_g were used to determine the amount of mobile and rigid amorphous fractions. Overall oxygen solubility was associated with the solubility of mobile and rigid amorphous fractions. The oxygen solubility of the RAF was determined and related to the specific volume of this fraction. The specific volume of the RAF showed a direct correlation with the crystallization temperature. It was shown that upon crystallization from either melt or glassy state, the constrained between crystalline lamellas PET chains consisting of the RAF, vitrify at the crystallization temperature and resemble the glassy behavior despite high temperature. When cooled to room temperature, the RAF preserves a memory about the melt state of polymer, which is uniquely defined by the crystallization temperature.     

The crystallization behavior and mechanical properties of polylactic acid in the presence of a crystal nucleating agent

The crystallization behavior of polylactic acid (PLA) was studied in the presence of a crystal nucleating agent, ethylenebishydroxystearamide (EBH). The crystallization rate and crystallinity were significantly increased with addition of EBH. The isothermal crystallization half-time at 105°C was decreased from 18.8 minutes for neat PLA to 2.8 minutes for PLA with 1.0 wt % of EBH. The crystallinity of PLA with 1.0 wt % EBH was about 35% after 5-minute annealing at 105°C. Like neat PLA, the double melting peaks were also observed for nucleated PLA. The changes of the double melt peaks were investigated with various crystallization temperatures, heating rates, and annealing times. The heat deflection temperature (HDT) of nucleated PLA was up to 93°C after annealing. The correlation between crystallinity and HDT was demonstrated. A percolation threshold of crystallinity was found corresponding to HDT. The crystal size of nucleated PLA was significantly decreased with addition of EBH. The mechanical properties of annealed PLA blends simultaneously; showed improved modulus and impact strength. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Tailoring the rigid amorphous fraction of isotactic polybutene-1 by ethylene chain defects

The effect of different amounts of ethylene co-units in the butene-1 chain, on the fold-surface structure of crystals of isotactic polybutene-1, has been probed by analysis of the rigid amorphous fraction (RAF). The exclusion of ethylene co-units from crystallization in random butene-1/ethylene copolymers and their accumulation at the crystal basal planes leads to a distinct increase of the RAF with increasing concentration of co-units. A specific RAF was determined by normalization of the RAF to the crystal fraction. While in the butene-1 homopolymer a specific RAF of 20–30% is detected, it increases to more than 100% in copolymers with 5–10 mol% of ethylene co-units, being in accordance with the previously observed increase of the free energy of the crystal fold-surface due to copolymerization. It has also been shown that the specific RAF increases with decreasing temperature of crystallization, due to formation of a fold-surface of lower perfection, containing an increased number of chain segments traversing the crystalline-amorphous interface.     

Annealing effects on the crystallinity of polyetheretherketone (PEEK) and its carbon fiber composite

The degree of crystallinity of polyetheretherketone (PEEK) has been measured using both the density gradient technique (DGT) and differential scanning calorimetry (DSC). The difference in results between the methods was shown to depend on crystallization taking place during the heating scan in the DSC. By freezing the sample at different stages of the DSC thermogram and measuring its crystallinity in the density gradient column, the existence of induced crystallization for PEEK was established. Though this induced crystallization is not visible in the DSC thermogram, it must be taken into account when comparing the degree of crystallinity measured by the two methods. The induced crystallization was in turn interpreted as a result of an increase in crystal perfection that is also commonly observed during the initial stages of the annealing process. Accordingly, the effect of annealing on the crystallinity was also investigated. DSC scans on annealed samples exhibited a small endothermic peak at approximately 10°C above the annealing temperature. This peak was observed in both neat PEEK and its carbon fiber-reinforced composite. Annealed PEEK shows, therefore, two melting transitions, a low one which depends on the annealing temperature and a high one which is independent of annealing temperature conditions. Collectively, the results of this study demonstrate that processing conditions and morphological features must be considered in characterizing semicrystalline-based matrix polymers for high performance composites.     

Tailoring the thermal and mechanical properties of injection-molded poly (lactic acid) parts through annealing

The effect of cold-crystallization on poly (lactic acid) (PLA) injection-molded parts was systemically investigated at different annealing temperatures (80/100/120°C) and annealing times (0.5/1/1.5/2 hr). The relative crystallinity (X_c) and crystal form (α' and α) of samples was investigated by differential scanning calorimetry (DSC) and wide X-ray angle diffraction (WAXD). The dependence of the thermal and mechanical performance on relative crystallinity and crystal form/morphology was discussed in detail. A linear relationship between the increment of heat distortion temperature (HDT) and that of X_c was found. The tensile strength, tensile modulus and storage modulus all increased with annealing time and annealing temperature, while the tensile toughness presented a different behavior. The elongation at break for specimens reached a maximum value of 16.9% after annealing at 80°C for 2 hr, which is a threefold improvement compared to PLA samples prepared without annealing. This work suggests that annealing is an effective method for tailoring the physical properties of PLA products.     

Effects of thermal history on isotactic polypropylene

The effect of past thermal history on the melting behavior of isotactic polypropylene is investigated in some detail. It is shown that a series of stepwise annealing treatments at steadily increasing temperatures will raise the final melting point and will result in a double endothermic peak if the final anneal temperature is at or close to 160°C. It is also shown that a series of stepwise annealing treatments at steadily decreasing temperature will lead to multiple DSC peaks. The number of such separate peaks is equal to or greater than the number of annealing steps. Even low-temperature anneals (100–130°C) affect the melting endotherm, while high-temperature anneals have a marked effect on both the degree of crystallinity of the sample and the final melting temperature. For a 3-min anneal, the highest degree of crystallinity is produced by an anneal temperature of 155°C. The highest melting temperature (～182°C) is produced by a 30-min, or longer, anneal at about 160°C. The implications of these results in terms of crystal thickening and perfection are discussed.     

Low‐frequency sound absorption of organic hybrid comprised of chlorinated polyethylene and N,N′‐dicyclohexyl‐2‐benzothiazolyl sulfenamide

We investigated the sound absorption characteristics of an organic hybrid material comprised of chlorinated polyethylene (CPE) as the matrix polymer and N,N′‐dicyclohexyl‐2‐benzothiazolyl sulfenamide (DBS) as the second component of an organic low‐molecular‐weight compound. We found specific crystallites, obtained by annealing, that generated new absorption for a low‐frequency sound in a CPE/DBS blend. We observed two sound absorption peaks, around 300 and 1000 Hz, in the annealed CPE/DBS (50 : 50 w/w) blends, whereas those peaks were not observed in the untreated sample. There were two kinds of crystals with different melting points in the annealed samples. It was confirmed that the crystals with the lower melting point brought about sound absorption at a low frequency. The crystals that had the lower melting point were smaller and/or more disordered than the crystals that had the higher melting point. We calculated the fraction of these two types of crystals from differential scanning calorimetry and wide‐angle X‐ray diffraction measurements. The annealing or reannealing temperature specified the fraction of the crystal with the lower melting point, and the obtained crystal fraction characterized sound absorption frequency. Therefore, it is possible to control the sound absorption frequency of an organic hybrid by heat treatment such as annealing. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

Annealing effects in poly(phenylene sulfide) as observed by dynamic mechanical analysis

Poly(phenylene sulfide) (PPS) is a semicrystalline engineering resin with exceptional solvent resistance and thermal performance. Properties such as these are directly related to the high crystallinity of PPS. In order to exploit its crystalline nature, PPS should be molded at a high tool temperature (hot oil tool) to allow for the highest crystallization rate, and therefore the highest percent crystallinity. Alternately, if a low mold temperature is used, the molded parts should be annealed. This latter process has been studied for injection molded neat PPS resin for various annealing temperatures. Two different grades of PPS were studied that represent cured and linear types. Samples were studied as-molded, and annealed at 160, 180, 200 and 220°C. Increases in glass transition temperature were noted upon annealing. An increase in storage modulus was also noted for annealed samples. This increase persisted up to approximately the annealing temperature. Differential scanning calorimetry has been used to show that annealing PPS allows for a secondary crystallization to occur whereby an endotherm appears that corresponds to the secondary crystalline phase melting near the annealing temperature. As the annealing temperature is increased, the area of the endotherm increases. The secondary crystallization explains the higher storage modulus that persists up to the annealing temperature. These results are discussed in terms of crystallinity and overall effect on heat distortion temperature.     

Effect of crystallinity on enthalpy recovery peaks and cold‐crystallization peaks in PET via TMDSC and DMA studies

Poly(ethylene terephthalate) (PET) sheets of different crystallinity were obtained by annealing the amorphous PET (aPET) sheets at 110°C for various times. The peaks of enthalpy recovery and double cold‐crystallization in the annealed aPET samples with different crystallinity were investigated by a temperature‐modulated differential scanning calorimeter (TMDSC) and a dynamic mechanical analyzer (DMA). The enthalpy recovery peak around the glass transition temperature was pronounced in TMDSC nonreversing heat flow curves and was found to shift to higher temperatures with higher degrees of crystallinity. The magnitudes of the enthalpy recovery peaks were found to increase with annealing times for samples annealed ≤30 min but to decrease with annealing times for samples annealed ≥40 min. The nonreversing curves also found that the samples annealed short times (≤40 min) having low crystallinity exhibited double cold‐crystallization peaks (or a major peak with a shoulder) in the region of 108–130°C. For samples annealed long times (≥50 min), the cold‐crystallization peaks were reduced to one small peak or disappeared because of high crystallinity in these samples. The double cold‐crystallization exotherms in samples of low crystallinity could be attributed to the superposition of the melting of crystals, formed by the annealing pretreatments, and the cold‐crystallizations occurring during TMDSC heating. The ongoing crystallization after the cold crystallization was clearly seen in the TMDSC nonreversing heat flow curves. DMA data agreed with TMDSC data on the origin of the double cold‐crystallization peaks. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

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.     

Role of additives in influencing heating and melting of polymers during thermoforming

The heating stage in thermoforming of amorphous and semi-crystalline polymers was analyzed for constant heat flux conditions using an energy balance model; candidate polymers were high impact polystyrene and isotactic polypropylene. Using an analytical solution, temperature differences as large as 100°C were predicted to arise between the surface and the interior of the sheet being thermoformed for conditions chosen in this work, and these can limit the heat flux being used. A Matlab program was used to compute temperature and crystallinity profiles for crystal melting. Melting took almost as much time as required to heat the surface of the film to the crystal melting point. High thermal conductivity additives, such as calcium carbonate and graphene, can provide temperature uniformity, and the additive uniformity can be verified using thermogravimetric analysis. The ability of these additives to provide temperature uniformity and to reduce energy consumption and heating time is determined in a quantitative manner. Both additives improve heat transfer, and, at the same added volume fraction, graphene is more effective. However, calcium carbonate has a lower cost. The role of density, specific heat, thermal conductivity, and amount of the polymers and additives in influencing temperature and crystallinity profiles was explored, and methods of carrying out thermoforming in an energy efficient manner are proposed.     

Crystallinity,thermal diffusivity,and electrical conductivity of carbon black filled polyamide 46

Polyamide 46 (PA 46) with carbon black (CB) has been subjected to a heat treatment. Crystallinity, specific heat capacity, crystalline melting peak temperature, thermal diffusivity, and electrical conductivity were measured. The crystallinity increases with duration of thermal treatment. The maximum value is dependent on the filler fraction. A lower CB content leads to a higher crystallinity at maximum tempering time. The crystalline melting peak temperature increases with decreasing filler fraction and duration of thermal treatment due to different crystal types and/or diverging geometric forms of the crystals. Thermal diffusivity and electrical conductivity act positively proportional to each other and increase with CB content and tempering time. The thermal diffusivity decreases with increasing temperature. The volume resistance of PA 46 is lowered by heat treatment. By CB addition in combination with a tempering process, the PA 46 can be transferred into a conductor. CB is moved by PA 46 crystals into amorphous regions forming conductive pathways. © 2019 The Authors. Journal of Applied Polymer Science published by Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48882.     

注塑级PPES/PPS共混合金热性能及结晶性能的研究

通过熔融挤出、注塑成型的方法制备了配比不同的含二氮杂萘酮结构的聚芳醚砜(PPES)和聚苯硫醚(PPS）的共混物，对材料的热性能及结晶性能进行了研究。热失重研究表明，在所组成范围内PPS的加入并未降低共混物的热性能，但使共混物的热变形温度有所降低；PPES的加入使PPS的结晶受到阻碍，当PPES质量分数达80％时，PPS产生了晶格缺陷，从而不能形成完善的球晶，退火过程有利于提高共混物的热变形温度，使PPS形成更完善的球晶。     

The melting process and the rigid amorphous fraction of cis-1,4-polybutadiene

A thorough analysis of the melting behavior of cis-1,4-polybutadiene (cis-PBD) is detailed in this contribution. Isothermal crystallization at −26 °C, followed by cooling, provides a three-phase structure composed of a mobile amorphous fraction equal to 0.41₃, a crystallinity of 0.27₇, and a rigid amorphous fraction of 0.31₀. Similar to many other polymers, cis-PBD displays multiple melting after isothermal crystallization, and up to three main endotherms can be evidenced by calorimetry, in dependence of the scanning rate. The results of conventional and temperature-modulated calorimetry analyses presented in this contribution suggest a link between multiple melting and devitrification of the rigid amorphous fraction in cis-PBD. The small endotherm located a few degrees above the crystallization temperature appears to be caused by concurrent partial mobilization of both the crystal and the rigid amorphous portions. Additional partial mobilization of rigid amorphous segments seems to take place at around −11 °C, and it is only above this temperature that large reorganization of the crystal phase becomes possible, allowing partial melting and recrystallization/annealing/crystal perfection.     

Effect of annealing on fracture behavior of poly(propylene‐block‐ethylene) using essential work of fracture analysis

The influence of annealing conditions on the fracture behavior of poly(propylene‐block‐ethylene) sheets was investigated by means of the essential work of fracture method, and was complemented by the study of the effect of annealing on crystal structure, using differential scanning calorimetry. It was shown that both the crystal perfection degree and crystallinity could be improved substantially as annealing temperature (Ta) increased, while the prolonged annealing time at 80°C mainly resulted in the improvement of crystallinity. The reasons for an increase in the specific essential work of fracture and a decrease in the specific plastic work item as crystal perfection degree and crystallinity grew are discussed. The displacement to failure of double edge notched tension specimens decreased gradually with increasing T_a, and the double‐plastic zone could be observed in all specimens. In addition, a novel method to aid the accurate measurement of intense stress‐whitening outer plastic zone height by adjustment of illumination conditions is proposed. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3438–3446, 2007