热历史对尼龙1212熔融行为的影响

利用DSC方法研究了不同热历史条件对尼龙1212熔融行为的影响.不同的热历史条件下,在DSC曲线上,观察到尼龙1212产生2个或3个熔融峰,依据聚合物结晶理论,对各峰的来源进行了分析.在160℃下不同温度退火120 min的尼龙1212样品DSC曲线上,低温结晶熔融峰主要由低温结晶形成的一些微晶体或者片晶熔融产生,其晶体完善程度较差,熔融峰值较低,峰面积较小;主熔融峰是由样品在淬火过程中形成的晶体和升温过程中低温结晶形成的晶体的熔融重结晶形成较为完善的晶体熔融所产生,熔融峰值较高,峰面积较大.在不同的升温速率条件下,熔融峰温度有所移动,表明不同升温速率条件下产生的熔融峰的结晶晶型是相同的.在不同结晶时间下结晶,延长结晶时间对较高完善程度晶体的生长有利.在不同温度下依次退火处理的样品,熔融产生两个附加峰,这两个附加峰的峰温都比它们相应的退火温度高,而峰高和峰面积随退火温度降低而减小.根据等温结晶结果,由Hoffman方法确定了尼龙1212的平衡熔融温度为202.8℃.     

尼龙1010结晶与熔融行为的研究

用DSC研究了降温速率R对尼龙10 10结晶与熔融的影响,以及室温(RT)和液氮(LN)骤冷退火样品的熔融.降温时结晶温度随R增大线性降低;T_g以上可完成结晶时结晶度相同;结晶起始温度>181℃生成的晶体有三个熔融峰,对应于环状和放射状球晶的转化与熔融;在181℃和T_g间结晶,无放射球晶转化峰;T_g下有结晶放热峰样品加热时有冷结晶发生.RT未退火样品三个熔融峰,退火温度T_α≥180℃样品两个峰,结晶度C∝T_a;LN未退火样品单一熔融峰,T_a>160℃双峰,T_a≤160℃三峰,低温峰温与C均∝T.     

Crystallization and melting of cold-crystallized poly(phenylene sulfide)

In this study, we report the melting behavior of poly(phenylene sulfide), PPS, which has been cold-crystallized from the rubbery amorphous state. We find that the crystallization kinetics are faster for cold-crystallized PPS than for melt-crystallized material, due to formation during quenching of a short-range ordered, but noncrystalline, structure. We observe that the endothermic response of cold-crystallized PPS at a large undercooling consists of a low temperature endotherm, followed by an exothermic region, and by the main higher melting endotherm. The lower melting peak temperature of cold-crystallized PPS increases as the crystallization temperature increases, but the main upper melting peak temperature remains almost the same. The size of the exothermic region is strongly related to the degree of undercooling, and must be taken into account in order properly to determine the degree of crystallinity of the material prior to the scan. When the crystallization time is varied, we see a systematic decrease in the size of the main endotherm, and an increase in size of the lower melting endotherm. This suggests that a portion of the main endothermic response is due to reorganization during the scan. Annealing will not only increase the degree of crystallinity but also improve the crystal perfection; therefore the ability of an annealed sample to reorganize decreases as the annealing time increases. However, an additional third melting peak is seen when a cold-crystallized sample is annealed at high temperature for a sufficiently long residence time. The existence of the third melting peak suggests that more than one kind of distribution of crystal perfection may occur when PPS has been cold-crystallized and subsequently annealed.     

PC/PET/EPDM共混体系的结晶与熔融行为

本文用解偏振光法与DSC法分别测定并研究了PC/PET/EPDM共混体系的结晶速度、结晶度、Avrami指数(n)和熔融温度及其影响因素,共混物中PET的结晶速度、结晶度均随PC含量增加而下降;EPDM用量不超过10%时,可提高PET的结晶速度,但不影响结晶度和成核与增长方式,n值不变。当EPDM为5%时,结晶速度呈现极大值。经退火处理的共混物呈现熔融双峰,PC量增加,高温熔融峰略移向高温方向;热处理温度升高或时间延长,则低温熔融峰移向高温方向。     

热处理对尼龙1010固态结晶和玻璃态热松驰的影响

本文用DSC首先论证淬火尼龙1010试样在DSC曲线上出现的放热峰是冷结晶峰,然后研究淬火尼龙1010在不同热处理条件下,冷结晶峰和玻璃态热松驰峰的变化规律。实验结果表明,等温结晶时间较短,试样的固态结晶速率较快;等温结晶时间较长,固态结晶速率较慢,这可能与在Tg区域等温所形成的新氢键有关。当升高等温温度时,固态结晶速率加快。在低于Tg的不同温度退火,玻璃态热松弛峰的峰高及热焓在281K达最大值,进而确定对玻璃态热松驰影响最敏感的温度区间是277～284K。     

The double melting behavior of a liquid crystalline polyimide derived from PMDA and 1,3‐bis[4‐(4′‐aminophenoxyl) cumyl] benzene

The double melting behavior of a thermotropic liquid crystalline polyimide was studied by means of differential scanning calorimetry (DSC), polarized light microscopy (PLM), transmission electron microscopy (TEM), wide‐angle X‐ray diffraction (WAXD), and small‐angle X‐ray scattering (SAXS). This liquid crystalline polyimide exhibited a normal melting peak around 278 °C and transformed into a smectic A phase. The smectic A phase changed to nematic phase upon heating to 298 °C, then became isotropic melt around 345 °C. The samples annealed or isothermally crystallized at lower temperature showed double melting endotherms during heating scan. The annealing‐induced melting endotherm was highly dependent on annealing conditions, whereas the normal melting endotherm was almost not influenced by annealing when the annealing temperature was low. Various possibilities for the lower melting endotherm are discussed. The equilibrium melting points of both melting peaks were extrapolated to be 283.2 °C. Combined analytical results showed that the double melting peaks were from the melting of the two types of crystallites generated from two crystallization processes: a slow and a fast one. Fast crystallization may start from the well‐aligned liquid crystal domains, whereas the slow one may be from the fringed or amorphous regions. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3018–3031, 2000     

PA13N的合成和非等温结晶动力学研究

采用DSC方法研究了PA13N在不同降温速率下的结晶过程,并利用Avrami方程研究了其非等温结晶动力学。在非等温结晶过程中,随着降温速率的增大,结晶温度向低温偏移。综合利用Avrami方程得到Avrami指数为1.35~1.88和结晶速率常数Zc≈1;并求得其结晶活化能为-58.42kJ/mol。结果表明,PA13N的结晶能力小于其他脂肪族尼龙。     

Relationship between melting behavior and morphological changes of semicrystalline polymers

The present study on the case of poly(hexamethylene succinate) is to provide a basis for a better understanding of the subtle relationship between melting behavior and morphological changes of semicrystalline polymers. The melting behavior and morphological changes of poly(hexamethylene succinate) during both isothermal secondary crystallization and annealing processes were investigated by DSC and SAXS. DSC results showed that, with increasing crystallization time or annealing time, the melting endotherm continuously shifted to higher temperature, which suggested that some minor structural or morphological changes must occur. However, almost no changes at all on the crystal thickness were observed from SAXS measurements. The observed evidence confirmed that the increase in the melting temperature is not attributed to crystal thickening but crystal perfection. More exactly, the rearrangement and smoothing of tie molecules at the folding surface result in the reduction of the fold surface free energy, which dominantly contributes to the increase in the melting peak temperature. The origin of the new endothermic peak observed after annealing at elevated temperature was also discussed. TMDSC results indicated that the annealing peak resulted from the enthalpy relaxation and devitrification transition of rigid amorphous fraction formed by the driving force of thermodynamic nonequilibrium, rather than usually regarded as the melting of thin lamellae or imperfect crystals formed by annealing secondary crystallization.     

Crystallization behavior of Mg–Cu–Y amorphous alloy

Amorphous Mg₆₁Cu₂₄Y₁₅ ribbons were manufactured by melt-spinning at wheel speeds in the range 5?C20?ms^?1. The crystallization behavior of amorphous ribbons was investigated by a combination of differential scanning calorimetry (DSC) and X-ray diffractometry. DSC measurements showed that the amorphous ribbons exhibit distinct glass transition temperature and wide supercooled liquid region before crystallization. During continuous heating three exothermic peaks and two endothermic peaks were observed. The characteristic thermodynamic parameters such as T _g, T _x , ??T _x , and T _rg are around 432?C439, 478?C485, 46?C54?K, and 0.55?C0.56, respectively. Isothermal annealing DSC traces for this amorphous alloy, the first crystallization peak showed a clear incubation period and Avrami exponent was found to be 2.30?C2.74, which indicate that the transformation reaction involved nucleation and three-dimensional diffusion controlled growth. Mechanical properties of the as-quenched and subsequently annealed ribbons were examined by Vickers microhardness (HV) measurements. Results showed that microhardness of the as-quenched ribbons were about 309?HV. However, the results also showed that microhardness of the rapidly solidified ribbons increases with the increasing temperature.     

MELTING CRYSTALLIZATION BEHAVIOR OF NYLON 66

Analysis of isothermal and nonisothermal crystallization kinetics of nylon 66 was carried out using differentialscanning calorimetry (DSC). The commonly used Avrami equation and that modified by Jeziorny were used, respectively, tofit the primary stage of isothermal and nonisothermal crystallizations of nylon 66, In the isothermal crystallization process,mechanisms of spherulitic nucleation and growth were discussed. The lateral and folding surface free energies determinedfrom the Lauritzen-Hoffman treatment are σ= 9.77 erg/cm~2 and σ_e= 155.48 erg/cm~2, respectively; and the work of chainfolding is q = 33.14 kJ/mol. The nonisothermal crystallization kinetics of nylon 66 was analyzed by using the Mo methodcombined with the Avrami and Ozawa equations. The average Avrami exponent n was determined to be 3.45, Theactivation energies (ΔE) were determined to be -485.45 kJ/mol and -331.27 kJ/mol, respectively, for the isothermal andnonisothermal crystallization processes by the Arrhenius and the Kissinger methods.     

Stepwise annealing of poly(butylene terephthalate)

Annealing of poly(butylene terephthalate) (PBT) was studied by differential scanning calorimetry (DSC) and small angle X‐ray scattering (SAXS) measurement. A PBT sample was annealed at a recrystallization temperature where recrystallization occurs with a maximum rate in the heating process of the sample. In the subsequent annealing steps, the annealed sample was annealed repeatedly at the recrystallization temperatures, and the stepwise annealing sample was obtained. Peak melting temperature (T_m) and sharpness of DSC peak of the stepwise annealing sample increased with the annealing step. A high melting‐temperature sample was obtained in a short time, and T_m increased up to 238.5°C which is higher than all the T_m values that appear in the literature. The long period calculated from SAXS curves of the stepwise annealing sample increased with the annealing step. The increase of crystallite size and perfection of the crystal in the stepwise annealing process is suggested. Annealing experiment indicated that T^°_m should be higher than about 235°C. T_m increased linearly with the annealing temperature of the final step in the stepwise annealing (T_a). The equilibrium melting temperature (T^°_m) for PBT was estimated to be 247°C by the application of a Hoffman–Weeks plot to the relation between T_m vs. T_a. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2420–2429, 1999     

Crystallization and thermal properties of PLLA comb polymer

Poly(L ‐lactide) (PLLA) on poly(2‐hydroxyethyl methacrylate) (PHEMA) backbone was prepared by a combination of atom transfer radical polymerization (ATRP) and ring‐opening polymerization (ROP). The structure of the comb polymer was analyzed by wide angle X‐ray diffraction (WAXD), small angle X‐ray scattering (SAXS), and differential scanning calorimetry (DSC). WAXD result indicates that the comb polymer has α crystalline modification with a 10₃ helical conformation. Lamellar parameters of the crystalline structure were obtained by one‐dimension correlation function (1DCF) calculated from SAXS results. The calculations show that the thickness of crystalline layer is controlled by annealing temperature and comb structure. DSC was applied to study kinetics of the crystallization and melting behavior. Two melting peaks on melting curves of the comb polymer at different crystallization temperature were detected, and the peak at higher temperature is attributed to the melt‐recrystallization. The equilibrium melting temperature is found to be influenced by the comb structure. In this article the effects of the comb structure on Avrami exponent, equilibrium melting point and melting peak of the comb polymer were discussed. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 589–598, 2008     

Melting behavior of isotactic polystyrene

The melting behavior of isotactic polystyrene, crystallized from the melt and from dilute solutions in trans-decalin, has been studied by differential scanning calorimetry and solubility measurements. The melting curves show 1, 2, or 3 melting endotherms. At large supercooling, crystallization from the melt produces a small melting endotherm just above the crystallization temperature T_c. This peak originates from secondary crystallization of melt trapped within the spherulites. The next melting endotherm is related to the normal primary crystallization process. Its peak temperature increases linearly with T_c, yielding an extrapolated value for the equilibrium melting temperature T_c° of 242 ± 1°C as found before. By self-seeding, crystallization from the melt could be performed at much higher temperature to obtain melting temperatures as high as 243°C, giving rise to doubt about the value of T_c° found by extrapolation. For normal values of T_c and heating rate, an extra endotherm appears on the melting curve. Its peak temperature is the same for both melt-crystallized and solution-crystallized samples, and independent of T_c, but rises with decreasing heating rate. From the effects of heating rate and partial scanning on the ratio of peak areas and of previous heat treatment on dissolution temperature, it is concluded that this peak arises from the second one by continuous melting and recrystallization during the scan.     

Effect of annealing on thermal properties and crystalline structure of polyamides. Nylon 12 (polylaurolactam)

Summary Thermal and crystalline behaviour of nylon 12 annealed at low supercooling for 10–2500 hours were studied by DSC, SAXS, WAXS and electron microscopy. The melting temperature and the heat of melting increased on annealing time and temperature from 174 °C and 10 cal/g to 187 °C and 23 cal/g, and at the same time the long period was enhanced from 80 to 160 Å. Long time annealing caused a partial transformation of the-crystal structure to the-crystalline modification.Annealed nylon 12 consisted typical folded chain spherulites of increased latteral dimensions.The extrapolated heat of meltingH _m ^* of the crystalline phase of nylon 12 in the-form is 50 cal/g.With 9 figures     

Nonisothermal crystallization kinetics and melting behavior of bimodal medium density polyethylene/low density polyethylene blends

Nonisothermal crystallization kinetics and melting behavior of bimodal-medium-density- polyethylene (BMDPE) and the blends of BMDPE/LDPE were studied using differential scanning calorimetry (DSC) at various scanning rates. The Avrami analysis modified by Jeziorny and a method developed by Mo were employed to describe the nonisothermal crystallization process of BMDPE. The BMDPE DSC data were analyzed by the theory of Ozawa. Kinetic parameters such as the Avrami exponent (n), the kinetic crystallization rate constant (Z_c), the peak temperatures (T_p) and the half-time of crystallization (t_1/2) etc. were determined at various scanning rates. The appearance of double melting peaks and the double crystallization peaks in the heating and cooling DSC curves of BMDPE/LDPE blends indicated that the BMDPE and LDPE could crystallize respectively.     

Isothermal and nonisothermal crystallization kinetics of nylon‐46

Isothermal and nonisothermal crystallization kinetics of nylon‐46 were investigated with differential scanning calorimetry. The equilibrium melting enthalpy and the equilibrium melting temperature of nylon‐46 were determined to be 155.58 J/g and 307.10 °C, respectively. The isothermal crystallization process was described by the Avrami equation. The lateral surface free energy and the end surface free energy of nylon‐46 were calculated to be 8.28 and 138.54 erg/cm², respectively. The work of chain folding was determined to be 7.12 kcal/mol. The activation energies were determined to be 568.25 and 337.80 kJ/mol for isothermal and nonisothermal crystallization, respectively. A convenient method was applied to describe the nonisothermal crystallization kinetics of nylon‐46 by a combination of the Avrami and Ozawa equations. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1784–1793, 2002     

Thermal analysis of the multiple melting behavior of poly(butylene succinate‐co‐adipate)

The melting behavior of poly(butylene succinate‐co‐adipate) (PBSA) isothermally crystallized from the melt was investigated by differential scanning calorimetry. Triple, double, or single melting endotherms were observed in subsequent heating scan for the samples isothermally crystallized at different temperatures. These endothermic peaks were labeled as I, II, and III for low‐, middle‐, and high‐temperature melting endotherms, respectively. The independence of endotherm III to the crystallization temperature, the existence of an exothermic crystallization peak just below the endotherm III, and the heating rate dependence of endotherm III indicated that endotherm III was due to the remelting of recrystallized lamellar during a heating scan. The influence of crystallization time on the melting behavior of PBSA showed that endotherms II and III developed prior to endotherm I; endotherm III developed rather simultaneously with endotherm II. Further investigation showed that the peak temperature of endotherm I increased linearly with the logarithm of the crystallization time. It suggested that endotherm II was attributed to the melting of the primary lamellae, while endotherm I was due to the melting of secondary lamellae. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3077–3082, 2005     

Multiple endotherm melting behavior in relation to polymer morphology

The two endotherms found during DSC analysis of annealed or drawn poly(ethylene terephthalate), PET, are discussed in greater. detail. Earlier workers proposed that the endotherms were the result of separate morphologies, i.e., extended-chain and folded-chain crystals, but more recently Roberts and others have presented data on the effect of DSC heating rate on annealed PET endotherm areas which indicate that the higher temperature endotherm is the result of recrystallization in the DSC. The present work explains the reasons for recrystallization, and presents data showing that samples cooled at various rates from the melt also exhibit recrystallization in the DSC, in much the same manner as samples annealed for different lengths of time. Further, by prolonged annealing before analysis, part of the recrystallization exotherm can be observed in the DSC scan. Drawn nylon 66 also exhibits recrystallization in the DSC, in a manner similar to annealed or slowly crystallized PET. The amount of material that recrystallizes is determined by the time and supercooling available between first melting and the ultimate recrystallization temperature, i.e., a temperature at which there is too little time and temperature driving force for further recrystallization to occur. Infrared absorption data show an increase in “regular” fold content during prolonged annealing of PET, while dynamic mechanical data show a marked decrease in a dispersion that is likely associated with the looser fold crystal morphologies. Annealed PET does superheat in the DSC, leaving unanswered the question as to whether any partially extended material is present along with the regular-fold material. For cold-drawn PET, the infrared data indicate disappearance of regular folds and the dynamic mechanical data indicate disappearance of the looser folds. Cold-drawn PET also superheats. These data indicate a likelihood of at least partially extended morphologies in cold-drawn PET; these observations do not apply to PET drawn at high temperatures or to polyethylene.     

Isothermal and nonisothermal crystallization kinetics of irradiated nylon 1212

The crystallization behavior of nylon 1212, irradiated at ⁶⁰Co γ‐rays (50 kGy), was studied by a rheometer, polarized optical microscopy (POM), and differential scanning calorimeter (DSC). The results showed that irradiated nylon 1212 samples exhibited abnormal crystallization behavior during the crystallization process: The Avrami exponent n was calculated and was found to be in the range from 2.06–2.41 for isothermal crystallization, and from 2.67–4.91 for nonisothermal crystallization; the spherulite morphology also changed largely by polarized optical microscopy (POM); the crystallization activation energy ΔE for isothermal and nonisothermal crystallization process of irradiated nylon 1212 are determined to be 57.4 kJ/mol and 78.65 kJ/mol, respectively, which are lower than that of nonirradiated nylon 1212. At the same time, a new method by a combination of the Avrami and Ozawa equations was successfully applied to analyze the noncrystallization process of irradiated nylon 1212. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2326–2333, 2005