高含量碳纳米粒子填充EVA结晶行为研究

在以前大多数聚合物结晶的工作中,为了强调CNT和GNS成核作用,纳米填料的含量都尽可能地低,质量分数基本都是小于2%;然而,在实际的纳米填料增强工作中,足够多(大于5%)的纳米填料和其均匀的分散是复合物获得理想性能的必要条件。不同于低含量的纳米填料体系,高含量的纳米粒子不仅可以提供更多的异相成核点,而且意味着特别是在纳米粒子含量足够高以致形成受限网络时,结晶成核和生长过程中对分子链更加强烈地限制作用,最终导致更高的成核位垒和扩散活化能。低填料含量体系的研究结果不能代表高含量填料体系,特别是对于聚酰胺复合材料而言。同时,高含量下一维CNT和二维GNS之间的显著几何维度差异也会导致聚合物结晶行为的变化。     

β成核剂对 PP／EPDM 复合材料固化及结晶行为的影响

分别以β成核剂二苯基己二酞二胺(NT–C)和芳酰胺类化合物(TMB–5)增韧 PP／EPDM 复合材料,制备了 PP／EPDM／NT–C 复合材料与 PP／EPDM／TMB–5复合材料。研究了β成核剂与 EPDM 的协同增韧效应,以及两种β成核剂对 PP／EPDM 复合材料固化行为及结晶行为的影响。结果表明,在 TMB–5含量相同时,随着 EPDM含量的增加,PP／EPDM／TMB–5结晶速率先增大后减小,在 EPDM 质量分数为10%时达到最佳。在 EPDM 含量相同时,随着 NT–C 含量的增加,PP／EPDM／NT–C 复合材料的结晶速率逐渐提高。     

纤维素纳米纤维对聚乳酸结晶、流变和拉伸性能的影响

采用熔融共混⁃注射成型制备了聚乳酸（PLA）/纤维素纳米纤维（CNFs）可生物降解纳米复合材料,利用差示扫描量热仪、流变测试、拉伸性能测试等手段,考察了CNFs含量对PLA/CNFs复合材料结晶行为、流变特性和力学性能的影响规律。结果表明,少量的CNFs能均匀分散在PLA基体中,CNFs可作为PLA的异相成核剂,提高结晶速率常数,缩短半结晶时间,CNFs的含量为5 %（质量分数,下同）时,半结晶时间由纯PLA的10.4 min缩短至2.9 min;CNFs体现出润滑作用,使PLA/CNFs复合材料的储能模量和损耗模量均低于纯PLA;CNFs的含量为3 %时,复合材料的断裂伸长率较纯PLA提高了41.2 %。     

纳米氧化物作为成核剂对PET结晶速率的影响

研究了成核剂纳米氧化镁和纳米氧化硅对聚对苯二甲酸乙二醇酯(PET)结晶速率的影响。通过等温结晶差热分析(DSC)研究了纳米氧化镁在不同含量、不同温度下对PET等温结晶行为的影响。用纳米氧化镁和纳米氧化硅填充PET体系的非等温结晶DSC,由所得冷结晶峰温度值和热结晶峰温度值的对比,探索纳米成核剂对PET结晶速率的影响及其规律。研究结果表明:纳米成核剂均能明显提高PET的结晶速率,而纳米氧化镁比纳米氧化硅对促进PET的结晶效果更好;添加不同含量的纳米氧化镁对PET在不同温度下的等温结晶影响不同,在所研究的范围内,1.0%的添加量较有利于PET的结晶。     

增韧剂及成核剂对PP–R复合材料性能的影响

采用注射成型法制备无规共聚聚丙烯(PP–R)复合材料,结合差示扫描量热仪、扫描电子显微镜、X射线衍射仪及偏光显微镜等技术,分析了乙烯–辛烯共聚物+高密度聚乙烯增韧剂及WBGⅡ型β成核剂对PP–R复合材料力学性能和结晶行为的影响。结果表明,增韧剂与成核剂对复合材料的综合性能有明显的影响,β成核剂和增韧剂同时加入到PP–R材料中,协效提高了复合材料的冲击强度,为78.7 k J/m~2,与纯PP–R材料比较提高了175%,而对拉伸及弯曲强度影响较小。同时,加入增韧剂及成核剂均能诱导α晶向β晶转变,晶粒细化,进而改善其冲击韧性。     

聚丙烯／纳米碳酸钙原位聚合复合材料的结晶特性研究

采用DSC方法，研究了四种不同纳米CaCO3含量的PP／CaCO3纳米原位聚合复合材料的结晶特性，并通过偏光显微镜观察了其结晶结构，同时和纯PP进行了比较。结果表明，纳米CaCO3在PP中具有成核作用，PP以异相成核方式结晶，使PP的结晶温度提高，结晶速率增大，球晶颗粒变小。从纳米CaCO3含量对PP结晶特性的研究可知，其含量在2％～3％时，对PP的上述结晶行为影响最为明显。     

聚乳酸/可反应性纳米SiO2复合材料的等温结晶动力学

采用熔融共混法制备了聚乳酸/可反应性纳米二氧化硅(PLLA/RNS)复合材料。利用差示扫描量热仪研究了RNS对PLLA等温结晶行为的影响;用Avrami 方程研究了PLLA及其复合材料的等温结晶动力学。结果表明,加入RNS对PLLA结晶起到了异相成核作用,随着RNS含量的增加,PLLA的结晶速率(K)提高,半结晶时间(t1/2)减小,而Avrami指数(n)变化不大,说明RNS没有改变PLLA结晶的成核机理;利用Arrhenius方程和Lauritzen-Hoffman理论分别对PLLA及其复合材料的结晶活化能(ΔE)、成核参数(Kg)和折叠链端表面自由能(σe)进行计算后发现,复合材料的ΔE比纯聚乳酸的小,Kg 、σe略有增加。这表明加入RNS降低了复合材料的ΔE,从而有效地促进了PLLA基体的结晶。     

PP/纳米SiO2复合材料的非等温结晶动力学

采用差示扫描量热法研究了聚丙烯（PP）/纳米SiO2复合材料的非等温结晶动力学,研究了纳米粒子的成核活性及复合材料的结晶有效能垒。研究结果表明,纳米SiO2起到异相成核的作用,使PP的结晶峰温升高,结晶总速率增大;增容剂马来酸酐接枝聚丙烯(PP-g-MAH)提高了纳米SiO2的成核活性;添加纳米SiO2使复合材料的结晶有效能垒降低,PP-g-MAH使复合材料的结晶有效能垒增大,但低于纯PP的结晶有效能垒。     

聚甲醛/纳米SiO2复合材料的力学性能、结晶行为及热稳定性研究

以改性纳米SiO2为填料,通过熔融共混工艺制备聚甲醛/纳米SiO2复合材料,对其力学性能、结晶行为及热稳定性进行了研究。结果表明:复合材料的拉伸强度和缺口冲击强度随着SiO2含量的增加呈先增大后减小的趋势,二者分别在SiO2质量分数为3%和1%时达到最大;而弹性模量的情况则有所不同,其随着SiO2含量的增加不断增大。DSC测试结果显示,纳米SiO2具有较好的形核作用,能够促进聚甲醛的结晶温度升高,但会抑制晶粒的生长,导致复合材料结晶度的降低。此外,纳米SiO2还能显著提高聚甲醛的热稳定性。与纯聚甲醛相比,复合材料的最大热分解温度在氮气和空气气氛下分别提高了约41.1℃和24.5℃。     

PTT／纳米CaCO3复合材料流变行为及结晶性能研究

以毛细管流变仪研究了聚对苯二甲酸丙二酯(PTT)/纳米CaCO3复合材料的流变行为,讨论了复合材料的组成、剪切应力和剪切速率及温度对熔体流变行为、熔体黏度的影响,测定了不同配比的复合材料熔体的非牛顿指数 n。结果表明,PTT/纳米CaCO3复合材料熔体为假塑性流体,表观黏度随着剪切速率增加而降低。纳米CaCO3的加入量较少(1％)时,熔体黏度较纯PTT迅速下降;随着纳米CaCO3含量增加(2％-20％),熔体黏度随之上升,但都小于纯PTT的;直到含量为30％时,熔体黏度才超过纯PTT的。差示扫描量热仪测定复合材料的结晶和熔融性能发现,复合材料的熔体结晶温度Tpc和熔融温度Tm较纯PTT、都有所升高,说明纳米CaCO3的加入对PTT的结晶起到了异相成核作用。     

聚丙烯在溶液中的等温结晶行为

采用差示扫描量热仪（DSC）分析聚丙烯（PP）在2种不同溶剂中的等温结晶过程,并使用Avrami方程研究等温结晶动力学。 结果表明,晶体生长速率随着结晶温度的升高而降低。 同时,根据Lauritzen⁃Hoffman 二次成核理论,PP在溶液中的成核常数K_g和折叠表面自由能σ_e低于PP原料,反映了其在溶液中的结晶速率较高。     

Effect of long chain branching on nonisothermal crystallization behavior of polyethylenes synthesized with constrained geometry catalyst

Nonisothermal crystallization behavior of linear and long chain branched (LCB) polyethylene (PE) samples having similar molecular weight but different long‐chain branching densities (LCBD) up to 0.44 C per 1000 carbons was investigated using differential scanning calorimetry (DSC) at various scanning rates. The LCB PE samples were prepared in our high‐temperature, high‐pressure continuous stirred‐tank reactor (CSTR) system using the constrained geometry catalyst. The existence of LCB was found to affect the PE crystallization behavior considerably. The enthalpy of crystallization and the ultimate degree of crystallinity decreased with the increase of LCBD. At the relatively low cooling rates, the small amount of LCB promoted nucleation but restrained chain movement and reduced the crystal growth rate. There was ～ 17% of crystallinity generated from a secondary crystallization. The energy barrier became significant with the LCB structure, resulting in chain diffusion limitations and lower LCB PEs overall crystallization rates than their linear counterpart. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

Kinetic analysis on spherulite growth rate of polypropylene catalloys

The kinetic analysis on melt-crystallization of polypropylene catalloys (PP-cats) was conducted through measuring their spherulite growth rates. A multiple melting behavior of PP-cats was found through differential scanning calorimetry (DSC) and the corresponding crystalline microstructures of PP-cats were studied by wide-angle X-ray diffraction (WAXD). The calculated T_m^o value of propylene homopolymer (PP) suggests an obvious melting point depression of PP-cats. Moreover, it is found that the existence of ethylene-propylene copolymer could result in the changes of crystalline microstructure of PP and the PP crystal is in favor of growing along (040) lattice plane of α-monoclinic crystal. The crystal growth rate of PP-cats decreases with the increase of ethylene-propylene copolymer content in PP-cats. A comparison of crystallization kinetics between PP-cats and virgin iPP through a modified Lauritzen-Hoffman model indicates that there appears a transition from regimes II to III in iPP and PP-cats containing low ethylene-propylene copolymer content. However, for the PP-cats containing high ethylene-propylene copolymer content, crystallization always processes in regime II. In addition, both calculated nucleation parameter (K_g) and the fold surface free energy (σ_e) for PP-cats increase with the increase of ethylene-propylene copolymer content, implying that the existence of ethylene-propylene copolymer is unfavorable for the surface nucleation of PP and regular folding of the molecule chain. It is believed that an increase in viscosity of the melts induced by different compositions could remarkably slow crystallization growth down, because under this condition surface nucleation dominates as compared with crystal growth.     

The overall crystallization behavior of polyethylene/wax blends as phase change materials for thermal energy storage: A mini review

This review paper deals with the overall crystallization behavior of polyethylene/wax blends as phase change materials (PCMs) for thermal energy storage with the determination of their thermal properties. The addition of molten wax to the polyethylenes decreases the crystallization and melting temperatures of the blends. However, incorporating fillers to the polyethylene/wax blends can either decrease or increase the crystallization and melting temperatures of the composites depending on the filler type. The normalized enthalpy values of linear low-density polyethylene showed no significant change when increasing the wax content. On the contrary, the normalized enthalpy values of the wax in the blends were lesser than that of pure wax and increased with increasing wax content. Since the wax in the blend had a lower crystallinity compared to pure wax, this influences its effectiveness as a PCM for thermal energy storage. The effect of different polyethylenes on the wax morphology gave rise to enhance phase separation when wax was blended to high-density polyethylene as compared to the other polyethylenes. On the contrary, the effect of various waxes on the morphology of polyethylene resulted in different morphologies due to the molecular weight of the wax used and the structure of the polyethylene chain. The addition of fillers to the polyethylene (PE)/wax samples resulted in enhanced phase separation. The overall isothermal crystallization rate and the equilibrium melting temperature of PEs in the PEs/wax blends were depressed by wax addition due to the wax dilution effect.     

Crystallization behavior and nucleation analysis of poly(l‐lactic acid) with a multiamide nucleating agent

To accelerate the crystallization of poly(L ‐lactic acid) (PLLA) and enhance its crystallization ability, a multiamide nucleator (TMC) was introduced into the PLLA matrix. The thermal characteristics, isothermal and nonisothermal crystallization behavior of pure PLLA and TMC‐nucleated PLLA were investigated by differential scanning calorimetry. The determination of thermal characteristics shows that the addition of TMC can significantly decrease the onset temperature of cold crystallization and meanwhile elevate the total crystallinity of PLLA. For the isothermal crystallization process, it is found that the overall crystallization rate is much faster in TMC‐nucleated PLLA than in pure PLLA and increases as the TMC content is increased, however, the crystal growth form and crystalline structure are not influenced much despite the presence of TMC. In the case of nonisothermal crystallization, the nucleation efficiency and nucleation activity were estimated and the results indicate that excellent nucleation‐promoting effect could be achieved when the weight percentage of TMC is chosen between 0.25% and 0.5%. Polarized optical microscopy observation reveals that the nuclei number of PLLA increases and the spherulite size reduces greatly with the addition of TMC. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

Crystallization kinetics of polypropylene composites filled with nano calcium carbonate modified with maleic anhydride

The subject of this study was the crystallization behavior and thermal properties of polypropylene (PP)/maleic anhydride (MAH) modified nano calcium carbonate (nano‐CaCO₃) composites. In this study, 5 wt % nano‐CaCO₃ modified with different contents of MAH was filled into a PP matrix. X‐ray diffraction and differential scanning calorimetry were used to characterize the crystal morphology and crystallization kinetics of a series of composites. The results demonstrate that the nano‐CaCO₃ modified with MAH had an important effect on the thermal and morphological properties of the nanocomposites. The Avrami exponent of the pure PP was an integer, but those of the composites were not integers, but the crystallization rate constant decreased as the content of MAH in the nano‐CaCO₃ filler increased in isothermal crystallization. In nonisothermal crystallization, the kinetic parameter F(T) and the degree of crystallinity of pure PP were compared with those of the PP composites filled with nano‐CaCO₃. We suggest that heterogeneous nucleation existed in the PP composites and that the transformation and retention of the β‐form crystal into the α‐form crystal took place in the composite system and the β‐form crystal had a higher nucleation rate and growth process than the α‐form crystal in the PP composites. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Crystallization behavior of carbon nanotubes‐filled polyamide 1010

The nanocomposites of polyamide1010 (PA1010) filled with carbon nanotubes (CNTs) were prepared by melt mixing techniques. The isothermal melt‐crystallization kinetics and nonisothermal crystallization behavior of CNTs/PA1010 nanocomposites were investigated by differential scanning calorimetry. The peak temperature, melting point, half‐time of crystallization, enthalpy of crystallization, etc. were measured. Two stages of crystallization are observed, including primary crystallization and secondary crystallization. The isothermal crystallization was also described according to Avrami's approach. It has been shown that the addition of CNTs causes a remarkable increase in the overall crystallization rate of PA1010 and affects the mechanism of nucleation and growth of PA1010 crystals. The analysis of kinetic data according to nucleation theories shows that the increment in crystallization rate of CNTs/PA1010 composites results from the decrease in lateral surface free energy. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3794–3800, 2006     

Melting behavior and isothermal crystallization kinetics of PP/mLLDPE blends

The melting/crystallization behavior and isothermal crystallization kinetics of polypropylene (PP)/metallocene-catalyzed linear low density polyethylene (mLLDPE) blends were studied with differential scanning calorimetry (DSC). The results showed that PP and mLLDPE are partially miscible and interactions mainly exist between the mLLDPE chains and the PE segments in PP molecules. The isothermal crystallization kinetics of the blends was described with the Avrami equation. Values of the Avrami exponent indicated that crystallization nucleation of the blends is heterogeneous, the growth of spherulites is almost three-dimensional, and the crystallization mechanism of PP is not affected much by mLLDPE. The Avrami exponents of the blends are higher than that of pure PP, showing that the mLLDPE helps PP to form perfect spherulites. The crystallization rates of PP are decreased by mLLDPE because the crystallization temperature of PP was decreased by addition of mLLDPE and consequently the supercooling of the PP was correspondingly lower. The crystallization activation energy was estimated by the Friedman equation, and the result showed that the activation energy increased by a small degree by addition of mLLDPE, but changed little with increasing content of mLLDPE in the blends. The nucleation constant (K _g) was determined by the Hoffman–Lauritzen theory. Supported by the Science Foundation of Hebei University (2006Q13).     

Nonisothermal crystallization kinetics of polyamide 6 and ethylene‐co‐butyl acrylate blends

The nonisothermal melt‐crystallization behavior of PA6 and EBA blends at varying EBA content was investigated using differential scanning calorimetry at different scanning rates. Several macrokinetic models such as Avrami, Jeziorny, Ozawa, Liu, Ziabicki, and Tobin were applied to analyze the crystallization behavior thoroughly under nonisothermal conditions. The Avrami and Tobin model predicted that, for pure PA6 and PA6/EBA blends, simultaneous growth of all forms of crystal structures such as fibrillar, disc‐like, and spherulitic proceeds at an increasing nucleation rate. However, when applied to blends for isothermal crystallization, the Avrami model predicted that the crystallization process is diffusion‐controlled for pure PA6 and PA6/EBA blend containing higher content of EBA (50 phr), where the nylon‐6 chains were able to diffuse freely to crystallize under isothermal conditions. Liu model predicted that, at unit crystallization time, a higher cooling rate should be used to obtain a higher degree of crystallinity for both PA6 and PA6/EBA blends. The kinetic crystallizability of PA6 in the blends calculated using Ziabicki's approach varies depending upon the nucleation density and PA6‐rich regions present in the blend compositions. Nucleation activity of the blends estimated by Dobreva and Gutzowa method reveals that the EBA particles are inert at lower concentrations of EBA and do not act as nucleating agent for PA6 molecules in the blends. The activation energy of nonisothermal crystallization, calculated using Augis–Bennett, Kissinger, and Takhor methods indicated that the activation energy is slightly lower for the blends when compared to the neat PA6. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

山梨醇衍生物对聚乙烯结晶行为的影响

分别以山梨醇与对甲基苯甲醛、对乙基苯甲醛、3,4-二甲基苯甲醛为原料,利用缩醛反应机理,合成出3种不同的山梨醇衍生物——1,3-2,4-二亚(对甲基)苄基山梨醇(MDBS)、1,3-2,4-二亚(对乙基)苄基山梨醇(EDBS)、二(3,4-二甲基二苄叉)山梨醇(DMDBS)。使用溶液沉淀法制备含DMDBS、EDBS、MDBS质量分数分别为0.3%的3种粉末聚乙烯(PE)样品;采用X衍射仪、红外光谱仪、偏光显微镜、差示扫描量热仪对其进行了结晶结构及性质考察;用Avrami模型、Kissinger方程计算了各PE样品的结晶动力学参数。结果表明,山梨醇衍生物没有改变PE的晶体结构,保持了PE晶体的稳定性,提高了成核密度,使PE球晶尺寸由65μm平均降至53μm。动力学数据表明,山梨醇衍生物能提高PE的结晶速率,降低结晶活化能,PE的Avrami指数由4减小至3~3.6。在添加量相同的条件下,DMDBS、EDBS和MDBS可分别令PE结晶度提高4.83%、4.42%、1.25%。DMDBS对PE的改性效果最明显,EDBS次之,MDBS最小。