Study on the nonisothermal crystallization behavior of poly(vinyl alcohol)/attapulgite nanocomposites by DSC analysis

Attapulgite (AT)‐reinforced poly(vinyl alcohol) (PVA) nanocomposite films were prepared by solution‐casting technique. The nonisothermal crystallization behaviors of PVA bulk and PVA/AT nanocomposites have been investigated by differential scanning calorimetry (DSC). It has been found that the uniformly dispersed AT nanorods in the matrix have great influence on the glass transition temperature and crystallization behavior of PVA matrix. The Jeziorny method has been employed to analyze the DSC data. The results show that Jeziorny method could describe this system very well. Comparing with the PVA bulk, PVA/AT nanocomposites have higher crystallinity X_t, shorter semicrystallization time t_1/2, and higher crystallization rate constant Z_c. It can be concluded that AT can be used as an effective nucleating agent and has effects on the growth of crystallites in the crystallization process of PVA matrix. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 534–540, 2006     

原位聚合聚乙烯/蒙脱土纳米复合材料的结晶行为

用DSC研究了HDPE与MMT负载的催化剂熔融共混和原位聚合得到的两种纳米复合材料的熔融、 结晶行为和等温结晶动力学.  结果表明, HDPE与熔融共混样品的结晶度、 平衡熔点、 球晶生长速率和结晶能力大体相同; 原位聚合得到的HDPE/MMT纳米复合材料的结晶度和平衡熔点高于纯HDPE; 在相同过冷度条件下熔融结晶速率和结晶能力低于纯HDPE, 而在相同结晶温度Tc下, 熔融结晶速率和结晶能力则高于纯HDPE.  纯HDPE的晶体生长侧向单位面积表面自由能最小, 其次是熔融共混样品, 原位聚合样品最大, 且随ＭＭＴ含量的增加逐渐升高.     

Nonisothermal crystallization and morphology of poly(butylene succinate)/layered double hydroxide nanocomposites

Biodegradable poly(butylene succinate) (PBS) and layered double hydroxide (LDH) nanocomposites were prepared via melt blending in a twin-screw extruder. The morphology and dispersion of LDH nanoparticles within PBS matrix were characterized by transmission electron microscopy (TEM), which showed that LDH nanoparticles were found to be well distributed at the nanometer level. The nonisothermal crystallization behavior of nanocomposites was extensively studied using differential scanning calorimetry (DSC) technique at various cooling rates. The crystallization rate of PBS was accelerated by the addition of LDH due to its heterogeneous nucleation effect; however, the crystallization mechanism and crystal structure of PBS remained almost unchanged. In kinetics analysis of nonisothermal crystallization, the Ozawa approach failed to describe the crystallization behavior of PBS/LDH nanocomposites, whereas both the modified Avrami model and the Mo method well represented the crystallization behavior of nanocomposites. The effective activation energy was estimated as a function of the relative degree of crystallinity using the isoconversional analysis. The subsequent melting behavior of PBS and PBS/LDH nanocomposites was observed to be dependent on the cooling rate. The POM showed that the small and less perfect crystals were formed in nanocomposites.     

A study on the melting and crystallization of polyoxymethylene‐copolymer/hydroxyapatite nanocomposites

In this work, isothermal crystallization kinetics of polyoxymethylene copolymer (POM) in POM/hydroxyapatite (HAp) nanocomposites has been investigated. Melting behavior and crystalline structure formation were studied using TOPEM DSC, positron lifetime spectroscopy (PALS), atomic force microscopy (AFM) and ¹³C and ³¹P solid‐state NMR. The highest degree of crystallinity was found for POM/0.5% HAp nanocomposite and the lowest for POM/2.5% HAp. Isothermal crystallization analysis showed that an introduction of HAp nanoparticles led to effective heterogeneous nucleation and formation of crystals with higher Avrami exponent. Besides, changes in overall crystallization rate were observed – the highest overall crystallization rate was found for POM/0.5% HAp sample, while the lowest for POM/2.5% HAp was observed. Generally, for POM in POM/HAp nanocomposites, a significant decrease in nucleation activation energy (K_g), and the fold surface free energy (σ_e) was found. For nanocomposite containing 2.5% HAp, heterogeneous nucleation takes place as well, but too high concentration of nanoparticles hinders POM crystallization and enhances formation of more defected crystals as confirmed by AFM data. The presence of HAp nanoparticles in the POM matrix was confirmed by ³¹P MAS‐NMR, but their influence on the crystallization process was not observed in the ¹³C CP‐MAS‐NMR spectra. Copyright © 2012 John Wiley & Sons, Ltd.     

Understanding isothermal semicrystalline polymer drying: Mathematical models and experimental characterization

The drying mechanism of semicrystalline poly(vinyl alcohol) (PVA) was investigated. PVA samples of various molecular weights were crystallized by annealing at temperatures slightly above the glass transition temperature of PVA, and swollen in water for different time periods. The water volume fraction in the sample was measured using a buoyancy technique. The samples were dried in air at constant temperatures, and the drying kinetics were investigated using thermogravimetric analysis. The change in degree of crystallinity of the swollen polymer during drying was measured by differential scanning calorimetry (DSC) as well as by Fourier transform infrared spectroscopy (FTIR). The degree of crystallinity of the samples increased during drying, which in turn was found to alter the drying rate. The drying kinetics were faster at higher temperatures, for lower molecular weights, and for lower degrees of crystallinity. A mathematical model was developed to predict drying rates of semicrystalline polymers by considering the crystallization kinetics during drying. The model predictions included the thickness of the polymer sample, the degree of crystallinity of the polymer, and the water weight loss as functions of drying time. Model predictions were found to agree reasonably well with the experimental results. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2771–2780, 1998     

Nonisothermal crystallization behavior of polypropylene-clay nanocomposites

The influence of two concentrations of clay nanoparticles on the nonisothermal crystallization behavior of the intercalated polypropylene-clay nanocomposites is investigated here. It is observed that the crystallization peak temperature (T_p) of PP-clay nanocomposites is marginally higher than neat PP at various cooling rates. Furthermore, the half-time for crystallization (t_0.5) decreased with increase in clay content, implying the nucleating role of clay nanoparticles. The nonisothermal crystallization data is analyzed using Avrami, Ozawa and Mo and coworkers methods. The validity of kinetic models on the nonisothermal crystallization process of PP-clay nanocomposites is discussed. The approach developed by Mo and coworkers successfully describes the nonisothermal crystallization behavior of PP and PP-clay nanocomposites. The activation energy for nonisothermal crystallization of pure PP and PP-clay nanocomposites based on Kissinger method is evaluated.     

In situ polymerized poly(butylene succinate)/silica nanocomposites: Physical properties and biodegradation

A series of poly(butylene succinate)/silica (PBS/silica) nanocomposites were prepared by in situ polymerization. Solid-state ²⁹Si NMR and FTIR analysis indicated that silanol-bonded carbonyl groups are established within PBS/silica nanocomposite materials. Rheological effects inherent to the silica filler were evaluated by melt rheological analysis as a function of shear force in the molten state. Despite high shear force, PBS/silica nanocomposites maintained a relatively high melt viscosity, attributable to a network structure resulting from covalent bonding between silica and the polymer chain. Nanocomposite material containing 3.5 wt% silica exhibited greatly improved mechanical properties. The tensile strength at break and elongation were ca. 38.6 MPa and 515%, while those of the parent PBS were 26.3 MPa and 96%, respectively. PBS/silica nanocomposites showed composition dependency on biodegradation ascribable to reduced crystallinity and preferential microbial attack.     

Enhancement of water barrier properties and tribological performance of hybrid glass fiber/epoxy composites with inclusions of carbon and silica nanoparticles

Water barrier properties and tribological performance (hardness and wear behavior) of new hybrid nanocomposites under dry and wet conditions were investigated. The new fabricated hybrid nanocomposite laminates consist of epoxy reinforced with woven and nonwoven tissue glass fibers and two different types of nanoparticles, silica (SiO₂) and carbon black nanoparticles (C). These nanoparticles were incorporated into epoxy resin as a single nanoparticle (either SiO₂ or C) or combining SiO₂ and C nanoparticles simultaneously with different weight fractions. The results showed that addition of carbon nanoparticles with 0.5 and 1 wt% resulted in maximum reduction in water uptake by 28.55% and 21.66%, respectively, as compared with neat glass fiber reinforced epoxy composites. Addition of all studied types and contents of nanoparticles improves hardness in dry and wet conditions over unfilled fiber composites. Under dry conditions, maximum reduction of 47.26% in weight loss was obtained with specimens containing 1 wt% carbon nanoparticles; however, in wet conditions, weight loss was reduced by 17.525% for specimens containing 0.5 wt% carbon nanoparticles as compared with unfilled fiber composites. Diffusion coefficients for different types of the hybrid nanocomposites were computed using Fickian and Langmuir models of diffusion. Copyright © 2017 John Wiley & Sons, Ltd.     

Dependence of degree of crystallinity and melting point on time and temperature of annealing for γ-irradiated polytetrafluroethylene

The annealing behavior of polytetrafluoroethylene (PTFE) irradiated with 1 × 10⁵ R of γ rays has been studied in terms of the degree of crystallinity and the melting temperature. At short annealing times the crystallinity changes rapidly to a level characteristic of the annealing temperature. It seems that the crystallization arises from transport of chain segments in noncrystalline regions. The increase in crystallinity at long times of annealing is linear in the logarithm of time. This process is mostly due to elimination of voids formed during γ irradiation, but it also involves slight lamellar thickening and crystallization to form extended-chain regions.     

冷冻/解冻制备的聚乙烯醇水凝胶的结构和流变性研究

研究了冷冻/解冻法制备的不同浓度(5wt%～25wt%)聚乙烯醇(PVA)水凝胶的结构和流变行为之间的关系.由XRD确定了凝胶中PVA的结晶度和晶粒尺寸.用应力流变仪研究了凝胶的流变行为,包括动态模量和蠕变等.在频率为1Hz和低应力的条件下,测量了凝胶的储能模量和损耗模量.在该试验条件下,PVA水凝胶的形变是完全可以回复的.低频率区和低应变区的储能模量随浓度增加而变大,但当浓度超过20wt%时,储能模量增加速率明显降低.由PVA水凝胶在1Hz时的储能模量和结晶度的数据,理论分析得到了形成PVA水凝胶的最低PVA浓度和最小结晶度.当PVA浓度低于15wt%时,储能模量主要由PVA的微晶控制,分子链间的氢键影响很小.通过低应变区储能模量的数值计算出了凝胶网孔尺寸的结构参数.同时对不同温度下PVA水凝胶的储能模量数据进行了标度分析.PVA水凝胶的蠕变行为显示,随浓度提高,凝胶的蠕变黏弹性由线性向非线性转变.     

聚乙烯醇/聚乙烯基吡咯烷酮共混体系的结晶行为

用DSC、WAXD和SAXS研究了聚乙烯醇(PVAl)/聚乙烯基吡咯烷酮(PVP)共混体系的结晶行为.PVAl的结晶度随PVP含量增加而减少,并存在结晶度为零的组成(PVAl)的重量分数约为50％.与纯PVAl相比,共混物的温度区间T_m-T_g减小,表明PVP对PVAl的结晶起抑制作用.共混物中PVAl的结晶速度下降,具体表现为PVAl过冷区随PVP含量增加而扩大,动力学速度常数减小,球晶增长速度下降.纯PVAl和共混体系的等温结晶速率均遵循Avrami方程.退火样品的长周期、片晶厚度和过渡层厚度大于相同组成未退火样品.两者长周期随PVP含量增长加显著增大,片晶厚度增长次之,过渡层厚度变化不大.     

β成核剂修饰多壁碳纳米管增强聚丙烯纳米复合材料的分散及结晶行为

利用非共价键改性方法,用芳香二羧酸酰胺类的β成核剂(β-NA)改性多壁碳纳米管(MWCNTs),采用溶液法制备了聚丙烯(PP)/β-NA-MWCNTs复合材料.通过广角X射线衍射(WAXD)分析了复合材料的结晶形态,结果表明β-NA-MWCNTs诱导聚丙烯生成大量β晶,同时提高了复合材料的结晶度.含5.0 wt％β-N...     

Preparation of PLA-based nanocomposites modified by nano-attapulgite with good toughness-strength balance

Polylactic acid (PLA) was modified by poly (butylene adipate-co-terephthalate) (PBAT) and nano-attapulgite (AT) using the melt blending technique. Ethylene-butyl acrylate-glycidyl methacrylate (E-BA-GMA) was used as a compatibilizer which can bond the AT nanoparticles with PLA/PBAT matrix by interaction between the epoxy and hydroxyl groups. The effects of the AT content on the mechanical properties, thermal properties, crystallinity and morphology of PLA/PBAT/ATT nanocomposites were investigated. The results showed that the tensile strength, elongation at break and impact strength of PLA/PBAT could be simultaneously increased by incorporating AT nanoparticles. PLA/PBAT/AT nanocomposites possessed higher thermal stability than pure PLA/PBAT. In the ternary composite system of PLA/PBAT/AT, AT acted as a heterogeneous nucleating agent and was able to increase the crystallization temperature. When the AT content was low (≤2.5 wt%), AT nanoparticles could uniformly disperse in the PLA/PBAT matrix. In general, AT was an effective filler to reinforce and toughen PLA/PBAT blend simultaneously, and the PLA/PBAT/AT nanocomposite with 2.5 wt% AT exhibited a good combination of strength and toughness.     

Dissolution mechanism of semicrystalline poly(vinyl alcohol) in water

Changes occurring in the degree of crystallinity and lamellar thickness distribution of poly(vinyl alcohol) (PVA) samples during dissolution in water were investigated. PVA samples of three different molecular weights were crystallized by annealing at 90, 110, and 120°C. The initial degrees of crystallinity measured by differential scanning calorimetry (DSC) and by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) varied from 43 to 60% and the average lamellar thicknesses measured by DSC ranged from 50 to 400 Å. PVA dissolution was followed at 25, 35, and 45°C from 30 s up to 195 min. Lamellar thicknesses were determined as a function of dissolution time using DSC. There was an initial drastic decrease in the degree of crystallinity, which leveled off to a fairly constant value before reaching zero by the time the polymer dissolved completely. Increase in molecular weight led to lesser number of crystals, but with larger average lamellar thickness, which were more stable in the presence of water. Increase in crystallization temperature or decrease in dissolution temperature led to larger average lamellar thickness. Based on these findings, a dissolution mechanism involving unfolding of the polymer chains of the crystal was proposed. © 1996 John Wiley & Sons, Inc.     

Hydrogen bonding and mechanical properties of thin films of polyether-based polyurethane-silica nanocomposites

Two series of thin films of polyether-based polyurethane-silica nanocomposites having hard segment content of 51% and 34% and different concentrations of SiO₂ nanoparticles (0, 0.5, 1.0 and 3.0 vol.%) have been prepared. Infrared linear dichroic (LDIR) ratio, mechanical and differential scanning calorimetry (DSC) measurements were performed in order to determine the influence of hydrogen bonding on their mechanical and thermal properties. The degree of phase separation (DPS) and orientational functions in dependence on strain were calculated from the polarized IR spectra. The presence of silica nanoparticles gives rise to significant differences in the mechanical (stress-strain) properties of the nanocomposites with regard to the pure polymer. The nanocomposite thin films with lower hard segment content (HSC) displayed decreased stiffness and tensile and increased elongation at break in comparison to the nanocomposites with higher HSC. There was no distinctive influence of nanoparticles on the glass transition temperatures of soft segments. Nanosilica significantly affected the melting behavior of the hard phase only in samples with higher HSC.     

Effect of poly(amid–imide)/Al2O3 hybrid with various ratios on the physicochemical properties of poly(vinyl alcohol) nanocomposites films

To study the effect of the various ratios of poly(amide–imide)/Al₂O₃ nanocomposites (PANC)s on the mechanical and thermal properties of nanocomposites films, poly(vinyl alcohol) (PVA)/PANCs based on various ratios of 2, 4, and 6 wt% were prepared and characterized. In the first step, the surface of alumina nanoparticles was treated with 15 wt% of biosafe diacid and consequently, about 10 wt% of these modified nanoparticles were loaded into the poly(amide–imide) matrix. Then, various contents of the obtained PANCs were incorporated into a PVA solution using a sonochemical treatment. The effects of PANC on the structure and morphology of PVA matrix were studied using powder X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, atomic force microscopy (AFM), and thermal gravimetric analysis (TGA). The results show that the tensile strength and decomposition temperature were improved as the portion of PANC into PVA matrix are increased from 2 to 6 wt%. Also, AFM pictures of the fracture surfaces of PVA/PANCs showed a significantly rougher surface than the neat PVA.     

Influence of the nanoparticle type on the thermal decomposition of the green starch/poly(vinyl alcohol)/montmorillonite nanocomposites

In this study, some aspects concerning the thermal decomposition of starch/poly(vinyl alcohol) (PVA)/montmorillonite (MMT) nanocomposites with 2 wt% nanoclay, prepared by melt mixing method, were studied. For these loadings, the inorganic fillers are well dispersed through the PVA/starch matrix, i.e., the nanocomposites formed are mostly intercalated hybrids. The aim of this article is to establish the effect of the nanofiller nature on the thermal decomposition of the starch/PVA/MMT nanocomposites. The thermal behavior of the 50 wt% starch/50 wt% PVA blend and its nanocomposites with 2 wt% nanoclay has been investigated by thermogravimetric analysis coupled with Fourier transform-infrared spectroscopy and mass spectrometry (MS). The volatile compounds resulting during the thermal degradation were studied by in situ vapor phase FT-IR spectroscopy and MS technique under a controlled temperature/time program. Apart from the identification of the volatile compounds, some conclusions on the nanoclays effect on the degradation mechanism and formation of the volatile compounds in accordance with the previously developed general mechanisms for PVA and starch degradation have been formulated. The clay–PVA/starch nanocomposites show completely different degradation product distribution patterns, which may be attributed to the presence of the head-to-head structures and Si–O–C linkages formed between clay and blend components.     

The crystallization behavior of biodegradable poly(butylene succinate) in the presence of organically modified clay with a wide range of loadings

The crystallization behavior of poly(butylene succinate)(PBS) nanocomposites with a wide range of contents of clays was revealed. It was of interest to find that the crystallization rate of PBS was accelerated obviously at relatively low contents of clays; while a retarded crystallization kinetics and a decreased crystallinity of PBS were found in the nanocomposites with higher clay contents. Two interplaying effects existed in the nanocomposites, i.e., a suppression effect of clays on nucleation and a templating effect of clays on crystal growth, were clarified to contribute to this intriguing crystallization behavior.     

Synthetic preparations,thermal properties and crystallization characterization of PLLA‐PPG block copolymers/clay nanocomposites

In this study, intercalation of the polymer or pre‐polymer from solution was used to blend different proportions of polylactic acid‐propylene glycol (LPG) copolymers (polypropylene glycols (PPG) of : 700, 1000, 2000) and lipophilic montmorillonite (clay) in order to investigate the melting and the crystalline nature of LPG copolymers/clay nanocomposites via a differential scanning calorimeter (DSC). In addition, changes in the intermolecular force and crystal morphology of the nanocomposites under different crystallization conditions were also studied. For the results, it was observed from a thermogravimetric analyzer that increasing the clay content elevated the weight loss temperature. In non‐isothermal experiments using a DSC, it was discovered that the melting temperature and crystallization temperature of the LPG copolymers also increased with increasing amounts of added clay. The crystallinity of LPG2000 + 1.5 wt% clay was enhanced by 17.00%; in addition, it was found in the crystallinity study that adding clay slowed down the crystallization rate of the LPG copolymers. Moreover, it was found via X‐ray diffractometer (XRD) that the intensity of the diffraction peaks of the 1.5 wt% specimen was stronger than that of the 0.5 wt% specimens. The results imply that copolymers with a longer chain length provide greater space for the crystals to grow, thus making it easier for larger crystals to grow. Conversely, the added clay generates an inhibitory effect in copolymers, reducing the d‐spacing (d) in the XRD. Therefore, adding clay would change the crystallization behavior and the morphology of the LPG copolymers. Copyright © 2015 John Wiley & Sons, Ltd.     

Effect of a post‐annealing process on microstructure and mechanical properties of high‐density polyethylene/silica nanocomposites

High‐density polyethylene (HDPE) and nanosilica nanocomposites were prepared for SiO₂ content up to 15 wt%. Microstructural characterization evidenced a homogenous distribution of silica aggregates with a mean size increasing with the filler content finally resulting in a rheological percolation between 7.5 and 10 wt%. Nanoparticles did not induce any significant impact on the matrix crystallinity but led to a real improvement on elastic properties accompanied with a large embrittlement above the percolation threshold. The effect of annealing near HDPE melting temperature was studied. Differential scanning calorimetry, X‐ray diffraction, and small‐angle X‐ray scattering analyses showed a significant change in the HDPE microstructure after annealing at 125°C. A large increase in the crystallinity (from 68 to 76%) and a clear improvement of Young's modulus (by 55%) were observed prior to polymer degradation. A valuable impact of silica particles on thermal stability was also obvious regarding the evolution of elastic properties for extended exposure times (850–1,200 h). © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 535–546