聚丙烯腈凝胶的结晶性研究

利用动态流变测试、差示扫描量热法(DSC)和广角X射线衍射(WAXD)等实验方法研究了由聚丙烯腈(PAN)/二甲基亚砜(DMSO)和PAN/DMSO/水体系形成的凝胶的结晶性质。通过动态应变扫描结果发现PAN凝胶结构在受到破坏后可以迅速恢复,说明PAN凝胶的交联点不是由微晶构成的,而很可能是氰基之间的偶极-偶极相互作用;对内含溶剂的凝胶样品进行DSC测试的结果表明,单纯通过降温形成的PAN凝胶不会发生结晶,结晶不是其形成的原因;PAN溶液在凝胶化转变过程中可能会发生相分离;X射线衍射实验的结果与流变和热分析实验的结果一致,即仅依靠温度变化而形成的PAN凝胶是无定形的,但是,在非溶剂水的作用下形成的PAN凝胶是可以结晶的,结晶度和平均晶粒尺寸随凝胶中水含量的减少而增大。     

落球法研究PAN/DMSO溶液的溶胶凝胶化转变

该研究测定不同温度下PAN/DMSO溶液落球粘度的变化,以活化能的转变为判据,确定了不同条件下PAN/DMSO溶液的溶胶凝胶化转变温度。将落球法的结果与动态流变法结果相比较,发现两者十分接近,所以改进的落球法是一种研究溶液凝胶化转变的简单、可信的方法。     

水对聚丙烯腈溶液热致变凝胶化的微观促进机理研究

使用分子动力学模拟作为研究方法，从微观层面上研究了聚丙烯腈（PAN）/二甲基亚砜（DMSO）/水（H2O）混合溶液的热致变凝胶化过程，并明确了水的微观促进机理。结果表明，模拟计算得到的含水量为4%（质量分数，下同）的三元体系的凝胶点温度在332.75 K附近，与文献报道的实验结果吻合；水会增加PAN分子链的振动幅度，增加氰基之间发生交联作用的概率，从而促进PAN溶液快速发生凝胶化转变；在一定的范围内，含水量越大，PAN的振动幅度越大；水减少了PAN周围DMSO分子的数量，增强PAN分子链的聚集程度，导致氰基之间更容易发生交联作用。     

PAN/DMSO/H2O溶液的流变学性质

用HAAKE公司RSISOL型流变仪对聚丙烯腈（PAN）／二甲基亚砜（DMSO）／去离子水（H2O）溶液的流变行为进行了测试。结果表明：PAN／DMSO／H2O溶液受到剪切作用再经过短时间松弛后有较好的保持取向的能力;随着温度的降低,溶液的粘、弹性模量都会升高;结合溶液凝胶点的判定和logG′-logG″,对体系在不同温度时的状态和均一性进行了研究。     

PAN基碳纤维纺丝原液的均一性和分散性研究

运用动态流变手段，通过绘制logG′-logG″图和测定体系的凝胶点，对不同温度和非溶剂含量的PAN／DMSO／H2O体系的均一性和分散性进行了研究，并通过凝胶点的测定对体系所处的状态进行了验证。结果表明，当水含量为5％时，体系在60℃～70℃之间由均一液态转变成均一凝胶态；对于水含量不同的其他体系，70℃时均未发生凝胶化转变，依然处于均一的溶液态。通过理论分析，实验采用的常规分子量和较高浓度的PAN溶液均匀稳定，呈现单分散性，是较为理想的纺丝原液。     

凝胶化理论初探及PAN原丝的凝胶法纺丝

提出了一种聚丙烯腈(PAN)原丝的新型纺丝方法—凝胶法纺丝技术。根据凝胶化理论,从溶液浓度、体系的凝胶化温度和非溶剂水的影响3个方面对PAN原丝凝胶法纺丝进行深入的研究,并与干湿法纺丝进行比较,得出重要结论:凝胶法纺丝所得的PAN原丝皮芯结构差更小,平均孔洞半径更小,结构更致密,孔洞相对体积分数大幅度下降,力学性能更佳。     

温度对PAN-DMSO-H_2O体系凝胶化行为的影响

丙烯腈(AN)和衣康酸(IA)经自由基溶液聚合合成相对分子质量中等的聚丙烯腈(PAN)共聚物,用流变学方法研究了温度对PAN-DMSO-H2O三元体系凝胶化过程的影响,以及三元体系储能模量G′、损耗模量G″等流变特性在凝胶化过程中的变化,得出不同温度下该体系的凝胶化转变与时间的变化规律。结果表明:温度越低,凝胶化转变所需时间越短。此外,还发现凝胶成形是逐渐变化的过程,凝胶点之后,网络结构仍继续生长形成更致密的结构。     

文摘

提出了一种聚丙烯腈（PAN）原丝的新型纺丝方法——凝胶法纺丝技术。根据凝胶化理论,从溶液浓度、体系的凝胶化温度和非溶剂水的影响3个方面对PAN原丝凝胶法纺丝进行深入的研究,并与干湿法纺丝进行比较,得出结论：凝胶法纺丝所得的PAN原丝皮芯结构差更小,平均孔径半径更小,结构更精密,孔洞相对体积分数大幅下降,力学性能更佳。     

添加高相对分子质量聚丙烯腈对纺丝溶液的流变性能与可纺性的影响

在聚丙烯腈/二甲基亚砜(PAN/DMSO)纺丝溶液体系中添加高相对分子质量PAN,通过锥板流变仪对该体系的稳态和动态流变行为进行了研究.结果表明:纺丝溶液为切力变稀流体,在较高剪切速率下,溶液出现明显的剪切变稀现象;在较低的剪切速率下,随温度的升高,溶液粘度逐渐变低.通过添加高相对分子质量PAN,溶液粘度增大,有利于表...     

通过硅氧烷凝胶吸附剂的羧化改性高效去除铁(Ⅲ)

研究了使用羧基化改性硅氧烷凝胶从模型水溶液中去除Fe3+离子的方法.制备了五种不同组分的羧基化改性硅氧烷凝胶,并通过FT-IR、SEM和BET对凝胶进行结构表征,探究了初始浓度、pH、温度对吸附的影响.结果显示,随着甲基丙烯酸比例的增大,比表面积逐渐增大.通过增加溶液的初始浓度,凝胶吸附量逐渐增大.在最初的120min...     

聚丙烯腈纺丝原液三元体系的流变行为

用流变测试方法研究了非溶剂水的含量对聚丙烯腈/二甲基亚砜/水三元纺丝原液溶胶-凝胶的转变过程的影响,并测定了不同组成的原液的凝胶点。结果表明:随着非溶剂含量的提高,原液的凝胶点提高。聚丙烯腈纺丝原液凝胶转变的行为与非溶剂的含量密切相关。     

Rheological study on thermal-induced gelation behavior of polyacrylonitrile solution

Thermal-induced gelation process of different polyacrylonitrile (PAN) solutions was investigated through dynamic rheological measurements. The rheological material parameters characterizing the gelation behavior of the PAN solutions were measured and the effects of such factors as water content, solvent type and concentration on the gelation process were analyzed. It is found that the gel point T _gel of PAN/DMSO (dimethyl sulfoxide)–water solution increases with increased water content in the solution. Also, T _gel becomes higher when the solvent has a lower solvency for PAN. A linear relationship between logG′ and logω, logG″ and logω indicates the beginning of gelation. The PAN concentration of the solution affects T _gel in a way similar to that of the water content. The relaxation exponent n is found to relate to the fractal dimension of the polymer gel. For PAN/DMSO–water solution, this exponent n remains almost constant when the water content differs. The solvent type and concentration respectively influences n value in their own way. The PAN/DMSO–water solution systems in our work exhibit similar characteristics of gel structure, concluded from the n and d _f values of these systems. A logarithmic plot between G′ and G″ can be used to evaluate the structural evolution of the solution, and a comparatively steep slope of the curve is an indication of gelation.     

Rheological properties of syndiotacticity-rich ultrahigh molecular weight poly(vinyl alcohol) dilute solution

Viscoelastic behavior of the dilute solution of ultrahigh molecular weight syndiotactic poly(vinyl alcohol) (UHMW s-PVA)/dimethyl sulfoxide (DMSO)/water was investigated through rheological response. Below a critical temperature, the dynamic storage modulus (G′) of the solution became greater than the dynamic loss modulus (G″) and the viscoelastic exponent for G′ became smaller than that for G″ before macroscopic gelation, which indicates the evolution of viscoelastic solid properties at the sol state. Also, the loss tangent (tan δ) of the solution below the critical temperature increased with increasing frequency. Consequently, the dilute solution of UHMW s-PVA/DMSO/water showed the rheological behaviors as can be observed in general chemical or physical gel systems below the critical temperature. These results suggest that solid-like heterogeneity prevailed in the solution before macroscopic gelation with decreasing temperature. Such heterogeneity was considered as phase-separated domains attributed to spinodal decomposition. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 569–576, 2001     

Dynamic rheological behavior and microcrystalline structure of dioctyl phthalate plasticized poly(vinyl chloride)

The dynamic rheological behavior of poly(vinyl chloride) (PVC)/dioctyl phthalate (DOP) systems were studied as a function of DOP content and melting temperature. The dynamic rheological behavior of the PVC/DOP systems was found to be remarkably affected by the DOP content. The observed curves of storage modulus (G′) versus frequency were well fitted to an empirical equation (G′ = G′₀ + Kωⁿ, where G′₀ is the low‐frequency yield value of the storage modulus, the exponent n is a dependent index of frequency, K is a constant coefficient, and ω is the angular frequency). The loss tangent and/or phase angle increased remarkably at a higher DOP content. There was an apparent critical DOP content transition where the dynamic rheological behavior of the PVC/DOP systems changed greatly. Scanning electron microscopy observations revealed the existence of a multiscale particle structure in the PVC/DOP systems. For the PVC/DOP (100/70) system, with increasing melting temperature, its dynamic rheological behavior showed an apparent mutation at about 190°C. Differential scanning calorimetry (DSC) analysis confirmed that the high elastic networks in the PVC/DOP systems were closely related to the microcrystalline structure of PVC. The transitions in the curves of the gelation degree and crystallinity versus the DOP content corresponded well to the DOP content transition in the dynamic rheological behavior. DOP could inhibit the secondary crystallite of PVC and almost had no effect on the primary crystallite of PVC. The coexistence of the microcrystalline structure of PVC and the plasticizer (DOP) resulted in high elastic networks in the PVC/DOP systems. The DSC results explained the DOP content transition and the temperature transition in the dynamic rheological behavior of the PVC/DOP systems well. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Molecular and mechanical properties of hydroxypropyl methylcellulose solutions during the sol:gel transition

This paper outlines the latest findings in our work to understand the fundamental interactions within hydrated hydroxypropyl methylcellulose (HPMC) at elevated temperature. ATR-FTIR spectroscopy was used to relate molecular interactions to the rheological changes in aqueous HPMC solutions during the sol:gel transition. Sol:gel transition temperatures determined using ATR-FTIR spectroscopy, oscillatory rheology and turbidimetry were in agreement to within experimental error. ATR-FTIR spectroscopy provided direct evidence of increased hydrophobic interactions within the gel network through a shift to lower wavenumber of ν_as(CH) vibrations observed during the gelation process. In addition, the FTIR spectra provide evidence that the structure of the polymer network is different in the thermo-formed gel, to that which exists in viscous solution. Both the rheological and ATR-FTIR data confirmed the supposition that thermal gelation is a two stage process. The first stage has been attributed to the disruption of native cellulosic bundles and this is supported by the changes in both the storage modulus and intensity of the ν(CO) band at low temperatures. The second stage corresponded to phase separation and gelation resulting from increased hydrophobic interactions between polymer chains at elevated temperatures.     

Viscoelastic sol–gel state of the chitosan and alginate solution mixture

The rheological behavior of chitosan/alginate solutions was investigated in relation to gelation and polyelectrolyte complex (PEC) formation. Before mixing, the chitosan and the alginate solutions were both homogeneous fluids. However, heterogeneity developed after mixing, accompanied by a serious increase of viscosity. To determine the sol–gel state of the solutions, the viscoelastic variables, such as the dynamic storage modulus (G′) and loss modulus (G″), the loss tangent, and the viscoelastic exponents for G′ and G″, were obtained. Depending on the concentration, the chitosan/alginate solutions revealed unexpected rheological behavior. At a polymer concentration of 1.0 wt %, the chitosan/alginate solution was in a viscoelastic gel state, whereas, at higher concentrations, viscoelastic sol properties were dominant. A viscoelastic gel state for the chitosan/alginate solution was induced based on the weak formation of fiber‐shaped precipitates of a PEC at a low polymer concentration. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1408–1414, 2007     

碳纳米管/聚丙烯腈复合纤维的制备及结构研究

通过原位聚合的方法制备了碳纳米管/聚丙烯腈(CNTs/PAN)聚合液,用湿法纺丝工艺制备了CNTs/PAN复合纤维,分析了复合纤维流变性能、热性能及截面形貌。结果表明:CNTs的加入使得聚合物溶液出现了假凝胶化,粘度和弹性均有所上升,纺丝时溶液细流的表层遇水迅速凝固成致密的皮层,影响了纤维芯部的二甲基亚砜(DMSO)和水的双扩散作用,凝固丝出现了很明显的皮芯结构,CNTs的加入还使得纤维预氧化放热过程得到了缓和。     

Viscoelastic behavior of thermosetting epoxy mixtures modified with syndiotactic polystyrene during network formation

The rheological behavior of thermosetting epoxy mixtures modified with thermoplastic syndiotactic polystyrene (sPS) was monitored during the curing of the epoxy resin. The selected thermosetting system was diglycidyl ether of bisphenol A cured with 4,4′‐methylene bis(3‐chloro‐2,6‐diethylaniline) in the presence of various compositions of sPS (from 2.5 to 12.5 wt %). The storage and loss shear moduli of the systems were monitored during network formation. The validity of the Winter–Chambon criterion for the accurate determination at the gelation point from rheological data was demonstrated. The influence of the sPS concentration on the dynamic rheological properties of the samples was investigated. The experimental data showed that at sPS concentrations lower than 7.5 wt %, phase separation induced a quick increase in the viscosity, which was related to a crystallization‐induced phase separation of sPS. For sPS concentrations higher than 7.5 wt %, near the phase‐inversion composition, the rheological behavior of the mixtures was characteristic of a cocontinuous structure. After the viscosity jumped at the onset of phase separation, a decrease in the viscosity was found, and later on, the viscosity increased again because of gelation. Additionally, the influence of the cure temperature on the rheological properties was studied. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2348–2355, 2006