The dependence of shear and elongational viscosity on the molecular weight of poly(vinylidene fluoride)

The dependence of shear and elongational viscosity on the molecular weight of poly(vinylidene fluoride) has been studied using a capillary rheometer. The elongational viscosity was evaluated based on Cogswell's method with two types of capillaries: capillary length (L)/capillary diameter (D) = 10 mm/1 mm and L/D = 0 mm/1 mm. We used the ratio P₀/P_L that indicates the contribution of elongational flow to the total flow involving both the shear and elongational flows. P_L and P₀ are the pressure losses in the capillary and the converging flows, respectively. P₀/P_L increased with molecular weight and shear rate. This corresponds to decreasing the number of entanglements of molecular chain under a large displacement, especially high shear. Thus, we suggest using P₀/P_L as the parameter of the entanglement interaction on the molecular chain under a large displacement. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 2381–2384, 1999     

Polymer molecular weight,zero shear viscosity,steady state compliance and binary blending law

Viscoelastic properties of binary blends of polystyrenes with a narrow distribution of low and high molecular weights (M₂ > M₁ > M_c) were examined. By combining the theoretical work of Montfort et al., Kurata, and Schausberger, a binary blending law was developed and was used to calculate the zero shear viscosity and steady state compliance of the blend of two monodisperse polymers. The blending law was also used to calculate the molecular weight distribution of a polydisperse polymer. The calculated results were compared with those obtained from viscoelastic ] and GPC measurements, with good agreement.     

窄分布低乙烯基线型液体聚丁二烯的环氧化反应

采用负离子聚合法设计合成了分子量分布(MWD)窄、乙烯基含量低的线型液体聚丁二烯(NDLPB),然后用过氧甲酸原位法制备了不同环氧度的环氧化线型液体聚丁二烯(ENDLPB),并以其为偶联剂合成了梳状聚丁二烯,通过核磁共振法对ENDLPB的环氧度和结构进行了表征,考察了ENDLPB环氧度的影响因素.结果表明,随着ENDLPB环氧度的增加,环氧基特征峰逐渐增强,顺式-1,4-结构和反式-1,4-结构含量减小,1,2-结构含量不变;随着NDLPB数均分子量(~￣M_n)的增加,ENDLPB的环氧度减小,且~￣M_n略有增大,MWD约为1.08;以~￣M_n为2 450、MWD为1.07的NDLPB为反应物,在H_2O_2/NDLPB(摩尔比)为0.5、甲酸/NDLPB(摩尔比)为 0.4、反应温度为50 ℃的条件下反应2 h,可获得环氧度约为33%的ENDLPB,且实验的重复性良好;改变ENDLPB用量可获得不同偶联度的梳状聚丁二烯.     

Viscous behavior of ultrahigh molecular weight polyethylene solution

The viscous behavior of the decalin solution of ultrahigh molecular weight polyethylene (UHMEPE) was studied. The influence of the concentration of polymer as well as the temperature was investigated. The flow curve can be described by the power-law model. The dependence of the viscosity on the temperature can be described by the Arrhenius–Frenkel–Eyring equation. The dependence of viscosity on the concentration can also be described by a power-law correlation. The addition of aluminum stearate increased the activation energy of flow of the solution. The viscosity of UHMWPE solution was decreased at lower concentration and increased at higher concentration of UHMWPE. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65:289–293, 1997     

Well-mixed blends of HDPE and ultrahigh molecular weight polyethylene with major improvements in impact strength achieved via solid-state shear pulverization

Compared with conventional polyolefins, ultrahigh molecular weight polyethylene (UHMWPE) possesses outstanding impact strength and crack resistance that make it desirable for a wide variety of applications. Unfortunately, UHMWPE has an ultrahigh viscosity that renders common, continuous melt-state processes ineffective for making UHMWPE products. Attempts to overcome this problem by blending UHMWPE with lower molecular weight high-density polyethylene (HDPE) by melt processing have typically led to poorly dispersed blends due to the vast viscosity mismatch between blend components. Here, we present solid-state shear pulverization (SSSP) as a mild, continuous, and simple approach for achieving effective and intimate mixing in UHMWPE/HDPE blends. These SSSP blends are easily processed by post-SSSP melt extrusion; for an SSSP blend with 50 wt% UHMWPE, we observe more than a factor of 1000 increase in viscosity at a shear rate of 0.01 s⁻¹ but less than a factor of 5 increase at 100 s⁻¹, the latter being more typical of melt-processing operations. Using extensional rheology, we confirm the strain hardening behavior of SSSP blends. Shear rheology and crystallization data show that the mixing between UHMWPE and HDPE can be improved with subsequent passes of SSSP and single-screw extrusion. Finally, we show that blending via SSSP leads to dramatic improvements in impact strength: as compared to literature results, injection-molded sample bars made from SSSP blends with 30–50 wt% UHMWPE exhibit very high values of notched Izod impact strength, 660–770 J/m (the impact strength of neat HDPE was 170 J/m).     

Comments on the accuracy of zero shear intrinsic viscosity of high molecular weight polyacrylamide

The intrinsic viscosity of a polymer is traditionally measured with a capillary tube viscometer where the shear rate range is moderately high. Such method is valid when the polymers are non-ionic and have low to moderate molecular weight. The viscosity-shear rate curves obtained for dilute aqueous solutions of two high molecular weight polyacrylamides using two rotational viscometers indicate a strong shear-dependent viscosity in the medium to high shear rate regions. The zero shear intrinsic viscosity of the polymers determined by extrapolation from the high shear rate region to the zero shear condition may result in large errors. Its implication in predicting the molecular weight of polymers using the Mark-Houwink-Sakurada equation is discussed. A rheological equation for intrinsic viscosity as a function of shear rate is proposed.     

Detection of low levels of long‐chain branching in polydisperse polyethylene materials

Creep experiments have been applied to probe the zero‐shear viscosity, η₀, of polyethylene chains directly and precisely in a constant‐stress rheometer at 190°C. Such experiments, when combined with precise measurements of the weight‐average molecular weight, M_w, calibrated relative to linear chains of high‐density polyethylene, are shown to provide a very sensitive approach to detect low levels (0.005 branches per 1000 carbons) of long‐chain branching (LCB). This detection limit is shown to be insensitive to whether the molecular weight distribution (MWD) breadth, M_w/M_n, rises from about two to ten. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

超高相对分子质量聚乙烯特性黏数的测试方法

采用毛细管黏度计法测定超高相对分子质量聚乙烯(UHMWPE)的特性黏数([η]),对[η]的准确测定的影响因素进行了探讨。结果表明:以十氢萘为溶剂,使用前进行蒸馏提纯;在配制UHMWPE溶液时,加入质量分数为0.2%的抗氧剂1010,按预溶胀、溶胀、溶解3步骤进行溶解,溶解温度150℃,溶解时间0.5 h,配制UHMWPE溶液浓度为100~600 mg/L;严格控制温度(135±0.2)℃,所测UHMWPE的[η]平行性较好,相对标准偏差为1.08%。     

The breaking strength of ultra-high molecular weight polyethylene fibers

Ultimate mechanical properties of polyethylene fibers were measured. Results are in close agreement with the stress-induced melting theory of fracture (for finite molecular weight polymers). The perfect fiber work of rupture W_c, modulus K_c, strength σ_c, and strain _c are found to be W_c=0.084±0.003 GPa; σ_c=7.5 GPa; K_c=335±12 GPa; _c=0.0225±0.0005. The activation energy of fracture is measured as ≈108 kJ/mol—the activation energy of polyethylene fusion and one-third the activation energy of bond scission. Non-uniformity of fibers necessitates averaging properties over several test lengths. Actual stress-strain curves are decomposed into thermodynamic and irreversible components. Fusion theory applies to the thermodynamic component..     

Relationship between molecular structure and zero‐shear viscosity of polymers

The relationship between molecular structure and zero‐shear viscosity of polymers was studied. In this study we propose a new equation, which is based on Berry and Fox's equation. This new equation is constructed from some molecular parameters, such as mean square length and average molecular weight of statistical skeletal unit, characteristic ratio, entanglement molecular weight, glass‐transition temperature, free volume fraction at glass‐transition temperature, and thermal expansion coefficient of free volume. It is proposed that some of these molecular parameters could be predicted by group contribution methods, except for the free volume parameters. We also propose new empirical relations between free volume parameters and molecular structures of polymers, which make it possible for free volume parameters to be obtained from molecular structure. Using these relationships, it is possible that the zero‐shear viscosity and its temperature dependence are obtainable from the molecular structure of polymers. We applied this formula to some polymers, including both amorphous and semicrystalline polymers. Comparison between the measured and calculated zero‐shear viscosity showed quite good agreement. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1609–1618, 2001     

超高分子量聚乙烯成型加工及改性

介绍了近些年超高分子量聚乙烯（UHMWPE）的成型加工方法，由于UHMWPE熔体粘度极高，成型加工困难，通常采用模压成型，限制了其应用领域；通过综述中低分子量聚乙烯，聚丙烯，液晶聚合物及无机填料等改性UHMWPE所取得的成绩，指出解开UHMWPE的链缠结是改性最核心的问题。     

超高分子量聚乙烯的研究现状

介绍了超高分子量聚乙烯（UHMWPE)的性能，用途和工艺特点以及国内外关于流动改性UHMWPE的研究现状，阐述了共混改性、润滑剂改性和液晶高分子原位复合材料改性三种加工性能改性方法。     

用旋转剪切法测量聚合物熔体零切粘度时应注意的几个问题

本文研究了不同测试条件下用旋转剪切法测量聚合物熔体零切粘度值的变化规律,并通过比较旋转剪切法所得数据与蠕变法及动态法数据,得出对于均聚物或均相共混体系零切粘度的测量,旋转剪切法与蠕变法及动态法同样适用。     

超高分子量聚乙烯纤维的表面处理

综述了超高分子量聚乙烯（UHMWPE）纤维复合材料界面的重要性,总结了表面改性方法对UHMWPE纤维以及UHMWPE／树脂界面的影响。     

High molecular weight tail and long-chain branching in SRM 1476 polyethylene

The composition of the high molecular weight tail in the branched polyethylene standard reference material SRM 1476 was studied in detail using size exclusion chromatography coupled with a refractive index, a viscosity, and a light scattering on-line detector. The light scattering determinations of both molecular weight and radius of gyration point at a difference in kind between the molecular species below and above 10⁶ Da. In particular, above 10⁶ Da, increases in molecular weight resulted in a marked “densification” process. This is consistent with a greater concentration of long-chain branching and/or with a change in molecular architecture. Previous literature results for this polymer and a critical comparison of the performance of the different detectors are also discussed. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2807–2812, 1999     

Mechanism of secondary crystallisation in irradiated and aged ultra high molecular weight polyethylene

Ultra high molecular weight polyethylene has been irradiated in air and aged for 168 months. On melting and recrystallisation in the differential scanning calorimeter a secondary crystallisation event is observed around 80 °C. This event has been studied using isothermal crystallisation and the results analysed using the Avrami equation and the Lauritzen-Hoffman approach. This suggests that during this event growth occurs through regime II kinetics whereby large numbers of surface nuclei form on the substrate, with multiple nucleations acts commencing before the previous ones have finished. It is postulated that this secondary crystallisation event involves the development of a diffuse semi-ordered interface between the well-developed lamellae and the amorphous phase.     

Tuning the water permeability of ultra-high molecular weight polyethylene microporous membrane by molecular self-assembly and flow field

We have systematically studied phase separation behavior in ultra-high molecular weight polyethylene/liquid paraffin/dibenzylidene sorbitol (UHMWPE/LP/DBS) ternary blends. The aim of this paper is to investigate the combined effect of DBS and flow field on the structure and water permeability of UHMWPE microporous membrane. The experimental results show that DBS molecules self-assemble into fibrils firstly during cooling and the blends exhibit a gel-like state before liquid–liquid phase transition. The relaxation time of DBS fibrils is quite long, which shows a great sensitivity to flow field as compared to UHMWPE chain. UHMWPE microporous membrane was prepared via thermally induced phase separation method. DBS fibrils, as in situ formed nucleating agent, decrease the pore size and water permeability and enhance mechanical properties of membrane remarkably. Shear flow can result in alignment of DBS fibrils, which facilitates the nucleation of UHMWPE and induces the lamellae aligned perpendicular to flow direction. This feature was used to design thermal and mechanical histories and obtained oriented UHMWPE microporous membrane. In comparison to the isotropic UHMWPE microporous membrane, the oriented UHMWPE microporous membrane provides low tortuous paths across the membrane and produces high water permeability.     

Optimal process operation for the production of linear polyethylene resins with tailored molecular weight distribution

An optimization model is presented to determine optimal operating policies for tailoring high density polyethylene in a continuous polymerization process. Shaping the whole molecular weight distribution (MWD) by adopting an appropriate choice of operating conditions is of great interest when designing new polymers or when improving quality. The continuous tubular and stirred tank reactors are modeled in steady state by a set of differential‐algebraic equations with the spatial coordinate as independent variable. A novel formulation of the optimization problem is introduced. It comprises a multi‐stage optimization model with differential‐algebraic equality constraints along the process path and inequality end‐point constraints on product quality. The resulting optimal control problem is solved at high computational efficiency by means of a shooting method. The results show the efficiency of the proposed approach and the benefit of predicting and controlling the complete MWD as well as the interplay between operating conditions and polymer properties. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

超高分子量聚乙烯溶解均匀性研究

超高分子量聚乙烯纤维生产工艺中,超高分子量聚乙烯溶解设备普遍使用双螺杆挤出机。文章研究了如何使用双螺杆挤出机,具体涉及到螺纹元件的组合、双螺杆挤出机的转速以及各区温度控制等关键技术,制备溶解均匀的超高分子量聚乙烯溶液,以达到纺丝要求。     

氯化聚丙烯甲苯溶液的特性黏数及其黏均分子量

分别用乌氏黏度计和旋转黏度计测定了氯化聚丙烯甲苯溶液的特性黏数和黏度,把特性黏数和黏度进行关联,发现ηsp/C与浓度C(ml/g)的关系用多项式ηsp/C=[η]+k1[η]2C+k2[η]2C2拟合,可以得到很好的结果。并且用特性黏数和Mark-Houwink方程求出了不同氯化聚丙烯(CPP)的黏均分子量。