聚乳酸成型加工过程中的熔体粘弹特性和结晶行为调控

聚乳酸(PLA)是目前合成生物可降解高分子材料中应用量最大的品种,可望逐渐部分取代聚烯烃而更广泛应用于各个领域。但PLA树脂结构决定的松弛特性导致其加工过程特殊的黏弹特性,使其熔体强度低、成型工艺特性不稳定并进而导致产品尺寸和性能不稳定。此外,PLA极低的结晶速率,使其在挤出和注射成型等较高冷却速率的实际加工条件下呈无定型态,进一步影响了其加工和使用性能。这些问题已成为PLA更大规模商品化应用的瓶颈。本文从通过调控PLA熔体加工过程的黏弹特性而提高其可加工性出发,综述近年来本课题组在PLA成型加工过程中熔体粘弹特性和结晶行为(结晶速率和结晶结构)调控方面的研究进展。     

P3/4HB和滑石粉复合增韧填充PLA的研究

通过熔融共混法制备了可完全生物降解的聚乳酸/聚(3-羟基丁酸-co-4-羟基丁酸共聚酯)/滑石粉(PLA/P3/4HB/滑石粉)复合材料,利用万能试验机、扫描电子显微镜、熔体流动速率仪、差示扫描量热仪及热失重分析仪等测试研究了偶联剂处理以及滑石粉含量对PLA/P3/4HB/滑石粉复合材料的力学性能、界面相容性、熔体流动性和结晶性能的影响。结果表明,当滑石粉填充量较少时,偶联剂处理对复合材料的性能影响不大,但当滑石粉填充量较高时,经过偶联剂处理后的滑石粉能显著提高复合材料的熔体流动速率和冲击强度,PLA/P3/4HB/处理滑石粉复合材料的拉伸强度也保持在36.9MPa以上,结晶性能也有所提升,复合材料具有良好的综合性能。     

聚乳酸熔体纺丝、粘接和吸附性能的初步研究

针对制备香烟过滤嘴用丝束和过滤嘴棒的特殊需要,研究了聚乳酸的纺丝、粘接和吸附性能,通过流变学测量考察了各种加工因素对熔体流动性和可纺性的影响,特别是残留水分的影响,用分子量、DSC和力学性能测量跟踪了熔体纺丝和后加工过程.所获结果对确定聚乳酸树脂的规格范围,确定纺丝和后加工工艺,具有参考价值.     

提高聚丙烯熔体强度的研究进展

熔融状态下聚丙烯的熔体强度对其发泡效果有着举足轻重的作用,高熔体强度有利于聚丙烯的热成型,从而得到优质的发泡材料。本文综述了聚丙烯泡沫塑料的特点、性能及聚丙烯熔体强度较低对发泡过程的影响,分别从共混改性法、交联改性法、填充改性法这三种提高聚丙烯发泡性能的途径上对目前国内外致力于聚丙烯熔体强度的研究进展进行了重点介绍,指出聚丙烯化学交联改性与共混改性、共聚改性等方法并用乃是改善聚丙烯发泡性能的重点及热点。     

聚合物体系拉伸流变行为的表征及研究进展

聚合物的拉伸流动在吹膜、纺丝、热成型等加工中扮演着支配的角色,因此掌握聚合物熔体在拉伸条件下的流动行为对于控制和预测其加工性能具有重要意义。相对于剪切流动,拉伸粘度对于大分子的结构、填充粒子的各向异性、共混物中两相的结构等更加敏感。本文简要介绍了当前用于拉伸流变研究的常用装置及其原理,并举例描述了单一组分聚合物、聚合物纳米复合材料和聚合物共混物等体系拉伸流变研究的现状和成果,最后指出了当前拉伸流变研究领域存在的一些不足之处并进行了展望。     

高熔体强度聚丙烯的制备与表征*

高熔体强度聚丙烯（HMSPP）相比于普通聚丙烯具有优越的综合性能,其良好的加工性能使其具有广阔的应用前景,并成为许多国家近年来竞相开发的热点。本文从HMSPP的结构特点、制备方法、性能测试等方面总结了近年来的研究进展,着重介绍了反应器合成长链支化型高熔体强度聚丙烯（LCB-HMSPP）的方法以及其结构表征,并在总结国内外现状的基础上展望了高熔体强度聚丙烯的发展前景。     

高分子材料动态加工新技术

介绍了振动力场作用下高分子材料动态挤出成型、动态注射成型以及动态混炼成型的技术及装备,简要阐述了动态挤出中动态固体输送、动态熔融塑化、动态熔体输送过程的机理及实验研究成果,分析了脉动压力对注射过程的影响以及动态混炼过程中强制混炼、强制分散的原理,对各种动态成型技术辅以相应的制品性能测试.研究结果表明振动力场的引入在产量相同并保证甚至改善制品综合性能的同时,可有效缩短成型历程,降低功率消耗,因此高分子材料动态成型加工方法是一种新型的低能耗成型加工技术.     

非缠结缔合高分子的熔体拉伸性能

缔合高分子是指链间具有吸引作用(如离子键、氢键作用)的高分子.理解缔合高分子的熔体拉伸性能对于缔合高分子材料的开发和加工成型具有重要的指导意义.本综述聚焦本课题组在非缠结缔合高分子熔体拉伸行为研究中的最新进展,对熔体拉伸行为进行分类(如脆性行为、韧性行为和超韧性行为),并进一步总结了熔体拉伸行为对于缔合程度、缔合强度和缔合种类的依赖性.研究表明,脆韧行为不仅取决于拉伸流场下缔合网络结构的破坏,而且取决于网络结构破坏后的重整行为.     

聚丙烯纺丝成型与结构性能的关系第一报降低纺丝温度的研究

由于聚丙烯纺织温度太高,使防老助剂的分解、流失以及由此造成污染环境等缺陷。本文通过分析熔融纺丝过程中,造成熔体挤出膨化的原因,认为采用在喷丝板下方增添加热套筒,可减少或消除熔体挤出膨化,从而达到降低纺丝温度30～40℃的目的,也可提高纤维的均匀性并有刊十提高纺丝速度。与此同时也进行了加与不加热套筒和不同热套筒温度对初生纤维晶态结构、密度和双折射的影响;不同分子量的聚丙烯对纺丝温度的影响,以及与此有关的成型与纤维结构性能的关系的研究。     

滑石粉和PLA-g-MAH对聚乳酸结晶性能和熔体强度的改善

采用混沌混炼单螺杆挤出机,制备马来酸酐接枝聚乳酸(PLA-g-MAH),进而制备PLA/滑石粉(5%,10%和20%,质量分数)和PLA/滑石粉(20%)/PLA-g-MAH(5%和10%)复合材料.复合材料样品中滑石粉的分散状态良好,滑石粉含量高达20%时未发生团聚.20%滑石粉和10%PLA-g-MAH使复合材料中PLA的α晶含量明显增加,结晶度提高至31.6%.在175℃下,PLA样品的熔体强度仅为3.6 mN,20%滑石粉明显提高了PLA的熔体强度(11.6 mN),这是由于分散较均匀的片状滑石粉对PLA熔体起增强效应并可提高PLA结晶速率,对PLA结晶有促进效应.与PLA样品对比,PLA/滑石粉(20%)/PLA-g-MAH(5%)复合材料的杨氏模量和冲击强度分别提高了51.7%和16.9%.     

Effect of elongational flow on morphology and properties of polymer/CNTs nanocomposite fibers

Carbon nanotubes (CNTs) have been attracting increasing interest for the fabrication of polymer‐based nanocomposites because of their excellent properties. Traditional methods for the preparation of polymer/CNTs nanocomposites are in situ polymerization, solution blending, and melt mixing. The achievement of a good CNT dispersion and a percolation network is important in order to obtain better mechanical and electrical properties. However, the rheological behavior of polymer/CNTs systems, in particular regarding the extensional flow, has not been much investigated so far. In this work we present, for the first time, rheological data in non‐isothermal extensional flow and an investigation on the effect of the extensional flow upon the final properties of several polymer/CNTs systems was carried out as well. Extensional flow led to higher mechanical properties and higher melt strength, but only a slightly reduced breaking stretching ratio. This result could be particularly interesting in the view of potential industrial applications such as film blowing and spinning. Morphological analyses also showed higher degrees of dispersion and variation in the CNTs final dimensions. Copyright © 2010 John Wiley & Sons, Ltd.     

Boosting PLA melt strength by controlling the chirality of co-monomer incorporation

Bio-based and degradable polymers such as poly(lactic acid) (PLA) have become prominent. In spite of encouraging features, PLA has a low melt strength and melt elasticity, resulting in processing and application limitations that diminish its substitution potential vis-a-vis classic plastics. Here, we demonstrate a large increase in zero shear viscosity, melt elasticity, elongational viscosity and melt strength by random co-polymerization of lactide with small amounts, viz. 0.4–10 mol%, of diethylglycolide of opposite chiral nature. These enantiomerically pure monomers can be synthesized using one-step zeolite catalysis. Screening of the ester linkages in the final PLA chains by the ethyl side groups is suggested to create an expanding effect on the polymer coils in molten state by weakening of chain–chain interactions. This effect is suspected to increase the radius of gyration, enabling more chain entanglements and consequently increasing the melt strength. A stronger melt could enable access to more cost-competitive and sustainable PLA-based biomaterials with a broader application window. Amongst others, blow molding of bottles, film blowing, fiber spinning and foaming could be facilitated by PLA materials exhibiting a higher melt strength.

Melt strength improvements of PLA by co-polymerizing lactide with co-monomers of opposite chirality were discovered. Stronger melts can translate in less plastic usage, paving the way towards more sustainable bioplastics with broader applicability.     

A facile LED backlight in situ imaging technique to investigate sub-micron level processing

Many laboratory experimental techniques used for investigating fine fluid structure, such as fiber spinning, microfluidic flow, and electrospinning, require high quality images with good contrast. Common processes of observation and image recording rely heavily on highly technical light and camera setups which can be difficult to operate in some processing conditions and expensive as well. Here, we report a facile technique using LED backlight imaging to investigate ultrathin fluid profile in two different processes, melt electrospinning and tubeless siphoning. The setup comprises of a simple LED light source facing toward the camera, directly shining into the camera lens. The object under investigation was placed between the camera and the light source. The high-quality captured images and video recordings enable the precise analysis of the cone diameter and jet solidification in case of melt electrospinning, and extensional behavior profiles for tubeless siphoning. The LED backlight setup with high resolution camera is a useful tool to investigate sub-micron scale dimensions in fiber spinning, microfluidic flow, solution electrospinning, contact angle measurement for surface properties analysis, etc.     

Three‐dimensional printing of poly(lactic acid) bio‐based composites with sugarcane bagasse fiber: Effect of printing orientation on tensile performance

The cellulose fiber was extracted from the abandoned crop sugarcane bagasse (SCB) by means of chemical treatment methods. Poly(lactic acid) (PLA) bio‐based composites with SCB were prepared through fused deposition modeling (FDM) 3D‐printing technology, and the morphologies, mechanical properties, crystallization properties, and thermal stability of 3D‐printed composites were investigated. Compared with the neat PLA, the incorporation of SCB into PLA reduces the tensile strength and flexural strength of 3D‐printed samples but increases the flexural modulus. The difference in tensile performance and bending performance is that the tensile strength of 3D‐printed samples is best when the SCB content is 6 wt%, while the flexural modulus continuously decreases as the SCB content increases. Furthermore, the effects of various printing methods on the tensile performance of 3D‐printed samples were explored via modifying G‐code of 3D models. The results indicate that the optimum SCB fiber content is identical for all printing methods except method “vertical.” Due to the fibers and molecular chains are oriented to varying degrees with altering raster angle in 3D‐printed samples, the fully oriented sample printed by method “parallel” has a better tensile strength. Besides, SCB exhibits enough high thermal decomposition temperature to meet requirements for melt extrusion processing of PLA composites, and SCB fiber is capable of promoting the crystallization of PLA.     

Simultaneously reinforce and toughen polypropylene by in‐situ introducing polylactic acid microfibrils

A petroleum‐based polymer, isotactic polypropylene (iPP), and a biodegradable polymer, poly(lactic acid) (PLA), were compounded and molded into parts through the micro‐injection technique. A systematic structural investigation indicated that the microfibrillation of PLA minor phase depended on the operation parameter of inter‐mixer, ie, rotor speed. The higher rotor speed, the lower viscosity ratio of the PLA/iPP pair was favorable for microfibrillation occurred during micro‐injection process. The PLA microfibrils with high aspect ratio was successfully introduced into iPP matrix, and the tensile strength and strain at break of iPP/PLA blends were simultaneously improved. This study suggests a promising method for designing special microfibrillar morphology in polymer blend by using conventional melt processing techniques.     

Theoretical analysis of texture dependent extensional viscosity of discotic mesophases

Rheological functions for uniaxial extensional flows predicted by a previously selected and validated constitutive equation (CE) for discotic mesophases are presented. The predicted relations between extensional viscosities, flow-induced microstructure, processing conditions, and material parameters of discotic mesophases are characterized and discussed. It is found that, in contrast to rod-like nematics, two distinct uniaxial extensional viscosities need to be defined to characterize the extensional rheological functions of discotic mesophases completely. The model predicts non-Troutonian extensional viscosities of discotic nematics, such as strain thinning and strain thickening, depending on the process temperature, and the ratio of viscous to elastic stress contributions. The uniaxial extensional viscosities are also found to depend strongly on the flow-induced microstructure. The rheological analysis is then used to characterize the relations between extensional flow viscosities and the classical microstructures that arise during the industrial fiber spinning of discotic mesophase pitches.     

Nanostructural modifications of polyamide/MMT hybrids under isothermal and nonisothermal elongational flow

The aim of this work is to investigate the effects of elongational flow on the nanoscale arrangement of the silicate inside polyamide‐based nanocomposites. Hybrids, at different loadings of a commercial organoclay, were produced by melt compounding using two polyamide matrices, a nylon‐6, and a copolyamide with similar molecular weight and rheological properties. The elongational flow characterization was performed under both isothermal and nonisothermal conditions by using, respectively, an elongational rheometer (SER) and a fiber‐spinning technique. The extensional rheological response of melt‐compounded nanocomposites, correlated to TEM and X‐ray analyses, was used to probe the nanostructural modifications developed during the uniaxial stretching. The results demonstrated that isothermal and nonisothermal elongational flow can modify the nanomorphology of the nanocomposite hybrids affecting the degree of silicate exfoliation as well as the extent of silicate orientation upon the stretching direction. The entity of structural modifications induced by the stretching were highly dependent on the initial nanomorphological state and on the polymer‐clay affinity. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 981–993, 2009     

Melt elongation flow behaviour of LDPE/LLDPE blends

The extensional rheological properties of low density polyethylene (LDPE)/linear low density polyethylene (LLDPE) blend melts were measured using a melt spinning technique under temperatures ranging from 160 to 200 °C and die extrusion velocities varying from 9 to 36 mm/s. The results showed that the melt elongation stress decreased with a rise of temperature while it increased with increasing extensional strain rate and the LDPE weight fraction. The dependence of the melt elongation viscosity on temperature roughly obeyed the Arrhenius equation, it increased with increasing extensional strain rate and the LDPE weight fraction when the extensional strain rate was lower than 0.5 s⁻¹, and it reached a maximum when the extensional strain rate was about 0.5 s⁻¹, which can be attributed to the stress hardening effect.