Effect of Drawing Temperature on Structure and Properties of a＇-Phase of Poly（lactic acid）

a＇-晶型聚乳酸（PLA）膜被制备和单轴拉伸．通过凝胶渗透色谱仪（GPC）、全反射红外光谱（ATR-IR）、差示扫描量热仪（DSC）,X射线衍射（XRD）及Raman光谱等测试技术研究了拉伸温度梯度变化对a＇-晶型PLA膜的分子量及其分布、分子链构象、结晶度、晶型转变和取向行为的影响．在恒定拉伸速度与应变下,拉伸温度对PLA膜的应力．应变曲线,特别是屈服强度、拉伸模量产生了较大的影响,其值随拉伸温度的增加而降低．GPC测试结果表明,在不同的温度下拉伸后,PLA会发生一定程度的降解,分子量降低;ATR-IR,XRD,DSC和Raman光谱测试结果表明,在不同的温度下拉伸后a’-型PLA没有发生晶型的转变,即没有由a＇-晶体转变为a-或β-晶体．结果表明PLA的结晶度、分子链取向程度强烈依赖于拉伸温度：当拉伸温度低于100℃时,a’-型PLA膜 的结晶度与沿着拉伸方向的变形程度随拉伸温度的增加而增加,分子链的高度取向诱导了PLA结晶;当拉伸温度超过100℃后,PLA的分子链沿着拉伸方向上的有序度与结晶度将降低．     

拉伸温度对α’-型聚乳酸结构与性能的影响

α’-晶型聚乳酸(PLA)膜被制备和单轴拉伸.通过凝胶渗透色谱仪(GPC)、全反射红外光谱(ATR-IR)、差示扫描量热仪(DSC),X射线衍射(XRD)及Raman光谱等测试技术研究了拉伸温度梯度变化对α’-晶型PLA膜的分子量及其分布、分子链构象、结晶度、晶型转变和取向行为的影响.在恒定拉伸速度与应变下,拉伸温度对PLA膜的应力-应变曲线,特别是屈服强度、拉伸模量产生了较大的影响,其值随拉伸温度的增加而降低.GPC测试结果表明,在不同的温度下拉伸后,PLA会发生一定程度的降解,分子量降低;ATR-IR,XRD,DSC和Raman光谱测试结果表明,在不同的温度下拉伸后α’-型PLA没有发生晶型的转变,即没有由α’-晶体转变为α-或β-晶体.结果表明PLA的结晶度、分子链取向程度强烈依赖于拉伸温度:当拉伸温度低于100℃时,α’-型PLA膜的结晶度与沿着拉伸方向的变形程度随拉伸温度的增加而增加,分子链的高度取向诱导了PLA结晶;当拉伸温度超过100℃后,PLA的分子链沿着拉伸方向上的有序度与结晶度将降低.     

成核剂对应力诱导聚乳酸薄膜结晶的影响

为使PLA薄膜的结晶速度进一步加快,提高PLA薄膜的结晶度,提高薄膜的耐热性和力学强度,研究了成核剂对应力诱导PLA薄膜结晶的影响.利用XRD、DSC、FTIR等表征手段,对PLA薄膜拉伸前后的结晶状态进行了表征.研究表明,简单的引入成核剂,并不能实现成核剂和应力诱导促进PLA结晶的协同作用.相反,成核剂的引入会在很大程度上阻碍应力诱导结晶.即使成核剂在与拉伸温度相同的等温冷结晶过程中能够显著的提高PLA结晶速率,但在应力诱导结晶过程中,成核剂的引入使得拉伸后薄膜结晶度较纯PLA体系拉伸后薄膜明显更低.在相对较低的拉伸温度进行实验,成核剂同样起到阻碍应力诱导结晶进行的作用.同时,实验发现相对较低的拉伸温度,更有利于应力诱导结晶的进行,薄膜在较低温度拉伸后结晶度更高且更稳定.     

钛硅沸石的结晶动力学研究

 在TPABr-正丁胺体系中合成了钛硅沸石TS-1, 研究了此体系中的结晶动力学,求出了不同温度下的成核速率、晶体生长速率及表观活化能. 还研究了晶种类型和用量对钛硅沸石晶化的影响. 结果表明,随着晶化温度的升高, TS-1成核诱导期缩短,成核速率及晶体生长速率加快; 加入晶种可明显缩短TS-1成核诱导期,并减小晶粒粒度; TS-1, M, ZSM-11和β沸石等均可作为晶种合成TS-1, 并存在一个最佳晶种用量; 不加晶种时TS-1成核活化能和晶体生长活化能分别为44.4和75.7 kJ/mol.     

聚苯硫醚纤维的抗张强度与工艺和结构的关系

以熔融纺丝法制备出不同结晶度的各向同性聚苯硫醚纤维作为样品,根据密度和声速测定值确定出PPS晶相和无定形相的本征横向声模量E0⊥,c(4.40 GPa)和E0⊥,am(1.99 GPa).利用密度梯度法测定出的结晶度Xc和X-衍射法测定的晶区取向因子fc,按照Samules模型计算出不同牵伸和定型工艺下制备的PPS纤维样品的非晶区取向因子(fam),在此基础上分析PPS纤维抗张强度与牵伸定型工艺参数、结构之间的关系.结果表明,PPS纤维的最佳牵伸温度及紧张热定型温度分别在90℃和190℃附近;提高PPS纤维的牵伸温度及紧张热定型温度可以增加纤维的结晶度,在一定范围内对纤维抗张强度的增加有促进作用;但较高的牵伸温度及紧张热定型温度不利于纤维非晶区取向的提高,造成PPS纤维抗张强度降低.牵伸倍数的增加可以有效提高PPS纤维的非晶区取向程度,抗张强度也随着增加.     

β晶型成核剂N,N’-二环己基环己对二甲酰胺的合成与应用研究

合成了新型成核剂N,N’-二环己基环己对二甲酰胺(DCHCHDA),用广角X射线衍射仪、差示扫描量热仪和偏光显微镜对其进行了表征,并研究了其对等规聚丙烯(IPP)的力学性能、结晶行为、β晶含量和熔融行为的影响。结果表明,成核剂DCHCHDA主要是改变球晶的形态,可明显改善IPP的抗冲击性能,并可以大幅度提高IPP的结晶度、β晶含量与结晶峰温度。当DCHCHDA添加量为0.05(wt)%时,IPP的悬臂梁缺口抗冲击强度可以提高137%,结晶度提高47%,β晶含量提高3.3倍。当降温速率为10℃/min时,添加DCHCHDA的IPP的结晶峰温度可从空白样的115.6℃提高到127.5℃。DCHCHDA的成核性能优于传统的芳香酰胺型β晶型成核剂DC26NDCA。     

快速增压制备非晶聚乳酸的等温冷结晶行为研究

通过快速增压法(RC)和自然冷却法(CN)分别制备出完全非晶的聚乳酸(PLA)样品,利用差示扫描量热仪(DSC)、偏光显微红外光谱仪(FTIR)和广角X-ray(WAXD)研究了2种非晶样品的初始结构、不同结晶温度下的等温冷结晶行为及最终的微观结构.结果表明,在温度为110、115、125、130、135及140℃下等温结晶时,RC样品的结晶速率明显高于CN样品;WAXD数据显示,结晶完成后RC样品的结晶度明显高于CN样品,但两者最终的晶粒尺寸大小相近,说明RC样品结晶过程中具有较高的成核密度;实验发现,PLA熔体在快速增压过程中很可能形成了某些类似物理老化样品中存在的局部有序结构,这些局部有序结构促进了晶核的形成,使冷结晶速率明显加快,结晶度提高.     

SSBR/TPI共混体系中TPI的结晶、成核及动力学研究

研究了不相容共混体系溶聚丁苯橡胶(SSBR)/反式-1,4-聚异戊二烯(TPI)共混体系的结晶行为.当SSBR含量较低时,SSBR/TPI共混物的成核密度降低,晶体生长速率降低,等温结晶速率降低,非等温结晶峰移向低温,SSBR抑制了α-TPI的生成,SSBR/TPI共混体系总结晶度降低,但TPI的相对结晶度升高,结晶行为与热力学相容的结晶/非晶共混体系一致;当SSBR含量较高时,随着SSBR含量增加,体系的成核密度增加,球晶的生长速率显著降低,SSBR显著抑制了α-TPI的生成,共混体系以β晶为主.     

温度对PbO2电沉积形核生长过程的影响研究

为了探讨温度对PbO₂电沉积形核生长过程的影响,在25℃、35℃、45℃、55℃、65℃使用电化学工作站测试了PbO₂在玻碳电极上沉积过程的循环伏安曲线、计时电位曲线及计时电流曲线,并对不同温度下电沉积的PbO₂镀层进行了SEM和XRD分析. 结果表明,在不同温度下PbO₂都经历了成核和核生长过程. 温度的升高没有改变PbO₂电沉积三维连续成核的模式,降低了沉积过程溶液阻力,对成核和晶体生长速率均有促进作用,在晶核密度达到饱和晶核密度以前,是以促进成核速率为主,减小PbO₂颗粒尺寸. 达到饱和晶核密度后,电沉积后期以促进晶体生长速率为主,不利于形成细小PbO₂颗粒.高温使得析氧速率提高,能耗增大.由实验结果得出,在55℃时得到的PbO₂镀层粒径最小.     

β晶型成核剂对聚丙烯力学及结晶性能的影响

研究了N,N′-二环己胺基对苯二酰胺作为β成核剂对聚丙烯(PP)力学性能及结晶性能的影响。结果表明,成核剂具有使聚丙烯增韧的优异性能,当添加质量分数为0.3%时,其抗冲击强度由原来的36.06 J/m2提高到65.79 J/m2。DSC研究表明,添加β成核剂可以诱导PP中β晶生成。考察了冷却速率对结晶温度的影响,当冷却速率为10℃/m in时,结晶温度从118.38℃提高到124.53℃,表明该成核剂的加入使结晶向高温方向偏移,结晶速度加快。     

分级聚丙烯的取向结晶过程

聚丙烯熔体在剪切或应变应力作用下 ,分子链发生取向形成伸直链纤维晶 ,这些先取向形成的纤维晶成为其后结晶的晶核 .这种线形排列的特殊自晶核被称作排核 ( Row nuclei) [1] .排核诱导的结晶温度高于异相核和均相核 .折叠链片晶在排核上附生生长 ,形成具有柱状对称性的超分子结构 ,称为柱状晶 ( Cylindrite) [2 ,3] .聚合物的分子量 ,剪切温度和剪切速度等因素对柱状晶的生成有很大影响 [4 ,5] .本文选用不同级分的聚丙烯样品 ,利用高分子 (特别是取向结晶 )的记忆效应 [6,7] ,研究了剪切后薄膜试样在熔融重结晶过程中柱状晶的形成和发展…     

Correlations between microstructure of α‐row nuclei and polymorphism of shear‐induced iPP/carbon fiber cylindrite

Herein, we reported the formation mechanism of hybrid crystalline (cylindrite) in isotactic polypropylene (iPP)/carbon fiber (CF) via pulling a CF within the iPP melt. The α‐row nuclei layer closely attached to the surface of CF acts as a self‐nucleation site, rather than a heterogeneous nucleation one, to grow cylindrites. As a result, the polymorphic feature of iPP/CF cylindrite is significantly influenced by the microstructure of α‐row nuclei. With decreasing crystallization temperature (T_c), the polymorphic cylindrite changes from pure α‐form to mixed α‐/β‐form and to β‐rich form. The main characteristics of this change include: (a) the outlines of α‐row nuclei layer correspond to wave‐like, saw‐like, and straight lines; (b) the orientation level of iPP molecules in the α‐row nuclei layer become higher; (c) the α‐lamellae rearrange from loose to compact; and (d) the distance between the growth sites of β‐sectors and the surface of CF is evidently longer than in the case of α‐sectors. Moreover, this study provides a guideline for developing the interfacial enhanced iPP/CF composites through manipulation of polymorphic structure in cylindrites. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 368–377     

Shear‐Induced Crystallization in a Blend of Isotactic Poly(propylene) and Poly(ethylene‐co‐octene)

Summary: Shear‐induced crystallization in a blend of isotactic poly(propylene) and poly(ethylene‐co‐octene) (iPP/PEOc) has been investigated by means of in‐situ optical microscopy and a shear hot stage under various thermal and shear histories. Cylindrites are observed after shear in the phase‐separated iPP/PEOc blends for the first time. The nuclei (shish) come from the orientation of the entangled network chains, and the relationship between the shear rate and the network relaxation time of the oriented iPP chains is a very important factor that dominates the formation of the cylindrites after liquid‐liquid phase separation. The cylindrites can grow through phase‐separated domains with proper shear rate and shear time. In addition, the number of spherulites increases with shear rate, which is consistent with the notion of fluctuation‐induced nucleation/crystallization.

Phase‐contrast optical micrograph of the iPP/PEOc = 50/50 (wt.‐%) sample sheared during cooling with shear rate of 10 s⁻¹ and isothermally crystallized at 140 °C for 142 s after isothermal annealing at 170 °C for 420 min. The shear time is 180 s.     

聚对苯二甲酸乙二酯非晶态膜片单轴热——拉伸中的结晶与松弛

用动态力学损耗温度谱作为测试手段,研究了非晶态PET膜片在78—112℃温度范围内的单轴拉伸。实验结果说明,在较低温度下所得结晶的拉伸试样,完全由于应变诱发结晶,发生在应力-应变曲线的屈服后应力开始上升的阶段。在较高温度下(90℃或更高)拉伸可得非晶态而且光学各向同性的试样,是由于分子链的小尺度取向在拉伸过程中已完全热松弛所致,而分子链的大尺度取向要通过高弹态流动而松弛,其速率较慢,用拉伸后试样两端固定时的应力松弛进行了观察。在较低温度下应力松弛后仍为非晶态,在较高温度下应力松弛到起始应力的1O％下才开始结晶。FTIR研究表明在这种状态下的结晶有一结晶诱导期,其时间尺度与应力松弛阶段相当。     

Drawing of nylon 6 fibers (bristles)

By means of electron microscopy of surface replicas and both small-angle and wide-angle x-ray scattering, nylon 6 fibers were investigated in the as-spun state, after drawing at 180°C to a draw ratio up to 4.95, and after subsequent annealing. As spun, the fiber exhibits a small fraction of row-nucleated cylindrites and a great many spherulites (with an average diameter of a few microns) side by side. Drawing deforms the spherulites into spindle-shaped structures (λ = 2) and subsequently produces well-aligned microfibrils. Small-angle x-ray scattering yields a two-point diagram at small λ and a fourpoint diagram at high λ. The long period seems to decrease slightly with draw ratio. Annealing at temperatures above the temperature of drawing increases the long period to a greater extent with samples of lower λ. The crystal lattice orientation is nearly complete at λ = 4.95.     

Shear-induced crystallization in phase-separated blend of isotactic polypropylene and poly (ethylene-co-octene)

Isothermal crystallization after shear in a blend of isotactic polypropylene (iPP) and poly (ethylene-co-octene) (PEOc) was investigated by in situ optical microscopy and shear hot stage under various thermal and shear histories. Crystalline cylindrites during growth were observed in phase-separated iPPPEOc blends for the first time. According to our results, the very long cylindrites are formed which are much longer than the dimensions of the liquid-liquid phase-separated domains under shear, and the cylindrites appear to grow through noncrystallizable domains, as well as through crystallizable ones. Obviously, the nuclei ("shish") come from the oriented and entangled network strands instead of pulled-out long chains. The number of cylindrites and the distortion and breakup of the cylindrites are related to the shear rate and shear time. On the other hand, the number of spherulites increases not only with shear rate but also with liquid-liquid phase separation time. Spherulites always form with longer induction time than cylindrites due to the different nucleation mechanism. The shish is nucleated through the shear-induced mechanism, and most of the spherulites are nucleated through liquid-liquid spinodal decomposition and crossover after the cessation of shear. During the process of experiments, we also found three kinds of shish-kebab structures, which provide further physical insights into the mechanism of the shish formation in polymer blend after liquid-liquid phase separation under shear.     

Hydrolytic Degradation of Polylactic Acid Fibers as a Function of pH and Exposure Time

Polylactic acid (PLA) is a widely used bioresorbable polymer in medical devices owing to its biocompatibility, bioresorbability, and biodegradability. It is also considered a sustainable solution for a wide variety of other applications, including packaging. Because of its widespread use, there have been many studies evaluating this polymer. However, gaps still exist in our understanding of the hydrolytic degradation in extreme pH environments and its impact on physical and mechanical properties, especially in fibrous materials. The goal of this work is to explore the hydrolytic degradation of PLA fibers as a function of a wide range of pH values and exposure times. To complement the experimental measurements, molecular-level details were obtained using both molecular dynamics (MD) simulations with ReaxFF and density functional theory (DFT) calculations. The hydrolytic degradation of PLA fibers from both experiments and simulations was observed to have a faster rate of degradation in alkaline conditions, with 40% of strength loss of the fibers in just 25 days together with an increase in the percent crystallinity of the degraded samples. Additionally, surface erosion was observed in these PLA fibers, especially in extreme alkaline environments, in contrast to bulk erosion observed in molded PLA grafts and other materials, which is attributed to the increased crystallinity induced during the fiber spinning process. These results indicate that spun PLA fibers function in a predictable manner as a bioresorbable medical device when totally degraded at end-of-life in more alkaline conditions.     

Effect of multiscale structure on the gas barrier properties of poly(lactic acid)/Ag nanocomposite films

Poly(lactic acid) (PLA) is the most suitable for biodegradable packaging film because of its excellent integrated property, but the poor gas barrier property is its weakness. In this study, a nanocomposite film based on PLA incorporated with 0‐, 1‐, 3‐, 5‐, 10‐, or 15‐wt% nano‐Ag was developed. Effect of multiscale structure on the barrier properties of PLA/nano‐Ag films was studied. The PLA nanocomposite film with 5‐wt% nano‐Ag had the lowest water vapor permeability (WVP) value. Oxygen transmission rate (OTR) value for PLA nanocomposites with 3‐wt% nano‐Ag was found to be the lowest among all the samples. Multiscale structure was demonstrated by the scanning electron microscopy, Fourier transform‐infrared spectroscopy, X‐ray diffraction measurement, and differential scanning calorimetry results. The crystallinity of the PLA phase increased with the content of nano‐Ag in the PLA composites. The evolution of the PLA phase crystallinity could improve the barrier properties of PLA/nano‐Ag composite films for food packaging applications. From the view of multiscale structure, it is better to achieve a balance among short‐range conformation in the amorphous region, long‐range‐ordered structure, and ordered aggregated structure to improve the barrier properties of PLA/nano‐Ag composite films.     

Effects of physical aging,crystallinity, and orientation on the enzymatic degradation of poly(lactic acid)

The effects of physical aging, degree of crystallinity, and orientation of poly(lactic acid) (PLA) were studied using differential scanning calorimetry (DSC) and wide angle X-ray scattering (WAXS). The samples of PLA with 96% [L] and 4% [D] contents were prepared by injection molding. The physical aging of PLA strongly depended on time and temperature. The change of rate of physical aging was very fast initially and slowed down as time increased. The enzymatic degradation of PLA was carried out with proteinase K at 37°C at a pH value of 8.6 in a Tris/HCl buffer solution. The enzymatic degradation rate was found to decrease as a function of physical aging (i.e., excess enthalpy relaxation). The rate of enzymatic degradation of PLA decreased with the increase in crystallinity. A threshold was observed when the heat of fusion was less than 20 J/g. The weight loss of PLA with a low level of crystallinity had no apparent change during any period of testing time. The rates of enzymatic degradation of stretched and injection-molded specimens were comparable. © 1996 John Wiley & Sons, Inc.