沥青混合料黏弹性主曲线模拟及换算

基于时间-温度等效原理将不同温度下沥青混合料的储存模量和蠕变柔量进行平移形成主曲线,采用广义Maxwell和Kelvin模型分别模拟沥青混合料的松弛和蠕变特性,利用Prony级数表达式和Laplace变换实现黏弹性主曲线间的相互换算,并将换算结果与试验结果进行了比较。结果表明,广义Maxwell和Kelvin模型可以较好地模拟沥青混合料黏弹性主曲线,储存模量和蠕变柔量主曲线相互换算结果与试验结果规律基本一致,松弛模量实际结果应介于由储存模量和蠕变柔量换算的松弛模量主曲线之间,这可为沥青混合料黏弹性力学研究及黏弹性参数的获取提供有益参考。     

聚丙烯/尼龙6共混物熔体的松弛时间

用扫描电镜对聚丙烯(PP)/尼龙6(PA6)共混物断面形貌进行了表征,用旋转流变仪小振幅剪切模式测试了PP/PA6共混物的动态模量,并采用广义Maxwell模型对于损耗模量-角频率曲线进行了拟合。扫描电镜结果表明,聚丙烯接枝马来酸酐(PP-g-MAH)有效提高了PP和PA6之间的相容性,使得分散相PA6粒径尺寸减小。广义Maxwell模型中,当松弛模数为N=5和6时,拟合曲线与实验数据在低频下存在明显的偏差,而松弛模数为N=7时,广义Maxwell模型能够较好地拟合实验曲线。固定PP/PP-g-MAH质量比为90/6时,随着PA6含量的增加,共混物PP/PP-g-MAH/PA6的松弛时间延长,松弛时间谱曲线以短时区为轴心逆时针旋转,向长时区偏移。     

面向残余应力检测的聚碳酸酯在溶剂环境下的初始应力-开裂时间关系

本研究利用四点弯曲应力松弛实验装置,获得了聚碳酸酯(PC)材料在不同比例甲苯/正丁醇混合溶剂中的初始应力-开裂时间实验数据;依据离散数据的几何相似性,建立了一个指数关系模型,拟合得到了初始应力-开裂时间的连续分布,并基于对拟合曲线的抽样误差分析,建立了残余应力与初始应力之间的关系式,可用于提高PC制品残余应力的检测精度。     

聚合物损耗行为的分数阶粘弹模型

分析了广义Zener模型的粘弹性损耗特征,用该模型对聚酯树脂的损耗因子进行了拟合,对其损耗行为给出了非常好的描述,且表明主转变区衰减指数随温度线性增加,松弛时间随温度指数衰减。用并联分数Maxwell模型对拉伸高密度聚乙烯的损耗因子进行了模拟,表明用两个具有不同特征时间的双松弛过程能对结晶度较高的HDPE的松弛特征进行很好的描述。     

热致非晶态形状记忆聚合物的热粘弹性参数及回复行为

分别采用非线性拟合应力松弛主曲线及单个常频率条件下变温动态力学分析(DMA)实验中储能模量与损耗角正切曲线的方式,获取热致非晶态形状记忆聚合物(SMP)多重松弛模型中的热粘弹性参数。结合广义有限变形粘弹性理论和KAHR33参数热变形模型在有限元软件平台研究了该SMP的自由回复与约束回复行为。仿真结果与实验数据相符较好,说明该建模方法的合理性及获取的热粘弹性参数的准确性。相比构建应力松弛和储能模量主曲线而言,单个DMA实验数据更易得到,因此对其拟合能大幅减少实验成本。     

基于LM算法的粘弹性材料剪切松弛模量的拟合

在对粘弹性材料进行有限元计算时,需要材料的剪切松弛模量,必须对经验公式或实验数据进行转换。根据KWW经验公式与广义Maxwell剪切松弛函数的对应关系,列出了求解松弛时间与剪切松弛模量的超定非线性方程组,用LM优化算法得到了方程组的最优解。初步讨论了初值的选取方法。根据此方法得到的松弛时间与剪切松弛模量的拟合曲线的拟合度达到了0.99以上。所提出的方法也适用于用实验数据(随时间变化的应力应变)求解材料的剪切松弛模量。     

WJ-8扣件系统中弹性垫板的应力松弛性能研究

对高速铁路轨道WJ-8扣件中的聚氨酯弹性垫板分别进行长时限压缩15％、20％、25％试验,并测定应力松弛过程.应用五元件广义Maxwell为模型,对应力松弛过程进行了数学回归.结果表明,五元件Maxwell模型可以完美拟合弹性垫板的应力松弛过程.随着应变增加,松弛时间呈下降趋势,即垫板抗形变能力减弱,垫板恢复原有形状能力下降.为WJ-8扣件承载限量预测、模拟工作特性研究和垫板分子结构设计提供理论依据.     

基于广义Maxwell模型的非球面光学镜片成型模拟

介绍了玻璃材料模压过程及其存在的问题,针对玻璃模压仿真过程中所表现粘弹性力学特性,提出了采用广义Maxwell模型来模拟,分析了模压过程中的热传递形式及其模型.采用MSC.MARC软件对玻璃L-BAL42圆柱模压实验进行仿真,结果表明,广义Maxwell模型能够较好地模拟实际模压中的粘弹性力学特性,在600℃时利用此模型得到的松弛曲线与理论上的松弛曲线十分吻合.通过对非球面光学镜片的模压成型过程进行模拟,发现玻璃的最大应力出现在与上下模接触处的侧边,随着温度的降低,最大应力变大、模压力逐渐升高,在上下模闭合时,模压力快速上升,通过分析得到了透镜边缘处最易破碎失效的结论,实际透镜模压结果证实了结论.最后对非球面的形状偏差进行了预测,表明随着中心距离的增加,偏差变大,偏差最大值超过了5μm.研究的结果对玻璃模压建模及非球面透镜的实际加工具有一定的指导意义.     

92负刚度蜂窝结构压缩性能分析

为准确开展以黏弹性材料为载体的负刚度蜂窝结构数值模拟研究,提出一种基于黏弹性广义Maxwell模型的负刚度蜂窝结构压缩性能数值模拟法。进行了尼龙12的动态机械分析(DMA)测试,基于广义Maxwell模型拟合实测动态模量数据,得到反映尼龙12动态黏弹性的无量纲模量g_i和松弛时间τ_i。建立了负刚度蜂窝结构的有限元模型,基于实测的动态黏弹性参数,进行压缩性能的数值模拟研究,并同压缩试验结果进行比较,验证了数值模拟的准确性,利用数值模拟研究几何参数对结构压缩性能的影响规律。结果表明,基于黏弹性广义Maxwell模型的负刚度蜂窝结构数值模拟分析法可较准确模拟结构的压缩性能,为预测负刚度蜂窝结构的力学性能提供帮助。     

一种新的聚合物基复合材料应力松弛经验模型

视黏弹性元件弹簧和粘壶的特性参数为时间的函数,建立双变参Maxwell模型,求解其本构方程及松弛函数,探讨了其松弛函数与经验KWW函数的关系,提出了一种新的应力松弛经验模型。运用最小二乘原理,建立该模型参数的确定方法。开展了玻璃纤维增强聚合物(GFRP)复合材料的长期应力松弛实验,并通过实验数据的分析验证了该模型的正确性。研究表明,新应力松弛经验模型拟合曲线与实测曲线吻合较好,相关系数达到96%以上,说明了该模型适合描述GFRP复合材料的黏弹性松弛特性。     

Effect of mixing ceramics with a thermosensitive biodegradable hydrogel as composite graft

The composite of methoxy polyethylene glycol (mPEG) and poly(lactic-co-glycolic acid) (PLGA) thermosensitive hydrogel mixed with various portions of hydroxyapatite (HAP) or β-tricalcium phosphate (β-TCP) were used as bone graft substitutes. The physical properties of a series of composite gels, including the critical micelle concentration (CMC), particle sizes, zeta potential, rheological behavior, morphology of composite gels, and sol–gel transition, were characterized in vitro. These composite gels could form a gel at body temperature and could be controlled easily at room temperature, but showed only a small decline in pH, to between 6.33 and 6.66, whereas mPEG–PLGA gel without ceramic exhibited a more significant decrease in pH over a period of 5 days. The dissolution of ceramics results in an increase in the concentration of calcium and phosphate, which can buffer the degradation of mPEG–PLGA. Higher cell viability was observed in the composite gels with more bioceramics, as shown in the MTT assay and the live and dead stain. Mixing mPEG–PLGA with HAP or β-TCP may hold greater promise than mPEG–PLGA alone for repairing bone defects.     

Effect of mixing ceramics with a thermosensitive biodegradable hydrogel as composite graft

The composite of methoxy polyethylene glycol (mPEG) and poly(lactic-co-glycolic acid) (PLGA) thermosensitive hydrogel mixed with various portions of hydroxyapatite (HAP) or β-tricalcium phosphate (β-TCP) were used as bone graft substitutes. The physical properties of a series of composite gels, including the critical micelle concentration (CMC), particle sizes, zeta potential, rheological behavior, morphology of composite gels, and sol–gel transition, were characterized in vitro. These composite gels could form a gel at body temperature and could be controlled easily at room temperature, but showed only a small decline in pH, to between 6.33 and 6.66, whereas mPEG–PLGA gel without ceramic exhibited a more significant decrease in pH over a period of 5 days. The dissolution of ceramics results in an increase in the concentration of calcium and phosphate, which can buffer the degradation of mPEG–PLGA. Higher cell viability was observed in the composite gels with more bioceramics, as shown in the MTT assay and the live and dead stain. Mixing mPEG–PLGA with HAP or β-TCP may hold greater promise than mPEG–PLGA alone for repairing bone defects.     

PLGA–PEG–PLGA hydrogel for ocular drug delivery of dexamethasone acetate

Aim: This study aims to investigate the suitability of thermosensitive triblock polymer poly-(dl-lactic acid-co-glycolic acid) (PLGA)–polyethylene glycol (PEG)–PLGA as a matrix material for ocular delivery of dexamethasone acetate (DXA). Methods: The copolymer was synthesized and evaluated for its thermosensitive and gelation properties. DXA in situ gel-forming solution based on PLGA–PEG–PLGA copolymer of 20% (w/w) was prepared and evaluated for ocular pharmacokinetics in rabbit according to the microdialysis method, which was compared to the normal eye drop. Result: The copolymer with 20% (w/w) had a low critical solution temperature of 32°C, which is close to the surface temperature of the eye. The C_max of DXA in the anterior chamber for the PLGA–PEG–PLGA solution was 125.2 μg/mL, which is sevenfold higher than that of the eye drop, along with greater area under the concentration–time curves (AUC). Conclusion: These results suggest that the PLGA–PEG–PLGA copolymer is potential thermosensitive in situ gel-forming material for ocular drug delivery, and it may improve the bioavailability, efficacy of some eye drugs.     

新型改性的n-HA与PLGA复合材料的制备及性能研究

采用硅烷偶联剂(KH550)和L-丙交酯LLA联合接枝处理的新方法对纳米羟基磷灰石(n–HA)进行表面改性, 然后将其与聚乳酸-羟基乙酸(PLGA)作不同比例复合(n-HA为3wt%、10wt%、20wt%及30wt%), 得到改性n-HA/PLGA复合材料(g-n-HA/PLGA)。将其与未改性n-HA及未改性n-HA/PLGA复合材料作对比检测。结果表明, 该联合处理方法是n-HA进行表面接枝改性的新型有效方法。且改性处理后的n-HA与未改性处理的n-HA相比, 能更好地在PLGA基体中分散均匀, 并能提高PLGA结晶能力和PLGA的力学性能。当改性处理后的n-HA添加量为10wt%时, 其复合材料抗弯强度和拉伸强度分别比未改性n-HA/PLGA提高14.4%和11.3%。该新型g-n-HA/PLGA复合材料有望用作骨折固定材料。     

Synthesis and characterization of novel biodegradable tetra-amino-terminated PLGA telechelic copolymer

A new method for the synthesis of biodegradable multifunctional group-terminated copolymers is explored, by which a series of novel biodegradable tetra-amino-terminated PLGA telechelic copolymers are synthesized for the first time. The composition, structure, and thermal property of the telechelic copolymer were characterized by FTIR, NMR, GPC, and MDSC. The results show that by careful selection of the synthesis parameters, tetra-amino-terminated PLGA telechelic copolymer with desired molecular weight and glass transition temperature can be prepared. This versatile synthetic platform can provide a large family of biodegradable, tetra-amino-terminated telechelic copolymers with tunable physiochemical and biological properties useful for the preparation of network biodegradable polyester elastomer with tailorable properties.     

In vitro degradation and drug release from polymer blends based on poly(<Emphasis Type="SmallCaps">dl</Emphasis>-lactide), poly(<Emphasis Type="SmallCaps">l</Emphasis>-lactide-glycolide) and poly(ε-caprolactone)

Bioresorbable materials are extensively used for a wide range of biomedical applications. Accurately modifying and evaluating the degradation rate of these materials is critical to their performance and the controlled release of bioactive agents. The aim of this work was to modify the physical properties, degradation rate and drug delivery characteristics of thin films for medical applications by blending poly(dl-lactic acid) (PDLLA), poly(l-lactide-co-glycolide) (PLGA) and poly(ε-caprolactone) (PCL). The thin films were prepared using solvent casting and compression moulding and the in vitro degradation study was performed by immersing the films in a phosphate-buffered saline at elevated temperature for a period of 4 weeks. The degradation rate of the materials was analysed by differential scanning calorimetry, tensile testing and weight loss studies. The thermal analysis of the blends indicated that the presence of PLGA or PDLLA in the film resulted in increased degradation of the amorphous regions of PCL. It was observed that the samples consisting of PDLLA with PCL demonstrated the greatest weight loss. The decrease in mechanical properties observed for both sets of polymer blends proved to be similar. The solvent cast technique was selected as the most appropriate for the formation of the polymer/drug matrices, due to the potentially adverse thermal processing effects associated with compression moulding. It was found that modulation of drug release was achievable by altering the ratio of PCL to PDLLA or PLGA in the thin film blends.     

Porous nano-hydroxyapatite/collagen scaffold containing drug-loaded ADM–PLGA microspheres for bone cancer treatment

To develop adriamycin (ADM)-encapsulated poly(lactic-co-glycolic acid) (PLGA) nanoparticles in a porous nano-hydroxyapatite/collagen scaffold (ADM–PLGA–NHAC). To provide novel strategies for future treatment of osteosarcoma, the properties of the scaffold, including its in vitro extended-release properties, the inhibition effects of ADM–PLGA–NHAC on the osteosarcoma MG63 cells, and its bone repair capacity, were investigated in vivo and in vitro. The PLGA copolymer was utilized as a drug carrier to deliver ADM–PLGA nanoparticles (ADM–PLGA–NP). Porous nano-hydroxyapatite and collagen were used to materials to produce the porous nano-hydroxyapatite/collagen scaffold (NHAC), into which the ADM–PLGA–NP was loaded. The performance of the drug-carrying scaffold was assessed using multiple techniques, including scanning electron microscopy and in vitro extended release. The antineoplastic activities of scaffold extracts on the human osteosarcoma MG63 cell line were evaluated in vitro using the cell counting kit-8 (CCK8) method and live-dead cell staining. The bone repair ability of the scaffold was assessed based on the establishment of a femoral condyle defect model in rabbits. ADM–PLGA–NHAC and NHAC were implanted into the rat muscle bag for immune response experiments. A tumor-bearing nude mice model was created, and the TUNEL and HE staining results were observed under optical microscopy to evaluate the antineoplastic activity and toxic side effects of the scaffold. The composite scaffold demonstrated extraordinary extended-release properties, and its extracts also exhibited significant inhibition of the growth of osteosarcoma MG63 cells. In the bone repair experiment, no significant difference was observed between ADM–PLGA–NHAC and NHAC by itself. In the immune response experiments, ADM–PLGA–NHAC exhibited remarkable biocompatibility. The in vivo antitumor experiment revealed that the implantation of ADM–PLGA–NHAC in the tumor resulted in a improved antineoplastic effect and fewer adverse side effects than direct intraperitoneal injection of ADM. The ADM–PLGA–NHAC developed in this study exhibited excellent extended-release drug properties, bone repairing and antineoplastic efficacy, which make it a promising osteoconductivity material with the capability to inhibit osteosarcoma.     

Mechanical and rheological properties and injectability of calcium phosphate cement containing poly (lactic-co-glycolic acid) microspheres

To enhance tissue ingrowth and promote rapid resorption, efforts were made to build macropores into calcium phosphate cement (CPC); however, this led to a decrease in its mechanical properties. In this study, poly (lactic-co-glycolic acid) (PLGA) microspheres were incorporated into CPC to impart macroporosity and maintain early strength. The influences of the content of PLGA microspheres on the mechanical strength, rheological properties, injectability, setting time, and microstructure of CPC were also systematically investigated. At the PLGA to CPC mass ratios of 20/80 and 30/70, the compressive strength of the composites was similar to that of CPC without PLGA microspheres. The rheological results indicated that PLGA microspheres/CPC pastes showed plastic and shear-thinning behaviors. The addition of PLGA microspheres to CPC resulted in the increase of viscosity and yield stress of the pastes. Simultaneously, the injectability of the pastes decreased with the addition of PLGA microspheres. When the PLGA to CPC ratio was 20/80, the injectability of the paste was still higher than 95%. The calcium phosphate cement containing 20 wt.% PLGA microspheres exhibited excellent injectability and satisfactory setting time without strength degradation. Obviously, such an in situ macropores-generable CPC should have potential prospects for the wider applications in orthopedics, oral, and maxillofacial surgery.     

Increased osteoblast functions on nanophase titania dispersed in poly-lactic-co-glycolic acid composites

The design of nanophase titania/poly-lactic-co-glycolic acid (PLGA) composites offers an exciting approach to combine the advantages of a degradable polymer with nano-size ceramic grains to optimize physical and biological properties for bone regeneration. Importantly, nanophase titania mimics the size scale of constituent components of bone since it is a nanostructured composite composed of nanometre dimensioned hydroxyapatite well dispersed in a mostly collagen matrix. For these reasons, the objective of the present in vitro study was to investigate osteoblast (bone-forming cell) adhesion and long-term functions on nanophase titania/PLGA composites. Since nanophase titania tended to significantly agglomerate when added to polymers, different sonication output powers were applied in this study to improve titania dispersion. Results demonstrated that the dispersion of titania in PLGA was enhanced by increasing the intensity of sonication and that greater osteoblast adhesion correlated with improved nanophase titania dispersion in PLGA. Moreover, results correlated better osteoblast long-term functions, such as alkaline phosphatase activity and calcium-containing mineral deposition, on nanophase titania/PLGA composites compared to plain PLGA. In fact, the greatest collagen production by osteoblasts occurred when cultured on nanophase titania sonicated in PLGA at the highest powers. In this manner, the present study demonstrates that PLGA composites with well dispersed nanophase titania can enhance osteoblast functions necessary for improved bone tissue engineering applications.     

Thermal-softening properties of water-swollen wood: The relaxation process due to water soluble polysaccharides

To clarify the thermal-softening properties of water-swollen wood, the viscoelastic properties of wood specimens conditioned to various moisture contents and of water solutions of amorphous cellulose, hemicellulose and milled wood lignin, have been studied in the temperature range from –150 to 0°C at frequencies ranging between 0.5 and 10 Hz. A relaxation process around –40°C was observed in wood specimen with high moisture content. It was observed in wood specimens with lower moisture content at higher temperature. The value of apparent activation energy for the relaxation process was 50 kJ/mol. On the other hand, a similar relaxation process was observed in water solutions of amorphous cellulose and hemicellulose around –40°C, but it was not observed in water or water solution of milled wood lignin. The values of apparent activation energy for the relaxation process were 230–545 kJ/mol. Therefore, we considered that the relaxation process was due to the water-swollen polysaccharides and affected by the situations of water molecules.