聚合物表面的纳米力学研究

综述了近些年才开展的采用原子力显微技术,在聚合物表面进行纳米力学测量的实验方法和基本理论的进展,内容包括分子链的纳米强度测量,纳米力学各向异性的表征,表面分子间的纳米相互作用,表面形貌的纳米测量以及表面微区的纳米粘弹性研究。     

Quantitative mapping of elastic moduli at the nanoscale in phase separated polyurethanes by AFM

The micro phase separated nanoscale morphology of phase separated polyurethanes (PUs) was visualized by atomic force microscopy (AFM) height and phase imaging of smooth surfaces obtained by ultramicrotonomy. PUs were obtained from 4,4′-methylenbis (phenyl isocyanate) (MDI), 1,4-butanediol (BD) and poly(tetrahydrofurane) polyether polyol (PTHF). The segmented polyether PUs with varying stoichiometric ratio of the isocyanate and hydroxyl groups were prepared to investigate the effect of molar mass, as well as the type and number of end-groups on their morphology and mechanical performance.The PU samples studied show characteristic “fingerprint” AFM phase images. Novel dynamic imaging modes of AFM, including HarmoniX material mapping and Peak Force Tapping were used to assess the mechanical performance of phase separated polyurethanes quantitatively as a function of their molecular structure. The values of surface elastic moduli were determined with nanoscale resolution and were in excellent agreement for both AFM modes. While tensile testing provides a bulk average value for the elastic modulus of the elastomers, the novel AFM based elastic moduli mappings introduced enable the study of surface stiffness with nanoscale resolution in a quantitative way.     

Attachment of nanoparticles to the AFM tips for direct measurements of interaction between a single nanoparticle and surfaces

Here we report a universal method of attachment/functionalization of tips for atomic force microscope (AFM) with nanoparticles. The particles of interest are glued to the AFM tip with epoxy. While the gluing of micron size particles with epoxy has been known, attachment of nanoparticles was a problem. The suggested method can be used for attachment of virtually any solid nanoparticles. Approximately every other tip prepared with this method has a single nanoparticle terminated apex. We demonstrate the force measurements between a single approximately 50 nm ceria nanoparticle and flat silica surface in aqueous media of different acidity (pH 4-9). Comparing forces measured with larger ceria particles ( approximately 500 nm), we show that the interaction with nanoparticles is qualitatively different from the interaction with larger particles.     

有机HTDIOO分子LB膜结构的AFM研究

利用原子力显微镜（AFM）对有机分子HTDIOO单层和多层LB膜结构进行了观察·实验结果表明，针尖与LB膜表面分子间的相互作用力会对成像的膜结构有影响．当悬臂针尖与LB膜表面分子的相互作用力较大时，针尖会扰动HTDIOO分子在单层LB膜中的有序排列．HTDIOO单层LB膜具有有序结构；而在多层LB膜中，HTDIOO分子则聚集在一起形成了一定的畴结构．     

有机HTDIOO分子LB膜结构的AFM研究

利用原子力显微镜（ＡＦＭ）对有机分子ＨＴＤＩＯＯ单层和多层ＬＢ膜结构进行了观察。实验结果表明，针尖与ＬＢ膜表面分子间的相互作用力会对成像的膜结构有影响。当悬臂针尖与ＬＢ膜表面分子的相互作用力较大时，针尖会扰动ＨＴＤＩＯＯ分子在单层ＬＢ膜中的有序排列。ＨＴＤＩＯＯ单层ＬＢ膜具有有序结构；而在多层ＬＢ膜中，ＨＴＤＩＯＯ分子则聚集在一起形成了一定的畴结构。     

磷酯酰乙醇胺与硬脂酸和十八醇二元混合体系单层膜的热力学特性和AFM观测

利用表面压力-平均分子面积(π-A)曲线的关系, 分别研究了在水/空气界面上形成的磷酯酰乙醇胺(PE)与硬脂酸(SA)和十八醇(OD)二元混合体系的热力学特性. 根据对弹性模量(C_S^-1)、过量分子面积(A_ex)以及表面过量吉布斯自由能(ΔG_ex)等热力学参数的数据计算, 定量分析了混合单层膜分子之间的相互作用. 实验结果表明, PE/SA和PE/OD单层膜两种组分之间摩尔比对其热力学参数有影响. 热力学分析表明, 在X_SA=0.2, 0.8和X_OD=0.8处, PE/SA和PE/OD二元混合体系的热力学参数[过量分子面积(A_ex)和表面过量吉布斯自由能(ΔGex)]相对理想状态均表现为负偏差作用, 说明分子之间的作用为引力作用. 相反, PE/OD二元体系在X_OD=0.2, 0.4, 0.6处, 表现为正偏差作用, 这说明分子之间的作用为斥力作用. AFM观测为PE/SA和PE/OD单层膜热力学特性的理论分析提供了有力的支持.     

Orientation control of ferroelectric polymer molecules using contact-mode AFM

We have developed an orientation control technique for polymer molecules utilizing contact-mode atomic force microscopy (AFM). In this technique, the molecular chains were directly modified by scanning an AFM cantilever tip in contact with the film surface at the temperature just below its melting point. We call this process “modification scan”. Here, we applied this technique to poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)) thin films on graphite and glass. We prepared a 75-nm thick copolymer crystalline film on graphite whose lamellar plane was perpendicular to the substrate (edge-on), and also prepared a film of the same thickness on glass whose lamellar plane was parallel to the substrate (flat-on). After applying this technique on both films, molecular chains were stretched and aligned to the modification scan direction, and new edge-on crystals were obtained, whose lamellar planes were well-aligned perpendicular to the modification scan direction.     

Images of Azurin Protein Studied by AFM

Azurins, a wild type and a genetically mutant K27 altered one. were immobilized on annealed gold sur-face and investigated by means of atomic force microscopy. It was found that the surface coverage and height distribution of the adsorbed protein molecules are different from each other, which is possibly the result of the different orientation on the surface. It is believed that the wild type azurin is connected to gold surface by the disulphide bridge;while the mutant, K27C, might be through the thiol groups of the cysteine residues on their surface.     

Using AFM to explore food nanostructure

The major recent advances in the past year are all in the use of atomic force microscopy as an analytical tool to answer questions related to real food systems. These samples are bio materials and interfaces, all fall in one of the most challenging environments for AFM imaging. The include food dispersions, gels, and raw materials of plant and marine origin.     

Mechanism of soft solution processing formation of alkaline earth metal tungstates: an electrochemical and in situ AFM study

The mechanism of alkaline earth metal tungstate formation during soft solution processing was studied by cyclic voltammetry, electrochemical impedance spectroscopy and by direct in situ observation of the surface changes using atomic force microscopy. The electrochemical oxidation of W to WO₃ was followed by dissolution of WO₃ and, with some delay, by precipitation of tungstates at the metal surface. The same Tafel slopes observed in Li⁺, Ba²⁺, Sr²⁺ and Ca²⁺ containing solutions indicate that the course of the oxidation process is independent of the cation present in solution. The observed differences in the current-voltage curves outside the Tafel region are accounted for by the different film-forming tendencies of the various alkaline earth metal cations. The growth of tungstate layers at the W substrate decreases the electrochemically active area and limits the production of WO₄ ^2– at later stages of deposition. At low potentials (E<0.2 V) the oxidation of W is the rate-controlling step. At higher potentials, however, the dissolution process slows down due to a relative decrease of the pH in the electrode vicinity and dissolution becomes the rate-limiting step. Electronic Publication     

Depth-sensing indentation applied to polymers: A comparison between standard methods of analysis in relation to the nature of the materials

Mechanical data (hardness and elastic modulus) from instrumented indentation testing are often extracted assuming linear elasticity in the initial portion of the unloading. The method is nowadays widely accepted as a convenient tool to interpret depth-sensing data, however it is a matter of controversy when applied to polymer materials due to their time-dependent behavior. More recently, Loubet and co-workers applied continuous stiffness measurements (CSM), consisting of superimposing a small oscillation to the quasi-static component of loading, to the study of the mechanical properties of polymers and proposed a new model to account for the apparent increase in the contact area detected at the first stages of contact. The present work offers a comparative study between the Loubet’s model using CSM and the procedure yielding a single reading from the onset of unloading. A wide range of thermoplastic polymer materials including glassy and semicrystalline polymers have been investigated. The most important equations employed for each method are summarized and the advantages and disadvantages of employing one procedure or the other are discussed. The differences found between the results obtained from both approaches are discussed in relation to the nature of the polymer material. A comparison between mechanical data extracted from indentation measurements and from classical dynamic mechanical analysis is offered.     

原子力显微镜原位观察球晶界面上片晶的生长

当结晶聚合物由熔融冷却或从浓溶液中析出结晶时 ,在不存在应力和流动的情况下 ,一般形成球晶 .球晶在一定的生长时期内呈现球形外观 ,在偏光显微镜下通常呈现Maltese黑十字消光图样 .球晶作为一种常见的结晶形态 ,由片晶堆积而成[1,2 ] .Keith和Padden认为形成球晶的体系包含杂质和聚合物链 ,由于杂质在片晶生长界面的富集导致片晶产生小角度分叉 ,这样片晶能填满球状的空间[1,2 ] .近年来的研究表明球晶是由一个片晶开始生长 ,片晶在生长过程中不断的诱导成核使片晶分叉 ,首先形成片晶捆束 ,然后片晶向各个方向发散生长 ,最终形成球晶[3…     

AFM诱导正十八硫醇在金基底上的选择性生长

扫描探针显微镜（SCCnningPF0boMICCOSCOPy,SPM）由于其极高的空间分辨能力和高度的可控性,已成为纳米尺度加工的有力工具［‘·’j．自Schneir等［’j报道原子级平整金基底的制备和用装备An针尖的扫描隧道显微镜（ScanningTunnelingMicroscoPy,STM）在基底上制备金纳米点以来,有关在All和HOPG等基底上制备由金点构成的任意图案的方法及用导电原子力显微镜（AtomicForceM卜roscopy,AFM）在HOPG和St基底上制备金点阵的工作已有许多报道［‘·’‘．用导电AFM和TaPPingmodeAFM”,’‘对St进行直接氧化可在其表面加…     

DNA共价结合在化学修饰云母片上的AFM研究

原子力显微镜(AFM)自1986年发明以来,已经成为生物学研究领域中的一个有效工具,尤其在核酸及其它生物大分子结构方面的应用已成为普遍关注的热点.原子力显微镜要求基底达到原子级平整,硅片和玻璃表面的起伏很大,因而原子级平整的云母具有重要的价值.     

Force measurements on platelet surfaces with high spatial resolution under physiological conditions

Investigations on platelets are essential to understanding the regulation of hemostasis and thrombosis. Activated platelets undergo dramatic conformational and morphological changes mediated by numerous plasma proteins. AFM techniques can combine high spatial resolution with measurements of the mechanical properties of platelet surfaces. Here, we demonstrate two-dimensional force mapping over a human platelet adsorbed on glass under physiological buffer. The best resolution of platelet membrane elasticity we obtained was at 15.6×15.6 nm² pixel⁻¹. In addition, quantitative information on platelet surface charge density was extracted from individual force curves with the aid of DLVO theory.     

Smoothing and passivation of special Si(111) substrates: studied by SPV,PL, AFM and SEM measurements

Surface sensitive techniques, the field-modulated surface photovoltage, photoluminescence measurements, atomic force microscopy and scanning electron microscopy, were employed to yield detailed information on the influence of wet-chemical treatments on the preparation induced microroughness and electronic properties of wet-chemically passivated Si(111) substrates with special surface morphology. Stepped substrates with evenly distributed atomically flat terraces were prepared and passivated by thin oxide layers, which were used as a starting point for the subsequent H-termination after long storage in air. It was shown that their surface morphology and electronic properties do not degrade. Applying this preparation method to solar cell substrates with randomly distributed Si(111) pyramids, we achieved significantly lower densities of surface states and reduced recombination loss at a-Si:H/c-Si interfaces, compared with conventional pretreatments. The surface microroughness, the density of rechargeable states and the resulting recombination loss on a-Si:H/c-Si heterojunctions were found to be mainly influenced by two steps of surface pretreatment: firstly, the wet-chemical smoothing procedure of structured substrates and, secondly, the removal of native and wet-chemical oxides during the final etching in HF- or NH₄F- containing solutions. Figure After wet-chemical oxidation in H₂SO₄/H₂O₂ and storage in air     

Sub-micron scale mechanical properties of polypropylene fibers exposed to ultraviolet and thermal degradation

Nanoindentation using atomic force microscopy (AFM) was conducted to investigate the affect of accelerated ultraviolet (UV) and thermal degradation on the mechanical properties of polypropylene fibers. The affect of degradation on Young’s modulus across fiber cross-sections was investigated with progressive nanoindentation from the surface to the center of the fiber. UV degradation initially increases the Young’s modulus both at the center and the surface of the fibers until 120 h of exposure with the increase being more rapid at the surface. Moduli started to decrease beyond 120 h of exposure. Wide angle x-ray scattering shows an increase of crystallinity up to 120 h of exposure and total destruction of crystallinity at 144 h. Infrared spectra showed the formation of carbonyl bonds with UV exposure. To investigate thermal degradation, the fibers were exposed to 125 °C for four weeks. Young’s modulus increased near the surface after four weeks exposure. These results support the idea that surface degradation may lead to embrittlement of textile fibers.     

Anodic Stripping Voltammetry: An AFM Study of Some Problems and Limitations

The use of anodic stripping voltammetry for quantitative analytical measurements using solid electrodes is addressed in the light of generic limitations arising from i) electrode heterogeneity, ii) electrode morphology, iii) inhibited electrodeposition, and iv) incomplete stripping of deposited metal in the anodic sweep. It is shown, using direct imaging of electrode surfaces via AFM and optical microscopy, that each of the preceding factors may produce significant deviations from ideal electrode behavior. The use of atomic force microscopy to fully characterize any developed ASV procedures is strongly recommended. To ensure reproducible and accurate stripping voltammetry, steps should be taken to minimize the effects discussed.     

单分子水平高分子相互作用的AFM成像与单分子力谱研究

结合作者近期的研究工作,重点介绍了如何把原子力显微镜(AFM)成像及单分子力谱结合(包括原位结合或者离位结合)起来,研究高分子之间的相互作用.本文涉及生物高分子(主要是核酸-蛋白质体系)以及合成高分子体系(如聚氧乙烯,PEO)的相关研究工作.对于生物高分子体系,主要以长链核酸(如双螺旋DNA及RNA)为探针,首先利用A...     

利用原子力显微镜和分子技术研究海水微生物腐蚀(英文)

生物膜在自然界无处不在,但生物膜造成的腐蚀却基本上被忽视.本文展示了几种化学和微生物学新方法在海水微生物腐蚀研究中的应用.原子力显微镜用来揭示生物最初形成的机理和钢在受污染海水中的腐蚀程度,16SrDNA/RNA技术则用来分析生物膜中的微生物组成.试验结果表明,微生物腐蚀在6d内就已经开始了,腐蚀体积与时间的2.83次方成正比;腐蚀生物膜中的微生物以硫酸还原菌(脱硫弧菌科)为最多,其次是梭状芽孢杆菌.