Crystallisation and molecular mobilities in liquid and glassy states of a MXD6 polyamide

The dynamic of molecular mobilities occurring above and below the glass transition of a polyamide called MXD6 have been investigated on samples with different degrees of crystallinity. The fragility index (m) has been evaluated for a fully amorphous material and it was found that m = 180 classifies this material as fragile in regard to the strong-fragile glass forming liquid concept. For semi-crystalline samples, the results show two relaxation processes at the glass transition. One can be attributed to the molecular mobility existing in the free amorphous domain, and the second peak to a molecular mobility existing in a rigid amorphous domain as proposed by the '3-phase model'. The existence of these two phenomena does not allow a correct determination of the fragility index for such semi-crystalline materials by means of calorimetric investigation.     

Amorphous solidification in polymer-platelet nanocomposites

Computer simulations are used to understand the molecular basis of the rheology changes in polymer melts when loaded with platelet filler particles, specifically when the polymer and nanofiller interact attractively. With decreasing temperature, there is increasing aggregation between chains and filler and an increase in the polymer matrix structural relaxation time. These lifetimes are predicted to diverge at an extrapolated temperature, which we identify with the emergence of an amorphous solid state. Our findings suggest that filled polymers are phenomenologically similar to solutions of associating polymers and to supercooled liquids near their glass transition.     

Vanillin Phosphorescence as a Probe of Molecular Mobility in Amorphous Sucrose

Changes in molecular mobility are important in defining the stability and quality of amorphous solid foods, pharmaceuticals, and other solid biomaterials. Predictions of stability must consider matrix mobility below and above T_g (the glass transition temperature); measurement of molecular mobility in amorphous solids over time scales ranging from <10⁻⁹ s to >10⁸ s requires specialized methods. This research investigated how the steady-state and time-resolved emission and intensity of phosphorescence from vanillin (4-hydroxy-3-methoxy benzaldehyde), a common flavor compound, can be used to probe molecular mobility when dispersed within amorphous pure sucrose films. Phosphorescence emission spectra and time-resolved intensity decays, measured in sucrose as a function of temperature in the absence of oxygen, were strongly modulated by matrix molecular mobility. Temperature had a significant effect on vanillin phosphorescence peak frequency and bandwidth, intensity, and lifetime both in the glass and in the melt. Time-resolved phosphorescence intensity decays from vanillin were multiexponential both below and above the glass transition temperature, indicating that the pure (single component) amorphous matrix was dynamically heterogeneous on the molecular level. These data show that vanillin is a promising intrinsic probe of molecular mobility and dynamic heterogeneity in amorphous solid foods and perhaps pharmaceuticals.     

典型高分子材料的固体核磁共振研究

本论文通过固体核磁共振（NMR）谱及动力学参量的测量,并结合X-射线衍射技术和DSC测量等研究了两种典型高分子材料的相结构、链的运动以及相与相之间的关系.  乙烯-醋酸乙烯共聚物( EVA) 是最主要的乙烯共聚物之一. 研究发现,EVA的相组成非常复杂,共有5个不同的组分. 除了PE中所观察到的常规单斜晶相和刚性的正交晶相外,我们发现还存在第三个晶相分量－运动性较强的晶相（SOCP,可能是转动相）. 它不仅拥有自己的熔点,而且它的化学位移和分子运动性不同于刚性正交晶相（LOCP）. 另一方面,非晶相也由两种不同的分量组成：运动受限的各相异性的非晶界面相和高度可动的橡胶型的非晶相. 我们进一步详细研究了EVA中的晶区链动力学和非晶区的低温冻结行为. 实验发现,在正交晶相中,高分子链以180° flip-flop方式运动,同时伴随沿链方向的平移型跳跃运动,并引起正交晶相和非晶相之间的长程链扩散,通过NOE的测量证实了这种相间链扩散的存在,并进一步通过实验证实这种相间链扩散是一种受限扩散而不是自由扩散. 同时非晶相的两个组分具有不同的低温冻结行为：当温度低于-弛豫转变温度时,橡胶型的非晶相中的长程分子运动被冻结,但仍存在分子的局域运动;而界面非晶在低温时冻结成一种有序取向结构,并用质子自旋扩散实验证实该有序结构与正交晶相相邻近.  少量纳米级片层状粘土分散在聚合物中就可赋予材料许多优异的性能,我们用固体NMR技术对EVA/REC复合材料的结构和其中粘土的分散性质进行研究,发现上述复合材料中所形成的晶体类型不仅依赖于各组分的性质还依赖于所形成的复合材料的类型.  偏氟乙烯/三氟乙烯共聚物（P（VDF-TrFE））是最主要的铁电高聚物之一. 我们利用变温固体¹⁹F MAS NMR 谱及弛豫数据的测量详细研究了电子辐照对P(VDF-TrFE)共聚物的分子结构、构型、运动性以及相变等的影响. 发现,电子辐照不仅改变了分子链段的构型和运动性,同时也改变了局部分子化学结构. 电子辐照促使铁电相向顺电相(或者非晶相)转变,与此同时诱发了富含VDF和含-TrFE链段从全反式的构型到混合的反式-旁式构型的转变. 电子辐照加剧顺电区域中的分子运动而在高温熔融态中(>100 ℃）,分子的运动反而受限.     

竹基纤维素Ⅱ氢键结合的近红外光谱研究

纤维素是一种可再生天然亲水性高聚物,其庞大的氢键网格形成多种不同的晶体结构形式。在其五种结晶变体（纤维素Ⅰ,Ⅱ,Ⅲ,Ⅳ和Ⅹ）中,纤维素Ⅱ由纤维素Ⅰ（天然纤维素）经再生或丝光化处理获得,是表面自由能最低、性能最稳定的纤维素,这主要归因于纤维素Ⅱ具有与纤维素Ⅰ晶型平行链结构相反的反平行链结构,且相比于纤维素Ⅰ有附加的分子间氢键。基于近红外光谱（NIRS）对含氢基团的敏感性及纤维素的结晶结构中有大量氢键,使通过NIRS定性检测、定量评价纤维素的结晶结构成为可能。目前,用NIRS对纤维素结晶变体氢键结合的研究甚少,针对竹材纤维素Ⅱ及其衍生材料氢键结合的研究国内外尚未见相关报道。用竹材制备纤维素Ⅰ,经丝光化处理得到竹基纤维素Ⅱ,通过NIRS研究其氢键结合状况,结果与竹粉及竹基纤维素Ⅰ相比较。此外,研究还通过NIRS对竹粉及竹基纤维素的结晶度做了定量评价。结果表明：(1）在无定形区,竹基纤维素Ⅰ、Ⅱ和竹粉相比光谱差异不大,氢键结合只有量的变化,而无质的差异;(2）在半结晶区,与竹粉相比,竹基纤维素Ⅰ晶型结构保持不变,而竹基纤维素Ⅱ形成双峰;(3）在纤维素结晶区的近红外谱带范围内,反映竹基纤维素Ⅰ结晶表面纤维素分子内氢键O2-H2···O6的强氢键结合的羟基伸缩振动的一次倍频吸收峰由6 292 cm-1向高波数转移到6 354 cm-1,该处与竹基纤维素Ⅱ形成的强氢键结合的分子间氢键O2-H2···O2反平行构造相对应;(4）NIRS预测的结晶度与XRD分析结果有良好相关性。上述结果表明,纤维素结晶区内的氢键结合在近红外特征谱带出现转移且在半结晶区形成双峰,是区别竹基纤维素Ⅱ和Ⅰ的主要特征。研究也表明NIRS对探讨纤维素多种变体的氢键结合及结晶度预测是可靠的。     

Effect of compatibilizers on the crystallization kinetics of cellulose-filled high density polyethylene

High density polyethylene (HDPE) is a ubiquitous material with versatile properties. It is produced and used in greater volume than any other thermoplastic. HDPE is often filled with a variety of materials for various applications. Glass fiber and wood flour are two common fillers for HDPE. This study investigated microcrystalline cellulose (MCC) as a filler in HDPE. The use of compatibilizers, or coupling agents, was investigated as a means of improving the dispersion of the cellulose filler in the HDPE matrix and the mechanical properties of the resulting composites. One compatibilizer was shown to improve the strength of the resulting composite. The stiffness was unaffected, as expected. Thermal properties were measured by means of differential scanning calorimetry. Analysis of the crystallization kinetics indicated that the Avrami coefficient was altered by the filler and was also modified by the presence of the compatibilizer. The presence of cellulose and/or compatibilizer increased the matrix degree of crystallinity. The two compatibilizers studied did not behave similarly and may have different mechanisms of compatibilization.     

Influence of ultrasonic processing on the macromolecular properties of poly (d,l-lactide-co-glycolide) alone and in its biocomposite with hydroxyapatite

In this work poly(d,l-lactide-co-glycolide) (PLGA) and a poly(d,l-lactide-co-glycolide)/hydroxyapatite (PLGA/HAp) composite processed in an ultrasonic field at higher (25 °C) and lower (8 °C) temperatures were studied with respect to the molecular properties of the obtained materials. The processing of the PLGA and the PLGA/HAp composite in an ultrasonic field resulted in a change of molar mass averages of the polymer/polymeric part of these materials, while an amorphous structure and a 50:50 lactide-to-glycolide co-monomer ratio were preserved without the formation of crystalline oligomers. However, mobility of polymeric chains obtained after ultrasonic processing was lower indicating ordering the structure of polymeric chains as a result of processing. Additionally, it was observed that the mobility of the PLGA macromolecules was lower within the composite in comparison with the mobility of the chains within the PLGA alone in the case when both were obtained after ultrasonic processing. This was a consequence of the structure formation through the interactions between the PLGA and the HAp. Based on these results different degradation rate of PLGA in composite can be expected, which is important in the application of this material for the controlled drug delivery of medicaments.     

Surface science studies of environmentally relevant iron (oxy)hydroxides ranging from the nano to the macro-regime

In this prospective, new developments in the study of the structure and reactivity of iron oxyhydroxides are reviewed. These materials are of particular interest, since their surfaces control an extraordinary amount of environmental chemistry. Understanding the environmental interfaces at a molecular level often appears to be a daunting scientific endeavor at first glance. Surfaces of interest range from the nano to micron regime and appear in the environment in varying shapes and sizes. Often the powerful suite of vacuum-based surface science tools are not applicable, since the surfaces of environmental particles can vary from amorphous to semi-crystalline and their surface reactivity is often affected by varying levels of surface hydration. However, the introduction of new and powerful surface probes and advancements in computational chemistry are allowing surface scientists to shed light on these hidden interfaces and how they control environmental chemistry.     

Effect of Spherical Nanoparticles on the Motion of Macromolecular Chains and Segments of Isotactic Polypropylene. I. Dynamic Mechanical and Thermal Properties

The role of spherical nano-CaCO₃ particles treated with 2 wt% and 6 wt% stearic acid (SA), respectively, on the motion of macromolecular chains and segments of isotactic polypropylene (iPP) was studied through the dynamic mechanical analysis and nonisothermal crystallization. Higher nucleation activity of the particles and more nucleating sites were achieved in the 6 wt% SA treated particle nanocomposites with respect to the 2 wt% SA counterpart. The increased nucleation efficiency caused high inhomogeneity and thus large mobility of the amorphous phase of iPP, which favored a low glass transition temperature (T_g ) in the nanocomposites. However, the spherical nanoparicles also spatially restrained the motion of macromolecular chains and segments, and the better the nanoparticles dispersed, the stronger the restriction was. Thus the glass transition temperature (T_g ) of the nanocomposites decreased with increasing filler loading but recovered at a certain particle concentration. At this filler content, the maximal α-transition temperature (T_α ) and the main melting peak temperature (T_m1 ) as well as the lowest degree of crystallinity (X_PP ) also occurred. This critical filler loading appeared at lower value (20 wt%) in 6 wt% SA treated nano-CaCO₃ composites with respect to 2 wt% SA counterpart (25%) due to the better dispersion of particles in the former. It was concluded that the mobility of the macromolecular chains and segments of iPP was dominated by the competition of the spatial confinement and nucleation effect of nano-CaCO₃ particles in the matrix.     

Nanoclay and Cu Nanoparticles Loaded Polyethylene Nanocomposites for Natural Gas Transfer Applications

High density polyethylene nanocomposites loaded with a reinforcing filler (Cloisite 20A as a modified nanoclay) and an electrically conductive filler (Cu nanoparticles) were prepared by a melt blending method. The morphological, mechanical, thermal, and electrical properties of the prepared nanocomposites were investigated to evaluate their performances as appropriate materials for production of reinforced conductive polymeric pipes to be used in natural gas distribution and transportation pipelines. A random and uniform dispersion of both nanoparticles in the polyethylene matrix, with a nanoclay intercalated morphology, was observed by scanning electron microscopy and X-ray diffraction techniques. The results revealed ca. 117, 13 and 21% increases in the Young’s modulus, tensile strength and yield stress of the polyethylene matrix by adding 3 wt.% of Cloisite 20A into it. For the similar conditions, however, more than a 71% decrease was observed for the elongation at break. Thermal analysis demonstrated that the melting points of the nanocomposites were increased by incorporating both fillers and the crystallinity of polyethylene chains was decreased by incorporating Cloisite 20A and then slightly increased by adding Cu nanoparticles. Moreover, the results revealed the creation of conductivity inside the non-conductive polyethylene matrix due to the presence of the conductive Cu nanoparticles.     

Estimation of the lateral dimensions of cellulose crystallites using 13C NMR signal strengths

Differences in proton rotating-frame spin relaxation rates were exploited to edit the 13C NMR spectra of solid lignocellulosics, separating signals assigned to cellulose crystallites from signals assigned to amorphous material. Clusters of signals at 89 and 85 ppm were assigned to C-4 in the interiors and on the surfaces of cellulose crystallites, respectively. Relative signal areas were used to estimate the weight-averaged lateral dimensions of crystallites, using a model in which crystallites have approximately square cross sections. The same 10 samples were also characterized by wide-angle X-ray scattering (WAXS). There was a strong correlation (r2 = 0.988) between the two sets of lateral dimensions, but those estimated by WAXS were typically 10% lower than those estimated by NMR. The deviations were attributed to differences in molecular conformations between interior and surface chains, causing broadening of the WAXS peaks. In the case of an eleventh sample containing well-ordered xylan, the NMR and WAXS methods were in good agreement only after exclusion of a xylan signal at 82.6 ppm from the NMR data.     

Molecular dynamics simulation of wetting on modified amorphous silica surface

The microscopic wetting of water on amorphous silica surfaces has been investigated by molecular dynamics simulation. Different degrees of surface hydroxylation/silanization were considered. It was observed that the hydrophobicity becomes enhanced with an increase in the degree of surface silanization. A continuous transformation from hydrophilicity to hydrophobicity can be attained for the amorphous silica surfaces through surface modification. From the simulation result, the contact angle can exceed 90° when surface silanization percentage is above 50%, showing a hydrophobic character. It is also found that when the percentage of surface silanization is above 70% on the amorphous silica surface, the water contact angle almost remains unchanged (110–120°). This phenomenon is a little different from the wetting behavior on smooth quartz plates in previous experimental report. This change in the wettability on modified amorphous silica surfaces can be interpreted in terms of the interaction between water molecules and the silica surfaces.     

Thermoplastic Elastomer Phase Changing Materials for Stiffness Modulation

Abstract

Four natural rubber-based phase changing materials (PCMs) were synthesized and evaluated with the goal of achieving controlled stiffness modulation at elevated temperatures (～100-120?°C). The phase changing was achieved through either the glass transition or melt transition of one of four phase changing fillers: polystyrene, poly(methyl methacrylate), low density polyethylene, and indium tin alloy. The PCM stiffness was analyzed using dynamic mechanical analysis and the results showed that all four PCMs exhibited two thermal transitions: a low temperature transition at ?58?°C due to the glass transition of the natural rubber matrix and a high temperature transition at ～100-120?°C due to the thermal transitions in the four filler materials. The degree of stiffness change of all PCMs at 100-120?°C was found to be strongly influenced by the type of the phase changing filler as well as the type of thermal transition (glass transition versus melt transition). The compatibilities of the phase changing materials were analyzed using the Flory-Huggins interaction parameter and it was shown that natural rubber and low density polyethylene had the best compatibility. All systems exhibited effective stiffness modulation with the change in temperature, enabling them to be used for applications requiring variable stiffness control.     

聚倍半硅氧烷以不同方式改性纤维素绝缘纸的机理研究

本文运用分子动力学方法,建立了纯纤维素模型和一系列聚倍半硅氧烷以不同方式改性的纤维素绝缘纸模型,并对其热力学性能进行了计算、对比和讨论. 实验结果表明,各聚倍半硅氧烷纤维素改性模型的性能均优于未改性模型,并且当两条纤维素分子链作为取代基通过化学键接枝在聚倍半硅氧烷分子上(M2模型)时,改性效果最佳,其内聚能密度与溶解度参数的值相较于未改性模型提高了9%,拉伸模量、体积模量、剪切模量、柯西压的值分别提升了38.6%、29.5%、41.1%和29.5%,此外本文还计算并分析了各模型的自由体积分数和均方位移,结果显示,相较于其他模型,M2模型中的化学键在避免使纤维损失强度的同时,增加了分子链间的纠缠,使纤维素分子链占据了更多自由体积,从而使体系具有更小的自由体积分数,抑制了纤维素的链运动,进而提升了纤维素绝缘纸的热稳定性,这从微观角度解释了化学处理影响改性效果的机理.     

Methods to determine surface energies of natural fibres: a review

To tailor the interaction across composite interfaces especially for the development of green composites, i.e. composites made completely from renewable materials, information about the fibre surfaces is required. We review the current state of the art of methods to determine the surface tension of natural fibres and discuss the advantages and disadvantages of techniques used. Although numerous techniques have been employed to characterise surface tension of natural fibres, it seems that commonly used wetting techniques are very much more affected by the non-ideal character of natural fibres. Inverse Gas Chromatography (IGC) is a much better suited technique to determine the surface energetic properties of natural fibres than wetting techniques. The surface tension of natural reinforcements, determined using IGC, was reported for nanosized bacterial cellulose as well as bamboo, cornhusk, flax, hemp and sisal, covering a wide range of cellulose content. The effect of methods to separate/extract fibres from the plants as well as of a few surface modification procedures on the fibre surface properties is also reviewed. The dispersive part of the natural fibre surface tension γ ^d _S varies from 32 to 61 mJ/m². The fibre surface tension increases with increasing cellulose content of natural fibres. We also found that a higher basicity (Donor Number, K _B to Acceptor Number, K _Aratio) was observed for fibres containing more cellulose. This may be reflective of higher crystalline cellulose content in the surfaces of the fibres, as only the ether linkage of the cellulose is labile for hydrogen bonding.     

Effects of Rice Husk Derived Amorphous Silica on the Thermal-Mechanical Properties of Unsaturated Polyester Composites

Rice husk is rich in amorphous silica which has found various applications as a filler in rubbers and plastics. In the research described here silica was extracted from rice husk ash in the form of sodium silicate which was used to produced amorphous precipitated silica (PS) and silica aerogel (SA) using a sol – gel process and supercritical drying. These materials were then physically mixed with unsaturated polyester (UP) resin and cured at room temperature to form polymer composites. The experimental results showed that the UP composites with 30% (volume percent) of SA filler had lower density and better thermal insulation than the composites with the same amount of PS. Thermogravimetric analysis (TGA) results showed that the T_onset of the PS and SA composites were slightly delayed by 15 and 10°C, respectively. The tensile stress-strain curves showed that addition of the fillers reduced the tensile strength, but increased the elastic moduli of the UP matrix. PS filled UP composites exhibit higher moduli (higher stiffness) than that of SA filled UP composites. This was due to agglomeration and poor adhesion of the SA particles to the UP matrix while better dispersion was observed for the PS filled composite.     

Poly(3-Hydroxybutyrate) Matrices Modified with Iron(III) Complexes with Tetraphenylporphyrin. Analysis of the Structural Dynamic Parameters

The effect of small additions of the iron(III) complex with tetraphenylporphyrin (0–5%) on the structure and properties of ultrathin fibers based on poly(3-hydroxybutyrate) (PHB) was studied by differential scanning calorimetry (DSC), X-ray diffraction analysis (XRD), EPR probe method, and scanning electron microscopy. When tetramethylporphyrin was added to the PHB fibers, the crystallinity significantly increased, and the molecular mobility in the amorphous regions of the polymer decreased. The thermal treatment of the fibers (annealing at 140°C) led to significantly increased crystallinity and decreased molecular mobility in the amorphous regions of the PHB fibers. The addition of tetramethylporphyrin to the PHB fibers led to a sharp decrease in crystallinity. Ozonolysis of the fibers at small treatment times caused a considerable decrease in their molecular mobility (to 5 h), while prolonged ozonolysis led to increased mobility. The obtained fibrous materials have bactericidal properties and will find use in the development of antibacterial and antitumor therapeutic systems.     

Optically transparent composites reinforced with plant fiber-based nanofibers

The fibrillation of pulp fiber was attempted by two methods, a high-pressure homogenizer treatment and a grinder treatment. The grinder treatment resulted in the successful fibrillation of wood pulp fibers into nanofibers. The nanofibers demonstrate promising characteristics as reinforcement material for optically transparent composites. Due to the size effect, the nanofiber-reinforced composite retains the transparency of the matrix resin even at high fiber content such as 70 wt %. Since the nanofiber is an aggregate of semi-crystalline extended cellulose chains, its addition also contributes to a significant improvement in the thermal expansion properties of plastics while maintaining its ease of bending. Cellulose nanofibers have tremendous potential as a future resource since they are produced in a sustainable manner by plants, one of the most abundant organic resources on earth. PACS 81.05.Lg; 81.05.Qk     

基于缠结微结构的非晶态玻璃态高分子材料屈服行为的分子动力学研究

非晶态玻璃态高分子材料作为结构材料在工程领域应用广泛,其机械力学性能特别是屈服变形行为受到热处理、加载应变率和环境温度的影响.采用分子动力学模拟方法研究非晶态玻璃态高分子材料不同工况下的单轴拉伸变形,基于分子链缠结微结构的概念,阐明了非晶态玻璃态高分子材料屈服和应变软化过程的内在变形机制.结果表明,拓扑缠结具有较为稳定的空间结构,难以发生解缠,决定了非晶态高分子材料屈服后的软化平台.由相邻分子链的局部链段相互作用形成的次级缠结在一定外界条件下可发生破坏或重新生成,次级缠结微结构及其演化是非晶态高分子材料发生屈服及软化的内在物理原因.     

Real-time monitoring of graphene oxide reduction in acrylic printable composite inks

This work reports the electrical characterization of a water-based graphene oxide/acrylic composite material, which was directly inkjet printed to fabricate dissipative patterns. The graphene oxide filler, which is strongly hydrophilic due to its heavily oxygenated surface and can be readily dispersed in water, was reduced by UV irradiation during photo-curing of the polymeric matrix. The concurrent polymerization of the acrylic matrix and reduction of graphene oxide filler was demonstrated by real-time resistance measurements during UV light irradiation. The presence of graphene filler allowed decreasing the resistance of the pure polymeric matrix by nearly five orders of magnitude. This was explained by the fact that clusters of reduced graphene oxide inside the polymer matrix act as preferential pathways for the mobility of charge carriers, thus leading to an overall decrease of the material’s resistance.