纳米纤维素/聚丁二酸丁二醇酯母粒改性聚乳酸的结晶与力学性能研究

以聚乳酸(PLA)为基体,聚丁二酸丁二醇酯(PBS)为增韧相,纳米纤维素(CNF)为增强相,采用不同的挤出温度,利用双螺杆挤出机熔融共混制备出一系列CNF/PBS母粒改性PLA复合材料。采用扫描电子显微镜、广角X射线衍射仪、差示扫描量热仪、偏光显微镜和万能试验机以及悬臂梁冲击试验机对复合材料的结晶和力学性能进行测试。结果表明:CNF可以起到异相成核的作用,但含量过多易引起团聚;与纯PLA相比,当CNF/PBS复合母粒的添加量为20%时,低温挤出的复合材料的结晶度、冲击强度分别提高了10.66%和141.51%,拉伸强度仅下降14.86%;当CNF/PBS母粒的添加为20%时,低温挤出的PLA基复合材料的结晶度、拉伸强度和冲击强度分别较高温下挤出的复合材料提高了11.61%、3.82%和16.37%。     

Easy Cases of Probabilistic Satisfiability

The Probabilistic Satisfiability problem (PSAT) can be considered as a probabilistic counterpart of the classical SAT problem. In a PSAT instance, each clause in a CNF formula is assigned a probability of being true; the problem consists in checking the consistency of the assigned probabilities. Actually, PSAT turns out to be computationally much harder than SAT, e.g., it remains difficult for some classes of formulas where SAT can be solved in polynomial time. A column generation approach has been proposed in the literature, where the pricing sub-problem reduces to a Weighted Max-SAT problem on the original formula. Here we consider some easy cases of PSAT, where it is possible to give a compact representation of the set of consistent probability assignments. We follow two different approaches, based on two different representations of CNF formulas. First we consider a representation based on directed hypergraphs. By extending a well-known integer programming formulation of SAT and Max-SAT, we solve the case in which the hypergraph does not contain cycles; a linear time algorithm is provided for this case. Then we consider the co-occurrence graph associated with a formula. We provide a solution method for the case in which the co-occurrence graph is a partial 2-tree, and we show how to extend this result to partial k-trees with k>2.     

Catecholaminergic and acetylcholine esterase containing nerves of cranial and spinal dura mater in humans and rodents

The innervation of cranial and spinal dura mater in humans and rodents was studied by examining several dural zones (vascular, perivascular, intervascular) in different regions. Characterization and distribution of dural acetylcholinesterase-positive nerve fibers, catecholaminergic nerve fibers, and mast cells are analyzed and discussed. The results of chemical and surgical sympathectomy as well as the relationships between catecholaminergic nerve fibers and mast cells are studied. Our results are discussed in the light of possible implications in the physiopathology of dural algic syndromes including cephalalgia and spinal pain.     

Effects of surface modification,carbon nanofiber concentration,and dispersion time on the mechanical properties of carbon‐nanofiber–polycarbonate composites

The time effect of ultrasonication was investigated for dispersing carbon nanofibers (CNFs) into a polycarbonate (PC) matrix on the mechanical properties of thus‐produced composites. The effects of CNF surface modification by plasma treatment and the CNF concentration in composites on their mechanical properties were also explored. The plasma coating was characterized by HRTEM and FT‐IR. Furthermore, the plasma polymerization (10 w) treatment on the CNF enhanced the CNF dispersion in the polymer matrix. The mechanical properties of the CNF–PC composites varied with the dispersion time, at first increasing to a maximum value and then dropping down. After a long ultrasonic treatment (24 h), the properties increased again. At a high concentration, the CNF‐PC suspension became difficult to disperse. Additionally, the possible mechanisms for these behaviors are simply proposed. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3792–3797, 2007     

Polyamide‐12/Functionalized Carbon Nanofiber Composites: Evaluation of Thermal and Mechanical Properties

This work aims at improving the interfacial bonding between polyamide‐12 and CNFs. CNFs were oxidized and dispersed in polyamide‐12 giving rise to polymer nanocomposites. The oxidation caused an increase in the specific surface area and structural defects of the fibers, as indicated by surface area and Fourier‐transform Raman spectroscopy. The nanocomposites exhibited improved thermal and thermo‐oxidative stabilities. The oxidized nanofibers had marginal effect on the crystallinity and crystallization of the polyamide‐12. An over‐proportional enhancement of stiffness due to the fibers could be achieved. In spite of these improvements the fiber/polymer adhesion should be further improved.

     

Electromagnetic interference properties of carbon nanofiber–reinforced acrylonitrile–styrene–acrylate/natural graphite composites

Acrylonitrile–styrene–acrylate/natural graphite/carbon nanofiber composites (ASA/NG/CNF) were prepared using a melting blending method. The effects of CNFs on the morphology, rheological properties, dynamical mechanical properties, electrical resistivity, and electromagnetic interference shielding effectiveness (EMI SE) were studied using a scanning electron microscope, a rotational rheometer, and dynamic mechanical analysis (DMA). The addition of CNFs changed the oriented and laminated structure of the ASA/NG composite. The flexural strength of the ASA composite reached a maximum at 6% CNF, and then it began to decrease. The addition of CNFs did not alter the glass‐transition temperature of ASA, but it largely increased the storage modulus of the composite in DMA tests. In the rheological measurements, the complex viscosity and storage modulus of the composite increased as CNF content increased, and the resistance to creep of the composites was significantly increased by the addition of CNFs. The electrical resistivity of the ASA composites decreased from 49.8 Ω cm to 2.3 Ω cm as the CNF content was increased from 0 to 12%. At the same time, the EMI properties of the composites rose from 15 dB to 30 dB in the frequency range 30–1500 MHz. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45455.     

Fundamentals, processes and applications of high-permittivity polymer-matrix composites

There is an increasing need for high-permittivity (high-k) materials due to rapid development of electrical/electronic industry. It is well-known that single composition materials cannot meet the high-k need. The combination of dissimilar materials is expected to be an effective way to fabricate composites with high-k, especial for high-k polymer-matrix composites (PMC). This review paper focuses on the important role and challenges of high-k PMC in new technologies. The use of different materials in the PMC creates interfaces which have a crucial effect on final dielectric properties. Therefore it is necessary to understand dielectric properties and processing need before the high-k PMC can be made and applied commercially. Theoretical models for increasing dielectric permittivity are summarized and are used to explain the behavior of dielectric properties. The effects of fillers, fabrication processes and the nature of the interfaces between fillers and polymers are discussed. Potential applications of high-k PMC are also discussed.     

碳纳米纤维对玻纤增强环氧树脂基复合材料机械及耐磨性能的影响

采用真空辅助喷涂的方法制备了相对于玻纤质量分数1%、3%的碳纳米纤维/玻纤/环氧树脂基复合材料,并对其机械性能、耐固体粒子冲蚀磨损性能与纯环氧树脂(EP)、玻纤增强环氧树脂(FRP)进行了对比研究。结果表明:1%、3%碳纳米纤维的加入使得复合材料的拉伸强度达到72MPa、70MPa,相对于EP分别增加了20%、16.7%,相对于FRP分别降低了32.7%、30.7%;弯曲强度达到150MPa、178MPa,相对于EP分别增加了40.2%,66.4%,相对于FRP分别降低了19.8%、4.8%;侵蚀磨损试验中质量损失相对于FRP分别降低了75%、86.7%,在对复合材料机械强度影响较小的前提下大大提高了复合材料的耐磨损性能。     

喷涂碳纳米纤维对玻纤预制体渗透率的影响

采用真空辅助喷涂的方法制备了相对于玻纤质量分数为0.5%、2%的碳纳米纤维/玻纤复合材料,并对其饱和渗透率、不饱和渗透率进行了对比研究。结果表明:0.5%、2%碳纳米纤维的加入均使得预制体的饱和渗透率、不饱和渗透率降低;不含CNF的玻纤织物垂直Z向上的饱和渗透率约为水平Y向渗透率的20%,喷涂CNF后,下降至10%左右;当孔隙率小于47%时,喷涂CNF的预制体Z向渗透率接近0;0.5%CNF喷涂后对预制体径向不饱和渗透率影响较小,但2%CNF喷涂后,其径向不饱和渗透率约为玻纤织物基体的20%。     

Testing satisfiability of CNF formulas by computing a stable set of points

We show that a conjunctive normal form (CNF) formula F is unsatisfiable if and only if there is a set of points of the Boolean space that is stable with respect to F. So testing the satisfiability of a CNF formula reduces to looking for a stable set of points (SSP). We give some properties of SSPs and describe a simple algorithm for constructing an SSP for a CNF formula. Building an SSP can be viewed as a natural way of search space traversal. This naturalness of search space examination allows one to make use of the regularity of CNF formulas to be checked for satisfiability. We illustrate this point by showing that if a CNF F formula is symmetric with respect to a group of permutations, it is very easy to make use of this symmetry when constructing an SSP. As an example, we show that the unsatisfiability of pigeon-hole CNF formulas can be proven by examining only a set of points whose size is quadratic in the number of holes. Finally, we introduce the notion of an SSP with excluded directions and sketch a procedure of satisfiability testing based on the construction of such SSPs.