4,795 research outputs found
Nanocellulose as building block for novel materials
This thesis describes the fabrication of novel green materials using nanocellulose as
the building block. Bacterial cellulose (BC) was used as the nanocellulose
predominantly in this work. BC is highly crystalline pure cellulose with an inherent
fibre diameter in the nano-scale. A single BC nanofibre was found to possess a
Young’s modulus of 114 GPa. All these properties are highly favourable for using
BC as a nanofiller/reinforcement in green nanocomposite materials.
In this work, the surface of BC was rendered hydrophobic by grafting organic acids
with various aliphatic chain lengths. These surface-modified BC was used as nanofiller
for poly(L-lactide) (PLLA). Direct wetting measurements showed that the BC
nanofibre-PLLA interface was improved due to the hydrophobisation of BC with
organic acids. This led to the production of BC reinforced PLLA nanocomposites
with improved tensile properties. Nanocellulose can also be obtained by grinding of
wood pulp, producing nanofibrillated cellulose (NFC). The surface and bulk
properties of one type of NFC and BC were compared in this work. Furthermore, the
reinforcing ability of NFC and BC was also studied and it was observed that there is
no significant difference in the mechanical performance of NFC or BC reinforced
nanocomposites.
A novel method based on slurry dipping to coat sisal fibres with BC was developed
to modify the surface of natural fibres. This method can produce either (i) a densely
BC coating layer or (ii) “hairy” BC coated sisal fibres. Randomly oriented short BC
coated sisal fibre reinforced hierarchical composites were manufactured. It was
found that hierarchical (nano)composites containing BC coated sisal fibres and BC
dispersed in the matrix were required to produce composites with improved
mechanical properties. This slurry dipping method was also extended to produce
robust short sisal fibre preforms. By infusing this preform with a bio-based
thermosetting resin followed by curing, green composites with significantly
improved mechanical properties were produced. BC was also used as stabiliser and
nano-filler for the production of macroporous polymers made by frothing of
acrylated epoxidised soybean oil followed by microwave curing
Aggregate quasi rents and auditor independence : evidence from audit firm mergers in China
Using a sample of audit firm mergers in China\u27s audit market, this paper provides evidence on the way auditor independence can be improved following audit firm mergers as a result of a change in the aggregate quasi rents that are exposed to risk (i.e., the quasi rents at stake). This setting allows us to examine the relationship between auditor independence and the aggregate quasi rents at stake directly after controlling for the confounding effects of auditor competence, audit firm brand name, and the self-selection problem that may exist in previous studies. We hypothesize that auditors become more independent in the post-merger period only if the mergers increase the aggregate quasi rents at stake. Proxying audit quality by the frequency of modified audit opinions (MAOs) and using a \u27\u27difference-in-differences\u27\u27 research design, we conduct separate tests for two types of mergers under the institutional arrangements in China: one with an increase in the aggregate quasi rents at stake and the other with little change in these rents. Consistent with our hypothesis, we observe an improvement in auditor independence, but only for mergers that increase auditors\u27 aggregate quasi rents at stake. Moreover, the post-merger increase in the propensity for MAOs in this type of merger is positively associated with the magnitude of the change in the aggregate quasi rents at stake. Our empirical findings support the theory that auditor independence is a positive function of the aggregate quasi rents at stake
Interdependence of Income between China and ASEAN-5 Countries
This paper examines the interdependence of income between China and ASEAN-5 countries by resorting to the time series econometrics analysis from 1960 to 2000 of the real Gross Domestic Product (GDP). Empirical results are found to support the strong interdependence of income between China and ASEAN-5 countries. With the increasing interest of economic integration around the globe especially the proposed China-ASEAN Free Trade Area (CAFTA), the interdependence and synchronization movements of income between member countries is an important characteristic for suitability toward the regional common currency goal.
Optimal Control for Holonomic and Nonholonomic Mechanical Systems with Symmetry and Lagrangian Reduction
In this paper we establish necessary conditions for optimal control using the ideas of Lagrangian reduction in the sense of reduction under a symmetry group. The techniques developed
here are designed for Lagrangian mechanical control systems with symmetry. The benefit of such
an approach is that it makes use of the special structure of the system, especially its symmetry
structure and thus it leads rather directly to the desired conclusions for such systems.
Lagrangian reduction can do in one step what one can alternatively do by applying the
Pontryagin Maximum Principle followed by an application of Poisson reduction. The idea of
using Lagrangian reduction in the sense of symmetry reduction was also obtained by Bloch and
Crouch [1995a,b] in a somewhat different context and the general idea is closely related to those
in Montgomery [1990] and Vershik and Gershkovich [1994]. Here we develop this idea further
and apply it to some known examples, such as optimal control on Lie groups and principal
bundles (such as the ball and plate problem) and reorientation examples with zero angular
momentum (such as the satellite with moveable masses). However, one of our main goals is to
extend the method to the case of nonholonomic systems with a nontrivial momentum equation in
the context of the work of Bloch, Krishnaprasad, Marsden and Murray [1995]. The snakeboard
is used to illustrate the method
The geometric structure of nonholonomic mechanics
Many important problems in multibody dynamics, the dynamics of wheeled vehicles and motion generation, involve nonholonomic mechanics. Many of these systems have symmetry, such as the group of Euclidean motions in the plane or in space and this symmetry plays an important role in the theory. Despite considerable advances on both Hamiltonian and Lagrangian sides of the theory, there remains much to do. We report on progress on two of these fronts. The first is a Poisson description of the equations that is equivalent to those given by Lagrangian reduction, and second, a deeper understanding of holonomy for such systems. These results promise to lead to further progress on the stability issues and on locomotion generatio
The Hamiltonian and Lagrangian Approaches to the Dynamics of Nonholonomic Systems
This paper compares the Hamiltonian approach to systems with nonholonomic constraints
(see Weber [1982], Arnold [1988], and Bates and Sniatycki [1993], van der Schaft and Maschke
[1994] and references therein) with the Lagrangian approach (see Koiller [1992], Ostrowski [1996]
and Bloch, Krishnaprasad, Marsden and Murray [1996]). There are many differences in the
approaches and each has its own advantages; some structures have been discovered on one side
and their analogues on the other side are interesting to clarify. For example, the momentum
equation and the reconstruction equation were first found on the Lagrangian side and are useful
for the control theory of these systems, while the failure of the reduced two form to be closed
(i.e., the failure of the Poisson bracket to satisfy the Jacobi identity) was first noticed on the
Hamiltonian side. Clarifying the relation between these approaches is important for the future
development of the control theory and stability and bifurcation theory for such systems. In
addition to this work, we treat, in this unified framework, a simplified model of the bicycle (see
Getz [1994] and Getz and Marsden [1995]), which is an important underactuated (nonminimum
phase) control system
Poisson reduction for nonholonomic mechanical systems with symmetry
This paper continues the work of Koon and Marsden [1997b] that began the
comparison of the Hamiltonian and Lagrangian formulations of nonholonomic
systems. Because of the necessary replacement of conservation laws with the
momentum equation, it is natural to let the value of momentum be a variable
and for this reason it is natural to take a Poisson viewpoint. Some of this
theory has been started in van der Schaft and Maschke [1994]. We build on
their work, further develop the theory of nonholonomic Poisson reduction, and
tie this theory to other work in the area. We use this reduction procedure
to organize nonholonomic dynamics into a reconstruction equation, a nonholonomic
momentum equation and the reduced Lagrange d’Alembert equations in
Hamiltonian form. We also show that these equations are equivalent to those
given by the Lagrangian reduction methods of Bloch, Krishnaprasad, Marsden
and Murray [1996]. Because of the results of Koon and Marsden [1997b],
this is also equivalent to the results of Bates and Sniatycki [1993], obtained by
nonholonomic symplectic reduction.
Two interesting complications make this effort especially interesting. First
of all, as we have mentioned, symmetry need not lead to conservation laws
but rather to a momentum equation. Second, the natural Poisson bracket fails
to satisfy the Jacobi identity. In fact, the so-called Jacobiizer (the cyclic sum
that vanishes when the Jacobi identity holds), or equivalently, the Schouten
bracket, is an interesting expression involving the curvature of the underlying
distribution describing the nonholonomic constraints.
The Poisson reduction results in this paper are important for the future
development of the stability theory for nonholonomic mechanical systems with
symmetry, as begun by Zenkov, Bloch and Marsden [1997]. In particular, they
should be useful for the development of the powerful block diagonalization
properties of the energy-momentum method developed by Simo, Lewis and
Marsden [1991]
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