Polymer-liquid crystal interface: a molecular dynamics study

Liquid crystals (LCs) are an interesting class of soft condensed matter systems characterized by an unusual combination of fluidity and long-range order, mainly known for their applications in displays (LCDs). However, the interest in LC continues to grow pushed by their application in new technologies in medicine, optical imaging, micro and nano technologies etc. In LCDs uniaxial alignment of LCs is mainly achieved by a rubbing process. During this treatment, the surfaces of polymer coated display substrates are rubbed in one direction by a rotating cylinder covered with a rubbing cloth. Basically, LC alignment involves two possible aligning directions: uniaxial planar (homogeneous) and vertical (homeotropic) to the display substrate. An interesting unresolved question concerning LCs regards the origin of their alignment on rubbed surfaces, and in particular on the polymeric ones used in the display industry. Most studies have shown that LCs on the surface of the rubbed polymer film layer are lying parallel to the rubbing direction. In these systems, micrometric grooves are generated on the film surface along the rubbing direction and also the polymer chains are stretched in this direction. Both the parallel aligned microgrooves and the polymer chains at the film surface may play a role in the LC alignment and it is not easy to quantify the effect of each contribution. The work described in this thesis is an attempt to find new microscopic evidences on the origin of LC alignment on polymeric surfaces through molecular dynamics (MD) simulations, which allow the investigation of the phenomenon with atomic detail. The importance of the arrangement of the polymeric chains in LCs alignment was studied by performing MD simulations of a thin film of a typical nematic LC, 4-cyano-4’-pentylbiphenyl (5CB), in contact with two different polymers: poly(methyl methacrylate)(PMMA) and polystyrene (PS). At least four factors are believed to influence the LC alignment: 1. the interactions of LCs with the backbone vinyl chains; 2. the interactions of LCs with the oriented side groups; 3. the anisotropic interactions of LCs with nanometric grooves; 4. the presence of static surface charges. Here we exclude the effect of microgrooves and of static surface charges from our virtual experiment, by using flat and neutral polymer surfaces, with the aim of isolating the chemical driving factors influencing the alignment of LC phases on polymeric surfaces.

A new method for continuous quality control of nsm cfrp systems

In the last decade the near-surface mounted (NSM) strengthening technique using carbon fibre reinforced polymers (CFRP) has been increasingly used to improve the load carrying capacity of concrete members. Compared to externally bonded reinforcement (EBR), the NSM system presents considerable advantages. This technique consists in the insertion of carbon fibre reinforced polymer laminate strips into pre-cut slits opened in the concrete cover of the elements to be strengthened. CFRP reinforcement is bonded to concrete with an appropriate groove filler, typically epoxy adhesive or cement grout. Up to now, research efforts have been mainly focused on several structural aspects, such as: bond behaviour, flexural and/or shear strengthening effectiveness, and energy dissipation capacity of beam-column joints. In such research works, as well as in field applications, the most widespread adhesives that are used to bond reinforcements to concrete are epoxy resins. It is largely accepted that the performance of the whole application of NSM systems strongly depends on the mechanical properties of the epoxy resins, for which proper curing conditions must be assured. Therefore, the existence of non-destructive methods that allow monitoring the curing process of epoxy resins in the NSM CFRP system is desirable, in view of obtaining continuous information that can provide indication in regard to the effectiveness of curing and the expectable bond behaviour of CFRP/adhesive/concrete systems. The experimental research was developed at the Laboratory of the Structural Division of the Civil Engineering Department of the University of Minho in Guimar\~aes, Portugal (LEST). The main objective was to develop and propose a new method for continuous quality control of the curing of epoxy resins applied in NSM CFRP strengthening systems. This objective is pursued through the adaptation of an existing technique, termed EMM-ARM (Elasticity Modulus Monitoring through Ambient Response Method) that has been developed for monitoring the early stiffness evolution of cement-based materials. The experimental program was composed of two parts: (i) direct pull-out tests on concrete specimens strengthened with NSM CFRP laminate strips were conducted to assess the evolution of bond behaviour between CFRP and concrete since early ages; and, (ii) EMM-ARM tests were carried out for monitoring the progressive stiffness development of the structural adhesive used in CFRP applications. In order to verify the capability of the proposed method for evaluating the elastic modulus of the epoxy, static E-Modulus was determined through tension tests. The results of the two series of tests were then combined and compared to evaluate the possibility of implementation of a new method for the continuous monitoring and quality control of NSM CFRP applications.