الفهرس 
Polymer nanocompositesOnly 14 pages are availabe for public view
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Abstract
While epoxy resin is the most used adhesive in structural applications, it suffers from brittle behaviour at failure and relatively low energy absorption (toughness). The main objective of this work is to examine the effect of incorporating a mixture of reactive rubber nanoparticles (RRNP) and organically modified nanoclay (Cloisite-30B) into the epoxy matrix with the aim of improving material toughness without altering its desired strength and stiffness.
The functionalized RRNP and Cloisite-30B have been impregnated in the epoxy matrix using ultrasonic homogenization technique targeting to create tough and stiff epoxy nanocompos ites. Each nanomaterial has been studied separately and this was followed by combining the optimal percentage of each to form hybrid. Morphological information of hybr id materials is important for perfect understanding of the relationship between the structure and mechanical properties.
Firstly, SBR elastomer is dissolved in a low boiling point solvent, then emulsified in an aqueous solution through a soap-in-situ procedure and then homogenized by sonification. After forming uniform latex particles with sub-micron size, most of the solvent was stripped off, and
cross-linking reaction was performed by simply increasing the emulsion temperature to form crosslinked rubber particles that are insoluble yet completely dispersible.
Overall, during the cross-linking reaction to form stable styrene-co-butadiene (SBR) rubber nanoparticles; the internal double bond units reacted with each other by the enhancement of a crosslinking agent (p-divinylbenzene DVB) via radical addition reaction. However, most of external double bond units on or near the surfaces are much more difficult to find each other and remained on the surfaces. As a result the presence of these unreacted double bonds is valuable in the subsequent epoxidation reactions.
The epoxidized (RRNP) was characterized by 1H-NMR spectrum where the formation of epoxy groups is assured by the appearance of two new signals corresponding to the protons of the epoxy group (at 2.45 and 2.7 ppm )with the reduction in area of the signal of vinyl double bond protons(at δ = 4.9 ppm). In addition; FT-IR spectrum of epoxidized SBR showed the bonds of stretching and contracting in phase of all epoxy rings observed at about
1258 and 842 cm−1 .
After functionalization, the epoxy groups created on
RRNP surface have been chemically connected into the
epoxy matrix. The incorporation of the surface epoxy groups of the particles during the epoxy curing reaction enhanced the adhesive force between the epoxy matrix and the particles and exceeds the matrix cohesive force. Consequently, the resulting core-shell reactive rubber nanoparticles (RRNP) are homogeneously embedded in the epoxy matrix with chemical interfacial interactions. All of these are assured by complete morphological study via SEM&TEM.
Secondly, Cloisite30B/epoxy nanocomposite specimens have been prepared by treating the epoxy base with different percentages of clay (2, 4, 6, 8 and 10 wt %). Epoxy resin monomer readily entered into the gallery of Cloisite30B by the help of organically modified clay surface. Morphological studies for all specimens having these Cloisite30B percentages have been performed using XRD and TEM. In XRD, almost disappearance of the nanoclay main peak (at 2θ=4.62°) was observed, this is an evidence of complete dispersion of the nanoclay in the epoxy matrix. As the wt% of clay increases from 2 wt% to
10 wt. %; there is a noticeable increase in 2θ and consequently an obvious decrease in d-spacing values. In TEM, it can be concluded that most of the clay platelets have been dispersed in the exfoliation mode for nanoclay
percentage up to 6wt%. Up to this weight percentage, nanoclay has fully delaminated to silicate layers giving rise to mostly exfoliated nanocomposites with partial intercalated nanoplatelts, which is in consistent with XRD results. On the other hand, increasing the weight percentage of nanoclay in the epoxy matrix resulted in the difficulty of dispersion. This can be noticed from the TEM image of the epoxy–clay nanocompos ite (10 wt. % specimen), where intercalated clay nanoplatelets were observed with some agglomerates.
Finally, the mechanical properties were studied by 3-d bending test (flexural test) to obtain stress –strain curve for epoxy nanocomposites. Epoxy with different percentages of RRNP (5, 7, and 10) wt. % give higher flexural strain than neat epoxy with lower flexural strength due to plastic effect of rubber nanoparticles. The best performance was normally achieved with 5 wt% of RRNP which gives higher strain than neat epoxy. As a result, it was found that the optimal wt. % of RRNP is 5wt. % .This percentage shows the highest toughness value (area under the curve), compared with all other wt. % of RRNP specimens. This could be attributed to the fact that, at higher RRNP percentage, the rubber particles agglomerate and consequently reduce their toughe ning effect.
Flexural (3-d bending) test had also been performed for specimens prepared by epoxy with different nanoclay percentages (2, 4, 6, 8, and 10) wt%. It gave higher strength than neat epoxy with lower flexural strain due to the highly modulus of clay particles with good dispersion in the matrix . It was found that the optimal load percentage of nanoclay to be (~6 wt. %).Up to this percentage ,the study shows well dispersion and the best mechanical enhancement. Increasing the load percentage resulted in a decreasing in flexural modulus .This is attributed to the presence of unexfoliated aggregates which affect the overall mechanical properties.
Therefore, in the present work; the optimal percentages of nanoclay and RRNP have been incorporated into the epoxy matrix for obtaining the best hybrid flexural properties.
By investigating the storage modulus of nanocomposites using DMA, it was found that due to chemical bonds between epoxide groups of RRNP or that of pristine epoxy with the hardener give higher crosslink density leading to a modest reduction in storage modulus than a neat epoxy. On the other hand, incorporation of clay increases the stiffness of the epoxy system. It was found that the storage modulus increases with increasing clay content up to 6 wt% at
rubbery region. Further increase in clay loading decreases the modulus due to agglomeration. The improvement in modulus can be directly ascribed to the stiffening effect of clay fillers since the clay has a higher modulus than epoxy. The storage modulus of hybrid nanocomposite is about five times higher than that of unfilled epoxy. This is a strong advantage of addition of both additive nanoparticles over the neat polymer.
By studying the mechanical properties in nanoscale using nanoindentation technique, increasing clay load percentage showed depth values lower than neat epoxy. This was noticed for clay wt. % up to 6wt. %.Increasing the clay load percentage over than this value was found to increase displacement depth. This is also attributed to the same agglomeration behaviour mentioned before.