Material CharacterisationCharacterisation of poly(vinyl alcohol)–montmorillonite composites with higher clay contents
Introduction
Composites comprised of poly(vinyl alcohol) (PVA) and montmorillonite (MMT) have been reported in a number of studies to determine their suitability for a range of practical applications, such as wound dressings [1], [2], [3], [4], [5], [6], [7], [8], [9]. PVA is a water soluble, hydrophilic, non-toxic polymer with the ability to form good quality films. MMT is a smectite clay with a layered structure. The studies thus far have focused on comparatively low proportions of clay in the composites for the applications of interest. However, for this study, the potential of producing a polymer-clay composite with a greater amount of clay has been investigated.
The need for the examination of such compositions arose out of studies of the weathering of sandstones used in heritage buildings. Previous investigations have demonstrated that the degradation can be associated with changes occurring to the clay component of sandstone [10], [11], [12], [13]. The use of consolidants is recognised as a means of protection for heritage buildings. A variety of consolidants have been investigated for use in the protection of different building materials and polymeric materials have been widely used to minimise the rate of decay [14]. Previous work has demonstrated that polymeric nanocomposites based on organic polymers and clay may provide improved consolidation properties compared to the polymer alone [15], [16]. Such materials have the advantage of potentially providing greater compatibility with the stone surface.
Both the PVA and MMT are hydrophilic and the incorporation of the clay into the polymer can be achieved via a relatively easy dispersion process. A higher loading of clay (up to 90 w/w %) was used in this study in an attempt to minimise the proportion of polymer, while producing a cementing material with similar characteristics to the original clay binder. A solution intercalation method was chosen as the method for preparation of the PVA-MMT composites in this study. This approach is based on utilising a solvent in which a polymer is soluble and the silicate layers are capable of swelling. The MMT was initially swollen in water and then the PVA and MMT were mixed, the polymer intercalates, that is, locates in the clay interlayer and expands the structure.
For this study, a series of PVA-MMT composites with higher clay loadings were prepared. The degree of intercalation produced has been investigated using scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) and x-ray diffraction (XRD).
Section snippets
Materials
PVA (molecular weight 78,000) was supplied by Sigma Aldrich, Australia. ActiveGel 150 (drilling and civil engineering grade), an activated sodium bentonite with a high montmorillonite content, was supplied by Unimin Ltd, Australia. Arumpo montmorillonite was provided by Arumpo Bentonite, Australia. With a pH of 4.6 in solution, it contains 98–99% montmorillonite and trace elements.
PVA-MMT composites
MMT samples were ground in a ring mill for 30 s and stored in an oven at 50 °C until use. For the acidified clay
SEM
The ESEM micrographs of PVA-MMT (Unimin) show the clay particles immersed in the polymer matrix. Fig. 1 illustrates the micrograph for 70 w/w % PVA - 30 w/w % MMT. The clay particles are agglomerated and exfoliation was not achieved. The particle size of 3 μm for the 70 w/w % PVA – 30 w/w % MMT (Unimin) and 50 w/w % PVA – 50 w/w % MMT (Unimin) samples suggests that the clay layers are not separated by the polymer matrix to form either intercalated or exfoliated composites. No obvious changes
Discussion
Several techniques were applied to the characterisation of polymer-clay composites in this study. Although the results obtained from the SEM investigation offer more limited insight into the polymer-clay interactions due to melting, some micrographs provide information about the clay orientation and the film profile of the composites. The detection of non-intercalated clay particles in PVA-MMT (Unimin) samples formed using untreated clay indicate that the relatively strong interlayer attraction
Conclusions
PVA-MMT systems with higher concentrations of clay were prepared and characterised. Acidified MMT samples gave better intercalated/exfoliated polymer-clay composites. Two different types of MMT platelets exhibited different particle orientation and shape in an identical polymer matrix. MMT from Unimin exhibited hexagonal clay platelets within the polymer matrix, while rod-shaped nanoscale particles were observed for the Arumpo MMT.
The observed intercalation/exfoliation resulting in the
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