Enhancing gas-barrier properties of polymer-clay nanocomposites

20 August 2012
Sabu Thomas, A. P. Meera, and Hanna Joseph Maria
The volume fraction of constrained polymer regions correlates with a reduction in permeability.

The automotive and packaging industries are increasingly demanding high-performance fillers, plastics, and composites for novel applications. For instance, nanoclay polymer composites show considerable promise for food packaging that can keep the contents fresh, and their excellent gas-barrier properties will likely significantly boost their use as packaging materials. In addition, incorporating nanoclay fillers into certain polymers has been shown to reduce loss (by transmission) markedly, and there is considerable interest in these materials as both fuel tank and fuel line components for cars. Using nanofillers to prepare nanocomposites with high barrier properties can make it possible to use less material, which will considerably reduce costs. Additionally, important rubber engineering products used in high-performance applications, such as tire inner tubes, air springs, and cure bladders, demand a highly effective barrier to gas permeation. It would enhance their performance if we could improve the gas-barrier properties of natural rubber (NR).

The theory of how nanocomposites reduce permeability is well established. Toyota researchers were the first to report that their polyamide 6-clay hybrid absorbs water at a 40% slower rate compared with the pristine polymer.1 Until recently, this impressive decrease in permeability was attributed to the large aspect ratio (length to thickness ratio) of the clay platelets, which should increase the tortuosity (length) of the path of the gas as it diffuses into the nanocomposite (see Figure 1). The tortuosity factor is defined as τ= d/d', where d is the actual distance that the penetrant must travel in the absence of clay and d' is the tortuous path length covered in the presence of clay. The tortuosity factor increases with the aspect ratio.


Model describing the path of the diffusing gas through the nanocomposite. d: Actual distance travelled in the absence of clay. d': Tortuous path length in the presence of clay.

Organically modified layered silicates have been widely studied for the past decade for enhancing the barrier properties for polymeric materials.2,3 Dispersing nanoclay layers in the rubber matrix should reduce the gas permeability. The platelet structure of layered silicates can improve the barrier properties of polymer materials according to the tortuous path model.4 We consider the nanocomposite as a permeable polymer phase in which impermeable nanoplatelets are dispersed. Three main factors influence the permeability of a nanocomposite: the volume fraction of the nanoplatelets; their orientation relative to the diffusion direction; and their aspect ratio. Depending on the clay-polymer interaction and the clay loading, different dispersion regimes occur.5,6 Enhanced barrier properties depend on the degree of exfoliation of the clay platelets. In the fully exfoliated state, individual clay platelets have the largest possible aspect ratio and, therefore, we expect the greatest barrier improvement. We investigated the gas transport behavior of NR/clay nanocomposites. We measured the gas-barrier properties of the nanocomposites for oxygen (O2), nitrogen (N2), and carbon dioxide (CO2) gases. For each gas we found that the permeability was significantly reduced as the organoclay content increased, reaching a minimum for a composition of 10phr (parts per hundred resins) clay. We reduced permeability by 57% for O2, 44% for N2, and 38% for CO2.7

Although this is consistent with the tortuous path model, we also related the results to the volume of the constrained region of the nanocomposites.7 The constrained region is where the polymer matrix meets the clay platelets. Polymer-clay interactions alter the properties of the polymer from those of the bulk polymer matrix. Where the polymer-clay interaction is large, the polymer adjacent to the clay will show low segmental mobility. This occurs with a partially or fully exfoliated morphology, which also creates a tortuous path. The reduced segmented mobility obstructs the movement of gas molecules through the permeable polymer, reducing the rate of diffusion through the nanocomposite.

We used dynamic mechanical analysis (DMA) measurements to detect changes in the molecular mobility of the polymer segments in the vicinity of nanoclay. A height depression in the tan δ(damping, or the ratio of the loss modulus to the storage modulus) peak indicates a reduction in mobile polymer chains during the glass transition. Hence it can be used to estimate the amount of constrained chains.8DMA studies confirm that some portions of polymer chains are immobilized in NR/organoclay nanocomposites. Zhang and Loo correlated the modulus enhancement of the organoclay nanocomposites with the volume of the constrained region and proposed a constrained region model for polymer nanocomposites.9

To understand the contribution of the constrained region to enhancing the gas barrier properties of the nanocomposites, we used DMA measurements to quantitatively estimate the volume fraction of constrained regions in the nanocomposites. We observed that the volume fraction of the constrained region (C) increases with clay content in the NR matrix (see Figure 2). We further investigated the correlation between the volume of the constrained region and the gas barrier properties by plotting the normalized permeability against calculated C (see Figure 3). This shows that the contribution of the matrix permeability, Pm, to the total (nanocomposite) permeability increases, as the volume ratio of the constrained polymer region increases.


Plot of the volume fraction of constrained polymer region (C) versus clay content for the nature rubber/clay nanocomposites. The line represents the curve of best fit.


Nanocomposite/matrix permeability ratio (P/Pm) as a function of the volume fraction of the constrained region, C. The line represents the curve of best fit.

In summary, the enhancement in gas-barrier properties of rubber/organoclay nanocomposites reveals that there is a significant role of immobilized or constrained polymer regions in the permeation process. Furthermore, the permeability decrease of the nanocomposites bears a good correlation with the volume of the constrained region. Polymer rubber nanocomposites with reduced permeability are used to make tires. Clay-reinforced rubber nanocomposites with low permeability are in demand for superior inner tubes. We found that natural rubber nanocomposites with exfoliated or partially exfoliated clay morphology had significantly enhanced gas-barrier properties. A partially (or fully) exfoliated morphology reduces the diffusion rate in the nanocomposite by creating a tortuous path. The constrained model explains the effect of the polymer-filler interaction on the segmental mobility and hence the permeability of the polymer matrix in the region of the clay. In future studies, we will study the effect of using different organically modified clays to enhance the barrier properties.


Authors

Sabu Thomas
Mahatma Gandhi University

A. P. Meera
Mahatma Gandhi University

A.P. Meera holds an MSc from Cochin University of Science and Technology, India, and a PhD from Mahatma Gandhi University. She is a lecturer and her main research interests include polymer nanocomposites containing spherical as well as layered fillers.

Hanna Joseph Maria
Mahatma Gandhi University

Hanna Joseph Maria holds an MSc and MPhil from Mahatma Gandhi University, where she is a doctoral student. Her research includes polymer blends and polymer blend nanocomposites.


References

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DOI:  10.2417/spepro.004266