Novel Approach of Evaluating Polymer Nanocomposite Structure by Measurements of the Freezing‐Point Depression

Abstract

IntroductionIn recent years, polymer/layered silicate (PLS) nanocomposites have attracted great interest, both in industry and in academia, because they exhibit unique properties not shared by their micro counterparts or conventional filled polymers. [1][2][3][4][5] The addition of just a few weight percent (<5 wt.-%) of nanolayered inorganic material gives rise to sensible increases in the stiffness and strength with minimal loss of ductility and impact resistance, [6,7] decreased gas permeability [8,9] and flammability, [10] improved abrasion and heat resistance, [11] and increased biodegradability of biodegradable polymers. [12] The physical mixture of a polymer and a layered silicate not necessarily gives rise to a nanocomposite. Depending on interfacial interactions between the polymer and layered silicates, three types of morphologies can occur: immiscible, intercalated, and exfoliated.The improved properties are only achieved when an exfoliated nanocomposite is formed. For this reason, it is very important to characterize the polymer nanocomposite structure prior to the analysis of their properties. The structure of the nanocomposites has typically been established by means of X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. Because of their easiness and availability, X-rays are most commonly used to determine the nanocomposite structure. However, one needs to be very careful with the interpretation of the results. Lack of sensibility in the analysis and limits of the equipment can lead to wrong conclusions about the nanocomposite structure. That is, the nanocomposites generally contain a fairly small amount of clay (less than 10 wt.-%), therefore the XRD analysis must be sensitive enough to detect the crystalline structure of the clay in the polymer. If this is not the case, no peaks appear in the diffraction pattern and a false conclusion, that a delaminated nanocomposite has been synthesized, might be drawn. Moreover, the analysis is Summary: A novel experimental approach based on the freezing-point depression of a solvent in a swollen gel has been developed to characterize the structure of nanocomposites. A higher depression in the freezing point has been related with an exfoliated nanocomposite. This increased depression not only depends on the formation of a tighter network but also on the decrease of the size of the solvent cages where the nucleation takes place.The nucleation process of unfilled and organoclay-filled natural rubber with the same crosslinking density.Communication 1309 performed at low angles in order to detect the (001) reflection and evaluate the spacing between the clay layers. It means that the irradiated surface may include not only the sample but also the sample holder. This might create a large amount of noise and, as a consequence, complicate the interpretation of the XRD patterns. Therefore, TEM is a necessary complement to confirm results obtained by XRD about the organization of the clay layers in the nanocomposite. TEM allows a qualitative u...

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