Dependence of the Dynamic Mechanical Properties and Structure of Polyurethane-Clay Nanocomposites on the Weight Fraction of Clay

The effect of clay and chemical cross-linking on the dynamic mechanical properties of polyurethane reinforced with different concentrations of organically modified montmorillonite clay is investigated in this study. The polyurethane matrix is constituted of polytetrahydrofuran soft segment and 4,4′-methylenebis(phenyl isocyanate) hard segment. Glycerin was used as the chemical crosslinking agent, while Cloisite 30B clay was the reinforcing filler. The nanocomposites containing up to 1 wt.% clay showed a uniform dispersion of clay; however, the nanocomposites containing higher concentrations of clay showed the presence of heterogeneities. Dynamic mechanical spectroscopy, DMS revealed that the nanocomposites containing between 2 and 10 wt.% clay had two glass transition temperatures, Tg,1 and Tg,2. The higher-temperature glass transition temperature, Tg,2 increased with increasing clay concentration, while the low-temperature glass transition temperature, Tg,1 decreased with increasing clay concentration. The nanocomposites containing low clay concentrations up to 1 wt.% showed only one glass transition temperature with a narrow glass transition region. The crosslink density for the nanocomposites increased with increasing wt.% clay.

Compatibilizer Polarity Parameters as Tools for Predicting Organoclay Dispersion in Polyolefin Nanocomposites

Nanocomposites give an innovative method to increase the mechanical, thermal, and barrier performance of polymers. However, properly dispersing the nanoparticles in the polymer matrix is often key in achieving high performance, especially in the case of hydrophilic nanoparticles and hydrophobic polymers. For that purpose, nanoparticles may be functionalized with organic groups to increase their affinity with the polymer matrix. Compatibilizing agents may also be included in the nanocomposite formulation. This paper aims at identifying parameters relative to the compatibilizer polarity that would allow predicting nanoparticle dispersion in the polymer nanocomposite. The analysis used published data on nanocomposite samples combining clay nanoparticles, polyolefins, and various compatibilizing agents. We studied the correlations between the nanoclay exfoliation ratio and five different parameters describing the compatibilizer hydrophilic-lipophilic balance: the acid value, the mole, and weight fraction of polar groups, the number of polymer chain units per polar group, and the number of moles of polar groups per mole of compatibilizer. The best correlation was observed with the number of polymer chain units per polar group in the compatibilizer. This parameter could be used as a tool to predict the dispersion of organoclay nanoparticles in polyolefins. Another important result of the study is that, among the compatibilizers investigated, those with a low acid value provided a better nanoclay exfoliation compared to those with a high acid value. This may indicate the existence of a maximum in the nanoclay exfoliation/compatibilizer polarity curve, which would open new perspectives for nanocomposite performance optimization.

Application of the Experimental Results to the Modified Halpin-Tsai Micromechanical Model to Evaluate the Clay Dispersion in Clay-Reinforced Polyethylene Nanocomposites

Low-density polyethylene (LDPE)- and linear low-density polyethylene (LLDPE)-based polymer nanocomposites (PNC) containing organically modified montmorillonite (MMT) were prepared by melt mixing process. Itaconic acid (IA) and monomethyl itaconate (MMI) grafted polyethylene (PE) were used as compatibilizers. Na-MMT was modified by dodecyl-, hexadecyl-, and octadecyl-ammonium chlorides and then the compatibilizers were synthesized in solution reaction. 5 wt% organoclay and different concentration of compatibilizers (5, 10, and 15 wt%) were used for the PNC preparations to compare the effects of compatibilizers. The interlayer spacings, d001, and the mechanical properties of samples were measured by XRD analysis and universal testing machine, respectively. The experimental tensile moduli were used in Halpin-Tsai micro mechanical model to obtain “l/t” ratios and the results were compared experimental d001 values obtained from XRD data. It was found that the experimental results are in good agreement with the calculated results.

Composites of cationic nanofibrillated cellulose and layered silicates: water vapor barrier and mechanical properties

Composites of trimethylammonium-modified nanofibrillated cellulose and layered silicates (TMA-NFC/LS) were prepared by high-shear homogenization followed by pressure filtration and vacuum hot-pressing, which gave rise to particularly homogeneous dispersion of the silicate particles. Thirteen different clays and micas were employed. Water vapor barrier and mechanical properties (tensile strength, E-modulus, strain at break) of the composite films were investigated, considering the effects of layered silicate types and their concentration (in the range of 0 to 85 wt %). Good interactions between TMA-NFC and LS were obtained due to electrostatic attraction between cationic fibrils and anionic silicate layers, and even favored by high-shear homogenization process. Furthermore, oriented TMA-NFC/LS composite structure was achieved. Layered silicates exerted a pronounced influence on the water vapor barrier and mechanical properties; however, there was no common trend reflecting their types. The transport of water molecules through TMA-NFC/LS composites was studied considering both diffusion and adsorption mechanisms. As a result, diffusion pathways were proposed based on two new and one well-known models: the "native network", "covered fiber composite", and "fiber-brick composite" models. Importantly, it was found that the insertion of layered silicate particles did not improve automatically the barrier properties as indicated by the commonly used "fiber-brick composite" model. Mica R120 at a 50 wt % loading in composites with TMA-NFC matrix showed 30-fold improved water vapor permeability and 5-fold higher E-modulus compared to commercially used base paper.

Enhancing the Mechanical Properties of Cross-Linked Rubber-Toughened Nanocomposites via Electron Beam Irradiation

Improving the mechanical properties of a pristine system is the main target of developing nanocomposites. The nanocomposites systems were first prepared via intercalation technique with different organophilic montmorillonite (OMMT) loading. Two types of cross-linking techniques were applied, namely, as maleic anhydride polyethylene (MAPE) and electron beam (EB) irradiated system. The effectiveness of these systems was then compared with the control one and analyzed based on the mechanical tests and morphological examination. The mechanical tests revealed that control, MAPE, and EB irradiated systems had attained the optimum mechanical properties at 4 vol% OMMT content. EB irradiated unit of a dose of 100 kGy showed excellent mechanical properties with higher crosslinking degree which were proved by gel content analysis. X-ray diffraction (XRD) analysis confirmed the existence of delamination structure with MAPE and EB irradiation techniques based on the disappearance of characteristic peak. The degree of delamination was further investigated by transmission electron microscope (TEM).