Constrained polymer region

Adame, D. and Beall, G. W. Direct measurement of the constrained polymer region in polyam-ide/clay nanocomposites and the implications for gas diffusion. Appl. Clay Sci., 42, 545-552... 

Figure 4.9 Schematic diagram of the constrained polymer region around the clay plate.

The large changes observed in solvent uptake in these nanocomposites provide strong indirect evidence of the constrained polymer region and its size. In the work on solvent uptake by ethylene-vinyl acetate nanocomposites, the solvent uptake effect levels off at around 5 wt.% clay nanoparticles. This indicates that nearly all the polymer in the composite is constrained at this loading level. At 5 wt.%, assuming that the clay plates are evenly dispersed, the distance between plates would be approximately 50 nm. The constrained polymer region would be required to extend at least 25 nm from the surface. 

Figure 4.10 AFM pictures of the topography of a swelled MXD6 nanocomposite at various clay loadings showing valleys and hills owing to constrained polymer regions.

The difficulty of aligning montmorillonite in these high-viscosity polymers is discussed later in this chapter. This alignment difficulty as a function of viscosity results in a random distribution of the montmorillonite. An additional feature of this work is a prediction of the benefit of the constrained polymer region in relation to the surface of the montmorillonite. A measurement on the constrained polymer region is found in the chapter on barrier properties. 

The last paper in this series [15] focused on the measurement of the mechanical properties of the nylon 6-montmorillonite nanocomposites prepared above. The control nylon 6 (1013B, Ube Industries) was reported to have a = 13 000. The montmorillonite content in the nylon 6 polymer nanocomposites varied from 1.9% to 7.1%. In the previous article, the montmorillonite content at 1.5% in the nylon 6 nanocomposite produced M = 62000 the nylon 6 nanocomposite at 6.8% produced M = 29 000. The influence of molecular weight on the Young s modulus was not compensated for in the comparisons of the pure nylon with the nylon nanocomposites. The Young s modulus values were measured at 23 °C and 120 °C. The modulus values at 120 °C increased from about 0.19 GPa for pure nylon to about 0.7 GPa for the nylon 6 nanocomposite with 6.8% montmorillonite content. The heat distortion temperature climbed from approximately 65 °C to approximately 150 °C for the polymer nanocomposite with a 6.8% montmorillonite content. The authors argue the applicability of the mixing law (Equation 5.3) coupled with a constrained polymer region associated with the montmorillonite as the mechanism for reinforcement. Identification of the proper mechanism for reinforcement of nylon 6-montmorillonite is provided above by Paul et al. [5j. 

SAXS was run during the stress-strain evaluations of the samples. The presence of montmorillonite in the PP apparently prevents the change in the order of the PP crystals to the direction of the applied stress. This behavior is consistent with the restrictive influence of montmorillonite in forming a constrained polymer region around the montmorillonite as discussed in Chapter 4 on barrier performance. 

Very interesting studies of natural rubber reinforcement with ZnO nanoparticles were performed by scientists from India, under the direction of Sabu Thomas [62]. The goal of these studies was to characterize the viscoelastic behavior and reinforcement mechanism of ZnO nanoparticles introduced into the rubber matrix. They have presented a constrained polymer model based on a rubbery region and a ZnO nanoparticle. Very interestingly, the authors presented a core-shell morphology model and constrained polymer model to explain the constrained polymer chains in NR/ZnO nanocomposites [62]. Thanks to this research and the proposed models, it is possible to understand the behavior of nanofillers in the polymer matrix and maybe in the future to develop an ideal nanofiller for use in the rubber matrix. 

For accounts of DRS studies of other crystalline polymers, including poly-oxymethylene, polyvinylidene difluoride, polyvinyl fluoride and the nylons, the reader is referred to the text by McCnim et al. [3], the reviews [12-14,17,23,44, 45] and references therein. In all cases multiple dielectric relaxations are observed, arising from motions within crystals, on crystal surfaces and in the constrained amorphous regions within crystals. These processes are also observed in NMR and mechanical relaxation studies of such polymers. 

It has been recognised for several decades that most of the characteristic properties of polymers (except the untypical globular proteins), such as their high solution and melt viscosities, their rubber-like elasticity, and their viscoelastic behaviour generally, can be ascribed to the fact that their molecules, unless constrained as in the crystal, are free to adopt extended but coiled configurations, so that each molecule affects a region of space of many times its own volume ... 

Results and Discussion. The application of infrared spectroscopy to the analysis of SAN copolymers in solution has two limitations the poor solubility of the polymers, which constrains the analysis to those regions where the solvents are transparent and the poor sensitivity of the infrared at low concentrations. This last problem is more severe when the characteristic absorption bands have medium or weak intensities (typically -C = N stretching). Of particular interest are the bands between 3-7 jim, some of which can be detected in a variety of solvents and provide information on the AN/St ratio. The bands at 13 and 14 fim are very strong and amenable to detection particularly in SEC applications. These bands contain information on the styrene concentration and the molecular structure (Hummel 1974). 

In one example, the tensile strength of polyamide 6 was increased by 55% and the moduli by 90%, with the addition of only 4wt% of delaminated clay. The enhanced tensile property of PCN suggests that nanocomposite performance is related to the degree of clay delamination, which increases the interaction between the clay layers and the polymers. Several explanations, based on the interfacial properties and the mobility of the polymer chains, have been given for this reinforcement. Kojima et al. reported that the tensile modulus improvement for polyamide 6-clay hybrid originated from a constrained region, where the polymer chains have reduced mobility. The dispersion and delamination of the clay were the key factors for the reinforcement. The delaminated nanocomposite structure produces a substantial increase in modulus. 

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