Constrained polymer

P. J. Ludovice and U. W. Suter, Cotnput. Polym. Set., 1, 69 (1991). Molecular Dynamics of Geometrically Constrained Polymer Systems in Generalized Coordinates Basic Formalism. 

FIGURE 10.2 Schematical representation of the viscoelastic behavior of an elastomer in a single frequency DMA experiment performed by temperature sweep. The star marks a melting peak, and the black dotted line indicates the viscoelastic behavior for a more constrained polymer. 

Therefore, it is a reasonable approach to consider at first very concisely the physical basis for the interpretation of the glass transition anomalies in complex systems. This relates basically to three topical problems (1) polymer dynamics in nanoscale-conflning geometries or at free surface (2) constrained polymer dynamics and (3) the notion of the common segmental nature of the a- and p-relaxations in flexible-chain polymers. 

Constrained Polymers. The conformation of polymers constrained in various ways, for example, grafted to a flat surface ( brush ), adsorbed on a spherical colloidal particle, or tethered to a central branch point as in star polymers. All such problems involve potentially large nonideal conformational effects and also introduce additional complications associated with site inequivalence within the PRISM formalism. Progress for star polymers is briefly described in the next section. 

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. 

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... 

AFM can, however, be operated in one mode where this kind of sample preparation can be avoided. In this mode the point of the stylus is pressed into the surface with a specific force and the modulus of the material is measured. In this mode the polymer modulus is normally much lower than that measured for the clay particles. Figure 3.8 is an example of the type of image that can be obtained from AFM. In this image the sample has been exposed to a swelling solvent and the constrained polymer around the clay swells less than the unconstrained polymer and, therefore, the clay plates reside at the bottom of the valleys. 

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

Nanocomposites that deviate positively from the tortuous path model and the constrained polymer model... 

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.

It would be highly unhkely that these different crystalline forms would have radically different solvent uptake behavior. In the case of MXD6, which is amorphous, the effect of constrained polymer could radically alter the solvent uptake. In general, the radical changes in solvent uptake have mainly been observed in amorphous polymers. With further study, it may turn out that only amorphous polymers will exhibit this constrained polymer effect on solvent uptake and large increases in barrier. 

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. 

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