Polymer/nucleating agent interactions

Several analyses of polymer/nucleating agent interactions have been performed over the years. As indicated, when the analysis is made at a sufficient level of detail and when the partners in the epitaxy have well-defined crystal structures, (that is, excluding, e.g., liquid-crystalline phases), it appears that most epitaxial interactions belong to the class of hard epitaxy. However, a major characteristic of these analyses is that they deal with polymer/nucleating agent systems that have been, for the most part, discovered empirically. 

Remarkable stabilization may also be achieved by the use of chemically inactive fillers or nucleating agents. ITie nature of the interaction of fillers and polymer... 

Auxiliary agents silanes, titanates fatty acids polymer grafting with maleic anliydride to increase interaction," " stearic acid, nucleating agents octamethylcyclotetrasiloxane amino silane dimeric aluminates acrylic acid "... 

In another study by Di et al. [47] revealed that the exfoliation of MMT in PLA matrix has been achieved due to the strong interaction between the MMT and polymer matrix. They also stressed that the high surface area of MMT layers had reduced molecular mobility of PLA leading to unique properties for the PLA/ MMT nanocomposites. The exfoliated MMT platelets act as nucleating agents at... 

The effect of PE-g-MAH as compatibilizer of PE/OLS nanocomposites was investigated by varying its concentration. PE/PE-g-MAH/OLS samples showed an increase in both the exothermic peak temperature and activation energy compared with PE/OLS composites prepared with the same clay concentration, but without the compatibilizer. Hence, the clay was effective as nucleating agent and the composite system with PE-g-MAH was more active in nucleation process (Kim 2006). These effects were correlated with the increase of the melt viscosity in the case of PE/PE-g-MAH/OLS samples due to the confinement effect on the motion of the polymer chains and stronger interactions between polymer and clay. 

Due to their rigid nature, clay platelets can function as nucleating agents that are able to modify the ciystallization behavior of the polymer matrix such as PVDF-HFP. Organically modified clay promotes an a- to 3-transformation of the polymer crystals. The degree of transformation depends on the nature of the clay surface modifier and the strength of the interactions between the clay and the polymer. ... 

Although a number of nucleating agents are used for simple polymers, such as polyethylene (PE), and for iPP and sPP, the real nature of the interactions has been worked out only in the last ten years or so. These interactions are essentially of epitaxial character, i.e. do not involve chemical reactions. For polymers with a linear envelope (such as PE), the epitaxial relationship involves, as a rule, the chains lying flat on the substrate (with the helix axis parallel to the substrate). The major dimensional match involves the interchain distance in the contact plane the latter may differ for different substrates. As a consequence, the polymer lamellae stand edge on on the substrate [1]. 

The development of metallocene catalysts has made it possible to synthesize syndiotactic polypropylene (sPP). sPP has nearly the same melting temperature as aiPP, but its industrial development seems hindered by its lower overall crystallization rate. As a consequence, the search for appropriate nucleating agents may be essential. sPP has been shown to interact with low molecular weight organics and polymers, and notably aiPP. 

The issue of nucleating agents has been a recurring one in the previous sections. This industrially important issue is developed in its own right (cf Chapter 4) and only aspects relating to the role of epitaxy will be considered here. Indeed, epitaxy is a physical interaction that helps favor the nucleation of a polymer or any other material for that matter. In polyolefins only weak physical interactions are to be considered, thus the emphasis put so far on an understanding of these interactions, and their structural consequences and manifestations. 

Polar polymers and polymers with loose, irrational helices (e.g., poly(ethylene oxide) with a 72 helical conformation, POM with a 9s helical conformation) are difficult to nucleate only few nucleating agents are known for these polymers (cf Chapter 13). The irrational helical conformations are probably to blame since they imply indeed relatively large chain axis repeat distances, thus precluding a high density of any favorable local polymer-substrate interaction. 

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