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s first commercial production plant in 1960. At its simplest, POM represents a repeating unit of CH2O. Whereas the DuPont? Delrin? acetal homopolymer manufacturing process maintains a straight chain of CH2O monomers with end caps, other manufacturers of acetals add one of several possible comonomers that appear in the chain on average every

70 -100 repeating units (see Fig. 1: molecular structure). Figure 1: Molecular structure of acetals White paper - DuPont? Delrin ? acetal homopolymer C Page

3 In addition, the typical chemistry of acetal copolymer polymerization produces roughly 2-8 weight percent of cyclic low molecular weight chains (oligomers) that, in general, do not participate in the structure and function of the material. The consequences of that molecular difference are exhibited in the crystalline packing of the polymer (Fig. 2). The purity of the uniform backbone in Delrin? homopolymer allows for a more organized stacking of the polymer into larger crystalline domains as the polymer solidifies. Meanwhile, the additional comonomer units in the copolymer disrupt this organization, ending the stacking locally and, ultimately, limiting the size of the crystalline domains. Figure 2: Delrin ? homopolymer vs. acetal copolymer crystalline structure in 2-D. On zooming out, the full significance of having a pure uniform backbone can be seen. As the diagram of the 3-D crystalline domains shows (Fig. 3), the larger the domain, the more entanglement strands emerge from each domain. These entanglements engage in a more extensive and intricate network with neighboring domains, reducing their mobility relative to each other. This highly networked structure is key to the increased level of mechanical properties extended by the uniform backbone of Delrin? homopolymer versus acetal copolymer. The cyclic oligomers are shown in bold blue. They cannot participate in the crystalline organization of the molecule and may even negatively affect the crystallization and mechanical properties. White paper - DuPont? Delrin ? acetal homopolymer C Page

4 Figure 3: The relationship between crystalline blocks and entanglements in 3-D. The uniform backbone of Delrin ? homopolymer allows the formation of larger blocks with more entanglements leading to a tighter network and, subsequently, superior mechanical properties. The mechanical properties that are exhibited by the acetal resins can be envisioned as resulting largely from the interactions of the large crystalline domains with the entanglements ― elongation across any axis will first stretch/straighten the entanglements along that axis before the polymer chain can begin peeling off from the crystalline domains. As the number of entanglements between each adjacent pair of domains increases, more stress will be required. This is akin to stretching

2 rubber bands at the same time compared to stretching

4 rubber bands at the same time. More entanglements can also absorb more energy during impact and help reset the structure after impact energy is absorbed, retaining the original shape. As the network of crystalline domains becomes more complex, there is more limited mobility of domain relative to each other. If mobility is difficult, it becomes more difficult for the domains to shift during elongation, resulting in higher modulus and stiffness. The same phenomenon of increasingly limited mobility can contribute to higher creep and fatigue resistance, and better spring back after momentary tensile or flexural deformation, or after fast deformation during an impact, as the crystalline domains relax back into their original configuration. White paper - DuPont? Delrin ? acetal homopolymer C Page