Morphological Control of Segmented Polyurethanes via Crystallization Confinement of Soft Segments

摘要

Polyurethane’s structural capabilities have been well established over the past 50 years (1,2).Polyurethane foams comprise two thirds of current polyurethane production in the United States, mostlyas support structures within couches and cars, due to their high crosslinking density and strength.Segmented polyurethanes (SPUs) are a high molecular weight polymer comprised of linear regions ofboth “soft” and “hard” segments, which polymerize into chains with minimal to no crosslinking (1).The linear structure of SPUs, unlike their foam counterparts, allow for the formation of transparent filmsthat display similar, if not superior, mechanical properties to polycarbonates (2). During the synthesis ofSPUs phase separation occurs between the hard and soft segments spontaneously (3). This phaseseparation is the origin of SPU’s superior mechanical strength as the hard domains reinforce the softdomains preventing crack propagation, while the softdomains provide the flexibility and toughness that allow forgood energy adsorption (4). An additional increase inmechanical strength is created at the interface between thedomains due to their contrasting mechanical properties (5).Figure 1 demonstrates the morphology of SPUs withdomains between 2-10 nm in size. Domains of amorphoussoft segments surround the crystallized hard domains whichare held together via hydrogen bonding. Hard segments arecomposed of diisocyantes and short diols which react toform urethane bonds. Each urethane bond has both an amideand carbonyl group creating the right conditions forhydrogen bonding. The longer the hard segment lengths themore hydrogen bonding will be due to an increase in the number of urethane bonds along the segment.By increasing the interface density of the SPU and enhancing the contrasting properties of the hard andsoft segments it might be possible to produce a transparent SPU film possessing structural capabilitiessuitable for transparent body armors.One key problem in the design of SPU systems is finding a structurally capable soft segment thatis also amorphous at room temperature. It is necessary that the soft segment is amorphous so that themechanical properties between the hard and soft domains are drastic thus increasing the interfaceadsorption phenomenon dramatically. We believe that since the soft segment, which is composed of amacrodiol, will crystallize at a much lower temperature than the hard segment that we can confinecrystallization of the soft segment by increasing hard segment domains.