Thermal, mechanical, and fracture properties of copolyureas formed by reaction injection molding: Effects of hard segment structure

Abstract

SYNOPSISA series of copolyureas containing 50% by weight hard segment have been formed by RIM. The hard segment structure was systematically varied to investigate the effects of urea group density, hard segment crosslinking, relative reaction rates, and to compare the properties of aromatic and aliphatic hard segment materials. In each case the soft segment was based on a 2000 molecular weight polyether diamine. The RIM materials formed ranged from flexible elastomers to brittle plastics depending on composition and were characterized by SAXS, DSC, DMA, tensile stress-strain and fracture mechanics studies. SAXS, DSC, and DMA showed that microphase separation had occurred to give materials with a nonequilibrium morphology. DMA and tensile stress-strain studies showed the small strain properties to be very sensitive to the volume fraction of glassy material whereas the ultimate properties were dependent on chemical structure of the hard segment. Fracture properties were determined using the single-edge notch technique. In most cases ductile failure occurred with G, > 2.5 k J m-* and the fracture surfaces showed gross yielding and tearing. In the case of the copolyurea with the highest urea group content, brittle fracture occurred with G, = 0.06 kJ m-'.

Theoretical modeling of the relationship between Young's modulus and formulation variables for segmented polyurethanes

“…It is known that in segmented PUs, the main reinforcement mechanism comes from the aggregation of the “HSs” into “hard phase” nanodomains 3, 4, 12–16. Thus, our polymer effectively becomes a nanocomposite, with glassy “fillers” (hard phase) in a rubbery matrix (soft phase).…”

“…It is well established that by varying chemical formulations, one can prepare PU elastomers with Young's modulus (“stiffness”) ranging from “soft” (∼1–2 MPa) to “hard” (>100 MPa). Understanding the dependence of the properties of PU elastomers on the formulation chemistry is a critical issue in PU science and technology 3–28…”

“…A significant number of studies were also devoted to the relationship between the morphology and mechanical properties. Ryan et al12–16 demonstrated that for several model PUU copolymers, shear modulus G as a function of HSF qualitatively can be described by Davies' equation, G 1/5 = ϕ H G H 1/5 + ϕ S G S 1/5 (where G H , G S , and ϕ H , ϕ S are shear moduli and volume fractions of the hard and soft phases) 35–37. Since Davies' equation assumes a cocontinuous morphology (both hard and soft phases percolate), it is valid primarily in the intermediate‐to‐high HS content and is expected to fail at low HS weight fractions.…”

Polyurethane elastomers based on amphiphilic poly(caprolactone)‐b‐poly(butadiene)‐b‐poly(caprolactone) triblockcopolyols

“…Polyurethane generated from diisocyanates and a mixture of long chain and short chain diols tend to yield segmented block copolymers as the result of a phase separation during synthesis . The hard domains contain the combination of diisocyanate and the short chain diol (chain extender) monomers, which are referred to as the hard segment components.…”

Relationships between synthesis and mechanical properties of new polyurea materials

“…Generally, the bulk synthesis of segmented polyureas is carried out by reacting a mixture of aliphatic and aromatic diamines with aromatic isocyanates by reaction injection molding (RIM), because the rates of reaction are such that RIM is the only suitable process to form these engineering polymers. [1][2][3][4][5][6][7] Small angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), and dynamic mechanical analysis showed that microphase separation between the soft and hard segments occurred by spinodal decomposition during the reaction and gave materials with a non-equilibrium morphology. 3,4 Compared to classical polyurethanes, the use of an amine terminated instead of alcohol terminated chain extender and soft segment led to the introduction of urea linkages, which enhanced interaction through hydrogen bonds and preserved properties up to a higher temperature.…”