The Effect of Polyurethane Composition and Processing History on Mechanical Properties

Abstract

Uniaxial stress-strain behavior of a wide range of polyurethane elastomers was compared with current models of rubber-like elasticity. Although the data could be described well by a semi-empirical model, a systematic discrepancy was observed with more theoretically based models. This took the form of an additional energy term during deformation, which depended on the polyurethane composition. Microdomain fragmentation may provide a possible explanation. Alternatively, it may be due to the effects of segmental interactions on the relaxation of polymer chain entanglements during deformation.This combination of useful mechanical properties and ease of processing has led to the use of TPUs in many diverse applications, such as conveyer belts, rollers, solid tires, upholstery, elastomeric textile fibers, and shoe soles. [8 -11] Moreover, many TPUs exhibit good biocompatibility, making them promising candidates for medical and surgical applications such as gloves, catheters, arterial stents, and replacement heart valves. [12,13] Uniaxial tensile stress-strain curves of TPUs generally exhibit sigmoidal shapes characteristic of rubber-like elastomers. A relatively steep increase in stress (s) at low strain (1) is followed by a gradual decrease in slope (i.e., strain-softening), a less steep section of curve at intermediate strain and a second region of increased slope (i.e., strain-hardening) at higher strain. [1 -7,9] Young's modulus (E) generally increases with HS content, which has been attributed to HS microdomains acting as reinforcing filler, [1 -9] while the pronounced strain-softening behavior, mechanical hysteresis, and "tensile set" (i.e., the relaxed sample is elongated, compared with its original unstrained length) observed with some formulations may be caused by microdomain fragmentation. Strain hardening has been attributed to strain-induced crystallization; [2,3,7 -9] however, observations of similar behavior with cross-linked rubbers and other elastomers [14] suggest that alternative mechanisms may be possible in some cases.The work presented here investgated the mechanical properties of thermoplastic poly(ether-urethane) (PEU) and poly(carbonate-urethane) (PCU) copolymers at moderate strain. In particular, the effects of composition, processing history, and morphology on Young's modulus and the stress-strain behavior were studied. This forms part of an ongoing project to develop polyurethanes with improved characteristics and performance for medical applications. Rubber-like ElasticityRubber-like elasticity was discussed extensively by Heinrich et al., [15] Treloar, [16] and Dušek and Prins. [17] Recent developments in theory were reviewed by Mark. [14] and Beda and Chevallier. [18] Therefore, only a few key points need be discussed here.The stress-strain relationship of a rubber-like elastomer in uniaxial extension at low strain (i.e., before strain hardening is observed) can generally be described by the semiempirical equation derived by Mooney [19] and Rivlin: [20]

Hydrogen Bonds under Stress: Strain-Induced Structural Changes in Polyurethane Revealed by Rheological Two-Dimensional Infrared Spectroscopy

“…Figure D shows stress–strain measurements where we deform a thin polyurethane film up to a final strain of 500%. We observe an elastically rigid response up to 100% (elastic regime) and then a more compliant response at higher strain (strain softening), which are consistent with descriptions based on rubber elasticity theory . Above 400%, the polyurethane again stiffens (strain hardening), due to the limited extensibility of polymer chains as it approaches the fracture point.…”

“…We observe an elastically rigid response up to 100% (elastic regime) and then a more compliant response at higher strain (strain softening), which are consistent with descriptions based on rubber elasticity theory. 17 Above 400%, the polyurethane again stiffens (strain hardening), due to the limited extensibility of polymer chains as it approaches the fracture point. The mechanical response of the sample during the second cycle is much more compliant compared to the first cycle.…”

Stress Relaxation Behaviour Modeling in Rigid Polyurethane (PU) Elastomeric Materials

“…Many works address the topic of polymer rheology much more frequently than metallic materials [14][15][16][17][18][19]. Particularly in the automotive industry, a lot of effort has been put into rubber/composite materials for rheological studies of passive damping systems, such as the studies in [18][19][20][21][22][23][24].…”