Effects of Heat Exposure and Volumetric Compression on Poisson's Ratios, Young's Moduli, and Polymeric Composition During Thermo‐Mechanical Conversion of Auxetic Open Cell Polyurethane Foam

• Published in: physica status solidi (b) 2019-01, Vol.256 (1), p.n/a
• Main Authors: Duncan, Olly, Clegg, Francis, Essa, Abdusalam, Bell, Anthony M. T., Foster, Leon, Allen, Tom, Alderson, Andrew
• Format: Article
• Language: English
• Subjects: auxetic; digital image correlation; foam; polyurethane; shape memory
• ISSN: 0370-1972; 1521-3951
• DOI: 10.1002/pssb.201800393

The effects of thermo‐mechanical auxetic foam conversion parameters on the Young's modulus and Poisson's ratio of open‐cell polyurethane foam are related to changes in chemical bonding and cell structure. Applied volumetric compression, conversion temperature, and duration are reported on foam Young's modulus, Poisson's ratio, and structural stability. Fourier transform infrared spectral analysis on samples converted at and above 160 °C strongly indicates a hydrogen bond interaction increase in urea groups (C=O‐‐‐H‐N) and an increase in hydrogen bonding population. Spectral changes inferred soft segment degradation following extensive heat exposure (200 °C, 180 min), specifically a shift and intensity change in CH2 and C‐O‐C polyol bands and a broad baseline increase between 3600 and 2400 cm−1. These changes are linked to (i) resistance to dimensional recovery over time and during re‐heating; (ii) Poisson's ratio becoming negative, then zero in tension or marginally positive in compression; (iii) Young's Modulus reducing then increasing; (iv) mass loss, evidenced by thermogravimetric analysis increasing from 150 °C. The minimum mean values of Poisson's ratio of ≈−0.2 (to 10% compression/tension) are comparable to other studies. All samples that retain their imposed compression over time are isotropic, with near constant Young's moduli and Poisson's ratio (to 10% compression/tension). Temperature, duration of heating, and amount of compression during thermo‐mechanical conversion of open cell polyurethane foam are linked to Poisson's ratios, Young's moduli, and polymeric composition. Detailed experimentation includes a wide range of processing conditions and comparison of produced mechanical properties, polymeric composition, cellular structure, and dimensional stability. Blocked shape memory from a single shape memory cycle is demonstrated.