Development of Polyurethane-Based Sheets by Phase Inversion Method for Therapeutic Footwear Applications: Synthesis, Fabrication, and Characterization G. Saraswathy, 1 Gautham Gopalakrishna, 1 B. N. Das, 1 Ganga Radhakrishnan, 1 S. Pal 1,2 1 Polymer Laboratory, Central Leather Research Institute, Chennai 600020, India 2 School of Bioscience and Engineering, Jadavpur University, Kolkata 700032, India Received 18 May 2008; accepted 9 August 2008 DOI 10.1002/app.29256 Published online 21 November 2008 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: It has been proved that polyurethane (PU) foam and viscoelastic PUs are offering better cushioning and shock absorption properties than other materials such as foam rubbers, polyethylene, ethylene vinyl acetate, and polyvinyl chloride which are used currently as insole materials in therapeutic footwear for diabetic and orthope- dic patients to ‘‘offload’’ or redistribute high pressure under the foot. The aim of this research work was to pre- pare viscoelastic materials based on PUs having the high- est degree of phase separation that provides for the elastomeric nature of these polymers. Polymer structures with a high concentration of amide groups can be made with the addition of hydrazine or a diacid hydrazide to a diisocyanate. We had prepared various PUs by chain extending the isocyanate-terminated prepolymer with ter- epthalic dihydrazide, 5-hydroxy isothalic dihydrazide, and 1,4-butanediol. Polymers were developed into sheets by phase inversion method using dimethyl formamide as sol- vent and water as nonsolvent. To improve the mechanical properties of PU sheets the polymer solution was blended with polyester-based PU Desmopan 8078 (CPU) in 1 : 1 ra- tio and the solution mixture was developed into sheet by the same method. Further PU sheets based on only CPU were also developed with various concentrations of PU. The synthesized PU and their blends with CPU were char- acterized by infrared spectroscopy, differential scanning calorimetry, thermo gravimetric analysis, gel permeation chromatography, and dynamic mechanical analysis. Mor- phological characteristics of PU sheets were studied by scanning electron microscopy. V V C 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 2387–2399, 2009 Key words: block copolymers; polyurethanes; viscoelastic polymers; therapeutic footwear; phase inversion method INTRODUCTION Polyurethanes (PUs) are used in wide physical forms, such as resins, elastomers, gels, adhesives, laminates, reinforcements, and soft and rigid foams, because they can be synthesized into materials with a variety of functions from various combinations of diisocyanates and diol monomers depending on the application. 1,2 A number of therapeutic approaches to ‘‘offload’’ or redistribute high plantar pressure in diabetic and orthopedic patients have been reported in the last 10 years. 3–6 Usage of therapeutic footwear turns out to be an effective tool for managing nor- mal gait and reduction of peak plantar pressures. Flexible, custom-made orthotic shoe insoles, which reduce elevated local pressures at the plantar foot- shoe interface, can be of substantial preventive and/ or therapeutic value to a variety of patients. Com- mon materials include foam rubbers, such as latex, and cellular polymers such as polyethylene (PE), ethylene vinyl acetate (EVA), PU, and polyvinyl chloride (PVC). Essentially solid materials such as viscoelastic polymers and natural cork are also used effectively. PU foam and viscoelastic PUs are offer- ing good cushioning and shock absorption proper- ties without suffering compression set. EVA offers good cushioning and shock absorption, but tends to suffer high compression set, meaning these proper- ties deteriorate quite rapidly in wear. PE and PVC can provide reasonable cushioning and shock absorption but PE, like EVA, suffers high permanent compression set. Latex rubber foams tend to be too soft and ‘‘bottom out’’ under low loads. They offer little cushioning or shock absorption and they pri- marily serve to cosset the foot. The unique elastic nature of segmented polyetherurethane (SPEU) materials and versatile fabrication techniques allow researchers to design and fabricate SPEU with bio- mechanical properties that are required. The two- phase micro-domain structure exhibited by SPEU is responsible for their superior physical and Journal of Applied Polymer Science, Vol. 111, 2387–2399 (2009) V V C 2008 Wiley Periodicals, Inc. Correspondence to: G. Radhakrishnan (gangaradhakrishnan@ hotmail.com). Contract grant sponsors: Department of Science and Technology, Government of India.