Cellulose Nanocrystals as Reinforcement for Shape Memory Polyurethanes

AUAD, ML; NUTT, S; ARANGUREN MI; MARCOVICH, N. E.

Chicago, IL, USA

Simposio; ACS (American Chemical Society); 2007

Shape memory polymers (SMPs) are functional polymers, which find applications in a broad range of temperature sensing elements and biological micro-electro-mechanical systems (MEMS). These polymers are capable of fixing a transient shape and recovering to their original shape after a series of thermo-mechanical treatments. However in some applications, SMPs may not generate enough recovery force to be useful. In this work we explore a completely new approach to increase the recovery force of the shape memory polymer through the incorporation of cellulose nanocrystals as nanofillers. In this work, a thermoplastic polyurethane (TPU) prepared from 4,4-diphenylmethane diisocyante (MDI), 1,4-butanediol (BD) and poly(ethylene glycol) (PEG) with shape memory properties was reinforced with cellulose nanocrystals. Three sets of TPUs were synthesized containing a constant soft-segment molecular weight of 2000 and a variable MDI/BD-based hard-segment content ranging 23-45 wt%. Another TPU, based in a soft-segment molecular weight of 650, with a hard-segment content of 48% was also synthesized. Reinforced polyurethanes were also obtaining by adding different amounts of cellulose nanocrystals during the first step of the polymer synthesis. DSC and DMA measurements show that hard-segment crystalline melting temperature was in the range of 175-200 ºC and soft-segment melting temperature was in the range of 10-25 ºC and both increased with increasing cellulose content. The mechanical and thermo-mechanical behavior of neat polyurethane and resulting composites was characterized through tensile and thermal cyclic tensile test. Tensile modulus and strength increased as cellulose fibers content increases. The cellulose nanocrystal composites displayed essentially the same shape memory properties as those corresponding to the neat polyurethane. Thermal cyclic tensile tests indicated that the shape fixity capacity increases and the shape recovery ability decreases with hard segment concentration, but the opposite trend was found as the soft-segment molecular weight decreases.