Unraveling the impact of phase separation induced by thermal annealing on shape memory effect of polyester‐based thermoplastic polyurethane

Abstract

In this study polyester‐based thermoplastic polyurethane (TPU) samples were annealed at 110°C for 0, 8, 16, and 24 h. To elucidate the relationship between the hard and soft phase separation and the shape memory properties, the samples were characterized within a rheometer, by selecting well defined thermal cycles. The results showed an enhancement of the shape recovery ratio (Rr ~ 65%) for all the annealed samples, when compared to the non‐annealed TPU (Rr ~ 60%). This behavior was attributed to the modifications of hard‐soft domain morphology occurring during the thermal annealing treatment, as it was shown both by differential scanning calorimetry with the shift in thermal transitions toward higher temperature, due the formation of new short‐range ordered hard domains and infrared spectroscopy, with the increase in degree of phase separation. Analyzing small‐angle X‐ray scattering, the decrease of the invariant also confirmed the enhancement of short‐range domains. The formation of these new short‐range domains acted as crosslinking points in TPU's morphology leading to an increase in stiffness, presented by higher Young and storage modulus. Based on these results, a model mechanism is proposed to correlate the morphology and structure of the annealed materials with their shape memory effect.

Influence of annealing‐induced phase separation on the shape memory effect of graphene‐based thermoplastic polyurethane nanocomposites

“…In noncrystallizable polyols (SS), as the one used in this work, event I can be also attributed to the glass transition (T g ) from the HSs dispersed in the amorphous phase and weakly bonded with SSs phase. 16,53,56 Thus, the shift at higher temperatures of T I may indicate a reduction of the chain mobility regarding this event. The nonannealed TPU + GNP nanocomposite shows the highest T I value (Table 1) suggesting that GNP with less functional groups than mGO, is mainly disperse in the SS, interfering in the enthalpic relaxation of weak interactions between the soft and HSs.…”

“…Thermomechanical characterization SME was evaluated by a thermomechanical characterization in a rotational Rheometer Anton Paar MCR 702 Twin Drive, as developed and previous published by the authors. 16,43 The test in a tensile mode was driven with rectangular samples with the dimension of (1 Â 5 Â 15) mm. The thermomechanical cycles were set in temperature range (T trans ± 25 C) from 40 to 90 C, in which T trans is 65 C (defined in the following DSC measurements) and a tensile stress of 2 MPa.…”

“…DSC curve of pristine TPU recorded during first heating show three thermal events: I-about 70-, associated to an enthalpic relaxation due to breaking of weak interactions (hydrogen bonding) between the soft and HSs; II-around 100 C, corresponding to the melting of short-range HD ordered structure (ordered HSs); III-around 180 C, ascribed as the melting of largerange HD ordered structure (micro-crystalline regions of HSs). 13,16,[51][52][53][54][55][56] When GNP and mGO were incorporated into TPU matrix a shift at higher temperature of the event I (T I ) is observed (Figure 6 and Table 1). In particular, T I increases of about 5 and 3 C for TPU + GNP and TPU + mGO respectively.…”

“…13,15 This morphological phase separation process is highly sensitive to annealing, 13 and leads to an increase in stiffness, which is highly influential on the SME since HD works as a net point, responsible for the returning of the shape, and SD assigns the flexibility during SME programming and switch unit. 16 However, the low stiffness, tensile strength, thermal and electrical conductivity are still some of SMP limitations. In addition, the use of temperature as the main type of stimulus for the returning into the original shape narrows its application.…”

Effect of hydrogen bonds on phase structure and crystallization behavior of UPy‐functionalized polyurethane

“…19 For example, the PU obtained by two-step synthesis method in solution possesses a more regular arrangement of hard and soft segments and a higher degree of microphase separation in comparison to one-step bulk synthesis, and thermal annealing is an effective way to improve the phase separation. 20 In fact, the PU's properties are greatly determined by the size, distribution, and degree of microphase separation, and a moderate microphase structure is an important guarantee for desired PU materials. 10,14,21 The external factors take effect through changing the internal driving forces.…”