Strain-induced structural and dynamic changes in segmented polyurethane elastomers

doi:10.1016/j.polymer.2018.12.062

Polymer

Volume 163, 1 February 2019, Pages 154-161

https://doi.org/10.1016/j.polymer.2018.12.062 Get rights and content

Highlights

•
Strain-induced structural and dynamic changes were revealed.
•
Structural changes at room temperature were revealed after sample fractures.
•
Strain-induced reorientation and restriction of amorphous chains were revealed.
•
Structural heterogeneity was revealed by low-resolution proton NMR spectroscopy.

Abstract

Polyurethane elastomers have been widely used in the industry and daily life due to their versatile physical and chemical properties. Therefore, a fundamental understanding of the structures and dynamics at a molecular level will provide piercing insights into the precise design and application of new polyurethane materials. In this study, we mainly focused on investigating the strain-induced structural and dynamic changes in a typical polyurethane elastomer composed of poly(ε-caprolactone) (PCL) and 4,4′-diphenylmethylene diisocyanate (MDI) as the soft and hard segments, respectively. Obvious strain-hardening phenomenon was observed during the mechanical tensile test, and a systematic comparison was performed on the fractured and pristine samples. DSC results revealed that the crystallization of PCL chains was still going on after the sample was fractured, and the crystallite structures became stable after physical aging at 25 °C for two days. ¹H solid-state nuclear magnetic resonance (NMR) experiment was further employed to determine the fraction of mobile PCL chains that were converted to crystallites during stretching. Besides, the microphase separation was also significantly enhanced in the fractured sample. The mobility of amorphous PCL chains was largely reduced due to the strain-induced crystallization of nearby PCL segments, as revealed by the ¹H magic-sandwich echo (MSE) NMR experiments. ¹H multiple quantum (MQ) NMR experiments also quantitatively revealed the strain-induced orientation of amorphous PCL chains in the fractured sample, indicating that the PCL crystallites were acting as the physical cross-linkages to prevent the contraction of the elongated PCL chains when the sample is fractured.

Graphical abstract

Introduction

Over the past decades, there has been a growing demand for the development of high-performance polyurethane materials, due to the excellent and tunable chemical and physical properties [[1], [2], [3], [4], [5], [6], [7], [8]]. Polyurethane materials are generally comprised of alternating hard segments (HS, diisocyanates and small molecule chain-extender diols or diamines) and soft segments (SS, macro-polyols), leading to a microphase-separated morphology due to the thermodynamic incompatibility [9,10]. As a result, thermoplastic polyurethane elastomer generally has superior mechanical properties with respect to other elastomers, such as the higher elongation at break, tensile strength and Young's modulus. Typically, at room temperature the soft domains of polyurethane elastomer are in a rubbery state due to their lower glass transition temperature (T_g), while the hard domains are semi-crystalline or glassy with a higher T_g; therefore, increase in the hard segment content will boost the strength and stiffness, while the extensibility and toughness will be enhanced by increasing the content of soft segments. These unique mechanical properties have made polyurethane a good choice for a wide range of applications, ranging from the medical device coating to sport products and even aerospace engineering [[11], [12], [13], [14]]. As a result, it has also attracted dramatic attention to optimize the combination of a variety of different soft and hard segments in order to tailor the secondary properties of polyurethane, such as shape-memory [[15], [16], [17]], thermal reversibility [[18], [19], [20], [21]], biodegradability [22,23], while at the meantime maintaining the superior mechanical properties. For example, dynamic covalent bonds were introduced into the cross-linked polyurethane to render the polyurethane thermal-recyclable without compromising the outstanding mechanical properties [4,18,24,25]. A very low volume fraction of nanoscale rod-like cellulose nanocrystals could be dispersed into the polyurethane matrix to strengthen and reinforce the materials without decreasing the extensibility [26]. Hydroxy telechelic poly(β-methyl-δ-valerolactone) was utilized as a replacement for petroleum-derived polyols in the synthesis of thermoplastic polyurethane to make the polyurethane recyclable and degradable, while the mechanical performance could still rival that of petroleum-derived polyurethanes [27]. In terms of the development of polyurethane materials with unique properties for specific applications, it is necessary and significant to gain a fundamental understanding of the inter-relationship among molecular structures, segmental dynamics and macroscopic chemical and physical properties. In particular, understanding the strain-hardening behaviors [[28], [29], [30]] of polyurethanes could offer insights into the design and preparation of smart materials with superior mechanical properties. Since the strain-hardening, especially the strain-induced crystallization, has important effects on the strength and anti-fatigue properties of polymers, it can help avoid fracture or break if the polyurethane material suddenly suffers from strong strikes or severe deformation. Although the strain-hardening phenomenon has been well known for several decades, there is still no a universal conclusion as the origin of strain-hardening in polymers [[31], [32], [33], [34]]. Since the polymer deformation generally comes with the remarkable changes in structures (molecular packing, network entanglement, phase-separation, etc.) [[35], [36], [37], [38], [39]] and dynamics (segmental relaxation, molecular motions, etc.) [32,[40], [41], [42]], it is important to probe the strain-induced structural and dynamic changes in order to better understand strain-induced enhancement of mechanical performance.

In this study, we had a detailed investigation on the structures and dynamics of both fractured and pristine samples, aiming to understand the strain-induced structural and dynamics changes, and thus to provide insights into the origin of strain-hardening in the polyurethane elastomer. A new crystalline structure, similar to the bulk PCL crystals, was found in the fractured polyurethane sample, whereas it was absent in the pristine sample. Besides, the pre-existing microphase separation was also enhanced by the stretching. Interestingly, PCL crystallization was still going on after the sample was fractured, and the structures became stable after two days' physical aging at 25 °C. Finally, it was also revealed that the segmental mobility of the amorphous PCL chains was greatly restricted, and the strain-induced molecular orientation was retained in the fractured sample even after physical aging at 25 °C for a couple of days.

Section snippets

Materials

Poly(ε-caprolactone) diol (PCL, M_n = 2000 g/mol), 4,4′-diphenylmethylene diisocyanate (>98%, MDI), and Tin (II) 2-ethylhexanoate (Sn(Oct)₂, 95%) were purchased from Sigma-Aldrich (Shanghai, China). PCL was degassed for two hours at 110 °C to remove moisture prior to use, and the others were used as received without further purification. 1,4-Butanediol (>99%, BDO) was bought from TCI (Shanghai, China), which was drived over calcium hydride prior to distillation under reduced pressure, and then

Strain-hardening of polyurethane

The stress-strain curve of the pristine polyurethane sample obtained from the mechanical tensile experiments is shown in Fig. 1, which clearly shows the presence of strain-hardening phenomenon after the yielding point. No necking is observed during the stretching. Although the Young's modulus (12.0 ± 8.0 MPa) and the yield stress (3.5 ± 3.0 MPa) are both small, the stress at break is relatively high, around 40 ± 2 MPa, and particularly, the elongation at break reaches 950 ± 10%. It is worth

Conclusion

In this study, we present a systematical study on the strain-induced structural and dynamics changes of a typical segmental polyurethane elastomer. The slight structural changes with increasing the physical aging time at room temperature was observed due to the slow reorganization of polymer segments in the fractured sample, including both the PCL chains and hard segments. However, the structure became stable after 4 days' aging at 25 °C. Strain-induced crystallization of PCL was also found

Acknowledgement

The authors are grateful for financial support by the National Natural Science Foundation of China (NSFC) (Nos. 21704046 and 21534005).

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¹: These authors have contributed equally to this work.

View full text

Strain-induced structural and dynamic changes in segmented polyurethane elastomers

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Strain-hardening of polyurethane

Conclusion

Acknowledgement

Polymer

Prog. Polym. Sci.

Prog. Polym. Sci.

Chem. Sci.

Polymer

Polym. Chem.

Polymer

Polymer

Polymer

Polymer

Polymer

Solid State Nucl. Magn. Reson.

Prog. Nucl. Magn. Reson. Spectrosc.

Comput. Phys. Commun.

Comput. Phys. Commun.

Solid State Nucl. Magn. Reson.

Polymer

Polymer

Angew. Chem. Int. Ed.

Advances in Polyurethane Biomaterials

Adv. Mater.

Adv. Funct. Mater.

Adv. Mater.

J. Am. Chem. Soc.

Chin. J. Polym. Sci.

Polyurethane Elastomers: from Morphology to Mechanical Aspects

Macromolecules

Biomacromolecules

The Polyurethanes Book