Effect of the degree of soft and hard segment ordering on the morphology and mechanical behavior of semicrystalline segmented polyurethanes
Introduction
An examination of the molecular architecture of natural materials, such as spider silk, nacre, and collagen, has offered a new perspective on the development of materials with enhanced mechanical properties. Nacre, a natural composite, is arranged in a brick-and-mortar configuration with staggered layers of calcium carbonate (aragonite) platelets and a complex mixture of proteins [1], [2], [3], [4], [5] with ultimate mechanical properties superior to that of the individual components [6]. Anisotropic collagen fibrils are traversed by a layered arrangement of apatite mineral; the elastomeric collagen matrix provides a reinforcing effect within the hierarchical, core–shell morphology which influences the toughness of bone [7], [8], [9], [10]. Native spider silk, a thermoplastic elastomer, consists of a majority matrix of alternating soft (amorphous, glycine-rich) with hard (β-pleated sheets of alanine) blocks [11], [12], [13], [14]. The glycine-rich, continuous matrix also contains an ‘oriented’ amorphous component that is thought to play a key role in the extreme toughness of natural spider silk [12], [15], [16], [17].
Thermoplastic elastomers, specifically multi-block copolymers such as segmented polyurethanes, provide a unique template for the design of synthetic materials with hierarchical microstructures. Here, we explore the influence of ordering within the continuous soft segment domain on material properties. Although many researchers have focused on the effect of soft segment (SS) type and length, hard segment (HS) type and length, hard domain crystallinity, and the extent of microphase segregation on structure–function relationships in segmented polyurethanes [18], [19], [20], [21], [22], [23], only a few investigators have systematically targeted the role of soft domain ordering (both crystallization and orientation of non-crystalline soft segment) on the morphological and mechanical behavior of segmented polyurethanes, particularly those containing highly crystalline hard domains. Kloss et al. studied the role of soft segment crystallinity for poly(caprolactone) (PCL)-based polyurethanes with amorphous hard domains, proposing a morphological model of PCL crystallites sequestered within the soft segment rich domain based on thermal analysis [24]. Skarja and Woodhouse analyzed a series of biodegradable polyurethane elastomers with varying PCL soft segment molecular weights. As the PCL soft segment crystallinity increased, these researchers suggested that the crystalline soft segment regions contributed to the overall reinforcement of the continuous domain, leading to enhanced mechanical behavior, including tensile strength, tensile modulus, and elongation-at-break [25]. Similar behavior has also been reported by Yen and Cheng upon increasing the crystalline poly(butylene adipate glycol) soft segment content in segmented polyurethanes [26]. Most recently, Sonnenschein et al. probed the enhancement of material properties using semicrystalline polyester diols as soft segments in low hard segment content polyurethanes, concluding that crystalline soft segments reinforce the hard phase according to a continuous reinforcement model [27].
Another way to alter the extent of order in the soft segment is to employ a side-chain LC soft segment. Nair et al. [28] modified a poly(siloxane) soft segment using liquid crystalline ordering groups. The deformation of these materials coupled the smectic A layers and the paracrystalline hard domains, suggesting that the incorporation of liquid crystalline moieties within the soft block may provide additional energy-absorbing mechanisms [29], [30]. Yet, despite extensive studies of segmented copolymers, unanswered questions still exist. Specifically, we wish to address the interplay between hierarchical microstructure, deformation and mechanical function in materials with both high and low degrees of soft segment crystallinity.
We approach this study by developing polyurethane elastomers that incorporate several types of order within the soft phase. The soft segment is designed to contain a crystallizable segment, which is an additional load-bearing component and can undergo additional rearrangements during the deformation process. Poly(ethylene oxide) (PEO) and poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) were chosen as the crystallizable soft segments. While PEO-based polyurethane soft segments have been used in prior investigations [31], [32], [33], [34], the focus of these studies has been primarily devoted to structural evolution and product development without an emphasis on systematic evaluation of property enhancement through soft domain ordering. The glass transition temperature of PEO is well below room temperature, but, depending on its molecular weight, the melting temperature may vary from 30 to 60 °C, which may moderate the modulus of the soft phase regions. PEO may also be copolymerized with an amorphous component, such as poly(propylene oxide) (PPO), to tune the degree of order exhibited within the soft domains.
Section snippets
Materials
All materials were purchased from Sigma–Aldrich. Dibutyltin dilaurate (DBTDL) was stored under dried 0nitrogen and used as received. Anhydrous N,N′ -dimethylacetamide (DMAc), packaged in a sure/seal™ bottle, was used as received. The soft segments, (PEO) (1000 and 4600 g/mol) or (PEO–PPO–PEO) (1900 g/mol, 50 wt% PEO, each PEO block=475 g/mol), were dehydrated and degassed under vacuum at 60 °C for 3–4 h and stored under dried nitrogen. The hard segment, 1,6-hexamethylene diisocyanate (HDI), was
Results and discussion
High molecular weight thermoplastic polyurethanes of varying soft segment content and type were synthesized by varying the stoichiometry of the macrodiol, chain extender, and diisocyanate. Table 1 details the soft segment type, molecular weight, and hard segment content (wt%) for these multi-block polymers. PEO-based polyurethanes with varying soft segment molecular weight were synthesized to study the effect of soft segment crystallinity on mechanical behavior. The segmented polyurethanes are
Conclusions
A series of high molecular weight polyurethanes containing PEO–PPO–PEO and PEO soft segment and HDI–BDO hard segments were developed to examine the relative roles of hierarchical order via hard and soft segment crystallinity in polyurethanes. The PEO–PPO–PEO soft segments did not exhibit crystallinity when incorporated into polyurethanes; however, PEO soft segments were semicrystalline. An increase in the PEO soft segment molecular weight contributed to increased incompatibility between the
Acknowledgements
This research was supported by, the US Army through the Institute for Soldier Nanotechnologies, under Contract DAAD-19-02-0002 with the US Army Research Office. This work is based upon research conducted at the Cornell High Energy Synchrotron Source (CHESS), which is supported by the National Science Foundation and the National Institutes of Health/National Institute of General Medical Sciences under award DMR-0225180. Research carried out (in whole or in part) at the National Synchrotron Light
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