Elsevier

Polymer

Volume 54, Issue 26, 13 December 2013, Pages 6910-6917
Polymer

Synthesis and properties of segmented polyurethanes with triptycene units in the hard segment

https://doi.org/10.1016/j.polymer.2013.10.028Get rights and content

Abstract

Segmented polyurethanes based on polytetramethylene glycol (PTMG) of 1000 g/mol were synthesized using a two-step polymerization procedure. Various hard segments were obtained using hexamethylene diisocyanate (HDI) or 4,4′-methylenebis(phenyl isocyanate) (MDI) as the diisocyanates and hydroquinone bis(2-hydroxyethyl)ether (HQEE) or triptycene-1,4-hydroquinone bis(2-hydroxyethyl)ether (TD) as the chain extenders. The effect of rigidity and bulkiness of the hard segments on morphology, thermal and mechanical properties were studied. Fourier transform infrared (FTIR) suggested that hydrogen bonding interactions were weakened in the presence of the bulky triptycene-containing hard segments. Variable temperature FTIR demonstrated that hydrogen bonds completely dissociate at around 170 °C for polyurethanes chain extended by HQEE compared to around 110 °C for their TD analogs. Polyurethanes from MDI and TD displayed microphase mixing behavior based on atomic force microscopy (AFM) and small angle X-ray scattering (SAXS). When HDI was used as the diisocyanate in the TD chain extended polyurethane, enhanced microphase separation was observed with comparable mechanical properties to those of the MDI analogs with HQEE.

Introduction

Polyurethanes (PU) constitute a broad class of polymers for various applications such as elastomers, adhesives, foams, etc. Among different types of polyurethanes, linear segmented polyurethanes are of importance since their physical and chemical properties can be easily tailored by changing the chemical composition, segment molecular weight, etc [1]. The segmental structure of these polyurethanes, with alternating hard and soft segments along the backbone, offers control of unique morphologies and properties. The hard segments, which are of relatively low content, can organize to form domains. These hard domains are dispersed in the soft matrix and act as physical or virtual crosslinks. The soft segment regions can benefit mechanical properties at low temperature [2]. Despite numerous studies focusing on preparation and characterization of new polyurethanes, improvements in polyurethane properties, such as mechanical properties, still receive considerable attention since their scientific implications and commercial applications keep expanding [3]. The most common approach to prepare high performance polyurethanes is incorporating unique chemical structures into the polymer chains to tailor various properties for specific applications [4], [5], [6].

The effect of the hard segment on morphology and properties of segmented polyurethanes has been widely studied. Polyurethanes derived from various diisocyanates and chain extenders can result in different morphological structures and physical properties. Chu and coworkers investigated the dependence of microphase separation on the flexibility of diisocyanates [7], [8]. They observed that the 4,4′-methylenebis(phenyl isocyanate) (MDI) gave rise to a partially microphase-mixed morphology while 1,6-hexamethylene diisocyanate (HDI) resulted in increased microphase separation. Due to the larger solubility parameter difference between MDI and soft segments such as poly(tetramethylene glycol) (PTMG) or polycaprolactone (PCL), thermodynamic effects suggest a more distinct microphase separation for the MDI based polyurethane. However, greater mobility introduced by the incorporation of flexible HDI can enhance microphase separation, which suggests that microphase separation is at least also greatly influenced by kinetic factors such as hard segment mobility rather than purely thermodynamic factors. Lee and coworkers studied the comparison among polyurethanes based on a series of diisocyanates [9]. They found that the hard segments containing aromatic diisocyanates such as toluene diisocyanate (TDI) and MDI were normally more miscible with the PTMG soft segments than aliphatic diisocyanates such as isophorone diisocyanate (IPDI) and HDI. It is also reported that chain extenders can be specially designed to impart particular chemical and physical properties to the polyurethanes [10], [11], [12]. Gong et al. synthesized polyurethane with calix[4]arene derivatives as new types of chain extenders [6]. These large bulky chain extenders provide potential applications in metal ion sequestrants and ion selective substrates. Polyurethanes containing calix[4]arene derivatives exhibited lower tensile strength and modulus when compared to polyurethanes chain extended by 3,3′-dichloro-4,4′-diaminodiphenylmethane (MOCA). It was proposed that this behavior was due to the bent chain conformation of calix[4]arene structure which introduced free volume and decreased the intermolecular forces especially the hydrogen bonding interactions. Polyhedral oligomeric silsesquioxanne (POSS) was also introduced into the hard segments of polyurethanes either along the backbone or as the side chains [13], [14]. It was found that the polyurethanes containing POSS in the hard segments as side chains exhibited a wider rubbery plateau regime than the non-POSS analog. It was proposed that the crystallized POSS hard blocks acted as physical crosslinks and provided further reinforcements.

Previous research has reported that triptycene and its derivatives can be introduced into various polymers to prepare new functional materials [15], [16], [17], [18], [19]. The study of triptycene-containing polymers started in the 1960s, for the purpose of preparing thermally stable polymers, a variety of polymers based on triptycene units were synthesized and characterized [20]. In this early research, bridgehead substituted triptycene derivatives were used to prepare polyesters, polyurethanes, and polyamides. Polyurethane films obtained in this initial work were brittle due to stiffness of polymer chains and the relatively low molecular weight indicated by the inherent viscosity. Recently, Swager and his coworkers have incorporated triptycenes into polyesters, polycarbonates, and other polymers [21], [22], [23], [24], [25]. In most of their work, 1,4-benzene substituted triptycene rather than bridgehead substituted triptycene was used which can increase free volume. Due to the three-dimensional propeller shape of triptycene units with large mass (254.11 g/mol), polymers with the triptycene structures are proposed to allow the flexible chains to thread through the clefts in the triptycene units for the purpose of minimizing free volume. Simultaneous enhancement of strength and ductility can also be obtained when triptycene units can interlock with each other in polyester and other particular polymer systems [26]. In our group, a new triptycene primary diol, triptycene-1,4-hydroquinone-bis(2-hydroxyethyl) ether (TD) (Scheme 1), was prepared and incorporated into copolyesters via melt polymerization. It was found that there were increases in thermal stability and glass transition temperature in comparison to the non-triptycene controls [27]. These results suggested highly aromatic content of triptycene units could impart rigidity and bulkiness into the copolyester system.

To our knowledge, triptycene-containing segmented polyurethanes have not been explored. In this paper, TD is utilized as the chain extender for polyurethanes in an effort to understand the effect of the bulky triptycene unit on the morphologies and properties of segmented polyurethanes with triptycene units being part of the hard segments. Polyurethanes using hydroquinone bis(2-hydroxyethyl)ether (HQEE) as a chain extender were also synthesized for the purpose of comparison. It is worth mentioning that the large molecular weight of triptycene unit can increase hard segment content significantly when compared to the non-triptycene analogs. Two types of diisocyanates, HDI and MDI, were also used to prepare the hard segments with different flexibility. FTIR was used to investigate the hydrogen bonding interactions. TGA, DSC, DMA, SAXS and AFM were employed to study the properties and morphologies of the DMF solution cast polyurethane films.

Section snippets

Materials

Poly(tetramethylene glycol) oligomer (Terathane, DuPont) with a number average molecular weight of 1000 g/mol, and hydroquinone bis(2-hydroxyethyl)ether (HQEE, 98%) were purchased from Aldrich. 4,4′-methylenebis(phenyl isocyanate) (MDI, 99.5%) was kindly provided by Bayer MaterialScience and used as received. 1,6-Hexamethylene diisocyanate (HDI, 98%) was purchased from Alfa Aesar and used as received. 1,4-Triptycene hydroquinone was kindly provided by ICx technologies, Inc (now FLIR Systems,

Hydrogen bonding behavior

Hydrogen bonding interactions are important driving forces for microphase separation of polyurethanes and significantly affect the thermal and mechanical properties [28]. Hydrogen bonds are formed in the polyurethanes between the active hydrogen atom in the urethane group and the oxygen atom of the carbonyl group in the hard segments or the ether oxygens in the soft segments, such as PTMG, in this study. With such interactions, the stretching vibration of the carbonyl groups and N–H groups can

Conclusions

Four segmented polyurethanes based on PTMG (1000 g/mol) with MDI or HDI were successfully chain extended with HQEE or TD. The incorporation of the bulky triptycene structure in the hard segment was found to disrupt the hydrogen bonding, which resulted in a significant influence on the thermal and mechanical properties. SAXS measurements showed little microphase separation for the MDI–TD polyurethane sample from the information of the scattering vector q vs intensity plot. The HDI–TD showed

Acknowledgments

This research was sponsored by the Army Research Laboratory and was accomplished under Cooperative Agreement Number W911NF-06-2-0014. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing official policies, either expressed or implied, of the Army Research Laboratory or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation

References (40)

  • P. Król

    Prog Mater Sci

    (2007)
  • Y.V. Savelyev et al.

    Polymer

    (1998)
  • N.T. Tsui et al.

    Polymer

    (2008)
  • I. Yilgor et al.

    Polymer

    (2006)
  • D.K. Chattopadhyay et al.

    Prog Polym Sci

    (2009)
  • F.M.B. Coutinho et al.

    Polym Degrad Stab

    (2003)
  • J.P. Sheth et al.

    Polymer

    (2004)
  • R. Gao et al.

    Polymer

    (2012)
  • D.B. Klinedinst et al.

    Polymer

    (2012)
  • K. Kojio et al.

    Polymer

    (2007)
  • P.R. Laity et al.

    Polymer

    (2004)
  • A. Aneja et al.

    Polymer

    (2003)
  • C.B. Wang et al.

    Macromolecules

    (1983)
  • M.E. Rogers et al.

    Synthetic methods in step-growth polymers, chapter 4

    (2003)
  • Q. Zhang et al.

    Macromolecules

    (2011)
  • S.R. Williams et al.

    Macromolecules

    (2008)
  • Q. Zheng et al.

    Aust J Chem

    (2007)
  • Y. Li et al.

    Macromolecules

    (1993)
  • Y. Li et al.

    Macromolecules

    (1994)
  • D.-K. Lee et al.

    J Appl Polym Sci

    (2000)
  • Cited by (0)

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