Structural development during deformation of polyurethane containing polyhedral oligomeric silsesquioxanes (POSS) molecules

doi:10.1016/S0032-3861(00)00389-X

Polymer

Volume 42, Issue 2, January 2001, Pages 599-611

https://doi.org/10.1016/S0032-3861(00)00389-X Get rights and content

Abstract

A unique polyurethane (PU) elastomer containing inorganic polyhedral oligomeric silsesquioxane (POSS) molecules as molecular reinforcements in the hard segment was investigated by means of wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) techniques. The mechanical properties of POSS modified polyurethane (POSS-PU) were also compared to those of polyurethane without POSS. The crystal structures of two different POSS molecules were first determined by X-ray powder diffraction analysis, yielding a rhombohedral cell with a=11.57 Å, α=95.5° for octacyclohexyl-POSS (1,3,5,7,9,11,13,15-octacyclohexylpentacyclo[9.5.1.13,9.15,15.17,13] octasiloxane) and a=11.53 Å, α=95.3° for hydrido-POSS (1-[hydridodimethylsiloxy]-3,5,7,9,11,13,15-heptacyclohexylpentacyclo [9.5.1.13,9.15,15.17,13] octasiloxane). WAXD results showed that reflection peaks distinct to POSS crystal diffraction were seen in POSS-modified polyurethane, which suggests that POSS molecules formed nanoscale crystals in the hard domain. During deformation, the average size of POSS crystals in POSS-PU was found to decrease while elongation-induced crystallization of the soft segments was observed at strains greater than 100%. The SAXS results showed microphase structure typical of segmented polyurethanes, with an initial long spacing of 110 Å between the domains. At high strains, the average length of strain-induced microfibrillar soft-segment crystals was estimated to be about 60 Å by SAXS. The TEM analysis of highly stretched samples showed a preferred orientation of deformed hard domains perpendicular to the stretching direction, indicating the destruction of hard segment domains by strain.

Introduction

Segmented polyurethane-based elastomers have excellent mechanical and thermophysical properties, such as high tensile strength, large reversible elongation, and enhanced rubbery modulus. It is now widely accepted that the superior properties in these copolymers are directly related to the formation of microphase separation from the thermodynamic incompatibility (immiscibility) of solid-like hard segments and rubbery soft segments [1], [2], [3]. The hard segments usually involve interchain interactions by means of van der Waals forces and hydrogen bonding, which determine the macroscopic properties. It is the intermolecular association of the hard segments that provides the physical crosslinking to the system.

Considerable academic and industrial efforts have been focused on studying the structure-property relationships in segmented polyurethanes. A commercially important class of polyurethanes containing crystallizable 4,4′-methylenebis (phenylisocyanate) (MDI) hard segments, polytetramethylene glycol (PTMG) soft segments and a chain extender of 1,4-butanediol (BD) has been studied most extensively. Several studies have successfully developed similar structural models for the microphase separation of hard and soft segments in this polymer system, based primarily on results from wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) [4], [5], [6], [7], [8].

In this study, a polyurethane system containing polyhedral oligomeric silsesquioxane (POSS) cage-like molecules pendent to the polymer chain has been investigated. The copolymer consists of random sequences of PTMG soft segments and 4,4′-methylenebis (phenylisocyanate) (MDI) hard segments. The MDI hard segment was chain-extended by 1-[3-(propylbisphenolA)propyldimethylsiloxy]-3,5,7,9, 11,13,15 heptacyclohexylpentacyclo-[9.5.1.13,9.15,15.17,13] octasiloxane (BPA-POSS) (Fig. 1). The unique properties of this polymer are brought about by the POSS molecules reinforcing the hard segment domains at the molecular level. Previous studies have shown that such organic-inorganic hybrid polymers possess improved properties such as higher T_g, increased oxygen permeability, reduced flammability and enhanced mechanical strength [9], [10], [11], [12], [13], [14].

The POSS molecule contains a polyhedral silicon–oxygen nanostructured skeleton with intermittent siloxane chains (general formula (SiO_3/2)_n) [15], [16], [17], [18], [19], [20], which was first reported in 1946 [21]. Silsesquioxanes have the empirical formula RSiO_1.5, with their name being derived from the non-integer (one and one-half or sesqui) ratio between oxygen and silicon atoms, and the organic substituent. A variety of substituents can be incorporated on the silicon atom, with recent interest focusing on the incorporation of a polymerizable group on one of the silicon atoms, and aliphatic hydrocarbon group on the remaining silicon atoms, to impart desirable solubility properties. These molecules have come to be known as polyhedral oligomeric silsesquioxanes, or POSS monomers and can be polymerized to the corresponding POSS macromers and polymers.

The POSS molecules have been successfully incorporated into different polymers such as styryls, acrylics, liquid crystalline polyesters, siloxanes and polyamides, etc. by scientists from the Air Force Research Laboratory (AFRL) [9], [10], [11], [12], [13], [14], [15]. In this study, the chosen POSS molecule (SiO_3/2)_n has n=8 (cage-like), with the corner group as cyclohexyl (molecular weight greater than 1000 amu). This molecule is considered large (approximately 15 Å in molecular axis, with an inner Si–Si diameter of 5.4 Å) as compared to the regular size of polyurethane hard domains (25 Å) [22]. Our goal in this study is to understand the effect of POSS molecules on both microscopic and macroscopic properties.

Section snippets

Experimental

The chosen basic POSS molecular structure (SiO_3/2R)₈ contains R=cyclohexyl (octacyclohexyl-POSS). We have also investigated its derivative with one corner group substituted by: (1) hydridomethylsiloxy group (hydrido-POSS) and (2) 3-(allylbisphenol-A) propyldimethylsiloxy group (BPA-POSS). The BPA-POSS compound is a diol, which was used in the polyurethane synthesis as a chain extender. The POSS-polyurethane (POSS-PU) contains soft segments of PTMG (M_w=2000), and hard segments of

Mechanical behavior

The tensile properties of segmented polyurethane depend upon the chemical structure, the copolymer composition, and the microphase morphology. Typical stress–strain curve of the compression-molded POSS-PU-34 sample obtained from the Instron tensile machine is shown in Fig. 2A(solid line). The reference polyurethane sample without POSS is shown in Fig. 2B for comparison. The maximum applied strain of POSS-PU-34 was about 400% (the maximum attainable strain was about 700%), and the maximum

Conclusion

We have studied the structural development of a unique polyurethane system having inorganic POSS molecules attached to the hard segments as nanoscale reinforcing agents. The tensile test showed that the incorporation of POSS molecules greatly enhanced the tensile modulus and the strength. The X-ray powder diffraction method was used to determine the crystal structure of two kinds of POSS monomers. One was octacyclohexyl-POSS, which showed a rhombohedral unit cell with a=11.57 Å, α=95.5°. The

Acknowledgements

BH would like to acknowledge the financial supports of this work in part by a Young Faculty Grant from DuPont and in part by NSF-MRSEC (DMR9632525). We thank Dr Zhigang Wang for assistance of data analysis in this study. PTM and HGJ acknowledge support from AFRL Materials and Manufacturing Directorate and AFOSR/NL. Research carried out in part at SUNY X3 beamline at the National Synchrotron Light Source at Brookhaven National Laboratory, which is supported by the US Department of Energy.

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Structural development during deformation of polyurethane containing polyhedral oligomeric silsesquioxanes (POSS) molecules

Abstract

Introduction

Section snippets

Experimental

Mechanical behavior

Conclusion

Acknowledgements

Top Curr Chem

J Macromol Sci -Phys B

Macromolecules

Macromolecules

J Macromol Sci -Phys B

J Macromol Sci -Phys B

J Macromol Sci -Phys B

J Macromol Sci -Phys Ed

J Macromol Sci -Phys Ed

Mat Res Soc Symp Proc

Macromolecules

Soc Plast Eng Symp Proc

Am Chem Soc Polym Prep

Am Chem Soc Polym Prep

Macromolecules