Soft shape memory in main-chain liquid crystalline elastomers

The field of shape memory polymers (SMPs) has been dominated by polymeric network systems whose fixing mechanism is based on crystallization or vitrification of the constituent chains, rendering such systems stiff in comparison to elastomers, gels, and living tissues. In this report, we describe the synthesis and characterization of liquid crystalline elastomers that exhibit both bulk and surface shape memory effects with compositionally dependent transition temperatures that determine the shape fixing and shape recovery critical temperatures. Main-chain, segmented liquid crystalline elastomers were synthesized using hydrosilylation linking of poly(dimethylsiloxane) oligomers with mesogenic dienes of two compositions and a tetrafunctional crosslinker. Calorimetric and dynamic mechanical analyses revealed two composition-determined thermal transitions for the LCEs, including a glass transition event at low temperatures (33 °C < Tg < 48 °C) and a first-order isotropization transition at higher temperatures (57 °C < Ti < 71 °C), each increasing with an increase in the concentration of the more slender, unsubstituted mesogenic diene, 5H. Despite the existence of a glass transition event, the materials remain soft at low temperature, a finding explained by vitrification of only the mesogen-rich layers within the smectic phase. Shape memory behavior was evaluated quantitatively and revealed excellent shape fixing and shape recovery values, generally in excess of 98%, with the recovery temperature depending on composition in a manner determined by the LCE phase behavior, particularly Tg. The temperature-dependent kinetics of shape memory were analyzed for a selected LCE composition, revealing exponential time dependence with rate constants that depended on temperature in an Arrhenius manner. Finally, the softness of the LCE SMPs was exploited to fix an embossed, micron-scale pattern on the surface and then recover the equilibrium flat state quite completely. We envision application of this surface shape memory phenomenon in the areas of soft lithography, especially microcontact printing and microfluidics.

[1]  C. Tanford.  Macromolecules , 1994, Nature.

[2]  R. Larson,et al.  Interdependence of shear deformations and block copolymer morphology , 1993 .

[3]  R. Larson,et al.  Morphology of a Lamellar Diblock Copolymer Aligned Perpendicular to the Sample Plane: Transmission Electron Microscopy and Small-Angle X-ray Scattering , 1993 .

[4]  R. Larson,et al.  The rheology of layered liquids: lamellar block copolymers and smectic liquid crystals , 1993 .

[5]  Yoshihito Osada,et al.  Shape memory in hydrogels , 1995, Nature.

[6]  B. Chisholm,et al.  Morphology and Physical Properties of Poly(styrene-b-isobutylene-b-styrene) Block Copolymers , 1996 .

[7]  E. Kramer,et al.  Deformation of a Polydomain, Smectic Liquid Crystalline Elastomer , 1998 .

[8]  Banahalli R. Ratna,et al.  Liquid Crystal Elastomers with Mechanical Properties of a Muscle , 2001 .

[9]  Spontaneous thermal expansion of nematic elastomers , 2001, cond-mat/0106138.

[10]  Andreas Lendlein,et al.  Cover Picture: Angew. Chem. Int. Ed. 12/2002 , 2002 .

[11]  P. Mather,et al.  Shape memory effect exhibited by smectic-C liquid crystalline elastomers. , 2003, Journal of the American Chemical Society.

[12]  Ian Charles Sage,et al.  Liquid Crystal Elastomers , 2003 .

[13]  E. M. Terentjev,et al.  Liquid Crystal Elastomers , 2003 .

[14]  Ping Xie,et al.  Liquid crystal elastomers, networks and gels: advanced smart materials , 2005 .

[15]  Hairpin rubber elasticity , 2005, The European physical journal. E, Soft matter.

[16]  Patrick T. Mather,et al.  Tailored Phase Transitions via Mixed-Mesogen Liquid Crystalline Polymers with Silicon-Based Spacers , 2005 .

[17]  J. Watanabe,et al.  Temperature-Induced Reversible Distortion along Director Axis Observed for Monodomain Nematic Elastomer of Cross-Linked Main-Chain Polyester , 2006 .

[18]  P. Mather,et al.  Responsive materials: soft answers for hard problems. , 2007, Nature materials.

[19]  Rudolf Zentel,et al.  Monodomain Liquid Crystal Main Chain Elastomers by Photocrosslinking , 2007 .

[20]  K. Urayama.  Selected Issues in Liquid Crystal Elastomers and Gels , 2007 .

[21]  Patrick T. Mather,et al.  Review of progress in shape-memory polymers , 2007 .

[22]  Macromol. Rapid Commun. 7/2007 , 2007 .

[23]  P. Mather,et al.  Two-way reversible shape memory in a semicrystalline network , 2008 .

[24]  J. Watanabe,et al.  Elongation behavior of a main-chain smectic liquid crystalline elastomer , 2008 .

[25]  H. Finkelmann,et al.  Main-Chain Liquid Crystalline Elastomers : Monomer and Cross-Linker Molecular Control of the Thermotropic and Elastic Properties , 2008 .

[26]  Robert Weiss,et al.  New Design of Shape Memory Polymers: Mixtures of an Elastomeric Ionomer and Low Molar Mass Fatty Acids and Their Salts , 2008 .

[27]  H. Finkelmann,et al.  Nematic main-chain elastomers: Coupling and orientational behavior , 2009 .

[28]  H. Finkelmann,et al.  Crosslinking Topology and Chain Anisotropy of Smectic Liquid-Crystalline Elastomers , 2009 .

[29]  P. Mather,et al.  Shape Memory Polymer Research , 2009 .

[30]  R. Rosenfeld.  Nature , 2009, Otolaryngology--head and neck surgery : official journal of American Academy of Otolaryngology-Head and Neck Surgery.

[31]  Xiaofan Luo,et al.  Preparation and Characterization of Shape Memory Elastomeric Composites , 2009 .

[32]  Heng Zhang,et al.  A novel type of shape memory polymer blend and the shape memory mechanism , 2009 .