WSU Material Science Engineering Capstone: Senior Thesis on Liquid Crystal Elastomers

Course Description and Premise

The WSU course MSE 416: Senior Thesis allows students to partner with university professors to dive into research they have been conducting or would like to conduct. I partnered with Dr. Narasimha Boddeti and Dr. Kaiyan Qiu to conduct research on 3D printing Liquid Crystal Elastomers (LCEs) using direct ink writing techniques (DIW). This course generally followed three stages:

  1. Conduct the background research to understand the premise and the details of all components necessary to the project.

  2. Determine and order the products and necessary equipment

  3. Begin experimentation while continuing to research

Stage 1: Background Research on Liquid Crystals and Liquid Crystal Elastomers

The following is research I conducted in preparation of creating my own LCE. This was accomplished over several weeks by reading numerous research papers and initial documentation of preliminary attempts by Heino Finkelmann. I first looked into liquid crystals (LCs), which are often called mesogens, then began research on their incorporation into LCEs. Both LC and LCE function and premises are described below, in Stage 1.

Introduction to Liquid Crystals (LCs)

Phase transition in LCs as a result of temperature change, showing the transition from a solid crystal, to the mesophase of liquid crystal, and finally to the liquid phase.

Takashi Kato et. al. “Liquid-crystalline physical gels”, Chem Soc Rev, Vol. 12, 2007

4-Cyanophenyl 4-((6-(acryloyloxy)hexyl)oxy)benzoate

“4-Cyanophenyl 4-((6-(acryloyloxy)hexyl)oxy)benzoate”, https://www.ambeed.com/products/83847-14-7.html

Example LC with rigidity and polarity

This liquid crystal shows all properties of typical LCs. Rigidity originates from the benzyne rings on the left side of the molecule in the image shown. Polarity is provided by the triple bonded nitrogen and carbon. Due to their large difference in electronegativity (~0.49), it provides the molecule with the capability of influence by polar fields.

Introduction to LCEs: Incorporation into the elastomeric network, function, and production

Two types of LCEs: Main Chain (MC) and Side Chain (SC)

MC-LCE

MC-LCEs incorporate the LC into the primary chain of the polymer. This requires two reaction sites on the LC and polymer chains that terminate in reaction sites.

SC-LCE

SC-LCEs attach the LC to the sides of the polymer chain. This requires that the LC has one reaction site on the side and the polymer chain has numerous throughout.

LCE Functioning Mechanism and Production

Anna Boczkowska, “Advanced Elastomers…”, 2012.

LCE contraction with temperature change

During the curing process of the LCE, the LCs can be can be aligned with a direction. Once aligned, the LCs will hold the polymer chains in a stretched state. Upon heating, the LCs will lose their crystalline order and allow the polymer chains to curl, resulting in overall contraction, as shown.

Jennifer A. Lewis et. al. “3D Printing of Liquid Crystal Elastomeric Actuators…”, AM, Vol. 30.​ Jan. 2018

Typical programming of LCEs with DIW and UV

Typical production of LCEs consist of using an LCE ink loaded into a DIW 3D printing setup. This ink is heated to reduce the viscosity and pushed through a nozzle where shear alignment occurs. Using UV light, the polymer chain finishes curing in a cooled state with the LCs holding the long chains stretched to allow contraction with heating.

Stage 2: Ordering and preparing Equipment

The material used during this research primarily consisted of an Ender 6 Desktop printer, Ecoflex 00-30, and our chosen liquid crystal: 4-Cyanophenyl 4-((6-(acryloyloxy)hexyl)oxy)benzoate. Existing equipment in the laboratories included a Thinky Mixer ARE-310 and a Nordson pressure dispenser. I redesigned a syringe attachment model from the graduate student Yiran Guo, such that it could adapt the 3D printer for attachment of a 3cc syringe.

Ecoflex 00-30

Ecoflex was chosen due to its availability and expected simplicity. As a two part RTV silicone, it operates on platinum catalyzed hydrosilylation, which would allow connection of the vinyl group of our LC and the hydrogen sites on the polymer chain sides (hydrogen sites can be seen below).

Ender 6 Desktop Printer

The Ender 6 was chosen for its familiarity within the lab and ease of modification. This allowed direct control of the syringe position using G-code.

10CC Syring Adapter

A re-design of Yiran Guo's syringe mount. Adapted from the 10cc syringe he was working with and modified to interface with the Ender 6 extruder mounting plate.

4-Cyanophenyl 4-((6-(acryloyloxy)hexyl)oxy)benzoate

Our chosen LC consisted of the necessary polarity (found on the left side with the triple bonded Nitrogen and Carbon) that all LCs must have. As our goal was to create a SC-LCE, the single reactive site on the right side could then undergo platinum catalyzed hydrosilylation. This reaction is described in the drop down.

Shigeyoshi Sakaki et. al. “Theoretical Study of Platinum(0)-Catalyzed…”, Organometallics, 17, March, 1998​, pp. 2511

Pt-Catalyzed reaction mechanism, where the Si-H bond (top) and the double C-C bond (right side) both break and attach to the Pt atom. These then connect to one another and result in the attachment of the C to the Si, leaving behind a methyl group.

“4-Cyanophenyl 4-((6-(acryloyloxy)hexyl)oxy)benzoate”, https://www.ambeed.com/products/83847-14-7.html

Stage 3: Research Progress and Conclusion

Beginning of research consisted of testing various mixtures of the LC, Ecoflex 00-30 Part A, Ecoflex 00-30 Part B, and fumed silica. These first three components were essential for creating the LCE, while the fumed silica served as a rheological modifier for DIW printing. Below are examples of the testing mixture as well as test print samples with the rheology modified Ecoflex 00-30 test prints. Due to this research only lasting ~4 months, it was not able to come to complete fruition. Conclusion occurred during the mixing and preparation of the LCE ink, where I began incorporating dichloromethane as a polar solvent for the LC to ensure an even disbursement throughout the ink. This is discussed more in depth below.

Test prints with rheology modified Ecoflex 00-30

An important stage in the research was determining the optimal mixture of parts A and B of the Ecoflex 00-30 and the fumed silica. Additionally, the printing pressure was another component in preventing over or under extrusion when coupled with the printing speed. The ideal inclusion of fumed silica was 2wt% at an extrusion pressure of 450kPa, moving at 300 mm/min. Due to the 30 min. pot-life of the mixture, printed samples 1-8 as shown, served as a test to ensure repeatability of the overall printing process at these parameters.

Project conclusion and future work

Due to the constraint provided by the end of the course, the project concluded during the incorporation of the LC. The vial shown depicts the separation of the Ecoflex 00-30 components (horizontal oval) from the LC/fumed silica (vertical oval). This work could continue by using pure polymer chain components as opposed to Ecoflex or incorporating a difunctional LC as a crosslinker.

This exploratory course taught me how to tackle the highs and lows of research. Understanding that there can always be unexpected results or unaccounted for variables. Most importantly, being able to acknowledge when something is unlikely to work and thus pursue an alternative solution. In this experiment, the use of Ecoflex left many different open-ended questions regarding what the true components were of the two part mixture. Had we found this information prior to the start of the experiment, it would have shaped the bulk of our approach. This is why the suggested continuation of this work discusses new approaches and directions entirely.

Sources

  1. “Phase transitions in liquid crystals”, 23 Nov. 2011. http://softmatter.seas.harvard.edu/index.php/Phase_transitions_in_liquid_crystals.

  2. Takashi Kato et. al. “Liquid-crystalline physical gels”, Chem Soc Rev, Vol. 12, 2007