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Updated: 15 hours 35 min ago

Ask for a Ph.D. position

Sat, 2018-01-06 04:10

In reply to Postdoc position in nonlinear multiscale simulation and optimization @ SUTD Singapore

Dear KenvinGE,

Thank you for your post.

I'm very interested in the position. Unfortunately, currently I need a Ph.D. position. 

Could you help me ask the leader of the project if there is a similar job for Ph.D. candidates? 

Thank you very much.

Best regards,

Nhan

 

Hi Jiawei, 

Sat, 2018-01-06 00:41

In reply to Then the LCE is an

Hi Jiawei, 

the anisotropicity of the fracture toughness in LCE is actually not very large as shown in our paper. 

we do not fully understand it. 

shengqiang

Dear Zhigang, 

Fri, 2018-01-05 13:59

In reply to Dynamic covalent chemistry for LCE

Dear Zhigang, 

A simple summary of the advantages of disulfide bond: 

1. catalyst/photoinitiator free (we tested in a LCE sample stored for several months, dynamic reaction can still be easily triggered)

2. bond exchanging reaction in disfulfide bond can be triggered by both UV and heat (our recent work has shown such reaction can be also probably triggered by force alone)

3. The temperature to trigger such reaction is very tunable (which is important for LCE application, since actuation of LCE is often thermally triggered also.) The reaction temperature can be tuned by varying the chemical groups linked to disfuldie bond. The temperature can be also lowered down by simply diffusing in a small amount of catalyst.

shengqiang 

 

Dear Zhigang, 

Fri, 2018-01-05 13:49

In reply to The choices of dynamic covalent chemistry

Dear Zhigang, 

thanks for your question. Zhijian and I have been talking about writting a review paper focusing on dynamics bond in LCE. 

shengqiang

Dear Rong, 

Fri, 2018-01-05 13:38

In reply to Very interesting review!

Dear Rong, 

Thanks for your interest and questions. 

For question 1, yes, we see much larger hystereis loop in polydomain LCE sample than more alligned monodomain LCE. 

see the figure below (unpublished raw data). The black curve is for a polydomain LCE, while the red one if for a monodomain LCE. Both LCE are synthesized in the same condition. The only difference is the allignment of LC molecules. I did a rough quantitative estimation. The hysteresis can indeed lead to the  fracture toughness increase measured in our fracture experiments.  

 

We also had a paper published in EML last year, using the transparency transition in LCE to visulize the strain. 

http://caigroup.ucsd.edu/pdf/2016-07.pdf

For question 2, it is a great question. For a LCE, a mechanics (chemo-mechanical) model to link its synthesis procedure and performance is highly desired, which is also (in my opinion) a great opportunity for us mechnician. In particular, during the synthesis of  LCE, mechanical load is often applied. If you have interest in modelling it, we can discuss about it in details.  

DCC-based LCE

Fri, 2018-01-05 12:30

In reply to Dear Zhigang,

Dear Rong,

I am impressed by your excellent work on modeling of DCC. Most DCCs are mainly used for make thermoset polymer network recyclable and malleable. It is a big breakthrough for the fabrication. I think most of the work focuses on the chemistry. 

Processing monodomain LCE is a special case of the malleable properties of DCCs. The polymer network must be stable around the actuation temperature.

Thanks.

Zhijian

Dynamic covalent chemistry for LCE

Fri, 2018-01-05 12:22

In reply to The choices of dynamic covalent chemistry

Dear Zhigang,

Thanks for your interest. We can simply regard the LCE as two parts: liquid crystal and polymer network. The dynamic covalent chemistry is used for processing LCE from polydomain to monodomain through exchange reaction. The actuation property of LCE is attributed to the phase transiton of liquid crystal. When we want to use  LCE as actuators, LCE should be thermoset without any exchange reaction. Otherwise, the actuation properties will disappear after several cycles.

We tried several kinds of dynamic covalent bonds to make LCE. Like RM257, most liquid crystals have liquid crystal-isotropic temperature (Ti). 

For thermo-triggered dynamic covalent bonds like epoxy-acid system, they have reaction temperature (Tr). Tr should be much larger than Ti. We can process LCE from polydomain to monodomain above Tr. When cooling down, the monodomain state is fixed. When the monodomain LCE is used as actuator around Ti, the polymer network is thermoset and doesn't rearrange. The actuation behavior results from the phase transition of liquid crystal mesogens. Ji and her coworkers discussed it in Ref [19].

If Tr is close or lower than Ti, when LCE is heated above Ti, it will contract. However, the exchange reaction also happens andhe polymer network rearranges. After several times or heating above Ti for enough time. LCE will change back to polydomain state. The Tr for imine chemistry and olefin metathesis are lower than 80 oC. They are not suitable for RM257-type liquid crystal mesogens. We tried imine chemistry and didn't obtain monodomain LCE.

The disadvantage of thermo-triggered DCC for LCE is that when we heat LCE above Tr which is much larger than Ti, LCE will contract and generate large stress. Usually, people use creep experiment to align it slowly. It is diffcult to control the actuation strain, stress.

For photo-triggered DCC based LCE, the situation is much simpler. In Ref 24, Bowman and his coworkers reported RAFT-based DCC to make LCE. The RAFT exchange reaction can be triggered by light in the presence of photoinitiator. After initiaor is consumed, LCE is thermoset and can not change from polydomain to monodomain.

Disulfide exchange reaction can be triggered either under heat and UV light. The reaction temperature (Tr) depends on the substitute groups (R), catalyst and base. In our work, Tr for disulife exchange reaction is much higher than Ti of liquid crystal mesogens we used. So disulfide LCE can be processed through heating.

Disulfide exchange reaction can also be triggered under UV light without any photoinitiator. It can be processed for many times and stored for long time.

Best,

Zhijian

 

 

Dear Zhigang,

Fri, 2018-01-05 09:37

In reply to The choices of dynamic covalent chemistry

Dear Zhigang,

I have been interested in dyanmic covalent chemistry (DCC) for some time and can add a few references that I know. Prof. Leibler and colleagues at ESPCI used transesterifcation reactions in epoxy networks to achieve DCC (Montarnal et al. 2011). Prof. Guan's group at UC Irvine used olefin metathesis in polybutadiene networks (Lu & Guan 2012). Prof. Zhang's group at CU Boulder used imine chemistry to acheive DCC (Taynton et al. 2016). These works mainly aim to make the covalently crosslinked network malleable and recyclable. I am also curious about what are the limitations on combining various DCC mechanisms with LCE?

Very interesting review!

Fri, 2018-01-05 09:14

In reply to Journal club for January 2018: Recent advances in liquid crystal elastomer

Dear Shengqiang,

Thanks for this fascinating and informational review! I am particularly interested in the fracture of LCE and the combination of dynamic covalent chemistry (DCC) with LCE. I have two questions and would appreciate your insights:

1. You mentioned that the polydomain-to-monodomain transition is a possible toughening mechanism for LCE. Is there any bulk mechanical testing data (e.g. uniaxial tension) showing hysteresis and dissipation solely caused by this transition? I would guess this may be not straightforward in experiments since viscoelasticity is also present, as you pointed out, and can also lead to hysteresis. I ask this question because even in elastic networks, stretch can also cause alignment of chains as shown in a few Molecular Dynamics simlations (e.g. Bergstrom & Boyce 2001, Yang et al. 2015), but such chain alignment does not cause any energy dissipation.

Also, the opaque-to-transparent transition during a fracture test is very interesting! Is it possible to engineer this transition and use it as a strain/stress indicator?

 

2. The usage of DCC to help align the monodomain in LCE is inspiring. Are there any models to guide this process, e.g. the stretch, irradiation intensity, and irradiation time required to achieve a certain degree of the monodomain? 

The choices of dynamic covalent chemistry

Fri, 2018-01-05 08:44

In reply to Journal club for January 2018: Recent advances in liquid crystal elastomer

Dear Zhijian and Shengqiang: Many dynamic covalent chemistries exist.  You noted that people have used various dynamic covalent chemistries for LCE.  You introduced disulfide for LCE.  In your paper you discussed briefly about the attributes of disulfide for LCE.  Can expand this discussion and compare various dynamic covalent chemistries for LCE?

SF=2

Fri, 2018-01-05 02:02

In reply to Damage and Reliability

Thanks for your comment.  SF = 2 is not too big, if you consider in composites they would need to take SF=LF=13 (from Weibull modulus = 1.25). 

However, in principle, SF=2 is corresponds to alfa weibull = 4 or 5 which is still lower than I would expect from typical aluminum specimen with long crack (see the classical Virkler tests which I have recently reconsidered, and soon I'll share a paper on this).  They show alfa = 12 or so, so I agree SF could be reduced.   

But there are other issues.  Convincing EASA and FAA would not be easy.

 

 

The focus of the Damage Tolerance concepts is an inspection focus, a certification of tolerance to Barely Visible Damage, BVD, and other size damages, to be established by the OEM and a demonstration of Residual Strength capability, Flutter, ect. and a nearly no-damage-growth in a defined usage environment for a minimum of 2 Design Service Lives. Demonstrate the Safety margins on the last flight for the Design Life Goal. The Full Scale Fatigue Test verifies the nearly no-damage-growth objective and the Fail Safe damage demonstration is in the next test increment. The "Service Life" can be extended by continuing the test duration. Operators are authorized to fly for 1/2 of the demonstrated test life. The factor of 2 is explained in the briefing as assuring multiple inspections to assure a very high probability of damage detection (typically 3 or 4 chances to see the damage) in the operational word. The Full Scale Fatigue test supplements the Full Scale Static Strength test demonstration that includes the Ultimate Strength capability of BVD and some Fail Safety demonstrations. 

 

The implanted damage sites are determined by the OEM's and industries service experience not by an arbitrary regulatory criterion. The policy is "you must demonstrate safety with damage states that you cannot inspect for and repair".

 

In metallic aiging a/c issue is that damage sites, typically intergranular voids develop,combine into microcracks  and then into the dominate fracture mechanisms size cracking. The concern is the generalized cracking micro and macro will eventually compromise the Fail Safe design provisions. This is a topic that the composite community is also working. 

Hope this helps.

 

FEM for LCEs

Fri, 2018-01-05 01:28

In reply to Hi Jinxiong, 

Jinxiong and Shengqiang, we implemented a LCE model into a FEM code in the following paper. Unfortunately, at that time, I wasn't good at Abaqus, so I implemented based on the software, FEPG, which you can essentially write the weak form. I know Prof Yongzhong Huo is still working on FEM implementation, but I don't know whether it's based on ABAQUS.

https://www.sciencedirect.com/science/article/pii/S0020768312002417

Hi Jinxiong, 

Fri, 2018-01-05 01:04

In reply to Hi Shengqiang, I have a

Hi Jinxiong, 

Yes. I agree in general. It is probably the limitation for most light-sensitive material. 

In some senarios, if light-induced inhomgenous deformation is desired, LCE with the thickness larger than the light-peneration depth can still work. 

We made LCE samples typically with diameter for rod or thickness for thin film around several mms. Large, fast, precisely-controllable and reversible bending can be easily realized in the material using laser. 

shengqiang

Hi Jinxiong, 

Fri, 2018-01-05 00:57

In reply to Hi, Lihua, I read

Hi Jinxiong, 

We did some search on this. We cannot find any abaqus subroutine for LCE. It can be generally difficult, because of the involvement of phase transition. 

shengqiang

Hi Shengqiang, I have a

Fri, 2018-01-05 00:56

In reply to Dear Xuanhe, 

Hi Shengqiang, I have a thought on possible limitation of light-sensitive LCE. One unique feature of LCE is that it can be triggered by remotely controlled light source. I thick light sensitive LCE material should be synthesized into thin films to make it more effective. For bulk materials, it is hard to use its light-sensitivity because only its surface layer would work. Am I right?

Jinxiong

Hi, Lihua, I read

Fri, 2018-01-05 00:49

In reply to Model of LCE

Hi, Lihua, I read yourexcellent  paper with Prof. Huo. I agree with you that modeling inhomogeneous deformation of LCE is very interesting since several work on wrinkling of LCE was also reported. Researchers can follow the strategy on modeling inhomogeneous deformation of hydrogel and write a UMAT subroutine and embed it into ABAQUS. Do dou know anyboy has tried to do this?

Jinxiong

Hi, Xuanhe, let me try to

Fri, 2018-01-05 00:42

In reply to Dear Shengqiang,

Hi, Xuanhe, let me try to partly answer your first question on the application of LCE. Compared with dielectric elastomer and hydrogels, LCE shows higher modulus but still can exhibit pretty large deformation, up to several hundreds percent in the record. This unique to other soft materials with relatively low modulus. Actuators made of high modulus materials thus can give higher output forces, which is needed for many applications.

JX

Thanks, Lihua. I also knew

Thu, 2018-01-04 23:45

In reply to Model of LCE

Thanks, Lihua. I also knew your eariler efforts in modelling LCE with Yongzhong. I should have cited those papers in the review. 

Model of LCE

Thu, 2018-01-04 23:41

In reply to Model of LCE

Thanks, Zhigang. In our paper, we model the free energy of LCEs as the combination of an elastic energy and a phenomenological Landau phase transition energy. The LCEs are modeled as completely 'soft', i.e. the director rotates following the mechanical deformation. However, in reality, researchers observe 'semi-soft' behavior, i.e., large enough energy is needed to rotate the director. 

There are indeed efforts to model the 'semi-soft' behavior, and to better understand the mechanics of polydomain-monodomain transition. Other than that, it is needed to develop a physical model for the phase transition energy, to take into account in the model the fabrication history and rate-dependence (ref 26) of LCEs. Modeling inhomogeneous deformation of LCEs is also interesting, such as all kinds of actuations, instability and fracture.

Model of LCE

Thu, 2018-01-04 18:05

In reply to 3D constitutive model

Lihua, thank you for the link to your paper. What is needed for modeling beyond your paper?

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