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Insight into the thermal degradation behaviour and degradation products of cross-linked polydimethylsiloxanes

Ogliani, Elisa; Yu, Liyun; Hvilsted, Søren; Skov, Anne Ladegaard

Publication date:2017

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Citation (APA):Ogliani, E., Yu, L., Hvilsted, S., & Skov, A. L. (2017). Insight into the thermal degradation behaviour anddegradation products of cross-linked polydimethylsiloxanes. Poster session presented at 7th InternationalConference on Electromechanically Active Polymer (EAP) Transducers & Artificial Muscles (EuroEAP 2017),Cartagena, Spain.

https://orbit.dtu.dk/en/publications/cf40f207-a633-4c9c-8cac-927d8b827165

Insight into the thermal degradation behaviour and degradation products of cross-linked polydimethylsiloxanes

Materials The samples were prepared by a hydrosilylation reaction (Figure 1) between the linear vinyl terminated polydimethylsiloxane DMS-V35 (Mw= 49500 g/mol) and the hydride-functional cross-linker HMS-301 (Mw= 1950 g/mol), catalyzed by a platinum catalyst (cyclovinylmethyl-siloxane complex, 511). The uniform mixture was cast on the frame with a gap of 1 mm and fully cured at 80 ˚C. The stoichiometric imbalances (r= ratio between the reactive groups of the PDMS and cross-linker) of the films were 1, 1.5 and 2.

References 1. F. B. Madsen et al., “The current state of silicone-based dielectric elastomers transducers”, Macromol. Rapid Commun. 2016 DOI: 10.1002/marc.201500576. 2. G. Camino et al., “Polydimethylsiloxane thermal degradation Part. 1 Kinetic aspects”, Polymer 42 (2001) 2395-2402. 3. G. Camino et al., “Polydimethylsiloxane thermal degradation Part. 1 Kinetic aspects”, Polymer 42 (2001) 2395-2402.

Elisa Ogliani (1), Liyun Yu (1), Søren Hvilsted (1), Anne Ladegaard Skov* (1)

(1) Technical University of Denmark, Department of Chemical and Biochemical Engineering, Kgs Lyngby, Denmark

Poster ID:

2.3.7

EuroEAP 2017 International conference on Electromechanically Active Polymer (EAP) transducers & artificial muscles

Cartagena, 6-7 June 2017

Contact e-mail: al@kt.dtu.dk

Figure 1 – Preparation of the silicone elastomers.1

Abstract Silicone elastomers are extensively used as dielectric elastomers transducers[1], due to their unique features such as a remarkable thermal stability. This resistance to high temperatures is fundamental for the reliability and the performance of silicone-based devices and it originates from the high bond dissociation energy of the siloxane bond and the pronounced flexibility of the chain backbone. Keeping in mind the vast majority of work done so far on the degradation of silicones[2, 3], the goal of the present work is to achieve a deeper insight into the thermal degradation mechanism of commercial silicone

elastomers with the main aim of translating it into the complex, coupled thermal and electrical breakdown processes that dielectric elastomers undergo. Furthermore, this study may pave the way towards designing more reliable dielectric elastomers transducers.

Investigation of the thermal stability of commercial silicone elastomers:

setting up a standard experimental procedure

• Destructive TGA of the pristine samples Inspection of the degradation performance

• Extraction of the samples in heptane Removal of the non-bonded PDMS chains

• Thermal treatment of the samples: isothermal TGA

Analysis of the degradation products

Sample T at Max DTG (˚C) Additional

degradation stage (˚C) Residual weight (%)

V35 540 -- 0,10

r = 1 481 550 0,47

r = 1,5 565 499 1,79

r = 2 519 591 1,48

Destructive TGA of the pristine samples Preliminary thermogravimetric measurements were carried out on the pristine samples with ratio 1, 1.5 and 2 (Figure 2); the pure non-crosslinked V35 was tested as the reference. Experiments were performed upon N2 flow with a heating rate of 10 ˚C/min, from room temperature up to 700 ˚C; the weight of the loaded samples was between 15 and 20 mg.

Table 1. Thermal degradation properties of the samples.

Figure 2 – TG and DTG curves of the different samples.

Extraction in heptane Two consecutive extractions in heptane (24h each) were carried out to ensure the removal of the non-bonded PDMS chains from the polymer network (Figure 3).

Figure 3 – Weight% lost by the samples after the extractions in heptane .

Size Exclusion Chromatography of the liquid of extraction confirmed that non-reacted DMS V35 was extracted from the elastomers.

Destructive TGA of the extracted samples TGA measurements were repeated on the samples extracted in heptane with the same procedure previously reported (Figure 4a, b): one and well-defined degradation stage was detected for all the elastomers (T at Max DTG ~ 500 ˚C).

a) b)

Figure 4 – TG and DTG curves of the different samples after the extraction (a) and comparison of the TG

curves before and after the extraction (b).

Conclusions This work is meant to be relevant in a design guide towards reliable and robust dielectric elastomers. As a matter of fact, the presented experimental procedure is effective in investigating the thermal stability of silicone elastomers and elucidating the thermal degradation mechanism through the systematic analysis of the collected degradation products.

Thermal treatment of the samples After the systematic inspection of the general thermogravimetric behaviour of the elastomers, isothermal TGA analysis was carried out for 12 hours (N2 flow, sample weight ~ 100 mg) at a constant temperature of respectively 300 ˚C (Figure 5a) and 400 ˚C (Figure 5b).

Figure 5 – Plot of weight% against time for the sample r=1 thermally treated at 300 ˚C (a) and 400 ˚C (b) before and after the extraction.

a) b)

This last step is fundamental in order to achieve a complete overview of the thermal stability of the samples. Furthermore, the volatile and soluble degradation products are collected respectively through a cold finger connected to the TGA and a further extraction in heptane. The collected degradation products are then analyzed through SEC and NMR.

• Destructive TGA of the samples after the extraction in heptane Is there any difference between the degradation profiles before and after the extraction?