Sulfur-rich Polymers with Heating/UV Light-responsive Shape Memory and Temperature-modulated Self-healing

Synthesis of an inverse vulcanised polymer with bended mono- and di-olefins, adjusting the ratio of which allows control of the shape memory response and self-healing temperatures.


Chemicals and materials
Dicyclopentadiene,99.0%; Zinc diethyldithiocarbamate 99.0%; a-pinene, AR; Hexadecene, AR; 1,3-Divinylbenzene, 99.0%; Xylene,90.0%, purchased from Energy chemical, used directly without any further purification. Sulfur ,99%; Styrene , AR, 1 wt%) were added into a round bottom flask, the mixture was then stirred at 140 °C until a homogeneous solution formed. Then styrene (0.6 g) and DCPD (0.9 g) were added and the mixture was stirred at the same temperature for 30 min. The resulted polymer poured into silicone mould while it was still hot and cured in an preheated oven at 130 °C for 12 h. After cooling down to room temperature, the resulted polymer was taken out the oven, washed with distilled water, and dried in the air as the final product. Elemental analysis(%): C: 33.66,H: 3.12,S: 62.27. yield: 3.98 g (> 99%).
Poly (S-St): Elemental sulfur (2.5 g) and dihexyl zinc dithioformate (25 mg, 1 wt%) were added into a round bottom flask, the mixture was then stirred at 140 °C until a homogeneous solution formed. Then styrene (1.5 g) were added and the mixture was stirred at the same temperature for 30 min. The resulted polymer poured into silicone mould while it was still hot and cured in an preheated oven at 130 °C for 12 h. After cooling down to room temperature, the resulted polymer was taken out the oven, washed with distilled water, and dried in the air as the final product. Elemental analysis(%): C: 18.86,H: 1.76,S: 78.24. yield: 3.88 g (97%).
Poly (S-DCPD): Elemental sulfur (2.5 g) and dihexyl zinc dithioformate (25 mg, 1 wt%) were added into a round bottom flask, the mixture was then stirred at 140 °C until a homogeneous solution formed. Then DCPD (1.5 g) were added and the mixture was stirred at the same temperature for 30 min. The resulted polymer poured into silicone mould while it was still hot and cured in an preheated oven at 130 °C for 12 h. After cooling down to room temperature, the resulted polymer was taken out the oven, washed with distilled water, and dried in the air as the final product. Elemental analysis(%): C: 34.72,H: 3.20,S: 61.65. yield: 3.96 g (99%).
Poly (S-alpha-pinene-DCPD): Elemental sulfur (2.5 g) and dihexyl zinc dithioformate (25 mg, 1 wt%) were added into a round bottom flask, the mixture was then stirred at 140 °C until a homogeneous solution formed. Then alpha-pinene (0.9 g) and DCPD (0.6 g) were added and the mixture was stirred at the same temperature for 30 min. The resulted polymer poured into silicone mould while it was still hot and cured in an preheated oven at 130 °C for 12 h. After cooling down to room temperature, the resulted polymer was taken out the oven, washed with distilled water, and dried in the air as the final product.
Poly (S-Styrene-1, 3-Divinylbenzene): Elemental sulfur (2.5 g) and dihexyl zinc dithioformate (25 mg, 1 wt%) were added into a round bottom flask, the mixture was then stirred at 140 °C until a homogeneous solution formed. Then styrene (1.2 g) and 1,3-Divinylbenzene (0.3 g) were added and the mixture was stirred at the same temperature for 30 min. The resulted polymer poured into silicone mould while it was still hot and cured in an preheated oven at 130 °C for 12 h. After cooling down to room temperature, the resulted polymer was taken out the oven, washed with distilled water, and dried in the air as the final product.
Poly(S-Hexadecene-DCPD): Elemental sulfur (2.5 g) and dihexyl zinc dithioformate (25 mg, 1 wt%) were added into a round bottom flask, the mixture was then stirred at 140 °C until a homogeneous solution formed. Then Hexadecene (0.6 g) and DCPD (0.9 g) were added and the mixture was stirred at the same temperature for 30 min. The resulted polymer poured into silicone mould while it was still hot and cured in an preheated oven at 130 °C for 12 h. After cooling down to room temperature, the resulted polymer was taken out the oven, washed with distilled water, and dried in the air as the final product.

Characterization methods
1 H NMR analysis: use Brucher AM 600 M nuclear magnetic resonance instrument, produced by Brucher Company in the United States, use DMSO and DCl 3 as solvent, and the sample concentration is 1 g·L -1 . FTIR analysis: Use Digilab Merlin FTS 3000 FTIR from the United States. XRD analysis: Rigaku D/Max-2400 XRD ray scanner, scanning angle: 5°-90°. CHNS elemental analysis: it was determined by Elementar UNICUBE organic element Analyzer (EA) in Germany, DSC analysis: use NETZSCH TA differential scanning calorimeter, nitrogen atmosphere, heating rate 10°C/min, test temperature -50°C-150°C, sample weight 0.05 g. TGA analysis: use Seiko EXSTAR 7200 TGA/DTA instrument. Thermal scans were conducted on 3-5 mg samples under nitrogen atmosphere. All samples were heated from 25 °C to 800 °C at a standard rate of 10 °C min -1 . Mechanical test：Suzhou Shimadzu AGS-X mechanical tester was used to test the tensile property and Young's modulus, according to the national standard GB228-76, the standard sample is made, and the sample is tested between the testing machine.

Experimental for the H 2 S gas determination Hot press
Experimental process: Elemental sulfur (2.5 g) either with or without dihexyl zinc dithioformate (25 mg, 1 wt%) were added into a round bottom flask, the mixture was then stirred at 140 °C until a homogeneous solution formed. Then styrene (0.6 g) and DCPD (0.9 g) were added and the mixture, before sealing the flask with a rubber septum connected to an outlet tube. The outlet of the tube was placed to flow over silver nitrate paper, and the reaction continued at the same temperature for 30 min.
The generation of H 2 S gas is determined by observing the discoloration of the test paper.
The experimental results are shown in Figure 1 below.
Without catalyst) Figure S1 photos of the silver nitrate test paper before and after the reaction. 1 is the color of the test paper after the catalyst is added, and it can be seen that only less H 2 S gas is produced by the color of the test paper; 2 is the color of the test paper after the reaction without catalyst. Figure S2. 1

H NMR of poly(S-St-DCPD)2 at different reaction time
The reaction was carried out under the standard conditions in current research.
As shown in Figure S2, both Styrene and DCPD were fully converted into the polymer in 3 h reaction time. However, there is no identical peaks, normally should be at 6.25-6.60 ppm, for CPD observed during the entire reaction time from 10 min to 3 h, and even in a prolonged reaction time of 6 and 12 h. This observation indicated that there was little decomposition of DCPD into CPD under the current reaction conditions.  Compared three different polymers and their reaction at different reaction time, there were no CPD observed for all the polymerization with S8 involved. The decomposition of DCPD into CPD was only observed in the DCPD self-polymerization in 12 h, with around 1.5% CPD peaks in the NMR at 6.25 -6.50 ppm appeared.

Hot press
The obtained polymer at 200 °C, 10 MPa for half an hour, and finally the polymer film.

Shape memory behavior based on thermoelasticity [1, 2]
The polymer film is cut into strips and folded into a u-shape at 40°C. Then, the ushaped sample was cooled at -40 °C for 90 minutes to fix its shape. Subsequently, observe whether the u-shaped sample can be restored to its original shape at different temperatures.

Based on light-induced shape memory behavior [2, 3]
The polymer film is folded into a u-shape, irradiated under ultraviolet rays, and then cooled at -40 °C for 90 minutes to fix the shape. Then put the sample in the oven and record the recovery process.

Gel amount
q 0 is the quality of the filter paper soxhlet backflow loss, q 1 is the quality of the iron wire. q 2 is the quality of the iron wire and the sample package, q 3 is the quality of the sample package, and q 4 is the mass of the sample package after Soxhlet extraction and drying in an oven at 50 °C.

Sample Preparation
The sample was cut into small slices about 1mm in size within 5mm×5mm. The mass of the sample was between 2.00 g-3.00 g, xylene used as the extractant.

Measurement process
The sample was cut into small pieces, and the filter papers and iron wire were dried in the oven for 6 hours (50°C) prior to use, then cooled down in another drying oven for later use. After accurately weigh out the sample (0.20000 g -0.30000 g), the sample was wrapped with filter paper, subsequently tightened with iron wire; the mass of the obtained sample package was accurately weighed out. The sample package was then Soxhlet extracted with xylene for 24 hours at 200 °C. After cooled down to room temperature, the sample package was dried in a vacuum oven overnight. The mass of the sample, sample pack, filter paper, and iron wire were measured after Soxhlet extraction, respectively.

Characterization
It can be seen from the above table that as the ratio of diolefin increases, the gel amount of the polymer will also increase; the increase in the degree of crosslinking will result in an increase in the T g of polymers with a high ratio of diolefins. This is also the reason why reverse vulcanization polymers with different response temperatures can be produced.