Sefton, Joseph
(2024)
Synthesis of ω-vinyl methacrylic polymers via in situ catalytic chain transfer polymerisation for photopolymer additive manufacturing applications.
PhD thesis, University of Nottingham.
Abstract
Photopolymer-based additive manufacture (PAM) typically relies on the use of crosslinking acrylate monomers, with very little existing literature on PAM of thermoplastics. New materials and understanding of the polymerisation that occurs in PAM is key to enabling further advancement toward PAM of thermoplastics. In this work, ω-vinyl polymers were utilised as chain transfer agents (CTAs) in digital light projection (DLP) of thermoplastic parts in order to gain an understanding of the empirical polymer characteristics required for successful thermoplastic PAM.
ω-vinyl polymers were synthesised utilising in situ catalytic chain transfer polymerisation (CCTP). The catalyst dibromo bis(diphenylglyoximate)cobalt(II) in solution with 2-methyltetrahydrofuran was found to provide excellent control over the degree of polymerisation (DPn) of a range of methacrylate monomers bearing alkyl, aryl and hydrophilic pendant groups. As the products of CCTP were of low volatility and high solubility in traditional antisolvents, purification of crude CCTP reaction mixtures was investigated. A solution of poly(methyl methacrylate) (pMMA) was effectively recrystallised from methanol (MeOH) at -78 °C. The recrystallised polymer fraction contained virtually no residual monomer or solvent by 1H nuclear magnetic resonance spectroscopy (1H NMR), Ɖ < 1.2 by gel permeation chromatography (GPC), and 90% less residual cobalt catalyst than the crude solution. As the DPn¬ of this polymer was 25 by 1H NMR analysis, this was termed pMMA25.
pMMA25 was copolymerised with the acrylate monomers methyl acrylate (MA), isobornyl acrylate (IBA) and lauryl acrylate (LA) in order to evaluate it’s efficacy as a CTA. MMA dimer (pMMA2) was also utilised as a pure ω-vinyl polymer of low Mwt for comparison. Thermal copolymerisation of either pMMA2 or pMMA25 with MA resulted in apparent chain transfer constants (Csapp) of 0.012 and < 0.001 respectively, based on the Mp value of the resultant copolymers. This was rationalised by the grafting of pMMAx species to pMA chains competitive with AFCT due to sterics of the methyl pendant groups on each reactant. Thermal copolymerisation of pMMA2 or pMMA25 with IBA yielded Csapp = 0.116 and 0.205 respectively, confirming sterically bulkier acrylate pendant groups promoted AFCT. Photo-copolymerisation of pMMA2 or pMMA25 with MA, IBA or LA yielded Csapp values of 0.525 – 0.833 and 0.300 – 0.531 respectively. These values were significantly higher than for thermal copolymerisation, rationalised by performing the reaction at a lower temperature and progressing to a much lower conversion (ca. 5 – 60% conversion compared to ≥ 95% in thermal polymerisations). These results displayed that pMMA2 and pMMA25¬ were anticipated to act as CTAs in PAM.
Lithographic PAM was undertaken utilising a commercial DLP apparatus. A resin of IBA and 2.5 wt% photoinitiator was used as a baseline thermoplastic resin, and a minimum per-layer exposure time (ET) of 6 s was required to print a defect-free cuboid of pIBA. The resulting pIBA print was insoluble, attributed to light crosslinking of high Mwt polymer chains. Modifying this IBA resin with varying amounts of MMA monomer, pMMA2 or pMMA25 resulted in increased minimum ETs required to yield a successful print in all cases. This was thought to result from a slower overall rate of polymerisation due to either a) the lower kp of MMA monomer, or b) AFCT retarding propagation where pMMA2/pMMA25 were used. The polymer produced from these resins was solvent-soluble, and GPC analysis revealed that the polymer Mwt typically decreased upon increasing concentration of any methacrylic additive in DLP. Below a plateau of ca. 120000 g mol-1 no successful prints were observed, which was taken to mean that polymer below this Mwt was prone to re-dissolve into the surrounding liquid resin during PAM. Csapp values were calculated for MMA, pMMA2 and pMMA25 to be 0.006, 0.247 and 0.108 respectively, indicating that all additives resulted in some degree of Mwt reduction with increasing concentration. Resins which failed to print were predicted to produce polymer < 120000 g mol-1 based upon the Csapp values and the resin composition. Conversion in successful prints was relatively constant at 80 – 88%, and a minimum conversion is also proposed to be required for successful thermoplastic PAM, though conversion and Mwt did not correlate in this study.
These results indicate that PAM of thermoplastics is only attainable for systems which meet threshold criteria for Mwt and conversion of the liquid resin to polymer. ω-vinyl polymers were effectively utilised as diagnostic additives in order to modulate the Mwt of PAM-produced parts, and build an understanding of the behaviour of the liquid resins within the DLP apparatus. ω-vinyl polymers may find future application in the modulation of crosslinking networks via AFCT and the in situ synthesis of block copolymers in PAM.
| Item Type: |
Thesis (University of Nottingham only)
(PhD)
|
| Supervisors: |
Irvine, Derek
Wildman, Ricky |
| Keywords: |
polymer chemistry, polymers, chain transfer, additive manfuacturing, photopolymerisation, copolymerisation, thermoplastic, catalytic chain transfer polymerisation |
| Subjects: |
T Technology > TS Manufactures |
| Faculties/Schools: |
UK Campuses > Faculty of Engineering > Department of Mechanical, Materials and Manufacturing Engineering |
| Item ID: |
77907 |
| Depositing User: |
Sefton, Joseph
|
| Date Deposited: |
18 Jul 2024 04:40 |
| Last Modified: |
18 Jul 2024 04:40 |
| URI: |
https://eprints.nottingham.ac.uk/id/eprint/77907 |
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