Functional siloxanes with photo-activated, simultaneous chain extension and crosslinking for lithography-based 3D printing
Graphical abstract
Introduction
Additive manufacturing (AM) or 3D printing (3DP) enables the creation products featuring complex geometries that are not attainable with traditional manufacturing methods, such as injection molding and machining [1]. Vat photopolymerization (VPP), also referred to as stereolithography, is widely considered the most accurate and highest-resolution AM technique [2]. VPP creates solid objects upon irradiation of an ultraviolet (UV) light-activated, liquid polymer, or photopolymer, with patterned light and in a layer-by-layer fashion. This layerwise, selective curing converts the photopolymer into a three-dimensional (3D), insoluble network. Photopolymers typically contain UV-reactive functional groups that either excite directly upon UV irradiation at appropriate wavelengths, or more commonly, a low concentration of photoinitiator forms a propagating intermediate upon UV irradiation. Photoinitiators for AM are selected based on the particular printing conditions, generate a variety of active centers including free radicals, cations, or bases (anions), and are generally identical to those employed in polymer synthesis [3]. After early AM discoveries in the 1980s, the recent expiration of several key patents [4] catalyzed a renaissance of fundamental and applied research, as well as the founding of several AM-based startup companies focused both on production-ready capabilities and inexpensive, consumer-grade printers. AM enjoys applications in self-healing materials [5,6], biomaterials [[7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]], high-strength metamaterials [20,21], microfluidics [[22], [23], [24]], and AM-derived ceramics [25].
VPP enjoys an extensive catalog of commercially available photopolymers [6,26,27]. Originally designed as UV-curable coatings, many of these photopolymers produce high modulus and often mechanically inferior objects as a result of high crosslink density due to low molecular weight (MW), often monomeric, precursors. Though appropriate for rapid prototyping, objects printed from these photopolymers suffer from brittleness, low strain at break, and sensitivity to light and humidity [25,27]. According to the 2016 Wohler's report (Appendix E: Material Properties), only 7% of commercially available photopolymers for vat photopolymerization possess a photocured tensile modulus of ≤20 MPa [27]. A low modulus is one characteristic of elastomers or rubbers, which enjoy a wide variety of applications including tires, athletic wear, seals, and damping elements. Most elastomers possess a low tensile modulus and high tensile strain at break (i.e. > 100%) [6,28]. An examination of rubber elasticity theory and photopolymer viscosity requirements offers some explanation for this disproportionately small offering of commercially available photopolymers for the production of elastomeric objects.
As shown in Equation (1), there is an inverse relationship between molecular weight between crosslinks (Mc), or the statistical average distance between network junctions, and plateau shear modulus as measured in the gel state () [29]. These two parameters are related by ρ (density), R (universal gas constant), and T (temperature).
The affine network model captures this theory of rubber elasticity and states that the macroscopic network deformation is equal to the sum of the deformations of the individual network strands [29]. The elastic restoring force upon macroscopic deformation/elongation is entropic in nature and results in these elongated network strands regaining degrees of freedom and their Gaussian distribution of random coil conformations. In the context of vat photopolymerization, a photocured elastomeric object with high Mc typically requires a high MW photopolymer, ensuring long polymer chains that can uncoil upon loading. These higher MW precursors also provide a deleterious, concomitant increase in pre-cure viscosity, as zero-shear viscosity (η0) ∝ MW1 below the entanglement molecular weight (Me) and η0 ∝ MW3.4 above Me for non-associating, linear polymers [30]. Hence, a high viscosity photopolymer will require a comparatively long time to recoat a previously printed layer, versus a low viscosity photopolymer, upon the build stage lowering one-layer thickness at a time into the photopolymer vat. A practical upper viscosity target for photopolymer processing with VPP is 5 Pa s [26]. Knowledge of MW and MW distribution is highly relevant to photopolymer processing in VPP. In previous work, a variety of techniques including supercritical fluid chromatography and size exclusion chromatography with appropriately sized columns facilitated determination of MW and MW distribution as relevant to the AM process [16,18,31,32].
In top-down vat photopolymerization, processing of higher molecular weight photopolymers is facilitated in many ways. First, a ‘recoat blade’ smooths the photopolymer surface to ensure subsequent printed layers are uniformly flat [2]. The ‘recoat blade’ is held one-layer thickness above the previously printed layer to ensure the uncured photopolymer is uniform prior to UV irradiation. Second, reactive diluents (e.g. monomers) are admixed and reduce the viscosity of higher MW precursors [33]. However, these monomers are often classified as volatile and toxic, and the comonomer alters properties of the printed network. Third, unreactive diluents (e.g. solvents) reduce photopolymer viscosity but do not participate in the crosslinking reaction. Similar issues of volatility and toxicity exist, while solvent removal from the printed objects remains challenging [34]. Finally, heating of the vat remains the simplest method, but promotes thermal polymerization, especially for acrylate- and methacrylate-containing systems [35].
The thiol-ene reaction is classified as a click reaction due to rapid rates and high yield, absence of by-products, and insensitivity to water or oxygen, and occurs under mild reaction conditions [36]. It also can be self-initiated with 254 nm UV irradiation, demonstrating a particular utility for photoinitiator-free systems [36]. Due to step-growth polymerization kinetics, systems employing thiol-ene chemistry for crosslinking produce more homogenous networks compared to conventional free radical homopolymerization (FRP), as higher overall conversion is reached prior to vitrification [37]. Conversely, FRP is widely employed for network formation in vat photopolymerization, due to the fast polymerization rates and low cost of acrylates and methacrylates. However, unlike the thiol-ene reaction, FRP proceeds in a chain-growth manner, often producing highly heterogeneous networks and significant polymerization shrinkage. Bowman et al. investigated the photocuring kinetics of combinations of thiols and acrylates, finding the acrylate propagation constant to be ∼1.5 times the rate for hydrogen abstraction from the thiol [38]. This was due to an increase in chain transfer events as each individual polyacrylate chain possesses a lower degree of polymerization than would normally occur in the absence of thiols [37,38].
Thiol-ene chemistry enjoys abundant use in vat photopolymerization applications [23,24,39,40]. Furthermore, Cramer and Bowman demonstrate that small additions of monomeric dithiols, e.g. 1,6-hexanedithiol, to low MW diacrylates, e.g. 200 g/mol poly(ethylene glycol) (PEG) diacrylate, enable networks with highly tunable glass transition temperatures and plateau moduli [38]. However, to the best of our knowledge, this strategy remains unprecedented for polymeric systems or vat photopolymerization. In this work, a polymeric system that employs simultaneous MW growth via the radical-mediated, step-growth thiol-ene reaction and FRP via acrylamide homopolymerization is applied to a VPP AM process. This system achieves low viscosity before printing, through use of relatively low MW precursors, and achieves relatively high Mc after photocuring. This approach leverages the extremely low glass transition temperature (−123 °C) [41,42] and nonpolar nature of poly(dimethylsiloxane)s and avoids the use of reactive diluents, heat, and solvent (beyond a low level of solvent that is required to dissolve a photoinitiator, e.g. 1 wt %, oligomer basis). Fast gelation times (<5 s) and high gel fractions (>90%) are confirmed. Finally, mechanical properties of the photocured films and 3D-printed objects are comparable to photocured, higher MW PDMS telechelic acrylamide.
Section snippets
Materials
2,2-Dimethoxy-2-phenylacetophenone (DMPA, 99%), diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO, 97%), 3-mercaptopropionic acid (≥99%), acryloyl chloride, (≥97%), potassium hydroxide solution (1 N in water), p-toluenesulfonic acid monohydrate (ACS grade, ≥ 98.5%), and magnesium sulfate (≥98.0%) were purchased from Sigma Aldrich and used as received. Bis(3-aminopropyl)-terminated poly(dimethyl siloxane) (PDMS-NH2) was purchased from Sigma Aldrich (Mn ∼ 2500 and ∼27,000 g/mol), Gelest
Results and discussion
This work demonstrates the synthesis, processing, and characterization of a novel, low viscosity photopolymer for vat photopolymerization that possesses the lower modulus of a photocured, higher MW precursor. This approach employs a photopolymer that contains thiol- and acrylamide-functional telechelic oligomers. When irradiated with UV light in the presence of a photoinitiator, two radical-mediated processes occur: thiol-ene coupling, which represents a linear chain extension event, and
Conclusions
Synthesis, characterization, and VPP AM is reported for a low viscosity PDMS-based photopolymer that provides the properties of a photocured, higher MW precursor. This is accomplished without the use of reactive diluents (i.e. monomers), unreactive diluents (i.e. solvents), or heat. Photorheology enabled determination of curing kinetics and gel-state moduli (). In this system, thiols serve a dual purpose, both undergoing thiol-ene coupling with acrylamide moieties to lengthen network strands
Conflicts of interest
The authors declare no conflicts of interest.
Acknowledgements
The authors would like to acknowledge Wacker Chemie for their gracious donation of a 12 kg/mol bis(3-aminopropyl)-terminated PDMS and Dr. Jamie Messman at the Department of Energy's Kansas City National Security Campus (managed by Honeywell) for funding. The authors would also like to thank Jonathan E. Seppala (NIST) for helpful discussions.
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