Photoinduced Plasticity in Cross-Linked Polymers

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Science  10 Jun 2005:
Vol. 308, Issue 5728, pp. 1615-1617
DOI: 10.1126/science.1110505


Chemically cross-linked polymers are inherently limited by stresses that are introduced by post-gelation volume changes during polymerization. It is also difficult to change a cross-linked polymer's shape without a corresponding loss of material properties or substantial stress development. We demonstrate a cross-linked polymer that, upon exposure to light, exhibits stress and/or strain relaxation without any concomitant change in material properties. This result is achieved by introducing radicals via photocleavage of residual photoinitiator in the polymer matrix, which then diffuse via addition-fragmentation chain transfer of midchain functional groups. These processes lead to photoinduced plasticity, actuation, and equilibrium shape changes without residual stress. Such polymeric materials are critical to the development of microdevices, biomaterials, and polymeric coatings.

Cross-linked, gelled polymers have an “infinite” molecular weight and are described as thermosets, implying a network that cannot be melted or molded (1). This description is true for most chemically cross-linked polymers; however, several cross-linked networks are known to undergo bond cleavage or depolymerization at high temperatures or under various chemical or other treatments (2). Although such treatments are useful for recycling purposes, there is an associated degradation in the mechanical properties of the polymers. “Crack-healing” networks, such as those that use groups in the polymer backbone able to undergo thermoreversible Diels-Alder reactions (3), are able to relieve stress without mechanical degradation. However, this reaction must be performed at elevated temperatures, making it unsuitable in thermally sensitive applications such as dental composites. Internal stress buildup during polymerization is typical when shrinkage occurs. This stress decreases the ultimate mechanical properties of the cured polymer, which is highly detrimental in fields such as polymeric coatings, fiber-reinforced composites, and dental materials, or it may introduce birefringence, unwanted in optical materials. Additionally, given that the equilibrium shape of conventional cross-linked polymers is defined by the shape at gelation, stress relief would enable a material to be “molded” and subsequently destressed, allowing for arbitrary equilibrium shapes to be attained after cure.

We describe a covalently cross-linked network that is able to undergo photomediated, reversible cleavage of its backbone to allow chain rearrangement for rapid stress relief at ambient conditions without mechanical property degradation. The key to this reversible backbone cleavage is addition-fragmentation chain transfer. Reaction diffusion of radicals through the cross-linked matrix occurs initially by reaction of a radical with an in-chain functionality, forming an intermediate, which in turn fragments, reforming the initial functionality and radical. Allyl sulfides have been used as efficient addition-fragmentation chain transfer agents (46). This addition-fragmentation process alters the topology of the network, but the polymer chemistry and network connectivity remain unchanged. In the absence of radical termination events or other side reactions, the number of allyl sulfide groups, and hence network strands, remains unchanged (Scheme 1), although relaxation of the stresses in each bond is facilitated by the alternating cleavage and reformation reactions.

Scheme 1.

Reaction mechanism for chain transfer within the polymer backbone.

The monomers used to produce the networks are shown in Scheme 2. The base network studied was formed from a stoichiometric mixture of pentaerythritol tetra(3-mercaptopropionate) (PETMP) and triethyleneglycol divinylether (TEGDVE), which produces a rubbery network with a glass transition temperature (Tg) of about –25°C. This monomer system was modified by the addition of varying concentrations of the ring-opening monomer 2-methyl-7-methylene-1,5-dithiacyclooctane (MDTO) (7) as a comonomer. Addition of a stoichiometric amount of 1,6-hexanedithiol (HDT) and TEGDVE to the tetrathiol/divinylether was used to produce an alternative traditional network with a lower cross-link density and rubbery modulus. Typically, thiol-ene polymerizations follow a step-growth radical mechanism (8) with alternating thiol and ene monomer units. Addition of the ring-opening monomer does not alter the stoichiometry of this network, because the carbon-centered vinyl ether radical only abstracts a hydrogen from the thiol and the ring-opening monomer propagates via a sulfur-centered radical (7, 911). Thus, whereas ring-opening monomers are more typically used to reduce the shrinkage due to polymerization (12), we used MDTO to conveniently introduce the reversibly cleavable allyl sulfide functionality regularly throughout the polymer backbone.

Scheme 2.

Monomers used to produce the networks.

Confirmation of the cross-linked nature of fully cured specimens was obtained by dynamic mechanical analysis, in which a specimen is sinusoidally deformed during a temperature ramp to determine the real (storage) and imaginary (loss) components of the modulus. The value of Tg was systematically reduced as the concentration of ring-opening monomer was increased [Tg = –34°C for 75 weight percent (wt %) MDTO] as a result of the reduced cross-link density; however, all specimens displayed a rubbery plateau modulus typical of cross-linked polymers. All the specimens were clearly within the rubbery regime at ambient temperature. Additionally, although all the specimens were readily swollen with common organic solvents, none were soluble—again a defining feature of cross-linked polymers.

Strain profiles of the specimens during and after irradiation are presented in Fig. 1. During irradiation, homolytic photolysis of residual photoinitiator produces radicals in the specimens. Diffusion of these radicals occurs via addition-fragmentation chain transfer through the allyl sulfide functionalities. As a result, the polymer backbone is repeatedly cleaved, stress is alleviated, and the backbone is reformed in a less stressed conformation. Neither of the neat thiol-ene specimens contain any groups in the backbone capable of this cleavage; thus, the small amount of deformation that occurs upon irradiation is primarily due to the minimal heating induced by the light exposure. The sequential cleavage and reformation in the remaining networks repeats as long as radicals are produced, because the addition-fragmentation chain transfer reaction does not consume functional groups. The ductility or malleability of these cross-linked networks is limited only by the radical generation and termination reactions.

Fig. 1.

Strain profiles of tetrathiol/divinyl ether specimens with varying concentrations of MDTO (solid line, 0 wt %; dashed line, 25 wt %; dotted line, 50 wt %; dashed-dotted line, 75 wt %). The specimens were under tensile stress of ∼105 Pa throughout the experiment and were irradiated (320 to 500 nm, 30 mW cm–2) from t = 2 min to t = 62 min.

The variation in the degree of strain for specimens with differing MDTO concentrations (Fig. 1) is due to the increasing concentration of allyl sulfide groups in the network strands. At elevated MDTO concentrations, the concentration of allyl sulfide groups in the network increases, and there is a small, corresponding increase in the length of the average network strand and a decrease in the cross-link density (Table 1). Consequently, the rate at which strands are broken and stress is relieved is substantially higher at elevated MDTO concentrations.

Table 1.

Tensile moduli of specimens before and after experiments performed in Fig. 3A.

MDTO (wt%) Ratio of cross-links to allyl sulfide groups Modulus before extension and irradiation (MPa) Modulus after extension and irradiation (MPa)
0 1: 0 11.5 11.8
25 1.17: 1 7.33 7.72
50 0.390: 1 4.58 5.17
75 0.130: 1 2.38 2.92
0 (75 wt % dithiol/divinyl ether) 1: 0 3.21 3.38

The reversible strain in the neat thiol-ene polymer networks (Fig. 1) was attributed to a small thermal expansion effect due to heating during irradiation. Temperature rise measurements indicated an increase in temperature of ∼5°C during the first minute; the temperature then remained at 5°C above room temperature for the remainder of the irradiation. The measured temperature rise profile was unaffected by the specimen composition. The apparent strain recovery of the neat thiol-ene polymer networks upon cessation of irradiation was due to the specimens cooling in the dark.

In the absence of radiation, upon application of stress, each specimen underwent a degree of strain that was dependent on the modulus and therefore the concentration of MDTO in the specimen. When the stress was released, the specimen returned to its original length. This situation was altered when the specimens were irradiated during the stress application. The reversible and irreversible components of the strain during the application of stress and irradiation are readily observed in Fig. 2. The strain is completely reversible when a cyclic stress is applied without irradiation (Fig. 2A); however, an irreversible component is clearly observed when irradiation is applied at the same time as the stress (Fig. 2B) for all samples that contain MDTO. As shown in Fig. 2 for both the lower and higher modulus control materials that do not contain MDTO, no alteration of the equilibrium strain is possible in the absence of the allyl sulfide linkages.

Fig. 2.

Strain/recovery profiles of tetrathiol/divinyl ether specimens with varying concentrations of MDTO (solid line, 0 wt %; dashed line, 25 wt %; dotted line, 50 wt %; dashed-dotted line, 75 wt %) and a 25 wt % tetrathiol/divinyl ether–75 wt% dithiol/divinyl ether specimen (short-dashed line). The stress was alternated between 0 and 105 Pa. (A) Without irradiation. (B) Irradiation (320 to 500 nm, 30 mW cm–2) during stress application after the first load cycle (irradiation indicated by the shaded areas).

Stress relaxation experiments were performed by deforming the specimens to a certain strain and subsequently irradiating them. The results (Fig. 3A) directly show the relaxation of stress in the specimens during irradiation. A small decrease in the stress experienced in the unmodified thiol-ene specimens was again attributed to thermal expansion due to heating from the lamp; however, the specimens containing the allyl sulfide groups all show substantial stress relaxation. The stress relaxation is accompanied by a variation in the specimen dimensions when the specimens are removed from the instrument clamps (greater than 1 mm lengthwise for the 75 wt% MDTO specimen in Fig. 3A), demonstrating the induced plasticity via the introduction of radicals in the system.

Fig. 3.

Stress versus time for four MDTO concentrations (solid line, 0 wt %; dashed line, 25 wt %; dotted line, 50 wt %; dashed-dotted line, 75 wt %). (A) Constant strain (irradiation started at t = 330 s). (B) Constant initial stress (offset to align the start of irradiation at t = 0, irradiation stopped at 900 s). The specimens were irradiated at 320 to 500 nm, 20 mW cm–2.

A clearer picture of the effect of the MDTO concentration on the rate and degree of stress relaxation is seen in Fig. 3B. At a constant applied stress, both the rate and degree of stress relaxation increase with MDTO concentration. We again attribute this response to an increased probability of addition-fragmentation chain transfer groups in the network strands. After cessation of irradiation, the stress actually rises slightly because of specimen shrinkage upon cooling.

If the stress relaxation had been the result of photodegradation, the results in Figs. 1 and 3 would have been similar. It is possible to determine whether the network is simply undergoing photodegradation during irradiation by measuring the modulus of the material before and after irradiation. The elastic moduli (determined in tension) of the specimens before and after the irradiation experiments shown in Fig. 3A are presented in Table 1. The slight increase in the modulus after irradiation clearly indicates that photodegradation is not responsible for the stress relaxation. Additionally, the results in Fig. 2 would appear different if photodegradation were responsible for the observed behavior, because the magnitude of the reversible strain would vary with irradiation; however, such variation is not observed. Fourier transform infrared spectroscopy analysis of the allyl sulfide linkages in the polymer also shows no measurable net degradation of these groups as a result of irradiation.

One of the many interesting applications of this phenomenon is the deliberate introduction of stress gradients in a cross-linked material. Irradiation of an optically dense specimen under stress leads to the release of stress only on the exposed side. As a result, once the stress is released, the specimen warps away from the direction of irradiation (Fig. 4). This curvature is released by irradiation of the previously unexposed side. As a result, a shape-change or actuation phenomenon may be effected without the typical increase in temperature required by shape-memory polymers (13, 14).

Fig. 4.

Specimens with stress gradients through their thickness on a 2 mm by 2 mm grid. Specimens from left to right: 0, 25, 50, and 75 wt % MDTO. The direction of irradiation (365 nm, 20 mW cm–2 for 15 s) used for the creation of the stress gradient for each specimen was from left to right.

Although our study involved model rubbery networks, this process may also be applied to a vast array of other applications for which the control or elimination of stress is critical. Specifically, it is feasible to produce low residual stress in high-Tg materials by incorporating this relaxation process throughout the curing reaction. Groups capable of undergoing these chain transfer reactions need not be introduced via the copolymerization of a ring-opening monomer as performed in this work. Rather, cross-linking monomers (e.g., diacrylates, dimethacrylates) can be envisaged that contain linear addition-fragmentation functionalities incorporated in the monomer structure between the multiple polymerizable functionalities. Thus, networks with high cross-link density and high Tg that contain the relevant functionalities may be readily synthesized, with the concomitant benefit of reduced stress resulting from the polymerization.

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