(517f) Modification of Linear Prepolymers to Control Polymerization-Induced Phase Separation in a Free-Radical Photo-Polymerization

Multi-phase, heterogeneous polymer networks have the potential to alleviate the stress experienced during a polymerization reaction[1-3]. One approach to network heterogeneity is through polymerization induced phase separation (PIPS). Here we present a method to adjust and expand the limits at which PIPS leads to effective stress reduction by varying the molecular weight of the prepolymer added to induce phase separation during a free-radical polymerization.

Polymerization stress leads to both internal and interfacial defects of a network, which manifest as cracks within a network or delamination of the material from the substrate to which it has been applied. To improve the utility and longevity of polymeric materials, our research has focused on understanding and optimizing both materials-based and processing-based techniques to reduce and eliminate polymerization shrinkage and stress, while maintaining critical properties such as mechanical strength, overall conversion and appearance.

Through PIPS, phase separation is favored as the entropy of mixing decreases during reaction, causing a multi-phase network to be formed from the polymerization of an initially homogeneous multi-component monomer/prepolymer mixture. Previously, we have reported a method to introduce network heterogeneity via PIPS in a one-step, purely photo-initiated, free radical polymerization[4].  Polymer networks formed via photo-initiation have the advantage of spatial and temporal control of the reaction - which is favored, and often necessary for application in stereo-lithography, coatings and biomaterials. In our approach, a bulk dimethacrylate polymerization (TEGDMA-based) was modified by the addition of linear non-reactive prepolymers: poly-methyl, ethyl, and butyl methacrylates (PMMA, PEMA, and PBMA) to induce phase separation shortly after photo-irradiation. The use of photo-initiation and ease of implementation of this technique broadens the scope of applications for PIPS, especially those where biological compatibility is essential.

One advantage of PIPS is that the phase separation process and the resulting phase structure can be modified through tuning of the kinetics and thermodynamics of the polymerization reaction. In a photo-initiated system, simple kinetic tuning can be achieved by varying the rate of initiation through irradiation intensity. Thermodynamics of the reaction can be modified through the design of prepolymers and selection of the monomer(s). Unfortunately, the kinetic and thermodynamic factors cannot be probed and tested separately, since they are co-dependent. The degree and extent of phase separation that occurs during PIPS is a direct result of the dynamic competition between kinetic formation and thermodynamic instability in the network.

In our studies utilizing PMMA, PEMA, and PBMA to modify TEGDMA-based polymerizations, we observed differences in resulting phase structure, domain size, efficiency at stress reduction, and kinetics of phase separation based on the prepolymer utilized and the loading level thereof. Many of these differences arise from the varying molecular weight and glass transition temperatureas well as the composition of the prepolymers. In this work, we present systematic studies of prepolymer physical properties, specifically molecular weight, and how it can be used to manipulate the balance between kinetics and thermodynamics during PIPS, the resulting phase structure, and stress reduction efficiency.

Three different PMMA prepolymers were synthesized, with increasing molecular weights (1.5K, 11K, and 120K Da). As the molecular weight decreases, the minimum loading level at which two distinct phases exist in the final polymer network increase (1.5K – 10wt%, 11K – 5wt%, 120K – 1wt%), as indicated by the tan delta profile post cure. This indicates that as molecular weight decreases, the prepolymer behaves more like filler within the network. Only at sufficiently high loading levels will thermodynamic instability be encountered during the polymerization, thus promoting PIPS.

Using real-time tensometry, we measured the efficiency of polymerization stress reduction as a function of loading level, molecular weight of the prepolymer, and irradiation light intensity. By increasing the irradiation light intensity, the rate of polymer network formation increases, and the time for diffusion of incompatible phases decreases. This results in a decrease in the extent of phase separation, and can lessen the amount of stress reduction. For PMMA with a molecular weight of 1.5 KDa, the kinetic network formation dominates and eliminates stress reduction via PIPS at irradiation intensities equal to or greater than 300μW/cm². However, this threshold occurs at 5mW/cm² for 11 KDa prepolymer, and 20mW/cm² for 120 KDa prepolymer, indicating that by increasing the molecular weight of prepolymer, stress reduction via PIPS can still occur in high intensity photo-polymerizations. This is a favorable result for applications such as dental restorative materials where minimum reaction time and maximum overall conversion is favored. Our future work will investigate approaches to alter the order of polymerization of incompatible phases, and whether this can be achieved through prepolymer modification. Additionally, we will apply this understanding to work in co-polymer based systems appropriate for dental restorative applications.

Works Cited

1.         Velazquez R, Sanchez F, Yanez R, and Castano VM.Journal of Applied Polymer Science 2000;78(3):586-591.

2.         Velazquez R, Ceja I, Guzman J, and Castano VM.Journal of Applied Polymer Science 2004;91(2):1254-1260.

3.         Li W and Lee LJ.Polymer 2000;41(2):685-696.

4.         Szczepanski CR, Pfeifer CS, and Stansbury JW. Polymer 2012;53(21):4694-4701.