Casting and synthesis
Casting, or die casting, is a common form of manufacture in thermosets and has been successfully utilized for SMPs such as acrylic co-polymers developed by MedShape, Inc. It should be noted that, much like regular polymers, SMPs with Tgs higher than their curing temperatures do not typically reach full conversion. This is due to the Trommsdorf effect in which, as the polymer chains continue to grow, the motion of the chains becomes more and more hindered [114]. Eventually, a glassy polymer is formed, which severely restricts the motion of propagating active chains to find unreacted monomer. This is especially true for any approach to the casting of SMPs whose monomer constituents include high molecular weights [115].
As such, thermoset polymerization processes should include a postpolymerization thermal treatment above Tg to help aid the complete conversion of these restricted chains. When bulk casting SMP materials, postpolymerization can be followed by removal from synthesis containers and machining to the desired final, or zero-strain, geometry. Machining of the outside surfaces of the cast SMP may also aid in improving homogeneity of the material properties for the final “machined” part. This also allows for a higher degree of accuracy in producing the final (zero strain) geometry, though unintended thermal conditioning can occur if processing (machine tool) temperatures are not closely monitored and controlled. The authors have significant experience with bulk and near-net casting, whereby final geometries are produced using standard machining processes. Refer to Fig. 2.17 for an example of bulk synthesis of an acrylic SMP.
Figure 2.17 Example thermoset shape-memory polymer formed into a stock rod for secondary machining;the authors have successfully produced bulk rod stock of thermoset SMPs in excess of 1 min length. Reproduced with permission from MedShape, Inc.
Figure 2.18 Example silicone mold (upper) used to polymerize samples near to their final shape (lower). Reproduced with permission from MedShape, Inc.
Polymerization into shapes near to size (also referred to as “near-net” polymerization; refer to Fig. 2.18) results in significantly less waste and reduces the impact of the exothermic reaction, common to photopoly- merized thermosets, on the potential formation of voids and other internal defects to the finished device. Utilization of this type of process also allows for a reduction in the amount of secondary processing, such as machining the zero-strain geometry, than is required by bulk polymerization. However, this approach has a variety of challenges associated with it. These include control of the natural shrink that occurs during polymerization which may impact the finished device compliance to geometric specifications, and the risk of any poorly polymerized layers on the outside surface of the device which could easily be removed during machining of bulk synthesized SMP.
As mentioned previously, activation of recovery is highly dependent on Tg, while recovery stress is controlled by the RM. These two important thermomechanical parameters can be independently tailored by controlling the amount and molecular weight of the cross-linker during polymer synthesis and casting. This is an extremely useful attribute in SMP manufacture. Fig. 2.19 demonstrates how both the Tg and RM of a polymer can be varied independently in a thermoset acrylic co-polymer SMP. In the figure, the solid lines show the glass transition shifting with
Figure 2.19 Independently tailored thermomechanics of a shape-memory polymer. Reproduced with permission from MedShape, Inc.
respect to temperature, while holding a constant RM. The dashed lines show an increasing RM while the Tg remains constant.
