Post-Polymerization Crosslinked Polyurethane Shape-Memory Polymers

Abstract

Novel urethane shape-memory polymers (SMPs) of significant industrial relevance have been synthesized and characterized. Chemically crosslinked SMPs have traditionally been made in a one-step polymerization of monomers and crosslinking agents. However, these new post-polymerization crosslinked SMPs can be processed into complex shapes by thermoplastic manufacturing methods and later crosslinked by heat exposure or by electron beam irradiation. Several series of linear, olefinic urethane polymers were made from 2-butene-1,4-diol, other saturated diols, and various aliphatic diisocyanates. These thermoplastics were melt-processed into desired geometries and thermally crosslinked at 200°C or radiation crosslinked at 50 kGy. The SMPs were characterized by solvent swelling and extraction, differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), tensile testing, and qualitative shape-recovery analysis. Swelling and DMA results provided concrete evidence of chemical crosslinking, and further characterization revealed that the urethanes had outstanding mechanical properties. Key properties include tailorable transitions between 25 and 80°C, tailorable rubbery moduli between 0.2 and 4.2 MPa, recoverable strains approaching 100%, failure strains of over 500% at T(g), and qualitative shape-recovery times of less than 12 seconds at body temperature (37°C). Because of its outstanding thermo-mechanical properties, one polyurethane was selected for implementation in the design of a complex medical device. These post-polymerization crosslinked urethane SMPs are an industrially relevant class of highly processable shape-memory materials.

Figure 1

Comparison of synthesis and processing of a traditional SMP and a post-condensation crosslinked SMP

Figure 2

Plots of gel fraction versus % DCHMDI for 1H and 1R series.

Figure 3

DSC results for Series 1 thermally crosslinked samples

Figure 4

Storage modulus plots for thermoplastic, radiation crosslinked, and heat crosslinked 1-a urethane sample

Figure 5

DMA storage modulus (G′) plots for thermally crosslinked samples in Series 1H

Figure 6

Tan delta plots for thermally crosslinked samples in Series 1H

Figure 7

Effect of heating time and temperature on rubbery modulus of Series 4 and 5 samples.

Figure 8

Effect of Increasing DCHMDI composition on radiation crosslinking of select samples in Series 1R

Figure 9

A comparison of the storage moduli of samples 1a-R (radiation crosslinked 50% 2-butene-1,4-diol sample) and 1f-R (50% 1,4-butanediol)

Figure 10

Cyclic Free strain recovery plots of recovered strain versus temperature for (a) thermally crosslinked 20% DCHMDI sample and (b) thermoplastic 20% DCHMDI sample

Figure 11

Constrained recovery plot of recoverable stress versus temperature for thermoplastic and radiation crosslinked (1R_a) 0% DCHMDI sample.

Figure 12

Strain to Failure Results for Sample 1H-d

Figure 13

Images of the shape recovery at 37°C of sample 1R_a over a 12-second time period.

Figure 14

Proposed chemical mechanism for the radiation crosslinking of samples containing 2-butene-1,4-diol

Figure 15

Artificial oropharyngeal airway device made from molding Sample 1a and then exposing it to radiation