Novel triol-crosslinked polyurethanes and their thermorheological characterization as shape-memory materials

Abstract

A new family of crosslinked polyurethanes was synthesized and characterized as shape-memory polymers. Three-arm network junctions are provided by 1,1,1-trimethylol propane with an isocyanate group on each arm. Three diisocyanates are used: 4,4′-methylene bis(phenyl isocyanate), toluene diisocyanate, and 4,4′-dibenzyl diisocyanate. They are linked together by macrodiol soft segments formed from either polytetrahydrofuran with molar mass of 650, 1000 or 2000 g mol⁻¹ or polycaprolactone glycol with molar mass of 830 or 1250 g mol⁻¹. Thermorheological response of each polymer was characterized by tensile creep tests through the glass transition of the soft segments, to obtain the linear viscoelastic retardation spectrum, limiting compliances and time–temperature shift factor. These were used to predict significant features of shape-memory performance. With a decrease in soft segment chain length, the temperature of maximum shape recovery rate increased and the width of the recovery window decreased, consistent with loss of soft segment chain mobility remote from the crosslinks. Tensile modulus in the switched condition (above T_g) was 8–16 MPa, increasing with crosslink density and hard-segment rigidity. The results confirmed the potential of these polyurethanes as a new family of tunable shape-memory materials.

Introduction

Polymers have great potential as shape-memory materials. The source of “memory” is their capacity to store preferred molecular orientation on large molecular length scales. The mechanism by which the shape memory is locked-in is usually cooling through the glass transition T_g (and consequent lengthening of mechanical relaxation times) or formation of a hard phase such as crystal domains. Shape recovery is triggered by subsequent heating, causing passage through T_g (and hence shortening of relaxation times) or softening of the hard phase. Lendlein and Kelch [1] provide a comprehensive review of the possibilities and of many of the shape-memory polymers proposed so far. It is clear that shape-memory behaviour is intrinsic to any polymer where molecular orientation arising from a prior stretch can be retained sufficiently at the orientation temperature. This will be the case where there exist chemical crosslinks, physical crosslinks provided by an entanglement network (with sufficiently long reptation time τ_d) or by a dispersed phase that is rigid to sufficiently high temperatures.

In practical applications of polymers as shape-memory materials, that is where they are acting as “shape-memory polymers” (SMP), a small set of parameters determine the usefulness, or otherwise, of a given polymer system for a given application. (1) The fractional recoverable strain: the maximum fraction of the imposed strain that can be recovered when triggered. (2) The temperature of maximum recovery rate T_max for a given rate of heating: a measure of the temperature required to trigger recovery of shape. (3) The width ΔT of the window of temperature within which shape recovery occurs. (4) The tensile modulus E_R at the temperature of full recovery: this determines the maximum shape-recovery restoring force if shape recovery is resisted. Since different applications make different demands on SMPs, it is particularly desirable to find polymer systems where parameters T_max, ΔT, and E_R are variable, so the same basic system can be tuned to suit different applications. Preferably, these will be systems where all the imposed strain is recoverable, that is the fractional recoverable strain is 100%.

Previous authors have highlighted the merits of polyurethane polymers as potential SMPs [1]. This arises largely from the chemical versatility of segmented polyurethane copolymer systems. Thus by suitable choice of diisocyanate and macrodiol phase-segregated materials may be produced, for example where one component comprises hard segments that provide physical crosslinking, to enable the retention of shape memory, while the other component provides a softer phase with a lower softening temperature, either glass transition or melting temperature, to enable the locking-in and release of shape memory at convenient temperatures. The softening temperature, which acts as the shape-memory trigger temperature (to be precise, T_max), may be changed systematically by variation of the hard-segment content [2], [3], [4], [5] or the chain length of the macrodiol employed [6], [7]. A feature of practical importance for biomedical applications of SMPs is the fact that these systems may be designed with trigger temperatures in the convenient range 20–50 °C [2], [5], [6], [7], [8].

The width of the trigger window ΔT is also important in applications of SMPs, but it has received little attention in previous published work. Nevertheless, published shape-recovery measurements, for example as reported by Lin and Chen [7] for polyurethanes, suggest values in the range 28–46 K, when T_max exceeds room temperature.

The third parameter, E_R, poses the greatest challenge for SMPs relative to shape-memory metal alloys, which are orders of magnitude stiffer. Shape recovery in polymers is usually driven by entropy-elastic recoil of molecular chains, but this is associated with extremely low elastic moduli, for example Young's moduli in the range 1–10 MPa. Thus there has been interest in reinforcing SMPs with hard particles such as chopped glass fibres [9], sub-micron SiC particles [10], [11] or molecular-level mixing with silica [12]. However, the price for a small increase in E_R (e.g. to 10 MPa) may be a significant deterioration in the recoverability of shape, as observed by Ohki et al. [9]. Even 20% by weight of sub-micron SiC in a crosslinked epoxy gave only a 50% increase in shape restoring force [10].

The purpose of the present paper is to present a new family of potential polyurethane SMPs, addressing the issues discussed above. They are all produced by the prepolymer route using diisocyanate (DII) hard segments and macrodiol (MD) soft segments. Thus they have the usual versatility with respect to varying hard-segment content and macrodiol chain length, allowing the systematic variation of T_max and E_R (without using reinforcing particles). However, they are also single phase, chemically crosslinked polymers, and hence have the advantage of 100% recoverability of shape. This is achieved by employing a triol as chain extender, giving a network structure with tri-functional crosslinks. These polymers are characterized with respect to their thermorheological response in tension, in the linear viscoelastic regime. Then their performance in a shape-memory sequence is computed rigorously from the theory of temperature-dependent linear viscoelasticity, allowing comparisons to be made between them.