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标题:自愈合热塑性弹性体

1楼
上海北京顺德 发表于:2011/4/29 16:48:00

Self-healing Polymer Composites

SpecialChem - Apr 26, 2011


 

Mark T. DeMeuse


 

Polymer composite systems are often exposed to small-scale damage. If this damage occurs in the form of cracks that are on a small enough scale, it is possible for the composite to undergo autonomous repair or "self-healing". This process of self-healing without input from the outside is meant to mimic the ability of biological materials to repair themselves.

Healing in the composites is achieved through the addition of a microencapsulated healing agent and a catalyst to the matrix polymer. Completion of the self-healing process requires a suitable chemistry to polymerize the healing agent in the fracture plane. Small fractures in the composites move through the material and rupture the microcapsules, releasing the healing agent. The healing agent, then, fills the crack through capillary action. After filling the crack, the healing agent reacts with the catalyst which is embedded in the matrix polymer. The concept of the manner in which a self-healing composite is shown pictorially in Figure 1.

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Fig. 1: Schematic of self-healing composite concept

There are several technical challenges involved with designing this type of material. First, the catalyst must maintain its reactivity even after it is encapsulated. Further, the viscosity of the reactive monomer must be low enough to cover the entire crack before polymerization occurs. Otherwise, full healing capacity will not be obtained. Finally, the catalyst must quickly dissolve into the monomer so that the reaction happens efficiently to prevent the crack from spreading further.

Typically, the healing agent is a liquid dicyclopentadiene (DCPD) and the solid catalyst is Grubbs catalyst (1). The catalyst initiates a ring opening metathesis polymerization of DCPD. The monomer on its own is relatively unreactive and polymerization does not take place. The metathesis reaction of the DCPD involves the severance of two double bonds in favor of the production of new bonds. The main driving force of this reaction is a relieving of the ring strains in the DCPD monomer to form a polymer. The microcapsules are made from a urea-formaldehyde shell.

2楼
上海北京顺德 发表于:2011/4/29 16:48:00

Grubbs' catalyst (benzylidene - bis (tricyclohexyphosphine) dichororuthenium is a good choice for this type of process because it is insensitive to air and water, thus robust enough to maintain reactivity within the material. The presence of the catalyst allows the energy barrier to be lowered, and the polymerization reaction can proceed at room temperature. The major drawback to the use of Grubbs' catalyst is cost. Ruthenium is quite costly and it was shown that using more of the catalyst corresponded directly to higher degree of healing.

The matrix polymer that has been primarily investigated in self-healing composites are epoxies. There are several reasons for this. First, self-healing occurs best over planar surfaces, such as those that are created by delamination or by small brittle fractures. Epoxies tend to be involved in laminated structures and also they tend to fail in a brittle manner. In addition, the nature of processing of thermosets, like epoxies, allows for the microcapsules to be added during the material formation process.

The rate at which the DCPD polymerization occurs is a limiting factor in the self-healing process. Typically for DCPD to provide a sufficient amount of toughness, times up to 48 hours and quantities of up to 5 wt. % catalyst can be required (2). In addition, in a dynamic fracture situation, the mechanical rate of fracture must be balanced with the chemical rate of recovery. If the fracture forms too quickly, recovery will not have time to occur. If the polymerization reaction occurs faster than the rate of dissolution of the Grubbs catalyst, there will be inadequate mixing, putting a limit on the polymerization.

Methods of microcapsule processing which were initially used put lower limits on the sizes of the urea-formaldehyde spheres. The ability to be able to produce capsules on a nanoscale is highly desirable, as it would permit self-healing to be applied on a nanoscale. With microemulstion polymerization, the current range of microcapsules is 10-1000 microns (3).

New processing techniques need to be established to deal with the encapsulation of healing agents at the nanoscale. A technique called colloidal templating is one approach which may be useful in that regard. In that method, material is deposited onto the surface of a colloid to achieve a shell in the nanometer scale range. This means that nanocapsules may be constructed.

There has been work reported (4) which involves an in-situ encapsulation method, allowing for over an order of magnitude reduction for the preparation of the urea-formaldehyde capsules. Capsules with diameters as small as 220 nanometers are achieved using sonication techniques and an ultrahyrophobe to stabilize the DCPD droplets. The capsules possess a uniform urea-formaldehyde shell wall of 77 nanometer average thickness and have excellent thermal stability. The capsules can be uniformly dispersed in an epoxy matrix and cleave rather than debond when the matrix fractures.

3楼
上海北京顺德 发表于:2011/4/29 16:49:00

In addition, healing agents other than DCPD need to be investigated. The rate of healing of DCPD does not represent the fastest healing agent. 5-ethylamine-2-nonbornene (ENB) is a faster healing agent that requires lower concentrations. It was determined (2) that the time it took for the healing polymer to go from the liquid state to gelation and from gelation to vitrification for DCPD with 1.0 wt. % catalyst were 5 minutes and 50 minutes, respectively. On the other hand, for ENB with 0.1 wt. % catalyst, the times were 0.8 and 8 minutes, respectively. Thus, ENB is a more effective healing agent in terms of both speed and efficiency than DCPD.

A limitation to the use of self healing materials is the resources which are available for the healing process within the material. Using the method described in this article of microcapsules and a dispersed catalyst, there is only so much of each healing material that can be used to polymerize over the cracks in a material. This means that the self-healing process may not be complete in a damaged composite. In fact, typical values of as much as 75 % recovery in toughness is reported in the literature (5).

Thus far, the majority of self-healing composites work has been done on epoxy-based composites. This is primarily because these materials tend to be involved in laminated structures and also tend to fail in a brittle manner. As already mentioned, self-healing is most effective in planar surfaces, such as those which are created by delamination or by small brittle fractures. In addition, the manner in which thermosets, like epoxies, are processed allows for the easy addition of the self-healing microcapsules during formation.

In order for the concept of self-healing composites to be more widespread, approaches to deal with matrix materials other than epoxies need to be developed. For example elastomers represent an opportunity to apply the principles of self-healing. The failures of these materials are tears that, in general, represent planar sections that can be recovered by polymerization. In addition, there is a similar processing opportunity as in epoxies to add capsules and the catalyst to elastomer systems.

References

  1. M.S. Sanford, L.M. Henling and R.H. Grubbs, Organometallics, 17, 5384-5389 (1998).
  2. K. Lee, X. Liu and S. Yoon, "Characterization of Diene Monomers as Self-Healing Agents for Polymer Composite and its Microcapsules. 2005.
  3. John Wiley and Sons, "Recent Advances in Self-Healing Materials Systems. 2007.
  4. B.J. Blaiszik, N.R. Sottos and S.R. White, Composites Science and Technology, 68, 978-986 (2008).
  5. S.R. White, N.R. Sottos, P.H. Geubelle. J.S. Moore, M.R. Kessler, S.R. Sriam, E.N. Brown and S. Viswanathan, Nature, 409, 794-797 (2001).
4楼
上海北京顺德 发表于:2011/4/29 16:55:00

Polymer composite systems are often exposed to small-scale damage. If this damage occurs in the form of cracks that are on a small enough scale, it is possible for the composite to undergo autonomous repair or "self-healing". This process of self-healing without input from the outside is meant to mimic the ability of biological materials to repair themselves.

 

聚合物复合体系常常受到小尺度的损伤。假如这种损伤以足够小的尺度的裂纹形式出现,聚合物复合体系可能要经历自动修复或者叫“自愈合”过程。这种没有外界参与的自愈合过程意味着具有模拟生物材料修补自身的能力。

5楼
上海北京顺德 发表于:2011/4/29 17:08:00

Healing in the composites is achieved through the addition of a microencapsulated healing agent and a catalyst to the matrix polymer. Completion of the self-healing process requires a suitable chemistry to polymerize the healing agent in the fracture plane. Small fractures in the composites move through the material and rupture the microcapsules, releasing the healing agent. The healing agent, then, fills the crack through capillary action. After filling the crack, the healing agent reacts with the catalyst which is embedded in the matrix polymer. The concept of the manner in which a self-healing composite is shown pictorially in Figure 1.

 

在复合体系中的愈合作用是通过添加微包覆的愈合剂和一种催化基体聚合物的催化剂实现的。完成自愈合过程要求具有一个合适的聚合开裂平面上的愈合剂的化学反应。复合体系中的小裂纹穿过材料并使微包覆物破坏,从而释放出愈合助剂。然后愈合剂通过毛细作用而充填裂纹。充填裂纹后,愈合剂与包埋在基体聚合物中的催化剂反应。这种自愈合复合物的行为概念如图1所示。

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