Fracture polymethylmethacrylate

The second significant stage in the direct production of polyimide structures involves the thermal conversion of the patterned crosslinked film to the patterned polyimide film. It is important to understand how and under what condition the photo-crosslinked polyimide precursor is converted into polyimide as well as how completely. Mechanistically it is intriguing to determine wether the crosslinking fractures are split into small pieces or escape as pure hydroxyethylmethacrylate comparable to the zip-off depolymerization of polymethylmethacrylate. 

Since polymers with tetrasubsltuted centers are difficult to synthesize, two main types of positive resists have been dlsclosed. Polymethylmethacrylate r polymethyl Isopropenyl ketone hich undergo side chain elimination and subsequent main chain fracture have been used as positive electron beam resists (cf. 

Percutaneous cementoplasty (PC) consists of injecting acrylic cement (polymethylmethacrylate) in bone and is a perfect example of a minimally invasive technique in the musculoskeletal system. PC has been performed since 1984. Its role is to consolidate weight-bearing bone and to treat pain. Bone packing with acrylic cement is a procedure aimed at preventing pathological fractures and pain in patients with vertebral body lesions and acetabular metastasis. 

Evans AJ, Jensen ME, Kip KE, DeNardo AJ, Lawler GJ, Negin GA et al Vertebral compression fractures pain reduction and improvement in functional mobility after percutaneous polymethylmethacrylate vertebroplasty retrospective report of 245 cases. Radiology 2003 226 366-72... 

Vertebroplasty and kyphoplasty using polymethylmethacrylate (PMMA) have become a common procedure for treatment of vertebral compression fractures (Layton et al. 2007). Usually, the procedure is performed under fluoroscopy. Up to now, control CT was necessary to confirm correct distribution of PMMA after the procedure. FD-CT renders this control CT uimeces-sary it provides all the information needed. A considerable advantage is the possibility to perform FD-CT in the angiography suite without moving the patient. This allows confirmation of correct needle position prior to PMMA injection as well (Fig. 40.19). Furthermore, in primarily intended mono-pedicular procedures, this allows accurate, three-dimensional assessment of PMMA distribution. This may be helpful for the decision whether an additional needle placement into the contralateral side is required. 

Polymers show a similar effect in the presence of solvents. Solvents pre-ferredly enter the material near the crack tip because the distance between the molecules is increased there by the large tensile stresses. If, for instance, a rod made of polymethylmethacrylate (Plexiglas) is bent and the tensile side is wetted with acetone or alcohol, brittle fracture can occur after a short exposure time. In this case, the cleavage strength is reduced because the dipole bonds between the molecules are replaced by bonds formed with the solvent (see also section 8.8). 

Jensen ME, Evans AJ, Mathis JM et al (1997) Percutaneous polymethylmethacrylate vertebroplasty in the treatment of osteoporotic vertebral body compression fractures technical aspects. AJNR Am J Neuroradiol 18 1897-1904... 

Organic polymeric materials include both nonpolar hydrocarbons, such as polyethylene, and substances containing polar groups, which generate the mosaiclike surface, such as polymethylmethacrylate (PMMA). The fracture of these materials may involve the cleavage of the intermolecular bonds between polymer chains, as well as the polymer chains themselves. In media with strong surface activity, one may expect to see the prevailing role of weakened intermolecular interactions. 

Figure 12.17 Measured values of the compressive yield stress a j (true stress) plotted against applied tensile stress a2 (nominal stress). The full circles denote ductile yield, the crosses, brittle fracture, and the combined points, tests where ductile yielding occurred, followed immediately by brittle fracture. (Reproduced from Bowden, RB. and Jukes, J.A. (1968) The plastic yield behaviour of polymethylmethacrylate. J. Mater. Sci., 3, 183. Copyright (1968) Springer Science and Business Media.)...

Borduas, H.F, Culver, L.E. and Bums, D.J. (1968) Fracture-mechanics analysis of fatigue-crack propagation in polymethylmethacrylate. /. Strain Anal., 3, 193. 

Fig. 9.27. Fracture surface of polymethylmethacrylate (M = 163000) showing the transition from a smooth surface (crack speed a = 300 ms ) to a coarse one a > 400 ms l) crack propagation is from left to right. (Courtesy W. Doll, IFKM Freiburg), Specimen thickness 3.75 mm.

This procedure was tested on both linear (brittle) and moderately non-linear (semiductile) fractures, such as exemplified by polyvinylchloride (PVC) (Figure 4a) and rubber-toiighened polymethylmethacrylate (RTPMMA) (Figure 4b), respectively, and it was found to be effective. 

The 7 round robin used a sample of polyvinylchloride (PVC) producing fairly linear loading curves up to the maximum load, followed by unstable (brittle) fracture, as illustrated in Figure 4. To assess the applicability of the protocol to moderately nonlinear fractures, a sample of rubber-toughened polymethylmethacrylate (RTPMMA) showing some ductility and limited stable crack propagation before the load drop, as seen in Figure 5, was used. 

Table II. Effective Fracture Surface Knergles> of Polymethylmethacrylate. ... 

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