Impression recovery of PMMA
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Impression recovery of PMMA Fuqian Yang and J. C. M. Li Department of Mechanical Engineering, University of Rochester, Rochester, New York 14627 (Received 4 April 1996; accepted 19 February 1997)
Impression recovery in which the deformation temperature is the same as recovery temperature is studied for the first time. PMMA is deformed by impression above the glass transition temperature to a depth of less than 0.3 mm and recovered at the same temperature. Almost complete recovery of dimension is observed every time. The dimensional changes obey second order kinetics and the temperature dependence of the rate constant shows two consecutive processes with activation energies 440 kJymole (between 104 and 113 ±C) and 95 kJymole (between 113 and 140 ±C). Two pairs of defects of opposite signs are believed to be involved in the dimensional recovery processes.
I. INTRODUCTION
Dimensional stability of structural components is important in precision instruments and devices. Extensive studies have been reported on the dimensional recovery process of materials, such as metals1–7 and polymers.7–22 Dimensional changes during recovery are usually absent for crystalline materials. However, there is at least one exception5 : the indentation made by impacting a steel ball on a cleaved surface of a zinc single crystal disappeared completely on heating to 400 ±C. Dimensional changes during the annealing of polymers are common. Examples are nylon,13 poly(ethylene terephthalate),14–16 polypropylene,17 and polystyrene.11,12,18 There are many ways to study the recovery process of materials, such as the tensile/compression test11–19 and the indentation test.11 However, for most studies of the recovery processes, samples were deformed at room temperature and recovered at high temperatures. With the development of the impression test,23 impression creep has been used to measure the creep properties of materials such as ABS polymers24 and Pb–Sn eutectic alloys.25 Recently, the RSA II (Rheometric Solid Analyzer II, Rheometrics, Inc., Piscataway, NJ) was modified to carry out the impression test such as impression creep and stress relaxation in our lab.25 Based on the procedure of two-step loading of RSA II, it is possible to study directly the recovery properties of polymers above glass transition and to carry out deformation and recovery at the same temperature.
Its glass transition temperature was about 100 ±C. A block of material about 25 3 25 mm2 was cut from the sheet. Each specimen block was further ground and polished metallurgically using a final polish of 0.05 mm fine alumina powders to obtain a pair of flat surfaces across a thickness of about 6.5 mm. After that, each specimen was annealed at 113 ±C for 10 h and furnace cooled down to room temperature to relieve the residual stresses from polishing. B. Impression test
A modified RSA II (Rheometrics Solids Analyzer, Rheometrics, Inc., Piscataway, NJ)25 was used to carry out the impression test using a 0.5 mm punch made of stainless st
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