Physical Aging of Polycarbonate: Elastic Modulus, Hardness, Creep, Endothermic Peak, Molecular Weight Distribution, and Infrared Data Victor A. Soloukhin,* ,† Jose ´ C. M. Brokken-Zijp, † Otto L. J. van Asselen, ‡ and Gijsbertus de With † Laboratory of Solid State and Materials Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands, and Laboratory of Polymer Technology, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands Received March 10, 2003; Revised Manuscript Received July 23, 2003 ABSTRACT: For the first time, load and depth sensing indentation (DSI) was used in order to monitor physical aging of bisphenol A polycarbonate for 30 months at room temperature and for 1 month at an elevated temperature. The DSI experiments were combined with differential scanning calorimetry, gel permeation chromatography, and infrared spectroscopy. The endothermic peak of polycarbonate shifted toward higher temperatures upon aging at an elevated temperature and did not change its location upon aging at room temperature. The elastic modulus and hardness of polycarbonate increased in a stepwise fashion during aging at room temperature. The molecular weight distribution broadened slightly, and the trans-trans conformational population increased during annealing. No simple correlation between changes in the mechanical properties and the shift of the endothermic peak during annealing was found. These changes seem to be caused by phenomena of different nature; namely, the changes in the mechanical properties appeared to have a reasonable correlation with free volume relaxation of the polymer, whereas the changes in the endothermic peak may be associated with internal energy changes. The similarities and differences between our results and the results of others are discussed. 1. Introduction Presently, polycarbonate of bisphenol A (4,4′-isopro- pylidenediphenol) is an engineering material of major importance. Its good optical transparency and superb toughness have promoted it for a variety of industrial applications. 1,2 Over time, however, properties of poly- carbonate change, causing, for example, embrittlement, or in other words the polycarbonate ages. Though these changes are typically minor, they still can be undesir- able for a plastic part intended for long-term use, particularly, when failure of the part can be a result of these changes. 3,4 It is generally believed that these changes are related to the structural modifications occurring upon aging. As polycarbonate, or any other glassy polymer, is cooled below its glass transition temperature, molecular mobility decreases significantly, and the molecules are unable to reach an equilibrium packing density and conformational structure with respect to temperature. 5 The process of relaxation toward equilibrium is commonly referred to as physical aging. It can occur at an elevated temperature, e.g., during an experimentally carried out annealing pro- cess, 5,6 as well as at room temperature. 7 The phenomena taking place in polycarbonate upon physical aging have been observed by means of a large number of analytical techniques such as tensile and creep measurements, 6,8,9 dynamic mechanical thermal analysis (DMTA), 3,10-13 differential scanning calorime- try (DSC), 6,14-17 positron annihilation lifetime spectro- scopy (PALS), 5,9,18 Doppler broadening spectroscopy (DBS), 18 dilatometry, 3,8,15,19 Fourier transform infrared spectroscopy (FTIR), 7,12,20-24 Raman difference spectro- scopy (RDS), 20 and solid-state nuclear magnetic reso- nance spectroscopy (NMR). 3 Although significant sci- entific knowledge has been gathered on the matter, the precise physical origin of physical aging phenomena is not fully understood. One of the remaining questions is the reason for occurrence of the enthalpy and volume relaxation changes in different time frames with respect to aging of polycarbonate at different temperatures. In the meantime, a new analytical technique capable of providing information for elastic, plastic, and creep properties of materials has emerged, known as nanoin- dentation. 25 Also referred to as load and depth sensing indentation (DSI), this technique has proven itself as a powerful tool to characterize the above-mentioned me- chanical properties and has been broadly used for inorganic substances for over a decade. On the contrary, for polymeric substances this technique has been limit- edly applied. It will be shown in this paper that DSI can be used to accurately estimate the elastic modulus, hardness, and creep behavior of polycarbonate and even to observe subtle changes occurring in these properties upon physical aging. In the present work we examine physical aging of polycarbonate by means of DSI, DSC, gel permeation chromatography (GPC), attenuated total reflection in- frared (ATR-IR), and FTIR transmission spectroscopes in combination. The information obtained is compared with literature data. The results indicate a possible cause for different findings after volumetric and en- thalpy relaxation experiments. 2. Experimental Section 2.1. Material and Specimens. Polycarbonate of bisphenol A used in this work was a commercially available optically transparent Lexan 9030 sheet (G.E. Plastics, Bergen op Zoom, The Netherlands) with an average molecular weight of ap- proximately 30 000 according to PS standards. The chemical structure of polycarbonate of bisphenol A is given in Figure 1. † Laboratory of Solid State and Materials Chemistry. ‡ Laboratory of Polymer Technology. * Corresponding author: e-mail V.A.Soloukhin@tue.nl; Fax +31 40 2445619; Tel +31 40 2473053. 7585 Macromolecules 2003, 36, 7585-7597 10.1021/ma0342980 CCC: $25.00 © 2003 American Chemical Society Published on Web 08/30/2003