Indentation Creep Studies of Cross-Linked Glassy Polymer Films
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INDENTATION CREEP STUDIES OF CROSS-LINKED GLASSY POLYMER FILMS KEVIN M. O'CONNOR AND PAMELA A. CLEVELAND Eastman Kodak Company, Imaging Research Laboratories, Rochester, NY 14650-2116.
ABSTRACT Indentation creep testing was done on cross-linked glassy polymers based on polystyrene, specifically poly(styrene-co-divinylbenzene) (PS-DVB) and poly(divinylbenzene) (PDVB). The continuous depth-sensing capabilities of the NanoIndenter II were used to measure the time-dependent response to indentation at constant applied load. The raw creep data in terms of indentation depth vs time showed that PDVB was about 20% more resistant to penetration than PS-DVB. A data analysis program was developed that converted the raw data to stress a and plastic strain rate and generated the plastic flow curve that was observed to follow the power law £ = Bea. The stress exponent a for PS-DVB increased with applied loads between 1 and 27 mN and was generally larger in magnitude than the exponent for PDVB. When compared at equivalent stress and indentation depth, the plastic strain rate for PDVB was observed to be about 100 times slower than for PS-DVB. This was attributed to the higher degree of cross-linking increasing the characteristic relaxation time governing creep flow in these polymers.
INTRODUCTION Creep is the time-dependent deformation of a material under constant load and is an important plastic or viscoelastic response, particularly of polymeric materials.1 The conventional methods of creep testing have several shortcomings: fairly large amounts of material are necessary to fabricate the required specimens, and some classes of polymers are difficult if not impossible to mold or machine free of defects due to brittleness caused by low molecular weight or the presence of cross-linking. Indentation creep testing using the Nano-Indenter II is a desirable method because of its intrinsic advantages of load-controlled operation, continuous depth sensing, small sample size requirements, and insensitivity to edge defects. In addition, it has the benefit of isolating the creep response of small volumes of material such as surface coatings or discrete phases within a multiphase material. Although use of the Nano-Indenter for creep studies on metals has been reported, 2 4 to our knowledge the method has not yet been applied to polymeric materials. The work described here focused on the characterization of indentation creep in polystyrene (PS) as an example of a glassy polymer. The effects of two widely different cross-link densities were also investigated. The brittleness of such cross-linked glassy polymers, particularly at high degrees of cross-linking, made indentation testing an attractive choice.
SAMPLE PREPARATION AND TESTING Two polystyrene-based samples were prepared using the difunctional monomer divinylbenzene (DVB) as the cross-linking agent: a polystyrene-divinylbenzene copolymer having 5 mol% divinylbenzene (designated PS-DVB) and a divinylbenzene homopolymer (designated PDVB). These polymers were chosen to cover a wide range
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