Rheological Measurements of the Thermoviscoelastic Response of Ultrathin Polymer Films

Abstract

Measurement of the thermoviscoelastic behavior of glass-forming liquids in the nanometer size range offers the possibility of increased understanding of the fundamental nature of the glass-transition phenomenon itself. We present results from use of a previously unknown method for characterizing the rheological response of nanometer-thick polymer films. The method relies on the imaging capabilities of the atomic force microscope and the reduction in size of the classical bubble inflation method of measuring the biaxial creep response of ultrathin polymer films. Creep compliance as a function of time and temperature was measured in the linear viscoelastic regime for films of poly(vinyl acetate) at a thickness of 27.5 nanometers. Although little evidence for a change in the glass temperature is found, the material exhibits previously unobserved stiffening in the rubbery response regime.

Onset of Plasticity in Thin Polystyrene Films

“…In addition to an overall small value of ǫ p , measurements show the critical strain increases in films of thicknesses below 100 nm. While anomalous effects have often been measured in polymer films of this same thickness range [9,10,[12][13][14][15][16]25], our measurement is novel in several respects. First, this is a change in a heretofore untested material property relevant to failure, elastic theory and the basic sample preparation used in countless thin film experiments.…”

“…Polystyrene is often used as a model polymeric material because thin films are easily produced and manipulated, they are relatively rigid, and polystyrene is a comprehensively studied glassforming polymer with well known bulk material properties. However, as the thickness of a polymer film is reduced to nanoscopic dimensions it is now well established that the glass transition is altered [9][10][11][12][13][14][15], the number of entanglements between chains are reduced [1,2], and mechanical properties can change [16,17]. Remarkably few studies have considered how failure processes might be altered in thin films, or how plasticity might alter a films response to other stimuli.…”

“…The change we observe could be related to a reduction of inter-chain entanglements [1,2], or to a change in the glass transition temperature of the film [9][10][11][12][13]17]. Bending gives the opportunity to make a clear distinction between the two possibilities as it preferentially probes a film's free surface and the applied strains can be accurately measured in the vanishingly small strain limit.…”

“…Thin polymer films are materials that are uniquely suited for countless goals, finding use in studies ranging from fundamental polymer physics [1,2], to elasticity [3][4][5][6][7][8], and glass-phase physics [9][10][11][12][13][14][15]. Polystyrene is often used as a model polymeric material because thin films are easily produced and manipulated, they are relatively rigid, and polystyrene is a comprehensively studied glassforming polymer with well known bulk material properties.…”

Relaxation and intermediate asymptotics of a rectangular trench in a viscous film

“…Thin films are prevalent in applications such as lubricants, coatings for optical and electronic devices, and nanolithography to name just a few. However, it is also known that the mobility of polymers can be altered in thin films [1][2][3][4][5]. Thus, understanding the dynamics of thin films in their liquid state is essential to gaining control of pattern formation and relaxation on the nanoscale [6,7].…”