Rheological measurements of the thermoviscoelastic response of ultrathin
polymer films
(Paul O'Connell,
Shanhong Xu)
In recent years,
considerable effort has been put into understanding the change in the
behavior of polymeric materials when they are reduced to the nanometer
size scale. In particular it has generally been observed that the
material at the nano-scale shows a reduced glass transition temperature
(Tg), though it should be noted that this remains a
controversial area of research with often-conflicting results on the
presence and magnitude of such changes. The aim of my current research
is to develop and apply a new nanorheological measurement method that
allows the determination of the viscoelastic response of thin films in
which the film thickness and lateral constraints can be varied
systematically. The approach we have taken is to scale down the
mesosopic bubble
inflation biaxial test method such that polymer films of nanometer
thickness can be tested.
* O'Connell P.A. and McKenna G.B., Science, Vol. 307, No. 5716,
1760-1763 (2005)
Our
results on the creep compliance of a polystyrene and polycarbonate show
reductions in Tg of approximately 60 °C down to a thickness of 13nm and
some 80 °C down to a thickness of 9nm respectively. Interestingly.
poly(vinyl acetate) (PVAc) shows no significant change (<3 oC)
in the Tg down to a film thickness of 23nm suggesting the
reduction in Tg as the film thickness is reduced is not a universal
phenomenon. The results do show, however, surprising stiffening in the
rubbery response regime, with the material stiffness scaling as
approximately the square of the thickness. This is true for all three
materials.
Future
work will extend the measurements to holes of varying diameter and
shapes to vary the confinement conditions.
Funding: National Science Foundation
U.S Army Research Office
We are performing nanorheological studies using embedment of micro and
nanospheres to study the mechanical properties of polymeric surfaces. We
are performing experiments with nanospheres of
different surface energies and radii to determine if there is a size
effect (study of nanoheterogeneity of polymeric surfaces). The
experiments will be accompanied by finite element simulations of embedding
spheres.
Funding: National Science Foundation
Instrumentation
I am involved in the evaluation and comparison of a Sensotec strain gage
based and 2KFRT transducers in the ARES Rheometer.
Funding: John R.Bradford endowment at TTU
Back to Top
Distribution of the Surface Mechanical Properties in Nanocomposites by
Spontaneous Embedment of Submicron Particles (Taskin
Karim)
There is considerable interest in the distribution of
material properties in heterogeneous materials such as composites. Here
we are examining one nanocomposite system using a novel particle
embedment method that allows the determination of the spatial
distribution of the surface mechanical properties of heterogeneous
materials. We use Universal SPM atomic force microscopy (AFM) from
Ambios Technology and elastic models of contact mechanics to evaluate
the modulus of the surface.
Schematic of Particle Embedment
AFM Images and the Height Distribution of Particles
Our initial result
shows that the calculated modulus values were found to be significantly
lower than expected from macroscopic measurements on the epoxy
nanocomposite.
Back to Top
Determination of the elastic properties of spin coated lipid
multibilayer films by embedment of sub-micro sized particles
(
Jinhua Wang)
In
recent years, much work has focused on the behavior of materials at the
micro-, nanometer size scales. Teichroeb and Forrest [1]
proposed a novel nano-particle embedment technique to study the
properties of polymer surfaces. The purpose of the present work is to
use this method to examine the mechanical properties of lipid
multibilayer films. The standard JKR model [2] relates the
shear moduli of the film to the embedment depth of the particle into the
surface. In our experiments, particle embedment depth can be obtained by
atomic force microscopy (AFM). This is used to obtain moduli of the
lipid multibylayer films.
AFM image of 200nm polystyrene particles dispersed on a
1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) multibylayer
film.
References:
[1] J. H. Teichroeb and J. A. Forrest, “Direct imaging of nanoparticle
embedding to probe viscoelasticity of polymer surfaces”, Phys. Rev.
Lett., 91, 016104(1-4) (2003).
[2] K. L. Johnson, K.
Kendall and A. D. Roberts, “Surface energy and the contact of elastic
solids”, Proc. R. Soc. Lond. A, 324, 301-313 (1971).
Funding:
National Science Foundation
John R.Bradford endowment at TTU
Back to Top
Viscoelastic properties of ultrathin polymer films on a liquid substrate
(
Jinhua Wang)
There is considerable interest in the behavior of polymer films of
nonometer thickness. In particular, the mechanical response has been
investigated in our lab using a novel bubble inflation method to obtain
the creep response for films as thin as 10nm. [1] Yet the
results of this method have yet to be replicated by other techniques.
[2] The purpose of the present work is to investigate the
reasons for the different results between the bubble inflation
measurement and the liquid dewetting method of Bodiguel and Fretigny. To
this end we have constructed a similar device to that of Bodiguel and
Fretigny and are in the process of beginning measurement to replicate
their results on polystyrene. Efforts will be made to compare bubble
inflation results directly with those of the thin film dewetting
experiment.
The
picture of the thin film dewetting device
References:
[1] O’ Connell P. A. and McKenna G. B., “Rheological Measurements of the
Thermoviscoelastic Response of Ultrathin Polymer Films”, Science,
307, 1760-1763 (2005).
[2] H.
Bodiguel and C. Fretigny, “Viscoelastic dewetting of a polymer film on a
liquid substrate,” Eur.Phys. J. E., 19, 185-193 (2006).
Back to Top
Mechanical
properties of PETN crystal
(Ben
Xu)
Our objective is to determine the mechanical properties of PETN crystals
at the nanoscale by using the AFM for either direct nanoindentation or
microparticle embedment methods. Currently, our research has focused on
establishing temperature and impurity effects. Figure 1 shows an AFM
image of a Zn-doped PETN crystal.
Figure 1. The three-dimensional image of Zn-doped PETN crystal
Instrumentation:
Atomic Force Microscopy
Back to Top |
