Thermal lens versus DSC measurements for glass transition analysis
of polymer
J. H. Rohling, A. N. Medina, J. R. D. Pereira, A. F. Rubira
a
A. C. Bento, L. C. M. Miranda and M. L. Baesso
*
Departamento de Física, Departamento de Química
a
, Universidade Estadual de Maringá.
Av. Colombo 5790, CEP 87020-900, Maringá, PR, Brazil
In this work thermal lens spectrometry is applied to investigate the thermo-optical properties of
polymers as a function of the temperature. The method is applied in poly(vinyl chloride) as a testing
sample. It is proposed that thermal lens spectrometry with minor change in its experimental
configuration, could be adapted to develop a new tool, called differential thermal lens scanning,
especially designed for the investigation of phase transition in polymers.
Keywords: Thermal lens method, glass transition, poly(vinyl chloride ), thermo-optical properties,
glass transition.
(Received on June 29, 2000, accepted on November 5, 2000)
In the last two decades we have witnessed the development of a
number of techniques for non-destructive characterization of the
thermal, optical and structural properties of materials based upon
the photothermal techniques [1,2]. Despite this growing interest
and the importance of the applications of these techniques to the
polymer area [3,4], so far the photothermal measurements have
been carried out mostly at near room temperature conditions.
This apparent limitation is essentially dictated by the fact that
most of the photothermal polymer measurements reported so far
were based upon the use of the photoacoustic technique. The use
of an electret microphone, in a conventional photoacoustic, is the
main reason why applications to polymers have been restricted
to near room temperatures.
In this work we discuss the use of an alternative
photothermal technique for measurements of the thermo-optical
properties of polymers as a function of temperature. The
proposed technique is based upon the use of the thermal lens
(TL) technique [5]. The room-temperature TL technique has
been proved to be a valuable method for investigating not only
the complete thermal and spectroscopic properties of transparent
materials, such as, glasses [6-12], liquid crystals [13,14] and
polymers, but also for the sensitive monitoring of the kinetics of
fast chemical reactions [15], percolation in microemulsions [16],
and dynamics of water-surfactant interaction [17]. Since the TL
technique is an intrinsically remote technique the measurements
on a sample placed inside a harsh environment presents, in
principle, no extra difficulty. It is precisely this aspect we take
advantage of to carry on the thermal properties measurements of
polymers as a function of temperature.
Experimental
In the two beam arrangement the TL effect is created
when an excitation laser beam passes through the sample and the
absorbed energy is converted into heat, changing the refractive
index of the sample and therefore producing a lens-like element
within the sample. The propagation of the probe beam laser
*
Corresponding author: mlbaesso@dfi.uem.br
through the TL results in either a defocusing (dn/dT < 0 ) or a
focusing (dn/dT > 0) of the beam center. The theoretical
treatment of the TL effect considers the aberration of the thermal
lens as an optical path length change to the probe laser beam,
which can be express as an additional phase shift on the probe
beam wave front after its passing through the sample. The
analytical expression for absolute determination of the
thermo-optical properties of the sample is given by [6-14,18]:
() ()
2
2
2 1 ) 2 / )(
2 2
) 2 1 ((
2
1
tan
2
1 0
+ + + + +
-
- =
V m t
c
t V m
mV
I t I
θ
(1)
Where
p
dT
ds
p
K
l
e
A
e
P
- =
λ
θ
0
(2)
c
Z
Z
V
1
= ,
2
=
e
p
m
ω
ω
,
D
t
e
c
4
2
ω
=
(3)
In Eq. (1) I(t) is the temporal dependence of the probe
laser beam at the detector, I(0) is the initial value of I(t), θ is the
thermally induced phase shift of the probe beam after its passing
through the sample, ω
p
and ω
e
are the probe beam and excitation
beam spot sizes at the sample, respectively, P
e
is the excitation
beam power, A
e
is the optical absorption coefficient of the
sample at the excitation beam wavelength (cm
-1
), Z
c
is the
confocal distance of the probe beam, Z
1
is the distance from the
probe beam waist to the sample, l
0
is the sample thickness, K is
the thermal conductivity, λ
p
is the probe beam wavelength, t
c
is
the characteristic thermal lens time constant, and (ds/dT)
p
is the
temperature coefficient of the optical path length change at the
probe beam wavelength, which can be rewritten as [6,19]:
( )( )
dT
dn
T
n
dT
ds
+ + - = α ν 1 1
(4)
s103
2001 © The Japan Society for Analytical Chemistry
ANALYTICAL SCIENCES APRIL 2001, VOL.17 Special Issue