Handbook of Solvents - George Wypych - ChemTech - Ventech!

10.2 Effect of polymer/solvent acid-base interactions 581
10.2.3 INFLUENCE OF THE NATURE OF THE SOLVENT ON THE α AND
β-RELAXATIONS OF CONVENTIONAL PMMA
10.2.3.1 Introduction
Thin polymer films are generally obtained by solution casting, in many applications, such as
paints, varnishes, or adhesives. The properties of polymer films obtained from a solution
differ from the original bulk properties, and this effect can have some significant consequences
on the expected behavior of the final film.
Recent studies have analyzed the influence of the nature of the solvent on the relaxation
temperatures of conventional PMMA solid films. 39,40 PMMA was dissolved in various
solvents and the solid films (after solvent evaporation) were analyzed by dielectric spectroscopy.
The purpose of the study was to investigate the influence of the nature of the solvent
on the α and β-relaxations of PMMA solid films. The α-relaxation is related to the glass
transition of PMMA and corresponds to the rotation of lateral groups around the main chain
axis. 41 The β-relaxation, which occurs at a lower temperature, is induced by the rotation of
the acrylate groups around the C-C bonding which links them to the main chain. Different
solutions of conventional (atactic) PMMA in good solvents (chloroform, acetone, toluene
and tetrahydrofuran) are cast on a metallic substrate (aluminum). After solvent evaporation
at room temperature, solid films of PMMA are analyzed by dielectric spectroscopy (DETA)
at 100 Hz and 10 KHz from -40 to 135°C with a scanning temperature of 2°C/min. The variation
of the loss factor tan δ is studied as function of temperature. The temperatures of the
tan δ peaks are related to the relaxations temperatures of the polymer. 42 The reference
DETA spectrum is obtained by analyzing a bulk PMMA film obtained from heating press
(without solvent).
At 10 KHz, only one peak is detectable (whatever the sample) which is attributed to
the α-relaxation. However, the peak is not symmetrical, with a broadening towards the
lower temperatures, due to the contribution of the β-relaxation. At 100 Hz, two peaks are
present, the major at lower temperature, corresponding to the β-relaxation and the minor at
higher temperature, attributed to the α-relaxation.
10.2.3.2 Dielectric spectroscopy results
Table 10.2.6 reports the temperatures of the α-relaxation (measured at 10 KHz) and theβ-relaxation
(measured at 100 Hz) for the reference sample (bulk PMMA) and the different solution-cast
films.
Table 10.2.6. Temperature of α andβ-relaxations of bulk PMMA and solution-cast films
(after reference 40)
PMMA PMMA/ PMMA/ PMMA/ PMMA/
bulk chloroform toluene acetone THF
T° of α peak, °C at 10KHz 108 75 96 85 82
T° of β peak, °C at 100 Hz 40 34 49 45 47
The results show that the temperature of the α-transition is significantly reduced for
cast films compared to the bulk polymer, with a lower value for chloroform samples. The
decrease of the α-transition temperature can be explained by the presence of residual solvent,
inducing a plasticizing effect. On the contrary, an increase of theβ-transition tempera-