Investigation of Thermal Properties of Radiation Grafted Polystyrene / Polytetrafluoroethylene (PTFE) Copolymer Films

Grafting was carried out using 7-radiation from a 60Co source at dose rates of 1.32-
15.0 kGyh"! at room temperature. The chemical structure and the crystallinity ofthe
grafted PTFE films were studied by means of Fourier transform infrared (FTIR).
Structural investigation of polystyrene grafted polytetrafluoroethylene) (PTFE)
membranes prepared by radiation-induced grafting of styrene onto commercial
PTFE films carried out by Fourier transform infrared spectrometer (FTIR) and the
thermal stability of thermal gravimetric analyzer (TGA). The effect of the structural
changes taking place in the film by grafting and the variation of the degree of
grafting on melting temperature (Tm), glass transition temperature (Tg), heat of
melting (AHm), anddegree of crystallinity was studied by using differential scanning
calorimetry (DSC). Polystyrene grafted films prepared by radiation-induced grafting
of styrene onto PTFE films were found to undergo considerable structural changes.
The degree of crystallinity decreased upon grafted as well as deviation of the degree
of grafting. The changes in melting temperature (Tm) was found to be insignificant
regardless of the degree of grafting unlike glass transition temperature (Tg). The
decrease in crystallinity was suggested mainly due to the effect of dilution on the
original crystalline structure by incorporation of the amorphous polystyrene grafts.
Furthermore based on the results and discussion and those on the original and
grafted films is that the PTFE film surface undergoes structural changes in terms of
chemical composition as a result of styrene grafting. The effect of grafting of
polystyrene onto PTFE membrane on the thermal stability of the PTFE matrix is
studied. It is found that the fluorinated-structure of the PTFE matrix has a thermal
stabilityup to ~550°C followed by one-step degradation. Grafting of styrene, which
led to the formation of polystyrene grafts in PTFE matrix introduces a two-step
degradation pattern. The polystyrene grafts start to degrade at ~400°C and continue
until ~480°C. This is followed by the degradation ofPTFE backbone, which starts at
~550°C and continues to ~680°C. Styrene, which radiochemically grafted onto
PTFE films leading to the formation of polystyrene grafted PTFEfilms. The grafted
films were found to be very stiff compared to the original PTFE fimi.From DSC
thermograms, the high value of Tg compared to that reported in literature (>20°C)
[10,11], thus indicates that commercial PTFE film contains certain additive (filler
such as glass fibres) that decreases the polymer chain mobility and gives rise to Tg.

The incorporation of polystyrene side chain grafts into PTFE film caused a shift in
the Tg of the grafted film to higher temperature while significant changes took place
in Tm. Grafting of styrene were found to reduce the heat of melting and the degree of
crystallinity of PTFE films. In addition, the degree of crystallinity decreased in the
original PTFE film to the grafted films.