Commercial poly(p-phenylene sulfide) (PPS) was thermally cured, which resulted in an increase of molecular weight due to cross-linking. Non-isothermal crystallization studies of samples cured for up to 7 days at 250?C showed a monotonous increase of crystallization temperature compared to pure PPS. However, a further increase of curing time decreased the crystallization temperature. The change in the half-crystallization time (t1/2) was similar to the crystallization temperature. Thus, the cross-linking of PPS affected crystallization behaviors significantly. To a certain extent, crosslinks acted as nucleation agents, but excessive cross-linking hindered the crystallization. Morphologies observed by polarized optical microscopy suggested that thermal curing for as little as 1 day contributed to the spherulitic structure having a smaller size, that was not observed with pure PPS.
High performance thermoplastics have been developed and commercialized to satisfy the increasing demand for polymeric materials with such properties as thermal stability, high strength and modulus, and chemical resistance. Poly(p-phenylene sulfide) (PPS) is one of the high performance polymers, having outstanding properties compared to conventional thermoplastics such as polyethylene and polypropylene [
One of the interesting characteristics of PPS is curability, which means that PPS undergoes chemical reactions on exposure to an oxygen-containing atmosphere at high temperatures, leading to an increase of viscosity due to cross-linking [
The change of the internal structure of PPS that results from the curing reactions inevitably causes a change in the crystallization behavior, which is closely related to the morphology developed during processing. In this paper, the effect of the curing of PPS on its thermal properties and crystalline morphologies was studied. A commercial PPS resin was thermally cured from 1 to 16 days at 250˚C. To investigate the change of molecular weight during curing, the intrinsic viscosities of as-received and cured PPS were measured. Non-isothermal crystallization kinetics were investigated by differential scanning calorimetry (DSC). Furthermore, polarized optical microscopy (POM) was used to observe the morphologies of pure and cured PPS.
PPS powders (W316 grade, density and molecular weight of 1.35 g/cm3 and 35,000 g/mol, respectively) supplied by Kureha Chemical Industry Co., Ltd (Japan) were used for this study. The PPS powders were encased in a 20 mL vial and then the samples were cured at 250˚C for 1 to 16 days in an air atmosphere.
The intrinsic viscosities of pure and cured PPS were obtained using a Brookfield viscometer with a Thermosel system and a S18 spindle (DV-II + Pro, Brookfield Co., UK). Samples were dissolved in 1-chloro naphthalene for preparing 4 wt% polymer solutions at 210˚C. The intrinsic viscosities were calculated using the Solomon-Ciuta relation as follows [
where k is 0.302 for PPS as a material property, C is the concentration of polymer in 1-chloro naphthalene, [η] is the intrinsic viscosity, and ηsp is the specific viscosity. The viscosity-average molecular weight was calculated using the Mark-Houwink-Sakurada (MHS) equation as follows:
where K and a are 8.91 × 10−5 and 0.747 for PPS, respectively [
Thermal analysis was performed in a differential scanning calorimeter (DSC Q20, TA Instruments, USA). Samples were heated up to 350˚C at a rate of 10˚C/min and held at that temperature for 10 min to erase the thermal histories of the samples. Subsequently, samples were cooled down to 50˚C at a rate of 10˚C/min and then heated up to 350˚C again at a rate of 10˚C/min to study the effect of curing on the non-isothermal crystallization. The crystalline morphologies were examined in an Olympus BX-60 optical microscope (Olympus, Japan) with cross-polarizers. The samples were loaded on a hot stage (Mettler Toledo FP82HT, USA), and heated up to 350˚C at a rate of 10˚C/min. Samples were squeezed to be a thin film with holding at 350˚C for 10 min. Then, samples were cooled down to 50˚C at a rate of 10˚C/min, and the crystalline morphologies of the samples observed.
The intrinsic viscosities of as-received and thermally cured PPS at 250˚C were obtained using the SolomonCiuta relation. Since samples cured for more than 7 days did not dissolve in 1-chloro naphthalene, intrinsic viscosities [η] of pure samples and samples cured for 1, 3, or 5 days were measured. Note that the curing took place below the melting temperature (280˚C from the manufacturer), so that a solid-state cure was applied in this study.
The crystallization behavior of pure and cured PPS was investigated using non-isothermal DSC (
Scheme 1. Crosslinking mechanism of PPS during the curing process.
first heating scan curve in
The PPS samples were held at 350˚C for 10 min after the first heating scan to remove the prior thermal history and then slowly cooled at 10˚C/min. In the first cooling, the crystallization temperature (Tc) of pure PPS was 213.9˚C. There was a significant increase of Tc for the 1 day cured sample (224.9˚C), and then a continuous further increase for samples cured for up to 7 days (228.8˚C). It is interesting to note that the further curing resulted in a decrease of crystallization temperature with the 16 days cured sample showing a Tc of 177.3˚C. It is likely that the curing induced cross-linking structure led to a higher Tc in the system because cross-linking structure acted as a nucleating agent. However, the further increase of curing time beyond 7 days caused the cross-linking structure to become dominant, which would hinder the formation of a crystalline structure.
Inoue and Suzuki described the effects of the crosslinking of ethylene-propylene-diene terpolymer (EPDM) particles on the crystallization of the polypropylene (PP) matrix [
The second heating scans were also obtained to investigate the melting behavior after the first controlled cooling in DSC because the first heating scans reflect the thermal history experienced during the thermal curing and initial processing. The second heating scans are presented in
the samples were determined by dividing ∆Hf of crystallization by the value of 80.4 J/g for PPS [
The non-isothermal crystallization kinetics of pure PPS and the cured samples were analyzed. The relative degree of crystallinity, X, at crystallization time t can be obtained by the following equation:
where To and Te are the onset and end crystallization temperatures, respectively, and dHc/dT is the heat flow rate. In non-isothermal crystallization, the time t is related with the temperature T as follows [13,14]:
where T is the temperature at time t, To is the temperature at which the crystallization begins (t = 0) and is the cooling rate. Using the above two equations, the X vs. t plots of pure PPS and the cured samples in
The crystalline morphologies of pure and cured PPS were observed using polarized optical microscopy (POM). To mimic the thermal history of the non-isothermal study in DSC, the samples were loaded on a hot stage and heated up to 350˚C at a rate of 10˚C/min before being held at 350˚C for 10 min. Subsequently, the samples were cooled down to 50˚C at a rate of 10˚C/min, and the crystalline morphologies of the samples observed. The pure PPS (
Commercial PPS was thermally cured, resulting in an increase of its molecular weight due to cross-linking as compared to the original sample; the viscosity-average molecular weight of samples cured for 5 days was ~58,000 g/mol. The crystallization temperature (Tc) of pure PPS was found to be 213.9˚C by non-isothermal DSC studies. The values of Tc increased monotonously for samples cured for up to 7 days (228.8˚C), but a further increase in curing time up to 16 days led to a decrease of Tc down to 177.3˚C. In addition, the change of the half-crystallization time (t1/2) was similar to that of the crystallization temperature. Pure PPS, and samples cured for 7 days and 16 days, showed half-crystallization times of 1.46, 1.01, and 1.97 min, respectively. Thus, the cross-linking of PPS affected the crystallization behaviors significantly; a certain amount of cross-linking structure acted as a nucleation agent, but excessive cross-linked hindered the crystallization. Crystalline morphologies observed by polarized optical microscopy suggested that thermal curing for as little as 1 day contributed to the formation of smalller size crystalline structures, that the larger spherulites were not observed for pure PPS.
This work was supported by a grant from Korea Institute
of Science and Technology Institutional program and the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea.