Effects of Molecular Weight on the Structure of Poly(phenylene sulfide) Crystallized at Low Temperatures

Structural, Thermal, and Mechanical Characterization of a Thermally Conductive Polymer Composite for Heat Exchanger Applications

Polymer composites are being considered for numerous thermal applications because of their inherent benefits, such as light weight, corrosion resistance, and reduced cost. In this work, the microstructural, thermal, and mechanical properties of a 3D printed polymer composite with high thermal conductivity are examined using multiple characterization techniques. Infrared spectroscopy and X-ray diffraction reveal that the composite contains a polyphenylene sulfide matrix with graphitic fillers, which is responsible for the high thermal conductivity. Furthermore, differential scanning calorimetry determines that the glass transition and melting point of the composite are 87.6 °C and 285.6 °C, respectively. Thermogravimetric analysis reveals that the composite is thermally stable up to ~400 °C. Creep tests are performed at different isotherms to evaluate the long-term performance of the composite. The creep result indicates that the composite can maintain mechanical integrity when used below its glass transition temperature. Nanoindentation tests reveal that modulus and hardness of the composite is not significantly influenced by heating or creep conditions. These findings indicate that the composite is potentially suitable for heat exchanger applications.

Distinct dynamics of structural relaxation in the amorphous phase of poly(l-lactic acid) revealed by quiescent crystallization

Fast scanning calorimetry (FSC) experiments were performed to investigate physical aging in amorphous and semi-crystalline poly(l-lactic acid)s (PLLAs) that were thermally crystallized under conditions leading to the α'- or α-crystalline form, and either favouring or inhibiting the development of a rigid amorphous fraction (RAF). The enthalpy of recovery was calculated after two procedures of rescaling to the content of the whole amorphous phase and also to the only content of the mobile amorphous fraction (MAF), which helped in clarifying the contribution of the RAF. From the dependence of the structural relaxation rate on the aging temperature, two regimes were evidenced for all samples. In the aging temperature domain situated close to the glass transition, the structural relaxation occurs significantly faster in the MAF. Its rate is independent of the aging temperature and is not influenced by the microstructure. However, the distance to equilibrium is higher in samples for which the coupling is strong between crystal and amorphous, implying that the time to reach equilibrium is also higher. In contrast, at low aging temperatures, for which the whole amorphous phase can be considered as solid, MAF and RAF exhibit the same structrural relaxation rate. This convergence in the relaxation kinetics by decreasing the temperature of physical aging was interpreted as the evolution of relaxation dynamics in the MAF from segmental to local. This change is highlighted by the comparison between MAF and RAF relaxation kinetics, but it occurs similarly in a pure amorphous system.

Thermal, viscoelastic and mechanical properties' optimization of polyphenylene sulfide via optimal processing parameters using the Taguchi method

Thermal, viscoelastic and mechanical properties of polyphenylene sulfide (PPS) were optimized as a function of extrusion and injection molding parameters. For this purpose, design of experiments approach utilizing Taguchi's L27 (37) orthogonal arrays was used. Effect of the parameters on desired properties was determined using the analysis of variance. Differential scanning calorimeter (DSC) tests were performed for the analysis of thermal properties such as melting temperature (Tm) and melting enthalpy (ΔHM). Dynamic mechanical analysis (DMA) tests were performed for the analysis of viscoelastic properties such as damping factor (tan δ) and glass transition temperature (Tg). Tensile tests were performed for the analysis of mechanical properties such as tensile strength and modulus. With optimized process parameters, verification DSC, DMA and tensile tests were performed for thermal, viscoelastic and mechanical properties, respectively. The Taguchi method showed that 'barrel temperature' and its level of '340°C' were seen to be the most effective parameter and its level; respectively. It was suggested that PPS can be reinforced for further improvement after optimized thermal, viscoelastic and mechanical properties.

Synthesis and characterization of silane-grafted polyphenylene sulfide

A novel polyphenylene sulfide (PPS) modified by silane grafting and water cross-linking technology has been prepared using suspension grafting. Fourier transform infrared and x-ray photoelectron spectroscopic spectra demonstrated the achievement of silane grafting and subsequent water cross-linking. The grafting degree could be adjusted by altering the silane/PPS molar ratio, reaction temperature, reaction time, and so on. Wide angle x-ray diffraction analysis showed that the crystallinity and crystal size of the modified PPS decreased with increase in the grafting degree. Differential scanning calorimeter results also confirmed that the crystallinity and crystallization temperature decreased with the increase in the grafting degree. The silane-grafted PPS exhibited two weight loss stages in thermogravimetric curves, and the initial thermal decomposition temperature was much higher than the processing temperature. Therefore, the silane-grafted PPS is thermally stable enough to be used as a high-performance polymer and polymer matrix for structural applications. The extent of silane grafting plays a key role in the improvement of the impact toughness of the silane-grafted PPS. With the increasing silane grafting degree, the impact strength of the silane-grafted PPS/neat PPS blends obviously increased.

Glass transition and the origin of poly(p-phenylene sulfide) secondary crystallization

The effect of low temperature cold-crystallization on quenched poly(p-phenylene sulfide) (PPS) amorphous phase behaviour was systematically investigated by differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) over the entire range of the process, from its early stage to the end. For the first time a well resolved double glass transition of partially cold-crystallized samples was evidenced. A T(g) steady increase was observed during the primary crystallization process, due to the reduction of amorphous chain segmental mobility imposed by the growing rigid phase. A shift of the relaxation temperature of about 10 degrees C was recorded at the end of the primary crystallization process. As the secondary crystallization takes place a new glass transition appears at higher temperature. For longer annealing time the lower T(g) disappears while the intensity of the upper one increases. The upper temperature glass transition of semi-crystalline PPS is explained as a consequence of the PPS secondary cold-crystallization process. In the light of the thermal and dynamic mechanical results, an interpretation is given of step-wise double crystallization, evidenced in non-isothermal cold-crystallization experiments carried out at low heating rate and followed by means of FT-IR techniques.

Thermodynamic properties of several soil- and sediment-derived natural organic materials

Improved understanding of the structure of soil- and sediment-derived organic matter is critical to elucidating the mechanisms that control the reactivity and transport of contaminants in the environment. This work focuses on an experimental investigation of thermodynamic properties that are a function of the macromolecular structure of natural organic matter (NOM). A suite of thermal analysis instruments were employed to quantify glass transition temperatures (Tg), constant-pressure specific heat capacities (Cp), and thermal expansion coefficients (alpha) of several International Humic Substances Society (IHSS) soil-, sediment-, and aquatic-derived NOMs. Thermal mechanical analysis (TMA) of selected NOMs identified Tgs between 36 and 72 degrees C, and alphas ranging from 11 mum/m degrees C below the Tg to 242 mum/m degrees C above the Tg. Standard differential scanning calorimetry (DSC) and temperature-modulated differential scanning calorimetry (TMDSC) measurements provided additional evidence of glass transition behavior, including identification of multiple transition behavior in two aquatic samples. TMDSC also provided quantitative measures of Cp at 0 and 25 degrees C, ranging from 1.27 to 1.44 J/g degrees C. Results from TMA, DSC, and TMDSC analyses are consistent with glass transition theories for organic macromolecules, and the glass transition behavior of other NOM materials reported in previous studies. Discussion of the importance of quantifying these thermodynamic properties is presented in terms of improved physical and chemical characterization of NOM structures, and in terms of providing constraints to molecular simulation models of NOM structures.