Investigation on curing reaction of phthalonitrile resin with nanosilica and the properties of their glass fiber-reinforced composites

In this work, Phthalonitrile containing benzoxazine (BA-ph) and Bisphenol A based cyanate ester (CE) were chosen as the matrix resin. Various amount of nano-SiO₂ was incorporated into BA-ph/CE and their glass fiber-reinforced composite laminates were fabricated. Curing reaction and processability of BA-ph/CE/SiO₂ blends were studied by differential scanning calorimetry and dynamic rheological analysis. Results showed that BA-ph and CE exhibited good processability and curing reaction of BA-ph/CE was not obviously affected by SiO₂. Scanning electron microscope images of the composites showed that SiO₂ particles were well dispersed in BA-ph/CE matrix. Moreover, SiO₂ could act as physical crosslinking points and diluent in matrix as well as between the glass fibers to improve the mechanical properties of composite laminates. As the results of dynamic mechanical analysis and thermogravimetry analysis, composite laminates possessed satisfactory T_g and good thermal stability. With incorporation SiO₂ particles into matrix resin, dielectric constant and dielectric loss of BA-ph/CE/SiO₂/GF composites were increased and showed frequency dependence.     

Phase transitions and glass transition in a hyperquenched silica‐alumina glass

We investigate phase transitions, glass transition, and dynamic behavior in the hyperquenched 69SiO₂–31Al₂O₃ (mol%) glass (SA glass). Upon reheating, the SA glass exhibits a series of thermal responses. Subsequent to the sub‐T_g enthalpy release, the glass undergoes a large jump in isobaric heat capacity (ΔC_p) during glass transition, implying the fragile nature of the SA glass. The mullite starts to form before the end of glass transition, indicating that the SA glass is extremely unstable against crystallization. After the mullite formation, the remaining glass phase exhibits an increased T_g and a suppressed ΔC_p. The formation of cristobalite at 1553 K indicates the dominance of silica in the remaining glass matrix. The cristobalite gradually re‐melts as the isothermal heat‐treatment temperature is raised from 1823 to 1853 K, which is well below the melting point of cristobalite, while the amount of the mullite remains unchanged.     

New methodologies for separating glass transitions from energy of film formation in polystyrene latexes

Polystyrene latexes were synthesized to study water evaporation details, and to separate the energy of film formation from the glass transition temperature. Three experimental techniques were employed in the present work to study the formation of polystyrene latex films: standard differential scanning calorimetry (DSC), modulated DSC, and the Mahr method applied to modulated DSC. High-resolution thermogravimetric analysis (TGA) was used to study the water evaporation. The TGA experiments revealed evaporation curves consistent with the notion of surface tension and tortuosity effects being the controlling factors. The method of Mahr was combined with the new software package for DSC to provide an improved method of separating the glass transition of polystyrene from its surface tension-based free energy of film formation. The result was that the films formed under the extremely mild conditions employed had 30–70% of their surfaces obliterated by interfacial adhesion. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1763–1768, 1999     

差示扫描量热法测定聚丙烯腈纤维玻璃化转变温度

本文采用差示扫描量热法,两段升温模式,以10℃/min加热速度,从30℃加热到150℃,测定了4个聚丙烯腈纤维样品的玻璃化转变温度,探讨了加热方法和样品量试验条件,得到了准确可靠的玻璃化转变温度。     

粉末涂料的玻璃化温度

重点讨论了粉末涂料玻璃化温度、玻璃化转变理论,并对玻璃化温度与粉末涂料稳定性、熔融黏度、热应力、分子量、化学结构和聚合物的关系做了全面的阐述。     

Interrelation between dyeing and thermal properties of PET fibers

The interrelation between dyeing and the thermal properties of micro and conventional poly(ethylene terephthalate) fibers is studied with conventional and modulated differential scanning calorimetry. X‐ray diffraction, density, and birefringence studies are used to confirm the obtained results. It is shown that three studied anthraquinone dyes, in contrast to three studied benzodifuranone dyes, act as plasticizers for the fibers. A comparison between fibers and partially crystallized bulk samples is made. The specific fiber morphology makes the fibers more susceptible to plasticization than are bulk samples. This ability of a dye to lower the glass transition of the fibers will influence the dye diffusion and this information is needed to optimize the dyeing process for a specific dye–fiber combination. Further, the presence of an anthraquinone dye in the fiber alters the melting endotherm by changing the stability of the original crystals. The onset of the melting and recrystallization process is lowered. Although still well above the dyeing temperature, this lowering may be critical for the dimensional stability of the fabrics during any subsequent high‐temperature process. A first investigation of the effect of variations in the thermal properties on the obtained color strength after dyeing is reported. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 105–114, 2003     

Confinement effects at nanoscale in natural rubber composites: Influence on macroscopic properties

The study of molecular level interactions in elastomer composites has got very scant attention even though a large number of studies are going on in this topic. This work embodies the understanding of confinement effects in natural rubber (NR)/ ZnO composites from micro to nano length scales, and their influence on macroscopic properties of the composites. The interactions between fillers and matrix are characterized from the Donth's approach, which allows estimating the cooperativity size N_α at the glass transition, N_α being directly related to the filler-matrix interactions. The improved properties of NR-nano ZnO composites can be attributed to microstructural and morphological changes due to nano ZnO in the NR matrix. A correlation between N_α, the constrained volume C_v and the thickness of immobilized polymer chains χ_m at the glass transition has also been established. Thus, this work proves that the Donth's approach is a powerful probe to estimate the enhance of mechanical properties in nanocomposites from calorimetric investigations.     

The glass transition temperature of poly(N-vinyl pyrrolidone) by differential scanning calorimetry

Previously a wide range of values have been reported for the glass transition temperature, T_g, of poly(N-vinyl pyrrolidone), PVP, and it was suggested that lower values are due to variable uptakes of water caused by the hygroscopic nature of the polymer. Now it has been found that there are large variations in T_g, even in carefully dried specimens of PVP. Other factors found to influence T_g are residual monomer and the molecular weight of PVP. Polymers prepared by bulk polymerization, either by γ-irradiation or by heating with 2-azobisisobutyronitrile, have much lower values of T_g than dried ones prepared containing 30% water. The difference is mainly due to depression of T_g by residual monomer which, in the absence of water during polymerization, fails to react completely because of conversion to a glassy state. An unexplained observation is that even when all residual monomer has been removed, polymers prepared by bulk polymerization still have a lower T_g than would be expected from their molecular weight.     

Studies of thermal,mechanical properties,and kinetic cure reaction of carboxyl-terminated polybutadiene acrylonitrile liquid rubber with diepoxy octane

Epoxy resins, despite their unique properties, have limitations in many applications due to their low fracture toughness. One of the most effective methods to overcome this limitation is to use toughening agents, such as carboxyl terminated poly butadiene-acrylonitrile (CTBN) in the epoxy matrix. CTBN can react with various compounds, such as epoxies. In this study, we investigated the severity of CTBN sedimentation with di epoxy octane (DEO) in the presence basis catalysts. The studied of the physical properties of the synthesized copolymer in the presence of pyridine compared to other catalysts increases mechanical properties (248.43% elongation, 0.63 MPa strength, and 32 hardness with Shore A) and decreases the glass transition temperature (−45.1) of the copolymer. Investigated the cure kinetics of the CTBN-DEO reaction was in the presence of pyridine using a nonisothermal technique of differential scanning calorimetry, and the curing kinetic parameters, such as activation energy (E_a), pre-exponential factor (A), and rate constant (k), were calculated by different kinetic methods. The obtained curing kinetic values with different kinetic methods are well-matched, the E_a values are in the range of 91.3–97.1 kJ.mol⁻¹ and the A values are in the range of 0.48 × 10¹¹–1.51 × 10¹¹ S⁻¹.     

温度调制热分析技术对预吸热峰的研究

以快速的冷却速率冷却熔体获得的金属玻璃通常远离平衡状态。如果在适当温度经过合适时间退火后,在热流曲线上玻璃转变温度之前会产生明显的预吸热峰。使用常规差示扫描量热法(DSC)和温度调制差示扫描量热法(TMDSC)对Pd₄₀Ni₁₀Cu₃₀P₂₀和Au₄₉Ag_5.5Pd_2.3Cu_26.9Si_16.3两种金属玻璃进行研究。结果表明,预吸热峰从玻璃态到过冷液态是一个连续演变过程,当预吸热峰出现在玻璃转变温度之下时,预吸热峰向高温移动过程中对玻璃化转变过程没有影响。然而,当预吸热峰出现在玻璃化转变温度以上时,玻璃转变开始温度会随着预吸热峰值的增加而增加,因此金属玻璃的动力学稳定性得到了提高。实验结果对理解预吸热峰从玻璃态到过冷液态过程中的稳定变化具有重要意义。     

Temperature memory effect from below glass transition to up to melting range in an ethylene-vinyl acetate copolymer

In this paper, the temperature memory effect (TME) in a commercial ethylene-vinyl acetate copolymer (EVA) is characterized via differential scanning calorimetry (DSC) tests. Three temperatures, which are 35, 60, and 85°C representing temperatures below glass transition (T_g), within T_g and within melting (T_m), respectively, are included for the investigation. It is found that TME in polymers is not as reported to be limited at around either T_g or T_m only, the effective temperature range for TME could be actually much wider that covers from below T_g to up to T_m. In addition, it is concluded that higher heating stop temperature (T_s) erases the memory of previous lower ones in this EVA. Hysteresis (described by ΔT) between the temperature of turning points (T_ts) and their corresponding T_ss is always observable. However, the ΔT decreases at higher T_s.     

Thermal and mechanical properties of PES/PTFE composites and nanocomposites

Polyethersulphone/polytetrafluoroethylene (PES/PTFE) nanocomposites and composites were prepared by precipitation of PES into a PTFE latex‐containing nanoparticles. Different samples were obtained by varying the relative ratio between PES and PTFE. The complex crystallization process, discussed within the fractionated crystallization frame, allowed to identify and quantify different dispersion degree of the PTFE nanoparticles within the PES matrix. The different samples were thus divided into nanocomposite and composites. The effect of crystalline PTFE domains on the mobility of PES was investigated and discussed. The dynamic‐mechanical behavior was explained in terms of the particle aggregation state. The mechanical properties of the PES/PTFE composites were found to depend on both the dispersion and the concentration of the PTFE nanoparticles. In the glassy state the stiffness of the materials was found to increase with the dispersion degree, resulting higher for the nanocomposite with respect to composites. On the contrary, in the rubbery state the modulus was found proportional to the PTFE nanoparticles concentration, resulting higher in the composites with respect to the nanocomposite. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3624–3633, 2013     

Optical properties of injection‐molded polystyrene scintillators. II. Distribution of dopants

The distribution of fluorescing dye solutes in scintillating tiles for the Tilecal/Atlas project is assessed, and a link between the homogeneity of the dopant distribution and the optical yield and nonuniformity is established. The effect of the injection‐molding parameters on the dye distribution is also analyzed, as well as the actual dye incorporation into the scintillators. This incorporation has been assessed with a set of experiments performed with laboratory samples with controlled amounts of additives and with samples obtained from injection‐molded scintillators. Differential scanning calorimetry has been used to characterize the raw material and to establish a link between the thermophysical properties and the processing conditions, and it is proven to be a quite appropriate technique. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2714–2718, 2003     

Factors influencing the glass transition of DGEBA-anhydride epoxy resins

A number of chemical and physical factors influence the glass transition behaviour of anhydride-cured diglycidyl ether of bisphenol A (DGEBA) type epoxy resins. Several of these were investigated using differential scanning calorimetry to assess the nature of the glass transition, the transition temperature and the glass- and liquid-phase specific heats. The influence of molecular weight and chemical functionality, cooling rate and silica fillers were investigated for unreacted epoxy prepolymers. Two anhydride-cured systems were also considered and the influence of cure with and without silica fillers and the effect of phase separation and cooling rate were investigated.     

Thermal properties relevant to the processing of PET fibers

The thermal properties of poly(ethylene terephthalate) (PET) conventional fibers and microfibers are measured and compared to bulk samples. It is shown that the glass transition temperature (T_g) of the fibers can be monitored with modulated differential scanning calorimetry (MDSC). The T_g region is about 30°C wide and shifted to approximately 110°C for conventional as well as for micro‐PET fibers. The T_g of these fibers is compared to the T_g of cold‐crystallized bulk samples. Upon crystallization, a shift and even a split up of T_g is observed. The second T_g is much broader and is situated around 90°C. This T_g is related to the appearance of a rigid amorphous phase. In comparison, the mobility of the amorphous phase in fibers is even more restricted. The whole multiple melting profile observed on the fibers is the result of a continuous melting and recrystallization process, in contrast to bulk PET. The heat‐set temperature is shown to trigger the start of melting and recrystallization. It is seen in the MDSC as an exotherm in the nonreversing signal and an excess contribution in the heat‐capacity signal. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3840–3849, 2003     

The glass transition of amorphous polymers: About a continuous equation of state and its derivatives

Describing the pressure-volume-temperature (PVT) behavior of polymers is often hindered due to limitations of models and theories. In this work, we propose the logistic function to derive a phenomenological equation of state (EOS) that overcomes these limitations. Without dividing the temperature domain into several intervals, this continuous EOS describes the PVT behavior of amorphous polymers in the equilibrium state, the vitreous state and at glass transition. Discontinuities at glass transition are inherently eliminated. The EOS is continuously differentiable with respect to temperature and pressure. The thermal expansion coefficient and compression modulus thus can be determined at any valid reference state. The EOS linearly depends on temperature both in the equilibrium state and vitreous state. Furthermore, a parameter of the EOS is interpreted as the pressure-dependent glass transition temperature. Using PVT data of polyethersulfone (PES), measured in the temperature range of 20 − 360 °C and pressure range of 10 − 200 MPa in both the isobaric and standard isothermal mode, the validity of the EOS is discussed. Polymer processing simulation and solid mechanics benefit from the EOS and its derivatives, respectively. In addition, the EOS can be used to determine the glass transition temperature from PVT data.     

Detection of sub-glass transition events in amorphous polymers using differential scanning calorimetry

The variation of the heat capacity (C_p) as a function of temperature has been measured for a sample of poly(di-cyclo-octyl itaconate) using a differential scanning calorimeter. Two inflexions in the C_pT curve have been detected. One, observed at 390K has been assigned to the glass-rubber transition, the other, located at 140K, is a consequence of intramolecular motion in the pendant cyclo-octyl rings. The heat capacity contribution calculated from the partition function describing a series of cyclo-octane conformers agreed closely with the experimental values associated with this lower inflexion, and supported our interpretation of the molecular origins of this transition.     

Evolution of the amorphous fraction of PEEK during annealing at atmospheric and high pressure above the glass transition temperature

The constrained fraction of the amorphous phase of semi‐crystalline polymers is in an out‐of‐equilibrium state so that "physical aging"‐like features can be observed (e.g., by calorimetry) even above the glass transition temperature. This was already addressed in the literature in several semi‐crystalline polymers at atmospheric pressure. Despite the well‐known influence of pressure on molecular mobility, the pressure‐sensitivity of these microstructure rearrangements has never been tackled. This study focuses on annealing in highly pressurized Poly‐Ether‐Ether‐Ketone (PEEK), compared with atmospheric pressure. The phenomenon is tracked by ex‐situ Differential Scanning Calorimetry (DSC). A significant influence of pressure is evidenced, without any complete equivalence with temperature. Indeed, pressure seems to confine rearrangements within spatially limited domains. The stability and coexistence of reorganization processes upon successive annealings is also investigated. Finally, relationships between constrained and free amorphous phase rearrangements are discussed via the different glass transition shifts observed after atmospheric or high pressure annealing. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1148‐1157, 2013     

Effect of cooling rate and frequency on the calorimetric measurement of the glass transition

The glass transition of thermoplastics of different polydispersity and thermosets of different network structure has been studied by conventional differential scanning calorimetry (DSC) and temperature modulated DSC (TMDSC). The cooling rate dependence of the thermal glass transition temperature T_g measured by DSC, and the frequency dependence of the dynamic glass transition temperature T measured by TMDSC have been investigated. The relation between the cooling rate and the frequency necessary to achieve the same glass transition temperature has been quantified in terms of a logarithmic difference Δ=log₁₀[|q|]−log₁₀(ω), where |q| is the absolute value of the cooling rate in K s⁻¹ and ω is the angular frequency in rad s⁻¹ necessary to obtain T_g(q)=T(ω). The values of Δ obtained for various polymers at a modulation period of 120 s (frequency of 8.3 mHz) are between 0.14 and 0.81. These values agree reasonably well with the theoretical prediction [Hutchinson JM, Montserrat S. Thermochim Acta 2001;377:63 [6]] based on the model of Tool–Narayanaswamy–Moynihan with a distribution of relaxation times. The results are discussed and compared with those obtained by other authors in polymeric and other glass-forming systems.     

Evolution of mechanical properties during cure for out‐of‐autoclave carbon‐epoxy prepregs

Extent of cure and rheological properties were obtained for out‐of‐autoclave materials, Cycom 5320‐8HS and Cycom 5320‐PW, for the manufacturer recommended cure cycle using differential scanning calorimeter and encapsulated sample rheometer (ESR), respectively. Rheological properties from ESR were further used in designing the cure cycles to study the evolution of mechanical properties. Five panels were cured at different cure stages using the designed cure cycles and coupons were tested for short beam shear and combined loading compression properties at different cure stages. To correlate the mechanical properties with its respective glass transition temperature, dynamic mechanical analyzer was used to obtain the glass transition temperature for the coupons obtained from the respective panels. Statistical results showed significant difference in short beam shear and combined loading compression properties up to vitrification, however, no significant difference was observed on these mechanical properties after vitrification. The observed linear trend between degree of cure (DOC) and glass transition temperature (T_g) was validated using DiBenedetto relation. Linearly increasing trend between DOC and glass transition temperature (T_g) for different cure states suggests that both DOC and T_g can be used interchangeably to define the state of material. A good correlation was observed between material cure state and the mechanical properties. A mathematical model was also proposed to determine the short beam shear and combined loading compression properties based on material cure state. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41548.