Temperature dependence of the thermal expansivity in polymer

The effects of temperature on the specific volumes and thermal expansivities for a range of amorphous polymers, above and below the glass transition temperature, are treated on the basis of the physical properties of polymers. The results are found to be in good agreement with observed data. The analysis of the results shows that the temperature derivative of the zeropressure thermal expansivity of the liquid polymer increases with increasing temperature. The change in the thermal expansivity, Δα = α_OL ? α_OG, decreases with increasing temperature.     

Morphology and thermal properties of two polymethacrylates modified by a polymer liquid crystal

We have studied blends of a polymer liquid crystal (PLC) with poly(cyclohexylethyl methacrylate) (PCHEMA) or poly(cyclohexylpropyl methacrylate) (PCHPMA). The PLC is PET/0.6PHB where PET = poly(ethylene terephthalate), PHB = p-hydroxybenzoic acid and 0.6 is the mole fraction of the latter in the copolymer. The microstructure was studied by scanning electron microscopy (SEM). PCHEMA + PLC (20 wt% of the latter, blend E) has a fine texture with LC islands evenly distributed in the matrix and good adhesion between the phases resulting from their partial miscibility. The PCHPMA + PLC (20 wt% of the latter, blend P) shows only limited compatibility. The SEM results are confirmed by values of the glass transition temperatures T_g determined via thermal mechanical analysis. The T_g value of the blend E is shifted towards the T_g of PLC; T_g of blend P is practically equal to that of PCHPMA. The linear isobaric expansivity α_L values for both blends are lower than the respective values for pure PCHPMA and PCHEMA. Thermal stabilities of the blends determined by thermogravimetry are also better than those of pure polymethacrylates. The temperature of 50% weight degradation for blend E is higher than that for pure PCHEMA by more than 60 K Copyright © 2004 Society of Chemical Industry     

Thermal conductivity and thermal expansivity of in situ composites of a liquid crystalline polymer and polycarbonate

The thermal conductivity and thermal expansivity of extruded blends of a liquid crystalline polymer (LCP) and polycarbonate (PC) with volume fraction (V_f) of LCP between 0.09 and 0.8 have been measured as functions of draw ratios λ ranging from 1.3 to 15. At V_f < 0.3, the LCP domains are dispersed in a PC matrix and the aspect ratio of the domains increases with increasing λ. At V_f > 0.55, phase inversion has occurred and the LCP becomes the continuous phase. The axial thermal conductivity K_∥ increases while the axial expansivity α_∥ decreases sharply with increasing λ, as a result of the higher aspect ratio of the LCP fibrils and the improved molecular orientation within the fibrils. Since the transverse thermal conductivity and expansivity are little affected by drawing, the blends exhibit strong anisotropy in the thermal conduction and expansion behavior at high λ. At V_f < 0.3, the behavior of K_∥ is reasonably modeled by the Halpin-Tsai equation for short fiber composites. At high draw ratio (λ = 15), all the blends behave like unidirectional continuous fiber composites, so K_∥ and α_∥ follow the rule of mixtures and the Schapery equation, respectively.     

Strain‐induced crystallization behavior of natural rubber and trans‐1,4‐polyisoprene crosslinked blends

The strain‐induced crystallization (SIC) behaviors of crosslinked blends based on natural rubber (NR) and trans‐1,4‐polyisoprene (TPI) with different content of TPI were probed explored by using synchrotron two‐dimensional wide angle X‐ray diffraction and dynamic mechanical analysis. The results showed that when TPI content is less than 70% no reflection peak of TPI but NR crystallite diffractions can be observed and the diffractions of TPI βform appear when TPI content is 70 wt % in the cocured blend. SIC of cocured blends started at smaller strain ratio than the pure NR. By calculating ΔS_def, it is found that the drop in entropy upon strain decreased when TPI is incorporated into NR due to the reduction of molecular mobility of NR. The degree of SIC and crystallization rate index in crosslinked blends monotonously decreased with the increase of TPI content. The apparent crystallite size exhibited some surprising variations. L₂₀₀ and L₁₂₀ decreased with the increase of TPI content in the cocured blends. These observations were usually caused by two factors: (i) Less number of polymer chains could involve in crystal growth due to the lower mobility of polymer chains in the cocured blends which is proved by dynamic mechanical analysis results; (ii) The mean distance between nuclei decreases, which was caused by the fluctuation of crosslink density in NR phase derived from the heterogeneous distribution of curatives in two phases supported by the varying tendency of curing degree and crosslink density. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Thermal expansivity of oriented nylon-6 and nylon-6,6

Measurements on the thermal expansivities along (α_∥) and normal (α⊥) to the draw direction have been carried out from 120 to 380K for nylon-6 and nylon-6,6 with draw ratio λ between 1 and 3.6. The sharp drop in α∥ and the slight increase in α⊥ with increasing λ can be attributed to the gradual alignment of chain segments in both the crystalline and amorphous regions. Using the presently available expansivity data on nylon-6 crystals it is found that the observed orientation dependence below the glass transition can be quantitatively described by a two-phase aggregate model. Water acts as a plasticizer and so its absorption leads to a drop of about 80K in the glass transition temperature. At low temperature, however, water appears to form bridges between molecular chains. This gives rise to a stronger interchain interaction and hence a lowering of the thermal expansivity.     

Effect of processing route on thermomechanical properties of low temperature firing ceramic for electronic packaging

Abstract

Mechanical properties and thermal expansivities of two compositionally identical ceramics intended for use in 'low temperature cofired ceramic' (LTCC) technology were investigated. Both were based on a commercial MgCaTiO₃ dielectric ceramic with the sintering temperature reduced by addition of ZnO-B₂O₃-SiO₂, either in the glassy state or as separate glass forming oxides. Although in each case the additions were accompanied by decreases in elastic modulus, flexural strength, hardness, fracture toughness, and linear thermal expansivity, the values remained close to those for commercial LTCC materials. The route which involved mixing the separate oxides produced a slightly tougher material, which also had a thermal expansivity closer to the optimum value. The study demonstrates that an acceptable LTCC ceramic can be produced starting from glass forming oxides as sintering aids, thus avoiding the need for glass melting and comminution steps.     

λ-PHASE TRANSITIONS IN ICE IH NEAR THE MELTING POINT

This study gives our predictions for the thermal expansivity α _p , isothermal compressibility κ _T , and specific heat Cp near the melting point for ice Ih. We calculate those thermodynamic quantities as functions of temperature and pressure near the melting point using the experimental data for the thermal expansivity as a function of pressure from the literature. The Pippard relations are validated by plotting C_p against α _p and also α _p against κ _T linearly close to the melting point in ice Ih. We deduce the values of the slope ?P _m /?T from those linear plots, which are very close to the experimental value obtained from the P-T phase diagram of ice Ih along the melting curve.     

Retardation of cold flow in immiscible rubber blends by tailoring their microstructures

Cold flow is a well‐known characteristic and also an unresolved drawback for uncured rubber materials. In this paper, a simple approach of retarding the cold flow of cis‐1,4‐polybutadiene rubber (BR) elastomer is reported by controlling the phase separation and crystallization that occurs in immiscible BR/trans‐1,4‐polyisoprene (TPI) blends. The BR/TPI blends showed an untypical phase diagram below 150 ^oC. Upon crystallization the amorphous BR facilitates the nucleation of TPI. The higher the BR content is, the less the surface roughness of the TPI crystals, and then a larger dendritic pattern that resulted from the cold flow of BR was observed in the microstructure. BR/TPI blends with the highest resistance to cold flow were obtained by optimizing the composition and thermal treatment in such a way that small soft amorphous BR domains were entrapped in the rigid TPI crystalline phase. It is expected that this study could provide a simple way for the prevention of cold flow of rubber materials. © 2017 Society of Chemical Industry     

Multiple shape memory effects of trans‐1,4‐polyisoprene and low‐density polyethylene blends

A novel series of shape memory blends of trans‐1,4‐polyisoprene (TPI) and low‐density polyethylene (LDPE) were prepared using a simple physical blending method. The mechanical, thermal and shape memory properties of the blends were studied and schemes proposed to explain their dual and triple shape memory behaviors. It was found that the microstructures played an important role in the shape memory process. In TPI/LDPE blends, both the TPI crosslinking network and LDPE crystalline regions could work as fixed domains, while crystalline regions of LDPE or TPI could act as reversible domains. The shape memory behaviors were determined by the components of the fixed and reversible domains. When the blend ratio of TPI/LDPE was 50/50, the blends showed excellent dual and triple shape memory properties with both high shape fixity ratio and shape recovery ratio. © 2017 Society of Chemical Industry     

Semicrystalline thermoplastic polyimide + polymer liquid crystal blends: Nonisothermal calorimetry and thermogravimetry

Blends of a thermoplastic polyimide (TPI) and a polymer liquid crystal (PLC) were studied using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The presence of PLC enhances orientation in the system and lowers the crystallization temperature-—a manifestation of the channeling effect predicted theoretically (38). The degradation studies show high temperature stability of all blends with the degradation onset consistently above 520°C. The onset decreases with PLC concentration but reaches a plateau above 30 wt% PLC when the PLC-rich islands are formed. The amount of moisture absorbed decreases with the PLC concentration while the moisture does not affect the degradation of the samples significantly. A phase diagram is constructed for the PLC + TPI blends from the DSC and TGA data. A comparison of amorphous and semicrystalline TPI and the characterization of amorphous TPI + PLC blends will be reported later.     

Relationship between crystal structure and properties and irradiation behavior of reactor graphites

Nineteen graphites manufactured with a wide variety of raw materials and processes were characterized and correlations sought between structure and properties. The raw materials of the graphites ranged from highly isotropic cokes to anisotropic needle cokes and processing from conventional molding and extrusion to proprietary processes that produce high-strength isotropic materials. Strength, thermal expansivity, and the degree of anisotropy were found to correlate with crystallite size. The materials with relatively large mean apparent crystallite sizes $\text{L}\text{?}{}_{\mathrm{c}}$ were anisotropic and weak and had low thermal expansivity values, whereas those with smaller $\text{L}\text{?}{}_{\mathrm{c}}$ values were isotropic and strong and had large thermal expansivities. The graphites were irradiated at 625°–1625°C to fluences of up to 2 × 10²²n/cm². The only strong correlation found was between preirradiation strength and dimensional and volumetric changes. High-strength isotropic graphites with intermediate $\text{L}\text{?}{}_{\mathrm{c}}$ values were most stable with lowest initial contraction rates and lowest expansion rates after turnaround. Low-strength isotropic graphites were in the same range of stability with low-strength anisotropic graphites, except distortion was minimized due to their isotropic dimensional changes     

Study on mechanical and thermal properties of composites of cis‐ and trans‐polyisoprene blends filled with polyaniline

In this study, the effect of polyaniline (PANI) as filler has been investigated on the mechanical and thermal properties of blends of cis‐polyisoprene (CPI) and trans‐polyisoprene (TPI) by dynamic mechanical analyzer and transient plane source technique, respectively. The samples blend composites with different concentration of PANI have been prepared by solution casting method and characterized through X‐ray diffraction. Experimental results from mechanical and thermal measurements show that incorporation of PANI in CPI/TPI blends increases glass transition temperature, mechanical properties and thermal properties whereas decreases damping property. This increase in different properties is only upto a certain amount of PANI and over this amount an entirely apposite behavior is observed. This observed behavior of mechanical and thermal properties is explained on the basis of crystallinity and crosslink density. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Thermogravimetry of Thermoplastic Polyimide Powders under Four Different Atmospheres

Thermal degradation of a thermoplastic polyimide ( TPI ) fine powder was studied in nitrogen, helium, argon, and air, from room temperature to 790°C by a high‐resolution thermogravimetry (TG) at a variable heating rate in response to changes in the weight‐loss rate of the sample and also by traditional TG. In the three inert atmospheres, the high‐resolution TG found a two‐step degradation process with higher resolution for the TPI , which was hardly ever revealed by a traditional TG for the TPI and other similar polyimides. On the contrary, only a traditional TG in air observed a two‐step degradation process for the TPI . The initial thermal degradation temperature T_d, the temperatures at the maximum weight‐loss rate T_dm1 and T_dm2, the first maximum weight‐loss rate (dα/dT)_m1, as well as the degradation activation energy of the TPI all increase with the variation of testing atmosphere in the following order: in nitrogen < in helium < in argon < in air, but the char yield at 700°C appears to increase in a different order: in air < in helium < in argon < in nitrogen.     

High-temperature neutron irradiation of highly oriented carbons and graphites

Highly oriented massive pyrolytic carbons were annealed at 2900–3300°C to produce apparent crystalline heights (L_c) ranging from 220 Å to 1660 Å. Transmission electron microscopy showed that annealing progressively increased the crystallite width, while the height between twist boundaries was little changed over most of the annealing range. The pyrolytic carbons, together with samples of hot-worked polycrystalline graphite, were irradiated with fast neutrons at 1300–1500°C to 2 × 10²² n/cm² (E > 0·18 MeV). Changes in dimensions, thermal expansivities, apparent crystallite heights (L_c), and unit cell heights (C_a) were measured. At 1300–1500°C the c-axis growth and a-axis contraction of the pyrolytic carbons increased linearly with neutron fluence, while the dimensional change rates of the hot-worked graphite decreased with increasing fluence. The dimensional change rates of the annealed pyrolytic carbons decreased systematically with increasing crystal perfection. During irradiation at 1300–1375°C the thermal expansivity in the principal c-direction of as-deposited massive pyrolytic carbon increased rapidly to nearly the theoretical single-crystal value, while that of the hot-worked graphite initially increased, passed through a maximum, and then declined to less than half its initial value.     

The influence of pitch-binder coke content on the properties and irradiation behavior of molded graphite

A series of needle-coke graphites were prepared with various fractions of pitch-binder coke in the range 5.5–14.2 wt-%. The specimens were irradiated to 9.70 × 10²¹n/cm² (E > 0.18 MeV) at 1225°C. Apparent bulk density and apparent crystallite size L_c were dependent on the binder-coke content. Crystallite orientation and thermal expansivities were not affected. Dimensional and volumetric changes were sensitive to binder-coke content; specimens deficient in binder-coke showed volumetric turnaround at lower fluences and higher expansion rates at high fluences. Specimens of 5.5 wt-% binder coke expanded 20 vol-%, whereas those of 11.7 wt-% binder coke expanded only 4 vol-%. Thermal expansivity increases closely followed the volume changes and were large in specimens deficient in binder coke. Unbonded coke particles expanded at three times the rate of coke particles bonded with pitch. The needle-coke particles were restrained by the binder coke during expansion at high fluences. Large cracks formed between filler particles during expansion and were responsible for the large volume expansions.     

Thermal properties and non‐isothermal crystallization behavior of poly(trimethylene terephthalate)/poly(lactic acid) blends

Thermal properties and non‐isothermal melt‐crystallization behavior of poly(trimethylene terephthalate) (PTT)/poly(lactic acid) (PLA) blends were investigated using differential scanning calorimetry and thermogravimetric analysis. The blends exhibit single and composition‐dependent glass transition temperature, cold crystallization temperature (T_cc) and melt crystallization peak temperature (T_mc) over the entire composition range, implying miscibility between the PLA and PTT components. The T_cc values of PTT/PLA blends increase, while the T_mc values decrease with increasing PLA content, suggesting that the cold crystallization and melt crystallization of PTT are retarded by the addition of PLA. The modified Avrami model is satisfactory in describing the non‐isothermal melt crystallization of the blends, whereas the Ozawa method is not applicable to the blends. The estimated Avrami exponent of the PTT/PLA blends ranges from 3.25 to 4.11, implying that the non‐isothermal crystallization follows a spherulitic‐like crystal growth combined with a complicated growth form. The PTT/PLA blends generally exhibit inferior crystallization rate and superior activation energy compared to pure PTT at the same cooling rate. The greater the PLA content in the PTT/PLA blends, the lower the crystallization rate and the higher the activation energy. Moreover, the introduction of PTT into PLA leads to an increase in the thermal stability behavior of the resulting PTT/PLA blends. Copyright © 2011 Society of Chemical Industry     

Blends,hydrogen bonds,and orientation: Understanding the role of interactions

A review of past and present studies on orientation, rheology, and FTIR investigations on a hydrogen bond–forming polymer, poly(vinyl phenol) (PVPh), and its blends with polyethylene oxide (PEO), poly(methyl methacrylate) (PMMA), and poly(vinyl methyl ether) (PVME) is presented. Orientation is analyzed on the basis of deformation‐induced orientation and relaxation. For deformation, it is proposed from recent molecular modeling studies that orientation is similar for flexible backbone polymers of the types studied. To investigate relaxation, dynamical rheology analysis was performed previously on PVPh/PEO blends and global molecular weight between entanglement, M_e, and chain friction ζ were estimated. M_e remained close to that of the polymer forming the dominant network, a discontinuity being observed near 50 mole percent. Friction coefficient exhibited a maximum near that of the orientation function of this system. Near‐infrared measurements also showed a maximum in the number of interchain hydrogen bonds at this concentration, although broader than that of orientation or of the friction coefficient. For strongly interacting blends, it is proposed that a break in orientation behavior would be associated with the dominant network present, and therefore to M_e, whereas ζ will dictate whether orientation decreases or increases in a given network domain.     

Mechanical and thermal characterization of <Emphasis Type="Italic">cis</Emphasis>-polyisoprene and <Emphasis Type="Italic">trans</Emphasis>-polyisoprene blends

Measurements of mechanical and thermal transport properties have been made on the blends of cis-polyisoprene (CPI) and trans-polyisoprene (TPI) prepared by a solution casting method. Characterization of these blends has been done using wide angle X-ray scattering. Thermo-mechanical, mechanical, and thermal transport properties have been determined employing dynamic mechanical analyzer (DMA) and transient plane source. Storage modulus and tan δ as determined from DMA have been found to increase and decrease with the increase in TPI content, respectively. Mechanical properties such as Young’s modulus and tensile strength, as determined from strain–stress behavior of CPI/TPI blends, have been found to increase with increasing TPI content. This increase in properties has been explained on the basis of the crosslink density, calculated using theory of rubber elasticity. Thermal transport properties such as thermal conductivity, thermal diffusivity, and volumetric heat capacity are higher for all the three blends as compared to their pure components.     

Thermal properties and non‐isothermal crystallization behavior of biodegradable poly(p‐dioxanone)/poly(vinyl alcohol) blends

Blends of two biodegradable semicrystalline polymers, poly(p‐dioxanone) (PPDO) and poly(vinyl alcohol) (PVA) were prepared with different compositions. The thermal stability, phase morphology and thermal behavior of the blends were studied by using thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). From the TGA data, it can be seen that the addition of PVA improves the thermal stability of PPDO. DSC analysis showed that the glass transition temperature (T_g) and the melting temperature (T_m) of PPDO in the blends were nearly constant and equal to the values for neat PPDO, thus suggesting that PPDO and PVA are immiscible. It was found from the SEM images that the blends were phase‐separated, which was consistent with the DSC results. Additionally, non‐isothermal crystallization under controlled cooling rates was explored, and the Ozawa theory was employed to describe the non‐isothermal crystallization kinetics. Copyright © 2006 Society of Chemical Industry     

Volumetric mass transfer coefficients in an internal loop airlift reactor with low‐density particles

The influence of organic additives (propanol, benzoic acid, isoamyl alcohol and carboxymethylcellulose) on the volumetric mass transfer coefficient, k_La, in an internal loop airlift reactor with low‐density particles (nylon‐6 and polystyrene) was investigated. The k_La values increased with increase in superficial gas velocity, U_sg, and decreased with increase in solid loading. A draft tube to reactor diameter ratio, D_R/D, of 0.4 gave maximum k_La values. The addition of benzoic acid and propanol increased the k_La values owing to their coalescence inhibiting characteristics. The addition of isoamyl alcohol decreased k_La, owing to the formation of rigid bubbles and recirculation of small bubbles having a low oxygen content. The k_La values decreased with increase in the concentration of the non‐Newtonian fluid carboxymethylcellulose (CMC). The proposed correlations predicted the experimental data well. Copyright © 2006 Society of Chemical Industry