Free Volume Thermal Expansion Research Articles

Thermal expansion of free volume in "classic" and regulated dimethacrylates: photocured directly and via a mask to study pillar formation.

The temperature dependence of free volume in dimethacrylates (poly2M), cured by direct irradiation (poly2M-A) or via a mask (poly2M-B), and in a thiol-based 2M sample (poly2M-co-EDDT), was investigated by positron annihilation lifetime spectroscopy (PALS) and dilatometry (DIL) to study the influence of thiol regulation on the microstructure via free volume characteristics. It was found that the free volume fraction as determined from experimental data by using the standard spherical approach for the hole shapes showed systematic differences from the analogous quantity as evaluated from the lattice-hole theory. Much better results were obtained for cylindrical holes, which expand 'anisotropically' in poly2M samples and 'isotropically' in the poly2M-co-EDDT resin. In addition, the hydrogen bond changes and the conversion of monomers in cured samples studied by near infra-red spectroscopy (NIR) revealed spectrum-structure correlations for the final cured thermosets.

Thermal Expansion of Free-Volume in Hexadecane Confined in the Silica Gel Nanopores

Temperature dependence of the free-volume of voids in hexadecane (HXD) con ned in the 3 nm silica gel pores (Develosil-30) was measured by positron annihilation lifetime spectroscopy. Di erent amount of HXD was lled into the pores of matrix. The anomalous thermal expansion was observed in the partially lled pores with extremely low lling. The thermal expansion coe cient of free-volume voids in this solidi ed HXD layer has a negative sign at temperatures below 180 K compared to highor fulllled pores of silica gel. At these temperatures, the positron annihilation lifetime spectroscopy measurements demonstrate the appearance of a new type of the free volume within HXD which has been interpreted as the cracks in the HXD layer on the inner walls of matrix cavities. The di erential scanning calorimetry method con rmed that the processes were enacted within pores and not on the outer surfaces of silica gel grains. In the sample with the extremely low lling, only the one HXD solidi cation/melting e ect connected with con ned states is manifested.

Change of structure and free volume properties of semi-crystalline poly(3-hydroxybutyrate-co-3-hydroxyvalerate) during thermal treatments by positron annihilation lifetime

The structure and free volume properties of semi-crystalline poly(3-hydroxybutyrate-co-3-hydroxy valerate) (PHBV) were investigated in this study. The structure change and conformational motion of PHBV during melting and crystallization process were discussed by in situ FTIR. The free volume within the amorphous phase and its temperature dependence in the PHBV membrane, which was prepared by the compression molding method with isothermal crystallization processes, were characterized by using positron annihilation lifetime (PAL) spectroscopy. From the lifetime parameters, the temperature dependence of free volume size, amount, distribution, and fractional free volume, and the thermal expansion of free volume and/or polymer were discussed. Furthermore, the knee temperature was first observed in the melting process of the crystallized PHBV membranes. It indicated that there was structural transition of polymer chains during melting as the corresponding results observed with in situ FTIR measurement.

Thermodynamics and structure of the ordered amorphous phase in polymers

Experimental data are analysed to show that the activation enthalpy and the structural entropy appear to follow empirical relations for structural relaxation of polymeric systems. The relations indicate that melting temperature, glass transition temperature, relaxation time, coefficient of thermal expansion of free volume are interrelated in polymers. The parameters of structural relaxations, measured by mechanical and dielectric spectroscopies, are reviewed for polyethylene, poly(4-methyl-1-pentene) and a liquid-crystalline polynorbornene derivative. The thermodynamic parameters obtained from calorimetric measurements, are reported for zero heating rate extrapolation and they are used in the empirical relation, which combines the Arrhenius and the Vogel-Fulcher formulae.

Effects of crosslinking on physical properties of phenol–formaldehyde novolac cured epoxy resins

AbstractTo clarify the relationship between crosslinking density and physical properties of phenol–formaldehyde novolac cured epxy resin and factors governing their physical properties, we studied various properties of cured resins having different crosslinking densities. The resins were prepared with various curing accelerators and raw epoxy resins having different molecular weights. We found that as the crosslinking density of a cured resin increases, glass transition temperature (Tg) rises and the relaxation time becomes longer. Furthermore, in the rubbery region, the coefficient of linear thermal expansion drops and the elastic modulus become larger, while, in the glassy region, the coefficient of linear thermal expansion, specific volume, water absorption, diffusion coefficient, and permeability all increase but the elastic modulus becomes smaller. The WLF analysis on the relaxation behaviors of typical cured resin showed that cured resin with a higher crosslinking density decreases in the fractional free volume. This behavior is completely opposite from the relationship predicted from the temperature dependency of specific volume. While the coefficient of thermal expansion of free volume decreases as the crosslinking density increases for the cured resin, it coincides well with the tendency predicted from the difference in coefficient of cubic thermal expansion in the rubbery and glassy regions of each cured resin. That the free volume obtained from WLF analysis shows a relationship opposite to the predicted free volume as based on the temperature dependency of specific volume is explained as follows: Namely, the free volume obtained from the WLF analysis is a hole free volumeVhwhich contributes to fluidity andVhdecreases with the crosslinking density. On the other hand, the free volume predicted from the specific volume is a sum of the interstitial free volumeViandVh.Viincreases with the crosslinking density and thisViincrease exceeds the decrease ofVh. Therefore, the free volume predicted from the specific volume increases with the crosslinking density. Consequently, the influence of free volume on the relationship between the crosslinking density and physical properties of cured resin can be interpreted as follows. As the crosslinking density increases on cured resins,Tgrises, the relaxation time is lengthened, and the coefficient of linear thermal expansion becomes smaller in the rubbery region because, as the crosslinking density increases,Vhdecreases. Since crosslinking density increases on cured resins, the coefficient of linear thermal expansion, water absorption, diffusion coefficient, and permeability become larger, and the elastic modulus becomes smaller in the glassy region because, as the crosslinking density increases,Viincreases and, accordingly, molecular chain packing becomes looser; i.e., the specific volume increases. © 1993 John Wiley & Sons, Inc.

Dynamic mechanical properties of concentrated polystyrene-tricresyl phosphate solutions

The components of the complex shear modulus for polystyrene-tricresyl phosphate solutions have been measured between 6 and 1,000 c.p.s. A polystyrene of broad and known molecular weight distribution (weight average molecular weight of 434,000) was used to prepare six solutions from 5% to 67% polystyrene by weight. The method of reduced variables was found to be applicable, and the temperature dependence of relaxation times of all solutions could be described by the WLF equation with a fractional free volume at the glass temperature of 0.0255 and a thermal expansion of free volume of 7 × 10 −4 deg. −1. An equation of the WLF form was derived and used to calculate the concentration dependence of relaxation times and was applied to the transition regions of the relaxation spectra. Above a concentration of 10%, the relaxation spectra were proportional to the 2.0 power of concentration rather than the first power predicted by molecular theories. The number of bonds between entanglements was estimated from minima in the loss tangent-frequency curves and found to be proportional to the −0.75 power of concentration. The components of the complex shear modulus of the 5% solution were compared with the molecular theories of Zimm and Rouse as modified for polymolecularity by Menefee and Peticolas. Good agreement was observed between the experimental storage modulus and the theoretical Rouse curve, whereas the loss modulus occupied an intermediate position between Rouse and Zimm theories.

Dynamic mechanical properties of concentrated solutions of poly- n-butyl methacrylate in diethyl phthalate

The real ( J′) and imaginary ( J″) components of the complex compliance have been measured between 10 and 2400 cycles/sec. for four solutions of a fraction of poly- n-butyl methacrylate with molecular weight 3.05 × 10 6. The solvent was diethyl phthalate. The temperature range for each polymer concentration was as follows: 30%, −49° to 0°; 40.3%, −49° to 25°; 50%, −15° to 47°; 60%, −44° to 40°C. The method of reduced variables gave superposed curves for J′ and J″ except for a low-temperature anomaly in the 60% solution and a high-temperature anomaly in each solution near the maximum in J″. The temperature dependence of relaxation times followed an equation of the WLF form in each case. The value of α f (thermal expansion of free volume derived from the WLF equation) ranged from 2.2 to 2.8 × 10 −4 deg. −1. The glass transition temperatures, T g , of the 50% and 60% solutions were determined refractometrically as −67° and −46°, respectively; f g (fractional free volume at T g from the WLF equation) was 0.024 for both. The high-temperature reduction anomaly was resolved by a treatment used earlier for several undiluted methacrylate polymers, formally equivalent to assuming a dissociation of entanglement points with increasing temperature; the apparent heat of dissociation was 2.1 kcal./mole for all four solutions. The relaxation and retardation spectra are similar in shape except for differences in time scale and in magnitude. The monomeric friction coefficient ζ 0 at 0°C. ranges from 10 −6.28 for the 30% to 10 −2.90 for the 60% solution, and is closely represented by the equation ζ 0 = 5.8 × 10 −10 e 24 c , where c is grams polymer per cubic centimeter solution. From the relative magnitudes and positions of the retardation spectra near their maxima, the average degree of polymerization between entanglement points is concluded to be proportional to c 2.3.