On the relationship between microhardness and glass transition temperature of some amorphous polymers

On the basis of microhardness (H) data measured at room temperature only for a number of polymers in the glassy state, a linear correlation between H and the glass transition temperature T_g has been found (H = 1.97T_g − 571). By means of this relationship, the deviation of the H values from the additivity law for some multicomponent and/or multiphase polymeric systems can be accounted for. The latter usually contains a liquidlike soft component and/or phase with T_g below room temperature. A completely different deformation mechanism in comparison to systems with T_g above room temperature is invoked. A novel expression for the hardness of polymers in terms of crystallinity of the single components and/or phases, the T_g values, and the mass fraction of each component is proposed. This expression permits the calculation of (i) the room‐temperature H value of amorphous polymers, mainly containing single bonds in the main chain, provided T_g is known, and of (ii) the contribution of the soft liquidlike components (phases) to the hardness of the entire multiphase system. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1413–1419, 1999     

Estimation of the parameter of the main equation of the glass transition of amorphous polymers and other amorphous substances

Parameter C from the main glass-transition equation qτ_g = C according to Nemilov’s theory has the meaning of temperature bandwidth δT_g in which the freezing of the structure of the glass-forming liquid occurs (where q is the cooling rate of a melt and δ_g is the time of structural relaxation at the glass-transition temperature). The currently used method to estimate C results in inflated values, a circumstance that is due to the assumption of the constancy of the activation energy of the glass transition in the derivation of the calculation formula. Methods of estimation of C that are in agreement with the experimental data have been considered. A calculation of the time of structural relaxation, δ_g, on the basis of the values of the parameters of the Williams–Landel–Ferry equation has been proposed.     

Anharmonicity of lattice vibrations and the fluctuation volume of amorphous substances near the glass transition temperature

The fraction of the fluctuation volume (in a model of the excited state) at the glass transition temperature depends linearly on the Gruneisen parameter, i.e., the degree of the anharmonicity of lattice vibrations in amorphous polymers and glasses.     

Surface glass transition of miscible amorphous polymers blends

The surface glass transition temperature (T _g ^surface) of the bulk samples of miscible blends formed of amorphous polystyrene (PS) and poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) has been characterised in terms of an adhesion approach we proposed recently. T _g ^surface has been measured as the temperature transition “occurrence of autoadhesion–nonoccurrence of autoadhesion” by employing a lap-shear joint mechanical testing method. The effect of the reduction in T _g ^surface with respect to the glass transition temperature of the bulk (T _g ^bulk), which had been observed earlier in pure homopolymers, has been found to exist in the blends of PS with PPO as well. The values of this effect for the blends have been compared with those for pure homopolymers, and the differences found have been discussed.     

On the molecular mobility in amorphous polymers, inorganic glasses, and amorphous metal alloys in the glass transition range

The transition of kinetic units (atoms or groups of atoms) in amorphous media from one quasi-equilibrium state to another is determined by fluctuations of both energy and entropy of the system. In the glass transition range of liquids and polymers, the entropic mechanism plays a determining role: the fluctuation of packing of particles turns out to be more important than accumulation of energy. Above the glass transition range, the energy mechanism begins to play a dominant role. The procedure that is currently used to calculate the constant for the Bartenev equation, which relates the relaxation time to the cooling rate at the glass transition temperature, leads to overestimated values. A procedure for the calculation of this parameter was proposed with allowance for the temperature dependence of the entropy of activation in the region of the liquid-glass transition. The use of this equation in the relaxation spectrometry of amorphous polymers, inorganic glasses, and amorphous metal alloys is discussed.     

Influence of the glass transition on solvent self-diffusion in amorphous polymers

The free-volume theory of diffusion is used to analyze the temperature dependence of solvent self-diffusion coefficients both above and below the glass transition temperature at concentrations removed from the pure polymer limit. The glass transition can have a pronounced effect on the temperature dependence of solvent self-diffusion coefficients at small solvent concentrations, but the theory predicts a decreased effect of the transition on the diffusion process with increasing solvent concentration.     

Correlation between glass transition temperatures and entanglement statistics in amorphous polymers

Glass transition temperatures Tg in a series of 24 linear polymers with different chemical structure and high molecular weight beyond entanglement limit are correlated in the framework of freely jointed chain model with the statistical characteristics of chain parts between entanglements.     

Crystallization and glass transition of chlorobenzene/toluene binary amorphous systems

Crystallization and glass-transition phenomena were studied for amorphous chlorobenzene (CB)/toluene (TL) binary systems as the function of composition. Samples were prepared by vapor-deposition onto cold substrates, and their structural changes due to temperature elevation were monitored with Raman scattering and light transmission. It was found that the crystallization temperature (T _c) of CB-rich amorphous samples increases as the TL concentration is increased. This is similar to the linear dependence of glass-transition temperatures (T _g) of many organic compounds on the concentration of additive. Also found was that T _c of TL-rich supercooled-liquids decrease as the CB concentration is increased. Issues related to the two kinds of T _c are discussed briefly. This revised version was published online in August 2006 with corrections to the Cover Date.     

The proper glass transition temperature of amorphous polymers on dynamic mechanical spectra

Glass transition is crucial to the thermal and dynamical properties of polymers. Thus, it is important to detect glass transition temperature (T _g) with a sensitive and proper method. Dynamic mechanical analysis (DMA) is one of the most frequently used methods to determine T _g due to its advantage of high sensibility. However, there is controversy in the past literatures to determine the proper glass transition temperature among three transition temperatures, i.e., T _g1, T _g2 and T _g3 in the dynamic mechanical spectra, which correspond to the temperature abscissa of intersect value of two tangent lines on storage modulus (E′), the peak of the loss modulus (E″) and the peak of the loss tangent (tan δ). In this work, these three transition temperatures were compared with the glass transition temperature determined by DSC (T _gDSC). Based on the discussion of different modes of molecular motion around the glass transition region, it is demonstrated that T _g1 and T _g2 have the same molecular mechanism as T _gDSC, i.e., local segmental motion which is enthalpic in nature and determines the proper glass transition temperature, while T _g3 is assigned to the transition temperature of entropic Rouse modes, thus cannot be used as the proper glass transition temperature.     

The glass transition behaviors of low-density amorphous ice films with different thicknesses

The glass transition behaviors of amorphous ice with different thicknesses are studied by determining the heat capacity of low-density amorphous ice without crystallization using first principle molecular dynamics (FP-MD) and classical MD methods. The behaviors are also studied by analyzing hydrogen-bond network, the radial distribution functions, and relationship between hydrogen bond and electronic structures. It is found that the glass transition temperature (T(g)) in the range of 90 K < T < 100 K for 4 nm amorphous ice film by FP-MD method, and 120 K < T(g) < 130 K for 8 nm amorphous ice film by MD method. Meanwhile, T(g) decreases with the decreasing thickness of amorphous ice film, which is also validated by the theoretical model.     

Creep behavior of amorphous ethylene–styrene interpolymers in the glass transition region

The viscoelastic behavior of amorphous ethylene–styrene interpolymers (ESIs) was studied in the glass transition region. The creep behavior at temperatures from 15°C below the glass transition temperature (T_g) to T_g was determined for three amorphous ESIs. These three copolymers with 62, 69, and 72 wt % styrene had glass transition temperatures of 11, 23, and 33°C, respectively, as determined by DMTA at 1 Hz. Time–temperature superposition master curves were constructed from creep curves for each polymer. The temperature dependence of the shift factors was well described by the WLF equation. Using the T_g determined by DMTA at 1 Hz as a reference temperature, C₁ and C₂ constants for the Williams, Landel, and Ferry (WLF) equation were calculated as approximately 7 and 40 K, respectively. The master curves were used to obtain the retardation time spectrum and the plateau compliance. The entanglement molecular weight obtained from the plateau compliance increased with increasing styrene content as 1,600, 1,870, and 2,040, respectively. The entanglement molecular weight of the ESIs was much closer to that of polyethylene (1,390) than to that of polystyrene (18,700); this was attributed to the unique chain microstructure of these ESIs with no styrene–styrene dyads. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2373–2382, 1999     

Heat capacity functions of polystyrene in glassy and in liquid amorphous state and glass transition

The heat capacity or the specific heat is for any crystalline, partially amorphous or completely amorphous substance or material a significant thermodynamic property. The glass transition may be regarded as the melting point of amorphous substances and materials, a transition property of an outstanding technical importance. A crucial point is the fact that the presence of a glass transition is an unequivocal proof of an amorphous content of a material. Furthermore, the change of the specific heat at the glass transition temperature enables the quantitative determination of the amorphicity on a relative or absolute level of any substance or material. The absolute determination of the amorphicity affords a calibration with a reference corresponding to the material under investigation. The crystallinity for this reference substance must be known from the preparation and or by any independent analytical method. The literature data for the specific heat and the glass transition of polystyrene were collected and evaluated. Data were found for the specific heat in literature from 10 to 470 K. The data were unified for each of the reported temperature in a mean value and the corresponding standard deviation was determined. An excellent conformity was found in the glassy state of polystyrene with standard deviations lower than 0.7%. The standard deviations above the glass transition were considerably higher.     

Self-bonding in an amorphous polymer below the glass transition: A T-peel test investigation

Films of amorphous polystyrene (PS) with a weight-average molecular weight (M_w) of 225 × 10³ g/mol were bonded in a T-peel test geometry, and the fracture energy (G) of a PS/PS interface was measured at the ambient temperature as a function of the healing time (t_h) and healing temperature (T_h). G was found to develop with (t_h)^1/2 at T_h = T_g-bulk − 33 °C (where T_g-bulk is the glass-transition temperature of the bulk sample), and log G was found to develop with 1/T_h at T_g-bulk − 43 °C ≤ T_h ≤ T_g-bulk − 23 °C. The smallest measured value of G = 1.4 J/m² was at least one order of magnitude larger than the work of adhesion required to reversibly separate the PS surfaces. These three observations indicated that the development of G at the PS/PS interface in the temperature range investigated (<T_g-bulk) was controlled by the diffusion of chain segments feasible above the glass-transition temperature of the interfacial layer, in agreement with our previous findings for fracture stress development at several polymer/polymer interfaces well below T_g-bulk. Close values of G = 8–9 J/m² were measured for the symmetric interfaces of polydisperse PS [M_w = 225 × 10³, weight-average molecular weight/number-average molecular weight (M_w/M_n) = 3] and monodisperse PS (M_w = 200 × 10³, M_w/M_n = 1.04) after healing at T_h = T_g-bulk − 33 °C for 24 h. This implies that the self-bonding of high-molecular-weight PS at such relatively low temperatures is not governed by polydispersity. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1861–1867, 2004     

On the relaxation theory of glass transition in liquids

A method for calculating the constant in the main equation of glass transition (which relates the relxation time and the cooling rate near the glass transition temperature) with consideration given to the temperature dependence of the activation energy in this region is proposed. A modification of the main glass transition equation is considered. Application of this equation to the relaxation spectrometry of amorphous polymers and inorganic glasses is discussed.     

Free volume microstructure of amorphous polymers at glass transition temperatures from positron annihilation spectroscopy data

The analysis of annihilation characteristics of ortho-positronium at conventional calorimetric glass transition temperatures for a series of amorphous polymers reveals empirical correlations of average lifetime of o-Ps , and of its product with a relative intensityI _3g with appropriateT _g ^DSC values. These trends in terms of free volume mean that both the average size of free volume hole entityv _hg and the fractional free volume grow with increasingT _g ^DSC . The results are discussed considering the chemical microstructure as well as possible mechanisms acting in glass transition. A relation is indicated between geometric and flexibility characteristics of chains and thev _hg andf _g parameters of free volume microstructure on the one side and potential motional processes responsible for solidification of the amorphous system on the other side.     

The effects of co-lyophilized polymeric additives on the glass transition temperature and crystallization of amorphous sucrose

The purpose of this study was to measure the effect of co-lyophilized polymers on the crystallization of amorphous sucrose, and to test for a possible relationship between the ability of an additive to raise theT _g of a sucrose-additive mixture, relative to theT _g of pure sucrose, and its ability to inhibit crystallization. Differential scanning calorimetry was used to measure the glass transition temperature,T _g, the non-isothermal crystallization temperature,T _c, and the induction time for crystallization,Q, of sucrose in the presence of co-lyophilized Ficoll or poly(vinylpyrrolidone) (PVP). The effect of these polymers on the crystallization of sucrose was significant as demonstrated by a marked increase inT _c, and in the induction time (Q) in the presence of relatively small amounts (1–10%) of additive. Surprisingly, small amounts of polymeric additive had no effect on theT _g of sucrose, although at higher concentrations, theT _g increased proportionally. Thus, it appears that the inhibition of sucrose crystallization by the additition of small amounts of a higher-T _g component cannot be attributed solely to changes in molecular mobility associated with an increase inT _g.