Structural relaxation of stacked ultrathin polystyrene films

The T_g depression and kinetic behavior of stacked polystyrene ultrathin films is investigated by differential scanning calorimetry (DSC) and compared with the behavior of bulk polystyrene. The fictive temperature (T_f) was measured as a function of cooling rate and as a function of aging time for aging temperatures below the nominal glass transition temperature (T_g). The stacked ultrathin films show enthalpy overshoots in DSC heating scans which are reduced in height but occur over a broader temperature range relative to the bulk response for a given change in fictive temperature. The cooling rate dependence of the limiting fictive temperature, T_f′, is also found to be higher for the stacked ultrathin film samples; the result is that the magnitude of the T_g depression between the ultrathin film sample and the bulk is inversely related to the cooling rate. We also find that the rate of physical aging of the stacked ultrathin films is comparable with the bulk when aging is performed at the same distance from T_g; however, when conducted at the same aging temperature, the ultrathin film samples show accelerated physical aging, that is, a shorter time is required to reach equilibrium for the thin films due to their depressed T_g values. The smaller distance from T_g also results in a reduced logarithmic aging rate for the thin films compared with the bulk, although this is not indicative of longer relaxation times. The DSC heating curves obtained as a function of cooling rate and aging history are modeled using the Tool-Narayanaswamy-Moynihan model of structural recovery; the stacked ultrathin film samples show lower β values than the bulk, consistent with a broader distribution of relaxation times. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2741–2753, 2008     

Subglass-transition-temperature annealing of poly(ethylene terephthalate) studied by FTIR

Using FTIR spectroscopy we have examined conformational changes in the quenched and slowly cooled amorphous PET films during physical aging process. It was observed that the amount of trans conformers for quenched sample decreased upon sub-T_g annealing. For the slowly cooled sample that corresponds to a state closer to equilibrium, the amount of trans conformers hardly decreased, but increased gradually during sub-T_g annealing process. The conformational populations of these two samples tend to be identical with annealing time. These results demonstrate that sub-T_g annealing will lead to closer interchain packing and result in the formation of new cohesional entanglements along the chains. In situ FTIR studies on the conformational changes of these samples were also carried out during heating up of the sample through the glass transition region. The results showed that incremental changes of the amount of trans conformers in Samples Q and SC were gradual, while an abrupt change of trans conformers occurred in the sub-T_g annealed samples. These results were in agreement with the formation of the interchain cohesional entanglement due to sub-T_g annealing. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 783–788, 1998     

Effects of annealing above Tg on the physical aging of quenched PLLA studied by modulated temperature FTIR

The effect of the annealing few degrees above the glass transition temperature (T_da = 62 °C) on the physical aging (T_pa = 51 °C) of amorphous quenched poly(l ‐lactide) is investigated by an implementation of variable temperature Fourier transform infrared (FTIR). By using a temperature program composed of a linear heating ramp superimposed to a temperature modulation (modulated temperature FTIR), the reversing and nonreversing intensity variation of selected bands, related to high‐energy gg and low‐energy gt conformers, is investigated. It is observed that the annealing above T _g changes irreversibly the conformation distribution of the liquid polymer. The glasses obtained from annealed and nonannealed liquids behave differently, evolving in the physical aging toward their own liquid state and retaining the memory of their original condition before the vitrification. The recovery through T _g of the relaxation occurred in the physical aging depends not only from aging conditions but also by the thermal history of the sample above the T_g. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 174–181     

Physical transitions and crystallization phenomena in poly(ethylene terephthalate) studied by microhardness

The crystallization behavior of poly(ethylene terephthalate) both with and without sodium montanate, a crystal nucleating agent, has been studied using the microhardness technique. The kinetics of crystallization from the glassy state were investigated in real time by measuring the microhardness H at different crystallization temperatures. Results are discussed in terms of the Avrami equation. Values of the Avrami exponent n of about 3 are observed for samples irrespective of nucleating agent. For samples with nucleant two crystallization ranges are observed: a first range which corresponds to a fast crystallization from nucleating agent particles and a second range which is associated with a slow self-crystallization mode. New transitions evidenced by the presence of a small maximum in H as a function of annealing time and temperature are detected at temperatures above T_g for physically aged samples. The kinetics of this transition have also been examined. It is further shown that the presence of nucleating agent induces a hardening at room temperature which is similar to the effect produced by the physical aging of the samples below T_g. Finally, it is found that aging reduces the rate of creep of the material under the indenter. © 1993 John Wiley & Sons, Inc.     

Epoxy resins (DGEBA): The curing and physical aging process

Differential scanning calorimetry (DSC) and infrared spectroscopy (IR) were used to monitor the degree of cure of partially cured epoxy resin (Epon 828/MDA) samples. The extent of cure, as determined by residual heat of reaction, concurred with that determined by monitoring the infrared radiation absorbance of the epoxide group near 916 cm^?l. The fictive temperature T_{f, g} was found to increase with the degree of cure, increasing rapidly during cure until reaching a value near the cure temperature T_c of 130°C (approximately 80% cure) where the material vitrified. The greatly reduced reaction rate during the final 20% of cure was not only a consequence of vitrification but, as revealed by infrared spectroscopy, the result of the depletion in the number of reactive epoxide groups. The endothermic peak areas and peak temperatures evident during the DSC scans were used as a measure of the extent of “physical aging” which took place during the cure of this resin, and after, fully cured samples were aged 37°C below their ultimate glass temperature for various periods of time. The rate of physical aging slowed as the temperature increment (T_t,g ? T_c) increased. Although an endothermic peak was evident after only 1 h of cure (T_{f, g} = 138.3°C), such a peak did not appear until fully cured samples were aged for 16 h or more. Enthalpy data revealed that for partially cured material, the fictive temperature T_{f, a}, reflecting physical aging, increased with curing time. In contrast, the T_{f, a}, for fully cured samples decreased with sub-T_g aging time. The characteristic jump in the heat capacity ΔC_p which occurred at the T_{f, g} decreased as curing progressed. This decrease appears to be dependent upon the rotational and vibrational degrees of freedom of the glass. Finally, a graphical method of determining the fictive temperature T_{f, a}, of partially and fully cured epoxy material from measured endothermic peak areas was developed.     

Volume and enthalpy relaxation response after combined temperature history in polycarbonate

Summary Volume and enthalpy relaxation in polycarbonate subjected to double temperature jumps in the T_g region has been analysed. It concerns both initial T_down-jump from equilibrium above T_g to consolidation temperature below T_g and fina1 T_up-jump to relaxation temperature, also below T_g. The measured H and V data after T_up-jump were compared with respect to aging time calculating (dH/dV) ratio denoted as aging bulk modulus, K_a. According this new methodology H and V relaxation response after T_up-jump demonstrates differences in relaxation responses.     

Effect of aging time on properties of acrylic impact modifier modified bisphenol A polycarbonate

Polycarbonate (PC) was blended with acrylic impact modifiers (AIMs). The effects of modifiers weight fraction on the Izod impact strength and yield strength of PC/AIM blends were investigated. The samples with 4% modifiers were aged under the T_g of PC in an air‐circulating oven, and the effects of aging time on impact strength, yield strength, modulus, elongation at break, post yield stress drop (PYSD) values, and morphology of fracture surface were investigated. The effects of aging time on the shape of stress–strain curve were also investigated. The aged samples were heat‐treated over the T_g of PC to erase the effects of physical aging. It was found that the drop of impact strength caused by physical aging can be recovered, the increment of yield strength and PYSD value caused by physical aging can only be partly recovered, and the heat‐treatment over the T_g of PC caused further increment of modulus. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2715–2724, 2005     

Changes of mechanical properties in cold-crystallized syndiotactic polypropylene during aging

Many semicrystalline polymers undergo a process of aging when they are stored at temperatures higher than their glass-transition temperature (T _g). Syndiotactic polypropylene was quenched from the melt to −40 °C, crystallized from the glassy state at 20 or 40 °C and stored at the respective temperature for different aging times up to 7200 h. A significant increase in the tensile modulus and stress at yield and a decrease in strain at yield were observed for both aging temperatures. Differential scanning calorimetry (DSC) scans of aged material showed an endothermic annealing peak 15–30 °C above the previous aging temperature, the maximum temperature and enthalpic content of which increased with aging time. The position and the shape of the melting peak were not affected by aging. Scans of the storage modulus obtained from dynamic mechanical analyser measurements indicated a softening process starting at about 20 °C above the aging temperature and correlating with the annealing peak detected by DSC. Density measurements and wide-angle X-ray scattering investigations revealed that neither the crystallinity increased significantly nor did the crystal structure change. So the observed property changes induced by aging are attributed to microstructural changes within the amorphous phase. Furthermore, it could be shown by annealing experiments carried out at 60 °C, that aging above T _g is, analogous to aging below T _g (physical aging), a thermoreversible process. Received: 18 September 2000 Accepted: 2 January 2001     

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     

Influence of local chain dynamics on diffusion of gases in polymers as determined by pulsed field gradient NMR

Diffusion of gases in polymers below the glass transition temperature, T_g, is strongly modulated by local chain dynamics. For this reason, an analysis of pulsed field gradient (PFG) nuclear magnetic resonance (NMR) diffusion measurements considering the viscoelastic behavior of polymers is proposed. Carbon‐13 PFG NMR measurements of [¹³C]O₂ diffusion in polymer films at 298 K are performed. Data obtained in polymers with T_g above (polycarbonate) and below (polyethylene) the temperature set for diffusion measurements are analyzed with a stretched exponential. The results show that the distribution of diffusion coefficients in amorphous phases below T_g is wider than that above it. Moreover, from a PFG NMR perspective, full randomization of the dynamic processes in polymers below T_g requires long diffusion times, which suggests fluctuations of local chain density on a macroscopic scale may occur. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 231–235, 2010     

Dielectric studies of the effects of thermal history on secondary relaxation in bisphenol-a-polycarbonate

The dielectric permittivity and loss of Bisphenol-A-polycarbonate (PC) was measured over the frequency range 100 Hz to 200 kHz and temperature range 77–383 K. One sub-T_g relaxation peak is observed which rapidly broadens with a decrease in temperature. This is attributed to a progressive separation of the γ and β peaks, which at high temperatures are merged to form one peak of high strength. The strength of the sub-T_g relaxations decreases on physical aging of PC but is increased if the sample is quenched from a temperature above its T_g. Slowly cooled PC has a lower strength of its sub-T_g relaxation than a quenched specimen. The thermal history of PC affects the magnitude of its sub-T_g relaxation.     

Viscoelastic characteristics of plasticized cellulose nanocomposites

The plasticization effects of cellulose diacetate composite systems including nanoparticles (montmorillonite, MMT) and plasticizers(diethyl phthalate, DEP) were investigated by the time–temperature superposition technique and viscoelastic modeling. Exhibiting the highest modulus value in the glass state, the viscoelastic modulus of the MMT nanocomposite rapidly decreased above the glass‐transition temperature (T_g). The Arrhenius‐type activation energy of pristine cellulose acetate showed the lowest value of activation energy and both DEP‐plasticized and MMT‐reinforced systems exhibited increased values of activation energy. Although the free volume fraction at the T_g decreased with the plasticizer content, it increased with the incorporation of MMT, seemingly preventing the polymer chains from being arranged in an ordered structure. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 59–65, 2005     

Comparison of enthalpy relaxation between two different molecular masses of a bisphenol-A polycarbonate

Summary The present work is an extension of an earlier study that compared the stress relaxation between two molecular masses of a bisphenol-A polycarbonate due to thermal aging. The enthalpy relaxation of the same materials has been characterized. First, by measuring the change in enthalpy loss (ΔH_a) and fictive temperature (T_f) as a function of aging temperature (T_a) ranging from -25 to 120°C, using differential scanning calorimetry. For the limited aging time of 120 h, ΔH_a and T_f changes were only appreciable for (T_g -70 K)<T_a<T_g . While the influence of molecular mass was somewhat discernible, enthalpy measurements were not as sensitive as stress relaxation tests in differentiating molecular mass effects. In a second investigation, the kinetics of enthalpy relaxation upon isothermal aging at 130°C was evaluated using the peak shift method and found to be comparable to literature values. The plot of ΔH_a as a function of log (aging time) showed two distinct regions: a brief non-linear portion (less than 1 h aging) which is followed by a linear relationship as typically reported in the literature. In contrast to the linear region, the non-linear relaxation behaviour of the poorly aged state does not appear to be dependent on molecular mass.     

Effects of thermal treatment on biaxially oriented poly (ethylene terephthalate). III. Creep behavior following various thermal histories

Elongational creep measurements were carried out on a biaxially oriented poly (ethylene terephthalate) (PET) film parallel to, orthogonal to, and at 45° to the principal optic axis. Measurements made after various thermal treatments which were intended to stabilize the physical state of the PET were shown to be ineffective. Samples were annealed at 140°C for 12 days and aged at 95°C for over 24 days before measurement without success. Thermal cycling between 41 and 91°C which was also employed to stabilize the mechanical response also failed. Significant deceleration of the creep rate caused by densification of amorphous regions of the samples during storage below the glass temperature T_g is illustrated. Because of physical aging below T_g and morphological changes occurring above T_g during the various thermal treatments and histories, time-scale shift factors were found to be not unique.     

Relationship between morphology and glass transition temperature in solvent-crystallized poly(aryl ether ketones)

The relationship between semicrystalline morphology and glass transition temperature has been investigated for solvent-crystallized poly(ether ether ketone) (PEEK) and poly(ether ketone ketone) (PEKK). Solvent-crystallized specimens of both PEEK and PEKK displayed a sizeable positive offset in T_g compared to quenched amorphous specimens as well as thermally crystallized specimens of comparable bulk crystallinity; the offset in T_g for the crystallized samples reflected the degree of constraint imposed on the amorphous segments by the crystallites. Small-angle X-ray scattering studies revealed markedly smaller crystal long periods (d) for the solvent-crystallized specimens compared to samples prepared by direct cold crystallization. The strong inverse correlation observed between T_g and interlamellar amorphous thickness (l_A) based on a simple two-phase model was in excellent agreement with data reported previously for PEEK, and indicated the existence of a unique relationship between glass transition temperature and morphology in these poly(aryl ether ketones) over a wider range of sample preparation history and lamellar structure than was previously reported. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36 : 65–73, 1998     

Sub‐glass‐transition temperature interface formation between an immiscible glass rubber pair

We used neutron reflectivity to measure the interfacial width in the immiscible system polystyrene/poly(n‐butyl methacrylate) (PS/PnBMA). Measurements were made on the same samples at temperatures ranging from below the glass‐transition temperature (T_g) of PS to slightly above. We observed significant broadening of the interface at temperatures below the T_g of PS, indicating chain mobility below the bulk T_g value. The interfacial width exhibited a plateau at a value of 20 Å in the temperature range of 365 K < T < 377 K. A control experiment involving hydrogenated and deuterated PS films (hPS/dPS) showed no such broadening over the same temperature region. The results are consistent with a reduction of the T_g of PS in the interfacial region of ～20 K. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2664–2670, 2001     

Influence of entanglements on glass transition of atactic polystyrene

Freeze‐dried samples were prepared from dilute solutions of atactic polystyrene (a‐Ps) in benzene in a concentration range of 1 × 10^?1 to 2 × 10^?5 g/mL, and their glass transition temperatures (T_g) were determined by differential scanning calorimetry. It was found that below a critical concentration (C), the T_g of samples decreases linearly with decreasing logarithmic concentration of solutions. The freeze‐dried samples were annealed at a constant temperature for various times or at various temperatures for the same period of time. The T_g of samples prepared from solutions of concentration below C was observed to shift to higher temperature with increasing annealing temperature or annealing time, finally approaching that of the bulk sample. Fourier transform infrared (FTIR) spectra of the freeze‐dried samples were determined and were compared with the bulk sample. Significant changes in line width and absorption intensity were observed in the spectra of the freeze‐dried samples, indicating that the packing of segments is in a more dilatant state. During annealing, the packing of segments in the samples gradually approaches that of the bulk sample, diminishing the differences in FTIR spectra between the freeze‐dried sample and the bulk sample. The experimental observations are discussed, and it is assumed that intrachain and interchain entanglements may play an important role in glass transition. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2243–2251, 2005     

Time–temperature-dependent behavior of a substituted poly(paraphenylene): Tensile,creep, and dynamic mechanical properties in the glassy state

The linear viscoelastic behavior of a poly(paraphenylene) with a benzoyl substituent has been examined using tensile, dynamic mechanical, and creep experiments. This amorphous polymer was shown to have a tensile modulus of 1–1.5 Msi, nearly twice that of most common engineering thermoplastics. The relaxation behavior, which is similar to that of common thermoplastics, can be described by the WLF equation. Outstanding creep resistance was observed at low temperatures, with rubbery-like behavior being exhibited as the temperature approached T_g. Physical aging was shown to interact with long-term creep, rendering time–temperature superposition invalid for predicting the long-term properties. The effect of physical aging on the creep behavior was characterized by the shift rate μ. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 70: 2971–2979, 1998     

Effect of sample preparation on the glass‐transition of thin polystyrene films

We have investigated the effect of sample preparation on the glass‐transition temperature (T_g) of thin films of polystyrene (PS). By preparing and measuring the glass‐transition temperature T_g of multilayered polymer films, we are able to assess the contribution of the spincoating process to the reduced T_g values often reported for thin PS films. We find that it is possible to determine a T_g even on the first heating cycle, and that by the third heating cycle (a total annealing time of 15 min at T = 393 K) the T_g value has reached a steady state. By comparing multilayered versus single layered films we find that the whole T_g depends only on the total film thickness, and not on the thickness of the individual layers. These results strongly suggest that the spincasting process does not contribute significantly to T_g reductions in thin polymer films. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4503–4507, 2004